Design, Fabrication, and Measured Performance of Anti

Document technical information

Format pdf
Size 3.2 MB
First found May 22, 2018

Document content analysis

Category Also themed
Language
English
Type
not defined
Concepts
no text concepts found

Persons

John Mossman
John Mossman

wikipedia, lookup

Organizations

Places

Transcript

SPIE 5786-40
March 29, 2005
Design, Fabrication, and Measured Performance of Anti-Reflecting
Surface Textures in Infrared Transmitting Materials
Douglas S. Hobbs* and Bruce D. MacLeod
TelAztec LLC, 15 A Street, Burlington, Massachusetts 01803 USA
ABSTRACT
Rugged infrared transmitting materials have a high refractive index, which leads to large reflection losses. Multi-layer
thin-film coatings designed for anti-reflection (AR), exhibit good performance, but have limited bandwidths, narrow
acceptance angles, polarization effects, high costs, and short lifetimes in harsh environments. Many aerospace and
military applications requiring high optical transmission, durability, survivability, and radiation resistance, are
inadequately addressed by thin-film coating technology.
Surface relief microstructures have been shown to be an effective alternative to thin-film AR coatings in many infrared
and visible-band applications. These microstructures, etched directly into the window surface and commonly referred
to as “Motheye” textures, impart an optical function that minimizes surface reflections without compromising the
inherent durability of the window material. Reflection losses are reduced to a minimum for broad-band light incident
over a wide angular range. For narrow-band applications such as laser communications, a simpler type of AR surface
structure called a sub-wavelength, or "SWS" surface, is used. In general, both the Motheye and SWS surface textures
will exhibit the same characteristics as the bulk material with respect to durability, thermal issues, and radiation
resistance. The problems associated with thin-film coating adhesion and stress, are thus eliminated by design.
Optical performance data for AR structures fabricated in fused silica, sapphire, Clear ZnS, ZnSe, cadmium zinc telluride
(CZT), silicon, and germanium, will be presented.
Keywords: Anti-reflection, Motheye, Infrared, Rad-hard, Micro-Structures, sapphire, CLEARTRAN, SWS, ZnSe
1. INTRODUCTION
The use of infrared (IR) light in military, industrial, space and commercial applications has expanded significantly in
recent years. Laser communication systems, active and passive imaging sensors, industrial cutting, welding, and
marking lasers, and a variety of security devices, typically require durable infrared transmitting windows and optics
made of materials such as zinc selenide (ZnSe), zinc sulfide (ZnS or Cleartran®), germanium (Ge), sapphire, ALONTM,
silicon, and gallium arsenside (GaAs). In most applications, the region of the IR light spectrum employed is not
absorbed by these materials. However, reflected IR light is a major problem particularly with IR cameras and laser
radar. For example, just one surface of a ZnSe window will reflect 17% of the long wave IR (LWIR – 7 to 14 micron
wavelength) light incident on-axis, a cadmium zinc telluride (CZT) window reflects 21%, and a Ge window or optic
will reflect over 36%. The problem gets worse for IR light incident at higher angles off the normal to the window.
Such large reflections produce stray light and can lead to superimposed images that can reduce the contrast or even
blind security cameras.
The conventional approach to suppressing reflections from optics and windows is to employ multiple thin layers of
dielectric materials deposited onto the external surface of the window or optic. Each deposited layer of material is
designed to affect destructive interference for a particular IR wavelength reflecting from the window or optic surface. A
great number of thin-film layers are needed to increase the range of wavelengths over which reflections are suppressed.
For adequate anti-reflection (AR) over the LWIR range, a typical design would call for as many as 25 layers of material
with a total deposited thickness of over 10 microns. In addition, the performance of thin-film AR coatings is limited to
applications where the IR light is incident along or near the system axis (normal to the window or optic external
surface). For stray light incident off-axis, thin-film coating stacks can produce an increase in reflected light and
undesirable polarization effects.
*
Correspondence: [email protected], Voice: 781-229-9905, Fax: 781-229-2195 www.telaztec.com
SPIE 5786-40
March 29, 2005
Thin-film AR coatings are typically deposited by high temperature evaporation of the coating materials within a
vacuum chamber, a costly process that is problematic for some temperature sensitive materials used in IR cameras.
Durability and thermal cycling are a concern with thin-film AR coatings where inherent stress and adhesion problems
are found due to dissimilar thermal expansion coefficients of the layer materials. Loss of adhesion from temperature
cycling has resulted in catastrophic failure of space-based IR cameras and industrial lasers. Lastly, thin-film AR stacks
suffer from degradation and short lifetimes in the presence of solar radiation – mainly high energy protons. However,
the use of some types of thin-film material layers to absorb high-energy protons, can impart a degree of protection.
