WO2000037217A1

WO2000037217A1 - Method for cleaning an abrasive surface

Info

Publication number: WO2000037217A1
Application number: PCT/US1999/028816
Authority: WO
Inventors: Rajeev Bajaj; Bradley Withers; Rahul Surana; Stephen Jew; Wilbur C. Krusell
Original assignee: Lam Research Corporation
Priority date: 1998-12-21
Filing date: 1999-12-03
Publication date: 2000-06-29
Also published as: TW431940B

Abstract

A method of cleaning an abrasive surface, such as a polishing pad in a chemical-mechanical-polishing apparatus, includes the application of an acidic cleaning solution to the abrasive surface, while applying an abrasive force to the abrasive surface for the removal of reaction by-products and processing debris from the abrasive surface. The acidic cleaning solution chemically reacts with the reaction by-products, and in conjunction with the abrasive force, removed the reaction by-products from the abrasive surface. In a preferred embodiment, a polishing pad used to remove copper from a semiconductor substrate is cleaned by applying a dilute hydrochloric acid solution to the polishing pad, while rotating a cleaning disk against the polishing pad.

Description

METHOD FOR CLEANING AN ABRASIVE SURFACE

FIELD OF THE INVENTION

This invention relates to a method for cleaning a polishing pad in a chemical-mechanical-polishing apparatus during semiconductor device fabrication.

BACKGROUND

The increasing need to form planar surfaces in semiconductor device fabrication has led to the development of process technology known as chemical-mechanical-polishing (CMP). In the CMP process, semiconductor substrates are rotated against a polishing pad in the presence of an abrasive slurry. Generally, the layer within the semiconductor device to be planarized is either an electrically insulating layer, or an electrical interconnect layer. As the substrate is rotated against the polishing pad, the abrasive force mills away the surface of the layer being planarized. Additionally, chemical compounds within the slurry undergo a chemical reaction with the components of the layer being planarized. The combination of the abrasive force created by the polishing pad and the chemical reaction within the slurry enhances the polishing rate. By carefully selecting the chemical components of the slurry, the polishing process can be made more selective to one type of material than to another. For example, in the presence of potassium hydroxide, silicon dioxide is removed at a faster rate than, for example, boron nitride. For the removal of metals, such as copper, commercially available copper slurry compositions typically include iron nitrates, ammonium salts, hydrogen peroxide, and the like. The ability to control the selectivity of a CMP process has led to its increased use in the fabrication of advanced integrated circuit devices.

A common requirement of all CMP processes is that the substrate be uniformly polished. A uniform polishing rate is essential for the formation of a planar surface in the semiconductor device. However, uniform polishing rates can be difficult to obtain, because, typically, there is a strong dependence of the polished removal rate on both the surface topography of the semiconductor substrate and equipment factors, such as the condition of the polishing pad, polishing pressure, and the like. In particular, changes in the surface texture of the polishing pad during the polishing process reduce the degree of abrasiveness of the polishing pad. During the polishing process, reaction by-products generated in the polishing slurry, and other debris, collect on the surface of the polishing pad. The by-product materials fill micro- pores in the surface of the polishing pad, which is known as glazing. Also, during the polishing process the abrasiveness of the pad is reduced by wearing of the pad surface. When the micro-pores become filled with residue from the polishing process, the polishing rate and removal variance decline. In extreme cases, a decline in polish removal rate can result in an incomplete removal of material from the semiconductor substrate. In CMP processing, as in many other areas of semiconductor device fabrication, a change in the polishing rate and within wafer-non-uniformity make process control extremely difficult. Ideally, a constant polishing rate is desired in order to maintain precise controls on the polishing process.

