Opinion paper
Processing study of SU-8 pillar profiles with high aspect ratio by
electron-beam lithography
Yaqi Ma, Yifan Xia, Jianpeng Liu, Sichao Zhang, Jinhai Shao, Bing-Rui Lu, Yifang Chen
⁎
Nanolithogrophy and Application Research Group, State Key Lab of ASIC and System, School of Information Science and Engineering, Fudan University, Shanghai 200433, China
abstractarticle info
Article history:
Received 8 February 2015
Received in revised form 27 July 2015
Accepted 19 October 2015
Available online 20 October 2015
Keywords:
SU-8 pillar
Profile control
Electron beam lithography
High aspect ratio
Nanofabrication
We report the fabrication of micro-pitched SU-8 pillar arrays with height up to 5 μm and aspect ratio of 7.14:1 by
electron beam lithography (EBL) at 100 keV, combined with a hot developing process. Careful study of processing
latitude for geometry parameters of SU-8 pillars was conducted to achieve three different profiles, vertical pillar
for biosensing application, trapezoidal shape for antireflection of light in solar cells as well as in displays, and final
pillars with thick residuals in the gaps. It was found in our work that SU-8 is particularly a good candidate for tall
micro/nano structures with various shapes when the unavoidable proximity effect in the EBL was exploited,
which was enhanced by the ultra-high sensitivity of epoxy groups in SU-8. Optical properties of the fabricated
structures have been characterized. To the best of our knowledge, pillar like structures of SU-8 in the height
range of 5 μm have not been addressed sufficiently despite their broad potential applications in bioscience ,
environment protection, display and renewable energy sources etc.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Large area submicron structures with high aspect ratio (HAR) can
find extensive applications in cross disciplinary areas, such as scaffolds
for trapping biomaterials [1], cells and liquids [2], antireflection layers
in solar cells [3] and metallic pillar arrays for local surface plasmonic
reson ators (LSPRs) as sensors when they are covered by a layer of
noble metals as suggested by our earlier work [4]. For constructions of
these devices, resists with 5–10 μm thic kness are usually nee ded.
However, the majority of applications with electron beam lithography
(EBL) deals with resists below 1 μ m in thickness for nanostructure s.
It is believed that EBL at high tension such as 100 kV should also be
capable of replicating submicron structures in thick resists such as
chemical amplified resist (CAR), SU-8. Negative tone resist SU-8 was
initially designed for deep ultraviolet lithography (DUV) [5–7]. It was
then successfully applied in micro-electro-mechanical systems (MEMS)
[8]. Owing to its ultra-high sensitivity (roughly 150 times of PMMA)
leading to sever proximity effect, SU-8 had not been applied in EBL
until the end of 1990s [9–10]. Even though, the reported study of EBL
on SU-8 are limited to the resist thickness below 500 nm, resulting in
an aspect ratio up to 3.8:1 [2]. Few works on HAR SU-8 structures by
EBL have been reported.
In this paper, we proposed a process by e-beam direct write at
100 kV to fabricate desired HAR SU-8 pillars with diffe rent profiles.
The processing parameters such as implanted charge dosage, pattern
design, baking and developing temperature for desired profiles were
carefully studied. Optical measurements of the reflectance by HAR
SU-8 pillars were carried out in near infrared region.
Fig. 1 schematically illustrates the process flow for the replication of
micro pitched SU-8 pillar arrays by EBL. A layer of SU-8 resist supplied
by MicroChem Ltd., with two different thickness of 5 μ mand10μm
respectively, was spin coated on a fresh Si wafer after a layer of HMDS
as adhesion layer, followed by a precisely contr olled two-step pre-
bake at 65 °C for 3 min and 95 °C for 10 min on a hot plate, respectively.
It was found in our experiments that such a two-step bake is necessary
for reducing the strain between SU-8 and Si substrate, which may cause
collapse of the tall SU-8 pillars. E-beam exposure was carried out by a
state-of-the-art beam writer, JEOL 6300FS at the electron beam energy
up to 100 KeV with a typical beam current of 500 pA and an 8 nm
beam-spot size. Post-exposure bake (PEB) was carried out by a similar
two-step bake at 65 °C for 1 min and 95 °C for 3 min on a hot plate. A
developing process was immediately carried out by the SU-8 developer
(supplied by TELTEC Ltd.) at 45 ± 2 °C for 10 min, and finally the sample
was thoroughly rinsed in IPA (also heated at 45 ± 2 °C).
The lithography p roperty of such a thick SU-8 layer was first
measured by contrast curves under the developing temperatures of
20 °C, 30 °C and 40 °C respectively, as shown in Fig. 2.Thefilled symbols
depict the remaining thickness of SU-8 after developing, whilst the open
symbols represent the thickness of the scum between the lithography
features. Extremely high sensitivity as 1 μ
C/cm
2
was measured. With
such a high sensitivity, proximity ef fect by both forward scattering
and backscattering of electrons is the main issue in EBL, as reflected by
the scum thickness in Fig. 1. Therefore, dealing with the proximity effect
is the central concern in the processing study of this work.
Microelectronic Engineering 149 (2016) 141–144
⁎ Corresponding author.
E-mail address: yifangchen@fudan.edu.cn (Y. Chen).
http://dx.doi.org/10.1016/j.mee.2015.10.013
0167-9317/© 2015 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Microelectronic Engineering
journal homepage: www.elsevier.com/locate/mee