POLISHING PAD TREATMENT FOR SURFACE CONDITIONING

Abstract: A polishing pad is treated for surface conditioning by exposing a polishing surface on the pad to a chemical solvent having a solubility parameter that differs by less than about twenty percent from a solubility parameter of the material of the polishing pad that provides the polishing surface, wherein the polishing surface is softened relative to a remainder of the material to minimize the time consumed by surface conditioning of the polishing surface.

Description: POLISHING PAD TREATMENT FOR SURFACE CONDITIONING The invention relates to a polishing pad for use in a chemical-mechanical polishing operation, wherein the polishing pad is subjected to a process known as surface conditioning . Surface conditioning is performed by making numerous passages or sweeps of abrasive material against the polishing surface of the pad. Surface conditioning either creates or restores a micro-texture on the polishing surface that is debris-free and defect-free, which is ideal for polishing a wafer in a CMP operation. Further, surface condition is used to true the polishing surface to a desired plane. A disadvantage to be overcome, is that surface conditioning is time consuming, and increases the cost- of producing polished wafers. A process is needed for treating a polishing pad, which reduces the time required for surface conditioning of the polishing pad. Further, a polishing pad is needed that has undergone a treatment that reduces the time required for surface conditioning of the polishing pad. The invention resides in a process of treating a polishing pad for surface conditioning, wherein the process comprises, softening a polishing surface on a material of the polishing pad by exposing the polishing surface to a chemical solvent having a solubility parameter that differs by less than about twenty percent from a solubility parameter of the material of the polishing pad that provides the polishing surface, wherein the polishing surface is softened relative to a remainder of the material to reduce the time required for surface conditioning of the polishing surface . According to the invention, a polishing pad treated for surface conditioning comprises, a polishing surface being softened by a chemical solvent, wherein the polishing surface is softened relative to a remainder of the material that provides the polishing surface, to reduce the time required for surface conditioning of the polishing surface. Embodiments of the invention will now be described, by way of example, with reference to the following description. A new pad undergoes surface conditioning, i.e., preconditioning, to create the desired micro-texture. A polishing pad is pre-conditioned prior to initial use to achieve a stable polishing rate. During use of a pad in a CMP operation, the micro-texture can experience unwanted plastic flow and can be fouled by debris, which requires surface conditioning, i.e., post-conditioning to restore the desired micro-texture that is debris-free and defect-free. A polishing pads is post-conditioned when the polishing rate declines or falls off, such that the post-conditioned pad attains a higher level of polishing rate. A polishing pad is post-conditioned periodically during its useful life to restore an optimal micro-texture . A chemical-mechanical polishing, CMP, operation is described as, urging a semiconductor wafer, on which integrated circuits are to be fabricated thereon, against a moving polishing pad to remove a deposited layer of metal and to produce an extremely smooth and flat, planar surface on the wafer. A polishing fluid having, de-ionized water and/or a chemically active reagent with the metal and with or without a slurry of abrasive particles, is applied to the interface of the wafer and the polishing pad during the CMP operation. A rate at which material is removed from the wafer surface is described as the polishing rate. Higher polishing rates are generally desired to reduce polishing time and consequent production costs. Polishing rates are initially low for an untreated polishing pad. As the polishing pad is broken in, by polishing successive wafers, the polishing rate increases, or will ramp up, to a stable maximum level. After polishing numerous wafers, the polishing rate tapers off and eventually declines to such an extent that the polishing pad must be renewed or replaced. Pre-conditioning is especially necessary for molded polymeric polishing pads because these have a surface skin that must be disturbed or broken-in by conditioning the pad surface to attain a higher polishing rate. It is desirable to reduce the time required for the pre-conditioning process and to reduce the time required for the post-conditioning process. Similarly, it is also desirable to extend the time between post-conditioning operations. Therefore, a polishing pad that is subjected to a treatment that reduces the duration of the