Sep 16, 2014

Graphics script provides quick classification of CaAs wafers

Infrared transmission topography has long been used to detect variations in gallium arsenide wafers that can cause dark-line defects that limit lifetime of GaAs lasers and solar cells. In the past, infrared transmission was measured over a whole wafer by scanning a small spot mechanically. Absorption was calculated at each location across the surface of the wafer and used to produce colour-coded plots that allow the wafer’s characteristics to be determined at a glance. The program ran on VAXNMS computers, but these are being taken out of service due to obsolescence. To overcome these problems, the author developed a graphics script using a state-of-the-art data analysis program which provides quick classification of CaAs wafers based on traps and defects, but runs on inexpensive personal computers and, as a bonus, produces bit-map plots that can be cut  and pasted into Windows word processing and presentation software.

Polished wafers of semiinsulating undoped GaAs or doped conducting GaAs are important for the manufacture of semiconductor devices and integrated circuits that operate at very high frequencies. Semi-insulating GaAs wafers are typically used as substrates for electronic devices, while silicon-doped wafers are used in the fabrication of solar cells and edge-emitting lasers. The advantage of GaAs is that it is capable of operating at 5 to 10 times the maximum frequency level of silicon circuits.These devices are currently used in three major consumer markets: wireless (including PCS and cellular), fibre-optic communications, and television (including cable and direct-broadcast satellite TV). There are also many military communications applications for GaAs.Unfortunately, the EL2 trap is an electronic defect in the GaAs crystal that is not yet fully understood in terms of its atomic structure. It is, however, instrumental in producing semi-insulating GaAs crystals by pinning the Fermi level near mid-gap. Uneven distributions of EL2 can cause problems in GaAs by affecting the resistivity and  device isolation. Another type of defect, crystalline dislocations, can be mapped by etching the wafer (for semi-insulating GaAs) or nondestructively (for GaAs:Si). The effects of dislocations on electronic devices fabricated on active layers grown on semi-insulating GaAs are unclear, but dislocations are unlikely to improve device characteristics. Dislocations in GaAs:Si are known to cause dark-line defects in lasers and solar cells, leading to premature failure. 

Wafer scanning

The Air Force Research Laboratory has developed an automated method of accurately measuring the infrared transmission and therefore the absorption (or scattering) at all locations across a GaAs wafer. From this, the EL2 density (or dislocation density) can be calculated. T3he wafer is mechanically scanned past a beam from a tungsten-halogen light source. The collimated  light is focused through a monochromator that passes only 1.1 l_trn wavelength light, which is absorbed b) the EL2 trap. Measurement at 1.1 urn wavelength gives neutral EL2 density, while using 1.2 nm wavelength  gives total  EL2  density  Dislocation  density  is measured  at  a  wavelength  where  the  EL2  trap  does  not  absorb;  we use  1.45  urn  wavelength  (see Figures  l-4).  Measurement  at other  wavelengths  is  required  for  other sample wafer compositions.  For example, total  iron  density  in  indium phosphide  wafers  requires  measurement  at  1.0  urn wavelength,  as shown  in Figure 5. The  light  passes  through  an electromechanical chopper  and  is  focused  into a 0.5  mm2 spot  on the wafer. A germanium diode detector operating  in the  low-noise  zero-bias mode  detects  the  infrared  light passing through  the  samp1e.A com- mercial lock-in amplifier detects  the light, digitises  its  intensity, and  the acquisition  computer  program stores  the  intensity  in  a  file  along with on-wafer coordinates. Measurement  of  the  16,597  locations  required  to  map  a  3”  wafer  takes about  an  hour;  100  mm  wafers (measured  at 28,593  locations)  and 150  mm  wafers  (measured  at 68,444  locations)  take longer. Comprehending  the  meaning  of these  large data-sets can be very difficult.  Our  analysis  constructs  a colour  histogram by ranking the  da- ta  into  14  bins  and  assigning  a colour  to  each  bin, then  plotting  a square  of  that  colour  at  each  location where  the measured value corresponds  to  a  bin  range.  This provides  an  easily  interpreted colour-map  of the  measured  values keyed to the  colour  histogram. This  provides  an  excellent method  of  investigating  relatively obscure  correlations  between  mate- rials properties  and  device  properties.The  plotted  colour-map  of  the dataset  can  easily  be  compared  to the  properties  of  semiconductor device  test  structures  fabricated  on the  wafers.  Such  measurements  as Hall-effect  for  free  carrier  density and  mobility,  sourcedrain  resistance,  source-drain  saturation  cur- rent,  pinch-off  voltage  and microwave  characteristics  such  as cut-off  frequency  can  also  be plotted as wafer-maps. Visual inspectionof the colour-maps quickly
reveals any rough correlations; more detailed mathematical correlations can be carried out as desired.

New program replaces old

Commercial data plotting packages do not use this scheme of a colour histogram with the colours keyed to locations ofmeasurements on a semiconductorwafer.A decade ago, the author began
using a custom program developed by co-workers D.Elsaesser, S. Dudley and J. Sewell, who implemented the scheme in a FORTRAN wafer-mapping program on a Digital Equipment Corp VAX/VMS super-minicomputer. Unfortunately, their

Refining plots

The user can then easily refine the plots. LabTalk usually draws too many numbers on each axis. The author corrects this by clicking the “Format” tab to get the drop-down menu, choosing “axis” and “X” (or
“Y”) and entering lower values for the number of tic marks.The axes may also be labelled by clicking on “X axis label” (or “Y axis label”) and entering new axis-label text. Origin has a layout screen that
may be used to combine the histogram and wafer-map plots on a single page for printing or plots may be saved and combined on any word processor or graphics program that supports colour.

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