Microelectromechanical pump utilizing porous silicon

A microelectromechanical (MEM) pump is disclosed which includes a porous silicon region sandwiched between an inlet chamber and an outlet chamber. The porous silicon region is formed in a silicon substrate and contains a number of pores extending between the inlet and outlet chambers, with each pore having a cross-section dimension about equal to or smaller than a mean free path of a gas being pumped. A thermal gradient is provided along the length of each pore by a heat source which can be an electrical resistance heater or an integrated circuit (IC). A channel can be formed through the silicon substrate so that inlet and outlet ports can be formed on the same side of the substrate, or so that multiple MEM pumps can be connected in series to form a multi-stage MEM pump. The MEM pump has applications for use in gas-phase MEM chemical analysis systems, and can also be used for passive cooling of ICs.

FIELD OF THE INVENTION

The present invention relates in general to microelectromechanical (MEM) devices, and in particular to a MEM pump (also termed a thermal transpiration pump, or a Knudsen pump) comprising a porous silicon region. The MEM pump has applications for use in gas-phase MEM chemical analysis devices, and for passive cooling of integrated circuits (ICs).

BACKGROUND OF THE INVENTION

Recently, interest has been rekindled in forming thermal transpiration pumps since these pumps have no moving parts, do not require oil, and can operate in any orientation. Additionally, thermal transpiration pumps are amenable to miniaturization for use with microelectromechanical (MEM) devices. Previous attempts to form Knudsen pumps have utilized an aerogel comprising suspended silicon dioxide particles, a photopolymer, a plurality of stacked spherical particles, or very shallow channels etched into a substrate (see U.S. Pat. Nos. 5,871,336 and 6,533,554; and U.S. Patent Publication No. 2004/0179946).

The present invention provides an advance over the prior art by forming a microelectromechanical (MEM) Knudsen pump (hereafter referred to as a MEM pump) using a porous silicon region formed in a silicon substrate.

An advantage of the MEM pump of the present invention is that the porous silicon can be formed with a pore size (i.e. a cross-section size of each pore) that can be predetermined to be anywhere in the range of 10 nanometers to 10 microns or more.

Another advantage of the present invention is that the MEM pump can be integrated with other gas-phase MEM devices including chemical preconcentrators, gas chromatographs, detectors, etc.

Yet another advantage of the present invention is that a multi-stage MEM pump can be formed on a common substrate by connecting together in series multiple MEM pumps each tailored to operate in a different gas pressure regime.

These and other advantages of the present invention will become evident to those skilled in the art.

SUMMARY OF THE INVENTION

The present invention relates to a microelectromechanical (MEM) pump for pumping a gas. The MEM pump comprises an inlet chamber for receiving the gas; an outlet chamber in thermal communication with a heat source; and a silicon substrate separating the inlet chamber from the outlet chamber, with the silicon substrate comprising a porous silicon region having a plurality of pores extending between the inlet chamber and the outlet chamber, and with a cross-section dimension of each pore being substantially equal to or smaller than a mean free path length of the gas to pump the gas from the inlet chamber to the outlet chamber in response to a thermal gradient provided along a length of each pore by the heat source. The cross-section dimension of the pores can be, for example, in a range of 10 nanometers to 10 microns.

An inlet port can be located on one side of the silicon substrate and connected to the inlet chamber; and an outlet port can be located on the same side of the silicon substrate and connected to the outlet chamber. This can be done by providing a channel formed through the substrate to connect the outlet chamber to the outlet port, or alternately by providing a channel formed through the substrate to connect the inlet chamber to the inlet port. Locating the inlet and outlet ports on the same side of the silicon substrate using the channel formed through the substrate can facilitate making external connections to the MEM pump (e.g. with tubing). Also, the provision of the channel through the substrate is useful for connecting a plurality of MEM pumps in series to form a multi-stage MEM pump.

The heat source can comprise an electrical resistance heater. In some embodiments of the present invention, the electrical resistance heater can be supported by a lid which forms one or more walls of the outlet chamber. In other embodiments of the present invention, the electrical resistance heater can be supported on a suspended membrane (e.g. comprising silicon nitride or silicon dioxide).

In yet other embodiments of the present invention, the heat source can comprise an integrated circuit (IC) which is in thermal communication with a lid which forms at least one wall of the outlet chamber. In these embodiments of the present invention, the heat generated by the IC can act to pump a gas (e.g. air) through the MEM pump, with the gas being heated and thereby removing heat from the IC. In this way, the IC can be passively cooled without requiring any electrical power for the MEM pump, or any external pump to flow the gas through the porous silicon region.

The present invention also relates to a MEM pump for pumping a gas which comprises a silicon substrate having a plurality of pores formed therethrough with each pore having a first end in fluid communication with an inlet chamber located on a first major surface of the silicon substrate, and with each pore having a second end in fluid communication with an outlet chamber located on a second major surface of the silicon substrate. Each pore is substantially straight and aligned substantially perpendicular to the major surfaces of the silicon substrate, and can have a cross-section dimension which is substantially equal to or less than a mean free path of the gas. An electrical resistance heater is located proximate to the second end to provide a thermal gradient between the first and second ends of each pore to draw the gas through each pore. The cross-section dimension of each pore is generally in a range of 10 nanometers to 10 microns, with the exact cross-section dimension of each pore depending upon a pressure of the gas being pumped.

