The Different Kinds of Photomasks
Our customer’s processes and the lithography tools vary significantly. The scale factor (1x through 5x reduction ratios), mask to substrate printing proximity (full contact to close proximity aligners, projection steppers or scanners), and the wavelength of light used (‘broadband’ through to deep-UV), all influence the physical characteristics of the photomasks.
When our customers use hard contact printing to transfer the design to their substrates, the photomask can quickly deteriorate due to mechanical damage. When the feature size and specifications allow, the most cost effective solution can be to use a copy photomask made from a ‘master’ or ‘sub-master’ which we retain for making additional copies as required. Copy masks are usually made on soda-lime (SL) glass substrates.
When our customers use close proximity printing or projection aligners to transfer the design to their substrates, little or no attrition of the photomask occurs. Like hard contact printing, these systems typically use broadband or near-UV light (300-450nm) to expose the wafer or substrates. While still at the same scale factor (1x) as the final device, higher pattern fidelity and tighter specifications can be met using photomasks made directly by our mask-writers. 1x masks can be made on either soda-lime (SL) glass or fused silica (QZ) substrates.
When our customers use an optical projection stepper or scanner with a reduction ratio of 2.5:1, 4:1 or 5:1, the photomasks used in these systems are usually called ‘reticles’. These systems use single wavelengths from i-line (365nm) to deep-UV (248nm or 193nm). Reduction stepper or scanner reticles, typically support the most stringent lithography requirements. In some of the advanced fabs, they are imaging features smaller than the wavelength of light. Reticles are typically made on fused silica (QZ) substrates.