This invention relates to solid-state gyroscopes and in particular to methods of assembling solid-state gyroscopes and to gyroscopes whenever assembled by the methods of this invention.

There have been various proposals for solid-state gyroscopes - that is to say, electronic devices which are able to detect angular movement of the device without the device having any moving parts, unlike a mechanical gyroscope. A solid-state gyroscope can exploit the characteristics of piezo-electric materials, by generating a rotating electric field in a piece of such material, and then detecting a change in the stress of the material when the piezo-electric material is subjected to angular movement. For example, there is disclosed in GB2154739A (NRDC) a proposal which employs a cylinder or disc of piezo-electric material which is subjected to a rotating field by electrodes formed on the surface of the cylinder or disc. Such a device is relatively large, complex and so expensive to make and in any event is insufficiently small for general use. Further, a complex mounting arrangement is required mechanically to decouple the resonating sensor from the surrounding environment.

More recently, there have been proposals for a gyroscope having a relatively small disc-shaped piezo-electric sensor with an electrode pattern formed on the surface of the disc. Using micro-fabrication techniques it becomes possible to produce a sufficiently small piezo-electric sensor disc with an electrode pattern able to detect small angular accelerations. It has proved to be very difficult to mount the disc in such a way that the mounting technique does not influence the operation of the device. The usual technique is to provide a central hole in the disc and then to plant a pin-like stem in the hole, such as is described in WO 99/22203 (BTG Int. Ltd.) but such an arrangement is susceptible to out-of-plane resonances.

In WO 03/023323 (European Technology for Business) this has been addressed by providing an internal central earth electrode parallel to the disc surfaces, and exactly-corresponding electrode patterns on the opposed outer faces of the disc. Though this permits bending modes of the disc to be cancelled out, there is a greatly increased complexity and hence cost for the final assembly. In

addition, there are considerable difficulties in mounting the sensor disc on a stem and completing connections to the electrodes on the disc.

The disc of WO 03/023323 has a central hole and a stem extends thereinto, conductors being provided on the pin to connect with the electrodes of the disc. Unfortunately, it has proved almost impossible to produce such a gyroscope on a production basis, at an economic price, in view of the difficulties of connecting the electrodes to the tracks on the stem.

The present invention results from further research into micro-fabricated solid-state gyroscopes using a piezo-electric sensing disc, with the aim of improving the mounting technique so as not to have a significant influence on the performance of the finished device, thereby enabling the mass production of a small, low-cost gyroscope suitable for large volume applications. A particular aim is a mounting arrangement which allows the required electrical connections to be made to the disc as well as providing a mechanical support for the disc, whilst decoupling the resonating disc structure from the surrounding environment.

In its broadest aspect, this invention provides a method of assembling a solid-state gyroscope which comprises the steps of:

- providing a disc of a piezo-electric material;

- forming an electrode pattern on a disc surface, each electrode having a respective termination point in a predefined array in a central region of the disc;

- providing a mounting substrate having a mount surface;

- providing on the mount surface a series of tracks each leading away from a mount area whereat each track has a respective connection point arranged in said predefined array thereby to form with the termination points respective pairs of points;

- furnishing a respective upstanding conductive element on at least one or both of the connection and termination points of each said pair of points;

- accurately aligning the pairs of points of the disc and mounting substrate and then moving the disc towards the mount surface until the conductive elements interconnect the connection points and the termination points; and

- bonding the disc to the substrate with the electrodes electrically connected to the tracks by the conductive elements.

There are two very closely related but distinct species of mounting methods of this invention as defined above. According to the first, a pad of a curable adhesive is applied to one of said mount area of the mount surface or the central region of the disc. The disc is mounted on the substrate with said disc surface facing said mount surface, by pressing the disc towards the mount surface until the adhesive pad is compressed between the disc and the substrate and the conductive elements interconnect the connection points and the termination points. Then the adhesive of the pad is cured or is allowed to cure, thereby bonding the disc to the substrate with the conductive elements electrically connecting the electrodes to the tracks of the substrate.

With this first assembling method of the invention, the disc is mechanically mounted on a mounting substrate by means of the adhesive pad, disposed solely in the central region of the disc. Electrical connections are made to the electrodes of the disc by means of conductive elements which connect between the electrodes and tracks formed on the mount surface.

According to the second mounting method of this invention, no adhesive pad is provided. Rather, when the disc has been moved towards the mount surface sufficiently for the conductive elements to make electrical connections between the respective connection and termination points, each conductive element is bonded to the opposed point or element, as appropriate, of each pair of points, thereby to attach the disc to the mounting substrate solely by the conductive elements with the electrodes in electrical communication with the respective tracks.

