WO2015159864A1 - Bulk quality evaluation method and device for semiconductor wafer - Google Patents

Abstract

In order to establish a wafer evaluation technique for measuring the lifetimes of carriers in all semiconductor wafers when the semiconductor wafers are cut and manufactured from a semiconductor crystal ingot, the wafer evaluation technique being practical and more significantly improved in measurement speed, accuracy, device operation, and the like than conventional methods, the present invention is a method and device for evaluating the lifetime of free carriers (a general term for free electrons and free holes) within a silicon wafer (3). First, free carriers are generated (excited) by irradiating the silicon wafer (3) to be evaluated with a free carrier generation (excitation) laser beam (R21). Next, the silicon wafer (3) in which the concentration of the generated free carriers is non-uniformly distributed is irradiated with an observation laser beam (R11). An angle at which the observation laser beam (R11) is emitted after being transmitted through the silicon wafer (3) is measured. Consequently, the spread (half-value width) of the distribution curve (FD) of electrons and holes can be measured, thereby making it possible to evaluate the quality of the silicon wafer (3).

Description

Semiconductor wafer bulk quality evaluation method and apparatus

The present invention relates to a bulk quality evaluation method and apparatus for semiconductor wafers for manufacturing electronic devices.

In response to the rapidly expanding demand for power semiconductors, power converter performance and manufacturing yields are improved by improving the quality and management of materials used in the manufacturing process (semiconductor crystal wafers and semiconductor crystal ingots). Improvement has become a major issue. Among the power semiconductors that are currently estimated to be 3 trillion yen, the fields of high-voltage, high-current devices such as IGBTs (Insulated Gate Bipolar Transistors) and PiN diodes are the result of the emergence of HEVs (hybrid electric vehicles) and the use of inverters in air conditioners. In this area, it is known that material quality and its management are directly linked to improving the performance and reliability of power converters.

However, since there was no device for measuring material quality at high speed and with high accuracy, only material sampling inspections, so-called sample inspections, were performed, and all inspections at the time of shipment and delivery were not performed. In other words, for all wafers made from a single semiconductor crystal ingot, the production system was not based on a management system in which quality data including the wafer acquisition site (site in the ingot) was complete and stored. . For this reason, traceability to investigate the cause back to crystal ingot production (whether due to crystal or subsequent process) in terms of material quality even if poor performance power semiconductor or failure occurs Even the situation could not be secured.

Conventionally, as a method used for sampling inspection as described above regarding wafer quality, the process in which free carriers generated by laser light annihilation with time are regarded as time decay of microwave reflection by free carriers on the wafer surface. There is a method (μ-PCD method, for example, see Non-Patent Document 1). However, in this method, in order to evaluate the internal quality (crystallinity) of the semiconductor wafer, a special surface treatment for erasing the surface recombination center is required due to the measurement principle. It has become a hindrance in improving the evaluation speed. Further, the problem that the quality evaluation results vary greatly due to the fact that the surface treatment cannot be performed stably under the same conditions has not been solved.

Further, since the evaluation itself is based on the measurement of the reflectance of the laser beam irradiated, if it is attempted to increase the resolution of high-precision measurement, that is, quality judgment, it takes a long time for the measurement of one point of the wafer and the wafer. The entire surface scan measurement takes about 20 minutes per sheet. 100% inspection of wafers was practically difficult due to the problem of the measurement principle that stands for the solution to the improvement of the measurement speed.

As an apparatus for evaluating a semiconductor wafer, Patent Document 1 discloses that a first photon beam is generated, and a wave of charge carriers is generated in the region when the first photon beam is incident on a certain region of the wafer. A first source that provides the first photon beam with a first intensity modulated with a sufficiently low frequency so as not to generate a second photon beam, and the second photon beam is said region of the wafer. A second beam of energy that is sufficiently lower in energy than the photons of the first beam so that no more than a negligible number of charge carriers are generated in the region. And a photoelectric element disposed in an optical path of a portion of the second beam reflected by the region and modulated by the frequency after being reflected by the region. To generate a first signal representing a first concentration of charge carriers formed in the region by the incidence of the first beam device is disclosed.

Patent Document 2 discloses a light irradiation unit that irradiates at least two types of light having different wavelengths to different first and second regions in a semiconductor to be measured, and a predetermined amount for each of the first and second regions. A measurement wave irradiating unit for irradiating a measurement wave, and a first reflected wave of the measurement wave reflected by the first region or a first transmitted wave of the measurement wave that has passed through the first region and the reflected by the second region A difference measurement wave that is a difference between the second reflected wave of the measurement wave or the second transmitted wave of the measurement wave transmitted through the second region is used as the first reflected wave or the first transmitted wave and the second reflected wave. Alternatively, based on the difference measurement wave generation unit that is generated by using the second transmitted wave as it is, the detection unit that detects the difference measurement wave generated by the difference measurement wave generation unit, and the detection result detected by the detection unit Find the carrier lifetime in the semiconductor to be measured Semiconductor carrier lifetime measuring apparatus having a computation unit is disclosed that.

Special Table 2002-517915 JP 2011-233742 A

In the conventional semiconductor wafer evaluation methods and apparatuses disclosed in Non-Patent Document 1 and Patent Documents 1 and 2 described above, the state of free carriers in the surface layer portion of the semiconductor wafer is evaluated by observing reflected waves. is doing. Such a method can be applied to LSI (Large Scale Integration) where the crystal quality of the surface layer portion of the semiconductor wafer is a problem, but for semiconductors such as IGBTs and PiN diodes whose crystal quality is questioned inside the semiconductor wafer. Has poor measurement accuracy and cannot be evaluated correctly.
Therefore, the present invention is practical, which measures the carrier life inside all semiconductor wafers when cut out from a semiconductor crystal ingot, and is greatly improved in terms of measurement speed and accuracy, device operation, etc. compared to the conventional method. The objective is to establish an improved wafer evaluation technology.

