Method for evaluating semiconductor wafers

Abstract: Method and its device for evaluating semiconductor wafers that evaluates semiconductor wafers by estimating the dopant level which is equivalent to the critical value at which the excess minority carrier injection density reaches the high injection state, and that measures the minority carrier lifetime at a low-injection-state exposure condition adapted to said dopant level. Excitation light (emitted by excitation light generator 4 ) is emitted onto a semiconductor wafer 2 at varying exposure conditions as imposed by an exposure condition controller 9 . Detector 6 detects the change in the level of reflected radiation from microwaves emitted by microwave generator 5 onto the wafer 2 . The dopant level in the semiconductor wafer 2 is estimated by estimation circuit 10 ' based on the change in the exposure conditions and the change in the minority carrier lifetime as determined by the change in the microwave level. The minority carrier lifetime is measured by measurement circuit 12 at the exposure condition adapted to said dopant level as detected by exposure condition detection circuit 11 . This structure makes possible the estimation of the dopant level in the semiconductor wafer while obtaining the lifetime based on Shockley-Read-Hall statistics, and is thus highly qualified for evaluating the semiconductor wafer 2 .

Description: Background of the Invention The present invention relates to a method for evaluating semiconductor wafers. More specifically, the present invention relates to a method of estimating the dopant level in semiconductor wafers. The recent trend toward ultrahigh levels of accuracy in semiconductor devices, as typified by ultralarge-scale integration, requires that the semiconductor wafers used for this purpose be subjected to an evermore rigorous quality control. In achieving this control, it is highly desirable that semiconductor wafer evaluation be carriedout by anon-contact measurement method that does not risk contaminating or damaging the semiconductor wafer. An example of such a method is the use of microwaves to measure semiconductor properties. See for example the following journal articles: S. Deb and B. R. Nag: J. Appl. Phys. 33 (1962) 1604. H. Furuta, T. Kotsuzaki, N. Yamada, A. Usami, T. Wada and M. Imura: Technical Report of IEECE. 87-187 (1988) 27. M. Hourai, T. Naridomi, Y. Oka, K. Murakami, S. Sumita, N. Fujino and T. Shiraiva; Jpn. J. Appl. Phys. 27 (1988) L2361. Figure 1 contains a block diagram of the measurement principle for one example of the prior art devices for measuring semiconductor properties. Figure 2 contains an input/output time chart of this device for measuring semiconductor properties. Figure 3 contains a graph of the relationship between semiconductor wafer resistivity and the variation in the value of the microwave detection current. Figure 4 contains an attenuation curve as displayed on a synchroscope. As demonstrated on Figure 1, the prior device A for measurement of semiconductor properties is constituted of the following: a sample support 1 , an excitation light generator 4 that generates excitation light that is emitted onto a semiconductor wafer 2 (measurement sample) held on sample support 1 , a microwave generator 5 that generates microwaves that are emitted onto semiconductor wafer 2 , a waveguide 3 that conducts the microwaves from microwave generator 5 onto semiconductor wafer 2 , a detector 6 that detects (via waveguide 3 ) the microwaves reflected from the surface of the semiconductor wafer 2 , a synchroscope 7 that displays the microwave level variation as detected by the detector 6 , and a control circuit 8 that controls excitation light generator 4 , microwave generator 5 , detector 6 , and synchroscope 7 . The corresponding measurement principle is explained below with reference to Figure 2. A known microwave level b generated by the microwave generator 5 is emitted from the emitter tip of waveguide 3 onto the surface of semiconductor wafer 2 . A light pulse a , of known level, width, and interval, is generated by excitation light generator 4 and is emitted as excitation light onto the surface of the semiconductor wafer 2 that is also receiving microwave radiation b , thereby exciting the carriers (free electron and positive hole pairs) within the semiconductor wafer 2 . Since the majority carrier is originally present in the semiconductor wafer 2 in very large numbers, it undergoes only a relatively small change in concentration due to exposure to this excitation light. However, the minority carrier undergoes a substantial change in concentration, and the properties of the semiconductor wafer 2 can be very accurately detected by detecting the change in the minority carrier concentration. Thus, the discussion that follows will generally focus on the minority carrier. Because the minority carriers excited as above represent an excess over the carrier concentration when semiconductor wafer 2 is at thermal equilibrium, the minority carrier concentration d increases. Then, between pulses (interruption in photoexposure), these excess carriers