Patterned substrate, referred to as PSS (Patterned Sapphire Substrate), commonly known as patterned substrate, that is, a mask for dry etching is grown on a sapphire substrate, and the mask is patterned by a standard photolithography process, and etched by ICP. The etch technique etches sapphire, removes the mask, and then grows GaN material thereon, causing the longitudinal epitaxy of the GaN material to become laterally epitaxial, etched (in sapphire C-face dry etch/wet etch) on a sapphire substrate Designed to produce micro- or nano-scale micro-structure-specific patterns to control the output light form of the LED (the embossing pattern on the sapphire substrate produces light scattering or refraction that increases the light output rate) while the GaN film grows A lateral epitaxial effect is produced on the patterned sapphire substrate, which reduces the difference between the GaN grown on the sapphire substrate, improves the epitaxial quality, and improves the internal quantum efficiency of the LED and increases the light extraction efficiency. Compared to LEDs that grow on a typical sapphire substrate, PSS can increase brightness by more than 70%.
Do nGGEUN KO, JA COB YOON, JANGHO SEO describes how to reduce the defect density and total reflection loss by patterning the wafer, thereby improving the LED light extraction efficiency.
Manufacturers are rapidly adopting nitride-based LEDs as standard light sources for a wider range of products, from general lighting, headlights, traffic lighting to background lighting for consumer electronics such as HDTVs, smartphones, and tablets. Computer and size display. LED performance and cost make LED technology widely available. In fact, low cost and high light efficiency can promote the acceptance of the consumer market. LED chip manufacturers are looking for patterned sapphire substrate production technology to maximize light extraction efficiency and promote the use of LEDs.
Applying patterning to an LED substrate or wafer can improve light output in two ways. This technique can increase the light exit of the active quantum well layer by reducing the epitaxial defect density. Moreover, the patterned sapphire substrate can reduce the light loss due to total reflection by the photon scattering effect.
Researchers have designed patterns of periodically varying structures of different shapes and sizes on the surface of sapphire substrates, including cones, domes, pyramids, and columnar structures. Such a sapphire substrate is referred to as a patterned sapphire substrate.
There are currently two methods in the industry for producing patterned sapphire substrates: dry plasma etching and wet chemical etching, but most of the patterned sapphire substrates are produced using dry plasma etching techniques. The accuracy and uniformity of the example etching, such as dry etching, is easier to control than wet chemical etching. The fabrication of patterned sapphire substrates discussed herein will focus primarily on the use of inductively coupled plasma dry reactive corrosion.
By collaborating with 100-mm and 150-mm patterned sapphire substrates with many of the world's most advanced LED manufacturers, Rubicon has the opportunity to understand the range of effective patterned sapphire substrates. The most critical requirements are pattern size, shape, aspect ratio (eg, aspect ratio of the pattern), wafer uniformity, and wafer-to-wafer uniformity.
Because of the high customization of the epitaxial process in the LED industry, we are unable to obtain an optimal solution for patterned sapphire substrates. The design of the pattern is ever-changing, and there is no convergence in the design of the patterned sapphire substrate in the future. Typical pattern shapes include cones, domes, square pyramids or triangular pyramids. Even academic studies have shown that the smaller the pattern size (100-1000 nm), the better the light efficiency, but the LED industry still dominates the 3-4 μm pattern.
Process parameters affecting key characteristics are dimensional accuracy, uniformity of photoresist mask, selectivity of sapphire etching to photoresist mask, RF power, inductively coupled plasma process pressure, RF coil Design plasma consistency, ratio of trifluoromethane to boron trifluoride, and substrate temperature.
Increase light extraction rate
Low light extraction rates are a big challenge for producing high brightness LEDs. The patterned sapphire substrate allows photons outside the totally reflective vertices to be scattered into the total reflection vertebral (Fig. 1a), thereby improving light extraction efficiency. This effect is equivalent to increasing the critical angle of photon overflow (Figure 1b). The study found that by this means the light extraction efficiency can be increased by up to 30%.
