JPH0868802A - Conductive cantilever and holder of compound microscope - Google Patents

Abstract

PURPOSE: To measure the change in capacitance between a probe and a sample accurately when the capacitance between the probe and the sample is detected by providing the probe at the lower part of the tip of a cantilever. CONSTITUTION: The entire body of a holder 2 is made of an insulator, and a metal line 3 is formed at a part. The capacitance between the holder 2 and a sample is decreased. Holes are made in a cantilever 1 made of metal, and the area is reduced. Longitudinal holes and the holes corresponding to the shape of the triangular cantilever 1 are formed. Or a thin metal line is evaporated on the insulating cantilever. Thus, the capacitance of the cantilever 1 can be reduced. Since the capacitance between the cantilever 1 and the holder 2 and the sample becomes less, the change in capacitance between the probe and the sample can be detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive cantilever and holder used as a sensor section of a compound microscope. Here, the compound microscope is a combination of the following three types of microscopes. Scanning tunneling microscope (ST
M), atomic force microscope (AFM), and scanning capacitance microscope (SCaM). By combining the functions of these three microscopes, it is possible to investigate various properties and characteristics of a sample with one microscope.
However, the atomic force microscope and the capacitance microscope are directly related to the present invention.

A scanning tunneling microscope has a sharp probe brought close to a sample by a few nm, a voltage is applied between the sample and the probe, and a tunnel current flowing between the sample and the probe is measured.
Observe the fine shape and electronic state of the sample surface. By keeping the current at a constant value, it is possible to observe the state of irregularities on the sample surface at the atomic level. The detection unit is a probe. A sharp, short metal needle. There is no deflection. The probe is attached to the piezoelectric element, and can move vertically (Z direction) and horizontally (XY). The distance between the sample and the probe is about 0.5 nm to 1.5 nm. The tunnel current is of the order of a few nA. This is a scanning type because the shape of the sample surface is examined by moving the sample relatively.

A scanning electrostatic capacitance microscope measures the electrostatic capacitance formed between a conductive probe and a charge on the surface of a sample by bringing a conductive probe close to the sample to obtain a capacitance distribution on the surface of the sample. Is to detect. Also in this case, the sensor unit is a short probe made of metal. No need to bend. Since a current can be passed perpendicularly to the surface of the sample, the capacitance generated between the sample and other parts of the probe can be reduced. The principle diagram is shown in FIG.

The atomic force microscope is designed to bring an insulative and flexible member close to a sample and optically or electrostatically detect the member deflection due to the atomic force acting between the member and the sample atom. is there. This member is called a cantilever because it is a cantilever and bends well. The cantilever must be insulative and flexible. A thin film made of a material having a low Young's modulus such as Si ₃ N ₄ , SiO ₂ or Si is used. Some people call cantilevers either leaf springs or springs. When the probe is brought close to the sample surface, a force is generated at the tip of the sample atom and the cantilever, and this force is measured. Since no electrical mechanism is used, the cantilever may be insulating. The deflection of the cantilever is obtained by detecting the displacement of light reflected by the laser light by a light receiving element. The principle diagram is shown in FIG.

[0005]

2. Description of the Related Art These three microscopes have different purposes and structures. The present inventor has proposed a compound microscope in which these three parts are combined together. In this case, there are two types of probe and cantilever that face the sample and detect the state of the sample. In order to make a compound microscope, it is necessary to combine both properties. Therefore, the present inventor has adopted a sensor having a structure in which a short probe is attached to the tip of the cantilever. The atomic force microscope does not have to be an electrical measurement, but can detect the deflection of a member. This is detected optically. However, both electrostatic capacitance and tunnel current require an electrical signal. To measure the capacitance, a high frequency voltage is applied to detect the capacitance between the probe and the sample. In the case of tunnel current, a small tunnel current is measured by applying a DC voltage between the probe and the sample.

Since the form of the applied voltage is different, the same probe cannot be used for two purposes at the same time. Therefore,
A high-frequency voltage and a DC voltage are selectively applied by using a changeover switch. The present inventor has made diligent efforts to combine the three microscope functions in this manner. Another problem is the leaf spring or cantilever. The cantilever used in the atomic force microscope is an insulator, and it is easy to select a flexible material having a low Young's modulus. However, in the case of a tunnel microscope or a capacitance microscope, it is not possible to provide a probe other than the cantilever, so the probe is attached to the tip of the cantilever.

