Patent Application
20150059254
This specification relates to polishing pads useful for polishing and planarizing substrates and particularly to planarizing polishing pads producing low defect levels.
Polyurethane polishing pads are the primary pad-type for a variety of demanding precision polishing applications. These polyurethane polishing pads are effective for polishing silicon wafers, patterned wafers, flat panel displays and magnetic storage disks. In particular, polyurethane polishing pads provide the mechanical integrity and chemical resistance for most polishing operations used to fabricate integrated circuits. For example, polyurethane polishing pads have high strength for resisting tearing; abrasion resistance for avoiding wear problems during polishing; and stability for resisting attack by strong acidic and strong caustic polishing solutions.
The production of semiconductors typically involves several chemical mechanical planarization (CMP) processes. In each CMP process, a polishing pad in combination with a polishing solution, such as an abrasive-containing polishing slurry or an abrasive-free reactive liquid, removes excess material in a manner that planarizes or maintains flatness for receipt of a subsequent layer. The stacking of these layers combines in a manner that forms an integrated circuit. The fabrication of these semiconductor devices continues to become more complex due to requirements for devices with higher operating speeds, lower leakage currents and reduced power consumption. In terms of device architecture, this translates to finer feature geometries and increased metallization levels. These increasingly stringent device design requirements are driving the adoption of copper metallization in conjunction with new dielectric materials having lower dielectric constants. The diminished physical properties, frequently associated with low k and ultra-low k materials, in combination with the devices' increased complexity have led to greater demands on CMP consumables, such as polishing pads and polishing solutions.
In particular, low k and ultra-low k dielectrics tend to have lower mechanical strength and poorer adhesion in comparison to conventional dielectrics, rendering planarization more difficult. In addition, as integrated circuits' feature sizes decrease, CMP-induced defectivity, such as, scratching becomes a greater issue. Furthermore, integrated circuits' decreasing film thickness requires improvements in defectivity while simultaneously providing acceptable topography to a wafer substrate—these topography requirements demand increasingly stringent planarity, dishing and erosion specifications.
Casting polyurethane into cakes and cutting the cakes into several thin polishing pads has proven to be an effective method for manufacturing polishing pads with consistent reproducible polishing properties. M. J. Kulp, in U.S. Pat. No. 7,414,080, discloses the use of low-free toluene diisocyanate-based polishing pads to improve product uniformity. Unfortunately, polyurethane pads produced from these formulations lack the planarization and copper dishing properties necessary for the most demanding low defect polishing applications.
An aspect of the invention provides a polishing pad suitable for planarizing at least one of semiconductor, optical and magnetic substrates, the polishing pad comprising a cast polyurethane polymeric material formed from a prepolymer reaction of a polypropylene glycol and a toluene diisocyanate to form an isocyanate-terminated reaction product, the toluene diisocyanate having less than 5 weight percent aliphatic isocyanate and the isocyanate-terminated reaction product having 5.55 to 5.85 weight percent unreacted NCO, the isocyanate-terminated reaction product being cured with a 4,4′-methylene-bis(3-chloro-2,6-diethylaniline) curative agent, the cured polymer as measured in a non-porous state having a tan delta of 0.04 to 0.10 from 20 and 100° C. with a torsion fixture (ASTM 5279), a Young's modulus of 140 to 240 MPa at room temperature (ASTM-D412) and a Shore D hardness of 44 to 56 at room temperature (ASTM-D2240).
Another aspect of the invention provides a polishing pad suitable for planarizing at least one of semiconductor, optical and magnetic substrates, the polishing pad comprising a cast polyurethane polymeric material formed from a prepolymer reaction of a polypropylene glycol and a toluene diisocyanate to form an isocyanate-terminated reaction product, the toluene diisocyanate having less than 5 weight percent aliphatic isocyanate and the isocyanate-terminated reaction product having 5.55 to 5.85 weight percent unreacted NCO, the isocyanate-terminated reaction product being cured with a 4,4′-methylene-bis(3-chloro-2,6-diethylaniline) curative agent, the cured polymer as measured in a non-porous state having a tan delta of 0.04 to 0.10 from 20 and 100° C. with a torsion fixture (ASTM 5279), a Young's modulus of 180 to 240 MPa at room temperature (ASTM-D412) and a Shore D hardness of 46 to 54 at room temperature (ASTM-D2240).
