CN117538341B - Defect detection based on tunable wavelength laser source induced second harmonic - Google Patents
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Abstract
The invention provides a defect detection scheme based on tunable wavelength laser source induced second harmonic, which solves the problem of limitation of detection range caused by different detection requirements of materials with different forbidden bandwidths and is a barrier to realizing the scheme. The invention has the advantages of wide application range. The dual-light source system not only can accurately measure the energy band structure of the material, but also can pertinently adjust the wavelength of the excitation light, so that the applicable material range of the invention is wider. Second, the measurement parameters are increased. The double light source structure can still use the detection light to detect the recombination of electrons after the excitation light is turned off, and can observe the recombination process, so that the service life and diffusion length of the carriers can be analyzed. Finally, by ensuring that the energy distribution of the area detected by the detection light spot is uniform, the measurement accuracy is improved. The invention improves the comprehensiveness and accuracy of sample detection and has great promotion to the existing second harmonic detection system.
Description
Technical Field
The invention relates to a defect detection technology of a semiconductor material, which mainly uses tunable wavelength laser to induce second harmonic waves to measure or monitor interface characteristics or oxide layer quality of the semiconductor material.
Background
The second harmonic is a nonlinear effect, meaning that under certain conditions a material can emit light at twice the frequency of the incident light. Many papers have found that the second harmonic has higher sensitivity in quality detection of the surface/interface where the inversion symmetry of the intrinsic material is broken, and has unique application value as a qualitative analysis method in the field of quality detection based on silicon-based semiconductors. The current second harmonic measurement method uses a laser source with fixed wavelength to excite the second harmonic, and has the greatest advantages of convenient light path design, less parameters to be regulated, easy measurement and data analysis completion, but the method has the defects of limitation of application fields. This is because when the forbidden bandwidth of the measured material is wider, more photons are required to be absorbed by the bound electrons to ionize, which can cause a great decrease in the conversion efficiency of the second harmonic signal, a decrease in the signal strength, and an influence on the accuracy of the measurement result.
The present invention relates in part to patent application number 202210328447.6, a method and apparatus for measuring a semiconductor multilayer structure based on a second harmonic, hereinafter referred to as patent 1.
Disclosure of Invention
The fundamental problem solved by the invention is the problem of limitation of detection range caused by different detection requirements of materials with different forbidden bandwidths, and therefore, the invention provides a second harmonic measurement technology based on a tunable wavelength laser source and solves the obstacle for realizing the technology. The difficulties encountered in implementing this technique, addressed by the present invention, are specifically set forth below.
If the fixed wavelength laser source is directly replaced by the tunable wavelength laser source, the wavelength of the second harmonic signal received will also change with the wavelength of the incident light because the wavelength of the second harmonic generated by the sample to be measured is half of the wavelength of the incident light. Because the wavelength has a large variation range, the design of the outgoing light path and the theoretical calculation of the signal curve are also changed accordingly, which makes it difficult to ensure the stability of the light path and the accuracy of the measurement calculation.
Therefore, the invention proposes a dual light source structure (i.e. a combination of an excitation light source and another detection light source of tunable wavelength for inducing the second harmonic, the wavelength variation range being different when the wavelengths of both light sources are tunable, and the wavelength of the detection light source being kept constant during the measurement after tuning) to ensure that the wavelength of the collected second harmonic is fixed. The optical path design of the dual light source structure is also difficult in the implementation process. First, the spot calibration problem of the dual light source is the problem. Because the incident light paths of the double light sources are not coincident, the light spots of the double light sources are difficult to fully coincide; secondly, the size and intensity density of the light spot can change along with the wavelength adjustment of the laser, which also makes it more difficult to keep the light spot coincident, and the change of the intensity density can increase the difficulty of second harmonic analysis. In addition, since the current second harmonic theory knowledge is mainly directed to the case of inducing the second harmonic by the fixed wavelength laser source, no deep research has been made on how to induce the second harmonic by the variable wavelength excitation light and how to process the data. In the existing patent or literature, laser light emitted by a single laser is generally divided into two light beams to be incident on the surface of a sample, electron hole pairs are excited by the light beam with higher power, and the second harmonic is excited by the light beam with lower power. Although the scheme reduces the difficulty in the light path design, the scheme has no difference from a single light source to excite the second harmonic, and the meaning of the double-incidence light path is lost. The method utilizes a low-power detection light source to reach saturation quickly, so that the measurement of an initial value is realized; in fact, however, the measurement of this initial value can also be achieved using a single light source of lower power. During the measurement of the initial value, the redistribution of charge occurs soon to saturation. Thus, when measuring with a larger power, a time-varying second harmonic signal is seen. However, the two-harmonic of the dual light source provided by the invention does not induce charge redistribution by utilizing photons with lower energy, so that the initial value of the two-harmonic is not changed, and therefore, no matter how high-power detection light source is used, the two-harmonic which changes with time is not generated.
To overcome the aforementioned difficulties, to achieve a second harmonic measurement based on a tunable wavelength laser source, the present patent presents embodiments from the following five aspects, respectively:
In a first aspect, an embodiment of the present invention provides a defect detection device for inducing a second harmonic based on a tunable wavelength laser source:
the light source of the device comprises a detection light source and an excitation light source;
The detection light source emits detection incident light; the wavelength of the detection incident light is the detection incident wavelength; the detection light path of the detection light source comprises a converging lens, and the converging lens is used for adjusting the appearance of a light spot of the detection incident light;
The excitation light source is a tunable wavelength laser source, and emits excitation incident light; the wavelength of the excitation incident light is the excitation incident wavelength; the photon energy of the excitation incident light is equal to or greater than the energy required for ionization of the electrons of the semiconductor material and not less than 1/3 of the minimum energy that the electrons need to overcome from the valence band of the semiconductor material to the conduction band of the insulating material; the excitation light source comprises a shaper in an excitation incident light path for homogenizing the excitation incident light; the excitation light spot formed by the excitation light source should be capable of covering the detection light spot emitted by the detection light source;
The incident paths of the detection incident light and the excitation incident light are not coincident;
the signal receiving system is used for independently receiving the detection second harmonic signal excited by the detection incident light.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic:
the photon energy of the detected incident light is lower than the forbidden bandwidth of any material in the detected sample.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic:
the detection light source also comprises a 1/2 wave plate in the detection incident light path for changing the polarization direction of the detection incident light; the second harmonic signal is separated into a polarized signal P and a polarized signal S through a spectroscope; the signal receiving system comprises a signal receiving system P and a signal receiving system S, which respectively receive the polarized signal P and the polarized signal S.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic:
the incident angle of the excitation incident light is 0 DEG
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic: the excitation incident light path also comprises an optical compensator.
