A. H. Verbruggen

High-temperature bulge-test setup for mechanical testing of free-standing thin films

We developed a bulge-test setup which enables measurement of the elastic and plastic properties of free-standing thin film samples between room temperature and at least 300 °C. Mechanical stress is applied to the film by a differential gas pressure across the sample and the bulge height is measured by a scanning laser beam technique. To prevent sample oxidation the pressure cell containing the sample is mounted in a vacuum chamber. The correct operation of the setup is demonstrated by measurement of the thermal expansion of free-standing Al films. Creep experiments and tensile tests demonstrate measurement of the plastic deformation of these films at temperatures up to 200 °C.

Transient creep in free-standing thin polycrystalline aluminum films

We studied room-temperature transient creep in polycrystalline, free-standing Al films with a thickness between 220 and 550 nm using a high-resolution bulge test technique. A transient logarithmic creep strain is observed. The time and stress dependence of the creep strongly support the idea that dislocation glide, limited by forest dislocation cutting, is the prevailing rate limiting mechanism. This is in contradiction with the misfit dislocation model for thin-film strengthening but in agreement with recent work on plasticity in thin Ag and Cu films on a substrate. A comparison is made with data on bulk Al. Both the transient creep strain and the initial fast strain are at least three orders of magnitude smaller for the thin-film samples. We argue that the strain hardening coefficient is the key parameter distinguishing thin film from bulk creep.

High-temperature tensile tests and activation volume measurement of free-standing submicron Al films

Tensile tests on free-standing, 200-nm-thick Al films were conducted between room temperature and 200 °C. Applied strain rates were in the range 1×10−8–2×10−6/s. At a temperature of 200 °C a saturation of the flow stress was observed. The strain required to achieve steady-state saturation is of the order of 2×10−3, which is much smaller than for bulk Al. From the observed strain rate sensitivity of the saturation stress we calculate an activation volume Ωact=60b3, which is small compared to the value of 700b3 observed in bulk Al (b is the magnitude of Burgers vector).

Resistance changes induced by the formation of a single void/hillock during electromigration

Electromigration induced resistance changes in short Al lines have been studied by high-resolution AC bridge measurements. The samples were pure, unpassivated Al lines having a length of 3, 5, 8, 12, 17 or 100 μm, a width of 2 μm and a film thickness of ∼100 nm. Depending on current density and sample length the induced resistance changes fully recover or do not recover after removal of the DC stressing current. The transition from recoverable to non-recoverable behavior is clear-cut and is characterized by a constant critical current density—sample length product. Inspection of the lines after current stressing by atomic force microscopy revealed that non-recoverable resistance changes are caused by the growth of a single void, hillock or hillock/void pair. Negative resistance changes correspond to the growth of a hillock and positive resistance changes to the growth of a void. The characteristic time of the relaxation process of the recoverable resistance changes scales with the sample length squared. T...

Fabrication of submicron single-crystalline and bamboo Al lines by recrystallization

Abstract We have developed a process for the fabrication of single-crystalline and bamboo Al lines by recrystallization. A groove pattern of submicrometer dimensions is fabricated into an oxide layer by electron-beam-lithography and reactive-ion-etching. Blanket Al sputter deposition and heating, led to melting and subsequent filling of the grooves. The microstructure of the Al was examined with Backscatter Kikuchi Diffraction (BKD) and TEM. The presence of an in-plane temperature gradient during heating and cooling had a profound influence on the microstructure of the resolidified Al lines. The lines fabricated without a temperature gradient were ‘bamboo’ and had the common (111) fiber texture. Those fabricated with a temperature gradient were ‘distorted’ single-crystals. Both types of lines have a low resistivity, indicating a high purity. The lines are electromigration lifetime-tested at 200 °C and with a current density of 2 or 8 MA/cm 2 . After 1800 hrs, one of the 15 single-crystalline lines had failed. Two out of the 7 bamboo lines had failed. In these two, facetted voids were observed.

Stress relaxation and creep in free-standing thin Al films studied using a bulge tester

We constructed a second generation micro tensile test apparatus (“bulge tester”) to study mechanical properties of free standing thin films. The apparatus is insensitive to the initial bulge height, allowing for measurements on films deposited under compressive stress. We have checked the performance of the set-up by measuring the elastic properties of thin films. We found that our set-up displays an excellent linearity without hysteresis. We measured the Young’s modulus of Al thin films with submicron thickness. We obtain a Young’s modulus of 62±4 GPa. We studied stress relaxation in thin Al films after furnace anneal. The membrane samples cool down quickly after removal from the furnace, inducing a high thermal stress. The measurements could be started about 3 minutes after removal from the furnace, which enables us to study the first stage of the stress relaxation, which is thusfar not achieved by stress relaxation studies using the substrate curvature method. In the set-up we are able to perform creep...

Electromigration and Diffusion in Short Al-Ni-Cr Lines

To investigate why the formation of electromigration damage in Al lines alloyed with both 0.1 at.% Ni and 0.1 at.% Cr is so effectively suppressed we studied electromigration induced changes in the electrical resistance of short lines. The length of the lines is 4, 6, 8, or 12 µm, the width is 4 µm and the film thickness is 0.3 µm. As previously reported for pure Al lines, depending on current density and sample length, the induced resistance changes fully recover or do not recover after DC stressing for a few hours. The recoverable resistance changes correspond to the build-up or relaxation of mechanical stress and the permanent resistance changes to the formation of a void or hillock. Electron microprobe analysis and resistance measurements after prolonged current stressing provided no evidence for changes in local solute concentration. Therefore diffusion of Al along grain boundaries is regarded as the dominating diffusion process. The activation energy and the pre-exponential factor of this process are determined from the temperature dependence of the relaxation time of the recoverable resistance changes. The activation energy of this process is 0.84±0.03 eV and the pre-exponential factor δD 0 = (1.8±0.8)×10 −8 cm 3 /s. At 175 °C the resulting diffusion coefficient is about a factor 70 smaller than that of pure Al. The transition from recoverable to permanent resistance changes is characterized by a constant critical current density -sample length product. It is found that the value of this product for the Al-Ni-Cr lines is two times larger than the value found for pure Al lines. This means that the threshold current density at given sample length for the formation of hillocks or voids is two times larger. Both the slower solvent diffusion and the higher threshold current density contribute positively to a long electromigration lifetime.

Aluminium via-fill at elevated pressure and temperature

In the present paper the filling of vias with aluminium at elevated temperatures and pressures is discussed. The role of via wall material is discussed in terms of wetting or surface energy. A simple one-dimensional stress driven diffusion model is presented. An activation energy and pre-exponential factor are derived by fitting the model to the data on temperature and pressure needed to fill the via in 30 minutes.