CHIN.PHYS.LETT.
Vol. 20, No. 3 (2003) 338
Shear Force Detection Using Single-Tine Oscillating Tuning Fork for Scanning Near-Field Optical Microscopy
TAN Xiao-Jing(ª«¥)1 , SUN Jia-Lin(©¤¦)1 , LIU Sheng(§¬)1 , GUO Ji-Hua( £¢)1 , SUN Hong-San(©¡¨)2 ;
1
Key Laboratory of Atomic and Molecular Nanosciences of Education Ministry and Department of Physics, Tsinghua University, Beijing 100084 2 CAAD Laboratory, Tsinghua University, Beijing 100084
(Received 29 October 2002) We propose a new method to detect near- eld by using a single-tine oscillating tuning fork with mechanically asymmetric excitation that exhibits the sensitivity and stability better than that by using a double-tine oscillating one. Comparison of shear forces for the two methods demonstrate that the single-tine oscillating tuning fork provides a simpler and more sensitive method for near- eld measurements. A theoretical analysis is presented for explanation to the greater sensitivity. The method is demonstrated by imaging a sparse-packed layer of micro-spheres in size of 200 nm. PACS:
07. 79. Fc
In recent years, non-optical detections are widely used in near- led measurements. Among these methods, shear force detection is commonly used as a key one for distance control in scanning near- eld optical microscopy (SNOM) and related techniques. The rst method of shear force detection is realized by xing a bre probe to one tine of a tuning fork that is excited by a ceramic piezoelectric tube.[1] Subsequently, the piezoelectric tube was removed and the tuning fork was excited directly.[2] These techniques work well, but they require hard e orts to attach the bre probe to a tine of the tuning fork. In addition, the high-Q factor limits the scan speed to relatively low values. A few alternative methods without tuning fork have also been applied.[3 8] The present one is realized by attaching an optical bre probe directly to a piezoelectric bimorph cantilever.[6] The main advantages of this method show that it is easy to attach probe to the bimorph and the device can work well at a low Q factor (40{50). However, it requires a very thin ( 0:3 mm) and narrow ( 1 mm) piezoelectric bimorph cantilever to obtain the comparative sensitivity as tuning fork. It is diÆcult to machine such a cantilever practically. In this Letter, we present a new method of shear force detection combining tuning fork and piezoelectric bimorph cantilever. The piezoelectric bimorph cantilever is used as the mechanical oscillator and the tuning fork is used as the detector. Di erent from the conventional tuning fork detection, only one tine of the tuning fork is excited, so the detection sensitivity can be improved greatly. In addition, it does not require awful e orts to attach the probe. With the Q factor of about 240, the detection system can work
well. Figure 1 shows the schematic diagram of our method. The piezoelectric bimorph cantilever for the mechanical excitation is in dimensions of 8 mm 2 mm 0.8 mm, which is made in Institute of Acoustic, Chinese Academic of Science. One of piezoelectric layers is used as a stimulation piezo-layer and the central electrode of the bimorph is grounded. A reference signal generator incorporated on a lock-in ampli er (Perkin Elmer) is connected to the stimulation piezo and the central electrode to drive the cantilever to vibrate. The other layer is used to attach and to excite the tuning fork. The tuning fork is commercially available with an electrical resonance quality Q of 80 000 (in vacuum) and resonance frequency of 32.768 kHz. One tine of the tuning fork is fully xed on one side of the piezoelectric bimorph cantilever by cyanoacrylate adhesive. An optical bre probe fabricated by a chemical, etching technique[9 10] is attached by cyanoacrylate adhesive on the edge of the other tine. Because the quantity of glue has little in uence on Q, it is easy to attach a probe to the tine using a 20 magni er with the probe tip sticking out about 1 mm. The two electrodes of the tuning fork are connected through a low noise voltage preampli er to the lock-in ampli er. In the experiment, the piezoelectric bimorph vibrates at the resonance frequency of the single-tine oscillating tuning fork to make the tine resonate. The mechanical excited tuning fork in our experimental set-up is a highly asymmetric system. One tine of the tuning fork is fully xed on the piezoelectric bimorph and the other one can vibrate freely. This asymmetry causes a voltage di erence between ;
Supported by the National Natural Science Foundation of China under Grant Nos 19890380-7 and 10174043 and THSJZ. To whom correspondence should be addressed: Email:
[email protected].
