US20060070883A1

US20060070883A1 - Fixtureless vertical paddle electroplating cell

Info

Publication number: US20060070883A1
Application number: US10/958,161
Authority: US
Inventors: Lincoln Bejan; Douglas Crafts; Steve Swain; Duy Trang
Original assignee: Chemical Safety Tech Inc
Current assignee: Chemical Safety Tech Inc
Priority date: 2004-10-04
Filing date: 2004-10-04
Publication date: 2006-04-06

Abstract

A submersible assembly for exposing respective front surfaces of different work-pieces to an electroplating bath as a partial respective electrodes thereof, while protecting opposing, rear surfaces of the work-pieces from the bath. The assembly includes a wall, having a front surface also partially forming a first electrode of the bath, and having an opening smaller in size than the perimeter of a work-piece while substantially corresponding in size to the portion of the front surface of the work-piece to be exposed to the bath. The assembly also includes a chuck mechanism for applying pressure against the rear surface of the work-piece to thereby hold the work-piece against a rear surface of the wall and create a fluid-tight seal between the rear surface of the wall and the perimeter of the front surface of the work-piece, thereby exposing the front surface of the work-piece to the bath while protecting the rear surface of the work-piece from exposure to the bath.

Description

TECHNICAL FIELD

The present invention relates in general to electroplating a work-piece such as a silicon, integrated circuit wafer. More particularly, the present invention relates to a technique which allows easy insertion, positioning, and removal of a work-piece from an electroplating cell.

