Effect of annealing on the surface microstructural evolution and the electromigration reliability of electroplated Cu films

This paper presents the effects of annealing, performed over a temperature range from 200°C to 400°C, on the surface microstructural evolution and the electromigration reliability of electroplated Cu films. After annealing, a substantial increase in surface roughness was observed, while variations in mean grain size and nanoindentation hardness were minor. Given the annealing temperature, the surface roughness was larger for the films annealed in forming gas, due to the existence of hydrogen. In particular, the films annealed at 400°C in forming gas demonstrated severe grain-boundary grooving and surface voiding. The defective nature of the annealed surface can be alleviated by chemical-mechanical polishing (CMP), when annealing is conducted prior to the CMP. However, it appears that a sequential thermal excursion at relatively high temperatures re-aggravates the integrity of the Cu surface. This argument may be supported by the electromigration-test results on dual-damascene interconnects fabricated using two different thermal profiles. The electromigration lifetimes were longer by more than a factor of two for the interconnects that skipped a post-passivation anneal at 400°C. The experimental evidence presented in this work suggests that controlling the integrity and quality of the Cu surface is an important step in ensuring good electromigration reliability.     

The effects of dielectric overcoating on electromigration in aluminum interconnections

The phenomena of electromigration is self-diffusion effect, and has an associated activation energy. Although the activation energy for bulk self-diffusion in aluminum is 1.48 eV, experimentally measured energies for films range approximately from 0.5 to 1.2 eV. This lower observed energy may be or has been ascribed by investigators to crystalline imperfections in and on the surface of the aluminum film. Of the three contributions to the effective activation energy, i.e., the surface, grain boundary, and bulk component, the surface component is here investigated. In light of the requirements for LSI and multilevel integrated circuitry, i.e., narrower, thinner metallizafion implying a larger surface to bulk atom ratio, correlation is made between surface treatment and effective activation energy of the conductor. Surface treatment is accomplished through vapor plating of dielectric overcoatings of P2O5-SiO2and Al2O3-SiO2glass. A study is made of the effects of dielectric overcoating as a function of aluminum thickness. It is seen that surface treatment is effective for ahtminum less than 5000 Å thick. It is further pointed out that for aluminum thickness greater than 6000 Å, surface passivation will have no effect with respect to electromigration. It has been shown that the type of aluminum deposition technique and deposition and post-deposition heat treatment is critical in obtaining the maximum bulk component of the activation energy. The dielectric overcoating, in combination with optimal bulk properties, increases the mean time between failure of the aluminum stripes by one to two orders of magnitude at high current densities and elevated ambients, than that found in many conventional integrated circuits.     

Activation energies for the different electromigration mechanisms in aluminum

The activation energies concerning all electromigration contributions in aluminum are theoretically determined starting from the activation energy for self-diffusion. For grain boundary electromigration the activation energy was found to be E_g = 0.4–0.5 ± 0.04 eV, whereas all other contributions except the surface electromigration are described by higher activation energies. For comparing with experimental results the grain boundary activation energy was determined by direct measurement of aluminum drift velocities. It is shown that the known measuring method is valid only in combination with some modifications. The measured activation energies agree well with the estimated ones.     

Effect of oxide and W-CMP on the material properties and electromigration behaviour of layered aluminum metallisations

It has been found that chemical mechanical polishing increases the electromigration resistance of a Ti/AlCu/Ti/TiN metal stack. (111) X-ray diffraction polefigures indicate an increased (111) texture of AlCu deposited on a substrate that received at least one CMP step. This improved (111) texture can be attributed to a decreased roughness of the underlying oxide when chemical mechanical polishing is used.     

Effect of surface finish on the failure mechanisms of flip-chip solder joints under electromigration

Two substrate surface finishes, Au/Ni and organic solderable preservative (OSP), were used to study the effect of the surface finish on the reliability of flip-chip solder joints under electromigration at 150°C ambient temperature. The solder used was eutectic PbSn, and the applied current density was 5×10³ A/cm² at the contact window of the chip. The under bump metallurgy (UBM) on the chip was sputtered Cu/Ni. It was found that the mean-time-to-failure (MTTF) of the OSP joints was six times better than that of the Au/Ni joints (3080 h vs. 500 h). Microstructure examinations uncovered that the combined effect of current crowding and the accompanying local Joule heating accelerated the local Ni UBM consumption near the point of electron entrance. Once Ni was depleted at a certain region, this region became nonconductive, and the flow of the electrons was diverted to the neighboring region. This neighboring region then became the place where electrons entered the joint, and the local Ni UBM consumption was accelerated. This process repeated itself, and the Ni-depleted region extended further on, creating an ever-larger nonconductive region. The solder joint eventually, failed when the nonconductive region became too large, making the effective current density very high. Accordingly, the key factor determining the MTTF was the Ni consumption rate. The joints with the OSP surface finish had a longer MTTF because Cu released from the substrate was able to reduce the Ni consumption rate.     