The silicon substrate can have a channel formed therethrough to transport the gas from the outlet chamber to the first major surface of the silicon substrate. The electrical resistance heater can be supported on a suspended membrane, or by a lid which forms at least one wall of the outlet chamber.

The present invention further relates to a MEM pump for pumping a gas which comprises a silicon substrate having a first major surface and a second major surface, with an inlet chamber being formed on the first major surface of the silicon substrate, and with an outlet chamber being formed on the second major surface of the silicon substrate, and with the outlet chamber being in fluid communication with an outlet channel which extends through the silicon substrate to the first major surface thereof. A porous silicon region is formed in the silicon substrate, with the porous silicon region comprising a plurality of pores extending between the inlet chamber and the outlet chamber. Each pore is substantially straight and has a cross-section dimension in the range of 10 nanometers to 10 microns. The MEM pump also comprises means for providing a thermal gradient across the porous silicon region along a length of each pore to draw the gas from the inlet chamber through the porous silicon region to the outlet channel.

The means for providing the thermal gradient across the porous silicon region can comprise an electrical resistance heater located in the outlet chamber to heat the porous silicon region on the second major surface of the silicon substrate. Alternately, the means for providing the thermal gradient across the porous silicon region can comprise an integrated circuit in thermal communication with the porous silicon region on the second major surface of the silicon substrate.

The present invention also relates to a MEM pump for pumping a gas which comprises a silicon substrate having a pair of major surfaces; a plurality of porous silicon regions formed in the silicon substrate between the pair of major surfaces, with each porous silicon region further comprising an inlet end located proximate to one of the major surfaces, an outlet end located proximate to the other major surface, and a plurality of substantially straight pores extending through each porous silicon region between the inlet end and the outlet end. In the MEM pump, each adjacent pair of the porous silicon regions can be interconnected by a flow channel which extends through the silicon substrate from the outlet end of one porous silicon region of the pair to the inlet end of the other porous silicon region of the pair. An electrical resistance heater is located proximate to the outlet end of each of porous silicon region to provide a thermal gradient across that porous silicon region to pump the gas therethrough.

The pores in each porous silicon region can have a cross-section dimension which is substantially equal to or smaller than a mean free path of molecules of the gas being pumped through that porous silicon region. The pores in one or more of the porous silicon regions can also have a cross-section size which is different from the cross-section size of the pores in another of the porous silicon regions. By providing different pore sizes for the various pump stages, each pump stage can be optimized for an expected gas pressure therein.

Each electrical resistance heater can be disposed on a lid which is attached to the major surface of the silicon substrate wherein the outlet end of each porous silicon region is located. Alternately, each electrical resistance heater can be supported on a suspended membrane.

The present invention further relates to a MEM pump for pumping a gas which comprises a plurality of pump stages connected together in series. Each pump stage can comprise an inlet chamber and an outlet chamber separated by a porous silicon region, with the porous silicon region comprising a plurality of pores formed in a silicon substrate. Each pore is substantially straight and has a cross-section size which is substantially equal to or smaller than a mean free path of the gas therein. An electrical resistance heater is located within the outlet chamber of each pump stage to provide a thermal gradient directed along a length of the pores of that pump stage to draw the gas through the pores of that pump stage. Each adjacent pair of the pump stages can be connected together in series by a channel extending from the outlet chamber of a first pump stage of the pair through the silicon substrate to the inlet chamber of a second pump stage of the pair.

Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1Ashows a schematic plan view of a first example of a microelectromechanical (MEM) pump10according to the present invention; andFIG. 1Bshows a schematic cross-section view of this device10along the section line1-1inFIG. 1A. InFIG. 1A, a lid12has been omitted from the schematic plan view of the MEM pump10in order to show details of the interior of the pump10including a porous silicon region14which is responsible for providing a thermal transpiration pumping of a gas100when a thermal gradient is provided across the porous silicon region14. The porous silicon region14comprises a plurality of substantially straight pores16which are etched through a silicon substrate18, with the pores16all being of substantially the same size, and with a cross-section dimension of the pores16that generally ranges from about 10 nanometers (nm) to about 10 microns (μm) depending upon a pressure of the gas100in the MEM pump10.

By forming the pores16substantially perpendicular to the silicon substrate18as shown inFIGS. 1A and 1B, a large number of pores16can be provided in a relatively small space in the MEM pump10. This can save a significant amount of space, especially when a number of MEM pumps10are to be formed on a common substrate (e.g. as a multi-stage MEM pump). This configuration with the pores16substantially perpendicular to the silicon substrate18can also provide a relatively high flow rate since the pores16can be closely packed together.

In the MEM pump10ofFIGS. 1A and 1B, an inlet chamber20is located on one side of the porous silicon region14as shown inFIG. 1B; and an outlet chamber22is located on the other side of the porous silicon region14. The gas100being pumped can be admitted into the inlet chamber20through an inlet port24which can be open to the ambient, or connected to tubing26as shown inFIG. 1B. The gas100is drawn into an inlet side of the porous silicon region14which forms one wall of the inlet chamber20.

