高解像度UVA マイクロLEDがもたらすディスプレイの未来

マイクロLEDは、高解像度、高輝度、低消費電力、製品寿命の長さなどの利点から、次世代ディスプレイ技術として注目されています。LEDinsideの推計によると、マイクロLEDの全体市場価値は2023年までに42億ドルに達する見込みです。2014年にAppleがスタートアップ企業のLuxVueを買収したことは、ディスプレイアプリケーションの未来へのシグナルであり、マイクロLED市場に新たな競争の波が到来したことを示すものでした。現在では、グーグル、インテル、サムスン、ソニーなど、多くのグローバル企業が、M&Aや共同開発などさまざまな戦略を用いて、技術的なリーダーシップを発揮しています

他のディスプレイ技術とは異なり、マイクロLEDは小型であるため、コンポーネントも小さくできます。そのため、テレビや車などの大型ディスプレイの超高解像度化に加え、多くの新興産業への応用に適しています。このため、携帯機器やウェアラブル機器（フレキシブル、折りたたみ、透明スクリーン）、メタバースAR/VRディスプレイ（ピクセルサイズの要件が1～8μm）、マスクレスリソグラフィー、光ピンセット、光人工内耳、デジタル光通信などのさまざまな先進技術といった分野で、大きなビジネスチャンスの鍵を握る存在となっています。

しかし、現時点では、マイクロLEDの技術開発はまだ未成熟です。製造コストが高いことに加え、マストランスファーの問題、低すぎる外部量子効率（EQE）、複雑で多様な部品を使った回路でテストが必要など、克服すべき技術的な問題や課題がまだ多く残っています。実際、マイクロLEDの製造工程は、従来のディスプレイとは大きく異なり、むしろ、半導体技術とより共通しています。台湾はすでに半導体分野で高度なプロセス技術と完全な産業チェーンを有するため、業界で主導的な地位を獲得する絶好のチャンスがあります。産官学が一体となって、台湾のマイクロLED技術の明るい未来と1兆ドル規模のビジネスチャンスの開拓を加速させることができればと願っています。

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今回の「新技術チャンネル｜コラボコラム」では、呉教授をお招きし、マイクロLED研究の最新情報をお伝えします。2017年に科学技術部の優秀研究賞を受賞した呉教授は、主に「III-V族化合物半導体プロセス」と「パワーデバイス」の分野を専門としています。高い評価を得ている革新的な研究プロジェクトを数多く担当し、その成果は電子部品分野でトップの国際学術誌に掲載されています。この紹介記事を通じて、台湾における学術的および応用的なマイクロLED研究の進展を理解していただければ幸いです。

Director of R&D Center & Marketing Division, Chris Chen, 2021/12/20

The Future of Display Technology Lies with the High-Resolution UVA MicroLED

1920×1080 high-resolution UVA MicroLED and Maskless Lithography Applications

Professor Meng-Chyi Wu
The Institute of Electrical Engineering, Tsing Hua University

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High-efficiency III-nitride light-emitting diodes have been shown to significantly improve solid-state lighting and are well on the way to replacing florescent bulbs and other forms of lighting. This is important as, with the ever-decreasing size of portable and mobile electronics, display technology has been required to move towards smaller component sizes and ultra-high resolution while simultaneously maintaining high efficiency and brightness uniformity and reducing power consumption. Virtual reality and augmented reality (VR/AR) was a hot topic at America’s Consumer Electronics Show (CES) this year. It is anticipated that these technologies will soon be widely used in numerous fields from film and television, entertainment and games to education and medical care. However, existing display technologies will find it difficult to meet the high resolution, high brightness, and low power requirements of future VR/AR applications. As such, the high brightness and power-saving characteristics of MicroLED promises to become the mainstream of future display technology.

To meet the needs of developing technologies, gallium nitride-based MicroLED was born. It can greatly improve brightness, luminosity, efficiency, responsiveness and lifetime. According to the 2020 LED Inside the report, the size of a MiniLED is defined as being between 50 to 130 μm, and the size of a MicroLED is less than 50 micrometers. As such, tiny pixel size is only one of the basic requirements for MicroLED display and production. It is also necessary to have hundreds of thousands to millions of pixels with high uniformity. Table 1 shows a comparison between current, mainstream display technologies and MicroLED.

