JP3650917B2 - Semiconductor surface evaluation method and apparatus using surface photovoltage - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor element manufacturing field in which a semiconductor element is formed on a silicon substrate. In other words, the present invention is for inspecting whether an element manufacturing process is appropriate, in other words, whether a manufacturing process satisfies a desired specification. The present invention relates to a semiconductor surface evaluation method and apparatus using surface photovoltage.
[0002]
[Prior art]
In recent years, high-performance semiconductor elements have a so-called planar structure, and the surface of the semiconductor and the oxide film formed thereon often influence the element performance. As an example, in MOS (metal / oxide / semiconductor) transistors widely used in large scale integrated circuits (LSIs), the number of semiconductor surfaces (the interface between the oxide film and the semiconductor and the crystal surface toward the inside is several. It is well known that the characteristics (meaning a range up to a depth of about μm) greatly affect the performance of the transistor. Further, in a charge coupled device that has been widely used as an imaging device such as a television camera recently, the characteristics of the semiconductor surface directly affect the transport efficiency of a so-called photocarrier (excess carrier) excited by light.
[0003]
Specifically, interface traps present at the interface between the semiconductor and the oxide film and crystal imperfections near the semiconductor surface determine the characteristics of the semiconductor surface. That is, when there are many interface traps, for example, the optical carriers near the semiconductor surface recombine via the interface traps and disappear. This means that in the charge coupled device, the optical carrier is lost during transportation, and the characteristics of the imaging device are significantly deteriorated. In the case of a charge coupled device, a so-called surface potential (see FIG. 1) is generated on the semiconductor surface, and the photocarrier is stored in a potential well formed at the surface potential, that is, in a depletion layer formed in accordance with the surface potential. However, if the surface properties are poor, the amount of photocarrier will decrease soon after storage. In a MOS transistor, the current between the source and drain is less likely to flow. In general, some of the interface traps are represented by the level (large or small) of the interface trap density Dit.
[0004]
As described above, the level of the interface trap density Dit is extremely important in the sense that it influences the device performance in many planar type devices. This interface trap density Dit naturally depends on the polishing state of the semiconductor surface, and also extremely sensitively depends on the oxide film formation process. For example, when the oxide film is formed by the dry oxidation method, the interface trap density Dit is extremely high, whereas in the wet oxidation method, the interface trap density Dit is extremely low.
[0005]
The generation of the interface trap depends on the state in which oxygen is bonded to the semiconductor atoms, and thus depends on the thickness of the oxide film. In recent years, with the miniaturization of elements, the oxide film gradually becomes thinner in accordance with the similarity law. However, the change in the oxide film thickness naturally affects the generation of interface traps. Since the natural oxide film contains pollutants in the atmosphere, the number of interface traps increases from the viewpoint of contamination. Even if the interface trap density Dit is low at the beginning of the oxide film formation, if the semiconductor substrate (wafer) is exposed to gas plasma widely used in the semiconductor device manufacturing process, the interface trap density Dit is reduced due to radiation damage or the like. Increase significantly.
[0006]
As described above, there are many causes for the increase in the interface trap in the element manufacturing process, and the increase / decrease in the interface trap is extremely fluid. Similar concerns are associated with the crystallinity of the semiconductor surface. That is, if impurities reach the crystal surface through a thin oxide film in various processes, the surface characteristics of the crystal naturally change in the direction of deterioration. Therefore, it is apparent that it is desirable to inspect the surface characteristics during the device manufacturing process in order to guarantee the integrity of the device manufacturing process.
[0007]
Actually, a method for inspecting the size and surface characteristics of the interface trap density Dit has been put into practical use and is used for evaluation of a semiconductor manufacturing element process. Two typical methods currently used in the semiconductor manufacturing process are described below with the main purpose of evaluating interface traps.