In a second aspect, an embodiment of the present invention proposes a fixed wavelength second harmonic measurement method for a sample with a known material forbidden bandwidth using the tunable wavelength laser source-induced second harmonic-based defect detection apparatus according to the first aspect, the fixed wavelength second harmonic measurement method comprising:
The first stage: turning on only the detection light source so that the detection second harmonic signal received by the signal receiving system is constant, which is called an initial state;
And a second stage: the detection light source is kept on, the excitation light source is turned on, the required excitation incident wavelength is determined according to the forbidden bandwidth of the sample to be detected, the excitation incident wavelength is kept unchanged, at the moment, the charge distribution of the sample changes along with the change, and meanwhile, the detection second harmonic signal received by the signal receiving system also changes correspondingly and is called an excited state; when the charge distribution eventually reaches a new dynamic balance, the detected second harmonic signal also eventually stabilizes.
In some embodiments, the present invention provides a method for measuring a fixed wavelength second harmonic of a sample with a known material forbidden bandwidth using the tunable wavelength laser source-induced second harmonic-based defect detection apparatus according to the first aspect, the method further comprising:
And a third stage: keeping the detection light source on, turning off the excitation light source, and returning the captured electrons to the substrate layer again to be compounded, wherein the detection second harmonic signal received by the signal receiving system is gradually recovered and is called a recovery state; when the recovery time is long enough, the detected second harmonic signal is recovered to the initial state.
In a third aspect, an embodiment of the present invention proposes a tunable wavelength second harmonic measurement method for a sample with unknown material forbidden bandwidth using the defect detection device based on tunable wavelength laser source induced second harmonic according to the first aspect, where the tunable wavelength second harmonic measurement method includes:
The first stage: turning on only the detection light source so that the detection second harmonic signal received by the signal receiving system is constant, which is called an initial state; keeping the detection incident wavelength constant;
and a second stage: the excitation incident wavelength is tunable during the test, decreasing from long to short; keeping the detection light source on and simultaneously turning on the excitation light source; when the minimum forbidden bandwidth of the sample is the same as the photon energy of the excitation light source, the detected second harmonic signal undergoes critical change, namely the excitation incident wavelength at the moment is called as the forbidden bandwidth wavelength, and the excitation incident wavelength is kept unchanged; when the charge distribution finally reaches new dynamic balance, the detected second harmonic signal also finally tends to be stable;
And a third stage: selecting more than two wavelengths near the forbidden bandwidth wavelength, namely test forbidden bandwidth wavelength, sequentially setting the excitation incident wavelength as each test forbidden bandwidth wavelength, measuring according to the second harmonic measurement method as described in the second aspect, and receiving the detection second harmonic signal to obtain a test result under successive test forbidden bandwidth wavelengths; after each setting of the excitation incident wavelength, the size and the light intensity density of the excitation light spot are ensured to be the same;
The following processing is performed on the obtained successive specific wavelength test results:
The detected second harmonic signal curve for this excited state is fitted using equation 1 as follows:
Wherein a 0,a1,a2 is a constant; τ 1 is the time variable corresponding to the electron excitation process; τ 2 is the time variable corresponding to the hole excitation process; i 2ω is the intensity density of the detected second harmonic;
The factor affecting 3 1 can be described by equation 2 as follows:
wherein, Energy of photons of the excitation incident light; n is a natural number, which is used to represent the number of photons absorbed in the multiphoton absorption process; k n is the n photon absorption coefficient; i ω is the intensity density of the excitation incident light;
The data of the corresponding group number can be obtained by the formula 2 Obtaining a data graph of the number of absorbed photons and the energy of the corresponding photons, and selecting the nearest integer from the data graph as n 1;
The minimum energy that the electron needs to overcome to reach the conduction band of the insulating material from the valence band of the semiconductor material can be expressed by equation 3 as follows:
Egap＝n1E1
The detected second harmonic signal curve for this recovery state is fitted using equation 4 as follows:
Where a 4,a5 is a constant and τ 3 is a time constant associated with charge recombination.
In a fourth aspect, an embodiment of the present invention provides an electronic device, including:
At least one processor; and
A memory communicatively coupled to the at least one processor; wherein,
The memory stores instructions executable by the at least one processor to enable the at least one processor to perform the methods of the second and third aspects.
In a fifth aspect, embodiments of the present invention provide a non-transitory computer-readable storage medium storing computer instructions for causing a computer to perform the methods of the second and third aspects.
The beneficial effects of the invention are as follows:
the invention provides a measuring device and a measuring method based on tunable wavelength laser-induced second harmonic aiming at sample defect detection. The invention has great improvement compared with the prior single light source excited second harmonic system.
Firstly, the application range is improved. Because the laser energy of the single light source is fixed, the applicable material is necessarily limited, and the double-light source system not only can accurately measure the energy band structure of the material, but also can pertinently adjust the wavelength of the excitation light, so that the applicable material range is wider.
Second, the measurement parameters are increased. In the single light source structure, after the laser is turned off, the electrons are still compounded, but the compounding process cannot be detected. The double-light source structure can still use the detection light to detect the excitation light after the excitation light is turned off, and can observe the recombination process of the excitation light, so that the service life and the diffusion length of the current carrier can be analyzed.
Finally, the morphology of the excitation light spot is optimized from a Gaussian light spot to a flat-top light spot, and the diameter of the circular excitation light spot is larger than the long axis of the elliptical detection light spot, so that the excitation light spot completely covers the detection light spot, the energy distribution of the detected area of the detection light spot can be ensured to be uniform and consistent, the assumption premise in a theoretical formula is more met, and the measurement accuracy is improved.
