Investigating the Use of Replica Morpho Butterfly Scales

UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
1
Investigating the Use of Replica Morpho Butterfly
Scales for Colour Displays
Rebecca E. Coath
Abstract—The reflective properties and bright colours of butterfly wings are of great interest in many applications. The
brilliant blue of the Morpho genus is due to multilayer interference effects originating from tiny lamellae on ridges present
on the butterfly’s scales. Diffraction also occurs due to the ridges
themselves. The combination of these two effects leads to a high
intensity reflection spectrum with a strong angular dependency.
In this paper, the physics behind the structure is first discussed,
followed by an introduction to and simulation of a replica
structure which mimics the Morpho butterfly’s scale structure.
By varying the periodicity of the replica ridge structures, many
different colours can be produced, demonstrating the potential
of these structures for use in applications such as high intensity
colour displays.
I. I NTRODUCTION
This work details the investigation and analysis of the
structural colour of Morpho butterflies, and the design and
fabrication of replica structures for use in colour displays.
Male Morpho butterflies are bright blue iridescent butterflies, whose colour has been reported to be so intense that they
can be seen from low flying aircraft or “up to a quarter of a
mile off” in their natural habitat in the rainforests of South
America [1]. In order to achieve such a bright colour, the
butterfly incorporates structural colour via corrugated ridges
present on the scales of its wings, which will be described in
detail in Sec. II.
The investigation and reproduction of animal structural
colour is part of the field of biomimetics, where natural
phenomena are replicated by humans for innovative designs.
Such mimicry has already been implemented in the replication
of tiny cones found on moth’s eyes for use as antireflective
surfaces to improve the efficiency of solar cells [2]. This work
discusses the replication of the scale structure of the Morpho
Rhetenor butterfly using a simple cleanroom process involving
multilayer deposition and a combination of anisotropic and
selective isotropic etching.
An analysis of both the butterfly and the fabricated structures was performed via rigorous coupled wave analysis
(RCWA) and is presented in Sec. IV. This method can determine the reflection spectrum of any arbitrarily sized diffraction
grating with few or zero approximations [3].
Finally, the results of the analysis are used in Sec. VI
to determine the suitability of the replica structures for use
in colour displays by discussing their benefits over existing
technologies.
II. T HE M ORPHO B UTTERFLY - I NTERFERENCE AND
D IFFRACTION C OMBINED
Morpho butterflies are bright blue iridescent butterflies
found in the rainforests of South America, with wingspans be-
tween 10cm and 12cm. It is believed that the butterflies utilise
their high intensity colouration for polarization-dependent long
range communication, or in selecting a mate [4]. A picture of
a Morpho Didius butterfly, displaying the bright blue colour
of its wings, is given in Fig. 1.
Fig. 1.
Morpho Didius Butterfly
The wing material (or cuticle) is a dull brown, indicated by
the ventral side of the wing, suggesting that the bright blue
arises not from a pigment but from a unique structure present
on the scales, consisting of an array of ridges with asymmetric
corrugated edges, forming lamellae on either side of the ridge.
This leads to a tree-like cross-section as depicted in Fig. 2. The
three SEM images show the scales present on the wing (2a),
their ridge structure (2b), and a horizontal view showing the
tree-like cross-section (2c), whilst (2d) is a diagram of the
tree-like cross-section highlighting its main features. Typical
values of the parameters illustrated in this figure are given in
Table I [5]–[7].
This structure produces a combination of three effects;
multilayer interference, diffraction, and scattering. Multilayer
interference is the source of the blue colour present on the
wing and is due to the interference between the lamellae
and the air gaps inbetween them, whilst diffraction from the
periodic ridge array aids to broaden the angular dependency of
the colour and make it polarization-dependent [8]. Scattering is
also present due to irregularity of the height of the structures,
resulting in the disruption of coherence between ridges and
producing uniform colour [9].
Two layers of scales are present in the butterfly’s wings; the
ground scales, responsible for the colour, and the transparent
cover scales, which act as optical diffusers for the incident
light and broaden the angular dependency even further [10].