BACKGROUND OF THE INVENTION

Electroplating is a common process for depositing a thin film of metal or alloy on a work-piece such as a silicon wafer. During the electroplating process, the work-piece is placed in a suitable electrolyte bath (in a “plating cell”) containing ions of the metal to be deposited. The work-piece forms a first electrode (e.g., cathode) connected to the negative terminal of a power supply. A separate electrode (e.g., anode) separately positioned in the bath is connected to the positive terminal of the power supply. The anode and cathode are electrically charged to create an electric potential or field through the electrolyte. This electric field drives the metal ions to migrate towards and deposit onto the work-piece, thus electro-plating its surface.
Certain challenges exist in electroplating systems. For example, it is desirable that only the front surface of a silicon wafer, and not the back surface, be exposed to the electrolyte solution.
In addition, some systems require the use of complex fixtures to hold the work-piece in place during electroplating. One such fixture includes a polyvinylidene fluoride (PVDF) “holder” rigidly affixed to the wafer. This holder is then affixed to a surrounding frame or other structure to provide proper placement of the work-piece in the plating cell. This technique has certain drawbacks—including the need to assemble and disassemble the PVDF holder—resulting in decreased production throughput.
What is required, therefore, is a technique, including an apparatus and method, for positioning, holding, and sealing a work-piece in an electrolyte bath during electroplating, and which improves on the more complex structures and methods of the prior art techniques.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantages are provided by the present invention (apparatus and method) which in one aspect involves a submersible assembly for exposing respective front surfaces of different work-pieces to an electroplating bath as a partial respective electrode thereof, while protecting opposing, rear surfaces of the work-pieces from the bath. The assembly includes a wall, having a front surface also partially forming a first electrode of the bath, and having an opening smaller in size than the perimeter of a work-piece while substantially corresponding in size to the portion of the front surface of the work-piece to be exposed to the bath.
The assembly also includes a chuck mechanism for applying pressure against the rear surface of the work-piece to thereby hold the work-piece against a rear surface of the wall and create a fluid-tight seal between the rear surface of the wall and the perimeter of the front surface of the work-piece, thereby exposing the front surface of the work-piece to the bath while protecting the rear surface of the work-piece from exposure to the bath.
In one embodiment, the opening has a recess formed about its perimeter at the rear surface of the wall, facing away from the bath, and into which the work-piece can be removably positioned. The recess may be deep enough to allow the front surface of the work-piece to be substantially co-planar with the front surface of the wall. At least one thief may be installed on the front surface of the wall, substantially co-planar with the front surface of the work-piece.
Additional walls of the assembly may form a submersible compartment for fluid-tight submersion into the electrolyte bath and into which the work-pieces can be removably positioned. A bellows surrounds the chuck mechanism and for sealing the chuck mechanism within the compartment from any fluid which may inadvertently invade the compartment.
The retractable, bellow-sealed chuck; the wall/cathode contact with recess seal; and the overall submersible cathode compartment assembly are the operative elements of the vertical paddle plating cell of the present invention. This unique combination eliminates the need for complicated, manually assembled fixturing. The elimination of manually assembled fixturing enables the present invention to be very well suited for automated loading and unloading of a work-piece (e.g., an integrated circuit wafer), immediately from/to adjacent process steps, thus increasing production throughput.
Further, additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed out and distinctly claimed at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken with the accompanying drawings in which:
FIG. 1 is a perspective view of the primary electroplating cell and submersible cathode compartment, in accordance with the present invention;
FIG. 2 is a perspective view of the submersible cathode compartment of the present invention;
FIG. 3 is a sectional, perspective view of a section of the compartment of FIG. 2;
FIG. 4 is a side view of the section of FIG. 3;
FIG. 5 is an enlarged view of the recess/work-piece interface of FIG. 4;
FIG. 6 is a further enlarged view of the recess/work-piece interface of FIG. 4; and
FIG. 7 is a flow diagram of one exemplary process for electroplating a work-piece, in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As discussed above, the present invention is directed to a fixture-less electroplating apparatus which facilitates easy insertion, positioning, and removal of a work-piece from an electroplating bath. The invention includes in one embodiment a submersible cathode compartment assembly serving as the “vehicle” for carrying the work-piece into the bath. Advantageously, the front surface of the work-piece is exposed to the bath, while the back surface of the work-piece is protected from exposure within the submersible compartment; and the technique for handling the work-piece can be implemented without any significant pre-fixturing of the work-piece, thus substantially increasing production throughput.