Table 1　A comparison of LED, OLED, and MicroLED display technologies

Passive Matrixes enable pattern display control with a low-cost development driver architecture (it can use commercially available multi-bit LED driver ICs like R8A66162SP matched with a microcontroller). However, the limitations of core architecture size and pattern speed (considering its use as a communication or excitation light source) as well as the difficulty of lead/wire bonding for high-density pixels means it is only suitable for low pixels and small and medium matrixes (for example, a 128×128 matrix or below). Active matrixes feature high-speed, synchronized and independent drives; read-In IC (RIIC) and MicroLED matrix are bonded via “Flip-Chip” technology. Each MicroLED matrix pixel corresponds to a single IC driver, significantly reducing circuit packaging and space.

The Active Matrix driving method involves connecting the MicroLED matrix to the CMOS chip. Each MicroLED pixel corresponds to a specific circuit, and each MicroLED pixel size should be the same as that of the driver IC unit. For fix-sized displays, such as 0.5in or 0.2in displays, the smaller and more numerous the MicroLED pixels, the higher the display resolution. When it comes to the MicroLED displays for virtual reality and augmented reality, the number of matrixes is often 1920×1080 or even higher. Such high-resolution MicroLED displays are more difficult to manufacture.

The team at the Institute of Electrical engineering at Tsing Hua University has successfully created a high-performance 960×540 blue light Active MicroLED display. The resolution is equivalent to 1900 ppi (pixels per inch). It has a single pixel size of 8µm and a pixel pitch of 12µm. This MicroLED display can already be used in smartphones, smartwatches, head-mounted and near-eye displays and optogenetic stimulation, etc.

Each pixel for the 1920×1080 UVA Active MicroLED is plated with indium, which plays the role of an adhesive metal. The pixel is then bonded with the CMOS IC via Flip-Chip technology. This device opens up many possibilities for improving white LED solid-state lighting applications and is expected to have a wide range of applications in areas such as high-density data storage, chemical and biological sensors, sterilization, lithographic equipment and more. The use of UV light in lithographic exposure equipment has gained a lot of attention in recent years. Using the 370-nm UV light as an exposure tool, the patterns on the display can be exposed directly onto the photoresist. This practice can eliminate the need to use expensive photomasks as tools for defining patterns. In addition, this would provide a simple method for building 3D structures using stereolithography. Therefore, such equipment is considered to be a more efficient exposure tool because it is capable of both exposing and defining a pattern at the same time.

The Institute of Electrical Engineering at Tsing Hua University has succeeded in developing this high-resolution microdisplay and demonstrated the transferring of an image using the SDK module. This 1920×1080 UVA MicroLED display has an operability rate of 80.9% and is the first demonstration of a high resolution UV MicroLED display with a 16:9 aspect ratio, which is the current mainstream in high-definition displays. Finally, the specific pattern is shown on the display via direct transfer from the photoresist, successfully demonstrating the application of maskless lithography.

The basic process behind most MicroLEDs is to first define the plateau (mesa) pattern, in other words, the pixel area. The vertical sidewalls are then etched into the n-layer. Next, the mesh electrodes are placed sequentially on the n-layer, and the p-electrodes are placed on the p-layer and grow the passivation layer. Finally, indium balls are grown as a bonding metal. Note that a new mask is needed before each of the aforementioned processes. However, due to the limitations of the contact aligner in the academic laboratory, it is difficult to make this type of MicroLED structure with high uniformity. First of all, the first pattern has a pixel size of only 5 microns, so it is easy for the mesa pattern to float away during the development or de-ionized water cleaning processes. Furthermore, the minimum resolution line width of the exposure machine is only one micron, and it is manually aligned. This makes it extremely difficult to open a 2 micron width wiring pattern in a mesa space of only 3 microns.

Therefore, the Institute of Electrical Engineering at Tsing Hua University has developed a MicroLED with a special, trapezoidal sidewall. Trapezoidal sidewalls allow metal n-electrodes and p-electrodes to be formed automatically when deposited using e-beam evaporators with low step coverage. This is the auto-alignment technique. By combining trapezoidal sidewall etching technology with automatic alignment technology, the number of masks can be reduced by two—that of the n-electrode and p-electrode. This not only speeds up the process but also eliminates alignment issues.