2. ENGINEERED SURFACES FOR ANTI-REFLECTION
[1-4]
Surface relief microstructures, commonly known as Motheye textures
, have been shown to be an effective
[5-8]
alternative to thin-film AR coatings in many infrared and visible-band applications
where durability, radiation
resistance, wide viewing angle, or broad-band performance are critical. These microstructures are built into the surface
of the window or optic material, imparting an optical function that minimizes reflections without compromising the
inherent properties of the material. An array of pyramidal surface structures provides a gradual change of the refractive
index for light propagating from air into the bulk optic material. Reflection losses are reduced to a minimum for broadband light incident over a wide angular range. In general, these surface relief structures will exhibit similar
characteristics as the bulk material with respect to durability, thermal issues, and radiation resistance. The problems
associated with thin-film coating adhesion, stress, abrasion resistance and lifetime, are eliminated.
To achieve high performance AR with Motheye surface textures, optical phenomena such as diffraction and scattering
must be avoided. This requires that the surface structures be fabricated with a spacing smaller than the shortest
wavelength desired. In addition, the height of the surface structures should be sufficient to ensure a slowly varying
density change. Motheye textures can be designed to yield reflectance levels of less than one half percent over IR
wavelength ranges from 7 to 14 microns, or from 3 to 12 microns, the ranges needed for advanced multi-color IR
cameras. In addition, Motheye textures can also be designed and fabricated to suppress the reflection of IR light
incident at large angles (45 to 60 degrees or more) with little to no polarization dependence.
As general design guidelines, the relief height of a Motheye
texture should exceed 40% of the longest operational
wavelength, and the distance between structures should be
less than 25 to 30% of the shortest operational wavelength
to avoid free-space diffraction losses. A Motheye texture
designed to suppress LWIR light reflections from a CZT
window used in mercury-cadmium-telluride (HgCdTe) IR
cameras, is shown on the right. These micrographs, taken
by a scanning electron microscope (SEM), show flat-top, or
truncated pyramid structures arranged on a hexagonal, or
honeycomb grid pattern. The feature height is 5 micron and
the grid spacing is 2.4 micron. This Motheye texture
reduces the typical LWIR reflection of 21% for an
untreated CZT window, to a level below 1%. Even further
reductions are possible with a more detailed understanding of the design parameters. TelAztec has designed and built
Motheye textures for near IR communications, mid-wave IR (MWIR - 3 to 5 micron) windows, LWIR filters, and
visible-band prescription eyeglasses, that exceed the performance specifications for the given application.
For narrow-bandwidth applications such as laser
communications, a simpler type of AR surface structure
called a sub-wavelength, or "SWS" surface, is used. An
SWS is a porous texture that reduces the effective
refractive index of the material surface, creating the
equivalent of a textbook quarter-wave AR treatment at the
design wavelength. A typical SWS texture fabricated in
the surface of a fused silica window is shown in the SEM
micrographs on the right. Holes in the surface of the material are proportioned to create an effective refractive index
SPIE 5786-40
March 29, 2005
equal to the square root of the window material refractive index. With fused silica the effective index created is 1.21, a
value that is not attainable with thin-film coating materials. The depth of the holes in the array is set to one quarter of
the design wavelength divided by the effective index. Reflection of NIR light from the surface of the SWS textured
window was reduced to levels below the stringent requirements of optical fiber telecommunications (-30dB, 0.1%).
3. PERFORMANCE MODELING
TelAztec has developed sophisticated computer models to guide the fabrication of, and predict the performance of our
Motheye and SWS AR structures. Using a rigorous vector diffraction calculation, our software can predict the spectral
reflectance and transmittance of infrared light through a user defined three-dimensional surface texture composed of
multiple structured and uniform materials. The model accounts for arbitrary polarization states and light incident
angles. Measured data for the optical constants of a library of materials is included. As further demonstrated below and
in the Measured AR Performance Section 6, our modeled performance has proven to be a good match to measured AR
performance. This ability to predict device behavior and to analyze the impact of fabrication errors is essential to the
practical commercialization of surface structure AR technology.
When working with the high refractive index materials required by IR systems, the cross-sectional profile of the
[6]
structures in the AR surface texture has a significant impact on the AR performance . For example, Figure 1 below
shows the results of performance modeling of three types of AR textures built into the surface of a silicon window used
in a MWIR application. (Note that the model results for GaAs windows are nearly identical). Cross sectional profiles of
each modeled structure are shown to scale for two periods (one period is 780nm) of the full texture. The pyramidal and
sinusoidal structures protrude from the surface whereas the SWS structures are cylindrical holes in the window surface.