With reference to FIG. 1, electroplating assembly 10 includes a primary electroplating cell 100 containing the volume of electrolyte in its tank 102 and an anode 104 held in place within the tank, here vertically against one wall. The tank in one embodiment includes bottom, front, back and side walls forming a sealed box for holding the liquid electrolyte. The anode 104 is positioned in a parallel orientation with the cathodic, metallized surface of the work-piece held in submersible compartment 200 (discussed below). However, the anode can be typically placed in any orientation within the electrolyte suitable for electro-deposition to occur.
The work-piece can be any material/structure having an electrically conductive front surface, amenable to electroplating. The particular work-piece shown here is a silicon wafer held vertically in the compartment, with an electrically conductive metal seed layer deposited onto its front surface. Electrical current flow between the anode and the cathodic, metallized surface of the wafer, through the elecrolyte, deposits the metal or alloy on the article by an electro-chemical reaction.
In one embodiment the primary cell 100 also includes a paddle (not shown) which agitates the electrolyte near the surface of the work-piece surface by reciprocation via moving belt 106. The paddle is used to agitate the electrolyte in very close proximity to the cathodic surface of the work-piece. This significantly improves mass transfer of metal ions from the electrolyte to the cathodic surface of the work-piece. Moreover, mass transport of metal ions can be increased while still achieving high deposition uniformity the closer the paddle can be placed to the work-piece. The thin profile and precise parallelism of the thieves (discussed below) to the cathodic surface of the work-piece allows the paddle to be placed in closer proximity to the cathodic surface of the work-piece, thus, higher deposition rates and faster throughput are achieved at a given deposition uniformity.
In accordance with the present invention, submersible cathode compartment 200 secures and seals the work-piece, raises and lowers the work-piece into the electrolyte, and provides electrical contact to the cathodic, metallized front surface of the work-piece exposed to the electrolyte.
Exemplary compartment 200 includes a bottom (not shown), front wall 216/217, back wall 210 and side walls 212, 214 forming an inner chamber 202. Compartment 200 may be guided in and out of primary plating cell 100 along guides 112, which are in electrical contact with the front surface 216 of the compartment. The guides, and therefore the front surface 216 of the front wall, can be energized by cathode connections 110, to effect electroplating action in tank 102.
As shown in the perspective view of the compartment 200 of FIG. 2, additional cathode surfaces 231, 232, 234, 235 can be arranged around a circumference of an opening 220 of the front wall of compartment 200. (As discussed below, the front surface of the work-piece is exposed to the bath through this opening). The electrical potential can be independently controlled for each of these cathode surfaces, known as “thieves,” using separate sets of contacts 230, 233. The purpose of the thieves is to expand the electrical field beyond the outer circumference of the work-piece, thus resulting in improved uniformity of the plating deposit. Using independent control on a multiplicity of thieves allows the deposition uniformity around all quadrants of the work-piece to be adjusted. Advantageously, the thieves can be attached in very close proximity to the perimeter of the wafer without actually contacting the cathodic, metallized surface of the work-piece; and the thieves can be embedded in very nearly the same plane as the cathodic surface of the work-piece, since the front surface 216 may have recessed pockets in which the thin thieves can be inserted.
Since the submersible cathode compartment does not require any fixturing on the work-piece, and since the thieves are attached to the submersible cathode compartment, there is no need for complicated manual attachment of the thieves around the wafer. The thieves are thus held in place independently of the mechanism that secures the wafer into position, thus simplifying the overall system.
A single thief or multiple thieves can laterally surround the work-piece in any configuration—thus providing considerable control of the shape of the electrical field surrounding the work-piece.
Like the primary tank 102, the submersible compartment 200 can also form a box, with the exception that the front wall has an opening 220, and, in one embodiment, a recess in rear surface 217 of the front wall in which the work-piece is inserted and pressed against—forming a fully enclosed and sealed chamber 202. The particular opening 220 shown in the figures is a circular shaped, for accommodating a circular wafer.
With reference to the sectional view of the compartment 200 in FIG. 3, as discussed above, the front wall of compartment 200 includes a front surface 216 and back surface 217, and opening 220. Compartment 200 also contains a retractable chuck 246, which is adapted to removably hold the back surface of the work-piece on chuck surface 242 (e.g., by vacuum). Additionally, chuck surface 242 may include an o-ring or other suitable sealing mechanism about its perimeter, to form a fluid-tight seal over the back surface of the wafer, upon activation of the vacuum. The chuck actuating mechanisms 246 may also be enclosed by flexible, teflon bellows 244.
The fluid-tight sealing of the back surface of the work-piece, and the sealing bellows 244, though not required, provide extra levels of protection against any fluid which may unintentionally enter submersible compartment 200.
