Rotary laser thinning of silicon wafer via diffractive optical element: Effects of process parameters on surface integrity

As the demand for computing power and processing speed continues to increase, integrated circuits require greater circuit layer density and better heat dissipation capabilities. Therefore, wafer thinning technology is crucial. Traditional laser thinning technology can obtain a silicon wafer surface with a surface roughness of 749 nm. Here, we show a novel rotary laser thinning technique for silicon wafers utilizing diffractive optical element (DOE) is presented, wherein conventional circular laser spots with Gaussian-distributed energy are transformed into linear spots with uniform energy distribution. This method overcomes critical limitations of conventional laser thinning, offering controllable thinning operations. Experiments demonstrate that DOE-based laser thinning reduces the surface roughness to 196.6 nm with a maximum reduction of 50.34 % amorphous silicon formation and 5.96 % oxide formation. When applied to flat surfaces, DOE-based linear spots effectively mitigate the formation of raised redeposited material and sunken removed materials created by Gaussian spot thinning, resulting in a flat and uniform surface. When applied to uneven surfaces, the method mitigates approximately 50 % of the regenerative effects common in conventional laser thinning. Notably, the surfaces thinned by the DOE-based linear spots exhibit consistent hydrophilicity in both feed and tangential directions, in contrast to the anisotropic wettability of Gaussian-thinned surfaces. The influence of pulse frequency and rotational speed on machining depth and surface roughness was further explored, and optimal thinning parameters were identified. This innovative approach provides a promising alternative thinning method for silicon wafers, with potential applications in semiconductor device fabrication and other high-precision fields requiring smooth and precise surfaces. Furthermore, the multi-pass capability of the DOE-based method shows promising advantages in surface quality control and machining uniformity.