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ULTRAPURE MATERIALS DELIVERYEvaluating the use of MEMS-based gas and fluid delivery systemsAlbert K. Henning, Edward B. Dehan, Errol B. Arkilic, and James M. Harris, Redwood MicroSystems The advent of microelectromechanical systems (MEMS) technology enables dramatic changes in high-purity semiconductor processing. As the industry's manufacturing requirements become increasingly stringent, new technologies must be developed to meet the challenging performance, cost, yield, and reliability specifications of 300-mm wafer manufacturing.1 Several conventional solutions have been proposed to improve gas and liquid control and distribution systems; only precision MEMS-based systems have the potential to meet and exceed these critical requirements. Silicon microfabricated valves, sensors, and orifices can be integrated to form complex MEMS-based systems for many purposes, including process gas and liquid delivery for dry etch and chemical vapor deposition (CVD) applications. MEMS technology greatly reduces the internal volume, dead space, and contact surface area in fluidic components. Integration of MEMS-based components on common manifolds further reduces the overall system footprint beyond the capabilities of conventional technology.2 Using MEMS-based components in certain applications will make it possible to shrink the size of conventional gas and liquid delivery panels by an order of magnitude. In addition to the obvious benefits in reducing system size, MEMS technology decreases system costs while improving performance and reliability. Low-cost silicon components are microfabricated using high-yield semiconductor manufacturing techniques. Smaller component sizes and advanced semiconductor packaging methods reduce the number of fittings and the amount of manifold material required. The precision, solid-state components are capable of highly repeatable, reliable performance relative to their conventional mechanical and electromechanical counterparts. With recent MEMS innovations, the list of conventional gas and liquid control components that can be replaced with smaller, MEMS-based equivalents includes mass-flow controllers (MFCs), pressure regulators, and positive shutoff valves. It is now possible to replace complete gas panels with integrated MEMS-based systems. Building Blocks In order to create true gas and liquid flow control systems using MEMS technology, a number of advances, or building blocks, are necessary. For semiconductor processing equipment, these building blocks include normally open proportional valves; normally closed, positive shutoff valves; silicon microfabricated temperature and pressure sensors; and flow restriction devices. In the study presented in this article, critical flow orifices were used. In addition, thin films to expand the chemical compatibility of silicon structures may need to be developed. These technological building blocks will be discussed here in detail. Silicon Microfabricated Proportional Control Valves. Proportional control microvalves that use thermopneumatic actuation have been reported since 1987.3 Recently, the applications have grown to include control of liquids and gases over wide ranges of temperature and pressure.4 For high-purity applications involving corrosive materials, these valves can be fabricated so that silicon is the primary material in the wetted path. Figure 1 shows a cross section of a normally open valve. The fabrication process relies on both silicon-to-silicon fusion bonding and silicon-to-Pyrex anodic bonding. Typical lateral dimensions are 6.0 x 6.3 mm. The vertical dimension of a three-layer device is typically 1.6 mm with a Pyrex layer of 0.8 mm, a silicon membrane layer of 0.4 mm, and a silicon flow channel layer of 0.4 mm. The membrane itself measures 4.0 x 4.0 mm with a thickness of 50 µm. Dimensions for flow channels and inlet/outlet ports range from 25 to 1000 µm.
The thermopneumatic actuation principle relies on a cavity that is etched into the membrane layer of the device. When bonded to the Pyrex layer, the cavity is rigid on five sides but is flexible in the direction of the single-crystal silicon membrane. The cavity is filled with a control liquid, an environmentally safe fluorocarbon called Fluorinert (3M, St. Paul, MN). The cavity is sealed hermetically with silicon caps covering two ultrasonically drilled holes in the Pyrex. These holes also serve as electrical feedthroughs to a platinum resistor that is deposited on the Pyrex surface in order to provide controlled heat transfer to the control liquid. Heating the control liquid causes it to expand. The membrane moves in response to this expansion and extends outward to form a constriction in tandem with the silicon valve seat etched in the bottom silicon layer. From a design perspective, the valve must achieve several objectives. It must meet ambient temperature requirements, close and open against specified system pressures, consume minimal power, respond quickly, and proportionally control relevant flow rates. All of these requirements must be met within the fracture strength limits of crystalline silicon membrane. Unlike other methods, the thermopneumatic microvalve actuation allows operation throughout the typical range of pressures and flow rates found in present and future semiconductor processing tools (see Table I).
