Achieving comprehensive fab monitoring with optical particle counters
Contamination monitoring bolsters semiconductor manufacturers’ efforts to keep up with shrinking critical dimensions and cost and energy reductions
By Morgan Polen and Bill Shade, Lighthouse Worldwide Solutions
Contamination control in today’s production facilities is specifically targeted at loss prevention, cost reduction, and process control. As critical dimensions shrink and pressure increases for cost and energy reduction in manufacturing, contamination monitoring can be a useful tool to ensure that the cost reduction measures do not impact the production environment.
Various contamination monitors have been used in the past as indicators or watchdogs for contamination events. These have informed facilities, equipment, or process personnel about changes in contamination levels.
These same instruments are starting to be used to fine-tune the facilities, equipment, and utilities for cost and energy reduction. From the high purity water systems and bulk gas systems to the air handling systems and the air itself, contamination monitoring has become a more useful tool in cost and energy management.
Particle monitoring has long been a part of contamination control in semiconductor manufacturing. Optical particle counters are used to measure particle levels in the cleanroom ambient air environments as well as the gases, water, and chemicals used in the manufacturing process. As semiconductor manufacturing advances, new requirements are placed on optical particle counting technology and monitoring systems. This article explores some current trends in semiconductor manufacturing, optical particle counter technology, and monitoring systems, as well as contamination control applications in modern fabs.
Airborne particle counting
The ambient environment in today’s semiconductor fab is often comprised of a centralized cleanroom and the minienvironments for the process tools. This approach provides a high level of isolation that prevents contaminants from entering the wafer processing area. The cleanliness of these environments is strictly controlled to ensure that contamination does not impact yield and throughput in the manufacturing process.
Most fabs certify cleanliness periodically according to the ISO 14644-1 standard. In addition, today’s semiconductor manufacturing facilities frequently employ a centralized particle monitoring system for monitoring particle levels in the process tool environments. Typically these systems include multiple remote particle counters sampling in real time from several locations simultaneously or a single particle counter sequentially sampling from multiple locations via a manifold. The particle sensitivity of the instrument utilized depends on the cleanliness levels of the environments that are monitored.
The real-time application of remote particle counters places extreme requirements on the design for lower cost and high reliability since a fab typically employs a significant number of these in a monitoring system. In addition, as these instruments are used to monitor inside minienvironments, process tools, or stocking systems, their size can often become an issue. Figure 1 shows a 0.2-µm remote particle counter that can be used as part of a real-time monitoring system. The current state-of-the-art remote particle counters can have laser diode lifetimes of 20 years or greater.
A sequential sampling manifold system includes a manifold and portable particle counter. The manifold contains several ports that are connected to individual tubes routed to various locations in the fab. Figure 2 is a layout of a sequential sampling manifold system. The manifold mechanically samples from the various ports and delivers the sample to the portable particle counter. The current trend with these systems is to be able to control sample locations down to a few particles at 0.1 µm. Therefore, the manifold must be very carefully designed with flow efficiency in mind and no right angles in the flow path. This approach allows for better particle transport. In addition, the design must not generate any particles from either the internal mechanisms or from cross-contamination.
The portable particle counters employed in these systems are often used interchangeably in numerous other fab applications. These universal instruments are used as portable devices in a cleanroom monitoring plan or can be used for troubleshooting particles in a particular process tool or area. In addition, these devices are used for measuring particles in gases on-line or in a portable manner. Because these instruments are used in so many applications throughout the fab, they must be portable in nature and, as such, be as lightweight as possible and capable of running on a battery. They also must be easy to operate for personnel who are required to use them on infrequent occasions.
Today’s semiconductor manufacturing is composed of fully automated material handling and process equipment as well as minienvironments for isolating the product at all times from the central facility. These advances in the manufacturing process have created the need for optical particle counting instrumentation to facilitate the measurement of particles in minienvironments and stockers as well as to facilitate the troubleshooting and monitoring of complex automated process equipment. The result has been the advent of smaller sampling devices that permit the sampling of a handful of locations in a minienvironment, stocking system, or in and around a process tool using a single portable particle counter. Some of these devices use an ensemble averaging technique to draw in the sample from multiple locations simultaneously. Such sampling techniques cannot identify the specific location of the particle source. Other devices use sequential sampling techniques to monitor multiple locations. These devices have the ability to easily partition the particle measurement between different locations, helping pinpoint the source of particle issues rapidly. Figure 3 is an example of one such sampling device, which permits multipoint sequential measurement in a portable manner.
Gas particle monitoring
Another potential source of particle contaminants that can affect semiconductor product is the bulk gases used throughout the semiconductor manufacturing process. For this reason, many leading-edge fabs monitor critical high pressure gases for particles on-line. Historically this has been accomplished using particle counters specifically designed for measuring high pressure gases or using airborne particle counters in conjunction with high pressure diffusers. Current particle counters for measuring high pressure gases have relatively low flow rates and limited sensitivity. In addition, the complex design required for high pressure gas measurement results in a high cost-of-ownership due to periodic maintenance costs. High pressure diffusers use an orifice or series of orifices to reduce gas to atmospheric pressure. As such, these can be easily contaminated and can have long cleanup times. In addition, high pressure diffusers typically exhaust a significant percentage of the gas sampled, which is an added cost for this method of sampling.
