silicon carbide micro-devices for combustion gas sensing under harsh conditions (1.16mb)英文精品课

1000700SiC7+I+I Gate 43900800Fully ActivatedT = 800 K650600550SiC2-II+I Gate 2Not ActivatedT = 600 KH2 = 3x10-6 torrO2 = 4x10-5 torrCsens (pF)Csens (pF)70060050040030020000.511.525004504003503002502000H2 = 5x10 torr-7O2 = 9x10 torr-70.511.52Vgate (V) Vgate (V) Fig. 3.3.2. 1 MHz C-V curves measured in UHV. Left panel: New sample, fully activated at atmospheric pressure before installation in UHV, measured with new LabView program. Right panel: Previous sample, not activated at atmospheric pressure, measured with earlier (Asyst) program. The fully activated sample shows a much larger voltage shift. 16 3.4 Gas pressure calibration Measurements of gas partial pressure in UHV are based on ion current measurements from a quadrupole residual gas analyzer (RGA) (Balzers Prisma 200). Because of differences in molecular mass and ionization efficiency, the sensitivity of the RGA can be different for different gases. To obtain more accurate measurements of gas partial pressure, we calibrated the RGA signals against the ion gauge total pressure reading. Ion gauge pressure readings were corrected for different gas species’ sensitivity [Summers 1969]. The ratio of true pressure to RGA signal was 0.4 for hydrogen and 1 for oxygen. Fig. 3.4.1 shows the calibration results for hydrogen and oxygen. This method could not be used for hydrogen sulfide, because the pressure of other gases was always greater than that of the H2S, due to chemical interactions in the vacuum pumps and on the chamber walls. All that could be determined was that for H2S the RGA reading and actual partial pressure are consistent within an order of magnitude. 1.E-051.E-04H2 CalibrationPressure (torr)O2 Calibration Pressure = 1 RGA signalPressure (torr)1.E-051.E-061.E-071.E-081.E-061.E-071.E-08 Pressure = 0.4 RGA signal1.E-091.E-091.E-081.E-071.E-061.E-051.E-091.E-091.E-081.E-071.E-061.E-051.E-04 Fig. 3.4.1. Calibration of RGA signal against actual pressure for hydrogen and oxygen. RGA signalRGA signal 17 3.5 High Temperature Reliability of SiC n-MOS Devices up to 630 oC 3.5.1 Capacitance- voltage characterization We have used high frequency capacitance – voltage measurements extensively to characterize the electrical properties of the SiC MOS capacitors. The 1MHz C-V characteristic of a 1000 μm diameter n-MOS capacitor at 330 oC is shown in Fig. 3.5.1.1. We operate the Pt-SiO2-SiC devices as sensors for hydrogen containing gases in the 400 - 600 oC range. The optimum sensor bias point with respect to response time and device to device repeatability is near mid-gap [Ghosh 2002]. Fig. 3.5.1.1: 1MHz capacitance-voltage characteristic of a 1 mm diameter 6H- SiC n-MOS capacitor. For gas sensing the optimum device bias is at mid-gap. 3.5.2 Gate reliability – oxide leakage measurements The gate leakage measurements were made in air using a commercial current - voltage characterization system, consisting of a Keithley 236 Source Measurement unit and the Interactive Characterization Software (Metrics). The samples were placed inside a shielded probe station with a temperature controlled chuck that can be heated up to 320 °C. To make ± 2 pA measurements at high temperature, care was taken to electrically isolate the SiC sample mounted on its alumina header from the heater coils of the hot chuck. At a given temperature both a current - voltage (I-V) and capacitance - voltage (1MHz C-V) scan were taken for each device. The gate leakage characteristic as a function of gate voltage was obtained by subtracting out the “probe up” current from the I-V characteristic. The leakage current density was evaluated at a gate voltage corresponding to the capacitor being biased at midgap as follows. First, we calculated the midgap capacitance from the measured device area, oxide thickness and doping density of the epitaxial layer (obtained from 1/C2 analysis of the C-V curve in depletion). Then using the measured C-V characteristic, we obtained the midgap voltage. 