Table I: Specifications and design limits for thermopneumatic MEMS valves. The current design limit of each parameter was maximized independent of other specifications. Silicon Microfabricated Shutoff Valves (Vacuum Leak Rate). Using the same thermopneumatic actuation concept, electronically controlled MEMS-based shutoff valves have been designed. These values provide direct on/off control, in contrast to the indirect control provided by conventional, pneumatically driven valves. These valves typically are designed in a normally closed configuration in which there is no flow when power is not applied. Requirements for shutoff valves differ in several ways from the requirements for proportional control valves. Pressure drop across the valve, power consumption, and response time must be minimized so that the overall system budgets for speed, pressure, and power are not compromised (see Table I).
Figure 2: Cross section of a normally closed, positive shutoff MEMS valve with dimensions of 8 x 6 x 2 mm. In order to provide extremely low leak rates across the valve seat, a polymer is inserted into the valve seat to form a compression seal, as shown in Figure 2. The normally closed design is accomplished by attaching a silicon cantilever to the silicon membrane. The cantilever retracts from the valve seat when power is applied, and the membrane expands outward. Leak rates below the industry standard of 1 x 10—9 atm cm3/sec of helium can be achieved. At the same time, because of a comparative decrease in the surface area of the valve seat, long-term leakage associated with diffusion through the valve seat material can be substantially less than that provided by conventional valves (see Figure 3).
Figure 3: MEMS shutoff valve leak rate versus conventional shutoff valve leak rate. Silicon Microfabricated Pressure and Temperature Sensors. The automotive and medical industries' need for high-volume, small (several millimeters square) and inexpensive (<$2) temperature and pressure sensors has made these devices readily available from many suppliers. With minor structural adjustments to accommodate the high-purity requirements of semiconductor manufacturing, these commodity devices can be used in applications for flow and pressure regulation of process gases and liquids. When incorporated into a final-flow control system, the information they provide becomes the basis for the temperature and pressure calibration data that are stored locally on an EEPROM. The small geometries of the microfabricated components are integrated using equally small flow channels, a factor essential to achieving high degrees of accuracy and repeatability in a complete-flow or pressure control system. For example, in a pressure-based MFC, sensing must occur as close as possible to the calibrated flow orifice. Silicon Microfabricated Flow Orifices. To obtain flow measurements using pressure sensing, a calibrated flow orifice must be placed downstream of the pressure measurement. These precision flow orifices can be manufactured repeatedly and inexpensively using silicon micromachining. Wet anisotropic etching is used to create sharp beveled orifices, thus yielding the near-ideal coefficients of discharge required for accurate inference of flow rates. As with microfabricated sensors, a key advantage of machining orifices in silicon is the ability to package MEMS orifices very close to MEMS sensors and valves in closed-loop systems. Chemical Compatibility of Silicon Structures. For high-purity applications in the semiconductor equipment market, materials must be compatible with various process gases and liquids. The primary difference in materials for MEMS-based systems versus conventional systems is the introduction of silicon into the wetted path. The corrosion resistance of silicon is quite good by itself. Very few semiconductor process gases attack silicon actively.5—9 In general, no organic solvents or organic acids will attack silicon. Some organic and inorganic bases (such as the KOH etch used in bulk silicon micromachining) are used purposely to etch silicon, but are not typically used in integrated circuit fabrication.5 Halogens represent a special class of materials. Aqueous or gaseous mixtures of HF, HCl, HBr, or HI will not, in general, etch silicon. In studies conducted by Redwood MicroSystems (Menlo Park, CA), silicon microvalves were immersed in HBr for more than 1000 hours, and no measurable increase in surface roughness was detected. Materials compatibility tests have also been performed in the presence of fluorine-based hydrocarbons, at elevated pressures and temperatures.4 In these experiments also, no detectable, adverse effects were noted. Pitting of silicon, however, can be observed in HBr or HI under certain conditions.9 Atomic fluorine will attack silicon at room temperature; for instance, if the process gas is XeF2. Interestingly, atomic fluorine will not attack SiO2—even native SiO2 on silicon is sufficient to stop the reaction.5 Neither chlorine gas nor atomic chlorine has been found capable of etching silicon without the assistance of a plasma. Only in combination with fluorine ions, (M)F, have strong oxidizing mineral acids (e.g., HNO3) been reported to etch silicon. Silicon is impervious to aqua regia (HCl + HNO3), Piranha (H2SO4 + H2O2), and other mineralized solutions used to clean wafers. For MEMS-based control of gases and liquids that do attack silicon, a thin-film coating must be provided over the silicon valves, orifices, and sensors. Silicon carbide (SiC) can be used to create a barrier between the process fluid and the silicon. SiC can be deposited economically over the devices during manufacturing at the wafer level before the devices are sawn into component die. Only a limited number of gases, including ClF3 and N2F4, require SiC-coated components. Interestingly, NH3, Cl2, HBr, HF, and other gases that are difficult for conventional systems to handle do not require an SiC coating and can directly contact silicon. The moisture content of the gas stream also can be a critical factor in the corrosion resistance of most materials. The passivity of typical stainless steel greatly depends on the amount of water present. In the presence of halides, passivity declines sharply as the water content increases above 1 ppm.10 In contrast, moisture content has little effect on the corrosion resistance of silicon. Small amounts of water in a gas, of which atomic fluorine (e.g., ClF3) is the decomposition product, however, can have a deleterious effect on silicon if it is protected only by SiO2 or S3N4. Mass-Flow Control Using MEMS Using the building blocks previously described, a pressure-based MFC can be built using MEMS components. In Figure 4, the configuration of the MEMS-Flow MFC (Redwood MicroSystems) is provided. The unit uses a proportional control valve, two absolute pressure sensors, a temperature sensor, and a calibrated orifice to control mass-flow rates of gases and liquids. Flow passes through the proportional valve and then to a pressure sensor that reports the pressure immediately above the orifice. Media temperature and outlet pressures (downstream of the orifice) also can be included in the flow measurement algorithm. The control valve is modulated based on the calculated pressure required to achieve a desired flow set point.
