(103a) Microchip-Based Sensitive and Fast Gas Analysis System by Gas/liquid Two Phase Flow and Thermal Lens Microscope

In modern semiconductor industry, ultra-clean environment is increasingly desired. Especially, control of ammonia gas concentration in a clean room is quite important for fine patterning process because ammonia is a basic gas that can form salt with acidic gases and the salts deposited on a photomask turn into haze and deteriorates photo-patterning. In the next decade, control of ammonia concentration in 0.1 ppb level will be required as is shown in a semiconductor road map. In addition, a portable and continuous monitoring system is highly required. However, conventional methods which utilized macroscale chemical processes did not meet these demands due to long chemical processing time for concentration of ammonia gas in liquid and the large system size. Previously, we reported a basic method for sensitive gas (formaldehyde) analysis on a microchip with gas/liquid two phase flow and TLM (thermal lens microscope) detection. In this work, we further developed a sensitive (0.1ppb w/w) and fast analysis method on a microchip for ammonia gas utilizing microchip technology. The basic performance of the proposed method was investigated.

2. Experimental

We designed a glass microchip which included gas/liquid two phase flow formation, gas/liquid adsorption and concentration, gas/liquid separation, colorimetric reaction, and TLM detection processes. All these processes were integrated into a single microchip.

3. Results and discussion

In order to realize concentration (~100 times in weight unit) for sensitive detection, gas/liquid flow condition was investigated. Investigating liquid evaporation, volume flow rates of gas and liquid were set at 100mL/min and 3.7µL/min (~1µL/min after evaporation), respectively. In this condition, gas flow through the center of the microchannel, while liquid flow around the channel wall due to the viscosity difference. Next, gas/liquid adsorption and concentration was investigated with these flow conditions. For efficient gas adsorption, 25mM oxalic acid was added to the water. After 15ms contact time of gas with liquid, 97% ammonia gas was effectively adsorbed into the liquid due to the large surface-to-volume ratio. High concentration (97 times in weight unit) was achieved. Then, gas-liquid separation method was investigated. A reservoir (diameter of 2mm) was formed on the microchannel, and gas permeable PTFE membrane (pore size 5µm) was covered on the reservoir to avoid liquid leakage. The gas was effectively removed from the reservoir, and the liquid was introduced to downstream for colorimetric reaction and TLM detection. Finally, colorimetric reaction (indophenol method) and TLM detection were investigated. Two step colorimetric reaction (10 min) and TLM detection (633 nm excitation) was conducted. From the calibration curve, lower limit of detection (LOD) of approximately 1ppb ammonium solution was obtained. By considering these performances, 0.01ppb sensitivity in less than 15 min was obtained, and the applicability was verified.