Design of LCOS microdisplay backplanes for projection applications

De evolutie van licht emitterende diodes (LED) heeft ervoor gezorgd dat het op dit moment interessant wordt om deze componenten als lichtbron te gebruiken in projectiesystemen. LED’s hebben belangrijke voordelen vergeleken met klassieke booglampen. Ze zijn compact, ze hebben een veel grotere levensduur en ogenblikkelijke schakeltijden, ze werken op lage spanningen, etc. LED’s zijn smalbandig en kunnen een groterekleurenbereik realiseren. Ze hebben momenteel echter een beperkte helderheid. Naast de lichtbron is het type van de lichtklep ook bepalend voor de kwaliteit van een projectiesysteem. Er bestaan verschillende lichtkleptechnologieën waaronder die van de reflectieve LCOS-panelen. Deze lichtkleppen kunnen zeer hoge resoluties hebben en wordenvaak gebruikt in kwalitatieve, professionele projectiesystemen. LED’s zijn echter totaal verschillend van booglampen. Ze hebben een andere vorm, package, stralingspatroon, aansturing, fysische en thermische eigenschappen, etc. Hoewel er een twintigtal optische architecturen bekend zijn voor reflectieve beeldschermen (met een booglamp als lichtbron), zijn ze niet geschikt voor LED-projectoren en moeten nieuwe optische architecturen en een elektronische aansturing ontwikkeld worden. In dit doctoraat werd er hieromtrent onderzoek gedaan. Er werd uiteindelijk een driekleurenprojector (R, G, B) met een efficiënt LED-belichtingssysteem gebouwd met twee LCOS-lichtkleppen. Deze LEDprojector heeft superieure eigenschappen (zeer lange levensduur, beeldkwaliteit, etc.) en een matige lichtopbrengst

Low-Power and High-Performance Drivers for OLEDoS Microdisplays

The rapid growth of the microdisplay market, driven by the demand for smartwatches, head-mounted displays in Virtual Reality (VR) and Augmented Reality (AR), and other portable devices, has presented a need to enhance their energy efficiency. This thesis focuses on reducing the power and energy consumption of microdisplays while maintaining display luminance, and image quality; and enhancing key features such as resolution, refresh rate, and color depth. First, a novel driving method and pixel circuit are proposed that reduces the number of subframes in a digitally-driven display. The dual-driver method offers flexibility in different design modes, allowing for the enhancement of various display characteristics. In the low-power mode, the operating frequency is reduced, resulting in decreased dynamic power consumption by the drivers. Experimental results on a proof-of-concept array fabricated using TSMC 65 nm technology demonstrate a significant 39% reduction in power consumption compared to a conventional array. Furthermore, designing the display in other modes yields remarkable improvements, with up to 8.5 times enhancement in refresh rate or resolution. In addition, the high color depth mode presents an opportunity to increase color depth from 8 bits to 14 bits, enhancing the visual experience. Additionally, this thesis investigates power reduction techniques specific to row drivers in microdisplays. Circuit techniques are proposed to recycle energy in the row driver, thereby reducing dynamic power consumption. Measurement results on proof-of-concept arrays implemented in TSMC 65 nm technology reveal substantial reductions of up to 30% in the power consumption of the row driver using different energy recycling techniques. Applying these techniques led to a significant reduction in the dynamic power consumption of the row driver. For instance, employing the direct energy restoration technique resulted in a remarkable decrease of over 45% in the dynamic power consumption of the row driver. Finally, a digital data driver with a data energy recycling feature is presented to further reduce the dynamic power consumption of microdisplays. Measurement results obtained from a proof-of-concept array fabricated using TSMC 65 nm technology demonstrate an average power consumption reduction of 16% in the display’s data driver when subjected to randomly generated test images. This thesis addresses the pressing need for energy-efficient microdisplays, offering innovative driving methods, pixel circuit design, and dynamic power reduction techniques. The proposed solutions provide significant power savings while preserving display quality and enabling enhancements in resolution, refresh rate, and color depth, contributing to extended battery life and improved user experience in portable electronic systems

