Physical Vapor Deposition EVAPORATION SPUTTERING Typically used for metallization of semiconductors. Both Evaporation & Sputtering are done in vacuum environments. Typically: y Evaporation Pressures are < 10-6 Torr (1Torr = 133.32 Pa) Sputtering Pressures are ~ 10-3 Torr. Optimization of Physical Vapor Deposition process includes: Film Quality Film Uniformity (Thickness) Film Stress & Adhesion Film Stoichiometry (For Multi-component Films) Film Step Coverage (Conformality) Film Deposition Rates
Why Metallization Quality is Important? Metallization is used for Gates & Interconnection in IC s s. It can easily affect the speed of a circuit by virtue of the RC-Time constant of the signal line. EXAMPLE: For a 1cm long Polysilicon interconnection runner on 1μm thick SiO 2 (ε ox /ε 0 = 3.9), Polysilicon thickness of 5000Å and resistivity it ρ of 1000 μω-cm. 2 L W. L ρ ε RC = R = oxl s ε ox W d d d Since ε 0 = 8.86 x 10-12 F/m 5 1x10 RC = 10 5x10 = 710 7x10 8 s - = ox 3.9x8.86x10 6 1x10 70 ns poly 12 ox ( 2 10 ) Reduction in R s is essential for high-speed circuits. 2
Vacuum & Vacuum Pumps Rough hvacuum: 01T 0.1 Torr 760 Torr Rotary Vane Pump Diaphragm Pump Medium Vacuum: 10-4 Torr 10-1 Torr Diffusion Pump High Vacuum: 10-8 Torr 10-4 Torr Turbomolecular Pump Cryopump Ultrahigh Vacuum: < 10-8 Torr This division is based upon the pumps and technology required to attain this vacuum. Terminology: The mass flow rate of a gas is given by: dg d = = ( ρv ) ρ = mass density, V=Volume q m dt The throughput of a gas Q is given by: dt P Q = qm And has the units of ρ Pressure - Volume Time
Vacuum Basics Gas flows are measured typically in standard volume i.e., volume that an equivalent amount of gas will occupy @ 273K and 1 atmosphere pressure. 1 Standard Liter is the amount of gas that would occupy 1 liter @ 1 atm. and 273K. Since 1 mole of a gas occupies 22.4 liters at standard conditions, 1 Standard liter = (22.4) -1 moles 1 Standard liter/minute = a throughput of 760 (Torr-liter)/min. The conductance C of a vacuum component: Q Q C = = P 1 P 2 Δ P P 1 P 2
Vacuum Basics Conductance in parallel simply add i.e., Chamber C = C 1 + C 2 + C 3 + Conductance in Series add as inverse: C 1 C 2 C 3 1 1 1 1 = + + +... C Series C1 C2 C3 For a tube of diameter D and dlengthll in viscous flow regime (<1mTorr) 4 5 D C = 1.8x10 P av (Volume/second) L P av is the average pressure of P 1 & P 2 in Torr. Pumps are usually specified in terms of pumping speeds S p as: S Q P p = = p where P p is the inlet pump pressure. A pump of 1000 slm pumps 1000 slm at an inlet pressure of 1 atmosphere. dv dt p
Vacuum System Pressure Gage Roughing Line Rough Pump Vacuum Chamber Vent Baffle Valve High Vacuum Liquid N 2 Pump Trap Backing Line q 2 A vacuum station of volume V with no leaks, the chamber pressure P at any time t after pump down has been initiated is given by: S pt P P exp 0 + V Q Outgassing Q outgassing is the outgassing rate within the chamber After ~ 1 hour of pumping the second term dominates S p
Load Locked Vacuum System Pressure Gage Main Vacuum Chamber Sample Load Lock Chamber Sample Transfer Arm Baffle Valve Roughing Line High Vacuum Pump High Vacuum Pump Liquid N 2 Trap Rough Pump Backing Line Rough Pump Backing Line
Rotary Vane Pump Rough vacuum pumps involve the positive displacement of gas through the mechanical movement of a piston, vane, plunger etc. The process involves: Capture of a volume of gas Compression of the captured volume Gas expulsion For an ideal gas the pressure differential is just the ratio of the fully expanded to fully compressed volumes. For an exhaust pressure of 1 atmosphere, and a compression ratio of 100:1 the lowest pressure that can be achieved is 0.01atm=7.6Torr.
Diffusion Pump Turbomolecular Pump High Vacuum Pumps Both these pumps work by transferring momentum to gas molecules. Oil is heated at the bottom and the vapor rises through hthe center stack and is ejected through vents at very high speeds. They then strike cooled walls of the pump and condense. Compression ratio of 10 8 is achieved. Turbomolecular pump blades rotate at ~20,000 rpm. The stator and rotor are spaced by ~1mm. Compression ratios of 10 9 can be achieved due to the many stages. In both pumps, mass of molecules play a very important role in determining the compression ratio
Pumping Speed of Turbomolecular Pumps Pumping Speed is a function of the gas being pumped: For example for Alcatel ATP 80, the nominal pumping speeds are: Source: Alcatel