US20120325671A2 - Electroplated lead-free bump deposition - Google Patents

Abstract

A method of forming a metal feature on a workpiece with deposition is provided. The method includes providing an under bump metal layer for solder of an electronic device on the workpiece, depositing a substantially pure tin layer directly to the under bump metal layer, and depositing a tin silver alloy layer onto the substantially pure tin layer.

Description

BACKGROUND
• 1. Field
• The disclosed embodiments relate generally to a method and apparatus for applying metal structures to a workpiece, and more particularly to a method and apparatus for depositing a lead-free solder into micro-scale patterns in the surface of a workpiece coated with a photo-resist patterning film, and more particularly to a method and apparatus for electroplating tin-silver alloy solder bumps.
• 2. Brief Description of Related Developments
• The semiconductor industry has been working towards eliminating lead in electronics, as required under the European Union's Restriction of Hazardous Substances (RoHS) Directive. The industry is moving faster than the regulation to offer “green” consumer's electronics with lead-free packaging. Electrodepositon of lead-free solder such as using through mask patterned deposition, is a technology capable eof providing tight pitch bumping (connection pitch less than approximately 300 microns) or microbumping for advanced electronic packaging. An alloy of tin (Sn) and silver (Ag) is the leading candidate metal for these applications. Substantially pure tin has many desirable properties of a solder metal, for example fatigue resistance, thermal cycling and ductile mechanical properties, however the industry has found that tin whisker growth in substantially pure tin solder makes it an unreliable joining solder for advanced packaging applications. It has been found that a small addition of silver, between approximately 1% and 4% Ag by weight, may significantly reduce the likelihood of Sn whisker formation in the solder joint. Tin-silver alloy (SnAg) solder plating in a conventional manner is more difficult than substantially pure tin electroplating or lead-tin (PbSn) electroplating because of the large difference in electrochemical reduction potential between tin (−0.130 volts SHE) and silver (+0.799 volts SHE). This reduction potential difference causes Ag⁺ ions in the solution to spontaneously react with metallic Sn and or the stannous ion (Sn⁺²) oxidizing the Sn or Sn⁺²to Sn⁺²or Sn⁺⁴and thereby immersion depositing metallic Ag on the Sn surface. Similarly the Ag⁺ ion in the plating solution can immersion deposition other metals such as nickel or copper. Chemical suppliers have developed organic molecules that are to complex the Ag⁺ ion to bring its reduction potential close to that of Sn⁺²and thereby stabilize the Ag⁺ ion in the plating solution. The organic Ag⁺ ion complex in the plating solution does not eliminate the likelihood of unwanted Ag immersion deposition on the Under Bump Metal (UBM), which is typically Nickel or Copper, when electroplating SnAg lead free solder on such UBM structures. This unwanted immersion deposition may cause void defects at the UBM/SnAg interface, said voids are observable after reflowing the solder, and such voids can cause mechanical and electrical failures of the chip to package joint. There is therefore a need for an alternate method of electroplating SnAg solder to form reliable lead-free bump attachment to the underlying metal to solve the problem facing the electronics industry as it moves toward eliminating all lead from integrated circuit products. Further, the industry also needs to develop economical methods of replacing the lead-tin (PbSn) plated bump structures with a lead-free (SnAg) plated bump structures. Due to the thigh cost of the Ag-complexor and other components in commercial SnAg plating chemistries, the typical cost of SnAg plated bumps is several multiples of the PbSn bumps. Existing methods of electrodepositing SnAg bumps involve expensive control systems in the manufacturing equipment, for example as described in U.S. patent application Ser. No. 11/840,748, which is hereby incorporated by reference in its entirety discloses a commercial plating equipment with a control system to ensure that a constant alloy compositions is provided in the solder metal throughout the deposition. There is therefore a need for a method of SnAg electroplating that minimizes the use of expensive chemistry while providing a reliable interface between the SnAg and the underlying metal.
BRIEF DESCRIPTION OF THE DRAWINGS
• The foregoing aspects and other features of the embodiments are explained in the following description, taken in connection with the accompanying drawings. The technology described above may be better understood by referring to the following description taken in conjunction with the accompanying drawings. In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the technology.
• FIG. 1 shows a cross sectional view of prior art after the deposition step;
• FIG. 2 shows a cross sectional view of prior art after the deposition step;