Predicted Transmission, %
The transmission of normally incident light propagating from an air environment through the surface texture into the
bulk window material is predicted. For reference, the model for an untreated flat silicon surface predicts a maximum
transmission of 70% into the bulk material. Figure 1 shows that for narrow-band, single wavelength applications, the
SWS textures are the best choice due to their ease of fabrication. The highest performance over the widest bandwidth is
afforded by the sinusoidal textures. To obtain nearly equivalent AR performance to the sinusoidal profile textures, the
pyramidal profile structures need to be fabricated 33% taller.
Diffraction Loss
100
99
Pyramidal
1600nm Deep
Sinusoidal
1200nm Deep
98
97
SWS
450nm deep
96
780nm
95
2500
3000
3500
4000
4500
5000
5500
Wavelength, nm
Figure 1: Predicted transmission of MWIR light propagating from air through
an AR texture into silicon for various profile structures.
Another factor must be considered when designing AR structures for durability in harsh environments. Figure 2 shows
the predicted performance of AR structures fabricated in CZT designed for operation over the LWIR. The predicted AR
performance of textures consisting of truncated pyramidal profile cones are compared to textures made up of parabolic
SPIE 5786-40
March 29, 2005
profile holes. Cross sections of each modeled profile are drawn to scale in the figure. Intuition suggests that due to the
connected web-like nature of the hole structures, the texture may be more resistant to damage caused by extreme
temperature cycling (like that found in space environments). TelAztec is currently investigating this possibility under
an ongoing project sponsored by the MDA.
Predicted Transmission, %
100.0
7
8
9
10
11
12
13
14
Holes
5.5 µm deep
99.5
2.4 µm
2.4 µm
99.0
Posts
5.0 µm deep
98.5
98.0
7
8
9
10
11
12
13
14
Wavelength, µm
Figure 2: Predicted transmission of LWIR light propagating from air through hole- and cone-type AR textures into CZT.
100
Flat-Top Posts
99
900nm
98
900nm
900nm
Predicted Transmission, %
The same reasoning also suggests that hole textures may be less susceptible to abrasion due to the impact of sand and
rain as found with aircraft and automobile windows. Figure 3 shows the predicted performance of hole- and cone-type
Motheye textures fabricated in the surface of a sapphire window. Again the transmission of normally incident light
propagating from an air environment through the surface texture into the bulk window material is predicted. The
window is designed for very broad-band performance over the NIR to MWIR range. For reference, the model for an
untreated flat sapphire surface predicts a maximum average transmission of 92.5% into the bulk material. The models
show that hole-type structures must be fabricated with a larger depth than the cone-type structures to obtain nearly
equivalent performance (cross sections drawn to scale are also shown in the figure). TelAztec is fabricating both holeand cone-type structures in sapphire to determine the surface texture configuration with the largest resistance to
abrasion damage. The Measured AR Performance Section 6 below lists details of this ongoing work.
U-Shaped
Holes
97
1500
2000
2500
3000
3500
4000
4500
Wavelength, nm
Figure 3: Predicted transmission of NIR to MWIR light propagating through hole- and cone-type textures into sapphire.
SPIE 5786-40
March 29, 2005
Predicted Transmittance, %
Motheye AR textures can be designed to operate over an even wider wavelength range. For example, Figure 4 shows
the predicted performance of cone-type structures in ZnSe, ClearTran, and CZT designed to suppress reflections over
both the MWIR and LWIR wavelength ranges. As in the previous examples, the transmission of normally incident light
propagating from an air environment through the surface texture into the bulk window material is predicted. Cross
sections of the truncated sinusoidal profile structures modeled are drawn to scale in the figure. (Hole-type structures
would show a similar predicted performance). The spacing of the structures is set to allow free-space diffraction loss
only at wavelengths below 3 micron. Note that a depth of just 3 microns is sufficient to suppress reflections below 1%
over a 9 micron range spanning 3 to 12 microns.
100
ZnS
Λ =1.2
h=2.8
99
98
97
96
ZnSe
Λ =1.2
h=3.0
95
3
4
1.2µm
CdTe
Λ =1.0
h=2.8
1.0µm
Truncated Pyramid AR Structures
5
6
7
8
9
10
11
12
13
Wavelength, µm
Figure 4: Predicted transmission of MWIR and LWIR light propagating through cone-type textures in CdTe, ZnS & ZnSe.
