The optimization of a tungsten trioxide film for application in a surface acoustic wave gas sensor

1
The optimization of a tungsten trioxide film for
application in a surface acoustic wave gas sensor
2
Introduction

• The films coated on SAW sensors contain metal,
  polymer and semiconducting metal oxide (SMO).
• Metal films are extremely thin and are very
  sensitive to certain target gases but have a very
  limited dynamic range.
• Polymer or organic films is unstable over
  extended periods of use. They are prone to
  interaction with environmental factors.

3

• SMO films are stable and reversible properties
  when they are properly deposited and annealed.
  These films are well suited for use in high
  temperature industrial processes and combustion
  monitoring. They are also potentially the most
  sensitive for SAW sensor application.
• To take full advantage of the high sensitivity
  offered by the SAW sensor, the SMO film
  conductivity must be in a particular range.

4
Theory of operation

• The dominant film property change of SMO films
  when the film is exposed to the target gas is the
  electrical conductivity.

5

• The conductivity range depends only upon the
  substrate properties.
• When the sensing film is exposed to the range of
  gas concentrations of interest, it will exhibit a
  corresponding range of conductivity.
• This change in conductivity upon exposure to the
  target gas can be in either direction.

6
Experimental

• Substrate 220Y-cut (RYC) quartz
• Coating Au doped WO3 (1.5 mm?2 mm) Au
• Target gas H2S

7

• Thickness of the deposited films 50 nm (WO3)
  1.6nm (Au)
• Film deposition?measure the electrical resistance
  at 200oC?anneal in 50RH air at 250oC ( 300oC,
  350oC, and 400oC) for 1 h? measure the electrical
  resistance during exposure to 10 ppm (0 ppm and
  15 ppm) H2S at 200oC

8
Results and discussion

• The annealing has an affect on the conductivity
  even for annealing temperatures well below the
  deposition temperature.
• The 500oC deposition produces conductivities in
  the optimum film conductivity range for the
  quartz SAW sensor.

9

• The baseline and exposed conductivities increase
  with increasing deposition temperature.

10

• For the deposition in 50 Ar/50 O2, a deposition
  temperature of 350oC produces the best
  conductivity range for the quartz SAW application.

11

• The films produced in 80/20 Ar/O2 atmosphere are
  less suitable for quartz SAW application.
• Fig.s 2-4 imply that the films deposited at
  higher temperatures may be more stable than those
  deposited at lower temperature.

12

• The 450oC deposition film gives the best range of
  baseline to exposed conductivity for a quartz SAW
  application.
• The 450oC deposition film also has the greatest
  sensitivity to the change in H2S concentration
  between 10 ppm and 15 ppm.

13

• The exposed conductivity is stable to within 4
  for the remainder of the test.

14

• The films produced in the 0/100 Ar/O2 atmosphere
  with a high deposition temperature and low
  deposition rate are expected to be nearly
  stoichiometric and are stable at temperatures
  below the deposition temperature.
• Low rate, high deposition temperature films are
  presumed to be polycrystalline as-deposited.

15

• The films produced in the 80/20 Ar/O2 atmosphere
  and at a high deposition rate are likely to be
  atmosphere and possibly sub-stoichiometric
  as-deposited.
• The increasing conductivity with increasing
  deposition temperature is likely due to large
  polycrystalline grain sizes associated with
  higher deposition temperatures.
• Water can be incorporated into WO3 films during
  deposition and expelled during annealing.
• Annealing can cause gold to diffuse into metal
  oxide films along grain boundaries.