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Dual-technology optical sensor head for 3D surface shape
measurements on the micro and nano-scales
Roger Artigas, Ferran Laguarta, and Cristina Cadevall1
Center for Sensors, Instruments and Systems Development (CD6)
Technical University of Catalonia (UPC)
Rambla Sant Nebridi 10, E-08222 Terrassa, Spain
ABSTRACT
New material applications and novel manufacturing processes are driving a systematic rise in market demands
concerning surface inspection methods and the performance of non-contact profilers. However, analysis of the
specifications and application notes of commercial optical profilers shows that no single system is able to offer all the
features a general purpose user would like simultaneously. Whereas white light interferometers can achieve very fast
measurements on the micro and nano-scale without any range limitation, they can not easily deal with steep smooth
surfaces or structured samples containing dissimilar materials. PSI techniques allow the user to perform shape and
texture measurements even below the 0.1 nm scale, but they have an extremely short measurement range. Imaging
confocal profilers overcome most of these difficulties. They provide the best lateral resolution achievable with an optical
profiler, but they have a resolution limit, which is dependent on the NA and cannot achieve the 0.1 nm vertical
resolution. In this paper we introduce a new dual-technology (confocal & interferometer) illumination hardware setup.
With this new sensor head it is possible to choose between standard microscope imaging, confocal imaging, confocal
profiling, PSI and white light interferometry, by simply placing the right objective on the revolving nosepiece.
Keywords: Optical surface metrology, shape measurement, confocal profiler, PSI, white light interferometry.
1. INTRODUCTION
In recent years, there has been considerable competition among the manufacturers of interferometers and confocal
imaging profilers in order to conquer the non-contact surface metrology market. Both systems are capable of accurately
and reliably measuring surface topographies on the scale of millimeters to nanometers. However, the measurement
principles involved in these techniques are very different and, as a result, the capabilities of interferometers and confocal
profilers seem to be more complementary than coincident.
In this paper we will focus on a novel dual-technology (confocal & interferometer) sensor head for 3D surface shape
measurements on the micro and nano-scales. This concept is a real breakthrough in non-contact optical profiling.
In Section 2 we review and compare the specifications of the different image-based profiling technologies: Confocal
Imaging Profilers, Phase Shift Interferometers (PSI) and White light Vertical Scanning Interferometers (VSI). The
analysis of the specifications of the various systems shows that none of the commercial optical profilers is able to offer
all the features that a general-purpose user might require. Whereas white light interferometers can perform very fast
measurements on the micro and nano-scale without any range limitation, they cannot easily deal with steep, smooth
surfaces or stratified samples that contain dissimilar materials. PSI techniques allow the user to perform shape and
texture measurements even below the 0.1 nm scale, but they have an extremely short measurement range, and are subject
to the limitations inherent to all interferometers. Imaging confocal profilers overcome most of these difficulties. They
1 For further information: rartigas@oo.upc.es, laguarta@oo.upc.es, and cristina.cadevall@upc.es
provide the best lateral resolution that can be achieved with an optical profiler. However, vertical resolution, which is
dependent on the NA, is still limited and the 0.1 nm figure cannot be achieved.
In Section 3 we introduce the new dual-technology illumination hardware. This sensor has been developed by UPC-CD6
and Sensofar using microdisplay technology without any moving part inside the measuring head. With this system it is
possible to choose between standard microscope imaging, confocal imaging, confocal profiling, PSI and VSI. The user
simply has to choose the right objective and select the appropriate acquisition and data processing algorithms. Moreover,
the compact design of the sensor head breaks the traditional microscope appearance of most of optical profilers and
makes it possible to build up many different configurations; from the simple stand set-up for R&D and quality
inspection laboratory to the most complete and sophisticated arrangements for on-line process control.
