THIN FILM HYDROGEN SENSORS: A MATERIALS PROCESSING APPROACH

Open Access
Author:
Jayaraman, RaviPrakash
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 17, 2002
Committee Members:
  • Osama O Awadelkarim, Committee Member
  • Mark William Horn, Committee Member
  • Russell Messier, Committee Chair
  • Lawrence Joseph Pilione, Committee Member
  • Robert T Mc Grath, Committee Chair
Keywords:
  • Sputtering
  • Pulsed DC
  • Hydrogen
  • Palladium
  • Thin film
  • Sensor
  • Morphology
Abstract:
Hydrogen (H2) is consumed and produced in large quantities by chemical, petroleum, plastic, space and glass industries. Detection and quantitative estimation of H2 in a reliable and efficient manner is of great value in these applications, not only from a safety stand point but also economically beneficial. Hence the requirement for a simple but efficient hydrogen sensor. The simplest hydrogen sensors are based on monitoring changes in electrical properties of group VIII transition metals, especially palladium (Pd). Hydrogen adsorbs on Pd surface and diffuses into its bulk altering its electrical and optical properties. This variation is used to detect/estimate hydrogen in the ambiance. However, at high hydrogen concentrations palladium undergoes a phase change. This causes an expansion of the lattice - a problem for fabricating reliable sensors using this metal. This problem was overcome by alloying palladium with nickel. Currently, sensors made from palladium alloy thin films (resistors and FET's) can detect/estimate hydrogen from ppm to 100% concentrations. However, these sensors are affected by the total gas pressure and other gases like carbon monoxide (CO), sulfur dioxide (SO2), hydrogen sulfide (H2S). This work, for most part deals with resistors (chemiresistors). Resistors estimate hydrogen by correlating the change in resistance to the hydrogen concentration. Magnetron sputtering enables the deposition of films of different compositions and morphology. In this work, Pd and Pd/Ni alloy thin films resistors were fabricated by sputtering. Morphology was seen to have a significant effect on the hydrogen sensing property of these films. In the presence of CO, the response of these sensors are extremely sluggish, however by employing SiO2 barrier layer the response was greatly improved. It was noted that despite the sluggish response, the signal from the chemiresistors did saturate to same level as seen in absence of CO from gas mixture, contrary to the earlier results. Also, the geometry of the resistors did not have any effect on the sensor sensitivity or response. Mass spectroscopy and ion energy distribution function (IEDF) analysis are important tools for characterizing processing plasmas. In this work, the sputter discharges were studied using energy and quadrupole mass spectrometer from Hiden. The IEDF of pulsed DC sputter discharges indicated a higher energy peak (~65 eV) and a broad distribution in addition to the low energy peak (~5 eV). The high energy peak was absent from the DC sputter discharge. This high energy peak was correlated to the pulsing of the DC source and was found to be independent of the target material.