Evaluation method for surface defects on SiGe substrates

Abstract

To provide a method for evaluating surface defects of a SiGe substrate, which allows defect evaluation to be performed easily and with high accuracy. [Solution] A method for evaluating surface defects in a SiGe substrate, comprising: a step of preparing a SiGe epitaxial substrate for cross-sectional observation; a first defect manifestation step of using selective etching to reveal defects on the surface of the cross-sectional observation substrate; a cross-sectional observation step of evaluating the cross-sectional shape of etching pits formed by the selective etching in advance for the cross-sectional observation substrate and determining a reference ratio of the width size to the depth of the defect to be evaluated; a step of preparing an evaluation substrate; a second defect manifestation step of using selective etching to reveal defects on the surface of the evaluation substrate; and an evaluation step of observing the surface of the evaluation substrate that has been subjected to the second defect manifestation step, counting only defects whose ratio of the width size to the depth is equal to or less than the reference ratio, and evaluating the surface defects.

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating crystal defects present on the surface of a SiGe substrate. [Background technology]

[0002] SiGe and Ge are materials widely used in various devices such as electronic, optical, and RF devices. In particular, SiGe has recently been proposed as a replacement for the Fin structure currently used in logic, and for the next generation and beyond, GAA and even CFETs, which stack NMOS and CMOS, have been proposed, and SiGe is a material that plays an important role in the manufacturing process of these devices (Non-Patent Document 1).

[0003] However, SiGe is known to be prone to polycrystallization because its liquidus and solidus are far apart in its equilibrium phase diagram, with a large distribution coefficient of 2 to 5. Even if single crystals can be grown, the growth rate is slow and it is difficult to grow them consistently (Non-Patent Document 2).

[0004] Therefore, for semiconductor devices, SiGe is grown on Si substrates, sometimes referred to as virtual SiGe substrates. This SiGe growth (heteroepitaxial growth) requires addressing the difference in lattice constants between Si and Ge. The lattice constant of Si crystals is 0.5431 nm, while that of Ge crystals is 0.56754 nm, resulting in a difference of approximately 4.5%. To mitigate this difference in lattice constants, SiGe alloys are used. If the Ge composition ratio is x, the lattice constant of the SiGe alloy is 0.5431 nm + x × 0.02 nm + x squared × 0.0027 nm. For example, if x is 0.3, the lattice constant is 0.5493 ​​nm, resulting in a minimal lattice mismatch of 0.14%. This lattice mismatch can lead to dislocations and defects in the subsequently grown epitaxial layer, resulting in degradation of quality. However, it is believed that there is a critical thickness, and even if there is lattice mismatch, defects will not occur unless the critical thickness is exceeded (Non-Patent Document 3).

[0005] However, in reality, this difference in lattice constant causes lattice-like irregularities on the surface, known as crosshatching, and dislocations from the interface between the Si substrate and the SiGe epitaxial layer reach the surface, creating defects. Therefore, evaluating these defects is important. For example, when optimizing SiGe epitaxial growth conditions, the number of defects can be used as an index to find conditions with fewer defects.

[0006] Regarding this defect evaluation method, it is of course possible to evaluate the epitaxial layer directly using techniques such as light scattering, but in addition to this, a more common method is to perform selective etching (either liquid phase or gas phase) to reveal defects and then evaluate them.

[0007] At this time, defects revealed by selective etching generally take the shape of pits, but it is particularly difficult to distinguish whether these pits are due to dislocations, which are the intended cause of detection, or other disturbances. This is for the reasons explained below. Because the parent phase of a silicon substrate is a single silicon substrate and is extremely uniform, the presence of precipitates, impurities, or crystal defects changes the etching rate, making it possible to detect defects. However, with SiGe on a silicon substrate, stress due to differences in lattice constants and compositional fluctuations are selectively etched, resulting in pits caused by factors other than dislocations, making accurate defect evaluation difficult.

