Application of Chelating Cleaning Solution for Semiconductors Electronic/EL Grade
How Does Chelation Control Post-CMP Copper and Tungsten Residues in Sub-10 nm Dual-Damascene Clearing?
In copper/low-k dual-damascene interconnect fabrication, post-chemical mechanical planarization cleaning is the first unit operation where metallic contamination is thermodynamically fixed by slurry chemistry. Residual glycine, benzotriazole and colloidal silica form a mixed organic–inorganic film on copper lines. Tungsten plug loss and oxide erosion during barrier clearing release tungstate anions. A chelating cleaning solution formulated with 2.0–3.5 wt% citric acid and 0.5–1.2 wt% diglycolic acid in ultrapure water is metered into a brush scrubber at 22–25 °C. The pH is buffered between pH 3.0 and pH 4.2 with ammonium hydroxide. This range keeps Cu²⁺ solubility high enough for complexation while limiting copper dissolution below 0.5 nm/min on blanket wafers. Chelator stoichiometry is maintained at a molar excess of 3:1 over total Cu²⁺ plus Fe³⁺, with periodic titration using inductively coupled plasma mass spectrometry. PVA sponge rollers and soft pad backside cleaning operate at 80–120 rpm. Process water quality follows ASTM D5127 Type E-1.2 resistivity and total organic carbon specifications. Drying is performed with an IPA vapor dryer under 40–60 °C nitrogen. Defect inspection on a KLA Surfscan SP5 at 90 nm threshold rejects lots above 150 defects per 300 mm wafer. The sequenced application of chelating chemistry before final rinse reduces Cu-BTA residue that would otherwise decompose in subsequent low-k dielectric cap deposition. For sub-10 nm nodes, contact resistance measurements at test structures formed after cobalt cap deposition show sensitivity to residual tungsten oxides. The chelating solution is therefore qualified for simultaneous tungsten residue removal by extending brush dwell time to 35–45 seconds. Published data for this specific configuration is limited. Process qualification typically uses three-wafer split lots on polished copper blanket and patterned dual-damascene wafers, with total reflection X-ray fluorescence metal mapping at 1E10 atoms/cm² detection limits. Terminal products include logic application-specific integrated circuits, mobile processors and baseband modems fabricated with copper interconnect at 7 nm, 5 nm and 3 nm design rules.
FEOL Surface Conditioning Prior to High-k Atomic Layer Deposition
In front-end-of-line transistor manufacturing, metallic contamination on the silicon surface before high-k atomic layer deposition affects equivalent oxide thickness and channel mobility. Traditional SC-1 and SC-2 cleaning sequences remove particles but leave trace aluminum, iron and zinc from the cleaning bath. A dilute electronic-grade chelating cleaning solution based on ethylenediaminetetraacetic acid tetrasodium salt and triethanolamine at 0.1–0.5 wt% total chelator is applied in a single-wafer spin processor after diluted hydrofluoric acid etching. The pH is adjusted to pH 6.5–7.5 with ammonium hydroxide to avoid excessive surface roughening. Spin speed is ramped from 800 rpm during dispense to 1800 rpm during rinsing. Contact time is limited to 30–60 seconds. Chelation of Fe³⁺ and Al³⁺ occurs through hexadentate ligand coordination, forming soluble complexes that are removed by centrifugal force. The process is monitored by vapor phase decomposition inductively coupled plasma mass spectrometry with recovery standards at 5 ppb for iron, aluminum, nickel, copper and zinc. Gate oxide integrity requires metallic contamination below 1E10 atoms/cm². Surface roughness is checked by atomic force microscopy over a 1 µm × 1 µm scan area. The chelating solution must meet SEMI F63 metal contamination limits for approved semiconductor process chemicals. In some fabrication lines, ozonated water is combined prior to chelation to regenerate surface oxide and improve ligand accessibility. The process chamber temperature is held at 23 °C to prevent ammonia loss. Batch-to-batch variation in chelator active concentration is controlled by conductivity and refractive index. Terminal devices produced after this clean include fin field-effect transistors, gate-all-around nanosheet transistors and dynamic random access memory access transistors with high-k metal gate stacks.