[0008]
(1) CV (capacitance-voltage) method
As a standard method for measuring the interface trap density, the CV (capacitance-voltage) method has been put into practical use from an early stage. In this method, the interface trap density Dit is measured using the same configuration as the gate electrode portion of the MOS transistor, and the capacitance (capacitance; C) between the gate electrode and the semiconductor layer is changed to a high frequency voltage while changing the gate voltage. And the quasi-static voltage, and the interface trap density Dit is determined from the dependence of the capacitance (C) on the gate voltage (V). Since the interface trap traps the optical carrier, a kind of electric capacity is formed at the interface. Since the relationship between the electric capacity and the interface trap density Dit is theoretically determined, the interface trap density Dit can be determined from the correspondence between theory and experiment.
[0009]
Although this method can accurately determine the interface trap density Dit, it is necessary to form a metal electrode on the oxide film in order to perform the measurement. Thus, if a semiconductor substrate (wafer) in the process of device manufacturing is extracted from the process and CV (capacitance-voltage) measurement is performed, a metal electrode unnecessary for device manufacturing is added to the wafer. The same wafer cannot be returned to the manufacturing process again. That is, this method is basically a destructive inspection method, and it is impossible to inspect all the wafers during the manufacturing process.
[0010]
Recently, a non-contact type CV method has been proposed in which a metal electrode is not formed on an oxide film, but the electrode is opposed to the oxide film through a gap. This method is a nondestructive inspection in that a metal electrode is not formed on the wafer, but a strong DC electric field is applied to the oxide film as long as the CV method is used. This means that the charge in the oxide film is forcibly moved, and the state of the wafer taken out from the process is changed. However, if ions or the like reach the semiconductor surface, the interface trap state changes, and the conventional CV method has a risk of changing the interface state in the measurement process. Therefore, the non-contact type CV method is essentially a destructive inspection.
[0011]
Further, the characteristics of the semiconductor surface are not determined solely by the interface traps, and the element characteristics are significantly affected by the crystallinity of the depletion layer. However, no information about the crystal perfection of the depletion layer can be obtained by the CV method. This is also a drawback of the CV method.
[0012]
(2) μ-wave photoconductive decay method
Recently, the μ (micro) wave photoconductive decay method has been used as an evaluation method of the interface trap density Dit. Originally this method was developed to measure minority carrier lifetime in semiconductor crystals. There are several definitions of lifetime, but the lifetime inside the crystal that is not affected by the crystal surface is generally called bulk lifetime or volume lifetime, and simply referred to as minority carrier lifetime means this volume lifetime. To do.
[0013]
Light is irradiated onto a wafer or the like to generate an optical carrier (excess carrier), and the disappearance process of the optical carrier is detected by reflection of μ waves. Although the conductivity of the wafer locally increases due to the generation of the optical carrier, the reflection of the μ wave depends on the conductivity of the wafer. Therefore, the magnitude of the reflection of the μ wave represents the abundance of the optical carrier. The time over which the reflection intensity of μ waves is measured and the reflection intensity decreases to 1 / e (where e is the base of natural logarithm) is defined as the photoconductive decay time. Often referred to as carrier lifetime.
[0014]
It has been found that this method evaluates and measures the (effective) minority carrier lifetime of the wafer and is strongly influenced by the wafer surface (including the interface if an oxide film is present) as a secondary phenomenon. It was done. That is, the volume life time can be measured by this method only in a limited case. Generally, it is strongly influenced by an interface trap and the like. Minority carrier lifetime (which is defined as surface lifetime versus volume lifetime) is strongly affected. That is, the effect that the optical carrier rapidly disappears on the surface and the effect that the optical carrier is stored for a long time on the surface are mixed. Therefore, depending on the surface condition of the wafer, the photoconductive decay time may be longer than the volume life time, or a photoconductive decay time much shorter than the volume life time may be observed.
[0015]
In this way, by utilizing the fact that the μ-wave photoconductive decay method is strongly influenced by the surface, this method may be used for evaluating the interface trap density Dit in special cases. Actually, when the interface trap density Dit increases due to irradiation damage due to plasma or the like, the μ-wave photoconductive decay time is shortened. This phenomenon occurs as a result of a significant decrease in the surface lifetime of the minority carriers, but the same phenomenon appears not only when the interface trap density Dit increases but also when defects in the crystal in the depletion layer increase. That is, a wider range of information than the CV method can be obtained by the μ-wave photoconductive decay method.
