Miniature LED arrays produce incredibly bright, colourful displays that are suitable for many applications. Opportunities include exposing resists; confining and manipulating cells; and probing and controlling genetically targeted cells, says Jim Bonar from mLED.

 

The LED is, in general, getting bigger and bigger. Until recently, the most common dimensions for an LED were 300 μm by 300 μm, a size that can generate enough light for backlighting the displays of handsets, laptops and other screens. But the killer applications for this decade and beyond, general lighting, requires far higher light levels, and this means chips with sides of at least 1 mm.

Making chips bigger, however, is not the only route to increasing the number of applications that LEDs can serve. New markets also beckon when the device’s dimensions are reduced substantially, so that its sides are less than 100 μm. Some markets can be addressed with single emitters of this size, but most of the opportunities require a battalion of them that form an LED array. These can be used to expose resists, confine and manipulate cells, play a role in measurements of fluorescent lifetimes, aid the acquisition of depthresolved microscopic images and help to build directwrite photolithography systems.

One of the most exciting opportunities of all is in optogenetics, a nascent field in neuroscience that involves using high-speed optical methods to probe and control genetically targeted cells with intact neural circuits. Miniature LED arrays could help 1.5 billion sufferers of neurological disorders, such as Alzheimers, Parkinsons, depression and chronic pain. In addition, there are opportunities for miniature LEDs in visible light communications. This technology has already realized data transmission rates of up to 1 Gbit/s from a single pixel at 450 nm, using on-off keying non-return to-zero modulation.

At mLED, which is headquartered in Glasgow, UK, and was founded in July 2010, we are starting to tap into the many exciting opportunities associated with miniature LEDs. One of our core strengths is our exclusive licence that allows us to exploit patented research from Martin Dawson’s group at the Institute of Photonics (IoP), University of Strathclyde. The IoP has been at the forefront of research on micro-pixellated LEDs for more than a decade, and during that time it has demonstrated many technical achievements in this area, in collaboration with partners at several UK universities, including Edinburgh, Glasgow, St. Andrews and Imperial College London. In particular, the IoP group has pioneered the use of these micro-pixellated sources in optical microsystems.

Our mission is to create a range of industry-leading, high-brightness micro-displays that can be controlled by computer to provide pattern-programmable sources. We aim to work alongside system integrators, first developing prototype capabilities and then scaling up to production volumes. This way, we can provide bespoke designs that address specific applications and enable products to get to market fast. To make this happen, efforts have focused on taking a patented technology and turning it into a robust commercial product. This quest has been aided by securing access rights to facilities at the West of Scotland Science Park in Glasgow. Here we can access a range of equipment dedicated to III-V manufacturing and development. This allows us to reduce variations in our processes and minimize capital expenditure. In addition, this approach reduces the risk of cross-contamination and process instability, while the standard process flows and building blocks that we have established give us the opportunity to scale to high-volume production.



 

Figure 1. Graphical user interface for matrix addressable 64x64 demonstrator array, permitting simple installation and turn-key operation for the customer’s specific application

 

Shrinking the size of LEDs 

 

At the heart of our displays are high-density arrays of miniature GaN LEDs, which can span the ultraviolet through to the blue and green (recent research at the IoP, however, shows that it is also possible for GaN LEDs to even emit in the green-yellow and amber – see the box “Stretching GaN beyond the green”). These arrays are the foundation of a high-brightness monochromatic GaN microdisplay or ‘pico-projector’ technology, which can be used to form full-colour microdisplays with the addition of pixel colour conversion.

The LEDs in our arrays are typically just 2 μm to 80 μm in diameter. To form these devices, we use a series of processing steps that were developed at the IoP. These involve the growth of a GaN LED epiwafer on sapphire, which is processed into an array of pillars with a combination of photolithography and etching with an inductively coupled plasma.

As with conventional LEDs, a flip-chip architecture holds the key to far higher light extraction efficiency. This is realised by adding several layers to the processed epiwafer: An n-contact; a silicon dioxide insulator layer; and then a highly reflective, common p-contact. After the devices are formed, chemical mechanical polishing reduces the thickness of the sapphire substrate.

Efforts at the IoP demonstrate that it is possible to make thousands of miniature LEDs into arrays that are collectively addressable and have a total active area equivalent to that of one ‘conventional’ LED chip. With this passive matrix approach, only 2N contacts are required for an N x N matrix. Using this, back in 2004 IoP demonstrated a 128 x 96 passive matrix device with 12 μm x 16 μm pixels on a 20 μm center-to-centre spacing. Occupying an active area of 3 mm x 2 mm, displays based on blue and green devices were produced with a luminance in excess of 30,000 Cd/m2 (This is brighter than today’s state-of-the-art microdisplays based on organic LEDs, a technology described in the box “What are the alternatives?”).