In silicon wafer reclaim operations processing 300 mm test wafers, metallic residues from prior polishing, copper electrolysis and photoresist ashing must be removed before re-polishing. The chelating cleaning solution is used in an immersion batch tank at 50–60 °C with recirculation filtration at 0.1 µm absolute rated polypropylene filters. The working bath contains 1.5–2.5 wt% tetramethylammonium hydroxide and 0.5 wt% ethylenediaminetetraacetic acid disodium salt. Ultrasonic agitation at 40–80 kHz is applied for 10–15 minutes. The high pH of pH 11.5–12.5 lifts organic residues, while the chelator binds aluminum, copper and iron released from backside etching. Wafer cassettes are oscillated vertically at 5–10 mm/s. After chelation, wafers are rinsed in cascading ultrapure water baths and dried with a Marangoni dryer. Bath exhaustion is monitored by inductively coupled plasma mass spectrometry for copper, iron, aluminum and manganese. The bath is dumped when total metal concentration exceeds 50 ppb. This control prevents redeposition of metal hydroxides onto wafer surfaces. Reclaimed wafers are subsequently double-side polished and inspected by surface particle counters. Wafers that meet SEMI M1 flatness and surface roughness specifications are returned as monitor wafers for lithography track qualification, diffusion furnace contamination checks and wet bench particle testing. Some reclaim lines blend the chelating solution with hydrogen peroxide at 0.5–1.0 wt% to oxidize residual copper. The terminal use for such wafers excludes device production but includes process monitoring, equipment requalification and thickness uniformity mapping in chemical vapor deposition and electrochemical deposition tools.
BEOL Low-k Damage Containment and Organosilicate Porosity Effects
For back-end-of-line structures using porous organosilicate low-k dielectrics with k-value below 2.4, post-etch residue removal must not degrade film density or create moisture adsorption. A chelating cleaning solution modified with 10–20 wt% ethylene glycol ether is dispensed on a single-wafer resist strip tool. The pH is held at pH 4.0–4.8. The chelating agent, typically 0.2–0.8 wt% methylglycinediacetic acid or ethylenediaminedisuccinic acid, complexes copper and titanium residues without decomposing the methyl-silsesquioxane matrix. Solvent fraction reduces surface tension below 35 mN/m, preventing capillary collapse in pores with mean diameter below 2 nm. The plasma-induced damaged layer is removed by a preceding 0.25% HF dip. Chelation is then applied to prevent residual metal re-adsorption. Precise chelator-to-metal ratio is controlled by monitoring rinse effluent with ultraviolet absorbance. Low-k damage is measured by ellipsometric porosimetry before and after cleaning. Acceptable shift in refractive index is 0.003 or less. Adhesion of subsequent tantalum/tantalum nitride barrier and copper seed is tested per ASTM D3359 Class 5B. The process is qualified on patterned dielectric wafers with line spacing at 40 nm pitch. If chelator concentration exceeds 1.0 wt%, copper is over-complexed and the dielectric surface becomes hydrophilic, causing copper seed voiding. Titration and pH monitoring are therefore interlocked with the chemical delivery pump. Terminal products include high-performance server processors, graphic processing units and networking switches using dense low-k copper interconnects.
After chrome hard mask etching on photomask blanks, trace-metal contamination from chromium, molybdenum and iron can cause printable defects in 193 nm and extreme ultraviolet masks. A chelating cleaning solution for photomask substrates is applied in a spin rinse tool with quartz chucks and chemical-resistant EPDM seals. The product is diluted with ultrapure water at 1:5 to 1:10. The working temperature is 20–23 °C. The formulation contains sulfonic acid, oxalic acid and a sodium-free chelant. Total chelant content is 0.1–0.3 wt%. The pH is kept below pH 2.0 to dissolve metal oxides while the chelator prevents redeposition onto quartz. Spin speed is set to 1200 rpm for dispense and 2400 rpm for drying. Contact time is 45–90 seconds. Ultrapure water rinse resistivity is monitored to 18.2 MΩ·cm. The process is followed by hot sulfuric acid-peroxide cleaning if organic haze remains. Haze level is measured with an inspection system at 70 nm pixel size. Photomask blanks with haze above 0.02 ppm are rejected. For extreme ultraviolet mask substrates with ruthenium-capped multilayer stacks, the chelating solution is screened for absorbance at 13.5 nm wavelength. Published data for this specific configuration is limited. Blank suppliers generally qualify cleaners by film thickness loss, contact angle and particle adder data. Terminal products include 248 nm and 193 nm binary masks, attenuated phase-shift masks and extreme ultraviolet mask blanks used in high-volume wafer lithography.
