JPS63121740A - Surface analyzing method and its device - Google Patents

Abstract

PURPOSE:To attain accurate and highly reliable surface analysis by allowing a probe to scan the surface of a sample in its height direction by photoirradiating the surface or without photoirradiating, detecting tunnel current, moving the probe in the height direction so that the current is fixed, and measuring the moving distance of the probe in the height direction correspondingly to the scanning position. CONSTITUTION:A fixed voltage is impressed from a power supply 21 to an interval between a sample 13 and the probe 14, and while vertically moving the probe 14 by a piezo-electric element 15a, the surface of the sample 13 is scanned in the prescribed direction so that the tunnel current IP is fixed. Relation between the horizontal driving distance of the scanning and the height direction moving distance of the probe 14 is measured. Then laser beams 32 projected from a laser light source 31 irradiate the surface of the sample 13 through a window 32 and similar measurement is executed. The results of both measurements are compared and the contribution of a work function included in a signal can be separated from the contribution of surface ruggedness in accordance with a change in the height of a peak.

Description

[Detailed Description of the Invention] [Purpose of the Invention (Industrial Application Field) The present invention relates to a surface analysis technique for measuring the fine structure and surface elemental composition of a solid surface such as a semiconductor or a metal.
In particular, the present invention relates to a surface analysis method that utilizes a tunnel current flowing between a sample to be measured and a metal probe, and a surface analysis device used therefor.

(Prior Art) In recent years, research on crystal structures and surface compositions specific to the surfaces of solids has become active. Such surface research is essential for the development of electronic devices and solid catalysts that utilize molecular adsorption on surfaces such as gas sensors. It is also extremely important for elucidating the basic mechanisms of dry etching and thin film growth.

By the way, methods for researching solid surfaces include spectroscopic methods that are sensitive to the surface element composition, such as Auger electron spectroscopy, photoelectron spectroscopy, and secondary ion mass spectrometry, and methods such as backscattered electron diffraction and electron microscopy. There are structure-sensitive methods. A common problem with these methods is that they all provide only average information from a relatively large area on the surface.

In contrast, scanning tunneling electron microscopes, which have been attracting attention recently, can identify individual steps on the atomic order. In a scanning tunneling electron microscope, a positive voltage is applied to a metal probe with a sharp tip, the probe is brought very close to the sample surface, and tunneling electrons flowing from the sample into the probe are detected. Taking advantage of the fact that the tunneling current is sensitively dependent on the distance between the tip of the probe and the sample surface, by scanning while keeping the height of the probe constant so that the tunneling current remains constant, it is possible to increase the atomic density of the sample surface. The unevenness can be measured as the vertical movement of the probe. Therefore, it becomes possible to measure the fine structure of the sample surface.

However, this type of method has the following problems. That is, the tunneling current depends not only on the distance between the probe tip and the sample surface but also on the work function of the sample surface. For this reason, there is a problem in that it is not possible to distinguish whether the measured vertical movement of the probe directly reflects the unevenness of the sample surface or indicates a local change in the work function. Furthermore, in the measurement of elements adsorbed on the surface of compounds and Alloy 9, there was a problem in that it was not possible to distinguish whether a signal change was due to a change in the surface element or a change in irregularities.

(Problem to be solved by the invention) In this way, in conventional scanning tunneling electron microscopes, it is difficult to determine whether the measured vertical movement of the probe directly reflects the unevenness of the sample surface or indicates local changes in the work function. It was difficult to perform accurate surface analysis. Furthermore, with other analysis methods, only average information from a fairly large area on the surface can be obtained, making it difficult to perform surface analysis with high resolution.

The present invention has been made in consideration of the above circumstances, and its purpose is to be able to separate the contribution of the work function contained in the signal of a scanning tunneling electron microscope and the contribution of surface irregularities, and to The object of the present invention is to provide a surface analysis method that can obtain more detailed information regarding surface composition and surface structure, and can perform accurate and reliable surface analysis.

