Facility-wide ionization monitoring and control systems maintain yield and productivity
By David Feindel, MKS, Ion Systems
Ionization is a facilities requirement that is critical for the maintenance of production yield and output. Unfortunately, it can be very difficult to monitor and control. Typical systems include hundreds of ceiling emitter air ionizers scattered throughout cleanroom areas in semiconductor, hard disk drive, liquid-crystal display (LCD), medical device, MEMS, or nano facilities. Emitters can be connected to six or more controllers, and settings and alarm levels can vary from area to area within a facility. The emitters and ancillary equipment are usually mounted in remote places where they operate without intruding on the process. As a result, an ionization system alarm may not be noted until manufacturing yield or throughput drops, well after the problem was first detected by the ionizer or controller.
To improve the monitoring and control capabilities of ionization systems, the industry is shifting to the use of facility-wide software packages that continually monitor the performance of ionizers and provide appropriate alarms for timely corrective action.
Room ionization systems
Cleanroom ionization systems are designed to eliminate any static charge buildup on the surfaces within a facility by flooding the air with both positive and negative ions. Charged surfaces attract ions of the opposite polarity and are rapidly neutralized. Leading-edge manufacturing facilities such as those for semiconductor front-end, hard disk drive media, and flat-panel display arrays require particle-free environments since particles even 50 nm or 100 nm in size can cause a yield loss. By neutralizing the charge on product surfaces, ionization systems prevent airborne particles from being attracted to and adhering to the product. A second function of neutralizing static charge is the prevention of electrical damage in the product. In-process semiconductor wafers, reticles, LCD arrays, and other products can suffer direct or latent damage from electrostatic discharge (ESD) events resulting from charge accumulating on devices. ESD events can also cause process and tool downtime.
Figure 1. A magnified view of a dirty emitter point, causing reduced ion output. Photo courtesy of MKS, Ion Systems.
A cleanroom ionization system typically consists of 50-1,000 ionizers, mounted to the ceiling just below the HEPA filters. One ionizer typically covers 20-60 sq. ft. (approximately 2-6 m2) of floor space. Groups of up to 80 ionizers are connected to a controller, which is usually mounted on a wall in the fab. Prior practice in these systems used a 4-20 mA loop circuit to connect each controller to a central monitoring computer, called a factory monitoring system, or FMS. The presence of this signal indicates that a warning in the system has been triggered by a controller or the ionizers connected to it. This arrangement makes it difficult for the responsible engineer to detect an ionization alarm and to identify what equipment or procedures would be required to fix it.
An examination of the facilities history within a highly dynamic environment such as an operating fab shows that numerous situations occur that can defeat the remediating action of an ionization system. These include accidental turn off/disconnect of the ionizers, damaged interconnect cables as a result of other facility work, inappropriate operating settings, or temporary or permanent system damage from process equipment electrical interference (EMI). The most common fault lies in a poorly monitored and maintained system that suffers reduced ion output over time due to a buildup of contaminants on the emitter points from the presence of particles in the environment.
Monitoring the system
A good monitoring system will observe ionization activity. However, it can, and should, do more. Key capabilities of a monitoring system should include the following.
Providing a graphical map of the system. This allows maintenance personnel to quickly pinpoint exactly where alarms or other events occur, so they can rapidly fix the issue. The ideal system will provide a graphical summary view of system status, as well as easily understood tools such as panning and zooming for drilling down on suspected problems or issues.
Figure 3. Map of the ionization system magnified to display a bay. Image courtesy of MKS, Ion Systems.
Remote monitoring and control capability. Process and maintenance engineers want the ability to instantly see the entire system’s status at a glance. Ideally, this information can be viewed outside the cleanroom area, and even in other buildings (firewall and IT policies permitting). Being able to remotely view a detailed system status also allows the manufacturer’s support technicians to better diagnose problems.
Providing helpful information on what the problem is. This may take the form of detailed error messages, a display of a parameter’s history over the last several days, or the identification of the probable cause of problem (i.e., next to a recently installed tool).
Figure 4. A detailed view showing the status of all emitters connected to a controller. Image courtesy of MKS, Ion Systems.
Error notification. During initial installation, users often ask that the system be configured to report all errors as soon as they are detected. After the installation, however, users realize that the ability to filter the errors that trigger notifications is important. For example, the user probably does not want to receive a communications error on its first occurrence, but rather only if the same attempted communication fails consecutive times. Users also find it beneficial to set up filter systems for clearing errors and escalating notifications.
Ionizer control. Users need the ability to determine the values for the various ionization parameters of each emitter and to change them where appropriate. Normally, ionization settings should not be changed unless measurements are made using a charged plate monitor (CPM) at the site of the ionizer. However, it can be useful to be able to remotely turn off the ionization in a bay while a tool is being moved in, to change ionization settings as a result of fab-wide environmental changes, or to place an alarming ionizer in standby mode.
Managing scheduled maintenance. The ionizer system must be able to monitor performance over elapsed time in order to issue preventative maintenance (PM) notifications or, if the ionizers in a particular bay are becoming less efficient, to note that they are approaching a needed cleaning cycle.
Monitoring the hardware in the ionization system, providing status updates at user-set intervals. Change occurs slowly in ionization systems and there is typically no need for historical, minute-by-minute records. User-set data logging periods allow system managers to make an intelligent tradeoff between tracing problems with the system and sifting through unchanging data logs.
System scalability. The software needs to accommodate smaller installations (for example, 20 to 30 ionizers in one room) as well as multi-facility installations, with 1,000+ emitters in several buildings.
Monitoring electrostatics in the environment. As manufacturing processes become even more sensitive, using a monitoring system that can also monitor ESD events, grounding, and other electrostatics-related conditions makes sense. For example, it is now possible to detect voltage on wafers as they pass through the front end of a process tool. Measuring this voltage and adjusting the ionization dynamically to compensate for its presence improves the process.
A high-end, industry-standard database. Monitoring and controlling a key manufacturing system requires highly reliable components, including an underlying database engine that logs data over many months or years and allows users to retrieve the data quickly, easily, and without error. The best systems will use a Microsoft® SQL Server or equivalent database.
The presence of a fully capable cleanroom ionization monitoring and control system-whether as part of an initial install or as a retrofit to an existing system-ensures the electrostatic quality of an environment. Such systems are of immediate and quantifiable benefit to facilities and maintenance staff, allowing them to make rapid, fact-based evaluations of the criticality of any ionization problems and to determine the most effective solutions to these problems. The best systems deliver a concise picture of the ionization equipment and conditions in a cleanroom to the manager’s desk, often eliminating the need for cleanroom entry for problem evaluation. The reduction in the number of ESD events that can be achieved by using a properly functioning ionization system can improve facility yield through reductions in tool disruptions and shutdowns.
As the design and fabrication processes for high-tech products become ever more complex, facility-wide ionization monitoring and control systems that ensure proper static control are a requirement for maintaining yield and productivity.
David Feindel is vice president of product marketing at MKS, Ion Systems (Alameda, CA; www.mksinst.com), responsible for all phases of product performance-from development to release to market. He holds a BS in management science and an MBA.