NO316776B1 - Package solution for fingerprint sensor - Google Patents

Description

Package solution, especially for the fingerprint sensor

This invention relates to a sensor device for carrying out measurements on an at least partially conductive surface. More specifically, it relates to a package solution for a substantially 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) in addition to international patent applications no. PCT/NOO1/00238 and PCT/NOO 1/00239.

The package solution for this type of fingerprint sensor is important since the packaging, while it must take care of all the "usual" functions of an electronics package, in most cases also acts as a contact surface between the sensor elements and the finger. While allowing contact between the finger and the sensor, the package must therefore protect the sensor element and the signal processing electronic circuits (amplifiers, signal processing, logic) inside the package against external influences such as wear, mechanical forces, moisture, chemicals, ESD discharges, etc.

However, recognition principles for fingerprints often require that the sensitive element, often a silicon chip or a substrate type, is in direct contact with the finger. Since the silicon crystal is fragile, it can break along the crystal planes if a concentrated load is applied to the surface. Although the surface is normally covered by a protective layer, wear and scratches can also be a problem.

For capacitive sensors, wear in the upper dielectric (which often constitutes the sensitive capacitance dielectric) can additionally change the measured capacitance and thus the sensor's characteristic.

Patent application no. PCT/NO98/00182 describes a mechanically robust package solution in which the amplifier circuit (the silicon chip, "ASIC") is placed away from the finger so that the chip is well protected from external influences. At this price point, the one-dimensional array is realized as two arrays of conductors formed on opposite sides of the printed circuit board. These conductors lead from the part of the sensors that is in close contact with the finger to the input parts of the silicon chip.

For capacitive sensors that use this package solution, it is specified that the surface of the conductor ends must be covered by a dielectric material. Since this material will wear down over time when using the sensor, the characteristic of the sensor will change.

This problem is avoided in the resistive variant of the sensor, where the conductor ends are directly exposed to the finger surface. Since the conductors are oriented normal to the sensor surface, the galvanic contact with the finger will be maintained despite wear on the surface.

Sensors of this type, but galvanic connection between the finger and a conductor, which in turn is connected to an electronic circuit, have several disadvantages. For example, if a shunt impedance is not connected between the finger and the amplifier, the current or voltage in the input signal to the amplifiers can become very dependent on the resistivity of the finger, which can vary several orders of magnitude depending on the moisture of the finger, etc. Such a large variation is undesirable from a signal processing perspective due to the large dynamic gain range needed to cover all humidity levels.

To minimize the difference between different humidity levels, the shunt impedance series should be approximately the same order of magnitude as between the inside of a finger and the sensor element with the least conductive finger to be measured. This ensures that a dominant or at least significant part of the voltage from the finger to the amplifier always falls across the measurement shunt, so that the signal is not determined by the resistivity of the finger alone. The effect of the upper dielectric layer on many capacitive sensors is precisely to serve as a measuring shunt impedance.

A possible solution could be to implement a capacitance or shunt impedance on the electronic circuit. However, since a dominant part of the voltage drop from the finger to the amplifier can fall across this impedance, this means that the conductors operate at a fairly significant AC voltage. Since long, parallel conductors with a relatively small mutual distance are always connected capacitively, this can lead to unsustainably 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 wear, however, as discussed above, it should preferably not be realized as a capacitance if the dielectric layer is in direct contact with the finger.

The present invention relates to a package solution for an AC impedance based on a fingerprint sensor as described in the above-mentioned patent applications, with galvanic contact between the finger and the sensor element (wire end), and comprising a measuring shunt impedance which can be placed suitably close to the sensor surface so that crosstalk can is minimized. The method provides a way to mechanically isolate the silicon surface from the finger while maintaining the necessary electrical coupling between the sensor and the finger. Furthermore, the method ensures that wear on the surface does not change the electrical properties of the sensor.

This invention thus relates to a sensor unit as described above for carrying out measurements on an at least partially conductive surface comprising electrical circuits and an AC current source, where the device has an outer surface for contact with the at least partially conductive surface. The sensor device comprises a number of outer conductors extending from the outer surface, where the outer conductors are connected to measuring electrodes via a shunt impedance at an inner end of the conductors, where the outer conductors are separated by an insulating material. More precisely, the invention is characterized as stated in claim 1.

The packaging concept is suitable for mass production and can be realized using a combination of packaging technology that is already common in the electronics industry, such as plastic molding and "wire bonding" techniques.

