Functional Microcantilever for a Novel Scanning Force Microscope

Transcription

1 Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008, pp Functional Microcantilever for a Novel Scanning Force Microscope Dong-Weon Lee School of Mechanical Engineering, Chonnam National University, Gwangju Adrian Wetzel and Ernst Meyer Institute of Physics, University of Basel, Basel, CH-4056, Switzerland (Received 17 July 2007) This paper presents a novel scanning probe microscope that is combined with a time-of-ight mass spectrometer for analyzing material properties of solid surfaces. Chemical analysis on the nanometer scale is achieved by transferring material from surfaces via the probing tip to the mass spectrometer under an ultrahigh vacuum condition. The instrument based on a rotatable probe holder or an actuator-integrated microcantilever allows quasi-simultaneous topographical and chemical analyses of solid surfaces to be performed in the same way as with the conventional scanning probe technique. The basic characteristics of the instrument are evaluated using the motorized rotatable probe holder and electrochemically etched tungsten tips. A Further increase of the switching speed between the scanning probe and mass analysis operation is realized by using the functional microcantilever, instead of a motorized rotatable holder. PACS numbers: Lh, Cm, Rt Keywords: Scanning probe microscope, Time-of-ight mass spectrometer, Chemical atomic force microscope I. INTRODUCTION Scanning probe microscopy has become an important tool in surface science due to its exceptional spatial resolution. However, its major drawback is the lack of chemical sensitivity. In previous report [1{8], a mass spectrometer (MS) that converts components of a sample into gaseous ions and measures their mass in a molecular level is proposed as a powerful instrument for chemical analysis applications. To increase the spatial resolution, the MS has been modied with various types of analyzers such as magnetic type, quadrupole type, time-of-ight type, the Fourier transform ion cyclotron type [9{14]. Recently, a time-of-ight(tof) type MS was shown to be an ultimate MS for chemical analysis of solid surfaces because it has some advantages in comparison with others. However, a serious drawback of those approaches is related to the requirement that the sample have a very sharp tip. An additional diculty in the process is that sample materials are limited to semiconductors or conductors. Moreover, the area being analyzed is limited to an extremely small area of the apex of the sharp tip. A combination of scanning probe microscopy (SPM) with time-of-ight (TOF) mass spectrometry in ultrahigh vacuum (UHV) gives an opportunity to overcome the limitations mentioned above. The basic idea is to transfer material from the sample surface to the SPM tip and to mems@chonnam.ac.kr; Fax: analyze it in the TOF mass spectrometer. The target material is removed from the tip and is ionized by means of eld evaporation. In this paper we describe two dierent types of instruments that have the capability of chemical analysis. The basic congurations of the two instruments, STM combined with TOF-MS and SPM combined with TOF- MS, are exactly the same except for the probe holder. A central part of the instrument for STM and TOF-MS operation is a motorized rotatable tip holder that allows one to move the probing tip from the STM position at the sample to a position in front of the entrance aperture of the mass spectrometer. A main part for SPM and TOF- MS operation is a functional microcantilever. Employing the functional microcantilever instead of the motorized rotatable holder provides many more advantages in switching speed, ion extraction voltage, SPM operation, etc. II. CONFIGURATION OF THE INSTRUMENT A photograph and a schematic drawing of the instrument are given in Figures 1(a) and (b). A typical experiment starts with imaging a sample in either the STM or the AFM operation mode. At a certain spot of interest, the tip is brought into contact with the surface in order to transfer material to the scanning probe tip. This

