US20080179288A1 - Process for wafer backside polymer removal and wafer front side scavenger plasma - Google Patents

Abstract

A process is provided for removing polymer from a backside of a workpiece. The process includes supporting the workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed. The process further includes confining gas flow at the edge of the workpiece within a gap at the edge of the workpiece on the order of about 1% of the diameter of the chamber, the gap defining a boundary between an upper process zone containing the front side and a lower process zone containing the backside. A first plasma is generated in a lower external chamber from a polymer etch precursor gas, and an etchant by-product is introduced from the first plasma into the lower process zone. A second plasma is generated in an upper external plasma chamber from a precursor gas of a scavenger of the etchant by-product, and scavenger species are introduced from the second plasma into the upper process zone.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application claims the benefit of U.S. Provisional Application Ser. No. 60/898,645, filed Jan. 30, 2007.
BACKGROUND
• Plasma processing of a workpiece or semiconductor wafer, particularly dielectric etch plasma processing, typically employs carbon-containing process gases (e.g., fluorocarbon or fluoro-hydrocarbon gases) that enhance the etch selectivity of dielectric materials, such as silicon dioxide, relative to other materials such as silicon. These processes are used to treat the front (top) side of the wafer on which the microelectronic thin film structures are formed. The opposite (back) side of the wafer is typically unpatterned. One problem is that the carbon-containing process gases tend to form polymer precursors in the plasma, which can leave a polymer residue on the front side of the wafer and on the exposed portion of the backside of the wafer, and even some distance under the unexposed portion of the wafer backside. Such residues should be removed to avoid contamination of later processing steps. The polymer residues deposited on the wafer front side tend to be easily removed with plasma ion bombardment using appropriate chemistry. However, the wafer edge is beveled, and the curved surface on the backside of the wafer edge is also exposed and therefore susceptible to polymer deposition during plasma processing. The backside of the curved surface of the wafer edge is shadowed from ion bombardment during plasma processing so is more difficult to remove, but can be removed in an oxygen plasma at high temperature (e.g., above 300 degrees C.). Such difficult-to-remove polymer films require a post-etch polymer removal step using (for example) an oxygen-rich plasma for thorough polymer removal.
• In many applications, the plasma etch process is used to form openings (e.g., trenches or contact holes) through multiple thin films on the wafer front side. Such thin film structures can include (for example) a special carbon-containing dielectric film having an ultra-low dielectric constant (ultra low-K film). The ultra low-K film is exposed in cross-section at the side wall of each trench or contact opening formed by the etch process step. Attempting to remove the backside polymer film by heating and exposing the wafer to an oxygen-rich plasma (during a post-etch polymer removal step) will damage the ultra low-K film by removing carbon from it. In semiconductor structures having 60 nm features sizes (or smaller), such damage to the ultra-low K film is permitted only to a depth of about 3 nm beyond the exposed surface (e.g., 3 nm beyond the sidewall of the opening). In contrast, the polymer film deposited on the wafer backside edge is about 700 nm thick. It is generally difficult if not impossible to avoid damaging the ultra low-K (ULK) film beyond the permissible 3 nm depth while exposing the wafer to an oxygen-rich plasma of a sufficient density and for a sufficient time to remove 700 nm of polymer from the backside of the wafer edge or bevel. The required polymer-to-ULK film etch selectivity (over 200:1) for such a polymer removal process in general cannot be maintained reliably in conventional processes.
• In conventional plasma reactor chambers, the wafer support pedestal includes an annular collar surrounding the edge of the wafer. Such a collar tends to shield the wafer edge, but cannot be sufficiently close to the wafer edge to prevent polymer deposition on the backside of the wafer edge. This is because some finite gap between the wafer edge and the collar is required to accommodate variations in the robot wafer placement and tolerance stackup. Moreover, the wafer edge-to-collar gap tends to increase as successive wafers are etched in the chamber, since the collar is (typically) formed of a process-compatible material (e.g., quartz, silicon or silicon carbide) that is gradually etched away during plasma etch processing of successive wafers. Therefore, it has seemed inevitable that unwanted polymer is deposited on the wafer, including the backside edge of the wafer.
• The foregoing problems might be avoided by using a rich mixture of oxygen in the plasma during the initial etch process. However, this approach is not practical if the thin film structure on the wafer includes an ultra-low K film that is exposed on a sidewall of an etched opening. Such a rich oxygen mixture in the etch plasma would cause unacceptable damage to the ultra-low K film.
• There is a need for a way of removing polymer from the backside of the wafer (i.e., the backside of the wafer edge) without harming or damaging any low-K film layers in thin film structure.
SUMMARY OF THE INVENTION
• A process is provided for removing polymer from a backside of a workpiece. The process includes supporting the workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed. The process further includes confining gas flow at the edge of the workpiece within a gap at the edge of the workpiece on the order of about 1% of the diameter of the chamber, the gap defining a boundary between an upper process zone containing the front side and a lower process zone containing the backside. A first plasma is generated in a lower external chamber from a polymer etch precursor gas, and an etchant by-product is introduced from the first plasma into the lower process zone. A second plasma is generated in an upper external plasma chamber from a precursor gas of a scavenger of the etchant by-product, and scavenger species are introduced from the second plasma into the upper process zone.
• In one embodiment, the process further includes confining the upper process zone between the front side and a ceiling of the reactor to an upper process zone height on the order of about 1% of the diameter of the chamber. In certain embodiments, the process includes evacuating the upper process zone through a slit opening surrounding the upper process zone. In a related embodiment, the process includes evacuating the lower process zone through a slit opening surrounding the lower process zone near the edge of the workpiece. In one embodiment, the gap and the upper process zone height are both about 2 mm.
• In certain embodiments, the polymer etch precursor gas comprises oxygen gas and in related embodiments the scavenger precursor gas comprises hydrogen or nitrogen gas.
• In one embodiment, the process includes promoting dissociation of oxygen in the first plasma by furnishing nitrogen gas to the lower external plasma source. In a related embodiment, the process includes removing photoresist from the workpiece front side by supplementing the hydrogen gas supplied to the upper external plasma source with an oxygen-containing gas.
BRIEF DESCRIPTION OF THE DRAWINGS
• So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
• FIG. 1A depicts a backside polymer removal reactor chamber in which polymer etch species are furnished from a first external plasma source toward the backside of the wafer.
• FIGS. 1B and 1C are plan and elevational views, respectively, of an implementation of the workpiece support pedestal in the reactor ofFIG. 1A that can be used in each of the reactors described herein.
• FIG. 2 depicts a modification of the backside polymer removal reactor chamber ofFIG. 1A in which etchant scavenger species are supplied from a second external plasma source toward the front side of the wafer.
• FIG. 3 depicts another backside polymer removal reactor chamber in which a concentrated stream of hot radicals or ions are directed to the wafer backside edge from a separate plasma source that is near the wafer.
• FIG. 4 is an enlarged view of a portion of the chamber ofFIG. 3 depicting the placement of special materials to contain the concentrated stream of hot radicals or ions.
• FIG. 5 depicts a backside polymer removal process carried out with the reactor chamber ofFIG. 1A.
• FIG. 6 depicts a backside polymer removal process carried out with the reactor chamber ofFIG. 2.
• FIG. 7 depicts a group of additional steps for the process ofFIG. 6 for removing photoresist from the wafer front side.
• FIG. 8 depicts an alternative group of additional steps for the process ofFIG. 6 for removing photoresist from the wafer front side.
• FIG. 9 depicts a backside polymer removal process carried out with the reactor ofFIG. 3.
• FIG. 10 depicts an alternative process carried out in the reactor ofFIG. 2 for simultaneously removing backside polymer and removing frontside photoresist from the wafer.
• FIG. 11 depicts a modification of the reactor ofFIG. 3 in which the external plasma source of the plasma stream is replaced by an internal inductively coupled source.
• FIG. 12 depicts a modification of the reactor ofFIG. 11, in which the internal inductively coupled source is replaced by an internal capacitively coupled source.
• FIG. 13 depicts an alternative approach in which a ring plasma is generated beneath the wafer backside edge by an inductively coupled source.
• FIG. 14 depicts a modification of the reactor ofFIG. 13 in which the inductively coupled source is replaced by an internal capacitively coupled source electrode for generating the ring plasma.
• FIG. 15 depicts a modification of the reactor ofFIG. 14 in which the internal capacitively coupled source electrode is replaced by an external capacitively coupled source electrode.
• FIG. 16 depicts a feature of the ceiling for shielding the wafer front side during backside polymer removal.
• FIG. 17 is a block diagram of a process for backside polymer removal and front side photoresist strip involving temperature switching.
• FIG. 18 illustrates a first reactor adapted to perform the process ofFIG. 17.
• FIG. 19 illustrates a second reactor adapted to perform the process ofFIG. 19.
• FIGS. 20 and 21 depict a modification of the reactor ofFIG. 2.
• To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The drawings in the figures are all schematic and not to scale.
DETAILED DESCRIPTION OF THE INVENTION