In the present invention, prior to the evaluation of bulk quality, a method is realized in which preparation for evaluation is completed if the wafer itself is set in an evaluation apparatus without pre-processing the wafer.
As an evaluation principle, a method of using a response from the inside of the wafer (using a method reflecting the internal state) is used instead of using a response from only the surface layer of the semiconductor wafer.
Compared with the conventional microwave PCD method (μ-PCD method), a new evaluation principle that can significantly reduce the quality evaluation time of one point of the wafer (ingot) is used. Specifically, instead of the conventional signal intensity measurement that requires repeated measurement (data integration) to improve accuracy, the principle that accuracy can be obtained by a single measurement is adopted.

That is, the present invention is a method and apparatus for evaluating the lifetime of free carriers (generic term for free electrons and free holes) inside a semiconductor wafer, and includes photons having energy larger than the energy band gap of the semiconductor to be evaluated. Photons that are smaller than the energy band gap of the substance to be evaluated are generated (excited) by irradiating excitation light on the portion of the wafer where the concentration of the generated free carriers is unevenly distributed. It is characterized by irradiating energy observation light and measuring the angle at which the observation light exits after passing or reflecting through the wafer.
Here, “measurement of the outgoing angle” means not only the direct angle measurement but also the arrival point when the observation light is emitted without being influenced by the excitation light and the observation light is affected by the excitation light. In this case, the distance from the reaching point when the light is emitted is included because the distance and the angle are in a proportional relationship.

When the time-dependent change in the angle of emission after the observation light is transmitted through or reflected by the semiconductor wafer without measuring the excitation light that has been irradiated on the semiconductor wafer is observed From this, the free carrier life can be calculated and evaluated.

The excitation light irradiation means can use laser light, particularly YAG laser, as the excitation light.
The observation light irradiation means may use a laser beam, particularly an infrared laser as the observation light. Further, the observation light irradiation means can be formed of an LED.

The excitation light irradiating means can irradiate the observation light with the excitation light so that the excitation light is parallel and reverse, or parallel and in the same direction.
The excitation light irradiation unit can irradiate the observation light so that the excitation light is parallel. Or the said excitation light irradiation means can irradiate the said excitation light so that it may incline with respect to the said observation light.

The excitation light irradiation means and the observation light irradiation means make the excitation light and the observation light incident on the semiconductor wafer at an incident angle of 45 degrees with respect to the front surface or the back surface of the semiconductor wafer. Can be irradiated.

At least one of the excitation light irradiation means and the observation light irradiation means is configured so that the excitation light and the observation light are changed from a non-crossing state to a non-crossing state inside the semiconductor wafer. When scanning is performed, the half width of the free carrier can be measured.
At least one of the excitation light irradiating means or the observation light irradiating means is such that the excitation light and the observation light pass through a state in which the excitation light and the observation light coincide with each other from a state in which they are parallel and separated from each other inside the semiconductor wafer. It is possible to scan so as to be in a state of being parallel and separated from each other.

The direct effect of the invention is to significantly improve the quality evaluation speed of the semiconductor wafer as compared with the prior art. This significant increase in evaluation speed enables the highest level of quality assurance and troubleshooting not possible before, such as:
In the production of power semiconductors, it is possible to manage the total number of non-defective and defective products in all processes from the semiconductor wafer (crystal ingot) that is the raw material to the final product production, and the quality assurance of the final product is greatly improved. When a defect or the like occurs in a product, it becomes possible to investigate the test data by going back to the crystal ingot (part), so that the cause of the defect can be investigated quickly and accurately.
Furthermore, through the investigation of the cause of troubles that occur very rarely at the product level, clues for improving the product manufacturing technology itself can be obtained.

It is a schematic diagram which shows the structure and operation | movement of the bulk quality evaluation apparatus of the semiconductor wafer which concerns on embodiment of this invention. It is a graph which shows the relationship between the amount of refraction which becomes a function of the irradiation position of the laser beam for excitation in this invention, and carrier concentration. It is a figure which shows arrangement | positioning of the bulk quality evaluation apparatus of the semiconductor wafer used by the preliminary experiment of this invention. It is sectional drawing of the silicon wafer which shows the state of the reflected light when irradiating the excitation laser beam and the observation laser beam, and it is the figure seen from the direction along the Z-axis. It is sectional drawing of the silicon wafer which shows the state of the reflected light when irradiated with the laser beam for excitation, and is the figure seen from the direction along the Y-axis. It is sectional drawing of the silicon wafer which shows the state of the reflected light when irradiated with the laser beam for excitation and the laser beam for observation, and is the figure seen from the direction along the Y-axis. It is a block diagram of the modification of the bulk quality evaluation of the semiconductor wafer shown in FIG. It is a graph which shows the relationship between the state which stopped irradiation of the excitation laser beam, and the refraction angle of the observation laser beam. FIG. 5 is a diagram illustrating an example of an arrangement in which laser beams are parallel, and is a diagram illustrating a relationship between an observation laser beam and an excitation laser beam. It is an example of the arrangement in which the laser beams are parallel, and is a diagram showing the refraction of the observation laser beam. It is an example of the arrangement in which the laser beams are parallel, and is a diagram showing the carrier concentration distribution. It shows an example of a three-dimensional intersection beam arrangement, and is a view seen from a cross-sectional direction of a silicon wafer. It shows an example of a three-dimensional intersection beam arrangement, and is a view seen from the front of a silicon wafer. It is a figure which shows arrangement | positioning of the apparatus at the time of implementing the bulk quality evaluation of a semiconductor wafer by the reflection method. It is sectional drawing of the silicon wafer which shows the state of reflection of the observation laser beam when not irradiating the excitation laser beam, and is a view seen from the direction along the Z axis. It is sectional drawing of the silicon wafer which shows the state of reflection of the observation laser beam when not irradiating the excitation laser beam, and it is the figure seen from the direction along the Y-axis. It is sectional drawing of the silicon wafer which shows the refractive state of the reflected light of the observation laser beam at the time of irradiating the excitation laser beam, and is the figure seen from the direction along the Z-axis. It is sectional drawing of the silicon wafer which shows the refractive state of the reflected light of the observation laser beam at the time of irradiating the excitation laser beam, and is the figure seen from the direction along the Y-axis.

