JPH0544187B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring carrier lifetime in a depletion layer when the junction resistance of a pn junction in a semiconductor is dominated by recombination current.

Generally, it is well known that the current-voltage characteristics of a pn junction diode are expressed by the following equation.

J _d = J _p (e ^q/mkTVd −1) …(1) where, J _d is the diode current density, J _p is the saturation current density, V _d is the diode applied voltage, q is the electron charge, k is the Boltzmann constant, T represents temperature, and m represents a number of 1 or more. In an ideal diode, current flows due to carrier diffusion, and m=1. However, when the intrinsic carrier concentration n _i is small like S _i , the current due to carrier recombination becomes dominant, and the characteristic is given by m=2. In the end, the current-voltage characteristics of a diode, in which the current due to recombination is dominant, are expressed as qV _d /kT on the horizontal axis and log on the vertical axis.
If (J _d /J _p ) is taken, it becomes as shown in Figure 1 (in the figure,
The curve (J _d ) _F shows the case of forward current, and (J _d ) _R
The curve shows the case of reverse current. ).

That is, in the region where recombination current is dominant, the slope of the straight line part is m = 2, and J _d at the point where the straight line intersects the vertical axis where V _d = 0 is the saturation current density J _pr due to recombination. give. Such an increase in recombination current deteriorates the diode characteristics at a small signal level.

If V _d ≪kT/q, the recombination current density
J _dr is given by the following formula.

J _dr = (qn _i WV _d ) / (√ _{po pp} V _b ) ...(2) Here, τ _po is the carrier life in the p layer near the junction, τ _pp is the carrier life in the n layer near the junction, W
is the depletion layer width, and V _b is the junction diffusion potential.

In equation (2), when τ _re =√ _{po pp} , τ _re
represents the effective carrier lifetime in the depletion layer. From this, by measuring the current-voltage characteristics of the diode and fitting the calculated value of equation (1) including equation (2) to the measured value, τ _re =√ _{po pp} can be evaluated. As a result, the quality of the diode characteristics can be evaluated based on the magnitude of τ _re . [Reference: Proceedings of the IR (Proc.
IRE), Vol. 45, Sept/1957, pp. 1228-1243] However, in this conventional method, in order to measure the current-voltage characteristics, it is necessary to form a resistive electrode on the junction diode. Therefore, when evaluating the device during the manufacturing process, there is a problem in that an extra electrode forming step is required. Furthermore, since the effects of contamination and damage associated with electrode formation occur, this method has the disadvantage of being virtually a destructive measurement.

An object of the present invention is to provide a carrier life measuring device that can non-contact and non-destructively measure the carrier life in a depletion layer of a pn junction of a semiconductor before forming an electrode on a sample.

Hereinafter, the present invention will be described in detail using the drawings.

First, the principle of the present invention will be described.

It is well known that when a pn junction of a semiconductor is irradiated with light having a photon energy greater than the energy gap, a photocurrent flows through the junction and a photovoltage is generated. This can be expressed as an electrical equivalent circuit as shown in FIG. Here, C _j represents junction capacitance, and R _j represents junction resistance.

Next, when the intensity of light is stepped by frequency, the photovoltage also changes by frequency accordingly. At this time, the open-circuit voltage _Vpp of the photovoltage generated at both ends of the junction is given by the following equation, where _Jp is the photocurrent density and S is the junction area.

V _pp =R _j /1+jωC _j R _j J _p S...(3) Here, ω=2π. Therefore, V _pp becomes an alternating current signal, and its amplitude is given by the following equation.

｜V _pp ｜=R _j /√1+ω ² C _j ² R _j ² ｜J _p ｜S...(4) By the way, the wavelength of the irradiated light is a region where the absorption of light is sufficiently shorter than the amount of carrier diffusion of the semiconductor substrate. It is known that if it is chosen so that J _p does not change with ω and can be considered constant. Therefore, in this case, the frequency dependence of |V _pp is as shown in FIG. 3 when the horizontal and vertical axes are expressed logarithmically. From now on, |
The frequency characteristic of V _pp | has a cutoff frequency _c given by the following equation.

_c = 1/2πC _j R _j …(5) Now, C _j is the depletion layer capacitance of the junction, so C _j =
It is given by ε _p ε _s S/W (where ε _p is the dielectric constant in vacuum, and ε _s is the relative dielectric constant of Si). Here, when the impurity concentration of the p layer is N _A and the impurity concentration of the n layer is N _D , the depletion layer width W is given by the following equation.

