Health parameters measurement is the focus of many researches because of their wide application area. Mechanical sensors have also many interests, especially if their sensitivity is enough high to measure small deformations. Their application can then address pulse measurement or muscle and tendons movements (fig. 1). Mechanical sensor are widely developed. Low temperature technologies allow to fabricate them on many types of substrates including flexible substrates. These sensors require the use of piezoresistive materials. We previously develop mechanical sensors based on microcrystalline silicon resistances.

Microsensors are fabricated on 25 µm thick Kapton. The piezoresistive material is N-type doped microcrystalline silicon deposited by PECVD. The main problem of this thin substrate is the stress induced during the process that deforms and creates cracks in each layer (fig. 2). In order to avoid this phenomenon, various possibilities were studied, first using different structures with nitride silicon layers. A specific cleaning process was also defined and the design of a shadow mask that reduce the surface of the microcrystalline silicon deposit was proposed. The combination of the three methods made that our samples are flatten and the cracks practically disappear. Different designs were then studied in order to obtain a good sensitivity to deformation.

Resistances are characterized under mechanical stresses. Their response to variable deformations was measured. In order to increase the sensitivity of the sensor, a specific electronic circuit is used, including a variable amplification of the measured signal (typically equal to 7). Sensors are placed on a thin layer which shape can be change with pressure variation. Several deformations can then be applied. Sensors are able to response to variable frequencies and different excitation waves like simple delta pulse or complicated signals like the ECG wave. The sensor response to deformation is studied with those different signals and at different frequencies (fig. 3). Several geometries are compared with the aim of obtaining the most sensitive design. The sensitivity to deformation as well as the response time are studied. This shows the feasibility and development of a highly sensitive system for small deformation measurements.