The semiconductor industry uses a wide range of specialty gases during the processing of silicon wafers. Standard atmospheric gases include helium and nitrogen, along with argon, oxygen and hydrogen. Other gases may include dichlorosilane, disilane, silane, silicon tetrachloride and silicon tetrafluoride. There are additional gases used during the doping and etching states including boron trifluoride, diborane, phosphine, boron trichloride, chlorine, sulfur hexafluoride, and a wide variety of halocarbons. There are many other gases that may also be used. These gases are generally stored in a compressed, liquefied form. This allows a significant amount of the gas to be stored in a standard gas cylinder. The cylinders are often placed in a specialty gas cabinet. These cabinets are connected into the semiconductor wafer process equipment. When a specific gas is required during the processing, the appropriate valves are opened to allow for the flow of gas. Pressure regulators assure that the correct amount of gas is dispensed. It is extremely important to assure the proper flow of these gases. If this flow should be interrupted for any reason, the batch of semiconductor wafers being processed is likely to be completely ruined. As each wafer can represent hundreds of completed integrated circuits, and as these devices can individually be very expensive, a single incident of inadequate gas flow can cause tens of thousands of dollars of loss to the semiconductor company. For this reason, it is imperative to monitor the contents of each gas cylinder. In many situations, the amount of compressed gas in a container can be determined by measuring the gas pressure. There are a number of different types of pressure transducers that can be used for this purpose. Some of the simplest devices incorporate a diaphragm that can move a spring loaded potentiometer. The voltage output is proportional to the gas pressure. There are a large number of fully electronic devices where the diaphragm is actually a semiconductor material. The change in strain on this material will cause a change in resistance, which can be converted into a digitally displayed value. Because some gases are very corrosive, the pressure transducer may have to be isolated with stainless steel or some other material. But none of these methods will work for the liquefied gases described. Almost all of the gas remains in the liquefied condition. A small amount will evaporate to the top of the gas cylinder. When the process calls for this gas, the valve opens to dispense the gas. The liquid will evaporate as long as more gas is needed. But the gas pressure will always remain the same until there is no more liquid available. At that point, the pressure will fall very quickly as the gas is depleted. Therefore a pressure transducer is inappropriate. Instead, a gas cylinder scale is placed underneath the cylinder. After subtracting the weight of the cylinder itself, the remaining weight value will be an accurate indication of the amount of liquefied gas still in the cylinder. As the liquid boils into the gaseous state, and is dispensed for the process, the industrial scale will monitor the remaining contents. When the contents falls below a pre-determined value, an alarm will notify the staff that the cylinder should be replaced before it is completely depleted. This avoids a costly process problem. The gas cylinder scales must be designed with a number of important features. Arlyn Scales produces these digital scales with capacities that are appropriate for a full gas cylinder, which may weigh in the range of 250 pounds. The scale platform should be very thin, because the cylinders are often placed onto and off of the scale manually. While many cylinder scales are as high as 2 å_‰Û, the Arlyn Scales design is just over 1‰Û thick. A load sensor inside the electronic scale detects the weight of the gas cylinder and converts it into an electronic signal. This signal is sent to the instrumentation of the gas cabinet. The load sensor must be extremely rugged, as it will experience very high instant shock loads when the cylinder is pushed onto the platform. The scale described above uses a load cell that is fabricated from a solid bar of a special stainless steel alloy. The design capacity is rated at 600 pounds. This results in a gas cylinder scale that is quite capable of sustaining the type of usage that would damage other scales.
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