US20070246443A1 - Process using combined capacitively and inductively coupled plasma process for controlling plasma ion dissociation - Google Patents

Abstract

A method of processing a workpiece in the chamber of a plasma reactor includes introducing a process gas into the chamber, simultaneously (a) capacitively coupling VHF plasma source power into a process region of the chamber that overlies the wafer, and (b) inductively coupling RF plasma source power into the process region, and adjusting the extent of dissociation of species in the plasma by adjusting the ratio between the amounts of the capacitively coupled VHF power and the inductively coupled power. The method may further include controlling plasma ion density in the chamber by controlling the total amount of plasma source power capacitively and inductively coupled into the process region.

Description

BACKGROUND OF THE INVENTION
• In semiconductor fabrication processes, conventional sources of plasma source power, such as inductively coupled RF power applicators or capacitively couple RF power applicators, introduce inherent plasma density non-uniformities into the processing. In particular, inductively coupled plasma sources are characterized by an “M”-shaped radial distribution of plasma ion density over the semiconductor workpiece or wafer. As device geometries have continued to shrink, such non-uniformities become more critical, requiring better compensation. Presently, the non-uniformity of an overhead inductively coupled source is reduced or eliminated at the wafer surface by optimizing the coil design and ceiling-to-wafer distance, aspect ratio, of the chamber. This distance must be sufficient so that diffusion effects can overcome the effects of the nonuniform ion distribution in the ion generation region before they reach the wafer. For smaller device geometries on the wafer and the inductive plasma source located near the ceiling, a large ceiling-to-wafer distance is advantageous. However, a large ceiling-to-wafer distance can prevent the beneficial gas distribution effects of a ceiling gas distribution showerhead from reaching the wafer surface, due to diffusion over the large distance. For such large ceiling-to-wafer distances, it has been found that the gas distribution uniformity is not different whether a gas distribution showerhead is employed or a small number of discrete injection nozzles are employed.
• In summary, the wafer-ceiling gap is optimized for ion density uniformity which may not necessarily lead to gas delivery optimization.
• One limitation of such reactors is that not all process parameters can be independently controlled. For example, in an inductively coupled reactor, in order to increase reaction (etch) rate, the plasma source power must be increased to increase ion density. But, this increases the dissociation in the plasma, which can reduce etch selectivity and increase etch microloading problems, in some cases. Thus, the etch rate must be limited to those cases where etch selectivity or microloading are critical.
• Another problem arises in the processing (e.g., etching) of multi-layer structures having different layers of different materials. Each of these layers is best processed (e.g., etched) under different plasma conditions. For example, some of the sub-layers may be best etched in an inductively coupled plasma with high ion density and high dissociation (for low mass highly reactive species in the plasma). Other layers may be best etched in a capacitively coupled plasma (low dissociation, high mass ions and radicals), while yet others may be best etched in plasma conditions which may be between the two extremes of purely inductively or capacitively coupled sources. However, to idealize the processing conditions for each sub-layer of the structure being etched would require different process reactors, and this is not practical.
SUMMARY OF THE INVENTION
• A method of processing a workpiece in the chamber of a plasma reactor includes introducing a process gas into the chamber, simultaneously (a) capacitively coupling VHF plasma source power into a process region of the chamber that overlies the wafer, and (b) inductively coupling RF plasma source power into the process region, and adjusting the extent of dissociation of species in the plasma by adjusting the ratio between the amounts of the capacitively coupled VHF power and the inductively coupled power. The method may further include controlling plasma ion density in the chamber by controlling the total amount of plasma source power capacitively and inductively coupled into the process region.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a simplified block diagram of a plasma reactor in accordance with an embodiment of the invention.
• FIGS. 2A and 2B together constitute a block diagram depicting a method of one embodiment of the invention, and these drawings are hereinafter referred to collectively as “FIG. 2”.
• FIG. 3A is a graph depicting a radial distribution of plasma ion density that is typical of an inductively coupled plasma.
• FIG. 3B is a graph depicting the radial distribution of plasma ion density that is typical of a capacitively coupled plasma.
• FIG. 3C is a graph depicting the radial distribution of plasma ion density obtained in the reactor ofFIG. 1 in accordance with a method of the invention.
• FIG. 4 illustrates ion radial distribution non-uniformity (deviation) as a function of the ratio of the power levels of inductively and capacitively coupled power.
• FIG. 5 illustrates ion radial distribution non-uniformity (deviation) as a function of the ratio of the pulse duty cycles of inductively and capacitively coupled power.
• FIG. 6 is a graph illustrating lines of constant plasma ion density for pairs of values of inductively and capacitively coupled power levels.
• FIG. 7 is a graph illustrating lines of constant plasma ion density for pairs of values of inductively and capacitively coupled power pulsed duty cycles.
• FIG. 8 is a graph illustrating the dependency of electron density in the bulk plasma as a function of source power levels for different VHF frequencies of the capacitively coupled power.
• FIGS. 9A and 9B together constitute a block diagram depicting a method of another embodiment of the invention, and are hereinafter referred to collectively as “FIG. 9”.
• FIG. 10 is a graph illustrating different bulk plasma electron energy distribution functions obtained for different mixtures of capacitively and inductively coupled power.
• FIG. 11 depicts the change in electron energy distribution functions for different source power levels obtained when capacitively coupled power is added to inductively coupled power.
• FIG. 12 depicts different optical emission spectra obtained for different degrees of dissociation (electron energy distributions).
• FIG. 13 is a graph depicting how the degree of dissociation (e.g., population of free carbon or free fluorine) increases with increasing ratio of inductively coupled power to capacitively coupled power.
• FIG. 14 is a graph depicting how the degree of dissociation (e.g., population of free carbon or free fluorine) increases with increasing ratio of inductively coupled power pulsed duty cycle to capacitively coupled power duty cycle.
• FIGS. 15A and 15B illustrate the contemporaneous waveforms of pulsed inductively coupled power and capacitively coupled power, respectively.
