摘要:

Insituhigh voltage scanning electron microscope (HVSEM) examinations of accelerated electromigration tests were performed. The samples tested were Al‐1% Si lines 3 &mgr;m wide, 1 &mgr;m high, and 300 &mgr;m long. Voiding processes were observed through passivation and the early stages of voiding were documented. Voids nucleated at the sidewall of the line at the metal‐passivation interface. The voids then grew in a wedge‐like manner. The later stages of movement and coalescence of voids, leading to eventual failure, were also imaged. A simple model of the early stages of voiding is proposed.

ISSN:0094-243X    

DOI:10.1063/1.45709

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We outline a microstructure‐based statistical model for passivated near‐bamboo interconnect structures to obtain quantitative predictions of stress migration and electromigration failures, and their stress, temperature and current dependence. In particular, we find the failure distributions to be multimodal, and the mode responsible for failure at the early failure level is often not significant in the experimentally accessible regime of the cumulative failure, about 5–95%. We show that the early and longer term failure modes, both at vias and the straight line portions, depend differently on the stress, temperature and current density, which necessitates devising improved extrapolation procedures for the prediction of the time to early failure in service conditions. As far as electromigration lifetimes are concerned, the effect of refractory metal barrier layers is simply to allow larger void sizes at failure.

ISSN:0094-243X    

DOI:10.1063/1.45706

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A systematic investigation of the effects of oxygen contamination in Al on microstructures and stress‐migration failures in Al lines has been made using Al lines intentionally contaminated with oxygen to various degrees. The results of stress migration tests at 150 °C, 200 °C, 250 °C and 295 °C show that stress‐migration characteristics, i.e., void shape and statistical lifetime distribution, are strongly influenced by oxygen‐contamination levels. Two characteristic failure types were derived from the results: one, the low‐oxygen type, is characterized by open failures caused by large (∼2 &mgr;m2) wedge‐shaped voids, small (∼0.15) logarithmic standard deviations, and a large (∼1.2 eV) activation energy, and the other, the high‐oxygen type, is characterized by open failures caused mostly by crack‐like voids and large (≳1) logarithmic standard deviations. Instantaneous failure rates at operating temperatures, estimated from the storage test results, seem to be negligible until the wearout period begins for the low‐oxygen type, while those for the high‐oxygen type seem to be unacceptably high in the early and random failure periods. Therefore, reducing oxygen contamination is useful for lowering the stress‐migration failure rate at operating temperatures.

ISSN:0094-243X    

DOI:10.1063/1.45699

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Accurate modeling of stresses and strains and prediction of stresses leading to stress‐assisted diffusive voiding in interconnection metallizations requires measurement of the stress/strain state of the lines during thermal cycling. X‐rays are a proven method for measuring strains and calculating stresses in lines and yield precise results when properly interpreted. We highlight two x‐ray geometries we have used to successfully determine the strain and stress state of unpassivated and passivated aluminum alloy lines. There is good agreement between the two techniques. Additionally, we have measured strain relaxation in the lines. We correlate the relaxation in passivated Al alloy lines with direct observation of stress‐induced voiding performed using a scanning transmission electron microscope.

ISSN:0094-243X    

DOI:10.1063/1.45700

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The effects of oxide confinement on the thermal stress and yield behavior of Al(Cu) line structures have been studied as a function of linewidth to submicron dimensions. Measurements have been carried out on unpassivated and passivated lines during thermal cycling using a bending‐beam technique. By measuring periodic line structures with lines oriented parallel and perpendicular to the beam direction, the principal stresses in the line structures were deduced based on a micromechanical analysis of a unit of the periodic structure. The analysis was extended to incorporate plastic deformation, enabling the determination of the metal line yield stresses. The stress evolution in the unpassivated AlCu lines during thermal cycling exhibit a marked anisotropy in the stresses perpendicular &sgr;⊥and parallel &sgr;∥to the line direction, which can be attributed to the effect of stress relaxation at the line edges. In contrast, the principal stress components obtained for the passivated lines constitute a near‐hydrostatic stress state with the stress levels increasing for decreasing linewidth. The yield stress of the passivated lines was also found to increase with decreasing line width, rendering a more elastic behavior of the passivated lines. All of these characteristics tend to favor void formation in the passivated lines. The stress behavior of the oxide passivation has also been deduced and the result raises the possibility of crack formation in the oxide during thermal cycling.

