First created in 1973 by Hughes Research, Liquid Crystal on Silicon (LCoS) technology now allows hi-resolution, rapid switching of light pixels, improving on the existing liquid crystal display (LCD) and digital light processing (DLP) technologies.
LCoS combines elements of DLP technology and LCD technology. Whereas the DLP technology uses tilting micro-mirrors as opto-mechanical elements that deflect light (typically +/-12º), the LCoS technology uses liquid crystals to switch light and to determine how much light reaches the reflective silicon backplane (and finally the user).
LCoS devices use either artificial light placed behind the panel, or reflected ambient light for illumination. The latter solution is more power-efficient and offers a better contrast ratio than its artificial cousin, but is more expensive to manufacture.
To give some perspective, the maximum number of pixels in a DLP system is around 2 million. LCoS has military simulation applications with 3 million pixels, and some implementations use 9 simultaneous projectors providing a total of 27 million pixels. This gives extremely realistic imagery, suitable for F-16 fighter jet simulators, with 360º ‘wrap-around’ technology.
The LCoS Stack
As we learned in our previous article, liquid crystals are anisotropic materials which alter the polarisation of light (and hence its direction) when driven by an electrical voltage.
The liquid crystals in LCoS devices are either ferroelectric or nematic, giving switching times as low as 40 microseconds, which allows for very rapid light modulation.
LCoS shares some core elements with smartglass and switchable windows, such as the use of indium tin oxide (ITO) transparent conductors to apply the voltage on the liquid crystal layer. This helps LCoS to achieve high contrast ratios (2000:1) and superior quality in intense black levels.
The LCoS stack breakdown looks like this:-
Liquid Crystal on Silicon (LCoS) Stack
- Cover Glass: Serves as protection
- Transparent Electrodes: normally indium tin oxide (ITO)
- Alignment layer: allows the liquid crystals to accurately direct the light
- Liquid crystals: electrically control light reaching the lower reflective layer
- Reflective coating: reflects incoming light
- CMOS layer: thin film transistors (TFTs) based on Complementary Metal Oxide Semiconductor technology regulate each pixel of the liquid crystal, and sit on top of a silicon substrate
- Printed circuit board (PCB): mechanical substrate and signal circuitry layer
The incoming light is initially polarised before penetrating the Cover Glass and passing into the device. A voltage is applied to the liquid crystals via the upper transparent electrodes and the lower CMOS layer which contains fixed aluminium micro-mirrors (one per pixel).
The upper transparent electrodes are normally made of a conductor such as indium tin oxide, which is also used in smartglass technologies such as polymer-dispersed liquid crystal (PDLC) and suspended particle devices (SPD). In LCoS implementations, this layer is held at a static voltage level.
The CMOS silicon layer has on its surface fixed aluminium mirrors, typically 2 to 10 microns (thousandths of a millimetre) in size, electrically isolated from each other and with a pixel pitch (space between pixels) in the order of 200 nm to 500 nm, as shown in the image below.
Liquid Crystal on Silicon (LCOS) backplane under Reflection Electron Microscopy (REM)
In the image below, we see a focused ion beam (FIB) section of a LCOS backplane, showing pixel electrodes with passivation dielectrics, and connected with the pixel vias (two of them per pixel). The light blocking elements reduce light leakage through the interpixel gaps into the underlying CMOS structure which can otherwise generate leakage photocurrent and affect the voltage applied to the pixels.
Focused ion beam (FIB) section of a Liquid Crystal on Silicon backplane
Each pixel is individually addressable with a variable voltage level applied by the thin-film transistors (TFTs) sitting in the body of the CMOS layer.
LCoS reflects up to 80% of all incident light and achieves high Pixel Aperture Ratios (light-sensitive area to total pixel area), albeit at a cost premium. LCoS is available in monochrome and colour.
Applications for LCoS include pico projectors, holographic projectors, realistic military simulations, near-eye wearables, optical telecommunications, lithography, medical simulations, heads-up displays (HUDs) and augmented / virtual reality.
Spatial Light Modulator (SLM)
The liquid crystals and silicon backplane form part of a component called a Spatial Light Modulator (SLM) which can control the intensity, phase and polarisation of a light beam.
LCoS devices function either via amplitude modulation or phase modulation of light. In amplitude modulation, the polarisation of the incident light is modulated, just as in flat panel LC televisions. In phase modulation, electrical control circuitry adjusts the refractive index along the light path and this results in a phase delay.
In a phase-only LCoS SLM, no light is absorbed by the polarisers, which maximises light efficiency.
The LCoS SLM schematic shown below applies a voltage to the liquid crystal cell which changes the refractive index seen by the incident wave due to the fact that liquid crystals are birefringent, thus delaying the phase of the reflected wave.
Liquid Crystal on Silicon (LCoS) based Spatial Light Modulator (SLM)
An applied voltage across the LCoS cell results in a variation in tilt of the LC molecules due to their electrical anisotropy. As they also exhibit optical anisotropy, this changes their refractive index which affects the optical path length within the LC cell.
One major area of growth for LCoS technology is holographic wearables and headsets. VR headsets based on LCoS include those from Meta, HTC and Microsoft. Manufacturers of LCoS microdisplays include Jasper Display, Hamamatsu Photonics, Meadowlark Optics, Santec and Himax.
How LCoS Display Projectors Work
The original driver (and still burgeoning market) for LCoS technology is display projection, not just for consumer devices, but also industrial: think medical, military, manufacturing, etc.
Both LCD and LCoS technologies use 3 imaging panels, one for each of the three primary colours: red, blue and green, which are combined before being output as the final image.
The source light of the Illumination System is first condensed, then separated into the three primary colours by a beam splitter (dichroic mirror) which also polarises the light.
Liquid Crystal on Silicon Display Projector
The coloured beams reach their respective LCoS panel, where each input channel has a liquid crystal layer atop the silicon wafer. The liquid crystals act as window blinds that transmit or reflect the light hitting the pixels sitting on the surface of the silicon chip.
Just as LCD projectors, LCoS projectors change the voltage applied to the liquid crystals ‘pixels’ to modulate the light produced, to output either no light at all, or some portion of it, or by varying the degree of translucence of the liquid crystals to produce colours or greyscale.
Outlook
With the surge in demand for realistic military and medical simulators, high-resolution industrial display solutions, as well as augmented reality, holographics and near-eye wearables, the 2023 to 2028 market growth estimations of 16% CAGR look very reasonable. It remains to be seen whether production costs can be controlled to allow LCoS to realise its true market potential.
References
1. Optimisation of Liquid Crystal on Silicon Technology for Laser Microprocessing Applications, URL
2. Holoeye Microdisplay Technology, URL
3. Liquid Crystal on Silicon (LCoS), Advanced Materials and Liquid Crystal Institute, Kent State Univ., URL
4. How an LCoS Projector Works, URL
5. Pursuing High Quality Phase-Only Liquid Crystal on Silicon (LCoS) Devices, MDPI, URL
6. Ferroelectric Liquid Crystals on Silicon, Forth Dimension Displays, URL
7. Liquid Crystal on Silicon, International Defence Security and Technology, URL
8. Exploring the Potential of LCoS Microdisplays, AZOM, URL
9. Introduction to ±12 Degree Orthogonal Digital Micromirror Devices (DMDs), Texas Instruments, URL
10. Fundamentals of phase-only liquid crystal on silicon (LCoS) devices, Nature, URL
11. Beyond the display: phase-only liquid crystal on Silicon devices and their applications in photonics, URL
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