High-Resolution Scanning Electron Microscopy

Károly Havancsák

 

Eötvös Loránd University, Department of Materials Physics

Hungary,1117 Budapest, Pázmány Péter sétány 1/a.

 

 

INTRODUCTION

 

The high resolution scanning electron microscope is a versatile tool for nanotechnology and advanced materials science, moreover, opens up new dimensions in archaeology, biology, earth science, meteorite research, hydrology and many other research areas.

 

 

Fig. 1. Quanta 3D FEG dual beam scanning electron microscope at ELTE

 

The Technoorg Linda Co. Ltd. works in close cooperation with the Scanning Electron Microscopy Laboratory of the Eötvös Loránd University (ELTE), Budapest, Hungary [1]. In this laboratory we have a FEI Quanta 3D FEG scanning electron microscope. This is why the possibilities of high resolution scanning electron microscopy will be demostrated in this paper through this microscope.

The FEI Quanta 3D FEG scanning electron microscope is a high resolution dual beam device. Dual beam means that besides the electron beam it has a focused ion beam (FIB) as well. The electron and the ion beams are suitable for taking microscopic images while the ion beam allows simultaneously tailoring the sample surface. A picture of the Quanta 3D FEG scanning electron microscope can be seen in Fig. 1.

 

ELECTRON BEAM – SAMPLE INTERACTION

 

The image formation in a scanning electron microscope is different from that in a conventional optical microscope. In SEM a focused mono-energetic electron beam scans the sample surface producing various "products" from the surface. These are secondary electrons, backscattered electrons and X-ray photons (see Fig. 2.). These products are collected by dedicated detectors and utilizing their signals, an intensity map is constructed on the screen; this is the microscopic image. Due to this image formation mechanism in SEM there is no need to have a transparent to the electrons specimen as in case of a transmission electron microscope (TEM). The sample thickness is arbitrary within reasonable limits and quite often sample preparatory measures are not needed at all. Of course, the sample should be cleaned in order to remove the surface contamination, dust and grease if they are. In case of microstructural or micromechanical analysis the sample can need more sophisticated cleaning and polishing. The proper preparation method is extremely important because the true microstructure of the sample may be obscured or altered by the use of poor technique.

As the energies of emerging products are different, they provide information from different depths. The secondary electrons (SE) are specimen electrons, knocked out of the surface by inelastic collisions with the incoming beam electrons and their majority are emitted with energies of a few electron volts (3-5 eV) providing only near-surface information. In this case, the resolution is determined by the diameter of the focused electron beam. The ultimate resolution in case of secondary electrons is ~1 nm.

 

 

Fig. 2. The products due to the electron-material interaction

 

The backscattered electrons (BSE) are beam electrons experiencing large angle (or multiple) elastic scattering. The order of magnitude of their energy is some tens of kilo electron volts (10-30 keV). Owing to their energy range they provide information from deeper layers. Accordingly, in this case the resolution is poorer, ~2-4 nm.

The energy of X-ray photons is characteristic of the atoms they originate from, thus measuring their energy allows for local elemental analysis. The analysis can be point- or surface analysis (elemental mapping).

 

THE SET-UP OF A SCANNING ELECTRON MICROSCOPE

 

The main parts of the scanning electron microscope are as follows: electron source, focusing- and scanning magnets, detectors and sample stage. The diagram of the set-up can be seen in Fig. 3. The source of electrons in an electron microscope is the electron gun. In the gun the electrons are emitted from the cathode either due to heating (thermionic source) or due to expelling electric field (field emission gun = FEG).

In the Quanta 3D FEG microscope the electron source is a field emission gun which is heated as well, called Schottky source. The material of the filament is tungsten covered by zirconium oxide (ZrO2) in order to lower the work function of the cathode material.

The electrons emitted from the cathode are accelerated by an electric field up to the required energy. The maximum electron energy in scanning electron microscopes is usually 30 keV which can be adjusted towards the smallest energies.

After acceleration the electrons are focused and scanned by magnetic lenses. The electron lenses work on the basis of the Lorentz force. The focal length of magnetic lenses and the size of the beam spot can be changed by changing the current flowing through the coils. The minimum diameter of the beam spot is ~1 nm.

The beam scans the sample surface point by point and row by row. The detectors positioned above the sample surface collect the generated by the beam secondary electrons (SE), backscattered electrons (BSE) and the X-ray photons. The simplest scanning electron microscopes have only SE detector, but the SEM installed at ELTE has all of these three ones.

 

 

Fig. 3. The set-up of a scanning electron microscope

 

The secondary electron detector most commonly used in scanning electron microscopy has been developed by Everhart and Thornley (1960). The Everhart‒Thornley detector (ETD) is so popular that it is extremely rare to find a conventional SEM without one. The schematic drawing of the ETD detector can be seen in Fig. 4. At the entrance of the detector there is a grid biased by a potential typically selectable in the range -250 V to +250 V. Since the secondary electrons have low energy the positive bias easily attracts them towards the grid for subsequent acceleration to hit a scintillator material. The photons emitted by the scintillator are conducted by a light guide to a photomultiplier (PM) where the photons are converted back to electrons. These electrons are accelerated onto the successive electrodes of the PM producing an ever-increasing cascade of electrons until the final collector is reached. This process provides high gain with little noise and high bandwidth.

 

 

 

Fig. 4. Schematic diagram of the Everhart‒Thornley detector

 

The backscattered electron detector is usually a solid state semiconductor diode operating on the principle of electron-hole production induced by energetic electrons. It has the form of a flat, thin plate with an annular opening in the middle. Backscattered electron detectors are typically placed on the polepiece (end of the electron column) around the incoming beam as shown in Fig.3.