conditioning process would be advantageous. One embodiment of a polishing pad is made of a polymeric material that provides a polishing surface. The pad may be produced by any suitable process including thermoplastic injection molding, thermoset injection molding (often referred to as "reaction injection molding" or "RIM"), thermoplastic or thermoset injection blow molding, compression molding, casting, or any similar-type process in which a flowable material is positioned and solidified. According to an embodiment, a polishing pad is treated with a chemical solvent which modifies the polishing surface. A surface layer defined by a depth of the material beneath the polishing layer is also modified as determined by the depth of penetration by the chemical solvent. The polishing pad is treated by simply contacting the polishing pad with the solvent. According to one method, the solvent is applied by wiping the polishing pad with a lint-free applicator that has been soaked in the solvent. The solvent is applied as a uniform wet coating to the polishing surface of the polishing pad. Subsequently, the pad is air dried prior to use for CMP. Alternatively, the solvent is sprayed onto the surface of the polishing pad using a suitable spray gun or atomizer. Alternatively, the solvent is combined with a preconditioning liquid that is applied to the polishing pad during a pre-conditioning cycle prior to a polishing operation. Alternatively, the chemical solvent is combined with a polishing fluid, which is used along with the polishing pad during a polishing operation. The chemical solvent is required to modify or alter a surface layer of the polishing pad, yet be non-reactive with any polishing slurry and a semiconductor wafer with which the polishing pad will be used. To determine an effective chemical solvent for the polishing pad, one factor which should be considered is solubility parameter. Solubility parameter is a value relating to cohesive energy density of a solvent or a polymer. A solubility parameter can be calculated for each different solvent and each different polymer. The difference between the solubility parameters of two substances relates to how well the substances will mix. As the difference between solubility parameters is reduced, substances can be more readily mixed, and two substances having the same solubility parameter will be completely miscible. A discussion of methods for calculating solubility parameter and a table of solubility parameters for various solvents and polymers can be found in the Polymer Handbook, second edition, Brandrup and Immergut editors, Interscience Publishers, John Wiley and Sons, 1975, pages 341-368. A suitable solvent for application to a polymeric polishing pad should have a solubility parameter that differs by less than about twenty percent from the solubility parameter of the polishing pad material. A solubility parameter that differs by less than about ten percent from the solubility parameter of the polishing pad material is further suitable. According to an embodiment, a polymeric polishing pad is made of a polyurethane material having a solubility parameter of approximately 10 (cal/cm 3 ) . Preferred solvents for use with this polishing pad are N- methyl pyrrolidone (NMP) and dimethyl formamide (DMF) , which have solubility parameters of 11.3 and 12.1 (cal/cm 3 ) , respectively. The chemical solvents described herein, further have a medium to low range of hydrogen bonding capability or infinity for hydrogen bonding. Solvent treatment as described herein, softens the surface layer of the polishing pad. The softer surface makes the pad easier to condition and reduces both the preconditioning time and post-conditioning time, significantly. An optimum micro-texture is more easily achieved, which is desired for producing higher, maximizing, polishing rates and increased, maximizing, uniformity of a polished wafer surface. According to one embodiment, the chemical solvent has a solubility parameter that differs by less than about twenty percent from a solubility parameter of the material that provides the polishing surface. One embodiment of a chemical solvent for polyurethane polishing pads is N-methyl pyrrolidone (NMP) . Another embodiment of a chemical solvent for polyurethane polishing pads is dimethyl formamide (DMF) . A method of treating a polishing pad made of polymeric material comprises contacting a surface of the polishing pad with a chemical solvent, wherein the surface and a layer of the polishing pad adjacent to the surface are softened. According to an embodiment, the chemical solvent is integrated into a pre-conditioning liquid which is applied to the polishing pad during a pre-conditioning cycle prior to a polishing operation. Alternatively, the chemical solvent is integrated into a polishing slurry with which the polishing pad is used during a polishing operation. EXAMPLE A test was conducted to determine the effect of polishing