Microelectronic Engineering 88 (2011) 994–998
Contents lists available at ScienceDirect
Microelectronic Engineering
journal homepage: www.elsevier.com/locate/mee
Development of metal etch mask by single layer lift-off for silicon nitride
photonic crystals
Kang-mook Lim, Shilpi Gupta, Chad Ropp, Edo Waks ⇑
Department of Electrical and Computer Engineering, IREAP, and JQI, University of Maryland, College Park, MD 20742, USA
a r t i c l e
i n f o
Article history:
Received 29 October 2010
Received in revised form 29 December 2010
Accepted 30 December 2010
Available online 8 January 2011
Keywords:
Silicon nitride (SiN)
Photonic crystals (PC)
Nanofabrication
Metal liftoff
Nanophotonics
a b s t r a c t
We present a method for fabrication of nanoscale patterns in silicon nitride (SiN) using a hard chrome
mask formed by metal liftoff with a negative ebeam resists (maN-2401). This approach enables fabrication of a robust etch mask without the need for exposing large areas of the sample by electron beam
lithography. We demonstrate the ability to pattern structures in SiN with feature sizes as small as
50 nm. The fabricated structures exhibit straight sidewalls, excellent etch uniformity, and enable patterning of nanostructures with very high aspect ratios. We use this technique to fabricate two-dimensional
photonic crystals in a SiN membrane. The photonic crystals are characterized and shown to have quality
factors as high as 1460.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
The ability to pattern dielectric structures with high resolution
electron beam lithography has enabled a broad range of photonics
device applications. One important example is the fabrication of
photonic crystal (PC) devices that exploit Bragg reflection in multiple dimensions to achieve strong confinement of electromagnetic
fields [1,2]. The engineering of photonic devices has mostly focused
on high index materials such as silicon (Si) or Gallium Arsenide
(GaAs) that typically operate at infrared wavelengths. Recently
there has been great interest in extending these devices into the
visible and ultraviolet (UV) wavelengths. Silicon nitride (SiN) is
an ideal material for these applications due its large transparency
bandwidth that spans the entire visible and part of the UV spectrum. SiN has already been used to develop low loss optical waveguides [3,4], one-dimensional resonant tunneling structures [5–7],
and two-dimensional PCs [8–10] for studying cavity quantum electrodynamic (QED). SiN also exhibits a large nonlinear coefficient
[11] and is CMOS compatible, making it useful for on-chip nonlinear optical devices such as multiple-wavelength oscillators [12].
The fabrication of these devices in a SiN material system has relied on direct patterning of a polymer etch mask from ebeam resist,
followed by dry etching using fluorine chemistry. However, SiN
generally exhibits poor etch selectivity relative to ebeam resist under dry etching, which results in significant degradation of the
mask pattern. This degradation typically results in sloped sidewalls
⇑ Corresponding author. Tel.: +1 301 4055022.
E-mail address: [email protected] (E. Waks).
0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.mee.2010.12.113
that greatly reduce the quality (Q) factor of the device [9]. One
method to improve the sidewall profile of the structures is to use
a hard etch mask that does not degrade under dry etching. Hard
etch masks based on silicon dioxide (SiO2) have been demonstrated in GaAs and indium phosphide (InP) systems [13,14]. However, the etching rate ratio of SiN to SiO2 has been reported to be
only 2.3 using standard fluorine chemistry [15], which significantly
limits the etch depth. Metallic masks such as chrome (Cr) and nickel (Ni) would have the advantage of much higher selectivity. Such
metals are usually patterned by a liftoff technique where the metal
is deposited on a patterned resist mask that is subsequently removed. The majority of work on high resolution metal liftoff utilizes positive tone resists such as polymethyl methacrylate
(PMMA) [16,17]. But in many photonics applications (such as
PCs) the device must be patterned on thin suspended membranes.
In these cases the use of positive tone resist would require exposing the negative pattern of the device, which necessitates exposure
of an extremely large area in order to support the membrane structures. This disadvantage makes the use of positive tone resists
impractical for patterning of such devices.
Methods for patterning of metallic structures with negative
tone resists have been reported in previous works. In particular,
high resolution liftoff using Hydrogen Silses Quioxane (HSQ) negative tone resist has been previously demonstrated to pattern germanium and platinum [18]. HSQ has the advantage of very high
spatial resolution, but is difficult to liftoff resulting in pattern irregularities. To overcome this problem, an HSQ/PMMA bilayer technique has been employed [19] to improve the ease of liftoff.
However, the bilayer liftoff method can result is significant distor-
K.-m. Lim et al. / Microelectronic Engineering 88 (2011) 994–998
tion of the pattern when transferred from resist to the metal mask
by dry etching.
