Patent Publication Number: US-9419147-B2

Title: Electronically controlled squishable composite switch

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/291,016, filed on Nov. 7, 2011, which claims the benefit of U.S. Provisional Application No. 61/410,611, filed on Nov. 5, 2010, the contents of which are hereby incorporated by reference herein. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under Grant #ECCS-0939514 awarded by the National Science Foundation. The Government has certain rights in this invention. 
    
    
     FIELD OF INVENTION 
     This application is related to electronic devices. 
     BACKGROUND 
     Micro-Electromechanical Systems (MEMS) devices are starting to be used in a variety of applications. In particular, there is great demand for MEMS switches due to their low power consumption, very small size, low cost, reliable, wide tuning range, low loss digital switching, low phase noise, low insertion loss, higher isolation, better linearity and single chip packaging which are almost impossible with standard semiconductor switches. 
     SUMMARY 
     A method and apparatus for making analog and digital electronics which includes a composite including a squishable material doped with conductive particles. A microelectromechanical systems (MEMS) device has a channel made from the composite, where the channel forms the primary conduction path for the device. Upon applied voltage, capacitive actuators squish or squeeze the composite, causing it to become conductive. The squishable device includes a control electrode, and a composite electrically and mechanically connected to two terminal electrodes. By applying a positive or negative voltage to the control electrode relative to a first terminal electrode, an electric field is developed between the control electrode and the first terminal electrode. This electric field results in an attractive force between the control electrode and the first terminal electrode, which compresses the composite and enables electric control of the electron conduction from the first terminal electrode through the channel to the second terminal electrode. The degree of conduction of the composite may be controlled by the control electrode voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding may be had from the following description, given by way of example in conjunction with the accompanying drawings, wherein: 
         FIGS. 1A and 1B  illustrate examples of a composite in two states; 
         FIG. 2  illustrates an embodiment of a three-terminal squishable or squeezing switch (“squitch”); 
         FIG. 2A  illustrates an embodiment of a two gate squitch; 
         FIG. 2B  illustrates an embodiment of another two gate squitch; 
         FIG. 3  illustrates example squitch drain-to-source resistances as a function of gate-to-source voltage for different gate capacitor air gaps; 
         FIG. 4  illustrates an example stress-strain characteristic of an example nickel-doped-polymer composite; 
         FIG. 5  illustrates an example resistance strain of the experimental nickel-doped-polymer composite; 
         FIG. 6  illustrates an example squitch drain-to-source resistance as a function of gate-to-source voltage; 
         FIG. 7  illustrates an embodiment of a digital inverter using a squitch; 
         FIG. 8  illustrates an embodiment of a common source analog amplifier; 
         FIG. 9  illustrates an example fabrication process for the squitch; 
         FIG. 10  illustrates low crosslinking and high crosslinking; and 
         FIG. 11  illustrates an example fabrication process for a second gate electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     It is to be understood that the figures and descriptions of embodiments of the electronically controlled squishable composite switch (“squitch”) have been simplified to illustrate elements that are relevant for a clear understanding, while eliminating, for the purpose of clarity, many other elements. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. 
     Analog and digital electronic devices may include a squishable material doped with conducting particles (herein referred to as “squitch material” or composite). Such squitch materials may exhibit a dramatically decreasing resistivity as the squitch material is compressed. The squishable material may include, but is not limited to, silicones, polymers, organic polymers, aerogels and the like. The squishable material may be doped with conductive particles including, but not limited to, nickel nano-particles, gold nano-particles, carbon black or carbon nanotube fragments and the like. In one embodiment, a combination of the different conductive particles may be used. Such squitch materials conduct via tunneling from particle to particle, (where the particles may never touch), and the tunneling currents grow exponentially as the particles become closer together. In some embodiments, conduction may occur via percolation, (where conduction follows a meandering connected path). Squitch material conductivity may vary by 12 orders of magnitude or more over a 40% strain. 
