Method and measuring instrument for identifying the diffusion length of the minority charge carriers for non-destructive detection of flaws and impurities in semiconductor crystal bodies
BACKGROUND OF THE INVENTION 
     The present invention relates to a method and to a measuring instrument for identifying the diffusion length of minority charge carriers and of their topical fluctuations for non-destructive detection of flaws and impurities in a semiconductor crystal body. 
     For qualitative characterization of the material of a semiconductor crystal, it is known to identify the diffusion length of the minority charge carriers therein since this is valid measurement of the number of recombination centers and traps in the semiconductor crystal. Two methods are standard for identifying the diffusion length of the minority charge carriers in a semiconductor crystal, measuring photoconductivity (photoconductive decay) and measuring the surface photovoltage. Both methods are disclosed in the text &#34;Library of Congress Cataloging&#34; in &#34;Publication Data of Texas Instruments Elec. Ser.&#34;, edited by Tyler G. Hicks and Lester Strong, 1975, pages 105 through 129. 
     In measuring the photoconductivity, the semiconductor is irradiated with light such that electron hole pairs are generated and the change in conductivity is subsequently measured. The life time and, thus, the diffusion length of the minority charge carriers are capable of being identified therefrom. The shape of the semiconductor crystal is thereby of significance for the measurement and the semiconductor crystal must usually be destroyed before the measurement. 
     A surface photovoltage occurs when a semiconductor crystal is irradiated and can be measured by capacitative coupling. The surface photovoltage can be used to measure the diffusion length of the minority charge carriers. This method, however, has the disadvantage of a long measuring time since measurements with different light wavelengths must be successively carried out. Further, the topical resolution of the measurement is low since the irradiated area must have a size of at least 1 cm 2 . 
     SUMMARY OF THE INVENTION 
     An object of the present invention is the measuring of the diffusion length of minority charge carriers in a semiconductor crystal body with optimum precision. 
     The invention is characterized by a method of the type initially cited wherein: 
     (a) the semiconductor crystal body is positioned between two electrolyte-filled measuring chamber halves such that a front side and a backside of the semiconductor crystal body are each in contact with the electrolyte of the measuring chamber; 
     (b) the semiconductor crystal body is connected to a voltage source via an ohmic contact and a voltage is applied between the backside of the semiconductor crystal body and an electrode of the measuring chamber half connected to the backside of the semiconductor crystal body, so that a blocking space charge zone arises at the backside of the semiconductor crystal body; 
     (c) the front side of the semiconductor crystal body is irradiated with light; 
     (d) the photocurrent I G  of the minority charge carriers occurring at the backside of the semiconductor crystal body upon irradiation is measured; 
     (e) a voltage is applied between the front side of the semiconductor crystal body and an electrode of the measuring chamber half connected to the front side of the semiconductor crystal body, so that a blocking space charge zone is formed at the front side of the semiconductor crystal body; 
     (f) the photocurrent I 0  of the minority charge carriers occurring at the front side of the semiconductor crystal body upon irradiation is measured; and 
     (g) the diffusion length L is calculated from the the mathematical equation: ##EQU1## 
     Where α is the absorption coefficient and D is the specimen thickness. 
     The method is implemented with a measuring instrument that is characterized by: 
     (a) two measuring chamber halves held by a frame that can be individually filled with electrolyte and each of which contains an electrode; 
     (b) a holding mechanism for the semiconductor crystal body which is designed such that the semiconductor crystal body can be positioned between the measuring chamber halves, so that the front side and the backside of the semiconductor crystal body are each in contact with the content of a measuring chamber half; 
     (c) a terminal for producing an ohmic contact to the semiconductor crystal body; 
     (d) at least one light source such that a light ray is directed onto the surface of the semiconductor crystal body; 
     (e) a dc voltage source for applying a dc voltage between the semiconductor crystal body and the electrolyte; and 
     (f) an ammeter means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel, are set forth with particularity in the appended claims. The invention, together with further objects and advantages, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in the several Figures in which like reference numerals identify like elements, and in which: 
     FIG. 1 is a schematic illustration of a measuring instrument for implementing the method of the present invention; 
     FIG. 2 schematically depicts a crystalline semiconductor wafer having impurities; 
     FIG. 3 shows the topical distribution of the impurities of the semiconductor wafer of FIG. 2. 
     FIG. 4 is a perspective view of a measuring chamber half having a holding device for a semiconductor crystal body. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a semiconductor crystal body 3 is situated between two electrolyte-filled measuring chamber halves 1 and 2. A connection between the semiconductor crystal body 3 and a controlled constant voltage source 6 (for example, a potentiostat) is produced via an ohmic contact 5. A constant voltage is produced between the ohmic contact 5 and the electrode 9 that is situated in the electrolyte 12 of the second measuring chamber half 2. The backside of the semiconductor crystal body 3 thus is in contact with the electrolyte 12 of the second measuring chamber half 2. A surface-proximate electrode 7 for measuring the voltage at the surface of the semiconductor crystal body 3 is located at the backside of the semiconductor crystal body 3, so that the constant voltage can be controlled with the measured voltage. The front side of the semiconductor crystal body 3 that is in contact with the electrolyte 11 of the first measuring chamber half 1 is irradiated with laser light from a laser 4. The light ray is deflectable by an apparatus (not shown) such that individual points on the surface of the semiconductor crystal body 3 can be designationally irradiated. For example, the deflection device for the light ray can be composed of an arrangement of mirrors. 
