Appl. Phys. A 66, S295–S297 (1998)
Applied Physics A Materials Science & Processing Springer-Verlag 1998
Stable operation mode for dynamic noncontact atomic force microscopy H. Ueyama, Y. Sugawara, S. Morita Department of Electronic Engineering, Faculty of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565, Japan (Fax: +81-6/879-7764, E-mail:
[email protected],
[email protected],
[email protected]) Received: 25 July 1997/Accepted: 1 October 1997
Abstract. We investigated the force interaction between tip and surface in the constant-vibration mode and the constantexcitation mode. We found that the distance dependence of the frequency shift reverses in the constant-vibration mode, but it does not in the constant-excitation mode. The repulsive force interaction in the cyclic-contact region weakens in the constant-excitation mode because of the decrease of the vibration amplitude of the cantilever, whereas the repulsive force interaction does not weaken in the constantvibration mode. Besides, it is quite effective in avoiding the degradation of the initial sharp tip and the destruction of the sample surface when the sudden contact between the tip and the sample surface occurs. We conclude that the constant-excitation mode is stable and gentle compared with the constant-vibration mode.
Recently, several groups including ourselves have reported true atomic resolution images using noncontact atomic force microscopy (AFM), where clean semiconductor surfaces such as a Si(111) 7×7 reconstructed surface [1–4] and an InP(110) 1×1 surface [5, 6] have been observed under ultrahigh vacuum (UHV). In these experiments, a frequency modulation (FM) detection method [7] was used. In the FM detection method, the cantilever for the force sensor is vibrated at the mechanical resonant frequency and its frequency shift due to the force interaction is detected as the force gradient acting on the tip. The sensitivity of the force gradient measurement increases drastically because the high Q value of the oscillating cantilever is used in UHV. In order to obtain stable AFM images with high signalto-noise ratio, it is important to realize the most suitable operation conditions for the noncontact AFM imaging using the FM detection method. In the noncontact AFM, as the feedback signal, the frequency shift of the oscillating cantilever due to the force interaction between tip and sample is used [1–6]. In the noncontact AFM, pure atomic force is measured and hence both insulators and conductors can be measured. The time-averaged tunneling current between the conductive AFM tip and conductive sample is also used as the feedback signal [8, 9], but this is not noncontact AFM.
In the noncontact AFM, two operation modes (namely, the constant-vibration mode [1, 3–6] and the constant-excitation mode [2]) have been used. In the constant-vibration mode, the vibration amplitude of the cantilever is maintained constant. On the other hand, in the constant-excitation mode, the excitation voltage supplied to the piezo-transducer that is used to vibrate the cantilever is maintained constant. In this paper, we discuss the stability of the two operation modes during noncontact AFM imaging by comparing the distance dependence of the frequency shift of the oscillating cantilever and tip–sample force interaction from the noncontact region to the cyclic-contact region. 1 Experimental Figure 1 shows a schematic diagram of the noncontact-mode AFM using the FM detection method. We used a home-built UHV-AFM described briefly elsewhere [10]. The deflection of the cantilever is detected by an optical fiber interferometer, which is one of the most sensitive displacement sensors. The cantilever scanning and excitation was done by a piezoelectric tube scanner. The two operation modes (constant-vibration mode and constant-excitation mode) can be changed by switching the signal into the automatic gain control (AGC) circuit. In the constant-vibration mode (switch 1 is “ON”), the input signal to the AGC circuit is the deflection of the cantilever, as shown in Fig. 1. The AGC circuit controls the gain of the variable-gain amplifier to maintain the vibration amplitude of the cantilever constant. In the constant-excitation mode (switch 2 is “ON”), the input signal to the AGC circuit is the output signal of the variable-gain amplifier. The AGC circuit maintains the excitation voltage supplied to the piezoelectric tube scanner constant. As a force sensor, a conductive silicon cantilever (n+ -type, 0.01–0.02 Ω) was used. The spring constant and the mechanical resonant frequency were 36 N/m and 161 kHz. The Q-factor was about 29 000 in UHV. The sample was Zn-doped p-type GaAs(001) wafer (carrier concentration 1.4 × 1019 /cm3 ). The GaAs(001) wafer was cleaved in situ along the (110) plane. A GaAs(110) cleaved surface was used to avoid the chemical bonding between the Si tip and
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Fig. 1. Schematic diagram of noncontact AFM using the FM detection method
the surface, because it has no surface state in the band gap and is chemically inert compared to the Si(111) surface. The noncontact AFM measurement was performed at room temperature. 2 Results and discussion Figure 2 shows simultaneously measured approaching traces of the frequency shift (a) and the excitation voltage supplied to the piezoelectric tube scanner (b) in the constant-vibration mode. Figure 3 shows simultaneously measured approaching traces of the frequency shift (a) and the vibration amplitude (b) of the cantilever in the constant-excitation mode. The initial vibration amplitude of the cantilever without the force interaction was 180 Å in both operation modes. The sample bias-voltage was set to be +1 V to compensate for the contact potential difference between the tip and the sample surface.