With this second assembling method, the disc is secured to the substrate solely by the upstanding conductive elements which (after the assembly has been completed) are attached to both the connection point and the termination point of each pair thereof. As such, there is no need to provide an adhesive pad to hold the disc to the substrate. In turn, this further isolates the disc from the substrate

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and thus assists the mechanical decoupling of the disc from its surrounding environment while still providing the required electrical connections.

In a preferred embodiment, each conductive element comprises one or more conductive balls which can be made very accurately to a required diameter, so ensuring that contact is made simultaneously by all of the balls with the respective connection and termination points, as the disc is pressed towards the mounting surface. In another embodiment, the conductive elements comprise pillars or columns upstanding from the respective points, either formed integrally therewith or subsequently attached thereto. These possibilities will be discussed in greater detail, below.

Though all the conductive elements could be provided on either the termination points or on the connection points, respectively on the disc or the mount surface, it would be possible to provide elements on some (but not all) termination points and also on some (but not all) connection points -for example in each case on alternate points so that on aligning the disc correctly with the substrate and then moving the disc towards the substrate, all of the required connections will be made. Yet another possibility is to provide conducting elements on all of the termination points and also on all of the connection points, so that the ends of those respective conducting elements remote from the points will engage each other in respective pairs on moving the disc and mounting surface towards each other. The preferred technique is however to provide the conducting elements initially on the connection points of the mounting substrate. Whichever method is chosen, at the completion of the assembly the elements should electrically communicate between the opposed connection and termination points, of each pair thereof.

In a case where the conductive elements comprise balls, each termination point (or each connection point, as appropriate) may be furnished with only a single conductive ball or with more than one conductive ball, arranged in a stack such as of two or three such balls, so as to form a column upstanding from the respective point. Each ball typically may have a diameter of about 40μm. Thus, when a stack of three balls is used, the facing surfaces will have an initial separation of about

120μm as the contact with the opposed point is first made, so long as there is no axial compression of the balls. The balls may be secured to the respective points and, if appropriate, to each other by means of one of ultrasonic welding, laser welding, thermal fusion, chemical fusion, brazing, soldering, alloying, metal deposition or other bonding technique which allows conduction through the bond. The current preferred technique is to use a laser welding or thermo-compression bonding process.

Instead of using conductive balls, columns or pillars of a conducting material may be used at each termination or connection point. Such a column or pillar may be separately fabricated and then bonded by an appropriate conducting process to the required point, as with conductive balls as described above. In the alternative, each such column or pillar may be produced during the manufacture of the component (that is, the mounting substrate or the disc) for example by electrolysis or an electroplating process. Such a process may build up a pillar for example of copper, which subsequently may be treated to resist corrosion, for example by flashing with nickel and/or gold. The diameter of each pillar may be in the region of 40μm to 60μm and when complete may upstand above the surface by a height of about 60μm to 80μm. In a preferred method, such pillars are built up on the substrate, during the production of the electrodes thereon, using micro-fabrication techniques.

Once the pillars have been fully formed and the disc is to be mounted on the mount surface, the arrays of points are accurately aligned and the disc is then moved until the pillars interconnect the electrodes of the disc to the tracks on the mount surface. The free end of each pillar (that is, the end not already connected to an electrode or track) is then bonded to the opposed connection or termination point or the free end of an opposed pillar, as appropriate, in such a way that there will be a conductive path through the bond. For this purpose, one of a thermal fusion, chemical fusion, welding, brazing, soldering, alloying, metal deposition or other bonding technique which allows conduction through the bond may be used. The current preferred technique is to use a laser welding or thermo-compression bonding process.

In the case of the first assembling method of this invention, the adhesive pad may be formed of an anisotropic conducting film, for example comprising a film of a curable resin containing randomly-disposed conducting spheres each separated from the others by the resin. Each such sphere may have a diameter of about 5μm, with the spacing between the spheres being of the order of 20μm to 50μm.

Such an adhesive pad may have an initial thickness of slightly greater than the height of the upstand of each conductive element from the respective point, so that the pad is compressed when the conductive elements interconnect the points of each pair thereof. In the alternative, the adhesive pad may have a thickness which is less than the height of each conductive element and sufficient force is applied to the substrate and disc plastically to deform each upstanding conductive element and to reduce the height thereof to less than the initial thickness of the adhesive pad. For example, and in a case where a column of three balls having an initial height of 120μm is used for each conducting element, the adhesive pad may have an initial thickness of about 100μm.

Though the adhesive pad could have an effective size smaller than that of the array of points and so lie therewithin, it is preferred for the adhesive pad to be larger than the array of points so that on completion of the assembly, the upstanding conductive elements are embedded within the adhesive pad itself. In this way, and following completion of the assembly and curing of the adhesive, the disc is held to the substrate by the pad and the electrical connections are assured by the compression of the balls, in conjunction with the conducting spheres contained within the matrix of the adhesive pad. Further, the adhesive pad then protects the connections against external influences such as corrosion.

