High-performance MEMS RF passive components and silicon micromachined technology
Lei Gu
ABSTRACT
On-chip inductors, transformers and tunable capacitors are most important passive components in RF integrated circuits and play key roles for designing VCO (voltage control oscillator), LNA (low noise amplifier) and passive filter or resonator. Besides, personal mobile communication, GPS (global position system), and satellitic signal receiver also need these high-performance devices, namely high Q-factor of inductors/tunable capacitors and high available gain of transformers. Presently, it is a hotspot for studying RF passive components by using MEMS technology. Bulk, surface micromachining and LIGA technique are three essential method of fabrication. Several kinds of processes infused and integrated are also the direction for further MEMS research.
In this dissertation, we mainly study the 3-D micromachined process based on silicon substrate and research of key RF passive components. On the one hand, some high performance RF components are implemented for telecommunications by using a concise process, such as inductors, transformers and tunable capacitors. The novel process could be compatible with CMOS process with low temperature. Moreover, the substrate loss is suppressed deeply by selective removing silicon surrounding the devices. On the other hand, a nano-precision XY-stage is designed and fabricated with trench-sidewall technology. Moreover, the XY-stage is formed on a single wafer instead of SOI wafer to reduce the cost of the fabrication. The stage is an elementary operation method for further research from MEMS (microelectricmechanical system) to NEMS (nanoelectricmechanical system) research. The main work and contributions are as follows:
1) In order to improve the performance of inductors and transformers, concave-suspended solenoid structures are proposed and formed by low temperature CMOS compatible process. To our best knowledge, these structures are reported firstly in the world. To facilitate the package and post-fabrication, the coils of inductors or transformers are embedded into the silicon substrate. The silicon surrounding the coils are etched and removed by gaseous XeF2. Therefore, the substrate loss of the device is suppressed due to the silicon surrounding coils removed. Moreover, the ohmic loss also reduced by using the copper plated process. Both factors improve the Q-factor and resonant frequency. The whole structure is sustained with protruding silicon oxide film on the both sides of the structure. The robust structure can be survived even under a shock of 10000g acceleration. By using finite element software ANSYS, the deformation of coil is one order smaller than other suspended inductors under 100g acceleration and 80 centigrade temperature changed. According to these proposed concave-suspended solenoid inductors and transformers, lumped and quasi-distributed circuit models are given. The simulated results based on circuit model can agree well with the measurement and be used to design and optimize the inductors and transformers. The measured Q-factor, the inductance and resonant frequency of the concave-suspended solenoid inductor are 54@2GHz, 2.47nH at low frequency and more than 15GHz, respectively. These performances are better than most of other micromachining inductors. We also fabricate some transformers which formed with the similar process. For the transformer, available gain can be defined as the ratio between the power delivered to the load and the power input to the network. It can be considered as an important figure-of-merit for overall evaluation of the RF transformers. The available gain of our present transformer is as high as 0.89 which is better than all other reported transformers. The available bandwidth is from 14 to 16GHz.
2) Different layouts of inductors and transformers are proposed for different applications. For the high inductance with small area occupation, a compact structure made by connecting several one column inductors in series is better choice. For the high inductance with high performance, an “S”shaped structure is a suitable choice with a little bigger area occupation. In order to eliminate the effect of electromagnetic interference and noise coupling with neighboring RF components, several closed loop solenoid inductor are implemented. With simulation of HFSS (Ansoft company software), these structure can confine the magnetic flux into a close-path. The structure can also avoid the flux intersecting with metals and semiconductor materials where eddy currents are easily induced. In order to improve the uniformity of the performance, a kind of structure with silicon oxide refilled trenches is given, which can define the region for XeF2 gaseous etching. The oxide block beneath the test pad can suppress the substrate parasitics. This structure can be used not only on-chip integration but as an off-chip for flip-chip package.
3) Some new structures and process are presented for solving problems existed in conventional MEMS tunable capacitors. In order to reduce the series resistance and simply the fabrication, low stress multi-metal electroplated technology (also called metal MEMS technology) is proposed and used to fabricate the suspended tunable capacitor with comb drivers. Electroplated nickel is used as the structure material due to its excellent mechanical properties. A thin gold film is covered on the surface of the nickel for long-term antioxidation in the air. After finishing the whole structure, the silicon substrate beneath the capacitive combs is also removed by XeF2 dry etching to for the same reason of suppressing the substrate loss. The structures are sustained by surrounding silicon oxide anchors. A suspension gap distance from the silicon substrate is formed. The temperature during the whole fabrication is below 120 centigrade. Only two masked are required. Under 4V driving voltage, the tuning ratio is 2.98:1. The Q-factor is 103 at 1GHz and 46.2 at 2GHz, which is better than most of micromachining tunable capacitor.
