Real-Time pH Monitoring of Ultra-Diluted Chemistry with a Micro-Sampling pH Monitor

At wet cleaning and etching process in semiconductor manufacturing, several ppm of ultra-dilute ammonia is sometimes used. When CO₂ dissolves into the ammonia solution, it changes to carbonic acid, and neutralizes ammonia solution. It means, CO₂ dissolution leads to pH change and impact to process performance. In order to know the chemical condition, pH is one of the key parameters. From its principle, however, internal solution (KCl) of pH probe dissolves into sample solution and lead to contamination, so the sample needs to be drained after measurement. Schematic system of pH probe is shown in Fig.1.  Measurement using pH probe requires several tens to hundreds mL sample, so waste chemical amount and cost impact is significant.

In order to solve such problems, we developed a real time, in-situ use pH monitor. This monitor equips micro flow pH sensor, which samples 0.5 mL per one monitoring, minimum interval is 1 min, i.e. sample consumption is maximum 30 mL per hour, and it enables to obtain real time pH data.

Using this monitor, ca. 1ppm ammonia solution was continuously monitored at every 1 minute. Test system is shown in Fig.2. From 0 to 0.9 h, sample solution was bubbled with N2, from 0.9 to 1.9 h bubbled with air, then from 1.8 to 4 h bubbled with N₂.  Monitor result is shown in Fig.3.

[0 to 0.9 h] bubbled with N₂ gas, CO₂ does not dissolve to the solution, and pH is kept stable.

[0.9 to 1.9 h] bubbled with air, pH drops from 9.3 to 7.0 approximately in 10 minutes, which is caused by CO₂ dissolution from the air.

[1.9 to 4 h] bubbled with N₂ gas again, pH value recovered because CO₂ is put out.

In conclusion, micro-sampling pH monitoring system, it enabled to detect clear pH change of ultra-diluted ammonia solution caused by CO₂ dissolution with real-time bases.