## Spin-on doping ### A brief introduction Spin-on doping is a low-cost, easy and robust diffusion doping method when accurate doping profile, controlled doping region and low junction depth is not the primary concern. Otherwise, ion-implantation, monolayer doping or in-situ doping by LPCVD/MOCVD, etc. The chemicals of commercially available spin-on doping agents are mostly a mixture of silicon oxide polymer (like TEOS) and dopant chemicals with phosphorus or boron or any required dopant species. Typical procedure includes depositing the SOD solution onto the substrates, spin coating, then heat to remove the solvent, solidify the spin-on glass. Following by a furnace annealing or rapid thermal annealing (RTA), the dopant atoms will diffuse from the spin-on glass into the substrates and a doped region is achieved. Since it is a diffusion process, the doping concentrations is highly dependent on precursor concentrations, temperatures and annealing time, as well as the substrate type, material, and crystallinity. ### Spin-on doping applied chemicals and related papers | Years | Title | SOD solution | | ----- | ---------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------- | | 1982 | Diffusion in Silicon from a Spin‐On Heavily Phosphorus‐Doped Oxide Source | Ethyl alcohol, ethyl acetate, TEOS, water, vinyl trichloro silane (US patent; liquid silica source with 3–10% water and 1–8% vinyl trichloro silane) | | 1985 | Phosphorus Diffusion Using Spin‐On Phosphosilicate‐Glass Source and Halogen Lamps | OCD (spin-on glass, Tokyo Ohka Kogyo Company, Limited) + P₂O₅ | | 1992 | Shallow-junction formation on silicon by rapid thermal diffusion of impurities from a spin-on source | P: 1 or 5 g of P₂O₅ dissolved in ethanol and tetrahydroxysilane; B: B₂O₃ in ethoxyethyl ether | | 1994 | Rapid thermal annealing of thin doped and undoped spin-on glass films | Filmtronics P509 and P507 | | 1994 | Silicon doping from phosphorus spin‐on dopant sources in proximity rapid thermal diffusion | 5 wt% phosphosilicate-type polymer in alcohol/ester solvents (Allied Signals, Inc.) | | 1995 | Kinetics of phosphorus proximity rapid thermal diffusion using spin‐on dopant source for shallow junctions fabrication | Alcohol/ester solution of phosphosilicate polymer (Allied Signal Inc.) containing ~3 wt% P₂O₅ | | 1996 | Influence of baking conditions of doped spin-on glass sources on the formation of laser assisted selective emitters | Filmtronics P509 | | 1997 | Rapid thermal annealing of spin-coated phosphoric acid films for shallow junction formation | Laboratory-grade H₃PO₄ (61% P₂O₅ in aqueous solution) | | 1998 | Phosphorus diffusion from a spin-on doped glass (SOD) source during rapid thermal annealing | Filmtronics P505, P506, P507, P509 | | 2003 | Low cost fabrication for high efficiency monocrystalline silicon solar cells | Filmtronics P509 | | 2016 | [[Study of the spin on dopant technique as alternative for the fabrication of c-Si solar cells]] | Filmtronics P509 | | 2017 | [[A comparative study of the emitter formation of a c-si solar cell using gas and spin on dopant sources]] | Filmtronics P509 | | 2020 | [[Engineering of the spin on dopant process on silicon on insulator substrate]] | Filmtronics P508 | | 2020 | Phosphorus-doped polycrystalline silicon passivating contacts via spin-on doping | P-250 (Desert Silicon), [P] ≈ 5 × 10²¹ cm⁻³ | Some tendencies can be noticed from the change of chemical applied in the SOD process. - **1980s–early 1990s**: Self-mixed phosphoric or phosphosilicate solutions were used, often with limited process control. - **Mid-1990s**: Commercial SOD products like Allied Signal and Filmtronics P505–P509 began to replace lab-made formulations. - **Late 1990s–2010s**: Filmtronics P509 became the dominant standard for consistent, high-concentration phosphorus spin-on doping. - **2020s**: Newer high-concentration solutions (e.g., Desert Silicon P-250) support advanced device applications such as passivated contacts. - **Overall trend**: A shift from exploratory chemistry to industrial-grade, standardized doping solutions with high reproducibility. *Note: Only papers explicitly focusing on doping strategies are included here. Some studies may have applied self-made SOD solutions but primarily emphasized other aspects of device fabrication.* ### A simple diffusion model