Liquid crystal on silicon
Liquid crystal on silicon (LCOS or LCoS) is a reflective active-matrix microdisplay technology, and also the microdisplay manufacturing process associated with it, in which a layer of liquid crystal is formed directly on top of a silicon backplane. It is a type of spatial light modulator (SLM). LCoS was initially developed for projection televisions, but has since been used in near-eye displays.[1]
Because the pixel-driving electronics are buried in the silicon underneath each reflective pixel rather than alongside it, LCoS panels achieve a high fill factor and very small pixel pitches, which makes the technology attractive for compact projectors and for head-mounted display and augmented reality (AR) optical engines.[1] Unlike a conventional liquid-crystal display, which is transmissive (light passes through it), an LCoS panel is reflective: light enters the front of the device, passes through the thin liquid-crystal layer, reflects off a mirrored aluminium electrode on the silicon, and exits again, so the light makes a double pass through the liquid crystal.[2][1]
How it works
An LCoS device is an array of independently addressable pixels. Each pixel sits over a CMOS cell on the silicon backplane that stores and applies a voltage to a reflective aluminium electrode; the liquid crystal between that electrode and a common transparent front electrode reorients in response to the applied field.[3][2]
Most LCoS imagers used for displays are operated as amplitude (intensity) modulators. Light is first polarized; the voltage on a pixel changes the rotation of polarization imparted by the liquid crystal (commonly a twisted nematic liquid crystal arrangement), and a polarization-selective optic such as a polarizing beam splitter then converts that change in polarization state into a change in brightness for that pixel. As the liquid crystals "open" and "close" under control of the backplane voltages, the reflected light for each pixel is either passed or blocked.[3][2][1]
A separate, important use of LCoS is the phase-only spatial light modulator. Here the device is not used to make a picture; instead the birefringence of the liquid crystal is exploited so that the applied voltage changes the optical path length (the phase delay) of the reflected light at each pixel, without intentionally changing its amplitude. Phase-only LCoS SLMs are used for beam shaping, holography, optical pulse shaping, and optical-tweezer and wavefront-correction applications.[3]
Color
Because the liquid crystal itself does not produce color, LCoS systems generate full color in one of two ways. In a three-panel architecture, white light is split into red, green and blue paths, each illuminating its own LCoS panel, and the three images are recombined; this is common in high-end projectors. In a single-panel field-sequential color (FSC) architecture, one panel is illuminated in rapid succession by red, green and blue light sources (typically LEDs), and the eye integrates the sequence into a full-color image. FSC removes the need for a per-pixel color filter, which allows smaller pixels and a more compact, more light-efficient engine, and it is the approach generally favored for near-eye AR optics.[4]
Construction
Layers
An LCOS module is layered.
The first layer is the top layer. It is the counter electrode (CE). It is made of a glass substrate covered by a transparent layer of indium tin oxide (ITO).
There is a top layer of ITO and a bottom layer of ITO. Between them is the liquid crystal. Underneath the bottom layer of ITO is a CMOS backplane.
Parameters
The counter electrode glass must have a thermal expansion coefficient that matches the backplane's as closely as possible.
The liquid crystal material used in LCOS devices is typically a nematic liquid crystal.
The cell gap between the backplane and the glass substrate is typically a few micrometers.
Manufacturing
Manufacturing of LCOS gadgets can be separated from the process of etching the silicon. A manufacturing facility may be able to receive pre-etched and pixelated backplane CMOS wafers from a fab. Then the facility can simply fill the liquid crystal, do layer alignment, and packaging.
Inputs
The inputs to the manufacturing process are:
- CMOS silicon wafers with a TFT pattern etched into them
- cover glass wafers
- indium tin oxide
- liquid crystal
- polyimide or inorganic material for an alignment layer
The CMOS silicon wafers already have CMOS patterns in them.
The CMOS silicon wafer is known as a backplane.