• Exemplary embodiments of the invention pertain to removing polymer from the backside edge of a wafer without damaging critical films, such as an ultra low-K dielectric film, by heating the wafer in a chamber while exposing only the backside of the wafer to polymer etch radicals or plasma by-products, such as atomic or free oxygen, from an external plasma source. The oxygen radicals may be provided by an external plasma source which is supplied with an oxygen-containing gas or vapor, such as O2, H2O, N2O, CO2, or CO, for example. The oxygen-containing gas may be combined or diluted with other gases such as H2, N2 or Ar. Other fluorine-containing gases (such as CF4 or NF3) may be added to allow removal of polymer films that contain other materials (such as Si) and are not etched efficiently in O chemistry alone. The critical films in the thin film structure on the wafer front side are protected from damage by the polymer etch species by pumping purge gases across the wafer front side. In addition, the wafer edge and the chamber side wall are separated by a very narrow gap to define a lower process zone with the wafer backside and an upper process zone with the wafer front side. The narrow of the gap is configured to resist or minimize migration of polymer etch species from the lower to the upper process zone where it would attack the ultra low-K film on the wafer front side. The external plasma source is coupled to the lower process zone so that the polymer etch species are delivered to the wafer back side. Delivery rate and residence time of polymer etch species are minimized in the upper process zone by restricting the height of the upper process region to a very narrow gap between the wafer and the chamber ceiling. The purge gases pumped across the wafer front side may be inert or non-reactive. Such purging reduces the etch rate of the front side critical films relative to the backside polymer removal rate.
• In one embodiment, to further reduce the etch rate of the critical films relative to the backside polymer removal rate, the purge gases may be supplemented with or replaced by reactive scavenger gases that chemically scavenge the backside polymer etch species in the upper process region.
• In another embodiment, a further increase in the backside polymer etch rate is attained by employing a second (upper) external plasma source coupled to the upper process zone. Gases that are precursors of species that scavenge the polymer etchant are introduced into the upper external plasma source, to produce scavenger radicals for the upper process zone that reduce the amount (partial pressure) of polymer etchant species in the upper process zone. In one embodiment, pressure is maintained sufficiently low pressure in the upper process zone to enable the upper external plasma source to generate plasma efficiently, while simultaneously achieving a sufficiently high flow rate of scavenger species to protect the wafer front side thin films. In another embodiment, unused polymer etch species are removed from the lower process zone before they migrate to the upper process zone. In one embodiment, the upper and lower process zones are separately evacuated near the wafer edge through separate pumping evacuation ports in the sidewall near the wafer edge. In addition, the scavenger species from the upper external plasma source may be heated before they enter the upper process zone.
• The scavenger species furnished by the upper external plasma source that removes the polymer etch species from the upper process zone (e.g., hydrogen) may also serve to remove photoresist from the wafer front side. In such a case, photoresist removal may be performed in a separate step in which no polymer etch species are introduced into the lower process zone and the wafer-to-ceiling gap (height of the upper process zone) is increased. In addition, an agent gas (e.g., nitrogen) that promotes the etching of photoresist from the wafer frontside may be furnished in a small quantity to the upper external plasma source. In an alternative mode, none of the front side thin film layers is susceptible to damage by the polymer etch species, and the steps of backside polymer etch removal and front side photoresist removal are performed simultaneously using the upper and lower external plasma sources. In this case, the height of the upper process zone is increased by lowering the wafer support pedestal.
• In one embodiment, an increase in polymer etch rate relative to etch rate of the critical (ultra low-K) film by the polymer etch species is achieved by locating the lower external plasma source very close to the wafer edge and directing a concentrated stream or jet of plasma products from the lower external plasma source directly at the wafer backside edge while rotating the wafer. By reducing the lower external plasma source pressure, the concentrated stream may consist of polymer etchant ions, radicals and neutrals, while at a higher pressure the stream consists of etchant radicals and neutrals.
• Referring now toFIG. 1A, a plasma reactor for removing polymer residue from the backside of a semiconductor wafer includes areactor chamber100 having aside wall102, aceiling104 that is a gas distribution plate and afloor106. The ceiling orgas distribution plate104 has aninterior gas manifold108 and pluralgas injection orifices110 opening from the manifold108 into the interior of thechamber100. Awafer support pedestal112 in the form of a disk-shaped table has a diameter less than the diameter of a workpiece to be supported on thepedestal112, so as to expose the backside peripheral annulus of the workpiece. Thepedestal112 is supported on alift member114 elevated and lowered by alift actuator116. A workpiece such assemiconductor wafer118 can be supported with a center portion of its backside resting on thepedestal112. The front side of the wafer118 (the side on which the microelectronic thin film structures are formed) faces the ceilinggas distribution plate104. Thepedestal112 is sufficiently small to leave an annular periphery of the wafer backside exposed, for backside polymer removal. Often, thewafer118 has a rounded or beveled edge as depicted inFIG. 1A. Such a beveled feature may make it difficult to avoid polymer deposition on the wafer backside during plasma (e.g., etch) processing of thin films on the wafer front side. In one embodiment,radial arms113 are provided to position theworkpiece118 on thepedestal112. In one embodiment, three symmetrically spaced thinradial arms113 shown inFIG. 1B are provided and extended outwardly from the periphery of thepedestal112. As shown inFIG. 1C, theradial arms113 are below the workpiece support surface of thepedestal112 so as to leave the entire peripheral annulus of the workpiece backside exposed for backside polymer removal. Eachradial arm113 supports a thinaxial tab113aat its distal end, thetabs113aserving to locate theworkpiece118 in coaxial alignment with the pedestal.
• Thechamber side wall102 can include a removable liner orprocess kit120. Hereafter, theterm sidewall102 is used to include aliner120 if one is present. Agap122 between thewafer edge118aand theside wall102 is very small, e.g., about 0.2-2 mm, so as to resist migration of gases through thegap122. The gap is configured to be sufficiently narrow to present a gas flow resistance that is on the order of about one hundred times greater than gas flow resistance in other portions of the chamber. Thegap122 may be on the order of about 1% of the chamber diameter. In this way, thewafer118 divides thechamber100 into anupper process zone130 bounded in part by the front side or top surface of thewafer118 and alower process zone132 bounded in part by the backside or bottom surface of thewafer118. A bottomexternal plasma source134 receives a polymer etch precursor gas from agas supply136 and furnishes polymer etch radicals (e.g., oxygen radicals or atomic oxygen) into thelower process zone132 through aport138 in thechamber floor106.
• Some polymer etchant (e.g., oxygen) radicals can migrate from thelower process zone132 into theupper process zone130 through thegap122 and pose a risk of damage to critical layers on the wafer front side, such an ultra low-K thin film. In order to prevent this, a non-reactive purge gas, namely a gas that does not react with the thin film materials on the wafer front side, (e.g., nitrogen gas or argon gas) is supplied to the ceilinggas distribution plate104 from agas supply140 in order to flush out theupper process zone130 and keep it free of etchant species. In order to facilitate a thorough and fast purge of theupper process zone130, theupper process zone130 is restricted to a very small height corresponding to a small wafer-to-ceiling gap144, e.g., about 0.2-2 mm. Thegap144 may be sufficiently small to confine the cross-section of theupper zone130 to an aspect ratio greater than 100. The upper process zone height (gap144) is sufficiently small so that residence time of gas in theupper process zone130 is less than about one tenth to one hundredth of the gas residence time in thelower process zone132. Also, the upper process zone height (gap144) is sufficiently small so that gas flow resistance through thegap144 is on the order of 100 times the gas flow resistance through thelower process zone132. Such confinement of the upper process zone height may be accomplished by raising thewafer support pedestal112 with thelift actuator116.
• In one embodiment, pressure in thechamber100 is controlled by avacuum pump146 that maintains thelower process zone132 at a sufficiently low pressure to draw plasma by-products out of the lowerexternal plasma source134 and maintain theexternal plasma source134 at a sufficiently low pressure to enable it to efficiently generate plasma. Alternatively, the upper andlower process zones130,132 may be evacuated separately through separate slit openings near the wafer edge byseparate pumps210,216. In this case, thevacuum pump146 may not be necessary.
• In one embodiment, the polymer removal process is quickened by heating thewafer118 to on the order of 300 degrees C., for example, either by anelectrical heating element150 inside thepedestal112 or by radiant lamps (not shown). An electricalheater power supply152 is coupled to theheating element150 through wires in thelift member114. By raising the wafer temperature to about 300 degrees C., the backside polymer removal rate is significantly increased.
