Here are the fingerprint sensing techniques involving electrons interacting with the skin.
Several configurations exists:
Making a fingerprint sensor based on capacitance is easy with a CMOS standard process, and many companies are proposing capacitance-based sensors, active or passive.
Passive Capacitance / Capacité passive |
After optical sensing, the measurement of the capacitance between the skin and the pixel
is the most physical effect used to acquire fingerprints.
Where there is a ridge or a valley, the distance varies, as does the capacitance.
Because an electrical field is measured and the distance between the skin and the pixel must be
very low to provide enough sensitivity, the coating must be as thin as possible (a few microns).
A significant drawback is vulnerability to strong external electrical fields,
the most dangerous being ESD (Electro-Static Discharge).
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Companies proposing (or having proposed) capacitance sensing:
Compagnies proposant (ou ayant proposé) des capteurs d'empreintes capacitifs:
Laboratories working (or having worked) on silicon capacitance sensors:
Laboratoires travaillant (ou ayant travaillés) sur les capteurs d'empreintes capacitifs:
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500 dpi, 32x32 pixels. |
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160x192 pixels. |
TFT / Flat panelFlat panel technology rather than bulk silicon has been studied by Philips Korea and some others, using capacitive sensing. La technologie des écrans plats a été étudiée par Philips comme substitut du silicium,
en utilisant un effet capacitif.
Alps Electric développe 2 capteurs d'empreinte. L'un d'eux est transparent, électrostatique (capacitif). |
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RF field - Active capacitance / transmission RF - capacitif actif |
Sometimes confused with capacitance sensing, the common point between RF field sensing and
capacitance sensing is the "capacitance" connection of the signal.
A low radio frequency (RF) signal is injected into the finger,
then read by the pixels on the silicon acting like antennas.
The signal strenght depends on the capacitance/resistive connection, so from the distance
between the skin and the pixel.
Many clever variations exist, particularly about the readout circuitry to increase to signal to noise ratio, especially when the coating is becoming thick (greater than the pixel pitch, so 50 microns). Because of the required sensitivity, very often pixels are done directly on a silicon chip: it helps to have many good transistors directly where we need to read the signal. But the silicon is expensive, and it is better to have large area sensor to enhance the recognition levels. So some companies proposed to use simple copper lines (or equivalent) over a low cost substrate such as Kapton, and connect the lines to a silicon chip: the silicon area is not depending on the captured fingerprint area. But at the cost of a higher noise, so a lower image quality. Active RF sensing requires to send a signal in the skin: very often, this is done using a metal ring (e.g. Apple). |
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Sensing pixels are directly done over a silicon chip.
Companies proposing (or having proposed) RF/AC- active capacitance sensing:
Compagnies proposant (ou ayant proposé) des capteurs d'empreintes RF:
Labs working (or having worked) on RF capacitance sensing:
Laboratoire travaillant (ou ayant travaillé) sur les capteurs d'empreintes RF:
Sensing pixels are done on an unexpensive substrate, and connected to a further silicon chip.
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When the frequency is zero = a constant voltage, this is the simple impedance/resistance that is used. |
Electrochemical - Prussian Blue / Bleu de Prusse |
Prussian blue is known to visualize latent prints by selective electrochemical deposition: It is also possible to capture a fingerprint in one roll: |
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Electro-optical readers / La lecture électro-optique |
Some polymers are able to emit light when properly excited with the proper voltage
(generally quite a high voltage is required. This polymer is directly contacting a CMOS camera,
which is necessarily the size of the finger. Generally, the finger acts as the ground,
and the polymer emits light where the ridges touch. Ethentica and TesTech propose such a solution.
Certains polymères sont capables d'émettre de la lumière lorsqu'ils sont convenablement polarisé
(généralement une assez haute tension est requise). Ce polymère est directement déposé sur une caméra CMOS,
qui est nécessairement de la taille du doigt. En général, le doigt sert de masse, et le polymère émet
de la lumière là où les crêtes touchent. Ethentica et TestTec propose ce genre de capteur.
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Impedance (coil) |
Like capacitance, it is possible to use the skin impedance characteristics, and this requires a coil to read the signal, something no so easy to manufacture with a 50 microns pitch (500 dpi).
(The first sensor with impedance detection, one of my patents is describing this for aliveness detection a few years ago :) |