Showing papers by "Paul S. Ho published in 2001"

Electromigration critical length effect in Cu/oxide dual-damascene interconnects

Abstract: Electromigration tests at temperatures between 340 and 400 °C and current densities between 1.0 and 3.0 MA/cm2 have been performed to determine the temperature dependence of the critical length effect in 0.5-μm-wide Cu/oxide dual-damascene interconnects with 0.1 μm silicon nitride (SiNx) passivation. A focused-ion-beam-induced contrast imaging technique is used to locate failure sites of critical length test structures. Statistical analysis [E. T. Ogawa et al., Appl. Phys. Lett. 78, 18 (2001)] yields a threshold-length product (jL)c, of 3700 A/cm, and a temperature dependence is not observed within the temperature range 340–400 °C.

Statistical analysis of early failures in electromigration

Abstract: The detection of early failures in electromigration (EM) and the complicated statistical nature of this important reliability phenomenon have been difficult issues to treat in the past. A satisfactory experimental approach for the detection and the statistical analysis of early failures has not yet been established. This is mainly due to the rare occurrence of early failures and difficulties in testing of large sample populations. Furthermore, experimental data on the EM behavior as a function of varying number of failure links are scarce. In this study, a technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge is presented. Three types of structures with a varying number of Ti/TiN/Al(Cu)/TiN-based interconnects were used, starting from a small unit of five lines in parallel. A serial arrangement of this unit enabled testing of interconnect arrays encompassing 480 possible failure links. In addition, a Wheatstone Bridge-type wiring using four large arrays in each device enabled simultaneous testing of 1920 interconnects. In conjunction with a statistical deconvolution to the single interconnect level, the results indicate that the electromigration failure mechanism studied here follows perfect lognormal behavior down to the four sigma level. The statistical deconvolution procedure is described in detail. Over a temperature range from 155 to 200 °C, a total of more than 75 000 interconnects were tested. None of the samples have shown an indication of early, or alternate, failure mechanisms. The activation energy of the EM mechanism studied here, namely the Cu incubation time, was determined to be Q=1.08±0.05 eV. We surmise that interface diffusion of Cu along the Al(Cu) sidewalls and along the top and bottom refractory layers, coupled with grain boundary diffusion within the interconnects, constitutes the Cu incubation mechanism.

Direct observation of a critical length effect in dual-damascene Cu/oxide interconnects

Abstract: Electromigration results have provided clear evidence of a short or “Blech” length effect in dual- damascene, Cu/oxide, multilinked interconnects. The test structure incorporates a repeated chain of Blech-type line elements and is amenable to failure analysis tools such as focused ion beam imaging. This large interconnect ensemble provides a statistical representation of electromigrationinduced damage in the regime where steady-state interconnect stress is manifest. Statistical analysis yields a critical length of 90 μm for interconnects with line width 0.5 μm at j=1.0×106 A/cm2 and T=325 °C.

Thermal conductivity and interfacial thermal resistance of polymeric low k films

Abstract: The effective thermal conductivity of four polymeric thin films with distinct molecular morphologies has been measured as a function of film thickness down to 70 A using a 3ω technique. Comparing to SiO2, the intrinsic thermal conductivity of polymers is about 5–10 times smaller while the interfacial thermal resistance is about 2–10 times larger. The interfacial thermal resistances are explained in terms of the mismatches of acoustic and mechanical properties between polymers and crystalline materials. Both elastic and inelastic scatterings at the interface are examined.

Statistics of electromigration early failures in Cu/oxide dual-damascene interconnects

Abstract: Electromigration (EM) study at temperatures from 325-400/spl deg/C and current densities from 1-2 MA/cm/sup 2/ has determined the failure time characteristics and failure behavior of submicron dual-damascene Cu/oxide interconnects. The test structures used are based on statistical concepts potentially suitable to address the early failure issue in sub-/spl mu/m interconnects and are designed to examine failures occurring only in dual-damascene interconnects. A combination of single and repeated (N=1, 10, 50, and 100) serial chains of nominally identical interconnects are used in conjunction with statistical analysis based on weakest-link concepts (Nelson, 1990) to identify differences in the failure distribution as larger collections of interconnect elements are sampled. In total, nearly 10,000 interconnects were tested using this configuration. Through the use of these multiply-linked interconnect ensembles, statistical evidence of two distinct (weak and strong) failure modes in dual-damascene Cu/oxide interconnects is first reported. The bimodal failures have also been identified with distinct void formation mechanisms that appear characteristic of the dual-damascene interconnect architecture. The weak mode is found to be void formation within the dual-damascene via, while the strong mode is associated with voiding in the dual-damascene trench. The weak mode activation energy is found to be about 1 eV and seems consistent with void formation controlled by interface diffusion between Cu metal and Ta diffusion barrier. These observations using this type of testing methodology confirm the utility of the multi-link approach in electromigration reliability analysis and the detection of early failures.