A cantilever to guide the current from the probe.
It will have to be electrically conductive. It is not possible to make a cantilever with an insulator as in the past. This problem is caused by combining the atomic force microscope and the capacitance microscope. Conductive cantilevers are required in two types of compound microscopes that lack tunneling microscopes.

[0008]

SUMMARY OF THE INVENTION Conductive cantilevers
That is something that has never existed before. The need arises to couple the capacitance microscope to the atomic force microscope. However, it has been found that it is not just that the insulator is made of metal. The cantilever is a long rod-shaped member, but if it is made of metal, a capacitance C ₂ is generated between the cantilever and the sample. It is necessary to accurately capture the change in the capacitance C ₁ between the probe and the sample. When the distance between the sample and the probe is short and the charge distribution state of the sample changes, the capacitance changes greatly.
C ₁ between samples. Capacity C ₂ between sample and cantilever
Does not fluctuate much depending on the charge distribution of the sample. However, when a high frequency is applied between the cantilever and the sample to measure the capacitance, most of the capacitance comes from C ₂ . This becomes noise for the signal C ₁ .

In the atomic force capacitance microscope prepared by the present inventor, the applied voltage was changed to measure the change in the thickness of the depletion layer of the semiconductor with respect to the voltage.
It was not possible to observe the change in capacity within the range of applied voltage. Why is this? The inventor is a cantilever
The capacitance C ₂ between the samples is too large, and the capacitance C between the probe and the sample is
_I think it may be masking the change of ₁ . When the applied voltage changes, the depletion layer increases or decreases. If the depletion layer expands, the capacitance C ₁ between the probe and the sample should decrease. However, since the capacitance C ₂ between the cantilever and the sample is large, it is considered that the fluctuation of the signal is unknown.

The capacitance between the cantilever and the sample was generated for the first time by using a conductive cantilever. Since the cantilever + probe serves as a sensor for an atomic force microscope, a capacitance microscope, and a tunnel microscope, the cantilever must be made conductive. The conductive cantilever was first introduced by this. Conventionally, in the case of the capacitance microscope, the high frequency voltage was applied only to the small probe, so that the capacitance between other portions and the sample did not pose a problem. However, if you attach a large area such as a cantilever on the probe,
The capacitance between the cantilever and the sample becomes a problem.

Then, the capacities between the probe and the sample and between the cantilever and the sample are evaluated. The lower part of the probe is assumed to be hemispherical. So, as shown in Figure 3,
Calculate the capacitance between a sphere (conductor sphere) and an infinitely spread conductor surface (ground). In the case of a hemisphere, the charge is distributed in the lower half, so it is the same even if a sphere exists. In FIG. 3, the radius of the sphere is r, the distance between the lower end of the sphere and the conductor surface is s, and the distance between the center of the sphere and the conductor surface is d (d = s +
r). The conductor surface is the XZ surface. The direction perpendicular to this is the Y-axis. The coordinates of the center of the sphere are (0, d, 0). Suppose this sphere has + Q charge. Consider a mirror image sphere in order to set the potential of the conductor surface to zero. -Q at position (0, -d, 0)
There is a mirror image sphere of charge. A linear point P (0, y,
The electric field strength E at 0) is (ε ₀ is the dielectric constant of vacuum)

E = Q (4πε ₀ ) ⁻¹ {(d−y) ⁻² + (d + y) ⁻² } (1) The voltage V is obtained by integrating this from y to 0 to s.

V = ∫Edy = Q (2πε ₀ ) ⁻¹ {r (2s + r)} ⁻¹ s (2). The capacitance C ₁ as a capacitor is

C ₁ = (2πε ₀ ) {r (2s + r)} s ⁻¹ (3) This is the capacitance between the probe and the sample (conductor surface).

Consider an example. The tip of the probe is a distance from the sample s =
5 nm apart, the radius of curvature r of the tip of the probe is 0.5 μ
Let m be. Substituting this into (3), C ₁ =
It is 2.8 fF. It is a very small value. Next in FIG.
Consider the capacitance between the cantilever and the sample (conductor surface). The capacitance of the parallel plate electrodes having the distance d and the area S is εS / d. Distance d on infinitely spread conductor surface
Similarly, the capacitance formed between the conductor pieces (area S) existing apart from each other with the conductor surface is εS / d. Cantilever
In the case of, the one end A is extremely close to the sample, and the other end (portion supported by the holder) B is far from the sample. The diagonal conductor has a charge + Q. Consider a mirror image with the sample plane as the XZ plane. The mirror image has a charge of -Q.

