Si cryogenic etch Sub 60 nm features with PMMA mask - NanoLabStaff/nanolab GitHub Wiki

by Bjørn A. Strøm ([email protected])

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1. Apply etch mask

      PMMA A4, 208 nm 
   

2. Chamber clean

      O2: 40 sccm
      SF6: 40 sccm
      RF power: 100 W
      ICP power: 1500 W
      T= 20 °C
      Chamber pressure: 15 mTorr
   

If Carbon chemistry has been used it could be necessary to wipe down the chamber (contact Nanolab engineer)

3. Chamber condition

      O2: 16 sccm
      SF6: 34 sccm
      RF power: 6 W
      ICP power: 1000 W
      T= -120 °C
      Helium pressure: 10 torr
      Chamber pressure: 6 mTorr
      Time: At least 15 minutes
   

4. Insert sample

      Carrier type: Si, 2"
      Sample mounting: Fomblin oil
   

5. Strike

      Could be necessary to start off with lower ICP and immediately increase it once the plasma is ignited
   

6. Standard etch recipe:

      O2: 16 sccm
      SF6: 34 sccm
      RF power: 6 W
      ICP power: 1000 W
      T= -120 °C
      Helium pressure: 10 torr
      Chamber pressure: 6 mTorr
      Etch rate: 654 nm/min.
      Selectivity: 1.5:1
   

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See papers:

• Liu et al. "Super-selective cryogenic etching for sub-10 nm features"
• Wu et al. "Cryogenic etching of nano-scale silicon trenches with resist masks"

Using laser endpoint detection and manually ending the etch process based on the observed reflection value change proved sufficient for obtaining consistent etch depth (not super accurate). Multiplying the initial reflection value by 0.65 gives a residual PMMA layer and an Si etch depth of approximately 220 nm.
The ICP power is extremely important for the anisotropic profile, and needs to be high.

Discuss this protocol!