Choong Un Kim

CMOS-compatible fabrication of room-temperature single-electron devices

Devices in which the transport and storage of single electrons are systematically controlled could lead to a new generation of nanoscale devices and sensors. The attractive features of these devices include operation at extremely low power, scalability to the sub-nanometre regime and extremely high charge sensitivity. However, the fabrication of single-electron devices requires nanoscale geometrical control, which has limited their fabrication to small numbers of devices at a time, significantly restricting their implementation in practical devices. Here we report the parallel fabrication of single-electron devices, which results in multiple, individually addressable, single-electron devices that operate at room temperature. This was made possible using CMOS fabrication technology and implementing self-alignment of the source and drain electrodes, which are vertically separated by thin dielectric films. We demonstrate clear Coulomb staircase/blockade and Coulomb oscillations at room temperature and also at low temperatures.

The influence of Cu precipitation on electromigration failure in Al‐Cu‐Si

This paper reports a study of the effect of Cu precipitation on electromigration failure in Al‐2Cu‐1Si thin‐film conducting lines. The films were 0.5 μm in thickness, and patterned to widths of 1.3 and 4 μm, providing width‐to‐grain‐size ratios (W/G) of approximately 0.5 and 2. The lines were aged for various times at 226 °C, and were then tested to failure at a current density of 2.5×106 A/cm2. Scanning and transmission electron microscopy were used to study the Cu precipitate distribution, its evolution during aging and electromigration, and the microstructural failure mechanism. Aging produces a dense distribution of intragranular θ’ (Al2Cu; coherent), with stable θ (Al2Cu; incoherent) in the grain boundaries. The θ’ is replaced by θ as aging proceeds. In the wide lines (W/G≊2), the mean time to failure (MTF) increases slowly and monotonically with prior aging time. The failure happens through the growth and coalescence of intergranular voids. In the narrow lines (W/G≊0.5), both the MTF and the time to...

The mechanism of electromigration failure of narrow Al‐2Cu‐1Si thin‐film interconnects

This work is principally concerned with the microstructure of electromigration failure in narrow Al‐2Cu‐1Si conducting lines on Si. Samples were patterned from 0.5‐μm‐thick vapor‐deposited films with mean grain size of 2.4 μm, and had linewidths of 1.3 μm (W/G≊0.5), 2 μm (W/G≊0.8), and 6 μm (W/G≊2.5). The lines were tested to failure at T=226 °C and j=2.5×106 A/cm2. Other samples were tested over a range of substrate temperatures and current densities to test the effect of these variables, and 1.3 μm lines were tested after preaging at 226 °C for various times to change the Cu‐precipitate distribution prior to testing. Three failure modes were observed: The 6 μm specimens failed by separation along grain boundaries with an apparent activation energy of 0.65 eV; the 1.3 μm specimens that were preaged for 24 h failed after very long times by gradual thinning to rupture; all other narrow lines failed by the transgranular‐slit mechanism with an activation energy near 0.93 eV. Microstructural studies suggest that the transgranular‐slit failure mechanism is due to the accumulation of a supersaturation of vacancies in the bamboo grains that terminate polygranular segments in the line. Failure occurs after Cu has been swept from the grain that fails. Failure happens first at the end of the longest polygranular segment of the line, at a time that decreases exponentially with the polygranular segment length. Preaging the line to create a more stable distribution of Cu lengthens the time required to sweep Cu from the longest polygranular segment, and significantly increases the time to failure. In the optimal case the transgranular‐slit failure mechanism is suppressed, and the bamboo grain fails by diffuse thinning to rupture. Preaging is particularly effective in increasing the lifetimes of lines that contain very long polygranular segments, and has the consequence that the time to first failure in an array of lines is much longer than predicted by a log‐normal fit to the distribution of failure times.

Mechanism of reliability failure in Cu interconnects with ultralow-κ materials

This letter presents evidence of an oxidation-driven failure mechanism in Cu interconnects integrated with ultralow-κ materials. It is found that the open pore structure of ultralow-κ materials allows oxidants in the ambient to reach the interconnect structure and induce oxidation of Cu. In contrast to a normal oxidation process where Cu is in contact with the oxidant, oxidation is controlled by the outdiffusion of Cu through the barrier layers, Ta and SiCN, to form Cu oxide in the pores of the dielectric material. The loss of Cu by outdiffusion induces extensive voiding and subsequent failure in Cu interconnects.

