Quantitative detection method and system for content of inorganic trace elements in high-purity cadmium telluride

Abstract

The invention discloses a quantitative detection method and system for the content of inorganic trace elements in high-purity cadmium telluride. According to the detection method, a blank control group sample is introduced in the sample preparation process, so that the influence degree of impurities in a container on a measurement result in the sample preparation process can be reduced, and the accuracy of the test result is further ensured; meanwhile, the linear correlation coefficient of a standard curve can be improved by adopting an internal standard method, so that the accuracy and the credibility of a solution test result are improved to a certain extent, that is, the quantitative determination of the content of the trace elements in the high-purity compound CdTe is fundamentally realized by adopting the detection method disclosed by the invention; quality monitoring can be carried out on the process application of the high-purity CdTe raw material.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of chemical test analysis, in particular to a method and system for quantitatively detecting the content of inorganic trace elements in high-purity cadmium telluride. BACKGROUND

[0002] In the field of infrared materials, silicon-based mercury cadmium telluride grown by molecular beam epitaxy (MBE) is used to prepare infrared focal plane detectors. The source materials are mainly high-purity cadmium telluride and high-purity mercury, and the purity of the source materials directly determines the number of defects and deep energy levels in the thin film. In addition, due to the large lattice mismatch between silicon and mercury cadmium telluride, the lattice structure of the grown thin film material is poor, with many dislocations, and cannot be used for subsequent device processing. Therefore, CdTe is introduced as a buffer layer on the silicon surface to reduce the lattice mismatch and improve the quality of the epitaxial thin film. The trace elements such as Mg, Al, Ni, Cu, Cr, Fe and Ca in CdTe can affect the number and movement of carriers in semiconductors by gaining or losing electrons, and the content determines the quality of the CdTe buffer layer and the mercury cadmium telluride thin film, and further affects the performance of the detector. The current industry still does not have an effective sample preparation method and testing method for trace elements in high-purity (>7N) CdTe compounds, so it is particularly important to develop a quantitative testing method suitable for trace elements in the material.

[0003] High-purity cadmium telluride requires higher sensitivity of the testing instrument. Compared with glow discharge mass spectrometry (GD-MS), atomic absorption spectrometer (AAS), inductively coupled plasma optical emission spectrometry (ICP-OES) and other testing instruments, inductively coupled plasma mass spectrometry (ICP-MS) is the preferred testing method due to its high sensitivity (up to ppt level) and multi-element measurement. However, among the commonly used sample preparation methods matched with it, digestion method, melting method and zone melting method can cause strong matrix effect, resulting in a decrease in the detection rate of trace elements. That is, in the actual testing process, the response rate of a single element is easily affected by the matrix elements and fluctuates greatly, thereby reducing the accuracy of the test results. SUMMARY

[0004] The application provides a method and system for quantitatively detecting the content of inorganic trace elements in high-purity cadmium telluride, to solve the problem that the content of inorganic trace elements in high-purity cadmium telluride cannot be accurately and simply detected.

[0005] In a first aspect, the application provides a method for quantitatively detecting the content of inorganic trace elements in high-purity cadmium telluride, which comprises: preparing a CdTe sample by a saturated vapor pressure method, and setting an empty container as a control group at the same time of preparing the CdTe sample; adding HNO3 solution into the CdTe sample and the empty container of the control group respectively to obtain a CdTe sample solution and a control group solution, and determining the content of inorganic trace elements in the CdTe sample solution and the control group solution after dissolution of the HNO3 solution based on a standard curve of inorganic trace elements in a standard solution drawn by an internal standard method; and taking the difference between the content of inorganic trace elements in the CdTe sample solution and the control group solution as the actual content of the inorganic trace elements, wherein the inorganic trace elements are Mg, Al, Ni, Cu, Cr, Fe and Ca elements.

[0006] Optionally, the preparation pressure of the CdTe sample is 0.01 Pa-10 Pa, the preparation temperature is 350°C-490°C, and the heating time is 3h-12h.

[0007] Optionally, the adding of the HNO3 solution into the CdTe sample and the empty container of the control group respectively to obtain the CdTe sample solution and the control group solution comprises: The prepared CdTe sample and the empty container of the control group are dissolved with concentrated nitric acid respectively, and the dissolved CdTe sample and the control group are diluted equally to obtain CdTe sample solution and control group solution with the same concentration.

