JPH0351701A - Measuring method for surface of sample using tunnel current - Google Patents

Abstract

PURPOSE:To enhance resolution by laminating a conductive monomolecular film which has a hydrophilic part and a hydrophobic part and comprises organic molecules, and measuring the roughness of the surface by using a tunnel current. CONSTITUTION:A conductive monomolecular film 2 which has a hydrophilic part and a hydrophobic part and comprises organic molecules is laminated on the surface of an insulating sample 1. Leading electrodes 3 to the outside are attached. The irregularities on the surface of the sample are observed. A bias voltage is applied across a probe and the sample through the electrodes 3. A tunnel current flowing between the probe and the sample is inputted into a control circuit. Thus, a minute movement controlling mechanism is controlled. The distance between the electrodes 3 is controlled with the control circuit so that the tunnel current is constant all the time. Thus, the probe is controlled, and the surface of the sample is observed. In this way, the surface roughness of the sample can be measured readily for the large area with high resolution.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for measuring the surface of a sample using a tunneling current using a scanning tunneling microscope or a precision measuring device applying its principle.

[Prior Art] Recently, a scanning tunneling microscope (hereinafter abbreviated as STM), which can directly observe the electronic structure on and near the surface of a material, has been developed [G., Binning et al., He1vet.
icaPhysica, Acta, 55,726 (1
982)], it has become possible to measure real space images with high resolution regardless of whether they are single crystal or amorphous, and it also has the advantage of being able to observe with low power without damaging the medium due to electric current. It operates not only in ultra-high vacuum but also in the atmosphere and in solutions, and can be used for various materials, so it is expected to have a wide range of applications.

In such an STM, when a voltage is applied between a metal probe (probe electrode) and a conductive substance (sample) and they are brought close to a distance of about 1nI11, a current flows between them (tunnel current).
I'm taking advantage of that. This current is very sensitive to changes in the distance between the two, and surface information in real space can be obtained by scanning the probe while keeping the current or the average distance between the two constant.

When observing a sample surface with STM, most of the voltage (bias voltage) applied between the tip and sample needs to be applied to the tip-sample gap, so the resistance of the sample is several LOOKΩ.
Must be less than or equal to

Therefore, it has been impossible to directly observe the surface of an insulator or a high-resistance semiconductor using STM. Therefore,
In such a case, Au, Pt, Pt-P is coated on the sample surface.
Coating with conductive metal such as d by sputtering method, vapor deposition method, etc.AppliaPhysics Le
tter P, 1656-165752 (20) Ma
y198g) Alternatively, a method was adopted in which a conductive metal replica of an insulating sample was created and this replica was observed using STM (Science P, 1013
vol.

239198g).

[Problems to be Solved by the Invention] However, surface observation of insulators using STM using the metal coating method, metal replica method, etc. described above is difficult due to the grain shape of the conductive metal used during coating, etc. Its resolution is limited, and it has been considered difficult to obtain a resolution of the order of lOn+a or higher. Also,
On a sample surface with surface irregularities or fine bumps on steps, uniform coverage of the steps was desirable and it was difficult to coat with metal.

That is, one object of the present invention is to provide a method for measuring the surface of a sample using tunneling current, which solves the above-mentioned problems and coats the surface of the sample with a conductive layer and measures the sample. It is in.

[Means for Solving the Problems] The present invention is characterized in that when measuring the surface of a sample using a tunneling current, such as a scanning tunneling microscope or a pattern line width measuring device applying the principle of such a device, A conductive monomolecular film or monomolecular cumulative film 9 made of organic molecules having a hydrophilic site and a hydrophobic site is applied to the surface of the sample.
The purpose of this method is to laminate and measure polymeric membranes.

That is, the present invention provides a method for observing the surface of a sample, especially an insulating sample, using the principle of STM, using a conductive monolayer or a monomolecular cumulative film of amphiphilic molecules as a conductive coating layer on the surface of the sample, or a polymerization thereof. By using a film, the surface of an insulating sample is to be evaluated with a resolution on the nanometer order.

Here, examples of the target sample include insulating samples, samples in which insulating parts and conductive parts coexist, and the like.

Examples of the conductive organic material forming the conductive film coated on the sample include molecular crystals of charge transfer complexes represented by TTF-TCNQ, conductive polymers represented by polyacetylene, and the like.

When the amphiphilic molecules of these compound groups are dissolved in a solvent and spread on the water surface, a monomolecular film is formed, and Langmui
By transferring this monomolecular film onto a substrate (sample surface) by the r-Blodgett method (La method), a monomolecular cumulative film or the like is formed. The in-plane conductivity at room temperature of a conductive La film with such a monolayer cumulative structure is l s/cm
It is possible to form a coating layer with sufficient electrical conductivity.

The monomolecular film, monomolecular cumulative film, etc. used in the present invention are
It is formed by the apparatus shown in the figure.

In the figure, 26 is a surface pressure needle, and the surface control device 2
7, the movement of the moving barrier 28 is controlled to maintain a constant surface pressure. 29 is an aqueous phase, which is pure water or water containing metal ions. 30 is a film forming substrate (sample), and 31 is a film forming substrate holder which can be moved up and down.

