Development of a piezoelectric self-excitation and self-detection mechanism in PZT microcantilevers for dynamic scanning force microscopy in liquid Chengkuo Lee a) Tokyo 153, Japan, and Department of Manufacturing Systems, Mechanical Engineering Laboratory, AIST, MITI, Tsukuba, Ibaraki 305, Japan Toshihiro Itoh and Takahiro Ohashi b) Tokyo 153, Japan Ryutaro Maeda Department of Manufacturing Systems, Mechanical Engineering Laboratory, AIST, MITI, Tsukuba, Ibaraki 305, Japan Tadatomo Suga Tokyo 153, Japan Received 12 September 1996; accepted 4 March 1997 We report on the first successful operation of a cyclic contact scanning force microscope SFM using a self-excited cantilever in liquid. Based on a new structure including a PZT reference pattern and a free-standing PZT cantilever, the piezoelectric self-excitation and self-detection mechanism for a PZT microcantilever was developed successfully. The topography is recorded by measuring the piezoelectric current variation, which corresponds to the vibration. Since the acoustic excitation from the external oscillator no longer exits, a clear single resonance peak can be obtained by using this self-excited force sensing PZT cantilever. Clear grating imaging in liquid was demonstrated, and it is compatible with the image taken in air. The future potential of applying this piezoelectric SFM to the characterization of biological samples in liquid is very promising. 1997 American Vacuum Society. S0734-211X 97 05904-0 I. INTRODUCTION Since scanning force microscopy SFM was introduced by Binnig et al. in 1986, 1 its applications in biological science have progressed rapidly. 2 Among those applications, imaging of DNA and living cells in solution has become one of the most importance. Many researchers have employed contact mode SFM in such applications, but the cantilever tip acting on the surface could be a major cause of possible deformation and the slight modification of soft biological samples. The use of the cyclic contact mode SFM, i.e., the so-called tapping mode SFM, for imaging soft biological samples can reduce the influence of lateral forces and incur less deformation on soft biological samples. 3,4 In the cyclic contact mode, the cantilever is oscillated near its resonance frequency and only periodically touches the sample surface at the bottom of each swing; then the variation of vibrational amplitude is recorded as the image signals for sample topography. A microcantilever, an external piezoelectric oscillator, and an optical displacement sensing component are necessary constituents for a cyclic contact mode SFM system. a Present address: Microsystems Lab., ITRI, Bldg. 52, 195 Sec. 4, Chung Hsing Rd., Chutung, Hsinchu, Taiwan 310, R.O.C.; Electronic mail: 860004@hq.itri.org.tw b Present address: Toto Ltd., 2-8-1, Honson, Chigasaki-city, Kanagawa Prefecture 253, Japan. Three force sensing schemes, i.e., piezoresistive, piezoelectric, and capacitive, are applied to activate the force sensing cantilevers. 5 7 Use of these force sensing cantilevers can eliminate the optical displacement sensor that occupies most of the space in a SFM and that also imposes operational difficulty for the SFM due to unavoidable alignment problems. An external piezoelectric oscillator will induce a severe damping influence on the cantilever when it operates in a liquid environment because of its relatively large volume. Only the self-excited force sensing microcantilever based on the piezoelectric scheme can overcome the drawbacks caused by the optical sensor and external oscillator without loss of performance and resolution in the SFM. 8 11 For a SFM equipped with a self-excited force sensing piezoelectric microcantilever, the essential component for cyclic contact mode SFM becomes simply the piezoelectric cantilever itself. In this study a piezoelectric PZT microcantilever for the cyclic contact SFM in liquid is proposed. Based on the successfully developed piezoelectric self-excitation and selfdetection mechanism for this PZT microcantilever, the operation of such cantilevers in liquid is characterized. Clear images of a grating sample are obtained by a SFM using this self-excited force sensing PZT microcantilever operated in a liquid environment. The images are compatible with the images taken in an ambient air environment. 1559 J. Vac. Sci. Technol. B 15(4), Jul/Aug 1997 0734-211X/97/15(4)/1559/5/$10.00 1997 American Vacuum Society 1559
