Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2003-06-20
2004-12-14
Young, Brian (Department: 2819)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S185000
Reexamination Certificate
active
06831531
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a time base, i.e. a device comprising a resonator and an integrated electronic circuit for driving the resonator into oscillation and for producing, in response to this oscillation, a signal having a determined frequency.
Time bases, or frequency standards, are required in a large variety of electronic devices, ranging from wristwatches and other timepieces to complex telecommunication devices. Such time bases are typically formed by an oscillator including a quartz resonator and an electronic circuit for driving the resonator into oscillation. An additional division chain may be used to divide the frequency of the signal produced by the oscillator in order to obtain a lower frequency. Other parts of the circuit may serve to adjust the frequency, for example by adjusting the division ratio of the division chain. The components of the electronic circuit are advantageously integrated onto a single semiconductor substrate in CMOS technology. Other functions, not directly related to the frequency processing, may be integrated onto the same substrate.
Advantages of quartz resonators are their high quality factor Q leading to good frequency stability and low power consumption as well as their good temperature stability. A disadvantage of typical time bases using quartz resonators however resides in the fact that two components, namely the quartz resonator and the integrated electronic circuit, are required in order to provide a high-precision frequency. A discrete quartz resonator requires board space which is scarce in many cases. For instance, a standard quartz resonator for wristwatch applications requires space of the order of 2×2×6 mm
3
. Moreover, additional costs are caused by the assembly and connection of the two components. Yet, space and assembly costs are major issues, especially in the growing field of portable electronic devices.
It is thus a principal object of the present invention to provide a solution to the above-mentioned problems by providing a time base comprising an integrated resonator.
SUMMARY OF THE INVENTION
Another object of the present invention is to provide a time base that may be fully integrated on a single substrate, that is suitable for mass production and that is compatible with CMOS technology.
Still another object of the present invention is to provide a time base comprising a resonator having an improved quality factor Q and thereby a greater frequency stability and low power consumption.
Yet another object of the present invention is to provide such a time base which is low-priced and requires only a very small surface area on a semiconductor chip.
Accordingly, there is provided a time base comprising a resonator and an integrated electronic circuit for driving said resonator into oscillation and for producing, in response to said oscillation, a signal having a determined frequency, characterised in that said resonator is an integrated micromechanical tuning fork resonator supported above a substrate and adapted to oscillate, according to a first oscillation mode, in a plane substantially parallel to said substrate, said tuning fork resonator comprising a base member extending substantially perpendicularly from said substrate, a free-standing oscillating structure connected to said base member and including at least a first pair of substantially parallel fork tines disposed in said plane, and an electrode structure disposed adjacent to said fork tines and connected to said integrated electronic circuit.
An advantage of the time base according to the present invention lies in the fact that the micromechanical tuning fork resonator exhibits a high quality factor Q.
Quality factors as high as 50'000 have been measured which is of the same order as those obtained using conventional quartz resonators. The quality factor Q is determined by air friction and by intrinsic losses in the vibrating resonator material. Air friction can be neglected if the resonator is operated under vacuum conditions. Intrinsic losses depend on the material as well as on the design of the resonator. Resonators made of crystalline materials, like quartz or silicon, are known to be capable of high-Q oscillation. In addition, the clamping, i.e. the mechanical support of the resonator part, strongly influences the dynamic behaviour. According to the present invention, the tuning fork resonator is designed and driven in such a way that the centre of gravity of the entire structure remains motionless during oscillation and that the bending moments of the fork tines can be compensated in a relatively small region of the base member. Different design features favouring a high quality factor Q are the object of the dependent claims and will be described hereinafter in detail.
In addition, for a given resonant frequency, the surface area required on the substrate to form the tuning fork resonator is small in comparison with other resonators. For instance, a tuning fork resonator according to the present invention designed for a frequency of 32 kHz requires a chip area of approximately 0.2 mm
2
which is smaller than the chip area required by the silicon ring resonator described in pending international application No. PCT/CH 00100583 filed on Nov. 1, 2000 by the same Applicant.
According to one aspect of the invention, the electronic circuit is advantageously integrated on the substrate together with the micromechanical tuning fork resonator, thereby leading to a low-priced time base. A lower price is also obtained by vacuum-sealing of the resonator at the wafer-level in a batch-process using wafer-bonding technology.
Tuning fork structures have been proposed as resonating structures for different types of sensor applications, such as acceleration, rotation or strain sensors. These sensor structures are, however, not optimised according to the same guidelines as in the present invention where a high quality factor is a primary goal in order to obtain a highly precise time base.
U.S. Pat. No. 5,747,691 to Yoshino et al. for instance describes a tuning fork-shaped vibratory element made of a single crystalline silicon substrate. The tines of the tuning fork have thin and thick regions in order to allow a bending of the arm in a direction perpendicular to the oscillation when an external force is applied. The resonating element has been optimised with respect to the sensor application.
GB Patent No. 2,300,047 to Fitzpatrick et al. describes an assembly of tuning fork sensors in order to provide a three-dimensional motion detection.
Yet other documents, e.g. WO 91103716 to Jensen et al., GB 2,162,314 to Greenwood et al., U.S. Pat. No. 4,912,990 to Norling et al., or the article of Beeby et al. in the Journal of Microelectromechanical Systems, Vol. 9, No. 1 (2000), pp. 104 ff., describe micromachined silicon resonant strain gauges in the form of a double-ended tuning fork.
None of the above-cited documents however indicates or suggests using such a type of tuning fork resonator in an oscillator circuit to act as a frequency standard or time base. Moreover, a number of design features of the tuning fork resonators disclosed in these documents render them less suitable for horological applications where frequency stability and low power consumption are essential.
Anisotropically etched oscillating tuning fork structures have been described previously by the present inventors. Anisotropic etching of the structure inevitably yields, however, a different length of the fork tines, which results, in turn, in a low quality factor for such a tuning fork resonator. The fabrication method and disadvantages of such anisotropically etched tuning fork resonators are discussed in greater details in the article by M. Giousouf et al., published in Proc. of Eurosensors XII, Vol. 1 (1998), pp. 381-384, or the article by M. Giousouf et al., published in Sensors and Actuators 76 (1999), pp. 416-424, both entitled “Structuring of Convex Corners using a Reoxidation Process-Application to a Tuning Fork Resonator made from (110)-Silic
Giousouf Metin
Kück Heinz
Platz Rainer
Eta Sa Manufacture Horlogere Suisse
Lauture Joseph
Sughrue & Mion, PLLC
Young Brian
LandOfFree
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