When Citric Acid–DTPA Blends Replace Ammonium Peroxide Mixtures in Bump and TSV Cleaning
In wafer-level packaging, copper pillars, lead-free solder bumps and through-silicon vias create different metal corrosion risks for cleaning chemistry. Ammonium peroxide mixtures remove organic flux but aggressively etch copper. A chelating cleaning solution based on citric acid and diethylenetriaminepentaacetic acid at 1.0–2.0 wt% total active agent is applied after dry film resist stripping and before under-bump metallization deposition. The pH is adjusted to pH 5.0–5.8 with potassium hydroxide or tetramethylammonium hydroxide. At this pH, Cu₂O and SnO₂ dissolve slowly, but the DTPA ligand binds Cu²⁺, Sn²⁺ and Pb²⁺, preventing redeposition. The process is run in a batch spray processor with chemical recirculation at 30–40 °C. Continuous filtration through 0.2 µm PTFE membranes removes suspended residues. The cleaner is compatible with photosensitive polyimide and polybenzoxazole dielectrics when exposure time is kept below 3 minutes. Bump shear strength is tested per JEDEC JESD22-B117A. Post-clean surface roughness on copper pillars is measured by laser confocal microscopy. A root mean square roughness increase of less than 5 nm is required. Titanium-tungsten adhesion layers are not attacked due to chelator selectivity. If the bath pH drifts above pH 6.2, tin oxide redeposition can occur on sidewall passivation. The bath is monitored by acid-base titration and inductively coupled plasma optical emission spectroscopy.
Compliance matrix for a high-volume bump cleaning line| Parameter | Control range | Analytical method / standard |
|---|
| Chelator concentration | 1.0–2.0 wt% | ASTM D5127 / acid-base titration |
| Working pH | pH 5.0–5.8 | ASTM E70 |
| Copper in bath | ≤ 10 ppm | ICP-MS per SEMI C63 |
| Bump shear | Pass / fail | JEDEC JESD22-B117A |
| Roughness increase | < 5 nm | ISO 25178 laser confocal microscopy |
| Surface metal contamination | < 1E10 atoms/cm² | VPD ICP-MS per SEMI F63 |
Terminal products include high-density flip-chip ball grid array packages, chip-scale packages, interposers and high-bandwidth memory stacks.
Managing Isoelectric Point Drift and Metal Residues in Silicon MEMS Release Cleaning
In microelectromechanical systems manufacturing, sacrificial oxide release in vapor HF leaves aluminum, copper and alkali residues that affect resonator performance. A chelating cleaning solution formulated with tetramethylammonium hydroxide, sorbitol and ammonium citrate is used after vapor HF release. The pH of pH 9.0–10.0 prevents stiction by maintaining negative zeta potential on silicon surfaces. The solution is dispensed in a single-wafer megasonic tool at 25 °C. Megasonic energy at 928 kHz and 0.5–2.0 W/cm² dislodges residues from comb structures. Chelation of aluminum prevents precipitation of Al(OH)₃ on released microstructures. The solution is compatible with thin-film aluminum nitride and silicon nitride because chelator concentration is kept below 0.8 wt%. Process time is limited to 60–120 seconds. The wafer is then rinsed with isopropyl alcohol and dried with supercritical carbon dioxide to avoid capillary stiction. Resonator quality factor is measured before and after cleaning using laser Doppler vibrometry. Any shift greater than 5% indicates residual contamination or surface charge damage. Terminal products include accelerometers, gyroscopes, pressure sensors, micro-mirrors and silicon microphones used in automotive, medical and consumer electronics.
