JP3665207B2 - Semiconductor evaluation equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor evaluation apparatus for evaluating a carrier lifetime and a surface energy band state inside a semiconductor.
[0002]
[Prior art]
The carrier lifetime of a semiconductor material is an important physical parameter that determines the characteristics of many semiconductor devices such as transistors and solar cells. In addition, silicon wafers are widely used for quality control because they are greatly affected by minute crystal defects and impurities.
Conventionally, the photoconductive decay method has been widely used to measure the carrier lifetime of such a semiconductor wafer. In this method, a semiconductor wafer is irradiated with pulsed light to excite excess carriers in the semiconductor wafer, and the carrier lifetime is measured based on the attenuation of the number of excess carriers. The attenuation state of the above excess carrier number is
(1) Measuring resistance by attaching an electrode (2) It can be known by a method of irradiating a microwave and measuring the intensity of the reflected wave or transmitted wave. Among them, the method (2) using the microwave is widely used because it can measure without contact and at high speed. However, the photoconductive decay method has a problem that the internal (bulk) properties cannot be evaluated separately from the surface effects. That is, excess carriers excited by pulsed light recombine with majority carriers inside and on the surface of the semiconductor wafer and disappear, but it is often the case that the above-mentioned internal and surface attenuation due to two types of recombination is distinguished. Have difficulty. Therefore, in order to measure the lifetime inside the semiconductor wafer (hereinafter referred to as the bulk lifetime), it has been necessary to suppress recombination on the surface by some method.
Here, as one method for suppressing the surface recombination of the semiconductor wafer, it is conceivable to thermally oxidize the surface or to apply an appropriate chemical treatment to the surface. However, such a method is not only troublesome, but also has a risk of changing the state of the semiconductor wafer. For example, thermal oxidation can change the internal defect density, and chemical treatment is likely to contaminate the surface as a result. In addition, the effect of such treatment varies greatly depending on the conditions, and there is a problem that a sufficient effect cannot be obtained if performed under inappropriate conditions. Moreover, it is almost impossible to ensure that surface recombination is completely suppressed by the above treatment.
[0003]
Therefore, as an alternative method, methods for suppressing surface recombination by applying an electric field to the surface are disclosed in, for example, Japanese Patent Application Laid-Open Nos. 64-37843 (referred to as Reference 1) and Japanese Patent Application Laid-Open No. 4-289442 (reference) 2), JP-A-5-264473 (referred to as publication 3), and the like.
The method proposed in each of the above publications is such that flat electrodes are installed on the front and back surfaces of a semiconductor wafer, and a voltage is applied between the two electrodes to drive off majority or minority carriers from the surface of the semiconductor wafer and recombine the surfaces. It is intended to suppress. In the above publication 1, as shown in FIG. 6, a device is described in which two electrodes 102a and 102b are installed on the upper and lower sides of a semiconductor wafer 101, and a power source 103 is connected to these electrodes. Further, in the above publications 2 and 3, as shown in FIG. 7, a device for applying a voltage by a power source 203 to electrodes 202a and 202b installed on the upper and lower sides of a grounded semiconductor wafer 201 is described. At this time, the polarity of the applied voltage is the direction in which accumulation of majority carriers occurs on the surface in the publication 2 and the orientation in which a depletion layer forms on the surface in the publication 3.
[0004]
[Problems to be solved by the invention]
However, the methods proposed in the above publications 1 to 3 cannot always sufficiently suppress surface recombination. That is, in order to suppress surface recombination, it is necessary to apply a voltage having an appropriate polarity and strength according to the energy band state on the surface of the semiconductor wafer (details will be described in detail in the description of the present invention). In the methods proposed in the above publications, a voltage that can sufficiently suppress surface recombination is not always applied.
For example, in the apparatus described in the above publication 1 (FIG. 6), a reverse polarity voltage is always applied to the front and back of the semiconductor wafer 101. Therefore, even if the surface recombination can be suppressed on one side, the opposite side is changed. There is a possibility of promoting surface recombination.
Further, in the apparatus described in the above publications 2 and 3 (FIG. 7), although the voltage of the same polarity is applied to the front and back surfaces, the polarity of the applied voltage is limited to one side. Depending on the energy band state at the surface of the semiconductor wafer, the surface recombination may not be reduced at the specified polarity, but may be increased. Further, the surface recombination cannot be sufficiently suppressed unless the intensity of the applied voltage is set to an appropriate value according to the energy band state on the surface.