Many applications require the suppression of reflected light incident at large angles. For example, windows covering
aircraft night vision cameras are often mounted at steep angles due to aerodynamic considerations. At these angles,
stray reflected light becomes a significant problem. Thin-film AR coatings that operate over wide bandwidths and wide
viewing angles do not exist. Motheye AR structures can be designed to meet this demand while simultaneously
providing a longer operational lifetime. Figure 5 shows the predicted performance of a cone-type Motheye texture
etched in one surface of a sapphire window. The transmission of MWIR light propagating from an air environment
through the surface texture into the bulk window material is predicted for angles of incidence (AOI) of ±30°, ±45° and
±60°. A cross section of the truncated sinusoidal profile structure modeled is drawn to scale in the figure. (Hole-type
structures show a similar predicted performance). Depth of the cones was 1.4 micron, and the cone spacing was set at
1.1 micron in the model. For light propagating perpendicular to the window and at angles up to 30 degrees off the
normal to the window, reflected light is suppressed to an average level below 1%. No polarization dependence is
predicted for light incident at 30 degrees or less. As the AOI increases, the effects of polarization become evident,
particularly at the longer wavelengths. (Note that the long-wave absorption in sapphire is not included in the model).
The predicted performance at 45° is degraded slightly for S-polarized light at wavelengths beyond 4.5 micron. For Ppolarized light incident at 45°, the transmission has been enhanced due to the Brewster angle effect (~30°). For an AOI
of 60°, the polarization sensitivity is more pronounced with the amount of S-polarized light transmitted reduced to just
94% at 4.5 micron. Additional modifications to the Motheye structure design can improve the long wave performance
at the cost of a more difficult fabrication process. For comparison, the transmission of MWIR light into an untreated
sapphire window is 92% for light incident from 0 to 30°, 87% for S-polarized light incident at 45°, and just 77% for Spolarized light incident at 60° (the transmission of P-polarized light approaches 100% for AOIs beyond 30°).
SPIE 5786-40
March 29, 2005
Predicted Transmission, %
100
Diffraction Loss
AOI±30°,S or P
99
AOI±45°
P-Pol
AOI±60°
P-Pol
98
97
AOI±45°
S-Pol
1.1µm
96
1.4µm
95
AOI±60°
S-Pol
Truncated, Sinusoidal-Profile Cone Structures
94
2.5
3
3.5
4
4.5
5
Wavelength, µm
Figure 5: Predicted transmission of MWIR light propagating through cone-type textures in sapphire designed
to suppress reflection of light incident at high angles up to 60 degrees off the normal.
4. FABRICATION
Surface relief microstructures designed for high performance AR in the visible or NIR spectral range, are molded
[12]
directly into plastic or sol-gel glass surfaces using high volume replication of master molds . For IR transmitting
materials, lower-volume batch processing is used to directly etch the AR structures into the surface of each window or
optic. Fabrication is then a two-stage process whereby lithography is used to pattern the microstructure, and
conventional etching methods are employed to transfer the patterns into the surface of the final product. (TelAztec
provides AR structure fabrication as a service – at costs similar to thin-film coating services).
Figure 6 shows a process flow diagram typical of Motheye fabrication. The process begins with coating a specified
substrate with a conventional positive photoresist such as one of the AZ1500 series (steps 1 and 2). Next a non-contact,
maskless lithography technique is employed to expose a latent image of the Motheye texture in the photoresist layer
(step 3). The structure lithography is completed by a wet development step that delineates the image as a surface relief
texture in the photoresist layer (step 3).
AR Structure Process
1
Substrate Clean
4
Re-flow(Optional)
2
5
Spin Coat Photoresist
Etch
3
6
Interference Lithography
Strip: Resist Mask
Figure 6: General process flow diagram used in the fabrication of AR microstructures.
SPIE 5786-40
March 29, 2005
TelAztec employs a sophisticated patterning method known as
interference, or holographic lithography (HL) to record Motheye
textures[10,11]. A bench-top HL tool is shown on the right. Multiple
beams of light are split from a laser source (typically emitting in the
blue or violet), expanded and redirected to overlap in a region of space
where the resulting interference pattern can be recorded. The platform
in the lower part of the photograph is illuminated by three overlapping
beams derived from the laser source located at the back edge of the
work-bench. An HL system can pattern large field sizes, limited only
by the size of the beam that can be created, in a single rapid exposure.
