Elsevier

Microelectronic Engineering

Volume 84, Issues 5–8, May–August 2007, Pages 945-948
Microelectronic Engineering

3D structures for UV-NIL template fabrication with grayscale e-beam lithography

https://doi.org/10.1016/j.mee.2007.01.015Get rights and content

Abstract

The individual steps in fabrication of templates for UV-NIL processes are described. After spin coating a conductive copolymer (ESPACER 300) on top of the resist, insulating substrates have been structured by use of electron beam lithography at 20 keV beam energy. A three-dimensional (3D) pattern has been created in a low contrast positive tone resist PMMA 35k. By RIE in a CHF3 – O2 – process, the pattern has been transferred into the quartz substrate. Finally, the 3D structures have been replicated in a UV-NIL process.

Introduction

The main advantage of nanoimprint lithography (NIL) based on ultra violet light (UV) [1], [2] compared to other lithography techniques is the ability to fabricate three dimensional (3D) structures in a single replication process. One major application of 3D NIL is the dual damascene approach to fabricating interconnected structures [3] where a wiring level and a via level are created simultaneously, thereby reducing the total number of processing steps. However, manufacturing of high quality molds is elementary because their inverted surface replica is directly replicated during the imprint process. The fabrication process of 3D structured templates for UV-NIL applications by means of electron beam lithography (EBL) faces three major challenges. (1) E-beam exposures on UV transparent and therefore non-conductive substrates like quartz, (2) usage of thick resists layers with a low contrast for three dimensional patterning and (3) reliable transfer of 3D structures into the template. Within the following article we describe a possible approach to solve all these different tasks.

Section snippets

Pattern generation

The pattern designed in GDSII format contains a variety of different test structures. As some of them were found to be sensitive to proximity effect, their dose distribution had to be conditioned by use of the proximity correction package NanoPECSTM. In order to achieve 3D features after development, the dose of each GDSII element has to be adjusted carefully corresponding to the desired depth in the resist. The relation between dose and resist depth is illustrated in the contrast curve shown

Sample preparation and EBL

As substrate for the UV-NIL templates a 0.5 mm thick SiO2 wafer was diced in 15 × 15 mm2 samples. For EBL the samples have been coated with 500 nm positive tone resist PMMA 35k. Resist thickness measurements have been carried out by use of an optical white light interferometer. Due to the low contrast (γ) of PMMA 35k (γ = 1.2) the resist depth can be varied between 0% and 100% by modifying the dose applied during the EBL exposure. This makes the resist well suited for grey scale EBL.

After baking at 170

Pattern transfer

Pattern transfer has been achieved via dry etching (RIE) in CHF3 plasma. However, the use of CHF3 as only reactant leads to curing and hardening of the PMMA mask, making full pattern transfer of the 3D features impossible. To enhance the ablation of the PMMA mask O2 was added to the etch process. While the etch rate f of SiO2 stays nearly constant at least for small amounts of O2 (f(O2) < f(CHF3)), the PMMA removal was enhanced. Although selectivity between PMMA and SiO2 is reduced, the height of

Imprint results

As proof of concept, the sample has been used as a mold in a UV-NIL process. The substrate consisted of a silicon wafer coated with 300 nm thick AMONIL resist (AMO GmbH, DE). The mold has been pressed into the resist under a pressure of 300 mbar while the environmental pressure before mold to resist contact was about 20 mbar. After imprint the resist was cured through the backside of the mold under UV light at a dose of 2 J/cm2. Fig. 6 illustrates an scanning electron microscopy (SEM) micrograph

Summary and outlook

Examples of the whole process chain applied during 3D template fabrication for UV-NIL applications have been presented. Through the use of low contrast PMMA resist, grey scale lithography could be performed for the definition of the molds pattern using a RAITH150 EBL system. The features could be transferred into the SiO2 substrate via RIE while the addition of O2 enables a modulation of the selectivity between the resist mask and the substrate. The mold was replicated via UV-NIL in one single

Acknowledgement

This work has been supported by the Bundesministerium für Bildung und Forschung, FKZ: 13N8401.

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