• FIG. 16 is a graph illustrating how the degree of dissociation decreases with increasing frequency of capacitively coupled power.
• FIGS. 17A, 17B and17C are graphs of sheath ion energy distribution for the cases in which only low frequency bias power is applied, only high frequency bias power is applied and both low and high frequency bias power is applied to the wafer, respectively.
• FIG. 18 illustrates a multi-layer gate structure which is to be etched in the process ofFIG. 2 orFIG. 9.
DETAILED DESCRIPTION OF THE INVENTION
• FIG. 1 depicts a plasma reactor for processing aworkpiece102, which may be a semiconductor wafer, held on aworkpiece support103, which may (optionally) be raised and lowered by alift servo105. The reactor consists of achamber104 bounded by achamber sidewall106 and aceiling108. Theceiling108 may comprise agas distribution showerhead109 having smallgas injection orifices110 in its interior surface, theshowerhead109 receiving process gas from aprocess gas supply112. In addition, process gas may be introduced throughgas injection nozzles113. The reactor includes both an inductively coupled RF plasmasource power applicator114 and a capacitively coupled RF plasmasource power applicator116. The inductively coupled RF plasmasource power applicator114 may be an inductive antenna or coil overlying theceiling108. In order to permit inductive coupling into thechamber104, thegas distribution showerhead109 may be formed of a dielectric material such as a ceramic. The VHF capacitively coupledsource power applicator116 is an electrode which may be located within theceiling108 or within theworkpiece support103. In an alternative embodiment, the capacitively coupledsource power applicator116 may consist of an electrode within theceiling108 and an electrode within theworkpiece support103, so that RF source power may be capacitively coupled from both theceiling108 and theworkpiece support103. (If the electrode is within theceiling108, then it may have multiple slots to permit inductive coupling into thechamber104 from an overhead coil antenna.) AnRF power generator118 provides high frequency (HF) power (e.g., within a range of about 10 MHz through 27 MHz) through an optionalimpedance match element120 to the inductively coupledsource power applicator114. AnotherRF power generator122 provides very high frequency (VHF) power (e.g., within a range of about 27 MHz through 200 MHz) through an optionalimpedance match element124 to the capacitively coupledpower applicator116. The efficiency of the capacitively coupledpower source applicator116 in generating plasma ions increases as the VHF frequency increases, and the frequency range preferably lies in the VHF region for appreciable capacitive coupling to occur. As indicated symbolically inFIG. 1, power from bothRF power applicators114,116 is coupled to abulk plasma126 within thechamber104 formed over theworkpiece support103. RF plasma bias power is capacitively coupled to theworkpiece102 from an RF bias power supply coupled to (for example) anelectrode130 inside the workpiece support and underlying thewafer102. The RF bias power supply may include a low frequency (LF)RF power generator132 and anotherRF power generator134 that may be either a medium frequency (MF) or a high frequency (HF) RF power generator. Animpedance match element136 is coupled between thebias power generators132,134 and theworkpiece support electrode130. Avacuum pump160 evacuates process gas from thechamber104 through avalve162 which can be used to regulate the evacuation rate. The evacuation rate through thevalve162 and the incoming gas flow rate through thegas distribution showerhead109 determine the chamber pressure and the process gas residency time in the chamber.
• The plasma ion density increases as the power applied by either the inductively coupledpower applicator114 or VHF capacitively coupledpower applicator116 is increased. However, they behave differently in that the inductively coupled power promotes more dissociation of ions and radicals in the bulk plasma and a center-low radial ion density distribution. In contrast, the VHF capacitively coupled power promotes less dissociation and a center high radial ion distribution, and furthermore provides greater ion density as its VHF frequency is increased.
• The inductively and capacitively coupled power applicators may be used in combination or separately, depending upon process requirements. Generally, when used in combination, the inductively coupledRF power applicator114 and the capacitively coupledVHF power applicator116 couple power to the plasma simultaneously, while the LF and HF bias power generators simultaneously provide bias power to thewafer support electrode130. As will be discussed below, the simultaneous operation of these sources enables independent adjustment of the most important plasma processing parameters, such as plasma ion density, plasma ion radial distribution (uniformity), dissociation or chemical species content of the plasma, sheath ion energy and ion energy distribution (width). For this purpose, asource power controller140 regulates thesource power generators118,122 independently of one another (e.g., to control their ratio of powers) in order to control bulk plasma ion density, radial distribution of plasma ion density and dissociation of radicals and ions in the plasma, as will be described in a later portion of this specification. Thecontroller140 is capable of independently controlling the output power level of eachRF generator118,122. In addition, or alternatively, thecontroller140 is capable of pulsing the RF output of either one or both of theRF generators118,122 and of independently controlling the duty cycle of each, or of controlling the frequency of theVHF generator122 and, optionally, of theHF generator118. In addition, abias power controller142 controls the output power level of each of thebias power generators132,134 independently in order to control both the ion energy level and the width of the ion energy distribution, as will be described below. Thecontrollers140,142 are operated to carry out various methods of the invention.
• In accordance with a first method of the invention depicted inFIG. 2, plasma ion density, plasma ion density uniformity, sheath ion energy and ion energy distribution (width) are controlled independently of one another. The method ofFIG. 2 includes introducing process gas, preferably through the ceiling gas distribution showerhead109 (block202 ofFIG. 2). The method continues by capacitively coupling VHF source power to the bulk plasma (block204) while inductively coupling RF source power to the bulk plasma (block206). The user establishes a certain plasma ion density in accordance with a particular process step. This is accomplished by maintaining the combined total of the VHF capacitively coupled source power and the inductively coupled source power at a level providing the desired plasma ion density for the process step to be carried out (block208). At the same time, the radial distribution of plasma ion density at the wafer surface is customized (e.g., to make as uniform as possible) while maintaining the desired plasma ion density. This is accomplished by adjusting the ratio between the amounts of the VHF capacitively coupled power and the inductively coupled power (block210). This apportions the radial ion distribution between the center-low distribution promoted by the inductively coupled power and the center-high distribution promoted by the VHF capacitively coupled power. As will be described below in this specification, this can be accomplished without perturbing the ion density by maintaining the total RF power nearly constant while changing only the ratio between the power delivered by the HF andVHF generators118,122.