In Section 4 we show several measurements performed with the new optical profiler. The advantages of confocal
technology in combination with the interference techniques provide a unique wide spectrum of applications. The
examples show that the dual-technology sensor head is suitable for repeatable and accurate profiling of very flat and
smooth surfaces with shape features on the nanometer scale, but also for the measurement of very rough surfaces, high
aspect ratio features, steep smooth surfaces and steeply sloping samples. Step height measurements can also be made
using confocal technology in structured or stratified samples containing dissimilar materials.
Finally, some conclusions related to the implementation and the modularity of such an optical imaging profiler are
drawn.
2. TECHNOLOGY AND SPECIFICATIONS REVIEW
2.1 Technology review
Confocal profilers have been developed to measure the surface height of surfaces ranging from smooth to very rough1-4.
The sample is scanned vertically in steps so that every point on the surface passes through the focus. The height of the
surface at each pixel location is found by detecting the peak of the narrow axial response. Because only one or a few
points of the surface are illuminated at the same time, in-plane raster scanning is also required in order to build the axial
response (i.e. the confocal image) at each vertical step.
In an interferometer, a light beam passes through a beam splitter, which directs the light to both the surface of the sample
and a built-in reference mirror. The light reflected from these surfaces recombines and a fringe interference pattern is
formed.
Phase Shift Interferometers (PSI) have been developed to measure the surface height of very smooth and continuous
surfaces with sub-nanometer resolution. The sample, which must be in focus, is scanned vertically in a few steps, which
are a very precise fraction of the wavelength. The profiling algorithms produce a phase map of the surface, which is
converted to the corresponding height map by means of a suitable unwrapping procedure5-7.
White-light vertical scanning interferometers (VSI) have been developed to measure the surface height of smooth to
moderately rough surfaces. Maximum fringe contrast occurs at the best focus position for each point on the surface of
the sample. The sample is scanned vertically in steps so that every point on the surface passes through the focus. The
height of the surface at each pixel location is found by detecting the peak of the narrow fringe envelopes8-9.
2.2 Specifications review
Concerning the microscope objectives available today, confocal profilers offer a very broad range of magnifications
(from 5X to 200X) and numerical apertures (from 0.15 NA to 0.95 NA). Super long working distance objectives (from
20X to 100X SLWD) can also be used to measure high aspect ratio features, large steps and steeply sloping samples. In
PSI and VSI interferometers a range of magnifications (from 1.5X to 50X) and numerical apertures (from 0.02 NA to
0.42 NA) are available. Magnifications higher than 50X can be obtained using Linnik objectives, but they are extremely
expensive and unpractical.
The magnification of the objective used defines the field of view that can be measured on the surface of the sample. To
overcome this limitation on the lateral range, most of imaging profilers can also measure extended topographies beyond
a single field of view by acquiring and stitching several contiguous measurements.
As to lateral resolution, because of the illumination-detection setup, confocal technology provides the highest lateral
resolution achievable by an optical profiler (close to 0.3 µm for 0.95 NA and wavelength of 500 nm). This makes it
possible to reduce the spatial sampling down to 0.10 µm, which is ideal for critical dimensions measurements.
Vertical measurement using confocal and VSI technologies has intrinsically unlimited measurement ranges. In practice,
however, VSI ranges are limited to 1mm. PSI technology has a vertical measurement range limited by the coherence
length of the light source (i.e. just a few micrometers when standard bulb light sources and interference filters are used).
Concerning the vertical resolution and repeatability, in confocal profiling the axial response becomes broader as the NA
decreases. Thus, resolution and repeatability are dependent on the objective used (1 nm repeatability can only be
achieved with the highest available NA). Conversely, PSI & VSI profiling provide the same vertical resolution and
repeatability for all NA (1 nm for VSI and 0.1 nm for PSI). Very low magnifications can be employed to measure large
fields of view with the same height resolution. Independently of the profiling technology, closed-loop z-scan stages are
always necessary in order to achieve the accuracy that provides calibration to traceable standards.
