Why Flip-Chip UBM Wet Etching Demands Separate Ni and Ag Chemistries

In flip-chip and wafer-level chip-scale package bumping, the Ni/Ag etchant is introduced after copper pillar or solder bump plating, when sputtered seed and adhesion layers are exposed across the wafer surface. The etch step must remove the Ag cap and Ni barrier from field regions without attacking the electroplated Cu pillar or the underlying Ti/Cu seed, and without producing lateral undercut exceeding the design allowance for ≤40 µm pitch interconnects. On production-scale spray processors equipped with 0.3–1.2 bar fan nozzles and 300 mm wafer handling, bath temperature is typically maintained at 25–35 °C; acceptable lot-to-lot endpoint repeatability requires edge-zone temperature control within ±1 °C because nitric-acid-based nickel etchants are strongly exothermic and local temperature excursions at the wafer edge shift the Ni etch rate non-uniformly. Split-chemistry processing is standard: a halogen-free ammoniacal peroxide mixture removes Ag capping material first, followed by a nitric-acetic acid Ni etch that provides Ni:Cu selectivity above 8:1 when the oxidizer-to-acid ratio is held within the supplier-qualified control band. Endpoint detection relies on open-circuit potential monitoring at the wafer surface; the potential transient between Ag, Ni, and underlying Cu allows feed-forward termination but requires signal baselining within the same cassette because spray bath ageing shifts the absolute millivolt values. SEMI C1 analytical procedures quantify trace metals in the etch bath, and particle counts for 0.5 µm and larger are controlled below 100 particles/mL through point-of-use filtration to prevent bump-surface defects. The finished wafer enters subsequent reflow or wafer-level moulding with a residual metal-ion level compatible with the packaging cleanroom classification of ISO 14644-1 Class 5.

The principal process conflict in UBM seed etching is not etch-rate speed but the simultaneous requirement for complete Ag removal and minimal Ni undercut on Cu. If the Ag etchant remains active at the Ag-Ni interface after the cap film has cleared, the pendent Ni layer acts as a local cathode and accelerates Cu oxidation at the pillar base, producing notch defects that are only detectable by cross-sectional scanning electron microscopy after wafer singulation. Production lines therefore insert a short deionized-water rinse and nitrogen-gas purge between the Ag and Ni etch modules, and the Ni etch time is derived from a characterisation wafer rather than from a fixed recipe. The distinction between Electronic/EL-grade and lower-purity industrial etchants is most visible here, because parts-per-billion-level copper, zinc, and iron contamination in the Ni bath changes the open-circuit potential endpoint by several tens of millivolts and reduces lot-to-lot endpoint repeatability. Where a fab operates 80–120 µm pitch copper pillar geometries, the etch depth loss is typically budgeted at 0.5–1.0 µm of Cu per side; for tighter pitch, the budget is reduced and the process window narrows to ±1 °C around the qualified setpoint.

Electroluminescent Bus-Bar Patterning on ITO-Coated PET

The ITO-coated PET web used for electroluminescent backlights carries screen-printed silver bus bars that must be etched without attacking the underlying transparent conductor. The EL-grade Ni/Ag etchant removes residual silver and nickel flash between electrode fingers at a bath temperature of 20–28 °C, and total contact time is limited to 30–120 s depending on silver paste thickness, because prolonged exposure delaminates the ITO edge and degrades luminous uniformity. Chloride-containing etchants are excluded from qualified process lines because dissolved chloride at concentrations as low as 1 ppm is known to induce pitting corrosion of ITO and can increase bus-bar-to-bus-bar leakage current after lamination. A post-etch rinse sequence of deionized water at 18 MΩ·cm resistivity, followed by air-knife drying, prevents mineral residues from embedding in the phosphor layer during later screen printing. RoHS Directive 2011/65/EU must be considered for the finished EL lamp assembly; the etchant itself is selected to leave no regulated heavy-metal residue on the flexible substrate. Terminal products include backlit membrane switches, automotive dashboard indicators, and portable appliance keypads where the patterned silver electrodes must survive repeated mechanical flexing and accelerated aging at 60 °C/90 % RH for 500 h without visible migration.