     As described herein, the squitch material may be used as the active element in electronically-controlled switches and may have application in both analog and digital electronics. In particular, the squitch material may be used to make an electrostatically actuated or electronically-controlled squishable switch, or “squitch” that may function as a gated transistor. Although the squitch may be referred to as a switch herein, (as applicable for digital circuits), it may be referred to as a transistor for purposes of both digital and analog circuits. The term squitch may be used herein to generically refer to an electronic device using such squitch material or composite materials. The squitch may exhibit a very large on-to-off conduction ratio, (up to 10 7  to 1), and may exhibit a voltage-controlled conduction with a gain greater than 1 decade per 60 mV, a fundamental limit for silicon-based semiconductor switches. Moreover, these electronic devices may not use silicon, which can be an expensive substrate. They may be fabricated through printing or photolithography techniques and may be fabricated on, for example, flexible substrates. 
       FIGS. 1A and 1B  illustrate a composite  100  connected to a pair of electrodes  105  and  110 . The composite  100 , in a non-limiting example, may be a mixture of conductive particles  115  in a squishable material such as an elastomer matrix  120 , where the elastomer matrix  120  may be doped near the percolation threshold. In a first state shown in  FIG. 1A , the pair of electrodes  105  and  110  are in a non-compressive mode with respect to the composite  100 . The composite  100 , in this state, is highly resistive and a poor conductor that has little current flow. In a second state shown in  FIG. 1B , the pair of electrodes  105  and  110  are in a compressive mode, (as shown by arrows  125  and  126 ), with respect to the composite  100 . As shown by the arrow  130 , the compressed composite  100  conducts via tunneling from one conductive particle  115  to another conductive particle  115  and provides decreased resistivity. In another embodiment, a compressed composite may conduct via percolation. In another embodiment, a compressed composite may conduct via percolation as well as tunneling. The type of conduction may depend on the type of dopant selected. As described herein below, the squishable material composition, curing period and other factors may be selected to determine the resistance and mechanical characteristic of the composite  100 . For example, by making the material softer, it may take less force, and hence less actuation voltage, to make the squitch move. 
       FIG. 2  shows an embodiment of an electronically-controlled or electrostatically actuated squishable-composite switch or squitch  200 . As illustrated, the squitch  200  is a three-terminal device. For purposes of illustration, the terminals may be labeled as per the comparable terminals in a metal-oxide-semiconductor field-effect transistor (MOSFET). For example, a gate electrode may refer to or be a control electrode, and a source and drain may refer to or be terminal electrodes on each side of the squitch material or composite. The squitch may also be implemented as a four-terminal device as discussed herein below. 
     The squitch  200  may include a squitch material such as a doped polymer composite  205  that is connected electrically and mechanically to a source  210  and a drain  215 . The source  210  and drain  215  may be, for example, metal conductors that form the two electrodes of the primary conduction path through the squitch  200 . The squitch may further include a gate  220 , which may also be, for example, a metal conductor. An insulator  225  may be positioned between the source  210  and gate  220  to prevent a short circuit between the source  210  and gate  220  upon application of a voltage as described herein. Although  FIG. 2  shows that the insulator  225  may be situated on the gate  220 , in another embodiment, the insulator  225  may be situated on the source  210 . Although metal conductors are described herein for the gate  220 , source  210  and drain  215 , other applicable conductors may be used. The gate  220 , source  210  and drain  215  are fabricated on a substrate  230 . In a non-limiting example, the substrate  230  may be silicon, glass, plastic, flexible materials or the like. Although not shown, an insulating film may cover the substrate  230 . 
