CN115910833A - Method for testing thickness of silicon epitaxial layer on lightly doped silicon substrate - Google Patents
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Abstract
The invention provides a method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate, which comprises the following steps: preparing a standard sample and a sample wafer to be tested, wherein the impurity elements of the standard sample are the same as the elements to be tested of the epitaxial layer in the sample wafer to be tested; measuring the intensities of the impurity ions and the silicon ions of the standard sample by using a secondary ion mass spectrometer, and obtaining a relative sensitivity factor RSF of the secondary ion mass spectrometer under the current test condition through the standard sample; testing the depth of a sputtering pit on the surface of the sample by using a step instrument, and dividing the depth of the sputtering pit by the testing time to obtain the sputtering rate; and calculating the concentration of the element to be detected of the sample wafer to be detected by using the RSF value of the standard sample, finding the mutation time of the mutation of the concentration of the element to be detected, and calculating the thickness of the epitaxial layer of the sample wafer to be detected. The method has the advantages that the direct test of the thickness of the silicon epitaxial layer on the lightly doped substrate is realized, more selectivity is provided for test analysis, the concentration of the doping element of the epitaxial layer is tested while the thickness of the epitaxial layer is obtained, and more references are provided for analyzing the quality of the epitaxial wafer.
Description
Technical Field
The invention belongs to the technical field of analysis and test of silicon epitaxial wafers, and particularly relates to a method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate.
Background
The epitaxial wafer refers to a substrate on which an epitaxial layer grows, the silicon epitaxial wafer generally comprises a silicon substrate and a silicon epitaxial layer deposited on the substrate, and the deposited epitaxial layer is used for eliminating defects generated in the crystal growth and machining processes of a traditional silicon wafer.
In the actual production, the thickness of the epitaxial layer is usually tested by an infrared reflection method, the instrument is simple and convenient to operate, belongs to nondestructive detection, cannot damage the epitaxial surface, and is widely applied to the process inspection of epitaxial manufacturing. Subject to the test principle, the infrared reflection method is used for measuring the growth of lightly doped epitaxial silicon on a heavily doped substrate, and requires that the substrate resistivity is less than 0.02 omega.cm (heavily doped substrate) and the epitaxial layer resistivity is greater than 0.1 omega.cm (lightly doped epitaxial layer); the thickness of the epitaxial layer of the epitaxial wafer of some lightly doped silicon substrates cannot be directly tested by using an infrared interference method, and the common method in the industry is as follows: growing a silicon epitaxial layer on the heavily doped silicon substrate under the same condition, testing the silicon epitaxial layer to be used as a monitoring wafer, and indirectly representing the thickness of the epitaxial layer on the lightly doped silicon substrate by testing the monitoring wafer.
Disclosure of Invention
The invention aims to provide a method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate, and effectively solves the problems that the thickness of the epitaxial layer can only be indirectly represented, the experiment speed is low and the conversion efficiency is low in the prior art.
In order to solve the technical problems, the invention adopts the technical scheme that: a method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate comprises the following steps:
preparing a standard sample and a sample wafer to be detected, wherein the impurity elements of the standard sample are the same as the elements to be detected of the epitaxial layer in the sample wafer to be detected, and the concentration of the impurity elements in the standard sample is known; measuring the intensities of impurity ions and silicon ions of the standard sample by using a secondary ion mass spectrometer, and obtaining a relative sensitivity factor RSF under the current test condition of the secondary ion mass spectrometer through the standard sample;
measuring the ion strength and the silicon ion strength of the sample wafer to be measured by using a secondary ion mass spectrometer, obtaining the sputtering rate through the depth of a sputtering pit and the test time, calculating the concentration of elements to be measured of the sample wafer to be measured by using the RSF value of the standard sample, finding the mutation time of the mutation of the concentration of the elements to be measured, and calculating the thickness of the epitaxial layer of the sample wafer to be measured by using the sputtering rate and the mutation time.
Further, the method for testing the ionic strength and the silicon ionic strength of the standard sample and the sample wafer to be tested comprises the following steps: bombarding the surfaces of the standard sample and the sample wafer to be detected by a primary ion source to excite secondary ions, screening the mass-to-charge ratio of the secondary ions, detecting the intensity of the secondary ions by a detector, and obtaining the intensities of the ions to be detected and silicon ions of the standard sample and the sample wafer to be detected after signal conversion.
