REVIEW OF SCIENTIFIC INSTRUMENTS 81, 063706 共2010兲
Implementation and characterization of a quartz tuning fork based probe consisted of discrete resonators for dynamic mode atomic force microscopy Terunobu Akiyama,1,a兲 Nicolaas F. de Rooij,1 Urs Staufer,2 Manfred Detterbeck,3 Dominik Braendlin,4 Simon Waldmeier,4 and Martin Scheidiger4 1
IMT SAMLAB, Ecole Polytechnique Fédérale de Lausanne (EPFL), Rue Jaquet-Droz 1, 2002 Neuchâtel, Switzerland 2 3mE Faculty, Micro and Nano Engineering Lab, Delft University of Technology, NL-2628 CD Delft, The Netherlands 3 Nanoworld AG, Rue Jaquet-Droz 1, 2002 Neuchâtel, Switzerland 4 Nanosurf AG, Grammetstrasse 14, 4410 Liestal, Switzerland
共Received 15 March 2010; accepted 26 May 2010; published online 25 June 2010兲 The quartz tuning fork based probe 兵e.g., Akiyama et al. 关Appl. Surf. Sci. 210, 18 共2003兲兴其, termed “A-Probe,” is a self-sensing and self-actuating 共exciting兲 probe for dynamic mode atomic force microscope 共AFM兲 operation. It is an oscillatory force sensor consisting of the two discrete resonators. This paper presents the investigations on an improved A-Probe: its batch fabrication and assembly, mounting on an AFM head, electrical setup, characterization, and AFM imaging. The fundamental features of the A-Probe are electrically and optically characterized in “approach-withdraw” experiments. Further investigations include the frequency response of an A-Probe to small mechanical vibrations externally applied to the tip and the effective loading force yielding between the tip and the sample during the periodic contact. Imaging of an electronic chip, a compact disk stamper, carbon nanotubes, and Si beads is demonstrated with this probe at ambient conditions in the so-called frequency modulation mode. A special probe substrate, which can snap on a receptacle fixed on an AFM head, and a special holder including a preamplifier electronic are introduced. We hope that the implementation and characterization of the A-Probe described in this paper will provide hints for new scanning probe techniques. © 2010 American Institute of Physics. 关doi:10.1063/1.3455219兴
I. INTRODUCTION
Atomic force microscope 共AFM兲 共Ref. 1兲 became an indispensable instrument for visualizing, monitoring, and characterizing micro- and nano-scale materials and structures. One of the challenges in AFM instrumentation is the implementation of nonoptical sensing systems, which will enable many new opportunities to use AFM in different conditions and environments. Cantilevers featuring piezoresistors2 have opened the epoch of integrated nonoptical sensing in an AFM. Thanks to this probe concept, many years later, human has successfully sent and operated an AFM on the red planet Mars.3 The “Mars AFM” was equipped with an array of eight piezoresistive levers to enable autonomous operation of the AFM. A special subgroup of self-actuating and self-sensing probes is based on quartz tuning forks 共TF兲. Several designs and concepts for TF based AFM have been demonstrated.4–9 We have introduced a unique probe in previous works,10,11 where the TF is used to simultaneously drive and sense a cantilever. A microfabricated U-shaped cantilever with an integrated sharp tip is assembled on a commercially available quartz TF such that the two legs of the cantilever are atTel.: ⫹41-32-720-55-71. FAX: ⫹41-32-720-57-11. Electronic mail:
[email protected].
a兲
0034-6748/2010/81共6兲/063706/8/$30.00
tached in a symmetrical way to the two prongs of the TF. We designated this probe “A-Probe” 共“A” is taken from the name of the lead兲. Based on the A-Probe techniques, two more variants have been developed. The first one was applied for three different applications: scanning gate experiments at low temperature,12 local sensitivity measurements of the source-drain current of an ion sensitive field effect transistor 共ISFET兲 as a function of the probe position,13 and local oxidation of titanium in the dynamic operation mode.14 The second one was designed and used for local transport measurements on quantum devices below liquid-4He temperature.15 In this paper, we present recent results on the implementation and characterization of an improved A-Probe. The first part of this paper focuses on the design, fabrication, and assembly of the probe, as well as on mounting a setup for AFM imaging under ambient conditions. The second part presents experimental results on the performance of the new probe. II. A-PROBE CONCEPT AND PRINCIPLE OF OPERATION
Figure 1共a兲 shows the basic implementation of the A-Probe as previously reported.10,11 The essential parts of the probe are the U-shaped cantilever and the TF. These compo-
81, 063706-1
© 2010 American Institute of Physics
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-2
Akiyama et al.
