Paul A. Flinn

Measurement and Interpretation of stress in aluminum-based metallization as a function of thermal history

Mechanical stress in interconnections is a problem of growing importance in VLSI devices. The origins of this stress are discussed, and a measurement technique based on the determination of wafer curvature with a laser scanning device is described. The changes in stress observed during thermal cycles are interpreted quantitatively in terms of a simple model of elastic and plastic strain in the metal. The effects of changes in deposition conditions, film composition, and film structure are discussed.

The Role of Indentation Depth on the Measured Hardness of Materials

Ultra micro-indentation tests on Ni and Cu samples showed increasing hardness with decreasing penetration depth over a range from 200 to 2000 nm. The results suggest increased strain hardening with decreased indentation depth. To establish that this is a real material effect, a series of tests were conducted on amorphous materials, for which strain hardening is not expected. The hardness of Metglas ® was found to be independent of depth. A simple model of the dislocation densities produced under the indenter tip describes the data well. The model is based on the fact that the high density of dislocations expected under a shallow indentation would cause an increase in measured hardness. At large depths, the density of geometrically necessary dislocations is sufficiently small to have little effect on hardness, and the measured hardness approaches the intrinsic hardness of the material.

X‐ray diffraction determination of the effect of various passivations on stress in metal films and patterned lines

Direct x‐ray diffraction determination of elastic strain and stress in aluminum and aluminum‐silicon films and patterned lines has been used to investigate the effect of various passivations. Passivation over uniform metal films has very little effect. Passivation over patterned metal results in substantial triaxial tensile stress. Contrary to the conventional wisdom, high compressive stress in the passivation does not result in additional tensile stress in the metal. The deleterious effects of highly compressive silicon nitride on metal is probably due to the effect of excess hydrogen in the silicon nitride.

Mechanical stress as a function of temperature in aluminum films

Mechanical stress in interconnection is a problem of growing importance in VLSI devices. Open circuits due to metal cracking and voiding and short circuits due to hillocks are stress-related phenomena. The origins of this stress are discussed including intrinsic stresses from the synthesis of the films and thermally induced stresses. A measurement technique based on the determination of wafer curvature with a laser scanning device is utilized to directly measure the film stress in situ as a function of temperature during thermal cycling. The changes in stress observed during thermal cycles are interpreted quantitatively and mechanisms that lead to plastic deformation and their relationship to hillocks are discussed. In the stress vs. temperature measurements, several regions have been identified including elastic and plastic behavior both under compression and tension, the yield strength, recrystallization, gain growth, hardening, and solid-state reactions. The effects of deposition conditions on these regions are also examined. >

Measurement and interpretation of stress in copper films as a function of thermal history

Since copper has some advantages relative to aluminum as an interconnection material, it is appropriate to investigate its mechanical properties in order to be prepared in advance for possible problems, such as the cracks and voids that have plagued aluminum interconnect systems. A model previously used to interpret the behavior of aluminum films proves to be, with minor modification, also applicable to copper. Although the thermal expansion of copper is closer to that of silicon and, consequently, the thermally induced strains are smaller, the much larger elastic modulus of copper results in substantially higher stresses. This has implications for the interaction of copper lines with dielectrics.

Stress in metal lines under passivation; comparison of experiment with finite element calculations

The elastic strain in Al‐0.5% Cu metal lines under silicon nitride passivation has been determined by x‐ray diffraction. The experimental stress tensor calculated from these strain values is in excellent agreement with the results of a finite element model calculation. The intrinsic stress in the dielectric plays no role in influencing the stress in the metal; only thermal stress effects are important.

Mechanical properties and microstructural characterization of Al-0.5%Cu thin films

Using a wafer curvature technique we have studied the variation of stress with tem-perature in Al-0.5%Cu thin films deposited on oxidized silicon wafers. Concurrently, the microstructural changes in the films induced by the thermal cycling inherent to this technique were studied with in-situ transmission electron microscopy heating experi-ments. On heating an as-sputtered film a stress drop occurs, corresponding to the onset of grain growth. The in-situ TEM experiments indicate that the extent of grain growth is significantly altered by the presence of compressive stresses in the film. During cool-ing, dislocation loops nucleate on {111} planes inclined to the film surface, although the grain size plays an important role in determining the extent to which this mechanism can account for the deformation. A native oxide can influence the stress levels in the film by pinning one end of the dislocation loops. Upon cooling below 200° C a rapid increase in stress occurs. Although this increase has been attributed to hardening due to the precipitation of excess copper, no evidence of precipitate-dislocation interactions were observed.

Mechanical stress as a function of temperature for aluminum alloy films

Aluminum alloys have virtually replaced aluminum thin films for interconnections in very large‐scale integration because of their improved reliability. Mechanical stress is a problem of growing importance in these interconnections. Stress as a function of temperature was measured for thin aluminum films on an oxidized silicon substrate and several aluminum alloys and layered films consisting of silicon, copper, titanium, tungsten, tantalum, vanadium, and TiSi2. Solid‐state reactions of the aluminum with the additives and with the ambient during thermal cycling will occur, and depending on what compounds have formed and at what temperature, this will determine the morphology and reliability of the metallization. The measurement technique, based on determination of wafer curvature with a laser scanning device, directly measures the total film stress and reflectivity in situ as a function of temperature during thermal cycling. Changes in stress were detected when film composition and structure varied and wer...

Principles and Applications of Wafer Curvature Techniques for Stress Measurements in Thin Films

Measurement of the curvature induced in a wafer (or other flat plate) by the stress in a thin film has long been used as a convenient and accurate technique for the determination of the stress. Numerous improvements over the years have led to instruments that provide simple and rapid measurements of stress as a function of the time and temperature for any desired thermal history. A computer controlled instrument using laser scanning will be briefly described and its capabilities and limitations discussed. Applications of the technique to a variety of thin film materials will be discussed. In addition to the effects of differences in thermal expansion, stresses associated with various deposition techniques, gain or loss of material, phase transformations and flow will be considered. In aluminum based systems, themal expansion, plastic flow and phase transformation play major roles. Refractory metals show, in addition, large stresses associated with the deposition process. In inorganic dielectric systems thermal expansion effects are usually relatively small; deposition effects and the gain or loss of material are the dominant effects. Silica based glasses formed by chemical vapor deposition, for example, show large stress changes due to gain or loss of water, and plasma deposited silicon nitride films show large effects associated with hydrogen. Overall, determination of the stress as a function of time and temperature is a valuable part of the evaluation of a thin film material for use in a VLSI device.

In situ scanning electron microscopy observation of the dynamic behavior of electromigration voids in passivated aluminum lines

The dynamic behavior of electromigration (EM) voids has been studied in situ using a field‐emission scanning electron microscope fitted with a Robinson backscatter detector. A high‐temperature stage has been used to minimize the temperature gradients associated with Joule heating and to allow independent control of temperature and current density. No evidence of pre‐existing voids was found. The formation, growth, and motion of electromigration voids were observed and recorded photographically. The voids moved dynamically against the electron wind. No correlation between void size and void velocity was found. The static growth of EM voids was observed in some instances; however, this did not precede void motion nor did it lead to failure. Moving voids formed late in the test dominated final failure. Comparison of experimental results with void motion models reveals that the models for dynamic void motion are not consistent with experimental observations.