WO2001060736A2

WO2001060736A2 - Methods and apparatus for a vertically-integrated sensor structure

Info

Publication number: WO2001060736A2
Application number: PCT/US2001/004850
Authority: WO
Inventors: William G. Mcdonald; David J. Monk; Mark Layne Shaw
Original assignee: Motorola, Inc.
Priority date: 2000-02-18
Filing date: 2001-02-15
Publication date: 2001-08-23
Also published as: WO2001060736A3; AU2001241499A1

Abstract

A vertically integrated sensor structure (10) includes a sensor device (50) having a sensing element (54) formed thereon. The sensor device (50) is bonded directly to a control device (42), which itself is bonded to a mounting area (13) of a package (12). The sensing element (54) is electrically coupled to the control circuitry (46), which includes control circuitry (46) configured to receive input signals from the sensor device (50).

Description

METHODS AND APPARATUS FOR A VERTICALLY- INTEGRATED SENSOR STRUCTURE

Background of the Invention

The present invention relates, in general, to semiconductor devices and, more particularly, to solid state sensor devices.

Pressure sensors, accelerometers, and the like are increasingly being used in automotive, industrial, and biomedical applications. Such devices may be used, for example, for monitoring tire-pressure, oil pressure, a -conditioning systems, manifold pressure sensors, airbag systems, and any other application where an environmental condition requires monitoring.

Sensor devices often require calibration and/or signal processing in order to produce a useful signal. Such functional modules, however, are often implemented using device technologies which pose fabrication challenges when attempts are made to integrate these elements (e.g., CMOS signal processing devices) with the sensing elements themselves. Manufacturers typically obviate this problem by connecting a second chip (i.e., with a microcontroller or other control chip) either external to the sensor structure or adjacent to the pressure sensor in a side-by- side configuration.

Side-by-side controller/sensor configurations increase manufacturing cost, and necessarily require a larger package — i.e., a package with a relatively large footprint. As board-space is often very limited (particularly in modern coiταnunication devices), this increase in overall device footprint makes the package prohibitively large for many applications.

Accordingly, it would be advantageous to provide a sensor package which includes a sensor device and a control device vertically integrated in a highly- manufacturable sensor structure. Brief Description of the Drawings

FIG. 1 illustrates an enlarged cross- sectional view of a vertically integrated sensor structure in accordance with the present invention; FIG. 2 illustrates an enlarged cross- sectional view of an alternate embodiment of the present invention; and

FIG. 3 illustrates an enlarged cross- sectional view of another embodiment of the present invention.

Detailed Description of the Drawings

Generally, methods and systems in accordance with the present invention provide a vertically integrated sensor structure which addresses the disadvantages of the prior art. Referring now to FIG. 1, one embodiment of the present invention is shown in an enlarged cross- sectional view. Vertically-integrated sensor structure, or sensor structure, 10 generally comprises a sensor device 50, an adhesive 48, a control device 42, an adhesive or die bond 40, conductive leads 14, bond wires 24 and 26, and a package body 12 having a cover 16 and a cavity 20 optionally filled with a protective gel material, or gel, 22. Sensor device 50 includes a sensor substrate 52 in which or on which is provided a sensing element 54 capable of sensing an environmental state. Sensor device 50 also includes bond pads 30 (only one shown) which allow electrical connectivity to bond wires 24. Such bond pads 30 comprise any suitable conductive material, for example, aluminum, aluminum alloy, or the like. In the illustrated pressure- sensor embodiment, sensing element 52 comprises a diaphragm formed in conjunction with a cavity and suitable electronic components in order to sense variations in absolute pressure. Sensing element 52 may include one or more piezo -resistive and/or capacitive electronic components configured to sense relative movement of a diaphragm and provide an electronic signal suitable for processing by control device 42. In a preferred embodiment, sensor device 50 comprises an absolute capacitive pressure sensor in a wafer of approximately 380-640 microns thick and having lateral dimensions of about 1270x1270 microns. Non-square die, e.g., rectangular die or other die shapes may also be used.

While an absolute pressure sensor is described herein, it will be appreciated that device 50 may comprise a wide range of devices, e.g., accelerometers, gas- sensors, remote keyless entry devices, differential pressure sensors, or the like.

Control device 42 includes a substrate 44 having appropriate control circuitry 46 formed therein and/or thereupon. Control device 42 is configured to receive input signals from sensor device 50 (e.g., through bond wires 24 and 26, or chip-to-chip wires as described further below) and process this input to provide a suitable output (e.g., through leads 14). In this regard, control device 42 may include various, signal conditioning components. Control device 42 also includes bond pads 36 comprising any suitable conductive material (e.g., aluminum).

Control device 42 includes electronic devices and/or integrated circuit devices formed on and/or extending from major surface 45, including, for example, passive and active components integrated to provide, inter alia, signal conditioning, span compensation, temperature compensation, signal amplification, data storage, data processing, load control, and/or a combination thereof. For example, electronic devices may include capacitors, thin film resistors, diffused resistors, diodes, CMOS and bipolar logic devices, insulated gate field effect transistors (IGFET) and/or bipolar transistors functioning as amplifier and/or switching devices, JFET devices, and the like.

