Use of atmospheric pressure spatial chemical vapor deposition to create spatially variant metal oxide semiconductor films for use in gas sensing arrays

Abstract

Manufacturing gas sensor arrays is a key roadblock in commercially viable electronic nose systems as sensor arrays require large numbers of unique sensors. Atmospheric-pressure spatial chemical vapor deposition (APSCVD) is a fabrication method that can be utilized to lower manufacturing costs. In this thesis, APSCVD is used to create gradients of sensing materials which are then used to create an array of sensors with unique physical properties. Materials explored using APSCVD are SnO2 thickness gradients, SnO2 and Cu2O heterojunction gradients, and zinc-tin-oxide composition gradients. These materials are created using a combination of a stainless steel atmospheric-pressure spatial atomic layer deposition reactor head and a custom APSCVD reactor head designed to create metal-oxide-semiconductor thin films with physical property gradients. The custom APSCVD reactor head implements a substrate-reactor spacing gradient to achieve physical property gradients, building upon a previous work showcasing that tilting a stainless steel reactor head leads to a thickness gradient [1], [2]. The heterojunction gradient consists of a uniform Cu₂O layer with a thickness of ~103 nm and a SnO₂ layer with a thickness gradient from ~22 nm to ~12 nm, measured using ellipsometry. The ellipsometry thickness measurements show an R² value of 0.95. The energy-dispersive x-ray spectroscopy measurements of the composition gradient film show the tin to zinc ratio ranging from 0.86 to 0.21 with a R² value of 0.96. The fabricated gradient films are converted to sensors using photolithography. Interdigitated electrodes are fabricated on the top surface, and chips with 8 sensors are placed on chip carriers. A custom gas sensor testing system is created to continuously run experiments and generate response data. The test system consists of control software for heating, an Arduino-based relay for recording up to 8 sensors at a time, and mass flow controllers which auto adjust to cycle through different experiments and analytes. Ethanol, isopropyl alcohol, acetone, and water are used as analytes in this thesis. The data recorded showcases that APSCVD can be used to create functional gas sensors with thickness, heterojunction, and composition gradients. The composition gradient exhibits a response-direction inversion, resulting in an increase in resistance at room temperature and a decrease at 200 °C. Additionally, heterojunction gradient showcases a parabolically varying response across the film. Principal component analysis of heterojunction gradient sensor data shows that combining multiple sensors improves selectivity relative to individual sensors, as reflected by an increase in silhouette score from -0.02 to 0.38, corresponding to a transition from overlapping to distinct response clustering.