• In one embodiment, etching of films (e.g., an ultra low-K film) on the wafer front side is minimized or eliminated by maintaining a very high flow rate of a non-reactive purge gas (e.g., nitrogen or argon) through the ceilinggas distribution plate104 into theupper process zone130. This improves the polymer etch selectivity, i.e., the ratio of the polymer etch rate to the ultra low-K film etch rate. The purge gas flow rate may be as high as necessary to achieve a desired etch selectivity, raising the pressure of theupper process zone130 to a very high level. The pressure in thelower process zone132 is maintained at a sufficiently low level (e.g., a few Torr or less) to ensure efficient operation of theexternal plasma source134. In order for theexternal plasma source134 to generate a plasma, the chamber interior pressure of theexternal plasma source134 typically should not exceed a few Torr, and because theexternal plasma source134 is coupled directly to thelower process zone132, the pressure in thelower process zone132 should be maintained at a correspondingly low level. This requirement is met by the main chamber vacuum pump146 (or by the vacuum pump216) regardless of the high flow rate of purge gas into theupper process zone130 through thegas distribution plate104. This permits the purge gas flow rate and upper process zone pressure to be as great as necessary to eliminate or minimize etching of any ultra low-K film on the wafer front side.
• In one embodiment, to further increase the rate of polymer removal from the wafer backside, a dissociation agent gas (e.g., nitrogen) that promotes dissociation of the polymer etch precursor gas (e.g., oxygen) may be supplied at a low flow rate to theexternal plasma source134 from agas supply156.
• In one embodiment, to further reduce the amount of polymer etch species (e.g., oxygen) in theupper process zone130, agas supply158 furnishes an etchant scavenger gas (e.g., hydrogen or carbon monoxide) to the ceilinggas distribution plate104. This may be instead of or in addition to the non-reactive purge gas from thegas supply140. Some polymer etchant (e.g., oxygen) atoms or molecules that have migrated into theupper process zone130 are chemically consumed by combining with the scavenger gas (e.g., H2 or CO). Optionally, this scavenger reaction may be accelerated by heating the scavenger gases furnished to thegas distribution plate104 with anelectrical heater159. If the polymer etchant precursor gas is oxygen, then the scavenger gas may be carbon monoxide or hydrogen. Carbon monoxide is less reactive with a carbon-containing ultra low-K film than the oxygen gas that it scavenges from theupper process zone130. Hydrogen gas may be a good choice for the scavenger gas because it may not deplete carbon from the carbon-containing ultra low-K film, and therefore fulfills a requirement of being less reactive with the ULK film than the polymer etch species (oxygen) that it scavenges. The scavenger gas is selected so that the product of the chemical reaction between the scavenger and the polymer etchant (e.g., oxygen) does not react at a high rate with the ultra low-K film. In the case of a hydrogen scavenger and oxygen as the polymer etchant, the product is water and in the case of a carbon monoxide scavenger, the product is carbon dioxide, satisfying the requirement of a scavenging reaction product that is safe for the ultra low-K film.
• In an optional mode, the reactor ofFIG. 1A is used to remove photoresist from the wafer front side. In this mode, thewafer pedestal112 may be lowered to the dashed-line position ofFIG. 1A, to enlarge theupper process zone130 with a wafer-to-ceiling gap of about 0.5 to 5 cm. If none of the thin film materials on the wafer front side include a ULK film or other material susceptible to damage from oxygen, then the oxygen radicals from thelower process zone132 are permitted to migrate into theupper process zone130 by halting the purge gas flow from thegas supply140 and/or the scavenger gas flow from thegas supply158. Also, such migration may be enhanced if the sidewall-wafer gap122 is greater at the lowered (dashed-line) wafer position. In this optional mode, the backside polymer and the frontside photoresist are removed simultaneously.
• FIG. 2 depicts a modification of the reactor ofFIG. 1A. The reactor ofFIG. 2 may be particularly advantageous in providing even greater protection for ultra low-K films on the wafer front side during backside polymer removal. In the reactor ofFIG. 2, a secondexternal plasma source200 is provided. The amount of polymer etchant species (e.g., oxygen) in theupper process zone130 is reduced more efficiently because the second (upper)external plasma source200 provides plasma by-products (e.g., radicals) of a scavenger species (e.g., hydrogen or nitrogen) to the ceilinggas distribution plate104. The scavenger species radicals (hydrogen or nitrogen) chemically scavenge or combine with some polymer etchant species (e.g., oxygen) that may migrate into theupper process zone130. Such radicals tend to have a faster rate of reaction with the etchant species (than the molecular gas scavenger of the reactor ofFIG. 1A) and therefore provide a higher rate of removal of etchant species (e.g., oxygen) from theupper process zone130. This provides superior protection from attack upon thin film structures (e.g., an ultra low-K film) on the wafer front side. Agas supply202 furnishes a molecular gas form of a scavenger precursor (e.g., hydrogen or nitrogen gas) to the chamber of the topexternal plasma source200. By-products of the plasma generated by the source200 (e.g., either hydrogen radicals or nitrogen radicals) are scavengers of the polymer etch species (e.g., oxygen), and are delivered to thegas distribution plate104 to reduce or eliminate oxygen from theupper process zone130 and thereby protect the thin film structures on the wafer front side.
• As depicted inFIG. 2, the topexternal plasma source200 may consist of a dielectric (e.g., quartz)tube chamber204 encircled by anRF coil antenna206 that is driven by an RF plasmasource power generator208 through animpedance match element209. The quartz material is compatible with the hydrogen or nitrogen chemistry of the topexternal plasma source200. The topexternal plasma source200 produces radicals of the scavenger species that are fed to the ceilinggas distribution plate104 through acenter port212. In order to provide uniform distribution of the scavenger species across the gas distribution plate, abaffle214 is provided in the center of thegas manifold108 that blocks direct gas flow from thecenter port212 togas injection orifices110 near the center of thegas distribution plate104. A toroidal plasma chamber may be used for either (or both) the top and bottomexternal plasma sources200,134. Such a toroidal chamber consists of reentrant conduit of a conductive material. To accommodate the hydrogen chemistry used in the topexternal plasma source200, such a toroidal plasma chamber may include an insulating liner protecting the conductive chamber or conduit.
• The chamber pressure within the topexternal plasma source200 should be sufficiently low (e.g., not exceeding several Torr) to ensure efficient plasma generation within theexternal chamber204. Since the topexternal plasma source200 is coupled to the upper process zone130 (through the gas distribution plate104), the upper process zone pressure cannot be too high without extinguishing plasma in theexternal source200. Meeting this limitation may prevent a sufficiently high flow rate of scavenger species into theupper process zone130 necessary to protect the wafer front side. In one embodiment, an upperzone vacuum pump210 is coupled directly to theupper process zone130 through an upper zone vacuum slitpassage217 that is near (but slightly above, by a few mm or less) the wafer edge and passes through the side wall102 (andliner120 if present). The upperzone vacuum pump210 facilitates or ensures a sufficient flow rate through the very thin wafer-sidewall gap122. With this feature, themain vacuum pump146 may be eliminated in the reactor ofFIG. 2, as will be discussed below. In one embodiment, the pumping rate of theupper zone pump210 is maintained at a sufficient level to keep the pressure in theupper process zone130 below a few Torr, for example. This permits a high flow rate of radicals from the topexternal plasma source200 and ensures quick removal of scavenger-etchant reaction by-products from theupper process zone130. The present embodiment provides a low chamber pressure inside the topexternal plasma source200 to facilitate efficient plasma generation within the topexternal plasma source200. The present embodiment also reduces the migration of scavenger or purge species into thelower process zone132 that would otherwise dilute the polymer etch species (e.g., oxygen) at the wafer backside.
• In one embodiment, to reduce migration of polymer etchant species (e.g., oxygen) from thelower process zone132 through the wafer-sidewall gap122 into theupper process zone130, a lowerzone vacuum pump216 is coupled to thelower process zone132 through a lowerzone slit passage218 that is near (but slightly below, by a few mm or less) the wafer edge. The upper andlower slit passages217,218, are within a few (or several) mm of each other along the rotational axis of symmetry of the chamber. In the illustrated reactor ofFIG. 2, both slitpassages217,218 are at heights above and below (respectively) the wafer by about 1 mm, although this distance may be in a range of about 0.5 to 2 mm, for example. Theslit passage217,218 may be axially displaced from one another by about 1-2 mm. Generally the distance is less than the wafer-to-ceiling gap (the height of the upper process zone130). The upper and lowerzone vacuum pumps210,216 operate simultaneously to remove throughseparate slit passages217,218 etchant-scavenger reaction by-products (from theupper zone130 through the slit passage217) and etchant species and etchant-polymer reaction by-products (from thelower zone132 through the slit passage218).
• Theslit openings217,218 have a narrow (e.g., 0.2-2 mm) axial height and extend around at least nearly the entire circumference of theside wall102. Theslit openings217,218 are each completely enclosed except for their connections to therespective pumps210,216.
• In one embodiment, with the upper andlower vacuum pumps210,216 providing optimum performance, themain vacuum pump146 is eliminated in the reactor ofFIG. 2. Providing the upper andlower vacuum pumps210,216, increases the flux of etchant species to the wafer backside.
• The upper andlower vacuum pumps210,216 and theirslit passages217,218 may also be included in the reactor ofFIG. 1A, although they may not be required in the reactor ofFIG. 1A because of the lack of an upperexternal plasma source200 inFIG. 1A. Because the upperexternal plasma source200 is not present in the reactor ofFIG. 1A, the purge gases may be pumped through thegas distribution plate104 at very high pressure to protect the wafer front side. Therefore, thelocal pumps210,216 and theirslit passages217,218 are not necessarily required in the reactor ofFIG. 1A.
