Developing dual-beam methodologies for the study of heterogeneous polymer-based systems

Authors


A.M. Donald. Tel: +44 1223 337382; fax: +44 1223 337000; e-mail: amd3@cam.ac.uk

Summary

The use of a combined focused ion beam/environmental scanning electron microscope (FIB/ESEM) offers new possibilities for imaging the internal structure of complex heterogeneous polymeric samples. The use of the focused ion beam, using positively charged gallium ions in conjunction with a measured ‘defocused’ low-energy primary electron beam, has permitted milling through the heterostructure to be achieved in a controlled way, exposing the inner structure, without introducing significant ion beam damage/destruction into the sample. The subsequent use of the environmental scanning electron microscope for imaging the revealed internal structure has then enabled insulating polymer structures to be imaged, without charging problems. Cross-sections of a 900-nm-thick spun cast film of phase-separated polystyrene–polybutadiene blends have been successfully milled and imaged; the morphology agreeing with previous results produced using ultramicrotomy and transmission electron microscopy.

Introduction

The dual beam is a combination of a focused ion beam and scanning electron microscope (FIB-SEM). The FIB and SEM columns are angled typically at 52° from each other (Fig. 1A), enabling the FIB to mill into the bulk of the sample, and the subsequent use of the SEM to immediately analyse the exposed internal structure. Dual beams are able to perform all of the functions of the single-beam systems, while enhancing the capabilities by operating the columns concurrently. The conventional dual beam's main applications include material deposition (Tao et al., 1990), transmission electron microscopy (TEM) sample preparation (Kirk et al., 1989), defect characterizations, mask-less lithography (Kubena et al., 1989) and circuit edits and repairs. Most dual-beam work has concentrated on conventional FIB materials such as silicon. This is because of FIB's origins, which lie mainly in the semiconductor industry, from the late 1970s to early 1980s.

Figure 1.

Schematic diagram of dual-beam FIB-SEM. (A) Conventional FIB milling in high vacuum and (B) SEM imaging of milled sections. The two columns are positioned at 52° from each other. The sample surface is normally kept normal to the FIB column.

FIB columns follow principles similar to conventional SEM, in which the ion column can be divided into four separate sections. The ion sources of FIBs, known as liquid metal ion sources (LMIS; e.g. a tungsten coil dipped in liquid gallium), are positioned at the top of the column and resistively heated. The ion gun emits ions over a small spatial volume, with small angular spread and selectable energy. The ions then go through the optical system, consisting of several ion lenses, which direct the ions towards the specimen surface, before being deflected by the scan unit, which moves the beam in a raster pattern. The beam is typically focused to diameters of approximately 7 nm.

A major advantage of using the finely focused beam of ions is that it allows the site-specific milling and deposition of materials in a very controllable way (Tseng, 2004). The site-specific deposition of material is achieved by introducing a gas that chemisorbs onto the sample, and the FIB causes the gas to decompose into volatile and non-volatile components. The non-volatile components remain on the sample and are deposited precisely. FIB deposition has served as a simpler and faster, resistless (non-lithographic) technique (Marzi et al., 1999; Ziroff et al., 2003) for producing patterned material with sub-micrometre resolution (Vila et al., 2006). Traditionally, these depositions are used to connect or isolate wires when conducting device edit operations.

Platinum, which has a thermal conduction of 72 Wm−1 K1 and an electrical resistivity of 10.0 μΩ cm−1 at 300 K, can also be deposited on insulating surfaces (Tao et al., 1990; Marzi et al., 1999; Telari et al., 2002). A thin layer of platinum is applied with a low beam current. A low beam current enables the deposition of material onto insulating surfaces without causing any major charge-induced artefacts. However, low beam currents mean long deposition times. Hence, a further thicker layer of platinum is applied on top of the initial layer with a higher beam current. This ensures a ‘flat uniform surface’. This two-step process is important for rough insulating surfaces. A layer of platinum is able to protect insulating surfaces that are to undergo milling and enhances charge and heat transfer during the process.

Sputtering is the physical process by which material is removed by FIB. Site-specific atoms in a solid target material are ejected into the gas phase due to bombardment by energetic focused ions, largely driven by momentum exchange between the ions and the atoms in the material. The sputtering yield, defined as the number of atoms ejected per incident ion, is essentially a measure of the efficiency of material removal. For conventional dual-beam materials, the yield is normally in the range of 1–50 atoms per ion and is a function of the mass of ion and target atoms, ion energy, temperature and ion flux (Tseng, 2004). The yield is controlled usually by altering the energy of the beam. Initially, sputtering yield increases as ion energy increases, but the yield starts to decrease as the energy is increased past a certain level, for which ions can penetrate deeper into the material (Brodie & Murray, 1992). If the energy is too high, it can cause a variety of unwanted interactions including swelling, deposition, implantation, backscattering and nuclear reaction (Brodie & Murray, 1992). The milling yield dependence on incident angle, defined as the angle between the beam of ions and the surface of the target material, is optimal at 90° (Tseng, 2004).