The ridge structure in question is present on the ground scales,
and is comprised of between 2 and 12 thin lamellae running
along both sides of the length of each ridge, tilted upwards
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
2
TABLE I
TABLE S UMMARISING M ORPHO R HETENOR S TRUCTURE D IMENSIONS
[5]–[7]
a
b
c
θi θr
Γ
Parameter
Dimensions
Lamella thickness (d2)
62nm
Air gap thickness (d1)
145nm
Lamella length (l)
(from centre of ridge)
308nm
Lamella periodicity (d1+d2)
207nm
Width of centre of ridge (w-2l)
60nm
Total ridge width (w)
676nm
Number of lamellae on each side of
ridge
8 (16 lamellae in total)
Total ridge height (h)
1801nm
Ridge periodicity (Γ)
746nm
Refractive index (n2)
(both substrate (scale) and ridge)
1.56+0.06i
Tilt Angle of Lamellae (Φ)
<20°
at a slight angle from the surface of the scale. The lamellae
are positioned asymmetrically around the centre of the ridge,
separated by a netting of trabeculae to form the tree-like crosssection [5]. The periodicity of the ridges lies between 300nm
and 2000nm, with a total ridge width of between 500nm and
700nm, but despite this, little variation is seen in the colour
of different species of Morpho butterfly; it is the intensity of
the colour which varies, from a high intensity blue to a pearly
blue-white hue [11].
The lamellae are approximately 60nm in thickness, with a
spacing of 200nm [5]. The lamellae and the spacings between
them act as a multilayer structure leading to constructive
and destructive interference from the reflections between each
lamella-air interface as shown in Fig. 3. The refractive index of
the cuticle is approximately 1.56 + 0.06i [6], whilst that of air
is 1, and it is this difference in refractive index which allows
the bright blue colour to be produced and high intensities to
be reached due to the number of layers involved and the high
refractive index contrast.
Φ
d2
h
d1
n1
no
l
θi
n2
w
n1
d
Fig. 2. SEM Images of Morpho Butterfly. a) Scale Structure b) Ridges
Present on Scales c) Close-up Showing Tree-Like Cross-Section at Ends of
Scales d) Diagram of Tree-Like Cross-Section showing Relevant Parameters
Fig. 3.
θr
d
Constructive Interference in a Multilayer Structure
For constructive interference at a wavelength λ, the following condition must be met by both of the materials used in
the multilayer stack,
mλ = 2n1 d cos θr
(1)
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
where m = n + 1/2 (where n is an integer) and represents
the diffraction order, n1 is the refractive index of the material
and d its thickness, and θr is the angle of refraction in the
material, given by Snell’s law (n0 sinθi = n1 sinθr , where n0
is the refractive index of the incident medium).
This expression can be substituted into Eq. 1 to obtain
the wavelength for constructive interference as a function of
incident angle,
p
(2)
mλ = 2d n1 2 − n0 2 sin θi
Si
SiO2
Si3N4
Resist
Diffraction is also present on the wing due to the periodicity
of the ridge structures, which can be modelled by Eq. 3 for a
reflection grating,
mλ = d(sin θi − sin θd )
3
1.Silicon
substrate
4.Expose
resist
2. Deposit
60nm Si3N4
followed by
80nm SiO2;
repeat five
times (Total
height 700nm).
5.
Anisotropic
etch to form
ridges
3. Spin-coat
with resist,
ready for
transferring
ridge pattern
6. HF etch
SiO2 to
form treelike crosssection
(3)
where m in this case is an integer, d is the periodicity of the
grating and θd is the diffracted angle. Light incident on the
grating is split into various diffracted orders indicated by m,
dividing it into its component colours. If the periodicity is
varied, the grating will start to display longer wavelengths at
smaller angles of diffraction.
III. R EPLICA M ORPHO B UTTERFLY S TRUCTURES F OR
FABRICATION USING S IMPLE C LEANROOM P ROCESSING
T ECHNIQUES
Eq. 1 and Eq. 3 predict that by altering the periodicity
or thickness of the lamellae on the ridges of the Morpho
butterfly’s scales, varying the refractive index of the multilayer
material or altering the ridge parameters, a change in reflected
colour can be observed. This shows potential for the structure
to be reproduced and optimized for use as pixels in highintensity colour displays.
