Resonator resonant frequency generated means of electronic components, commonly divided into the quartz crystal resonators and ceramic resonators. Played the role of production frequency, with a stable, good anti-jamming performance characteristics, widely used in various electronic products, quartz crystal resonator frequency accuracy than ceramic resonators, but the cost is higher than the ceramic resonator. From the resonator frequency control the important role of all electronic products related to frequency of transmitter and receiver are required resonator. Type of resonator-line according to shape and can be divided into two of SMD.
Reliability of Silicon Resonator Oscillators Wan-Thai Hsu Discera Inc. Ann Arbor, Michigan, U.S.A. email@example.com 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 , 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 . 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. II. MICROMECHANICAL RESONATORS 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 . At the same time, several breakthroughs have been made on temperature compensation . 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 (a) (b) Figure 7 Power spectral density of a MEMS oscillator at 50G rms (c) 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. ACKNOWLEDGMENT 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. REFERENCES  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  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.  W.-T. Hsu, and Ken Cioffi, “Low Phase-Noise 70MHz Micromechanical Oscillators,” International Microwave Symposium, D. Thermal cycling June 2004, pp. 1927-1930.  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  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.  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)  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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