Proceedings of the 39th European Microwave Conference
Phase Error and Nonlinearity Investigation of Millimeter-Wave MEMS 7-Stage Dielectric-Block Phase Shifters N. Somjit1, G. Stemme, J. Oberhammer Microsystem Technology Laboratory, School of Electrical Engineering, KTH-Royal Institute of Technology SE 10044, Stockholm, Sweden 1
[email protected]
Abstract— This paper reports on phase error and nonlinearity investigation of a novel binary-coded 7-stage millimeter-wave MEMS reconfigurable dielectric-block phase shifter with best performance optimized for 75-110-GHz W-band. The binarycoded 7-stage phase shifter is constructed on top of a 3D micromachined coplanar waveguide transmission line by placing Ȝ-long high-resistivity silicon dielectric blocks which can be displaced vertically by MEMS electrostatic actuators. The dielectric constant of each block is artificially tailor-made by etching a periodic pattern into the structure. Stages of 15°, 30° and 45° are optimized for 75 GHz and put into a coded configuration of a 7-stage phase shifter to create a binary-coded 15°+30°+5x45° 7-stage phase shifter with a total phase shift of 270° in 19x15° steps. The binary-coded phase shifter shows a return loss better than -17 dB and an insertion loss less than -3.5 dB at the nominal frequency of 75 GHz, and a return loss of -12 dB and insertion loss of -4 dB at 110 GHz. The measurement results also show that the binary-coded phase shifter performs a very linear phase shift from 10-110 GHz. The absolute phase error at 75 GHz from its nominal value has an average of 2.61° at a standard deviation of 1.58° for all possible combinations, and the maximum error is 6° (for 240°). For frequencies from 10-110 GHz, all possible combinations have a relative phase error of less than 3% of the maximum phase shift at the specific frequencies. The 7-stage binary-coded phase shifter performs 71.1°/dB and 490.02°/cm at 75 GHz, and 98.3°/dB and 715.6°/cm at 110 GHz. From the measured self-modulation behavior the third-order intermodulation (IM) products level are derived to -82.35 dBc at a total input power of 40 dBm with the third-order IM intercept point (IIP3) of 49.15 dBm, employing a mechanical spring constant of 40 N/m. In contrast to conventional MEMS phase shifters which employ thin metallic bridges which limit the current handling and show fatigue even at slightly elevated temperatures, this novel phase-shifter concept is only limited by the power handling of the transmission line itself, which is proven by temperature measurements at 40 dBm at 3 GHz and skin effect adapted extrapolation to 75 GHz by electro-thermal FEM analysis.
I. INTRODUCTION Microelectromechanical systems (MEMS) phase shifters are known for low insertion loss and high linearity over a large bandwidth as compared to solid-state technology. At millimeter wave frequencies, including the 75-110 GHz Wband, switched MEMS true-time delay phase shifters are not suitable anymore because of high losses created by switches and discontinuities of the transmission line [1]. Distributed
978-2-87487-011-8 © 2009 EuMA
MEMS transmission line phase-shifters (DMTL) by a periodically capacitive-loaded high-impedance transmission line using MEMS bridges perform well in the W-band [2], [3] but are composed of thin metal bridges which cannot handle large induced current densities at high RF power because of limited heat-conductivity to the substrate due to airsuspension of the bridges. This results in reliability issues due to buckling (plastic yielding) or even melting of the thin metal layer [4]. Additionally, thin gold bridges as employed in these types of phase-shifter are subject to drastically losing their elastic behavior even at slightly elevated temperatures of just around 80°C. The authors presented recently the basic principle of a phase-shifter concept based on dielectric blocks moving by MEMS electrostatic actuators above a 3D micromachined transmission line [5]. Based on this novel concept the authors have designed a very broadband W-band binary-coded phase shifter with 19 phase-shift steps, i.e. 4.25 bits, with phase shift resolution of 15° at the nominal frequency of 75 GHz. The binary coding is achieved in a unique way by artificially tailor-making the dielectric properties of each block by etching a periodic pattern into the structure, resulting in stages of 15°, 30° and 45° [6]. The present paper reports on the millimeter-wave performance, especially the absolute and relative phase errors and on nonlinearity distortion. The nonlinearity characteristics of the phase shifter were characterized by large signal network analyzer with RF power of 40 dBm at 3 GHz; the measured results were extrapolated to the design frequency at 75 GHz. II. CONCEPT OF DIELECTRIC-B LOCK PHASE SHIFTER The dielectric-block digital-type millimeter-wave MEMS phase shifters consist of a 3D high-impedance (>50 micromachined CPW loaded by a high-resistivity monocrystalline silicon block. The relative phase shift between the upstate and the downstate position is achieved by vertically moving the dielectric block above the CPW t-line by MEMS electrostatic actuation, which results in different propagation constants of the microwave signal depending on the vertical displacement of the dielectric block (Fig. 1). The additional deep-etched slots of 50 µm into the high-resistivity silicon substrate (HRSS) decreases substrate loss and increases the sensitivity of the propagation speed to changes