In this paper, we report an approach for fabrication of nanoscale
patters on SiN using a hard Cr mask. The mask is deposited by single layer metal liftoff of ma-N 2401 negative tone ebeam resist. A
variant of this resist, maN-2410, has previously been used to pattern gold structures with feature sizes as small as 200 nm
[20,21]. Here we use maN-2401to fabricate a robust Cr etch mask
for ebeam lithography. Using this approach we demonstrate the
ability to fabricate feature sizes as small as 50 nm, which are then
transferred to a 200 nm thick SiN membrane. The fabricated structures exhibit straight sidewalls, superb pattern regularity, and very
high aspect ratios. The fabrication technique we demonstrate is
ideally suited for patterning PCs due to its high spatial resolution
995
and the fact that we are patterning the negative tone, enabling
us to pattern structures that can be undercut to form membranes.
As a demonstration, we fabricate a PC cavity on a thin SiN membrane that exhibits straight sidewalls and Q factors as high as 1460.
2. Fabrication procedure
Fig. 1 illustrates our fabrication procedure for patterning of the
metal Cr mask and subsequent transfer to SiN. The initial substrate
is composed of a 200 nm thick SiN layer grown by low pressure
chemical vapor deposition (LPCVD) on a 500 lm thick silicon wafer. After the LPCVD process, the wafer is cleaned with standard
acetone, methanol, iso-propanol and de-ionized water rinse. The
Fig. 1. Sketch of the fabrication process, (a) negative ebeam resist spun on SiN layer, (b) ebeam exposure and development to define patterns in the resist, (c) 20 nm Cr
deposition with ebeam evaporator, (d) hard metal mask formed via Lift-off technique, (e) RIE to transfer patterns into the SiN, (f) KOH wet etching to remove the Si
underneath the PCs, (g) removal of the Cr mask.
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K.-m. Lim et al. / Microelectronic Engineering 88 (2011) 994–998
negative tone ebeam resist, ma-N 2401 (Micro Resist Technology),
is then spin coated at 3000 rpm for 60 s and baked on a hot plate at
90 °C for 2 min. The thickness of resist after spin coating was measured to be approximately 100 nm with a surface profilometer
(Tencor TP-20). The resist layer is then patterned by ebeam lithography. After ebeam exposure the resist is developed (MF CD-26,
Microposit) to form the negative mask pattern. A 20 nm Cr film
is then deposited onto the sample by using an ebeam evaporator.
After Cr deposition, a metal mask is obtained by lifting off the resist
layer with acetone assisted by ultrasonic agitation for 1 h. Once the
Cr etch mask is formed, the pattern is transferred to the SiN layer
using reactive ion etching (RIE). For samples requiring suspended
structures, the sacrificial Si layer is etched for 5 min at 80 °C in a
solution of 20 g of solid KOH in 300 ml DI water. Once the structure
is finalized, the Cr mask can be removed by chemical etchant (Ceric
Ammonium Nitrate and Nitric Acid).
3. Characterization of fabrication
We have carried out the fabrication procedure described in the
previous section and examined the resulting structures. Fig. 2
shows several scanning electron microscope (SEM) images of the
fabrication results taken at various steps in the process. Fig. 2(a–
d) were taken after completion of the steps illustrated in panels
(c), (d), (f) and (g) of Fig. 1. Fig. 2a shows an angle view of cylindrical negative resist patterns on a SiN layer defined by ebeam lithography (RAITH E-Line at 30 kV acceleration voltages). After
development, a 20 nm Cr layer was deposited on the sample, followed by liftoff. Fig. 2b shows a top view image of the Cr mask
on SiN after liftoff, showing that the written pattern has been
cleanly transferred to the Cr mask. Every hole was successfully
lifted off from the structure, and the mask exhibits superb regularity. Fig. 2c shows a cross section of patterned SiN membrane after
RIE and undercut, but prior to removal of the Cr mask. The circular
patterns on the Cr etching mask have been transferred through the
SiN layer using an RIE dry etching process. The cross cut shows a
straight side wall profile and well defined bottom edge of etched
air holes. In Fig. 2d, an SEM image of a final structure after Cr removal is shown, where the image is taken at a 30° tilt angle. Gold
particles were applied on the surface to enhance the SEM image
quality, resulting in a rough surface appearance.
Using this technique we fabricated a series of test patterns of
different sizes to determine the resolution limit of the fabrication
process. Fig. 3a shows an array of air holes etched into the SiN layer
with varying sizes. The 4 4 array consists of groups of air holes
with varying sizes ranging from 250 nm (top left) down to 60 nm
(bottom right) in diameter. Lift-off was successful for every hole
in the structure, including the smallest one. These results demonstrate that liftoff can be reliably applied to structures with many
different disconnected features in a highly repeatable way.
