JP7608146B2 - Polishing Pad - Google Patents

Description

The present invention relates to a polishing pad.

Polishing pads made of polyurethane foam are widely used in mirror polishing of workpieces such as silicon wafers used as semiconductor substrates. In recent years, there has been an increasing demand for improved planarization characteristics of workpieces to improve the yield of semiconductor chip products.

Polishing pads made of urethane foam have many roughly spherical cavities (hereafter also referred to as "pores"). In particular, the pores that open on the surface have functions such as holding the slurry and forming minute contact points with the workpiece to be polished. The pore structure is characterized by the size and number distribution, the proportion of closed cells and open cells (hereafter also referred to as "communicating pores"), etc., and various polishing pads have been proposed for controlling these.

For example, Patent Document 1 discloses a polishing pad that achieves a good balance between slurry retention and the rigidity of the polishing layer by adjusting the proportion of interconnected pores. Patent Document 2 discloses a polishing pad that improves the flatness of the workpiece and suppresses edge drooping by increasing the proportion of interconnected pores.

Furthermore, Patent Document 3 discloses a polishing pad that uses unfoamed heat-expandable microspheres to prevent edge droop and to have uniform pore diameters without containing large pores.

JP 2015-193057 A JP 2011-020234 A JP 2010-274361 A

However, in the polishing pads of Patent Documents 1 and 2, the size of the pores is not controlled, and the pore diameter is not uniform, which causes variation in the amount of resin around the pores. As a result, the amount of deformation during polishing, and therefore the stress on the workpiece to be polished, also varies locally, which can cause deterioration in the site flatness of the workpiece to be polished.

On the other hand, the polishing pad of Patent Document 3 uses unfoamed heat-expandable microspheres to achieve uniform pore diameters, and the foam is only able to retain a small amount of slurry inside due to the shell of the heat-expandable microspheres. As a result, even if slurry is supplied to the pad surface during polishing, most of it is expelled due to the sliding of the workpiece. As a result, the slurry is depleted towards the center of the workpiece, reducing the polishing rate, and the center is prone to becoming convex, which causes a problem of deterioration in the global flatness of the workpiece.

As such, in order to improve both the site flatness and global flatness of the polished object, it was difficult to increase the proportion of interconnected pores to improve slurry retention while controlling the pore diameter uniformly.

In view of the current situation, the present invention aims to provide a polishing pad that has a high proportion of interconnected pores, improves slurry retention, and allows the pore diameter to be uniformly controlled.

The polishing pad according to the present invention is a polishing pad comprising a polyurethane resin foam having a large number of pores formed therein, and the polyurethane resin foam satisfies the following requirements (1) and (2).
(1) The filling rate of the pores with pure water is 80% or more.
(2) The difference between the cumulative volume 84% diameter of the pores and the cumulative volume 16% diameter of the pores is 120 μm or less.

In this polishing pad, the pores are filled with pure water at a rate of 80% or more, which increases the proportion of interconnected pores and improves the retention of slurry. This allows sufficient slurry to be supplied to the center of the workpiece being polished, suppressing a decrease in the polishing speed, and thus improving the global flatness of the workpiece.

In addition, with this polishing pad, the difference between the pore diameter at 84% cumulative volume and the diameter at 16% cumulative volume is 120 μm or less, so the pore diameter can be controlled uniformly. This makes it possible to suppress the variation in local stress on the workpiece and improve the site flatness of the workpiece.

As described above, the present invention can provide a polishing pad that has a high proportion of interconnected pores, improves slurry retention, and allows the pore diameter to be uniformly controlled.

One embodiment of the present invention is described below.

The polishing pad according to this embodiment includes a polyurethane resin foam having a large number of pores formed therein. The polyurethane resin foam satisfies the following requirements (1) and (2).
(1) The filling rate of the pores with pure water is 80% or more.
(2) The difference between the cumulative volume 84% diameter of the pores and the cumulative volume 16% diameter of the pores is 120 μm or less.

The rate at which pure water fills the pores is the ratio of the volume of water that penetrates the pores when submerged to the total volume of the pores, and indicates the slurry retention during polishing. The higher the rate of interconnected pores, the greater the rate at which pure water fills the pores, improving the slurry retention.

In the polyurethane resin foam, the filling rate of the pores with pure water is 80% or more, and preferably 90% or more.

