2-1 Introduction
The structures are made the Metal-Insulator-Metal (MIM), top-contact bottom-gate Thin Film Transistors (TFTs), and a novel porous structure of pentacene-based TFT. We will introduce our devices how to fabricate, and the pattern of devices is shown in the Fig. 2-1 we use in this thesis. And the parameter of device extracting method and material analysis we use.
2-2 Cleaning the Glass Substrate
There are two steps to clean and remove particle and organic pollution on Glass substrate (CORNING EAGLE 2000). The roughness of glass substrate is important for metal deposition and metal adhesion, or could cause point defect to influence the interface between organic layer and metal.
The glass cleaning steps are as follow:
Steps:
(1) Glass substrates are place of the DI water current flow for 5 minutes in order to remove the particles.
(2) Put the glass substrates in the acetone with ultrasonic resonance for 5minutes to remove the organic pollution
(3) Put the glass substrates under the DI water current flow for 5 minutes to remove the solvent.
(4) Put the glass substrates in the KG detergent with ultrasonic resonance for 1
(5) Put the glass substrates under the DI water with ultrasonic resonance for 1 minute twice to remove the residual solvent.
(6) Dry the glass substrates with N2 flow to blow off the water drop on the substrates
(7) Put the glass substrates on the hot plate at 120℃ for 3 minutes to remove the moisture.
(8) Naturally cool down to the room temperature.
2-3 Metal-Insulator-Metal Fabrication 2-3.1 Bottom Metal Deposition
Before metal deposition, the glass substrates should be cleaned. The 50 nm Aluminum (99.999%) was deposited by thermal evaporator around 5x10-6torr. The deposition rate was controlled at 2Å/sec and substrate heater was hold at 60℃, the adequate deposition rate and temperature were benefited to help metal film have good adhesion to glass substrate. The metal pattern was defined by shadow mask.
2-3.2 Dielectric Fabrication
In this section, introduce each of the dielectric layer deposition respectively:
1. Poly (4-vinyl phenol) (PVP):
PVP was obtained from Aldrich with molecular weight about 20000. PVP have OH function groups on its molecule structure, poly(melamine-co-formaldehyde) (PMF) was used to be cross-linking agent for PVP, and propylene glycol monomethyl ether acetate (PGMEA) as the solvent. Their chemical structures of PVP and PMF are schematically shown in Fig. 2-2. First, we dissolved PVP and PMF in the PGMEA solvent with a magnetic stirrer for overnight. PVP was 8wt% in the solution, and fixed
PVP and PMF weight ratio was 11:4. We then filtered mixed solution through a syringe filter with 0.2μm PTFE membrane and spin coating on the substrate at 1200 rpm for 1min. To avoid metal pad or bonding result in unnecessary leakage current through the insulator, we removed additional solution on the substrate by the cotton swabs with PGMEA. Put the substrates on the hot plate and cured at 200℃ for 1 hour in the ambient air. The cross-link reaction was more completed at high temperature and for a long time, let OH functional groups on the PVP can be cross-linked by PMF as more as possible.
2. Poly(4-vinyl phenol-co-methyl methacrylate) (PVP-PMMA):
PVP-PMMA was obtained from Aldrich, PVP-PMMA copolymer both have phenol and methyl methacryl functional group chain. The chemical structure of PVP-PMMA is schematically shown in Fig. 2-2. We dissolved 15wt% PVP-PMMA in the N, N-dimethyl formamide (DMF) and put the solution on the hot plate at 70℃ until spin-casting the solution on the substrate. After spin coating of the polymer solution, to avoid metal pad or bonding result in unnecessary leakage current through the insulator, we removed additional solution on the substrate by the cotton swabs with DMF. The polymer film was baked on the hot plate at 150℃ for 1 hour. Because PVP-PMMA solution didn’t mix cross-linking agent, the film could be removed easily by acetone, ethanol, etc.
2-3.3 Top Metal Deposition
We deposited Gold (99.999%) as the top metal on the film. The deposition was started at 5×10-6torr and rate was controlled at 1Å/s. Au electrodes are defined through shadow mask and orthogonal to bottom metal. The destination of gold evaporation was used to simulate the actual structure for source/drain & gate metals of pentacene TFT.
