Development of a new heating stage equipped thermal electron filter for scanning electron microscopy M. Nakamura, T. Isshiki, M. Tamai and K. Nishio Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585, Japan ( mailto:isshiki@dj.kit.ac.jp )
Introduction Microscopic observations of ma- terials at high temperature give useful information to elucidate sintering and phase transition behaviors of materials. Kamino and Saka devel- [1] oped a heating stage for high- resolution TEM, which has a nar- row tungsten filament to hold and heat specimen. This direct heating method has merits that the temperature of the filament can be raised by small current and is quickly stabilized within a few ten seconds.
Although a weak point of the heating method was indirect measurement of specimen tem- perature, we make it possible to measure the temperature pre- cisely by using radiation ther- mometer [2] . In this paper, we discuss a prob- lem caused in the case of apply- ing the heating method to high- temperature scanning electron microscopy (HTSEM) and sug- gest a solution of it.
� High-temperature SEM Problems of HTSEM Thermal electrons Heated filament for HTSEM emits a lot of thermal electrons ( Fig. 1 ). 10 25 m −2 s −1 ] 10 20 Electron density [ 10 15 10 10 10 5 10 0 400 800 1200 1600 2000 Temperature [ o C] Fig. 1 Dependence of electron density emitted from tungsten on temperature.
The emitted thermal electrons are detected by the secondary elec- tron detector. They act as back- ground noise and decrease con- trast of the SEM image. Temperature measurement Measurement of specimen temper- ature by converting heating cur- rent into specimen temperature is indirect method and a troublesome job. The conversion table should be rebuilt every time the heating stage is changed to keep accuracy of the measurement.
� Solution Thermal electron filter Secondary and thermal electrons have different energy distribution. Secondary Thermal electrons ( Fig. 2 ) electrons 20-30 eV < a few eV 6 × 10 −4 400 o C Velocity distribution [%] 800 o C 5 × 10 −4 1200 o C 1600 o C 4 × 10 −4 2000 o C 0.5 eV 1.0 eV 3 × 10 −4 2 × 10 −4 1 × 10 −4 0 2 × 10 5 4 × 10 5 6 × 10 5 8 × 10 5 0 Velocity of electron [m/s] Fig. 2 Velocity distribution of thermal electrons.
These electrons can be separated by energy filtering method. Fig. 3 shows an overview of a new heating stage equipped with a ther- mal electron filter. A grid electrode inserted between the heating stage and the detector is as follows. Voltage range: 0 - − 30 V Grid wire tungsten (20 µ m φ ) - Material: - Winding density: 4 turns/mm Dimension: 40 mm in width 10 mm in height A small light bulb removing glass cover is employed as a disposable heating filament.
Fig. 3 An overview of a new heating stage for high-temperature SEM. Radiation thermometer A radiation thermometer (Japan Sensor, FTZ2), installed by the side of a specimen chamber to detect
infrared rays emitted from the heat- ing filament, is used for precise measurement of specimen temper- ature. The heating stage equipped an SEM (JEOL, JSM-845) is schemat- ically illustrated in Fig. 4 . secondary electron detector 10 kV thermal electron filter secondary electrons Voltage controller (0V - 30V) thermal electrons Current controller infrared rays (0mA - 150mA) radiation sapphire thermometer glass heating stage (tungsten filament) Fig. 4 A schematic diagram of a new heating stage for high-temperature SEM.
Results & Discussion Effect of the new heating stage is verified by high-temperature SEM observation of SiC particles ( Fig.5 ). with the filter without the filter
JEOL Equipment: JSM-845 Accelerating voltage: 15 kV Probe current: 1 nA Magnification: 10,000 Grid voltage: − 20 V Fig. 5 High-temperature SEM images of SiC particle with ( a - e : left) and without ( f - i : right) thermal electron filter.
Thermal electron filter without the filter ( right, Fig. 5 ) > ∼ 800 ◦ C ( Fig. 5 f, g): Observation is hindered by the necessity of drastic adjustment of contrast and brightness. > ∼ 1,300 ◦ C ( Fig. 5 h, i): The effect of the thermal electrons is beyond the margin of adjust- ment and it is difficult to observe. with the filter ( left, Fig. 5 ) < ∼ 1,200 ◦ C ( Fig. 5 a): The filter brings smooth observa- tion without drastic adjustment.
< ∼ 1,400 ◦ C ( Fig. 5 b-e): It is possible to observe images. Temperature measurement Accuracy of temperature measure- ment using the radiation ther- mometer is verified by observation of a melting point of metals (Ag and Au). Use of the radiation thermome- ter brings real-time measurement of specimen temperature precisely without the influence of an individ- ual difference of heating filaments.
Application to In-situ SEM Fig. 6 shows an example of high- temperature SEM observation of sintering process of Ca-deficient hydroxyapatite (Ca-def HAp) with the heating stage.
Fig. 6 In-situ SEM observation of Ca-deficient hydroxyapatite whiskers at various temperatures.
The same sintering process as that in the air [3] is observed, i. e. , < 900 ◦ C: Whiskers keep their shape. 900 - 1,000 ◦ C: Whiskers are rapidly sin- tered and fattened. > 1,000 ◦ C: Size of particles and pores increases but surface area decreases. In-situ HTSEM observation of Ca-def HAp can reveal the rela- tion between heat treatment and characteristics of porous HAp sintered in the air.
Summary The thermal electron filter is ef- fective to prevent the distur- bance by the thermal electrons and the radiation thermometer brings accurate and convenient observation. They are needed HTSEM observation by the direct heating method. References [1] T. Kamino and H. Saka, Microsc. Microanal. Mi- crostruct. 4 (1993) 127 [2] T. Isshiki, K. Nishio, Y. Deguchi, H. Yamamoto and M. Shiojiri, In : Proc. 14th ICEM, Cancun, Mexico, 3 , pp. 521 (1998) [3] M. Tamai, S. Miki, G. Pezzotti and A. Nakahira, J. Ceram. Soc. Japan 108 (2000) 915
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