Highly uniform Motheye textures have been fabricated over 6-inch
diameter windows. A significant advantage to HL patterning is the very
large depth of field, on the order of inches, which eliminates depth of
focus problems that spherical optics present to conventional lithography
equipment such as image projection steppers and contact mask aligners.
Figure 7 shows multiple SEM images of various micro-structures
recorded in photoresist using an HL system. The left side of the figure
shows both elevation and overhead views of a photoresist mask
containing post-type structures defined on a silicon window intended
for MWIR operation. The structures have a pattern pitch of 780nm and a height of about 650nm. Hole-type structures
in a photoresist mask defined on a ClearTran window intended for LWIR operation are shown in the center area of the
figure. The holes are about 3 micron deep and spaced on a 2.9 micron grid. Lastly on the right side of the figure, posttype structures in resist on a ZnSe window intended for LWIR operation are shown. These posts are about 2.8 micron
high with a grid spacing of 2.4 micron.
Figure 7: SEM micrographs of various photoresist masks used in the fabrication of AR microstructures.
Once the Motheye texture has been recorded in the photoresist mask layer, the mask can be optionally reshaped to suit
the particular process via plasma etching or re-flow (step 4 in Figure 6). Next the photoresist mask is used to transfer
the Motheye texture into the surface of the substrate material using standard dry etching techniques (step 5). If required,
removal of the residual photoresist mask material completes the process (step 6). A thorough understanding of the
interaction between lithography and etching is essential to fabricating antireflective surface structures, as the finished
etched structure will be different than the starting resist profile. This is because the physical and chemical aspects of
etching are quite variable for different materials, and process development must account for this.
SPIE 5786-40
March 29, 2005
5. CHARACTERIZATION
Surface structure Motheye and SWS AR textures are characterized using SEM analysis, FTIR transmission
measurements, and both transmission and reflectance measurements in the NIR using an Agilent optical spectrum
analyzer. In addition, precise microstructure pitch and symmetry are configured using diffraction measurements
obtained with a fiber-coupled white light spectrometer arranged in the Littrow configuration. Data collected for a
variety of AR texture designs is presented next.
6. MEASURED AR PERFORMANCE
6.1 Fused Silica, SiO2, (n = 1.46)
Fused silica is a high purity glass that has extensive use at telecommunication wavelengths. Many communications
products require a specific wavelength or narrow wavelength band within the NIR range from 1520 to 1620nm. High
performance SWS structures have been designed and fabricated in fused silica targeting specific wavelengths within this
spectral region. These AR surfaces were engineered with a symmetric hexagonal grid of holes etched to a specified
design depth, effectively mimicking an ideal thin film AR coating, as shown in the SEM images included in Figure 8.
Figure 9 shows both theoretical and experimental data for SWS structures fabricated in fused silica windows targeting
lossless performance at 1530nm (a product specification of less than 0.05% reflectance). Four of the windows
fabricated are shown in the inset photograph. The measured performance is shown as the solid dark line in the figure
(an absorption band of fused silica is seen as the dip in transmission centered around 1390nm). For comparison, the
dashed line in the figure shows the predicted performance of an idealized single-layer thin-film AR coating. The thinfilm model and measured SWS performance show good agreement, illustrating that SWS structures can be designed to
mimic single-layer quarter-wave thin-film coatings – but with a key advantage that SWS structures can be fabricated in
any substrate without the material constraints found with thin-film AR coatings. Note that by changing the depth of the
SWS structures, the peak performance wavelength can be shifted to meet any target, and that as with quarter-wave thinfilm AR coatings, harmonics of the target wavelength can be produced (dual-band AR).
Figure 8: SEM photographs of SWS-type AR textures fabricated in a fused silica window.
SPIE 5786-40
March 29, 2005
97.0
Transmission, %
Max Transmission
Ideal AR One Side
96.5
Measured
Transmisson
SWS 19
96.0
95.5
Thin-Film Model:
Idealized n=1.21
Material, 1/4-wave
thick (305nm)
95.0
1100
1200
Fused Silica
1300
1400
1500
1600
1700
1800
Wavelength, nm
Figure 9: Measured reflectance of SWS AR textures fabricated in a fused silica window.
