TITLE DEVICE AT PIEZOELECTRIC CRYSTAL OSCILLATOR DESCRIPTION Technical field The present invention intends to offer a monolithitc, piezoelectric crystal device which simultaneously (i) has several oscillators on the same crystal, the resonance frequency of which can be chosen in a flexible way, (ii) do not show any problems related to acoustical overhearing between the oscillators, and (iii) facilitate a simple addressing of the individual oscillators.

Background of the invention It is previously known to use crystal micro balancing means at the weighing of small amounts of material. Such micro balancing units are based upon a so called piezoelectric effect of the sensor material (cf below). Consequently all materials which possess piezoelectric properties can be used as sensoric material. Example of suitable sensoric materials are quartz, lithium niobate, and gallium arsenide. In the following we will briefly describe the fonction of a micro balancing unit and present our invention starting from quartz as sensoric material. However, the following description is generally due for piezoelectric materials.

The quartz crystal micro balancing unit, QCM, (from the English Quartz Crystal Microbalance) is an extremely sensitive balance intended to weigh small amounts of material, often in the form of a film, which is added to or removed from the electrodes of the QCM. For details, cf the references (Guilbaut 1980; Alder and McCallum 1983; Czanderna and Lu 1984; Lu 1984; Buttry 1991). The QCM consists of a disc of a single crystal of a piezoelectric material, usually quartz. As an example we choose a so called AT-cut quartz crystal. Such a crystal oscillates in shearing mode with an amplitude of 1-10 rim and a basic frequency of f = constant t-' (1) where t is the thickness of the crystal. Having a t = 0.17 mm f is-10 MHz. The crystal will

start to oscillate when an AC field is applied perpendicular to the surface of the crystal. The frequency of the AC field shall be centred around the intrinsic frequency of the crystal. The more far away the frequency of the AC field is from the intrinsic frequencies of the crystal (a crystal can have more than one intrinsic frequency) the less will the oscillation amplitude of the crystal will become. If one draws the oscillation amplitude of the crystal as a function of the driving frequency one will obtain a peak at the resonance frequency of the crystal.

In pactise the AC filed will be applied over electrode pairs of metals, one on each side of the crystal, which electrodes have been deposited by means of evaporation of in any other suitable manner. The pair of electrodes are then brought into electrical contact with an external oscillation circuit or device that can generate a frequency close to the intrinsic frequency of the oscillator, e. g., an frequency generator, and which can excite the oscillator.

Such a device can manage to measure very small mass changes on the electrodes of the quartz crystal, in the size of 1 ng/cm2, or less under advantageous conditions.

Under ideal conditions the shift in resonance frequency, $ ; is proportional to the change of mass, Am, i. e., Af=-constant-Am (2).

The size of the constant will depend on the type of crystal material used, the resonance frequency, and the crystallographic cut of a given material.

An oscillator is hereby defined as that part of a piezoelectric crystal which can be brought into resonance at a certain frequency. The lowest frequency as the oscillator can be brought into resonance (into a desired mode of oscillation) is defined as the basic resonance frequency. In a conventional piezoelectric crystal oscillator the physical extension of the oscillation is substantially (but not exact, cf. e. g., Rodahl and Kasemo, 1997) the overlap of the electrodes of the underside and the overside.

There are several situations when it is desirable/necessary to be able to obtain more information at the time of determination. Below two examples of determination situations are described where the present invention can be used to increase the information determined

and significantly improve the measurement.

Case 1: Viscoelastic systems Equation 2 presumes that the mass, represented by Am, is fixedly attache to the electrode, and accompanies the oscillation of the QCM without any dissipative losses. Equation 2 does not work when the mass applied is a liquid drop or consists of a viscous material which is not rigidly attache to the crystal and/or is deformed elastically or plastically during the oscillation movement. In such cases the mass-frequency relation becomes more complex.

Then it is important to obtain more information of the properties of the film applied in order to correct characterize the film.

