| HS Code | 428260 |
| Product Name | Au Etchant Electronic/EL Grade |
| Composition | Aqueous solution of iodine (I2) and potassium iodide (KI) |
| Physical State | Liquid |
| Appearance | Dark red-brown transparent liquid |
| Odor | Characteristic pungent iodine odor |
| Density | 1.00 to 1.05 g/cm³ at 25°C |
| Ph | Approximately 5 (mildly acidic) |
| Boiling Point | Approximately 100°C (water-based) |
| Melting Point | Approximately 0°C |
| Solubility | Miscible with water; soluble in common organic solvents with caution |
| Etch Rate For Gold | Typically 20-50 nm/s at room temperature, dependent on temperature and agitation |
| Purity Grade | Electronic/EL grade with ultra-low trace metal ion content |
| Shelf Life | Typically 6 to 12 months when stored properly |
| Storage Conditions | Store in a tightly sealed container in a cool, dark area at 2-8°C |
| Hazard Class | Corrosive and oxidizing liquid; harmful if swallowed and irritating to skin and eyes |
As an accredited Au Etchant Electronic/EL Grade factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Packaged in a securely sealed, corrosion-resistant high-density polyethylene bottle to ensure purity. Available as Electronic/EL grade gold etchant in 1 L quantity. |
| Container Loading (20′ FCL) | 20′ FCL shipment of Au Etchant (Electronic/EL Grade), packed in sealed containers on pallets, secured and labeled for safe transport. |
| Shipping | Au Etchant (Electronic/EL Grade) ships as a hazardous, corrosive chemical. It requires leak-proof, chemically compatible packaging with proper UN-rated containers and labeling. Transport follows strict IATA/DOT regulations, segregated from incompatible materials. Handle with protective equipment; observe temperature controls to maintain solution integrity and prevent container degradation during transit. |
| Storage | Store in a tightly sealed original container in a cool, dry, well-ventilated area, away from direct sunlight, heat, and incompatible substances such as strong acids, bases, oxidizers, and metals. Maintain electronic-grade purity by using dedicated, clean equipment. Provide secondary containment to prevent spills and check manufacturer expiry before use. |
| Shelf Life | Shelf life is typically 6–12 months when stored tightly sealed in a cool, dark area. Avoid light and contamination. |
Production-scale wafer bumping lines use Au Etchant Electronic/EL Grade to strip the gold cap over nickel or nickel-copper under-bump metallization after electroplating and before solder paste printing. The wet etch bath is prepared from EL-grade potassium iodide and iodine at a KI:I₂ molar ratio between 4.5:1 and 5.5:1, diluted with Type E-1 water conforming to ASTM D1193 to an iodine concentration of 15 g/L to 25 g/L. Bath temperature is held at 32 °C ± 1 °C in a fully recirculated PTFE-lined etch module with 0.05 μm point-of-use filtration. On 300 mm single-wafer spin processors, edge-of-wafer etch rate nonuniformity increases when bowl exhaust falls below 80 m³/h, because iodine vapor accumulates at the gas-liquid boundary and locally accelerates gold dissolution. Etch rate on evaporated gold is qualified each shift by four-point probe sheet resistance per ASTM F1529-97, with complete removal of a 120 nm gold cap indicated when sheet resistance rises beyond the measurable range for the underlying nickel film. PH is controlled at 7.2 to 7.8 by metered addition of dilute potassium hydroxide; iodide-to-iodate side reactions and free iodine evaporation shift bath stoichiometry over 72-hour continuous operation, requiring replenishment based on redox potential rather than visual color. Undercut of the gold edge beneath photoresist sidewalls is held below 0.25 μm per side by limiting post-endpoint immersion to 90 s and by adding 1.0 to 2.0 vol% non-ionic wetting agent. Metal impurity control for bumping requires sodium, potassium, calcium, iron, nickel, and copper each below 50 ppb by ICP-MS, with particle counts no greater than 100 particles/mL at 0.1 μm in an ISO 14644-1:2015 Class 4 chemical handling zone. Rinse uses overflow deionized water at 18.2 MΩ·cm for 10 minutes, followed by nitrogen spin-dry at 1,200 rpm to suppress water marks on exposed copper. Residual gold after dry-out is measured by X-ray photoelectron spectroscopy survey scan and is kept below 0.5 at.% on the nickel surface before the substrate proceeds to selective nickel etching or flux application.