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in the displacement of the dielectric block. The length of the dielectric EORFN LV FKRVHQ WR EH Ȝ DW WKH QRPLQDO IUHTXHQF\ of 75 GHz. Periodically etched patterns of the dielectric block with three different sizes are introduced for artificially tuning the macroscopically effective dielectric constant, resulting in relative phase shift of 45°, 30° and 15°. For digital-type operation, an optimum operation point of high phase-shift sensitivity at medium movement was chosen resulting in an actuator design with a displacement of 5 µm. The binarycoded 1x15°+1x30°+5x45° phase-shifter with a total phaseshift of 270° in 15° steps, i.e. 19 different phase states, were successfully fabricated and evaluated (Fig 2). surpentine flexure
dielectric block etched patterns for different phase shifts 15°: 38X38 µm2 30°: 17X17 µm2 45°: 5X5 µm2
anchor
III. FABRICATION The 1-µm thick sputtered-gold coplanar-waveguide (CPW) LVIDEULFDWHGRQKLJKUHVLVWLYLW\VLOLFRQVXEVWUDWH !ȍÂFP 100-nm thick Si3N4 bumps are deposited on the metal layer as distance keepers to prevent stiction and DC short circuit of the pulled-in block. The slots of the CPW are etched down into the substrate by deep-reactive-ion-etching (DRIE) to create deep trenches. A high-resistivity silicon-on-insulator (SOI) wafer is transfer-bonded to this wafer by adhesive polymer bonding with mr-I 9250xp thermal nanoimprint lithography photoresist, and the SOI handle wafer is afterwards removed by plasma etching. Its 35-µm thick device layer is patterned by two DRIE steps to shape the thick dielectric block, the release-etch holes of varying sizes to determine the tailormade dielectric properties and the 9-µm thick serpentine flexures. The structures are released by O2-plasma etching of the bonding layer. Fig.3 shows a top-view SEM picture of prototype of a multi-stage phase shifter. 15° stage
30° stage
45° stage
length: 15°: 850 µm 30°: 800 µm 45°: 760 µm
CPW 20/60 µm gap/signal line with 50 µm deep slots 100nm thick Si3N4 distance keeper down state
up state 380 338 80 µm µm
35 µm
5 µm 50 µm
Fig. 3. SEM picture of fabricated phase shifter blocks within 7-stage phase shifter: 15° (left), 30° (middle) and 45° (right).
ǻSKL -45°
phi=-135°
phi=-180°/ Ȝ
IV. CHARACTERIZATION AND ANALYSIS
Fig. 1. Working principle of a single stage of the phase shifters.
phase resolution:15°
200 µm
15°
30°
45°
45°
stage numbers combinations PD[ǻࢥ (°) resolution (°) ǻࢥ/loss at 75 GHz (°/dB) ǻࢥ/length at 75 GHz (°/cm) ǻࢥ/loss at 110 GHz (°/dB) ǻࢥ/length at 110 GHz (°/cm) total length (mm)
45°
45°
45°
A. S-parameter Characterization Fig. 4 shows the measured S-parameters for the binarycoded 15°+30°+5x45° phase shifter, for actuated states from a single to all 7 stages in the downstate position. Over the Wband the multi-stage binary-coded phase shifter performs with the maxi mum return loss better than -12 dB and an insertion loss of less than -4 dB, and with -17 dB and -3.5 dB, respectively, at the nominal frequency of 75 GHz.
binary-coded 15°+30°+5x45° 7 8 270 15 71.05 490.02 98.3 715.6 5.51
Fig. 2. Microscope pictures and key performance data of fabricated 7-stage phase shifters consisted of 15°+30° +5x45 °.
Fig. 4. Measured return and insertion loss of the binary-coded 15°+30°+5x45°.
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less than 3% of the maximum phase shift at the specific frequencies. Table I lists the nominal and measured phase shift for all combinations, as well as the absolute error and the relative error (here in relation to the phase shift of the specific state) for the nominal frequency of 75 GHz. The absolute value of the phase deviation from its nominal value has an average of 2.61° at a standard deviation of 1.58° for all possible combinations, and the maximum deviation is 6° (for 240°).
B. Phase Shift and Phase Error Characterization
TABLE I MEASURED PHASE SHIFT AND PHASE ERROR AT 75 GHZ ǻĭD = NOMINAL DESIGN PHASE SHIFT ; ǻĭM =MEASURED PHASE SHIFT )
ǻࢥD(°) ǻࢥM(°)
(a)
err. (°) err.(%) ǻࢥD(°) ǻࢥM(°) err. (°) err.(%) ǻࢥD(°) ǻࢥM(°) err. (°) err.(%)
15 12.95 -2.05 -13.67 105 101.4 -3.6 -3.34 195 189.9 -5.1 -2.58
30 32.98 2.98 9.93 120 121.3 1.3 1.13 210 209.6 -0.4 -0.16
45 44.33 -0.67 -1.49 135 132.85 -2.15 -1.59 225 221.2 -3.8 -1.68
60 57.25 -2.75 -4.58 150 145.7 -4.3 -2.87 240 234 -6 -2.5
75 77.19 2.19 2.92 165 165.5 0.5 0.34 255 253.6 -1.4 -0.52
90 88.58 -1.42 -1.58 180 177.1 -2.9 -1.61 270 266.56 -3.44 -1.27
(b)
C. Nonlinearity Analysis with Large Signal Network Analyzer As for every MEMS phase shifter, the self-modulation of the device in the upstate of the dielectric block, by actuation due to the electrostatic force of the RF signal voltage, results in a phase modulation.