     The doped polymer composite  205 , as fabricated and in a relaxed state, would be a poor conductor and permit little if any electron current to flow from the source  210  to the drain  215 . In one embodiment, the doped polymer composite may have at least 0.5 wt %, (by weight percentage), particles. In another embodiment, the doped polymer composite may have up to 50 wt % particles. In general, the amount of dopant needed may be dependent on the type/size/shape of the conductive particles. The resistance of this conduction path would be very large, putting the squitch  200  in an “off state”. The doped polymer composite  205  may start to conduct as it is compressed, in the vertical direction, for example. When compressed sufficiently, the doped polymer composite  205  would conduct very well, putting the squitch in an “on” state, (the “squished state”). The direction of compression may be defined by the placement or positioning of the respective electrodes, and the vertical direction described herein is a non-limiting example. 
     The degree of conduction of the doped polymer composite  205  may be controlled by the gate  220 . By applying a voltage to the gate  220  relative to the source  210 , either positive or negative, an electric field may be developed between the gate  220  and the source  210 . This electric field may result in an attractive force between the gate  220  and source  210 , which may compress the doped polymer composite  205  and enable electric control of the electron conduction from the source  210  to the drain  215 . The squitch  205 , therefore, may be a voltage-controlled conductor in the same manner as a field effect transistor (FET) or a bi-polar junction transistor (BJT) is a voltage-controlled conductor. 
     The stair-case shape of the source  210  may serve two purposes. First, the stair-case shape indicates that it may be desirable to reduce the spring constant of the source  210  since this spring must also be compressed in order to compress the doped polymer composite  205 . For example, a straight metal (or other material) structure may have a larger stiffness than a bent one. Second, by moving the middle step in the source  210  closer to the gate  220 , the field strength may be enhanced for a given gate-to-source voltage. This may permit compression of the doped polymer composite  205  with lower gate-source voltages. However, it may limit the extent of compression of the doped polymer composite  205  since the source  210  must stop upon reaching the gate  220 . A tradeoff in the design of the gate electrode therefore exists. For example, as shown in  FIG. 5 , by the time a composite is squished by 30% of its original length in the squishing direction, it may have exhibited the majority of its conduction change and the composite may also get much harder to squish at this point, as shown in  FIG. 4 . The middle step may therefore be moved as close to the gate as possible while still permitting a 20-30% strain (squishing) of the composite.  FIG. 3  shows example squeezing switch drain-to-source resistances as a function of gate-to-source voltage for different gate capacitor air gaps. In this plot, the term “3× vertical scaling” means that all vertical dimensions, (with reference to  FIG. 2 ), including for example the squisbable material thickness, gate-source electrode gap, insulator bumper-stop gap and the like, were reduced by a factor of 3 but the lateral dimensions (area) were not changed. 
       FIG. 2A  illustrates an embodiment where the drain  215  of  FIG. 2  may be split in half to become the drain and the source. In particular, the squitch  240  may include a drain  242 , a source  244 , a first gate  246  and a second gate  248 . All of which, may be made on a substrate  250 . The squitch  240  may include a squitch material such as a doped polymer composite  255  that may be electrically isolated from, but still mechanically connected to the second gate  248 . An insulating layer (not visible) may be placed between the doped polymer composite  255  and second gate  248 . This isolates the second gate  248  from the doped polymer composite  255  to make a true four terminal device. The second gate  248  may have stepped shape, (the staircase configuration described herein above), so that the regions attracted to the first gate  246  are closer to first gate  246 . 
     In this embodiment, electron conduction occurs laterally through the doped polymer composite  255 . The compression of the doped polymer composite  255  may be controlled by the voltage between the two gates, while the conduction occurs between the independent source  244  and drain  242 . Thus, the conduction path is separated from the control electrodes, i.e., the first gate  246  and a second gate  248 . In particular, the conduction is from the source  244  into the doped polymer composite  255  and back out the drain  242 . In this embodiment, the attraction between the two gates actuates the device. Although not shown in  FIG. 2A , the second  248  gate may have a spring structure similar to source  210  in  FIG. 2 . The second gate  248  may be stepped to reduce the actuation voltage. The electrical connection to the second gate  248  may be made via the stepped spring. 
