DE19532838A1 - Non contact scanning force microscope - Google Patents

Abstract

The microscope has an electrically conducting probe (1) that is moved across a specimen surface (4) without any contact and with varying distances between them. The probe is mounted at the end of a cantilevered element (2) that has a piezo element (23) to cause it to vibrate. The specimen is table mounted (5) and this has a piezo element (18) for height adjustment and this connects with a distance measuring unit. Coupled to the probe is voltage sensor (7) and this connects to a voltage supply (15). An ac signal is produced for the displacement and a compensation is applied to avoid contact.

Description

The invention relates to a non-contact, scanning, force-interaction microscope is equipped by an additional electronic circuit so that despite electrical Charging the sample enables measurements of the topography and the method of operation and the Signal detection of this charge on such a microscope.

As is known, a scanning force interaction microscope (also: Atomic force microscope (AFM), scanning force microscope (SFM), im further text: RKM) a device with which an extremely fine probe tip atomic interaction forces can be measured depending on the location.

It has ended since Binnig and Rohrer's basic work on the scanning tunneling microscope known in the 1970s that it is possible to achieve such fine tips and such precise measuring technology, that atomic resolution can be achieved [Binnig, Rohrer, Gerber, Weibel, Appl. Phys. Lett. 40 (1982) p. 178]. The measuring principle of the scanning tunneling microscope is based on the approximation of one under Voltage-bearing, conductive atomically fine tip on a conductive sample until a measurable Tunnel current begins. By extremely fine positioning in the sub-nm range, you can Record the tunnel current in a grid pattern and thus also achieve atomic spatial resolutions.

Based on this development, the RKM was developed in the mid-1980s [U. Binnig, C. Quate, C. Gerber; Phys. Rev. Lett: 56 (1986) p. 930]. The measuring principle of the atomic force microscope relies on approaching a tip of the specimen suspended from a bending beam and the To register the force interaction via the bending of the bending beam. When approaching the The tip of the test is to first reach an area in which the sum of all forces (van der Waals Interaction, writable u. a. through the Lenard-Jones potential, Coulomb interaction through Ionic species or dipoles, dispersion forces, short-range forces, etc.) attract the tip (see graphic representation).

The tip does not yet touch the sample surface. If you approach the tip further, bump the sample and tip come off and come into contact (e.g. repulsive interaction of the electro of the tip and sample atoms).

Through a control circuit with feedback, either the attractive or the repelling forces are kept constant, the former as an attractive RKM (without contact), the latter as repulsive RKM (with contact) called (attractive non-contact mode, repulsive contact mode). Emotional now taking the measurement sample in the lateral (i.e. x, y) direction, the atomic forces can be scanned (to scan). The result is a two-dimensional map (image) of the forces in the raster Area. More information can be found e.g. B. in the review by E. Meyer and H. Heinzelmann in Scanning Tunneling Microscopy II, Springer-Verlag Berlin 1992, p. 99 ff (Springer Series in Surface Sciences 28, ISBN 3-540-54555-7).

Another, more refined measurement method uses a vibrating tip that is close to one the resonance frequency excited bending beam. Because of an attractive force interaction the amplitude of the oscillation to be evaluated changes between the tip and the sample. You stop the frequency of the forced oscillation of the bending beam is constant when approaching the attractive range an amplitude depression of the vibration. If you carry this information Control loop, which changes the amplitude of the vibration by varying the distance between the tip and the sample constant, you have a second non-contact mode when scanning the sample Atomic force microscopy realized. The distance correction size is above the location as Topogra  Phyinformation written. With the help of computer technology, it is easy to correct this distance generate size-resolved images of the topography.

In addition to the van der Waals forces, the interaction of the Tip used with electrical or magnetic forces for additional information, about magnetic, dielectric or ferroelectric domains, potentials and localized charges to win. Here is an example of the summary article R. Lüthi u. a., Progress in Non-contact Dynamic Force Microscopy, J. Vac. Sci. Technol., 1994, Vol. B. 12, pp. 1673 ff.

In addition to the possibility of additional information from the effect of magnetic or electrical Gaining strength, however, is at the same time the problem that when such Forces the actual topography can be distorted because these forces are atomic Force overlay. The tip attached to the bending beam reacts to all interaction forces (atomic, electrical, magnetic, etc.) initially unselective.

The load on the sample can be pointed to the extreme in the RKM via its force interaction demonstrate: In the publication J.E. Star u. a., Deposition and Imaginging of Localized Charge on Insulator Surfaces Using a Force Microscope, Appl. Phys. Lett., 188, Vol. 53, p. 2717, e.g. B. First, the topography was recorded with a non-contact RKM. Then it is lowered A voltage of 100 V is applied to the tip for 25 ms and a charge is applied. The force distribution is then scanned again. From the comparison of both force measurements you get the information about the additional cargo. By creating an existing one Charging-modifying DC voltage of different polarity can be connected or disconnected Decreasing the force also determine the polarity of the charge.

