if a forming gas ambient is utilized, hydrogen diffuses through the dielectric network to the underlying silicon-silicon dioxide interfaces, passivating silicon dangling bonds and reducing the interface state density .
Positioned immediately after contact formation (W CMP and post-CMP clean) and
before the first metal routing (M1), this low-temperature anneal serves as a critical stabilization and passivation phase . During the preceding contact metallization, the deposition of titanium, titanium nitride liners, and tungsten plugs introduces substantial thermo-mechanical stress, which is further exacerbated by the shear forces of chemical-mechanical polishing (CMP) . The subsequent anneal is required to relieve this film stress, densify the metal plug microstructure, and passivate process-induced interface states before the subsequent inter-metal dielectric (PMD 5) seals the structure . In the specific context of a CMOS Image Sensor (CIS), minimizing interface states at this stage is essential to suppress trap-assisted tunneling and dark current generation . The primary physical mechanism involves the thermal relaxation of the contact metals and the micro-structural stabilization of the metal-to-silicide interfaces . At the bottom of the contact, the titanium liner interacts with the underlying silicon or existing silicide to ensure a low-resistance ohmic contact . The total contact resistance is dictated by the Schottky barrier height at the metal-semiconductor interface, which depends on the work function of the metal and the electron affinity of the semiconductor, as defined by contact physics . The thermal energy provided by this anneal promotes localized atomic rearrangement at the W/TiN and TiN/Ti interfaces, minimizing micro-voids and reducing interfacial contact resistance . Concurrently, if a forming gas ambient is utilized, hydrogen diffuses through the dielectric network to the underlying silicon-silicon dioxide interfaces, passivating silicon dangling bonds and reducing the interface state density . A strictly low-temperature regime is selected because the front-end-of-line (FEOL) structures already feature self-aligned silicides, which are highly sensitive to thermal budgets . For instance, advanced contact schemes often employ nickel silicide (NiSi) due to its low formation temperature and minimal line-width dependence . However, NiSi suffers from morphological instability and tends to agglomerate or undergo phase transformation if subjected to temperatures exceeding its narrow thermodynamic stability window . Therefore, the annealing temperature must be kept below the degradation threshold of these critical silicide phases, ensuring that the low-resistance properties of the source/drain contacts are preserved . The precise control of temperature, time, and ambient gases determines the trade-off between sufficient stress relief and the prevention of undesirable metal diffusion or silicide agglomeration . In 40nm technology, the extremely scaled contact hole dimensions elevate the proportion of barrier and liner materials relative to the bulk tungsten, intensifying localized stress and increasing the risk of contact voiding . Furthermore, in Backside Illuminated (BSI) CMOS Image Sensors, the front-side metallization must maintain exceptional integrity and low defectivity, as any front-side stress or interface traps will significantly degrade the pixel's noise performance and dark current characteristics . The low-temperature anneal acts as a final front-side interface conditioning step before entering the back-end-of-line (BEOL) copper interconnect modules, carefully balancing electrical activation needs against the stringent thermal limits of nanoscale silicides .
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