[0008] Optionally, the method further comprises: determining the concentration gradient of the standard solution, configuring the standard solution with the corresponding concentration, and drawing a standard curve with concentration as the horizontal coordinate and response rate as the vertical coordinate, and taking a linear standard curve with a linear correlation coefficient of the inorganic trace elements greater than 0.999 as the final standard curve.

[0009] Optionally, the determination of the concentration gradient of the standard solution comprises: determining the concentration gradient of the standard solution according to the content of inorganic trace elements in the CdTe sample.

[0010] Optionally, the internal standard element of the internal standard method is Re / Rh, and the standard curve of inorganic trace elements in the standard solution is obtained in a preset quantitative detection device.

[0011] Optionally, the standard curve of the inorganic trace elements in the standard solution drawn based on the internal standard method comprises: in the internal standard method, siphon sampling is adopted, the sampling time is set to 10s-40s, the reading number is 1-7 times, the concentration gradient of the standard curve is 1ppb-60ppb, and the standard curve of each inorganic trace element is drawn in turn.

[0012] Optionally, the method further comprises: correcting and adjusting the quantitative detection equipment. The correcting and adjusting the quantitative detection equipment comprises: adjusting the position, correcting the resolution (higher than 0.8) and correcting the mass number (response rate higher than 10 4 ) of the inductively coupled plasma mass spectrometer, so that the inductively coupled plasma mass spectrometer reaches the best state; and setting the equipment parameters of the inductively coupled plasma mass spectrometer according to the test requirements, adjusting the equipment parameters to the best state, so as to call the test method to complete the test of the inorganic trace elements in the CdTe sample solution and the control group solution.

[0013] In a second aspect, the present application provides a quantitative detection system for implementing any one of the above-mentioned methods, and the system comprises: A sample preparation tube is used to prepare the CdTe sample and the control group by the saturated vapor pressure method, and the control group is a blank container. An inductively coupled plasma mass spectrometer is used to draw the standard curve of the inorganic trace elements in the standard solution based on the internal standard method, and determine the content of the inorganic trace elements in the CdTe sample solution and the control group solution after being dissolved by HNO3 solution. A processor is used to take the difference between the inorganic trace elements in the CdTe sample solution and the control group solution as the actual content of the inorganic trace elements. Wherein, the inorganic trace elements are Mg, Al, Ni, Cu, Cr, Fe and Ca elements.

[0014] In a third aspect, the present application provides a computer readable storage medium, and the storage medium stores a computer program, and the program is executed by a processor to implement any one of the above-mentioned methods.

[0015] The present application has the following beneficial effects: The detection method of the present application can reduce the influence of impurities in the container during sample preparation on the measurement results, thereby ensuring the accuracy of the test results of the present application. Meanwhile, the use of the internal standard method can improve the linear correlation coefficient of the standard curve, which also improves the accuracy and reliability of the test results of the solution to some extent. That is, the detection method of the present application fundamentally realizes the quantitative determination of trace elements in high-purity CdTe, and can realize the quality monitoring of the process application of high-purity CdTe raw materials.

[0016] The above description is only a summary of the technical solutions of the present application. In order to more clearly understand the technical means of the present application, the content of the specification can be implemented, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0017] Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The accompanying drawings are intended to only illustrate preferred embodiments and are not considered limiting of the present application. Moreover, throughout the drawings, like reference numerals are used to designate like parts. In the drawings: Figure 1 is a flowchart of a quantitative detection method for inorganic trace element content in high-purity cadmium telluride provided by an embodiment of the present application; Figure 2 is a standard curve diagram of inorganic trace elements provided by an embodiment of the present application; Figure 3 is a multi-mode internal standard recovery rate curve diagram at different concentrations provided by an embodiment of the present application. DETAILED DESCRIPTION

