Richard P. Vinci

Thermal strain and stress in copper thin films

Abstract Desired improvements in the performance and reliability of integrated circuit interconnects may necessitate a move from aluminum alloys to copper. Before copper is adopted, however, characterization of the thermal stress behavior of copper thin films is necessary in order to identify mechanical reliability concerns and to determine differences from aluminum behavior. In this study, the behavior of copper films is evaluated to determine effects of film texture, thickness, and the presence of a passivation layer. Mechanistic models based on bulk deformation maps and interface-controlled dislocation glide are compared with the measured behavior. A preferred [111] grain orientation is found to slightly increase the stress throughout a thermal cycle as compared with a film with random grain orientation. An inverse relationship between film thickness and strength, similar to that seen in aluminum, is quantified. The presence of a passivation layer significantly reduces stress relaxation at high temperatures, resulting in behavior that closely resembles that of unpassivated aluminum films. Neither model adequately predicts the thickness and passivation effects over the entire temperature and stress range, emphasizing the need for more characterization of the flow processes active in metallic thin films.

Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nanopatterned AGOG Sapphire Substrate by Abbreviated Growth Mode

Metalorganic vapor phase epitaxial (MOVPE) growth of GaN on nanopatterned AGOG sapphire substrates was performed, and characteristics of the light-emitting diode (LED) devices grown on patterned sapphire and planar substrates were compared. The nanopatterned sapphire substrates were fabricated by a novel process (AGOG) whereby aluminum nanomesas were epitaxially converted into crystalline Al2O3 via a two-stage annealing process. The GaN template grown on the nanopatterned sapphire substrate was done via an abbreviated growth mode, where a 15-nm thick, low-temperature GaN buffer layer was used, without the use of an etch-back and recovery process during the epitaxy. InGaN quantum wells (QWs) LEDs were grown on the GaN template on the nanopatterned sapphire, employing the abbreviated growth mode. The optimized InGaN QW LEDs grown on the patterned AGOG sapphire substrate exhibited a 24% improvement in output power as compared to LEDs on GaN templates grown using the conventional method. The increase in output power of the LEDs is attributed to improved internal quantum efficiency of the LEDs.

Nanoindentation measurements on Cu–Sn and Ag–Sn intermetallics formed in Pb-free solder joints

Nanoindentation testing has been used to measure the hardness and elastic modulus of Ag 3 Sn, Cu 6 Sn 5 , and Cu 3 Sn intermetallics, as well as Sn-Ag-Cu solder and pure Sn and Cu. The intermetallics were fabricated by solid-state annealing of diffusion couples prepared from a substrate (Cu or Ag) and a solder material (Sn or Sn-Ag-Cu solder), providing geometries and length scales as close as possible to a real solder joint. Nanoindentation results for the intermetallics, representing penetration depths of 20-220 nm and loads from 0.7 to 9.5 mN, reveal elastic/plastic deformation without evidence of fracture. Measured hardness values of Cu 6 Sn 5 (6.5 ′ 0.3 GPa) and Cu 3 Sn (6.2 ′ 0.4 GPa) indicate a potential for brittle behavior, while Ag 3 Sn (2.9 ′ 0.2 GPa) appears much softer and ductile. Using a bulk Cu 6 Sn 5 sample, Vickers hardness testing revealed an indentation size effect for this compound, with a hardness of 4.3 GPa measured at a load of 9.8 N. An energy balance model is used to explain the dependence of hardness with load or depth, where the observation of an increasing amount of fracture with applied load is identified as the primary mechanism. This result explains discrepancies between nanoindentation and Vickers results previously reported.

Effects of barrier layer and annealing on abnormal grain growth in copper thin films

Abnormal (100) grain growth has been characterized in predominantly (111)‐textured Cu thin films as a function of deposition temperature, annealing temperature and the presence of a Ta or W underlayer. For films deposited at room temperature, bimodal grain size distributions are observed at annealing temperatures at or above 150 °C for Cu on Ta and 100 °C for Cu on W. Suppression of (100) abnormal grain growth was achieved by depositing Cu on either barrier layer at 150 °C. A bimodal grain size distribution was still observed for the film deposited on W at 150 °C but the large grains forming this distribution were found to be (111) oriented. These results are explained as the result of competition between strain energy minimization and surface and interface energy minimization. The (100) growth is shown to be driven by a reduction of the orientation‐dependent strain energy that builds up due to the elastic anisotropy of Cu. Films deposited at higher temperatures have a lower yield stress which limits the ...

