Artificial tactile receptor
This project aims to realize to deploy smart tactile sensor chip on the whole body of a humanoid robot. the tactile sensor for a robot is currently highly desired as it enables not only grabbing an object by hand but also detection of unexpected collision to people or unorganized environment such as a home.
If we look at our skin, there are millions of tactile receptors are distributed in our whole body. These invisible sensors are very small and designed so that autonomously transmit the chemical impulse as if it has an intelligence.
In this project, high sensitive MEMS force sensor and CMOS are integrated into one single die to mimic the function of the tactile receptor.Among many ways to integrate MEMS and CMOS[ Link], we chose the method using wafer bonding technology of two separately fabricated wafers. This method enables us to optimize the manufacturing process of MEMS and LSI individually.
Design of packaging and integration process
As you can see in the picture on the right, the integration of MEMS and CMOS device can be done by wafer bonding. The Si membrane can deform due to an external force and the sub-micron displacement of the diaphragm is detected by LSI underneath. This force sensor also works as cap structure to protect I/O pads and active layer of LSI. It is notable that I/O pads are extended along tapered groove of LSI to establish interconnection with pads on backside.
I will introduce only overview as this work is going to be publish. The notable feature of this process is using BCB (Benzocyclobutene) polymer for wafer bonding and insulator of re-distribution layer. The BCB enables low temperature wafer bonding with small out gas, and filling of three-dimensional structure with it.
The ASIC designed by Masanori Muroyama [Link] is used for prototyping and we obtain an autonomous transmission from integrated tactile sensor as shown in the right figure. The functions and details of LSI will be described soon.
The blue colored plot shows applied force onto chip and you can see impulses are transmitting only when applied force exceed some threshold. In contrast to chemical impulse of tactile receptor, integrated CMOS generates digital packets which contains sensor ID, 32bit sensing data, CRC code for error collection and other bits for packet receiving (Clock recovery).
Track of improvements
This project involved an intense effort to realize acceptable yield of this device structure in an un-automated clean room in university. The fabrication process required 12 photomasks and 60 steps. Process integration and modification realize smaller package and higher yield.