"The Technical Issues Associated with Highly Miniaturized Vacuum Systems"
Phil Muntz (University of Southern California)

Vacuum pumps for miniaturized instruments must be reliable, have an appropriately small size, and have a power consumption or energy requirement consistent with the associated instruments. All non-capture pumping systems exhibit a trade-off between the volume or mass flow of the working gas and the pumping pressure ratio; at what point this balance is struck depends on the pump and, in the present context, on the relative weighting between the constraints on pump volume and power consumption. Attempts to date have shown that it is extremely difficult to provide microscale or even mesoscale vacuum pumping systems satisfying projected volume and power constraints; reliability has hardly had a chance to surface as an issue . First of all, it is already clear that there will have to be a significant, continuing dialog between instrument and pump designers; it will be essential to identify carefully the minimum pumping requirements that will satisfy the particular scientific purposes of each application. The subject of appropriate volume and energy performance indicators for meso- and microscale vacuum pumps will be discussed. Typical values of these indicators for conventional macroscale vacuum systems are compared to projected requirements of meso- and microscale instruments. The scaling suitabilities of several conventional vacuum pumping technologies to meso- and microscale are discussed. One result of the scaling analyses is that there is an apparent need to develop unconventional phenomena for application as meso- and microscale vacuum pumping technologies. Estimated values of the volume and energy performance indicators for several unconventional technologies will be presented. The presentation will conclude with a brief summary of the major technical issues confronting the successful achievement of meso- and microscale vacuum pumping systems, that will be compatible with meso- and microscale instrumention.
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"Meso-Scale Scroll Pump Array Fabricated using LIGA Technology for Portable, High-resolution Mass Spectrometer"
Beverley Eyre, Kirill Shcheglov, Otto Orient, Nosang V. Myung, Dean Wiberg (Jet Propulsion Laboratory)

A scroll pump is a pump whose action requires planar, rotary movement of two intelocking and complimentary Archimedian Spirals. In the initial position a volume of gas is trapped in the outer ring of the scroll, and as the inerlocking spirals move relative to each other the gas is compressed and pushed in towards the center of the spiral. When the gas reaches the center, it is pushed out of a hole, and the motion starts again with a new volume of gas. A scroll pump array is being built at JPL consisting of nine individual scrolls. Each scroll has an approximate diameter of one centimeter with a varying wall thickness as the walls spiral in towards the center. The precise matching of the complimentary sidewalls and the target dimensions of this device make it a good candidate for fabrication by LiGA technologies. The sidewalls of the scrolls will be approximately 3mm in height, and must be straight to within a very fine tolerance. This requires special filtering to achieve the necessary top to bottom dose ratio in during the exposure step of the LiGA process. The exposures for this device have been done at three synchrotron radiation sources around the country: The Advanced Light Source at Lawrence Berkeley National Laboratory, The Standford Synchrotron Radiation Laboratory, and the National Synchrotron Light Source at Brookhaven National Laboratory. The differences in these three light sources and the results in exposing ultra-thick PMMA will be discussed.
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"Performance Analysis for Meso-Scale Scroll Pumps"
Eric Moore, E. Phillip Muntz (University of Southern California); Francis Eyre, Nosang Myung, Otto Orient, Kirill Shcheglov, Dean Wiberg (Jet Propulsion Laboratory)

The scroll pump is an interesting positive displacement pump that is currently being investigated as a potential micro-scale backing pump. The pump uses a circular motion with pairs of fixed and orbiting scrolls to form a peristaltic pumping action. As the moving scrolls follow an orbital trajectory, pockets of trapped gas that continuously decrease in volume are forced along the fixed scrolls, eventually reaching the center and being discharged. A complete scroll pump can be defined by one fixed scroll and one orbiting scroll. In most scroll pumps an Archimedes spiral is used to determine the shape of the fixed scrolls. For the orbiting scrolls an Archimedes spiral 180° out of phase relative to the fixed scrolls is used. The scrolls typically have from a few up to ten turns about their origins. As the number of turns increases so does the compression ratio of the pump. The pump being studied has two and a half turns. It is intended to be used as one stage of a multi-stage roughing pump for a meso-scale turbo molecular pump, in order to provide the vacuum system for a mobile, sampling mass spectrometer. Governing equations for the meso-scale scroll pump have been developed, taking into account the losses, due to leaks, of trapped gasses as they are transported from the pump inlets at the outer perimeter of the scrolls to the centrally located discharge port. A modeling of energy losses in the scaled pumps is also included. The main purpose of the paper is to present and apply a size scaling analysis; in order to determine if a multi-stage, meso-scale scroll pump can operate with sufficient efficiency to meet the specifications for the sampling mass spectrometer's pumping system. The modeling includes arbitrary numbers of cascaded pump stages.
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"The Knudsen Compressor as an Energy Efficient Micro-Scale Vacuum Pump"
Marcus Young, E. P. Muntz, G. Shiflett (University of Southern California); A. Green (Jet Propulsion Laboratory)

