Thanks for all the responses to my questions regarding temp/humidity control. It looks like, as of last Friday, we have authorization to pursue a desiccant type outside air conditioning system to augment our existing chilled water system for humidity control. Historically, we have been trying to control to 35% RH +/- 5% and 68F +/- 2F. Based on your responses, inquiries to other fabs, Clariant AZ Electronic Materials, and others, and having a long term knowledge base at 35%, we will probably change the humidity control spec and try to achieve 38% +/- 2%. We would also like to tighten the temperature spec to +/- 1F in the photolith areas. At this point it would be big a big improvement just to have reliable chilled water all summer, but hey, "I can dream can't I ?" Actually I'm very excited about the proposed project.
I will try to summarize the competing issues driving a university research environment microfabrication cleanroom temp/humidity spec, as I see them. Any disagreements, corrections or additions would be appreciated.
a.. Photoresist Sensitivity: photoresist begins to lose its photosensitivity below about 30% RH.
b.. Photoresist Adhesion: the lower the humidity the better (HMDS or not).
c.. Photoresist Minimum Linewidth Control/Repeatability: there are many factors here but they all add up to the tighter the temp/humidity spec, the better. What is good enough depends on the minimum uniform and repeatable linewidth that must be routinely accomplished. Beyond a reasonable level environmental chambers are used or small isolated rooms with separate temperature/humidity control systems.
d.. High Vacuum Chamber Pumpdown: the lower the humidity the better (In our case we have 20 machines that operate at high vacuum and only 6 have loadlocks).
e.. Water Condensation on Cooled Surfaces: the lower the humidity, the lower the temperature that you can cool to without dripping water or adding insulation or a warm water cycle.
f.. Electrostatic Discharge: this is a general impression but I think this begins to be more of a problem below 30% RH, but in general, the higher the humidity, the better (we have a grounded conductive vinyl floor system for whatever it's worth).
g.. Comfort of Occupants: 45 - 55% RH is typical for office/classroom space, but this can probably swing as far as 30 - 70% before people really start to complain, depending on the temperature.
These are the unfortunate controlling factors:
a.. Cost of Initial Purchase: lower the better, means in general higher humidity and a wider swing in humidity tolerance.
b.. Long Term Operating Cost: same as previous.
c.. Size/Space Availability Constraints: same as previous.
d.. Equipment compatibility with existing systems in place.
Our cleanroom was constructed in 1983. The original control specs were 72F +/- 1F and 35% RH +/- 5%. We relatively quickly went to 68F due to the warmth of the cleanroom garments. However, even at 72F, it is questionable, according to the psychrometrics (or psychometrics), if 35% could be controlled reliably during the summer months with only 38F chilled water. In fact we had a rental chiller that could deliver 36F water two summers ago and we still could not maintain 35% RH. It seems that the need to tolerate swings in RH during the summer was designed into the system. We are now pulling in about 2,000 cfm less than the designed for total of outside air, so the problem is not due to excess exhaust growth syndrome.
Our chilled water coils are 5 row units as opposed to 6 or 8 rows of coils, so this also severely limits the system. There were severe space constraints on the initial design ... we wanted as much clean space as we could get (10 pounds in a five pound box). There was also a minimal budget, which must have controlled many of the design decisions ( 4,000 sq. ft. overall for $850,000 --- not bad).
During the summer our one chiller is being challenged to put out 38F water. It can't. It can typically keep up with 40F, but is running on both compressors almost continuously during the day. The nominal tonnage of a chiller is reduced significantly when the operating chilled water temperature is reduced from 45F to 38F. The lifetime of the compressors is reduced far from nominal in these conditions. The predictable shut downs of the chiller in the middle of the summer for compressor rebuilds, and the resultant complete lose of chilled water, in our case, reeked havoc on the operation of the cleanroom. Chilled water redundancy is a big desire to insure any kind of reliability and ease of maintenance.
We are planning to install a packaged desiccant system into the outside air intake for the cleanroom. We hope (pray) that this will take care of several problems; but, as usual, "no good deed goes unpunished":
a.. The desiccant dehumidification system will precondition all of the incoming outside air. The existing chilled water coils at each air handler will then be used only for controlling sensible heat.
a.. We will be able to use the 45F campus wide chilled water system for our primary source of chilled water. This is a non-glycol system. There are some advantages to removing the glycol including better heat transfer, no environmental waste problem, and less impact when leaks occur. The problem with it, in our case, is freezing of chilled water coils in the winter months. A preheat coil will be part of the design for this purpose. The existing chiller will be retained for redundancy. The campus system has some inherent redundancy with multiple chillers and towers, but may drift up to 47F in the worst part of the summer. This, combined with the question of what the temperature of the water will actually be when it reaches the coils, must be tolerated by the design.
a.. This system should reduce our operating costs somewhat. The outside air will be cooled with chilled water before entering the desiccant wheel, but not to the the very low temperatures required to drop out the water. This eliminates the steam that was necessary for reheat after dehumidification.The desiccant wheel will be dried with hot air heated by steam, so we will actually be dehumidifying with steam in the middle of summer when peak electrical energy costs are the highest. Steam is plentiful on this campus and comes at a relatively lower cost. The air will be cooled again after leaving the desiccant wheel with another set of coils to remove the heat generated by the hot desiccant wheel. A preliminary energy analysis projects a reduction of chilled water from 158 tons to 95 tons, and a reduction of steam from 1271 MBH to 625 MBH.
a.. The system will provide us with chilled water redundancy. The campus chilled water system has some inherent redundancy. Our existing stand alone chiller will be valved into the campus system to establish a redundant system seperate from the campus system in case the campus system must be shut down for leaks or plumbing in of new components. The existing chiller plumbing will be heat traced to make it compatible with the non-glycol chilled water system.
a.. The system should reduce or maintenance costs. We will no longer challenge the chiller to make 38F water, so its compressor lifetime should be extended. The condensate water generated in the summer by the chilled water coils in each air handler will be eliminated. This will eliminate pan overflows caused by plugged drains, drain pipe leaks, and the maintenance required to eliminate these problems.
a.. The system should be designed to provide us with some reasonable amount of head room in the maximum volume of outside air that can be brought into the cleanroom.
a.. There are always some unknowns when installing a new system, but we do have a new desiccant system on campus that has been in operation for a year. Maintenance and reliability must be proven. There are more filters, but I think this will help us overall. There is some concern of introducing a new contaminate into the airstream from the desiccant material.
Thanks again for the input....please add to this discussion.
Chris Bowman
Carnegie Mellon University
ECE Dept/Data Storage Systems Center
Pittsburgh, PA 15213
P(412) 268-2471
F(412) 268-4323
ccbowman@ece.cmu.edu
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