Low-damage Micromachining for 4H-SiC Pressure Sensitive Diaphragm by Femtosecond Laser

As a representative material of the third-generation semiconductor, silicon carbide (SiC) demonstrates excellent physical properties and chemical stability, which is an ideal material for harsh environment operating devices and high power electronics. To fabricate SiC pressure sensitive components,blind holes are processed on the surface of the SiC substrate to obtain sensitive diaphragms. Laser ablation is an efficient way to process SiC materials, and femtosecond laser processing has been widely studied because of its small thermal effect, low damage to materials, fast processing rate, insensitivity to crystal orientation, and ability to form complex structures. To investigate the characteristics of 4H-SiC material processed by femtosecond laser,the effects of fabrication process parameters such as step spacing in the depth direction, scanning direction, single pulse energy and scan line spacing on the surface morphology and ablation rate of 4H-SiC material are investigated. The 4H-SiC blind hole with a diameter of 1 600 mu m, a depth of 250 mu m and a thickness of 100 mu m is prepared. To investigate the effect of depth direction step spacing on the ablation depth and surface morphology, the single pulse energy of the femtosecond laser is set to be 30 mu J; the scanning line spacing is set to be 20 mu m; the angle between the laser scanning path and the laser polarization direction (theta) is set to be 90 degrees. The parallel line scanning path is used to process 4H-SiC. The step spacing in the depth direction is set as 2.9 mu m and 15 mu m. These two samples are marked as sample 2 and sample 3, respectively. The ablation depth of sample 2 was 15.6% larger than the expected ablation depth, and holes appeared on the surface. The ablation depth of sample 3 was 10.2% smaller than the expected ablation depth, and no holes appeared. To investigate the effect of scan path on surface morphology,the single pulse energy is set to be 30 mu J; the scanning line spacing is 20 mu m, the step spacing is 2.9 mu m; theta is 90 degrees, 60 degrees, 30 degrees,and 0 degrees. The surface roughness of the sample gradually increases with decreasing theta and the number of surface holes gradually increases. This is because the angle between the microgrooves and the scan path gradually decreases with the decreasing theta, leading to an increase in the probability of overlapping microgrooves on the scan path and thus the generation of holes on the sample surface. To investigate the effect of single pulse energy on the ablation depth and surface roughness, the single pulse energy is set to be 5, 10, 15, 20, 25, 30 mu J; theta is 90 degrees; the scanning line spacing is 20, 15, 10, 8, 5, 4, 3, 2 mu m. The laser light intensity increases linearly with the increase of the laser single pulse energy. As a result,the ablation depth gradually increases with the increase of the single pulse energy. With the increase of laser single pulse energy,the light intensity distribution is more uneven, resulting in a gradual increase in the surface roughness. Furthermore,the superimposed light intensity of the femtosecond laser decreases exponentially with the increase of the scan line spacing by changing the scanning line spacing. As a result,the ablation depth decreases exponentially with the increase of the scan line spacing. In this paper, a 1 028 nm,190 fs femtosecond laser system is used to process 4H-SiC sensitive diaphragms. The experimental results show that the formation of holes on the surface of 4H-SiC samples is related to the overlap of laser-induced microgrooves. The final setting of the femtosecond laser with a single pulse energy of 30 mu J, a scan path angle of 90 degrees to the laser polarization direction,and a scan line spacing of 2 mu m is used to process blind holes with a diameter of 1 600 mu m and a depth of 250 mu m using a circular table-shaped material removal method. The ablation depth of a single turn is 85.7 mu m, and the 4H-SiC sensitive diaphragm with a thickness of 100 mu m is obtained after three turns of processing. The resulting 4H-SiC pressure-sensitive diaphragm has no obvious holes on the surface, the edge over-ablation depth is less than 10 mu m, and the low-damage femtosecond laser processing of 4H-SiC pressure-sensitive diaphragm is obtained.