Deep anisotropic etching of silicon

Abstract

We are interested in etching very deep anisotropic trenches (∼100 μm) with high aspect ratios (depth/width) (∼20–50) and high etch rates (∼5 μm/min). A high density plasma helicon reactor using SF6/O2 chemistry and a cryogenic chuck has been used for etching very narrow trenches from 1.2 to 10 μm wide on n-type Si wafers with a SiO2 mask. The first results show significant features that demonstrate the feasibility of this method. Two-micron-wide trenches have been etched to a depth of 80 μm at an average etch rate of 2.7 μm/min. The resulting profiles are highly anisotropic and selectivity of Si/SiO2 is remarkably high (>500).

Effects of deep reactive ion etching parameters on etching rate and surface morphology in extremely deep silicon etch process with high aspect ratio

“…This range covers most of the normal sizes of micromechanical devices. 6 Four controlled parameters were carefully controlled and investigated: etching bias power, chamber pressure, gas flow rate of SF 6 , and gas flow rate of C 4 F 8 . The etching rate, sidewall angle, and sidewall damage were all measured and compared in detail.…”

“…During the passivation period, C 4 F 8 gas is used to generate polymer materials on the sidewalls and bottom, as a passivation layer which will block chemical reactions between the silicon and SF 6 gas during the etching period. During etching, SF 6 /O 2 gas is excited and turned into ions by the RF coil, and then the ions are accelerated to bombard the passivation layer at the bottom. The polymer materials at the bottom are removed by this ion bombardment, and the silicon is etched by the chemicals reacting with the F ions.…”

“…Moreover, the etch rate is very low, and the photoresist may crack during the prolonged process. 8 For the gas mixture process, which involves mixing SF 6 /C 4 F 8 or other etching and passivation gases such as SF 6 /CHF 3 , the low selectivity between the silicon and the photoresist (4:1) and the low etching rate (150 nm/min) are the two main shortcomings. 9,10 Compared to the above three methods, the Bosch process has been more widely used in the fabrication of HARSE structures in recent years.…”

“…However, unlike ICs or memory devices in which the trench width is on the submicron-scale, the width of trenches in micromechanical devices is between 10 and 1000 mm. 6 This means it requires an etch of several hundred microns to meet the aspect ratio demands in micromechanical device fabrication. Nevertheless, when the silicon etching reaches a depth of more than 200 mm, problems such as etching stop, poor sidewall angles, and damage to the surfaces of the sidewall and bottom appear.…”

“…9,10 Compared to the above three methods, the Bosch process has been more widely used in the fabrication of HARSE structures in recent years. It is a time-multiplexed process which uses gaseous SF 6 for etching and gaseous C 4 F 8 for passivation. Its most notable merits include a high etching rate (up to 5 mm/min), high selectivity in the photoresist-mask process, and the fact that it can be conducted at room temperature.…”