W=√2 _{p s} ^-1 ( _A ^-1 + _D ^-1 ) _b …(6) On the other hand, if V _d is sufficiently small and the junction is characterized by a recombination current, R _j can be expressed as is given by

R _j = V _d / (J _dr S) = τ _re V _b / (qn _i WS) ...(7) From this, use equations (6) and (7) to find _c from equation (5). be able to. As a result, τ _re is given by the following equation.

τ _re =(n _i /π _c )(N _A ^-1 +N _D ^-1 ) (8) Here, the intrinsic carrier concentration n _i becomes a value specific to the semiconductor once the temperature is determined. Therefore, if N _A and N _D are known, the effective carrier life τ _re can be determined by measuring _c .

Furthermore, since this method irradiates light instead of applying voltage, it can be performed non-contact and non-destructively. In other words, since the light is being stepped, the generated photovoltage is in the form of an alternating current. Therefore, the optical voltage of the signal can be measured without contact via the electrical capacitance. Therefore, there is no need to form electrodes on the diode.

Note that it is known that the values of N _A and N _D can be measured in advance in a non-contact and non-destructive manner by methods such as the eddy current method, the light reflection method, and the polarization analysis method.

Next, the present invention will be explained in detail with reference to specific examples.

FIG. 4 shows an example of a carrier life measuring device according to the present invention. 1 is a metal sample stage which also serves as an electrode. 2 is a wafer sample having a pn junction.
A glass plate 5 on which transparent electrodes 4 are attached at a distance of several tens of μm to form electrical capacitive coupling is placed on the spacer 3 so that the distance between the sample 2 and the transparent electrodes 4 is constant. . One method, light emitting diode (LED) light source 7 is a frequency sweep oscillator 8 and LED
The light is modulated by the drive circuit 9 and pulsed light having a certain frequency is emitted. The light emitted from LED7 is
The light is focused onto the sample 3 by the lens 6. The photovoltage generated on the sample 3 by the irradiation of light is transmitted through the electrodes with reference numbers 1 and 4, and the input impedance is
It is detected by a phase detection type amplifier 10 of 100MΩ or more. Next, the frequency of the oscillator 8 is swept to measure the frequency characteristics of the optical voltage. The output of the phase detection type amplifier 10 is connected to a 3 dB voltage drop detection circuit indicated by reference numeral 11, and the sweep of the frequency sweep oscillator 8 is stopped when the output drops by 3 dB. At the same time, open the gate circuit with reference number 11',
The current frequency of the oscillator 8 is connected to the interface circuit 13 as a digital signal by an A-D converter reference number 12. On the other hand, the data N _A and N _D are input to the microcomputer 14 through the interface circuit 13, and calculations are performed every time the value of _c is input. The value of the determined effective carrier life τ _re is output by the printer 15.

As explained above, according to the present invention, the carrier lifetime in the p-n junction depletion layer, which is dominated by recombination current, can be measured non-contact and non-destructively without forming an electrode on the sample. . Therefore, in the manufacturing process, the quality of the device can be determined before electrode formation, and the measured sample is not destroyed.
This can be highly effective in improving yield.

[Brief explanation of the drawing]

Figure 1 is a curve diagram to explain the current-voltage characteristics of a p-n junction diode, Figure 2 is a diagram showing the electrical equivalent circuit of a p-n junction, and Figure 3 is a diagram showing the frequency characteristics of a photovoltage. The curve diagram shown in FIG. 4 is a diagram showing an embodiment of the carrier life measuring device according to the present invention for measuring the cutoff frequency from the frequency characteristics of the optical voltage and determining the carrier life. 1...Metal sample stand and electrode, 2...Sample, 3...
Spacer, 4...Transparent electrode, 5...Glass plate, 6
... Lens, 7 ... LED light source, 8 ... Frequency sweep oscillator, 9 ... LED drive circuit, 10 ...
Phase detection amplifier, 11...3dB voltage drop detection circuit, 11'...gate circuit, 12...A-D converter, 13...interface circuit, 14...microcomputer, 15...printer.

Claims (1)

[Claims] 1. A conductive sample stage, a semiconductor sample having a bond placed on the sample stage, a transparent electrode provided in close proximity to face the sample, and a means for irradiating a pulsed photon beam with a variable frequency onto the sample; a means for generating the pulsed photon beam with a variable frequency; an amplification means connected to the transparent electrode and the sample stage for extracting a photovoltage, and a signal processing means for determining a cutoff frequency of a junction in the semiconductor sample from an output signal of the amplification means, the cutoff frequency and the semiconductor sample being A carrier life measuring device comprising a calculation means for calculating the carrier life in the depletion layer of the semiconductor sample based on a predetermined relational expression from the impurity concentration of the sample.