Stress Simulation

Stress simulation in semiconductor manufacturing computes the mechanical stress and strain induced in the wafer, films, and device structures by fabrication processes — predicting how stress affects device performance, reliability, and structural integrity.

Why Process-Induced Stress Matters

• Every fabrication step introduces mechanical stress:
• Film Deposition: Different materials have different thermal expansion coefficients and intrinsic stress.
• Thermal Processing: Heating and cooling create thermo-mechanical stress due to CTE mismatch between materials.
• STI (Shallow Trench Isolation): Oxide-filled trenches compress the silicon channel — affects transistor performance.
• Contact/Metal Fill: Filling trenches and vias with different materials creates local stress concentrations.
• Stress is not always bad — it is deliberately engineered in modern transistors to enhance performance (strained silicon).

Intentional Stress Engineering

• NMOS: Benefits from tensile stress in the channel direction — increases electron mobility by up to 70%.
• Methods: Tensile silicon nitride liner (SiN capping), tensile SiGe in source/drain areas (embedded SiC), SMT (stress memorization technique).
• PMOS: Benefits from compressive stress in the channel direction — increases hole mobility by up to 50%.
• Methods: Embedded SiGe source/drain (compresses the channel), compressive nitride liner.

What Stress Simulation Calculates

• Stress Tensor: The full 3D stress state (σxx, σyy, σzz, τxy, τxz, τyz) at every point in the structure.
• Strain: The deformation of the material — directly related to mobility enhancement in strained channels.
• Wafer Bow/Warp: Overall wafer deformation due to the cumulative stress of all deposited films — affects lithographic focus if excessive.
• Film Cracking/Delamination Risk: Stress exceeding the adhesion strength or fracture toughness causes mechanical failure.
• Via/Interconnect Stress: Stress concentration at metal-barrier-dielectric interfaces that drives electromigration and stress voiding.

Simulation Methods

• Finite Element Analysis (FEA): The standard method. Mesh the device structure, apply boundary conditions, solve the equilibrium equations. Tools: ANSYS, COMSOL, Sentaurus Process.
• Atomistic Simulation: For nanoscale stress effects — molecular dynamics or tight-binding methods model stress at the atomic level.
• Process Simulation Integration: Stress is tracked incrementally through each process step — the stress state evolves as layers are deposited, patterned, etched, and annealed.

Semiconductor Applications

• Strained Silicon Optimization: Model the stress transfer from SiGe S/D regions to the channel — optimize Ge concentration, recess depth, and proximity for maximum mobility enhancement.
• STI Stress: Predict compressive stress from STI on adjacent transistors — important for narrow-width effects.
• 3D Integration: Model thermal stress in TSV (through-silicon via) structures — CTE mismatch between Cu fill and Si creates significant stress.
• Packaging: Predict die stress from package assembly — affects device parameters and reliability.

Stress simulation is fundamental to modern transistor design — without accurate stress modeling, predicting device performance at advanced nodes is impossible.