Fuel cell and power chip technology

A fuel cell is disclosed which is formed on a semiconductor wafer by etching channel in the wafer and forming a proton exchange membrane PEM barrier in the etched channel. The barrier divides the channel into two. A hydrogen fuel is admitted into one of the divided channels and an oxidant into the other. The hydrogen reacts with a catalyst formed on an anode electrode at the hydrogen side of the channel to release hydrogen ions (protons) which are absorbed into the PEM. The protons migrate through the PEM and recombine with return hydrogen electrons on a cathode electrode on the oxygen side of the PEM and the oxygen to form water.

BACKGROUND OF THE INVENTION
 The electrochemical fuel cell is not new. Invented in 1839 by Alexander
 Grove, it has recently been the subject of extensive development. NASA
 used its principals in their 1960's space program, but the latest push
 into this technology is being driven largely by the automotive industry.
 Daimler-Chrysler and Ford Motor Co. together have invested about $750
 million in a partnership to develop fuel cell systems. As environmental
 concerns mount and legislation toughens, development of "green" energy
 sources becomes more justified as a course of action, if not required.
 The information age has ushered in the necessity for new ways to examine,
 process, manage, access and control the information. As the basic
 technologies and equipment evolve to handle these new requirements, there
 is a growing need for a smaller, lighter and faster (to refuel/recharge)
 electrical energy source. Portable computing and communications, in
 particular, would benefit greatly from a miniature fuel cell based power
 source.
 SUMMARY OF THE INVENTION
 In accordance with the invention, a method and apparatus is provided which
 uses a combination of SAMs (self-assembled monolayers), MEMS (micro
 electrical mechanical systems), "chemistry-on-a-chip" and semiconductor
 fabrication techniques to create a scalable array of power cells directly
 on a substrate, preferably a semiconductor wafer. These wafers may be
 "stacked" (i.e. electrically connected in series or parallel, as well as
 individually programmed to achieve various power (V*I) characteristics and
 application driven configurations.
 One preferred embodiment of the invention is formed by fabricating a
 plurality of individual fuel cells on a planar semiconductor wafer into
 which flow channels are formed by etching or other well-known
 semiconductor processes. Oxygen is admitted into one side of a channel and
 hydrogen into the other side; with the two gases being separated by a
 membrane. Electrodes are formed on opposite sides of the membrane and a
 catalyst is provided in electrical communication with the electrode and
 membrane on both sides. Lastly, a gas impermeable cover or lid is attached
 to the cell.
 Preferably, the membrane is a PEM (Proton Exchange Membrane) formed by
 depositing or otherwise layering a column of polymers into etched channels
 in the substrate to create a gas tight barrier between the oxygen and
 hydrogen, which is capable of conveying hydrogen ions formed by the
 catalyst across the barrier to produce electricity across the contacts and
 water when the H-ions combine with the oxygen in the other channel.
 In addition, a number of fuel cells can be electrically interconnected and
 coupled to gas sources on a portion of the same wafer to form a "power
 chip". Traditional electrical circuitry can be integrated on the wafer
 along with the chips to provide process monitoring and control functions
 for the individual cells. Wafers containing multiple chips (power discs)
 or multiple cells can then be vertically stacked upon one another.
 A further understanding of the nature and advantages of the invention
 herein may be realized with respect to the detailed description which
 follows and the drawings described below.

DETAILED DESCRIPTION OF THE INVENTION
 A description of preferred embodiments of the invention follows.
 Referring now to FIG. 1, there is shown in plan view a conventional
 semiconductor wafer 10 upon which a plurality of semiconductor fuel cells
 12 have been fabricated. A plurality of cells may be electrically
 interconnected on a wafer and provided with gases to form a power chip 15.
 For simplicity, fuel cells 12 and chips 15 are not shown to scale in as
 much as it is contemplated that at least 80 million cells may be formed on
 a 4" wafer. One such cell is shown in fragmented cross-section in FIG. 2.
 In its simplest form, each cell 12 consists of a substrate 14, contacts
 16A and B, and a conductive polymer base 18 formed on both sides of a
 first layer 20(a) of nonconductive layered polymer support structure 20
 and in intimate contact with the metal electrical contacts.
