JP7600659B2 - Epitaxial wafer defect inspection method - Google Patents

Description

The present invention relates to a method for inspecting defects in epitaxial wafers.

A surface inspection device is known that spirally scans the surface of a semiconductor substrate with laser light, obtains specific scattered light information in a specific direction of scattered light from the surface, and uses this specific scattered light information to detect the presence or absence of crystal defects occurring along the crystal slip plane of the semiconductor substrate (for example, Patent Document 1). Also known is a method of irradiating a semiconductor substrate with polarized parallel light and evaluating the crystal quality of the semiconductor substrate from an image obtained from the light transmitted through or reflected by the semiconductor substrate (for example, Patent Document 2).

JP 2007-214491 A Special table 2017/078127 publication

Incidentally, crystal defects in silicon epitaxial wafers include slip dislocation defects caused by external thermal or physical stress, and misfit dislocation defects caused by lattice mismatch at the interface between the substrate and the epitaxial layer. However, with the surface inspection device described in Patent Document 1, both slip dislocation defects and misfit dislocation defects are observed as similar linear defects, which creates the problem of being unable to distinguish between them.

The problem that this invention aims to solve is to provide an epitaxial wafer inspection method that can distinguish between slip dislocation defects and misfit dislocation defects.

In the present invention, when an epitaxial wafer has dislocation defects including slip dislocation defects and misfit dislocation defects, a residual stress of the epitaxial wafer is obtained by wafer stress measurement;
The problem is solved by judging that the defect is the slip dislocation defect when the residual stress is equal to or greater than a predetermined value, and judging that the defect is the misfit dislocation defect when the residual stress is less than the predetermined value. Note that the wafer stress measurement is performed by irradiating the wafer with infrared light, analyzing the change in polarization state caused by the photoelastic effect in the stressed portion, and measuring it as strain.

In the above invention, the inspection surface of the epitaxial wafer may be irradiated with inspection light, and the presence or absence of dislocation defects, including slip dislocation defects and misfit dislocation defects, may be determined based on the scattered light.

According to the present invention, when dislocation defects, including slip dislocation defects and misfit dislocation defects, are found in an epitaxial wafer, the residual stress is measured by wafer stress measurement, and if the residual stress is large, it is determined to be a slip dislocation defect, making it possible to distinguish between slip dislocation defects and misfit dislocation defects.

1A to 1C are process diagrams showing an embodiment of an epitaxial wafer inspection method of the present invention. FIG. 2 is a configuration diagram showing an example of a surface inspection device used in the surface inspection process of FIG. 1 . FIG. 2 is a configuration diagram showing an example of a wafer stress measuring device used in the residual stress inspection process of FIG. 1 . FIG. 2 is a diagram showing an example of dislocation defects in an epitaxial wafer. FIG. 4 is a diagram showing an output waveform of the wafer stress measuring device of FIG. 3 . FIG. 5B is a diagram showing a waveform obtained by filtering the output waveform of FIG. 5A. FIG. 5C is a diagram showing a method for determining a relative distortion amount from the waveform of FIG. 5B using the GBA method.

In one embodiment of the epitaxial wafer defect inspection method of the present invention, the objects of inspection are slip dislocation defects caused by external thermal or physical stress, and misfit dislocation defects caused by lattice mismatch at the interface between the substrate and the epitaxial layer.

Here, slip dislocations refer to linear steps that occur when stress exceeding the yield value acts in the circumferential direction, such as when there is temperature non-uniformity in the radial direction of the wafer. Since it is a partial slip of the crystal region caused by thermal factors, in the case of epitaxial wafers, slip dislocations can be suppressed by adjusting the heater temperature conditions of the growth furnace during epitaxial growth.

In contrast, in silicon substrates with a high impurity concentration, the atomic radius of the impurity atoms differs from that of the silicon atoms, causing a slight change in the crystal lattice constant and film distortion in the epitaxial layer with a low impurity concentration. Misfit dislocations are linear steps that occur when the distortion in the film reaches a critical value. Since dislocations occur due to the difference in the crystal lattice constants between the epitaxial film and the silicon substrate, misfit dislocations can be suppressed by adjusting the impurity concentrations of the epitaxial film and the silicon substrate, or by providing a buffer layer of intermediate concentration between the epitaxial film and the silicon substrate.

