In this chapter, the fabrication processes, structures of devices and material analysis tools used will be briefly introduced. During the processes, glove boxes filled with nitrogen were used to prevent our devices from oxygen and vapor.
3.2 Space Charge Limited Transistor (SCLT) Sensor
In the section, the processes of vertical P3HT-based space charge limited transistor (SCLT) sensors based on ITO glass substrate are demonstrated. The electrodes of SCLT are emitter (E), base (B), and collector (C). Indium tin oxide (ITO) is used as emitter electrode, while aluminum (Al) is used as base and collector electrodes. Active region of SCLT sensor is 1x1 cm2. Three-dimensional and two-dimensional SCLT sensor schematic diagrams are shown in Fig. 3-1; a scanning electron microscope (SEM) image of SCLT is shown in Fig. 3-2. All the fabrication steps were processed in glove box, except for glass substrate cleaning, ITO
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cleaning was to remove particle and organic pollution on glass substrate. The followings were procedures of glass substrate cleaning. First, Immerse the glass substrate in the streaming DI water for 5 minutes: to remove particles. Second, put glass substrates into acetone (ACE) under ultrasonic resonance for 10 minutes: to remove organic pollution. Third, immerse the glass substrate in the streaming DI water for 5 minutes: to remove the residue of ACE. Fourth, put glass substrates into isopropanol (IPA) under ultrasonic resonance for 10 minutes: to remove organic pollution. Fifth, immerse the glass substrate in the streaming DI water for 5 minutes: to remove the residue of IPA. Sixth, use N2 to quickly blow off water drops remained on the substrate to prevent spots of water. Finally, put glass substrates on the hot plate at 120℃
for 5 minutes to remove the residual moisture and then cool down to the room temperature.
3.2.2 ITO Patterning
To define the ITO electrodes, the shadow mask was used. The processes of ITO patterning are as follow. First, put the cleaned ITO glass substrates on the hotplate at 170℃for 10 minutes. Then rapidly stick the negative photoresist (PR) onto the glass substrates once ITO substrates leave the hot plate. Second, use the defined shadow mask and exposure to UV lithography to define ITO patterns. Third, immerse the patterned substrates into K2CO3
solution of which the formula ratio is 50mg K2CO3 to 1000ml water for 50 to 80 seconds to develop. After that, use DI water to clean, and then remove the residue PR by tissues. Fourth, use 36wt% Hydrogen chloride (HCl) solution at 50℃ to 60℃ for 50 seconds to etch the
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un-developed ITO, and then use DI water to clean. Finally, use 5 wt% sodium hydroxide (NaOH) solution to remove PR and then use DI water to clean.
3.2.3 ITO Surface Planarization
Before forming insulator layer, Oxygen plasma at power of 100 W was used to treat the ITO surface for 10 min. The purpose was to planarize the roughness of ITO surface.
3.2.4 Insulator Layer
Cross-linkable poly(4-vinyl phenol) (PVP) (8 wt.%) (Mw approx. 20000, Aldrich) and cross-linking agent poly(melamine-co-formaldehyde) (PMF) were dissolved in propylene glycol monomethyl ether acetate (PGMEA) with a PVP:PMF mass ratio of 11 : 4.
With 8wt% PVP solution, insulator layer was formed with PVP solution by spin-coating on the patterned ITO glass substrate in the glove box. Then annealing at 200℃ for 1 hour spin-coating at 5000 rpm for 30 seconds on the PVP layer in the glove box. After that, 200℃
annealing for 10 minutes was used. Thickness of the layer was about 20 nm.
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3.2.6 Spin-Rinse
p-Xylene (4 drops) was used to spin-rinse the sacrificing layer by spin-coating at 7000 rpm for 60 seconds. After this process, it decreased the thickness of sacrificing layer by 10 nm and increased the P3HT surface polarity.
3.2.7 Adherence of Polystyrene Balls
In this step, 0.4 wt% negatively charged polystyrene (PS) balls (Fluka-90517, diameter = 100 nm; Fluka-95581, diameter = 200 nm) solution dissolved in ethanol was used. After spin-rinse, immerse the substrate into the 0.4 wt% PS balls solution for 90 seconds at first.
Then take the substrate out off the PS balls solution and use ethanol to clean. Finally, immerse the cleaned substrate into boiling 220℃ IPA for 10 seconds, and then use nitrogen to blow the substrate dry immediately.
3.2.8 Porous Base Electrode
With PS balls adhering to the sacrificing layer, 40 nm-thick Al served as the base electrode (B) was deposited onto the substrate by thermal evaporation at the rate of 0.2 nm/s.
Then scotch tape (3M) was used to remove the PS balls covered with Al, and the left Al comprised porous base electrode with revealed PVP insulator layer.
3.2.9 Vertical Channel
With porous base electrode as hard mask, oxygen plasma at 100 W for 10 minutes was used to etch the exposed PVP down to ITO electrode. After oxygen plasma treatment,
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cylindrically vertical channels were formed.
3.2.10 Active Layer
A 350 nm active layer was composed of 4.5 wt% P3HT solution dissolved in CB by spin-coating at 1500 rpm for 40 seconds. After spin coating, the substrate with P3HT-based active layer was annealed at 200℃ for 10 minutes.