The polishing pad is suitable for planarizing at least one of semiconductor, optical and magnetic substrates. Most preferably, the pad is useful for polishing semiconductor substrates. The polishing pad includes a cast polyurethane polymeric material formed from a prepolymer reaction of a polypropylene glycol, toluene diisocyanate that forms an isocyanate-terminated reaction product. The toluene diisocyanate is cured with a 4,4′-methylene-bis(3-chloro-2,6-diethylaniline) curative agent. The non-porous cured product has a tan delta of 0.04 to 0.10 as measured between 20 and 100° C. for consistent polishing behavior up to high temperatures. In addition, the non-porous cured product has a Young's modulus of 140 to 240 MPa. This modulus provides an excellent combination of planarization, TEOS erosion and copper dishing performance. Preferably, the non-porous cured product has a Young's modulus of 180 to 240 MPa. For low defectivity, the non-porous cured product has a Shore D hardness of 44 to 56. Most preferably, the non-porous cured product has a Shore D hardness of 46 to 54.
The polymer is effective for forming non-porous; and porous or filled polishing pads. For purposes of this specification, filler for polishing pads include solid particles that dislodge or dissolve during polishing, and liquid-filled particles or spheres. For purposes of this specification, porosity includes gas-filled particles, gas-filled spheres and voids formed from other means, such as mechanically frothing gas into a viscous system, injecting gas into the polyurethane melt, introducing gas in situ using a chemical reaction with gaseous product, or decreasing pressure to cause dissolved gas to form bubbles. The porous polishing pads contain a porosity or filler concentration of at least 0.1 volume percent. This porosity or filler contributes to the polishing pad's ability to transfer polishing fluids during polishing. Preferably, the polishing pad has a porosity or filler concentration of 0.2 to 70 volume percent. Most preferably, the polishing pad has a porosity or filler concentration of 0.25 to 60 volume percent. Optionally, the pores have an average diameter of less than 200 μm. Preferably, the pores or filler particles have a weight average diameter of 10 to 100 μm. Most preferably, the pores or filler particles have a weight average diameter of 15 to 90 μm. The nominal range of expanded hollow-polymeric microspheres' weight average diameters is 15 to 50 μm.
Optionally, the pad is non-porous. Non-porous pads are particularly useful for applications requiring excellent pad life and planarization. In particular, non-porous pads having macrogrooves and a roughened surface from a diamond conditioner are effective for copper and tungsten applications. Generally, increasing macrotexture or microtexture increases the removal rate for the non-porous pads.
Controlling the unreacted NCO concentration is particularly effective for controlling the pore uniformity for pores formed directly or indirectly with a filler gas. This is because gases tend to undergo thermal expansion at a much greater rate and to a greater extent than solids and liquids. For example, the method is particularly effective for porosity formed by casting hollow microspheres, either pre-expanded or expanded in situ; by using chemical foaming agents; by mechanically frothing in gas; and by use of dissolved gases, such as argon, carbon dioxide, helium, nitrogen, and air, or supercritical fluids, such as supercritical carbon dioxide or gases formed in situ as a reaction product.
The polymeric material is a polyurethane formed with polypropylene ether glycol [PPG] and 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) [MCDEA]. For purposes of this specification, “polyurethanes” are products derived from difunctional or polyfunctional isocyanates, e.g. polyotherureas, polyesterureas, polyisocyanurates, polyurethanes, polyureas, polyurethaneureas, copolymers thereof and mixtures thereof. An approach for controlling a pad's polishing properties is to alter its chemical composition. In addition, the choice of raw materials and manufacturing process affects the polymer morphology and the final properties of the material used to make polishing pads.
Preferably, urethane production involves the preparation of an isocyanate-terminated urethane prepolymer from a polyfunctional aromatic isocyanate and a prepolymer polyol. For purposes of this specification, the term prepolymer polyol is polypropylene ether glycol [PPG], copolymers thereof and mixtures thereof. Preferably, the polyfunctional aromatic isocyanate toluene diisocyanate that contains less than 5 weight percent aliphatic isocyanate and more preferably, less than 1 weight percent aliphatic isocyanate.