As such, both defects are linear steps and cannot be distinguished by visual observation, but because the countermeasures are completely different as mentioned above, identifying these dislocation defects is important for increasing the operation rate and product yield of the epitaxial wafer manufacturing process. Of course, these slip dislocation defects and misfit dislocation defects can be identified by measuring and observing the cross section of the epitaxial wafer with an X-ray measuring device. However, destructive testing cannot be performed in real time during the manufacturing process, so the operation rate of the manufacturing process and product yield will decrease until the identification results are obtained.

Therefore, in the inspection method of this embodiment, attention is focused on the differences in the causes of dislocation defects, and when an indistinguishable slip dislocation defect or misfit dislocation defect is observed, the residual stress of the epitaxial wafer is obtained by wafer stress measurement, and if the obtained residual stress is equal to or greater than a predetermined value, it is determined that the defect is a slip dislocation defect caused by stress, and if the residual stress is less than the predetermined value, it is determined that the defect is a misfit dislocation defect caused by a difference in crystal lattice constant that is unrelated to stress.

Figure 1 is a process diagram showing one embodiment of the epitaxial wafer inspection method of the present invention. In the epitaxial wafer inspection method of this embodiment, in step S1, a surface inspection of the epitaxial wafer is performed, and in step S2, it is determined whether or not there are dislocation defects, including slip dislocation defects and misfit dislocation defects, on the surface of the epitaxial wafer. Dislocation defects DF, including slip dislocation defects and misfit dislocation defects, are observed as linear steps as shown in Figure 4, so they can be inspected visually, but if the surface inspection device 1 shown in Figure 2 is used, the inspection of dislocation defects can be performed more accurately.

Figure 2 is a configuration diagram showing an example of a surface inspection device 1 used in the surface inspection process of step S1 in Figure 1. The epitaxial wafer WF to be inspected is placed on a movable table 11, and the entire surface of the epitaxial wafer WF is irradiated with laser light by moving the movable table 11 on the XY plane. The laser light, which is the inspection light, is generated by a first laser oscillator 12 and a second laser oscillator 13, and the laser light generated by the first laser oscillator 12 is irradiated as vertical incident light onto the surface of the epitaxial wafer WF, and the laser light generated by the second laser oscillator 13 is irradiated as oblique incident light onto the surface of the epitaxial wafer WF. Although not particularly limited, it is preferable to inspect scratches, epitaxial defects, defects within a film, etc. using vertical incident light, and it is preferable to use oblique incident light for high sensitivity measurement, measurement of wafers with high roughness, haze, etc.

The reflected light of the laser light irradiated onto the surface of the epitaxial wafer WF is received by the first photomultiplier tube 15 via the ellipsoidal shaped collector 14, and is simultaneously received by the second photomultiplier tube 16 via the lens 16 and mirror 16b. Although not particularly limited, the laser light received by the first photomultiplier tube 15 is preferably used to measure minute particles and haze, and the laser light received by the second photomultiplier tube 16 is preferably used to measure linear defects, scratches, COPs (void defects), epitaxial defects, etc.

When the surface of an epitaxial wafer WF is scanned using the surface inspection device 1 of this embodiment, the reflected laser light from the mirror-like surface is received as is, but if there is any step such as a foreign matter or pits on the surface, the laser light becomes scattered light and is received by the first photomultiplier tube 15 and the second photomultiplier tube 16. This makes it possible to determine whether or not there are dislocation defects DF, including slip dislocation defects and misfit dislocation defects.