• FIG. 3 shows a cross sectional view of solder bump after thermal treatment;
• FIG. 4 shows a top-down section of prior art showing the presence of voids at the UBM to SnAg interface;
• FIG. 5 shows a top-down section of the present disclosed embodiments showing absence of voids at the UBM to SnAg interface;
• FIG. 6 shows a cross sectional view of the present disclosed embodiments after the second deposition step;
• FIG. 7 shows a commercial wafer electro-deposition machine suitable for a manufacturing process using the present disclosed embodiments;
• FIG. 8 shows a electro-deposition module; and
• FIG. 9 shows a process flow diagram.
DETAILED DESCRIPTION OF THE EMBODIMENTS
• Although the present embodiments will be described with reference to the embodiments shown in the drawings, it should be understood that the embodiments can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. The present disclosed embodiments provide a method of providing a reliable interface between an electrodeposited lead-free solder bump and an underlying bump metal (UBM).
• Referring now toFIG. 1, there is shown a cross section of single bump at the workpiece surface where the workpiece has been prepared for electrodeposition. Anelectrical contact element101 is substantially surrounded by aninsulating film100, these type of features are disposed in a semi-periodic array over the integrated circuit workpiece, for example a 300 millimeter silicon wafer may have 1,000 to 100,000 of such electrical contact, elements distributed across the surface. It is noted that any suitable workpiece or substrate may be provided, for example, gallium arsenide or otherwise. The workpiece is coated with aseed layer102 and then coated withphotoresist104 which is photo patterned to provide openings into which an underbump metal106, such as nickel (Ni) or copper (Cu) or a series of Ni and Cu layers, is electrodeposited.Solder metal120 is electrodeposited onto the underbump metal106 using the same resistpattern mask layer104. For example, U.S. Pat. No. 7,012,333 which is hereby incorporated by reference herein in its entirety teaches deposition of a SnAg solder alloy with the alloy being deposited at lower than the SnAg eutectic point which is about 3.5% by weight.
• Referring now toFIG. 2, there is shown another prior art method of providing lead-free bump, for example a SnAg or SnAgCu alloy where more noble substantially pure metal layer(s)131 are deposited on the under bump metal prior to the deposition of a substantiallypure tin layer130. U.S. Pat. No. 6,596,621, which is hereby incorporated by reference in its entirety, teaches forming a lead-free SnAgCu bump by using a underbump metal layer106 comprised of about 2 micron thick Ni and thencoating layer106 with Ag/Cu131 in proportions to the substantiallypure Sn130 necessary to form a SnAgCu alloy bump with proportions of about 3.5% Ag and about 0.6% Cu and with the balance Sn.
• Referring now toFIG. 3, the potential drawbacks of these prior art approaches will be discussed whereFIG. 3 shows a cross section of the solder bump after the thermal reflow process. A thermal reflow process is advantageous to stabilize the solder bump structure prior to subsequent processing. After the electrodeposition step, the photoresist104 (not shown) is removed and theseed layer102 is etched away everywhere except where it is protected by the underbump metal106. Subsequently the wafer is thermally treated in a so-called reflow process step. Briefly described, reflow involves heating the workpiece in a controlled atmosphere so that the tin-oxides are substantially removed before the solder melts, which may occur between about 221° C. and about 232° C. for SnAg alloy; about 221° C. being the SnAg eutectic at composition of about 3.5% Ag and about 232° C. being the substantially pure Sn melting point, when the solder changes phase from solid to liquid the surface tension causes the metal volume to change shape, transforming into a substantiallyspherical shape126 as the liquid surface tension minimizes the surface area. Also occurring at the elevated temperature is the formation of a layer of intermetallic compounds (IMCs)128 which are a mixture of several alloy phases, for example at a Cu/Sn interface the IMCs will be a combination of Cu₅Sn₆and Cu₃Sn alloy phases. Also occurring at the elevated temperature is the vaporization and outgassing of various organic molecules that may be incorporated into the solder during the deposition process. These elevated temperature processes are halted by cooling down the wafer or substrate, causing the solder to solidify, wherein the solid solder is composed of many sub-micron sized grains which can have different sizes and compositions. For example, U.S. Pat. No. 6,805,974, which is hereby incorporated by reference herein in its entirety, teaches the importance of controlling the alloy composition and the cool-down rate to avoid the unwanted formation of large Ag₃Sn plate shaped grains and instead form a fine grained dispersion of Sn grains and Ag₃Sn small grains.
