Patent Publication Number: US-6984163-B2

Title: Polishing pad with high optical transmission window

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to polishing pads for chemical mechanical planarization (CMP), and in particular, relates to polishing pads having windows formed therein for performing optical end-point detection. 
     In the fabrication of integrated circuits and other electronic devices, multiple layers of conducting, semiconducting and dielectric materials are deposited on or removed from a surface of a semiconductor wafer. Thin layers of conducting, semiconducting, and dielectric materials may be deposited by a number of deposition techniques. Common deposition techniques in modern processing include physical vapor deposition (PVD), also known as sputtering, chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and electrochemical plating (ECP). 
     As layers of materials are sequentially deposited and removed, the uppermost surface of the wafer becomes non-planar. Because subsequent semiconductor processing (e.g., metallization) requires the wafer to have a flat surface, the wafer needs to be planarized. Planarization is useful in removing undesired surface topography and surface defects, such as rough surfaces, agglomerated materials, crystal lattice damage, scratches, and contaminated layers or materials. 
     Chemical mechanical planarization, or chemical mechanical polishing (CMP), is a common technique used to planarize substrates, such as semiconductor wafers. In conventional CMP, a wafer carrier is mounted on a carrier assembly and positioned in contact with a polishing pad in a CMP apparatus. The carrier assembly provides a controllable pressure to the wafer, urging it against the polishing pad. The pad is optionally moved (e.g., rotated) relative to the wafer by an external driving force. Simultaneously therewith, a chemical composition (“slurry”) or other fluid medium is flowed onto the polishing pad and into the gap between the wafer and the polishing pad. The wafer surface is thus polished and made planar by the chemical and mechanical action of the pad surface and slurry. 
     An important step in planarizing a wafer is determining an end-point to the process. Accordingly, a variety of planarization end-point detection methods have been developed, for example, methods involving optical in-situ measurements of the wafer surface. The optical technique involves providing the polishing pad with a window to select wavelengths of light. A light beam is directed through the window to the wafer surface, where it reflects and passes back through the window to a detector (e.g., a spectrophotometer). Based on the return signal, properties of the wafer surface (e.g., the thickness of films) can be determined for end-point detection. 
     Birang et al., in U.S. Pat. No. 6,280,290, discloses a polishing pad having a window in the form of a polyurethane plug. The pad has an aperture and the window is held in the aperture with adhesives. Unfortunately, these prior art windows have light transmission properties that hamper effective endpoint detection or measurement for a wide variety of planarizing conditions. This is due, in part, to the high degree of crystallinity of aromatic diisocyanate-based materials, such as, toluene diisocyanate (TDI), diphenylmethane (MDI) and its derivatives. These aromatic diisocyanates (TDI, MDI) are the two most commonly used in polyurethane manufacture. Furthermore, the use of aromatic diamine curatives, such as methylene bis 2-chloroaniline (MBOCA), increase crystallinity. Also, curatives such as MBOCA are colored, typically yellow to green, and impart a color to (i.e., cause absorption in) the finished polymer. 
     For example, typical prior art windows provide only about 50% transmission at 450 nm, and to just over 40% at 430 nm. At 400 nm, the transmission steeply declines to about 13% making robust in-situ endpoint detection or measurement difficult. This is particularly problematic due to the demand for shorter wavelength endpoint detection requirements (e.g., at 400 nm). 
     Hence, what is needed is a polishing pad and method for robust end-point detection or measurement during CMP over a wide range of wavelengths, and in particular, the shorter wavelengths. Also, there is a need for a polishing pad and method that can reduce the use of curatives. 
