Detecting Contamination in Semiconductor Processing is Key for Particle Counters
Surface particle detection is the “first line of defense” against killer surface particles unknowingly introduced into the cleanroom on the surfaces of equipment, parts, supplies, garments, and even on packing materials.
By SUSAN ENGLISH
Today, the world`s most advanced semiconductor chips are manufactured in a 0.35-micron process. That includes Intel`s Pentium Pro microprocessor, which runs at 200 MHz and packs 5.5 million transistors on a single slice of silicon. The cleanroom standards of cleanliness required to protect those minuscule geometries may seem stringent today, but consider where the technology is projected to lead us tomorrow. Analysts at research firm DataQuest (San Jose, CA) predict that as we enter the next century, microprocessors will be sporting 20 million transistors and will be fabricated with a 0.15-micron process. Particles that are not even detected, or detectable, in today`s fabs will become killer contaminants in the year 2000.
Will technology have reached its outermost limits at that time? Don`t bet on it, says Frank Granzeier, a marketing consultant with Dryden Engineering Co., Inc. (Fremont, CA). The company has long viewed contamination control as a “key enabling technology for the 21st century” and particle detection as the first line of defense against cleanroom contamination. Demanding as it is today, warns Granzeier, the job is only going to get tougher. DataQuest is also projecting that in 10 years, microprocessors will have moved to a 0.05-micron process, will run at 3,000 MHz, and have as many as 50 million transistors on a single chip. In order to stay ahead of this demanding curve, Dryden`s research team has been directing its attention to a neglected area of contamination control technology– surface particle detection. The development of instrumentation to detect and eliminate airborne contaminants has taken off, and it is common in today`s best fabs to read zero airborne particles at 0.1 µm. throughout ultra clean environments.
The hidden culprit: “Killer” surface particles
Despite improved air purity, however, wafers continue to be contaminated. Yields still drop suddenly and mysteriously. The industry continues to struggle with efforts to maintain Overall Equipment Effectiveness (OEE) at acceptable levels. According to Gene Sullivan, Dryden`s president, a primary cause of continuing contamination problems can be traced not to airborne particles, but rather to killer surface particles unknowingly introduced into the cleanroom on surfaces that were thought to be clean. Sullivan says this includes the exterior surfaces of process equipment, as well as chamber interiors, equipment parts, boats and boxes used to port the wafers, and even on cleanroom garments and other supplies brought into the cleanroom.
Dryden`s research on surface particle contamination culminated in 1992 in the development of the QIII Surface Particle Detector, which was later enhanced in 1995. The Model DE3496 SPD, Rev. provides particle concentrations per in.2 or cm2 in five sizes from 0.3 µm to 10 µm. It measures surface contamination in and on process equipment, product containers, minienvironments, tables, benches, walls and any nominally flat surface. It detects particles sizes from 0.3, 0.5 1.0, 5.0 to 10 microns. All five channels of data are gathered at the same time, while one channel is displayed. Selectable sample times are 1, 3 and 6 seconds, with a flow rate of 1 cfm. Options include an external printer, carrying case with casters, portable cart, an exhaust ULPA filter, custom probes and a 90-degree handle.
The system`s initial application was to replace time-consuming rinse tests designed to ensure cleanliness of wafer boxes. The instrument provides particle-count readout in a matter of one to six seconds by passing a probe-head over the surface to be tested. “Users of the surface-particle detector,” says Sullivan, “rapidly become strong proponents for the instrument, and understandably so, since semiconductor value has advanced to the point where a single contaminated box containing 25 wafers could possibly ruin product valued in excess of $1,000,000.”
According to Sullivan, protection against surface particles should start with certification of the cleanroom itself, not only ensuring that ceilings, walls, floors and all other surfaces are initially clean, but that their component materials are not subject to rapid deterioration, spewing out killer particles in the process. Once a cleanroom has been certified as clean, it is critical to eliminate the possibility of particles entering the cleanroom on the surfaces of equipment, parts, supplies and personnel. These particles must first be detected before they can be eradicated. In many instances, equipment and parts that have been cleaned and shipped to a fab arrive in a contaminated state, due to packing materials that have been used. Instruments such as Dryden`s QIII Surface Particle Detector can detect contamination, identify its source, and signal for additional cleaning procedures before equipment is even installed.
Documenting particle detector applications
During a particle contamination control meeting for users of the Dryden Engineering QIII Surface Particle Detector, held during Semicon West in July, a series of case histories documented the instrument`s ability to increase OEE for process equipment in cleanroom operations. Presentations from diverse disciplines, including process equipment manufacturers, semiconductor fabs, testing laboratories, laser optics processing and component suppliers covered applications ranging from evaluating the cleanliness of equipment surfaces and chambers to components and varied cleanroom items, such as disposable garments and gloves. The following emerged as the three most pervasive in a broad spectrum of applications for the particle-metering system: (1) validation of cleaning procedures (2) troubleshooting contamination problems, and (3) establishing benchmarks for cleaning procedures.