Most of the previously reported MEMS tunable capacitors share a common electric terminal with the electrostatic actuator. This common-ground configuration confines the flexibility of RFIC design. Moreover, the DC driving voltage interferes with the input RF signal especially in high power application. To overcome this limitation, the present technique electrically isolates the two parts at the middle of the linking bars with a patterned SiO2 bridge. The bridge just connects the two terminals mechanically but isolates each other electrically. This allows a greater flexibility in RF circuit layout. Under 4V driving voltage, the tuning ratio is 3.18:1. The Q-factor is 115 at 1GHz and 40.2 at 2GHz
4) To date, a drawback of the developed micromachined tunable capacitors lies in the mechanical instability induced by the straight-line-driven comb structures. For achieving a large tuning ratio under a low driving voltage, the researches have always designed the suspension springs as flexible as possible. When used in a mobile system like cell phone, such a mechanically instable capacitor will be inevitably influenced by the environmental vibratory/acceleration. On the contrary, rigid suspended beams require high driving voltage for getting wide tuning range. This is a dilemma in the design of micromaching tunable capacitors. To solve this problem, this dissertation develops, for the first time, a rotationally driven tunable capacitor. The rotational structure features a low stiffness in tangential direction for capacitance tuning but a much higher stiffness along radial directions for anti-vibration, thereby, exhibiting both high tuning-range under low-voltage and high resist against the axial vibration and acceleration. Generally, the environmental acceleration is applied along the straight direction. Therefore, the rotational structure can effectively suppress the environmental vibratory/acceleration. By using ANSYS simulation, the comb moving distance along the electrostatic driving direction is about two orders of magnitude lower than that of the conventional straight-moving tunable capacitors. Under 12V driving voltage, the tuning range of rotational capacitor is 2.1:1. The Q-factor is 35.2 at 2GHz.
5) Besides the RF passive components, a XY-stage is firstly presented on a single silicon wafer with trench-sidewall MEMS integrated technology. The trench-sidewall technology can be used to integrated piezoresistive, capacitive sensing and electrostatic actuator on a single silicon wafer instead of expensive SOI or SOG wafer. Moreover, the reliability of device can be improved by self-testing and feed back control. By using measured microscope, the moving range of the XY-stage is tested as 20×20μm2 under a driving voltage of 23V. In order to get a precision of movement, a nano-print system is designed and used to measure the XY-stage. Limited by the radius of the nanometric tip, the resolution of the tip position is about 18 nm. With this error bar denoted, all five data are on the straight fitting line. Therefore, in the small displacement region, the positioning precision of the stage can be considered better than the resolution of the nano tip, i.e. better than ±18 nm. The XY-stage provides an acceptable measured method for NEMS research.
The work of this thesis is creating innovation to develop 3-D MEMS integrating fabrication technology that is well known a very difficult work. Especially the CMOS-compatible integration technology for high-performance RF passives has been highly appraised by international academic field. The developed techniques have been transferred to microelectronics industry for production applications. Two papers have been published in the famous “International Electron Device Meeting (IEDM)”. Quite a number of papers are also published in the top-level journals of the field, such as IEEE E-D Lett., IEEE T-ED, IEEE J-MEMS, IEEE T-MTT, etc. The related results are invited to present as an invited paper on 2008 IEEE International RFIC Symposium. Based on the related achievement. Some famous scientists, including the vice president of IEEE ED Society and IEEE Fellow, have collaborated with us to develop RF Ics. Three patents are now pending. Some microelectronics companies like ASMC have collaborated with us, working on patent technology transfer for production applications. Based on this R&D content, recently Shanghai government granted an key project to support this industrial application research.
In conclusion, the investigation in this thesis realizes an important breakthrough in integration of complicated MEMS 3-D structures. The achievements are of great signification for getting high performance RF IC, telecommunication and nano-operation. These efforts will bring remarkable and prominent development in information and technology field and gain more economic benefit.
Relative publication and patent list:
Publications:
19. Lei Gu, Xinxin Li, Variable Capacitors and Tunable LC-Tanks Formed by CMOS-Compatible Metal MEMS for RF ICs, IEDM 2007 --- 53th IEEE International Electron Device Meeting, Washington DC,USA, 10-13 Dec., 2007, pp.427-430.
20. Lei Gu, Xinxin Li, A Post-CMOS Concave-Suspending MEMS Process in Standard Silicon Wafers for High-Performance Solenoidal-DNA-Configured Micro-Transformers, IEDM 2006 --- 52th IEEE International Electron Device Meeting 2006, San Francisco, CA, USA, Dec. 11-13, 2006, pp. 521-524.
21. Lei Gu, Xinxin Li, Rotational-driven RF Variable Capacitors with Post-CMOS Processes, IEEE Electron Device Letters, Vol.29, No.2, 2008, pp.195-197.
22. Lei Gu, Xinxin Li, High-Q Solenoid Inductors With a CMOS-Compatible Concave-Suspending MEMS Process, IEEE Journal of Microelectromechanical Systems, VOL. 16, NO. 5, 2007, pp.1162-1172.
