Monolithic silicon smart tactile image sensor with integrated strain sensor array on pneumatically swollen single-diaphragm structure

In this paper, a novel concept of silicon-microelectro-mechanical system tactile image sensor aimed at fingertip tactile applications and evaluation results of a prototype device are presented. Array of strain-sensitive sensor pixels is integrated on a pneumatically swollen silicon diaphragm with signal processing circuits in monolithic configuration. Elastic surface of the tactile image sensor is realized using repulsive force of the air pressure applied to diaphragm backside. Contact force distribution of an object is detected from the stress distribution change on the diaphragm using piezoresistor array. Force range and force sensitivity can be controlled by the pressure even after device packaging step. In this principle, fine pitch of pixels and a large-scale sensing array can be easily realized using the abilities of silicon CMOS technology. In addition, the movable stroke of sensor surface can be made much longer than the individually formed micromechanical pixels. A prototype device, of the new concept, with 3040 x 3040 mu m(2) sensing diaphragm was fabricated with integrated 6 x 6 array of 420-mu m pitch pixels. Increased movable stroke of the swollen diaphragm from the original surface is around 30 mu m at 23-kPa pressure at the top. Realized spatial resolution of the prototype device is approximately 400 mu m, which is determined by the relationship between the pixel pitch and mechanical crosstalk among the pixels. Positions of touch and their contact force amplitudes were detected as a two-dimensional distribution of the output voltage from the pixel array in a multipoint contact test. The maximum input-force range can be controlled from 21 to 176 mN by changing the backside pressure from 5 to 64 kPa. Through the evaluation of the fabricated device, major advantages of this tactile image sensor have been demonstrated successfully.