Field of the Invention The present invention relates to flexures created in silicon wafers and the like, to produce structures that can be used in micromachined sensors such as accelerometers.

Background of the Invention In one well known type of aceelerometer design, a proof mass is connected to a support by one or more flexures that permit the proof mass to rotate in response to an acceleration along a predetermined sensitive axis. In a conven¬ tional aceelerometer, flexures may be fabricated by mechanically thinning a portion of a blank from which the proof mass is cut. However, in a micromechan- ical aceelerometer of the type that is formed from a single crystal of silicon, the flexures must generally be created by etching techniques. Because of the relative inflexibility of etching processes, as compared to mechanical formation, the options for creating flexures in a silicon micromechanical device are somewha limited. One known technique is to create an epitaxial layer on one surface of silicon wafer, and then etch an opening in the opposite wafer surface, using th epitaxial layer as an etch stop. Although this a highly reproducible technique, i suffers from the fact that the effective hinge axis of the flexure is located in th plane of one surface of the wafer, rather than at a point midway between th wafer surfaces. As a result, the effective sensitive axis of the aceelerometer i not parallel to the wafer surfaces, and alignment of the instrument with an exter nal housing is thereby made more difficult. Summary of the Invention

The present invention provides novel flexures, and a novel method for creat ing flexures, particularly adapted for use in silicon micromechanical devices. I

an aceelerometer, the technique of the present invention permits the creation of a flexure system having an effective hinge axis midway between the surfaces of the silicon wafer.

In one aspect, the present invention provides an improvement for a sensor fabricated from a wafer having upper and lower surfaces, the sensor being of the type comprising a first member pivotally connected to a second member. The improvement comprises first and second flexures connecting the first member to the second member, the flexures permitting rotation of the second member with respect to the first member about a common hinge axis. The first flexure is connected to the first member at a position closer to the upper than to the lower wafer surface, and is connected to the second member at a position closer to the lower than to the upper wafer surface. The second flexure is connected to the first member at a position closer to the lower than to the upper wafer surface, and is connected to the second member at a position closer to the upper than to the lower wafer surface. The result is a highly stable "crossed flexure" arrange¬ ment.

In a second aspect, the present invention provides a method for processing a wafer having upper and lower surfaces, so as to produce a first wafer portion flexurally connected to a second wafer portion. The method comprises etching first and second grooves in the upper and lower wafer surfaces, respectively, the first and second grooves having respective first and second longitudinal axes. The step of etching the first groove comprises removing wafer material on one side of a first plane that is parallel to the first longitudinal axis, and that intersects the upper surface of the. wafer at an acute angle. The step of etching the second groove comprises removing wafer material on one side of a second plane that is parallel to the second longitudinal axis, and that intersects the lower surface of the wafer at an acute angle. The positions and depths of the first and second grooves are selected such that a section of the wafer between the first and second planes forms a flexure connecting the first and second wafer portions to one another. This method can be extended to create crossed flexures of the type described above.

Brief Description of the Drawings FIGURES 1-4 illustrate the fabrication of a single flexure in accordance with the present invention. FIGURES 5-8 illustrate the formation of a crossed flexure pair, in accor¬ dance with the present invention.

FIGURES 9 and 10 illustrate an aceelerometer formed using two crossed flexure pairs.

Detailed Description of the Invention

The general technique of the present invention can be described with refer- ence to FIGURES 1-4. FIGURE 1 presents a side or cross-sectional view of a silicon wafer 12. Wafer 12 includes upper surface 14 and lower surface 16. Upper surface 14 is covered with a masking (e.g., silicon dioxide) layer 20, and lower surface 16 is similarly provided with a masking layer 22. It is assumed that upper surface 14 and lower surface 16 have 100 orientations, i.e., that these surfaces are parallel to the 100 crystal plane.

In a preferred embodiment, the method of the present invention proceeds by creating rectangular openings 24 and 26 in masking layers 20 and 22, respectively, as shown in FIGURES 2 and 3. Opening 24 is aligned with a central longitudinal axis 30, and opening 26 is aligned with a central longitudinal axis 32. The longitu- dinal axes are parallel to one another and to wafer surfaces 14 and 16. FIGURE 3 shows axis 32 in phantom, but does not show opening 26, to simplify the illustra¬ tion. Axis 32 is offset from axis 30 in a lateral direction, perpendicular to axes 30 and 32 and parallel to the wafer surfaces 14 and 16. Axes 30 and 32 are prefer¬ ably aligned parallel to the 110 crystal plane, which plane is typically defined by a flat portion or notch on the silicon wafer.

