Determining Tetramethylammonium Hydroxide in Water by NIR

For semiconductor wafer processing, the level of tetramethylammonium hydroxide in water is very critical in the developer blend system. For many reaction processes, the solution of tetramethylammonium hydroxide must be kept at 2.38%. Near-infrared (NIR) spectroscopy can be used for in-situ monitoring of reaction conditions.

A feasibility study was conducted with 13 sample points. Solutions of (CH3)4 NOH were made by diluting a 25% solution of aqueous tetramethylammonium hydroxide, electronic grade, in distilled water by volume. The samples were placed into a beaker, which was immersed in a temperature controlled circulating bath. A 2-mm optical path length o-ring sealed SST probe was inserted into the sample along with a Pt-RTD temperature sensor. Spectral data were collected with thermally stable 500 μm diameter single strand low-OH fused silica fiber optic cables on an extended range (1000 nm-2100 nm) Full Spectrum NIR-O Process Analyzer. All spectra were collected at an average of 22.0°C with less than a range of 0.15°C over a 3-hour period. Each spectrum consisted of an average of 16 scans for a total data collection time of 47s (Figure 1).

Figure 1 Raw Absorbance Spectra of Tertramethylammonium hydroxide

Summary of Conclusions on the Vaibility of Measuring Tetramethylammonium Hydroxide in Water with Process Spectroscopy

The concentrations of tetramethylammonium hydroxide can be easily measured at 2.38% by near-infrared (NIR) spectroscopy with a Guided Wave fiber optic coupled NIR-OTM Process Analyzer to better than ±0.1 % v/v. Analysis time is under 50 seconds. This study did not investigate the effect of varying bath temperature on the results. Water is a highly polar molecule, hence its spectrum is very temperature sensitive. Since the bath is 98% water, temperature will be a factor in any online installation. Experience indicates that it can be corrected spectroscopically. (See Low Levels of Organics in Water; Saccharin— Guided Wave Publication #3014).

What is Process Spectrscopy?

Modern NIR spectrometers have extremely high signal-to-noise ratios and superb long-term stability plus multi-channel capability making them ideal for monitoring of chemicals in process applications. Fiber optics permit the probe to be located remotely from the analyzer. Probes may be inserted directly into the process (insertion probes) or located in side streams (flow cells). In most cases, sample conditioning is not required eliminating the need for costly and failure-prone sample conditioning systems. Coupled with a computer, these systems are fast and reliable.

Previous Guided Wave studies have shown that low levels of organics can be measured in water. Guided Wave demonstrated this is previous application: Low Levels of Organics in Water: Saccharin. Sodium saccharin could be measured to 60-ppm by taking into account the temperature effect on the water spectrum from 16 to 32°C. In the saccharin study, we demonstrated that water spectra are about 10 times more sensitive to water temperature than platinum RTDs. The temperature can be inferred from the spectra, eliminating the need for costly temperature control or highly accurate thermometers.

Data Analysis Tetramethylammonium Hydroxide NIR Spectra

Baseline Corrected Spectra
Figure 2 Baseline Corrected Spectra
Difference Spectrum of Water
Figure 3 Difference Spectrum of Water

The raw spectra were baseline corrected at 1625 and 1795 nm (5 pt averages) (Figure 2) and the spectrum of distilled water was subtracted (Figures 3). The peak height at 1660 nm was correlated to the percent tetramethylammonium hydroxide (Figure 4). Figure 5 shows this correlation extended out to 0 and 25%. The zero point fits well on the linear regression but the 25% point does not. This is due to density changes in the sample with increasing concentration. In Table I (page 3) the predictions based on this single wavelength analysis are given. Note that when the prediction for 25% is multiplied by its density, the prediction improves significantly. No corrections were made on these spectral data for temperature changes, which could impact the accuracy of the predictions.

NIR Absorbance at 1660nm versus trace tetramethylammonium hydroxide concentration
Figure 4 Absorbance at 1660nm versus trace tetramethylammonium hydroxide concentration
Figure 5 Absorbance at 1660nm versus full scale tetramethylammonium hydroxide concentration

Measuring tetramethylammonium hydroxide in water using NIR spectroscopy is quite feasible. We have shown measurements levels equal to ±0.008% for one standard deviation. This yields a 95% confidence level (3x) of about ±0.03% and a conservative lower detection limit (10x) of 0.08% v/v.

From our experience, we know the temperature will have a significant impact on spectroscopy. We have also learned that controlling the temperature, or even measuring it, is not accurate enough by about a factor of ten. Fortunately, the thermal information is accurately recorded in the spectra, therefore it is sufficient to measure the spectra and deduce directly from it, the proper corrections for the thermal changes.

Sample Interfaces Designed for Process

Since semiconductor processes are very sensitive to metals contamination, Guided Wave recommends using our mechanically stabilized Teflon Flow Cell or PFA/PEEK Flow cell for corrosive environments when the probe needs to be constructed completely metal-free. With sample wetted components of synthetic sapphire optics and Teflon, no metals are exposed so metal contamination is eliminated. The cell is seen below with its front entry port exposed with a cleaning port on top. Stainless steel metal parts stabilize the unit against thermal distortion.