Patent Application
20180238792
This disclosure relates to a method of conducting an analysis in the depth direction by irradiating ions to a sample in vacuum atmosphere and analyzing mass of the secondary ions ejected by sputtering of the sample, and simultaneously separating the emission spectrum emitted from the sample to thereby analyze the sample in the depth direction. The disclosure also relates to an analysis device provided with such method.
When the surface of a sample is irradiated with ions (a bundle of accelerated ions), the atoms at and near the surface are agitated, and some of the atoms are ejected into the vacuum. It has been found in the 1970's that atomic emission is observed in that process (C W White, D L Simms, N H Tolk; Science, 1972, 177: 481), and the process has been referred to as ion impact radiation (Surface Composition by Analysis of Neutral and Ion Impact Radiation; SCANIIR) (Takeshi Okuya, “Spectroscopy of Ion Excitement and Its Application,” Chapter 3, Surface Composition Analysis by Neutral and Ion Impact Radiation (SCANIIR) (1987)). That method, however, was not practically used in those days due to the low ion impact radiation intensity. Instead, use of secondary ion mass spectrometry (SIMS) has been popular since then because of the availability of the highly sensitive mass spectrometers. SIMS is a method wherein components in the sample are qualitatively and quantitatively analyzed by conducting the secondary ion mass spectrometry in the particles that had been ejected by the irradiation of the sample with the sputtering ion (primary ion). The type of the SIMS where minute amount of impurities in the solid are mainly analyzed is called Dynamic SIMS (D-SIMS) and, in that method, oxygen and cesium ions are mainly used for the sputtering ion. The mass spectrometers used are quadrupole and sector (double-focusing) mass spectrometers. Unlike the D-SIMS where the sputtering ion also plays the role of primary ion, the primary ion of low current density (1×10−12 ions/cm2) is used apart from the sputtering ion in time-of-flight mass spectrometer (TOF-SIMS). In addition, in SIMS, an electron beam (a bundle of accelerated electrons) is generally used to neutralize charging of the sample by the sputtering ion, and an electron gun is provided for this purpose.
SIMS is an analytical technique used to analyze impurities, and concentration of the impurities can be quantitatively determined from intensity of the impurities obtained from the sample to be evaluated by determining relative sensitivity constant from the intensity obtained for the samples of known concentration. The composition cannot be quantitatively determined while such determination is possible in some systems. This is because the change in the composition that occurs when the target element reaches the level of the main component, and sensitivity changes by this change in the composition. Linear formation of the secondary ion intensity in relation to the concentration of the target element is no longer retained at the level of approximately 1% although this level differs by the main component or the target element.
In the analysis in the depth direction conducted by the sputtering with ion, a factor that greatly affects the resolution in the depth direction is convergence of the spot diameter of the sputtering ion because the convergence affects the bottom shape of the sputtering mark (crater) formed by the sputtering ion. When the spot diameter of the sputtering ion is large, the crater bottom will have a mortar shape, and this shape has considerable influence on the area of the crater wall seen from the top, and this invites poor resolution in the depth direction due to the difference in the depth of the information acquired.
An exemplary technique capable of conducting the analysis in the depth direction by using atomic emission is GD-OES (Glow Discharge Optical Emission Spectrometry). In that technique, the sample is sputtered with Ar plasma, and the emitted particles undergo atomic emission in the plasma, and the emission is observed for the analysis in the depth direction. In GD-OES, the composition is quantitatively determined by using the relation that the product of the sputtering speed and the concentration is in correlation with the atomic emission intensity. However, the resolution in the depth direction is poor when sputtering is conducted with Ar plasma because of the frequent deformation of the crater shape and inability to separate the information from the crater bottom from the information from the crater wall.
Cathodoluminescence microscopes where the light emitted from the sample upon the electron beam irradiation is analyzed for evaluation of crystallinity and defects of the substance are already commercially available. However, the damage given to the sample by the electron gun used as the source of excitation electrons in the cathodoluminescence device is serious due to the high acceleration voltage, and such high acceleration voltage also results in the increase of the electron beam intrusion into the sample. In addition, information in the depth direction cannot be acquired by the sputtering of the sample with the electron beam.
It could therefore be helpful to provide a device capable of obtaining emission spectrum in a device used for surface analysis technique as well as an analytical technique using such a device that conducts surface analysis technique such as SIMS and simultaneously obtain atomic emission spectrum or cathodoluminescence with the resolution level of nanometer in the depth direction.
We thus provide:
A method of analyzing a sample wherein the analysis is conducted in the vacuum atmosphere of a surface analyzer device at a degree of vacuum of up to 2×10−9 mbar, and at least one type of ion selected from argon ion, oxygen ion, cesium ion, and cluster ion at an irradiation energy of up to 20 keV is irradiated for sputtering of the sample, and emission spectrum from the sample is separated to analyze the sample in the depth direction simultaneously with any one of the following methods (1) to (3):
The device enables simultaneous analysis of the minute impurities in the material and the analysis in the depth direction of the element of the main component in the resolution in the depth direction at the nanometer level and, also, analysis in the depth direction of defects, impurities, electron structure, and the like is enabled by the measurement of cathodoluminescence using ion sputtering.
