Yoshihisa Iba

Effects of Etch Rate on Plasma-Induced Damage to Porous Low-

We investigated the effects of etch rate on low-k damage induced by dry etching under CF4, CF4/O2, and C4F6/O2/Ar chemistry conditions. The amount of damage increases with decreasing etch rate in all chemistries. This is because the amount of fluorine or oxygen radical diffusion increases with plasma exposure time. These radicals extract CH3 groups from the low-k film or oxidize the film. To reduce damage to the lowest level possible, it is necessary to suppress the effect of the damage diffusion using etching conditions where the etching speed is higher than the diffusion speed of the damage.

k

45 nm-node multilevel Cu interconnects with porous-ultra-low-k have successfully been integrated. Key features to realize 45 nm-node interconnects are as follows: 1) porous ultra-low-k material NCS (nano-clustering silica) has been applied to both wire-level and via-level dielectrics (what we call full-NCS structure), and its sufficient robustness has been demonstrated; 2) 70-nm vias have been formed by high-NA 193 nm lithography with fine-tuned model-based OPC and multi-hard-mask dual-damascene process - more than 90% yields of 1 M via chains have been obtained; 3) good TDDB (time-dependent dielectric breakdown) characteristics of 70 nm wire spacing filled with NCS has been achieved. Because it is considered that the applied-voltage (Vdd) of a 45 nm-node technology will be almost the same as that of the previous technology, the dielectrics have to endure the high electrical field. NCS in Cu wiring has excellent insulating properties without any pore sealing materials which cause either the k/sub eff/ value or actual wire width to be worse.

Films

Synthetic antiferromagnetic (SAF) free layers consisting of [Co (0.3 nm)/Pd (0.7 nm)]n/Ru/Ta/CoFeB were investigated for use in the free layers of top-pinned magnetic tunnel junctions (MTJs). We found that interlayer exchange coupling properly provides thermal stability when the saturation magnetization of the CoPd layer is less than that of the CoFeB layer. Furthermore, we demonstrated that the thermal stability factor of the top-pinned MTJ with [Co (0.3 nm)/Pd (0.7 nm)]n=1/ Co (0.3 nm)/Ru/Ta/CoFeB- SAF free layers was improved without increasing the intrinsic switching current.

45 nm-node BEOL integration featuring porous-ultra-low-k/Cu multilevel interconnects

In the stress control of an X-ray mask absorber, the repeatability of control and stability are important. We found that the change in the stress in a Ta film resulting from annealing depends on the oxygen concentration in the film; the magnitude of the stress change is determined by the annealing temperature and time. Using this characteristic of Ta film, we have successfully controlled the stress in the Ta absorber to less than 5 MPa with good repeatability. In our mask fabrication process, Al 2 O 3 film was used as an etching mask. We found that the Al 2 O 3 film prevented the Ta absorber stress from changing in high-temperature atmospheres because the Al 2 O 3 film prevented oxygen diffusion into the Ta film. Utilizing Al 2 O 3 films, we succeeded in preventing changes in Ta absorber stress in the thermal processes after Ta stress control, such as frame bonding and resist baking. Consequently, we were able to precisely control the Ta absorber stress in X-ray masks with good repeatability and stability in a realistic X-ray mask fabrication process.

Enhanced Thermal Stability in Perpendicular Top-Pinned Magnetic Tunnel Junction With Synthetic Antiferromagnetic Free Layers

According to the 45 nm BEOL technology node, we demonstrated that a homogeneous interlayer dielectric with dielectric constant of 2.25 has a substantial advantage in terms of RC delay reduction compared to other potential architectures such as hybrid and tri-level dielectrics. Combination of the homogeneous interlayer dielectric and ultra-thinned barrier metal lowered the RC delay to 86 % compared to that listed in the ITRS 2006 update.