[0008] Of course, each pit could be checked using a cross-sectional TEM or SEM, but this method requires time and places a limit on the number of pits that can be evaluated. Although it is possible to accurately classify the morphology, there are significant challenges in adapting this method as an evaluation method that represents the quality of the SiGe epitaxial layer.

[0009] Patent Document 1 discloses a method for quickly and simply evaluating the type (substance) of defects present in a semiconductor wafer. Specifically, it discloses a process of determining in advance the relationship between the defect form and the defect type of the semiconductor wafer, measuring the defect form of the semiconductor wafer to be evaluated, and determining the type of defect of the semiconductor wafer from the measured defect form of the semiconductor wafer based on the relationship between the defect form and the defect type. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-176943 [Non-patent literature]

[0011] [Non-Patent Document 1] Japan Society of Applied Physics, Industry-Academia Collaboration Committee on Semiconductor Crystal Growth, Processing, and Evaluation, 1st Workshop Materials, "Crystal Technology Supporting the Revival of Semiconductors," (June 21, 2023) [Non-patent document 2] Ichiro Yonenaga, "Growth of high-quality SiGe crystals and elucidation of their fundamental properties," Materia, 47(1),3(2008) [Non-patent document 3] Katsuaki Sato, "Fundamentals and Challenges of Heteroepitaxy: 1st Workshop of the 3C-SiC Technology Study Group for IoT in Harsh Environments" (2019) Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention has been made to solve the above problems, and has an object to provide a method for evaluating surface defects on a SiGe substrate that can easily and accurately evaluate defects. [Means for solving the problem]

[0013] In order to solve the above problems, the present invention provides a method for evaluating surface defects of a SiGe substrate, comprising the steps of: preparing a SiGe epitaxial substrate for cross-section observation by vapor-depositing SiGe on a silicon substrate; a first defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for cross-section observation by selective etching; a cross-sectional observation step of evaluating in advance the cross-sectional shape of an etching pit formed by the selective etching of the SiGe epitaxial substrate for cross-sectional observation, and determining a reference ratio of the size in the lateral width direction of the defect to be evaluated to its depth; preparing an SiGe epitaxial substrate for evaluation by vapor-phase growing SiGe on a silicon substrate, separately from the SiGe epitaxial substrate for cross-section observation; a second defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for evaluation by selective etching; an evaluation step of observing the surface of the SiGe epitaxial substrate for evaluation that has been subjected to the second defect revealing step, and counting only defects whose ratio of width to depth is equal to or less than the reference ratio from among the defect images observed by the surface observation, thereby evaluating the surface defects; The present invention provides a method for evaluating surface defects of a SiGe substrate, comprising:

[0014] With this type of method for evaluating surface defects on a SiGe substrate, a reference ratio for the width / depth ratio of the defect to be evaluated is determined in the cross-sectional observation process, and this reference ratio is used as an index to count only defects whose width / depth ratio is equal to or less than the reference ratio from among the defect images observed by surface observation of the SiGe epitaxial substrate for evaluation.This makes it possible to easily and accurately evaluate surface defects on a SiGe substrate without performing cross-sectional observation of the SiGe epitaxial substrate for evaluation.

[0015] For example, a selective etching method using an acidic chemical solution can be used as the selective etching in each of the first defect revealing step and the second defect revealing step.

[0016] Alternatively, a selective etching method using gas can be used as the selective etching in each of the first defect revealing step and the second defect revealing step.

[0017] Thus, the selective etching method is not particularly limited.

[0018] A polishing step can be performed before each of the first defect revealing step and the second defect revealing step to reduce the unevenness of the surface of the SiGe epitaxial substrate for cross-section observation or the surface of the SiGe epitaxial substrate for evaluation by polishing.

[0019] If the crosshatching on the surface of the SiGe epitaxial layer has a large effect, it is possible to reduce the surface irregularities by polishing before the selective etching, but this is just one option and is not essential.

[0020] In the evaluation step, the number of defects can be automatically counted based on the reference ratio.

