Kou-Chiang Tsai

High-Reliability Ta2O5 Metal–Insulator–Metal Capacitors with Cu-Based Electrodes

The properties of tantalum oxide (Ta 2 O 5 ) metal-insulator-metal (MIM) capacitors with Al/Ta/Cu/Ta bottom electrodes were investigated. An ultrathin Al film successfully suppresses oxygen diffusion in the Ta 2 O 5 MIM capacitor with the Cu-based electrode. The electrical characteristics and reliability of Ta 2 O 5 MIM capacitors are improved by addition of ultrathin Al films. Ta 2 O 5 MIM capacitors have low leakage current density (1 nA/cm 2 at 1 MV/cm) and high breakdown field (5.2 MV/cm at 10 -6 A/cm 2 ). The decrease in leakage current is attributed to the formation of a dense and uniform Al 2 O 3 layer, which has self-protection property and stops further oxygen diffusion into the tantalum contact. The dominant conduction mechanism of leakage current is the Poole-Frenkel effect at electric fields above 1.5 MV/cm.

Improving Electrical Properties and Thermal Stability of (Ba,Sr)TiO3 Thin Films on Cu(Mg) Bottom Electrodes

Cu(Mg) alloy films have replaced pure Cu as bottom electrodes for (Ba,Sr)TiO3 (BST) capacitors used in high-frequency devices. A combined BST/Cu(Mg) structure reduced the leakage current density to 3.0×10-8 A/cm2 at 1 MV/cm, and increased the breakdown field from 0.4 to 2.4 MV/cm at 10-6 A/cm2, from the corresponding values of the BST/Cu structure. High-quality characteristics probably follow the formation of a self-aligned MgO layer following the deposition of a Cu(Mg) alloy by annealing in an oxygen ambient, yielding an electrode with an excellent diffusion barrier and electrical characteristics, which is therefore effective in a BST thin-film capacitor. Additionally, the bias temperature stress and time-dependent dielectric breakdown in ambient nitrogen at an electric field of up to 2 V at temperatures between 100 and 200 °C were considered to accelerate Cu+ ion drift.

Numerical and Experimental Analysis of Cu Diffusion in Plasma-Treated Tungsten Barrier

As device dimensions decrease toward 180 nm and below, the necessity for interconnections with resistivity as low as possible is pressing. Copper is now being used in advanced metallization technology due to its low resistivity and high electromigration and stress migration resistance. However, Cu can diffuse into and react with Si or dielectric materials during annealing, and create deep trap levels that cause degradation of device performance and reliability. 1,2 Therefore, a qualified diffusion barrier inserted between Cu and Si or dielectric materials is necessary for the Cu metallization. Refractory metals and their nitrides are used as diffusion barriers in copper metallization. 3-6 Among these materials, tungsten nitride ~WN! has received the most attention owing to its high thermal stability and excellent chemical mechanical polishing ~CMP! process compatibility. 7 However, resistivity of tungsten nitride film is higher than that of tungsten film. As the technology node moves to 130 nm and below, a barrier layer with low resistivity is necessary to lower the resistance of the total line interconnect and/or via. Most refractory metal films used as diffusion barriers in Cu metallization are polycrystalline rather than monocrystalline. Moreover, columnar grain structure is frequently found in sputtered refractory metal and metal nitride films. 8-10 In general, atomic diffusion along grain boundaries is much faster than in the bulk of grains. Grain boundaries may presumably serve as fast diffusion paths for copper. Coupled lattice and grain boundary diffusion is different from lattice diffusion. Whipple had given formulas for the concentration in a semi-infinite region of low diffusion coefficient bisected by a thin well-diffusing slab. 11,12 This is of interest in grain boundary diffusion. In this study, a method of forming nitrogen and oxygen incorporated W films with low resistivity was investigated. N 2O plasma was used to post-treat the W diffusion barrier. The effectiveness of asdeposited and N2O plasma treated W films as diffusion barriers between Cu and Si was evaluated. A composite diffusion barrier was formed after N2O plasma treatment. Cu diffusion in the N2O plasma treated W barrier was further analyzed by numerical calculation using Whipple’s and Fick’s models.