• The importance of providing a repeatable and well controlled intermetallic structure (IMC) between the underbump metal (UBM) and the solder, along with a well controlled grain structure within the solder, may influence both the mechanical and electromigration reliability of the solder bump. In addition, during cooldown the nucleation and growth of the solder grain structure is strongly influenced by the IMCs that were formed. Prohibiting the presence of Ag away at the underbump metal interface during the initial phase of reflow is advantageous as is demonstrated by comparingFIGS. 4 and 5 which showoptical microscope images230,240 of bumps that have been lapped and polished to theinterface region232,242 between the underbump metal and the solder, where light and dark colors correspond to the different materials of solder, UBM, and IMC, where the very dark spots are voids. Using a nickel UBM layer and about a 2.5% Ag alloy single step electrodeposition of SnAg shown inFIG. 4 for example, frequent occurrence ofinterface voids234,236,238 in the region between the UBM and SnAg may occur. By contrast, the disclosed embodiments using a first layer of substantially pure tin and a second layer of tin-silver repeatedly as shown inFIG. 5, no such occurrence of interface voids occur. The substantially, pure Sn layer/bath may be referred to as, for example, a commercially available substantially pure Sn material or bath such as available from Dow Chemical.
• Referring now toFIG. 6, there is shown a single bump structure in cross section.Workpiece250 is prepared with astructure252 havingelectrical contact element101 that is substantially surrounded by an insulatingfilm100, where these type of features are disposed in a semi-periodic array where the workpiece is coated with aseed layer102 and then coated withphotoresist104 which is photopatterned to provide openings into which anunderbump metal106, such as nickel (Ni) or copper (Cu) or a series of Ni and Cu layers, is electrodeposited. It is noted that any suitable underbump metal may be provided. A substantiallypure tin layer121 is electrodeposited using an electroplating bath with a metal ion content containing no other metal ion besides tin. It is noted that theworkpiece250 may be rinsed to remove the electroplating bath. A tin-silver layer122 is then electrodeposited using the same resistpattern mask layer104 in another plating bath having a metal ion content including tin and silver ions. The thicknesses of the substantially pure Sn layer, T_Sn, and of the SnAg layer, T_SnAg, and the % Ag in the SnAg layer, C_SnAg, are adjusted to provide a final composition % Ag according to the following equation:
  % Ag=CSnAg×TSnAg/(TSnAg+TSn).
• For example, to achieve a final composition % Ag equal to 1.5% Ag the T_Sn=T_SnAgand C_SnAg=3.0%.
• It has been considered to apply substantially pure silver (Ag) and substantially pure tin (Sn) to facilitate fabrication of the SnAg alloy, or even to apply Ag, then Cu, then Sn which would then be reflowed to form a SnAgCu alloy, this method could have particular cost advantages since substantially pure Ag and substantially pure Sn plating materials are less expensive then SnAg alloy plating. When using a combination of substantially pure metal layers it is necessary to apply the more noble metals prior to applying the substantially pure tin for two reasons: (1) electrodeposition of Ag onto a Sn surface is difficult to control because of the problem of uncontrolled Ag immersion deposition on Sn, thereby producing an unstable Sn/Ag interface which will cause production control problems between the deposition step and the thermal treatment reflow step; (2) during the thermal reflow process the substantially pure Ag doesn't melt, instead it dissolves into the Sn, and therefore a Ag metal layer would be unstable on the melted tin solder ball, drifting around during the period between Sn melting and Ag fully dissolving into the Sn. However, to apply the Ag directly on top of the UBM material during the reflow process where the intermetallic layer is formed, the presence of Ag between the Sn and the UBM causes the formation of voids in the intermetallic layer, and these voids reduce the reliability of the solder joint. Because the SnAg materials are several times more expensive than Sn materials the present disclosed embodiments provide some of the economic benefit of the substantially pure Ag and substantially pure Sn method, for example reducing the solder deposition cost by approximately 50% or more, without the associated disadvantage of worsening the solder joint reliability.