[0016]
Since the μ-wave photoconductive decay method does not bring a metal electrode or the like into contact with an oxide film, the measurement is completely non-contact and non-destructive. Therefore, when the semiconductor surface is evaluated by the μ-wave photoconductive decay method, unlike the CV method described above, the inspection can be performed without changing the state of the wafer extracted from the element manufacturing process. It is not impossible to return the inspected wafer to the manufacturing process.
[0017]
However, the observed photoconductive decay time or effective minority carrier lifetime is naturally strongly influenced by the minority carrier lifetime (volume lifetime) in the wafer. Therefore, in general, when the photoconductive decay time is long, it cannot be distinguished whether the cause is the crystal surface (surface life time) or the volume life time inside the wafer. The same determination cannot be made when the photoconductive decay time is short.
[0018]
[Problems to be solved by the invention]
As described above, when the interface trap density Dit is evaluated by the CV method, the interface state is disturbed because a strong electric field is applied to the oxide film from the outside, and the interface trap density before the CV method is applied. There is a risk that Dit information will be lost. In order to avoid this danger, it is necessary not to give a large electric field change to the interface at the measurement stage.
[0019]
Moreover, although the crystallinity of the depletion layer cannot be evaluated by the CV method, it is the characteristics of the surface layer (depletion layer) including the interface that actually affects the element characteristics. Therefore, an evaluation method including the state of the depletion layer is required.
[0020]
Although the method of evaluating the crystal (wafer) surface by the μ-wave photoconductive decay method is simple, as described above, the surface cannot be accurately evaluated unless the effect of the volume life time inside the wafer is separated.
[0021]
The present invention has been made in view of such problems, and an object of the present invention is to provide a semiconductor surface evaluation method and apparatus using a surface photovoltage that can easily and accurately evaluate a crystal surface.
[0022]
[Means for Solving the Problems]
  The semiconductor surface evaluation method by the surface photovoltage according to the present invention uses laser light as a pulse signal.Rectangular wave with a pulse width of 100 μs or lessModulating into pulsed light, irradiating the surface of the semiconductor crystal with the pulsed light, detecting the generated surface photovoltage by capacitive coupling, and evaluating the crystal characteristics of the surface of the semiconductor crystal based on the decay time of the surface photovoltage It is characterized by that.
[0023]
In this case, the laser light preferably has a light transmission length shorter than the diffusion length of minority carriers in the semiconductor crystal.
[0024]
  In the semiconductor surface evaluation method using the surface photovoltage, the laser beam preferably has a wavelength of 488 nm.New. Further, the surface photovoltage is such that two electrodes are arranged above the semiconductor crystal via a gap or an insulating film, the electrode far from the semiconductor crystal is grounded, and the photovoltage is applied from the near electrode. Can be detected by capacitive coupling. In this case, the electrode can be a transparent electrode, and the pulsed light can be incident on the semiconductor crystal through the transparent electrode.
[0025]
  A semiconductor surface evaluation apparatus using surface photovoltage according to the present invention is an apparatus for evaluating crystal characteristics of a surface of a semiconductor crystal., Les-A light source that emits laser light and the laser light as a pulse signalRectangular wave with a pulse width of 100 μs or lessA pulsed light irradiating means for modulating the pulsed light and irradiating the surface of the semiconductor crystal with the pulsed light, a surface photovoltage detecting means for detecting the surface photovoltage generated in the semiconductor crystal by capacitive coupling, and the decay time of the surface photovoltage. And means for evaluating the crystal characteristics of the surface of the semiconductor crystal.
