Page 438 -

Single-Crystal Silicon Carbide MEMS: Fabrication, Characterization, and Reliability 7-27

1.2

1.1
Input resistance (kΩ) 1.0

0.9

0.8

0.7

0.6
0 100 200 300 400 500 600 700
Temperature (°C)

FIGURE 7.19 Bridge resistance of 6H-SiC piezoresistive pressure sensor as function of temperature.

0.10

0.05

0
TCR (%/°C) −0.05

−0.1

−0.15

−0.2

−0.25
0 100 200 300 400 500 600
Temperature (°C)

FIGURE 7.20 Temperature coefficient of resistance of 6H-SiC (calculated over 100°C increments) as function of
3
19
temperature (epilayer doping level, N 2 10 cm ).
d
is shown in Figure 7.19. It indicates a gradual decrease from a room temperature bridge resistance value
of 1.13kΩ to about 750Ω at 300°C caused by carrier ionization. The upward swing of the resistance is
associated with the growing dominance of the lattice scattering mechanism [Streetman, 1990]. From this
result, the TCR from Equation (7.20) was calculated over 100°C increments and is shown in Figure 7.20.
The negative TCR characteristic, relative to the room temperature resistance, was consistent with an n-
3
19
type 6H-SiC epilayer of this doping level (2 10 cm ). For more heavily doped crystals, the negative
TCR will extend to higher temperatures, thereby allowing for a less-complex compensation scheme.

7.6 Reliability Evaluation

Recently, Masheeb et al. (2002) reported the demonstration of a leadless (no wire-bond), SiC-based pres-
sure transducer at 500°C. The elimination of the gold bonds, and the protection of the metallization from
the harsh environment, offer the potential for long-term survival of SiC pressure transducers at high
temperature. These developments have increased the possibility of direct insertion of uncooled SiC pres-
sure sensors into high-temperature environments. However, for SiC pressure sensor technology to tran-
sition from the laboratory to commercial production, several reliability challenges, including

© 2006 by Taylor & Francis Group, LLC