by Mike Misuris, Brooks Automation
Contamination control is widely regarded as the largest single issue affecting yields in semiconductor manufacturing. While once limited in its definition to control of particles, today it includes airborne molecular contamination (AMC) control, temperature and humidity control, vibration control, electrostatic discharge (ESD) and electromagnetic interference (EMI) control, pressure and exhaust control and containment. A more comprehensive term might be “environmental control,” because the goal is to provide reliable and robust process environments.
With the advent of 300 mm processing and the adoption of minienvironments for nearly every tool in the fab, comes increased responsibility and challenge for toolmakers. Because of the variety of factors involved and their increasing complexity and interdependency, traditional approaches to the problem may no longer be effective. As the industry proceeds through the current downturn and anticipates the growth ahead, toolmakers may need to explore alternative approaches that offer tool-based environmental control solutions while lowering their overall costs.
To date, the particle control challenge has been raised and largely met. It is no longer regarded as the limiting issue it once was. Still, control of particles can be a cause for concern, specifically because it is not just a function of filtration media, but is largely influenced by overall design and implementation.
Industry guidelines1 for design and manufacture of tool-integrated environments have been in existence since 1999. However, for a variety of reasons, their implementation is still not widespread. The end result is that a substantial number of tools are burdened with marginal particle performance. Initially, it can affect tool qualification and acceptance; in production, it can lead to reduced process yields.
AMC is regarded of as one of the primary emerging environmental control challenges. ITRS characterizes it as a critical issue, in many cases, for semiconductor manufacturing. AMCs have been defined and categorized,2 and there is a considerable body of literature surrounding AMC.
However, there is not yet a widespread understanding of the quantitative effects of the various contaminants on specific process steps. Still, AMC-related process effects have been identified and preliminary limits have been established for various process steps.3 Fortunately, AMC can be measured and filtered with existing technology.
AMC control requires a unified approach from the fab-level to minienvironments to the FOUPs that transport wafers. Tool minienvironment enclosures represent the last “line-of-defense” against AMC and offer a cost-effective opportunity to protect against specific molecular contaminants at individual process and metrology steps. However, traditional approaches to cleanroom (and, by extension, minienvironment) design have not accounted for AMC. Retrofit of designs can be time-consuming, costly and, in some cases, impossible due to physical constraints.
Controlling AMC will be dependent upon identifying the need during tool development and designing AMC control systems up-front. Materials selection will also play an important role, as the improper selection could actually contribute to AMC (primarily through outgassing) rather than controlling it.
Temperature and humidity control are also emerging as key environmental technology concerns, particularly for lithography processes. Steppers, mask writers and tracks require environments finely tuned with respect to temperature and humidity. Photoresist is affected by variations in humidity and temperature. The precision optics in steppers and mask writers are particularly sensitive to minute variations in temperature. By extension, optics systems inside wafer and reticle inspection and metrology tools, while not as precise as those in steppers and mask writers, are also affected by temperature variations. Humidity control as tight as ±0.1 percent RH is required in some cases. Temperature control requirements for process and metrology equipment can be as tight as ±0.05° C.
ESD/EMI and containment
To combat ESD/EMI damage, SEMI (through E78) has established test methods and acceptable levels for static charge to assist equipment manufacturers in controlling static charge. Controlling ESD/EMI damage at the tool level is well understood, although the implementation of such solutions is subject to subtlety of application-the most popular method being incorporation of ionizers at strategic locations in the tool minienvironment.
In sharp contrast to most areas of the fab where the environmental technology challenge inherent in minienvironments is isolation of the tool/process from the fab ambient, the environmental technology challenge for wet processes, such as those of wet benches, spray processors, CMP and electrochemical deposition, is one of containment.
Keeping the raw materials, chemicals and process byproducts contained within the tool until they can safely be exhausted and scrubbed is the primary objective. More importantly, because these process tools are all connected to house exhaust, they are subject to fluctuations (typically ±30 percent), instantaneous spikes and long-term drift in house exhaust.
Environmental technology is complex and multifaceted, demanding a concerted, systems-level approach to environmental control. Disjointed or sporadic effort applied as a tool approaches the release phase of its development cycle is likely to produce disappointing results, creating problems whose solutions will be time consuming and costly.
Broad expertise must be developed to address the wide variety of concerns.
This would include capability in computational fluid dynamics (CFD). CFD allows the modeling of complex tool geometries under a variety of scenarios; virtual iteration of the design reduces cycle time and decreases the need to build costly mock-ups for verification. This necessitates acquisition of tools and expertise for design verification. These include laser-based airborne particle counters and CO2 fogging equipment for airflow visualization.
For temperature and humidity control, a thorough understanding of thermodynamics, controls and refrigeration systems is required. To address vibration control requires expertise in modal analysis, modeling software and instrumentation and measurement capabilities. Lastly, sufficient depth of knowledge is required with respect to ESD/EMI to allow effective interaction with suppliers of these solutions.
While many toolmakers possess part or all of this capability already, there is often not a steady enough supply of environmental control work within any given toolmaker to justify dedicated resources. As such, resources are often diverted, leading to a lack of sufficient development of the necessary expertise, particularly as complexity increases. Given the cyclical nature of the industry, such resources, where they exist, are often decimated in downturns.
In addition, toolmakers may need to expand their supply base to include vendors of chemical filtration, ESD control products, and additional instrumentation and software. While this is by no means overwhelming, its costs must not be underestimated either. Finally, additional systems integration breadth will likely be required. Again, this is not an overwhelming proposition for any toolmaker but one whose costs should be taken into account.
- International SEMATECH Technology Transfer #99033693A-ENG.
- SEMI F21-95.
- International SEMATECH Technology Transfer #95052812A-TR.
- Pletner, and Brown, Enabling advanced lithography through active vibration control, Spangler, Vipperman, Micro, October 2001.
- Levit and Guon, ESD-Induced EMI and its Effect on Fab Automation, Ion Systems White Paper.
Mike Musuris is senior product marketing manager for Brooks Automation's Environ mental Technology Business Unit. He can be contacted at [email protected].