18 4. RESULTS AND DISCUSSION 4.1 Effect of activation on sensor morphology and structure We have characterized the change in the gate surface morphology due to surface activation processes. In the characterization process, we collect both Bright-field (BF) and Dark-field (DF) images of a few selected gates; before and after activation. BF images are convenient to demonstrate the surface morphology of the Pt gates before activation. DF images of smooth and shiny surfaces are featureless, but provide an excellent way to bring out the texture from rough surfaces created during the activation process. The left hand side image shown in Fig. 4.1.1 is the BF images of 3.5 gates (2 bonded) and on the right hand side, the DF images from the same area of non-activated gates is illustrated. Bright-field Image Dark-field Image 300 μm Before Activation Before Activation Fig. 4.1.1. High magnification micrographs of a few Pt gates before activation. The left image shows the BF image of the gates and the right image shows the DF image of the same gates. As the Pt surface is mirror smooth and shiny prior to activation, the DF image is featureless. Shown in Fig. 4.1.2 is a DF image of a 500 μm gate after activation. The gate morphology has changed to a textured surface morphology that is opaque (cloudy), i.e. no longer perfectly reflective in the visible. This image is typical for the majority of our activated Pt gates. Note that in some cases, the area around the bonded gold wire has a different surface morphology from that of the rest of gate. We postulate that direct bonding to the Pt gate may change the crystal structure of the point of contact as well as producing a strain in the area surrounding the point contact, which in turn depends on the specifics of the bonding parameters. 19 200 μmDark-field Image After Activation Fig. 4.1.2. A Dark field image of a 500 μm in diameter Pt gate after activation. The Pt surface is now grainy (textured) and non-reflective, whereas it was shiny and featureless prior to activation (see Fig.4.1.1). The structure of the Pt gates was investigated by collecting x-ray diffraction (XRD) patterns before and after activation process. The XRD pattern from the Pt gates before activation is shown in Fig. 4.1.3a. As seen in the figure we observe only the Pt (111) peak at 2θ ? 39.7°, the Pt (002) peak at 2θ ? 46.2° was not seen. This indicates that the Pt film is textured Pt (111). In Fig. 4.1.3b, shown is the XRD pattern collected from an activated gate. Three peaks (1-3) appeared in this XRD pattern with Pt (111) (#2 at 2θ ? 39.7°) as the dominant peak. The spectrum has a higher background below 39.7°, suggestive of an amorphous material on the surface. There are two additional small peaks, (1) at 2θ ? 38° and (3) at 2θ ? 44°. The lattice parameters of peaks 1 & 3 (see Table 4.1.1) do not match the first or higher order orientations of Pt, PtO and PtO2. Note that the after activation XRD spectra were taken on a gate that had a bonded Au wire. The Au (111) peak is at 2θ ?38.11° and the (002) peak is at 2θ ?44.29°, therefore we attribute peaks 1 and 3 to residual gold from the wire bonding processes. (h, k, l) Pt-dhkl (?) ? 2θ PtO-dhkl (?) ? 2θ PtO2-dhkl (?) ? 2θ 001 ---- ---- 4.342 ? 20.43° 100 ---- 3.077 ? 29° 2.696 ? 33.19° 002 1.962 ? 46.2° 2.670 ? 33.52° 2.171 ? 41.54° 101 ---- 2.666 ? 33.57° ---- 110 ---- 2.176 ? 41.45° 1.556 ? 59.32° 111 ---- 2.266 ? 39.73° 1.465 ? 63.42° 200 ---- 1.538 ? 60.08° 1.348 ? 69.67° 202 --- 1.387 ? 67.44° 1.333 ? 70.57° Table 4.1.1 X-ray diffraction parameter for Pt, PtO and PtO2 from [EMC] and [McBride 1991]. 20