Thin Film Deposition Processes Usually source materials are transported to a substrate in the gas phase Exceptions: LPE Source Material Tsource Vapor Phase Mass transport to substrate Substrate Tsubstrate < Tsource Condensation, Reaction

Thin Film Deposition Technology PVD CVD Source material enters the vapor phase by physical means (e.g., evaporation, sputtering) No chemical reactions Source material is supplied in the form of a precursor gas Chemical reactions

(Thermal Evaporation) PVD PVD Resistance-Heated (Thermal Evaporation) Sputtering E-beam Evaporation

Thermal Evaporation Tungsten Filament Sheet/Boat Sources Crucible Molten beads of metal are retained by surface tension forces on W wires Sheet/Boat Sources W, Ta, or Mo form boats that contain the evaporant Crucible Sources Oxide, pyrolytic boron nitride (PBN), or refractory metal containers Heated by W wire wrapped around the crucible from Mahan, Fig. I.1, p. 2

Thermal Evaporators from Ohring, Fig. 3-11, p.97

Thermal Evaporators Typical bell jar evaporator From http://www.tedpella.com/cressing_html/crs308.html

Thermal Evaporators Requirements : Heater, crucible, etc. must not contaminate, react, or alloy with the source material Heater, crucible, etc. must not vaporize Typical materials: graphite, Ta, Mo, W, SiO2, Al2O3, BeO, ZrO2, pyrolyric boron nitride (BN)

Thermal Evaporators Typically need Tsource ~ Tmelt for most metals to achieve reasonable VP for deposition (10-4 to 10-2 Torr) from Mahan, Fig. V.14, p. 143

Thermal Evaporators from Ohring, Table 3-3, p. 102

Thermal Evaporators from Ohring, Table 3-3, p. 103

Effusion cell = Knudsen cell (1909) A special type of crucible source Effusion Cells Effusion cell = Knudsen cell (1909) A special type of crucible source charge from Panish & Temkin, Fig. 3.2, p. 58

Resistively heated using Ta or Mo wire Effusion Cells Resistively heated using Ta or Mo wire Uses inert containers; e.g., pyrolytic BN from Mahan, Fig. I.2, p. 2

Effusion Cells Used as solid sources in molecular beam epitaxy (In, Ga, Al, Sb, Si, Be) from Ohring, Fig. 7-21(a), p. 338

Effusion Cells Effusion: the process by which molecular flow occurs from a region of high vapor pressure toward a region of low vapor pressure Effusion Cell Molecular flow P1 P2 < P1

Effusion Cells P1 P2 < P1 No collisions Dss Effusion Cell Fluid Flow Molecular (MFP) >> Dss (no collisions in the gas) Viscous << Dss No collisions Dss Effusion Cell Molecular flow (molecular beam) P1 P2 < P1

Molecular Flow vav radius, r density, n · Collision rate, R = # collisions per unit time R ~ (volume per unit time swept out by particle) x (gas density) = (particle cross-section) x (average particle speed) x (gas density) = pr 2 vav n vav radius, r density, n

Mean Free Path Collision rate, R ~ vav Pc vav = average particle speed = distance traveled per unit time Pc = probability of a collision per unit distance traveled Therefore, Pc = R/vav = pr2 n

· Mean free path of gas particles before a collision, l: l = 1 / Pc   l = 1 / Pc Pc = probability of a collision per unit distance traveled l = 1 / Pc = (pr2 n)-1

Mean Free Path Ideal gas law: n = P / kT gives : l = (kT / pr2) ~ 5 x 10-3 / P for ambient air (N2) with [l] = cm, [P] = Torr e.g., at 760 Torr, l = 650 Å P

molecular flow: Dss < 5 x 10-3 / P Mean Free Path molecular flow: Dss < 5 x 10-3 / P from Ohring, Fig. 2-3, p. 56

Mean Free Path · For a source-to-substrate distance >10 cm, molecular flow requires chamber pressure < 10-3 Torr from Ohring, Fig. 2-3, p. 56