Among the seven technical presenters, Lynn Armentrout, senior engineer at Advanced Micro Device`s Fab 25 in Austin, TX, included the following in his list of applications for the surface particle detector: monitoring cleanroom construction during fab construction; tool acceptance; installation of all tools, tool parts and piece parts; and piece-part management for items such as wafer boxes, bubble wrap, and foams. Technicians using the particle detection system learned that many shipping materials cause as much contamination as anything else, particularly on component parts for tools and equipment. In building and qualifying any type of cleanroom operation, in some instances, users pay as much as $50,000 to have a single tool shipped in a specific manner. This cost is more than recovered in the amount of time saved in qualifying tools in the fab and bringing them up to production status. Armentrout said AMD is saving time simply by insisting on a particular methodology to ensure that tools are delivered clean.
In one instance, the time required for wet clean product maintenance on a particular tool was reduced from 15 hours to three hours, based on data derived from the particle detector. In another instance, the instrument led to dramatic OEE improvements for a particular semiconductor process equipment tool, extending the average cleaning cycle from a daily occurrence to a maximum of six days without requiring any cleaning maintenance of any kind. The QIII Surface Particle Detector was used to identify the sources of particle contamination and then eliminate them, resulting in increased up-time for the tool.
Among experienced users attending the meeting, there was consensus that data can be skewed through improper usage of the instrument and that operators need to be trained for precision repeatability of measurement methodologies. Attendees also agreed that absolute numbers of particle-count are less important than relative numbers, which are sufficient to evaluate the effectiveness of cleaning procedures and to identify trouble spots.
Facility monitoring systems
Daily monitoring is a regulation in the pharmaceutical industry. This regulation creates the need for smaller, less expensive particle counters that can be easily moved or permanently installed as multi-point facility monitoring systems. All pharmaceutical manufacturers must protect their products from contamination by dust. Even non-viable particles can affect the clarity of liquids or the passage of dry powders. Severely contaminated parenteral solutions may cause capillary blockage, and chemically reactive materials in dust can affect a product`s chemical purity. Monitoring for airborne particles is a proven method of controlling product quality. Airborne contamination monitoring is also necessary to comply with the U.S. FDA`s cGMP requirements, as well as those of voluntary standards organizations. The requirements regarding accepted monitoring procedures promise to become even more specific in the future.
According to a study by The McIlvaine Company (Northbrook, IL), the two types of facility monitoring systems in use today are tube transport systems and remote sensor systems. Tube transport systems are the most commonly used and the least expensive. One particle counter may be used to monitor up to 30 remote locations using a multiport sampler, or manifold system. However, sampling is sequential instead of real time, or continuous. Remote systems are used for continuous monitoring of up to several hundred locations simultaneously. Sensors are downloaded into a personal computer directly, or through a multiplexer. Although most remote sensor systems are configured to monitor two sizes at the same time, usually there is no capability to obtain distributions of sizes.
Lighthouse Associates (Milpitas, CA) offers a real-time cleanroom monitoring system with complete data management to track cleanroom performance 24 hours a day. The system supports sensors from any vendor: particle, ESD, temperature, humidity, differential pressure, air velocity, water or chemical. Using a unique method of data reduction called adaptive data control, the system allows customized specification of recording rates for each sensor. Data is then analyzed by means of an MS-Windows 3.1 graphical user interface that makes both the presentation and manipulation of the data easy and intuitive. Proactive, the system provides current status–alarm, warning, normal–of every sensor in the cleanroom through real-time maps and graphs, through-lights and enunciators. The system is modular and can be used in small fabs with a few sensors or networked to multiple buildings even across the world.
Laser particle counting systems
Most recommended practices, in-house specifications, military specs and standards bodies recommend light-scattering particle counters for monitoring airborne contaminants. The light source is either white light, laser or laser diode. The least expensive is white light sensor, which usually has a 0.3 1 µm lower sensitivity and a relatively short lamp life. The most expensive light source–the laser– is capable of much lower sensitivities and has a very long life. The newer, low-powered laser diodes last longer than white light or regular gas lasers, but at the same flow rates, they are not cost effective for sensitivities lower than 0.3 µm. Terra Universal, Inc. offers a comprehensive line of laser particle counters for monitoring cleanrooms, workstations, labs and other critical environments. Available in stock are counters for measuring particle diameters of 0.1, 0.3, 0.5, 3 and 5 µm. Lightweight and compact handheld particle counters and concentration meters provide on-the-spot particle detection. The larger Airborne Particle Counter, available with or without a printer, is ideal for cleanroom certification. The Portable Airborne Particle Counter, which weighs less than 7 lbs., can display a count of total particles or particles within several selected size ranges.