In order to avoid degradation in the polish removal rate caused by glazing a surface of the polishing pad, the pad can be abraded by a conditioner, such as a steel brush or, a nylon brush. Additionally, periodic cleaning of the pad can be carried out using conditioning disks having a diamond abrasive embedded in a polymer matrix. In one abrasion cleaning process of the prior art, metal by-products and polishing debris are removed from the surface of the pad by a mechanical grinding process in the presence of de-ionized water. If carried out vigorously, this process can result in removing material from the pad itself, in addition to reaction by-products and debris from the polishing process. However, increased mechanical conditioning, while maintaining the polishing rate, can reduce both planarization and pad life. Frequent cleaning of the polishing pad can be beneficial in maintaining a constant polishing rate. For example, in a dielectric polishing process, the polishing rate will typically be stable for process loads of about 500 to 1000 substrates. However, the polish rate stability can decline rapidly where a metal, such as copper, is polished. Many CMP processes show significant polish rate variability with as few as 25 to 50 semiconductor substrates even with frequent cleaning. The rapid decline in the copper polishing rate can be very detrimental to the throughput and quality of a copper CMP process. The copper CMP process is especially critical in copper metallization technology used in high performance microprocessor devices. Copper polishing slurries are formulated to oxidize copper and polish away localized areas of increased copper thickness, while not removing copper in localized areas of reduced thickness. Accordingly, in the copper polishing process, planarization is therefore achieved by preferential removal of copper-copper oxide from the localized areas of increased copper thickness. Given the importance of the copper polishing process to the fabrication of advanced integrated circuit devices, improved processes are needed to maintain uniform copper polishing rates, and to increase the throughput of the copper CMP process.

BRIEF SUMMARY

The preferred embodiment described below is a method of operating a polishing apparatus in which a polishing slurry is applied to a polishing pad. A semiconductor substrate having a metal layer thereon is subjected to metal polishing to remove portion of the metal layer. Then the semiconductor substrate is removed from the polishing apparatus, and an acidic cleaning solution is applied to the polishing pad. An abrasive force is then applied to the polishing pad in the presence of the acidic cleaning solution to remove metallic by-products and polishing debris from the surface of the polishing pad. The foregoing description provides a summary of an embodiment of the invention only and is not intended to limit the scope of the invention in any way whatsoever. BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 illustrate, in cross-section, processing steps for the removal of a copper layer from a semiconductor device using a CMP process in accordance with the invention; FIG. 3 is a perspective view of the operative portions of a single substrate CMP apparatus useful for practicing the invention; and

FIG. 4 is a top view of the operative portions of a batch CMP apparatus useful for practicing the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment, the present invention is for cleaning an abrasive surface, such as a polishing pad, in an abrasive metal removal apparatus. In one form, the present invention is for a method of operating a polishing apparatus that provides an improved metal polishing process. The operating method of the invention includes a pad cleaning process in which an acidic solution is used to remove metallic by-products, such as copper oxide, and corrosion inhibitors typically used in commercial copper polish slurries. By periodically carrying out a chemical cleaning process, in accordance with the invention, stable and increased copper polishing rates can be realized. Shown in FIG. 1 , in cross-section, is a portion of a semiconductor substrate 10 having various device components formed at the surface of the substrate. The device components shown in FIG. 1 are merely representative of a vast number of different components used in an integrated circuit device. Accordingly, these components are only generally illustrated and are not meant to describe any particular type of semiconductor device. The device components include, for example, gate electrodes 12 and 14, and a resistor 16 all residing on a surface 18 of semiconductor substrate 10. A doped region 20 separates gate electrodes 12 and 14 along surface 18. A first thick dielectric layer 22 overlies gate electrodes 12 and 14 and resistor 16. A patterned metal interconnect layer 24 overlays dielectric layer 22 and a portion thereof makes contact with doped region 20 in semiconductor substrate 10. A second dielectric layer 26 overlies metal interconnect layer 24 and first dielectric layer 22. A copper layer 28 overlies second dielectric layer 26 and fills vials 30, 32, and 34.

Those skilled in the art will recognize the structure illustrated in FIG. 1 as a general depiction of a multi-level metal interconnect structure. In such a device structure, electrical connections are provided in multiple overlying layers and each interconnect layer is separated by an inter level dielectric (ILD) material. Typically, a CMP process is carried out to form in-laid copper interconnects in vias and in trenches formed within the various ILD layers. In accordance with one embodiment of the invention, once copper layer 28 has been deposited, a CMP process is carried out to form the various in-laid copper interconnects, as illustrated in FIG. 2. Upon completion of the copper CMP process, a planar surface 34 is formed across second dielectric layer 26. The CMP process also forms an in-laid copper interconnect 36 and via studs 38, 40 and 42. The CMP process removes substantially all of copper layer 28 overlying the upper surfaces of second dielectric layer 26, while not removing substantial portions of second dielectric layer 26. This is known in the art as a selective polishing removal process, in which the polish removal rate of copper is substantially greater than the polish removal rate of a dielectric material, such as second dielectric layer 26.

