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Micron Vol. 28, No. 3, pp. 217 219, 1997 ~) 1997 Elsevier Science Ltd

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Ultramicrotomy for Cross-sections of Nanostructure

CARMEN QUINTANA

Instituto de Microelectronica de Madrid ( CNM-CSIC), Parque Tecnoldgico de Madrid. 8 Isaac Newton, 28760 Tres Cantos, Madrid, Spain

(Received 25 May 1996; accepted 10 March 1997)

Abstrac~ Ultramicrotomy is a thin sample preparative method for TEM observations. In materials science, it can be used as an alternative method to mechanical polishing and ion-milling for cross-section specimen preparation. This short communicat ion reports results obtained for two different materials: a laser structure based in semiconductor technology (InP/GaInAs) prepared by MBE and a Au-SiFe multilayer prepared by co-sputtering. In both cases, Epon embedding medium and a diamond knife were used. The resulting cross-sections revealed good preservation of the different regions of the laser device and the presence of defects in the layers of the Au FeSi structure. It was concluded that ul t ramicrotomy can be used as a routine preparative method for TEM observations of nanostructures. ~) 1997 Elsevier Science Ltd

Key words: ultramicrotomy, cross-sections, microelectronics devices.

Ultramicrotomy is a thin sample preparative method for light and transmission electron microscope observations (Hayat, 1989). Since the 1950s, ultramicrotomy has been routinely applied to soft biological materials using glass knives. About 20 years later the commercialisation of diamond knives allowed the use of ultramicrotomy to obtain sections of hard materials such as mineralised tissues (Quintana and Sandoz, 1978). In materials science, the methods generally used for TEM specimen preparation are mechanical and ion-sputtering (ion- milling) thinning of the samples rather than ultramicro- tomy. However, since 1990 (Malis and Steele, 1990) there has been an increased interest in this method. Ultramicrotomy has been used to obtain specimens of oxidic films on glass (Becker and Bange, 1993), thin sections of metals (Howell et al., 1995; McMahon and Malis, 1995) and semiconductors (Albu-Yaron et al., 1993; Glanvill, 1995).

The advantages of ultramicrotomy over ion-milling are: no irradiation damage, no chemical mixing, no differential thinning rates and the ease of preparation of many serial sections with large, thin areas of uniform thickness in a relatively short time.

To test the possibilities of ultramicrotomy for cross- section TEM observation of samples prepared in the laboratory, this preparative method was applied to two different materials: a laser structure based on III V semiconductor technology (GaInAs/InP), prepared by Molecular Beam Epitaxy on InP, and a Au-SiFe multilayer, prepared by co-sputtering on GaAs (Fig. 1).

In both cases, Epon (EMbed 812) embedding medium (medium hardness) was used. Before the embedding, samples were mechanically polished to a thickness of around 0.1 mm and small fragments of 0 .2x0.5mm were cut. Unclosed flat embedding moulds were used.

Tel.: +34 I 8060705. Fax: +34 1 8060701.

After removing from the moulds, the blocks were mounted in the microtome chuck of a Reichert-Jung Ultracut F ultramicrotome for trimming, on a special trimming block, using a razor blade. The blocks were then transferred to the ultramicrotome and the face to be sectioned was aligned perpendicular to the axis of the specimen holder. The multilayer to be cross-sectioned was aligned parallel with the cutting direction and normal to the cutting edge. Prior to sectioning, the knife was slightly rotated so that only the multilayer and a small portion of the substrate were sectioned. As a consequence, the interface between the multilayer and the substrate was not always easily observed. Sections of 60-70 nm nominal thickness were then performed using standard conditions (cutting speed of 1 mm/s, clearance angle of 8 °) with our 18-year-old, 45 ° diamond knife (Quintana and Sandoz, 1978). Sections were picked up on carbon-coated copper grids and observed at 200 kV in a Jeol 200FX TEM microscope.