The most accurate flow measurements can be obtained from conditions of critical (sonic) flow through the calibrated orifice. In this configuration, the ratio of the pressures upstream and downstream of the orifice must be kept above a certain value (2:1 for nitrogen) to achieve sonic flow through the orifice. The algorithm for this measurement is simplified because the dependence on MFC outlet pressure is often negligible. This method of flow measurement is particularly relevant in the semiconductor industry, since most applications (e.g., metal etch) require MFC outlet pressures near vacuum and inlet pressures above 30 psia. Such external pressure conditions allow a pressure-based MFC to operate in sonic mode over a wide dynamic range of flow rates. Alternatively, for APCVD and other atmospheric outlet pressure applications, the flow may become subsonic, requiring measurements of pressure on both sides of the calibrated flow orifice and introducing more uncertainty into the calculation. In addition to the different algorithm requirements for subsonic versus sonic flow, different flow models are required for compressible (gas) versus incompressible (liquid) media. Pressure-based mass-flow control relies on the analytical expressions for the flow through the fluidic elements of the device. The various flow models are represented in the following equations: Sonic compressible flow:
Subsonic compressible flow:
Incompressible liquid flow:
The pressure upstream and downstream of a flow element (whether valve or orifice) is related to a calibrated flow model in order to determine the flow. For gas flow, if there is no viscous loss, then the compressible flow model in the subsonic regime is used, in which The pressure sensed by the sensor Px determines the intersection of the flow curves for the valve and orifice flow components. As shown in Figure 5, as the valve flow is modulated from 100% to a lower flow value, the value of Px will also decrease; the MFC inlet pressure is 50 psia.
Eutectic bonding or a variety of Teflon-type polymers can be used to attach MEMS components to a chip-carrier substrate material such as ceramic, nickel, or chrome. Electronic control signals are communicated via metal lines patterned on the substrate. A welded lid over the MEMS component ensures hermeticity by providing a helium leak rate to the ambient of <1 x 10—10 atm cm3/sec. Outgassing of the polymers and leak permeation of gas through the polymers will be limited because of the small quantities of polymer required. The interface to the chip-carrier module is accomplished with standard C-type metal seals. The orifice also lies within this C-seal boundary, which prevents any leaks to atmosphere from occurring at the attachment point between the orifice and the chip-carrier module.
In Figure 6, the sealing lid of a mass-flow controller chip-carrier module has been removed so the internal components are visible. The largest of the four silicon chips is the microvalve, the smallest component is the temperature sensor, and the remaining two components are the pressure sensors used for flow control. An EEPROM, which contains the calibration information for the flow module, is also shown. The flow control orifice, which is found underneath the module, is not pictured. The MFC's pressure-based configuration provides state-of-the-art accuracy and repeatability performance levels. In particular, the design enables revolutionary performance for low flow rates of gas (<10 std cm3/min). In sonic mode, the capability for accurate control is > ±1.0% of reading over a 10:1 dynamic range with repeatability exceeding ±0.2% of reading (see Figures 7 and 8). For a full-scale flow rate of 1 std cm3/min, this mode allows highly accurate and repeatable flow for rates as low as 0.1 std cm3/min. The drift of the devices is < 1% of full scale (1%/F.S.) per year and limited only by pressure sensor stability. Sensor design modifications have the potential to improve drift performance as low as 0.1% F.S./year. Specifications for the MEMS-Flow MFC are listed in Table II.
