Wafer-Level Vacuum-Encapsulated Ultra-Low Voltage Tuning Fork MEMS Resonator

Junjun Huan 1, George Xereas 2 and Vamsy P. Chodavarapu1,* 1Department of Electrical and Computer Engineering, University of Dayton,

300 College Park, Dayton, Ohio, 45469, USA

2Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, QC, H3A 0E9 Canada

E-mails: huanj1@udayton,edu; george.xereas@mail.mcgill.ca; vchodavarapu1@udayton.edu

Abstract—We present the design of a wafer-level vacuum-encapsulated silicon tuning fork resonator operating at 32 kHz, along with a low power Complementary Metal-Oxide Semiconductor (CMOS) sustaining amplifier towards an oscillator. The resonator is designed using MEMS Integrated Design for Inertial Sensors (MIDIS) process developed by Teledyne DALSA Semiconductor Inc. (TDSI). The MicroElectroMechanical Systems (MEMS) resonator is designed to operate with an ultra-low DC polarization voltage, as low as 1V, and low motional resistance. This is achieved by using a transduction gap reduction technique based on electrostatic deflection of movable electrode and subsequent localized melting of welding pads. A transimpedance operational amplifier is used as sustaining amplifier to help with continuous oscillation. The oscillator operates with an average power consumption of 1.86 mW that is suitable for mobile electronics.

Keywords—MEMS tuning fork resonator, MIDIS Process, 32 kHz Oscillator, Transduction gap reduction, Wafer-level vacuum encapsulation

I. INTRODUCTION

Quartz crystal oscillators are currently widely used for timing and frequency reference applications [1, 2]. However, this technology has reached its intrinsic limitations in terms of miniaturization and system integration for use in mobile devices, wearable devices and Internet-of-Things applications. In particular, these emerging applications impose stringent requirements in terms of low cost, low power consumption, low motional resistance that quartz crystal oscillators struggle to meet. MicroElectroMechanical Systems (MEMS) resonators, based on piezoelectric or electrostatic operation principles, have shown to produce performances equivalent to quartz devices, but without the size and integration limitations [1].

32.768 kHz oscillators are a fundamental building block of most low-power electronics systems. Their frequency can conveniently be divided by 215 to get a one second time period. Their applications include almost all battery powered devices, as they are generally characterized by ultra-low power consumption. Because of their importance in portable electronics systems, considerable research effort has been put into MEMS enabled oscillators that operate in this frequency range. Key considerations include an ultra-small footprint, very low power consumption and an accurate frequency

reference. In the first generation of 32.768 kHz MEMS resonators, laser trimming was used to reduce the initial offset [3]. More recently, active tuning electrodes were used to increase the stability of the device and reduce the initial offset, and in that study, the MEMS oscillator measured just 0.0154 mm2 which is over two orders of magnitude lower than an equivalent quartz oscillator [4]. In a different implementation, a highly stable and low-footprint 32.768 kHz oscillator was presented in Ref. [5]. The device employs a fractional-N phase-locked-loop to increase the temperature stability and reduce any offsets.

Fig. 1. Cross-sectional view of MIDIS fabrication process

While the MEMS oscillators mentioned above surpass

their quartz competitors in terms of performance, their potential is much higher than what has been achieved to date. A key challenge is that, the MEMS resonators have relatively large transduction gaps due to microfabrication process limitations. This results in a relatively high motional resistances on the order of tens to hundreds of kΩ. A high motional resistance results in increased phase noise, increased power consumption, and the requirement of a higher DC polarization voltage.

Here, we present the design of an electrostatic low-voltage wafer-level vacuum-encapsulated silicon MEMS tuning fork resonator operating at 32 kHz. To reduce the transduction gap, a new methodology with welding pads is employed [6]. The resonator is designed using the MIDIS process from TDSI, an ultra-clean vacuum encapsulation process that helps to achieve a high Quality factor (Q), and in turn, high spectral purity. The

vacuum leak rate in MIDIS process has been shown to be as low as 5-18 Torr-L/s in several fabrication runs [7]. The MEMS resonator described this work is integrated with a CMOS sustaining amplifier in order to create a frequency reference oscillator. The device is designed to operate with an ultra-low polarization voltage of 1 V and with an average power consumption of 1.86 mW.

