Scatter Measurements and Instrumentation

This chapter reviews the processes and equipment for taking scatter measurements. As pointed out in Sec. 1.5, the BSDF is defined in differential form, but is measured with the incremental limitations imposed by real instrumentation. The finite detector aperture, scatter created within the instrument, calibration inaccuracies, and other practical equipment limitations such as noise, detector nonlinearity, and mechanical errors, all produce noticeable deviations between the true BSDF and the measured BSDF. In order to generate meaningful scatter specifications and fully utilize the data, it is important to understand the source and magnitude of these deviations. System calibration, an often-discussed issue, is described in detail. The chapter progresses from these basic concepts to discussions of other measurement techniques (curved samples, area raster scans, retroscatter, TIS, and out-of-plane scatter) and concludes with a section on error analysis. Chapter 1 provides necessary background information for understanding this material. 6.1 Scatterometer Components The simple plane-of-incidence scatterometer outlined in Fig. 6.1 contains most of the components typically found in more-sophisticated systems. These may be grouped into four categories: light source, sample mount, receiver (detection system), and computer/electronics package. This section outlines the need for and general operation of these modules. The light source is shown as a laser beam that is chopped, spatially filtered, expanded, and finally brought to a focus on the detector path. Lasers are convenient, but not necessary, sources for scatter measurements. The beam is chopped to reduce both optical and electronic noise. This is accomplished through the use of lock-in detection in the electronics package that suppresses all signals except those at the chopping frequency.