520.495/580.495 Microfabrication Laboratory

September 13, 1998

Laboratory Assignment #2

Objectives: This week, we will become familiar with the CAD tools that are available to design the masks used in the project. If you ever do a project like this from scratch, you will have to design your own masks! The masks used in the class were designed by Dr. Norman Sheppard.

The CAD program LEDIT used for mask layout and editing can be found in the Computer Laboratory in Barton Hall. It runs on Windows machines.

The diffusion mask patterns are green, the metalization mask patterns are blue, the contact cuts are black and the membrane etch mask is coded by purple grid pattern. The design is done hierarchically and you can navigate through the individual cells. If you make changes in a leaf cell, these changes will manifest themselves to all instances of the leaf cell. Note that since our process has limited steps the number of masks are also limited. A typical CMOS process has a minimum of nine mask layers and more advance processes will have eleven or twelve.

The design is done on a grid and the size of the basic grid spacing is one lambda. This is a parameter that depends on the process. For example, state of the art CMOS processes today have a lambda of 0.15 micron.

Question 2: Calculate the dimensions of all structures on the mask. Get a hardcopy of the design by printing it and indicate on it these dimensions. What do you think lambda is for our process.

Question 3: Define another layer in the masks menu and give it a CIF name NA. This will be a mask layer that will define n-diffusion areas on the n-doped wafer.

Question 4: Modify the design of the project to include an n-diffusion ring around the sensor membrane. This will provide for a nice ground plane and also electrical access to the wafer. Don’t forget to include a nice big bonding pad and contact cuts to access the diffusion area.