Abstract:
A micromechanical device includes a single crystal micromechanical structure where at least a portion of the micromechanical structure is capable of performing a mechanical motion. An epitaxial layer covers at least a portion of the micromechanical structure. In one embodiment, the micromechanical structure and the epitaxial layer are formed of different materials.

Description:
BACKGROUND OF THE INVENTION 
     Miniaturized devices, such as actuators, micro-optics, micro-fluidics, tunable electronics (filters), scanning probe microscope tips, micropower generators, and sensors (for example, temperature, pressure, acceleration, flow, radiation, chemical species etc.) sometimes include micromechanical structures formed from semiconductor materials. The micromechanical structures can be, for example, a membrane, cantilever beam, or tethered proof mass, etc., which is designed to be perturbed by external stimuli when used as a sensor, or to produce a motion when used as an actuator. Typically, the micromechanical structures are micromachined by an etching process. In some instances, a film of polycrystalline material is deposited over the micromechanical structure to provide the micromechanical structure with additional properties. For example, the film may have piezoresistive, piezoelectric, etc., properties. A drawback of polycrystalline films is that it is difficult to produce polycrystalline films that have constistent or uniform properties. In addition, some desirable functional properties are not provided by polycrystalline films. Furthermore, polycrystalline films are usually less stable at high temperatures and corrosive environments which restricts operation of devices including these films to lower temperatures and less corrosive environments. 
     SUMMARY OF THE INVENTION 
     The present invention provides a micromechanical device with highly reproducible properties and improved functionality that is capable of operating at higher temperatures and in more corrosive environments than previous devices. The micromechanical device includes a single crystal micromachined micromechanical structure. At least a portion of the micromechanical structure is capable of performing a mechanical motion. An epitaxial layer covers at least a portion of the micromechanical structure. The micromechanical structure and the epitaxial layer are formed of different materials. 
     In preferred embodiments, the micromechanical structure and epitaxial layer are each preferably formed from a material selected from the group consisting of group IV, III-V, II-VI and IV-VI semiconductors. In particular, the micromechanical structure is preferably formed from a material selected from the group of solids consisting of Si, Ge, SiC, GaAs, InAs, InP GaP, GaSb, InSb, GaN, ZnO, CdTe and ZnTe. The epitaxial layer is preferably formed from a material selected from the group of solid solutions consisting of SiGeC, AlGaInPAsSb, AlGaInN and ZnCdHgOSSeTe. In one embodiment, the epitaxial layer is formed before the micromechanical structure is micromachined. In another embodiment, the epitaxial layer is formed on the micromechanical structure after the micromechanical structure is micromachined. The function of the epitaxial layer is dependent upon the material selected. In one embodiment, the epitaxial layer is formed of a material that provides a protective layer for the micromechanical structure. In another embodiment, the epitaxial layer is formed of a material that provides a measurable response to external stimulation of the micromechanical device. A secondary layer or layers of material may be formed on the epitaxial layer such that the epitaxial layer acts as an intermediate layer. Depending upon the application and the materials chosen, the micromechanical device may be a sensor, an actuator, an electronic device or an optoelectronic device. 
     The present invention also provides a micromechanical device including a single crystal micromachined micromechanical structure. At least a portion of the micromechanical structure is capable of performing a mechanical motion. An epitaxial layer is formed on at least a portion of the micromechanical structure after the micromechanical structure is micromachined. 
     In the present invention, since the micromechanical structure and the epitaxial layer are each preferably formed from a single crystal material, the resulting properties (mechanical, electronic, optical, etc.) of the micromechanical device are more readily reproduced employing known micromachining and thin film deposition techniques. In addition, single crystal materials which are stable at high temperatures and in harsh environments can be used. This allows the present invention micromechanical device to be used in a wider range of applications than is possible with devices having polycrystalline films which are susceptible to microstructure changes and preferential attack at grain boundaries. For example, with the appropriate selection of materials, devices of the present invention can operate with greater functionality at temperatures exceeding 1000° C. as well as in environments found in gas turbines, internal combustion engines and munitions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1A is a plan view of an embodiment of a micromechanical device of the present invention. 
     FIG. 1B is a side sectional view of the micromechanical device of FIG.  1 A. 
     FIGS. 2A-2C are side sectional views depicting the formation of a micromechanical device according to one method. 
     FIGS. 3A-3E are side sectional views depicting the formation of a micromechanical device according to another method. 
     FIGS. 4A and 4B, are side sectional views depicting the formation of a micromechanical device according to still another method. 
     FIGS. 5A and 5B are side sectional views depicting the formation of a micromechanical device according to yet another method. 
