Reactive Ion Etching of SiO2:HSQ

Motivation

While developing the etch process for PMMA:HSQ bilayer, we decided to use those samples to also characterize the etch rate of SiO2 and HSQ. This experiment is designed to confirm the etch rate of SiO2 that was developed earlier and learn how the ICP power affects etch rate.

Objective

Characterize the etch rate of SiO2 and HSQ for an oxide etch process. Determine how ICP power influences etch rate of SiO2.

Design of experiment

The experiment is described below:

  1. Use the PMMA-HSQ bilayer process to prepare a sample
    1. Start with oxide wafer
    2. Spin on PMMA and bake at 180 °C for 2 minutes
    3. Spin on HSQ and bake at 90 °C for 1 minute
    4. Expose gratings at 700 uC/cm2 and develop in TMAH 25% for 1 minute
    5. Etch away all the PMMA using the anisotropic PMMA etch
  2. Measure the height of the 400 nm grating using an atomic force microscope
  3. Etch patterns in a reactive ion etcher
    • Oxford DRIE 180
    • Parameter Units Value
      RF Power [W] 70
      ICP Power [W] 200 to 1500
      Pressure [mTorr] 6
      C4F8 [sccm] 45
      O2 [sccm] 5
      DC Bias [V] 132
      Temperature [C] 20
      Helium [Torr] 10
  4. Measure the height the 400 nm grating with an atomic force microscope
  5. Strip the grating patterns by immersing the sample in acetone for 5 minutes
  6. Measure the height the 400 nm grating with an atomic force microscope

Results

Figure 1 is a graph of the etch rates for SiO2 and HSQ at various ICP power. After realizing that the etch rates for both SiO2 and HSQ is the same at an ICP power of 200 W, we decided to run the experiment again to confirm our results. From the second set, we confirmed that the data results at an ICP power of 200 watts is repeatable and therefore real. It will be interesting to perform another experiment in the future to determine what the etch rates at ICP power between 0 and 350 watts.

Figure 1: The etch rate increases monotonically with ICP power and the two sets of measurements are in agreement.

Figure 2 is a graph of the etch selectivity of SiO2 and HSQ. At 350W and above the selectivity of SiO2 to HSQ is approximately 0.6 to 1. An anomaly occurs at an ICP power of 200 W, where the selectivity is almost 1 to 1.

Figure 2: The etch selectivity of SiO2 and HSQ is approximately 0.6 for all ICP power values except 200 W.

Figure 3 is a graph of the DC bias that is observed at each etch process. Normally, the DC bias is deliberately decreased to increase selectivity. In this case, we have the highest selectivity at the highest DC bias. What is going on?

Figure 3: The etch rate increases monotonically with ICP power and the two sets of measurements are in agreement.