recombine and their population gradually declines. As a result, the minority carrier concentration d also gradually declines until it reaches the level at thermal equilibrium. The exponential function exp(-t/τ) governs the time required for restoration of the thermal equilibrium condition after this type of displacement of the minority carrier from thermal equilibrium (t = time elapsed from cessation of photoexposure). The parameter τ is called the minority carrier lifetime, and it is one of the parameters that can represent the impurity concentration in a semiconductor wafer. Due to the change in electrical conductivity (i.e., resistivity) in semiconductor wafer 2 that accompanies the change in minority carrier concentration d , the relationship depicted in Figure 3 is obtained between the resistivity and the detection current value. This detection current value corresponds to changes in the level of the microwave radiation that has been emitted onto the semiconductor wafer 2 . After the level change, the reflected microwave radiation c (reflected by the surface of the semiconductor wafer 2 ) again passes through waveguide 3 and travels to detector 6 . The microwave (reflected wave) attenuation detected by synchroscope 7 is expressed by an attenuation curve, for example, as in Figure 4. The level of impurities such as, for example, heavy metals, in semiconductor wafer 2 can be evaluated from measurement of the minority carrier lifetime based on this attenuation curve. In the above-described microwave-based method for measuring semiconductor properties, the detection accuracy for the change in microwave level is reduced when the carrier injection density (corresponds to the exposure) is reduced at the point of production of the excess minority carriers by exposing semiconductor wafer 2 to the excitation light. In other words, a diminution in the carrier injection density also diminishes the magnitude of the variation in minority carrier concentration between the thermal equilibrium and injection states. Due to this, the level change in the reflected microwaves becomes smaller, the S/N ratio is diminished, and the detection accuracy is reduced. For this reason, based on considerations of the detection accuracy of the device A for measuring semiconductor properties, the carrier injection density in prior measurement methods has been established taking into account a safety factor with respect to the detection limit. The semiconductor characteristics are then evaluated by measuring the minority carrier lifetime while fixed at the carrier injection density established on the preceding basis. However, when the carrier injection density is established at a high injection state at or above some specified value, the injected carrier density can become dominant or controlling, with the result that the density values for the majority carrier and minority carrier within semiconductor wafer 2 become approximately equal. This generates a carrier migration phenomenon known as bipolar diffusion. Bipolar diffusion substantially affects minority carrier lifetime. In addition to this bipolar diffusion, the following nonlinear phenomena are produced at high injection states: variations in the diffusion constant (indicates the relationship between carrier migration distance {diffusion path} and carrier lifetime), Auger recombination in which recombination of electron-positive hole pairs occurs by a phenomenon other than the aforementioned photoexcitation recombination, and saturation of the occupied states of the recombination centers by excess carriers. Due to the complex interactions among these phenomena, the risk arises that the measured minority carrier lifetime will differ from the native minority carrier lifetime as derived from the known theoretical equation according to Shockley-Read-Hall statistics [W. Shockley and W. T. Read, Phys. Rev., 87, 835 (1952); R. N. Hall, Phys. Rev., 87, 387 (1952)]. As a result, the semiconductor wafer 2 may not be accurately evaluated in some cases. Furthermore, the change in the microwave level extends into the nonlinear segment of the microwave detection curve at high injection states, and this creates additional risk of error in measurement of the minority carrier lifetime. Summary of the Invention : The present invention was developed in order to solve the problems associated with the prior art as discussed above. The object of the present invention lies in the introduction of a method of estimating the dopant level in semiconductor wafers that estimates the dopant level which is the initially occurring impurity concentration in p-type and n-type semiconductors and which is equivalent to the critical value at which the excess minority carrier injection density enters the high injection state. The present invention essentially comprises a method of estimating the dopant level in a semiconductor wafer, wherein the semiconductor wafer is continuously irradiated with microwave radiation and the following steps are carried out: a) exposing the wafer to a pulse of excitation light to excite charge carriers in the semiconductor wafer and detecting the change