Since the pores16have a cross-section dimension (i.e. a diameter or width) which can be about equal to or smaller than a mean free path of the molecules of the gas100, molecule-to-wall interactions will dominate the flow of the gas100in the pores16. In this so-called free molecular regime, a thermal transpiration pumping (also termed thermal creep) of the gas100will occur when a thermal gradient is provided along the length of the pores16. If the size of the pores16is increased to larger than the mean free path of the molecules of the gas100, the gas flow will transition to viscous dominated molecule-to-molecule interactions, and a thermal creep portion of the gas flow will decrease. Such viscous dominated molecule-to-molecule interactions are characteristic of the inlet and outlet chambers20and22which have dimensions much larger than the mean free path of the gas100.

In the MEM pump10ofFIGS. 1A and 1B, an electrical resistance heater28is provided in the outlet chamber22to heat an outlet side of the porous silicon region14which forms one wall of the outlet chamber22. This establishes a thermal gradient along the length of the pores16due to a temperature difference between the inlet and outlet sides of the porous silicon region14which draws the gas100through the pores16from the inlet chamber20to the outlet chamber22. In a closed system, the relation between temperature and pressure in the inlet and outlet chambers20and22, respectively, is given by:

P2P1=T2T1
where P1and T1are the pressure and temperature in the inlet chamber20, and P2and T2are the pressure and temperature in the outlet chamber22. Thus, the pumping of the gas100from the inlet chamber20into the outlet chamber22depends on the absolute temperatures T1and T2of the gas100in these two chambers. The temperature T2in the outlet chamber22can be, for example, up to a few hundred degrees Kelvin (e.g. 400-600° K); and the temperature T1in the inlet chamber20can be, for example, about room temperature (e.g. 300° K). This allows the MEM pump10to be operated as a vacuum pump, or as a compressor, or both depending upon how connections are made to the chambers20and22. When used as a vacuum pump, the MEM pump10can evacuate the gas100from an external chamber which is connected by the tubing26to the inlet port24, or from the input chamber20when the inlet port24is sealed. When used as a compressor, the MEM pump10can provide an increased pressure of the gas100at an outlet port30which is shown connected to additional tubing26inFIG. 1B. The MEM pump10can also be used to draw the gas100through a MEM chemical analysis system to detect one or more chemical species of interest in the gas100.

In the example ofFIGS. 1A and 1B, the outlet port30is located on the same side of the silicon substrate18as the inlet port24by providing a channel32through the silicon substrate18, with the channel32generally being located outside the porous silicon region14. The cross-section size of the channel32, which can be circular, rectangular or any arbitrary shape, is much larger than that of each pore16so that viscous dominated molecule-to-molecule interactions will occur in the channel32.

The provision of the inlet and outlet ports24and30on the same side of the silicon substrate18facilitates making external connections to the MEM pump10through tubing26as shown inFIG. 1B. It also facilitates packaging of the MEM pump10and facilitates coupling of the MEM pump10to other MEM devices which can be formed separately or on the same silicon substrate18. Additionally, by providing the inlet and outlet ports24and30on the same side of the silicon substrate18, multiple MEM pumps10can be fabricated on a common silicon substrate18and connected together in series to form a multi-stage MEM pump (seeFIGS. 4A and 4B).

Fabrication of the MEM pump10ofFIGS. 1A and 1Bwill now be described with reference toFIGS. 2A-2Kwhich show a series of schematic cross-section views that illustrate different steps in the formation of the MEM pump10. Only the essential steps of the fabrication process will be described herein. Those skilled in the art will understand that many additional process steps are required which are well known in the micromachining art, including photolithographic mask design and fabrication, substrate cleaning steps, photolithographic mask exposure, development and stripping, impurity dopant diffusion, material deposition, device packaging, etc.

InFIG. 2A, a <100>-oriented monocrystalline silicon substrate18is provided which comprises an upper major surface34(also referred to herein as an upper surface34) and a lower major surface34′ (also referred to herein as a lower surface34′). The silicon substrate18is preferably lightly n-type doped (e.g. with phosphorous) with the exact doping level depending upon the cross-section size of the pores16to be formed in the substrate18. Generally, the cross-section size or diameter of the pores16in microns will be approximately equal to a resistivity of the silicon substrate18as measured in units of Ohm-centimeters (Ω-cm).

InFIG. 2B, layers36and36′ of silicon nitride are blanket deposited over the two major surfaces34and34′ of the silicon substrate18. The silicon nitride can be conformally deposited over the entire substrate18to a layer thickness of, for example, up to about 1-2 μm. This can be done by low-pressure chemical vapor deposition (LPCVD) at a substrate temperature of about 850° C.

InFIG. 2C, a plurality of generally micron-sized openings38can be formed through the silicon nitride layer36at the locations where the pores16are to be formed. This can be done by providing a photolithographically-defined etch mask over the silicon nitride layer36and then etching down through the layer36using reactive ion etching to expose the underlying upper surface34of the silicon substrate18. Although only a few openings38are shown inFIG. 2C, those skilled in the art will understand that up to thousands or more individual openings38can be formed, with the exact number of openings38depending upon the size and spacing of the pores16being formed to define the porous silicon region14of the silicon substrate18. The openings38can be round or square, or arbitrary shaped.

