Patent Description:
In accordance with a first aspect of the present disclosure, a fingerprint sensing device is provided, comprising a plurality of sensor cells, wherein each sensor cell comprises: at least one capacitive sense plate and a discharge electrode insulated from the capacitive sense plate; a first discharge path for discharging a first static electricity charge from the capacitive sense plate to a first electric potential terminal; a second discharge path for discharging a second static electricity charge from the capacitive sense plate to a second electric potential terminal; a charge reservoir coupled between the first electric potential terminal and the second electric potential terminal, and configured to take up electrostatic charge originating from the capacitive sense plate, wherein the charge reservoir is implemented as a capacitor; wherein the first electric potential terminal and the second electric potential terminal are electrically connected to the discharge electrode through the first discharge path and the second discharge path; and wherein the capacitor is formed by a first capacitor plate in a polysilicon layer and a second capacitor plate in an n-well layer of said device as two parallel plates.

In an embodiment, the first discharge path, the second discharge path and the charge reservoir are implemented in a voltage clamping circuit.

In an embodiment, the plurality of sensor cells is organized as a matrix.

In an embodiment, the device further comprises a shield plate positioned between the sense plate and a substrate of the device, wherein the shield plate and the sense plate are separated from each other by an insulator.

In an embodiment, the discharge electrode forms part of a grid of discharge electrodes.

In an embodiment, said grid is connected to an external ground potential via one or more bonding wires and/or bumps.

In an embodiment, multiple bonding wires are connected in parallel.

In an embodiment, the device further comprises a metal layer between the sense plate and the discharge electrode.

In an embodiment, the sense plate and the discharge electrode are implemented in adjacent metal layers of said device.

In an embodiment, the device further comprises a protective layer that covers the plurality of sensor cells, wherein said protective layer has a thickness of at least <NUM>.

In an embodiment, the device further comprises a first readout stage and a second readout stage, wherein the second readout stage is configured to receive a programmable common mode voltage input.

In an embodiment, a smart card, a wearable device, an Internet of Things device, or a smart grid device comprises a fingerprint sensing device of the kind set forth.

In accordance with a second aspect of the present disclosure, a method of producing a fingerprint sensing device is conceived, comprising providing said fingerprint sensing device with a plurality of sensor cells, wherein each sensor cell comprises: at least one capacitive sense plate and a discharge electrode insulated from the capacitive sense plate; a first discharge path for discharging a first static electricity charge from the capacitive sense plate to a first electric potential terminal; a second discharge path for discharging a second static electricity charge from the capacitive sense plate to a second electric potential terminal; a charge reservoir coupled between the first electric potential terminal and the second electric potential terminal, and configured to take up electrostatic charge originating from the capacitive sense plate, wherein the charge reservoir is implemented as a capacitor; wherein the first electric potential terminal and the second electric potential terminal are electrically connected to the discharge electrode through the first discharge path and the second discharge path; and wherein the capacitor is formed by a first capacitor plate in a polysilicon layer and a second capacitor plate in an n-well layer of said device as two parallel plates.

Fingerprint-based user authentication is usually convenient and fast, and provides a positive user experience. Currently, fingerprint-based authentication is mainly enabled in high-performing mobile computing devices. However, it is expected that there exists a growing market potential for such authentication also in the domain of cost-sensitive and low-performing computing platforms. Thus, it would be useful to enable low-cost fingerprint-based user authentication. One factor that contributes to the cost of a fingerprint sensor is the ESD protection circuitry. ESD protection is often provided by external protection devices that require assembly next to the fingerprint sensing device. A more advanced ESD protection method has been disclosed by patent application <CIT>. Such a more advanced ESD protection method may also have disadvantages. For example, the method disclosed in <CIT> may result in a sense plate with a higher parasitic capacitance. A higher parasitic capacitance will generally result in a lower sensitivity of the fingerprint sensor, and thus in a lower contrast in a fingerprint image, which is undesirable. <CIT>, discloses a fingerprint sensor having electrostatic discharge protection.

