REVIEW OF SCIENTIFIC INSTRUMENTS 77, 083701 共2006兲
Interface circuits for quartz crystal sensors in scanning probe microscopy
applications
Johann Jersch,a兲 Tobias Maletzky,b兲 and Harald Fuchsc兲
Physikalisches Institut, Universität Münster, Wilhelm-Klemm-Strasse 10, Münster D48149, Germany
共Received 26 April 2006; accepted 2 July 2006; published online 7 August 2006兲
Complementary to industrial cantilever based force sensors in scanning probe microscopy 共SPM兲,
symmetrical quartz crystal resonators 共QCRs兲, e.g., tuning fork, trident tuning fork, and needle
quartz sensors, are of great interest. A self-excitation scheme with QCR is particularly promising
and allows the development of cheap SPM heads with excellent characteristics. We have developed
a high performance electronic interface based on an amplitude controlled oscillator and a
phase-locked loop frequency demodulator applicable for QCR with frequencies from 10 up to
10 MHz. The oscillation amplitude of the sensing tip can be set from thermal noise level up to
amplitudes of a tenth of nanometers. The device is small, cheap, and highly sensitive in amplitude
and frequency measurements. Important features of the design are grounded QCR, parasitic capacity
compensation, bridge schematic, and high temperature stability. Characteristic experimental data of
the device and its operation in combination with a commercial SPM and a homemade scanning
near-field optical microscope are reported. By using the 1 MHz needle quartz resonator with a
standard atomic force microscope tip attached, atomic scale resolution in ambient conditions is
achieved. Furthermore, reproducible measurements on very soft materials 共Langmuir-Blodgett
layers兲 with a very stiff needle quartz 共⬃400 000 N / m兲 are possible. © 2006 American Institute of
Physics. 关DOI: 10.1063/1.2238467兴
INTRODUCTION
Quartz crystal resonators 共QCRs兲 belong to the most frequently used sensors in engineering because of their low internal energy dissipation, high temperature as well as mechanical and chemical stabilities, and relatively high
piezoconstants. Recently, various QCR probe systems such
as tuning fork 共TF兲, trident TF, and needle quartz were introduced as a platform for an attached force-sensing tip in different scanning probe microscopy 共SPM兲 techniques such as
acoustic,1 force,2 near-field optical, and magnetic force microscopies 共see review article3兲. A high quality factor, even
in air, and high spring constants are preconditions for resolving short range forces and stable operation in force microscopy. Furthermore, very low energy dissipation at low excitation amplitudes, the possibility to maintain a subnanometer
tip-sample gap without repulsive mechanical contact 共snap
in兲 and low dielectric losses even at high frequencies are
crucial factors in specific cases, e.g., in scanning microwave
or capacitance microscopy.
There are two excitation modes in QCR based techniques: first, the mechanical excitation 共QCR is typically attached to a dither piezo and the mechanically induced piezoelectric voltage is measured兲 and second, the electrical
excitation mode 共both electromechanical and mechanicelectrical piezoeffects are used for excitation and detection,
respectively兲. The mechanical excitation mode is more popua兲
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c兲
Electronic mail: [email protected]
b兲
0034-6748/2006/77共8兲/083701/5/$23.00
lar because of the simplicity of the principle. However, one
of the main advantages of the symmetrical quadruple configuration of the tuning forks—the immunity to environmental mechanical and electrical distortions, higher Q factor—in
the mechanical excitation mode is sacrificed. The so-called
qPlus sensor3 can be seen as limiting case: here one prong of
the TF is fixed to a large substrate and the free prong with a
tip attached is now a mechanical dipole. Our experience with
an electrical QCR used in the self-excitation mode showed
no ringing problems using a proper designed circuit. The
change of Q factor through tip-sample interaction by operating in environmental conditions is relatively low and may be
taken in account. Nevertheless, the first experiments with
qPlus sensor were successful, yielding atomic force microscopy 共AFM兲 images of silicon with atomic resolution.4
This example emphasizes the potential of QCR. It seems
plausible that in the electrical excitation mode the quadruple
symmetry of the tuning fork or the needle quartz positively
manifests in the ultimate sensitivity, stability against acoustic
and electromagnetic interferences, and stability in time. Consequently, by using a self-excitation high-resolution QCR
sensor a SPM system with less expensive acoustical and
electromagnetic 共em兲 isolations can be developed. Very stiff
QCRs such as the needle quartz or the trident tuning fork
driven at subnanometer amplitudes are particularly appealing
for use in ambient conditions. Mass balancing of the QCR
prongs should be done.
