What is Liquid Crystal on Silicon?

Introduction

Originally created in 1973 by Hughes Research, Liquid Crystal on Silicon (LCoS) allows high-resolution, rapid pixel switching. LCoS improves upon existing liquid crystal display (LCD) and digital light processing (DLP) technologies.

Whereas DLP uses tilting micro-mirrors to deflect light, LCoS instead employs liquid crystals to determine how much light reaches the reflective backplane.

Regarding illumination, LCoS uses either artificial light behind the panel or reflected ambient light. While the latter is more efficient and offers a better contrast ratio, it costs more to manufacture.

To give some perspective, the maximum number of pixels in a DLP system is around 2 million. LCoS has military applications with 3 million pixels; some implementations use 9 simultaneous projectors giving 27 million pixels.

This gives extremely realistic imagery, suitable for F-16 fighter jet simulators, with 360º ‘wrap-around’ technology.

The LCoS Stack

Liquid crystals are anisotropic materials which alter the polarisation of light (and hence its direction) when driven by an electrical voltage.

The crystals 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, such as the use of indium tin oxide (ITO) transparent conductors. This layer distributes the electrical charge to the liquid crystals, achieving high contrast ratio (2000:1) and superior quality in intense black levels.

LCoS Stack

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

LCOS Functionality

Incoming light is polarised before penetrating the Cover Glass and passing into the device. The upper transparent electrodes apply voltage to the liquid crystals, and the lower CMOS layer contains aluminium micro-mirrors (one per pixel).

The electrodes are a transparent conductor such as indium tin oxide, also used in smart glass 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 in size, electrically isolated and with a pixel pitch of 200 nm to 500 nm.

Liquid Crystal on Silicon (LCOS) backplane under Reflection Electron Microscopy (REM)

LCOS Backplane

The image below shows a focused ion beam (FIB) section of a LCOS backplane, showing pixel electrodes with passivation dielectrics, 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. Specifically, manufacturers offer LCoS in both monochrome and color versions.

Applications for LCoS include holographic projectors, realistic military simulations, near-eye wearables, medical simulations, heads-up displays (HUDs) and augmented reality.

Spatial Light Modulator (SLM)

The liquid crystals and silicon backplane form part of a component called a Spatial Light Modulator (SLM) which controls the intensity, phase and polarisation of light.

LCoS devices function via amplitude modulation or phase modulation of light. 

• In amplitude modulation, the polarisation of incident light is modulated, just as in flat panel televisions.
• In phase modulation, control circuitry adjusts the refractive index, giving 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.

LCoS Display Projectors

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. Pursuing High Quality Phase-Only Liquid Crystal on Silicon (LCoS) Devices, MDPI, URL

5. Ferroelectric Liquid Crystals on Silicon, Forth Dimension Displays

6. Liquid Crystal on Silicon, International Defence Security and Technology, URL

7. Exploring the Potential of LCoS Microdisplays, AZOM, URL

8. Introduction to ±12 Degree Orthogonal Digital Micromirror Devices (DMDs), Texas Instruments, URL

9. Fundamentals of phase-only liquid crystal on silicon (LCoS) devices, Nature, URL

10. Beyond the display: phase-only liquid crystal on Silicon devices and their applications in photonics, URL