The filling rate of the pores with pure water can be calculated using the pad apparent density (hereinafter, normal density ρ _a ) and the pad apparent density in liquid (hereinafter, in liquid density ρ _b ). Here, the normal density ρ _a is obtained by dividing the pad weight by the pad volume. The in liquid density ρ _b is obtained from the in liquid weight W' when immersed in pure water by the following formula (i) using Archimedes' principle. At this time, if the pad is simply immersed in pure water, it takes a long time to sufficiently wet the pores due to surface tension. Therefore, the immersion in pure water is performed for 10 minutes under a reduced pressure of -0.1 MPa.

なお、（ｉ）式中、Ｗは乾燥重量、Ｗ’は液中重量、ρ_ｌｑは溶液密度を意味する。

In the formula (i), W is the dry weight, W' is the weight in liquid, and ρ _lq is the solution density.

If there are many interconnected pores and large pores that penetrate in the thickness direction, and there are no independent bubbles inside the pad, the liquid density will be equal to the resin true density. On the other hand, if there are only independent bubbles inside the pad, the liquid density will be smaller due to the buoyancy caused by the contained air bubbles. From this difference, the filling rate can be calculated. Specifically, the filling rate can be obtained by the following formula (ii).

なお、（ｉｉ）式中、ａは充填率、φは空隙率（ポア体積率）を意味する。

In the formula (ii), a represents the packing ratio, and φ represents the porosity (pore volume ratio).

In the polyurethane resin foam, the porosity is preferably 45 to 75%, more preferably 50 to 70%, from the viewpoint of increasing the proportion of interconnected pores and improving the rigidity of the pad to improve flatness. The porosity is the ratio of the pore volume excluding the resin to the pad volume, and can be measured using an X-ray CT scanner. That is, an X-ray CT scanner (e.g., TDM1000H-I manufactured by Yamato Scientific Co., Ltd.) is used to capture a cross-sectional image (measurement range, for example, 1.6 mm x 1.6 mm x 0.7 mm) of the polyurethane resin foam in a direction perpendicular to the polishing surface. Then, the cross-sectional image is binarized using image processing software (e.g., VG StudioMAX 2.1 manufactured by VOLUME GRAPHICS Co., Ltd.) to clearly separate the pore portion from the matrix portion (portion other than the pore portion). Next, the porosity can be obtained by summing the volumes of each pore and dividing the volume of the pore portion by the volume of the measurement range.

The difference between the cumulative 84% volume diameter and the cumulative 16% volume diameter of the pores indicates the width of the distribution based on the pore volume. The cumulative 84% volume diameter and the cumulative 16% volume diameter refer to the diameters at which the pores are arranged in ascending order and the volumes are accumulated to 84% and 16%, respectively, of the total pore volume. The smaller this difference is, the narrower the distribution width on a volume basis, that is, the more uniform the pore diameter can be controlled. The cumulative 84% volume diameter and the cumulative 16% volume diameter of the pores can be calculated by measuring the volume of each pore using an X-ray CT scanning device. That is, an X-ray CT scanning device (e.g., TDM1000H-I manufactured by Yamato Scientific Co., Ltd.) is used to capture a cross-sectional image (measurement range is, for example, 1.6 mm x 1.6 mm x 0.7 mm) of the polyurethane resin foam in a direction perpendicular to the polished surface. The cross-sectional image is then binarized using image processing software (e.g., VG StudioMAX 2.1 manufactured by VOLUME GRAPHICS Co., Ltd.) to clearly separate the pore portion from the matrix portion (portion other than the pore portion). Next, the volume of each pore is measured, arranged in ascending order, and the volumes are accumulated. When the volumes are divided by the volume of all the pore portions, the pore volumes that reach 84% and 16%, respectively, are identified. The diameters of true spheres with the same volume as this pore volume are defined as the accumulated volume 84% diameter and the accumulated volume 16% diameter.

In the polyurethane resin foam, the difference between the pore diameter at 84% cumulative volume and the pore diameter at 16% cumulative volume is 120 μm or less, and preferably 110 μm or less. On the other hand, the difference between the pore diameter at 84% cumulative volume and the pore diameter at 16% cumulative volume is preferably 20 μm or more.

In the polyurethane resin foam, the cumulative 50% volume diameter of the pores is preferably 150 μm or less. This configuration reduces the amount of resin around the pores, suppressing the abrasive dust from getting caught between the workpiece and the resin on the surface, thereby reducing the occurrence of scratches. On the other hand, the cumulative 50% volume diameter of the pores is preferably 30 μm or more from the viewpoint of suppressing the collapse of open pores due to the stretching of the resin during polishing. The cumulative 50% volume diameter of the pores can be calculated using an X-ray CT scanner, in the same way as the cumulative 84% volume diameter and the cumulative 16% volume diameter.