Next, a summary of the device and the analysis method are described by referring to
The device has a vacuum chamber D1 associated with the SIMS device, a stage and a sample holder D2, a primary ion source D4, an electron beam source D5, a secondary ion optical system D6, and a sample D3 to be measured, and emitted beam D7 from the sample is taken out from a vacuum window D8 allowing confirmation of the position where the sample is irradiated with the ion, the electron beam, or both, guided to a spectrometer D10 through an optical system D9 to thereby conduct the analysis. While the angle of stage and sample holder D2 is preferably adjustable in relation to each of the direction of sputtering ion source D4, electron beam source D5, and the vacuum window D8, the angle of the stage and the sample holder D2 may be fixed. Alternatively, the angle may be adjusted in the course of mounting of the sample D3 to the sample holder.
The vacuum chamber D1 in the device is preferably at a degree of vacuum of up to 2×10−9 mbar, and more preferably up to 1×10−10 mbar. Use of a lower degree of the vacuum results in the lower limit of the detection in the surface analysis and reduced effects of the noise.
Various types of sputtering ion sources D4 may be used in the device, and examples include oxygen, cesium, argon, gas cluster, water cluster, helium, xenon, and neon, and of these, the preferred are argon, oxygen, cesium, and gas cluster. Acceleration energy of the ion irradiated is preferably up to 20 keV. When the acceleration energy is high, resolution in the depth direction will be sacrificed by the effect of the mixing by the ion. Accordingly, the acceleration energy should be adjusted according to the film thickness of the sample to be measured or depth to be measured.
In the device, when the sample is irradiated with the ion, the atoms constituting the sample are sputtered, and atomic emission spectrum is observed in the transition of the sputtered atom to the ground state. This observation enables elemental analysis in the depth direction. While the only information available by the conventional SIMS had been distribution of the impurities and the main component element in the depth direction, the device has enabled simultaneous measurement of the crystallinity and defect distribution in the depth direction in addition to the information of composition and elements of the sample. This simultaneous measurement of compositional or element information, crystallinity, and defect distribution is also applicable to other analytical techniques where ion sputtering is conducted in the ultra-high vacuum device. Accordingly, the surface analysis technique is not limited to the SIMS, and the technique is also applicable to XPS, AES, and TOF-SIMS.
When the surface analysis technique is SIMS and the mass spectrometer used is sector type, the sputtering ion is preferably up to 20 keV. Preferably, the irradiation energy is further reduced depending on the film thickness of the sample to be measured or depth to be measured.
When the surface analysis technique is SIMS and the mass spectrometer used is quadrupole type, the sputtering ion is preferably up to 8 keV. Preferably, the irradiation energy is further reduced depending on the film thickness of the sample to be measured or depth to be measured.
When the surface analysis technique is SIMS and the mass spectrometer used is time-of-flight type, the irradiation energy is preferably up to 5 keV when the sputtering ion is oxygen or cesium ion. The irradiation energy is preferably up to 20 keV when the sputtering ion is cluster ion.
The sputtering ion preferably has a radius of less than 50 μm when the irradiation energy is at least 8 keV. The radius is preferably less than 100 μm when the irradiation energy is less than 8 keV, and the radius is preferably less than 200 μm when the irradiation energy is less than 5 keV.
When the composition is analyzed by using atomic emission by using the device, provision of an electric gate as in the case of the SIMS is difficult. Accordingly, outer periphery of the target crater is preferably dug in advance to exclude the influence of the crater edge. This reduces influence of the crater edge and improves the resolution in the depth direction.
In the device, the electron beam source D5 for charge compensation used to compensate the charge resulting from the irradiation of the ion or X-ray is not particularly limited for its type, and exemplary types include thermionic emission type, field emission type, and Schottky Emission type (also called “thermal field emission” type). When the cathodoluminescence emitted from the sample is evaluated by using this electron beam, the irradiation energy of the electron beam is preferably up to 3 keV. Use of such electron beam enables collection of the information from the area much shallower than typical cathodoluminescence. Accordingly, use of a lower acceleration energy is desirable as long as the emission intensity can be obtained. Since the cathodoluminescence of the sputtered surface can be measured simultaneously with the sputtering of the sample with ion, the crystallinity, defects, and the like of the sample can be evaluated with the resolution in the depth direction in nanometer level.
The vacuum window D8 used in the device is preferably made of a material permit-ting penetration of the beam with the wavelength corresponding to the subject to be measured.