• Referring now toFIG. 7, there is shown a commercial wafer electro-deposition machine suitable for a manufacturing process using the present disclosed embodiments. The disclosed embodiments may be implemented in a commercially available electrodeposition machine such as the Stratus from NEXX Systems in Billerica Mass.System200 may incorporate features as disclosed in the International Application WO 2005/042804 A2 published under the Patent Cooperation Treaty and having publication date May 12, 2005 which is hereby incorporated by reference herein in its entirety.System200 is shown in block diagram form as an exemplary system. It is noted that more or less modules may be provided having different configurations and locations. Theindustrial electrodeposition machine200 may containload ports206 by which substrates previously patterned with photoresist as described above are inserted and withdrawn from the system.Loading station204 may have a robotic arm which transferssubstrates278 into substrate-holders270,272,274 which are then transferred bytransport280 tomodules210,212,214,216,260,262,264,266 and processed in succession, The succession may include a copper (Cu)electrodeposition module216, a nickel (Ni)electrodeposition module214, a tin (Sn)electrodeposition module212, a tin-silver (SnAg)electrodeposition module210. The substrates may then be returned to theloading station204 which unloads the substrates and passes them through asubstrate cleaning module202 from which they are returned to theload ports206. Cleaning steps, using de-ionized water for example, may be disposed before and after the electrodeposition steps, for example, cleaningmodules260,262,264,266 may be provided. Alternately,modules260,262,264 and266 may be rinse or thermal treatment modules as well as clean modules. Controller(s)220 may be provided within each station or module to sequence the process and/or transport within the station or module. A system controller(s)222 may be provided within thesystem200 to sequence substrates between the stations or process modules and to coordinate system actions, such as, host communication, lot loading and unloading or otherwise those actions that are required to control thesystem200.Controller222 may be programmable to plate the workpiece with substantially pure tin inprocess module212 disposed to support a plating bath having a suitable metal ion content (e.g. such as that described above). It is noted that theprocess module212 may include either a pure tin anode or an insoluble platinum-titanium (Pt—Ti) anode.Controller222 may be further programmable to rinse the workpiece in a rinse tank disposed to support rinsing substantially all of the substantially pure tin plating chemistry from the workpiece.Controller222 may further be programmable to plate the workpiece with tin and silver inprocess module210 disposed to support a plating bath with a suitable metal ion content (e.g. such as that described above). It is noted that the process module may include, for example, an insoluble Pt—Ti anode or any other suitable anode.Controller222 or any other suitable controller may further be programmable to thermally treat the workpiece in a thermal treatment module disposed to thermally treat the workpiece to cause the tin and tin-silver layers to intermix and form a substantially, uniform tin-silver alloy feature.Controller222 may be further programmable to deposit copper on the workpiece withcopper electrodeposition module216.Controller222 may further be programmable to deposit nickel on the workpiece withnickel electrodeposition module214.Controller222 may further be programmable to clean the workpiece withclean module260. In the embodiment shown, fourelectrodeposition modules210,212,214,216 and fourcleaning modules260,262,264,266 are shown. It is noted, however, that more or less modules may be provided. By way of example, only tin (Sn) electrodeposition module(s) and tin-silver (SnAg) electrodeposition module(s) may be provided. As a further example, separate tools having tin (Sn) electrodeposition module(s) and tin-silver (SnAg) electrodeposition module(s) may be provided. As a further example, multiple duplicate electrodeposition modules may be provided to allow multiple workpieces to be processed in parallel to increase the throughput of the system. As such, all such variations, alternatives and modifications of system configurations are embraced.
• Referring now toFIG. 8, there is shown a block diagram of an exemplaryelectrodeposition process module210.Electrodeposition module210 may incorporate features as do modules found in Stratus tools from NEXX Systems in Billerica Mass. and may incorporate features as disclosed in the International Application WO 2005/042804 A2 published under the Patent Cooperation Treaty and having publication date May 12, 2005 which is hereby incorporated by reference herein in its entirety. Exemplary electrodeposition module hashousing300 which containsfluid302 where fluid302 may flow throughhousing300 and where fluid302 may be a circulated electrolyte.Workpiece holder272 may be removable fromhousing300 byhandler280 and may holdsubstrates278. Although two substrates are shown, holder may hold more or less substrate(s).Anodes310,312 are provided withshield plates314,316 and paddle orfluid agitation assemblies318 and320. It is noted that more or less assemblies may be provided. For example, a single anode may be provided. By way of further example, the anode may be part ofhousing300 orshield plates314,316 and paddle orfluid agitation assemblies318 and320 may not be provided.