 A conductive polymer 22 with embedded catalyst particles 28 on both sides
 of the central structure 20 forms a PEM barrier separating the hydrogen
 gas on the left side from the oxygen gas on the right side. Etched
 channels 50B and 50A respectively for admittance of the O.sub.2 and
 H.sub.2 gas and a heatsink lid 40 over the cell 12 is also shown in FIG.
 2.
 FIGS. 3a-3h are a series of schematic sectional views showing the relevant
 fabrication details of the PEM barrier 30 in several steps. FIG. 3a shows
 the bottom of a power cell channel which has been etched into the
 semiconductor substrate 14. It also shows the metal contacts 16 which are
 responsible for conveying the electrons out of the power cell 12 to the
 rest of the circuitry. These metal contact are deposited by wellknown
 photolithographic processes in the metalization phase of the semiconductor
 fabrication process.
 FIG. 3b shows the conductive polymer base 18 as it has been applied to the
 structure. Base 18 is in physical/electrical contact with the metal
 contacts 16 and has been adapted to attract the conductive polymer 22 of
 the step shown in FIGS. 3a-3h.
 FIG. 3c shows the nonconductive polymer base 20(a) as it has been applied
 to the structure. It is positioned between the two conductive polymer base
 sites 18 and is adapted to attract the nonconductive polymer 20.
 FIG. 3d shows a polymer resist 21 as applied to the structure. Resist 21 is
 responsible for repelling the polymers and preventing their growth in
 unwanted areas.
 FIG. 3e shows the first layer 20B of nonconductive polymer as it has been
 grown on its base 20A. This is the center material of the PEM barrier. It
 helps support the thinner outer sides 22 when they are constructed.
 FIG. 3f shows the subsequent layers of nonconductive polymer 20 which are
 laid down, in a layer by layer fashion to form a vertical barrier. This
 vertical orientation allows for area amplification.
 FIG. 3g shows the first layer 22a of conductive polymer grown on its base
 18. This is the outside wall material with catalyst of the PEM barrier.
 FIG. 3h shows the subsequent layers of conductive polymer 22 laid down, in
 a layer by layer fashion on to the structure. FIG. 2 shows the completed
 structure after removal of the polymer resist layer 21 and the addition of
 lid 40 and the pre-existing sidewalls 52 left out of FIG. 3a-3h for
 simplicity. This resist removal may not be necessary if layer 21 was
 originally the passivation layer of the final step in the semiconductor
 fabrication process.
 Referring now to FIG. 2 again further details of the elements forming the
 fuel cell 12 will be explained. The proton exchange membrane shown
 generally at 30 forms a barrier between the fuel H.sub.2 and the oxidant
 O.sub.2.
 The PEM barrier 30 is made up of three parts of two materials. There is the
 first outside wall 22B, then the center 20, and finally the second outside
 wall 22C. It is constructed with a center piece 20 of the first material
 in contact with the two outside walls which are both made of the second
 material.
 The material 20 forming the center piece, is preferably an ionic polymer
 capable of passing the hydrogen ions (protons) through from the hydrogen
 side to the oxygen side. It is electrically nonconductive so that it does
 not, effectively, short out the power cell across the two contacts 16A and
 16B. It may be made of Nafion.RTM. or of a material of similar
 characteristics. An external load 5 as shown in dotted lines may be
 coupled across the contacts to extract power.
 The second material 22, forming the two outside walls, is also a similar
 ionic polymer capable of passing the hydrogen ions. In addition, it is
 doped with nano catalyst particles 28 (shown by the dots), such as,
 platinum/alloy catalyst and is also electrically conductive.
 By embedding the catalyst particles 28 into the polymer 22, maximum
 intimate contact is achieved with the PEM 30. This intimate contact
 provides a readily available path which allows the ions to migrate freely
 towards the cathode electrode 16B. Catalysis is a surface effect. By
 suspending the catalytic particles 28 in the polymer 22, effective use of
 the entire surface area is obtained. This will dramatically increase the
 system efficiency.
