CoSi2 Contacts - Wet Etching

CoSi2 Contacts - Wet Etching

Material Name: CoSi2
Recipe No.: 10320
Primary Chemical Element in Material: Co
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: To study cobalt silicide formation between Co films and heavily doped poly-Si for low contact resistance contacts, very thin (target thickness of ~20 nm) Co films were sputtered on heavily P+ implanted (~60 nm thick) poly-Si/SiO2/Si wafers, followed by a TiN and Ti film cap layer. Three hundred millimeter (300 mm) p- Si wafers were used as substrates in this study.

The wafers were annealed to form cobalt silicide in the two types of RTA systems (the cold wall, tungsten halogen lamp-based RTA system1 and the hotwall, furnace-basedRTAsystem1) in the temperature range of 350 ~ 900 C in 1 atm N2 ambient. The residual O2 concentrationwas kept below 30 ppm during RTA steps. The cobalt silicide formation process consists of two annealing steps: the first step at lower temperature, ~500 C, for the higher resistivity monosilicide (CoSi) formation, and the second step at higher temperature, >800 C, for the lower resistivity disilicide (CoSi2) formation. The annealing time was 15 ~ 30 s for the cold wall RTA and 50 ~ 150 s for the hot wall RTA system. Extensive design of experiment (DOE) utilizing over 200 samples was implemented. The 'annealing time' refers to the 'soak time' at annealing temperature for the cold wall RTA system and the 'residence time' (from wafer-in to wafer-out) in the annealing chamber for the hot wall RTA system. Details of RTA systems have been reported elsewhere.

During the low temperature 1st step annealing for CoSi formation, Co film was partially converted into CoSi with any unreacted Co film remaining as a metal layer. The TiN/Ti capping layer and the unreacted Co film were stripped off by chemical etching before the 2nd step annealing, at higher temperature, for CoSi2 formation. TiN capping layer was removed in a H2O2 + H2O (1:1) solution at 6 0C. The Ti and unreacted Co layers were etched off using a HNO3 + H2O (1:1) solution at 60 C. Sheet resistance (Rs) was measured at various stages (as prepared, after 1st step anneal, after metal film strip, and 2nd step anneal) using a four point probe to characterize the cobalt silicidation process and the low resistivity CoSi2 formation annealing process optimization.
Procedure (Condition): No data
Note: The effect of annealing temperature and time on the resulting resistivity of CoSi2 contacts with P-doped poly-Si was investigated using a single wafer furnace-based (hot wall) rapid thermal annealing (RTA) system. To achieve low resistivity contacts, CoSi2 formation process optimization was done by detailed design of experiment (DOE). Sheet resistance (Rs) response surface plots as a function of annealing temperature and time for 1st step and 2nd step RTA processes showed a very wide process window. The hot wall RTA resulted in significantly (>20%) lower Rs, over a very wide process window, compared to the equivalent RTA process using conventional tungsten halogen lamp-based (cold wall) RTA systems. Dopant (P) depth profiling results by secondary ion mass spectroscopy (SIMS) revealed that the P atoms in the CoSi2 film and at the CoSi2/P-doped poly-Si interface, redistribute very differently under different RTA conditions. The CoSi2 formation process was optimized utilizing characteristics of P pile-up near the CoSi2/P-doped poly-Si interface to suppress P depletion from the P-doped poly-Si layer for contact resistance minimization.
Reference: Jin Yul Lee, et al., Towards Contact Resistance Minimization through CoSi2 Formation Process Optimization on P-Doped Poly-Si by Hot Wall-Based Rapid Thermal Annealing, ECS Journal of Solid State Science and Technology, 4 (7) P220-P225 (2015).

Figure 1: (a) ~ (c) Schematic illustrations of cross-sections of TiN/Ti/Co/P-doped poly-Si/SiO2/Si wafers along process sequences. (d) Temperature and time dependence of sheet resistance of TiN/Ti/Co/P-doped poly-Si/SiO2/Si wafers annealed in a hot wall RTA system, before and after TiN capping layer and unreacted Co layer etching.