ABOUT DISPLAY , TFT and Wire Bonding

This study is based on research and experimental work conducted on display devices and thin-film transistors (TFTs). The manuscript is written from the perspective of hands-on experience, describing the characteristics of each device as well as the wire bonding process, focusing on the concepts and practical knowledge acquired through experimentation.

PROJECT DISPLAY & TFT

1. Glass Cleaning
2. OLED Manufacturing Process
3. OLED Measurement – (I-V-L Measurement system)
4. TFT Measurement - (Probe Station)
5. OLED & TFT Wire Bonding

** Glass Cleaning**
This process is designed to remove foreign substances and impurities from the glass substrate prior to the attachment of the PET film.

It can classify total 9 steps.

1. Ultrasonic Cleaning (Acetone) - Submerge the glass in an acetone solution. Place it in the ultrasonic cleaner and process at 40℃ for 10 minutes.
2. Rinsing - After 10 minutes, rinse the glass thoroughly with DI (Deionized) water.
3. Ultrasonic Cleaning (IPA) - Submerge the glass in an IPA (Isopropy Alcohol) solution. Place it in the ultrasonic cleaner and process at 40℃ for 10 minutes.
4. Rinsing - Rinse the glass again with DI water.
5. Ultrasonic Cleaning (DI Water) - Submerge the glass in DI water. Place it in the ultrasonic cleaner and process at 40℃ for 10 minutes.
6. Drying - After the final DI water rinse, hold the glass firmly with tweezers and remove all residual moisture and solution using N_2gas.
7. Dehydration Bake - Place aluminum foil on a hot plate and set the glass on top (face up). Heat at 270℃for 30 minutes to ensure complete dehydration.
8. NOA Coating (Planarization) - To ensure a smooth and level OLED surface, coat the glass with NOA (Norland Optical Adhesive). This step prevents potential contamination during the PET attachment process. The substrate is secured via vacuum. Ensure the center is properly fixed to prevent the substrate from detaching. Apply only enough NOA to cover the surface without overflowing. Start the device using the dial; the coating process is complete once the waveform is confirmed on the display.
9. UV Treatment - Place the glass on a temporary foil tray and transfer it to the UV chamber. Position the glass so its center aligns with the exposure mark. Operate the UV machine for 30 to 40 minutes to remove any remaining organic residues.

OLED Manufacturing Process
Substrate Preparation
Glass is used as the base substrate. Once the glass cleaning is complete, a PET film is attached to the deposition side of the glass substrate using adhesive tape.

Deposition Process(CVD)
OLED fabrication utilizes CVD (Chemical Vapor Deposition). CVD is a process where organic and inorganic materials are deposited onto OLED devices through chemical reactions. These reactions are triggered by heating materials in a high-temperature, negative-pressure (vacuum) environment.

Chamber Configuration and Material Handling
When facing the equipment, the left chamber is dedicated to organic material deposition, while the right chamber is used for metal deposition.
Transfer System - The interior of the transfer chamber, which houses the robotic arm, is filled with Nitrogen and maintained at positive pressure. The devices are moved between chambers exclusively by the robotic arm.

Patterning with Masks
Patterns are created by placing the substrate and overlaying it with a mask of the desired design. When organic and inorganic materials are deposited, they pass through the common openings of the masks, ensuring the materials are applied only to the specific intended areas.

System Control and Precision
Process parameters such as recipes, pressure, and temperature are configured and managed via a PC. This stage requires extreme precision, as even minute variations in these conditions can significantly impact the final quality and performance of the OLED device.

** TFT(Thin Film Transistor) Manufacturing Process **
A Thin Film Transistor (TFT) is a fundamental component of modern displays, acting as a microscopic switch that controls each individual pixel. By regulating the flow of electricity, TFTs determine when a pixel turns on or off and how much light it emits, which is essential for rendering high-resolution images.

Substrate Preparation and Dehydration
The process begins with thorough glass cleaning (using the method previously described) to remove all foreign particles. After cleaning, the glass is baked in an industrial oven to eliminate any residual microscopic moisture. This dehydration step is critical to minimizing impurities that could compromise the thin film's integrity.

Masking and Alignment
A patterned mask is placed over the cleaned device and secured firmly. Before loading it into the sputtering system, a transparent acrylic plate is used to fix the masked device in the optimal position for IGZO (Indium Gallium Zinc Oxide)deposition.
Purpose of Rigid Fixing - These multiple securing steps are implemented to prevent any displacement or vibration of the substrate during the process, ensuring maximum precision and accuracy.