Two further important methods for determining the charge on surfaces by means of RKM are to be described: In the publication Weaver et al. (JMR Weaver and DW Abraham, "High Resolution Atomic Force Microscopy Potentiometry", J. Vac. Sci. Technol. B9 (3) (1991), p. 1959), a sinusoidal AC voltage of frequency ω _{el is} applied between the conductive tip and the sample . This results in an oscillation of the bending beam with the frequency 2ω _el and, in the presence of electrical charge in the system, additionally with the frequency ω _el . The signal at frequency ω _el is regulated to zero by superimposing a DC voltage (null-force method). The level of the DC voltage and its polarity then corresponds to the charge on the surface. This method is based on the measurement of the electrical force interaction (via the oscillatory movement of the tip) and does not provide any information about the topography.

In the publication Said et al. (RA Said, M Mittal, GE Bridges and DJ Thomson, "High Frequency Potential Probe using Electrostatic Force Microscopy", J. Vac. Sci. Technol. A 12 (4) (1994), p. 2591) uses the same method as in Weaver et al. supplemented by mechanical excitation of the tip at the resonance frequency ω _mech , as is common in conventional non-contact RKM. This method is based on a measurement of the force interaction (via the oscillatory movement of the tip) and is able to measure both topography and charge distribution. The force interaction is described by:

There are:
F _z : sum of all forces in the z direction
z: location coordinate, distance tip-sample
V _Ext : External voltage applied to the system
A _el : Externally selected amplitude of the AC voltage
t: time
ω _mech : frequency of mechanical vibration excitation
H: Hamaker constant
C: capacity
Φ: potential due to the charge on the surface
ω _el : frequency of the applied AC voltage
A _mech : amplitude of the vibration of the bending beam
The first addition term describes the Van der Waals interaction, the second term describes all electrical interaction forces and the third term describes the mechanical excitation.

As a result of all forces acting simultaneously on the tip, it vibrates with three different, overlapping frequencies ω _el , 2ω _el and ω _mech . The occurrence of the frequency ω _el contains the information about the charge and is measured by superimposing a DC voltage until the amplitude of the frequency ω _{el is} zero. The system can then be controlled to maintain the amplitude of the frequency ω _mech , with which topography information is obtained. While this method provides the information you want, it has major disadvantages:

• - The method is complex and complicated because the tip movement consists of the superposition of three frequencies. These three frequencies have to be filtered out with high accuracy. In the publication Said et al. this problem is pointed out directly and a complex heterodyne measuring principle is described as a technical reading. This variable must be detected with the greatest accuracy close to zero, since the control condition is amplitude ω _el = 0. An error affects both the charge determination and the topography signal.
• - Since the excitation with ω _{el occurs} outside the resonance range of the bending beam at ω _mech , the resulting tip movement is small and the system is insensitive to the charge measurement.
• - The excitation of the bending beam at ω _Res and additional excitation with a further frequency ω _el outside the resonance area can lead to non-linear behavior of the vibrating system, which has an influence on the so-called Q-factor (quality factor). However, this must not change during the measurement. In addition, the vibration excitation at frequencies outside the resonance area can lead to energy dissipation due to the damping and thus to the heating of the vibrating system. Even the smallest changes in geometry at the RKM lead to dramatic incorrect measurements. B. topography and charge distribution.

Especially in atomic force microscopy on insulating samples, such as. B. plastics has in our measurements it was found that they very often carry a charge right from the start. This can sometimes be so high that the Coulomb forces actually interest the Van der Waals forces (Topography information) completely masked. This is shown in practical work in the Extreme case in that an approximation to the critical, necessary for topography measurement, Distance becomes impossible. Due to the dominance of the Coulomb forces, the regulation already sets in one such a large distance between the tip and the sample, possibly only for a short time, that the proportion of the Van der Waals forces and therefore the topography information can no longer be measured. All Try to unload the samples evenly before the measurement to avoid the problem, generally do not produce satisfactory results or have a lasting effect on the structure of the Surface.

The invention has for its object to a simple device and a method create the unadulterated topography measurements for samples with electrical charges enables the height and polarity as additional information to the topography with high accuracy the charge can be evaluated as a location at the same time.  

According to the invention, this object is achieved in that a classic non-contact RKM with a vibrating, electrically conductive tip and a sample mounted on an electrically conductive carrier is measured with an alternating current or alternating voltage measuring device with which amplitude or amplitude and phase position or real and imaginary parts are measured can be expanded in such a way that an alternating current or alternating voltage signal can be tapped and measured between the tip and the sample carrier ( Figure 1).