Precise stress control of Ta absorber using low stress alumina etching mask for X-ray mask fabrication

Patterning of an X-ray mask absorber after Si back-etching is desirable from the viewpoint of the pattern placement accuracy. We investigated Ta X-ray absorber etching on an SiC membrane equipped on a mask frame using a low-stress CrN hard mask and an ICP etcher with a He cooling system. In this system, the membrane temperature and the self-bias voltage could be controlled. A 40-nm-thick CrN film was etched using a 200-nm-thick resist and Cl2 and O2 gases with a selectivity of 0.72 and a vertical sidewall. A 400-nm-thick Ta film was etched using Cl2 gas at an electrode temperature of -10°C and a low gas pressure of 0.1 Pa. A high selectivity of Ta to CrN, 42, was obtained, and lines and spaces patterns below 0.1 µm with vertical sidewalls could be fabricated.

Strategies of RC Delay Reduction in 45 nm BEOL Technology

We evaluated MgO-based magnetic tunnel junctions (MTJs) for magnetic field sensors with spin-valve-type structures in the CoFeB sensing layer capped by an MgO film in order to obtain both top and bottom interfaces of MgO/CoFeB exhibiting interfacial perpendicular magnetic anisotropy (PMA). Hysteresis of the CoFeB sensing layer in these MTJs annealed at 275 °C was suppressed at a thickness of the sensing layer below 1.2 nm by interfacial PMA. We confirmed that the CoFeB sensing layers capped with MgO suppress the thickness dependences of both the magnetoresistance ratio and the magnetic behaviors of the CoFeB sensing layer more than that of the MTJ with a Ta capping layer. MgO-based MTJs with MgO capping layers can improve the controllability of the characteristics for magnetic field sensors.

PATTERN ETCHING OF TA X-RAY MASK ABSORBER ON SIC MEMBRANE BY INDUCTIVELY COUPLED PLASMA

Proximity X-ray lithography (PXL) is a promising technology for fabricating ultra-large-scale integrated (ULSI) devices smaller than 100 nm because it offers high-resolution capabilities and dimensional control with high throughput. We used PXL at five levels (mark, isolation, gate, contact, wiring) and fabricated 100-nm complicated dense patterns and sub-100-nm channel length MOSFETs. In this paper, we discuss lithographic performance and the performance of the MOSFET device.

Magnetic tunnel junctions for magnetic field sensor by using CoFeB sensing layer capped with MgO film

In the semiconductor manufacturing of below 100 nm devices, overlay accuracy is a critical issue for high resolution on proximity x-ray lithography (PXL). We applied PXL for five layers (mark, isolation, gate, contact, and wiring) to fabricate a 100 nm n-type metal–oxide–semiconductor field effect transistor and evaluated the accuracy. Alignment exposure was done with optical heterodyne alignment and magnification correction. The overall overlay accuracy (mean±3σ) was 25–45 nm, and the best was about 25 nm for the contact layer. The overlay error was classified into several components to estimate the error budget. The analysis revealed that the alignment error of the x-ray stepper is less than 20 nm under various conditions in device fabrication. Individual x and y scaling errors caused by mask image placement and the difference of wafer-to-mask gap was from 18–35 nm: the error of common in-plane deformation was about 20 nm. The present accuracy may make it possible to fabricate devices with a 100 nm grou...

Sub-100 nm device fabrication using proximity X-ray lithography at five levels

We proposed that the stress of the side walls of an absorber pattern causes the pattern distortion of an X-ray mask. The surfaces of the pattern side walls have a high compressive stress layer because they are easily oxidized in air after absorber pattern etching. This stress becomes a serious problem in proportion to the high degree of pattern integration. To lower the surface oxide layer stress of the film, we developed a TaGe nitride (TaGeN) absorber. The oxide layer stress of TaGeN films becomes lower with increasing the N2 content in the sputtering gas, and the stress value of Ta0.35Ge0.20N0.45 can be decreased to 1/17 that of TaGe. We fabricated a TaGeN X-ray mask and practically demonstrated that the TaGeN absorber was effective for high image placement accuracy to an X-ray mask having high dense patterns with a 0.1 µm pattern size.