Typically, the prepolymer reaction product is reacted or cured with 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) or mixture thereof; such as with other polyamines. For purposes of this specification, polyamines include diamines and other multifunctional amines. Examples of other curative polyamines include aromatic diamines or polyamines, such as, 4,4′-methylene-bis-o-chloroaniline [MOCA]; dimethylthiotoluenediamine; trimethyleneglycol di-p-aminobenzoate; polytetramethyleneoxide di-p-aminobenzoate; polytetramethyleneoxide mono-p-aminobenzoate; polypropyleneoxide di-p-aminobenzoate; polypropyleneoxide mono-p-aminobenzoate; 1,2-bis(2-aminophenylthio)ethane; 4,4′-methylene-bis-aniline; diethyltoluenediamine; 5-tert-butyl-2,4- and 3-tert-butyl-2,6-toluenediamine; 5-tert-amyl-2,4- and 3-tert-amyl-2,6-toluenediamine and chlorotoluenediamine. Preferably, the prepolymer reaction product is reacted or cured with a single 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline) curative. Optionally, it is possible to manufacture urethane polymers for polishing pads with a single mixing step that avoids the use of prepolymers.
The polyurethane polymeric material is preferably formed from a prepolymer reaction product of toluene diisocyanate and polypropylene ether glycol with 4,4′-methylene-bis-(3-chloro-2,6-diethylaniline). Preferably, the prepolymer reaction product has a 5.55 to 5.85 weight percent unreacted NCO. Preferably, the prepolymer has less than 0.1 weight percent free TDI monomer and has a more consistent prepolymer molecular weight distribution than conventional prepolymers, and so facilitate forming polishing pads with excellent polishing characteristics. This improved prepolymer molecular weight consistency and low free isocyanate monomer give an initially lower viscosity prepolymer that tends to gel more rapidly, facilitating viscosity control that can further improve porosity distribution and polishing pad consistency. In addition, low molecular weight polyol additives, such as, diethylene glycol, butanediol and tripropylene glycol facilitate control of the prepolymer reaction product's weight percent unreacted NCO.
In addition to controlling weight percent unreacted NCO, the curative and prepolymer reaction product preferably has an OH or NH2 to unreacted NCO stoichiometric ratio of 80 to 120 percent; and most preferably, it has an OH or NH2 to unreacted NCO stoichiometric ratio of 100 to 112 percent.
If the polishing pad is a polyurethane material, then the polishing pad preferably has a density of 0.5 to 1.25 g/cm3. Most preferably, polyurethane polishing pads have a density of 0.6 to 1.15 g/cm3.
For non-porous pads, typical circular or circular plus radial groove patterns are effective. Preferably the groove pattern is a an overlay of two groove patterns, a first larger pattern for removing debris and a second smaller channel for increasing removal rate. For example, circular grooves having a depth of 30 mils (0.760 mm), width of 20 mils (0.508 mm) and a pitch of 120 mils (3.05 mm) represents the first larger channel and second set of three circular grooves having a depth of 15 mils (0.381 mm), width of 10 mils (0.254 mm) and a pitch of 30 mils (0.760 mm) provides the smaller channel. This combination of large and small channels can contribute toward an effective combination of low defectivity, process stability and high rate.
Cast polyurethane cakes were prepared by the controlled mixing of (a) an isocyanate terminated prepolymer at 51° C. (or desired temperatures based on various formulations) obtained by the reaction of a polyfunctional isocyanate (i.e., toluene diisocyanate) and a polyether based polyol (for example, Adiprenee LF750D and others listed in Tables commercially available from Chemtura Corporation); (b) a curative agent at 116° C. and optionally, (c) a hollow core filler (i.e., Expancel® 551DE40d42, 551DE20d60, 461DE20d70, or 920DE80d30 available from Akzo Nobel). The ratio of the isocyanate terminated prepolymer and the curative agent was set such that the stoichiometry, as defined by the ratio of active hydrogen groups (i.e., the sum of the —OH groups and —NH2 groups) in the curative agent to the unreacted isocyanate (NCO) groups in the isocyanate terminated prepolymer, was set according to each formulation as listed in Tables. The hollow core filler was mixed into the isocyanate terminated prepolymer prior to the addition of the curative agent. The isocyanate terminated prepolymer with the incorporated hollow core filler were then mixed together using a high shear mix head. After exiting the mix head, the combination was dispensed over a period of 5 minutes into a 86.4 cm (34 inch) diameter circular mold to give a total pour thickness of approximately 8 cm (3 inches). The dispensed combination was allowed to gel for 15 minutes before placing the mold in a curing oven. The mold was then cured in the curing oven using the following cycle: 30 minutes ramp of the oven set point temperature from ambient temperature to 104° C., then hold for 15.5 hours with an oven set point temperature of 104° C., and then 2 hour ramp of the oven set point temperature from 104° C. down to 21° C.