(c) Fig. 5. (a) measured phase shift for all states of binary-coded 15°+30°+5x45° phase-shifter; (b) absolute phase error; (c) frequency-normalized phase error: phase error of all states relative to the maximum phase shift at the given frequency.
Fig. 5 shows the phase-shift analysis of the binary-coded 15°+30°+5x45° phase shifter, derived from measurements over the spectrum from 1 to 110 GHz. Fig. 5a shows the very linear relative phase shift over the whole spectrum for all 19 possible phase states. The phase-shift performance in terms of phase shift per loss and phase shift per length, along with other performance parameters, are summarized in the table in Fig. 2. The absolute phase error for all states is shown in Fig. 5b. It is increasing with frequency since the phase shift increases with frequency as well. Fig. 5c shows the relative phase error, calculated by the absolute phase error for all combinations, divided by the maximum phase shift (all stages down) for the given frequencies. For frequencies from 10-110 GHz, all possible combinations have a relative phase error of
Fig. 6. Phase shift modulation depending on input RF power.
The measurement setup involves a power amplifier, filters for filtering the nonlinearities of the amplifier, and two 50-ȍ high-power impedances and two circulators at the ports to protect the signal generator and the power amplifier from the reflected mismatched power. Fig. 6 shows the deviation in the relative phase shift of a 45° block depending on the input power level, measured at 3 GHz up to 40 dBm and extrapolated to the nominal frequency of 75 GHz. The nonlinearity characteristics of the upstate of the 45° stage can be calculated from the capacitance modulation derived from the measurements and by using the models
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introduced in [7]. The third IM level and IIP3 with different RF input power of the transmitted signal are plotted in Fig. 7, showing an IIP3 of 49.3 to 48.0 dB m for total input power levels of 10 to 30 dBm. With larger mechanical stiffness of the structure, the nonlinearity decreases. The spring constant of 40 N/m and initial gap of 5 µm of the presented design are a good compromise between low actuation voltage of 30 V and good RF performance. In contrast to conventional MEMS phase shifters, no thin metal bridges are employed, which means that the power handling is only limited by the power handling of the transmission line and the substrate heat sink. HFSS simulations indicate that the current densities of the 1-µm thick sputtered gold CPW reaches the maximum sustainable limit (20-35 GA/m2 for sputtered gold [8]) at approximately 34 dBm at 75 GHz. Temperature measurements at 40 dBm at 3 GHz were done with infrared cameras and extrapolated to 75 GHz by simulations taking the skin depth into account, resulting in a temperature increase of the hottest spot of only 30°C at a power level of 40dBm and a frequency of 75 GHz.
ACKNOWLEDGMENT This work is financially supported by The Swedish Governmental Agency for Innovation Systems (VINNOVA) through the NORDITE ICT program. The authors would like to thank also VTT Technical Research Centre, Espoo, Finland and Microwave Electronics Laboratory, Chalmers University of Technology, Göteborg, Sweden, for helpful assistance in microwave measurement and large-signal network analysis, respectively. REFERENCES [1]
[2]
[3]
[4]
[5]
[6]
[7]
(a)
[8] Fig. 7. Nonlinearity characteristic of a single 45° stage phase shifter with twotone signal at 75 GHz + 10 kHz: third IM level and IIP3 with different RF input power (for 40 dBm total signal power).
V. CONCLUSIONS This paper reports on the phase errors and nonlinearity characteristic of a novel millimeter-wave dielectric-block phase shifter with best performance optimized for 75-110GHz W-band. The relative phase shift is controlled by vertically moving a dielectric block above the 3D micromachined coplanar waveguide. 7-stage phase shifters with binary coded, 19 different phase states, were successfully fabricated and characterized to a return loss of less than -17 dB and an insertion loss less than -3.5 dB for 75 GHz, and -12 dB and -4 dB at 110 GHz, respectively. The absolute phase error at 75 GHz from its nominal value has an average of 2.61° at a standard deviation of 1.58° for all possible combinations, and the maximum error is 6° (for 240°). For frequencies from 10-110 GHz, all possible combinations have a relative phase error of less than 3% of the maximum phase shift at the specific frequencies. The binary-coded phase shifter performs with 71.05°/dB and 490.02°/cm at 75 GHz, and 98.3°/dB and 715.6°/cm at 110 GHz. The nonlinearity was characterized to an IIP3 of 49.15 dBm at 40 dBm total power.
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