Kazuo Kohmura

Influence of Humidity on Electrical Characteristics of Self-Assembled Porous Silica Low-k Films

The influence of humidity on the dielectric constant and leakage current of self-assembled porous silica films which have two-dimensional hexagonal periodic porous structures was investigated quantitatively by proposing anew water adsorption model. The amount of H 2 O adsorption was calculated by the modified Rayleigh model, where H 2 O molecules are assumed to be adsorbed on the inner surface of cylindrical porous silica structures and form a dispersal concentric double-layer dielectric cylinder system. The amount of H 2 O calculated by the proposed model was consistent with the measured dielectric constant and thermogravimetry data. It suggests that inner-surface coverage of the cylindrical porous silica wall with a hydrophobic group is the most effective way to suppress water adsorption. Hexamethyldisilazane as a surface coverage molecule was introduced to the periodic cylindrical porous silica film and the leakage current was suppressed by a factor of 1/100 even below 0.5% relative humidity, resulting in the improvement of time-dependent dielectric breakdown lifetime by a factor of 30.

Theoretical Investigation of Dielectric Constant and Elastic Modulus of Three-Dimensional Isotropic Porous Silica Films with Cubic and Disordered Pore Arrangements

The dielectric constant (k) and elastic modulus (E) of self-assembled three-dimensional porous silica films were investigated by analytical and numerical calculations to reveal the relationship between k and E. It was found that cubic pore arrangements have E values higher than those of random pore arrangements and two-dimensional periodic hexagonal pore arrangements for the same k. It was also found that disordered isotropic porous silica films having cylindrical pores with well-controlled pore size distributions exhibit an E vs k relationship similar to that of two-dimensional hexagonal periodic porous silica films. The elastic modulus of the skeletal silica was determined to be 40 GPa from the combination of the calculated results and experimental data on ultralow-k disordered porous silica film with a k value of 2.0 and a modulus of 8 GPa.

A Novel Organosiloxane Vapor Annealing Process for Improving Elastic Modulus of Porous Low-k Films

A novel organosiloxane-vapor-annealing method has been developed for improving the mechanical strength of porous silica films with a low dielectric constant. Treatment of a porous silica film with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) under atmospheric nitrogen above 350 °C significantly enhanced the mechanical strength (i.e., elastic modulus and hardness) of the film. Results of Fourier transform infrared spectroscopy (FT-IR) and thermal desorption spectroscopy (TDS) suggested the formation of cross-linked poly(TMCTS) network on the porous silica internal wall surfaces by the TMCTS treatment. Such TMCTS cross-linked network is thought to enhance the mechanical strength of the low-k film.

Electrical Reliabilities of Highly Cross-Linked Porous Silica Film with Cesium Doping

A highly cross-linked porous silica dielectric (PoSiO) film was fabricated at a low temperature of 350°C. PoSiO films were derived by the sol-gel method and their pore surface silanol groups were silylated with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) by vapor phase treatment. To promote the degree of siloxane cross-linkage of the film, cesium (Cs) was added to the precursor solution with the amount of 0, 5, 15, and 30 wt ppm as a base catalyst. Then, the amount of methyl-silicon-three oxygen (Me-Si T-type) and hydrogen-silicon-three oxygen (H-Si T-type) bridged structures of the chemisorbed TMCTS were increased, and the amount of surface silanol groups was decreased markedly with the increasing amount of Cs concentration. Leakage current and dielectric constant were measured under various humidity conditions, which were hardly degraded for the highly cross-linked PoSiO owing to its small amount of residual silanol groups and adsorbed water. It was also shown that the amount of mobile protons originated from the silanol groups became negligible. Time zero dielectric breakdown field strength was improved to 6.7 MV/cm and a projected time-dependent dielectric breakdown lifetime satisfied 10 years for Cs 30 ppm-doped PoSiO under a stress condition of 220°C and |E| = 1 MV/cm.

Effect of Water Adsorption on Electrical Characteristics of Porous Silica Films

The effects of water adsorption on the dielectric constant and leakage current of porous silica thin films were investigated. The films were prepared by the sol–gel method and their pore surfaces became hydrophobic with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) or hexamethyldisilazane (HMDS) vapor phase treatment. Permittivity and leakage current were measured within the relative humidity condition range from 3 to 50%. The dielectric constant increased with increasing relative humidity, and water adsorption properties were calculated from the dielectric constant. Then, the number densities of water adsorption sites were nearly the same in the two films. Leakage current began to increase at a relative humidity of 8%. The slope of leakage current relative to electric field was described well by the Pool–Frenkel leakage current model above an electric field strength of 0.3 MV/cm. To consider the effect of water adsorption on the leakage current model, we used the Pool–Frenkel current equation for ionic conduction. This model revealed that the ionization energy of proton emission from the silanol group decreased with increasing relative humidity; thus, leakage current increased with increasing relative humidity.

