Photomasks – Critical in Microlithography

When it comes to the microlithography process, photomasks play an integral role in the manufacture of micro-electro-mechanical systems or MEMS, integrated circuits or ICs and phototonic devices.  According toproximity photomasks experts, a typical photomask consists of a fused silica or glass substrate that is then coated with an opaque film where an accurate replication of the device designer’s design is then etched.

Quality

Aside from an excellent degree of pattern fidelity, photomasks need to adhere to a number of factors like linewidth control or critical dimension, pattern placement, defectivity and lastly customer expectations.

Masters Sub Masters and Copy Masks

When we talk about “masters, “sub-masters” and “copy mask” we usually refer to the 1x photomask. Reticles on the other hand refer to 2.5x, 4x and 5x stepper or scanner photomasks. A typical functioning device may need between five to forty distinct photomasks wherein one mask is used for each step in the process of fabrication. 

How are they Made?

According to PARCAM software specialists, a typical photomask is created by the exposure of the designer’s pattern onto ta resist coated chrome mask blank. The latent image in the resist is then developed in order to create the needed pattern.  The resist image then behaves like a mask during the process of etching. The pattern is then transferred into the chrome film when the resist layer is then removed.  The pattern undergoes an inspection process to check for any fault that could have an impact on its precision and functionality and if so warranted is then repaired as needed.  Finally, a protective pellicle is connected in order to complete the overall process.

Photomasks were first used in the creation of integrated circuits in the 60s and 70s. In those days no one had an inkling that forty plus years later it would still be routinely used in optical lithography for the purpose of printing structures that are smaller than the wavelength of light.