JPH0448542A - Surface analyzer - Google Patents

Abstract

PURPOSE:To get information of electrons (a band structure, chemical bond state and elemental species) of energy deeper than information obtainable from a tunnel current in the case of no light irradiation b detecting an electron in a sample excited by a monochromatic light as tunnel current using a scanning tunneling microscope. CONSTITUTION:A tungsten chip 22 is brought near to a sample 22 to adjust the space between a chip and the sample such that tunnel current becomes 1nA at a fixed tunnel voltage, for example, at 1V. Next, while light from a light source 21 irradiates the surface of a sample, it scans the chip 22 on x-y face (in parallel with the surface of the sample), and this feed back loop is turned off at each fixed quantity of horizontal displacement to stop the chip 22, and by sweeping at a chip voltage within the range from, e.g. -3V+3V to measure a tunnel current, the measurement is stored in memory. Next, the measurement which is the same as above is repeated in the same region without irradiation of light so that distribution of electron density of deep energy can be detected by obtaining the difference between irradiation and non-irradiation of a tunnel current at each measuring point.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field 1] The present invention relates to a surface analysis device that can be used for ultrafine surface analysis in all fields, such as the semiconductor industry, metal industry, and pharmaceutical industry.

1. Prior Art] Photoelectron spectroscopy has conventionally been used as a method for investigating surface band structures, chemical bond states, and element species. In conventional photoelectron spectroscopy, X
Line photoelectron spectroscopy (XPS), vacuum ultraviolet photoelectron spectroscopy (U
PS), and separate light sources have been used for each. Recently, synchrotron radiation light (SOR light) has also come to be used as a light source that continuously emits light with wavelengths from X-rays to infrared rays.

The principle of photoelectron spectroscopy is shown in FIG. 2. The horizontal axis 1 is the electron density inside the sample, the vertical axis 2 is the electron energy, and 3 is the vacuum level, which is usually taken at the zero point of energy. 4 is the electron distribution before excitation.-15 is the electron distribution that is excited to high energy by absorbing the energy of monochromatic light. In this example, all excited electrons are higher than the vacuum level, so if they are at a position shallower than the escape depth, they will fly out from the sample surface and enter the detector. That is, the incident light excites electrons within the sample by the energy of the light, and only electrons near the surface that can overcome the vacuum level barrier (work function) fly out from the sample surface. By detecting the relationship between the number of electrons and energy, information on the band structure, chemical bond state, and element species can be obtained. Electrons excited inside the sample move toward the surface and then escape from the surface, but the escape depth is determined by the energy of each electron, and is usually in the range of several nanometers to several tens of nanometers.

XPS is suitable for investigating electrons in deep levels such as inner-shell levels because the energy of light is high, and UPS is suitable for investigating shallow levels (bands).

FIG. 3 is a block diagram of a conventional photoelectron spectrometer.

In this figure, 8 is a vacuum chamber, 7 is an exhaust pump,
8 is a sample, 9 is an excitation light source, IO is a hemispherical energy analyzer, 11 is a photoelectron detector, and 12 is an amplifier.

In such a photoelectron spectrometer, it is difficult to narrow down the incident light, and the photoelectron detector is located several C degrees or more away, and although a window (slit) is provided at the entrance of the detector, it covers a wide area. Even the most advanced equipment had a lateral resolution of only a few square meters at most. In other words, the information in this area is obtained by averaging it. Such analytical area resolution is gradually being improved by reducing the diameter of the slit on the detector side and placing an electron lens between the sample and the slit, that is, by narrowing down the detection area rather than the irradiation area. , no significant improvement is expected. However, in the field of semiconductors, various types of processing are currently being carried out on dimensions of 1 μm or less, and analysis on the atomic order is becoming necessary in other fields as well. For this reason, the conventional technology has the disadvantage that it cannot meet the demand for analysis in such a microscopic region.

On the other hand, the scanning tunneling microscope (STML method) uses a sharp metal tip and a sample as shown in Figure 4.
X
+3' + The chip is scanned using a piezo element driven in the Z direction.

In FIG. 3, 13 is a sample, 14 is a chip, 15 is a piezo element, 16 is a voltage source, and 17 is a current source for detecting tunnel current.

