Method for preparing standard samples for evaluating surface metal contamination

Abstract

To provide a method for producing a standard sample for surface metal contamination evaluation, which suppresses generation of an oxide film or attachment of an organic substance such as hydrocarbons on a surface, and is excellent in repeatability and quantitativity.SOLUTION: A method for producing a standard sample quantitatively contaminated with predetermined metal impurities used in surface metal contamination analysis, includes a wafer surface cleaning step of removing the surface impurities of a silicon wafer for the standard sample, an oxidation reduction potential adjustment step of dissolving the metal impurities in an HF aqueous solution to reduce oxidation reduction potential until the metal impurities come into a neutral atom state, then a step of immersing the silicon wafer after the wafer surface cleaning step in the HF aqueous solution, and then a rinse step of removing the HF aqueous solution attached to the silicon wafer, and a step of drying the silicon wafer. The method for producing the standard sample for surface metal contamination evaluation is for producing the standard sample quantitatively contaminated with the predetermined metal impurities on the surface of the silicon wafer for the standard sample.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a standard sample (standard wafer) contaminated with a quantitative amount of a predetermined metal species (metal impurities) for use in surface metal contamination analysis (surface metal contamination evaluation) of silicon wafers. [Background technology]

[0002] As semiconductor devices become smaller, the cleanliness of the silicon wafer surface is becoming more stringent. In particular, metal contamination is a cause of device malfunction and deterioration of element characteristics, so it is being strictly controlled. On the other hand, when quantitatively analyzing metal contamination present on the surface of a silicon wafer, a standard sample in which the wafer surface has been contaminated with a predetermined amount of metal impurities is required.

[0003] For this reason, conventional methods for preparing standard samples have been disclosed, which include a method in which a wafer is immersed in an alkaline hydrogen peroxide aqueous solution containing a fixed concentration of metal impurities (Patent Document 1); a method in which the wafer surface is made hydrophilic by processing the wafer surface with an ammonia-hydrogen peroxide aqueous solution → hydrofluoric acid aqueous solution → hydrochloric acid-hydrogen peroxide aqueous solution, a solution containing a fixed concentration of metal impurities is dropped onto the surface, and after maintaining the surface for a fixed period of time, the wafer is rotated at high speed to remove the solution remaining on the wafer (Patent Document 2); a contamination treatment in which the wafer surface is contaminated with a specified amount of a metal impurity solution containing a volatile alcohol, and an oxidation / reduction / chemical reaction treatment or the like is performed on the contaminating metal impurities (Patent Document 3); and a method in which the amount of metal impurities adhering to the surface is controlled by adjusting the pH based on the solubility product of the desired metal oxide (Patent Document 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-249764 [Patent Document 2] Japanese Patent Application Publication No. 2-156636 [Patent Document 3] JP 2000-243798 A [Patent Document 4] Japanese Patent Application Publication No. 10-178077 Summary of the Invention [Problem to be solved by the invention]

[0005] When analyzing metal contamination on a wafer surface with a secondary ion mass spectrometer, especially when using a time-of-flight secondary ion mass spectrometer (hereinafter referred to as TOF-SIMS), the uniformity of the contamination concentration, the surface condition of the contaminated surface, and stability during analysis are extremely important. Specifically, if the sample surface is covered with an insulator such as an oxide film, there is a problem that charge-up causes deterioration of mass resolution and variation in detection intensity, and if a large amount of organic matter such as hydrocarbons is present, there is a problem that false detection and variation in detection intensity occur due to mass interference. Therefore, a metal contamination standard sample that is not affected by oxide films or hydrocarbons is required.

[0006] However, according to the method of Patent Document 1, since the wafer is treated with an alkaline hydrogen peroxide solution, an oxide film is formed on the wafer surface, and metal impurities are embedded in the oxide film formed as oxides or hydroxides. According to the method of Patent Document 2, an oxide film is formed on the wafer surface by treating the wafer with an ammonia-hydrogen peroxide solution → hydrofluoric acid solution → hydrochloric acid-hydrogen peroxide solution, and metal impurities are adsorbed on the oxide film. Both of the methods of Patent Documents 1 and 2 have the above-mentioned problems because an oxide film exists on the wafer surface. According to the method of Patent Document 3, there is a risk that a high concentration of hydrocarbons will remain on the wafer surface due to the process of contaminating the wafer surface with a metal impurity solution containing volatile alcohol, and even if the oxide film is removed by reduction treatment, metals that diffuse quickly, such as Ni and Cu, have a different concentration on the sample surface from the expected concentration due to the diffusion of impurities into the wafer bulk caused by the treatment conditions (temperature, time), and therefore are not suitable for use as a standard sample. Furthermore, according to the method of Patent Document 4, the pH is adjusted by using a solvent selected from hydrochloric acid, nitric acid, ammonium hydroxide, and hydrogen peroxide, either alone or in combination. However, all of these methods have the problem that an oxide film is produced during the treatment, and the metal oxide becomes embedded in the oxide film or is adsorbed onto the oxide film.

