Electron Source (Gun)

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1 Electron Optics The SEM uses a highly focused electron beam to strike and interact with a sample which is contained in a high vacuum environment to form a high resolution image. Different types of images can be formed in the SEM. These are images from secondary electrons, backscattered electrons, characteristic x-rays, Auger electrons, and others that are emitted by the sample. A typical SEM is comprised of the following: o Electron Gun or Electron beam generation o Tungsten filament cathode olab 6 cathode o Field emission gun o Cathode comparison o Deflection coils o Condenser lenses o Vacuum system o Detectors

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3 Electron Source (Gun) The electron gun is used to generate a small, bright source of electrons that can be focused onto the surface of the specimen. These electrons will then interact with the sample and produce electrons or x-rays which will be detected and produce an image of the sample. There are three typical types of electron guns used in SEM. They are: Tungsten hairpin (most common) (thermionic) Lanthanum hexaboride (LaB 6 ) (thermionic) Field emission electron gun (field emission) Electrons may be emitted from a conducting cathode material either by heating it to the point where outer orbital electrons gain sufficient energy to overcome the work function barrier of the conductor (thermionic sources) or by applying an electric field sufficiently strong that electrons "tunnel" through the barrier (field emission sources).

4 Most of the electron guns used in microprobes employ the thermionic method, in which electrons are effectively evaporated from a resistively-heated tungsten filament; some alternative names for the filament include cathode or emitter i.e. all Electron Microscopes utilize an electron source of some kind with the majority using a Thermionic Gun. A Thermionic Electron Gun is a triode gun (3 electrode gun) functions in the following manner: A positive electrical potential is applied to the anode. The filament (cathode) is heated until a stream of electrons is produced. The electrons are then accelerated by the positive potential down the column. A negative electrical potential (~500 V) is applied to the Wehnelt Cap. As the electrons move toward the anode they are repelled by the Wehnelt Cap toward the optic axis (horizontal center)

5 A collection of electrons occurs in the space between the filament tip and Wehnelt Cap. This collection is called a space charge Those electrons at the bottom of the space charge (nearest to the anode) can exit the gun area through the small (<1 mm) hole in the Wehnelt Cap These electrons then move down the column to be later used in imaging. This process insures several things: That the electrons later used for imaging will be emitted from a nearly perfect point source (the space charge) The electrons later used for imaging will all have similar energies (monochromatic) Only electrons nearly parallel to the optic axis will be allowed out of the gun area. The size and shape of the apparent source, beam acceleration and current are the primary determining factors in the performance and resolution of a SEM.

6 Electron Gun Components: Filament Wehnelt Cap Anode Two types of sources are available: -Thermoionic (Tungsten Filament and LaB 6 ) - Field Emission Thermionic Emission: occurs when enough heat is supplied to the emitter so that electrons can overcome the work-function energy barrier E W of the material and escape from the material. E W E F Metal E Vacuum E is the energy work necessary to take an electron from its lowest energy state to the vacuum. E F is the electrons Fermi Energy. The highest energy state that an electron can have at 0K. E W (work function) energy required for a small number of electrons (located close to the Fermi level) to escape into the vacuum.

7 The cathode current density is given by the expression: where i e = emission current density in Amp/cm 2, i e 2 = Α Τ exp( A = constant with a theoretical value of 120 A/cm 2.K (in reality it depends on the material), E W = work function, in ev (electron voltage); k B = Boltzmann s constant = 8.62x10-5 ev/k and T in K. It is desirable to operate the electron gun at the lowest possible temperature to reduce the evaporation of the filament: Materials with low work function are required. Metal E W (ev) A (Amp/10-4 cm 2 K 2 ) Melting Temp. (K) Fe x Ni x Ta x W x LaB x Εw k Τ B )

8 Choice of Emitter Materials: W: Does not need a good vacuum (~ mbar) as it has good oxidation resistance. It has a high A value and high melting point and therefore a high emission temperature can be used (~2700K). i e = 3.4 A/cm 2. Two classes: hairpin and point filament (up to 4 times brightness than hairpin). LaB 6 : (lanthanum hexaboride) It needs a high vacuum condition (~10-6 mbar). It has a low E W value and therefore higher brightness (10 to 100 times higher than W) can be obtained at its operational temperature of 1873K. Some Important Definitions: Emission Current ( i e ): total current emitted from the filament Beam Current ( i b ): portion of electron current that leaves the gun through the hole in the anode (at each lens and aperture along the column the beam current becomes smaller). Probe Current ( i p ): electron current measured at the specimen. It is several orders of magnitude smaller than the beam current.

