SiO2 Mold Fabrication

Motivation

Nanoimprint molds provide low cost access to nanopatterning technology. With a mold in hand, a new user can be trained to perform thermal imprinting within an hour and will be able to replicate the mold pattern on their sample within 30 minutes. However, the cost of purchasing a custom mold from companies is obstructive for academic researchers. Thus, UHNF has developed a core technology to develop custom nanoimprint molds at significantly lower cost. UHNF members may use this process to fabricate their own molds or contract staff members to do it for them.

Objective

Describe the process for fabricating a nanoimprint mold in a single attempt.

Process

The fabrication of a nanoimprint mold requires proficiency with the following equipment: electron beam writer, electron beam evaporator, deep reactive ion etcher, scanning electron microscope and atomic force microscope.

  1. Begin with a 3" Si wafer with 500nm of thermal oxide
  2. Coat with 85nm of PMMA
    • PMMA 950 A3
    • Spin at 3000 rpm at 1250 rpm/s for 60s
    • Bake at 180 °C for 2 minutes
  3. Coat with 20nm of DisCharge
    • DisCharge from DisChem Inc
    • Spin at 4000 rpm at 1250 rpm/s for 60s
    • The DisCharge coating is used to prevent charging artifacts during electron beam lithography
  4. Perform electron beam lithography
    • Perform the following experiments as needed
      • Perform a dose series to determine the best dose
      • Perform a beam current series to determine the fastest writing speed
      • Perform a focus series to assess pattern uniforming across a wafer
  5. Strip the DisCharge coating
    • Rinse the wafer with DI water for 30 seconds
  6. Develop the pattern
    • Immerse wafer in IPA:Water (2:1) for 1 minute
  7. Inspect the pattern with an optical microscope, SEM and AFM as needed
  8. Ebeam evaporate 25 nm of Cr
  9. Perform lift-off
    1. Half-fill a glass container with acetone
    2. Immerse the wafer in the acetone
    3. Cap the container with aluminum foil
    4. Ultrasonicate for 30 minutes
    5. Prepare a second glass container and fill it half-way with acetone
    6. Immerse the wafer in the second glass container
      • Continuously rinse the wafer with acetone squirt bottle during transferring to avoid drying
      • The Cr in the solution will stick permanently to the wafer if the acetone is allowed to dry
    7. Cap the container with aluminum foil
    8. Ultrasonicate for 30 minutes
    9. Rinse thoroughly with acetone followed by isopropyl alcohol
    10. Dry with nitrogen
  10. Inspect the pattern with an optical microscope, SEM and AFM as needed
  11. Etch the mold halfway
    1. Calculate the etch time for half the desired etch depth
      • The etch rate is approximately 4.7 nm/s
    2. Modify the etch time for the recipe "LVC - SiO2:Cr Mold"
    3. Run "LVC - Chamber Clean" on a dummy wafer
    4. Run "LVC - SiO2:Cr Mold" on a dummy wafer
    5. Use double sided tape to adhere the wafer to the dummy wafer
    6. Run "LVC - SiO2:Cr Mold" to etch the wafer
  12. Use AFM to measure the pattern height
    • The total pattern height includes the height of Cr and SiO2
    • The total pattern height minus 7nm is the height of SiO2
    • Use the height of SiO2 to determine the actual etch rate of SiO2
  13. Etch the mold to the desired height
    1. Calculate the etch time to reach the final pattern height
    2. Modify the etch time for the recipe "LVC - SiO2:Cr Mold"
    3. Run "LVC - Chamber Clean" on a dummy wafer
    4. Run "LVC - SiO2:Cr Mold" on a dummy wafer
    5. Use double sided tape to adhere the wafer to the dummy wafer
    6. Run "LVC - SiO2:Cr Mold" to etch the wafer
  14. Strip the Cr
    • Immerse the wafer in CEP-200 for 5 minutes
    • Rinse with DI water and dry with N2
  15. Perform inspection as needed