• The adjustment ofstep210 can be carried out by any one (or a combination) of the following steps: A first type of adjustment consists of adjusting the RF generator power levels of the inductively and capacitively coupledpower sources118,122 (block210aofFIG. 2). Another type consists of pulsing at least one or both of the inductively and capacitively coupledRF power generators118,122 and adjusting the duty cycle of one relative to the other (block210bofFIG. 2). A third type consists of adjusting the effective frequency of the capacitively coupled power VHF generator122 (block210cofFIG. 2), in which plasma ion density increases as the VHF frequency is increased. Adjusting the effective VHF frequency of the capacitively coupled plasma source power may be accomplished in a preferred embodiment by providing twoVHF generators122a,122bof fixed but different VHF frequencies (i.e., an upper VHF frequency f₁output by thegenerator122aand a lower VHF frequency f₂output by thegenerator122b) whose combined outputs are applied (through impedance matches124a,124b) to the capacitive power applicator. Changing the effective VHF frequency f_effwithin a range bounded by the upper and lower frequencies f₁, f₂, is performed by varying the ratio between the output
• power levels a₁, a₂, of the twogenerators122a,122b. The effective frequency f_effmay be approximated to first order as a function of the frequencies f₁and f₂of the twoVHF generators122a,122b, respectively, and their respective adjustable output power levels, a₁and a₂, as follows: f_eff=(a₁f₁+f₂a₂)/(a₁+a₂). While the foregoing example involves two VHF generators, a larger number may be employed if desired.
• The VHF capacitive source can efficiently create plasma density without creating high RF voltages in the plasma, which is similar to an inductively coupled plasma (ICP) source. In contrast, the LF and HF bias sources efficiently create high RF voltages in the plasma but contribute little to plasma density. Therefore, the combination of the VHF source (or VHF sources) and the ICP source allows the plasma to be produced without the side effect of creating large RF voltages within the plasma. As a result, the RF voltage produced by the LF of HF source applied to wafer pedestal can operate independently from the plasma density creating source. The VHF source can be operated independently from the ICP source, with an ability to create plasma density in combination with the ICP (whereas the traditional ICP source employs an HF or LF capacitively coupled power source connected to the wafer pedestal to create RF voltage on the wafer only).
• The method further includes coupling independently adjustable LF bias power and HF bias power supplies to the workpiece (block212). Thecontroller142 adjusts the ion energy level and ion energy distribution (width or spectrum) at the workpiece surface by simultaneous adjustments of the two RFbias power generators132,134 (block214). This step is carried out by any one of the following: One way is to adjust the ratio between the power levels of the HF and LF biaspower sources132,134 (block214aofFIG. 2). Another (less practical) way is adjusting or selecting the frequencies of the LF and HF bias power sources (block214bofFIG. 2). In a first embodiment, the LF and HF frequencies are applied to theESC electrode130 while the VHF source power is applied to the gas distribution showerhead110 (in which case theshowerhead110 is the CCP applicator116) while the ICP applicator114voverlies theshowerhead110. In a second embodiment, the VHF source power is applied to theESC electrode130 along with the HF and LF bias frequencies, while theICP power applicator114 overlies theshowerhead110.
• If the method is used in an etch process for etching successive layers of different materials of a multilayer structure, the plasma processes for etching each of the layers may be customized to be completely different processes. One layer may be etched using highly dissociated ion and radical species while another layer may be etched in a higher density plasma than other layers, for example. Furthermore, if chamber pressure is changed between steps, the effects of such a change upon radial ion density distribution may be compensated in order to maintain a uniform distribution. All this is accomplished by repeating the foregoing adjustment steps upon uncovering successive layers of the multilayer structure (block216).
• The superior uniformity of plasma ion radial distribution achieved in the step ofblock210 makes it unnecessary to provide a large chamber volume above the wafer. Therefore, the distance between the wafer and the plasma source may be reduced without compromising uniformity. This may be done when the reactor is constructed, or (preferably) thewafer support103 may be capable of being lifted or lowered relative to theceiling108 to change the ceiling-to-wafer distance. By thus decreasing the chamber volume, the process gas residency time is decreased, providing independent control over dissociation and plasma species content. Also, reducing the ceiling-to-wafer distance permits the gas distribution effects of thegas distribution showerhead109 to reach the wafer surface before being masked by diffusion, a significant advantage. Thus, another step of the method consists of limiting the ceiling-to-wafer distance to either (a) limit residency time or (b) prevent the showerhead gas distribution pattern from being masked at the wafer surface by diffusion effects (block218 ofFIG. 2). One advantage is that inductive coupling can now be employed without requiring a large ceiling-to-wafer distance to compensate for the center-low ion distribution characteristic of an inductively coupled source. In fact, the ceiling-to-wafer distance can be sufficiently small to enable an overhead gas distribution showerhead to affect or improve process uniformity at the wafer surface.
• The chemical species content of the plasma may be adjusted or regulated independently of the foregoing adjustments (e.g., independently of the adjustment of the radial ion density distribution of the step of block210) by adjusting the degree of dissociation in the plasma, in the step ofblock220 ofFIG. 2. This step may be carried out by adjusting the rate at which thechamber104 is evacuated by the vacuum pump160 (block220aofFIG. 2), for example by controlling thevalve162, in order to change the process gas residency time in the chamber. (Dissociation increases with increasing residency time and increasing chamber volume.) Alternatively (or additionally), the adjustment of dissociation may be carried out by adjusting the ceiling-to-wafer distance so as to alter the process gas residency time in the chamber (block220bofFIG. 2). This may be accomplished by raising or lowering theworkpiece support103 ofFIG. 1. The foregoing measures for adjusting dissociation in the plasma do not significantly affect the ratio of inductive and capacitive coupling that was established in the step ofblock210 for adjusting ion distribution or uniformity. Thus, the adjustment of the dissociation or chemical species content ofstep220 is made substantially independently of the adjustment of plasma ion density distribution ofstep210.
