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edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 2 lasers. The LIVA signals are, in general, stronger than the TIVA signals. This paper illustrates that the TIVA signals can be the dominant signals in some special cases; both 1064-nm and 532-nm lasers can generate stronger TIVA signals with a better spatial resolution compared to those from a 1340-nm laser. WHEN 1064-NM AND VISIBLE LASERS ARE BETTER SUITED FOR TIVA Visible and 1064-nm lasers are better suited for looking at test structures with no underlying transistors, such as metal combs, metal interconnect chains, and vias. Because there are no underlying transistors in these test structures, there will be no electrical junctions to create photocurrent and LIVA signals. Visible and 1064-nm lasers can also be used to look at devices with very densemetal- lization. Due to this dense metallization, the laser beam cannot access the underlying transistors. Consequently, the dominant effect in these devices is the localized heating in the top-level metal layers. Examples of TIVA measurementswith 532-nmor 1064-nm lasers inboth test structures and dense-metallization devices are presented later in this article. WHY 1064-NM AND VISIBLE LASERS ARE BETTER SUITED FOR TIVA The key to understanding the advantages of using visible and 1064-nm lasers is to examine the relationship between laser absorption and TIVA signals as illustrated in Fig. 2. In TIVA measurements, a device is biased in a constant-current configuration using a power supply. TIVA signals are generated by measuring ΔV (the change in voltage) of the power supply as a function of laser positions. The strength of TIVA signals (ΔV) depends on the local tem- perature rise (ΔT). The temperature rise, in turn, depends on the heat generated at the defect site. The heat generation depends on the laser absorption. In other words, when more laser power is absorbed, more heat is generated at the defect site, leading to a larger temperature rise and a stronger TIVA signal. Laser absorption and reflectance are strongly dependent on the wavelengths of the laser. Figure 3 shows the reflectance of three different metals as a function of laser wavelength at normal incidence. The reflectance is the percentage of the laser beam that is reflected at the metal surface. For example, a reflectance of 90% indicates that 90%of the laser power is reflected fromthemetal surfacewith only 10%available for absorption by the metal. In general, the reflectance at visible wavelengths is lower than that at near-infrared (NIR) wavelengths. Therefore, there is more laser absorp- tion at visiblewavelengths. The reflectance at 1064 nm is, in turn, lower than that at 1340 nm. For aluminum, the reflectance at 532 nm is about 92% compared to ~ 95% at 1064 nm and ~ 97% at 1340 nm. The lower reflectance at 532 nm and 1064 nm means that there is more laser absorption at these two wavelengths, compared to the absorption at 1340 nm. More laser absorption indicates that more heat is generated at the defect site, resulting in a higher temperature rise and a stronger TIVA signal. Fig. 2 A schematic showing the relationship between laser absorption and TIVA signals. Fig. 3 Reflectance curves for aluminum, gold, and silver. [3]