In semiconductor photomask fabrication, ceric ammonium nitrate (CAN)-based chromium etchant is used to pattern chromium absorber films on fused silica or quartz blanks. The sputtered chromium layer is typically 50–110 nm thick for binary masks and may include a CrON anti-reflective coating of 15–25 nm. Electronic/EL-grade etchant is specified because sodium and potassium must remain below 100 ppb on the mask surface, as required by SEMI P1 reticle blank cleanliness guidelines. In a recirculating PTFE etch bath, the working temperature is controlled at 23±1 °C. That narrow band prevents the nitric acid carrier from dissolving the resist–chromium interface while maintaining stable etch rate. A common working bath consists of 20–25 wt% CAN, 4–7 wt% nitric acid, and ASTM D5127 Type E-1 water. The etch rate for sputtered chromium at 23 °C in low-particle immersion is approximately 2–3 nm/s; single-wafer spray etchers may reach 5 nm/s but enlarge etch bias and CD non-uniformity. The bath is changed after 8–12 hours because cerium(III) accumulation reduces oxidation potential and causes rate drift.

Line edge roughness after wet etch is influenced by resist adhesion and etchant surface tension. Addition of 0.1–0.5 vol% perfluoroalkyl surfactant reduces pinhole attack, but this surfactant must be removed by a 18 MΩ·cm ultrapure water rinse at 25 °C to avoid organic haze. Bath-to-bath variation is controlled by oxidation-reduction potential measurement; a drop of 50 mV from the initial value signals cerium(IV) depletion and requires replenishment. In production mask shops, etch CD bias is measured on a dedicated CD SEM after each bath change, using a 50 µm isolated line pattern. The terminal product is a pellicle-mounted photomask reticle with critical dimension uniformity below 3 nm 3σ over the patterned array, qualified by AIMS aerial image analysis and reticle CD SEM.

How Does Chromium Etching Scale to Gen 8.5 Flat Panel Display Substrates?

Chromium sputtered on Gen 8.5 glass substrates serves as the gate electrode in amorphous silicon TFT-LCD backplanes. The film thickness ranges from 100–300 nm. Electronic/EL-grade chrome etchant is used to prevent particle-driven gate shorts on 2200 mm × 2500 mm sheets. The spray etch tool delivers etchant through nozzle arrays at 1.5–2.5 L/min per nozzle with substrate temperature 30–35 °C. A working formulation contains 15–20 wt% CAN and 3–5 wt% free nitric acid. The lower acid concentration protects underlying indium tin oxide or glass from pitting. The chromium etch rate under production spray conditions is 4–8 nm/s, measured by sheet resistance endpoint. Process conflict occurs when chromium overlaps molybdenum or aluminum interconnect layers; localized galvanic coupling increases the etch rate and produces taper angles below 30°. The critical dimension control target on gate lines is ±0.3 µm across the full sheet. The process line is maintained in an ISO 14644-1 Class 5 cleanroom with airborne particles ≥0.5 µm controlled to ≤3,520/m³.

Wastewater from the chromium etch line is treated by chromium reduction and precipitation at pH 8.5–9.5. Final display modules are evaluated under EU RoHS 2011/65/EU, while process chemical handling follows REACH Regulation (EC) No 1907/2006. The terminal product is an active-matrix TFT backplane with gate linewidth uniformity within ±0.3 µm and leakage current below panel specification, verified by array test.

For MEMS devices, chromium serves as an adhesion promoter between gold metallization and silicon, silicon dioxide, or borosilicate glass. The chromium film is usually 10–30 nm thick. Electronic/EL-grade etchant removes this adhesion film without roughening structural gold or silicon. The bath formulation contains 10–15 wt% CAN and 2–4 wt% nitric acid at 25–30 °C. Etch rate is controlled at 1–2 nm/s so lateral undercut of chromium under plated gold remains below 100 nm. Selectivity to gold exceeds 100:1, and selectivity to thermal silicon dioxide exceeds 50:1 under these conditions. The etchant is filtered through 0.05 µm PTFE cartridges and certified for sodium, potassium, calcium, and iron below 10 ppb each.

Process conflict arises when chromium etchant is used after hydrofluoric acid vapor release of silicon dioxide sacrificial layers. Residual fluoride in the bath accelerates chromium attack and pits aluminum bond pads. The etch station is segregated from HF processing, and wafers receive an intermediate overflowing DI water rinse of at least 10 min at 18 MΩ·cm. The terminal product is a released MEMS sensor or actuator wafer with residual chromium below 5 × 10¹⁰ atoms/cm², confirmed by total reflection X-ray fluorescence, prior to wafer bonding or hermetic encapsulation.

