Fundamentals of Vacuum Technology (1248463), страница 54
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The enlarged symmetrygreatly reduced the number of possible oscillation modes. A second groupof improvements involved providing one of the surfaces with a contour andmaking the excitation electrode smaller. The two together ensure that theacoustic energy is recorded. Reducing the electrode diameter limits theexcitation to the middle area. The surface contour consumes the energy ofthe moving acoustic waves before they reach the crystal edge. It is notreflected into the center where it could interfere with new incoming waves.Such a small crystal behaves like an infinitely expanded crystal. However, ifthe crystal vibrations remain restricted to the center, one can clamp theouter edge to a crystal holder, without engendering undesired side effects.Moreover, contouring reduces the resonance intensity of undesiredanharmonics. This limits the capacity of the resonator to maintain theseoscillations considerably.log Relative intensyUse of an adhesive coating has enhanced the adhesion of the quartzelectrode.
Even the rate spikes occurring with increasing film stress (strain)and caused by micro-tears in the coating were reduced. Coating materialremains at these micro-tears without adhesion and therefore cannotoscillate. These open areas are not registered and thus an incorrectthickness is indicated.Fig. 6.4 shows the frequency behavior of a quartz crystal shaped as in Fig.6.3. The ordinate represents the amplitude of the oscillation or also thecurrent flowing through the crystal as a function of the frequency on theabscissa.Frequency (MHz)Fig. 6.4Shape of LEYBOLD-Inficon quartz crystalsUsually an AT cut is chosen for the coating thickness measurementbecause through the selection of the cut angle the frequency has a verysmall temperature coefficient at room temperature.Frequency resonance spectrum126HomeThin film controllers/control unitsSince one cannot distinguish between• coating: frequency reduction = negative influence• temperature change:negative or positive influence• temperature gradients on the crystal, positive or negative• stresses caused by the coatingit is important to minimize the temperature influence.
This is the only way tomeasure small differences in mass.6.4Period measurementAlthough the instruments that functioned according to equation 6.2 werevery useful, it soon became obvious that for the desired accuracy their areaof application was typically limited to ÆF < 0.02 Fq. Even at a relativefrequency change of (Fq Ð Fc) / Fq < 2 %, errors of around 2 % occurred inthe coating thickness measurement so that the "usable service life" of thecoating in the case of a 6-MHz monitor crystal was about 120 kHz.In 1961 Behrndt discovered that:Mf (Tc − Tq) ∆F==MqTqFcwith (6.3)measurements on the order of magnitude of the frequency measurementaccuracy, good rate regulation becomes impossible (rate regulation:regulation of the energy supply to the coating source so that a specifiedcoating thickness growth per time unit is maintained).
The greatmeasurement uncertainty then causes more noise in the closed loop, whichcan only be countered with longer time constants. This in turn makes thecorrections due to system deviation slow so that relatively long deviationsfrom the desired rate result. This may not be important for simple coatings,but for critical coatings, as in the case of optical filters or very thin, slowlygrowing single-crystal coatings, errors may result. In many cases, thedesired properties of such coatings are lost if the rate deviations are morethan one or two percent. Finally, frequency and stability of the referenceoscillator determine the precision of the measurement.6.5Miller and Bolef (1968) treated the quartz oscillator and coating system as asingle-dimensional, coherent acoustic resonator.
Lu and Lewis (1972)developed the simplified Z match equation on that basis. Simultaneousadvances in electronics, particularly the microprocessor, made it possible tosolve the Z match equation in real time. Most coating process control unitssold today use this sophisticated equation, which takes into account theacoustic properties of the quartz oscillator/coating system: π ⋅ (Fq − Fc) N ⋅d Tf = AT q ⋅ arctg Z ⋅ tg Fqπ ⋅ df ⋅ Fc ⋅ ZTc = 1 / Fc ... oscillation period, coatedTq = 1 / Fq ... oscillation period, uncoatedThe period measurement (measurement of the oscillation duration) was theresult of the introduction of digital time measurement and the discovery ofthe proportionality of crystal thickness Dq and oscillation duration Tq.
