Fabrication of capacitive micromachined ultrasonic transducers by local oxidation

A method of fabricating a step on a substrate of a device, wherein said substrate comprises an oxidation-enable material, said method comprising:

• (a) determining a maximum allowed second oxidation thickness t2, wherein said determining is at least partially based on a desired height of said step and a size of said device;
• (b) calculating a first oxidation thickness t1, wherein said calculating is at least partially based on said determined maximum allowed t2;
• (c) thermally oxidizing said substrate to form an oxide layer on a surface of said substrate, wherein the thickness of said oxide layer is based on said calculated t1;
• (d) patterning said oxide layer to form an open region and a step region, wherein said patterning removes said oxide layer from said substrate at said open region;
• (e) thermally oxidizing said substrate and said patterned oxide layer based on said maximum allowed t2;
• and
• (f) removing approximately all of said oxide, whereby said substrate remaining has a step.

A method of fabricating a device having a vertical critical dimension, said method comprising:

• (a) oxidizing a substrate to form an oxide layer on a surface of said substrate, wherein said substrate comprises an oxidation-enable material;
• (b) depositing an oxidation-blocking layer on said oxide layer;
• (c) patterning said oxidation-blocking layer and said oxide layer, wherein said patterning forms a post region and a cavity region of said surface of said substrate, and wherein said patterning removes said oxidation-blocking layer and said oxide layer from said substrate at said post region;
• (d) thermally oxidizing said substrate, wherein said thermally oxidizing grows one or more oxide posts from said post region, and wherein said post defines said vertical critical dimension of said device;
• and
• (e) bonding a membrane layer onto said post, wherein said membrane layer forms a membrane of said device.