In summary, the measuring method and the measuring device provided by the invention improve the comprehensiveness and the accuracy of sample detection, and have great promotion to the existing second harmonic detection system.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following description will briefly explain the drawings used as needed in the embodiments or the description of the prior art. However, it should be understood by those skilled in the art that the drawings in the following description are only some examples of the present application and do not limit the scope thereof.
FIG. 1 is a schematic diagram of a defect detection apparatus based on tunable wavelength laser source induced second harmonic according to an embodiment of the present invention.
FIG. 2 is a schematic diagram showing the effect of homogenizing the excitation incident light of an embodiment of the defect detection apparatus of the present invention based on tunable wavelength laser source induced second harmonic.
FIG. 3 is a schematic diagram of an optical path design of a defect detection device based on tunable wavelength laser source induced second harmonic according to an embodiment of the present invention.
FIG. 4 is a flow chart of a method for measuring fixed wavelength second harmonic of a sample with known material forbidden bandwidth using a tunable wavelength laser-based defect detection apparatus according to the first aspect of the present invention.
Fig. 5, which includes fig. 5a, 5b, 5c, 5d, 5e, and 5f, is a schematic diagram of internal charge distribution variation of a specific embodiment of a fixed wavelength second harmonic measurement method of a sample with known material bandgap using a tunable wavelength laser-induced second harmonic based defect detection device as described in the first aspect and a tunable wavelength second harmonic measurement method of a sample with unknown material bandgap using a tunable wavelength laser-induced second harmonic based defect detection device as described in the first aspect.
Fig. 6A and 6B are schematic diagrams of measurement data curves of a specific embodiment of a fixed wavelength second harmonic measurement method of a sample with a known material bandgap of a defect detection device based on tunable wavelength laser source induced second harmonic according to the first aspect of the present invention and a tunable wavelength second harmonic measurement method of a sample with a known material bandgap of a defect detection device based on tunable wavelength laser source induced second harmonic according to the first aspect of the present invention.
FIG. 7 is a flow chart of a method for measuring tunable wavelength second harmonic using a sample with unknown material forbidden bandwidth based on a defect detection apparatus with tunable wavelength laser induced second harmonic according to the first aspect of the present invention.
Fig. 8 is a schematic diagram of a probability change during excitation of a specific embodiment of a tunable wavelength second harmonic measurement method of the present invention using a defect detection device based on a tunable wavelength laser source for inducing a second harmonic with an unknown material bandgap.
Fig. 9 is a schematic diagram of a plurality of curves obtained by performing a plurality of fixed wavelength measurements according to the scheme described in the second aspect according to a specific embodiment of the tunable wavelength second harmonic measurement method of the present invention for a sample with unknown material forbidden bandwidth using the defect detection device based on tunable wavelength laser source induced second harmonic according to the first aspect.
FIG. 10 is a graph of the absorption photon count versus the photon energy of the excitation incident light for a specific embodiment of a tunable wavelength second harmonic measurement method of the present invention using a sample with unknown material forbidden bandwidth for a tunable wavelength laser source induced second harmonic based defect detection apparatus as described in the first aspect.
FIG. 11 is a schematic diagram of a defect detection apparatus based on tunable wavelength laser source induced second harmonic according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. It will be appreciated by those of ordinary skill in the art that the embodiments described are some, but not all, of the embodiments of the application. Based on the embodiments in the present application, any suitable modification or variation may be made by those skilled in the art to obtain all other embodiments.
In a first aspect, an embodiment of the present invention provides a defect detection device for inducing a second harmonic based on a tunable wavelength laser source:
the light source of the device comprises a detection light source and an excitation light source;
The detection light source emits detection incident light; the wavelength of the detection incident light is the detection incident wavelength; the detection light path of the detection light source comprises a converging lens, and the converging lens is used for adjusting the appearance of a light spot of the detection incident light;
The excitation light source is a tunable wavelength laser source, and emits excitation incident light; the wavelength of the excitation incident light is the excitation incident wavelength; the photon energy of the excitation incident light is equal to or greater than the energy required for ionization of the electrons of the semiconductor material and not less than 1/3 of the minimum energy that the electrons need to overcome from the valence band of the semiconductor material to the conduction band of the insulating material; the excitation light source comprises a shaper in an excitation incident light path for homogenizing the excitation incident light; the excitation light spot formed by the excitation light source should be capable of covering the detection light spot emitted by the detection light source;
The incident paths of the detection incident light and the excitation incident light are not coincident;
the signal receiving system is used for independently receiving the detection second harmonic signal excited by the detection incident light.
In this embodiment, as shown in fig. 1, the defect detecting device is schematically configured. The light source of the device comprises the detection light source and the excitation light source. Wherein the excitation light source is the tunable wavelength laser source on which the device is based. By "excitation" is meant that photons injected by the excitation incident light within the detection region are absorbed by electrons, so that bound electrons in the valence band of the sample semiconductor layer are likely to gain sufficient energy to be excited into the semiconductor conduction band as free electrons. The electrons may be trapped by interface state defects or absorb more photons with sufficient energy to reach the oxide layer across the barrier, eventually forming a large charge build-up at the interface or surface of the sample, a process known as electron build-up. This process can be "detected" by the second harmonic. "detecting" means that the detected incident light from the detection light source is coupled to a structure having non-central symmetry to generate a second harmonic signal, and the lattice defect (such as interface state, fixed charge, impurity atom, etc.) is usually a coupling center, so that the generated detected second harmonic signal can be used to characterize the lattice defect density of the sample. The technical scheme of the embodiment uses double light sources to respectively realize the two functions of excitation and detection.
In practical measurement, the probe light source should be required not to change the charge distribution inside the sample to be measured, so as to faithfully measure the real situation of the semiconductor material; the excitation light source should be able to be wavelength tunable to ensure that the valence band electrons absorb one photon, i.e. have sufficient energy to transition into the conduction band.
