Reliability of Silicon Resonator Oscillators by bestt571


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									            Reliability of Silicon Resonator Oscillators
                                                            Wan-Thai Hsu
                                                           Discera Inc.
                                                    Ann Arbor, Michigan, U.S.A.

Abstract—Low-cost, CMOS oscillators with miniaturized                 such as electrostatics, piezoelectric transducers, magnetic
silicon-based MEMS resonators have been out of research               field, or even laser. Micro machining technologies inherit
laboratories for high volume production. Silicon resonators not       integrated circuit processes such as deposition, lithography,
only cover all the size reduction roadmaps of quartz crystals,        and etching, and include more MEMS specific technologies
but also lead to a fully-integrated oscillator solution in the near   such as bonding, laser machining, and releasing.
future. The major concerns about silicon resonators are their
reliability, including frequency stability over temperature               Figure 1 shows a micromechanical resonator fabricated
cycling, aging, vibration operation, and shock resistance. This       via a low cost 1µm, 2-poly-1-metal surface micromachining
paper demonstrates, for the first time, reliability testing results   process. The resonator is designed to have optimal
based on volume manufacturing silicon based oscillators.              performance balanced between low dc bias voltage, high Q,
                                                                      and good power handling. As described in previous paper
                                                                      [2], micromechanical resonators require a dc bias voltage
                      I. INTRODUCTION                                 and an ac excitation signal to generate electrostatic force.
    Micromechanical silicon resonators with tens of square            While the frequency of the ac excitation signal matches with
microns in size have been showing great quality factors (Q)           natural resonance frequency of the resonator, the resonator
of more than 10,000 in research lab for more than a decade.           vibrates perpendicular to substrate. While it vibrates, it acts
As high-Q is one of the major parameters for frequency                like a time-varying capacitor, which then outputs ac current
control components, it is apparent that micromechanical               that can be traced through frequency as a spectrum shown in
silicon resonators exhibit advantages over quartz crystals            Figure 2. The spectrum can be characterized by an RLC
especially in terms of size, cost, and system integration.            model shown in the inset of the Figure 2.
Researchers have been pursuing various vibration modes
                                                                           The resonator Q’s versus vacuum level is characterize
with various geometries of the resonator in order to expand
the range of applications [1]. However, to make silicon               from 1µTorr to 100Torr. As shown in Figure 3, for low
                                                                      frequency 32kHz resonators, their Q’s started degrading at
resonator oscillators a reliable source for frequency
reference, not only resonator design needs to be an                   around 2mTorr. On the other hand, due to larger mechanical
                                                                      stiffness, hence less sensitive to air molecular damping,
optimization between Q and power handling, both of which
are critical to the phase noise of the oscillator, but also the       19MHz resonators hold up their Q till 2Torr. As a result, for
                                                                      MHz resonators have less vacuum packaging requirements.
reliability of packaged resonators and oscillators needs to
meet or exceed industrial standards.
    This paper presents reliability data of MEMS oscillators
based on the data from volume manufacturing lots. At first
the mechanical bonding process was evaluated by a set of
mechanical tests. Hermeticity of wafer level sealed package
was gauged by the Q of micromechancial resonators. Q
degradation is evaluated by high temperature storage and
autoclave based on JSED standards. For oscillator reliability,
aging, shock resistance, vibration operation as well as
thermal cycling were tested. All the reliability data proved
that silicon resonator oscillator has met the requirements of
oscillators for timing and clock applications.

    Micromechanical resonators are micro machined devices              Figure 1 SEM photograph of a micromechanical resonator designed
                                                                       for frequency reference
that vibrate at a specific frequency due to external excitations
                                                                       and 168 hours. The testing data shows the Q’s of the
                                                                       resonators did not degrade and the frequency shift is within
                                                                       measurement error. Based on the permeation data and
                                                                       pumping capacity of getter material embedded inside the
                                                                       cavity, this package has no problem maintaining the vacuum
                                                                       within 100mTorr for 10 years.

