Fabricating a rigid mask from a flexible mask

Flexible masks are much cheaper than rigid masks and they can be acquired the day after submitting the layout to the manufacturer. The limitation of a flexible mask is the 10 um resolution and it is difficult to clean when contaminated. This article describes a process to fabricate a rigid mask from a flexible mask. It is arguably cheaper than purchasing a rigid mask.

  1. Acquire a flexible mask from a vendor
    1. Design mask using a layout editor such as KLayout
    2. Purchase mask flexible mask from a manufacturer such as outputcity.com
  2. Inspect the flexible mask under a microscope
  3. Figure 1: An image of a 5"x7" flexible mask

    Figure 1: A 5"x7" flexible mask.

  4. Acquire a photomask blank
    1. Vendor: nanofilm.com
    2. Material: soda-lime (cheap) or quartz (UV transparent)
    3. Dimensions: 2.5"x2.5"x0.060" or 5"x5"x0.090"
    4. Metallization: Low Reflective Chrome (LRC), 12% reflectivity, 100 [nm] thick
    5. Photoresist: AZ1518, 530 [nm] thick
  5. Prepare the mask for exposure, figure 2
    1. Place photomask blank on a mask aligner
    2. Place flexible mask on the photomask blank, emulsion side down
    3. Place a glass sheet on top of flexible mask
    Figure 2: An image of a sandwich of photomask blank/flexible mask/glass sheet

    Figure 2: A sandwich of photomask blank/flexible mask/glass sheet

  6. Expose the photoresist (AZ1518) at 70 mJ/cm2
  7. Bake the exposed resist on a hotplate at 105 °C for 50 seconds
  8. Develop the resist by immersing with in AZ MIF 300 for 60 seconds
  9. Etch the Chrome by immersing with agitation in CEP-200 for 180 seconds, figure 3
  10. Figure 3: An image of a mask after chromium etch

    Figure 3: Etch chromium in CEP200

  11. Strip the residual resist by immersing in acetone
    1. The mask can be further cleaned by immersing in piranha
  12. Inspect the mask for defects
  13. Figure 4: An image of the finished soda-lime (rigid) mask

    Figure 4: The completed soda-lime (rigid) mask