JPS6253944B2 - - Google Patents

Description

[Detailed description of the invention]

SPV (Surface Photovoltaic) is currently used as a lifetime measurement method for semiconductors.
Law, EBIC (Electron Beam Induced Current)
method, accumulation effect measurement method, MOS method, photoconductive decay method, etc. These measurement methods each have excellent features in terms of resolution, measurement accuracy, etc., but since they require electrodes to be attached to the measurement sample, there is a risk that the measurement sample may be defective. In order to avoid this, a non-contact measurement method based on the photoconductive attenuation method is being developed in which changes in conductivity are detected using microwaves (microwave method). There are two types of microwave methods: a method for detecting transmitted microwaves (transmission microwave method) and a method for detecting reflected microwaves (reflection microwave method).
The latter has a wider range of applications than the transmission microwave method because it has no restrictions on wafer shape and can measure even relatively low-resistance samples. The measurement system used for reflection microwave non-contact lifetime measurement consists of an injection light system (irradiation light system) that excites excess carriers and a microwave system that detects changes in the conductivity of the semiconductor wafer. . Such a measurement method is promising as a non-contact, non-destructive measurement method for semiconductor wafers. Generally, when measuring the minority carrier lifetime of a semiconductor wafer, the actual measured lifetime value τ _n is not only the bulk lifetime τ _b determined by the purity of the semiconductor crystal and crystal defects, but also the dirt and surface treatment layer of the semiconductor wafer. The relationship is 1/τ _n =1/τ _b +1/τ _S , where τ _S is related to the surface recombination rate _S. Therefore, in order to know the value of τ _S included in the actual measurement value of minority carrier lifetime of a semiconductor wafer, it is necessary to simultaneously measure and evaluate the surface recombination rate. In the minority carrier lifetime measurement method for semiconductor wafers using the reflected microwave method,
There is a known method to detect the attenuation characteristics of minority carriers excited by irradiating light with a specific wavelength and specific pulse width, and to separate and obtain τ _b and S by comparing the results with the results of numerical analysis (Usami et al. , Kandatsu, Kudo applied physics
49 (1980) 1192-1197). However, the first method has disadvantages in that it is necessary to perform lifetime measurements by changing the thickness of the sample wafer, which takes time to process the sample, and the sample cannot be reused after measurement. In addition, in the second method, the calculation is based on the deviation from the exponential change of the attenuation curve of the minority carriers, so wafers with a small S value (for example, S is 1000
In the case of wafers of approximately cm/sec or less, measurement errors were large and accurate separation evaluation was difficult. On the other hand, in recent years, as part of measures to improve device yield and quality through non-contact, non-destructive testing in the LSI manufacturing process, τ _b and S
In-process, non-contact, non-destructive measurement and evaluation has become important. SUMMARY OF THE INVENTION An object of the present invention is to provide a non-contact measurement method that can determine the values of τ _b and S even for a semiconductor wafer with a relatively small S value, which has been difficult using conventional methods. In the method of the present invention, when measuring the lifetime of minority carriers in a semiconductor wafer using the reflection microwave method, the semiconductor wafer is irradiated with light with a pulse width t ₀ to excite the minority carriers. Detects the situation in which the attenuation differs depending on the concentration distribution shown in the depth direction of the semiconductor as a transient phenomenon of the reflected microwave signal, and calculates τ _n as a function of the wavelength of the irradiated light and the pulse width t ₀ of the irradiated light. It is measured spectroscopically, and by setting the wavelength of the measurement irradiation light arbitrarily and measuring τ _n while changing t ₀ , a curve showing the relationship between τ _n and t ₀ is obtained and prepared. This is a non-contact measurement method for semiconductor wafers in which τ _b and S are determined separately from τ _n using the results of numerical analysis as a measure. That is, to briefly describe the present invention, a semiconductor wafer is irradiated with light having an energy greater than the forbidden band width, and while changing the pulse width t ₀ of the irradiated light, the state of recombination attenuation of the excited minority carriers is reflected. In the method of measuring the lifetime of a minority carrier, which is detected as a change in the microwave intensity signal, the saturation value τ _ST of the actual measurement value τ _n of the lifetime and the pulse width t _ST of the irradiation light that gives τ _ST are actually measured. τ _ST ”
S of τ _b obtained from the intersection with "graph of surface recombination rate S versus arbitrary τ _ST obtained by numerical analysis using bulk lifetime τ _b as a parameter"