Rev. Sci. Instrum. 81, 063706 共2010兲
characteristic of the cantilever, which is influenced with an external force, e.g., tip-sample interaction. In our experience, the A-Probe worked better if it was operated at the in-phase resonance. In practical operation, the A-Probe is self-excited 共oscillating兲 at its first resonance frequency 共in-phase peak兲, which is determined by the probe itself. As soon as any forces, e.g., due to tip-sample interactions, are applied to the tip, the resonance frequency changes. It is electrically detected and tracked by a phase-locked loop 共PLL兲.18,19 The frequency is kept at a set value by adjusting the tip-sample separation with z-feedback. The so-called frequency modulation 共FM兲 detection20 is the best operation mode for an A-Probe. The amplitude of the piezoelectric-current of the TF is also maintained at a set point during the operation. FIG. 1. 共Color online兲 共a兲 Scheme of the standard A-Probe: two discrete resonators, quartz TF, and U-shaped microfabricated cantilever are combined and form an oscillatory force sensor. The piezoelectricity of the quartz TF enables self-sensing and self-actuating of the probe. The spring constant of the A-Probe is determined by the cantilever. 共b兲 A simplified model of the A-Probe. Two resonators are serially coupled. 共c兲 The working principle. In-plane movements of the TF prongs are translated into the vertical motion of the silicon cantilever.
nents are used as discrete resonators. Figure 1共b兲 illustrates a simplified model of the A-Probe, where the cantilever corresponds to resonator 1 共m1 , k1 , c1兲 and the TF to resonator 2 共m2 , k2 , c2兲. F is the driving force created by the piezoelectric effect of the TF. This model was numerically investigated and published elsewhere.16 The working principle of the A-Probe is as follows. In operation, an electrical driving signal is directly applied to the electrodes of the TF to excite it at its lowest resonance frequency 关Fig. 1共c兲, left兴. In this mode, the ends of the two prongs are moving in-plane and have opposite phases, meaning that they approach and withdraw from each other. The vibration amplitude is typically in the order of tens of nanometer. Since the cantilever disturbs symmetry of the TF, the twisting motion of the prongs occurs 关Fig. 1共c兲, left兴. This motion results in a small vibration in the z-direction 共i.e., the axis of the tip兲 of the glued ends of the cantilever 关Fig. 1共c兲, middle兴. The cantilever amplifies the vibration according to its mechanical property, and a large out-of-plane motion of the tip is obtained 关Fig. 1共c兲, right兴. The correlation between the moving directions of the TF prongs, outward or inward, and that of the tip, positive or negative in z-direction, is determined by the initial condition of the probe. A small factor, e.g., misalignment between the prongs and the cantilever, can influence the determination of the correlation. The ratio between the vibration amplitude of the TF prong and that of the cantilever depends on a property of the probe. A practical number is presented in Sec. V B. The A-Probe is a system with two coupled resonators that are moderately matched in terms of resonance frequency. Like other similar systems,17 one obtains two typical resonances in theory: in-phase and antiphase peaks. In our case, the TF is used as an oscillatory force sensor. Hence, the probe is designed such that one of the two resonance peaks largely changes its frequency corresponding to the resonance
III. A-PROBE DESIGN AND IMPLEMENTATION
The quartz TF used in this study is a commercially available “watch crystal,” the resonance frequency of which is 32.768 kHz. An unpackaged product from Micro Crystal 共Div. of ETA SA, Grenchen, Switzerland兲 without any modifications was used. The dimensions of the prongs are 2.4 mm in length, 220 m in width, and 130 m in thickness. The cantilever and its pads for gluing were designed to match the dimensions of the TF. Different cantilever designs were evaluated by finite element 共FE兲 simulations to obtain best performance. As described above, the main focus was on the frequency shift optimization of the first resonance for a given interaction-change applied on the tip. In the FE simulation, a small spring with variable spring constant is attached to the tip to emulate the tip-sample interaction. The width of the cantilever and the separation of the legs were also important parameters because they significantly influence the stiffness to the prongs of the TF. The optimum design was 300 m in length, 3 m in thickness, and 30 m in width 共each leg兲. The separation of the two legs was 30 m. The spring constant of the silicon cantilever was approximately 3.5 N/m. The size of the pads was determined considering manual assembling and gluing. The implementation of the A-Probe has two stages: 共i兲 microfabrication of the cantilevers and 共ii兲 attachment to the TFs. Since, the TF is a relatively large piece not difficult to be picked up by tweezers, we decided to place and glue the TFs, piece by piece, onto the cantilevers. Figure 2 shows a micrograph of the fabricated cantilevers before assembly. The inset explains the assembly concept. The wafer was fabricated such that the cantilever was still connecting on its base-wafer with narrow support beams and the tip apex was pointing toward the base-wafer. Alignment guides, or grooves, for the TFs were formed on the top surface to ensure good alignment between the pieces and to facilitate assembling. The cantilever wafer was fabricated by using Micro Electro Mechanical Systems 共MEMS兲 microfabrication technologies. 4-in. highly doped silicon wafers were used. First, the alignment guides were formed on the surface by anisotropic etching with KOH. The cantilever pattern was then transformed on the top wafer surface, and the whole surface
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-3
Akiyama et al.