• The illustrated process may be performed, such as will be described further below withapparatus200 for example. As may be realized, controller(s)220 may be suitably programmed to effect the process at least in part in an automatic manner.
• Referring now toFIG. 9, there is shown an exemplary process flow diagram400 showing a method for forming a lead free solder bump on a workpiece. In accordance with the exemplary embodiment, for example, a workpiece with an electrically conducting seed layer covered by a patterned resist mask layer having a plurality of openings may be provided, block402, for instance in the apparatus. The workpiece may be immersed, block404, in a tin plating bath containing, for example, a substantially pure tin anode or an insoluble platinum-titanium anode. Inblock404, electrical contact to the seed layer may be formed and electrical potential applied between the workpiece and the anode to cause substantially pure tin to be deposited, for example, between about 2 and about 150 microns of tin to deposit tin the resist pattern features. Inblock408, the workpiece may be moved to a rinse tank. Inblock410, substantially all of the substantially pure tin plating chemistry from the workpiece may be rinsed. The workpiece may be removed from the rinse tank, block412, and immersed in a plating bath containing tin and silver ions and an anode (e.g., such as, for example, an insoluble platinum-titanium anode), block414. Electrical contact to the seed layer may be formed as perblock416, and electrical potential applied between the workpiece and the anode to cause tin-silver alloy to deposit. For example, between about 2 and about 150 microns of a tin-silver alloy may be deposited in the resist pattern features. Inblock418, the photoresist patterning layer may be removed, and substantially all of the seed layer not covered by the plated tin and tin-silver alloy may be removed, perblock420. Thermally treating the workpiece such as inblock422, for example, at between about 210° C. to about 230° C. (degrees centigrade), may, cause the tin and tin-silver layers to intermix and form a substantially uniform tin-silver alloy feature as desired. In theexemplary process400, the tin and tin-silver layers may have any suitable thickness or composition, for example, the tin layer may be about 30 microns and the tin-silver alloy layer is about 30 microns and the tin-silver alloy composition may be between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment. By way of further example, the tin layer may be about 10 microns and the tin-silver alloy layer may be about 10 microns and the tin-silver alloy composition may be between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment. By way of further example, the tin layer may be about one-fourth the thickness of the tin-silver layer. Further, in the embodiments,process400 may provide more or less steps or one or more steps may be combined in one or more step or process. By way of further example, the tin layer may be about 1 micron or 10 microns and the tin-silver layer may be between about 20 microns to about 120 microns.
• In accordance with an embodiment, a method of forming a metal feature on a workpiece with deposition is provided. The workpiece is provided with an under bump metal layer for solder of an electronic device. A substantially pure tin layer is deposited directly to the under bump metal layer. A tin silver alloy layer is deposited onto the substantially pure tin layer.
• In the embodiment, substantially all of the substantially pure tin plating chemistry from the workpiece may be rinsed.
• In the embodiment, the deposition is accomplished by electrodeposition.
• In the embodiment, the under bump metal comprises either copper or nickel.
• In the embodiment, an apparatus for forming a lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer covered by a patterned resist mask layer having a plurality of feature openings is provided. The apparatus has a first plating bath with a metal ion content adapted to deposit a substantially pure tin layer in the resist pattern features. A rinse tank may be provided and adapted to rinse substantially all of the substantially pure tin plating chemistry from the workpiece. A second plating bath is provided with a metal ion content adapted to deposit at tin-silver, alloy layer in the resist pattern features.
• In the embodiment, a copper electrodeposition module is provided.
• In the embodiment, a copper electrodeposition module and a nickel electrodeposition module are provided.
• In the embodiment, a cleaning module is provided.
• In the embodiment, an electronic device having a lead free solder feature is prepared by a process having a step of depositing a substantially pure tin layer directly to a layer of under bump metal for solder of the electronic device. A step of depositing a tin silver alloy layer onto the substantially pure tin layer is provided.
• In the embodiment, a step of rinsing substantially all of the substantially pure, tin plating chemistry from the electronic device may be provided.
• In the embodiment, the deposition is accomplished by electrodeposition.
• In the embodiment, the under bump metal comprises either copper or nickel.