Although detection of metal contamination on the surface of a semiconductor wafer is also important for wafer quality control, the inventions described in the above publications are aimed at obtaining a bulk lifetime, and information on the surface (energy band). A method for evaluating (status) is not disclosed.
The present invention has been made in view of the above circumstances, and it is a first object to provide a semiconductor evaluation apparatus capable of reliably measuring a bulk lifetime obtained in a state in which surface recombination is sufficiently suppressed. The second object is to provide a semiconductor evaluation apparatus capable of measuring the energy band state on the surface.
[0005]
[Means for Solving the Problems]
The energy band state on the surface of the semiconductor sample takes one of accumulation, depletion, inversion, or an intermediate state depending on the surface level and the amount of surface charge. Of these, surface recombination is most likely to occur in the depletion state. Minority carriers are removed from the surface in the accumulated state and majority carriers are removed from the surface in the inverted state, so that surface recombination is less likely to occur. Further, when a voltage is applied to the surface of the semiconductor sample to generate an electric field on the surface, the energy band state of the surface changes under the influence. Therefore, surface recombination can be sufficiently suppressed if the surface energy band state can be changed to either the accumulation or inversion state by applying a voltage to the surface of the semiconductor sample.
Here, the polarity of the applied voltage must be set appropriately according to the energy band state of the surface when no voltage is applied. That is, in a state where no voltage is applied, it is necessary to apply a voltage in a direction that enhances the accumulation state when the surface is close to accumulation, and a direction that further enhances the inversion state when the surface is close to inversion. There is. When a voltage with the opposite polarity is applied, the surface approaches the depletion state, resulting in the promotion of surface recombination. FIG. 4 shows the relationship between the energy band state of the surface and the polarity of the voltage for approaching the accumulation or inversion state from that state for each conductivity type of the semiconductor wafer. For example, in a p-type semiconductor, if the surface energy band state in the state where no voltage is applied is a state close to accumulation, it can be made closer to the accumulation state by applying a negative polarity voltage. It can be said that the binding can be suppressed.
Further, if surface recombination is suppressed, the lifetime obtained by the photoconductive decay method tends to increase. Therefore, it can be considered that the saturation value (or maximum value) of the obtained lifetime is the lifetime in the state where surface recombination is most suppressed, that is, the bulk lifetime.
As described above, the lifetime is measured by the photoconductive decay method while changing the polarity and magnitude of the voltage applied to the semiconductor sample, and the measurement lifetime τ is plotted as a function of the applied voltage V. By determining the maximum lifetime, the bulk lifetime can be determined reliably.
[0006]
Accordingly, the present invention for achieving the above Symbol purpose had an excitation light irradiating means for irradiating the pulsed excitation light in the semiconductor sample, detection of emitting detection waves in the illumination area of the pulsed excitation light by the excitation light illumination means Electromagnetic wave radiating means, detection means for detecting the reflected or transmitted wave of the detection electromagnetic wave in the semiconductor sample, and time variation of the reflected or transmitted wave detected by the detecting means In a semiconductor evaluation apparatus comprising a lifetime measuring means for measuring the lifetime of minority carriers, voltage applying means for applying a voltage to the front and back surfaces of the semiconductor sample, and the front and back surfaces applied by the voltage applying means Voltage variable means for changing the polarity and magnitude of each voltage, change in the applied voltage by the voltage variable means, and correspondingly Based on the saturated or maximum value in the change of the measurement lifetime by the lifetime measuring means, a semiconductor characterized by comprising comprises an internal lifetime evaluation means for evaluating the lifetime of the interior of the semiconductor sample It is configured as an evaluation device .
[0007]
By the way, if the result obtained by plotting the measurement lifetime τ as a function of the applied voltage V and the relationship shown in FIG. 4 are used, the energy band state of the semiconductor sample surface in the state where no voltage is applied can be known. . For example, if the measurement lifetime increases with respect to a negative applied voltage and decreases with respect to a positive applied voltage in the above plot result for a p-type semiconductor, the energy of the surface of the semiconductor sample in the state where no voltage is applied. It can be said that the band state is close to the accumulation state. That is, it is possible to evaluate the energy band state on the surface of the semiconductor sample in the state where no voltage is applied, based on the sign of the value of dτ / dV when the applied voltage of the plot result is 0. FIG. 5 shows the relationship between the positive / negative value of dτ / dV and the energy band state of the semiconductor sample surface when no voltage is applied, for each conductivity type of the semiconductor wafer.
[0008]
Therefore, a change in the voltage applied by the upper SL voltage varying means further based respect to the change in the measured lifetime by the lifetime measurement means that the surface state evaluation hand to evaluate the energy band state of the surface of the semiconductor sample One comprising steps is conceivable .
