JPS6237532B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a device for measuring the surface potential of a semiconductor, particularly a wafer-shaped semiconductor.

[Background of the invention]

The surface potential of semiconductors is determined by wafer surface treatment (e.g. oxide film formation, chemical cleaning treatment, gas adsorption treatment)
It is a major factor in evaluating surface treatment. Therefore, it is important in the early stages of large-scale integrated circuit (LSI) processing. In addition, in recent years, special (p
-N junction) is also considered important in the field of solar power sources.

Generally, the vibration capacitance method is known as a means of measuring the potential on the surface of a substance. For example, Bell System Technical Journal Vol. 32, No. 1, 1953, Section 1.
No.1 1953 P1~).

In the case of a semiconductor, especially when an oxide film is present on the surface, it is virtually impossible to measure the potential of the semiconductor surface (covered by the oxide film) using the oscillatory capacitance method. Another method uses photoelectron radiation in a vacuum, but it is not practical.

The only way known from a practical point of view is
A metal electrode is formed on the oxide film, so-called MOS
(Metal−Oxide−Semiconductor) structure and none,
This is a method of determining the so-called flat band voltage from the relationship between bias voltage and capacitance change. Flat band potential is a voltage applied externally to flatten the slope of the energy band formed by the surface potential, and this value does not directly indicate the surface potential. However, the surface potential can be determined from the flat bang voltage using certain assumptions.

However, the flat band voltage measurement method described above has the disadvantage that the metal that serves as the electrode is directly connected to the SiO ₂ film, resulting in contamination and damage to the surface of the film. Furthermore, when measuring the capacitance, it is necessary to know in advance the thickness of the oxide film, which increases the time required for the measurement. Metal electrodes are often formed by vapor deposition, etc., so the flat band voltage measurement method requires less labor and effort from the time a sample is given to the time the results are obtained.
Time is enormous. Moreover, since the measurement corresponds to a break inspection, it is impossible to return the sample to the process line again, and considering the recent trend towards online inspection, it must be said that this method has major drawbacks. do not have.

[Purpose of the invention]

Therefore, the present invention provides a method for measuring semiconductor characteristics that can measure the surface potential of semiconductors in a short time without contact and without breaking the surface, in place of the flat band voltage measurement method, which has conventionally been considered the only method for measuring surface potential. The purpose is to provide equipment. As a result,
The present invention provides a semiconductor characteristic measuring device that can be applied to semiconductor device manufacturing processes on-line (or in-line). The target semiconductors are:
The main target is p-type Si wafers with an oxide film, but
Needless to say, it is also applicable to other semiconductor materials such as Ge and GaAs.

[Summary of the invention]

The present invention is made by applying the photovoltaic effect. When a surface potential exists (ie, the energy band is tilted), a relatively high electric field exists at the surface of the semiconductor. Therefore, when a photon beam is irradiated onto a semiconductor in this state, the electron-hole pairs generated inside the semiconductor are separated in the high electric field region on the surface.
Therefore, a so-called photovoltage is generated between the front and back surfaces of the wafer. This phenomenon appears conspicuously when an oxide film is formed on a p-type Si wafer, and its characteristics are very similar to the state in which a photovoltage is generated when a pn junction is irradiated with light. Therefore, in recent years, p
A solar cell that simply forms an oxide film on Si type has been reported.

Therefore, in the following explanation, p-type
Take a Si wafer as an example. In the present invention, a semiconductor is irradiated with a pulsed photon beam, and an alternating current photovoltage generated at that time is detected. One of the features of the present invention is that, unlike the case of solar cells, the irradiation light is pulsed. As a result, it becomes possible to detect an alternating current photovoltage via the electrical capacitance, thereby realizing non-contact measurement.

Another important reason for using pulsed light is that it makes it possible to know the dependence of the light voltage on the pulsed light frequency. In other words, not only the phase change of the photovoltage (when the photon beam is continuously modulated) can be used, but also the decay time of the photovoltage (when the photon beam is a single pulse of light) can be used as an effective parameter. It is.

First, the basic principle of the present invention will be described.
According to the findings obtained from the inventor's experimental and theoretical studies, the surface potential V _S is given by the following equation.

However, the above equation holds true when V _S /2 is smaller than the Fermi potential inside the semiconductor (which can be easily determined from the equation shown in semiconductor textbooks). If this is not the case, the above equation requires some modification, but to avoid complexity, it is omitted here.