[0007] The present invention has been made in consideration of the above circumstances, and aims to provide a method for preparing a standard sample for evaluating surface metal contamination, which suppresses the formation of an oxide film on the surface and the adhesion of organic matter such as hydrocarbons, and has excellent reproducibility and quantitativeness. [Means for solving the problem]

[0008] In order to achieve the above object, the present invention provides a method for preparing a standard sample contaminated with a quantitative amount of a predetermined metal impurity for use in surface metal contamination analysis of a silicon wafer, comprising the steps of: a wafer surface cleaning step for removing surface impurities from the silicon wafer prepared for the standard sample; an oxidation-reduction potential adjusting step of dissolving the predetermined metal impurity in an aqueous hydrofluoric acid solution and lowering the oxidation-reduction potential until the predetermined metal impurity in the aqueous hydrofluoric acid solution is in a neutral atom state; a step of immersing the silicon wafer after the wafer surface cleaning step in the hydrofluoric acid aqueous solution after the oxidation-reduction potential adjustment step; a rinsing step for removing the hydrofluoric acid aqueous solution adhering to the silicon wafer after the immersion treatment step; A step of drying the silicon wafer after the rinsing step is included, The present invention provides a method for producing a standard sample for evaluating surface metal contamination, comprising the steps of: producing a standard sample by contaminating a surface of the standard sample silicon wafer with a quantitative amount of the specified metal impurities.

[0009] The method for preparing a standard specimen for evaluating surface metal contamination of the present invention can provide a standard specimen for evaluating surface metal contamination with excellent reproducibility and quantitativeness, without forming an oxide film on the surface of a silicon wafer and suppressing adhesion of organic matter such as hydrocarbon ions. In particular, even when subjected to precise mass analysis using a time-of-flight secondary ion mass spectrometer, since there is no (or is suppressed) formation of an oxide film or adhesion of organic matter on the surface, the influence of these on the analysis is suppressed, and a constant amount of standard specimen can be obtained with no variation in the analytical values ​​of surface metal impurities.

[0010] In this case, in the oxidation-reduction potential adjustment step, hydrogen gas is bubbled into the aqueous hydrofluoric acid solution by blowing hydrogen gas, thereby reducing the aqueous hydrofluoric acid solution and lowering the oxidation-reduction potential.

[0011] In this way, the oxidation-reduction potential in the aqueous hydrofluoric acid solution can be effectively lowered with almost no change in pH.

[0012] In the oxidation-reduction potential adjustment step, the predetermined metal impurities dissolved in the aqueous hydrofluoric acid solution may be one or more of Ni, Co, and Sn.

[0013] Ni and Co cause leakage defects during standby, and Sn causes defects in the oxide film characteristics, so these are metal impurities that need to be quantified by quantitative analysis to investigate the contamination state.

[0014] Furthermore, the amount of the predetermined metal impurities on the surface of the silicon wafer after the drying step can be quantified by a secondary ion mass spectrometer, and the silicon wafer thus quantified can be used as a standard sample for the secondary ion mass spectrometer.

[0015] In this way, it can be used as a standard sample for quantitative analysis in a secondary ion mass spectrometer.

[0016] In this case, the secondary ion mass spectrometer may be a time-of-flight secondary ion mass spectrometer.

[0017] In this way, the standard sample can be particularly suited for quantitative analysis using a time-of-flight secondary ion mass spectrometer capable of accurate mass analysis. Effect of the Invention