In order to meet the foregoing functional requirements, the technical solution of this embodiment requires: 1) The photon energy of the detected incident light during defect detection should be lower than the energy required for ionization of the electrons of the semiconductor material; 2) The photon energy of the excitation incident light is equal to or greater than the energy required by the electron ionization of the semiconductor material, for example, the photon energy of the excitation incident light is equal to or greater than the forbidden band width of the semiconductor substrate, so that the excitation incident light can ionize bound electrons in the semiconductor layer to form electron hole pairs, and in particular, if the forbidden band width of the semiconductor material is 2.2eV, excitation light with wavelength less than 564nm can be selected; 3) The photon energy of the excitation incident light is not less than 1/3 of the minimum energy required by the electrons from the valence band of the semiconductor material to the conduction band of the insulating material, so as to ensure that the photon absorption process can excite electrons in the semiconductor layer into the oxide layer, and the electrons in the semiconductor layer can enter the insulating layer as much as possible under the condition of not ionizing the electrons in the insulating layer.
The detection light source is typically a fixed wavelength laser, or at least a tunable wavelength laser that remains fixed in wavelength during detection, to avoid frequent adjustments to the detection light path due to wavelength changes. And the detection incident wavelength is generally longer, so that the photon energy is lower, and the energy required by the ionization of the electrons of the semiconductor material is lower, or not far higher than the forbidden bandwidth of the semiconductor material. When the detection wavelength is larger than 563nm, electrons of the semiconductor layer can be ionized only by absorbing at least two photons, so that the influence of the detection light source on the detected sample is reduced. It is emphasized here that even if the detection light source is a tunable wavelength laser source, its tunable wavelength should be as long as possible and the tunable range should be small. The wavelength is as long as possible in order to make its photon energy low. The tunable range is small, and frequent adjustment of the detection light path can be avoided. In order to minimize the influence of the detection light source, it is preferable that the detection light source uses a 1030nm laser.
To make the device applicable to a wider range of materials, a laser with a wider tuning range can be selected as the excitation light source. Preferably, detection of materials of different forbidden bandwidths can be achieved by adjusting the range of excitation incident wavelengths, for example using the laser with an adjustment range of [390nm,1080nm ].
In order to adjust the shape of the light spot of the detection incident light, the detection incident light path comprises the converging lens. The appearance of the light spot after adjustment can meet the detection requirement. The shaper in the excitation incident light path is used for homogenizing the excitation incident light. The intensity change of the excitation incident light after adjustment is shown in fig. 2. The left side of fig. 2 shows the original state of the excitation incident light, showing a gaussian light intensity distribution. The right side of fig. 2 shows the light intensity distribution of the excitation incident light after shaping, and the light intensity distribution is obviously more uniform, so that photons can be uniformly injected into the measurement area by the excitation light spot, so as to ensure that the charge change in the measurement area is consistent. Therefore, the excitation light spot formed by the excitation light source should be capable of covering the detection light spot emitted by the detection light source, and it is preferable that the detection light spot falls within a range where the intensity of the excitation light spot is relatively uniform.
The incident paths of the probe incident light and the excitation incident light are different, and thus the generated second harmonic signals are also separated from each other. The signal receiving system is used for independently receiving the detection second harmonic signal excited by the detection incident light, so that the interference of the second harmonic signal excited by the excitation incident light is effectively avoided, and the measurement accuracy in detection is ensured.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic:
the photon energy of the detected incident light is lower than the forbidden bandwidth of any material in the detected sample.
In this embodiment, the photon energy of the incident light is lower than the energy required by the ionization of the electrons of the semiconductor material, and lower than the forbidden bandwidth of any material in the sample, so that the second harmonic signal excited by the incident light is avoided as much as possible.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic:
the detection light source also comprises a 1/2 wave plate in the detection incident light path for changing the polarization direction of the detection incident light; the second harmonic signal is separated into a polarized signal P and a polarized signal S through a spectroscope; the signal receiving system comprises a signal receiving system P and a signal receiving system S, which respectively receive the polarized signal P and the polarized signal S.
In this embodiment, a half-wave plate is added to the detection incident light path to adjust the polarization direction of the detection incident light. On the detection exit light path, not only the reflected light with the same wavelength as the detection incident light appears, but also the second harmonic wave with the wavelength half of the detection incident light is generated. The P-polarized signal and the S-polarized signal in the second harmonic can be separated by the spectroscope so as to be measured respectively, and the method can be applied more flexibly, such as being used for analyzing different characteristics of the measured material, performing noise reduction treatment and the like in patent 1. The beam splitter may also be a beam splitter group. In order to obtain better effect, a filter and a converging lens can be added before the spectroscope or the spectroscope group to obtain converging second harmonic. As shown in fig. 3, the optical path design of this embodiment is shown. The half wave plate is not shown in fig. 3, nor is the S polarization direction shown, since the S polarization direction is perpendicular to the paper surface. In order to receive the polarized signal P and the polarized signal S, respectively, the signal receiving system includes the signal receiving system P and the signal receiving system S.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic:
the incident angle of the excitation incident light is 0 DEG
In the present embodiment, preferably, the incident angle of the excitation incident light is 0 °. Therefore, the polarization direction of the second harmonic generated by the exciting incident light is always perpendicular to the surface of the sample of the semiconductor material, and as long as the incident angle of the detecting incident light is not 0 degrees, a certain inclination angle is maintained, the detecting second harmonic signal can be smoothly separated from the second harmonic generated by the exciting incident light, no interference occurs, and a better detection effect is obtained. Preferably, the angle of incidence of the detected incident light is 45 °, which may be cheaper in terms of installation, etc.
In some embodiments, the present invention provides a defect detection device based on tunable wavelength laser source induced second harmonic: the excitation incident light path also comprises an optical compensator.
In this embodiment, the optical compensator in the excitation light path is used to make the spot size of the excitation light consistent during the wavelength tuning process of the excitation light source, so as to obtain a more accurate measurement result. The specific principles and functions will be described hereinafter and will not be described in detail.
