Silane and arsine, and their often equally nasty siblings, comprise a family of specialty gases used extensively in the treatment of semiconductor media during the integrated-circuit fabrication process. Silicon wafer fabrication facilities, known as "fabs," apply these gases to silicon wafers in arcane processes such as "etching," "doping," and "deposition."

  The control of process gases during wafer fabrication is hypercritical for three reasons: 1) life safety; 2)material cost (the hazardous nature and high purity requirements of these gases make them expensive to produce); and 3) quality of the final semiconductor product (contamination by only one errant molecule in a critical process gas can defect a single chip; a batch-wide, gas-purity mishap can ruin a million dollars worth of chips).

  Air Liquide Electronics (ALE, Fremont, Calif.), supplies specialty gas and gas distribution hardware to the electronics industry. "Distribution hardware" refers to manifolding, valving, tubing, and related controls that actually convey the gas to the fab processes.

  The distribution system's primary purpose is process-related because it supplies a continuous flow of expensive and dangerous gas to process tools. Its secondary and substantially more complicated task is batch related, and begins when gas flow is stopped for empty cylinder replacement, system maintenance, or emergency shutdown.

  It is then that safety and process-purity--less than one ppb of contamination--issues require that molecular residue from the gases be removed from the system. This clean-in-place (CIP) or "purging," is done via precisely executed inertgas (usually nitrogen) pressurizations and evacuations.

Mechanical overview

The standard mechanical system within a fab's process area consists of a gas distribution module, which includes the "tool(s)," gas source, and gas panel. The tool is any of the wafer fabrication machines that actually apply the gas to the semiconductor media during various stages of the fabrication. Actually, one tool might have as many as 15 separate gas panels supplying it with different gases. Or, one gas panel might supply as many as eight tools with a single gas.

  The gas source is a standard 44-liter gas cylinder. It supplies gas to the gas panel, which, in turn, conducts gas to wafer processing tools. A second cylinder is used for back-up gas supply, thus ensuring uninterrupted gas flow.

  The gas panel, a matrix of stainless-steel tubing and pneumatically actuated, normally closed diaphragm valves, is mounted above the gas cylinders (see diagram). The valves are monitored and controlled using a human-machine interface (HMI) and a programmable logic controller (PLC). Control functions include switching the gas supply cylinder as needed, providing locations for process and safety sensors and implementing the "batch-like" CIP purges.

  Because of variances in process requirements, many purge routines are user-specific and must configured by the operator. Purge requirements also vary widely with the type of gas used. High specific gravity gases are kinetically sluggish and require longer purge times. Other gases have tenacious residues. Process gases such as hydrogen chloride are particularly "sticky," requiring longer purge times. Maintaining higher gas purity also requires longer purges.

  Cylinder changeouts are the most common reasons for purges. They require the simplest sequences because only a short section of tubing has to be purged before reintroduction of the process gas.

  At the other extreme are system-wide purge routines. These can have over 300 discrete, timed steps that include venturi-generated evacuations, purge gas pressurizations, and sequenced valve actuations. A menu of application-specific purge routines is needed--rather than having "one big one do them all"--because some routines take hours to complete, whereas others take minutes. Using the correct purge routine is necessary because shutting down a fab's processes for longer than necessary is expensive. Downtime costs can approach $100,000 per hour!

Redefining the process

In order to improve CIP efficiencies, ALE introduced its Reduced Purge Volume (RPV) gas distribution hardware to decrease by half--both from time taken and gases lost during a typical CIP purge. ALE chose a control system from DST Controls Inc. (Benica, Ca.) to provide the flexibility needed for the new "plumbing." DST's OEM control system was developed to monitor and execute all of the complex purge--or gas batching--routines that ALE's new hardware could now implement.

  Additionally, wafer fabs also required that the gas batching system meet some less precisely defined generic criteria. These include flexibility, reliability, low cost, and fast-track development. A premium was also placed on user-friendliness, aesthetic appearance, a small hardware footprint, and on-time delivery.

  The reasons for these requirements are due to constraints and market pressures, unique to the semiconductor industry.

• Flexibility: Fabs are global, so a new controller had to be easily configurable to accommodate a wide range of international end-users' needs. Customer-specific "moving targets" included unique computer/software systems and local area networks, requiring bidirectional, plug-and-play communication. System security requirements varied from complex multi-level password access, to key switches, to no restrictions at all. Even the PLCs to be used were optional.

• Fast-track development: Because of the competitiveness in the chip industry and the short commercial life of many semiconductor products, new fabs typically must come on-line in a year or less. Vendor equipment availability must keep pace with delivery requirements.

• Cost: OEM products aimed at fabs are subject to competitive forces in a marketplace already flirting with overcapacity. New equipment must be "strategically priced."

• Size: Cramped spaces typify fab environments, so the size of all equipment matters.

