Center for Materials for Information Technology an NSF Materials Science and Engineering Center Vacuum Evaporation Lecture 8 G.J. Mankey

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Monolayer Time The monolayer time is the time for one atomic layer to adsorb on the surface:  = 1 / (SZA). At 3 x Torr, it takes about one second for a monolayer to adsorb on a surface assuming a sticking coefficient, S = 1. At Torr, it takes 1 hour to form a monolayer for S = 1. For metals at room temperature S = 1, so the vapor pressure should be >10 -6 Torr. Impingement rate for air: Z = 3 x P(Torr) cm -2 s -1 Sticking Coefficient S = # adsorbed / # incident Area of an adsorption site: A  1 Å 2 = cm 2

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Vapor Pressure Curves The vapor pressures of most materials follow an Arrhenius equation behavior: P VAP = P 0 exp(-E A /kT). Most metals must be heated to temperatures well above 1000 K to achieve an appreciable vapor pressure. For P VAP = mbar, the deposition rate is approximately 10 Å / sec.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Physical Evaporation A current, I, is passed through the metal boat to heat it. The heating power is I 2 R, where R is the electrical resistance of the boat (typically a few ohms). For boats made of refractory metals (W, Mo, or Ta) temperatures exceeding 2000º C can be achieved. Materials which alloy with the boat material cannot be evaporated using this method. High Current Source Substrate Flux Boat Evaporant

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Limitation of Physical Evaporation Most transition metals, TM, form eutectics with refractory materials. The vapor pressure curves show that they must be heated to near their melting points. Once a eutectic is formed, the boat melts and the heating current is interrupted.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Electron Beam Evaporator B Substrate Flux Crucible e-beam e-gun The e-gun produces a beam of electrons with 15 keV kinetic energy and at a variable current of up to 100 mA. The electron beam is deflected 270º by a magnetic field, B. The heating power delivered to a small (~5mm) spot in the evaporant is 15 kV x 100 mA = 1.5 kW. The power is sufficient to heat most materials to over 1000 ºC. Heating power is adjusted by controlling the electron current. Evaporant cooling

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Wire Evaporator This is a "mini" version of the electron beam evaporator. The entire assembly fits through a 2 3/4 " OD Flange. Electrons from the heated filament bombard a 2 mm wire that is held at a large positive bias. The power supply is operated in a current limiting mode and the heating power is P = V bias I emission. 0-12V 1-2 kV cooling shroud filament substrate

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Wire Basket Direct or alternating current is passed through a pre-fabricated helical wire container. Evaporant placed in the helix is heated by contact and irradiation. Heating power is of the order of 100 W or more with a refractory helix with mm diameter wire. Works for Ag, Au, Cu, Cr, Mn, etc V cooling shroud

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Knudsen Cell The crucible is heated by a coil or heater surrounding it. Crucibles are usually made of boron nitride, alumina, or graphite. Since there is a large amount of heat, the device is constructed of low outgassing materials and a large amount of cooling is necessary V cooling shroud

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Measuring and Calibrating Flux Many fundamental physical properties are sensitive to film thickness. In situ probes which are implemented in the vacuum system include a quartz crystal microbalance, BA gauge, quadrupole mass spectrometer, Auger / XPS, and RHEED. Ex situ probes which measure film thickness outside the vacuum system include the stylus profilometer, spectroscopic ellipsometer, and x-ray diffractometer. Measuring film thickness with sub- angstrom precision is possible. ?

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Quartz Crystal Microbalance The microbalance measures a shift in resonant frequency of a vibrating quartz crystal with a precision of 1 part in f r = 1/2  sqrt(k/m)  f 0 (1-  m/2m). For a 6 MHz crystal disk, 1 cm in diameter this corresponds to a change in mass of several nanograms. d = m / (  A), so for a typical metal d  10 ng / (10 g/cm 3 *1 cm 2 ) = 0.1 Angstroms. Quartz Crystal Substrate Frequency Measurement Conversion to Thickness Display Flux