Anne Sauter Mack

Hardness and Modulus Studies on Dielectric Thin Films

We report hardness and Youngs modulus measurements on various dielectric thin films. Hardness and modulus information was derived from indentation experiments with a Berkovich triangular-based diamond indenter in an ultra micro-indentation instrument (UMIS). We studied the effect of moisture content and phosphorous doping on hardness and Youngs modulus of low temperature Chemical Vapor Deposition (CVD) Si-oxides and found that dehydration and densification tend to harden samples, whereas increased P-doping results in a lower hardness. Hardness values of silicon nitride, silicon oxynitride, sputtered oxide, spin-on-glass and APCVD Si-oxides are compared. We also discuss how deposition conditions and chemical compositions correlate to dielectric properties such as stress as well as moisture uptake, thermal expansion coefficients and hardness and modulus values. Using these results, thermal stresses in encapsulated Al lines have been calculated and the calculated stress in Al is higher when encapsulated with dielectric films with higher moduli.

Strain Measurement and Calculation in Passivated Cu Lines Deposited by Three Different Methods

Passivated Cu lines deposited by CVD, electroplating, and sputter-reflow were investigated using x-ray diffraction. Blanket films of the three types were measured for strain and texture post-deposition and after an anneal step to mimic the passivation temperature step. Texture in the CVD films was random, while the electroplated and sputtered films showed a strong {111} texture. Lines were then measured of each type. The measured strain was modeled using finite element calculations. While the strain in Cu was high compared to Al lines of similar geometry, no stress voiding was observed using high voltage scanning electron microscopy.

Measurement and Modeling of Intrinsic Stresses in CVD W Lines

We have studied stress states in chemical vapor deposited (CVD) tungsten (W) for both blanket films and lines, to understand better the mechanical implications of intrinsic stress for interconnection structures. Since W has a low mobility at its deposition temperature, a very large intrinsic stress develops during deposition. Intrinsic strains in blanket W films were measured with an X-ray technique. The measured strains correspond to a biaxial tensile stress of the order of 1 GPa. This result was used to provide an initial strain input in a finite element calculation to obtain intrinsic stress states in W lines. SEM observation of cross sections of the metal lines enabled us to determine the growth pattern of the W, and infer the boundary conditions during growth. Finite Element Method (FEM) calculations of the room temperature stress in the lines, including both intrinsic and thermal components, are in good agreement with X-ray determinations.

The Effect of Intrinsic Passivation Stress on Stress in Encapsulated Interconnect Lines

The stress in an encapsulated metal line has been calculated using the finite element technique for two passivation stress conditions. It is found that the stress-state in the passivation has a negligible effect on the stress in the metal line. The dominant factors affecting metal line stress are the passivation temperature, the thermal expansion coefficient difference between the metal line and the substrate, and the passivation modulus.

Thermal Conductivity Measurements of Interlevel Dielectrics

The thermal conductivity of interlevel dielectrics (ILD) in interconnect structures is an important parameter in determining the temperature rise in the interconnects during use. Numerous researchers have previously shown that the thermal conductivity of thin film dielectrics can be significantly lower than that of bulk materials. As new materials, such as low-dielectric constant materials, are considered for use as ILDs, methods are needed for measuring the thermal conductivity of the these materials to determine whether they can adequately conduct heat away from interconnect lines. Many methods reported in the literature use patterned metal lines atop the dielectric on a Si substrate as combination Joule heaters and temperature sensors, and extract the thermal conductivity from a model of heat conduction through the dielectric to the substrate. One drawback of these methods is the lack of agreement of the conductivity determined from the different techniques. For example, a thermal conductivity ranging from 0.6 to 1.4 W/m-K was calculated for a 1.25 μm thick PTEOS oxide using five different methods on the same test structure. In this paper we present a unique combination of test structures, experimental methods, and heat conduction models that highlight the limitations of some of the models and methods. We also show good agreement in the thermal conductivity determined from both an experimental method and a finite element model, and suggest that these two techniques yield an accurate measure of the thermal conductivity of thin film dielectrics.

The Stress Change in Passivated Al Lines Due to the Reaction Between Ti and Al

The thin film reaction between Ti and Al-0.5%Cu to form TiAl 3 is common in the microelectronics industry. In this paper the stress changes in Al-0.5%Cu films at elevated temperatures during the reaction are measured. The changes are measured in blanket films as well as in passivated interconnect lines. Results show that in blanket films the Al-0.5%Cu does not experience any stress change due to the reaction. However in passivated lines, where the layers are not allowed to relax in the normal direction, tensile stresses build up in the Al-0.5%Cu due to the volume shrinkage that happens when these films react.