In GaN LED front-end processing, the p-contact stack is typically Ni/Au or Cr/Au with gold thickness between 50 nm and 150 nm, and the gold layer must be removed selectively from the mesa field while preserving the p-GaN surface. The etchant is diluted to a lower iodine strength of 10 g/L to 14 g/L with a KI:I₂ molar ratio of 5:1 to reduce attack on photoresist sidewalls at the mesa edge. Immersion is performed in a batch spray tool at 25 °C to 28 °C, with megasonic agitation at 0.8 MHz to disperse iodine-rich boundary layers without lifting patterned photoresist. Adhesion failure is the main process conflict: thin positive resists on patterned sapphire lose footing after 90 s exposure to the iodide/iodine solution, producing local undercut of 0.5 μm to 1.0 μm along the gold pad perimeter. A postbake at 110 °C for 60 s after lithography, with adhesion qualified by tape test per ASTM D3359-17 class 5B, and the addition of 0.3 vol% to 0.5 vol% surfactant reduce the undercut to below 0.3 μm. The cyanide-free formulation avoids cyanide use restrictions and allows aqueous neutralization with sodium thiosulfate before drain discharge, supporting EU industrial wastewater permits. Gold etch rate is checked daily on thermally evaporated gold coupons by stylus profilometry against a qualified rate of 0.8 nm/s to 1.4 nm/s at 25 °C; if the rate falls below 0.6 nm/s, the bath is recharged or replaced because prolonged immersion increases photoresist swelling. The terminal LED wafer after strip shows p-GaN roughness below 1.0 nm RMS by atomic force microscopy on 10 μm × 10 μm scans, which is required before indium tin oxide deposition.
| Parameter | Control value | Analytical method |
|---|---|---|
| Iodine content | 10 g/L to 14 g/L | titration with 0.1 N sodium thiosulfate |
| Potassium iodide content | 35 g/L to 45 g/L | ion chromatography |
| pH | 6.8 to 7.5 | calibrated glass electrode |
| Na, Fe, Cu, Ni | ≤ 50 ppb each | ICP-MS |
| Particles ≥ 0.2 μm | ≤ 150 counts/mL | laser particle counter |
Sacrificial gold layers in MEMS electrostatic actuator fabrication are etched after the release holes are defined by deep reactive-ion etching, so the etchant must penetrate high-aspect-ratio trenches with widths between 2 μm and 5 μm while leaving silicon dioxide and silicon nitride stops intact. The bath is mixed at a lower total iodine concentration of 8 g/L to 12 g/L and a KI:I₂ ratio of 6:1, with pH buffered at 5.8 to 6.4 using acetic acid to minimize attack on sputtered aluminium bond pads. The process is run at 20 °C to 23 °C to slow the gold etch rate to 0.3 nm/s to 0.6 nm/s, which is necessary because the gold traces undercut the isolation trenches and the release must stop before the sacrificial oxide is exposed. Batch cassettes are agitated by slow reciprocal motion at 10 cycles/min to avoid bubble entrapment in 5:1 aspect-ratio cavities; ultrasonic agitation is not used because acoustic cavitation damages released thin-film membranes. After gold removal, the wafer is transferred to a cascade rinse with deionized water at 18.2 MΩ·cm, then dried with isopropanol vapor at 85 °C to avoid capillary-force stiction of released cantilevers. Silicon dioxide loss after 15 min exposure is held below 2 nm by spectroscopic ellipsometry, and silicon nitride loss below 1 nm, to preserve actuator gap dimensions. Released structures are tested by laser Doppler vibrometry; resonant frequency shift outside ±5% of design indicates incomplete release or stiction-related mechanical coupling. Gold residue on the sidewall is inspected by focused ion beam cross-section at one location per wafer, with acceptable residue below 1 at.% by energy-dispersive X-ray spectroscopy.
Rework of ENIG-finished package substrates before gold wire bonding requires removal of immersion gold from bond pads without changing the underlying electroless nickel morphology. The etchant is applied as a 1:1 volumetric dilution of EL-grade stock with Type E-1 water at 22 °C to 25 °C, giving an immersion gold removal time of 20 s to 45 s for 0.05 μm to 0.10 μm gold thickness. The potassium iodide/iodine chemistry does not contain cyanide, so the nickel surface remains smooth if immersion is terminated immediately after the gold color disappears. Surface roughness of the electroless nickel pad is measured by stylus profilometry after rework and must remain below 0.1 μm Ra; if roughness increases beyond that limit, the nickel has been microetched and the substrate lot is rejected for wire bonding. The process is used on strip-level substrates in a conveyorized wet station with air-knife drying at 60 °C. Because the bath lifetime is shortened by organic residues from soldermask, activated carbon filtration at 0.2 μm and daily top-up of 5 vol% fresh etchant are applied. The reworked pads are subsequently gold wire-bonded with pull strengths above 6 cN for 25 μm wire, meeting MIL-STD-883 Method 2011.9 bond strength requirements.
