In photolithography, haze prevention is of critical importance to integrated circuit chip manufacturers. Numerous
studies have established that the presence of ammonia in the photolithography tool can cause haze to form on
optical surfaces resulting in permanent damage to costly deep ultra-violet optics. Ammonia is emitted into wafer
fab air by various semiconductor processes including coating steps in the track and CMP. The workers in the
clean room also emit a significant amount of ammonia. Chemical filters are typically used to remove airborne
contamination from critical locations but their lifetime and coverage cannot offer complete protection.
Therefore, constant or periodic monitoring of airborne ammonia at parts-per-trillion (ppt) levels is critical to
insure the integrity of the lithography process. Real time monitoring can insure that an accidental ammonia
release in the clean room is detected before any optics is damaged.
We have developed a transportable, highly accurate, highly specific, real-time trace gas monitor that detects
ammonia using Cavity Ring-Down Spectroscopy (CRDS). The trace gas monitor requires no calibration gas
standards, and can measure ammonia with 200 ppt sensitivity in five minutes with little or no baseline drift. In
addition, the high spectral resolution of CRDS makes the analyzer less susceptible to interference from other
gases when compared to other detection methods.
In this paper we describe the monitor, focus on its performance, discuss the results of a careful comparison with
ion chromatography (IC), and present field data measured inside the aligner and the reticule stocker at a semiconductor fab.
In the semiconductor industry, control of ammonia at the parts-per-billion concentration level is critical to insure the integrity of the lithography process. Ammonia is emitted into wafer fab air by various semiconductor processes including CVD, wafer cleaning, coater tracks, and CMP, as well as from outdoor air. Exposure to even low parts-per-billion concentrations of ammonia during the photolithography process can lead to yield loss and unscheduled equipment downtime. Picarro, Inc. has developed a field-deployable, real time, ambient gas analyzer capable of continuously monitoring parts-per-trillion levels of ammonia in situ, and in real-time, thereby allowing a user to directly monitor ammonia levels in sensitive photo-lithography equipment.