Jie-Hua Zhao

MEASUREMENT OF ELASTIC MODULUS, POISSON RATIO, AND COEFFICIENT OF THERMAL EXPANSION OF ON-WAFER SUBMICRON FILMS

A bending beam method has been developed to measure the elastic modulus E, the coefficient of thermal expansion (CTE) and the Poisson ratio ν for on-wafer dielectric films with thicknesses in the submicron range. The method was demonstrated for 0.5 μm thick silicon dioxide films made from tetraethylorthosilane (TEOS). First, the biaxial elastic modulus E/(1-ν) and CTE were measured on blanket TEOS on Si and GaAs substrates and found to be 77 GPa and 1.0 ppm/°C, respectively. The Poisson ratio ν was determined by combining the finite element calculation and the experimental result of the thermal stresses of TEOS fine lines on the Si substrate. The Poisson ratio of TEOS was determined to be 0.24 and, as a consequence, the Young’s modulus was 59 GPa. Fourier transform infrared spectra were obtained for TEOS films on the Si and GaAs substrates to ensure that the chemical structure of the film is independent of the substrate.

Simultaneous measurement of Young’s modulus, Poisson ratio, and coefficient of thermal expansion of thin films on substrates

A method of measuring the Yong’s modulus, Poisson ratio, and coefficient of thermal expansion (CTE) is presented. The method uses a wafer curvature technique to measure thermal stresses of thin films of the same material deposited on two different substrates, one isotropic and the other thermomechanically anisotropic. By analyzing the thermal stress data as a function of temperature, the Young’s modulus, Poisson ratio and CTE can be simultaneously determined. The method is demonstrated for Al (0.5 wt % Cu) and Cu thin films by performing measurements on (100) Si wafers and Y-cut single-crystal quartz wafers. The CTE, Young’s modulus, and Poisson ratio are found to be 24.3 ppm/°C, 58.9 GPa, and 0.342, respectively, for Al (Cu) thin film, and 17.7 ppm/°C, 104.2 GPa, and 0.352, respectively, for Cu thin film. They are in good agreement with those measured by other methods. This method is generally applicable to other on-wafer films with in-plane isotropy.

Thermomechanical properties and moisture uptake characteristics of hydrogen silsesquioxane submicron films

This letter describes measurement of the biaxial modulus, coefficient of thermal expansion (CTE), and moisture uptake characteristics of hydrogen silsesquioxane (HSQ) thin films. The biaxial modulus and CTE were determined using a bending beam method, and moisture uptake was studied using a quartz crystal microbalance method. The biaxial modulus and CTE of a 0.5 μm HSQ film were measured on Si and Ge substrates and found to be 7.07 GPa and 20.5 ppm/°C, respectively. The value determined for the diffusion constant of water in a 0.7-μm-thick HSQ films is 3.61×10−10 cm2/s at room temperature.

Thermomechanical property of diffusion barrier layer and its effect on the stress characteristics of copper submicron interconnect structures

Abstract Thermal stresses of thin titanium nitride (TiN) films on Si and Ge substrates have been measured by the bending beam method. The biaxial modulus and coefficient of thermal expansion of TiN thin film were deduced from the thermal stress data and found to be 355 GPa and 7.4 ppm/°C, respectively. Finite element analysis was used to study the effect of TiN diffusion barrier on the stress state of Cu lines in submicron damascene interconnects. The diffusion barrier was found to have a significant effect on the stress state of the Cu lines, especially for those embedded in interlevel dielectrics of low dielectric constant (k) materials. For the Cu/oxide interconnect structure, the metal lines with diffusion barrier were found to have a high near-hydrostatic triaxial stress state as expected. For the Cu/low k interconnect structure, a near-hydrostatic stress state was found to exist in the presence of the diffusion barrier; without the diffusion barrier, the stress state was not near-hydrostatic, instead it was dominated by a shear behavior. The implication of the diffusion barrier effect on the thermomechanical reliability of Cu interconnects is discussed.

Thermal stress and glass transition of ultrathin polystyrene films

The thermal stress of thin and ultrathin polystyrene (PS) films on Si substrate has been studied and the glass transition temperature (Tg) is determined from the thermal stress data. Tg of PS turned out to be thickness independent for thick films but decreases when the film thickness is comparable to the end-to-end distance of the polymer chains (<100 nm). The thermal stress level and the slope of the stress temperature curve of the film also decrease as the film thickness decreases. The slope reduction indicates that the product of the biaxial modulus E/(1−ν) and the coefficient of thermal expansion (CTE) of the film decreases with film thickness. Assuming that the CTE increases for ultrathin films, the modulus is found to decrease significantly with respect to the bulk value.

Thermal deformation analysis on flip-chip packages using high resolution moire interferometry

Solder reliability has been an issue with many fine-pitch, area-array packages because of the large thermal expansion (CTE) mismatch between the silicon die and the substrate. One solution to flip-chip plastic ball grid array (FCPBGA) package is to underfill the solder bumps to improve the reliability by reducing the solder bump shear stresses. However, for an underfilled flip-chip package, large thermal stresses will develop along the solder bump-underfill during thermal cycling due to the materials discontinuity. Delamination along the die-underfill interface has often been found in reliability test. In this study, high-resolution moire interferometry was used to investigate the thermal deformations for some experimental flip-chip packages. Experimental details of high-resolution moire interferometry are presented. Using a phaseshift technique, the resolution of moire interferometry is achieved at 26 nm per fringe order. Displacement and, especially, the strain distribution can be obtained accurately at this resolution. This experimental technique can analyze deformations with small features, such as the C4 bumps and high density interconnect (HDI) structure. Experimental results for HDI FCPBGA packages are presented and discussed.

On-wafer characterization of thermomechanical properties of dielectric thin films by a bending beam technique

A bending beam technique has been developed for on-wafer characterization of thermomechanical properties of dielectric thin films including Young’s modulus (E), the coefficient of thermal expansion (CTE), and the Poisson ratio (ν). The biaxial modulus E/(1−ν) and CTE were determined by measuring the thermal stresses of the dielectric film as a function of temperature on two different substrates. The Poisson ratio and Young’s modulus were determined by measuring the temperature dependence of the thermal stress of periodic line structures of the dielectric film. Three dielectric thin films were selected for this study, consisting of silica made from tetraethylorthosilane (TEOS), hydrogen silsesquioxane (HSQ), and biphenyltetracarboxylic dianhydride-p-phenylene diamine (BPDA-PDA). The deduced biaxial modulus and CTE are 77 GPa and 1.0 ppm/°C for TEOS, 7.07 GPa and 20.5 ppm/°C for HSQ, and 11.1 GPa and 3.4 ppm/°C for BPDA-PDA. The Poisson ratio is determined to be 0.24 and Young’s modulus is 59 GPa for the TE...