A stereo technique for measuring the depth of specimens in transmission electron microscopy

Micron, 1976, Vol. 7: 171-173. Pergamon Press. Printed in Great Britain.

A stereo te~hn;que for measur/ng the depth of spec/mens in transmlss/on electron m|croscopy H. F. PREMSELA Philips Research Laboratories, Eindhoven, The Netherlands Manuscript received January 8, 1976

SHORT COMMUNICATION With the aid of stereo pairs, measurements in depth of specimens used in transmission electron microscopy can be carried out in a way which is both relatively simple and accurate. By this procedure, various di~culties inherent in other measuring methods are avoided. Using a thin.film of SiO coated on both su~'aces with gold as an example, its thickness has been calculated using the eucentric goniometer of the Philips E M 301 trammission electron microscope and a stereoscope of the Wheatstone type and applying simple geometry. The method has been shown to be applicable to measurements of the order of" l Onm.

Since the early stages of electron microscopy it has been possible to obtain stereoscopic images (stereo pairs) of specimens examined in the transmission electron microscope (TEM). However, relatively little use has been made of this technique. Yet it provides a means of measuring the thickness of a thin film, difference in height of an uneven surface or distances in depth between details within a thin section in a direct way which is both relatively simple and accurate. The stereo technique makes it possible to carry out quantitative studies on processes taking place in situ, such as etching or contamination of the specimen. Moreover, other measuring methods may be less reliable or give relative values only. For example, when we attempted to measure the thickness of a thin film of SiO by micro-weighing we were unable to obtain reproducible results. On the other hand, reliable results were obtained when we applied the oscillating quartz crystal method during the evaporation of the SiO in the shadowcaster. However, this only produced relative values although these were found to be quite useful for the controlled production of films of a required thickness once we had obtained absolute values by the stereo technique described below. For accurate measurements with the stereo method it is essential to use a tilting stage in which (a) the focus remains constant during tilting so that the micrographs are obtained at the same initial magnification within a tolerance of a few percent, (b) the angle of tilt is accurately known and reproducible and (c) there 171

is minimal drift and vibration, especially when working at high magnification. For optimal results a high quality specimen stage is clearly necessary and in our studies we used the Philips E M 301 transmission electron microscope equipped with a eucentric goniometer. The procedure we adopted is most easily explained by reference to a particular example. For the purposes of this short communication, therefore, reference is made to films of SiO prepared in the usual way but with both surfaces covered with particles of gold to assist with the measurements. The specimen was inserted into the microscope and tilted about an angle ~ of ~ 6 °. Stereo pairs were obtained at an initial magnification of ×220,000 and enlarged photographically to a final magnification of ×300,000. A typical result is shown in Fig. 1. For measuring purposes, the negatives were enlarged to a final (print) magnification M of × 550,000 and examined in a stereoscope of the Wheatstone type (Haanstra, 1966). A schematic view of the arrangement of the components is shown in Fig. 2 where M 1 and M e are the semitransparent mirrors and where PhL and PhR are the left-hand and right-hand stereo micrographs respectively. Each of the micrographs is mounted on a disc that can be rotated about its axis thus permitting adjustment of the mutual angular position of the images as well as the angular position of both images required to correct for parallax (i.e. parallel to the line connecting the eyes of the observer). The viewing distance can be varied. As

172

tl. 1". Premsela

Fig. 1. Stereo pair of a thin film ~t" SiO covered on both sides wittl particles of gold. . 300,000. shown in Fig. 2 the stereoscopic image of detail P, represented by the mirror images P~. and P~, is located at Pster. A marker is placed at a distance from the observer approximately equal to the distance between him and the micrographs. T h e marker can then be translated along the x and y axis to make it coincide

with selected details in the tield of view and moved in the direction z to m a t c h the depth of the detail. T h e depth Z at which Pste,. is seen (Fig. 3) depends on: 1. the distance b between the observer's eyes, 2. the distance L between the line connecting the observer's eyes and the

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pLPhL IMI/~ 11M2 PRPh,,~~_~ ,:>

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Fig. 2. Arrangement of components in the stereoscope.

A Stereo Technique for Measuring the Depth of Specimens in TEM

173

at which Pster is located in the specimen can then be calculated as follows:

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Z =

P Ster

d =

L b / 2 M d sin a - - 1 b

Z

For measuring the thickness of the SiO support film in our example, the marker in the stereoscope was moved from the upper to the lower surface. In a particular case, this distance Z proved to be 20ram. As mentioned previously, M was × 550,000. (A check of the magnification read-out of the microscope showed that the indicated figure was correct within 2 ~ . ) The tilt angle ~ was 6*, the viewing distance L was 500mm, and the distance between the eyes of the observer was 60ram. Using equation (2) the thickness of the film was found to be 21nm. Equation (2) is valid only for exposures in which the magnification is the same from the top of the specimen to the bottom. In transmission electron microscopy this will almost always be the case since the specimens are relatively thin and the illuminating electron b e a m can be considered to be collimated (telecentric projection).

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I 1.2Mx.4

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Fig. 3. Geometry of stereoscopic image formation.

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mirror images of the micrographs, and parallax M x in the micrographs. Since: tan

13 --

Z --

3.

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L

Mx

b/2 - - Mx d

Z--

Z _.

LMx b/2 - - Mx L b/2Mx-

1

(1)

I f the specimen is tilted about an angle ~, the parallax x equals d sin ~ (Fig. 4). The depth d

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Fig. 4. Determination of parallax on tilting.