The Knudsen Compressor suggested by Pham-Van-Diep et al and demonstrated by Vargo et al is a modern version of the original thermal transpiration compressor described by Knudsen in 1910. The Knudsen Compressor can be applied as either a vacuum pump or compressor for gases. A single stage of a Knudsen Compressor is comprised of a transpiration membrane, with pore diameters such that the gas flow in them is in the rarefied regime, and a continuum connector section, with a diameter such that the gas flow in it is in the continuum regime. A temperature gradient is applied across the transpiration membrane, driving the flow from the cold to the hot side of the transpiration membrane due to thermal transpiration. The temperature is then returned to its original value in the connector section where the flow is in the continuum regime and no thermal transpiration takes place. This process is repeated for many stages until the required pressure difference is achieved. Earlier investigations on the MEMS Knudsen Compressor have indicated that there are several interesting potential applications of the Knudsen Compressor because it has no moving parts and requires no lubricants or supplementary working fluids. One of these applications, a micro-scale roughing pump for MEMS based sensors such a mass spectrometers, optical spectrometers, and gas chromatographs, will be discussed. The practical low and high-pressure pumping limits of the MEMS Knudsen Compressor have been previously identified as 10 mTorr and 10 atm, respectively. This indicates that the Knudsen Compressor can operate as a roughing pump for micro scale instruments from above atmospheric pressure down to 10 mTorr. It was concluded in the earlier work that the low-pressure stages of the Knudsen Compressor use the largest amount of energy, indicating that they require special considerations. Using a transitional flow model, an investigation was conducted into optimizing the Knudsen Compressor configuration to minimize the energy consumption of the low-pressure stages. Possible modifications for the low-pressure stages of the MEMS Knudsen Compressor operating as a roughing pump have been identified and will be discussed. Estimates of the effects of the modifications on the pumping performance of the Knudsen Compressor at low pressures have been estimated. One such modification is to etch carefully sized capillaries into the aerogel transpiration membrane to optimize the flow Knudsen number in the transpiration membrane pores. Another primary concern, efficiently transitioning from the capillary section to the connector at constant temperature will also be discussed.
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"MEMS-based Low-Flow Meters"
Tom Tsao, Fukang Jiang, Edward Chiu (Umachines, Inc.)

MEMS-based pressure, flow, and temperature sensors are well suited for monitoring gas flows in applications where system and component volume and weight are critical. Additionally, MEMS sensors can be very effective in measuring the low flow rates and volumes associated with small sample sizes. These combined factors make it worth investigating the use of such sensors for harsh-environment mass spectrometry applications. Umachines has developed generic pressure, flow and temperature sensors for a wide range of applications and environments, ranging from supersonic air flow across leading edges of airplane wings to oil flow in the harsh (pressure and chemical) environment existing 2 miles underground. Umachines has also customized both sensor and packaging technology specifically for use in space-based low flow (< 10 sccm) mass flow meter. The mass flow meter consists of three pieces: a sensor chip, a channel chip, and fluidic connections. The sensor chip consists of 5 clusters of sensors, with each cluster containing one shear, one temperature, and one pressure sensor. Both pressure and shear measurements are used because it was uncertain which measurement would prove superior. Temperature measurements were used to provide any needed temperature compensation. These sensors are all fabricated monolithically within the same process. The core technologies upon which Umachines flow sensors are based is surface micromachining. More specifically, various thin films are deposited, patterned, and then etched to form the desired structures. The pressure sensor measures absolute pressure using piezoresistors (arranged in a Wheatstone bridge configuration) sitting atop a nitride diaphragm covering a vacuum cavity. The shear stress sensor is a thermal sensor - a resistor sitting atop a vacuum cavity, which is used to provide thermal isolation and improve sensitivity by orders of magnitude compared with resistors sitting atop solid substrates. This thermal sensor is typically biased in a constant temperature mode for greatest sensitivity. The temperature sensor sits atop the substrate, not over a vacuum cavity, because the overall system temperature is a composite of the flow temperature and substrate temperature. Thermal isolation distorts this composite and skews the result. The channel chips can either be micromachined using deep reactive ion etching or can be conventionally machined using precision tools. One of the most difficult elements in packaging the final devices is the flow interconnection, which, if done incorrectly, can lead to contamination of the system. Experimentally, each sensor in a system needs to be well calibrated. This is an issue that needs to be addressed should such a device enter mass production. Our system has been tested for flows ranging from 5-100 sccm nitrogen, with a sub sccm resolution. Both pressure and shear stress measurements have been confirmed to work well, although using pressure drop appears to be a simpler method. Upon further experimentation and packaging improvement, field deployment is possible.
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"The Issues Limiting Large-scale Commercialization of Miniature Vacuum Systems"
Peter Kardok (Alcatel Vaccum Products, Inc.)