[0018] The present application will be further described in detail below in combination with the drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0019] When using inductively coupled plasma mass spectrometer (ICP-MS) to quantitatively detect the content of trace elements in CdTe compounds with a purity higher than 7N, the following problems exist: firstly, during the sample preparation process, placing the sample container and exposing it in the air can introduce trace elements, resulting in deviation of the final test result; secondly, the industry standard adopts a proper proportion of acid to dissolve CdTe, and the content of trace elements is extremely low compared with CdTe, so the method has a strong matrix effect, cannot obtain accurate response rate of the trace elements and thus cannot obtain accurate test results, and in the process of using saturated vapor pressure to prepare the sample, the saturated vapor pressure of CdTe as a compound and the preparation time are non-fixed values, and the saturated vapor pressures of some trace elements and CdTe are close, so it is necessary to determine appropriate pressure, temperature and preparation time to leave the trace elements to be detected on the basis of removing the CdTe matrix; in addition, the trace elements to be detected contain multiple isotopes, which are interfered by mass spectrometry of multiple atoms or ions with the same mass number, cannot show the real response rate and cannot measure the actual content of the solution to be detected; in order to solve the above problems, the embodiment of the present application provides a quantitative detection method for inorganic trace element content in high-purity cadmium telluride, which is described as follows Figure 1 The method provided by the embodiment of the present application comprises the following steps: S101, preparing a CdTe sample by using the saturated vapor pressure method, and setting an empty container as a control group at the same time of preparing the CdTe sample; Specifically, the embodiment of the present application uses the saturated vapor pressure method in the sample preparation process, and introduces a blank control group sample, so as to reduce the influence degree of impurities in the container on the measurement result in the sample preparation process, thereby greatly improving the accuracy of the test result. In the embodiment of the present application, the preparation pressure of the CdTe sample is 0.01 Pa to 10 Pa, the preparation temperature is 350 DEG C to 490 DEG C, and the heating time is 3h to 12h.

[0020] Further, in the embodiment of the present application, the preparation pressure of the CdTe sample is 1 Pa to 6 Pa, the preparation temperature is 400 DEG C to 450 DEG C, and the heating time is 6h to 10h.

[0021] In specific implementation, the preparation pressure of the CdTe sample is set to 4 Pa, the preparation temperature is 430 DEG C, and the heating time is 8h in the embodiment of the present application. It has been proved in practice that by continuously adjusting and refining the preparation parameters of the CdTe sample, a better CdTe yield can be obtained.

[0022] S102, respectively adding HNO3 solution into the CdTe sample and the empty container of the control group to obtain the CdTe sample solution and the control group solution, and determining the content of the inorganic trace elements in the CdTe sample solution and the control group solution after dissolution based on the standard curve of the inorganic trace elements in the standard solution drawn by the internal standard method; That is, the embodiment of the present application is to dissolve the prepared CdTe sample and the empty container of the control group with concentrated nitric acid respectively, and to dilute the dissolved CdTe sample and the control group equally to obtain the CdTe sample solution and the control group solution with the same concentration.

[0023] In specific implementation, the embodiment of the present application also needs to determine the standard curve of the inorganic trace elements in the standard solution, which mainly includes: determining the concentration gradient of the standard solution, configuring the standard solution with the corresponding concentration, and drawing the standard curve with the concentration as the horizontal coordinate and the response rate as the vertical coordinate, and taking the linear correlation coefficient of the inorganic trace elements greater than 0.999 linear standard curve as the final standard curve, for details, see Figure 2 .

[0024] It should be noted that the embodiment of the present application draws the concentration curve during the sample test process, and the concentration gradient coordinate needs to be close to the actual content of the trace elements, otherwise a larger test error will be introduced. Therefore, the concentration gradient of the standard solution in the embodiment of the present application is determined according to the content of the inorganic trace elements in the CdTe sample.

[0025] In the embodiment of the present application, the internal standard element of the internal standard method is Re / Rh, and the standard curve of the inorganic trace elements in the standard solution is obtained in the preset quantitative detection equipment.

[0026] Specifically, the standard curve of the inorganic trace elements in the standard solution drawn based on the internal standard method includes: the internal standard method uses siphon sampling, sets the sampling time to 10s~40s, the reading number to 1~7 times, and the concentration gradient of the standard curve to 1ppb~60ppb, and sequentially draws the standard curve of each inorganic trace element. The internal standard method is used in the test process of the embodiment of the present application to improve the linear correlation coefficient of the standard curve, thereby finally improving the accuracy and reliability of the test result of the solution.