The side, sectional views of the compartment 200 shown in FIGS. 4, 5 and 6, also include the work-piece 300 shown in three successive positions designated A (work-piece being lowered into compartment), B (attached to chuck, but prior to chuck activation) and C (in place for electroplating). The smallest diameter defining opening 220 is its inner diameter 260 (FIG. 6). The back surface 217 of the front wall includes a recess 270 that is slightly larger in diameter 262 than the outer diameter the work-piece 300, and a depth that is nearly (but not completely) the thickness of the wall. In one embodiment, a ledge 250 (FIGS. 4, 5) can also be included having an inner diameter that is equal to or slightly smaller than the work-piece, and capable of providing a temporary, cradling vertical support for the work-piece in position B, prior to chuck activation. This ledge can either be a unitary structure, or multiple structures (e.g., pegs) distributed about the bottom of opening 220.
Annular recess interface 272 is adapted to provide sufficient electrical contact between the compartment wall and the work-piece, while also providing a sealing function. Sealing can be enhanced without impacting the electrical contact using a small silicone bead at the outer perimeter of the recess.
The retractable, sealed chuck; the wall/cathode contact with recess seal; and the overall submersible cathode compartment assembly are the operative elements of the vertical paddle plating cell of the present invention. This unique combination eliminates the need for complicated, manually assembled fixturing that would otherwise provide the functions described previously (wafer backside sealing, electrical contact to the front side of the wafer). The elimination of manually assembled fixturing enables the present invention to be very well suited for automated loading and unloading of the wafer, immediately from/to adjacent process steps, thus increasing production throughput.
More particularly, the submersible cathode compartment 200 described herein provides a number of functions. The compartment vertically supports the work-piece, and moves the wafer up or down vertically into the primary tank at the onset of plating and out of the primary tank upon completion of the plating process. This movement is provided by the chuck 246 which can be activated pneumatically, electro-mechanically or by any number of means known to those skilled in the art. The particular method shown in the figures is through the use of pneumatic cylinders.
Another function of the submersible cathode compartment is to provide a water-tight seal between the wafer and the inside of the compartment, thus protecting the back surface of the substrate from exposure to the electrolyte. The wafer can be placed manually or automatically into the compartment. Once the wafer is placed into the compartment, the wafer is pressed into the recess by chuck 246.
The cylinder, chuck and all of its associated sub-components are protected from the outside environment using a cylindrically shaped elastomer bellows. The bellows seals the chuck, and also the backside of the wafer. In one implementation, the bellows is made of molded Teflon, though it could be made of any elastomeric or flexible material. The bellows, chuck and cylinder sub-assembly provide three functions:
1. Secure the outer perimeter of the front surface of the wafer against an electrically conductive, continuous wall/cathode contact having a recess sized to accommodate the wafer. When a metalized wafer is secured to the wall/cathode contact, the plating cell's electrical circuit is completed and the wafer is ready for immersion into the electrolyte for plating.
2. Precisely locate and fix the wafer within the plating cell relative to all other critical components of the plating cell, e.g., the anode and thieves.
3. Protect the wafer back surface from electrolyte. The recess 270 built into the wall provides the first barrier against undesirable electrolyte migration around the wafer to the backside. The wall/cathode contact is slightly compliant which enables it to conform around the wafer edge slightly under the force of the chuck. This provides a water-tight seal, while providing the electrical contact described in function (1) above.
4. Provide a continuous, compliant water-tight seal against the perimeter of the backside of the wafer. The bellows provides this function along with the other functions previously described. This backside seal provides a second barrier against any electrolyte that may have migrated into submersible cathode compartment 200.
5. Ensures that the cathode contact surfaces never come in direct contact with the electrolyte. This is important because some formulations of electrolyte are capable of dissolving commonly used cathode contact materials such as copper, aluminum, platinum, gold other preferred low electrical-resistance metals. Electrical contact to the wafer is needed to close the circuit of the electroplating cell. In order for electro-deposition to take place, a negative charge (cathode) must be applied to a conductive layer on the wafer surface. A key challenge addressed by the present invention is creating both a sealing surface and a good cathodic contact to the wafer, simultaneously.
With reference to the flow diagram of FIG. 7, an exemplary silicon wafer electroplating process is now described:
700: The process flow begins with the submersible cathode compartment raised above the surface of the electrolyte bath 700. The work-piece has not yet been loaded into the compartment and the cathode contact is dry—i.e., position “A” above.
710: The fixtureless wafer is vertically lowered into the holder, vertically supported by ledge 250—i.e., in position “B” above.
720: A vacuum is activated to hold wafer 300 against chuck surface 242.
730: The chuck is pressurized to compress the wafer into the recess 270 of the compartment wall/cathode contact 216/217. The contact area between the wafer and the wall/cathode contact completely seals, thus preventing exposure of the wafer's back surface the electrolyte. Once the seal is made, contact can be verified electrically—i.e., position “C” above.