The presentation, prepared from a demand perspective, defines the issues affecting the commercialization of miniature vacuum systems. Counter to electronics technology, moving on a steady path toward miniaturization, vacuum systems have made only modest gains in this direction. Most vacuum systems use roughing pumps larger than 0.5 cfm and high vacuum pumps greater than 7.5 l/s. This presentation will address the reasons behind these facts and the requirements for change. A review of the vacuum pumps world market and the major applications within each market will be presented. Analysis of this information will be used to reveal the reasons why "large" pumps are used (eg. outgassing rate, gas flow or substrate size) resulting in an estimate of the actual current market for miniature vacuum systems. This market review points to analytical instrumentation as the primary market for miniature vacuum systems. This market is significant in size however, miniature vacuum systems may be limited to portable instruments. Additionally, the presentation will outline potential future applications for miniature vacuum systems and describe some of the hurdles to overcome for these products to gain wide acceptance. Such hurdles include limited availability of associated miniature vacuum components. (eg. miniature gauges, flanges and fittings)
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"Development of Turbomolecular Pumps for Demanding Environments"
Marc Kenton (Creare, Inc.)

For efficient operation, the rotor tip speed of a turbomolecular pump must be significant compared to the molecular speed of the gas species being pumped. For a miniaturized pump, this requirement inevitably leads to very high rotational speeds, which can compromise bearing life and the pump's ability to withstand shock and vibration. Under NASA sponsorship, Creare is developing two pumps to meet these challenges. The current efforts leverage technology developed in an earlier NASA-funded project. The earlier turbomolecular pump has a diameter of 4.6 cm, a length of 11 cm, a mass less than 400 grams, a measured compression ratio for nitrogen greater than 1,000,000 and a measured pumping speed of 4.5 L/s. Power consumption is about 1 Watt at discharge pressures of 10 mTorr or less. Successful operation of this pump was demonstrated for several months running with a rotational rate of 100,000 rpm. The ruggedized pump being developed in one of the current programs is similar in size and pumping speed to the earlier pump, but it incorporates a unique motor design to achieve a very high resistance to shock and vibration. In a second effort, an ultra-miniaturized pump is being built that is approximately the size of a C-cell battery and has a pumping speed of 1.5 L/s at 200,000 rpm. Both of these pumps incorporate a molecular drag stage to achieve discharge pressures of a few Torr Further, as part of the development programs, both will be mated with rough pumps to form complete, miniaturized, vacuum systems for portable instruments such as mass spectrometers. This paper will summarize the advantages and challenges of miniaturized turbomolecular pumps and the experimental program employed to confirm the designs. Possible applications for the pumps will also be discussed.
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"Miniature Turbo-molecular Pump"
Rob Rowan, Mark Johnson (Phoenix Analysis & Design Technologies)

Phoenix Analysis and Design Technologies (PADT) is developing a miniature Turbo-Molecular Pump (TMP) for space and terrestrial applications. The pump is intended for applications requiring 10 l/s or less pumping speed where cost, weight, and power consumption are of high importance. The baseline specifications for the pump are: weight = 200gm, power consumption ~ 5 W, ultimate pressure = 10-6 Torr at 10 l/s, and MTBO = 10000 hours. This technology employs a number of novel concepts, which enable low-cost rotor manufacturing, very high drive efficiency, and an adjustable flow path. This adjustable flow path allows PADT to deliver custom machines designed to balance trade-offs between pumping speed, life, and ultimate pressure depending on application requirements. PADT is now in the late stages of prototype development and is forecasting first prototype delivery to NASA in mid 2002.
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"KSC Miniature, Rugged Mass Spectrometer Applications and Development Progress" Frederick Adams, Duke Follistein (NASA/Kennedy Space Center); Richard Arkin, Tim Griffin (Dynacs, Kennedy Space Center)

