Diamond Films Synthesized at Ultralow Filament Temperatures

Abstract

In chemical vapor deposition of diamond films, a recent research challenge is to push the limits of boundary conditions. [1,2] These attempts may lead to the development of new deposition techniques and applications, and can also be expected to give insight into diamond growth mechanisms. In the hot-filament CVD method, it is known that filament and substrate temperature, chamber pressure, and gas concentration are the main quality control variables for diamond films. Typically, these parameters are 2000 C, 800 C, 40 mbar, and 1 % CH 4 in H 2 , respectively. Limiting deposition conditions have been investigated by many researchers. For example, the deposition pressure can be varied in either direction: higher up to nearly one atmosphere [3] to improve technical application, or lower to about 0.2 mbar [4] for basic research to reach conditions at which standard plasma deposition can be applied. The lowest substrate temperature is of special interest for many industrial applications: it is about 100 C. A high filament temperature of about 2890 C has been used to increase growth rate by using higher methane concentration. [5] Similarly, a question of common interest is the possibility of depositing diamond films at lower filament temperatures, because the growth of diamond films under these conditions might give a new insight into the deposition process. A lower filament temperature would have several advantages. Filaments would hold their shape better, resulting in stable deposition conditions. Moreover, filaments operating at a lower temperature would be better able to withstand mechanical disturbance and therefore have longer lifetimes.In the present study, diamond films were deposited in the low filament temperature range of 1300±1700 C. The synthesized diamond crystals were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM).The experiments were conducted in a typical hot-filament CVD system. The hot filament was made of a 0.5 mm diameter tantalum wire wound into 10 circles of inner diameter 2 mm. Si substrates were abraded with 0.1 lm diamond paste, then ultrasonically cleaned in acetone for 10 min before fixing to a boron nitride holder. During the deposition process, the total ambient pressure was kept at 40 mbar, while the flow rates of hydrogen and methane were controlled by mass flow controllers with 100 sccm total flow rate. The concentration of methane was fixed at 0.5 %. The temperatures of the filament and the substrate were 1300±1700 C and 650±950 C measured by an optical pyrometer and a K-type thermocouple, respectively. The distance between the filament and the substrate was 2±5 mm. Atomic hydrogen concentration, [H], as a function of filament temperature was measured by using the calorimetric method. [6] [H] was determined after calibration of the temperature difference DT measured by two thermocouples, one with a silver-coated tip and the other with a non-catalyzing quartz tip. The films produced under different filament temperature were characterized by SEM and by XRD usi...