[0018] The method for preparing a standard specimen for evaluating surface metal contamination of the present invention can provide a metal standard specimen for surface analysis that is excellent in reproducibility and quantitativeness, without forming an oxide film on the silicon wafer surface and suppressing the adhesion of hydrocarbon ions. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a flow chart showing steps in a method for producing a standard sample for evaluating surface metal contamination according to the present invention. [Diagram 2] FIG. 1 is a correlation diagram showing the relationship between the oxidation-reduction potential of Ni and pH in an aqueous solution at 25° C. [Diagram 3] 1 is a graph showing the surface Ni concentration versus the Ni concentration in a 1% HF aqueous solution (25° C.) (fixed solution potential: −0.4 V, pH: 2.8). [Figure 4]1 is a graph showing the surface Co concentration versus the Co concentration in a 1% HF aqueous solution (25° C.) (fixed solution potential: −0.42 V, pH: 2.8). [Diagram 5] 1 is a graph showing the surface Sn concentration versus the Sn concentration in a 1% HF aqueous solution (25° C.) (fixed solution potential: −0.4 V, pH: 2.8). [Figure 6] 1 shows the results of measuring 58Ni mass spectrum (Example) by TOF-SIMS. [Figure 7] 1 shows the results of measuring 59Co mass spectrum (Example) by TOF-SIMS. [Figure 8] 1 shows the results of measuring a 120Sn mass spectrum (Example) by TOF-SIMS. [Figure 9] 1 shows the measurement results of 58Ni mass spectrum (Comparative Example 1) by TOF-SIMS. [Figure 10] 1 is a graph showing a comparison of average integrated intensity values ​​when 10 samples were measured (Example vs. Comparative Example 1). [Figure 11] 1 is a graph showing a comparison of the variation in average integrated intensity when 10 samples are measured (Example vs. Comparative Example 1). [Figure 12] 1 shows the results of measuring a 59Co mass spectrum (Comparative Example 2) by TOF-SIMS. [Figure 13] 1 shows the measurement results of a 120Sn mass spectrum (Comparative Example 2) by TOF-SIMS. [Figure 14] 1 is a graph showing a comparison of average integrated intensity values ​​when 10 samples were measured (Example vs. Comparative Example 2). [Figure 15] 1 is a graph showing a comparison of the variation in average integrated intensity when 10 samples are measured (Example vs. Comparative Example 2). BEST MODE FOR CARRYING OUT THEINVENTION

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited thereto. 1 is a flow diagram showing an example of the steps of the method for producing a standard sample for evaluating surface metal contamination in the present invention, which is roughly divided into a wafer surface cleaning step, an oxidation-reduction potential adjustment step (solution potential adjustment step), an immersion treatment step, a rinsing step, a drying step, and a step of evaluating metal impurities on the wafer surface.

[0021] (Wafer surface cleaning process) To prepare a standard sample for evaluating surface metal contamination, first, a silicon wafer is prepared as a standard sample (standard sample wafer), and the silicon wafer is cleaned to remove impurities such as metals from the surface. The silicon wafer used here refers to, for example, a mirror-finished silicon wafer that has been subjected to final cleaning in the silicon substrate manufacturing process and has no metal contamination on the wafer surface or inside, and is preferably a p-type or n-type silicon wafer with a resistivity of 0.1 Ω cm or more.

[0022] To clean this standard sample wafer, for example, it is placed in an 80°C ammonia-hydrogen peroxide solution to remove particles, then a 1% hydrofluoric acid solution (hereafter also referred to as HF solution) is used to remove the native oxide film containing metal impurities, and then it is placed in an 80°C hydrochloric acid-hydrogen peroxide solution to remove the metal impurities, and finally the native oxide film is removed with a 1% HF solution.

[0023] (Oxidation-reduction potential adjustment process) Next, a contamination treatment is performed to contaminate the surface of the standard sample wafer with a predetermined amount of predetermined metal impurities. At this time, the metal impurities are dissolved in a separately prepared HF aqueous solution, and the HF aqueous solution is reduced by, for example, bubbling hydrogen gas into the HF aqueous solution by blowing hydrogen gas, thereby lowering the redox potential until the metal impurities in the HF aqueous solution become neutral atoms. As long as the redox potential of the HF aqueous solution can be lowered, the method is not particularly limited, but if hydrogen gas is blown as described above, the redox potential can be effectively lowered with almost no change in pH.

[0024] The above-mentioned predetermined metal impurities (i.e., contaminant metals) are preferably at least one of Ni, Co, and Sn, for example. The reason is that Ni and Co cause leakage defects during standby, and Sn causes defects in oxide film characteristics, so quantitative analysis requires quantitatively contaminated wafers that are precisely contaminated with Ni, Co, and Sn. On the other hand, Ni, Co, and Sn are usually ionized in the HF aqueous solution, and in this state they cannot be attached to the standard sample wafer in the subsequent immersion treatment process. However, by reducing the Ni, Co, and Sn ions in the HF aqueous solution to neutral atoms as described above, they can be attached to the wafer surface, and a quantitatively contaminated wafer can be produced.

[0025] Here, we will explain the pH of an aqueous solution, the redox potential (solution potential), and the form of the metal using Ni as an example. Figure 2 is a redox potential-pH diagram for an aqueous solution containing 1 ppm Ni at 25°C. Figure 2 shows the form of Ni in an aqueous solution when the pH and solution potential of the aqueous solution are at certain values.

