Abstract:
An accelerometer device having a proof mass, a support base, a hinge that flexibly connects the proof mass to the support base, a double-ended fork (DETF) having two tines. The tines are made of only piezoelectric material. A plurality of electrode surfaces surround at least portions of the tines for inducing electric fields at the first tine is opposite a direction of the induced electric field at the second tine at similar locations along a longitudinal axis of the tines. This causes the tines to resonate in-plane and out of phase.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Conventional double-ended tuning forks (DETFs) have metallization on the tines, which is used for receiving the actuation signal. Stress-relief, plastic deformation and hysteresis are inherent in the metallization components. In high-performance accelerometers the inherent features of the metallization on the tines provides a source of error in the sensed signal. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention provides an accelerometer device having a proof mass, a support base, a hinge that flexibly connects the proof mass to the support base, one or more double-ended forks (DETF) having two tines. The tines are made of only piezoelectric material. A plurality of electrode surfaces surround at least portions of the tines. 
         [0003]    In one aspect of the invention, the electrode surfaces include a bottom section located adjacent a first side of the tines, a post received between the tines, a side section located adjacent a second side of the first tine and a second side of the second tine, the side section being planar with the tines and a top section. Each of the sections and post include a plurality of electrodes. The metalized sections and post induce an electric field between the top section and the bottom section, between the side section and the top section, between the post and the bottom section, and between the post and the top section. 
         [0004]    In another aspect of the invention, a direction of the induced electric field around the first tine is opposite a direction of the induced electric field around the second tine at similar locations along a longitudinal axis of the tines. This causes the tines to resonate in-plane and causes the tines to resonate out of phase. 
         [0005]    In still another aspect of the invention, the metalized sections along a longitudinal axis of each of the sections and the post include three separate metalized subsections, two of the metalized subsections comprise the same electrical charge. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0007]      FIG. 1-1  illustrates a side view of an accelerometer that includes a pair of double-ended tuning forks (DETFs) formed in accordance with an embodiment of the present invention; 
           [0008]      FIG. 1-2  illustrates a perspective view of a portion of an electrodeless DETF formed in accordance with an embodiment of the present invention; 
           [0009]      FIG. 1-3  illustrates a cross-sectional view of the DETF shown in  FIG. 1-2 ; 
           [0010]      FIG. 1-4  illustrates a partial cross-sectional top view of the DETF shown in  FIG. 1-2 ; 
           [0011]      FIGS. 2-1  through  2 - 3  illustrate cross-sectional views of stages in a process for forming a bottom portion of an exemplary DETF; 
           [0012]      FIGS. 3-1  and  3 - 2  illustrate cross-sectional views of stages in a process for forming the tines and side electrode sections of an exemplary DETF; 
           [0013]      FIGS. 4-1  and  4 - 2  illustrate cross-sectional views of stages in a process for forming an optional top portion of an exemplary DETF; 
           [0014]      FIG. 5  illustrates a planar view of the underside of the top layer of the DETF shown in  FIG. 1-2 ; 
           [0015]      FIG. 6  is a view of a DETF in an exaggerated resonance position; and 
           [0016]      FIGS. 7-1  and  7 - 2  illustrate electric field flow within the tines of the double-ended fork of  FIG. 6  at various locations along the length of the tines. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 1-1  illustrates a side view of an accelerometer  12  that includes a proof mass  16  that is flexibly attached to a base support structure  18  via a hinge  14 . Connected on top and bottom surfaces of the proof mass  16 , between the proof mass  16  and the support structure  18 , are double-ended tuning forks (DETFs)  20  formed in accordance with an embodiment of the present invention. Not shown in  FIG. 1-1  is a housing structure that surrounds the accelerometer  12  and is attached to the support structure  18 . 
         [0018]    The DETFs  20  are made of only a piezoelectric material, such as quartz (SiO 2 ), Berlinite (AlPO 4 ), gallium orthophosphate (GaPO 4 ), thermaline, barium titanate (BaTiO 3 ), or lead zirconate titanate (PZT), zinc oxide (ZnO), or aluminum nitride (AlN), etc. The electrodes are formed within the material that surrounds the tines of the DETF  20 , thereby causing the tuning forks to resonate. The electrodes adjacent to the tines will see the field generated by the motion of the tines at the frequency at which they are resonating. 
         [0019]      FIG. 1-2  illustrates a perspective view looking at one end of the DETF  20 . The DETF  20  includes two tines  46 ,  48  that are attached at either end to pads  50  that are connected to either the proof mass  16  or the support structure  18  indicated in  FIG. 1-1 . Located below the tines  46 ,  48  is a base electrode layer  26  that includes a center post (shown in  FIG. 2-3 ) that is received between the two tines  46 ,  48 . In the same plane as the tines  46 ,  48  is a middle electrode layer  28 . Located above the tines  46 ,  48  is a top electrode layer  30  that is attached to at least the center post and may also be attached to the middle electrode layer  28 . Surrounding at least portions of the tines  46 ,  48  on the bottom layer  26 , the center post, the middle layer  28 , and top layer  30  are electrodes that have been applied to the surfaces of the layers  26 ,  28 ,  30  and the post adjacent the tines  46 ,  48 . Electrodes  60 - 66  are located on the inner surface of the top layer  30 . The application and location of the electrodes are described in more detail below. The electrode layers attach to the support structure  18  either above, below, or separately from the pads  50  of the accelerometer  20 . In one embodiment, the layers are attached via direct fusion bonding to achieve the best available expansion coefficient matching. In one embodiment, the layers are attached using braze materials or epoxies. Charges are sent to the electrodes from an attached electronics via wirebonding and metallization patterns applied to the electrode support structures. 
