Ti Etchant Electronic/EL Grade is a low-sodium, electronic-grade wet chemistry formulation intended for the isotropic removal of sputtered, evaporated, or electroplated titanium and titanium-tungsten films in semiconductor, MEMS, and compound semiconductor manufacturing. The product is supplied under the designation TI-ELG-100 in 2.5 L and 20 L fluoropolymer containers. Each lot is filtered through a 0.05 µm PTFE cartridge prior to filling and is certified for 42 metal cations by ICP-MS, with a combined transition-metal ceiling of 500 ppb and individual alkali-metal limits of 50 ppb for sodium and 50 ppb for potassium. Typical physical property limits include a specific gravity of 1.115 to 1.125 at 25 °C and an appearance of clear to very pale amber liquid. The formulation is supplied as a ready-to-use mixture; dilution is not recommended because it shifts free fluoride activity and alters selectivity to underlying dielectrics.

In successive production lots, free fluoride concentration is controlled by ion chromatography to a tolerance of ±2% relative, and nitric acid concentration is maintained within 20.0 ± 0.8 wt%. This control reduces batch-to-batch variation in titanium etch rate to less than 5% for a 200 nm sputtered titanium film. The free acid values are balanced so that etching remains isotropic in immersion mode, with a lateral etch component of approximately 0.9 to 1.1 times the vertical etch depth. That relationship is important when defining metal lines of 5 µm to 50 µm width, where undercut must remain predictable.

What distinguishes Electronic/EL Grade from technical-grade titanium etchants?

The primary separation occurs in the contaminant, particulate, and packaging chain rather than in the bulk acid composition alone. A technical-grade etchant may contain sodium, iron, and calcium at 1 ppm to 10 ppm because it is often derived from industrial acid feedstocks and stored in unlined steel or standard polyethylene containers. In contrast, the Electronic/EL Grade material is blended in an ISO 14644-1:2015 Class 5 cleanroom and transferred through high-purity PFA lines to fluoropolymer drums that have been leached with ultrapure water. Metal extraction from the packaging is validated by a 7-day accelerated storage test at 40 °C; the acceptance criterion is an increase in total trace metals of less than 10 ppb.

Other titanium etchants based on ammonium fluoride and hydrogen peroxide are sometimes selected for their lower acidity, but those systems show a lower titanium etch rate at room temperature and a higher sensitivity to dissolved oxygen and cupric residues. For this nitric–hydrofluoric system, titanium removal is sustained by the formation of soluble fluorotitanate species while the nitric acid component regenerates the titanium oxide surface. This dual mechanism avoids the strong pH drift observed in peroxide-based baths after 120 min of continuous use. The chloride specification is also relevant: the product is formulated without chloride, sulfate, and phosphate ions, and chloride is held below 100 ppb. Industrial hydrofluoric acid feedstocks can contain chloride at 1 ppm to 20 ppm, which is sufficient to initiate galvanic pitting on exposed aluminum bond pads when titanium is etched above them.

On 150 mm and 200 mm immersion wet benches, the product is maintained at 22 °C to 28 °C with constant recirculation through a 0.2 µm PTFE cartridge filter at a flow rate of 12 L/min to 18 L/min. Cassettes are typically high-density PVDF with wafer spacing of 6 mm to 12 mm; the bath must be shielded from ambient light because ultraviolet exposure accelerates nitrate reduction and shortens bath life. Under these conditions, a fresh bath removes sputtered titanium at 28 nm/min to 32 nm/min and titanium-tungsten at 22 nm/min to 26 nm/min. Center-to-edge nonuniformity on a 200 mm wafer is held below 5% when the wafer rotates at 20 rpm during immersion. A recirculating chiller with proportional-integral-derivative control is required because a temperature deviation of ±2 °C changes the etch rate by approximately 10%.