The cured polyurethane cakes were then removed from the mold and skived (cut using a moving blade) at a temperature of 30 to 80° C. into multiple polishing layers having an average thickness of 2.0 mm (80 mil). Skiving was initiated from the top of each cake.
Table 1 includes the formulations for a series of pads manufactured to the above method with various prepolymers, isocyanate amounts and curatives.
Several samples from Table 1 prepared as above were tested for physical properties with an initial screen. The test method for Young's modulus (ASTM-D412) specimen geometry was as follows: dumbbell shape with 4.5 inch (11.4 cm) in total length, 0.75 inch (0.19 cm) in total width, 1.5 inch (3.8 cm) in neck length and 0.25 inch (0.6 cm) in neck width. The grip separation was at a rate of 20 inch/min. (50.8 cm/min.). The hardness measurements were in accordance with ASTM-D2240 to measure Shore D hardness using a Shore S1, Model 902 measurement tool with a D tip. Table 2 below compares hardness and modulus based upon prepolymer as a function of NCO and curative.
As illustrated in
A DMA comparison between samples 1 and I-2 was done in accordance with ASTM 5279 at a rate of 10 radians/s and a heating rate of 3° C. per minute using non-porous samples having specimen dimensions of 40 mm×6.5 mm×1.27 mm after five days of conditioning at room temperature in 50% humidity chamber using Torsion Rectangular fixture on a Rheometric Scientific RDA3 DMA tool. As seen from
Porous formulations of pad samples used in bulk copper polishing tests were modified as illustrated in Table 3.
A polishing defect comparison was then completed between formulation 1A and comparative formulation E-4. The pads Polishing conditions were with grooves having a depth of 30 mils (0.760 mm), width of 18 mils (0.457 mm) and a pitch of 70 mils (1.778 mm) on an Applied Materials Reflection LK tool, with a wafer velocity of 87 rpm and a platen velocity of 93 rpm using in-situ conditioning with a Kinik AD3BG-150855 diamond conditioner using Planar Solution CSL9044C slurry. Copper blanket wafers were inspected using KLA-Tencor Surfscan SPITBI with a threshold at 0.07 microns and the defect map was output by KLARF v 0.2 for further review using KLA-Tencor eDR5210 Review SEM for defect classification.
These data show that despite the similar modulus, formulation I-A provided a low defectivity. In particular, formulation 1-A provided a significant decrease in microscratches in comparison to the MOCA-containing comparative E-4.
The pads of Table 3 were then tested for dishing on an Applied Material Reflexion LK tool. Tables 5 and 6 below provides dishing at various densities after 60 seconds over polishing.
Tables 5 and 6 show the MCDEA pad of the invention having the best dishing performance at the densities tested. Since pads with low defects often have higher dishing, this represents an unexpected feature of the invention. Additional tests have shown that a stoichiometry of 100 to 112 percent provides the best dishing performance and exhibits the best topography performance.
In addition, the non-porous version of the formulation has a particular affinity to tungsten polishing. Polishing conditions were with grooves having a depth of 30 mils (0.760 mm), width of 20 mils (0.508 mm) and a pitch of 120 mils (3.05 mm) on an Applied Materials Mirra tool, with a wafer velocity of 111 rpm and a platen velocity of 113 rpm using ex-situ conditioning with a Saesol AM02BSL8031C1-PM diamond conditioner using Cabot SS2000 tungsten slurry. In particular, it out preformed the industry standard, IC1010 in a head to head comparison as follows:
Table 7 shows a significant improvement in tungsten removal rate for the MCDEA formulation of the invention. Furthermore, combining the low TEOS defectivity of Table 4 with the increased removal tungsten removal rate provides an excellent polishing combination not achieved with conventional polishing pads.
In summary, the specific combination of 5.55 to 5.85 wt % NCO polypropylene glycol in combination with a MCDEA curative provides an excellent combination of planarization, low defects and low copper dishing for copper polishing applications. Furthermore, this formulation possesses a stable tan delta between 20 and 100° C. for consistent polishing with minor temperature variations. Finally, the formulation provides non-porous pads having excellent tungsten removal rate in combination with low TEOS defectivity.