[0026] For example, the pH of a 1% HF solution is about 3.0, and the solution potential at that time is +0.6 V, so the form of Ni in the solution is Ni 2+ In the HF solution, Ni 2+ This indicates that Ni exists as an ion. In other words, when Ni exists as an ion in an aqueous solution, even if a silicon wafer (standard sample wafer) is present in the aqueous solution, Ni does not adhere to the silicon wafer and remains in the aqueous solution, and a quantitatively contaminated sample cannot be prepared by performing an immersion process in this state. In order to attach metal in an aqueous solution to a silicon wafer, the form of Ni in the aqueous solution must be in the neutral atomic state. When HF is used as the solvent for the aqueous solution, the pH is in the acidic range, so Ni in the HF aqueous solution 2+ Those that exist in the state of Ni 0 (neutral Ni atoms), the solution potential is Ni 2+ and Ni 0 It is necessary to lower (reduce) the potential below the boundary potential.

[0027] Ni in aqueous solution 2+ and Ni 0 The boundary potential can be calculated from the following Nernst equation: E = E0 + (2.303 × RT / nF) × logA The solution potential E of Ni under certain conditions can be calculated using the above formula and E0:Ni(Ni 2+ ⇔Ni 0 ) Standard redox potential: -0.257V R: Gas constant (8.314 J / K / mol) T: Temperature (K) F: Faraday constant (96485C / mol) A: Concentration (mol / L) n: number of electron transfers It can be found from. For example, the solution potential E of an HF solution of 1 ppm Ni at 25°C is -0.398 V. If the HF solution is reduced so that the oxidation-reduction potential (solution potential) E is lowered to less than -0.398 V, as shown in Figure 2, Ni will dissolve in the HF solution. 0 This indicates that it exists in a state of

[0028] Therefore, while measuring the solution potential, hydrogen gas is bubbled into the HF solution by blowing hydrogen gas as described above, lowering the redox potential of the HF solution. Although a small amount of hydrogen gas is dissolved in the water, it causes almost no change in pH. On the other hand, since the solution potential can be lowered to about -0.45 V, this method can be used to effectively contaminate silicon wafers with Ni even in the HF solution. This method is not limited to Ni, and has been shown to be capable of quantitatively contaminating metals such as Ni, as long as the boundary potential in the standard oxidation-reduction potential of the ionized / neutral atom is around -0.4 V. Furthermore, because the treatment is performed in an HF aqueous solution, there is an advantage that no oxide film is formed on the standard sample wafer, and no organic matter adheres to it.

[0029] In preparing the standard sample in the present invention, since there is no adhesion of organic matter (interfering ions) such as hydrocarbons on the wafer surface as described above, it is possible to prevent these from affecting and interfering with the spectrum of the element ions to be detected, such as Ni, and being detected at a higher intensity than would be the case (the actual amount of contamination). In addition, because no oxide film is formed on the wafer surface during metal contamination, it is possible to prevent the effects of charge-up (electrostatic charge) from affecting the analytical accuracy of surface metal impurities, thereby preventing variations in analytical values ​​and the increased effects of interfering ions due to a decrease in mass resolution.

[0030] (Immersion treatment process) In the above process, the oxidation-reduction potential is lowered until the metal impurities in the HF aqueous solution are in a neutral atom state, and then the standard sample wafer is immersed for a predetermined time. This immersion time is not particularly limited, but for example, immersion for 3 to 5 minutes can sufficiently attach the metal impurities to the wafer surface.

[0031] (Rinse process) Next, the HF aqueous solution (contamination treatment solution) adhering to the surface of the standard sample wafer is rinsed with pure water. The rinsing method is not particularly limited, and for example, the wafer can be spun and then rinsed with pure water.

[0032] (drying process) The wafer is then dried. The drying method may be spin drying or natural drying, but it is preferable to dry without using an alcohol solvent, in order to prevent a large amount of hydrocarbons from adhering to the wafer surface.

[0033] (Evaluation of metal impurities on wafer surface) After the drying process, the standard sample wafer is subjected to quantitative analysis of the specified metal impurities using a secondary ion mass spectrometer (SIMS), and this quantitatively analyzed standard sample wafer is used as the standard sample for the secondary ion mass spectrometer. In particular, it can be quantitatively analyzed using a time-of-flight secondary ion mass spectrometer (TOF-SIMS) and used as the standard sample. In this way, the optimal standard sample for a precision mass analysis system can be obtained.

[0034] In particular, when quantitatively analyzing metal impurities on a wafer surface by TOF-SIMS, it is important to require a standard sample that has little effect of interfering secondary ions on the target metal secondary ions, and has a wafer surface covered with an insulator such as an oxide film, which reduces deterioration of mass resolution due to charge-up and variation in detection intensity. The present invention can provide a standard sample that has no oxide film or organic matter on its surface, is particularly suitable for quantitative analysis of wafer surface metals by TOF-SIMS, and has excellent reproducibility and quantitativeness.

