Method for analying an impurity on a semiconductor substrate
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for analyzing an impurity present on the surface of a semiconductor substrate and, in particular, to a method for measuring an amount of impurity on a semiconductor substrate surface at those local areas of a repeated pattern. 
     2. Description of the Related Art 
     It has been known that a metal impurity is present on the surface of a semiconductor substrate or a semiconductor wafer and that the metal impurity, even if being very small in amounts, adversely affects the electric characteristics of a device. With a recent tendency toward realizing a high-density device, more and more exacting demand is made for the analysis of an impurity. There is also a high demand for the high-accurate analytical technique. 
     In the conventional analytical method, a solution for impurity collection, such as an acid solution, is dropped on a whole surface area of a semiconductor substrate or on a nearly liquid-drop-size area on the surface of a semiconductor substrate where an impurity is collected. Then an amount of impurity collected in that solution used is measured by a chemical analytic apparatus such as a flameless atomic absorption apparatus. 
     It has been known by an accelerated test that, on a semiconductor substrate having a surface pattern similar to a device surface pattern, the surface distribution pattern of an impurity concentration depends upon the surface pattern of the semiconductor substrate. The device surface pattern has become very small with the microminiaturization of the semiconductor element so that the surface distribution pattern of take impurity concentration becomes very small. Recently, the surface pattern of the device is of the order of a micron or submicron. 
     On the other hand, the surface resolution by the conventional analytical method is of the order of a few cm 2 , an ordinary size of to-be-collected liquid drops at best, thus failing to achieve high-accurate analysis. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the present invention to provide a method capable of accurately analyzing an amount of impurity on the surface of a semiconductor substrate. 
     According to one aspect of the present invention, a method is provided for analyzing an impurity on the surface of a semiconductor substrate, comprising the steps of: 
     oxidizing those to-be-measured areas on a surface of a semiconductor substrate to provide corresponding oxide films and trapping an impurity in the oxide film at each area of the semiconductor substrate; 
     removing an impurity from other than the areas on the surface of the semiconductor substrate; 
     dissolving the oxide films on the to-be-measured areas into droplets; 
     collecting the droplets into a drop; and 
     measuring an amount of impurity in the drop. 
     According to another aspect of the present invention, a method is provided for analyzing an impurity on a semiconductor substrate, comprising the steps of: 
     depositing nitride films on those to-be-measured areas on a surface of a semiconductor substrate and trapping an impurity in the nitride film at each area of the semiconductor substrate; 
     removing an impurity from other than the areas on the surface of the semiconductor substrate; 
     dissolving the nitride films on the to-be-measured areas into droplets; 
     collecting the droplets into a drop; and 
     measuring an amount of impurity in the drop. 
     According to a further aspect of the present invention, a method is provided for analyzing an impurity on a semiconductor substrate, comprising the steps of: 
     oxidizing those to-be-measured areas on a surface of a semiconductor substrate to provide corresponding oxide films and trapping an impurity in the oxide film at each area of the semiconductor substrate; 
     removing an impurity from other than the areas on the surface of the semiconductor substrate; 
     dissolving the oxide films on the to-be-measured areas into droplets; 
     collecting the droplets into a drop; 
     measuring an amount of impurity in the drop; 
     repeating the oxidizing step through the measuring step; and 
     plotting the amount of impurity obtained at each measuring step in a depth direction of the semiconductor substrate. 