Therefore, in accordance with a first aspect of the present disclosure, a fingerprint sensing device is provided, comprising a plurality of sensor cells, wherein each sensor cell comprises: at least one sense plate and a discharge electrode insulated from the sense plate; a first discharge path for discharging a first static electricity charge to a first electric potential terminal; a second discharge path for discharging a second static electricity charge to a second electric potential terminal; a charge reservoir coupled between the first electric potential terminal and the second electric potential terminal. In this way, a low-cost ESD protection may be realized - for example not requiring external protection devices - without negatively the sensitivity of the fingerprint sensing device.

In a practical and efficient implementation, the first discharge path, the second discharge path and the charge reservoir are implemented in a voltage clamping circuit. The charge reservoir is configured to take up electrostatic charge originating from the sense plate. In a practical and efficient implementation, the charge reservoir is implemented as a capacitor. Furthermore, in a practical and efficient implementation, said capacitor is formed by a first capacitor plate in a polysilicon layer and a second capacitor plate in an n-well layer of the fingerprint sensing device. Furthermore, in an embodiment, the plurality of sensor cells is organized as a matrix. This facilitates controlling the sensor cells.

<FIG> show illustrative embodiments of a fingerprint sensing device. The fingerprint sensing device <NUM> shown in <FIG> comprises a sensor integrated circuit (IC) <NUM>. The sensor IC <NUM> comprises a plurality of sensor cells <NUM>. The sensor cells <NUM> comprise sense plates <NUM>. Furthermore, the plurality of sensor cells <NUM> is organized as a sensor matrix <NUM>. Furthermore, in accordance with the present disclosure, the sensor IC <NUM> comprises a discharge electrode <NUM>. In this embodiment, the discharge electrode is common to all sensor cells <NUM> and forms a grid. Furthermore, the sensor IC <NUM> comprises a circuit <NUM> that is common to all sensor cells <NUM> and that may be used, for example, to control the sensor cells <NUM> and to collect measurement results from the sensor cells <NUM>. In this example, the ESD stress may be first lowered by the grid connected to system ground, by flowing a discharge current through the system ground. If any residual charges are still left, then the electrical conduction through the first and second discharge paths may provide additional ESD protection.

<FIG> shows another illustrative embodiment of a fingerprint sensing device <NUM>. <FIG> shows a further illustrative embodiment of a fingerprint sensing device <NUM>. In the embodiments illustrated in <FIG>, the fingerprint sensing device <NUM>, <NUM>, <NUM> comprises a common circuit <NUM> and an array of sensor cells <NUM> configured to measure the capacitance between a sense plate <NUM>, <NUM> of the sensor cells <NUM> and the surface of a finger (not shown). The sensor cells <NUM> are organized as a matrix having i*j sensor cells <NUM>, arranged in i rows and j columns. Each sensor cell <NUM> has at least one sense plate <NUM>, <NUM> operationally coupled to at least one capacitance-to-digital-data-converter (not shown), an ESD electrode <NUM> being insulated from the at least sense plate <NUM>, <NUM>, a first discharge path <NUM> for electrostatic charge from each sense plate <NUM>, <NUM> to a first voltage potential <NUM>, a second discharge path <NUM> for electrostatic charge from each sense plate <NUM>, <NUM> to a second voltage potential <NUM>, a charge reservoir <NUM> coupled between the first voltage potential <NUM> and the second voltage potential <NUM> and configured to take up electrostatic charge originating from the at least one sense plate <NUM>, <NUM>, and a control circuit <NUM> that operationally couples the at least one sense plate <NUM>, <NUM> to a corresponding capacitance-to-digital-data-converter. In an embodiment, a shield plate <NUM> may be positioned between the at least one sense plate <NUM>, <NUM> and a device substrate (not shown), separated by an insulator (not shown). In operation, the shield plate <NUM> may be coupled, by the control circuit <NUM>, to a DC voltage, to cancel the parasitic capacitance between the at least one sense plate <NUM>, <NUM> and the device substrate. Thus, the shield plate <NUM> facilitates cancelling the parasitic capacitance on the sense plate <NUM>, <NUM>.