FIG. 2. 共Color online兲 Micrograph of fabricated Si cantilevers before assembly with TFs. The inset sketches the conception of assembly. The TFs are individually placed in alignment grooves.
was covered with silicon nitride. To release the cantilevers and to form the tips, the wafer was partially etched from the back surface in KOH. The etching was stopped when the thickness of the cantilever achieved the desired value. Consequently, a sharp trigonal tip was formed at the end of the cantilever 共Fig. 3兲. After removing all the etch masks, a low temperature oxidization was performed to sharpen the tip apex. Finally, the oxide was removed in buffered HF. Figure 3共a兲 shows a micrograph of the tip. Its height is about 15 m. A typical tip radius of curvature of 15 nm and an aspect ratio of 4:1 at the last 1.5 m of the tip were achieved. Half cone angles are less than 12° 共viewed along the cantilever axis兲 and 8° 共seen from the side兲. The tip protrudes from the very end of the cantilever, which allows for easy positioning of the tip above a target place on the sample. Once the microfabrication process completed, the cantilevers were freestanding but still connected to the base-wafer by narrow support beams 共Fig. 2兲. In the assembling step, a small amount of nonconductive epoxy resin 共Araldite AY105/HY991兲 was applied to the prongs of the TF. The TF was manually placed into the alignment groove so that it is automatically aligned to the cantilever. The epoxy resin was then cured at 110 ° C. Finally, the TF was picked up by tweezers. At this moment, the supporting beams were broken, and the cantilever could be removed together with the TF. Figure 3共b兲 shows a micrograph of the cantilever of an assembled probe. The both pads of the cantilever were glued on the ends of the TF prongs. IV. PROBE MOUNTING SYSTEM AND OPERATION SETUP
As most AFM users might argue, handling of tiny probes with tweezers is a delicate task. To facilitate handling of the
FIG. 3. SEM pictures: 共a兲 side view of the tip and 共b兲 front view of the cantilever after the assembly.
Rev. Sci. Instrum. 81, 063706 共2010兲
FIG. 4. 共Color online兲 共a兲 Top view of the assembled A-Probe. The dotted lines show the outlines of the metal pieces and the TF, respectively, 共b兲 close up view of the cantilever, and 共c兲 developed probe substrate 共left兲, which can be plugged onto the kinematic mount of receptacle 共right兲. It is kept in place by means of the small magnet, visible in the center of the receptacle.
