JPS6237531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an apparatus for non-contact and non-destructive measurement of the minority carrier lifetime of a semiconductor sample such as a Si wafer.

[Background of the invention]

Conventionally, the lifetime of minority carriers in a semiconductor has been determined by attaching an electrode having ohmic contact to a semiconductor sample, irradiating it with impulse light from a xenon lamp, and measuring the attenuation of the output signal. This measurement is a contact measurement and is a type of damage measurement, so it is suitable only for sampling inspections. Therefore, measurements cannot be performed online.

On the other hand, as a non-contact measurement method, there is a method in which the minority carriers are injected by irradiating light, and then the lifetime of the minority carriers is determined from the time response of absorption by applying microwaves. For example, Journal of Applied Physics Vol. 30, No. 7, July 1959, pp. 1054-1060.
See July 1959pp1054-1060). However, this method has the disadvantage that it requires expensive and complicated microwave transmitters, receivers, etc., making the entire device complicated and expensive. Furthermore,
This method has the disadvantage that when the signal is small, reading errors from the attenuation characteristics tend to become large.

[Purpose of the invention]

An object of the present invention is to provide a simple and inexpensive semiconductor characteristic measuring device for online non-contact and non-destructive measurement of the lifetime of minority carriers in a semiconductor wafer such as silicon.

[Summary of the invention]

In order to achieve the above object, the present invention irradiates a sample with a modulated light beam that passes through a transparent electrode provided to be capacitively coupled to a sample (silicon wafer) placed on a sample stage. The method is characterized in that the photovoltage generated in the sample is extracted, the phase difference with the modulation frequency of the light is detected, and the lifetime of the minority carrier is determined from the phase difference.

That is, the present invention has been made based on the following operating principle. When a wafer with shallow junctions on its surface, such as a solar cell, is irradiated with light, a photovoltage is induced. If this is considered as an electrical equivalent circuit, it becomes a circuit in which a junction impedance is connected in parallel to a photocurrent source generated by a light carrier generated by light irradiation. Here, the junction impedance consists of a parallel connection of a junction resistance and a junction capacitance. The photovoltage is the open circuit voltage when a photocurrent flows through the junction impedance. It is known that this photovoltage is generated not only in Si wafers having junctions but also in p-type Si on which an oxide film is formed and on n-type Si that has been cleaned with alkali.

When light is tripped at frequency f, both the photocurrent and photovoltage generated become alternating current. In the present invention, this alternating current photovoltage (hereinafter simply referred to as photovoltage) is detected by capacitive coupling formed by a gap between the transparent electrode and the wafer surface. If the capacitance of the air gap is sufficiently larger than the input capacitance of the phase meter, and the frequency is higher than the cutoff frequency of the high-pass filter circuit consisting of the capacitance of the air gap and the input impedance of the phase meter, the measured signal will become a photovoltage. Become. Here, if the input resistance of the phase meter is made sufficiently large, the aforementioned cutoff frequency can be reduced to several Hz or less.

Now, as can be seen from the explanation of the equivalent circuit, the photovoltage is given as the product of the photocurrent and the junction impedance. When the absorption coefficient α of light in Si is sufficiently smaller than the reciprocal of the diffusion length L of minority carriers, the photocurrent is proportional to the diffusion length L. In the case of alternating current, the photovoltage V ₀ is expressed as follows using the effective diffusion length L _f .

V ₀ =KZ _j α·L _f ...(1) Here, K is a constant that is related to the reflectance of light, quantum yield, and number of photons, but does not depend on frequency.

In equation (1), impedance Z _j is expressed by the following equation using junction resistance R _j and junction capacitance C _j for an AC signal of frequency f.

Z _j =R _j /1+i2πfC _j R _j ...(2) Here, i=√-1. In addition, the effective carrier diffusion length L _f is the diffusion coefficient of minority carriers D
It is well known that there is a relationship with the minority carrier lifetime τ as follows.