The Morpho butterfly structure has already been reproduced
by Watanabe et al [12], who recreated the structure using
diamond-like carbon via a process incorporating focused ion
beam chemical vapor deposition which can be used to create
a wide range of “nano-objects”. However, this technique is
fairly complex and not widely available.
This work describes a much simpler and more widely
available processing technique of multilayer deposition and
etching for replicating the Morpho structure. A test wafer was
produced using this technique, which concentrates on mimicking the effects of the multilayer structure alone. It ignores
complex effects such as that of the cover scales to diffuse the
light, irregularity in ridge height, and the asymmetry of the
lamellae to increase colour uniformity.
The fabrication technique in question involves deposition
of 60nm of silicon nitride followed by 80nm of silicon
dioxide repeated five times (to give ten layers in total) on
a silicon wafer, followed by an electron-beam lithography and
anisotropic etch technique to create a ridge structure and a
selective isotropic etch of the silicon dioxide to form a treelike cross section. The use of multilayers allows both the
interference effect to be created as well as the formation of
the cross-sectional shape by selective etching. A diagram of
the fabrication process is given in Fig. 4.
Using this process, multilayer interference effects will occur
between the air and the nitride layers, and between the oxide
Fig. 4.
Fabrication Process Used to Create the Replica
and the nitride. Silicon dioxide has a refractive index of
approximately 1.56, whilst silicon nitride has a refractive index
of approximately 2.09. This results in a difference in refractive
index between each of the interfaces mentioned.
As the number of deposited layers is large, high intensity
reflection can be obtained if the thickness of each deposited
layer is carefully chosen via use of Eq. 1. For an oxide
thickness of 80nm, at normal incidence for m = 1/2, the
wavelength for constructive interference is 4nSiO2 dSiO2 =
499.2nm and for a nitride thickness of 60nm, 4nSi3 N4 dSi3 N4
= 501.6nm. The resulting multilayer stack should produce a
peak in reflection in the blue-green region, shifting to shorter
wavelengths as the angle of incidence increases.
As mentioned, the colour of the Morpho butterfly also
comprises diffraction, which broadens the angular distribution
of the bright colour. The lithography and anisotropic etch stage
was used to define ridges in fourteen regions of 1mm squares
on each quarter of the test wafer at periodicities ranging
between 330nm and 2500nm and for two ridge widths of
400nm and 500nm. This equated to fill factors (the ratio of
ridge width to periodicity) of between 0.2 and 0.8. The designs
were labelled (in order of decreasing periodicity) 31, 32, 33,
34, 35, 21 and 22 for the 500nm wide ridges, and 11, 12,
13, 14, 15, 25 and 24 for the 400nm wide ridges. A table
containing the design parameters for each square is given in
Table II.
The isotropic etch defines the lamellae on the ridges by
selectively etching the oxide layers laterally using a 20:1
HF solution. The resulting gaps between the ridges would
also lead to multilayer interference similar to that of the
Morpho butterfly, and so the width of these gaps would also
be important in determining the structural colour. To a rough
approximation, around the region close to each structure,
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
TABLE II
TABLE S UMMARISING D ESIGN PARAMETERS
Design
Number
Periodicity
(nm)
Ridge Width
(nm)
Fill Factor
(ridge width/
periodicity)
Layers
only
-
-
1
31
2500
500
0.2
32
1670
500
0.3
33
1250
500
0.4
34
1000
500
0.5
35
830
500
0.6
21
720
500
0.7
22
630
500
0.8
24
500
400
0.8
25
570
400
0.7
11
2000
400
0.2
12
1330
400
0.3
13
1000
400
0.4
14
800
400
0.5
15
670
400
0.6
4
could be obtained, analysed and compared to the simulation
results given in this paper.
a
b
the refractive index of each layer should be dependent on
the fill factor of the grating, and the colour reflected by
the structure should change. Each quarter of the wafer was
therefore isotropically etched for a different amount of time
to study the effects for increasing lamella lengths. One quarter
was left as a ridge structure with no selective isotropic etching,
and the other three quarters were labelled A, B and C and
etched to produce lamellae of 50nm, 100nm and 150nm in
length respectively.