Since the tunneling current in this STM method strongly depends on the distance between the sample and the tip, only the fine sample surface area directly under the tip contributes to the tunneling current, and keeping this constant means keeping the distance between the tip and the sample constant. It corresponds to keeping.

Therefore, the surface (xy plane) is scanned while moving the tip of the tip in two directions in accordance with the unevenness of the sample surface. The movement of this chip is entirely determined by the voltage applied to the piezo element, so if the piezo voltage in two directions at each position on the surface is displayed as an image, the irregularities on the sample surface can be visualized. With current technology, it is possible to detect irregularities with atomic-order resolution in both the vertical and horizontal directions. In this way, STM has extremely fine resolution both horizontally and vertically on the sample surface.

STM can also obtain information about the band structure from the tunnel current-tunnel voltage characteristics at each location on the surface.

[Problem to be Solved by the Invention] However, in this STM method, bands and levels with deep energy have a high tunnel barrier height (and therefore do not contribute to the tunnel current; therefore, only shallow band information can be obtained). Another drawback was that it was difficult to identify elements using conventional STM because deep energy bands reflected the characteristics of individual elements.

An object of the present invention is to solve the above-mentioned conventional problems by analyzing the band structure and chemical bonding state near the surface of a sample in an extremely small region with an analytical area resolution of the atomic order.

An object of the present invention is to provide a surface analysis device capable of obtaining information on element types.

1 Means for Solving the Problem 1 The surface analysis device of the present invention includes means for irradiating the sample surface with monochromatic light in a wavelength range from Xi to infrared rays, and a means for irradiating the sample surface with light by the light irradiation means. The present invention is characterized in that it includes means for measuring a tunneling current between a tip whose surface is scanned and a sample.

[Function 1] In the present invention, the light irradiation area on the sample surface is the same size as in the case of conventional photoelectron spectrometers (several 11 meters square), and a deep energy level excited to a range not exceeding the vacuum level is detected. By scanning the surface with an STM chip, the electrons from the STM and BASID can be detected as a tunnel current. Electrons (photoelectrons) excited above the vacuum level jump out of the surface in the entire region of light irradiation and flow into the chip, so they cannot be used as a signal for local analysis, but the electrons (photoelectrons) cannot be used as a signal for local analysis, but they must be sufficiently high within the range that does not exceed the vacuum level. Excited electrons contribute to tunneling current only from the sample surface region closest to the tip of the tip, making it possible to obtain local information on the sample surface, etc. This electron energy distribution can be obtained by changing the voltage applied to the chip and recording the tunneling current value.

In addition, in the present invention, although the light irradiation area is wide, the detection area should be an extremely small area directly under the chip, for example, at the corner of the mouth, because the tunnel current decreases rapidly as the barrier thickness increases. I can do it. Furthermore, since electrons excited by monochromatic light are detected as tunnel current, electrons in deep energy bands and levels can be detected. Since deep levels strongly depend on individual elements, it is also possible to identify the elements. Analysis of each atom is also possible by adjusting the distance between the tip and the sample. Furthermore, since the chip can scan while maintaining a constant distance from the sample, the elemental distribution and chemical bond state distribution on the surface can be obtained two-dimensionally. Note that in order to measure the energy distribution of excited electrons, it is necessary to stop scanning the tip at each location on the sample surface, sweep the voltage applied to the tip, and record the current. In this case, it is necessary to interrupt scanning and temporarily turn off the feedback loop that maintains a constant tip-sample distance.

Furthermore, in the present invention, the configuration of the detector part is completely different from that of conventional surface analyzers, and only electrons excited within a range that does not exceed the vacuum level are detected as tunnel current, and the distance from the sample surface is The biggest difference is that the surfaces can be manipulated in close proximity while maintaining a constant value.

However, since the incident light needs to reach the sample surface directly under the chip without being blocked by the chip, it is necessary to irradiate the sample surface at a fairly low angle.

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

FIG. 1 is a diagram for explaining one embodiment of the present invention, and shows the configuration of a surface analysis device.

18 is a vacuum chamber, 19 is an ion pump for vacuum evacuation, 20 is a sample, and 21 is a monochromatic S used as a light source.
OR light introduction boat, 22 is a tungsten (W) chip for detecting tunnel current, 23 is a chip with X+
This is a piezo element for driving in the y+ two directions. 2
Reference numeral 4 denotes an electronics section outside the vacuum chamber, which incorporates a voltage application mechanism to the chip, a tunnel current detection section, a feedback mechanism for applying voltage to the tunnel current and the piezo element, and the like.