Influence of High-G Mechanical Shock and Thermal Cycling on Localized Recrystallization in Sn-Ag-Cu Solder Interconnects

The impact of isothermal aging and recrystallized grain structure distribution on mechanical shock and thermal cycling performance of solder joints with 1% and 3% silver content Sn-Ag-Cu interconnects were investigated. Localized recrystallized grain structure distributions were analyzed to identify correlations between the microstructure evolution and shock performance. The results reveal that the shock tolerance depends on the amount of shock energy that can be absorbed during each shock cycle, which depends on microstructural features. Based on the recrystallized grain distribution, additional isothermal aging in 1% silver Sn-Ag-Cu interconnects shows improved shock performance, whereas degraded shock performance was observed in 3% Sn-Ag-Cu interconnects. Using the same grain boundary distribution analysis on thermally cycled samples, relationships between the particle size distribution, localized recrystallized grain structure development, shock, and thermomechanical performance were identified: finer particle spacing is beneficial for thermal cycling as it resists grain boundary generation, while conversely, wider particle spacing facilitates recrystallization and grain boundary mobility that allows Sn to absorb shock energy.

Fatigue properties of lead-free solder joints in electronic packaging assembly investigated by isothermal cyclic shear fatigue

This paper reports the fatigue properties of Sn-Ag-Cu (SAC) and Pb-Sn solder alloys determined from isothermal shear fatigue testing and analysis of resulting data using modified Coffin-Manson fatigue model. In our study, a series of cyclic shear fatigue testing was conducted on the solder joints in PBGA assembly with variation in testing temperature, strain range, and strain rate (or cycle frequency). The number of cycles to the first joint failure in the assembly was determined from the resistance of each solder joint and taken as the fatigue life of the assembly. Analysis of the resulting data reveals that isothermal fatigue behavior of both SAC (Sn-Ag-Cu) and Pb-Sn alloys follow the classic Coffin-Manson fatigue model and provide constants indicative of fatigue properties of the alloy, namely fatigue ductility coefficient and ductility exponent. These fatigue constants are consistent with what is generally expected from a metallic fatigue system except for their dependence on temperature. This deviation, attributable to the involvement of the thermal strain that is added to the applied mechanical shear strain, suggests that consideration of the thermal strain effect needs to be included in the fatigue analysis of solder joints even if the major fatigue mode appears to be pure mechanical shear. The fatigue constants gained from our study provide insights helpful in understanding the mechanism behind variation in fatigue reliability with solder alloy compositions and solder microstructure.

Electromigration in Cu thin films with Sn and Al cross strips

Electromigration in Cu thin films is studied in a cross-strip configuration. Cu lines with isolated areas of Cu(Al) or Cu(Sn) are tested between 250 and 390 °C with the following results. The hillock and void marker motion indicates that Sn moves in the direction of electron flow. The marker polarity indicates that it decreases the grain boundary electromigration of Cu, in agreement with previous studies. This study also finds evidence of active surface migration in Cu. During tests in forming gas, hillocks and voids form adjacent to a native Al2O3 layer at all temperatures, indicating the likelihood that Cu migrates faster through the Cu free surface than the interface between the surface layer of Al2O3 and Cu(Al). Active surface migration in Cu thin films is also evidenced by the growth of hillocks with highly developed facets, most of which are attached to the underlying film by narrow necks.

The metallurgical control of electromigration failure in narrow conducting lines

Electromigration is a serious potential source of failure in the narrow, thin-film Al-Cu conductors used in modern microelectronic devices. The problem has become more acute as line widths have shrunk to below one micrometer, creating lines with quasi-bamboo microstructures. The usual mechanism of internal electromigration failure in such lines involves the formation of a transgranular void across a bamboo grain at the upstream end of a long, polygranular segment, preceded by the depletion of copper from both the polygranular segment and the upstream bamboo grain. At least three metallurgical mechanisms are available to inhibit this failure mechanism and improve the useful lifetime of the line, Each of these methods has been demonstrated in the laboratory environment.

Observation of space charge limited current by Cu ion drift in porous low-k/Cu interconnects

This letter reports the observation of the space charge limited current (SCLC) induced by injection and drift of Cu ions into porous low-k dielectrics. The SCLC, characterized by the momentary rise and fall of current with time, is found in all Cu interconnects having defective Ta barrier while it is absent in interconnects with intact barrier. This observation, combined with existing model on SCLC, leads to the conclusion that Cu ions can be injected through defects in Ta barrier and drift under electric field with the mobility as high as an order of 10−13 cm2/sec V at room temperature.

INFLUENCE OF W VIA ON THE MECHANISM OF ELECTROMIGRATION FAILURE IN AL-0.5 CU INTERCONNECTS

This letter reports the effects of via current density on electromigration (EM) failure in Al–0.5 Cu conductors. Two-level metallization structures, differing in the number of feeding vias (1, 6, and 15), were made with the same pattern of Al lines at two levels to allow simultaneous EM testing of upper- and lower-level lines. It was established that the lower-level lines were more susceptible to the impact of the via, resulting in a failure by the formation of a local void beneath a via and a strong dependence of EM lifetime on the via current density. The results led to a phenomenological equation that incorporates via structure into failure kinetics.