Yi-Wen Cheng

TEM-Based Analysis of Defects Induced by AC Thermomechanical versus Microtensile Deformation in Aluminum Thin Films

Thin films of sputtered aluminum were deformed by two different experimental techniques. One experiment comprised passing high electrical AC current density through patterned Al interconnect lines deposited on SiO2/Si substrates. The other consisted of uniaxial mechanical tensile deformation of a 1 μm thick by 5 μm wide free standing Al line. In the electrical tests approximately 2 x 10 W/cm was dissipated at 200 Hz resulting in cyclic Joule heating, which developed a total thermomechanical strain of about 0.3 % per cycle. The tension test showed a gauge length fracture strain of only 0.5 % but did display ductile chisel point fracture. In both experiments, certain grains exhibited large, > 30°, rotation away from an initial <111> normal orientation toward <001>, based on electron backscatter diffraction (EBSD) measurements in the scanning electron microscope (SEM). Transmission electron microscopy (TEM) analysis of specimens from both experiments showed an unusually high density of prismatic dislocation loops. In the mechanically-tested samples, a high density of loops was seen in the chisel point fracture zone. In cross sections of highly deformed regions of the electrical test specimens, very high densities, > 10/cm, of small, < 10 nm diameter, prismatic loops were observed. In both cases the presence of a high density of prismatic loops shows that a very high density of vacancies was created in the deformation. On the other hand, in both cases the density of dislocations in the deformed areas was relatively low. These results suggest very high incidence of intersecting dislocations creating jogs and subsequently vacancies before exiting the sample.

Mechanical Behavior of Electrodeposited Copper Film at Elevated Temperatures

The temperature dependence of the strength of a thin copper electrodeposit has been measured, by microtensile testing, from room temperature to 150 °C. The ultimate tensile strength decreased from around 240 MPa at room temperature to just above 200 MPa at 150 °C. The yield strength followed a similar trend. Elongation to failure increased slightly with temperature. The Young’s modulus, as measured by the unload-load slope, was well below the values expected based on averaging single-crystal elastic constants at all test temperatures. The effect of strain rate on strength at room temperature, using a range of over a decade, was low, with a weak trend upward.Copyright

Electric Current Induced Thermomechanical Fatigue Testing of Interconnects

We demonstrate the use of electrical methods for evaluating the thermomechanical fatigue properties of patterned aluminum and copper interconnects on silicon‐based substrates. Through a careful selection of alternating current frequency and current density, we used controlled Joule heating to simulate in an accelerated manner the type of low frequency thermal stress cycles that an interconnect structure may undergo. Sources of such stressing may include power cycling, energy‐saving modes, or application‐specific fluctuations, as opposed to stressing at chip operating frequencies. The thermal stresses are caused by differences in thermal expansion properties between the metal and constraining substrate or passivation. Test conditions included a frequency of 100 Hz and current density of 11 – 16 MA/cm2, which led to a cyclic temperature amplitude of approximately 100 K, and corresponding cyclic stress amplitude in excess of 100 MPa for Al‐1Si and Cu lines on oxidized silicon. The failure mechanism differs f...

Microstructure Evolution During Alternating-Current-Induced Fatigue

Subjecting electronic interconnect lines to high-density, low-frequency alternating current creates cyclic thermomechanical stresses that eventually cause electrical failure. A detailed understanding of the failure process could contribute to both prevention and diagnostics. We tested unpassivated Al-1Si traces on the NIST-2 test chip; these are 3.5 μm wide by 0.5 μm thick by 800 μm long, with a strong (111) as-deposited fiber texture and an initial average grain diameter of approximately 1 μm. We applied rms current densities of 11.7 to 13.2 MA/cm2 at 100 Hz. Resistance changes in the lines indicated that such current densities produce temperature cycles at 200 Hz with amplitude exceeding 100 K. Open circuits occurred in under 10 minutes, with substantial surface damage seen after only one minute. A few failures initiated at lithography defects initially present in the lines, but most were produced by the current alone. In one detailed example presented in this paper, we monitored the damage process by interrupting the current at 10, 20, 40, 80, 160, and 320 s in order to characterize an entire line by scanning electron microscopy and automated electron backscatter diffraction (EBSD); failure took place after 697 s. Results are described in terms of deformation, grain growth, and orientation changes.© 2004 ASME

Microtensile Testing of Thin Films in the Optical and Scanning Electron Microscopes

Because thin films are formed by processes different from those used to produce bulk materials, their microstructures, and hence their mechanical properties, are quite different from those of bulk materials of the same chemical composition. While the general principles of conventional mechanical testing are applicable to thin films, special test equipment and techniques are required. These are briefly described here. Present specimen sizes are near the limit of what can be tested in the optical microscope, so techniques useful in the scanning electron microscope are of interest. Test techniques adapted for use in the SEM are presented. These test methods have been applied to pure aluminum films deposited in our laboratory, aluminum films made in a commercial CMOS fab facility, electrodeposited copper, polyimide films, and polysilicon films. The differences among the stress‐strain curves for these very different materials were as dramatic as would be expected. Now that some experience with these test techn...