Sep 16, 2014
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.
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
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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