ITO-Rework Electronic/EL Grade, model designation E/EL-100, is a wet-chemical rework formulation intended for controlled removal of indium tin oxide transparent conductive films from rigid glass and flexible polymer substrates in thin-film transistor array, touch sensor, and electroluminescent display manufacturing. The product is supplied as a ready-to-use acidic solution with a specific gravity of 1.09–1.13 g/cm³ at 25 °C by ASTM D4052, free acidity expressed as HCl at 18.5–20.5 wt% by acid-base titration, and dissolved chloride at 11.7–13.1 wt%. It is filtered at point-of-use through a 0.1 µm polytetrafluoroethylene membrane and is intended for immersion, puddle, and low-pressure spray rework tools. On 100 nm sputtered ITO deposited on 0.7 mm alkali-free boroaluminosilicate glass, immersion at 25 ± 0.5 °C with 200 rpm wafer rotation produces endpoint sheet resistance transition from 10–15 Ω/sq to >10⁶ Ω/sq in 70–95 s as measured by four-point probe. Fresh-bath vertical etch rate at 25 °C is therefore 10.5–14.3 Å/s. The material is qualified for rework of ITO films on color filter and array substrates but is not approved for selective active-channel patterning where channel length control below 2 µm is required.

The primary differences from commodity technical-grade ITO etchants and standard electronic grade strippers are simultaneous suppression of mobile ion contamination, particulate burden, post-strip tin residue, and isotropic undercut without reducing etch rate below inline rework throughput limits. Technical-grade materials may contain sodium at 5–20 ppm and iron at 10–50 ppm; this product restricts sodium and potassium to ≤0.20 ppm each, dissolved iron to ≤0.50 ppm, and total transition metals by inductively coupled plasma mass spectrometry to ≤1.0 ppm. Anion contamination is controlled to ≤3.0 ppm sulfate and ≤1.5 ppm phosphate because residual sulfate and chloride can form hygroscopic inclusions at the ITO-phosphor interface in electroluminescent stacks under 100–400 V alternating-current drive conditions.

Why Do Electronic and Electroluminescent Lines Require a Separate Rework Grade?

Commodity technical-grade ITO etchants carry alkali and transition metal burdens that are acceptable for decorative or architectural glass but incompatible with active-matrix backplane and electroluminescent device integration. In thin-film transistor gate dielectric stacks, mobile sodium and potassium ions migrate under bias at operating temperatures of 60–85 °C, shifting flat-band voltage by 0.1–0.3 V per 1×10¹² atoms/cm² of interfacial alkali contamination. Electroluminescent phosphor layers are similarly sensitive to iron and copper because these metals quench radiative recombination centers and reduce luminance stability. The Electronic/EL Grade product controls sodium and potassium by ASTM E2371 inductively coupled plasma mass spectrometry to ≤0.20 ppm each, dissolved iron to ≤0.50 ppm, and total transition metals to ≤1.0 ppm. After a 30 min immersion test on ITO-coated glass, electron spectroscopy for chemical analysis shows chloride residue below 0.05 at%, whereas technical-grade mixed-acid etchants leave chloride residue at 0.3–0.6 at%. This difference is observable as post-anneal haze on electroluminescent lamps and as contact resistance drift in display modules.

For electroluminescent lines processing polyethylene terephthalate substrates with sputtered ITO, the formulation is operated at 28–32 °C to control substrate haze and to avoid thermal stress cracking of the polyester film. In puddle tools with single-wafer dispense, a 60–90 s puddle residence time removes 100–150 nm ITO layers without measurable attack on the underlying hard-coat or SiO₂ barrier layer. Bath temperature above 35 °C accelerates etch rate to 12–18 Å/s but reduces the process window because isotropic undercut increases to 1.2–1.5 µm per side on patterned ITO features. Production data from 150 mm square glass cassette processing indicate that replacement of technical-grade stripper with Electronic/EL Grade reduces sodium residue on post-strip glass from 1.8×10¹³ atoms/cm² to 8×10¹¹ atoms/cm² after a single deionized water rinse and spin dry.

When Bath Ageing Shifts Undercut Beyond ±0.5 µm

Mixed-acid ITO etchants lose dissolved nitrate through reduction during repeated metal loading, which alters the redox potential and produces progressively slower, less uniform etching. In this product, bath life is specified as 8 production shifts or 14 days, whichever occurs first, at a sustained metal loading of up to 800 mg/L total dissolved indium and tin. Beyond that point, endpoint time measured on a 100 nm ITO coupon increases by 35–50%, and sidewall undercut on 2 µm line/space test structures widens from 0.8 ± 0.2 µm to >1.4 µm. For spray tools with in-line filtration, a bleed-and-feed replenishment rate of 10–15% of initial working volume per shift maintains endpoint time within ±10% of fresh-bath performance. The relationship between bath age and etch rate is monitored by measuring sheet resistance of a 50 mm square ITO-coated witness coupon after a fixed 60 s immersion at 25 °C; the coupon must reach >10⁶ Ω/sq before production substrates are released.