Returning to FIG. 1, if the surface inspection of steps S1 and S2 determines that there are no linear defects, the process proceeds to step S5, where it is determined that there are no slip dislocation defects or misfit dislocation defects. On the other hand, if the surface inspection of steps S1 and S2 determines that there are linear defects, the process proceeds to step S3, where the residual stress of the epitaxial wafer is inspected by wafer stress measurement. FIG. 3 is a configuration diagram showing an example of a wafer stress measurement device 2 (SIRD (Scanning InfRed Depolarization) measurement device) used in the residual stress inspection process of step S3 in FIG. 1.

The wafer stress measurement device 2 of this embodiment includes an infrared laser oscillator 21, and infrared light emitted from the infrared laser oscillator 21 passes through a lens 22 and enters a polarizer 23. The infrared light is linearly polarized by the polarizer 23 and then enters the epitaxial wafer WF to be inspected approximately perpendicularly. If the epitaxial wafer W is distorted, the infrared light that enters the epitaxial wafer WF is circularly polarized and attenuated in accordance with the distortion.

The infrared light transmitted through the epitaxial wafer WF passes through the objective lens 24 and enters the polarization separation element 25. The infrared light is separated by this polarization separation element 25 into a linearly polarized component and a circularly polarized component at the distorted portion of the epitaxial wafer WF. The separated infrared light components enter separate photodiodes 26 and 27.

Each photodiode 26, 27 detects the intensity of the incident infrared light component and outputs the detection result to the processing unit 28. Based on the input intensity, the processing unit 28 detects the difference between the linearly polarized component and the circularly polarized component as the amount of distortion. This process of detecting the amount of distortion is performed by rotating the epitaxial wafer WF and changing the radial position of the epitaxial wafer WF irradiated with infrared light, thereby detecting the amount of distortion at each position on the surface of the epitaxial wafer WF. In this detection process, the waveform of the detected amount of distortion (distortion waveform) has a waveform as shown in FIG. 5A. This distortion waveform includes distortion components caused by the dopant concentration of boron, phosphorus, etc. in the epitaxial wafer WF and distortion components caused by the overall warping of the epitaxial wafer WF.

Therefore, in order to extract localized distortion components from the distortion waveform, the processing unit 28 performs a filtering process, i.e., a floating average difference process, to remove long-wavelength distortion components corresponding to the dopant concentration and distortion components due to the overall warping of the wafer from the distortion waveform. When the floating average difference process is performed on the distortion waveform shown in Figure 5A, a distortion waveform from which the long-wavelength components have been removed is obtained, as shown in Figure 5B.

Next, the processing unit 28 uses the GBA method to find the relative strain amount based on the strain waveform. That is, the processing unit 28 applies a predetermined grid to the entire surface of the wafer W, and detects the relative strain amount that indicates the ratio of the number of cells in which strain exceeding a predetermined threshold occurs to the number of all cells in a predetermined area. Figure 5C shows the grid positions in the strain waveform, and the gaps between the grids indicate the cell strain.

The grid may be a Cartesian coordinate grid (X-Y grid) or a polar coordinate grid (R-T grid). In this embodiment, for example, a grid consisting of 1 mm x 1 mm cells is used, and the relative distortion amount is detected for an area that is, for example, a 5 mm wide annular area on the outermost periphery of the wafer W excluding a 0.5 mm wide annular area on the outermost periphery.

By using such a wafer stress measuring device 2, it is possible to obtain a relative strain amount indicating the residual stress of the epitaxial wafer WF. Returning to FIG. 1, in step S4, it is determined whether the obtained residual stress is equal to or greater than a preset residual stress threshold value _S0 . This residual stress threshold value _S0 can be obtained in advance, for example, as follows. That is, using epitaxial wafers containing dislocation defects manufactured under various manufacturing conditions, their residual stresses are measured, and then the cross sections are measured and observed with an X-ray measuring device to identify whether each dislocation defect is a slip dislocation defect or a misfit dislocation defect, and the residual stress value that is the identification boundary of this identified epitaxial wafer is set as the threshold value _S0 .

