Chapter 4 Results and Discussions
4.3 Conclusions
The ISFETs of PECVD SiO2 as the sensing membrane and the REFETs with a sensing layer of sputtered Ta2O5 were fabricated. The sensitivities of SiO2-gate ISFETs showed no consistency among the devices while the Ta2O5-gate REFETs were good in stability and linearity of the sensitivity characteristics. The sensitivities of
PECVD SiO2-gate ISFETs was relatively low in pH 9-13, but was relatively high in the same pH range through differential sensing with Ta2O5-gate REFETs. The drift characteristics of the four thickness conditions of PECVD SiO2 suggested that the hydration depth, or the space for exchanging of H+ ions, was around 500Å. This depth was obviously larger than other materials prepared by other methods, e.g. sputtered Ta2O5, LPCVD Si3N4, thermal SiO2, sputtered Al2O3, etc. The drift rate in the first hour was large because of the large density of Si-OH sites near the SiO2 surface, while the density of buried OH sites below the surface was relatively small to the surface.
Chapter 5 Future Work
5.1 Electric Field Enhanced Migration of Hydrogen Ions
The H+ ions may be influenced by the electric field applied by the gate electrode when migrating in the insulator. Therefore, the observation of the migration behavior of H+ ions under different electric field strength conditions is a critical subject. Figure 5-1 is the illustration of the H+ ions contamination influenced by different electric field strength.
References
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electrolyte
Drain Source Gate
(a) (b)
Fig. 2-1 Schematic representation of (a) MOSFET, (b) ISFET
(a) (b)
Fig. 2-2 Series combination of the (a) initial (b) hydrated insulator capacitance
Silicon
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Fig. 3-1 Fabrication process flow
Fig. 3-2 Measurement setup
Gate
Drain1 Source Drain2
V
GSV
DS1V
DS2Fig. 3-3 Detection principle of pH
Fig. 3-4 Detection principle of drift
(a) (b)
(c) (d)
(e)
Fig. 4-1 Sensitivities of ISFET and REFET with xU=100Å, W/L=400μm/20μm.
(a) (b)
(c) (d)
(e)
Fig. 4-2 Sensitivities of ISFET and REFET with xU=100Å, W/L=400μm/30μm.
(a) (b)
(c) (d)
Fig. 4-3 Sensitivities of ISFET and REFET with xU=100Å, W/L=400μm/40μm.
(e)
(a) (b)
(c) (d)
(e)
Fig. 4-4 Sensitivities of ISFET and REFET with xU=300Å, W/L=400μm/20μm.
(a) (b)
(c) (d)
(e)
Fig. 4-5 Sensitivities of ISFET and REFET with xU=300Å, W/L=400μm/30μm.
(a) (b)
(c) (d)
(e)
Fig. 4-6 Sensitivities of ISFET and REFET with xU=300Å, W/L=400μm/40μm.
(a) (b)
(c) (d)
(e)
Fig. 4-7 Sensitivities of ISFET and REFET with xU=500Å, W/L=400μm/20μm.
(a) (b)
(c) (d)
(e)
Fig. 4-8 Sensitivities of ISFET and REFET with xU=500Å, W/L=400μm/30μm.
(a) (b)
(c)
(e)
Fig. 4-9 Sensitivities of ISFET and REFET with xU=500Å, W/L=400μm/40μm.
(d)
(a) (b)
(c) (d)
(e)
g. 4-10 Sensitivities of ISFET and REFET with xU=1000Å, W/L=400μm/20μm.
Fi
(a) (b)
(c)
(e)
Fig. 4-11 Sensitivities of ISFET and REFET with xU=1000Å, W/L=400μm/30μm.
(d)
(a) (
(e)
Fig. 4-12 Sensitivities of ISFET and REFET with xU=1000Å, W/L=400μm/40μm.
b)
(c) (d)
Sensitivity
Fig. 4-13 Drift ch iO2-gate ISFET.
Fig. 4-14 Drift characteristics of the 300Å PECVD SiO2-gate ISFET.
aracteristics of the 100Å PECVD S
Fig. 4-15 Drift characteristics of the 500Å PECVD SiO2-gate ISFET.
Fig. 4-16 Drift characteristics of the 1000Å PECVD SiO2-gate ISFET.
Fig. 4-17 Average drift of the PECVD SiO2-gate ISFET during the last 5 hour.
Fig. 4-18 Drift characteristics of the 300Å Ta2O5-gate ISFET.
100Å PECVD
SiO2
300Å PECVD
SiO2
500Å PECVD
SiO2
1000Å PECVD
SiO2
300Å Sputtered
Ta2O5
Average Gate-Voltage
Drift (mV/hour)
7.4 12.8 15.4 16.2 1.6
Table 4-2 Average gate-voltage drift during the last 5 hour.
Fig. 4-19 ID-VG curve of the 100Å SiO2-gate ISFET.
Fig. 4-20 ID-VG curve of the 300Å SiO2-gate ISFET.
Fig. 4-21 ID-VG curve of the 500Å SiO2-gate ISFET.
Fig. 4-22 I -V curve of the 1000Å SiO -gate ISFET. D G 2
Fig. 5-1 The H+ ions contamination influenced by the electric field strength.