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Diamond tools used in the field of semiconductor processing

November 10,2023

1、 CMP dresser

When the feature size of a semiconductor chip is less than 0.35um, global flattening must be performed, and CMP is currently the only global flattening technology. The polishing fluid used in CMP is composed of chemical components and sub micron or nano sized abrasives, which remove processed materials through a combination of chemical corrosion and mechanical force. The polishing pad is directly in contact with the polishing solution and wafer, and the grooves on the surface of the polishing pad play a role in distributing the polishing solution and removing waste liquid. The surface roughness and flatness of the polishing pad directly affect the results of CMP.

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Polishing pads are prone to aging and surface grooves are prone to clogging during CMP machining, resulting in the pads losing their polishing function. At this point, it is necessary to use a  diamond  trimmer to trim the surface of the polishing pads. The CMP trimmer plays a role in removing waste liquid from the polishing pad groove, improving the surface roughness of the polishing pad, and improving the flatness of the polishing pad. Therefore, the performance of the CMP dresser directly affects the effect of global flatness on the wafer surface.

CMP dressers generally use a metal substrate and combine diamond abrasive particles with the substrate through high-temperature brazing technology. A CMP dresser with excellent performance should have characteristics such as high material removal rate, difficulty indiamondabrasive detachment, equal height, and uniform distribution, in order to ensure that the wafer achieves qualified global flatness.

2、 Back grinding wheel

The thinning grinding wheel plays an important role in the chip preparation process: 1) Through the thinning process, the overall thickness of the chip can be reduced, which is conducive to heat dissipation and integration; 2) Reduce the thickness and surface roughness of the damaged layer on the wafer surface, release internal stress caused by various processes before thinning, and reduce the degree of single chip breakage during the wafer scribing process.

Wafer thinning generally adopts wafer self rotating grinding, and the processing process includes rough grinding and precision grinding. The axial feed speed of rough grinding is large, and diamond grinding wheels with larger particle sizes are used to quickly remove about 90% of the machining allowance; The axial feed speed of precision grinding is relatively small, and diamond grinding wheels with extremely small particle sizes are used. The purpose of precision grinding is twofold: 1) to remove the remaining 10% machining allowance; 2) Eliminate damage layers caused by rough grinding. The commonly used diamond grinding wheels in wafer thinning industrial production are shown in the table.

Binder is one of the important factors affecting the performance and grinding effect of diamond grinding wheels. The third-generation semiconductor materials such as SiC and Si3N4 have the characteristics of high hardness, high brittleness, and easy burning, which makes it difficult to meet the processing requirements using traditional ceramic, resin, and metal bonded diamond grinding wheels. Composite binders (metal/resin, metal/ceramic composite binders) combine the advantages of two types of binders. Currently, metal/resin composite binders have been extensively studied, which have the advantages of long service life, good self sharpening, small deformation, and high processing quality. They have shown excellent performance in the processing of precision ceramic components. Composite bonded diamond grinding wheels have broad application prospects in the thinning of third-generation semiconductor materials.

In the grinding process of semiconductor materials such as silicon and silicon carbide, resin binders (mainly phenolic resins) are commonly used as wheel bonding materials. However, resin bonding agents are not suitable for use in ultra fine grained diamond grinding tools. On the one hand, due to the poor thermal conductivity of resin materials, the heat generated during grinding is not easily dissipated; On the other hand, resin bonded diamond grinding wheels have a lower holding force on diamonds. When the diamond particle size is finer, resin bonded diamond grinding wheels require a denser structure to ensure a certain holding force on diamonds, but this also leads to a decrease in the porosity of the grinding wheel.

The thickness of the subsurface damage layer on a wafer is an important indicator of machining quality. Research has shown that the thickness of the subsurface damage layer on a wafer is approximately half the size of diamond abrasive particles. High end products will use 4000-8000 mesh diamond grinding wheels, and some may even use diamond grinding wheels exceeding 12000 mesh. Ceramic binders have the advantages of high temperature resistance, high hardness, good wear resistance, adjustable porosity, elastic modulus about 4 times that of resin binders, and high grip on diamond abrasives.

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3、 Scribing knife

Scoring knives can be divided into hard knives and soft knives according to the installation method. The soft knife is fixed with a flange, which has the characteristics of large tool exposure, low price, and replaceable flange use. However, it requires the use of a sharpening knife to achieve a true round effect and there will be slight jumping at the contact between the knife and the flange. Hard knife refers to the integration of the blade body and flange, with small tool exposure and inability to process thicker products. Scoring is the final process of wafer processing, and the quality of the scoring knife directly affects the quality of a single chip. Semiconductor materials are typical hard and brittle materials with high hardness, high brittleness, and low fracture toughness. The most important problem in scribing is cracking. Once cracking occurs, it will cause the chip to be scrapped, leading to the failure of the entire processing process.

Cracking is caused by the mechanical force and load phenomenon on the surface of the wafer when the scriber cuts it at high speed. When the load exceeds the elastic limit of the wafer, cracking occurs. There are three main factors that affect the machining quality of the cutting tool: diamond particle size, diamond density, and bond strength. Diamond has a small particle size and good self sharpening of the cutting tool. After cutting with a sharp tool, the degree of wafer cracking is small, but the processing efficiency is low and the lifespan is short; Diamond has a large particle size, high processing efficiency, and long service life, but its processing quality is poor. Diamond has a high density, poor load resistance of the cutting blade, and a large degree of wafer cracking after cutting, but with a long lifespan; The diamond density is small, the load resistance of the scoring tool is improved, and the processing quality is high, but the service life will be shortened.