Problem 3: Photolithography using Shipley 1813
You want to create the photo pattern for an array of square etch windows (3um width, 6um
center-to-center spacing) onto a silicon wafer with a 1μm thick SiO2 barrier layer. You are given
the positive photoresist Shipley S1813 and wish to coat a 1.2μm thick layer of the PR material
onto the silicon.
(a) Draw a top view of the mask you would use, highlighting the opaque and transparent
regions, and explain the design based on the requirements of the photoresist. How would
your mask change if your photoresist was replaced with one that has a negative polarity?
(b) With the provided mask, outline the step-by-step photolithography process to get an
S1813 PR pattern with the desired thickness that exposes the SiO2 layer where necessary.
Each step should include a cross-section diagram like the figures below. The data sheet
for the available formulations of S1813 photoresist is given in the following link:
https://amolf.nl/wp-content/uploads/2016/09/datasheets_S1800.pdf.
Assume you have access to a spinner that allows you to input the following parameters:
TIME1-3 [seconds], ROTATION1-3 [rpm], and RAMP1-3 (acceleration) [seconds]
Your process should include up to three steps for each parameter. Also, assume you have access
to a contact aligner with a UV-lamp intensity of 5 mW/cm2 in the i-line region (365 nm
wavelength). The input parameter for the contact aligner is time. Recall that the exposure dose is
dependent on the thickness of the PR.
Make sure to include all steps, based on the data sheet information, throughout the entire
development process. Other sources may be used though must be cited.
(c) How would you change the photolithography process if you wanted your window width
to be 0.5 μm and 0.05 μm? Describe qualitatively any changes to your process, including
masks and equipment (no need to specify a new photoresist).
