What is a Quartz Crystal Microbalance - QCM

KSV Instruments Ltd.

Category: QCM Technology

Piezoelectricity did not receive lot of interest in the beginning and a moredetailed study of piezoelectricity was not started until 1917 when it was showed that quartz crystalscould be used as transducers and receivers of ultrasound in water. In 1919 several devices ofeveryday interest based on the piezoelectricity of Rochelle salt was described i.e. loudspeakers,microphones and sound pick-ups. In 1921 the first quartz crystal controlled oscillator wasdescribed. These first quartz crystal controlled oscillators were based on X-cut crystals, which havethe drawback of being very temperature sensitive. Therefore, the X-cut crystals are nowadays usedin applications where the large temperature coefficient is of little importance, such as transducers inspace sonars.

The dominance of the quartz crystal for all kind of frequency control applications started in 1934when the AT-cut quartz crystal was introduced. The advantage with the AT-cut quartz crystal is thatit has nearly zero frequency drift with temperature around room temperature. From the verybeginning of using quartz crystal resonators as frequency control elements it was common toincrease the frequency of the resonator by drawing pencil marks on the electrodes, or decreasingthe frequency by rubbing of some electrode material with an eraser. The understanding of thismass induced frequency shift was only known on a qualitative basis. However, in 1959 Sauerbreypublished a paper that showed that the frequency shift of a quartz crystal resonator is directlyproportional to the added mass. Sauerbreys work is generally taken as the breakthrough and thefirst step towards a new quantitative tool to measure very small masses i.e. the quartz crystalmicrobalance.

Hence, one can describe the QCM to be an ultra-sensitive mass sensor. The heart of the QCM isthe piezoelectric AT-cut quartz crystal sandwiched between a pair of electrodes. When theelectrodes are connected to an oscillator and an AC voltage is applied over the electrodes thequart crystal starts to oscillate at its resonance frequency due to the piezoelectric effect (see figurebelow). This oscillation is generally very stable due to the high quality of the oscillation (high Qfactor).

Quartz Crystal

If a rigid layer is evenly deposited on one or both of the electrodes the resonant frequency willdecrease proportionally to the mass of the adsorbed layer according to the Sauerbrey equation:

Df = -[2´fo²´Dm] / [A´(rqmq)½], where

Df = measured frequency shift,

fo= resonant frequency of the fundamental mode of the crystal,

Dm = mass change per unit area (g/cm²),

A = piezo-electrically active area,

rq= density of quartz, 2.648 g/cm³,

mq= shear modulus of quartz, 2.947´1011g/cm´s².

There are situations where the Sauerbrey equation does not hold, for example, when the addedmass is a) not rigidly deposited on the electrode surface(s), b) slips on the surface or c) notdeposited evenly on the electrode(s). Therefore, the Sauerbrey equation is only strictly applicableto uniform, rigid, thin-film deposits. Due to this the QCM was for many years just regarded as agas-phase mass detector. Not until the beginning of 1980´s scientists realized that a quartz crystalcan be excited to a stable oscillation when it was completely immersed in a liquid. Much of thepioneering work in liquid phase QCM measurements have been done by Kanazawa and coworkers,who showed that the change in resonant frequency of a QCM taken from air into a liquidis proportional to the square root of the liquid´s density-viscosity product:

Df = -fu2/3[(rLhL) / (p ´(rqmq)]½, where

Df = measured frequency shift,

fu= resonant frequency of the unloaded crystal,

rL= density of liquid in contact with the crystal,

hL= viscosity of liquid in contact with the crystal,

rq= density of quartz, 2.648 g/cm³,

mq= shear modulus of quartz, 2.947´1011g/cm´s².

After it was found out that an excessive viscous loading would not prohibit use of the QCM inliquids and that the response of the QCM is still extremely sensitive to mass changes at the solidliquidQCMs have been used in direct contact with liquids and/or visco-elastic films to assesschanges in mass and visco-elastic properties. Even in air or vacuum, where the damping of layershas been considered to be negligible or small the QCM has been used to probe dissipativeprocesses on the quartz crystal. This is especially true for soft condensed matters such as thick polymer layers deposited on the quartz surface.


