The present invention relates to a sensor for sensing the surface shape of a human finger or animal nose having a small three-dimensional (i.e., surface shape) pattern, and a collation/recognition technique.
In the social environment of today where the information-oriented society is developing, the security technology has taken a growing interest. For example in the information-oriented society, a personal authentication technology for constructing an electronic money system is an important key. In fact, authentication technologies for implementing preventive measures against burglary and illicit use of cards are under active research and development (for example, Yoshimasa Shimizu, “A Study on the Structure of a Smart Card with the Function to Verify the Holder”, Technical Report of IEICE, OFS92-32, pp. 25-30, (1992)). Such authentication techniques include various schemes using a fingerprint or voiceprint. Especially, many fingerprint authentication techniques have been developed.
Fingerprint authentication schemes are roughly classified into optical read schemes, schemes using human electrical characteristics, and schemes of detecting a three-dimensional pattern on the skin surface of a fingertip and converting it into an electrical signal. In an optical read scheme, fingerprint data is received mainly using light reflection and a CCD and collated (for example, Japanese Patent Laid-Open No. 61-221883). A scheme using a piezoelectric thin film to read the pressure difference on a finger skin surface has also been developed (for example, Japanese Patent Laid-Open No. 5-61965). As a scheme of converting a change in electrical characteristics due to contact of skin into an electrical signal distribution to detect the fingerprint, an authentication scheme of detecting a resistance change amount or capacitance change amount using a pressure sensitive sheet has been proposed (for example, Japanese Patent Laid-Open No. 7-168930).
However, of the above techniques, the scheme using light is hard to achieve size reduction and versatility, and its application purpose is limited. The scheme of sensing a three-dimensional pattern on the skin surface of a fingertip can hardly be put into practical use and is poor in reliability because of special materials and difficulty in working. A capacitive fingerprint sensor using an LSI manufacturing technology has also been proposed (for example, Marco Tartagni and Roberto Guerrieri, A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme, 1997 IEEE International Solid-State Circuits Conference, pp. 200-201 (1997)).
In this method, small sensors two-dimensionally arrayed on an LSI chip detect a three-dimensional pattern of a skin using a feedback electrostatic capacitance scheme. For this capacitive sensor, two plates are formed on the uppermost layer of LSI interconnections, and a passivation film is formed thereon. The skin surface functioning as a third plate is spaced apart by an insulating layer formed by air. Sensing is performed on the basis of the distance difference, thereby detecting the fingerprint. As characteristic features of this structure, no special interface is required, and the size can be reduced, unlike the conventional optical scheme.
However, such a conventional recognition apparatus has a large surface area occupied by the sensor portion and suffers several problems.
As the first problem, cost of a system construction becomes high when the yield of LSI development is taken into consideration. As the second problem, although an apparatus with a small contact area is hard to break and can stand mechanical stress, the apparatus readily breaks from the viewpoint of reliability because of the large volume. More specifically, in a conventional recognition apparatus, a sensor circuit device 81 whose sensor portion has a large surface area, a collation processing circuit device 82, and a memory circuit device 83 are mounted as a multi-chip structure to perform authentication, as shown in FIG. 17.
In this case, for example, authentication processing for a fingerprint is done using a sensor having a large finger contact area, and the image area used for authentication equals the contact area. Hence, the area at the sensor-circuit device 81 increases to impede cost reduction. Even when not multi-chip mounting but one-chip mounting is employed, the chip area inevitably increases. Obviously, the cost increases from the viewpoint of yield of LSIs, and the structure also poses a problem of reliability.
As illustrated in FIG. 18, a scheme has also been proposed, in which a number of pixel circuits 92 are provided to oppose a number of sensor elements 91, respectively, and a sensor circuit for driving a sensor element 91, a memory circuit for storing collation data, and a collation processing circuit are mounted in a corresponding pixel circuit 92, thereby integrating the authentication unit and sensor (for example, S. Shigematsu, H. Morimura, Y. Tanabe, and K. Machida, “A 15×15-mm2 single-chip fingerprint sensor and identifier using pixel-parallel processing”, 1999 IEEE International Solid-State Circuits Conference, (1991)).
In this case, pixel circuits in number corresponding to the finger contact area are necessary. The chip becomes large depending on the mounting area. For this reason, the cost cannot be reduced from the viewpoint of yield of LSIs.