The configuration according to the invention is to be described in detail below:

The RKM is provided with an electrically conductive or electrically conductive measuring tip ( Fig . 1, point 1 ). This is located on an electrically conductive or an electrically conductively coated bending beam ( Fig. 1, No. 2 ). On this bending beam, the contact to the tip is led to the base of the bending beam, so that the tip can be contacted here ( Fig. 1, point 3 ). The sample ( Figure 1, Item 4 ) is located on an electrically conductive sample carrier ( Figure 1, Item 5 ), which can also be contacted ( Figure 1, Item 6 ). This prior art arrangement is extended so the inventive circuit with an AC or Wechselspannungsmeßgerät (Figure 1, Zif. 7), in that this measuring device to the one pole (Figure 1, Zif. 3) contact to the conductive tip, and with the other pole ( Fig. 1, No. 6 ) is in contact with the conductive sample holder.

It is achieved by connecting the tip and sample holder via an AC or AC voltage measuring device that in the presence of electrical charges in or on the sample, over which a ladder moves, d. H. in this case the vibrating tip over the charged one Sample that the image charges influenced in the conductor (tip) in the form of the released compensa tion charges can be measured. The free compensation charges lead to high impedance Measuring tap for the occurrence of a voltage, with low-resistance tap for the occurrence of a current in the outer circle. Accordingly occurs when the tip approaches the sample on the invention according to measuring tap an AC voltage or an AC signal, the tip must located above a charged area, which is why the topography measurement is definitely is falsified. The level of the signal in the outer measuring circuit is dependent on the level of the charge average distance between the tip and the oscillation amplitude of the tip. Are distance and Known vibration amplitude, the level of the charge can be deduced directly. There in particular the distance from sample to tip, however, is not known, there is no direct one Access to the amount of charge, so that one does not rely on the share of electrical power in the Can calculate back total force.  

The presence of a charge can be measured in the same way in any system by changing the distance between the conductor and the sample. For example, this applies to the optical scanning near-field microscope (SNOM), in which an optical fiber with a fine tip ( Figure 2, Item 8 ), which carries a conductive coating ( Figure 2, Item 9 ), swings in an arc at a short distance above the sample surface. A variant is also conceivable in which the distance change is implemented by a second, independent height adjuster ( Figure 3, Item 10 ).

Since the occurrence of the measurement signal in the outer circle alone the presence of an electrical Charging indicates, in the embodiment of the invention, the problem of charging Undistorted topography measurement solved:

According to the invention, a phase-sensitive voltage or current meter ( Figure 4, Item 7 ) is used, the phase position of which with the vibration of the tip via the vibration transmitter ( Figure 4, Item 11 ) for the vibration transmitter (usually a piezo element, Figure 4, Item 7 ) . 23 ) or the measuring device for the oscillation amplitude is correlated (e.g. via trigger or lock-in principle, Figure 4, Item 12 ). Here you can use commercially available ammeters or voltmeters ( Fig. 4, No. 7 ), such as. B. Impedance analyzer, gain phase analyzer or leak-in amplifier. The polarity of the charge can be derived from the sign of the voltage or the direction of the current as the tip approaches or is removed due to the vibration. Thus, one has the possibility, in one process step by superimposing a in the external measuring circuit (connection for the tip: Figure 4, Zif 13, connection for the sample. Figure 4, Zif. 14) fed voltage (voltage generator: Figure 4, Zif 15 ) to reduce the charge difference between tip and sample and thus the electrical force. The decrease in induction effect can be to reduce the current or voltage signal (taps: amplitude (Figure 4, Zif 16) and phase (Figure 4, Zif 17) or real part of amplitude (Figure 4, Zif 16) and Imaginärteilamplitude (fig... 4, item 17 ) when selecting the device components for the current or voltage meter ( Figure 4, Item 7 ) and the voltage transmitter ( Figure 4, Item 15 ) or by means of circuitry measures based on the state of the art Make sure that the current or voltage meter is not short-circuited via the voltage sensor so that the measured variable alternating current or alternating voltage can be measured in an unadulterated manner. In the simplest case, a high-resistance series resistor is sufficient.