• In an alternative embodiment, the capacitively coupledsource power applicator116 consists of electrodes in both theceiling108 and theworkpiece support103, and VHF power is applied simultaneously through the electrodes in both theceiling108 and theworkpiece support103. The advantage of this feature is that the phase of the VHF voltage (or current) at the ceiling may be different from the phase at the workpiece support, and changing this phase difference changes the radial distribution of plasma ion density in thechamber104. Therefore, an additional step for adjusting the radial distribution of plasma ion density is to adjust the phase difference between the VHF voltage (or current) at theworkpiece support103 and the VHF voltage (or current) at theceiling108. This is indicated in block230 ofFIG. 2. This adjustment may or may not require changing the ratio between capacitive and inductive coupling selected in the step ofblock210.
• FIGS. 3A, 3B and3C show how the combination of a center-low or “M”-shaped inductively coupled plasma ion density distribution (FIG. 3A) with a center-high capacitively coupled plasma ion density distribution (FIG. 3B) results in a more ideal or more nearly uniform plasma ion density distribution (FIG. 3C) that corresponds to the superposition of the distributions ofFIGS. 3A and 3B. The ideal distribution ofFIG. 3C is achieved by a careful adjustment of the amount of inductive and capacitive coupling of the twosources118,122 ofFIG. 1. A high ratio of capacitively coupled power leads to a more center-high distribution, while a high ratio of inductively coupled power leads to a more center-low distribution. Different ratios will result in the ideal distribution at different chamber pressures. One way of apportioning inductive and capacitive coupling is to apportion the amount of RF power of the twogenerators118,122.FIG. 4 depicts how the ratio between the output power levels of thegenerators118,122 affects the radial ion distribution. The minimum or dip in the curve ofFIG. 4 corresponds to an ideal power ratio at which the non-uniformity or deviation in ion distribution is the least. Another way of apportioning between inductively and capacitively coupled power is to pulse at least one (or both) of the twogenerators118,122, and control the pulse duty cycle. For example, one of them (the inductive source118) may be pulsed and the other (the capacitive source122) may be continuous, and the two are balanced by adjusting the duty cycle of thecapacitively couple source122. Alternatively, both may be pulsed, and apportioning is done by controlling the ratio of the duty cycles of the two sources. The results are depicted inFIG. 5, in which a high ratio of inductively coupled-to-capacitively coupled duty cycles results in more inductively coupled power reaching the plasma and a more center-low distribution, A high ratio of capacitively coupled power-to-inductively coupled power results in more capacitively coupled power in the plasma, providing a center-high distribution.
• The foregoing adjustments to the ion density distribution can be carried out without changing plasma ion density.FIG. 6 illustrates how this is accomplished in the embodiment ofFIG. 4 in which uniformity adjustments are made by adjusting RF generator output power.FIG. 6 depicts lines of constant ion density for different combinations of inductively coupled power (vertical axis) and capacitively coupled power (horizontal axis). Provided that the values of inductively and capacitively coupled power from thegenerators118,122 respectively are constrained to lie along a particular one of the lines of constant density, the inductive-capacitive power ratio may be set to any desired value (in order to control uniformity) without changing the plasma ion density. The lines of constant density are deduced for any given reactor by conventional testing.FIG. 7 illustrates how this is accomplished in the embodiment ofFIG. 5 in which uniformity adjustments are made by adjusting RF generator pulsed duty cycle.FIG. 7 depicts lines of constant ion density for different combinations of inductively coupled duty cycle (vertical axis) and capacitively coupled duty cycle (horizontal axis). Provided that the values of inductively and capacitively coupled duty cycles from thegenerators118,122 respectively are constrained to lie along a particular one of the lines of constant density, the inductive-capacitive power ratio may be set to any desired value (in order to control uniformity) without changing the plasma ion density. The lines of constant density are deduced for any given reactor by conventional testing.
• FIG. 8 is a graph depicting the effect of the selection of the frequency of the VHF capacitively coupledpower source122 upon ion density, in the step ofblock210cofFIG. 2.FIG. 8 shows that ion density (and hence power coupling) increases with applied source power at a greater rate as the frequency is increased (e.g., from 27 MHz, to 60 MHz and then to 200 MHz). Thus, one way of affecting plasma ion density and the balance between capacitive and inductively coupled power is to select or control the VHF frequency of the capacitively coupledsource RF generator122.
• FIG. 9 depicts a modification of the method ofFIG. 2 in which a desired plasma ion density is maintained while the inductive-to-capacitive coupling ratio discussed above is employed to achieve a desired level of dissociation or chemical species content of the plasma. The method ofFIG. 9 includes introducing process gas, preferably through the ceiling gas distribution showerhead109 (block302 ofFIG. 9). The method continues by capacitively coupling RF source power to the bulk plasma (block304) while inductively coupling RF source power to the bulk plasma (block306). The user establishes a certain plasma ion density in accordance with a particular process step. This is accomplished by maintaining the combined total of the capacitively coupled power and the inductively coupled power at a level providing the desired plasma ion density for the process step to be carried out (block308). At the same time, the degree of dissociation in the bulk plasma is determined (e.g., to satisfy a certain process requirement) while maintaining the desired plasma ion density. This is accomplished by adjusting the ratio between the amounts of the VHF capacitively coupled power and the inductively coupled power (block310). This fixes the dissociation (kinetic electron energy in the bulk plasma) between a very high level characteristic of an inductively coupled plasma and a lower level characteristic of a VHF capacitively coupled plasma. Such apportionment can be accomplished without perturbing the ion density by maintaining the total RF power nearly constant while changing only the ratio between the power delivered by the HF andVHF generators118,122, in accordance with the methods described above with reference toFIG. 6 and (or)FIG. 7.