In order to determine the sidewall profiles, we fabricated a series of trenches. Fig. 3b shows the cross sectional profile of the
trenches, obtained by imaging the cleaved edge of the fabricated
sample. The trenches are measured to have widths ranging from
80 nm on the left down to 50 nm on right. The vertical side wall
of the trenches are found to be very straight, even for the thinnest
trenches that are only 50 nm in width, exhibiting a very small aspect ratio. Such aspect ratios would be very challenging to achieve
using only a soft polymer resist mask.
4. Design and fabrication of photonic crystals in silicon nitride
To demonstrate that this procedure can be used to fabricate real
photonic devices, we fabricated a PC optical resonator. The design
of the resonator, illustrated in Fig. 4a, is based on a three holes defect L3 cavity structure [22]. The Q factor of the device was numerically optimized by sequentially shifting three groups of air holes
(labeled A, B, and C in Fig. 4a) and performing Finite Difference
Time Domain (FDTD) simulations. The thickness of the membrane
was set to t = 200 nm while the refractive index of SiN was set to
n = 2.01. The optimal device was attained when the radius of air
holes and lattice constant were set to r = 70.2 nm and
Fig. 2. (a) SEM image of resist pillows on SiN layer after ebeam expose and development. (b) SEM image of top view of Cr mask formed via Lift-off technique. (c) Cross
sectional image of etched holes after RIE. (d) SEM image of 30° angle of fabricated structure after removal of Cr mask.
K.-m. Lim et al. / Microelectronic Engineering 88 (2011) 994–998
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Fig. 3. (a) SEM image showing an array of fabricated air holes with diameter size ranging from 250 nm down to 60 nm. (b) Cross-sectional SEM image of trenches etched into
SiN layer. Trench widths range from 50 nm (rightmost trench) to 80 nm (leftmost trench).
Fig. 4. (a) Schematic diagram of PC cavity (r = 70.2 nm and a = 250.9 nm). The three groups of air holes marked A, B, C are moved outwards respectively. (b) FDTD calculation
of the y component of the electric field of the cavity mode for the hole shifts of 0.07a (A), 0.04a (B) and 0.24a (C). (c) SEM top view of fabricated PC nanocavity. (d) PL spectra
measured from sample displayed in panel (c).
a = 250.9 nm, respectively. A maximum Q of 3280 was attained at a
wavelength of k = 620 nm when the hole shifts for groups A, B and
C were displaced by a distance of 0.07a, 0.04a and 0.24a, respectively. Fig. 4b plots the y-component of the electric field of the cavity mode that achieves the optimal Q.
The optimized PC design was fabricated onto a SiN membrane
using the described fabrication procedure. An SEM image of the fabricated structure is shown in Fig. 4c. To characterize the repeatability and degree of disorder of the fabrication, we measured the radius
of all 122 air holes shown in the panel. The average radius was found
to be 69.9 nm, which is very close to the target value of 70.2 nm,
while the standard deviation was only 1.5 nm. Thus, the fabricated
structure comes very close to the targeted design and exhibits su-
perb hole regularity along the entire PC structure. After fabrication
photoluminescence (PL) measurements of the fabricated SiN PCs
were carried out using a confocal microscope setup. The SiN substrate was pumped with a 532 nm laser at 30 mW pumping power,
producing a weak broadband PL emission that is filtered by the cavity mode. This emission was collected by an objective lens with a
numerical aperture of 0.7and spatially filtered by a 250 lm pinhole
to eliminate unwanted background emission from the surrounding
material. After spatial filtering, the collected light was sent to a high
resolution grating spectrometer (Princeton Instrument SpectraPro
2750) for spectral measurements.
Fig. 4d plots a typical cavity spectrum from a fabricated SiN PC
device. The mode resonance peak is centered at a wavelength of
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K.-m. Lim et al. / Microelectronic Engineering 88 (2011) 994–998
678.39 nm. The full width half maximum (FWHM) linewidth is calculated by numerically fitting the peak to a Lorentzian lineshape,
and is given by 0.462 nm. The Q factor, defined as the center wavelength divided by the FWHM linewidth, is calculated to
be1460.The measured Q factors are lower than those calculated
by FDTD simulations due to a number of practical imperfections
such as air hole roughness and material losses. The optical spectrum alone does not provide enough information to determine
which of these factors is the most significant source of reduction
in Q. From Fig. 3b we see that the fabricated sidewalls are very
straight, while the standard deviation in hole radii for the fabricated structure in Fig. 4c is only 1.5 nm, which is comparable to
hole regularities seen in GaAs and SiPCs. These numbers suggest
that cavity Q is likely not limited by fabrication, and that material
losses potentially play a major role in determining the cavity linewidth. One way to investigate this possibility, and simultaneously
improve the cavity Q, is to use stoichiometric SiN instead of the
low-stress SiN used in this work. Stoichiometric SiN exhibits lower
losses which should result in optical devices that perform much
closer to the ideal limit.
