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 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.

Low temperature pure boron layer deposition for Si diode and micromachining applications

“…This section gives a review, based on the reference [43] that was published as part of the present thesis research, of the properties of boron layers that are known from earlier work on PureB diode fabrication. So far, the 400℃ and 700℃ depositions in the Epsilon reactor have been the most interesting layers for electrical applications.…”

“…In Fig. 2.5 [43] the main structural and electrical differences between the layers are illustrated. Both layers were deposited in the ASM Epsilon CVD reactor.…”

“…Although the resistivity is very high [86] charging effects were not observed on the membranes presented here. An important advantage with respect to graphene is that B thin films are highly manufacturable [18,43].…”

“…Moreover, B-layer membranes of only 4 nm in thickness were fabricated. Patterning of the boron was straightforward, being achieved by standard resist masking and removal by aluminum etchant [43].…”

“…B-membranes would also have many of these advantages: B has an even lower atomic number than C and the CVD layers are conductive, albeit with very high resistivity. Also, for application as membranes, an important advantage with respect to graphene is that B thin films are highly manufacturable [18,43,44]. The XUV group at University of Twente was also devoting some time to studying boron as a material for capping EUV pellicles for EUV lithography systems [45].…”