As the industry moves to smaller linewidths, there has been a drive to improve the performance of optical particle counting instrumentation for gas sampling, including the sensitivity and sample flow rate. Recent advances in gas-sampling optical particle counting technology include the development of high pressure controllers. These devices reduce the pressure of the gas to atmosphere without any wasted gas and are less prone to contamination than high pressure diffusers. In addition, these devices have extremely short cleanup times. The result is efficient, reliable on-line gas measurement at 0.1-µm sensitivity and a 1.0 cfm flow rate using a universal airborne portable particle counter described previously.
In addition, because of the simplicity and rapid cleanup of these devices they can now be used easily in portable applications like checking incoming gas quality and filter performance as part of process tool startup and qualification. The use of these devices for tool startup and qualification can be expected to dramatically reduce startup costs and time.
High-purity water particle monitoring
In a typical fab several thousand gallons of high purity water come in contact with each wafer. For this reason, the output of the main high purity water plant is monitored extensively for several forms of contamination to ensure that the water does not impact product quality. Particles, TOC, dissolved oxygen, and silica are some of the forms of contamination monitored. The 2007 International Technology Roadmap for Semiconductors (ITRS) states the recommended particle levels at 50 nm are 0.3 particles/ml. This specification places extreme demands on the design of instrumentation to measure particles at these sensitivities and low concentration levels.
There are several industry trends that are likely to shape the future of particle monitoring for high purity water. Water conservation is one trend. The industry is developing specific goals to reduce incoming water consumption in the years to come, which is likely to lead to more recycling and reuse of water where possible. At the same time, local water purification solutions are emerging for specific processes where the purity of the water is critical. These trends are likely to drive the need for additional monitoring including point-of-use monitoring in key process steps.
In response to these changing requirements, optical particle counting instrumentation is advancing to reduce the background zero count levels to improve measurements at current and future concentration levels. In addition, instruments are now available that are truly portable so their application can be extended beyond the main DI plant into the fab and process.
Chemical particle monitoring
Optical particle counting is also used to measure particle levels in critical chemicals that are supplied to the fab by the bulk chemical delivery system. Current measurements are made at 0.065 and 0.1 µm with existing instrumentation. As the industry moves to smaller device geometries, it places demands on the instrumentation to improve sensitivities and/or background levels to provide reliable measurements in the near future. Given the current ITRS recommendation for critical particle size and number of particles, a 0.065-µm particle counter must measure particle concentrations at or below 1 particle/ml today. Clearly liquid particle counter instrumentation must continue to advance to meet the industry’s needs.
Fab-wide environmental monitoring systems
Fab-wide monitoring for particles and other environmental parameters is commonplace in most fabs today. These computer-based systems are composed of the various instruments previously discussed and other environmental instruments connected into a centralized data acquisition and analysis hardware and software system. Historically these systems have been used for environmental monitoring of the cleanroom.
At present, a number of additional applications for these systems are emerging that are in part being driven by a need to reduce costs and energy consumption. One such application is continuous monitoring per ISO 14644-2. ISO 14644-2 states that if a continuous or frequent monitoring system for particle counts and differential pressure is employed and these parameters are within the specified limits, then the cleanroom is in a state of compliance and does not require bi-annual or annual recertification. Therefore, users are starting to employ these continuous or frequent monitoring systems as part of their monitoring plan to reduce certification costs.
Another application that is being explored is realizing energy savings by using particle counts as feedback to control air handling units, thereby reducing energy consumption of the air filtration as much as possible. Given the cost of energy today and the present concern about preserving our global environment, this is an exciting application for these systems that is sure to be realized in the near future.
As fab-wide monitoring systems are utilized for more advanced, mission-critical applications, it drives the need to improve the overall performance of these enterprise systems. These systems must maintain higher uptimes, and that is now being realized through a number of system features being put in place.
At the instrument server data acquisition level, redundant controller architecture provides assistance in the event of any hardware or software failure on the main controller. At the database level, a clustered database deployed on multiple machines can dramatically improve the uptime of the database engine.
Another improvement in these systems is the development of open standardized interfaces. Modbus is commonplace at the data acquisition level and OPC is often an available interface to the data and instruments. These open interfaces permit these systems to be readily interfaced to building management systems that may need to perform advanced functions such as controlling fan filters or air handlers based on the collected data.
Semiconductor manufacturing is continuously changing as device geometries shrink in an effort to reduce cost and improve device performance. As the industry moves to smaller linewidths, there is continued focus on contamination control, including the use of optical particle counters and their fab-wide monitoring systems. The industry’s continuous adoption of these instruments and systems is in turn driving investment in the innovation of the technology and its application to realize additional cost savings and yield improvements.
Morgan Polen is vice president of applications technology and Bill Shade is vice president of marketing and engineering at Lighthouse Worldwide Solutions, headquartered in Fremont, CA (www.golighthouse.com). They can be reached at (510) 438-0500.