NO318882B1 - Substrate multiplexing - Google Patents

Description

This invention relates to a sensor module that can sense a topological monster in a surface that is in direct contact with it, for example a fingerprint.

The market for biometrics is developing rapidly. In order for biometrics to enter the consumer market, however, strict requirements are set with regard to, for example, sensor price, compact design, the imaging quality of the fingerprint and power consumption.

Capacitive fingerprint sensors represent one of the most promising technologies for realizing compact low-cost fingerprint sensors for the consumer market, and several concepts have been proposed in recent years. The sensor concepts can be roughly divided into two categories: Matrix sensors, where the fingerprint is placed on a two-dimensional sensor surface, and scanners or swipe sensors, where the user has to drag his finger over the sensor for it to register the image.

US patent. 6,069,070 describes a typical matrix type AC capacitive fingerprint sensor. This sensor is basically a silicon chip (IC) which is equipped with a two-dimensional matrix of sensor elements (pixels) in addition to amplifiers and other circuits. A driver electrode placed on the sensor package, outside the active sensor surface, is used to connect an AC voltage to the finger. The AC signal penetrates the finger and is connected through a dielectric layer to the sensor elements (Pads) on the sensor's surface. The sensor elements are connected to amplifier circuits in the silicon chip.

This sensor configuration has several disadvantages: For obvious reasons, the active surface of the matrix sensor must be as large as the part of the finger to be imaged, in other words on the order of 100 mm<2>. Due to the fact that the price of silicon ICs increases proportionally with the chip area, such a large chip will be too expensive for many consumer applications.

In addition, the fact that the finger is in direct contact with the surface of an active silicon chip places strict demands on the materials used to protect the IC against wear, mechanical stress, moisture, chemicals and electrostatic discharges (ESD). This necessitates non-standard and expensive protective materials, or can lead to reduced reliability and lifetime of the sensors.

Fingerprint scanners, where the user has to draw their finger over the sensor, do not have the same length as the fingerprint and can therefore be smaller and cost-effective. Because of their reduced size and lower price, scanner sensors may be a better choice for the typical mass-produced applications. Scanners only require a limited number of sensor lines, and while the width of the sensor still needs to match the width of the fingerprint, the sensor length only needs to be a few millimeters or even less. The linear arrangement of sensor elements also provides more flexibility in the design of the connection between the sensor and the finger - it does not necessarily have to be a flat surface as the matrix sensors often are.

Patent application PCT/NO01/00238 shows an example of a scanning sensor where the integrated circuit with amplifiers etc. (ASIC) is mounted on the back of a substrate. The substrate is, for example, silicon, ceramic or glass. The upper side of the substrate is equipped with a number of conductive elements (pads) for sensing capacitance and conductive vias/holes are made through the substrate to connect each sensor element (pad) to a corresponding input contact of the ASIC. The sensor elements are covered with a dielectric material. The the described sensor comprises a substantially linear array of sensor elements with one full, single line, and additional sensor elements to determine the velocity of the finger. The velocity measurements are necessary to build up a two-dimensional image based on the readings from the sensor. Examples of other scanning sensors are shown in PCT/NO98/00182, PCT/NO01/00239, US 6,289,114, and EP 0 735 502 A2.

The substrate-based sensor in PCT/NO01/00238 has several advantages: It is possible to draw in signal paths on the substrate, which makes it possible to decouple the size of the ASIC from both the width and the length of the fingerprint to be read. This makes it possible to design a much smaller IC chip and thus save costs. In addition, the substrate, which is far cheaper to make per area, will act as a protection against mechanical impact or the environment for the sensitive ASIC chip on the back. The sensor has a very low profile, and if delivered with, for example, BGA balls, it will not have to be packaged further before it is mounted on a main board, for example in a phone.

The sensor also has the advantage that the driver electrodes for stimulating the finger with an AC signal can be integrated directly on the surface of the substrate. These driver electrodes can be connected to ground through an ESD-protected semiconductor alignment, so that any ESD discharge from the finger will go to the electrode rather than to the sensor elements.

However, the device in PCT/NOO1/00238 requires that a large number of paths from the sensor elements are passed through the substrate and connected to the ASIC. This has several disadvantages. First and foremost, the substrate technology used must allow a large number of through current conductors (vias) within a relatively small area. This excludes the use of a number of different cheap substrate processes. For example, current technology for both ceramic, glass and laminate substrates has problems with the requirements for density.

The large number of connections also practically excludes the possibility of removing the connections on the side of the sensor by wire bonding.

