Sapphire is one of the main LED substrate materials. Diamond wheels are used for sapphire wafer thinning before using.

1 Introduction of sapphire:

The sapphire crystal is extremely hard, not only has excellent optical and mechanical performance and good thermal conductivity but also has good wear resistance and good wind erosion resistance. It is an ideal LED substrate material.

 

The sapphire substrate has mature processing technology and stable quality and can be used in the high-temperature growth process. In addition, the sapphire substrate has high mechanical strength, which is good for treatment and cleaning.

Table.1.1 The major properties of sapphire monocrystalline

Chemical formula	a-Al2O3
Density（g/cm3）	3.98
Mon’s hardness	9
Poisson ratio	0.25
Young’s modulus（GPa）	345
Modulus of rigidity（GPa）	148
Ultimate stress（GPa）	448
Specific heat capacity（cal/℃）	0.1
Thermal conductivity（W/m·K）	41.9
Melting point（℃）	2300

Due to the poor thermal conductivity of sapphire, the sapphire substrate should be back-thinned to improve the heat dissipation performance of the device after finishing the electrode preparation process, in order to prevent the excessive temperature rise of the LED active region from affecting its light output features and lifetime.

In addition, with the Mohs hardness of the sapphire of 9.0, the sapphire substrate should be back-thinned to a certain extent to meet the requirements of subsequent processes such as dicing, splitting, and etc.

There is surface damage layer on the backside of the thinned substrate, and the residual stress causes the thinned epitaxial wafer to be bent and deformed, and it is easily crushed in the subsequent process, thereby affecting the yield rate. Therefore, the backside of the substrate after thinning should be polished to remove the surface damage layer and eliminate residual stress. Usually, the thickness of the epitaxial wafer should be thinned from 400mm to 100mm or thinner.

2 Lapping process of the sapphire substrate

2.1 Technological process chart of the sapphire substrate

2.2 Sapphire lapping

2.2.1 Lapping mechanism of a sapphire substrate

There are 3 types of material removal lapping. They are mechanical abrasion of the abrasive grains, slight melting and plastic flow of the ground surface, and chemical action of the surface active material in the lapping slurry.

The melting point of the sapphire is as high as 2045 ° C, and the mechanical abrasion is the main way for thinning the sapphire substrate. Under the action of the abrasive, there are many microcracks on the surface of the sapphire substrate and they extend below the surface to form the surface damage layer. During the lapping process, these microcracks are continuously extended.

If they cross each other, the sapphire material surrounding the area will fall off, forming small pits and grooves, thereby achieving the lapping effect. With a high hardness of the sapphire, the removal rate is small during polishing. Therefore, should reduce the thickness of the damaged layer on the backside of the epitaxial wafer to the suitable level before polishing.

The formation and extension of microcracks, as well as the thickness of the surface damage layer,  are closely related to the lapping process parameters. Therefore, the surface damage layer of the sapphire substrate directly affects the surface condition during lapping and the surface roughness. So, the surface damage layer can be analyzed by studying the change in surface roughness with process parameters.

2.2.2 Selection of lapping process:

Usually, diamond wheel or ultrafine diamond powder is used for the sapphire substrates processing, as well as SlO2-based chemical-mechanical planarization (CMP) which can obtain small subsurface damage (SSD).

This paper mainly introduces conventional grinding- diamond grinding.

Advantage:

High grinding efficiency and high output.

Disadvantages:

There is a deep metamorphic layer during the lapping process, which affects the subsequent process. At the same time, there are scratches, microcracks, residual stress and lattice distortion on the surface, which affects optical performance.

Selection of diamond wheel in sapphire wafer thinning:

Choose resin bond diamond wheels in sapphire substrate thinning

Rein bond grinding wheel has good self-sharpness, good toughness, small grinding heat, and good surface finish of the workpiece.

Choose good quality resin bond diamond grinding wheel to achieve the high-quality surface finish, low subsurface damage, high production efficiency and reduce production costs.

2.2.3 Selection of polishing process.

After the sapphire is lapped, the next process is polishing. The purpose is to treat deep holes after lapping, or deep scratches after grinding. Usually, the depth of the hole after lapping is about 10um, and the scratch depth after grinding is 15um~20um.

In the polishing process after lapping, the polishing disc is polyurethane pad, that is called soft polishing. The soft polishing can make the surface as bright as a mirror, but the cutting rate is extremely low, and it is about 0.2 um/min. Another polishing method is to use tin and leads plates. Because the disc surface is made of metal, it is called hard polishing. The cutting rate of hard polishing is 0.7~1 um/min, and the processing speed is faster than that of soft polishing. However, the risk of hard polishing with a metal disc is higher. Although tin and lead are soft metals, pay attention to the condition of the disc surface, especially the dressing of the disc surface. If the metal particles are not removed after dressing, they are brittle after polishing.

Therefore, there has been a new type of polishing disc in recent year, the disc surface of which is a resin and the base is copper in order to increase the cutting rate and the stability of the disc surface. It is also called resin copper disc. Because the hardness of the disc surface material is from polyurethane to tin, it is also called a way of hard polishing. The resin copper disc is used for polishing with special diamond slurry and spray volume per second, the cutting rate can reach 2.3~2.8um/min. With the process of grinding, it can increase production output. Of course, the consumption of diamond slurry will also increase, but the production cost of each sapphire may not increase under the production mode with product improvement and lower risk of loss.

Conclusions:

In the process of LED substrate thinning should choose high-quality resin bond diamond grinding wheels in sapphire wafer thinning to ensure a good finish and low surface damage during the grinding process.  It is a good foundation for the sapphire lapping and polishing, which can greatly improve the production efficiency and reduce costs.