Characteristics of IGZO Material
IGZO is a cutting-edge semiconductor material with a unique atomic arrangement that ensures stable performance. Its key advantages include:
High Electron Mobility - It offers approximately 30 times higher electron mobility than conventional amorphous silicon (a-Si).
Low Leakage Current - It significantly reduces power consumption, contributing to longer battery life in portable devices.

Sputtering Process (Physical Vapor Deposition - PVD)
The sputtering equipment utilizes the PVD (Physical Vapor Deposition)method. This is a physical deposition technique where high-energy ions collide with the target material, causing atoms to be ejected and subsequently form a thin film on the substrate.
Operational Note - Due to the nature of this physical mechanism, the substrate must be loaded into the sputtering chamber with the deposition side facing the source material (IGZO) to ensure effective film formation.

** * OLED I-V-L Characterization (Device Performance Measurement)**
The I-V-L system is used to measure and analyze the electro-optical characteristics of the fabricated OLED devices.

System Startup and Calibration
Power on the measurement equipment in the designated sequence.
Launch the control software on the PC and verify that all three main components are successfully connected (indicated by green status lights).

Sample Loading and Configuration
Identify whether the device is Top Emission or Bottom Emission.
Place the OLED device into the Sample Test Fixture (Dark Box)and toggle the corresponding switches to align with the device's pixel positions.
Adjust the camera focus to monitor the emission area clearly.

Functional Verification and Measurement (Sweep)
Functional Check - In the 'One Point' session, apply a voltage of approximately 3V to 4V (Cell On)to verify that the device emits light normally. Once confirmed, turn the cell off.
IVL Sweep Setup - Access the IVL Sweep mode to configure the measurement parameters:
Start/Stop Value - Set the initial and final voltage levels.
Step Value - Define the voltage increment for the sweep.
Execution - Start the measurement to capture the electrical and optical data.

Data Analysis and Visualization
Upon completion, the measured data is exported to an Excel file to generate the following characteristic graphs Current-Voltage (I-V), Luminance-Voltage (L-V), Luminance-Current Density (L-J), Electroluminescence (EL) Spectrum
In this session, measurements were conducted for Blue, QD (Quantum Dot) Blue, and Red OLED devices.

** Analysis of OLED Characterization Results**

Current-Voltage (I-V) Characteristics
Comparison of Blue and QD Blue - The current flow for both Blue and QD Blue devices is nearly identical. This confirms that the application of a QD (Quantum Dot) film is a passive process that does not alter the fundamental electrical properties of the underlying OLED device.
Blue/QD Blue vs. Red - A significant difference in current behavior was observed between the Blue and Red devices. While the current for Blue and QD Blue increased sharply starting from 4V, the Red device did not show a substantial increase within the measured range (0V to 5V).

Luminance - Current Density (L-J) and Efficiency
Efficiency Metric - The Luminance vs. Current Density graph serves as a key indicator of luminous efficiency, showing which device produces the most light for a given amount of electrical current.
Device Performance - The data reveals that the Red device is the most efficient, as it reaches the benchmark luminance of 30cd/m² at the lowest current density among the three.
Impact of QD Film - The QD Blue device exhibited the lowest luminance. This is attributed to the presence of the QD film on top of the Blue device, which causes some light loss or absorption, resulting in a dimmer appearance compared to the bare Blue device.

Spectral Analysis and Color Conversion
Wavelength Distribution - The spectral peaks follow the order of Blue < QD Blue < Red, with Blue having the shortest wavelength.
Color Shifting - It was observed that the Blue light shifted toward the red spectrum upon the application of the QD film.
Conclusion - This experiment successfully demonstrated that attaching a QD film to a standard Blue OLED device allows for color conversion (wavelength shifting)without modifying the device's basic electrical characteristics, such as the I-V profile.

** TFT Characterization using a Probe Station**
The Probe Station is utilized to measure the electrical characteristics (such as I–V curves)of the fabricated Thin Film Transistor (TFT) devices.

Device Loading and Stabilization
Place the TFT substrate onto the stage inside the probe station.
Activate the vacuum system to securely fix the device in place, ensuring stability during the measurement.

Microscopic Alignment and Probing
Using the integrated microscope, identify the precise locations of the Drain, Source, and Gate electrodes.
Carefully adjust the probe arms to ensure that each probe tip makes accurate electrical contact with its corresponding electrode.

Software Configuration and Parameter Setting
Once a stable contact is confirmed, configure the measurement software on the PC by assigning the Drain, Gate, and Source terminals.
Set the graph display to a Logarithmic (Log) Scale to clearly observe the subthreshold behavior and off-current.
Define the specific measurement conditions, including the Drain Voltage V_D and Gate Voltage VGs weep ranges.