When Chromium Acts as the Adhesion Layer in Wafer-Level UBM Stacks

The removal of field-area chromium after copper/nickel/tin-silver bump plating requires a low-acid EL-grade etchant that does not roughen the solder cap. Chromium thickness in under-bump metallization is usually 50–150 nm. A single-wafer spray processor applies a formulation of 12–18 wt% CAN and 1–3 wt% nitric acid at 22–26 °C. The chromium etch rate is 1–3 nm/s, and the low acid concentration keeps the post-etch solder wetting contact angle below 20° after flux-assisted reflow. Endpoint detection uses optical emission spectroscopy at 520 nm chromium emission, preventing over-etch into the copper seed layer. If the bath pH drifts above 1.5, the chromium etch rate falls below 0.5 nm/s, causing delayed endpoint and increased wafer throughput loss. The etch must not increase nickel surface roughness above 2 nm Rq, measured by atomic force microscopy, because rougher nickel reduces flip-chip standoff height uniformity. The terminal product is a bumped wafer with bump shear strength above 50 g per bump, tested according to JEDEC JESD22-B117, and solderability verified by wetting balance.

Waste segregation for UBM chromium etching is required because the drain stream contains cerium, chromium, and trace lead from solder. The acid waste is neutralized to pH 7.0–8.5 and precipitated with sodium hydroxide before ion exchange. EU RoHS 2011/65/EU applies to the finished bumped die, not the etchant. The process bath is recirculated through 0.1 µm filtration and analyzed for cerium(IV) concentration by redox titration every 4 hours. Bath replenishment is triggered when cerium(IV) falls below 80% of the initial value, corresponding to a redox potential drop of approximately 50 mV.

When optical encoder discs and diffraction gratings require low-stray-light chromium patterns, the etchant must limit micro-void formation and chrome oxide redeposition on clear apertures. The chromium layer is typically 80–200 nm thick and is spin-coated with a positive photoresist before laser direct imaging. Electronic/EL-grade etchant is used at 24 ± 0.5 °C in a quartz immersion bath with megasonic agitation at 950 kHz. The working bath contains 18–22 wt% CAN and 5–8 wt% acetic acid instead of nitric acid alone. Acetic acid buffers the oxidation potential and reduces chromium oxide redeposition on the glass clear aperture. The etch rate is held at 2–4 nm/s, and the bath is exchanged after cumulative chromium loading of 0.5 m²/L to prevent particle decoration. Bath aging is monitored by optical densitometry at 350 nm; when absorbance increases by 0.05 AU, the bath is discarded.

Unlike FPD spray etching, optical grating processing cannot tolerate backside chromium contamination. The glass backside is protected with a strippable polymer coating before immersion etch. Any pinhole on the backside reduces transmitted wavefront quality and increases scattered light. A post-etch mask blank is inspected under 50× dark-field microscopy for particles larger than 0.5 µm over the clear aperture. The terminal product is a chrome-on-glass grating with sidewall roughness below 50 nm and stray light below 0.1% over a 10 mm clear aperture, measured by interferometric microscopy and ISO 10110-7 scratch/dig 60/40.

Reticle Repair and Chromium Residue Removal Chemistry in Mask Shops

Laser or focused ion beam repair of clear defects generates residual chromium, gallium staining, and redeposited quartz damage. A localized jet-etch tool delivers diluted EL-grade chromium etchant through a 50–100 µm nozzle at 0.5–2.0 mL/min and 25 °C. The formulation is diluted to 5–10 wt% CAN, and free nitric acid is capped at <1 wt%. Higher acid concentration micro-roughens quartz and increases transmission loss at 193 nm. The etch rate is reduced to 0.5–1.5 nm/s for controlled removal of 1–5 nm residual chromium islands. Endpoint monitoring uses in-situ reflectometry at 405 nm; the chromium absorbance peak drops before the clear substrate signal rises. Published data for femtosecond laser repaired CrON stacks compatibility with wet chromium etchants is limited. Mask shops therefore qualify each repaired plate by AIMS aerial image metrology and reticle CD SEM. The terminal product is a repaired photomask with post-repair critical dimension offset below 5 nm at the repair site and no printable transmission loss at 193 nm.

Because repair etch tools process one site at a time, bath turnover is frequent. The etchant is refreshed every 30 minutes or after 20 repair sites, whichever occurs first. Waste from the jet tool is collected in a separate container and treated as chromium-bearing acid waste. No pellicle is mounted until residual etchant has been removed by a 5-minute ultrapure water rinse and a heated nitrogen dry at 80 °C. The post-repair reticle is inspected under 100× dark-field microscopy for chromium redeposit larger than 0.2 µm.