How can cutting and grinding fluids be made to work effectively in sapphire grinding?

Generally, cutting and grinding fluids consist of aviation gasoline, kerosene, transformer oil, various vegetable oils, animal oils, hydrocarbons, and several additives. Depending on the abrasive material used, cutting and grinding fluids can be classified into diamond-based grinding fluids, silica-based grinding fluids, and cerium oxide-based grinding fluids. Among these, diamond-based grinding fluids can be further divided into single-crystal diamond grinding fluids, polycrystalline diamond grinding fluids, and detonation-produced nanodiamond grinding fluids.

The sapphire cutting and grinding fluid leverages the properties of polycrystalline diamond to maintain high cutting efficiency during grinding and polishing, while minimizing the risk of scratching workpieces. It is suitable for grinding and thinning sapphire substrates, as well as for the grinding and polishing of optical crystals, hard glass and crystals, ultra-hard ceramics and alloys, magnetic heads, hard disks, chips, and other similar materials.

To address the heat dissipation issue of sapphire, the thickness of the sapphire substrate needs to be reduced from 450 nm to 100 nm. This process involves two main steps: First, use a grinding wheel with a grit size of 50–70 μm on a lateral thinning machine to grind down the substrate from a thickness of approximately 300 μm; then, use a polishing machine equipped with different polishing pads and an appropriate sapphire polishing slurry to polish the back side of the chip, reducing its thickness from around 150 nm to about 100 nm.

Different cutting and grinding fluids should produce varying effects on the grinding of sapphire. Cutting and grinding fluids have shown promising results in enhancing both the grinding efficiency and surface quality of sapphire crystals. Due to their superior physical and optical properties, sapphire crystals are widely used as LED substrate materials, solid-state laser gain media, and in display panels. Consequently, the broad application of sapphire places ever-higher demands on ultra-smooth, non-damaging surface grinding and polishing technologies. However, because of sapphire's relatively stable physicochemical properties, it remains challenging to process.

In today’s industry, the processing of sapphire crystal involves a series of steps—including slicing, chamfering, grinding, and polishing—among which grinding plays a particularly significant role. This is because grinding is crucial for achieving excellent surface flatness and ensuring both the quality and efficiency of subsequent polishing operations. In the grinding of single-crystal sapphires, the grinding feed rate is controlled by gradually reducing the vertical diameter of the abrasive particles in the grinding fluid, thereby attaining a superior surface roughness. During the grinding process, hard abrasives rolling between the workpiece and the grinding wheel tend to cause substantial surface damage layers, thus prolonging the polishing time. Moreover, such abrasives exhibit low utilization rates and high costs. Currently, fixed-abrasive tools have emerged as an effective and low-damage approach, thanks to their ability to leverage protruding abrasive particles for fine material removal.

During the consolidation grinding process, the chemical effects of the slurry cannot be overlooked. When polishing silicon substrates with fixed-abrasive tools, we compared the effects of different polishing liquids on material removal rate and surface quality. We found that variations in the composition and pH value of the polishing liquid significantly influenced the polishing performance. Therefore, it can be inferred that the cutting and grinding fluid undergoes certain chemical interactions with the workpiece surface.

A certain chemical substance in cutting and grinding fluids can soften the surface layer of crystal glass, thereby enhancing the material’s polishing rate. However, due to sapphire’s high chemical stability, different polishing fluids yield varying polishing effects on single-crystal sapphire. Even subtle chemical interactions can have an imperceptible yet significant impact on the polishing efficiency and surface damage of sapphire wafers.