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This overview examines some types of semiconductor-based pressure sensors used in clean room environments, their operating principles, and their applications in wafer manufacture. Reno Suffi, Pressure sensors are indispensible to semiconductor wafer fabrication. The high-vacuum process equipment is enclosed in hermetically sealed chambers that permit the introduction of various process gases. Other operations carried out in a clean room environment include ion implantation and ion beam etching. Each precision process requires fine pressure control over the chamber's internal climate and its consequential outgas. To ensure process integrity, a distributed control system is used to monitor air recirculation and handling units, scrubbed exhaust systems, and the heating and cooling loops of the process apparatus. We will examine some types of semiconductor-based pressure sensors used in clean room environments, their operating principles, and their applications in wafer manufacture. Pressure Sensor Types Four types of semiconductor-based pressure sensors are widely available: absolute pressure, differential pressure, gauge pressure, and negative pressure (also known as vacuum pressure). Absolute pressure sensors measure pressure with respect to a zero pressure reference value (vacuum reference). Differential pressure sensors measure the difference between two pressure environments. Gauge pressure sensors behave much the same way as differential pressure sensors, but measure positive pressure in relation to the environment's ambient pressure (as opposed to a second pressurized environment). Gauge pressure sensors use atmospheric pressure as their reference points for measurement. Negative pressure sensors measure vacuum pressure with respect to atmospheric (ambient) pressure. Because absolute pressure sensors use a zero pressure reference point, they are the most suitable for measuring the integrity of vacuums or other low-pressure systems. A differential pressure sensor measures pressure with reference to itself. In this case, the two elements monitor different aspects of a pressurized system. For example, pressure can be monitored upstream and downline in a machine's exhaust system. If the exhaust system is clean, the two pressures will be similar; if it becomes clogged or has a leak, the two pressures will be different. Gauge and negative pressure sensors are better suited to applications in which pressure references become closer to ambient pressure. A negative pressure sensor could be used to monitor an air intake valve, where the pressure is lower than atmospheric, while a gauge pressure sensor would be used to measure the system's exhaust, whose pressure is higher than atmospheric. Operating Principles The two cutting-edge precision pressure-sensing technologies of interest here are piezoresistive and electrostatic capacitance.
Piezoresistive materials have the unique ability to convert mechanical energy into electrical impulse. A high-density diffusion layer of piezoresistive material is formed to make a resistive layer on a film (see Figure 1). Compression of this semiconductor film element bends the crystalline form of the piezo material so as to change its resistance value in proportion to the amount of stress on its surface. The signal changes are then amplified and interpreted as pressure readings. An electrostatic capacitor is formed when two electrodes are separated by a thin dielectric spacer (see Figure 2). One electrode is a thin film that has been deposited on a glass substrate where it remains in a fixed position. The other electrode is a movable diaphragm. When a pressure difference between the isolated "positive" and "negative" pressure ports causes a stress on the movable electrode, the distance between the two electrodes changes. This difference in distance alters the capacitance of the circuit. The signal changes are then amplified and interpreted as pressure readings. Advantages and Features Semiconductor-based pressure sensors have many advantages over older pressure detection technologies. Longer life is one example: semiconductor MTBF levels at 100,000+ hr. (11.4 yr.) are not uncommon. These pressure elements are extremely sensitive in stable conditions such as those in clean rooms, and have high resolutions as well. Repeat accuracy of sensor outputs is on the order of 1 kPa (~0.1psi). The sensors are also highly energy efficient. Maximum current draw is on the order of 70 mA, with energy saving modes in the 40 mA range. Microelectronics give these sensors the ability to pack full functionality into a subcompact package, the most common of which is the cube style (see Photo 1, page 62). This style typically features an LED pressure output display with silicone rubber operator keys on the face, pressure ports on the back, and a wire harness protruding from the bottom. The unit's small size and display allow it to be mounted into a control panel, and the pressure control system to be operator accessible.
The user interface on cube devices such as Omron's E8F2 gauge pressure sensor offer such advanced features as a teach functionality that automatically sets the on and off points based on actual readings at the pressure port. Other interface features include the 2-point teach mode, called hysteresis, that allows the operator to manually set the output on and off pressure points. Touch-point teaching in the sensor's onboard memory shortens equipment startup time and ensures that startup operations are performed only once. The intelligent support circuitry in these sensors permits the operator to change the display output unit (e.g., kPa, mmH2O, psi) or to adjust display refresh rates and even measurement averaging rates. These and other intelligent features make smart pressure sensors a natural choice for designers of advanced technology semiconductor fabrication equipment. Industry Applications Pressure monitoring is essential in wet-cleaning machines, which use a wash to remove particles from the surface of silicon wafers. Gauge or differential (with one port set to atmosphere) pressure sensors monitor the output of the exhaust system. A high exhaust pressure could indicate wafer-coating failures; a low exhaust output could indicate the presence of a gas leak in the exhaust line. The pressure sensors are set to monitor an acceptable operating range and to trigger an alarm when the pressure goes out of tolerance. Pressure sensors are also incorporated into chemical vapor deposition (CVD) systems, which are used to deposit or grow thin films on silicon wafers. As semiconductors become more complex, the number of film layers deposited or grown necessarily increases. The rate of film growth can be dependent on the pressure of the environment. For example, the oxide growth rate is faster at higher pressures. Differential pressure sensors are used in CVD systems to monitor and control the pressure difference between diluting gas and inert gas chambers, ensuring a stable deposition process. Distributed control systems in clean rooms are monitored and controlled by gauge pressure sensors that monitor positive pressure in the fan towers and air recirculation pumps to eliminate impurities that might reduce yield. Differential pressure sensors monitor the status of clean room air filters. Pressure ports are connected to either side of the filter; as the filter becomes clogged, the difference in pressure between the two sides increases. The differential pressure sensor is programmed to detect the point at which the pressure difference indicates the time for filter replacement, and when it is reached, the sensor triggers an output alarm. Summary Incorporating advanced semiconductor technology into pressure sensors provides the semiconductor/electronics manufacturing industry with the ability to precisely monitor and control critical semiconductor fabrication systems. Reno Suffi is Product Marketing Manager, Omron Electronics, Inc., 1 Commerce Dr., Schaumburg, IL 60173; 847-843-7900, x-260, fax 847-839-2260, reno_suffi@omron.com
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