Unfortunately, the test wafer exhibited some undesirable
effects following fabrication. One example is the formation
of tapered ridges during the anisotropic etch rather than
the desired square ridges. The tapered shape of the ridges
would lead to a slower change in effective refractive index
between the incident medium and the structure, resulting in
lower reflectivity. In some areas, the ridges were too close
together to form correctly during lithography, leading to ridges
merging into each other and destroying the diffraction effect.
The test wafer also showed formation of unwanted oxynitride
during fabrication. This prevented the isotropic etch from
successfully occurring. This typically occurred during the
switching between oxygen and nitrogen during the fabrication
process. Under a more controlled technique, this could easily
be avoided. Pictures of the test wafer are shown in Fig. 5.
In the following sections, the desired “ideal” structures
rather than those produced on the test wafer will be analysed.
Although experimental analysis was undertaken on the test
wafer, the reflectivity of each design was extremely low, and
the processing technique would have to be improved and any
unwanted effects removed before actual experimental results
c
d
Fig. 5. SEM Images of Test Wafer Structures. Although larger periodicities
such as in a) (design C35) are formed well, those with smaller periodicities
such as in b) (design A24) were not as successful, although, following the
isotropic etch, these malformed designs did begin to separate into ridges, as
shown in c) (design C24), which also demonstrates the desired isotropic etch
beginning to appear on the sides of the ridges. d) (design b34) demonstrates
the typical tapering of the cross-section of the ridges due to overetching.
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
1.0
IV. A NALYSIS OF THE ACTUAL AND R EPLICA M ORPHO
B UTTERFLY S CALE S TRUCTURES
1 http://software.kjinnovation.com/GD-Calc.html
Morpho Structure
Grating Only
Multilayer Only
P Polarization
Reflectivity
0.8
0.6
0.4
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
1.0
Morpho Structure
Grating Only
Multilayer Only
S Polarization
0.8
Reflectivity
Each of the designs mentioned in Sec. III was characterized
by simulation using GD-Calc, a MATLAB-based Grating
Diffraction Calculator1 . This program incorporates the technique of rigorous coupled wave analysis (RCWA), which
can calculate the diffraction efficiencies for each diffraction
order for any arbitrarily sized grating, with few assumptions.
The basic method involves dividing a grating into a number
of thin “slabs”, computing the electric field in the incident
medium and substrate, and expanding the fields inside each
slab in terms of the space harmonics of the fields inside the
periodic grating structure to form the rigorous coupled wave
equations [3]. The solution of these equations correspond to
diffracted orders outside of the grating.
In GD-Calc, P polarization (parallel to the plane of incidence) is taken as being perpendicular to the ridges, and
parallel to the ridges for S polarization (perpendicular to the
plane of incidence). In the results which follow, the reflection
for both P and S polarizations for each design are plotted
for normal incidence (unless otherwise stated) up to the third
diffraction order. The reflectivity is expressed as the reflection
coefficient for either polarization, and the overall reflectivity
is defined as the average reflectivity from both polarizations.
A simplified version of the Morpho butterfly structure,
which excluded the irregularity present in the actual butterfly
scale, was first simulated. The simplified structure assumed
that the lamellae were symmetrically distributed about the
centre of the ridge, that the overall structure was rectangular
in shape and that all ridges were of identical height. All
parameters for this simulation were taken from Table I. A
multilayer structure consisting of air and cuticle multilayers
(impossible to recreate in real life due to the lack of structural
support) and a diffraction grating structure formed purely of
cuticle and of the same width as the actual Morpho ridges were
modelled alongside the Morpho structure to determine where
the shape of the reflection spectrum arose from. The results
from GD-Calc for normal incidence are given in Fig. 6.
Fig. 6 shows that the Morpho’s reflection spectra is greatest
between 430nm and 580nm, corresponding to the colours
blue and green. The peak of the reflection spectrum has been
specified as approximately 450nm for Morpho Rhetenor [11],
agreeing well with this result. Any difference is most likely
due to the omitted effects of irregularity in the simulated
structure compared to that of the butterfly itself.
