Mask making

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Mask making
Mask making.jpg
Mask making[1]
Technology Details
Other Names Photomask making, mask fabrication, photomask fabrication
Technology Lithography
Equipment 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

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 chrome 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.[2]

Method of operation

Layouts are created in Computer Aided Design (CAD) editors such as L-Edit, AutoCAD, K-Layout, etc.

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.[3]

The energy delivered by the mask maker to the substrate surface is intended to react with the resist coating on the chrome 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.[3]

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 chrome. Etching can be accomplished by using liquid (wet) or plasma (dry) etch chemistry. After complete removal of unwanted chrome, the mask is stripped of all remaining resists,[3] typically in Nanostrip heated to 60 °C.

Applications

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
  • Optics
  • Photonics
  • Quantum devices
  • Thin film devices and solar cells

Parameters

Exposure time

The energy needed to expose the photoresist on the mask plate depends on the type and the thickness of the photoresist used.

Defocus

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.

Developer

The type of developer affects sensitivity, resolution, and development window.

Development time

Development time affects sensitivity, resolution, and exposure window.

Materials

  • Photoresist coated glass or quartz mask plate. Maximum size of plate is 5" X 5".
  • Layout in GDSII, DXF, CIF, or Gerber format.[dubious ]

Equipment

Heidelberg µPG 501 Mask Maker

The μPG 501 is a micro pattern generator for direct writing applications and low volume mask making.

CEE Developers

Main articles: CEE Developer 1 and CEE Developer 2

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

Main article: 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

Main article: YES-310TA

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.

See also

References

  1. Franssila, Sami. Introduction to microfabrication. John Wiley & Sons, 2010.
  2. Heidelberg Instruments
  3. 3.0 3.1 3.2 Rizvi, Syed, ed. Handbook of photomask manufacturing technology. CRC Press, 2005.

Further reading

  • Other stuff, e.g. technology workshop slides
  • External links (can be in another section below, if appropriate)