Some of this information can be obtained by measuring the response of the film at several different frequencies. Then it is important to be able to choose these frequencies in a flexible way starting from the conditions as the film requests. It is, using common technology, possible to excite a QCM in its harmonics, but these are situated in a great and fixed distance from each other. (The frequencies of the harmonics run as 30, 50 etc., whereinfOis the basic frequency). The present invention facilitates excitation/measurement of the film applied at many more frequencies which can be chosen in a flexible manner.

Case 2: Multiple sensors All sensors give undesired responses due to variations in the measuring conditions. QCM is no exception: small variations in, e. g., the surrounding pressure and/or temperature can seriously affect the measurement in a unpredictable way, if they are not controlled. One way of reducing the consequences of variations in temperature etc. is to use two independent oscillators: one used as an active sensor, and one used as a reference sensor. Both oscillators are then made subject to the same conditions of e. g., temperature and pressure, but only the active oscillator interacts with that one is interested of measuring on. If the oscillators react in the same way on changes of temperature etc. the difference signal between the oscillators then depends only on what one wants to mesure. In this situation it is important that: * the active oscillator is matched against the reference oscillator so that it reacts in the same way on changes in temperature etc. E. g., a very small deviation of the cutting angle or quality between the crystals can provide great differences in the temperature response

* to design the measurement in such a way that both oscillators are made subject to exactly the same conditions. The more far away the oscillators are placed from each other the more difficult it is to keep the conditions exactly the same, i. e. gradients in the surrounding conditions cause faults in the measurement.

If the two oscillators should be placed on one and the same crystal plate their properties will become practically the same, e. g., with reference to the quality of the crystal. It is then desirable to place the oscillators as close as possible on the crystal plate. The closer the oscillators are situated to each other, the more like can their measurement conditions be made. Less material consumption and size are two further important avantages by placing the oscillators close together.

The number of oscillators per crystal can be more than two. For example an array of oscillators can be placed upon a crystal. If the surface chemistry or another property of the different oscillators can be made different so that they will react differently on different substances in a sample several parameters can be registered simultaneously and/or pattern recognition can be utilized to identify a substance which does not react specifically with any of the oscillators but provides different (unspecific) signals for each oscillator. Even in such application it is desirable to place the oscillators as close as possible to each other so that the sample volume will become as small as possible and so that the registration conditions for the different oscillators will become as similar as possible.

In order to be able to utilize a multi-oscillator each individual oscillator has to be able to be addressed (registered) individually. Using conventional technique it is required either a series of drive and monitoring electronics for each oscillator which is connecte directly to each oscillator or a system of multiplexing which sequentially connects the oscillators to drive and monitoring electronics by means of one or more relays (or the similar device).

A strongly restriction factor for how close the oscillators can be placed to each other is acoustic superhearing (acoustic connection). The piezoelectric oscillation is excited by the electrical field between a pair of electrodes but the oscillation is not fully restricted to the overlap between the electrodes but is damped approximately in an exponential fashion

outside the electrode overlap (Rodahl and Kasemo, 1997). This"oscillation tail"can connect to another oscillator if it is situated too close (Kaushik, Chattopadhyaya et al. 1981). The oscillators will then not become independent but joined, and the whole idea using multiple oscillators on one crystal will fall apart.

There are several publications (e. g. US-A-5,495,135,3,898,489,5,032,755 and 4,894,577) concerning different embodiments to reduce undesired resonance modes (spurious modes) in piezoelectric crystals using one or more oscillators but these do not discuss acoustic connection between the resonance desired in QCM application. J Vig has patente (US-A- 5,686,779) a design of a temperature sensor using a matrix of piezoelectric oscillators on one and the same crystal. He describes how these can be disconnected thermally but in the present case a good thermal conductivity between the oscillators is desired so that they will have the same temperature. Many previous works (Kaushik, Chattopadhyaya et al. 1981) which relates to undesired acoustic connection between multiple oscillators on the same piezoelectric crystal have only propose, as a counteraction to this connection, to place the pairs of electrodes far enough from each other in that no connection occurs. A work (PCT/JP96/02125) discusses ways of reducing acoustic connection by building barries between oscillators. This work only discusses filter applications and does not solve the problem described in case 1 above or the addressing of the different oscillators.

