WO2003049012A2

WO2003049012A2 - Packaging solution, particularly for fingerprint sensor

Info

Publication number: WO2003049012A2
Application number: PCT/NO2002/000467
Authority: WO
Inventors: Jon Nysaether
Original assignee: Idex Asa
Priority date: 2001-12-07
Filing date: 2002-12-06
Publication date: 2003-06-12
Also published as: NO20016008D0; WO2003049012A3; NO316776B1; AU2002364494A8; NO20016008L; AU2002364494A1

Abstract

Sensor device for performing measurements on an at least partially conductive surface comprising electronic circuitry having a number of interogation electrodes for measuring impedance between the electrodes and a power supply, the device having an outer surface for contact with the at least partially conductive surface comprising a number of outer conductors stretching from the outer surface having an outer an an inner end, said outer ends being adapted to electrical coupling to the conductive surface and said inner ends of said outer conductors being coupled to the interrogation electrodes through a shunt impedance, said outer conductors being mutually separated by an insulating material.

Description

Packaging solution, particularly for fingerprint sensor

This invention relates to a sensor device for performing measurements on an at least partially conductive surface. More specifically it relates to a packaging solution for an essentially linear, AC impedance based fingerprint sensor. An example of such a sensor is presented in Norwegian patent no 304 766 (corresponding to international patent application No PCT/NO98/00182.) as well as international patent applications Nos. PCT/NO01/00238 and PCT/NOOl/00239,

The packaging solution for this kind of fingerprint sensor is essential as the package, while fulfilling all the functions of an "ordinary" electronics package, in most cases also serves as the interface between the sensitive elements and the finger. While providing the necessary finger-sensor interface, the package must therefore protect the sensor element and the signal processing electronical circuits (amplifiers, signal conditioning, logics) inside the package against external impacts such as wear, mechanical forces, humidity, chemicals ESD discharges etc.

Fingerprint recognition principles however often require that the sensitive . element, often a silicon chip or some kind of substrate, is directly exposed to the finger. As the silicon crystalline material is fragile, it may break along its crystal planes if a concentrated load is applied on the surface. Although the chip surface is normally covered with a protective coating, wear and scratching may also be a problem.

For capacitive sensors, wear of the top dielectric (which often constitutes the sensing capacitance dielectric) may in addition change the measuring capacitance and hence the characteristics of the sensor.

Patent application no PCT/NO98/00182 describes a mechanically robust packaging solution where the amplifying circuitry (silicon chip, "ASIC") is placed away from the finger, so that the chip is well protected against external impacts. In this principle, the essentially one-dimensional array of sensor elements are realized as two arrays of conductors, fabricated on each side of a printed circuit board. These conductors lead from the part of the sensor being in close contact with the finger to the input pads of the silicon chip.

For capacitive sensors using this packaging solution, it is specified that the surface of the conductor ends must be covered by a dielectric material. As this material will be worn down over time with use of the sensor, the characteristics of the sensor may be altered.

This problem is avoided in the "resistive" variety of the sensor, where the conductor ends are directly exposed to the finger surface. As the conductors are oriented perpendicularly to the surface of the sensor, the galvanic contact with the finger will remain despite any wear of the sensor surface.

Sensors of this kind, with a galvanic coupling between the finger and a conductor, which is again connected to an electronic circuit, however have several disadvantages.

For instance, if a measuring shunt impedance is not introduced between the finger surface and the amplifiers, the current or voltage input signal to the amplifiers will be extremely dependent on the resistivity of the finger, which may vary over several decades of magnitude depending on the humidity of the finger etc. Such a as a large variation is undesired from a signal conditioning point of view as a large dynamic amplification range is needed to cover all humidity levels. To minimize the difference between different humidity levels, the series shunt impedance should be approximately in the same order of magnitude as the impedance from the finger interior to a single sensor element of the least conductive finger to be measured. This will assure that a dominant or at least significant portion of the voltage from finger to amplifier always falls across the measuring shunt, so that the signal is not determined by the resistivity of the finger alone. The effect of the top dielectric layer of many capacitive sensors is precisely to act as such an measuring shunt impedance.