     As stated earlier, the squitch may be a four terminal device. This may be clearly shown by squitch  240  which includes two conduction terminals, (i.e., drain  242  and source  244 ), and two control electrodes, (i.e., first gate  246  and second gate  248 ). A relay may be built using this configuration as opposed to a transistor. The relay may have a smooth variable conduction through a squitch material so that the transistor behavior is exhibited. 
       FIG. 2B  illustrates an embodiment of a squitch  260 . The squitch  260  may include a drain  262 , a source  244 , a first gate  266 , a second gate  268 , and a floating electrode  270 . All of which may be made on a substrate  275 . The squitch  260  may include a squitch material such as a doped polymer composite  280  that may be electrically isolated from but mechanically connected to the floating electrode  270 . An insulating layer (not visible) may be placed between the doped polymer composite  280  and the floating electrode  270 . This isolates the second floating electrode  270  from the doped polymer composite  280 . 
     In this embodiment, the first gate  246  of  FIG. 2A  may be split at the back end of the horseshoe into two electrically separated halves, (i.e., the first gate  266  and the second gate  268 ). The second gate  248  of  FIG. 2A  may now become a floating electrode and no connection may need to be made to this electrode. A voltage may be applied between the first gate  266  and the second gate  268  and the floating electrode  270  may still be attracted to the first gate  266  and the second gate  268  as before. This embodiment may alleviate the need to have a connection to the floating electrode  270 . For example, the floating electrode  270  may be simply positioned on the doped polymer composite  280 . In another embodiment, a spring may be used to enhance tipping stability. In another embodiment, as described herein above, the floating electrode  270  may have a stepped shape so that those regions of the floating electrode  270  that are attracted to first gate  266  and second gate  268  are closer to the first gate  266  and second gate  268 . 
     In implementing and fabricating the squitch embodiments described herein, different squishable materials may be used. In one embodiment, an extremely soft elastomer such as silicone, (i.e., Ecoflex® 00-10), has been used for the polymer matrix. Other like materials may be used. The term extremely soft or mechanically soft may refer to a squishable material having a Young&#39;s modulus in the range of 1-2 MPa. In another embodiment, the squishable material may have a Young&#39;s modulus of at least 100 KPa. The term extremely soft or mechanically soft may also refer to a squishable material having a low crosslinking density, (as illustrated in  FIG. 10  for Ni-polydimethysiloxane (PDMS)), where low cross-link densities decrease the viscosities of polymer melts and high cross-link densities may cause materials to become very rigid or glassy. Using a soft polymer permits achieving the desired strain with the least possible stress, and hence the least possible control voltage. This increases the voltage-control gain of the squitch. In this embodiment, the PDMS has been uniformly mixed with nickel particles approximately 2.5 μm in diameter. For example, the term uniformly mixed may refer to equally distributed particles, equally spaced particles or both. In general, uniformly mixed may mean that the particles are not clumped, but are individually dispersed in a generally uniform, (as measured by their relative spacing), manner. In a non-limiting example, mixing may be done using a planetary mixer. Although nickel is used in this embodiment, other conductive nano-scale and micro-scale particles may be used such as carbon nanotube fragments, graphene, graphite, metal particles, conductive metal oxide particles, and others.  FIG. 4  shows the stress-strain characteristic of an example nickel-doped-polymer composite.  FIG. 5  shows the measured resistance of a nickel-doped-polymer composite sample as a function of strain. 
     On the basis of the data in  FIGS. 4 and 5 , a simulated controlled conduction is shown in  FIG. 6  of an example squitch. The plot is a function of the applied gate-to-source control voltage and in particular shows the squitch drain-to-source resistance as a function of gate-to-source voltage. In this example squitch, the polymer is assumed to be a factor of 10 more compliant than shown in  FIG. 4 . Note that the resistance of the squitch shown in  FIG. 5  varies on average by one decade per 48 mV, which exceeds the one decade per 60 mV typically exhibited by a silicon BJT transistor. 
