Developing dual beam methods for the study of polymers

The use of a combined Focussed Ion Beam/Environmental Scanning Electron Microscope (FIB/ESEM) offers new possibilities for imaging complex heterogeneous polymeric structures. The use of the focussed 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 charge damage into the sample. The use of the Environmental Scanning Electron Microscope for imaging the revealed milled sections has then enabled insulating polymer structures to be imaged without charging problems, despite the absence of a conductive coating. Cross sections of a 900nm thick spun cast film of phase separated polystyrene-polybutadiene blends have been successfully milled and imaged, the results agreeing with previous experiments produced using ultramicrotomy and TEM. In addition, elliptical shaped titanium dioxide particles approximately 220nm in diameter have been dispersed in commercial film forming latices at concentrations between 0 and 100 percent volume. The films have been cast, milled 2μm deep and imaged. Results on the arrangement of titanium dioxide particles in the polymer matrices are presented. 1.1 ESEM Conventional scanning electron microscopy has allowed the structural characterisation of various material systems [1]. However, the high vacuum employed by SEM restricts the imaging of insulating and wet specimens. Coating of insulating specimens is usually necessary for the prevention of imaging artefacts introduced as charge accumulates in the sample [2]. The development of environmental scanning electron microscopy [3] allows imaging of insulators, despite the lack of a coating. In addition to topography and variation in atomic number, contrast also arises from charge distribution and electronic structure [4]. ESEM employs a gas in the specimen chamber [2]. Typically water vapour is used at pressures of 1-2torr. As in conventional SEM, the electron source is kept in high vacuum; 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) which allow the electron beam to pass through, but maintain the pressure difference between each zone. The vacuum arrangement along the column is shown in Figure 1(a). 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 i.e. a cascade amplification effect [2] and positive ions. The signal is amplified as a result of the cascade, which is detected by a gaseous secondary electron detector (GSED). The cascade effect is demonstrated in Figure 1(b) [5]. Positive ions produced drift toward the specimen surface and compensate the negative charge built up on the surface. 1.2 Focused Ion Beam Milling (FIB) The finely focused beam of ions allows site specific milling of materials, in a very controllable way [6], in addition to being able to provide high resolution imaging and defect characterisation. FIB Electron Microscopy and Analysis Group Conference 2007 (EMAG 2007) IOP Publishing Journal of Physics: Conference Series 126 (2008) 012079 doi:10.1088/1742-6596/126/1/012079 c © 2008 IOP Publishing Ltd 1 columns follow similar principles to conventional SEM where the ion column can be divided into four separate sections. The ion sources of FIB known as liquid metal ion sources LMIS (for example a tungsten coil dipped in liquid Gallium) are positioned at the top of the column and resistively heated. The ion gun emits ions with small spatial volume, small angular spread and selectable energy. The ions then go through the optical system, consisting of several ion lenses which direct the ions toward 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 ~7nm. Sputtering is the physical process by which material is removed by FIB. 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 ions and 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. The yield is normally in the range 1-50 atoms per ion, and is a function of the mass of ion and target atoms, ion energy, temperature and ion flux. 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 where ions can penetrate deeper into material [7]. If the energy is too high, it can cause a variety of unwanted interactions including swelling, deposition, implantation, backscattering and nuclear reaction [7]. Milling yield due to incident angle, defined as the angle between the beam of ions and surface of the target material is optimal at 90 degrees [6]. Figure 1 (a) Schematic diagram of ESEM column, pressure zones and various pressure limiting apertures, (b) The cascade amplification effect in ESEM [5]. Gas molecules are ionised by electrons which have been emitted at the surface of sample, these collisions gives rise to daughter electrons which cause further ionisation of gas molecules en route to the positively charged detector Figure 2 Schematic diagram of dual beam (a) FIB milling revealing inner morphology (b) non-destructive ESEM imaging of crosssection, note the electron beam is in use during milling, charge neutralisation is explained in the next section 1.3 Charge Neutralisation FIB exposure causes electrostatic discharge damage in insulating material; the higher the current the greater the risk of FIB induced charging. Under irradiation, ions are implanted into the sample and several particles (electrons, ions and neutrals) are emitted from the surface. Since emission yield of secondary electrons is approximately ten times greater than secondary ion emission [8], the sample surface is charged up positively; which in turn deflects the positively charged ion beam. The use of FIB in conjunction with a measured ‘defocused’ low energy primary beam of electrons has permitted milling through the heterostructure to be achieved in a controlled way, exposing the inner structure without introducing charge damage into the sample [9]. Experimentally, it has been found 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 Electron Microscopy and Analysis Group Conference 2007 (EMAG 2007) IOP Publishing Journal of Physics: Conference Series 126 (2008) 012079 doi:10.1088/1742-6596/126/1/012079