τ _b obtained from the intersection of the dependence curve a, “actually measured t _ST ” and “graph of surface recombination rate S versus arbitrary t _ST obtained by numerical analysis using bulk lifetime τ _b as a parameter” This is a non-contact measurement method for semiconductor wafers in which the bulk lifetime τ _b and the surface recombination rate S are determined from the intersection of the S dependence curve b with the S dependence curve b. The temporal change in the concentration △p of minority carriers excited when a semiconductor is irradiated with light with a pulse width t ₀ is schematically shown in Figure 1, and the carrier concentration increases from generation until it reaches the maximum point. It consists of a region where the light is cut off and a region where the light is attenuated from the maximum point due to recombination. Generally speaking, the lifetime of minority carriers relates to the attenuation characteristics of a region, and indicates the time it takes for the carrier concentration to decay to 1/e of its maximum value. The concentration distribution of minority carriers in the region can be obtained by solving the following equation. ∂△p/∂t+△p/τ−D∂ ² △p/∂x ² = {U(t)−U(t−t ₀ )}Rexp(−αx)
...(1) △p: Excess minority carrier concentration t: Time τ: Lifetime x: Depth distance within the semiconductor D: Diffusion constant U: Function related to pulse waveform R: Number of photons of light injected into the semiconductor α: Light absorption coefficient R in equation (1) is expressed as follows. R=qαI(1-Rs)/hν q: Quantum efficiency I: Number of irradiated light photons per unit area Rs: Surface reflectance of irradiated light Expression (1) is changed to the initial condition △p(x, o)=0 and By solving based on the boundary conditions (2a) and (2b), the carrier concentration distribution in the region shown in Figure 1 can be obtained. D∂△p/∂x| _{x=0 =} S _A △p(o, t) …(2a) D∂△p/ _∂x | ) W: Wafer thickness S _A =S _a /D, S _B =S _b /D S _a and S _b represent the surface recombination speeds on the front and back sides of the wafer, respectively. The concentration distribution of minority carriers in the region can be obtained by solving the following equation. ∂△p/∂t+△p/τ−D∂ ² △p/∂x ² =0…
(3) Expression (3) as initial condition △p obtained from expression (1)
By solving under (x, t ₀ ) and boundary conditions (2a) and (2b), the concentration distribution of minority carriers in the region shown in FIG. 1 can be obtained. The method of the present invention sets the bulk lifetime τ _b by giving the absorption coefficient α of the irradiated light, the diffusion constant D, and the wafer thickness W for the attenuation curve in the above region, and obtains it by numerical analysis. It will be done. The value of the pulse width t _ST of the irradiation light that gives the lifetime saturation values τ _ST and τ _ST of the curve group of lifetime vs. t ₀ (e.g., Figure 2) with the surface recombination rate S as a parameter (Figure 2) τ _ST and t _ST ) corresponding to each point indicated by an arrow in the graph are plotted to produce the following graphs: (a) A graph of S versus any τ _ST with the bulk lifetime τ _b as a parameter ( (Example, Figure 3), (b) Prepare a graph of S vs. arbitrary t _ST with the bulk lifetime τ _b as a parameter (Example, Figure 4), and use the reflection microwave method to measure the lifetime. The relationship curve between the actual measured lifetime value τ _n of the obtained semiconductor wafer and the pulse width t ₀ of the irradiation light (for example, the sixth
Figure: Actual τ _S detected from τ _n vs. t ₀ graph)
The value of _S for the same τ _{b is detected by comparing the values of T and the measured t ST with the graphs of S vs. any τ ST and S vs. any t ST obtained by the} above-mentioned numerical analysis. . Examples Next, examples of the method of the present invention will be described.
FIG. 5 shows an example of the configuration of an apparatus for measuring minority carrier attenuation characteristics of a semiconductor wafer using reflected microwaves, which is suitable for carrying out the method of the present invention. Fifth
The irradiation light source 1 shown in the figure can select and set the wavelength of the generated light to two or more types, and the pulse width of the generated light is changed by a pulse generator 9. The irradiation light is irradiated onto the semiconductor wafer 2 loaded on the moving stage 3. The microwave emitted from the microwave generator 5 oscillated by the DC power supply 6 and radiated via the circulator 4 changes the concentration of minority carriers excited in the semiconductor wafer by this irradiation light. The state of reflection by the wafer 2 is observed and measured through a detector 7 using an oscilloscope 8 as a display. P-type silicon etched wafer (thickness
500μm, specific resistance 45Ω・cm) as a sample, the fifth
With the measurement system shown in the figure, the wavelength is 940 nm (α = 230 cm ^-1
), and measured τ _n while changing the pulse width t ₀ to obtain τ _n vs. _{t 0} as shown in Figure 6.
I got a graph of From Figure 6, the saturation value τ _ST of τ _n is
Pulse width t that gives 23.5 μsec, τ _ST is 19.5 _μ
It read as sec. This value of τ _ST was compared with the graph of S versus arbitrary τ _ST (Figure 3) with τ _b as a parameter prepared by numerical analysis to obtain the values shown in the table below.

