Mechanical (pressure) fingerprint sensing
Capture mécanique d'empreintes digitales

Mechanical fingerprint sensing is very often pressure sensing. Pressure is a force applied on a surface, so a mechanical constraint. We expect to see a difference where the ridges apply with the valleys.

This is one of the oldest ideas, because when you put your finger on something, you apply a pressure. Piezo-electric material has existed for years, but unfortunately, the sensitivity is very low. Moreover, when you add a protective coating, the resulting image is blurred because the relief of the fingerprint is smoothed.

These problems have been solved, and now some devices using pressure sensing are available.

C'est une des plus anciennes idées, car lorsque vous posez votre doigt, vous appliquez une pression. Les matériaux piézo-électriques sont connus depuis longtemps, mais malheureusement leur sensibilité est très faible. De plus, lorsque vous ajoutez une protection surfacique, l'image résultante est floue car le relief de l'empreinte est atténué.
Ces problèmes ont été résolus, et maintenant quelques dispositifs utilisant la pression sont disponibles.

Several solutions, depending on the material, have been proposed:

• Conductive membrane on a CMOS silicon chip, on TFT
• Micro-electromechanical (MEMS, NEMS) switches
• Micro-electromechanical (MEMS, NEMS) + electrostatic reading
• Piezo-electric nanowires
• Photonic crystals

(1994 Jul) Opsis (a french company) offered a device using a conductive membrane deposited on a CMOS chip.
Opsis (France) a offert un dispositif utilisant une membrane conductrice déposée sur une puce CMOS (juillet 1994).

• Since 2002, BMF is offering a product using a TFT substrate (developed with Sanyo)
• Fidelica offers since mid-2004 the FIS-3002, also using a TFT substrate from Sanyo.
• (April 2004) Alps Electric develops 2 fingerprint sensors. One is based on pressure.
• The Fraunhofer IKTS is working on 1-3 piezocomposites to create a fingerprint sensor (for CrossMatch, 2004).
• Micron Technology: Tactile sensor using an insulated flexible matrix loaded with filler particles US 6,561,044

Micro electro-mechanical devices allow engineers to make extremely tiny silicon switches. When a ridge touches a switch, it closes. But the coating remains a significant problem, and moreover, a binary image is the result, leading to minimal information. No further development has been done with this technique beyond the laboratory.

Les dispositifs micro-usinés permettent la réalisation de très petits interrupteurs. Lorsque qu'une crête touche un interrupteur, elle le ferme. Mais la protection surfacique reste un problème délicat, et de plus on obtient une simple image binaire, contenant peu d'information. Il n'existe pas encore de développement ayant dépassé le stade du laboratoire.

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NTT

• (2001) NTT Microsystem Integration Laboratories
• NTT research group

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Michigan University

• (1997) Very high density bulk micromachined capacitive tactile imager Michigan University (Wise)

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TIMA

• Tima homepage
• La thèse de Fabien Parrain (2002)
• N.Galy thesis page and the thesis
• CNRS: zoom sur le micro et nano systèmes: Tima

• (2010) TIMA & Department of Electronic & Computer Engineering HKUST HONG KONG The Design, Fabrication and Characterization of a Piezoresistive Tactile Sensor for Fingerprint Sensing. 1 pixel, 100 to 250 microns

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CEA-LETI

• (1997) Leti, capacitance pressure sensors (Rey)

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LighTuning

• (2004) LighTuning

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Universiti Kebangsaan Malaysia

Universiti Kebangsaan Malaysia / Mitra Damghanian, Burhanuddin Yeop Majlis

• Institute of Microengineering and Nanoelectronics (IMEN) (2006) (simulations) Design of a High Sensitivity Structure for MEMS Fingerprint Sensor / ICSE2006 Proc. 2006, Kuala Lumpur, Malaysia
• (2009) (simulations) Analysis and design of a wide micro beam as a pressure gauge for high sensitivity MEMS fingerprint sensors

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National TsingHua University

National TsingHua University, Hsinchu, Taiwan:

• (2006) A Highly Sensitive CMOS-MEMS Capacitive Tactile Sensor. 1 pixel, 200 microns.
• (2006) A CMOS Micromachined Capacitive Tactile Sensor With High-Frequency Output. idem
• (2011) A CMOS capacitive pressure sensor chip for fingerprint detection. 8x32pixels, 65 microns.

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Fidelica >> Lenovo

• (2013) Fidelica is now part of Lenovo.
• (2014) Lenovo proposes some MEMS to make fingerprint sensors: Mems on alternate substrates.

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BIT Beijing Institute of Technology 北京理工大学

• BIT Beijing Institute of Technology
• (2018 Sep) A Novel Fingerprint Sensing Technology Based on Electrostatic Imaging. A MEMS is vibrating, modulating the electrostatic field to read the fingerprint. Capable of sensing through 439 microns. (local copy Tang, K.; Liu, A.; Wang, W.; Li, P.; Chen, X. A Novel Fingerprint Sensing Technology Based on Electrostatic Imaging. Sensors 2018, 18, 3050. ).
  80x80 pixels @500dpi.

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Georgia Institute of Technology

• (2013 Feb) High-resolution electroluminescent imaging of pressure distribution using a piezoelectric nanowire LED array (Georgia Institute of Technology).
  When pressure is applied to the nanowires, piezoelectricity is generated, which stimulates the production of photons at the root of the nanowire — where the n-type ZnO meets the p-type GaN. The harder you press, the stronger the current, the brighter the light. These LED nanowires have a spatial resolution of just 2.7 micrometers (micron), resulting in a pixel density of 6350 dpi.

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CEA-LETI

• (2014) PiezoMat proposes a new technology of high-resolution fingerprint sensors based on a matrix of interconnected piezoelectric nanowires

(2006) University of Toronto
"You can elastically deform these crystals and produce different colours," says lead author André Arsenault, a PhD candidate in the laboratory of Geoffrey Ozin, a University Professor in the Department of Chemistry and a Canada Research Chair in materials chemistry.
Photonic crystals are a relatively new development in the scientific quest to control light. Ozin's lab first created photonic crystals in 2002, using spherical particles of silica mere micrometers in diameter that self-assemble into neat layers, creating what's known as an opal. After filling the space between the spheres with silicon, they used acid etching to remove the silica balls. This left an ordered sponge of air bubbles in silicon known as an inverse opal. This photonic crystal material, the first of its kind, did indeed trap light. These photonic crystals can produce color based on how an electromagnetic wave interacts with the structure -- meaning that it could be tuned to produce any color.
In the new study, the team injected an elastic compound between the spheres, which were then etched away, leaving an orderly and compressible elastic foam that can be transferred onto virtually any surface, such as glass, metal or plastic. The material changes color based on how far the spheres are separated.