Atomic Layer Deposited TiO2 as Sacrificial Layers and Internal Coatings for Nanoscale Gaps

Abstract

A key component of the MEMS cryogenic cooling systems we are developing is a MEMS compressor. Its function is to compress the refrigerants used in the cooling cycle. Layers of polyimide are stacked and patterned on a silicon wafer to create micro check-valves and a compression chamber over which a diaphragm is suspended. To achieve the high (4:1) pressure ratio needed for the refrigeration cycle, the polyimide diaphragm needs to be fabricated with minimal dead volume beneath it, hence the need for a sacrificial layer with thickness of 100–300 nm. The topography created by the check-valves and valve-seats makes atomic layer deposition (ALD) ideal due to its conformality. Furthermore, following sacrificial layer release, the inside of the compression chamber will also need to be coated with a hermetic moisture barrier layer to enable the device to operate at 4 atmospheres without leaking. ALD is therefore also ideal for the final internal coating because it does not require line-of-sight. Towards this end, we demonstrate here the concept of using ALD TiO2 as a sacrificial layer to create a 5 mm × 5 mm × 20 m-thick polyimide membrane, suspended by ∼ 100–350 nm above a silicon wafer, followed by a second thinner ALD coating of the interior surfaces bounded by wafer and membrane. The air gap under the membrane, defined by the released sacrificial layer, was measured at about 130–370 nm using two independent methods: reflectometry, and FIB cross sectioning followed by SEM imaging of the air gap’s cross section. The membrane was removed from one chip and the thickness of the internal coating on the underlying silicon was measured with the reflectometer to be about 40 nm. We thus demonstrate the use of ALD TiO2 as both a sacrificial layer for fabricating nanoscale gaps, as well as for coating nanoscale internal cavities and channels.