Processing Magazine

Flowthrough conductivity sensing helps TI improve productivity while reducing material, maintenance costs

September 24, 2007

The challenge

Semiconductor fabrication involves a carefully orchestrated sequence of intricate procedures, in a pristine “clean room” environment. Maximizing yield of acceptable product requires keeping all process materials at their precise original specification, from the point of delivery through application. Frequent and accurate monitoring of process material quality is key to profitable semiconductor production and achieving reliable monitoring, without interrupting production or jeopardizing worker safety, is an ongoing challenge for the industry.

Texas Instruments (TI) met this challenge at its Stafford, Texas facility in monitoring quality of the photo resist stripper ST-26, which is a key material in its wafer production applications. The on-line process viability of the ST-26 solution is dependent on its water content. Thus, it is critical to monitor the ongoing viability of the solution during production, especially given that TI uses hundreds of gallons of ST-26 a day, at approximately $30.00 per gallon. If undetected and used, ST-26 with an unacceptable water balance, can cause substantial material loss, as well as significant lost production.

Traditionally, analyzing ST-26 water balance required taking samples to a processing lab, and results might not be available for hours or sometimes even days. When the results did arrive, they may have showed unacceptable water balance after the fact. Texas Instruments has now found the right solution, using flowthrough conductivity sensing technology from Foxboro.

The solution

After evaluating many other analytical approaches, engineers at TI Houston, concluded that conductivity sensing would be the right solution if they could find a sensor with the right combination of measurement capability and process compatible components. After discussions with the conductivity product manager at Invensys Process Systems, they selected the Foxboro FT10 conductivity sensor. Given the uniqueness of this application, Invensys worked closely with TI to set up a trial loop on-site. The trial period began in August 2006 and the results were evident early on. By February, 2007 TI declared the trial a complete success.

The FT10 sensors feature all virgin PFA for compatibility with TI’s requirements for measuring water in ST-26. By monitoring the change in conductivity as an index of percent water, the sensors make an on-line, real-time determination of their process solution viability.

Texas Instruments installed the FT10 flowthrough sensor in their process on a 3/4-inch line using TI’s preferred Flaretek connections. TI has found that the allowable range of water content is between 400 to 500 microsiemen/cm. When conductivity reaches the lower limit, the FT10 sensor, with the Foxboro 875EC Intelligent analyzer, triggers an alarm and the process is remedied to restore the ST-26 solution back to process acceptable parameters with no loss of production. The TI application requires a measurement capability in the 500 uS/cm range, but the FT10 is suitable for applications with measurements ranging from less than 50 uS/cm to 2000 mS/cm.

Another significant advantage of Foxboro FT10 sensors is in-line calibration. The FT10 features a calibration port that eliminates the need to break into the line and associated costs and safety concerns.” The FT10 calibration plugs permit quick, precise calibration with a shirt-pocket tool, which significantly reduces scheduled maintenance time and related exposure of personnel or the environment to aggressive and dangerous chemicals.

Since installing the FT10, TI has eliminated lost product resulting from unacceptable water levels. Flow-through conductivity sensing technology has allowed TI to take a proactive role by heading off problems before they happen.