[0026]
  In this semiconductor surface evaluation apparatus using surface light voltage, the laser light preferably has a wavelength of 488 nm.New. Further, the surface photovoltage detection means can be configured to have two electrodes disposed above the semiconductor crystal via a gap or via an insulating film. In this case, the electrode closer to the semiconductor crystal The photovoltage is taken out of the electrode, and the far electrode is grounded. The electrode can be a transparent electrode, and the pulsed light can be incident on the semiconductor crystal through the transparent electrode.
[0027]
In the present invention, a laser whose light transmission length is sufficiently shorter than, for example, the diffusion length of minority carriers in the semiconductor crystal to be evaluated, and is comparable to or less than the depletion layer formed on the crystal surface. After modulating the light into rectangular pulse light, the semiconductor crystal is irradiated with this pulsed light, so that a surface photovoltage is generated from the surface of the semiconductor crystal, and this is detected to non-destruct the crystal characteristics of the surface of the semiconductor crystal. Can be detected.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification, p-type silicon is used as an example of a semiconductor crystal. However, this is for ease of understanding, and the present invention is not limited to application to p-type silicon. is there.
[0029]
First, the principle of the present invention will be described. The present invention evaluates a semiconductor surface by surface photovoltage. In the present invention, a weak voltage generated on the surface of the semiconductor wafer when the semiconductor surface is irradiated with light of a specific wavelength is detected without applying an external voltage to the oxide film or the interface. Since this generated voltage is generated on the semiconductor surface by light irradiation, it is generally called “surface photovoltage”. However, the magnitude of the generated voltage is 1 mV or less, as will be mentioned later, and the CV method. Compared to the value of 1V or higher applied at 1, the effect of the surface photovoltage on the state of the interface is negligible.
[0030]
The surface photovoltage itself was discovered by scholars in the United States in the 1950s, but because the phenomenon is complicated, it was hardly elucidated at that time, and the generation mechanism was revealed after 1970. According to the reported research results, the surface photovoltage is generated due to the oxide film charge existing in the oxide film formed on the surface of the wafer or the like. This oxide film charge forms an electric field on the wafer surface, but the resulting cause of the surface photovoltage is the electric field formed on the wafer surface. Therefore, even if light is applied to the wafer to generate a photocarrier, a surface photovoltage is not generated unless an electric field is present on the wafer surface. This is a significant difference from the fact that the μ-wave photoconductive decay method can obtain a μ-wave reflected signal regardless of the presence or absence of an oxide film charge (surface electric field). That is, the μ-wave photoconductive decay method can obtain a signal even when the surface electric field (oxide charge) is zero, but no surface photovoltage is generated.
[0031]
As can be seen from the generation mechanism of the surface photovoltage described above, the surface photovoltage is a phenomenon unique to the surface. As long as the wavelength of the light that irradiates the wafer satisfies a certain condition described later, unlike the μ-wave photoconductive decay method, information inside the wafer Will not be mixed. Therefore, when the interface trap density Dit is evaluated using the surface photovoltage, it is possible to remove the drawbacks of the μ-wave photoconductive decay method that is disturbed by the volume life time, as will be described later.
[0032]
Next, the relationship between the interface trap density Dit and the surface photovoltage will be described. FIG. 1 is a diagram for explaining generation of a surface potential due to oxide film charges. A surface potential is generated on the wafer surface by the oxide film charge, and an electric field is formed. In other words, a depletion layer (space charge layer) is formed on the wafer surface as shown in FIG.
[0033]
There is a natural oxide film on the surface of the semiconductor wafer that has not been heat-treated, and a thermal oxide film is present on the wafer that has been heat-treated. In any case, an oxide film charge exists in the oxide film. This charge is often positive for p-type wafers and negative for n-type wafers. Therefore, in many cases, a surface potential is generated on the wafer surface, and there is a depletion layer in which majority carriers are depleted in accordance with the charge of the oxide film. For example, in the case of a p-type silicon wafer having a resistivity of 0.1 Ωm used for LSI manufacturing, when a thermal oxide film is formed, 1 mC / m is contained in the oxide film.²Positive charge (fixed oxide film charge) is naturally generated, and a depletion layer having a width of about 1 μm is formed. When this is irradiated with an argon laser beam, the light transmission length is almost equal to the depletion layer width, so that most of the optical carriers are generated in the depletion layer. As a result, almost all minority carriers (in this case, electrons having a negative charge) of the photocarriers are absorbed by the positive oxide film charges, and almost all remain inside the depletion layer. On the contrary, the majority carriers (positive holes in this case) of the photocarriers are repelled by the positive oxide film charges and discharged out of the depletion layer.
