Showing papers by "Paul S. Ho published in 2000"

Low Dielectric Constant Materials for ULSI Interconnects

Abstract: ▪ Abstract As integrated circuit (IC) dimensions continue to decrease, RC delay, crosstalk noise, and power dissipation of the interconnect structure become limiting factors for ultra-large-scale integration of integrated circuits. Materials with low dielectric constant are being developed to replace silicon dioxide as interlevel dielectrics. In this review, the general requirements for process integration and material properties of low-k dielectrics are first discussed. The discussion is focused on the challenge in developing materials with low dielectric constant but strong thermomechanical properties. This is followed by a description of the material characterization techniques, including several recently developed for porous materials. Finally, the material characteristics of candidate low-k dielectrics will be discussed to illustrate their structure-property relations.

Thermal conductivity study of porous low-k dielectric materials

Abstract: An experimental method based on the 3ω technique has been developed to measure thermal conductivity of porous Xerogel films as a function of porosity. The results show that the thermal conductivity of these porous dielectric films can be an order of magnitude smaller than that of SiO2. To account for the porosity dependence of thermal conductivity, two porosity weighted semiempirical models are introduced. These models suggest the scaling rule expressing the thermal conductivity as a function of porosity. The decrease observed in thermal conductivity of porous films suggests that the tradeoff between thermal and electrical performance is an important consideration when implementing porous dielectric materials as interlevel dielectrics for on-chip interconnects.

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

Abstract: 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.

Thermal stress and glass transition of ultrathin polystyrene films

Abstract: 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.

Adhesion measurement for electronic packaging applications using double cantilever beam method

Abstract: Multilayers and interfaces are ubiquitous in microelectronics devices, interconnect and packaging structures. As the interface integrity becomes the major concern of performance, yield, and reliability, the need to evaluate the fracture and delamination behavior of various interfaces increases. This work focused on quantifying interfacial adhesion performance of a typical electronics packaging structure, flip-chip-on-organic-substrate. A series of experiments and analyzes were conducted to investigate the adhesion and fracture behaviors of the underfill/silicon and underfill/organic substrate interfaces. The experimental techniques for the interfacial fracture experiments were developed to produce the double-cantilever-beam (DCB) specimens and to establish a reproducible testing protocol. To extract the interfacial fracture energies, a closed-form solution was developed based on a beam-on-elastic-foundation model. A two-dimensional elastoplastic finite element analysis (FEA) model was also implemented to examine effects of mode-mixity, thermal/residual stresses, and underfill plasticity. The techniques allow for reproducible determination of underfill/printed circuit board (PCB) and underfill/silicon chip interfacial adhesion strength. The developed techniques are also readily applicable to evaluate interfacial adhesion performance for many other similar electronic packaging systems. This provides capabilities in optimizing material selections and process conditions to improve interfacial adhesion performance, Additionally, the interfacial fracture energy measured with high accuracy can provide a basis for realistic modeling of thermo-mechanical reliability of electronic components.

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

Abstract: 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...

Adhesion and toughening mechanisms at underfill interfaces for flip-chip-on-organic-substrate packaging

Abstract: The flip chip-on-organic-substrate packaging technology utilizes a particulate reinforced epoxy as the underfill (UF) to adhere the chip to the package or board, Although the use of underfill encapsulation leads to improved reliability of flip-chip solder interconnections, delamination at various interfaces becomes a major concern for assembly yield loss and package reliability. In spite of their importance, the adhesion and fracture behaviors of the underfill interfaces have not been investigated until recently. Considerable controversy exists over the effects of underfill formulation and the adhesion and toughening mechanisms of the interfaces. The present work focuses on investigating the effects of several key variables on the interface adhesion strengths for UF/chip and UF/organic substrate systems. These variables are underfill organosilane content, filler particle content, rubber particle content, surface morphology and chemistry of the chip and organic substrates. The approach of this study is to measure the effect of these variables on the interfacial fracture energy using the double-cantilever-beam (DCB) techniques. The results demonstrate that the underfill interfacial adhesion and fracture characteristics are controlled by several distinct but competing mechanisms, such as formation of primary bonds, crack-pinning by glass fillers, debonding of glass filler from epoxy matrix (defect formation), and cavitation and shearing induced by rubber particles. Fundamental understanding of the interfacial adhesion and toughening mechanisms can provide guidance for developing new processes and materials to enhance interfacial adhesion and reliability.