In addition, there is a need for an ASIC input channel, and possibly a preamplifier, for each sensor element on the top surface. Since both i/o areas (pads), amplifiers and related circuits for ESD protection use space on the ASIC, it will be difficult to reduce the size of the chip below a certain limit. This can make it difficult to reach the extremely low production costs needed for a mass-produced sensor.

Several of the above principles require the finger to be stimulated directly by an AC voltage. This does not always have to be an advantage. For example, the signal from the unshielded driver electrode can in some cases create electrical interference with other electronic equipment.

The AC signal from the driver electrode can also connect capacitively to the planes with a fixed potential (for example ground) in the substrate and thus increase the current consumption of the sensor. This can be a disadvantage, especially for hand-held equipment such as mobile phones where battery life is important.

Thus, it is an object of this invention to provide a sensor module in which the number of necessary connections between the sensor surface and the ASIC, and the number of input channels on the ASIC, is significantly reduced, thereby reducing the production costs of the sensor.

Moreover, it is an object of the invention to carry out this reduction of channels using a sensor element multiplexing which is carried out on the substrate, and which preferably does not use active switches or other semiconductor-based components.

The present invention is aimed at the principle of a scanning sensor that requires low power consumption, high reliability against mechanical or other influences from the environment, simple packaging and assembly, low electromagnetic radiation to the environment and high tolerance for ESD. The invention is characterized as stated in the independent claim.

According to a preferred embodiment of the present invention, the sensor is a scanner with a substantially linear array of sensor elements formed on a substrate. The sensor elements are arranged in a number of subgroups. For each subgroup there is a driver (excitation or activation) electrode that can be used to enable or excite elements in only that subgroup.. An element from each subgroup is connected to a common signal trace, connected to an input channel of an ASIC or in a discrete electronic system. This is done in such a way that the signal from a signal current or voltage in said signal track is received from only one element belonging to the subgroup with an activated electrode, and that no signals are received from the other elements belonging to other (non-activated) subgroups.

According to a preferred embodiment, the activation electrode is placed near the associated subgroup of sensor elements, so that there is a capacitive signal connection from said electrode to the sensor elements in said subgroup when an AC signal is applied to the activation electrode. From each sensor element in the subgroup there is a capacitive signal that is connected to a signal path that leads to one input channel on the ASIC. From each sensor element there is also a capacitive or galvanic connection to an external earth or other reference potential through the finger, so that the signal received at the ASIC's input channel is modulated by the presence of a fingerprint ridge or valley located directly above the sensor element.

The invention is described below with reference to the attached drawings, which describe the invention by means of examples.

Figure 1 schematically illustrates the circuit associated with a sensor element.

Figure 2 schematically illustrates the circuit according to the invention.

Figure 3 illustrates a longitudinal section of a preferred embodiment of the sensor module. Figure 4 illustrates a longitudinal section of an alternative embodiment of the sensor module.

Figure 5 illustrates a sensor according to the invention seen from above.

Figure 6 illustrates a detail of a cross-section of the top layers of the sensor module according to

the invention.

Figure 7 illustrates the cross-section of the top layers in the sensor.

In a preferred embodiment, detailed in Figures 3, 5, 6 and 7, the invention consists of a substrate 2 which can be manufactured from, for example, silicon, glass, flexprint or a laminated card. On the upper surface of the substrate, over which the finger to be imaged is guided, a number of sensor elements S are defined, for example as conductive areas (pads) made with a thin-film or a thick-film process. The sensor elements are organized in an essentially linear configuration. The sensor elements can preferably be covered by a dielectric 8 (such as SiO 2 , SiN, Polyimide, epoxy or ceramic) or be in galvanic contact with the finger. The substantially linear array may for example be arranged as a single line with additional sensors or groups of elements 6 to detect the speed or direction of movement, including rotation of the finger over the sensor. These movement measurements can either be used for correction of the image with regard to variances in finger speed, direction of pull or rotation, or for example to move a cursor on a screen ("Mouse").

Preferably, the sensor is equipped with a number of activation electrodes 11 and signal conductors 10 leading to the input channels of the ASIC 4. Said electrodes and conductors can, for example, be formed in a metal layer directly below the sensor elements, and separated from the latter by a dielectric material to achieve a capacitive coupling between the sensor elements and said electrodes and conductors.

The sensor elements 5 are preferably grouped in a number of subgroups, each with its activation electrode, which is shared between the elements in the subgroup. It is noted that the elements of a subgroup can be arranged in a number of different ways. For example, the elements of a subgroup may consist of a number of elements next to each other, or the elements of a subgroup may be spread out along the array. The elements in a subgroup can also be organized into smaller groups located in different places in the array.

