Estimation of Temperature Distribution in Silicon During Micro Laser Assisted Machining

Abstract

Silicon is machined using a diamond tool and the process is assisted with an IR Laser for the purpose of heating and thermal softening the work piece material. The laser beam passes through the tool and into the work piece, where the material is both thermally heated (by the laser) and mechanically deformed (by the tool). The laser is used to increase the work piece temperature (up to the softening temperature of silicon, about 500–800°C [10]), while the tool deforms and cuts the heated and softened silicon in the ductile regime, without producing cracks. This hybrid laser assisted machining process results in a smooth plastically deformed surface and extends the life of the diamond tool when cutting a hard and abrasive material, e.g. silicon. Scratch tests were done using the micro laser assisted machining method with diamond tools, which demonstrated enhancement in the depth of cut from 60 nm to 120 nm with (a 2x increase in depth of cut, at a constant load) while the cutting speed varied from 0.305 mm/sec to 0.002 mm/s. An analytical and numerical method was used to estimate the temperature rise in the vicinity of the diamond tool due to laser irradiation and absorption by the silicon work piece. It is assumed that the layer of silicon that absorbs the heat from the laser radiation is silicon II. Silicon II is a metallic phase of silicon, commonly referred to as the beta-tin structure, formed by a high pressure phase transformation (HPPT). In this context, the analytical and numerical models are solved using the heat conduction equation for semi-infinite solid over time with a Gaussian laser beam intensity distribution. The temperature rise for different cases (laser intensity, depth of cut, cutting speed, etc.) was modeled using point, and plane heat source method with Gaussian intensity distribution. These results are discussed in detail to estimate the temperature distribution while machining.