Introduction:The Scanning Electron Microscope wasinvented by Charles Oatley and co-workers at Cambridge University in the late1940s. The SEM is typically a type of microscope which uses beam of electronsto scan the surface of the specimen unlike the regular optical microscope whichuses beam of light to scan the surface and resolve the image in Millie scale.
The typical electron microscopeconsists of a pair of lenses which are magnetic in nature and is helpful inresolving the surface of the specimen by narrowing down the electron beam tofocus the specimen. The resolution of the electron microscope is limited by thewavelength of the electrons and the image quality of the lenses. Constructionof the SEMThe Emission GunThe Scanning Electron Microscopeconsists of the regular tungsten fiber which is the source of the electronswhich is the main constituent of the emission gun. Diverse types of emission gunsare used to produce the beam of electrons. The two common types are thermionicemission and field emission electron gun.
The thermionic emission takes placewhen the tungsten filament in the emission gun which is the cathode is heatedto a temperature of about 2800K. Due to the increase in the temperature theelectrons from the valence shell is emitted. The electrons thus generated isgathered as an electron beam, flowing into the metal plate (anode) by applyinga positive voltage to the anode. If a hole is drilled in the anode the electronbeam flow through this hole. This produces a beam of electron which is requiredto produce image of the specimen surface.The field emission electron gun workson the principle that under the influence of strong electromagnetic field,electrons are emitted from the surface of the conductor. Thin tungsten wire isused as the conductor and the tip of the tungsten wire is welded with a tungstensingle crystal and is shaped to be a curvature radius of about 100nm.
The onlydraw back in this type of emission gun is that it requires a vacuum of 10-8Pa. Wehnelt ElectrodeThe Wehnelt electrode is placedbetween the cathode (tungsten filament) and the anode and applying a negativevoltage to it can be used to adjust the current of the electron beam produced.Lens systemThe SEM consists of a pair of lenssystem which is the condenser lens, and objective lens.
The lens system usedhere is a magnetic lens system. When electric current is passed through a coilwound electric wire, a magnetic field is formed which behaves like a lens pushingthe electrons inwards which is like an optical lens. The optical lens convergesthe light beam whereas the magnetic lens bends the electron beam inwardsproducing a narrower beam.
Condenser lensThe condenser lens is right below theelectron emission gun to produce the fine electron beam to scan the surface ofthe specimen. By increasing the strength of the magnetic condenser lens, theelectron probe becomes narrower whereas if the strength of the electron beam isweakened, it might produce a broader electron probe due to the magnetic effectof the lens. A thin metal plate called aperture is placed between the condenserlens and objective lens that allows a part of the electron beam to be focused onthe objective lens.
Objective lensThe key role of the objective lens isto determine the final diameter of the electron. If the performance of the objectivelens is not good, an optimally-fine electron probe cannot be produced despite allthe efforts before the action of the objective lens. Thus, it is crucial tomake the objective lens with the best performance.
Specimen stage and specimen surfaceThe specimen stage of the electronmicroscope should can hold the specimen without drifting during the wholemicroscopy process. For this purpose, a eucentric specimen stage is used. Byusing this type of stage, the observation area does not shift even when thespecimen is tilted and the focus on the specimen does not change after shiftingthe field view while the specimen is tilted.The specimen surfacesThe surface of the specimen must be conductive.If the specimen is conductive, the electrons flow through the specimen stage.But, when the specimen is nonconductive, the electron stops in the specimen andthere is no outflow. The number of electrons thus flowing into the specimen isnot equal to the exit.
If this condition continues then negativecharge is accumulated resulting in a large negative potential. At a threshold limit,the electrons start discharging and comes back to original potential and thisis called charging. So, the specimen should be conducting. If the specimen is non-conductinga thin layer of conducting material is coated on the specimen to avoidcharging.Working of a SEMThe SEM is used to produce magnifiedimages of the specimen which is in the micro to nanometer scale. The image ofthe specimen is captured by the different signals produced when the electronbeam interacts with the specimen surface. To produce the signals the electronbeam needs to be finely focused on the specimen.
