Rate control in dual source evaporation

Two-component thin films are deposited in a high-vacuum system from two close sources, heated by an electron beam which is deflected between them. By using quartz-crystal monitors the evaporation rates are measured separately, which is usually considered to be problematical. One rate signal is used to regulate the appropriate deposition rate by controlling the emission current. The other deposition rate is regulated by using the rate signal to control the dwell-time ratio of the electron beam. With this system it is possible to deposit any composition with a good homogeneity, as in our case Gd-Fe and Gd-Co films. Evaporation from two close sources by a deflected electron beam has the advantages, compared with evaporation from two electron-beam sources (Edgecumbe et al 1964), of less composition variation in the plane of the deposited film (Maissel and Glang 1965) and the possibility of preparing any composition by simply adjusting the dwell-time ratio. For a good performance it is necessary to measure the evaporation rates of the two sources separately and to control the emission current and dwell time ratio of the electron beam. The rates are measured with Kronos quartz-crystal oscillators. For separate measurements of the two rates a plate of stainless steel is mounted between the two sources in such a way that each crystal can 'see' only one source (see figure 1). The minimum distance between plate and sourceholder should be about 4 cm to avoid interference with the electron beam. The distance between the crystals is also limited because of the crystal holders. From these limitations and the other dimensions given in figure 1, it can be calculated that the length of the plate has to be at least 30 cm to ensure full separation of the vapour beams with respect to the crystals. The substrate of course can 'see' both sources. To avoid systematic errors, symmetry between crystals and substrate is necessary and therefore their deposition angles must be the same; in our case this angle was about 5'. The monitors used were the Kronos QM-331 with rate output and the Balzers QSG 101 monitor, modified for a Kronos 5 MHz quartz-crystal oscillator, in combination with the Balzers rate monitor QRG 201B, which also contains a proportional-integral-differential (PID) regulator for rate control (see figure 2). The output of the latter is directly connected to the emission rate control unit of the power supply for the electron gun (Leybold Heraeus ESV 6). In this way rate RI is regulated by controlling the total emission power independent of the dwell-time ratio. The other rate R2 can be regulated by controlling the dwell-time ratio, i.e. the division of the total beam power on both sources. Therefore (see figure 2) the rate R2 is compared with the desired value, represented by an adjustable -k Water cooled source nslder Sources