GB2418283A

GB2418283A - Array of switches having triangular electrodes

Info

Publication number: GB2418283A
Application number: GB0420956A
Authority: GB
Inventors: William Leslie Ross
Original assignee: Personal Biometric Encoders Ltd
Current assignee: Personal Biometric Encoders Ltd
Priority date: 2004-09-21
Filing date: 2004-09-21
Publication date: 2006-03-22
Also published as: GB0420956D0

Abstract

An switch array including, a switch (1) comprising first and second contact portions (3, 6) separated by a gap, and a bridge member (7, 8) for connecting the first and second portions across the gap. The first contact portion has a first edge facing the second contact portion, and at least one other edge facing a contact portion of at least one adjacent switch. The area of the gap between the first edge and the second contact portion is less than the area of the gap between the at least one other edge and the contact element of the at least one adjacent switch. The switch array is used to digitise fingerprints.

Description

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ARRAY OF SWITCHES

The present invention is concerned with an array of switches.

Embodiments of the present invention are particularly concerned with an array of switches defining a digitiser including a matrix or array of individual sensing ceils for sensing the presence or absence of an epidermal fingerprint ridge and thereby sensing fingerprint patterns.

Embodiments of the present invention are concerned with improvements to fingerprint sensors of the type described in EP 459,808, EP 699,325, GB 2,243,235, WO 98/11499 and Now 98/11500.

Both WO 98/11500 and WO 98/11499, for example, describe digitisers defined by a matrix of sensing cells where each sensing cell is defined by a switch which can be closed in response to the presence of a fingerprint epidermal ridge. A problem with the prior art systems is the possibility that current can sneak between adjacent sensing cells. This can distort the sensor output and restricts how close adjacent sensing cells can be.

The present invention provides an array of switches as set out in claim 1.

Preferred features of the invention are defined in the dependent claims.

Embodiments of the invention will now be described, by way of example only, with reference to the attached figures (which are not to scale), in which: Figures 1 and 2 are cross-sectional views through fringerprint sensors embodying the present invention; Figures 3 and 4 illustrate the sensing (i.e. upper) surface of the sensors of figures 1 and 3 respectively; and Figures 5a and 5b illustrates the relative resistance of the different current paths available from the contact pads of figures 3 or 4.

As discussed above, the present invention is concerned with digitisers comprising a matrix of individually actuable sensing cells 1. The cells are defined by the cross-over points of two sets of parallel conductors 2, 3 A second set of parallel conductors 3 overlies and is separated from a first set of parallel conductors 2.

In order to create a switch at each cross-over point, a conductive via through the insulating material 5 separating the two sets of conductors 2, 3 connects a lower conductor 2 with a contact pad 6 in the same plane as the second conductor 2 (or alternatively in an arrangement not illustrated, a second contact pad itself connected to the second conductor). One set of conductors defines input on drive buses and the other set of conductors defines output or sensing buses.

The present invention is essentially only concerned with the shape of the contact pads 6, and how this relates to the conductor with which it is connected when the respective switch is closed. The construction of digitisers embodying the present invention is in all other aspects identical to one or other of the construction described in EP 459, 808, EP 699,325, GB 2243,235, WO 98/11499 and/or WO 98/11500. For efficiency of the process, the applicant therefore expressly incorporates the disclosure of these documents in this application and confirms that the described digitisers are or may be constructed in one or other of the manners described in all of these publications.

As described in, for example, WO 98/11499 and/or 98/11500, each sensing cell 1 comprises a second conductor 3 overlying and separated from a first conductor 2 with the first conductor 2 being connected to a contact pad 6. A resiliently deformable membrane 7 including a continuously conductive film 8 of the type described in WO 98/11499 overlies the switch contacts defined by the contact pads 6 and second conductors 3. Downward pressure on the continuously conductive membrane 7 caused by, for example, pressure from an epidermal fingerprint ridge causes the film 8 to contact the second conductor 3 and contact pad 6 at a respective cell and thereby allows current to flow between the first and second conductors 2,3 (one of which defines an input bus and the other an output bus).