5. Conclusion
In conclusion, we have demonstrated a method to fabricate a
hard Cr mask for nanofabrication using metal liftoff with negative
tone ma-N 2401. The spatial resolution of this technique was demonstrated to be as small as 50 nm. The Cr mask enabled transfer of
the mask pattern into 200 nm of SiN, enabling fabrication of structures with very high aspect ratios. The fabrication procedure was
used to etch PC cavities into SiN that exhibited Q factors as high
as 1460. Although we focused on patterning of SiN, the proposed
fabrication procedure should be compatible with virtually any
material substrate and most types of deposited metals. It thus provides a very versatile technique for high aspect ratio nanofabrication for broad range of applications in photonics.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
S. John, Physical Review Letters 58 (23) (1987) 2486–2489.
T.F. Krauss, R.M. DeLaRue, S. Brand, Nature 383 (6602) (1996) 699–702.
W. Stutius, W. Streifer, Applied Optics 16 (1977) 3218–3222.
N. Daldosso, M. Melchiorri, F. Riboli, M. Giradini, G. Pucker, M. Crivellari, P.
Bellutti, A. Lui, L. Pavesi, Journal of Lightwave Technology 22 (2004) 1734.
D. Amans, S. Callard, A. Gagnaire, J. Joseph, F. Huisken, G. Ledoux, Journal of
Applied Physics 95 (2004) 5010.
F. Iacona, G. Franzo, E.C. Moreira, F. Priolo, Journal of Applied Physics 89 (2001)
8354.
L. Dal Negro, J.H. Yi, V. Nguyen, Y. Yi, J. Michel, L.C. Kimerling, Applied Physics
Letters 86 (2005) 261905.
M. Makarova, J. Vuckovíc, H. Sanda, Y. Nishi, Applied Physics Letters 89 (2006)
221101.
M. Barth, J. Kouba, J. Stingl, B. Löchel, O. Benson, Optics Express 15 (2007)
17231–17240.
M. Barth, N. Nüsse, J. Stingl, B. Löchel, O. Benson, Applied Physics Letters 93
(2008) 021112.
K. Ikeda, R.E. Saperstein, N. Alic, Y. Fainman, Optics Express 16 (2008) 12987–
12994.
J.S. Levy, A. Gondarenko, M.A. Foster, A.C. Turner-Foster, A.L. Gaeta, M. Lipson,
Nature Photonics 4 (2010) 37–40.
Y. Peng, B. Xu, X. Ye, J. N, R. Jia, Z. Wang, Optical and Quantum Electronics
(2009) 41:151–158.
R. Colombelli, K. Srinivasan, M. Troccoli, O. Painter, C. Gmachl, D.M. Tennant,
A.M. Sergent, D.L. Sivco, A.Y. Cho, F. Capasso, Nanotechnology 15 (2004) 675–
681.
Y. Kataoka, M. Kanoh, N. Makino, K. Suzuki, S. Saitoh, H. Miyajima, Y. Mori,
Japanese Journal of Applied Physics 39 (2000) 294–298.
W. Chen, H. Ahmed, Journal of Vacuum Science & Technology, B:
Microelectronics Processing and Phenomena 11 (1993) 2519.
M.A. McCord, R.F.W. Pease, Journal of Vacuum Science & Technology, B:
Microelectronics Processing and Phenomena 6 (1988) 293.
V. Passia, A. Lecestreb, C. Krzeminskib, G. Larrieub, E. Duboisb, J. Raskina,
Microelectronic Engineering 87 (10) (2010) 1872–1878.
H. Yanga, A. Jina, Q. Luoa, J. Lia, C. Gua, Z. Cuib, Microelectronic Engineering 85
(2008) 814–817.
A. Perentes, I. Utke, B. Dwir, M. Leutenegger, T. Lasser, P. Hoffmann, F. Baida,
M.-P. Bernal, M. Russey, J. Salvi, D. Van Labeke, Nanotechnology 16 (2005)
S273–S277.
Y. Poujet, M. Roussey, J. Salvi, F.I. Baida, D. Van Labeke, A. Perentes, C. Santschi,
P. Hoffmann, Photonics and Nanostructures – Fundamentals and Applications
4 (2006) 47–53.
Y. Akahane, T. Asano, B. Song, S. Noda, Nature 425 (2003) 944–947.