As previously described, during a polishing process reaction by-products and polish residues collect on the polishing pad of the CMP apparatus. In the polishing process illustrated in FIGS. 1 and 2, the reaction by-product formed during the polishing process is a copper oxide material in which copper bonds with oxygen in both the +1 and +2 oxidation states.

Accordingly, the copper oxide can be copper (I) oxide (Cu₂O) and copper (II) oxide (CuO). In addition to copper oxide by-products, other residues can accumulate on a polishing pad during a copper polishing process, such as corrosion inhibitors, and the like. In many commercially available copper polishing slurries, benzotriazole (BTA) is included as a copper corrosion inhibitor. As previously described, once the polishing pad loads with reaction by-products and polishing residues, the polishing rate substantially declines. ln a preferred embodiment of the invention, the rapid decline in the copper polishing rate is combated through the use of an acidic cleaning solution during operation of a CMP apparatus. Upon addition of an acidic cleaning solution to the polishing pad during copper polishing operations, the copper oxide by-products are removed from the surface of the polishing pad, and either suspended within the polishing solution, or dissolved in the cleaning solution.

In a preferred embodiment of the invention, a CMP polishing apparatus is operated, in which a periodic cleaning process is carried out using an acidic solution. In the preferred embodiment for the operation of a copper CMP apparatus, a dilute hydrochloric acid solution is employed to remove copper oxide by-products from the surface of the polishing pad. Although a dilute hydrochloric acid solution is preferred, those skilled in the art will appreciate that other inorganic and organic acid solutions can also be used to remove metallic residues, such as copper oxide. For example, in addition to hydrochloric acid solutions, other inorganic acid solutions can be used, such as a nitric acid solution, a sulfuric acid solution, a hydrofluoric acid solution, and the like. Furthermore, in applications where an inorganic acid solution is undesirable, an organic acid solution can also be used. Preferably, a dicarboxylic organic acid solution can be used to dissolve metal oxide residues. Examples of dicarboxylic acid solutions include an oxalic acid solution, a tartaric acid solution, a malonic acid solution, and a phthallic acid solution, and the like. Additionally, a non-dicarboxylic acid solution can be used, such as a citric acid solution, and the like. In yet another embodiment of the invention the metal oxide removing agents of the CMP cleaning solution can be an acid salt. The acid salt can be an inorganic acid salt, or an organic acid salt. Preferably, the cleaning solution of the invention can be an inorganic or organic ammonium salt solution. Those skilled in the art will recognize that other metal oxides possess a similar reactivity to copper (I) oxide and copper (II) oxide. Accordingly, the present invention contemplates the removal of other transition metal oxides possessing similar chemical reactivity with an acidic solution. Specifically, transition metals within the group IB and MB groups of the periodic table can be expected to react in a similar manner in the presence of the inventive cleaning solution. Accordingly, it is within the scope of the present invention that any such group IB and IIB metal oxides can be removed from the surface of a polishing pad in accordance with the CMP operating method and cleaning process of the invention.

Those skilled in the art will recognize that the cleaning process of the invention can be carried out with a variety of different CMP machines. By way of illustration, the inventive process will be described in the context of two commonly-used types of CMP polishing machines. Shown in FIG. 3 is a perspective view of the operative portions of a single-substrate polishing apparatus 44. Polishing apparatus 44 includes first and second drive cylinders 46 and 48 that turn a polishing pad 50 in a continuous looping motion. A polishing wheel 52 is mechanically attached to and rotated by a shaft 54. A workpiece, such as semiconductor substrate 10, is mounted to a flat surface of polishing wheel 52 and brought into abrasive contact with polishing pad 50 by a vertical motion of rotating shaft 54. A platen resides directly beneath polishing pad 50 and resists the downward motion of rotating shaft 54. A dispenser 58 is cantilevered over polishing pad 50 in proximity to first drive cylinder 46.

