Nanometer-Thin Pure Boron Layers as Mask for Silicon Micromachining

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 wetetchant. The selectivity to Si was extremely high, much greater than 10
⁴
. Cavities 70 μm deep were etched without measurable etching of the boron layers, and with (100)/(111) 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.