A typical computer system includes at least a microprocessor and some form of memory. The microprocessor has, among other components, arithmetic, logic, and control circuitry that interpret and execute instructions necessary for the operation and use of the computer system. In addition to the microprocessor and memory, a computer system has integrated circuits (ICs) that have various functionalities and communication paths, i.e., buses and wires that are necessary for the transfer of data among the aforementioned components of the computer system.
An integrated circuit (IC) may have hundreds of thousands up to millions of individual devices (e.g. transistors) deposited on a single semiconductor substrate. Such an IC is commonly referred to as a “Very Large Scale Integration” (VLSI) IC or “chip.”
A common building block of an IC, such as a VLSI chip, is a device known as a “transistor.” A transistor is a semiconductor device often partially made from silicon that regulates current or voltage and acts as a switch or gate for electronic signals. One type of transistor has three terminals that are electrically attachable to other electrical devices, such as, for example, resistors and power supplies, and even to other transistors in order to carry out an electrical circuit's function. A transistor's attributes, for example, length (L), width (W), and carrier mobility (μ) (a ratio of a carrier's velocity to an applied external voltage) dictate how well the transistor conducts electrical current and under what circumstances particular behavior occurs.
One factor that may affect the operation of a transistor is the stress on the transistor. Stress is a measure of the force producing or tending to produce deformation in a body. Components of stress may be measured in units of pressure. The metric unit for pressure is the Pascal (Pa). Stress on a silicon-based transistor affects the transistor's carrier mobility (μ). Stresses as large as 108 Pa may produce mobility changes (Δμ/μ) on the order of 10%. The change in carrier mobility (Δμ/μ) has a component dependent on the direction in which the transistor is positioned (δ) (also referred to as a “direction-dependent mobility change”) and a component that is independent of the direction in which the transistor is positioned (α) (also referred to as a “direction-independent mobility change”), i.e., Δμ/μ is equal to (α+δ):
            Δ      ⁢                          ⁢      μ        μ    =      α    +          δ      .      The above expression may often be approximated for computational simplicity as:
            Δ      ⁢                          ⁢      μ        μ    =            α      +      δ        ≈          δ      .      
A transistor may be designed such that the orientation of its parts cancels the direction-dependent mobility change (δ=0) effect of a stress. In such cases, only the direction-independent mobility change (α) is present:
            Δ      ⁢                          ⁢      μ        μ    =      α    .  
When in use, VLSI chips may be subject to a variety of stresses. Typical stresses include mechanical loading due to the packaging of these chips, thermal loading due to heat generated by circuits through power dissipation, and stresses applied to materials in close proximity to the VLSI chip. These stresses may lead to mechanical failure of the chip and/or mechanical failure of the material in close proximity to the chip. The larger the stress, the higher the chance for mechanical and/or circuit failure.