Dispenser 58 is configured to dispense both polishing slurry and cleaning solution during operation of polishing apparatus 44. In a semiconductor polishing process in accordance with the invention, semiconductor substrate 10 is polished to remove a metal layer, such as copper layer 28. Upon completion of polishing semiconductor substrate 10, the substrate is removed from polishing apparatus 44, and a cleaning disk (not shown) is attached to polishing wheel 52. Next, previously described acidic cleaning solution is dispensed by dispenser 58 onto polishing pad 50, as a cleaning disk abrasively contacts the polishing pad. Preferably, cleaning solution is dispensed at a rate of about 50 to 100 ml per minute. During the dispensation of acidic cleaning solution, polishing pad 50 is rotated at a rate of about 20 to 80 rpm. A disk pressure of about 1 to 8 psi is applied by rotating shaft 54, while rotating the cleaning disk at about 5 to 80 rpm. Under these operation conditions, the cleaning process is carried out for a length of time sufficient to remove metal oxide residues and other polishing debris from the surface of polishing pad 50.

Those skilled in the art will appreciate that, although the process has been described in a sequence in which a semiconductor substrate is polished followed by a cleaning cycle, the sequence can be reversed. Accordingly, it is also contemplated by the present invention that a cleaning process be carried out prior to the polishing of a semiconductor substrate. Further, in a semiconductor fabrication facility, the high volume operation of a polishing machine, such as polishing apparatus 44, is typically carried out by the sequential polishing of many semiconductor substrates prior to the use of a cleaning cycle. Additionally, the process of the invention can be employed to initially condition a new polishing pad that has not previously been used to remove metal from semiconductor substrates. In this case, the polishing process of the invention is carried out to insure removal of any debris and metal oxides that are inadvertently present on the surface of a new polishing pad. Those skilled in the art will recognize that the cleaning process of the invention can also be carried in a CMP operation using a batch polishing apparatus. Shown in FIG. 4 is a top view of an exemplary batch CMP apparatus 60. A rotating polishing wheel 62 has a circular polishing pad 64 mounted thereon. A retaining wall 66 is disposed about the periphery of polishing wheel 62. A polishing slurry 68 is confined to a region about polishing pad 64 by retaining wall 66. A plurality of semiconductor substrates 70 are positioned on two rotating support assemblies 72. Movable arms 74 bring semiconductor substrate 70 into contact with polishing pad 64.

In operation, polishing pad 64 rotates about an axis 78, while substrate supports 72 rotate as shown in FIG. 4. The rotating motion of polishing pad

64 and substrate supports 76, together with pressure applied by movable arms 74, create abrasive action for removal of metal layers from semiconductor substrates 70. A dispenser 80 is positioned to dispense polishing slurry and cleaning solution onto polishing pad 64. In accordance with the invention, either immediately before or after polishing semiconductor substrates 70, polishing slurry 68 is replaced by an acidic cleaning solution and semiconductor substrate 70 are replaced by cleaning disks. Then, CMP apparatus 60 is operated, while dispensing about 20 to 1000 ml per minute of cleaning solution onto polishing pad 64. The cleaning process is carried out using a cleaning disk pressure of about 1 to 2.5 psi and CMP apparatus 60 is operated for a period of time of about 10 to 120 seconds to remove metal oxides and other polishing debris from the surface of polishing pad 64.

Those skilled in the art will recognize that the polishing apparatus described above are generally descriptive of CMP machines commonly used for semiconductor device fabrication. Both the single-substrate apparatus and the batch apparatus typically have a polyurethane type polishing pad. Poly- urethane polishing pads can be abrasively cleaned by conditioning disks, or alternatively by a cleaning brush. The cleaning method of the invention can be carried out to clean a polyurethane type polishing pad with either conditioning disks, or with an abrasive brush. Further, although the method of the invention has been described with respect to CMP machines employing a polishing pad, the invention can be applied to any abrasive removal process in which an abrasive surface is used in conjunction with a chemically reactive solution to remove a metal from a substrate surface. Preferably, an acidic solution, such as hydrochloric acid, nitric acid, sulfuric acid, hydrofluoric acid, and the like is applied to the abrasive surface and an abrasive force is generated against the abrasive surface. In a preferred method, the abrasive force is applied in the presence of a dilute hydrochloric acid solution to remove copper oxide from the abrasive surface. The cleaning process is carried out for a length of time sufficient to remove substantially all of the copper oxides from the abrasive surface. Thus, it is apparent that there has been provided, in accordance with the invention, a method for cleaning an abrasive surface, such as a polishing pad in a CMP apparatus that fully provides the advantages set forth above. Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. For example, the cleaning process disclosed herein can be used with different slurry compositions and different polish pad materials than those described. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.