2 Functional Microcantilever for a Novel Scanning Force Microscope { Dong-Weon Lee et al Fig. 1. (a) Photograph and (b) schematic diagram of a scanning probe microscope combined with a mass spectrometer. transfer is easily realized by applying a voltage pulse of a few volts for a minimal duration of 10 ms between the tip and the sample surface. Subsequently, the tip is moved in front of an extraction electrode that acts as the entrance aperture of the mass spectrometer. A negative pulse is applied to the extraction electrode(ee) for 10 ns by using a Blumlein-type ns-kv pulser while a positive dc high voltage of several hundreds to several kilovolts is biased to the tip. The negative pulse amplitude is kept at 20 % to 30 % of the positive dc voltage value, which is constantly and slowly increased until the rst ion impact is detected on a multichannel plate (MCP). The ions are accelerated to their nal velocity, which is given by the total potential dierence between tip and extraction electrode. The mass-to-charge state ratio of the ions is determined by measuring the time of ight in the eldfree drift region between the extraction electrode and the MCP detector, which is mounted at a distance of 165 mm from the tip. Two types of tip holds were developed for the demonstration of the instrument. In the case of the STM conguration, the basic elements of the instrument are the sample scanner with a stick-slip motor for coarse approach and sample exchange, the motorized rotatable tip holder with a tunneling current preamplier and tip exchange mechanism, the extraction electrode with high-voltage pulse lines and the ion detector. The tip holder can be rotated and displaced along the rotation axis through a stick-slip motion. By applying saw tooth-shaped voltages to the four electrodes of the four tubes, are can move between the two operational positions within 30 s. Furthermore, the motorized holder allows exact positioning of the tip sit here in front of the extraction electrode or at varying spots of the sample surface. The funnel-shaped extraction electrode with an opening diameter of 2 mm is made of aluminum and is held by an adjustable arm. The distance between the tip and the extraction electrode can be manually adjusted with the help of a wobble stick. Other details of the instrument with the motorized rotatable tip holder are described in Ref. 15. For the second version of the instrument, we develop a functional microcantilever with an integrated actuator, a piezoresistive deection sensor and a local electrode. A functional microcantilever is employed as a main element instead of a motorized rotatable tip holder. The local electrode with a tip-electrode distance comparable to the dimensions of the cantilevers signicantly reduces the eld desorption voltage needed. The switching time between the two dierent instruments, SPM and TOF- MS, is reduced by orders of magnitude by using an integrated bimorph microactuator. The functional microcantilever will potentially provide much better chemical and topographical information on the solid surfaces [16, 17]. III. EXPERIMENTS In order to test the rst version based on the rotatable tip holder, we cleaved a highly oriented pyrolytic graphite (HOPG) sample in air and inserted it into the UHV chamber. Prior to their use in the STM or AFM, all tips were analyzed by recording the eld emission of electrons as a function of applied voltage, as shown in Figure 2 and by mass spectrometry through eld evaporation. The initial eld-emission behavior of a freshly etched tungsten tip at a base pressure of mbar was found to be very unstable. Subsequently, mass spectra of the tip were recorded by applying 50 voltage pulses at U dc = 2995 V and U P = 1000 V. The signals for W 3+ and W 4+ observed in the experiment were used to recalibrate the TOF mass spectrometer for this particular tip. After the mass analysis, the tip showed a stable eld emission behavior. It is interesting to note that eld evaporation cleans and stabilizes the tips in a way very similar to what ashing the tips to 1000 C for a few seconds, a procedure frequently used for the preparation of tips for STM experiments, does. The slope of

3 Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008 Fig. 2. Field emission behaviors as a function of distance between the extraction electrode and the tip. Fig. 4. (a) Concept and (b) optical microscope image of a functional microcantilever with an integrated actuator, a piezoresistive sensor and a local electrode. Fig. 3. Time-of-ight mass spectra from (a) a tungsten tip and (b) after bring the tip into contact with the HOPG surface. the Fowler{Nordheim plot of the eld-emission data allows one to estimate the tip radius to be 23 nm. The tip was then brought in close contact with the HOPG surface by controlling the tip-sample distance at a tunneling current of 200 pa and a 1 V sample bias. Contact with the sample was established by further approaching the tip until the I V converter was saturated for a very short time. The sample was then retracted and the tip was again aligned towards the extraction electrode. The eld-emission behavior after this manipulation was characterized by moderately higher voltages and two steps in the emission current, which clearly indicated that the tip had been modied. A mass spectrum subsequently recorded at the same settings as before, shown in Figures 3(a) and (b), exhibits two peaks, 6 and 12 amu/esu, which can be assigned to C 2+ and C +, respectively. Tungsten evaporation can no longer be observed during the experiment. The carbon signal did not disappear for 350 vol pulses. Even on the following day, the mass spectrum still proves the occurrence of carbon at the tip. Due to contamination of the tip, further peaks were observed, mainly H + at 1 amu/esu, O 2+ or O + at 16 amu/esu and N 2+ or CO + at 28 amu/esu. Figures 4(a) and (b) show the functional microcantilever used in the experiments. The values of the piezoresistor and the three metal lm resistors used in the Wheatstone bridge are 650. A network analyzer measures the frequency dependence of the SC. The measured resonance frequency of the SC is 17 khz, which is very close to the predicted value of 20 khz. The cantilever deection by the integrated actuator is evaluated as a function of an applied voltage, as shown in Figure

4 Functional Microcantilever for a Novel Scanning Force Microscope { Dong-Weon Lee et al Fig. 5. Thermal actuator characteristics (a) in air and (b) in vacuum. Fig. 7. Field emission behaviors of cantilevers with (a) the integrated extraction electrode and (b) a funnel shaped electrode. Fig. 6. Topographic images of a sample surface obtained by using a piezoresistive deection sensor. 5 (a), in air and (b) in UHV conditions, respectively. When the integrated heater is heated with more than 100 mw, Al melting is observed at 110 mw. The Al melting point (660 C) is used as a reference to calibrate the temperature vs. electrical power. The frequency response of the cantilever deection of the SC upon application of a 25 mw actuation power is measured with an optical microscope and a function generator to decide the maximum switching speed of the bimorph actuator. The resulting switching speed of 10 msec is very attractive for the TOF-SFM application. It is more than 100 times faster than currently available bulky actuators. To obtain a surface image in the non-contact mode, we installed a functional cantilever into the conventional SFM system. Two electrical wires from the piezoresistor are connected to an external Wheatstone bridge circuit, which senses the cantilever deection by measuring the change of resistance value. The measured sensitivity is R/R is / nm. Figure 6 shows a topographic image of a sample surface. The Pt-coated tip turns on at only 15 Vdc thanks to the low Pt work function (line 1 in Figure 7(a)), the eld enhancement due to the sub-20-nm tip radius and the small tip{ee distance of less than 10 m. The turn-on voltage of the cantilever with an integrated EE is less than 50 times that of the cantilever with a funnel-shaped Al electrode,

