MEMS for RF, Micro Optics and Scanning Probe Nanotechnology Applications

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Gyles West
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1 MEMS for RF, Micro Optics and Scanning Probe Nanotechnology Applications

2 Part I: RF Applications Introductions and Motivations What are RF MEMS? Example Devices

3 RFIC RFIC consists of Active components Transistors and diodes High level of integration Passive components Capacitors, inductors, waveguides, antenna and filters Discrete components used

4 Toward Higher Level of Integration The major possible contribution of MEMS in RF IC is to enable new types of RF passive components (RF MEMS) Monolithic integration with active components Small size and highly functional (e.g. tunable) Higher performance (e.g. higher quality factor and lower loss) Reducing the number of discrete components and the cost of packaging (enhancing the reliability)

5 Typical RF MEMS devices Tunable capacitors vs. (PN junction and MOS capacitor) Advantages: much wider tuning range (DC/C), lower loss, insensitive to RF interference Challenges: high driving voltage, slow tuning speed (10 s µs) Inductors Advantages: much higher quality factor and also inductance Challenges: more complex fabrication and integration process Switches and relays vs. (PIN diodes) Advantages: much higher ON/OFF ratio and lower loss Challenges: high driving voltage, slow switching speed, reliability Resonators vs. Quartz resonators Advantages: Integrated and comparable quality factor Challenges: stability and reliability

On-chip (integrated) Inductors Electromagnetic coupling causing loss and parasitics 1 st Metal 2 nd Metal layer layer SiO 2 Si On-chip inductors are one of the major

6 On-chip (integrated) Inductors Electromagnetic coupling causing loss and parasitics 1 st Metal 2 nd Metal layer layer SiO 2 Si On-chip inductors are one of the major performance-limiting passive components in current RF ICs. Low quality factor due to substrate loss and parasitics (Q<20) Q should be as high as possible Large footprint (100s 100smm 2 )

7 Micromachined Spiral Inductors Polyimide Air Gap Si Applying a thick polyimide layer underneath Glass Levitating the inductor structure above the substrate Si Si Completely removing substrate material underneath Si Partially removing substrate material underneath Still Large footprint Involving complex microfabrication steps, possibly not compatible with IC fabrication

8 A 3D Solution Fabricate inductor on IC substrates Integrated circuits Put the spiral inductor into vertical position Reduce substrate loss and parasitics Higher Q Zero footprint But how?

9 The Prototype PDMA

10 Typical Electrostatic Actuated RF MEMS Switches: Capacitive ON State OFF State Co-planar waveguide (CPW) configuration ON state: C off = 25~50fF OFF state: C on = 3~4pF (serving as a short between the signal line and the ground line. Driving voltage larger than pull-in voltage

11 Typical Electrostatic Actuated RF MEMS Switches: Contact Co-planar waveguide (CPW) configuration Issue related the operation of the contact RF MEMS switch Stress in the gold films The contact point could fatigue and wear after continuous operation.

MEMS Resonators Current electronic filters used in RFICs RLC filters based on electrical resonance Low quality factor and frequency selectivity Difficult to integrate Filters using quartz based on

12 MEMS Resonators Current electronic filters used in RFICs RLC filters based on electrical resonance Low quality factor and frequency selectivity Difficult to integrate Filters using quartz based on mechanical resonance of the piezoelectric quartz crystal High quality factor (30,000) Difficult to integrate Developing MEMS resonators for integrated, high quality factor filters based on the resonance of micromechanical structures. electrical input signal Mechanical vibration and resonance Filtered electrical output signal

13 MEMS Resonators The input voltage causes the left comb drive (longitudinal or transverse?) to vibrate. The amplitude of the vibration depends on the frequency of the input signal and it will reach the maximum at the resonant frequency of the entire movable comb drive structure. The vibration cause the capacitance within the right comb drive to change, which can read out at the output. Frequency: 1~100kHz, Quality factor: ~8000 (in air or in vacuum?) The driving and signal read-out do not have to be electrostatic.

14 Part II: Micro Optics Applications

15 Introduction MEMS applications in optics Phase correction Pattern generation and projection Switching and scanning of optical beams

16 Adaptive Optical MEMS Devices Transitional mirrors Adjust the travel distance of light Deflection of rigid plates Piston motion membranes

17 Adaptive Optical MEMS Devices Deformable micro mirror array Piston motion micro mirror array Micromachined deformable mirror array

18 Rotational Scanning Mirrors Bent cantilever beams Top surface serves as reflector Mirror surface supported by torsional structures Able to generate large angles with reasonable actuation force

19 Digital Mirror Device +/-10 o

20 MEMS Optical Switches

21 Rotational Scanning Mirrors

Endoscopic Probe with Micromotor: 3D Helix

2.5 cm Liner transversal stage pulling of

22 Endoscopic Probe with Micromotor: 3D Helix Scan Rabbit esophagus Specifics Outer diameter: 2.7 mm Motor diameter: ~2.0mm with cycle feedback control Rigid length: 2.5 cm Liner transversal stage pulling of flexible part J. P. Su, Optical Express, 15, 16, 10390, 2007 Rabbit trachea

23 Part III: Scanning Probe Application Introduction and Motivation Dip-pen Nanolithography Examples of Application 1-D passive scanning probe array 1-D active scanning probe array 2-D active scanning probe array

24 Scanning Probe Microscopy Atomic force microscope

25 Scanning Probe Array

Dip-Pen Nanolithography The process Utilize a sharp scanning probe Coat the probe tip with ink Contact sample surface and form water meniscus Scan (using AFM machine) The advantages Nanometer

26 Dip-Pen Nanolithography The process Utilize a sharp scanning probe Coat the probe tip with ink Contact sample surface and form water meniscus Scan (using AFM machine) The advantages Nanometer resolution Direct patterning, no mask and photoresist involved Compatible with a wide range of chemicals (e.g. bio-chemicals) Suitable for both bottom-up and top-down approaches LFM images of 1-octadecanethiol (ODT) patterns by DPN (60nm)

27 Scanning Probe Array for DPN To develop large DPN probe arrays to increase the writing speed. Passive probe array A A A A A A A A Duplicate identical features Passive probe array A B C D E F G H Each probe has a micro actuator and is individually addressable. Flexibility for complex features

28 1D Passive DPN Probe Arrays Silicon nitride probe array Duplicate identical features Silicon probe array p++ doped layer

29 1D Active SiN DPN Probe Arrays Gold * David Bullen, et al, "Design, Fabrication, and Characterization of Thermally Actuated Probe Arrays for Dip Pen Nanolithography," JMEMS, 2004.

30 2D Active Probe Array 7 7 array packaged on a micromachined silicon chip holder

31 2D Active Probe Array Probe chip SiN Gold Writing sample Bimetallic Thermal Actuator

32 Actuation of 2D Active Probe Array Microscope Cool down Heat up Gold Si Nitride

33 Experimental Result 150nm LFM images of ODT patterns written simultaneously by the scanning probe array

MEMS in ECE at CMU. Gary K. Fedder

MEMS in ECE at CMU. Gary K. Fedder MEMS in ECE at CMU Gary K. Fedder Department of Electrical and Computer Engineering and The Robotics Institute Carnegie Mellon University Pittsburgh, PA 15213-3890 fedder@ece.cmu.edu http://www.ece.cmu.edu/~mems