Specify transmission at the wavelength and thickness you use
A material described as transparent in visible light may be unsuitable at a UV exposure wavelength. Transmission depends on glass grade and optical path length. Give the wavelength band and finished thickness when comparing substrate options, and distinguish uncoated-substrate transmission from the transmission of the coated part.
| Material family | Why it may be considered | What still needs confirmation |
|---|---|---|
| Synthetic fused silica | UV use and low thermal expansion where the grade is suitable | Grade, wavelength, thickness and required surface quality |
| Borosilicate glass | Instrument components where the thermal and optical properties fit | Actual grade data, coating process and mounting temperature |
| Soda-lime or specified optical glass | Visible-light patterns when the specification permits | Thermal drift, transmission and batch requirements |
Turn thermal expansion into a dimensional requirement
The first-order dimensional change is ΔL = α × L × ΔT. A longer scale experiences more absolute expansion than a short pattern of the same material. Use the coefficient for the actual grade and temperature range, then consider whether the mounting constrains that expansion or introduces stress.
Separate flatness, parallelism and thickness variation
Flatness describes the departure of a surface from a reference plane. Parallelism compares the two surfaces; thickness variation describes the change in their separation. A flat front surface does not prove that the back surface is parallel. State the clear aperture and whether the part is inspected free or in a defined support condition.
Thin or large substrates can change shape under clamping and coating stress. Where the assembled image plane matters, define the installed condition as well as the incoming-glass requirement. Tightening only the polishing value may not resolve a fixture-induced error.
Opacity and low reflectance are different film functions
Optical density in transmission is OD = −log10(T), where T is a fraction. OD 3 corresponds to T = 0.001, or 0.1%, at the stated wavelength and measurement conditions. That number does not specify reflectance, pinholes or edge quality. Those requirements must be stated separately where they affect reading or exposure.
For chrome and chromium-compound stacks, identify which side is illuminated and what the bright/dark regions must do. An anti-reflection coating also needs a wavelength band, incident angle and, where relevant, polarization. A coating name without those conditions is not an adequate optical acceptance requirement.
Specify the delivery state of a mask blank
Bare glass, chrome-coated glass and resist-coated chrome blanks serve different starting points. A resist-coated blank needs a compatible resist type and thickness, together with the requested process state and handling requirements. The same substrate and chromium film do not make two resist configurations interchangeable.
- State whether patterning will be performed by the customer or whether a finished pattern is required.
- Connect any baking, development or storage requirement to the selected resist system and agreed process.
Capture the selected configuration in the DWG
Put the material grade, substrate thickness, coated side and usable optical area on the DWG drawing. Reference a separate film specification if it contains wavelength-dependent curves or a multilayer stack. Keep its identifier and revision tied to the same part drawing so that a later coating change does not silently alter an approved configuration.