[0021] In this way, the defect counting can also be automated. [Effects of the Invention]

[0022] As described above, the method for evaluating surface defects of a SiGe substrate according to the present invention makes it possible to easily evaluate surface defects of a SiGe substrate with high accuracy. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a schematic cross-sectional view of a SiGe epitaxial substrate on a silicon substrate in an embodiment of the present invention. [Figure 2] 1 is an example of a cross-sectional TEM observation of defects in a SiGe epitaxial layer on a silicon substrate in an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] As described above, there has been a demand for the development of a defect evaluation method for SiGe substrates that allows defect evaluation to be performed easily and with high accuracy.

[0025] As a result of extensive research into the above-mentioned problems, the inventors have found that by focusing on the size and depth of pits after selective etching and classifying and counting defects based on the ratio of this size to depth, the accuracy of defect detection can be improved. In particular, they have found that pits caused by factors other than dislocations, such as stress or local compositional variations, tend to be large in size, and that in some cases they can be distinguished based on size alone. Based on these findings, the inventors have completed the present invention.

[0026] That is, the present invention is a method for evaluating surface defects of a SiGe substrate, comprising: preparing a SiGe epitaxial substrate for cross-section observation by vapor-depositing SiGe on a silicon substrate; a first defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for cross-section observation by selective etching; a cross-sectional observation step of evaluating in advance the cross-sectional shape of an etching pit formed by the selective etching of the SiGe epitaxial substrate for cross-sectional observation, and determining a reference ratio of the size in the lateral width direction of the defect to be evaluated to its depth; preparing an SiGe epitaxial substrate for evaluation by vapor-phase growing SiGe on a silicon substrate, separately from the SiGe epitaxial substrate for cross-section observation; a second defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for evaluation by selective etching; an evaluation step of observing the surface of the SiGe epitaxial substrate for evaluation that has been subjected to the second defect revealing step, and counting only defects whose ratio of width to depth is equal to or less than the reference ratio from among the defect images observed by the surface observation, thereby evaluating the surface defects; The present invention relates to a method for evaluating surface defects of a SiGe substrate, the method comprising:

[0027] It should be noted that Non-Patent Document 1 does not refer to counting, nor does it refer to a method for improving the accuracy of counting.

[0028] The present invention will be described in detail below with reference to the drawings, but the present invention is not limited thereto.

[0029] First, the SiGe substrate that is the object of evaluation in the method for evaluating surface defects of a SiGe substrate of the present invention will be described.

[0030] 1 is a schematic cross-sectional view of a SiGe substrate (SiGe epitaxial substrate) 1, which is the subject of evaluation of the present invention, and which has SiGe (SiGe layer) 3 grown on a silicon substrate (silicon single crystal substrate) 2. As described above, defects occur in the SiGe 3 grown on the silicon substrate 2 due to the difference in lattice constant. There are no particular restrictions on the growth conditions for the SiGe, but it is generally grown under reduced pressure conditions at a temperature of 600 to 800°C using monogermane (GeH4) and monosilane (SiH4) or dichlorosilane (H2SiCl2) gas as raw materials.

[0031] The method for evaluating surface defects of a SiGe substrate of the present invention generally includes a step of preparing a SiGe epitaxial substrate for cross-section observation, a first defect revealing step, a cross-section observing step, a step of preparing a SiGe epitaxial substrate for evaluation, a second defect revealing step, and an evaluation step. Each step will be described in order below.

[0032] <Process for preparing SiGe epitaxial substrates for cross-sectional observation> First, a SiGe epitaxial substrate for cross-section observation is prepared by vapor-depositing SiGe on a silicon substrate. The SiGe epitaxial substrate for cross-section observation can be fabricated, for example, by the same method as that for fabricating the SiGe substrate to be evaluated, as described above.

[0033] <First defect revealing process> Next, in the first defect revealing step, defects are revealed by selective etching on the surface of the SiGe epitaxial substrate for cross-section observation.

[0034] The selective etching at this time may be a selective etching method using an acidic chemical solution, for example, a mixed solution of hydrofluoric acid and nitric acid, or a gaseous selective etching method using a gas such as hydrochloric acid gas or a reducing gas, but in this case, the selective etching method is not particularly limited.