Usually a SPM based on QCR sensors uses a low-noise
preamplifier, an industrial lock-in amplifier, and a frequency
synthesizer. For example, in this way atomic resolution in
ambient conditions with a trident5 and a needle6 QCR is
77, 083701-1
© 2006 American Institute of Physics
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083701-2
Jersch, Maletzky, and Fuchs
Rev. Sci. Instrum. 77, 083701 共2006兲
FIG. 2. Bridge oscillator principle. Oscillation condition is achieved if
R2 / R1 = R3 / RQCR.
FIG. 1. Self-excitation oscillator with amplitude control 共Barkhausen loop兲.
AGC, automatic gain control; Uref, reference voltage defining the amplitude.
achieved, Ferrara7 introduced a simple, cheap, and compact
amplitude controlled oscillator based on 32 kHz TF for contact lateral force microscopy.
Here we introduce a high performance electronic interface based on an amplitude controlled bridge oscillator and a
phase-locked loop 共PLL兲 frequency demodulator applicable
for noncontact SPM measurements with a QCR platform at
frequencies from 10 kHz up to 10 MHz.
OPERATION PRINCIPLES
If mechanical properties of QCR are represented by the
impedance of an equivalent series circuit 共with elements L,
C, and Rs兲, the simplified characteristic equation 共without
considering noise and parasitic capacitance兲 of the selfexcited Barkhausen loop shown in Fig. 1 is
s2LC + sC共Rs − GR f 兲 + 1 = 0,
with G representing the variable gain of the automatic gain
control 共AGC兲 stage and R f the transimpedance resistance of
the low-noise amplifier 共A兲. It can be seen easily that in the
excited steady state 共Rs = GR f 兲 the dissipative and conservative interactions are fully separated in a first approximation.
Indeed, changes of the Rs value through the dissipative interactions ⌬R are immediately compensated through the corresponding gain changes ⌬G. Thus, the second term in the
equation is zero and the changes in AGC voltage represent
the dissipation changes. Conservative interactions modifies
the spring constant of the sensor 共the C value in the equivalent resonance circuit兲, and resulting in a frequency change
共1 / f = 2␲ 冑 LC兲 originating from the tip-sample interactions
in force measurements. Another mechanism of frequency
variations involving changes in effective oscillating mass of
the sensor corresponds to the L value changes. A closer look
on the interplay between tip-sample interactions, QCR impedance, measurement, and electronic parameters shows a
more complex picture.8,9
If optimal imaging and circuit parameters are used, the
noise limit at a given tip-sample interaction is defined by the
physical parameters of the QCR 共dissipative resistance, material constants, and dimensions兲. Consequently, the analysis
of the noise limit is similar to that in the AFM self-excitation
mode.10–12
Note that application and analyses of oscillators in
quartz crystal microbalance technique are similar and were
introduced long before tuning fork sensors in SPM techniques were introduced. The main difference is that in quartz
crystal microbalance technique tiny changes in the effective
oscillating mass of the sensor are measured rather than forces
as in the case of a SPM. Reference 13 gives good introduction and overview of quartz crystal microbalance technique.
BRIDGE OSCILLATOR CIRCUIT DESCRIPTION
The design of suitable interface circuits for QCR requires special circuit configurations, which cannot be obtained by simply modifying standard applications. An appropriate interface electronic should deliver fast and precision
QCR oscillation amplitude and frequency 共or phase兲 data in
order to investigate dissipative and conservative tip-sample
forces. Parallel capacity compensation and a grounded QCR
共for minimizing parasitic effects and EM interferences and
allowing the operation in conductive liquids兲 are further requirements. Miniaturization and component selection are
also crucial for minimizing parasitic effects within the electronic circuit
We present a precision bridge oscillator circuit with
grounded QCR and capacitance compensation. The principle
of the bridge oscillator is shown in Fig. 2. Figure 3 shows a
variant of the bridge oscillator circuit with Analog Devices
共US兲14 elements used. Stable oscillation amplitudes from approximately ten rms of the QCR mechanical noise level up to
some nanometers are possible. Low-noise wideband operation amplifiers in transimpedance stage 共such as AD8066,
AD8067 from Analog Devices or OPA2380, OPA656, and
OPA657 from Texas Instruments兲 allow high performance
operation with various QCRs. Note that high frequency elements 共AD8067, OPA657, and AD603 bandwidth
ⲏ100 MHz兲 in the circuit that operate at relatively low frequency ⱗ1 MHz should be used to obtain low phase delay
and noise. Output signals of this circuit are the amplitude of
the oscillation and the sinusoidal oscillation signal itself.
