A key feature of a photomask is the transmission (Tr) property of its many surfaces. Typical advanced 6" masks have 4
surfaces: back side Quartz (Qz), Front side pattern, inside pellicle and outside pellicle. In addition to the surfaces
themselves the bulk of the transparent materials- fused silica which is the material out of which the blank Qz is made and
fluoropolymer out of which the pellicle is made, have specific optical Tr properties which contribute to the total Tr
properties of the mask. Also surface coating materials like Cr, MoSi and Anti Reflective (AR) coatings have their
specific Tr contributions. Figure 1 (see paper) shows a schematic drawing with all the different contributors to Tr loss in a
photomask exposure system. Overall the wafer printed pattern fidelity to the design depends both on the physical size of the etched lines and spaces
and on the Tr properties of the spaces and of the coating material in the lines.
Factors which may contribute to transmission deviations may be:
1. Virgin Qz raw material non homogeneity.
2. Contamination by haze growth on any of the surfaces (Qz, absorber, pellicle).
3. Contamination by metal and oxide ions absorbed in the Qz and adsorbed on the Qz surface during mask
4. Photochemical degradation of the pellicle and fused silica substrates.
5. Degradation of absorber thickness, particularly of MoSi, due to clean processes.
6. Other factors.
Accumulated contributions of all those factors can give rise to several percents of transmission variation. Every percent
of exposure dose change may result in 1-2 nm CD change on wafer depending on exposure and process conditions. All the above raise the need for an advanced transmission measurement system that will be able to measure transmission
at the exposure wavelength with sensitivities better than 0.1%, preferably better than 0.01% (100 ppm). Such systems are
not currently available.