• Various kinds of plasma sources may be used for the upper and lowerexternal plasma sources134,200, such as microwave, conventional ICP or toroidal. The process chemistries that are to be used in the upper andlower sources134,200 limit the choice of materials. Toroidal reactors typically have metallic chambers or conduits, such as anodized aluminum, which is incompatible with the hydrogen chemistry of theupper plasma source200. However, toroidal plasma sources are also available with quartz liners or quartz toroidal shaped (round or square) vacuum vessels. If non-metal, non-coated metal, and non-quartz material is required for compatability with the plasma chemistry, then the choice of external plasma source may be more limited to conventional inductively coupled plasma sources, such as a quartz, alumina, sapphire or Yittria tube wrapped with an RF-driven coil, for example. Sources may also be electrostatically shielded to reduce plasma ion bombardment and subsequent erosion or particle/contamination issues. In one example, the lowerexternal plasma source134 may be a toroidal plasma source, consisting of atoroidal chamber220 fed by theprocess gas supply136, a coiledRF power applicator222 coupled to thetoroidal chamber220 and apassage224 from thetoroidal chamber220 to theport138. Thecoil222 may be driven by an RF generator through an impedance match, or may simply driven by a switched power supply (We need to discuss). Thetoroidal chamber220 is typically formed of metal with a dielectric external film, such as anodized aluminum, which is compatible with the oxygen and nitrogen gases employed in the lowerexternal plasma source134. Because the upperexternal plasma source200 is supplied with hydrogen gas, anodized aluminum is not a practical material for theupper source200, and therefore itschamber204, in one example, is formed of another material (such as quartz) that is compatible with hydrogen.
• In an optional mode, the reactor ofFIG. 2 is used to remove photoresist from the wafer front side. In this application, oxygen (polymer etchant) flow from thegas supply136 is halted (or the plasma in thelower source134 is extinguished). Preferably, this step is carried out with the height of theupper process zone130 being in the narrow regime (0.2-2 mm) discussed above, to enhance the photoresist removal rate. Alternatively, uniformity of the photoresist removal may be enhanced by increasing the upper process zone height, in which case thewafer pedestal112 is lowered to the dashed-line position ofFIG. 2, to enlarge theupper process zone130 with a wafer-to-ceiling gap of about 2.5 to 5 cm. Hydrogen radicals or related plasma by-products from the upperexternal plasma source200 fill theupper process zone130 and remove photoresist from the wafer front side in a reactive etch process. This reaction is promoted by supplementing the hydrogen gas flow to the upperexternal plasma source200 with an oxygen-containing gas (H2O or N2O) at a lower flow rate from anothergas supply240. The flow rate of the oxygen-containing gas into theupper plasma source200 may be less than 5% of the hydrogen flow rate. This photoresist removal step may be performed before or after the backside polymer removal step.
• In an alternative embodiment of the optional frontside photoresist removal mode, a capacitively coupled plasma is generated from the hydrogen in theupper process zone130 by anRF power generator250 coupled through animpedance match252 across the ceilinggas distribution plate104 and the wafer support pedestal112 (in its lowered dashed-line position ofFIG. 2). In this embodiment, hydrogen ions are generated in theupper process zone130 to conduct reactive ion etching of the photoresist on the wafer front side.
• The initial placement of the reactor ofFIG. 2 in a plasma etch system may require replacing one of two single wafer load locks normally present in such a system with the reactor ofFIG. 2. An etch system typically includes four plasma etch reactors, two single wafer load locks and a factory interface. For greater versatility, the reactor ofFIG. 2 may be configured to perform the functions of the single wafer load lock that it replaces in the plasma etch system. For this purpose, wafer ingress/egress slitvalves270,272 are provided on opposite sides of the reactor through the side wall102 (and liner120). The pair ofslit valves270,272 enables the reactor ofFIG. 2 to function as a single wafer load lock.
• Referring toFIG. 3, in one embodiment, a localized stream or jet of plasma radicals, neutrals, and ions of an etchant species is used to provide an even higher rate of backside polymer removal. The stream or jet of plasma species is directed onto a small target area or window of the wafer backside edge while rotating the wafer. For this purpose, a localexternal plasma source300 may be located near the wafer edge, and ashort conduit302 directs plasma ions from the interior of the localexternal plasma source300 as a localized stream of plasma ions, radicals and neutrals to the small target region at the wafer backside edge. In one embodiment, theconduit302 is sufficiently short and its output end is sufficiently close to the wafer backside to enable to enable ions from thesource300 to reach the wafer backside. For example, the transit distance between the output end of the short conduit and the wafer support plane of thepedestal112 may be 5% or less of the pedestal or wafer diameter. Agas supply136 furnishes a polymer etch precursor gas to the localexternal plasma source300. In one embodiment, to expose the entire backside periphery or edge of the wafer to the localized plasma stream, thewafer pedestal112 is rotated by arotation actuator304 coupled to the support member orleg114 of thepedestal112. By operating the localexternal plasma source300 at a low chamber pressure, it becomes a rich source of plasma ions and electrons, and the concentrated stream from theconduit302 consists of a large proportion of ions in an ion/radical mixture. By placing theexternal plasma source300 close thewafer118 and keeping theconduit302 short, ion loss through recombination is minimized, and the particle stream emanating from theconduit302 remains rich in ions.
• In one embodiment, the backside polymer etch rate is increased by the ion jet stream in the reactor ofFIG. 3; the ion energy at the wafer backside edge surface may be increased by applying RF bias power between the localexternal plasma source300 and the wafer support pedestal. For this purpose, an RFbias power generator310 is coupled through animpedance match element312 across the wafer support pedestal and the localexternal plasma source300. Agas supply156 can furnish to the local external plasma source300 a dissociation agent gas (e.g., nitrogen) that promotes the dissociation of the etchant species (e.g., oxygen) in the plasma of the local external plasma source3.00.
• If a radical stream is desired rather than an ion stream, then the chamber pressure in the local external plasma source may be increased. Raising the chamber pressure of the localexternal plasma source300 reduces the proportion of ions and raises the proportion of radicals in the stream of particles ejected by theconduit302. Furthermore, if a purely radical stream is required, then it is not necessary to locate theexternal plasma source300 near the wafer. Instead, it may be located (for example) near the bottom of the main chamber100 (as indicated in dashed line inFIG. 3) and theconduit302 may be relatively long (as shown in dashed line inFIG. 3). The concentrated jet stream from theplasma source300 can be very hot (e.g., as much as 600 degrees C.), and this heat can expedite the reaction between the etch species and the backside polymer. Preferably, the entire wafer is initially heated to about 300 degrees C. before exposing the backside to the concentrated radical or ion stream from thelocal plasma source300 andconduit302.
• Referring toFIG. 4, special precautions may be taken to minimize metal contamination from the ion stream emanating from theplasma source300/conduit302. Specifically, metal surfaces can be protected with a dielectric (e.g., quartz)liner320 that covers the bottom surface of the ceilinggas distribution plate104 and adielectric liner120 that covers the interior surface of theside wall102. The enlarged view ofFIG. 4 shows how each slitopening217,218 may open into a larger passage, but is completely enclosed within the chamber wall except for the connection to therespective pump210,216.
• FIG. 5 depicts an exemplary method that can be carried out in the reactor ofFIG. 1A. A first step (block402) is to support the wafer (using the pedestal112) so as to expose a peripheral portion of the wafer back side, while beginning to heat the wafer to on the order of 300 degrees C. A next step (block404), which may begin before the final wafer temperature (e.g., 300 degrees C.) is reached, is to define anupper process zone130 above the wafer front side and alower process zone132 below the wafer back side with minimal migration of gas between the two zones by maintaining a wafer-to-sidewall gap at less than 2 mm. This gap should be sufficiently small to produce a gas flow resistance that exceeds that of other portions of the chamber by a factor on the order of 100. A further step (block406) is to prevent accumulation of etch species at the wafer front side, or (equivalently) facilitate fast evacuation of theupper zone130, by maintaining a wafer-to-ceiling gap (the height of the upper process zone130) at a value at which a high gas flow resistance is established, e.g., less than 2 mm. This gap should be sufficiently small to confine theupper process zone130 to a cross-sectional aspect ratio greater than on the order of 100. Another step (block408) is to generate a plasma in anexternal plasma chamber134 with a polymer etchant precursor gas (e.g., oxygen), and introduce by-products (e.g., radicals, free oxygen) from the plasma into thelower process zone132 so as to etch polymer from the wafer back side. A related step (block410) is to enhance dissociation of the polymer etchant precursor species (oxygen) by introducing a dissociation agent (nitrogen gas) into theexternal plasma chamber134. In order to avoid or minimize etching thin films on the wafer front side, a further step (block412) consists of reducing the amount of polymer etchant precursor species (oxygen) in theupper process zone130 by injecting a purge gas (e.g., N2 or Ar) into theupper process zone130. A related step (block414) consists of further reducing etchant species in theupper process zone130 by introducing into the upper process zone a scavenger gas (e.g., H2 or CO) that scavenges the etch species (e.g., oxygen). The scavenger gas may be used in addition to or instead of the non-reactive purge gas.