3.2.11 Porous Collector Electrode
After annealing the active layer, the substrates were treated with spin-rinse mentioned in step (6) and adherence of PS balls mentioned in step (7) in order. With PS balls adhering to the active layer, 40 nm-thick Al served as collector electrode (C) was deposited onto the substrate by thermal evaporation at the rate of 0.2 nm/s. Finally, Scotch tape (3M) was used to remove the PS balls covered with Al, and the left Al comprised porous collector electrode with revealed P3HT-based active layer.
3.3 Amorphous IGZO TFT Hybrid Sensor
In this section, the fabrication of a-IGZO TFTs hybrid sensor including processes of a-IGZO TFTs and capping of W18O49 as sensing layer are demonstrated. Two-dimensional a-IGZO TFT hybrid sensor schematic diagram is shown in Fig. 3-4. The a-IGZO TFT here is bottom gate top contact with heavily p-doped Si wafer as gate electrode, 100 nm silicon nitride (SiNx) as gate dielectric, and 100nm Al as source and drain (S/D) electrodes. Channel length and width of a-IGZO TFT are 200 μm and 1000 μm respectively. Besides, W18O49 is
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from Prof. Chi-Chuang Hu of National Tsing Hua University. A scanning electron microscope (SEM) image of W18O49 is shown in Fig. 3-5.
3.3.1 Gate Dielectric Layer
A gate dielectric layer was deposited on a 6-inch heavily p-doped silicon wafer by low pressure chemical vapor deposition (LPCVD) of National Nano Device Laboratories at 780℃
with gases of NH3 and SiH2Cl2. After deposition of SiNx, there was SiNx on both sides of the p-doped silicon wafer. Reactive ion etching (RIE) was used to remove the SiNx on the back side of the wafer and the revealed silicon was used as gate electrode. Process gases during RIE were oxygen (O2) 5 sccm and tetrafluoromethane (CF4) 80 sccm. Besides, process pressure and RF power were 15.0 Pa and 100W.
3.3.2 Substrate Cleaning
The 6-inch heavily p-doped Si wafer with SiNx on one side was split into 3×3 cm2 squares at first. Before deposition of active layer – IGZO film, the spilt substrates should go through the standard clean composed of SC1 and SC2. The capability of SC1 and SC2 were to remove microscopic particles and alkali metal ions on the surface of substrates respectively.
The first step of standard clean was using N2 to blow off macroscopic particles and flushing the substrates with DI water for 5 minutes. Second, immerse substrates into SC1 solution for 10 minutes. SC1 solution was composed of ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2) as oxidant, and 75 ~ 85 ℃ DI water with the formula: NH4OH: H2O2: H2O =
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1: 4: 20. Third, flush the substrates with DI water for 5 minutes. Fourth, immerse the substrates into SC2 solution for 10 minutes. SC2 solution was composed of HCl, H2O2 as oxidant, and 75 ~ 85 ℃ DI water with the formula: HCl: H2O2: H2O = 1: 1: 6. Finally, flush the substrates with DI water for 5 minutes and use N2 to quickly blow off water drops remained on substrates to prevent spots of water.
3.3.3 a-IGZO Film Deposition
Radio-frequency (rf) sputter and a 3-inch circular IGZO target (In: Ga: Zn: O = 1: 1: 1: 4 at%) were used to deposit 30 nm a-IGZO film at room temperature with rf power of 100W, working pressure of 0.009 torr, and O2/(Ar+ O2) ratio of 0.098%. The pattern of a-IGZO was defined by shadow masks.
3.3.4 Post-Annealing
After rf sputtering a-IGZO, 400℃ the substrates with a-IGZO film were post-annealed at atmospheric pressure for 1 hour in a nitrogen furnace. The flow rate of nitrogen was 10 liter per minute (L/m).
3.3.5 Source/Drain Deposition
100 nm Al was deposited as source/drain electrodes by thermal evaporator. Patterns of source/drain were defined by shadow masks. The deposition was started at the pressure <
5×10-6 torr and depositing rate was controlled at 0.2 nm/s.
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3.3.6 Sensing Layer Capping
Through step 1 to 5, the a-IGZO TFT was fabricated. Basing on a-IGZO TFT, W18O49 was capped as sensing layer. There were two methods to cap W18O49 which are drop and spin-coating. They would be introduced respectively as follow.
Drop-coating
Before drop coating W18O49, a-IGZO TFTs were heated to 85℃. Then W18O49 solution dissolved in ethanol was dropped onto the channel of a-IGZO TFTs. Finally, wait for the evaporation of ethanol.
Spin-coating
Before spin-coating W18O49, a-IGZO TFTs, a-IGZO TFTs were irradiated by ultraviolet (UV) rays for 5 minutes. After irradiation of UV rays, W18O49 were spun onto the channel of a-IGZO TFTs at 500 rpm for 30 seconds. Then put the a-IGZO TFTs on 85℃ hotplate for 5 minutes to evaporate ethanol, and one layer of W18O49 was formed. By repeating the above processes, more than one layer of W18O49 could be formed.