[0035] Furthermore, if the crosshatching on the surface of the SiGe epitaxial film has a large effect, a polishing step can be performed before the selective etching to reduce the surface irregularities, but this polishing step is only an option and is not essential, as the size of the crosshatching is greatly affected by the SiGe epitaxial conditions.

[0036] <Cross-section observation process> Next, a cross-sectional observation step is performed in which the cross-sectional shape of the etching pit formed by the selective etching is evaluated in advance for the SiGe epitaxial substrate for cross-sectional observation, and a reference ratio of the width direction size of the defect to be evaluated to the depth is determined.

[0037] The method for observing the cross-sectional shape is not particularly limited, but for example, a transmission electron microscope (TEM) can be used.

[0038] For example, when the cross-sectional structure of defects in a selectively etched SiGe epitaxial substrate for cross-sectional observation is evaluated using cross-sectional TEM, three structures are found (Fig. 2). The first is a simple depression caused by a localized imbalance in the Ge concentration (left), the second is the same depression but where stress is applied (mainly cross-hatched) (center), and the third is actually caused by a defect (dislocation) (right).

[0039] For each of the depressions (left), stress (center), and dislocations (right), the width (W) and depth (D) of the defects in the area below the reference line indicated by the arrow were measured. The results are shown in Table 1 below.

[0040] [Table 1]

[0041] From the results shown in Table 1, it is possible to set the reference ratio (W / D) of defects caused by dislocations, which are the defects to be evaluated, to, for example, 10 or less.

[0042] <Process for preparing SiGe epitaxial substrate for evaluation> Meanwhile, separate from the SiGe epitaxial substrate for cross-section observation described above, a SiGe epitaxial substrate for evaluation is prepared by vapor-phase growth of SiGe on a silicon substrate.

[0043] The SiGe epitaxial substrate for evaluation can be fabricated by the method described above, similar to the SiGe epitaxial substrate for cross-section observation.

[0044] <Second defect revealing process> Next, in the second defect revealing step, defects are revealed by selective etching on the surface of the SiGe epitaxial substrate for evaluation.

[0045] As in the first defect revealing process, the selective etching at this time may be a selective etching method using an acidic chemical solution, such as a mixture of hydrofluoric acid and nitric acid, or a selective etching method using a gas such as hydrochloric acid or a reducing gas, but in this case, the selective etching method is not particularly limited.

[0046] Furthermore, as in the first defect revealing process, if the crosshatching on the surface of the SiGe epitaxial film has a large effect, a polishing process can be performed before the selective etching to reduce the surface irregularities, but this polishing process is only an option and is not essential, since the size of the crosshatching is greatly affected by the SiGe epitaxial conditions.

[0047] <Evaluation process> Next, a surface observation of the evaluation SiGe epitaxial substrate subjected to the second defect revealing step is performed, and from the defect images observed by the surface observation, only defects whose width-to-depth ratio is equal to or less than the reference ratio determined in the cross-sectional observation step are counted to perform an evaluation step of evaluating the surface defects. In other words, for example, after the selective etching is completed, etching pits can be observed from the surface.

[0048] Although there are no limitations on this observation method, scanning electron microscopes (SEM) and optical microscopes are suitable. In particular, by arbitrarily changing the stage height with a confocal microscope, it is possible to obtain data on pit size (for example, the width W of the defect) and, at the same time, data on pit depth (depth D of the defect). In this way, it is possible to identify the defect to be evaluated in the SiGe epitaxial substrate to be evaluated from the defect image viewed from the surface of the SiGe epitaxial substrate to be evaluated and the reference ratio obtained from the cross-sectional image. In other words, the defect image to be actually detected is one with a small ratio of width W to depth D (W / D) (a defect with a small width W and deep depth).

[0049] In the evaluation process, the number of defects may be automatically counted based on the reference ratio described above, for example.

