TY - JOUR
AU1 - Terauchi, Masami
AU2 - Satou, Futami
AU3 - Sugizaki, Hideo
AU4 - Suganuma, Katsuaki
AB - Abstract A specimen-cooling device has been designed, manufactured and integrated into a commercial ion-milling instrument for transmission electron microscopy. The instrument enables us to prepare section specimens of tin-plated Cu without forming intermetallic compound particles and/or voids. The results show that cooling of specimen during ion-milling process is necessary for fine structure investigations of low melting temperature materials. cooling stage, ion-milling, Sn-plating Transmission electron microscopy (TEM) is a powerful method for investigating fine structures of materials. Before TEM observations, thin-film specimens with a thickness of an order of 100 nm or less have to be prepared for electron transmission. For that purpose, ion-beam thinning instruments have been widely used. In recent years, identification of small specimen areas for thinning has been greatly improved by focused ion-beam instruments combined with scanning electron microscopy. In those instruments, suppression of irradiation damage and/or temperature increase during the thinning process is very important, because these can cause artificial structures. A temperature increase is especially serious for low melting temperature materials, because it can accelerate atom diffusion and sometimes changes the quality of the specimen. Tin-plating technology is practically significant for connecting electronic circuits in all electronic devices. To suppress discharge problems of electronic circuits due to the formation of tin-whiskers, Pb-doped tin plating has been widely used. Recently, Pb-free tin plating is required for preventing Pb pollution problems. It re-activates the tin-whisker problem in the recent fine-spaced dedicated electronic devices. To overcome the problem, investigation of tin-whisker formation mechanism from tin-plating layer has been expanded [1,2]. However, structural investigation of tin-plated layer by TEM is rather difficult, because the tin-plated structure easily changes its quality in the process of thin specimen preparation for TEM experiments. Conventional ion-beam thinning instruments usually cause a temperature increase in the specimen. This causes the formation of alloys and/or intermetallic compounds in the tin-plating specimen, because tin has a low melting temperature of 505 K. The temperature increase also causes the problem of accelerating Cu atom diffusion, which is a usual base material for tin plating. Figure 1 shows a TEM image of a section specimen of tin-plated Cu. The specimen was prepared using an Ar-ion milling instrument (JEOL EM-09100IS) without cooling the specimen. It is clearly seen the formation of grains of intermetallic compounds along the interface between the Sn layer and the Cu base. Energy-dispersive spectroscopy (EDS) analysis showed that the grains are composed of tin and Cu atoms. From this image, it is impossible to judge the existence of intermetallic compounds before the ion-milling or formation of those during the thinning process. It is believed that the intermetallic compounds may be formed by diffusion of Sn and/or Cu atoms even at room temperature [2]. Thus, an increase in temperature of tin-plated Cu has to be avoided before TEM observation. Then, a specimen-cooling device has been designed, manufactured and integrated into an ion-milling instrument (JEOL EM-09100IS). Fig. 1. View largeDownload slide TEM image of a section specimen of tin-plated Cu prepared by ion-milling without cooling. It clearly shows the formation of intermetallic compound grains along the interface between the Sn layer and the Cu base. Fig. 1. View largeDownload slide TEM image of a section specimen of tin-plated Cu prepared by ion-milling without cooling. It clearly shows the formation of intermetallic compound grains along the interface between the Sn layer and the Cu base. Figure 2 shows a schematic drawing of a section of the cooling instrument integrated into the ion-milling instrument. The cooling device is composed of a liquid-nitrogen vessel, conducting Cu rods, a flexible conductor (knitted Cu wires) covered with resin beads for thermal insulation and a modified specimen holder. For introducing the conducting Cu rod into a vacuum chamber with a minimum thermal leak, a thin metal pipe was co-axially welded to the conducting Cu rod. The flexibility of the conductor between the conducting rod and the specimen holder is necessary because the holder is fixed on a stage which swings during ion-beam milling process. For thermal insulation between the specimen holder and the swing stage, small pieces of glass plates of 0.1 mm thickness were pasted at each corner of the bottom plane of the holder to minimize the area of contact and a thermal leak. Furthermore, Ag paste was used for better thermal connection between the specimen holder and the specimen. This cooling device can lower the specimen stage temperature down to 140–150 K. Fig. 2. View largeDownload slide Schematic drawing of a section of the