Production-scale EL lamp tanks typically use a single-pass conveyorised spray configuration with etchant sump filtration through 0.2 µm absolute-rated cartridges. The filtration requirement is driven not by the silver itself but by the phosphor dielectric: particulate carryover from the etched silver lines into the subsequently printed dielectric layer produces local capacitive breakdown visible as dark spots after energising at 100–200 Vrms and 400–1000 Hz. A limitation of the wet etch process on ITO-coated PET is the absence of a self-limiting chemistry at the ITO surface; the line speed and nozzle overlap must therefore be qualified for each new lot of ITO film because sheet-resistance drift in the incoming ITO shifts the galvanic potential at the ITO-silver interface and alters the effective etch rate. Published data for this specific configuration is limited, but the industrial practice of segregated rinsing and neutralisation before waste discharge is standard.

Roll-to-roll patterning of silver nanowire transparent conductive films for projected-capacitive touch sensors places the etch step between film coating and UV-cured overcoat deposition. The etchant is applied in a web cleaner with gravure or slot-die coating and immediately quenched by a two-stage deionized water rinse; line speeds in production are commonly held between 0.5 m/min and 3.0 m/min, but the speed window narrows when sheet resistance after patterning must remain at 30–100 Ω/sq. For a 20–40 nm silver nanowire film on polyethylene terephthalate, the wet etch must remove all nanowire material from non-electrode areas while leaving no silver chloride or silver oxide residue that can act as a nucleation site for electrochemical migration under 5 V DC bias. The ferric-nitrate-based bath is replenished by controlled specific-gravity dosing; dissolved silver accumulates in the bath and is removed by plate-out cells on the recirculation loop to keep the etch rate within the qualified ±10 % band. After etching, the transparent conductive film is tested according to ASTM D1003-21 for haze and luminous transmittance, ASTM F390-21 for sheet resistance uniformity, and ASTM D3359-23 for adhesion of any subsequent hard-coat layers. The finished capacitive touch sensor stacks require a post-etch sheet resistance uniformity of better than ±8 % across a 1.5 m web width; batch-to-batch variation in nanowire diameter and binder adhesion is a larger contributor to nonuniformity than etchant temperature drift in closed-loop systems.

When Ag/Ni Mirror Contacts Are Patterned After GaN LED Metallization

After ohmic mirror deposition on GaN light-emitting diode wafers, a photoresist-masked etch removes the exposed Ag/Ni or Ag/Ni/Au stack while preserving the ohmic contact interface beneath the mask. In this application the etch chemistry must distinguish between the Ag layer, which dissolves readily in ammoniacal peroxide mixtures at pH 9–11, and the Ni layer, which requires a nitric-acid-based formulation to clear the field without roughening the underlying GaN. Immersion etching in cassette-to-cassette wet benches with megasonic agitation at 800–1200 kHz is common, but single-wafer spray tools are favoured when the wafer diameter exceeds 150 mm and edge-spec undercut must remain below 2 µm. The critical endpoint is the Ag/Ni interface: overetching the Ni layer after Ag removal initiates galvanic corrosion at the mirror edge because residual Ag ions in the bath redeposit on exposed Ni and create a local Ag-Ni cell that increases contact resistance in subsequent thermal budget tests. Control strategies include separate Ag and Ni etch tanks, point-of-use silver removal columns, and a final deionized water rinse with 18 MΩ·cm resistivity. The processed LED wafers are then measured for forward voltage shift; a shift of more than 0.1 V at 350 mA drive current is taken as evidence of mirror edge damage. Published data for this specific Ag/Ni configuration is limited, but the general requirement of a halogen-free Ag etch and a low-chloride Ni etch is consistent with compatibility data for GaN contact metallurgies.

