Femtosecond laser micromachining of crystalline silicon for ablation of deep macro-sized cavities for silicon-on-insulator applications

Abstract

We demonstrate the use of femtosecond laser micromachining for ablating macro-sized cavities in crystalline silicon. The method employed is laser milling in which the focused laser beam is raster scanned over the area to be removed. We report the achievement of very high volume ablation rates for the cavity of up to 8.48x10 6 µm 3 s -1 . To achieve such high rates, we make use of a high average power fiber laser source of 1030 nm wavelength and variable per pulse energy of up to 100 µJ. By carefully controlling the process variables such as pulse energy, repetition rate and scanner speed, the tradeoffs between micromachining quality and ablation rate are quantified. The developed process is applied on Siliconon-Insulator (SOI) wafers for improving performance of RF devices. By making use of laser removal and an additional step of selective silicon etch using XeF2, handler silicon is removed completely under RF circuits such as SP9T switch. The local removal of silicon under such circuits completely eliminates the losses and non-linearities caused by the coupling of RF signals to the semi-conducting substrate. Small-signal and large-signal RF measurements are performed before and after substrate removal to quantify the performance gain. The obtained performance after substrate removal is better than specialized RF-SOI substrates such as trap-rich SOI. This is of practical significance for next generation wireless technologies like 5G which operate at higher frequencies with stringent specifications. The proposed method is also potentially useful for fabricating membrane based devices in SOI technology such as pressure sensors.

Large-area femtosecond laser milling of silicon employing trench analysis

“…By locally removing handler silicon under the active area of circuits, they can be suspended on the BOX in the form of a membrane. The motivation for creation of such membranes is to attain better RF performance with reduced loss and improved linearity as it has been reported previously in numerous similar studies [15][16][17][18][19][20][21][22].…”

Substrate Engineering of Inductors on SOI for Improvement of Q-Factor and Application in LNA

“…By using an etch mask to protect the unablated regions, membranes are obtained in the ablated areas after the completion of XeF2 etch step leaving rest of the die intact. The process has been described in detail in previous works where mm sized membranes have been demonstrated with material removal rates of up to 8.5x10 6 μm 3 s -1 [16], [17]. At the end of the process, silicon is removed completely under the inductor area (ABCD in Fig.…”

“…They are grinded to reduce the starting thickness to ~250 µm before applying the FLAME process. A two-step laser milling strategy is used to balance the removal rate and milling quality as described in [17]. The laser parameters used for processing along with the attained removal rates are shown in Table IV.…”

“…A local handler substrate removal method enables suspension of RF circuits freely on the BOX and substrate related losses can be brought to a minimum. In this context, we have developed the Femtosecond Laser Assisted Micromachining and Etch (FLAME) process to fabricate membranes of RF circuits [16], [17]. Substrate removal has already been studied in numerous works as a means of improving Q-factor [18]- [22].…”

“…Additionally, high material removal rates are possible making the process time-efficient. For instance, in our work on RF switches, volume removal rate of 8.5 x 10 6 µm 3 s -1 was reported [17]. In the following, this paper is hierarchically organized from process description to device and circuit characterization.…”