     STATEMENT OF THE INVENTION 
     The present invention provides a chemical mechanical polishing pad comprising a window formed therein, wherein the window is formed from an aliphatic polyisocyanate-containing material. In particular, the window is formed from a reaction of an aliphatic polyisocyanate, a hydroxyl-containing material and a curing agent. The window of the present invention shows unexpected, improved transmission of laser signals for end-point detection during chemical mechanical polishing processes. 
     In a first aspect of the present invention, there is provided a chemical mechanical polishing pad comprising: a polishing pad having a window for end-point detection formed therein; and wherein the window is formed from a reaction of an aliphatic polyisocyanate, a hydroxyl-containing material and a curing agent. 
     In a second aspect of the present invention, there is provided an apparatus for chemical mechanical polishing comprising: a platen for supporting a polishing pad, the polishing pad having a window for end-point detection formed therein; a wafer carrier for pressing a wafer against the polishing pad; means for providing a polishing fluid between the wafer and the polishing pad; and wherein the window is formed by reacting an aliphatic polyisocyanate, a hydroxyl-containing material and a curing agent. 
     In a third aspect of the present invention, there is provided a method of forming a chemical mechanical polishing pad, the method comprising: providing a polishing pad having a window for end-point detection formed therein; and wherein the window is formed by reacting an aliphatic polyisocyanate, a hydroxyl-containing material and a curing agent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a polishing pad having a window of the present invention; and 
         FIG. 2  illustrates a CMP system utilizing the polishing pad of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a polishing pad  1  of the present invention is shown. Polishing pad  1  comprises a bottom layer  2  and a top layer  4 . The bottom layer  2  may be made of a felted polyurethane, such as SUBA-IV™ manufactured by Rodel, Inc. of Newark, Del. The top layer  4  may comprise a polyurethane pad (e.g., a pad filled with microspheres), such as, IC 1000™ by Rodel. A thin layer of pressure sensitive adhesive  6  holds the top layer  4  and the bottom layer  2  together. 
     In an exemplary embodiment, an intact bottom layer  2  (i.e., without an aperture formed within the layer  2 ) has its top surface coated with the pressure sensitive adhesive  6 . An intact top layer  44  is then provided over the bottom layer  2  and on the pressure sensitive adhesive  6 . Alternatively, the top layer  4  may already include an aperture  8  prior to the top layer  4  being being joined with the pressure sensitive adhesive  6 . Next, an aperture  10  is formed in the bottom layer  2 . Formation of this aperture  10  removes the pressure sensitive adhesive  6  within the aperture  10  so that an open channel exists through the polishing pad  1 . The aperture  8  in the top layer  4  is wider than the aperture  10  in the bottom layer  2 . This creates a shelf  12  covered with pressure sensitive adhesive  6 . Thereafter, a transparent window block  14 , is disposed over the pressure sensitive adhesive  6  on the shelf  12 . The transparent window block  14  completely fills the aperture  8  in the top layer  4 . Accordingly, laser light from a laser spectrophotometer (not shown) may be directed through the aperture  10  and transparent window block  14 , and onto a wafer or substrate to facilitate end-point detection. 
