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Page 1 1 Mechanical TEM Sample Preparation Pablo Mendoza Jessica Enos Allied High Tech Products, Inc. Page 2 2 Overview • Basics – What is a TEM sample? – What are the requirements? – What are the methods of preparing TEM samples? • Thin Film Preparation Process Page 3 3 What is a TEM Sample? • TEM – Transmission Electron Microscopy is a microscopy technique in which a beam of electrons is transmitted though a specimen to form an image. • TEM Sample – A specimen that is usually ultra thin (<100 nm) so that electrons can be transmitted through it. Zeiss HRTEM [3] Page 4 4 TEM Sample Requirements • Must be electron transparent • Must have a submicron thickness • Must have an area of ≤3 mm to fit various TEM grids and holders • Any deformation from previous processing must be removed. Two-specimen holder with double-tilt [3] Specimen support grids of different mesh sizes and shapes [3] Page 5 5 Common Methods of Preparation • Ion Milling/FIB • Electropolishing • Mechanical Preparation –Thin Film Preparation Page 6 6 Ion Milling/FIB • Bombarding a TEM sample with energetic ions or neutral atoms and sputtering material from the sample until it is thin enough to study in the TEM [3] • A versatile thinning process that can be used for a wide variety of materials • Is expensive to purchase and run • Implantation of source material and an amorphous layer is created Schematic of a two-beam FIB and stages in making TEM samples using a FIB [3] Page 7 7 Electropolishing • Immersing a sample in an electrolyte and subjecting it to a direct electrical current – Keep the sample anodic with a cathodic connection to a nearby metal conductor [2]. – The anodic dissolution of the sample polishes the surface. • Relatively quick and can produce samples with no mechanical damage • Can only be used for electrically conductive metals and alloys The Window Method [3] Page 8 8 • Smoothing the sample surface using abrasives and mechanical tools • Common types: – Manual: Uses hand tools (tripods) that allow the user to make angle adjustments – Semi-Automatic: Uses precision polishers with digital indicators and micrometers – Dimpling: Mechanical dimplers use a small-radius tool to grind and polish samples to a fixed radius of curvature in the center. Mechanical Preparation Page 9 9 • Advantages – TEM Wedge Capabilities – All damage removed using proper material removal procedures – No foreign material implanted – Repeatable and fast with experience – Lower cost of required equipment – Decreases milling time and costs • Disadvantages – Requires careful handling throughout the process – Greater learning curve than other techniques – Some samples may require additional processing using one of the other TEM preparation techniques Mechanical Preparation Page 10 10 Thin Film Preparation Process – Preparing the Sample – Fixturing – Grinding the Pyrex® – Thinning the First Side • Thin film samples are commonly prepared using mechanical methods. The process includes: – Flipping the Sample – Stopping Point – Inducing a Wedge – Color and Fringes Page 11 11 • Wafers are scribed down to a manageable size. • Using an appropriate adhesive, such as M-Bond 610, the thin film surfaces are “sandwiched” together. Preparing the Sample Page 12 12 • The surface of the Pyrex® piece on a thinning fixture requires grinding to ensure it is parallel to the platen, or the grinding plane. • Pyrex® can be ground using diamond lapping films. – A 15-9 µm finish is acceptable. – Do not polish the surface; having some scratches will help with adhesion. Grinding the Pyrex® Page 13 13 • Secure the sample on a thinning fixture with mounting wax. • Place the fixture on a hot plate to melt the wax. • Apply light pressure using a cotton-tipped applicator to assist in parallel registration of the sample to the fixture. Fixturing Page 14 14 • Abrasive: Diamond Lapping Films – Provide excellent edge retention and maintain coplanarity – Typically used for unencapsulated cross-sectioning, TEM preparation, backside polishing, etc. Thinning the First Side Page 15 15 • The sample surface is damaged throughout the grinding process. – Scratch patterns – Micro-cracks that can propagate further into the sample Sample Damage (micro-cracks) 30 µm DLF 15 µm DLF AOI 9 µm DLF 30 µm DLF Thinning the First Side Page 16 16 • Remove 3 times the previous abrasive to completely remove any damage. – Ex: If a 30 µm lapping film is followed by a 15 µm lapping film, the 15 µm film must remove at least 90 µm (3 x 30 µm) to completely remove any damage. Current Step (DLF) Current Distance from Target Previous Step Remove At Least 3x Prev. Step Size Distance to Target After Current Step 30 µm Varies N/A N/A 192 µm 15 µm 192 µm 30 µm 90 µm 102 µm 9 µm 102 µm 15 µm 45 µm 57 µm 6 µm 57 µm 9 µm 27 µm 30 µm 3 µm 30 µm 6 µm 18 µm 12 µm 1 µm 12 µm 3 µm 9 µm 3µm 0.5 µm 3 µm 1 µm 3 µm At Target 3X Rule Page 17 17 • Certain materials, such as ductile steels, may only require a “2X Rule.” • Fragile, brittle materials, such as ceramics, may require a “4X Rule” since cracks can propagate further into the sample. • Main Concept: No matter what steps follow, all damage introduced by the previous abrasive must be removed to obtain an accurate representation of the microstructure. 3X Rule Page 18 18 • Reheat the fixture on a hot plate. • Carefully remove and clean the sample, and then place it back on the fixture with the polished side face down. • Thin the sample according to the 3X rule. Flipping the Sample Page 19 19 Pre-FIB Thinning • Remove the sample to prepare for FIB thinning by placing the paddle into a piece of filter paper, and then into a container of acetone to remove the wax. Thinned to electron transparency Thick enough for handling TEM Wedge • Induce an angle to create a wedge sample. • The degree of the angle depends on the material being prepared. Goal: Pre-FIB or TEM Wedge? Page 20 20 0.02° • Micrometers on precision polishers are used to induce an angle on the sample and create a wedge. Parallel contact plane Inducing a Wedge Thinned to electron transparency Thick enough for handling Page 21 21 • Bulk – Homogeneous material – Can stop anywhere; only a useable area is needed • Thin Film – Film on material deposited on surface – Prepared properly, can stop anywhere Areas of Interest When is the Sample Complete? Page 22 22 • Certain TEM samples can display a series of colors in regions <10 µm thick with a transmitted light microscope. • Fringes can also occur in regions <2 µm thick. • Colors that correspond with different thicknesses vary based on materials; however, some are well documented, such as silicon. Color and Fringes Page 23 23 • Color and fringe information can be used to assist in the preparation of TEM samples, as they are a guide to overall progress during a TEM wedge procedure. Correspondence between Si wafer thickness and color of transmitted light [4] Color and Fringes Page 24 24 Page 25 25 Page 26 26 References • 1. Allied High Tech Products, Inc., http://www.alliedhightech.com/. Accessed 16 September 2019. • 2. Delstar Metal Finishing, Inc., Electropolishing A User’s Guide to Applications, Quality Standards and Specifications, https://www.delstar.com/assets/pdf/epusersguide.pdf. Accessed 23 May 2018. • 3. Williams, David B, and C B. Carter. Transmission Electron Microscopy: A Textbook for Materials Science. New York: Plenum Press, 1996. Print. • 4. Yougui Liao. Practical Electron Microscopy and Database. URL: http://www.globalsino.com/EM/page2805.html. GlobalSino 2007. Page 27 27 Questions Pablo Mendoza Laboratory Supervisor, Technical Services Allied High Tech Products, Inc. pmendoza@alliedhightech.com Jessica Enos Sr Materials Engineer, Technical Services Allied High Tech Products, Inc. jmenos@alliedhightech.com Technical Services Allied High Tech Products, Inc. (800) 675-1118 (North America) (310) 635-2466 (Worldwide) lab@alliedhightech.com