Heterogenous Integration of MEMS Gas Sensor using FOWLP : Personal Environment Monitors

The exponentially increasing global population has led to environmental pollution which drastically affects human health; emphasizing the need for personal environmental monitoring. This demands the development of wearable devices capable of sensing the local environment and wirelessly transmitting data to cloud for spatial pollution tracking.In this paper, we have demonstrated the integration of MEMS gas sensors on flexible PDMS substrate using the fan out wafer packaging technique called FlexTrate™. One of the main issues in FOWLP involving the integration of MEMS sensors with a released membrane is the stability of the membrane during the molding process which results in poor yield. We have optimized the process for integrating released MEMS devices by protecting the membrane prior to the molding process and thus improving the stability of the released membranes and improving the yield by >80%. If the membrane is not protected, during curing the cavity which is filled by PDMS leads to membrane cracking due to generation of stresses. Simulation studies on the temperature profile of the microheater after protecting the membrane shows that the power consumption for 300
^o
C of heater temperature is 0.1W as compared to 0.091W where the PDMS fills the cavity of the membrane, which is <; 10% increase. Thus, this proves that the membrane protection process improves stability without affecting the thermal characteristics of the heater. Furthermore, there is an effort to integrate rechargeable flexible batteries to power the system wirelessly. Adding to this, is the capability of wireless communication achieved by integrating a Bluetooth die in the system to transmit data to a mobile phone.The MEMS sensors along with the other electronic components such as transimpedance amplifiers, analog-to-digital converters and Bluetooth will be integrated on the same PDMS platform with interconnect pitches of 40 μm. This greatly reduces the form factor of the system and enables higher density of sensor arrays resulting in reliable and repeatable output. The intention of the work is to demonstrate a wearable gas sensor for local pollution monitoring with wireless communication capability using Bluetooth. This will provide a means to spatially map the pollution levels globally.