Abstract:
A method and system for measuring whole-wafer etch pit density (ρ D ) is disclosed in which an etch GaAs wafer is tested for fractional transmission at a plurality of points over its surface. The fractional transmission (T) of light through the wafer is detected, amplified and fed to a computer where at least two points of transmission measurement are selected for calibration. From these measurements, together with an estimate of the average etch pit size (area), the values for fractional transmission in regions of low etch pit density T O  and high etch pit density T E  may be calculated, and used to convert transmission data directly to etch pit density (ρ D ) according to the equation ##EQU1##

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to GaAs wafers and particularly to a method and system for automated measurement of whole-wafer etch pit density in GaAs wafers. 
     2. Description of the Prior Art 
     One of the prime factors affecting large-scale device development in GaAs is the etch pit or dislocation density. Although there is some disagreement on whether or not a single dislocation near a device such as a field-effect transistor (FET) can significantly affect the electrical behavior of that device, there seems to be general agreement that a large number of dislocations within the region surrounding the device will indeed affect its electrical properties. A good way to study such effects is to compare the variations in dislocation density over a large area, such as a 2-inch or 3-inch wafer, with the corresponding variations in some critical device parameter, such as the threshold voltage. Then standard statistical techniques can be used to determine the degree of correlation. Modern autoprobe techniques can be used to map the particular device parameter, even up to many thousands of devices. However, there is no such convenient technique to map the dislocation density. The standard method is to place the wafer in hot KOH, forming small etch pits where dislocations intersect the surface, and then to count the pits within a given area about the point of reference. This procedure is very tedious, so that seldom are more than lo positions analyzed on a given wafer, making good correlation with device properties nearly impossible. 
     U.S. Pat. No. 4,637,726 discloses a system for mapping GaAs and other semiconductor wafers for areas of nonuniformity. The wafer is positioned in a magnetic field and illuminated by a light source at a plurality of points on the surface thereof. A computer initializes the light level and the electronic gain of each detector preamp associated with a fiber optic link from an analyzer. The magnetic field direction is then reversed by computer command, causing a localized change in intensity cf light passing through the wafer due to Faraday Rotation. The resulting change in detector output together with location and wavelength data can be used to compute a map of the wafer. 
     U.S. Pat. No. 4,740,708 discloses a system and procedure for the inspection of the surface of a semiconductor wafer which ascertains that particulate contaminants have been adequately cleaned from the surface during the manufacture of integrated electric circuits The wafer is advanced in a first direction and is optically scanned in a second direction, transverse to the first direction, for recording intensities of light reflected normally from the wafer surface as a function of location on the scan line. 
     However, none of the above references teaches a method or system for obtaining whole-wafer dislocation (etch pit density) maps of a GaAs surface. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method and system for measuring whole-wafer etch pit density in GaAs. 
     It is another object of the invention to provide an automated, digitized method and system for realizing dense, whole-wafer etch pit maps of a GaAs surface. 
     According to the invention, an etched GaAs wafer is tested for fractional transmission at a plurality of points over its surface. A light beam from a suitable source such as a tungsten-halide lamp is passed through a monochromator and focused by a lens on the wafer. The fractional transmission (T) of light through the wafer is detected, amplified and fed to a computer where at least two points of transmission measurement are selected for calibration. One means of calibration is to compare the fractional transmission with at least two visually observed etch pit density points at identical locations. From these measurements, together with an estimate of the average etch pit size (area), the values for fractional transmission in regions of low etch pit density T O  and high etch pit density T E  may be calculated. 
     Having determined T O  and T E , the transmission data can be converted directly to etch pit density (ρ D ) according to the equation ##EQU2## By using this equation on all the measured transmission points, a total etch pit density map can be generated for the wafer under test. 
