Nanoimprint Lithography: An Enabling Technology for Future HB-LEDs

Topics Covered

Introduction

Increasing the efficiency of white LEDs offers the biggest potential right now for cost reduction. Growth of more perfect GaN layers and extraction of the generated light from the LED are the two most important issues to focus on. Nano-patterned sapphire substrates (NPSS) and photonic crystals are two emerging technologies to achieve this. Nanoimprint lithography (NIL) is an ideally suited replication method of such nanometer structures, needed for both these novel LED processing technologies.

The recent emergence of GaN-based LEDs in LCD TV backlight units has resulted in a very fast development of the market. This hype is going to be extended by the use of HB-LEDs for general lighting. Depending on local government regulations for the banning of incandescent lamps, this is expected to happen in two to four years.

While the moderate output power of LED TVs allows the usage of medium efficiency LEDs, increased efficiency is needed for general lighting to reach high enough lumen output. The efficiency derives from four individual terms, the internal quantum efficiency, the extraction efficiency, the conversion efficiency of the phosphors and the packaging efficiency. Conversion and packaging efficiency are mainly determined by backend processing and packaging. Internal and extraction efficiency values on the other hand are determined by epitaxial growth and chip processing.

Overview of GaN LEDs

Generally, sapphire is used as a growth substrate for GaN LEDs, where material parameters, such as lattice constant and thermal expansion coefficient, are quite different. As a result, grown GaN layers are strained and feature a high dislocation density. The basic obstacle to light extraction from the LED heterostructure is the refractive index difference between GaN and air. Therefore, only a limited amount of the light which is emitted from the LED can actually exit the LED. A major part of the light, however, is trapped due to total inner reflection and hence reabsorbed, resulting in a decreased overall efficiency and increased heat generation. In order to increase light extraction, different LED chip designs have been developed over the last years, namely lateral, vertical, flip-chip and thin-film flip-chip designs.

Besides the different chip layouts, different patterning and roughening techniques are employed for increased light extraction, namely patterning of the GaN-sapphire interface and surface roughening of the GaN interface to the ambient (air, encapsulant or phosphor layer).

Patterned Sapphire Substrates (PSS)

Patterned sapphire substrates (PSS) are dominantly used in lateral LEDs, where the sapphire wafer remains and is part of the final device. The predominantly used PSS features are micron-sized with pyramidal or cone shape.

Advantages of PSS

The advantages of PSS when compared to flat sapphire wafers are:

• The internal quantum efficiency is increased by more perfect epilayers with a reduced dislocation density.
• The PSS features effectively scatter light and hence reduce the total inner reflection of light. Nonetheless, these features used nowadays are still in the range of 1-3 mm, which means etching of sapphire and coalescent growth of AlInGaN compounds is quite time consuming. Furthermore, laser lift-off technology, which is widely used for vertical and thin-film-flip-chip LEDs cannot be used. Shrinking the PSS feature-size to the nanometer range has been shown to tackle most of the above mentioned issues.
• Indeed, improved light extraction has been shown for sub-wavelength feature size of such nano patterned sapphire substrates (NPSS), even outperforming micron-sized PSS.
• Furthermore, throughput for etching and growth is increased. The latter can be attributed to faster coalescence, and to abbreviated nucleation of GaN on NPSS substrates.

Photonic Crystals

Photonic crystals have been widely discussed in the previous years throughout the LED industry to further enhance light extraction. Photonic crystals (PhC) are dielectric perturbations in the surface of the LED. Periodic as well as quasi-periodic patterns, with a periodicity in the range of the emitted wavelength, increase light extraction by allowing guided light modes to exit the LED heterostructure. While PhCs on the LEDs’ surface are mainly effective for higher order light modes, buried photonic crystals can also be tuned to extract lower order light modes, and therefore further increase the extraction ef f iciency. Quasiperiodic PhCs additionally allow the control of the LEDs’ light emission pattern. This is another advantage over random surface roughening, where the emission pattern is purely Lambertian shape.

Nano Imprint Lithography

Nano imprint lithography is an ideally suited replication method of such nanometer structures, needed for both these novel LED processing technologies - NPSS as well as PhC. The typical properties of LED substrates – high bow, varying TTV and defects after epi-growth – have been shown to be challenging for conventional lithography methods. Same is true for high resolution capability of less than 100nm, which is needed for quasiperiodic PhCs. EVG’s lineup of NIL tools fulf ils all above requirements, in order to provide an effective, fullfield imprint.

Figure 1. EVG620NIL nano-imprint lithography tool for the generation of LED nanostructures, such as nano-patterned sapphire substrates (NPSS) or photonic crystals. A imprint of a full-area photonic crystal on a 3" wafer is shown as a inset. (Courtesy of EV Group)

Soft Molecular Scale Nanoimprint Lithography (SMS-NIL)

The new “Soft Molecular Scale Nanoimprint Lithography” (SMS-NIL) provides customers with a repeatable, cost-effective process for producing ultra-high-resolution patterns on large-area surfaces. UV-NIL offers a significantly lower processing cost than other nano-patterning techniques, making it an attractive solution for LED manufacturing.

SMS-NIL takes this approach a step further, employing soft polymeric working stamps to avoid damaging costly master stamps. The working stamps’ relatively low surface energy facilitates separation from the substrate after the imprinting process, while their flexibility allows the stamps to be used for multiple imprints. NIL is emerging to an enabling technology for GaN-based white LEDs to further boost the overall efficiency. Besides these two presented processing solution, NIL has been already shown its potential for other applications.

Recently, researchers from Rensselaer Polytechnic Institute have demonstrated a big step forward to boost efficiency of green LEDs by using nano patterned sapphire substrates. Even for the growth of semi-polar and non-polar GaN LEDs, which show less parasitic effects compared to today’s common polar GaN LEDs, NPSS substrates show high potential. Same is valid for the growth of GaN on silicon, which could lead to a further drop in LED cost, as required for the general adoption of solid state lighting.

About the Author

Dr. Thomas Uhrmann is the Business Development Manager for Compound Semiconductors and Si-based Power Devices at EV group. In his current role, he is responsible for introducing and managing technological innovations for the fabrication of high-brightness light emitting diodes (HB-LEDs) at EVG.

Source: EV Group.

For more information on this source, please visit EV Group.