The Basics of Photomasking Process

Photomasks have a crucial role in the process of microlithography as it is used in the creation of integrated circuits or ICs, phototonic devices as well as micro-electro-mechanical systems also known as MEMS.  Photomasks are composed of a fused silica or it could be a glass substrate coated with an opaque film wherein a precise replication of the device designer’s pattern is etched.

How is it made?

Basically, writing the pattern of the designer’s image onto a resist coated chrome mask blank creates a photomask. The latent image is then developed as to form the needed pattern.  The function of the resist is it acts as a mask during the etching process.  The pattern is conveyed into the chrome film and then the resist layer is removed. Lastly and if the need requires a protective pellicle is attached and the manufacturing process is done.

Types of Photomasks

Copy Masks

This kind of photomask is used for hard contact printing so as to transfer the design to their substrates. However, the photomask is prone to deteriorate from mechanical damage. If the feature size and specifications allow, the solution is to use a copy photomask created from a “master” which is then retained if more copies are needed. The copy mask is usually made on soda-lime glass substrates.

1x Masks

If there is a need for close proximity printing or projection aligners to transfer the design to their substrates, there is little damage to the photomask. Similar to hard contact printing this type utilizes broadband or near-UV light in order to expose the wafer. While at the same scale factor (1x) as the final device, higher pattern fidelity and tighter specs can be achieved.

Reticles

When there is a need to use an optical projection stepper or scanner using a reduction ratio of 2.5:1, 4:1 or maybe 5:1, these are called reticles. They use single wavelengths from i-line (365nm) to deep-UV (248). Reticles are capable of supporting the strictest lithography requirements and in some advanced fabrications the imaging details are tinier than the wavelength of light.