[0034]
The entry of minority carriers into the depletion layer means the generation of surface photovoltage. This is because, when minority carriers enter the depletion layer, the effect of the positive oxide film charge is canceled, and the surface potential indicated by φs in FIG. 1 slightly decreases, and this is caused as a surface photovoltage by an external circuit. Because it is observed. However, the electrode that detects the surface photovoltage is electrically separated from the oxide film, which is an insulator, and cannot contact the wafer surface. Therefore, the surface photovoltage is measured by capacitive coupling through the oxide film.
[0035]
There are almost no majority carriers (positive holes in this case) according to the surface potential (in other words, the amount of oxide film charge) inside the depletion layer. Therefore, the minority carrier (minority carrier portion of the optical carrier) that has entered the depletion layer survives for a long time because there is no partner to recombine. That is, the surface photovoltage once generated does not decrease easily.
[0036]
However, when the interface trap density Dit is high, minority carriers easily recombine with holes generated thermally through the interface trap and disappear. As a result, the surface photovoltage decreases rapidly. Therefore, the interface trap density Dit can be evaluated by measuring the decay time of the surface photovoltage.
[0037]
Next, the relationship between depletion layer characteristics and surface photovoltage will be described. The (small number) photocarriers attracted to the depletion layer are not only annihilated via the interface trap, but there are other recombination processes. One of them is recombination through crystal defects in the depletion layer. That is, when a foreign substance such as a metal atom or a crystal defect is present in the depletion layer, a recombination center is formed, and excess carriers are bonded to holes inside the wafer and disappear. Thus, the decay time of the surface photovoltage reflects the state of the semiconductor surface.
[0038]
Therefore, the longer the surface photovoltage decay time, the closer the crystallinity of the wafer surface is.
[0039]
The present invention is configured according to the above principle, and an embodiment of the present invention will be specifically described below with reference to FIG. FIG. 2 is a diagram showing a semiconductor surface evaluation apparatus using surface light voltage. A sample wafer 2 is disposed on a grounded metal stage 1, and a laminate of a mylar sheet 3, a transparent electrode 4, a glass plate 5, and a transparent electrode 6 is disposed on the sample wafer 2. Yes. The uppermost transparent electrode 6 is grounded. Above this laminated body, a lens 7, an ND filter 8, and a mirror 9 are arranged so as to be positioned on an optical axis perpendicular to the surface of the sample wafer 2. On top, an AOM (acousto-optic modulator) 11 and an Ar laser light source 10 are arranged. The laser light source 10 emits argon laser light having a wavelength of 488 nm, for example. The AOM 11 modulates the argon laser light in synchronization with the pulse from the pulse generator 13 to obtain, for example, a pulse having a pulse width of 100 μs or less. Therefore, after the laser beam transmitted from the argon laser light source 10 is reflected by the mirror 9 via the AOM 11, the optical axis thereof is changed in a direction perpendicular to the surface of the sample wafer 2, and the ND filter 8 and the lens 7 are further changed. Then, the sample wafer 2 is irradiated through the transparent electrode 6, the glass plate 5, the transparent electrode 4 and the mylar sheet 3.
[0040]
The surface photovoltage generated on the surface of the sample wafer 2 by the incidence of the pulsed light is detected by the lower transparent electrode 4 facing the sample wafer 2 through the insulating mylar sheet 3, and this surface photovoltage is transparent. The signal is input from the electrode 4 to the preamplifier 15, amplified, and then input to the oscilloscope 14. On the other hand, the oscilloscope 14 receives a pulse signal from the pulse generator 13. The output pulse of the pulse generator 13 is also input to the AOM driver 12, and the AOM 11 is driven by the driver 12.