As an evacuation pump, a diffusion pump or a Talio bomb can be used, but a non-vibration pump is preferable because this device avoids vibration.As a light source, a normal solid-state X-ray source or a rare gas ultraviolet source such as He can also be used. You can, but
Since SOR light is a continuous wavelength light source, it has the advantage that light of a desired wavelength (energy) can be irradiated by placing a spectrometer at the front stage.

To operate this, the tip 22 is brought close to the sample 2o and the tunneling current is set to In at a constant tunneling voltage, for example, IV.
Adjust the distance between the chip and the sample so that
Scan in the −y plane (parallel to the sample surface), turn off this feedback loop every time a certain amount of horizontal movement, stop the tip, and sweep the tip voltage in the range of, for example, −3■ to ÷3■ to create a tunnel current. is measured and stored in memory. Next, repeat the same measurement in the same area without irradiating light,
The electron density distribution at deep energy can be determined by taking the difference between when the tunnel current is irradiated and when it is not irradiated at each measurement point. Here, the reason for taking the difference is to eliminate the contribution of tunnel current caused by electrons that are not excited during light irradiation, if any. However, if this contribution can be estimated theoretically or experimentally in advance, the necessary information can be obtained only by measurement during light irradiation. When the wavelength of the incident light is in the appropriate X-ray region, core electrons appear as a peak in the tunneling current, so it is possible to identify the element by knowing the tunneling voltage at that time and the wavelength (energy) of the incident light. When the incident light is appropriate ultraviolet light, electrons in a band shallower than the core electrons appear in the tunnel current, so tunnel current/voltage characteristics that reflect the band structure can be obtained. Electrons excited higher than the vacuum level still fly into the tip at a certain rate, but this can be removed as a background component because it has almost no dependence on the voltage applied between the tip and sample.

[Effects of the Invention 1] As explained above, the surface analysis device according to the present invention can detect mold particles in a sample excited by monochromatic light as a tunnel current using STM. It is possible to obtain information on electrons with deeper energy (band structure, chemical bond state, element type) than that obtained from tunnel current, and (if) electrons ejecting from a wide area with energy exceeding the vacuum level can be obtained. Unlike photoelectron spectroscopy, which uses tunneling current to detect electrons, it has the advantage of being able to detect electrons from extremely minute areas directly beneath the chip.

Further, the present invention can be applied to, for example, Ge/SL, AρGaAs/
If applied to samples with thin heterostructures such as GaAS, it can be used to design band structures important for electronic devices, and if applied to materials with unknown element types, the distribution of each element can be determined on an atomic scale. be able to.

[Brief explanation of the drawing]

FIG. 1 is a schematic sectional view showing an embodiment of the present invention, and FIG. 2 is a diagram showing the principle of photoelectron spectroscopy. ! ...Horizontal axis showing electron density inside the sample, 2...Vertical axis showing electron energy, 3...Vacuum level, 4...Electron distribution before excitation, 5...Light energy Distribution of electrons excited by 6...
- Vacuum chamber 7... Exhaust pump, 8... Sample, 9... Excitation light source, 10... Hemispherical energy analyzer. DESCRIPTION OF SYMBOLS 11... Photoelectronic detector, 12... Amplifier, 13... Sample, 14... Chip, 15... Piezo element, 16... Voltage source, 17... Current for detecting tunnel current Source, 18...
Vacuum chamber, 19... Ion pump for evacuation, 20... Sample, 21... SOR light introduction port, 22... Tungsten (W) chip, 23... Piezo element, 24...・Electronics section outside the vacuum chamber. Honkabuto Roki Iohosu Curse 1 M Fig. 3 2 Energy 21, Gi ↑ Scanning type 1 Nenrenreshi I dai m/) Oval claw m Fig. 4

Claims (1)

[Claims] 1) A means for irradiating the sample surface with monochromatic light in a wavelength range from X-rays to infrared rays, and a tunnel between the sample and the chip that scans the sample surface while the light irradiation means is irradiating the sample surface with light. A surface analysis device characterized in that it includes means for measuring current.