A device as described above is operated as follows.

First, the liquid surface is cleaned, and a solution of conductive molecules dissolved in a solvent such as benzene, chloroform, andandhryl-benzene (1:1) is dropped onto the liquid surface to form a gas film6.
Next, the moving barrier 28 is gradually moved to the left to gradually reduce the area of the liquid surface where the molecules are spread, thereby increasing the areal density and forming a solid film. The state of this monomolecular film is determined by the surface pressure sensor 32.
It is detected by measuring the surface pressure of a monomolecular film spread on the liquid surface. The horizontal movement of the movable barrier 28 is controlled based on the measured value of this surface pressure sensor. In general, the surface pressure of a monomolecular film suitable for transferring to the film-forming substrate 30 is said to be 15 to 30 dyn/ca+, but depending on the chemical structure of the film structure material and temperature conditions, for example, the suitable surface pressure value may vary as above. The above range is just a guideline, as it may exceed the range.

By moving the film-forming substrate 30 up and down under the above conditions, the monomolecular film that has become a solid film can be attached to the surface of the substrate and transferred. Furthermore, a monomolecular cumulative film can be obtained by stacking and transferring a monomolecular film a plurality of times onto the same film-forming substrate 30.

The vertical movement of the film forming substrate 30 is usually 0.1-1 cm/w.
This is done at a speed of in.

As a method for transferring the monomolecular film to the substrate, in addition to the above-mentioned vertical dipping method, a horizontal adhesion method, a one-rotation drum method, etc. can be used.

[Function] For example, when measuring the surface of an insulating sample using a scanning tunneling microscope using a tunneling current, by coating the sample surface with the above-mentioned conductive film, the resistance of the sample can be reduced to several tens of thousands.
The bias voltage is on the order of Ω or less than 0, making it possible to apply a sufficient bias voltage between the STM probe and the sample.

Furthermore, the LB film is a uniform thin film with a molecular-sized thickness ranging from ns+ to sub-thickness, has good step coverage, and can be coated over a relatively large area, making it possible to reflect the surface roughness information of the sample at the molecular size. I can do it. Therefore,
Compared to conventional metal coating methods, the surface resolution is significantly improved.

[Example] Specific examples of the present invention are listed below.

A conductive LB film was coated on an insulating sample of blood and the surface roughness was observed for 37M. The conductive LB film was formed using the apparatus shown in FIG.

As a constituent molecule of the LB film, bis-tetracyanoquinodimethane docosyl biridium (CI+3-+-C1l-+-
2+0 (■[TCNQl m) was added to a 1:1 mixed solution of acetonitrile and benzene at a concentration of 1 sg/ml in a blast furnace, then KHCOm was adjusted to pH 6.8LC and CdC1z 711 degrees 4 x 10-' mol/
i' *Developed on the aqueous phase of the apparatus shown in Figure 5 at a water temperature of 17°C.

After the solvents acetonitrile and benzene were removed by evaporation, the surface pressure was increased to 20 dyn/cm, and a monomolecular film was formed.The hydrophobically treated insulating material, which had been immersed in the water phase in advance, was kept at a constant zero surface pressure. The sample 30 is gently pulled across the surface of the water at a rate of 5 am/win, followed by a gentle immersion at 5 mm/a+in. This process was repeated to accumulate 10 monolayer layers.

The sample coated with the conductive LB film in this way is provided with an electrode 3 for taking it out to the outside as shown in Figure 1, and used as an observation sample to observe the surface irregularities of the sample.
I went with M. FIG. 2 shows a configuration diagram of the STM used in this embodiment. In the figure, reference numeral 4 denotes a probe for monitoring tunnel current, which is a tungsten needle whose tip has been electrolytically polished.

Then, the insulating sample coated with the conductive film shown in FIG. Needle-
A bias voltage is applied between the samples by an application circuit 6. 7 in the figure indicates the in-plane direction of the sample (X, Y directions) of the probe made of a cylindrical piezoelectric element, and the depth probe-sample amount direction (Z direction).
It is a fine movement control mechanism. 8 is a circuit for detecting the tunnel current flowing between the probe and the sample, and the output of this circuit is input to the control circuit 9 to control the fine movement control mechanism 7.

To observe the sample, the coarse movement mechanism 5 is controlled with a voltage of IV applied by the bias voltage application circuit 6, and the sample is brought close to the distance where a current of lpA flows between the two poles (between the tip and the sample).
Next, the fine movement control mechanism 7 is set so that lnA flows between the two poles.
control. The control circuit 9 controls the distance between the two poles via the fine movement control mechanism 7 so that the tunnel current is always constant.

In this state, when the probe 4 was scanned in the X-Y plane using the control circuit 9 and the surface was observed via the fine movement control mechanism 7, the sample surface could be observed with a resolution on the order of nanometers.

A second embodiment of the present invention is shown in FIG. 3, which applies the principle of STM in measuring the line width of a fine pattern formed on an insulating sample. This is a line width measurement device that can secure nanometer-order resolution by detecting the edges of a pattern using tunneling current from the tip of the probe and measuring the edge spacing using an optical measurement method.

