As a method of evaluating metal contamination in a silicon substrate manufacturing process or a device manufacturing process, the measurement of a recombination lifetime by a microwave photoconductivity decay method (μ-PCD method) has been widely used. According to the μ-PCD method, a wafer is first irradiated with pulses of light having energy larger than that of a bandgap of a silicon single crystal to generate excess carriers in the wafer. The generated excess carriers increase the conductivity of the wafer, and thereafter the excess carriers disappear with time due to the recombination, with the result that the conductivity decreases. This change is detected as a change with time in reflected microwave power and analyzed so that a recombination lifetime can be determined. The recombination lifetime becomes shorter when metal impurities and defects forming a level to be a recombination center in a forbidden band are present. Therefore, the metal impurities, crystal defects, and the like in the wafer can be evaluated by measuring the recombination lifetime (for example, Non Patent Document 1).
In the case where a sample to be evaluated has a wafer shape, the excess carriers generated by the pulses of light not only disappear due to the recombination in the wafer but also diffuse to a front surface and a back surface of the wafer to disappear due to the surface recombination. Thus, in order to evaluate metal contamination and crystal defects in the wafer, it is necessary to suppress the surface recombination on the front surface and the back surface. As a method of suppressing the surface recombination, heat oxidation treatment (oxidation film passivation) and electrolyte solution treatment (chemical passivation processing, sometimes abbreviated as “CP processing”) have been generally used. In the oxidation film passivation, care needs to be taken so as not to cause metal contamination and crystal defects in a heat treatment step for forming an oxidation film. Therefore, in the case of evaluating metal contamination in heat treatment furnaces other than an oxidation furnace, for example, an epitaxial growth furnace for manufacturing an epitaxial wafer, the chemical passivation processing is used.
As a solution for the chemical passivation processing, an alcoholic iodine solution (for example, Non Patent Document 2) and an alcoholic quinhydrone solution (for example, Patent Document 1) are known. In the case of the alcoholic quinhydrone solution, it takes time for surface passivation effects to be stabilized (for example, Non Patent Document 3). Therefore, in the case where it is desired to obtain evaluation results of metal contamination as fast as possible, the alcoholic iodine solution is used.
However, in the chemical passivation processing using the alcoholic iodine solution, there is a problem in that a measurement value decreases with time after the processing, and hence the measurement value varies. In order to solve this problem, in Patent Document 2, there is disclosed a method of correcting a measurement value through use of a correction formula that expresses a correction value for correcting a change with time in a measurement value as a function of time. However, this method has a problem in that it is necessary to measure a change with time in a recombination lifetime, and hence it takes time and labor.
Along with an increase in performance of a semiconductor device, even a small amount of metal contamination has an adverse effect on the performance of the device, and hence it has become a crucial issue to reduce metal contamination. In particular, in imaging elements such as a charge-coupled device (CCD) and a CMOS image sensor (CIS), a faint white flaw, a dark current, and the like become problems along with the enhancement of light-receiving sensitivity and resolution, and there is a risk in that a slight amount of metal contamination may have an adverse effect on the imaging elements. Therefore, in an epitaxial wafer used widely as a substrate for an imaging element, it is strongly desired to reduce not only metal contamination in a device manufacturing process but also metal contamination in a process of manufacturing the epitaxial wafer.
In order to reduce metal contamination in the silicon substrate manufacturing process or the device manufacturing process, a method of evaluating a slight amount of metal contamination with high sensitivity and high accuracy is required. Further, whether or not products are to be manufactured depends on results obtained by evaluating metal contamination, and hence it is required that evaluation results be obtained as fast as possible.
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