Abstract:
Electro-mechanical structures and methods for forming same are disclosed. The structures are integratable onto an integrated circuit. The structures have a deformeable element formed in a plane substantially perpendicular to the substrate of the integrated circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to electromechanical structures and in particular to electromechanical structures having a deformable element, the structure being integratable onto an integrated circuit. 
     BACKGROUND OF THE INVENTION 
     Many types of electromechanical structures have been incorporated into integrated circuits. For example, the concept of a structure comprising a sealed cavity formed using semiconductor processing techniques with a suspended membrane whose deformation or deflection allows its use as a sensor or actuator is known in the literature. U.S. Pat. Nos. 4,744,863 and 4,853,669 granted to Guckel et al., U.S. Pat. Nos. 5,321,989 and 5,431,057 granted to Zimmer et al., and U.S. Pat. No. 5,316,619 granted to C. H. Mastrangelo discloses some form of this approach to realise pressure sensors. In all known cases, including the above, the sensor is a planar structure in parallel to the silicon substrate which typically forms the bottom plate of the sensor. 
     In the patents listed above, a pressure sensing diaphragm is formed using a deposited layer onto the chip surface. The use of silicon over a sacrificial buried oxide layer to create micromechanical elements is also known in the literature, such as that described in U.S. Pat. No. 5,677,560, granted to Zimmer et al. which etches out a buried oxide to create micromechanical structures. This structure uses the active silicon layer above the buried oxide to create the micromechanical diaphragm. 
     In all of the prior art described above, the plane of the deformeable element (the diaphragm) is parallel to the surface of the integrated circuit. 
     Known implementations of micromechanical structures that are fabricated in the same plane as the surface of the integrated circuit (e.g. diaphragm-type pressure sensor) consume considerable surface area on the integrated circuit. In addition, known methods of implementing sensors on silicon are highly process dependent. Accordingly, a need arises for an electromechanical structure that occupies minimal surface area, and can be easily scaled into different process generations. 
     SUMMARY OF THE INVENTION 
     These needs and others are addressed by the electromechanical structure of the present invention. The electromechanical element of the present invention includes a deformeable element. In one embodiment, the invention provides an electromechanical structure being a transducer that realises a mechanical diaphragm in silicon. The structure can be incorporated onto an integrated circuit manufactured through a standard semiconductor manufacturing process. The diaphragm can be used as a sensor or actuator. 
     The deformeable element is formed in a plane perpendicular to the surface of the integrated circuit by etching one or more trenches into the silicon. In the case where two parallel trenches are etched the silicon “wall” between the two trenches forms a diaphragm. The top of one of the trenches is sealed under vacuum conditions to form an evacuated cavity while the other trench is left open. The silicon “wall” now has a vacuum cavity on one side and the other side is open to ambient pressure. 
     This wall forms a “deflectable” or “deformeable” diaphragm of a sensor. Ambient pressure changes deflect the diaphragm, and the deflection is proportional to the pressure difference between the ambient pressure and the vacuum cavity. The deformation of the diaphragm can be measured directly as a variation in the electrical characteristics of the structure, thereby serving as a sensor. Alternatively, the diaphragm can be excited electrically to serve as an actuator. In the case where both trenches are sealed under vacuum conditions the silicon wall between the two can be excited electrically to serve as a resonator or mechanical filter. 
     In the case where the etch forms a single column of silicon surrounded by a trench the trench can be sealed under vacuum and the column can be contacted electrically. This sealed column can be used as a resonator or mechanical filter. 
     A main advantage of the structure arises from the fact that the deformeable element is substantially perpendicular to the surface of the integrated circuit. This allows large deformeable elements to be manufactured while consuming very little surface area of the integrated circuit. This approach is very cost effective and will scale with all process technologies. 
     Accordingly the invention provides an integrated circuit comprising: 
     a substrate having an upper surface defining a first plane; 
     an electromechanical element including a deformeable element defining a second plane; 
     such that the first and second planes intersect. 
     The first and second planes are preferably substantially perpendicular with respect to one another. 
     The deformeable element is typically insulated from the upper surface of the substrate. 
     In one embodiment the deformeable element is preferably responsive to applied pressure, said applied pressure effecting a change in the electrical characteristics of said electromechanical element. 
     In another embodiment the deformeable element can be electrostatically actuated to create a resonator or mechanical filter. 