1560 Lee et al.: Development of piezoelectric self-excitation 1560 FIG. 1. Schematic drawing of the self-excited force sensing PZT microcantilever for SFM used in liquid. II. PIEZOELECTRIC EXCITATION AND DETECTION MECHANISM The design and structure of the self-excited force sensing PZT microcantilever are schematically illustrated in Fig. 1. The 1.25- m-thick PZT film is deposited by a sol-gel process. 12 Ar ion beam etching is used to pattern the electrodes and the PZT layer, while reactive ion beam etching of C 3 F 8 /O 2 is applied to pattern the SiO 2 layer. Details of the microfabrication process of the self-excited force sensing PZT microcantilever are reported elsewhere. 10 Characteristics of the microfabricated PZT cantilever are outlined in Table I. The piezoelectric PZT cantilever is able to be excited by an applied ac voltage via the inverse piezoelectric effect, while the force sensing is executed by recording the piezoelectric current change due to the fact that the PZT layer may give a sensitive field response to weak stress through the direct piezoelectric effect. In order to measure the piezoelectric current caused by the cantilever vibration precisely, the capacitance current output from the piezoelectric cantilever must be negated because the piezoelectric layer of the cantilever is a capacitor as well. 8 A piezoelectric excitation and detection method for piezoelectric ZnO microcantilevers was proposed by Itoh and Suga. 8 A reference CR circuit was used to compensate for the capacitance current from the ZnO cantilevers. For these the capacitor and resistor, which have the same values as the capacitance and resistance of the ZnO cantilevers, were chosen as the reference circuit. TABLE I. Characteristics of the self-excited force sensing PZT microcantilever. Lever Lever PZT layer Calculated length width length spring constant 160 m 50 m 135 m 10.5 N/m Resonance frequency in air Resonance frequency in 2-propanol Quality factor in air Quality factor in 2-propanol 112.8 khz 60 khz 200 6 The capacitance of the ZnO layer is about 1 pf, whereas the capacitance of the PZT layer is about 50 pf. The capacitance current from the PZT cantilever is higher than the capacitance current from the ZnO cantilever. In addition, the capacitance value of the PZT layer is found to be influenced by the value and frequency of applied ac voltages in the high frequency range. 13 The capacitance value of the PZT layer also varies slightly for each different PZT cantilever. These factors make the offset canceling operation via outside electronics difficult and clear piezoelectric current signals for the PZT cantilevers become impossible to be read. As a result, in the present study the new structure includes a free-standing cantilever, and a reference pattern is developed for making the offset cancellation. Since this PZT reference pattern is the same size as the PZT cantilever, it is expected to possess the same capacitance and resistance as the cantilever. A differential current amplifier is used to record the current output coming from the reference pattern and the microcantilever when they both receive the same ac voltage. The piezoelectric current signals can then be recorded by deducting the capacitance current, which is the output from the reference pattern, from the output signals of cantilever. The variation of the vibrational amplitude can be measured by recording the piezoelectric current signals from the differential current amplifier. Figure 2 a shows the current output versus the driving frequency from the cantilever and reference pattern when the current output from both is recorded separately. The piezoelectric current versus the driving frequency recorded from the output of a differential current amplifier is shown in Fig. 2 b. Successful offset cancellation was done easily using this cantilever reference pattern structure. Compared to use of outside electronics for cancellation of the different piezoelectric cantilevers, the outside electronics themselves have to be modified depending on the capacitance and resistance of the different cantilevers. The differential current amplifier used here is sufficient for different cantilevers. III. DYNAMIC PIEZOELECTRIC SFM IN LIQUID A. Vibration versus driving frequency characteristics For the cyclic contact mode operation in liquid, the cantilever has to be excited at its resonance. Hansma et al. used the piezoelectric scanner to excite the cantilever by acoustic waves through the surrounding liquid, 3 while Putman et al. used a cantilever holder with an external piezoelectric oscillator to oscillate the cantilever. 4 Due to the strong viscous damping effect caused by the external oscillator, the cantilever is acoustically excited rather than mechanically excited by the oscillator. 4 In present study, in order to compare the two methods to each other, the PZT cantilever is either excited by an external oscillator or self-excited. The cyclic contact mode SFM using the self-excited force sensing PZT cantilever in liquid is schematically depicted in Fig. 3. The liquid used is 2-propanol, CH 3 CH OH CH 3, which has a boiling point of 82.3 C and a density of 0.78 g/cm 3 from Wako Chemicals, Co., Ltd., Tokyo. In Fig. 3 the PZT cantilever is self-excited J. Vac. Sci. Technol. B, Vol. 15, No. 4, Jul/Aug 1997