In view of what has been said above the object of the present invention is to offer a monolithic, piezoelectric crystal device which simultaneously (i) has several oscillators on the same crystal, the resonance frequencies of which can be selected in a flexible way, (ii) does not show any problem with regard to acoustic superhearing between the oscillators, and (iii) facilitates a simple addressing of the single oscillators.

Short description of the figures Figure I (lateral view, cross-section A-A in Figure 2) and Figure 2 (shall not be interpreted as a cross-section but is a top planar view) show an example of an embodiment of the present invention.

Figures 3 to 5 illustrate schematically other examples of embodiment of the present

invention where the crystal has been cut from the side (as in Figure 1).

Common to Figures 1 to 5 is that they are not scale proportional in order better to illustrate the art of the invention. Further, common to the figures is that all figures show three independent oscillators but this number can of course be varied both up and down.

Figure 6 shows the registration of the frequency spectrum from a crystal manufactured in accordance with the embodiment shown in Figure 5.

Detailed description of the invention In the following examples of embodiments of the invention are presented which shall be interpreted within the frame work of the accompanying claims. In the following examples of embodiments of the invention three oscillators per crystal is shown but this number can be varied up and down.

In a first embodiment of the present invention shown in Figures 1 and 2, there is piezoelectric crystal (5) on its upper side designed with staircase like pattern provided with a number of electrodes (1,2,3). The electrodes of the upper side (1,2,3) are provided with contact means using electrode wings (11,12,13). In this embodiment there is only one electrode (71) which advantageously can be connecte to earth, on its under side functions as counter electrode to all electrodes (X) on the upper side.

The manufacture of the piezoelectric crystal (5) can be done by known technique, such as by means of wet etching, dry etching, (reactive ion etching) MITE (micro machining by iron track etching), abrasion and/or other technique suitable for the purpose.

The arrangement using only one counter electrode (71) on the under side of the crystal is advantageous i. a. when the crystal is used as a QCM in liquid phase registrations where one side of the crystal is brought into contact with a liquid. In order to avoid that the conductivity of the liquid influences the resonance properties of the QCM that part of the crystal which is brought into contact with the liquid should be completely covered by an electrode (Rodahl, Höök et al. 1996). Furthermore, there is only one wire needed to bring the counter electrode

into contact..

When using the invention as a sensor in liquid phase registration it is in many cases an avantage if that part of the sensor which is brought into contact with the liquid is smooth.

This will be of no problem in the present invention as one side can be kept smooth (as shown in the Figures). If this restriction does not apply, it is possible to structure both sides of the crystal if this should be an avantage.

The basic tone frequency of the oscillators depends of the thickness of the oscillator in accordance with equation 1. By choosing a suitable thickness of the crystal for each oscillator one can thereby determine the basic tone frequencies of the oscillators. In the present invention the resonance frequencies of the oscillators in such away that they will not have any overlapping frequency peaks which drastically reduces the acoustic superhearing which means that the oscillators can be placed closer than would have been possible in a conventional crystal with several oscillators. Two resonance peaks are overlapping if a drive frequency can bring both oscillators into a significant oscillation, and if that is not the case, then the resonance peaks are not overlapping.

Figure 3 shows another example of an embodiment of the present invention where the resonance frequency of each oscillator have been adjusted by coating the crystal with films (31,32,33) of varying thickness in connection to each electrode on the upper side of the crystal. The thickness of the film and the density thereof will then determine how much the resonance frequency will be changed in relation to the areas where no film has been applied, (cf equation 2). If the thickness of the crystal corresponds with a desired resonance frequency of one of the oscillators, no film is needed for one oscillator.