• FIG. 6 depicts an exemplary method that can be carried out in the reactor ofFIG. 2. A first step (block416) is to support the wafer on thepedestal112 so as to expose a peripheral portion of the wafer back side while heating the wafer to on the order of 300 degrees C. A next step (block418) is to define anupper process zone130 above the wafer front side and alower process zone132 below the wafer back side with minimal migration of gas between the two zones by maintaining a wafer-to-sidewall gap at less than 2 mm. A further step (block420) is to prevent accumulation of etch species or plasma at the wafer front side by maintaining a wafer-to-ceiling gap (the height of the upper process zone) at less than 2 mm. Another (block422) step is to generate a first plasma in a lowerexternal plasma chamber134 with a polymer etchant precursor gas (e.g., oxygen), and introduce by-products (e.g., radicals, free oxygen) from the plasma into thelower process zone132 so as to etch polymer from the wafer back side. A related step (block424) is to enhance dissociation of the polymer etchant precursor species (oxygen) by introducing a dissociation agent (nitrogen gas) into the lower external plasma chamber, at a flow rate of 1-10% of the oxygen gas flow rate. Another step (block426) is to generate a second plasma in an upperexternal plasma chamber200 with a scavenger species (H2 or N2) that scavenges the polymer etch species, and introduce by-products of the second plasma (H radicals or N radicals) into theupper process zone130. In order to reduce or eliminate etching of thin films on the wafer front side, a further step (block428) is to evacuate theupper process zone130 at a pumpingport217 near the wafer edge to remove polymer etchant species (oxygen) from the upper process zone at a sufficiently high rate to avoid damage of critical (carbon-containing or low-k) films on the wafer front side. A related step (block430) is to evacuate thelower process zone132 at a pumpingport218 near the edge of the wafer at a sufficiently high rate to minimize migration of polymer etchant species (oxygen) from thelower process zone132 into theupper process zone130 and to maximize delivery of polymer etchant species to the backside edge of the wafer. For maximum delivery of polymer etch species to the wafer backside edge, only the front side and backside pumps210,216, are used, themain pump146 being eliminated or unused.
• FIG. 7 depicts an exemplary method carried out in an optional mode of the reactor ofFIG. 2, in which the reactor is employed to etch photoresist from the wafer front side. A first step (block432) is to stop the flow of polymer etchant species (oxygen) from the lowerexternal plasma source134 to the lower process zone. Although it is preferable to continue to restrict the height of theupper process zone130, optionally this height may be increased in preparation for the frontside photoresist removal step, in which case the next (optional) step (block434) is to increase the wafer-to-ceiling gap to a distance (e.g., 0.5-5 cm) at which etchant species can accumulate in theupper process zone130. However, it is not necessarily required to increase the upper process zone height in order to perform photoresist stripping on the wafer front side. The next step (block436) is to introduce at a reduced flow rate an oxygen-containing species (H2O or N2O) along with the H2 gas into the upper external plasma source200 (at a flow rate of less than 1-10% of the hydrogen gas flow rate) so as to enhance the etch rate of photoresist from wafer front side. The process ofFIG. 7 may be performed either before or after the process ofFIG. 6. The removal rate of photoresist in this step is enhanced if the height of theupper process zone130 is restricted to the narrow (0.2-2 mm) range. In the other hand, uniformity is enhanced by increasing this height, and the step ofblock434 may only increase the upper process zone height by a fractional amount.
• FIG. 8 depicts an alternative method for the reactor ofFIG. 2 to etch photoresist from the wafer front side, in which a capacitively coupled plasma is generated in theupper process zone130. A first step (block438) is to stop the flow of polymer etchant species (oxygen) from the lowerexternal plasma chamber134 to the lower process zone, and then (block440) increase the wafer-to-ceiling gap to about 2 to 5 cm. A next step (block442) is to introduce a photoresist removal species gas (H2) into theupper process zone130. A further step (block444) is to introduce at a reduced flow rate an oxygen-containing species (H2O or N2O) into the upper process region. This reduced flow rate may be about 1-10% of the hydrogen gas flow rate. A next step (block446) is to apply RF power into the upper process zone to produce a plasma that removes photoresist from the wafer front side.
• FIG. 9 depicts an exemplary method that can be carried out in the reactor ofFIG. 3, in which backside polymer is removed by a concentrated or localized stream of plasma ions, radicals and neutrals from an external plasma source. A first step (block448) is to support the wafer on thepedestal112 so as to expose a peripheral portion of the wafer back side while heating the wafer to on the order of 300 degrees C. A next step (block450) is to define anupper process zone130 above the wafer front side and alower process zone132 below the wafer back side with minimal migration of gas between the two zones by maintaining a wafer-to-sidewall gap at less than 2 mm. A further step (block452) is to prevent (or minimize) flow or delivery rate of etch species or plasma at the wafer front side by maintaining a wafer-to-ceiling gap (the height of the upper process zone) at less than 2 mm. A further step (block454) is to generate a first plasma in a localexternal plasma chamber300 with a polymer etchant precursor gas (e.g., oxygen), and direct a narrow stream of by-products from the first plasma through aninjection orifice302 near the wafer backside directly at the wafer back side, while rotating the wafer. A related step (block456) is to enhance dissociation of the polymer etchant precursor species (oxygen) by introducing a dissociation agent (nitrogen gas) into the localexternal plasma chamber300. Another related step (block457) is to enhance the backside polymer etch rate by applying RF bias power across the local external plasma chamber and the wafer. Another step (block458) is to generate a second plasma in an upperexternal plasma source200 with a scavenger species (H2 or N2) that scavenges the polymer etch species, and introduce by-products of the second plasma (H radicals or N radicals) into theupper process zone130. Another step (block460) is to evacuate theupper process zone130 at a pumpingport212 near the wafer edge to remove polymer etchant species (oxygen) from the upper process zone at a sufficiently high rate to avoid damage of critical (carbon-containing or low-k) films on the wafer front side. A related step (block462) is to evacuate thelower process zone132 at a pumpingport218 near the edge of the wafer at a sufficiently high rate to minimize migration of polymer etchant species (oxygen) from thelower process zone132 the into theupper process zone130.
• FIG. 10 depicts an exemplary process carried out in the reactor ofFIG. 2 for simultaneously removing polymer from the wafer backside while removing photoresist from the wafer front side. The process ofFIG. 10 may be carried out, for example, in cases in which there is no film on the wafer front side (such as a ULK film) that is particularly susceptible to damage from polymer etchant species, or in cases in which any critical or ULK films that are present can tolerate the limited flow of polymer etch species escaping from thelower process zone132 to theupper process zone130. A first step (block464) is to support the wafer so as to expose a peripheral portion of the wafer back side while heating the wafer to on the order of 300 degrees C. A next step (block466) is to define anupper process zone130 above the wafer front side and alower process zone132 below the wafer back side with minimal migration of gas between the two zones by maintaining a wafer-to-sidewall gap at less than 2 mm. A further step (block468) is to generate a first plasma in a lowerexternal plasma chamber134 with a polymer etchant precursor gas (e.g., oxygen), and introduce by-products (e.g., radicals, free oxygen) from the plasma into thelower process zone132 so as to etch polymer from the wafer back side. A related step (block470) is to enhance dissociation of the polymer etchant precursor species (oxygen) by introducing a dissociation agent (nitrogen gas) into the lowerexternal plasma chamber134. A step (block472) carried out simultaneously with the step ofblock468 is to generate a second plasma in an upperexternal plasma chamber200 with a scavenger precursor gas (H2) and a small proportion of an oxygen-containing species (H2O or N2O). By-products of the second plasma, e.g., scavenger species (H radicals) and oxygen-containing radicals, are introduced into theupper process zone130. The scavenger species removes etch species (e.g., oxygen) from theupper process zone130, and with the help of the oxygen-containing species-removes photoresist from the wafer front side. During this process, it is preferable to maintain the height of theupper process zone130 in the narrow range (0.2-2 mm) to enhance the rate of photoresist removal. The result of this step is to simultaneously remove backside polymer using the bottomexternal plasma source134 and remove front side photoresist with the upperexternal plasma source200. The flow rate of the oxygen-containing (H2O or N2O) into the upperexternal plasma source200 species may be 1-10% of the hydrogen flow rate into the upperexternal plasma source200, for example.
• FIG. 11 depicts a variation of the reactor ofFIG. 3, in which the plasma by-product stream or jet directed at the wafer backside edge is produced by components within the reactor itself rather than an external plasma source. For this purpose, an external plasma source such as theexternal plasma source300 shown inFIG. 3 is replaced in the embodiment ofFIG. 11 by aninternal plasma source500 consisting of a cylindrical sealedenclosure502, which may be formed of an insulating material such as quartz, and acoil504 wrapped around a portion of thecylindrical enclosure502. In the illustrated reactor, thecoil504 is outside of the chamber. Theenclosure502 is closed at itsbottom end502aand forms a nozzle or conical shapedoutlet502bat its top end facing and close to the backside edge of thewafer118. Agas supply506 storing a polymer etch gas species is coupled to theenclosure502 through thebottom end502a. AnRF generator508 is coupled to the coil504 (through an optional impedance match, not shown) and furnishes sufficient power to produce an inductively coupled plasma inside theenclosure502. The pressure inside theenclosure502 is greater than that of thelower process zone132. This pressure difference may be controlled by themain vacuum pump146 and anoptional vacuum pump509 coupled to theenclosure502. Plasma by-products, for example radicals, neutrals and/or ions, escape through thenozzle outlet502band form a concentrated or localizedstream510 that impinges upon a target area of the wafer backside edge. In order to control the ion energy of thestream510, an optional RF biaspower generator512 may be connected between aninterior electrode514 inside theenclosure502 and thewafer pedestal112. While the upperexternal plasma source200 ofFIG. 3 may be employed in the reactor ofFIG. 11, this option is depicted inFIG. 11 only in dashed line. Instead, the solid line image ofFIG. 11 shows that (optionally) thepurge gas supply140 may furnish purge gases through the ceilinggas distribution plate104 with no external plasma source. The purge gas may be either non-reactive or may be a reactive scavenger species, as discussed with reference toFIG. 1.
