If you want to invest in expensive LED backlight modules, LCD TVs will be a promising application market. However, if LED backlight module manufacturers use high-volume photonic crystal LEDs based on imprint lithography, the price of such a light source can be greatly reduced.
Author: Faiz Rahman and Ali Khokhar Glasgow University
The cathode ray tube has basically been eliminated. Now walk into any electronics store and you'll see rows of TVs and computer LCD monitors. Even plasma display technology, previously considered a better and more competitive competitor, is now challenged by LCD TVs based on the latest technology, which is characterized by higher efficiency, brighter and clearer images.
Unlike plasma displays, LCDs require a backlight module (BLU) to project white light onto the LCD panel. Cold cathode fluorescent lamps (CCFLs) are often chosen, but LED backlights offer an alternative to localized darkening to improve contrast. Because LED backlights are more efficient, they extend battery life and are therefore popular on laptops.
The advantages of LEDs include further reduction in the weight, thickness and energy efficiency of the backlight module. Solid-state light sources are more compatible with dynamic contrast control, and the technique used to turn off the LEDs in the black areas of the displayed image, resulting in contrast ratios of up to 10,000:1.
LED selection
The white light produced by the BLU can be achieved by carefully mixing the light emitted by the red, green and blue LEDs, or by using blue LEDs and yellow phosphors. Both options require GaN-based blue LEDs. All of these different types of devices have a common disadvantage: most of the light is trapped in the active area and cannot be efficiently extracted.
This inefficiency means that the backlight module requires more LEDs, which increases the cost. To improve light extraction efficiency, researchers have developed several techniques to extract light that is trapped in the chip by internal total reflection.
Commercial LED manufacturers prefer wafer top surface roughening technology. This simple and cost-effective random surface structure increases the light escape angle and significantly improves light extraction efficiency.
This technique is not suitable for backlight modules because of the lack of directional control of the light distribution. The surface-roughened LED chip actually has a non-directional light intensity distribution within the light-emitting cone. This feature is suitable for most general lighting applications, but it is not suitable for use as an LED backlight module because it requires a more regular light intensity distribution to direct light to the most suitable place.
Take out the light
How is light bound, how to remove it from the LED? It is good to study these. Calculations and simulations show that there are horizontal bound states in the GaN epitaxial layer and the sapphire substrate, and the addition of a regular porous grating structure on the LED generating end surface can effectively extract light. The most popular approach is to use a periodic or quasi-periodic array of shallow blind holes to form a two-dimensional photonic crystal grid.
Studies have shown that LEDs that use an etched photonic lattice can double the surface brightness and change the spatial luminescence distribution of the device compared to a non-surface treated control. The Lambertian distribution produced by the planar LED is not suitable for the backlight module, but by optimizing the structure of the photonic crystal, the radiation intensity distribution tends to be characteristic of the special diffuser and the brightness enhancement film.
American company Luminus Device Inc. is enjoying the success of the photonic crystal PhatLight LED. The device has been used in some high-end TVs, such as the Samsung 56-inch rear projection TV. The manufacturer believes that the introduction of such high-priced LEDs into flagship products is worthwhile. However, if photonic crystal LEDs have an impact on the entire market, especially the key LCD TV industry, their manufacturing costs must be significantly reduced to the same level as CCFL.
In May 2007, the “PQLDI†project for display lighting was officially launched, and its primary goal was to develop a low-cost production technology.
The project, which lasted two years and cost £1.2 million (about $2.4 million), was funded by the UK Science and Technology Strategy Committee. Project members are from the Electronics and Electricals team at the University of Glasgow, the University of Strathclyde Photonics Research Center, and the Sharp European Laboratory.
Advantages of quasicrystals
Traditional photonic crystal LEDs, such as those manufactured by Luminus, have attracted a strong patent portfolio. Therefore, we evade its direct and direct research on quasi-periodic photonic crystal structures. When used as a display backlight, the main advantage of this photonic crystal compared to conventional photonic crystals is that the quasi-periodic void structure arrangement provides greater freedom in optimizing the output light distribution, which in turn simplifies the design. .
Forming a photonic crystal structure is a key process step in the fabrication of such devices. Modern "deep submicron" lithography systems are a natural choice, but not optimal. Although it produces deep submicron-scale photonic crystal patterns, most of the LED wafer fabs do not have such expensive equipment.
Figure 1. (Left) The main steps of photosensitive nanoimprint lithography include pressing a resist coating on a wafer using a transparent quartz stencil, sensitizing it to ultraviolet light, and then etching to form a photonic crystal structure.
Figure 2. (Top) The quasi-photonic crystal LED used to display the illumination project still uses a conventional LED epitaxial structure.
A lower-cost alternative is electron beam lithography, which is widely used by researchers in the field of photonic crystal devices. However, we must point out that its writing speed is extremely slow, which means that the manufacturing cost of graphics is quite high.
It takes dozens of hours to write an area of ​​only a few square inches, in other words, a cost of several thousand pounds - and this is absolutely unbearable for the manufacture of large-scale commercial chips.
So we used nano-imprint lithography (NIL). The technique can be formed into a stencil, such as quartz or silicon, using some suitable hard material to form a pattern on the LED epitaxial wafer . High-resolution techniques such as direct-write electron beam lithography can be used to fabricate stencils, and since each stencil can be stamped to produce many wafers, the cost is unacceptably low. In a real manufacturing environment, a motherboard can be used to make several effective daughter boards.