• Reliability and user-friendliness: Fabs must deal with dangerous gases (arsine, silane, etc.) and the high scrap costs of products.

• Aesthetics: The customers fabs deal with are sophisticated and high tech. All production equipment must meet these criteria in both performance and appearance.

• On-time delivery: Because the fab industry is currently the "mother-of-all-buyers' markets," with high financial and operating safety risks, poor vendor performance is not tolerated. If there was a fab bumper sticker, it would say "Break a promise, go away."

  Purging, or clean-in-place, of gas panels is undeniably characterized by batch process elements. In fact, it was only after ALE and DST Controls viewed the system requirements from a "batch control" perspective that the current solution could be developed. •

  For more information, contact DST Controls: Tel: 707/745-5117; Fax: 707/745-8952; or Circle 227 on the Reader Service Card.

The 'Batch' Solution

Treating the control design as a batch problem embedded in a process environment was the approach used by DST Controls. DST viewed the defining characteristic of "batch" as the ordered sequence of controlled actions required to produce repeatable final products--in this case, molecularly spotless gas vessels.

  In fact, ALE's purging application had all three components of a batch recipe: formula, procedure, and equipment. Although the same generic purge routines would be used for normal cylinder changes and panel maintenance, each would be implemented with a different formula, configurable by the user for different gases in unique physical installations.

  Within these parameters, the controller would then repeatedly execute its purge routines, often with over 300 precise steps. The steps involved the actuation of multiple valves while monitoring variable transducer values to confirm each step's completion. The control system was to have the look and feel of a Pentium/Intellution batching package--at a monochrome flat-panel price.

  The controller's physical design took priority so that its assembly could pace software development. Close attention to ergonomic principles guided the design, especially for the service access doors and panel openings. Electrical design required that all purge options be supported with a minimum of I/O points and without module changes, rewiring, or DIP-switch reselection.

Software development

For software development, ALE and DST reverse-engineered an existing purge procedure, and recreated it as a 50-page flowchart. New software developed from these flowcharts focus on guiding the operator through abnormal purging situations by providing detailed on-screen text and icons. Operators are alerted to problems that can be addressed and solved during a long purge operation, avoiding possible restarting of the sequence.

  Purge sequencing contributed the most to program size and complexity. Some purges required over 300 steps, no small task for the unit's mid-range PLC. Because the required functions hogged PLC memory, use of efficient coding was paramount; for example, resetting an index value to zero saved two bytes by using a "block clear" instruction instead of a 'move' instruction. The current program is one byte short of the 16K limit for the selected CPU. Communication between the unit's touchscreen and controller has also been optimized, allowing for greatly improved screen update times.

  Ease of use and system security also concerned the fab operators. Although it has long been considered easiest to navigate these options like climbing along the branches of a tree, changes in PC-based operating systems now allow operators to "jump sideways to an adjacent branch," when necessary. Layout and hierarchy of screens were redesigned, allowing quicker, more intuitive navigation and reducing operator training time. Since no two customers had the same interpretation of which functions should be secured or how, the batch application's recipe was extended one further step, implementing a fully customizable security system.

  Manual mode is typically the most secure function in any customer's eyes. It is assumed that if operators have satisfied all the requirements to access manual mode, then they can take responsibility for operating the controller safely. However, in order to (continued on p. xx)

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assure absolute system safety, a set of manual interlocks, which step an operator through especially hazardous valve sequences, was implemented.

A look at the controller

The controller cabinet is rated NEMA 4. It features a 10-in. touchscreen display, alarm horn, LED indicators, key-switch, and an emergency-stop button-- mounted in the hinged front cover. The default PLC is a GE Fanuc Series 90-30 PLC. It is mounted on a pull-out tray behind the shock-mounted cover. Cost, size, memory capacity, expanded communication protocol capability, availability of worldwide service, and acceptance by the semiconductor industry all played a part in GE Fanuc's selection.

  Because the controller must operate in a Class I, Div. 2 environment, a 16-station, electropneumatic interface card transforms the PLCs 24 V dc output signals into nitrogen gas pulses. These pulses then actuate the intrinsically safe gas panel valves. The panel's sensor/transducers are 4-20 mA or 1-5 V dc, and input directly to the PLC. They monitor pressure, flow, and gas cylinder weights, detecting remaining gas quantities.

  Other controller features include:

• Operator configuration and execution of default and custom purge routines.

• Fail-safe and interlock gas panel valving, to ensure safe states during operator-executed cylinder changes, manual purges, and maintenance.

• Capability of monitoring low-pressure (empty cylinder) alarms or other out-of-spec conditions.

• Capability of monitoring, alarming, and initiating automatic shutdowns for fire, seismic events, gas and fluid leaks, E-stops, and panel malfunctions.

• Communication of system conditions to end-user's fab-wide, life-safety LANs.

• Access to 50 interactive screens for real-time system status indications and operator control.

• Reconfigurability of all purge and monitoring functions.