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Model CCS-EL-9X is an aqueous chelating cleaning solution supplied at semiconductor Electronic/EL grade for post-etch and post-chemical mechanical planarization residue removal on copper, tungsten, and cobalt interconnect structures. The product is compounded from polyaminocarboxylic acid chelating agents and ammonium hydroxide, filtered through a 0.05 µm polytetrafluoroethylene membrane, and filled into perfluoroalkoxy alkane containers under ISO 14644-1 Class 4 cleanroom conditions. As supplied, pH is 3.8 ± 0.2, specific gravity is 1.06 ± 0.01 at 25 °C, and the product contains no added hydrofluoric acid, no added hydrogen peroxide, and no non-ionic surfactant. Lot-release testing includes inductively coupled plasma mass spectrometry per ASTM D5673, ion chromatography per ASTM D4327, and particle counting per ISO 21501-4. The solution is intended for point-of-use dilution with ultrapure water at volume ratios from 1:10 to 1:50 in immersion wet benches or single-wafer spin processors.
Product Model and Lot-Release Specification Matrix
| Parameter | Acceptance Criterion | Test Method or Equipment |
|---|
| Chelator active content | 18.0–22.0 wt% | Potentiometric titration against 0.1 M Cu(NO₃)₂ |
| pH as supplied | 3.6–4.0 | Glass electrode per ASTM E70 |
| Specific gravity at 25 °C | 1.05–1.07 | Oscillating U-tube density meter per ASTM D4052 |
| Chloride | ≤ 100 ppb | Ion chromatography with conductivity detection per ASTM D4327 |
| Sulfate | ≤ 100 ppb | Ion chromatography per ASTM D4327 |
| Individual trace metal | ≤ 10 ppb | ICP-MS, matrix matched, per ASTM D5673 |
| Total trace metals | ≤ 50 ppb | ICP-MS, sum of 18 elements |
| Particles ≥ 0.1 µm | ≤ 50 particles/mL | Light-scattering particle counter per ISO 21501-4 |
| Particles ≥ 0.2 µm | ≤ 10 particles/mL | Light-scattering particle counter per ISO 21501-4 |
Trace-metal limits are lot-release maxima, not typical values. Certificate-of-analysis data are supplied with each lot, and retained samples are held for 24 months. Sodium and potassium are individually controlled to ≤ 5 ppb to minimize mobile-ion contamination at gate dielectric interfaces. Wetted packaging and distribution components conform to SEMI F57 polymer compatibility requirements. The formulation is ammonia-buffered, not alkali-metal buffered, so no potassium or sodium salt is introduced during pH adjustment.
Point-of-use dilution is performed through a chemical blending module with PTFE/PFA wetted surfaces and a 0.02 µm point-of-use filter. In single-wafer cleaning, the diluted solution is dispensed through a 950 kHz megasonic nozzle at 0.6–1.2 L/min for 45–90 s at wafer surface temperature 25–45 °C. The recommended starting dilution is 1:20 by volume. In immersion wet benches, the bath is held at 35 °C and recirculated at 30 L/min through a 0.05 µm PFA filter; contact time ranges from 5 min to 8 min. Post-rinse with ultrapure water at 40 °C or higher removes ligand-metal complexes and prevents carryover into subsequent sulfuric acid/hydrogen peroxide strip baths.
What Differentiates the Chelation Mechanism from Fluoride-Containing Cleaners?
In fluoride-containing cleaning formulations, the primary surface loss mechanism is conversion of titanium nitride hard masks, tantalum/tantalum nitride barriers, and titanium-containing liners to hexafluorometallate species. That etch reaction continues at low process pH and narrows the process window for high-aspect-ratio features. The chelating solution generates no free fluoride; its cleaning action depends on multidentate ligand coordination of ionic contaminants. Polyaminocarboxylic acid ligands of the type used in this product exhibit copper(II) complex stability constants in the range log β 16–20 and iron(III) stability constants in the range log β 22–26 in published thermodynamic compilations. This thermodynamic driving force is sufficient to solubilize metal ions without measurable oxide etching of dense SiCOH or TiN after 30 min immersion at 35 °C when thickness is monitored by X-ray reflectivity. Dilute mineral acid cleaners, by contrast, reduce pH below 2.0 and induce copper surface roughening and galvanic corrosion at noble-metal interfaces. Semi-aqueous solvent cleaners remove organic residues but do not complex free metal ions and may leave benzotriazole films intact.