     During the measurement, the front side of the semiconductor crystal body 3 is scanned with the laser beam 4. The scan speed is typically 1 through 100 cm/sec for a spot size of 1 mm 2 . The backside of the semiconductor crystal body 3 is located in the dark during the measurement and is positively biased relative to the electrolyte 12 before the measurement for an n-silicon semiconductor and is negatively biased relative to the electrolyte 12 for a p-silicon semiconductor. Constant voltages between 2 and 5 volts are typically selected. The silicon electrolyte contact then behaves like a diode polarized in a non-conducting direction, i.e., only a small leakage current flows. No voltage is applied between the front side of the semiconductor crystal body 3 and the electrolyte 11 of the first measuring chamber half 1. The electrolyte 11 serves the purpose of providing saturating surface conditions. The electrolyte 11, 12 is the same in both measuring chamber halves and, for example, can be composed of two percent hydrofluoric acid that has a few drops of a wetting agent added. Before the measurement it may be necessary to strip undesired surface layers on the semiconductor crystal body by electro-polishing. 
     The minority charge carriers generated by irradiation of the front side of the semiconductor crystal body 3 diffuse through the semiconductor crystal body and recombine at potential recombination centers. The charge carriers that proceed up to the backside of the semiconductor crystal body 3 are acquired by the electrical field present there and are measured as photocurrent I G  by the ammeter 10. It is advantageous to use a lock-in amplifier in the detection of the photocurrent I G . The level of the photocurrent I G  detected at the backside of the semiconductor crystal body 3 in relationship to the photocurrent I 0  generated at the front side of the semiconductor crystal body 3 is a measure of the density of the recombination centers of the semiconductor crystal body 3 and is thus linked to the minority charge carrier diffusion length L derived from the mathematical equation: ##EQU2## where D is the thickness of the semiconductor crystal body and α is the absorption coefficient. The portrayal of the photocurrent I G  as a function of the coordinates of the laser beam incident on the front side produces an image of the distribution of the recombination centers in the volume of the semiconductor crystal body 3. 
     The photocurrent I 0  is measured while a constant voltage is applied between the front side of the semiconductor crystal body 3 and the electrolyte 11 of the first measuring chamber half 1 while the front side of the semiconductor crystal body is being irradiated. No voltage is applied to the back side of the semiconductor crystal body 3 during the measurement of I 0 . 
     FIG. 2 depicts two lines of an iron impurity 22 situated on a crystalline semiconductor wafer 21. The line 23 marks the edge of the measuring area that is established by the geometrical dimensions of the measuring chamber halves 1 and 2. 
     FIG. 3 depicts a topically resolved measurement of the photocurrent I G  which exhibits minimums 31 at those locations where the lines of iron impurity 22 are situated on the crystalline semiconductor wafer 21. 
     FIG. 4 depicts an embodiment of a measuring instrument having a central frame 41, havinga holding mechanism having a holding frame 42 and an adaptor 43 for different semiconductor crystal bodies and having two measuring chamber halves of which only one (41) is shown in FIG. 4. The adaptor 43 can accept semiconductor crystal bodies having diameters from 10 mm through 200 mm and having thicknesses of a few 100 .sub.μ m up to several millimeters. The carrier plate 44 of the measuring chamber half 1 is pressed against the central frame 41 by a pneumatic means (not shown in FIG. 4) such that sealing lips 45 of the measuring chamber half 1 adhere firmly to the semiconductor crystal body. The sealing lips 45 of the measuring chamber half 1 compensate for small irregularities of the semiconductor crystal body so that the crystal body need not be either round or entirely planar. Like the adaptor 43, the carrier plate 44 can be adapted to the respective semiconductor crystal body size. 
     The outer cover plate 46 of the measuring chamber half 1 is optically transparent and can be mounted on the carrier plate 44. The entire part of the measuring instrument containing electrolyte is resistant to acid, particularly hydrofluoric acid, and is preferably manufactured of polyvinyl chloride or Teflon. The transparent cover plate 46 can be fabricated of acrylic or silica glass that is either periodically changed or is coated with a thin protective layer. When the measuring chamber half 1 is pressed onto the semiconductor crystal body, an ohmic contact is produced by ring a 47 of measuring tips. The ring 47 of measuring tips can be provided with a plurality of measuring instruments, so that a check can be carried out therewith to determine if the electrical contact to the semiconductor crystal body is ohmic. The ring 47 of measuring tips can be optionally used instead of the ohmic contact shown in FIG. 5. Leads 48 provide for delivery of electrolyte, vacuum and compressed air. 
     The invention is not limited to the particular details of the apparatus depicted and other modifications and applications are contemplated. Certain other changes may be made in the above described apparatus without departing from the true spirit and scope of the invention herein involved. It is intended, therefore, that the subject matter in the above depiction shall be interpreted as illustrative and not in a limiting sense.