     In an exemplary embodiment of the present invention, window  14  is made from an aliphatic polyisocyanate-containing material (“prepolymer”). The prepolymer is a reaction product of an aliphatic polyisocyanate (e.g., diisocyanate) and a hydroxyl-containing material. The prepolymer is then cured with a curing agent. Preferred aliphatic polyisocyanates include, but are not limited to, methlene bis 4,4′ cyclohexylisocyanate, cyclohexyl diisocyanate, isophorone diisocyanate, hexamethylene diisocyanate, propylene-1,2-diisocyanate, tetramethylene-1,4-diisocyanate, 1,6-hexamethylene-diisocyanate, dodecane-1,12-diisocyanate, cyclobutane-1,3-diisocyanate, cyclohexane-1,3-diisocyanate, cyclohexane-1,4-diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane, methyl cyclohexylene diisocyanate, triisocyanate of hexamethylene diisocyanate, triisocyanate of 2,4,4-trimethyl-1,6-hexane diisocyanate, uretdione of hexamethylene diisocyanate, ethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, dicyclohexylmethane diisocyanate, and mixtures thereof. The preferred aliphatic polyisocyanate has less than 14% unreacted isocyanate groups. 
     Advantageously, the hydroxyl-containing material is polyol. Exemplary polyols include, but are not limited to, polyether polyols, hydroxy-terminated polybutadiene (including partially/fully hydrogenated derivatives), polyester polyols, polycaprolactone polyols, and polycarbonate polyols. 
     In one preferred embodiment, the polyol includes polyether polyol. Examples include, but are not limited to, polytetramethylene ether glycol (“PTMEG”), polyethylene propylene glycol, polyoxypropylene glycol, and mixtures thereof. The hydrocarbon chain can have saturated or unsaturated bonds and substituted or unsubstituted aromatic and cyclic groups. Preferably, the polyol of the present invention includes PTMEG. Suitable polyester polyols include, but are not limited to, polyethylene adipate glycol, polybutylene adipate glycol, polyethylene propylene adipate glycol, o-phthalate-1,6-hexanediol, poly(hexamethylene adipate) glycol, and mixtures thereof. The hydrocarbon chain can have saturated or unsaturated bonds, or substituted or unsubstituted aromatic and cyclic groups. Suitable polycaprolactone polyols include, but are not limited to, 1,6-hexanediol-initiated polycaprolactone, diethylene glycol initiated polycaprolactone, trimethylol propane initiated polycaprolactone, neopentyl glycol initiated polycaprolactone, 1,4-butanediol-initiated polycaprolactone, PTMEG-initiated polycaprolactone, and mixtures thereof. The hydrocarbon chain can have saturated or unsaturated bonds, or substituted or unsubstituted aromatic and cyclic groups. Suitable polycarbonates include, but are not limited to, polyphthalate carbonate and poly(hexamethylene carbonate) glycol. The hydrocarbon chain can have saturated or unsaturated bonds, or substituted or unsubstituted aromatic and cyclic groups. 
     Advantageously, the curing agent is a polydiamine. Preferred polydiamines include, but are not limited to, diethyl toluene diamine (“DETDA”), 3,5-dimethylthio-2,4-toluenediamine and isomers thereof, 3,5-diethyltoluene-2,4-diamine and isomers thereof, such as 3,5-diethyltoluene-2,6-diamine, 4,4′-bis-(sec-butylamino)-diphenylmethane, 1,4-bis-(sec-butylamino)-benzene, 4,4′-methylene-bis-(2-chloroaniline), 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) (“MCDEA”), polytetramethyleneoxide-di-p-aminobenzoate, N,N′-dialkyldiamino diphenyl methane, p,p′-methylene dianiline (“MDA”), m-phenylenediamine (“MPDA”), methylene-bis 2-chloroaniline (“MBOCA”), 4,4′-methylene-bis-(2-chloroaniline) (“MOCA”), 4,4′-methylene-bis-(2,6-diethylaniline) (“MDEA”), 4,4′-methylene-bis-(2,3-dichloroaniline) (“MDCA”), 4,4′-diamino-3,3′-diethyl-5,5′-dimethyl diphenylmethane, 2,2′,3,3′-tetrachloro diamino diphenylmethane, trimethylene glycol di-p-aminobenzoate, and mixtures thereof. Preferably, the curing agent of the present invention includes 3,5-dimethylthio-2,4-toluenediamine and isomers thereof. Suitable polyamine curatives include both primary and secondary amines. 
     In addition, other curatives such as, a diol, triol, tetraol, or hydroxy-terminated curative may be added to the aforementioned polyurethane composition. Suitable diol, triol, and tetraol groups include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, lower molecular weight polytetramethylene ether glycol, 1,3-bis(2-hydroxyethoxy) benzene, 1,3-bis-[2-(2-hydroxyethoxy)ethoxy]benzene, 1,3-bis-{2-[2-(2-hydroxyethoxy) ethoxy]ethoxy}benzene, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, resorcinol-di-(beta-hydroxyethyl) ether, hydroquinone-di-(beta-hydroxyethyl) ether, and mixtures thereof. Preferred hydroxy-terminated curatives include 1,3-bis(2-hydroxyethoxy) benzene, 1,3-bis-[2-(2-hydroxyethoxy)ethoxy]benzene, 1,3-bis-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}benzene, 1,4-butanediol, and mixtures thereof. Both the hydroxy-terminated and amine curatives can include one or more saturated, unsaturated, aromatic, and cyclic groups. Additionally, the hydroxy-terminated and amine curatives can include one or more halogen groups. The polyurethane composition can be formed with a blend or mixture of curing agents. If desired, however, the polyurethane composition may be formed with a single curing agent. 
