When indium tin oxide (ITO) films are wet-patterned in flat-panel display, touch sensor, and organic light-emitting diode backplane processing, the etchant bath is typically an aqueous solution of electronic/EL-grade oxalic acid at 1–3 wt% heated to 40–50 °C. The oxalic acid solution dissolves the mixed indium oxide and tin oxide network through protonation of oxide surface sites and simultaneous chelation of lattice metal ions. Indium is carried into solution predominantly as the tris(oxalato)indate complex [In(C₂O₄)₃]³⁻, while a tin-rich residue may remain if the ITO film has been annealed to polycrystalline form above 200 °C. Amorphous ITO, commonly deposited by DC magnetron sputtering with substrate temperature below 150 °C, etches up to twice as fast as polycrystalline ITO in the same bath. Published data for exact etch rates vary with film stoichiometry, crystallite size, and sputter pressure; process developers rely on film thickness measurements by spectroscopic ellipsometry or stylus profilometry to bracket each batch. The rate window is commonly reported in the range 20–80 nm/min within the above concentration and temperature envelope. Bath temperature must be maintained within ±2 °C of the control setpoint because dissolution follows an Arrhenius-type dependence, and etch uniformity across a generation 8.5 glass substrate degrades when heat exchanger output oscillates.

Critical impurity control differentiates electronic/EL-grade oxalic acid from industrial material. Chloride is typically specified at ≤5 ppm because free chloride in an acidic ITO etchant initiates pitting on molybdenum or aluminium capping layers used as source-drain electrodes in thin-film transistor arrays. Sodium and calcium are limited to ≤1 ppm because mobile cations migrate into silicon nitride or oxide gate dielectrics under bias-temperature stress and shift threshold voltage. The bath is circulated through point-of-use filtration with polypropylene or PVDF cartridge filters rated at 0.05–0.10 μm. Particle counts are monitored by optical particle counters, and bath replacement is commonly triggered by particle count excursions rather than acid depletion. Conductivity-based concentration control replenishes the acid based on a calibration curve established for the specific etch tool. The etching step is followed by deionized-water rinsing to a resistivity endpoint of not less than 18 MΩ·cm at 25 °C.

Why Does Post-CMP Copper Cleaning Demand Low-Chloride Oxalic Acid?

Oxalic acid functions as a dual-action chelator and passivating agent in post-chemical mechanical planarization cleaning of copper interconnects. After copper CMP on 300 mm silicon wafers, the surface carries residual silica abrasive, organic slurry additives, and trace copper ions. A dilute electronic-grade oxalic acid solution, typically 0.1–1.0 wt% at pH 2.5–3.5, solubilizes cupric ions as [Cu(C₂O₄)₂]²⁻ while a sparingly soluble copper oxalate layer forms on the copper surface. The thin oxalate film inhibits oxidation of exposed copper lines during the interval between CMP and subsequent dielectric deposition. The pH is controlled with electronic-grade ammonium hydroxide or nitric acid; excursions below pH 2.0 dissolve the passivating film and increase copper loss, while excursions above pH 4.0 risk precipitation of basic copper salts on the wafer.

Low chloride content is required because residual chloride in the cleaning solution destabilizes the copper oxalate film and promotes local anodic dissolution at the copper/barrier interface. Electronic-grade oxalic acid with chloride at ≤5 ppm prevents pit-related yield loss in dense low-k interconnect structures. The cleaning tool can be a single-wafer scrubber or a batch immersion tank with megasonic transducers operating in the range 28–40 kHz. Published data for this specific configuration is limited; however, cleaning efficiency is evaluated by SP2 particle counts, scanning mobility particle counting, and X-ray photoelectron spectroscopy for residual copper oxidation state. Final rinsing with ultrapure water must achieve a total organic carbon level below 10 ppb and resistivity of 18.2 MΩ·cm at 25 °C per ASTM D5127-13. Bath temperature is usually maintained between 22 °C and 35 °C; higher temperatures accelerate oxalic acid decomposition and increase carbonaceous residue risk.

Oxalic acid is not combined with hydrogen peroxide in this cleaning step because the redox reaction consumes the acid and generates gas. If a peroxide-based post-CMP clean precedes it, the wafer must be rinsed to neutral pH before oxalic acid exposure. The cleaning sequence is validated by split-lot electrical tests on comb structures; acceptable shift limits are determined by process integration groups rather than by a single universal specification.

