How Are Silicon Carbide Wafers Obtained?

How Are Silicon Carbide Wafers Obtained?

Silicon carbide has very stable characteristics, so that it can work stably in some harsh environments. Because of the stable chemical bonds, the technical threshold for silicon carbide production is very high. The growth conditions of silicon carbide crystal ingots are harsh, requiring high temperature (~2600℃) and high pressure (>350MPa) growth environment; crystal growth speed is slow, production capacity is limited, and quality is relatively unstable. Limited by the size of the wafer growth furnace, the size of the crystal ingot is restricted. Silicon carbide is a hard and brittle material. The hardness is second to that of diamond. Cutting is difficult, and the grinding accuracy is difficult to control. Therefore, the silicon carbide wafer manufacturing process slicing from silicon carbide ingot is very difficult.

1. Industry Requirements for Silicon Carbide Wafer Production

Corresponding industry requirements are put forward for silicon carbide wafer manufacturing process:

ProcessParametersRequirements
Single crystal growthSize>4 inch
Crystal form4H-SiC
Microtubule density<2/cm2
Resistivity0.015~0.03Ω*cm(Conductive N type)
10^5 Ω*cm(Semi-insulated)
Processing requirementsTTV<15 μm
Bow<40 μm
Warp<60 μm
Ra<0.3 nm

* The silicon carbide(SiC) wafers produced by Xiamen Powerway Advanced Materials Co., Ltd.( short for PAM-XIAMEN) meet the industry requirements. For more details, please visit: https://www.powerwaywafer.com/sic-wafer/sic-wafer-substrate.html.

The parameters of TTV, bow, warp, and Ra in the above table are not as easy to achieve. The reasons are listed as follows:

The quality of the crystal itself determines the subsequent processing;

The hardness of silicon carbide is 9.2, which can only be processed by diamond;

If you simply use diamond processing, too much stress will cause damage to the silicon carbide wafer substrates.

2. Silicon Carbide Wafer Manufacturing Process

For above reasons, the entire silicon carbide wafer manufacturing process is designed as shown in the Figure below:

silicon carbide wafer manufacturing process

Silicon Carbide Wafer Manufacturing Process

Diamond wire multi-wire cutting is used to control warp, bow, and TTV; double-sided grinding is used to remove the cutting damage layer and raise warp, bow, TTV and LTV; double-sided polishing is used to reduce the roughness less than 2nm. Chemical mechanical polishing is used to improve the surface quality, make the roughness<0.2nm and no scratches. SiC wafer cleaning and SiC wafer packaging require no stickiness under strong light.

The all procedures generate the corresponding key technology of SiC wafer processing:

# Technique

# Multi-wire saw

# Control warp/bow/TTV/LTV

# CMP

# Wafer cleaning

The silicon carbide wafer manufacturing process is described in detail below.

2.1 Dicing Silicon Carbide Ingot by Multi-wire Cutting

To prevent warpage, the thickness of the wafer after dicing is 350um. Generally, it will be thinned after it is fabricated into a chip.

2.2 Silicon Carbide Wafer Grinding

Use diamond slurry for grinding. The particle size of the diamond powder in the slurry affects the removal rate and the surface damage layer. Using the method that combine the coarse grinding with a larger particle size and fine grinding with a smaller particle size can achieve better grinding results. The rough grinding disc is a resin copper disc/glass disc, and the fine grinding disc is a tin disc.

Grinding pressure and grinding disc speed also affect the SiC wafer grinding quality:

when the grinding pressure is high, the grinding speed is fast, but the TTV value will increase accordingly;

when the pressure is small, the grinding speed becomes slower;

increasing the grinding disc speed within a certain range can improve the removal rate, but the higher the speed, the worse the flatness of the substrate surface.

The grinding pressure is generally controlled at 0.5-0.8 kN (0.025 MPa), the grinding head speed is 60-80 r/min, and the grinding disc speed is about 60 r/min.