Various integrated sensor device circuits may be found in a number of standard references, for example: L. Ristic ed., Sensor Technology and Devices, Artech House

(1994); Sensor Device Data, Motorola Inc., Rev. 4, 1998; U.S. Pat. No. 5,132,559, issued to I. Baskett et al.; "MOS Integrated Silicon Pressure Sensor," LEEE Transactions on Electron Devices, vol. ED-32, no. 7, pp. 1191-1195, July 1985.

In a preferred embodiment, wherein sensor device 50 comprises a capacitive pressure sensor, control circuitry 46 is configured to provide capacitance-to-voltage conversion, calibration, and temperature compensation. The size of control device 42 may vary depending upon the application; however, in one embodiment, control device is lateral dimensions of about 3000x3505 microns and a wafer thickness of about 510 microns.

Control device 42 and sensor device 50 may consist of any suitable substrate material upon which or within which semiconductor devices may be formed. Suitable materials include, for example, group IV semiconductors (i.e., Si, Ge, and SiGe), group III-V semiconductors (i.e., GaAs, InAs, and AlGaAs), and other less-conventional materials, such as SiC, diamond, and sapphire. Control device 42 and sensor device 50 may comprise single crystal material, or may comprise one or more polycrystalline or amorphous epitaxial layer formed on a suitable base material. Adhesive 48 comprises any suitable material for bonding major surface 49 of sensor device 50 to major surface 45 of control device 42. In one embodiment, adhesive 48 comprises a layer of air-cured di-methyl silicone adhesive having a thickness of approximately 75-130 microns. Other materials such as any of the various room temperature vulcanizing (RTV) compounds may also be used. Die bond 40 is used to mechanically affix major surface 47 of control device 42 to a die mounting area 13 of package body 12. In one embodiment, die bond 40 comprises a standard heat-cured epoxy. Depending upon the nature of package body 12, however, a wide variety of die bond materials may be used, including, for example, soft or hard solders (e.g., various lead-tin alloys), gold/silicon eutectics, elastomeric adhesives, silver-filled epoxy, and the like.

Conductive leads 14 provide electrical communication from external circuitry (e.g., printed circuit boards and the like) to sensor device 50 and control device 42. Such leads, which are configured in accordance with the input/output and power requirements of package 10, comprise any convenient conductive material, e.g., copper, Alloy 42, or the like. Conductive leads 14 are suitably secured to package body 12 via any convenient method (i.e., via a molding process).

Bond wires 26 provide a conductive path from leads 14 to bond pads 45, and bond wires 24 provide a conductive path from leads 14 to sensor device 50. A number of wire materials and sizes are appropriate for electrically coupling these components, for example, a standard gold wire having a diameter of about 12 to 40 microns. Package body 12 in the illustrated embodiment corresponds to a standard small outline package (SOP), covered by a cap 16 (having an aperture 18), having a die mounting area 13 defined therein, and optionally filled by a gel 22. It will be appreciated, however, that a variety of other package configurations may also be employed. Indeed, any plastic, ceramic, metal, or other package having a die mounting area 13 which can accommodate a sensor device 50 vertically integrated with control device 42 can benefit from the present invention — for example, an over-molded epoxy package including a localized coating to relieve stress on sensor device 50.

Gel 22 shown in FIG. 1 comprises any convenient material suitable for protecting sensor device 50 and control device 42 from debris, moisture, and the like, while allowing proper functioning of sensing element 54. In one embodiment, gel 22 comprises a conventional fluoro silicone gel.

Having thus described an exemplary vertically integrated sensor package in detail, a method of forming such a package will now be described. It should be understood that the exemplary process illustrated may include more or less steps or may be performed in the context of a larger processing scheme. Furthermore, the illustrated process should not be construed as limiting the order in which the individual process steps may be performed.

First, control device 42 is attached to package body 12 using die bond 40. In the illustrated embodiment, die bond 12 comprises a standard epoxy material which is heat-cured in accordance with conventional techniques.

Next, adhesive 48 is dispensed onto a portion of major surface 45 of control device 42. In a preferred embodiment, adhesive 48 comprises di-methyl- silicone which is dispensed onto surface 45 using known dispensing schemes. Bottom surface 49 of sensor device 50 is then placed in intimate contact with adhesive 48 using any convenient pick-and-place and/or alignment technique.

Adhesive 48 is then optionally cured, for example, through air curing at about 150 C for about one hour. The volume and placement of adhesive 48 depends upon, among other things, the lateral dimensions of sensor device 50 and the desired thickness of adhesive 48. Optimization of such parameters can be readily performed by those skilled in the art.