The inclusion of a precursor gas during milling increases the volatility of the products significantly, reducing the degree of re-deposition. Material re-deposition has been envisioned as a process in which initially momentum is transferred from the ion to the surface, with the subsequent ejection of surface and near-surface species that form a crater (milled section in which the walls are slanted) (Lindquist et al., 1993). As milling proceeds, the slant of the walls increases, and the whole milled section starts to resemble a cosine distribution (Lindquist et al., 1993), as shown in Fig. 2. When the ion beam is applied in a raster pattern over a small area, that is, when the milling depth is greater than the width (known as aspect ratio >1), re-deposition is very likely to occur in all materials.

Figure 2.

Progression of the resemblance of cosine distribution of sputtered material when milling high aspect ratio structures. The amount of re-deposition can be significantly reduced in the presence of selective carbon milling gas, as described in the text.

The advantages of using a precursor gas include enhancement of the FIB etch rate, hence a reduction in processing times, a reduction of unwanted implantation from primary ion species onto the substrate and a reduction of re-deposition on side walls (Young et al., 1993; Harriott, 1994). Traditionally, halogen-based chemical etching has been typically used for materials in the semiconductor industry; these include the use of Cl2 with GaAs, Si and InP (Young et al., 1990, 1993) and XeF2 for SiO2, W (Harriott, 1994) and diamond (Russell et al., 1998; Datta et al., 1999). Very briefly, the reaction of the irradiated surface species with absorbed halogen produces volatile species and re-deposition of surface material is essentially quenched. However, although halogen-based chemical etching is useful for many material systems, halogens do not universally enhance removal rates for most carbon-containing specimens (Stark et al., 1995). It has been observed that the introduction of water vapour increases the removal rate of carbon-containing materials such as polymethyl methacrylate (PMMA) and other resists by a factor of 20 (relative to physical sputtering) (Stark et al., 1995). Selective carbon mill SCM™ (FEI, Eindhoven, the Netherlands) is a novel special water-based chemical (MgSO7H2O) that has been developed for removing carbon-based materials effectively.

As stated earlier, the FIB was originally developed in the 1970s and 1980s for the requirements of the semiconductor industry, in which it was mainly used for applications such as circuit edit and repair. Today, when milling conventional FIB materials such as silicon, the ion beam is able to precisely mill cross-sections, without any electrostatic discharge damage. However, this is of course not the case with insulating material; the higher the ion current, the greater the risk of FIB-induced charging. Under irradiation, ions are implanted into the sample and electrons and ions are emitted from the surface. Because the emission yield of secondary electrons is approximately 10 times greater than secondary ion emission for insulators (Komoda et al., 2005), the sample surface is charged up positively, which in turn deflects the positively charged ion beam.

The use of the FIB in conjunction with a measured ‘defocused’ low-energy primary beam of electrons has permitted milling through insulating structures to be achieved in a controlled way, exposing the inner structure, without introducing charge damage into the sample (Stokes et al., 2007) (see Fig. 3A). Experimentally, it has been found that if specimen current ISP is measured during ion irradiation, it measures roughly three times the primary ion beam current. This has led to the suggestion that for each positive charge implanted, there are two positive charges remaining as a result of secondary electron emission δ, that is, Q=QPI+ 2Qδ. This has provided the criteria for choosing the electron beam current for neutralization IPE∼3IPI (Stokes et al., 2007).

Figure 3.

Schematic diagram of dual-beam FIB-SEM/ESEM. (A) FIB milling revealing inner morphology. Note: The electron beam is in use during milling–SEM mode. In comparison, electron beam is not in use in Fig. 1(A). (B) Non-destructive environmental/ESEM imaging of freshly revealed cross-section. A gas is now employed in the chamber for imaging.

Furthermore, during charge neutralization, the electron beam is totally ‘unblanked’, that is, a constant flux of electrons is directed onto the sample surface. Also, by raising the focus plane of the electron beam, the electron beam is spread over the sample. The incident surface area of the electrons (Fig. 4) is controlled and measured by vertically raising the focus plane of the electron beam by distance l in millimetres and is calculated via the formula (Stokes et al., 2007):

image(1)

where r is the radius of the electron beam calculated from a =πr2, where a is the area of the mill.