• The adjustment ofstep310 can be carried out by any one (or a combination) of the following step: A first type of adjustment consists of adjusting the RF generator power levels of the inductively and capacitively coupledpower sources118,122 (block310aofFIG. 9). Another type consists of pulsing at least one or both of the inductively and capacitively coupledRF power generators118,122 and adjusting the duty cycle of one relative to the other (block310bofFIG. 9). A third type consists of adjusting the effective frequency of the capacitively coupled power VHF generator122 (block310cofFIG. 9), in which plasma ion density increases as the VHF frequency is increased. Changing the effective VHF frequency can be carried out by providing a pair of fixedfrequency VHF generators122a,122bhaving respective frequencies and adjusting the ratio between their output power levels.
• The method further includes coupling independently adjustable LF bias power and HF bias power supplies to the workpiece (block312). Thecontroller142 adjusts the ion energy level and ion energy distribution (width or spectrum) at the workpiece surface by simultaneous adjustments of the two RFbias power generators132,134 (block314). This step is carried out by any one of the following: One way is to adjust the ratio between the power levels of the HF and LF biaspower sources132,134 (block314aofFIG. 9). Another way is to adjusting or selecting the frequencies of the LF and HF bias power sources (block314bofFIG. 9).
• The method is useful for performing plasma enhanced etch processes, plasma enhanced chemical vapor deposition (PECVD) processes, physical vapor deposition processes and mask processes. If the method is used in an etch process for etching successive layers of different materials of a multilayer structure, the plasma processes for etching each of the layers may be customized to be completely different processes. One layer may be etched using highly dissociated ion and radical species while another layer may be etched in a higher density plasma than other layers, for example. Furthermore, if chamber pressure is changed between steps, the effects of such a change upon radial ion density distribution may be compensated in order to maintain a uniform distribution. All this is accomplished by repeating the foregoing adjustment steps upon uncovering successive layers of the multilayer structure (block316).
• The superior uniformity of plasma ion radial distribution achieved by combining inductively coupled source power and VHF capacitively coupled source power makes it unnecessary to provide a large ceiling-to-wafer distance. Therefore, the ceiling-to-wafer distance may be reduced without compromising uniformity. This may be done when the reactor is constructed, or (preferably) thewafer support103 may be capable of being lifted or lowered relative to theceiling108 to change the ceiling-to-wafer distance. By thus decreasing the chamber volume, the process gas residency time is decreased, providing independent control over dissociation and plasma species content. Also, reducing the ceiling-to-wafer distance permits the gas distribution effects of thegas distribution showerhead109 to reach the wafer surface before being masked by diffusion, a significant advantage. Thus, another step of the method consists of limiting the ceiling-to-wafer distance to either (a) limit residency time or (b) prevent the showerhead gas distribution pattern from being masked at the wafer surface by diffusion effects (block318 ofFIG. 9).
• The chemical species content of the plasma may be adjusted or regulated independently of the foregoing adjustments by adjusting the process gas residency time in the chamber, in the step of block320 ofFIG. 9. This step may be carried out by adjusting the rate at which thechamber104 is evacuated by the vacuum pump160 (block320aofFIG. 9), for example by controlling thevalve162, in order to change the process gas residency time in the chamber. (Dissociation increases with increasing residency time.) Alternatively (or additionally), the adjustment of dissociation may be carried out by adjusting the ceiling-to-wafer distance so as to alter the process gas residency time in the chamber (block320bofFIG. 9). This may be accomplished by raising or lowering theworkpiece support102 ofFIG. 1. The foregoing measures for adjusting dissociation in the plasma do not significantly affect the ratio of inductive and capacitive coupling that was established in the step ofblock310. Thus, the adjustment of the dissociation or chemical species content of step320 is made substantially independently of (or in addition to) the adjustment of dissociation ofstep210.
• In an alternative embodiment, the capacitively coupledsource power applicator116 consists of electrodes in both theceiling108 and theworkpiece support103, and VHF power is applied simultaneously through the electrodes in both theceiling108 and theworkpiece support103. The advantage of this feature is that the phase of the VHF voltage (or current) at the ceiling may be different from the phase at the workpiece support, and changing this phase different changes the radial distribution of plasma ion density in thechamber104. Therefore, the radial distribution of plasma ion density may be adjusted independently of the dissociation (i.e., without changing the capacitive-to-inductive coupling ratio selected in the step of block310) by adjusting the phase difference between the VHF voltage (or current) at theworkpiece support103 and the VHF voltage (or current) at theceiling108. This is indicated in block330 ofFIG. 9.
• FIG. 10 is a graph depicting how the ratioing of inductive and capacitive coupling controls dissociation in the bulk plasma in the step ofblock308. Dissociation is promoted by an increase in electron energy within the bulk plasma, andFIG. 10 depicts the electron energy distribution function for four different operating regimes.
• The curve labeled410 depicts the electron energy distribution function in the case in which only the HF bias power is applied to the wafer and no source power is applied. In this case, the electron population is confined within a low energy spectrum, well below an energy at which the cross-section for a typical dissociation reaction (represented by the curve420) has an appreciable magnitude. Therefore, less (if any) dissociation occurs.
• The curve labeled430 depicts the electron energy distribution function in the case in which VHF power is applied to the capacitively coupledsource power applicator116 and no power is applied to any other applicator. In this case, the electron population has a small component coinciding with thecollision cross-section420 and so a small amount of dissociation occurs.
• The curve labeled440 depicts the electron energy distribution function in the case in which HF power is applied to the inductively coupledsource power applicator114 and power is applied to no other applicator. In this case, the electron population has a component coinciding with a high value of thecollision cross-section420, and therefore a very high degree of dissociation occurs in the bulk plasma.