     FIGS. 6A and 6B are side sectional views depicting the formation of a micromechanical device such as a membrane according to another method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1A, and  1 B, micromechanical device  10  is an embodiment of a temperature sensor which is one example of a micromechanical device of the present invention. Micromechanical device  10  includes a single crystal semiconductor base substrate  18  having an upper region  18   a  and a lower region  18   b  where one region is n-type and the other region is p-type. A pattern  19  is etched through the upper region  18   a  of base substrate  18 . A cavity  16  extending from pattern  19  is etched within the lower region  18   b  and under the upper region  18   a , thereby forming a cantilevered beam  14 . An epitaxial layer or film  12  of single crystal material is formed over the cantilevered beam  14 . The epitaxial layer  12  closely registers with the crystalline structure of base substrate  18  and for all practical purposes is considered a single crystalline film. However, it is understood that minor defects may exist in epitaxial layer  12  such as inclusions or low angle grain boundaries. In addition, epitaxial layer  12  may have a highly oriented columnar structure with some lattice mosaic in the film. The epitaxial layer  12  is formed of a material that has a different thermal expansion coefficient than the base substrate  18 . A piezoresistive element  22  is formed over the epitaxial layer  12 . Electrical contacts  24  are electrically connected to the opposite ends of piezoresistive element  22  allowing the measure of its change in resistance. 
     The difference in thermal expansion coefficient between the epitaxial layer  12  and the cantilevered beam  14  induces varying degrees of deflection of the cantilevered beam  14  in response to varying temperatures. In other words, for a particular temperature, the cantilevered beam  14  has a particular deflection. Changes in deflection of the cantilevered beam  14  causes changes in the resistance of the piezoresistive element  22  so that piezorestive element  22  has a particular resistance for a particular deflection of cantilevered beam  14 . As a result, the resistance of the piezoresistive element  22  for a particular deflection of cantilevered beam  14  can be measured and then correlated to the appropriate temperature. 
     The base substrate  18  of micromechanical device  10  in one embodiment is silicon carbide (SiC) and the epitaxial layer  12  is a solid solution of aluminum gallium nitride (Al X Ga 1−X N). In addition, the upper region  18   a  of base substrate  18  from which the cantilevered beam  14  is formed, is doped to be p-type and the lower region  18   b  is n-type. The epitaxial layer  12  provides more than one function. As previously mentioned, the eptaxial layer  12  causes deflection of cantilevered beam  14  in response to the temperature. In addition, the epitaxial layer  12  forms an intermediate layer onto which piezoresistive element  22  is formed and also influences the mechanical characteristics of the cantilevered beam  14 . 
     As mentioned above, the temperature sensor of micromechanical device  10  is one example of a micromechanical device of the present invention. In addition, the cantilevered beam  14  is one example of a micromechanical structure. Other examples of micromechanical structures which may be included in a micromechanical device of the present invention are membranes, microbridges, tethered proof masses, etc., the use of each, being dependent upon the desired application. For example, typically, membranes are suitable for use in pressure and acoustic sensors, microbridges are suitable for use in flow sensors, cantilevered beams are suitable for use in temperature, chemical, biochemical and inertial sensors, and tethered proof masses are suitable for use in inertial sensors. It is understood that each micromechanical structure may be suitable for more than one purpose. It is also understood that the micromechanical structures can be made from a wide range of single crystal semiconductor materials. 
     The epitaxial layer  12  of micromechanical device  10  is one example of an epitaxial layer included in a micromechanical device of the present invention. In the present invention, the material of the epitaxial layer is selected to meet the requirements of the desired application and the micromechanical structure employed, as well as to be compatible with the material of the base substrate from which the micromechanical structure is formed. As with the base substrate, the epitaxial layer may be made from a wide range of single crystal semiconductor materials having a wide range of properties and functions. 