DESCRIPTION OF SYMBOLS 1 Laser beam source for observation 2 Light control part 3 Silicon wafer 31 Irradiation surface 32 Reflection surface 4 Laser beam detector 5 Laser source for free carrier generation (excitation) 6 Light control part 7 Reflection part 8 XYZ stage 9 Shutter device FD Concentration distribution R11 Observation laser beam R12 Transmitted laser beam when not refracted R13 Transmitted laser beam when refracted R14 Laser beam when reflected R21 Laser beam for free carrier generation (excitation) (excitation laser beam)
θ Refraction angle φ, φ 'Incident angle

Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.
Fig. 1 shows the quality evaluation technique of the present invention in the most basic form for MCZ (Magnetic field applied Czochralski method) silicon wafers for power devices (phosphorus-doped, specific resistance = about 20 ± 3Ωcm) It is a schematic diagram of the structure and operation | movement description of the bulk quality evaluation apparatus of the semiconductor wafer (Hereafter, it may only be called a wafer.) At the time of doing.

As shown in FIG. 1, the observation laser light source 1 is located on a perpendicular line from the point Pc on the silicon wafer 3. From this observation laser light source 1, an infrared observation laser beam R11 having a wavelength of 1550 nm (0.787 eV) is emitted as observation light. The observation laser beam R11 is focused to a desired beam diameter by the light adjusting unit 2 such as a condenser lens, a slit or an aperture, and is irradiated onto the silicon wafer 3 to be evaluated.
This observation laser beam R11 passes through the silicon wafer 3 easily and straight and reaches the laser beam detector 4 functioning as a measuring means. The arrival point on the laser beam detector 4 where the observation laser beam R11 has reached is assumed to be a point Pa. As the laser beam detector 4, a CCD (Charge Coupled Device) or a CMOS (Complementary Metal-Oxide Semiconductor) image sensor can be used.

In FIG. 1, a laser beam light source for free carrier generation (excitation) that functions as excitation light irradiation means in a state in which the laser beam R12 transmitted through the silicon wafer 3 reaches the point Pa of the laser beam detector 4 straightly. 5 (wavelength 1064 nm: photon energy 1.165 eV: output 1.3 W), excitation laser beam R21 emitted as excitation light, point Pc at which observation laser beam R11 reaches silicon wafer 3 (see FIG. 1) Is irradiated so as to be inclined with respect to the observation laser beam R11.
Here, the inclined state indicates a state in which the excitation laser beam R21 is three-dimensionally inclined with respect to the observation laser beam R11 in a direction in which the thickness of the silicon wafer 3 is viewed.
By this laser irradiation, free electrons and free holes (free carriers) are generated in the vicinity of the point Pc on the silicon wafer 3, and the free carriers are diffused toward and around the silicon wafer 3. Hereinafter, the free carrier generation (excitation) laser light source 5 is abbreviated as the excitation laser light source 5.

In FIG. 1, the portion where the concentration of electrons and holes increases is shown as a portion shaded with a dark color of the silicon wafer 3 (concentration distribution FD). The generated electrons and holes recombine during diffusion and disappear (pair annihilation). The average time from the generation of electrons and holes to the disappearance of recombination is called the carrier lifetime, and the crystal quality determines this.
In the steady state of excitation laser irradiation from the excitation laser light source 5, the speed at which free electrons and free holes, that is, free carriers are generated by the irradiation of the excitation laser beam R21 is balanced with the speed of recombination annihilation. As shown in the silicon wafer 3 of FIG. 1, a time-independent distribution of free carrier concentration occurs.
The shape of this distribution concentration FD reflects the quality of the silicon wafer. For example, the better the crystal quality, the larger the spread (half width) of the distribution curve of the distribution concentration FD. Therefore, it is possible to evaluate the quality of the silicon wafer by measuring the spread (half-value width) of the distribution curve of the distribution concentration FD.

The principle that the observation laser beam R11 is refracted by free carriers generated by light irradiation (laser irradiation) is used to measure the half width of the distribution concentration FD. That is, as shown in FIG. 1, when the observation laser beam R11 passes through the inclined portion (the portion where the concentration gradient exists) of the distribution concentration FD of the free carrier concentration, it depends on the concentration gradient and concentration of the free carrier. Refract. The observation laser beam refracted and transmitted in this manner is indicated by R13 in FIG. The magnitude of refraction corresponds to the distance to the point Pb with respect to the refraction angle θ and the point Pa in FIG.

1 is fixed, and the position of the excitation laser beam light source 5 is moved from the left to the right (or vice versa) with respect to the point Pc. Eventually, the distance between the points Pa and Pb (in other words, the refraction angle θ) changes as a function of the movement distance. FIG. 2 shows this state together with the carrier concentration distribution.
In the upper graph of FIG. 2, the X axis is the distance (relative position) between the irradiation positions of the observation laser beam R11 and the excitation laser beam R21 on the silicon wafer 3, and the Y axis is the refraction angle θ (point with respect to the point Pa). Pb displacement).
In the lower graph of FIG. 2, the X axis is the distance (relative position) between the irradiation positions of the observation laser beam R1 and the excitation laser beam R21 on the silicon wafer 3, and the Y axis is the carrier concentration. The origin 0 on the X-axis is a place where the irradiation positions of the excitation laser beam R21 and the observation laser beam R11 are coincident when the irradiation positions of the excitation laser beam R21 and the observation laser beam R11 are translated. It shows that there is.