Each signal track 10 is capacitively connected to several sensor elements 5, preferably one element from each subgroup.

In a preferred embodiment, the sensor surface is at least partially covered by a metal electrode 7 or another conductive plane held at a fixed potential, for example earth. This electrode can be in direct contact with the used finger 1 or be covered by a dielectric material. .1 a preferred embodiment, the substrate further comprises a number of via holes 3 through which the connections from the signal traces 10 and the activation electrodes 11 are passed through the substrate 2 to the rear. The via holes 3 are made by laser or mechanical drilling, or an etching process, and must then be electrically isolated from the substrate if the latter is conductive. This can be done by e.g. oxidation or deposition techniques. An electrically conductive material can be placed in the vias to provide electrical contact through the substrate. Preferably, the holes are hermetically sealed. If necessary, additional routing can be included between the bottom end of the via holes and the substrate's bonding areas for an ASIC 4 mounted on the back of the substrate (e.g. by flip-chip or wire bonding).

Preferably, parts of the signal paths or an activation electrode leading to different sensor elements are connected to electrically conductive layers on the top surface of the substrate, so that the required number of through holes is minimized. These connections 15 can, for example, be made using electrical vias between different conductive layers on the top surface. This can alternatively be done via 3 through the substrate for each sensor element, and the connection 15 can be made in the control layer on the back of the substrate.

In an alternative embodiment shown in Figure 4, there is no via 3 through the substrate, but connections are made on the edge of the substrate 2, for example by using wire bonding 9. Due to the limited number of necessary connections, the wire bonding can in some cases be more practical and cheaper than using vias 3.

In the case of silicon or other conductive or semi-conductive substrate types, the substrate 2 can be kept at a fixed potential.

The ASIC (or an equivalent electronic signal processing system) comprises a number of amplifiers 16 for amplifying the signal corresponding to each signal channel, in addition to other circuits for signal processing.

Finally, the ASIC implements at least one activation circuit for applying a suitable signal to the activation electrodes 11. The activation circuit can for example be in the form of an AC driver circuit connecting to n output channels through a multiplexer. The number n can here, for example, be equal to the number of activation electrodes 11 on the substrate. For example, the multiplexer can be programmed so that within a certain time frame corresponding to a limited movement for movement of the finger (eg less than 50um), it can activate each subgroup of the sensor array through its associated electrode, one subgroup at a time. For each time one subgroup is activated, each of the input channels of the ASIC is read at least once. In this way, a complete line scan of all the sensor elements will be achieved within the mentioned time frame.

A sensor with a line of 256 elements with a 50um distance can, for example, have 8 subgroups of 32 sensor elements. In addition, there may for example be two additional subgroups 6 of 32 elements that are used for speed calculation and pointer functionality, as described in patent application no. PCT/NOO1/00244. The sensor requires an ASIC with 32 input channels and amplifiers, and 10 output contacts connecting the different activation electrodes. In other words, only 42 connections are required between the ASIC and the top side of the substrate, instead of 320 channels for the principle described in PCT/NO01/00238. In addition, there is an earth connection and possibly other individual connections. The sensor elements belonging to different functionalities can share the same activation electrode.

A schematic version of the preferred measurement principle is shown in Figure 1. An AC voltage signal from the activation electrode 11 is connected to the sensor element 5.1 in the case shown in Figure 1, the input signal current to the ASIC amplifier 16 is determined by the capacitance 12 in series with the total capacitance from the sensor area to to ground potential. Said total capacitance is given by 17 in parallel with the series of capacitances at 13 (through the sensor dielectric 8) and 14 (through the back of the finger or the air space in a valley). The additional series impedance 18 through the finger to the external potential is for this discussion assumed to be negligible. Because all other elements are fixed, the magnitude of the input signal will change depending on the magnitude of the capacitance 14, which varies depending on whether a finger ridge or valley is present. The signal stream can be amplified and demodulated (e.g. synchronously) by the ASIC.

Figure 6 shows an example of how the capacitances 12, 13 and 17 in Figure 1 can

is implemented in practice in a multi-layer metallization process with at least two metal layers. When an AC voltage is applied to the activation electrode 11, there will be a reactive (capacitive) current from the activation electrode 11 to the signal conductor 10 through the sensor element 5. Due to the geometry and relative position of the electrodes 5,10, and 11, all parasitic

capacitance be small compared to 12,13 and 17, and can be neglected in the following discussion.