• The curve labeled450 depicts the electron energy distribution function for a case in which RF power is apportioned between the capacitive and inductively coupledapplicators116,114. In this case, the resulting electron energy distribution function is mixture of the twofunctions430,440 and lies between them, so that a lesser amount of ion dissociation occurs in the bulk plasma. Thecurve450 representing the combined case has a somewhat smaller electron population at or above an energy at which thecollision cross-section420 has a significant magnitude, leading to the lesser degree of dissociation. Thecombination case curve450 can be shifted toward greater or lesser energy levels by changing the ratio between the amounts of capacitive and inductive coupled power. This is depicted in the graph ofFIG. 11 in which each solid line curve corresponds to the electron energy distribution function for purely inductively coupled power at a particular power level. The dashed line curves extending from the solid line curves depict the modification of those curves as more power is diverted away from inductive coupling and applied to capacitive coupling. Essentially, this causes the electron population to shift to lower energy levels, thereby decreasing dissociation.
• FIG. 12 illustrates the effects of different levels of dissociation upon the chemical content of the plasma. The vertical axis represents the optical emission spectrum intensity and the horizontal axis represents wavelength. Different peaks correspond to the presence of certain radicals or ions, and the magnitude of the peak corresponds to the population or incidence in the plasma of the particular species. The solid line curve corresponds to a low degree of dissociation (capacitive coupling predominant), in which larger molecular species are present in large numbers. The dashed line curve corresponds to a high degree of dissociation (inductive coupling predominant), in which smaller (more reactive) chemical species are present in large numbers (depending upon the parent molecule). In the example illustrated inFIG. 12, a large molecular-weight species with high incidence in the predominantly capacitively coupled regime is CF2, while a low molecular-weight species with high incidence in the predominantly inductively coupled regime is free carbon C. In some cases, the presence of C (free carbon) is an indicator of the presence of very light and highly reactive species, such as free fluorine, which may be desirable where a high etch rate is desired. The presence of the larger species such as CF2 is an indicator of less dissociation and an absence of the more reactive species, which may be desirable in a plasma etch process requiring high etch selectivity, for example.
• FIG. 13 is a graph illustrating one way of carrying out the step ofblock310aofFIG. 9. The vertical axis ofFIG. 13 corresponds to the degree of dissociation in the bulk plasma, and may represent the optical emission spectrum intensity of a highly dissociated species such as free carbon inFIG. 12. The horizontal axis is the ratio of inductively coupled plasma (ICP) power to capacitively coupled plasma (CCP) power (the power levels of the ICP andCCP generators118,122 ofFIG. 1).FIG. 13 indicates that the dissociation is a generally increasing function of this ratio, although it may not be the simple linear function depicted inFIG. 13.
• FIG. 14 is a graph illustrating one way of carrying out the step ofblock310bofFIG. 9. The vertical axis ofFIG. 14 corresponds to the degree of dissociation in the bulk plasma, and may represent the optical emission spectrum intensity of a highly dissociated species such as free carbon inFIG. 12. The horizontal axis is the ratio of inductively coupled plasma (ICP) pulsed duty cycle to capacitively coupled plasma (CCP) pulsed duty cycle (the pulsed duty cycles of the ICP andCCP generators118,122 ofFIG. 1).FIG. 14 indicates that the dissociation is a generally increasing function of this ratio, although it may not be the simple linear function depicted inFIG. 14. TheCCP generator122 may not be pulsed, in which case its duty cycle is 100%, while only the ICP duty cycle is varied to exert control.FIGS. 15A and 15B illustrate one possible example of the contemporaneous waveforms of the pulsed ICP generator output and the pulsed CCP generator output. In this illustrated example, theCCP generator122 has a higher duty cycle than theICP generator118, so that the plasma is likely to exhibit more the characteristics of a capacitively coupled plasma, such as a low degree dissociation. The ratio between the duty cycles of the capacitively and inductively coupled power sources affects the proportion between inductively and capacitively coupled power in the plasma in the following way. First, the shorter the duty cycle of the inductively coupled power source, the longer the idle time between the pulsed bursts of RF inductive power. During the idle time, the highest energy electrons in the bulk plasma loose their energy faster than other less energetic electrons, so that the electron energy distribution function (FIG. 10) shifts downward in energy (i.e., to the left inFIG. 10). This leads to a more capacitively coupled-like plasma (i.e., less dissociation) during each idle time. This effect increases as duty cycle is decreased, so that the plasma has (on average over many cycles) less high energy electrons, leading to less dissociation. During the idle time, the higher energy electron distribution decays, and (in addition) spatial distribution of the higher energy electrons has an opportunity to spread through diffusion, thus improving process uniformity to a degree depending upon the reduction in inductively coupled power duty cycle.
• FIG. 16 is a graph depicting one way of carrying out the step ofblock310cofFIG. 9. The vertical axis ofFIG. 16 corresponds to the degree of dissociation in the bulk plasma, and may represent the optical emission spectrum intensity of a highly dissociated species such as free carbon inFIG. 12. The horizontal axis is the frequency of the capacitively coupled plasma (CCP)generator122 ofFIG. 1.FIG. 16 corresponds to the case in which both CCP and ICP power is applied simultaneously, as in the previous examples, and the frequency of theCCP power generator122 is increased. For a fixed level of ICP power and a fixed level of CCP power, increasing the effective VHF frequency increases the plasma dissociation, as indicated inFIG. 16. The dissociation behavior may not be the simple linear function depicted inFIG. 16.
• FIGS. 17A, 17B and17C illustrate how the step ofblock214 ofFIG. 2 (which corresponds to or is the same as the step ofblock314 ofFIG. 9) is carried out. Each of the graphs ofFIGS. 17A, 17B,17C depicts the population of ions at the plasma sheath (at the workpiece surface) as a function of ion energy, or the sheath ion energy distribution.
• FIG. 17A depicts the ion energy distribution in the case in which the only bias power that is applied to the wafer is a low frequency (e.g., 1 MHz) bias voltage or current. (InFIG. 1, this corresponds to the case in which only the LFbias power generator132 applies bias power.) This frequency is substantially below the sheath ion transit frequency, which is the highest frequency at which the sheath ions can follow an oscillation of the sheath electric field. Therefore, the sheath ions in the example ofFIG. 17A can follow the peak-to-peak oscillations of the sheath electric field imposed by the bias power. This results in a peak ion energy that coincides with the RF bias power peak-to-peak voltage (labeled eVp-p inFIG. 17A). The ion energy distribution is bi-modal and has a second peak at a much lower energy, as depicted in the graph ofFIG. 17A. The ion distribution between these two peaks is relatively low.
