Japanese Journal of Applied Physics
Vol. 45, No. 3B, 2006, pp. 2368–2371
#2006 The Japan Society of Applied Physics
Measurement of Cantilever Displacement Using a Compact Disk
/Digital Versatile Disk Pickup Head
En-Te HWU1;2 , Kuang-Yuh H UANG1 , Shao-Kang H UNG2;3 and Ing-Shouh H WANG2 1
Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan
Institute of Physics, Academia Sinica, Taipei, Taiwan
3
Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan
2
(Received July 4, 2005; revised November 13, 2005; accepted December 2, 2005; published online March 27, 2006)
We use the optical pickup head of a commercial compact disk (CD)/digital versatile disk (DVD) read only memory (ROM)
drive to detect the vertical displacement of micro fabricated cantilever in atomic force microscopy (AFM). Both the contact
and AC modes of AFM are demonstrated. The single atomic steps of graphite can be resolved, indicating that atomic
resolution in the vertical displacement detection can be achieved with this new setup. The low cost, compact size, and the light
weight of CD/DVD optical pickups may offer new advantages in future AFM designs. [DOI: 10.1143/JJAP.45.2368]
KEYWORDS: CD, DVD, AFM, cantilever
1.
Introduction
In atomic force microscopy (AFM),1) the interatomic
forces between the tip and the surface induce a vertical
displacement of microfabricated cantilever. Currently, two
major optical methods, the beam deflection method2–4) and
interferometry,5–7) are adopted in the detection of this
displacement, because they both have a sensitivity better
than 0.1 nm. In this study, we demonstrate for the first time
that AFM using a commercial compact disk (CD)/digital
versatile disk (DVD) read only memory (ROM) optical
pickup is also sufficiently sensitive for detecting single
atomic steps of graphite surfaces. This implies that the
optical system in the CD/DVD pickup head may have a
sensitivity comparable to that of the beam deflection method
and interferometry in terms of the displacement measurement of the cantilever. Most of the current AFM designs
adopt the beam deflection method, which makes the optical
detection system bulky, heavy, and awkward to use.
Commercial CD/DVD pickups are small, compact, lightweight, reliable, and economical, and thus may greatly
simplify the design of future atomic force microscopes.
Inside an optical pickup of a CD/DVD-ROM drive, a
laser beam emitted from a laser diode is first diffracted and
collimated into three beams, which are then focused onto an
optical disk by an objective lens controlled by a voice coil
motor (VCM).8) The beams reflected from the disk are then
directed back through the pickup and, after passing through
an astigmatic element, impinge onto a photo detector
integrated chip (PDIC). The PDIC is composed of six split
photosensors A–F, with the center having four quadrant
photosensors (A–D) similar to the quadrant position-sensitive detectors used in standard atomic force microscopes,
and six current preamplifiers for the six photosensors. The
detection of the focusing condition of the laser beam on the
optical disk adopts the astigmatic method, which uses a
cylindrical lens as a beam-shaping device.9) When the laser
beams are perfectly focused on the disk surface, the laser
spot on the four central quadrant photosensors (A–D) is
circular [Fig. 1(b)], but when the disk surface is slightly
higher or lower than the focus of the laser beam, the laser
spot on the central quadrant sensors appears more elongated,
Corresponding author. E-mail address: [email protected]
Fig. 1. Shape of laser spot and S-curve of FES. (a) Off focus, FES < 0.
(b) On focus, FES ¼ 0. (c) Off focus, FES > 0. (d) Linear region of FES.
as illustrated in Figs. 1(a) and 1(c). The shape change of the
laser spot on the PDIC can be detected by the focus error
signal (FES), which is defined as ðSA þ SC Þ ðSB þ SD Þ,
where SA –SD are the preamplifier output voltages of photosensors A–D, respectively. For a circular laser spot, FES ¼
0. Figure 1(d) shows the well-known S-curve for the
relationship of the FES vs the vertical distance of the
disk.10) By measuring the FES, the VCM can drive the
objective lens to make the laser beam focused on the disk
surface (i.e., FES ¼ 0). In this work, we demonstrate that the
linear region of FES (6 mm) can also be used to detect the
vertical displacement of the AFM cantilever with a high
sensitivity.
