Method of analyzing metal impurities in surface oxide film of semiconductor substrate

Abstract: A method of analyzing metal impurities in a surface oxide film of a semiconductor substrate enables a detection sensitivity of the order of 10¹¹ atoms/cm² in a simple technique. This method comprises measuring a quantity of oxide charge resulting from specified metal impurities existing in the surface oxide film formed on the surface of a semiconductor substrate, obtaining a correlation between the quantity of metal impurities and the quantity of oxide charge, and measuring the quantity of metal impurities in the surface oxide film from the quantity of oxide charge of the surface oxide film, based on this correlation. Further, it is possible to obtain the distribution of concentration of the metal impurities in the thermal oxide film on a silicon substrate by obtaining a correlation between a depth of the thermal oxide film and a quantity of oxide charge resulting from the metal impurities in the thermal oxide film and by obtaining a correlation between a quantity of the metal impurities in the thermal oxide film and the quantity of oxide charge resulting

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: This invention relates to a method of analyzing metal impurities in an oxide film formed on the surface of a semiconductor substrate, and more particularly to a method of analyzing iron and aluminium impurity, which method is useful for quality control chiefly in the cleaning process during the production of LSI. 2. Description of the Related Art: The conventional means for analyzing metal impurities in the surface layer of a semiconductor substrate is exemplified by SIMS (Secondary Ion Mass Spectrometry, see R.K. Lowery and R.G. Masters, 19th Annual Proc. Reliab. Phys., 157, 1981) and TRXRF (Total Reflection X-ray Fluorescence, see R.S.Hockett and W. Katz, J. Electrochem. Soc. 136, 3418, 1989). However, these conventional analyzing methods need a vacuum, which requires a large-sized and expensive analyzing apparatus. Therefore in Lhe conventional methods, a simple analysis cannot be achieved. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a method of analyzing metal impurities in a surface oxide film of a semiconductor substrate, which method does not need a vacuum and therefore can be carried out in an inexpensive small-sized apparatus and which enables a detection sensitivity of the order of 10¹¹ atoms/cm². This object is achieved with a method as claimed. Preferred embodiments of the invention are subject-matter of the dependent claims. A significant feature of the analyzing method of this invention is that the quality and quantity of the metal impurities can be analyzed by measuring the quantity of charge of the metal impurities. The present inventor has discovered that certain metal impurities contained in the surface oxide film of a semiconductor substrate have a charge, and more particularly that iron and aluminum in a natural oxide film have a negative charge and that when iron and aluminum from a natural oxide film diffuse into a thermal oxide film formed on the natural oxide film, only aluminum has a charge. This invention has been made based on these phenomena. Thus a metal impurity which has a charge in an oxide film can be analyzed by the method of this invention. Such particular metal impurity is exemplified by iron and aluminium. Copper and nickel do not have such characteristics. It has been discovered that in a natural oxide film both iron and aluminium have a charge and that in a thermal oxide film only aluminium has a charge. In this specifications the term "natural oxide film" means an oxide film developed on an active silicon surface such as in an atmosphere containing oxygen or a chemical oxide film formed such as by RCA cleaning, especially the latter normally having a small thickness of about 10 Angstrom. The term "thermal oxide film" means an oxide film of about 10 nm (100 Angstrom) formed by oxidation in an oxygen gas at a high temperature of 800 to 1.200°C in a furnace for oxidation. In the first analyzing method of this invention, it is possible to obtain the concentration of metal impurities by obtaining a quantity of oxide charge in a surface oxide film of the sample substrate if a correlation between a quantity of metal impurities existing in the surface oxide film in the form of a natural oxide film or a thermal oxide film and the quantity of oxide charge resulting from the metal impurities is known. In the second annalyzing method, a quantity of oxide charge in a thermal oxide film formed on a silicon substrate is measured, whereupon a surface layer of the thermal oxide film is removed such as by etching; in this stage, the quantity of oxide charge in the remaining thermal oxide film is measured. By repeating this process one or more times, it is possible to obtain a correlation between the depth of the thermal oxide film and the quantity of oxide charge resulting from the metal impurities in the thermal oxide film as well as a correlation between the quantity of metal impurities in the thermal oxide film and the quantity of oxide charge resulting therefrom. Based on these correlations, it is possible to obtain a correlation between the concentration of the metal impurity and the depth of the thermal oxide film, from which correlation the distribution of concentration of the metal impurities in the thermal oxide film can be determined. Then the concentration of the metal impurity (aluminium) can be obtained by obtaining the quantity of oxide charge in the thermal oxide film of the sample substrate from the concentration distribution. In the third analyzing method, the existence of iron or aluminium in a natural oxide film can be determined by comparing the pre-measured quantity of oxide charge in the natural oxide film with the premeasured quantity of oxide charge resulting from aluminium in the thermal oxide film, based on the fact that of iron and aluminum after they move from the natural oxide film into the thermal oxide film, only aluminum has a charge. As described above, according to the analyzing method of this invention, it is possible to specify the kind of a metal impurity in the surface oxide film and to detect the concentration of the metal impurity with high precision by simply measuring the quality of oxide charge in the surface oxide film. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a characteristic graph showing the correlation between the Fe concentration and the quantity of oxide charge in a natural oxide film or a thermal oxide film: FIG. 2 is a graph showing the distribution of Fe in the depthwise direction of the thermal oxide film; FIG. 3 is a characteristic graph showing the correlation between the Al concentration and the quantity of oxide charge if a thermal oxide film is formed on a natural oxide film; FIG. 4 is a graph showing the distribution of quantity of oxide charge of Al in the depthwise direction of the thermal oxide film; FIG. 5 is a flow diagram showing the forming and measuring a sample for obtaining the correlation diagram of FIG. 4; and FIG. 6 is a graph showing the distribution of Al concentration in the depthwise direction of the thermal oxide film. DETAILED DESCRIPTION Embodiments of this invention will now be described in detail with reference to the accompanying drawings. FIG. 1 shows the correlation (curve 1) between the iron (Fe) impurity concentration (atoms/cm²) and the quantity of oxide charge (×10¹¹g/cm²) in a natural oxide film formed on the surface of a silicon substrate, and also shows the correlation (curve 2) between the Fe impurity concentration (atoms/cm²) and the quantity of charge oxide charge (×10¹¹/cm²) in a thermal oxide film. For obtaining the correlation shown by the curve 1, as a pre-treatment process, the surface of an n-type silicon substrate is wet-cleaned by a first cleaning process (hereinafter called "SC-1'), which is performed in the following order: (a) etching with an HF cleaning solution (HF:water=1:10) for three minutes. (b) rinsing with pure water for three minutes. (c) cleaning with an oxide agent (HCl: H₂O₂:water=1:1:6) at 60°C for three minutes. (d) rinsing with pure water for three minutes. By the wet-cleaning of the first cleaning process, a natural oxide film (chemical oxide film) of about 0.1 nm (10 Angstrom) is formed on the silicon substrate. An oxide agent containing HCl is useful in removing metal impurities on the substrate. Then, the surface of the natural oxide film is coated with an Fe solution by rotating the substrate while dropping onto the surface of the substrate an Fe solution containing Fe ions at a predetermined concentration, namely, by the spin-coat method, whereupon the natural oxide film is dried. The resulting sample contains Fe in the natural oxide film at a controlled concentration. In this experiment, as shown in Fig. 1, for three kinds of samples or different Fe concentrations, the concentration of Fe and the quantity of oxide charge resulting from Fe in the natural oxide film are obtained. The more the points of measurement, the more precise the impurities-oxide charge correlation curve can be obtained. Subsequently, the natural oxide film is perfectly dissolved in a clean HF solution (HF:water=1:200) containing neither Fe ions nor Al ions by etching the natural oxide film with the HF solution, and this solution is collected, whereupon the quantity of Fe impurities in the natural oxide film is measured by measuring the quantity of Fe in the solution by atomic absorption. The quantity of oxide charge in the natural oxide film is obtained by measuring the relation (C-V) between a D.C. bias voltage V and a depletion layer capacitance C by measuring the surface photoelectric voltage (SPV) generated by LED intermittent light in proportion to the width of a semiconductor depletion layer while varying the D.C. bias voltage. In the C-V measurement of this SPV method, the inversion voltage measurement method (initially a positive d.c. bias voltage is applied to an n-type silicon substrate) is adopted. The SPV method is disclosed in a publication: E. Kamieniecki. SEMICON/EUROPA 91 TECHNICAL PROCEEDINGS, pages 85-95, March 5-6 (1991). As is apparent from the curve I of FIG. 1, along with the increase of the quantity of Fe impurities in the natural oxide film on the silicon substrate, the quantity of negative charge in the natural oxide film increased. This it was observed that Fe would have a negative charge as incorporated in a natural oxide film. Further, according to this measuring method, measurement of charge is possible even if the quantity of Fe is around 10¹¹ atoms/cm². Thus it was observed that Fe in a natural oxide film on a silicon substrate surface could be detected in the order of 10¹¹ atoms/cm² from the C-V analysis using the SPV method. The correlation, which is represented by the curve 2, between the Fe concentration and the quantity of oxide charge in the thermal oxide film was obtained by the following method. A natural oxide film contaminated with Fe was formed in the same manner as in the case of the sample formed to obtain the above-mentioned curve 1, and a thermal oxide film was formed on the natural oxide film. This thermal filin was formed by heating the substrate in an atmosphere of oxygen gas at 900°C for 15 