Wide lip seal for electroplating

Abstract

PROBLEM TO BE SOLVED: To provide a lipseal allowing optimal electric connection while an electroplating device is operating.SOLUTION: There is provided a lipseal 212 for an electroplating device wherein a clamshell engages and supplies electrical current to a semiconductor substrate during electroplating. The lipseal includes an elastomeric body having an outer portion configured to engage a cup 201 of the lipseal assembly and an inner portion configured to engage a peripheral region of the semiconductor substrate. The inner portion includes a protrusion having a width in a radial direction sufficient to provide a contact area with the semiconductor substrate which inhibits diffusion of acid in an electroplating solution used during the electroplating. The protrusion is located at the inner periphery of the lipseal.SELECTED DRAWING: Figure 2

Description

Field of the Technology

[0001] The present invention relates to the formation of damascene interconnects for integrated circuits and an electroplating apparatus used during integrated circuit processing.

Background Art

[0002] Electroplating is a common technique used in integrated circuit (IC) processing to deposit one or more layers of conductive metal. In some processing operations, it is used to deposit single or multiple levels of copper interconnects between various substrate features. Apparatus for electroplating typically comprises an electroplating cell having a pool / bath of electrolyte solution and a clam shell designed to hold a semiconductor substrate during electroplating.

[0003] During operation of the electroplating apparatus, the semiconductor substrate is immersed in the electrolyte pool such that one surface of the substrate is exposed to the electrolyte solution. One or more electrical contacts established with the substrate surface are used to drive a current through the electroplating cell and deposit metal onto the substrate surface from metal ions available in the electrolyte solution. Typically, the electrical contact element is used to form an electrical connection between the substrate and a bus bar that functions as a current source. However, in some configurations, a conductive seed layer on the substrate where the electrical connection contacts may become thinner towards the edge of the substrate, making it more difficult to establish an optimal electrical connection with the substrate.

[0004] Another problem that occurs in electroplating is the potential corrosiveness of the electroplating solution. Thus, in many electroplating apparatuses, a lip seal is used at the interface of the clam shell and the substrate for the purpose of preventing leakage of the electrolyte solution and contact of the electrolyte solution with elements of the electroplating apparatus other than the inside of the electroplating cell and the surfaces of the substrate designated to be electroplated.

Summary of the Invention

[0005] This specification discloses a lip seal for use in a lip seal assembly of an electroplating clamshell that engages with a semiconductor substrate during electroplating to supply current to the semiconductor substrate. The lip seal comprises an elastomer body having an outer portion configured to engage with a cup of the lip seal assembly and an inner portion configured to engage with a peripheral region of the semiconductor substrate. The inner portion is provided with a projection having a radial width sufficient to suppress the diffusion of acids in the electroplating solution used during electroplating. The projection is provided with an annular rim that extends completely around the inner circumference of the lip seal. [Brief explanation of the drawing]

[0006] [Figure 1] A diagram showing an electroplating apparatus that can utilize the lip seal described herein to prevent acid from reaching the contact elements.

[0007] [Figure 2] Figure 1 shows a detailed diagram of the lip seal assembly that can be used with the device shown in Figure 1.

[0008] [Figure 3] Figure 2 shows a detailed view of the lip seal assembly.

[0009] [Figure 4] Figure 3 shows a detailed view of the lip seal assembly.

[0010] [Figure 5] A graph showing the relationship between acid concentration and lip seal width in the lip seal region.

[0011] [Figure 6a] A photograph showing the copper seed layer after electroplating a dried wafer. [Figure 6b] Photograph showing severe corrosion of the copper seed layer after electroplating a wet wafer using a 0.028-inch (1 inch = 25.4 mm, same applies below) wide lip seal. [Figure 6c] Photograph of the copper seed layer after electroplating a wet wafer using a 0.034-inch wide lip seal, showing minor corrosion of the copper seed layer. [Modes for carrying out the invention]

[0012] The following explanation includes numerous specific details to facilitate a complete understanding of the presented concepts. The presented concepts can be implemented without some or all of these specific details. Furthermore, detailed descriptions of well-known processing operations have been omitted to avoid unnecessarily obscuring the concepts. While some concepts are described in relation to specific embodiments, it should be understood that these embodiments are not intended to be limiting.

