Abstract:
A bimaterial microelectromechanical system (MEMS) solar power generator device converts radiant energy received from the sun, or other light source, into heat, which is then used to produce electricity through the piezoelectric effect. As the efficiency of piezoelectric materials can often reach up to 90%, the efficiency is greater than that of conventional solar cells.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/432,101, filed Jan. 12, 2011, which is hereby incorporated in its entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to microelectromechanical systems (MEMS) and solar power generators. 
     2. Description of the Related Art 
     Harvesting energy from the sun offers clean solutions to growing energy challenges and can help decrease dependence on fossil fuels. Solar power is also a promising candidate for powering autonomous systems, such as sensors and actuators in remote areas. However, in order to meet the demanding energy needs of modern devices, high solar conversion efficiencies are needed. 
     Prior art solar cell technologies suffered from low efficiencies and were spectrum-dependent as the photoactive semiconductor materials dictated the energies required to create an electron-hole pair and therefore transform solar energy into electric energy. Some prior art multi-junction solar cells employed a variety of layered materials in order to broaden the range of photoactive wavelengths. However, this significantly complicated the manufacturing process and the devices became economically unfeasible for commercial applications. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment, a bimaterial microelectromechanical system (MEMS) solar power generator device converts radiant energy received from the sun, or another light source, into heat, which is used to produce a deformation, which in turn produces electricity through the piezoelectric effect. As the efficiency of piezoelectric materials can often reach up to 90%, the efficiency of some embodiments in accordance with the invention can be greater than that of conventional solar cells. In some embodiments, the bimaterial MEMS solar power generator device is spectrum-insensitive and converts a large fraction of incident solar radiation into electricity. 
     Bimaterial MEMS solar power generators in accordance with the invention are highly efficient and adaptable for powering small sensors as well as for powering large arrays allowing for the replacement of costly, less-efficient solar cells. 
     Embodiments in accordance with the invention are best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a bimaterial MEMS solar power generator device in accordance with one embodiment. 
         FIG. 2  illustrates the bimaterial piezoelectric structure of  FIG. 1  in accordance with one embodiment. 
         FIGS. 3A-3D  illustrate the conversion of solar radiation into electrical energy using the bimaterial MEMS solar power generator device of  FIG. 1  in accordance with one embodiment. 
         FIG. 4  illustrates an example voltage profile of one of the bimaterial multifold legs sets of  FIG. 1  in accordance with one embodiment. 
         FIG. 5  illustrates the deformation of bimaterial multifold legs, due to temperature increase, in accordance with one embodiment. 
         FIGS. 6A-6G  illustrate a method for forming the bimaterial MEMS solar power generator device of  FIG. 1 . 
         FIG. 7  illustrates a process flow diagram of a method for forming the bimaterial MEMS solar power generator device of  FIG. 1 . 
     
    
    
     Embodiments in accordance with the invention are further described herein with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a bimaterial MEMS solar power generator device  100  in accordance with one embodiment. Herein a solar power generator is defined as a device that produces electricity when powered by radiation from the sun or other radiant source. As shown in  FIG. 1 , in one embodiment, bimaterial MEMS solar power generator device  100  includes a bimaterial piezoelectric structure  102  in contact with a heat sink structure  104 , such as a silicon (Si) substrate. In one embodiment, support legs  108  of bimaterial piezoelectric structure  102  support it above heat sink structure  104  at an offset distance  106 . When exposed to radiative energy, such as sunlight (shown as waves  110 ), bimaterial MEMS solar power generator device  100  converts radiative energy into heat, which is then used to produce deformation through the bimorph effect and then electricity through the piezoelectric effect as further described herein. Although not shown, in operation, electrical circuit connections can be placed in contact with bimaterial piezoelectric structure  102 , such as at support legs  108  in this embodiment, to allow the electricity produced by bimaterial MEMS solar power generator device  100  to be transferred from device  100  and utilized, such as by a load (not shown). 