It is also interesting to note that the multilayer structure
is the dominant source of colour for the Morpho butterfly,
with diffracted colours playing a negligible part in the overall
reflection spectrum. The reflection spectrum of the ridge
structure itself is negligible in comparison, with an overall
reflection varying between 0.04 and 0.06.
The Morpho genus over time has gradually adapted itself
to optimising its multilayer structure whilst maintaining structural support by utilising a grating-like structure with a very
high grating fill factor. However, the use of a periodic array of
5
0.6
0.4
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
Fig. 6. GD-Calc Results for Simplified Morpho Ridge Structure, Multilayer
Structure and Diffraction Grating Structure
ridges is not purely to provide as much multilayer structural
coverage as possible over the entire scale.
Fig. 7 shows the differences between the multilayer structure and that of the Morpho butterfly. It can be seen that
although for shallow angles of incidence the reflectivity of
the multilayer structure is the greatest, the Morpho structure
maintains its high reflectivity for larger angles of incidence.
The reflection spectrum shifts to shorter wavelengths for both
structures for increasing angle of incidence, as expected from
Eq. 1. It is clear from this diagram that introduction of a
diffraction grating aids to increase the angular dependency
of the structural colour, whilst the multilayer structure is
responsible for the overall colour and high reflectivity.
Following the simulation of the Morpho structure, the
multilayer structure formed from alternating layers of nitride
and oxide was analysed using GD-Calc with and without a
silicon substrate for a wide range of angles. The results are
given in Fig. 8.
The peak of the multilayer structure alone lies in the bluegreen region with a peak at 500nm as predicted from Eq. 1.
Addition of a substrate material shifts and splits this peak
almost to the extent of producing peaks where troughs formed
in the reflectivity of the stack alone. These peaks reside
at approximately 425nm and 550nm, corresponding to blueviolet and green. The angular dependency of a multilayer
stack is also shown in this figure, with the reflectivity strongly
decreasing for angles larger than 45o .
Following characterization of the multilayer structure, design 21 was analysed to determine the typical reflectivity
expected from the process described in the previous section.
This design has a ridge periodicity of 720nm and a ridge
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
width of 500nm, with a high fill factor (0.7) and is the
closest design in its dimensions to the Morpho structure. The
refractive indices of silicon, silicon dioxide and silicon nitride
were found from tabulated values2 .
The ridge structure’s reflection spectrum for both polarizations at normal incidence, and for a wide range of incident
angles is given in Fig. 9.
1.0
0
15
30
45
60
75
Morpho Butterfly
Overall Reflectivity
0.8
6
0.6
0.4
0.8
P Polarization
S Polarization
Multilayer
0.2
0.6
0.45
0.50
0.55
0.60
0.65
0.70
Reflectivity
0.0
0.40
Wavelength (µ m)
1.0
0
15
30
45
60
75
Multilayer Structure
Overall Reflectivity
0.8
0.4
0.2
0.6
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength(µ m)
0.4
0.8
0.2
0
15
30
45
60
75
0.7
0.45
0.50
0.55
0.60
0.65
Overall Reflectivity
0.6
0.0
0.40
0.70
Wavelength (µ m)
Fig. 7. GD-Calc Results for Morpho Ridge Structure and Multilayer Structure, Demonstrating Angular Distribution for Increasing Angles of Incidence
0.5
0.4
0.3
0.2
0.1
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
1.0
0
15
30
45
60
75
Multilayer with Substrate
Overall Reflectivity
0.8
Fig. 9.
0.6
0.4
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
1.0
0
15
30
45
60
75
Multilayer Only
Overall Reflectivity
0.8
0.6
0.4
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
Fig. 8.
GD-Calc Grating for Multilayer Structure Both With (top) and
Without (bottom) a Silicon Substrate
GD-Calc Grating for Design 21 Prior to Isotropic Etch
Fig. 9 shows that the shape of the reflectivity corresponds
closely to that of the multilayer structure at this periodicity,
similar to what was observed for the Morpho structure. There
is also a strong angular dependency, with the reflectivity barely
changing in magnitude for large angles of incidence.
