ADVANCEMENTS IN LAPPING AND POLISHING WITH DIAMOND SLURRIES

Kan-Yin Ng and Timothy Dumm

Kan-yin.ng@diamondinnovations.com, 614-438-5620 Timothy.dumm@diamondinnovations.com, 614-438-2903

Diamond Innovations 6325 Huntley Road Worthington, OH 43085

Keywords: Diamond Slurry, HYPERION™, Lapping, Polishing, Sapphire, Silicon Carbide

Abstract A new diamond micron powder HYPERION™ and diamond slurry contains such powder was developed to improve fine lapping and polishing of advanced compound semiconductor materials such as sapphire and silicon carbide (SiC). Testing showed that the novel diamond and diamond slurry significantly increased effectiveness of fine lapping and polishing thereby allowing reduction in cost and increase in productivity. INTRODUCTION

The compound semiconductor materials used for making light emitting diode substrates are among the hardest materials on earth. Therefore, it is necessary to use diamond abrasives for slicing, edge grinding, lapping, polishing and dicing of the wafers. From the growth of the boule to final chip production, a considerable amount of cutting, grinding, lapping and polishing is performed on these materials. In substrate production alone, a significant portion of the manufacturing cost can be attributed to these material removal processes as well as long cycle times. Improvements in the use of abrasives for achieving substrate dimensions and surface finish can significantly reduce costs and cycle time. This paper will focus on advancements made in diamond slurry products used for lapping of sapphire wafers.

In the production line of silicon carbide or sapphire substrates, the fine lapping process is used for establishing the proper thickness of the wafers after the wire-sawing and rough lapping processes. Diamond slurries are used with lapping plates made from tin, copper or copper composite plates. Typical lapping times can range from 20 minutes to 3 hours depending on wafer diameter, required stock removal and surface finish. In the chip making production line, grinding process remove the bulk thickness from the backside of the substrates and fine lapping follows to achieve certain surface finish. Within the past two years, new diamond slurries have been

developed that significantly reduce the cycle time without sacrificing surface quality. DIAMOND TYPES AND SLURRY The type of diamond used in lapping and polishing slurries is an important factor in determining how the product will perform. Industrial diamond can be from natural or synthetic sources and can be produced in different ways. Natural diamond powder is derived from low-grade diamond that is of too poor quality for gemstones. Natural diamond is in relatively short supply and is subject to the variability inherent in natural sources. Synthetic industrial diamond can be produced using high-temperature/high-pressure processes that convert graphite to diamond using the help of metal catalysts. This type of diamond is commonly referred to as mono-crystalline diamond grit and varies in quality from very pure to highly impurity-included. Mono-crystalline micron diamond is made by crushing the grit into finer sizes and micronizing the fines into narrow size fractions. Mono-crystalline micron diamond is widely available. Another type of synthetic diamond is produced by detonating explosives that are packed around a graphite rod. The intense heat and pressure of the explosion directly converts the graphite to a polycrystalline form of diamond. The diamond from this process is generally less than 10 microns in size. This material is also micronized into narrow fractions for lapping and polishing. Polycrystalline diamond is not as widely available as mono-crystalline diamond and is generally more expensive. Nano-crystalline diamond is another type of diamond powder that is derived by exploding plastic explosives in a controlled environment – usually a sealed chamber. The residue that results from the explosion contains a small amount of low-grade diamond commonly referred to nano-diamond. This material is usually less than 1um in size when cleaned and dispersed. Although not as difficult to make as either mono-crystalline or polycrystalline diamond, it lacks the performance that

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micron-sized diamond provides in a lapping and polishing application.

Of the characteristics that impact the ability of slurry to perform lapping or polishing, particle size plays a primary role. Particle mean size determines the workpiece material removal rate and the resulting surface roughness or extent of scratching. Both mono-crystalline and polycrystalline diamond can be graded to extremely precise mean size and percentile tolerances.

