Managing CMP Contamination with environmentally enhanced enclosures
06/01/1997
Managing CMP contamination with environmentally enhanced enclosures
Christopher Young, Intelligent Enclosures Corp., Norcross, Georgia
Contamination associated with CMP slurry, solvents, and ambient impurities must be controlled to reduce related defects. Most current enclosure designs, however, are insufficient for total control of the polishing process environment. Both operators and products benefit from an integrated enclosure system that minimizes contamination and controls environmental conditions and exhaust in the CMP tool. Laminar filtered airflow, dynamic chemical exhaust, high relative humidity regulation, and engineered airflow design are required.
The chemical mechanical planarization (CMP) market is maturing more quickly than the original forecast by SEMI in 1994, and its growth is exceeding supplier expectations [1, 2]. Shrinking design rules, increasing numbers of metal interconnect layers, and increasing demand for planarization from DUV lithography systems are spreading the use of CMP in the microelectronics industry from microprocessors to DRAM, flat panel displays, and multichip modules [3].
Integrated CMP systems
Industry experts predict that the CMP process of the future will be a complete and integrated manufacturing solution featuring a "dry in/dry out" process flow [4]. A complete and integrated CMP production line will consist of:
a polisher,
consumables,
chemical distribution,
cleaning systems,
measurement,
process isolation,
environmental control, and
a material handling/facility-layout strategy.
An optimized and fully integrated CMP production line will also include an enclosure system to manage and control the process environment. Currently, most CMP tools either do not provide enhanced process environments or fail to control cross-contamination and environmental conditions fully.
Slurry and contamination
The slurry used in wafer planarization makes CMP a "dirty" process. Both cross-contamination and dried slurry ultimately result in wafer defects and low production yield. In an aqueous solution, slurry particles can be successfully managed for wafer polishing and planarization. Stagnant slurry and slurry particles that fall out of the solution, however, dry to a powder or cement-like state. Dried slurry particles deposited during processing scratch and gouge the wafer surface, eventually forming metal shorts after successive metal deposition processes [4]. Wafers, carriers, boxes, or personnel leaving the CMP production area can cross-contaminate other areas and processes in the fab.
Chemicals used in the CMP and post-CMP cleaning process can be hazardous. According to Occupational Health and Safety Administration (OSHA) guidelines, chemicals and their vapors (e.g., sodium hydroxide, potassium hydroxide, and acids) and solvents used in slurry and cleaning solutions in the CMP process pose a health risk to operators. If not contained, these chemicals can also cross-contaminate other production areas that are supported by the CMP process. Lithography processes using DUV photoresist, for example, require planarization. Ammonia, which is present in certain CMP applications, is a "killer" contaminant at ppb levels for ultrasensitive DUV photoresist, as it neutralizes the photoresist acids.
Slurry and environmental conditions
Planarization rate, equipment downtime, and maintenance activity all affect wafer throughput in the polishing process. The planarization rate and uniformity are affected by the slurry viscosity and the ability of the polishing pad to transport liquid and remove excess slurry [5]. Slurry viscosity is affected by the liquid content of the solution and environmental conditions. In uncontrolled enclosures, environmental conditions, such as temperature, relative humidity, airflow rate, and air turbulence, may vary from application to application.
Dry production environments and environments with high airflow rates tend to dry out the slurry. Wet slurry may be easily rinsed away, but dried and hardened slurry is much more difficult to remove. Dried slurry buildup on the tool and in delivery lines must be routinely cleaned. Clogged slurry delivery lines must be flushed and cleaned, thus requiring additional tool maintenance.
Facility layout requirements
Contamination issues present logistical problems for integrating the CMP sector with other production tools. Class 1 cleanroom space is unnecessary and costly for the process area. A common practice is to separate the polisher physically from clean/dry and input/output (I/O) areas and metrology areas with cleanroom walls. Alternatively, stand-alone polishers may be placed in gray cleanroom areas (ranging from Class 100 to Class 10,000), but a special material-handling procedure to stage, transfer, and transport wafers safely to and from other parts of the production line must be used. In either case, personnel must access both the clean fab space and the gray CMP production areas. The separation of the tools often requires that personnel change garments between each area to prevent contamination of the clean areas of the fab [6].
While any proposed layout strategy will have advantages and disadvantages, a robust plan will minimize or even eliminate the impact of cross-contamination from slurry and other chemicals used in the CMP process in any environment. For example, a well-designed, integrated CMP system with clustered cleaning and metrology tools employing dry in/dry out process flow should perform without problems in environments ranging from Class 1 to Class 100,000.
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Figure 1. Typical CMP enclosures a) consist of a fume hood-type system to capture and exhaust contamination. Enhanced CMP enclosure systems b) provide airflow and environmental control for both the I/O and polishing chambers.
