The most common, and cost-effective, method of delivering white light from an LED is to combine a blue “pump” LED source under a layer of phosphor or phosphor blends to achieve the desired spectrum. While there are relatively few manufacturers of high-quality blue LED die, there are a substantial number of companies that purchase those raw blue die and package them into finished white LEDs, ready for sale and eventual integration into a luminaire or replacement lamp, signage, or backlight, such as we see in our smartphones, laptops, and flat panel TVs.
A principal concern for any white LED is the distribution of the phosphor over the surface of the LED. The final color characteristics of a white LED, including its correlated color temperature (CCT), depends heavily upon the prescribed amount of blue photons being down-converted into the additional colors within the spectrum, such as green, yellow, and red. If the phosphor layer is too thin over a portion of the LED, then too much blue will bleed through unconverted, while too little of the other spectral components are produced. In addition, we have all heard that one of the virtues (and vices, depending upon your need) of LEDs is that they are uni-directional emitters. It turns out that’s not strictly true, especially since many higher-output LEDs are etched onto a carefully crafted wafer made up of compound semiconductor materials that are themselves laying upon a transparent substrate, such as sapphire. Since the photon generation layer is parallel to the plane of the chip, it shouldn’t come as a surprise that those blue photons escape this layer in a variety of directions, including downward into the transparent substrate, where they may be absorbed or scattered.
In a typical cavity-based LED package, the LED chip can be mounted onto a reasonably reflective surface, such as ceramic or metal, which will reflect many of those blue photons and contain the rest within the package, mitigating any efficiency or quality-of-light effects. Using cavity-based designs, LED packagers have developed effective techniques to evenly cover the blue chip within such a package, with the result being relatively even phosphor coverage and smooth delivery of uniformly colored directional light.
Unfortunately, such packages don’t lend themselves to lower-profile designs, such as a smartphone or laptop backlight, where just millimeters of height matter a great deal. In a lower-profile package, the LED chip needs to be mounted in a more “stand-alone” fashion without side reflectors, directly to the primary heat sinking structure. In these packaging scenarios, standard dispensing and spray-coating techniques tend to deliver a domed distribution rather than a flat, uniform coating, with the result being variations in the color output at different points on and around the chip. The issue is especially pronounced in standard horizontal chips, where the “blue leak” is no longer contained within the package. In a vertical chip structure, with the anode on top, and the cathode consisting of a reflecting metalized layer that makes up the entire bottom of the chip, the blue leak is effectively eliminated, but the typical domed phosphor distribution still creates a problem.
Current phosphor application techniques also present a substantial challenge when it comes to creating innovative package-level implementations, such as variable-CCT single-package solutions, or combinations of white and single-color LED chips to create RGBW, WWRA, WWWR, or WWGR solutions that enhance color rendering (CRI) and provide higher-efficiency red/amber/green augmentation. By leaving the phosphor-application challenges to the packagers, not only have they been burdened by the additional capital requirements, expertise, processing time, overspray, and wastage issues, but it has also limited packaging options and unnecessarily driven up costs.
A recent innovation from a vertical-metal LED manufacturer, SemiLEDs Optoelectronics Co., Ltd., may be turning the trend in a new direction. The company’s recently introduced EV-W chips incorporate a proprietary phosphor technology, designated ReadyWhite™, which delivers a highly uniform phosphor coating across the emitter surface, greatly increasing uniformity and improving color precision. While the process can be applied to both horizontal and vertical structures, SemiLEDs found that the more desirable result came when used on metal-vertical structures that did not exhibit blue leakage.
In addition to the previously mentioned challenges, standard wafer-scale approaches to coating the wafer use a single-phosphor composition, whether by spray, spin-on, or sheet phosphor film, which cannot allow for the normal wavelength variation of each chip across a wafer. The result is a much wider CCT variance from chip to chip. The ReadyWhite™ process avoids this challenge by pairing a proprietary phosphor-impregnated matrix to each chip, enabling individual optimization, and much more consistent color and output results. The company has been able to demonstrate an impressive level of color uniformity across the range of CCT options, from warm to cool white. This unique approach opens the long-closed door to truly tight binning options for low-profile applications, as well as in multi-color white packaged LEDs.