6.2 Sapphire, (n = 1.70)
Sapphire is a hard durable optical window material used in infrared applications out to 5.5 microns. Sapphire can
withstand large thermal shock loads and mechanical stresses, while providing protection against high velocities,
abrasion (sand, rain, hail), and high temperatures. Military applications for sapphire are numerous and include missile
domes, MWIR windows, and lightweight transparent armor. The ultimate goal of producing AR structures in sapphire is
to replace thin-film AR coatings, which have survivability issues in harsh environmental conditions. It is expected that
Motheye textures will maintain their AR function much longer than thin-film coatings in abrasive environments, and
thus provide a greater operational lifetime for a window or optic. This expectation of minimal transmission loss for
Motheye structures subjected to rain and sand erosion, is a common concept referred to as graceful degradation- the
Motheye post structures that are chipped by physical damage remain sub-wavelength and non-scattering, and retain a
significant portion of their AR function[9,13-15]. With thin-film AR coatings, physical damage and de-lamination often
leads to significant scattering losses. Standard rain and sand erosion tests on Motheye AR textures in sapphire will be
made this spring.
Motheye AR structures were fabricated in sapphire intended for use in the NIR to MWIR region. Typical etched
surface structures, shown in the SEM images of Figure 10, were designed with a blunt tip profile for optimum
durability. The pattern period is 780nm and the structure depth achieved to date is 530nm (the goal is 800nm).
Transmission measurements given in Figure 11 show an average transmission increase of over 5% in the NIR region at
1550nm (upper chart), but only about 3% in the MWIR region (lower chart). This compares to a maximum
transmission increase of about 6.5% as indicated by the solid gray curves in each plot. The measured transmission of a
sapphire window (2mm thick) with no AR treatment is also included in each plot.
SPIE 5786-40
March 29, 2005
Measured Transmission, %
Figure 10: SEM photographs of AR textures fabricated in a sapphire window.
Max Transmission With
Idealized AR One Side Only
93
92
91
Diffraction Loss
S16
Motheye AR
One Side Only
90
89
88
Untreated Sapphire, 2mm
87
86
1200
1250
1300
1350
1400
1450
1500
1550
1600
1650
1700
Measured Transmission, %
Wavelength, µm
94
Estimated Maximum
Transmission With
Idealized AR
One Side Only
93
92
S16 Motheye AR
One Side Only
91
90
89
88
87
86
Untreated Sapphire, 2mm
85
84
2
2.2
2.4
2.6
2.8
3
3.2
3.4
3.6
3.8
4
4.2
4.4
4.6
4.8
5
Wavelength, µm
Figure 11: Measured transmission through a sapphire window with a Motheye AR texture fabricated in one surface only.
SPIE 5786-40
March 29, 2005
6.3 Zinc Sulfide, ZnS (Cleartran®), (n = 2.22)
Clear ZnS, known by its trade name of Cleartran®, exhibits low absorption and scatter throughout its broad
transmission range from 400nm to 12 microns. Cleartran® is well-suited for multi-spectral applications that require a
single aperture for multiple wavelength bands, such as simultaneous infrared and visible imaging, along with target
designation. We have fabricated Motheye structures in Cleartran® for use as a durable optical window in the LWIR
region. The etched surface structure is shown in Figure 12 and was designed with a blunt tip profile for increased
durability. The pattern period is 2.9 microns and the structure has a depth of 3.4 microns. Transmission measurement
taken on the sample shows a flat 2% reflectance loss across the 8 to 12 micron region in the LWIR. (Even lower
reflectance losses are expected when the structure height reaches 3.8 microns.) Figure 13 shows this transmission
measurement over the wavelength range of 7.5 to 12.5 micron. The solid black line is the measured data, the solid gray
line is the maximum transmission attainable with zero reflection from one surface, and the dashed line is the
transmission of a 2 mm thick Cleartran® window with no AR treatment.
Measured Transmission, %
Figure 12: SEM photographs of AR textures fabricated in a Cleartran® window.
88
Diffraction Loss
Max Transmission With
Idealized AR One Side Only
86
84
82
80
ClrTrn M11a
Motheye AR
One Side Only
78
76
74
72
Untreated ClrTrn, 1mm
70
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
Wavelength, µm
Figure 13: Measured transmission through a Cleartran® window with a Motheye AR texture fabricated in one surface only.
SPIE 5786-40
March 29, 2005
6.4 Zinc Selenide, ZnSe, (n = 2.43)
ZnSe is commonly used for high power CO2 laser focusing lenses, night vision FLIR applications, and infrared
spectroscopy windows. Motheye structures were fabricated in ZnSe windows designed for LWIR imaging applications.
Figure 14 shows the hexagonal-grid array of pyramidal structures in the ZnSe surface that comprise the Motheye
texture. The texture has a structure spacing of 2.9 micron and structure height of 4.0 microns. Figure 15 shows broadband AR performance from 7 to 14 microns, with an average reflectance loss of less than 0.5% in the 8 to 12 LWIR
band. Even higher AR performance could be obtained for single wavelength applications, such as with high power CO2
laser optics, using an SWS AR structure. ZnSe is also a good candidate for demonstrating very wide-band Motheye AR
performance. Work is under way to produce ZnSe Motheye windows capable of AR performance over both the MWIR
and LWIR bands using the design of Figure 4 above.