• In the embodiment, a method for forming a lead free solder bump on a workpiece is provided, the method comprising providing a step of providing the workpiece with an electrically conducting seed layer, the electrically conducting seed layer covered by a patterned resist mask layer having a plurality of feature openings. The workpiece is immersed in a first plating bath with a metal ion content. The method comprises providing electrical contact to the seed layer and providing an electrical potential through the metal ion content of the first plating bath to cause between about 2 and about 150 microns of substantially pure tin to deposit in the resist pattern features. The workpiece is immersed in a second plating bath with a metal ion content. Electrical contact to the seed layer is formed and an electrical potential between through the metal ion content in the second plating bath is provided to cause between about 2 and about 150 microns of a tin-silver alloy to deposit in the resist pattern features is provided.
• In the embodiment, the method may include moving the workpiece to a rinse tank, rinsing substantially all of the substantially pure tin plating chemistry from the workpiece is provided, and removing the workpiece from the rinse tank is provided.
• In the embodiment, removal of the photoresist patterning layer is provided.
• In the embodiment, substantially all of the seed layer not covered by the plated tin and tin-silver alloy is removed.
• In the embodiment, thermally treating the workpiece at between about 210 to about 230 degrees centigrade to cause the tin and tin-silver layers to intermix and form a substantially uniform tin-silver alloy feature is provided.
• In the embodiment, the tin layer is about 30 microns and the tin-silver alloy layer is about 30 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.
• In the embodiment, the tin layer is about 1 micron or about 10 microns and the tin-silver alloy layer is between about 20 microns to about 120 microns.
• In the embodiment, the tin layer is 10 microns, and the tin-silver alloy layer is about 10 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.
• In the embodiment, the tin layer is about one-fourth the thickness of the tin-silver layer.
• In the embodiment, an apparatus for forming a lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer covered by a patterned resist mask layer having a plurality of feature openings is provided. The apparatus has a controller programmable to plate the workpiece with substantially pure tin in a first process module disposed to support a first plating bath having a metal ion content adapted to deposit a substantially pure tin layer on the workpiece. The controller is further programmable to plate the workpiece with tin and silver in a second process module disposed to support a second plating bath with a metal ion content adapted to deposit a tin and silver layer on the workpiece.
• In the embodiment, the controller is further programmable to rinse the workpiece in a rinse tank disposed to support rinsing substantially all of the substantially pure tin plating chemistry from the workpiece.
• In the embodiment, the controller is further programmable to deposit copper on the workpiece with a copper electrodeposition module.
• In the embodiment, the controller is further programmable to deposit nickel on the workpiece with a nickel electrodeposition module.
• In the embodiment, the controller is further programmable to clean the workpiece with a clean module.
• In the exemplary embodiment, a method for processing one or more workpieces to electrochemically form a pattern of lead-free bumps on a workpiece is provided. In one embodiment the lead-free bump is formed by a substantially two step deposition process, the first step being through mask deposition of substantially pure tin from an electroplating solution containing tin-ions (e.g. a metal ion content), and a second step being through mask deposition of tin-silver alloy from an electroplating solution containing a controlled mixture of tin-ions and silver ions (e.g. a metal ion content), the two steps being controlled to provide target layer1 and layer2 thicknesses, T1 and T2, along with the second step being controlled to provide X % alloy composition, such that after a subsequent thermal treatment the two layers intermix and form a substantially uniform alloy of tin-silver (SnAg), said alloy having a concentration intermediate between the deposited X % Ag in the alloy deposition step and the 0% Ag in the substantially pure tin deposition step. The disclosed embodiments prevent the immersion deposition of noble metal ion, such as Ag, and organic complexor on the Under Bump Material (UBM) surface to eliminate the potential forming of voids between the UBM and solder interface. A less noble metal layer, such as substantially pure Tin, is electrodeposited on the UBM before the lead-free solder alloy of Sn and more noble metal, such as Ag and/or Cu is co-deposited with Sn as a SnAg or SnAgCu alloy to form a bump for electronic packaging.
• It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.