Gold front-grid contact caps on lattice-matched GaInP/GaAs/Ge triple-junction cells are etchbacked after electroplating to reduce series resistance and remove bridging filaments between grid lines. The process uses a mid-range iodine concentration of 14 g/L to 18 g/L at a KI:I₂ ratio of 5:1, operated at 30 °C ± 1 °C in a manual immersion vessel with Teflon cassettes. Etch uniformity across a 100 mm wafer is sensitive to dissolved oxygen and iodine depletion; nitrogen sparging at 0.5 L/min stabilizes the etch rate at 1.0 nm/s ± 0.2 nm/s. By-product precipitation of potassium tetraiodoaurate can form if the gold loading exceeds 1.5 g/L, so the bath is changed after processing 25 wafers per liter. The front-grid width after etchback is measured by scanning electron microscopy and held at 12 μm ± 1.5 μm. Cell efficiency is then verified under AM1.5G illumination at 1000 W/m² per IEC 60904-3, with series resistance below 5 Ω·cm² for cells above 30% conversion efficiency. Published data for this specific configuration is limited; therefore, the etch rate is re-established on process monitor wafers before each production batch.
Wafer-level chip-scale package lines use the etchant to correct gold bump height distribution after electroplating by removing 2 μm to 5 μm from high bumps while low bumps are protected by a co-planarization film. The etchant is diluted to 6 g/L to 9 g/L iodine and maintained at 18 °C to 20 °C to keep etch rate low at 0.15 nm/s to 0.25 nm/s. The diluted bath is dispensed through a nozzle array onto a rotating wafer at 500 rpm, with total contact time limited to 240 s. Thickness mapping by X-ray fluorescence per ASTM B568 before and after etch shows within-die bump height range reduced from 8 μm to 3 μm. The terminal wafer proceeds to wafer-level test and ball drop, with coplanarity within 3 μm across a 200 mm wafer.
Competitive Au Etchant Electronic/EL Grade prices that fit your budget—flexible terms and customized quotes for every order.
For samples, pricing, or more information, please contact us at +8615365186327 or mail to admin@ascent-chem.com.
We will respond to you as soon as possible.
Tel: +8615365186327
Email: admin@ascent-chem.com
Flexible payment, competitive price, premium service - Inquire now!
Au Etchant Electronic/EL Grade is supplied as a ready-to-use, cyanide-free aqueous preparation for patterning gold films on silicon, glass, and compound semiconductor substrates. The formulation is based on the iodine–triiodide redox couple in a potassium iodide matrix; gold dissolution proceeds through formation of the linear AuI₂⁻ complex. Electronic/EL Grade is not a single stoichiometric product but a supply specification that sets upper limits for alkali and transition-metal contamination, submicrometre particle burden, and halogen stabiliser content. The solution is used after positive photoresist development to remove sputtered or evaporated gold layers in bumping, redistribution-layer, MEMS electrode, and LED pad fabrication. Because the chemistry contains no free cyanide, air extraction and waste treatment are configured for iodine vapour and iodide-containing rinse water rather than for HCN. This distinction changes the safety interface: bath tanks are fitted with condensate traps and scrubbers, and respiratory protection is normally limited to cartridges rated for iodine and acid gases under EN 14387.
Compatibility with production equipment is defined by wetted materials. Immersion baths constructed of PVDF, PTFE, or quartz with PFA heater sheaths are used; stainless steel components are removed or isolated because dissolved iodine attacks austenitic stainless steel at operating temperature and releases iron and chromium, which then nucleate insoluble iodide salts. The etch rate of a freshly prepared bath at 25 °C is typically 0.2–2.0 nm/s on evaporated gold when measured by sheet resistance or contact profilometry. The rate is not fixed by the manufacturer alone; it is a function of triiodide activity, bath age, gold loading, and local agitation. Published data for this specific configuration is limited, but the range is consistent with open literature for acidic and near-neutral triiodide gold etchants.
