Copper Pillar Resist Removal Across Fine-Pitch Flip-Chip Wafers

Electroplated copper pillar interconnects require thick positive-tone novolak/diazonaphthoquinone resists patterned at 40 μm to 120 μm thickness. The stripping step follows acidic copper sulfate electroplating, which induces a cross-linked crust at the resist surface and leaves cupric ion residues within the resist bulk. A working bath for this application is prepared by metering an EL-grade solvent concentrate into 18.2 MΩ·cm deionized water at 1:3 to 1:4 dilution. Published formulations for copper-compatible stripping concentrates commonly contain 20 wt% to 35 wt% dimethyl sulfoxide, 1 wt% to 3 wt% tetramethylammonium hydroxide, 0.5 wt% to 1.5 wt% alkanolamine, and the balance water. The dissolved copper concentration must be held below 30 ppm in single-wafer spray recirculation loops to prevent re-deposition on exposed Cu pillar sidewalls.

Processing is conducted on 300 mm single-wafer spray platforms with dispense temperature maintained at 70 °C ± 2 °C. A deviation above 74 °C shortens bath life through exothermic degradation of the quaternary ammonium hydroxide, while a deviation below 66 °C leaves resist residues at the pillar base. Dispense rate is set between 1.0 L/min and 1.4 L/min, with chuck rotation from 800 rpm to 1,200 rpm. The puddle dwell time is 120 s to 180 s. After stripping, deionized water rinsing at 22 °C for 90 s removes dissolved resist fragments. Drying uses filtered nitrogen at 40 psi with an oxygen concentration below 1 vol% to limit copper oxidation.

The terminal structure is a copper pillar bump with diameter 20 μm to 80 μm, height 40 μm to 100 μm, and pitch 40 μm to 150 μm. A critical failure mode observed on production lines is incomplete stripping at the resist foot adjacent to the Cu seed layer because the spray shadowing effect reduces local solvent replenishment. This is corrected by lowering chuck acceleration and adding a low-speed backside rinse step. Copper corrosion is suppressed by an aromatic inhibitor in the concentrate; open-circuit potential shift measured on a Cu coupon should remain below 50 mV versus the as-received coupon. Workpiece exposure above 10 min at process temperature is not recommended because inhibitor desorption from high-purity copper increases surface roughness.

Vacuum degassing of the working bath before dispense removes dissolved oxygen to below 2 ppm, reducing metal-assisted oxidation. Metal impurity control follows EPA Method 6020B by inductively coupled plasma mass spectrometry. Each cation in the as-supplied EL-grade concentrate is controlled to 10 ppb maximum. Particle loading is limited to 100 counts/mL for particles at or above 0.2 μm. Filtration through a 0.1 μm polytetrafluoroethylene membrane upstream of the dispense nozzle prevents particulate transfer. Bath replacement is triggered by refractive index fall below 1.3700 or dissolved Cu above 30 ppm, whichever occurs first.

In redistribution-layer patterning, the strip module follows a copper/titanium seed etch. Residues entering the stripper are not simple photoresist fragments but an agglomerated etch polymer containing copper chlorides, titanium fluoride species, and oxidized novolak. The EL-grade stripper used after seed etch is diluted 1:5 with ultrapure water to reduce solvent viscosity and improve penetration into narrow lines. A typical bath proportion is 12 wt% to 18 wt% aprotic solvent, 0.5 wt% to 1 wt% fluoride ion source, and 3 wt% to 5 wt% corrosion inhibitor. Fluoride concentration is verified with an ion-selective electrode and held between 0.08 wt% and 0.12 wt% to dissolve titanium fluoride residue without undercutting the titanium barrier layer.

The process runs in immersion wet benches with megasonic agitation at 950 kHz and 0.12 W/cm². Liquid temperature is maintained at 50 °C ± 1 °C for 6 min. The narrow temperature window is required because titanium fluoride dissolution rate doubles above 54 °C, causing adhesion loss at the Cu/Ti interface. After rinse-dry, the wafer proceeds to dielectric curing at 200 °C. The terminal redistribution layer has line width/space dimensions of 5 μm/5 μm to 2 μm/2 μm and copper trace thickness from 3 μm to 8 μm. Adhesion is verified by tape pull test after barrier metal etch; adhesion loss greater than 10 % by area is considered a strip bath chemistry fault. Bath pH is controlled at 6.8 to 7.2 because acidic drift increases Ti attack while alkaline drift destabilizes dissolved fluoride complexes.