c 2003 Chinese Physical Society and IOP Publishing Ltd
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the two electrodes of the tuning fork under excitation. Therefore, the resonance frequency of the tuning fork produces somewhat o set from 32.768 kHz. While the free tine oscillates without the bre probe in the air, the resonance frequency is about 31.7 kHz, shifting about 1 kHz, and the Q factor is about 400. When the stimulating volt on the bimorph is 10 mV, the oscillating amplitude of the detection system is about 20 nm, which is mainly decided by the amplitude of tuning fork because the resonance frequency of piezoelectric bimorph cantilever is 6kHz and the bimorph vibrates slightly at 31.7 kHz compared with that of the tuning fork. If a bre probe is glued to the free tine, the resonance frequency shifts to about 33.4 kHz and the Q factor decreases to 240 or so, but the detection system can work well with such a low Q factor.
Fig. 1. Scheme construction of the single-tine oscillating tuning fork detection set-up
Figure 2 shows the typical voltage di erence measured between the two electrodes of the tuning forks under the conditions: (a) a single-tine oscillating fork without probe, (b) a single-tine oscillating fork with a bre probe, (c) a double-tine oscillating fork without probe and (d) with a bre probe. Each curve is wonderfully linear. The most important point here is the response of the single-tine oscillating mode with or without the probe presented a greater output voltage, i.e., provided that there is greater sensitivity than the double-tine oscillating mode. For a driving voltage of 10 mV, the output peak values after enlarged 25 times by the preampli er were 22.10, 10.52, 4.65, and 1.10 mV, respectively, for the conditions mentioned above. As a result of attaching a bre probe, a 53% decrease of peak value was measured in the singletine oscillating mode but a 77% decrease of voltage was measured in double-tine oscillating mode. The reason is that in the double-tine oscillating mode, the two tines oscillate with the opposite phases so that the force between two tines is much greater than that between the fork and the holder, and the excitation
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energy should distribute in both the tines. While in the single-tine oscillating mode, one tine is adhered fully to the holder (the bimorph) and only the other tine oscillates, hence the excitation energy fastens on the oscillating tine only. Therefore, the single-tine oscillating tuning fork oscillates more greatly than the double-tine one. Because attaching a probe to the double-tine oscillating tuning fork causes much asymmetric distribution of mass between the two tines, its oscillating amplitude decreases more than that of the single-tine tuning fork. Figures 3(a) and (b) show the approach curves and the retracting curves of the double-tine oscillating and single-tine oscillating tuning forks, respectively. If we de ne 10%{90% of the transition as the interaction zone, we can observe a 20 and 6 nm interaction zone for Figs. 3(a) and (b), respectively. The shapes of these two curves are similar to each other, both the approximate beelines in the near- eld range, but the curve of single tine oscillating fork is steeper than that of the double-tine oscillating one. The slope of the curve represents the sensitivity of detection system. The slopes of the beeline ports in Figs. 3(a) and (b) are 0.041 and 0.152 V/nm, respectively, i.e., the sensitivities are 0.0141 and 0.152 V/nm, respectively. To obtain the plain near- eld images, 50% amplitude dumping is needed in the double-tine oscillating system but 10% amplitude dumping in the single-tine oscillating system and the corresponding tip-sample distance is 30{50 and 60{70 nm, respectively, which show that a much smaller force can be measured by the single-tine oscillating mode other than the double-tine one. In addition, because the signals of excitation and detection are separated, the interference between them can be avoided and the single oscillating system works more steadily, even for a whole day.