Conveniently, the termination points and connection points are arranged in a circular array, the array of termination points on the disc being centred on the axis of the disc. The pitch circle of both the circular array of termination points and the corresponding pitch circle of the connection points may be less than 1mm diameter, and preferably about 0.8mm so as to achieve a pitch circle to disc

diameter ratio of at least 1:14 but more preferably 1 :17, for a gyroscope sensor disc having a diameter of about 12mm to 14mm.

The array of termination and connection points may simply be circular. In one embodiment, there is an additional central pair of points, preferably used for an earth connection to the earth electrode of the disc.

It is highly preferred that the sensor disc used in the assembly method of this invention is solid, without a central hole. In turn, this leads to significant operational advantages, by reducing out-of-plane sensitivity and enhancing the disc output for in-plane angular accelerations. As such, the mounting methods of this invention have significant advantages over those of the prior art described above.

The disc is preferably manufactured by a micro-fabrication technique, by depositing a metal layer on the surface of the piezo-electric disc, and then using a photo-etching or screen printing process to define on the disc surface the required pattern of electrodes and connection tracks. Advantageously, the array of electrodes formed on the disc surface comprises a plurality of discrete sectors, each having a radial extent smaller than the radius of the disc and being spaced from the outer periphery thereof. On the disc surface opposed to that on which is formed the electrode array there may be provided an earth electrode which substantially covers that surface. That earth electrode advantageously connects to the substrate by means of a track which extends from the electrode, over the outer edge of the disc, and then between two electrodes on the opposed disc surface to a termination point on the same pitch circle as the other termination points, or for the second assembly method, centrally within that pitch circle. This invention extends to a solid state gyroscope whenever assembled by a method of this invention as described above.

By way of example only, two specific assembly methods of this invention for a solid-state gyroscope will now be described in detail, reference being made to the accompanying drawings, in which: Figure 1 is a plan view on the face of the disc which confronts a mounting substrate, in a completed assembly;

Figure 2 is a side view of the completed assembly including the disc of Figure 1 ;

Figure 3 is a detail view on an enlarged scale of the mounting region of the disc; Figure 4 is a plan view on the face of the substrate on which the disc is mounted;

Figures 5A and 5B show two steps in the mounting of the disc to the substrate for the first method;

Figure 6 is an enlarged side view of a part of the disc, with ball bonds applied thereto; and

Figure 7 is an isometric view of the central region of the disc, ready for mounting on the substrate.

The sensor disc used in this invention is essentially conventional in that it comprises a disc of a piezo-electric ceramic material produced in any suitable manner, for example by a sintering process. Unlike conventional discs, the disc 10 used in this invention is solid - i.e. without a central hole. The disc is formed to be accurately circular and to have exactly parallel radial surfaces on which is deposited a metal suitable for etching into the required electrode pattern by a photo-litho-gravure process such as is used in the production of semiconductor chips.

On one side 11 of the disc 10 there are formed eight separate electrodes 12 each of a general sector shape and each having an individual track 13 leading from the electrode to a respective termination point 14. The termination points 14 together with a ninth termination point 15 are arranged in a circular array, on a common pitch circle centred on the disc axis, equi-spaced around that pitch circle.

A track 16 leads from the ninth termination point 15 between two electrodes 12 to the edge of the disc, over the edge and then to a continuous circular electrode extending over the entire other side 18 of the disc. A pad 19 is provided on the track 16, spaced from the ninth termination point, and serves to assist the alignment of the disc when performing the assembly method.

In this example of the invention, the disc diameter is about 13mm, the disc thickness about 0.5mm, and the common pitch circle of the termination points has a diameter of about 0.8mm. The circularity of the disc as well as its accuracy and concentricity and also the positioning of the electrodes are all critical to minimise errors in the finished device, and hence micro-fabrication techniques are required to achieve these.

A mounting substrate 20 (Figure 4) has a mounting surface 21 having nine tracks 22 formed thereon, each terminating in a connection point 23 arranged on a common pitch circle. The connection points 23 are disposed in exactly the same array as are the termination points 14,15 of the disc. Each track 22 leads from its central termination point to an edge region of the substrate, whereat a conductor may be connected thereto, for example by soldering wires to the tracks. The mounting of the disc on the substrate by the first preferred method will now be described. The electrical connections to the disc are integrated with the mechanical mounting of the disc on the substrate, to provide both the required mechanical strength and also to minimise the interference with the sensor operation by the electrical connections. The steps to achieve this mounting are: (a) - A first layer of 40μm diameter balls of a conducting metal (so-called "ball bonds") 24 are secured by ultrasonic welding, one to each termination point of the disc, and then second and third layers of similar balls 24 are secured by a similar process to the applied first layer, as shown in Figure 6.