9 Current Density d is the cross over diameter and the intensity distribution at the cross over is taken to be Gaussian. The current density of the beam (J b ) at the cross-over (in Amp/cm 2 ) is: J b = ib do 2 π Two important parameters for any electron gun are: -Amount of current it produces (# of e- interacting with the sample) -Current stability (information is recorded as a function of time) 2 d α i b

10 Brightness: It measures gun performance. It takes into account the current density and the changes in the angular spread of electrons as they are focused. It is defines as the current density per solid angle, where the solid angle is in steradians. Brightness equation (A/cm 2 sr): The grid cap is set at a voltage slightly more negative than the filament to provide a focusing effect on the beam, forcing the electrons to a cross over of diameter d. β = current area solid _ angle = 4 ib π d α

11 The theoretical maximum value for the brightness was given by Langmuir in 1937; where i e is the emission current, E O is the accelerating voltage, k B is the Boltzmann s constant J b = i b do 2 π 2 β = β = MAX 4 i b π d α i i e E. e. π. k. e. EO. k. T 2 Following the above 2 e αo equations: Jb, MAX = π. βmax. αo = B Where J b,max is the maximum current density at the cross over. It can be seen that the brightness depends on the filament s nature, thus typical values for a W filament are: T=2700K, i e =3.4A.cm -2, E O =100kV. It gives a brightness of about 10 5 A.cm 2.sr.. The search for materials that produce higher brightness led to the development of LaB 6 giving ten times more brightness than W. O B. T

12 Tungsten Filament (Thermionic) A bent tungsten wire filament, with a diameter of around 100 μm, is spot welded to metal posts. These posts are embedded in a ceramic holder and extend out the other side to provide electrical connections. In operation, the filament will be heated by passing an electrical current through it. Optimum filament temperature for the thermionic emission of electrons is around 2700 degrees Kelvin. LaB 6 Filament (Thermionic) The Lanthanum Hexaboride Filament is a sharpened rod/block of single crystal LaB 6. It is about 50 μm in diameter and about 0.5 mm long. The LaB 6 crystal is both supported and resistively heated by either Carbon or Rhenium, two materials that do not react to form a compound with LaB 6. To equal the current density of W the temperature of the LaB 6 can be reduced to 1500K.

13 Field Emission Filament Another type of electron source (non-thermoionic) is field emission. The filament used is usually a wire of single-crystal tungsten fashioned into a sharp point (tip radius about 100 nm or less) and spot welded to a tungsten hairpin. An electric field can be concentrated to an extreme level at the tip of the filament. The electric field at the tip is very strong (10 7 V/cm) due to the sharp effect. Thus, the potential barrier for e-s becomes reduced and the e-s leave the cathode (filament) without requiring any thermal energy to lift them over the work function barrier.

14 Ef Ec Ei Φ m Field Emission Φ(x) + - Φ t (x) Φ s (x) Φ e (x) x An electron at an energy E i is free to move inside the solid. When it reach the surface it tries to move away and out of the surface. The solid then lacks a negative charge and the resulting Coulomb force attracts the electron back into the solid. The potential energy is Φ s. If an applied electric field directed toward the emitting surface. The electron will experience an additional potential energy.φ e The total potential energy is Φ t.

15 There are different types of field emission guns: Cold Field Emission (FE) e-s excited only by the presence of an electric field. Thermal Field Emission (TF) e-s excited by an electric field + temperature Schottky Emission (SE) e-s excited by electric field + temperature + a reduction of the work function (E W ) by coating. Examples of E W : W = 4.5 ev LaB 6 = 2.5eV W/ZrO (ZrO coating on a <100> tungsten facet) = eV

16 Summary of filament properties: Tip radius β (A/cm 2.sr) Current density (relative) Vacuum (torr) Point filament 1~ 10μm 2x Hairpin filament 30μm 5x LaB 6 1~10μm 7x Field Emission 50 nm 10 7 ~

17 Filament Saturation A condition of beam current saturation must be established to ensure a stable beam. A beam current saturation is reached when a small increase or decrease in the filament heating current do not change the electron beam current. A False Peak appears when there is an uneven temperature distribution and some other part of the filament surface reaches emission temperature before the filament tip. The beam current rises, and then falls, before the saturation condition is established.

