This dissertation describes the initial development of a polymer-based, microcantilever infrared sensor. The development of the sensor is bio-inspired and based upon the long-range infrared sensor found in the pyrophilous jewel beetle Melanophila acuminata, which is able to seek out forest fires from more than 50 km away. Based on several proposed models of the infrared detector found in Melanophila acuminata, as well as published in vivo experiments, the feasibility of polymer-based infrared thermal sensors was explored and developed. Polymer materials were chosen due to their high absorptivity in the infrared range due to vibrational resonance modes characteristic of their organic bonds.
Polymeric materials investigated in the course of this work include the polysaccharide and biomaterial chitin, its deacetylated derivative, chitosan, and the work-horse polymer of the semiconductor industry, novolak-resin-based photoresist. Chitin and chitosan are particularly noteworthy polymers for exploration in infrared detection due to their natural absorbance of infrared radiation near the 3 um and 10 um bands, which are important for the detection of the temperatures of warm engines and human body temperature, respectively.
Because only limited work (primarily focused on electrodeposition) has been focused on the microscale patterning of chitosan, a photolithography process for chitosan and chitin was developed to allow the integration of the material into a variety of microelectromechanical systems processes. In addition to optical / infrared sensing, this process has a variety of potential applications in tissue engineering, protein engineering, and lab-on-a-chip devices. To demonstrate these areas of use, surface functionalization was demonstrated using bioconjugation to attach a protein to a patterned chitosan surface. Thin films of chitosan and chitin were characterized using laser profilometry to identify the effect of temperature on the film stress, and contact angle to determine the degree of hydrophobicity or hydrophilicity at the chitosan or chitin surface.
Chitin in the infrared sensor of Melanophila acuminata functions as a thermomechanical material that has a high coefficient of thermal expansion and strong optical absorption in the infrared due to its chemical composition and organic bonds (absorption which matches its forest fire target). Based on this infrared detection mechanism, the potential exists to develop a broad, novel class of polymer-based, biomimetic infrared sensors having high CTE and spectral absorption tuned to a target of interest.
Towards this objective, a photoresist-polysilicon cantilever bimorph prototype was designed, fabricated, and tested. Two types of this device were formed with different readouts; one required an optical readout while the other used a capacitive readout. Devices were characterized using optical and thermal methods. Prototypes were found to suffer from the presence of residual stress which caused significant out-of-plane deflections upon release. Such strain gradients should be minimized (by modifying the thermal processing steps or by annealing) to maximize the capacitive output signal. The optically-interrogated cantilever was observed to bend due to both stimulation with heat and with infrared radiation. The presence of a human finger in the vicinity of the silicon die was sufficient to cause cantilever deflection that was readily observable using optical techniques. The interdigitated cantilever array using capacitive readout was found to be highly sensitive, with a dynamic range >500:1, a peak spectral sensitivity from 600 nm to 1.2 um, and a threshold sensitivity of 2 uW/cm2.
