Mounting for a quartz crystal

Details Mounting for a quartz crystal Mounting for a quartz crystal

2000-03-20

2002-10-15

Williams, Hezron (Department: 2856)

Measuring and testing

Gas analysis

By vibration

C073S031060, C073S651000, C310S355000, C310S352000

Reexamination Certificate

active

06463787

ABSTRACT:

The invention relates to a mounting for a quartz crystal, which is configured as a disc and has metal layers on both sides as electrical contacting surfaces, and to a quartz crystal microbalance which includes such a mounting and a plug-in module adapted thereto.
The sensitive detection of alterations in mass with the aid of quartz crystal microbalances has been used successfully for several years already. For example such a measuring technique is used in the application of methods of surface coating, for example in the vapour deposition of metals or metal oxides. The method is based on the knowledge that a mass applied to a quartz crystal oscillating at its own natural frequency causes an easily measurable alteration in the oscillation frequency of the quartz with the frequency shift, alterations in mass which correspond to only a few atomic positions of the applied material can easily be detected. According to the quartz crystal used, the natural frequency of the quartz ranges from roughly 1 MHz to roughly 10 MHz. The frequency alteration which can here be identified, and which is in a large frequency range proportional to the amount of material applied, can be determined very exactly. The alteration in mass corresponding to the alteration in frequency can be calculated according to Sauerbrey's equation:
&Dgr;f
=−[2
f
o
Q
(
A &mgr;
Q
&rgr;
Q
)
−1
&Dgr;m]
here
f
o
is the measurable frequency alteration on the basis of the alteration in mass &Dgr;m of the quartz,
f
o
is the natural frequency of the quartz without additional mass,
A is the geometric electrode surface of the quartz,
&mgr;
o
is the modulus of rigidity of the quartz and
&rgr;
o
is the density of the quartz.
The principle of this measurement is based on the fact that the synthetic piezoelectric quartzes used when electrically excited in the megahertz range carry out a shear vibration. The vibration frequency depends on the thickness of the quartz and on the additional mass applied to the quartz, for example a vapour-deposited metal layer.
For roughly 35 years, the measurement of the alteration in mass &Dgr;m with the quartz crystal microbalance has been used not only with coating methods in vacuum but also in electrochemical experiments in which one side of the quartz crystal is in complete contact with a liquid “The Quartz crystal microbalance: A Novel Approach to the In-Situ Investigation of Interfacial Phenomena at the Solid/Liquid Junction” by R. Schumacher in Angew. Chemie, Int. Ed., volume 29 (1990), pages 329 to 438). In this application, up to now predominantly scientific questions have been in the forefront, which arise during the investigation of interfacial reactions in liquid media. Together with further investigation methods, for example the measurement of the differential capacitance and the charge flow through the interface, this technique makes possible greater insight into interfacial reactions. For example this technique is sensitive enough also to detect the presence of absorbates on the quartz.
The quartz crystals are manufactured as thin wafers which are excited with an electronic oscillator circuit in shear vibrations parallel to the surface of the wafers. In order to transfer the excitation vibration to the quartz, contacting electrodes are applied to the quartz surfaces. For this purpose, large-area metal layers are for example vapour-deposited or sputtered onto both surfaces of the wafer, which layers are electrically connected to the exciting circuit. The oscillation frequency of the quartz is registered by means of standard measuring methods. For example in the essay by R. Schumacher, ibid., references are made to suitable oscillator circuits.
In U.S. Pat. No. 4,561,286 is described a piezoelectric contamination detector which is used in a gaseous environment. A quartz crystal is used in the detector. The quartz crystal is contacted via contact springs which are in contact with the contacting surfaces on the quartz.
In U.S. Pat. No. 4,917,499 is also described a device for analysing contamination in a gaseous environment, in which device the quartz crystal used is contacted electrically via springs. In this publication it is mentioned that these springs are used in order to achieve even distribution of the forces acting on the quartz, and thus to avoid the formation of tensions in the quartz.
In contrast to applications in which the quartz crystals are used in a gaseous environment, specific problems arise when quartz crystals are used which are in contact with liquid: on the one hand, care must be taken that no liquid reaches the electrical leads to) the contacting electrodes on the quartz and certainly does not come into contact with the electronic exciting circuit. Simultaneously, however, it must also be guaranteed that the excitation of the quartz vibrations is not hindered by the mounting of the quartz (R. Schumacher, ibid.). Simultaneously satisfying these two requirements is frequently difficult, since expediently mountings are used in which the quartz is not glued in, but in which the liquid-tightness is intended to be achieved by means of easily fitted and detachable sealing means, for example toroidal sealing rings to seal against penetrating fluid. The quartz is mechanically fixed with these sealing means so that the oscillation can be prevented.
In U.S. Pat. No. 5,201,215 is described a quartz crystal microbalance, in which the quartz crystal is in contact with a liquid and is contacted via securely connected contacts.
What has proved to be problematic is that the ability of the quartzes to start vibrating in a liquid is much lower than when used in a gaseous environment or in vacuum. In the latter case, on the other hand, the quartzes can be easily excited. If the quartzes are dipped into a liquid, the latter acts like a brake and dampens the shear vibration. The measures taken to prevent liquid from penetrating into the interior of the measurement cell in addition prevent the excitation of the quartz, such that the quartz vibration can altogether become easily unstable.
In particular in the monitoring of electroplating baths, metal s constantly deposited on the quartz crystal such that the deposited metal has to be removed intermittently again and again from the quartz. During the quick dissolution process desired for this purpose, the vibration excitation regularly breaks down, such that the vibration excitation has to be started up again once the dissolution process is completed. After several cycles of these deposition and subsequent dissolution processes, the quartzes have to be regularly exchanged in order to apply fresh metal layer electrodes to both sides of the quartz for renewed contacting. During the renewed fitting of the quartz, notice must be taken both of the low mechanical stability of the quartz wafers, the liquid-tightness of the measuring apparatus against penetrating liquid and of the reproducible vibration excitation of the quartzes. From these points of view the previously available techniques for fitting the quartz wafers do not guarantee any problem-free exchange since at least some of the above-mentioned problems almost always arise.
The problem underlying the present invention, therefore, is to avoid the disadvantages of the previously known quartz crystal microbalances and especially to find a device with which reproducible vibration excitation of the quartz can be also guaranteed under the technical conditions which prevail when it is used for monitoring electroplating baths.
Above all it must also be ensured that the bath liquid does not penetrate into the device, that the quartz crystal is easily exchangeable and that, after a sufficiently long period of time during which metal is deposited on the contacting surfaces, it can be freed of deposited metal again without any problem by means of an electrolytic method.
This problem is solved by the mounting according to the invention and according to claim
1
and the quartz crystal microbalance according to claim
11
. Preferred embodiments of the invention are

Affiliated with

Also associated with

Rating

Mounting for a quartz crystal does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Mounting for a quartz crystal, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mounting for a quartz crystal will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2965514