Thus, when evaluating only the energy band state of the surface, it is not always necessary to apply a voltage to both the front and back surfaces of the semiconductor sample, and the voltage may be applied only to the front or back surface.
Further, the surface condition evaluating means is configured to determine the semiconductor sample based on a slope of the measured lifetime in the vicinity of the applied voltage 0 in a relationship between the applied voltage by the voltage varying means and a measured lifetime by the lifetime measuring means. It can be configured to evaluate the energy band state of the surface.
[0009]
Further, an electrode constituting the upper Symbol voltage applying means, when configured to transmit light, without pulsed excitation light is blocked by the electrode, the apparatus structure can be simplified. As an electrode that transmits light as described above, for example, an electrode that is made of a transparent material or that has a hole that allows light to pass therethrough can be used.
[0010]
[Action]
In the semiconductor evaluation apparatus according to the present invention, the lifetime corresponding to the voltage is measured by the lifetime measuring means while changing the polarity and magnitude of the voltage applied to the semiconductor sample by the voltage varying means. Then, the relationship between the applied voltage and the measurement lifetime is obtained by the internal lifetime evaluation means. Here, since it can be said that the surface recombination is suppressed as the lifetime increases, it is determined that the lifetime saturation value (or maximum value) in the above relationship is the internal lifetime of the semiconductor sample. . Thus, the internal lifetime can be reliably measured regardless of the surface state of the semiconductor sample.
Further, by the table surface state evaluating means, the relationship between the applied voltage and the measured lifetime is obtained. Here, the relationship between the energy band state of the surface and the polarity of the voltage for approaching the accumulation or inversion state from that state is as shown in FIG. That is, the surface energy band state of the semiconductor sample is evaluated based on the slope of the measurement lifetime near the applied voltage. As described above, it is possible to know the surface energy band state of the semiconductor sample without adding any special means to the lifetime measurement apparatus.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments and examples of the present invention will be described below with reference to the accompanying drawings for understanding of the present invention. It should be noted that the following embodiments and examples are examples embodying the present invention, and do not limit the technical scope of the present invention.
FIG. 1 is a block diagram showing a schematic configuration of the semiconductor evaluation apparatus A1 according to the embodiment of the present invention, and FIG. 2 applies a voltage only to one side of a p-type silicon wafer (without an oxide film and with an oxide film). FIG. 3 shows an example of the relationship between the applied voltage and the measurement lifetime when a voltage is applied to both surfaces of the p-type silicon wafer with an oxide film. FIG. 4 is a table showing the relationship between the energy band state of the surface and the polarity of the voltage for approaching the accumulation or inversion state from that state, and FIG. 5 shows dτ / dV for each conductivity type of the semiconductor wafer. 4 is a table summarizing the relationship between the positive and negative values of the value and the energy band state of the semiconductor sample surface when no voltage is applied, for each conductivity type of the semiconductor wafer.
[0012]
The semiconductor evaluation apparatus A1 according to the present embodiment is configured as shown in FIG. Transparent electrodes 5 (an example of electrodes) are installed above and below the semiconductor sample W via insulating sheets 6, and DC power supplies 7 and 7 are connected between the transparent electrodes 5 and 5 and the semiconductor sample W. Is done. As the insulating sheet 6, for example, a Teflon sheet having a thickness of about 50 μm can be used. The insulating sheet 6, the transparent electrode 5, and the DC power source 7 are examples of voltage applying means. The DC power source 7 is connected to a computer 8 (an example of voltage varying means), and the polarity and magnitude of the applied voltage to the semiconductor sample W can be freely changed by the control of the computer 8.
A microwave (an example of a detection electromagnetic wave) emitted from a microwave oscillator 1 (an example of a detection electromagnetic wave radiation means) made of a Gunn diode is guided through a waveguide 3, a circulator 2, and a waveguide 3. The light is guided to the tube antenna 3a and irradiated from the opening end to the surface of the semiconductor sample W. Further, pulsed light emitted from a pulsed laser 9 (an example of excitation light irradiation means) passes through the transparent electrode 5 and the insulating sheet 6 and is irradiated onto the surface of the semiconductor sample W. The reflected microwave from the sample W returns to the waveguide antenna 3 a and is detected by the detector 4 through the waveguide 3 and the circulator 3. The reflected microwave intensity signal detected by the detector 4 is input to the computer 8 (an example of internal lifetime evaluation means and surface state evaluation means).