Steps
- Liquid crystal fill and seal
- Electrode attachment
- Packaging
Applications
Projection displays
LCoS was originally developed for projection televisions and remains widely used in front and rear projectors, where its high fill factor yields a smooth, low screen-door image and supports 4K and HDR. Two long-standing branded LCoS projector technologies are JVC's D-ILA (Direct-drive Image Light Amplifier) and Sony's SXRD (Silicon X-tal Reflective Display).[3][1]
Near-eye and augmented reality displays
LCoS is a common imaging engine for smart glasses and waveguide-based augmented reality head-mounted displays. In such systems an LED- or laser-illuminated LCoS panel generates the image, projection optics relay it, and a waveguide combiner with diffraction gratings couples the light into the user's eye while letting real-world light pass through.[1][4]
- Google Glass (2013) used a reflective near-eye LCoS engine built around a Himax Display chip. The panel had an nHD resolution of 640 by 360 pixels, and its image was projected into a small prism that redirected the light to the eye. Google subsequently took an equity stake in Himax Display.[5]
- Microsoft HoloLens used an LCoS-imager-based display engine in its first generation (HoloLens 1, 2016), feeding stacked color waveguides. The second generation (HoloLens 2, 2019) switched to a laser MEMS scanning display engine rather than LCoS.[6]
Newer color-sequential, front-lit LCoS microdisplays have been demonstrated specifically for AR glasses. Himax, for example, has shown a color-sequential front-lit LCoS engine with a total volume on the order of 0.5 cm3 and very high panel luminance, aimed at compact AR glasses using 2D exit-pupil-expansion waveguides.[4]
Other uses
Beyond displays, LCoS phase-modulating SLMs are used in optical telecommunications, where wavelength-selective switches (WSS) use beam-steering LCoS panels to route and reshape spectral channels. LCoS SLMs are also used in structured-light 3D measurement, holographic projection, optical pulse shaping, and adaptive optics.[3]
Comparison with other microdisplay technologies
The main microdisplay technologies competing with LCoS for projection and near-eye use are transmissive LCD, Texas Instruments' DLP (Digital Light Processing, based on the DMD micromirror chip), OLED-on-silicon (also called Micro-OLED or OLEDoS), and microLED-on-silicon.
| Technology | Light type | Notes |
|---|---|---|
| LCoS | Reflective, needs external illumination | High fill factor and high resolution at small pixel pitch; good contrast and black levels relative to LCD and DLP; requires a separate light source and polarization optics.[1] |
| Transmissive LCD | Transmissive, needs external illumination | Pixel electronics share the aperture, lowering fill factor; LCoS hides electronics behind the reflective pixel for higher fill factor.[2] |
| DLP / DMD | Reflective (micromirrors), needs external illumination | Uses tilting micromirrors rather than liquid crystal; LCoS is generally credited with superior contrast and black levels.[1] |
| OLED-on-silicon (Micro-OLED) | Emissive, self-illuminating | No separate light source needed; favored where image fidelity, high pixel density and crisp text matter, and offers good efficiency for wearables. |
| microLED-on-silicon | Emissive, self-illuminating | Very high peak brightness, but full-color (especially red) microLED at microdisplay pixel sizes remains difficult and expensive; many AR microLED panels are still monochrome. |
A practical reason LCoS remains popular for full-color AR despite emissive alternatives is system efficiency with waveguides. Display-optics analyst Karl Guttag has noted that LED-illuminated LCoS engines can be very roughly an order of magnitude more power efficient than microLED-plus-waveguide systems when driving a full white screen, with microLED's efficiency advantage appearing mainly for sparse, low-average-brightness content.[7] Reflecting this, recent AR products reported to use LCoS imagers include Google's Avegant-based optical engine and Meta's full-color LCoS engine paired with Lumus reflective waveguides.[7]
History
Some of the first LCOS modulators were made at Hughes Research Laboratories. Hughes developed an early liquid crystal light valve (LCLV) in the 1970s, and the technology was later commercialized for projection through efforts such as the Hughes-JVC venture, JVC's D-ILA (introduced in 1997) and Sony's SXRD (introduced in 2003, with 4K panels following in 2004).[3]
Display product companies
Silicon foundries
- SMIC
- TSMC
- UMC
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 "Exploring the Potential of LCoS Microdisplays". 2021-09-29. https://www.azom.com/article.aspx?ArticleID=20844.
- ↑ 2.0 2.1 2.2 2.3 "How LCoS Works". https://electronics.howstuffworks.com/lcos.htm.
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 "Liquid crystal on silicon". https://en.wikipedia.org/wiki/Liquid_crystal_on_silicon.
- ↑ 4.0 4.1 4.2 "Himax Unveils Color Sequential Front-Lit LCoS at Display Week 2023". 2023-05-25. https://www.eetasia.com/himax-unveils-color-sequential-front-lit-lcos-at-display-week-2023/.
- ↑ "Proof That Google Glass Uses A Himax LCOS Microdisplay". 2013-06-14. https://seekingalpha.com/article/1504292-proof-that-google-glass-uses-a-himax-lcos-microdisplay.
- ↑ "Display engines based on an LCoS imager, as in the HoloLens 1 (top, 2016), and a laser scanner". https://www.researchgate.net/figure/Display-engines-based-on-an-LCoS-imager-as-in-the-HoloLens-1-top-2016-and-a-laser_fig4_346144822.
- ↑ 7.0 7.1 "Meta Hypernova and Google AR/AI Glasses - Lumus & Avegant Inside, Both Using LCOS MicroDisplays". 2025-05-11. https://kguttag.com/2025/05/11/meta-hypernova-and-google-ar-ai-glasses-lumus-avegant-inside-both-using-lcos-microdisplays/.