                                                                       B. High Temperature Storgae Life Test
                                                                           Based on JESD22-A103 high temperature standard, a
                                                                       batch of 64 resonators was stored under 150°±10°C up to
                                                                       1000 hours. Neither Q degradation nor frequency shift was
                                                                       observed. The yield of this particular wafer is as high as
                                                                       97.3% even after the HTSL test. This indicates that there is
                                                                       no stress degradation within the resonator package.
 Figure 2 Measured frequency spectrum of a 19.42MHz micro-
 mechanical resonator
                                                                       C. Mechanical Strength of Bonding
                                                                           The bonding needs to be mechanically strong enough so
                                                                       the cap will not be delaminated from the MEMS substrate
                                                                       during assembly as well as over-molding processes.
                                                                       Typically the backend process requires 5kgf of bonding
                                                                       strength to ensure a reliable assembly. The average of our
                                                                       glass frit bonding is as high as 9.3kgf. This type of
                                                                       singulated devices has gone through the severe QFN
                                                                       assembly process and we have not seen any delamination.

                                                                                      IV.   OSCILLATOR RELIABILITY
                                                                           MEMS-based Silicon oscillators have demonstrated very
 Figure 3 Q versus vacuum level for low (32kHz) and high (19MHz)
                                                                       good phase noise performance over these past few years
 frequency micromechanical resonators                                  [3][4][5]. At the same time, several breakthroughs have been
                                                                       made on temperature compensation [6][7]. Therefore,
                                                                       MEMS-based silicon oscillators have entered into an era of
  III.   RELIABILITY OF WAFER LEVEL VACUUM PACKAGE                     productionization. The following reliability tests of these
                                                                       oscillators support this claim.
A. Autoclave Test
                                                                       A. Aging
    A singulated micromechanical resonator is shown in
Figure 4. As shown, the bonding material seals the resonator              As a mechanical device that vibrates tens millions of
cleanly. In order to test the hermetic reliability of the              cycles per second, aging is one of the major concerns of
package, a batch of 53 resonators are tested prior the                 overall MEMS devices. The aging requirement based on
autoclave test. The testing condition is based on JESD22-              timing and clock applications is generally ±5ppm the first
A102 standard. The devices were placed under 121°±2°C,                 year, which is equivalent to the resonator traveling 6000km a
relative humidity of 100%, and 2atm (205kPa) for 24, 96,               year with ±30m of accuracy.

 Figure 4 SEM picture of a singulated, vacuum sealed micro-             Figure 5 Aging measurement from temperature compensated MEMS
 mechanical resonator. The inset shows the IR image of bonding ring.    oscillators
                                                                                Table 1 Stress generated due to 30,000g shock on a BGA assembled
                                                                                                         MEMS oscillator



                                                                              Figure 7 Power spectral density of a MEMS oscillator at 50G rms
                                                                                            Table 2 Vibration Power Spectral Density
                                                                              MIL-std-883, Method 2026             MEMS Osc. Test Results
                                                                              Overall    Output power              Overall  Output power
                                                                               rms      spectral density             rms   spectral density
                                                                               5.2            0.02                   2.84        0.02
                                                                     (d)       7.3            0.04                  14.53        0.06
                                                                               9.0            0.06                  19.75        0.13
 Figure 6 (a) Packaged MEMS/IC for g-shock evaluation, Plot of stress          11.6           0.1                   25.06        0.2
 due to a shock force at 30,000g at (b) x-axis, (c) y-axis, and (d) z-axis     16.4           0.2                   30.13        0.3
    Our aging testing was conducted with real temperature                      20.0           0.3                   35.00        0.4
compensated oscillator products instead of resonators alone.                   23.1           0.4                   40.13        0.5
Figure 5 shows the aging characteristic at 85°C, which is the                  28.4           0.6                   42.25        0.7
maximum operation temperature based on the specification.                      36.6           1.0                   50.13        0.8
The curves show that the oscillators have +1/-4.5ppm for the                   44.8           1.5                     x           x
first year and +1.2/-5.2ppm for 10 years. At room
temperature, on the other hand, the samples show +1/-2ppm                    package or plastic over-mold package. The failure
over 18 months.                                                              mechanism is that the smaller solder balls on the resonator
                                                                             break out from the UBM adhesion layer due to the stress
B. Shock Resistance                                                          generated along z-axis as well as the shear stress. FEM
                                                                             results show that the smallest stress and smallest shear stress
     One feature that micromechanical resonator oscillators                  that can pull the solder off the ASIC is 100MPa and 40MPa,
could win over quartz crystal is the shock resistance. MEMS                  respectively. If the oscillator experience 30,000g shock along
resonator typically has mass as small as 10-14kg and its                     x, y, and z-axis, the z-axis stress and two shear stresses are
stiffness is usually as high as tens thousand of N for high                  listed in Table I. Therefore, this type of oscillator will
frequency structures. The FEM simulation shows the                           survive 30,000g of shock.
resonator structure shown in Figure 1 only bends 23Å with
100,000g shock. The resonator itself will survive the g-shock                   Recent test has shown that MEMS oscillators survived
very well.                                                                   the g-shock of an air gun. Three out of three 125MHz
                                                                             MEMS oscillator did not show any performance degradation.
   However, the impact of the g-shock for the packaging
could be severe since the packaging contains most of the
                                                                             C. Vibration Operation
mass. In this particular evaluation, we flip resonator on top
of ASIC with solder balls as shown in Figure 6(a), which                         Silicon MEMS oscillators were tested under random
probably represent a worse case compared to ceramic                          vibration ranging from 2.84G to 50G. The output spectral
                                                                  out of the oven for a thermal cycle test. As shown in Figure
                                                                  9, the frequency deviation across the temperature did not
                                                                  change with time. As we know that the packaged resonator
                                                                  passed the HTSL, this data indicate that the
                                                                  resonator/oscillator package did not degrade with high
                                                                  temperature storage.