The maximum slope that can be profiled on a smooth surface is a key feature. Because of the high NA (0.95) of the
objectives that can be used, confocal profilers are able to measure smooth surfaces with extremely steep local slopes (up
to 70º). Interferometers can not reach these figures because of: (i) the limitations in the NA of the available objectives,
and (ii) the limitations of the CCD arrays to adequately sample the interference fringes that become closer and closer
together as the local slope of the surface is increased.
Unlike interferometers, confocal profilers can easily deal with flatness assessment and step height measurements in
structured or stratified samples containing dissimilar materials10.
Finally, as for the total measurement time, confocal profilers and interferometers are very similar. Whereas PSI and VSI
techniques are extremely fast in the acquisition, they require more time to focus and level the samples. On the other
hand, using the confocal technique it is very easy and fast to focus the surface and it is not necessary to level the sample,
but acquisition is slower than with the interferometers. In the present, very fast acquisition rates (up to 100 µm/s) are
available in commercial profilers, but these figures can only be achieved at the expense of a significant loss in vertical
resolution.
3. DUAL-TECHNOLOGY ILLUMINATION SETUP
The basic setup of the illumination hardware is shown in Fig. 1a. The picture of the first prototype of the dual-
technology sensor head, which has been developed by UPC-CD6 and Sensofar, is also shown in Fig. 1b. The light
source is a high power LED, which is emitting at a peak wavelength of 480 nm with a lambertian emission pattern. The
light beam coming from the LED is collimated before reaching a polarizing beam splitter cube (PBS). The resulting
polarized beam strikes the ferroelectric liquid-crystal-on silicon (F-LCOS) microdisplay, which is placed on the field
diaphragm position of the optical epi-illuminator setup. The F-LCOS is the key active device of the sensor head and the
information transferred to the microdisplay will be imaged on the surface of the sample by the f/100mm field lens and a
CFI60 infinity-corrected microscope objective. In this standard episcopic arrangement the surface of the illuminated
sample will be also imaged on the CCD array.
When a white image (i.e. full-on state for all pixels of the LCOS) is displayed, the sample is illuminated in the whole
field of view as in a standard microscope. Conventional microscope objectives will provide standard microscope
imaging, whereas interference objectives will provide interference imaging.
LED Light source
480 nm
PBS Microdisplay
CCD
Objective
Fig 1a. Dual-technology illumination setup
Fig 1b. Prototype of the dual-technology profiler
In order to obtain confocal images a binary pattern is displayed in the LCOS and imaged onto the surface of the sample.
The optical setup provides the required matching between pixels of the LCOS and pixels of the CCD array, which in
turn behave as confocal apertures2. The axial response at each pixel position is calculated from the CCD frames using
the appropriate algorithms3,4. Because only one or a few points of the surface are illuminated at the same time, in-plane
raster scanning is necessary to build the axial response (i.e. the confocal image) at all pixel locations. In order to carry
out the raster scan, a sequence of binary patterns is displayed in the LCOS until the complete field of view has been
completely filled. As a result, confocal imaging is slower in comparison with standard or interference imaging because a
sequence of CCD frames must be acquired to build one single confocal image.
Fig 2a. Fixed stand set-up for R&D and quality
inspection laboratories
Fig 2b. Robot-driven setup for on-line process control
As can be seen in Fig 1b, the complete epi-illuminator and the revolving nosepiece with the objectives are mounted on a
vertical motor-driven linear stage. This allows the sensor head to scan the sample vertically in steps so that every point
on the surface passes through the focus. The height of the surface at each pixel location is found by using the profiling
technologies discussed in Section 2.1. The user simply has to choose the right objective and select the appropriate
acquisition and data processing algorithms (confocal, PSI and VSI).
Moreover, the compact design of the sensor head breaks the trend in traditional microscope appearance for most optical
profilers and makes many different configurations possible. Anything from the simple stand set-up for R&D and quality
inspection laboratories, which is shown in Fig.2a, to the most complete and sophisticated arrangements for on-line
process control, such as the robot-driven setup shown in Fig 2b, are now possible.