2.
Experimental
In our experiments, we modified a commercial scanning
probe microscope (SPM; Nanoscope III, Digital Instrument)
by replacing its beam deflection module with a slim-type
CD/DVD pickup (TOP1100S, Topray Technologies) and a
home-made probe clamping module (PCM). The laser
module of the pickup emits light at two wavelengths of
655 and 790 nm, used for DVD and CD, respectively. Here,
we use only the laser beam of 655 nm, because it is visible
and the focus spot is smaller than that of the other beam. The
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Fig. 2. (a) Experimental layout of SPM and optical sensor system. (b) Schematic of PCM.
experimental layout is shown in Figure 2. The position of
the VCM is fixed by gluing it to the frame of the pickup. The
pickup laser beam reaches the cantilever through an
objective lens (a working distance of 1.28 mm). The axis
of the impinging laser beam is perpendicular to the cantilever surface. The PCM is very carefully attached to the
pickup, so that the focus spot of the laser beam is right on the
back surface of the cantilever. Before gluing our PCM to the
pickup, we ensure that the FES, X (defined as SA þ SB SC SD ) and Y (defined as SA þ SD SB SC ) signals are
close to zero. The entire pickup-PCM assembly is very
compact and rigid, so that it has a very good mechanical
stability during the AFM operation. In order to eliminate any
need for alignment when the cantilever is changed, an
alignment chip from NANOSENSORS is used in our PCM
[Fig. 2(b)]. The alignment chip has three ridges that fit right
into the corresponding grooves on the back surface of any
AFM probe purchased from NANOSENSORS. According to
the specification of the alignment chip, the AFM probe can
be changed with a repositioning accuracy within 2 mm. We
have changed the AFM probe more than 10 times and the
vertical deviation is less than 500 nm (calculated from the
FES) after a new probe is placed in the alignment chip. For
operation in the contact mode, we use the CONTR-50 probe
from NANOSENSORS, which has a cantilever with a
thickness of 2 mm, a width of 50 mm, a length of 450 mm, and
a force constant of 0.07 – 0.4 N/m. For operation in the AC
mode, we use the AR5-NCHR-50 probe from NANOSENSORS, which has a cantilever with a thickness of 4 mm,
a width of 30 mm, a length of 125 mm, and a resonance
frequency of 300 kHz. The focal laser spot is confirmed
with a diameter of 5 mm, which is smaller than the width
and length of the cantilevers.
Since the PDIC already has a preamplifier for each
photosensor, we only need to build a simple electronic
circuit to obtain the voltages of the FES, X and Y. In the
contact mode AFM, the FES voltage is fed into the feedback
system of the SPM controller. Topographic images can be
obtained by keeping the FES constant through the feedback.
In the AC mode, oscillation voltage is applied to the
bimorph of the PCM. The AC signals (amplitude and phase)
of the FES are connected to the electronics of Nanoscope III
through a signal access module. By keeping the oscillation
amplitude of the FES constant, topographic images can be
obtained; a phase image can also be produced similarly to
the conventional tapping mode.
Because the electronic circuit is not properly shielded, it
results in a noise level of 0:2 nm in the vertical height
measurement. All the experiments presented here are carried
out on a standard optical table. The room temperature is
22 C and the relative humidity is 50%.
3.
Results and Discussion
Figures 3(a) and 3(b) show the topographic images of an
AFM calibration grating sample taken using the above
described new setup in the contact and AC modes,
respectively. The sample has a pitch of 463 nm and a height
of 31 nm. Many fine structures can be resolved. The quality
of the images is comparable to that of images taken using
the standard beam deflection module of Nanoscope III
[Figs. 3(c) and 3(d)].
The single atomic steps of graphite surfaces can also be
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E.-T. HWU et al.