 By making the second material 22 electrically conductive, an electrode is
 produced. The proximity of the electrode to the catalytic reaction affects
 how well it collects electrons. This method allows the catalytic reaction
 to occur effectively within the electrode itself. This intimate contact
 provides a readily available path which allows the electrons to migrate
 freely towards the anode 16A. This will allow for the successful
 collection of most of the free electrons. Again, this will dramatically
 increase the system efficiency.
 In addition to the electrical and chemical/functional characteristics of
 the PEM 30 described above, there are some important physical ones, that
 are described below:
 This self assembly process allows for the construction of a more optimum
 PEM barrier. By design it will be more efficient.
 First, there is the matter of forming the separate hydrogen and oxygen path
 ways. This requires that the PEM structure to be grown/formed so that it
 dissects the etched channel 50 fully into two separate channels 50A, 50B.
 This means that it must be patterned to grow in the center of the channel
 and firmly up against the walls of the ends of the power cell. It must
 also be grown to the height of the channel to allow it to come into
 contact with an adhesive 42 on the bottom of lid 40.
 Second, there is the matter of forming a gas tight seal. This requires that
 the PEM structure 30 be bonded thoroughly to the base structures 18 and
 20A, the substrate 14 and the end walls (not shown) of the power cell and
 to an adhesive 42 which coats the lid 40. By proper choice of the
 polymers, a chemical bond is formed between the materials they contact in
 the channel. In addition to this chemical bond, there is the physical
 sealing effect by applying the lid 40 down on top of the PEM barrier. If
 the height of the PEM 30 is controlled correctly, the pressure of the
 applied lid forms a mechanical "O ring" type of self seal. Growing the PEM
 30 on the substrate 14 eliminates any fine registration issues when
 combining it with the lid 40. There are no fine details on the lid that
 require targeting.
 Turning now to FIG. 4, there is shown in simplified perspective an
 alternate embodiment of a PEM barrier involving a casting/injecting
 process and structure.
 Using MEMS machining methods three channels 60A, 60B and 60C are etched
 into a semiconductor substrate 140. The outside two channels 60A and 60C
 are separated from the middle channel 60B by thin walls 70A, 70B. These
 walls have a plurality of thin slits S.sub.1 - - - S.sub.n etched into
 them. The resultant tines T.sub.1 -T.sub.n+1 have a catalyst 280 deposited
 on them in the area of the slits. At the bottom of these thin walls, 70A,
 70B, on the side which makes up a wall of an outside channel 60A, 60C, a
 metal electrode 160A, 160B is deposited. A catalyst 280 is deposited on
 the tines after the electrodes 160 are in place. This allows the catalyst
 to be deposited so as to come into electrical contact and to cover to some
 degree, the respective electrodes 160 at their base. In addition, metal
 conductors 90 are deposited to connect to each electrode 160, which then
 run up and out of the outside channels.
 A lid 400 is provided with an adhesive layer 420 which is used to bond the
 lid to the substrate 140. In this way, three separate channels are formed
 in the substrate; a hydrogen channel 60A, a reaction channel 60B, and an
 oxygen channel 60C. In addition, the lid 400 has various strategically
 placed electrolyte injection ports or holes 500. These holes 500 provide
 feed pathways that lead to an electrolyte membrane of polymer material
 (not shown) in the reaction channel 60B only.
 The structure of FIG. 4 is assembled as follows:
 First, the semiconductor fabrication process is formed including substrate
 machining and deposition of all electrical circuits.
 Next, the lid 400 is machined and prepared with adhesive 420. The lid 400
 is bonded to the substrate 140. Then, the electrolyte (not shown) is
 injected into the structure.