Figure 4.

Schematic diagram showing the criteria for charge neutralization. The electron beam, with radius r, is required for milling the pattern/area shown.

Once uniform milling of the heterostructure is achieved, the use of the environmental scanning electron microscope (ESEM) (Danilatos, 1988) for imaging the revealed milled section surfaces then enables insulating materials to be imaged without charging problems (see Fig. 3B). ESEM employs a gas in the specimen chamber (Meredith et al., 1996; Thiel et al., 1997) in which water vapour is typically used at pressures of 1–2 torr (133–266 Pa). As in conventional SEM, the electron source is kept in high vacuum, and for this reason, a pressure gradient along the column is necessary. The column is separated into a series of zones, each of which is pumped individually and is separated from its neighbours by small limiting apertures (PLAs) that allow the electron beam to pass through but maintain the pressure difference between each zone. The vacuum arrangement along the column is shown in Fig. 5.

Figure 5.

Schematic diagram of ESEM column, pressure zones and various pressure-limiting apertures.

The emission of electrons in ESEM is specimen-dependent, and subsequent collisions between these electrons and gas molecules result in the generation of daughter electrons, that is, a cascade amplification effect (Meredith et al., 1996; Thiel et al., 1997), and positive ions. The signal is amplified as a result of the cascade, which is detected by a gaseous secondary electron detector (GSED). The positive ions produced drift towards the specimen surface and compensate the negative charge built up on the surface (Meredith et al., 1996).

In this study, we mill a cross-section of a model polymer system of 900-nm-thick spun cast film of phase-separated polystyrene–polybutadiene blends stained with osmium tetroxide (OsO4) with the aim of developing and optimizing dual-beam methodologies for application to polymeric samples generally. The three-dimensional morphology of polystyrene–polybutadiene thin films cast from a toluene solution has been fully characterized previously (Geoghegan et al., 1994) by combining results from three different techniques: nuclear reaction analysis, neutron reflectometry and ultramicrotomy/TEM. An asymmetric mixture of polystyrene and polybutadiene (molecular weights [MW] 5050 and 540 000 for polystyrene and polybutadiene, respectively) forms a blend in which polystyrene-rich phases wet both air and substrate interface and are separated by a polybutadiene-rich central phase. These layers are very well defined, and the interfaces between them are sharp, with a width of <20 Å. The blends also show regions in which isolated polybutadiene domains are surrounded by polystyrene shells (Geoghegan et al., 1994).

Experimental procedures

For the sake of direct comparison, a sample similar to the work of Geoghegan et al. was carried out. Polystyrene 0.5 g (MW 4000) and polybutadiene 0.5 g (MW 420 000) were added to excess toluene solution and mixed with a magnetic stirrer for 12 h. Two drops of the solution of the polystyrene–polybutadiene mixture were applied to the surface of glass slides, while being rotated at ∼1 g for 60 s, allowing a thin uniform film to be formed. The films were vapour-stained with 2% aqueous solution of OsO4 and kept between 50°C and 60°C for 2 h. The films were then left in a desiccator for 12 h for the removal of any excess solvent.

A platinum deposition rectangle 8 μm × 22.5 μm × 100 nm was applied on the surface of the polystyrene–polybutadiene blend at a magnification of ×2400. A 50-pA Ga+ ion beam current was used for deposition, which required a neutralization electron beam current of 150 pA (IPE∼3IPI) and a radius of 7.56 μm. The radius was achieved by vertically raising the focus plane of the electron beam by 2.04 mm (Stokes et al., 2007). A further layer of platinum, 8 μm × 10 μm × 1 μm was applied on top of the initial platinum layer.

Cross-section milling was conducted with ion beam currents of 1.0 nA and 50 pA and electron neutralization currents of 3 nA and 150 pA, respectively, with a milling depth of 1.5 μm. (The milling depth of 1.5 μm is the approximate depth calculated by the Quanta 3D DualBeam™[FEI company, Eindhoven, The Netherlands] for milling silicon. In reality, this will mill deeper into a softer material such as the polymer blend.) The milling area for the 1-nA milling stage measured 180 μm2; hence, an electron beam radius of 7.56 μm was used. The milling area for the 50-pA milling stage measured 16 μm2; hence, an electron beam radius of 2.25 μm was used. Milling was conducted with the selective carbon mill gas present. The films were transferred from the Quanta 3D DualBeam™ to a FEG ESEM XL30™ (FEI company) and imaged at a pressure of 0.98 torr. (ESEM in the Quanta employs a tungsten hair-pin filament that is resistively heated and is of relatively low brightness compared with the field emission gun ESEM, which offers higher resolution.) The films were also studied using a Dektat profilometer (Veeco Instruments, Cambridge, UK), which was used to measure the thickness and roughness of the polystyrene–polybutadiene blend.

