The Case of the Unexpectedly High Particle Counts: An “Inspector Jack Caper”

The Case of the Unexpectedly High Particle Counts: An “Inspector Jack Caper”

By Harold D. Fitch

Sue works the 11-to-7 night shift in the ultra clean development manufacturing area, monitoring air quality of the clean hoods and the room. She enjoys her job and knows that good air quality is essential to the success of her product. She takes more interest in her job and in higher yields than many of her co-workers.

Currently, Sue spends her shift measuring the air particle count with 0.2 micron-sensitivity portable laser particle counters. The hoods she measures are specified at counts below 10 at 0.5 microns, usually about 1 or less at 0.5 microns, and 3 to 4 at 0.2 microns. At counts below 1,000, the rooms are specified at 0.5 microns and usually run 100-200 at 0.5 microns and 500-600 at 0.2 microns. Sue is proud of her proficiency in completing measurements. She often accomplishes almost as much on the third shift as the first two shifts combined. This is partly due to Sue`s ability and to fewer interruptions on her shift.

Tonight, one of Sue`s assignments is to make some initial measurements on the first of a series of about 20 new CVD hoods. She looks forward to examining the new hoods, which will help this section of the line come up to speed rapidly. However, her initial measurements are disturbing. The measurements are in the thousands of particles at 0.2 micron sensitivity. After recalibrating her instrument, she double-checks a few counts made earlier and finds they are still the same. Then, she remeasures the new hood and again detects alarmingly high counts!

Inspector Jack to the rescue

Sue knows her engineering advisor, Inspector Jack, will be in early the next morning. He is the line control specialist from the site contamination control group. She carefully assembles her results on the new hood and leaves a message for Jack.

At 6 AM when Jack arrives, he immediately receives the message from Sue telling him that the new hood was way out of specification and that she thinks the HEPA filter must be damaged. Jack had worked for a considerable time with Sue and had been responsible for most of her training. He was very confident in her: if she said there was a problem, then there was a problem!

At 6:15 AM, Jack and Sue met at the new hood location, and Jack reviewed the data, taking additional measurements. There was something wrong all right. Around the edge of the filter, at 0.2 microns, counts were in the thousands.

The hood they were measuring was a new design. The HEPA filters had been placed into the unit from below, so they could be easily changed without opening the back of the hood. Jack checked the tightening mechanism to make sure it had been tightened correctly–everything seemed fine. He made arrangements for the facilities group to remove the HEPAs, install new ones, and bring the defective filters to the contamination control lab for further checking.

Panic sets in

What happened next was a general panic. The same high counts were found around the edges of the brand new filters that had just been installed in the CVD hood. In addition, when installed in the lab`s special test fixture, the original filters showed no signs of leakage, performing according to spec. It seemed the problem was not the filters, but something to do with the hood itself. If one hood was defective, then the other new hoods might also be defective. Installing and debugging of the hood stations needed to done immediately.

Jack was now under pressure to solve the problem. He worked with the facilities group to remove the new filters from the critical hoods, seal around the gasket material with RTV sealant, and reinstall the filters. His fast action did improve the situation, but did not solve it completely. The counts were much better, but they were still out of spec. The severity of the problem appeared worse than originally feared!

Again, the filters were removed. Jack starting making measurements of the flange that the filters were butted against. Finally, these measurements pinpointed the problem: the flange was out of square, and also, out of flatness–by nearly 1/2 inch. This explained why the 1/2- in. thick gasket did not seal the filter to the flange properly when the filter was bolted in position.

Another attempt at solving the problem–using a combination of a thicker gasket, coupled with a liberal application of RTV sealant–finally brought the particle counts under control and corrected the hood problem. Further investigation showed that several other hoods that were installed had the same problem. After checking with purchasing and with the supplier, it was found that all of the hoods had been shipped from the West Coast to the East Coast, although not all had arrived yet. Several hoods had not yet been unpacked.

It took nearly three months to resolve the whole problem. The supplier`s engineers visited the manufacturing site to confirm that the problem had been correctly identified and worked with site contamination control, facilities, manufacturing engineering and purchasing to resolve it. The final solution was the installation of special thick gaskets, RTV sealant, and special testing to ensure that all hoods functioned properly.

The contamination control department`s expertise allowed the problem to be identified rapidly, leading to an early solution. New tools were installed quickly– speed being a very important consideration, given the strict manufacturing demands and the steep product learning curve that had to be meet.

The one positive result of the incident was that new guidelines for acceptable conditions of flatness and squareness were generated and became part of the purchase specifications for future clean workstations.n

“Inspector Jack” cases are based on actual line troubleshooting incidents from Jack Hedman`s 23-year contamination control career.

Harold Fitch is president of Future Resource Development, a consulting firm in Burlington, VT, specializing in cleanroom education and problem-solving. He conducts international training seminars for CleanRooms` shows and seminars.


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