SCANNING ELECTRON MICROSCOPE EXAMINATION K. O. Leedy ABSTRACT An examination with a scanning electron microscope was made of the wire bonds of over 75 high-reliability microelectronic devices. The device interconnects were ultrasonically bonded aluminum wires. Of primary interest were the bonds themselves; their appearance and its significance are described. Also described is the appearance of the metallization and the wire. Comments and explanations are given where the phenomena are understood. Although the devices studied had passed preliminary electrical tests and pre-encapsulation visual examinations, many potential reliability problems were identified such as weak bonds, electrical shorts and contamination. Key Words: Aluminum wire; high reliability; integrated circuit; metallization; scanning electron microscope; transistor; ultrasonic bonding; wire bonding. SCANNING ELECTRON MICROSCOPE EXAMINATION OF WIRE BONDS FROM HIGH-RELIABILITY DEVICES K. O. Leedy 1. INTRODUCTION Wire bonds in a variety of integrated circuits and transistors from production lines dedicated to the production of high-reliability devices were examined with a scanning electron microscope (SEM). The study was undertaken initially to observe differences in appearance of bonds produced by 14 supposedly identical ultrasonic bonding machines on three controlled production lines. Subsequently, many devices from other highreliability production lines were examined and found to have similar characteristics. A variety of the commonly used package types and many different circuit types were represented among the more than 75 devices examined. A11 devices were ultrasonically bonded with 0.001-in. (25-μm) diameter, 99percent aluminum 1-percent silicon wire. The bonding pads were metallized with aluminum while the package leads had either aluminum or gold metallization. The devices examined had all passed preliminary electrical tests and pre-encapsulation visual examinations but were removed from the production lines before they were capped, sealed, or environmentally tested. Each of the devices examined would probably have functioned as intended in the final tests if allowed to continue down the production line. Although this examination was not a study of failed devices, many potential reliability problems were observed. Comments and explanations are given where the phenomena are understood; in other areas the observations are merely reported. In addition, even though the study primarily centered on wire bonding, several other device defects were observed; these are illustrated in the last section of the report. Of the several hundred SEM photographs made, the examples shown were selected to be indicative of particular faults. It is not intended to suggest that the faults illustrated are unique to a particular device type, production line, or circuit configuration. In fact, most of the faults were observed to some extent in all types of the devices investigated. 2. BACKGROUND In order to obtain perspective of the problem and to define some terms, let us look first at the devices and see why the wire bond is important. In Figure 1 are shown an integrated circuit (a) and a transistor (b) representative of the device types studied. Wire bonds are made to connect the bonding pad on the circuit chip electrically to the external lead: a post or finger of the package in which the chip is held. The welded interface between the wire and the pad, post, or finger, under the flattened areas at the ends of the wire, are the bonds. The bond which is made first is called the first bond. The second bond then completes the bond loop. The weld is formed by the combination of a force applied by the bonding tool on the wire and by the application of ultrasonic vibration of the tool in the direction parallel to the wire axis.* The welding that occurs at the wire-metallization interface can be seen in Figure 2 which shows the lift-off pattern of a partially removed bond which was made under laboratory conditions. A similar pattern can be seen on both the pad and the wire. metallization interface is not welded. The center portion of the wire Figure 3 shows a schematic illustration of the bonding operation. The wire is fed through a hole in the tool and passes under the foot of the tool. The foot is the part of the tool that exerts a static force on the wire during the bonding process. The front edge of the foot is rounded to prevent damage to the wire, but the back edge is sharp to facilitate the cut-off of the wire after the second bond is produced. There are many differences in appearance and in structure between the first and second bonds. Some of these may be seen in Figure 4 where the first bond is shown in (a) and the second bond in (b). The most obvious difference between the two bonds in the figure is in the shape of the toe or free end of the bonding wire. The second bond exhibits evidence of the cut-off operation which occurred after it was made. The first bond shows the tail, the unflattened portion of the free end of the wire which extended beyond the foot during bonding. A more important difference is in the heel of the bond, the juncture between the undeformed wire and the bonded portion. The sharp heel of the tool, which provides for cut-off at the second bond, causes an indentation or crack. in the heel of the first bond. This will be discussed in more detail below. In most of the devices examined, the first bond was made on the external lead (a post) and the second was on the pad. This accounts for the different appearances of the metallization in Figures 4a and 4b. 3. BOND APPEARANCE A comparison of the devices showed significant differences in bond appearance. Bonds differ mainly in the amount of deformation of the *All bonds examined in this study were formed ultrasonically. However ultrasonic bonding is not the only wire bonding technique. The other common wire bonding technique is thermocompression bonding, which combines pressure and heat to form the weld. It is used primarily with gold wire and, in some special cases, aluminum wire. In addition there are other techniques for making interconnections such as flip-chip and beam-lead bonding which do not include the use of wires. Figure 1. Typical devices examined. An integrated circuit is shown in Figure 2. The lift-off pattern of a partially removed bond made under |