In the above equation, q: Elementary charge k: Boltzmann constant T: Temperature μ: Mobility of majority carrier p ₀ : Density of majority carrier deep in the wafer ε _S : Permittivity of semiconductor, both of which are based on past knowledge. It can be easily obtained from

The problem is T _j . This has a time dimension and is determined by a so-called depletion layer generated inside the semiconductor by the surface potential V _S and is expressed by the following equation.

T _j = r _j C _j ...(2) As already mentioned, the generation of photovoltage due to the presence of surface potential is very similar to the p-n junction, but the p-n junction is As an electrical equivalent circuit, it is represented by a parallel combination of a resistance R _j and a capacitance C _j . formula
r _j shown in (2) is a type of junction resistance (per unit area) that was introduced by the inventor this time by imitating a p-n junction, and C _j is a depletion resistance already described in textbooks. Layer capacitance (per unit area).

Therefore, in the present invention, measuring the surface potential comes down to finding T _j .

Next, a method for measuring T _j will be described.

The methods for measuring T _j can be broadly classified into the following two types. The first method uses a photon beam that is continuously modulated at a predetermined frequency. The second method is a method using single pulse light. In the first method, an amplifier with an effectively narrow frequency band can be applied, so it can be applied even when the optical voltage as a signal is small, and is highly general. On the other hand, since the second method requires a wide band amplifier, it is difficult to apply when the optical signal is small (μV or less) from the viewpoint of the S/N ratio. However, since the configuration is simple for a sample with a relatively large photovoltage, the second method is effective when simple functionality is required.

First, a method for measuring T _j using a continuously modulated photon beam will be explained using FIG. When the photovoltage V _P is measured by changing the modulation frequency f of the photon beam, its amplitude (absolute value)
shows frequency dependence as shown in FIG. This curve can be divided into three regions.
The first region is a region where the photovoltage V _P does not depend on the modulation frequency f, and the second region is a region where the photovoltage V _P is 45
A region in which the photovoltage V P decreases with increasing modulation frequency f with a slope of 45° is followed by a third region in which the photovoltage V _P decreases with increasing modulation frequency f with a steep slope of more than 45°.

In FIG. 1, the modulation frequency f giving the first bending point T _j is indicated by f _j , which corresponds to the photovoltage V _P
is given by the modulation frequency f at the point where it has decreased to 1/√2 from the flat maximum value. According to the inventor's experimental results, this modulation frequency f _j corresponds to T _j and is given by T _j =1/2πf _j (3).

The second bending point τ in Fig. 1 is given by the lifetime of the minority carriers in the wafer, and if the lifetime is small (several tens of microseconds or less), the bending point moves to a high frequency region, and the modulation frequency It can be easily separated from f _j . Furthermore, if the wavelength of the photon beam used is shortened (for example, 500 nm), this bending point can be virtually ignored, so the second bending point τ is effectively used to find the first bending point T _j It will not be a hindrance. Incidentally, the modulation frequency f _j is almost 1 KHz or less. On the other hand, the second bending point τ is
Exists above 10KHz.

As the amplitude (absolute value) of the photovoltage _VP shown in FIG. 1 changes, the phase _φP of the photovoltage _VP , which is an AC signal, changes.
also changes depending on the modulation frequency f. This φ _P
-f characteristics are shown in FIG. That is, the frequency at the time when the phase φ _P is 45° gives f _j .

The method of determining the frequency f _j from the amplitude change of the photovoltage V _P and the method of determining the frequency f _j from the change of the phase φ _P are equivalent to observing the two sides of the same phenomenon, so it is important to consider either one of them. However, with the current state of the art, it is more reliable to examine changes in amplitude (absolute value).

Next, a method of measuring T _j using single pulse light will be explained. Figure 3 shows the waveform of single pulse light,
The waveform of the optical voltage corresponding to the pulsed light is shown. This waveform is obtained by charging the previously described C _j by light irradiation and observing the discharge waveform. Therefore, it is advantageous for the pulse width of the single pulse light to be sufficiently longer than T _j , and considering that T _j is several ms or less, the appropriate pulse width is several tens of ms.

As shown in FIG. 3, if the time required for the photovoltage V _P to reach 1/e is determined, this will give T _j , and unlike the case of the first method, T _j can be determined directly.