[0013]
The computer 8 sequentially changes the polarity and magnitude of the applied voltage to the semiconductor sample W by controlling the DC power source 7 and attenuates the reflected microwave by the detector 4 for each applied voltage. Measure the lifetime obtained based on the state. This provides a relationship between the applied voltage and the measurement lifetime.
FIG. 2 shows the relationship between the applied voltage and the measurement lifetime when a p-type silicon wafer is used as the semiconductor sample W and a voltage is applied to only one surface thereof. □ indicates a wafer without an oxide film, and ■ indicates a wafer with an oxide film. Here, the measured lifetime value is defined as the time to decay from the peak to 1 / e, and is normalized by the value when no voltage is applied. In a wafer without an oxide film, the measurement lifetime greatly increases when a negative voltage is applied, and decreases on the contrary when a positive voltage is applied. On the other hand, in the wafer with an oxide film, on the contrary, the measurement lifetime increases at a positive voltage and decreases at a negative voltage. From the relationship shown in FIG. 5, it can be determined that the surface energy band state of the wafer without the oxide film is close to accumulation, and that of the wafer with oxide film is close to inversion.
In the computer 8 as described above, the surface energy band state of the semiconductor sample W is evaluated based on the relationship between the obtained applied voltage and the measurement lifetime. When only the surface energy band state is evaluated in this way, it is not always necessary to apply a voltage to both sides of the semiconductor sample.
[0014]
FIG. 3 shows the relationship between the applied voltage and the measurement lifetime when a p-type silicon wafer with an oxide film is used as the semiconductor sample W and a voltage is applied to both surfaces thereof. The measurement lifetime increases with application of a positive voltage, and is saturated (maximum value) at 1500V. That is, the surface energy band state is close to inversion (see FIG. 5), and closer to the inversion state by applying a positive voltage (see FIG. 4). Inversion state at 1500 V, that is, the state where surface recombination is most suppressed. It is thought that it came to. Therefore, it can be determined that the value of the measurement lifetime at this time (applied voltage 1500 V), that is, 43 μS, is the bulk lifetime of this semiconductor sample.
As described above, the computer 8 evaluates the bulk lifetime of the semiconductor sample W based on the relationship between the obtained applied voltage and the measured lifetime. When evaluating the bulk lifetime, it is necessary to apply a voltage to both sides of the semiconductor sample to suppress surface recombination on both sides.
As described above, according to the semiconductor evaluation apparatus A1 according to the present embodiment, it is possible to measure the energy band state on the surface, and the bulk lifetime obtained in a state in which surface recombination is sufficiently suppressed. Can be measured.
[0015]
【Example】
In the above embodiment, the case where the measurement lifetime shows the maximum value, that is, the case where the measurement lifetime reaches saturation has been described. However, even when the measurement lifetime does not reach full saturation, curve fitting or the like may be performed. It is possible to predict the saturation value (or maximum value) and evaluate the bulk lifetime.
In the above embodiment, a transparent electrode is used as the electrode. However, the electrode may be a conductor that transmits light. For example, even if it is an opaque body, the same effect can be obtained by forming a mesh shape or micropores. It is possible to obtain
In addition, although the insulating sheet 6 is used as an insulator between the semiconductor sample W and the electrode 5, a spatial gap may be provided.
Furthermore, although microwaves are used as detection electromagnetic waves used for lifetime measurement, electromagnetic waves (infrared light, etc.) in other wavelength regions can be used by using appropriate transmission and detection systems.
Further, the semiconductor evaluation apparatus A1 according to the above embodiment is configured to measure both the surface energy band state and the bulk lifetime of the semiconductor sample. However, it is naturally configured to measure only one of them. Is possible.
[0016]
【The invention's effect】
The present invention includes an excitation light irradiating means for irradiating a semiconductor sample with pulsed excitation light, a detection electromagnetic wave radiating means for radiating a detection electromagnetic wave to a region irradiated with pulsed excitation light by the excitation light irradiating means, and the detection electromagnetic wave. A detecting means for detecting a reflected wave or transmitted wave in the semiconductor sample, and a lifetime measurement for measuring a minority carrier lifetime of the semiconductor sample based on a time change of the reflected wave or transmitted wave detected by the detecting means A voltage applying means for applying a voltage to the front and back surfaces of the semiconductor sample, and changing the polarity and magnitude of each of the voltages applied by the voltage applying means. The voltage variable means, the change in the applied voltage by the voltage variable means, and the lifetime measuring means corresponding thereto Based on the saturated or maximum value in the change of the constant lifetime, is configured as a semiconductor evaluation apparatus characterized by comprising comprises an internal lifetime evaluation means for evaluating the lifetime of the interior of the semiconductor sample Therefore, the internal lifetime can be reliably measured regardless of the surface condition of the semiconductor sample .