2-4 Pentacene-based Thin Film Transistors Fabrication 2-4.1 Bottom Gate Metal Deposition
Before gate metal deposition, the glass substrate should be cleaned. The 50-nm Aluminum (99.999%) was deposited by thermal evaporator around 5x10-6torr. The deposition rate was controlled at 2Å/sec at 60℃.The gate metal pattern and metal pad were defined by shadow mask. The width of gate metal pattern was set up to be 1.5mm.
The device has large contact resistance if the width of gate metal is less than channel length.
2-4.2 Dielectric Film Formation
After gate metals defined, we purged the N2 flow to keep the substrates clean. The solutions of PVP and PVP-PMMA were spin coating on the different substrates.
Similarly, we removed additional solution by the cotton swabs with PGMEA and DMF.
And put each other on the hot plates in the ambient air. (PVP at 200°C and PVP-PMMA at 150°C for 1 hour).
2-4.3 Pentacene Thin Film Growth
The pentacene material obtained from Aldrich without any purification. Put the substrates with PVP and PVP-PMMA dielectric into the thermal evaporator until high vacuum pressure (3x10-6torr), we evaporate 50-nm-thick pentacene film. The deposition rate of pentacene was controlled at 0.4~0.5Å/sec at room temperature and active layer was defined by shadow mask.
2-4.4 Source and Drain Electrodes Deposition
We deposited 70-nm-Au (99.999%) through shadow mask as source and drain electrodes of pentacene-based TFTs. The deposition was started at 5×10-6torr and rate was controlled at 1Å/s. The device channel width and length was 1000μm and 200μm.
We can get conventional top contact and bottom gate TFT schematic as Fig. 2-1 (b)
2-5 Encapsulation of Pentacene-based TFT in Anhydrous Oxygen Environment
When the devices have been fabricated, the devices are got from the vacuum chamber to the ambient air. We first investigate their characteristic under the ambient environment and the relative humidity is about 65%. Then we do encapsulation for the devices, action in the glove box which environment is filled with nitrogen and ultra low water vapor and oxygen (H2O<0.1ppm, O2<0.1ppm). Put the samples into glove box and send to vacuum chamber linked to glove box. By the turbo pump, the device is under 1×10-6torr (ultra high vacuum) to make sure the water vapor absorption have been excluded. Then take it out in the glove box and use UV binder to joint substrates and package glass under UV light exposure in one minute. As a result, the OTFTs are situated in anhydrous oxygen environment with encapsulation can be measured by wire bonding and metal pad, even though the devices are encapsulated with package glass isolation.
2-6 A Novel Porous Structure of Pentacene-based TFT
As before sections, we also fabricate the OTFT on the glass substrates for the
under high vacuum (5x10-6torr) deposit 50-nm-Al (99.999%) film as the bottom gate metals and defined by shadow mask. We mixed the polymer dielectric solution as of 8%PVP (PVP: PMF =11:4) with PGMEA. The dielectric layer of OTFT devices is spinning on the gate metal and bake at 200℃ 1 hour under ambient air. The thickness is measured to be 420nm by Alpha-step. After cross-link reaction is completed. We put the glass substrates exposed to UV/Ozone in 5 seconds to increase PVP surface polarity for enhancing sensitivity for NH3.
The next step is to fabricate nano-scale meshed pentacene structure. We use high density polystyrene spheres as shadow mask. Positively charged polystyrene particles (200 nm, tetra-methylammonium latex) were adsorbed onto the substrates from dispersion by electrostatic interactions. Particles concentration was diluted with ethanol to 0.8 wt%. Immersion time was 3 minutes to allow the adsorption to reach saturation.
Excess particles were rinsed off in a beaker with ethanol and then transferred to a beaker with boiling isopropanol(IPA) solution for ten seconds. The key procedure in this fabrication is that the substrate is then transferred to a beaker with boiling isopropanol solution for ten seconds. The substrate is finally blown dry immediately in a unidirectional nitrogen flow. The benefit of this method is the possibility to process large areas in a short processing time without photolithography.
Then under high vacuum (3×10-6torr) deposit 50-nm-pentacene thin film as active layer and defined by shadow mask. We deposited 100-nm-Au (99.999%) through shadow mask as source and drain electrodes of pentacene TFT. The deposition was started at 5×10-6torr and rate was controlled at 1Å/s. The device channel width and length was 1000μm and 200μm. The process flow of the porous pentacene-based OTFT and schematic diagram are shown in Fig. 2-1 (c).
