ReflectionÕtransmission confocal microscopy characterization
of single-crystal diamond microlens arrays
E. Gu,
a)
H. W. Choi, C. Liu, C. Griffin, J. M. Girkin, I. M. Watson, and M. D. Dawson
Institute of Photonics, University of Strathclyde, 106 Rottenrow, Glasgow G4 0NW, United Kingdom
G. McConnell and A. M. Gurney
Centre for Biophotonics, University of Strathclyde, 27 Taylor Street, Glasgow G4 0NR, United Kingdom
共Received 4 December 2003; accepted 28 January 2004兲
Using the method of photoresist reflow and inductively coupled plasma dry etching, we have
fabricated microlens arrays in type-IIa natural single-crystal diamond, with diameters down to 10
m. The surface profile of the microlenses was characterized by atomic force microscopy and was
found to match well with a spherical shape, with a surface roughness of better than 1.2 nm. To
characterize the optical properties of these diamond microlens arrays, a laser scanning reflection/
transmission confocal microscopy technique has been developed. This technique enabled the surface
profile of the microlenses to be measured simultaneously with optical parameters including focal
length and spot size, opening up an application area for confocal microscopy. © 2004 American
Institute of Physics. 关DOI: 10.1063/1.1695101兴
Refractive microlenses with diameters of a few mi-
crometers to a few hundred micrometers have received much
attention, due to their numerous applications in, for example,
optical communications, optical data storage, digital dis-
plays, and laser beam shaping.
1,2
Recently, microlenses made
of III-nitride materials and sapphire have become attractive
for such applications as integrating with micro-size light-
emitting diode 共micro-LED兲 arrays
3–6
and vertical cavity
surface-emitting lasers.
7,8
To broaden the scope of applica-
tions of these devices, high-quality microlens arrays with
wide optical transmission bandwidth and high thermal con-
ductivity are urgently required. Here, we report success in
fabricating and characterizing microlens arrays in single-
crystal diamond, with diameters as small as 10
m. These
single-crystal diamond microlenses would have an impact
where optical absorption and scattering need to be mini-
mized and/or maximum thermal conductivity is important.
As well as producing diamond microlenses, we propose and
demonstrate a characterization method based on confocal mi-
croscopy, which rapidly and accurately allows the optical
parameters of the lenses to be determined.
The diamond used in this study is type-IIa natural single-
crystal diamond in platelet form. Type-IIa diamond is virtu-
ally free of nitrogen impurities, and consequently, exhibits
superior optical and thermal properties.
9
The high purity re-
sults in high ultraviolet transmission down to approximately
230 nm and an absence of infrared absorption in the 7–10
m band. At room temperature, type-IIa diamond has an
exceptionally high thermal conductivity, approximately six
times that of copper 共by comparison, type Ia is only twice
that of copper兲.
10
Thus, when integrated with light emitters,
microlenses made of single-crystal diamond can also serve as
excellent heat spreaders to improve the heat dissipation of
the light emitters. Furthermore, in the short wavelength re-
gion 共green/blue to deep ultraviolet兲, type-IIa diamond has a
refractive index close to that of GaN, thus diamond micro-
lenses are a good choice to integrate with micro-size GaN
based photonic devices for short wavelength applications.
In order to precisely and directly characterize these
single-crystal diamond microlens arrays, a reflection/
transmission laser scanning confocal microscopy 共LSCM兲
technique has been developed. This system enabled the sur-
face profile of the microlenses to be recorded simultaneously
with the focal power of the lens. LSCM has been used ex-
tensively in biology, medicine, and materials science due to
its ability to produce extremely high-quality images of speci-
mens, termed optical sections, at various depths.
11
Due to its
point detection properties, confocal microscopy should be
able to measure the light intensity distribution inside and
outside a miniature optical component such as a micro-size
light emitter or lens. For these measurements, confocal mi-
croscopy has the advantage of providing three-dimensional
共3D兲 optical images with a high spatial resolution.
For microlens characterization, it is extremely beneficial
to use both confocal reflected and transmitted light imaging
modes, i.e., to collect reflection and transmission optical sec-
tions simultaneously. The experimental setup of the laser
scanning reflection/transmission confocal microscope is
shown schematically in Fig. 1. Light at 488 nm from a mixed
gas Kr/Ar laser was directed into a commercial 共Bio-Rad
MRC1024ES兲 scan head. The output from the scan head was
then coupled into an upright microscope 共Nikon E600FN兲.A
mirror was placed just below the microscope stage to direct
collimated green light into the back surface of the lens ar-
rays. This light was provided by a 10 mW miniature diode-
pumped, frequency-doubled Nd:YAG laser operating at 532
nm. The output from this laser was expanded 16 times using
a Galilean telescope expander, before being sent into the mi-
croscope. Both the reflected blue and transmitted green light
were collected by the objective lens 共20⫻, 0.75 NA兲 before
passing back through the scan head where the beams were
detected by two photomultipliers after a dichroic mirror. In
a兲
Author to whom correspondence should be addressed; electronic mail:
erdan.gu@strath.ac.uk
APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 15 12 APRIL 2004
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order to ensure that each detector only saw one light source,
narrow passband filters (488⫾ 5 and 540⫾ 20 nm) were
mounted before each detector. Using the pinhole apertures in
the scan head it was possible to reject light from outside the
focal plane. The software control of the system enabled com-
plete 3D images or X–Z optical sections through the lenses
and focused beam to be obtained. In our previous calibra-
tions of the confocal system, the axial and lateral resolutions
had been measured to be 0.8 共⫾0.1兲
m and 0.25 共⫾0.02兲
m, respectively, at a wavelength of 488 nm.