 The thin walls 70A, 70B of the reaction channel 60B serve to retain the
 electrolyte during its casting. The slits S.sub.1 - - - S.sub.N allow the
 hydrogen and oxygen in the respective channels 60A, 60B access to the
 catalyst 280 and PEM 300. Coating the tines T.sub.1 - - - T.sub.1+n with a
 catalyst 280 in the area of the slits provides a point of reaction when
 the H.sub.2 gas enters the slits. When the electrolyte is poured/injected
 into the reaction channel 60B, it fills it up completely. The surface
 tension of the liquid electrolyte keeps it from pushing through the slits
 and into the gas channels, which would otherwise fill up as well. Because
 there is some amount of pressure behind the application of the
 electrolyte, there will be a ballooning effect of the electrolyte's
 surface as the pressure pushes it into the slits. This will cause the
 electrolyte to be in contact with the catalyst 280 which coats the sides
 of the slits S.sub.1 - - - S.sub.N. Once this contact is formed and the
 membrane (electrolyte) is hydrated, it will expand even further, ensuring
 good contact with the catalyst. The H.sub.2 /O.sub.2 gases are capable of
 diffusing into the (very thin, i.e. 5 microns) membrane, in the area of
 the catalyst. Because it can be so thin it will produce a more efficient
 i.e. less resistance (1.sup.2 R) losses are low. This then puts the three
 components of the reaction in contact with each other. The electrodes 160A
 and 160B in electrical contact with the catalyst 280 is the fourth
 component and provides a path for the free electrons [through an external
 load (not shown)] while the hydrogen ions pass through the electrolyte
 membrane to complete the reaction on the other side.
 Referring now to the cross-sectional views of FIGS. 5-7, various alternate
 configurations of the PEM structure 30 of the invention will be described
 in detail. In FIG. 5, the central PEM structure 20 is formed as a
 continuous nonconductive vertical element, and the electrode/catalyst
 16/28 is a non-continuous element to which lead wires 90 are attached.
 FIG. 6 is a view of an alternate PEM structure in which the catalyst 28 is
 embedded in the non-conductive core 20 and the electrodes 16 are formed
 laterally adjacent the catalyst. Lastly, in FIG. 7, the PEM structure is
 similar to FIG. 5 but the center core 20.sup.1 is discontinuous.
 FIG. 8 is a schematic block diagram showing some of the possible circuits
 that may be integrated along with a microcontroller onto the semiconductor
 wafer 10 to monitor and control multiple cells performance. Several sensor
 circuits 80, 82, 84 and 86 are provided to perform certain functions.
 Temperature circuit 80 provides the input to allow the micro processor 88
 to define a thermal profile of the fuel cell 12. Voltage circuit 82
 monitors the voltage at various levels of the configuration hierarchy or
 group of cells. This provides information regarding changes in the load.
 With this information, the processor 88 can adjust the system
 configuration to achieve/maintain the required performance. Current
 circuit 84 performs a function similar to the voltage monitoring circuit
 82 noted above.
 Pressure circuit 86 monitors the pressure in the internal gas passages 50A,
 50B. Since the system's performance is affected by this pressure, the
 microprocessor 88 can make adjustments to keep the system running at
 optimum performance based on these reading. An undefined circuit 81 is
 made available to provide a few spare inputs for the micro 88 in
 anticipation of future functions.
 In addition, configuration circuit 94 can be used to control at least the
 V*I switches to be described in connection with FIG. 9. The output voltage
 and current capability is defined by the configuration of these switches.
 Local circuitry 92 is provided as necessary to be dynamically programmed,
 such as the parameters of the monitoring circuits. These outputs can be
 used to effect that change. Local subsystems 94 are used by the micro 98
 to control gas flow rate, defect isolation and product removal. A local
 power circuit 96 is used to tap off some part of the electricity generated
 by the fuel cell 12 to power the onboard electronics. This power supply
 circuit 96 will have its own regulation and conditioning circuits. A
 two-wire communications I/F device 98 may be integrated onto the chip to
 provide the electrical interface between communicating devices and a power
 bus (not shown) that connects them.
 The microcontroller 8 is the heart of the integrated electronics subsystem.
 It is responsible for monitoring and controlling all designated system
 functions. In addition, it handles the communications protocol of any
 external communications. It is capable of "in circuit programming" so that
 its executive control program can be updated as required. It is capable of
 data storage and processing and is also capable of self/system diagnostics
 and security features.