Execution and Data Management
Initiate the measurement to capture the transfer and output characteristics of the TFT.
Upon completion, the data is automatically exported and saved as an Excel file, allowing for the analysis of current and voltage values under various conditions.

** Analysis of TFT Characterization Results**
Our team conducted 5 to 6 measurements for each of the two fabricated TFT devices. Based on the resulting graphs, Device 1 (Left)demonstrated superior performance compared to Device 2 (Right). The analysis is based on the following observations:
Consistency of Vth and Turn-on Voltage - The graphs indicate that Device 1 has a highly consistent Turn-on voltage and Threshold Voltage Vth across multiple sweeps.
Stability in the Saturation Region - At a fully turned-on state Vgs = 30V, the Drain Current ID in Device 1 remained stable and uniform. In contrast, Device 2 exhibited fluctuating and inconsistent current flow in the same region.

Conclusion - Device 2 showed higher deviation and lower reliability compared to Device 1. Therefore, Device 1 is identified as the better-performing TFT due to its superior electrical consistency and stability.

** Wire Bonding (TFT-OLED Interconnection)**
Wire Bonding is the critical final stage where the TFT and OLED components are electrically connected. This step is vital as the quality of the interconnection directly determines the final device's overall performance.
Experimental Approach (Innovative Solution)
For this experiment, our team utilized the internal metal wire from a bread twist tie. We carefully removed the outer plastic casing and adhesive to extract the conductive metal core.
Conductivity Verification
Using an ohmmeter, we measured the resistance of the extracted wire, which was approximately 1.2Ω ~ 1.3Ω. We confirmed that this resistance level is low enough for effective signal transmission.
Interconnection Method
We used Silver Paste to establish a secure electrical contact between the components. Specifically, we bonded the Source electrode of the TFT to the Anode of the OLED to complete the integrated circuit.

** Analysis of OLED-TFT Wire Bonding Characterization Results**

• First Measurment(Failed) During the first attempt, the process required extreme caution as any leakage of silver paste into the channel could cause a short circuit. While we were able to measure the I-V curve of the TFT, the OLED device failed to emit light.

Failure Analysis
After the unsuccessful trial, our team discussed and identified the following potential causes for the failure

1. OLED Device Issues - Since current was flowing but the device remained dark, we suspect an issue with the OLED itself. It is possible that the required driving current was excessively high, suggesting a need for a more efficient OLED device.
2. Silver Paste Contamination (Short Circuit) - There is a possibility that the silver paste penetrated the channel, leading to leakage current where the current flows through unintended paths instead of the device.
3. Poor Probe Contact - The failure may have resulted from unstable electrical contact between the probes and the electrodes (Poor Contact), preventing proper signal delivery to the device.

• Second Measurment (Failed) Recognizing the issues encountered in the previous attempt, our team conducted re-measurements on a different day. We meticulously applied conductive silver paste to ensure a precise connection solely between the Source and the Anode, avoiding any short circuits. Additionally, the probe contact was verified by the teaching assistant (TA) before proceeding.

However, despite these efforts, only the I-V characteristic curves of the TFT were obtained, and the OLED device failed to emit light.
Upon analyzing the measurement data, we suspected that the current flowing through the circuit was lower than the required driving current for the OLED.

Consequently, we replaced the device with an OLED having a lower driving current requirement. Nevertheless, light emission was still not observed.

• Third Measurment (Succes) After seeking assistance from the TA, we inspected the device and observed that a specific section of the OLED was not turning on consistently. Hypothesizing that the operating voltage range was the issue, we shifted the Gate Voltage sweep range by +10V.

Instead of the original range of -30V to 30V, we adjusted the measurement range to -20V to 40V. Following this adjustment, all devices operated normally, and successful light emission was observed.

• Discussion & Conclusion Through the success of the final experiment, we clearly identified the root causes of the previous OLED emission failures
• Insufficient Current The driving current required for the initial OLED was too high for the supplied current.
• Device Inconsistency Certain parts of the OLED device showed inconsistent activation.
• Operating Voltage Range The OLED device required a Gate Voltage above 30V to operate normally (The initial 30V limit was insufficient).

Through the failure and subsequent success of the TFT & OLED Wire Bonding experiment, we gained a comprehensive understanding of the OLED's driving current, operating voltage ranges, and the critical importance of precise bonding. Furthermore, we concluded that observing the TFT I-V characteristic curve alone is insufficient to verify the wire bonding result; the operating Gate Voltage plays a decisive role in the actual activation of the OLED.