A micromachined vapor jet pump

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Abstract

Initial results from a micro-pump based on the principle of macroscopic vapor-jet- and diffusion-pumps are presented. A mathematical description of the underlying gas dynamics is given and equations for the calculation of pump geometry are derived. Several planar micro-vapor jet pumps with different geometries were designed and fabricated by means of silicon micromachining (STS Advanced Silicon Etch). The pumps consist of two Laval nozzles and water-cooled sidewalls. Pumping was achieved via momentum transfer of externally supplied nitrogen gas and water vapor respectively after expanding through the nozzles. To derive key parameters for future designs, various pump configurations with different geometries were tested.

Based on the results of the measurements, it is proven that the pump principle can be transferred to micro-systems. It is shown that a differential pressure between the fore vacuum side and the high vacuum side is achieved due to the pumping effect of the supersonic jet.

Introduction

In recent years, micro-systems have been applied in extending fields of application. In many cases, however, peripheral components to support such systems are not available in an equivalently miniaturized form, which both impedes the introduction of micro-systems and extends volume and media consumption considerably. The increasing importance of micro-systems demands miniaturization of peripheral devices as well. This holds true, especially for analytical and biomedical applications, where micro-pumps for fluids and gases are necessary.

Many approaches to realize micro-pumps have been successfully pursued [1], [2]. The proposed pumps require either mechanically moving parts or do not meet the requirements for high flow rate or low pressure. Moving parts, in most cases membranes fabricated of silicon or additionally required active or passive valves [3], [4], [5], may result in a short lifetime. The properties of those systems regarding long-term stability are not known. Besides the thermal transpiration-driven pumps (Knudsen compressor), a promising kind of micro-vacuum pump which uses a mechanism that can only be applied in micro-systems [6], [7], [8], no micro-pump seems to be capable of providing vacuum conditions.

This paper presents a first view on a micro-vacuum pump based on the well-known principle of vapor-jet- and diffusion-pumps. Advantages of this pump mechanism are:

  • No moving parts, i.e. no friction and wear.

  • Gas exhaust to atmospheric pressure should be possible.

  • Multiple stages in a stack arrangement can be used to reach the desired vacuum level.

  • No valves are required.

  • Small geometries lead to pure laminar flow even at atmospheric pressure, i.e. pump down from atmospheric pressure is possible.

Our approach is based on the scaling down of a well-known macroscopic pump type: The design resembles that of a macroscopic diffusion pump. These pumps are still widely used for vacuum generation. The functional principle of all ejector pumps (vapor jet pumps, diffusion pumps) is very simple: Elevated pressure of gas or vapor inside the jet assembly is converted to kinetic energy of a high velocity jet after it passes through a nozzle. The pumping is achieved via momentum transfer in the jet direction. A complete system consists of a heater unit to evaporate the working fluid, the jet assembly (high-pressure unit) from where the vapor expands through the nozzles, an area where the pumped gas mixes with the vapor jet, cooled side walls to condense the vapor and a mechanism to return the working fluid to the heater unit (see Fig. 1). To simplify our first approach, we concentrated on the jet assembly, pumping area and condenser. As other groups [9] already reported on mesoscale ejector pumps but failed to investigate a micromachined one, the initial objective was to prove the applicability of the underlying functional principle to micro-systems.

Section snippets

Gas dynamics of vapor jet pumps

Theoretical description of processes inside vapor jet pumps is based on gas dynamics. In the case of one-dimensional stationary flow, the Bernoulli equation applies in the following form:dpρ=12dv2Integration between points 1 and 2 along the stream results in12(v22v12)=12dpρTo evaluate this integral, an expression for the density ρ as a function of pressure p is required. Poisson’s equationp1p0=ρ1ρ0κdescribes this correlation for adiabatic changes of state for ideal gases (pressure p,

Design and fabrication

Starting from a foreline pressure pa and a minimum nozzle cross-sectional area b all other geometrical data can be calculated as a function of the Mach number of the jet: pressure inside the high-pressure unit (jet assembly) (p0), cross-sectional area of the nozzle exit (ba), nozzle length (l) and maximum distance between nozzle and pump body (z).

For our design, pa was defined as atmospheric pressure. Two different values for b were set: 150 and 40 μ m. As Mach number Ma=1.5 (p0=3.6 bar) and M

Measurements and discussion

All experiments were repeatable given the same device and experimental conditions. The results presented are given as average values of at least five measurements.

In a first step to verify the function of our micro-pumps, nitrogen was used as a working fluid. A controlled gas flow was directly fed into the jet assembly, expanded through the nozzles and pumped out of the pump body by a rotary pump. The differential pressure between the fore vacuum side (FVS) and the high vacuum side (HVS) was

Conclusion

A micromachined vapor jet pump is presented to meet the needs for a reliable micro-vacuum pump based on the assembly of diffusion pumps. To demonstrate the basic function, externally supplied water vapor was used to generate a vapor jet. Condensation of the vapor was achieved by cooling the pump body with water. An externally connected pressure sensor was used to measure the pump effect of the fabricated systems.

First results show that the operation principle of macroscopic scale ejector pumps

Marco Doms is a postgraduate member of the Department of Micro System Technology at the Hamburg-Harburg University of Science and Technology where he studied electrical engineering and received his Dipl.-Ing. degree in 2003. He is currently working on the development of a micromachined vapor jet pump with integrated pressure sensing.

References (13)

  • R. Rapp et al.

    LIGA micropump for gases and liquids

    Sens. Actuators A

    (1994)
  • N.-T. Nguyen et al.

    MEMS-micropumps: a review

    Trans. ASME

    (2002)
  • P. Woias

    Micropumps – summarizing the first two decades

    Proc. SPIE

    (2001)
  • R. Linnemann et al.

    A self-priming and bubble-tolerant piezoelectric silicon micropump for liquids and gases

  • C.G.J. Schabmueller et al.

    Design and fabrication of a self-aligning gas/liquid micropump

    Proc. SPIE – Int. Soc. Opt. Eng. (USA)

    (2000)
  • R.M. Young

    Analysis of a micromachine based vacuum pump on a chip actuated by the thermal transpiration effect

    J. Vac. Sci. Technol. B

    (1999)
There are more references available in the full text version of this article.

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Marco Doms is a postgraduate member of the Department of Micro System Technology at the Hamburg-Harburg University of Science and Technology where he studied electrical engineering and received his Dipl.-Ing. degree in 2003. He is currently working on the development of a micromachined vapor jet pump with integrated pressure sensing.

Joerg Mueller teaches semiconductor electronics and microsystems at the Hamburg-Harburg University of Science and Technology since 1983, where he is head of the Department of Micro System Technology. He works on thin film sensors, micro systems for micro total analysis, biological and medical applications, integrated optics for optical communication and metrology, polycrystalline silicon solar cells on glass and direct methanol fuel cells. He studied electrical engineering at Technische Universitaet Braunschweig, where he also obtained his PhD and Habilitation on thin silicon film microwave and optical semiconductor devices. Between 1979 and 1983 he headed the of microwave diode development and fabrication section of Siemens AG in Munich.

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