A unique self-actuating and self-sensing probe, which is based on a quartz tuning fork and a microfabricated cantilever, is presented for dynamic scanning probe ...
Novel Dynamic Scanning Microscope Probe and its Application to Local Electrical Measurement in an Ion Sensitive Field Effect Transistor T. Akiyama, K. Suter, N. F. de Rooij and U. Staufer Institute of Microtechnology, University of Neuchâtel Rue Jaquet-Droz 1, CH-2007 Neuchâtel, Switzerland ABSTRACT A unique self-actuating and self-sensing probe, which is based on a quartz tuning fork and a microfabricated cantilever, is presented for dynamic scanning probe microscopy. The probing tip can be electrically connected to an external source or measure unit. The sensitivity of the drain-source current of an ion sensitive field effect transistor (ISFET) was investigated as a function of the probe position in order to assess the potential of the probe in device testing, where its non-optical read-out mechanism may proof to be a particular advantage.
INTRODUCTION The atomic force microscope (AFM) [1] has become an indispensable instrument for visualising, monitoring, and characterising micro- and nano-materials. One of the research fields on which we have been working is the instrumentation of AFM, more specifically the microfabrication of cantilever and tip, which together form the probe. Two typical probes, namely silicon cantilevers [2] and silicon nitride cantilevers [3], both having integrated tips, have been developed at the early stage of the AFM. Such plane cantilevers are designed to be operated in an AFM with an optical detection system for deflection read-out. Microtechnology, originally developed for semiconductor devices and lately applied for micro electro mechanical systems (MEMS), is employed to fabricate such probes. This technology also provides a unique possibility for integrating sensors and actuators into the cantilever. Cantilevers featuring incorporated piezoresistors [4] have opened the epoch of integrated non-optical sensing in an AFM. Sometimes these and similar cantilevers are also referred to as "intelligent" probes. A special subgroup of this family of probes are self-actuating and self-sensing probes based quartz tuning forks. Several designs and concepts for tuning fork based scanning probe microscopes (SPM) have been introduced [5-10]. The lateral resolution of AFM images strongly depends on the quality of the tip. Therefore, it is essential what kind of tip is added to the tuning fork and how it is mounted onto these commercially available quartz oscillators. Various methods to implement the concept are summarized elsewhere [11]. Another critical point in the probe design is its stiffness and force sensitivity usually indicated by the spring constant. In some applications like for imaging with atomic resolution in high vacuum, it is preferable to directly use the stiff prong of the quartz tuning fork as lever, which has compliance in the order of a few kN/m. For other applications like imaging biological samples, better results will be achieve if the spring constant of the probe is less than 0.1 N/m. We have considered these two points and recently introduced a unique implementation, where the tuning fork is used to drive and sense the cantilever probe [11,12]: A microfabricated cantilever, exhibiting a monolithic, sharp tip is assembled to a commercially available quartz tuning fork. In a more advanced implementation, two additional, long and soft beams are added for electrically contacting the tip [13].
In this paper, we present an application of the advanced type, for which it is possible to apply an arbitrary potential to the tip. The sensitivity of the drain-source current of an ion sensitive field effect transistor (ISFET) was investigated as function of the probe position and applied tip potential.
SCHEME AND WORKING PRINCIPLE OF THE PROBE The basic implementation of the tuning fork driven probe [11,12] consists of a U-shaped microfabricated cantilever, exhibiting a sharp tip. The applied tuning fork is commercially available and was originally designed for wrist watches. These two pieces are assembled such that the two legs of the cantilever are attached in a symmetrical way to the two prongs of the tuning fork as shown in Fig. 1(a). The more advanced implementation, which is shown in this figure, features also two suspended, long, and soft beams for electrically contacting the tip. One end of such a contacting beams is connected to the U-shaped cantilever and the other end, which is shaped to form a bonding-pad, is fixed on the base of the tuning fork. In operation, the electrical driving signal is directly applied to the electrodes of the tuning fork in order to excite vibrations at its lowest resonance. In this mode, the ends of the two prongs are moving in-plane and out of phase, meaning that they approach and withdraw from each other. This motion forces the U-shaped cantilever to start oscillating at the same frequency but in an out-of-plane motion. The tuning fork is also used as an oscillatory force sensor similar to a quartz microbalance. Its frequency and amplitude governs that of the tip vibration, while the cantilever determines the spring constant and hence the interaction with the sample. Since a quartz tuning fork has a much higher Q-factor than a silicon cantilever of comparable resonance frequency, the so-called frequency modulation detection, which is expected to yield a higher resolution than the amplitude detection [14], can be applied for AFM imaging also in ambient condition. During dynamic mode scanning probe microscopy, the resonance frequency of the tuning fork, which is different from the eigenfrequency of the cantilever, is tracked by a phase locked loop (PLL) and kept at a set point by adjusting the sample height-position with a feedback loop.