The reflectivity was seen to deviate for different periodicities, with different peaks appearing in the spectrum or
peaks shifting as the periodicity varied, but in no definite
direction. This suggests that different colours are appearing as
the periodicity is varied. Fig. 10 shows this effect occurring
for designs 31, 33, 35 and 22, and 11, 13, 15 and 24 (in order
of decreasing periodicity).
The reason for the variation can be explained as follows.
For the Morpho structure, diffraction appeared to play a
negligible part in the overall reflected intensity due to the
continuation of the ridge material into the substrate. A change
in periodicity should not make a difference to the colour of
the Morpho structure, and many of the Morpho species exhibit
this difference in periodicity, but display similar hues [11].
For the replica structures, this is different. The substrate is
of a different material with a higher permittivity than the
ridge structures, increasing the magnitude of the diffracted
orders substantially as a further component of reflection is
2 http://www.luxpop.com/RefractiveIndexList.html
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
1.0
0.8
Multilayer
31
33
35
22
0.4
Ridge
50nm etch
100nm etch
150nm etch
No oxide
P Polarization
0.8
Reflectivity
Reflectivity
0.6
0.2
0.0
0.40
7
0.6
0.4
0.2
0.45
0.50
0.55
0.60
0.65
0.0
0.40
0.70
0.45
0.50
0.60
0.65
0.70
1.0
0.8
Multilayer
11
13
15
24
0.4
Ridge
50nm etch
100nm etch
150nm etch
No oxide
S Polarization
0.8
Reflectivity
0.6
Overall Reflectivity
0.55
Wavelength (µ m)
Wavelength (µ m)
0.6
0.4
0.2
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
Fig. 10. GD-Calc Results for Designs 31, 33, 35 and 22, and 11, 13, 15 and
24 Showing Varying Reflection Spectra
incorporated. The diffraction efficiencies for a pure oxide or
nitride grating were calculated to be in the region of 0.3-0.4,
a vast difference to that of the Morpho butterfly.
It can also be seen in Fig. 10 that the difference in ridge
width does not play a significant part in altering the overall
reflectivity for large periodicities (designs 31 and 11), but
as the periodicity decreases, the reflection spectrum changes
shape and decreases slightly in reflectivity for designs 1324 compared to designs 33-22. It appears that the second
peak present at between 500-650nm is more affected by the
difference in ridge width than the first peak between 400500nm. This suggests that the first peak is due to the multilayer
structure alone, whilst the second peak is a result of strong
diffracted orders, which would vary with a change in the
parameters of the diffraction grating. The results show a strong
dependency on the diffraction grating itself, showing that a
change in parameters leads to a change in colour.
The effect of an isotropic etch to produce multilayer interference between the nitride and air interfaces is demonstrated
in Fig. 11 for the three intended isotropic etch depths of 50nm,
100nm and 150nm on design 21. Also plotted on this graph
is the ridge structure with the oxide layers replaced by air
gaps to produce the structure with the greatest refractive index
contrast, and the ridge structure prior to isotropic etching. Note
that as the top layer of the stack is an oxide layer, this is also
removed during an isotropic etch, and has been taken into
account in the simulation.
It can be seen in this figure that the deeper the etch, the
higher the reflectivity. A shift to shorter wavelengths also
appears to occur. This is because the air-nitride interface has a
higher refractive index contrast than the oxide-nitride interface,
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
Fig. 11. GD-Calc Results for Design 21 For Increasing Isotropic Etch Depths
leading to higher reflectivity as the etch depth increases. The
angular dependency is just as broad as the unaltered ridge
structure for increasing angles of incidence, as demonstrated
by Fig. 12, where it can be seen that the reflectivity remains
high over a broad range of incident angles.
This is a promising result, as it shows that the reflectivity of
the structure increases as the isotropic etch depth increases due
to the introduction of a multilayer stack with a higher refractive
index contrast, but with no visible loss of angular dependency,
giving high reflectivity for a large range of incident angle.