AC and DC electromigration failure of aluminum contact junctions

Current flow across a typical aluminum contact interface occurs only where metallic junctions form at cracks in the insulating aluminum oxide. When the total metallic conducting area is small, current density may be high enough to initiate electromigration failure. These experiments use a solid aluminum specimen having a constricted current path, representing a single metallic contact junction. Specimens are tested with alternating or direct current flow at current density up to 10/sup 6/ A/cm/sup 2/. Electromigration failure, observed as a relatively abrupt increase of resistance, occurs in both ac and dc specimens. Electron microscope examination indicates that the deterioration is due to electromigration. Conditions under which electromigration deterioration may occur in practical aluminum power connections are discussed. A method is provided for estimating the minimum possible current density in aluminum connections. The implications for the theory of aluminum contact behavior and the practical considerations of effective life testing are discussed.     

The evolution of the resistance of aluminum interconnects during electromigration

Resistance monitoring is a traditional method to investigate electromigration failure. It is important to understand how much information can be extracted from the data generated by these experiments. To this end, precision resistance measurements were included as part of accelerated electromigration tests performed inside of a high voltage scanning electron microscope (HVSEM). Twenty-two passivated Al interconnects were tested at 30 mA/μm² and at two temperatures, half at 212°C and half at 269°C. During every test, our automated apparatus stored images of each 300 μm long structure several times per hour. The resistance of each line was also precisely measured and recorded. Changing the temperature affected only the time scale of the resistance evolution. There were resistance changes before voids formed that were neither due to temperature fluctuations nor solute effects. In most cases, the nucleation of the first void to form in a line was signaled by an increase in the time derivative of the resistance. Due to the strong effect of void shape, the void volume could not be determined by the magnitude of the resistance change. The width of a void (transverse to the line) rather than the volume largely determined the resistance change.     

Bulk electromigration and contact interface electromigration and conditions for eliminating both mechanisms in SiO2-covered aluminum lines

Electromigration tests were performed at 200°C with SiO₂-covered, large-grained, narrow Al interconnect lines ending on semiconductor contacts, which were marked by different barrier layers. An overlapping Mo barrier layer produced premature contact failures by interface electromigration. The interface threshold value (jl)_th,i had been reduced by the covering oxide to about 400 A/cm. For Al current densities of 8 × 10⁵A/cm² interface mass flow was avoided by using a short silicide-like barrier layer, which was restricted to the contact area only. The remaining mass flow mechanism was bulk electromigration, resulting in contact openings after extremely long stress periods. Bulk electromigration can be eliminated in oxide covered Al lines, if the positive biased ends are completely covered. The reason is a threshold value for bulk electromigration, which is increased considerably by the covering oxide to a time-dependent value of about 8 × 10⁷Ah/cm at 200°C.     

激光红外热成像铝合金表面裂纹检测表面处理的研究

通过搭建激光红外热成像检测平台,对已制备的铝合金光滑表面及表面处理后裂纹缺陷试样进行检测,对裂纹处的红外热图和温度数据进行分析,并且为了突出表面处理检测效果,用差动式检测方法对比在不同功率下表面处理前后温差变化。实验结果表明,随着功率的增大,裂纹缺陷表面处理前后的温差都存在温差增大的变化,但是表面处理后裂纹缺陷温差变化更加明显。相同功率条件下,裂纹缺陷表面处理后温差幅度远大于表面处理前的温差幅度。可见对激光红外热成像铝合金表面裂纹检测进行表面处理,明显提高其热吸收率,而且相较于表面处理前,经过表面处理后,所需更小的功率,就能取得更大温差,检测效果更好,可检测性更强。     

Microstructural and surface effects on electromigration failure mechanism in Cu interconnects

It is shown that changes in the microstructure of Cu interconnects lead to qualitative variation in electromigration damage kinetics - from the formation of the open circuit to continuous damage not leading to failure. Surface diffusion acting simultaneously with grain boundary mass transport is shown to be critical for damage formation. Activation energy of electromigration was measured to be 0.95 eV.     

Statistical model for electromigration failure at aluminum contacts and terminals

The statistical model for electromigration failure in stripes developed by Attardo, Rutledge, and Jack has been extended to cover the cases of terminals and contacts to silicon. By the use of computer simulation techniques, a model was developed that relates the time-to-failure and the distribution of failures to the magnitude of the structural divergence occurring at the diffusion barrier. The model is then used to predict the dependence of contact or terminal reliability on grain size distribution, device geometry, and current crowding. In particular, the model predicts the previously unknown fact that for a constant grain size, a large standard deviation of the grain size at the contacts will retard electromigration and that the current density distribution at the contact does not affect mean time-to-failure.     

