SPR 955

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SPR 955 is an I-line series of photoresists developed by microchem, it's similar to SPR 220 but is formulated for thinner viscosities and higher resolution. We've been able to consistently get 500nm gratings in 0.97µm of SPR 955 in the stepper, on silicon. The 0.97µm thickness was chosen because it's one of the peaks listed in the datasheets swing curve. With thin photoresist like this, substrates with low reflection of 365nm light will have a much wider process window then a moderate to high reflection such as Silicon or aluminum.

SPR 955 (0.97µm)

Process

  1. Vapor prime wafer with HMDS
  2. Spin 0.97µm in ACS, CEE 200X #1 or CEE 100CB
  3. Bake at 100°C for 90 sec
  4. Verify thickness if possible, for thin resists small thickness variations drastically change the necessary exposure dose.
  5. Expose
Material Stepper Contact Aligner Notes
Silicon 0.30 sec 5.5 sec
Low Reflection 0.34 sec - 0.50 sec
Borosilicate 0.30 sec - 0.34 sec Severe sidewall degradation <1µm due to back reflection
  1. Bake at 110°C for 90 sec
  2. Develop

Notes: When photoresist is this thin the swing curve will have a larger effect; small thickness variations will have a larger change in the necessary exposure. Also at the end of the process you will have closer to .9-.94µm of resist. The softbake needs to be at 100C, not 115. Doing the softbake at 115 will burn out a lot of the PAC's, increasing dose to clear by ~40% (.24 sec vs .17 sec on the stepper).

Silicon

These gratings are on silicon and exposed via the stepper:

borderless borderless

Low Reflection Substrate

A Silicon wafer with a stack of 5nm SiO2, 34nm Si3N4, 5nm SiO2 was used because it should be mostly absorbing at 365nm, with around 10% reflection.

Gratings

With gratings smaller then ~500nm it's very difficult to tell if they came out just via a microscope, the failed 350nm gratings below can look like they resolved just from an optical microscope.


350nm gratings low reflection.jpg

400nm gratings low reflection.jpg

500nm gratings low reflection.jpg

1000nm gratings low reflection.jpg

Isolated Lines

The minimum size of an isolated line in a stepper is really limited by how well the substrate is in focus since it's really two large features (that can pass many orders through the lens) next to each other. Especially below 500nm you start to get pretty strong CD loss. And although 350nm & 400nm lines can typically be printed, small changes if how well the substrate is focused will have large changes in CD. Notice how some of these lines are around half the height they should be, this is probably just variation in how focused it was at that location due to wafer flatness, topology, etc.

350nm lines low reflection.jpg 400nm lines low reflection.jpg 500nm lines low reflection.jpg 1000nm lines low reflection.jpg

Isolated Trenches

Although you can get a nominal 350 / 400nm isolated trench to print, the physical size of it is going to end up closer to 500nm, as shown below.


350nm trenches low reflection.jpg 400nm trenches low reflection.jpg 500nm trenches low reflection.jpg 1000nm trenches low reflection.jpg

Process Curves

This plot shows the ideal exposure time for several develops in the ACS. The 0.3 sec exposure with a AZ 300 25 sec develop in the ACS seems to be a good point.

SPR 955 0.97µm CD Loss vs Exposure.png

The next plot shows the CD loss vs the amount of defocus in the stepper. ±900nm is about the limit before it's no longer going to work. Larger features will of course have a wider focus range.

Stepper 500nm Grating SPR 955 0.97µm CD Loss vs Defocus.png

Technical Data Sheet

SPR 955 Datasheet

Safety Data Sheet