Full‐color SVGA+ OLED‐on‐silicon microdisplay

OLED‐on‐silicon microdisplays represent a new generation of microdisplay technology built on conventional integrated circuits. OLED‐on‐silicon offers significant benefits to the realization of active‐matrix microdisplays for a number of commercial and industrial applications. This paper reviews the design, fabrication, and characterization of a full‐color 852 × 600 active‐matrix OLED‐on‐silicon microdisplay.     

Microdisplay‐based industrial 3‐D and microstructure measurement systems

Abstract— Microdisplays, whether they are of the liquid‐crystal‐on‐silicon (LCOS) or organic light‐emitting diode (OLED) type, have been, up until now, mainly used in multimedia applications or head‐mounted displays. Due to their interesting possibilities, these displays open more and more alternative applications; for example, in optical metrology. Projection lenses for this application area need to be specially designed because the requirements on these systems differ completely from those for multimedia applications. The lenses must have very low geometrical image distortion and they have to be adapted to small objects and/or image distances. On the other hand, they often work with light sources with small spectral bandwidths; consequently, they do not need to be corrected for chromatic aberrations. In addition, the numerical aperture has to be large enough to collect and transfer as much light as possible, but also the size of the projection lens has to be as small as possible to ensure compact measurement systems. All these requirements lead to a compromise in optical lens design. Three optical system designs and realizations — one with an OLED microdisplay and two with an LCOS microdisplay — are presented.     

Laser + LCOS projection‐technology revolution

Abstract— The advent of affordable direct‐diode lasers changes all the rules for optical designs and the associated technologies that generate the images from laser light. These new lasers are forseen as driving fundamental changes in the size, power consumption, cost, resolution, and even the uses for pico‐projectors. This paper discusses these topics from the perspective of laser‐light‐illuminated LCOS microdisplays.     

Development of an 8‐bit VGA LCOS microdisplay with FSC modes using a low‐cost process

Abstract— A VGA‐resolution LCOS microdisplay operated in the field‐sequential color (FSC) mode was fabricated using a low‐cost process. An 8‐bit digital data driver with digital‐to‐analog converters, which can realize a 256‐level gray scale and gamma compensation, has been integrated into a color microdisplay. A top‐to‐bottom approach to the design of an FSC LCOS microdisplay is described. The design of the silicon‐backplane circuits is outlined in detail. Finally, a prototype of this display and its imaging performance are discussed.     

Invited Paper: LCOS microdisplay technology for advanced applications

Abstract— Some technology aspects of LCOS microdisplays that are important for their deployment in advanced projection applications are discussed. The selection of the liquid‐crystal parameters of the vertically aligned system as a function of the requirements (response speed, contrast, etc.) is addressed; a three‐dimensional simulation engine to evaluate fringe‐field effects between pixels is described, allowing the fine‐tuning of the LCOS design with respect to the optical output. Finally, some observations on the nature of the so‐called Vcom drift inside the asymmetrical LCOS cells are presented.     

Retardation‐controlling color filter with two directly stacked retardation layers

Abstract— A polymerizable liquid crystal (PLC), the orientation of which can be frozen, is useful for making retardation layers. In this paper, a new color filter (CF) with retardation‐controlling layers made of PLC is reported. It has a positive A‐plate and a negative C‐plate, both directly stacked on a color‐filter layer. These two retardation layers exhibit good orientation ability, and function well as retarders, even when they are only 1/10 or less as thick as ordinary retardation films. The new CF also has excellent thermal stability. The change in retardation after heat treatment at 200°C for 30 min is around 5%, and there is no observable peeling. A prototype VA‐LCD made with our new CF provides good optical compensation, with the light leakage being extremely low in all azimuthal directions. This technology is very useful for making thin, highly reliable color filters for LCDs, even with other modes.     

Microdisplay wafer‐flatness metrology by optical interferometry

Abstract— We report a microdisplay wafer‐flatness metrology technique based on digital high‐pass filtering of topography data obtained from a commercial optical interferometer. This technique discriminates against both wafer‐scale bow/warp and pixel‐scale roughness to reveal die‐scale flatness variations that are the most relevant to microdisplay gap uniformity. We report flatness measurements of a variety of live and test silicon wafers supporting VLSI microdisplay circuitry, and show how these measurements correlate with the performance of liquid‐crystal microdisplays assembled from similar wafers. The technique is sensitive to cross‐die flatness variations as small as 25 nm in the presence of wafer bow of tens of microns. The wafer flatness variations that make the greatest contribution to liquid‐crystal cell‐gap non‐uniformity arise from interactions between the chemical mechanical planarization (CMP) process and the VLSI circuit layout. Our metrology technique can help the VLSI designer optimize microdisplay layout, and provides an objective flatness specification for wafers purchased from third‐party foundries.     

