Thermal Expansion Of Thin Films Research Articles

Evaluating Free Thermal Expansion and Glass Transition of Ultrathin Polymer Films on Heated Liquid.

Thermomechanical properties of ultrathin films are crucial for fabrication and use of reliable thin electronic devices. Due to the lack of precise measurement techniques, the thermal deformation behavior of ultrathin films has not yet been clarified. Here, we propose a film on heated liquid (FOHL) method to simultaneously measure the coefficient of thermal expansion (CTE) and glass transition temperature (Tg) of multiple ultrathin polymer films. Free thermal expansion of thin films without substrate interaction can be guaranteed when the thin films are afloat on a liquid surface. To investigate the thermal behavior in a wide temperature range, glycerol is adopted as a thermally stable heating platform owing to its high boiling point of 290 °C. The thin films are transferred onto the glycerol surface from the water surface using the hygroscopic properties of glycerol. Highly accurate and high-throughput thermal strain measurement is achieved using three-dimensional digital image correlation (3D-DIC). The thermomechanical properties of ultrathin polystyrene thin films of various thicknesses (25-400 nm) are precisely characterized utilizing the FOHL and 3D-DIC method.

Thermal expansion behavior of thin films expanding freely on water surface

Coefficient of thermal expansion (CTE) for thin film has been measured only from change in thickness because thin film has to be constrained on a solid substrate. However, thin film CTE shows different values depending on the supporting solid substrate. Here, a novel measurement method is suggested to quantitatively measure the in-plane thermal expansion of thin films floating on a water surface. In-plane thermal expansion of thin films on water surface is achieved by heating the water. The CTE is measured through a digital image correlation (DIC) technique. The DIC tracks displacement marks deposited on the film surface, and the in-plane thermal strain is defined as the change in distance between the patterns. The method can be applied to measure the CTE of polymer, metal, and graphene with a thickness ranging from a micrometer to one-atom-thickness. The in-plane thermal expansion of the polystyrene (PS) thin film decreased as the film thickness decreased. The negative CTE of graphene is also successfully explored without any substrate effects or complicated calculations. The CTE measurement method can provide understanding of the intrinsic thermal expansion behavior of thin films including emerging two-dimensional materials.

Pyroelectric property of SrTiO3/Si ferroelectric-semiconductor heterojunctions near room temperature

A nonlinear thermodynamic formalism is developed to calculate the pyroelectric property of epitaxial single domain [Formula: see text] heterojunctions by taking into account the thermal expansion misfit strain at different temperatures. It has been demonstrated that the crucial role was played by the contribution associated with the structure order parameter arising from the rotations of oxygen octahedral on pyroelectricity. A dramatic decrease in the pyroelectric coefficient due to the strong coupling between the polarization and the structure order parameter is found at ferroelectric [Formula: see text]–[Formula: see text] phase transition. At the same time, the thermal expansion mismatch between film and substrate is also found to provide an additional weak decrease of pyroelectricity. The analytic relationship of the out-of-plane pyroelectric coefficient and dielectric constant of ferroelectric phases by considering the thermal expansion of thin films and substrates has been determined for the first time. Our research provides another avenue for the investigation of the pyroelectric effects of ferroic thin films, especially, such as antiferroelectric and multiferroic materials having two or more order parameters.

Micron-scale measurements of the coefficient of thermal expansion by time-domain probe beam deflection

We describe a pump-probe optical technique, time-domain probe-beam deflection, that enables measurements of the coefficient of thermal expansion (CTE) with micron-scale spatial resolution. Our quantitative model for the beam deflection includes contributions from thermal expansion of thin films deposited on a substrate, thermal expansion of the substrate, elastic deformation of the substrate, heating of air above the surface of the sample, and the lateral temperature gradient at the surface of the sample. The usefulness of this technique for high-throughput CTE measurement is demonstrated by two experiments: we are able to find the Invar alloy composition directly from a simple Fe–Ni diffusion couple and we find abnormal CTE increase in an ≈100μm region of dentin adjacent to the dentin-enamel junction of a human tooth. Such a CTE abnormality would be difficult to find using other techniques.

Investigation of thermal expansion of PI/SiO 2 composite films by CCD imaging technique from −120 to 200 °C

The thermal expansion of the reference sample, pure copper film in the temperature range of −120 to 200 °C was first measured using the newly improved CCD imaging technique for measurement of thermal expansion of thin films. The results showed good accordance with the recommended data given by TPRC (Thermophysical Properties Research Centre, USA) handbook, verifying that the present method is valid for measuring thermal expansion of films. Then, the thermal expansion TE (Δ L/ L 0) of silica/polyimide composite films with different SiO 2 fractions i.e. 0, 1, 3, 5, 8, 10 and 15 wt% prepared using the sol–gel technique was obtained in the temperature range of −120 to 200 °C using the newly improved CCD method and the differential coefficient thermal expansion (CTE) can be deduced by Δ L/ L 0 ∼ temperature relation. The CTE of SiO 2/PI composite films decreased with the increase of SiO 2 content and the decrease of temperature. An empirical equation of CTE of SiO 2/PI with SiO 2 content has been given in this paper.

