Quartz Crystal Microbalance Digital Controller

More documents

Recommendations

Info

26 Theory, Operation and Calibration Chapter 2Noise AnalysisV c varies logarithmically with R m over most of the range of interest. There is animportant advantage in this: the fractional resolution of the resistance is nearlyindependent of the resistance and so allows detailed and low-noise measurement of theviscous losses. To estimate the noise in resistance measurements, we can take thederivative of the equation for the motional resistance (units are Ohms and Ohms/Volt):Rm= 10,00010-Vc5- 75(eqn. 7)dRdVmcV5 1 =c Vc510 , 000 10 ln( 10) = -2,000ln( 10) 10(eqn. 8) 5 dRm Vc 5 -460510= -0. 4605( Rm+ 75)(eqn. 9)dVcNoise on the V c signal, V c , is typically ±50 µV (with one second averaging). R m for a5 MHz crystal in water is about 375 . The fractional noise in the resistancemeasurement is then:R mR m= V cR m dR mdV c= V c0.4605 [ ( R m+ 75)]=±28 ppm (eqn. 10)R mThis low noise floor for fractional resistance measurements allows very small changes indissipation losses to be measured. This is why a high precision voltmeter (i.e. with atleast six digits of resolution) is recommended for measuring the analog conductanceoutput and why a very high resolution (24 bit) A/D converter is used by the QCM200 tocalculate resistance readings.Calibration of R mResistance measurements for the QCM25 Crystal Oscillator/QCM200 are calibrated byreplacing the crystal with a precision resistor in parallel with a 18 pF capacitor. Tworesistor values are used: 25 and 925 . The equation for R m may be inverted todetermine the calibration value for V c . (Motional resistance, R m , in and conductancevoltage output, V c , in volts.)Rm-V 5 10,000 c= 10,00010- 75 and Vc = 5 log (eqn. 11)Rm+ 75 The low pass filter is adjusted so that the QCM25 Crystal Oscillator oscillates at 5 MHzwith the 25 resistor in place of the crystal. The varactor dial is adjusted so that theCrystal Controller oscillates at 5 MHz with the 925 resistor in place of the crystal.Calibration potentiometers in the QCM25 Crystal Oscillator are adjusted so that (P1 pot)V c = 10.000 ±0.005 Vdc with a calibration resistor of 25 , and (P2 pot) V c = 5.000±0.005 Vdc with a calibration resistor of 925 . Iterate P1 and P2 adjustments asnecessary.QCM200 Quartz Crystal Microbalance
Chapter 2 Theory, Operation and Calibration 27Gas-Phase MeasurementsThe first use of QCMs was as mass sensors and thickness monitors in gas phase, thin-filmdepositions. To this date, this continues to be an important area of application for thistechnology.Sauerbrey EquationSauerbrey’s equation (eqn. 12) is often used to calculate mass loadings and thin-filmthicknesses in vacuum depositions. The basic assumption is that the incremental changein mass from the foreign film is treated as though it were really an extension of thethickness of the underlying quartz. The foreign film is considered rigid and so thin that itdoes not experience any shear forces during vibration. As a result, the sensitivity factor,C f , is a fundamental property of the quartz crystal and does not consider any of theproperties of the foreign film (i.e. it is only dependent on the acousto-elastic properties ofquartz).Cf( µ ) 1 2q202nf= (eqn. 12)qwheren = number of the harmonic at which the crystal is driven,f o = the resonant frequency of the fundamental mode of the crystal in Hz, q = density of quartz = 2.648 g cm -3 ,µ q = shear modulus of quartz = 2.947 x 10 11 g cm -1. s -2 .The dependence of the frequency change on the mass coverage per unit area, emphasizesthe fact that, within certain limits, the sensitivity factor is independent of the electrodegeometry. Thus, in theory, the QCM mass sensor does not require calibration for thisapplication. This ability to calculate mass loading from first principles is obviously a veryattractive feature of these devices.Film thickness is often the parameter of interest in gas-phase thin-film depositions. If themass coverage is believed to be uniform, the thickness of the film is easily calculated bydividing the mass per unit area provided by Sauerbrey’s equation by the material’sdensity.mTf= (eqn. 13)fwhere f = density of film material in g/cm 3 ,m = change in mass per unit area in g/cm 2 (calculated from Sauerbrey’s equation),T f = thickness of the film in cm.QCM200 Quartz Crystal Microbalance
Page 1 and 2: Operation and Service ManualQCM200Q
Page 3 and 4: Safety and Preparation For UseiSafe
Page 5 and 6: ContentsiiiContentsSafety and Prepa
Page 7 and 8: OverviewvFront Panel OverviewFigure
Page 9 and 10: OverviewviiBack Panel OverviewFigur
Page 11 and 12: OverviewixQCM25 Crystal Oscillator
Page 13 and 14: SpecificationsxiSpecificationsQCM20
Page 15 and 16: Getting Started 1Chapter 1Getting S
Page 17 and 18: Chapter 1 Getting Started 3Quick St
Page 19 and 20: Chapter 1 Getting Started 5Paramete
Page 21 and 22: Chapter 1 Getting Started 7Liquid S
Page 23 and 24: Chapter 1 Getting Started 98. Place
Page 25 and 26: Chapter 1 Getting Started 11Flow Ce
Page 27 and 28: Chapter 1 Getting Started 13Operati
Page 29 and 30: Theory, Operation and Calibration 1
Page 31 and 32: Chapter 2 Theory, Operation and Cal
Page 39: Chapter 2 Theory, Operation and Cal
Page 63 and 64: Sensor Crystals and Holders 49Chapt
Page 65 and 66: Chapter 3 Sensor Crystals and Holde
Page 89 and 90: QCM Circuit Description 75Chapter 4
Page 91 and 92:
Chapter 4 QCM Circuit Description 7
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Frequency Counter Selection Criteri
Page 115 and 116:
QCM200 Remote Progamming 101Appendi
Page 117 and 118:
Appendix B QCM200 Remote Programmin
Page 119 and 120:
Appendix B QCM200 Remote Programmin
show all

Quartz Crystal Microbalance Digital Controller

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?