Fig. 2. a shows approaching trace of the frequency shift of the cantilever and b shows the simultaneously measured excitation voltage supplied to the piezoelectric tube scanner as a function of tip–sample distance z in the constant-vibration mode. The distance dependence of the frequency shift reverses at point A. Contact occurs at point B where the excitation voltage starts to increase
Fig. 3. a shows approaching trace of the frequency shift and b shows the simultaneously measured vibration amplitude of the cantilever as a function of tip–sample distance z in the constant-excitation mode. Contact occurs at point C where the vibration amplitude of the cantilever starts to decrease and the gradient of the frequency shift curve changes abruptly
It should be noted that the attractive force gradient acting on the tip increases with decreasing tip–surface separation in the noncontact region. Thus, the vertical axes of the frequency shift in Fig. 2a and Fig. 3a were taken so that upward and downward movements on the vertical axes corresponded to the increase and the decrease of the attractive force gradient, respectively. In the constant-vibration mode in Fig. 2, the excitation voltage was almost constant up to point B (region I) and then quickly increased (region II) with decreasing tip–surface distance. An increase in the excitation voltage means an increase in the energy loss of the oscillating cantilever as a result of the cyclic repulsive contact between the tip and the sample surface. Therefore, point B at which the excitation voltage starts to increase is the contact point between the tip and the surface, and hence regions I and II are the noncontact- and the cyclic-contact regions, respectively. On the other hand, the frequency shift, first gradually and then quickly, decreased (region I) and became a minimum at point A. Then it increased rapidly from point A (region II) with decreasing tip– surface distance. It should be noted that, in the cyclic-contact region II, the distance dependence of the frequency shift reverses at point A when the tip approaches the surface only at a distance of 3 Å from the contact point B. In the constant-excitation mode in Fig. 3, the amplitude of the cantilever was almost constant up to point C (region I), and then decreased almost linearly (region II) with decreasing tip–surface distance. A decrease in the vibration amplitude means an increase in the energy loss of the oscillating cantilever because of the cyclic repulsive contact between the tip and the sample surface. Therefore, point C at which vibration amplitude starts to decrease is the contact point between the tip and the surface, and hence regions I and II are the noncontact- and the cyclic-contact regions, respectively. On the other hand, the frequency shift gradually and then quickly decreased up to point C (region I), and decreased rather slowly from point C (region II). It should be noted that, in the cyclic-contact region II, the distance dependence of the frequency shift does not reverse when the tip approaches the surface even at a distance of 10 Å from the contact point C.
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In the cyclic-contact region, the rate of decrease of the distance between the tip apex and the surface weakens in the constant-excitation mode because of the decrease of the vibration amplitude, while it does not weaken in the constantvibration mode. Therefore, if the tip accidentally touches the surface during noncontact AFM imaging, the repulsive force interaction between the tip and the surface will weaken in the constant-excitation mode, whereas it will not weaken in the constant-vibration mode. This means that the constantexcitation mode AFM is much more gentle than the constantvibration mode AFM for the sudden repulsive contact, so that it is quite effective in avoiding the degradation of the initial sharp tip and the destruction of the sample surface. In the noncontact AFM imaging using the FM detection method, the frequency shift of the cantilever is used as a feedback signal to control the tip–sample distance. In order to obtain clear noncontact AFM images, the frequency shift is set at a slightly lower value than at the contact point (z > 0). In the cyclic-contact region, the distance dependence of the frequency shift reverses in the constant-vibration mode, but it does not in the constant-excitation mode. If the distance dependence of the frequency shift reverses due to the sudden feedback error, the negative-feedback loop to maintain the tip–sample distance becomes positive feedback and hence brings the tip closer to the surface until the cantilever oscillation stops. Thus, the reverse of the frequency shift means a strong mechanical contact between the tip and the surface, which induces irreversible damage to the tip apex and the surface. As described above, the distance dependence of the frequency shift reverses even at the distance z = −3 Å in the constant-vibration mode, whereas it does not reverse up to the distance z = −10 Å in the constant-excitation mode. This suggests that in order to avoid the strong mechanical contact between the tip and the surface, the tip–sample distance should be precisely controlled within the stability margin of 3 Å in the constant-vibration mode and 10 Å in the constantexcitation mode. The stability margin of 3 Å of the distance control is less than the height of monatomic step, 4.0 Å, of GaAs(110). It should be noted that the strong mechanical contact between the tip and the surface due to the reverse of the distance dependence of the feedback signal can be avoided by inserting an absolute-value circuit between the FM demodulator and the feedback and scan controller as shown in Fig. 1. If the frequency shift becomes positive (dashed line) as shown in Fig. 4, the absolute-value circuit will invert the sign of the frequency shift (solid line) and hence will reverse the distance dependence of the frequency shift. Here, the tip–surface distance will be stopped at Z C by the negative-feedback loop, and hence the irreversible strong damage of the tip apex and the surface can be avoided. 3 Conclusions We have investigated the force interaction between tip and surface in the constant-vibration mode and in the constantexcitation mode. We found that the distance dependence of the frequency shift reverses at a distance of 3 Å from the contact point in the constant-vibration mode, whereas it does not occur even at 10 Å from the contact point in the constant-
Fig. 4. Schematic of the frequency shift curves in the constant-vibration mode with and without the absolute-value circuit. The distance dependence of the frequency shift reverses at Z B . If the frequency shift becomes positive (– – –), the absolute-value circuit will invert the sign of the frequency shift (——) and hence will reverse the distance dependence of the frequency shift. Z A is the initial set point of the tip–surface distance. Here, the tip– surface distance will be stopped at Z C by the negative-feedback loop, and hence the irreversible strong damage of the tip apex and the surface can be avoided
excitation mode. Furthermore, we found that the repulsive force interaction in the cyclic-contact region weakened in the constant-excitation mode because of the decrease of the vibration amplitude of the cantilever, but the repulsive force interaction did not weaken in the constant-vibration mode. These results suggest that at present, the constant-excitation mode has the advantage of stability during noncontact AFM imaging compared to the constant-vibration mode. Further technical improvements of the precise control of tip–sample distance within the stability margin are expected to avoid strong mechanical contact in the constant-vibration mode, because this mode has an advantage in investigating the force interaction between tip and surface from the attractive-force region to the repulsive-force region. Acknowledgements. The authors would like to thank Mr. S. Mishima of Olympus Optical Co., Ltd. for construction of the AFM unit. A part of this work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture.
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