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    Thứ sáu, 04/07/2014

    SEM Sample Preparation
    The purpose of this laboratory is to acquaint the new user of the SEM to various sample preparation techniques and the advantages and limitations of different methods.
    Before you start preparation of your sample, you need to make sure you know exactly what information you intend to get during the SEM evaluation. For instance, if you are doing imaging plus EDS on a non-conductive sample, the conductive coating you select will be important.   In order to choose the best
    preparation method, you must also consider size, geometry, state, and
    conductivity of the sample.

    Don’t hesitate to ask our experienced engineering staff for advice, especially when you first start out!

    Drying
    Before doing any SEM characterization, the sample must be clean and completely
    dry (meaning that it will not outgas). Surface oils or dirt must be removed with
    solvents such as methanol or acetone. Dirt particles can also be removed by
    blowing a compressed gas. Samples that have been cleaned in water must be
    allowed to dry completely before examining in the SEM. Keep in mind that some samples such as salts or pharmaceutical drugs have natural tendency to absorb
    moisture from the regular environment and should be dried from water before
    use.

    Biological specimens must undergo a procedure known as “critical drying,” in
    which moisture is removed from the sample without damaging or altering the
    features of the specimen.   There is a chapter in your textbook dedicated to this
    topic.


    Sample holders/stubs
    Sample holders are available for 0.4”, 0.5”, 1”, and 1.25”diameter cylindrical
    mounts, and we also have a jaw holder for irregularly shaped samples.
    Remember that if the sample surface is flush with the top of the holder, the
    working distance (selected by using the “Z” control) will be correct. If the sample is taller than the top of the holder, the working distance will be reduced.
    Importantly, tall samples can damage the EDS detector or the objective lens pole piece if moved too high!

    We also have a selection of cylindrical stubs for mounting larger or smaller
    (including powder) samples. Most of these stubs are made to fit exactly into one of the sample holders. Samples are held in place by adhesives or tape. Very fine powders can occasionally be scattered onto the mount without adhesives.

    You should select the sample holder BEFORE you prepare the sample!

    Adhesives
    There is a wide selection of adhesives available for mounting the sample. Double coated conductive carbon tape is the one used most commonly. It requires no drying time, holds the sample firmly to the surface, and can be used as a ground strap from the sample surface to sample holder. Its only drawback is that for the Field Emission scope, it will continue to outgas enough to degrade the vacuum.
    In this case, you must mount the sample in a glue such as Duco cement,
    superglue, or carbon or silver paint, and give it adequate time to dry before
    imaging. Other mounting adhesive options are crystal bond, carbon putty (which can be formed around an unevenly shaped sample), 5-minute epoxy, and copper or aluminum tape. Remember that if the adhesive is not conductive, the sample will probably have to be coated.

    There is also a catalog with a wide range of mounting aids, in case none of these work for your application.

    Mounting
    • A section only
    If a conductive sample can be sectioned into a size that fits easily into the jaw
    holder, that is all that is necessary (except for cleaning). Sometimes a small
    section can be attached to the base of the sleeve insert   (for the large cylindrical
    holder) with conductive tape or adhesive. The base can move up and down
    along threads inside the sleeve, and can be positioned to the desired height
    inside the sleeve. The sleeve is held into the holder with a tightening screw.
    Most of the time, a sample will be mounted on a plug using tape or adhesive. If the sample is non-conductive, mount it before coating so the coating will cover both sample and plug.  Usually, you will also have to paint a conductive strap from the plug to the top of the sample, since the carbon coating settles onto the top (but not always the sides) of the sample. If the sample shape is extremely irregular, mount it in carbon putty or carbon paint to bring conductive material as far up the sides of the sample as possible.

    Remember that paints must dry thoroughly before putting them into the scope, so sample must be prepared well in advance of your sign up time!