To provide low current noise the value of the feedback
resistor R1 is approximately one order of magnitude higher
than the dissipative resistance of the QCR. The noise of the
transimpedance amplifier 共input current noise of
ⱗ10 fA/ Hz1/2 and input voltage noise of ⱗ10 nV/ Hz1/2兲 is
at room temperatures lower than the noise of the feedback
resistor and the thermal noise of the dissipative resistance of
the QCR. The capacitance compensation is achieved through
the variable elements C1 and R2. The gain control loop
based on a voltage-controlled amplifier 共VCA兲 共AD603兲 consists of the rectifier 共IC1B and IC2A兲; the proportionalintegral 共PI兲 regulator with reference comparator 共IC2B兲 and
the adjustable reference voltage source 共LM4431, R14兲 provides stable oscillating amplitude in the bridge circuit. The
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083701-3
Interface circuits for quartz sensors
Rev. Sci. Instrum. 77, 083701 共2006兲
FIG. 3. Schematic of the bridge oscillator circuit. Integrated circuit destinations: IC1A, transimpedance amplifier in bridge connection; AD603, voltage
controlled amplifier; IC1B, full rectifier; IC2B, comparator and PI regulator; LM4431, voltage reference; IC2D, bandpass filter; IC2A and IC2C, opertional
amplifiers.
amplified control signal on pin 8 IC2C is used as measure for
the dissipative resistance of the QCR. The drive amplitude of
the QCR can be adjusted with the potentiometer R14. A second order bandpass filter on IC2D improves the output sinusoidal signal 共pin 14兲. Finally, the compensation capacity C1
should be tuned 共roughly with C1, fine with potentiometer
R2兲 to achieve the lowest value of the gain control signal.
The most expensive part in the circuit is the gain control
stage on VCA 共AD603 or VCA810兲. Typically, for gain control in voltage-controlled oscillators cheap field effect transistors 共FETs兲 are used 共e.g., in SPM applications7,15兲. However, FETs suffer on strong temperature dependence and
nonlinearity resulting in low precision of data obtained.
We have miniaturized the layout in order to locate the
amplifier at the base of the QCR to reduce interferences and
parasitic capacitance due to the cable connecting the sensor
to the circuit. The used precision elements reduce the complexity of the circuit. The device is mounted on a 27⫻ 35
-mm-sized double-sided printed circuit board 共PCB兲 and
placed in a shield box. Both lateral and vertical force measurement configurations can be realized by appropriate attachment of the tip to the QCR prong.
chanical oscillation amplitude of the QCR and mechanical
dissipation can be estimated straightforwardly.
The frequency shift ⌬f and the tip-sample force gradient
⌬k at small oscillation amplitudes are in a particularly simple
connection: ⌬k = 2k⌬f / f 共k is the force constant and f the
resonance frequency of the used QCR兲. For precision frequency measurements, we developed a simple PLL demodulator circuit based on a 74HC4096 phase comparator 共an improved variant of the well-known CD4046兲 and special
voltage controlled crystal oscillator 共VCXO兲 共AXTAL
GmbH, Mosbach, Germany兲 with an extremely wide pulling
range. The frequency resolution of our demodulator was better as 0.1 Hz at all used frequencies, sufficient for SPM applications. Tests with 32, 100, and 200 kHz TFs and 1 MHZ
needle quartz 共Fig. 4兲 showed the high performance of the
demodulator circuit. For example, by using the 1 MHz
needle quartz with a force constant of k ⬃ 400 000 N / m the
minimal detectable force gradient ⌬k is ⬍0.1 N / m, quite
enough for nondestructive scanning on relative soft samples.
DATA ACQUISITION
The signal amplitude, the resonant frequency, and the
gain control signal are the measurable data in our circuit.