A-Probe, the assembly is glued on a special holder or “plug,” which connects through a kinematic mount to a receptacle fixed at the scanner. This concept features high precision exchange of probes, which leads to continue the measurements very closely to the previous position on the sample after probe exchange. Figures 4共a兲 and 4共b兲 show pictures of an assembled A-Probe, and Fig. 4共c兲 shows a top view of the plug 共left兲 and the receptacle to be fixed on the scanner 共right兲. In microfabrication terminology, the plug is the package for the A-Probe. It is based on a glass-epoxy substrate 共FR4, the same material as often used for printed circuit boards兲 bonded to metal plates featuring finely etched latches 共holes, groove, and plate兲 for the kinematic mounting. The receptacle has the same base but features, in addition, three metal balls and one magnet. The plug is put on the receptacle and magnetically held in place during operation. The dimensions of the three balls are all different, and the two openings on the plug have a round and an oval shape, respectively, in order to form the required mechanical constrains for latching 关see Fig. 4共c兲兴. The glass-epoxy substrate of the package is machined such that the balls can directly touch the metal plates of the package for electrical contacting. The mounting of the TF to the package is equally critical for positioning. We opted again for a self-aligned approach. The two metal plates of the plug were patterned such that the base part of the TF can only be glued at one specific position 关cf. dotted lines in Fig. 4共a兲兴. The TF probe was manually placed on the groove under optical control and fixed with nonconductive epoxy resin. For good electrical interconnections, conductive epoxy resin 共Epotek H20兲 was used between the pads of the TF and the metal plates. In the same step, a pad of the cantilever was electrically connected to one of the electrodes of the TF with the same resin to define the electrical potential of the cantilever 关Fig. 4共b兲兴. A special holder fitting the head of a commercial AFM 共Nanoscope III and IV, Veeco, Santa Barbara, USA兲 was developed. Figure 5 shows a full-view of the holder. Electronic preamplifiers and the abovementioned receptacle for the special mounting platform are incorporated into the holder. It
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-4
Rev. Sci. Instrum. 81, 063706 共2010兲
Akiyama et al.
FIG. 5. 共Color online兲 Special holder with incorporated preamplifier electronics for mounting the A-Probe on a commercial AFM.
was designed to be fit in the head and clamped by the original spring arms. The necessary electrical connections, power supply and in-out signal lines, are looped through the original six pin connector of the head. The internal power sources of the microscope were used to supply power to the special holder. In a normal configuration, a cable with connector joins all necessary electrical lines between the head and the main body of the microscope. We added “break-out” wires in between the male and female connectors of the cable, where the in-out signal lines from the special holder are tapped. A small switch was also connected into the lines for on/off switching of the laser used in the commercial AFM for cantilever deflection sensing. No physical modification was necessary on the microscope side. Figure 6 shows a block diagram of the overall electronics for dynamic mode scanning probe imaging. The circuit makes a loop to self-oscillate the TF at its resonance frequency with constant amplitude. The preamplifier circuit on the holder was designed based on the one in the report:21 the first stage attenuates the input signal by a factor of ten. The signal is then applied to the TF through a resistor 共100 k⍀兲 as well as to the parasitic capacitance compensation line 共cf. Fig. 6兲. The outputs from both lines are summed such that
FIG. 6. 共Color online兲 Electrical setup for dynamic mode AFM in the FM detection. The A-Probe is self-oscillated at its resonance. Changes in frequency are tracked by the PLL working in a passive mode. The electronics in the square with dotted lines is implemented on the special holder.
only the piezoelectric-current of the TF is amplified by means of a current-voltage converter. The cantilever is electrically connected to the virtual ground in this configuration. The signal is amplified and fed to the external modules: a phase shifter and an amplitude controller. The processed signals from these modules are multiplied and applied to the TF as driving signal. In practice, a commercial system with all those functionalities 共easyPLL sensor controller, Nanosurf AG, Switzerland兲 was used. The excitation frequency was tracked by a PLL 共easyPLL, Nanosurf AG, Switzerland兲 in a passive mode. The output of the PLL, which corresponds to the frequency deviation from the set point, is an analog signal and can be supplied to the microscope to control the tip-sample separation. Note that the amplitude of the piezoelectric-current of the TF, which is proportional to mechanical vibration amplitude of the prongs, is maintained at a set value, but this does not mean that the tip vibration amplitude is also kept at a constant value.