Results and discussion

Figure 6(A) defines the axes for ESEM image analysis. The xy plane represents the surface of the films, and the xz plane represents the milled cross section. (Milling is in the z direction.) A full field of view of the cross-section of the milled polystyrene–polybutadiene blend is shown in Fig. 6(B). The image shows the surface of the blend, the platinum laid on the surface, the milled cross-section that used ion beam current of 1 nA and the cross-sectioned polished area that used ion beam current of 50 pA. Figure 6(C) shows the blend at a higher magnification and a higher contrast setting. The darker regions in the centre of the film are the osmium-stained polybutadiene. The lighter surrounding regions are polystyrene. There is a clear indication that polystyrene-based surface wetting has occurred at the blend–air and blend–substrate interface. In Fig. 6(D), the polystyrene at the blend–air interface appears brighter than at the blend–substrate interface because the layer at the air surface is closer to the GSED used in ESEM. The secondary electrons further away from the surface in the xz plane need to travel a longer distance than those produced above.

Figure 6.

(A) Axis definitions for ESEM images in which z-axis is parallel to the milling direction. (B–D) Cross-sectional ESEM images of polystyrene–polybutadiene blend (osmium tetroxide-stained) at different magnifications (lowest to highest). Note: Panels (B) and (C) are shown with high contrasts in order to show the milled inner sections; hence, the surface and platinum depositions are difficult to see in panels (B) and (C). (E) Surface (xz plane) ESEM image of same blend with crack.

Figure 6(D) allows film thickness estimation of approximately ∼912 nm (corrected for imaging at an angle of 52°), which is consistent with subsequent measurements made with a Dektat profilometer. The Dektat profilometer also detected cracks running on the surface, which can also be seen in Fig. 6(B). These cracks were observed in the study by Geoghegan et al., who speculated that they are formed by the polystyrene layers fracturing as they pass through the glass transition. A close-up ESEM image (xy plane) of a surface crack in Fig. 6(E) allows us to see the morphology under the polystyrene-rich surface, which is similar to the morphology shown in Fig. 6(D).

Overall, from Figs. 6(B–E), it can be seen that FIB cross-sectioning and ESEM imaging are an effective method for studying the model structure of the polystyrene–polybutadiene blend. ESEM imaging of the blends has been conducted at a resolution of approximately 20 nm. In comparison to the previous study of the structure (Geoghegan et al., 1994), sample preparation steps such as the floating of the films from glass microslides in a water bath and the embedding in epoxy resin have been avoided. Sectioning using a microtome/diamond knife can also often cause cutting artefacts such as compression lines in images; however, these were not apparent in the study by Geoghegan et al.

In addition, in the images by Geoghegan et al., the interface between the polystyrene-rich surface layers and the embedding medium could not be resolved because the low-molecular-weight polystyrene easily dissolved in the resin. It was also difficult to know for sure at what angle exactly the microtome had cut the films, as similar films showed different thicknesses. In the work by Geoghegan et al. (1994), the different thicknesses in the polystyrene–polybutadiene blends are due to thermodynamic reasons. Nonetheless, on more general terms, angular errors can be introduced during microtomy. For these reasons, FIB-ESEM may well be the most reliable, efficient and precise method for studying the three-dimensional morphology of these blends.

Conclusions

FIB-ESEM offers the ability to study cross-sections of insulating material, despite the absence of a conductive coating. Images show little or no evidence of any major induced charging during the milling stage, indicating that optimization of charge neutralization conditions has been achieved.

Cross-sectional images clearly distinguish the platinum applied on the surfaces of the polystyrene–polybutadiene blends. This has shown that platinum deposition can be carried out on insulating samples, while still preserving the topology, as long as the beam currents are kept low. This in turn has allowed the surface of the blends to be milled without introducing significant surface artefacts.

Cross-sectional milling of the blends has been conducted with the novel SCM™ gas, and it has been observed that SCM™ enhances the milling process of polymeric samples, reducing the degree of re-deposition and displaying a morphology consistent with previous work in which polystyrene-rich and polybutadiene-rich areas have clearly been distinguished.

Ancillary