Another object of the present invention is to provide a surface analysis device for carrying out the above method.

[Structure of the Invention] (Means for Solving the Problems) The gist of the present invention is to apply a predetermined voltage to a probe facing close to the surface while irradiating the sample surface with radiation having a work function or less. , the amount of Fermi level due to photoexcitation
The point is to measure the tunnel emission current of electrons that have acquired two energies. Furthermore, by comparing the tunnel current measured in this way under radiation irradiation with the tunnel current measured without radiation irradiation (actually, by comparing the amount of variation in the height direction of the probe), it is possible to determine whether the sample surface The objective is to estimate the local work function distribution above.

That is, the present invention provides a surface analysis method in which a predetermined voltage is applied between a sample to be measured and a metal probe, and the sample surface is analyzed based on a tunnel current flowing between the sample and the probe. The probe is scanned in the in-plane direction of the sample surface to detect the tunnel current at that time, and the probe is moved in the height direction perpendicular to the sample surface so that the current remains constant, corresponding to the scanning position. Measure the amount of movement of the probe in the height direction, and further measure the amount of movement of the probe in the height direction in the same manner as above while irradiating the surface of the sample with radiation having a photon energy less than the work function of the sample surface. Alternatively, the tunnel current change is measured while holding the vertical movement amount of the probe to be the same as the vertical position at the same measurement point in the first measurement step, and then the change in the tunnel current is measured. In this method, the surface of the sample is analyzed based on each measurement value obtained.

The present invention also provides a surface analysis apparatus for carrying out the above method, including a metal probe held close to the surface of a sample to be measured, and means for scanning the probe in the in-plane direction of the sample surface. , means for applying a predetermined voltage between the sample and the probe; means for detecting a tunnel current flowing between the sample and the probe; means for moving the probe in a height direction perpendicular to the sample surface; means for measuring the amount of movement of the probe in the height direction corresponding to the scanning position; and a means for irradiating radiation having a.

(Function) According to the present invention, the tip height variation information obtained in the second measurement step when light is irradiated is higher than the tip height variation information obtained in the first measurement step when light is not irradiated. The height variation information of is largely contributed by the work function of the sample surface. Therefore, by comparing each piece of information obtained in the first and second measurement steps, it is possible to know the local change in work function on the sample surface. Furthermore, if the local change in work function is known, by subtracting the contribution of this work function from the detection information in the first measurement step, it is possible to accurately determine -9 monoatomic irregularities on the sample surface. can. Therefore, the atomic irregularities and local work function changes on the sample surface can be accurately known, and the sample surface can be analyzed accurately and with high reliability.

(Embodiment) First, before describing the embodiment, the basic principle of the present invention will be explained with reference to FIG. 4. Figure 4 shows the sample surface 4.
1 and a probe tip 42, and a schematic diagram showing potential states for electrons in a vacuum 43 between them.

Fermi level 44 of the sample and vacuum level 45 just above the sample surface
There is a barrier of work function φ between . Further, a voltage V 1 is applied from the outside between the Fermi level 46 of the probe and the Fermi level 44 of the sample. Note that φ is the work function of the tip.

Here, in the absence of radiation irradiation, electrons in the sample are tunneled and emitted into the vacuum through the line 47 and flow into the probe tip 42. At this time, the current I and the work function φ,
There is a relationship S between the probe-sample distance S r . pS to measure 1 by changing the distance r
If p, then the average work function can be measured. However, as the distance increases, the measurement target expands, so if there is a change in the work function at the atomic level, it is difficult to distinguish that change from surface irregularities.

On the other hand, when radiation is irradiated, in addition to the line 47, electrons below the Fermi level are once exposed to photon energy (h
After absorbing ν), it is released into vacuum via line 48. For this reason, it is expressed as a tunnel electric. However, A is a constant that depends on the light intensity and the density of states of the electronic level, and is a value that can be measured experimentally.