         [0020]      FIG. 1-3  illustrates a cross-sectional view of the DETF  20 . The tines  46 ,  48  are located on the same plane as the middle electrode layer  28 . A consistent gap exists between the tines  46 ,  48  and the middle layer  28 , the bottom electrode layer  26 , and the top electrode layer  30 . The cross-sectional shape of the tines  46 ,  48  may be various shapes, such as a rectangle. 
         [0021]      FIG. 1-4  shows a top partial cross-sectional view of the DETF  20 . The center post  34  protrudes between the tines  46 ,  48 . A gap is formed between the center post  34  and the tines  46 ,  48  that is approximately equal to the gap formed between the edges of the middle layer  28  as well as the top and bottom layers  26 ,  30 . 
         [0022]      FIGS. 2-1  through  2 - 3  illustrate an exemplary process for forming the bottom layer  26 . The process begins with a block of quartz that is etched to produce a post  34 , see  FIG. 2-2 . Then, as shown in  FIG. 2-3 , the etched piece from  FIG. 2-2  is masked and etched to produce gaps located around the post  34 . These gaps are sized in order to later receive the tines  46 ,  48  with proper lateral and vertical gaps between the tines  46 ,  48  and the surface. Then, metallization is applied to the bottoms of the grooves that were just etched, as well as to the sides of the post  34 , applied using standard metallization techniques. 
         [0023]      FIGS. 3-1  and  3 - 2  illustrate a side cross-sectional view for forming the tines  46 ,  48  and the middle electrode layer  28  out of a single layer of quartz. A masking and etching process is performed in order to etch away the material between the tines  46 ,  48  and the area outside of the tines  46 ,  48  to provide the proper gap between the tines  46 ,  48  and the side walls of the middle electrode layer  28 . Next, metallization is applied to the side walls of the middle electrode layer  28  adjacent the tines  46 ,  48 . Metallization is not applied to either of the tines  46 ,  48 . In one embodiment, the tines  46 ,  48  are first deflected away from the adjacent side walls, then the metallization is applied to those side walls without getting any material on the tines  46 ,  48 . In one embodiment, a pressure is applied to the tines  46   48  in order to deflect them away from the side walls to be metalized. In another embodiment, a very thin shadow mask is placed over the tines  46 ,  48  in order to shield them during the metallization process. 
         [0024]      FIGS. 4-1  and  4 - 2  illustrate an etch and metallization that have occurred from a substrate of quartz to form the top electrode layer  30 . A groove is etched into the substrate. The material of the top electrode layer  30  is similar to the material used in the other two layers  26 ,  28 . The width of the groove is equal to the width between the side walls of the middle electrode layer  28  and the vertical, outer walls of the gap formed in the lower layer  26 . Electrodes (metalized areas  60 - 70 ) are then deposited in the formed groove. 
         [0025]      FIG. 5  is a top x-ray view of the top electrode layer  30 . On the underside of the top section  30  are six sections of metalized areas  60 - 70 . Three of the metallization areas  60 ,  62 , and  68  share a common axis that is approximately directly over a center axis of the first tine  46 . The other three metallization areas  64 ,  66 , and  70  also share an axis that is approximately located directly over a center axis of the second tine  48 . 
         [0026]    The first and last metallizations  60 ,  68  that are above the first tine  46  and the center metallization  66  that is above the second tine  48  are configured to hold the same charge. The center metallization  62  over the first tine  46  and the first and last metallizations  64  and  70  over the second tine  48  have the same charge, that is, opposite polarity to the charges on the other metallizations  60 ,  66 ,  68 . Not shown on the top electrode layer  30  are circuit traces that connect the metallizations of like charge. Also not shown are traces that lead off of the top electrode layer  30  to sources that provide the charge associated with each group of the metallizations. The surface of the bottom layer  26  that faces the bottom surface of the top layer  30  includes a metallization pattern comparable to the metallization pattern on the top layer  30 . The bottom layer  26  also includes electrical traces for electrically connecting similarly charged traces. 
         [0027]    The walls of the middle layer  28  that are adjacent the tines  46 ,  48 , as well as the walls of the post  34  adjacent the tines  46 ,  48 , include metallization patterns that will hold an electrical charge that is opposite the charge held by the metallizations included in the top and bottom layers  26  and  30 . Also not shown within the middle layer  28  and the post  34  are electrical traces for linking the metallizations to a source of electrical charge. This produces electric fields within the tines  46 ,  48 , as shown in  FIGS. 7-1  and  7 - 2 . 
         [0028]      FIG. 6  illustrates an exaggerated image of a DETF  80  that has been induced to resonate thus deflecting the tines in an out-of-phase manner. The tines are deflected because of the forces produced by the piezoelectric material of the tines. The piezoelectric material either wants to contract or expand based on the direction of an electrical field through the tines. At approximately a center of the tines an elongation force is induced within the tines, due to the production of the electric fields between the metallizations surrounding the tines, as shown in  FIG. 7-1 . The tines alternately expand outward and inward as a function of the polarity of the applied voltages. If the voltage on the bottom electrode is positive and the voltages on the side electrodes are negative, the tine deflects to the left. Similarly, if the voltage on the bottom electrode is negative and the voltages on the side electrodes are positive, the tine deflects to the right. The electric field alternates between the electrodes in order to change a peizoelectric force between expand and contract. 
         [0029]    At upper and lower sections of the tines it is desired to have either a contracting or nonelongating force induced within the piezoelectric material of the tines. Therefore, as shown in  FIG. 7-2 , the electrical field induced within the tines is opposite of that induced within the tines at the center location ( FIG. 7-1 ). 
         [0030]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.