The first voltage potential rail <NUM> and the second voltage potential rail <NUM> shown in <FIG> are examples of the above-mentioned first electric potential terminal and second electric potential terminal. These terminals are electrically connected to the discharge electrode <NUM> through the discharge paths <NUM> and <NUM>, which may be implemented as a diode configuration. The first voltage potential rail <NUM> is connected to the negative terminal of first discharge path <NUM>. In this example, as the first voltage potential rail <NUM> is positive, it keeps the first discharge path <NUM>, e.g. through a diode, electrically non-conducting (i.e. reverse biased) during normal sequence of fingerprint sensing. But, during ESD stress, when the potential of the discharge electrode <NUM> may become more positive than the voltage, the first discharge path <NUM> electrically conducts through the diode, thereby clamping the discharge electrode <NUM> at the voltage on the first voltage potential rail <NUM>. Similarly, the second voltage potential rail <NUM> is connected to the positive terminal of the second discharge path <NUM>. In this example, as the second voltage potential rail <NUM> is at zero, it keeps the second discharge path <NUM>, e.g. through a diode, electrically non-conducting (i.e. reverse biased) during a normal sequence of fingerprint sensing. But, during ESD stress, when the potential of the discharge electrode <NUM> becomes more negative than the voltage on the second voltage potential rail <NUM>, the second discharge path <NUM> electrically conducts through the diode, thereby clamping the discharge electrode <NUM> at zero, which is the voltage on the second voltage potential rail <NUM>. In this way, the ESD protection may be realized during fingerprint sensing.

As shown in <FIG>, the ESD electrodes of all i*j sensor cells form an ESD electrode grid, which is connected through contact pads <NUM>, <NUM>, bonding wires and/or bumps to a ground potential external to the fingerprint sensing device. In an embodiment, multiple bonding wires are connected in parallel to reduce the coupling inductance. The charge reservoirs of all i*j sensor cells form a common charge reservoir that is configured to collect charge from at least one sensing electrode. The ESD grid provides coupling capacitance to the charged finger. Multiple bonding wires/bumps connected to the ESD grid also facilitate providing a higher current carrying capability for the ESD grid. The common circuit and also circuits within the sensor cell matrix are provided with separate analog and digital supply voltage and ground connection using separate IO pad cells AGND, DGND, AVDD, DVDD, and bonding wires/bumps to avoid noise coupling.

<FIG> illustrate various embodiments for constructing the sense plate, shield plate, ESD electrode and charge storage capacitor.

<FIG> shows an illustrative embodiment a sensor cell shown in cross-section <NUM>. In particular, <FIG> shows an embodiment of a cell constructed by means of a process that uses four metal layers. A first metal layer is used to construct the shield plate <NUM>. A first inter-metal dielectric layer is formed on top of the first metal layer. The sense plate <NUM> is constructed using the second metal layer. A second dielectric layer is formed on top of the second metal layer. A third metal layer is advantageously not utilized on top of the sense plate <NUM>. However, it can be used in other parts of the sensor. A third dielectric layer is formed on top of the third metal layer. A fourth metal layer is used to construct the ESD electrode <NUM>. A passivation layer <NUM> is formed on top of the fourth metal layer and finally a scratch protection coating <NUM> is placed above the passivation layer <NUM>. Accordingly, in an embodiment, the fingerprint sensing device further comprises a metal layer between the sense plate <NUM> and the discharge electrode <NUM>. This embodiment has the advantage of creating more distance between the sense plate <NUM> and the ESD grid <NUM>, which facilitates withstanding higher ESD stress and reduces the parasitic capacitance. The charge storage capacitor is formed using poly layer and the n-well as the two parallel plates <NUM>, <NUM>. The coating <NUM> may have a thickness of at least <NUM>. In this way, the voltage that builds up on the sense plate <NUM> when a finger comes into its proximity can be kept within limits. More specifically, as the thickness of the coating <NUM> increases the induced voltage on the sense plate <NUM> due to ESD stress reduces and can be kept within certain limits to avoid extreme severity of ESD stress on the sense plate <NUM>. Furthermore, as mentioned, the coating <NUM> may provide scratch protection.