From equations (1), (2), and (3), the phase of the photovoltage is T _j = C _j R _j
It can be seen that it changes depending on two time constants, τ and τ. Usually, the minority carrier lifetime τ is on the order of μsec, which is less than 1 msec, and the time constant T _j is on the order of more than 10 msec.

Therefore, when the frequency f is changed, the phase change due to the minority carrier life τ and the phase change due to the time constant T _j can be sufficiently separated. The frequency dependence of the phase change is, for example, as shown in FIG. The phase at the frequency of 1/T _j <f<1/τ is taken as the zero reference for the phase change. In this case, the low frequency (f
<1/T _j ), the phase changes by 90 degrees due to the junction impedance, and at high frequencies (f > 1/τ), the phase changes by 45 degrees due to minority carrier life, so in the end, the phase characteristic of the photovoltage V ₀ is the first It will look like the figure. Therefore, the frequency at which the phase changes by 45 degrees by the time constant T _j is f _j , and the minority carrier life τ is
If the frequency at which the phase changes by 22.5 degrees is f〓, then normally f _j ≪f〓. That is, if f〓 is known,
The lifetime τ of the minority carrier can be found from τ=1/2πf〓.

Since the photovoltage V ₀ is extracted through capacitive coupling, the measurement is non-contact and non-destructive, and can be measured without contaminating or destroying the surface of the silicon wafer.

As described above, according to the present invention, the minority carrier lifetime τ of a silicon wafer can be measured in a non-contact and non-destructive manner, and since the phase of the signal is measured, it is relatively easy to read even when the signal strength is small, and Since no waves are used, the device is simple and can be manufactured at low cost.

[Embodiments of the invention]

This will be explained below using examples. FIG. 2 shows the basic structure of a semiconductor characteristic measuring device according to the present invention. In FIG. 2, 1 is a silicon wafer, 2 is an electrode/sample stage (metal), and 3 is a transparent electrode (indium oxide is deposited on a glass plate). The transparent electrode 3 is several tens of μm thick with the silicon wafer 1
Silicon wafers 1
The photovoltaic voltage generated by the sensor can be picked up by capacitive coupling. The light emitted by the light source 8 is intensity-modulated by the modulator 7, partially split by the beam splitter 6, and converted into an electrical signal by the photoelectric converter 9.

The other light that has passed through the beam splitter 6 is reflected by the reflecting mirror 5 and is irradiated onto the silicon wafer 1 sample by the lens 4. The phase of the photovoltage generated between the electrodes 2 and 3 is detected by a phase meter 10 as a relative change with respect to the output signal of the photoelectric converter 9, and is output and displayed by a signal processing circuit 11 as a minority carrier life τ.

〔Effect of the invention〕

As described above, according to the present invention, non-contact, non-destructive measurement of the minority carrier life of a semiconductor wafer can be realized at low cost with a simple device.

[Brief explanation of the drawing]

Fig. 1 is a principle explanatory diagram showing the relationship between the phase change of the photovoltage and the frequency, and Fig. 2 is the basic configuration of a semiconductor characteristic measuring device for measuring the lifetime of minority carriers from the phase of the photovoltage according to the present invention. It is a diagram. 1... Silicon wafer, 2... Electrode (sample stage), 3... Transparent electrode, 4... Lens, 5... Reflector, 6... Beam splitter, 7... Modulator,
8...Light source, 9...Photoelectric converter, 10...Phase meter, 11...Signal processing circuit.

Claims (1)

[Claims] 1: a means for generating a pulsed light beam with a variable frequency; a semiconductor sample placed on a 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 through an electrode; means for extracting the photovoltage generated in the semiconductor sample as an electrical signal and detecting a phase difference between the light beam signal and the light beam signal; Measure the frequency characteristics, detect the frequency corresponding to the phase of -22.5 degrees in the phase change from 0 to -45 degrees that occurs on the frequency characteristics, and calculate the frequency from the frequency corresponding to the above phase based on a predetermined relational expression. A semiconductor characteristic measuring device comprising: a signal processing circuit for calculating the minority carrier life of the semiconductor sample.