1561 Lee et al.: Development of piezoelectric self-excitation 1561 FIG. 3. Schematic diagram of a dynamic SFM using the self-excited force sensing PZT cantilever in liquid. The xy scanning and feedback actuation of tip sample spacing are executed by the tube scanner. The 2-propanol is used as the liquid in the sample holder for imitating the high viscous environment. FIG. 2. a Current output of a 160- m-long PZT cantilever and its reference pattern vs the driving frequency. The curves from the cantilever and reference pattern are recorded separately when an ac voltage of 12.5 mv is applied. b Piezoelectric current spectrum of a self-excited force sensing 160- m-long PZT cantilever in air. The excitation ac voltage of 20 mv is directly applied to the cantilever and its reference pattern. The piezoelectric current is measured by subtracting the reference current from the current output of the cantilever via the differential current amplifier. The first mechanical resonance peak is at 112.75 khz. by an applied ac driving voltage. The mechanical vibrational amplitude is represented in terms of the piezoelectric current signals from the differential current amplifier based on the mechanism discussed in Sec. II. However the driving voltage can also be applied to a piezoelectric oscillator that is connected with the cantilever; then the cantilever will be excited. Thereafter the piezoelectric charge output will vary with the change of mechanical vibrational amplitude; the difference between charge output and the setup value is taken as the feedback signal to the scanner for z actuation. This is the piezoelectric charge detection method. 14 Using this method the piezoelectric charge output from the cantilever vibrated by the oscillator in liquid can be recorded as the function of driving voltage frequency. The resulting curve is shown in Fig. 4 a. Although we did not immerse the external oscillator into the liquid, the substantial vibration from the bulk silicon base still imposed a strong damping influence on the vibration of the PZT cantilever. The curve is basically similar to the curve demonstrated by Putman et al. 4 Figure 4 b, on the other hand, shows the curve of the piezoelectric current signals versus driving frequency that is measured by the SFM shown in Fig. 3. A broad, but very clear, resonance peak near 60 khz is observed. It indicates that acoustic excitation is almost avoided by using the self-excited cantilever. The incremental trend on the piezoelectric current output as driving frequency is increased is attributed to the fact that the piezoelectric current is proportional to the driving ac voltage frequency. A modified current output versus driving frequency can be calculated from the curve of Fig. 4 b by deleting the factor ascribed to ac voltage frequency, as shown in Fig. 4 c. The quality factor is then calculated as 6 from Fig. 4 c. Comparing the curve in air, i.e., Fig. 2 b, with the one in liquid, i.e., Fig. 4 b, the earlier resonance frequency drops from 112.7 to 60 khz, and the quality factor is 200 in air and 6 in liquid, while the driving voltage used in liquid is 7.5 times greater than that in air. The vibrational amplitude versus driving voltage at the resonance can be calculated from the output current based on the method discussed in Ref. 10, in which the vibrational amplitude at the resonance equals the product of the static actuation ability and vibrational quality factor. The corresponding values are 25.6 nm/mv in air and 0.31 nm/mv in liquid. The damping coefficient per unit length of the cantilever can be obtained from the mass of per unit length, m e, times the resonance frequency, f R, divided by the quality factor, Q, i.e., C m e f R /Q. The damping coefficient of the PZT cantilever in liquid, i.e., 2-propanol, can be calculated as 18 times larger than the one in air. A strong influence by viscous damping in liquid can be concluded. B. Force curve of the cyclic contact mode and the images derived Figure 5 a shows a curve of piezoelectric current signals versus driving frequency by another 160- m-long cantilever. JVST B - Microelectronics and Nanometer Structures
1562 Lee et al.: Development of piezoelectric self-excitation 1562 FIG. 5. a Piezoelectric current spectrum of a self-excited force sensing 160- m-long PZT cantilever in liquid with an applied voltage of 375 mv. b Force curve trace in liquid of the self-excited PZT cantilever used in a. FIG. 4. a Piezoelectric charge spectrum of a 160- m-long PZT cantilever in liquid. The cantilever is excited by an external oscillator. Details of the experimental setup of the piezoelectric charge detection method can be found in Ref. 14. b Piezoelectric current spectrum of a self-excited force sensing 160- m-long PZT cantilever in liquid. The excitation ac voltage is 150 mv. The earlier mechanical resonance peak drops from 112.75 khz in air Fig. 2 b to 60 khz in liquid. The cantilever used here is the same one used in a. c A modified spectrum of b. The quality factor calculated is 6. A slight shift of resonance frequency is