The next step in the illustrated embodiment is to etch the upper and lower surfaces of wafer 12 using an anisotropic etchant. As a result, as shown in FIGURE 4, V-shaped grooves 40 and 42 are formed in the upper and lower surfaces of wafer 12. The outer groove edges, i.e., the upper edges of groove 40 and the lower edges of groove 42, are aligned with openings 24 and 26 in masking layers 20 and 22, respectively. Each groove consists of four sidewalls, each sidewall extending along a 111 crystal plane that forms an angle of about 55° with the wafer surface in which the groove is etched. Thus each of grooves 40 and 42 has a V-shaped cross section in a plane normal to its longitudinal axis. The original positions and sizes of openings 24 and 26 are selected such that when grooves 40 and 42 have been etched, sidewall 44 of groove 40 and sidewall 46 of groove 42 cooperate to form flexure 50 between the grooves. Flexure 50 preferably has a uniform cross section along the direction parallel to longitudinal axes 30 and 32. The flexure connects first portion 52 and second portion 54 of wafer 12, such that portion 52 can rotate, relative to portion 54, about hinge axis 56 that is parallel to, and spaced midway between, the longitudinal axes 30 and 32 of respective

openings 24 and 26 (FIGURE 2). Hinge axis 56 is thus positioned midway between wafer surfaces 14 and 16. As a result, when the technique shown in FIGURES 1-4 is used to form a proof mass and associated flexure for an aceelerometer, the sensitive axis of the aceelerometer will be perpendicular to surfaces 14 and 16. The production of a crossed flexure pair according to the present invention is illustrated in FIGURES 5-8. Referring initially to FIGURES 5 and 6, wafer 62 includes upper surface 64 and lower surface 66 that in turn include masking layers 70 and 72, respectively. Masking layer 70 includes composite opening 80, shown in bold outline in FIGURE 5, that consists of side opening 81, central open- ing 82 and side opening 83. Similarly, lower masking layer 72 includes composite opening 90, shown in phantom outline in FIGURE 5, that includes side opening 91, central opening 92 and side opening 93. In the plane of surfaces 64 and 66, central openings 82 and 92 are coextensive with one another, and are symmetrical with respect to axis 96 that will become the hinge axis of the flexure. Side opening 81 has its rightmost edge aligned with the rightmost edge of central opening 82, but has a left edge that is displaced inwardly in comparison to the left edge of open¬ ing 82, towards axis 96. As a result, side opening 81 is somewhat offset to the right of axis 96. Side opening 91 in lower masking layer 72 is offset a similar distance to the left of axis 96, such that side openings 81 and 91 have an align- ment, relative to one another, similar to the alignment of openings 24 and 26 shown in FIGURE 2. Side openings 83 and 93 have a similar relative alignment, except that in the case of this pair of openings, side opening 83 in upper masking layer 70 is positioned to the left with respect to side opening 93 in lower masking layer 72. The result of etching the structure shown in FIGURES 5 and 6 with an aniso- tropic etching, similar to that used above in connection with FIGURES 1-4, and of then removing the masking layers, is shown in FIGURES 7 and 8. Etching through side openings 81 and 91 has produced V-shaped grooves 100 and 102 having respec¬ tive sidewalls 104 and 106 between which flexure 108 has been formed. Etching through side openings 83 and 93 has produced V-shaped grooves 110 and 112 having respective sidewalls 114 and 116 between which flexure 118 has been formed. Etching through central openings 82 and 92 has proceeded entirely through wafer 62, to create opening 120 between the flexures.

As with the embodiment of FIGURES 1-4, flexures 108 and 118 permit relative rotation of first and second portions 124 and 126 of wafer 62 about a hinge axis 128 that passes through the midpoint of the flexures, and that is

positioned halfway between wafer surfaces 64 and 66. In addition, the fact that flexures 108 and 118 cross one another, as viewed in FIGURE 7, produces a highly stable structure that eliminates flexure S-bending and sag, while maintaining the angular compliance of a single flexural pivot. FIGURES 9 and 10 illustrate an aceelerometer formed using flexures formed in accordance with the present invention. The aceelerometer includes proof mass 130 that is suspended from support 132 by crossed flexure pairs 134 and 136. A first dual vibrating beam force transducer 140 is formed at upper surface 142 of the wafer, while a second identical force transducer 144 is formed ° at lower surface 146 of the wafer. As a result, the effective hinge axis 150 of the proof mass lies midway between the upper and lower surfaces 142 and 146, as well as between force transducers 140 and 144. The flexures may be etched from a single crystal of silicon, in a manner similar to that described above in connection with FIGURES 1-8. 5 While the preferred embodiments of the invention have been illustrated and described, variations will be apparent to those skilled in the art. Accordingly, the scope of the invention is to be determined by reference to the following claims.