InFIG. 2D, an etch pit40is formed in the silicon substrate18at the location of each opening38. This can be done by exposing the upper surface34of the silicon substrate18to an anisotropic etchant such as potassium hydroxide (KOH) which preferentially etches the silicon, with the etching being terminated at <111> crystalline planes of the silicon which intersect to form an inverted pyramid structure for each etch pit40. The bottom of the etch pits40will define the locations where each pore16will be subsequently formed.

InFIG. 2E, the pores16can be formed by an anodic dissolution process whereby the silicon is anodized by a hydrofluoric acid (HF) electrolyte. The anodic dissolution process can be performed by exposing the etch pits40to the HF electrolyte with an electrical current being applied to the silicon substrate18using a potentiostat. The silicon substrate18can be used as an anode, and a counter electrode can be provided in the HF electrolyte. The current density from the potentiostat can be, for example, up to about 30 milliAmps per square centimeter, at a voltage of up to about 10 Volts. A backside illumination of the lower surface34′ can be provided with a lamp during the anodic dissolution process. The illumination from the lamp can be, for example, about 100 milliWatts per square centimeter of white light. Filtering of the white light with a high pass filter is useful to limit a penetration depth of the light into the silicon substrate18so that minority charge carriers (i.e. holes) generated by the backside illumination are produced away from the etch pits40and are then transported to the location of the etch pits40by the electrical current. This helps to limit the formation of the pores16to the location of the etch pits40, to form the pores16with a uniform width, and to prevent anodic dissolution of the silicon between the pores16.

The holes produced by the backside illumination of the silicon substrate18are necessary for the anodic dissolution of silicon to form the pores16. The holes are transported to the etch pits40by the electrical current, with the etch pits40acting as nucleation centers for the growth of the pores16downward in the substrate18. In addition to adjusting the n-type doping of the silicon substrate18to control the cross-section size of the pores16, the cross-section size of the pores16can also be adjusted by controlling the current density provided by the potentiostat. In general, to form smaller size pores16, a smaller current density can be used. The pores16in the MEM pump10ofFIGS. 1A and 1Bcan have a pore size (i.e. a diameter or cross-section dimension of the pores16) that is generally in the range of 10 nm to 10 μm, with the exact pore size being determined by the pressure of the gas100which will be pumped. As an example, the pore size can be about 60 nm when the gas100is at atmospheric pressure; and the pore size can increase from this value when the pressure of the gas100being pumped is reduced. The anodic dissolution process step can proceed until the pores16have been formed through the silicon substrate18to a predetermined depth, or entirely through the substrate18which can be, for example, about 500 μm thick. A growth rate of the pores16can be on the order of about 1 μm per minute.

Further details of the anodic dissolution process, which is well known in the art, can be found in U.S. Pat. No. 5,360,759; and in an article by S. Ottow et al. entitled “Processing of Three-Dimensional Microstructures Using Macroporous n-Type Silicon,” published in theJournal of the Electrochemical Society, vol. 143, pp. 385-390, January 1996; and in another article by V. Lehmann entitled “Porous Silicon—A New Material for MEMS” published in theProceedings of the Ninth Annual International Workshop on Micro Electro Mechanical Systems, MEMS '96, pp. 1-6, February 1996. Each of these references is incorporated herein by reference.

InFIG. 2F, once the porous silicon region14containing the pores16has been formed, another opening42can be etched through the silicon nitride layer36′ to expose the lower surface34′ of the silicon substrate18. This can be done using reactive ion etching with a photolithographically-defined etch mask (not shown). The etch mask can comprise photoresist, or can be a hard etch mask formed of a silicate glass such as TEOS which can be deposited from the decomposition of tetraethylortho silicate by LPCVD at a temperature of about 750° C. and densified by a high temperature processing step.

InFIG. 2G, the channel32can be formed through the silicon substrate18using a deep reactive ion etch (DRIE) process such as that disclosed in U.S. Pat. No. 5,501,893 to Laermer, which is incorporated herein by reference. The DRIE process utilizes an iterative Inductively Coupled Plasma (ICP) deposition and etch cycle wherein a polymer etch inhibitor is conformally deposited as a film over an exposed portion of the lower surface34′ of the silicon substrate18during a deposition cycle, and is then subsequently removed during an etching cycle. The polymer film, which is formed in a C4F8/Ar-based plasma, deposits conformally over the sidewalls of the channel32being etched. During a subsequent etch cycle using an SF6/Ar-based plasma, the polymer film can be preferentially sputtered from the channel32being formed so that the silicon can be etched by reactive fluorine atoms from the SF6/Ar-based plasma. After the polymer at the bottom of the channel32being formed has been sputtered away and a portion of the silicon at the bottom of the channel32has been etched, but before the polymer on the sidewalls of the channel32has been completely removed, the polymer deposition step using the C4F8/Ar-based plasma can be repeated. This cycle can continue until the channel32has been etched completely through the silicon substrate18as shown inFIG. 2G. Each polymer deposition and etch cycle generally lasts but a few seconds (e.g. up to 10 seconds). The net result is that the channel32can be etched completely through the silicon substrate18while maintaining substantially straight sidewalls. \

Although the channel32is shown inFIG. 1Aas having a circular cross-section shape, the channel32can alternately be polygonal or any arbitrary shape. A channel32with a circular cross-section shape is useful for coupling to cylindrical tubing26as shown inFIG. 1B; whereas other cross-section shapes (e.g. square or rectangular) for the channel32may be better suited for MEM pumps10formed integrally with other types of MEM devices, or when a multi-stage MEM pump is formed. The cross-sectional area of the channel32will generally be on the order of the total cross-sectional size of all the pores16or larger. As an example, the channel32can have a cross-sectional dimension in the range of 10-200 μm.