Pixel design and characterization of high-performance tandem OLED microdisplays

Organic Light-Emitting Diode (OLED) microdisplays - miniature Electronic Displays comprising a sandwich of organic light emitting diode over a substrate containing CMOS circuits designed to function as an active matrix backplane – were first reported in the 1990s and, since then, have advanced to the mainstream. The smaller dimensions and higher performance of CMOS circuit elements compared to that of equivalent thin film transistors implemented in technologies for large OLED display panels offer a distinct advantage for ultra-miniature display screens. Conventional OLED has suffered from lifetime degradation at high brightness and high current density. Recently, tandem-structure OLED devices have been developed using charge generation layers to implement two or more OLED units in a single stack. They can achieve higher brightness at a given current density. The combination of emissive-nature, fast response, medium to high luminance, low power consumption and appropriate lifetime makes OLED a favoured candidate for near-to-eye systems. However, it is also challenging to evaluate the pixel level optical response of OLED microdisplays as the pixel pitch is extremely small and relative low light output per pixel. Advanced CMOS Single Photon Avalanche Diode (SPAD) technology is progressing rapidly and is being deployed in a wide range of applications. It is also suggested as a replacement for photomultiplier tube (PMT) for photonic experiments that require high sensitivity. CMOS SPAD is a potential tool for better and cheaper display optical characterizations. In order to incorporate the novel tandem structure OLED within the computer aided design (CAD) flow of microdisplays, we have developed an equivalent circuit model that accurately describes the tandem OLED electrical characteristics. Specifically, new analogue pulse width modulation (PWM) pixel circuit designs have been implemented and fabricated in small arrays for test and characterization purposes. We report on the design and characterization of these novel pixel drive circuits for OLED microdisplays. Our drive circuits are designed to allow a state-of-the-art sub-pixel pitch of around 5 μm and implemented in 130 nm CMOS. A performance comparison with a previous published analogue PWM pixel is reported. Moreover, we have employed CMOS SPAD sensors to perform detailed optical measurements on the OLED microdisplay pixels at very high sampling rate (50 kHz, 10 μs exposure), very low light level (2×10-4 cd/m2) and over a very wide dynamic range (83 dB) of luminance. This offers a clear demonstration of the potential of the CMOS SPAD technology to reveal hitherto obscure details of the optical characteristics of individual and groups of OLED pixels and thereby in display metrology in general. In summary, there are three key contributions to knowledge reported in this thesis. The first is a new equivalent circuit model specifically for tandem structure OLED. The model is verified to provide accurately illustrate the electrical response of the tandem OLED with different materials. The second is the novel analogue PWM pixel achieve a 5μm sub-pixel pitch with 2.4 % pixel-to-pixel variation. The third is the new application and successful characterization experiment of OLED microdisplay pixels with SPAD sensors. It revealed the OLED pixel overshoot behaviour with a QIS SPAD sensor

A highly integrated adiabatic energy recovery digital to analog computer

Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (p. 113).by M. Josephine Ammer.M.Eng

A 128 × 96 pixel CMOS microdisplay utilizing hot carrier electroluminescence from junctions in reach through

Visible light from silicon junctions under avalanche breakdown can be used to create microdisplay systems with integrated light sources. Junctions available in standard CMOS usually breaks down at much larger voltages than the typical operating voltage for integrated circuitry. It is possible to reduce the operating voltage of by making use of techniques which changes the electric field profile in light sources based on hot carrier electroluminescence such as electric field reach through between two highly doped implant regions. This work successfully demonstrates the possibility of tailoring the operating voltage and quantifying the optical performance in an integrated microdisplay consisting of a 128 by 96 pixel array based on light sources in standard CMOS. Based on the approach followed it becomes possible to integrate light sources in such a manner that it can coexist and interact with other on-chip analog and digital circuitry. The requirements for architectural features of a microdisplay in standard CMOS is discussed and it is shown to be possible to create large scale integrated circuits containing integrated light sources in standard CMOS without the need for postprocessing or additional back end modifications.http://www.opticsinfobase.org/jdthb201

CMOS Backplane Pixel Circuit with Leakage and Voltage Compensation for an Micro-LED Display achieving 5,000 PPI or higher

Micro-displays based on micro-LEDs are becoming more and more attractive in AR/MR (Augmented/Mixed Reality) applications. A display size of 0.5 to 0.7-inch is preferred, with 5,000 PPI (Pixel Per Inch) or higher. Due to this pixel density and size, a CMOS (Complementary Metal-Oxide-Silicon) backplane is an ideal solution to drive these pixelized micro-LEDs. As the required pixel size gets smaller, the design of the appropriate pixel circuit becomes more challenging. The simplest 2T1C (2 transistors & 1 capacitor) pixel circuit has potential problems, due to the leakage current of the switch transistor and the voltage drop on the matrix array layout. In this paper, a pixel circuit is proposed as a solution to overcome these two issues. Our simulation results show that the variation of the driving current to the LED is improved by 95 %, and the IR drop error rate is around 2.2 % compared to the 2T1C circuit. The test results also show that the error rate of IPIXEL for the whole region of display is under 2.5 %. This work is verified using a test chip implementation with 180 nm CMOS process technology