V. CHARACTERIZATION
A. Resonance characteristics
Resonance characteristics of the A-Probe were investigated on an AFM instrument. While a sine wave with sweeping frequency was directly applied to the TF, both amplitude and phase of the cantilever were optically measured under ambient conditions. The electrical outputs from the preamplifier circuit were also recorded under two different conditions, with and without the capacitance compensation 共cf. Fig. 6兲. Figure 7共a兲 shows an optical measurement of the tip vibration from 10 kHz to 1 MHz. The first peak at 12 kHz was due to the vibration in z-direction of the whole freestanding part of the probe. The second peak at 15 kHz was the fundamental resonance of the TF prongs in z-direction. The third peak at 42.77 kHz was the “in-phase” resonance of the TF, which is the operation frequency of the A-Probe. The one at 50 kHz is the “antiphase” resonance. A higher resonance mode of the TF was observed at 202 kHz. Figure 7共b兲 shows optical and electrical measurements around the inphase resonance of the A-Probe. All measurements showed resonant peaks in the amplitude and corresponding shifts in the phase. The optical measurement showed a symmetrical peak in amplitude and a monotonous phase change. A qualify factor of 1650 was estimated from the measurement. The electrical signal without capacitive compensation showed an asymmetric peak at a frequency different from the optically detected resonance of the TF. When the capacitive compensation was tuned such that this peak becomes symmetric, its location shifted back to the optically detected resonance. The Q-factor as measured from this compensated peak was 1650, which was the same value as delivered from the optical measurement. The phase shift was also more pronounced in the compensated case. For a stable, electronically excited selfoscillation of A-Probe, a large phase shift at resonance is preferred. Hence, signal conditioning with the capacitor compensation is suitable for operation of A-Probe under ambient conditions.
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-5
Akiyama et al.
FIG. 7. 共Color online兲 Characterization of the tip dynamics. 共a兲 Optical measurement: tip vibration amplitude as a function of driving frequency from 10 kHz to 1 MHz and 共b兲 optical 共solid line兲 and electrical measurements 共dashed and dotted lines兲 around the first resonance 共in-phase peak兲 of A-Probe.
B. Approach-withdraw curves
We conducted “approach-withdraw” measurements on a freshly cleaved, highly oriented pyrolytic graphite 共HOPG兲 surface. A self-oscillating A-Probe was approached to the sample from far above the surface and brought into periodic contact and withdrawn. During this process, the frequency shift ⌬f and so-called “dissipation” 共cf. Fig. 6兲 were simultaneously registered. In addition, deflection and vibration of the cantilever were monitored simultaneously by the laser optical system of the AFM. Figure 8共a兲 shows the optical measurement of the tip vibration during the approach phase 共withdraw phase is not shown兲. It is the raw data from the photodiode. One can see an evolution of the tip oscillation during approaching. While the tip was not in contact, its oscillation was independent from the tip-sample distance. Beyond the z-position A in the figure, the tip oscillation started decreasing linearly, and the center of the vibration was shifting upward. These phenomena continued till the z-position B as shown in Fig. 8. It is obvious that the tip was periodically contacting the surface in the region A-B comparable to the Tapping mode. Beyond the z-position B, a large static deflection of the cantilever with a very small tip vibration was observed. We interpret this as follows. The tip remained in contact with the sample surface during the full oscillation cycle, while the cantilever was still vibrated by the TF. We defined this operation as a “quasi” contact mode. An interesting fact that we found from this optical measurement was that the z-displacement from A to B, over which the periodic contact region spread, is about a
Rev. Sci. Instrum. 81, 063706 共2010兲
FIG. 8. 共Color online兲 Approach-withdraw curves onto freshly cleaved HOPG surface: 共top兲 optical measurement of the tip vibration during the approach phase, 共middle兲 electronically measured frequency shift of the A-Probe and 共bottom兲 normalized dissipation. The region between A and B indicates the periodic contact phase.
half of the peak to peak amplitude of the tip vibration in free space. Figure 8共b兲 shows the evolution of the frequency shift during the approach-withdraw step. Frequency shift of the probe in the periodic contact region was found to be almost linear, though small frequency jumps were observed in both the approach and withdraw cycles. As the tip moved closer to the sample surface, a larger positive shift in frequency was observed. Beyond the z-position B, the frequency shift was saturating. We observed that the total frequency shift in the periodic contact region 共denoted as ⌬f in the figure兲 was an inherent value for each probe and independent from the amplitude of the driving signal, i.e., ⌬f remained unchanged when various amplitudes were used to vibrate the tip. This was one of the interesting features of the A-Probe. If we define the spatial sensitivity of the A-Probe as the frequency shift ⌬f divided by the amount of tip displacement in the z-direction, it varies depending on the amplitude of tip vibration. In the case of Fig. 8, where the displacement from A to B was approximately 175 nm and ⌬f of the probe was measured to be 178 Hz, a sensitivity of ⬃1 Hz/ nm was obtained. It could be increased/decreased by changing the tip amplitude. The resolution of the commercial PLL is 5.5 mHz for the demodulation range of 183 Hz. A spatial resolution of less than 0.1 nm can be expected. During experiments, we found that ⌬f largely varied depending on humidity and temperature and ⌬f decreased as humidity or temperature 共or both兲 became higher. This can be attributed to the nature of the small resonators, e.g., the
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-6
Akiyama et al.