In order to obtain a good resonance for each oscillator it is of importance that the film has good mechanical properties so that the oscillation of the oscillator will not become too much damped and that the film has a good adhesion to both the crystal and the electrode. It is, e. g., possible to use the same material in the film as in the electrode. Common electrode materials are gold and silver. Chromium and titan are common material in order to improve the adhesion between electrode and crystal.

Figure 4 shows an embodiment of the present invention where the oscillators are lowered into the crystal. The cavity can be made using already known technique such as by means of wet etching, dry etching, (reactive ion etching) MITE (micro machining by iron track etching), abrasion and/or other technique suitable for the purpose. The electrodes (1,2,3) are placed in the cavities (41,42,43). In the same way as above the resonance frequency of each oscillator can be changed by altering the depth of the cavity. If all cavities are made to the same depth the resonance frequency can be changed by coating the cavities with films of different mass in a similar way as in the foregoing example.

Figure 5 shows an example of the embodiment of the present invention which has only two electrodes (51,71) but in other respects the same as in Figure 4. When the oscillators of the crystal has non-overlapping resonance frequencies they can be driven/registered by addressing each oscillator at its resonance frequency. Example of equipment which can address each oscillator in the latter way is e. g., a net work analyser or the drive electronics described in the Swedish patent SE-C-504 199. It is of course possible to drive/register on the oscillators by individual wires in a conventional way (as described above).

The possibility of addressing the oscillators at their resonance frequencies is not restricted to crystals having cavities therein but can also be used in all embodiments given above.

Exemple Figure 6 shows a frequency spectrum of a quartz crystal having three lowered oscillators and only two electrodes, of the type as shown in Figure 5. The sample was manufactured from a 5 MHZ AT-cut crystal having a diameter of 25 mm. The cavities made by etching using MITE, were approximately 36,52, and 68 zip deep and had a diameter of 6 mm with a separation of about 1.5 mm. As evident each oscillator gives raise to its own unique resonance frequency, well separated from the others. In this case a film/sample which had been placed upon one of the electrodes (preferably the electrode which covers the smooth side of the crystal) is excited and registered at 5.75 MHZ, 6.05 MHZ, and 6.45 MHZ. Using traditional QCM technique this will not be possible but one should have needed three separate crystals. This means that there is needed three identical films/samples which have to be applied on each their crystal in an identical way. This problem (which can be very great) is avoided using the present invention.

REFERENCES Alder, J. F. and J. MeCallum (1983)."Piezoelectric crystals for mass and chemical measurements."The Analyst 108 (1291).

Buttry, D. A. (1991). Applications of the quartz crystal microbalance to electrochemistry.

Electroanalytical Chemistry. A.J. Bard. New York, Marcel Dekker, Inc. 17: 1-86.

Czanderna, A. W. and Lu (1984). Introduction, history, and overview of applications of piezoelectrie quartz crystal microbalances. Applications of piezoelectric guartz crystal microbalances. C. Lu and A. W. Czanderna. Amsterdam, Elsevier. 7: 1-18.

Guilbaut, G. G. (1980). "Uses of the piezoelectric crystal detector in analytical chemistry." Ion Selective Electrode Review 2: 3-16.

Kaushik, D., et al. (1981)."Thin film thickness monitoring using a doubly oscillating quartz crystal and measurement of growth rate."Journal of Physics E: Scientific Instruments 14: 345-348.

Lu, C. (1 984). Theory and practice of the quartz crystal microbalance. Applications of piezoelectric quartz crystal microbalance. C. Lu and A. W. Czanderna. Amsterdam, Elsevier. 7: 19-61.

Rodahl, M., et al. (1 996)."QCM operation in liquids: An explanation of measured variations in frequency and Q factor with liquid conductivity."Analytical Chemist 68 (13): 2219-2227.

Rodahl, M. and Kasemo (1997)."QCM frequency and dissipation factor response to localized liquid deposits."Sensors & Actuators B 37: 111-116.