A number of methods have been devised to detect damage or imperfections in monocrystalline semiconductor articles, such as silicon wafers. The detection of such imperfections is of particular interest in the manufacture of integrated circuits where imperfections in the surface layers of semiconductor substrates can reduce the yield of usable integrated circuit chips. These methods include, for example, silica bevel and etch, scanning oscillating topography, and capacitor leakage current measurements. Recently, cathodic current measurements have been employed to characterize P-type semiconductor substrates in which the substrate is placed in a dilute aqueous acid electrolyte solution and is biased a few volts negative with respect to the electrolyte. It is believed that under the influence of the electrical field, a layer of positive ions coming from the electrolyte solution develops at the semiconductor/solution interface while holes are pushed away from the semiconductor surface into the bulk of the substrate to a depth W. Only the acceptor negative ions remain in this depletion zone, and their charge equilibrates the positive charge of the ion layer at the semiconductor/solution interface. Under these conditions, if a defect generates electron/hole pairs inside the depletion zone, the holes are pushed into the bulk of the substrate by the electrical field and the electrons are drawn to the semiconductor/solution interface where they can react with the positive ions in the solution at the interface. This mechanism produces a current which can be measured outside the cell. The expected reaction between electrons and positive ions at the interface in the solution would be; EQU H.sub.3 O.sup.+ + e.sup.- .fwdarw. 1/2 H.sub.2 + H.sub.2 O
because the H.sub.3 O.sup.+ ion concentration is greater than that of the minority carriers at the semiconductor/solution interface the reaction is controlled by the generation rate of electron/hole pairs or, in other words, by the electrically active defects in the depletion area. The measurement of the cathodic current is then a characterization of the quality of the semiconductor substrate. THe semiconductor material is not a participant in the electrochemical reaction so that the substate is not altered. It was also noted that artificial defects in a silicon wafer could be produced by a high intensity light flash with a biasing voltage of about 5 volts. In this case when the cell was illuminated, an important gas bubbling was noticed on the wafer.
Although the cathodic current measurements are nondestructive and fast, such measurements do not give an indication of the nature or location of the defects. For example, a single large defect at one place on a semiconductor wafer and a large number of small but significant defects distributed over a relatively large area of a second wafer might give the same cathodic current. In the former case, the wafer would be suitable for integrated circuit manufacture because the damage is limited to one chip site or could even be in the Kerf area of the wafer resulting in, at most, a small loss in yield. The wafer would be considered a good wafer whereas the second wafer with a large number of small defects would be unsuitable for integrated circuit manufacture.
A method of mapping semiconductor wafer quality has been disclosed in an IBM Technical Disclosure Bulletin, Vol. 18, No. 12, May 1976, page 4012, article entitled "In-Line Wafer Quality Monitor," in which an array of light emitting diodes is employed in order to illuminate different areas of the wafer so that cathodic current information is obtained on different segments of the wafer. An IBM Technical Disclosure Bulletin, Vol. 18, No. 11, April 1976, page 3623, article entitled "Scanning Cathodic Current Spectroscopy," employs a laser to scan the semiconductor material in order to map the depletion region, with defect regions in this case producing a decrease in current, so that a map of semiconductor quality can be obtained. It is also known to locate breaks in coating layers on semiconductors or metals using an electrolyte containing biased cell so that hydrogen is produced at the places where the substrate material is exposed to the solution. Metals have been tested for stress in homogeneities by placing them in a sulfuric acid electrolyte solution at a bias of 6 volts to produce nascent hydrogen which is absorbed by the inhomogeneities. The metal surface is then covered with a plastic film and heated in order to desorb the hydrogen and form bubbles in the film at the location of the defects.
A non-destructive method of mapping damage sites on the surface of a semiconductor substrate has now been found which is rapid and which gives good correspondence to the more time consuming and/or destructive methods heretofore used to locate electrical defects in the surface of semiconductor substrates. This method is also capable of detecting defective junctions or defective portions of a large area junction.