matching between the cantilever and the TF was changed due to different thermal expansion coefficients. The maximum ⌬f that we have obtained was approximately 1 kHz. This was, however, a very rare case. It was also found that the ratio between the vibration amplitude of the TF prong and that of the cantilever is almost proportional to the ⌬f value of probe. To verify this, 18 probes with ⌬f lower than 200 Hz were tested. While the probe was excited at its resonance frequency, vibration amplitudes of the TF prongs in the x − y plane, ATF, and of the tip oscillation in the z-direction, ACL, were sequentially measured by laser interferometer. A simple relation of ACL / ATF = 0.038⌬f + 1 was obtained to calculate a first-order. If a probe has ⌬f = 178 Hz like the case in Fig. 8, ACL / ATF ⬇ 7.8 is obtained. As mentioned above, the A-B displacement is about a half of the peak to peak amplitude of the tip vibration. The probe used for the measurements in Fig. 8 could have a peak to peak amplitude of 350 nm. The amplitude of the TF prongs could be approximately 45 nm peak-peak. Figure 8共c兲 shows normalized dissipation, which is the signal to maintain the amplitude of piezoelectric-current of the TF at a set value 共refer to Fig. 6兲. We found that dissipation was not monotonous. Its characteristic in the periodic contact region strongly depends on both tip vibration amplitude and the property of the sample. For example, in case the of Fig. 8共c兲, the value decreased gradually as the tip was moved closer to the sample surface. When the tip vibration amplitude was larger by a factor of 4 with all other parameters unchanged, the curve showed the opposite sign, meaning that the dissipation increased as the tip moved closer to the sample surface. After some measurements, we found that the dissipation signal may contain additional information of the sample, which is useful, e.g., in identifying different materials or mechanical properties of a sample. We will, however, leave this point open in this paper for further investigations.
Rev. Sci. Instrum. 81, 063706 共2010兲
FIG. 9. 共Color online兲 Frequency and amplitude response of the A-Probe to an applied small vibration of varying frequency 共solid lines兲. The bare performance of the PLL 共dotted lines兲 is also included for comparison. It can be seen that the performance of the probe is limited by the PLL.
width of the probe is estimated to be 400 Hz from the lower graph. Note that the PLL was out of the self-oscillation loop 共refer to Fig. 6兲 and there was no influence to the results by the PLL. It was also found that the frequency response was almost independent from both tip vibration amplitude and engaging frequency set point in the periodic contact mode operation. D. Measurement of effective loading force
C. Response speed of the A-Probe
To measure the effective loading force during the periodic contact mode operation, the experiment as shown in the inset of Fig. 10 was performed. It is similar to the one in the report.22 A standard Si cantilever was mounted to a conventional AFM with optical read-out system. The A-Probe was then mounted on the sample side and brought into selfoscillation. The two levers were approached to each other such that the tip of the A-Probe tapped at the end of the optically read cantilever. Both frequency shifts of the
The response speed of the A-Probe was investigated as a function of a small displacement applied to the tip. The A-Probe was mounted on an AFM as for standard dynamic mode imaging. The electrical setup was the same as shown in Fig. 6. The tip was approached to an HOPG surface until a small frequency shift occurred, i.e., the probe was brought in the periodic contact region. X − Y scanning was deactivated. The HOPG sample was then shaken in the z-direction by the piezoscanner. Consequently, a small mechanical vibration was superimposed on the tip oscillation. While the vibration frequency was swept, the corresponding frequency shift was detected. A typical result of such an experiment is indicated by the solid line in Fig. 9. For comparison, the bare performance of the PLL is also shown in the figure by the dashed line. The upper graph in Fig. 9 shows the magnitude of the frequency shift and the lower one shows the phase delay. At a glance, one sees that the measurements and the bare performances of the PLL have similar curves. This hints that, in our case, the response speed of the probe was limited by the performance of the PLL. If we consider a phase shift of ⫺45°, a band-
FIG. 10. 共Color online兲 Experimental setup for measuring an averaged effective loading force between the tip of an A-Probe and a sample in the periodic contact operation. A standard Si cantilever is “tapped” by the A-Probe mounted on the tube scanner of a standard AFM. From the deflection of the sensing cantilever, the loading force was estimated.