[0013] To provide context to the various lip seal and contact elements disclosed herein, an example of an electroplating apparatus is shown in Figure 1. Specifically, Figure 1 is a perspective view of a wafer holding / positioning apparatus 100 for electrochemically processing semiconductor wafers. The apparatus 100 comprises wafer engaging components, which are also referred to as “clamshell components,” “clamshell assemblies,” or simply “clamshells.” The clamshell assembly comprises a cup 101 and a cone 103. As shown in subsequent drawings, the cup 101 holds the wafer and the cone 103 firmly clamps the wafer within the cup. Other cup and cone designs not specifically illustrated herein may be used. A common feature is that the cup has an internal region in which the wafer fits, and the cone presses the wafer against the cup to hold the wafer in place.

[0014] In the embodiment shown in the figure, a clamshell assembly (comprising a cup 101 and a cone 103) is supported by a support column 104, which is coupled to an upper plate 105. The assembly (101, 103, 104, and 105) is driven by a motor 107 via a spindle 106 connected to the upper plate 105. The motor 107 is mounted on a mounting bracket (not shown). The spindle 106 transmits torque (from the motor 107) to the clamshell assembly to rotate the wafer (not shown) held therein during plating. An air cylinder (not shown) within the spindle 106 further provides a vertical force to engage the cup 101 with the cone 103. When the clamshell is not engaged (not shown), a robot with an end effector arm can insert the wafer between the cup 101 and the cone 103. After the wafer is inserted, the cone 103 engages with the cup 101 to secure the wafer within the apparatus 100, exposing one side of the wafer's working surface for contact with the electroplating solution (but not the other side).

[0015] In certain embodiments, the clamshell assembly includes a spray skirt 109 to protect the cone 103 from splashed electrolyte. In the illustrated embodiment, the spray skirt 109 comprises a vertical perimeter sleeve and a circular cap portion. A spacer member 110 maintains separation between the spray skirt 109 and the cone 103.

[0016] For the purposes of this discussion, we will refer to the assembly comprising components 101–110 collectively as a “wafer holder” (or “substrate holder”) 111. However, it should be noted that the concept of “wafer holder” / “substrate holder” generally extends to various combinations and partial combinations of components that engage with the wafer / substrate to enable its movement and positioning.

[0017] An inclined assembly (not shown) may be connected to the wafer holder to allow the wafer to be immersed in the plating solution at an angle (as opposed to flat, horizontal immersion). In some embodiments, a drive mechanism and arrangement of plates and pivot joints are used to move the wafer holder 111 along an arc-shaped path (not shown), thereby inclining the proximal end of the wafer holder 111 (i.e., the cup and cone assembly).

[0018] Furthermore, the entire wafer holder 111 is lifted vertically either up or down by an actuator (not shown) to immerse the proximal end of the wafer holder in the plating solution. Thus, the two-component positioning mechanism provides the wafer with both vertical movement along a trajectory perpendicular to the electrolyte surface and inclined movement that allows deviation from a horizontal orientation (i.e., an orientation parallel to the electrolyte surface) (inclined wafer immersion function).

[0019] Note that the wafer holder 111 is used in conjunction with a plating cell 115 having an anode chamber 157 and a plating chamber 117 for containing the plating solution. The chamber 157 holds an anode 119 (e.g., a copper anode) and may have a membrane or other separator designed to maintain different electrolyte chemicals within the anode and cathode compartments. In the illustrated embodiment, a diffuser 153 is used to direct the electrolyte upward toward the rotating wafer with a uniform front surface. In certain embodiments, the flow diffuser is a high-resistance virtual anode (HRVA) plate, which is formed of a solid insulating material (e.g., plastic) piece and has a large number (e.g., 4,000 to 15,000) one-dimensional small holes (0.01 to 0.050 inches in diameter) connected to the cathode chamber above the plate. The total cross-sectional area of ​​the holes is less than about 5 percent of the total projected area and thus helps to introduce substantial flow resistance within the plating cell to improve the plating uniformity of the system. Further descriptions of high-resistance virtual anode plates and corresponding apparatus for electrochemically processing semiconductor wafers are provided in U.S. Patent Application Publication No. 2010 / 0032310, which is incorporated herein by reference in its entirety for all purposes. The plating cell may comprise a separate membrane for controlling and generating a separate electrolyte flow pattern. In another embodiment, the membrane is used to define an anode chamber, which contains an electrolyte substantially free of inhibitors, accelerators, or other organic plating additives.

