by Marjorie K. Balazs
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Correlating data to performance will save time and money.
Monitoring an ultra-pure water (UPW) system requires more than just watching meters, tabulating numbers and looking at charts.
It takes understanding of each skid and component and some comprehension of what each test is saying about overall performance. In general, the more you know what your testing results mean, the higher the probability for continuously producing ultra-pure water at the least cost.
To achieve this low-cost goal, everyone responsible for monitoring a UPW system needs to learn more than the basics presented in this article. Nevertheless, it is hoped that this article will be sufficient to get an operator started.
Before we look at monitoring and various testing requirements, we need to look at the system itself. UPW systems are made up of three fundamental parts: the makeup, the re-circulating polishing loop and the distribution to the points of use (POU).
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The makeup part of the system is where most of the money is spent. The re-circulating polishing loop is your last chance to purify the UPW to meet specifications. It also increases the purity level because it has more sensitive units in for that purpose. The POU is what the wafer is being exposed to and is frequently different than what was measured at the final filter on the polishing loop. There may be a fourth system if reuse of UPW is a part of your system. However, this article only concerns itself with the basic three systems and take sthem as independent units, although they are certainly interrelated.
Each water system has a variety of operating manuals and specifications. It is important to familiarize yourself with them. If you have a water service, you need to talk to it and review the reports it gives you. These reports should tell you how the system is functioning.
It is not adequate to only look at the numbers that are on a report form. You particularly need to know when and why units or parts of the system are changed out.
The makeup
The makeup portion is the workhorse of the system. It is responsible for removing almost everything that is in the source water. The better it performs and the closer it is monitored to make adjustments, the cheaper the cost of your water and maintenance. Neglecting this part of the system will only get you into serious trouble. In the makeup portion of the system we have source water holding tanks, multimedia, chemical-injection pumps, reverse osmosis, makeup de-ionization, vacuum degasifiers, ultraviolet lamps, various filters and a makeup holding tank. Each of these must be monitored. Lack of monitoring in this front end usually leads to breakdown in water quality, increases in cost to produce water and, ultimately, yield problems.
Polishing loop
The polishing loop contains the polishing de-ionization beds, more ultraviolet units and refined filters that can include ultra filtration. These units work in relationship to the makeup portion of the system. Therefore if the makeup portion is sending a lot of material to the polishing loop, the chances are high that the polishing loop will pass some of this on to the POU. Also, the more material they collect, the shorter their lifetime. Because the units in the polishing loop are so refined, they are expensive. If the makeup portion of the system is working well, the polishing portion should seldom need a change out.
Points of use
Finally, we have the POU. There are many components in a system that once removed will not appear again or increase at the POU. These are things like trihalomethanes, silica, boron and generally metals. The ionic carbonate concentration can change but other ions like sodium do not. The biggest difference in the UPW composition at the POU is bacteria and particle concentrations. Therefore, the POU should be closely monitored for these contaminants.
The number of tests and the monitoring of a UPW system is extensive. They include, but are not limited to, the following:
Source water: TOC, Chlorine, Conductivity, pH, % boron, % silica; Makeup Loop: Multimedia: TOC, TDS, THM's; Reverse Osmosis: Feed conductivity, Feed pH, Feed temperature, Concentrate and product pressure, Concentrate and product flow, Product conductivity, Salt rejection, Delta P across tubes, Feed/concentrate delta P; Chemical injection: Rate, Total amount used; De-ionization: %Boron, % Silica, pH, conductivity, resistivity, Flow, TOC, Bacteria, THM's; Ultraviolet lights: 185's TOC, 256's Bacteria, Lamp intensity; Filters: Particle count and size, Bacteria Pressure drop; Polishing loop: DI's Resistivity, Bacteria, Flow; Ultraviolet Units: Bacteria, Lamp intensity; Filters: Particles, Pressure drop; Post Final Filter: Resistivity, THM's, Particle count and size, Bacteria, % Boron, % Silica, Cation and Anion concentration, Metal concentration; Point Of Use (POU): Particle count and size, Bacteria, TOC; Other Monitoring Data needed: Total amount of water supplied to the fab, Daily average used by the fab, Total dischaged UPW, Check on maintenance schedule, Calibration of On-line meters.
The monitoring schedule is also important. Generally, there is too little monitoring, especially of the makeup loop. The units of the makeup loop need to be monitored daily. Some companies do it weekly, biweekly or monthly. These time schedules work adequately only if nothing is changing in the water supply. However, a more frequent testing schedule allows one to see and understand the variations that occur in source water during the year. There are major changes in source water depending on where your supplier is getting the water, after heavy rains and during droughts or at the end of the summer. By monitoring the source water frequently, one can determine the load the system can handle and the schedule for cleaning or replacing units.
As much of the collected data as possible should be graphed. Most UPW systems have meters that feed data directly to computer systems. This data can be graphed to immediately show variations from the norm. These variations should be taken seriously and a follow-up should be scheduled.
At the time of the occurrence, samples of the water should be taken even if not immediately analyzed. If the fab incurs yield problems, these samples can be tested to determine if the water was the cause of the yield problem. If the anomalies continue to occur, more testing is needed immediately. These variations, if real, can cause a total yield bust that will not be recognized until days or weeks later.
To do an adequate job of monitoring, one should walk the system daily. Look at the meters, listen to the system, look for leaks and constantly watch for modifications to the system. Be sure that what you see correlates with what you may find on your computer readout.
In conclusion, monitoring a UPW system is an active job that requires continual learning about how the system functions as well as its idiosyncrasies. Correlating data to performance and cost can be rewarding and, when done properly, can save a company a great deal of money.
Marjorie K. Balazs is founder and CEO of Balazs Analytical Laboratory, which has provided analytical services to the semiconductor industry since 1975. She is also the president of SPWCC (Semiconductor Pure Water and Chemical Conference), which is in its 21st year. In 1993 she received the SEMI Award for North America and was honored by President Reagan as an outstanding woman entrepreneur. She has published over 40 papers that apply to contamination-free manufacturing in semiconductor processing.
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Recommended UPW tests
Source water:
TOC
Chlorine
Conductivity
pH
% boron
% silica
Makeup loop:
Multimedia: TOC
TDS
THMs
Reverse osmosis:
Feed conductivity
Feed pH
Feed temperature
Concentrate and product pressure
Concentrate and product flow
Product conductivity
Salt rejection
Delta P across tubes
Feed/concentrate delta P
Chemical injection:
Rate
Total amount used
De-ionization:
%boron
% silica
pH, conductivity, resistivity
Flow
TOC
Bacteria
THMs
Ultraviolet lights:
185s TOC
256s Bacteria
Lamp intensity
Filters:
Particle count and size
Bacteria
Pressure drop
Polishing loop:
DI's resistivity
Bacteria
Flow
Ultraviolet units:
Bacteria
Lamp intensity
Filters:
Particles
Pressure drop
Post final filter:
Resistivity
THMs
Particle count and size
Bacteria
% boron
% silica
Cation and anion concentration
Metal concentration
Point of use (POU):
Particle count and size
Bacteria
TOC
Other monitoring data needed:
Total amount of water supplied to the fab
Daily average used by the fab
Total dischaged UPW
Check on maintenance schedule
Calibration of on-line meters