     According to a still further aspect of the present invention, a method is provided for analyzing an impurity on a semiconductor substrate, comprising the steps of: 
     depositing nitride films on those to-be-measured areas on a surface of a semiconductor substrate and trapping an impurity in the nitride film at each area of the semiconductor substrate; 
     removing an impurity from other than the areas on the surface of the semiconductor substrate; 
     dissolving the nitride films on the to-be-measured areas into droplets; 
     collecting the droplets into a drop; 
     measuring an amount of impurity in the drop; 
     repeating the nitride film depositing step through the measuring step; and 
     plotting the amount of impurity obtained at each measuring step in a depth direction of the semiconductor substrate. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a view showing a model showing a surface pattern on a semiconductor substrate; 
     FIG. 2 is a cross-sectional view, taken along line II--II in FIG. 1; 
     FIGS. 3A to 3D show the steps of a method for analyzing an amount of impurity on the surface of a semiconductor substrate in accordance with one example of the present invention, a view mainly showing a layer variation on the substrate surface; 
     FIGS. 4A to 4D show initial steps of a method for analyzing an amount of impurity on the surface of a semiconductor substrate in accordance with another example of the present invention; and 
     FIGS. 5A to 5D show initial steps of a method for analyzing an amount of impurity on the surface of a semiconductor substrate in accordance with another example of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be explained below with reference to the accompanying drawings. 
     FIG. 1 shows a model of a surface pattern on a semiconductor silicon substrate. A surface pattern layer 14 is formed of, for example, an insulating layer for semiconductor element isolation, providing a repeated pattern. Semiconductor element areas 16 are each formed by a semiconductor substrate surface area bounded by the isolation layer 14. In this embodiment, a respective local area 18 to be measured constitutes a portion of each element area, that is, those local areas 18 constitute mutually corresponding areas on the repeated pattern. FIG. 2 is a cross-sectional view, taken along line II--II in FIG. 1. In FIGS. 3A to 3D, the insulating pattern 14 for isolation is omitted for brevity&#39;s sake. 
     The analytical examination is made by the following way. 
     As shown in FIG. 3A, an ultraviolet radiation 22 is directed with a given pattern as a mask to those to-be-measured local areas 18 on the surface of a semiconductor silicon substrate 12 in a clean oxygen atmosphere. In FIG. 3A, an insulating layer pattern 14 for isolation is omitted for the sake of simplification. 
     Then only the local area 18 irradiated with the ultraviolet radiation is oxidized by a heating process to form an oxide film 24, as a few nm-thick SiO 2  film, only on the areas to be measured. By doing so, an impurity 26a in the area on the substrate surface is trapped in the oxide film 24. 
     The surface of the semiconductor substrate 12 is cleaned with a suitable detergent, such as an acid solution consisting of, for example, a hydrochloric acid/hydrogen peroxide-mixed aqueous solution. As shown in FIG. 3B, an impurity on other than the to-be-treated area on the surface of the semiconductor substrate is removed. 
     Thereafter, the resultant semiconductor substrate surface 12 is exposed with, for example, a hydrofluoric acid vapor to dissolve the oxide film 24, so that, as shown in FIG. 3C, very fine droplets (impurity drops) 30 containing the impurity are left on the surface of the substrate 12. 
     About 100 to 500 μl of a liquid is dropped to the semiconductor substrate 12 and the very fine droplets (impurity collecting drops) are moved on the semiconductor substrate like in a scanning way to collect fine droplets 30 on the substrate surface and hence to provide a gathered drop 32 as shown in FIG. 3D. The liquid may be water, preferably, a high-purity acid solution consisting of, for example, a hydrochloric acid/hydrogen peroxide aqueous solution. 
     Then an amount of impurity 26a in the gathered drop is measured by a known chemical spectrometer, such as a flameless atomic absorption spectrometer and inductively coupled plasma mass spectrometer. From the measured amount it is possible to calculate the concentration level of the impurity per unit area of the area to be measured. Any other measuring device may be used in place of the chemical spectrometer. 
     As evident from the embodiment, it is possible according to the present invention to achieve a high surface resolution and hence to measure, with high accuracy, an amount of impurity 26a in the local areas 18 on the semiconductor substrate 12. In other words it it possible to highly accurately obtain the distribution characteristic from the amount of impurity on the surface of the semiconductor substrate 12. Although, according to the present invention, the local areas on the substrate surface correspond to a regularly repeated pattern array on the semiconductor surface 12, the present invention is not restricted thereto and it is also possible to measure a single local area 16 or any plurality of local areas. 
