PECVD is often selected because the RF plasma supplies the necessary activation energy, allowing high-quality SiN deposition to occur at significantly reduced temperatures compared to conventional thermal LPCVD .
In depth
In the 28nm planar CMOS flow, the integration of compressively strained Silicon-Germanium (SiGe) in the PMOS source/drain (S/D) regions is a critical strain engineering module to enhance hole mobility . The SiGe Hardmask SiN Deposition step is inserted immediately after the
Cell VTP (Threshold Voltage Program) implants and prior to the S/D recess patterning steps to establish a robust, chemically inert protective barrier across the active device areas . This SiN layer serves a dual purpose: first, it acts as an anisotropic hardmask for the subsequent SiGe Trench Etch to define the recess geometry ; second, it functions as a highly selective deposition mask during the selective epitaxial growth (SEG) of the in-situ boron-doped SiGe layer, preventing unwanted nucleations on non-active regions . This step is fundamentally distinct from SiGe Deposition (#100), which is an epitaxial growth process on exposed silicon , and from SiGe SiN Hardmask Remove (#102), which selectively strips this hardmask post-epitaxy using hot phosphoric acid chemistry .
The physical deposition of the Silicon Nitride (SiN) hardmask typically relies on Plasma-Enhanced Chemical Vapor Deposition (PECVD) or Low-Pressure Chemical Vapor Deposition (LPCVD) techniques to achieve a conformal, high-density film . The chemical reaction mechanism involves the gas-phase transport and surface adsorption of silicon-containing precursors (such as silane or chlorosilanes) and nitrogen-containing reactants (such as ammonia or diatomic nitrogen) . Thermal energy or RF plasma excitation drives the decomposition and surface migration of these species, leading to film growth described by classic chemical vapor deposition kinetics, where surface reaction rate and mass transport boundary layers dictate uniformity . Because this step occurs after the critical VTP implants, minimizing the thermal budget is crucial to prevent the transient enhanced diffusion (TED) and deactivation of dopants . Thus, PECVD is often selected because the RF plasma supplies the necessary activation energy, allowing high-quality SiN deposition to occur at significantly reduced temperatures compared to conventional thermal LPCVD .
Silicon Nitride is selected as the hardmask material due to its exceptional etch selectivity against silicon during the subsequent reactive ion etching (RIE) of the recesses , as well as its superior chemical resistance to wet pre-cleans compared to oxide layers . Furthermore, during the selective epitaxial growth of SiGe, the dielectric properties of SiN prevent precursor adsorption and subsequent non-selective deposition on the isolation oxide and gate regions . The structural and mechanical properties of the deposited SiN—such as film density, intrinsic stress, and stoichiometry—are highly sensitive to the deposition parameters . Increasing the deposition temperature or the low-frequency RF power component enhances film density and decreases hydrogen content, which in turn reduces the wet etch rate of the hardmask in subsequent clean steps, preserving its structural integrity during the recess and pre-clean sequences . Conversely, managing film stress is critical: excessive tensile or compressive stress in the hardmask can induce localized defect propagation or modify the stress coupling in adjacent channel areas, influencing device performance .
Risks & Challenges
[Medium] Dopant Redistribution and Deactivation: Excessive thermal budget during the SiN hardmask deposition can trigger transient enhanced diffusion (TED) of the previously implanted Cell VTP species, shifting the threshold voltage of adjacent transistors . High-temperature processing can also lead to the deactivation of dopants in highly doped areas, increasing the parasitic contact resistance .
[Medium] Selectivity Loss and Parasitic SiGe Growth: If the SiN deposition parameters (such as low deposition temperature or inappropriate reactant ratios) result in a high hydrogen content and low-density film, the hardmask will exhibit a high wet etch rate in hydrofluoric acid pre-cleans . This degradation can cause partial or complete mask loss, leading to a loss of selectivity and subsequent parasitic, non-selective SiGe deposition on the isolation oxide or gate structures during epitaxial growth .
[Medium] Thinning at Top Corners (Key-holing and Bread-loafing): During deposition over high-aspect-ratio features like gate stacks, the film can exhibit poor conformality, leading to severe bread-loafing at the top corners and thinning at the sidewalls . These thin regions are easily breached during the subsequent SiGe Trench Etch, resulting in undesirable etching of underlying gate or spacer materials, which ultimately degrades the short-channel control and gate dielectric reliability .
[Low] Lattice Dislocation Nucleation from Stress Coupling: High intrinsic tensile or compressive stress in the deposited SiN film can couple into the underlying silicon substrate, creating localized stress gradients . When subjected to subsequent thermal cycles, these high localized stresses can exceed the critical resolved shear stress of silicon, initiating dislocations that act as defect propagation pathways during epitaxial growth and degrading channel carrier mobility .
A free account opens the full rationale, risk analysis, and the paper and patent citations behind them.