5 Journal of the Korean Physical Society, Vol. 52, No. 5, May 2008 IV. CONCLUSION Fig. 8. Time-of-ight spectrum of a functional cantilever with a Pt-coated tip. We present a functional microcantilever for a combination of scanning probe microscopy and time-of-ight mass spectrometry in an ultrahigh vacuum environment. The instrument can be easily adapted to a variety of scanning probe methods, such as STM and AFM, or to piezoresistive force sensors. Fast switching between the two operational modes is facilitated by means of a motorized rotatable tip holder. Further improvement of the instrument is achieved by using a functional microcantilever with an integrated actuator, a piezoresistive de- ection sensor and a local electrode. The instruments with two dierent tip holders have been tested on different sample surfaces. A chemical analysis of a highly oriented pyrolytic graphite sample and a Pt-coated tip has been performed using the instrument. We are currently trying to change the structure of the functional microprobe for further improvement of the TOF-SFM system, as shown in Figure 9. REFERENCES Fig. 9. SEM image of an advanced functional microscope. as shown in Figures 7(a) and (b). The tip-ee distance for the funnel-shaped Al electrode was approximately 1 mm. This eld-emission experiment is of particular interest because tips start to eld-evaporate at approximately the inverse tenfold electric eld used for the eld emission. To measure the ight time of evaporated ions from the tip apex, we place the cantilever with a Pt-coated tip into the TOF-SPM chamber. A negative pulse is applied to the EE by using a Blumlein-type ns-kv pulser while a positive dc high voltage of 800 V is biased to the tip. The negative pulse amplitude is kept at 20 % to 30 % of the positive dc voltage value. The pulser denes the departure time t 0 of ions and the MCP denes the arrival time, t, of the evaporated ions. The masses and the chemical properties of the ions can be calculated using a computer system with labview control. The capability of single-ion detection in our TOF-SFM system is about 60 %. The vacuum in the chamber during the TOF measurements was Pa. A TOF spectrum of the Pt-coated tip consisting of 30 pulses at V dc = 800 V and V P = {204 is shown in Figure 9. C + (12), Si + (28) and Pt 2+ (97.5) ions evaporated from the Pt-coated tip are observed in the mass spectrum when a negative pulse voltage is applied to the EE. Sucient electric eld for ion evaporation is achieved only at the moment of the negative pulse on the EE. [1] J. Spence, U. Weierstall and W. Lo, J. Vac. Sci. Technol. B 14, 1587 (1996). [2] U. Weierstall and J. Spence, Surf. Sci. 398, 267 (1998). [3] T. Shimizu, J.-T. Kim and H. Tokumoto, Appl. Phys. A: Mater. Sci. Process. 66, 771 (1998). [4] T. Shimizu and H. Tokumoto, Jpn. J. Appl. Phys., Part 1 38, 3860 (1999). [5] A. Fian, C. Ernst and M. Leisch, Fresenius, J. Anal. Chem. 365, 38 (1999). [6] M. Tortonese, R. C. Barrett and C. F. Quate, Appl. Phys. Lett. 62, 835 (1993). [7] R. S. Chambers and G. Ehrlich, J. Vac. Sci. Technol. B 13, 273 (1976). [8] O. Nishikawa, K. Kurihara, M. Nachi, M. Konishi and M. Wada, Rev. Sci. Instrum. 52, 810 (1981). [9] M. K. Miller, A. Cerezo, M. G. Hetherington and G. D. W. Smith, Atom Probe Field Ion Microscopy (Clarendon Press, Oxford, 1996). [10] E. W. Mueller and T. Sakurai, J. Vac. Sci. Technol. B 11, 878 (1974). [11] J. A. Panitz, Prog. Surf. Sci. 8, 219 (1978). [12] A. Crezo, T. J. Godfry and G. D. W. Smith, Rev. Sci. Instrum. 59, 862 (1988). [13] M. K. Miller, Surf. Sci. 246, 428 (1991). [14] O. Nishikawa, M. Kimoto, M. Iwatsuki and Y. Ishikawa, J. Vac. Sci. Technol. B 13, 599 (1995). [15] A. Wetzel, A. Socoliuc, E. Meyer, R. Bennewitza, E. Gnecco and C. Gerber, Rev. Sci. Instrum. 76, (2005). [16] D. Lee, A. Wetzel, R. Bennewitz, E. Meyer, M. Despont, P. Vettiger and C. Gerber, Microelectron. Eng , 635 (2003). [17] D. Lee, A. Wetzel, R. Bennewitz, E. Meyer, M. Despont, P. Vettiger and C. Gerber, Appl. Phys. Lett. 84, 1558 (2004).