One possible solution may be to implement a capacitor or shunt impedance on the electronic circuit. However, as a dominant part of the voltage drop from finger to amplifier may fall across this impedance, this will mean that the conductors operate at a quite significant AC voltage. As long, parallell conductors at a relatively low pitch are always capacitively coupled, this may lead to unacceptably high levels of crosstalk between different channels.

To reduce crosstalk, the shunt impedance must therefore be placed as close to the finger surface as possible. To eliminate the effect of wear it should, as discussed above, however preferably not be realized as a capacitor whose dielectric layer being in direct contact with the finger. The current invention relates to a packaging solution for a Ac impedance based fingerprint sensor as described in the abovementioned patent applications, with a galvanic contact between the finger and the sensor element (conductor end), and containing a measuring shunt impedance which can be placed arbitrarily close to the sensor surface so that crosstalk may be minimized. The method offers a way of mechanically isolating the silicon surface from the finger, while providing the necessary electrical interface between sensor and finger. More, the method assures that wear of the top surface will not alter the electrical performance of the sensor.

This invention thus relates to a sensor device as described above for performing measurements on an at least partially conductive surface comprising electronic circuitry having a number of interrogation electrodes for measuring impedance between the electrodes and an AC power supply, the device having an outer surface for contact with the at least partially conductive surface. The sensor device comprising a number of outer conductors stretching from the outer surface, said outer conductors being coupled to the interrogation electrodes through a shunt impedance at the inner end of said outer conductors, said outer conductors being mutually separated by an insulating material. The packaging concept is well suited for mass production, and may be realized using a combination of packaging technologies already being extensively used in the electronics industry, such as plastic molding and wirebonding techniques. The sensor can therefore be produced at low cost. This is an important feature that makes the sensor well suited for consumer products such as mobile telephones and PDAs. The invention also gives the possibility to achieve a sensor with a very low profile, in contrast to the sensor principle described in patent application no PCTYNO98/00182. The invention will be described below with reference to the accompanying drawings, illustrateting the invention by way of examples: Figure 1 shows a cross section of a preferred embodiment of the invention. Figure 2 shows a cross section of an alternive embodiment of the invention. Figure 3 a shows a cross section of another embodiment of the invention positioned in a sensor unit.

Figure 3b shows the sensor unit in figure 3 a shown from above. Figure 4 shows a detail of the cross section shown in figure 3 a. Figure 5 shows an yet another alternative embodiment of the invention. Figure 6 illustrates an embodiment adapted for use in so called "smart cards". Figure 7 shows a detail of figure 6.

Referring to figure 1 the main idea behind the concept is to add thin conductive wires 2, e.g made of gold, between the sensor surface (finger/sensor interface) and measuring shunt impedances 3 which are defined on a buried, essentially planar surface, the shunt impedances 3 also being coupled to interrogation electrodes on a sensor chip 4.

In figure 1 the shunt impedances are positioned on the sensor chip 4, e.g. a silicon chip, the silicon chip being positioned on a substrate or circuit board 6. Both the chip 4, the shunt impendances 3 and the outer conductors 2 are embedded in a wear resistant plastic compound 5. The sensor chip 4 is preferably an integrated circuit provided with amplifiers and other circuitry.

The wires 2 are preferably placed so that they are perpendicular to this planar surface. As mentioned above the wires are then molded in e.g. a durable and wear resistant plastic compound so that they extend at least up to the top surface of the mold. The wires now provide a direct galvanic contact between the finger structures and the shunt impedances, e.g. a metallic top plate of the sensing capacitors.

The sensitive and fragile substrate and/or chip is now buried below a layer of plastic that protects it against wear and mechanical impacts. More, any wear of the sensor surface caused by the finger will now only lead to a shorter wire, the galvanic contact to the finger will be maintained and the characteristics of the sensor will not be altered.