Rouge Dissolution in High-Purity Water Loops

Within semiconductor high-purity water distribution loops, stainless steel electropolished surfaces accumulate iron oxide deposits, commonly termed rouge, due to long-term contact with oxygenated ultrapure water at elevated temperature. A circulating cleaning solution of electronic/EL-grade oxalic acid at 3–5 wt% and 65–75 °C removes ferric oxide through acid-assisted reductive dissolution and chelation. Ferric ions released from the surface enter solution as [Fe(C₂O₄)₃]³⁻; the complex remains dissolved in the circulating stream until blowdown or discharge. The dissolution rate falls sharply below 60 °C, so return-line heat exchangers must maintain the loop above the lower setpoint throughout the cleaning campaign. A typical circulation time for a modest loop is 4–12 h, with endpoint established by iron concentration in return samples measured by inductively coupled plasma mass spectrometry or atomic absorption spectrometry.

Material compatibility is a primary engineering constraint. The cleaning skid is constructed from PVDF, PFA-lined steel, or polypropylene to avoid corrosion of stainless steel during the acid phase. Elastomer seals must be EPDM or perfluoroelastomer; nitrile and natural rubber are incompatible. After oxalic acid cleaning, the loop is rinsed with high-purity water until pH returns to neutral and conductivity at 25 °C is below 1.0 μS/cm. A subsequent passivation step using electronic-grade nitric acid at 20–25% and 49–60 °C is performed in accordance with ASTM A967/A967M-17. Electronic/EL-grade oxalic acid with chloride below 5 ppm is preferred because even trace chloride can initiate pitting on sensitized heat-affected weld zones during acid exposure. Oxalic acid is not a passivation agent; it is a derouging and descaling agent.

Rouge is classified into Type I and Type II according to iron oxidation state and hydration, with Type II hematite being more resistant to acid attack. Oxalic acid is selected over sulfuric or hydrochloric acid descaling because it combines acid dissolution with reducing action without chloride stress corrosion risk. The spent cleaning solution is neutralized with sodium hydroxide to pH 7–9, precipitating iron oxalate and iron hydroxide. The precipitate is separated by filtration, and the filtrate is checked for total organic carbon before discharge to the facility acid waste treatment system. Hydrofluoric acid is not added to oxalic acid derouging formulations because hydrofluoric acid dissolves silica-based passivation layers and can damage glass-lined sample ports.

Precipitation of rare earth oxalates for phosphor precursors is controlled by the addition of electronic/EL-grade oxalic acid to high-purity rare earth nitrate or chloride solutions. The reaction consumes acid and produces a sparingly soluble lanthanide oxalate, represented as 2Ln³⁺ + 3H₂C₂O₄ + 10H₂O → Ln₂(C₂O₄)₃·10H₂O + 6H⁺. Precipitation is carried out at 60–80 °C with a stoichiometric excess of oxalic acid of 10–20% over the rare earth charge. pH is maintained between 1.5 and 2.5 because lower acidities generate fine, filter-resistant particles, while higher acidities redissolve the precipitate. Agitation is typically provided by a glass-lined reactor with a retreat-curve impeller at 120–200 rpm; the oxalic acid solution is added below the liquid surface to avoid local nucleation bursts. After precipitation, the slurry is aged for 2–6 h to improve crystal habit and filterability.

Impurity transfer from the precipitant is the dominant reason for selecting electronic/EL-grade material. Iron, calcium, sodium, and potassium in oxalic acid are incorporated into the oxalate lattice during crystallization and remain as non-volatile oxides after calcination at 850–1000 °C. These impurities act as luminescence quenchers in yttrium oxide, yttrium aluminum garnet, and lanthanum oxysulfide phosphors used in solid-state lighting and display applications. Particle size distribution of the calcined oxide is measured by laser diffraction per ISO 13320:2020; surface area is determined by nitrogen adsorption per ISO 9277:2010. The final oxide is milled and classified before being dispersed into phosphor slurries.

Typical COA control limits for electronic/EL-grade oxalic acid used in the above processes are compiled below; values should be confirmed against the supplier lot release because harmonized SEMI or ASTM specification for oxalic acid dihydrate is not uniformly adopted across all regions.