<FIG> shows another illustrative embodiment a sensor cell shown in cross-section <NUM>. In particular, <FIG> shows an embodiment of a cell constructed by means of a process that uses three metal layers. A first metal layer is used to construct the shield plate <NUM>. A first inter- metal dielectric layer is formed on top of the first metal layer. The sense plate <NUM> is constructed using the second metal layer. A second dielectric layer is formed on top of the second metal layer. A third metal layer is used to construct the ESD electrode <NUM>. A passivation layer <NUM> is formed on top of the third metal layer and finally a scratch protection coating <NUM> is added on top of the passivation layer <NUM>. Accordingly, in an embodiment, the sense plate and the discharge electrode are implemented in adjacent metal layers of the fingerprint sensing device. This embodiment has the advantage of using less metal layers. The charge storage capacitor is formed using poly layer and the n-well as the two parallel plates <NUM>, <NUM>.

<FIG> shows a further illustrative embodiment a sensor cell shown in cross-section <NUM>. In particular, <FIG> shows another embodiment of a cell constructed by means of a process that uses three metal layers. A first metal layer is used to construct the shield plate <NUM>. A first inter-metal dielectric layer is formed on top of the first metal layer. A sense plate <NUM> is constructed using the second metal layer. The second metal layer is also used for constructing a part of the shield plate <NUM>. The parts of the shield plate <NUM> in the first metal layer and the second metal layer are connected to each other using vias. A second dielectric layer is formed on top of the second metal layer. The third metal layer is used to construct the ESD electrode <NUM>. A passivation layer <NUM> is formed on top of the third metal layer and finally a scratch protection coating <NUM> is added on top of the passivation layer <NUM>. This embodiment has the advantage of better shielding. The charge storage capacitor is formed using poly layer and the n-well as the two parallel plates <NUM>, <NUM>.

<FIG> shows a further illustrative embodiment a sensor cell shown in cross-section <NUM>. In particular, <FIG> shows a further embodiment of a cell constructed by means of a process that uses three metal layers. A first metal layer is used to construct the shield plate <NUM>. The first metal layer is also used for constructing an electrode attached to a first system voltage. The first system voltage may be a VDD supply voltage. A first inter-metal dielectric layer is formed on top of the first metal layer. The sense plate <NUM> is constructed using the second metal layer. The second metal layer is also used for constructing an electrode attached to a second system voltage. The second system voltage may be a system ground voltage GND. A second inter-metal dielectric layer is formed on top of the second metal layer. The third metal layer is used to construct the ESD electrode <NUM>. A passivation layer <NUM> is formed on top of the third metal layer and finally a scratch protection coating <NUM> is added on top of the passivation layer <NUM>. The charge storage capacitor is formed using poly layer and the n-well as the two parallel plates <NUM>, <NUM>.

<FIG> shows illustrative embodiments of discharge path implementations <NUM>. In particular, the first discharge path <NUM> and second discharge path <NUM> shown in <FIG> may be implemented in different ways. In particular, the discharge paths <NUM>, <NUM> may be implemented by diodes, Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), parasitic bipolar junction transistors (pBJTs) or combinations thereof. For example, in a first implementation <NUM> diodes are used, in a second implementation <NUM> MOSFETs are used, and in a third implementation <NUM> MOSFETs and pBJTs are used. In case of diodes, the anode-terminal may be connected to a positive supply voltage, and the cathode-terminal may be connected to circuit ground potential. In case of MOSFETs, the source terminal of the PMOS may be connected to a positive supply voltage, and the source terminal of the PMOS may be connected to circuit ground potential. In case of pBJTs, the emitter of the PNP transistor may be connected to a positive supply voltage, and the emitter of the NPN transistor may be connected to circuit ground potential.