     In one embodiment, the deformable element is a transducer diaphragm forming a first wall of an evacuated cavity, an outer portion of the first wall being exposed to ambient pressure conditions and an inner portion of the first wall being exposed to an evacuated cavity, such that changes in ambient pressure with respect to the evacuated cavity effects a deflection of the diaphragm in the vertical plane, such deflection being electrically measurable. 
     The transducer diaphragm preferably forms a first wall of an evacuated cavity, the cavity having a second wall, the second wall of the cavity being electrically insulated from the first wall, such that on application of a signal between the first wall and the second wall the diaphragm is actuated, the actuation of the diaphragm resulting in the diaphragm vibrating mechanically. 
     The frequency of the diaphragm vibration typically is modulated by the frequency of the applied signal. 
     The invention also provides a method of forming an electromechanical structure onto an integrated circuit comprising the steps of: 
     forming an evacuated cavity in an active device layer of the integrated circuit, and wherein a wall of the cavity or a column within the cavity is substantially perpendicular to the substrate of the integrated circuit and is deformable. 
     The deformation of the wall of the cavity or the column within the cavity is desirably actuated by 
     application of a signal to electrically excite the side wall or column, or 
     having one side of the wall being exposed to a sealed cavity and the other side responsive to ambient pressure such that any changes in ambient pressure effect a deformation of the side wall. 
     The method preferably further comprising the steps of electrically isolating the electrochemical structure from the remaining integrated circuit. 
     By sensing any deformation of the wall resultant from fluctuations in pressure between the ambient pressure and the evacuated cavity, the method of the invention may also be used to form a pressure sensor on an integrated circuit. 
     The application of a signal between the deformable side wall and a second wall of the evacuated cavity, or between a column within the cavity and the wall of the cavity may be used to actuate the deformable side wall or column. This actuation results in the vibration of a wall or column, the characteristics of the vibration being defined by the applied signal and the mechanical dimensions of the vibrating wall or column, and may be used to form an actuator or resonator on an integrated circuit. 
     In accordance with one embodiment of the present invention, a method of forming an evacuated cavity in the active device layer of the integrated circuit comprises the steps of: 
     i) etching a trench in the device layer, the trench being etched in a plane substantially perpendicular to the substrate, 
     ii) filling the trench with a sacrificial material, 
     iii) forming a cover over the filled trench, and 
     iv) etching the sacrificial material previously deposited from the trench, and sealing the trench. 
     The column resonator of the present invention is formed in a plane perpendicular to the surface of the integrated circuit by etching in a pattern that creates a standing silicon column surrounded on all sides by a trench. The trench surrounding the column can be sealed in a similar manner to the previous embodiments to form an isolated column standing in a vacuum cavity supported at the top and bottom only. 
     In this case, a filled isolation trench is formed in the silicon outside the etched trench, and encloses it completely. This allows outside walls of the trench to be electrically biased with respect to the silicon column, allowing the column to be actuated and resulting in the column vibrating mechanically. 
     These and other features and advantages of the present invention will be better understood with relation to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a cross-section through a transducer device in accordance with one embodiment of the present invention, 
     FIG. 2 is a flow chart of method steps used to fabricate the device of FIG. 1, 
     FIGS. 3 a  and  3   b  are sections through a silicon wafer showing the formation of an isolation trench, 
     FIG. 4 is a section through the device of FIG. 3 b  showing a first option for forming a device of the present invention, 
     FIG. 5 is a section through the device of FIG. 3 showing a second option for forming a device of the present invention, 
     FIG. 6 shows the device of FIG. 5 with a deposited protection layer and sacrificial layer, 
     FIG. 7 shows the device of FIG. 6 after the sacrificial layer has been patterned and etched, 
     FIG. 8 shows the device of FIG. 7 with a cap layer formed on the sacrificial layer, 
     FIG. 9 shows the device of FIG. 8 after the etching of the sacrificial layer to form a cavity, 
     FIG. 10 shows the device of FIG. 9 after the cavity has been sealed, 
     FIG. 11 shows the etched and patterned device of FIG. 10, 
     FIG. 12 shows the device of FIG. 11 after the removal of silicon to form an active surface, 
     FIG. 13 shows a second embodiment in accordance with the present invention, 
     FIG. 14 shows a plan view of a third embodiment in accordance with the present invention, 
     FIG. 15 is a further plan view of the device of FIG. 14 showing the application of electrical contracts, 
     FIG. 16 is a section through the device of FIG. 15 along the line  16 — 16 , 
     FIG. 17 is a section through the device of FIG. 15 along the line  17 — 17 , 
     FIG. 18 is a plan view of a modification to the device of FIG. 15, and 
     FIG. 19 is a plan view of a further modification to the device of FIG.  18 . 