740: Once electrical contact is verified, the power supply is activated thus energizing the wafer. This prior energizing will prevent dissolution of certain surface metals when submerged. This is a key capability as it is often advantageous for the cathode contact and metalized wafer to be electrically charged prior to immersion into the electrolyte. The conductive metal seed layer on the wafer is often soluble in some commonly used electrolytes. Likewise, some commonly used cathode contact metals are soluble in electrolytes.
750: Compartment 200 is lowered into the primary plating cell 100. As the compartment is lowered into the tank, it displaces an equivalent volume of electrolyte held in the tank, thus raising the level of the electrolyte above the top edge of the vertically oriented wafer.
760: The anode is then energized, and plating proceeds.
770: Plating completes when the electro-deposit reaches the desired thickness.
780: Upon completion of plating, the submersible cathode compartment is raised out of the tank. As it is being raised, a water-jet air knife washes the electrolyte from the surface of the wafer and the exposed edges of the inner diameter of the cathode contact, while simultaneously blow-drying the wafer with a heated gas such as air or nitrogen. The key advantage of this capability is that the cathode contact is rinsed and dried in preparation for the next wafer to be loaded—thus the cathode contact is protected from potential dissolution by exposure to residual electrolyte. Moreover, the wafer can be removed from the holder in a clean, dry state, thus minimizing the potential for residual electrolyte to be carried into a subsequent plating step. Moreover, this capability makes this plating cell very well suited for automation, since the wafer is clean and dry, and there is little risk that an automated wafer carrier would be contaminated with residual electrolyte.
790: Once the compartment is fully raised, the chuck retracts, thus presenting the wafer for removal.
800: The vacuum is released, and the work-piece is available for removal. Since the cathode contact has been sealed from exposure to the electrolyte through each of the previous steps, it remains dry upon removal of the plated wafer.
The submersible cathode compartment can then be used for repeated, fast and accurate electroplating of different work-pieces, repeating the process above for each, without complicated, fixtured attachment of each work-piece.
Additional advantages of the present invention include:
1. The vertical orientation of the work-piece minimizes the potential for particles or debris from lodging into the metal deposit.
2. The vertical orientation of the work-piece prevents gases evolved from the electro-deposition reaction from collecting on the work-piece surface topology, which could form voids in the metal deposit.
3. The vertical orientation of the work-piece is well suited to rinsing and drying with a minimal amount of water, which could dilute the electrolyte bath.
4. The submersible cathode compartment and chuck mechanism enable the cathode contact to be kept dry before and after the completion of the plating cycle, which prevents dissolution of either the cathode contact or the metal seed layer on the work-piece.
5. The invention enables fixture-less work-piece handling, which enables effective rinsing and drying of the work-piece, minimizing the potential for contamination of subsequent plating.
6. Fixture-less handling also makes this design very well suited for automated wafer handling.
7. The invention enables the work-piece to be immersed into the electrolyte with the wafer surface electrically charged. This provides a second measure to prevent dissolution of either the cathode contact or the metal seed layer on the work-piece.
8. The compartment provides a simple means of providing multiple thieves surrounding the front surface of the work-piece, thus providing extensive process control.
9. The invention enables the thieves to be precisely positioned in-plane and parallel to the work-piece surface which enables very close proximity of the paddle to the surface of the wafer. This enables effective agitation of the electrolyte at the boundary layer of the work-piece surface. This agitation significantly increases mass transport of the metal ions in the electrolyte to the work-piece surface, which enables very high rates of plating deposit (high current density) with high plating thickness uniformity. This enables very high wafer plating throughput.
10. The invention lowers and raises the work-piece vertically in and out of the plating tank. This enables the use of a water-jet air knife for effective wafer rinsing and drying.
11. The submersible compartment displaces a like volume of electrolyte while it is being lowered into the primary tank. This is important because it allows electrolyte to remain in the primary tank, constantly immersing the anode and it's associated anode osmotic membrane that surrounds the anode. The anode can release solid debris during plating which are contained from the electrolyte via this membrane. Once the membrane dries, it will not function again as it was intended. Additionally, even if the membrane does not dry, it would take considerable time for it to refill and to establish a stabilized ionic balance with the electrolyte. The submersible cathode compartment allows the anode to remain immersed, thus preventing these problems, while enabling the wafer to be loaded dry-to-dry and enables the wafer to be energized before it is lowered into the electrolyte.
12. The submersible cathode contact can be lowered very rapidly into the electrolyte that is contained in the primary tank, thus minimizing the risk of the base metal dissolving when it comes in contact with the electrolyte. The submersion of the wafer can happen much faster than if the electrolyte had to be pumped up into the primary tank, with much less churning, stirring up or gasifying the electrolyte in comparison to pumping.
13. Another advantage of the submersible cathode compartment is ease of maintenance—with the sub-assembly raised out of the tank, all of the maintenance intensive sub-components are readily accessible.
Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.