On molybdenum/aluminum/molybdenum data-line test stacks, immersion for 180 s at 25 °C produces an aluminum etch loss of <0.2 µm per exposed edge and no continuous film attack when the photoresist opening is ≤10 µm. Exposure to bare copper lines is not recommended; the nitrate component can produce copper dissolution at 0.4–0.8 µm/h under stagnant conditions. The material is compatible with novolac/diazonaphthoquinone resists used in i-line patterning, but blanket photoresist films should be removed before ITO rework because organic residues increase local etch nonuniformity and leave carbonaceous particulate on the substrate. When photoresist cannot be removed first, the etch rate may be reduced by diluting with 5–10% deionized water, which increases endpoint time by 20–35% but reduces lift-off of resist edges.

Selectivity Window Against Photoresist and Metallization Stacks

The formulation suppresses galvanic undercut through a nonionic fluorosurfactant package that lowers surface tension to 22–26 mN/m at 25 °C as determined by ASTM D1331. This surface tension reduction permits wetting of high-aspect-ratio trace edges without excessive intrusion under the resist foot. Selectivity data on 2 µm line/space ITO features with 1.2 µm thick i-line photoresist show sidewall undercut of 0.5–0.8 µm per side after complete removal of 100 nm ITO. Under identical test conditions, electronic-grade hydrochloric acid-only etchants produce undercut of 1.5–2.0 µm per side, and technical-grade mixed-acid strippers produce undercut above 2.5 µm per side with visible resist lifting. The product also leaves no insoluble tin oxide residue on the exposed glass surface; scanning electron microscopy with energy-dispersive X-ray analysis at 20 kV shows residual tin below 0.05 at% on the post-strip surface after deionized water rinse.

Trace Metals Are the Controlling Purchase Criterion

Electronic/EL Grade certification includes lot-by-lot documentation of trace metal concentrations by inductively coupled plasma mass spectrometry according to ASTM E2371 and anion content by ion chromatography according to EPA 300.1. The use of high-purity acids with sub-ppt metal specifications is not sufficient to control contamination because container closure liners and transfer tubing contribute measurable iron, zinc, and calcium after 30–60 days of storage. The product is packaged in fluoropolymer-lined high-density polyethylene containers with certified leachable metal content below 0.05 ppb per gram of liner material. Incoming quality control on the production line should include a 0.2 µm filtrate particle count from a 100 mL sample and a four-point probe endpoint check on a reference ITO wafer. Any lot failing endpoint by more than ±15% or showing particle counts greater than 40 particles/mL at ≥0.5 µm should be quarantined because these deviations precede nonuniform rework and contamination excursions.

Immersion tools configured with 2 mm thick polyvinylidene fluoride or polytetrafluoroethylene tanks and quartz heaters are preferred; stainless steel wetted parts are excluded because chloride attack releases chromium and nickel into the bath. Recirculation pumps should deliver 0.5–1.0 L/min per litre of bath volume through 0.1 µm polytetrafluoroethylene cartridge filters. In-line refractive index or conductivity monitors are not reliable for endpoint control because the dissolved indium concentration at the 70–95 s endpoint changes conductivity by less than 2 mS/cm. Instead, endpoint control should be derived from timed processing with periodic sheet resistance verification. Spray tools require a 1.5–2.0 bar nozzle pressure and fan-spray nozzle spacing of 50–75 mm to maintain uniform wetting; higher pressures generate aerosol that deposits corrosive droplets on adjacent equipment frames.

Operational boundaries are defined by the redox couple and the substrate stack. The product must not be combined with alkaline strippers, amine-based cleaners, or concentrated hydrogen peroxide; mixing with alkaline solutions generates exothermic gas evolution and may redeposit indium hydroxide. Storage of unopened containers at 10–25 °C away from ultraviolet exposure gives a use life of 24 months. Once a container is opened, the headspace should be purged with dry nitrogen and the closure resealed within 30 min to limit nitric oxide uptake. The product is not suited to selective etching of sub-1 µm ITO features without mask compensation because the lateral etch component is 0.4–0.6 µm/min at 25 °C. For rework of ITO films thicker than 300 nm, staged immersion with intermediate rinse is recommended to prevent local heating and resist delamination. For silver nanowire or PEDOT:PSS transparent electrodes, published data for this specific configuration is limited; compatibility must be verified on candidate stack coupons before production use.