Claims (28)

1. A method of forming a metal feature on a workpiece with deposition, the method comprising:

providing an under bump metal layer for solder of an electronic device on the workpiece;

depositing a substantially pure tin layer directly to the under bump metal layer; and

depositing a tin silver alloy layer onto the substantially pure tin layer.

2. The method ofclaim 1, wherein substantially all of the substantially pure tin plating chemistry from the workpiece is rinsed.

3. The method ofclaim 1, wherein the deposition is accomplished by electrodeposition.

4. The method ofclaim 1, wherein the under bump metal comprises either copper or nickel.

5. The method ofclaim 1, wherein the workpiece is thermally treated.

6. An apparatus for forming a substantially lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer being covered by a patterned resist mask layer having a plurality of feature openings is provided, the apparatus comprising:

a first plating bath with a metal ion content configured to deposit a substantially pure tin layer in the resist pattern features;

a second plating bath with a metal ion content configured to deposit a tin-silver alloy layer in the resist pattern features.

7. The apparatus ofclaim 6, further comprising a rinse tank configured to rinse substantially all of the substantially pure tin plating chemistry from the workpiece; and.

8. The apparatus ofclaim 6, further comprising a copper electrodeposition module.

9. The apparatus ofclaim 6, further comprising a copper electrodeposition module and a nickel electrodeposition module.

10. The apparatus ofclaim 6, further comprising a cleaning module.

11. A method for forming an electronic device having a lead free solder feature, the method comprising:

depositing a substantially pure tin layer directly to a layer of under bump metal for solder of the electronic device; and

depositing a tin silver alloy layer onto the pure tin layer.

12. The method ofclaim 11, wherein the deposition is accomplished by electrodeposition.

13. The method ofclaim 11, wherein the under bump metal comprises either copper or nickel.

14. The method ofclaim 11, further comprising rinsing substantially all of the substantially pure tin plating chemistry from the electronic device.

15. A method for forming a lead free solder bump on a workpiece, the method comprising:

providing the workpiece with an electrically conducting seed layer, the electrically conducting seed layer being covered by a patterned resist mask layer having a plurality of feature openings;

immersing the workpiece in a first plating bath, the first plating bath having a metal ion content;

providing electrical contact to the seed layer and providing an electrical potential through the metal ion content of the first plating bath to cause between about 2 and about 150 microns of substantially pure tin to deposit in the resist pattern features;

immersing the workpiece in a second plating bath with a metal ion content; and

forming a electrical contact to the seed layer to form an electrical potential through the metal ion content of the second plating bath to cause between about 2 and about 150 microns of a tin-silver alloy to deposit in the resist pattern features.

16. The method ofclaim 15, further comprising removing the photoresist patterning layer.

17. The method ofclaim 15, wherein substantially all of the seed layer not covered by the plated tin and tin-silver alloy is removed.

18. The method ofclaim 15, further comprising thermally treating the workpiece at between about 210 to about 230 degrees centigrade to cause the substantially pure tin and tin-silver layers to intermix and form a substantially uniform tin-silver alloy feature.

19. The method ofclaim 15, wherein the substantially pure tin layer is about 30 microns and the tin-silver alloy layer is about 030 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.

20. The method ofclaim 15, wherein the substantially pure tin layer is about 10 microns and the tin-silver alloy layer is about 10 microns, and wherein the tin-silver alloy composition is between about 1% and about 7% silver by weight before thermal treatment and about 0.5% to about 3.5% silver by weight after thermal treatment.

21. The method ofclaim 15, wherein the substantially pure tin layer is about 1 micron or about 10 microns and the tin-silver alloy layer is between about 20 microns to about 120 microns.

22. The method ofclaim 15, wherein the substantially pure tin layer is about one-fourth the thickness of the tin-silver layer.

23. The method ofclaim 15, further comprising moving the workpiece to a rinse tank, rinsing substantially all of the substantially pure tin plating chemistry from the workpiece, and removing the workpiece from the rinse tank.

24. An apparatus for forming a lead free solder bump on a workpiece having an electrically conducting seed layer, the electrically conducting seed layer being covered by a patterned resist mask layer having a plurality of feature openings, the apparatus comprising:

a first process module disposed to support a first plating bath having a metal ion content adapted to deposit a substantially pure tin layer on the workpiece;

a second process module disposed to support a second plating bath with a metal ion content adapted to deposit a tin and silver layer on the workpiece; and

a controller programmable to plate the workpiece with the substantially pure tin layer in the first process module and to plate the workpiece with the tin and silver layer in the second process module.

25. The apparatus ofclaim 24, further comprising a rinse tank disposed to support rinsing substantially all of the pure tin plating chemistry from the workpiece, wherein the controller is further programmable to rinse the workpiece in the rinse tank.

26. The apparatus ofclaim 24, further comprising a copper electrodeposition module, wherein the controller is further programmable to deposit copper on the workpiece with the copper electrodeposition module.

27. The apparatus ofclaim 24, further comprising a nickel electrodeposition module, wherein the controller is further programmable to deposit nickel on the workpiece with the nickel electrodeposition module.

28. The apparatus ofclaim 24, further comprising a cleaning module, wherein the controller is further programmable to clean the workpiece with the cleaning module.

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