Figure 1: The patterned sapphire substrate scatters photons (a) and effectively expands the escape cone (b), which increases the light extraction rate by 30%.
Photons are released from the activated quantum well layer by electron-hole recombination, thereby being directed from the LED chip to the space.
Ideally, all photons released from the active quantum well are extracted as the light output of the LED, but in reality most of the photons cannot be emitted from the LED chip due to various factors.
A key impediment to the ideal light extraction efficiency is total reflection caused by the high refractive index of gallium nitride and the refractive index of free space (approximately 2.5:1). A large amount of photons generated from the active layer are reflected into the chip and are confined within the chip and cannot be emitted, and are eventually consumed as thermal energy. Only those photons that are injected into the vertebral body defined by the light defined by the critical angle of total reflection can emit the LED chip, and the photons outside the vertebral body are confined in the LED chip.
More efficient light conversion
From the perspective of thermal energy, mechanical energy and chemical properties, sapphire is a very good substrate material for III/V nitride epitaxial growth. However, sapphire does not only have a different crystal structure than the III/V nitride, but also has a lattice mismatch of about 15% with the nitride. Thus, the nitride epitaxial layer naturally produces misfit dislocations of about 108-1010 per square centimeter (mismatch dislocations are linear defects of the epitaxial layer used to indicate the degree of degradation of the crystal film quality). Such defects are generally characterized by etch pit density under an optical microscope or FWHM ray under X-ray.
The patterned sapphire substrate enhances the epitaxial growth of the nitride by reducing misfit dislocations, which are increased laterally by the patterned sapphire substrate, ie, parallel growth with the substrate surface. Misfit dislocations typically pass through traditional flat sapphire substrates or patterned sapphires
The substrate occurs during the initial epitaxial growth nucleation stage. Many researchers have found by transmission electron microscopy that misfit dislocations can be reduced by increasing the lateral configuration of epitaxial growth of patterned sapphire substrates.
Since the recombination of electrons and holes (non-radiative recombination) in the misalignment line, the reduction of misfit dislocations in the activation layer is one of the most important factors for improving the light conversion efficiency (also referred to as internal quantum efficiency). In general, by increasing the quality of the epitaxial quantum well, the patterned sapphire substrate can increase the internal quantum efficiency by about 30%. Of course, the size, shape, quality of the pattern, and the optimization of epitaxial growth properties matched to various pattern designs have a large impact on the improved internal quantum efficiency of the patterned sapphire substrate.
Effective patterned sapphire sinking design
In the design of a patterned sapphire substrate, we need to consider two key points to optimize the light output of the LED chip. The first is how to maximize side growth to more effectively suppress misfit dislocations in epitaxial growth; the second is how to maximize the scattering effect to improve light extraction.
Figure 2: The ratio of the pattern area to the total substrate area is calculated by the ratio of the shaded area or the ratio of the gray area to the area of the hexagon (a), and the aspect ratio refers to the ratio of the height to the width of the pattern structure (b).
The ratio of pattern area to total area and aspect ratio are two key points in the patterned sapphire substrate design that increase lateral epitaxial growth parallel to the substrate plane (as shown in Figure 2). With respect to the total area, the higher the pattern area ratio, the more the side structure can be increased during epitaxial growth to reduce the misfit dislocation density. Such an effect can be observed by transmission electron microscopy or by FWHM reduction in x-ray rocking zone analysis.
At present, studies have shown that a higher aspect ratio can improve the lateral composition of epitaxial growth. Of course, this conclusion needs further elaboration. The broader and higher developments now may not be accidental, but related to these design rules. In addition, the shape and density of the pattern are design factors that affect the lateral growth.
For increasing the light extraction rate by scattering effects, pattern shape, aspect ratio, and pattern density are all major factors considered by LED designers. However, this article will focus on pattern density.
