Clay Mortensen

Low thermal conductivity in nanoscale layered materials synthesized by the method of modulated elemental reactants

We report the room-temperature, cross-plane thermal conductivities, and longitudinal speeds of sound of multilayer films [(TiTe2)3(Bi2Te3)x(TiTe2)3(Sb2Te3)y]i (x=1−5,y=1−5) and misfit-layer dichalcogenide films [(PbSe)m(TSe2)n]i (T=W or Mo, m=1–5, and n=1–5) synthesized by the modulated elemental reactants method. The thermal conductivities of these nanoscale layered materials fall below the predicted minimum thermal conductivity of the component compounds: two times lower than the minimum thermal conductivity of Bi2Te3 for multilayer [(TiTe2)3(Bi2Te3)x(TiTe2)3(Sb2Te3)y]i films and five to six times lower than the minimum thermal conductivity of PbSe for misfit-layer dichalcogenides [(PbSe)m(TSe2)n]i. We attribute the low thermal conductivities to the anisotropic bonding of the layered crystals and orientational disorder in the stacking of layered crystals along the direction perpendicular to the surface.

Lower limit to the lattice thermal conductivity of nanostructured Bi2Te3-based materials

We investigate the lower limit to the lattice thermal conductivity of Bi2Te3 and related materials using thin films synthesized by the method of elemental reactants. The thermal conductivities of single layer films of (Bi0.5Sb0.5)2Te3 and multilayer films of (Bi2Te3)m(TiTe2)n and [(BixSb1−x)2Te3]m(TiTe2)n are measured by time-domain thermoreflectance; the thermal conductivity data are compared to our prior work on nanocrystalline Bi2Te3 and a Debye–Callaway model of heat transport by acoustic phonons. The homogeneous nanocrystalline films have average grain sizes 30<d<100 nm as measured by the width of the (003) x-ray diffraction peak. Multilayer films incorporating turbostratic TiTe2 enable studies of the effective thermal conductivity of Bi2Te3 layers as thin as 2 nm. In the limit of small grain size or layer thickness, the thermal conductivity of Bi2Te3 approaches the predicted minimum thermal conductivity of 0.31 W/m K. The dependence of the thermal conductivity on grain size is in good agreement with...

Studies of the Coefficient of Thermal Expansion of Low-k ILD Materials by X-Ray Reflectivity

The electrical and mechanical properties of low-k dielectric materials have received a great deal of attention in recent years; however, measurements of thermal properties such as the coefficient of thermal expansion remain minimal. This absence of data is due in part to the limited number of experimental techniques capable of measuring this parameter. Even when data does exist, it has generally not been collected on samples of a thickness relevant to current and future integrated processes. We present a procedure for using x-ray reflectivity to measure the coefficient of thermal expansion of sub-micron dielectric thin films. In particular, we elucidate the thin film mechanics required to extract this parameter for a supported film as opposed to a free-standing film. Results of measurements for a series of plasma-enhanced chemical vapor deposited and spin-on low-k dielectric thin films will be provided and compared.

Investigation of the interdiffusion of Sb and Bi in [Bi/sub 2/Te/sub 3/]/sub x/{Sb/sub 2/Te/sub 3/]/sub y/ superlattices

[Bi2Te3]x[Sb2Te3]y superlattices have been shown to have decreased thermal conductivities as compared to bulk alloys, resulting in enhanced thermoelectric efficiency. A key to the preparation of these kinetically stable superlattices has been the use of low temperature epitaxial growth techniques. We are exploring the use of modulated elemental reactants to synthesize [Bi2Te3]x[Sb2Te3]y superlattices by controlling the diffusion distance through modulated layers of the elemental reactants. The modulated elemental reactants are made by depositing thin alternating layers of Bi and Te followed by alternating thin layers of Sb and Te onto ambient substrates to yield a series of superlattice precursors with varying periods. X-ray reflectivity, x-ray diffraction and Time of FlightSecondary Ion Mass Spectrometry have been used to characterize the superlattice precursors and their evolution as a function of temperature. For various x and y formation of the desired superlattice competes with interdiffusion of the Bi2Te3 and Sb2Te3 layers at elevated temperatures. This competition has been explored as a function of annealing temperature and time and will be discussed.

Synthesis of (Sb/sub 2/Te/sub 3/)/sub x/(TiTe/sub 2/)/sub Y/ superlattices

(Sb/sub 2/Te/sub 3/)/sub x/(TiTe/sub 2/) superlattices with varying values of x and fixed y were prepared using modulated elemental reactants. A procedure to calibrate the ratio of the elements and the absolute amount deposited is discussed. The structure of a reactant designed to form a (Sb/sub 2/Te/sub 3/)/sub 3/(TiTe/sub 2/)/sub 3/ superlattice is followed as a function of annealing temperature. The annealing conditions obtained from this study enabled us to prepare a series of (Sb/sub 2/Te/sub 3/)/sub x/(TiTe/sub 2/)/sub y/ superlattices. The samples were characterized using X-ray reflectivity, X-ray diffraction, and electron probe microanalysis (EPMA).