       FIG. 2  illustrates bimaterial piezoelectric structure  102  in accordance with one embodiment. As shown in  FIG. 2 , in one embodiment, bimaterial piezoelectric structure  102  includes a piezoelectric material  202 , a broad-band radiation absorptive material  204 , and a coating material  208 . In one embodiment, piezoelectric material  202  is formed of quartz, aluminum nitride (AlN), silicon carbide (SiC), or other material having piezoelectric properties. In one embodiment, coating material  208  is formed of material having a different thermal expansion coefficient from piezoelectric material  202 . In one embodiment coating material  208  is formed of a metal, such as aluminum (Al), however, other materials having a different thermal expansion coefficient from piezoelectric material  202  can also be used. 
     In one embodiment, broad-band radiation absorptive material  204  is formed of a carbon nanomaterial or other material having high broad-band radiation absorption properties for at least a portion of the wavelengths of energy emitted by the sun or other light source. In one embodiment, broad-band radiation absorptive material  204  is a photoactive material capable of absorbing wavelengths of light energy in the range of microwave to UV and converting them to heat. For further details on broad-band radiation highly absorptive materials, see, for example, “Super black and ultrathin amorphous carbon film inspired by anti-reflection architecture in butterfly wing”, by Q. Zhao, et al., Carbon, Vol. 49, Issue 3, pg. 877. 
     In one embodiment, broad-band radiation absorptive material  204  is formed on a top surface of piezoelectric material  202  and is in conductive contact with piezoelectric material  202 . This allows broad-band radiation absorptive material  204  to absorb radiative energy, such as from the sun, and transfer the radiative energy as thermal energy to piezoelectric material  202 . In some embodiments, broad-band radiation absorptive material  204  is formed on a central body portion  212  of structure  102  in a pad configuration as shown in  FIG. 2 . In other embodiments, broad-band radiation absorptive material  204  is formed across the entire top surface of structure  102 , or to selected portions of structure  102 . 
     In one embodiment, piezoelectric material  202  and coating material  208  have different thermal expansion coefficients. In accordance with embodiments further described herein, the differing coefficients of thermal expansion of coating material  208  and piezoelectric material  202  allow portions of bimaterial piezoelectric structure  102  to deform, creating electric energy through piezoelectric and bimorph effects. 
     In one embodiment, bimaterial piezoelectric structure  102  includes bimaterial multifold legs  206 . As shown in  FIG. 2 , in one embodiment, structure  102  includes two sets of bimaterial multifold legs  206 , each set having 6 legs (numbered in  FIG. 2  as  1 ,  2 ,  3 ,  4 ,  5 , and  6 ). In other embodiments, different numbers of sets of bimaterial multifold legs  206  can be utilized, and the number and or shape of each bimaterial multifold leg  206  can differ from the embodiment shown in  FIG. 2 . It can be appreciated that in different embodiments, the electrical output of device  100  may vary dependent upon the selected configuration. 
     In one embodiment, coating material  208  is located on alternating legs of bimaterial multifold legs  206 . Thus, in some embodiments, the formed bimaterial of coating material  208  on piezoelectric material  202  may be on selected portions of bimaterial multifold legs  206 . Thus for example in  FIG. 2 , coating material  208  is present on legs  1 ,  3  and  5  of bimaterial multifold leg  206 , and similarly, though not numbered, on the opposite bimaterial multifold leg  206  structure as well. This configuration causes the coated, bimaterial legs ( 1 ,  3  and  5 ) to bend upward toward the incident flux, and central body portion  212  of structure  102  displaces about uniformly downward toward heat sink structure  104 . This reversal of displacement is due to the deflection angle of the final piezoelectric leg, e.g., leg  6 . In one example test, utilizing a heat flux of 700 W/m 2  to simulate the solar flux on the surface of the earth with an ambient temperature of 293.15, the displacement of central body portion  212  is approximately 5 um. 
     As earlier described, in one embodiment, coating material  208  has a different thermal expansion coefficient from piezoelectric material  202 . In one embodiment, coating material  208  has a coefficient of thermal expansion in the range of about 10×10 −6 /K to about 30×10 −6 K. In one embodiment, the larger the differences of thermal expansion coefficients between piezoelectric material  202  and coating material  208 , the more deformation is calculated. For further details on deformation calculations, see for example, Grbovic, D., Imaging by detection of infrared photons using arrays of uncooled micromechanical detectors, PhD diss., University of Tennessee, 2008 (http://trace.tennessee.edu/utk/graddiss/404). 