In a second aspect, an embodiment of the present invention proposes a fixed wavelength second harmonic measurement method for a sample with a known material forbidden bandwidth using the tunable wavelength laser source-induced second harmonic-based defect detection apparatus according to the first aspect, the fixed wavelength second harmonic measurement method comprising:
The first stage: turning on only the detection light source so that the detection second harmonic signal received by the signal receiving system is constant, which is called an initial state;
And a second stage: the detection light source is kept on, the excitation light source is turned on, the required excitation incident wavelength is determined according to the forbidden bandwidth of the sample to be detected, the excitation incident wavelength is kept unchanged, at the moment, the charge distribution of the sample changes along with the change, and meanwhile, the detection second harmonic signal received by the signal receiving system also changes correspondingly and is called an excited state; when the charge distribution eventually reaches a new dynamic balance, the detected second harmonic signal also eventually stabilizes.
In this embodiment, a fixed wavelength second harmonic measurement method of a sample with a known material forbidden bandwidth based on a defect detection device of a tunable wavelength laser source induced second harmonic is given. Specifically, the flow chart is shown in fig. 4, the schematic diagram of the internal charge distribution change is shown in fig. 5, and the measured data graph is shown in fig. 6.
In the first stage, only the detection light source is turned on, so that the detection second harmonic signal received by the signal receiving system is constant. Here, when the detection light source is a laser light of a fixed wavelength, the detection incident wavelength is naturally constant; when the detection light source is a tunable wavelength laser source, the detection incident wavelength should be tuned first so that the detection second harmonic signal received by the signal receiving system is constant, and then the detection incident wavelength is fixed so as to be kept constant. During detection, the photon energy of the detected incident light should be lower than the energy required for ionization of the electrons of the semiconductor material. Accordingly, in fig. 5a, since the photon energy of the detection light source is weak, a large number of electron-hole pairs cannot be excited in each layer, and thus the charge distribution inside the sample or the built-in electric field cannot be affected. The detection light source will generate a constant second harmonic signal, which can be characterized as an initial state, as shown in region i of fig. 6. And the signal is the result of the co-excitation of the interface state and the internal charge within the film. The second harmonic signal at this time can be used to characterize the properties of the material itself, such as second order polarizability, initial built-in electric field, etc. This is far more accurate than characterizing the initial state in a single light source light path with only the first measurement point in the data plot.
In the second stage, the detection light source is kept on, and meanwhile, the excitation light source is turned on and the excitation incident wavelength is kept unchanged. It is emphasized here that the photon energy of the excitation incident light is equal to or greater than the energy required for ionization of the electrons of the semiconductor material and not less than 1/3 of the minimum energy required for the electrons from the valence band of the semiconductor material to the conduction band of the insulating material, which condition should still be satisfied. Accordingly, in fig. 5b and 5c, when photons of the excitation light source penetrate the upper medium to reach the interface, electrons in the valence band of the space charge region (the region in the semiconductor layer close to the interface) have a probability of absorbing enough photon energy to transit into the conduction band to become free electrons and gradually accumulate at the interface. Wherein, a part of free electrons are transited into the conduction band of the upper medium after absorbing more photon energy, and the part of electrons move in the upper medium and are trapped by fixed charges or surface states with the double functions of concentration gradient and built-in electric field. The charge distribution of the sample changes throughout the process from ionization of the electrons to capture by the surface until a new dynamic equilibrium is eventually reached, i.e. the excitation speed of the electrons is the same as the recombination speed. During the secondary process, the built-in electric field changes, and the detection incident light generates the detection second harmonic signal which changes with time until the detection second harmonic signal finally stabilizes. The portion of the detected second harmonic signal variation shown in region ii, i.e., the excited state, is reflected in fig. 6. The excited state is the most complex and most information-reflecting stage, and the detected second harmonic signal can be in various forms of monotonic increment, monotonic decrement, increment-then-increment, and the like. The speed, the magnitude and the like of the trend change of the detected second harmonic signal are related to the thickness of a sample film layer, the forbidden bandwidth of a material, the frequency of photons of the excitation incident light and the like.
In some embodiments, the present invention provides a method for measuring a fixed wavelength second harmonic of a sample with a known material forbidden bandwidth using the tunable wavelength laser source-induced second harmonic-based defect detection apparatus according to the first aspect, the method further comprising:
And a third stage: keeping the detection light source on, turning off the excitation light source, and returning the captured electrons to the substrate layer again to be compounded, wherein the detection second harmonic signal received by the signal receiving system is gradually recovered and is called a recovery state; when the recovery time is long enough, the detected second harmonic signal is recovered to the initial state.
In this embodiment, a third stage of a fixed wavelength second harmonic measurement method of a sample with a known material forbidden bandwidth based on a defect detection device with a tunable wavelength laser source inducing a second harmonic is presented. The specific flow is shown in fig. 4, the schematic diagram of the internal charge distribution change is shown in fig. 5, and the measured data graph is shown in fig. 6.
In the third stage, the detection light source is kept on, and the excitation light source is turned off. Since the second stage is a dynamic process, electrons are accumulated until dynamic balance is reached, so that when the excitation light source is turned off, the dynamic balance is broken, and the trapped electrons return to the substrate layer again for recombination. In this process, the second harmonic signal generated by the detection light source is gradually restored to the initial state, as shown in fig. 5 f. Reflected in fig. 6, corresponds to the portion of the detected second harmonic signal variation shown in region iv.
It should be emphasized here that in fig. 6 there is also a final state, i.e. a critical state immediately after the end of the second phase and immediately before the start of the third phase. This phase may be steady state for a period of time as in region III in FIG. 6A or may be instantaneous as in point III in FIG. 6B. By stable, it is meant that the charge distribution inside the sample reaches a dynamic balance within the measurement time, and the built-in electric field is no longer changed. After this, even if the measurement time of the second stage is prolonged, no large change in the signal value occurs any more. By transient it is meant that under certain circumstances the charge dynamics inside the sample will appear at the end of the second phase, still not reach a steady state, after which if the measurement time is prolonged the second harmonic signal will still appear to trend over time, except that the last measurement point of the second phase is used to characterize the final state.