Our nanoimprint process uses Obducat's equipment, and at a certain pressure and temperature, the pattern on the stencil is "transferred" to a special NIL resist (Figure 1). The cross-linking between the polymers can occur in a few seconds and form a hard resist before the template is removed. Dry etching copies the pattern of the resist onto the underlying wafer (Figure 2).
We first form a silicon nanoimprint template pattern by electron beam lithography, and then use a dry etch to form a male mold. These templates work well and there is no wear in our limited number of R&D samples. However, we have found that silicon stencils cannot withstand high working pressures, so quartz or SiC stencils are a better choice.
Transparent quartz can also be used for photo-sensing nano-imprint (flash NIL), which allows the entire nanoimprint resist to be shaped through the stencil material so that the stencil is not heated. These advantages have led us to use a more advanced etch process to fabricate high-resolution NIL stencils and use them to imprint GaN LED wafers to achieve a device wavelength of 360 nm.
The nanoimprint-based patterning process requires two etches because the embossing leaves a thin layer of resist that extends even to the area where there should be no resist. This is because the resist material in a flexible state during the contact phase of the imprint is displaced. Fortunately, nanoimprint technology does not require a resist development process. It is therefore possible to remove the remaining thin layer of resist before etching the epitaxial layer.
Prior to dry etching of GaN, a one-step oxygen plasma process is typically performed. Several gases are used in combination. A combination of methane and hydrogen is one of them, but we and other teams have found that chlorine-containing gas mixtures produce better results.
Our dry etching uses an inductively coupled plasma (ICP) etch device that produces a nearly vertical surface profile. This method also increases the etch rate to a few hundred nanometers per minute, which means that it takes only about 5 minutes to form a photonic crystal structure on a monolithic wafer. The short process time makes the process suitable for large scale manufacturing requirements.
Developing a process that can be used for large-scale growth requires many challenges, including making a sharply defined template, ensuring that all of the pattern areas of the template are in good contact with the resist, preventing the template from sticking to the resist. The embossing and resist can be easily demolded after being shaped. The reflow of the soft resist causes a thin layer of resist to remain after imprinting, which must be removed in the fabrication process of the photonic crystal LED before the pattern is transferred to the substrate material.
The process we developed has a patent that solves all of the above problems, resulting in high quality graphics and ultimately etched onto the substrate material. The electron nanoscopy was used to observe the cross-section of the silicon nanoimprint template (Fig. 3) and a resist-coated wafer (Fig. 4). The picture shows the precision of our process.
These images show clear photonic crystal holes. The high fidelity reproduction of nanoscale graphics is a prominent feature of nanoimprinting, making it one of the strong competitors for next-generation lithography for CMOS chip manufacturing.
In order to speed up the nanoscale pattern transfer process in an actual large-scale production environment, nanoimprint templates must be used in step-and-repeat equipment, and thousands of chips can be imprinted in a few hours. This may not seem so difficult because it requires much less alignment than silicon chips.
Together with other members, we are exploring the possibility of roller embossing. It has the potential to form very large areas of graphics at high speeds, and it is simpler than step-and-repeat embossing because only one rotational motion is required.
The process imprints on the resist coating by using a hard roller with a male die pattern. The pattern transfer can be by hot pressing or photosensitive forming because the GaN epitaxial wafer is deposited on a sapphire substrate that is transparent to ultraviolet light, and ultraviolet light can be incident through the back side of the wafer.
Research work in this area is still ongoing, and initial results show different advantages for specific methods. Photosensitive imprinting works better on flat wafer surfaces, while flat surfaces are more suitable for hot pressing processes. Although these technologies still require more research, we believe that LED manufacturers will apply these technologies in the future, most likely when the GaN LED chip size exceeds 6 inches.
Figure 3. The quasi-crystal pattern avoids the patenting of traditional periodic structures and provides greater freedom to adjust the distribution of emitted light to suit a particular application.
Figure 4. By using a mixed gas containing chlorine as a process gas, the ICP process can generate a high-fidelity photonic crystal structure in just 5 minutes.
Figure 5. A quasi-photonic crystal LED chip with a side length of 350 Ã… has a very uniform emission intensity distribution on the top surface.
Once the pattern is formed, standard process steps can be used to complete the LED fabrication. We fabricated the array on a 350 x 350 Ã… chip and added a unique p-type current spreading layer to conduct current evenly across the top surface of the device. This is a critical step because the current spreading layer needs to be structurally compatible with the etched photonic crystal pattern.
Our device (Figure 5) has an operating current of 200 mA or more and emits light evenly in the device area. The angular distribution of light intensity is different from the Lambertian distribution of traditional LEDs and is now working with our backlight development team to further optimize it.
In the final year of the project, we will focus on optimizing processes, improving device performance, and seeking new soft mask methods for nanoimprint processes. In addition, we also hope to seek more support from the government and industry to continue research on large-size LED chips for mainstream lighting. These devices have special difficulties to overcome, such as effective thermal management, large-area current spreading, and efficient color conversion from blue to full-color spectroscopy.
About the author : Faiz Rahman (f..uk) is currently a lecturer in the Department of Electronics and Electrical Engineering at the University of Glasgow. His research interests include device processing technology, quantum effect devices, radiation detectors, imaging arrays, and computer system architectures. Ali Khokhar is engaged in postdoctoral research in the department.
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