InFIG. 2H, the silicon nitride layers36and36′ can be removed to expose each side of the porous silicon region14and to begin to form the inlet chamber20and the outlet chamber22. This can be done using a reactive ion etching step with a photolithographically-defined etch mask (not shown). If needed, additional layers of silicon nitride or silicon dioxide can be deposited over the substrate18to further build up the thickness of the inlet chamber20or the outlet chamber22.

InFIG. 2I, a lid12can be provided overtop the silicon substrate18to complete the outlet chamber22; and a base44can be provided underneath the substrate18to complete the inlet chamber20. The lid12and base44can comprise, for example, glass, ceramic or silicon which can be permanently attached to the silicon substrate18with an adhesive, or by anodic bonding.

The lid12can include an electrical resistance heater28which can be deposited on a bottom surface of the lid12(seeFIGS. 3A and 3B) so that the electrical resistance heater28will be located inside the outlet chamber22directly above the porous silicon region14to heat the outlet end thereof in order to provide the thermal gradient along the length of the pores16which is needed to pump the gas100through the pores16. Vias46(e.g. comprising a deposited, plated, or sintered metal such as gold, silver, copper, tungsten, platinum, aluminum, etc.) can be provided through the lid12to connect the electrical resistance heater28to a pair of contact pads48on a top surface of the lid12. This allows the electrical resistance heater28to be connected to an external power supply (not shown). The contact pads48on the top surface of the lid12can also be used to flip-chip bond the MEM pump10to a package having a plurality of electrical pins for making electrical connections to the MEM pump10.

The electrical resistance heater28can have a serpentine or spiral shape as shown in the schematic plan views of the bottom surface of the lid12inFIGS. 3A and 3B. The electrical resistance heater28can comprise a metal such as gold, platinum, tungsten, nickel-chromium (also termed nichrome), or alternately a semiconductor such as doped polysilicon. The material used to form the electrical resistance heater28can be deposited over the bottom surface of the lid12by evaporation, sputtering or chemical vapor deposition (CVD) and then patterned by etching or liftoff. The electrical power required to operate the heater28will, in general, depend upon the size of the heater28and the porous silicon region14and can be, for example, on the order of 100 milliWatts or less.

Returning toFIG. 2I, the base44can include openings for the inlet port24and the outlet port30which can be formed, for example, by etching or laser drilling. Optionally, the base44can include a recessed region50formed about each port24and30to facilitate positioning tubing26at these ports24and30. Once the tubing26is positioned in place in the recessed region50, the tubing26can then be permanently attached with an adhesive52(e.g. a UV-cured epoxy, or a silicon adhesive such as polydimethyl siloxane also referred to as PDMS) as shown inFIG. 2J. The adhesive52can be optionally built up around the tubing26to a predetermined layer thickness to strengthen the bond of the tubing26to the base44. This completes the fabrication of the MEM pump10ofFIGS. 1A and 1B.

In other embodiments of the present invention (seeFIG. 2K), the silicon nitride layers36and36′ can be completely removed after the step ofFIG. 2G, and the lid12and base44can be provided with molded or etched recesses which form the inlet chamber20and the outlet chamber22so that anodic bonding can be used to attach the lid12and base44to the silicon substrate18. In these embodiments of the present invention, the electrical resistance heater28can be located in the recess in the lid12as shown inFIG. 2Kand connected by electrical vias formed through the lid12to contact pads48on the top surface of the lid12.

Removing the silicon nitride layers36and36′ after the step ofFIG. 2Gcan also allow the pores16to be oxidized to form a lining of silicon dioxide therein and also to form a silicon dioxide region between the pores16on both of the major surfaces34and34′ of the silicon substrate18. This is useful to reduce the thermal conductivity of the porous silicon region14so that a larger thermal gradient can be generated across the porous silicon region14for a given amount of heating.

The silicon dioxide lining can be formed from the silicon in the pores16by oxidizing the silicon and thereby converting it into silicon dioxide. This can be done by a conventional thermal oxidation process in which the silicon substrate18is heated to a high temperature in the range of 800-1200° C. in an oxygen or steam ambient, at ambient pressure or higher. The extent of conversion of the silicon surrounding the pores16into silicon dioxide will depend upon the exact time, temperature and pressure used for the thermal oxidation process. In some cases, the porous silicon region14can be completely converted into silicon dioxide. Thus, the term “porous silicon region” as used herein also refers to a region wherein the porous silicon has been partially or completely converted into silicon dioxide with the pores16retaining their substantially straight shape.

If the porous silicon region14is oxidized as described above, this can narrow the cross-sectional size of the pores16; and this narrowing of the pores16must be taken into account to provide pores16of a predetermined size in the completed MEM pump10. The pores16can also be narrowed by depositing conformal coating of silicon nitride in the pores16and over the major surfaces34and34′ of the substrate18using LPCVD.