Because of the now reduced electrical force, one can start using the height adjuster, e.g. For example, a piezoelectric element (Figure 4, Zif. 18), driven by a distance controller (Figure 4, Zif. 19) with a scanning of the sample, the distance constant allows (Topographiesi gnal, read out at a tap Image 4, Zif. 22) to bring the tip closer. In a next step, the applied voltage is further increased, the electrical force continues to decrease, the tip can be approximated further, as a result of which the electrical signal becomes larger again and can therefore be measured more precisely. It may make sense to make the extent and speed of the approach of the tip by the distance controller ( Figure 4, Item 19 ) dependent on the level of the voltage or current signal (control signal: Figure 4, Item 24 ) to avoid the risk of Escaping touchdown. Finally, the condition is reached that the current or voltage signal ( Fig. 4, Nos. 16 and 17 ) becomes zero because of the voltage applied to the connections for tips and samples ( Fig. 4, Nos. 13 and 14 ) The charge generated at the tip corresponds to the charge on the sample, ie the field-free state has been reached and there is no longer any influential effect. The tip is now only influenced by the forces that have no charge as a cause, as in the classic RKM, so that the topography can be measured safely and without errors despite electrical charging. The sign and amount of the applied voltage ( Figure 4, Item 21 ) also contain information about the sign and amount of the charge on the sample. Since the charge does not have to be homogeneous, it is advisable to register the sign and value of the applied voltage ( Figure 4, Item 21 ) and the distance value ( Figure 4, Item 22 ) at any location and also use it for imaging.

These process steps are carried out for each individual location. Of course, it is advisable to regulate the sign and the level of the voltage fed in by suitable electronic circuits so that the zero point of the influenced voltage or current signal ( Figure 4, Item 21 ) is automatically reached. In a second control loop, depending on the remaining signal (control signal: Figure 4, item 20 ), the measuring tip can be further lowered until finally, when the voltage or current signal is adjusted to zero, the adjustment to an atomic interaction force preset for topography measurements takes place can. The tracking variable of the adjustment in the z direction ( Figure 4, Item 22 ) for the sample as well as the sign and value of the applied voltage are saved for each location and, as is usual with the RKM, used to generate a location image. In addition to the topography image, an image of the charges is also created.

For orientative measurements that can be carried out at higher grid speeds, it is advisable to do without zeroing the voltage or current signal, but instead to register the amplitude or, better still, the amplitude and phase position above the location with a constantly controlled vibration amplitude ( Figure 4 , Items 16 and 17 ). It gives an overview of the amount and the sign of the charge. Particularly in the case of charges whose electrical force significantly exceeds the atomic force, a largely authentic picture of the charge can be obtained in a simplified manner.

Claims (13)

1. Non-contacting, surface-scanning probe microscope with a varying distance between the electrically conductive probe and the sample surface, the sample being on a conductive sample carrier, and electrical contacts leading out to the tip and sample carrier, characterized in that the contacts of the tip and the sample carrier lead out Voltage or current measuring device, preferably an AC voltage or AC measuring device, is connected. 2. Probe microscope according to claim 1, characterized in that the distance variation as in Atomic force microscope is realized by a tip located on a vibrating bending beam. 3. probe microscope according to claim 1, characterized in that the distance variation as in optical scanning near-field microscope with an arc-shaped, metallized optical fiber fine tip is realized. 4. Probe microscope according to claim 1, characterized in that the distance variation of one vibrating sample carrier is realized. 5. Probe microscope according to claim 1, characterized in that the alternating voltage or In addition to amplitude, AC measuring device also relates to the phase relationship of voltage or current the vibration movement of the tip can register so that the polarity of an existing one Charging on the sample can be closed. 6. probe microscope according to claim 5, characterized in that starting from the determined Polarity of charging the sample and tip system by feeding an adjustable DC voltage is charged so that the AC voltage or AC amplitude reduced and finally becomes zero and this DC voltage is registered. 7. probe microscope according to claim 6, characterized in that when selecting the device com components for the current or voltage measuring devices and the voltage transmitter or by the stand Technically appropriate circuit measures are given that current or voltage measurement devices are not short-circuited via the voltage transmitter or the current or voltage measurement devices do not falsify the applied DC voltage.   8. Probe microscope according to claim 6, characterized in that the process of regulating the Voltage until the AC voltage or AC amplitude is zero by a suitable one electronic switching takes place automatically. 9. probe microscope according to claim 6, characterized in that in the course of the reduction of AC voltage or alternating current amplitude and thus the electrical force effect of the distance the peak is reduced and in case of meeting the condition "alternate or AC amplitudes are zero "the distance of the tip and stored as pristine Topography information is used for image generation. 10. Probe microscope according to claim 1, 6 and 9, characterized in that at a raster topography as well as quantity and polarity of a charge are registered, stored and can be used to generate separate or combined images. 11. Probe microscope according to claim 9, characterized in that the tip approximates as far until a presettable atomic force is reached and this distance as information is output and saved. 12. Probe microscope according to claim 9, characterized in that the tracking of the tips distance depending on the height by means of a suitable electronic circuit done automatically. 13. Probe microscope according to claim 1, characterized in that even at only roughly constant held swing amplitude of the tip when scanning the AC voltage or AC current plitude or additionally also evaluated the phase position in relation to the tip movement, stored and be used for image generation.