Further, when the element to be measured of the epitaxial layer of the sample wafer to be measured is phosphorus, the primary ion source is set as a cesium source.
Further, before the secondary ion mass spectrometer is used for detection, the light path of the secondary ion mass spectrometer is tuned.
Furthermore, the optical path conditions of the sample wafer to be measured and the standard sample and the impact energy of the primary ion source are completely the same.
Further, the sputtering rate is the ratio of the depth of the sputtering pit to the test time.
Further, the test time of the sample wafer to be tested starts with the sputtering of ions onto the surface of the epitaxial layer of the sample wafer to be tested, and the test elements gradually tend to be stable as the end time.
Further, a formula for calculating the concentration of the doping element of the sample wafer to be measured according to the RSF value of the standard sample is as follows:
wherein I is ionic strength and C is concentration.
Further, the depth of the sputtering pit is detected by using a step profiler.
By adopting the technical scheme, the thickness of the silicon epitaxial layer doped with the phosphorus element on the lightly doped borosilicate substrate is directly tested, the thickness of the epitaxial layer and the concentration of the epitaxial layer are synchronously tested in the testing process, and the resistivity is converted through the concentration of the doped element, so that the beneficial effect of comparing the resistivity of the epitaxial layer is realized.
By adopting the technical scheme, the direct test of the thickness of the silicon epitaxial layer on the lightly doped substrate is realized, more selectivity is provided for film thickness test analysis, the concentration of the doping element of the epitaxial layer is tested while the thickness of the epitaxial layer is obtained, more epitaxial product information is obtained, and more references are provided for analyzing the quality of an outer grinding wafer.
Drawings
FIG. 1 is a schematic diagram of a phosphorus concentration-sputtering depth curve of a sample wafer to be tested in a method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate according to an embodiment of the present invention
Detailed Description
The invention is further illustrated by the following examples and figures:
unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments and comparative examples only and is not intended to limit the scope of the present invention. It should be expressly noted that there may be a variety of names for the same organic structure, as long as the structure is within the scope of the present patent.
The main action principle of the invention is as follows: the method comprises the following steps of bombarding a silicon wafer material surface with a film by using a primary ion focusing beam with certain energy by using a secondary ion mass spectrometer device (note that the film is bombarded from the surface), wherein all molecular structures are destroyed, atom and molecular clusters are ejected, and a part of particles are ionized, namely secondary ions, and are used for mass spectrometry.
The concentration of the secondary ions is quantified by using a relative sensitivity factor method, and the relative sensitivity factor is obtained by using a standard sample, because the epitaxial layer is doped with the element to be detected and the lightly doped substrate is not doped with the element to be detected, the phosphorus ion strength is obviously reduced and tends to be stable when the sample wafer to be detected is bombarded and sputtered to the silicon substrate by the primary ion source.
A method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate comprises the following steps:
s1: preparing a standard sample and a sample wafer to be detected, wherein the standard sample is a silicon wafer sample with known doping element content;
the conditions given by the detector, the setting of the primary ion impact energy and the setting of the test light path conditions when the secondary ion tester tests the standard sample and the sample to be tested are completely the same, no adjustment is carried out in the test process, the sputtering process of ion bombardment on the surface of the sample is uniform sputtering, and the test conditions of the standard sample and the sample to be tested are the same;
the sputtering rate is the ratio of the depth of a sample sputtering pit to the testing time, the testing time starts with the bombardment of the primary ion source on the surface of the sample sputtering, and the content of the element to be tested is obviously reduced and tends to be stable as the end time;
s2: bombarding the surface of the standard sample by using a primary ion source to excite secondary ions, screening the mass-to-charge ratio of the secondary ions to obtain the doped ion strength and the silicon ion strength of the standard sample,
the ion intensity of the ions is detected by using a detector, the content of the doping elements is known and is calculated by a concentration formula,
the RSF was obtained from the test data of the standards.