Because some of these limits are not harmonized across regional suppliers, process owners should verify COA values against the specific bath life model and device reliability requirements. For phosphor applications, transition metal limits are commonly tightened further to ≤0.5 ppm for chromium, manganese, and nickel due to their strong absorption bands. Calcination of the wet oxalate cake is carried out in a two-step profile: first at 200–300 °C to remove hydration water, then at 900–1000 °C to decompose the oxalate to oxide and desorb carbonate species. The decomposition releases carbon monoxide and carbon dioxide; furnace off-gas is sent to an afterburner. The resulting rare earth oxide particle size is controlled by milling, and agglomerates are reduced by dispersion with polyacrylate dispersants. Electronic-grade oxalic acid reduces the need for post-calcination acid washing because the precipitant contributes minimal sodium, potassium, and iron. The final phosphor precursor is evaluated by X-ray fluorescence for rare earth stoichiometry and by photoluminescence screening.

When Aluminium Process-Chamber Components Require Anodic Barrier Coatings

Across semiconductor etch and deposition tool maintenance, aluminium chamber components are anodized in oxalic acid baths operating at 10–20 wt% acid concentration, 20–25 °C electrolyte temperature, and DC voltage from 20 V to 40 V. Current density is maintained between 1.0 A/dm² and 1.5 A/dm² during the initial constant-current phase. The resulting amorphous aluminium oxide film is porous and consists of hexagonal cells with central pores; the pore diameter and interpore distance scale with anodizing voltage. Film thickness is controlled by charge passing through the bath, with practical thicknesses for semiconductor chamber components ranging from 20 μm to 50 μm. Oxalic acid anodizing is selected when the aluminium oxide layer must provide both high dielectric strength and resistance to halogen-containing plasma by-products; the coating prevents reactive species from contacting the underlying high-purity aluminium.

Electronic/EL-grade acid is needed because sulfate in industrial-grade oxalic acid changes pore regularity and may cause local breakdown at thicknesses above 30 μm. Chloride is limited to ≤5 ppm to avoid pitting during the anodizing step. The voltage ripple from the rectifier must not exceed ±5% because voltage transients create non-uniform interpore distances and reduce dielectric strength. After anodizing, the porous layer may be sealed in boiling 18 MΩ·cm water or left unsealed for vacuum applications where trapped moisture must be avoided. Coating quality is inspected by eddy current thickness measurement and dielectric withstand testing; a typical acceptance criterion for oxygen plasma service is a breakdown voltage above 15 V/μm. Components are baked before installation to remove residual oxalate species, commonly at 150 °C for 4 h under flowing nitrogen.

Oxide Removal on Copper Leadframes Precedes Wire Bonding

Before wire bonding on copper leadframes, mixed cuprous and cupric oxides formed during storage and solder dipping are removed by an electronic-grade oxalic acid cleaning step. A typical immersion bath contains 2–5 wt% oxalic acid at 45–55 °C with a dwell time of 3–10 min. The acid dissolves copper oxides while the formation of copper oxalate limits base metal attack. Chloride is controlled to ≤5 ppm because residual chloride on silver-plated die attach pads causes galvanic corrosion between silver and copper after encapsulation. The cleaning tank is followed by a two-stage countercurrent rinse with ultrapure water; final rinse conductivity is monitored to below 1.0 μS/cm before drying in filtered nitrogen.

Electrode potential measurements of copper leadframes in 0.1 M oxalic acid show a passivation region that extends over several hundred millivolts; outside this region, base copper dissolves rapidly and the surface roughens. Process qualification includes wire-pull testing per MIL-STD-883 Method 2011.7 and ball shear testing to verify that the oxalic acid step does not degrade bond strength. Over-etching beyond 10 min is known to increase surface roughness and reduce wire-pull values due to copper undercut beneath residual oxide islands. The bath is filtered through 0.2 μm polypropylene cartridges, and dissolved copper concentration is maintained below 500 ppm by bath bleed-and-feed or dump-to-drain control. Published data for this specific configuration is limited; however, the passivation effect is reproducible across electroplated and rolled copper leadframe alloys when the acid concentration and temperature are maintained within the stated ranges.