T1D.005 HIGH PRECISION FREQUENCY SYNTHESIZER BASED ON MEMS PIEZORESISTIVE RESONATOR K.L. Phan1, T. van Ansem1, C. van der Avoort1, J.T.M. van Beek1, M.J. Goossens1, S. Jose2, R.J.P. Lander3, S. Menten1, T. Naass1, J. Sistermans2, E. Stikvoort1, F. Swartjes2, K. Wortel1, and M.A.A. in 't Zandt1 1Research & Development - NXP Semiconductors, Eindhoven, the NETHERLANDS 2Operations, NXP Semiconductors, Nijmegen, the NETHERLANDS 3Research & Development - NXP Semiconductors, Leuven, BELGIUM ABSTRACT middle of the structure. The heads of the dog-bone face In this paper, we present a detailed description of a two electrodes called the gate, over small gaps (g) of MEMS frequency synthesizer product, including the 200nm. The resonator is actuated electrostatically by a principle, processing, system architecture, and reliability combination of a DC voltage (VDC) and an AC voltage and characterization results. The synthesizer is based on a (vin) applied on the gate (Figure 1, right), which drives the MEMS piezoresistive dog-bone shaped resonator, having resonator into the extensional symmetrical resonance a resonant frequency of 56MHz, and a Q-factor of mode shape. To sense the vibration, a DC bias current >40,000. Using a specific temperature compensation (Id) is sent though the beams via the anchors, which are algorithm, the output frequency can be kept stable within also called the source and drain terminals. Thanks to the ±20ppm over an operating temperature from -20°C to piezoresistance effect, resistance of the beams is +85°C. Jitter over a bandwidth from 12kHz to 20MHz is modulated by their strain, which results in an AC signal typically 2.96ps. The product has been proven to be voltage at the drain. manufacturable using standard industrial processes, and to input (gate) Id output be reliable against various stress and life-time tests. gap v+Vin dc e (drain) d o tr anchor c e gap L l KEYWORDS e MEMS, piezoresistive resonator, frequency b e synthesizer, thin-film capping, reliability, fractional-N d o anchor tr PLL, temperature compensation, phase noise c le μ e g 10 m source (b) INTRODUCTION Figure 1: Left: SEM image of a dog-bone resonator, MEMS oscillators are considered a disruptive taken before the thin-film cap was created. During technology with the potential to replace quartz crystal resonance, the two heads of the dog-bone structure oscillators (XOs) for electronics. So far, MEMS-based vibrate laterally as shown by the arrows. Right: resonant frequency synthesizers have met all the requirements with mode-shape and electrical connections of the resonator. respect to phase noise, jitter, temperature stability, power The color map shows lateral strain during vibration. consumption, and reliability for the mainstream of 1 to 125 MHz XO segment, which covers high-volume Electrically, the resonator can be modeled as a field- applications including consumer electronics and effect transistor in the linear region (small signal model), computing [1, 2]. Unlike its peers [3, 4], NXP which has a frequency selection property [5-7]. The Semiconductors has been working on a unique oscillator transconductance gm of the resonator can be written as: concept based on piezoresistive MEMS resonators [5-7]. g g (ω) = m0 (1) This type of resonators overcomes the classical issue of m −ω 2 ω 2 + ω ω weak electro-mechanical coupling at high resonance 1 / 0 j /(Q 0 ) frequency, which is encountered in conventional ε hbKβ g = 0 I V (2) capacitive MEMS resonators. In this paper, a detailed m0 g 2 Lk d g description of NXP’s mature MEMS frequency ε synthesizer product, including the principle, processing, in which, 0 is the permittivity of vacuum, h is the system architecture, and reliability and characterization thickness of the resonator, b is the width of the heads, L is results, is presented. The product has been proven to be the length of the beams, K is the piezoresistive gauge manufacturable using standard industrial processes. factor of silicon, β is the ratio of strain in the beams to the total strain in the entire length of the resonator during PIEZORESISTIVE MEMS RESONATOR vibration, k is the stiffness of the resonator, Vg is the DC ω ω The heart of our frequency synthesizer is a MEMS bias voltage across the gaps, and 0 are the frequency dog-bone shaped resonator, having a resonant frequency of the signal and of the resonant frequency, respectively, of 55.8MHz, made in a 1.5μm-thick SOI layer (see Figure Q is the quality factor. The numerator of Eq. (1), gm0, 1, left). The dog-bone resonator consists of two describes the transconductance of the resonator symmetrical heads, connected together by four beams, irrespective of frequency, and the denominator describes which are attached to the substrate at two anchors at the the frequency selection of the