     FIG. 20 shows a plan view of a fourth embodiment in accordance with the present invention with trenches open, 
     FIG. 21 is a further plan view of the device of FIG. 20 showing sealed trenches and application of electrical contacts, 
     FIG. 22 is a section through the device of FIG. 21 along the line  22 — 22 , 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, an electromechanical element is described that provides distinct advantages when compared to those of the prior art. The diagrams included show the structure of four embodiments of the invention; in all embodiments described buried oxide silicon on insulator (SOI) wafers are used as starting material. 
     FIG. 1 is a partial perspective view illustrating a cross-section of a transducer  1  in accordance with one embodiment of the present invention. It comprises a mechanical diaphragm  2  formed in a vertical plane substantially perpendicular to a silicon substrate  3 . A sealed evacuated cavity is formed at one side of this diaphragm  2  by sealing the top of an etched trench  4  with a polysilicon layer  5 . The other side of the diaphragm is open to the ambient pressure, with the resultant effect that changes in ambient pressure (with respect to the evacuated reference cavity  4 ) will cause deflection of the diaphragm  2  normal to the vertical plane in which the diaphragm lies. The deflection of the diaphragm  2  can be sensed electrically as a capacitance change between the electrical contact  6  to the diaphragm  2  and the electrical contact  7  to the back plate or as a change in electrical resistance of the diaphragm  2 . The diaphragm  2  can also be electrostatically actuated by applying a signal between the electrical contact  6  to the diaphragm  2  and the electrical contact  7  to the back plate. To electrically isolate this transducer structure  1  from the substrate  3  it may be fabricated on a buried oxide layer  8 . To isolate it from surrounding circuitry, a vertical isolation trench  9  surrounds the diaphragm  2  and the capacitor back plate. Although not illustrated in FIG. 1, it will be appreciated by those skilled in the art that standard integrated circuits can be manufactured in the silicon area  10  outside the isolation trench and connected directly to the transducer. 
     FIG. 2 is a flow chart of method steps used to fabricate the device  1  in accordance with one embodiment of the present invention. FIGS. 3 to  12  illustrate the formation of the device  1  in accordance with the process steps of FIG.  2 . 
     Initially, as shown in FIGS. 3 a  and  3   b , an isolation trench  9  is formed in a silicon device layer  10  (Step  100 ). FIG. 3 a  shows the etching of the silicon  10  to form a trench  11  down to the buried layer  8 . The trench  11  is then filled with an insulating material, such as silicon dioxide, to form the isolation trench  9 . 
     The wafer is then processed, preferably through a standard IC process, to generate circuits in the surrounding silicon  10  (Step  110 ). Typically, the high temperature steps are performed at this stage before the remainder of the sensor processing. Alternatively, the sensor processing can be continued without interruption, in which case step  100  would be followed directly by step  120 . 
     FIGS. 4 and 5 illustrate two possible options for initiating the formation of the diaphragm  2  (Step  120 ). In FIG. 4, the silicon  10  is etched to form both a trench  4  that is used to form the sealed cavity, and the trench that remains open to ambient conditions so as to form the active surface  2  of the diaphragm. FIG. 5 shows a preferred option and embodiment wherein only the trench  4  that forms the sealed cavity is formed at this juncture. The trench that is open to ambient conditions is created at the end of the process. The advantage of this latter option is that, if both trenches are open, it is difficult to remove material from the open trench during subsequent processing. There is however a disadvantage in following this option, in that the opening of the second trench at the end of the process introduces added steps to the process and may result in variations in the diaphragm thickness due to alignment variations. The embodiment of the option of FIG. 5 will be assumed for the remaining process steps. 
     A nitride protection layer  12  is then applied prior to the filling of the trench  4  with a sacrificial oxide layer  13 , as shown in FIG. 6 (and Step  130 ). After the trench  4  is filled, the sacrificial oxide layer  13  is patterned and etched. A partial etchback of the oxide  13  may be used to thin the oxide  13  prior to patterning and etching it, as shown in FIG. 7 (and Step  140 ). 
     In Step  150 , a layer  5  is deposited, patterned and etched to create a “cover” over the trenches. This modification to the device of FIG. 7 is shown in FIG. 8 
     The sacrificial layer  13  of FIG. 8, is etched out of the trench (Step  160 ) and the trench is sealed with a sealing layer  14  creating a vacuum cavity  4 , as shown in FIGS. 9 and 10. It is then necessary to pattern and etch the sealant and protective nitride layers, as shown in FIG. 11 (Step  170 ). 