Ultra low-k dielectric mechanical property characterization

To meet electrical performance requirements, the industry is implementing ultra-low dielectric constant (ULK) materials in the back end of line interconnect structure. ULK dielectrics are inherently weak compared to traditional dielectrics and pose significant challenges to electronic packaging processes and reliability. Accurate mechanical properties are a prerequisite for upfront risk assessments associated with low-k integration using numerical simulations. In this paper, techniques used to characterize ULK dielectric elastic modulus and in-plane/out-of-plane coefficient of thermal expansion will be presented and the data for a candidate ULK dielectric will be summarized. Nanoindentation of ULK films on substrate was used to determine the plane strain modulus. In the direction normal to the film, the temperature gradient of the thermal expansion strain along the film thickness was measured by X-ray reflectivity. In the plane of the film, the temperature gradient of the biaxial thermal stress was obtained by the substrate curvature measurements. A method to deduce Poissons ratio of the thin ULK film is proposed using the data from the afore-mentioned characterization techniques.

The Evaluation of TiTe2 as a Diffusion Barrier in the Formation of Low Thermal Conductivity Nanolaminates with Bi2Te3 and Sb2Te3

The evolution of designed [(Ti-Te)]x[(Sb-Te)]y, [(Bi-Te)]x[(Sb-Te)]y, [(Ti-Te)]w[(Bi-Te)]x[(Sb-Te)]y and [(Ti-Te)]w[(Bi-Te)]x[(Ti-Te)]y[(Sb-Te)]z precursors were followed as a function of annealing temperature and time using both low and high angle x-ray diffraction techniques to probe the self assembly into nanolaminate materials. The [(Bi-Te)]x[(Sb-Te)]y precursors were found to interdiffuse at low temperatures to form a (BixSb1-x)2Te3 alloy. The [(Ti-Te)]x[(Bi-Te)]y and [(Ti-Te)]x[(Sb-Te)]y precursors formed ordered nanolaminates [{(TiTe2)}1.35]x[Bi2Te3]y and [{(TiTe2)}1.35]x[Sb2Te3]y respectively. The [(Ti-Te)]w[(Bi-Te)]x[(Sb-Te)]x precursors formed [{(TiTe2)}1.35]w[(Bi0.5Sb0.5)2Te3]2x nanolaminates on annealing, as the bismuth and antimony layers interdiffused. Over the range of TiTe2 thicknesses used in [(Ti-Te)]w[(Bi-Te)]x[(Ti-Te)]y[(Sb-Te)]z precursors, Bi and Sb were found to interdiffuse through the 2-4 nm thick Ti-Te layers, resulting in the formation of (BixSb1-x)2Te3 alloy layers as part of the final nanolaminated products. When the Bi-Te and Sb-Te thicknesses were equal in the amorphous precursors, symmetric [{(TiTe2)}1.35]m[(Bi0.5Sb0.5)2Te3]n nanolamiantes were formed. When the thicknesses of Bi-Te and Sb-Te layers were not equal in the amorphous precursor, asymmetric [(TiTe2)1.35]m[(BixSb1-x)2Te3]n[(TiTe2)1.35]m[(BixSb1-x)2Te3]p nanolaminates were formed. These results imply that to form (A)w(B)x(C)y nanolaminates using designed layered precursors all three components must be immiscible. To form (A)x(B)y(A)x(C)z nanolaminates, the components must be immiscible or the precursor to the A component and the A component itself must be an effective interdiffusion barrier preventing B and C from mixing.

Thermomechanical Property Characterization of Ultra Low‐k Materials

To meet electrical performance requirements, the industry is implementing ultra‐low dielectric constant (ULK) materials in the back end of line interconnect structure. ULK dielectrics are inherently weak compared to traditional dielectrics and pose significant challenges to electronic packaging processes and reliability. Accurate mechanical properties are a pre‐requisite for upfront risk assessments associated with low‐k integration using numerical simulations. In this paper, techniques used to characterize ULK dielectric elastic modulus and in‐plane/out‐of‐plane coefficient of thermal expansion will be presented and the data for a candidate ULK dielectric will be summarized. Nanoindentation of ULK films on substrate was used to determine the plane strain modulus. In the direction normal to the film, the temperature gradient of the thermal expansion strain along the film thickness was measured by x‐ray reflectivity. In the plane of the film, the temperature gradient of the biaxial thermal stress was obtai...

Progress towards the preparation of (TiTe/sub 2/)/sub 3/(Bi/sub 2/Te/sub 3/)/sub y/(TiTe/sub 2/)/sub 3/(Sb/sub 2/Te/sub 3/)/sub z/ superlattices

Superlattices based on the thermoelectric standard compounds Bi/sub 2/Te/sub 3/ and Sb/sub 2/Te/sub 3/ demonstrate enhanced ZT values. The formation of the superlattice (Bi/sub 2/Te/sub 3/)/sub x/(Sb/sub 2/Te/sub 3/)/sub y/ through the modulated elemental reactants (MER) approach is hindered by interdiffusion of the Bi/sub 2/Te/sub 3/ and Sb/sub 2/Te/sub 3/ layers at the necessary annealing temperatures. TiTe/sub 2/ has the potential to act as a diffusion barrier between the Bi/sub 2/Te/sub 3/ and Sb/sub 2/Te/sub 3/ layers of the superlattice. (TiTe/sub 2/)/sub 3/(Bi/sub 2/Te/sub 3/)/sub y/(TiTe/sub 2/)/sub 3/(Sb/sub 2/Te/sub 3/)/sub z/ superlattices were synthesized using the MER approach, successfully separating the different V-VI layers by TiTe/sub 2/. This study shows that the number of Bi/sub 2/Te/sub 3/ and Sb/sub 2/Te/sub 3/ layers can be precisely controlled using the MER method.