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Figure 2. Hood-type enclosures a) operate at negative differential pressure to prevent fumes from escaping the polishing chamber, which also permits infiltration of ambient contamination. Each color contour is associated with a particular value of pressure shown on the scale. In an enhanced enclosure b), precise control of differential pressure in the I/O and polishing chambers protects wafers from both ambient and internal contamination sources.
Standard process environments
Many CMP tool suppliers incorporate some type of enclosure or hood system to contain slurry and chemical vapors (Fig. 1a). The standard tool configuration supplies only filtered air to the I/O area, using positive air pressure to protect staged wafers from polluting sources. The polishing process area is usually operated with negative differential pressure (i.e., the pressure in the polishing area is lower than the pressure in the I/O area and the cleanroom), using facility exhaust to remove gaseous chemical contaminants.
This type of hood enclosure system is neither adequate nor optimized for total control of the environmental conditions that affect wafer throughput and yield. Operating a CMP tool enclosure with negative pressure allows infiltration of fab-generated impurities. A computer model (Fig. 2a) shows the differential pressure with respect to the ambient cleanroom resulting from the exhaust design in a typical CMP hood enclosure.
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Figure 3. Typical cleanroom layout for CMP equipment and metrology support.
If the polisher is located in a gray cleanroom area (Fig. 3), the chance for infiltration of contaminants from the degraded environment is increased. Macroscopic defects typically result when large particles enter the polishing chamber from either ambient sources or dried slurry buildup within the process area [7].
In addition, high air velocity (Fig. 4a) and excessive turbulent conditions due to the lack of airflow regulation increase the drying rate of chemical slurry and create more airborne slurry particles. The most effective way to capture and remove airborne contaminants from an enclosed process is by providing a laminar or unidirectional airflow path with minimal turbulence from an adjustable supply to the system exhaust. The nonlaminar airflow shown in Fig. 5a is typical of CMP equipment designed with a hood and exhaust system without airflow management. This design leads to stagnant and recirculating air zones in the polishing chamber that trap both chemical vapor and aerosol contaminants in the process area. Poor airflow and exhaust conditions also place operators at risk of exposure to chemical vapors while accessing the polishing area. Figure 6a shows the spread of vapors released from the polishing platens in a hood-type enclosure with exhaust located in the ceiling. An operator is at risk of exposure while accessing the polishing chamber.
Enhanced process environments
An enhanced enclosure can control and eliminate contamination, provide operator safety, protect wafers, and stabilize environmental conditions affecting the polishing process. Potential contaminants in the CMP process include slurry, cleaning solutions, chemical vapors, equipment-related contamination, and fab-generated pollution. Manipulating airflow conditions in both the I/O and process areas with controlled air supply and exhaust systems can fulfill these requirements (Fig. 1b).
Airflow variables that must be monitored include air velocity, airflow uniformity, turbulence, differential pressure, exhaust flow rate, and exhaust pressure. A complete system evaluation of these variables using computer modeling and physical testing is indispensable.
Airflow design and contamination control. Airflow design is critical, especially when the enclosed process produces most of the contaminants. Ambient contamination is eliminated with proper enclosure pressure, while internal contamination varies with air velocity, uniformity, and turbulence. Unfortunately, most CMP equipment was originally designed without airflow control in mind. CMP equipment suppliers usually provide a water spray
inse system to wash away excess slurry from the polishing platens and basin. While these spray nozzles may wash away most excess slurry particles, the high pressure and velocity disrupt normal airflow conditions within the polishing chamber. The resulting water spray can create areas of slurry buildup on the interior walls and components of the polishing chamber. This effect is clearly visible using standard techniques of airflow visualization, such as clean fog testing.
Even though the planarization process is "dirty," external particles that may end up in the slurry or on staged wafers can still be minimized. A Class 1 or better area is ideal for wafer-transfer operations and input (or output) to the polishing chamber. This is possible by precisely pressurizing the I/O area relative to both the ambient cleanroom and the polishing chamber. Excessive pressurization of the polishing chamber may result in slurry and vapor escaping the enclosure.
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Figure 4. In a hood enclosure system a), high differential pressure between the I/O area and the polishing chamber results in excessive air velocity through wafer pass-through ports and across polishing platens. In an enhanced enclosure b), precise air balancing of the filter velocities in the I/O and polishing chambers results in controlled airflow through pass-through ports and across platens.
CHRISTOPHER YOUNG received his BS degree in physics from Georgia State University. He joined Intelligent Enclosures Corp. in 1992 and was initially responsible for optimizing mini-environment airflow using computational fluid dynamics modeling. He is presently CMP product development manager and has just completed development of a new advanced CMP enclosure system for semiconductor ap-
plications. Intelligent Enclosures Corp. (iEAir), 1 Meca Way, Norcross, GA 30093; ph 770/564-5640 or 770/564-5681, fax 770/564-5548, e-mail [email protected], URL http://www.ieair.com.