• In one embodiment, to provide a desired (e.g., 300 degree C.) wafer temperature, thepedestal112 may be heated as inFIG. 3, or radiant lamps (not shown) above the ceiling may be employed. In one embodiment, aliner520 of a process-compatible material may cover the side and bottom edge surfaces of thepedestal112, and aliner522 of a process compatible material may cover theside wall102. Theliners520,522 may be useful in minimizing metal contamination due to etching of chamber surfaces by theplasma stream510. The process-compatible material may be, for example, quartz. In one implementation, theceiling104 may be formed of a process-compatible material such as quartz. In this case, the ceiling may be a smooth simple structure without the gas distribution plate features depicted inFIG. 11.
• In an alternative embodiment, theplasma source enclosure502 ofFIG. 11 may be in the shape of a torus, to form a toroidal plasma source.
• In another alternative embodiment, the inductive plasma source500 (the tube enclosure502) is replaced by a capacitively coupledsource530 as illustrated inFIG. 12. The capacitively coupledsource530 includes aconductive electrode532 having a small discharge portion or area532-1 close to and facing the backside edge of thewafer118 and at least one axially extending leg532-2. A polymer etch gas species is introduced into thelower process zone132 through anopening533 in the chamber floor from agas supply506. AnRF generator534 is coupled between the bottom end of the axially extending leg532-2 and thewafer pedestal112. A second axially extending leg532-3 parallel to the first leg532-2 may be provided. RF power from thegenerator534 produces a plasma discharge in the small gap between the electrode discharge portion532-1 and a corresponding area on the wafer backside edge. In one embodiment, the side of theelectrode532 facing the wafer118 (or the entire electrode532) may be covered by aliner535, which may be formed of a process-compatible material such as quartz, which is useful in minimizing or preventing metal contamination. As in the embodiment ofFIG. 11, the wafer is rotated so that the entire backside edge circumference is exposed to the localized plasma.
• If radiant heating is employed to heat thewafer118, then thepedestal112 may not be necessary, as will be discussed below.
• In the reactor ofFIG. 13, the need to rotate the wafer to expose its bottom circumference to a plasma stream is eliminated by instead generating a ring ofplasma538 beneath the backside edge extending around the entire circumference. This is accomplished by placing thewafer118 very close theceiling104, as in the foregoing embodiments, and then applying RF power to acoil antenna540 overlying the edge of thewafer118. Thecoil antenna540 may consist of solenoidal conductive windings, for example. While the wafer may be held in the elevated position depicted inFIG. 13 with theheated pedestal112 ofFIG. 11,FIG. 13 illustrates how the wafer may be elevated bylift pins542 suspended on thelift spider544 controlled by thelift actuator116. In this case, the wafer is heated byradiant lamps548 through theceiling104. Alternatively, the above-ceiling coil540 may be replaced by acoil540′ around thesidewall102.
• In certain embodiments, photoresist is removed from the wafer front side in a separate process. In such embodiments, the wafer is lowered to the dashed line position ofFIG. 13, and gases are introduced through the ceilinggas distribution plate104 capable of removing photoresist, as discussed previously in this specification. In other embodiments, a secondinner coil antenna550 is provided controlling plasma ion density near the center of the wafer. The presence of the secondinner coil antenna550 also improves uniformity of the photoresist removal. The twocoil antennas540,550 may be driven at independently adjusted RF power levels, to permit tuning of the plasma ion density radial distribution. This may be accomplished by providingseparate RF generators552,554 coupled to theseparate coil antennas540,550, or by providing asingle RF generator556 whose power is controllably apportioned between the twoantennas540,550 by apower splitter558. Thelamp heaters548 are placed in open spaces over the ceiling between the inner andouter coil antennas540,550.
• Theheated pedestal112 ofFIG. 11 can be used to hold thewafer118 in the elevated position ofFIG. 13. In this case, for a low chamber pressure and in the absence of radiant lamp heaters, efficient heat transfer requires the use of an electrostatic chuck on thepedestal112. An advantage of using the wafer support pedestal is that it enables bias power to be applied to the wafer in a highly uniform manner while at the same time effecting uniform heating or temperature control of the wafer. Aliner520 can also be provided having process-compatible materials to avoid metal contamination and excessive consumption of pedestal materials during plasma processing.
• An advantage of using theradiant lamps548 to heat the wafer is faster heat transfer (compared to a heated pedestal) in the case of a low chamber pressure where heat conduction or convection is poor.
• FIG. 14 depicts another embodiment of the reactor ofFIG. 13 in which thecoil antenna540 for generating a ring plasma is replaced by aring electrode560 surrounding the wafer edge, and anRF generator562 coupled to thering electrode560. RF discharge from thering electrode560 produces thering plasma538 by capacitive coupling.FIG. 15 depicts a modification of the reactor ofFIG. 14 in which aring electrode560′ is outside of thechamber100.
• FIG. 16 depicts a modification that can be implemented in any of the reactors described herein, in which theceiling104 has a shallow cylindrical hollow570 corresponding to the volume of thewafer118. In certain embodiments, the wafer can be lifted into the hollow570 to effectively shield the wafer front side from polymer etch gases during the backside polymer removal process.
• FIG. 17 illustrates a process in which the reactor chamber is used to perform reactive ion etch of polymer from the wafer backside in a raised wafer position and then photoresist stripping on the wafer front side in a lowered wafer position. Temperature switching is employed to enhance wafer front side protection. Specifically, backside polymer removal is performed at a low wafer temperature and then front side photoresist strip is performed at a high wafer temperature (the order may be reversed). The backside polymer reactive ion etch step may be performed at a sufficiently low wafer temperature to retard the reaction of its by-products (radicals) with thin films (e.g., photoresist) on the wafer front side. If the front side photoresist strip process uses radicals from a remote source, it is facilitated by raising the wafer temperature to a threshold at which the reaction rate of the radicals with photoresist is significantly increased. If the front side photoresist strip process is a reactive ion etch process, then the wafer temperature does not necessarily have to be increased for this step.
• Referring now to an exemplary process shown inFIG. 17, a first step (block600) is to support the wafer on the pedestal so as to expose a peripheral portion of the wafer back side. The wafer temperature is set under a threshold temperature (e.g., under 200 degrees C.) below which the reaction rate of polymer etch species radicals with wafer thin film materials is significantly retarded (block602). A next step (block604) is to define an upper process zone above the wafer front side and a lower process zone below the wafer back side with minimal migration of gas between the two zones by maintaining a wafer-to-sidewall gap at less than 2 mm. A further step (block606) is to prevent accumulation of etch species or plasma at the wafer front side by maintaining a wafer-to-ceiling gap (the height of the upper process zone) at less than 2 mm. Another step (block608) is to purge the upper process zone to remove any etch species radicals that may leak through the wafer-sidewall gap. In order to reduce or avoid etching of thin films on the wafer front side, a further step (block610) is to evacuate the upper process zone at a pumping port near the wafer edge to remove polymer etchant species (oxygen) from the upper process zone at a sufficiently high rate to avoid damage of critical (carbon-containing or low-k) films on the wafer front side. A related step (block612) is to evacuate the lower process zone at a pumping port near the edge of the wafer. Reactive ion etching of the polymer on the wafer backside is performed with a plasma close to the wafer backside edge (block614), until the backside polymer is completely removed. Application of plasma to the wafer backside is then stopped, and the wafer is lowered to increase the height of the upper process zone, in order to permit accumulation of plasma or radicals in the upper process zone (block616). Then, the wafer temperature is increased above a higher threshold temperature (e.g., above 300 degrees C.) in order to significantly increase the reaction rate of the radicals with photoresist on the wafer front side (block618). Typically, the reaction rate increase corresponding to the temperature increase from the lower threshold temperature to the higher threshold temperature is about a factor of 5. Radicals from a remote plasma source are employed to strip the front side photoresist (block620). Alternatively, the step (block618) of raising the wafer temperature may be omitted, and a reactive ion etch process is used to strip the photoresist (block622).
• FIG. 18 depicts a modification of the plasma reactor ofFIG. 13 that is capable of performing the process ofFIG. 17. In this modification, a front side (e.g., photoresist strip)process gas supply160 is provided in addition to thepurge gas supply140 through theceiling104. Theouter coil540′ is moved to an axial location below the wafer plane. Optionally, theheater lamps548 are moved from the ceiling to the floor at the bottom of the chamber. Theheater lamps548 ofFIG. 18 may be employed in carrying out the wafer temperature control step ofblock602 ofFIG. 17. Aquartz window549 is provided in the floor for theheat lamps548. The small wafer-ceiling gap ofblock606 ofFIG. 17 is realized at the raised (solid line) position of the wafer in the reactor ofFIG. 18. Thepurge gas supply140 ofFIG. 18 provides the gas for the purge step ofblock608 ofFIG. 17. The pumpingports217 and218 ofFIG. 18 are used in the steps ofblocks610 and612 ofFIG. 17. The reactive ion etch step ofblock614 ofFIG. 17 is carried out by thesplitter558 ofFIG. 18 applying RF power only to theouter coil540′. This creates a ring of plasma beneath the backside wafer edge, as described previously with reference toFIG. 13, to remove the backside polymer. Upon completion of this step, thewafer118 is lowered to the dashed line position ofFIG. 18, and process gas (e.g., a photoresist strip process gas) is introduced through theceiling104 from thegas supply160 and into now enlarged upper process zone. Thesplitter558 applies RF power to both the inner andouter coils540,550, with power being apportioned between the two coils to optimize plasma ion uniformity over the front side of the wafer. The RF power and gas flow is maintained until the completion of the reactive ion photoresist etch step ofblock622 ofFIG. 17.