Bistable FLCOS devices for doubled‐brightness micro‐projectors

Abstract— Optical output is of paramount importance to emerging ultra‐miniature projector products. Experimental bistable ferroelectric liquid‐crystal‐on‐silicon (FLCOS) projection microdisplay devices using newly developed FLC materials aligned on obliquely deposited SiO₂ have been developed. These devices enable the doubling of the illumination duty cycle, and hence doubling of the achievable projector light output, while maintaining a DC‐balanced electrical drive.     

New pixel driving circuit using self‐discharging compensation method for high‐resolution OLED micro displays on a silicon backplane

A new 4T2C pixel circuit formed on a silicon substrate is proposed to realize a high‐resolution 7.8‐μm pixel pitch AMOLED microdisplay. In order to achieve high luminance uniformity, the pixel circuit compensates its Vth variation of the MOSFET for the driving transistor internally by using self‐discharging method. Also presented are 0.5‐in Quad‐VGA and 1.25‐in wide Quad‐XGA microdisplays with the proposed pixel circuit.     

Diamond‐like‐carbon LC‐alignment layers for application in LCOS microdisplays

Abstract— To improve the lifetime and yield of LCOS microdisplays, non‐contact LC alignment techniques using inorganic materials are under investigation. This report focuses on oblique ion‐beam treatment of diamond‐like carbon (DLC) layers, and in particular on the influence of the ion dose on the LC alignment on DLC, keeping the ion‐beam angle (40°) and ion‐beam energy (170 eV) the same. LC alignment on ion‐milled DLC layers is uniform if the ion dose is between 3.8 × 10^?4 C/cm² and 5.5 × 10^?3 C/cm². Above and below this ion dose range, non‐uniform alignment is observed. NEXAFS experiments show that this is caused by lack of molecular anisotropy on the surface of the ion‐milled DLC layers. By varying the ion dose between 3.8 × 10^?4 C/cm² and 5.5 × 10^?3 C/cm², LC molecules have an average pre‐tilt between 3° and 5°, which is within the desired range for application in LCOS microdisplays. The lifetime of the LCOS microdisplays with ion‐milled DLC for projection‐TV application is, however, shorter than the lifetime of microdisplays with PI layers. Ion milling probably creates a reactive surface that is unstable under the high light fluxes used in projection TVs. A solution for this problem could be chemical passivation of the ion‐milled alignment layers. Initial experiments with passivation of ion‐milled PI resulted in an increase in lifetime, but the lifetime after passivation was still lower than the lifetime of rubbed PI layers (factor 0.7). Nevertheless, ion‐milling of DLC or PI can be a good alternative LC alignment technique in other LCD applications. LC‐alignment layers based on inorganic layers such as obliquely deposited SiO₂ films would be a better option for application in LCOS microdisplays due to their higher light stability.     

Eye‐safety analysis of current laser‐based LCOS projection systems

Abstract— A laser safety analysis for liquid‐crystal—on—silicon (LCOS) based imaging projection systems utilizing laser light sources is presented. It is shown that a typical laser‐based imaging projector is capable of providing a D65 white‐balanced luminous flux in excess of 20 lm while remaining Class 1 eye safe. By considering a Class 2 classification, it is shown that the same architecture is capable of providing several hundred lumens, a performance level which could potentially be applicable to a new class of high‐brightness miniature projection systems.     

Analog LCOS SLM devices for AR display applications

Liquid crystal on silicon (LCOS) device is a potential candidate for spatial light modulators (SLMs) due to its high resolution and the wide range of tunable phase retardation. Besides the telecom applications, SLM is also very promising for display applications, especially for the augmented reality (AR) displays. In this paper, we will discuss the advantages of analog LCOS SLM compared with the digital one. A 0.6‐in short‐loop sample test result with precise cell gap controlled is discussed. A fast liquid crystal with high birefringence that attains less than 2.5‐ms full‐on–full‐off response time is also introduced. Regarding the computer‐generated hologram (CGH), a new algorithm called multiconstraints angular spectrum algorithm (MASA) is proposed to enhance the image quality of dual‐plane holographic projection.     

A study of electro‐optical characteristics depending on LC pre‐tilt angle and cell gap in a full‐HD LCOS panel

Abstract— The goal of this work is to achieve a better understanding of the electro‐optical characteristics of a VA‐mode full‐HD LCOS panel via simulations and experiments. The optical parameters, such as reflectance, fill factor, and contrast that vary due to the pre‐tilt angle and cell gap, were also studied. Based on the simulations, the optical fill factor was the highest at an angle of 81° and 2.1 μm under the given conditions. The contrast ratio was the highest at an angle of 89° and 2.4 μm. Five different LCOS panels were fabricated; three different angles (85°, 87°, and 89°) at a 2.1‐μm cell gap and two different cell gaps (1.8 and 2.1 μm) at an angle of 87°. The measured reflected light intensity was compared to the calculated reflectance. The simulated and measured contrasts were compared with each other. The simulation results well‐matched the experimental results and the differences were less than a few percentage points. Based on the comparative studies on reflectance and contrast, the test panel under the condition of an 87° angle and 2.1‐μm gap showed the best performance results.     