After the thermal oxidation process or deposition of a conformal coating of silicon nitride to narrow the pores16, the MEM pump10can be completed by attaching the lid12and base44using an adhesive (e.g. epoxy), or by anodic bonding. The lid12and base44can be recessed as shown inFIG. 2K. Alternately, one or more layers of silicon nitride or silicon dioxide can be deposited over the major surfaces34and34′ of the silicon substrate18to build up the inlet chamber20and the outlet chamber22prior to attaching the lid12and base44in a manner similar to that described with reference toFIG. 2J.

FIG. 4shows a second example of a MEM pump10formed according to the present invention. In this example, two porous silicon regions14and14′ are formed on the same silicon substrate18and connected together in series to form a two-stage MEM pump10. The porous silicon regions14and14′ can be simultaneously formed as previously described with reference toFIGS. 2A-2H. The use of two porous silicon regions14and14′ allows a larger pressure difference to be developed between the input port24and the output port30. Although the device10ofFIGS. 4A and 4Bshows only two pumping stages (i.e. two porous silicon regions14), in other embodiments of the present invention additional pumping stages can be added in series to form a multi-stage MEM pump10which can include up to one hundred or more pumping stages.

In the example ofFIGS. 4A and 4B, the gas100is pumped through the MEM pump10along a direction indicated by the arrows. The gas100, upon entering the MEM pump10, initially flows into a first inlet chamber20where the gas100is drawn through the pores16of a first porous silicon region14and into a first outlet chamber22. The gas100then passes through a channel32formed through the silicon substrate18and into a second inlet chamber20′. The gas100is then drawn through the pores16of a second porous silicon region14′ and into a second outlet chamber22′. The gas100then passes through another channel32and into the outlet port30where the gas100exits the MEM pump10.

In this example of the present invention, an electrical resistance heater28for each pumping stage is located on a membrane54which is suspended over the porous silicon region14or14′ to provide thermal isolation from the lid12, thereby providing increased heating for a given electrical power input. The membrane54can comprise, for example, a layer of silicon nitride or silicon dioxide which can be a fraction of a micron thick (e.g. 0.2-0.5 μm). A blanket deposition of the membrane54over the bottom surface of the lid12can be performed by LPCVD. The electrical resistance heaters28can be blanket deposited over the membrane54and patterned by etching or liftoff to form a serpentine or spiral shape as shown inFIGS. 3A and 3B. A cavity56can then be etched into the lid12to form the suspended membranes54. This can be done by first etching a plurality of micron-sized openings through the membranes54to provide access to the bottom surface of the lid12so that a portion of the lid12can be etched away to form the cavities56. When the lid12comprises glass or fused silica, the cavities56can be etched with a selective etchant comprising HF. When the lid12comprises silicon, the cavities56can be etched with a selective etchant comprising KOH, or alternately with gaseous xexon difluoride. Each cavity56can be, for example, up to about 10 μm deep or more.

In some cases, the cavities56can be formed completely through the lid12from the top surface thereof. When the lid12comprises silicon, for example, a silicon nitride membrane54can be blanket deposited over the bottom surface of the silicon lid12followed by the deposition and patterning of the electrical resistance heaters28. A DRIE etch step can then be used as described previously to etch each cavity56completely through the silicon lid12from the top surface thereof. The open cavities56can then be closed, if needed, by attaching a cover (not shown) over the top surface of the lid12. The cover can comprise a glass or ceramic plate which can be attached to the lid12with an adhesive (e.g. epoxy), or by anodic bonding. When the cavities56are closed with a cover, a plurality of micron-sized openings can be optionally formed through the membrane54at the location of each cavity56to equalize the pressure between each cavity56and the adjacent output chamber22or22′.

Electrical connections to the heater28can be made using vias46through the lid12with contact pads48being formed on the top surface of the lid12as described previously. When the lid12comprises silicon, the vias46and contact pads48can be electrically insulated from the silicon lid12by forming a thermal oxide layer over the surfaces34and34′ of the silicon lid12and in the openings wherein the vias46are formed by depositing, plating, or sintering metal.

In other embodiments of the present invention, each cavity56can be etched or molded into the lid12and then filled in with a sacrificial material (e.g. polycrystalline silicon when the lid12comprises a glass or ceramic; or silicon dioxide, a silicate glass such as TEOS, or a spin-on glass when the lid12comprises silicon). The bottom surface of the lid12can then be planarized, if needed, with a polishing step (e.g. a CMP step). The membrane54and the electrical resistance heater28can then be deposited over the bottom surface of the lid12, with the heater28being patterned by liftoff or etching. The sacrificial material can then be removed with a selective etchant through a plurality of micron-sized openings which can be reactive ion etched through each membrane54. The selective etchant can comprise xenon difluoride or KOH when a polycrystalline silicon sacrificial material is used, or can comprise hydrofluoric acid (HF) when the sacrificial material comprises silicon dioxide, silicate glass or a spin-on glass. External electrical connections to the heater28can be made through contact pads48on the top surface of the lid12and vias46through the lid12.

