# Thermomechanical Stresses in Silicon Chips for
## 1 Introduction  
Semiconductor silicon chips are used for various applications, including consumer electronics, industrial purposes, telecommunication, and in the automotive field. Being the second most abundant element on Earth, silicon is a relatively inexpensive material and is highly reliable due to its versatility and physical-chemical properties [1]. Its thermal expansion coefficient is low compared to most metals, making most of the effects negligible due to thermal shock and fatigue. Silicon is fairly inert under standard environmental conditions, it does not react with most acidic compounds, but reacts with diluted alkalis and halogens, and its electrical conductivity can be tuned by adding small amounts of impurities [2]. For these reasons, semiconductor silicon remains the core component of many microelectronic devices, it is an essential component of integrated circuits (ICs), the building block of power devices such as computers and smartphones, and it is essential for photovoltaics to manufacture solar cells. In particular, single-crystal silicon is the most widely used semiconductor material in the micro-electromechanical (MEMS) manufacturing and electronic and optoelectronic circuits industry. In addition, the continuous demand for compact and miniaturized devices has increased the need for more functionalities from a single device. This means that today an IC chip is requested to house a larger number of components to support more features. A silicon-integrated circuit chip is realized by embedding and overlaying different elements of a wide variety of materials, each characterized by different elastic and thermal properties. Therefore, it follows that many structural elements, byvirtue of their formation processes, exhibit intrinsic stresses. Stress problems are prevalent in silicon technology and become critical, as they can impact the performance of the ICs. For these reasons, understanding the nature and origin of stresses is a fundamental issue. It is strategic to investigate the stresses developed during the assembly process, as a function of the working parameters, using an experimental technique and predictive theoretical models. Raman spectroscopy is an efficient tool to measure residual stress by correlating the effects with the optical change in the materials [1; 2; 3; 4; 5; 6; 7]. Moreover, it is a nondestructive and noncontact method, characterized by a high spatial resolution, and has already been widely used to investigate silicon, such as in cubic crystals [8; 9], microelectronic devices [10; 11; 12; 13], and nanostrips [14]. Selecting the right substrate materials for the production of microelectronics is one key point in the process. The substrate must be highly reliable, in configurations precisely chosen to meet the needs of a specific application. Several substrate materials are used, which are carefully chosen on the basis of the performances needed to reach, but also following other criteria like safety, environmental impact, economic costs, and process complexity. Organic-based composite materials in polymeric form have attracted considerable attention because they offer the opportunity to produce substrates with multiple customizable functionalities, they cover large areas, and are flexible, lightweight, and are relatively low in cost [15; 16; 17; 18]. There is an immense scope for the future of flexible organic electronics. However, the current performance and lifetimes of electronic devices based on organic substrates are still lower than those of traditional ceramic or other inorganic materials [19], such as aluminium nitride or copper. Ceramic substrates are well known for high-temperature electronic integration. Copper substrates are widely used in microelectronics because of the advantageous properties of copper, such as low electrical resistivity, high electromigration resistance, high thermal conductivity, excellent chemical and thermodynamic characteristics, antimicrobial properties, and low cost.