A carbon nanotube (CNT) is expected for use as a cold cathode electron emitter of vacuum electronic devices such as an ionization gauge, an ion source and a mass spectrometer. To fabricate CNT field emitter with low cost and mass production, screen-printing method has been regarded as one of the most promising routes.[1] However, the agglomeration of CNTs during purification process to prepare a paste for screen printing and the ununiformity in the length of commercial CNTs cause a degradation of emission properties. To overcome this issue, it is desirable to adopt vertically aligned CNTs grown by thermal chemical vapor deposition (CVD)[2] as a source material of a screen-printed emitter, since their lengths are unified upon the termination of the growth. In this study, we fabricated the screen-printed emitters made of purification-free CNTs grown by CVD and applied them to a cold cathode ionization gauge.
The Fe/Al multilayer catalyst film was deposited on a Si substrate by electron beam deposition. CVD was carried out at 700°C for 10 min using acetylene gas under 200 Pa. A transmission electron microscope observation of the CNTs revealed the formation of multiwalled CNTs occurs in the base growth mode. From a thermogravimetric analysis it was found that the amount of amorphous carbon and catalyst contained in CNTs were less than 1 and 0.5 wt.% respectively, so that no purification process was needed.
The CNT paste was prepared by mixing binder polymer, CNTs, and inorganic frits in terpineol. The CNT films with an active area of 1 cm2 were screen-printed. The field emission properties were measured using parallel plates with a 0.2 mm separation in a high-vacuum system. The threshold electric fields to produce a current density of 1 mA/cm2, ranged from 1.45 to 1.52 V/µm, showed good reproducibility.
Next, a cold cathode ionization gauge was fabricated by replacing the hot filament of a conventional ionization gauge with a CNT emitter. The gauge showed good measurement linearity between ion collection current and nitrogen gas pressure. The detailed gauge performance will be presented.
[1]H. Machida et al., Jpn. J. Appl. Phys. 45 (2006) 1044.
[2]K. -Y. Lee et al., J. Vac. Sci. Technol. B 23 (2005) 1450. |