Electrostatic capacitance detection device

#1. Field of Invention

#The present invention relates to an electrostatic capacitance detection device that reads the surface contours of a fingerprint or other target object having extremely small ridges and valleys, by detecting electrostatic capacitance, which changes according to a distance from the surface of the target object.

#2. Description of Related Art

#FIG. 1 is a schematic illustrating the principle of operation in a related art technology.

#FIG. 2 is a schematic illustrating the principle of operation in the present invention.

#FIG. 3 is a schematic illustrating the principle of operation in the present invention.

#The present invention provides an electrostatic capacitance detection device that reads the surface contours of a target object by detecting electrostatic capacitance which changes according to the distance from a target object. The electrostatic capacitance detection device is formed as an MIS thin-film semiconductor device including a metal-insulator-semiconductor layer. Since they are ordinarily fabricated on inexpensive glass substrates, thin-film semiconductor devices are generally known in the related art for their usefulness in inexpensively fabricating semiconductor integrated circuits that require a large area, specifically devices such as liquid crystal display devices. Therefore, fabricating thin-film semiconductor devices for use as an electrostatic capacitance detection device in a fingerprint sensor, for example, obviates the need to use an expensive substrate, such as a single-crystal silicon wafer, whose production consumes tremendous energy. Instead, the device can be fabricated inexpensively on glass without wasting precious global resources. In addition, a semiconductor integrated circuit made up of a thin-film semiconductor device can be fabricated on a plastic substrate by applying the transfer technology disclosed in Japanese Laid Open to the Public No. Hei 11-312811 and by S. Utsunomiya et. al in Society for Information Display, p. 916 (2000). Therefore, such electrostatic capacitance detection devices do not have to be produced on a single-crystal silicon wafer, and instead can also be formed on a plastic substrate.

#As shown in FIG. 1, using a thin-film semiconductor device to produce an electrostatic capacitance detection device that operates on related art principles is impossible with the related art thin-film semiconductor device technology. The charge Q that is induced between two capacitors connected in series is extremely small. The charge Q can be read if single-crystal silicon LSI technology, which enables high-accuracy detecting, is used. However, the charge Q cannot be accurately read with a thin-film semiconductor device. This is because the transistor characteristics in a thin-film semiconductor are not as good as the transistor characteristics obtained with single-crystal silicon LSI technology, and, moreover, because there is a large degree of deviation in characteristics between thin-film semiconductor devices. Therefore, the electrostatic capacitance detection device of the present invention has M number (M is an integer of 1 or more) of individual power-supply lines and N number (N is an integer of 1 or more) of individual output lines arranged in a matrix of M rows and N columns. An electrostatic capacitance detecting element is formed at the intersection between an individual power-supply line and an individual output line. The electrostatic capacitance detecting element includes a signal detection element and a signal amplifying element. The signal detection element includes a capacitance detecting electrode and a capacitance detecting dielectric layer. The charge Q is produced in the capacitance detecting electrode in response to an electrostatic capacitance. In the present invention, this charge Q is amplified by the signal amplifying element formed within each electrostatic capacitance detecting element and is converted to current. Specifically, the signal amplifying element includes an MIS thin-film semiconductor device for signal amplification, including a gate electrode, a gate insulator, and a semiconductor layer. The gate electrode of the MIS thin-film semiconductor device for signal amplification is connected to the capacitance detecting electrode. The operating principle of the present invention is shown in FIG. 2. A charge occurring between a capacitor having an electrostatic capacitance Cs and a capacitor having an electrostatic capacitance CF, which changes according to the surface contours of a target object, changes the gate potential of the MIS thin-film semiconductor device for signal amplification. Thus, if a predetermined voltage is applied to the drain region of this thin-film semiconductor device, the current I that flows between the source and the drain of the thin-film semiconductor device in proportion to the induced charge Q is markedly amplified. The induced charge Q itself does not flow elsewhere but is retained. Hence, the measurement of the current I is also facilitated by raising drain voltage or by extending the length of measuring time and so forth. Consequently, the surface contours of a target object can be measured with sufficient accuracy even if a thin-film semiconductor device is used.

#As mentioned above, in the present invention an MIS thin-film semiconductor device for signal amplification is used as the signal-amplifying element. In this case, the MIS thin-film semiconductor device for signal amplification itself can also serve as a capacitor having an electrostatic capacitance CS. In other words, the transistor capacitance CT of the MIS thin-film semiconductor device for signal amplification serves as a new electrostatic capacitance in place of the electrostatic capacitance Cs. Such a design enables a capacitor having electrostatic capacitance CS to be eliminated from the electrostatic capacitance detecting element. Not only does this simplify the structure, it also makes the fabrication process easier. In addition, consolidating the two power supplies depicted in FIG. 2 into a common power supply Vdd is an effective way to eliminate superfluous wires within an electrostatic capacitance detection device. The equivalent circuit diagram for the principle of operation in this condition is shown in FIG. 3. A capacitor having an electrostatic capacitance CF, which changes according to the surface contours of the target object, and a capacitor having the transistor capacitance CT are connected in series. Strictly speaking, the transistor capacitance CT is an electrostatic capacitance that is formed between the drain electrode and gate electrode of the MIS thin-film semiconductor device for signal amplification. To realize the configuration of FIG. 3, it is sufficient to connect the source region of the MIS thin-film semiconductor device for signal amplification to an individual output line and to connect the drain region of an MIS thin-film semiconductor device for signal amplification to an individual power-supply line, whereupon a voltage Vdd is applied to the individual power-supply line, and a current I, which changes according to the surface contours of the target object, is extracted.