Quartz crystal deposition monitor

Quartz, since it is a piezoelectric and not a ferroelectric, has no hysteresis loss when it oscillates, thus quartz crystal oscillators are widely used as frequency control devices in radios, computers, and watches. Since the frequency is a function of the mass of the crystal, they can serve as deposition monitors (quartz crystal microbalances) with sensitivities of less than 1 ng. By functionalizing the surface to absorb specific gases, they can also act as chemical sensors. The temperature sensitivity of a quartz crystal oscillator can be minimized by choosing the cut of the crystal relative to the optical axis, which is necessary for its use as a frequency standard. On the other hand, a cut can be chosen to maximize the frequency dependence on temperature and quartz crystal thermometers with millikelvin resolution are available. 

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. 

Sorption of gases in coatings deposited on quartz crystal microbalance (QMB) [29] or on surface acoustic wave (SAW) devices [30, 31] can be monitored by weight changes of the exposed layer. By applying that technique it is important to ascertain that the Sauerbrey equation is valid, i.e. that contributions to the frequency shift other than mass loading are negligible. By combined mass and admittance measurements the concentra-... 

The deposition rate is often an important processing variable in PVD processing. Not only can the rate affect the film growth along with the deposition time, it is often used to determine the total amount of material deposited. The quartz crystal deposition rate monitor (QCM) is the most commonly used in situ, real-time deposition rate monitor for PVD processing. 

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. 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Fig. 11. Photograph of the four-electrode, vacuum flange and dual, quartz crystal, microbalance assembly, (A) side view, and (B) front view, used for mixed Cr atom. Mo atom matrix depositions with simultaneous monitoring of the individual metal flows. (The resolution of the microbalance is 10 g) (113).

Fig. 2 (a) Edwards E308 evaporator. One quartz-crystal thickness monitor is pointed towards the Au source to monitor Au vapor deposition on chamber walls the other monitors Au deposited through the shadow mask atop the organic layer. In the cold Au deposition, a small amount of Ar gas is added to the chamber to cool the Au atoms to room temperature before they physisorb atop the cryocooled organic monolayer, (b) Geometry of an Au I monolayer I Au pad sandwich, with electrical connections made using a Ga/In eutectic... 

Monoatomic copper vapor was generated by directly heating a tungsten-rod assembly around which copper wire was wrapped. The rate of metal atom deposition (10-12 K) was continuously monitored with use of an in situ quartz-crystal microbalance assembly. Cu(C2H2) and Cu(C2H2)2 were detected by spectroscopic methods. 

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. 

Quartz crystals have a characteristic oscillation frequency which varies according to their mass. Although crystal wafers have been used as mass sensors in vacuum and gas-phase experiments for many years, it is only recently that they have been employed in contact with liquids or solutions. Quartz crystal wafers can be used as electrodes by depositing a thin film of electrode material on the exposed surface, and interfacial mass changes can then be monitored. It is then known as the electrochemical QCM or EQCM. It is a direct, but non-selective, probe of mass transport. 

The rotatable reactor can also be used for reactions in fluids having suitably low (< 10"3 Torr) vapor pressure. In this mode, metal atoms are evaporated upwards into the cold liquid, which is spun as a thin band on the inner surface of the flask. Reactions with dissolved polymers can then be studied. Specially designed electron gun sources can be operated, without static discharge, under these potentially high organic vapor pressure conditions (6). Run-to-nin reproducibility is obtained by monitoring the metal atom deposition rate with a quartz crystal mass balance (thickness monitor). 

Silver evaporations were done in the same preparation chamber using an inhouse-constructed evaporation assembly containing a resistively heated tungsten basket. Ag coverages were controlled by a quartz crystal thickness monitor. The deposition rates were typically 0.01 to 0.1 A/s depending on the desired coverage. Coverages were subsequently calibrated by XPS analysis of the Ag 3d and polymer core level (C Is and O Is) intensities as compared to known standards, and by neutron activation analysis (111. 

The test specimen was a PET substrate 6 pm or 12 pm thick, 2.5 mm wide and about 1.5 cm long with a 3 mm long strip of evaporated metal, 0.2 pm to 1.2 pm thick, deposited near its center. The film thickness was monitored, during evaporation, with a quartz crystal thickness monitor, and the substrate temperature was measured with a thermocouple. The substrate temperature always remained within within 10 C of room temperature. An ion gun could be used to treat the substrate surface before deposition to modify the film adhesion. After deposition, the specimen was removed from the deposition... 

Figure 1.5 Commercial deposition thickness monitor (courtesy Sloan, Inc.) employing AT-cut, 5-MHz quartz crystal in the sensor head at left. Digital control and readout equipment is shown at right.

When argon is used as the plasma gas, the sputtering rate measured by the deposition rate of aluminum on a quartz crystal thickness monitor was found to be linearly proportional to Wjp as depicted in Figure 9.9 [15]. In the domain of glow discharge, W increases by the increase of current at nearly constant discharge voltage, and Wjp could replace Vjp. 

The separation properties of the membranes vary not only with the thickness of the plasma polymer coatings but also with the conditions of LCVD. One such plot of the variation is shown in Figure 34.21 [14]. The upper portion of this figure shows how the specific conversion parameter, DRjFM, varies as a function of the composite energy input parameter, WjFM. DR is the nominal deposition rate of plasma polymer film on the surface of a quartz crystal thickness monitor located in the plasma reactor, adjacent to the hollow fibers. 

The metal coverage was monitored by Auger spectroscopy (AES) and with a quartz crystal oscillator. These data were calibrated by neutron activation analysis to ensure proper determination of the amount of material deposited. The change in frequency of the quartz oscillator was proportional to the film thickness. 

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