Micromachining helps guide light into new markets

Whereas many micro electro-mechanical system (MEMS) technologies are focused on sensing, silicon used for optical applications provides the ability to support actuation as well.

One feature that the telecom industry has been able to exploit for almost two decades is the routing, processing and control of light through silicon. The increasing demand for low-cost fibre-optic communications within data centres has provided new impetus for device markers to use MEMS and CMOS integration to build highly sophisticated switching and control devices.

One of the first techniques to be exploited in telecoms was the precise of alignment of fibres for optical connectors, using ferrules or V-shaped grooves etched into the surface of a silicon substrate. Precision etching also makes the use of selective interference possible, in order to filter out and separate channels in wavelength division multiplexing (WDM) systems. These components rely on the use of finely spaced gratings that are etched into the silicon.

These filters need not be fixed in frequency. The electro-mechanical properties of silicon microstructures have been harnessed by developers of micro-opto electro-mechanical system (MOEMS) technology to build tuneable dichroic filters. A displacement of the order of tens of nanometres is enough to tune a filter to the wavelengths needed in fibre-optic communications.

Typically, the filter grating is split into a fixed and moving part. The moving part is fabricated on a cantilever that can be moved electrostatically as charge is built up and removed from the circuitry underneath. Larger cantilevers have been exploited to build optical switches, in which mirrors are steered to reflect light from a source channel into different receiving channels.

Pure silicon has the advantage of being transparent to photons across a broad range of wavelengths but techniques such as electro-refraction can modulate the flow of photons through the substrate. This causes a change in the effective refractive index of the substrate based on an applied electric field. Unfortunately, for the wavelengths normally employed in telecom applications, the electro-refraction property is weak.

Instead, devices exploit an alternative effect - plasma dispersion. The technique works by injecting charge into the channel used to convey the photons from one point to another. The increase in hole or carrier concentration leads to a change in the refractive index of light and its absorption coefficient. By using this effect, it becomes possible to build solid-state optical modulators out of silicon.

The use of cantilevers for photon control has extended the reach of MOEMS into digital cinema and TV. Since beginning the search for a suitable technology in the late 1970s, Texas Instruments (TI) has pioneered the use of micromachining technologies in image projection through its digital mirror device (DMD) product line. The core of the DMD technology that exists today was invented by TI fellow Larry Hornbeck in 1987.

A DMD device or digital light projector (DLP) has on its surface hundreds of thousands of microscopic mirrors, generally arranged in a Cartesian array. Each mirror can rotate by around 10° under the control of a cantilever structure upon which the mirror is formed. Electrostatic attraction and repulsion is used to pull one edge of the mirror down onto support pillars.

By rotating the mirror in one direction, photons from a bright light source can be reflected onto a particular x-y position on the target surface. Rotated in the other direction, the light is reflected onto an absorbing surface instead of the target, effectively turning the pixel off at that point. To produce different levels of brightness, the mirror can be switched from one state to the other very quickly, with the ratio of on-time to off-time determining the shade.

Today the key technology behind digital cinema and as a mass-production device, DMD now offers the cost/performance characteristics required to transform the smart home with the help of on-demand displays. Already used in projection TVs for the domestic environment, the low cost of DMD technology makes it possible to design smart picture frames. These can replace static photographs and prints with dynamic content projected by a nearby DLP focused on the area inside the frame.

Thanks to DLP technology, active displays can be built into home automation devices that do not have enough surface area to host a conventional display. Through the use of ultra-short-throw projection optics a device, such as a home-security or thermostat control panel. can use a nearby wall to provide an intuitive interactive user interface. Through the employment of a camera input and gesture recognition software, the interface can interpret the user’s fingers reaching for icons on the projected displays as button presses or dial turns.

With the availability of a more advanced user interface, a DLP-based system can alert the user to problems. For example, as they go to set the alarm, the display can show the status of doors and windows around the home, and warn if they have been left open. In the kitchen, a DLP mounted above a countertop can help with cooking and preparation, displaying recipes and indications of serving sizes for users who want to be sure they are sticking to a diet.

A dishwasher can display its status on the floor - such as when it will finish its current cycle. This makes it possible to remove obvious front panel controls so that the unit can blend in more aesthetically with neighbouring kitchen units. A wave of a hand close to the unit can be sensed by a camera or a simple infrared proximity sensor, turning the projection on or off.

The DLP technology can even help in manufacturing. Because the devices can handle light beyond the visible spectrum and well into the ultraviolet range, they can be used in 3D printing machines. The DLP devices can steer the lasers that are used to form solid objects by selectively curing a liquid feedstock.

To suit the varied applications now emerging for DLP technology, TI has developed a wide range of DMD devices and controllers with resolutions ranging from 640x360 for simple user interfaces all the way up to 1920x1080 for more sophisticated home automation applications.

Many of these devices support algorithms such as IntelliBright, which can automatically manage brightness, contrast and power consumption to accommodate different projection surfaces and ambient lighting conditions. For larger video and active-picture display applications, DLP devices can support resolutions as high as the 4K formats that are now becoming standard in TV and computer monitor products.

Other applications are emerging for MOEMS devices. One example is in environmental sensing, making use of the technology’s ability to support interferometry. Potentially, MOEMS construction makes it possible to scale laboratory equipment down to the level of handheld sensors.

In chemical analysis, infrared spectroscopy provides a highly effective way of determining which compounds are present. Each molecule has a characteristic fingerprint of absorption bands in the infrared region of the spectrum that are caused by differences in bond energies between their constituent atoms.

In environmental sensing, infrared absorption is already widely used but the instruments are generally based on simple, single-frequency filters that focus on a particular part of the absorption spectrum. This makes the sensors relatively inflexible, they have to be tuned for specific molecules such as carbon dioxide, and prone to false positives.

Tuneable Fabry-Pérot interferometers similar to those employed in telecom filters provide the ability to analyse a more complete infrared spectrum in real time.

In typical implementations, the tuneable filter has two mirrors facing each other and an air gap in between. When a voltage is applied across electrodes attached to the mirror mounts, electrostatic attraction adjusts the size of the air gap. If the air gap is half the target wavelength, photons that are close to that wavelength will pass through, but others will be blocked.

Using this structure, the sensor can use a rising voltage to scan across a range of target wavelengths to build up a spectrum - showing which wavelengths have been absorbed by molecules in a cavity that lies between the light source and the interferometer.

Although MOEMS devices are not as widely used as MEMS sensors, the ability to steer and modulate light with high precision and at low cost means is clearly of value. It will therefore see them becoming fundamental parts of Internet of Things (IoT) implementations.