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Imprinting Technique Offers Low-cost Photonic Crystal LEDs

Rob Hershey from the Texan company Molecular Imprints explains how a simple and low-cost whole-wafer imprinting technique can increase the extraction efficiency of LED chips.

LED makers are always striving to cut the cost-per-lumen of their devices so that these chips can penetrate new markets like solid-state lighting. It s a goal that has led manufacturers to employ rougher surfaces, or to incorporate mirrors within the device, to boost extraction efficiency. However, greater gains can be made by using photonic crystals that can block light propagation in unwanted directions and redirect it to where it is needed.

The effectiveness of these photonic structures increases with the depth of these features, their density and their proximity to the active region of the device that generates the emission (see box "LED designs"). The performance can be modeled using supercomputers by companies like Mesophotonics, a spin-off of Southampton University, UK, which can even design quasi-crystal structures that are able to emit beams, cones and rectangles with specific angular directions. Yet today s LED makers use expensive package-level components such as index-matching sol-gels and beam-directing optics to construct their devices because the manufacturing methods associated with patterning and etching periodic structures are too slow and expensive.

Patterning optionsPatterned photonic crystals and other 2D periodic light extraction features have been produced by several university groups using "deep ultraviolet" interference lithography. However, this low-cost technique is ill-suited to manufacturing because it requires a controlled environment to produce stable nanometer-scale fringe patterns. The range of patterns that can be produced with interference methods is also limited to simple periodic multibeam interference patterns and precludes the fabrication of photonic quasi-crystals.

Consequently, many commercial researchers and universities prefer to use electron-beam (E-beam) lithography to create photonic crystal structures. This approach is used by semiconductor manufacturers to generate the fine features on leading-edge photomasks and semiconductor wafers. However, even commercial grade E-beam pattern generators can only process up to three 2 inch wafers per day and with a price tag of more than $5 million they have been restricted to proof-of-concept demonstrations.

Fortunately, these E-beam resources are not wasted because they can be used to produce a pattern on a master template that can be replicated on several wafers using nano-imprint lithography (NIL). The NIL technique is already proven in commercial niche applications such as specialized optical element production, which has higher margins and lower costs than commercial LED manufacturing. In the commodity-driven LED market, high-resolution NIL needs to deliver incremental costs of less than $20 per wafer (∼$0.01/ mm2), inclusive of incremental support equipment costs that may be required to implement the process.

At Molecular Imprints, a nano-imprint company based in Austin, TX, we have developed a new high-throughput imprint approach that meets the cost requirements for LED processing. A typical process will involve coating the epiwafer with an imprint material and then pressing a template or stamp with the master pattern into this coating (figure 2). Heating and high pressures are applied to high-viscosity imprint layers, but capillary forces are sufficient for low-viscosity films. Cooling or exposure to ultraviolet radiation solidifies the imprinted film before the template is removed to form an imprinted epiwafer.

Our approach uses a two-step template manufacturing process to deliver high throughput at a low cost. The master stamp is created using commercial semiconductor mask processing technology and is imprint-replicated to form a thin whole-wafer template that can imprint an entire LED wafer in a single step. With this approach the E-beam written master can be kept to a typical minimum size of 5 × 5 mm and replicated 100, 200 or 350 times to fully populate a thin whole wafer replicate template for 2, 3 or 4 inch material, respectively. The master template can produce hundreds of thin quartz replicas that can each imprint thousands of wafers before they require recycling, making it possible to imprint millions of wafers from a single master.

We have developed a tool that has the capacity to fill the features of the template while simultaneously producing a uniform thin residual layer and can process 2, 3 and 4 inch LED wafers with an undulating topography of more than 10 µm in under 3 min. The thin template is bowed and brought into contact with a low-viscosity imprint fluid, which is dispensed in seconds directly onto the wafer using a commercial inkjet technology (figure 3). The template is then relaxed, which allows capillary forces to pull the template into conformal contact with the surface and expel all of the air from the wafer.Manufacturing considerationsThe key to any lithographic process is the robustness of the pattern transfer process to the underlying material. To produce effective photonic crystals, this pattern needs to be etched to a depth of several hundred nanometers into the LED s upper region of GaN or GaP. Today s LED manufacturers commonly use a SiO2 hard mask and a reactive ion inductively coupled plasma etch, which can produce etch depths of 250 nm from SiO2 hard masks just 50 nm thick. We have used this process in conjunction with our imprinting technique to etch photonic structures into GaN epiwafers with a 150 nm thick SiO2 mask (figure 4).

Processes that are successful in high-volume manufacturing are reliable, repeatable and cost effective. If there is an Achilles heel to imprint technology, it is the need for a clean, flat surface on which to imprint. Unfortunately, many LED manufacturing facilities fall short of this requirement as they have class 10,000 or class 1000 cleanrooms that provide ample opportunity for particles to fall onto the wafers prior to imprinting. At relatively low imprint pressures sharper particles can damage the template and shorten its lifetime. Large particles can also cause problems, as they can prevent the template from conforming to the substrate surface, which leads to unpatterned areas on the epiwafer.

The surface protrusions on the imprint layer are often caused by particles falling on the wafers before or during the epitaxy or hard mask deposition (or during the GaN etchback process if laser lift-off is used). However, the density of these defects can be reduced to acceptable levels for most types of imprint lithography, which use reusable templates such as SiO2, by using class 100 clean room conditions. This level of cleanliness is achievable by using low-cost "mini-environments" in selected areas of existing cleanrooms and must be implemented in conjunction with regular preventive maintenance and cleaning of process chambers, which has been shown by the silicon industry to efficiently eliminate particles in deposition tools. Improving the cleanliness of these processes also benefits the overall yield.

Our imprint tool development shows that nano-scale light extraction features can be manufactured on LED wafers at a low cost. These machines are able to process at least 20 wafers per hour at an incremental cost of less than $20 per wafer and will empower chip makers to produce LEDs with photonic crystals or other nano-extraction features within the next 12–18 months.

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