Since the beginning of civilization, there has been a thrust to better ourselves, to enhance our performance and capabilities. With nature as a template, a variety of sensors were created. As technology improved, artificial sensors have mostly surpassed their natural counterparts in sensitivity and size except in several specific areas, one being infrared sensing. The Melanophila acuminata, also known as the jewel beetle, is one of several fire-loving beetles and has one of the most sensitive infrared organs amongst insects and animals alike. The organ has sensilla approximately 15 μm in width, is uncooled and allows the beetle to detect infrared radiation at distances in the tens of kilometers. Long studied by entomologists across the globe, scientists have been trying to understand these infrared organs for eventual creation of a biomimetic duplicate. In order to achieve this end, chitin, one of the chief materials used in the organ, must be made into an 'engineering material' - capable of being processed and fabricated into patterned films on the same nano and microscale as the beetle's sensilla - before proper characterization and incorporation of the material into a sensor platform can be accomplished.
Chitin, the second most naturally abundant polysaccharide on the planet, is found in insects, crustaceans and plants. Chitin is used to make shells and organs in a bottom-up approach; however, to incorporate the material into existing microfabrication processes, a top-down process flow was designed and developed. Furthermore, the deacetylated version of chitin, known as chitosan, was used due to the fact that it could be dissolved in solutions heavy with anions such as acids, while chitin remains insoluble in most acids and bases. Chitosan and chitin share a very similar structure and infrared absorption spectrum. Consequently, one can be converted to the other with pre-existing processing techniques.
In this report, 1) chitosan solutions safe for usage in a microfabrication facility (devoid of fast diffusers and other possible contaminants incompatible with normal IC processing), 2) a spin-casting process for these solutions and 3) photolithographic patterning recipes were developed. Techniques for solution preparation and thin-film development through spin-casting will be elaborated. Etch rates and process compatibility of chitosan with reactive-ion etching will be discussed. Future characterization of the material for use as an active infrared material for sensor pixels will also be examined.
September 30, 2006
Cheng, J. C. (2006). Chitosan as a MEMS Engineering Material. United States: University of California, Berkeley.