Rinse is performed in ultrapure water meeting ASTM D5127-13 Type E-1 requirements, with a resistivity of 18.2 MΩ·cm at 25 °C. A post-etch rinse time of 30 s to 60 s is typical for 200 nm titanium films; shorter rinse times can leave fluoride residues that continue to etch titanium after transfer to the next bath. The etchant is compatible with standard positive photoresists for process times up to 10 min, but resist adhesion loss may occur at temperatures above 30 °C or when the bath is aggressively agitated. For liftoff processes, complete removal of residual solvent is verified before immersion; incomplete solvent removal can cause local precipitation of titanium-containing residues at the wafer edge.

Etch uniformity and bath aging under mixed-metal production loads

Etch rate decays as dissolved titanium accumulates because free fluoride is progressively consumed by the formation of stable TiF₆²⁻ complexes. Six equivalents of fluoride are required per titanium atom to form the soluble hexafluorotitanate species. At a dissolved titanium concentration of 2.0 g/L, the etch rate typically falls to 24 nm/min; at 3.0 g/L, the rate is approximately 20 nm/min and microroughness on the titanium sidewall begins to increase. Bath replenishment with fresh concentrate can maintain a steady-state etch rate only if the specific gravity is held within 1.110 to 1.125. Replenishment calculations should not rely solely on specific gravity because dissolved titanium also increases density. Ion chromatography for free fluoride and ICP-MS for titanium are the recommended control methods. Published data for this specific formulation loaded with copper and silver from subsequent redistribution layers is limited; users should qualify the endpoint shift when etching titanium directly above copper because the nitric acid component can corrode exposed copper at 5 nm/min to 15 nm/min depending on bath age.

The product is formulated with low ammonium and chloride content to reduce residues after spin-drying. In aluminum-containing stacks, the etch rate of aluminum at 25 °C is 5 nm/min to 10 nm/min, so titanium overetch above aluminum bond pads should not exceed 100% of the clear endpoint. For a 50 nm titanium film, a 100% overetch removes less than 35 nm of underlying thermal oxide because the titanium-to-silicon dioxide selectivity is approximately 1.2:1 to 1.5:1. This selectivity is adequate for under-bump metallization and adhesion-layer removal, but it is not sufficient for gate-stack applications where oxide loss must remain below 1 nm; in those cases, dry plasma etching is used instead of wet chemistry.

When titanium etching is transferred from immersion tanks to single-wafer spray tools

Single-wafer processing imposes different requirements because the etchant is dispensed onto a rotating substrate and is heated by both the wafer chuck and the exothermic reaction of titanium with nitric and hydrofluoric acids. In a spray acid processor equipped with a 12-inch PTFE bowl and a temperature-controlled chuck at 25 °C, the observed titanium removal rate is typically 35 nm/min to 45 nm/min, higher than immersion rates by 20% to 40% because of continuous replenishment of fresh acid at the metal surface. The process must be tuned by reducing dispense time rather than by diluting the product, because dilution shifts free fluoride activity and can increase attack on silicon dioxide. Endpoint is best monitored by optical reflectance at 670 nm; the signal transition width increases when dissolved titanium from re-circulated spray remains in the bowl. For this reason, single-wafer tools are preferred when titanium is exposed over copper or nickel, while immersion tools are preferred for batch removal of titanium adhesion layers from backside films.

All wetted surfaces in the recirculation loop are restricted to high-density polyethylene, PVDF, PTFE, or PFA. Borosilicate glass, quartz, and 316L stainless steel are not used for storage or distribution because free fluoride attacks silica and metal passivation layers. Storage limitations include separation of oxidizer and reductant waste streams, and long-term storage above 30 °C that accelerates nitric acid decomposition and raises headspace pressure. Containers must be vented with a fluoropolymer pressure-relief cap, and partial containers should be double-sealed inside an outer HDPE bag. The product is not compatible with amine-based strippers, alkali hydroxide solutions, or isopropyl alcohol in a closed drain system because exothermic neutralization and gas evolution may occur. Operators validate each new lot by etching a 200 nm titanium monitor wafer and comparing sheet resistance endpoint time against the previous three lots; a shift greater than 5% requires recalibration of the etch time.