1. Field of the Invention
This invention relates to a surface pressure distribution sensor which is suitable for detecting microscopic asperity patterns such as fingerprint patterns by using a flexible conductive film, and also relates to a manufacturing method of such a sensor.
2. Description of the Related Art
FIGS. 14A–14B show an example of an active matrix surface pressure distribution sensor for detecting fingerprint patterns. FIG. 14A is a plan view of the device, and FIG. 14B and FIG. 14C are cross sectional views taken along line D—D shown in FIG. 14A.
A conventional sensor 200 for surface pressure distribution includes a substrate 201 which is made of a glass, a ceramic or the like, and a common electrode film 202. The device also has a number of TFTs (thin film transistors) 204a thereon as unit detection elements.
Each of the unit detection elements 204 includes TFT 204a and contact electrode connected thereto. The unit detection elements 204 are arranged in the form of a matrix on the substrate 201. The active layers of the TFTs of the unit detection elements 204 are made of an amorphous silicon film. The contact electrodes 204b are made of ITO (indium tin oxide).
The common electrode film 202 is provided so as to face the substrate 201, and includes a flexible insulator film 202a and a conductive film 202b deposited on the rear side of the film 202a (TFT side). The common electrode film 202 is fixed on a sealing agent 203 applied around the substrate 201 so as not to be in contact with the substrate 201.
An example of a manufacturing method of this surface pressure distribution sensor will be described. After the TFTs are formed on the substrate 201, the sealing agent 203 made of a low temperature thermosetting resin is applied around the substrate 201 in order to affix the common electrode film 202 thereon. The common electrode film 202 is then affixed on the substrate 201 and subjected to a heat treatment. Consequently, the substrate 201 and the common electrode film 202 are fixed to each other.
FIG. 14C shows an example of detecting fingerprint patterns by using this surface pressure distribution sensor. By placing a finger F to press slightly the top of the sensor 200, the common electrode film 202 as a whole is pressed down. However, the difference in pressure between the peaks and the valleys of the fingerprint pattern causes only the contact electrodes 204b of the unit detection elements 204 directly below and in the vicinity of the peaks to come into electrical contact with the common electrode film 202. On the other hand, the contact electrodes 204b of the unit detection elements 204 directly below and in the vicinity of the valleys of the fingerprint pattern are not in electrical contact with the common electrode film 202. Hence, the signals corresponding to the regions in which the common electrode film 202 and the unit detection elements 204 come into contact with each other are generated so as to detect fingerprint patterns.
It is known that a surface pressure distribution sensor with TFTs can be realized by the above-mentioned structure and manufacturing method. However, the reproducibility of such devices is poor when mass-produced.