with time, caused by the excitation of minority-charge carriers and their subsequent recombination following termination of the excitation light pulse, of the intensity of the microwave radiation transmitted or reflected by the wafer, and determining the lifetime of the minority-charge carriers in the semiconductor wafer from said change with time; b) repeating (a) for a number of pulses of varying height and/or width, thus determining the minority charge carrier lifetime for a number of different carrier injection densities; c) plotting a graph of minority charge carrier lifetime versus carrier injection density and determining the carrier injection density (A) where the lifetime first starts to increase with decreasing carrier injection density; d) multiplying said determined carrier injection density (A) by a factor in the range 0.8-20 to obtain an estimate of the semiconductor dopant level. The method estimates the dopant level in semiconductor wafers. Said dopant level is estimated to be equivalent to the injection density that is approximately 0.8 to 20 times the minority carrier injection density that corresponds to the photoexcitation dosage at the point at which an increase first appears in the course of the variation in the minority carrier lifetime in the semiconductor wafer which accompanies the aforesaid variation in exposure conditions. According to the method of the present invention for estimating the dopant level in semiconductor wafers, the semiconductor wafer is exposed to excitation light using various exposure conditions and is also irradiated with microwaves, and the reflected or transmitted microwave radiation is detected. The dopant level in the semiconductor wafer is estimated based on the aforesaid change in exposure conditions and the change in the minority carrier lifetime as determined from the change in the microwave level. As a result, the critical value can be estimated at which the excess minority carrier injection density reaches a high injection state. Brief Description of the Drawings : In support of a thorough comprehension of the present invention, the present invention is explained hereinafter through specific examples with reference to the attached drawings. The following examples are merely individual specific examples of the present invention and should not be construed as limiting the technical scope of the present invention. Figure 5A contains a block diagram that presents the measurement principle for a device which may be used for measuring semiconductor properties in accordance with the present invention. Figure 5B contains a block diagram that presents the measurement principle for a device for measuring further semiconductor properties. Figure 6 describes a graph that shows the relationship between carrier injection density and lifetime. Figure 7 reports the experimental results for various types of semiconductor wafers for the following ratio: injection density at point A/dopant level. Figure 8 is a block diagram that relates to another example concerning the region in the vicinity of the excitation light generator. Figure 9 is a block diagram that relates to another example concerning the region in the vicinity of the microwave waveguide. On those figures, the same numbers are used for those elements in common with the elements in the block diagram in Figure 1 (block diagram of the measurement principle for one example of a prior art device for measuring semiconductor properties). Detailed Description of the Invention : Considering first the device A 1 for measuring semiconductor properties in accordance with the invention, as demonstrated in Figure 5A this device A 1 is equipped with the following elements as in the prior art example: sample support 1 , semiconductor wafer 2 , excitation light generator 4 as one example of the exposure means, microwave generator 5 as one example of the emission means, waveguide 3 , detector 6 as one example of the detection means, and synchroscope 7 . In addition, device A 1 is also equipped with the following: an exposure condition controller 9 that is one example of the variation means for varying the excitation light's exposure conditions, and an estimation circuit 10 that is one example of the estimation means for estimating the dopant level in semiconductor wafer 2 . The measurement principle will now be explained in detail below with reference to Figures 6 and 7. This discussion omits a detailed explanation for those elements identical with the prior art since this has already been done above. Microwaves b , emitted at a known level by microwave generator 5 , are emitted onto the surface of semiconductor wafer 2 by waveguide 3 . Exposure condition controller 9 varies the exposure conditions, i.e., the pulse strength, pulse width, or pulse interval of the light pulse, individually or in combination, during exposure of the wafer surface to the excitation light output, e.g., the light pulse, from excitation light generator 4 . Figure 6 reports the correlation between the variation in carrier injection density and the variation in minority carrier lifetime as determined from the