Introduction

Precise microLED characterization is important for measuring parameters like luminance, chromaticity, uniformity, and carrier recombination lifetime to drive innovation in areas such as performance optimization, quality control, AR and VR, and other high-brightness displays. 

However, proper characterization can be challenging due to complications with the mass transfer technique, end-to-end fabrication, small size, and, most importantly, cost.

At present, these problems can be tackled with techniques like strain relaxation, surface recombination, and sidewall passivation. Also, there are key regional players from Taiwan, Mainland China, Japan, Korea, Europe, and North America who are actively working to advance the research in this field.

This article will offer new perspectives on microLED characterization and introduce some new research being conducted.

Advanced MicroLED Characterization Techniques

The latest developments in MicroLED characterization are as follows:

1. Laser Lift-Off (LLO)

LLO is a technique for detaching microLEDs from their growth substrate, especially sapphire. 

Technical Details

• Laser Wavelength and Power: The laser wavelength employed for LLO is in the ultraviolet (UV) spectrum, with a power of about 0.8 W. This wavelength was selected because the semiconductor material significantly absorbs it.
• Laser Absorption and Decomposition: The microLED chip and substrate contact is where the laser absorption takes place. This causes the semiconductor material to break down into its component parts, causing the chip to separate from the substrate.
• Laser Scanning and Venting: The laser beam is scanned across the interface line by line to create a clean lift-off surface. The nitrogen venting procedure should be optimized to ensure a clean separation and help eliminate any leftover gas.
• Laser Lift-Off Parameters: The main LLO parameters are the laser wavelength, power, fill density, pitch, frequency, and scan speed.

Applications and Examples

• Coherent UVtransfer Systems: Coherent’s UVtransfer systems for damaged pixels make high-throughput and effective microLED display manufacturing possible.
• Han’s DSI Laser Lift-Off Technology: Han’s DSI has developed advanced LLO technology that can strip microLED chips with high yields and precision.

2. Micro Transfer Printing (µTP)

With μTP, the transfer of various semiconductor materials takes place onto substrates like flexible polymers, glass, and metals. It is damage-free, enabling high-yield manufacturing. 

Researchers have developed advanced micro-vacuum Assisted Selective Transfer (mVAST), which offers visible adhesion switchability during the transfer process, providing damage-free manufacturing with high-yield production rates.

Researchers have also studied the fracture mechanics and mechanical models underlying various μTP methods, such as kinetic control transfer printing and laser-controlled seal temperature transfer printing. 

Nanomaterials in μLEDs

The integration of nanomaterials like nanorods and nanoparticles into μLEDs has been a game-changer. The luminous efficiency is affected through localized surface plasmon resonance (LSPR). The localized surface plasmon resonance (LSPR) coupling effect induced by metal nanoparticles (NPs) has also been initiated for the color conversion of quantum dot-μLEDs (QD-μLEDs).

3. Time-Resolved Photoluminescence (TRPL)

This technique measures the decay time of photoluminescence as a function of time. This information about the recombination processes and the device’s internal dynamics is useful.

Applications of TRPL

TRPL has many applications in microLED characterization:

• Carrier Recombination Lifetime: TRPL is used to measure the carrier recombination lifetime, which is the time it takes for carriers to recombine and emit photons. This helps in modulation bandwidth.
• Carrier Transport Effects: It neglects carrier transport effects, which are important for understanding the performance of microLEDs under electrical injection. 
• TREL: Time-Resolved Electroluminescence (TREL) is a technique that can characterize carrier transport effects and is essential for understanding the performance of microLEDs under real-world conditions.
• Carrier Density: TRPL provides information about the carrier density in the active region of the microLED. This information is important for understanding the performance of microLEDs and for optimizing their design.

Techniques Used in TRPL

Several techniques are used in TRPL, including:

• Time-Correlated Single Photon Counting (TCSPC): TCSPC is a popular method for carrying out TRPL measurements. The duration between the laser pulse’s stimulation of the sample and the photon’s arrival at the detector is measured.
• Bi-Exponential Model: A bi-exponential model is typically employed to fit the decay trace of InGaN, which is the material used in most microLEDs. 