 Referring now to FIG. 9, further details of the invention are shown. In
 this embodiment, the individual power cells 12.sub.1, 12.sub.2 - - -
 12.sub.n are formed on a wafer and wired in parallel across power buses
 99A and 99B using transistor switches 97 which can be controlled from the
 microprocessor 88 of FIG. 8. Switches 97B and 97A are negative and
 positive bus switches respectively, whereas switch 97C is a series switch
 and switches 97D and 97E are respective positive and negative parallel
 switches respectively.
 This allows the individual cells or groups of cells (power chip 15) to be
 wired in various configurations, i.e., parallel or series. Various
 voltages are created by wiring the cells in series. The current capacity
 can also be increased by wiring the cells in parallel. In general, the
 power profile of the power chip can be dynamically controlled to achieve
 or maintain a "programmed" specification. Conversely, the chip can be
 configured at the time of fabrication to some static profile and thus,
 eliminate the need for the power switches. By turning the switches on and
 off and by changing the polarity of wiring one can produce both AC and DC
 power output.
 To implement a power management subsystem, feedback from the power
 generation process is required. Circuitry can be formed directly on the
 chip to constantly measure the efficiencies of the processes. This
 feedback can be used to modify the control of the system in a closed loop
 fashion. This permits a maximum level of system efficiency to be
 dynamically maintained. Some of these circuits are discussed next.
 The quality of the power generation process will vary as the demands on the
 system change over time. A knowledge of the realtime status of several
 operational parameters can help make decisions which will enable the
 system to self-adjust, in order to sustain optimum performance. The
 boundaries of these parameters are defined by the program.
 For example, it is possible to measure both the voltage and the current of
 an individual power cell or group of power cells. The power output can be
 monitored and if a cell or group is not performing, it can be removed if
 necessary. This can be accomplished by the power switches 97 previously
 described.
 An average power level can also be maintained while moving the active
 "loaded" area around on the chip. This should give a better overall
 performance level due to no one area being on 100% of the time. This duty
 cycle approach is especially applicable to surge demands. The concept here
 is to split the power into pieces for better cell utilization
 characteristics.
 It is expected that the thermal characteristics of the power chip will vary
 due to electrical loading and that this heat might have an adverse effect
 on power generation at the power cell level. Adequate temperature sensing
 and an appropriate response to power cell utilization will minimize the
 damaging effects of a thermal build up.
 The lid 40 is the second piece of a two-piece "power chip" assembly. It is
 preferably made of metal to provide a mechanically rigid backing for the
 fragile semiconductor substrate 14. This allows for easy handling and
 provides a stable foundation upon which to build "power stacks", i.e., a
 plurality of power chips 15 that are literally stacked on top of each
 other. The purpose being to build a physical unit with more power.
 FIG. 10 illustrates how the fuel 50A and oxidant/product channels 50A (and
 50B not shown) may be etched into the surface of the substrate 14. These
 troughs are three sided and must be closed and sealed on the top side. The
 lid 40 and adhesive 42 provides this function of forming a hermetic seal
 when bonded to the substrate 14 and completes the channels. A matrix of
 fuel supply and oxidant and product water removal channels is thereby
 formed at the surface of the substrate.
 The lid 40 provides a mechanically stable interface on which the
 input/output ports can be made. These are the gas supply and water removal
 ports. The design must encompass the size transition from the large
 outside world to the micrometer sized features on the substrate. This is
 accomplished by running the micrometer sized channels to a relatively much
 larger hole H. This larger hold will allow for less registration
 requirements between the lid and substrate. The large holds in the lid
 line up with the large holes in the substrate which have micrometer sized
 channels also machined into the substrate leading from the large hold to
 the power cells.
 Each wafer may have its own manifolds. This would require external
 connections for the fuel supply, oxidant and product removal. The external
 plumbing may require an automated docking system.
 FIGS. 11 and 12 illustrates one of many ways in which several cells 12 (in
 this example three cells side-by-side can be formed on a wafer 14 to form
 a power chip 15. Power disks can be stacked vertically upon each other to
 form a vertical column with inlet ports, 50HI, 50OI respectfully coupled
 to sources of hydrogen and oxygen respectively. The vertical column of
 wafers with power chips formed therein comprise a power stack (93).
