High Resolution Processes

The high resolution process can be used for etch transfer of patterns or, with minor modifications also for high resolution lift-off. The process is based on a stack of three or four different resists (see Figure 107). Therefore, it allows for very good structure control in the nanometer range, down to sub-20 nm half pitches. A version of the process is described in more detail in [Wolf2015_processes].

An example of structures patterned using a high-resolution lift-off is shown in Figure 108.

hr_process

Figure 107 Process flow for the high-resolution pattern transfer process.

hr_example

Figure 108 Two examples of structures fabricated via a high-resolution lift-off process: golden plasmonic bowtie antennas (left) and electrodes with sub-20 nm gaps (right).

High resolution etch transfer

  1. Prepare a PPA solution of 0.5 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 25 mg PPA and 4.975 g solvent for about 5 mL of PPA solution.
  • Shake the solution well to fully dissolve the PPA and allow to stabilize for a couple of hours.
  1. Prepare the solution for hard mask (HM)
  • SwissLitho has found HM8006 (Honeywell), PMGI (SFG-2S, Microchem) and PMMA 950k (AR-P 672.02, Allresist) to work as polymer hard mask layers.
    • or HM8006, dilute the original solution with ethyl lactate (EL) with the ratio of 1 part HM8006 : 2 parts EL.
    • or PMGI, dilute the original solution with G-Thinner TM™ (GT, cyclopentanone with some PMGE) with the ratio of 1 part PMGI : 1 part GT.
  1. Depositing the hard mask on the sample
  • Clean the silicon sample using either mild oxygen plasma or a hydrofluoric acid (HF) dip.
  • Cover the sample with the 20 nm hard mask solution and spin-coat it. Use the diluted solutions described above.
    • HM8006: at 3000 rpm for 35 seconds, bake at 225 °C for 90 seconds.
    • PMGI: at 3000 rpm for 35 seconds, bake at 200 °C for 60 seconds.
    • PMMA: at 4000 rpm for 35 seconds, bake at 180 °C for 90 seconds.
  1. Deposit a ceramic layer (e.g. silicon oxide)
  • Deposit a thin (2.5 nm) silicon dioxide layer on the HM layer. Note that this layer has to be very smooth and has to have a very uniform thickness. Therefore, special attention has to be paid to the deposition process. SwissLitho has achieved good results using an electron beam evaporator (Evatec BAK 501). Depositing the layer by sputtering is another option, as well as atomic layer deposition.
  • Optional: clean the sample using a gentle oxygen plasma etch in a barrel asher (e.g. Tepla BA, oxygen plasma for 10 seconds at 100 W).
  • Deposit 0.5 w-% PPA solution on the sample and spin at 2,000 rpm for 35 seconds. Bake at 90 °C for 3 min. This results in a PPA layer with a thickness of 12 nm.
  1. Patterning using the NanoFrazor
  • Optional: clean the sample using a gentle oxygen plasma etch in a barrel asher (e.g. Tepla BA, oxygen plasma for 10 seconds at 100 W).
  • Optional: deposit a 3 nm PMMA layer, which protects the tip and acts as a heat barrier. (1 part PMMA AP-672.02 : 9 parts anisole, at 5000 rpm, no bake)
  • Deposit 0.5 w-% PPA solution on the sample and spin at 2,000 rpm for 35 seconds. This results in a PPA layer with a thickness of 12 nm.
  • These thin layers of PMMA or PPA do not have to be baked. They are thin enough so that the solvent can evaporate.
  1. Patterning with the NanoFrazor, using standard settings:
  • Use a pixel size of 5-10 nm
  • Set the target depth to 10 nm
  • increase the force such that it patterns deep enough.
  1. Etching the mask stack
  • The following process parameters have been found to work for Oxford Instruments PlasmaLab RIE NG80 tool. For other etchers, parameters might have to be somewhat varied.
  • Etching the residual PPA layer: (4 sccm O2 and 16 sccm N2, power 10 W, pressure 15 mTorr, etch rate approximately 20 nm/min).
  • Etching the silicon dioxide (20 sccm CHF3, power 100 W, pressure 15 mTorr, etch rate approximately 14 nm/min).
  • Etching the polymer hardmask (20 sccm O2, power 20 W, pressure 15 mTorr, etch rate approximately 20 nm/min).
  1. Etch transfer to silicon
  • A deep RIE (DRIE) etcher (e.g. Alcatel AMS 200) is used to etch the structures into Si.
  • Etching the silicon (SF6 1.5:1 C4F8, power 1,200 W, pressure 11 mTorr). Etch rate (Si): approximately 550 nm/min, etch rate (HM) approximately 80 nm/min.
  • Use a thicker polymer hardmask for larger etching depths.
  1. Cleaning of sample
  • To clean the sample from any polymer residual one can apply a mild oxygen plasma etch step.

High resolution lift-off

For high resolution lift-off the following steps replace step 3, 8 and 9 of the high resolution etch.

  1. Replace polymer hardmask with PMMA
  • For pattern transfer using lift-off, most commonly the PMMA is used instead of the polymer hardmask.
  • Good results have been achieved with PMMA 950k, AR-P 672.02. Different dilution grades for different thicknesses are available.
  • For lift-off in general, the resist thickness should minimal be twice as thick as the desired metal layer.
  • A maximum of 100 nm of PMMA can be etched using the SiO2 etch mask.
  1. Material deposition (instead of silicon etch)
  • Deposit the desired layer(s), preferably using an evaporation tool. Both metals and insulators can be deposited using lift-off.
  1. Lift-off
  • Immerse the sample in acetone for 30 minutes. The PMMA should dissolve in acetone very easily.
  • For a faster lift-off, scotch tape can be used as follows: press a piece of scotch tape on the sample and rip it off. Only the material deposited on top of the polymer should stick on the tape, not the pattern.
  • If 30 minutes is not enough to finish the lift-off, try leaving the sample in acetone or NMP overnight. Remember to cover the sample or the acetone will evaporate! As a last resort, placing the container with the sample immersed in acetone and using an ultrasonic bath can improve the result, but also damage the sample.
[Wolf2015_processes]Heiko Wolf et al., Sub-20 nm silicon patterning and metal lift-off using thermal scanning probe lithography, Journal of Vacuum Science & Technology B, 33, 02B102 (2015).