The main operational boundary on an LED line is the sensitivity of the exposed GaN surface to alkaline solutions after the Ag etch has cleared. Even brief contact with ammoniacal solutions can alter the surface stoichiometry and increase the specific contact resistance of subsequent passivation steps; therefore, the wafer must be transferred within 15 s from the Ag etch chamber to a deionized-water quench. In high-volume production, this transfer constraint is met by automated robots rather than manual cassettes, and nitrogen blow-off is installed before the Ni etch module to prevent drag-over of ammonia into the acidic bath, where ammonium nitrate formation would shift the Ni etch rate. The Ni etch itself is intentionally operated at the lower end of the aggressive range, often 20–30 °C, because exposed GaN and InGaN layers are chemically reactive toward acidic Ni etch chemistries. Qualification lots on 100 mm sapphire substrates are etched with dummy wafers and measured by atomic force microscopy for surface roughness increase; an increase above 5 nm root-mean-square is grounds for bath replacement or recipe adjustment.

Thick-film Ag and Ni/Ag multilayer conductors on 96 % alumina and low-temperature co-fired ceramic substrates require an entirely different etching sequence from thin-film silicon and polymer web processes. The etchant is used after laser ablation or screen-printing to clear the fired Ag/Ni residue from via capture pads and to sharpen conductor edge definition before electroless nickel/immersion silver finishing. Ceramic substrates absorb liquid from the etch bath; if the rinse is not completed within 10 min, the residual acid migrates along the ceramic grain boundaries and causes pad delamination after thermal cycling. Production baths are therefore run at lower acid concentration than semiconductor-grade formulations, with a specific gravity of 1.05–1.15, and a cascading overflow rinse using deionized water at 40–50 °C is used to reduce viscosity and improve desorption from porous alumina. The terminal RF resonator or power module substrate is tested for conductor adhesion by wire-pull testing after wire bonding, and for surface leakage current under 85 °C/85 % RH bias. A limitation of the wet etch route on co-fired ceramic is that it cannot compensate for screen-printing thickness variation; the etch clears the thin edge region faster than the centre and can remove 1–3 µm of conductor from the pad periphery. Published data for specific etching rates on co-fired Ag/Ni conductors is limited, and qualification runs are required for each ceramic lot.

Residue Ionic Contamination Determines UHF Antenna Edge Quality

In print-and-etch manufacturing of silver RFID antennas and sensor electrodes on polyimide or PET, the etchant functions as a pattern refinement step after screen printing or gravure of Ag flake pastes, removing inter-finger bridges and smoothing line edges that would otherwise cause capacitive coupling or corona discharge in high-frequency operation. The line is composed of a multi-chamber spray module, a neutralisation stage, and a final deionized-water rinse with conductivity monitoring; the etchant must be free of non-volatile ions because potassium, sodium, and chloride residues at single-digit ppm levels reduce the surface insulation resistance of the flexible substrate below the 1.0 × 10⁹ Ω threshold used in printed electronics qualification. Spray pressure in the etcher is constrained to 0.7–1.0 bar to avoid lifting the printed flake edges, while etchant temperature is held below 30 °C to prevent polyester film distortion. The finished antenna web is tested for sheet resistance, line width, and adhesion; ASTM F390-21 four-point probe measurements on the silver tracks verify batch-to-batch consistency after etching. A known limitation is that the isotropic nature of the wet etch removes sidewall material at the same rate as vertical material, so the minimum printed line width is set by the paste deposit thickness and etch time rather than by the screen mesh alone. For UHF RFID tags operating at 860–960 MHz, post-etch line edge roughness below 5 µm root-mean-square is necessary to prevent resonance frequency shift.

The process window for printed silver flake pastes is narrower than for sputtered or evaporated silver because the paste contains glass frit and organic binders that retard wetting at the flake-resin interface. If the etchant is too aggressive, it rapidly attacks the exposed flake surfaces but leaves undercut pockets beneath the organic binder, producing mechanical weakness at the edge of the printed line. If the etchant is too mild, inter-finger bridges remain and the antenna capacitance rises above the reader-modulation threshold. A two-step sequence is therefore used in some flexible hybrid electronics lines: a mild acid pre-dip removes the oxidized silver surface, followed by the main etchant at a lower oxidizer concentration. The printed substrate must then be rinsed within 20 s of leaving the main etch chamber to prevent residual silver thiosulfate complexes from drying into non-conductive stains.