Improved preparation of membrane surfaces for field-emission scanning electron microscopy

Improved preparation of membrane surfaces for field-emission scanning electron microscopy

Journal of Membrane Science 187 (2001) 85–91 Improved preparation of membrane surfaces for field-emission scanning electron microscopy M. Schossig-Ti...

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Journal of Membrane Science 187 (2001) 85–91

Improved preparation of membrane surfaces for field-emission scanning electron microscopy M. Schossig-Tiedemann∗ , D. Paul GKSS Forschungszentrum Geesthacht GmbH, Max-Planck-Str., D-21502 Geesthacht, Germany Received 29 August 2000; received in revised form 16 November 2000; accepted 16 November 2000

Abstract The separation characteristics of membranes depends greatly on their morphology. Thus, membrane investigation using scanning electron microscopy (SEM) is a standard method in membrane characterization. Careful specimen preparation methods are required to ensure excellent performance of field emission scanning microscopes and to minimize artifacts generated by the preparation process. For polymer samples with structural elements on a sub-micron scale, the surface structure can be significantly altered by energy impacts resulting from the preparation method. A method of conductive coating with reduced energy impact is demonstrated, which largely avoids the generation of artifacts during the coating process. This procedure generates images of membrane surfaces virtually artifact free. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning electron microscopy; Preparation; Polymer

1. Introduction SEM-investigation is a valuable tool used to characterize membranes during development and in industrial use. The separation characteristics and the mechanical strength of porous membranes are mainly determined by their morphology, which can be observed in the secondary electron image. It is possible to localize the concentration of elements on the surface investigated using the back scattered electron image mode and by means of elemental analysis. In this way we are able to identify the distribution of functional elements or materials in order to reinforce the membrane against mechanical stress. Using the results of SEM-investigation, membranes can be ∗ Corresponding author. E-mail address: [email protected] (M. Schossig-Tiedemann).

developed which are optimized for their prospective separation task. The membrane manufacturing process also profits from the results of SEM-investigation; quality control of randomly taken samples is a reliable method to find and identify changes in product quality. In addition the cause of damage to membranes or changes in their separation capacity can be investigated by SEM examination. Examples are fouling [1], damage caused by particles, or unsuitable cleaning methods. The separation power of porous membranes is characterized by their permeation rate and rejection. Typical pore diameters of ultrafiltration membranes are in the range of 2–100 nm. The number of pores and the pore size distribution determine the permeation behavior of these membranes. The characterization of membrane pores is usual accomplished by means of indirect measuring methods; distribution of pore sizes as well as the number of pores can be calculated from

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permeation experiments through the use of various types of solute [2]. For the calculation of pore size, the geometry of the pores must be determined. If the pore geometry differs, (e.g. caused by different production methods) the only way to show this is by using direct imaging methods. SEM-imaging of porous membrane surfaces allows the determination of pore entrance shape and size. With suitable image processing software all pores shown in the image can be characterized in one step. This technique is limited by the resolution of the microscope used as well as by the quality of membrane preparation. Although, polymers are generally insulators, however in principle, even uncoated, non-conductive polymer samples can be imaged via SEM with reduced charging effects [3]. For each type of non-conductive sample, there is a narrow range of acceleration voltage in which the surface charge is low enough for good images to be obtained [4]. However, under these conditions, the resolution of field emission microscopy is usually reduced due to aberration of the electron optical system and magnetic field disturbances [5]. Preparing polymer samples with “wet chemical” procedures always has the risk of additional artifacts from the chemical treatment, especially affecting high resolution micrographs. Conductive coatings can effectively eliminate the surface charging problems and additionally enhance the image contrast due to higher secondary electron yield. However, even with an artifact free and optimally thin surface coating the acceleration voltage should be kept as low as possible to reduce the penetration depth of the electrons. The resolution of scanning electron microscopes with field emission is limited (depending on the acceleration voltage) to 0.6–3 nm. In order to obtain this high resolution with real polymer samples, preparation artifacts have to be minimized. One reason for artificial changes to the specimen surface is the effect of impact energy during the conductive coating process. In the course of this, the sample is subjected to radiation impacts of various wavelengths, which may cause changes in morphology. Based on various methods for conductive coating of SEM samples, improved imaging and preparation methods are compared, with the aim of accomplishing optimal imaging within the field emission scanning electron microscope.

