We developed a high definition holographic display with two different approaches. First approach is to adjust a wellknown display technology using a liquid crystal as an optical modulator. While merit of display technology is a large panel size, the challenge is to define a small pixel pitch and to reduce a crosstalk effect of a liquid crystal. Second approach is to adapt a silicon technology with a new optical modulator, Ge2Sb2Te5. Although we can easily define a small pixel pitch, we have to confront a small panel size. We will explain our solutions to overcome these issues.
Holographic display is known as the most realistic 3D display by creating real 3D image in front of observer. But there are tremendous technical hurdles in realizing electronic hologram. It is required to achieve very small pixel pitch SLM (spatial light modulator) for realizing electronic hologram using flat panel display. All the components of the unit pixel should be carefully redesigned and improved to maintaining same performance in spite of reduction of pixel pitch. The size of hologram image depends on the size of SLM. Therefore, the resolution of the SLM increases very fast as the reduction of pixel pitch in SLM. The driver chip and architecture should be developed to manage the large number of pixels with high speed.
Novel unit pixel design for achieving 1μm pixel pitch will be presented. VST (vertically stacked transistor) structure will be introduced. In VST structure, the driving TFT is stacked on the data line. VST structure has the merits for reducing horizontal pixel pitch without introducing expensive high-definition lithography tool. The technology compatible with 0.5μm critical dimension can be used for fabrication of 1μm pixel pitch SLM.
Alternative structure for achieving small pitch pixel is the adopting VTFT (vertical channel thin film transistor). VTFT means that the channel direction is vertical to SLM plane. The footprint of VTFT is very small, for required area is only the overlap region of active layer and source/drain layer.
Cross-talk between adjacent pixels can interfere the rotation of LC molecule. This phenomenon will be very critical for accurate phase modulation for 1μm pixel pitch SLM. Novel concept for reducing cross-talk will be also presented with experimental results.
A spatial light modulator (SLM) is the key component for a digital holographic display system. It requires both ultrahigh- resolution fine pixel integration and large-area panel to get a wide viewing angle and large realistic 3D images. Because these requirements collide with each other, it is very difficult to develop the digital holography system satisfying the standard of the general public. Unlike conventional SLMs, which are based on small size semiconductor ICs, we have developed larger size, highresolution SLM on glass (SLMoG) using both semiconductor and display fabrication technologies. 1-μm channel length oxide TFTs are integrated for pixel switching and the liquid crystal (LC) of high refractive index anisotropy was used to modulate the phase of incident light. Multiple high-speed interface boards and large channel driver ICs are used to drive large size holographic image data. 2.16-inch 16K SLMoG panel with 3-μm horizontal pixel pitch was successfully developed for the first time. Without the aid of additional instruments, it was possible to observe a simple 3D objects with the naked eye. For even wider viewing angle and larger size SLMoG, the proof of concept SLMoG panel for still image hologram with smaller pixel pitches was developed to evaluate cross-talk between adjacent LC pixels.
SLM (Spatial Light Modulator) with ultra fine pixel pitch (circa 1 micron meter) has been thought as a big issue to realize electronic hologram with wide viewing angle. Two types of approach are proposed for accomplish SLM panel with 1 micron meter pitch pixel.
SLM with LC light modulator controlled by TFT based backplane constructed on glass substrate is proposed according to the methods of scaling down flat-panel display technology. Introduction of sub micron meter patterning processes, the SLM panel with 3 micron meter pitch pixel was successfully developed for the first time. The SLM with 2 inch diagonal length had the resolution of 16K by 2K. Hologram with depth was reconstructed with manufactured SLM. Oxide semiconductor TFTs of 1 micron meter channel length with high performance have been developed for the SLM. Technical issues to accomplish 1 micron meter pitch pixel will be discussed.
PCM (phase change material) has been used for memory devices and information recording devices. In former case, information is recorded and read using electrical signal. For latter case, information is recorded and read using light (laser) signal. We propose a SLM with PCM such that information is recorded using electrical signal and read using light signal. The light modulation of PCM pattern recorded by pulsed laser was successfully demonstrated using reconstruction of hologram images. Operation of arrayed pixels with PCM pattern driven by Si MOSFET is under development. Technical challenges for SLM with PCM will be discussed.
We developed a high-resolution active matrix spatial light modulator on a glass substrate. To integrate a switching device on the glass substrate, we designed a high-performance oxide thin-film transistor with a minimum channel length of 1 μm and a maximum processing temperature of 380°C. To drive a large number of data lines, we used multiple source drivers and data drivers as well. For an optical modulation, we optimized a liquid crystal of a high anisotropic refractive index of 0.25 with a cell gap of 2.5 μm, which was effectively operated until pixel pitch is 1.6 μm. Hologram was successfully reconstructed by fabricated SLM with 7-μm pixel pitch. For the other approach for a high-resolution spatial light modulator, we tested a phase change material of Ge2Sb2Te5 [GST]. The variation of refractive index between a polycrystalline phase and an amorphous phase of the GST film is used for a hologram reconstruction. By optimizing the underlying oxide thickness, we can show a color hologram without color filters.