       FIG. 7  is an embodiment of digital logic implemented using squitches. In particular, an inverter  700  is implemented using complementary metal-oxide-semiconductor (CMOS) logic design principles and more complex logic may follow directly here from. The inverter  700  has first squitch  705  and a second squitch  710 . The first squitch  705  may include a doped polymer composite  720 , a source  722 , a drain  724 , a gate  726  and an insulator  728  configured as shown in  FIG. 2 . The second squitch  710  may include a doped polymer composite  730 , a source  732 , a drain  734 , a gate  736  and an insulator  738  also configured as shown in  FIG. 2 . An input voltage V in    740  may be tied to gate  726  and gate  736 . The source  722  of first squitch  705  may be tied to ground  745  and the source  732  of second squitch may be tied to V DD    750 . The drain  724  of first squitch  705  and the drain  734  of second squitch  710  may be tied to an output voltage V out    760 . 
     As described hereinabove, a squitch may be turned on by applying either a positive or negative gate-to-source voltage and developing an attractive force between the gate and source electrodes. This makes it possible to implement CMOS-like logic using two identical squitches, as opposed to using complementary switches like the p-type and n-type FETs used in CMOS technology. For example, using the simulated resistance-voltage characteristics of the example squitch shown in  FIG. 6 , lower and upper gate-to-source threshold voltages may be defined, near 0.1 V and 0.6 V, respectively. In this situation, the squitch will turn on upon application of a high input voltage magnitude above the upper threshold, and the squitch will turn off upon application of a low input voltage magnitude below the lower threshold. 
     This switching characteristic, combined with the fact that the source  722  of first squitch  705  is tied to ground and the source  732  of the second squitch  710  is powered by V DD , allows the inverter  700  to function as a logic inverter. It is the absolute value of the gate-to-source voltage of the squitch that determines its conduction. Consequently, the first squitch  705  turns on with a high input voltage and the second squitch  710  turns off with a high input voltage. The reverse is true for a low input voltage. Thus, like CMOS logic, the static power consumption of squitch-based logic may be very small since one of the two squitches is always in an off state. 
       FIG. 8  is an embodiment of analog circuit using a squitch. In particular, an analog amplifier  800  may be implemented using a squitch  805 . A squitch  805 , as described herein, may include a doped polymer composite  810 , a source  812 , a drain  814 , a gate  816  and an insulator  818  configured as shown in  FIG. 2 . The gate  816  may be tied to an input voltage V in    820  and the source  812  may be tied to ground  830 . The drain  814  may tied to V DD    840  through a pull-up resistor  845  and to an output voltage V out    850 . 
     In this embodiment, the analog amplifier  800  may be a single-stage amplifier that mimics a common-source FET amplifier and a common-emitter BJT amplifier. However, more complex analog circuits, for example but not limited to, operational amplifiers, filters, multipliers, oscillators, power supplies and other analog devices may be built as multi-stage squitch circuits following the general principles of analog design. In analog electronics, a common-source amplifier is one of three basic single-stage amplifier topologies, typically used as a voltage or transconductance amplifier. In this circuit, the gate-to-source voltage of the transistor serves as the input, and the drain-to-source voltage serves as the output. The drain is connected to a power supply through a pull-up resistor, and the source is grounded. 
     In view of this and using, for example, the simulated resistance-voltage characteristics of the example squitch shown in  FIG. 6 , a lower and upper threshold voltage may be defined, near 0.1 V and 0.6 V, respectively. Below the upper threshold, as a transconductance amplifier, the input voltage V in    820  may smoothly modulate the resistivity of the doped polymer composite  810  by creating an electric field between the gate  816  and the source  812  that compresses the doped polymer composite  810 . The output voltage across the drain  814 -to-source  812  of the squitch  805  may then vary in accordance with the power-supply voltage divider formed by the series connection of the doped polymer composite  810  and the pull-up resistor  845 . 