2. Experimental 2.1. Microscope With exception of the image of an uncoated sample, all the micrographs were obtained using a Jeol 6400 F field emission scanning electron microscope. The acceleration voltage was 5 kV and the nominal current density 1 × 10−11 A. For secondary electrons, the standard Everhard–Thomley detector was used. The images were saved as TIFF files at 1024 × 768 pixel and 8 bit gray scale, using an image processing program by SIS [6]. The uncoated sample was studied using a DSM 682 Gemini microscope with E = E2 at 1 kV [4,7]. 2.2. Coating equipment Room temperature magnetron sputtering was performed using an Emitech K575. The cryo preparation was made using a Baltec SCU 20. The ion beam sputter coater used was a Gatan 681. The penning sputter coater was a Gatan EPA 101. The samples coated by electron beam evaporation were prepared in a BAF 060 freeze fracture system. Metals used for coatings were gold/palladium, chromium and platinum/carbon. 2.3. Specimen coating methods 2.3.1. Magnetron sputter coating This is the most popular way of applying a conductive layer on a non-conductive specimen. For this method, the sputter source and the sample are located in a common vacuum chamber. The working pressure is about 10 Pa. The noble gases argon or xenon are used as sputtering gases. The mean free path of target atoms at this pressure is only a few millimeter, so the metal atoms move and collide with other particles on their way towards the substrate. This allows an omnidirectional coating without much technical effort. However, some negative aspects must be considered. Studies from Renou and Gillet [8] have shown that argon ions can act as nucleation sites and in this way enlarge the target particles by an “in flight” nucleation. The result is an increased coated surface grain size. Another significant disadvantage of this method is the fact that the samples are in contact with the plasma during the coating process. Thus, various

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energetic particles, generated in the plasma can reach the specimen surface. A strong magnetic field placed near the sputter target can help to prevent the sample being treated with charged particles but this protection is not perfect. A strong magnetic field placed near the sputter target, can help to prevent the sample being treated with charged particles but this protection is not perfect. As the specimen, plasma and target are in the same volume, the treatment with charged particles is only reduced by this technique. 2.3.2. Ion beam sputter coating In contrast to the magnetron sputter coating, in this procedure the sample is placed under a much higher vacuum (8 × 10−3 Pa). An ion source generates a directed ion beam, which hits the target material, ejecting the atoms from the solid surface in towards the specimen surface [9]. At this significantly lower pressure, the mean free path of the target atoms is many times larger than the target-substrate distance. The extracted particles experience a lower number of collisions on their path to the substrate and therefore they maintain their energy level and direction. Complex structured specimens have to be tilted and rotated during the coating procedure to ensure an even coating. 2.3.3. Penning sputter coating (PSC) The penning sputter method described by Peters [10], combines plasma generation and ejection of target material in one piece of equipment. From the source, a directed beam of neutral particles is emitted with an energy comparable to the kinetic energy of an ion beam sputter coater. In addition, the emitted beam is directed through an electrical field, which filters out charged particles. Here, too, the sample is maintained under high vacuum and must be kept in motion for a continuous coating. The high kinetic energy of the coating atoms leads to an implantation of the coating material in the membrane surface which can result in lower surface diffusion. Using neutral particle source thin coatings are possible, even at room temperature, and thus in most cases polymers can be prepared at room temperature. 2.3.4. Electron beam evaporation Evaporation through heating of the target material in a high vacuum is a reliable and (especially in the transmission electron microscopy) a widely used

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procedure. The directed beam of uncharged particles hits the sample with low kinetic energy. The same conditions concerning the geometry of the coating are valid as in the ion beam and penning sputter procedures. Because of the high temperature of the target, however, a higher thermal damage is to be expected. If the heat is applied to the target material through an electron beam, the evaporation area can be concentrated in a very little spot, and the emitted heat can be significantly reduced. For heat sensitive specimens, a liquid nitrogen cooling is indispensable. 2.4. Investigated membranes In this work only the porous surface of the investigated membranes is shown. To investigate the different coating methods, a porous polyetherimide (PEI)-membrane was selected. The PEI-membrane is mechanically stable and usually applied to support composite-membranes. Polyetherimide has a glass transition temperature of 220◦ C (Table 1). This membrane is relatively stable against heat radiation and treatment with charged particles. To demonstrate the difference between cryo preparation and the same process at room temperature, a polyacrylonitrile (PAN)-membrane was selected. The PAN-membrane can also be used to support composite-membranes. Additionally, this membrane is an excellent ultrafiltration membrane. However, its stability under electron radiation is much poorer than that of PEI.