Why Does Post-Electroplating Residue Retain Tin After SnAg Bump Formation?

Dry film resist removal after SnAg electroplating presents a selectivity problem that does not exist in copper pillar processing. SnAg solder bumps are electroplated from methanesulfonic acid baths containing tin methanesulfonate and silver methanesulfonate. The dry film resist, commonly 40 μm to 80 μm thick, absorbs plating additives and retains tin ions after the wafer exits the electroplating cell. Alkaline stripping formulations dissolve the dry film rapidly but also etch the SnAg surface if the bath potential rises into the transpassive region. A tin-selective stripper for this application uses a working dilution of 1:3 and a controlled pH of 11.0 ± 0.3. The bath comprises 15 wt% to 20 wt% dimethyl sulfoxide, 2 wt% to 3 wt% quaternary ammonium hydroxide, 1 wt% to 2 wt% imidazole-based passivator, and ultra-pure water.

Processing is performed at 65 °C for 90 s to 150 s in single-wafer spray equipment. A puddle process with no mechanical brush is preferred because solder bumps are ductile and deform under contact scrubbers. The imidazole passivator adsorbs on tin and silver, maintaining SnAg etch rate below 0.05 μm/min during the strip dwell. If the bath temperature rises above 68 °C, tin etch increases above 0.15 μm/min and solder morphology degrades. If the bath drops below 62 °C, dry film residue remains between fine-pitch bumps. After stripping, the wafers are rinsed in 18.2 MΩ·cm deionized water with carbon dioxide injection to reduce pH shock and prevent tin oxide redeposition.

Terminal solder bumps have diameters from 60 μm to 120 μm, heights from 50 μm to 90 μm, and pitches from 100 μm to 200 μm. The as-stripped solder surface is measured by X-ray photoelectron spectroscopy for carbon and oxygen reduction. Carbon surface concentration below 20 at% after strip is considered acceptable for flux wetting in the subsequent reflow step. Silver content at the surface must remain within the alloy composition of 1.0 wt% to 4.0 wt% Ag. A production line failure mode is silver surface enrichment above 6 at%, which occurs when the passivator is depleted through drag-out. To maintain passivator concentration, the bath is analyzed by ultraviolet-visible spectroscopy at 282 nm and replenished when absorbance falls below the setpoint corresponding to 1.0 wt% active imidazole.

Through-silicon via reveal leaves a plasma-damaged resist shell along the via rim and embedded silicon dust inside the via. The stripping step after via reveal is required to remove this shell before barrier deposition and copper fill. Because the via aspect ratio is 8:1 to 12:1, wetting is limited by capillary pressure and dissolved gas entrapment. The EL-grade stripper is pre-wetted with a low-foaming nonionic surfactant to reach a surface tension of 29 mN/m to 33 mN/m at 60 °C. Working bath concentration is 10 wt% to 14 wt% aprotic solvent, 1 wt% to 2 wt% tetraalkylammonium hydroxide, and 5 wt% to 8 wt% glycol ether. The bath is degassed under vacuum to dissolved oxygen below 1 ppm before wafer immersion.

Immersion processing uses a single-bath tool with pulsed megasonic energy at 1.0 MHz for 10 min at 60 °C ± 1 °C. The megasonic pulses create acoustic streaming that promotes exchange inside the via. After the strip, a cascade rinse with hot deionized water at 55 °C removes redeposited silicon particles. The terminal via dimensions are 5 μm to 10 μm diameter and 50 μm to 100 μm depth. In-line particle inspection after dry is performed on the via field; particle defects above 50 counts/wafer traced to silicon dust require bath regeneration. Residual solvent inside the via is measured by thermal desorption–gas chromatography and must be below 0.1 ng/via before chemical vapor deposition barrier layer deposition.