APPLICATIONS OF QCM

The QCM is basically a mass sensing device with the ability to measure very small mass changeson a quartz crystal resonator in real-time. The sensitivity of the QCM is approximately 100 timeshigher than an electronic fine balance with a sensitivity of 0.1mg. This means that QCM´s arecapable of measuring mass changes as small as a fraction of a monolayer or single layer of atoms.The high sensitivity and the real-time monitoring of mass changes on the sensor crystal make QCMa very attractive technique for a large range of applications. Especially, the development of QCMsystems for use in fluids or with visco-elastic deposits has dramatically increased the interesttowards this technique. Major advantages of the QCM technique used for liquid systems are that itallows a label-free detection of molecules. A partial list of the application areas of the QCM isshown below, and it seems that the application areas are only limited by your imagination.

  • Thin Film thickness monitoring in thermal, e-beam, sputtering, magnetron, ion and laserdeposition.
  • Electrochemistry of interfacial processes at electrode surfaces

Biotechnology

  • Interactions of DNA and RNA with complementary strands
  • Specific recognition of protein ligands by immobilized receptors, immunologicalreactions
  • Detection of virus capsids, bacteria, mammalian cells
  • Adhesion of cells, liposomes and proteins
  • Biocompatibility of surfaces
  • Formation and prevention of formation of biofilms

Functionalized surfaces

  • Creation of selective surfaces
  • Lipid membranes
  • Polymer coatings
  • Reactive surfaces
  • Gas sensors
  • Immunosensors

Thin film formation

  • Langmuir and Langmuir-Blodgett films
  • Self-assembled monolayers
  • Polyelectrolyte adsorption
  • Spin coating
  • Bilayer formation
  • Adsorbed monolayers

Surfactant research

  • Surfactant interactions with surfaces
  • Effectiveness of surfactants

Drug Research

  • Dissolution of polymer coatings
  • Molecular interaction of drugs
  • Cell response to pharmacological substances
  • Drug delivery

Liquid Plating & Etching

In situ monitoring oflubricant and petroleum properties


WORKING PRINCIPLE OF KSV QCM-Z500 INSTRUMENT

The KSV QCM-Z500 measuring principle is based on impedance analysis of the quartz crystal. Inthis measuring principle the quartz crystal is not resonating all the time, but the crystal is swept withpotential perturbations with different frequencies close to the quartz crystals resonant frequencyand the potential (U) over the crystal and the electricity (I) flowing through the crystal are recorded.The ratio of U and I then gives the Impedance (Z), and the result of the sweep is the so-calledImpedance curve (the inverse curve is called Admittance). The Impedance or Admittance curveholds all the information concerning the properties of the quartz crystal and the layer deposited onthe crystal (see figure 2 a). This sweep can be done as a function of time, which enables either themeasurement of mass changes happening at the quartz crystal electrode surface or only once insteady state case studies. The KSV QCM-Z500 sweep generator technology is specially designedfor quartz crystal microbalance applications and it enables fast real-time monitoring of adsorptionprocesses and kinetics.

Normally an equivalent circuit model shown in figure 2b is fitted to the impedance curve and theobtained parameters can be used for calculating the resonant frequency and quality factor (Q) ordissipation (D) of the quartz crystal i.e. masses and visco-elastic properties of the deposited layers.Shortly, one can say that the frequency of the crystal is where the admittance peak has itsmaximum, whereas the broader and lower the admittance peak is the lower the Q-value and moredissipation is taking place. Figure 2a also shows how the Admittance curve is affected by a rigidlydeposited layer or a viscous liquid. Furthermore, the KSV QCM-Z500 can measure differentharmonics of the quartz crystal, which gives additional value for determining visco-elasticcoefficients of the coating. Due to the specially designed sweep generator technology of the QCMZ500the jump between the different overtones can be made very quickly, and up to the 11thovertone can be measured for a 5 MHz quartz crystal.

In most cases quartz resonators are integrated to oscillator circuits to form a QCM for microweighingapplications. While this is a cheap and convenient way of determining only resonancefrequencies of quartz crystals, it will not give any information about the quality of the resonance (Qvalue), which on the other hand would help for visco-elastic analysis. The advantages ofdetermining the Impedance or Admittance curve are:

  • Both Frequency (f) and quality of resonance (Q factor) or dissipation are accessible
  • Different harmonics can be quickly and sequentially be measured
  • Undesired anomalies including scewed resonance curves or distorted resonances due tothe interference of anharmonic sidebands are directly detected

 

Admittance curve for different overlayers on the quartz crystal electrodes
Figure A: Admittance curve for different overlayers on the quartz crystal electrodes

 

Figure B: Equivalent circuit model for determining f and Q of the quartz crystal
Figure B: Equivalent circuit model for determining f and Q of the quartz crystal