As shown in FIG. 2, the half width of the carrier concentration distribution FD corresponds to the free carrier lifetime, and is observed when the irradiation position of the excitation laser beam R21 for generating free carriers is changed. The distance between the point Pa and the point Pb. In other words, the half-value width of the distribution concentration FD is the interval between the irradiation position where the refraction angle θ is the maximum value and the irradiation position where the refraction angle θ is the minimum value, and is obtained as the angle emitted after passing through the silicon wafer 3.
Since the silicon wafer 3 having a wide interval between the irradiation position where the refraction angle θ is the maximum value and the irradiation position where the refraction angle θ is the minimum value indicates that the free carrier has a long lifetime, it can be evaluated that the quality is good.

The features of the bulk quality evaluation method of the present invention are as follows:
(1) performing light irradiation (laser irradiation) to generate free carriers;
(2) the observation laser beam is refracted and transmitted by the generated free carrier concentration gradient;
(3) A combination of the three elements of detecting the arrival position of the transmitted observation laser beam using a two-dimensional image sensor such as a CCD.
Obviously, the laser beam for observation reaches (transmits) the inside of the wafer, so that the object of evaluation is different from the conventional microwave reflection attenuation method (the object of evaluation is free carrier near the surface). Internal crystallinity. In other words, the “bulk lifetime” that is important for power device wafers is measured.

In general, in an evaluation method using a light beam such as a laser beam, it is better to treat the evaluation amount as the intensity of light (light absorption method, reflection method, luminescence method) as a change in the direction of the light beam. Measurement time is short and accuracy is improved. This is because in the method based on intensity measurement, it is necessary to repeat signal acquisition many times in order to improve s / n (signal to noise ratio). In contrast, in a method of measuring the traveling direction of light (for example, X-ray diffraction), measurement can be completed in a short time.

Furthermore, the merit of using the CCD as the laser beam detector 4 is very great, and it is possible to improve the evaluation accuracy and shorten the time by combining the two-dimensional image capturing and the image analysis method.

Here, FIG. 3 shows the arrangement of the semiconductor wafer bulk quality evaluation apparatus used in the preliminary experiment leading to the present invention. Hereinafter, a bulk quality evaluation apparatus for semiconductor wafers is abbreviated as a bulk quality evaluation apparatus.
In the bulk quality evaluation apparatus shown in FIG. 3, an infrared laser is used as the observation laser light source 1 with a light source lens, and a condensing lens is used as the light control unit 2 (first light control unit). In the bulk quality evaluation apparatus, a CCD is used as the laser beam detector 4, and a YAG laser is used as the excitation laser light source 5.
Further, the bulk quality evaluation apparatus includes not only a condensing lens for condensing the observation laser beam but also a light control unit 6 (for condensing the observation laser beams R12 and R13 transmitted through the silicon wafer 3). A condensing lens as a second light control unit) and a silicon wafer as a reflection unit 7 for guiding light to the laser beam detector 4. The excitation laser light source 5 is mounted on an XYZ stage 8 which is an example of a scanning device.

The XYZ stage 8 includes a stage horizontal feed adjustment unit (X-axis direction and Y-axis direction) and a stage height feed adjustment unit (Z-axis direction). With these adjustment units, the excitation laser light source 5 can be translated in the vertical direction, translated in the horizontal direction, or moved in the approaching direction or the separating direction with respect to the silicon wafer. Thus, the irradiation position to the silicon wafer 3 can be adjusted by moving the excitation laser light source 5 with high accuracy.
Then, the bulk quality evaluation apparatus controls the scanning of the XYZ stage 8 and obtains the angle emitted after passing through the silicon wafer 3 from the position of the observation laser beams R12 and R13 received by the laser beam detector 4, A control device that evaluates the quality of the silicon wafer 3 by calculating the half width of the carrier concentration distribution FD, for example, a computer (not shown), is provided.

By moving the excitation laser light source 5 by the XYZ stage 8 controlled by the control device, the excitation laser beam R21 passes through the observation laser beam R11 from the non-crossing state to the non-crossing state. Scan as follows.
The XYZ stage 8 moves the excitation laser beam light source 5 to scan the excitation laser beam R21, and the laser beam R13 refracted by the observation laser beam R11 is measured by the laser beam detector 4, so that FIG. The characteristics of the silicon wafer 3 shown can be obtained.

FIG. 4A and FIG. 4B show an example of laser beam irradiation to a silicon wafer that is particularly effective in the evaluation by the bulk quality evaluation apparatus shown in FIG.
4A to 4C, in the orthogonal coordinate system, the silicon wafer 3 is illustrated with the thickness direction (left and right direction) of the silicon wafer 3 as the X axis, the depth direction as the Y axis, and the height direction (up and down direction) as the Z axis. (See FIG. 3).

In FIG. 4A, the observation laser beam R11 (infrared laser beam) irradiated to the silicon wafer 3 from the observation laser light source 1 (see FIG. 3) and the silicon wafer 3 from the excitation laser light source 5 are irradiated. The state with the excitation laser beam R21 is shown.

