F E A T U R E

C o m p o u n d semicon- ductor device devel- o p m e n t and fabrica-

t ion techniques have been forced to progress beyond the point where low yield processes and loose device performance parameters can be tolerated.

Production of high per- formance equ ipmen t for the te lecommunica t ions industry and new applica- tions such as optical m e m - ories have increased the d e m a n d for semiconduc- tor lasers, detectors and other devices based on III-V materials. Wafer scale processing has helped pro- duce the economies of scale required to fulfil this demand, but in turn sets new standards for process reliability. More sophisti- cated device designs and t ighter tolerances have produced a need for a new generat ion of process anal- ysis tools. These analysis tools must be non-destruc- tive and provide relevant, accurate and reproducible data wi thin a reasonable measurement period.

In the past pho to lumines- cence systems designed primarily for use in research laboratories probed a single, or at most

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Spatially Resolved Photoluminescence as a Process Control Tool for Compound

Semiconductors By Dr. Chris Moore and Dr. John Hennessy

Photoluminescence (PL) has graduated from a single, point R&D measurement to whole wafer mapping. This article reveals how PL is applied to substrate problems and process induced damage. New generation tools such as the Waterloo Scientific SPM-200 provide the user with spatial information on epitaxial layers and substrates which is necessary to ensure good wafer yields. This system is capable of measuring a large number of points on a wafer, storing, rapidly analyzing and mapping the data. PL mapping has evolved into a useful diagnostic screening tool for advanced epitaxy processes.

a few, spatial points on a wafer. Typically these measurements were done at high incident probe power densities and low sample temperatures (down to 4.2K). Although the technique provided informat ion about layer composition 1 and quali- tative informat ion about

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use to measure a large number of spatial points on a wafer at rooln tem- perature and to store and analyze the resulting PI, intensity or spectral parameter maps. The SPM- 200 uses h)w probe power densities ot 10 to 100 Watts per square cent ime- tre and wa~ specifically designed to pn)vide accu- rate calibrated intensity profiles oi samples.

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T e r n a r y PL I n t e n s i t y

F i g u r e I Correlation o f PL o f ternary epilayer with leakage current of finished devices. (Adapted from Knight et aL 1990).

defect densities 2 and layer non-uniformit ies , it was cumbersome and t ime consuming.

Re-engineering of these systems to suit the demands of c o m p o u n d semiconductor product ion rather than pure research has led to a new genera- t ion of measurement tools such as the Waterloo Scientific SPM-200 3 This tool is designed for routine

In general, ~or epilayers ol III-V materials, the brighter the PL, the lower the defect in the layer and thus the better the devices which can be produced 4. p[. intensity real)ping can be used as an everyday in-pro- cess screen to determine the quality and uniformity of layers being produced by MOCVI) and MBE tech- niques. This information can be provided to the grower within mimites rather than waiting until devices have been pro- cessed and tested. Effective implementat ion of such a process c()ntro] schenle demands that the l'I, map- per be able t~ provide con- sistent and accurate rou- tine measurements of both wavelengths and PL emis- sion intensity. This allows the user to compare materi- al and process performance over extended periods (months and even years).

Fibre 2 PL map of an lnGaAs epilayer on hd'. I he non~uHi/ormitv seen is due to poor substrate preparatmn