MEMS-based electrochemical gas sensors and wafer-level methods

This thesis describes novel microelectromechanical system (MEMS) based

electrochemical gas sensors and methods of fabrication.

This thesis presents the research in two parts. In the first part, a method to handle a thin

silicon wafer using an electrochemically active adhesive is described. Handling of a thin

silicon wafer is an important issue in 3D-IC manufacturing where through silicon vias

(TSVs) is an enabling technology. Thin silicon wafers are flexible and fragile, therefore

difficult to handle. In addressing the need for a reliable solution, a method based on an

electrochemically active adhesive was developed. In this method, an electrochemically

active adhesive was diluted and spin coated on a 100 mm diameter silicon wafer (carrier

wafer) on which another silicon wafer (device wafer) was bonded. Device wafer was

subjected to post processing fabrication technique such as wafer thinning. Successful

debonding of the device wafer was achieved by applying a voltage between the two

wafers. In another part of the research, a fabrication process for developing a functional

nanoporous material using atomic layer deposition is presented. In order to realize a

nanoporous electrode, a nanoporous anodized aluminum oxide (AAO) substrate was used,

which was functionalized with very thin layers (~ 10 nm) of platinum (Pt) and aluminum

oxide (Al2O3) using atomic layer deposition. Nanoporous material when used as an

electrode delivers high sensitivity due to the inherent high surface area and is potentially

applicable in fuel cells and in electrochemical sensing.

The second part of the thesis addresses the need for a high performance gas sensor that

is applicable for asthma monitoring. Asthma is a disease related to the inflammation in the

airways of the lungs and is characterized by the presence of nitric oxide gas in the exhaled

breath. The gas concentration of above approximately 50 parts-per-billion indicates a

likely presence of asthma. A MEMS based electrochemical gas sensor was successfully

designed and developed to meet the stringent requirements needed for asthma detection.

Furthermore, to enable a hand held asthma measuring instrument, a miniaturized sensor

with integrated electrodes and liquid electrolyte was developed. The electrodes were

assembled at a wafer-level to demonstrate the feasibility towards a high volume fabrication

of the gas sensors. In addition, the designed amperometric gas sensor was successfully

tested for hydrogen sulphide concentration, which is a bio marker for bad breath.