Next, bond wires 24 and 26 are bonded to their respective bond pads 30 and 36. In a preferred embodiment, wherein bond wires 24 and 26 comprise gold wire, these bonds are produced through thermo-sonic bonding methods. Other techniques, e.g., ultrasonic, thermo -compression, and the like may also be employed depending upon the type of wire used for bond wires 24, the size of bond pads 30 and 36, and other factors known in the art.

In one embodiment, after wire bonding is complete, the cavity 20 of package body 12 is at least partially filled with gel 22, (e.g., fluorosilcone gel). As mentioned above, other materials may be employed for this purpose.

Next, package lid 16 is attached to package body 12 using a suitable adhesive material. As will be appreciated by those skilled in the art, the individual devices — which are typically processed in large groups connected via a common leadframe — will then be singulated (i.e., separated into individual packages 10) using conventional singulation equipment.

General information regarding electronic package technology and techniques such as those described above may be found in a variety of texts, including, for example, Donald Seraphim, Ronald Lasky, and Che-Yu Li, Principles of Electronic Packaging (1989).

Referring now to FIG. 2, an alternate embodiment of the present invention further includes a collar or dam 56 bonded to the top of sensor device 50 in order to substantially prevent gel 22 from contacting sensing element 54. The size and shape of collar 56 that is used may vary depending upon, among other things, the geometry of package body 12, the dimensions and placement of sensing element 54, and the dimensions of devices 42 and 50. Collar 56 is suitably attached to sensor device 50 using a suitable adhesive 57, e.g., a standard heat-cured epoxy.

In yet another alternate embodiment illustrated in FIG. 3, chip-to-chip wires 60 may be used to provide electrical connectivity between sensor device 50 and control device 42. That is, rather than using a pair of wires 24 and 26 connected to a common lead 14 to provide electrical connectivity (as shown in FIG. 1, wherein the sensing element 54 is electrically coupled to the control circuitry 46 with a first bond wire 26 connecting the control device 42 to the conductive lead 14, and a second bond wire 24 connecting the conductive lead 14 to the sensor device 50), a single bond wire 60 is used. It will be appreciated that wire 26 as shown in FIG. 3 is included for illustrative purposes, and that not all bond pads 36 on control device 42 will necessarily be connected via a wire 26 to a lead 14. Furthermore, while two separate wire bonds are shown at bond pad 36, other bonding schemes, e.g., stitch-bonding or the like, may also be employed where connectivity is required from bond pad 30 to bond pad 36 and then to lead 14. While specific embodiments of the invention have been shown and described, further modifications and improvements will occur to those skilled in the art. It is understood that this invention is not limited to the particular forms shown, and that the appended claims cover all modifications of the invention which fall within the scope of the invention.

Claims

1. A method for forming a vertically integrated sensor structure comprising the steps of: forming a package body (12), wherein the package body includes a die mounting area (13); forming control circuitry (46) on a control device (42), wherein the control device includes a first major surface (45), a second major surface (47) opposite the first major surface, wherein the control circuitry is formed on the first major surface, and wherein the control device comprises a semiconductor material; and forming a sensor device (50) having a sensing element (54) formed thereon, wherein the sensor device includes a third major surface, wherein the sensing element is formed opposite the third major surface, and wherein the sensor device comprises a semiconductor material.

2. The method of claim 1 further including attaching the second major surface of the control device to the die mounting area of the package; attaching the third major surface of the sensor device to the first major surface of the control device by at least one bondwire; and electrically coupling the sensing element to the control circuitry.

3. The method of claim 1 or 2, wherein the step of forming the sensing element on the sensor device includes foπning a pressure sensor device.

4. The method of claim 3, wherein the step of forming a pressure sensor device includes fomύng a capacitive pressure sensor.

5. The method of claim 1 or 2, wherein the step of forming the sensing element on the sensor device includes foπning a gas sensor.

6. The method of claim 1 or 2, wherein the step of foπning the sensing element on the sensor device includes forming an accelerometer device.

7. The method of claim 1 or 2 or 3 or 4 or 5 or 6, wherein the step of forming the control circuitry on the control device includes forming circuitry configured to provide temperature compensation of input signals from the sensor device.

8. The method of claim 1 or 2 or 3 or 4 or 5 or 6 or 7, wherein the step of attaching the third major surface to the first major surface includes bonding the third major surface to the first major surface using a silicone adhesive and wherein the step of attaching the second major surface to the die mounting area includes bonding the second major surface to the die mounting area using an epoxy.

9. The method of claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8, wherein the step of electrically coupling the sensing element to the control circuitry includes the steps of: forming a conductive lead; forming a first bond wire between the control device and the conductive lead; and forming a second bond wire between the conductive lead and the sensor device.

10. The method of claim 1 or 2 or 3 or 4 or 5 or 6 or 7 or 8 or 9, further comprising the steps of: attaching a collar to the sensor device; filling, at least partially, the package body with a gel such that the collar substantially prevents the gel material from contacting the sensing element.