[0035] As mentioned above, for example, the solution potential E of an HF solution of 1 ppm Ni at 25°C is -0.398 V, and if the HF solution is reduced so as to lower the oxidation-reduction potential (solution potential) E below -0.398 V, Ni will become a neutral atom in the HF solution and will be able to contaminate the immersed standard sample wafer, but other metal impurities can also be quantitatively contaminated in the same way if the boundary potential in the standard oxidation-reduction potential of ionized / neutral atoms is around -0.4 V. This point will be further explained below with experimental examples in which contamination was performed with Ni, Co, and Sn as metal impurities. In addition, the method of controlling the contamination concentration (the relationship between the concentration of metal impurities in the HF aqueous solution and the contamination concentration) will be explained by giving an experimental example.

[0036] (Experiment 1) Hydrogen gas was bubbled into a 1% HF solution (25°C) containing 1 ppm Ni, and the solution potential was adjusted to different values. The solution potential and pH were measured, and a clean p-type silicon wafer (resistivity 10 Ω cm) with a diameter of 150 mm was immersed in the solution. It was then rinsed with pure water for 10 minutes and spin-dried at 3000 rpm for 20 seconds. After the wafer surface was subjected to vapor-phase decomposition with HF gas, the entire wafer was scanned with a mixed aqueous solution of 2% HF and 2% H2O2 to recover Ni from the wafer surface. The Ni concentration in the recovered solution was then analyzed using an inductively coupled plasma mass spectrometer (ELEMENT2 manufactured by Thermo Fisher Scientific, hereafter referred to as ICP-MS) to obtain the surface Ni concentration. The above-mentioned processing and analysis were performed on 10 wafers, and the average Ni concentration and the variation (RSD) are shown in Table 1.

[0037] [Table 1]

[0038] As shown in Table 1, when the solution potential is -0.4 V or less, the surface Ni adhesion is 3.60 × 10 13 atoms / cm 2 The concentration of metal impurities is almost constant around this range. The variation in the analytical values ​​is also relatively small. This shows that the metal impurities can be contaminated at a constant level by adjusting the solution potential to the neutral atom state.

[0039] (Experiment 2) Next, the surface Ni concentration was measured by ICP-MS in a 1% HF aqueous solution (25°C) with the solution potential fixed at -0.4 V and the pH fixed at 2.8, while varying the Ni concentration. The samples were collected in the same manner as in Experiment 1 above, and the results are shown in Figure 3. As shown in Fig. 3, the surface Ni concentration can be increased as the Ni concentration in the HF aqueous solution increases. In this way, the surface Ni concentration can be controlled by changing the Ni concentration in the HF aqueous solution. When preparing a standard sample in the present invention, the Ni concentration in the HF aqueous solution can be appropriately determined according to the desired contamination concentration.

[0040] (Experiments 3 and 4) In a 1% HF aqueous solution (25°C) containing 1 ppm each of Co (Experiment 3) and Sn (Experiment 4) instead of Ni, hydrogen gas was bubbled by blowing hydrogen gas in the same manner as in Experiment 1, and the solution potential was adjusted so that the solution potential was different. The boundary potential (25°C) between ionized and neutral atoms of Co and Sn is -0.417 V for Co and -0.282 V for Sn. The solution potential and pH were measured, and a clean p-type silicon wafer with a diameter of 150 mm (resistivity 10 Ω cm) was immersed in the solution, rinsed with pure water for 10 minutes, and spin-dried at 3000 rpm for 20 seconds. The wafer surface was then subjected to vapor phase decomposition using HF gas, the entire wafer surface was scanned with a mixed aqueous solution of 2% HF and 2% H2O2, Co and Sn were recovered from the wafer surface, and the Co and Sn concentrations in the recovered liquid were analyzed using ICP-MS to obtain the surface Co and Sn concentrations. The above-mentioned processing and analysis were performed on 10 wafers, and the average concentrations and the variations (RSD) of Co and Sn are shown in Tables 2 and 3, respectively.

[0041] [Table 2]

[0042] [Table 3]

[0043] As shown in Table 2, when the solution potential was −0.42 V or less, the surface Co adhesion was 3.30 × 10 13 atoms / cm 2 It is almost constant around this range, and the variation is relatively small. As shown in Table 3, when the solution potential is -0.3 V or less, the amount of Sn attached to the surface is 5.00 × 10 13 atoms / cm 2 It is almost constant around this range, and the variation is relatively small.