2-7 Micro-Fluid Gas Sensing System
As shown in Fig. 5, various NH3 concentration is obtained by uniformly mixing various amount of NH3 (from a cylinder contains 100 ppm NH3) with high purity nitrogen (N2) into a 20-cm-long U-shape tube. The amount of N2 is controlled by a mass-flow controller. The amount of 100 ppm NH3 is controlled by an electrical syringe pump. The U-shape tube with mixed gas is connected to a micro-fluid measuring system made by Polytetrafluoroethylene (PTFE) (width = 1 cm, length = 3.5 cm, and height = 0.1 cm). Two Keithley 2400s are utilized to synchronously generate -5 V (drain terminal) and 30V (gate terminal) pulse each 50 millisecond and read out the currents
2-8 Devices Electrical Parameter Extraction
Field effect mobility, threshold voltage, subthreshold slope and Ion/Ioff ratio are usual used to compare different devices’ performance. In the following section, extraction methods would present how to extract parameters from electrical transfer characteristic of pentacene.
2-8.1 Field Effect Mobility (μ
FE)
The field effect mobility (μFE) was determined by the orientation of pentacene molecules near gate dielectric. Therefore, gate dielectric surface states strongly affect the device μFE. The device μFE variation can be used to compare the difference between PVP and PVP-PMMA dielectric layers. In our experiment, μFE were extracted by using the linear region equation. Because the electrical transfer characteristic of
transistor and can be expressed as
I - Saturation region
, where Ceff and VT are effective capacitance per unit area and the threshold voltage. W and L are device channel width and channel length. When operating device at low drain bias, the linear region equation can be modified to
I - Linear region
Generally, mobility can be extracted from the transconductance maximum GM in the linear region:
The field effect mobility can be extracted from the transconductance and this equation can be expressed as
In this study, we used max GM value to calculate and define the field effect mobility.
2-8.2 Threshold voltage (V
T)
Threshold voltage (VT) determines the device operation voltage and smaller VT
can help to lower power consumption. Because VT strongly dependents on dielectric surface states, environmental and fabrication process variations easily cause a shift on the VT. Based on this phenomenon, the device VT shift is usually used as an importance parameter when pentacene-based TFTs applied to photo-detector or biochemical detector. In this study, we used the linear region equation to extract the device VT.
2-9 Material Analysis Instruments
2-9.1 Atomic Force Microscope (AFM)
Since PVP treated surface may exist OH groups, its surface morphology can also be different compare with PMMA surface. In addition, pentacene deposits on PVP (hydrophilic) and PMMA (hydrophobic) influenced by these two kinds of surfaces may have different grain size or morphology, too. Atomic force microscope (AFM) is used to measure surface morphology on a scale from angstroms to 30 microns. It scans samples through a probe or tip, with radius about 20 nm. The tip is held several nanometers above the surface and using feedback mechanism that measured interactions between tip and surface on the scale of nanoNewtons. Variations in tip height are recorded when the tip is scanned repeatedly across the sample, then producing morphology image of the surface. In this experiment, the used equipment is Digital Instruments D3100 as shown in Fig. 2-3 and the used active mode is tapping mode.
2-9.2 Contact Angle System
Contact angle system is used to estimate wetting ability of a localized region on a solid surface. The angle between the baseline of the drop and drop boundary is measured. Comparing the result can tell the material surface is relatively hydrophilic or hydrophobic, then can further analysis surface chemical composition. Since PVP have OH groups on its molecular structure while PVP-PMMA have less. We prepared the glass substrate coating with PVP and PVP-PMMA. By the contact angle system, we found the water contact angle and surface energy of PVP and PVP-PMMA were similar.
Figure of Chapter 2
Fig. 2-1 (a) The schematic picture of MIM pattern and cross section
Fig. 2-1 (b) The schematic picture of pentacene-based TFT pattern and cross section
Fig. 2-1 (c) The process flow of the porous pentacene-based OTFT
Fig. 2-2 The molecular structure of PVP, PMF, and PVP-PMMA
Glass substrate Al Insulator
Au
Glass substrate Gate Insulator Pentacene
Au Au
Glass GatePVP Gold
Glass GatePVP
Pentacene
Glass GatePVP Polystyrene sphere
200nm
PVP-co-PMMA
Fig. 2-3 Micro-fluid sensing system
Fig. 2-4 The picture of Digital Instruments D3100
Fig. 2-5 The picture of contact angle system
N2
100ppm NH3 Mass
Flow Controller
Syringe Pump
Micro‐Fluid Sensing Chamber
Exhaust
Valve Valve
I‐V Measurement