Technical-grade strippers used for PCB gold recovery and decorative plating lines are not subjected to the same trace-metal and particle budgets as EL-grade chemistries. A circuit-board bath may tolerate sodium and potassium concentrations above 100 mg/L, whereas wafer fabs require total alkali metals below 1 mg/L because mobile ions migrate into interlayer dielectrics under bias-temperature stress and alter flatband voltage. The EL-grade product is filtered through membrane cartridges with retention ratings of 0.05–0.10 µm and is analysed for cations by ICP-MS according to EPA Method 6020B. Particle counts are measured with optical particle counters calibrated to ISO 21501-4 and are specified for particles ≥0.5 µm. This difference is not cosmetic; a single particle embedded in a 20 nm gold film can produce a localised galvanic cell that accelerates gold removal and leaves residue.
Another difference concerns the oxidant ratio. PCB strippers are often operated to exhaustion, where the iodide-to-triiodide ratio decays and the bath is replenished with bulk chemicals. In EL-grade use, the bath is monitored by potentiometric titration and redox potential, and it is replenished or replaced when the ratio moves outside a defined band. This maintains a stable undercut profile. Technical-grade cyanide strippers also remove gold at comparable or higher rates, but their pH above 10 attacks aluminium bond pads and their residual cyanide levels require segregated waste lines and cyanide destruction tanks.
The table below summarises representative differences across gold etching chemistries used in microelectronic processing.
| Chemistry | Operating temperature | Typical etch rate on evaporated gold | Photoresist compatibility | Primary limitation |
|---|---|---|---|---|
| Iodine–potassium iodide, EL grade | 20–40 °C | 0.2–2.0 nm/s depending on triiodide activity | Compatible with common novolac and acrylic resists up to 40 °C | Iodine vapour; attack on exposed copper and nickel |
| Cyanide stripper, technical grade | 25–35 °C | 0.5–3.0 nm/s | May attack some resists; pH above 10 | HCN hazard; high mobile-ion residual |
| Aqua regia, laboratory grade | 20–30 °C | Very high, non-linear | Destroyed by strong acid | Metal attack; chlorine gas |
| Thiosulfate-based, experimental | 30–50 °C | Low without catalyst | Limited published data | Bath life; copper cementation |
On a 300 mm bumping line, the transition from cyanide to iodine–iodide EL-grade chemistry is usually driven by wafer dielectric contamination and resist adhesion. Cyanide baths produce high alkalinity that saponifies exposed polyimide and can lift thick gold bump resists. Iodine–triiodide solution, by contrast, etches the gold surface without consuming the bulk novolac or acrylic resist shape, provided the bath temperature remains below 40 °C and the triiodide concentration is not allowed to rise through air oxidation. At 25 °C, an evaporated gold monitor of 0.5 µm thickness clears in approximately 4–25 min depending on agitation; endpoint detection on production equipment is commonly accomplished by optical transmittance through a patterned monitor wafer or by electrical resistance change on a sacrificial coupon.
Production-scale failure modes are dominated by bath contamination and heater overshoot. A quartz immersion heater with a PFA coating can develop pinholes after repeated thermal cycling, exposing a metal sheath that releases iron into the EL-grade bath. Iron concentrations above 50 µg/L in an iodide matrix form ferric iodide complexes that absorb triiodide and reduce etch rate; on some lines this appears as a drift of clear-point time on monitor wafers. For that reason, process control charts on a 300 mm gold bump line track clear-point time, bath density, oxidation–reduction potential, and particle counts from daily sampling. Batch-to-batch variance is addressed by diluting incoming product with high-purity water meeting ASTM D5127-13 Type E-1 requirements, but dilution also shifts the iodide-to-triiodide ratio and must be followed by analytical adjustment under the supplier’s documented replenishment protocol.
Impurity control begins with raw material selection and continues through packaging and point-of-use filtration. Potassium iodide used in EL-grade etching is specified for low calcium, magnesium, and iron because multivalent cations form insoluble iodates or hydroxides that precipitate and deposit on gold features. On a wafer fab production line, batch-to-batch variance is tracked by ICP-MS of a digested sample, ion chromatography for chloride and sulfate, and particle counting by laser light obscuration. A representative acceptance schedule appears in the following table.