Both the pattern array geometry and the pattern spacing should be considered in controlling the pattern density. Even though the patterns can be designed in different geometries, the hexagonal arrangement is the only universally applied geometry because of its close alignment. However, the arrangement density can be further increased by shortening the period distance of the pattern. Numerous researchers are working on the potential of nanoscale patterning. These studies focus not only on the efficiency of light extraction by increasing the alignment density, but also on the improvement of internal quantum efficiency caused by the increase in epitaxial quality.
Many researchers have reported a major breakthrough in achieving LED light efficiency improvements on nanopatterned substrates. These breakthroughs are not only superior to conventional planar substrates, but also superior to micropatterned substrates. However, nanoscale patterned sapphire substrates have not yet been used in the LED industry due to their highly customizable customization requirements.
Typically, nanoscale patterned sapphire substrates are fabricated by silkscreening on a sapphire substrate or high resolution optical exposure followed by inductively coupled plasma etching. High-resolution optical exposure requires a high degree of flatness of the substrate, which is a big challenge for today's sapphire substrate suppliers. As mentioned above, the LED industry is gradually gaining a consensus on the impact of various patterned sapphire substrates on LED performance. But these consensuses are limited by the reluctance of LED chip vendors to share sensitive patent information.
Key process parameters
We have discussed a lot about the effects of design parameters of patterned basket gemstone substrates on LED performance. The height, width, spacing (period distance between patterns) and shape are the most important. In addition, the consistency of edge parameters between these individual wafers and different wafers is important in both LED chip assembly and lean manufacturing.
Figure 3: The patterned sapphire substrate is maintained in a rigid structure by inductively coupled plasma dry reactive etching through a photoresist mask (Figure a), however, if the rigid structure of the photoresist mask is resistant If the etch process is not maintained, the result will be poor (Figure b).
The periodic pattern on the sapphire is achieved by a patterned photoetch mask in an inductively coupled plasma dry reactive etch. Creating an accurate and uniform high precision structure in a photoetch mask is the first step to success. In the subsequent steps, it is equally important to maintain the structural integrity of the photoetch mask during the inductively coupled plasma dry etch process. This structural rigidity can be achieved by hardening the resist or cooling the substrate during inductively coupled plasma dry etch production. As shown in Figure 3, if the complete accuracy of the resist structure is not well maintained during the etching process, the quality of the patterned sapphire substrate will be severely impaired.
Figure 4. The structure of the patterned sapphire substrate can vary. For example, a 1.3 micron high, 2.5 micron wide wafer (a) to a 1.9 micron high, 2.6 micron wide wafer (b).
Other key factors critical to patterned sapphire profile characteristics in inductively coupled plasma dry etching are the selectivity of sapphire etching for photoresist masks, RF power, inductively coupled plasma operated pressure, plasma consistency RF coil design, plasma chemistry and substrate temperature control. Successful implementation of the most efficient patterned sapphire substrate for the target application is determined by how intelligently combined and implemented all of these parameters. As shown in Figure 4, different combinations of these parameters have a large impact on the pattern design.
Today's patterned sapphire substrate market trends
Until now, the market for patterned sapphire substrates has dominated by LED chip manufacturers. They either produce patterned sapphire substrates themselves or outsource to other contract manufacturers. This situation has gradually changed, and the dominant position of patterned sapphire substrates has gradually shifted from LED chip manufacturers to sapphire substrate manufacturers.
Today, sapphire wafer manufacturers have begun working with LED chip manufacturers to pattern sapphire substrates. However, most sapphire producers focus on 2 to 4 inch small diameter patterns. Only a few manufacturers have begun to introduce 6-inch products. In 2013, Rubicon Technology launched 4-inch, 6-inch and 8-inch large-diameter patterned sapphire substrates with better quality control and vertical integration to differentiate it from other manufacturers in the industry.