[0027] S103, taking the difference between the contents of the inorganic trace elements in the CdTe sample solution and the control group solution as the actual content of the inorganic trace elements; That is, the detection method of the embodiment of the present application can reduce the influence of impurities in the container in the sample preparation process on the measurement results, thereby ensuring the accuracy of the test results of the present application. At the same time, the present application uses the internal standard method to improve the linear correlation coefficient of the standard curve, which also improves the accuracy and reliability of the test results of the solution to some extent. That is, the detection method of the present application fundamentally realizes the quantitative determination of trace elements in high-purity compound CdTe, and can realize the quality monitoring of the process application of high-purity CdTe raw materials.

[0028] It should be noted that the inorganic trace elements described in the embodiment of the present application are Mg, Al, Ni, Cu, Cr, Fe and Ca, and the like. The specific person skilled in the art can set it according to the needs, and the present application does not make specific limitations.

[0029] It should be noted that in order to better improve the accuracy of the detection, the quantitative detection equipment needs to be corrected and adjusted during specific implementation; specifically, the embodiment of the present application is to adjust the position of the inductively coupled plasma mass spectrometer, correct the resolution (higher than 0.8) and mass number (response rate higher than 10 4 ), take Be9 and Bi209 as the benchmark, and make the inductively coupled plasma mass spectrometer reach the best state; and set the equipment parameters of the inductively coupled plasma mass spectrometer according to the test requirements, adjust the equipment parameters to the best state, and call the test method to complete the test of the inorganic trace elements in the CdTe sample solution and the control group solution.

[0030] The method described in the embodiment of the present application will be explained and described in detail through a specific example: Figure 1 , Figure 2 and Figure 3 , , Figure 1 , Figure 2 and Figure 3 , the embodiment of the present application provides a method for one-time quantitative determination of trace element content in CdTe by using saturated vapor pressure and inductively coupled plasma mass spectrometry. In this method, the sample preparation process uses pre-concentration combined with saturated vapor pressure method, the CdTe sample quality is fixed, the preparation pressure is 0.01 Pa ~10 Pa, the preparation temperature is 350℃ ~490℃, and the heating time is 3h ~12h.

[0031] In order to eliminate the impurity problem introduced by the container and exposure in the sample preparation process, the present application also prepares a control group sample. The control group is an empty container, which is in the same preparation environment as the CdTe sample. The difference between the CdTe sample and the control group is the actual content of trace elements in the CdTe.

[0032] The prepared sample is dissolved with concentrated nitric acid, and the obtained solution to be measured is obtained after dilution.

[0033] The internal standard method is used in the test process, the internal standard element is Re / Rh, and due to the abundance value and interference term, the isotopes of the seven trace elements are selected as follows 24 Mg, 27 Al, 56 Fe, 70 Ni, 65 Cu, 52 Cr, 43 Ca, wherein 56 Fe is affected 40 Ar 16 O cannot show the normal response rate, and He gas collision test is used, and other elements are tested in a conventional manner.

[0034] In the internal standard method, the siphon sampling is used, the stable sampling time is 5s~40s, the reading number is 1~7 times, the standard curve concentration gradient is 1ppb~60ppb (at least three concentration gradients), 1% HNO3 solution is used as a blank solution, the corresponding concentration standard solution is configured into the instrument, the standard curve of a single element is obtained, and the linear correlation coefficient is required to be greater than 0.999.

[0035] Finally, the content values of the seven elements in the CdTe sample and the control group solution are tested, and the actual content of the inorganic elements in the CdTe can be calculated.

[0036] The specific test steps are as follows: 1) Sample preparation: take n g of CdTe sample and place it in the quartz basket in the sample preparation tube as the sample group, the sample group is the CdTe sample, and finally the CdTe sample solution is prepared, and the blank quartz basket is placed as the control group, and the two groups are placed in the sample preparation tube. The sample chamber is evacuated, the heating temperature is set, and after heating, the container is taken out, concentrated HNO3 solution is added until complete dissolution, and the sample to be measured is obtained, and it should be noted that the sample to be measured is a high-concentration CdTe sample solution and a control group solution.