Claims

1. In a metal polishing process for the removal of a metal from a semiconductor device, a method of operating a polishing apparatus comprising the steps of: applying a polishing slurry to a polishing pad; providing a semiconductor substrate having a metal layer thereon; polishing the semiconductor substrate to remove portions of the metal layer; applying an acidic cleaning solution to the polishing pad; and abrasively cleaning the polishing pad.

2. The method of claim 1 , wherein the step of providing a semiconductor substrate having a metal layer comprises providing a semiconductor substrate having a Group IB metal layer.

3. The method of claim 2, wherein the step of providing a semiconductor substrate having a Group IB metal layer comprises providing a semiconductor substrate having a copper layer.

4. The method of claim 1 , wherein the step of applying an acidic cleaning solution comprises applying a solution having a pH of less than about 7.

5. The method of claim 4, wherein the step of applying a solution having a pH of less than about 7 comprises applying a solution selected from the group consisting of an inorganic acid solution, an organic acid, a inorganic acid salt solution, and an organic acid salt solution.

6. The method of claim 5, wherein the step of applying an inorganic acid solution comprises applying a solution selected from the group consisting of a hydrochloric acid solution, a nitric acid solution, a sulfuric acid solution, and a hydrofluoric acid solution.

7. The method of claim 5, wherein the step of applying an organic acid solution comprises applying dicarboxylic acid solution.

8. The method of claim 7, wherein the step of applying a dicarboxylic acid solution comprises applying a solution selected from the group consisting of an oxalic acid solution, a tartaric acid solution, a malonic acid solution, a phthallic acid solution.

9. The method of claim 5, wherein the step of applying an organic acid solution comprises applying a citric acid solution.

10. The method of claim 5, wherein the step of applying an inorganic acid salt solution comprises applying an inorganic ammonium salt solution.

11. The method of claim 5, wherein the step of applying an organic acid salt solution comprises applying an organic ammonium salt solution.

12. A method of operating a chemical-mechanical-polishing apparatus having an abrasive surface upon which reaction by-products and processing debris can collect comprising the steps of: applying an acidic cleaning solution to the abrasive surface; applying an abrasive force to the abrasive surface in the presence of the acidic cleaning solution; and removing the reaction by-products and polishing debris from the abrasive surface.

13. The method of claim 12, wherein the step of applying an acidic cleaning solution comprises the step of applying a solution selected from the group consisting of an inorganic acid solution, an organic acid, a inorganic acid salt solution, and an organic acid salt solution.

14. The method of claim 13, wherein the step of applying an inorganic acid solution comprises applying a solution selected from the group consisting of a hydrochloric acid solution, a nitric acid solution, a sulfuric acid solution, and a hydrofluoric acid solution.

15. The method of claim 14, wherein the step of applying an organic acid solution comprises applying dicarboxylic acid solution.

16. The method of claim 15, wherein the step of applying a dicarboxylic acid solution comprises applying a solution selected from the group consisting of an oxalic acid solution, a tartaric acid solution, a malonic acid solution, a phthallic acid solution.

17. The method of claim 13, wherein the step of applying an organic acid solution comprises applying a citric acid solution.

18. The method of claim 13, wherein the step of applying an inorganic acid salt solution comprises applying an inorganic ammonium salt solution.

19. The method of claim 13, wherein the step of applying an organic acid salt solution comprises applying an organic ammonium salt solution.

20. A method for cleaning copper oxides and processing debris from an abrasive surface in a copper removal system comprising the steps of: applying an acid cleaning solution to the abrasive surface; applying an abrasive force to the abrasive surface; and removing the copper oxides and processing debris from the abrasive surface.

21. The method of claim 20, wherein the step of applying an abrasive force comprises rotating a cleaning disk against the abrasive surface while applying force to the cleaning disk.

22. The method of claim 21 , wherein the step of applying an acidic cleaning solution comprises applying a solution selected from the group consisting of a hydrochloric acid solution, a nitric acid solution, a sulfuric acid solution, and a hydrofluoric acid solution.

23. The method of claim 20, wherein the step of removing the copper oxides and processing debris comprises dissolving the copper oxides in a hydrochloric acid cleaning solution.