Mechanical characterization of low-K dielectric materials

Abstract: The implementation of materials in device interconnect structure is being driven by shrinking device geometries In order to meet customer demands for increasing electrical performance, the industry is adopting a solution that provides both lower resistance and lower capacitance Lower resistance is accomplished by switching from Al(Cu) to Cu interconnect and the capacitance is reduced by replacing SiO2 in the inter-level and inter-metal dielectric layers with lower dielectric constant materials (low-K materials) [1,2] A change in materials in a process as complex as IC manufacturing is inherently accompanied by an increase in reliability risk A thorough understanding of the low-K dielectric candidates is necessary for selection of the best candidate that has sufficient mechanical integrity to survive thermal stresses, CMP, packaging, and test, as well as allows for maximum extendibility to next generation devices Towards this end, the industry has adopted methods and tools to measure mechanical properties and adhesion energies associated with low-K films It is expected that porosity will significantly deteriorate the mechanical strength of ILD films compared to non-porous films and the effect on mechanical strength may be markedly different if the pores percolate together to form channels rather than remain isolated Understanding the mechanical properties of these thin films and choice of appropriate mechanical performance metrics is necessary for successful full-scale integration into a reliable packaged product

Characterization of thermal stresses of Cu/low-k submicron interconnect structures

Abstract: Thermal stress characteristics of damascene processed Cu lines passivated with TEOS and low-k (SiLK) were investigated using an X-ray diffraction method during thermal cycling. Lines with different aspect ratios were studied and the results were compared to that of an Al/low-k structure. The stress characteristics of Cu lines are quite different from those of Al lines, indicating that the diffusion barrier plays an important role in controlling the stress behavior of Cu damascene structures. The effect of low-k passivation and diffusion barrier were quantified according to materials properties. Finite element analysis was performed to verify the X-ray measurements and the results confirm the role of the diffusion barrier in stress behaviors of Cu lines.

Electromigration reliability of dual damascene Cu/CVD SiOC interconnects

Abstract: Electromigration (EM) characteristics were evaluated for multilevel copper test structures embedded in a CVD SiOC low k inter-metal dielectric. After electromigration stress testing, Cu extrusion along the interface between SiOC and the SiN dielectric diffusion barrier was revealed as the primary cause of EM failure. No evidence of cracking or mechanical weak points was observed in the bulk SiOC film; thus improved EM lifetime is expected from enhancement in the adhesion strength of SiN to SiOC. The calculated EM activation energies for 0.35 /spl mu/m via chains and 0.5 /spl mu/m via chains are 0.82 eV and 0.93 eV, respectively. The current density exponent (n) was measured to be about 1, which is consistent with the void growth mechanism in Cu. The critical length was found to decrease with increasing current density, and the j/spl middot/L/sub c/ product was determined to be approximately 7500 A/cm.

Electromigration in multilevel interconnects with polymeric low-k interlevel dielectrics

Abstract: The impact of low-k dielectrics on the reliability of advanced Cu interconnects is of growing importance. As a first step to understanding this impact, we have investigated the effect of two types of polymeric low-k materials on the electromigration (EM) behavior of multilevel Al(Cu) interconnects. The two polymers used as interlevel dielectrics in this work are a fluorinated polyimide and a poly(aryl) ether. Joule heating experiments and microstructural analysis were both conducted on Al(Cu) to ensure that there were no significant microstructural or thermal differences between the polymer samples and their oxide counterparts. The resulting EM behavior can then be directly attributed to differences in the mechanical properties of the low-k and oxide interlevel dielectrics. We have observed that the low-k samples had a higher steady-state drift rate and did not reach saturation, resulting in shorter EM lifetimes. These results indicate that the short length is greatly modified with the incorporation of me...