For example, when the observation laser beam R11 is incident on a point A on the front surface of the silicon wafer 3 with an incident angle φ = 45 °, the straight line AB, the straight line BC, and the straight line from the point A in the silicon wafer 3 thereafter. The reflection shown by the CD, the straight line DE, and the straight line EF is repeated, and the light path is emitted from the reflection position (for example, the point B, the point D, and the point F) to the outside shown by the straight line BP, the straight line DQ, and the straight line FR. .
Among such optical paths, there is an optical path in which the observation laser beam R11 exits the silicon wafer 3 from the point D and becomes a straight line DQ.
In this case, when the excitation laser beam R21 is viewed from the Z-axis direction, the point coincides with the point C (hereinafter, this point is also referred to as the point C using the same symbol C) and enters the silicon wafer 3 at an incident angle φ ′. Make it incident. However, the straight line UC of the excitation laser beam R21 is incident on a virtual plane (XY plane) formed by the straight line SA and the straight line AB along the optical path of the observation laser beam R11, or on a plane parallel thereto. The incident angle φ ′ is set to be the same as the incident angle φ = 45 ° of the observation laser beam R11. The excitation laser beam R21 incident on the silicon wafer 3 from the point C becomes an optical path indicated by a straight line CB when viewed from the Z-axis direction.

That is, the virtual plane (XY plane) including the straight lines UC and CB that are the optical paths of the excitation laser beam R21 is the virtual plane (XY plane) including the straight lines SA and the straight lines AB that are the optical paths of the observation laser beam R11. Then, scanning is performed from a state of being separated from each other in parallel to a state of being separated from each other in parallel by way of a state in which they are in agreement with each other. This scanning is performed by adjusting the Z-axis direction of the excitation laser light source 5 in the Z-axis direction of the XYZ stage 8.

In such an arrangement, the optical path portion indicated by the straight line BC of the observation laser beam R11 is completely parallel to the optical path portion indicated by the straight line CB of the excitation laser beam R21 (the direction is opposite). In addition, by adjusting the Z-axis direction of the XYZ stage 8, the mutual height (position of the plate surface of the silicon wafer 3) relationship can be changed or matched completely.
This is because two laser beams that are non-parallel to each other outside the silicon wafer 3 and part of the optical path of the excitation laser beam R21 and the observation laser beam (in FIG. 4A, the optical path of the section CB) are inside the silicon wafer 3. This shows that an ideal situation can be realized in which they are completely coincident when viewed from the Z-axis direction. In this way, in the bulk quality evaluation apparatus shown in FIG. 3, the observation laser beam R11 and the excitation laser beam R21 can be arranged in parallel.

In this case, as shown in the side view (viewed from the Y-axis direction) of the silicon wafer 3 shown in FIG. 4C, the concentration distribution FD of electrons and holes generated when the excitation laser beam R21 is irradiated. Thus, the optical path indicated by the straight line BC of the observation laser beam R11 is bent downward in FIG. 4C, reflected at the point C, followed by the optical path indicated by the straight line CD, and shown by the straight line DQ that is refracted downward. The optical path is emitted to the outside of the silicon wafer 3.

Of course, in FIG. 4A, if the condition of the incident angle φ ′ of the observation laser beam R11 = the incident angle φ of the excitation laser beam R21 is satisfied, the values of the incident angle φ and the incident angle φ ′ are angles other than 45 °. There may be.

In the present embodiment, the excitation laser light source 5 determines the emission direction of the excitation laser beam R21 by the XYZ stage 8, but the emission direction of the excitation laser light source 5 is determined by a commercially available device. You may make it set. However, many commercially available devices can be set only at an angle with good symmetry, for example, every 45 degrees, like an optical surface plate. Therefore, with respect to the incident angle to the silicon wafer 3, the arrangement of the incident angle φ ′ = incident angle φ = 45 ° is advantageous in that it can be realized by inexpensive apparatus (surface plate) processing while maintaining high accuracy. .

As shown in FIG. 4A, in the beam arrangement satisfying the condition that the incident angle φ ′ = the incident angle φ, the reflection of the excitation laser beam R21 is reflected by the observation laser beam R11 as a result of various reflections in the silicon wafer 3. The same (parallel) optical path may be traced, reaching the CCD of the laser beam detector 4 for measuring the amount of refraction or the observation laser light source 1 for irradiating the infrared laser light, thereby obstructing the measurement. As a countermeasure, an excitation laser beam R21 is provided by arranging a notch filter in front of the laser beam detector 4 on the optical path, the observation laser light source 1 and the front (in the middle of the optical path shown by the straight line SA shown in FIG. 4A). Can be prevented from reaching the laser beam detector 4 and the observation laser light source 1.

In the bulk quality evaluation apparatus shown in FIG. 3, the observation laser light source 1 is fixed, and the excitation laser light source 5 is scanned by the XYZ stage 8. However, the observation laser light source 1 may be scanned by a scanning device such as the XYZ stage 8.

In the embodiment described above, the non-uniform concentration distribution FD (non-uniform concentration as a function of the position in the wafer) of free carriers generated by irradiation with the excitation laser is constant regardless of time. Among the evaluation methods using this evaluation principle, which is performed under such controlled conditions (hereinafter referred to as a steady state), it is a relatively simple and simple method.

In addition, examples of the following effective quality evaluation methods based on the same principle can be given.
For example, there is an evaluation method for observing a temporal change of a refracted beam (refracted laser beam R13).
A state in which the non-uniform concentration distribution FD (non-uniform concentration as a function of the position in the wafer) generated by irradiation of the excitation laser beam R21 changes (decays) with time (hereinafter referred to as an unsteady state). Is used to calculate / evaluate the free carrier lifetime from the observation result that the arrival point of the observation laser beam (point Pb in FIG. 1) changes with time. FIG. 5 shows a bulk quality evaluation apparatus that calculates and evaluates the free carrier lifetime using this unsteady state. In FIG. 5, the same components as those in FIG.

