Elsevier

Microelectronic Engineering

Volume 87, Issues 5–8, May–August 2010, Pages 963-967
Microelectronic Engineering

UV enhanced substrate conformal imprint lithography (UV-SCIL) technique for photonic crystals patterning in LED manufacturing

https://doi.org/10.1016/j.mee.2009.11.134Get rights and content

Abstract

The global LED (light emitting diode) market reached 5 billion dollors in 2008 and will be driven towards 9 billion dollors by 2011 [1]. The current applications are dominated by portable device backlighting, e.g. cell phones, PDAs, GPS, laptop etc. In order to open the general lighting market doors the luminous efficiency needs to be improved significantly. Photonic crystal (PhC) structures in LEDs have been demonstrated to enhance light extraction efficiency on the wafer level by researchers [2]. However, there is still a great challenge to fabricate PhC structures on LED wafers cost-effectively. Nanoimprint lithography (NIL) [3] has attracted considerable attentions in this field due to its high resolution, high throughput and low cost of ownership (CoO). However, the current NIL techniques with rigid stamps rely strongly on the substrate flatness and the production atmosphere. Those factors hinder the integration of NIL into high volume production lines. UV-NIL with flexible stamps [4], e.g. PDMS stamps, allows the large-area imprint in a single step and is less-sensitive to the production atmosphere. However, the resolution is normally limited due to stamp distortion caused by imprint pressure.

A novel NIL technique developed by Philips Research and Süss MicroTec, substrate conformal imprint lithography (SCIL), bridges the gap between UV-NIL with rigid stamp for best resolution and soft stamp for large-area patterning. Based on a cost-effective upgrade on Süss mask aligner, the capability can be enhanced to nanoimprint with resolution of down to sub-10 nm on an up to 6 inch area without affecting the established conventional optical lithographic processes on the machine. Benefit from the exposure unit on the mask aligners, the SCIL process is now extended with UV-curing option, which can help to improve the throughput dramatically. In this paper, the fabrication of photonic crystal structures with SCIL technique on Süss MA6 mask aligner is demonstrated. In addition, the industrialization considerations of UV-SCIL process in high volume manufacturing are briefly discussed.

Introduction

The global LED (light emitting diode) market reached 5.5 billion dollars in 2008 and is dominated by portable device backlighting applications e.g. mobil phones, PDAs, GPS, Laptop, etc. Emerging applications including general lighting will drive the market towards 9 billion dollars by 2011, according to Yole development report. However, the luminous efficiency needs to be improved significantly in order to open the market for LEDs to replace traditional light bulbs [1]. Photonic crystal (PhC) structures in LEDs have been demonstrated to enhance light emission efficiency by diffractive waveguide structures. It is still a great challenge to fabricate PhC structures on LED wafers cost-effectively [2], [3].

Nanoimprint lithography (NIL) [4] is a simple method to fabricate nanostructures and it has attracted considerable attentions in many applications due to its high resolution, high throughput and low cost of ownership (CoO). There are several NIL candidates for mass productions in the market: hot-embossing lithography (HEL), UV-NIL [5] with rigid quartz stamps and UV-NIL with flexible stamps [6]. HEL has been discarded from PhC LEDs application due to its thermal cycles and therefore low throughput. UV-NIL with rigid stamps provides excellent resolution down to sub-20 nm. However, the process relies strongly on the substrate flatness and production environment. It is not suitable for patterning of most of the LED wafers, which have roughness of more than 10 μm. In addition, most of the existing LED factories are designed as class 1000 or above, which will limit the yield of the process and increase the CoO of imprint stamps dramatically. UV-NIL with soft stamps, e.g. PDMS stamps, allows the large-area imprint in a single step with less sensitivity to substrate flatness and particles. However, the resolution is normally limited due to the stamp distortion caused by imprint pressure.

Substrate Conformal Imprint Lithography (SCIL) [7], a novel NIL technique developed by Philips Research and Süss MicroTec, bridges the gap between UV-NIL using rigid stamps for best resolution and soft stamps for large-area patterning. Based on a cost-effective upgrade on Süss mask aligner, the aligner capability can be enhanced to nanoimprint with sub-50 nm resolution up to 6 inch diameter area without affecting the established conventional optical lithographic processes on the machine. In this paper, the process details of SCIL technology and the imprint tooling are briefly described. Additionally, the introduction of SCIL into high volume manufacturing of high brightness LED (HB LED) is discussed.

Section snippets

SCIL principle

In order to reduce the CoO (cost of ownership) of large-area imprint stamps, SCIL process uses composite working stamps consisting of a glass carrier with patterned rubber [8] which are replicated from the original master pattern. Philips Research developed a SCIL master replication tooling (MRT) and automatic separation tooling, which allow the end-users to produce high quality SCIL stamps themselves from their own masters; this tooling is available from SUSS MicroTec Lithography GmbH. Before

Experimental results

In this paper, a 6 inch silicon master (Fig. 4) from AMO GmbH (Aachen, Germany) with 2D hole array fabricated by laser interference lithography and etching processes has been used for stamp replication. A 6 inch multilayer SCIL imprint stamp was replicated from this master using the MRT tooling. Full field imprint with the replicated SCIL stamp has been carried out into a 120 nm thick imprint sol–gel layer on a 6 inch substrate. After removal of the residual layer (ca. 20 nm) by CF4 RIE, the

Conclusions

In this paper, a revolutionary NIL technique, SCIL, and the corresponding tooling solution on SUSS mask aligners has been introduced. The imprints of 2D holes array over 6 inch area in sol–gel and AMONIL resist with composite working stamp have been demonstrated. The structure depth and residual layer uniformity have been evaluated by measurements on the imprinted wafer. The capability of the UV enhanced SCIL process in AMONIL resist on different substrates has also been demonstrated. The

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