     Accordingly, the present invention provides a chemical mechanical polishing pad comprising a window formed therein, wherein the window is formed from an aliphatic polyisocyanate-containing material. In particular, the window is formed from a reaction of an aliphatic polyisocyanate, a hydroxyl-containing material and a curing agent. The window of the present invention shows unexpected, improved transmission of laser signals for end-point detection during chemical mechanical polishing. 
     Referring now to  FIG. 2 , a CMP apparatus  20  utilizing the polishing pad of the present invention is provided. Apparatus  20  includes a wafer carrier  22  for holding or pressing the semiconductor wafer  24  against the polishing platen  26 . The polishing platen  26  is provided with pad  1 , including window  14 , of the present invention. As discussed above, pad  1  has a bottom layer  2  that interfaces with the surface of the platen, and a top layer  4  that is used in conjunction with a chemical polishing slurry to polish the wafer  24 . Note, although not pictured, any means for providing a polishing fluid or slurry can be utilized with the present apparatus. The platen  26  is usually rotated about its central axis  27 . In addition, the wafer carrier  22  is usually rotated about its central axis  28 , and translated across the surface of the platen  26  via a translation arm  30 . Note, although a single wafer carrier is shown in  FIG. 2 , CMP apparatuses may have more than one spaced circumferentially around the polishing platen. In addition, a hole  32  is provided in the platen  26  and overlies the window  14  of pad  1 . Accordingly, hole  32  provides access to the surface of the wafer  24 , via window  14 , during polishing of the wafer  24  for accurate end-point detection. Namely, a laser spectrophotometer  34  is provided below the platen  26  which projects a laser beam  36  to pass and return through the hole  32  and high transmission window  14  for accurate end-point detection during polishing of the wafer  24 . 
     EXAMPLES 
     In the Examples, numerals represent examples of the invention and the letter represents a comparative example. In this experiment, the percent optical transmission for exemplary windows of the present invention was measured using a Gretag Macbeth 3000A spectrophotometer, for the wavelength range of 360 nm to 750 nm. In particular, windows formed from aliphatic diisocyanate-containing materials were tested against a window formed from an aromatic diisocyanate-containing material. For Test A, 100 parts of the prepolymer kept at 120° F., with 26 parts of the curing agent kept at 240° F., was mixed in a liquid tank and degassed under vacuum (&lt;1 torr). The mixture was then cast in a mold and cured at 220° F. for 18 hours. For Tests 1–8, 100 parts of the prepolymer kept at 150° F., with appropriate amounts of the curing agent kept at room temperature, was mixed in a liquid tank and degassed under vacuum (&lt;1 torr). The mixture was then cast in a mold and cured at 220° F. for 18 hours. Adiprene® LW520 and LW570 are registered trademarks of Uniroyal Chemical, Inc. and are commercially available aliphatic diisocyanate-containing prepolymers. The LW520 has an NCO of 4.6 to 4.9 wt % and LW570 has an NCO of 7.35 to 7.65 wt %. Adiprene® L325 is a registered trademark of Uniroyal Chemical, Inc. and is a commercially available aromatic diisocyanate-containing prepolymer. The L325 has an NCO of 8.95 to 9.25 wt %. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Curing 
                 Trans 
                 Trans 
                 Trans 
                 Trans 
                 Trans 
                 Trans 
               