• FIG. 17B depicts the ion energy distribution in the case in which the bias power consists only of a high frequency (HF) component (such as 13.56 MHz). (InFIG. 1, this corresponds to the case in which only the HFbias power generator134 applies bias power.) This frequency is well above the sheath ion transit frequency, and therefore the sheath ions are unable to follow the peak-to-peak sheath electric field oscillation. The result is that the ion energy distribution ofFIG. 17B is confined to a narrow energy band centered at half of the peak-to-peak voltage of the sheath. The ion energy distributions ofFIGS. 17A and 17B can be seen to be somewhat complementary to one another, with one distribution (FIG. 17B) being rich in a middle frequency band while the other (FIG. 17A) peaks at two extremes, has a wide distribution that is somewhat depleted at the middle frequencies.
• FIG. 17C illustrates an example of an ion energy distribution that can be realized by applying both LF and HF bias power simultaneously (by enabling both biaspower generators132,134 ofFIG. 1). This results in an ion energy distribution that is, in effect, a superposition of the two extreme distributions ofFIGS. 17A and 17B. The “combination” ion energy distribution ofFIG. 17C is therefore adjustable by adjusting the relative amounts of LF and HF bias power. This is accomplished by either (or both) apportioning the power levels of the LF and HF biaspower generators132,134 (as instep214aofFIG. 2) or pulsing one or both of them and apportioning their duty cycles (as instep214bofFIG. 2). Alternatively, or as an additional step, the frequency of either the HF or the LF bias power may be changed. For example, the LF bias power frequency may be increased to a value closer to the sheath ion transit frequency, which would reduce the ion energy distribution population near the maximum energy (eVp-p) inFIG. 17C (thereby narrowing the ion energy distribution as indicated by the dotted line curve ofFIG. 17C). As another example, the HF bias power frequency can be reduced to a value closer to the sheath ion transit frequency, which would decrease the distribution peak at the intermediate energies ofFIG. 17C (thereby broadening the ion energy distribution in the middle frequencies as indicated by the dashed line ofFIG. 17C).
• FIG. 18 depicts a multilayer thin film structure of a typical gate of a typical field effect transistor (FET). These layers include a high dielectric constantsilicon dioxide layer602 overlying asemiconductor substrate604, a polycrystalline siliconconductive layer606 on theoxide layer602, atitanium silicide layer608 on theconductive layer606, ahard mask layer610 over thesilicide layer608, an anti-reflective (AR) coating612 on thehard mask layer610 and aphotoresist layer614 on theAR coating612. In a plasma etch process for etching such a structure, the different materials of each of the layers602-614 is best etched in a different etch process. Some of the layers (e.g., thephotoresist layer614 and the polycrystalline siliconconductive layer606 are best etched in a plasma that is more inductively coupled than capacitively coupled, while other layers (e.g., the hard mask layer610) are best etched in plasma that is more capacitively coupled than inductively coupled. Using the methods ofFIG. 2 orFIG. 9, each of the different layers may be processed (e.g., etched) with the type of plasma process conditions that are optimal for that particular layer, by changing the process conditions, including the type of source power coupling (i.e., changing the ratio between inductively and capacitively coupled source power). Thus, in an etch process, as each successive layer602-614 is exposed, the adjustments described with reference toFIGS. 1 and 9 are repeated to change the process parameters to customize the process for each layer. This is the goal of the step ofblocks216 and316 ofFIGS. 2 and 9 respectively. In making such changes, other process parameters may be changed. For example, a predominantly inductively coupled plasma of the type used to etch thepolycrystalline layer606 may be better maintained at a lower chamber pressure (e.g., a several milliTorr), while a predominantly capacitively coupled plasma may be better maintained at a higher chamber pressure (e.g., tens of milliTorr). Plasmas having nearly the same amount of inductively and capacitively coupled power may be operated at chamber pressures intermediate the higher chamber pressure range of a capacitively coupled plasma and the lower pressure range of an inductively coupled plasma. Moreover, different bias power levels and ion energy distributions may be employed to etch different ones of the layers602-614, using the steps ofblocks214 or314 ofFIG. 1 or9 to make the adjustments.
ADVANTAGES
• The simultaneous application of both VHF capacitively coupled power and inductively coupled power to the plasma enables the user to independently control plasma ion density and either plasma uniformity or dissociation (or chemical species content of the plasma). Conventional reactors compensate for the center-low ion density distribution of an inductively coupled plasma by applying power from the ceiling using a high ceiling-to-wafer distance so that diffusion effects produce a uniform plasma ion distribution at the wafer. However, such a large ceiling-to-wafer distance would mask the desired effects of an overhead gas distribution showerhead at the wafer surface, so that the benefits of an overhead gas distribution showerhead could not be realized in an inductively coupled reactor. Another problem is that the large ceiling-to-wafer spacing renders the chamber volume very large, so that the process gas residency time is correspondingly large (unless an extremely high capacity vacuum pump evacuates the chamber), making it difficult to control dissociation in the bulk plasma below a minimum level. This has made it more difficult to minimize or solve etch processing problems such as etch microloading or lack of etch selectivity. These problems are all solved in the invention. The seeming inability to employ an overhead gas showerhead in an inductively coupled reactor to improve process uniformity at the wafer surface is solved by introducing an ideal amount of capacitively coupled power to make the ion distribution uniform in the ion generation region. This permits the ceiling-to-wafer spacing to be greatly reduced to the point that an overhead gas showerhead controls process uniformity at the wafer surface. Etch selectivity is improved and etch microloading is reduced by reducing dissociation in the plasma through the reduced gas residency time of the smaller chamber volume facilitated by the reduced ceiling-to-wafer distance. In addition, the etch microloading problem may be solved by independent means by selecting a desired chemical content of the plasma by promoting the degree of dissociation that promotes the desired chemical species. Certain chemical species can suppress the effects of etch microloading, and by adjusting the ratio of the capacitively coupled power to inductively coupled power, the dissociation may be varied to maximize the amount of the desired species present in the plasma. Another advantage is that all of this can be performed while maintaining the overall plasma ion density at a desired level, or independently adjusting plasma ion density.
• The foregoing methods are applicable to plasma processing of a semiconductor wafer or plasma processing of a plasma display substrate.