[0050] According to the evaluation method of the present invention described above, the accuracy of defect evaluation can be improved by observing the surface over a wide area using, for example, an optical microscope or an SEM, without performing cross-sectional observation of the SiGe substrate to be evaluated. In other words, according to the SiGe substrate defect evaluation method of the present invention, defect evaluation can be performed easily and with high accuracy.

[0051] Furthermore, it becomes easier to optimize the SiGe epitaxial growth conditions using the evaluation results obtained by the SiGe substrate defect evaluation method of the present invention as an index, and as a result, it becomes possible to provide a higher quality SiGe substrate. [Example]

[0052] The present invention will be specifically described below using examples, but the present invention is not limited to these examples.

[0053] (Example) First, a SiGe epitaxial substrate for cross-section observation was prepared as follows. A 300 mm (100) diameter, boron-doped, 10 Ω·cm single-crystal silicon substrate was prepared, and a 1 μm SiGe film (Ge composition ratio = 30%) was grown on the single-crystal silicon substrate in a low-pressure CVD apparatus using SiH2Cl2 gas and GeH4 gas as source gases, with a gas flow rate of 1000 sccm for each, a chamber pressure of 10 Torr, a temperature of 610°C, and a growth time of 60 minutes. In this way, a SiGe epitaxial substrate for cross-section observation was produced.

[0054] Using this SiGe epitaxial substrate for cross-section observation as a base, a first defect revealing step and a cross-section observing step were carried out in the following order to obtain a reference ratio for defect classification.

[0055] First, silicon was immersed in a chemical solution made by mixing hydrofluoric acid and nitric acid in a 1:1 ratio for 10 minutes to dissolve the silicon. After that, the SiGe epitaxial substrate for cross-sectional observation was immersed in the solution for 30 seconds to perform selective etching (first defect revealing process).

[0056] Next, cross-sectional TEM analysis was performed to evaluate dislocation-induced pits (defects) and other pits, and the ratio of their width (W) to their depth (D) was calculated. As a result, the SiGe epitaxial substrates used for cross-sectional observation were identified as follows: Dislocation-induced defects, where the pit size was 50 nm or less (W / D ratio of 10 or less) based on the aspect ratio (W / D) of dislocation-induced pits; stress-induced defects, where the pit size was greater than 50 and less than 100 nm (W / D ratio of 10 and less than 50) based on the aspect ratio (W / D) of stress-induced pits; and depression-induced defects, where the pit size was 100 nm or more (W / D ratio of 50 or more) based on the aspect ratio (W / D) of simple surface depressions. Therefore, defects with a pit size of 50 nm or less (W / D ratio of 10 or less) were defined as dislocation-induced defects. In other words, 10 was defined as the reference ratio.

[0057] In accordance with this definition, defect evaluation of the SiGe epitaxial substrate for evaluation, which was the evaluation target, was carried out as follows.

[0058] The SiGe epitaxial substrate for evaluation was fabricated as follows: A 300 mm (100) diameter, boron-doped, single-crystal silicon substrate with a resistivity of 10 Ω·cm was prepared, and a 1 μm SiGe film (Ge composition ratio = 30%) was grown on the single-crystal silicon substrate in a low-pressure CVD apparatus using SiH2Cl2 gas and GeH4 gas as source gases with a gas flow rate of 1000 sccm each, a chamber pressure of 10 Torr, a temperature of 610°C, and a growth time of 60 minutes. In this way, the SiGe epitaxial substrate for evaluation was fabricated.

[0059] Next, as the second defect revealing process, the silicon was immersed in a chemical solution made by mixing hydrofluoric acid and nitric acid in a 1:1 ratio for 10 minutes to dissolve the silicon. After that, the SiGe epitaxial substrate for evaluation was immersed in the solution for 30 seconds to perform selective etching.

[0060] After selective etching, the surface of the SiGe epitaxial substrate for evaluation was observed with an optical microscope. Defects below the previously obtained reference ratio were determined to be defects caused by dislocations. The defect density was evaluated to be 3 × 10 7 Ke / cm 2 The results obtained were as follows.

[0061] As described above, according to the embodiment, defect evaluation could be easily performed with high accuracy without observing the cross section of the SiGe epitaxial substrate for evaluation.

