Optimizing load lock operation

How can better pressure measurement improve the operation of your load lock?

If we consider the primary purpose of a load lock—to pass from an ambient condition to a vacuum process condition—it only makes sense that the better it can match those two environments, the better it can do its job. Measuring pressure effectively at these two levels is critical to the proper operation of a load lock. At ambient pressure, when the load lock is opened to the room or handler, it is ideal to have a slightly positive relative pressure. This reduces the flow of contaminated, humid air into the load lock. However, if the load lock pressure is too high, it can cause a symptomatic “popping” sound as a burst of gas exits the load lock, creating its own set of problems. Precise measurement near atmospheric pressure allows the load lock to open only when it has been properly vented.

At transfer pressures in the mTorr range, the measurement is also critical to effective load lock performance. If the gauge reads too high, the actual pressure will be lower than necessary when the transfer occurs. This situation results in the loss of valuable time as the load lock chamber remains in the pumping phase too long. On the other hand, if the gauge reads too low, the actual load lock pressure will be too high when the transfer valve opens. The resulting higher contaminant levels can migrate from the load lock during transfer, compromising the purity of the process environment.

The need for precise pressure measurement in these two very different ranges requires that two different sensing techniques be employed. Several approaches to this task have historically been used. One method has been to use a convection enhanced pirani vacuum gauge to measure both pressure regions. While this type of gauge functions relatively well for the low pressure transfer; it has significant issues at the vent pressure condition due to response time, gas type, attitude, and temperature. A mechanical force-based pressure switch has sometimes been used for the vent pressure indication; however, this technology has inherent variability which creates over-pressure or under-pressure events. Using two properly selected, separate gauges is an effective technical solution but requires more electronics, interfaces, and vacuum connections. The ideal approach to load lock pressure measurement combines two sensors, each targeted for the specific task, in a single instrument.

Objectives

• Ensure proper load lock vent and transfer conditions
• Avoid “popping” events when opening the load lock
• Reduce contaminant transport to the process

Method

Proper load lock operation requires an accurate measurement at ambient (atmospheric) pressure and at transfer pressure in the mTorr range. Pressure measurement in between these regions is useful for diagnostic reasons, but is not critical to the function of the load lock.

When considering the vent to atmospheric pressure, the load lock pressure must be above the ambient pressure, typically by a few Torr. This provides the proper condition for opening the load lock door to the room. Pressure switches based on mechanical linkages, springs, and contacts cannot achieve this level of precision and often allow the door to open before the load lock has attained positive relative pressure. To compensate for the variability of mechanical switches, some systems add a “wait period” to ensure the load lock chamber has vented enough. However, this practice causes an unnecessary delay and can often produce too much positive pressure.