     As earlier described, in one embodiment, bimaterial piezoelectric structure  102  is located above heat sink structure  104  ( FIG. 1 ) at offset distance  106 . Offset distance  106  is defined herein as the distance between the bottom surface of central body portion  212 , and the top surface of heat sink structure  104  ( FIG. 1 ). In one embodiment, offset distance  106  is about 80 to 90% of the maximum displacement of central body portion  212  obtained for a given dimension with moderate incoming heat flux, for example 300 W/m 2 . This allows central body portion  212  to touch heat sink structure  104  ( FIG. 1 ) when thermally deformed and still allow the motion necessary to generate a voltage in bimaterial piezoelectric structure  102 . 
       FIGS. 3A-3D  illustrate the conversion of solar radiation into electrical energy using bimaterial MEMS solar power generator device  100  of  FIG. 1  in accordance with one embodiment. Referring now to  FIGS. 1 ,  2  and  3 A, in one embodiment, bimaterial piezoelectric structure  102  initially begins at an initial position (IP), when not exposed to radiant energy, shown as IP  302  in  FIG. 3A . When exposed to radiant energy, such as sunlight, broad-band radiation absorptive material  204 , for example, carbon nanoparticles, absorbs a large fraction of radiant energy, e.g., solar radiation (shown as waves in  FIG. 2 ), and converts the radiant energy into heat. 
     Referring now to  FIGS. 1 ,  2 , and  3 B, as a consequence, the temperature of piezoelectric material  202 , and coating material  208  increases. The temperature increase induces a thermal expansion of piezoelectric material  202 , and coating material  208 . Due to a mismatch in the thermal expansion coefficients, both piezoelectric material  202  and coating material  208  expand at different rates causing bimaterial piezoelectric structure  102  to deform, e.g., bend. This mechanical deformation turns some of the heat energy into mechanical energy. The deformation generates a potential difference at ends of bimaterial MEMS solar power generator device  100  due to the piezoelectric effect turning some of the mechanical energy into electrical energy. More particularly, bimaterial multifold legs  206  begin to deform due to the mismatch in the thermal expansion coefficients between piezoelectric material  202  and coating material  208 . This generates electricity, e.g., voltage (V) greater than zero (V&gt;0V). 
     Referring now to  FIGS. 1 ,  2 , and  3 C, in one embodiment, when bimaterial piezoelectric structure  102  reaches a maximum displacement position (MP), shown as MP  304 , central body portion  212  of bimaterial piezoelectric structure  102  contacts heat sink structure  104  ( FIG. 1 ). Heat is then transferred from bimaterial piezoelectric structure  102  to heat sink  104 . 
     Referring now to  FIGS. 1 ,  2 , and  3 D, removal of heat from bimaterial piezoelectric structure  102  causes structure  102  to cool and relax to a state closer to IP  304  ( FIG. 3A ), e.g., un-deform. This process is repeated, with solar radiation heating bimaterial MEMS solar power generator device  100  again. This continued oscillation of bimaterial MEMS solar power generator device  100  generates electrical energy in bimaterial piezoelectric structure  102  which can then be drawn off through electrical connections and utilized. 
     In the present embodiment, a deformation-magnification technique is utilized in bimaterial MEMS solar power generator device  100  to increase the total deformation. In one embodiment, bimaterial multifold legs  206  are utilized and combine individual deformations into a large displacement, thus increasing the total amount of electricity that can be produced by device  100 . 
       FIG. 4  illustrates an example voltage profile  400  of one of bimaterial multifold legs  206  of device  100  in accordance with one embodiment. In one embodiment, voltage profile  400  results from deformation, e.g., bending, induced by the mismatch in thermal expansion coefficients of piezoelectric material  202  and coating material  208  when exposed to radiative energy, e.g., sunlight. In one embodiment, an exemplar bimaterial MEMS solar power generator device  100  is sized at about 200 μm×200 μm and exhibits an output voltage of −80 mV. The testing results indicate a potential difference of −40 mV per bimaterial multifold leg  206 , meaning about 80 mV per structure of about 200 μm×200 μm in size. 
       FIG. 5  illustrates the deformation of bimaterial multifold legs  206 , due to temperature increase, in accordance with one embodiment. 