     For example, some epitaxial layers are employed as a protective coating for protecting micromechanical structures. Such protection may be against chemicals, mechanical abrasion, heat, or to act as a diffusion and/or oxidation barrier. The protective coating may be employed to also improve or obtain desired mechanical properties, for example, to improve yield strength, increase toughness, tune resonant frequencies or provide vibration damping. In addition, some epitaxial layers are employed to provide a measurable response to external stimulation. Such epitaxial layers may have properties that are piezoelectric, piezoresistive, pyroelectric, electro-optic, magneto-resistive, have variable reflectivity or are sensitive to particular chemical or biological species, etc. These epitaxial layers may be employed to form a wide range of sensors, actuators, and electronic or optoelectronic devices when used in conjunction with the appropriate micromechanical structures. Furthermore, some epitaxial layers may be employed as an intermediate layer over which another layer of material is formed. Such an intermediate epitaxial layer may be employed as a buffer layer to prevent diffusion and/or oxidation between the separated materials. The intermediate epitaxial layer may also be employed to enable or improve the deposition of overlying materials for example, multiple quantum well structures. Although, the epitaxial layer may be formed of many different materials and perform many different functions, the material for a particular epitaxial layer must have a crystal structure which is compatible to that of the base substrate of the micromechanical structure in order for the layer to grow epitaxially on the base substrate. Alternatively, the material of the semiconductor base substrate may be selected to match a particular epitaxial layer material. The epitaxial layer is a single crystal or a highly oriented columnar structure with a certain degree of lattice mosaic in the film, depending on the lattice match (in dimensions as well as symmetry) between the epitaxial layer and the base substrate. For example, a GaN epitaxial layer with a hexagonal structure is compatible with and may be grown on a 6-H SiC (0001) substrate (which has the same structure but a 3% mismatch in lattice spacing) and on a (111) Si substrate (which has 17% lattice mismatch and cubic structure, but has the same in-plane symmetry as hexagonal GaN). 
     One method for forming a micromechanical device in accordance with the present invention using available micromachining techniques is depicted in FIGS. 2A-2C. Reference numeral  15  in FIG. 2C depicts a generic micromechanical structure which may be any suitable micromechanical structure such as the micromechanical structures described above. In FIG. 2A, a single crystal epitaxial layer  26  of semiconductor material having desired characteristics or properties is first grown on a single crystal semiconductor base substrate  18 . The base substrate has an upper region  18   a  and a lower region  18   b  with one region being n-type and the other region being p-type. The material of the epitaxial layer  26  is selected to address the application at hand and may have any suitable characteristic or property such as those previously mentioned. The epitaxial layer  26  is deposited by any suitable method such as molecular beam epitaxy, pulsed laser deposition, vapor phase epitaxy, liquid phase epitaxy, evaporation or sputtering. As previously mentioned, the material of the epitaxial layer  26  is also selected to have a crystal structure that is compatible to that of the base substrate  18  which enables the epitaxial layer  26  to grow in a single crystal over the base substrate. A selected pattern  28  of the epitaxial layer  26  is removed, by etching, as seen in FIG.  2 B. Referring to FIG. 2C, the portions of the upper region  18   a  of the base substrate exposed by the pattern  28  in the epitaxial layer  26  are etched to form a pattern  19  through the upper region  18   a  corresponding to pattern  28 . A cavity  16  is then etched into the lower region  18   b  of base substrate  18  through the pattern  19  in the upper region  18   a . Cavity  16  extends under portions of upper region  18   a  to form micromechanical structure  15 . The portion of the epitaxial layer  26  on micromechanical structure  15  forms the epitaxial layer  12  which provides the micromechanical structure with the desired properties, such as those previously discussed. 
     Semiconductor materials suitable for base substrate  18  and epitaxial layer  26  are those chosen from columns IV, III-V, II-VI and IV-VI of the periodic table. Examples of semiconductor materials suitable for base substrate  18  include silicon (Si), silicon carbide (SiC), gallium arsenide (GaAs), indium phosphide (InP) and gallium nitride (GaN). Other examples include Ge, InAs, GaP, GaSb, InSb, ZnO, CdTe, ZnTe, etc. Examples of semiconductor materials suitable for epitaxial layer  26  include solid solutions of silicon, germanium and carbon (SiGeC), aluminum, gallium, indium, arsenic, antimony and phosphorus (AlGaInAsSbP) and aluminum, gallium, indium and nitrogen (AlGaInN). Other examples include solid solutions of zinc, cadmium, mercury, oxygen, sulfer, selenium and tellurium (ZnCdHgOSSeTe). Epitaxial materials such as III-V nitrides (for example solid solutions of AlN—GaN—InN) on silicon micromechanical structures are useful as UV emitters and detectors, blue LEDs, high power devices and piezoelectric films. Epitaxial layers of MnTe are useful for registering a change in electrical resistance in response to a magnetic field. SiC and AlN—GaN semiconductors have excellent thermal, mechanical, chemical and electrical properties which are suitable for harsh environments. Such semiconductors are inert in most environments except molten metals and salts and have high melting/decomposition temperatures (1700-3000° C.). 
