Airborne molecular contamination can cause a number of problems, but Allen Humphries and Dan Rodier of PMT say technology can now bring early detection and identify sources
Airborne molecular contamination (AMC) is a concern in many industries, especially those involved in semiconductors, computer hardware, including hard disk drives and flat panel displays, and aerospace.
Although the critical molecular species and their specific impact vary, AMC has been found to cause yield losses, product degradation and product failures in all of these industries. For example, in the semiconductor industry AMC 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.4,5 In a cleanroom, AMC either comes from the outdoor "makeup air" or is generated within the facility. Such AMC sources include recirculated air (with inadequate filtration), cross-process chemical contamination, outgassing of cleanroom materials (filters, gel sealants, construction materials, etc.), people and personal care products, and equipment.6,7 The AiM monitor from Particle Measuring Systems provides real-time detection of AMC deposition on surfaces. The data is particularly useful for correlating AMC occurrences with cleanroom processes and events. By monitoring long-term trends in mass deposition, one can establish baseline contamination levels, helping to identify new contamination sources early. 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. AMC events typically last from seconds to weeks, with several simultaneous events involving different sources and molecular species. To effectively monitor, understand and resolve complex AMC issues – before electrical, optical or chemical degradation of critical surface occurs – high sensitivity, real-time information must be obtained.
Surface acoustic wave technology Acoustic wave sensors have been used since the 1950s in quartz crystal microbalances (QCM). In the late 1970s, surface acoustic wave (SAW) sensors were first used in chemical sensing, providing roughly 100 times more sensitive measurements than conventional QCM.1. As SAW and supporting technologies have advanced in the past two decades, sensitivity has continued to improve.2 The AiM, shown in Fig. 1, can measure mass changes of less than 0.2ng/cm2, roughly 0.75% of one monolayer,3 with measurements each minute. The AiM SAW sensors operate at an acoustic resonance frequency of 200MHz. The acoustic waves travel along the surface of the crystal (Fig. 2). As mass increases on the surface, the acoustic wave velocity is reduced and a frequency shift is observed. 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.
AMC deposition 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 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. Fig. 3 illustrates high, medium and low volatility surface contamination events.
AMC events occur very rapidly The AiM provides real-time monitoring of dynamic molecular contamination events in a range of environments. As shown in Fig. 3, AMC 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 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 (Fig. 4). Monitoring longer-term AMC mass deposition trends and rates (Figs. 5a and b) allows the user to document background contamination levels, identifying new AMC sources (such as chemical filter saturation, activities in the facility or process steps) earlier than before. Notice that for several of the days shown in Fig. 5b, the deposition rate was negative. This behaviour 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. 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. In summary, AMC interactions with surfaces are complex – physically and chemically – and occur on time scales ranging from seconds to days, and even 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 Systems' AIM monitor meets these measurement criteria and provides unique insights into the dynamic nature of trace level AMC on surfaces.