Figure 1. The advanced implementation of the self-actuating and self-sensing probe: (a) schematic illustration and (b) SEM picture, the inset shows a close up view of the tip. The basic type of the probe consists of a U-shaped microfabricated cantilever with exhibiting a sharp tip and a commercial quartz tuning fork. This advanced type features two additional suspended, long, and soft beams for electrically contacting the tip.
Fabrication of the sensor is separated in two parts, the microfabrication of cantilever probes and the assembling them with tuning forks. The quartz tuning forks used in this study are so-called “watch crystals” whose resonance frequency is 32.768 kHz (i.e. 215 Hz). We use a commercially available type, which is as small as standard AFM chips. The silicon cantilever to be glued on the tuning fork is designed and fabricated in a similar way as conventional silicon AFM probes [2]. The differences in the design are the large pads to be glued on the tuning fork and the long and soft beams for electrically contacting the tip. The finished cantilevers are freestanding but still connected to the base-wafer by narrow support beams. The tip of the cantilever is pointing towards the base-wafer. In the assembling step, a small amount of non-conductive epoxy resin is applied to each of the free ends of the tuning fork prongs. Then, the tuning forks are aligned to the cantilevers. After the epoxy resin is completely cured, the tuning forks are separated from the wafer together with the cantilevers. At this moment, the support beams are broken. Batch assembling of 10 to 20 pieces at a time was demonstrated. Figure 1(b) shows a scanning electron micrograph of the assembled sensor. The tuning forks have prongs which are 2.4 mm long, 130 µm thick and 220 µm wide. The silicon cantilever with a resistivity of 0.15 mΩ·cm, is 500 µm long, 12.5 µm thick, and each leg is 50 µm wide. The spring constant of the cantilever calculated from these values is 66 N/m. The inset in Fig. 1(b) shows a close-up view of the tip. The tip height is about 15 µm. A typical tip radius of curvature is 10 nm and an aspect ration of the last 1.5 µm of the tip is 4:1. Half cone angles is less than 12° (viewed along the cantilever axis) and 8° (seen from the side). The tip is positioned at the very end of the cantilever so that one can easily see where the tip is scanning during operation.
SIMULTANEOUS MEASUREMENTS OF LOCAL ELECTRICAL CHARACTERISTICS AND TOPOGRAPHY OF AN ISFET In the standard application of an ISFET, the source-drain (S-D) current is modulated by the specific adsorption of chemical species above the channel area of the FET. Our new probe with the two contact beams (i.e. Fig. 1) can be used to evaluate the sensitivity of an ISFET to local variations of the electrical potential, simulating an adsorption. Since ISFET’s are also light sensitive, the self-detecting property of the probe was instrumental for this experiment.
Figure 2. Schematic illustration of the experimental setup. The tuning fork probe is in a PLL so that its resonance frequency is tracked. An ac potential is applied to the tip while scanning above the channel area of an ISFET.