     Other features and advantages of the invention will be apparent from the following description, drawings and claims, which show and describe an embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system for achieving automated measurement of etch pit density in a GaAs wafer; 
     FIG. 2 is a flow diagram of the process for generating etch pit density maps according to the invention; 
     FIGS. 3a-d are a gray-scale map, a histogram, and horizontal and vertical slice patterns showing visually counted etch pit density in a low pressure liquid encapsulated-Czochralski (LEC) wafer; 
     FIGS. 4a-d are a gray-scale map, a histogram, and horizontal and vertical slice patterns showing fractional transmission (T) at 1.45 μm in the LEC wafer; 
     FIG. 5 is a graph showing fractional transmission (T) versus visually counted etch pit density at common points of measurement on the LEC wafer; and 
     FIGS. 6a-d are a gray-scale map, a histogram and horizontal and vertical slice patterns showing a complete etch pit density map for the LEC wafer as determined by the method and system of the invention. 
    
    
     DETAILED DESCRIPTION 
     The invention is an automated, digitized method and system for realizing dense, whole wafer etch pit maps of a GaAs surface. The invention is based on the fact that the etch pits scatter light strongly, and thus decrease the transmitted light. The situation is somewhat complicated at high etch pit densities because of overlapping pits, but an appropriate theory, which contains three parameters, can model this effect. One of the parameters is the average etch pit size, which can be measured with a calibrated microscope, while the other two parameters can be determined by visually or electronically counting the pits at two positions on the wafer. The pits can be electronically counted using a scanning electron microscope and electron scattering techniques. Even without this calibration, however, the degree of dislocation non-uniformity will be apparent in the transmission map itself. 
     FIG. 1 shows a system according to the invention for achieving the automated etch pit density map. A GaAs wafer 10, for example, typically of 2-3 inches in diameter and 0.1 to 1.0 mm in thickness, is positioned in a wafer holder 12 attached to an X-Z drive apparatus 14 containing a Pair of electromechanical positioning mechanisms (not shown). The positioning mechanisms such as Newport 850-4 positioners manufactured by Newport Corporation, 18235 Mt. Baldy Circle, Fountain Valley Calif. 92728, are mounted at right angles in order to move the wafer holder 12 in two orthogonal directions. The &#34;X&#34; direction indicates a horizontal direction extending into and out of the view of FIG. 1, and the &#34;Z&#34; direction indicates a vertical direction. Positioning accuracy within 10 microns is achievable using a Newport model 855C positioning controller. The X-Z drive apparatus is controlled by a computer 20 to move wafer 10 in the path of an infrared light beam 22 generated by a 1/4 meter monochromator 24. An optical lens 26 is used to focus the beam 22 into a small spot 30 of light on the surface of GaAs wafer 10. The 1/4 monochromator 24 is a grating instrument which divides the light emitted by a suitable source such as a 50W tungsten-halogen lamp (not shown) into the single wavelength light beam 22. The intensity and wavelength of the light beam 22 are controlled by computer 20 through a monochromator controller 32 such as may be constructed from a SloSyn model M061-FF-411C3A stepper motor and Modulynx IOD010 motion controller, both manufactured by Superior Electric Co., 383 Middle Street, Bristol, Conn. 06010. 
     The fractional transmission of light spot 30 through the GaAs wafer 10 is measured by a cooled germanium detector 40 and coupled to a detector readout amplifier 42 for input to computer 20. The cooled germanium detector 40 is a 3 mm diameter planar p-n germanium junction mounted to a TO-5 header with an infrared transmitting window and a Peltier-junction cooler to reduce thermal noise. The combination of germanium doping materials, temperature, and window material can give a response that is linear with intensity over six decades of intensity. 
     To enhance the stability of the measurement, the infrared light beam 22 may be passed through a mechanical chopper before passing through the GaAs wafer 10 so that narrow-band synchronous amplification can be used. This also minimizes noise. 
     The relationship between transmission and etch pit density is complicated by the fact that the etch pits can overlap, especially if the pits are large and their density is high. The relationship can be expressed mathematically as follows. Let A T  be the total illuminated-spot size, and let A E  be the area within this spot size covered by etch pits. If dn etch pits of average area (a) are added, then the increase in A E . i.e., dA E , is adn(1-A E  /A T ), to take account of the fact that some of the new etch pits will fall on areas already covered. If etch pit density is defined on a single surface as ρ D  ≡n/A T , the resulting differential equation yields A E  /A T  =[1-exp(-aρ D )]. 
