3D etch into silicon¶
The following process is an example on transferring 3D structures patterned by the NanoFrazor into the substrate. In 3D transfer, careful optimization of the dry etch processes is of paramount importance for achieving the targeted result. Special attention has to be paid to accurate calibration of the etching rates and to suitable layer thicknesses.
This recipe only serves as a rough guideline and the etch parameters have to be determined separately for every tool used. Ask your facility responsible for more details.
- Prepare PPA solution of 5 w-%
- Solvent: Anisole (CH3OC6H5, 0.995 g/mL, bp: 154 °C) or Cyclohexanone (C6H10O, 0.9478 g/mL, bp: 155.6 °C).
- For example: use 250 mg of PPA and 4.75 g of solvent for approximately 5 mL of PPA solution.
- Shake the solution well to fully dissolve the PPA and allow to stabilize for a couple of hours.
- Sample preparation
- Clean the silicon sample using either mild oxygen plasma or a hydrofluoric acid (HF) dip.
- Deposit 3 nm of PMMA (eg. PMMA 950k, AR-P 672.02, diluted 1:9 with anisole). This PMMA serves as a thermal barrier and protects the tip.
- Deposit 60-100 nm of PPA by spin coating. Bake the sample for 2 minutes at 110 °C.
- Use the sample within one week. By storing the sample in dry nitrogen atmosphere, you could use it also longer.
- Pattern the PPA using the NanoFrazor with the following standard settings:
- Pixel size: 20 nm.
- Activate the depth feedback.
- Optimal maximum patterning depth is 50 nm. But all the depths from 20 to 80 nm are fine (achievable depth resolution is in the order of 1 nm).
- A residual layer of 10 nm or less should be targeted. (this means that the PPA thickness should be about 10 nm thicker than the patterning depth).
- Transfer pattern into silicon or metal
- Use an anisotropic dry etch like DRIE to simultaneously etch the substrate and the PPA (at a lower rate than the substrate). The etching selectivity between PPA and the substrate determines the achievable depth.
- E.g.: With an etch rate selectivity of 3:1 (Si:PPA) and a PPA thickness of 50 nm, it is possible to achieve a thickness of 150 nm in silicon.
- The residual PMMA and PPA layer have to be considered. Often they do not disturb, since these residuals are etched in the very beginning, when the etch conditions might not yet be stable.
- For larger depths in silicon, two options are available:
- Increase the etch selectivity between silicon and PPA by tuning the etching parameters.
- Use an intermediate layer between the PPA and the substrate (e.g. silicon dioxide). This way, the pattern has first to be transferred from PPA into the silicon dioxide followed by a second etch where the pattern in the silicon dioxide is transferred into the silicon. This approach allows for etching depths of more than 4 microns.