Physical Vapor Deposition Evaporation Reading Chapter 12 Atomic layer deposition by who and when Deposition Simulation Other Does this seminar help your research Physical Vapor Deposition Physical methods produce the atoms that deposit on the substrate Evaporation Thermal Evaporation E beam Evaporation Sputtering Sometimes called vacuum deposition because the process is usually done in an evacuated chamber Evaporation Evaporation is usually used to deposit metal layers Evaporation is difficult to produce well controlled alloys Photos of source material for evaporation Types of evaporation according to heating method Three types crucible heating systems Thermal evaporator resistive heating Electron beam evaporator heated by electron beam most popular more expensive than thermal evaporator Inductive heating must be unpopular I have never seen one Evaporator Systems Thermal evaporation A resistively heated system is the simplest type of The charge is contained in a crucible that is heated source resistively The filament wire heat material to be evaporated Widespread use for materials whose vapor pressure can be reasonable at 1600oC or below Common evaporant materials Au Ag Al Ni Cu Sn Cr Sb Ge In Mg Ga CdS NaCl KCl AgCl MgF2 Evaporator Systems Thermal evaporation Thermal evaporation In resistively heated system the filament wire must be at least as hot as the material to be evaporated When a material such as aluminum is to be deposited adequate vapor pressures can be obtained with only moderate power input When a refractory metal must be deposited there is often no suitable resistive heating element Solution use electron beam evaporator One of the problems with resistively heated crucibles is evaporation and outgassing from the wire Inductive heating System One way to achieve at least moderate charge temperatures is the use of inductively heated crucibles A solid charge is placed in a crucible typically made of boron nitride BN A metal element is wound around the crucible and RF power is run through the coil The RF induces eddy currents in the charge causing it to heat The coil itself can be water cooled to keep its temperature below 100oC effectively eliminating any loss of material from the coil Inductive heating System is not common used Inductive heating can be used to raise the crucible temperature high enough to evaporate refractory materials But contamination of the charge from the crucible itself remains a serious problem Evaporator Systems Electron beam evaporation Electron beam e beam evaporation can avoid contamination of the charge from the crucible by heating only the charge and cooling the crucible Using a focused electron beam to heat and evaporate metals Electron temperature can be as high as 10 000 K Electrons are accelerated by DC 10kV and current 10s 100s of mA An electron gun under the crucible ejects an intense high energy beam A strong magnetic field bends the beam causing it to be incident on the surface of the charge The beam can be rastered across the charge to melt a significant fraction of the surface The hot portion of the charge is then effectively self contained by the cooler portion of the charge Evaporation occurs at a highly localized point near the beam bombardment spot on the source surface Top surface of metal is melted during evaporation so little contamination from the crucible not hot water cooled Suitable for refractory high Tmelt metals like W Ta Photos of e beam evaporator Mechanical shutter Evaporation rate is set by temperature of source but this cannot be turned on and off rapidly Cooling water Shutter Put crucible here Power density 10kV up to 1 5A 0 2 1cm2 15 75kW cm2 13 Evaporator Systems Electron beam evaporation Drawback When thermionic emission electron guns are used the hot electron filaments remain a source of contamination in the chamber Special care must be taken in e gun design when these systems are operated at very high vacuums A more serious concern particularly for silicon based technologies is radiation damage The radiation is due to highly excited electrons in the material being evaporated decaying back to ground levels Since x rays will damage the substrate and the dielectrics leads to trapped charge e beam evaporators cannot be used in Si MOS or other technologies that are sensitive to this type of damage because silicon bipolar technologies are susceptible to this type of damage correction later thermal annealing steps will be adequate to remove the damage But E beam evaporators are commonly used in GaAs technologies Because GaAs is very stable materials which can resist radiation damage Electron Beam Evaporation Electron Beam source Second Step Transport Evaporation The wafers are loaded into a high vacuum chamber that is commonly pumped with either a diffusion pump or a cryopump Material to be deposited charge is loaded into a heated container called the crucible In evaporation source material is heated in high vacuum chamber P 10 5 Torr hence the name vacuum deposition High vacuum is to minimize contamination and collisions of source atoms with background species Heating is done by resistive or e beam sources As the material in the crucible becomes hot the charge gives off a vapor Since the vacuum pressure in the vauum chamber is less than10 5 Torr the atoms of the vapor travel across the chamber in a straight line until they strike a surface where they accumulate as a film Evaporation Evaporation systems can contain up to 24 wafers suspended in a frame above the crucibles To deposit multiple layers of different materials Evaporation systems can contain up to four crucibles to allow the deposition of multiple layers without breaking vacuum To help start and stop the deposition abruptly mechanical shutters are used in front of the crucibles Evaporation is not widely used by industry sputter deposition For microfabrication R D evaporation is as important as sputter deposition Evaporation Vapor Pressure Figure shows the equilibrium vapor pressure as a function of temperature for a variety of elements As the temperature of the sample is raised the material typically goes through the solid liquid and gas phases At every temperature there exists an equilibrium pressure Pe of vapor above the material Evaporation with high vapor pressures produces acceptable deposition rates To obtain reasonable deposition rates the sample vapor pressure must be at least 10 mtorr Evaporation Vapor Pressure To obtain reasonable deposition