AVS 56th International Symposium & Exhibition
    Advanced Surface Engineering Tuesday Sessions
       Session SE+TF-TuA

Paper SE+TF-TuA9
Investigation of Surface Sensitivity of Relative Humidity Sensors through ALD Coated GLAD

Tuesday, November 10, 2009, 4:40 pm, Room C4

Session: Glancing Angle Deposition II
Presenter: M.T. Taschuk, University of Alberta, Canada
Authors: M.T. Taschuk, University of Alberta, Canada
K.D. Harris, NRC National Institute for Nanotechnology, Canada
J.M. Buriak, NRC National Institute for Nanotechnology, Canada
M.J. Brett, NRC National Institute for Nanotechnology, Canada
Correspondent: Click to Email
We have been investigating the performance and optimization of nanostructured relative humidity (RH) sensors produced by glancing angle deposition (GLAD) [1 – 2]. GLAD offers significant advantages for sensor applications, including extremely large surface areas, response times as low as 50 ms, and the use of any material compatible with physical vapour deposition. However, there remain a number of open questions regarding the underlying physics of GLAD RH sensor performance and response time. Response time and magnitude varies with the size and nature of pores in our sensors, and the interaction between the analyte and deposited material. To better understand the analyte-surface interactions, we have studied the performance of RH sensors with a thin coating of TiO2 deposited by atomic-layer deposition (ALD).

Columnar thin films of Si and TiO2 were produced by GLAD on interdigitated electrode substrates, creating a RH sensor. The GLAD sensing layer was conformally coated with TiO2 films by atomic layer deposition. In this process, the deposition chamber is evacuated and the reactive precursor, titanium isopropoxide (TIPO), is admitted. A thin layer of TIPO saturates the exposed substrate surfaces, and once complete, the deposition chamber is purged and oxygen is introduced. At the precursor-loaded substrate, this oxygen reacts with TIPO in a plasma-driven process to produce a thin layer of TiO2 conformally surrounding the high surface area GLAD film. In this work, the ALD process is repeated cyclically to build up TiO2 layers of different thicknesses.

Preliminary investigations have used 1.5 µm Si and TiO2 GLAD films, coated with ALD TiO2 films between 2 nm and 32 nm thick. The RH sensing properties of these devices were testing using a custom environmental chamber [1]. As expected, the uncoated Si and TiO2 thins films exhibited different responsivities. However, once the different sensors were coated with ALD TiO2 layers as thin as 2 nm, sensor response was very similar, indicating that sensor function is dominated by surface properties. To better investigate the transition between the GLAD post dominated response and ALD-layer dominated response, a series of films with thinner ALD coatings is underway. The sensor response will be characterized as a function of RH, electrical probe frequency, and ALD layer thickness. Current experimental results will be presented.

[1] J.J. Steele et al. IEEE Sensors Journal (2008) Vol 8. pp. 1422 - 1429

[2] M.T. Taschuk et al. Sensors and Actuators B (2008) Vol. 134, pp. 666 - 671.