Center for Materials for Information Technology an NSF Materials Science and Engineering Center Thin Film Processing Gary Mankey MINT Center and Department of Physics

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Vacuum A vacuum is defined as less than 1 Atmosphere of pressure. 1 Atm = 10 5 Pa = 10 3 mbar = 760 Torr Below Torr, there are more gas molecules on the surface of the vessel then in the volume of the vessel. High Vacuum < Torr Very High Vacuum < Torr Ultra High Vacuum < Torr 760 mm Hg Vacuum ATM

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Why do we need a vacuum? Keep surfaces free of contaminants. Process films with low density of impurities. Maintain plasma discharge for sputtering sources. Large mean free path for electrons and molecules ( = 1 7 x mbar). Mean free path for air at 20 ºC: = 7 x cm / P(mbar)

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Vacuum Systems A vacuum system consists of chamber, pumps and gauges. Chambers are typically made of glass or stainless steel and sealed with elastomer or metal gaskets. Pumps include mechanical, turbomolecular, diffusion, ion, sublimation and cryogenic. Gauges include thermocouple for 1 to mbar and Bayard- Alpert for to mbar.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Alabama Deposition of Advanced Materials All materials are either glass, ceramics, stainless steel, copper and pure metals. A turbomolecular pump and a cryo pump create the vacuum. Sputtering sources are used for deposition. Characterization methods include RHEED, and Auger electron spectroscopy.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center 1 cm e- Bayard-Alpert or Ionization Gauge Electrons, e-, produced by the hot filament are accelerated through the grid acquiring sufficient energy to ionize neutral gas atoms, n. The ionized gas atoms, I+, are then attracted to the negatively, biased collector and their current is measured with an electrometer. Typical ion gauges have a sensitivity of 1-10 Amp / mbar and range of mbar. Electrometer +150 V -45 V 6 V AC e- n n n n n n n n I+ Filament Collector Grid

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Residual Gas Analysis A quarupole mass spectrometer analyzes the composition of gas in the vacuum system. The system must be “baked” at ºC for 24 hours to remove excess water vapor from the stainless steel walls. The presence of an O 2 peak at M/Q = 32 indicates an air leak. At UHV the gas composition is H 2, CH 4, H 2 O, CO and CO 2.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Monolayer Time We define the monolayer time as the time for one atomic layer of gas to adsorb on the surface:  = 1 / (SZA). At 3 x Torr, it takes about one second for a monolayer of gas 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 most gases at room temperature S<<1, so the monolayer time is much longer. 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 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

Center for Materials for Information Technology an NSF Materials Science and Engineering Center The Sputtering Process Electrons (e-) are localized in the plasma by a magnetic field. The e- collide with argon gas atoms to produce argon ions. The Ar+ are accelerated in an electric field such that they strike the target with sufficient energy to eject target atoms. The target atoms, being electrically neutral, pass through the plasma and condense on the substrate. NS S SNN       - +   Magnets Target Plasma Discharge Substrate 1 mTorr Ar

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, 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

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Auger / XPS An x-ray source produces photoelectrons or a electron gun produces Auger electrons. The electrons have kinetic energies which are characteristic of the material. The attenuation of substrate electrons by the film is described by Beer’s law: I = I 0 exp(-dcos  /  ). Since,   10 Å, this technique has a high sensitivity. Photoelectrons & Auger Electrons Excitation: X-rays or keV Electrons Electron Energy Analyzer

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Auger Electron Spectroscopy The excitation knocks a core electron out producing a core hole. To lower the energy of the ion, an electron from an upper shell decays nonradiatively into the core hole. The Auger electron from the upper shell acquires an energy equal to the energy difference of the core hole and upper shell. The kinetic energy of the electrons are measured to identify the chemical species of the atoms. E VAC Excitation Core Hole Upper Shell Kinetic Energy

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Secondary Electron Energy Distribution The energy distribution is characterized by –An elastic peak at the incident electron energy. –A low energy peak which increases as 1/E 2 and drops off rapidly below 10 eV. –Auger electrons, which can be measured to determine chemical composition.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Cu Auger Scan in Pulse Counting Mode These transitions correspond to Auger electrons ejected from the valence band by a neighboring electron filling the L shell or 2p levels.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Cu Auger Spectrum for Analog Mode In analog mode, the analyzer energy is modulated and a lock-in amplifier detects the derivative of the number of electrons or dN(e)/dE vs. E. The high energy peaks correspond to those on the previous slide.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center The Universal Curve for  The Universal Curve describes the dependence of electron mean free path, , on energy for most materials. In most cases, it is accurate to within a factor of two.  = (35 / E)  E with  in Å and E in eV.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Reflection High-Energy Electron Diffraction 15 keV electrons reflect from the surface and are displayed as a spot on a phosphor screen. The angle is adjusted such that electrons reflecting from adjacent layers interfere destructively. When only one layer is exposed, the spot is bright. When the top layer covers half of the surface, the spot is extinguished. The time between two maxima in the intensity plot is the monolayer time. d = atomic spacing 15 keV Electron Gun Screen  Path difference = 2dsin  = (n+1/2) = [150 / E(eV)] 1/2

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Epitaxial Growth Epi-Taxi (greek) epi meaning “on” taxi meaning “arrangement in relation to a source of stimulation” The crystal structure of the film has a direct relationship to that of the substrate Film Substrate

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Growth Modes for Ultrathin Films The growing film surface can exhibit different behaviors depending on substrate temperature, interfacial strain, and alloy miscibility. The growth modes must be characterized using a combination of chemical tools such as Auger electron spectroscopy and structural tools such as RHEED and atomic force microscopy. Stranski-Kastranov Diffusion Limited Surface AlloySurface Segregation Volmer-Weber Layer by Layer