When there is an air-filled space located directly above the sensor element 5, the impedance from the sensor element to an external potential (at the electrode 7) through the finger 1 will in practice be infinite.

On the other hand, when a fingerprint ridge is present there will be a much lower finite impedance from the sensing element to the external potential through the finger. This will lead to a reduction in the signal current received at the signal track. This reduction in signal flow will give rise to a signal contrast between the ridges and valleys which can be visualized as a "grayscale" fingerprint image when the signals are amplified and digitized.

Figure 2 shows the situation where several sensor elements 5a-d are capacitively connected to the same signal path 10 in the manner described. Each of the sensor elements can be activated by its associated electrode, marked as 1 la-d. As can be seen in Figure 2, if an activation electrode (e.g. 1 la) is allowed to float or is held at a fixed potential, there will in practice be no signal flow to the signal path 10 from the corresponding sensor element 5a. This lack of signal flow is independent of whether there is a finger ridge or valley across the element. This means that only those sensor elements that are "activated" by an AC voltage will give rise to a signal current. This ensures efficient "multiplexing" between different subgroups using the activation voltage as a "control signal".

A possible disadvantage of the proposed principle is that the signal fed into the finger through an activated sensor may cross-couple to nearby, non-activated elements and create a parasitic signal from these elements. This effect can be reduced by increasing the thickness of the dielectric on top of the sensor elements. To reduce crosstalk effectively, the dielectric should provide an impedance between the sensor element and the surface to be measured that is significantly greater than the impedance between the reference potential and the surface. If such a thickness is not desirable due to the signal level, the effect can be counteracted by adding additional, local, shielding electrodes on the sensor surface, close to the sensor elements. These electrodes, one set for each subgroup, can for example be in galvanic contact with the finger or be covered by a thin dielectric material. The electrodes can, for example, be tied to ground when the elements in the subgroup are not active, and held at the driver potential when the group is active.

In the latter case, these shielding electrodes will have a similar function to the "local" or binding electrodes in PCT/NO02/00465, which is included here by reference. As described in the aforementioned patent application, these electrodes can thus be advantageous also for very wet fingers.

To further reduce the effect of crosstalk between neighboring elements, redundancy can be added, for example in the form of additional signal paths or activation electrodes. For a sensor with 8 activation electrodes and 32 elements in each subgroup, the number of activation electrodes can for example be doubled to 16 to avoid such crosstalk, for example by ensuring that two neighboring points are never connected to the same signal paths.

An important function of the plane 7 is to shield the signal paths 10 from parasitic coupling of AC voltage from the finger 1. However, the coupling of AC voltage from the activation electrodes to the finger through the sensor element in the proposed concept is very limited. Because of this, it may also be necessary to shield the signal paths by covering them with a sufficiently thick dielectric layer. This can reduce the number of required metal layers on the upper surface and thus the costs of the device.

In an alternative embodiment, the electrode 7 can be driven by an AC voltage with inverted polarity to increase the device's sensitivity.

In addition to the version described above, the invention can be in the form of a number of different embodiments. For example, the use of "current measurement", inverting amplifier coupling are just examples. Other principles, for example based on non-inverting amplifier coupling, possible. The invention is thus not limited to the connection described in figure 1, and other schemes, including voltage measurement techniques can be used to measure the size of the variable capacitance 14, which is the most important measured variable.

Also, the activation electrodes can use a DC voltage instead of an AC voltage, thus creating a measurement price pinch based on DC capacitance. In this case, for example, the amplifiers can be replaced or supplemented with a combination of a counter and a voltage level detector (comparator). An AC voltage with several frequencies can also be used, and several activation electrodes, each with a different voltage signal that is used on each sensor element. The capacitive elements in the figures can also be replaced by other impedance components or networks, or in the case of a semiconductor substrate, "active" switches can be used.

The geometric electrode arrangement is only intended as an example. There are several ways to arrange the electrodes to achieve the described functionality. The electrodes can, for example, be placed in the same layer so that the capacitive coupling goes "sideways". Using a sensor geometry as described in PCT/NO98/00182, which is incorporated herein by reference, the actuation electrode, sensor areas, and signal paths can be defined in different layers on a PCB board.

In an alternative embodiment, the combination of 11 and 12 can be replaced with, for example, a constant amplitude AC power source.

