Adopting controls solutions for auto chemicals management
08/01/2002
By Jay Jung, Josh H. Golden, Marc van den Berg, Microbar Inc., Sunnyvale, California
Overview
Semiconductor manufacturers still lag behind other industries in multitool process control and information-gathering capability, particularly for chemical handling. Implementation of general systems that integrate hardware and process control can greatly simplify wafer processing and process troubleshooting. In the area of chemical handling and distribution, off-the-shelf hardware and control systems produce both process data and a mass balance of chemical input and output.
Increasing tool and fab-wide process efficiency is driven by the introduction of more costly and sensitive materials used in IC production at the 130nm technology node and beyond. The higher costs of these new-generation materials result from increased R&D expense, which includes synthesis, integration testing, production, packaging, and handling. Now, specialty electronic materials costs are also more closely linked to their limited market lifetime within a given technology node [1].
As a result, reduction in materials waste has emerged as a critical issue. Consider, for example, the use of spin-on DUV photoresists. Recently, TSMC in Taiwan reported that 9% of its total materials cost is due to photoresist consumption [1]. Much of this reported cost is due to resist waste; typically, 99% of resist is lost as waste in the spin-on process. We have estimated that resist loss alone can exceed $500,000/track/year [2]. To address these issues, over the last several years, track manufacturers, such as Tokyo Electron and Dai Nippon Screen, have invested in R&D programs that decrease resist consumption by reducing dispense volume. Emerging technologies include piezoelectrically actuated microdispense, scan-coating, and sophisticated software algorithms that optimize fluid application and the subsequent spinning step.
Some of these concerns in the photochemical arena have crossed over to the emerging generation of spin-on materials such as low-k dielectrics and spin-on barriers [3]. Significantly, the general inefficiency of the spin-on process has catalyzed new efforts to reduce costs by streamlining spin-on related controls and automation systems.
Simple fab automation solutions
Paradoxically, while the semiconductor industry has rapidly adopted sophisticated cutting-edge design and fabrication technologies, many wafer fab factory control subsystems and infrastructure methods are not cutting-edge. In many cases, they are antiquated. This is in contrast to other more mature industries, such as food, paper, and pharmaceuticals. These industries have traditionally used factory-wide, prepackaged automation solutions to achieve process improvements and decrease materials consumption. Many of these engineered software packages are "off the shelf" and are standardized across different toolsets and factories.
Software providers specializing in industrial controls automation include Wonderware, Intellution, Rockwell, and GE Industrials. These companies can provide foundation platforms that can be built on, turnkey packages specific to a factory or process, or software for company-wide process automation linkage and electronic enterprise. The broad applicability and flexibility of these packages are reflected in their successful implementation in a variety of applications from ephedrine production to glass bottle manufacturing.
In contrast, semiconductor equipment manufacturers and fabs have largely developed in-house automation and software solutions, in the search for process control with optimized device yield, or in many cases, to attain less than optimal process goals.
Until recently, little attention was given to materials and chemicals management and consumption and their connection to new ideas in standardized fab-wide automation and process control. This is mostly due to the wide variety of tool sets adapted from different suppliers, each with a specific set of control and automation requirements.
For example, communication protocols can vary widely from manufacturer to manufacturer. Typical protocols include ASCII, but many are home-brewed, and need significant programming tweaks to be effective in disparate tool-to-tool communication. Moreover, the venerable semiconductor equipment communication standard (SECS) is limited in functionality and flexibility for peer-to-peer communication, and is thus cumbersome to implement in many cases.