Another key characteristic of diamond powder and subsequent slurry performance is particle shape or surface morphology. In a lapping or polishing application, the edges and corners interact with the workpiece and do the abrading. Particle shape also affects the ability of the particle to embed in the lapping plate. Once the particles are embedded in the plate, they become much more effective in abrading the workpiece as shown in Figure 1

FIGURE 1

ILLUSTRATION OF LAPPING MECHANISM

Mono-crystalline micron powders typically have sharp edges and corners as shown in Figure 3a. As can be seen, each particle will have a few corners that will have some potential or probability for interacting with the workpiece. Polycrystalline diamond powders have a rougher surface texture than mono-crystalline particles and therefore have more cutting points as shown in Figure 3b. This feature increases the probability of embedding into the plate and interacting with the workpiece. It is the surface texture of polycrystalline diamond that allows it to achieve a significantly higher material removal rate than mono-crystalline diamond. In addition to the higher material removal rates, the resulting surface roughness of the workpiece will be lower than with mono-crystalline diamond.

This paper describes a new type of diamond powder HYPERION™ and results obtained with it. HYPERION™ diamond has been developed by surface modifying mono-crystalline diamond that transforms the surface texture of mono-crystalline diamond from having just a few cutting points to one where there are many fine and distinct features as shown in Figure 2 [1]. Figure 3c is a SEM image of a modified HYPERION™ diamond powder. The surface-modified, mono-crystalline diamond with the increased numbers of pits and spikes out-performs polycrystalline diamond in lapping and polishing of semiconductor materials.

FIGURE 2

TYPICAL PROFILE OF MONO-CRYSTALLINE DIAMOND PARTICLE (2a) SHOWS A FEW EDGES THAT ACT AS CUTTING

POINTS IN LAPPING AND POLISHING. BUT SURFACE MODIFIED DIAMOND PROFILE HAS MANY FINE MICRO-POINTS

THAT INCREASES THE EFFECTIVENESS OF THE DIAMOND.

FIGURE 3

SEM IMAGES OF MONOCRYSTALLINE, POLYCRYSTALLINE AND HYPERION™ DIAMOND

Lapping and polishing slurries or suspensions are made from liquid media and diamond powder. The liquid component of the slurry has several important functions. The liquid provides the means for uniform dispersion and controlled distribution of the diamond onto the lapping plate or polishing cloth. The slurry vehicle also provides a lubricating effect that prevents heat build-up when the workpiece interacts with the embedded abrasives. Finally, the liquid media carries away the fine debris that results from abrasion. In addition to diamond size and shape effects, slurry vehicle composition can add significant value to the performance of slurries.

Tests have shown that different slurry formulation

works differently on various types of plates or pad. Specific slurry formulations with HYPERION™ diamond have been developed to be used on tin, copper or copper composite plates. Specific slurry was also developed to be used on a polishing pad. LAPPING AND POLISHING PERFORMANCE

Extensive lapping and polishing tests were performed to demonstrate the differences between mono-crystalline, poly-crystalline and surface-modified diamond. Table 1 is a list of general test conditions and parameters. The following are several case-studies that show how HYPERION™ diamond powder greatly advances lapping and polishing performance.

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TABLE 1 GENERAL LAPPING AND POLISHING TEST CONDITIONS

Parameters Lapping Polishing Lapper/Polisher Model 15”

LapMaster Model 15” LapMaster

Lapping Plate/Polishing Pad

Tin and Copper Composite

Suba 500

Workpieces 2” as-lapped sapphire wafers

3” as-cut SiC wafers

Table rotation speed

60rpm 60rpm

Wafer mounting Free Free Pressure 3.2psi 5.7psi Slurry flowrate 4ml/min 15ml/min Process time 60 minutes 90 minutes

