Batch In Situ Particle Monitoring
Batch In Situ Particle Monitoring offers a rapid and cost-effective requalification of wet process tools following chemical changes or filter replacement. In these applications, a particle counter is mounted on a cart and moved from one sample location to another. This mobile sampling approach maximizes the use of each particle counter and provides particle contamination information in real-time for improved process tool utilization. Unlike traditional surface scan test wafers, ISPMs characterize contamination in the individual baths being maintained. With an ISPM, samples can be taken directly from a process bath without special modification of the process tool, or lengthy down time. This article explains the use and benefits of portable liquid ISPMs especially their application within the semiconductor and like manufacturing industries.
The portable batch ISPM system is mounted on a cart so that the sampler is above the fluid level in the sample vessel or bath. This positioning prevents the possibility of forming a siphon in the event of a catastrophic plumbing failure. All sample tubing, including the inlet, outlet and purge should be contained to protect the operator and equipment from chemical contact. While sampling, the system as a whole must be protected from disturbance. If the sample lines are jarred or otherwise disturbed, particles can be shed from the fittings or the tubing walls, producing artificially high particle readings. These precautions ensure operator safety and maximize data accuracy and repeatability.
Batch Sampling Systems
Particle Measuring System’s CLS-700T batch ISPMs have successfully monitored a wide variety of processing chemicals. The system is controlled using MS Windows based data acquisition software. When sampling, chemical is drawn into the sampler under vacuum. Once proper fluid levels are reached the inlet valve is closed and the chemical is pressurized to eliminate bubbles. After a user specified pressurization delay, the chemical is forced through the particle counter at a constant pressure and flowrate. Several different particle counters may be used in the batch system to match particle sizing requirements of the chemical or process being monitored. The great flexibility provided by the batch sampling system allows the units to perform well in a number of varied applications. Their portability and bubble suppression capability are very useful in spot checking chemical delivery systems to verify chemical cleanliness before it can impact production yields.
ISPMs vs. Test Wafers
Like any complex device, wet process tools must be maintained to:
- replace consumables
- repair worn parts
- upgrade system components
The most frequent forms of maintenance are chemical and filter replacement. After performing this type of maintenance, the process tool is brought on-line to recirculate chemical through the integrated filtration system. Historically, after the recirculation system is running, particle test wafers are prepared and run through the various process baths. The test wafers are scanned with a surface particle counter and the number of particles added to their surfaces are recorded. If the total particle added are within quality limits, the process tool is qualified to run product again. If the total particles added exceed quality control limits, the test process is repeated before system troubleshooting begins.
Particle test wafer procedures are very resource intensive on personnel, equipment and raw materials. Particle test wafers are normally run through several process baths rather than the one or two which are being maintained. This procedure takes time and can mask problems in individual baths. An ISPM offers a faster and more cost-effective approach to tool qualification.
Monitoring after Maintenance
Following tool maintenance, a portable ISPM can be installed to sample directly from the process bath being maintained. Particle concentration can be measured within the bath in a matter of minutes. When chemical begins flowing through the re-circulated filtration system, there is a marked decrease in particle contamination.
When the particle concentration drops below quality assurance limits, the tool can be qualified to run product again. The ISPM identifies bath particle concentrations were within qualification limits well before surface scan data would typically be available. If a problem occurs in the recirculation system or bath, the particle concentration does not drop normally. These problems are quickly identified with an ISPM and can prevent product contamination, or additional lost processing time spent rerunning test wafers.
The ISPM’s ability to measure particle concentration changes in real-time naturally lend themselves to continuous process monitoring. As process monitors, ISPMs are very helpful in detecting catastrophic failures in fluid handling systems. Once notified of the system failure, operators can react to correct the problem before significant loss of product. Continuous process monitoring is best accomplished through dedicated ISPMs rather than mobile units.
When sampling, the system should use the same flowrate at which the sensor was calibrated. Although the system may be calibrated at slower flowrates, most semiconductor processing chemicals have not required it. It is common for the sensor flowrate to vary with chemical viscosity and compression pressure. Flowrate is displayed on the controlling software and may be adjusted as needed. Flowrate deviations less than 10% from the calibrated flowrate do not cause significant particle sizing errors.
The sample compression pressure should be set to the lowest pressure required to eliminate bubbles. This pressure setting will vary for different chemical combinations, dilutions and temperatures. To determine an appropriate compression pressure follow these steps while sampling a relatively stable sample source:
- Set at least 5 particle size bins covering the sizing range of your ISPM (0.2, 0.3, 0.5, 1.0, 2.0) or (0.3, 0.5, 0.7, 1.0, 2.0).
- Increase the compression pressure to 60 PSI and adjust the flow to remain constant.
- Run the system long enough to collect a statistically significant number of samples (20 or more).
- Reduce the system compression pressure by 5 PSI.
- Repeat steps 3 and 4 until you observe increased counts in the larger size channels.
CLS-700 Batch In-situ Particle Monitor
Using ISPMs like the CLS-700T can maximize wet process tool utilization by reducing requalification times, and more accurately characterizing the cleanliness of individual baths. Modern batch ISPMs can reliably overcome variances in chemical viscosity and bubble formation to accurately size and count particulate contamination. Through proper equipment use, portable ISPMs can provide an accurate and cost efficient means of testing many process tools.