[0062] The present specification includes the following aspects. [1] A method for evaluating surface defects of a SiGe substrate, comprising: a step of preparing a SiGe epitaxial substrate for cross-section observation, which is formed by vapor-phase growing SiGe on a silicon substrate; a first defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for cross-section observation by selective etching; a cross-section observation step of evaluating in advance the cross-sectional shape of an etching pit formed by the selective etching on the SiGe epitaxial substrate for cross-section observation, and determining a reference ratio of the width direction size of the defect to be evaluated to the depth; and A method for evaluating surface defects of a SiGe substrate, comprising: a step of vapor-phase growing SiGe on a silicon substrate to prepare a SiGe epitaxial substrate for evaluation; a second defect revealing step of selectively etching the surface of the SiGe epitaxial substrate for evaluation to reveal defects; and an evaluation step of observing the surface of the SiGe epitaxial substrate for evaluation that has been subjected to the second defect revealing step, and counting only defects whose ratio of width to depth is equal to or less than the reference ratio from among the defect images observed by the surface observation to evaluate the surface defects. [2] A defect evaluation method according to [1], characterized in that a selective etching method using an acidic chemical solution is used as the selective etching in each of the first defect revealing process and the second defect revealing process. [3] The defect evaluation method according to [1], wherein the selective etching in each of the first defect revealing step and the second defect revealing step is a selective etching method using gas. [4] A defect evaluation method according to any one of [1] to [3], characterized in that a polishing step is performed before each of the first defect revealing step and the second defect revealing step to reduce unevenness on the surface of the SiGe epitaxial substrate for cross-section observation or the surface of the SiGe epitaxial substrate for evaluation by polishing. [5] The defect evaluation method according to any one of [1] to [4], wherein in the evaluation step, the defects are automatically counted based on the reference ratio.

[0063] The present invention is not limited to the above-described embodiments, which are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that provides similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0064] 1...SiGe substrate (SiGe epitaxial substrate), 2...silicon substrate (single crystal silicon substrate), 3...SiGe (SiGe layer).

Claims

1. A method for evaluating surface defects on a SiGe substrate, comprising: preparing a SiGe epitaxial substrate for cross-section observation by vapor-depositing SiGe on a silicon substrate; a first defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for cross-section observation by selective etching; a cross-sectional observation step of evaluating in advance a cross-sectional shape of an etching pit formed by the selective etching of the SiGe epitaxial substrate for cross-sectional observation, and determining a reference ratio of the size in the lateral width direction of the defect to be evaluated to its depth; preparing an SiGe epitaxial substrate for evaluation by vapor-depositing SiGe on a silicon substrate separately from the SiGe epitaxial substrate for cross-section observation; a second defect revealing step of revealing defects on the surface of the SiGe epitaxial substrate for evaluation by selective etching; an evaluation step of observing the surface of the SiGe epitaxial substrate for evaluation that has been subjected to the second defect revealing step, and counting only defects whose ratio of width to depth is equal to or less than the reference ratio from among defect images observed by the surface observation, thereby evaluating the surface defects; A method for evaluating surface defects of a SiGe substrate, comprising:

2. 2. The method for evaluating surface defects of a SiGe substrate according to claim 1, wherein the selective etching in each of the first defect revealing step and the second defect revealing step is a selective etching method using an acidic chemical solution.

3. 2. The method for evaluating surface defects of a SiGe substrate according to claim 1, wherein the selective etching in each of the first defect revealing step and the second defect revealing step is a selective etching method using a gas.

4. 2. The method for evaluating surface defects of a SiGe substrate according to claim 1, characterized in that a polishing step is performed before each of the first defect revealing step and the second defect revealing step to reduce unevenness on the surface of the SiGe epitaxial substrate for cross-section observation or the surface of the SiGe epitaxial substrate for evaluation by polishing.

5. 5. The method for evaluating surface defects of a SiGe substrate according to claim 1, wherein in the evaluation step, the number of defects is automatically counted based on the reference ratio.

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