JOURNAL OF MATERIALS SCIENCE 20 (1985) 2169-2185 Applications of tilted illumination in transmission electron microscopy and diffraction of crystalline materials J. R. WHITE,* E. L. THOMAS Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, USA Diffraction contrast depends on the relative orientation of the electron beam and the crystals in a polycrystalline sample. Tilting the illumination will therefore alter image contrast, and the consequences of changing the illumination direction rather than the specimen attitude (using a goniometer) are examined. The electromagnetic tilt controls permit very accurate tilts to be set up very easily and quickly, a feature of special import- ance when dealing with radiation-sensitive crystals. The effect on both dark- and bright- field imaging is considered. The loss of resolution that happens when the beam is tilted does not seriously detract from the overall image quality when operating at low magnifi- cation, as is often the case with beam-sensitive specimens. Examples of tilted-beam operation are presented for polycrystalline gold and polyethylene samples. 1. Introduction Most transmission electron microscopes are pro- vided with a set of electromagnetic coils in the illumination system for tilting the electron beam. The primary reason for including this device is to permit tilted-illumination dark-field operation. In this mode it is arranged that the diffracted beam to be used to form the dark-field image passes along the optic axis of the objective lens, so minimizing spherical aberration. The appropriate illumination tilt is (minus) 20hk z where Oak z is the Bragg angle for the chosen beam, and the microscope is adjusted while viewing the selected- area diffraction pattern. The normal procedure is to note the position of the undeflected illumi- nation (the central transmitted beam) then to bring the chosen diffracted beam to this position using the tilt coils [1, 2]. A similar procedure can be employed when forming high-resolution images using two or more beams (which might include the transmitted beam) for imaging. In this case best resolution is obtained when the beams are symmetrically distributed about the optic axis, so that the same spherical aberration phase-shift is suffered by each beam [2-4]. For such oper- ation the tilt adjustment is again made with reference to the diffraction pattern, using the objective aperture to mark the position of the objective optic axis, but in this case the objec- tive aperture must be of quite critical dimen- sions and rather larger than is normally employed in single-beam dark-field operation as described here. The purpose of the current paper is to describe the consequence of using tilted illumination with crystalline specimens. Particular attention is given to the use of tilts other than 20hlel , to the effect of beam tilt on bright-field images, and to the combined effect of beam tilt and defocus. The procedures described here were developed initially for use with polycrystalline polymer specimens, but some of the results presented below were obtained with evaporated gold speci- mens which are simple to make, and which do not suffer from the problem of rapid deterioration in the electron beam. *Permanentaddress: Department of Metallurgyand EngineeringMaterials, Universityof Newcastle-Upon-Tyne, UK. 0022-2461/85 $03.00 + .12 9 1985 Chapman and Hall Ltd. 2169