Along with the advance of the information-oriented society and in the environment of the modern society, the security technology has received a great deal of attention. For example, in the information-oriented society, a personal authentication technology for constructing a system for, e.g., electronic cashing is an important key. Authentication technologies for preventing theft or unauthorized use of credit cards have also been extensively researched and developed. (Reference 1: Yoshimasa Shimizu et al., “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))
There are various kinds of authentication schemes such as fingerprint authentication and voice authentication. In particular, many fingerprint authentication techniques have been developed so far. Fingerprint reading schemes include an optical scheme which includes an optical system such as a lens and illumination, a pressure scheme using a pressure sheet, and a semiconductor scheme by which a sensor is formed on a semiconductor substrate. Of these schemes, the semiconductor scheme allows easy miniaturization and generalization. An example of the semiconductor sensor is a capacitive fingerprint sensor using the LSI fabrication technique. (Reference 2: Marco Tartagni and Robert Guerrieri, “A 390 dpi Live Fingerprint Imager Based of Feedback Capacitive Sensing Scheme”, 1997 IEEE International Solid-State Circuits Conference, pp. 200–201 (1997)) This fingerprint sensor senses the three-dimensional pattern of a skin by using the feedback capacitive scheme by a sensor chip in which small capacitive sensors are two-dimensionally arranged on an LSI.
This capacitive sensor will be described below with reference to a sectional view in FIG. 14. The sensor includes a sensor electrode 1403 formed on a semiconductor substrate 1401 via an interlevel dielectric 1402, and a passivation film 1404 which covers the sensor electrode 1403. Although not shown in FIG. 14, on the semiconductor substrate 1401 below the interlevel dielectric 1402, a sensing circuit which is an integrated circuit including a plurality of MOS transistors and an interconnecting structure is formed. When a finger as an object of fingerprint sensing touches the passivation film 1404 (sensing surface) of the sensor chip having the above arrangement, the sensor electrode 1403 and the skin of the finger function as electrodes to form a capacitance.
This capacitance is sensed by the sensing circuit described above via an interconnection (not shown) connected to the sensor electrode 1403. However, since the capacitive fingerprint sensor uses the skin as an electrode, the built-in integrated circuit of the sensor chip is electrostatically destroyed by the static electricity generated at the fingertip.
To prevent this electrostatic destruction of the capacitive fingerprint sensor described above, a surface shape recognition sensor including a capacitive sensor having a sectional structure as shown in FIG. 15 is proposed. The arrangement of the sensor shown in FIG. 15 will be explained below. This sensor includes a sensor electrode 1503 formed on a semiconductor substrate 1501 via an interlevel dielectric 1502, a deformable plate-shaped moving electrode 1504 positioned at a predetermined distance from the sensor electrode 1503, and a support member 1505 which is formed around the sensor electrode 1503 so as to be insulated and separated from the sensor electrode 1503, and supports the moving electrode 1504.
When a finger as an object of fingerprint sensing touches the moving electrode 1504, the pressure from the finger deflects the moving electrode 1504 toward the sensor electrode 1503, thereby increasing the capacitance formed between the sensor electrode 1503 and moving electrode 1504. This capacitance is sensed by a sensing circuit on the semiconductor substrate 1501 via an interconnection (not shown) connected to the sensor electrode 1503. In this surface shape recognition sensor, when the moving electrode 1504 is grounded via the support member 1505, the static electricity generated at the fingertip flows to ground via the support member 1505 even if the electricity is discharged to the moving electrode 1504. This protects the built-in sensing circuit incorporated below the sensor electrode 1503 from the electrostatic destruction.
In addition to the surface shape recognition sensor shown in FIG. 15, a structure having a cubic projection 1601 as shown in FIG. 16 is also proposed. (Reference 3: Japanese Patent Laid-Open No. 2002-328003) In this structure, a force from a finger 1602 can be transmitted to the moving electrode 1504 more efficiently than in the structure shown in FIG. 15.
Unfortunately, the above conventional fingerprint sensors have the problem that no desired high sensitivity can be obtained. For example, in the fingerprint sensor having the arrangement shown in FIG. 14, the sensitivity largely changes in accordance with the state of the finger surface, so it is not easy to stably obtain high sensitivity. Also, in the fingerprint sensor having the arrangement shown in FIG. 15, no large change in upper electrode can be obtained, and this makes it impossible to obtain high sensitivity.
Furthermore, in the structure shown in FIG. 16, the projection 1601 is readily damaged by a force applied sideways to the moving electrode 1504, e.g., a force produced by scratching, and this lowers the mechanical strength. In addition, in the structure shown in FIG. 16, the projection 1601 sinks into the finger 1602 if it is soft, and the force disperses in a region on the support member 1505 of the moving electrode 1504, thereby lowering the sensitivity.