The sensor can therefore be produced at low costs. This is an important feature that makes the sensor suitable for consumer products such as mobile phones and PDAs. The invention also provides the possibility of obtaining a sensor with a very low profile, in contrast to the sensor principle described in patent application No. PCT/NO98/00182.

The invention will be described below with reference to the attached drawings, which illustrate the invention by means of examples:

Figure 1 shows a cross-section of a preferred embodiment of the invention.

Figure 2 shows a cross-section of an alternative embodiment of the invention.

Figure 3a shows a cross-section of a further embodiment of the invention placed in a

sensor unit.

Figure 3b shows the sensor unit in Figure 3a seen from above.

Figure 4 shows a detail of the cross-section shown in Figure 3a.

Figure 5 shows yet another alternative embodiment of the invention.

With reference to figure 1, the main idea behind the concept is to add thin conductive wires 2, for example made of gold, between the sensor surface (finger/sensor contact surface) and measuring shunt impedances 3, which are defined on a covered, essentially flat surface, where the shunt impedances 3 are also connected

the interrogation electrodes on the sensor chip 4.

In Figure 1, the shunt impedances are placed on the sensor chip 4, for example a silicon chip, where the silicon chip is placed on a thin substrate or circuit board 6. Both the chip 4, the shunt impedances 3 and the outer conductors 2 are embedded in a wear-resistant plastic material 5. The sensor chip 4 is preferably an integrated circuit provided with amplifiers and other circuits.

The conductors 2 are preferably placed so that they are perpendicular to the flat surface. As mentioned above, the conductors are then molded into, for example, a durable and wear-resistant plastic material so that they extend at least up to the upper surface of the casting. The conductors thus provide galvanic contact between the finger structures and the shunt impedances, for example a metallic top plate of measuring capacitances.

The sensitive and fragile substrate and/or chip is now buried under a layer of plastic that protects it from wear and mechanical influences. In addition, any wear and tear on the sensor surface from the finger will now only lead to a shorter conductor, the galvanic contact with the finger is maintained and the sensor's properties will not change.

If the surface is a substrate 6, as illustrated in 2, 3 and 4, and not an electronic circuit, the conductive paths 7 are made on or through the substrate to connect the shunt impedances to the input channels or interrogation electrodes of the ASIC 4. The shunt impedances 4 are preferably capacitances form, for example with a planar technology on the surface (for example thin or thick film technology), but they can also be resistors or a combination of these. By using a combination of capacitors and resistors, it is possible to change the impedance by changing the AC frequency. Figure 2 illustrates a flat sensor type in which the conductors 2 and the electronic circuits 4 are mounted at different locations on a substrate 6. Internal conductive paths (not shown) can be formed in an electrically conductive layer on the substrate 6. Figures 3a and 3b illustrate a sensor which is based on the solutions discussed in the patent applications No PCT7NO98/00182, PC17NO01/00238 and PCT/NO01/00239, comprising stimulation electrodes 8 to provide a varying current or voltage to a finger which is moved over the sensor. In this case, the sensor with the substrate 6, the electronic chip 4 is completely encapsulated in a plastic material, leaving only conductors 2,8 for carrying out the measurements and the external connectors 13 for connecting external equipment to be in contact with the environment. The connections between the external connectors 13 and the ASIC are standard techniques and are not shown in the drawings.

Figure 4 shows a detailed image of the threads 2 with associated parts shown in Figure 3a. The outer conductors 2 extend from the sensor surface through the plastic mold S to the shunt impedances 3. the shunt impedances are made up of the conductor ends of the outer conductors 2 and in this case the internal conductors 7 separated by an insulating layer 9. A further conductive layer 11 can be provided to supply voltage to the stimulation electrodes 8 or be connected to ground.

The internal conductors 7 are also connected to the ASIC and can, as described in PCT/NOO 1/00238, consist of conductors extending through the substrate 6, in addition to conductive paths as shown in Figure 2 if the positions of the sensor points are defined by the the outer ends of the outer conductors 2 are different from the entry positions in the ASIC 4. As described in PCT/NOO 1/00238, the internal conductors 7 can be connected to the ASIC by using so-called "soldering bumps" 10.

In the embodiment shown in figure 1 and discussed in PCT/NO01/00239, the shunt impedances 3 can be connected directly to the inputs of the ASIC 4, or via conductive paths in a conductive layer corresponding to what is illustrated in figure 2.