Figure 2(a) and (b) show the waveguide structure of the laser device. In the selected area diffraction pattern [Fig. 2(a)], which corresponds to a plane next to the cleavage plane (110), the 002 reflections can be observed together with two satellite reflections in the [001] direc- tion due to the superlattice of (InP)s(Ga0.47 In0.s3 As) s (2.7 nm periodicity). The micrograph is a two-beam bright field image (transmitted beam and 002 plus superlattice reflections). Different regions of the waveguide structure, superlattices, barriers, quantum wells and the InP substrate can be visualised [Fig. 2(b)].

Figure 3(a) and (b) show bright field images of the Au-SiFe multilayer grown on the GaAs support. The 2.5nm periodicity discontinuities in the layers and bending of the multilayers are visible in these images.

Summing up, the cross-sections obtained with a 45 ° diamond knife reveal that ultramicrotomy can be used as a routine preparative method for TEM observation of

217

(a)

0.2 ~m cap layer InP:Be 5x10 Is cm "3

1 prn cladding InP: Be 5xl017cm "3

0.2448 Ima waveguide

1 ~ma cladding InP: Si lxl018 cm 3

Substrate

InP:S

SL

~Barriers

- ~ QW

. . . . . . . . . . . . . . . . SL

SL: (InP)5/(Ga0.47 In0.53 As)s: 1000 A

Barriers: (InP)5/(Ga0.47 I110.53 AS)5:175 A

QW: Ga0.47 In0.53 As: 32 A

t 1Au 16 x Fe60Si40

GaAs (100)

Fig. 1. (a) Scheme of the laser device on InP substrate. SL=superlattice, QW=quantum well. (b) Scheme of the superlattice Au-Fe6oSi4o on GaAs substrate.

@

(a )

Fig. 2. (a) Selected area diffraction pattern of waveguide of the laser device. (b) Bright field TEM image of waveguide of the laser device. Superlattices (2.7 nm periodicity), quantum wells and barriers are well visualised. The other features super- imposed on the image of the multilayer could be due to inhomogeneities of the carbon support. The scale bar indicates

50 nm.

W

Fig. 3. Bright field TEM images of the superlattice of Au- Fe6oSi40 (2.5 nm periodicity). (a) Scale bar indicates 100 nm, (b)

Scale bar indicates 10 nm.

C. Quintana 219

microelectronic devices. The quality of the sections was high enough to preserve the whole morphology. It allows us to measure the periodicity of the two multi- layers studied, the verification of the absence of defects in the waveguide structure of the laser device and the observation of defects in the Au-FeSi multilayers. How- ever, it is not possible to obtain very thin, high quality sections with our 'old' knife. The quality of the sections obtained is not high enough to assert whether the observed bending in the Au-SiFe multilayer is a cutting artefact or if it comes from the GaAs native oxide substrate corrugation. For the same reason, the complex structure observed at the interface between the first layer of FeSi and the GaAs substrate can not be studied in sections 60-70 nm thick.

In order to obtain thinner and better quality sections for high resolution observations it becomes compulsory (1) to use a high quality diamond knife and (2) in every specific case, to perform systematic studies of the opti- mal cutting conditions minimising the cutting artefacts such as microcleavage, microtwining, rumpling and bending.

Acknowledgements' The laser was prepared by M. L, Dotor, the Au-FeSi multilayer by P. Caro and A. Cebolleda. D. Gomez and A. Calle participated in specimen preparation. The ultramicrotome be- longs to the 'Centro de Ciencias del Medio Ambiente' (R. de-Felipe in

charge) and the TEM to the 'Instituto de Ciencias de Materiales de Madrid' (P. Herrero in charge). The author would like to express sincere gratitude to all of them and would also like to thank F. Briones for his encouragement and support. This work was supported by Grant MAT95-1042-C02-01 from the Comisi6n lnterministerial de Investigaci6n Cientifica y Tecnol6gica (CICYT).

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