Controlling AMCs in the Fab
Steam is frequently used as the water vapor source to adjust cleanroom humidity. Urea easily passes through most RO/DI water systems. This urea is both injected with the steam and converted to ammonia in the steam generator. This generates high levels of both ammonia and Urea in the cleanroom air, leading to haze on photolithography optics, and T-topping of Photoresist.
Instead of removing the sources of ammonia upstream, most fabs are spending money downstream with chemical filters to address the symptom instead the of the cause.
RASIRC Steamers use a non-porous hydrophilic membrane that selectively allows water vapor and steam to pass through it. The chart below shows some typical results. Significant reduction can be seen in all contaminants, even with difficult contaminants such as Urea and ammonia.
| Purification Performance Results (ppb) |
| |
DI Water Source |
Pre-Purified Steam |
Purified Steam |
| Total Metals |
19.8 |
0.15 |
0.009 |
| Total Organic Carbon |
1200 |
380 |
22 |
| Total Silica |
28 |
4.3 |
0.7 |
| Urea |
2200 |
48 |
2.6 |
| Ammonium |
1.468 |
1.117 |
0.116 |
|