4. MEASUREMENTS AND APPLICATIONS
As stated above, the advantages of confocal technology in combination with the interference techniques provide a
uniquely wide spectrum of applications. In this section we will show different applications with the aim of revealing the
limits of the different techniques and, at the same time, to demonstrate that they can be complementary solutions in order
to obtain the requested shape data for each sample.
Figures 3 and 4 show the results obtained in very smooth surfaces with nanometric features like a 50 nm step or a 90 nm
dent. In this kind of samples, PSI is the right solution in order to overcome the limitation of VSI and confocal techniques
to achieve resolution and repeatability figures on the sub-nanometric scale. Figures 5 and 6 show two smooth samples
with very steep surfaces, in which confocal is the only applicable technique. Figure 7 shows a confocal measurement of
a structured sample containing three layers of dissimilar materials on a silicon substrate. Confocal results are coincident
within 1% with contact stylus measurements, whereas VSI results show step height errors close to 10%. Finally, Fig. 8
shows very good measurements obtained on a MEM device with both, VSI and confocal techniques. However, because
of the better lateral resolution, confocal technique provides better results in the measurement of the finest lateral
features.
Fig. 3a. Surface of a sample showing a step with a height
of 50 nm. Materials of the top and the bottom are the
same, but roughness of the bottom is slightly higher than
the roughness of the top part of the sample.
Fig. 3b. Measurement of the 50 nm step using a 50X DI
interferential objective and a PSI algorithm.
Fig. 3c. 50 nm step height measurement using a 50X DI
interferential objective and a VSI algorithm.
Fig. 3d. 50 nm step height measurement using a 50X EPI
standard objective and a confocal algorithm.
Fig. 4a. Measurement of a dent on a supersmooth flat
surface using the 10X DI objective and PSI technique.
Fig. 4b. Measurement of the same dent on a supersmooth
flat surface using VSI or confocal techniques.
Fig. 5a. 3D shape measurement of a cylindrical lens
obtained with a 100X EPI objective and confocal technique.
Peak-to-valley is over 10 µm and maximum slope is 45º.
Fig. 5b. Profile of the cylindrical lens. The red arrow
shows the area of the surface that can be measured with a
50X DI objective and VSI technique.
Fig. 6a. Contour (top) and Profile (bottom) displays of an
aspherical microlens measured with a 50X EPI objective
and confocal technique.
Fig. 6b. Contour (top) and Profile (bottom) displays of the
same aspherical microlens measured with a 50X DI
objective and VSI technique.
Fig 7a. Structured sample containing three layers of
dissimilar materials measured with a 50X EPI objective
and confocal technique.
Fig. 7b. Profile of the sample sample showing the different
steps. Unlike VSI measurements, confocal results are
coincident with stylus contact measurements.
Fig. 8a. Contour (top) and Profile (bottom) displays of a
microgyroscope device measured with the 50X DI
objective and VSI technique. The white arrow shows the
position of the profile.
Fig. 8b. The same measurement obtained with the 50X EPI
objective and confocal technique. Note the sharper edges,
which demonstrate the better lateral resolution achieved
with the confocal technique.
5. CONCLUSIONS
A new dual-technology (confocal & interferometer) illumination hardware setup has been developed. This sensor
configuration provides the user with all the available imaging options (standard microscope, confocal and
interferometry), as well as the highest resolution surface profiling techniques (confocal, PSI and VSI), by simply placing
the right objective on the revolving nosepiece and selecting the appropriate acquisition and data processing algorithm.
The advantages of confocal technology in combination with interference techniques provide a unique wide spectrum of
applications. This means a real breakthrough in non-contact optical profiling on the micro and nano-scales.
ACKNOWLEDGMENTS
This research was supported by the Ministerio de Ciencia y Tecnología, Spain. The project (Ref. DPI2002-01018) was
partially financed by the EU-FEDER program. The authors would like to thank Dr. Andrei Skhel from the University of
California, Irvine for supplying the MEMs samples used in Fig. 8.
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