Table II: The pressure-based configuration provides state-of-the-art accuracy and repeatability performance levels. The MFC is being evaluated at various test sites for chemical compatibility and particle generation. In terms of long-term reliability, the MEMS-based device has no parts in sliding contact to cause mechanical wear. The single-crystal nature of the silicon membrane makes it deflect elastically to modulate flow. This results in microvalves that, under long-term life tests, have remained defect-free after more than 107 cycles. Liquid MFCs Liquid delivery is another application for MEMS-based flow control devices. Applications for accurate mass-flow control of water, tetraethylorthosilicate, trimethylphosphate, trimethylborate, and other precursors are not adequately served by conventional technology. Liquid MFCs based on conventional thermal mass-flow sensing tend to be inaccurate. At the same time, conventional vapor-delivery systems are expensive and do not perform as reliably as desired. MEMS-based liquid MFCs can be configured with the same building blocks as their gas counterparts. The only significant difference is in the control algorithms (subsonic and incompressible) used to compute mass flow. These devices can provide exceptional accuracy of < ±2% F.S. and repeatability of < 0.5% F.S. for flows as high as 25 ml/min and as low as 0.01 ml/min of water. MEMS-based Gas and Liquid Delivery Systems With a MEMS-based MFC providing the requisite flow accuracy and repeatability, additional MEMS modules can be used to design and create more complex systems. The same chip-carrier package used to support the MFC configuration can support many MEMS-based gas panel components. For example, omitting the orifice and the downstream pressure sensor from the MFC design provides a standard pressure regulator configuration. The same chip-carrier module can also be used to house shutoff valves or to house various sensor combinations for additional in situ calibration and diagnostic capabilities. The module also can be used for isolating gas lines and introducing purge gases. The advantage of common chip-carrier configurations for all functional components is in modular design flexibility. By matching the footprint of the SEMI specification for surface-mount components, it becomes easy to replace conventional components with their MEMS-based counterparts.11 The design progression for replacing a conventional gas stick with a MEMS-based gas stick is shown in Figure 9.2 While current technology is limited to a slow evolutionary reduction in the size and internal surface area of a conventional gas-stick system, MEMS technology would permit a revolutionary reduction. Ultimately, multiple component functions will be placed on the same chip carrier to eliminate all internal welds, fittings, and seals from the gas-stick, while driving the internal volume below 1 cm3.
Figure 7: MEMS-based MFC performance accuracy versus competing MFCs. The dotted line represents the target specification of ±1.0% of the reading. The integrated gas panel design uses the multiple sensor outputs of the system for self-calibration and self-diagnosis. MEMS-based panels will add additional functions to the system with no significant increase in system size or cost. For example, rate-of-rise or rate-of-fall calibration modules can be added to a panel and periodically referenced to ensure accurate MFC performance. Also, parameters such as gas temperature, flow, and pressure can be monitored at various locations in the system to identify potential problems. These problems can then be characterized as external to the system (changing inputs or outputs) or as internal to the system (constant inputs and outputs but changing performance), and alarms that describe the problem can be sent to the user. Using a digital protocol such as DeviceNet (Open DeviceNet Vendor Association, Boca Raton, FL), only the process engineer's imagination limits the quantity and quality of data that may be requested or provided.
Figure 8: MEMS-based MFC performance repeatability versus competing MFCs. The dotted line represents the target specification of ±0.2% of the reading.
Figure 9: Semiconductor equipment gas-stick technology progression. (*Source: Semiconductor International, January 1997.) Integrated gas panels for use in the semiconductor dry-etch market provide the largest application for MEMS-based gas and liquid delivery systems. These systems typically require 8 to 12 gas sticks per chamber. The process-gas flow rates and gas compatibility requirements are within the range of MEMS-based components. Figure 10 shows the schematic and isometric views of a fully integrated gas panel that comprises 10 gas sticks and a purge gas line. The manifold on which the MEMS modules reside is typically built of 316L stainless steel (ranging from 4 to 16 Ra finish) with connections to standard 1/4-in. VCR fittings.