Detection and analysis of early failures in electromigration

Abstract: The early failure issue in electromigration (EM) has been an unresolved subject of study over the last several decades. A satisfying experimental approach for the detection and analysis of early failures has not been established yet. In this study, a technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge is presented. A total of more than 20 000 interconnects were tested. The results indicate that the EM failure mechanism studied here follows lognormal behavior down to the four sigma level.

Electromigration Reliability of Dual-Damascene Cu/Oxide Interconnects

Abstract: An electromigration study has determined the lifetime characteristics and failure mode of dual-damascene Cu/oxide interconnects at temperatures ranging between 200 and 325 °C at a current density of 1.0 MA/cm2. A novel test structure design is used which incorporates a repeated chain of “Blech-type” line elements. The large interconnect ensemble permits a statistical approach to addressing interconnect reliability issues using typical failure analysis tools such as focused ion beam imaging. The larger sample size of the test structure thus enables efficient identification of “early failure” or extrinsic modes of interconnect failure associated with process development. The analysis so far indicates that two major damage modes are observable: (1) via-voiding and (2) voiding within the damascene trench.

Nonuniform temperature and strain fields in a powered package

Abstract: This paper analyzes the nonuniform temperature and strain fields resulting from power dissipation in an electronic package. A 208 lead plastic quad flat pack (PQFP) manufactured by Texas Instruments is used to show the temperature distribution and mechanical deformation resulting from power dissipation in the package. The package is tested experimentally and thermally modeled using finite element analysis to obtain the temperature distribution in the active package. The moire interferometry technique is used to acquire displacement contours of an active PQFP and the results are compared to a uniformly heated sample. The results revealed that the thermal loading due to internal power dissipation produces significantly different strains than a uniformly heated sample.

Dielectric anisotropy and molecular orientation of fluorinated polymers confined in submicron trenches

Abstract: Dielectric anisotropy of polymers with low dielectric constant is an important property to consider for developing interlevel dielectrics for advanced on-chip interconnects. The effect of molecular structure on dielectric anisotropy has been investigated for two low dielectric-constant polymers: a fluorinated polyimide, FPI-136M and a fluorinated poly(aryl ether), FLARE-1.51. Optical and electrical measurements have been carried out to determine the dielectric anisotropy in blanket thin films and trench line structures. Results from optical measurements show that the FPI-136M film has a larger birefringence than the FLARE-1.51 film, indicating a larger dielectric anisotropy. This reflects a higher degree of in-plane molecular orientation for the rigid rod-like fluorinated polyimide, while the fluorinated poly(aryl ether) has an ether linkage that allows greater chain flexibility. Electrical measurement of the dielectric anisotropy has been performed using a metal–insulator–metal structure to determine the...

How low-energy ions can enhance depositions on low-K dielectrics

Abstract: Ultrathin titanium nitride layers grown on three different dielectrics were studied to examine how low-energy ions change the chemical composition at and near their interface. Comparisons were made by growing titanium nitride under similar conditions both with (ion-assisted) and without (reactive) nitrogen ions. Although the chemical reactions between the titanium nitride and the three dielectrics under both growth conditions depend on the type of dielectric used, a few general observations were seen. In comparison with the reactively grown samples, all of the ion-assisted growths show a significant increase in the amount of nitride in the titanium nitride layer at and near the titanium nitride/dielectric interface. Moreover, the amount of chemical binding between the titanium nitride and dielectric is increased when low-energy ions are used. Finally, by using angle resolved x-ray photoemission it was determined that the enhancement in the deposition process from low-energy ions occurs without inducing significant intermixing between the titanium nitride layer and the dielectric.

Electromigration in multi-level interconnects with polymeric low-k interlevel dielectrics

Abstract: Electromigration (EM) characteristics were evaluated on multi-level Al(Cu) test structures with polymeric low k and standard oxide interlevel dielectrics. The two polymers used as interlevel dielectrics in this work are a fluorinated polyimide (FPI) and a poly(aryl) ether(PAE). Joule heating experiments and microstructural analysis were both conducted on Al(Cu) to insure that that there were no microstructural or other differences between the polymer samples and their oxide counterparts. The results show that the integration of polymeric low k dielectrics has a significant impact on EM performance of interconnect structures.