• FIG. 19 depicts a modification of the embodiment ofFIG. 18, in which theceiling104 is modified to form an upward extendingneck650 having a cylindrical side wall652 and aneck cap654, forming aneck volume656. The outputs from thepurge gas supply140 and theprocess gas supply160 are received in the neck volume. Theinner coil550 is wound around the neck sidewall652, so that theneck volume656 functions as the chamber of a remote plasma source. Thesplitter558 ofFIG. 19 applies power only to theouter coil540 during the backside etch process to form a ring of plasma beneath the wafer backside edge. Thesplitter558 applies power only to theinner coil550 during the front side etch process to provide a remote plasma source. Because of the distance of theremote source chamber656 from thewafer118 in its lowered (dashed line) position ofFIG. 19, ions from the plasma in the neck recombine before reaching thewafer118, and therefore the front side etch is a radical-based process, in accordance with the step ofblock620 ofFIG. 17. This makes it advantageous to use theheater lamps548 ofFIG. 19 prior to this step to increase the wafer temperature above the radical reaction threshold temperature, in accordance withblock618 ofFIG. 17.
• The reactor ofFIG. 11 may be adapted to perform the process ofFIG. 17 by adding aremote plasma source200 and its process gas supply. Theremote plasma source200 may be implemented with a coil wrapped around a neck or tube (as shown inFIG. 19) or it may be any other type of plasma source, such as a microwave plasma source, for example. Thewafer pedestal112 ofFIG. 11 may control the wafer temperature in accordance with the process ofFIG. 17, and thepedestal112 may move between the raised and lowered (dashed-line) positions ofFIG. 11, as an optional feature of the process ofFIG. 17. The reactor ofFIG. 12 is modified in the same manner to adapt it to perform the process ofFIG. 17, by adding theremote plasma source200 at the ceiling of the reactor ofFIG. 12, and moving thepedestal112 between the solid and dashed-line positions ofFIG. 12. The same modifications may carried out in each one of the reactors ofFIGS. 13,14 and15. Each one ofFIGS. 13,14 and15 shows that, while lift pins may be employed to control the wafer position, a movable heated pedestal112 (partially shown in dashed line in both its elevated and retracted positions) may be employed instead, in which case theheater lamps548 would not be required. In each of the reactors ofFIGS. 11-15, the optional use of awafer pedestal112 enables bias power to be applied to the wafer. Advantages of this feature include the enhancement of the backside polymer etch process, and the suppression of ions in theupper process zone130 if the wafer-ceiling gap is less than the plasma sheath thickness.
• Reactive ion etching for removal of backside polymer from the wafer can be accomplished in the process ofFIG. 9 using the reactor ofFIG. 3. In this aspect, thebackside plasma source300 ofFIG. 3 produces a sufficient flow of ions that reach the wafer backside. This condition is realized by holding the chamber pressure of theexternal plasma source300 to a low pressure. In this step, the wafer temperature is held under a low threshold temperature (e.g. below about 200 degrees C. below which the reaction rate of polymer etch radicals with wafer front side thin film materials is very slow (e.g., about 5 times slower than at about 300 degrees C.). The reactive ion etch process used to remove the backside polymer is not hampered at low wafer temperatures. Therefore, this low wafer temperature does not prevent the polymer etch ions from reacting with the backside polymer, but it does effectively slow down the reaction rate of polymer etch radicals that may escape from thelower process zone132 into theupper process zone130 with the wafer front side materials. The small wafer-to-ceiling gap employed during the backside polymer removal step essentially guarantees that most (or all) of the polymer etch species that can survive in theupper process zone130 are radicals or neutrals, not ions.
• During the front side photoresist strip step ofFIG. 7, in which there are no polymer etch species threatening the wafer front side, the wafer temperature is raised above a high threshold temperature (e.g., above 300 degrees C.) at which radicals can react at a faster rate with front side thin film materials, such as photoresist (e.g., at a rate about 5 times faster than at the lower threshold temperature of 200 degrees C.). Typically, the wafer temperature is constantly ramping. If the wafer is being heated to ramp its temperature upwardly, then the backside polymer removal step is performed prior to the wafer temperature exceeding the lower threshold temperature, and the front side photoresist strip step is not performed until after the wafer temperature has reached or exceeded the higher threshold temperature. For example, the wafer temperature may begin slightly above room temperature, ramps upwardly during the entire process. In the period before the temperature exceeds about 150 degrees C. or 200 degrees C., the backside polymer removal step is performed and halted upon completion. Then, after a pause to allow the wafer temperature to reach or at least get near 300 degrees C., the front side photoresist step is performed. The processes ofFIGS. 7 and 9 may be performed in any order, an advantage being that they are performed in the same reactor without having to remove the wafer. When lowering the wafer as required in the process ofFIG. 7, the backside plasma source (300 ofFIG. 3 for example) is moved so as not to obstruct wafer movement.
• The reactor ofFIG. 3 may be modified by replacing theexternal plasma source300 with an internal plasma source, such as the internalinductive source500,502,508 ofFIG. 11 or the internalcapacitive source530,532,534 ofFIG. 12. In this case,
• As previously described in detail in this specification, wafer front side protection is provided by establishing the narrow wafer-ceiling gap and pumping a purge or scavenger gas through that narrow gap. Such protection is improved upon by the feature of holding the wafer temperature below the threshold temperature during backside polymer removal.
• Typically, polymer etch radicals (in the absence of ions) react very slowly with photoresist below the low threshold temperature of about 200 degrees C., while a higher reaction rate is obtained above a high threshold temperature of about 300 degrees C. The polymer etch rate increases by about a factor of 5 as the wafer temperature increases from the low threshold temperature (e.g., 300 degrees C.) and the high threshold temperature (e.g., 200 degrees C.). Therefore, wafer front side protection is enhanced by holding the wafer temperature below the low threshold temperature of 200 degrees C. (e.g., at 150 degrees C.) during the backside polymer removal process. During the subsequent front side photoresist stripping, the wafer temperature is raised to about 300 degrees C.
• At a high chamber pressure on the order of a Torr, the heated electrostatic chuck orpedestal112 ofFIG. 3 may be employed to control the wafer temperature. One advantage of this is that RF plasma bias power may be applied to the wafer to enhance processing. A possible disadvantage of using aheated pedestal112 is that thepedestal112 may require a protective liner of a compatible material, such as quartz, alumina or yittria. At a lower chamber pressure, it may be necessary to use radiant lamps in order to attain a requisite heat transfer rate. One advantage of radiant lamps is that the wafer temperature may be switched between the two temperatures more quickly, particularly if the wafer is lifted above contact with thepedestal112 during processing.
• FIG. 20 illustrates a modification of the reactor ofFIG. 2 in which the plasma by-products (radicals) from theexternal plasma source134 are fed into anannular plenum630 inside the bottom of thepedestal112 and upwardly through axialcylindrical bores632 within thepedestal112 and is shot out throughejection orifices634 aimed generally at the peripheral edge of thewafer118. Theannular plenum630 supplies gas or plasma by-products to the bottom of each bore602 and the ejection orifices ornozzles634 receive the plasma by-products from the tops of thebores632. As shown inFIG. 21, thebores632 andnozzles634 are arrayed concentrically. In this way, the flow of plasma by-products from theexternal plasma source134 is collimated withininternal bores632 of thepedestal112 and then aimed as a circular array of directed spray patterns from the ejection orifices634. Thepedestal112 has a wafer-support surface636 having a diameter less than that of thewafer118 so as to leave a peripheral annular region of the wafer backside exposed. Thepedestal112 further has a peripheralannular surface638 that is parallel to but below the wafer support surface, the height difference between the twosurfaces636,638 providing agas flow space640 between the peripheralannular surface638 and the wafer backside.
• As in the embodiments ofFIGS. 1-3, a boundary between the upper andlower process zones130,132 is established by constricting gas flow at the peripheral edge of thewafer118. In the reactor ofFIG. 20, this is accomplished by providing aconfinement ring640 surrounding thepedestal112 and having aninner edge640aadjacent theedge118aof the wafer, the twoedges118a,640abeing separated by a small gap of about 0.5-5 mm. This gap is sufficiently small to limit gas flow between the upper andlower process zones130,132 and thereby protect the wafer front side during etching of polymer from the wafer backside edge. Evacuation of polymer etch species is performed by themain vacuum pump146 through a horizontalradial space642 between the annularperipheral surface638 of the pedestal and abottom surface644 of thering640. This evacuation extends through a verticalaxial space646 between theside wall648 of thepedestal112 and avertical surface649 of thering640.
• In the implementation depicted in the drawing ofFIG. 20, thechamber220 of theexternal plasma source134 is a toroid and the RFsource power applicator222 consists of a magneticallypermeable ring222awrapped around a section thechamber220 and acoil222bwrapped around thering222aand driven through animpedance match135 by an RFsource power generator137. Thechamber220 is coupled to theplenum630 via a conduit
• While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims (20)

