Evaporation, sources baffle

Evaporation source, baffle An evaporation source in which the vapor must colUde with several hot surfaces before it can leave the source. Used to evaporate materials such as selenium and silicon monoxide, which vaporize as clusters of atoms or molecules. 

Figure 5.6 Schematic drawing of the oven used for the evaporation of magnesium (Mg) and the essential apparatus for angle-resolved electron spectrometry around the oven. The source volume which also provides a centre of reference is marked by the symbol S. The oven itself is shown in the third quadrant for the symbols characterizing the oven see main text. The direct vapour beam is stopped by the baffle B (placed in the first quadrant). The dashed curves starting from S indicate the angular ranges in which the emitted electrons are accepted by the monitor analyser (second quadrant, mon. , fixed in space), and by the two extreme positions of a rotatable sector CMA (fourth quadrant, pos. 1 , pos. 2 , respectively). Reprinted from J. Electron Spectrosc. Relat. Phenom. 67, Kammerling et al, 363 (1994) with kind permission of Elsevier Science - NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

Fig. 4.4. A schematic cross-section of two evaporation boats showing the popcorn effect. In (a), the boat is open, and the evaporant material sublimes first at the hottest point of the assembly, which is the bottom of the material granules. This creates a vapor jet which can eject solid particles of evaporant from the boat. In a baffled arrangement (b), the source is constructed so as to prevent line of sight contact between the source and the substrate. Solid particles which are propelled are trapped, whereas material vapor can escape and deposit.

Most elements vaporize as atoms but some, such as Sb, Sn, C, and Se, have a significant portion of the vaporized species as clusters of atoms. For materials that evaporate as clusters, special vaporization sources, called baffle sources , can be used to ensure that the depositing vapor is in the form of atoms. It should be noted that, as a material is heated, the first materials that are volatilized are high vapor pressure surface contaminants, absorbed gases, and high vapor pressure impurities. These can contaminate a clean surface before the deposition begins. 

There are several modified forms of effusion sources that are used in certain process tools, especially in MBE machines, such as shown in Figure 11.6. For example, one may include a gas cracker which is simply a series of heated baffles or plates configured such that before a gas molecule can escape the source it must strike at least one of these surfaces. Group V and VI elements usually evaporate from then-own soHd/hquid surfaces as small clusters of four to eight atoms. The desorption rate equations tell us that the rate for such a process will depend upon a power of the species concentration. On the surface this concentration is constant and high. Therefore, evaporation of clusters is relatively easy. However, reaction of these large... 

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