Measured Transmission, %
Figure 14: SEM photographs of AR textures fabricated in a ZnSe window.
84
Max Transmission With
Idealized AR One Side Only
Diffraction Loss
80
ZnSe#7 Motheye AR
One Side Only
76
72
Untreated ZnSe, 2mm
68
7
8
9
10
11
Wavelength, µm
12
13
14
Figure 15: Measured transmission through a ZnSe window with a Motheye AR texture fabricated in one surface only.
SPIE 5786-40
March 29, 2005
6.5 Cadmium Zinc Telluride, CZT, (n = 2.67)
Cadmium Zinc Telluride (CZT) has been the substrate of choice for the epitaxial growth of HgCdTe, a critical detector
material for a wide range of infrared applications. HgCdTe detectors are typically used in a backside-illuminated
configuration where the detector array is bump bonded onto silicon readout circuitry. This requires the incident infrared
flux to pass thru the transparent but highly reflective CZT substrate. AR treatments are required on the CZT backside to
minimize signal loss and crosstalk. We have designed and fabricated Motheye AR surfaces in CZT substrates for
broadband LWIR imaging applications.
A Motheye structure was designed and fabricated with a structure spacing of 2.4 microns and structure height of 4.0
micron depth into a CZT substrate for operation in the 7.5 to 13 micron range. Again, the etched surface structure, as
shown in the SEM images of Figure 16, was designed with a blunt tip profile for increased durability. Transmission
measurements, shown in Figure 17 show minimal loss of incident radiation at the Motheye surface, with an average loss
of 1% across the broad 8 to 13 micron band in the LWIR, a significant improvement over the 21% loss for an untreated
surface.
Figure 16: SEM photographs of AR textures fabricated in a CZT window.
Measured Transmission, %
82
Max Transmission With
Idealized AR One Side Only
80
78
76
CZT #20
Motheye AR
One Side Only
74
72
70
68
66
Untreated CZT, DSP
64
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
Wavelength, µm
Figure 17: Measured transmission through a CZT window with a Motheye AR texture fabricated in one surface only.
SPIE 5786-40
March 29, 2005
6.6 Silicon, Si, (n = 3.42)
Silicon is a low cost optical material primarily used for MWIR windows and lenses. Silicon has a high refractive index,
which makes it attractive for lenses, but also makes AR treatments more critical. Motheye AR structures have been
fabricated in silicon for MWIR applications. The high index of silicon requires a Motheye design with a period much
smaller than the wavelength range of interest to avoid diffractive effects, which results in a high aspect ratio structure.
Figure 18 shows a silicon Motheye surface with an 800nm square grid period and 1800nm structure depth.
Transmission data for the sample from 2 to 6 microns is shown in Figure 19, with an average loss of 1% within the
target 3 to 5 micron spectral region. A slightly deeper structure would be required to get a flatter and lower loss
response at the longer wavelengths, where the performance is beginning to trail off.
Measured Transmission, %
Figure 18: SEM photographs of AR textures fabricated in a silicon window.
75
Diffraction
Loss
Max Transmission With
Idealized AR One Side Only
70
65
Silicon #15
Motheye AR
One Side Only
60
55
Untreated Silicon, DSP
50
2.5
3
3.5
4
4.5
5
5.5
Wavelength, µm
Figure 19: Measured transmission through a silicon window with a Motheye AR texture fabricated in one surface only.
SPIE 5786-40
March 29, 2005
6.7 Germanium, Ge, (n = 4.01)
Germanium is a preferred lens and window material for high performance infrared imaging systems in the LWIR. Its
high refractive index is ideal for imaging systems because of reduced surface curvature requirements. However, the
index also results in high surface reflections, resulting in the need for high performance AR treatments. Preliminary
fabrication and etching work with Ge has focused on an SWS structure. Figure 20 shows an etched SWS structure in a
Ge window. The etch depth has reached the target of 1250nm, however the hole diameter has been etched larger than
the target which reduces the effective index of the surface and results in the observed performance shift to shorter
wavelengths. The transmission of the SWS textured Ge window is shown in Figure 21. The reflection loss at 8.2
microns is only 1.5%, a significant improvement over an untreated Ge surface that losses 36% to reflection. Internal
process development is ongoing to optimize the structure for LWIR applications.