A line of electric force is formed between the real electrode and the mirror image electrode, but this line is not straight but curved in an arc shape. The distance from the surface of the electrode (cantilever) at the end A closest to the conductor surface is d. The angle of inclination of the electrode with respect to the conductor plane (XZ plane) is Θ. The distance from the edge A of the electrode to an arbitrary point P of the electrode is l. The length of the arc line of electric force between this and the conductor surface is given by (d + lΘ). Assuming that the electric field E is uniform along the lines of electric force, E (d + lΘ) becomes a constant c. Applying Gauss's theorem to the closed space near the electrodes, the surface charge η at point P is η = ε ₀ E. The total charge Q is obtained by integrating η from 0 to L by l and multiplying it by the electrode width w.

Q = w∫ηdl = cε ₀ w∫ (d + lΘ) ⁻¹ dl = cε ₀ wΘ ⁻¹ log {(d + LΘ) / d} (4) c = QΘ / [wε ₀ log {(d + LΘ) / d}] (5)

The voltage V is the electric field E integrated in the Y direction. V = −∫Edy = QΘ / [wε ₀ log {(d + LΘ) / d}] (6)

The capacitance C _{is, C 2 = wε 0 [log} {(d + LΘ) / d}] / Θ (7) which is like the will epsilon ₀ wL / d in the limit of theta is 0 epsilon ₀ Matches S / d.

D is the distance between the cantilever and the sample,
Since the probe is almost in contact with the sample, d is the length of the probe. Consider an example. The length, width, tilt angle and distance of the cantilever are assumed as follows. L = 2.
0 mm, w = 0.3 mm, Θ = 10 °, d = 5.0 μ
m. Then, C ₂ = 64 fF.

Since the cantilever is larger, the capacitance of the cantilever is much larger than the capacitance of the probe. For this reason, the capacitance between the sample and the probe is buried in the signal between the cantilever and the sample. Therefore, even if a bias voltage is applied between the probe and the sample to change the capacitance between the probe and the sample, this cannot be detected as a change in the capacitance.
Furthermore, since the cantilever bends, the capacitance between the cantilever and the sample changes. Since the signal is buried in the change in the capacity of the cantilever due to the bending, it is impossible to accurately capture the change in the signal. It is an object of the present invention to reduce the capacitance of such a conductive cantilever or holder so that a change in capacitance between the probe and the sample can be detected.

[0022]

SUMMARY OF THE INVENTION The present invention reduces the capacity of the holder by reducing the area of the metal portion of the holder. For this purpose, a metal wire is formed on a part of the insulating holder and used as a conductive wire. Furthermore, the area of the metal part of the cantilever itself is reduced, and the cantilever
Reduce the capacity of. Make a long hole in the elongated metal cantilever. Alternatively, a triangular cantilever is used to remove the central portion. The stray capacitance is reduced by reducing the area of the metal part of the holder or cantilever. Since the capacity of the probe relatively increases, it becomes possible to detect a change in the capacity between the sample and the probe.

[0023]

The floating capacitance due to the holder and the cantilever is reduced, and the capacitance between the probe and the sample is relatively increased.
The N ratio increases. When used as a capacitance microscope,
The charge distribution on the sample surface, the thickness of the depletion layer, etc. can be obtained more accurately. It is possible to improve the function as a compound microscope having three functions such as a scanning tunneling microscope, a scanning capacitance microscope, and an atomic force microscope.

[0024]

The gist of the present invention is to reduce the stray capacitance by reducing the area of the conductor portion of the cantilever. For this reason, for example, the following improvements are possible.

A metal cantilever with a hole (a small hole is made in the longitudinal direction. As shown in FIG. 5. A number of round holes are made.) A triangular cantilever with a wide hole in the center. -(Shown in Figure 6). A thin conductive wire is formed on the thin film of the insulator (shown in FIG. 7). A holder in which the cantilever is supported is provided with a thin metal wire to reduce the capacity of the holder.

In the example of FIG. 5, the capacity is reduced by cutting a groove in the elongated cantilever in the longitudinal direction. It has a strip shape with a sharp tip. This cantilever is obtained by cutting a metal foil. A probe (not shown) is attached to the lower surface of the front end. The rear end is fixed to the holder-2. If the whole holder is made of metal, the capacity will increase,
A holder is formed of an insulator. However, an electric signal cannot be transmitted by itself, so a metal wire is coated on the upper surface or the lower surface of the holder-2. It can be easily formed by vapor deposition.