 FIG. 12 illustrates how stacking of a number of power discs 15 may be used
 to form power stacks (93) with appreciable power. The use of the word
 "stacking" is reasonable for it suggests the close proximity of the
 wafers, allowing for short electrical interconnects and minimal plumbing.
 In reality, the stacking actually refers to combining the electrical power
 of the wafers to form a more powerful unit. They need only to electrically
 stacked to effect his combination. However, it is desirable to produce the
 most amount of power in the smallest space and with the highest
 efficiencies. When considering the shortest electrical interconnect (power
 bussing) alternatives, one should also consider the possibility of using
 two of the main manifolds as electrical power busses. This can be done by
 electrically isolating these manifold/electrical power buss segments and
 using them to convey the power from each wafer to the next. This reduces
 the big power wiring requirements and permits this function to be done in
 an automated fashion with the concomitant increased accuracy and
 reliability.
 A desirable manifold design would allow for power disc stacking. In this
 design the actual manifold 95 would be constructed in segments, each
 segment being an integral part of the lid 40. As the discs are stacked a
 manifold (tube) is formed. This type of design would greatly reduce the
 external plumbing requirements. Special end caps would complete the
 manifold at the ends of the power stack.
 In summary, one of the primary objects of this invention is to be able to
 mass produce a power chip 15 comprised of a wafer 10 containing multiple
 power cells 12 on each chip 15 utilizing quasi standard semiconductor
 processing methods. This process inherently supports very small features.
 These features (power cells), in turn, are expected to create very small
 amounts of power per cell. Each cell will be designed to have the maximum
 power the material can support. To achieve any real substantially power,
 many millions will be fabricated on a single power chip 15 and many power
 chips fabricated on a "power disc" (semiconductor wafer 10). This is why
 reasonable power output can be obtained from a single wafer. A 10
 uM.times.10 uM power cell would enable one million power cells per square
 centimeter. The final power cell topology will be determined by the
 physical properties of the constituent materials and their
 characteristics.
 The basic electro-chemical reaction of the solid polymer hydrogen fuel cell
 is most efficient at an operating temperature somewhere between 80 to
 100.degree. C. This is within the operating range of a common
 semiconductor substrate like silicon. However, if the wafers are stacked
 additional heatsinking may be required. Since a cover is needed anyway,
 making the lid 40 into a heatsink for added margin makes sense.
 The fuel and oxidant/product channels are etched into the surface of the
 semiconductor substrate. These troughs are three-sided and must be closed
 and sealed on the top side. The lid 40 provides this function. It is
 coated with an adhesive to form a hermetic seal when bonded to the
 semiconductor substrate and completes the channels. This forms a matrix of
 fuel supply and oxidant and product water removal channels at the surface
 of the semiconductor substrate. The power cells two primary channels are
 themselves separated by the PEM which is bonded to this same adhesive.
 Thus, removing any fine grain critical alignment requirements.
 EQUIVALENTS
 While this invention has been particularly shown and described with
 references to preferred embodiments thereof, it will be understood by
 those skilled in the art that various changes in form and details may be
 made therein without departing from the scope of the invention encompassed
 by the appended claims. For example, while silicon because of its
 well-defined electrical and mechanical properties is the material of
 choice for the substrate 14, other semiconductor materials may be
 substituted, therefore, such as Gd, Ge, or III-V compounds such as GaAs.
 Alternatively, the substrate for the cell may be formed of an amorphous
 material such as glass or plastic, or phenolic; in which case, the
 controls for the cells can be formed on a separate semiconductor die and
 electrically coupled to the cells to form a hybrid structure. The
 interface between the PEM's structure is preferably an assembled monolayer
 (SAM) interface formed of gold, however, other metals such as silver or
 platinum, may be used in place thereof. Likewise, although the PEM is
 formed of many molecular chains, it preferably has a base with an affinity
 for gold so that it will bond to the gold SAM feature. Again, other pure
 metals such as platinum and silver may be substituted therefore.