Here, if we assume that φ ) hν for simplicity, then the 0
The formula can be expanded as follows.

Ip' = Ip (1+Ae2φ!'>, ■,
■h ν ,・,φ8− □ ・・・・■4In ((
The formula Ip'-1p)/Alp1 indicates that the ratio of current 1' to I during radiation irradiation becomes larger as the work function becomes smaller. That is, during radiation irradiation, the signal intensity is emphasized in the portion where φ is small.

Furthermore, the equation (2) does not include the term r of surface unevenness, and shows that the work function φ8 can be directly determined from the comparison between s 1 ' and Ip. That is, by measuring the distribution p of 1' and 1, it is possible to measure the distribution of the work function on the sample surface.

In the manner described above, it is possible to separate and measure the work function term, which is sensitive to the type of element, in the signal intensity of a scanning tunneling electron microscope. Furthermore, the value of this work function is set to 0
By substituting pS into the equation, it becomes possible to measure the original unevenness of the sample surface, that is, r.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram showing a surface analysis device according to an embodiment of the present invention. In the figure, 11 is a vacuum container, and inside this container 11 is a sample holder 12. Sample to be measured 13. A metal probe 14 and a drive mechanism 15 using a piezo element are housed. The sample 13 is attached to the lower surface of the sample holder 12, which has a circular hole in the center. The base end of a drive mechanism 15 is fixed to the upper surface of the sample holder 12, and the probe 14 is fixed to the free end of this drive mechanism 15. The drive mechanism 15 is
Piezo element 15a for vertical fine movement, piezo element 1 for horizontal fine movement
5b and a piezo laminated element 15C for vertical coarse movement. Note that the vacuum container 11 is installed on a vibration isolation table (not shown) for preventing vibration.

A DC power supply 21 and an ammeter 22 are connected in series between the sample 13 and the probe 14. Further, a feedback circuit 23 is connected to the vertical fine movement piezo element 15H. This feedback circuit 23 inputs the current value detected by the ammeter 22 and controls the amount of expansion and contraction of the piezo element 15a so that this value remains constant. Thereby, the distance between the surface of the sample 13 and the tip of the probe 14 is maintained so that the tunnel current flowing between the sample 13 and the probe 14 is constant. In addition to the feedback circuit 23, the piezo element 15. The applied voltage information given to a and the amount of vertical movement of the probe 14 are stored in storage means (memory, plotter, etc.), not shown, in correspondence with the scanning position of the probe 14.

The configuration up to this point is the same as that of a conventional scanning tunneling electron microscope, and this embodiment differs from this in that a means for irradiating the surface of the sample 13 with light is provided. That is, a laser light source 31 is arranged outside the container 11, and the laser light 32 emitted from the laser light source 31 is directed toward the container 1.
The light is introduced into the container 11 through a light transmitting window 33 provided on the side wall of the probe 14, and the sample surface located below the probe 14 is irradiated. Here, as the laser light source 31, an Ar laser with a wavelength of 48 g [r+n+] was used, and its output was 10 [W/cm2]. Note that this light only needs to have energy greater than the work function of the sample surface, and other laser light may also be used. Furthermore, it is also possible to use radiation instead of light.

Next, a surface analysis method using the above device will be explained.

The present inventors attempted to analyze the (110) plane of GaAs in order to confirm the effects of this analysis method and analyzer.

Applying a voltage of 2 [V] between the sample 13 and the probe 14,
While moving the probe 14 up and down so that the tunneling current is constant at 1o-10 [A], the probe 14 is set to <111. ＞
scanned in the direction. Here, the vertical movement of the probe 14 was performed by the piezo element 15a, and the scanning of the probe 14 was performed by the piezo element 15b.

FIG. 2 is a characteristic diagram showing the measurement results. The horizontal axis is the horizontal driving distance, which is the probe scanning position, and the vertical axis is the vertical driving distance, which is the amount of movement of the probe in the height direction.