By evaluating pores using the cumulative volume 50% diameter, it is possible to take into account the influence of large diameter pores that occupy a large volume, compared to the conventional evaluation using the number average diameter.

The polyurethane resin foam is a cured product obtained by curing a polyurethane resin composition. The polyurethane resin composition contains a compound containing active hydrogen (hereinafter also referred to as an "active hydrogen compound") and a compound containing an isocyanate group (hereinafter also referred to as an "isocyanate compound"). That is, the polyurethane resin foam contains a polyurethane resin having a structure in which a first constituent unit of an active hydrogen compound and a second constituent unit of an isocyanate compound are alternately repeated by urethane bonding.

The active hydrogen compound is an organic compound having an active hydrogen group in the molecule that can react with an isocyanate group. Specific examples of the active hydrogen group include functional groups such as a hydroxyl group, a primary amino group, a secondary amino group, and a thiol group. The active hydrogen compound may have only one type of functional group in the molecule, or may have multiple types of functional groups.

Examples of the active hydrogen compound include polyol compounds having multiple hydroxyl groups in the molecule, and polyamine compounds having multiple primary amino groups or secondary amino groups in the molecule.

Examples of the polyol compound include polyol monomers and polyol polymers.

Examples of the polyol monomer include linear aliphatic glycols such as 1,4-benzenedimethanol, 1,4-bis(2-hydroxyethoxy)benzene, ethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, and 1,9-nonanediol; branched aliphatic glycols such as neopentyl glycol, 3-methyl-1,5-pentanediol, 2-methyl-1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol, and 2-methyl-1,8-octanediol; alicyclic diols such as 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, and water-added bisphenol A; and polyfunctional polyols such as glycerin, trimethylolpropane, tributyrolpropane, pentaerythritol, and sorbitol.

The polyol monomer is preferably ethylene glycol or diethylene glycol, from the viewpoints that it is likely to have higher strength during reaction, that it is likely to have higher rigidity in the polishing pad having the produced polyurethane resin foam, and that it is relatively inexpensive.

Examples of the polyol polymer include polyester polyol, polyester polycarbonate polyol, polyether polyol, polycarbonate polyol, etc. In addition, examples of the polyol polymer include multifunctional polyol polymers having three or more hydroxyl groups in the molecule.

Examples of the polyester polyol include polyethylene adipate glycol, polybutylene adipate glycol, polycaprolactone polyol, polyhexamethylene adipate glycol, etc.

The polyester polycarbonate polyol may be, for example, a reaction product of a polyester glycol, such as polycaprolactone polyol, with an alkylene carbonate. Also included is a reaction product obtained by reacting ethylene carbonate with a polyhydric alcohol, and then further reacting the resulting reaction mixture with an organic dicarboxylic acid.

Examples of the polyether polyol include polytetramethylene ether glycol (PTMG), polypropylene glycol (PPG), polyethylene glycol (PEG), and ethylene oxide-added polypropylene polyol.

The polycarbonate polyols include reaction products of diols such as 1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, polyethylene glycol, polypropylene glycol, or polytetramethylene ether glycol with phosgene, diallyl carbonate (e.g., diphenyl carbonate), or cyclic carbonate (e.g., propylene carbonate).

Other examples of the polyol compound include diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, and polyethylene glycol with a molecular weight of 400 or less.

The polyamine compounds include 4,4'-methylenebis(2-chloroaniline) (MOCA), 4,4'-methylenedianiline, trimethylene Bis(4-aminobenzoate), 2-methyl 4,6-bis(methylthio)benzene-1,3-diamine, 2-methyl 4,6-bis(methylthio)-1,5-benzenediamine, 2,6-dichloro-p-phenylenediamine, 4,4'-methylenebis(2,3-dichloroaniline), 3,5-bis(methylthio)-2,4-toluenediamine, 3,5-bis(methylthio)-2,6-toluenediamine, 3,5-diethyltoluene-2,4-diamine, 3,5-diethyltoluene-2,6-diamine, trimethylene glycol-di-p-aminobenzoate, 1,2-bis(2-aminophenylthio)ethane, 4,4'-diamino-3,3'-diethyl-5,5'-dimethyldiphenylmethane, etc.

Examples of the isocyanate compound include polyisocyanates and urethane prepolymers.

Examples of the polyisocyanate include aromatic diisocyanates, aliphatic diisocyanates, and alicyclic diisocyanates.

Examples of the aromatic diisocyanate include tolylene diisocyanate (TDI), 1,5-naphthalene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, and 1,4-phenylene diisocyanate. Examples of the aromatic diisocyanate include diphenylmethane diisocyanate (MDI) and modified products of diphenylmethane diisocyanate (MDI).