<FIG> and <FIG> show illustrative embodiments of a discharge path in operation <NUM>, <NUM>. The ESD voltage stress is first lowered by the ESD electrode grid <NUM> via providing a larger capacitive coupling to the finger. Due to the capacitive coupling, a positive electrostatic voltage causes a negative charge to be brought on the ESD electrode grid <NUM> from the system ground, which results in a discharge current towards the grid <NUM>. In case of a negative ESD potential, a positive charge is brought on the discharge grid <NUM> by pushing negative charge to the system ground, which results in a discharge current towards the system ground. Although the ESD stress is lowered by the ESD electrode grid <NUM>, it can still damage the devices connected to the sense plate <NUM> if no additional protection is provided. The positive ESD charge on the fingertip induces a positive voltage on the sense plate <NUM>. A discharge path is provided from the sense plate <NUM> for the positive potential as shown in <FIG> to keep the sense plate voltage at a safe level. The discharge path for the positive potential goes through the upper diode connected to the sense plate <NUM> and into the charge storage capacitor, which is present in each sensor cell. The negative electrostatic voltage induces a negative voltage on the sense plate <NUM>. <FIG> illustrates the discharge path from the sense plate <NUM> for the negative voltage to keep it at a safe level. The discharge path for the negative voltage is via the lower diode connected to the system ground potential. The negative potential discharge may also be taken up by the charge storage capacitor coupled between the positive supply voltage and circuit ground.

<FIG> shows illustrative embodiments of supply rail clamp circuits <NUM>, <NUM>. <FIG> shows an illustrative embodiment of an IO cell <NUM>. This IO cell <NUM> may be a IO pad cell AGND, DGND, AVDD, DVDD, as shown in <FIG>. In particular, <FIG> shows supply rail clamps <NUM>, <NUM> that may be required to clamp the supply rail to a safe potential if too much charge is collected by the charge reservoir. In other embodiments, a supply rail clamp may be embedded in an IO pad cell protection circuit <NUM> as shown in <FIG>.

<FIG> shows an illustrative embodiment of a coupling <NUM> from a fingertip to circuit ground. In particular, a schematic view of said coupling is provided. The outer skin of the finger (stratum corneum) may be modeled by parallel coupled capacitances CSCi and resistances Rsci. The resistances RSCi model the moisture dependency of the skin impedance. If the outer skin is dry it may exhibit preferably capacitance behavior, in case of a wet finger preferably resistive behavior may be exhibited. The outer skin components couple to the internal body resistance RB, which in turns couples to the body capacitance CB and to earth potential. In case of mains operated devices, coupling CTR between earth potential and circuit ground is established by the transformer's winding capacitance, thus closing the path from fingertip to circuit ground. In case of mains operated devices CTR may be in the order of hundreds of pF, in case of battery operated devices CTR may be in the order of tens of pF.

<FIG> shows an illustrative embodiment of coupling paths <NUM>. In particular, <FIG> shows the three coupling paths from the fingertip, which acts as a counter electrode for capacitance measurement, to the circuit ground. A first path is established by sense plates being connected by switches to circuit ground. In case of an <NUM>*<NUM>-pixel sensor up to <NUM> sense plates may be connected to ground, thus forming a relatively large coupling capacitor in the nF range. A second path is established, in accordance with the present disclosure, from circuit ground via the ESD grid to the fingertip, also forming a relatively large coupling capacitor in the nF range. A third path is established through the outer skin impedance, body resistance RB, body capacitance CB to earth and coupling capacitance CTR from earth to circuit ground.

<FIG> shows an illustrative embodiment of a capacitance measurement principle <NUM>. A capacitance-to-digital conversion is performed using a common readout circuit, which is connected to each sensor cell in a sequence. The readout circuit includes two stages <NUM>, <NUM>. The first stage <NUM> is a sense amplifier that primarily performs the capacitance-to-voltage conversion using a switched capacitor topology. The first stage voltage output is proportional to the parasitic capacitance between the sense plate and the ESD grid, along with the variations of the ridge/valley sensed capacitance. To ensure a proper dynamic range of the sensed voltage output before analog to digital conversion, the common mode voltage input for the second readout stage <NUM> can be positioned within the peak to peak value of first stage output, which is proportional to the ridge/valley sensed capacitance. The common mode voltage can be made programmable to compensate for the change in parasitic capacitance due to manufacturing process variations. The second readout stage <NUM> with switched capacitor topology incorporates programmable gain adjustment for controlling the brightness of the image proportional to the sensed capacitance. Both the readout stages <NUM>, <NUM> comprise an operational transconductance amplifier (OTA). So as to perform sense capacitance measurement using a common readout circuit, each sensor cell (i,j) in the matrix comprises dedicated switches attached to the sense plate and shield plate. Two dedicated switches are attached to the sense plate. A first switch Sija connects the sense plate to a common row read rail. A second switch Sijb connects the sense plate to system ground potential. Two dedicated switches are attached to the shield plate. A first switch Sijc connects the shield plate to the system ground potential. A second switch Sijd connects the shield plate to a common shield drive rail driven by a fixed reference potential meant for the first stage <NUM> of the readout system.

