HSQ is an electron beam resist with a composition that is very similar to SiO2. After HSQ is patterned, it is typically removed by immersing in HF. While developing the etch process for PMMA:HSQ bilayer, these samples are suitable for investigating the selectivity of SiO2 to HSQ in a reactive ion etcher.

Objective

  1. Determine the etch rate of SiO2 and HSQ using a reactive ion etcher
  2. Determine how ICP power influences the etch rate of SiO2

Design of Experiment

  1. Prepare samples using PMMA-HSQ bilayer process
    1. Begin with a 3″ wafer with 500nm of thermal oxide
    2. Spin on 107 [nm] of PMMA
      1. Dispense PMMA950 C2 across entire wafer
      2. Spin at 2500 [rpm] at 1250 [rpm/s] for 60 [s]
      3. Bake on a hotplate at 180 [ºC] for 120 [s]
    3. Spin on 73 [nm] of HSQ
      1. Dispense Dow Corning XR-1541 4% (HSQ) across the entire wafer
      2. Spin at 2500 [rpm] at 1250 [rpm/s] for 60 [s]
      3. Bake on a hotplate at 90 [ºC] for 120 [s]
    4. Pattern HSQ using electron beam lithography
      1. Grating patterns are 100, 200, 400, and 1000 [nm] wide lines with 50% duty cycle
      2. Dose is 700 [μC/cm2]
      3. Develop by immersing in 25% TMAH for 60 [s]
    5. Etch through PMMA in an reactive ion etcher
      1. Oxford DRIE 180
      2. RF Power at 70 [W]
      3. O2 at 70 [sccm]
      4. Pressure range from 10 [mTorr]
      5. DC bias was 250 [V]
      6. Temperature is 20 [C]
      7. Helium backpressure is 10 [Torr]
      8. Overetch PMMA by 50%
  2. Measure the height of the 400 nm gratings using an Atomic Force Microscope
  3. Etch the sample in a reactive ion etcher
    1. Oxford DRIE 180
    2. RF Power at 70 [W]
    3. ICP Power at 200 to 1500 [W]
    4. Pressure at 6 [mTorr]
    5. C4F8 at 45 [sccm]
    6. O2 at 5 [sccm]
    7. DC Bias was 132 [V]
    8. Temperature is 20 [°C]
    9. Helium Pressure at 10 [Torr]
  4. Measure the height of the 400 nm gratings using an Atomic Force Microscope
  5. Strip the grating patterns by immersing in acetone for 5 [min]
  6. Measure the height of the 400 nm gratings with an Atomic Force Microscope

Results and Discussion

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 likely real. Also shown in Figure 1 is the etch selectivity of SiO2 relative to HSQ. At 350 [W] and above, SiO2 etches 40% slower than HSQ. Interestingly, at 200 [W], the etch rate of SiO2 and HSQ is the same.

Figure 1: The etch rate of SiO2 and HSQ increases similarly at 350 [W] and above maintaining a selective of 0.6.