In the device, the emitted beam D7 is taken out from the vacuum window D8, and thus taken out emitted beam D7 should be efficiently guided to the spectrometer D10 and the optical system D9 such as lens, optical fiber, or converging mirror is used for this purpose. This converging means is preferably placed within 30 cm from the sample D3 to suppress the sensitivity loss. It is to be noted that a part of the optical system D9 may be placed in the vacuum chamber. In the case of a device such as XPS and AES, this distance is preferably within 50 cm when the irradiation energy of the sputtering ion is up to 10 keV.
The device has enabled simultaneous analysis in the depth direction by conducting the surface analysis using the secondary ion optical system D6 while separating the emitted beam D7.
Next, the merits of our devices and methods are described in further detail by referring to Examples.
SIMS 4550 manufactured by FEI (a quadrupole SIMS analyzer) was used. The normal ultimate vacuum degree of the vacuum chamber of the device was up to 1×10−10 mbar. An optical lens was placed at a position in the atmosphere to thereby collect the light from the sample stage in the vacuum chamber to the spectrometer through the vacuum window. The position of the optical lens was 25 cm from the sample stage. The optical lens was connected to a spectrometer microHR by HORIBA, Ltd. equipped with a CCD detector Sincerity manufactured by HORIBA, Ltd. The spectrometer was placed on the XYZ stage for fine adjustment of the converging lens position.
Of the oxygen ions, O2+ ion was used for the primary ion of the SIMS, and the scanning was conducted at an acceleration energy of 3 keV, an incident angle of 45°, a current amount of 100 nA, a spot diameter 20 μm, and a luster size of 150 μm. In this process, the position of the optical lens was at a position 90° to the sample. The evaluation was conducted without using the vacuum gauge to avoid the influence of the emission from the filament of the vacuum gauge.
A groove density of 300 gr/mm and a blaze wavelength of 500 nm were used for the diffraction grating of the spectrometer. With regard to the exposure of the CCD, leakage of exterior light or the like was measured as dark before each analysis after moving the sample to be measured to the position of the analysis, and this value was subtracted at the time of the analysis. The exposure was conducted for 5 seconds in total.
The sample evaluated was a compound semiconductor sample having a laminate structure. The laminate structure and the design value of the compositional ratio of each layer was In(Al0.25Ga0.75)P/In(Al0.50Ga0.50)P/In(Al0.70Ga0.30)P/GaAs-sub, and the laminate was the one comprising films each having a thickness of 200 nm.
CCD exposure was started at a timing slightly before the start of the sputtering by the ion. The timing at the start of the atomic emission spectrum was considered as the start time of the sputtering.
Depth profile of the atomic emission spectrum of the Al and Ga was obtained from the thus obtained spectra by assuming that Al is at 396 nm and Ga is at 403 nm. Sputtering speed of Al and Ga in the film obtained by separate measurement and calculation was used for the calculation of x axis. By using the relation of “Emission intensity=constant×sputtering speed×concentration,” concentration at each depth can be determined from the constant preliminarily determined by using samples of known concentration and the emission intensity and the sputtering speed at each depth.
The results of the actual evaluation using the sample are shown in
The following sample was evaluated by using device constitution as used in Example 1. The laminate structure was In(Al0.70Ga0.30)P/In(Al0.50Ga0.50)P/In(Al0.70Ga0.30)P/In(Al0.25Ga0.75)P/In(Al0.70Ga0.30)P/In(Al0.50Ga0.50)P/In(Al0.70Ga0.30)P/GaAs-sub, and the film thickness was respectively 100 nm/10 nm/10 nm/10 nm/10 nm/10 nm/300 nm/substrate.
The CCD exposure was started at a timing slightly before the start of the analysis by the SIMS. The timing at the start of the emission spectrum was considered as the start time of the SIMS analysis so that origin point in x axis is consistent with the SIMS data.
In
As shown in the data obtained in the SIMS, depth profile of the impurity Mg is simultaneously obtained, and this indicates that the evaluation of the main component and the depth profile of the impurities can be obtained at once.
In the device constitution as used in Examples 1 and 2, the position of the optical lens was changed to the position of 75° to the sample. It is to be noted that the sputtering ion was not used.
The electron beam used for obtaining the cathodoluminescence was at an acceleration energy of 500 eV, a current amount of 5 μA, an incident angle of 15°, and a tilt angle of 45°.
A groove density of 300 gr/mm and a blaze wavelength of 250 nm were used for the diffraction grating of the spectrometer. With regard to the exposure of the CCD, leakage of exterior light or the like was measured as dark before each analysis after moving the sample to be measured to the position of the analysis, and this value was subtracted at the time of the analysis. The exposure was conducted for 5 seconds in total.
The sample used for the evaluation was a sample prepared by depositing a SiO2 film of about 40 nm on a SiC substrate.
The ion beam-induced atomic emission spectrometer and analysis method using such device contribute not only to the analytical device industry, but also to the improvement in the quality and yield of the products through the analysis of the state of the products to their minute parts in the fields including the industry of electronic parts.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-022840 | Feb 2015 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/080216 | 10/27/2015 | WO | 00 |