[0044] (Experiments 5 and 6) In addition, the concentration of Co in a 1% HF aqueous solution (25°C) with the solution potential set to -0.42 V and the pH set to 2.8 (Experiment 5), and the concentration of Sn in a 1% HF aqueous solution (25°C) with the solution potential set to -0.4 V and the pH set to 2.8 (Experiment 6) were changed, respectively. The surface Co concentration and surface Sn concentration were collected in the same manner as in Experiments 3 and 4, and measured by ICP-MS. The results are shown in Figures 4 and 5. As shown in Figs. 4 and 5, similarly to Ni in Experiment 2, the surface Co and Sn concentrations can be controlled by changing the Co and Sn concentrations in the HF aqueous solution. EXAMPLES

[0045] The present invention will be described in more detail below with reference to examples and comparative examples of the present invention, but the present invention is not limited to these. (Example) As an embodiment, a method for preparing standard samples for evaluating surface metal contamination of Ni, Co, and Sn in the present invention and the results of surface metal analysis by TOF-SIMS will be described. The wafers used as standard samples for evaluating surface metal contamination were p-type PW wafers with a diameter of 150 mm and a resistivity of 9 to 12 Ω·cm that had been subjected to final cleaning in the silicon substrate manufacturing process. (Wafer surface cleaning process) To clean the wafer, it is first placed in an 80°C ammonia-hydrogen peroxide solution to remove particles, then in a 1% HF solution to remove the native oxide film containing metal impurities, then in an 80°C hydrochloric acid-hydrogen peroxide solution to remove the metal impurities, and finally in a 1% HF solution to remove the native oxide film.

[0046] (Oxidation-reduction potential adjustment process - immersion treatment process) Separately from the above cleaning step, the following HF aqueous solution containing a certain amount of metal impurities was prepared. The 1% HF solution at 25°C was adjusted to 10 ppb of Ni(NO3)2, CoCl2, and SnCl4, 1000 ppm standard solutions manufactured by Fujifilm Wako Pure Chemical Industries, and hydrogen gas was blown into the HF solution at a flow rate of 1 L / min to bubble it, and the solution potential and pH were measured while adjusting the solution potential by stirring. The solution potential and pH were measured using a HORIBA multi-digital water quality meter WQ-300. Hydrogen gas was blown continuously until the solution potential reached -0.4 V for the HF solution containing Ni and Sn, and -0.42 V for the HF solution containing Co, and then the wafer was immersed for 3 minutes.

[0047] (Rinse process - Dry process) The wafer subjected to the above immersion treatment was washed with ultrapure water for 10 minutes, and finally, spin-dried at 3000 rpm for 20 seconds. The Ni, Co, and Sn concentrations on the surfaces of these samples are as shown in the above-mentioned FIGS. 3 to 5, each solution concentration being 0.01 ppm. Ten of the above samples were prepared.

[0048] (Metal impurity evaluation process) Next, the prepared sample was introduced into a TOF-SIMS (ULVAC-PHI nano-TOF II) and positive secondary ion analysis was performed using a Bi ion source. The analysis was performed using Ni (mass number 58), Co (mass number 59), and Sn (mass number 120), and the mass spectrum of each was obtained. The results are shown in Figures 6 to 8.

[0049] When acquiring a mass spectrum, it is important to consider whether the sample surface is conductive and whether there are interfering ions near the target ions. In particular, when the sample surface is non-conductive and there are interfering ions near the target ions, mass interference and fluctuations in the sample potential can cause each mass peak in the mass spectrum to become broad, resulting in poor quantitation. Such problems are encountered in conventional methods that involve the formation of an oxide film and the adhesion of organic matter. However, in the present invention, since HF treatment is performed, there is no oxide film on the sample surface, and since no organic solvent is used, no excess organic components are detected. 29 Si2, 28 SiCH3O, C6O3) are clearly separated, indicating that there are no problems with quantification. In addition, as will be described later, the variance in the analytical values ​​is smaller than in Comparative Examples 1 and 2, and it can be said that the reproducibility is sufficiently high.

[0050] By using the wafer obtained in this way as a standard sample for TOF-SIMS, highly accurate impurity evaluation becomes possible when evaluating metal contamination in the silicon wafer to be actually evaluated.

[0051] Comparative Example 1 As Comparative Example 1, the method described in Patent Document 1 will be described. The wafer used as the standard sample for evaluating surface metal contamination was the same silicon wafer as in the example. To clean the wafer, it was first placed in an 80°C ammonia-hydrogen peroxide solution for 10 minutes to remove particles, then placed in a 1% HF solution for 5 minutes to remove the native oxide film containing metal impurities, and then placed in an 80°C hydrochloric acid-hydrogen peroxide solution for 10 minutes to remove the metal impurities and form a native oxide film on the wafer surface.