[0041]
Next, the operation of this embodiment will be described. The pulse generator 13 outputs a predetermined pulse signal to the AOM driver 12, and the AOM 11 modulates the laser light from the laser light source 10 with the pulse signal to obtain a predetermined rectangular wave pulse having a pulse width of 100 μs or less. This rectangular wave pulse is reflected by the mirror 9, attenuated to an appropriate amount of light by the ND filter 8, converged by the lens 7, and enters the surface of the sample wafer 2.
[0042]
By irradiation with pulsed light, a photovoltage is generated on the surface of the sample wafer 2 and immediately attenuates. FIG. 3 is a voltage waveform diagram (output diagram of oscilloscope bright lines) showing the generation and attenuation of the surface photovoltage. The surface light voltage attenuation characteristics of FIG. 3 are obtained by converting a voltage signal waveform into a digital signal and storing it, and then outputting the stored waveform to a printer. This voltage waveform shows that when a pulsed light with a pulse width of 100 μs is irradiated, a photovoltage is generated on the surface of the sample wafer at the rise of the pulse, and the photovoltage is attenuated at the fall of the light pulse. . In the voltage waveform as shown in FIG. 3, the time during which the voltage is 1 / e of the maximum value is defined as the surface light voltage decay time.
[0043]
The surface photovoltage is detected by the transparent electrode 4 and then input to the preamplifier 15, amplified by, for example, 100 times by the preamplifier 15, and then displayed on the oscilloscope 14. The pulse signal of the pulse generator 13 is also input to the oscilloscope 14, and the oscilloscope 14 displays the pulse signal and the photovoltage simultaneously.
[0044]
As described above, the decay time of the surface photovoltage reflects the state of the semiconductor surface. Accordingly, the longer the surface photovoltage decay time, the closer the crystallinity of the wafer surface is. Therefore, based on the decay time of the photovoltage displayed on the oscilloscope 14, the surface state of the sample wafer 2, that is, the crystallinity can be evaluated.
[0045]
In order to excite the photovoltage, it is necessary to excite the photocarrier in the semiconductor by light irradiation. In addition, at this time, the photocarrier generation region must be near the semiconductor surface. This can be expressed using mathematical formulas as follows.
[0046]
αL_n>> 1 (1)
[0047]
In the above formula, α is a light absorption coefficient, and is determined by the type of semiconductor and the wavelength of light. L_nIs the diffusion length of minority carriers (in this case electrons). 1 / α has a unit of length, and if this is Lα (= 1 / α), it means the transmission length of light in the semiconductor. Rewriting equation (1)
L_n>> Lα (2)
It becomes. That is, the light transmission length needs to be sufficiently shorter than the minority carrier diffusion length, but in the case of a p-type silicon crystal, blue light such as argon laser light (wavelength) is used except for a special case where Ln is several μm or less. 488 nm) satisfies this condition.
[0048]
The condition shown in equation (1) or (2) limits the photocarrier generation to the surface region, but this makes the photocarrier unaffected by volume life time. This is apparent from the fact that when the light transmission length is sufficiently smaller than the diffusion length, the light cannot distinguish the length of the diffusion length. Therefore, as long as short wavelength light is used, the surface photovoltage decay time is not affected by the volume life time unlike the μ-wave photoconductive decay method.
[0049]
Next, the time for irradiating light to generate a photocarrier will be described. As is well known in the CV method, the charge density in the depletion layer on the semiconductor surface does not respond to rapid voltage changes. That is, the charge inside the depletion layer does not leak outside when the voltage change is about 10 kHz or more. Therefore, if the light irradiation time is about 100 μs or less, minority carriers that have entered the depletion layer do not go out of the depletion layer and recombine with holes inside the depletion layer.
