ASICS for MEMS BRILLANT Grégory 13 th of October

ASICS for MEMS

BRILLANT GrégoryBRILLANT Grégory

1313thth of October of October

60 MHz Wine-Glass Micromechanical-Disk

Reference Oscillator

October 2006October 2006

IntroductionAmong off-chip components in a wireless communication circuit, the quartz crystal used in the reference oscillator is perhaps the most difficult to miniaturizeQ> 10 000 and thermal stabilities better than 35 ppm uncompensated over 0-70°C are generally unavailable on-chip. Recently, on-chip vibrating micromechanical resonators based on MEMS technology have become increasingly attractive as on-chip frequency selective elements for communication-grade oscillators and filters

Q >100 000 at 60 MHz, frequency temperature dependencies of only 18 ppm over 25-105°C at 10 MHz


IntroductionRecently, an on-chip vibrating clamped-clamped beam (CC-beam) micromechanical resonator based on MEMS technology has been demonstrated at 10MHz with a Q of 4,000 and a frequency stability of 34ppm over 0-70°C, which matches that of quartz.But, the problem is the far-from-carrier phase noise (only -120dBc/Hz )This value is caused by the insufficient power handling ability of the CC-beam micromechanical resonator device used This paper presents a work which achieves an effective 25dB improvement in phase noise performance over the previous 10MHz oscillatorReplacement of the wide-CC-beam resonator by a 60MHz MEMS-based wine glass disk micromechanical resonator


The wine glass resonator:results

Q > 48 000The combination of this resonator with a CMOS trans-resistance sustaining amplifier designed to accept the high impedance of the wine glass disk yields a 60MHz reference oscillator that achieves:

phase noise density of -100dBc/Hz at 1kHz offset from the carrier-130dBc/Hz at far-from-carrier offsets

Dividing down to 10MHz, these values correspond to:

-115dBc/Hz at 1kHz offset from a 10MHz carrier -145dBc/Hz at far-from-carrier value.


The wine glass oscillator:principle

The wine glass disk resonator consists of a 3μm-thick disk supported by two beams that attach to the disk at its nodal pointsThe nodal points are motionless when the disk vibrates in its wine glass mode shape



There is different vibrating modesThis modes are distinguishable by the phasingEach mode exhibits unique resonator phasingA single mode can be selected by choosing the input ac signal to match the phasing of the desired mode In the mode shape used in this work, the disk expands along one axis and contracts along the orthogonal axis



Wine-glass resonator Wine-glass resonator array array can be use in order to can be use in order to achieve better achieve better performancesperformances



To excite vibrations a dc-bias voltage Vp and an ac input signal Vi to oppositely located input electrodes are applied to the disk structureThese voltages result in a force proportional to the product VpVi that drives the resonator into its vibration mode shapeThis occurs when the frequency of Vi matches the wine glass resonance frequencyψ is a modified Bessel function quotient, fo is the resonant frequency, R is the disk radius, and ρ, σ, and E, are the density, Poisson ratio, and Young's modulus, respectively, of the disk structural materialSeen through its terminals, the whole device can be equated to a LCR circuit


Realization

The key to achieving improvements lies not only in the use of a wine glass disk resonator but also in the specific advances applied to its designThe wine glass disk of this work differs from that of a previous prototype in that its:

thickness is increased to 3μm and gap is reduced from 100nm to 80nm → it increases its power handling and lower its impedance The number of supports used is reduced from four to two, in order to decrease energy loss from the disk to the substrate through anchors → maximize the device QThe stiffness of this wine glass disk is 6.6e5N/m → more than 55X the 1.2e4N/m of the 10MHz wide-CC-beam device →This allows it to handle powers 55X higher.


RealizationEven with these enhancements, the resistance of the device is 1.5kΩ for a 64μm-diameter 60MHz wine glass disk with Vp=12V and Q=48,000It is larger than the 50Ω normally exhibited by a off-chip quartz crystalsA sustaining amplifier capable of supporting high tank impedance is required.A trans-resistance CMOS sustaining amplifier is used


RealizationA fully balanced differential CMOS op amp connected in shunt-shunt feedbackM1-M5: the basic differential OpampM11-M18: common-mode feedback circuit that sets bias point.MOS resistor MRf serves as a shunt feedback element that allows control of the trans-resistance gain via adjustment of its gate voltage


Realization

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ASICS for MEMS BRILLANT Grégory 13 th of October