Abstract

The AiM® monitor from Particle Measuring Systems provides real-time detection of molecular deposition on SiO2 surfaces. These data are particularly useful for correlating molecular contamination occurrences with cleanroom processes and events. By monitoring long-term trends in mass deposition, one can establish baseline contamination levels, facilitating early identification of new contamination sources.

The AiM can be used to detect events such as process chemical migration, chemical outgassing, and introduction of outdoor air contamination into the facility. It thereby provides the ability to track and resolve the yield-reducing molecular contamination of critical processes or product surfaces.

Molecular events typically last from seconds to weeks, with several simultaneous events involving different sources and molecular species. To effectively monitor, understand, and resolve complex molecular contamination (SMC) issues--before electrical, optical, or chemical degradation of critical surfaces occurs--high sensitivity, real-time information must be obtained.

Surface Acoustic Wave Technology

Acoustic wave sensors have been employed since the 1950s as quartz crystal microbalances (QCM). In the late 1970s, surface acoustic wave (SAW) sensors were first used in chemical sensing, providing approximately 100 times more sensitive measurements than conventional QCM1. As SAW and supporting technologies have advanced in the last two decades, sensitivity has continued to improve2. The AiM, shown in Figure 1, can measure mass changes of less than 0.2 ng/cm2, roughly 0.75% of one monolayer3, with measurements each minute.

The AiM SAW sensor operates at an acoustic resonance frequency of 200 MHz. The acoustic waves travel along the surface of the crystal (Figure 2). As mass increases on the surface, the acoustic wave velocity is reduced and a frequency shift is observed

where
D = change in frequency
a = constant
f = fundamental frequency
Dm/A = change in mass per unit area.

The AiM reports the difference in frequency between an exposed SAW sensor and a sealed SAW crystal. As mass accumulates on the exposed crystal, the difference in frequency between the two crystals increases proportionately.

Airborne Molecular Contamination

AMC is a concern in many industries, including: semiconductor, hard disk drive, flat panel display, and aerospace. Although the critical molecular species and their specific impact vary across industries, SMC has been found to be the cause of yield losses, product degradation, and product failures in all of these industries. For example, in the semiconductor industry SMC causes T-topping of the resist, defective epitaxial growth, unintentional doping, uneven oxide growth, changes in wafer surface properties, corrosion, and decreased metal pad adhesion.

In a cleanroom, molecular contamination either comes from the outdoor makeup air or is generated within the facility. Examples of AMC sources within a facility include: recirculated air (with inadequate filtration), cross-process chemical contamination, outgassing of cleanroom materials (filters, gel sealants, construction materials, etc.), people/personal care products, and equipment.

AMC deposition on surfaces (wafers, optics, AiM sensor chip, etc.) can occur in a reversible or irreversible manner. Reversible AMC is usually physically adsorbed on the surface. It is always either in equilibrium or trending toward equilibrium with the ambient air. When the chemical contamination levels increase in the ambient air, the mass on the surface will increase proportionally (and rapidly). When the air contamination decreases, the contamination mass on the surface will decrease proportionally. The rate of mass loss from the surface depends on the volatility of the chemical compound and any interactions with other surface contaminants. More volatile compounds re-equilibrate faster than less volatile compounds.

Irreversible AMC can be either physically adsorbed or chemically bonded to the surface. This type of AMC is either reactive with the surface or has a very low volatility. In either case, once it contacts the surface, it remains on the surface. Figure 3 illustrates high, medium, and low volatility surface contamination events.

Real-Time Monitoring

The AiM provides real-time monitoring of dynamic molecular contamination events in a range of environments. As shown in Figure 3, SMC events can occur very rapidly and often. If a test wafer or a chemical sorber is used alone to monitor contamination, the snap shot nature of their sampling means that their sample timing can significantly influence the results of the analysis. In contrast, AiM's high sensitivity, real-time measurements allow AMC events to be correlated with cleanroom activities or process steps (Figure 4).

Monitoring longer-term AMC mass deposition trends and rates (Figures 5a and b) allows the user to document background contamination levels, facilitating early identification of new AMC sources (such as chemical filter saturation, activities in the facility, or process steps). Notice that for several of the days shown in Figure 5b, the deposition rate was negative. This behavior is expected when volatile compounds desorb from the surface of the sensor chip (or from a test wafer).

Laboratory Analysis - TOF/SIMS

The value of the AiM's mass sensitivity and real-time monitoring can be further extended by conducting Time Of Flight/Secondary Ion Mass Spectroscopy (TOF/SIMS) on the sensor chip. In cold stage TOF/SIMS analysis, the sensor chip is cooled with liquid nitrogen to retain the more volatile contamination species. While chilled, the sensor chip is placed under a vacuum and an ion beam is directed onto its surface. The ion beam fragments the surface contamination (Figure 6), which is then categorized by its mass-to-charge ratio (m/z) to produce a fingerprint of the contamination (Figure 7). TOF/SIMS provides the ability to identify the chemical species on a surface even when present only at trace levels. Combining this molecular identification capability with the mass sensitivity and time resolution of the AiM creates a powerful tool for monitoring and diagnosing AMC problems.

Summary

Airborne Molecular Contamination (AMC) interactions with surfaces are complex physically and chemically and occur on time scales ranging from seconds to days to weeks. To fully understand AMC issues in an ultra-clean manufacturing environment, observations must be made:

- With high mass sensitivity.

- On time scales considerably shorter than brief AMC events.

- Over sufficient duration to be able to establish a baseline contamination level.

When these monitoring criteria are met, short-term AMC events can be distinguished from long-term contamination trends, and AMC events can be directly correlated to activities and process steps. Particle Measuring System's AiM monitor meets these measurement criteria and provides unique insights into the dynamic nature of trace level AMC contamination of SiO2 surfaces.

AiM® is a registered trademark of Particle Measuring Systems, Inc.

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