Before the chamber is sealed and pumped one must load source filaments with the desired evaporant. It is important to know how much material to load as one cannot simply reload the filaments while the system is under high vacuum. In addition it is NOT a good idea to overheat the filament to get that 'last drop' of evaporant as this frequenctly leads to deposition of the filament material itself ... in most cases Tungsten metal. Better the filament should have too much material than too little. Of course if the filament is too overloaded material tends to drip off it and you're back to square one!
So, before the chamber is sealed you need to figure out how much material is required on the filament and check the quartz thickness monitor as well ... that will be required for accuracy and must be tested prior to pumpdown.
Before quartz crystal thickness monitors were common the primary method used to obtain a desire thickness was to preload the filament with exactly the right amount of evaporant required then ensure it is all evaporated during the deposition. This method is not particularly accurate and has been superceeded by the use of an in-process quartz thickness monitor. It does, however, at least give us an idea of how much material is required to allow a proper amount to be loaded into the source filament prior to deposition.
The nomograph method uses two intersecting lines to determine the load required given the source-to-substrate distance and the evaporant density.
To start choose the evaporant used on the top line and draw a straight line with a ruler through the desired layer thickness (in Angstroms, one Angstrom being 0.1nm) and intersecting the middle line. Now draw a second straight line starting at the point where the first line intersects the middle line through the source-to-substrate distance and intersecting the bottom line. At the point where the line you have just drawn intersects the bottom line read the evaporant load in mg.
For example, in our lab we commonly use aluminum. The first line would start at "Al" on the top line and go through the desired thickness on the next line until it touches the middle line. We would then draw a line beginning at that point on the middle line through the source-to-substrate distance as measured on our equpiment to the bottom line. The load is then read from the bottom line. In our lab we use small 'horseshoes' of pure aluminum, each of which weighs 29.9 mg. If the nomograph told use we'd need 50mg for a particular deposit, we'd put two horseshoes (almost 60mg) on the filament to be sure we had enough.