[0017]
Furthermore, a change in the voltage applied by the voltage varying means, based for on the change in the measured lifetime by the lifetime measurement means that the surface state evaluation means to evaluate the energy band state of the surface of the semiconductor sample by providing, without adding a special device to device for lifetime measurement, it is possible to know the surface energy band state of the semiconductor sample.
Further, the surface condition evaluating means is configured to determine the semiconductor sample based on a slope of the measured lifetime in the vicinity of the applied voltage 0 in a relationship between the applied voltage by the voltage varying means and a measured lifetime by the lifetime measuring means. It can be configured to evaluate the energy band state of the surface .
[Brief description of the drawings]
FIG. 1 is a block diagram showing a schematic configuration of a semiconductor evaluation apparatus A1 according to an embodiment of the present invention.
FIG. 2 is a graph showing an example of the relationship between applied voltage and measurement lifetime when a voltage is applied only to one side of a p-type silicon wafer (without an oxide film and with an oxide film).
FIG. 3 is a graph showing an example of the relationship between applied voltage and measurement lifetime when a voltage is applied to both surfaces of a p-type silicon wafer with an oxide film.
FIG. 4 is a table summarizing the relationship between the energy band state of the surface and the polarity of the voltage for approaching the accumulation or inversion state from that state for each conductivity type of the semiconductor wafer.
FIG. 5 is a table summarizing the relationship between the positive and negative values of dτ / dV and the energy band state of the semiconductor sample surface when no voltage is applied, for each conductivity type of the semiconductor wafer.
6 is a schematic diagram showing a schematic configuration of a voltage application device for a semiconductor wafer described in the publication 1. FIG.
FIG. 7 is a schematic diagram showing a schematic configuration of a voltage application device to a semiconductor wafer described in the publications 2 and 3. FIG.
[Explanation of symbols]
1 ... Microwave oscillator (an example of electromagnetic wave radiation means for detection)
2 ... circulator 3 ... waveguide 4 ... detector 5 ... transparent electrode (an example of an electrode)
6 ... Insulating sheet 7 ... DC power supply 8 ... Computer (an example of voltage varying means, internal lifetime evaluating means, and surface condition evaluating means)
9. Pulse laser (an example of excitation light irradiation means)
W ... Semiconductor sample

Claims (6)

Excitation light irradiating means for irradiating a semiconductor sample with pulsed excitation light, detection electromagnetic wave radiating means for radiating a detection electromagnetic wave to a region irradiated with pulsed excitation light by the excitation light irradiating means, and the detection electromagnetic wave in the semiconductor sample Detecting means for detecting a reflected wave or transmitted wave; and a lifetime measuring means for measuring a lifetime of minority carriers of the semiconductor sample based on a time change of the reflected wave or transmitted wave detected by the detecting means. In the semiconductor evaluation apparatus,
Voltage applying means for applying a voltage to the front and back surfaces of the semiconductor sample;
Voltage variable means for changing the polarity and magnitude of each of the voltages applied by the voltage applying means;
An internal lifetime for evaluating the lifetime inside the semiconductor sample based on the change in the applied voltage by the voltage varying means and the saturation value or the maximum value in the change in the measurement lifetime by the lifetime measuring means. A semiconductor evaluation apparatus comprising an evaluation means. The change of the applied voltage by the voltage varying means, based for on the change in the measured lifetime by the lifetime measurement means that, provided the surface state evaluation means to evaluate the energy band state of the surface of the semiconductor sample The semiconductor evaluation apparatus according to claim 1 . The surface condition evaluating means is configured to detect the surface of the semiconductor sample based on the slope of the measured lifetime in the vicinity of the applied voltage 0 in the relationship between the applied voltage by the voltage varying means and the measured lifetime by the lifetime measuring means. The semiconductor evaluation apparatus according to claim 2 , wherein the energy band state is evaluated. The voltage applying means comprises an electrode that transmits light;
Semiconductor evaluating device according to any one of claims 1 to 3, pulsed excitation light from the excitation light irradiation means is irradiated onto the semiconductor sample passes through the electrode. The semiconductor evaluation apparatus according to claim 4 , wherein the electrode is made of a transparent material. The semiconductor evaluation apparatus according to claim 4 , wherein the electrode is provided with a hole through which light passes.

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