[0020] The plating cell 115 may also include piping or piping connections for circulating the electrolyte through the plating cell and, by extension, to the workpiece being plated. For example, the plating cell 115 includes an electrolyte inflow tube 131 that extends vertically to the center of the anode chamber 157 through the central hole of the anode 119. In another embodiment, the cell includes an electrolyte inflow manifold (not shown) that introduces fluid into the cathode chamber below the diffuser / HRVA plate at the peripheral wall of the chamber. In some examples, the inflow tube 131 includes outflow nozzles on both sides (anode side and cathode side) of the membrane 153. This configuration supplies the electrolyte to both the anode chamber and the cathode chamber. In another embodiment, the anode chamber and the cathode chamber are separated by a flow resistance membrane 153, and each chamber has a separate cycle of flow of the separated electrolyte. As shown in the embodiment of FIG. 1, the inflow nozzle 155 supplies the electrolyte to the anode side of the membrane 153.

[0021] Furthermore, the plating cell 115 includes a rinse drain line 159 and a plating solution recovery line 161, each of which is directly connected to the plating chamber 117. Also, a rinse nozzle 163 supplies deionized rinse water for cleaning the wafer and / or the cup during normal operation. The plating solution typically fills most of the chamber 117. To mitigate bubble bounce and generation, the chamber 117 includes an inner weir 165 for plating solution recovery and an outer weir 167 for rinse water recovery. In the illustrated embodiment, these weirs are peripheral vertical slots in the wall of the plating chamber 117.

[0022] As described above, an electroplated clamshell typically comprises a lip seal and one or more contact elements to provide sealing and electrical connection functions. The lip seal may be formed of an elastomer material. The lip seal forms a seal with the surface of the semiconductor substrate and excludes the electrolyte from the peripheral region of the substrate. No deposition occurs in this peripheral region, and it is not used for IC device formation; that is, the peripheral region is not part of the work surface. Sometimes this region is also called the edge exclusion region because the electrolyte is excluded from the region. The peripheral region is used to support and seal the substrate during processing, as well as to electrically connect with the contact elements. Generally, it is desirable to have a large work surface, so the peripheral region needs to be as small as possible while maintaining the functions described above. In certain embodiments, the peripheral region is between approximately 0.5 mm and 3 mm from the edge of the substrate.

[0023] During installation, the lip seal and the contact element are assembled with the other components of the clam shell. A person skilled in the art will understand that this operation is difficult, especially when the surrounding area is small. The entire opening provided by this clam shell corresponds to the size of the substrate (for example, an opening for accommodating a 200 mm wafer, a 300 mm wafer, a 450 mm wafer, etc.). Furthermore, the substrate has its own dimensional tolerances (for example, for a typical 300 mm wafer according to SEMI specifications, + / - 0.2 millimeters). A particularly difficult task is the alignment of the elastomeric lip seal and the contact element, since both are made of relatively flexible materials. These two components need to have a very precise relative position. If the sealing edge of the lip seal and the contact element are placed too far apart from each other, during the operation of the clam shell, the electrical connection between the contact and the substrate may become insufficient or disappear completely. At the same time, if the sealing edge is placed too close to the contact, the contact may interfere with the seal and cause leakage into the surrounding area. For example, conventional contact rings are often formed with a plurality of flexible "fingers" that are pressed onto the substrate with a spring-like action to establish an electrical connection, as shown in the clam shell assembly of FIG. 2 (cup 201, cone 203, and lip seal 212). These flexible fingers 208 are not only very difficult to align with the lip seal 212, but are also easily damaged during installation and difficult to clean when electrolyte enters the surrounding area.

