High selectivity cryogenic tungsten-boron-carbide etch

A method of selectively etching a hardmask layer formed on a device substrate, comprising:

• supplying an etching gas mixture to a process region of a processing chamber, wherein
• a device substrate is disposed in the process region when the etching gas mixture is supplied to the process region, wherein
• the device substrate comprises:
• a substrate,
• at least one cavity formed in the substrate, and
• a hardmask layer formed over the at least one cavity and over the substrate;
• and
• the etching gas mixture comprises at least a fluorine-containing gas and a hydrogen-containing gas;
• maintaining the device substrate at a cryogenic temperature;
• delivering radio frequency (RF) power to the etching gas mixture to form a plasma in the process region;
• and
• selectively etching exposed portions of the hardmask layer relative to the substrate at a ratio greater than 10:1 while in a presence of the plasma.