               
                   
                   
                 agent 
                 360 
                 400 
                 450 
                 550 
                 650 
                 750 
               
               
                   
                 Prepolymer (100 parts 
                 (parts by 
                 nm 
                 nm 
                 nm 
                 nm 
                 nm 
                 nm 
               
               
                 Test 
                 by weight) 
                 weight) 
                 (%) 
                 (%) 
                 (%) 
                 (%) 
                 (%) 
                 (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 A 
                 Adiprene L325 
                 MBOCA 
                 1 
                 13 
                 57 
                 74 
                 79 
                 82 
               
               
                   
                   
                   (26) 
                   
               
               
                 1 
                 Adiprene LW520 
                 DETDA 
                 89 
                 93 
                 93 
                 94 
                 93 
                 93 
               
               
                   
                   
                  (9.6) 
                   
               
               
                 2 
                 Adiprene LW570 
                 DETDA 
                 90 
                 95 
                 95 
                 95 
                 95 
                 95 
               
               
                   
                   
                 (15.1) 
                   
               
               
                 3 
                 Adiprene 
                 DETDA 
                 87 
                 93 
                 94 
                 94 
                 94 
                 94 
               
               
                   
                 75% LW570/ 
                 (13.8) 
                   
               
               
                   
                 25% LW520 
               
               
                 4 
                 Adiprene LW520 
                 MCDEA 
                 84 
                 92 
                 94 
                 95 
                 95 
                 95 
               
               
                   
                   
                   (20) 
                   
               
               
                 5 
                 Adiprene LW570 
                 MCDEA 
                 59 
                 87 
                 94 
                 94 
                 94 
                 94 
               
               
                   
                   
                 (31.7) 
                   
               
               
                 6 
                 Adiprene LW520 
                 MBOCA 
                 10 
                 55 
                 87 
                 92 
                 94 
                 94 
               
               
                   
                   
                 (14.3) 
                   
               
               
                 7 
                 Adiprene 
                 MBOCA 
                 5 
                 45 
                 87 
                 94 
                 94 
                 94 
               
               
                   
                 75% LW570/ 
                 (20.6) 
                   
               
               
                   
                 25% LW520 
               
               
                 8 
                 Adiprene LW570 
                 MBOCA 
                 1 
                 21 
                 69 
                 85 
                 90 
                 92 
               
               
                   
                   
                 (22.6) 
                   
               
               
                   
               
            
           
         
       
     
     As illustrated in Table 1 above, all of the windows made from an aliphatic diisocyanate-containing material provided an overall improved percent transmission over the wavelength range of 360 nm to 750 nm. Test 2 showed at least 90% transmission of the end-point signal over the entire wavelength range of 360 nm to 750 nm. Tests 1, 3 and 4 provided at least 84% transmission over the wavelength range of 360 nm to 750 nm. Tests 5–8 showed a transmission value of at least 69% over the wavelength range of 450 nm to 750 nm. In fact, Tests 5–7 provided transmission values of at least 87% over the wavelength range of 450 nm to 750 nm. In comparison, Test A showed a transmission value as low as about 57% over the wavelength range of 450 nm to 750 nm. At 400 nm, Tests 1–8 showed a transmission value of at least 21%, while Test A showed a transmission value of only 13%. 
     In addition, as shown in Table 1, the aliphatic diisocyanates typically achieves the desired hardness and transmission values, at lower levels of curative content, minimizing the detrimental effects of curatives as discussed above. For example, in Tests 1–4 and 6–8, the amount of curing agent to achieve the desired hardness was less than that required for Test A, which required 26 parts of the curing agent to achieve the same level of hardness. 
     Accordingly, the present invention provides a chemical mechanical polishing pad comprising a window formed therein, wherein the window is formed from an aliphatic polyisocyanate-containing material. In particular, the window is formed from a reaction of an aliphatic polyisocyanate, a hydroxyl-containing material and a curing agent. The window of the present invention allows for an optical signal strength (e.g., the relative intensity of beam as it exits/enters the window) greater than otherwise possible with prior art windows having less optical transmission over the wavelength range of in-situ optical endpoint detection or measurement systems. These improvements in signal strength lead to significant improvements in the in-situ optical measurement of wafer surface parameters. In particular, reliability and measurement accuracy for end-point detection are improved.