[Embodiments of the invention]

Hereinafter, the present invention will be described in detail with reference to embodiments. FIG. 4 shows the basic configuration of a semiconductor characteristic measuring device according to the present invention, which determines T _j from the amplitude or phase of the optical voltage V _P and also determines the surface potential V _S . A sample 2 is placed on a sample stage 1 which also serves as a back electrode, and a pulsed photon beam 10 is irradiated onto the sample 2 via a transparent electrode 3. The transparent electrode 3 is placed facing the sample 2 with an air gap in order not to damage the surface of the sample 2, but in many cases, a transparent insulating film such as a mylar film or a mica film is inserted therein. Good too. A photon beam 10 is emitted from a light source 6 and optionally a lens (not shown).
may be used to converge. A photon beam 10' emitted from a light source 6 is modulated to a predetermined frequency by an optical modulator 5, a part of which is split by a beam splitter 7, and converted into an electrical signal by a photodetector 8. This signal is used as a phase reference for the photovoltage. Most of the light, after being reflected by the reflecting mirror 4, passes through the transparent electrode 3 and enters the sample 2. The generated photovoltage is detected by the transparent electrode 3 and the electrode 1 which also serves as a sample stage, and its amplitude and phase are detected by the phase detection amplification processing device 9. Next, by scanning the driving frequency with the oscillator 11, the frequency dependence of the amplitude or phase of the optical voltage can be determined. Finally, the surface potential can be determined using equation (1) (or a modified equation as necessary),
This value can be displayed.

FIG. 5 shows that T _j
An example of finding the following is shown below. The arrangement of the sample 2, transparent electrode 3, sample stage 1, reflecting mirror 4, optical modulator 5, light source 6, etc. is the same as that shown in FIG. By generating a single pulse with the pulse generator 12 and using this voltage as a trigger signal to observe the attenuation waveform of the optical voltage with the synchroscope 13 having a high input impedance, T can be obtained as already explained using FIG. 3. _j can be found. Therefore, the surface potential can be calculated from equation (1).

Since the surface potential changes depending on the gas atmosphere, if an appropriate gas atmosphere is to be created around the sample, the sample may be placed in an appropriate container. Furthermore, it is clear that in order to select measurement points for research on the sample surface, it is sufficient to deflect and scan the photon beam, and this can be easily achieved by adding a conventionally known optical beam deflection device. Ru.

〔Effect of the invention〕

As explained above, according to the present invention, conventionally,
It becomes possible to provide a non-destructive mounting device for surface potential, which has been virtually impossible.

Moreover, according to the present invention, since the thickness of the oxide film does not intervene in the measurement, the measurement labor can be significantly reduced compared to the flat band voltage measurement method.

Further, according to the present invention, measurement can be performed without making any changes to a given sample, and as a result, the present measuring device can be introduced during the mounting process.

Therefore, its industrial and physical effects are enormous.

[Brief explanation of the drawing]

Figure 1 shows the relationship between the photovoltage and the modulation frequency of light, Figure 2 shows the relationship between the phase of the photovoltage and the modulation frequency, and Figure 3 shows how the sample was irradiated with light pulses. FIG _. 4 is a diagram showing the temporal relationship between the optical pulse waveform and the optical voltage signal at the time of the test. Basic configuration, 5th
The figure shows T _j measured from the temporal attenuation characteristics of the photovoltage,
FIG. 2 is a basic configuration diagram of an embodiment for determining surface potential. 1...Sample stand (electrode), 2...Semiconductor sample, 3
...transparent electrode, 4...reflector, 5...light modulator,
6...Light source, 7...Beam splitter, 8...Photodetector, 9...Phase detection amplification processing device, 10,1
0'... Photon beam, 11... Oscillator, 12...
Pulse generator, 13... synchroscope.

Claims (1)

[Claims] 1. means for generating a pulsed light beam with variable frequency; a semiconductor sample placed on a conductive sample stage; a transparent electrode disposed facing the semiconductor sample so as to be capacitively coupled; means for irradiating the semiconductor sample with the light beam passing through the transparent electrode; and a means for extracting the photovoltage generated in the semiconductor sample as an electrical signal by the transparent electrode and the conductive sample stage; Find a specific frequency of the light beam signal that gives a phase difference of approximately 45° between the signal and the light beam signal, or a drop in amplitude of the electrical signal of approximately 1/√2 from the maximum value, and calculate a predetermined frequency from the specific frequency. A semiconductor characteristic measuring device comprising means for calculating the surface potential of the semiconductor sample based on the relational expression.