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High resolution imaging at low acceleration voltages and low bem currents with MERLIN


To cite this article use: Stodolka, J., Jaksch, H., Marshman, J., Yang, J., Merard, J-C., Albiez, M.  High resolution imaging at low acceleration voltages and low bem currents with MERLIN. J. Nano Sc. Tech, 3(2015)48-52

Jorg Stodolka

Heiner Jaksch

Jeff Marshman

Jijin Yang 

Jean-Claude Menard

Michael Albiez

MERLIN® is Carl Zeiss NTS’ latest development in the field of emission scanning electron microscopes (FE-SEM).  It combines high resolution imaging, optimized at all energies, with unpre edented ease of use. The continuously adjustable current up to several hundred nA allows time-saving image acquisition and high-speed analytics. In-situ cleaning and local charge compensation add up to a system which is the most versatile and user friendly FE-SEM to date, offering endless possibilities for imaging and analysis of specimens.

Instrumentation

MERLIN® is the first system combining a double condenser with the well-proven Carl Zeiss NTS GEMINI® technology. The GEMINI® column offers good resolution at low acceleration voltages. Even at low energies the beam is kept stable with the help of a beam booster that accelerates the electrons in the column above 10 kV prior to deceleration on the sample surface. GEMINI®’s twin lens, consisting of an electrostatic and a magnetic part, limits the magnetic field to the column, making undisturbed investigations of magnetic samples possible. It is also used as a detection lens, separating secondary electrons from backscatter electrons. The latter are directly detected by an on-axis backscatter detector on an energy-specific basis. Without any additional adjustment,   it can be toggled between the various detectors. The implementation of an additional condenser (Fig.1) into the GEMINI® II column now offers the unique opportunity to continuously change the current from 4 pA up to 300 nA (depending on configuration and acceleration voltage) without having to change any apertures. A further improvement in the ease of use has even been achieved.  It was specifically developed for offering high resolution possibilities at all currents. While the upper condenser sets the current, the lower condenser accounts for optimum resolution by optimizing the aperture angle. For a larger depth of field the system can be switched to a different imaging mode and the convergence angle is decreased.

Figu1

Figure 1. Outline of the new GEMINI® II Column, unique to the MERLIN® FE-SEM.

Imaging Results

The following examples show the huge range of low energy and / or low current investigations that are now possible with MERLIN® without losing resolution.

In the simplest case imaging of conductive samples at low energies offers the opportunity to be extremely sensitive to surface information. Details can be revealed that normally would be blurred due to larger penetration depths of the electrons at higher voltages. Fig. 2 reveals grains on the gold islands of a gold-on-carbon sample (due to back sputtering of material) – a specimen that is normally used for resolution measurements. The image was acquired at 400 V and 7 pA only. Such surface detail  disappears with an increase of beam energy.

Figura2

Figure 2. Details of grains on gold islands of a gold-on-carbon sample, increased surface detail at low voltages: 400V, 7 pA, in-lens detector.

Carbon nanotubes are an important object of recent research. Although the diameters of the up to several centimeters long cylindrical structures are in the nanometer range these tubes are the strongest and stiffest material yet discovered. This toughness makes them ideal candidates for materials science with a wide range of applications such as in the clothing industry and even in architecture. In biology they have been used to transport RNA into cells while their electrical properties have led to the development of ultracapacitors and transitors. The imaging of carbon nanotubes can be done at all energies but only at low energies (1 kV and below) the tubes do not appear translucent and the cylindrical wall can be investigated. Fig. 3 shows an image acquired at an energy of only 30 V. Even at extremely low voltages the surface of the specimen shows fine details.

figura3

Figure 3. Carbon nanotubes investigated at 30V, 250 pA, in-lens detector: the bright areas are thin layers of polymer. Detailed imaging is possible even at lowest  nergies.

The thin layer of silica seen in Fig. 4 is another example for high surface sensitivity at low acceleration voltages. Silica is primarily used in the production of glass and glass fibers. Additionally, layers of Silica (SiO2) are used as highly stable electric insulators on silicon wafers. At higher energies the fine surface details would be lost. While the silica does not charge up at higher currents, it does so at higher acceleration voltages. However, for some samples a reduction of current is crucial to avoid charging.

figura4

Figure 4. Investigation of a silica layer at 30V, 250 pA, Everhart-Thornley detector, highest topographical detail at lowest voltages.

Fig. 5 shows the image of alumina spheres that more recently seem to become the new standard sample for testing the resolution capability of SEMs at low voltages. At energies below 500 eV and currents of about 50 pA the charging of these non-conductors (industrially used as grinding powder or carrier material in catalysts) can be sufficiently suppressed to show even the finest surface detail that was not accessible before.

figura5

Figure 5. Finest surface detail on Al2O3 spheres at low voltage and low beam current: 450V, 60 pA, in-lens detector.