       FIGS. 6A-6G  illustrate a method for forming bimaterial MEMS solar power generator device  100  in accordance with one embodiment shown in  FIG. 1 .  FIG. 7  illustrates a process flow diagram of a method  700  for forming bimaterial MEMS solar power generator device  100  in accordance with one embodiment. Individual general MEMS fabrication processes and terms used herein are well known to those of skill in the art. 
     Referring now to  FIGS. 6A and 7 , in operation  702 , a substrate  602 , such as a silicon wafer, is obtained and utilized on which to grow, i.e., fabricate, bimaterial MEMS solar power generator device  100 . 
     Referring now to  FIGS. 6B and 7 , in operation  704 , due to the bimaterial of piezoelectric material  202  ( FIG. 2 ) and coating material  208  ( FIG. 2 ) being on the bottom of device  100 , a sacrificial layer  604  is deposited on a top surface of substrate  602  as illustrated in  FIG. 6B . In one embodiment, sacrificial layer  604  is formed to have a thickness of the desired offset distance (offset distance  106 ,  FIG. 1 ). 
     Referring now to  FIGS. 6C and 7 , in operation  706 , following deposition of sacrificial layer  604 , a coating material layer (not shown), for example, aluminum (Al), is deposited over a top surface of sacrificial layer  604  and in operation  708 , the coating material layer is etched to create alternating coating material  208  portions ( FIG. 2 ), shown as  606  in  FIG. 6C , of bimaterial multifold legs  206  ( FIG. 2 ). 
     Referring now to  FIGS. 6D and 7 , in operation  710 , anchor holes  608  are etched in sacrificial layer  604  as shown in  FIG. 6D . In one embodiment, as shown in  FIG. 6D , two anchor holes  608  are formed at opposite corner sides of sacrificial layer  604 . In one embodiment, anchor holes  608  are formed with angled sides. 
     Referring now to  FIGS. 6E and 7 , in operation  712 , a piezoelectric material layer (not shown) is deposited over the top surface of sacrificial layer  604 . In one embodiment, the deposition is conformal. In one embodiment, the piezoelectric material is a material such as aluminum nitride (AlN) or silicon carbonate (SiC). As is well known to those of skill in the art, there are numerous methods for growing AlN and SiC on both silicon (Si) and silicon carbide (SiC). In the present embodiment, the growth of piezoelectric material layer is on the top surface of sacrificial layer  604 . Those of skill in the art can recognize that the polytype of SiC grown is dependent on the material on which it is grown. For example, in order to grow SiC on a Si substrate (or layer), single crystal 3C—SiC is used due to the dissimilarity in crystalline structure of SiC (6H) with Si. In operation  714 , following deposition of the piezoelectric material layer, piezoelectric material layer is etched to form piezoelectric material structure  610 . 
     Referring now to  FIGS. 6F and 7 , in operation  716 , sacrificial layer  604  is removed. As illustrated in  FIG. 6F , removal of sacrificial layer  604  results in the creation of the freestanding bimaterial piezoelectric structure  614 , elevated on top of substrate  602 . As shown in  FIG. 6F  coating material  606  is now present on alternating legs of bimaterial multifold legs  612 . 
     Referring now to  FIGS. 6G and 7 , in operation  718 , a broad-band radiation absorptive material layer  616  is formed on the top surface of structure  614  as earlier described, such as by deposition and etching, selective deposition, growth, or other method of formation or application. The resultant bimaterial MEMS device, shown as  600  in  FIG. 6G , is now complete and ready to be attached to electrical circuitry, such that when exposed to radiant energy, such as from the sun, electricity is generated as earlier described herein. 
     As can be understood by those of skill in the art, method  700  presents but one embodiment of a method of forming bimaterial MEMS solar power generator device  100 , and that other fabrication processes and techniques can also be utilized. 
     In other embodiments, a plurality of bimaterial MEMS solar power generator devices  100  can be connected, such as in an array, to harness the radiant energy from the sun by absorbing radiation over a wide spectral range, and converting the resulting heat into electricity using the bimorph and piezoelectric effects. In this fashion, a large area can be utilized. The number of the bimaterial MEMS solar power generator devices  100  formed in the array as well as the nature of their connection (in series or in parallel) can be customized to support the energy needs of a particular load. 
     Accordingly, this disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification or not, may be implemented by one of skill in the art in view of this disclosure.