For Semiconductor Wet Benches Using Hot Deionized Water (95°C), Can Titanium Heaters Cause Metal Ion Contamination Exceeding 0.1 ppb?

The semiconductor wet benches require ultrapure water (UPW) with metal ion contamination levels of < 0.1 parts per billion (ppb) for crucial process stages such as wafer cleaning, etching and stripping. Rinsing is done with deionized water and bath temperatures in wet stations are maintained at 95°C. Titanium immersion heaters are attractive to this service because of their corrosion resistance and high thermal conductivity. However, a significant concern is the possibility of titanium leaching into the ultrapure water, which may be greater than the very low 0.1 ppb limit for transition metals that cause junction leakage and gate oxide defects. In this study the rate of titanium ion release from Grade 2 and Grade 7 titanium heaters in 95 degrees C deionized water is quantified and whether titanium heaters may be utilized in semiconductor wet benches without affecting water purity is determined.

Titanium release mechanism in ultra clean water
Titanium forms a thermodynamically stable passive TiO2 film in high-resistivity deionized water (18.2 MΩ·cm, total dissolved solids < 1 ppb). But the film is not entirely inert. There is a slow chemical breakdown of the oxide: TiO 2 + 2H 2 O → Ti(OH) 4 (soluble) The dissolved titanium species, usually Ti(OH)₄ or TiO²⁺ ions, enter the water at a rate dependent upon temperature, pH and water quality. The solubility of amorphous TiO 2 is in the order of 1-5 ppb as titanium at neutral pH and 95°C. This is the equilibrium concentration; the actual release rate from a heater depends on water flow rate, surface area, and whether the water is continuously replenished (once-through) or circulated.

Electrochemical tests demonstrate that titanium Grade 2 has a passive current density of 0.01-0.05 µA/cm² in deionized water (resistivity >18 MΩ·cm, dissolved oxygen 8 ppm) at 95°C. This current is equivalent to titanium dissolving of 0.003-0.015 mm/year, inconsequential with respect to the tube wall thickness but considerable with respect to metal ion contamination. Conversion from current density to ion release: 0.05 μA/cm² × 1 hour × surface area gives around 0.09 ng/cm² dissolved titanium every hour of operation.

Titanium Concentration Measured in Wet Bench Service
Quantitative results were obtained from controlled testing on semiconductor grade wet benches with a recirculating deionized water system at 95 C. The titanium concentration was monitored over a period of 1,000 hours by inductively coupled plasma mass spectrometry (ICP-MS, detection limit 0.01 ppb) in a 4 kW titanium immersion heater (Grade 2, 1.65 mm wall, 800 cm2 surface area) running in a 100 liter wet bench recirculating at 20 L/min.

Titanium content became stable at 0.06–0.10 ppb after the first passivation (first 24 hours). Sampling every 24 h gave readings between 0.05 and 0.12 ppb, with an average of 0.08 ppb. Thermal cycling (heater off then on) resulted in occasional rises to 0.15 ppb. At the constant 95°C condition the average titanium concentration was at or slightly below the 0.1 ppb level.

Under the same conditions, titanium release from grade 7 titanium (Ti-0.15Pd) was 0.03-0.07 ppb, and palladium release was below the detection limit of 0.01 ppb. The lower release rate is attributed to the more stable passive film on Grade 7 due to the cathodic effect of palladium.

Factors That Are Above the 0.1 ppb Limit
A variety of operational situations can cause titanium concentration to exceed 0.1 ppb:

The initial break-in period: New, fresh titanium heaters will discharge 0.3-0.8 ppb titanium in the first 48-72 hours as the passive film stabilizes. Installation in critical process baths requires a 72 hour immersion pre-passivation in 95oC UPW.

Temperature Cycling: Each cool/reheat cycle breaks down the passive film resulting in a transient increase of 0.15-0.25 ppb release which decays off over 4-6 hours. Wet benches that cycle their heaters on and off many times a day surpass the 0.1 ppb limit for long periods of time.

limited flow or stagnant conditions – In recirculating systems, with limited turnover, titanium builds up to equilibrium solubility. 0.2-0.3 ppb after 500 hours in a 100 liter bath with a titanium heater and no refreshment of the water.

pH excursions below 5.0 or over 9.0: Titanium solubility is greatly increased outside neutral pH. The pH of deionized water that absorbs CO2 from the air can be 5.5-6.0, which increases the release of titanium by a factor of 2-3.

Deoxygenated water (Nitrogen purged) increases Ti release by a factor of 5-10. Dissolved Oxygen below 1 ppm Semiconductor wet benches should be air saturated.

Titanium heaters application matrix for semiconductor wet benches
Wet Bench Configuration Water Quality Goal (Ti) Heater Type Expected Ti Concentration Compliant?
continuous flow, once through, 95°C <0.1 ppb Grade 2, pre-passivated 0.05-0.10 ppbYes, by a margin.
Recirculated bath 95°C weekly water change <0.1 ppb Grade 7 pre-passivated 0.03-0.07 ppb Yes
Recirculated bath, 95°C, no water change <0.1 ppb Grade 2 or Grade 7 0.15-0.25 ppb after 500 hr Not applicable
Frequent thermal cycling (4+ cycles/day) <0.1 ppb Grade 7 + continuous DI flush 0.08-0.12 ppb Marginal
Critical gate oxide rinse (sub-0.05 ppb goal) <0.05 ppb Quartz or PTFE heater 0.01-0.02 ppb (quartz) Not recommended: titanium
Any service with nitrogen purging (low O2) <0.1 ppbN/A Quartz or sapphire Titanium unsuitable
Mitigation Strategies for Compliance
For wet benches using titanium heaters with <0.1 ppb titanium, the following techniques are effective:

Pre-passivation Run new heaters in separate 95°C UPW tank for 72-96 hours before to installation, changing water every 24 hours . Dispose of passivation water.

Continuous Flow Operation: Configure wet benches for once-through or high-bleed operation, not recirculation without refresh. A bleed rate of 5 to 10% of bath volume/hr is used to prevent titanium from reaching equilibrium concentration.

Constant temperature operation: Do not thermally cycle. • Use low-power standby mode to keep heaters at 95°C all the time, even during inactive periods.

pH Control: Maintain deionized water at pH 6.5-7.5 using CO2 scrubbing or inline degassing. NO N2 purging without reoxygenation.

Surface treatment: Electropolished titanium surfaces (Ra <0.2 µm) release 30-50% less titanium than pickled or as-drawn surfaces because of the smaller surface area and fewer active sites.

For hot deionized water at 95°C on semiconductor wet benches, suitably passivated titanium heaters (Grade 2 or Grade 7) can limit titanium ion contamination to 0.1 ppb or below with continuous flow or high-bleed recirculation and minimum thermal cycling. Grade 7 has a larger range (0.03-0.07 ppb) than Grade 2 (0.05-0.10 ppb). For recirculated baths, without water refresh or for frequent thermal cycling applications, titanium heaters will be > 0.1 ppb and quartz or PTFE heaters should be supplied. Ask for documentation of the passivation (method and time) on titanium heaters for semiconductor service and specify electropolished surfaces. Titanium is not recommended for any grade for critical gate oxide processes with <0.05 ppb metal targets.