This is our group's standard fabrication process for robotic mechanisms (as of 2021).

We start with SOI wafers purchased from an external vendor (e.g., in the past, we've ordered from SVM).

These are 150mm diameter with a 40um frontside, 2um buried oxide (BOX), and 550um backside (substrate). The silicon (both frontside and substrate) is P/boron doped to 15 to 20 ohm-cm. The wafer surfaces (front and back) should have standard <100> orientation with a single primary flat in the <110> direction. The frontside is polished; the backside may or may not be. We've considered ordering wafers with oxide on the backside to use as a hard mask.

The wafer frontside is about as thick as feasible to increase inchworm motor power while maintaining a 2um feature size in the later frontside DRIE etch. The backside thickness maintains structural rigidity during fabrication. The BOX is an etch stop for both the front and backside DRIE etches and may benefit from increased thickness. The wafer conductivity may or may not affect inchworm motor operation, and the crystal structure orientation is standard (structures in different orientations have, e.g., a different elastic modulus).

We start by patterning metal on the frontside via liftoff to improve wirebonding and make contact with micromanipulator probes easier (also, for silver epoxy to make an electrical connection to the gold; it doesn't form an ohmic contact to the silicon).

Thickness is typically 300 to 500nm of Cr (as an adhesion layer) then Au (for conductivity and preventing oxidation). Current designs are robust to metal variations.

See the liftoff recipe page for current best practices.

Next, we DRIE etch the 40um frontside layer to produce inchworm motors and other MEMS structures using the buried oxide as an etch stop. We typically achieve a 2um minimum feature size. Our inchworm motors are fairly robust to sidewall geometry (DRIE scallop size, draft angle, and overetch). It's important for the DRIE etch mask to completely cover the metal layer to avoid exposure to the DRIE etch (which could sputter the metal to contaminate the etch chamber).

See the DRIE silicon etch page for current best practices.

TODO