Measured voltage di erence between the two electrodes of the tuning fork in (a) a free single-tine oscillating tuning fork, (b) a single-tine oscillating tuning fork with probe, (c) a free double-tine oscillating tuning fork, and (d) a double-tine oscillating fork with a probe. Fig. 2.
We can illustrate the higher sensitivity of the
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single-tine oscillating mode using the main resonance equation.[11] The main resonance mode should be modelled by equations for the coupled linear oscillator: my1 + by_1 + k(y1 y2 ) = F exp(i$t); (1) (m + m )y2 + (b + b )y_2 k(y1 y2) = F exp(i$t);
(2) where y1 and y2 are the amplitudes of the tines without and with the probe respectively, m is the e ective mass of the free tine, b is the dumping coeÆcient, k is the spring constant of the tines for the interesting mode. The additional mass m and the dumping coeÆcient b represent the in uence of a probe glued to one tine. Force F here is not the complete exciting force putting on the tuning fork but a di erence between the two forces putting on the two tines. The two equations can be expressed in the following form: i h 2(m + m )y2 2b 1 + 2mm + b y_ + 4k 1 + 2mm y2 = 2F exp(i$t): (3) t
t
t
t
t
t
t
t
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where
k0 = 4k(1 + m =2m): t
Substituting b0 = 2(1+ m =2m)b + b for new dumping coeÆcient, in view of the energy, we can obtain p 0 $0 2k (m + m ) : 0 Q = $ = b0 In our design that only one tine is excited, the resonance mode equation reads (m + m )y + (b + b )y_ + k[y A exp(i$t)] = 0; where A is the amplitude of oscillation of excitation piezoelectric bimorph with the glued fork tine. Resonance frequency is p $0 = k=(m + m ) and p Q = k(m + m )=(b + b ): The spring constant k and quality factor Q decide the sensitivity of the system, p [12] i.e., the equivalent spring constant K = k= 3Q, the more the sensitivity, the less the K value. Since m is much less than m, we can obtain k0 4k and b0 = 2b + b as well as K 0 3K . Therefore, the single-tine oscillating tuning fork has sensitivity higher than the double-tine oscillating one. t
t
t
t
t
t
t
t
t
t
Approach curves of the amplitude signal at resonance frequency as a function of tip-sample gap for (a) double-tine oscillating tuning fork and (b) single-tine oscillating tuning fork. The curves here were the results in the same driving voltage of 10 mV and the output signals were enlarged 25 times by a preampli er. Fig. 3.
Its solution supplies the resonance frequency p $00 = k0 =2(m + m ); t
Shear-force (a) and near- eld optical (b) images (2 m 2 m) of a sparse-packed layer of spheres in size of 200 nm by the single-tine oscillating tuning fork detection technique. An uncoated bre probe working in collection mode is used in this case. Fig. 4.
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A sparse-packed layer of micro-spheres in size of 200 nm is imaged by our SNOM set-up using the single-tine oscillating detection system in the collection mode. Figures 4(a) and (b) show the shear-force image and the near- eld optical image, respectively, both in area of 2 m 2 m. The exciting voltage is 10 mV and the Q factor of the detection system is 226 as well as the oscillating amplitude of the single tine is estimated to be about 10 nm. It is worth pointing out that an uncoated bre probe whose tip is in size of 100 nm is used in this case. The resolution is also estimated to be about 100 nm. Because half of the spheres are immersed into the gelatin and the height detected by the probe is 120 nm or so. The two images show that the single-tine oscillating has an excellent sensitivity and the near- eld optical image can be achieved by an uncoated probe in nano-meter size. In summary, we have successfully developed a new shear force detection technique for near eld information. In our technique, only one tine is excited and the other is glued on a piezoelectric bimorph cantilever that is used as the mechanical oscillator. Force curves as well as shear force and near- eld optical images of a sparse-packed layer of micro-spheres show that the
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single-tine exciting detection techniques has a higher sensitivity than the double-tine exciting one. At the same time, a theory analysis is presented for the higher sensitivity.
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