Center for Materials for Information Technology an NSF Materials Science and Engineering Center Substrate Preparation Techniques Lecture 7 G.J. Mankey

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Types of Substrates Insulating and Conducting Amorphous and Single Crystal Native Oxide or Passivated Cleaved or Cut Mechanical Polished or Electropolished

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Characterization of Substrates Optical Microscopy Atomic Force Microscopy Spectroscopic Ellipsometry Electron Diffraction Auger or X-ray Photoelectron Spectroscopy

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Surfaces and Solvents Never touch the substrate surface without first determining that it must be cleaned or polished before use. If a solvent wash is necessary verify the purity of the solvent before use. Do not allow the solvent to form droplets and dry up on the surface--they will redeposit dirt on the surface. Remember--the best solvents always have dirt dissolved in them, so use them sparingly.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Surface Polishing For most applications, a surface with a mirror finish is best. Mechanical polishing should be performed with progressively finer diamond or alumina polishing compound. The substrate should be thoroughly washed between steps to avoid contamination the polishing compounds. Leave the polishing area cleaner than when you found it.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Electrochemical Polishing Refer to Tegart, Electrochemical Polishing Techniques for the proper chemicals and voltages. Carefully monitor the surface quality with a microscope at small time intervals. Test the polarities and voltages by polishing a small amount of a test material first. Always label the electropolishing mixture with the chemical formula and amounts of materials.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Passivated Surface A layer of adsorbate material on the surface that limits further oxidation or corrosion is called the passivation layer. The layer can be amorphous as in the case of SiOx on Si or crystalline as in the case of H-Si. If there are defects in the passivation layer, it will be less resistant to corrosion and it will degrade with time. SiOx H-Si

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Special Techniques Irradiation with UV light for a day or two to "break up" hydrocarbon contaminants. CO 2 "snow" a jet of frozen particles gently bombard the surface to remove contaminants. Plasma etching can be used to roughen a surface in a controlled manner.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Single Crystal Surfaces For thin film deposition only the ordering of the topmost layers matters. A layer of adsorbed gas or oxide on a single crystal generally prevents epitaxial growth--there are few exceptions to this rule. Oxide layers on metals usually cannot be removed by annealing alone. On Si the oxide must be heated to above 800ºC to remove it. In situ processing, Ar bombardment and/or annealing, is usually required to make a good clean single crystal surface.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Ar Bombardment and Annealing Bombarding the surface with an energetic beam ( eV) of Noble gas ions removes contaminated surface layers. A typical current density of a few microamps per square centimeter will etch a few atomic layers per minute. Some material is redeposited, so the efficiency of removing contaminant layers is reduced. Cycles of bombardment and annealing to about 2/3 of the melting point produce a smooth, ordered surface.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Optical Microscopy Use the optical microscope to check for scratches, spots and dirt. Use polarized light with the analyzer adjusted to close to extinction to detect small particles and imperfections. Vary the color of the incident light with the filters to highlight different features.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Atomic Force Microscopy Characterize the substrate roughness before and after each process step. Tapping mode is generally the best method for this task. A good substrate should have a root mean square roughness near the limit of detection (1 nm). Take scans at different scan sizes to fully characterize roughness scale (0.1, 1, 10, 40 micrometer).

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Spectroscopic Ellipsometry For future determination of film thickness, you must fully characterize the substrate optical properties first. Take data in the entire wavelength range ( nm) to insure future applicability of material optical constants. Compare a few substrates produced with the same process to identify possible variations.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Electron Diffraction RHEED and LEED probe only the topmost layers. An electron diffraction pattern indicates the surface has a periodic atomic arrangement. The appearance of a diffraction pattern does not guarantee good crystallinity. Analyze the diffraction pattern according to spot to background intensity ratio, dependence of peak width on energy, and dependence of peak intensity on diffraction conditions.

Center for Materials for Information Technology an NSF Materials Science and Engineering Center Auger / XPS Characterize the chemical composition of surfaces. Always perform a high statistics scan of the energies for C, N, and O-- these are the main contaminants of most surfaces. Depth profiling using Ar bombardment can be used to identify the location of contaminants--surface or bulk?