The X-ray detector is an energy dispersive silicon drift detector made of high purity silicon. Therefore it does not need continuous cooling as the conventional silicon pin detectors. The detector is cooled only when it is working by a Peltier cooler down to -60 oC to reduce the electronic noise. The dead-time of such a detector is short allowing to receive 105 photons in a second. The energy resolution at energy of Mn Kα (5.9 keV) is: ΔE=130 eV.

 

IMAGE FORMATION IN SEM

 

The lenses do not play direct role in image formation in SEM and this is why the Abbe resolution formulae is not relevant in case of scanning microscopes. In the scanning microscopes the main factors determining resolution are as follows: the size of the beam on the sample surface, the excited by the beam sample volume under the surface and the energy of the products escaping the sample surface.

How is the image formed? The principle of image formation is shown in Fig. 5. Electron beam scanning is controlled by a scan generator. The same scan generator controls the activation succession of the pixels on the screen. The products emitted from the sample surface are captured by dedicated detectors and their output signal, which is proportional to the product-yield, modulates the brightness of the corresponding pixels in the screen. If the emissivity of a surface object is different from its surrounding, then this difference will be seen in the screen. In this way the image of the surface is formed point by point and this image is an emissivity map of the surface relating to the product in question.

 

 

Fig. 5. The principle of image formation in a SEM

 

As it has been mentioned, the secondary electrons taking part in image formation originate from a shallow surface region, thus the SE image provides information of the surface topography. The backscattered electrons, having higher energy, originate from a thicker surface region and so the BSE image maps a deeper region.

The BSE image has another important feature. Since the backscattered electron yield depends on the atomic number of the specimen, the backscattered image has a so called Z-contrast. The higher the atomic number, the larger the BSE yield is. As a consequence, if different parts of the sample consist of elements with different atomic numbers then the corresponding image parts will have difference in brightness.

The mentioned features of SE and BSE images can be seen in Fig. 6. The Fig 6.a. is a SE image and Fig. 6b. is a BSE image of the same sample area, taken simultaneously. The SE image shows the surface topography, while in Fig. 6b. deeper surface details can be seen while small white dots indicate a heavier element.

Since the image size is always the same (size of the screen), the magnification depends on the size of surface area scanned by the electron beam. If a small part of the sample surface is scanned, the magnification is high, if the scanned area is increased then the magnification is getting smaller.

There is vacuum inside the microscope, but this is a dynamical vacuum, which means that the pressure is different in different parts of the device. The pressure in the area of the gun is in the range of 10-7 Pa and in the sample chamber its value is about 10-3 Pa. Between these two values the pressure changes continuously along the electron column.

  

a

 

b

 

Fig. 6. SE image (a) and BSE image (b) of the same sample area

 

Special features of Quanta 3D FEG scanning electron microscope

 

The extraordinary feature of Quanta 3D microscope is that insulating samples can be examined without any special sample preparation. The charge delivered by the electron beam tends to accumulate on the surface of an insulating sample, making further examination of the surface impossible. In a conventional scanning electron microscope this charge effect is usually avoided by coating the surface with a thin gold (or carbon) layer, converting sample surface into conductive. In many applications, such as in case of nano-objects, the surface coverage is not an eligible solution because it can substantially modify the properties of the sample. Quanta 3D microscope has a so called low vacuum operating mode, with water vapour of 30-100 Pa in the sample chamber, eliminating the need for conductive coverage of the surface since the ionised by the electron beam water molecules are capable to neutralize the surface charges.

Another exceptional feature of the Quanta 3D microscope is that biological samples sensitive to vacuum and water loss can be examined directly. This is the so-called environmental operating mode, with pressure in the 500-2000 Pa range, where the measurement is performed in saturated water vapour.

The second beam of the dual-beam microscope is a focused gallium ion beam (FIB), with maximum energy of 30 keV. Beam focusing and acceleration are similar to those of electron beam. The surface of the sample can be sputtered away by the ion beam and thus sample surface can be manipulated efficiently. The presence of the ion beam multiplies the capabilities of the microscope. FIB is applicable for several functions: cutting sample cross section, preparing thin TEM lamella or performing nanolithography etc.. As an example, cutting by the ion beam perpendicularly into the surface and preparing a new flat surface, allows to examine the sample along its cross section. Fig. 7 shows such a cross-sectional new surface prepared by FIB.

This versatile device is suitable for backscattered electron diffraction (EBSD) measurement as well. The diffraction image measured point by point enables to characterise the crystal structure and orientation in a small vicinity of the beam incidence point. Having been measured all points of a given area an orientation map can be constructed. Using the orientation map grain structure and texture can be studied.

The apparatus has also a transmission operating mode (STEM mode). If the sample is thin enough, bright-field and dark-field images can be taken by the STEM detector, information content of which corresponds to those of TEM images. The ultimate resolution at this operating mode in ideal circumstances is 0.9 nm.

 

 

 

Fig. 7. Cross-sectional new surface prepared by focused ion beam

 

FOR FURTHER READING

 

1. Home page of the SEM laboratory for a detailed description:

     http://sem.elte.hu/en/index.php?p=introduction

2. J. Goldstein, Ch. Lyman, D.E. Newbury et.al., Scanning Electron Microscopy and X-ray Microsanalysis, Springer, 2003.

3. S. Amelinckx, D. van Dyck, J. van Landuyt, G. van Tendeloo (Eds), Handbook of Microscopy, VCH, 1997. Vol. 2: Methods II, Electron Microscopy.