Microstructural evolution in lead-free solder alloys: Part I. Cast Sn–Ag–Cu eutectic

Coarsening of the ternary eutectic in cast Sn–Ag–Cu lead-free solder alloys was investigated. The process was found to follow r 3 ∝ t kinetics where r is the rod radius of the dispersed phase and t is time. The effective activation energy for the process is 69 ± 5 kJmol -1 . The two types of intermetallic rods, Cu 6 Sn 5 and Ag 3 Sn, in the eutectic structure coarsen at different rates, with each having a different rate-controlling mechanism. The overall coarsening kinetics for the Sn–Ag–Cu ternary eutectic is significantly slower than that found for the Pb-Sn eutectic, which has implications for long-term reliability of Sn–Ag–Cu solder joints.

Mechanical properties of intermetallic compounds in the Au-Sn system

The mechanical properties of intermetallic compounds in the Au–Sn system were investigated by nanoindentation. Measurements of hardness and elastic modulus were obtained for all of the confirmed room-temperature intermetallics in this system as well as the β phase (8 at.% Sn) and AuSn 4 . Overall, it was found that the Au–Sn compounds have lower hardness and stiffness than common Cu–Sn compounds found in solder joints. This finding is in contrast to common knowledge of “Au embrittlement” due to the formation of either AuSn 4 or (Au,Ni)Sn 4 intermetallic compounds. This difference in understanding of mechanical properties of these phases and the resulting joint strength is discussed in terms of reliability and possible failure mechanisms related to interface strength or microstructural effects. Indentation creep measurements performed on Au 5 Sn, Au–Sn eutectic (29 at.% Sn) and AuSn indicate that these alloys are significantly more creep resistant than common soft solders, in keeping with typical observations of actual joint performance.

The influence of strain energy on abnormal grain growth in copper thin films

Biaxial stress and strain in (100) and (111) oriented grains have been measured as a function of annealing temperature for a Cu film on an oxidized Si substrate which exhibits abnormal (100) grain growth. The observed behavior indicates isostrain averaging, which is consistent with grain growth that is controlled by strain energy density minimization. In contrast, two films which do not exhibit (100) abnormal grain growth appear to follow isostress averaging. Strain energy density minimization in this situation favors (111) grain growth.

Microstructural evolution in lead-free solder alloys: Part II. Directionally solidified Sn-Ag-Cu, Sn-Cu and Sn-Ag

The tin-silver-copper eutectic is a three-phase eutectic consisting of Ag 3 Sn plates and Cu 6 Sn 5 rods in a (Sn) matrix. It was thought that the two phases would coarsen independently. Directionally solidified ternary eutectic and binary eutectic samples were isothermally annealed. Coarsening of the Cu 6 Sn 5 rods in the binary and ternary eutectics had activation energies of 73 ′ 3 and 82 ′ 4 kJmol - 1 , respectively. This indicates volume copper diffusion is the rate controlling mechanism in both. The Ag 3 Sn plates break down and then coarsen. The activation energies for the plate breakdown process were 35 ′ 3 and 38 ′ 3 kJmol - 1 for the binary and ternary samples respectively. This indicates that tin diffusion along the Ag 3 Sn/(Sn) interfaces is the most likely the rate-controlling mechanism. The rate-controlling mechanisms for Cu 6 Sn 5 coarsening and Ag 3 Sn plate breakdown are the same in the ternary and binary systems, indicating that the phases evolve microstructurally independently of one another in the ternary eutectic.

Study of the effect of grain boundary migration on hillock formation in Al thin films

We have studied the effect of grain boundary migration on hillock formation in unpassivated Al thin films during thermal cycling. Hillocking occurs more frequently in Al films that experience grain growth during thermal cycling than in films with stabilized grain structures. The hillocking frequency is at least four times greater in the films that experience grain growth, as judged by the number of hillocks observed per initial grain boundary triple junction. This latter measure takes account of the smaller initial grain size in the film that experiences grain growth and shows that grain boundary migration itself must enhance the hillocking frequency.

Anelastic Stress Relaxation in Gold Films and Its Impact on Restoring Forces in MEMS Devices

In order to evaluate the importance of stress relaxation on device performance of capacitive RF MEMS switches, stress relaxation has been measured in 1.2-mum-thick Au films using a membrane bulge technique. When the residual stress in the films is small, the stress relaxation is fully recoverable and is well described by linear anelasticity (viscoelasticity) theory. A 27% reduction in the effective elastic modulus occurs over a three-day period under constant strain conditions at room temperature. The time dependence of the relaxation can be represented by a series of time constants with values extending from seconds to days. Linear superposition of the anelastic response can be used to accurately predict the stress under any time dependence of the strain. The prediction is accurate even during cyclic loading and unloading, and even when the strain is cycled at rates that are fast compared with any of the relaxation times. The restoring force available to open a capacitive RF MEMS switch is modeled for two different switch designs. The restoring force is shown to drop by approximately 7% or 20% at room temperature for the two cases presented.