3. Results and discussion When imaging sputter coated polymer membranes, nodular structures frequently can be observed on the membrane surface. Such nodules have been extensively reported in the membrane literature [12,13]. Table 1 Glass-transition temperature and flux data of the investigated membranes Polymers

Tg (◦ C)

Water flux (l/m2 h bar)

PEI PAN

220 [11] 90

350 1400

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Fig. 1. PEI-membrane, native specimen-surface.

Fig. 2. PEI-membrane, magnetron sputtered at room temperature.

3.3. Electron beam evaporation However, through comparative examinations of porous polymer membrane surfaces using SEM and SFM (scanning force microscope), it was possible to identify these nodular surface structures at least in some cases, as preparation artifacts [14].

Fig. 4 represents the PEI-membrane, coated with 2 nm Pt at 123 K by means of electron beam evaporation. This surface looks completely different from the

3.1. Comparison of coating procedures The surface of an uncoated porous polyetherimide membrane is shown in Fig. 1, investigated with an acceleration voltage of 1 kV. On this membrane preparation artifacts are not visible. 3.2. Magnetron sputtering The magnetron sputtered surfaces, represented in Figs. 2 and 3 are characterized by different nodule sizes. The PEI-membrane in Fig. 2 was prepared at room temperature and Fig. 3 represents a membrane coated at liquid nitrogen temperature. The nodule size reduces with the preparation temperature, but the result clearly indicate that cooling the specimen is not enough to reduce these preparation artifacts to an acceptable level.

Fig. 3. PEI-membrane magnetron sputtered at 143 K.

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Fig. 4. PEI-membrane, prepared with electron beam evaporation at 123 K.

magnetron sputter coated surfaces. In this case nodule structures are not visible. The pore shapes are similar to those in Fig. 1. In comparison to the uncoated specimen in Fig. 1, the surface still has a lack of fine structured details.

Fig. 5. PEI-membrane, ion beam sputtered at RT.

sputter coated surfaces. The ion beam sputter coating was not perfect, this resulted in specimen charging up at 5 kV acceleration voltage. The investigation of the ion beam sputter coated surface was thus performed

3.4. Ion beam and penning sputtering Both methods differ only in detail. During the coating process, the pressure in the specimen chamber is 1.5 × 10−3 Pa for the penning sputter coater (PSC) and 8 × 10−3 Pa for the ion beam sputter coating process [15]. In both cases, the pressure is low enough to prevent contamination and agglomeration of coating atoms before reaching the sample. A disadvantage of the ion beam sputter used here was that the preparation could not be made under liquid nitrogen cooling because this did not allow the possibility of tilting the large SEM-specimen during the coating process. Therefore, this result is not absolutely comparable to that of the PSC which was prepared at 140 K. Later works has shown, that for the PEI-sample cryo preparation is not necessary, is important if optimum results are to be obtained. Figs. 5 and 6 show the results from ion beam and penning

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Fig. 6. PEI-membrane, penning sputter coated at 140 K.

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Fig. 7. PAN-membrane, PSC at RT.

Fig. 8. PAN-membrane, PSC at 140 K.

3.5. The effect of cryo preparation at 1 kV. Under these conditions, the resolution of the microscope is reduced. The penning sputter coated sample was investigated at 5 kV without any charging effects. If electron beam evaporation is compared to PSC, it is apparent that the energy impact in the PSC is generally caused by the kinetic energy of the coating atoms, while heat is the main damage factor during electron beam evaporation. An increase of sample temperature also enhances the mobility of condensing atoms on the surface, and thus leads to growth of clusters and to a grainy structure of the conductive coating. Increased surface diffusion also encourages the condensation at preferred spots on the sample surface and thus causes a decoration. The higher the kinetic energy of the metal atoms from the ion beam or penning sputter sources, the greater the penetration of metal atoms into the sample surface. This results in a higher number of nucleation sites and thus to thinner closed coatings even at room temperature [16]. PSC i.e. presents the procedure with the lowest probability of causing damages to the sample surface through artifact buildup or decoration and thus allows excellent results.

