1. Field of the Invention
This invention relates to sensor device for performing measurements on an at least partially conductive surface, specially a sensor geometry to facilitate AC capacitive fingerprint measurements on wet and dry fingers.
2. Description of the Related Art
AC capacitive fingerprint sensors are known e.g. firm international patent applicaton WO 98/58342. These sensors utilize an array of capacitive sensor elements (sensor pads) to capture a fingerprint image from a finger which is held onto or pulled across the sensor surface. Capacitive fingerprint sensors roughly divide into two categories: matrix sensors, where the fingerprint is placed onto a two-dimensional sensor surface, and scanners or sweep sensors, where the user is required to pull his or her finger across the sensor to capture an image. An alternative linear type sensor is described U.S. Pat. No. 6,289,114, this does not, however, describe in any detail how the capacitance is measured.
Each capacitive sensor element normally consists of a conductive pad covered by a dielectric material (sensor dielectric) with a thickness ranging from around 1 μm to around 50 μm. When a ridge is present above a sensor element, the conducting skin we be in close contact with the sensor surface, and the capacitance between finger and sensor pad is determined mainly by the thickness and dielectric properties of the sensor dielectric. When a valley is present above the sensor element, there will be an air gap between the finger surface and the sensor surface, resulting in a much lower capacitance. By converting said capacitance to a voltage or current signal that can be amplified and further processed or digitalized, it will therefore be possible to distinguish a ridge from a valley, and consequently to build up a grayscale or black-white image of the ridge/valley pattern across the fingerprint.
Another example of such a concept is shown in WO 01/99035, WO 01/99036 and WO 01/94902, which show an the constructions and operation of a AC capacitive sweep sensor. This sensor is equipped with a so-called drive electrode or stimulus electrode for stimulating the finger with an AC signal. The stimulus electrode is located aside the sensor elements. In Norwegian application NO 2002 1031 (WO 03/075210) an alternative concept is described in which the stimulation electrode is grounded while the varying current or voltage is provided through the sensor elements.
One problem for capacitive fingerprint sensors is that the finger conductivity (both resistive and AC capacitive) vary strongly with the humidity of the finger. For dry fingers, the result may be that the outer part of the skin (stratum corneum) has a higher impedance (lower capacitance) than the sensor dielectric, so that the combined series capacitance is dominated by the finger impedance. This ridge may then seem more like a valley than like a ridge.
Another problem may be that the ridge itself is not perfectly smooth, resulting in ridge areas where there is a shallow air gap between the ridge and the sensor surface. Also in this case this past of the ridge may be mistaken for a valley, especially if the sensor dielectric is very thin (comparable to the depth of the air gap). It therefore necessary to find a means of amplifying the faint signal from dry fingers, and also be able to separate the “false” shallow valleys from the real valleys.
For wet fingers, where water or saline (sweat) fill the valleys, the problem is mainly that the saline is even more conductive than the stem corneum. With the described capacitive measuring technique this gives a high signal for both valleys and ridges, so that it is difficult to separate the two. When converted to a digital image of the finger, the result may be a low-contrast “inverted” image where the well-conducting valley appear as “black” and the slightly less conductive ridges appear as “dark grey”. Such an image does not easily lend itself to software algorithms aimed at fingerprint recognition.
Norwegian patent application 2002 1031 shows another principle where the outer electrode is grounded, and where the AC signal is applied to the sensor pads by drive circuits located elsewhere in the sensor module. This principle however has the same drawback: Sweat-filled valleys will give approximately the same signal as a fingerprint ridge, and the two will be practically impossible to distinguish.