(b) - A circular glue patch (or pad) 25 is applied to the substrate over the connection points and centred on the pitch circle thereof. The glue patch comprises an anisotropic conducting film of a curable resin having an initial thickness of slightly greater than about 100μm and containing randomly-disposed conducting spheres 26 each separated from the others by the resin. Each such sphere has a diameter of about 5μm, with the spacing between the spheres being of the order of 20μm to 50μm. The patch is of less than 1mm diameter and typically about 0.8mm diameter.

(c) - The disc is carefully aligned with the substrate to ensure the termination and connection points respectively on the disc and substrate are in near-perfect registration (Figure 5A) and then the disc is pressed down on to the substrate so that there is a pre-determined gap between underside 11 of the disc and mounting surface 21 of the substrate. That gap is such that the ball bonds are brought into electrical connection with the connection points on the substrate, with the spheres 26 within the glue patch 25 completing the connections as shown in Figure 5B. The adhesive pad being compressed slightly during this step.

(d) - The glue is left to cure, so leaving the disc held by the cured glue to the substrate, with the termination points in electrical communication with the connection points of the substrate and the glue enveloping the ball bonds 24.

(e) - Connections for the driving and sensing circuitry (not shown) are made to the tracks 22 on the substrate and the whole assembly is then packaged in a suitable manner for the intended application. In a typical assembly, the disc diameter d1 (Figure 2) is 13mm and the diameter d2 of the circular adhesive patch is 800μm. The size of each termination and connection point is 100μm, and the spacing between the disc and the mount surface of the substrate is also 100μm, when the disc is fully mounted and the adhesive is cured. The alignment of the termination and connection points is better than 10μm, and the separation between the disc and the mount surface is maintained to within 5μm across the entire disc surface. Further, the mechanical joint over the entire area of the glue patch 25 is uniform, with the nine connections having nominally the same mechanical and thermal stability as the non-conductive medium inter-dispersed between the electrical connections. The second preferred assembly method is very similar to that described above but omits the use of the circular glue patch 25. Figure 7 shows the central region of the mount surface of the substrate 30 and it can be seen that there are eight connection points 31 in a circular array, each connection point being at the radially inner end of a respective track 32 formed on the substrate 30. Upstanding from each connection point 31 is a pillar 33, formed integrally with the track 32 at the time of manufacture of the substrate, for example by an electroplating

operation which builds up the pillar in copper. Each pillar 33 is first flashed with nickel and then gold.

A star-shaped conductor 34 is also provided on the substrate, this conductor having tracks 35 extending between each pair of tracks 32 and all radiating from a central region 36 on which is provided a circular earth stud 37, centred on the circular array of connection points 31. This earth stud 37 may be built up at the same time and in the same manner as the pillars 33, or could be separately formed and then bonded in a conducting manner to the central region 36 of the conductor 34. The faces of the pillars 33 and of the stud 37 remote from the substrate 30 lie in a common plane parallel to the upper surface of the substrate, with a high degree of accuracy, to permit a disc (not shown, but similar to that of Figure 1) having correspondingly formed termination points to be connected thereto, with all of the required connections made simultaneously. A stack of conducting balls as described above may be used as an alternative to the pillars 33, as shown in Figure 6. If a stack of balls are used in this way, the balls of each stack are secured together for example by means of a laser welding technique which permits the highly localised application of heat to form a conducting bond between the balls and the termination points, or by a thermo- compression technique.

The assembly is completed by accurately aligning face 11 of the disc with the substrate and then moving the disc to engage the pillars on the substrate with the termination points of the disc. A laser welding process or a thermo- compression process is performed to connect the ends of the pillars remote from the substrate to the respective termination points of the disc. Simultaneously, the face of the earth stud 37 is also welded to a corresponding termination point on the disc, which termination point is connected to the earth conductor on the face of the disk opposed to that provided with the sector-shaped electrodes.

The second assembly method mounts the disc on the substrate solely by means of the pillars 33 and the stud 37, though depending upon the mechanical constraints of the assembly, the earth stud 37 could be replaced by a further pillar

arranged on the same pitch circle as the connection points 31, much as has been described above with reference to Figure 1. The need for an adhesive pad to provide the required mechanical support for the disc is obviated and the pillars (and optionally the stud) serve the combined purpose of effecting the electrical connections to the electrodes of the disc and also of supporting the disc.

In both methods, the disc is solid and so without a central hole. Tests have shown that by providing such a solid disc, highly advantageous results can be obtained, with enhanced sensitivity in the in-plane mode and reduced sensitivity in the out-of-plane mode. By effecting the mechanical mounting of the disc solely in the central region thereof but by omitting the central hole as used in prior embodiments, excellent decoupling from the surrounding environment can be achieved.