The present invention relates to an uneven pattern sensing device for sensing an uneven pattern such as fingerprints.
Personal computers containing a fingerprint recognition system have been widely used in financial institutions and public offices such as the police, who often deal with highly confidential information. Meanwhile e-commerce such as stock transactions is becoming pervasive in accordance with increase of Internet users via mobile phones and development of IT industry, so that not only personal computers but also mobile phones and other mobile terminals etc. start to include the fingerprint recognition system therein.
Conventionally developed devices which include this kind of fingerprint recognition system are such as an optical fingerprint sensing device by which light and dark patterns formed by unevenness of a fingerprint is converted to charge information so as to be sensed and a capacitive fingerprint sensing device for electrically sensing a difference of capacitances formed by unevenness of a fingerprint.
Both of these two types of fingerprint sensing devices generally obtain two-dimensional distribution information of unevenness of a fingerprint by means of an active element array in which each pixel includes switching elements, by sensing charge information of each pixel.
The optical fingerprint sensing device requires a light source such as an LED and hence it is impossible to down-size a sensor as a whole including the fingerprint sensing device. Moreover, a structure of the fingerprint sensing device is complex since photo sensing elements such as a photo diode and a photo transistor have to be provided on the active element array.
In the meantime, the capacitive fingerprint sensing device is superior to the optical fingerprint sensing device in terms of the size, manufacturability, production cost and so forth. On this account, these days the capacitive fingerprint sensing device is more popular than the optical fingerprint sensing device.
What is shown in FIG. 12 as an example is a cross section of a capacitive fingerprint sensing device described in U.S. Pat. No. 5,325,442 (disclosed on Jun. 28, 1994). FIG. 13 also shows a plan of the same capacitive fingerprint sensing device. As can be seen in FIG. 12, in the fingerprint detection device, sense electrodes 103 arranged in a two-dimensional array (see FIG. 13) are provided on an interlayer insulating film 102 on a chip-shaped silicon (Si) substrate 101 on which a signal sense circuit (row drive circuit, sense circuit) etc. are formed, and a protective film 104 is provided so as to cover these sense electrodes 103. The fingerprint sensing device is manufactured using a standard CMOS process.
In the case of this type of CMOS fingerprint sensing device, i.e. the fingerprint sensing device including a MOS transistor formed on the Si substrate 101, a surface of the protective film 104 has differences in level in accordance with irregularity of heights between under the protective film 104 such as the interlayer insulating film 102, unillustrated metal wiring, electrical members like the sense electrodes 103 and so forth. On account of the differences, the protective film 104 is ripped so that the element becomes prone to degradation, when a finger touches a surface of a sensing part to obtain a fingerprint.
Although the interlayer insulating film 102 under the sense electrodes 103 seems flat in FIG. 12, this figure is a schematic view so that the Si substrate 101 is portrayed as flat for the sake of convenience. In reality the surface of the interlayer film 102 has certain differences in level (0.5 to 1.0 xcexcm, for instance) in accordance with irregularity of heights between the MOS transistor and the metal wiring on the Si substrate 101.
Moreover, an SiO2 thin film is generally used as an interlayer insulating film of the MOS transistor using an Si substrate. However, it is difficult to smooth out differences in level on the surface of the interlayer insulating film made of the SiO2 thin film, since the shape of the film reflects that of underlying members.
Japanese Laid-Open Patent Application No. 2000-194825 (Tokukai 2000-194825; published on Jul. 14, 2000) discloses an arrangement of a fingerprint sensor capable of smoothing out a surface thereof using a protective film on sense electrodes. In this arrangement an SiNx film is adopted as the protective film to smooth out the surface of the protective film. As methods to smooth out the surface of the protective film, a method to etchback the surface after applying a resist thereon and a method to smooth out the surface using chemical mechanical polishing (CMP) are adopted. Alternatively it is possible to smooth out the surface of the protective film by applying a spin on glass (SOG) material as the surface protective film. By these methods it is possible to smooth out the surface so as to improve reliability of the fingerprint sensor.
However, the arrangement described in Japanese Laid-Open Patent Application No. 2000-194825 uses the SiNx film as the surface protective film so that the step to etchback the surface after applying the resist thereon or the CMP step are required. Thus the number of the manufacturing process is increased and the production yields are reduced, and hence the fingerprint sensor is manufactured at high cost.
An Si substrate is inherently expensive and hence manufacturing cost of the MOS transistor using an Si substrate tends to be high, thus cutting costs in manufacturing the same has been required. Especially this high manufacturing cost due to the increase of the manufacturing process becomes fatal flaw when one tries to mount the fingerprint detecting device on consumer-oriented cheep PDAs and mobile phones.
Also, a variety of materials available for the protective film is limited when the SOG material is adopted as the surface protective film. For instance, only a few types of SOG materials such as SiO2 are generally obtainable. Therefore in this case, as the protective film it is impossible to adopt a wide variety of functional protective films using materials with added value such as a material having characteristics for improving the sensitivity of the fingerprint sensing device and a material with pollution resistance, other than the few types of films.
It is a main object of the present invention to provide an uneven pattern sensing device capable of smoothing out a surface thereof without any increase of a manufacturing process and limitation of a choice of materials for a protective film.
To solve the aforementioned problems, the uneven pattern sensing device in accordance with the present invention includes:
scanning lines and signal lines formed on a substrate in a grid pattern;
switching elements formed on the substrate in each grid and connected to the scanning lines and the signal lines;
an insulating film, formed on the substrate so as to cover the switching elements, having contact holes each of which is a perforation;
sense electrodes formed on the insulating film and connected to the switching elements via the contact holes; and
a protective film formed on the insulating film so as to cover the sense electrode,
wherein the insulating film has a flat area, which excludes a surface where each contact hole is provided, on which the sense electrode is provided.
According to this arrangement, the insulating film has a flat area, which excludes a surface where each contact hole is provided, on which the sense electrode is provided. This makes it also possible to form a flat protective film on the insulating film. Thus in the protective film it is unnecesary to conduct smoothing processes such as a process to etchback by applying a resist and a CMP process, so that a choice of materials for the protective film is not limited. Consequently, as the protective film it is possible to adopt a wide variety of functional protective films using materials with added value such as a material having characteristics for improving the sensitivity of the uneven pattern sensing device and a material with pollution resistance.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.