ECE 6450 - Dr. Alan DoolittleGeorgia TechLecture 13 and 14Thin Film Deposition and Epitaxy (Chemical Vapor Deposition, Metal Organic CVD and Molecular Beam Epitaxy)Reading:Chapters 13 and 14ECE 6450 - Dr. Alan DoolittleGeorgia TechChemical Vapor DepositionChemical gas sources are thermally, optically, or electrically (plasma) reacted with a surface to “leave” behind deposits with reaction byproducts pumped out of the reaction tube or vacuum chamber.ECE 6450 - Dr. Alan DoolittleGeorgia Tech1.) Atmospheric Pressure CVD (APCVD)Advantages: High deposition rates, simple, high throughputDisadvantages: Poor uniformity, purity is less than LPCVDUsed mainly for thick oxides.2.) Low Pressure CVD (LPCVD at ~0.2 to 20 torr)Advantages: Excellent uniformity, purityDisadvantages: Lower (but reasonable) deposition rates than APCVDUsed for polysilicon deposition, dielectric layer deposition, and doped dielectric deposition.3.) Metal Organic CVD (MOCVD)Advantages.: Highly flexible—> can deposit semiconductors, metals, dielectricsDisadvantages: HIGHLY TOXIC!, Very expensive source material. Environmental disposal costs are high.Uses: Dominates low cost optical (but not electronic) III-V technology, some metalization processes (W plugs and Cu)4.) Plasma Enhance CVDPlasmas are used to force reactions that would not be possible at low temperature.Advantages.: Uses low temperatures necessary for back end processing. Disadvantages: Plasma damage typically results.Used for dielectrics coatings.Four Basic CVD ReactorsECE 6450 - Dr. Alan DoolittleGeorgia TechLow Pressure Chemical Vapor Deposition (LPCVD) can be used for a variety of materials:•Polysilicon for gate contacts•Thick oxides used for isolation between metal interconnects•Doped oxides useful for global planarization•Nitrides and other dielectrics for isolation or capacitors (higher K materials for larger capacitance)•Metals for seed layers for vias and interconnect lines (not typically used for the entire metal line due to slow deposition rate)PolysiliconUses: Gate contact in MOS (prevents metal/oxide reactions) and very short interconnect lines. Also, resistors in analog technologies.Typically uses Si containing compounds (typically 100% silane, SiH4, or 20-30% silane/ 80-70% inert gas) are reacted with the wafer at ~0.2 to 1 torr and ~575-650.The temperature range is limited by:•Limited by low deposition rate at low temperature end (insufficient thermal energy for the reaction)•Formation of particles in the gas phase (gas spontaneously reacting before it reaches the wafer) and poor adhesion on the upper temperature endLPCVD for Si TechnologyECE 6450 - Dr. Alan DoolittleGeorgia TechLPCVD for Si TechnologyDeposition rate is limited by reaction rate (controlled by temperature and pressure) and arrival rate (controlled by pressure (remember gas throughput is related to pressure by Q=CP)).ECE 6450 - Dr. Alan DoolittleGeorgia TechCrystalline Structure is also controlled by temperature.Poly-Si can be doped using Diborane (B2H6), arsine (AsH3) or phosphine (PH3), diffusion (lowest resistivity) or by implantation (highest resistivity). The resistivity can vary from ~10-3to 10+5Ω-cm. Doped poly-Si makes good short interconnect linesLPCVD for Si TechnologyECE 6450 - Dr. Alan DoolittleGeorgia TechLPCVD of OxidesUses:Undoped: Insulator between multilevel metalization, implantation or diffusion mask, increase thermal oxide thickness for high voltage devices.Doped: P-doped is used as a multilevel metalization insulator, final passivation layer (prevents ionic diffusion), or a gettering source.Undoped Oxide Deposition Methods:Silane SiH4+ O2—> SiO2+ 2H2< 500 °C (contain H2O, SiH, and SiOH impurities)DCS (Dichlorosilane) SiCl2H2+ 2N2O —> SiO2+ 2N2+ 2HCL (etches) ~900 °C (contains Cl)TEOS (tetraethoxysilane) Si(OC2H5)4—> SiO2+ many byproducts 650-750 °CTEOS + Ozone (O3) Ozone is more reactive and lowers deposition temperatures to ~400 °CECE 6450 - Dr. Alan DoolittleGeorgia TechDoped Oxide Deposition Methods:PSG - Phosphorosilicate Glass4PH3+ 5O2----> 2P2O5+ 6H2~950-1100 °C for flowed glass and <400 for passivationBPSG - Borophosphorosilicate GlassPH3+ B2H6+ O2—> Complex BXPYOZ~850-950 °C , Flows better than PSG, but can absorb moistureDoped Oxides (glasses) can be made to “flow” or smooth out. This is particularly useful for smooth interconnects (prevents sharp edges which tend to break metal lines) or for partial global planarization for subsequent lithography steps.LPCVD of Doped OxidesECE 6450 - Dr. Alan DoolittleGeorgia TechLPCVD of Silicon NitrideSilane 3SiH4+ 4NH3—> Si3N4+ 12H2~700-900 °CDCS (Dichlorosilane) 3SiCl2H2+ 4NH3—> Si3H4+ 6N2+ 6HCL (etches) ~700-800 °CContains up to 8% hydrogenLPCVD of NitridesSilicon Nitride is used for encapsulation (sealing up the circuit to prevent contamination from moisture, plastics used in packaging, or air, etc…). It is sometimes used for a dielectric isolation layer and rarely used as a gate dielectric. Oxide/nitride mixtures known as oxynitrides are sometimes used in FLASH memories.ECE 6450 - Dr. Alan DoolittleGeorgia TechAlternative CVD ChemistriesECE 6450 - Dr. Alan DoolittleGeorgia TechEpitaxyECE 6450 - Dr. Alan DoolittleGeorgia TechCompound Semiconductors allow us to perform “Bandgap Engineering” by changing the energy bandgap as a function of position. This allows the electrons to see “engineered potentials” that “guide” electrons/holes in specific directions or even “trap” them in specific regions of devices designed by the electrical engineer.Example: Consider the simplified band diagram of a GaN/ Ga0.75In0.25N/ GaN LED structure. Electrons and holes can be “localized” (trapped) in a very small region – enhancing the chance they will interact (recombine). This is great for light emitters!Classifications of Electronic MaterialsEconductionEvalenceLightECE 6450 - Dr. Alan DoolittleGeorgia TechCompound Semiconductors allow us to perform “Bandgap Engineering” by changing the energy bandgap as a function of position. This allows the electrons to see “engineered potentials” that “guide” electrons/holes in specific directions or even “trap” them in specific regions of devices designed by the electrical engineer.Example: Consider the band Diagram of a GaAs MODFET. Electrons in the “transistor channel” can be confined in a very thin (50-100 Angstroms) sheet known as a 2 dimensional electron gas (2DEG).