Thenecessary precision of thickness measurement permits application ofequation 6.3 up to about ÆF < 0.05 Fq.In period measurement a second crystal oscillator is essentially used as areference oscillator that is not coated and usually oscillates at a muchhigher frequency than the monitor crystal. The reference oscillatorgenerates small precision time intervals, with which the oscillation durationof the monitor crystal is determined. This is done by means of two pulsecounters: the first counts a fixed number of monitor oscillations m. Thesecond is started simultaneously with the first and counts the oscillations ofthe reference crystal during m oscillations of the monitor crystal.
Becausethe reference frequency Fr is known and stable, the time for m monitoroscillations can be determined accurately to ± 2/Fr. The monitor oscillationperiod is thennFr · mwhere n is the reading of the reference counter. The accuracy of themeasurement is determined by the frequency of the reference oscillator andthe length of the counting time that is specified through the size of m.For low coating rates, small densities of the coating material and fastmeasurements (that require short counting times), it is important to have areference oscillator with a high frequency. All of this requires great timeprecision so that the small coating-related frequency shifts can be resolved.If the frequency shift of the monitor crystal decreases between twoThe Z match techniqueZ=dq ⋅ Uqdf ⋅ U f(6.4)acoustic impedance ratioUq = shear module, quartzUf = shear module, filmThis led to basic understanding of the conversion of frequency shift intothickness which enabled correct results in a practical time frame for processcontrol.
To achieve this high degree of accuracy, the user must only enteran additional material parameter Zf for the coating material. The validity ofthe equation was confirmed for many materials and it applies to frequencyshifts up to ÆF < 0.4 Fq! Note that equation 6.2 was only valid up toÆF < 0.02 Fq. And equation 6.3 only up to ÆF < 0.05 Fq.6.6The active oscillatorAll units developed up to now are based on use of an active oscillator, asshown schematically in Fig.
6.5. This circuit keeps the crystal actively inresonance so that any type of oscillation duration or frequencymeasurement can be carried out. In this type of circuit the oscillation ismaintained as long as sufficient energy is provided by the amplifier tocompensate for losses in the crystal oscillation circuit and the crystal caneffect the necessary phase shift. The basic stability of the crystal oscillatoris created through the sudden phase change that takes place near theseries resonance point even with a small change in crystal frequency, seeFig.
6.6.127HomeThin film controllers/control unitsoutputphaselog .Z. (Ohm)| impedance |phase (degrees)amplifierseries resonanceFrequency (MHz)Fig. 6.5Fig. 6.6Circuit of the active oscillatorNormally an oscillator circuit is designed such that the crystal requires aphase shift of 0 degrees to permit work at the series resonance point. Longand short-term frequency stability are properties of crystal oscillatorsbecause very small frequency differences are needed to maintain the phaseshift necessary for the oscillation.
The frequency stability is ensured throughthe quartz crystal, even if there are long-term shifts in the electrical valuesthat are caused by Òphase jitterÓ due to temperature, ageing or short-termnoise. If mass is added to the crystal, its electrical properties change.Fig. 6.7 shows the same graph as Fig 6.6, but for a thickly coated crystal. Ithas lost the steep slope displayed in Fig. 6.6. Because the phase rise isless steep, any noise in the oscillator circuit leads to a larger frequency shiftthan would be the case with a new crystal. In extreme cases, the originalphase/frequency curve shape is not retained; the crystal is not able to carryout a full 90 ¡ phase shift.log .Z. (Ohm)phase (degress)The impedance |Z| can increase to very high values.
If this happens, theoscillator prefers to oscillate in resonance with an anharmonic frequency.Sometimes this condition is met for only a short time and the oscillatoroscillation jumps back and forth between a basic and an anharmonicoscillation or it remains as an anharmonic oscillation.
This phenomenon iswell known as "mode hopping". In addition to the noise of the rate signalcreated, this may also lead to incorrect termination of a coating because ofthe phase jump. It is important here that, nevertheless, the controllerfrequently continues to work under these conditions.
Whether this hasoccurred can only be ascertained by noting that the coating thickness isFrequency (MHz)Crystal frequencies near the series resonance pointsuddenly significantly smaller, i.e. by the amount of the frequency differencebetween the fundamental wave and the anharmonic adopted by theoscillation.6.7The mode-lock oscillatorINFICON has developed a new technology for overcoming theseconstraints on the active oscillator. The new system constantly analyzes theresponse of the crystal to an applied frequency: not only to determine the(series) resonance frequency, but also to ensure that the quartz oscillates inthe desired mode.
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