In yet other embodiments of the present invention, each membrane54and electrical resistance heater28can be formed on the layer36of silicon nitride. This can be done, for example, after completion of each porous silicon region as previously described with reference toFIG. 2Fand after etching each channel32almost entirely through the silicon substrate18except for a thin top portion which can be up to a few microns thick. The silicon nitride layer36overtop each porous silicon region14and14′ and overtop each almost-completed channel32can be removed by a reactive ion etching step. Then a sacrificial material such as silicon dioxide, a silicate glass such as TEOS, or a spin-on glass can be deposited over each porous silicon region14and14′ and over the thin top portion of each channel32. If needed, a CMP step can be used to planarize the sacrificial material to the level of the silicon nitride layer36. The electrical resistance heaters28can be deposited overtop the sacrificial material and patterned by etching or liftoff. A silicon nitride layer forming the membranes54can then be blanket deposited over the silicon nitride layer36and over the sacrificial material and heaters28. Alternately, the electrical resistance heaters28can be deposited and patterned overtop the silicon nitride layer forming the membranes54.

Electrical connections to the heaters28can be made through wiring which can be deposited at the same time as the heaters28. The wiring can be connected to vias46in the lid12, or to contact pads formed on the silicon nitride layer36, or to electronic circuitry formed on the silicon substrate18.

A plurality of micron-sized openings can then be etched down through the membranes54to provide access to the underlying sacrificial material which can then be removed using a selective etchant (e.g. comprising HF). A DRIE etch step can then be performed from the bottom of the silicon substrate18to complete each channel32so that each channel32opens into the outlet chambers22or22′. A lid12having a cavity56formed therein at the location of each heater28can then be attached (e.g. with epoxy) over the silicon nitride layer which forms the membranes54.

In the example ofFIGS. 4A and 4B, the pores16in each porous silicon region14and14′ can be of substantially the same size which is preferably substantially equal to or smaller than the mean free path of the gas100in each set of pores16. In other embodiments of the present invention where a number of porous silicon regions14are provided connected together in series to form a multi-stage MEM pump10, the pore size can be different for different of the porous silicon regions14.

To form different pore sizes in different porous silicon regions14, dopant diffusion can be used to selectively dope regions of the silicon substrate18to different dopant levels using thermal diffusion of an impurity dopant deposited on one or both major surfaces34and34′ of the substrate18. The dopant diffusion can extend partially or completely through the silicon substrate18. When the dopant diffusion extends only partially through the silicon substrate18so that a diffusion-doped thickness of the substrate18has a different doping level from the remainder of the thickness of the substrate18, the pores16in the diffusion-doped thickness can have a cross-section dimension which is different from the cross-section dimension for the remainder of the thickness of the substrate18. When the dopant diffusion extends through the entire thickness of the silicon substrate18, the pores16will have a substantially uniform cross-section dimension.

The locations where the pores16are formed by anodic etching can be defined using etch pits40as previously described. The different size pores16in different diffusion-doped regions of the substrate18can be simultaneously formed in a manner similar to that previously described with reference toFIGS. 2A-2E.

To account for different rates of anodic etching of different size pores16, the upper surface34of the substrate18can be masked off, for example, in certain regions to limit the anodic etching while the anodic etching proceeds in other regions. Alternately, the lower surface34′ of the substrate18can be masked off to control the amount of backside illumination reaching certain regions of the substrate18to limit the anodic etching of these regions while the anodic etching proceeds in the other regions.

As yet another example, the anodic etching can be allowed to proceed simultaneously for each differently-doped porous silicon region14being formed. If this results in different etch depths for the different sized pores16, then the substrate18can be polished or etched on the lower surface34′ to a depth which is sufficient to open up all the pores16in each porous silicon region14. The lower surface34′ of the substrate18can be polished by a CMP step; whereas etching of the lower surface34′ can be performed by DRIE, or by a KOH etch step. Multiple DRIE steps can be used to etch completely through the substrate18to form the channels32and also to etch to varying depths as needed to open up the pores16in each differently-doped porous silicon region14.

FIG. 5shows a schematic cross-section view of a third example of a MEM pump10according to the present invention. The device10ofFIG. 5is similar to the MEM pump10ofFIGS. 1A and 1Bexcept that no electrical resistance heater28is provided in the device10ofFIG. 5. InFIG. 5, the thermal gradient necessary to pump the gas100through the MEM pump10is provided by an integrated circuit (IC)110which is located on top of the lid12which can comprise, for example, ceramic, silicon or metal. In some embodiments of the present invention, a silicon or silicon-on-insulator substrate wherein the IC110is formed can be used as the lid12for the MEM pump10.

The IC110generates heat which can be utilized to drive the MEM pump10by heating the outlet side of the porous silicon region14. This heat from the IC110provides the thermal gradient along the length of each pore16which is necessary to draw the gas100through pores16of the MEM pump10so that the gas100flows from the inlet port24to the outlet port30. The gas100, which can be air, helium, or any other gas, also provides the beneficial effect of cooling the IC110as the waste heat from the IC110is transferred to the gas100upon passing through the pores16and outlet chamber22, with the heated gas100then being expelled through the outlet port30. The inlet side of the porous silicon region14can be in thermal contact with a heat sink which can form the base44of the MEM pump10. A closed-cycle cooling system can also be formed using the MEM pump10inFIG. 5, with the heated gas100exiting the outlet port30and being directed to a heat sink which cools the gas100so that the cooled gas100can be directed back into the inlet port24and recirculated.