4. Time-Correlated Single Photon Counting (TCSPC)

TCSPC is a highly sensitive technique used to measure the decay time of photoluminescence with high temporal resolution. This technique is based on the fact that the probability of detecting a single photon at a time after an exciting pulse is proportional to the fluorescence intensity at time t.

Applications of TCSPC

TCSPC has several applications in microLED characterization:

• Fluorescence Lifetime Measurement: TCSPC is used to measure the fluorescence lifetime of microLEDs.
• Carrier Recombination Lifetime: TCSPC can also be used to measure the carrier recombination lifetime of microLEDs.
• Dynamic Fluorescence Lifetime Sensing: TCSPC can be integrated with CMOS single-photon avalanche diode (SPAD) arrays to create a dynamic fluorescence lifetime sensing system that can measure fluorescence lifetimes in real time.

Techniques Used in TCSPC

Several techniques are used in TCSPC, including:

• Pulsed Excitation Sources: Pulsed LEDs and laser diodes are used as excitation sources to generate the fluorescence signal.
• Single-Photon Detectors: Fast, cooled photomultiplier tube detectors are used to detect the single photons emitted by the microLEDs.
• Time-to-Digital Converters: Time-to-digital converters are used to record the time between the excitation pulse and the arrival of the photon at the detector

5. Fluorescence Lifetime Imaging Microscopy (FLIM)

FLIM is a technique that combines fluorescence microscopy with lifetime measurements. It provides detailed spatial and temporal information about the photoluminescence dynamics in MicroLEDs, enabling the analysis of defects and inhomogeneities.

Key Points

• FLIM Principle: This measures the time a fluorophore remains in an excited state before emitting a photon, detecting molecular variations that are not apparent with spectral techniques alone.
• Instrumentation: FLIM instruments use a pulsed laser source and time-correlated single photon counting (TCSPC) electronics coupled to a confocal microscope. The microscope is equipped with a pulsed laser excitation source, a wavelength selector, and a single photon counting detector such as a photomultiplier tube (PMT).
• FLIM Acquisition: Fluorescence decays are acquired using TCSPC. The area of interest on the sample is split into pixels, and the fluorescence decay from each pixel is recorded using TCSPC. The laser spot is directed onto each pixel by moving the microscope stage or the excitation laser spot.
• FLIM Analysis: The fluorescence lifetime of each pixel in the dataset is calculated by least squares fitting a single exponential model to each decay. The range of fluorescence lifetimes obtained from the fitting is mapped onto a color map to create a color-coded fluorescence lifetime image.

Applications in MicroLED Characterization

• Carrier Recombination Lifetime: FLIM can measure the carrier recombination lifetime of microLEDs, which is critical for understanding their electrical properties.
• Fluorescence Lifetime Contrast: It also provides a contrast mechanism that can be used to image the variation in the carrier lifetime of nanomaterials, solar cells, and semiconductors.
• Quantitative Imaging: FLIM can be used for quantitative imaging of microLEDs, enabling the measurement of fluorescence lifetime and intensity simultaneously. 

Case Studies on MicroLED Characterization

Some case studies on microLED characterization are:

• High-Resolution Blue Micro-LED Arrays: A study demonstrated the fabrication of blue micro-LED arrays with an impressive 1692 PPI on sapphire substrates.
  
  By optimizing ITO layers for current spreading, researchers achieved displays with exceptional brightness and uniformity, setting a new standard for high-performance MicroLED screens.
• Porotech’s Vivid Displays: Porotech has made significant strides in creating micro-LED displays that are not only brighter but also offer sharper images and more vivid colors. 

Their innovative use of InGaN materials has resulted in displays with superior performance characteristics. DPT technology allows microLED pixels to emit any visible color when driven with a specific current density. This technology enhances display capabilities and enables the creation of full-color displays with high pixel density and minimum form factor.

End Note

MicroLED characterization can address problems with luminosity, color uniformity, and failure frequencies. With the expected expansion of the worldwide microLED business, sustained innovation will be essential. Subsequent research should focus on enhancing characterization methods such as Fluorescence Lifetime Imaging Microscopy and integrating nanomaterials to increase efficiency.