Examples of modified diphenylmethane diisocyanate (MDI) include carbodiimide modified, urethane modified, allophanate modified, urea modified, biuret modified, isocyanurate modified, and oxazolidone modified. Specific examples of such modified products include carbodiimide-modified diphenylmethane diisocyanate (carbodiimide-modified MDI).

Examples of the aliphatic diisocyanate include ethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, and hexamethylene diisocyanate (HDI).

Examples of the alicyclic diisocyanate include 1,4-cyclohexane diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, isophorone diisocyanate, norbornane diisocyanate, and methylenebis(4,1-cyclohexylene) diisocyanate.

The urethane prepolymer is a polymer formed by bonding a polyol and a polyisocyanate, and has an isocyanate group as a terminal group.

As one aspect of the polishing pad according to this embodiment, the polyurethane resin composition preferably contains 4,4'-methylenebis(2-chloroaniline) (MOCA). The polyurethane resin composition preferably further contains a foam stabilizer that is incompatible with the MOCA. The term "incompatible" means that the foam stabilizer is incompatible when 10% by mass of the foam stabilizer is added to MOCA melted at 125°C.

Examples of such foam stabilizers include silicone surfactants, fluorine surfactants, and ionic surfactants. Among these, from the viewpoint of foam stabilization and fine foaming, it is preferable that the foam stabilizer is a silicone surfactant. Examples of silicone surfactants include polyether-modified silicone.

The foam stabilizer is believed to assist in the dispersion of the polyurethane resin composition when the polyurethane resin composition is at a low viscosity in the early stage of the reaction. On the other hand, when the polyurethane resin composition is at a high viscosity in the later stage of the reaction, microphase separation progresses, and aggregates of hard segments containing MOCA occur. When this comes into contact with the foam film, the foam stabilizer concentration in that area decreases due to incompatibility, causing instability. This, combined with the increase in the internal pressure of the bubbles as the foaming reaction progresses, is believed to facilitate communication with adjacent bubbles. However, since the polyurethane resin composition has become sufficiently viscous, it is believed that the bubbles do not coalesce and become coarse. Therefore, it is believed that by including the foam stabilizer in the polyurethane resin composition, a structure in which the pores are interconnected while maintaining the fine pores is obtained.

When the isocyanate compound is a urethane prepolymer, the content of the foam stabilizer is preferably 0.2 to 5% by mass relative to the urethane prepolymer.

The polishing pad according to this embodiment is configured as described above. Next, we will explain an example of a method for manufacturing the polishing pad according to this embodiment.

The polishing pad according to this embodiment can be manufactured, for example, by the prepolymer method. Specifically, a urethane prepolymer having an isocyanate group as a terminal group, water as a foaming agent, a foam stabilizer, an active hydrogen compound as a curing agent, and a catalyst are mixed and polymerized to obtain a polishing pad with a polyurethane resin foam.

Examples of objects to be polished with the polishing pad of this embodiment include silicon wafers, optical materials, semiconductor devices, hard disks, glass plates, etc.

The polishing pad according to this embodiment is configured as described above and has the following advantages:

That is, the polishing pad according to this embodiment is a polishing pad comprising a polyurethane resin foam having a large number of pores formed therein, and the polyurethane resin foam satisfies the following requirements (1) and (2).
(1) The filling rate of the pores with pure water is 80% or more.
(2) The difference between the cumulative volume 84% diameter of the pores and the cumulative volume 16% diameter of the pores is 120 μm or less.

In this polishing pad, the pores are filled with pure water at a rate of 80% or more, which increases the proportion of interconnected pores and improves the retention of slurry. This allows sufficient slurry to be supplied to the center of the workpiece being polished, suppressing a decrease in the polishing speed, and thus improving the global flatness of the workpiece.

In addition, with this polishing pad, the difference between the pore diameter at 84% cumulative volume and the diameter at 16% cumulative volume is 120 μm or less, so the pore diameter can be controlled uniformly. This makes it possible to suppress the variation in local stress on the workpiece and improve the site flatness of the workpiece.

The polishing pad according to the present invention is not limited to the above embodiment. The polishing pad according to the present invention is also not limited by the above-mentioned effects. The polishing pad according to the present invention can be modified in various ways without departing from the gist of the present invention.

Next, the present invention will be explained in more detail with reference to examples and comparative examples.