Figure 1 shows the electrical characteristics of a single component from a 1920×1080 UV light MicroLED matrix. The logarithmic plot on the left shows that there is a current leakage of 9.48pA at a reverse bias of 10V. In addition, the turn-on voltage of a component is typically set at a current density of about 22.2A/cm². Since the size of a single component is about 5μm, the turn-on voltage after conversion can be determined to be a current of 4.36μA. Therefore, as shown in the linear graph on the right, the single LED turn-on voltage of a 1920×1080 UV MicroLED is about 3.29 volts.

After conducting an analysis and comparison of the data, it was found that MicroLED leakage current is mainly caused by sidewall defects created by the dry etching process, where the turn-on voltage required for each MicroLED element and the leakage current at reverse bias are both very small.

Figure 1　Diagram of the I-V characteristics of a single 5μm MicroLED (On the left is a logarithmic graph. On the right is a linear graph)

Figure 2 shows the optical characteristics of a single MicroLED. From the graph on the left of Figure 2, we can see that the optical output power is 70.4μW when the current is 1mA. According to the graph on the right of Figure 2, the maximum External Quantum Efficiency (EQE) is 3.74% and appears at a current density of 153.2A/cm². The maximum photoelectrical conversion efficiency (WPE) is 3.61% and appears at a current density of about 101.8A/cm². In general, we want the two above conversion efficiency maxima to appear at lower current densities. This allows the MicroLED components to be driven at lower currents at peak luminous efficiency. As such, the above results have met our expectations.

On the right of Figure 2, we also observe that there is a shift in the peak wavelength. This is because the built-in electric field that originally existed in the quantum well will cause the distortion and tilt of the energy band. This is known as the Quantum Confinement Stark Effect (QCSE). However, increasing the injected current will shield the built-in electric field to some extent, thus attenuating the Stark effect and resulting in a wavelength blue shift. However, as the current injection continues to increase, the thermal effect of current crowding will become more pronounced, resulting in a red shift of the wavelength.

Figure 2　On the left is a graph of light output power and luminosity versus the current of a single MicroLED. On the right is a plot of the External Quantum Efficiency (EQE), electro-optical efficiency and wavelength versus current density.

Figure 3 shows the electroluminescence (EL) spectrum. It was obtained by applying different levels of current to the same single element. The resulting wavelengths are plotted against the luminescence intensity from 5μA to 1,000μA. It can be seen that, for the 370nm UVA MicroLED, the peak stays at around 370nm as the injection current increases. This is also very close to the wavelength of the i-line (365nm) exposure light used in standard lithography processes. The full width at half the maximum of its luminescence spectrum also increases very slowly with the increase of the current, from 6.1nm at 10µA to 8.2nm at 300µA. This is ideal for monochromatic light applications.

Figure 3　Electroluminescence (EL) spectrum of a single 5μm MicroLED for input currents from 5μA to 1,000μA

However, it is worth noting that the diagram shows that the UVA MicroLED has a small defect light peak at 550nm. It is about a thousandth of the intensity at 370nm. This is due to the epitaxial defects of gallium nitride materials. The defect energy generated in the energy gap due to epitaxial defects will capture electrons and make the energy gap smaller, thereby producing defect light visible to the naked eye.

Due to the Flip-Chip technology used, light is emitted from the sapphire substrate in the back. Re-measuring the fully lit display, it was found that, at a current of 100mA, there is a maximum overall optical output power of 2.6mW. Furthermore, as mentioned earlier, we can see that, during full lighting, the defect light in the 550nm band, which is in the green light range, can be seen by the naked eye.

Figure 4　Scanning Electron Microscope photo of a MicroLED that has been bonded to an IC via Flip-Chip technology. The adhesive metal in the middle is In.

Figure 4 is a photo taken by an electron microscope after the LED and IC are bonded via Flip-Chip technology. The yield rate for MicroLEDs is and will likely continue to be of interest in both academia and the industry. At present, two main ways that we believe we can work to improve yield are to optimize the wafer production process and improve wafer quality.

Figure 5　Using a 1920×1080 UVA MicroLED display to show pictures; the picture on the left is a cheetah, and the picture on the right is the Tsing Hua University logo.

In Figure 5, a 1920×1080 UVA MicroLED is connected to a flexible cable and a drive circuit board. The computer is used to play pictures. It can also be used to play videos. To our knowledge, this is the first demonstration of a high-resolution UVA MicroLED display. It has a 16:9 aspect ratio, and the size of a single LED element is 5μm.

