Machining on Rear Surface of a Silicon Substrate by an Infrared Femtosecond Laser via Non-linear Absorption Processes

Silicon (Si) is a semiconductor material widely used in modern technologies such as microelectronics, MEMS, and photonics. Si is an opaque material in visible to near-infrared wavelength region but it transmits any light longer than 1127 nm, which corresponds to its band gap energy of 1.12 eV. Therefore, Si can be considered as transparent material for radiations longer than 1127 nm. Here we have proposed a new laser microprocessing method of Si, which increases the efficiency, accuracy and flexibility in machining. The thickness of the Si substrate used was 320 μm and a femtosecond laser at 1552 nm was focused by a 100x infrared objective lens on the rear surface of the substrate. However, only a shallow groove was formed: its depth was approximately 170 nm or less. It is expected that when the laser is focused on the Si rear surface, where it is contacting with an etchant, wet etching will occur due to the temperature rise caused by the laser irradiation. Therefore, to increase the machining rate, we tried laser-assisted backside wet-etching using KOH solution as the etchant. The maximum groove depth was increased to more than 3 μm. The effects of laser irradiation conditions on machined grooves were examined. The results achieved showed rather scattered values and indicated that the maximum groove depth was not always produced at high energy deposition conditions. One of the reasons might be the formation of hydrogen bubbles from the chemical reaction between Si and KOH, which block the contact of Si and KOH.