[0052] Separately from the above process, an alkaline hydrogen peroxide solution (a mixed solution of 29% ammonia water and 31% hydrogen peroxide water) containing a certain amount of metal impurities was adjusted to 10 ppb of Ni(NO3)2, and the solution was heated to 80° C. The above wafer was placed in the heated solution and immersed for 10 minutes. The immersed wafer was taken out, washed in pure water for 10 minutes, and then spin-dried at 3000 rpm for 20 seconds. Ten of the above samples were prepared and analyzed by TOF-SIMS in the same manner as in the Example, and the results are shown in Fig. 9. Regarding the analysis values ​​(integral intensity), the average value of the ten samples and the variation thereof are compared between the Example and Comparative Example 1, and the results are shown in Figs.

[0053] In the sample in Comparative Example 1, contaminating metals exist in the oxide film or on the oxide film surface, but in the TOF-SIMS analysis, the presence of the oxide film (insulating film) causes charge-up, which causes the sample potential to fluctuate, and the mass peaks in the mass spectrum become broad and cause mass interference (see FIG. 9). Therefore, the analysis values ​​are higher than the actual values ​​(Examples) (see FIG. 10). For the same reason, it is thought that the analysis values ​​vary more than in the Examples where no oxide film exists (see FIG. 11).

[0054] Comparative Example 2 Next, as Comparative Example 2, the method described in Patent Document 3 will be described. The wafer used as the standard sample for evaluating surface metal contamination was the same silicon wafer as in the example. To clean the wafer, it was first immersed in a 1% HF aqueous solution at 25°C for 5 minutes, then rinsed with pure water for 10 minutes, immersed in an ammonia-hydrogen peroxide aqueous solution (1% ammonia / 5% hydrogen peroxide) at 65°C for 10 minutes, then rinsed with pure water for 10 minutes, and then air-dried.

[0055] Next, 1 mL of an isopropanol-containing solution adjusted to 10 ppb each of CoCl2 solution and SnCl4 solution was dropped onto the surface of the wafer, and the wafer was allowed to dry naturally. After that, the wafer was subjected to a reduction treatment in a hydrogen gas atmosphere at 150°C. Ten of the above samples were prepared and analyzed by TOF-SIMS in the same manner as in the Example. The results are shown in FIGS.

[0056] It is presumed from the same document that the natural oxide film on the wafer surface of the sample in Comparative Example 2 disappeared due to the reduction treatment, and each metal chloride was changed to a neutral metal. On the other hand, it is considered that the organic components constituting isopropanol were volatilized by the reduction treatment, but the results of TOF-SIMS measurement showed that SiCH3O, an interfering molecular ion in Co analysis, and CO3, an interfering molecular ion in Sn analysis, were detected at higher concentrations than in the Example, although there was some variation, causing mass interference with Co and Sn, respectively (see Figures 12 and 13). This is considered to be the cause of the analysis value being higher than the actual value (Example) (see Figure 14) and the cause of the variation in the analysis value (see Figure 15).

[0057] The present specification includes the following aspects. [1]: A method for preparing a standard sample quantitatively contaminated with a specified metal impurity for use in surface metal contamination analysis of silicon wafers, comprising: a wafer surface cleaning step for removing surface impurities from the silicon wafer prepared for the standard sample; an oxidation-reduction potential adjusting step of dissolving the predetermined metal impurity in an aqueous hydrofluoric acid solution and lowering the oxidation-reduction potential until the predetermined metal impurity in the aqueous hydrofluoric acid solution is in a neutral atom state; a step of immersing the silicon wafer after the wafer surface cleaning step in the hydrofluoric acid aqueous solution after the oxidation-reduction potential adjustment step; a rinsing step for removing the hydrofluoric acid aqueous solution adhering to the silicon wafer after the immersion treatment step; A step of drying the silicon wafer after the rinsing step is included, A method for preparing a standard sample for evaluating surface metal contamination, comprising the steps of: preparing a standard sample by contaminating a surface of the standard sample silicon wafer with a quantitative amount of the predetermined metal impurities. [2]: The method for preparing a standard sample for evaluating surface metal contamination according to the above [1], wherein in the oxidation-reduction potential adjustment step, hydrogen gas is bubbled into the hydrofluoric acid aqueous solution by blowing hydrogen gas into the hydrofluoric acid aqueous solution to reduce the hydrofluoric acid aqueous solution and lower the oxidation-reduction potential. [3]: A method for preparing a standard sample for evaluating surface metal contamination according to [1] or [2] above, wherein in the oxidation-reduction potential adjustment step, the specified metal impurities dissolved in the hydrofluoric acid aqueous solution are one or more of Ni, Co, and Sn. [4]: A method for preparing a standard sample for evaluating surface metal contamination according to any one of [1] to [3] above, comprising quantifying the amount of the specified metal impurities on the surface of the silicon wafer after the drying step using a secondary ion mass spectrometer, and using the quantified silicon wafer as a standard sample for the secondary ion mass spectrometer. [5]: The method for preparing a standard sample for evaluating surface metal contamination according to [4] above, wherein the secondary ion mass spectrometer is a time-of-flight secondary ion mass spectrometer.

