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CEA-Leti and Partners in PiezoMAT Project developed New Fingerprint Technology for Highly Reliable Security and ID Applications
 
Ultra-high Resolution Pressure Sensing Uses Matrices of Vertical Piezoelectric Nanowire
to Reconstruct the Smallest Features of Human Fingerprints
 a research project funded by the European Commission in the Seventh Framework Program (FP7, grant No. 611019)
 

The PiezoMAT team

The European project PIEZOMAT proposes a new technology of very high-resolution fingerprint sensors based on a matrix of interconnected piezoelectric ZnO nanowires (NWs). 

The direct consequence of integrating nano-objects on microelectronic chips rather than conventional microsystems is to diminish the size of individual pixels, which enables larger integration densities and thus higher spatial resolutions (>1000 dpi).

PiezoMATexplores 3 possible configurations with the charge collections at the bottom of single nanowires or with top-bottom contacts.

Partners challenges were the integration of ZnOnanowires, onto microelectronics chips (200mm wafer), considering process compatibility at the wafer scale of the successive nanowire growth, contacting and encapsulation steps.

    Principle

    • Each nanowire or bundle of nanowires forms a pixel (size below 50 × 50 µm2)
    • Pixel encapsulated in a polymer
    • Generated potentials are proportional to the NW displacement
     
    Chip processing
    Challenges: High patterning density, choice of proper materials (especially seed layer), chouce of appropriete process (only dry processes due to seed layer sensitivity), integration
    Following steps on dies
    • Nanowire growth
    • Encapsulation
    • Top electrode
    • Bonding
    • Testing
    option - BENDING
    • 2 contacts in high potential area
    • 5000dpi – 8 × 8 pixels
    • Technologically challenging
       
    • First experimental demonstration of bottom-bottom contacted 'bending mode’ force sensor 
    • Very high gauge factor and force sensitivity on the active NWs 
    option - COMPRESSING
    • 2 top-bottom contacts
    • 1000dpi – 25 × 10 pixels
    • Technologically safer
     
    • Development of stable and reproducible process in clean-room
    • Successful NWs integration: localized and contacted NWs growth on a processed 200 mm silicon wafer
    • Development of a suitable UV-crosslinkable polymeric formulation and an accurate deposition process for the encapsulation
    • Development of highly conductive top electrodes
    • Demonstration of the feasibility of the compression option → full device setup for MORPHO’s application
       

    Results - Conclusions

    3 demonstrators by 3 partners: one live demonstration and 2 videos
    • Morpho: peak response to mechanical contact (250 pixels)
    • MFA: pattern recognition and position sensitivity (128 pixels, one by one)
    • Fraunhofer: the 8-bit detection is successfully demonstrated (80 pixels)
       

    Morpho characterisation: peak respons of 250 pixels

    MFA characterisation: position sensitive pattern recognition by 128 pixels