     As described herein, the squitches may be fabricated using photolithographic and printing techniques.  FIG. 9  shows an example method of fabricating a squitch. Drain electrodes  900  may be deposited and patterned on a silicon oxide layer  905  on a substrate  910  ( 990 ). In non-limiting examples, the drain electrodes  900  may be 50 nm gold (Au) electrodes, the silicon oxide layer  905  may be a 300 nm SiO 2  layer and the substrate  910  may be a p-type silicon substrate. A mask  915  may be placed on top of the drain electrodes  900  to establish a pattern for a doped polymer  920  ( 992 ). In a non-limiting example, the mask  915  may be a 120 μm stainless steel mask. 
     The doped polymer  920  may be deposited or spun onto the patterning mask  915  and the unnecessary portions of the doped polymer  920  may be removed from the mask surface ( 994 ). In one embodiment, the patterning may be done with reactive ion etching using a photo mask. In another embodiment, a photo-patterned resist may be used as a mask. This may produce a smaller patterned polymer-composite as the resist may allow for finer feature sizes. In an embodiment where carbon nanotube fragments may be used as the dopant, the carbon nanotube fragments may be etched away with the polymer using the same etchant (assuming the polymer and dopant are both carbon based), resulting in a clean device, (noting that reactive ion etching with metal nanoparticles may leave metal nanoparticle dust on the surface as the same etchant may not remove the metal nanoparticle dust). The doped polymer  920  may be a mechanically soft Ni-polydimethysiloxane (PDMS) uniformly mixed with nickel particles. For example, the ratio of Ni to PDMS may be 3:2 by weight. The PDMS, for example, may have low cross linking and a 2-10% curing agent. For example, a PDMS crosslinking reaction is shown in  FIG. 10 . The degree of crosslinking is directly proportional to both the percentage of curing agent added and the eventual Young&#39;s modulus. In a non-limiting example, the doped polymer may be in a liquid form at the patterning stage. In non-limiting examples, the diameter of the patterned doped polymer  920  may be 250 μm, 500 μm or 1 mm. 
     The patterning mask  915  may be removed and the doped polymer  920  may be cured ( 996 ). In the photo-patterned resist embodiment, the mask for the polymer patterning may be removed after curing. In a non-limiting example, the doped polymer  920  may be cured at 100° C. for 10 minutes. After the curing period, the source electrode  925  may be placed on top of the doped polymer  920  ( 998 ). In a non-limiting example, the source electrode may be aluminum. 
       FIG. 11  shows a fabrication process for the second gate electrode  248  of  FIG. 2A  or the floating electrode  270  of  FIG. 2B . In particular,  FIG. 11  shows a transfer pad fabrication process that has a PDMS transfer pad with raised mesas  1130  made using a mold ( 1191 ). The PDMS transfer pad  1130  may then be treated with oxygen plasma for 30 seconds. An organic release layer  1132  is thermally evaporated onto the transfer pad  1130  ( 1193 ). A metal electrode  1134 , for example gold, may be thermally evaporated on top of the organic release layer  1132  ( 1195 ). As described herein above, the gold electrode  1134  may have a spring shape or a stepped shape. 
     A squitch  1105  may include squitch material  1120  in contact with a drain  1110  and a source  1115 . The squitch  1105  may then be brought into conformal contact with the gold electrode  1134  on the transfer pad  1136  ( 1198 ). The transfer pad  1136  is lifted away rapidly from the squitch  1105  to transfer gold electrode  1134  to form, for example, a squitch  1105  with a second gate electrode, collectively squitch  1140  ( 1199 ). See, “Micro-contact Printed MEMS”, by Murarka, Packard, Yaul, Lang and Bulovic,  Micro Electro Mechanical Systems  ( MEMS ), 2011  IEEE  24 th International Conference , page 292-295, the entire contents of which are herein incorporated by reference. 
     Although features and elements are described above in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. 
     Those of ordinary skill in the art may recognize that many modifications and variations of the above may be implemented without departing from the spirit or scope of the following claims. Thus, it is intended that the following claims cover the modifications and variations provided they come within the scope of the appended claims and their equivalents.