Overview

Grating chemical etching produces the periodic line structure of a diffraction grating by dissolving metal through a photoresist mask rather than ruling or scribing it. The foil is coated on both faces, exposed through a film carrying the line pattern, developed, and etched until the alternate lines and spaces reach the required depth and duty cycle. Period, line width and wall angle are set by the artwork and the dwell time, so the grating is defined once and then reproduced, instead of being cut line by line with a mechanical stylus.

Optical performance depends on how evenly that period repeats. A grating with a nominal 40 µm pitch is only useful if the hundred-thousandth period is still within tolerance, because phase errors show up directly as stray light and reduced diffraction efficiency. Molybdenum, nickel 200, 304 stainless and copper are the common substrates, from 0.05 mm to 0.5 mm, with periods typically between 20 µm and 500 µm.

Parameter Window

• Available Materials: molybdenum, nickel 200, 304 stainless steel, copper C110
• Material Thickness Range: 0.05–0.5 mm
• Tolerance: line width ±1.5 µm; pitch ±0.5 µm per period, ±2 µm cumulative over 50 mm
• Minimum Feature: 8 µm line and 8 µm space at 0.05 mm stock
• Surface Options: optical polish, blackening for stray-light control, gold coating, passivation
• Compliance: ISO 9001 process control, RoHS/REACH material traceability

Manufacturing Process / Workflow (8 Steps)

1. Optical Requirement Review — Fix period, duty cycle, groove depth and target efficiency, and check that the requested line width is achievable at the chosen foil thickness.
2. Substrate Preparation — Select molybdenum, nickel or stainless with the flatness and surface finish the application needs, then polish and clean before coating.
3. Photoresist Coating — Apply resist at a controlled thickness on both faces; an uneven coat shifts line width across the panel and shows up as efficiency variation.
4. Exposure — Expose through a dimensionally stable film held in hard contact, indexing the panel so multi-field patterns stitch to a common origin.
5. Development — Wash out the unexposed resist and inspect line continuity under a microscope; broken lines cannot be recovered later in the bath.
6. Etching — Etch with chemistry and spray parameters tuned for a near-vertical wall, monitoring depth so the groove profile matches the designed duty cycle.
7. Stripping and Optical Finish — Remove resist, apply blackening or a reflective coating where specified, and handle the part only on its margins to protect the active field.
8. Metrology and Packing — Measure line width and pitch by microscope or interferometry, record efficiency on a sample, and pack in a rigid carrier with a protective cover.

Core Advantages

1. Period set by artwork, not by a tool: The pitch comes from the film, so a change from 40 µm to 25 µm is a new drawing rather than a new ruling engine setup.
2. Duty cycle held in the etch: Line width is controlled by lateral etch rate, letting 50/50 and 60/40 profiles be produced on the same substrate without changing the exposure.
3. No mechanical stress in a brittle material: Molybdenum and nickel gratings would chip under a ruling stylus; chemical removal leaves the surface free of micro-cracks.
4. Clean edges mean less scatter: An etched wall has no burr or roll-over, so stray light from edge roughness stays low and efficiency holds near the design value.
5. Whole field at once: Every line on a 100 mm panel etches simultaneously, so the period does not drift from one end of the part to the other as it would in sequential ruling.
6. Variable and chirped patterns: Non-uniform pitch, apodised duty cycle and curved lines are all just artwork, which is impractical for mechanical ruling.

Typical Applications

• Spectrometer gratings: Flat-field gratings in nickel and molybdenum for compact optical analysers.
• Optical encoder scales: Fine-pitch stainless scales for linear and rotary encoders used in motion stages.
• Diffractive optical elements: Beam splitters and fan-out elements with stepped or varied duty cycle.
• Laser beam sampling: Low-efficiency gratings that pick off a fixed fraction of a beam for monitoring.
• Displacement metrology: Reference gratings for interferometric and moiré-based position measurement.
• Security and decorative optics: Fine line structures that produce a controlled diffractive image under point light.

Design Notes & Process Considerations

1. Specify pitch and duty cycle separately: A nominal period with no duty cycle leaves line width open, and a 10% shift in line width changes efficiency noticeably.
2. Keep the aspect ratio sane: Grooves deeper than about three times the line width etch slowly and lose wall verticality, so depth should be chosen with the period, not against it.
3. State the acceptance method: Geometric line width and diffraction efficiency are different measurements; name the one that will be used at incoming inspection.
4. Plan the blackening: A blackened surface cuts stray light but adds a process step that can shift line width slightly, so it should be quoted with the part, not added later.
5. Define the active field and handling margin: Gratings should carry a border that can be touched, clamped or bonded without contaminating the line area.
6. Watch stitching on large parts: Where several exposures make one grating, cumulative pitch error at the seam is the critical number to agree before grating chemical etching begins.