Measured Transmission, %
Figure 20: SEM photographs of LWIR AR textures fabricated in a Ge window.
66
Max Transmission With
Idealized AR One Side Only
64
62
60
58
Diffraction
Loss
56
Ge #14B
SWS AR
One Side Only
54
52
50
48
Untreated Ge, 1mm
46
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
Wavelength, µm
Figure 21: Measured transmission through a Ge window with an SWS AR texture fabricated in one surface only.
SPIE 5786-40
March 29, 2005
7. SUMMARY
Engineered AR surface structures can be an attractive alternative to thin-film coatings for many applications requiring
mechanical durability, high laser damage thresholds, and wide-band performance. The practical design of Motheye and
SWS AR textures have been discussed for a variety of important infrared transmitting materials. Interference
lithography is utilized to pattern the repetitive structures because of the large field size, large depth of focus, and high
process throughput characteristics of the technique. Motheye and SWS AR structures were fabricated in materials used
throughout the infrared spectrum. SEM images of the structures are shown and discussed in relation to optical
performance and durability. Transmission data is presented for each material, with experimental results closely
matching the predicted performance. The wide-band AR performance presented demonstrate that rugged and durable
surface structures can replace thin-film AR coatings in applications where coatings fail to address performance or
durability requirements.
8. ACKNOWLEDGEMENTS
Janos Technologies of Vermont has generously supplied ZnSe, ClearTran, Ge, and sapphire substrates for our internal
Motheye development work. The work with CZT is funded under a 2004 Phase I SBIR sponsored by The Department
of Defense, Missile Defense Agency, Wright Patterson Air Base. All SEM analysis was performed by Mr. John
Knowles at MicroVision Laboratories, Inc., (781-272-9909).
9. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Bernhard, C. G., "Structural and functional adaptation in a visual system", Endeavour, 26, pgs. 79-84, 1967
Clapham, P.B. and Hutley, M.C., "Reduction of lens reflexion by the 'Moth Eye' principle", Nature, 244, 281-2,
Aug. 3, 1973)
Cowan, J. J., “Holographic Honeycomb Microlens”, Opt. Eng. 24, pp. 796-802 (1985)
Thornton, B.S., "Limit of moth's eye principle and other impedance-matching corrugations for solar-absorber
design." JOSA, Vol. 65, No. 3, pgs 267-270, March 1975
Wilson, S.J. & Hutley, M.C., "The optical properties of 'moth eye' antireflection surfaces", Optica Acta, Vol. 29,
No. 7, pgs 993-1009, 1982
Southwell, W. H., "Pyramid-array surface-relief structures producing antireflection index matching on optical
surfaces", JOSA A, Vol. 8, No. 3, pgs 549-553, March 1991
Raguin, D.H. & Morris, G.M., "Antireflection structured surfaces for the infrared spectral region", Applied Optics,
Vol. 32, No.7, pg 1154, March 1993
DeNatale, J. F., et. al., "Fabrication and characterization of diamond moth eye antireflective surfaces on Ge", J.
Appl. Phys.,71, (3), pg1388,Feb.1992
Harker, A.B. and DeNatale, J.F., "Diamond gradient index 'moth-eye' antireflection surfaces for LWIR windows.",
SPIE Vol. 1760, Window and Dome Technologies and Materials III, pgs. 261-267, July 1992
Cowan, J. J., U.S. Patent 4,496,216, “Method and Apparatus for Exposing Photosensitive Material” (Jan. 29,
1985).
Hobbs, D.S., et. al., “Automated Interference Lithography Systems for Generation of Sub-Micron Feature Size
Patterns”, SPIE Conference on Micromachine Technology for Diffractive and Holographic Optics, Proc. SPIE,
Vol. 3879, September 1999, pg 124-136
MacLeod, B.D., and Hobbs, D.S., “Low-Cost Anti-Reflection Technology For Automobile Displays”, Journal of
the Society for Information Display, Automotive Display Conference, November 2004.
Weimer, Wayne A. and Klocek, P. "Advances in low-cost long-wave infrared polymer windows.", SPIE Vol.
3705, Window and Dome Technologies and Materials VI, pgs. 276-281, July 1999
Harker, A.B., DeNatale, J.F. et. al. U.S. Patent 5334,342, August 2, 1994
Hobbs, D.S., and Dorschner, T.A., “Fourier Lithography: A New Manufacturing Tool for Optics”, Workshop For
Electro-Optics Manufacturing Science and Technology, Army Night Vision Labs, 95-498, May 1995

Similar documents

×

Report this document