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-7
Rev. Sci. Instrum. 81, 063706 共2010兲
Akiyama et al.
observing the scanned images. The procedure was exactly the same as for a conventional dynamic mode AFM. A typical scan speed was 1 Hz per line to ensure minimum errors and to obtain the best image quality. The scan speed could be increased by ⬃5 Hz, without serious image distortion. The smallest tip amplitude with which we could successfully perform AFM operation was about 100 nm peak-peak. Below that, the oscillation of the probe was heavily disturbed likely due to the trapping of the tip by water molecules on the sample surface. In contrast, we did not have any problems to perform AFM with amplitude larger than 10 m peak-peak. Operation in amplitude modulation mode was not so straightforward with the self-oscillation configuration. It was, however, feasible to perform it when the setup was similar to that of the Tapping mode, i.e., with fixed driving frequency. FIG. 11. 共Color online兲 AFM images taken at ambient conditions in frequency detection mode: 共a兲 electronic chip, 共b兲 compact disk stamper, 共c兲 carbon nanotubes, and 共d兲 Si beads.
A-Probe and the deflection of the sensing lever were simultaneously recorded as a function of z-position of the A-Probe. An averaged loading force can be estimated from the deflection of the sensing cantilever and its spring constant. The sensing cantilever in this experiment was 225 m in length, and the calculated spring constant was 9.9 N/m. Figure 10 shows the results under three different tip amplitudes: 230, 460, and 680 nm peak-peak. The figure only shows data obtained, while the two probes were separating each other. All three curves in the upper graph show the same total frequency shift, ⌬f = 110 Hz. This demonstrates the independency of ⌬f from tip amplitude settings mentioned earlier. In the lower graph of Fig. 10, one finds that the increments of the effective loading force under three different conditions were almost the same, at least, in the first two-thirds of the periodic contact region. The slope in the liner region is approximately 0.47 nN/nm. Beyond the linear region, tendency of the loading force is not evident. In the quasicontact region, the A-Probe was statically pushing the sensing cantilever, and a large deflection of the sensing cantilever was observed. In order to minimize the loading force, the engaging frequency set point should be as low as possible like in optical systems E. AFM imaging
Figure 11 shows four AFM images taken in ambient conditions: 共a兲 an electronic chip, 共b兲 a compact disk stamper, 共c兲 carbon nanotubes, and 共d兲 Si beads. All images were of comparable quality to standard AFM images. No particular features due to the use of an A-Probe could be found. A relatively large tip amplitude was used when we scan a sample for the first time, typically 400–600 nm peak-peak. A typical engaging frequency set point was equal to the resonance frequency of the probe plus 15% of ⌬f, or 10–15 Hz in case the ⌬f of the probe was unknown. We found that the P and I gains of the z-feedback had to be lower during tip approaching in some cases. Once the scan started, the feedback gains as well as the other parameters were optimized by
VI. SUMMARY
A practical implementation of a new model of the A-Probe was presented based on previous works. Design, fabrication, and assembly of the probe, which are suited for batch processing, were described. The frequency shift of the A-Probe and the evolution of the tip vibration were characterized as a function of the tip-sample distance. Frequency response to a small mechanical disturbance externally applied to the tip was measured. The dynamic response characteristic could be measured up to the maximum speed of the PLL electronics. The effective loading force between the tip and the sample during the periodic contact was experimentally assessed. A special probe package to be magnetically clipped on an AFM head and a special holder for mounting an A-Probe on a commercial AFM were introduced. The selfsensing and self-actuating capability of the A-Probe was demonstrated by taking various AFM images in ambient conditions in the FM mode. The described fundamental features of the A-Probe will stimulate the development of new scanning probe instruments and enable conducting AFM measurements under conditions where common optical read-out systems cannot be easily implemented. ACKNOWLEDGMENTS
The authors acknowledge technical support from the staff of Comlab, the CSEM clean room facility. In addition, we acknowledge valuable discussions with P. Vettiger, A. Baumgartner, A. Gildemeister, T. Ihn, K. Ensslin, and A. Tonin. The authors thank Professor H. Shea of EPFL for giving access to his characterization laboratory. This work was financially supported by the Commission for Innovation and Technology 共CTI兲 of the Swiss Federation, the NCCR Nanoscale Science of the Swiss National Science Foundation, and the State of Neuchâtel. G. Binnig, C. F. Quate, and Ch. Gerber, Phys. Rev. Lett. 56, 930 共1986兲. M. Tortonese, R. C. Barrett, and C. F. Quate, Appl. Phys. Lett. 62, 834 共1993兲. 3 M. H. Hecht, J. Marshall, W. T. Pike, U. Staufer, D. Blaney, D. Braendlin, S. Gautsch, W. Goetz, H.-R. Hidber, H. U. Keller, W. J. Markiewicz, A. 1 2
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
063706-8
Rev. Sci. Instrum. 81, 063706 共2010兲
Akiyama et al.
