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Experimental Techniques

# Volumetric Heat Capacity Enhancement in Thin Films of Amorphous Fluorocarbon Polymers

[+] Author and Article Information
Hongxiang Tian, Marc G. Ghossoub, Oksen T. Baris, Jun Ma

Department of Mechanical Science and Engineering,  University of Illinois, Urbana, IL 61801

Murli Tirumala

Intel Labs,Intel Corporation, Hillsboro, OR 97124

Sanjiv Sinha

Department of Mechanical Science and Engineering,  University of Illinois, Urbana, IL 61801sanjiv@illinois.edu

J. Heat Transfer 134(8), 081601 (Jun 05, 2012) (9 pages) doi:10.1115/1.4006205 History: Received March 04, 2011; Revised February 10, 2012; Published June 05, 2012; Online June 05, 2012

## Abstract

Plasma deposited amorphous fluorocarbon polymers find use in biopassivation, and as low-friction coatings, adhesion promoters, and interlayer dielectrics. Here, we exploit their ease of deposition into ultrathin layers (<50 nm thick) to explore their potential as thermal storage elements. We design and fabricate a microcalorimeter for measuring the heat capacity of thin fluorocarbons. Conventional thin film calorimetry assumes adiabatic conditions that lead to large errors as film thickness decreases. We propose a new data analysis procedure that incorporates a one-dimensional solution of the transient heat diffusion equation to account for conduction losses. The data for films with thicknesses in the range 12–27 nm reveal a lowering of the melting point and an increase in the volumetric heat capacity with decreasing thickness. We attribute this to change in the carbon to fluorine ratio in the films’ composition. The volumetric heat capacity approximately doubles at room temperature as the film thickness decreases from 27 nm to 12 nm.

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Copyright © 2012 by American Society of Mechanical Engineers
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## Figures

Figure 1

Specific enthalpy change for various thermophysical processes along with their typical temperatures highlighting the upper limit for capacitive thermal management at temperatures less than 400 K

Figure 2

A top-view schematic of the micro-calorimeter

Figure 3

(a) Schematic of the fabrication process: (A) Al2 O3 deposition by ALD to serve as the etch stop for later backside etching, (B) SiNx deposition by PECVD, (C) heater definition by metal evaporation and lift-off, (D) backside silicon etching by ICP-RIE and XeF2 . (b) A finished device showing the reference and sample calorimeters.

Figure 4

The film deposition rate is linear with time and allows 10 nm thin layers to be readily deposited

Figure 5

The refractive index n and the extinction coefficient κ of the fluorocarbon polymer for different film thickness measured using ellipsometry

Figure 6

A schematic of the electrical circuit. The label “Ref” indicates the reference device and “Sample” indicates the device with the deposited polymer. Rex 1 and Rex 2 are two known resistors used to measure the applied current. A DAQ card measures voltages.

Figure 7

A representative calibration curve for the reference and sample sensors between 300 and 400 K

Figure 8

A representative transient temperature profile for an 18 nm polymer film. (Inset) Temperature rise during two successive pulses. Measurements are averaged over thousands of cycles to reduce noise.

Figure 9

The domain for the one-dimensional transient heat diffusion model

Figure 10

Comparison of the transient temperature profiles calculated from the analytical solution and that from a two-dimensional finite element simulation. Dashed lines represent the finite element simulation, and the symbols represent calculations using the analytical solution.

Figure 11

A Theoretical dT/dt curve plotted for a range of thermal diffusion coefficients at different times of the heating curve. The solid lines are linear fits to the diffusivity dependence.

Figure 12

Measured volumetric heat capacities at different film thicknesses. The dashed lines show data extracted using the adiabatic analysis. The round symbols denote data extracted using the analytical solution to the heat diffusion equation. The difference between the two reflects error arising from neglecting heat lost to the substrate in the adiabatic analysis. The measured value of heat capacity is nearly 25% smaller for the thinnest film at the highest temperature.

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