Fig. 2. Top-view of the MEMS tuning fork resonator 3D model

II. MIDIS FABRICATION PROCESS

The cross-sectional view of a generic device in the MIDIS process is shown in Figure 1. A detailed description of the MIDIS fabrication process flow can be found in Refs. [7, 8] and a brief overview is provided here. The MIDIS process relies on high aspect ratio bulk micromachining of three Single Crystal Silicon (SCS) wafers. In the first step, Deep Reactive Ion Etching (DRIE) is used to pattern cavities on the handle and interconnect wafers. The top and bottom cavities measure 20 µm and 30 µm respectively; the resonant structure will be suspended over them. The second step involves fusion bonding the membrane and handle wafer. DRIE is then used to define the resonant structure and electrodes on the 30 μm membrane wafer.

Through Silicon Vias (TSVs) are formed on the interconnect wafer in order to provide an electrically isolated conductive path to the resonator and electrodes. The interconnect wafer is then fusion bonded to the wafer assembly under a high vacuum of 10 mTorr. This step takes place under a high temperature of 1100 oC in order to remove residual gases and ensure the long-term stability of the resonant device. The final assembled wafer stack is further processed to add a passivation layer, contacts, metal interconnects and bond pads.

III. TUNING FORK RESONATOR DESIGN

A 3D model of the 32 kHz tuning fork resonator is shown in Figure 2. The device is aligned with the <100> crystal orientation and covers a die area of 700 µm × 550 µm. Two electrostatic comb drives are used for sensing and actuation.

The resonant structure consists of a 30 μm thick movable shuttle that is suspended over the bottom cavity via four fixed-guided folded beam springs. A total of 179 finger gaps (89 shuttle fingers) are used between the electrode and shuttle structure in order to increase the electrostatic efficiency and lower the motional resistance. The tuning fork device in this work is a modified version of the comb-drive resonator design described in Ref. [9]. A key difference in our implementation is that the sensing and actuation electrodes are designed to be movable instead of fixed, at first. This is accomplished by providing serpentine spring supports. Our configuration enables a dramatic reduction in the transduction gap between the interdigitated fingers which in turn reduces the motional resistance of the resonator; this technique is discussed in detail in the following Section.

Fig. 3. Electrical equivalent circuit model of the MEMS resonator

IV. TRANSDUCTION GAP REDUCTION TECHNOLOGY

The minimum transduction gap size allowed in the MIDIS process is 1.5 μm, which is mainly limited by the 1:20 aspect ratio in DRIE processing considering a 30 μm thickness of the device wafer. However, in the case of electrostatic MEMS resonators, a much smaller gap would be needed if the devices were to be commercially viable. A sub-micron gap would greatly reduce the motional resistance of the resonator, and in turn, significantly improve the performance of the integrated oscillator. Additionally, a reduced gap would enable the operation of the devices with a much lower DC polarization voltage. A novel gap reduction technique which delivers transduction gaps as low as 50 nm is presented here.

The process is initiated after the fabricated devices are received from the foundry [6]. A DC voltage of about 50V is applied between the electrodes and strategically located stop anchors that are located within them; an illustration of the setup is shown in Figure 2. The induced electrostatic force pulls-in the movable electrodes towards the stop anchors. The transduction gap between the resonator and the electrode is now reduced to the difference between 1.5 μm and the gap between the electrode and stop anchor. A short current pulse of about 10 mA is then used to melt the welding-pad of the movable electrode and form a permanent connection. Using the MIDIS process, this technique allows for the formation of transduction gaps as low as 50 nm.

V. CMOS SUSTAINING OSCILLATOR CIRCUIT

The design of the MEMS tuning fork resonator in this work is integrated as an oscillator containing a sustaining amplifier circuit as presented in this Section.

A. Electrical Equivalent Circuit Model of the Resonator

In order to design the oscillator, the tuning fork resonator is converted into an equivalent electrical model consisting of a series connected motional resistor, Rm, capacitor, Cm, and inductor, Lm, along with a shunt capacitor, Cp, connected in parallel as shown in Figure 3 [10]. Here, Cp represents the total parasitic capacitance between the electrodes and contact terminals. The results of Rm, Lm and Cm are calculated using the Equations (1), (2) and (3) derived in Ref. [11] as following, = (1)

= (2) = (3)

where m is the dynamic mass of the movable device structure, Q is the quality factor of the resonator and η is a factor which describes how the electrodes and the resonator device are electrically coupled. The coupling factor,η, is a function of the polarization voltage, Vp and the differential capacitance

between fingers, , as given below in Equation (4),

η = = (4)

where N is number of fingers for electrical coupling, is dielectric constant of free space, t is the finger thickness, and g is the transduction gap size. Thus, assuming a nominal Q of 50,000 and Co of 1.6pF, for our resonator, the electrical equivalent model, , and are calculated, as 752 fF, 31.4 H and 130Ω, respectively.