Claims (22)

1. A method of processing a workpiece in the chamber of a plasma reactor, comprising:

introducing a process gas into the chamber;

simultaneously (a) capacitively coupling VHF plasma source power into a process region of the chamber that overlies the wafer, and (b) inductively coupling RF plasma source power into said process region;

adjusting the extent of dissociation of species in the plasma by adjusting the ratio between the amounts of said capacitively coupled VHF power and said inductively coupled power.

2. The method ofclaim 1 further comprising:

controlling plasma ion density in the chamber by controlling the total amount of plasma source power capacitively and inductively coupled into said process region.

3. The method ofclaim 2 wherein said ratio is adjusted along a line of constant plasma density.

4. The method ofclaim 1 further comprising applying independently adjustable LF bias power and HF bias power to said workpiece; and

5. The method ofclaim 4 further comprising adjusting the average value and population distribution of ion energy at the surface of said workpiece by adjusting the ratio between said LF and HF bias powers.

6. The method ofclaim 2 wherein the control of plasma ion density and the adjustment of dissociation are performed substantially independently of one another.

7. The method ofclaim 1 further comprising further adjusting the extent of dissociation of species in the plasma by adjusting the effective frequency of said capacitively coupled VHF source power.

8. The method ofclaim 7 wherein the adjusting of the effective frequency of said capacitively coupled VHF source power comprises adjusting the ratio between the output power levels of a pair of VHF power generators having different fixed frequencies and coupled to deliver VHF power to said process region.

9. The method ofclaim 1 wherein the step of adjusting said ratio comprises:

adjusting the power levels of said capacitively coupled RF plasma source power and said inductively coupled RF plasma source power.

10. The method ofclaim 1 wherein the step of adjusting said ratio comprises:

pulsing at least said inductively coupled RF plasma source power and adjusting the ratio between the duty cycles of said inductively coupled plasma source power and said capacitively coupled plasma source power.

11. The method ofclaim 1 further comprising adjusting the chemical species content of plasma in said process region by adjusting the residency time of said process gas in said chamber.

12. The method ofclaim 11 wherein the step of adjusting the chemical species content comprises adjusting an evacuation rate of process gas in said chamber.

13. The method ofclaim 11 wherein the step of adjusting the residency time comprises adjusting a distance between said workpiece and a ceiling of said chamber.

14. The method ofclaim 1 wherein the step of introducing a process gas comprises introducing the process gas through an overhead gas distribution showerhead having plural gas inlet orifices, said method further comprising:

limiting the distance between said workpiece and said gas distribution showerhead so that a gas distribution pattern of said plural gas inlet orifices affects plasma distribution at said workpiece.

15. The method ofclaim 2 wherein said workpiece comprises a multi-layer thin film structure comprising different materials in different layers of said structure, and wherein said method is employed to etch successive ones of said different layers in different plasma process conditions, said method further comprising:

repeating the step of adjusting whenever said method causes a successive one of said layers to be exposed, whereby to customize plasma process conditions for each of said different layers.

16. The method ofclaim 1 wherein said ratio is adjusted to maximize plasma ion radial distribution uniformity.

17. The method ofclaim 1 wherein said ratio is adjusted by changing said inductively coupled and capacitively coupled power levels along lines of constant plasma density.

18. The method ofclaim 10 wherein said ratio is adjusted by changing said inductively coupled and capacitively coupled power duty cycles along lines of constant plasma density.

19. The method ofclaim 5 wherein the step of adjusting the average value and population distribution of ion energy comprises adjusting the effective frequency of said HF bias power to change the ion population distribution about an intermediate energy.

20. The method ofclaim 5 wherein the step of adjusting the average value and population distribution of ion energy comprises adjusting the effective frequency of said LF bias power to change the ion population distribution about a maximum energy.

21. The method ofclaim 1 wherein said VHF power is capacitively coupled through one or both of said ceiling and said wafer support.

22. The method ofclaim 1 wherein the step of capacitively coupled RF source power into said process region comprises capacitively coupling VHF power from both a ceiling of said chamber and a wafer support of said chamber simultaneously, said method comprising:

performing a further adjustment of the radial distribution of plasma ion density by adjusting the difference between the phase of VHF voltage or current at the ceiling and the phase of VHF voltage or current at the wafer support.

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