The stimulation electrode 8 may be connected directly to the external connectors 13 or to the ASIC in any known manner and is not important for this invention. As mentioned in PCT/NO01/00238 and PCT/NOO1/00239, additional layers, for example connected to earth, can also be provided.

Sometimes it can be beneficial to have a sensor surface that is curved to better fit the finger surface, as illustrated in Figure 5. However, the surface of most sensor elements is flat. With the proposed method, a curved surface can be created based on a simple variation of the length of the conductive wires.

Figure 5 illustrates an embodiment of the invention based on the same solution as the embodiments shown in Figures 3a, 3b and 4, but also comprising a curved surface 12 which is thus arranged to obtain an image that covers a larger part of the finger surface.

The curvature of the sensor surface can be selected according to a general finger shape. In order to achieve this shape without having too much variation in the length of the outer conductors 2 between the surface 12 and the shunt impedance 3, the substrate can have a thickness that varies in steps. Such a step-by-step surface can be made in so-called low-temperature "Cofired" ceramic technology. Another solution that is more complicated in production is a substrate that has the same curvature as the surface 12.

The internal conductor 7 in Figure 5 extends through the substrate 6 and along the conductive layer (not shown) to the ASIC 4.

Plastic molding of electronic equipment and microsensors, for example "transfer moulding" are well-known and widely used packaging processes that combine low costs with high reliability. Plastic molding is often combined with a conductive frame technology where the conductor connections are pulled from the device to solderable and metal-wire compatible conductors before the molding process. After molding, the conductors extend out of the side of the package and can be used to solder the device to a PCT card or similar.

To make metal parts, several processes can be used, including "stud bumps", "electroplating" or attaching a plastic part that is already provided with embedded conductors. The latter can be used, for example, by using so-called anisotropic conductive glue to form contact with the shunt impedances. All these methods are standard methods used in the electronics industry. For example, in the "stud bump" method, rather short, vertically banded conductors are used placed on metal mounts on an IC or a sensor chip's surface. The metal mounts can, for example, be the top plates of the sensor capacitances or be connected to these through conductive paths, etc.

The proposed method works equally well where the sensor element has the following properties: 1. The sensor element is an integrated circuit 4 as illustrated in figure 1 and in international patent application No. PCT/NO01/00239. 2. The sensor element is a substrate 6 with reading electrodes 4 mounted on the back as illustrated in figures 3a, 3b and 4, in addition to PCT/NO01/00238. 3. The sensor element is a substrate 5 with readout electrodes 4 mounted on the upper side, as illustrated in figure 2. 4. The sensor element is a substrate 6 with readout electronics connected to wires, etc.

The described concept is not limited to fingerprint sensors, but can also be used on other types of AC impedance-based sensors that make use of the topology of a finger surface, for example for navigation or pointer/mouse functionality on a screen as described in international applications no PCT/ NOO1/00243 and PCT/NO01/00244.

Claims (10)

1. Sensor device for carrying out measurements on an at least partially conductive surface comprising electronic circuits with a number of interrogating electrodes for measuring the impedance between the electrodes and a power supply, where the device has an outer surface for contact with the at least partially conductive surface, comprising a number of outer electrodes extending from the outer surface with an outer end, wherein the outer end is arranged for electrical connection to the conductive surface, and where the outer conductors are separated from each other by an insulating material, characterized in that the inner end of the outer conductors are connected to the interrogation electrodes via shunt impedances placed electrically between the inner end of each of said outer conductors and the corresponding interrogation electrode in the electronic circuits. 2. Sensor device according to claim 1, where the shunt impedance consists of a dielectric layer. 3. Sensor device according to claim 1, where an internal electrical conductor is provided between the shunt impedance and the interrogation electrodes on the electronic circuits. 4. Sensor device according to claim 1, comprising a number of external conductors for connection to external equipment, where the device is encapsulated in the insulating material apart from the number of external conductors and external conductors. 5. Sensor device according to claim 1, also comprising a stimulation electrode for supplying a varying voltage to the surface to be measured and arranged for electrical connection to the surface. 6. Sensor device according to claim 1, where the outer members are placed in an essentially linear row. 7. Sensor device according to claim 1, where the outer conductors are placed in an essentially linear array, with a number of outer electrodes placed on the outside of this line and at selected distances from the line for measuring speed and direction on the surface being measured. 8. Sensor device according to claim 1, where the outer conductors are placed in a two-dimensional pattern. 9. Sensor device according to claim 1, where the outer conductors are placed perpendicular to the outer surface. 10. Sensor device according to claim 1, where the outer surface is curved.