The blocking barrier effect on aluminum electromigration due totitanium layers in multilayered interconnects of LSI's

Experimental evidence of an increase in the resistance of a cathode-side metal line without any void generation is presented for a multilayered metal structure terminated by via-holes during electromigration tests. This resistance increase is reversed to the initial value by high temperature storage after electromigration testing. The increase in the resistance of multilayered metal structures is attributed to the vacancy accumulation in the cathode side due to the blocking barrier effect of the refractory metal layer in the via-hole     

Bulk electromigration by eliminating grain boundary mass flow in continuous aluminum lines

Starting from electromigration threshold in single segments of narrow large-grained lines, grain boundary mass flow was eliminated in continuous Al lines. Mass flow divergences proceeding from grain boundaries on bonding pads were suppressed by another threshold, which is given by the test current per Al thickness. Narrow test lines should be free of tensile stress, which can produce premature failure. Lifetime tests were performed at 200°C with current densities of 8 × 10⁵ A/cm². The averaged stripe width was 1.3 μm. Within 16400 hours no failure occurred, due to homogeneous bulk electromigration.     

Electromigration in sputtered aluminum films

The electromigration behavior of sputtered aluminum films is discussed. Direct measurement of the aluminum drift velocity yielded a grain boundary activation energy of about 0.43 to 0.46 eV in the temperature range between 140 and 280°C. Time to failure and reliability were also tested and compared with electron-beam-evaporated aluminum, indicating a good quality for the sputtered films if sputter conditions are properly chosen.     

Electromigration threshold in aluminum films

Electromigration threshold was measured in oil bths at the temperature range 100–280°C, approaching operation temperature of integrated circuits. For uncovered unannealed films, threshold was constant in the temperature range below 200°C, yielding a low threshold value of about 420 A/cm (42 μm × 10⁵ A/cm²) for the product of current density and stripe length. Annealing does not affect temperature behavior, but threshold values grow with increasing annealing temperatures. Threshold does not depend on annealing atmospheres, such as N₂, O₂ and N₂/H₂. Different film compositions like Al, Al/Si and Al/Si/1% Cu are equivalent concerning threshold. Elevated, slightly temperature-dependent threshold values were measured using covering SiO₂ films. The threshold of covered films depends on test conditions. Reduced current densities and stress times result in increasing values which may be beneficial for electromigration reliability. Moreover, electromigration test results can be influenced by threshold, which has to be considered. The discrepancy to former threshold results is assumed to be due to N₂/H₂ test ambients, reducing electromigration mass flow. The reason for threshold is suggested to proceed from different bond energies for the transition from grain boundaries to surface.     

The energy of activation of electromigration in aluminum conductors tested by the drift-velocity method

Electromigration in pure aluminum films evaporatively deposited with the method of partially ionized flux was studied by the drift-velocity technique. In annealed films, the energy of activation of electromi-gration was found to be almost half as large as in as-deposited ones. The film structure and texture, as well as the grain-boundary structure, were examined. The electromigration front morphology and activation energy variations were analyzed in the context of film structure.     

离焦量对铝合金表面激光除漆损伤特性的影响

飞机激光除漆时铝合金基体保护及除漆效率与材料损伤阈值密切相关,而激光离焦量的变化将直接改变材料的损伤阈值,以及除漆效率。采用ISO 11254 1-on-1激光损伤阈值测试方法研究不同离焦量时的铝合金和漆层的损伤阈值,对激光单脉冲光斑的二维、三维形貌进行分析,将理论值与实测值得到的不同离焦量时的损伤凹坑直径及深度进行验证。结果表明：随激光离焦量增加,光斑面积增大的同时材料损伤阈值下降,可有效提升能量利用率及除漆效率;实际单脉冲作用的材料凹坑深度随离焦量的增大而减小,且面积增大;凹坑深度、凹坑直径的理论值、实测值具有较好的相关性,并从激光材料作用机理角度对其误差进行了解释说明。     

Bulk electromigration reliability tests of large-grained aluminum lines with regard to semiconductor contacts

A test structure was devised for investigating electromigration reliability. Considering electromigration threshold, narrow large-grained Al/Si interconnect lines failed on contacts due to homogeneous bulk electromigration. Failure times were dependent on the uniformity of the contact interfaces. Bulk electromigration was marked by an extremely low mass flow at the test temperature of 200°C. This mass flow will be reduced considerably with decreasing temperature due to the activation energy of 1.38 eV. Si electromigration can damage pn-junctions. A double layer metalization, like annealed Ti-Al, may delay or prevent Si mass flow, but a strong interface electromigration occurs. Comparable double layers are improper for future integrated circuits. The advantage of bulk electromigration is not restricted to narrow lines. Wide lines have to be divided into separated sections, which behave like narrow, large-grained lines.