18 Brightness and bias voltage At low bias, since little or no focusing takes place. The diameter d of the cross over is large and the brightness β obtained is not optimum. At high bias, most of the emitted electrons will return to the filament and it will shut down (cut-off) all the emission. We want good emission, good focus (small d) and high brightness. There is an optimum bias setting for maximum brightness. Two kinds of adjustment are possible: adjust the h (height) distance between the tip of the filament and the Wehnelt cap hole. adjust the bias voltage,

19 Comparison of Electron Sources Characteristic W LaB 6 Cold FE Thermal Schottky Type of Emission Thermoionic Thermoionic Field emission Field emission Field emission Vacuum (Pa) Electron source size 30μm 10μm 5nm 5nm 20nm Cathode Temp. K 2,800 1, ,800 1,800 Energy spread ev Brightness (A.cm -3.sr) Stability % Life hours ,000 2,000 2,000 6, Application Standard SEM. VP SEM. EDS WDS High Resolution SEM. EDS EBSP HR-SEM EDS,WDS CL EBSP

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29 Electron Lenses Electrons have a charge and their direction of travel can be altered by an electromagnetic field. An electron traveling in off-axis to a uniform magnetic field follows a helical path. Electrons can be brought to focus by engineering the electrostatic and/or magnetic fields. Electron lenses are used to demagnify the image of the beam cros-over in the electron gun (e.g. d o ~50μm for a heated tungsten gun) to the final spot size on the specimen (~10nm). This is a demagnification of 5000 times. In a filed emission gun the source is small and only requires demagnifications of times to produce a 1-2nm probe size. The electrons in the SEM are focused by electromagnetic lenses. These lenses have smaller aberrations, however these perform poorly compared to typical glass lenses.

30 In the electromagnetic lens, the intensity of the field (the magnetic flux) causes a radial vector along the optical axis, so when an electron is accelerated through the pole pieces, it takes a helical path through the lens. The rotational force is the product of the electron velocity and the density of the magnetic flux. This vector interaction also results in focusing as the strength of the lens is changed

31 The focal length of the electromagnetic lens is controlled by varying the lens current. The focal length is approximately proportional to V ( NI ) 2 where V is the accelerating voltage, N is the number of turns in the magnet coil and I is the current.

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33 Beam-controlling lenses The electron microscope has a number of electromagnetic lenses that are used for: Centering beam on column Adjusting and regulating microprobe current Focussing beam on sample surface Electromagnetic lenses are composed of coils of wire in a soft iron housing. These must be very symmetrical to avoid beam distortion. Current passed through coiled wire creates a magnetic field that deflects electrons and causes them to focus to a point. The stronger the current to the wires, the shorter the focal length.

34 These lenses are created with high precision and even a hairline scratch can distort their magnetic field and will have to be replaced. Most electromagnetic lenses are cooled with water to prevent extra heating. Their functions are similar to optical lenses. A condenser lens can condense electrons; an objective lens can focus electrons on the specimen, and a projector lens can project an image onto a screen.

35 1 f = 1 p + 1 q Demagnification = m = p q = d d O 1

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40 Defects in Lenses All lenses suffer of a number of defects or aberrations in their performance. In contrast to light optics, the effects of aberrations in electron optics can not be cancelled by using combinations of lenses. Chromatic Aberration Spherical Aberration Astigmatism Aberrations in the Objective Lens Aperture Diffraction Types of Defects

41 Chromatic Aberration Since the focal length (f) of a lens is dependent on the strength of the lens, it follows that different wavelengths will be focused to different positions. Chromatic aberration of a lens is seen as fringes around the image due to a zone of focus. In light optics wavelengths of higher energy (blue) are bent more strongly and have a shorter focal length. In the electron microscope the exact opposite is true in that electrons of higher energy (blue), or shorter wavelengths are less effected and have a longer focal length.

42 In light optics chromatic aberration can be corrected by combining a converging lens with a diverging lens. This is known as a doublet lens. A few manufacturers have combined an electromagnetic (converging) lens with an electrostatic (diverging) lens to create an achromatic lens LEO Gemini Lens The chromatic aberration however can not be totally cancelled and the best recourse is to try to minimize this effect.

43 The simplest way to correct for chromatic aberration is to use illumination of a single wavelength! This is accomplished in an EM by having a very stable acceleration voltage. If the e velocity is stable the illumination source is monochromatic The effects of chromatic aberration are most profound at the edges of the lens so by placing an aperture immediately after the specimen chromatic aberration is reduced along with increasing contrast.

44 Spherical Aberrations The fact that wavelengths enter and leave the lens field at different angles results in a defect known as spherical aberration. The result is similar to that of chromatic aberration in that wavelengths are brought to different focal points Spherical aberrations are worst at the periphery of a lens so a small opening aperture that cuts off the most offensive part of the lens is the best way to reduce the effects of spherical aberration

45 Aperture Diffraction Diffraction occurs when a wavefront encounters an edge of an object. This results in the establishment of new wavefronts. When this occurs at the edges of an aperture the diffracted waves tend to spread out the focus rather than concentrate them. This results in a decrease in resolution, the effect becoming more pronounced with ever smaller apertures.