                                                                                            V.      CONCLUSIONS
                                                                      With all the reliability data shown in previous sections,
 Figure 8   Frequency stability during thermal cycling            and with the fact these oscillators have met the XO
                                                                  specification including jitter, power consumption, voltage
                                                                  variation, and temperature stability, MEMS-based silicon
                                                                  resonator oscillators are now ready for timing and frequency
                                                                  reference applications.

                                                                      The author would like to express the deepest appreciation
                                                                  to Discera technical team that makes commercial grade
                                                                  MEMS oscillators possible. Also the author would like to
                                                                  thank Larry Burke and Stephen Oder at MA COM for
                                                                  vibration operation and shock resistance tests.

                                                                  [1]   W.-T. Hsu, “Vibrating RF MEMS for clock and frequency reference
 Figure 9 High temperature storage life + thermal cycling               applications”, Technical Digest, International Microwave Symposium
                                                                        2006, San Francisco, June 10-15, 2006
density of a MEMS oscillator at 50G rms is shown in Figure
                                                                  [2]   F. Bannon, J. Clark, and C.T.-C. Nguyen, “High-frequency micro-
7. Table 2 lists the power spectral density for different               mechanical filters,” IEEE, J. Solid-State Circuits, Vol. 35, no. 4, pp.
vibration amplitude. It is clear that the output power spectral         512-526, April 2000.
density is much lower than what is required in mil-std-883.       [3]   W.-T. Hsu, and Ken Cioffi, “Low Phase-Noise 70MHz
                                                                        Micromechanical Oscillators,” International Microwave Symposium,
D. Thermal cycling                                                      June 2004, pp. 1927-1930.
                                                                  [4]   V. Kaajakari et al, “Square Extensional Mode Single-Crystal Silicon
    Figure 8 shows the frequency variation of an oscillator             Micro Mechanical RF-resonator,” Transducers ’03, Jun. 8-12, Boston,
while the temperature switches between 40°C to 100°C. The               MA, U.S.A. pp. 951-954.
temperature cycles every 5 minutes and frequency                  [5]   Y. Lin et al, “60-MHz Wine-Glass Micromechanical-Disk Reference
measurement is taken every minute. As shown, the                        Oscillator”, Technical Digest, ISSCC 2004, San Francisco, Feb. 7-8,
frequency variation is with ±5ppm. No significant trend of              2004.
degradation was observed.                                         [6]   W.-T. Hsu, A.R. Brown, K. Cioffi, “A Programmable MEMS FSK
                                                                        Transmitter”, Technical Digest, International Solid State Circuit
                                                                        Conference 2006, sec. 16.2, San Francisco, Feb. 7-8, 2006.
E. High Temperature Storage Life (HTSL)                           [7]   W.–T. Hsu et al, “Geometric stress compensation for enhanced
    Five temperature compensated MEMS oscillators were                  thermal stability in micromechanical resonators,” IEEE Itnl.
tested initially. After the testing, oscillators are place into         Ultrasonic Symp, Sendai, Japan, Oct. 5-8, 1998 , pp. 945-948.
oven at 150°C for high temperature storage. Parts are taken

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