Oxygen and moisture: a costly combination

The sub-micron scale patterns etched on to microprocessor chips are complex and dense, with around 4km etched per square centimeter of device area. At this scale, even the presence of a few molecules of water or oxygen contaminating the inert process gases will cause loss of yield by reacting with the chemicals overlaid on the wafers.

With manufacturers under pressure to maximize yields to keep costs low, using oxygen analyzers to control the purity of the process gases is vital.

Typical requirements for trace impurity measurements in semiconductor manufacture

The requirements to avoid contamination are simple: no oxygen or moisture should be present in the gases used during key processes such as chemical vapor deposition or plasma etching.

Any analyzer or sensor selected for either moisture or oxygen must have a lower detection level (LDL) down to parts per billion. Measurements need to be made close to the point of use to reduce the chance of leaks occurring after the measurements have been taken, although checks of gas quality after storage may also be made.

Technologies commonly used for oxygen control in semiconductor manufacturing processes

Typically, zirconium oxide and electrochemical sensors are used for measurements of trace oxygen in these applications.

The electrochemical oxygen sensing options are galvanic/Pico-Ion or Hersch Cells. These offer the advantages of an LDL of 100 parts per trillion and a small footprint, making it easy to install close to each point of use. Both technologies are very reliable and fast to respond from low to high ppm readings. The cost-effective galvanic analyzers use sensors that deplete and need to be replaced in periods of 12 to 18 months, depending on the application and gas in which they measure. However, replacing the sensors is extremely easy to perform without the need of expert knowledge, and is often incorporated into a routine schedule. Hersch Cells do not need to be replaced, but the electrolyte needs to be topped up every few weeks.

For many customers a periodic check of control equipment is a standard quality routine that will be applied no matter what equipment is being used. Some, however, may see it as increased maintenance costs.

The advantage of the zirconium-oxide oxygen sensing technology is the fast speed of response to changing conditions, whether it’s from low to high ppm or from high to low – as well as the fact that the sensors are non-depleting. Unlike electrochemical sensors, exposure to air doesn’t consume zirconium oxide sensors.

The main disadvantage is the relatively high LDL (around 0.01ppm O2) which makes them unsuitable for some applications. Any hydrocarbons present will react with oxygen on the sensor due to its high temperature sensor and give a false low reading.

Technologies commonly used for moisture control in semiconductor manufacturing

There are three technologies typically selected for measuring trace moisture in semiconductor manufacture:

Cavity ring-down spectroscopy (CRDS) offers precise, stable and fast measurements due to the optical sensor technology. The instruments have an LDL down to 0.1 ppb depending on the gas measured. Due to the relatively high costs their use is mainly at central distribution points. If contaminated, maintenance can be extensive and costly.

Quartz Crystal Microbalance (QCM). Although not as fast as CRDS these analyzers have an LDL of 0.05 ppmV and offer reliable long-term measurements with auto-calibration functionality. Due to the analyzer enclosure size, and despite significant cost advantages vs. optical technologies, these analyzers will normally be installed in central locations e.g. at gas distribution. Many models offer very good lifetime cost ratio with extended maintenance periods.

Ceramic impedance dew-point transmitters. Very cost effective at fractions of the other technologies’ costs, the sensors are capable of measuring down to 1ppbV as well as being small and easily installed at point of use. To ensure consistent measurements an annual recalibration is needed. However, this can be carried out by the user and could be combined with other routine maintenance on the system. When participating in a manufacturer’s sensor exchange program the user effectively receives a lifetime warranty for his or her sensor.

Importance of sampling

No matter if it is a measurement of oxygen or moisture, making the right decision on sampling is just as important as selecting the correct analyzer since the speed and accuracy of the measurements relies on all parts of the system, not just the sensor technology. The type of sample tube used is paramount as is selecting suitable fittings for low level measurements. Ideally, electro-polished stainless-steel tube with high integrity fittings should be used.

If you are looking to buy a moisture or oxygen measurement system, always consider sampling in the early decision-making stages and take expert advice when possible. Only the right combination of analyzer and sampling system will offer the optimal results, preventing premature damage and ensure reliable and consistent measurement results.

Summary

We hope this brief article has given some insight into the many options currently available on the market for making precise measurements of trace moisture and oxygen for semi-conductor manufacture.

The wide range of technologies, analyzers and brands on the market give users the opportunity to select the best fit for their application, in terms of performance and budget. However, it is always a good idea to seek expert advice before making a decision, especially if it is for a system that will measure more than one parameter to be sure that the technologies and sampling components are compatible.




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