Overview

Custom grating etching writes a periodic line structure into metal foil by photochemical machining, so slit width, pitch and duty cycle are defined by the artwork instead of by a ruling engine or a saw. Photoresist is exposed through a high-resolution film, developed to open the slit areas, and the foil is etched to the depth the optical design requires, leaving freestanding bars between open slits. Typical substrates are molybdenum, nickel, stainless steel 304/316L and copper alloy between 0.02 mm and 0.3 mm, with pitch from 0.02 mm upward and bar widths down to 0.01 mm.

Gratings behave differently from ordinary sheet parts because a fraction of a micrometre in pitch directly shifts the diffraction angle. Etching is attractive here for two reasons: the whole pattern is imaged in one exposure, so pitch error is a function of film and registration rather than of a tool stepping across the part, and the process removes material without mechanical force, so thin foil stays flat and no burr stands up along a slit. Molybdenum and nickel are common where the grating sees heat or a corrosive beam environment; stainless is used for alignment marks, encoder scales and mechanical apertures in the same assembly.

Parameter Window

• Available Materials: molybdenum, nickel, stainless steel 304/316L, copper, brass, nickel-iron alloy
• Material Thickness Range: 0.02–0.3 mm; 0.05 mm and below handled on rigid carriers
• Tolerance: ±0.05 mm on outside profile, ±0.002 mm on cumulative pitch over 20 mm
• Minimum Feature: 0.01 mm slit with 0.01 mm bar, aspect ratio held near 1:1
• Surface Options: bright anneal, electrolytic polish, black oxide, anti-reflection coating, protective pellicle frame
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Optical Data Review — Take pitch, duty cycle, slit depth and aperture from the optical model and check that each value sits inside the etchable aspect ratio.
2. Substrate Selection — Match alloy to wavelength, beam power and atmosphere; molybdenum for heat, nickel for fine grain, stainless for scale and mark gratings.
3. Stress Relief — Anneal and flatten the foil before coating, because residual roll stress shows up later as pitch drift across the panel.
4. Resist Coating — Laminate or spin a resist film of uniform thickness; thickness variation translates directly into slit width variation after development.
5. Contact Exposure — Expose through a high-resolution film in hard contact, with the pattern aligned to datums so pitch error stays cumulative-free.
6. Development — Develop and inspect slit openings under magnification, measuring a sample of slits before the part reaches the etchant.
7. Controlled Etching — Etch in a chemistry matched to the alloy, stopping on depth while monitoring slit widening so the duty cycle holds.
8. Strip, Measure & Pack — Remove resist, measure pitch over a fixed span, record diffraction check data, and pack between optical-grade interleaves.

Core Advantages

1. Pitch set by artwork: Slit spacing comes from one exposure, not from a tool indexing across the part, so cumulative error over the panel stays small.
2. No mechanical load: Chemical removal applies no force, so 0.02 mm foil keeps its flatness and no bar is bent or rolled over.
3. Clean slit walls: Etched walls are smooth and taper-controlled, reducing scatter that would otherwise appear as stray light in the order pattern.
4. Duty cycle control: Slit width and bar width are adjusted independently on film, letting the transmission ratio be tuned without changing stock.
5. Refractory alloys: Molybdenum and nickel etch cleanly, opening grating options that sawing or stamping cannot handle at this thinness.
6. Prototype speed: A revised pitch is a revised film. Custom grating etching therefore supports optical designs that are still being modelled.

Typical Applications

• Encoder scales: Fine line tracks on stainless strip for linear and rotary encoders with index and reference marks on the same sheet.
• Spectrometer slits: Entrance and exit slits with defined width in molybdenum for instruments exposed to heat.
• Alignment reticles: Cross, box and vernier patterns etched into thin foil for optical assembly and beam alignment.
• Diffraction samples: Test gratings at several pitches for laboratory evaluation before a design is frozen.
• Beam apertures: Multi-slit aperture plates that clip a beam and define its profile in one plane.
• Sensor masks: Periodic masks over photodiode arrays, where stray light between pixels must be blocked.

Design Notes & Process Considerations

1. Aspect ratio limit: Keep slit depth at roughly 1:1 with slit width; deeper etches widen the slit and shift the duty cycle away from the optical model.
2. Pitch reference length: Specify the span over which pitch is measured, since a tolerance without a span cannot be inspected reproducibly.
3. Datum strategy: Put two datums on the part outline and reference the pattern to them, so downstream mounting does not reintroduce rotation error.
4. Bar integrity: Minimum bar width equals minimum slit width; thinner bars deflect under airflow and can touch a neighbouring bar.
5. Handling and cleaning: Gratings are cleaned by solvent rinse only; wiping along the bars packs debris into the slits and is difficult to reverse.
6. Tolerance stack: Divide the allowed angular error between pitch, mounting and detector before release. Custom grating etching can hold pitch, but it cannot absorb error created downstream.