[Table] Next, the values of _tST were prepared by numerical analysis.
The values shown in the following table were obtained by comparing with the graph of S vs. arbitrary _tST (Fig. 4) with the bulk lifetime τ _b as a parameter.

[Table] For numerical analysis to obtain Figures 3 and 4,
The following values were used as the absorption coefficient α of the irradiation light, the diffusion constant D, and the wafer thickness W, respectively. α=230cm ^-1 D=30cm ² /sec W=500μm Using the values in the two tables above, τ _ST is shown in Figure 7.
From the intersection of the S dependence curve a of τ _b obtained from the measured value of tST and the S dependence curve b of τ _b obtained from the measured value of _tST , we can find that τ _b = 84 (μsec) and S = 490 (cm/sec). Got the results. This value is in good agreement with the results of evaluating the same single crystal using the photoconductive attenuation method.
cm/sec or less), it was also found that τ _b and S can be determined separately. According to the method of the present invention, the lifetime of minority carriers of a semiconductor wafer can be measured and the surface recombination rate can be determined at the same time. By accumulating numerical analysis results through computer processing,
Measures various wafers non-contact and non-destructively.
Automatic evaluation is possible in an inline process.

[Brief explanation of the drawing]

Figure 1: Schematic diagram showing temporal changes in the concentration of minority carriers excited when semiconductor pulsed light is irradiated, Figures 2 to 4: Results obtained by numerical analysis for carrying out the method of the present invention. Figure 5: A block diagram showing a lifetime measurement system for carrying out the method of the present invention; Figure 6: Graph of τ _n vs. t ₀ measured in an embodiment of the present invention; Figure 7: S-dependence curves a and b of τ _b for embodiments of the invention. DESCRIPTION OF SYMBOLS 1... Irradiation light source, 2... Semiconductor wafer, 5... Microwave generator, 7... Detector, 9... Pulse generator.

Claims (1)

[Claims] 1 Irradiate a semiconductor wafer with light with energy greater than the forbidden band width, and while changing the pulse width t ₀ of the irradiated light, detect the state of recombination attenuation of the excited minority carriers as a change in the intensity signal of reflected microwaves. In the method of measuring the lifetime of a minority carrier, the saturation value τ _ST of the actual measurement value τ _n of the lifetime and the pulse width t _ST of the irradiation light that gives τ _ST are actually measured,
S dependence curve a of τ _b obtained from the intersection of “actually measured τ _ST ” and “graph of surface recombination rate S versus arbitrary τ _ST obtained by numerical analysis using bulk lifetime τ _b as a parameter” and the surface recombination rate S vs. arbitrary t obtained by numerical analysis using “actually measured t _ST ” and “bulk lifetime τ _b as parameters.
A non-contact measurement method for semiconductor wafers that determines the bulk lifetime τ _b and the surface recombination rate S from the intersection of τ _b determined from the intersection with the S-dependent curve b with _{the ST} graph.