The initial state, the excited state, the final state, and the recovered state are four of the most interest in the overall process including the first stage, the second stage, and the third stage. By collecting and analyzing the detected second harmonic signal generated by the detected light source as shown in fig. 6, defects related to electrical characteristics inside the sample can be evaluated.
In a third aspect, an embodiment of the present invention proposes a tunable wavelength second harmonic measurement method for a sample with unknown material forbidden bandwidth using the defect detection device based on tunable wavelength laser source induced second harmonic according to the first aspect, where the tunable wavelength second harmonic measurement method includes:
The first stage: turning on only the detection light source so that the detection second harmonic signal received by the signal receiving system is constant, which is called an initial state; keeping the detection incident wavelength constant;
and a second stage: the excitation incident wavelength is tunable during the test, decreasing from long to short; keeping the detection light source on and simultaneously turning on the excitation light source; when the minimum forbidden bandwidth of the sample is the same as the photon energy of the excitation light source, the detected second harmonic signal undergoes critical change, namely the excitation incident wavelength at the moment is called as the forbidden bandwidth wavelength, and the excitation incident wavelength is kept unchanged; when the charge distribution finally reaches new dynamic balance, the detected second harmonic signal also finally tends to be stable;
And a third stage: selecting more than two wavelengths near the forbidden bandwidth wavelength, namely test forbidden bandwidth wavelength, sequentially setting the excitation incident wavelength as each test forbidden bandwidth wavelength, measuring according to the second harmonic measurement method as described in the second aspect, and receiving the detection second harmonic signal to obtain a test result under successive test forbidden bandwidth wavelengths; after each setting of the excitation incident wavelength, the size and the light intensity density of the excitation light spot are ensured to be the same;
The following processing is performed on the obtained successive specific wavelength test results:
The detected second harmonic signal curve for this excited state is fitted using equation 1 as follows:
Wherein a 0,a1,a2 is a constant; τ 1 is the time variable corresponding to the electron excitation process; τ 2 is the time variable corresponding to the hole excitation process; i 2ω is the intensity density of the detected second harmonic;
the factor affecting τ 1 can be described by equation 2 as follows:
wherein, Energy of photons of the excitation incident light; n is a natural number, which is used to represent the number of photons absorbed in the multiphoton absorption process; k n is the n photon absorption coefficient; i ω is the intensity density of the excitation incident light;
The data of the corresponding group number can be obtained by the formula 2 Obtaining a data graph of the number of absorbed photons and the energy of the corresponding photons, and selecting the nearest integer from the data graph as n 1;
The minimum energy that the electron needs to overcome to reach the conduction band of the insulating material from the valence band of the semiconductor material can be expressed by equation 3 as follows:
Egap＝n1E1
The detected second harmonic signal curve for this recovery state is fitted using equation 4 as follows:
Where a 4,a5 is a constant and τ 3 is a time constant associated with charge recombination.
In this embodiment, a tunable wavelength second harmonic measurement method of a sample with unknown material forbidden bandwidth based on a defect detection device with a tunable wavelength laser source inducing a second harmonic is provided. The forbidden band width is unknown, and may be truly unknown; it is also possible that the possible range is too large and therefore it is unknown that some semiconductor materials, such as SiC, have larger variations in the forbidden band width due to the more homogeneous isomers. For the semiconductor sample with unknown forbidden bandwidth, the tunable wavelength second harmonic measurement can well determine the forbidden bandwidth, realize corresponding parameter setting and finish defect detection.
Specifically, the flow chart is shown in fig. 7, the schematic diagram of the internal charge distribution change is shown in fig. 5, and the measured data graph is shown in fig. 6.
In the first stage, only the detection light source is turned on, so that the detection second harmonic signal received by the signal receiving system is constant. Here, when the detection light source is a laser light of a fixed wavelength, the detection incident wavelength is naturally constant; when the detection light source is a tunable wavelength laser source, the detection incident wavelength should be tuned first so that the detection second harmonic signal received by the signal receiving system is constant, and then the detection incident wavelength is fixed so as to be kept constant. During detection, the photon energy of the detected incident light should be lower than the energy required for ionization of the electrons of the semiconductor material. Because the photon energy of the detection light source is weak, a large number of electron hole pairs cannot be excited in each layer, and therefore the charge distribution inside the sample or a built-in electric field cannot be influenced. The detection light source will generate a constant second harmonic signal. And the signal is the result of the co-excitation of the interface state and the internal charge within the film. The second harmonic signal at this time can be used to characterize the properties of the material itself, such as second order polarizability, initial built-in electric field, etc.
In the second stage, the excitation incident wavelength is tunable in the test process, the excitation incident light is set to be the maximum wavelength, the detection light source is kept on, the excitation light source is turned on, the energy of the excitation incident light is weaker at this time, the detection second harmonic signal cannot change, and then the wavelength of the excitation light source is continuously reduced until the wavelength is minimum, so that the measurement is completed. In the measurement process, the ionization probability of the bound electrons of the measured sample is related to the forbidden bandwidth of the sample and the energy of photons of the excitation incident light, and the shorter the forbidden bandwidth is, the shorter the excitation incident wavelength is, and the larger the ionization probability is. The critical conditions for the probability of the excitation process to change are therefore: when the minimum forbidden bandwidth of the sample to be measured is the same as the photon energy of the excitation incident light, the second harmonic signal starts to change, as shown in fig. 8.