[0024] As explained above, in an electroplating cell, electrical contact is made with the wafer around the wafer edge, and electroplating is performed on the rest of the wafer. However, when the plating solution reaches the contact point, the acid in the plating solution may corrode the metal seed layer on the wafer in the contact area, resulting in increased resistance distributed irregularly around the wafer, which in turn reduces plating performance and increases wafer non-uniformity. Metal ions in the solution can also precipitate on the contact point, further reducing plating efficiency. To prevent seed corrosion and plating on the contact point, the area where contact is made is separated from the plating solution by a lip seal. Previously, it was thought that complete separation of the contact point from the plating solution was sufficient unless there was overall damage to the lip seal (cracks, tears, etc.). However, recent studies have shown that when a wet wafer is placed on a lip seal (e.g., as in Sabre 3D's advanced pre-treatment process), a thin layer of water remains between the lip seal and the wafer, allowing the acid in the plating solution to diffuse through it and reach the contact area. At high temperatures and / or long plating times, this diffusion can occur to the extent that enough acid reaches the contact area, causing corrosion of the metal seed layer (seed corrosion). To address this challenge, a wide lip seal was designed to increase the distance the acid must diffuse and, accordingly, slow down the rate at which the acid reaches the contact area. In this way, seed corrosion at the wafer edges is reduced and the uniformity of the plating is improved.

[0025] In an electroplating cell, the wafer to be plated is held in a cup, which allows electrical contact with the wafer edge in the area enclosed by the lip seal, while the rest of the wafer is exposed to the plating solution. The cup is partially immersed in the plating solution within the plating cell during plating. However, as described above, the acid can diffuse across the liquid film between the wafer and the lip seal quickly enough to damage the metal seed layer on the wafer in the contact area.

[0026] According to one embodiment, a hardware design modification was made to increase the width of the lip seal (the width of the protrusions on the lip seal that seal the wafer) in order to reduce the rate at which acid diffuses through the liquid layer between the wafer and the lip seal. Increasing the width of the lip seal reduces etching of the metal seed layer by increasing the diffusion distance and, as a result, reducing the amount of acid that reaches the contact area.

[0027] The lip seal width can be increased by increasing the outer diameter of the lip seal projection or by decreasing the inner diameter of the projection. A preferred embodiment is to increase the outer diameter of the projection, as this does not reduce the area available for plating.

[0028] The shape of the projection on the lip seal may be enlarged to form a lip seal with a cross-section similar in shape to that of the previous design, where the projection is simply extended in the radial dimension. This is a preferred embodiment. The lip seal may comprise a single contact surface in the form of an annular rim with a cylindrical wall and a flat or angled surface that contacts the wafer.

[0029] Previously, lip seal designs for Sabre 3D did not prevent a significant amount of acid diffusion into the contact area at temperatures or higher acid concentrations in the plating solution than those present under past standard operating conditions (below 35°C, below 140 grams per liter of acid). The advantage of a wider lip seal is that it can provide sufficient sealing at more demanding operating conditions, such as temperatures above 35°C and / or acid concentrations above 140 grams per liter of acid.

[0030] In wafer processing, when a wet wafer is placed on a lip seal, a thin layer of water may remain, allowing acid to diffuse through this layer, reach the contact area, and damage the metal seed layer on the wafer. To avoid this problem, the lip seal is configured to provide a longer diffusion path for the acid, thus substantially increasing the time required for etching to occur in the contact area. A longer diffusion path can be achieved by a wider lip seal (longer linear length).

[0031] Diffusion beyond the lip seal can be modeled as one-dimensional diffusion with a constant source, which has the following equation: C / C s =erfc(z / 2√Dt), where z is the width of the lip seal, D is the diffusion constant of the acid, t is time, C s is the acid concentration at the source, C is the acid concentration at z, and erfc is the complementary error function. Therefore, 2√Dt (diffusion length) is given by C, C for the given conditions. s , and it is possible to estimate by knowing z, and z and C s We can find C as a function of , and we can predict that 2√Dt will remain approximately constant as long as time, temperature, and diffusing species remain the same.

[0032] For example, z = 0.020 inches and "C s Considering the case of 180 g / L sulfuric acid: Under these conditions, C is estimated to be about 8–9 g / L after 1.5 hours of plating, with 2√Dt ≈ 0.014, and 2√Dt can be used to estimate the sulfuric acid concentration after plating at other lip seal widths, as shown in the graph in Figure 5. C reaches 1 g / L (approximate level at which very slight corrosion occurs after 1.5 hours) at a lip seal width of about 0.028 inches, and thereafter decreases rapidly. A preferred lip seal width is at least 0.032 inches, and more preferably at least 0.034 inches.