In some cases, an in-house custom control approach has allowed semiconductor equipment manufacturers and fabs to achieve a higher level of sophistication and lower cost than commercially available products or solutions. Custom motor control in a lithography track requires sophisticated motion control to coordinate multiple axes. Many off-the-shelf motor drives are limited because they combine amplifier and position control. This ultimately limits the axes of control, hence the need for customization. In other cases, the lack of fab-wide or fab-to-fab standardization of controls and materials management can yield inflexibility and materials waste by limiting and stifling coordinated automation. Fragmented and nonstandardized home-brewed controls and automation schemes are also particularly susceptible to personnel turnover, where the required knowledge leaves when the responsible engineer changes jobs or companies. Perhaps, the IC industry suffers from this syndrome more than other industries because of its more extreme business cycles.
To overcome most of these problems, chemical management and control systems must be detached from process tools and proprietary control systems. In their place, off-the-shelf software and hardware packages should be integrated, either semilocally or fab-wide. These systems are designed and especially well-suited for chemical management, handling, and delivery in IC fabs.
Typically, standardization of local and fab-wide controls systems for materials handling and dispense yields simplicity of operation, enhanced flexibility, lower costs, and decreased raw material consumption. In addition to standardized tool hook up, other benefits can include:
- proactive process control with histograms of equipment uptime and performance;
- Internet tracking of process data and equipment performance data from remote sites;
- maintenance and service logs for reference and predictive maintenance;
- security for chemical validation;
- automated mass balance of chemical consumption and waste for environmental and operational expense measurements;
- automated customer support; and
- an automated procurement link for consumables.
One fab facility in California that manufactures power-amplifier ICs recently opted for a fab-wide photochemical delivery system (Fig. 1) instead of a localized delivery layout, to lower equipment costs and improve the safety and efficiency of its chemical-handling operations. Other benefits of this installation included the elimination of manual solvent and developer replenishment operations, containment of flammable materials, and an automated alert system with classification capability (i.e., chemical, maintenance, safety categories).
 Figure 1. General scheme for a fab-wide photochemical delivery system that services multiple photolithography track systems. |
In this specific example, 21 photolithography tracks were located within a 160-ft x 125-ft fab area. The setup included four process-dependent groups of five tracks each and one R&D process track. Each track group was allocated approximately a 40-ft x 20-ft space, one in each corner of the fab area. Each track tool had a traditional chemical dispense cabinet. A separate room containing bulk delivery equipment for solvent and developer was located 90 ft away from the closest fab wall with an elevation change >20 ft.
Prior to adopting the new system, the fab's management had to consider several key issues such as budget, fab space, equipment downtime for installation, and return on investment. A key feature was the elimination of the chemical-dispense cabinet for each track, resulting in a significant reduction in hardware, facilitization, and floor space. The move from individual chemical dispense cabinets per track to one central bulk dispense unit with redundancy per five tracks yielded a combined chase-and-fab footprint savings of 21 ft2.
Benefits realized from the adoption of such a fab-wide management system include equipment cost savings (at least $1 million), a 4% increase in output through the elimination of manual interventions, and a reduction in start-up-related problems through the use of collected data and remote diagnostic capability. These benefits were realized by using a system that combined programmable logic control (PLC) and supervisory control and data acquisition (SCADA), with automated alert and remote diagnostics capabilities.
Specifically, each bulk dispense unit is controlled by an industrially hardened PLC and graphical user interface with Ethernet communication capability. The controller is programmed in ladder logic language that was first developed for the automotive industry in the 1970s and has since been adapted as the benchmark by the majority in manufacturing industries. Commercially available SCADA software from Wonderware, Irvine, CA, served as the foundation for this application.
As-installed remote real-time process monitoring via Ethernet communications enables proactive real-time monitoring of multitrack dispense and handling and collection of historical data for all delivery and process tools. The process and hardware is controlled via a PC at the engineer's workstation outside the cleanroom or at specific tools. In this application, the SCADA system records every action and status change of all equipment on the network (11 different systems) at one-second intervals.
The infrastructure is similar to a typical office network, thus making it easily understood by the MIS or IT personnel. The SCADA system resides on a PC with Windows NT 4.0, and is certified for Windows 2000 for later changes, if desired. The SCADA system also has a telephone alert feature so that appropriate individuals may be alerted, depending on the alarm class and personal schedules. Remote access to the SCADA system is achieved by commercial software.