     The sensitivity of the present method under the chemical analysis, depending upon the size or area of the local area to be measured, is of the order of 10 9  to 10 10  atoms/cm 2 , a very high value than an ordinary sensitivity under a physical analysis of the order of 10 13  to 10 14  atoms/cm 2 . Even the impurity distribution characteristic of a pattern as small as a micron or submicron order under the pattern rule of a recent device can be obtained with high accuracy. 
     In the above embodiment, the position of the surface portion of the semiconductor substrate 12 at which the oxide film 24 has been dissolved, is lowered in the depth direction of the semiconductor substrate 12. Therefore, after the step of collecting the very fine droplets 30, an insulating film is formed on those local areas, to be measured, on the resultant semiconductor substrate structure by the application of an ultraviolet radiation with the above-mentioned mask again used and, by doing so repetitively, a corresponding distribution characteristic is formed, each time, relative to the amount of impurity, so that amount-of-impurity distribution characteristics can be gained at different positions in a depth direction of the semiconductor structure, that is, it is possible to obtain amount-of-impurity distribution characteristics in the depth of the semiconductor structure by plotting these distribution characteristics in that depth direction. 
     In the case where an impurity-free epitaxial layer is formed on a semiconductor wafer (semiconductor substrate), if those selected areas on the surface of the epitaxial layer are those spots to be measured, it is possible to, upon the formation of an oxide film, suppress the entry of an impurity from the semiconductor wafer into the oxide film. Therefore, those droplets collected contain only that impurity trapped in the epitaxial layer surface and more accurate analytic examination can be made for a metal impurity, such as Fe and Na, present on the surface of the semiconductor substrate. 
     Although, in the above-mentioned embodiment, the oxide film 24 has been explained as being formed by an irradiation with the ultraviolet radiation and hence application of heat, it may be possible to utilize the exposure and hence heating of, for example, a laser beam. The formation of the oxide film 24 is not restricted only to the exposure of light. For example, an SiN film or SiO 2  film 42 is formed, by a known chemical vapor deposition method, on the whole surface of a semiconductor silicon substrate 12 as shown in FIG. 4A and a photoresist layer 44 whose pattern is an inverted version of a pattern for those to-be-measured areas on the semiconductor substrate is formed by a known method on the SiO 2  film 42 as shown in FIG. 4B. Then only those SiO 2  film 42 portions on these areas are left, by a selective etching, on the semiconductor substrate 12 with the photoresist layer 44 used as a mask as shown in FIG. 4C, followed by the elimination of the photoresist layer 44. By doing so it may be possible to obtain those SiN or SiO 2  film 42 portions as shown in FIG. 4D. 
     The oxide film 42 may be replaced with a nitride film. Forming the nitride film can be achieved in substantially the same way as in the case where the SiO 2  film 42 is formed on the substrate as shown in FIGS. 4A to 4D. That is, an SiN film 52 is formed, by a known chemical vapor deposition method, on the whole surface of a semiconductor silicon substrate 12 as shown in FIG. 5A and a photoresist layer 54 whose pattern is an inverted version of a pattern for those to-be-measured areas on a semiconductor substrate 12 is formed, by a known method, on the SiN film 52 as shown in FIG. 5B. Then the surface of the resultant semiconductor structure is selectively etched with a photoresist layer 54 used as a mask to leave only those to-be-measured areas of the SiN film 52 on the surface of the semiconductor device as shown in FIG. 5C, followed by the elimination of the photoresist layer 54 as shown in FIG. 5D. The remaining processing steps are the same as those using the SiO 2  film set out in conjunction with FIGS. 3A to 3D. 
     As evident from the above explanation, according to the present method, it is possible to realize a high surface resolution. It is possible to highly accurately measure an amount of impurity on local areas on the semiconductor substrate 12 and hence to highly accurately obtain a distribution characteristic relating to an amount of impurity on the surface of a semiconductor substrate 12. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.