Discussion on scanning electron microscope sem technology
SEM test items
1. Material surface morphology analysis, micro-area morphology observation
2. Analyze the shape, size, surface, cross-section and particle size distribution of various materials
3. Surface morphology observation, film roughness and film thickness analysis of various thin film samples
SEM sample preparation is simpler than TEM sample preparation and does not require embedding and sectioning.
Sample request:
The sample must be solid; meet the requirements of non-toxic, non-radioactive, non-polluting, non-magnetic, anhydrous, and stable composition.
Preparation principles:
The sample whose surface is polluted should be properly cleaned without destroying the surface structure of the sample, and then dried;
Newly broken fractures or sections generally do not need to be treated, so as not to damage the structural state of the fracture or surface;
The surface or fracture of the sample to be eroded should be cleaned and dried;
Magnetic samples are pre-demagnetized;
The size of the sample should be suitable for the size of the sample holder dedicated to the instrument.
Common methods:
bulk sample
Block conductive material: no sample preparation is required, and the sample is bonded to the sample holder with conductive glue for direct observation.
Bulk non-conductive (or poorly conductive) materials: first use the coating method to treat the sample to avoid charge accumulation and affect image quality.
powder sample
Direct dispersion method:
The double-sided adhesive is stuck on the copper sheet, the particles of the sample to be tested are directly scattered on it with the help of cotton balls, and the sample is gently blown with the ear cleaning ball to remove the attached and not firmly fixed particles.
Turn over the glass piece loaded with particles, align it with the prepared sample stage, and gently tap with small tweezers or glass rods to make the fine particles evenly fall on the sample stage.
Ultrasonic dispersion method: put a small amount of particles in a beaker, add an appropriate amount of ethanol, and ultrasonically vibrate for 5 minutes, then add it to the copper sheet with a dropper, and let it dry naturally.
Coating method
Vacuum coating
The vacuum evaporation coating method (referred to as vacuum evaporation) is to heat the raw material to be formed in the evaporation container in a vacuum chamber, so that the atoms or molecules are vaporized and escaped from the surface, forming a vapor flow, which is incident on the solid (called the substrate). or substrate) surface, the method of condensation forming a solid film.
ion sputtering coating
principle:
Ion sputtering coating is a glow discharge in a partially vacuumed sputtering chamber to generate positive gas ions; under the acceleration of the voltage between the cathode (target) and anode (sample), the positively charged ions bombard the surface of the cathode, The cathode surface material is atomized; the formed neutral atoms are sputtered from all directions and fall to the surface of the sample, thus forming a uniform film on the surface of the sample.
Features:
For any material to be plated, as long as it can be made into a target, sputtering can be realized (suitable for preparing difficult-to-evaporate materials, and it is not easy to obtain thin-film materials corresponding to high-purity compounds);
The film obtained by sputtering is well bonded to the substrate;
Consumption of precious metals is less, only about a few milligrams each time;
The sputtering process has good repeatability, the film thickness can be controlled, and at the same time, a film with uniform thickness can be obtained on a large-area substrate.
Sputtering method: DC sputtering, radio frequency sputtering, magnetron sputtering, reactive sputtering.
1. DC sputtering
It is rarely used because the deposition rate is too low ~0.1μm/min, the substrate heats up, the target must be conductive, high DC voltage, and high air pressure.
Advantages: simple device, easy to control, good formwork repeatability.
Disadvantages: high working pressure (10-2Torr), high vacuum pump does not work;
Low deposition rate, high substrate temperature rise, only metal targets can be used (insulating targets cause positive ions to accumulate)
2. RF sputtering
RF frequency: 13.56MHz
Features:
The electrons do oscillating motion, which prolongs the path and no longer needs high voltage.
Insulating Dielectric Thin Films Can Be Prepared by Radio Frequency Sputtering
The negative bias effect of RF sputtering makes it similar to DC sputtering.
3. Magnetron sputtering
Principle: Use the magnetic field to change the direction of electron movement, restrain and extend the trajectory of electrons, improve the ionization probability of electrons to the working gas, and effectively use the energy of electrons. Therefore, the target sputtering caused by the bombardment of positive ions on the target is more effective, and sputtering can be carried out under lower air pressure conditions. on substrates that can only be deposited in time.
