ASICS for MEMS

1
ASICS for MEMS

• BRILLANT Grégory
• 13th of October

2
60 MHz Wine-Glass Micromechanical-Disk Reference
Oscillator
3
Introduction

• Among off-chip components in a wireless
  communication circuit, the quartz crystal used in
  the reference oscillator is perhaps the most
  difficult to miniaturize
• Qgt 10 000 and thermal stabilities better than 35
  ppm uncompensated over 0-70C 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 gt100 000 at 60 MHz,
• frequency temperature dependencies of only 18 ppm
  over 25-105C at 10 MHz

4
Introduction

• Recently, 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 0-70C, 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 oscillator
• Replacement of the wide-CC-beam resonator by a
  60MHz MEMS-based wine glass disk micromechanical
  resonator

5
The wine glass resonatorresults

• Q gt 48 000
• The 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.

6
The wine glass oscillatorprinciple

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

7
The wine glass oscillatorprinciple

• There is different vibrating modes
• This modes are distinguishable by the phasing
• Each mode exhibits unique resonator phasing
• A 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

8
The wine glass oscillatorprinciple

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

9
The wine glass oscillatorprinciple

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

10
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
  design
• The 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 Q
• The 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.

11
Realization

• Even with these enhancements, the resistance of
  the device is 1.5kO for a 64µm-diameter 60MHz
  wine glass disk with Vp12V and Q48,000
• It is larger than the 50O normally exhibited by a
  off-chip quartz crystals
• A sustaining amplifier capable of supporting high
  tank impedance is required.
• A trans-resistance CMOS sustaining amplifier is
  used

12
Realization

• A fully balanced differential CMOS op amp
  connected in shunt-shunt feedback
• M1-M5 the basic differential Opamp
• M11-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

13
Realization