results of such experiments. For a semiconductor wafer with an already known dopant level (point C), as the carrier injection density gradually declines from the high injection state, the minority carrier lifetime first declines in that region and then begins to increase (point A). Figure 7 reports examples of the density ratio between point A and point C as measured for 8 types of semiconductor wafers: the range for the A/C ratio (= ratio between carrier injection densities at points A and C) falls within the range of approximately 1.2 to 0.05. Furthermore, when in Figure 6 the carrier injection density reaches approximately ≤ 0.5 times point A, the minority carrier lifetime becomes saturated and no longer increases (point B). Accordingly, it can be understood from Figure 6 that measurement at carrier densities between point B and point C is conducted at a low injection state that is free of high-injection-state factors that impede measurement of the native minority carrier lifetime, e.g., bipolar diffusion, changes in the diffusion coefficient, Auger recombination, saturation of occupied states at recombination centers by high injection carriers, nonlinearity of the microwave detection curve, and so forth. The invention was implemented by considering the correlation between the change in carrier injection density and the change in minority carrier lifetime as described above. In other words, the minority carrier lifetime is measured at each different exposure condition (injection density) for the excitation light as imposed by exposure condition controller 9 . This results in detection of the point (point A) at which the lifetime begins to increase as the injection density declines, and the dopant level in the semiconductor wafer 2 is estimated based on the injection density at this point A. Estimation circuit 10 manages this series of processes. The dopant level can be obtained by multiplying the injection density at point A by approximately 0.8 to 20 (reciprocal of the above-described A/C). The device A 2 for measuring further semiconductor properties will now be taken up (see Figure 5B). In addition to the structures present in the device A 1 , device A 2 is also equipped with an exposure condition detection circuit 11 , which is one example of an exposure condition detection means that detects the exposure condition adapted to the dopant level in the semiconductor wafer, and with a measurement circuit 12 that is one example of a lifetime measurement means that measures the minority carrier lifetime in semiconductor wafer 2 at the exposure condition detected by exposure condition detection circuit 11 . With respect to the estimation circuit 10 ' in this case, functionality simply to the detection of point A is sufficient. In this case, the optimal low-injection-state exposure condition is detected by exposure condition detection circuit 11 based on the injection density at point A as detected by estimation circuit 10 '. The minority carrier lifetime is measured by measurement circuit 12 using carrier injection into semiconductor wafer 2 at the exposure condition detected in this manner. The minority carrier lifetime measured in this manner is the minority carrier lifetime based on Shockley-Read-Hall statistics due to impurity alone, and those factors discussed hereinbefore that can impair measurement of the native lifetime are eliminated. Accordingly, the impurity level in the semiconductor wafer, e.g., heavy metals and so forth, can be evaluated from this lifetime. The most suitable optimal low-injection-state exposure condition from exposure condition detection circuit 11 should provide an injection density that is approximately ≤ 0.5 times the carrier injection density at point A. In the aforementioned devices, a regulator 13 (one example of the regulatory means that interrupts and/or attenuates the excitation light) may be installed as depicted in Figure 8 between the semiconductor wafer 2 and excitation light generator 4 , thereby to function as an exposure condition controller 9 that is capable of varying the light pulse exposure conditions. This regulator 13 can be constituted of, for example, a filter, attenuator, lens, diffusor plate, optical shutter, or some combination thereof. In addition, in the aforementioned devices, a regulator 13 can be installed as a separate element from the exposure condition controller 9 . In this case, the exposure conditions can be micro-controlled through an interruption and/or attenuation by regulator 13 of the emitted excitation light whose exposure condition has already been subject to variation by exposure condition controller 9 . The conditions can thereby be set more precisely. Or, the task of varying the exposure conditions can be distributed over both the exposure condition controller 9 and regulator 13 . The detection accuracy of these devices A 1 and A 2 for measuring semiconductor properties at low injection states will be considered below. Thus, even in the aforementioned devices, the detection accuracy for the change in microwave level still declines as discussed above in the case of carrier injection into semiconductor