resonator, which is similar 978-1-4673-5983-2/13/$31.00 ©2013 IEEE 802 Transducers 2013, Barcelona, SPAIN, 16-20 June 2013 to a RLC resonant circuit. At resonance (ω=ω0), the The complete synthesizer product contains a MEMS transconductance becomes –jgm0Q, which implies that it resonator die, stacked on an ASIC die (CMOS circuitry of has the maximum amplitude, and the resonator behaves the frequency synthesizer), overmolded inside a standard 3 like an inductor. There is a 90° phase shift between the 4-pin SMD plastic package measuring 5×3.2×0.85mm AC output current and input voltage. Unlike its (see Figure 4), using all standard packaging processes at capacitive rival, the transconductance of the piezoresistive NXP’s back-end fabs. resonator can be “boosted” by increasing not only the bias Q-factor change voltage V , but also the bias current I , see Eq. (2), which g d -4000 -2000 0 2000 4000 results in a large output current at resonance and hence can easily be combined with an amplifier in an oscillation 99 loop. Furthermore, the output current does not depend on resonator size and makes this concept suitable for 90 realizing high frequency oscillators. 70 FABRICATION 50 The MEMS resonator is capped in vacuum using 30 NXP’s proprietary thin-film capping technology, which is a low-temperature (<400°C) and low-cost CMOS- 10 Freq. change, UHAST Cumulative percentage Q change, UHAST compatible process. Figure 2 shows a schematic cross- Freq. change, HTSL section of a capped resonator. The process starts with 1 Q change, HTSL patterning and release etch of the MEMS resonator in a -4 -2 0 2 4 SOI wafer. Next, a sacrificial layer is deposited and Frequency change (ppm) patterned to form the vacuum cavity shape. A PECVD silicon nitride capping layer is subsequently deposited, Figure 3: Cumulative distribution of resonant frequency followed by contact and release hole etching. In the next change (in ppm) and Q-factor change of dog-bone step, the sacrificial layer is etched through the release resonators after 96hrs of UHAST (at 130°C/85%RH) and holes to form the cavity, and after that, the cavity is sealed 168hrs of HTSL (at 200°C). by depositing and patterning a plug layer at low pressure. The cap is subsequently reinforced by an extra oxide/nitride double layer. Finally contact holes are etched and metallization is made to make contact pads. The cavity under the cap can sustain <40mbar of pressure, which is enough to enable resonance of a quality factor of >40,000. Reinforcement Plug Resonator Figure 4: Complete frequency synthesizer assembled in a layer Cap Actuation leadless 4-pin plastic package. Left: picture of a gaps Vacuum cavity decapped product, showing a stack of a MEMS resonator die glued on top of an ASIC die. Middle (and right): SOI bottom (and top) view of the product. Buried oxide Handle wafer FREQUENCY SYNTHESIZER Figure 2: Schematic cross-section of a thin-film capped Figure 5 shows the top-level system architecture of resonator. our frequency synthesizer. Basically, the system consists of an oscillator core connected to the resonator, a charge- The thin-film cap has been proven to yield excellent pump to generate a DC voltage bias for the resonator, a robustness against various standard accelerated stress fractional-N Phase-Lock-Loop (frac-N PLL), a dedicated tests, such as Unbiased Highly Accelerated Steam Test block for temperature compensation, an output buffer, a (UHAST), High Temperature Storage Life (HTSL), digital block with an embedded memory, and a number of Temperature Cycling (TMCL), as well as the standard high-performance low-dropout regulators (LDOs). grinding, dicing and overmolding packaging process with The resonance of the MEMS resonator is maintained a peak pressure of 80bar. As examples, Figure 3 shows by the oscillator core consisting of a two-stage amplifier, results of a UHAST test (at 130°C and 85% relative which amplifies the AC voltage swing at the drain of the resonator and feeds back the amplified signal to its gate. humidity, for 96hrs) and HTSL test (at 200°C for 168hrs), ° performed on full 8-inch wafers. The resonant frequency The phase of the feed-back signal shifted by +100 , in ° change after the tests has been found to be below 2ppm, which +90 is needed for compensating the phase shift of which is within the accuracy of the measurement setup. the piezoresistive resonator and +10° is for compensating After the tests, majority (99%) of devices show parasitic capacitance coming from the resonator structure insignificant change (<7%) in the Q factor, which and bond-wires. The resonator is biased at VDC= -5V indicates that the cavity remains hermetic and outgassing (supplied by the charge pump) at the gate, and Id= 1.5mA within the cavity is not significant. from the drain to the source.