minutes, to a thickness of about 95 Angstrom (9.5 nm). Then, the Fe concentration and the quantity of oxide charge in the thermal oxide film were obtained by the above-mentioned method. As is apparent from the curve 2 of FIG. 1, it was observed that in the thermal oxide film, Fe has almost no negative charge, irrespective of its concentration. FIG. 2 shows the distribution of Fe in the depthwise direction of the thermal oxide film. This distribution was obtained in the following manner. The sample used in this test is a thermal oxide film (thickness: about 95 Angstrom (9.5 nm)) formed by dry oxidation at 900°C for 15 minutes after cleaning a silicon substrate by the first cleaning method and then doping Fe into the natural oxide film at the concentration of 1× 10¹⁴ atoms/cm² by the spin coat method. Using this sample, the concentration of Fe at two or more positions in the depthwise direction of the thermal oxide film was obtained by the TRXRF method. More specifically, firstly the concentration of Fe at the surface of the thermal oxide film was obtained by the TRXRF method. Subsequently, the thermal oxide film was etched with an HF solution, and a surface layer of the thermal oxide film was removed, whereupon the concentration of Fe in the new surface layer was measured. By repeating this procedures, the distribution of Fe in the thermal oxide film was obtained. As is apparent from the curve 3, it was observed that the number of Fe atoms in the thermal oxide film increased with the depth of the thermal oxide film, peaking and localizing near the border of the thermal oxide film and the silicon substrate. From the curves 2 and 3, it was observed that Fe in the thermal oxide film concentrated near the silicon substrate and load no negative charge. FIG. 3 shows the correlation between the Al concentration and the quantity of oxide charge in the thermal oxide film, The pre-treatment process used in this test for the silicon substrate is exemplified by the following processes: (a) doping, onto the substrate wet-cleaned by the first cleaning method, Al atoms by the spin coat method (SC-1 + Al spin coat). (b) wet cleaning by a second cleaning method (SC-2) using the following solutions in the described order: 1) HF:water (1:10) for three minutes. 2) rinsing with pure water for three minutes. 3) NH₄OH:H₂O₂:WATER (1:1:6) at 60°C for three minutes. 4) rinsing with pure water for three minutes. (c) wet cleaning by a third cleaning method (SC-3) using the following solutions in the described order: 1) HF:water (1:10) for three minutes. 2) rinsing with pure water for three minutes. 3) NH₄OH:H₂O₂:water (1:1:6) at 60°C for three minutes. 4) HF:water (1:300) for one to two minutes. Firstly the correlation curve 4 of FIG. 3 was obtained by the following procedures: A silicon substrate underwent one of the foregoing pre-treatment processes including wet cleaning. By this pre-treatment process, a natural oxide film was formed on the silicon substrate by chemical oxidation. Then a thermal oxide film having a thickness of about 95 Angstrom (9.5 nm) was formed by dry oxidation at 900°C for 15 minutes. Subsequently, the quantity of oxide charge of this thermal oxide film was obtained by the SPV method, and the Al concentration, like the Fe concentration, in the thermal oxide film was obtained by atomic absorption. By plotting the thus obtained values of each sample on FIG. 3, the curve 4 representing the correlation between the Al concentration and the quantity of oxide charge in the thermal oxide film was obtained. The result represented by the curve 4 shows that Al originally existing in the natural oxide film had a negative charge after having moved into the thermal oxide film and that when the Al concentration was larger than 10¹¹ atoms/cm², the negative charge increased substantially linearly along with the increase of concentration. When the second cleaning method (SC-2) or the third cleaning method (SC-3) were performed as the pre-treatment process as when Al was positively doped into the natural oxide film (SC-1 + Al spin coat), it was also observed that Al was contained as impurities in the natural oxide film and diffused into the thermal oxide film. In this test, in the case of the second cleaning method (SC-2). the concentration of Al impurities was high. FIG. 4 shows the correlation between the thickness (depth of etching) of the thermal oxide film and the quantity of oxide charge, illustrating the distribution of Al in the depthwise direction of the thermal oxide film. The samples used in this test were a substrate whose surface was contaminated with Al at a concentration of about 1×10¹² atoms/cm² after performing the first cleaning method as the pre-treatment process (SC-1 + Al spin coat) and a substrate whose surface was treated by the second cleaning (SC-2) method as the pre-treatment process. The correlation curves 5 and 6 of FIG. 4 were obtained according to the flow diagram of FIG. 5. After an n-type silicon substrate was treated with the foregoing pre-treatment processes, dry oxidation was performed at 900°C for 15 minutes to form a thermal oxide film. Then the quantity of oxide charge and the Al concentration in this thermal oxide film were measured by the above-mentioned method. Subsequently, the surface layer (about 2 Angstrom (0.2nm)) of the thermal oxide film was removed by etching using an HF solution, whereupon the quantity of oxide charge and the Al concentration in the resulting thermal oxide film were measured again. And the foregoing etching and measuring processes were repeated one or more times. Thus it was observed that the correlation curve 5 (according to SC-1 + Al spin coat) and