Significance of Microstructure for a MOCVD-Grown YSZ Thin Film Gas Sensor
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electrolytes [6]. This approach provides current-voltage curves whose shapes and peaks are gas specific and can be read by pattern recognition methods. In order to create such a working device, the YSZ layer must be gas-tight, electrically insulating, and ionically conductive. The MOCVD YSZ film must therefore contain a minimum number of pinholes and voids to minimize electrical shorting between the bottom and top electrode. Hence, we focused our initial effort on the effect of growth conditions on the YSZ microstructure and texture and their relationship to sensor leakage current and gas-sensing response. EXPERIMENTAL Yttria-stabilized zirconia films (100 nm-2 gim thick) were grown in a low pressure cold wall horizontal quartz reactor. Commercially available Y(TMHD)3 and Zr(TMHD)4 solid precursors were employed. Source temperatures were chosen to give a final composition of 12-20% yttria as determined by energy dispersive x-ray analysis. We used UHP (99.9995%) 02 and N2 as the oxidizing and carrier gas respectively. Growth temperatures between 450'C and 800'C were investigated. Additional deposition conditions are listed in Table I. Films were grown on a variety of substrates, but for the current work we focus on results from films fabricated on (100) oriented Si (n-doped), sputter coated with a Pt (300 nm thick)/Ti (20 nm thick) bilayer. After film growth, a small region of the YSZ film was etched to expose platinum for use as a lower electrode. Electrical and gas-sensing tests were conducted on PtIYSZ/Pt thin film structures fabricated by evaporating a Pt overlayer (10-50 nm thick) to provide an upper electrode which was exposed to an atmosphere of controlled composition. 565 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
Table I. Typical conditions for MOCVD growth of YSZ thin films substrate Growth temperature
Pt/Ti/Si(100) 450-800'C
Growth rate
0.1-0.6 jim/hr 6-10 Torr 140-165°C 1 15-125'C 1 slm
Pressure Zr source temperature Y source temperature Total flow rate Oxygen flow rate
300 sccm
RESULTS AND DISCUSSION Film texture The texture and morphology of YSZ films grown at temperatures between 450*C and 800°C were studied using x-ray diffraction (XRD) and scanning electron microscopy (SEM). In general crystal quality as judged by the intensity and sharpness of the x-ray film peaks is better for films grown at higher temperatures. All films grown above 500'C are 100% cubic phased, polycrystalline, and strongly (200) oriented. A typical 0-20 x-ray pattern for a YSZ film grown at 600'C is shown in Figure 1.
C
(0
20
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40
50
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70
Two Theta (degrees)
80
Fig. 1 0-20 X-ray diffraction scan for a (200) highly textured YSZ film
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Although Pt and cubic YSZ have similar structures, they are not well lattice matched (24% average mismatch) and therefore epitaxial deposition is not expected. YSZ prepared below 500°C produced weak XRD peaks attributable only to Y203 suggesting this material is predominantly amorphous. Wavelength dispersive x-ray analysis indicated the films contai
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