The bulk quality evaluation apparatus shown in FIG. 5 is a shutter device 9 that is a means for making the excitation laser beam R21, which is excitation light, not reach the silicon wafer 3 between the excitation laser light source 5 and the silicon wafer 3. Is arranged.
When the excitation laser beam R21 is shielded by the shutter device 9, the concentration of free carriers generated by the excitation laser beam R21 decreases. Therefore, as shown in FIG. 6, the steady state is changed to the unsteady state, and the refraction angle θ is attenuated and gradually approaches zero. This is measured by the laser beam detector 4, and a free carrier lifetime is calculated and evaluated from a time constant of exponential decay (a decrease in the distance from the point Pa to the point Pb) by a control device (not shown). If the time when the refraction angle θ is 0 is long, it indicates that the free carrier life is long, so that the silicon wafer 3 can be evaluated as having good quality.

In the bulk quality evaluation apparatus shown in FIG. 5, the excitation laser beam R21 is shielded by the shutter device 9 so that it does not reach the silicon wafer 3. However, the excitation laser light source 5 is turned on. It is possible to make the laser beam non-reachable by cutting or preventing the excitation laser beam R21 from being emitted inside the excitation laser light source 5. Further, the direction of emission of the excitation laser beam R21 can be changed to make it non-reachable.

In this case, since it is necessary to assume a case where the time change of the arrival point occurs on a time scale of microseconds, in order to observe the state, the laser beam detector 4 is a photodiode having a faster response speed than the CCD. A high speed photoelectric device such as an array or a CMOS image sensor is used.

In the embodiment shown in FIGS. 3 and 5 described above, an example in which the surface of the silicon wafer 3 is irradiated with a free carrier generation (excitation) laser beam (hereinafter referred to as excitation laser beam R21) from an oblique direction. Although shown, this example is basically the same as the case where the laser beams shown in FIG. 7A are arranged in parallel (hereinafter, the laser beam in such an arrangement is referred to as a parallel beam). Show the effect. That is, the excitation laser beam R21 and the observation laser beam R11 are irradiated as parallel beams that are separated from the surface of the silicon wafer 3 by a distance d. As shown in FIGS. 7B and 7C, inside the silicon wafer 3, a free carrier concentration distribution FD is generated around the optical axis of the excitation laser beam R21, and the vicinity of the optical axis of the excitation laser beam R21 is used for observation. When the laser beam R11 passes, refraction occurs due to the gradient of the concentration distribution FD. The laser beam R13 transmitted through the silicon wafer 3 passes through the silicon wafer 3 at a refraction angle θ corresponding to the gradient of the free carrier concentration distribution, and is detected by the laser beam detector 4. The distance d between the observation laser beam R11 and the excitation laser beam R21 is changed from “+” (a state where the arrival point of the observation laser beam R11 is on the right side of the arrival point of the excitation laser beam R21 as shown in FIG. 7C). Refraction angle while moving relatively to “−” (state where the observation laser beam R11 arrival point is to the left of the arrival point of the excitation laser beam R21) via 0 ”(state where the arrival points of the two beams overlap) Measure θ. The distance d when the refraction angle θ is largest on the left side and the distance d when the refraction angle θ is largest on the right side are measured. The quality of the silicon wafer 3 can be evaluated from the value of the distance d.

8A and 8B show examples of beam arrangement in a state where the observation light and the excitation light are three-dimensionally crossed. In the parallel beam arrangement in the bulk quality evaluation apparatus shown in FIG. 5, the excitation laser beam R21 and the observation laser beam R11 are incident on the silicon wafer 3 from the same front surface side. In the three-dimensional cross beam arrangement in the bulk quality evaluation apparatus shown in FIG. 9, the excitation laser beam R21 is irradiated from the front surface side of the silicon wafer 3, and the observation laser beam is irradiated from the side portion (thickness direction) of the silicon wafer 3. Irradiates R11.

The excitation laser beam R21 and the observation laser beam R11 are changed from a non-crossing state where the optical axes do not intersect to a non-crossing state, and carriers generated around the optical axis of the excitation laser beam R21. The quality is evaluated by the angle θ of the observation laser beam R13 refracted by the concentration distribution.
Also in this example of the three-dimensional crossed beam arrangement, if the optical axes of the excitation laser beam R21 and the observation laser beam R11 are changed from the non-crossing state to the crossing state and become the non-crossing state, the excitation laser beam R21 The laser beam R21 is not necessarily irradiated perpendicularly to the surface of the silicon wafer 3, and may be irradiated so as to be inclined.

In the bulk quality evaluation apparatus shown in FIG. 3, even if the silicon wafer 3 is not perpendicular to the observation laser beam R11, the laser transmitted through the silicon wafer 3 due to the high parallelism of the silicon wafer 3. The beam R12 is slightly translated from the incident light beam, but its traveling direction does not deviate from the traveling direction of the incident light. In other words, in the evaluation, it is not necessary to pay special attention to accurately setting the angle of the surface of the silicon wafer 3 with respect to the direction of the observation laser beam R11. Does not occur. This point also contributes to an improvement in evaluation speed by simplifying the evaluation operation.

In the bulk quality evaluation apparatus in FIG. 3, the laser beam R12 transmitted through the silicon wafer 3 is measured by the laser beam detector 4, but in the bulk quality evaluation apparatus according to another embodiment shown in FIG. The laser beam R14 reflected from the silicon wafer 3 can also be measured by the laser beam detector 4.

Here, the bulk quality evaluation apparatus shown in FIG. 9 will be described. In FIG. 9, the same components as those in FIG.
In the bulk quality evaluation apparatus shown in FIG. 9, the laser beam detector 4 is disposed at a position where the reflected light (laser beam R14) from the silicon wafer 3 is measured.
In this bulk quality evaluation apparatus, the excitation laser beam R 21 from the excitation laser light source 5 is reflected by the silicon wafer 3. The irradiation surface 31 of the silicon wafer 3 can receive the excitation laser beam R21 and the observation light laser beam R11, but the back surface opposite to the irradiation surface is mirror-finished so that the laser beam cannot be emitted. For. For example, the mirror finish can be provided with a reflective film on the back surface of the silicon wafer 3. A reflective surface 32 is formed on the back surface of the silicon wafer 3 by this reflective film.
A method for evaluating the silicon wafer 3 with the laser beam R14 reflected by the observation laser beam R11 on the silicon wafer 3 will be described with reference to FIGS. 10A, 10B, 11A, and 11B.