pad treatment on material removal rate. In this test, treated and untreated OXP3000 polishing pads manufactured by Rodel, Inc., of Newark, DE were pre- conditioned by sweeps of a conditioning apparatus across each pad. Prior to pre-conditioning, the treated pad was treated by receiving an application of NMP at 50% concentration in de-ionized water. • The NMP solution was applied by soaking a cheesecloth in the solution and wiping the cheesecloth over the polishing surface of the pad so as to wet the polishing surface. The following Table shows material removal rates as a function of the number of pre-conditioning sweeps for each polishing pad. Table Removal Rate as a function of the number of preconditioning sweeps The Table illustrates that the treated pad achieves 95% of its final removal rate after only 30 sweeps, while the untreated pad requires 90 sweeps to achieve 95% of its final removal rate. This translates into a significant saving in time that is required for pre-conditioning. As a further benefit, the soft surface layer of a treated polishing pad reduces scratches and light point defects (LPD) on polished wafers compared to polishing with an untreated pad. Laboratory testing has shown that total defects produced by a treated pad are less than 2% of the total defects produced by an untreated pad. Another beneficial aspect of solvent treatment is increased hydrophilicity of the surface layer of the polishing pad. Hydrophilicity can be determined by measuring the contact angle which de-ionized water exhibits on the surface of the polishing pad. Lower contact angles are associated with increased hydrophilicity, i.e., better wetting of the surface which promotes slurry distribution across the polishing pad and improved polishing performance. In one example, contact angles were measured for de- ionized water on the surface of OXP3000 polishing pads. The contact angle for an untreated polishing pad was 111° . For a treated pad, the contact angle was 82°. After the treated pad was used for polishing, the contact angle was 79°, which shows that the treated surface does not deteriorate after polishing. It has been found that solvent treatment according to the invention modifies a surface and a layer beneath the surface, which is only about 5% of the thickness of the polishing pad. Thus, the bulk modulus and the stiffness of the polishing pad are not significantly reduced, which thereby produces no further deviations from planar polishing due to a softened polishing surface, thus, having no detrimental effect on the planarity of polished wafers as compared with an untreated pad.

Claims: Claims : 1. A process of treating a polishing pad for surface conditioning, wherein the process comprises, sweeping a polishing surface on the polishing -pad with an abrasive, the process further characterised by; softening the polishing surface on a material of the polishing pad by exposing the polishing surface to a chemical solvent having a solubility parameter that differs by less than about twenty percent from a solubility parameter of the material of the polishing pad that provides the polishing surface, wherein the polishing surface is softened relative to a remainder of the material to reduce the time required for surface conditioning of the polishing surface. 2. The method of claim 1 further characterised by; combining the chemical solvent with a polishing fluid, and polishing a semiconductor wafer by the polishing pad with the polishing fluid and the chemical solvent. 3. The method of claim 1 further characterised by; softening the polishing surface on a material of the polishing pad by exposing the polishing surface to a chemical solvent comprising, N-methyl pyrrolidone (NMP) . 4. The method of claim 1 further characterised by; softening the polishing surface on a material of the polishing pad by exposing the polishing surface to a chemical solvent comprising, dimethyl formamide (DMF) . 5. A polishing pad treated for surface conditioning comprises, a material providing a polishing surface on the polishing pad, and further characterised by: the polishing surface being softened by a chemical solvent, wherein the polishing surface is softened relative to a remainder of the material that provides the polishing surface, to reduce the time required for surface conditioning of the polishing surface. 6. The polishing pad of claim 5 wherein, the chemical solvent has a solubility parameter that differs by less than about twenty percent from a solubility parameter of the polymeric material. 7. The polishing pad of claim 5 wherein the chemical solvent is N-methyl pyrrolidone (NMP) . 8. The polishing pad of claim 5 wherein the chemical solvent is dimethyl formamide (DMF) .

Inventors: SHIDNER DAVID, VISHWANATHAN ARUN, SHIDNER, DAVID, VISHWANATHAN, ARUN
Applicants: RODEL INC, RODEL HOLDINGS, INC.
Application year: 2000
Main classification: B24B37
IPC codes: B24B37, B24B53, B24B57, B24D3, H01L21