The diameter of the electron beamcan be calculated with the following diagram. Figure1 (http://coen.boisestate.
edu/faculty-staff/files/2012/01/SEM.pdf)In this diagram the filament diameteris d0, the distance between the filamentand the condenser lens is u1 and v1 is given to be the distance between the demagnified electron beam diameterand the condenser lens.So, the demagnified electron beam diameter d1 can be found asd1= d0 x v1/u1From this equation it is apparent thatthe stronger the condenser lens, v1 becomes shorter, and the overalldemagnified electron beam diameter will be smaller. The final electron probediameter d on the specimen is (from the image)d = d1 x v2 /u2 = d1 x WD / u2Where, u2 is the distance between the demagnified source image and the objective lensv2 is the distance between the objective lens and the specimen which is alsocalled working distance (WD) of the SEM(v1 + u2) is a constant for a calibrated instrument so smallerworking distance results in smaller diameter. The probe size in an SEM isdecreased by either increasing the strength of the condenser lens or decreasingthe working distance.Interaction of electrons with SpecimensNow that electron beam is produced, thebeam is made to enter the specimen surface.
When some electron interacts withthe specimen surface, it gradually loses its energy and is absorbed. Some otherelectrons on interacting with the specimen produce a variety of signalsdepending on the type of interaction with the specimen. Some of the signalsthus produced are Backscattered electrons, Secondary electrons, Augerelectrons, X-rays. Each of the signal produces different type of informationabout the specimen. These electrons produce the topographic, morphologic,compositional and crystallographic information of the specimen.
Secondary electronsWhen the incident electron beamenters the specimen, secondary electrons are produced from the emission of the valenceelectrons of the constituent atoms in the specimen. Detecting the Secondary electronThe Secondary electrons are detected bythe secondary electron detector which has a scintillator (fluorescent substance)coated on the tip of detector and a high voltage of +10kV is applied togenerate light when the secondary electrons hit the scintillator. This light isused to produce the image of the specimen.
Backscattered electronsBackscattered electrons are thosescattered back when the electrons interact with the nucleus of the atoms in thespecimen. Since the backscattered electrons possess higher energy thansecondary electrons, information from a relatively deep region is contained inthe backscattered electrons. The detection of backscattered electrons is doneby a specific detector.ResolutionResolution is defined as the “theminimum distance that can be separated as two distinguishable points in theimage”. The resolution can be affected by factor such as structure of the specimen,wear and tear of the instrument and so on.In the scanning electron microscope,resolution is limited by the source brightness. db = (?/ 2?) (Ic /?) ½where, ? is the source brightness (controlled by theelectron gun), Ic is the minimumbeam current required So, to improve theresolution of an SEM it is necessary to have a large aperture, small probesize, and a bright electron gun; hence, for high resolution work, an SEM with afield emission electron gun (FEG SEM) is typically used.
Limitations of SEMSome of thelimitations of SEM is that the sample must be solid and conducting, and theemission guns should be kept at high vacuum to avoid contamination of theelectron probe. Electronand Ion Beam LithographyTo structurepatterns in the nanometer scale, we require an instrument that is little enoughto fit in. Since we have mastered in producing the electron beam throughdifferent emission guns, the same can be used to pattern structure exactly likein an optical lithography replacing the light with electron beam.Since electronsare charged particle, they can be controlled by magnetic waves and can focusedto write on a resist. This makes electron beam lithography easier andpatterning structures in the sub 100nm domain to 0.1nm resolution can beachieved.Like EBL, ion beamlithography can also be used.
Instead of the electron beam, an ion beam is used.The charged molecule utilized by IBL is bigger and heavier than electrons,which actuates less diffusion when being focused on the photoresist. Bothmolecules beam lithographic procedures have been utilized for designing.Limitations of EBL andIBLThe electrons are particles which have very lessmass and hence they are easily scattered by other molecules and hence can onlybe carried out under vacuum condition as gas molecules can easily scatter anelectron.
Both lithographic techniques lack efficiency. Since they requiregreat control over the magnetic field, it is not possible to write a lot of patternsat the same time