cooling instrument integrated into the ion-milling instrument. The cooling device is composed of a liquid-nitrogen vessel, conducting Cu rods, a flexible conductor (knitted Cu wires) covered with resin beads for thermal insulation and a modified specimen holder. Fig. 2. View largeDownload slide Schematic drawing of a section of the cooling instrument integrated into the ion-milling instrument. The cooling device is composed of a liquid-nitrogen vessel, conducting Cu rods, a flexible conductor (knitted Cu wires) covered with resin beads for thermal insulation and a modified specimen holder. Figure 3 shows a section specimen of tin-plated Cu prepared by using the Ar-ion milling instrument integrated with the cooling device. It is clearly seen that grains of Cu–Sn intermetallic compound observed in Fig. 1 are not seen in this specimen. This confirms that the grains shown in Fig. 1 were formed due to the temperature increase during thinning process. The thinning procedure was performed according to a conventional manner of this ion-milling instrument, except for cooling the specimen. The ion-milling conditions were 5 kV accelerating voltage at 0.2° incident angle for initial milling and subsequently 3 kV accelerating voltage at 3–5° for final milling. The specimen temperature under an ion-beam irradiation was not measured. However, the temperature of the specimen may be not higher than the room temperature during the ion-beam irradiation, because grains of intermetallic compound are not observed. Small dark-contrast regions thinner than 0.1 μm along the interface, indicated by an arrow, for example, may be due to the surface roughness of the Cu base or an early stage of the formation of intermetallic compounds. Fig. 3. View largeDownload slide Section image of tin-plated Cu prepared by using the cooling devise. It is clearly seen that intermetallic compound grains observed in Fig. 1 are not formed. Fig. 3. View largeDownload slide Section image of tin-plated Cu prepared by using the cooling devise. It is clearly seen that intermetallic compound grains observed in Fig. 1 are not formed. Figure 4 shows section images of Pb-doped tin-plated Cu specimen prepared without cooling (Figure 4a) and with cooling (Figure 4b). The specimen (a) shows an apparent formation of intermetallic compound grains, indicated by black arrows. Specimen prepared with cooling shows particles on the boundary, but smaller ones compared withthose in (a). The most important difference between specimen with and without cooling is the formation of voids around Pb particles, identified by EDS, in (Figure 4a). The specimen (b) does not show any void around Pb particles. This indicates that the voids are created by interdiffusion of Pb atoms (Kirkendall effect) promoted by a temperature increase during ion-beam irradiation [3]. This result again confirms the importance of specimen cooling during the ion-milling process, especially for tin-plating materials. This cooling instrument is also applicable for thin specimen preparations of other low melting temperature metals and also for organic compounds. Fig. 4. View largeDownload slide Section images of Pb-doped tin-plated Cu prepared without cooling (a) and with cooling (b). Specimen (a) without cooling shows a formation of void (Kirkendall effect) around Pb particles due to interdiffusion of Pb atoms, but not for specimen (b) prepared with cooling. Fig. 4. View largeDownload slide Section images of Pb-doped tin-plated Cu prepared without cooling (a) and with cooling (b). Specimen (a) without cooling shows a formation of void (Kirkendall effect) around Pb particles due to interdiffusion of Pb atoms, but not for specimen (b) prepared with cooling. Funding Present developments of the cooling device were partly supported by Materials Science & Technology Research Center for Industrial Creation (2005.4–2010.3) funded by the Ministry of Education, Culture, Sports, Science and Technology of Japan. References 1 Lee B Z,  Lee D N.  Spontaneous growth mechanism of tin whiskers,  Acta Mater. ,  1998, vol.  46 (pg.  3701- 3714) https://doi.org/10.1016/S1359-6454(98)00045-7 Google Scholar CrossRef Search ADS   2 Boettinger W J,  Johnson C E,  Bendersky L A,  Moon K W,  Williams M E,  Stafford G R.  Whisker and Hillock formation on Sn, Sn–Cu and Sn–Pb electrodeposits,  Acta Mater. ,  2005, vol.  53 (pg.  5033- 5050) https://doi.org/10.1016/j.actamat.2005.07.016 Google Scholar CrossRef Search ADS   3 Zeng K,  Stierman R,  Chiu T C,  Edwards D.  Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability,  J. Appl. Phys. ,  2005, vol.  97 pg.  024508-1-8  © The Author 2010. Published by Oxford University Press [on behalf of Japanese Society of Microscopy]. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
TI - Development of a cryogenic stage for an ion-milling instrument
JF - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfq069
DA - 2010-10-08
UR - https://www.deepdyve.com/lp/oxford-university-press/development-of-a-cryogenic-stage-for-an-ion-milling-instrument-Qo4hZ7d7WZ
SP - 25
EP - 27
VL - 60
IS - 1
DP - DeepDyve
ER -