10A and 10B, the silicon wafer 3 is illustrated with the thickness direction (left-right direction) of the silicon wafer 3 as the X-axis, the height direction (up-down direction) as the Y-axis, and the depth direction as the Z-axis.
As shown in FIG. 9A, when viewed from the Z-axis direction, the observation laser beam R11 indicated by the straight line SA is incident at the point A, and has the same reflection angle as the incident angle and in the direction indicated by the straight line AT. reflect. The observation laser beam R11 incident on the silicon wafer 3 is reflected between the reflecting surface 32 and the irradiation surface 31 while repeating reflection shown by the straight line AB, straight line BC, straight line CD, straight line DE, and straight line EF. Progress as. As shown in FIG. 10B, when this state is viewed from the Y-axis direction, the incident light, the reflected light, and the emitted light all overlap with one straight line.

In this state, as shown in FIGS. 11A and 11B, the point C is irradiated with the excitation laser beam R11 parallel to the XY plane.
The irradiation with the excitation laser beam R11 generates a free carrier concentration distribution FD. At this time, the incident angle of the excitation laser beam R11 is adjusted, and the laser beam indicated by the straight line BC reflected inside the silicon wafer 3 is one side from the center of the concentration distribution FD (in FIG. 11B, the concentration distribution FD It passes through the upper side from the center.
When the laser beam reflected at the point B on the reflecting surface 32 by the irradiation of the excitation laser beam R11 passes through the concentration distribution FD, the concentration distribution FD has a high electron / hole concentration as shown by a curve BC ′. The light is refracted to the center side (downward from the center of the concentration distribution FD in FIG. 11B) and reaches a point C ′.

A part of the laser beam reaching the irradiation surface 31 of the silicon wafer 3 is reflected and emitted in the direction indicated by the straight line C′U. The remaining laser beam is reflected by the irradiation surface 31 and returns to the silicon wafer 3 as a laser beam indicated by a straight line C′D.
Since the laser beam indicated by the straight line C′U emitted from the irradiation surface 31 of the silicon wafer 3 is refracted due to the influence of the free carrier concentration distribution FD, the laser beam is not emitted perpendicularly from the silicon wafer 3 but the XY plane. Deviate.

In this way, the irradiation position of the excitation laser beam R11 that affects the observation laser beam R11 reflected in the silicon wafer 3 is changed from the non-crossing state to the non-crossing state with the observation laser beam R11. Scan as follows.
First, when the irradiation position of the excitation laser beam R21 is sufficiently away from the position of the reflected light of the observation laser beam R11, the reflected light of the observation laser beam R11 is not affected by the concentration distribution FD. The optical path is the same as in FIGS. 10A and 10B.

Next, when the excitation laser beam R21 is scanned and the reflected light of the observation laser beam R11 enters the range of influence of the concentration distribution FD (one side from the center of the concentration distribution FD), as shown in FIGS. 11A and 11B. Refracts under the influence of the density distribution FD.
Next, when the excitation laser beam R21 intersects with the reflected light of the observation laser beam R11, that is, when the reflected light of the observation laser beam R11 coincides with the peak position of the concentration distribution, the concentration gradient is zero. Since there is no refraction, the optical path is the same as in FIGS. 10A and 10B.
Next, when the excitation laser beam R21 is scanned and the reflected light of the observation laser beam R11 moves from the center of the concentration distribution FD to the other side, the reflected light is opposite to the direction of refraction shown in FIGS. 11A and 11B. Begin to refract to the side.
Further, when the irradiation position of the excitation laser beam R21 is sufficiently separated from the position of the reflected light of the observation laser beam R11 and the influence does not reach the reflected light, the reflected light is emitted from the silicon wafer 3 to the outside. Sometimes it becomes perpendicular to the silicon wafer 3 and returns to its original optical path.

Thus, the excitation laser beam R21 is used to sequentially receive the refraction of the reflected light by the laser beam detector 4, and the half-value width of the distribution density FD is measured from the position, thereby increasing the carrier life of the silicon wafer 3. You can know and evaluate the quality.

In the embodiment of the present invention described above, an example in which an excitation laser beam having a wavelength of 635 nm and a photon energy of 1.95 eV is used as the excitation laser light source 5 has been described. However, a free carrier generation (excitation) laser light source 5 is used. For example, a YAG (Yttrium Aluminum Garnet) laser having a wavelength of 1064 nm capable of generating carriers to the inside of the silicon wafer can be used.

When light is applied to a semiconductor or an insulator, light is absorbed because electrons in the valence band are excited to the conduction band. In many crystalline solids, the absorption of light caused only by the excitation of electrons is the electron wave number vector in the valence band (initial state) and the electron in the conduction band after excitation (final state). Only when the wave vector is equal (this is called the vertical transition rule).
When this rule is applied to the case of light absorption in silicon, strong light absorption occurs when electrons in the highest energy state of the valence band are excited to the conduction band of the same wave vector. This excitation energy is called the direct transition absorption edge.
However, the actual light absorption of silicon occurs from a position lower than the direct transition absorption edge. This is because interband optical transition in silicon does not occur only by excitation of electrons, but lattice vibration (phonon) is involved in the excitation process of electrons.
That is, an interband transition may occur with the emission (generation) of phonons simultaneously with the absorption of light. In such a case, excitation of electrons by light can be performed between the highest energy state of the valence band and the lowest energy state of the conduction band, that is, between the band gaps, without being bound by the vertical transition law. Such excitation energy is called an indirect transition absorption edge.

