Fabrication of Area Effective Silicon Membrane By Frontend Bulk Micromachining for High Pixel Density Thermal Imager Array
Authors: Paul Nirupam , Reddy Ranga , Mohanty Radharamana , Vanjari Siva Rama Krishna , Singh Shiv Govind 
Published: May 2020

The fabrication of Silicon membrane by frontend bulk micromachining process has been proven to be one of the simple and cost-effective technique, for fabricating microelectromechanical structures. In this paper, we propose novel geometric mask designs, to be used as a pixel for Microbolometer application, for achieving highly reliable and area-efficient microstructures by frontend Silicon (Si) bulk micromachining. In the proposed design, all the rectangular openings are aligned at 45⁰ with respect to the wafer primary flat (100) of the Silicon wafer, such that only <100> planes are exposed and etched during anisotropic etching. However rectangular openings lead to high silicon area consumption which makes the process unworthy. Therefore, we proposed different geometries to minimize area consumption for achieving the same dimension of suspended structure. The fabrication of these Silicon membranes, to be used as Microbolometer pixels were simulated using Intellisuite FABSIM based physical simulator by two different techniques, one involving only wet chemical etching (called Method I) and the other involving dry-cum-wet etching process (called Method II). The simulation result shows an 86% reduction in footprint by Method II over the basic mask design when compared with the Method I, thus making it suitable for high pixel density thermal imager array. Thermal imaging is an established technique to convert the invisible radiation pattern emitted by an object into visible images, without establishing contact with the object, for feature extraction and analysis. Though Infrared thermal imaging was first developed for military purposes, it later gained a wide application in various fields such as aerospace, agriculture, civil engineering, medicine, and veterinary and many more, which is left to the researcher's imagination to explore its diverse applications in near future. It can be applied in all fields where temperature differences could be used to assist in the evaluation, diagnosis, or analysis of a process or product. Thus from agriculture and food industry perspective, thermal imaging technology can have the following potential applications: predicting water stress in crops, planning irrigation scheduling, disease and pathogen detection in plants, predicting fruit yield, bruise detection in fruits and vegetables, evaluating the maturing of fruits, detection of foreign bodies in food material, grain storage & drying, etc. Finally, as a proof of concept, we fabricated a single-pixel Silicon membrane using the proposed Method I as per the process flow outlined in the paper. Both simulation and experimental result shows very good agreement on all process steps. The fabricated Silicon membrane which acts as a mechanical platform is monolithic, as it does not involve any sacrificial layer, thus making it highly robust and reliable, as a result, it can be used for diverse sensing applications.

Figure 1