Dilution of SU 8 2000 series - NanoLabStaff/nanolab GitHub Wiki

by Kai Beckwith ([email protected])

As only a limited set of SU-8 resists exist in the cleanroom, to reach the desired thicknesses (e.g. for EBL) one must make custom dilutions. The available thinner is cyclopentanone, which is only compatible with SU-8 2000-series, of which SU-8 2100 is available in the cleanroom. SU-8 2100 (spins to roughly 100 µm thickness) contains 75% solids. The SU-8 2000 series has better adhesion to several substrates compared to the original SU-8 series. There is some limited information of the dilutions to reach certain resist thicknesses in the SU-8 2000-series datasheet.

More possible thickesses in the range of 0.1-3 µm have been investigated by making a series of dilutions and empirically measuring the resulting thicknesses. The results from dilutions in the range of 5-37% solids are summarized in the figures below. Layers thinner than about 0.1 µm were not always stable, they could easily fragment and tear, depending on the type and cleanliness of substrate.

Protocol:

  1. Prepare reagents:
  • Get out SU-8 2100 thinner from fridge, let it equilibrate to room temperature.
  • Move scale to fume hood in lithography finger.
  • Find a suitable container for your new resist.
  1. Calculate dilution:
  • Final weight = Initial weight * Initial concentration / Final concentration
  1. Perform dilution
  • Tare scale with container.
  • If starting from SU-8 2100, dispense into container by pouring carefully, or using some kind of stick/spoon. Note weight of SU-8 as initial weight.
  • Add SU-8 thinner until scale reaches final weight.
  • Mix until mixture is completely homogeneous. This is easier the more dilute solution you make, for the thicker solutions this is quite an effort.

Results

image

Detailed plots of the resulting thickness (in nm) of each dilution vs spin speed.

Values between 3000 and 6000 have been interpolated using a fit to t=a*O^-b, where t is the layer thickness, O is the speed in rpm and a and b are constants empirically determined by the values at 3000 and 6000 rpm.

SU-8 2000.1 (5% solids): image SU-8 2000.5 (14.5% solids): image SU-8 2000.7 (21% solids): image SU-8 2001 (25% solids): image SU-8 2002 (29% solids): image SU-8 2003.5 (37% solids): image