     If the second embodiment, i.e. the process flow illustrated in FIG. 5, is followed, it is now necessary to remove silicon  10  adjacent the sealed cavity  4  to form the diaphragm active surface  2 , as shown in FIG. 12 (Step  190 ). 
     The description above outlines the general structure. A specific implementation, which is very useful, is to build the structure adjacent to the saw line  15  (FIG. 13) of a silicon chip. This creates a diaphragm at the edge of the chip just inside the saw line  15 . The advantage is that a protective coating (e.g., gel coat  16 ) can be deposited on the chip after packaging without compromising sensitivity. Such coatings are necessary for many pressure sensor applications, and if deposited into an open cavity within a silicon chip, rather than at the edge, would fill the trench and reduce the sensitivity of the sensor. 
     FIGS. 14 to  19  illustrate a third embodiment of the present invention and the same reference numerals will be used with reference to equivalent structures. This third embodiment may be processed using the same process flow steps described above with reference to the description of the manufacture of the first embodiment, the difference being in the application of different masks so as to achieve a variation on the finished structure. 
     As shown in FIG. 14, the isolation trench  9  is formed initially as in the previous descriptions and will serve to isolate the final structure from surrounding circuitry. Then a trench  4  is etched down to the buried oxide in a pattern that results in a column of silicon  17  being formed. Using the sealing process already described, this etched trench is then sealed with a cap  5  to form a vacuum cavity  4  around the silicon column  17 . Standard IC processing techniques are preferably used to realise an electrical contact  18  to the isolated silicon column and an electrical contact  19  to the enclosing silicon  7  inside the isolation trench  9 , as shown in FIG.  15 . 
     FIG. 16 is a cross-section representation of FIG. 15 along the  16 — 16  plane, and FIG. 17 is a cross-section representation of FIG. 15 along the  17 — 17  plane. An electrical signal applied between the electrical contacts  18  and  19  generates an electrostatic field resulting in vibration of the column  17 . 
     In a further refinement, where it would be useful to separate the driving circuit and sensing circuit, FIGS. 18 and 19 illustrate an arrangement in which the isolation trench  9  is used to create two electrically isolated regions  7   a  and  7   b  enclosing the column  17 . Separate isolated contacts  19   a  and  19   b  make connections to these two regions. 
     FIGS. 20 to  22  illustrate a fourth embodiment of the present invention and the same reference numerals will be used with reference to equivalent structures. This fourth embodiment may be processed using the same process flow steps described above with reference to the description of the manufacture of the first embodiment, the difference being in the application of different masks so as to achieve a variation on the finished structure. 
     As shown in FIG. 20, the isolation trench  9  is formed initially previously described with reference to the other embodiments and will serve to isolate the final structure from surrounding circuitry. Trenches  4   a  and  4   b  are the etched down to the buried oxide in a pattern that results in a beam of silicon  20  being formed. Using the sealing process already described, these etched trenches are then sealed with a cap  5  to form two vacuum cavities  4   a  and  4   b  around the silicon beam  20 . Standard IC processing techniques are preferably used to realise an electrical contact  18  to the isolated silicon column and electrical contact  19   a  and  19   b  to the enclosing silicon  7   a  and  7   b  inside the isolation trench  9 , as shown in FIG.  21 . 
     FIG. 22 is a cross-section representation of FIG. 21 along the  22 — 22  plane. An electrical signal applied between the electrical contacts  18  and  19   a  or  19   b  generates an electrostatic field resulting in a vibration of the beam  20 . 
     The structure of the present invention can be used to realise a pressure sensor, i.e., utilising the feature that the deformation of the diaphragm is a function of applied pressure. As the diaphragm deflection increases with applied pressure, the effective dielectric thickness decreases so the capacitance of the structure varies with deflection. This capacitance variation as a function of pressure can be easily measured, thereby implementing a pressure transducer or sensor. 
     This structure may also be applied as an actuator circuit element where an electrical signal drives vibration of the diaphragm. This structure has applications as a resonator in a circuit or to produce ultrasound or other signals, as described in conjunction with the third embodiment. 
     An advantage of the device of the present invention is that only the “edge” of the sensor consumes active surface silicon. This is a very small area compared to the total size of the diaphragm, and therefore it costs very little in terms of silicon area to integrate this sensor onto an integrated circuit. 
     There has been described herein an electromechanical element which is improved over the prior art. It will be apparent to those skilled in the art that modifications may be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except as may be necessary in view of the appended claims.