S3: testing the sample wafer to be tested under the same conditions as the standard sample, enabling the sputtering rate and RSF of the sample wafer to be tested to be consistent with the standard sample, and screening the mass-to-charge ratio of the secondary ions to obtain the ion strength to be tested and the silicon ion strength of the sample wafer to be tested; starting from the surface of the sample wafer to be tested by primary ion sputtering, the content of elements to be tested is obviously reduced and tends to be stable as ending time;
calculating the concentration of the sample to be measured by the following formula:
the RSF value is calculated by the standard sample, and the concentration of the element to be tested in different time periods can be obtained in the test according to the ion intensity ratio of different time periods in the test;
and testing the depth of the sputtering pit by using a step profiler, dividing the depth of the sputtering pit by the testing time of the sample to obtain a sputtering rate Sr, and calculating the sputtering depth of the sample wafer to be tested by using the product of the sputtering rate and the sputtering time.
And drawing a relation curve of the concentration of the element to be measured and the sputtering depth, wherein the depth corresponding to the concentration of the impurity element to be measured when the concentration of the impurity element to be measured is obviously changed in the curve is the thickness of the epitaxial layer of the sample wafer to be measured.
Because the element to be detected in the epitaxial layer is different from the doping element of the silicon substrate, for example, the silicon epitaxial layer is doped with phosphorus element, and the lightly doped substrate silicon wafer of the epitaxial wafer to be detected is not doped with phosphorus element, the substrate silicon wafer has no phosphorus element in principle, but trace impurity elements always exist in nature, but the content of the trace impurity elements is extremely low, a small amount of signals are sporadically generated during testing, and the identification and the judgment are very easy. At this time, a secondary ion mass spectrometer is used for testing the phosphorus element of the sample, the phosphorus ion strength in the epitaxial layer is very high, when the substrate is tested, the phosphorus ion strength is rapidly reduced and stable, and the position of the phosphorus ion concentration change is the position of the epitaxial layer, so that the thickness of the epitaxial layer is obtained.
One specific example is listed below:
the silicon wafer sample to be tested used in this embodiment is an epitaxial wafer epitaxially doped with phosphorus element on a lightly doped borosilicate substrate, and phosphorus element is an element intentionally doped in an epitaxial processing process in order to obtain a desired resistivity and a desired conductivity.
Standard sample preparation: the known phosphorus concentration content is 9.65E17atoms/cm3, and is marked as # 1;
preparing a sample wafer to be tested: an epitaxial wafer (the thickness of a silicon wafer substrate is 625 um) doped with phosphorus on a lightly boron-doped substrate is marked as # 2;
cutting the sample wafer to be tested and the standard sample into sample blocks with the size of about 7mm x 7mm, marking the surface of the silicon wafer with the film as a surface to be tested, sequentially marking 1# and 2# on the non-testing surface of the silicon wafer by using a marking pen for distinguishing, preventing wafer mixing, and filling the sample into a sample rack for later use;
preparing a secondary ion mass spectrometer with the model number of SIMS 6FE7, wherein the test condition is cesium source, namely primary ions are cesium source, preparing the optical path and the test condition of the device, and uploading the sample racks loaded with the 1# and 2# samples to the SIMS device for testing;
setting basic analysis conditions: the element to be detected is 31P, the main element is 28Si or 30Si, and the detector is EM or FCs; the scanning sputtering area during sample analysis is 150um;
bombarding the No. 1 sample wafer and the No. 2 sample wafer respectively by using a secondary ion mass spectrometer, sputtering secondary ions, detecting the ion intensity of phosphorus ions and silicon ions by using a detector, wherein the sputtering time starts from the sputtering of the ions to the surface of the sample, the content of phosphorus elements obviously decreases and tends to be stable, and the test data is recorded and stored after the test is finished and is respectively stored as a No. 1 file and a No. 2 file;
and taking the standard sample out of the secondary ion mass spectrometer, testing the sputtering pit depth of the sample by using a step profiler, and obtaining the sputtering rate Sr according to the ratio of the sputtering pit depth of the sample to the testing time of the sample.
Sample analysis # 1: and calling out a No. 1 sample file through automatic analysis software to obtain a corresponding relation curve of the ionic strength and the sputtering time of the No. 1 sample, and analyzing to obtain a relative sensitivity factor RSF of the standard sample according to the known phosphorus concentration of the standard sample.
2# sample analysis: calling out the No. 2 sample file through automatic analysis software to obtain a corresponding relation curve of the ion intensity and the sputtering time of the No. 2 sample, substituting the obtained sputtering rate Sr and RSF into the No. 2 file, and converting the relation curve of the phosphorus ion intensity and the time into a phosphorus ion concentration-depth relation curve, as shown in figure 1.