Therefore, when considering the interband optical transition (by light absorption) of silicon while increasing the energy, first, weak absorption starts from the indirect transition absorption edge corresponding to the band gap energy (1.14 eV) and reaches the direct transition absorption edge. Then, it shows the behavior that strong absorption starts suddenly. Therefore, from the viewpoint of light absorption intensity, absorption (and hence excitation of electrons) occurs between the indirect transition absorption edge and the direct transition absorption edge, but is very weak. For light with energy higher than the direct transition absorption edge, optical properties similar to those of metal are exhibited, and the light is almost reflected and does not reach the inside of the crystal.

The energy of the light emitted from the YAG laser, which is the laser source 5 for free carrier generation (excitation), enters between the indirect transition absorption edge and the direct transition absorption edge, and can enter the crystal while appropriately exciting electrons. it can. Quantitatively, in a silicon crystal, the intensity (number of photons) per 0.5 mm light penetration (progress) length decreases to about 50%.
In the present invention, since the purpose is to evaluate the bulk lifetime, it is necessary to excite (generate) electrons (and thus holes) to the inside of the silicon wafer to be evaluated, but a YAG laser is used. This purpose can be achieved.

Furthermore, in addition to laser light, depending on energy and wavelength, LED light which is non-coherent light can be used as excitation light and observation light. A light source used to excite electrons and / or holes in a silicon wafer is required to have a band gap energy (1.14 eV) or more and a direct transition absorption edge energy (about 2 eV) or less. That is, photons that fall within the range are generated and the emitted light can be collected by a condenser lens or the like. In particular, coherence like a laser beam is not essential. There are LEDs that satisfy this condition, and more optimal LEDs can be produced with current technology.

The present invention is practical and significantly improved in terms of measurement speed and accuracy, apparatus operation, etc. compared to conventional methods, as a semiconductor wafer manufacturing technology for electronic device manufacturing and in the technical field of electronic device manufacturing. It can be suitably used.

Claims (13)

1. A step of generating free carriers in the semiconductor wafer by irradiating the semiconductor wafer with excitation light containing photons having energy larger than the energy band gap of the semiconductor wafer to be evaluated;
   Irradiating the portion of the semiconductor wafer where the concentration of the generated free carriers is unevenly distributed with observation light having a photon energy smaller than the energy band gap of the substance to be evaluated;
   And a step of measuring an angle at which the observation light is emitted after being transmitted or reflected through the semiconductor wafer.
2. Non-reaching the excitation light that has been irradiated to the semiconductor wafer;
   The method for evaluating a bulk quality of a semiconductor wafer according to claim 1, further comprising a step of measuring a temporal change in an angle at which the observation light is emitted after being transmitted or reflected through the semiconductor wafer.
3. Excitation light irradiation means for generating free carriers in the semiconductor wafer by irradiating the semiconductor wafer with excitation light containing photons having energy larger than the energy band gap of the semiconductor wafer to be evaluated,
   Observation light irradiation means for irradiating the portion of the semiconductor wafer in which the concentration of the generated free carriers is unevenly distributed with observation light having a photon energy smaller than the energy band gap of the substance to be evaluated;
   A bulk quality evaluation apparatus for a semiconductor wafer, comprising: measuring means for measuring an angle at which the observation light is emitted after being transmitted or reflected by the semiconductor wafer.
4. Means for making excitation light from the excitation light irradiation means non-reachable to the semiconductor wafer;
   4. The semiconductor wafer bulk quality evaluation apparatus according to claim 3, further comprising means for measuring a temporal change in an angle at which the observation light is emitted after being transmitted or reflected by the semiconductor wafer.
5. 4. The semiconductor wafer bulk quality evaluation apparatus according to claim 3, wherein the excitation light irradiation means irradiates a laser beam as the excitation light.
6. 6. The semiconductor wafer bulk quality evaluation apparatus according to claim 5, wherein the laser beam is a YAG laser.
7. 4. The semiconductor wafer quality evaluation apparatus according to claim 3, wherein the observation light irradiation means irradiates a laser beam as the observation light.
8. 4. The semiconductor wafer bulk quality evaluation apparatus according to claim 3, wherein the excitation light irradiation means irradiates the observation light so that the excitation light is parallel and reverse, or parallel and in the same direction.
9. The excitation light irradiation means and the observation light irradiation means are configured such that the excitation light and the observation light are not parallel to each other outside the semiconductor wafer, and are parallel, opposite, or parallel inside the semiconductor wafer. 4. The semiconductor wafer bulk quality evaluation apparatus according to claim 3, wherein the irradiation is performed so as to be in the same direction.
10. The excitation light irradiation means and the observation light irradiation means make the excitation light and the observation light incident on the semiconductor wafer at an incident angle of 45 degrees with respect to the front surface or the back surface of the semiconductor wafer. The apparatus for bulk quality evaluation of a semiconductor wafer according to claim 9, wherein irradiation is performed as described above.
11. The quality evaluation apparatus for a semiconductor wafer according to claim 3, wherein the excitation light irradiation means irradiates the excitation light so as to be inclined with respect to the observation light.
12. At least one of the excitation light irradiation means and the observation light irradiation means is configured so that the excitation light and the observation light are changed from a non-crossing state to a non-crossing state inside the semiconductor wafer. The apparatus for bulk quality evaluation of a semiconductor wafer according to any one of claims 3 to 9,
13. At least one of the excitation light irradiating means or the observation light irradiating means is such that the excitation light and the observation light pass through a state in which the excitation light and the observation light coincide with each other from a state in which they are parallel and separated from each other inside the semiconductor wafer. 13. The semiconductor wafer bulk quality evaluation apparatus according to claim 12, wherein scanning is performed so as to be in a state of being parallel and separated from each other.

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