Claims

1. A submersible assembly for exposing respective front surfaces of different work-pieces to an electroplating bath as a partial respective electrode thereof, while protecting opposing, rear surfaces of the work-pieces from the bath, the submersible assembly comprising:

a wall, having a front surface also partially forming a first electrode of the bath, and having an opening smaller in size than the perimeter of a work-piece while substantially corresponding in size to the portion of the front surface of the work-piece to be exposed to the bath;

a chuck mechanism for applying pressure against the rear surface of the work-piece to thereby hold the work-piece against a rear surface of the wall and create a fluid-tight seal between the rear surface of the wall and the perimeter of the front surface of the work-piece, thereby exposing the front surface of the work-piece to the bath while protecting the rear surface of the work-piece from exposure to the bath.

2. The submersible assembly of claim 1, wherein the opening has a recess formed about its perimeter at the rear surface of the wall, facing away from the bath, and into which the work-piece can be removably positioned.

3. The submersible assembly of claim 2, wherein the recess is deep enough to allow the front surface of the work-piece to be substantially co-planar with the front surface of the wall.

4. The submersible assembly of claim 3, further comprising at least one thief on the front surface of the wall.

5. The submersible assembly of claim 1, further comprising additional walls forming a submersible compartment for fluid-tight submersion into the electrolyte bath and into which the work-pieces can be removably positioned.

6. The submersible assembly of claim 5, further comprising a bellows surrounding the chuck mechanism and for sealing the chuck mechanism within the compartment from any fluid which may invade the compartment.

7. The submersible assembly of claim 5, in combination with a primary electroplating cell containing the electroplating bath, the combination comprising:

mechanical means to align the submersible assembly within the primary cell; and

electrical means to energize both the submersible assembly and the primary cell.

8. The submersible assembly of claim 7, wherein the work-piece comprises an integrated circuit wafer.

9. The submersible assembly of claim 1, wherein the work-piece comprises an integrated circuit wafer.

10. A method for exposing respective front surfaces of different work-pieces to an electroplating bath, while protecting opposing, rear surfaces of the work-pieces from the bath, the method comprising:

using a submersible assembly as a partial respective electrode of the electroplating bath; the assembly comprising:

11. The method of claim 10, wherein the opening has a recess formed about its perimeter at the rear surface of the wall, facing away from the bath, and into which the work-piece can be removably positioned.

12. The method of claim 11, wherein the recess is deep enough to allow the front surface of the work-piece to be substantially co-planar with the front surface of the wall.

13. The method of claim 12, wherein the assembly further comprises at least one thief on the front surface of the wall.

14. The method of claim 10, wherein the assembly further comprises additional walls forming a submersible compartment for fluid-tight submersion into the electrolyte bath and into which the work-pieces can be removably positioned.

15. The method of claim 14, wherein the assembly further comprises a bellows surrounding the chuck mechanism and for sealing the chuck mechanism within the compartment from any fluid which may invade the compartment.

16. The method of claim 14, in combination with a method for operating a primary electroplating cell containing the electroplating bath, the method comprising:

aligning the submersible assembly within the primary cell; and

energizing both the submersible assembly and the primary cell.

17. The method of claim 16, wherein the work-piece comprises an integrated circuit wafer.

18. The method of claim 10, wherein the work-piece comprises an integrated circuit wafer.