Fig. 4. Circuit schematic of CMOS oscillator circuit

B. CMOS Transimpedance Operational Amplifier

The sustaining amplifier used in the oscillator circuitry is a low-power CMOS operational amplifier to compensate power attenuation through the relatively small motional resistance of

the resonator. This operational amplifier circuit is comprised of a first-stage telescopic differential amplifier, a second-stage push-pull amplifier as an output buffer and a small capacitor in a compensation loop to attain good circuit operation stability [12]. The circuit topology of the CMOS operational amplifier is shown within the dash-line area in Figure 4, designed using Global Foundries 180 nm CMOS process. TABLE1. TRANSISTOR PARAMETERS OF THE TRANSIMPEDANCE OPERATIONAL

AMPLIFIER AND THE BIASING CIRCUIT

Device Type W (µm) L (µm) M1, M4, M6-7, M15-16, M18-19, M23-24,

M26-27, M30-32, M1a-4a, M14a-15a

N 1.8 0.36

M8-11, M14, M17, M20-22, M28-29, M33-34, M5a-8a,

M12a-13a, M17a-18a

P 3.6 0.36

M3 N 1.8 0.18 M5 N 7.2 0.36

M13, M11a N 1.8 1.8 M2 P 1.8 18 M12 P 18 18 M25 P 3.6 1.8

M9a-10a N 5.4 0.36 M16a N 0.9 0.36 M20a N 18 0.36 M19a P 1.8 0.36 M21a P 36 0.36

TABLE 2. PARAMETERS OF ALL RESISTORS, INDUCTOR AND CAPACITORS USED

IN THE OSCILLATOR

Parameter Value Unit Rm 130 Ω Cm 762 fF Cp 1.6 pF

C1, C2 10 pF Cc 240 fF Lm 31.4 H

TABLE 3. DC OPERATING POINT SIMULATION RESULTS

Device Value Unit R1 40 kΩ R2 50 kΩ R3 1000 M Ω R4 870 M Ω

The telescopic architecture of the differential amplifier at

the input stage provides a high gain, high output swing and consumes low power. The output stage is a class AB push-pull amplifier and provides rail-to-rail output swing. In addition, a 240fF capacitor is used for indirect compensation between the outputs of both stages to stabilize the amplifier circuit. The

amplifier circuit is biasing using a constant-gm circuit as shown in Figure 5. The parameters of all transistor devices can be found in Table 1.

Fig. 5. Circuit schematic of biasing circuit

Fig. 6. 3D modal shape analysis of the resonator with Vp=1V and g= 50nm

C. CMOS-MEMS Oscillator

Figure 4 presents the overall oscillator design with the sustaining CMOS transimpedance amplifier connected in a feedback loop with the MEMS resonator equivalent electrical model. The CMOS-MEMS oscillator design employs the basic concept of a Colpitts oscillator configuration [13]. The principle of operation is to utilize a capacitive voltage divider as a feedback source, which means that only a fraction of the

output feedback voltage will return to the input terminal to establish a stable oscillation with low distortion. An overall loop phase shift of 360° between output and input port must be satisfied for oscillation condition [4]. This is acheived as a result of a 180° phase shift of transimpedance amplifier with an additional 180 ° phase shift attributed to the two load capacitors in the voltage divider system. The oscillating frequency of the pure sine-wave output signal is determined by the resonance frequency of the resonator as a frequency setting element operating at 32 kHz. The parameters of all resistors, capacitors and inductor used in the circuit are given in Table 2.

Fig. 7. Bode plot of the CMOS operational amplifier circuit

Fig. 8. Bode plot of the oscillator circuit

Fig. 9. Output signal of the oscillator circuit in the time domain

VI. SIMULATION RESULTS AND DISCUSSION

The resonator layout design, 3D modeling, Finite Element Modelling (FEM) and modal analysis are performed using CoventorWare.

A. Resonance Frequency Simulation

Figure 6 shows the 3D mode shape result of the resonator with a reduced transduction gap of 50nm and 1V DC polarization voltage. The simulation result shows that the tuning fork resonator can operate at a resonance frequency of 32.81 kHz, which is very close to 32.768 kHz, a standard frequency for real-time clock applications in low power portable electronics [4].