the correlation curve 6 (according to SC-2) were substantially identical with each other. As is apparent from these correlation curves 5 and 6, almost all of the negative charge resulting from Al localized at the region of 2 Angstrom from the top surface of the thermal oxide film. Thus it was observed that part of Al existing in the natural oxide film (i.e., chemical oxide film) formed by the foregoing pre-treatment process was diffused and moved toward the surface of the thermal oxide film after thermal oxidation. FiG. 6 shows the correlation between the thickness (depth) and the Al concentration in the thermal oxide film. In this test, two kinds of samples treated by SC-1 + Al spin coat and SC-2 as pretreatment processes were used. The measuring method used in this test was as follows: The sample of two kinds treated by the different pre-treatment processes were formed under the same condition; the number of samples of each kind was equal to the number of points of measurement. For SC-2. a separate sample substrate which would have only a natural oxide film and would be devoid of any thermal oxide film should be formed in advance. Then, regarding each of the samples , the Al concentration in the individual thermal oxide film was measured. Describing this more specifically using SC-1 + Al spin coat by way of illustrative example, the al concentration in the thermal oxide film of one sample substrate was measured by atomic absorption. Then using another sample substrate. the surface portion of the thermal oxide film was removed by the thickness of about 6 Angstrom by etching with an HF solution, and the Al concentration of the remaining thermal oxide film was measured. By plotting the thus obtained result in FIG. 6, the correlation curve 7 was obtained. Likewise, the correlation curve 8 was obtained by plotting the result of measurement for SC-2. A plot 9 was obtained by measuring the Al concentration in the natural oxide film treated by SC-2 of the separate samples mentioned above. As is apparent from the correlation curves 7 and 8 of FIG. 6, it was observed that Al existing in the natural oxide film moved into the thermal oxide film to localize very near the top surface, proving the result shown in FIG. 4. By the cleaning process (SC-2) as pre-treatment process, Al impurities were incorporated in the natural oxide film to contaminate the thermal oxide film. In the case of SC-2, about 40% (concentration C B ) of Al (concentration C A ) existing in the natural oxide film was incorporated in the thermal oxide film. In the thermal oxide film, Fe had no charge. From the result shown in FIGS. 4 and 6, it was observed that the quantity of oxide charge in the thermal oxide film resulted from Al impurities and that the Al impurities localized at the very surface layer of the thermal oxide film. Thus the quantity of oxide charge resulting from Al can be determined by the different Q C between the quantity of oxide charge Q A when the etching depth was zero and when the depth of the thermal oxide film was zero, and the quantity of oxide charge Q B when the very surface layer (layer having a depth of about 2 to 6 Angstrom (0.2-0.6 nm) was removed. Therefore, the Al concentration in the thermal oxide film can be obtained from the differential quantity of oxide charge Q C , based on the Al concentration-oxide charge correlation curve of FIG. 3. Further, by pre-measuring the quantity of oxide charge in the natural oxide film formed by the pre-treatment process (wet cleaning) and then comparing this quantity of oxide charge with the quantity of oxide charge in the thermal oxide film, it is possible to qualitatively analyze Fe and Al in the natural oxide film. In other words, though both Fe and Al are moved from the natural oxide film into the thermal oxide film, only Al has a charge in the thermal oxide film. Therefore, if an oxide charge is detected in the thermal oxide film, it shows that Al is contained in the natural oxide film. By pre-detecting how much part of Al initially existed in the natural oxide film is moved to the thermal oxide film, it is possible to estiminate the approximate concentration of Al (i.e., the quantity of Al oxide charge) in the natural oxide film. By comparing this quantity of Al oxide charge with the total quantity of oxide charge in the natural oxide film, it is possible to presume the presence/absence and the approximate quantity of Fe in the natural oxide film. According to this invention, the following quantitative and qualitative analyses can be realized: (1) It is possible to obtain the concentration of sum of Fe and Al in a natural oxide formed on a silicon substrate. (2) By forming a thermal oxide film on the natural oxide film, it is possible to measure the concentration of Al moved from the natural oxide film into the thermal oxide film. (3) By evaluating the concentration of sum of Fe and Al in the natural oxide film of (1) above and the concentration of Al in the thermal oxide film of (2) above, it is possible to detect the presence of Fe and Al in the natural oxide film and also to presume their ratio. According to this invention, it is possible to detect Al atoms and Fe atoms, in the order of 10¹¹ atoms/cm², which are highly apt to cause contamination in various cleaning processes, such as RCA cleaning, in the LSI manufacturing process. Since the SPV method is used in the analyzing method, it is possible to carry out this invention easily and inexpensively on a small-sized apparatus. Regarding Ni and Cu atoms of metal impurities, because no oxide charge would be generated in a silicon oxide film, they cannot be detected by the analyzing method of this invention.