V. O PTIMIZATION OF THE M ULTILAYER S TACK
It has been found so far that the introduction of air into
the multilayer stack increases the reflectivity due to the higher
refractive index contrast observed. However, the width of the
air gaps in this multilayer stack are not optimized for the
highest reflectivity at a given wavelength. In the Morpho
structure, the width of the air gap is more likely to be
optimized to give the highest reflectivity possible. This is not
a trivial task, and requires repetitive rigorous coupled wave
analysis to be performed correctly and the results analysed
for each trial width, as Eq. 1 is not sufficient to optimise the
multilayer stack here due to the presence of the diffraction
grating.
However, the replica designs can be optimized in one simple
way from those presented in this report. The optimization
requires the stack order to be reversed, starting with an oxide
layer on the substrate instead of a nitride layer. This will allow
the substrate to form part of the stack. Currently, as nitride
is used as the bottom layer, the refractive index increases
(from oxide to nitride to silicon) leading to an unwanted
180o phase shift between the nitride and silicon and causing
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
1.0
Multilayer - Oxide First
Multilayer - Nitride First
0.8
Overall Reflectivity
0.8
Overall Reflectivity
1.0
0
15
30
45
60
75
50nm Wet Etch
0.6
0.4
8
0.2
0.6
0.4
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.0
0.40
0.70
Wavelength (µ m)
0.45
0.50
0
15
30
45
60
75
100nm Wet Etch
0.60
0.65
0.70
1.0
Replica - Oxide First
Replica - Nitride First
0.8
Overall Reflectivity
0.8
Overall Reflectivity
0.55
Wavelength (µ m)
1.0
0.6
0.4
0.6
0.4
0.2
0.2
0.0
0.40
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
1.0
0
15
30
45
60
75
150nm Wet Etch
0.8
Overall Reflectivity
0.0
0.40
Fig. 13. GD-Calc Results for Multilayer Stack and Design 21 with Reversed
Stack Ordering
0.6
0.4
0.2
0.0
0.40
Col 14 vs Col 15
Col 16 vs Col 17
Col 18 vs Col 19
Col 20 vs Col 21
Col 22 vs Col 23
Col 24 vs Col 25
0.45
0.50
0.55
0.60
0.65
0.70
Wavelength (µ m)
Fig. 12. GD-Calc Results for Design 21 For Different Isotropic Etch Depths
with Increasing Angle of Incidence
destructive interference to occur between the reflected light
from the nitride-substrate interface and that of the bottom
oxide-nitride interface, which dominates the reflectivity over
the constructive interference of the light reflected from the
multilayer stack. This leads to peaks in the reflectivity roughly
where troughs were initially expected from Eq. 1. This is the
cause of the shifted spectrum observed in Fig. 8.
If the stack order is reversed, no unwanted phase shift
will occur, constructive interference will occur between the
reflected light of the nitride-oxide interface and that of the
oxide-substrate interface, leading to higher reflectivity and
peaks appearing as expected from Eq. 1.
The results of reversing the stack ordering on the multilayer
structure alone and on design 21 following the 150nm isotropic
etch are clearly shown in Fig. 13. Reversing the stack ordering
also increases the number of multilayers in the replica structure
as the top layer is no longer an oxide layer subjected to the
isotropic etch. An increase in the number of layers generally
results in an increase in the overall reflectivity of a multilayer
structure.
The Morpho butterfly also incorporates irregularity such as
asymmetry and a random ridge height distribution to aid its
angular dependency and overall colour. This could be a future
continuation for this work, and could be implemented via use
of an uneven substrate prior to fabrication or changing the
order of processing steps to produce asymmetric structures.
This will give a more uniform colour appearance due to
scattering from the irregularities present in the structure similar
to the uniform colour exhibited by the butterfly itself.
VI. A PPLICATION OF M ORPHO B UTTERFLY R EPLICA
S TRUCTURES IN C OLOUR D ISPLAYS
A typical colour display consists of an array of pixels in
groups of threes. In the case of LCD screens, one of these
pixels is capable of transmitting green light, one blue, and
one red. Colour is a perception of the observer, and colours
can easily be mixed to form others.
The results show that the ridge structures in this work
can alter the colour based on their periodicity alone, unlike
the Morpho butterfly. This aspect is very promising for the
application of colour displays. If the peaks in the reflection
spectrum can be varied and understood, the three colour pixels
required for colour displays should be relatively simple to
fabricate.
