Thermal Transport in Layered Sytems and Micro-structured Semiconductor Devices

Investigation of Thermal Transport in Layered Sytems and Micro-structured Semiconductor Devices by Photothermal Techniques and Finite Element Simulations

1.1 Motivation
The generic term layered systems refers to samples consisting of a basic material or  substrate on which can be identified one or more layers of differing optical and thermophysical properties. Generally, these layered materials systems are built up to fulfill some
specific purposes, e.g. to serve as thermal barriers or alternatively as thermal conductors, and so a true knowledge of the individual characteristics of the participating components is beneficial to find out the best structuring. Thus, the performance of the entire structure depends upon the integrity of the individual components as well as the interface between layers. This is why thermal characterization of such systems is of great importance since it allows to point out the governing thermal transport properties and to detect the eventual structuring defects. However, one of the major problems often resides in the contamination and even in the deterioration of the samples under investigation by inappropriate experimental methods. These problems are overcome or avoided by using the photothermal techniques which are more suited for performing thermal investigations since they are non-destructive and do not require any sample preparation.

On the other hand, these layered systems are exploited for the design and manufacturing of micro-structured devices, e.g. micromechanical and microelectronic devices. As for the microelectronic devices, they operate very fast and thus produce high
power densities, implying necessarily new challenges in the field of thermal management. The situation is even worse as the dimensions of these devices are getting drastically smaller and smaller since in this case the phenomena of transport and dissipation become more complex.
For example, it has been established, that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thus aggravating problems of thermal management [Cahill et al., 2003]. Main obsessions in investigating –both in the theoretical and in the experimental point of view? these so tiny structures reside either in the determination of the local thermal conductivity/diffusivity [Milcent et al., 1995; Langer et al., 1996; Hartmman et al., 1997; Ruiz et al., 1998; Milcent et al., 1998; Hui et al, 1999;
Gervaise et al., 2000] or in the localization of hot spots [Bolte et al., 1998; Bolte, 1999] or even in the calibration of the absolute temperature [Schaub, 2001], by using different photothermal methods. The main experimental approaches in thermal microscopy are reported in [Price et al., 2000; Pelzl et al., 2001].

However, the veritable difficulty in dealing with these high power devices is the identification and detection of the location of high temperatures (hot spots) as well as the management of the dissipated heat. Another major difficulty comes from the fact that materials which are brought into contact with each other to make up a unique device have different deposition temperatures and thermal expansion coefficients [Prorok and Espinosa, 2002]. Such a materials composition with so different characteristics can unavoidably lead to damages including for example cracking and de-lamination. There is therefore an absolute
necessity to examine and study the mechanisms which can help to enhance the thermal performances of these extremely thin devices. In principle, two particular experimental techniques, namely the Scanning Thermal Microscopy (SThM) and the Scanning Thermal Expansion Microscopy (SThEM), are aimed at investigating and controlling the thermal movements in these kinds of devices but due to the limitations imposed to experimental measurements by the hostile dimensions of the structures, numerical simulations mostly based on the Finite Element Method (FEM) constitute a very important tool to predict the thermal
behaviour of devices and then discuss about their performances via faithful models. This option has at least the advantage of limiting the high costs related to the production of layouts and of avoiding hazardous industrial tests.

1.2 Objectives of the work
The present research work had three main objectives: The first task was to investigate several layered structures by means of photothermal techniques and to find out concrete solutions for a more rapid and efficient quantitative interpretation of the measured signals.
Another important task was to propose an alternative photothermal method which can help to get more reliable information on the lateral transport properties and allow the identification of heat sources in micro-scaled systems. The third objective of this contribution was to investigate by means of Scanning Thermal Expansion Microscopy and finite element simulations, the hot spots in some selected micro-structured semiconductor devices following their excitation by a modulated heat source.
1.3 Thesis overview
This work, which can be globally subdivided into two parts including the photothermal characterization of layered systems ?from macroscopic to microscopic scale? and the finite element investigation of thermal and thermo-elastic signals in micro-structured devices, is organized in the following way:
Chapter 2 recalls the basic concepts of thermal waves and reviews the different  experimental configurations which are designed for the generation and the detection of photothermal signals in solids.