To prevent a direct conduction of the heat from the IC110through the silicon substrate18and into the porous silicon region14which can be detrimental to the establishment of a large thermal gradient along the length of the pores16, a trench58can be formed around the porous silicon region14to thermally isolate the porous silicon region14from the remainder of the substrate18. The trench58, which can be, for example, 10-100 μm wide, can be formed by etching a majority of the way through the silicon substrate18from the lower surface34′ thereof as shown inFIG. 6A, or alternately by etching through the majority of the substrate18from the upper surface34as shown inFIG. 6B. In some cases, such as that shown inFIG. 6Bwhere the silicon nitride layer36extends under the trench58, the trench58can be etched completely through the silicon substrate18.

Etching the trench58from the lower surface34′ can be performed by a two-step DRIE process with a shallow DRIE step being used to etch a portion of the channel32, and with a deep DRIE step then completing the channel32and forming the trench58. Alternately, an etching delay layer as disclosed in U.S. Pat. No. 6,930,051, which is incorporated herein by reference, can be used to retard etching of the trench58so that only a single DRIE step is required to etch both the trench58and channel32.

Etching the trench58from the upper surface34can be performed with a DRIE step prior to forming the porous silicon region14. The trench58can then be filled in or lined with photoresist or silicon nitride prior to the anodic etching step used to form the porous silicon region14. The photoresist or silicon nitride can then be removed after the anodic etching step forms the pores16, or can be left in place in the trench58.

In other embodiments of the MEM pump10of the present invention, one or more additional channels32can be formed through the silicon substrate18to connect the inlet chamber20to the inlet port24. This can be useful, for example, when the MEM pump10is to be integrated into a gas-phase MEM chemical analysis system60which can comprise other types of MEM devices known to the art. Such a MEM chemical analysis system60can include a chemical preconcentrator62as shown inFIG. 7which can be used to selectively adsorb or absorb and concentrate a particular chemical species of interest from the gas100over time, and can then be triggered to suddenly release the chemical species into the gas100in a concentrated form (e.g. as a puff). Further details of chemical preconcentrators are disclosed in U.S. Pat. Nos. 6,171,378 and 6,902,701 which are incorporated herein by reference.

FIG. 7shows a schematic cross-section view of the gas-phase MEM chemical analysis system60to illustrate how the MEM pump10can be integrated together with other types of MEM devices such as the chemical preconcentrator62described above. In the example ofFIG. 7, the gas100is drawn into the chemical preconcentrator62through the entry port24and a channel32formed through the silicon substrate18by action of the MEM pump10. A chemical species of interest present in the gas100is absorbed or adsorbed onto a sorptive coating (e.g. a polymer or sol-gel material) which is disposed over a heating element64in the chemical preconcentrator62. The heating element64can comprise a serpentine or spiral electrical resistance heater similar to that previously described with reference toFIGS. 3A,3B and4B except that the heating element64is operated in a pulsed mode (for generally only a fraction of a second), and has a sorptive coating.

As the MEM pump10draws the gas100through the chemical preconcentrator62over time, the chemical species of interest is selectively concentrated into the sorptive coating. Upon a pulsed heating of the heating element64with an electrical current pulse, the chemical species of interest is released in a concentrated puff of gas which is then drawn through the MEM pump10and delivered to the output port30. The chemical preconcentrator62and MEM pump10can be co-fabricated in a manner similar to that described previously.

Other MEM chemical analysis and detection devices known to the art can be integrated into the gas-phase MEM chemical analysis system60as illustrated in the schematic cross-section view ofFIG. 8. In this example of the present invention, a MEM gas chromatograph66is located between the chemical preconcentrator62and the MEM pump10. Different chemical species of interest, which can be selectively concentrated with the chemical preconcentrator62and subsequently released as a concentrated puff of gas, can be separated in time in the MEM gas chromatograph66for detection or chemical analysis. The MEM gas chromatograph66can comprise a spiral or serpentine high-aspect-ratio channel68which can be etched into a separate glass or silicon substrate70by a DRIE step and then coated with a thin polymer stationary phase or packed with a particulate stationary phase. The channel68can have an overall length of, for example, up to about one meter.

The gas flow through the MEM chemical analysis system60in the example ofFIG. 8is indicated by the arrows. The gas100containing one or more chemical species of interest is drawn into the inlet port24by action of the MEM pump10. The gas100then enters the chemical preconcentrator62where the chemical species of interest are selectively absorbed or adsorbed from the gas100over time and concentrated into the sorptive material disposed upon the heating element64. Upon applying a heating current pulse to the heater64, the chemical species of interest are suddenly released as a concentrated puff of gas which then passes through the channel32in the silicon substrate18and into the MEM gas chromatograph66where different chemical species of interest are separated in time. Upon exiting the MEM gas chromatograph66, the different chemical species can be detected with a detector (not shown) which can be located before or after the MEM pump10. The gas100after passing through the MEM pump10can then be expelled through the outlet port30, or directed to an external detector connected to the output port30via the tubing26.

To assemble the MEM chemical analysis system60inFIG. 8, the substrate70containing the MEM gas chromatograph66can be attached to the silicon substrate18and the base44using an adhesive (e.g. epoxy) or anodic bonding. If the MEM gas chromatograph is fabricated with a spiral channel68having a side entry port and a central exit port, a channel (not shown) can be formed in the base44or in the layer36′ of silicon nitride to connect the central exit port to the inlet chamber20of the MEM pump10.

The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.