Mask making is a fabrication process where a computer-aided design (CAD) is transferred to a thin (80-100 nm) layer of metal in a glass or fused silica substrate, known as mask or photomask. The metal works as an absorption layer for light at different wavelengths. The standard photomasks use Chromium as absorption material for i-, g- and h-line UV light. The transparent image on the photomask is a master template, which is transferred by a contact mask aligner or projection lithography system into a photosensitive polymer known as photoresist by lithography. Typical applications require a set of photomasks to produce the complete device.
|Other Names||Photomask making, mask fabrication, photomask fabrication|
Heidelberg µPG 501 Mask Maker|
CEE Developer 1
CEE Developer 2
Mask Bench 13
YES Image Reversal Oven
|Materials||Positive and negative tone photoresists on glass and quartz mask blanks, and full wafers|
Method of operation
Masks are generally written on either of two classes of write tools. Electron beam (e-beam) writers precisely direct a focused beam of electrons onto the mask substrate while controlling the position of those electrons through the use of an interferometer controlled stage. Laser/LED writers essentially perform in the same fashion but use photon energy as opposed to charged electrons.
The energy delivered by the mask maker to the substrate surface is intended to react with the resist coating on the Chromium film. The resist is engineered to be sensitive to either e-beam or photon exposure. The locally cross linked molecules of the resist become either sensitive or insensitive to developers used in post-write processing. When the develop step removes exposed resist, the process is referred to as positive working, and when leaving behind exposed resist, negative working.
After development the mask moves on to the etch step. In this part of the process, the surface of the mask that has been left uncovered by resist becomes exposed to the etching medium. The resists are engineered to withstand the etching process and at the very least stand up to the etch chemistry with a removal rate that is slower than the removal rate of the underlying Chromium. Etching can be accomplished by using liquid (wet) or plasma (dry) etch chemistry. After complete removal of unwanted Chromium, the mask is stripped of all remaining resists, typically in Nanostrip heated to 60 °C.
The fabrication of functional micro- and nano-scale devices involve the pattern transfer on to the substrate. While direct writing on to the substrate is suitable for the fabrication of few devices, large scale fabrication require the utilization of a mask or several masks. Photomasks are used for the fabrication of devices including but not limited to:
- Integrated circuits and systems
- MEMS and Bio-MEMS
- Transport phenomena in materials and engineering systems
- Micro- and nano- technology to tissue repair and regeneration
- Quantum devices
- Thin film devices and solar cells
The energy needed to expose the photoresist on the mask plate depends on the type and the thickness of the photoresist used.
The best focus position for an exposure can depend on e.g., the resist thickness, or reflectivity. This parameter can be adjusted using defocus value.
The type of developer affects sensitivity, resolution, and development window.
Development time affects sensitivity, resolution, and exposure window.
Heidelberg µPG 501 Mask Maker
The μPG 501 is a micro pattern generator for direct writing applications and low volume mask making.
The CEE Developers are used to develop photoresist after it has been exposed by forming a puddle of developer on the wafer. They can accept pieces, 4" and 6" wafers as well as 4", 5" and 7" mask plates. Due to their automation, they are well suited for highly repeatable puddle developing of photoresist.
Mask Bench 13
Mask Bench 13 is a wet bench that is set up for wet etching and cleaning masks. There is a tank of chromium etch and a tank of Nanostrip for photoresist removal.
YES Image Reversal Oven
The YES-310TA (E) is an image reversal and vapor prime oven. It can vapor prime wafers with HMDS in order to improve photoresist adhesion. It can also perform image reversal on wafers and photomasks using NH3.
This online course covers the fundamentals of using the open-source K-layout software for designing photomasks.
- Franssila, Sami. Introduction to microfabrication. John Wiley & Sons, 2010.
- Heidelberg Instruments
- Rizvi, Syed, ed. Handbook of photomask manufacturing technology. CRC Press, 2005.