Recovery from Plasma-Process-Induced Damage in Porous Silica Low-k Films by Organosiloxane Vapor Annealing

It was demonstrated that recovery from dry etching and ashing damage in porous silica low-k films occurred by 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor annealing. The increase in k-value after Ar/C5F8/O2 plasma etching was reduced from 35 to 6.5% of the initial value (k=2.25) by TMCTS vapor annealing. Leakage current also returned to the initial level. Hydrofluoric acid wet etching revealed the sidewall damaged region in a porous silica trench due to plasma processes. The TMCTS vapor annealing was found to be effective for recovery from the sidewall damage. Fourier transformed infrared absorption spectroscopy indicated that the replacement of Si–CH3 bonds in low-k films by Si–O and Si–OH bonds occurred during plasma processes. The recovery mechanism involves hydrophobic bond (–CH3) reintroduction into the film followed by stable cross-linked poly(TMCTS) network formation on pore wall surfaces by TMCTS vapor annealing.

Pure Silica Zeolite Films Prepared by a Vapor Phase Transport Method

Pure silica zeolite films were prepared by a vapor-phase transport method using ethylenediamine (EDA), triethylamine (Et3N), and water as vapor sources. The optimum mixture ratio of EDA:Et3N:H2O was 1:2:1 for forming randomly oriented ZSM-48 zeolite crystals. Compared with samples formed by the sol–gel technique and spin-coating method, the vapor-phase transport method can suppress isolated Si–OH bonds and film shrinkage. Dielectric constant decreased with increasing vapor-phase transport time, and leakage current decreased to an order of 10-8 A/cm2.

Cu-Oxide-Assisted Selective Pyrolysis of Organic Nanolayer on Patterned SiO2–Cu Surface

Organic nanolayers attract much attention for the isolation and adhesion promotion of the Cu line and insulator in Cu interconnection of microelectronic devices. This paper proposes a strategy for selective formation of adhesion nanolayer on the insulator surface with etching it on Cu surface by Cu-oxide-assisted pyrolysis. After deposition of a uniform polyelectrolyte layer on both SiO2 and Cu surfaces, heat treatment at 350 °C in ambient nitrogen was applied. Then, a larger thickness decrease was observed on the polyelectrolyte layer on Cu when compared to that on SiO2. According to the TDS and XPS analysis, the polyelectrolyte layer was relatively stable on SiO2 up to the intrinsic decomposition temperature of the material, but on the Cu surface it decomposed to volatile small molecules at a lower temperature due to Cu2O-assisted oxidization. This substrate dependent selective pyrolysis was examined for 100 nm width Cu lines and SiO2 spaces, and then a patterned polyelectrolyte layer on the SiO2 surface was obtained with a single nanometer scale edge resolution.

Fabrication of mesoporous silica for ultra-low-k interlayer dielectrics

We have developed sol-gel self-assembly techniques to control the pore structure and diameter of ultra-low-k interlayer dielectric (ILD) films. Porous silica films have been fabricated using cationic and nonionic surfactants as templates, resulting in 2D-hexagonal and disordered pore structures, respectively. The disordered mesoporous silica film has a worm-hole like network of pore channels having a uniform diameter. Precursors of the mesoporous silica films were synthesized by use of tetraethyl-orthosilicate (TEOS), inorganic acid, water, ethanol and various surfactants. The surfactants used were cationic alkyltrimethyl-ammonium (ATMA) chloride surfactants for 2D-hexagonal pores and nonionic tri-block copolymer for disordered structures. Dimethyldiethoxysilane (DMDEOS) was added for forming the disordered mesoporous silica. The disordered cylindrical pore structure with a uniform pore size was fabricated by controlling the static electrical interaction between the surfactant and the silica oligomer with methyl group of DMDEOS. Tetramethylcycrotetrasiloxane (TMCTS) vapor treatment was developed, which improved the mechanical strength of mesoporous silica films. The TMCTS polymer covered the pore wall surface and cross-linked to passivate the mechanical defects in the silica wall. Significant enhancement of mechanical strength was demonstrated by TMCTS vapor treatment. The porous silica film modified with a catalyst and a plasma treatment achieved higher mechanical strength and lower dielectric constant than conventional porous silica films because the TMCTS vapor treatment was more effective for mechanical reinforcement and hydrophobicity.

Electrical reliabilities of porous silica low-k films

Electrical reliability of self-assembled porous silica films was investigated. Vapor phase silylation by use of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) was developed to reduce silanol groups and enhance siloxane cross-linkage, resulting in achieving lower dielectric constant and higher elastic modulus. To promote siloxane cross-linkage, Cs ion was doped to its precursor solution. The self-assembled porous silica low-k film was integrated in Cu damascene interconnects with ultraviolet (UV) irradiation and TMCTS vapor treatment, resulting in the highest elastic modulus of 9 GPa with the dielectric constant of 2.1. Sidewall protection layer was formed in the trench for Cu interconnects to improve time-dependent dielectric breakdown (TDDB) lifetime of more than 10 years at the electric field of 2.3 MV/cm.