The grinding disc needs to be sharpened during the grinding process to ensure the removal rate of the silicon carbide single crystal substrate. The grinding disc dressing system can make the grinding liquid evenly distributed and ensure the grinding removal effect. When the removal rate of the grinding disc is decreased and the removal rate cannot be guaranteed by online dressing, the grinding disc needs to be trimmed offline.

powerwaywafer

2.3 SiC Wafer Polishing by Chemical Mechenical During Silicon Carbide Wafer Manufacturing Pocess

Using mechanical action (pressure) and chemical/oxidation (hydrogen peroxide, pH value) to work together to make the surface smooth and clean. The balance between chemical action and mechanical action should be paid more attention. The type of polishing fluid, polishing pad, polishing pressure, polishing disk speed and many other conditions will determine the polishing quality:

  1. Influence of polishing solution concentration: the higher the concentration of polishing solution, the stronger the polishing removal ability.However, the substrate surface roughness will increase, the surface quality will decrease. The concentration decrease, the polishing removal ability will decrease, and the efficiency will be low;
  2. Harder polishing pads can get a better flatness, and softer polishing pads can get a surface with fewer defects;
  3. Increasing polishing pressure or rotation speed canimprove the material removal rate, but at the same time, it will increase the surface roughness of the material and the sub-surface damage layer, which will affect the surface quality.

When the removal amount is too large, due to the high hardness of the silicon carbide material, the abrasive particles of the grinding wheel are gradually rounded and dull under the action of friction and extrusion. Grinding debris embedded in the pores on the surface of the grinding wheel will cause blockage of the diamond grinding wheel, resulting in a decrease in the grinding capacity and efficiency of the grinding wheel, and unevenness on the surface of the workpiece.  

In order to solve this problem, the SiC wafer grinding process has been improved, and the oilstone online dressing process has been added. On the one hand, it can remove the abrasive debris clogged on the surface of the grinding wheel and make the abrasive particles protrude to the surface; on the other hand, when the grinding wheel becomes blunt, it can be sharpened again by sharpening, making the grinding and cutting process easier.

Maximum cutting thickness=2*workpiece speed/(grinding wheel speed*grinding wheel particle size number)*√(radial feed/grinding wheel diameter)

The smaller the feed of the grinding wheel, the better the surface roughness of the workpiece, and the higher the surface accuracy. During the machining process, you can choose the appropriate feed according to the surface roughness to be achieved.

The general grinding process is divided into: high-speed feed stage, empty feed stage, P1 stage, P2 stage, P3 stage, and high-speed return stage.

The material removal amount can be obtained through measuring the quality of silicon wafers before and after polishing by Sartorius CP225D precision electronic balance. The surface morphology characteristics of the silicon carbide wafers are analyzed by Olympus OLS4100 optical microscope. Measure the surface roughness of the silicon carbide wafer by Zygo Newview5022 Surface Profiler, and take 3 equal points on the 1/2 wafer diameter circle to measure the average value. After fine polishing, the surface roughness of the silicon carbide wafer is measured by XE-200 atomic force microscope.

2.4 Silicon Carbide Wafer Cleaning

The steps of RCA cleaning silicon carbide are:

  1. Use acetone (C3H6O) for 15min ultrasonic cleaning;
  2. Use deionized water for 3 times of ultrasonic cleaning, 10 minutes each time;
  3. After boiling a solution of H2O2+NH3H2O:H2O with a volume ratio of 1:1:5 for 15 minutes, clean the wafer(the concentration of H2O2 is 30%);
  4. Use deionized water for 3 times of ultrasonic cleaning, 10 minutes each time;
  5. After boiling a solution of H2O2+HCl:H2O with a volume ratio of 1:1:5 for 15 minutes, clean the wafer(the concentration of HCl is 37%);
  6. Use deionized water for 3 times of ultrasonic cleaning, 10 minutes each time;
  7. After taking out the wafer, dry it with high-purity nitrogen

RCA treatment can effectively remove the contamination layer and other impurities remaining on the surface of the silicon carbide wafer after annealing, and it will not affect the structure of the silicon carbide wafer surface.

PAM-XIAMEN strictly obeys silicon carbide wafer manufacturing process, aiming to provide customers with high-quality silicon carbide wafer.

For more information, please contact us email at [email protected] and [email protected].

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