[0058] The present invention is not limited to the above-described embodiment. The above-described embodiment is merely an example, and any configuration that is substantially the same as the technical idea described in the claims of the present invention and that provides similar effects is included in the technical scope of the present invention.

Claims

1. A method for preparing a standard sample contaminated with a quantitative amount of a predetermined metal impurity for use in surface metal contamination analysis of a silicon wafer, comprising the steps of: a wafer surface cleaning step for removing surface impurities from the silicon wafer prepared for the standard sample; an oxidation-reduction potential adjusting step of dissolving the predetermined metal impurity in an aqueous hydrofluoric acid solution and lowering the oxidation-reduction potential until the predetermined metal impurity in the aqueous hydrofluoric acid solution is in a neutral atom state; a step of immersing the silicon wafer after the wafer surface cleaning step in the hydrofluoric acid aqueous solution after the oxidation-reduction potential adjustment step; a rinsing step for removing the hydrofluoric acid aqueous solution adhering to the silicon wafer after the immersion treatment step; A step of drying the silicon wafer after the rinsing step is included, A method for preparing a standard sample for evaluating surface metal contamination, comprising the steps of: preparing a standard sample by contaminating a surface of said standard sample silicon wafer with a quantitative amount of said predetermined metal impurities.

2. 2. The method for preparing a standard sample for evaluating surface metal contamination according to claim 1, wherein in the oxidation-reduction potential adjustment step, hydrogen gas is bubbled into the aqueous hydrofluoric acid solution by blowing hydrogen gas into the aqueous hydrofluoric acid solution to reduce the aqueous hydrofluoric acid solution and lower the oxidation-reduction potential.

3. 2. The method for preparing a standard sample for evaluating surface metal contamination according to claim 1, wherein in the oxidation-reduction potential adjustment step, the specified metal impurities dissolved in the hydrofluoric acid aqueous solution are one or more of Ni, Co, and Sn.

4. 3. The method for preparing a standard sample for evaluating surface metal contamination according to claim 2, wherein in the oxidation-reduction potential adjustment step, the specified metal impurities dissolved in the hydrofluoric acid aqueous solution are one or more of Ni, Co, and Sn.

5. 2. The method for preparing a standard sample for evaluating surface metal contamination according to claim 1, further comprising the steps of: quantifying the amount of the predetermined metal impurities on the surface of the silicon wafer after the drying step using a secondary ion mass spectrometer; and using the silicon wafer thus quantified as a standard sample for the secondary ion mass spectrometer.

6. 3. The method for preparing a standard sample for evaluating surface metal contamination according to claim 2, further comprising the steps of: quantifying the amount of the predetermined metal impurities on the surface of the silicon wafer after the drying step using a secondary ion mass spectrometer; and using the quantified silicon wafer as a standard sample for the secondary ion mass spectrometer.

7. 4. The method for preparing a standard sample for evaluating surface metal contamination according to claim 3, further comprising the steps of: quantifying the amount of the predetermined metal impurities on the surface of the silicon wafer after the drying step by a secondary ion mass spectrometer; and using the quantified silicon wafer as a standard sample for the secondary ion mass spectrometer.

8. 5. The method for preparing a standard sample for evaluating surface metal contamination according to claim 4, further comprising the steps of: quantifying the amount of the predetermined metal impurities on the surface of the silicon wafer after the drying step by a secondary ion mass spectrometer; and using the quantified silicon wafer as a standard sample for the secondary ion mass spectrometer.

9. 6. The method for preparing a standard specimen for evaluating surface metal contamination according to claim 5, wherein the secondary ion mass spectrometer is a time-of-flight secondary ion mass spectrometer.

10. 7. The method for preparing a standard specimen for evaluating surface metal contamination according to claim 6, wherein the secondary ion mass spectrometer is a time-of-flight secondary ion mass spectrometer.

11. 8. The method for preparing a standard specimen for evaluating surface metal contamination according to claim 7, wherein the secondary ion mass spectrometer is a time-of-flight secondary ion mass spectrometer.

12. 9. The method for preparing a standard specimen for evaluating surface metal contamination according to claim 8, wherein the secondary ion mass spectrometer is a time-of-flight secondary ion mass spectrometer.

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