Control of the pixels, and the intensity of their reflection, is
harder to realise. Qualcomm3 have produced a colour display
based on multilayer structures alone, and vary the intensity
of the pixels via an air gap sandwiched between a thin
voltage-controlled membrane. When a voltage is applied to
3 http://www.qualcomm.com/technology/imod/index.html
UNIVERSITY OF SOUTHAMPTON, SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE, MAY 2007
the membrane, it is attracted to the thin film stack, changing
its reflectivity [13]. A similar idea could be applied to the
Morpho butterfly pixels to control their reflectivity.
Another alternative is to vary the refractive index of the
incident medium. It has been found that when a droplet
of colourless alcohol is placed on the wing of the Morpho
Rhetenor butterfly, the alcohol replaces the air in the system
and modifies the interference conditions, resulting in a bright
green colour [14]. If a liquid was found which could shift the
main reflection peak into the non-visible parts of the spectrum,
this could possibly be used to turn pixels on and off.
The benefits of this technology for use in colour displays
has been widely promoted by Qualcomm, who claim that
their device consumes little power, is high-contrast, has a
high switching speed (useful for videos), and only requires
a light when the device is in dark conditions, unlike LCDs
which require a consistent back light. Currently, Qualcomm
have produced only small displays suitable for use in mobile
phones and similar screen sizes, but are looking to increase
the size of their screens in the near future to produce large,
high intensity colour displays. Information about the future of
this technology can be found on their website.
If Morpho butterfly scale structures are to be implemented
for use in colour displays, they may provide a means of
increasing the angular dependency of the reflectivity compared
to Qualcomm’s displays, as multilayer structures alone, as
demonstrated in Sec. IV, do not exhibit a wide angular
dependency, with the reflectivity decreasing for increasing
angles of incidence.
VII. C ONCLUSIONS
It has been found that the idea of recreating the structures
responsible for the bright iridescent blue of Morpho butterflies
is perfectly feasible and looks very promising as an alternative
colour display, provided sufficient means of control can be
realised. The simulations show that the resulting structures
from this process are able to incorporate both interference
from multilayer structures and diffraction dependent on the
periodicity of the ridges present in the designs as a means of
producing colour, rather than producing colour via multilayer
interference alone and using diffraction just as a means of
increasing the reflectivity for large angles of incidence. Such
suggestions can only be verified experimentally or by further
simulations at this stage, but appear to be reflecting different
colours based on the combination of these two effects to the
naked eye from a top-down perspective.
The structures are subject to many improvements. In future
work, reversing the order of the multilayers will give a higher
reflectivity by avoiding destructive interference between the
bottom of the multilayer stack and the substrate. The multilayer structures themselves may benefit from multilayer stacks
with a thinner oxide layer, so that following the isotropic etch,
the dominant source of the multilayer reflection will arise from
and be optimized for the nitride-air interface rather than the
nitride-oxide interfaces, giving a larger contrast in refractive
index and increasing the reflectivity. These improvements were
demonstrated in Sections IV and V. Prior to any of these
9
improvements, the processing technique must first be perfected
to produce structures which agree with the simulation results
presented here.
Overall, the results appear to demonstrate that the Morpho
butterfly scale structure can be replicated by the proposed
cleanroom process, and by alteration of the periodicity of
the ridge structures, different peaks in the reflection spectrum
appear, leading to different colours. Intriguing ideas of how to
proceed with this work have arisen from the results presented
here, and further optimization could lead to a highly reflective,
tuneable pixel for use in colour displays.
ACKNOWLEDGMENTS
The author would like to thank Dr. Barbara Cressey and
Shuncai Wang of the Electron Microscope Centre in the
Department of Chemistry, University of Southampton, for use
of the JSM 6500F FEG-SEM which was used to take the SEM
images presented here, and Dr. Darren Bagnall for providing
the Morpho Didius specimen for analysis. Thanks also to
Graham Ensell and David Sager for all their help in producing
the test wafer, and to Stuart Boden for use of his template for
the wafer layout.
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