Claims: A method of analyzing metal impurities including at least one of iron and aluminum, in a surface oxide film formed on a semiconductor substrate, comprising: (a) measuring the quantity of oxide charge resulting from metal impurities existing in the surface oxide film; and (b) determining the quantity of metal impurities in the surface oxide film from the quantity of oxide charge measured in step (a) and a pre-established correlation between the quantity of oxide charge and the quantity of the metal impurities. The method according to claim 1, wherein the surface oxide film to be analyzed is a natural oxide film or a thermal oxide film. The method of claim 1 wherein the quantity determined in step (b) represents the total quantity of iron and aluminum in a natural oxide film and the method comprises the subsequent steps of: c) forming a thermal oxide film on said natural oxide film, d) measuring the quantity of oxide charge resulting from aluminum existing in the thermal oxide film, e) etching off a surface layer of the thermal oxide film, f) measuring the quantity of oxide charge resulting from aluminum existing in the etched thermal oxide film, g) determining from the difference between the quantities measured in steps d) and f) and a pre-established correlation between the quantity of oxide charge and a quantity of aluminum in a thermal oxide film a first value representing the quantity of aluminum in said thermal oxide film, h) calculating a second value representing the quantity of aluminum initially contained in said natural oxide film from said first value and a pre-established ratio between the quantity of aluminum initially contained in said natural oxide film and the quantity of aluminum moving from said natural oxide film to said thermal oxide film in response to step (c), and i) calculating the difference between the quantity determined in step b) and said second value to obtain a third value representing the quantity of iron in said natural oxide film. The method according to any one of the preceding claims, wherein the quantity of oxide charge resulting from the metal impurities existing in the surface oxide film is obtained by measuring a relation (C-V) between a d.c. bias voltage V to the semiconductor substrate and a depletion layer capacitance C by measuring a surface photoelectric voltage (SPV) generated by LED intermittent light in proportion to the width of a semiconductor depletion layer while varying the d.c. bias voltage to the semiconductor substrate. The method according to any one of the preceding claims, wherein said pre-established correlation in step (b) is obtained by preparing multiple samples of semiconductor substrate each having a surface oxide film with a different metal impurity concentration, measuring for each of said sample substrates the quantity of oxide charge resulting from metal impurities existing in the respective surface oxide film,and obtaining the quantity of metal impurities in said surface oxide films by measurement of atomic absorption after the respective surface oxide film has been dissolved in a clean hydrogen fluoride solution devoid of metal impurities and the metal impurities then collected. A method of analyzing metal impurities in a surface oxide film of a semiconductor substrate, comprising: (a) obtaining a correlation between a depth of a thermal oxide film formed on a silicon substrate and a quantity of oxide charge resulting from the thermal oxide film; (b) obtaining a correlation between a quantity of metal impurities in the thermal oxide film and the quantity of oxide charge resulting therefrom; and (c) obtaining a distribution of concentration of the metal impurities in the thermal oxide film, based on theses correlations. The method according to claims 6, wherein the metal impurities are aluminum. A method of analyzing metal impurities in a surface natural oxide film formed on a semiconductor substrate, wherein iron or aluminum in a natural oxide film is qualitatively analyzed by comparing a premeasured quantity of oxide charge in the natural oxide film with a quantity of oxide charge resulting from aluminum in a thermal oxide film formed on said natural oxide film, based on the fact that, of iron and aluminum, after they move from the natural oxide film into the thermal oxide film, only aluminum has a charge.

Inventors: KATO JURI, KATO, JURI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1991
Main classification: G01N33
IPC codes: G01N33, H01L21, G01R31