The experiment setup is shown in Fig. 2. An ISFET is mounted on a tube scanner. The tuning fork is operated at its resonance frequency in a PLL, which is a similar set up as in Ref. [8]. A phase shift in electrical signal from the tuning fork is tracked and kept at a set point by adjusting the height-position of the ISFET with a feedback loop. An ac potential is applied to the tip while scanning above the channel area of an ISFET, which is driven at a constant current. By doing so, the S-D current is perturbed. These small current variations are registered simultaneously with the topography image of the channel area. It should be noted that this measurement is performed in air and no chemical reactions were involved. Hence, we do not obtain a quantitative sensitivity of the ISFET with respect to specific species, but relative charge sensitivity in the channel of the FET, which is eventually visualized on a computer. The measurement conditions are as follows: The device under investigation was an ISFET covered with an Al2O3 film [15]. It has an n-type channel with width and length of 44µm and 16 µm respectively. The ISFET is driven at a constant voltage of + 0.5 V, which leads to a constant S-D current of 30 µA. On the probe side, the tip is vibrated at 58.4 kHz, which is the resonance frequency of the tuning fork with the cantilever. Since the cantilever gives an additional stiffness, the eigenfrequency of the tuning fork (i.e. 32.768 kHz) is enhanced. The resonance frequency of the cantilever is around 72 kHz. An ac signal of 2 VRMS at 2 kHz is applied to the tip from an external source. Since the channel of the ISFET is in the depression mode, no additional dc offset was applied. A small ac current at 2 kHz due to the gate effect, which is superimposed on the constant S-D current, is monitored by a lock-in amplifier. The AFM is fully covered so that no light can influence the results. In a preliminary test, we found that the S-D current is significantly increased, if a large surface area of the cantilever is hanging over the channel region of the FET. Hence, we decided to scan only a small area at the edge of the channel in order to suppress this undesired influence. Even though we used a relatively high aspect ratio tip, electric filed from the side wall of the tip would still give a large influence in the S-D current. Figure 3 shows an approach curve on to a point in the channel area. In the upper part, the frequency shift of the tuning fork and in the lower part the relative evolution of the S-D current is given versus the z position of the tip. The vibration amplitude of the tip was set to about 20 nm peak to peak in free space. It is clearly seen that the S-D current is already influenced when the tip is 400 nm away from the sample surface, while no frequency shift was observed at this
Figure 3. An approach curve on to a point in the channel area: the frequency shift of the tuning fork (up) and the relative evolution of the S-D current (down) are given versus the z position of the tip. The vibration amplitude of the tip was about 20 nm peak to peak in free space. The base S-D current was 30 µA.
Figure 4. A typical result of simultaneous measurement of topography and sensitivity of the ISFET : (a) optical view of the channel under investigation, (b) topography and (c) S-D current images, and (d) cross section views of the dotted line in (b) and (c), respectively. position. A continuous change in resonance frequency of the sensor as a function of z position was observed from the point where the tip starts tapping the surface until to the point where the vibration of the tip becomes zero and a constant contact was established. In this narrow region, corresponding to a 10 nm tip displacement in z direction, the resonance frequency was shifted about 85 Hz. A radical change in the S-D current was also observed, but it was relatively small amount. While the tip was permanently being in contact on the sample surface, the tuning fork was significantly less sensitive to a lateral displacement of the tip, or to a force applied on the tip and the ac current due to the gate effect was almost constant. No significant damage on the tip was observed after this moderate contact. In a withdrawing step, a small hysteresis (indicated by arrows) was observed. This is mainly due to a trapping of the tip by electrostatic force. Figure 4 shows a typical result of simultaneous measurement of topography and sensitivity of the ISFET. The maximum scan size is limited by the tube scanner in our case. A difference between the channel and the field region of the ISFET are clearly seen on both topography (Fig. 4(b)) and current (Fig. 4(c)) images. The most effective location turned out to be the interface to the source region.
SUMMARY AND CONCLUSIONS A tuning fork driven scanning microscope probe is introduced with references to previous work. The self-actuating and self-sensing capability of the sensor enables us to perform dynamic mode AFM in ambient condition with the frequency modulation detection schema. The advanced implementation of the probe is designed such that an arbitrary potential can be applied to the tip. Using this tip as a scanning gate AFM on an ISFET the potential of the new probe was assessed.
The local sensitivity of the channel was visualized and we found that the interface to the source region is most sensitive. One of the advantages of the probe, non-optical detection, could be demonstrated. It is conceivable that such probes could also be used for interrogating signals in other integrated circuits, e.g. in a prober-station, where a highly accurate positioning of the prober-tips is required, or for scanning gate microscopy at cryogenic temperature [16].
ACKNOWLEDGMENTS The authors would like to thank all technicians at Comlab for their technical supports. We acknowledge valuable discussions with A. Baumgartner, T. Ihn, K. Ensslin, and A. Tonin. This work has been partially supported by the NCCR Nanoscale Science of the Swiss National Science Foundation and by the Swiss Commission for Innovation and Technology (CTI).
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