The micro-LED roadmap: status quo and prospects

Micro light-emitting diode (micro-LED) will play an important role in the future generation of smart displays. They are found very attractive in many applications, such as maskless lithography, biosensor, augmented reality (AR)/mixed reality etc, at the same time. A monitor that can fulfill saturated color rendering, high display resolution, and fast response time is highly desirable, and the micro-LED-based technology could be our best chance to meet these requirements. At present, semiconductor-based red, green and blue micro-LED chips and color-conversion enhanced micro-LEDs are the major contenders for full-color high-resolution displays. Both technologies need revolutionary ways to perfect the material qualities, fabricate the device, and assemble the individual parts into a system. In this roadmap, we will highlight the current status and challenges of micro-LED-related issues and discuss the possible advances in science and technology that can stand up to the challenges. The innovation in epitaxy, such as the tunnel junction, the direct epitaxy and nitride-based quantum wells for red and ultraviolet, can provide critical solutions to the micro-LED performance in various aspects. The quantum scale structure, like nanowires or nanorods, can be crucial for the scaling of the devices. Meanwhile, the color conversion method, which uses colloidal quantum dot as the active material, can provide a hassle-free way to assemble a large micro-LED array and emphasis the full-color demonstration via colloidal quantum dot. These quantum dots can be patterned by porous structure, inkjet, or photo-sensitive resin. In addition to the micro-LED devices, the peripheral components or technologies are equally important. Microchip transfer and repair, heterogeneous integration with the electronics, and the novel 2D material cannot be ignored, or the overall display module will be very power-consuming. The AR is one of the potential customers for micro-LED displays, and the user experience so far is limited due to the lack of a truly qualified display. Our analysis showed the micro-LED is on the way to addressing and solving the current problems, such as high loss optical coupling and narrow field of view. All these efforts are channeled to achieve an efficient display with all ideal qualities that meet our most stringent viewing requirements, and we expect it to become an indispensable part of our daily life

Advanced Integration of Devices Enabled by Laser Crystallization of Silicon

The push for higher levels of performance drives research and innovation in all areas of electronics. Thus far, shrinking circuit sizes and development of new material systems have satisfied this need. Continued scaling and material improvements have become increasingly difficult; simultaneously, more functionality is needed in smaller spaces. Advanced integration techniques provide a solution by engineering together previously incompatible systems. The fabrication of high-performance devices typically requires high temperature processing steps. Since fabrication occurs sequentially, the high temperature prevents the direct integration of two high-performance layers, as completed devices cannot withstand the processing temperatures of subsequent steps. There are significant challenges to integrating process-incompatible systems, and techniques such as wafer bonding, heteroepitaxial growth, and various thin film technologies have shown limited success. In this work, advanced integration is achieved through laser crystallization processes. Unique to laser methods is the ability to locally heat the surface of a material while keeping the underlying substrate at room temperature. This property allows for high performance electronic materials to be integrated with substrates of different functionalities. This thesis focuses on three key components for advanced integration: 1. Laser-crystallized electronic devices, 2. Relevant substrates for integration, and 3. The feasibility of integrating of laser-crystallized devices with low-temperature substrates. Two types of laser-crystallized devices are explored. Thin-film, laser-crystallized silicon transistors are fabricated at low-temperatures and exhibit high mobilities above 400 cm2 2/Vs. Vertical structure diodes built from laser-crystallized silicon outperformed epitaxially-grown diodes of the same geometry. Light emitting diode (LED) arrays are fabricated from compound semiconductor substrates and tested for display applications. These LED arrays are envisioned to sit underneath the laser-crystallized devices, enabling new applications where both high brightness and high performance transistors are needed. Substrates of low-κ dielectric material are also of interest, as they are widely used for their low capacitance properties. Preliminary results suggest that laser crystallization of silicon can be successfully performed on a low-κ dielectric. In addition to enabling new device architectures, it is important for laser crystallization methods to leave the underlying layers unaffected. Simulations of the laser irradiation process predict substrate temperatures to reach only 70C even when the surface reaches the melting temperature of silicon (1400C). Integration feasibility is further investigated with measurements on conventional front-end field effect transistors. When comparing properties from wafers with and without laser processing, no changes in transistor characteristics are observed. In all three components of work, proof-of-principle devices and concepts lay out the groundwork for future investigation. The developed technologies have promising applications in both the microelectronics and display industry. In particular, the integration of LEDs and laser-crystallized silicon enables a high-brightness microdisplay platform for head-mounted displays, pico projectors, and head-up displays

Eurodisplay 2019

The collection includes abstracts of reports selected by the program by the conference committee