To calibrate the response of the sensor, a calibration capacitance can be connected between each signal trace and another AC electrode that can be turned on and off. When the electrode is turned on, the change in signal current will represent the total variations in the measurement system, for example gain in the amplifier. This reading can therefore be used to normalize the output signal from each channel. While the calibration capacitance contributes to a constant signal level (AC current with constant amplitude) it can also be used to shift the signal level up or down. Alternatively, an AC power source can also be used for calibration.

In other embodiments, the contact between the finger and the sensor areas can also be galvanic, and a shunt capacitance or impedance (equivalent to 13) placed between the area and the input terminal of the amplifier, as described in the patent application PC17NO02/00467, which is included here by reference. Moreover, the dielectric layer 8 may be provided with upper conductors molded into an insulating material such as plastic, for direct contact with the finger surface, for example to shape the upper surface of the finger or to provide a structure for the sensor elements that differs from the structure as shown of the positions of the sensors 5. This provides a sensor surface that retains its properties even if the surface wears down.

If a laminate or other flexible substrate type is used, the proposed principle is very suitable for integration of the sensor with a smart card, due to the very modest size of the ASIC.

If the external electrode 7 is grounded it will reduce the need for additional ESD protection since the discharges from the finger will go directly to ground. The use of an upper earthed layer can also be an advantage with regard to electromagnetic radiation to the surroundings.

List of reference numbers in the drawings:

1. finger surface

2. substrate

3. vias

4. ASICs

5. sensor element

6. Additional elements (pads)

7. Ground electrode

8. Dielectric

9. Wirebonds

10. Signal paths

11. Activation electrode

12. Cin

13. Csensor

14. C was

15. Connecting two other elements

16. Amplifier

17. C_out

18. Zfing

19. Cfeedback

Claims (15)

1. Sensor module for measuring structures in a surface, in particular a finger surface, comprising: a number of sensor elements which are located at selected positions in a trap surface, where each sensor element is connected to a driver device for providing an excitation current or voltage to each sensor element, thereby connecting a signal through the sensor element to an external reference potential, where the sensor elements are also connected to an electronic circuit which electronic circuit is arranged to measure the magnitude of the capacitance or impedance between each sensor element and the external reference potential at selected times, and where said electronic circuit comprises input means including at least two amplifier circuits, where each amplifier circuit is connected to a set of sensor elements containing at least two sensor elements for amplifying the signals from these and for transmitting them to said electronic circuits, where each sensor element in each set is connected one to different driver devices for excitation of each sensor element according to a predetermined sequence, so that the amplifier receives a signal from one sensor element at a time. 2. Sensor module according to claim 1, where the sensor elements form an essentially linear array to thereby provide a fingerprint sensor which is arranged to measure a finger surface that is moved over the sensor elements, where said linear array is also arranged to measure the movement between the sensor and the surface, and wherein the electronic circuits are arranged to combine the measured values to form a representation of the surface pattern. 3. Sensor module according to claim 1, where the sensor elements are placed on a substrate, said substrate comprising wires which connect the sensor elements to an electronic circuit. 4. Sensor module according to claim 3, where the substrate is provided with a number of openings through which electrical contacts connected to the sensor elements are led, and that the electronic circuit is positioned on the opposite side of the substrate in relation to the sensor elements. 5. Sensor module according to claim 3, where the substrate is made of silicon, glass, ceramics or a polymer material. 6. Sensor chip according to claim 3, where the substrate is a semiconductor, and where electronic circuits are defined on the top surface of the substrate. 7. Sensor module according to claim 1, where said electronic circuits comprise control means for controlling the driver means to thereby activate them in a selected sequence, thereby enabling the electronic circuits to associate the known positions with the activation times for each sensor element and thus determine position corresponding to the signal from each amplifier. 8. Sensor module according to claim 1, where each driver device is connected to a number of sensor elements, one in each set, thereby activating one sensor element in each sensor element set at the same time. 9. Sensor module according to claim 1, where the external reference potential is provided by an external electrode placed separately from the sensor elements and arranged to connect said potential to the surface. 10. Sensor module according to claim 1, where the reference potential is a variable voltage. 11. Sensor module according to claim 1, in which the module comprises one or more electrodes which can excite each sensor element, so that the response from each sensor element can be calibrated. 12. Sensor module according to claim 1, where the driver means are AC current sources. 13. Sensor module according to claim 1, where the sensor elements are provided with a dielectric layer. 14. Sensor module according to claim 1, where said driver means connects an AC signal to said sensor element through a capacitance, and where said sensor elements are connected to said amplifier through another capacitance. 15. Sensor module according to claim 14, where said capacitances are made in a multi-layer metallized structure.