B. Operational Amplifier Simulation

Figure 7 illustrates the Bode plot of the operational amplifier, which describes its performance in the frequency domain. The graph shows a DC gain of about 50 dB, which is sufficient for amplification of the resonator output. A bandwidth of over 100 kHz is measured at the -3 dB point which exceeds the required frequency of oscillation at 32 kHz. High stability of the oscillation is ensured under the condition of a high phase margin of about 60° for the amplifer [12].

C. CMOS-MEMS Oscillator Circuit Simulation

The frequency response of the oscillator is shown in Figure 8. The output signal oscillates at a frequency slightly over 32 kHz with high Q factor. The time domain signal is shown in Figure 9 with the output sine wave signal. The power consumption of the system is an important characteristic. A DC operating point simulation is conducted, which gives the amplitude of the current flowing in each branch sourced through the biasing circuit. The total average power consumption is about 1.86 mW.

VII. CONCLUSIONS

A 32 kHz wafer-level vacuum-encapsulated silicon MEMS tuning fork resonator is designed using the MIDIS process. A novel gap-reduction technology is described that reduces motional resistance and allows ultra-low DC polarization voltage of only 1V. A CMOS sustaining transimpedance amplifier is designed using Global Foundries 180 nm CMOS process.

REFERENCES [1] C. T. Nguyen, "MEMS technology for timing and frequency control,"

IEEE Trans Ultrason Ferroelectr Freq Control, vol. 54, pp. 251-70, Feb 2007.

[2] J. T. M. van Beek and R. Puers, "A review of MEMS oscillators for frequency reference and timing applications," Journal of Micromechanics and Microengineering, vol. 22, Jan 2012.

[3] K. R. Cioffi and H. Wan-Thai, "32KHz MEMS-based oscillator for low-power applications," in Proceedings of the 2005 IEEE International Frequency Control Symposium and Exposition, 2005., 2005, pp. 551-558.

[4] H. G. Barrow, T. L. Naing, R. A. Schneider, T. O. Rocheleau, V. Yeh, Z. Y. Ren, et al., "A Real-Time 32.768-kHz Clock Oscillator Using a 0.0154-mm(2) Micromechanical Resonator Frequency-Setting Element," 2012 IEEE International Frequency Control Symposium (Fcs), 2012.

[5] S. Zaliasl, J. C. Salvia, G. C. Hill, L. Chen, K. Joo, R. Palwai, et al., "A 3 ppm 1.5 x 0.8 mm(2) 1.0 mu A 32.768 kHz MEMS-Based Oscillator," IEEE Journal of Solid-State Circuits, vol. 50, pp. 291-302, Jan 2015.

[6] M. Nowack, S. Leidich, D. Reuter, S. Kurth, M. Kuechler, A. Bertz, et al., "Micro arc welding for electrode gap reduction of high aspect ratio microstructures," Sensors and Actuators A-Physical, vol. 188, pp. 495-502, Dec 2012.

[7] G. Xereas and V. P. Chodavarapu, "Wafer-Level Vacuum-Encapsulated Lame Mode Resonator With f-Q Product of 2.23 x 10(13) Hz," IEEE Electron Device Letters, vol. 36, pp. 1079-1081, Oct 2015.

[8] MEMS Integrated Design for Inertial (MIDIS) [Online]. Available: http://www.teledynedalsa.com/semi/mems/applications/midis/

[9] E. Carr, S. Olivier, and O. Solgaard, "Large-stroke self-aligned vertical comb drive actuators for adaptive optics applications," Mems/Moems Components and Their Applications Iii, vol. 6113, 2006.

[10] One-Pin 32 kHz Low-Power Crystal Oscillator [Online]. Available: http://digitalcommons.calpoly.edu/cgi/viewcontent.cgi?article=1097&context=eesp

[11] C. T. C. Nguyen and R. T. Howe, "An integrated CMOS micromechanical resonator high-Q oscillator," IEEE Journal of Solid-State Circuits, vol. 34, pp. 440-455, Apr 1999.

[12] R. J. Baker and Institute of Electrical and Electronics Engineers., CMOS circuit design, layout, and simulation, Rev. 2nd ed. Piscataway, NJ, IEEE Press, 2008.

[13] X. Y. Meng, Z. H. Wang, and B. Y. Chi, "A 180 GHz differential Colpitts VCO in 65 nm CMOS," Analog Integrated Circuits and Signal Processing, vol. 86, pp. 25-31, Jan 2016.