Tutorial - How Photomasks are Made

If you don't know how photomasks are made, it is easy to make a mistake in your order or tell the photomask vendor something that confuses them but makes sense to you. Glass photomasks are made using the same fabrication processes as your wafer lithography, except with direct-write lithography.

There are essentially 2 sections to the mask plate fab: (1) The data preparation and (2) the mask production.

Mask Production

Fabublox Schematic of mask production

1. Vendors buy a glass plate, that is purchased pre-coated with Chrome and Photoresist.
   1. The glass plate, called a "mask blank", is the "substrate" that they will process on.
   2. The size and thickness and type of glass specifies which "mask blank" they choose to start with.
   3. Soda Lime is the cheapest glass, and is recommended for more lithos. Quartz is more expensive, but is more transparent to UV, so is required for DUV exposures.
   4. Photomasks mostly use positive photoresist.

      

      The "Mask blank" that photomask vendors start with.

2. The photomask is loaded into the mask writer with Chrome + Photoresist Side Up in order for the mask writer to write on the photoresist directly.
   1. Mask writers are tools very similar to the Heidelberg MLA150 or EBL tools.
   2. Most photomasks are used in the fab with Chrome Down. Don't confuse these two!
   3. To avoid confusion, photomask vendors will use the terms
      1. "Right Reading Chrome Down" (RRCD), which means TEXT you inserted into tour CAD should be legible/readable ("right reading") when the chrome is Down, for example on a table.
         1. This is how most NanoFab users would define their CAD - their CAD is of their actual desired wafer, which is when the Mask Plate's Chrome is Down, so any text should be readable with Chrome Down.
      2. "Wrong reading chrome down" means that TEXT in your CAD file would be mirrored when the chrome is down. Some users might need that!
   4. Since the mask is written Chrome Side Up but most NanoFab users want their patterns to be correct with Chrome Side Down (the mask is flipped over in the mask writer compared to how it'll be used in the fab), the vendors usually have to Enable the "Mirror" option on their mask writer's software.

      

      Direct-write onto mask blank, with Photoresist and Chrome UP.

   5. The mask writing software usually shows you Black or filled shapes wherever the laser will be On, meaning the positive photoresist gets removed and Cr exposed in those regions.
      1. Some Dataprep teams are kind enough to (a) mirror and (b) invert the software screenshots, so you can see what a Fab user cares about - chrome down + dark=Chrome (opaque), but neither of these are what the photomask vendor cares about!
   6. This is a very long write, because the laser writehead raster-scans (zig zags) like a regular ink printer, except with 0.10µm grid size (a home printer is ~50µm spot size - 500x larger) - so it takes hours for a mask plate to get written! Since time is money (eg. companies are paying staff by the hour), the longer the write, the more expensive the mask plate.
3. Develop - just like wafer lithography. The Chrome is now exposed wherever the photoresist is removed.
4. Etch the Chrome & Strip the Resist. Wet chemical etch is common, but many vendors also have dry etch for tight specs. Std. solvent or O2 plasma for PR strip.
5. Metrology - measure/confirm whether the requested resolution and defect levels were met.
   1. The Data Prep team will have either added their own in-line measurement patterns to your mask, on the edge of the mask plate (preferred), or you will have told them which exact feature they should measure to confirm that they met the Critical Dimension (CD) you requested.
   2. Automated measurement of defects, or general microscope inspection, to ensure they are below the quoted spec.
6. If CD was not met, or defects too high, then they will have to re-run the mask fab - which is expensive for the vendor!
   1. So if you request an unusually tight spec, they will charge you more to cover the cost of any potential remakes they sometimes might need to do.

Data Preparation

Users often send their CAD files in varying states of disarray. The data needs to be wrangled so the production team doesn't get confused or perform a bad write - which is an expensive mistake!

• The patterns might be at some random coordinate, or centered around (0,0), or with the (0,0) in the bottom-left corner of their design. That coordinate may or may not have anything to do with the mask plate size!
  • We recommend you always put (0,0) in the center of your desired mask plate!
• The patterns may be on some random GDS layer, or there may be many layers. Which one do they write?
  • We recommend you always export a version of the CAD file with ONLY the layers to be written.. All constructions and non-write layers removed.
• The user-provided patterns are usually only the wafer pattern, but might need other patterns added, such as:
  • Outer templates for specific stepper machines, including alignment marks for the stepper's mask alignment
  • Barcodes for automatic readers (also in found Steppers),
  • Human-readable "Titles" printed onto the plate (outside the wafer exposure areas)
  • Metrology test-patterns - for automated measurement of Critical Dimension, for example - also placed outside the wafer exposure area on the edge of the plate.

    

    Example of patterns added by photomask vendors - in this case a fictitious stepper mask (these regions are *not* correct, just an example).

• For steppers, user-provided patterns may need to be magnified (scaled-up) from 1x wafer-scale to the 5x or 4x reticle-scale, and inserted into the stepper template.
• You can specify multiple layers for multiple individual plates (for example, a full "set" of contact mask plates, one for each process step/Layer) - where each layer will be a unique photomask plate. The data prep team needs to export each layer into their own photomask writer job.

Afterword

You can see that the needs of the photomask vendor's production team are different, and often conflict, with the needs of a NanoFab user! Keeping this in mind, you can often avoid a lot of confusion, or even expensive photomask mistakes, by making the photomask vendor's job as easy and clear-cut as possible.

They don't want options ("do this or that, whichever you prefer"), they'd much rather get exact instructions. So make up your mind, or discuss the options with NanoFab Processing staff before you submit you mask, and don't change anything mid-flow, especially not via email!

And Never "approve" the plots they send you for approval if you see something you don't understand! Once you "approve" it, they'll start the expensive write; so make sure it's 100% correct before you say "looks good"!.

Demis D. John, 2025-08-04