(Examples 1 to 6)
A prepolymer, water as a foaming agent, a foam stabilizer, a curing agent, and a catalyst were mixed in a closed system using a stirring blade, and the prepolymer was polymerized to obtain a polishing pad (φ25 mm × thickness 1.3 mm) that is a polyurethane resin foam. The above-mentioned materials were specifically as follows. The stirring was performed in a closed system so that coarse pores would not be created due to air entrainment. In addition, the foam stabilizer in each example was dispersed incompatible with the curing agent.
Prepolymer: a urethane prepolymer in which polytetramethylene ether glycol (PTMG) and tolylene diisocyanate (TDI) are bonded together and which has an isocyanate group as a terminal group. Foam stabilizer: a silicone foam stabilizer SZ-1671 (manufactured by Dow Corning Toray Co., Ltd.)
Hardener: 4,4'-methylenebis(2-chloroaniline) (MOCA)
Catalyst: Tertiary amine catalyst

(Comparative Example 1)
Except for using a foam stabilizer that was compatible with and dispersed in the curing agent, a polishing pad was obtained in the same manner as in Example 1. Specifically, the following foam stabilizer was used.
Foam stabilizer: Silicone foam stabilizer SF-2937F (manufactured by Dow Corning Toray Co., Ltd.)

(Comparative Examples 2 and 5)
A polishing pad was obtained in the same manner as in Example 1, except that no foam stabilizer was used.

(Comparative Example 3)
A polishing pad was obtained in the same manner as in Comparative Example 2, except that stirring was carried out in an open system.

(Comparative Example 4)
A polishing pad was obtained in the same manner as in Comparative Example 1, except that stirring was carried out in an open system.

The filling rate of the pure water in the pores of the polishing pads of each example and comparative example, and the cumulative volume of each pore were measured using the method described above. The results are shown in Table 1.

Using the polishing pads of each of the Examples and Comparative Examples, polishing of an object to be polished was carried out under the following polishing conditions.
Object to be polished: Etched wafer
Polishing machine: DMS 20B-5P-4D, manufactured by Speed FAM Slurry flow rate: 5 L/min
Slurry type: NP6610 diluted with water (NP6610:water = 1:30 (volume ratio))

Flatness was evaluated by measuring the GBIR (global backside ideal range) ratio and SFQR (site front least square) ratio using a Nanometro 300TT-A (Kuroda Precision Industries, Ltd.). The GBIR ratio indicates global flatness and is for the entire wafer surface defined excluding the periphery. On the other hand, the SFQR ratio indicates site flatness and relates to the flatness of a limited area of the wafer, which roughly corresponds to the area of the semiconductor components to be fabricated. The results are shown in Table 1.

As can be seen from the results in Table 1, the polishing pads of each embodiment have a pore filling rate of 80% or more of pure water, which increases the proportion of interconnected pores and improves slurry retention. In addition, the difference between the pore diameter at 84% cumulative volume and the diameter at 16% cumulative volume is 120 μm or less, so the pore diameter can be uniformly controlled.

As can be seen from the results of Examples 1 and 2, a polishing pad that meets all of the constituent requirements of the present invention can improve the global flatness and site flatness of the workpiece.

On the other hand, the polishing pads of each comparative example are unable to retain the slurry well because the pores are filled with less than 80% pure water. Also, the polishing pads of comparative examples 3 and 4 are unable to uniformly control the pore diameter because the difference between the pore diameter at 84% cumulative volume and the pore diameter at 16% cumulative volume exceeds 120 μm.

As can be seen from the results of Comparative Examples 1, 2, and 4, polishing pads that do not satisfy the constituent requirements of the present invention cannot improve the global flatness and site flatness of the workpiece.

Claims (4)

A polishing pad having a polyurethane resin foam having a large number of pores formed therein,
The polyurethane resin foam satisfies the following requirements (1) and (2),
The polishing pad , wherein the polyurethane resin foam is a cured product obtained by curing a polyurethane resin composition, and the polyurethane resin composition contains a foam stabilizer dispersed in an incompatible manner .
(1) The filling rate of the pores with pure water is 80% or more.
(2) The difference between the cumulative volume 84% diameter of the pores and the cumulative volume 16% diameter of the pores is 120 μm or less. The polishing pad according to claim 1, wherein the polyurethane resin foam has a cumulative 50% volume diameter of the pores of 150 μm or less. The polishing pad according to claim 1 or 2, wherein the polyurethane resin foam has a porosity of 45 to 75%. 4. The polishing pad according to claim 1, wherein the polyurethane resin foam is a cured product obtained by curing a polyurethane resin composition containing 4,4'-methylenebis(2-chloroaniline).