Spatial distribution of copper vapour from a two-dimensional source using a strip electron beam at aspect ratio greater than 3

We present results on the spatial distribution of copper vapour as a function of temperature, from a 2D evaporating source generated using 130 mm strip electron beam at aspect ratio >3. At this aspect ratio vapour flow is expected to appear approximately as a point source flow. The 2D evaporating source during these studies can be described in terms of inverse Knudsen number (1/K_n) that is proportional to source temperature, ranging from 0.75 to 6.67. These experimental results could be fitted with accuracy of ±5% using a spatial distribution function with different beaming coefficients along the width and the length of a 2D source. It is observed that the beaming exponent along the width of a 2D source saturates at about 2, which is marginally higher than that observed for the aspect ratio in the range of 1 to 2 for the similar range of 1/K_n, whereas, the beaming exponent along the length direction of a 2D source was observed to increase up to 6.34, which is substantially higher than that observed at the aspect ratio in the range of 1-2 for the similar range of 1/K_n. These results clearly indicate that, for given evaporating source conditions, the widthwise atomic collisions, collimation of atomic beam and beaming exponent along the width remains constant. At the same time lengthwise collisions, atomic beam collimation and hence beaming coefficient do not freeze and the wedge-shaped atomic beam does not run out of collisions in the lengthwise direction even at aspect ratio >3. Thus, the atomic vapour from a 2D source does not attain the free molecular regime even at radial distance in excess of 390 mm above the source.