Other opportunities that have been identified for MEMS-based systems require the mixing of liquids and gases on the same panel and the vaporization of the liquid into the process chamber. In many applications, the fluid delivery function can be placed in close proximity to the process chamber because of the smaller MEMS-based designs. In these instances, it is also possible to separate the electronic control functions from the chip-carrier modules and place the electronics remote from the chamber. Conclusion The full impact of MEMS-based gas and liquid delivery systems on semiconductor manufacturing will not be realized until toolmakers begin to redesign equipment based on the unique benefits provided by this technology. The most obvious area for improvement is decreasing tool footprint, which can reduce required cleanroom space by up to a factor of 10. Less-obvious benefits are derived by reducing the amount of gases and liquids consumed, increasing process yields through improved pressure-based MFC performance, and decreasing system downtime via self-diagnostic and self-calibration capabilities. All of these benefits will decrease cost of ownership of process fluid delivery systems by a factor of five, while enabling new processes for liquids as well as high precision for low-flow-rate gases. References 1. The National Technology Roadmap for Semiconductors, San Jose, SIA, 1997. 2. Cestari J, Laureta D, and Itafuji H, "The Next Step in Process Gas Delivery: A Fully Integrated System," Semiconductor International, 20(1):79—87, 1997. 3. Zdeblick MJ, and Angell JB, "A Microminiature Electro-Fluidic Valve" in Tranducers '87, Proceedings of the 1987 International Conference on Solid State Sensors and Actuators, Piscataway, NJ, IEEE Press, pp 827—829, 1987. 4. Henning AK, Fitch J, Hopkins D, et al., "A Thermopneumatically Actuated Microvalve for Liquid Expansion and Proportional Control" in Transducers '97, Proceedings of the 1997 International Conference on Solid State Sensors and Actuators, Piscataway, NJ, IEEE Press, pp 825—828, 1997. 5. Williams K, and Muller R, "Etch Rates for Micromachining Processing," IEEE Journal of Microelectromechanical Systems, 5:256—269, 1996. 6. Jansen H, Gardeniers H, de Boer M, et al., "A Survey of the Reactive Ion Etching of Silicon in Microtechnology," Journal of Micromechanics and Microengineering, 6:14—28, 1996. 7. Eriksen G, and Dyrbye K, "Protective Coatings in Harsh Environments," Journal of Micromechanics and Microengineering, 6:55—57, 1996. 8. Electronic Materials Chemistry, Pogge HB (ed), New York, Marcel Dekker, 1996. 9. CRC Handbook of Metal Etchants, Walker P, and Tarn W (eds), Boca Raton, FL, CRC Press, 1991. 10. Flaherty E, Herold C, Wojciak J, et al., "Reducing the Effects of Moisture in Semiconductor Gas Systems," Solid State Technology, 30(7):69, 1987. 11. "Specification for Surface Mount Interface of Gas Distribution Components," SEMI Draft Document #2787, Mountain View, CA, SEMI, 1997. Albert K. Henning, PhD, is director of technology at Redwood MicroSystems (Menlo Park, CA), where he has worked for two years. He formerly was a device physicist at Intel and later an assistant and associate professor of engineering at Dartmouth College (Hanover, NH). He spent one year as a visiting scientist in the MicroStructures Technology Laboratory at Massachusetts Institute of Technology (Cambridge, MA). He holds AB and AM degrees in physics from Dartmouth College and a PhD in electrical engineering from Stanford University (Palo Alto, CA). He has published more than 50 articles and has received one U.S. patent. (Henning can be reached at henning@redwoodmicro.com.) Edward B. Dehan is vice president of business development at Redwood MicroSystems, where he has spent six years identifying and qualifying applications for MEMS valves and MEMS-based fluid control systems. He holds an AB in economics from Dartmouth College and an MBA from the Stanford Graduate School of Business. (Dehan can be reached at 650/617-1209 or bdehan@compuserve.com.) Errol B. Arkilic, PhD, is principal engineer at Redwood MicroSystems, where he has worked since August 1997. He received his PhD for work on gaseous flow in micromachined channels at the department of aeronautics and astronautics at the Massachusetts Institute of Technology. James M. Harris, PhD, has been president and CEO of Redwood MicroSystems since 1996. He previously was CEO at Akashic Memories and Purus. His background also includes work at IBM and National Semiconductor. He holds a PhD in materials science from MIT. He has been granted nine patents and has published six articles.
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Figure 4: Configuration of a pressure-based MFC that uses MEMS components.
is a parameter related solely to the ratio 
of specific heats (at constant pressure and volume) for the particular gas under control; R is the universal gas constant divided by the molecular weight of the gas; Cd is the coefficient of gas discharge for the flow element; and T is the fluid temperature. The sonic flow relations for compressible gas flow are also shown, in which 
is a parameter similar to 
Figure 5: Flow curves for the valve and orifice flow components. The black portions of the curves depict sonic behavior, the blue portions represent subsonic behavior.
Figure 6: MEMS-based MFC chip-carrier module.
Figure 10: Schematic view (a) of integrated MEMS-based gas panel that comprises 10 channels, or gas sticks, with purge capability; and isometric view (b) of the gas panel.