(a)
(b)
(c)
(d)
Fig. 3. Topographic images of calibration grating sample, a 607-AFM waffle grating replica purchased from Ted Pella, Inc. The pitch
and the grating height are 463 and 31 nm, respectively. (a) Contact mode: scan size = 5 5 mm2 , scan rate = 0.6 Hz. (b) AC mode:
scan size = 5 5 mm2 , scan rate = 0.6 Hz, working frequency of cantilever = 296.94 kHz. (c) DI Nanoscope III contact mode:
scan size = 5 5 mm2 , scan rate = 0.79 Hz. (d) DI Nanoscope III AC mode: scan size = 5 5 mm2 , scan rate = 0.35 Hz, working
frequency of cantilever = 298.85 kHz.
resolved with our new setup using the CD/DVD pickup.
Figure 4(a) shows a topographic image and a cross-sectional
height profile of a graphite surface taken in the contact
mode. Markers indicate one step with a height of 0.35 nm
and another step with a height of 1.00 nm. Figure 4(b) shows
a topographic image and a cross-sectional height profile of a
graphite surface taken in the AC mode. Markers indicate one
step with a height of 0.36 nm and another step with a height
of 0.69 nm. Both modes achieve a resolution of single
atomic steps,11) which indicates that the FES voltage is
sufficiently sensitive for the detection of the cantilever
displacement of one atomic height. We note that our
electronics for the FES voltage was not optimally built. No
proper electronics shielding was applied. The noise level in
our topographic images is 0:2 nm, which is larger than that
taken using the beam deflection module of Nanoscope III
[Fig. 4(c)]. We believe the electronics noise can be
significantly improved if a better electronics is used.
Therefore, our experiments successfully demonstrate that
the optical system in the CD/DVD pickup may have a
sensitivity better than the atomic scale in the displacement
measurement of the cantilever.
Previously, Quericioli et al.12) used a CD pickup as the
cantilever position sensor in an AFM. However their entire
mechanical design was very large and induced a mechanical
instability. The resolution was only tens of nm. We use a
CD/DVD pickup and greatly improve the compactness and
rigidity in our pickup-PCM assembly. Therefore, we achieve
a much better resolution.
There are several advantages in using CD/DVD pickups
in the displacement measurement of the AFM cantilever.
First, commercial CD/DVD pickups are manufactured in
large quantities for the consumer electronics market; consequently, they are of very low cost. Second, some optical
pickups (e.g., those of the slim type used for notebook
computers) are small, compact, reliable, and of light weight.
One can attach the pickup-PCM assembly to a piezo scanner
to build a stationary-sample type AFM, which is usually
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(a)
(b)
(c)
Fig. 4. Topographic image and cross-sectional profile of graphite surface. (a) Contact mode: scan size = 1 1 mm2 , scan rate =
0.5 Hz. (b) AC mode: scan size = 1 1 mm2 , scan rate = 0.5 Hz, working frequency of cantilever = 297.04 kHz. (c) AC mode
taken using the beam deflection module of Nanoscope III: scan size = 1 1 mm2 , scan rate = 0.5 Hz, working frequency of
cantilever = 285.97 kHz. The noise level is 0.064 nm.
used for large samples. In the beam deflection method, the
optical detection system is bulky and heavy. A sophisticated
optical design is needed in order to attach the cantilever
directly to the scanner, and the resolution usually becomes
worse than that of the scanning-sample type AFM.
4.
Summary
We have demonstrated the capability of the CD/DVD
pickup-PCM assembly to detect the vertical cantilever
displacement in an AFM. The resolution of single atomic
steps of graphite surfaces was achieved in both the contact
and AC modes. Not only CD/DVD pickups, other commercial pickups, such as those used in blu-ray drives (BDs) or
high-definition (HD) DVDdrives, could also be adapted to
measuring the vertical displacement of cantilevers. The
compact, lightweight and integrated structure of the pickups
will open more possibilities in future AFM design.
Acknowledgments
The authors would like to acknowledge the Topray Co. for
providing the CD/DVD pickup heads used in our experiments. We also thank Mr. Cherng-Yin Lin of the Institute of
Physics, Academia Sinica for machining mechanical parts.
This research is supported by the National Science Council
of Taiwan (contract # NSC93-2120-M-001-007) and Academia Sinica.
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