In Fig. 6 small gold particles can be seen on the surface of titanosilicate, a material used as a heterogene catalyst for oxidations. Due to the highly charging nature of the sample this information is only available at low currents. Another catalyst is Gold-Palladium nanoparticles commonly used for the production of hydrogen peroxide.

figura6

Figure 6. Titanosilicate: small gold particles on crystal form of silicon dioxide: 1 kV, 80 pA, in-lens detector. Fine surface detail at low current.

Fig. 7 shows an analysis of these particles in alumina as a carrier material. The use of the Carl Zeiss NTS specific on-axis energy selective backscatter detector (EsB®) proves that material contrast images do not need high currents to be impressive. With the MERLIN®, materials science is no longer limited to certain ranges of parameters. Apart from academics, this versatility may also be of crucial importance for service laboratories.

figura7

Figure 7.  AgPd catalyst in Al2O3 carrier, material information even at low current and low voltage conditions: 800V, 80 pA, EsB® detector.

In the semiconductor industry failure analysis is of crucial importance in order to optimize the performance of a device or to identify problems in its production. For integrated circuits the chip has to be delayered by means of reactive ion etching and/or lapping and is then inspected layer by layer. Investigating the layers with an SEM offers the challenge of imaging low-k dielectrics that are used to reduce the time constant of the chip. These dielectrics are non-conductive and beam-sensitive. Hence, low beam currents have to be used in order to avoid charging (Fig. 8-10). Additionally, the surface of a layer containing low k dielectrics can be investigated without encountering any significant beam damage or delamination of metal lines at low voltages and low beam currents (Fig. 11).

Figura10

Figure 8. AgPd catalyst in Al2O3 carrier, material information even at low current and low voltage conditions: 800V, 80 pA, EsB® detector.

With low voltage/current options at hand, the staining of samples in life sciences is no longer needed. Investigations of tissue slices are becoming more timeefficient and even reveal more detail: Fig. 12-14 show the fine nanostructures of the collagen fibrils and the matrix of proteoglycans and other proteins in bovine cornea and sclera, which have never been seen with an SEM before.

The cornea and sclera together form the outer fibrous shell of the eye globe and withstand both internal and external forces to maintain the shape of the eyeball. While the cornea is transparent, the sclera is not. Both the cornea and sclera are mainly composed of collagen fibrils surrounded by a matrix containing proteoglycans and other proteins. High resolution SEM imaging of biological samples is often challenged by charging and beam damage. Strategies for dealing with these two issues involve the use of low acceleration voltage and probe current. However, low kV and low probe current introduce other problems during imaging. For example, low kV impairs resolution and causes faster contamination buildup at high magnification. The signal-to-noise ratio is another concern when using low probe current. With the high resolution capabilities of the MERLIN® at low energies and beam currents these strategies can be pursued without encountering the aforementioned issues. The uncoated and unstained samples provide information that was never accessible before.

 Figure 9. Detail of the same semiconductor device, 2 kV, 57 pA: even at high magnifications no charging is observed due to low beam current.

Figure 9. Detail of the same semiconductor device, 2 kV, 57 pA: even at high magnifications no charging is observed due to low beam current.

 

Figure 10. Semiconductor device polished to the gate oxide level, 1 kV, 20 pA: prevention of charging and beam damage.

Figure 10. Semiconductor device polished to the gate oxide level, 1 kV, 20 pA: prevention of charging and beam damage.

FIgure 11 Semiconductor device polished to the contact level just before exposing the polysilicon level, 300V, 70 pA: high surface sensitivity without charging and beam damage.

FIgure 11 Semiconductor device polished to the contact level just before exposing the polysilicon level, 300V, 70 pA: high surface sensitivity without charging and beam damage.

Figure 12. Bovine scleral collagen fibrils with D-periodicity and matrix with nanostructure of ~5 nm in diameter (bright color), 500V, 30 pA: details that were never seen with an SEM before.

Figure 12. Bovine scleral collagen fibrils with D-periodicity and matrix with nanostructure of ~5 nm in diameter (bright color), 500V, 30 pA: details that were never seen with an SEM before.

FIgure13. Bovine corneal collagen fibrils showing a characteristic Dperiodicity, 500V, 30 pA.

Figure13. Bovine corneal collagen fibrils showing a characteristic Dperiodicity, 500V, 30 pA.

FIgure 14. Collagen fibrils with D-periodicity which are imbedded in a 3-D cross-bridge network containing nanostructures of ~5 nm in diameter, 500V, 30 pA: highly detailed images at low kV / low beam current conditions.

Figure 14. Collagen fibrils with D-periodicity which are imbedded in a 3-D cross-bridge network containing nanostructures of ~5 nm in diameter, 500V, 30 pA: highly detailed images at low kV / low beam current conditions.

Summary

MERLIN® offers high resolution imaging even at low acceleration voltages and low beam currents. Charging and beam damage of specimens can be prevented. The GEMINI® II column – unique to the MERLIN® SEM – ensures optimum resolution under all conditions, making MERLIN® the most versatile tool for imaging and analysis with unmatched ease of use.

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