| Parameter | Test method | Representative limit |
|---|---|---|
| Total alkali metals, sodium and potassium | ICP-MS, EPA Method 6020B | ≤100 µg/L each |
| Total transition metals, iron, nickel, copper, zinc | ICP-MS, EPA Method 6020B | ≤50 µg/L each |
| Calcium and magnesium | ICP-MS, EPA Method 6020B | ≤50 µg/L each |
| Chloride and sulfate | Ion chromatography | ≤5 mg/L each |
| Particles ≥0.5 µm | Optical particle counter, ISO 21501-4 | ≤100 particles/mL |
| Free iodine | Redox titration | 3–8% of total iodine |
When batch data are evaluated, particle growth is not linear. A bath aged beyond 72 h or exposed to air in an open tank forms iodine crystals at the meniscus because the vapour pressure of iodine is significant above 20 °C. These crystals redissolve in the bath and create locally high triiodide zones that etch gold at accelerated rates and produce undercut at resist edges. The operational boundary is therefore not the bulk temperature alone but the combination of temperature, exhaust velocity, and bath cover status. Enclosed systems with nitrogen blanketing extend bath life, but the gain is process-specific and published data for this exact formulation is limited.
The EL-grade product is also distinguished from technical acid strippers by its effect on thin-film adhesion layers. Gold films deposited on titanium or chromium adhesion layers are undercut when the etch chemistry penetrates pinholes or grain boundaries and attacks the underlying metal. With iodine–iodide solutions, titanium is passivated by oxide formation and is etched slowly, while chromium may be oxidised to soluble chromate if the bath becomes strongly oxidising. Process integration therefore verifies the selectivity on a patterned stack of Au/Ti/SiO₂ before lot acceptance; published data for this specific configuration is limited, and the selectivity range is highly dependent on triiodide concentration and pretreatments.
The pH of an EL-grade gold etch bath is usually 6.0–8.5. In this range, the solution is less aggressive to aluminium and silicon oxides than aqua regia or alkaline cyanide. However, pH is not a primary process control because iodine reduction is relatively insensitive to mild pH changes; the controlling variable is triiodide activity measured by redox potential. A redox potential below 250 mV versus Ag/AgCl indicates depletion of triiodide and a slowing etch rate, while values above 450 mV indicate excessive free iodine and increased undercut. The exact set point is determined by the supplier and by fab qualification, but the bracketed range is used in continuous replenishment systems.
If an immersion bath is operated above 40 °C, the etch rate does not increase indefinitely; it becomes limited by diffusion of triiodide through the solution boundary layer to the gold surface. The apparent activation energy shifts from a surface-kinetic value near 40–60 kJ/mol at low temperature to a mass-transfer value below 20 kJ/mol under strong agitation. At the same time, novolac photoresist adhesion to gold weakens because the iodine–iodide solution plasticises the resist surface and expands the gold–resist interface. Undercut values measured from cross-sectional scanning electron microscopy show a widening beyond 0.5 µm per side on a 1 µm gold film when temperature exceeds 40 °C and the bath is not periodically replenished. This is the primary reason EL-grade baths are specified for 20–35 °C rather than the 45–55 °C sometimes used for technical-grade stripping.
Agitation also influences undercut. In a quartz immersion tank with recirculating flow of 10–20 L/min through a 0.1 µm filter, gold layers patterned with resist openings below 5 µm may exhibit centre-to-edge etch variation if the wafer is not rotated or if gas bubbles collect on the surface. Spray processors reduce the boundary layer and improve uniformity, but they increase iodine vapour emission. Local exhaust capture velocities at the tool opening are therefore set by exposure assessment rather than by etch rate; iodine has an established threshold limit value of 0.01 ppm as an 8 h time-weighted average under ACGIH. Exhaust design and air monitoring are integrated into the process specification.
Compared with aqua regia, which attacks gold through free chlorine and nitrosyl chloride intermediates, the iodine–iodide system does not dissolve silicon dioxide or silicon nitride at practical process times. This selectivity is essential for gold patterning on passivated wafers. However, the EL-grade solution attacks exposed copper, nickel, and silver metallisations because iodide forms soluble complexes with those metals. Wafers with Cu/Ni/Au stacks therefore require a masking scheme that protects the copper and nickel layers, or the process is reordered so that the gold etch occurs before base-metal deposition.
Spent EL-grade gold etchant contains AuI₂⁻ and unreacted triiodide. It is compatible with gold recovery by electrolytic deposition or by reducing agents such as sodium metabisulfite or hydroxylammonium chloride. Waste classification must follow local regulations; iodide-bearing liquid waste is not discharged to municipal drains without treatment because of aquatic toxicity and because dissolved gold represents a recoverable material. The absence of free cyanide simplifies the segregation of anion exchange resins and pH adjustment systems on the fab support deck, but the iodine content requires closed-loop collection and prevents discharge to reclaim systems that use activated carbon only.