As described above, slip dislocations are generated due to thermal or physical stress, whereas misfit dislocations are generated due to crystal lattice constants unrelated to stress. Therefore, in step S4, if the residual stress found is equal to or greater than a predetermined threshold value _S0 , it is determined that the defect is a slip dislocation caused by stress (step S6), and if the residual stress is less than the predetermined threshold value _S0 , it is determined that the defect is a misfit dislocation caused by a crystal lattice constant unrelated to stress (step S7).

As described above, according to the epitaxial wafer inspection method of this embodiment, when a linear defect that appears to be a dislocation defect is observed on the surface of the wafer, it is possible to identify whether this is a slip dislocation defect or a misfit dislocation defect by non-destructive testing, thereby shortening the time from discovery of the dislocation defect to dealing with it. As a result, the operating rate and product yield of the epitaxial wafer manufacturing process can be improved.

Identifying Wafer Stress and Dislocations
Epitaxial wafers were manufactured under conditions 1 to 4, which have different levels of heater heating conditions in the epitaxial growth furnace, and when the presence or absence of dislocations was inspected, linear dislocation defects were observed in all cases. Therefore, the residual stress of each epitaxial wafer was measured using wafer stress measurement device 2. The results are shown in Table 1. The residual stress is a normalized value with the residual stress value under condition 1 set to 1.

Next, epitaxial wafers were manufactured using silicon wafers with reduced impurity concentrations, without changing the above conditions 1 to 4. The presence or absence of dislocations in these epitaxial wafers was observed, and the respective residual stresses were measured using wafer stress measurement device 2. The results are shown in Table 2. The residual stresses are normalized by setting the residual stress value under condition 1 to 1.

The results in Tables 1 and 2 reveal the following. First, the dislocation defects in Table 1 observed in the epitaxial wafers under conditions 1 and 2 were no longer observed by adjusting the impurity concentration as shown in Table 2, and therefore can be determined to be misfit dislocation defects. In addition, the dislocation defects in Table 1 observed in the epitaxial wafers under conditions 3 and 4 were observed even when the impurity concentration was adjusted as shown in Table 2, and therefore can be determined to be slip dislocation defects.

As shown in the residual stresses in Tables 1 and 2, it can be understood that the residual stresses of the epitaxial wafers under Conditions 3 and 4, which are considered to be slip dislocation defects, are significantly larger than the residual stresses of the epitaxial wafers under Conditions 1 and 2, which are considered to be misfit dislocation defects. Therefore, in step S4 of Fig. 1, if the residual stress found is equal to or greater than a predetermined threshold value _S0 , it is appropriate to determine that the defect is a slip dislocation defect caused by stress (step S6), and if the residual stress is less than the predetermined threshold value _S0, it is appropriate to determine that the defect is a misfit dislocation defect caused by a crystal lattice constant unrelated to stress (step S7).

Reference Signs List 1: Surface inspection device 11: Movable table 12: First laser oscillator 13: Second laser oscillator 14: Condenser 15: First photomultiplier tube 16: Second photomultiplier tube 16a: Lens 16b: Mirror 2: Wafer stress measurement device 21: Infrared laser oscillator 22: Lens 23: Polarizer 24: Objective lens 25: Polarization separation element 26, 27: Photodiode 28: Processing section WF: Epitaxial wafer DF: Dislocation defect

Claims (2)

When the epitaxial wafer has dislocation defects including slip dislocation defects and misfit dislocation defects, the epitaxial wafer is irradiated with infrared light, a change in polarization state caused by a photoelastic effect is analyzed, and a residual stress in the epitaxial wafer is obtained by a wafer stress measurement that measures the distortion of a portion subjected to stress ;
An epitaxial wafer inspection method, comprising: determining that the defect is a slip dislocation defect when the residual stress is equal to or greater than a predetermined value; and determining that the defect is a misfit dislocation defect when the residual stress is less than the predetermined value. The epitaxial wafer inspection method according to claim 1, in which an inspection light is irradiated onto the inspection surface of the epitaxial wafer, and the presence or absence of dislocation defects, including slip dislocation defects and misfit dislocation defects, is determined based on the scattered light.

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