Diamond and Related Materials 10 Ž2001. 557᎐560
CVD-diamond optical lenses
E. Woerner U , C. Wild, W. Mueller-Sebert, P. Koidl
Fraunhofer-Institut fur
Tullastr. 72, 79108 Freiburg, Germany
¨ Angewandte Festkorperphysik,
¨
Abstract
Diamond lenses are highly desirable for a variety of optical applications including the operation under high-power, high
thermal load and under harsh environmental conditions. We report on the realization of CVD-diamond lenses by deposition on
structured substrates. Lenses with diameters between 2 and 5 mm and focal lengths between 3.2 and 5.2 mm were prepared.
Substrates with a spherical surface or with an array of spherical impressions have been used. CVD-diamond of high phase purity
was deposited on these structured substrates using microwave plasma CVD. The growth side of the diamond layer was polished
and the substrate removed. Laser cutting separated the lenses. The plano-convex CVD-diamond lenses prepared with this
‘molding’ technique were characterized regarding flatness and spherical aberration of the individual surfaces as well as optical
and thermal properties. 䊚 2001 Elsevier Science B.V. All rights reserved.
Keywords: Diamond properties and applications; Optical lenses; Molding technique
1. Introduction
Diamond lenses were already made in 1824 by Andrew Pritchard w1x. In those days diamond lenses were
needed for microscopes w2x making use of the large
refractive index of diamond. However, when compound
lenses became available in 1835, the interest in diamond lenses faded. With the development of CVDtechniques to produce flat diamond wafers and the
increasing demand of optics for high-power radiation,
diamond optics became again a topic of high interest.
Nowadays, CVD-diamond with properties matching
those of the best natural diamonds w3᎐5x is available
and used for a variety of optical applications, e.g. as a
window material for high-power lasers or microwave
generators Žgyrotrons..
Since polycrystalline CVD-diamond is by far more
difficult to machine than single crystalline diamond,
U
Corresponding author. Tel.: q49-761-5159-384; fax: 49-761-5159400.
E-mail address: woerner@iaf.fhg.de ŽE. Woerner..
CVD-diamond focusing optics have not yet been realized although affordable diamond lenses would be
highly desirable for a variety of applications. In order
to fabricate diamond lenses while avoiding the difficulties related with polishing spherical diamond surfaces, we have deposited CVD-diamond on substrates
with spherical impressions. Since the CVD-diamond
substrate side is a perfect copy of the substrate w6,7x,
plano-convex diamond lenses have been obtained by
polishing the CVD-diamond growth side and removing
the diamond structure from the substrate.
2. Preparation of CVD-diamond optical lenses
To prepare CVD-diamond optical lenses, first the
substrate has to be machined. An array of spherical
impressions can be formed in various substrates including silicon either by mechanical polishing or by photolithography and ion milling. We used grinding and
polishing techniques to form the structured substrates.
The spherical impressions were between 2 and 5 mm in
0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 5 - 9 6 3 5 Ž 0 0 . 0 0 3 9 3 - 9
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E. Woerner et al. r Diamond and Related Materials 10 (2001) 557᎐560
Fig. 3. Two-inch CVD-diamond wafer with diamond lenses from 3 to
5 mm in diameter and sag heights up to 380 ␮m.
Fig. 1. Processing steps to prepare CVD-diamond optical lenses.
3. Surface characterization
diameter and had a curvature radius of 4᎐7 mm. This
corresponds to a sag height of the individual lens
performance of up to 380 ␮m. Between 70 and 500
structures fit on the 2-inch substrate that is usually
used for the preparation of CVD-diamond lenses.
After substrate preparation, CVD-diamond of high
phase purity is deposited ŽFig. 1b.. For the deposition
of thick optical grade CVD-diamond a microwaveplasma-CVD reactor with ellipsoidal cavity w8x, that
provides stable deposition conditions, is used. With
standard growth conditions, we use a microwave power
of 6 kW at a frequency of 2.45 GHz, temperatures
between 700 and 900⬚C and pressures ranging from 100
to 200 mbar. The feed-gas is 1᎐2% methane in hydrogen. After deposition, the growth side of the diamond
wafer is polished and the diamond film is removed
from the substrate ŽFig. 1c᎐d.. In a final step the
individual lenses are separated using a Nd:YAG-laser
ŽFig. 1e..
