CHARACTERIZATION OF III-V MATERIALS BY OPTICAL INTERFEROMETRY

Digital interference microscopy is a new measuring technique with subm icron horizontal resolution and nanometric vertical resolution, that c an be used for the three-dimensional analysis of surface defects and d evice features in many microelectronics applications on bulk materials and epitaxial layers. In this paper we show how certain defects can b e analysed on III-V materials and devices using two different interfer ometric techniques. The choice of the technique depends on the height and the slope of the surface features to be measured. We show that sma ll defects less than lambda/2 in height, or surfaces with shallow cont inuous slopes upto one or two microns high are best profiled with the phase stepping technique (PSM) because of the high vertical resolution of 1 nm and the higher speed and precision. This is illustrated by st udies of the surface polish of InP wafers, defects after chemical etch ing of tin doped InP, defects on an epitaxial layer of GaAs on InP and quantum dot structures on GaAs. For measuring devices which contain m esas and grooves with step heights greater than lambda/2, the peak fri nge scanning (PFSM) method is the better choice. The vertical resoluti on is slightly less (4 nm), but the vertical range is higher (upto 15 mum) as demonstrated with the measurement of an etched groove in a las er/detector device on a quaternary layer on InP, and a MESFET device o n GaAs. Compared with electron microscopy and the new near field scann ing techniques, digital interference microscopy has the advantages of ease of use and speed of analysis and being able to resolve certain pr oblems that are difficult or not possible by other means, such as prof iling deep narrow etched grooves, or measuring the relief of a surface hidden under a transparent layer. The main disadvantages are that the horizontal resolution is limited to the resolving power of the object ive and that errors due to variations in the optical properties of the sample need to be taken into account.