ISSN:0094-243X    

DOI:10.1063/1.45701

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

From the perspective of microelectronic chip reliability, the goal of understanding stress‐induced voiding is to be able to evaluate the impact of void formation on chip lifetime. This requires a reliable acceleration factor for chip failure which relates failure under accelerated stress conditions to reliability under use conditions. However, a well‐defined acceleration factor has not been available for stress‐induced voiding, as is used, for example, for electromigration. Without such an acceleration factor, the effectiveness of any set of stress conditions for revealing potential wearout risks is not clear. A suitable acceleration factor must be consistent with a physically descriptive model which contains all of the necessary variables. Part of the difficulty in producing a reliable acceleration factor for chip failure has been the sheer number of variables upon which failure and void growth depend. These include mechanical and thermal properties of passivation materials, intrinsic stress in the passivation, metallization microstructure, metal composition, void nucleation density and distribution, void growth rate, and void shape, among others. Predicting chip failure due to voiding therefore depends upon understanding the contribution of each of these variables. This paper describes an expression for void growth which includes most of the relevant variables. Predictions are compared with data.

ISSN:0094-243X    

DOI:10.1063/1.45708

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Mechanical stress in aluminum metallizations during thermal cycling has been investigated byin‐situhigh‐temperature X‐ray diffraction and optical curvature measurements. The X‐ray diffraction method delivers true stress in thin aluminum films even if barriers and passivation layers are present. Plastic flow of aluminum starts at temperatures above 200 °C. Local stress—caused by precipitations in the AlSiCu alloy—disappears at increasing temperature. This is indicated by increasing sharpness of the diffraction peaks. Si dissolves in aluminum between 450–500 °C and recrystallizes in the same temperature range. On cooling, the diffraction peaks stay sharp until plastic flow freezes at around 200 °C and Si grains create local stress in aluminum by lattice deformation again. In pure aluminum films the effect of peak sharpening is, as a consequence, much smaller. The macrostress in thermally cycled aluminum obtained by optical curvature measurements is nearly identical to that measured by the X‐ray technique. In passivated aluminum lines the Si‐recrystallizations preferentially precipitate near the line edges. Typical degradation of aluminum during electromigration tests corresponds to void‐hillock formation. The critical void‐hillock distances decrease with increasing current density, indicated by increasing ohmic resistance. The degradation process of passivated structures starts with crack formation in the passivation and is followed by hillock growth through the cracks and coincident void formation in the metal lines until interruption. The requirements to a suitable quick response test method for characterization of passivated metal lines led us to the development of an accelerated measurement technique at the wafer level using a new designed test structure. Monte Carlo simulations confirmed by experimental results demonstrate a reduced Median Time to Failure and lowered Standard Deviation.

ISSN:0094-243X    

DOI:10.1063/1.45702

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

As feature size enters the sub‐micron range in Si based solid‐state electronic devices, void formation leading to open circuits in Al interconnect metallizations becomes an increasingly important reliability problem. Techniques for the fast determination of stress in Al thin films in the presence of SiO2passivation or line patterning were developed and were used to investigate triaxial and biaxial stress states. Stresses were determined as a function of temperature (from 30 °C to 400 °C) with average heating rates on the order of 5 °C/min and with data points every 15–20 °C. Results from a number of 0.6 &mgr;m Al(1%Cu)‐film systems are presented, including: blanket unpassivated films; blanket, SiO2passivated films; passivated, patterned lines with widths ranging from 0.5 to 6.0 &mgr;m.

ISSN:0094-243X    

DOI:10.1063/1.45703

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Simple theoretical models are used to investigate the nucleation and growth of voids in passivated interconnect lines. It is shown that voids are likely to nucleate if the line is cooled below a critical temperature after passivation. Current manufacturing methods appear to exceed this critical temperature change by a considerable margin. In addition, models are presented for four possible mechanisms of void growth in interconnect lines with a bamboo grain structure. The mechanisms considered are: creep deformation by dislocation motion; grain boundary diffusion; diffusion along the interface between the line and the surrounding passivation; and void growth by lattice diffusion. The results show that all four mechanisms may cause the void to grow, but any one mechanism operating alone gives rise to slow or negligible rates of growth. However, plastic creep flow and grain boundary diffusion may act in a co‐operative manner, which causes rapid void growth. It is shown that the extent of coupling between diffusion and creep increases as the line width is reduced.

ISSN:0094-243X    

DOI:10.1063/1.45704

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Stress migration phenomena in submicron aluminum interconnect structures have been studied using electrical measurements at elevated temperatures. Highly accurate resistance measurements on metal snake structures were used to measure degradation resulting from stress migration creep. A semi‐empirical model for time‐to‐failure as a function of temperature was derived from the measured data. The model reflects the reduction in stress migration both at low temperatures due to reduced vacancy diffusivity, and at high temperatures due to reduced stress levels. The results indicate that the peak degradation occurs at ≊180 C.

ISSN:0094-243X    

DOI:10.1063/1.45705

出版商:AIP    

年代:1994

数据来源: AIP