    • Bakelite/epoxy mounts
    Frequently you will mount a sample in bakelite or epoxy and polish it for optical examination before SEM analysis. These will fit into the cylindrical holders exactly. A spring in the bottom holds the surface flush with a retaining ring on the top of the holder.  When you mount the sample, it must be short enough to fit between the spring and the ring - which means about 25 ml of bakelite for most samples. (A note about polishing - be sure to ultrasonically clean the surface after the final polish to remove any abrasive on the surfaces or in cracks, especially if you are doing EDS analysis. Also, watch for embedded abrasive -especially in soft materials.)

    Remember that bakelite and epoxy are not conductive, and you must have a
    ground strap from the conductive sample to the sample holder if it is not carbon coated. Also, imaging where the edges of the sample interface with bakelite will be a problem if the sample is not coated.

    • Powders
    Powders are some of the hardest materials to mount. They agglomerate, are hard to distribute evenly on a surface, and can charge easily and fly off the mount in the vacuum and under the beam. You need to try to get one layer only of powder onto the mount. If it is piled up, only the top will be coated, and when the sample shifts, the conductive surface will be disrupted. It is also important to make sure that you get powder that is representative of your material, since settling can occur. Ideally, a riffle splitter should be used for powders, but in practice, mixing the powder thoroughly with a spatula is usually enough.

    Carbon tape or paint makes a good adhesive base for most powders. Sprinkle the powder lightly with a spatula, press lightly to seat (if possible - with carbon
    paint you might totally submerge the powder in the paint!), and spray with
    canned air to remove loose material from the top.
    If you are going to do particle sizing using image analysis, it is critical that particles do not touch. Owen has a jet dispersion device that works well to disperse particles in a stream of air, onto an adhesive surface.

    Another option for fine powders is to disperse them in a polar liquid (such as
    alcohol). Put several drops on a glass slide and allow the alcohol to evaporate.
    Some of the supply catalogs have liquids designed to both disperse particles and act as an adhesive.

    • Biological specimens
    Most biological specimens need to be totally dried before they can be imaged
    using the SEM. Depending on the sample, the drying procedure can be quite elaborate and it is suggested that the student search the literature for recommended methods for the sample to be imaged.

    We have developed a fairly simple method for preparing insects for our
    demonstration mounts. First, the insect is held in absolute ethanol for at least a month. You must check periodically to make sure the alcohol does not evaporate from the container. The insect is then removed, washed gently with absolute ethanol, and allowed to air dry in a covered dish (to avoid dust) for at least several days. This step allows the alcohol that has replaced the insect’s blood to evaporate. Then the insect is placed on a mount on a spot of carbon paint to hold it in place, and coated with a layer of gold.


    Coating and Conductivity
    Most samples will need to be coated to make them conductive; you might even
    want to coat a conductive material, especially if it is mounted in bakelite or
    epoxy. Carbon is the most common coating material because it is cheap and
    almost invisible to most x-rays. Gold or gold-palladium is preferred if the sample has a very irregular surface and you are absolutely certain you will not need compositional analysis. If you are not sure whether will need to do
    compositional analysis, opt for carbon because it is impossible to remove the
    gold (other than by polishing) once it is on.  Both coatings are usually applied at
    a thickness of about 20 nanometers, which is too thin to interfere with
    dimensions of surface features. Carbon at that thickness will have little or no
    effect on elemental analysis.
    The coating evaporates onto the sample surface, but if the sample is tall or the
    surface is very irregular (such as spheres or powders), lower portions of the
    sample will not be coated and the coating will not be continuous. Mounting the
    samples in carbon paint before coating is sometimes a good idea since the sample is somewhat embedded in the carbon paint. This will cover the lower sides that the evaporated coating might not reach. If a tall sample is mounted on a conductive plug, it is necessary to paint a “ground strap” of carbon paint from the plug, up the side of the sample, to the coated surface to make a continuous path. Finally, even if you have a flat surface held by the self-referencing spring and ring, you should make a small ground strap out of carbon tape or paint from the surface of the sample to the ring, just in case the underside of the ring does not have a good contact with the sample surface.

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