Three‐panel LCOS projection systems

Abstract— Three‐panel liquid‐crystal‐on‐silicon (LCOS) projection systems are presented with an emphasis on the commercially successful shared retarder‐stack‐filter (RSF) polarizing‐beam‐splitter (PBS) architectures. The design and operation of the specific CQ90 projection core is presented in detail, and its contrast and transmission derived. alternative three‐PBS/X‐cube LCOS architectures are briefly introduced and their performance is compared to that of the CQ90.     

A field‐sequential‐color display with a local‐primary‐desaturation backlight scheme

Abstract— Field‐sequential‐color technology eliminates the need for color filters in liquid‐crystal displays (LCDs) and results in significant power savings and higher resolution. But the LCD suffers from color breakup, which degrades image quality and limits practical applications. By controlling the backlight temporally and spatially, a so‐called local‐primary‐desaturation (LPD) backlight scheme was developed and implemented in a 180‐Hz optically compensated bend (OCB) mode LCD equipped with a backlight consisting of a matrix of light‐emitting diodes (LEDs). It restores image quality by suppressing color breakup and saves power because it has no color filter and uses local dimming. A perceptual experiment was implemented for verification, and the results showed that a field‐sequential‐color display with a local‐primary‐desaturation backlight reduced the color breakup from very annoying to not annoying and even invisible.     

High‐resolution and high‐brightness full‐colour “Silicon Display” for augmented and mixed reality

High‐brightness micro‐LED display bonded onto silicon backplane has been successfully demonstrated. The 0.38‐inch full‐colour active matrix LED microdisplay system consists of 352 × 198 pixels. Each pixel is 24 μm square composed of red, green, and blue (RGB) subpixels corresponding to a pixel resolution of 1053 ppi. Quantum‐dot materials are formed on III‐nitride blue micro‐LED array to convert blue light into red and green for full‐colour operation. We have confirmed that this microdisplay, which we call “Silicon Display” has wide colour gamut exceeding 120% of sRGB. We describe the advantage of this colour‐converting approach for the full‐colour micro‐LEDs. Progress toward higher resolution is also described. Brightness of more than 30 000 cd/m² has been confirmed at a driving current density of 4 A/cm² for 3000 ppi blue monochrome micro‐LED prepared for full‐colour Silicon Display. We believe our “Silicon Display” is ideally suited for near‐to‐eye displays for augmented and mixed reality.     

Room‐temperature deposition of thin‐film indium tin oxide on micro‐fabricated color filters and its application to flat‐panel displays

Abstract— Direct deposition of indium tin oxide (ITO) thin film on color filters is of practical use in the fabrication of state‐of‐the‐art flat‐panel displays. Room‐temperature dc magnetron sputtering of thin‐film ITO and issues related to the integration of ITO‐on‐glass panels containing micro‐fabricated color filters and other functional materials have been investigated. The resulting polycrystalline ITO exhibited good adhesion to the underlying color filters, as well as good optical transparency and high electrical conductivity. Application of this ITO deposition technology to color liquid‐crystal and organic light‐emitting diode displays will also presented.     

Application‐specific integrated filters for color‐sequential microdisplay‐based projection applications

Abstract— Application‐specific integrated filters (ASIFs), based on a unique holographic polymer‐dispersed liquid‐crystal (H‐PDLC) material system offering high efficiency, fast switching, and low switching voltage, are being developed for microdisplay‐based projection applications. The basic properties and key benefits of ASIFs in projection displays are reviewed.     

A lifetime model for LCOS panels under intense illumination

Abstract— The mechanisms of photochemical reactions occurring during the operation of LCOS panels under intense illumination are analyzed. Regardless of the liquid‐crystal‐mixture formulation, light may generate radicals in the liquid crystal and the polyimide alignment layer. The interaction of these radicals modifies the PI surface and causes alignment degradation. Using kinetic equations for these processes, a lifetime model for the LCOS panels is developed. This new model of lifetime dependence versus light intensity is found to be in agreement with experimental data. This model verifies the TN LCOS panel lifetime to be in excess of 50,000 hours for RPTV applications.     

GaN‐based emissive microdisplays: A very promising technology for compact,ultra‐high brightness display systems

High‐brightness GaN‐based emissive microdisplays can be fabricated with different approaches that are listed and described. They consist either of hybridizing a GaN LED array on a CMOS circuit or building a monolithic component on a single substrate. Using the hybridization approach, two types of 10‐μm pixel pitch GaN microdisplay prototypes were developed: (1) directly driven, 300 × 252 pixels and (2) active‐matrix, 873 × 500 pixels. Brightness as high as 1 × 10⁶ and 1 × 10⁷ cd/m² for blue and green arrays, respectively, were reached. GaN‐based emissive microdisplays are suitable for augmented reality systems or head‐up displays, but some challenges remain before they can be put in production.