What is important is that stripe-shaped holes (grooves) 4 are formed in the longitudinal direction of the cantilever-1. Due to this flute, the area of the metal part is reduced.
Therefore, the capacity between the cantilever and the sample is reduced. When it is used as an atomic force microscope, light is applied to a cantilever and the deviation of reflected light is observed. In this case, since the light strikes the front end of the metal foil cantilever-1, the existence of the groove does not hinder the reflection of the light. Even with such a structure, the rigidity of the cantilever must be maintained to some extent. The groove 4 can be formed by etching. By cutting the groove 4, the capacity of the cantilever can be reduced by about 1/3.

In the example of FIG. 6, the metal is cut into a triangular shape and the inside is removed. For example, a triangular hole is formed by a method such as press cutting. The cantilever 6 has a triangular frame shape. Inside is a hole 7. The end of this is fixed to the front end of the insulator holder-8. The holder 8 has a thin metal wire coat 9. Current is the metal wire coat 9,
Can flow along the cantilever-6. When the atomic force microscope is used, the light 10 is applied to the front end of the cantilever 6, so that the light is not reflected due to the hole 7.

In the example of FIG. 7, a metal wire 12 is coated in a longitudinal direction on a part of an insulator cantilever 11.
The holder 13 is partially covered with a metal wire. The front end of the cantilever is entirely covered with the metal film 15. A current flows from the metal wire 14 to the cantilever metal wire 12 and the front end metal film 15. The probe 16 is fixed below the metal film at the tip.

[0030]

According to the present invention, the capacitance between the sample, the cantilever and the holder can be reduced by narrowing the area of the metal portion of the holder and the cantilever.
Therefore, it is relatively easy to detect the capacitance change between the probe and the sample. Therefore, when used as a capacitance microscope, S
/ N ratio increases.

Using a cantilever and a holder made entirely of metal, using a sample of Si, a DC voltage of -10 V to +
The voltage was applied in the range of 10 V and the capacitance change between the sample and the probe was measured. However, when the bias is in the range of -10V to + 10V,
The capacity change could not be detected. For Si,
If the impurities are heavily doped, the depletion layer should have a thickness of several tens of μm, and if the impurities are doped at a low concentration, the depletion layer should have a thickness of several hundreds of μm. This should change with the voltage and cause a change in capacitance. However, the output remained unchanged. Using the cantilever of FIG. 5, the same -10V to +
A change in capacitance could be detected with respect to a change in bias of 10V.

[Brief description of drawings]

FIG. 1 is a diagram showing the measurement principle of an atomic force microscope.

FIG. 2 is a measurement principle diagram of a capacitance microscope.

FIG. 3 is an explanatory diagram for calculating a capacity of a sphere facing a plane conductor.

FIG. 4 is an explanatory diagram for calculating a capacity of a rectangular plate facing a plane conductor.

FIG. 5 is a perspective view showing an embodiment of the present invention in which the area is reduced by forming a stripe hole in the longitudinal direction of the cantilever.

FIG. 6 is a diagram showing a triangular cantilever according to an embodiment of the present invention.
FIG. 3 is a perspective view showing what is cut into a triangle to reduce the area.

FIG. 7 is a perspective view showing a metal wire formed on a thin film of an insulating material according to the embodiment of the present invention.

[Explanation of symbols]

 1 Strip-shaped cantilever 2 Insulating holder 3 Metal wire coat 4 Striped hole 5 Light spot when used as an atomic force microscope 6 Triangular cantilever 7 Triangular hole 8 Insulator Holder 9 Metal wire coat 10 Light exposure place 11 Insulating cantilever 12 Metal wire 13 Insulator holder 14 Metal coat 15 Metal film 16 Probe

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H01J 37/28 Z

Claims (3)

[Claims] 1. A composite cantilever which combines the functions of at least a scanning capacitance microscope and an atomic force microscope and supports a conductive cantilever which is opposed to a sample and is bent by an atomic force acting between the sample atoms. A conductive cantilever and a holder for a compound microscope, characterized in that it is made of an insulating material, a thin metal wire is coated, and the tip of the metal wire is connected to a cantilever. . 2. A conductive cantilever and a holder for a compound microscope, wherein the cantilever is made of metal and has holes perforated in a direction perpendicular to its surface. 3. The cantilever is made of an insulating material,
A conductive cantilever and a holder for a compound microscope, wherein a metal wire is formed on a surface of an insulator, and the metal wire is connected to the metal wire of the holder.