wafer 2 at a low injection state. In order to exceed the detection limit of devices A 1 and A 2 in light of this detection accuracy and realize a further reduction in injection, a beam size regulator 14 (one example of a beam-size-regulatory means) can be installed in order to make the beam size of the excitation light approximately equal to the area of the emitter tip of the microwave waveguide 3 (see Figure 9). The reason for this is considered below. When the illumination area S 2 by the excitation light is smaller than the area S 1 of the microwave emitter tip of waveguide 3 , reflected microwave radiation is also detected from the range ΔS (= S 1 - S 2 ) in which there is no excitation of excess minority carriers by the illumination of the surface of semiconductor wafer 2 by excitation light. When the contribution by this ΔS is relatively large compared to (or larger than) the contribution by the reflected microwaves in which excitation has produced a level change, the reflected microwaves travelling to the detector 6 are very noisy. The S/N ratio declines as a result and the device's detection accuracy is reduced. Accordingly, the device's accuracy can be improved by approximately equalizing the two areas S 1 and S 2 . Specific examples of the beam size regulator 14 are considered below. Thus, as depicted in Figure 9(a), a beam size approximately equal to the area of the emitter end of the waveguide 3 can be obtained by focussing the excitation light by, for example, an optical lens 14 ', and then introducing it into the microwave waveguide 3 , where the excitation light is irregularly reflected from the interior wall of the waveguide 3 . In this example, the optical lens 14 ' and the waveguide 3 are equivalent to the beam-size-regulatory means. As an alternative, as depicted in Figure 9(b), a beam size approximately equal to the area of the emitter tip of waveguide 3 can be produced by the introduction of the excitation light into the microwave waveguide 3 and diffusing it with diffusor plate 14 ''. In this example, the diffusor plate 14 '' and the waveguide 3 are equivalent to the beam-size-regulatory means. As another alternative, the emitter tip of the waveguide 3 can be contracted to agree with the beam size. In addition, the functionalities in the preceding examples, e.g., the control circuit 8 , estimation circuit 10 ( 10 '), exposure condition detection circuit 11 , and measurement circuit 12 , can be centralized in and executed by one or more computers. While the preceding methods concern measurement of the minority carrier change using reflected microwaves as the probe, any method that can sense a minority carrier increase/decrease can be used in the practical implementation, for example transmitted microwaves, and so forth. As discussed hereinbefore, the present invention can estimate the dopant level in a semiconductor wafer and thereby estimate the critical value at which the carrier injection density in the semiconductor wafer reaches the high injection state. As a consequence, the present invention is highly qualified for the evaluation of semiconductor wafers. Because the present invention's method for the evaluation of semiconductors wafers has the form as discussed hereinbefore, the dopant level in the semiconductor wafer can be estimated and the critical value can thereby be estimated at which the carrier injection density in the semiconductor wafer reaches the high injection state. As a consequence, the present invention is highly qualified for the evaluation of semiconductor wafers.

Claims: A method of estimating the dopant level in a semiconductor wafer, wherein the semiconductor wafer is continuously irradiated with microwave radiation and the following steps are carried out: a) exposing the wafer to a pulse of excitation light to excite charge carriers in the semiconductor wafer and detecting the change with time, caused by the excitation of minority charge carriers and their subsequent recombination following termination of the excitation light pulse, of the intensity of the microwave radiation transmitted or reflected by the wafer, and determining the lifetime of the minority charge carriers in the semiconductor wafer from said change with time; b) repeating (a) for a number of pulses of varying height and/or width, thus determining the minority charge carrier lifetime for a number of different carrier injection densities; c) plotting a graph of minority carrier lifetime versus carrier injection density and determining the carrier injection density (A) where the lifetime first starts to increase with decreasing carrier injection density; d) multiplying said determined carrier injection density (A) by a factor in the range 0.8-20 to obtain an estimate of the semiconductor dopant level.

Inventors: FRIEDT JEAN, FUJIHIRA CHIYO, HASHIZUME HIDEHISA, HIROSE MASATAKE, MORIN MICHEL, NAKAI YASUHIDE, FRIEDT, JEAN, FUJIHIRA, CHIYO, HASHIZUME, HIDEHISA, HIROSE, MASATAKE, MORIN, MICHEL, NAKAI, YASUHIDE
Applicants: AIR LIQUIDE, KOBE STEEL LTD, L'AIR LIQUIDE, SOCIETE ANONYME POUR
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, G01N22, G01R31