Quartz crystal monitor

Fig. 2.5. Quartz crystal monitor response when a silicon surface, previously exposed to a high dosage of CF ions, is subjected to 500 eV O2 ion bombardment. The regions of this curve are dominated by the following phenomena ...

Thickness controllability (Table 9.1, no. 6) and reproducibility in OVPD is achieved by accurate adjustment of the flow of carrier gas by means of mass-flow controllers whereas in VTE quartz crystal monitors are used to control the rate of deposition by adjustment of the evaporation temperature. In VTE small deviations of the evaporation temperature are known to affect the stability of the deposition rate and consequently the layer thickness, which may also affect the roughness and morphology of the VTE-deposited layer. 

An optical monitor and/or a quartz crystal monitor is used to control the film thickness and deposition rate. Quarter-wave films in the visible range can be deposited within 3-4 min. 

For control of the rate of deposition, a quartz crystal monitor can additionally be used. Because of the importance of this method, it will be treated in more details in the next section. 

The difference between Cf and C f is easy to calculate. For a mass corresponding to a 2% shift in starting frequency, the difference in areal density using C f instead of C, is about 4%. For accurate mass determinations, this variable sensitivity has to be considered. The quartz crystal monitor can also record the rate of deposition, as already mentioned above. Rate measurements are very important especially in reactive deposition. The rate is obtained by electronic differentiation of the mass-dependent frequency change with respect to time. The slightly varying mass sensitivity with increasing mass load need not be considered for rate measurements. 

The high sensitivity, excellent mechanical stability and the controllable thermal influences make the quartz-crystal microbalance a valuable tool. Determinations of the physical film thickness, however, may be more of a problem since the density pf of a thin film, depending on deposition method, chosen parameters and film thickness, is generally different from the density of the bulk material being considered. At film thicknesses below 100 nm, the density discrepancy is greater than with thicker films. This seems to be valid for metals as well as for dielectrics. It is, however, usually possible to reproduce the film density. For the deposition process, this demonstrates that the chosen vacuum and deposition parameters must be carefully controlled and maintained exactly to always attain the same film density. In its manner and after calibration, the geometrical film thickness can be exactly reproduced by quartz-crystal monitoring. 

Similar problems exist during the reproducible production of optical thickness and electrical characteristics of thin films by quartz-crystal monitoring. 

Transmittance curve of a special low pass multilayer filter. nL = 1.46, nH = 2.40, The stack consists of 23 layers, 6 of them are correcting layers. To control thickness a quartz crystal monitor was used. Full line = measured values, dotted line = calculated values. 

The disks are reweighed to determine the total mass of deposited metal on each disk. The exact effective thicknesses of the metal layers on each disk are derived assuming that the metals are pure and of bulk density and that the quartz crystal monitor is only off by an unknown but constant scale factor relative to the values it should have read if placed at the location at which each disk was during the depositions. 

Quartz crystal monitor (QCM) (deposition rate) Quartz crystal deposition monitors measure the change in resonant frequency as mass (the film) is added to the crystal face. 

Tooling factor The ratio of the observed condition, using sensors, during processing to the measured condition after processing. Example The ratio of the film thickness on a quartz crystal monitor to the measured thickness of the film deposited on the substrate. 

QCM Quartz crystal microbalance Quartz crystal monitor... 

Deposition rate monitors include devices such as quartz crystal monitors and RHEED tools that measure the rate of deposition of a film on a surface and... 

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