23. Lei Gu, Xinxin Li, High-Performance CMOS-compatible Solenoidal Transformers with a Concave-suspended Configuration, IEEE Transactions on Microwave Theory and Techniques, VOL. 55, NO. 6, 2007, pp.1237-1245.
24. Lei Gu and Xinxin Li, Concave-suspended High-Q Solenoid Inductors with an RFIC-compatible Bulk-micromachining Technology, IEEE Transactions on Electron Devices, Vol.54, 2007, pp.882-885.
25. Lei Gu, Xinxin Li, Haifei Bao, Bin Liu, Yuelin Wang, Min Liu, Zunxian Yang, Baoluo Cheng, Single-Wafer-Processed Nano-Positioning XY-Stages with a Trench-Sidewall Micromachining Technology, J. Micromech. Microeng., 16 (2006) 1349–1357.
26. L. Gu and X. Li, Post-CMOS micromachined RF varactor with mutual-isolated large-tuning capacitor and low-voltage actuator, Electronics Letters, Vol. 43, No. 15, 2007, pp.808-809.
27. Lei Gu, Xinxin Li, On-chip embedded toroidal-solenoid inductors and transformers formed by post-CMOS micromachining techniques, Microelectronic Engineering, Vol. 85, 2008, pp.697-703.
28. Lei Gu, Zhengzheng Wu, Xinxin Li, Post-CMOS compatibly micromachined high-performance on-chip air-core solenoidal inductors, Microwave and Optical Technology Letters, Vol. 50, 2008, pp.2442 – 2446.
29. Lei Gu, Zhengzheng Wu, Xinxin Li, Post-CMOS micromachined nickel tunable-capacitors with a large tuning-range under low actuating voltage, Microwave and Optical Technology Letters, Vol. 50, 2008, pp.2469 – 2472.
30. Lei Gu, Xinxin Li, An wide-range tunable on-chip radio-frequency LC-tank formed with a post-CMOS compatible MEMS fabrication technique, Microelectronics Journal, (Available online 23 July 2008, doi:10.1016/j.mejo.2008.06.065)
31. Lei Gu, Xinxin Li, CONCAVE-SUSPENDED HIGH-Q SOLENOID INDUCTORS WITH A POST-CMOS MEMS PROCESS IN STANDARD SILICON WAFERS, IEEE MEMS-07 --- the 20th IEEE International Conference on Micro Electro Mechanical Systems, Kobe, Japan, Jan. 21-25, 2007, pp.771.
32. Lei Gu, Xinxin Li, Haifei Bao, Bin Liu, Yuelin Wang, Min Liu, Zunxian Yang, Baoluo Cheng, A Single-Wafer-Processed XY-Stage Fabricated with Trench-sidewall Doping and Refilled-Trench Isolating Technology, IEEE NEMS-06 --- the 1st IEEE International Conference on Nano/Micro Engineered and Molecular Systems, Jan. 18-21, 2006, Zhuhai, China, pp.860-863.
33. Xinxin Li, Lei Gu, (INVITED) Hi-Performance RF passives Using Post-CMOS MEMS Techniques for RF SoC, IEEE RFIC-2008 --- 2008 IEEE Radio Frequency Integrated Circuits Symposium, June 15-17, 2008, Atlanta, GA, USA, pp.163-166.
34. Xinxin Li, Lei Gu, Yuelin Wang, Heng Yang, Single-wafer-processed Self-testable High-g Accelerometers with Both Sensing and Actuating Elements Integrated on Trench-sidewall, IEEE Sensors Journal, Vol. 8, 2008, pp.1992-1999.
35. Xinxin Li, Baoluo Chen, Yuelin Wang, Lei Gu, Jian Dong, Xiaohong Ge,A trench-sidewall single-wafer-MEMS technology and its typical application in high-performance accelerometers,IEDM 2004---50th IEEE International Electron Device Meeting, San Francisco, CA ,13-15 Dec. 2004,pp.43-46.
36. Xinxin Li, Wei Xu, Ying Chen, Le Luo, Lei Gu, Min Liu, Baoluo Cheng, and Yuelin Wang, Single-wafer-processed trench-sidewall integration and its application in a micro resonant detector for vacuum monitoring, J. Vac. Sci. Technol. B 24 (1), Jan/Feb 2006, pp16-19.
专利:
4. A fabrication method for CMOS compatible concave-suspended solenoid inductors and transformers, CN200610029276.8, X. Li and L. Gu (Granted)
5. A fabrication method for CMOS compatible suspended variable-capacitors, CN200710044283.3, X. Li and L. Gu (Pending)
6. A kind of MEMS on-chip varactors with anti-vibration capability for movable telecommunication, CN200710045457.4, X. Li and L. Gu (Pending)
Key words: MEMS, RF passive components, inductors, transformers, tunable capacitors, Tunable LC-tank, XY-stage, micromaching process.
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