We have improved the performance of these displays with matrix-addressing schemes that are suitable for high-resolution applications. All pixels along one column share a common anode and all pixels along one row share a common cathode. We now offer a turn-key 64 x 64 demonstrator unit. This light engine is supplied with a simple-to-use graphical user interface, offering the ability to interface easily with a host of system applications.



 

Figure 2. A number of device configurations highlighting the flexibility of the technology. (a) 128 microLED stripes each 20 μm x 500 μm on a centre-to-centre spacing of 23 μm. (b) 200 μm x 200 μm checkerboard device with pixel edge separation of 2 μm and (c) a magnified view of a single pixel in operation



 

Figure 3. 8 x 8 individually addressable microLED array demonstrator unit operating at 450 nm



Figure 4. Colour conversion process for ultraviolet microLED emitter array using direct inkjet processing

 

It is also possible to address every pixel individually. We can do this with our bespoke CMOS backplane technology and flip-chip bonding techniques, which combine to control the output power of microLEDs. By employing a novel device arrangement, heat-sinking improves, opening the door to increased current density handling. Thanks to the possibility to use emission through the polished sapphire substrate (inert window), devices can be used in close proximity to an object, or the microLED emitter plane can be optically relayed/ imaged. Alternatively, aligned microlenses can be monolithically integrated and formed in the sapphire substrate.

Driving the arrays

To drive these pixels, we use a CMOS design based on standard low-voltage 3.3 V logic. However, we have developed a technique for the driver array that enables biasing above or below ground prior to an excitation signal being applied by the driver. This means that for higher output power density applications each diode can be biased at voltages greater than 5 V to allow the LED to be driven at high currents and therefore provide higher output power. LEDs can be driven continuously, or with excitation pulse widths that can be as short as just 300 ps (FWHM). A collaboration involving the IoP and other universities integrated such devices with arrays of single-photon avalanche diodes, and in 2010 the partnership claimed that it had made the smallest reported solid-state microsystem for fluorescence decay analysis.

 Flexibility of the lithographic mask design enables the manufacture of various structures, including micro-disc, micro-stripe and chequerboard arrangements. Applications requiring structured illumination can be catered for with a stripe configuration, and high fill factors in excess of 98 percent are possible with chequerboard structures. We currently offer a turn-key 8 x 8 microLED demonstrator unit for system integration. The IoP has also fabricated arrays with colour conversion, realised by integrating photocurable nanocomposites and polymer blends onto the micro-LEDs. These are added by ink-jet printing, and convert ultraviolet or blue output of the array into a ‘RGB’ display. This team of researchers has also demonstrated self-aligned direct writing and colour conversion with a colloidal quantumdot nanocomposite. In this case, the ultraviolet microLED cures the nancomposite in registry with the underlying pixels, and is then down-converted by this composite film.

Many applications have minimum levels for brightness, and if the lumen output exceeds this figure, so much the better. Our LEDs are outstanding in this regard, with power densities in excess of 3250 mW/mm2 per pixel produced by 8 x 8 individually addressable 14 μm emitters operating at 450 nm. In comparison, conventional highbrightness LEDs are typically 700 mW/mm2.

Our miniature LEDs have operated at a current density of 18 kA/ cm2 when driven in DC; that’s two orders of magnitude higher than that for high-brightness LEDs. Operating at this very high current density does not require any specific heat-sinking arrangements, and junction temperatures are low, thanks to the architecture of our arrays. What’s more, there is still room for further improvement in the performance of arrays of miniature LEDs: We are currently developing more efficient light extraction techniques that will boost lumen output and enable this technology to target the general lighting market.

One area where our novel LEDs could soon start making an impact is the field of optogenetics. Neurological disorders affect more than one in five people across the world, and the total bill for treatment exceeds $1 trillion per year. Drugs, neuromodulation, surgery and talk therapy are all used today either to improve or control a patient’s condition. However, in future, optogenetics may be added to that list. Although optogenetics research is at an early stage, rapid progress is being made. Our devices have a great deal to offer here, because they can deliver light of the required wavelength at sufficiently high power densities using very high switching speeds. We plan to launch specific products for this growing market over the next 24 months, and also investigate other opportunities for LED arrays. Their success will highlight that making LEDs smaller, just like making them bigger, opens the door to new and lucrative applications for these solid-state emitters.



 

Figure 5. Advanced processing equipment used in the formation of microLED arrays.

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