Two goals are of interest to the Kennedy Space Center (KSC). The first is to install mass spectrometers close to the T0 disconnect umbilicals (Shuttle) to eliminate the delay in getting the sample. These would have to operate and supply reliable concentration data during main-engine pressurization and the first six seconds of main-engine operation just prior to liftoff. The second is to be sufficiently rugged to supply gas concentration data during lift-off and ascent to orbit while the vehicle is still within the atmosphere. The goal is to disable a leaking component of the propulsion system during flight or prevent lift-off if the leak occurs during main engine pressurization. A secondary goal related to the second issue is to be able to gather gas concentration data for second-generation (flying 10 years in the future) and third-generation (25 years in the future) test vehicles. KSC will require rugged mass analyzers made from small, reliable components that can be integrated into a usable sensor (ionizers, mass filters and detectors). We will require small, rugged, reliable high-vacuum pumps with adequate compression for both hydrogen and helium (slightly less than for nitrogen and oxygen). Sample delivery hardware requires miniature or microscopic pressure regulators (absolute/ balanced against vacuum as opposed to atmospheric pressure) and flow controllers to switch and feed sample and calibration gas mixtures to the gas analyzer. These require low dead or un-swept volume to minimize calibration gas and increase speed. Miniature or micro-miniature valves and manifolds are required for sample and calibration gas selection. Techniques for welding micro-miniature tubing in small sizes and tight spaces or otherwise fabricating miniature gas transport and switching functions into manifolds are necessary. Point sensors for hydrogen and oxygen are needed for incorporation into a dedicated vehicle health monitoring system that are small in size, require low power that can be distributed about a vehicle propulsion system to identify leak locations quickly Development in work consists of miniature turbo-molecular pump development, a time-of-flight instrument to reduce size and mass, fiber-optic point sensors for hydrogen and oxygen, chemical point sensors for hydrogen and oxygen, solid state ionizers to use physical mechanisms to generate free electrons to eliminate thermionic emission based ionizers. Areas where JPL might help are development of miniature, high-efficiency ruggedized ion engines to enhance the performance of miniature turbo-molecular pumps. Analysis of vibration effects to estimate the increase in limits of detection as a function of mechanical vibration would be useful to us also.
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"Miniature Peristaltic Vacuum Pump with Magnetic Actuation"
Sabrina Feldman, Danielle Svehla (Jet Propulsion Laboratory)

An increasing number of portable scientific instruments require a means of producing a vacuum of several mTorr or less. However, commercially available vacuum pumps which provide a lower pressure limit of ~ 1 mTorr and are capable of venting to atmosphere have sizes, masses, and power consumptions incompatible with portable applications. We are developing a novel miniature peristaltic vacuum pump which uses magnetically-actuated pump chambers to control gas flow. Our estimates of pump parameters and calculations of the expected pump performance indicate that if we are successful, this pump will provide new capabilities for portable instrumentation and will also be suitable for use in extreme environments such as those encountered in space and planetary exploration. Pump description: The pump body will consist of multi-staged pump chambers manufactured in silicone rubber enclosed within aluminum housing. Permanent magnets will be mounted in a fixed position above the pump chambers as well as on a rotating wheel below the chambers. We estimate a total pump mass of ~ 0.2 kg, a volume of ~ 1"x2"x2", a power consumption of ~ 5 W, and a lower pressure limit of several mTorr. The calculated mass and power consumption are an order of magnitude lower than those of the smallest currently available portable vacuum pump. In addition, our pump design offers the following desirable features: very low dead volume, robust oil-free design, low cost fabrication through mold replication of the pump body, and ease of multi-staging. Applications: Many widely used scientific instruments require a means of producing a vacuum with pressures in the sub-Torr range. Such instruments include secondary ion mass spectrometers, laser ablation mass spectrometers, cooled infrared sensing detectors, microwave spectrometers using Stark cells, instruments containing unsealed low-pressure lasers, instruments containing ion or electron sources, trace gas concentration systems, leak detectors, scanning electron microscopes, etc. In particular, instruments which involve charged particle generation or detection typically require vacuum operation. Portable applications of these instruments, including field analysis, environmental monitoring, and use in outer space, require minimizing the size, mass, and power consumption of components in order to increase the instrument's portability and utility.
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"Development of a Miniature Lightweight Ion Pump"
Mahadeva Sinha (Jet Propulsion Laboratory)

A miniature ion pump is being developed in our laboratory. In the design of the pump, its outer shell (pump housing) is made of titanium which also works as the pumping element. Argon stability is achieved by placing pin electrodes in the cylindrical anodes of the pump. The mass of the magnet for the ion pump is reduced by using rare earth magnet material, and high permeability alloy for the yoke. The pump has been fabricated and its performance is presently being measured. The details of the pump design and the results of the performance measurements will be presented.
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