If the surface is a substrate 6, as illustrated in figure 2, 3 and 4 and not an electronical circuit 4, conducting tracks 7 are made on or through the substrate to connect the sensor shunt impedances to the ASIC 4 input channels or interrogation electrodes. The shunt impedances 4 are preferably capacitors fabricated e.g. in a planar technology on the surface (e.g. thin- or thick film), but they may also be resistors or a combination thereof. Using a combination of capacitors and resistors may give a possibility to alter the impedance by altering the AC frequency. Figure 2 illustrates a flat sensor type in which the wires 2 and the electronic circuits 4 are mounted in different positions a substrate 6, the internal conducting tracks 7 being produced from an electrically conductive layer on the substrate 6.

Figures 3 a and 3b illustrates a sensor being based on the solutions discussed in the international patent applications No PCT/NO98/00182, PCT/NO01/00238 and

PCT NOOl/00239, comprising stimulation electrodes 8 for providing a varying voltage or current to the finger 1 being moved over the sensor. In this case the sensor with substrate 6, electronic chip 4 is completely enclosed by the plastic compound, leaving only the conductors 2,8 for performing the measurements and the external couplings 13 for connecting to external circuitry in contact with the surroundings. The couplings between the external couplings 13 and the ASIC are standard techniques and are not shown in the drawings.

Figure 4 shows a detailed view of the wires 2 with associated parts shown in figure 3 a. The outer conductors 2 stretches from the sensor surface through the plastic mould 5 to the shunt impedances 3. The shunt impedances are constituted by the wire ends of the outer conductors 2 and in this case the internal conductors 7 being separated by an insultating layer 9.

The internal conductors 7 are also coupled to the ASIC and may, as described in PCT/NO01/00238, be constituted by conductors leading through the substrate 6, as well conductive track as shown in figure 2 if the positions of the sensor points defined by the outer ends of the outer conductors 2 are different from the input positions in the ASIC 4. As described in PCTYNO01/00238 the internal conductors 7 may be coupled to the ASIC using soldering bumps 10.

In the embodiment shown in figure 1 and discussed in PCT/NOOl/00239 the shunt impedances 3 may be coupled directly to the inputs in the ASIC 4, or through conductive tracks in a conductive layer similar to the layer illustrated in figure 2.

The stimulation electrodes 8 may be coupled directly to the external couplings 13 or to the ASIC in any well known way and is not important to this invention. As mentioned in PCT/NOOl/00238 and PCT/NOOl/00239 several additional layers, e.g. being coupled to earth may be provided as well.

It may sometimes be advantageous that the sensor surface has a "curved" form to match better with the finger surface, as illustrated in figure 5. However, the surface of the sensor element is in most cases flat. With the proposed method, a curved surface can be made based on a flat surface merely by varying the length of the wires.

Figure 5 illustrates an embodiment of the invention being based on the same solution as the embodiments shown in figures 3 a, 3b and 4, but also having a curved surface 12 thus being adapted to provide an image covering a larger part of the finger surface. The curvature of the sensor surface may be chosen according to general finger shape. For obtaining this shape without having to large variation in the lengths of the outer conductors 2 between the surface 12 and the shunt impedances 3 the substrate 6 has a thickness varying in steps. Such a stepped surface can be made in Low temperature Cofired ceramics technology. Another solution being more complicated in production could be a substrate with a similar curvature as the surface 12.

The internal conductors 7 in figure 5 stretches through the substrate 6 and along a conductive layer (not shown) toward the ASIC 4.

Plastic moulding of electronic devices and microsensors, e.g transfer moulding, is a well-known and extensively used packaging process that combines low cost with high reliability. Plastic molding is often combined with lead frame technology, where wire bonds are drawn from the device to solderable and wire bondable leads before the moulding process. After moulding, the leads protrude the side of the package and can be used for soldering the device to a PCT board etc. For making the metal wires several processes may be used, including "stud bumps", electroplating and attachment of a plastic part already furnished with moulded wires. The latter can be attached e.g. by use of so-called anisotropically conductive adhesive for achieving electrical contact with the shunt impedances. All these methods are standard methods used in the electronics industry. As an example, in the "stud bump" method, quite short, vertical bond wires are added to metal pads on an IC or a sensor chip surface. The pads may e.g be the top plates of the sensing capacitors or coupled to these via routing tracks etc.