Fig. 2. Highly transparent CVD-diamond lenses with focal lengths
ranging from 3.2 to 5.2 mm.
Figs. 2 and 3 show photographs of a highly transparent 2-inch CVD-diamond wafer containing diamond
lenses with diameters between 3 and 5 mm and radii of
curvature ranging from 4.5 to 7.3 mm.
Optical lens applications require low spherical aberration. To investigate surface profile and roughness, a
laser-profilometer ŽUBM Expert. has been used. Fig. 4
shows the height profile of a perfectly radially symmetric diamond lens with 7.3 mm radius of curvature. The
height difference between contour lines is 10 ␮m.
In Fig. 5 a line scan through the center of the lenses
and a numerical fit to a spherical surface is shown. The
deviation between the measured profile and a perfect
spherical surface is below 1% for all lenses measured.
Fig. 4. Contour map of a CVD-diamond lens surface Žradius of
curvature: 7.3 mm. measured with a laser-profilometer. The height
difference between contour lines is 10 ␮m.
E. Woerner et al. r Diamond and Related Materials 10 (2001) 557᎐560
Fig. 5. Surface profile of three plan-convex CVD-diamond lenses
Ž=.. For comparison, the solid lines represent a perfect spherical
surface with 7.3, 5.1 and 4.5 mm radii, respectively.
The roughness of both the curved surface as well as the
flat surface is below 5 nm Žrms..
4. Thermal and optical characterization
Like CVD-diamond windows, CVD-diamond lenses
can be used as an optical component for high-power
CO 2-lasers. To avoid a distortion of the transmitted
wave front due to thermal lensing, this application
requires both a low absorption coefficient, and a high
thermal conductivity w4x. Since the techniques we used
to measure these properties require planoparallel
559
CVD-diamond films, the measurements were not made
at the lenses themselves, but at planoparallel material
surrounding the lenses.
For the thermal characterization a laser-induced
thermal transient technique was applied. This uses a
pulsed Nd:YAG-laser focused at the front side of the
titanium-coated sample to launch a thermal wave. Its
transient is measured optically with an MCT-detector
that is focused to the rear side of the sample. Like the
front side it has to be coated since diamond is highly
transparent in the IR. The temperature transient is
detected at several radial displacements between
laser-focus and detection point. To derive the thermal
conductivity of the sample, the measured transients are
fitted with an analytical expression taking into account
the finite pulse duration and focal diameter as well as
the sample thickness.
Reference samples of the lens material reached a
thermal conductivity of 20.8 WrcmK. This is almost as
high as the best values reported for natural type IIa
diamond Ž20᎐25 WrcmK w3x.. To measure the IR-absorption coefficient we used an improved calorimetric
technique described elsewhere w9x. With 0.06 cmy1 at
10.6 ␮m the IR-absorption coefficient is nearly as low
as that of type IIa diamond Ž0.03᎐0.05 cmy1 w4x..
5. Applications
By growing high quality CVD-diamond on substrates
with spherical impressions, affordable diamond optical
lenses with excellent thermal and optical properties are
Fig. 6. CVD-diamond lenses used as tips for length gauges.
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E. Woerner et al. r Diamond and Related Materials 10 (2001) 557᎐560
becoming available. These lenses can be used for a
variety of applications. Typical examples are focusing
lenses for fiber-optic-laser-surgery w10x that are transparent not only for the CO 2-cutting-laser but also for
the HeNe-positioning-laser. Besides purely optical applications, many mechanical or mechano-optical applications are possible. Frequent mechanical applications
where natural diamond lenses have been used so far
are tips for length gauges to be used under highly
abrasive conditions. These tips can be most easily produced by using CVD-diamond lenses as shown in Fig.
6.
Acknowledgements
The authors would like to thank P. Ruther of
Freiburg University for the laser-profilometer measurements.
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