In the third stage, the data of the second stage are analyzed, the wavelength near the forbidden bandwidth wavelength is selected and sequentially set as the excitation incident wavelength, and the measurement of the fixed wavelength is completed according to the scheme of the second aspect of the invention, so as to obtain a complete curve as shown in fig. 6. However, it has been ensured in the first stage of the present embodiment that the detected second harmonic signal received by the signal receiving system is constant, and therefore in the course of performing the fixed wavelength measurement according to the scheme described in the second aspect, even if the detected light source is tunable, it is not necessary to tune, and the detected incident wavelength that has been kept constant in the first stage of the present embodiment is not required. In this process, after each setting of the wavelength of the excitation light, it should be ensured that the spot size and the intensity density of the excitation incident light are the same, and the means used may be to adjust the excitation incident light path simultaneously. The measurement result of this process is shown in fig. 9. Fig. 9 shows the results of a plurality of measurements, the specific number of times being the same as the number of wavelengths near the selected wavelength of the forbidden bandwidth. Fitting the excited states (II areas) of the multiple curves, wherein the fitting formula is as follows:
Wherein a 0,a1,a2 is a constant, which can be obtained by fitting; τ 1,τ2 is related to the excitation mechanism, also called time constant. Since electrons have much greater mobility than holes and require less absorbed photon energy for electron excitation than holes, the electron excitation process is faster than the hole excitation process, and τ 1<τ2. There are many factors that can influence this time constant τ 1、τ2. And (3) fitting τ 2 to obtain the product. Tau 1 can be used for representing the electrical characteristics of the semiconductor material to be detected, and is an important detection content in the detection method of the invention; the influencing factor for tau 1 can be described by equation 2 below,
Wherein,Energy of photons of the excitation incident light; n is the number of photons absorbed in the multiphoton absorption process and is a natural number; k n is the n photon absorption coefficient; i ω is the intensity density of the excitation incident light.
When the intensity densities of the excitation light in different wavebands are similar or even the same, the relationship between the number of absorbed photons and the photon energy of the excitation incident light can be obtained by fitting the specific wavelength test result through the formula 1 and combining the formula 2, as shown in fig. 10. In fig. 10, the X-axis represents the photon energy of the excitation light, and the Y-axis represents the number of absorbed photons; when the photon energy is less than E 1, at least n 1 photons are required (n 1 should be the nearest integer in the figure. That is, the forbidden band width of the sample, i.e., the minimum barrier that electrons on the valence band in the semiconductor layer need to overcome to reach the insulating layer is:
E gap＝n2E2 (equation 3)
The energy level structure of the sample is known and the excitation incident wavelength with photon energy greater than 1/3E gap can be selected for fixed wavelength second harmonic measurement to obtain the detected second harmonic data plot shown in FIG. 6.
It should be noted that, in order to implement the measuring method described herein, and in the foregoing and the following, it is necessary to maintain the light intensity density stable. To achieve this, it is first of all necessary to determine that in the measurement, the two spots are to coincide, or that the excitation spot is to fully cover the detection spot, and that in the measurement the intensity density of the two spots does not change significantly. This is not difficult to achieve for the probe light source where the wavelength is fixed during the measurement. The excitation light source with variable wavelength needs to have an adjusting function, because the size, the intensity density, etc. of the excitation light spot can change along with the change of the wavelength. Therefore, in the optical path design, the shaper and the optical compensator need to be used on the excitation optical path. The functions of the shaper and the optical compensator have been described above. Here, when the change in the excitation incident wavelength is small, the change in the photon energy of the excitation incident light is also small, and the influence on the measurement result is not large. However, if desired to obtain a better measurement result, the combination of the shaper and the optical compensator should be used simultaneously to ensure that the intensity density of the excitation spot remains consistent during adjustment of the excitation incident wavelength.
To characterize the electrical properties of the sample under test, the recovery state is fitted using equation 4 as follows:
the time constant τ 3 is inversely proportional to the defect density inside the sample and the cross section of the recombination center, so that the value can also be used as a parameter for measuring the defects of the sample, and can be obtained by fitting for measuring the service life, diffusion length and the like of carriers. Similarly, constants a 4 and a 5 can also be obtained by fitting.
It should be noted that the final object of the present invention is to detect electrically related defects in the sample structure, such as interface states (i.e., dangling bonds) near the interface of the sample, and various charge defects such as mobile charges, trapped charges, fixed charges, etc. existing inside the film layer. These several drawbacks, while generating second harmonics, are different in their mechanism of generation. The interface states at the interface are of two types, donor-type interface states and acceptor-type interface states. Both interface states have no electrical property in the empty state, but the donor-type interface state can lose one electron to cause the positive electrical property at the place, and the acceptor-type interface state can capture one electron to cause the electronegativity at the place. The density of interface states can thus be calculated using a model of the electric dipole generating the second harmonic. The charges in the film layer have electrical property and can generate a strong built-in electric field, so that a model of generating second harmonic by using electric quadrupoles can be used for calculating the density of interface states. The technical scheme provided by the invention is an alternative scheme with better effect and more convenient use.
In order to further explain the structure of the defect detection device of the present invention that induces the second harmonic based on the tunable wavelength laser source and the use in combination with patent 1, a description will be given again with reference to fig. 11. Fig. 11 is a view showing the light path of the excitation light of the present invention, in comparison with fig. 10 of patent 1, with the addition of an excitation light source (70), a collimator lens group (80), a reflecting mirror (90), and a light shaper (100). The rest, still the same as that represented in fig. 10 of patent 1: the detection light source is emitted by a laser (10), reaches the sample (90) through a polarizer (20) and a collimating lens group (30), excites second harmonic wave and generates reflected light; after passing through the collimating lens group (40) and the filter (50), the reflected light is filtered, and only the excited second harmonic wave generates the second harmonic wave of two polarization directions through the beam splitting prism (60); wherein the second harmonic in the P direction is directly received by the signal receiver (110) after passing through the beam-splitting prism (60), and the second harmonic in the S direction is received by the other signal receiver (110) after passing through the beam-splitting prism (60) through the reflecting side of the reflecting mirror (51). During the measurement, the information of the measured area is monitored by a transmission electron microscope (80) and a lens group (81,70), and the height, coordinates and moving speed of the sample are controlled by the carrier (100); all information, including measurement data, is ultimately collected for processing by the central processing system (120).
The foregoing describes in detail preferred embodiments of the present invention. It should be understood that numerous modifications and variations can be made in accordance with the concepts of the invention without requiring creative effort by one of ordinary skill in the art. Therefore, all technical solutions which can be obtained by logic analysis, reasoning or limited experiments based on the prior art by the person skilled in the art according to the inventive concept shall be within the scope of protection defined by the claims.
