Porous silicon exhibits distinct mechanical properties compared to bulk silicon due to its unique microstructure characterized by nanometer- to micrometer-sized pores. These properties are critical for its application in microelectromechanical systems (MEMS), where mechanical reliability under operational stresses is paramount. The mechanical behavior of porous silicon is governed by porosity, pore morphology, and surface chemistry, which influence Young’s modulus, fracture toughness, and thermal stability.

Young’s modulus of porous silicon is significantly lower than that of bulk silicon (approximately 169 GPa for single-crystal silicon). Experimental studies using nanoindentation and ultrasonic techniques reveal that Young’s modulus decreases with increasing porosity. For example, a porosity of 50% can reduce Young’s modulus to 50–70 GPa, while highly porous layers (80–90% porosity) exhibit values as low as 5–20 GPa. This reduction follows power-law or exponential scaling models, where the modulus (E) relates to porosity (p) as E = E₀(1 - p)ⁿ, with E₀ being the modulus of bulk silicon and n an empirical exponent typically between 1.5 and 3.

Fracture toughness, a measure of crack resistance, is also porosity-dependent. Bulk silicon has a fracture toughness of about 0.8–1.1 MPa·m¹/², but porous silicon shows lower values due to its weakened skeletal structure. For porosities above 60%, fracture toughness can drop below 0.3 MPa·m¹/², making the material prone to brittle fracture. Crack propagation in porous silicon often occurs along pore walls, where stress concentrations are highest. Surface passivation, such as oxidation or hydrosilylation, can improve fracture resistance by reducing surface defects and enhancing interfacial cohesion.

Thermal stability is another critical factor for MEMS applications. Porous silicon undergoes structural changes at elevated temperatures due to surface diffusion and oxidation. Above 300°C, sintering of pore walls can occur, leading to pore collapse and reduced porosity. Oxidation in ambient air begins at 200–300°C, forming a silicon dioxide layer that alters mechanical properties. The coefficient of thermal expansion (CTE) of porous silicon differs from bulk silicon (2.6 ppm/K) due to its porous network. Measurements indicate CTE values ranging from 1.5 to 4 ppm/K, depending on porosity and pore distribution.

Reliability concerns in MEMS applications stem from these mechanical and thermal characteristics. Cyclic loading can induce fatigue in porous silicon, with crack initiation observed at lower stress amplitudes than in bulk silicon. Stress corrosion cracking is another issue, particularly in humid environments, where water molecules weaken Si–Si bonds at crack tips. To mitigate these risks, design strategies include optimizing porosity gradients, reinforcing pore walls with coatings, or integrating porous silicon with bulk silicon supports for hybrid structures.

Comparisons with bulk silicon mechanics highlight key differences. Bulk silicon’s mechanical behavior is isotropic and predictable, governed by its crystalline perfection. In contrast, porous silicon exhibits anisotropic properties if pores are directionally aligned, and its stochastic pore distribution introduces variability in local stiffness and strength. Finite element modeling of porous silicon often requires homogenization techniques or explicit pore geometry representation to capture these effects accurately.

In summary, the mechanical behavior of porous silicon is defined by its porosity and microstructure, leading to reduced stiffness, fracture resistance, and altered thermal response compared to bulk silicon. For MEMS applications, careful material selection and structural design are necessary to ensure reliability under mechanical and thermal loads. Advances in surface modification and composite integration offer pathways to enhance performance, but the inherent trade-offs between porosity and mechanical integrity must be carefully managed.

Tables for key properties:

Property | Porous Silicon Range | Bulk Silicon
Young’s Modulus | 5–70 GPa | 169 GPa
Fracture Toughness | 0.1–0.8 MPa·m¹/² | 0.8–1.1 MPa·m¹/²
CTE | 1.5–4 ppm/K | 2.6 ppm/K
Thermal Stability Limit | <300°C (unpassivated) | >1000°C

This analysis underscores the importance of tailoring porous silicon’s mechanical properties for specific MEMS applications while addressing its reliability challenges through material and design innovations.