When Fluoride-Bearing Strippers Are Required for UBM Seed-Layer Breakthrough

Under-bump metallization stacks of titanium/copper or titanium-tungsten/copper create inorganic residues that resist conventional alkaline solvent stripping. A fluoride-bearing EL-grade stripper is introduced after seed layer etch to remove titanium fluoride residues, tin oxide, and copper chloride without attacking the underlying aluminum pad. The working solution is mixed at 1:4 and contains 0.2 wt% to 0.3 wt% free fluoride, 10 wt% to 15 wt% dimethyl sulfoxide, and 2 wt% to 4 wt% buffering agent to stabilize pH at 6.5 ± 0.2. Free fluoride is monitored with a fluoride ion-selective electrode every 4 h. A drift above 0.35 wt% causes aluminum pad roughening; a drift below 0.15 wt% leaves Ti residue at the UBM edge.

The residual UBM strip is executed in a spray processor at 45 °C for 3 min to 5 min. Low temperature is required because fluoride attack on aluminum accelerates above 50 °C. The etch amount on an aluminum 99.5 wt% coupon must not exceed 0.02 μm/min under process conditions. Terminal bump structures use UBM layers of 0.1 μm Ti and 0.3 μm Cu before nickel plating. Sheet resistance on exposed Cu after strip changes less than 5 % compared to pre-etch baseline. Adhesion loss at the UBM edge is inspected by die shear after bumping; a shear strength below 20 N on a 80 μm bump indicates incomplete residue removal or over-etch of the tie layer.

Panel-level fan-out substrates processed in 600 mm × 600 mm formats have non-uniform thermal history and higher warpage than silicon wafers. The stripping step for organic interposers uses a lower temperature and a higher solvent fraction to avoid delamination of the molded area. A typical working bath for panel dry film resist strip is 1:3 dilution with 15 wt% to 20 wt% aprotic solvent, 3 wt% to 5 wt% alkanolamine, and the balance deionized water. No fluoride is included because the panel substrate lacks Ti or TiW. Processing is carried out in a horizontal conveyorized spray chamber at 55 °C for 120 s to 180 s. Panel bow is controlled below 5 mm across the 600 mm diagonal by temperature ramping below 2 °C/min.

After solvent stripping, the panel is flooded with 18.2 MΩ·cm deionized water at 40 psi to remove swollen resist fragments from narrow via openings. Terminal dielectrics on panel-level fan-out packages are epoxy-based build-up films with via openings 30 μm to 60 μm in diameter. Post-strip desmear is not required if the stripper is replenished before refractive index falls below 1.3750. A field failure mode is residual resist at panel edges where spray shadowing occurs. This is addressed by edge conveyor speed compensation and additional top-side spray bars positioned at 15° incidence to the panel surface. Particle contamination is controlled by in-line filtration at 0.2 μm and cleanroom operation per ISO 14644-1:2015 Class 5. All wetted surfaces in the dispense system are specified to SEMI F57 for ultrapure water and liquid chemical distribution compatibility.

Temporary Bonding Adhesive Removal Once Laser Release Is Completed

Laser debond separates a thinned device wafer from its carrier, leaving a carbonized adhesive residue and partially ablated release layer on the device backside. The cleaning formulation differs from photoresist stripping because the residue is a voided polyimide or polyvinyl alcohol-based temporary bonding material. The EL-grade strip bath is prepared at 1:2 dilution for higher solvency, with 25 wt% to 30 wt% aprotic solvent, 5 wt% to 8 wt% alkanolamine, and 0.5 wt% surfactant. Bath temperature is held at 75 °C ± 2 °C. The higher temperature is required to swell the polyimide backbone, but it reduces bath life to 24 h because of solvent evaporation.

Single-wafer cleaning uses a soft-jet spray at 600 rpm with dispense rate 1.2 L/min for 6 min. The low rotation speed prevents fracture of thinned wafers, which are typically 50 μm to 100 μm thick. After the solvent dwell, a warm deionized water rinse at 50 °C removes re-solidified adhesive flakes. The cleaned wafer surface is inspected for adhesive residue by optical microscopy under dark field; residue larger than 5 μm is considered a strip defect. Backside metallization layers on thinned wafers are protected by pH control below 11.5. Aluminum metallization shows pit densities below 10 defects/cm² at the specified bath composition and time.