     Another method of forming a micromechanical device  15  is depicted in FIGS. 3A-3E. Referring to FIG. 3A, a layer of masking material  30 , for example, amorphous or polycrystalline oxide, is deposited on a single crystal semiconductor base substrate  18 . Base substrate  18  has an upper region  18   a  and a lower region  18   b  of p and n types as in FIGS. 2A-2C. The layer of masking material  30  is formed of a material such as silicon dioxide (SiO 2 ), on which a single crystal epitaxial layer will not grow. Referring to FIG. 3B, the layer of masking material  30  is selectively etched to form a pattern  30   a . Referring to FIG. 3C, a single crystal epitaxial layer  26  of semiconductor material having desired characteristics is deposited upon base substrate  18  and around the masked pattern  30   a  due to the fact that the epitaxial layer  26  cannot grow on the masked pattern  30   a , and either does not deposit at all, or deposits as an amorphous or polycrystalline film which can be selectively removed. Referring to FIG. 3D, the masked pattern  30   a  is removed by an etching process to form a pattern  28  through the epitaxial layer  26 . The pattern  28  exposes the upper region  18   a  of base substrate  18 . Referring to FIG. 3E, the micromechanical structure  15  can then be etched in a manner similar to that described relative to FIG.  2 C. The portion of the epitaxial layer  26  on micromechanical structure  15  forms epitaxial layer  12 . 
     FIGS. 4A and 4B depict another method for forming a micromechanical device  15 . Referring to FIG. 4A, a single crystal semiconductor base substrate  18  having upper  18   a  and lower  18   b  regions of p and n types, is etched to form a pattern  19  through the upper region  18   a . A cavity  16  is etched in the lower region  18   b  of base substrate  18  extending through pattern  19  and under the upper region  18   a  to form the micromechanical structure  15 . Referring to FIG. 4B, a single crystal epitaxial layer  26  of semiconductor material having desired characteristics is then deposited upon the micromechanical structure  15  and base substrate  18 , thereby forming the epitaxial layer  12  on the micromechanical structure  15 . The preferred methods of forming the epitaxial layer  26  are pulsed laser deposition and molecular beam epitaxy, but alternatively, can be any physical vapor epitaxy that provides a shower of ions for depositing material only on top surfaces. 
     Yet another method of forming a micromechanical device is depicted in FIGS. 5A and 5B. Referring to FIG. 5A, the micromechanical structure  15  is micromachined from a single crystal semiconductor base substrate  18  in a similar manner as depicted in FIG. 4A. A single crystal epitaxial layer  26  of semiconductor material having desired characteristics is then applied to the single crystal base substrate  18  to cover all the exposed surfaces including those within cavity  16 . The preferred method of depositing epitaxial layer  26  is by chemical vapor deposition and vapor phase epitaxy. As a result, the exposed top, bottom and sides of upper region  18   a , micromechanical structure  15  and cavity  16  are all covered by the epitaxial layer  26 , such that micromechanical structure  15  is encapsulated by an epitaxial layer  12 . Consequently, micromechanical structure  15  has minimal residual stress and is temperature compensated since a film is present on opposing sides of the micromechanical structure  15 . Depending upon the material selected, the epitaxial layer  26  may be suitable for providing chemical, oxidation, mechanical and thermal protection to the micromechanical structure  15  and base substrate  18  due to the encapsulation. 
     FIGS. 6A and 6B depict a method for forming a micromechanical structure such as a micromechanical membrane  34  having a single crystal epitaxial layer  12 . Referring to FIG. 6A, a single crystal epitaxial layer  26  of semiconductor material having desired characteristics is deposited upon a single crystal semiconductor base substrate  18  having an upper region  18   a  and a lower region  18   b  of p and n types. Referring to FIG. 6B, the lower region  18   b  of base substrate  18  is etched on the side opposite to the epitaxial layer  26  to form a recessed area or trough  32  which extends to the upper region  18   a  of base substrate  18 . The upper region  18   a  of base substrate  18  exposed by the trough 32 forms a thin membrane  34  and the portion of epitaxial layer  26  covering the membrane  34  forms epitaxial layer  12 . Although the base substrate  18  depicted in FIGS. 6A and 6B preferably has both n-type and p-type regions, alternatively, base substrate  18  can be entirely n-type or entirely p-type. 
     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, the micromachining techniques employed for forming the micromechanical devices  15  can be any known micromachining techniques such as chemical, electrochemical, photo-electrochemical and reactive ion etching, or any combination thereof. In addition, the use of single crystal materials allows the micromechanical devices of the present invention to be incorporated on the same wafer with other electronic devices such as amplifiers, processors etc., or more than one micromechanical device. Furthermore, although the single crystal epitaxial layer in the present invention is typically formed of a different material than the base substrate, in some cases, the epitaxial layer is formed from the same material. In such a case, the epitaxial layer and the base substrate may be formed so that one is p-type and the other is n-type. Also, more than one epitaxial layer may be deposited upon a micromechanical structure. The multiple layers may be on top of each other or side by side.