The solid line is measured without laser irradiation, and two peaks appear at regular intervals. In addition,
This measurement result is equivalent to the output signal of a conventional scanning tunneling electron microscope.

FIG. 3 is a schematic diagram of the crystal structure of GaAs viewed from the <110> direction. GaAs has a zincite-type crystal structure, consisting of atom A represented by a circle and atom B represented by a circle with a cross.The single circle represents the atoms in the outermost layer of the surface, and the double circle represents the atoms in the outermost layer of the surface. It represents the atoms of the second layer. Comparing FIG. 2 and FIG. 3, it can be seen that the probe was scanned along the dashed line in FIG. In Figure 2, two peaks with different heights can be seen corresponding to the A atoms in the outermost layer of the surface and the B atoms in the second layer of the surface, but when looking only at the solid line data, the difference in height indicates that the elements are different. It cannot be determined whether this is due to the difference in height between the A and B atoms due to surface rearrangement.

On the other hand, the broken line shown in FIG. 2 is the result of measurement while irradiating the laser beam. In this case, it can be seen that the peak height is reversed compared to the case without irradiation. Examining this result based on the above equation 0, the work function of the sample surface is
It can be seen that it is small on the B atom and large on the A atom. Although the work function of A atoms is larger, the peak in the case of no irradiation is higher, which indicates that A atoms are ejected due to surface rearrangement. Furthermore, considering the ionization potentials of Ga and As, it can be inferred that the A atom is As and the B atom is Ga.

Thus, according to this embodiment, tunneling current detection in a state where no light is irradiated, similar to the conventional scanning tunneling electron microscope (the height corresponding to the scanning position when the probe is moved up and down so that the tunneling current is constant) is possible. The contribution of the work function contained in the output signal of the scanning tunneling electron microscope can be determined by measuring the amount of height movement) and detecting the tunneling current under light irradiation (measuring the amount of height movement in the same way as above). It is possible to separate and measure the contribution of the surface roughness and the contribution of the surface unevenness. Therefore, it is possible to accurately measure the atomic irregularities on the sample surface and local changes in the work function. Therefore, it is extremely effective for determining the surface structure of compounds and analyzing the surface composition. Further, the apparatus has the advantage that it can be easily realized by simply adding a light irradiation means to a conventional scanning tunneling electron microscope.

Note that the present invention is not limited to the embodiments described above. In the example, in the second measurement step while irradiating light, the probe was moved up and down so that the tunneling current remained constant and the amount of vertical movement was measured. can be done as follows. That is, the vertical movement position of the probe in the first measurement step when not irradiated is memorized, and the vertical movement position of the probe is stored in the first measurement step in the second measurement step.
Measure the tunnel current while maintaining the same measurement process as in . This makes it possible to more accurately measure changes in the work function based on the above equation 0.

Further, the radiation irradiated onto the sample surface is not limited to single wavelength light such as laser light, but may be any monochromatic radiation. Furthermore, the light is not limited to continuous light, and may be pulsed light. In this case, the current detection means is
The tunnel current can be detected in synchronization with the pulsed light. This makes it possible to measure the work function distribution with greater precision m<. Further, as the radiation, a laser beam other than an Ar laser, another continuous light source, or the like may be used.

Further, the photon energy of the radiation may be appropriately set within a range below the work function of the sample surface.

Furthermore, the driving means for the probe is not limited to piezoelectric elements;
Any device that can move the probe with high resolution is sufficient. Furthermore, the DC voltage applied between the sample and the probe can be made negative on the probe side if the work function on the probe side is sufficiently large. In addition, various modifications can be made without departing from the gist of the present invention.