With the knowledge of the electromechanical coupling constant and the dimensions of the used QCR, the mechanical
tip-sample interaction parameters can be estimated from the
electrical measurements in usual way 共e.g., see review
article3兲. For example, from the amplitude signal and the
gain control signal it is easy to calculate the current through
the QCR and the dissipative resistance of the QCR. The me-
FIG. 4. Various tuning forks 共100, 32, and 200 kHz兲 and 1 MHz needle
quartz tested with developed circuit.
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083701-4
Rev. Sci. Instrum. 77, 083701 共2006兲
Jersch, Maletzky, and Fuchs
FIG. 5. Image of a fresh cleaved graphite surface in ambient conditions,
atomic monolayer is reasonable resolved.
The data are comparable with the previous results16,17 obtained with the 1 MHz needle quartz in an Omicron ultrahigh
vacuum system. It should be noted that in measurements on
soft samples at ambient environment, images obtained in the
dissipative channel 共AGC control signal兲 are generally have
higher quality as those obtained with the frequency modulation mode.
The developed electronic is applicable with nearly all
industrial SPMs with optional signal inputs. For example, we
used two external analog inputs display dissipative and conservative data channels in an NT-MDT system 共Moscow,
Russia兲 and Solver SPM as well as in a homebuilt scanning
near-field optical microscopy 共SNOM兲 based on PI 共Waldbronn, Germany兲 XYZ piezosystem.18 In the latter system,
the complex self-excitation circuit with dither piezo was successful replaced by the described circuit.
FIG. 7. Polymer film 关polymethylmethacrylate 共PMMA兲兴 with embedded
gold islands 共calibration pattern for SNOM兲. The structure was created by
using latex bead adsorbed in a dense packed monolayer on mica prior to
evaporating gold on this structure. After removing the latex beads, the remaining gold projection pattern was spin coated with PMMA and glued onto
a cover glass. The resulting holes have a depth of about 4 nm. The remaining gold triangles have a height about 2 nm. The image was taken with the
tip of a glass prism attached to a 32 kHz tuning fork.
FORCE MICROSCOPY IMAGES
Tapping mode force image made on a graphite surface
关highly oriented pyrolytic graphite 共HOPG兲兴 in ambient conditions by using a 1 MHz needle quartz with a standard cantilever tip attached is shown in Fig. 5. On a 4 ⫻ 4 ␮m2 area
monoatomic terraces are resolved in the dissipative mode.
The noise in cross section line 共Fig. 6兲 is dominated by the
mechanical distortions and our SPM 共NT-MDT兲 resolution.
Furthermore, in Fig. 7 we present images obtained by using
of developed circuits in the SNOM distance control. As a
force sensor, the relatively large tetrahedral glass probe18 attached on a 32 kHz tuning fork is used. In order to test the
capability in measurements on soft samples we used
monomolecular dipalmitoylphosphatidylcholine 共DPPC兲
Langmuir-Blodgett 共LB兲 strips.19 Figure 8 shows that the interaction is gentle enough to made nondestructive images on
this fragile samples. Multiple scans across LB strips revealed
no changes.
SUMMARY
We have reported on a self-excitation interface circuit
for force detection based on different QCRs. A bridge oscil-
FIG. 6. Image section 共black line on the top, right in Fig. 5兲 shows an
atomic step approximately corresponds to graphite 共interlayer spacing
334.7 pm兲.
FIG. 8. Image of monomolecular LB strips on plasma etched Si surface
made with 1 MHz needle quartz.
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083701-5
lator configuration with grounded QCR for precision distance control was developed and tested in various applications 共SNOM, SCM, and other兲. An optional precision PLL
demodulator is developed for the detection of frequency
shift. Frequency shift and QCR loss representing conservative and dissipative interactions in noncontact 共NC兲-AFM
measurements are used to demonstrate the device capability.
Highly resolved graphite and LB stripe images in AFM application using the needle quartz and SNOM applications
using 32 kHz TF demonstrate the performance of the device.
It is shown that a very stiff mechanical sensor allows probing
very soft samples without damage. The inherent fast operation in both dissipative and conservative force measurement
modes and the capability of applying the sensor in liquid
environment open wide applications.
ACKNOWLEDGMENT
This work is supported by European Community
through ASPRINT project 共under the Sixth Framework Programme of the European Community 2002-2006兲.
1
Rev. Sci. Instrum. 77, 083701 共2006兲
Interface circuits for quartz sensors
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