Mazer, T. P. Meloy, J. M. Morookian, C. Mogensen, D. Parrat, P. Smith, H. Sykulska, R. J. Tanner, R. O. Reynolds, A. Tonin, S. Vijendran, M. Weilert, and P. M. Woida, J. Geophys. Res., 关Planets兴 113, E00A22 共2008兲. 4 H. Edwards, L. Taylor, W. Duncan, and A. J. Melmed, J. Appl. Phys. 82, 980 共1997兲. 5 F. J. Giessibl, Appl. Phys. Lett. 73, 3956 共1998兲. 6 M. Todorovic and S. Schultz, J. Appl. Phys. 83, 6229 共1998兲. 7 J. Rychen, T. Ihn, P. Studerus, A. Herrmann, and K. Ensslin, Rev. Sci. Instrum. 70, 2765 共1999兲. 8 W. H. J. Rensen, N. F. van Hulst, A. G. T. Ruiter, and P. E. West, Appl. Phys. Lett. 75, 1640 共1999兲. 9 H. Göttlich, R. W. Stark, J. D. Pedarnig, and W. M. Heckl, Rev. Sci. Instrum. 71, 3104 共2000兲. 10 T. Akiyama, U. Staufer, N. F. de Rooij, P. Frederix, and A. Engel, Rev. Sci. Instrum. 74, 112 共2003兲. 11 T. Akiyama, U. Staufer, and N. F. de Rooij, Appl. Surf. Sci. 210, 18 共2003兲. 12 K. Suter, T. Akiyama, N. F. de Rooij, A. Baumgartner, T. Ihn, K. Ensslin, and U. Staufer, AIP Conf. Proc. 696, 227 共2003兲.
13
T. Akiyama, K. Suter, N. F. de Rooij, and U. Staufer, Mater. Res. Soc. Symp. Proc. 838E, O11.1.1 共2005兲. K. Suter, T. Akiyama, N. F. de Rooij, and U. Staufer, Jpn. J. Appl. Phys., Part 1 45, 2099 共2006兲. 15 T. Akiyama, K. Suter, N. F. de Rooij, A. Baumgartner, A. E. Gildemeister, T. Inn, K. Ensslin, and U. Staufer, Jpn. J. Appl. Phys., Part 1 45, 1992 共2006兲. 16 D. Bayat, T. Akiyama, N. F. de Rooij, and U. Staufer, Microelectron. Eng. 85, 1018 共2008兲. 17 X. Li, T. Ono, R. Lin, and M. Esashi, Microelectron. Eng. 65, 1 共2003兲. 18 T. Ihn, “Scanning Probes for Semiconductor Nanostructures,” Encyclopedia of Nanoscience and Nanotechnology, edited by H. S. Nalwa, 共American Scientific, Jan. 2004兲, ISBN 1-58883-001-2 19 A. Baumgartner, Ph. D. thesis, Swiss Federal Institute of Technology Zurich, Zurich, 2005. 20 T. R. Albrecht, P. Grütter, D. Horne, and D. Rugar, J. Appl. Phys. 69, 668 共1991兲. 21 M. Kageshima, H. Jensenius, M. Dienwiebel, Y. Nakayama, H. Tokumoto, S. P. Jarvis, and T. H. Oosterkamp, Appl. Surf. Sci. 188, 440 共2002兲. 22 C. Su, L. Huang, and K. Kjoller, Ultramicroscopy 100, 233 共2004兲. 14
Downloaded 27 Jul 2010 to 194.225.237.18. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/rsi/copyright.jsp
