Investigation On Double-side Lapping Process Of Single Crystal Sapphire Substrates

Single crystal sapphire possesses superior optical, physical and chemical properties, which make it an excellent material to be used to produce substrates for high temperature superconducting thin films, infrared optical materials and microelectronic devices, etc. No matter what the application is, it is a series of high demanding process to manufacture sapphire substrate in terms of machining precision and surface quality. Due to its remarkable hardness and brittleness, Single crystal sapphire is a typical material difficult to machine. Currently it is mainly through the processes of Double-side lapping with free abrasives to obtain sapphire substrates with high precision and low surface damage, and then the process of the single-sided polishing to get super-smooth surface. However, there are a number of factors that affect the precision and quality of sapphire substrates surface during the processes of Double-side lapping, including the concentration, viscosity, flow rate, particle size, and pressure of lapping slurry, and the wheeling path of wander star wheel, etc. Therefore, this paper focuses on key substrate indicators for LED, the most important application for sapphire substrate, including the total thickness variation(TTV),local thickness variation(LTV), Warp, Bow, and surface roughness(Ra), and evaluates the effect of the processing parameters on those key indicators as well as the processing costs and yield rates during the Double-side lapping process.This paper discussed the influence of the flow rate of lapping slurry, rotating speed and pressure on the substrates in terms of surface roughness and machining precision, during the lapping process of the 4-inch sapphire substrates on the Speedfam lapping machine. In the process, FRT surface measure device is used to detect the TTV, LTV, Warp and Bow, and Mitutoyo surface profiler is used to measure the surface roughness.First, according to the requirements for surface roughness, the particle size of boron carbide in the lapping slurry was initially set at 240#. Then various machining parameters were set up, including flow rate of lapping slurry, rotating speed and pressure to evaluate the effects on the sapphire substrates during the Double-side lapping process. With flow rate between 20ml/min and 60ml/min, no obvious effect was detected on substrate's TTV, LTV, Warp, Bow and other main indicators, while surface damage was increased. Rotating speed was proved to have obvious influence on substrate's TTV and LTV. With rotating speed rising from 15r/min up to 30r/min, TTV decreased, however excessive speed could also cause scratches and damage. Pressure affects the removal rate of the substrate. And as pressure increases, Bow and Warp increase significantly, and therefore results in scratches and uneven roughness.Secondly, focusing the yield and cost for the production process, an orthogonal experiment of multivariate, including pressure, rotating speed and flow rate, was designed.Finally, the optimum processing parameters were obtained with most key indicators being reduced effectively: TTV down by 2.5?m, LTV down by 0.6?m, Warp down by2.1?m, Bow absolute average value down by 1.27?m, while machining time was shortened by and lapping slurry was save by 35%.