First, on an insulating sample on which a fine pattern is formed,
A conductive LB film was coated using the apparatus shown in FIG.

As a constituent molecule of the L8 membrane, tetramethyltetratifulvalene-octadecyltetracyanoquinodimethane (TMT
TF/Cl8TCNQ); to l mg/mI! After melting at 1 degree, it was spread on the aqueous phase of the apparatus shown in FIG. 5 at a water temperature of 20 degrees Celsius.

After removing the solvent benzene by evaporation, the surface pressure was reduced to 25 dyn.
/ca+ to form a monomolecular film.While keeping the surface pressure constant, the hydrophobically treated insulating sample 30, which had been previously immersed in the water phase, was moved at a speed of 10 in the direction across the water surface.
Gently pull up at mm/lll1n, then at a speed of 10o
Gently soak in +a+/lll1n. Through this process, two monolayers were accumulated. This sample was also provided with the extraction electrode 3 in the same manner as in Example 1.

Next, the line width of this fine pattern was measured.

This measurement will be explained using FIGS. 3 and 4.

A bias voltage is applied to the sample to be measured lO coated with the conductive LB film by a bias voltage application circuit 6, and a bias voltage is applied between the sample to be measured lO and the probe 4 which is placed close to the sample lO at a distance of 1 nm or less. A tunnel current detection circuit 8 detects the flowing tunnel current 11.

The LPF 12 and the probe longitudinal position control circuit 13 are installed so that the average tunneling current is constant. Probe longitudinal position control means 1
4, the position of the probe 4 in the Z direction is controlled. The stage 16 is moved laterally by the stage driving means 15,
The sample to be measured lO is scanned. The edge of the butterfly is detected by the edge detection circuit 17 from this desired probe longitudinal position control signal 3a (see FIG. 4), and the edge detection circuit 17
b (see FIG. 4) is sent to the line width calculation circuit 18. On the other hand, the coherent light from the coherent light source 19 is split into two by the beam splitter 20, the first light is reflected via the mirror 21 onto the mirror 22 fixed to the stage 16, and the reflected light is sent to the beam splitter 20. Re-inject it. The other light is reflected by a mirror 23 fixed near the probe 4 and made to enter the beam splitter 20 again.

The two reflected lights are combined again by the beam splitter 20 and enter the photodiode 24 as interference light. At this time, due to the lateral relative movement between the stage 16, that is, the sample to be measured 10, and the probe 4, a change in brightness and darkness occurs in the interference light incident on the photodiode 24.For example, if the wavelength of the coherent light is λ, the relative movement amount is In the case of cancer/2, the brightness changes for one cycle. Therefore, the relative lateral movement amount can be read with nm precision from the light intensity change signal 3c (see FIG. 4) from the photodiode 24, and the lateral movement amount detection circuit 25 detects the lateral movement amount signal 3d (see FIG. 4). (see FIG. 4) is obtained and sent to the line width calculation circuit 18. The line width calculating circuit 18 calculates a measured pattern line width value 3e (see FIG. 4) based on the edge detection signal 3b and the lateral movement amount signal 3d. In addition, in this example,
Although an example has been shown in which Michelson type optical interferometry using a single frequency laser is applied as the coherent light source 19, optical heterogain interferometry using a dual frequency orthogonally polarized laser such as a Zeeman laser may also be applied.

[Effects of the Invention] As described above, according to the measurement method of the present invention, the surface roughness etc. of an insulating sample or a sample in which insulating parts and conductive/conductive parts are mixed can be measured using STM etc. Compared to conventional metal coating methods, measurements can be easily made over a large area and with high resolution.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of an insulating sample coated with and inked a conductive film, which is a feature of the present invention. FIG. 2 shows a configuration diagram of the STM used in Example 1. FIG. 3 shows a configuration diagram of the line width measuring device used in Example 2. FIG. 4 shows signals obtained by the device used in Example 2. FIG. 5 shows an apparatus for producing an LB film. l... Insulating sample 2... Conductive film 3... Extraction electrode 4... Probe 5... Coarse movement mechanism
6... Bias voltage application circuit 7... Fine movement control mechanism 8... Tunnel current detection circuit 9...
・Control circuit IO...Measurement sample 11...
Tunnel current 12... LPF 13... Probe longitudinal position control circuit 14... Probe longitudinal position control means 15... Stage driving means 16... Stage 17
...Edge detection circuit 19...Coherent light source 18.
... Line width calculation circuit 21, 22, 23 ... Mirror 20 ... Beam splitter 24 ... Photodiode 25 ... Lateral movement amount detection circuit 26 ... Surface pressure needle 27 ... Surface control device 28...Movement barrier 29...Aqueous phase 30...Film formation substrate (sample) 31...Film formation substrate holder 32...Surface pressure sensor

Claims (1)

[Claims] (1) In measuring the sample surface using tunnel current,
The method is characterized in that a conductive monomolecular film, a monomolecular cumulative film, or a polymeric film consisting of organic molecules having a hydrophilic part and a hydrophobic part is laminated on the surface of the sample to be measured. A method for measuring the surface of a sample using tunneling current.