[0050]
  Therefore, it is desirable that the duration of the light pulse for irradiating light is about 100 μs or less. Conversely, when the pulse width exceeds 100 μs, the charge inside the depletion layer leaks from the wafer surface toward the inside, and finally the waferBack sideReaching the depletion layer on the backside of the wafer. As a result, the effect of the back surface of the wafer is mixed and analysis becomes difficult.
[0051]
The measurement of the surface photovoltage can be performed by capacitive coupling. Therefore, the surface photovoltage detection electrode may be disposed on the wafer via a gap, and in principle, the surface photovoltage measurement can be performed in a non-contact manner.
[0052]
However, if the electrodes are arranged on the wafer through the gap, the apparatus becomes expensive. In order to reduce the device cost, the sensing electrode may be brought into contact with the oxide film through a thin polymer insulating film or the like. In general, since the oxide film is not destroyed by contact with the polymer insulating film, it may be brought into contact with the oxide film through the polymer insulating film or the like in a clean environment. In general, since the oxide film is not destroyed by contact with the polymer insulating film, even if the polymer insulating film is brought into contact with the oxide film in a clean environment, the destruction measurement is not performed.
[0053]
The electrode for detecting the surface photovoltage needs to be transparent to light or to be partially transparent to light, such as a metal net. Otherwise, the wafer cannot be irradiated with light. It is necessary to bring the back surface of the wafer into contact with a metal or conductive material base in the widest possible area, and the metal or conductive base must be electrically grounded. Moreover, when irradiating light from a back surface, you may irradiate a wafer from a wafer back surface using a transparent electrode.
[0054]
When the optical power is about 20 μW, the surface photovoltage signal is about 1 mV or less, but this voltage signal is amplified about 100 times with a wide-area amplifier, and the output of the amplifier is introduced into a broadband oscilloscope. For example, when the light irradiation is stopped after 100 μs, the surface photovoltage that once rises sharply decreases.
[0055]
The surface photovoltage decay time indicates how long minority carriers among the photocarriers that enter the depletion layer survive in the depletion layer. Conversely, the surface photovoltage decay time is the time for the photocarrier to disappear upon recombination. The disappearance of the photocarrier includes a state in which charges are effectively neutralized. As described above, the recombination of minority carriers depends on the interface trap density Dit and the crystal defects in the depletion layer. However, the amount of minority carriers (holes) entering the depletion layer from the inside of the wafer is Dependent.
[0056]
In the 1950s, it was predicted that the surface photovoltage decay time might give the volume life time of the wafer (EOJohson 1957: Journal of Applied Physics, volume 28, Number 11 pp.1349-1353). . However, since the decay time of the surface photovoltage generally depends on the wavelength of the excitation light, it is not possible to interpret an experiment in which the wavelength is not specified. In addition, when the wavelength is not short, the surface life time and the volume life time are measured in parallel as in the case of the μ-wave photoconductive decay method, so that an accurate volume life time cannot be obtained. Therefore, the interpretation of the surface photovoltage attenuation characteristic shown in FIG. 3 is a novel one based on the research results of the present inventors, unlike the interpretation of the above-mentioned literature, and the present invention is excellent based on the theory of the present inventors. It is effective.
[0057]
In addition to the above method, there are various methods for generating pulsed light. For example, pulsed light can be generated by driving a semiconductor laser and a light emitting diode with a rectangular short pulse current. Alternatively, short pulse light may be formed by selecting short wavelength light from a light source such as a halogen lamp with an appropriate filter and using a mechanical shutter.
[0058]
【The invention's effect】
According to the present invention, the state of the semiconductor crystal surface defined by the depletion layer and the interface can be easily evaluated without being disturbed by the carrier lifetime in the crystal. Since the present invention can be carried out without bringing the metal electrode into direct contact with the wafer having the oxide film, it is a nondestructive inspection, and a total inspection of the wafer is also possible.