1. A process for removing polymer from a backside of a workpiece, comprising:

supporting said workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed;

confining gas flow at the edge of said workpiece within a gap at the edge of said workpiece on the order of about 1% of the diameter of the chamber, said gap defining a boundary between an upper process zone containing said front side and a lower process zone containing said backside;

generating a first plasma in a lower external chamber from a polymer etch precursor gas, and introducing an etchant by-product from said first plasma into said lower process zone; and

generating a second plasma in an upper external plasma chamber from a precursor gas of a scavenger of the etchant by-product, and introducing scavenger species from said second plasma into said upper process zone.

2. The process ofclaim 1 further comprising:

confining said upper process zone between the front side and a ceiling of the reactor to an upper process zone height on the order of about 1% of the diameter of the chamber.

3. The process ofclaim 1 further comprising:

evacuating said upper process zone through a slit opening surrounding said upper process zone.

4. The process ofclaim 3 further comprising:

evacuating said lower process zone through a slit opening surrounding said lower process zone near the edge of said workpiece.

5. The process ofclaim 2 wherein said gap and said upper process zone height are both about 2 mm.

6. The process ofclaim 1 wherein said polymer etch precursor gas comprises oxygen gas.

7. The process ofclaim 6 wherein said scavenger precursor gas comprises hydrogen or nitrogen gas.

8. The process ofclaim 6 further comprising promoting dissociation of oxygen in said first plasma by furnishing nitrogen gas to said lower external plasma source.

9. The process ofclaim 7 further comprising:

removing photoresist from the workpiece front side by supplementing the hydrogen gas supplied to the upper external plasma source with an oxygen-containing gas.

10. The process ofclaim 9 wherein said oxygen-containing gas comprises one of H2O, N2O, CO.

11. The process ofclaim 1 wherein the upper process zone height is sufficiently small to limit residency time of gases in said upper process zone to less than 1% to 5% of the residency time of gases in said lower process zone.

12. The process ofclaim 1 wherein the step of confining is a carried out by one of: (a) defining said gap between the workpiece edge and the sidewall of the chamber, (b) defining said gap between the workpiece edge and a ring surrounding the workpiece.

13. The process ofclaim 1 further comprising:

confining the flow of the etchant by-product from said first plasma through axial conduits underlying the backside of the workpiece.

14. A process for removing polymer from a backside of a workpiece, comprising:

supporting said workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed, said workpiece corresponding to a boundary between an upper process zone and a lower process zone of said chamber;

confining the front side of said workpiece by a ceiling of said chamber to as to establish a height of the upper process zone that is less than 1% of the diameter of said workpiece;

generating a first plasma in an external chamber from a polymer etch precursor gas, and introducing an etchant by-product from said first plasma into said lower process zone; and

generating a second plasma in an upper external plasma chamber from a precursor gas of a scavenger of the etchant by-product and introducing scavenger by-product from said second plasma into said upper process zone.

15. The process ofclaim 14 further comprising:

evacuating said upper process zone through a first slit opening surrounding said upper process zone.

16. The process ofclaim 15 further comprising:

evacuating said lower process zone through a second slit opening surrounding said lower process zone near the edge of said workpiece.

17. The process ofclaim 13 wherein said polymer etch precursor gas comprises oxygen gas.

18. The process ofclaim 13 wherein said scavenger precursor gas comprises hydrogen or nitrogen gas.

19. The process ofclaim 18 further comprising:

removing photoresist from the workpiece front side by supplementing hydrogen gas furnished to the upper external plasma source with an oxygen-containing gas.

20. A process for removing polymer from a backside of a workpiece, comprising:

supporting said workpiece on the backside in a vacuum chamber while leaving a peripheral annular portion of the backside exposed, said workpiece corresponding to a boundary between an upper process zone and a lower process zone of said chamber;

confining the front side of said workpiece by a ceiling of said chamber to as to establish a height of an upper process zone that is sufficiently small to limit residency time of gas in said upper process zone to less than 1% to 5% of the residency time of gases in said lower process zone;

generating a first plasma in a first external chamber from a polymer etch precursor gas, and introducing an etchant by-product from said first plasma into said lower process zone; and

generating a second plasma in a second external plasma chamber from a precursor gas of a scavenger of said etchant by-products and introducing a scavenger by-product from said second plasma into said upper process zone.

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