To demonstrate the maskless lithography process, we first played the NTHU typeface (mirror image) on a 1920×1080 UVA MicroLED display. The target wafer is then spin-coated with a negative photoresistor and soft baked to complete direct contact bonding of the silicon wafer to the display. After 200 seconds of exposure, we conducted post-exposure baking and development, successfully placing the NTHU graphic on the silicon wafer.

Figure 6　The letters NTHU were successfully developed on the silicon wafer that with light-exposed photoresist.

As shown in Figure 6, the letters NTHU were successfully developed on the silicon wafer. However, it can also be seen that the line width of the typeface and the sharpness of the corners are slightly different from those of the image shown on the display. The reason is that, unlike a real exposure machine, this device lacks a focusing lens. This causes light to scatter in the gap between the display and the silicon wafer.

Therefore, the combination of standard lithography exposure equipment and high-resolution UVA MicroLED displays promises great improvements for future lithography equipment and maskless lithography processes. However, the minimum line width this device can achieve depends on the size of the MicroLED components. Still, if the size of MicroLED components can continue to be scaled down, it could reduce the cost of lithographic processes in the industry in addition to providing other significant advantages, such as improved process yields and shortened processing times.

Light-emitting diodes have many advantages and therefore are widely used today in everything from general lighting, traffic lights and car headlights to LCD backlights, microdisplays and virtual reality. Miniature light-emitting diode displays are likely going to become the mainstream display technology in the future. Even now, there is a growing demand in the market for products such as augmented reality or smart glasses with higher and higher resolutions. So, how do we scale down the size of the LEDs, get a high-resolution matrix, make it emit light evenly and improve the yield? All of these are important questions that need to be addressed.

The high resolution 1920×1080 UVA MicroLED display successfully developed by the Institute of Electrical Engineering at Tsing Hua University has a single LED component size of 5μm and a component spacing of 8μm on a wafer with a diagonal measurement of 0.69in. The resolution is equivalent to 3200ppi. Measurement of the electrical characteristics of a single element of the MicroLED matrix show that said element has an ideal turn-on voltage and advantages such as a very small leakage current. In addition, a measurement of the optical properties of a single element showed that it had better optical output power and higher photoelectric conversion efficiency. These excellent characteristics can be attributed mainly to the advanced manufacturing processes used, including the auto-alignment lithography process and optimized dry etching technology. These advanced processes contribute greatly to the development of high ppi UVA displays.

In addition, the UVA energy is greater than that of the three primary colors. Taking advantage of this, the Institute of Electrical Engineering at Tsing Hua University is currently trying to use the three primary colors of perovskite phosphors and photo excitation to develop full color MicroLED displays. We anticipate making even greater breakthroughs for the next generation of display process technologies in the future.

マイクロLEDディスプレイは、液晶、LED、光電材料、半導体、精密機器などの技術を統合したものです。フレキシブル、透明、曲面など、高度なディスプレイを実現する可能性を秘めています。近年、多くの国際的なメーカーがマイクロLEDの開発への投資を急いでいます。スマートライフにおけるハイエンドディスプレイシステムの需要の高まりや、「メタバース」とも呼ばれる仮想現実や拡張現実における高解像度アプリケーションの進化が、マイクロLED産業の爆発的成長に寄与し、新たな市場機会の波が押し寄せているのです。

今回、この分野の第一人者である吳孟奇教授が、「新技術チャンネル」でマイクロLED技術の応用例やご自身の研究内容を紹介し、台湾におけるこの次世代ディスプレイ技術の研究開発について深く理解していただけることを大変嬉しく思います。同時に、MA-tekは、マイクロLEDの研究に必要な分析サービスを提供することで、呉教授と手を携えて今年の産学連携を実施できることを非常に光栄に思っています。MA-tekは、先進的な試験設備と専門的な技術経験を有し、マイクロLED部品の構造および材料開発に関するさまざまな分析・試験ニーズに対応します。詳細については、以下のリンクを参照するか、ページの右下をクリックして直接チャットでお問い合わせください。MA-tekは、分析サービスを提供するグローバルリーダーです。一緒に技術の鼓動を感じていきましょう!

現在、次号の「科学技術の新チャネル｜コラボレーションコラム」を企画中ですので、MA-tekの技術記事にご期待ください。さまざまな最先端技術の最新情報を入手し、グローバルサプライチェーンで、より競争力を高めましょう!

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