Nanometer-Thin Pure Boron Layers as Mask for Silicon Micromachining

Abstract

The properties of nanometer-thin pure boron layers deposited by chemical vapor deposition were investigated for use as a barrier against tetramethyl ammonium hydroxide (TMAH) and potassium hydroxide (KOH) etching of Si. Deposition temperatures of 400 °C and 700 °C were applied to form layers of a few nanometer thick. Down to 2-nm thickness, they were all found to be resistant to these wet Si etchants. Patterning of the layers was achieved with resist masking and standard aluminum wet-etchant. The selectivity to Si was extremely high, much greater than 104. Cavities $70~\mu \text{m}$ deep were etched without measurable etching of the boron layers, and with $\langle 100\rangle /\langle 111\rangle $ etch-rate ratios of about 35 for TMAH and larger than 50 for KOH. Stress levels were measured to be 490 MPa tensile for 400 °C and 1250 MPa compressive for 700 °C deposition. Hundreds of micron wide membranes that remained intact were fabricated. [2017-0175]