Transcript ch10-1x
Specimen Preparation T R A NSMIS SION E LE CT RON MI CR OS COPY Introduction • Broad subject with several techniques • Preparation technique mustn't affect measurement • Preparation times range from 5 minutes to 2 days • Specimen must be electron transparent Safety • Liquids used for polishing solutions can be poisonous, corrosive, or explosive • For example, extremely poisonous hydrofluoric acid (HF) • Important to be up-to-date with laboratory safety before preparing the specimen Before preparing the specimen • Different samples will require different preparation paths • Specimens either selfsupported or on a grid • Mechanical stability is crucial • Different sample geometries Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY) Self-supporting disks and grids • Grid may contribute to the X-ray spectrum • The best result is achieved with thin specimens • Nominal disk diameter is 3.05 mm • Specimen should be located at the center of a disk Pre-thinning • Initial thinning involves a slice of 100 and 200 mm thick • Cutting the 3-mm disk from the slice • Pre-thinning the disk to a few micrometers Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY) Final thinning • Electropolishing of electrically conducting samples • Specimen is bombarded and sputtered in Ion milling • Ion beam will penetrate the specimen • Ion implantion will occur Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY) Cross-section specimens (1/2) • Preparation technique in order to study interfaces • Special type of self-supporting disk • Interface is parallel to the electron beam • To study structural and chemical variations close to an interface Cross-section specimens (2/2) • Sample can be cut and glued together • “Sandwich” is sectioned to see the layers • Low-temperature epoxies to be used in gluing • Glued sections can be cut into 3 mm rods Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY) Electropolishing • Used for a thin sheet of metal • Sheet is cut into approx. 10 mm square and edges are sealed • Exposed ‘window’ is immersed in electrolyte • Surrounded by a cathode and voltage is applied • Viscous layer will build up on the surface • The sheet will thin in the center Ultramicrotomy • Slices less than 100 nm thick • Used for biological samples and polymers • Recently used also for crystalline materials • Leaves chemistry of the sample unchanged • Fractures and/or deforms the sample Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY) Grinding and Crushing • Brittle materials are easily prepared by crushing • Crushing done in a clean pestle and mortar in an inter liquid • Liquid can be ultrasonically stirred • Drop of the liquid placed on a holey carbon film on a grid • Liquid will evaporate leaving particles on the support film Replication and extraction • Direct replication used to study fracture surfaces or surface topography • Carbon film is evaporated on the surface of interest • Underlying film is etched away • Alternatively, soft plastic can be pressed on the surface • Plastic replicate is then pulled off and coated with carbon • Plastic is dissolved and carbon replicate is picked on a support grid Cleaving • Oldest technique • Adhesive tape is attached on both sides of the sample • Two pieces of tape is pulled apart • Special variation: small-angle cleaving technique (SACT) The 90 degree wedge • Used for compound semiconductors such as GaAs • Semiconductors that are grown with a (001) surface • Cleaved on planes that are perpendicular to growth surface Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY) Lithography and etching • Technique used in microelectronics • Preferential area is masked • Other parts of the substrate is chemically etched Focused-ion beam (FIB) • Prices are going down, becoming more readily available • FIB an SEM with a built-in ion mill • Well-controlled beam of Ga ions • Polish the specimen thin for TEM Specimen storing • Periods up to 1 month: dry, inert atmosphere, and inert container • For long-term storing: no gelatin capsules for delicate materials, no slotted grid-holders for anything that might deform • Always use vacuum tweezers • Old specimens can be cleaned by ion polishing or chemical cleaning Conclusions • Most tedious aspect of TEM work • Thinner is always better • Quality of data directly proportional to quality of specimen • Need to find method that works best for your material • Safety is important Reference • Williams, D.B. and Carter, C.B., 2009, Transmission Electron Microscopy: A Textbook for Materials Science (2nd Ed, Springer, NY)