Tsung-g Chen

Nano-IGZO layer for EGFET in pH sensing characteristics

In-Ga-Zn-O (IGZO) was widely applied in the substrate of TFT to replace a-Si in recent year. In this study, IGZO layer with thickness of 70 nm is first proposed as a pH sensing membrane directly on P-type Si substrate acting as an extended gate of conventional extended-gate field-effect Transistor (EGFET). Material criteria of extended gate electrode are low resistance and high capacitance. Therefore, Ar/O2 ratio was modified in the rf sputtering with IGZO target. Post deposition anneal was also performed to check the sheet resistance and pH sensing performance. EGFETs were measured in standard pH buffer solution by using B1500A and constant voltage constant current (CVCC) circuit. Similar IDS-VGS curves including transconductance (Gm) and substrate swing (S.S.) are obtained in various sputtering conditions of IGZO compared to commercial NMOSFET in CD4007. pH application range is only between pH 2 to pH 10. IGZO-EGFET prepared by Ar/O2 ambience of 24/1 in sputtering can have a sensitivity of 59.5 mV/pH. Lower sensitivity and linearity can be observed in the samples with RTA treament at higher temp and in O2 ambience. N2 anneal at 500°C can be used to improve pH sensing performance for IGZO-EGFET prepared by Ar/O2 ambience of 20/5 in sputtering. Nano-IGZO layer is verified to be the sensing membrane in EGFET to have a high sensitivity of 59.5 mV/pH for the first time. More studies on enlargement pH application range and minimization of non-ideal effect still need to be investigated.

Ultra-high scanning speed chemical image sensor based on light addressable potentiometric sensor with analog micro-mirror

In chemical image sensor, spatial resolution and scanning speed are the most important factors which may need to optimize and trade off. Before the minimization of spatial resolution, scanning speed of system needs to be improved to have an image in a short time within few seconds. In this study, ultra-high scanning speed of light addressable potentiometric sensor (LAPS) could be achieved by means of the combination of laser diode and single analog micro-mirror in light source. In the constant bias operation, an U-shape of pH solution can be detected within 8.5 sec for 14×121 points in the area of 3.2 mm×6.12 mm by using 5 kHz of ac signal in laser diode. The image resolution of the present LAPS system with 10 μm-thick Si3N4/3 nm-thick SiO2 on 500 μm-thick silicon wafer is 0.0116 mm2 which could be further improved by LAPS structure optimization with thinner substrate. A chemical image sensor with ultra-high scanning speed is demonstrated by the application of analog micro-mirror for LAPS.

Thin silicon light-addressable potentiometric sensor by Deep reactive-ion etching

To have a high photocurrent and spatial resolution of 2-dimensional (2D) chemical image in light-addressable potentiometric sensor (LAPS), silicon substrate is thinned down to about 100 μm by Deep reactive-ion etching (DRIE). Compare to commercial Si substrate with thickness of 350 μm, photocurrent is increased for 4.5 times and operation frequency is increased to 20 kHz. pH sensitivity is 67.09 mV/pH for NbOx sensing membrane. A clear 2D image to recognize a pattern with width of 100 μm can be obtained. Higher signal to noise ratio is benefit for high-speed and spatial resolution of 2D chemical image.

High photocurrent and operation frequency for light-addressable potentiometric sensor by thinner Si substrate

LAPS with different thickness of P-type silicon wafers are first investigated for photocurrent, operation frequency of applied ac signal and pH sensing performance, respectively. A high dielectric constant material, niobium oxide (NbOx), was directly deposited as the sensing membrane by reactive radio frequency sputtering. pH sensitivity is around 60.3 mV/pH with linearity of 99.1%. Thin Si substrate has higher photovoltage, especially with high-frequency ac signal. With ac signal of 20kHz, photovoltage of 350 um Si shows comparable to 500 um Si. Even in frequency higher than 50 kHz, fast scanning speed could be achieved by analog mirror and red laser for chemical image sensor.

Extended titanium nitride gate field-effect transistor with PVC selective membrane for hydrogen and potassium ion detection

A 50 nm-thick TiN sensing membrane with nitrogen (N2) ratio modification was firstly used in EGFET for pH sensing. The highest pH sensitivity of 61 mV/pH with linearity of 99.9% could be found in N2 to N2+Ar ratio of 20%. Similar performance could be maintained at least for 250 days. With additional PVC selective membrane of K+ ionophore, sensitivity could be increased to 48.7mV/pK. This developed TiN EGFET could be a potential candidate for pH and pK sensing application.