Reactive Ion Etching of PMMA:HSQ bilayer

Motivation

PMMA and HSQ bilayer is a useful combination. HSQ is a negative tone resist with almost negligible etch rate in an oxygen plasma. On the contrary, PMMA etches very quickly in an oxygen plasma. In addition, the process pressure can be adjusted to control the etch to achieve vertical sidewalls or undercut. Since this is a dry process, smaller structures with higher aspect ratios can be achieved. The pattern can also be easily reversed using electron beam deposition and lift-off.

Objective

Characterize the vertical and lateral etch rate of PMMA in an oxygen plasma.

Design of experiment

We know that HSQ does not significantly etch in an oxygen plasma from prior experience. Since HSQ is also a negative tone resist, this makes it a good candidate for pursuing our objective. Therefore, we designed an experiment to characterize the vertical and lateral etch rate of PMMA as a function of process pressure assuming that the etch rate of HSQ is zero. The experiment is described below:

  1. Use a Silicon substrate with oxide
  2. Spin on 107 nm of PMMA
    • PMMA 950 C2
    • Spin at 2500 rpm, 1250 rpm/s, for 60s
    • Bake at 180 °C for 2 minutes on a hotplate
  3. Spin on 73 nm of HSQ
    • Dow Corning XR-1541 4% (HSQ)
    • Spin at 2500 rpm, 1250 rpm/s, for 60s
    • Bake at 90 °C for 2 minutes on a hotplate
  4. Expose grating patterns using electron beam lithography
    • Grating patterns are 100, 200, and 400 nm wide with 50% duty cycle
    • Dose is 700 μm/cm2
  5. Develop in 25% TMAH for 60 seconds
  6. Measure the height of each grating with an atomic force microscope
  7. Etch patterns in a reactive ion etcher
    • Oxford DRIE 180
    • Parameter Units Value
      RF Power [W] 70
      O2 [sccm] 50
      DC Bias [V] 250
      Temperature [C] 20
      Helium [Torr] 10
      Pressure [mTorr] 3.3 to 40
  8. Measure the height of each grating with an atomic force microscope
  9. Measure the amount of undercut using a scanning electron microscope
    • Tilting sample in a scanning electron microscope by 45 degrees is sufficient to measure undercut width
    • Using a focused ion beam to image a cross-section of the sample is not significantly better

A key parameter that needs to be determined is the vertical etch rate of PMMA. The etch time is selected via trial and error. The etch time needs to be low enough such that some PMMA remains after the etch. After the vertical etch rate is determined for each process pressure, we can proceed to measure the lateral etch rate.

Results

Figure 1 shows a FIB cross section of the 400 nm grating sample after it has been etched and the edge of a 100 nm grating sample. By measuring the width of the grating and the width of the undercut and knowing the etch time, we can calculate the undercut rate. All of our measurements are recorded using the cross-section method, but it is questionable whether the extra effort produces more accurate results.

Figure 1: An image of the etched PMMA HSQ bilayer after an oxygen etch

Figure 1: The undercut can be measured by a FIB cross-section or by simply imaging the edge of the grating

Figure 2, 3 and 4 are graphs showing the vertical etch rate, lateral etch rate and etch anisotropy as a function of the process pressure. The vertical and lateral etch rate increases monotonically as pressure increases. The lateral etch rate does not appear to be influenced by the size of the gratings. The etch does not produce an angled sidewall, but instead produces a near vertical sidewall underneath the etch mask (HSQ) as shown in Figure 1. A combination of a lower pressure anisotropic etch followed by a higher pressure lateral etch can be used to control the amount of undercut for various PMMA film thickness.

Figure 2: The vertical etch rate increases monotonically with pressure.

Figure 3: The lateral etch rate increases monotonically with pressure.

Figure 4: The lateral etch rate increases monotonically with pressure.

At the beginning of the process development, we used expired HSQ. Figure 5 shows an image of artifacts/defects that results from using expired HSQ. The shelf life of HSQ is specified to be 6 months. From our experience, the first sign of expired HSQ is the observation of HSQ residue speckled all over the sample after the sample has been exposed and developed. The shapes of these residue are then transferred into the PMMA during the reactive ion etch process. Since the etch is not perfectly anisotropic and the residue is small, the PMMA underneath the residue etches away leaving the residue on the sample surface. All measurements were made using samples with new HSQ.

Figure 5: An image of defects caused by expired HSQ

Figure 5: Expired HSQ leaves residue after development.