[0033] Figure 3 shows one embodiment of a lip seal 212 attached to a cup 201, where an electrical contact 208 engages with the underside of a semiconductor substrate (such as a wafer W). As shown in Figure 4, the lip seal 212 comprises an inner portion 218 having a projection 220 with an upper surface 220a that contacts the underside of the wafer W, and an outer portion 230 having a rim 232 that engages with a recess 201a of the cup 201. The projection 220 extends axially upward and has a width (width measured radially between the inner and outer cylindrical walls of the projection) sufficient to prevent the diffusion of acid in the plating solution from reaching the contact point between the electrical contact 208 and the wafer W. For processing a 300 mm diameter wafer, the width of the projection 220 may be at least about 0.032 inches, and preferably at least about 0.034 inches. The lip seal 212 is preferably a single, fully elastomerized component configured to mesh with the cup 201. Thus, the lip seal 212 is a separate consumable part that can be easily replaced when needed.

[0034] Figures 6a–c are photographs of the outer edges of wafers processed under different conditions. Figure 6a shows a wafer plated without pre-wetting, where the lip seal provided sufficient sealing to prevent acid diffusion beyond it, and the copper seed layer at the wafer edge is not corroded. Scratches caused by electrical contacts used during electroplating are visible on the copper seed layer. Figure 6b shows a wafer plated with pre-wetting, using a 0.028-inch wide lip seal that does not provide sufficient sealing. Pre-wetting forms a film of water between the wafer and the lip seal, allowing acid to diffuse beyond it and corrode the copper seed layer at the wafer edge. In Figure 6b, the copper seed layer is severely corroded, and only a thin layer of copper oxide (black) and a tantalum barrier layer (silver) is visible. Figure 6c shows a wafer plated with pre-wetting, using a 0.034-inch wide lip seal that provides sufficient sealing. In the image, the copper seed layer is visible, and a very thin layer of copper oxide on the surface of the copper seed layer forms small stains. This indicates that the copper seed layer has only suffered minimal corrosion due to a wide lip seal that created longer diffusion pathways.

[0035] In this specification, the term "approximately" is often used with numerical values ​​to indicate that mathematical precision is not intended for such values. Therefore, when the term "approximately" is used with a numerical value, a tolerance of ±10% is assumed for that value.

[0036] While embodiments and applications of the present invention are illustrated and described herein, many modifications and variations are possible within the scope and spirit of the invention, and such modifications will be apparent to those skilled in the art through careful reading of this application. Therefore, these embodiments are considered illustrative and not limiting, and the invention is not limited to the details shown herein and may be modified within the scope of the appended claims and equivalents.

Claims

1. A lip seal used in a lip seal assembly of an electroplating clamshell that engages with a semiconductor substrate during electroplating to supply current to the semiconductor substrate, The elastomer body comprises an outer portion configured to engage with the cup of the lip seal assembly and an inner portion configured to engage with the surrounding region of the semiconductor substrate, The inner portion comprises a projection having a radial width sufficient to suppress the diffusion of acid in the electroplating solution used in the electroplating, and the projection comprises an annular rim that extends completely around the inner circumference of the lip seal.

2. A lip seal according to claim 1, wherein the width is the width between the inner and outer walls of the projection, and the width is at least about 0.8128 mm (0.032 inches).

3. A lip seal according to claim 2, wherein the width of the lip seal is approximately 0.8636 mm (0.034 inches).

4. A lip seal according to claim 1, wherein the outer portion comprises a downwardly extending rim configured to be received in a recess of the cup.

5. A lip seal according to claim 1, wherein the inner surface of the projection defines the inner diameter of the lip seal.

6. A method for electroplating a semiconductor substrate using the lip seal of claim 1, comprising the steps of: supporting a pre-wet semiconductor substrate in an electroplating clamshell such that the projection of the lip seal contacts the outer periphery of the semiconductor substrate; and bringing the exposed surface of the semiconductor substrate inside the projection into contact with an electroplating solution.

7. A method according to claim 6, wherein the projection has a width of about 0.8636 mm (0.034 inches).

Images