[Effects of the Invention] As detailed above, according to the present invention, tunneling current is detected in two cases: when no radiation is irradiated and when radiation is irradiated (in the height direction of the probe corresponding to the scanning position). By measuring the amount of movement) and comparing these two pieces of measurement information, it is possible to separate the contribution of the work function contained in the signal from the scanning tunneling electron microscope and the contribution of surface irregularities.

Therefore, more detailed information regarding surface composition and surface structure can be obtained, and accurate and reliable surface analysis can be performed.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing a surface analysis device according to an embodiment of the present invention, FIGS. 2 and 3 are for explaining the operation of the above embodiment, and FIG. A characteristic diagram showing the vertical movement position of the needle, Figure 3 is a schematic diagram showing the (110) plane of a GaAs crystal, and Figure 4 is for explaining the present invention in detail, and shows the potential between the sample and the probe. It is a schematic diagram showing a state. 11... Vacuum container, 12... Sample holder, 13...
- Sample to be measured, 14... Metal probe, 15... Drive mechanism, 15a, ~, 15b... Piezo element, 21...
- DC power supply, 22... Ammeter, 23... Feedback circuit, 31... Laser light source, 32... Laser light, 33... Light transmission window. Applicant's agent Patent attorney Takehiko Suzue = 21-

Claims (10)

[Claims] (1) A surface analysis method in which a predetermined voltage is applied between a sample to be measured and a metal probe, and the sample surface is analyzed based on a tunnel current flowing between the sample and the probe. The probe is scanned in the in-plane direction of the sample surface to detect the tunnel current at that time, and the probe is moved in a height direction perpendicular to the sample surface so that the current is constant, corresponding to the scanning position. a first measurement step of measuring the amount of movement of the probe in the height direction; and a step of irradiating the surface of the sample with radiation having a photon energy less than or equal to the work function of the sample surface, in the same manner as the first measurement step. Measure the amount of vertical movement of the probe, or move the probe in the tunnel while keeping the vertical movement of the probe the same as the vertical position at the same measurement point in the first measurement step. A surface analysis method comprising: a second measurement step of measuring a current change; and an analysis step of analyzing the surface of the sample based on each measurement value in the first and second measurement steps. (2) A patent claim characterized in that the voltage applied to the probe is a positive voltage with respect to the sample and is larger than the value obtained by subtracting the work function of the sample from the work function of the probe. The surface analysis method according to item 1. (3) The surface analysis method according to claim 1, characterized in that the radiation is irradiated in a pulsed manner and the detected current is synchronously detected. (4) The surface analysis method according to claim 1, wherein monochromatic light or monochromatic continuous light such as a laser or an excimer laser is used as the radiation. (5) A metal probe held close to the surface of the sample to be measured, a means for scanning the probe in the in-plane direction of the sample surface, and a predetermined voltage applied between the sample and the probe. means for applying an electric current, means for detecting a tunnel current flowing between the sample and the probe, and means for moving the probe in a height direction perpendicular to the sample surface so that the current is constant; comprising means for measuring the amount of movement of the probe in the height direction corresponding to the scanning position, and means for irradiating the surface of the sample with radiation having a photon energy less than or equal to the work function of the sample surface. A surface analysis device featuring: (6) A patent claim characterized in that the voltage applied to the probe is a positive voltage with respect to the sample and is larger than the value obtained by subtracting the work function of the sample from the work function of the probe. The surface analysis device according to item 5. (7) The surface analysis apparatus according to claim 5, wherein the sample and the probe are installed in a vacuum container. (8) The means for moving the probe in the height direction is composed of a piezoelectric element, and the means for measuring the amount of movement of the probe in the height direction is for measuring the voltage applied to the piezoelectric element. A surface analysis device according to claim 5, characterized in that: (9) The surface analysis device according to claim 5, wherein the radiation is applied in a pulsed manner, and the tunnel current is detected in synchronization with this pulse. (10) The surface analysis device according to claim 5, wherein the radiation is monochromatic light such as a laser or excimer laser, or monochromatic continuous light.