[0037] In specific implementation, the preparation pressure in the sample chamber of the embodiment of the application is 0.01 Pa ~10Pa, the preparation temperature is 350℃~490℃, and the heating time is 3h~12h; Most preferably, the preparation pressure of the embodiment of the application is 1Pa, the preparation temperature is 420℃, and the heating time is 8h.

[0038] 2) Tuning: adjust the position, resolution correction and mass number correction of the inductively coupled plasma mass spectrometer device, take low mass elements (represented by Be9) and high mass elements (represented by Bi209) as the benchmark, and make the device reach the best state.

[0039] 3) Method call: according to the test requirements, set the parameters, adjust the instrument to the best state, call the stored test method, and select isotopes 24 Mg, 27 Al, 43 Ca, 52 Cr, 60 Ni, 65 Cu, 56 Fe, 56 Fe adopts He gas collision mode, and other elements adopt normal working mode. That is, according to the test requirements, the method parameters are set, the parameters are adjusted to the best state to better complete the test of the element content.

[0040] 4) Solution configuration. Take the standard solution with the measured element, configure the corresponding concentration of the standard solution according to the preset concentration gradient, and wait for use.

[0041] That is, the standard solution is configured according to the different concentration gradients set in the method, so as to obtain standard solutions with different concentration gradients.

[0042] It should be noted that the concentration gradient in the embodiment of the application is set according to the concentration of the measured element, that is, the concentration of the measured element falls within the concentration gradient range. In other words, the concentration of the measured element is between the preset initial concentration and the final concentration. For example, the gradient range can be set to 0.5-60 or 1-50, etc. Those skilled in the art can set it arbitrarily according to the concentration of inorganic trace elements, and the concentration gradient described in the application can be various gradient points in the gradient range, such as 0.5, 1, 10, 20, 40, 60, etc. Of course, the more the values of the concentration gradient are set, the higher the accuracy of the calculation result is.

[0043] In short, the embodiment of the application dilutes the existing standard solution to obtain standard solutions with different concentration gradients, and matches the concentration of the actual standard solution with the concentration set in the method for sampling.

[0044] 5) Draw a standard curve. Take 1% HNO3 solution as a blank solution, input the standard solution with the corresponding concentration in turn, obtain the corresponding response rate, draw a linear standard curve with concentration as the horizontal coordinate and response rate as the vertical coordinate, and check that the linear correlation coefficient of each element is greater than 0.999.

[0045] That is, the embodiment of the present application is to compare the same concentration of CdTe sample solution and the control group for detection, and the linear correlation coefficient of each element is greater than 0.999 to be identified as test qualified.

[0046] 6) Sample determination. The control group and sample group solution (i.e. blank control group CdTe sample solution) is taken into the detection instrument, and the actual content of each element in the solution is obtained according to the standard cancellation, and the actual content m of the CdTe sample is calculated. i (i is Mg, Al…). That is, the difference between the inorganic trace element value of the CdTe sample solution under the same solubility and the inorganic trace element value of the control group solution is taken as the actual content of the inorganic trace element in the CdTe solution.

[0047] Table 1 is the linear standard curve equation and the measurable range of seven elements formed in the CdTe trace element quantitative test process, which shows that the standard curve and the test result have very high credibility.

[0048] Table 1 CdTe trace element standard curve equation parameters Table 2 is the actual content calculation method of seven trace elements in CdTe. The ICP-MS test results of the control group and the sample group are shown in the table, and the actual content is obtained through the test results in example one.

[0049] Table 2 CdTe trace element actual content Unit: ppb Correspondingly, the embodiment of the present application also provides a quantitative detection system for realizing any one of the above-mentioned methods, and the system comprises: A sample preparation tube is used to prepare CdTe samples and a control group by saturated vapor pressure method, and the control group is a blank container; An inductively coupled plasma mass spectrometer is used to draw a standard curve of inorganic trace elements in a standard solution based on an internal standard method, and determine the content of inorganic trace elements in CdTe sample solution and control group solution after dissolution by HNO3 solution; A processor is used to take the difference between the inorganic trace elements of the CdTe sample solution and the control group solution as the actual content of the inorganic trace elements; Wherein, the inorganic trace elements are Mg, Al, Ni, Cu, Cr, Fe and Ca elements.

[0050] In addition, the embodiment of the present application also provides a computer readable storage medium, and the storage medium stores a computer program, and the program is executed by a processor to realize any one of the methods in the method embodiment of the present application.