<FIG> shows an illustrative embodiment of processing steps <NUM>. In particular, <FIG> shows processing steps for converting the capacitance between the sense plate and a finger surface into a computer readable format. The sense cell matrix scanning is started upon receiving a command from the host controller. First a row is selected by closing the corresponding row select switch Sir. This switch remains closed until all sensor cells within the row are scanned. For each sensor cell within the row, in a first step the switch Sija is opened, the reset switch S3 in the first readout stage <NUM> of the common readout circuit is closed to reset the first readout stage <NUM> and the preset switch Sijb is closed to connect the sense plate to the system ground potential. Then, in a second step reset switch S3 is opened, switch Sijb is opened and switch Sija is closed to connect the sense plate to the common row read rail. The first readout stage output starts to build a voltage corresponding to the sense capacitance. Once the first stage output stabilizes, a programmable gain adjustment is performed using the second readout stage <NUM>. After the second readout stage output is stabilized, it is stored in a sample-and-hold circuit <NUM> by closing switch S4. The sample-and-hold output is converted to a digital code using an analog-to-digital converter. After sensing all sensor cells in the matrix, the matrix scanning is stopped.

<FIG> shows an illustrative embodiment of a fingerprint sensing system <NUM>. In particular, it shows how a matrix of sensor cells of the kind set forth can be integrated into a fingerprint sensing system <NUM>. A fingerprint sensing device may comprise, in addition to said matrix (i.e., sensor array <NUM>), a synchronous communication interface and command decoder block <NUM>, a shift register and clock control block <NUM>, a shift register preset block <NUM>, a row select block <NUM>, and capacitance-to-digital conversion and data conditioning block <NUM>. The communication interface <NUM> may be configured to receive commands from a host device <NUM>. An embedded command decoder may be configured to control the shift register and clock control unit <NUM> in response to the commands and associated data received from the host device <NUM>. Read access to individual sensor cells is controlled by the outputs of shift-registers, wherein one shift-register may control access to sensor cells being arranged in one row. Multiple of said shift-registers may enable accessing sensor cells in multiple rows. The shift register and clock control unit <NUM> may be a state machine that in conjunction with said shift-registers may be configured to sequentially select and read individual sensor cells of the sensor array <NUM>. A row-select unit <NUM> may, under control of the shift-register and clock control unit <NUM>, connect an individual sensor cell to a central readout unit included in the row-select unit <NUM>, which may perform a capacitance-to-voltage conversion. The capacitance-to-digital conversion and data conditioning block <NUM> may be configured to convert a voltage level provided by said readout unit into its numerical representation. Said numerical representation may be communicated by the communication interface unit <NUM> to the host device <NUM>.

<FIG> shows another illustrative embodiment of a fingerprint sensing system <NUM>. In particular, <FIG> shows how a sensor unit <NUM> cooperates with a processing unit <NUM> that executes a fingerprint feature extraction function and a fingerprint template generation function. The sensor unit <NUM> may comprise the sensor array <NUM>, synchronous communication interface and command decoder block <NUM>, shift register and clock control block <NUM>, shift register preset block <NUM>, row select block <NUM>, and capacitance-to-digital conversion and data conditioning block <NUM> shown in <FIG>. The processing unit <NUM> may be included in the host device <NUM> shown in <FIG>. In operation, the processing unit <NUM> may recreate a fingerprint image by performing calculations on measurement results received from the sensor unit <NUM>, extract relevant features (e.g., minutiae such as ridge endings and ridge bifurcations), and generate a fingerprint template based on the extracted features, which can be compared with a reference template, for example.

<FIG> and <FIG> illustrate how a generated fingerprint template (i.e., biometric template) can be used to advantage in different applications.