Figs. 7 and 8 show porous PAN-membrane surfaces, coated in the same way as the PEI-membrane samples using PSC. The membrane was coated at room temperature (300 K) in Fig. 7 and at 140 K in Fig. 8. Charged particles are effectively filtered by means of the electric field applied in the particle outlet of the PSC. Thus, it can be assumed, that any specimen surface damage results from a surface temperature increase. Cooling the specimen surface before starting the coating process can reduce this temperature increase to an acceptable level.

4. Conclusions For high resolution images of polymer membranes with sufficient contrast, a conductive coating is indispensable. The recent developments in the area of conductive coating should suppress surface charging, minimize radiation damage and increase electron emission from the surface. It was shown that the selection of a suitable coating procedure could lead to significantly improved results in SEM images. Heat and the impact of charged

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particles can cause a significant alteration to the surface of fine structured polymer surfaces. The use of a neutral particle source was particularly suitable because the impact of charged particles was less than that obtained using other plasma-based processes. Heat damage could be avoided by cooling the sample with liquid nitrogen. The preparation method demonstrated is a reliable way to prepare sub-micron scaled heat- or radiation-sensitive samples for high resolution scanning electron microscopy.

Acknowledgements Thanks are due to the support of Fund of the Chemical Industry; Stephan Pfeiffer, Beiersdort and U. Nietschke, Gatan for the preparation works; H. Jacksch — LEO, for LVFESEM investigation of the uncoated specimen; Suzana Nunes, Jonathan Paul and Matthias Schossig for critical reading. References [1] K.J. Kim, A.G. Fane, C.D.J. Fell, D.C. Joy, Fouling mechanisms of membranes during protein ultrafiltation, J. Membr. Sci. 68 (1992) 79–91. [2] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Dordrecht, 1996, pp. 183–188. [3] K.-J. Kim, A.G. Fane, Low voltage scanning electron microscopy in membrane research, J. Membr. Sci. 88 (1994) 103–114.

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[4] L.C. Sawyer, D.T. Grubb, Polymer Microscopy, 2nd Edition, Chapman & Hall, London, 1996, pp. 141–142. [5] L. Reimer, G. Pfefferkorn, Rasterelektronenmikroskopie, Springer, Berlin, 1977, pp. 63–70. [6] Soft Imaging System GmbH, Hammer Str. 89, D-48153 Münster, Germany. [7] D.C. Joy, Control of charging in low voltage SEM, Scanning 11 (1989) 1–4. [8] A. Renou, M. Gillet, Formation of gold particles in a flowing argon system: electon microscopy of the density, size distribution and size dispersion, J. Cryst. Growth 44 (1978) 190–196. [9] P. Echlim, Recent advances in specimen coating techniques, Scanning Electron Microsc. 1 (1981) 79–90. [10] K.-R. Peters, Penning sputtering of ultra thin films for high resolution electron microscopy, Scanning Electron Microsc. 1 (1980) 143–154. [11] Manufacturer Information, General Electric, Das Eigenschaftsprofil von ULTEM Polyetherimid. [12] R.E. Kesting, The four tiers of structure in integrally scinned phase inversion membranes and their relevance to the various separation regimes, J. Appl. Polym. Sci. 41 (1990) 2739– 2752. [13] I.M. Wienk, Th. Van den Boomgaard, C.A. Smolders, The formation of nodular structures in the top layer of ultrafiltration membranes, J. Appl. Polym. Sci. 53 (1994) 1011–1023. [14] H. Kamusewitz, M. Schossig-Tiedemann, M. Keller, D. Paul, Characterization of polymeric membranes by means of scanning force microscopy (SFM) in comparison to results of scanning electron microscopy (SEM), Surf. Sci. 377–379 (1997) 1076–1081. [15] Manufacturer information, http://www.gatan.com/nav specimen.html. [16] I. Wildhaber, H. Gross, H. Moor, Comperative studies of very thin shadowing films produced by atom beam sputtering and electron beam evaporation, Ultramicroscopy 16 (1985) 321– 330.