Thermal Expansion of Low Dielectric Constant Thin Films by High-Resolution X-Ray Reflectivity

AbstractBy introducing high precision sample alignment technique, repeatability of incident angle to the sample surface for x-ray reflectivity (XRR) measurement is achieved to be within 0.3 arcsec. As a result, film thickness and density are possible to be measured repeatability within 0.03% and density within 0.26%. This accuracy realized to detect very small change of thermal expansion of thin films. The coefficient of thermal expansions (CTE) for porous low-k films deposited by CVD method were measured up to 400°C. The obtained values are in the range from 40 to 80 x10-6 K-1 and they are very large compare to that of copper (16-20 x10-6 K-1).

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.

Determining thermal expansion coefficients of thin films using micromachined cantilevers

In this research, the coefficient of thermal expansion (CTE) of thin films was studied through analytical and experimental approaches using micromachined beams. Both single layer and bilayer micromachined cantilevers (microcantilevers) were exploited in measuring the thermal expansion of the thin films. It was obtained that both single and bilayer micromachined cantilevers would exhibit an out of plane deflection after subjected to temperature changes. Thus the thermal expansion of thin film materials can be determined using optical interferometric techniques on these heat-deformed microcantilevers. The contributions of the proposed techniques are that they can be used to increase the sensitivity and accuracy of CTE measurements. Furthermore, the distribution of the thin film CTE across the entire substrate can also be determined through the proposed approaches. Since the microcantilever structure used in this study is very simple, both modeling and fabrication processes are simplified. Thus the proposed technique can be applied to supplement other techniques used in determining the CTE of thin films.

Noncontact method for measuring coefficient of linear thermal expansion of thin films

We measured the coefficient of linear thermal expansion of ZnSe thin films with a temperature regulated spectroscopic ellipsometer. The change in the film thickness was found to be a quadratic function of temperature whereas the coefficient of linear thermal expansion varied linearly in the temperature range between 295 and 413 K. While spectroscopic ellipsometry is recognized as a measurement technique to determine thin film optical constants and film thickness, the present work expands upon this technique to measure temperature induced dimensional change in the Å range.

Extraction of the Coefficient of Thermal Expansion of Thin Films from Buckled Membranes

AbstractWe report a new method to measure the temperature-dependent coefficient of thermal expansion α(T) of thin films. The method exploits the temperature dependent buckling of clamped square plates. This buckling was investigated numerically using an energy minimization method and finite element simulations. Both approaches show excellent agreement even far away from simple critical buckling. The numerical results were used to extract Cα(T) = α0+α1(T−T0 ) of PECVD silicon nitride between 20° and 140°C with α0 = (1.803±0.006)×10−6°C−1, α1 = (7.5±0.5)×10−9 °C−2, and T0 = 25°C.

Thermal Expansion of Thin Diblock Copolymer Films

AbstractThe thermal expansion of thin films of symmetric diblock copolymers of polystyrene (PS) and poly(methyl methácrylate) (PMMA) was investigated by X‐ray reflectivity. The confinement of the copolymer to the substrate, coupled with the multilayering of the copolymer where PS and PMMA layers are oriented parallel to the substrate, gives rise to unusual thermal expansion characteristics. The total thickness of the film increases as 3αL, where αL is the linear thermal expansion coefficient of the copolymer. Unlike homopolymer films, the thermal expansion of an ordered block copolymer film results in an excessive stretching of the copolymer chains at the interface between the PS and PMMA layers. This excess stretching is a result of the confinement of the junction points of the copolymer chains to the interfaces and the suppression of the lateral expansion of the copolymer. When the stretching of the chains becomes too high, relaxation occurs by transporting copolymer chains to the surface. This is evidenced by a reduction in the period of the multilayer. After the copolymer chains have relaxed, the change in the multilayer period with temperature closely follows αL.

A new apparatus for measuring thermal expansion of thin films

An apparatus for measuring the thermal expansion of thin films has been developed. The film, deposited on a substrate, was heated with hot nitrogen gas and the warp of the substrate was measured by an optical lever method. Films of iron, chromium, vanadium, and cobalt ferrite were deposited by sputtering and their thermal expansions were measured between 25 degrees C and 100 degrees C. The values of the thermal expansion coefficients were in good agreement with reported data for the bulk specimens.

An electrical method for the measurement of thermal expansion of thin films

A two-terminal capacitance method has been devised to measure the coefficient of linear thermal expansion ( alpha ) of thin films as conventional methods are unsuitable. The formula to evaluate the observed value of the thermal expansion coefficient ( alpha obs) has been obtained and the relevant correction factor has been incorporated in the final formula alpha true= alpha obs+ alpha el where alpha el and alpha true are the thermal expansion coefficients of electrode film and sample thin film respectively. The design of the dilatometric cell, the preparation of the thin-film specimen and the electrode film configuration are described. The change in capacitance with temperature of the vacuum capacitor, where the specimen film is the spacer, is used to evaluate alpha obs. This technique has been standardised using thin platelets ( approximately 100 mu m) of a single crystal of NaCl. The other considerations which are important for the precision of this measurement technique are also discussed.