It is foreseen that Smart cards will be one of the main markets for fingerprint sensors in the future. For this application, a sensor with a very low profile, e.g with a thickness in the range of 200 - 500 μm, is required. It will be advantageous if the sensor is quite flexible, e.g. for a large part made of a plastic materials. Figure 6 shows a possible embodiment of the invention suited for smart card applications. Figure 6 shows a substrate-based sensor as in PCT/NOOl/00238, where the substrate 6 is based on a multi-layer flex or laminate (PCB) process. The galvanic contact with the finger is obtained through a via in the top layer(s) of the circuit, the via being filled with a conductive material. The metal-filled vias 2 are mutually insulated with the laminate board material. In the drawing, the via is completely filled with a conductive material, which will be advantageous to planarize the top surface. Filled vias are not standard in circuit board production, but via filling can e.g. be obtained by electroplating or by printing a conductive paste in the via. The shunt impedance 3 can for instance be implemented in lower layers of the structure as a plate capacitor. Preferably, the dielectric of this capacitor has a high K (dielectric constant) or a low thickness to achieve the desired shunt capacitance in a limited area. The top layers of the laminate substrate may e.g. be of the "thin film" type to allow for fine-line capabilities and small via sizes. On one of the top metal layers the stimulation electrode 8 can be defined. In a multilayer laminate process, the via connections through the substrate 6 to the

ASIC 4 can e.g. be realised in a staggered fashion as illustrated in figure 6.

Figure 7 shows a more close-up drawing of the area around the via and shunt capacitance of figure 6. Similar structures can also be realised in ceramic technology, e.g. LTCC. The proposed method works equally well where the sensor element has the following characteristics:

1. The sensor element is an integrated circuit 4 as illustrated in figure 1 and in international patent application No PCT/NOOl/00239.

2. The sensor element is a substrate 6 with readout electronics 4 mounted on the back side as illustrated in figures 3a,b and 4, as well as in PCT/NOOl/00238.

3. The sensor element is a substrate 5 with readout electronics 4 mounted on the top side as illustrated in figure 2.

4. The sensor element is a substrate 6 with readout electronics connected by wires etc. The described concept is not limited to fingerprint sensors, but can also be used for any kind of AC impedance based sensors that make use of the topology of a finger surface, e.g. for navigation, pointer/mouse or touchpad functionalities related to a display as described in international applications No PCT/NOO 1/00243 and PCT/NOOl/00244.

Claims

C l a i m s

1. Sensor device for performing measurements on an at least partially conductive surface comprising electronic circuitry having a number of interogation electrodes for measuring impedance between the electrodes and a power supply, the device having an outer surface for contact with the at least partially conductive surface comprising a number of outer conductors stretching from the outer surface having an outer an an inner end, said outer ends being adapted to electrical coupling to the conductive surface and said inner ends of said outer conductors being coupled to the interrogation electrodes through a shunt impedance, said outer conductors being mutually separated by an insulating material.

2. Sensor device according to claim 1, wherein the shunt impedance is constituted by a dielectric layer positioned electrically between the inner end of said conductor and the interrogation electrodes of the electronic circuitry.

3. Sensor device according to claim 1, wherein an inner electrical conductor is provided between the the shunt impedance and the interrogation electrodes of the electronic circuitry.

4. Sensor device according to claim 1 , comprising a number of external conductors for coupling to external circuitry, the device being enclosed in said insulating material except for said number of outer conductors and external conductors.

5. Sensor device according to claim 1, also comprising a stimulation electrode for applying a varying voltage to the surface to be measured and being adapted to electrical coupling to the surface.

6. Sensor device according to claim 1, wherein the outer conductors are positioned in an essentially linear array.

7. Sensor device according to claim 1, wherein the outer conductors are positioned in an essentially linear array, constituting an essentially linear sensor array, with a chosen number of outer conductors being positioned outside the line at chosen distances from said line for measuring the velocity and direction of the surface to be measured.

8. Sensor device according to claim 1, wherein the outer conductors are positioned in a two dimensional pattern.

9. Sensor device according to claim 1, wherein the outer conductors are positioned perpendicular to the outer surface.

10. Sensor device according to claim 1 , wherein the outer surface is curved.