Acoustic Microscopy for Imaging and Characterization
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tion of the appropriate technique will depend on the answers. The basic principle of a scanning acoustic microscope is that a short pulse of acoustic waves is generated by a transducer and is brought to a focus by a spherical lens surface.1 The waves propagate through a suitable coupling medium (almost always water) to a sample where they are reflected either by features within the material or at its surface. The reflected waves are then detected using the same lens-transducer assembly and appropriate electronic circuits. The lens is scanned in a raster pattern over an area of the sample to build up an image, and the reflected signal is used to modulate the brightness at each spot. One of the most recently available acoustic microscopes, the KSI SAM 100, is shown in Figure I.2 Earlier acoustic microscopes sometimes had a reputation for being 30
difficult to use and for producing pictures the interpretation of which was obscure. The new generation of commercial instruments is much more compact; they are convenient to use for both research and routine quality control. As will become clear, there are several important variations on the basic method. However in each case, the key interest lies in understanding how the reflected signal depends on the elastic structure and properties of the sample. Interior Imaging A growing number of international inspection codes for semiconductor packaging call for inspection by scanning acoustic microscopy.3 Defects that can be detected by acoustic microscopy include delaminations between die and heatsink, lid-attach failures (the lid is not fully attached to the body of the package), and so-called popcorn defects (blister defects due to moisture at an interface that expands during heat treatment).4 Some of these can be detected by other techniques such as radiography and leak tests, but acoustic microscopy is proving increasingly popular because of its convenience and reliability. For this kind of application, a frequency of about 100 MHz is often appropriate. At this frequency, the lateral resolution in a typical solid is generally better than 0.1 mm, and the ultrasound is able to penetrate through several millimeters. Depth resolution can be obtained by setting a time gate so that only signals from a certain range of depths are displayed in an image, thus enabling a plane or an interface of interest to be selected for inspection. An acoustic microscope for this purpose uses a focused transducer with a modest opening angle, typically with less than a 16° semiangle, which is mounted on a motor-driven scanning system so as to
permit scan ranges of several millimeters or more, with the sample placed in a water bath to give acoustic coupling. The instrument performs somewhat like a high-resolution ultrasonic C-scan. Figure 2 shows two images at different depths of a flip-chip component obtained using a commercial KSI SAM 100 scanning acoustic microscope. Interior delaminations are visible in the central region of Figure 2b. Other important applications of acoustic microscopy with thi
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