Many types of systems use sensors to detect the value of a property of a physical system and to generate a corresponding electrical signal representative of the sensed value. The electrical signals from sensors are commonly provided to electrical circuitry off chip or in adjacent locations on chip in order to enable a desired function to be performed. For example, inertial guidance systems use gyroscopes and accelerometers to monitor the orientation of an object in flight and the direction and magnitude of forces experienced by the object. The gyroscopes and accelerometers rely on electromechanical sensors that translate particular types of motion, such as rotational or linear acceleration, into corresponding electrical signals.

It has become possible to fabricate very small or micromechanical sensors out of silicon wafers of the type used in the manufacture of integrated circuits.

Micromechanical sensors are comparable in size to integrated circuits (IC's). During fabrication of micromechanical sensors, silicon wafers containing numerous sensors are diced into individual sensors, and the sensors are individually mounted into packages of the type used for integrated circuits, such as sealed leadless ceramic chip carriers. The packaged sensor is then mounted on a printed circuit board along with an associated control circuit, usually an IC itself and packaged in a similar manner. The sensor and control circuit, along with other circuit components as required, collectively perform the desired sensing function.

One of the drawbacks of the above-described method of packaging sensor-based systems is that individual unprotected sensors must be handled during packaging operations, potentially reducing yield and increasing manufacturing costs. Also, the sensor package is generally significantly larger and more expensive than the sensor, so the package contributes notably to the dimensions and cost of the assembled printed circuit board. Also, mounting the sensor in a package such as a chip carrier limits how close the sensor can be placed to the control integrated circuit, which in turn can unnecessarily limit the electrical performance of the sensor-based system or increase the susceptibility to noise.

It would be desirable to improve the cost, size, and performance of systems employing micromechanical sensors and associated control circuitry.

BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, a method of fabricating and packaging micromechanical sensors and their associated control circuitry is disclosed that eliminates the need for a separate carrier package for the sensor. The disclosed packaging method advantageously eliminates the handling of exposed sensors during packaging operations, and results in closer placement of the sensor and its associated control circuit, so that costs are reduced and greater system performance can be achieved.

In the disclosed fabrication method, a micromechanical sensor is fabricated on a semiconductor wafer, and a control circuit is fabricated on another semiconductor wafer. A cavity is etched on the back side of the control circuit wafer, the cavity being formed such that the raised portion of a sensor fabricated on the other wafer fits within the cavity when the wafers are brought together in an adjoining relationship.

Through-holes are etched through the back side of the control circuit wafer to allow access to electrical contact points of the sensor circuit, and a patterned layer of metal is deposited to form electrical interconnections between the electrical contact points and termination points on the back side of the control circuit wafer via the through-holes. The termination points are arranged such that electrical contacts of the sensor contact the termination points when the wafers are placed in the adjoining relationship. The wafers are then cleaned and bonded together in the adjoining relationship. In a typical process the respective wafers contain multiple sensors and control circuits, respectively, and thus the bonded wafers are diced to yield individual bonded sensor-circuit pairs. The bonded pairs are then packaged in a known manner.

The fabrication method eliminates the need for

separate packages for the sensor and control circuit.

Also, processing is done at the wafer clean room level so that the handling of individual exposed sensors is eliminated. The sensor and control circuit in each bonded pair are placed in intimate contact, thus reducing overall size and improving electrical performance, and the pairs can be packaged using conventional integrated- circuit packaging technology.

Other aspects, features, and advantages of the present invention are disclosed in the detailed description which follows.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING Figure 1 is a flow diagram of a sensor packaging method according to the present invention; Figures 2 through 5 are schematic depictions of a control-circuit wafer at various stages in the packaging method of Figure 1 ; and Figure 6 is a schematic depiction of a bonded sensor and control circuit pair resulting from the packaging method of Figure 1.

DETAILED DESCRIPTION OF THE INVENTION In the process of Figure 1, an array of control circuits are fabricated on a semiconductor wafer at step 10 using conventional integrated-circuit fabrication techniques. The resulting control circuits may be completely custom-designed, or may be of the type known as"application-specific integrated circuits"or ASICs.

The layout of the array of control circuits mirrors that of a corresponding array of sensors on a separate wafer, so that when the wafers are placed in an aligned adjoining relationship each control circuit is aligned with a corresponding sensor. This arrangement facilitates a subsequent fabrication step in which the wafers are bonded together, as described below.

Figure 2 shows a site on a wafer 30 where a single control circuit 32 is formed. Figures 2 through 6 show the results of steps from the process of Figure 1 at the site for a single sensor and control circuit pair.

However, it is to be understood that the same results are obtained for each sensor and control circuit pair from the two wafers being processed, so that a complete picture of the process results would simply show an array of identical sites on the wafers.

In step 12 of Figure 1, the upper portions of the control circuit wafer 30 are removed by known mechanical and/or chemical means, resulting in a thinned wafer 30 as shown in Figure 3 In step 14, through-hole vias 34 are etched in the backside of the control circuit wafer 30 to allow access to sensor interconnection points or metallizations 33 from the backside of the control circuit wafer 30. In step 16, a second etch process is conducted to form cavities 36 for the sensors on the backside of the control circuit wafer. The results of these etching steps are shown in Figure 4.

In step 18, the back side of the control circuit wafer 30 is metallized with a patterned layer 38 of metal to create electrical interconnections in the previously- etched vias to electrical input and output points of the control circuit. The interconnections extend to include contact pads on the back side of the control circuit wafer as a part of layer 38. Corresponding contact points 39 on the sensors for input and/or output signals (I/O) are pressed against the contact pads during subsequent bonding of the wafers to complete the electrical interconnection between them. The result of the metallization step 18 is shown in Figure 5.

In step 20, the control circuit wafer 30 and a sensor wafer 40 are cleaned under high vacuum conditions.

While this high vacuum is maintained, the wafers are

bonded together in step 22 using heat and/or pressure in known bonding techniques. The result of the bonding is a bonded component as shown in Figure 6 including a control circuit 32 and an associated sensor 42 interconnected with the control circuit via pads in layers 38 that contact the sensor contacts 39 and disposed in the etched cavity 36 on the back side thereof.

In the remaining steps 24 and 26, the bonded wafers are diced and the individual bonded components are packaged in an integrated circuit package such as a leadless chip carrier. The packaged component can then be mounted on a printed circuit board or other circuit substrate that provides power inputs and receives instrument outputs as required by the system in which it is used.

While the above description is of a preferred embodiment and best mode, the sensor circuit and control circuit can be placed both on outer wafer surfaces, or the control circuit placed in a back side cavity of the sensor wafer.

A method of fabricating and packaging micromechanical sensors and their associated control circuits has been shown. It will be apparent to those skilled in the art that modification to and variation of the above-described methods and apparatus are possible without departing from the inventive concepts disclosed herein. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.