Transcript ASICS for MEMS

ASICS for MEMS
BRILLANT Grégory
13th of October
60 MHz Wine-Glass
Micromechanical-Disk
Reference Oscillator
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
Q> 10 000 and thermal stabilities better than 35 ppm
uncompensated over 0-70°C are generally unavailable onchip.
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 25105°C at 10 MHz
October 2006
Introduction
Recently, an on-chip vibrating clamped-clamped beam (CCbeam) 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 oscillator
Replacement of the wide-CC-beam resonator by a 60MHz
MEMS-based wine glass disk micromechanical resonator
October 2006
The wine glass resonator:
results
Q > 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.
October 2006
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
points
The nodal points are
motionless when the disk
vibrates in its wine glass
mode shape
October 2006
The wine glass oscillator:
principle
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
October 2006
The wine glass oscillator:
principle
Wine-glass resonator array
can be use in order to
achieve better
performances
October 2006
The wine glass oscillator:
principle
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
ψ 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 material
Seen through its terminals, the whole device can be equated to a
LCR circuit
October 2006
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.
October 2006
Realization
Even with these
enhancements, the resistance
of the device is 1.5kΩ for a
64μm-diameter 60MHz wine
glass disk with Vp=12V and
Q=48,000
It is larger than the 50Ω
normally exhibited by a offchip quartz crystals
A sustaining amplifier capable
of supporting high tank
impedance is required.
A trans-resistance CMOS
sustaining amplifier is used
October 2006
Realization
A fully balanced differential CMOS op amp connected in shuntshunt 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
October 2006
Realization
October 2006