12
Type-IIa natural single-crystal diamond microlens arrays
were fabricated by the technique of photoresist reflow and
inductively coupled plasma 共ICP兲 etching. ICP dry etch tech-
nology allows control of selectivity between the diamond
substrate and the photoresist mask, permitting adjustment of
the lens properties. In this work, high-density O
2
/Ar plasma
gas was used for ICP etching. By optimizing etch param-
eters, a high etch rate of 230 nm/min of diamond has been
achieved. More details of the fabrication process will be re-
ported elsewhere.
A 3D atomic force microscopy 共AFM兲 image of repre-
sentative diamond microlenses fabricated as above is shown
in Fig. 2. The diameter of these particular lenses is 18
mat
substrate surface with a height of 1.5
m. By fitting the
observed lens surface profile, it was confirmed that the mi-
crolenses had a spherical shape. For optical applications, the
microlenses should have a very smooth surface. The AFM
measurements show that the single-crystal diamond micro-
lenses have a rms surface roughness value of 1.2 nm for a
scanned area of 1.0
m⫻ 1.0
m. This measured roughness
is in the same range as for the untreated single-crystal dia-
mond and is much smaller than that obtained from the ICP
etched polycrystalline diamond lenses 共15 nm兲,
13
showing
the high surface quality of these single-crystal lenses.
Using the paraxial ray approximation, the focal length f
of a thin plano-convex lens can be calculated using the rela-
tion
f⫽R/
共
n⫺ 1
兲
, 共1兲
where n is the refractive index of the lens material and R is
the curvature radius. For type-IIa single-crystal diamond, n is
2.42 at a wavelength of 530 nm.
14
The curvature radius R can
be estimated from the measured values of the microlens di-
ameter d at substrate surface and height h via
R⫽
共共
d/2
兲
2
⫹ h
2
兲
/2h. 共2兲
Using this relation, the curvature radius of the micro-
lenses shown in Fig. 2 is estimated to be R⫽ 27.8
m. The
optical parameters of the microlenses can be directly mea-
sured by using the reflection/transmission laser scanning
confocal microscope. Whereas the plane-view-reflection sec-
tions reveal the structural topography of the microlenses, the
plane-view-transmission sections provide information about
FIG. 3. 共a兲 Confocal plane view reflection image of the diamond microlens
array. 共b兲 Confocal plane view transmission section at the focal plane of the
microlens array.
FIG. 1. Schematic diagram of the laser scanning reflection/transmission
confocal microscope.
FIG. 2. A three-dimensional AFM image of diamond microlenses with
diameter 18
m.
2755Appl. Phys. Lett., Vol. 84, No. 15, 12 April 2004 Gu
et al.
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the focal properties of the lenses, such as the light intensity
distribution. A reflection section focused at the substrate sur-
face of the diamond microlens array of lens diameter d
⫽ 18
m is shown in Fig. 3共a兲. Figure 3共b兲 shows the trans-
mission section collected at the focal plane of the diamond
microlens array. It is evident that at this plane, all the lenses
have a sharp focal spot, demonstrating high uniformity of the
single-crystal microlens array. From Fig. 3共b兲, the beam spot
size at the focal point was measured to be ⬃1.4
m.
To fully determine the focal power of the lens, cross-
sectional (X–Z sections兲 reflection and transmission scans
have also been performed. Whereas a cross section of the
microlens is shown in the reflection X –Z section, the trans-
mission image reveals how light rays propagate through and
are focused by the microlenses. For full lens characterization,
it is useful to merge these cross-sectional reflection and
transmission images into one image. Such a merged cross-
sectional image of the diamond microlens array of lens di-
ameter d⫽ 18
m is shown in Fig. 4. It should be noted that
in this image, the vertical and horizontal length scales are
slightly different. It can be seen clearly that collimated light
rays, after they pass through the lenses, converge on a focal
point. The distance from this focal point to the lens is the
focal length of the lens. From this image, the focal length of
the diamond lenses is measured to be 21⫾ 1.0
m. This
measured focal length is quite close to the calculated value
of 19.6
m, confirming that the fabricated diamond lenses
have a high-quality surface profile. A diamond microlens ar-
ray with lens diameter d⫽ 10
m has also been characterized
by the confocal microscope. For these lenses, the measured
focal length is 10⫾ 1.0
m, which also agrees well with the
calculated value. Using this confocal microscope, it is also
possible to measure the optical aberrations of microlenses.
In summary, type-IIa single-crystal diamond microlens
arrays have been fabricated by using the method of photore-
sist reflow and ICP dry etching. AFM measurements show
that these diamond microlenses are of high structural quality.
With excellent optical and thermal properties, these micro-
lenses will enable applications in photonics and optoelec-
tronics. A laser scanning reflection/transmission confocal mi-
croscopy technique was developed to characterize
microlenses. It was demonstrated that this is a tool for char-
acterizing miniature optical components and devices, open-
ing up an application area for confocal microscopy.
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FIG. 4. Confocal reflection/transmission X– Z scan image of the diamond
microlens array.
2756 Appl. Phys. Lett., Vol. 84, No. 15, 12 April 2004 Gu
et al.
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