Repairing of Etching-Induced Damage of High-k Ba0.5Sr0.5TiO3 Thin Films by Oxygen Surface Plasma Treatment

Ba0.5Sr0.5TiO3 (BST) thin films were patterned for fabricating BST capacitors in a helicon-wave plasma system. The optimization conditions were an Ar (80%)/Cl2 (20%) gas mixture with a helicon-wave plasma power and a substrate bias rf power of 1500 and 90 W, respectively. From results of X-ray photoelectron spectroscopy, physical ion bombardment is more effective than chemical reaction for removing Sr, while Ti can be removed by the formation of volatile TiClx. Ba was primarily removed by chemically assisted physical etching (such as that using BaClx). Some etching residues consisting of Ba and Sr were found after the BST films were etched and increased leakage current density. Oxygen surface plasma treatment can effectively repair surface damage caused by etching, and it reduced the leakage current density of the BST capacitor from 4.0×10-7 to 3.0×10-8 A/cm2 at 1 MV/cm and increased the breakdown field to ~2 MV/cm at 1.0×10-6 A/cm2.

Enhancement of electrical property and thermal stability of (Ba,Sr)TiO/sub 3/ thin films on Cu(Mg) bottom electrodes

The feasibility of using Cu (Mg) alloy films as bottom electrodes for (Ba,Sr) TiO/sub 3/ (BST) capacitors has been investigated for application of high-frequency devices at interconnect levels. When Cu was used as the bottom electrode of the BST capacitor, severe interdiffusion occurred between Cu and BST and resulted in serious oxidation after annealing in an oxygen ambient at elevated temperatures. BST/Cu(Mg) structure reduced the leakage current density down to 3.0 /spl times/ 10/sup -8/ A/cm/sup 2/ at 1 MV/cm and increased the breakdown filed up to 2.4 MV/cm from 0.4 MV/cm at 10/sup -6/ A/cm/sup 2/ compared to BST/Cu structure. The enhancing characteristics of the BST/Cu(Mg) structure are most likely due to the formation of a self-aligned MgO layer, which results in excellent diffusion barrier properties and electrical characteristics. In addition, the bias temperature stressing under an electric field of 2 MV/cm and temperatures between 100 and 200/spl deg/C was used to accelerate the Cu ion drift. The BST films using Cu(Mg) as electrodes are found to have lower Cu drift than those using Cu electrodes. Therefore, a self-aligned MgO layer could be a good Cu drift barrier layer for reliable ULSI interconnects.

Repairing of etching-induced damage of high-k Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3/ thin films by oxygen surface plasma treatment

Ba/sub 0.5/Sr/sub 0.5/TiO/sub 3/ (BST) thin films were patterned for fabricating BST capacitors in a helicon-wave plasma system. The etching parameters were optimized to minimize the leakage current density and maximize etch rate. The optimization condition was Ar(80%)/Cl/sub 2/(20%) gas mixture, with a helicon-wave plasma power and a substrate bias rf power of 1500 W and 90 W, respectively. From results of the X-ray photoelectron spectroscopy, the physical ion bombardment is more effective than chemical reaction for removing Sr, while Ti can be removed by formation of volatile TiCl. Ba was mainly removed by chemically assisted physical etching (such as BaCl/sub x/). Some etching residues consisting of Ba and Sr were found after the BST films were etched and increased leakage current density. Oxygen surface plasma treatment can effectively repair surface damage caused by etching, and reduce the leakage current density of the BST capacitor from 4.0 /spl times/ 10/sup -7/ to 3.0 /spl times/ 10/sup -8/ A/cm/sup 2/ at 1 MV/cm and increase the breakdown field to /spl sim/2 MV/cm at 1.0 /spl times/ 10/sup -6/ A/cm/sup 2/.