<FIG> shows an illustrative embodiment of a payment system <NUM>. The payment system <NUM> comprises a biometric template generation unit <NUM>, for example the template generation unit comprised in the processing unit <NUM> shown in <FIG>. Furthermore, the payment system <NUM> comprises a secure element <NUM> and a payment network <NUM>. The biometric template generation unit <NUM> and the secure element <NUM> may be embedded, for example, in a mobile phone. The secure element <NUM> comprises a matching component that is configured to compare a biometric template received from the biometric template generation unit <NUM> with a stored reference template. If the received template matches the reference template, then a payment application comprised in the secure element <NUM> may initiate a payment through the payment network <NUM>.

<FIG> shows an illustrative embodiment of an Internet-of- Things (IOT) system <NUM>. The IOT system <NUM> comprises a biometric template generation unit <NUM>, for example the template generation unit comprised in the processing unit <NUM> shown in <FIG>. Furthermore, the IOT system <NUM> comprises a communication controller <NUM> and an IOT network <NUM>. The biometric template generation unit <NUM> and the communication controller <NUM> may be embedded in an IOT device. The communication controller <NUM> comprises a matching component that is configured to compare a biometric template received from the biometric template generation unit <NUM> with a stored reference template. If the received template matches the reference template, then an accounting application comprised in communication controller <NUM> may initiate an accounting operation through the IOT network <NUM>.

<FIG> shows an illustrative embodiment of a smart card <NUM>. The smart card <NUM> comprises a fingerprint sensor <NUM>, which may be a fingerprint sensing device of the kind set forth herein. The fingerprint sensor <NUM> is embedded into a smart card <NUM> together with a processing module <NUM> that may perform different functions. The processing module <NUM> comprises a processing unit (i.e., microcontroller) <NUM>, a secure element <NUM>, and a power management unit <NUM>. An image capture unit of the fingerprint sensor <NUM> is configured to capture a fingerprint image. The secure element <NUM> may execute a payment application requesting authentication from a fingerprint-match-on-card application, wherein said fingerprint-match-on card application communicates with the MCU <NUM> to obtain a fingerprint feature list for matching against a fingerprint reference feature list that is securely stored in the secure element <NUM>. The MCU <NUM> communicates with the fingerprint sensor <NUM> in order to receive an electronic representation of a fingerprint pattern. The MCU <NUM> is further configured to process said electronic representation of a fingerprint pattern in order to extract features from said representation and converting them into said feature list in a machine-readable format. The MCU <NUM> may furthermore provide user feedback through a user feedback device, to guide the fingerprint imaging process. The secure element <NUM> may communicate through an ISO-<NUM> and/or ISO-<NUM> interface with a payment network and/or an identification network (not shown).

Claim 1:
A fingerprint sensing device (<NUM>) comprising a plurality of sensor cells (<NUM>), wherein each sensor cell (<NUM>) comprises:
at least one capacitive sense plate (<NUM>) and a discharge electrode (<NUM>) insulated from the capacitive sense plate (<NUM>) ;
a first discharge path (<NUM>) for discharging a first static electricity charge from the capacitive sense plate (<NUM>) to a first electric potential terminal (<NUM>);
a second discharge path (<NUM>) for discharging a second static electricity charge from the capacitive sense plate (<NUM>) to a second electric potential terminal (<NUM>);
a charge reservoir (<NUM>) coupled between the first electric potential terminal (<NUM>) and the second electric potential terminal (<NUM>), and configured to take up electrostatic charge originating from the capacitive sense plate (<NUM>),
wherein the charge reservoir (<NUM>) is implemented as a capacitor (<NUM>);
wherein the first electric potential terminal (<NUM>) and the second electric potential terminal (<NUM>) are electrically connected to the discharge electrode (<NUM>, <NUM>) through the first discharge path (<NUM>) and the second discharge path (<NUM>); and
wherein the capacitor (<NUM>) is formed by a first capacitor plate (<NUM>) in a polysilicon layer and a second capacitor plate (<NUM>) in an n-well layer of said device (<NUM>) as two parallel plates (<NUM>, <NUM>).