IC sensor directly records fingerprints electronically

At this year`s CardTech/SecurTech conference in Orlando, FL, SGS-Thomson Microelectronics (Lincoln, MA) demonstrated a prototype integrated circuit (IC) that can register the pattern of a human fingertip by detecting variations in the electrical currents running along the ridges and valleys of the skin. Its electronic fingerprint imaging sensor design differs substantially from existing technologies. Through direct physical contact, the sensor array captures a fingerprint pattern without using a

Aug 1st, 1997

IC sensor directly records fingerprints electronically

At this year`s CardTech/SecurTech conference in Orlando, FL, SGS-Thomson Microelectronics (Lincoln, MA) demonstrated a prototype integrated circuit (IC) that can register the pattern of a human fingertip by detecting variations in the electrical currents running along the ridges and valleys of the skin. Its electronic fingerprint imaging sensor design differs substantially from existing technologies. Through direct physical contact, the sensor array captures a fingerprint pattern without using a scanner or a camera.

When a finger is placed onto the chip`s silicon surface, capacitive sensors register the fingerprint pattern by interacting with electric-field variations produced by the skin`s ridges and valleys. The chip then creates an electrical representation of the fingerprint.

"Some of the applications for this electronic fingerprint imaging sensor include electronic security, physical identification for access to automobiles and buildings, and personal-identification-number replacement," says Alan Kramer, director of the Innovative Systems Design Group of SGS-Thomson`s Central Research and Development Division.

The fingerprint imager works via a feedback capacitive sensing scheme. Two metal plates are placed adjacently in the cell area and separated from the skin surface by a silicon oxide layer. In touching the imager, the finger forms a third layer or plate opposed to the metal layer.

System operation

In operation, each charge amplifier on the imager is first reset, shorting the input and the output of an inverter device. By shorting the input and output, the inverter settles to its logical threshold. After settling, a fixed amount of charge is sinked from the input, causing an output voltage swing inversely proportional to the feedback capacitance value. This feedback capacitance, being inversely proportional to the distance of the skin from the sensor, makes the output voltage linearly dependent on this distance.

Because the distance between the skin and sensor identifies the presence of ridges and valleys, an array of cells is used to sample the fingerprint pattern. This array is addressed in a raster mode by means of horizontal and vertical readouts.

The current prototype can capture fingerprint images of 390 dpi, enough to provide reliable fingerprint matching. Future prototypes are expected to increase the resolution to 750 dpi.

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