The depth corresponding to the data point before the data point where the phosphorus concentration starts to stabilize in the phosphorus concentration-depth relation curve in fig. 1 is the epitaxial layer thickness, the epitaxial layer thickness read in fig. 1 is 4.77um, and the P concentration of the epitaxial layer obtained by SIMS analysis software analysis is 1.098e16atoms/cm3.
The thickness of the lightly doped P epitaxial layer on the heavily doped silicon substrate of the same epitaxial process was measured to be 4.75um using an infrared method, and the resistivity measured using an epitaxial layer resistivity measuring instrument was 0.479 Ω · cm (theoretical concentration of 1.092e16).
The deviation of the film thickness tested by the invention and the thickness tested by the monitoring wafer is 0.42 percent, which meets the requirement of 1 percent of internal control of the dispersion difference between the epitaxial wafers and also meets the requirement of +/-5 percent of the thickness tolerance of the epitaxial layer of the common epitaxial silicon wafer.
It should be noted that the above-mentioned contents are only embodiments of the present invention, and those produced by those skilled in the art with appropriate modifications and alterations according to the main idea and related contents of the present invention shall also fall into the protection scope of the claims of the present invention. And the technical terms and other materials referred to in the present invention are only used for clearly illustrating the advantages and effects of the present invention and should not be taken as limitations to the novelty of the present invention. The above embodiments are part of the description of the practical application effect of the present invention, and the embodiments are not intended to limit the scope of the present invention, and all modifications and substitutions made by those skilled in the art based on the present invention are within the protection scope of the present invention.
Claims (9)
1. A method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate comprises the following steps:
preparing a standard sample and a sample wafer to be detected, wherein the impurity elements of the standard sample are the same as the elements to be detected of the epitaxial layer in the sample wafer to be detected, and the concentration of the impurity elements in the standard sample is known; measuring the intensities of impurity ions and silicon ions of the standard sample by using a secondary ion mass spectrometer, and obtaining a relative sensitivity factor RSF under the current test condition of the secondary ion mass spectrometer through the standard sample;
testing the ion strength and the silicon ion strength to be tested of the sample wafer to be tested by using a secondary ion mass spectrometer, obtaining the sputtering rate through the depth of a sputtering pit and the testing time, calculating the concentration of the element to be tested of the sample wafer to be tested by using the RSF value of the standard sample, finding the mutation time of the mutation of the concentration of the element to be tested, and calculating the thickness of the epitaxial layer of the sample wafer to be tested by using the sputtering rate and the mutation time.
2. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to claim 1, wherein the method comprises the following steps: the method for testing the strength of ions to be tested and silicon ions in the epitaxial layers of the standard sample and the sample wafer to be tested comprises the following steps: bombarding the surfaces of the standard sample and the sample wafer to be detected by a primary ion source to excite secondary ions, screening the mass-to-charge ratio of the secondary ions, detecting the intensity of the secondary ions by a detector, and obtaining the intensities of the doped ions, the ions to be detected and the silicon ions of the standard sample and the sample wafer to be detected after signal conversion.
3. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to claim 1, wherein the method comprises the following steps: and when the element to be tested of the epitaxial layer of the sample wafer to be tested is phosphorus, the primary ion source is set as a cesium source.
4. A method for testing the thickness of a silicon epitaxial layer on a lightly doped silicon substrate as claimed in any one of claims 1 to 3, wherein: and before the secondary ion mass spectrometer is used for detection, the secondary ion mass spectrometer is subjected to light path tuning.
5. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to claim 4, wherein the method comprises the following steps: and the optical path conditions of the sample wafer to be detected and the standard sample and the impact energy of the primary ion source are completely the same.
6. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to claim 1, wherein the method comprises the following steps: the sputtering rate is the ratio of the depth of the sputtering pit to the test time.
7. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to claim 1, wherein the method comprises the following steps: the testing time of the sample wafer to be tested starts from the sputtering of ions to the surface of the epitaxial layer of the sample wafer to be tested, and the testing elements gradually tend to be stable as the end time.
8. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to any one of claims 1 to 3 and 5 to 7, wherein: and calculating the doping element concentration of the sample wafer to be detected according to the RSF value of the standard sample by the following formula:
9. The method for testing the thickness of the silicon epitaxial layer on the lightly doped silicon substrate according to claim 1, wherein the method comprises the following steps: the depth of the sputtering pit is detected by using a step profiler.
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