This system can be configured to include both data mining and e-diagnostics. Such data collection and sorting capability allows an engineer to search for and quickly identify current and past process parameters, alerts, and alarms. For example, with the installation discussed above, a problem arose that initially appeared to be a random event and was not easily identified by visual inspection.
 Figure 2. Troubleshooting protocol, initiated by the proactive controls system, rapidly traces a problem related to incorrect valve position. |
As illustrated in Fig. 2, the engineer was alerted at his desktop PC that a "fill time-out" alarm was issued, with a time and date stamp, for one of the developer bulk chemical delivery modules. The alarm indicated that too much time had passed for chemical transfer from the bulk supply to a local tool. Examination of the time-stamped data and the region of the system where the alarm occurred enabled the engineer to systematically isolate the problem to a given portion of the system. Ultimately, it was discovered that a valve was only partially open, even though, externally, it appeared to be in working order.
Another situation involved an operator alert for the switching of an empty solvent container for a full container. This operation is triggered when a sensor detects an air bubble in the supply line. Typically, a short delay is required to prevent false air-bubble detection, which will have a short lifetime and will be followed by more fluid. This mundane yet critical operation parameter that discriminates between a false air bubble and an empty container can cause annoying premature warnings. Through the collection and analysis of historical data, the operator was able to optimize the delay, rather than settling for a "guesstimation." This ultimately reduced operator anxiety and process interruption.
In addition to hardware troubleshooting and preventive maintenance alerts, the proactive controls system is a valuable tool for process monitoring. When real-time monitoring is coupled with data archiving, information is yielded about track use and chemical consumption. The system may be also configured for automatic access to sorted or plotted data, in Microsoft Excel, for example.
 Figure 3. Tracking photochemical use variation helps eliminate waste. |
Figure 3 shows photochemical usage and a noticeable variation over the last 22 days. During this period, the wafer fab was engaged in qualifying a process in which the photoresist dispense volume was increased from 2 to 5ml. The noticeable decrease in slope between April 5th and April 19th is due to the decrease in the number of pump strokes per bottle as the volume per dispense increased. This type of information is useful in tracking the amount of photochemicals used per track, in the short or long term, and may be used to decrease waste and monitor track uptime.
Such tracking of chemical consumption and equipment uptime enables automated reordering of materials, as well as the determination of mass balance of consumables and yield.
This data is monitored in real time so the process support equipment can detect if the process stops or cycle time changes. Even the range of acceptable variation can be set for automated alerts. Depending on the chemicals and processes, unexpected variation in chemical dispense rate can have significant effect. The system can alert end users with this critical information.
Conclusion
Our recent work in the implementation of fab-wide controls platforms and in the development of chemical dispense hardware and apparatus has yielded reduced costs through more efficient materials handling and waste reduction. In some cases, this has led to the elimination of individual or localized chemicals dispense cabinets and less materials consumption. Finally, both safety and environmental concerns are diminished by the reduction of manual chemical handling.
References
1. J. Tremblay, Chemical & Engineering News, pp. 30-24, Nov. 19, 2001.
2. Calculations based on 1cc dispense volume and a resist cost of $1250/liter.
3. L. Peters, "Tailoring Tracks for Spin-on Dielectrics," Semiconductor International, April 2002.
Jay Jung received his BS in electrical engineering from California State Polytechnic Institute, San Luis Obispo. He is director of controls engineering at Microbar Inc., 1252 Orleans Dr., Sunnyvale, CA 94089; ph 408/542-9072, fax 408/541-1441, e-mail [email protected].
Josh Golden received his PhD in chemistry from Cornell University, Ithaca, NY. He is director of process technology at Microbar.
Marc van den Berg received his BS in electrical engineering from Santa Clara University, CA. He is VP of engineering at Microbar.