[0051] The related content of the device embodiments and the storage medium embodiments of the present application can be understood by referring to the method embodiments of the present application, which will not be discussed in detail herein.

[0052] Although the preferred embodiments of the present application have been disclosed for exemplary purposes, those skilled in the art will realize that various modifications, additions and substitutions are possible, therefore, the scope of the present application should not be limited to the above embodiments.

Claims

1. A method for quantitatively detecting the content of inorganic trace elements in high-purity cadmium telluride, characterized in that, The method includes: CdTe samples were prepared by saturated vapor pressure method, and an empty container was set up as a control group during the preparation of CdTe samples. HNO3 solution was added to the empty containers of CdTe sample and control group respectively to obtain CdTe sample solution and control group solution. The content of inorganic trace elements in the standard solution after dissolving in HNO3 solution was determined by plotting the standard curve of inorganic trace elements in the standard solution based on the internal standard method. The difference in the content of inorganic trace elements between the CdTe sample solution and the control group solution is taken as the actual content of the inorganic trace elements, wherein the inorganic trace elements are Mg, Al, Ni, Cu, Cr, Fe and Ca.

2. The method according to claim 1, characterized in that, The CdTe sample was prepared under a pressure of 0.01 Pa to 10 Pa, at a temperature of 350℃ to 490℃, and for a heating time of 3h to 12h.

3. The method according to claim 1, characterized in that, The step of adding HNO3 solution to empty containers of the CdTe sample and control group respectively to obtain CdTe sample solution and control group solution includes: The empty containers of the prepared CdTe sample and the control group were dissolved in concentrated nitric acid, and the dissolved CdTe sample and the control group were diluted equally to obtain CdTe sample solution and control group solution of the same concentration.

4. The method according to claim 3, characterized in that, The method further includes: Determine the concentration gradient of the standard solutions, prepare standard solutions of corresponding concentrations, and plot a standard curve with concentration as the x-axis and response rate as the y-axis. The linear standard curve with a linear correlation coefficient greater than 0.999 for inorganic trace elements is taken as the final standard curve.

5. The method according to claim 4, characterized in that, Determine the concentration gradient of the standard solution, including: The concentration gradient of the standard solution was determined based on the content of inorganic trace elements in the CdTe sample.

6. The method according to any one of claims 1-6, characterized in that, The internal standard element in the internal standard method is Re / Rh, and a standard curve of inorganic trace elements in the standard solution is obtained using a preset quantitative detection device.

7. The method according to any one of claims 1-6, characterized in that, The standard curves for inorganic trace elements in standard solutions plotted using the internal standard method include: The internal standard method employs siphon injection, with an injection time of 10s to 40s and 1 to 7 readings. The concentration gradients of the standard curves are 1ppb to 60ppb, and the standard curves for each inorganic trace element are plotted sequentially.

8. The method according to claim 6, characterized in that, The method further includes: calibrating and adjusting the quantitative detection equipment; The calibration and adjustment of the quantitative detection equipment includes: The position, resolution, and mass number of the inductively coupled plasma mass spectrometer were adjusted, and Be9 and Bi209 were used as references to bring the inductively coupled plasma mass spectrometer to its optimal state. The equipment parameters of the inductively coupled plasma mass spectrometer were set according to the test requirements and adjusted to the optimal state to call the test method to complete the test of inorganic trace elements in the CdTe sample solution and the control group solution.

9. A quantitative detection system for implementing the method according to any one of claims 1-8, characterized in that, The system includes: Sample preparation tubes are used to prepare CdTe samples and control groups by saturated vapor pressure method, wherein the control group is an empty container; Inductively coupled plasma mass spectrometry was used to plot standard curves of inorganic trace elements in standard solutions based on the internal standard method, and to determine the content of inorganic trace elements in CdTe sample solutions and control solutions after dissolving in HNO3 solution. A processor is used to take the difference in inorganic trace elements between the CdTe sample solution and the control solution as the actual content of the inorganic trace elements; The inorganic trace elements are Mg, Al, Ni, Cu, Cr, Fe and Ca.

10. A computer-readable storage medium storing a computer program that, when executed by a processor, implements the method of any one of claims 1-8.

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