Example 1 – Polishing of Silicon Carbide Wafers Polishing tests were performed using a LapMaster 15” lapping machine with conditions listed in Table 1. One wafer was polished at a time for 90 minutes and tests were repeated 3 times. Wafers were weighed before and after polishing. Surface roughness on polished workpieces was measured on Veeco Wyko. Figure 4 shows the test results of SiC wafers polished with K285T slurries containing mono-crystalline, poly-crystalline and HYPERION™ diamond powder with the same particle size and diamond concentration. The results in Figure 4 show that HYPERION™ slurry had an average material removal rate of nearly 350 mg in 90 minutes using the test setup. This is an increase in the removal rate of over two times compared to the mono-crystalline diamond slurry and an increase of eighty percent as compared to poly-crystalline diamond slurry. Figure 4 also shows that even though the material removal rate of the HYPERION™ slurry is significantly higher than the poly-crystalline slurry, the resulting surface finish on the wafers was equivalent or better.

FIGURE 4 SLURRY PERFORMANCE ON POLISHING SIC WAFERS

Example 2 – Lapping Sapphire Wafers on Tin Plate Lapping tests were performed on a LapMaster 15” lapping machine with conditions listed in Table 1. For each test, three wafers were lapped at a time for 60 minutes and the tests were repeated. Wafers were weighed before and after lapping to determine material removal rate (MRR). The surface roughness on lapped workpieces was measured on a Veeco Wyko. Figure 5 shows the test results on sapphire wafers with DI3300 slurries containing mono-crystalline, poly-crystalline and HYPERION™ diamond powder with the same particle size and diamond concentration.

FIGURE 5 DIAMOND SLURRY PERFORMANCE OF LAPPING SAPPHIRE

WAFERS ON TIN PLATE

Results showed that the HYPERION™ slurry achieved a material removal rate of over 170mg in 60 minutes. In the same test, the mono-crystalline slurry only achieved a removal rate of 30mg and the poly-crystalline slurry was 80mg. Therefore, the HYPERIONTM slurry almost doubled the removal rate compared to the poly-crystalline diamond slurry and the surface finish was significantly improved. Example 3 – Lapping Sapphire Wafers on Copper Composite Plate

Similar lapping tests were performed using a copper composite lapping plate with DI950 slurries containing mono-crystalline, poly-crystalline and HYPERIONTM slurry having the same particle size and diamond concentration. As shown in Figure 6, the material removal rate with the HYPERION™ slurry was 270mg in 60 minutes whereas the mono-crystalline slurry provided a rate of 50mg per hour and the poly-crystalline slurry a rate of 180mg per hour. Even though the removal rate of the HYPERIONTM slurry was nearly double that of the poly-crystalline diamond, the resulting surface roughness of the wafers was lower compared with the mono-crystalline slurry and about the same as the poly-crystalline slurry.

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FIGURE 6 DIAMOND SLURRY PERFORMANCE ON LAPPING SAPPHIRE

WAFERS ON TIN PLATE

CONCLUSIONS

Diamond slurry is a critical material for lapping and polishing advanced compound semiconductor materials. Diamond Innovations has successfully developed a novel diamond and diamond slurry that significantly increases lapping and polishing effectiveness, allowing reduced processing costs through reduced cycle times, improved capacity without sacrificing surface quality. Table 2 shows a general guideline for choosing the proper HYPERION™ slurry for each lapping and polishing system.

TABLE 2 SELECTION OF HYPERION DIAMOND SLURRY FOR LAPPING

AND POLISHING

DI3300 Series

DI900/950 Series

K285T Series

Application Precision lapping

Precision lapping

Polishing

Workpieces Sapphire, SiC, GaN

Sapphire, SiC, GaN

Sapphire, SiC, GaN

Plate Types Tin Copper and Copper composite

Polishing pad

Slurry type Water-soluble

Water-soluble Water-soluble

Diamond size

2 – 10um 2 – 10um 2 – 10um

Handling Easy Cleaning and dispersing

Easy Cleaning and dispersing

Good suspension and easy cleaning

ACKNOWLEDGEMENT The authors would like to thank all colleagues who have performed lapping and polishing tests and analyzing diamond powder, slurry and workpieces.

REFERENCES [1]. US Patent Application #US20100064593A1 and #US20100068974A1

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