observed between Fig. 5 a and Fig. 4 b, and the piezoelectric current outputs are somewhat different from the data in Fig. 4 b. These differences may not be due only to the deviation of cantilever characteristics from different cantilevers, but may also be due to the amount of liquid used in experiment setup because the initial amount might be different, and this amount may decrease depending on an increase in the operation time. The force curve derived is shown in Fig. 5 b. It is measured by recording the current signals from the current amplifier while the dc actuation voltage is applied to the scanner to control the spacing between the sample surface and the cantilever end. The sensitivity of a 160- m-long PZT cantilever in liquid can be defined as the slope of the force curve and its value is 1.2 na/nm. The minimum piezoelectric current output is also derived from the magnified area of a segment in the force curve. Then the amplitude resolution can be estimated by the minimum detectable current output/slope of force curve, i.e., 0.32 na/1.2 na/nm 0.27 nm at a bandwidth of 125 Hz. Because there is an apparatus setup angle, here, 30, the vertical resolution equals the vertical vector of the amplitude resolution and its value is 2.3 Å. This vertical resolution is sufficient to image most of the biological samples. For imaging the sample, a feedback bias is needed to keep the vibrational amplitude at the set point. This bias signal is then recorded as the trace of sample displacement in the z direction, i.e., its topography. Figure 6 a shows a SFM image of a Au film coated 1.0 m pitch SiO 2 grating in 2-propanol, taken by cyclic contact SFM with a self-excited force sensing 160- m-long PZT cantilever. Figure 6 b shows an image for a similar sample taken by cyclic contact SFM with a self-excited force sensing 125- m-long PZT cantilever operated in air. 10 In general, these images agree. J. Vac. Sci. Technol. B, Vol. 15, No. 4, Jul/Aug 1997
1563 Lee et al.: Development of piezoelectric self-excitation 1563 structure, the piezoelectric current, which corresponds to the vibration, can be precisely measured. We also demonstrated cyclic contact mode SFM in liquid using this self-excited force sensing PZT cantilever. Since the acoustic excitation from the external oscillator no longer exists, a clear single resonance peak can be observed. A clear grating image in liquid was demonstrated, and it is compatible with the image taken in air. The potential of applying this piezoelectric SFM to the characterization of biological samples in a solution environment is optimistic. Because this piezoelectric SFM of the cyclic contact mode can offer a fast and convenient way for operation in liquid, it may also be possible to reduce the influence of lateral forces and limit deformation on soft biological samples. ACKNOWLEDGMENT The authors would like to thank the Japan Science and Technology Corporation ~JST! for offering the National Institute postdoctoral fellowship to one of the authors ~C.L.!. 1 G. Binning, C. F. Quate, and C. H. Gerber, Phys. Rev. Lett. 56, 930 ~1986!. M. Radmacher, R. W. Tillmann, M. Fritz, and H. E. Gaub, Science 257, 1900 ~1992!. 3 P. K. Hansma, J. P. Cleveland, M. Radmacher, D. A. Walters, P. E. Hillner, M. Bezanilla, M. Fritz, D. Vie, H. G. Hansma, C. B. Prater, J. Massie, L. Fukunaga, J. Gurley, and V. Elings, Appl. Phys. Lett. 64, 1738 ~1994!. 4 C. A. J. Putman, K. O. Van Der Werf, B. G. De Grooth, N. F. Van Hulst, and J. Greve, Appl. Phys. Lett. 64, 2454 ~1994!. 5 M. Tortonese, R. C. Barrett, and C. F. Quate, Appl. Phys. Lett. 62, 834 ~1993!. 6 T. Itoh and T. Suga, Nanotechnology 4, 218 ~1993!. 7 T. Goddenhenrich, H. Lemke, U. Hartmann, and C. Heiden, J. Vac. Sci. Technol. A 8, 383 ~1990!. 8 T. Itoh, T. Ohashi, and T. Suga, IEICE Trans. Electron. E78-C, 146 ~1995!. 9 T. Itoh and T. Suga, Sens. Actuators A 54, 477 ~1996!. 10 C. Lee, R. Maeda, T. Itoh, and T. Suga, Proceedings of the 3rd France Japan/1st Europe Asia Congress on Mechatronics, Besancon, France, 1 3 Oct. 1996, Vol. 1, p. 285. 11 T. Itoh, C. Lee, and T. Suga, Appl. Phys. Lett. 69, 2036 ~1996!. 12 C. Lee, S. Kawano, T. Itoh, and T. Suga, J. Mater. Sci. 31, 4559 ~1996!. 13 J.-F. Li, D. Viehland, C. D. E. Lakeman, and D. A. Payne, J. Mater. Res. 10, 1435 ~1995!. 14 C. Lee, T. Itoh, and T. Suga, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 43, 553 ~1996!. 2 FIG. 6. ~a! A cyclic contact SFM image of a Au film coated 1.0 mm pitch SiO2 grating in 2-propanol. The scanning rate is 2 Hz and the sampling points are 256 3 256. ~b! A cyclic contact SFM image of a sample similar to that used in ~a! taken in air. The scanning rate is 0.5 Hz and sampling points are 256 3 256. IV. CONCLUSIONS A new device that includes a PZT reference pattern and a free-standing PZT cantilever is proposed for the realization of a self-excited force sensing PZT cantilever. Using this JVST B - Microelectronics and Nanometer Structures