     The fraction of light scattered by area A E  can be defined as S≡βA E  /A T  =β[1-exp(-aρ D )], where the parameter β can range from 0 to 1, depending on the scattering strength of the etch pits. If β=0, then none of the light is scattered, whereas, if β=1, then all of it is scattered and assumed to miss the detector. Since two etched surfaces are involved, multiple scatterings and reflections must be considered. If S is the same for both surfaces (i.e., both sides etched) then it can be shown that the transmission T is given by ##EQU3## where α is the absorption coefficient, d the wafer thickness, and R the reflectivity. In general, R 2  (1-S) 2  &lt;&lt;1, and the wavelength is chosen such that α≈0. By replacing the unknown parameters R and β by more convenient parameters T O  and T E , where T O  =T(aρ D  &lt;&lt;1), and T E  =T(aρ D  &gt;&gt;1), then Eq. 1 can be shown to yield 
     
         T.sup.1/2 =T.sub.E.sup.1/2 +(T.sub.O.sup.1/2 -T.sub.E.sup.1/2)e-aρ.sub.D                           (2) 
    
     For wafers which have both very high (ρ D  a&gt;&gt;1) and very low (ρ D  a&lt;&lt;1) etch pit density regions, T O  is simply the maximum, and T E  the minimum measured transmission. However, in general, it is better to carry out an actual etch pit count at two points. Then, T O  and T E  can be related to the actual counts (ρ D1  and ρ D2 ) and the fractional transmissions (T 1  and T 2 ) at these two points can be expressed as ##EQU4## With these values of T O  and T E , the transmission data can be converted directly to etch pit data by inverting Eq. 2, such that ##EQU5## 
     The invention may be illustrated by reference to the flow diagram of FIG. 2 in conjunction with the following example in which an etch pit map was generated from a GaAs low pressure liquid encapsulated-Czochralski wafer etched in molten KOH, at 450° C. for 40 minutes, in a zirconium crucible. Etch pits were visually counted at 166 locations, along horizontal [011], vertical [011], and diagonal [010] directions. The counting was carried out on 0.41×0.41 mm areas, centered on points 1 mm apart in the horizontal and vertical directions and 4.2 mm apart in the diagonal direction. The average etch pit size a was estimated, with a calibrated mircoscope to be a=1.5×10 -5  cm 2 . These data are summarized in FIG. 3, and show strong &#34;W&#34; patterns in the horizontal and vertical data slices. 
     After visually counting the etch pits, the wafer was mounted in the holder 12 shown in FIG. 1 and the transmission at 1.45 μm was measured at 3851 points on the wafer surface. The results are shown in FIG. 4, in which the horizontal and vertical slices are now &#34;M&#34; patterns rather than &#34;W&#34; patterns, because the transmission varies inversely with the etch pit density. 
     To calibrate the transmission data shown in FIG. 4, two points, T 1  and T 2 , were calibrated against two visually counted etch pit density points, ρ D1  and ρ D2 , at identical locations. In this particular case, more points were available for the calibration, since 166 points were visually counted. However, in general, it is necessary to obtain only two visual points to obtain the parameters T O  and T E . It is usually advantageous to read T O  in a region of low etch pit density (high transmission) and T E  in a region of high etch pit density (low transmission). If, further, the wafer has regions of very low etch pit density, such that aρ D1  &lt;&lt;1, and regions of very high etch pit density, such that aρ D2  &gt;&gt;1, then T O  ≈T 1 , and T E  ≈T 2 . In such a case, it is not necessary to visually count any points. 
     For this example the two counting locations shown in FIG. 3 were chosen and thus determined T O  and T E . Since the average etch pit size a was already determined, Eq. 2 can be plotted, and is shown as the solid line in FIG. 5. Also represented are the 166 visually counted points, and it is seen that they scatter evenly about the theoretical relationship. Thus, Eq. 2 is validated as a calibration curve, and is used in inverted form (Eq. 5) to obtain the complete etch pit map from the transmission map, as shown in FIG. 6. A program listing for the software used in computer 20 to generate the etch pit density map of FIG. 6 is incorporated herein as Appendix A. 
     Thus, while preferred features of the invention are described and illustrated herein, it is understood that changes and variations may be made by those skilled in the art without departing from the spirit and scope of the invention. ##SPC1##