Thin film technology for chemical sensors

Microfabrication and thin film technologies were applied in the fabrication of miniaturized chemical sensors. Two types of devices were developed: an electrochemiluminescence device utilizing tunnel-emitted hot electrons, and a microhotplate semiconductor gas sensor with an atomic layer deposited (ALD) tin dioxide sensing film. The hot electron-induced electrochemiluminescence (HECL) device is an integrated microelectrode device that combines an insulator-covered working electrode and a platinum counter electrode on a single chip. Two types of fluidic systems were integrated on the same type of electrode chip: either an enclosed sample chamber made of polydimethylsiloxane (PDMS) elastomer, or hydrophobic sample confinement on the chip surface. Different metals were tested as the working electrode, and different types of insulator films made by various methods were tested as the tunneling dielectric, to determine the optimal working electrode structures for HECL. These were then used in the integrated microelectrode devices, which were fabricated on silicon and glass substrates. A variety of electrode geometries were tested with the different fluidic systems, and sub-nanomolar sensitivity and wide dynamic range were demonstrated with the best devices. Ongoing work with polymeric substrates is briefly presented. In the review, latest results are presented on the restoration of the hydrophilic properties of enclosed PDMS microfluidic channels. While a PDMS surface quickly reverts to its naturally hydrophobic state, thus preventing capillary filling, this plasma treatment enables capillary filling even after extended periods of storage.The gas sensor is a microhotplate (MHP) device, utilizing a tin dioxide sensing layer deposited by ALD for the first time in a MHP sensor. Unconventional solutions were developed for the fabrication sequence to accommodate the demands of the deposition method. Also, metallizations and intermetal dielectrics not commonly used in MHP devices were tested to enable rapid processing of prototype devices with available methods and equipment. Fast response to various analyte gases, as well as good recovery and short-term stability were observed, demonstrating the potential of ALD tin dioxide films in gas sensor applications.