[0059]
Further, according to the present invention, it is possible to detect the surface potential, and hence the presence and sign of the oxide film charge. That is, when the surface potential is positive, that is, when the oxide film charge is positive, the surface photovoltage appears in the negative direction, and when the sign of the oxide film charge is negative, the sign of the surface photovoltage is positive. The sign of charge and surface potential can be determined. Accordingly, when measuring the lifetime of the wafer by the μ-wave photoconductive decay method, it is possible to separately grasp whether the length of the decay time is due to the influence of the surface potential. As a result, it is possible to correctly interpret the measurement result by the μ-wave photoconductive decay method.
[0060]
In particular, in the case of silicon crystal, the positive oxide film charge contributes to the generation of the surface photovoltage in the case of p-type and the negative oxide film charge in the case of n-type. Therefore, the conductivity (p or n) of the wafer can be determined from the sign of the surface photovoltage. In addition, real-time measurement is possible and results can be determined immediately.
[Brief description of the drawings]
FIG. 1 is an energy level diagram for explaining a mechanism of generating a surface photovoltage.
FIG. 2 is a diagram showing a semiconductor surface evaluation apparatus according to an embodiment of the present invention.
FIG. 3 shows an optical voltage waveform detected by a method of an embodiment of the present invention, and is an output diagram of an oscilloscope emission line, showing attenuation characteristics of a surface photovoltage excited by pulsed light.
[Explanation of symbols]
1: Metal stage
2: Sample wafer
3: Mylar sheet
4, 6: Transparent electrode
5: Glass plate
10: Ar laser light source
14: Oscilloscope

Claims (10)

The laser light is modulated into a rectangular wave pulse light with a pulse width of 100 μs or less with a pulse signal, the surface of the semiconductor crystal is irradiated with this pulse light, the generated surface photovoltage is detected by capacitive coupling, and the decay time of this surface photovoltage is determined. A method for evaluating a semiconductor surface by a surface photovoltage, characterized in that crystal characteristics of the surface of the semiconductor crystal are evaluated.   2. The semiconductor surface evaluation method according to claim 1, wherein a transmission length of the laser beam is shorter than a diffusion length of minority carriers in the semiconductor crystal.   3. The semiconductor surface evaluation method according to claim 1, wherein the laser beam has a wavelength of 488 nm. In the surface photovoltage, two electrodes are arranged above the semiconductor crystal via a gap or an insulating film, the electrode far from the semiconductor crystal is grounded, and the photovoltage is taken out from the near electrode. semiconductor surface evaluation method by surface photovoltage according to any one of claims 1 to 3, characterized in that to detect by capacitive coupling by. The electrode is a transparent electrode, a semiconductor surface evaluation method by surface photovoltage according to any one of claims 1 to 4, wherein the incident the pulsed light through the transparent electrode on the semiconductor crystal. An apparatus for evaluating the crystal properties of the surface of the semiconductor crystal, said light source for emitting a record laser light, the pulse width the laser light pulse signal is modulated in the following square wave pulse light 100 [mu] s, the pulse light semiconductor The pulsed light irradiation means for irradiating the surface of the crystal, the surface photovoltage detection means for detecting the surface photovoltage generated in the semiconductor crystal by capacitive coupling, and the crystal characteristics of the surface of the semiconductor crystal based on the decay time of the surface photovoltage A semiconductor surface evaluation apparatus using a surface photovoltage. 7. The semiconductor surface evaluation apparatus according to claim 6, wherein the laser light has a light transmission length shorter than a diffusion length of minority carriers in the semiconductor crystal. 8. The semiconductor surface evaluation apparatus according to claim 6, wherein the laser light has a wavelength of 488 nm. The surface photovoltage detecting means has two electrodes arranged above the semiconductor crystal via a gap or an insulating film, and takes out the photovoltage from the electrode closer to the semiconductor crystal, 9. The semiconductor surface evaluation apparatus using a surface photovoltage according to claim 6 , wherein the electrode is grounded. 10. The semiconductor surface evaluation apparatus according to claim 6, wherein the electrode is a transparent electrode, and the pulsed light is incident on the semiconductor crystal through the transparent electrode. 11.

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