Particle Measuring Systems`s LASAIR aerosol optical particle counters are specifically designed for monitoring contamination in cleanroom facilities, using a passive laser cavity and sample flow cell as an integral assembly, which is easily removable for servicing. Minimum sizing thresholds are available at 0.1, 0.2, 0.3 and 0.5 µm at sample flow rates from 0.001 to 1 cfm. FS-209E air cleanliness certification is available, along with an optional 3.5-in. disk drive and electroluminescent CRT display. The LASAIR particle counter can record critical contamination trends, as well as short term bursts, and will display the data in a time series histogram. Automatic sampling and storage of up to 99 data samples can be programmed into the system, and printed reports are available in full screen or compress print.
The Laser Particle Sensor Model LPS-C offers the microcontamination control industry a family line of compact sensors for measuring airborne particles. The small size of the LPS-C makes it suitable for particulate monitoring where individual sensors are required and space is at a premium. Sizing sensitivities for the LPS-C particle sensors are 0.2, 0.3 and 0.5µm, with a flow rate of 1.0 cfm, using an external vacuum pump. Data collection from the LPS-C is provided by the STAR NODE data logger, designed to collect information from up to eight LPS-C sensors. It communicates with a facility monitoring system on a LAN with RS-485 capabilities.
The HIAC/ROYCO Division of Pacific Scientific Co. (Silver Spring, MD) supplies particle measurement systems to the fluid power, industrial machinery, semiconductor, pharmaceutical and potable water markets. Its 8103 particle counting system is a “no frills” particle analysis package ideally suited for well-defined tasks such as the USP (788) small volume injectable test tool for applications such as USP (788) particle limits testing and general particulate analysis. Comprised of an ABS-2 bottle sampler, an 8000A counter and an HRLD 400 laser diode sensor, the system is self-contained, has no computer, requires minimal operator training and features reduced system validation activities.
HIAC/ROYCO`s 9103 particle counting system is designed for pharmaceutical applications such as particle count limit testing [USP(788)], small-volume injectables, large-volume injectables, plus blood products, dialysate, opthalmics, research samples, saline solution and vaccines. The BR8 (Beta Ratio 8) standalone particle counter system for on-line liquid filter testing of hydraulic fluids as well as fluids used in the food and beverage, pharmaceutical and chemical industries. BR8 simultaneously samples upstream and downstream fluid to prevent misinterpretation of data due to batch sampling. The BR8 is pre-programmed with NAS 1638, Mil-Std-1246, ISO 4406 and user-definable measurement standards.
Touted as “the economical choice” in a particle counter and the “smallest and lightest weight 1.0 particle counter available in the market,” the MicroPro, from Climet Instruments (Redlands, CA) uses a 1 cfm blower to take air samples 10 times faster than handheld products, according to the company. The instrument utilizes a laser diode sensor which requires no routine maintenance. Particle counts can be displayed in total counts or concentration. At the conclusion of sampling activity, the MicroPro can quickly deliver hard copy data via an internal thermal printer. Standard on the MicroPro are two particle channels, automatic flow control, a quiet DC blower, 200 sample memory, time/date stamped samples, selectable alarm preset, variable time delays and variable sample volumes.
System management software
Particle Measuring Systems` Facility-View software package provides a comprehensive account of all environmental conditions within a cleanroom. A Windows-based program, it lets users simultaneously view tabular displays, real-time or retrieval time plots, 3-D histograms, status conditions, event logs and a facility map for every monitoring instrument. Networking capabilities are available, allowing communications with other computers using the TCP/IP protocol to distribute data among many users.
Each instrument can support user-defined sample intervals and can be grouped to an alarm device. The software program also has a paging function that uses a modem to send numerical codes to specified pagers to notify of alarm events or other occurrences. Other features are powerful calculation capabilities that enable an operator to combine any number of sample points using a mathematical or logical expression to generate a new sample point and a flexible security system that allows password access for any number of users. Facility-View also provides data management functions such as the ability to control the amount of data to be stored for each monitoring instrument. All data is automatically compressed into a format that reduces storage requirements.
HIAC/ROYCO has developed PharmSpec, a Windows-based software package designed for its pharmaceutical customers to ensure current and future system operation. PharmSpec performs all USP (788) testing requirements, while at the same time controlling the operation of the company`s enhanced Model 9064 particle counter and Model 3000A liquid syringe sampler. Minimum computer requirements are an IBM-compatible 386 processor, DOS 5.0, Windows 3.1, a VGA color screen, a mouse, 4 MB RAM and 20 MB disk space. Copies of system documentation are available to customers for validation of the product within their own process. n
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