Claims (10)
1. A defect detection device based on tunable wavelength laser source induced second harmonic is characterized in that:
the light source of the device comprises a detection light source and an excitation light source;
The detection light source emits detection incident light; the wavelength of the detection incident light is the detection incident wavelength; the photon energy of the detected incident light should be lower than the energy required for electron ionization of the semiconductor material; the detection light path of the detection light source comprises a converging lens, and the converging lens is used for adjusting the appearance of the light spot of the detection incident light;
the excitation light source is a tunable wavelength laser source and emits excitation incident light; the wavelength of the excitation incident light is the excitation incident wavelength; the photon energy of the excitation incident light is equal to or greater than the energy required for ionization of electrons of the semiconductor material and not less than 1/3 of the minimum energy that the electrons need to overcome from the valence band of the semiconductor material to the conduction band of the insulating material; the excitation light source comprises a shaper in an excitation incident light path for homogenizing the excitation incident light; the excitation light spot formed by the excitation light source should be capable of covering the detection light spot emitted by the detection light source;
the incidence paths of the detection incident light and the excitation incident light are not coincident;
the signal receiving system is used for independently receiving the detection second harmonic signal excited by the detection incident light.
2. The defect detection apparatus for inducing second harmonic based on tunable wavelength laser source according to claim 1, wherein: the photon energy of the detected incident light is lower than the forbidden bandwidth of any material in the detected sample.
3. The defect detection apparatus for inducing second harmonic based on tunable wavelength laser source according to claim 1, wherein:
the detection light source also comprises a 1/2 wave plate in the detection incident light path for changing the polarization direction of the detection incident light; the second harmonic signal is separated into a polarized signal P and a polarized signal S through a spectroscope; the signal receiving system comprises a signal receiving system P and a signal receiving system S, and the polarized signal P and the polarized signal S are respectively received.
4. The defect detection apparatus for inducing second harmonic based on tunable wavelength laser source according to claim 1, wherein:
The incident angle of the excitation incident light is 0 °.
5. The tunable wavelength laser source induced second harmonic defect detection apparatus according to any one of claims 1-4, wherein: the excitation incident light path also comprises an optical compensator.
6. A fixed wavelength second harmonic measurement method for a sample with a known material forbidden bandwidth using the tunable wavelength laser source induced second harmonic based defect detection apparatus according to any one of claims 1-5, characterized in that the fixed wavelength second harmonic measurement method comprises:
The first stage: turning on only the detection light source so that the detection second harmonic signal received by the signal receiving system is constant, which is called an initial state;
And a second stage: the detection light source is kept on, the excitation light source is turned on at the same time, the required excitation incident wavelength is determined according to the forbidden bandwidth of the sample to be detected, the excitation incident wavelength is kept unchanged, at the moment, the charge distribution of the sample changes along with the change, and meanwhile, the detection second harmonic signal received by the signal receiving system also changes correspondingly and is called an excited state; when the charge distribution eventually reaches a new dynamic balance, the detected second harmonic signal also eventually stabilizes.
7. The method for fixed wavelength second harmonic measurement of a sample of known material forbidden bandwidth of claim 6, further comprising:
And a third stage: keeping the detection light source on, turning off the excitation light source, and enabling the captured electrons to return to the substrate layer again to be compounded, wherein the detection second harmonic signal received by the signal receiving system is gradually recovered and is called a recovery state; when the recovery time is long enough, the detected second harmonic signal is recovered to the initial state.
8. A tunable wavelength second harmonic measurement method for a sample with unknown material forbidden bandwidth using the tunable wavelength laser source induced second harmonic-based defect detection apparatus according to any one of claims 1-5, characterized in that the tunable wavelength second harmonic measurement method comprises:
the first stage: turning on only the detection light source so that the detection second harmonic signal received by the signal receiving system is constant, which is called an initial state; keeping the detection incident wavelength constant;
And a second stage: the excitation incident wavelength is tunable during the test, and gradually decreases from long to short; keeping the detection light source on and simultaneously turning on the excitation light source; when the minimum forbidden bandwidth of the sample is the same as the photon energy of the excitation light source, the detected second harmonic signal is subjected to critical change, the excitation incident wavelength at the moment is called as forbidden bandwidth wavelength, and the excitation incident wavelength is kept unchanged; when the charge distribution finally reaches new dynamic balance, the detected second harmonic signal also finally tends to be stable;
and a third stage: selecting more than two wavelengths near the forbidden bandwidth wavelength, namely test forbidden bandwidth wavelength, sequentially setting the excitation incident wavelength as each test forbidden bandwidth wavelength, measuring according to the second harmonic measurement method of claim 7, and receiving the detection second harmonic signals to obtain test results under successive test forbidden bandwidth wavelengths; after each setting of the excitation incident wavelength, the same size and light intensity density of the excitation light spot should be ensured;
the following processing is performed on the obtained successive specific wavelength test results:
The detected second harmonic signal curve for the excited state is fitted using equation 1 as follows:
Wherein a 0,a1,a2 is a constant; τ 1 is the time variable corresponding to the electron excitation process; τ 2 is the time variable corresponding to the hole excitation process; i 2ω is the light intensity density of the detected second harmonic;
the factor affecting τ 1 can be described by equation 2 as follows:
wherein, Energy of photons of the excitation incident light; n is a natural number, which is used to represent the number of photons absorbed in the multiphoton absorption process; k n is the n photon absorption coefficient; i ω is the intensity density of the excitation incident light;
The data of the corresponding group number can be obtained by the formula 2 Obtaining a data graph of the number of absorbed photons and the energy of the corresponding photons, and selecting the nearest integer from the data graph as n 1;
The minimum energy that the electrons need to overcome from the valence band of the semiconductor material to reach the conduction band of the insulating material can be expressed by equation 3 as follows:
Egap＝n1E1
The detected second harmonic signal curve for the recovery state is fitted using equation 4 as follows:
Where a 4,a5 is a constant and τ 3 is a time constant associated with charge recombination.
9. An electronic device, comprising:
At least one processor; and
A memory communicatively coupled to the at least one processor; wherein,
The memory stores instructions executable by the at least one processor to enable the at least one processor to perform the method of any one of claims 6-8.
10. A non-transitory computer readable storage medium storing computer instructions for causing the computer to perform the method of any one of claims 6-8.
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