When Post-CMP Copper and Benzotriazole Residues Remain
Post-CMP wafers carry silica or ceria abrasive particles, organic additives, and benzotriazole-passivated copper. The benzotriazole-copper film is difficult to remove with dilute organic acids because its solubility remains low below pH 4.0; the chelating agent competes for surface copper and destabilizes the benzotriazole film. The recommended condition is 1:20 dilution, pH 4.2 ± 0.2, 35 °C, and 6 min immersion or 60 s single-wafer spray, followed by a 2 min ultrapure water rinse at 40 °C. On production-scale immersion tools, the bath is recirculated at 30 L/min through a 0.05 µm PFA filter; low-flow dead zones above wafer cassettes reduce particle removal efficiency and require periodic flow mapping. If pH exceeds 5.0, benzotriazole redeposition on copper increases; if pH drops below 3.6, copper surface roughness increases. Published data for this specific configuration is limited.
Particle redeposition in recirculated immersion baths is controlled by maintaining pH at 4.2 ± 0.2, which places silica particles above their isoelectric point near pH 2.0 and ceria particles below their isoelectric point near 6.5–7.0. Zeta potential for silica under these conditions is typically -20 mV to -40 mV by electrophoretic light scattering per ISO 13099-1, and the resulting electrostatic repulsion reduces particle adhesion to the wafer surface. In-line particle monitoring between the filter and process tank triggers a system alarm when cumulative counts at ≥ 0.1 µm exceed 100 particles/mL. A point-of-use filter with 0.05 µm rating on the recirculation loop removes agglomerated slurry particles; filter change-out is based on differential pressure increase above 0.2 MPa or 24 h, whichever occurs first.
Accumulated copper loading in the immersion bath establishes the primary bath-life limit. When dissolved copper concentration exceeds 2 ppm as determined by ICP-MS per ASTM D5673, cleaning rate declines and particle adhesion increases. The bath is replaced when particle counts at ≥ 0.1 µm exceed 100 particles/mL or after 8 production hours, whichever occurs first. The solution is incompatible with strong oxidizing acids, including hot nitric acid; mixing generates heat and may degrade the chelating ligand. It is also incompatible with cationic coagulant polymers used in wastewater treatment; neutralization should use anionic flocculants. Storage must be maintained in vented containers at 15–25 °C. Freeze-thaw cycling may cause phase separation; if freezing occurs, the material should be discarded rather than rehomogenized.
Compatibility Limits with Low-k Dielectrics and Capping Layers
Low-k organosilicate glasses and porous SiCOH dielectrics are sensitive to acidic chemistries that hydrolyze Si—O—Si bonds. The product after 1:20 dilution has pH 4.2 ± 0.2 and contains no surfactant micelles. For lot acceptance, a dense SiCOH coupon is immersed for 30 min at 35 °C; allowable thickness loss is 0.1 nm by X-ray reflectivity, and allowable k-value shift is 0.1 by capacitance-voltage measurement at 1 MHz. Cobalt and tungsten witness films are subject to the same 0.1 nm thickness-loss criterion. Avoid use on aluminum bond pads except at 1:50 dilution and contact time below 2 min; aminopolycarboxylates can chelate aluminum at low pH. The solution is not recommended for exposed tin-silver solder bumps; published data for this configuration is limited.
On a 300 mm single-wafer cleaning platform, sequential exposure to 1:20 chelating solution and then 0.5% hydrogen peroxide in ultrapure water can remove copper and organic residues in a 60 s main dispense step before final rinse. The hydrogen peroxide step must not be mixed in the same chemical line as the chelating solution because oxidation of the ligand reduces copper binding capacity. Separate PFA delivery lines and dedicated point-of-use filters are required. Rinse water conductivity below 0.1 µS/cm after processing confirms removal of ionic cleaning residues.