In the prior art digitizers, the contact pads are rectangular.In the digitisers of Figure 3 and 4, the contact pad 6 is substantially triangular. The inventor has appreciated that this results in a lower likelihood of sweat currents when using a continuously conductive membrane of the type described in WO 98/11499 to form the contact bridge for the array of switches making up the digitiser.

The resistance of a continuously conductive membrane of the type described in WO 98/11499 is defined as ohms per square (n/1 per square.

Referring to Figure 5, the use of a triangular contact pad 6 means that the effective resistance of the continuously conductive membrane between the contact pad and its respective conductor can be made significantly lower than that between the contact pad and adjacent contact pads and/or just conductors of adjacent sensing cells. For the example, in the configuration of Figure 5a there are 6 squares 9 in parallel between the contact pad and the second conductor, 6 squares in series between the contact pad and adjacent contact pads, and 4 squares in series between the contact pad and the sensor conductor of all adjacent sensing cell. As shown in Figure 5b this means that for a continuously conductive membrane having a resistance of RO/l, the resistance between the contact pad and its second conductor is significantly lower that that between the contact pad and other elements. This reduces the likelihood of sneak currents and allows one to eliminate or reduce the effect of any sneak currents by setting appropriate measurements thresholds in the circuitry which monitors the status of the sensing cells.

Claims

1. An array of switches including, a switch comprising first and second contact portions separated by a gap, and a bridge member for connecting the first and second portions across the gap, wherein the first contact portion has a first edge facing the second contact portion, and at least one other edge facing a contact portion of at least one adjacent switch, and wherein the area of the gap between the first edge and the second contact portion comprises less squares than the area of the space between the at least one other edge and the contact element of the at least one adjacent switch.

2. An array of switches according to claim 1 wherein the first contact portion is substantially triangular

3. An array of switches according to any preceding claim, the array being defined by the cross-over points of a plurality of parallel first conductors and a plurality of parallel second conductors overlying the first conductors, and switch means being associated with each cross-over point, each switch means including a first contact portion connected to one of the first conductors and a second contact portion connected to one of the second conductors wherein the first and second contact portions of each switch are separated from one another, and the array further including an overlying resiliently deformable surface layer providing, for each switch, a contact bridge for connecting the contract portions of the switch means in response to an applied pressure on the surface layer.

4. An array of switches according to claim 3 wherein the plurality of parallel first conductors are substantially orthogonal to the plurality of second conductors, and the said first edge of the first contact portiont is substantially parallel to the second conductors.

5. An array of switches according to any preceding claim wherein a switch includes a first contact pad defined by said first contact portion and being electrically connected to one of said first conductors.

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6. An array of switches as claimed in claim 5 including an insulative layer overlying the first conductors, and having respective first vies for each first contact pad.

7. An array of switches as claimed in claim 6, wherein the second conductors are supported upon the insulative layer, the second contact portions are formed by second contact pads connected to the second conductors, and wherein the first and second contact pads are supported by the insulating layer.

8. An array of switches according to any preceding claim wherein the surface layer is a resiliently deformable membrane.

9. An array of switches according to claim 8 wherein the membrane has a continuously conductive film.

10. An array of switches according to any of claims 3 to 9 wherein the first and second conductors are formed on opposite sides of an insulating support, the first contact portions are on the same side of the support as the second conductors and each first contact portion is connected to its respective first conductors by a conductive via passing through the insulating support.

11. A fingerprint sensor including an array of switches according to any preceding claim.

12. A digitiser substantially as hereinbefore described with reference to the attached figures

13. A fingerprint sensor substantially as hereinbefore described with reference to the attached figures

14. A contact pad substantially as hereinbefore described with reference to the attached figures.