Slide 1: Getting In Touch with MEMS: The Electromechanical Interface
ISSCC 2012 Tutorial Getting In Touch with MEMS: The Electromechanical Interface Dr. Aaron Partridge ap@sitime.com SiTime, Corp. February 19, 2012
Slide 2: Overview
Overview These slides accompany the 2012 ISSCC Tutorial, Getting In Touch with MEMS: The Electromechanical Interface. The tutorial is written for practicing IC engineers and students. No MEMS background is needed. The goal is to expand the attendee’ s potential role from circuit designer to system d
Slide 3: Outline
Outline o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 3 of 70 The Electromechanical Interface
Slide 4: Outline: MEMS Materials, Processes, and Applications
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 4 of 70 The Electromechanical Interface
Slide 5: Materials
Materials o Standard Semiconductor Materials n Silicon (single crystal and poly). n Oxide (thermal and deposited). n Nitride. n Aluminum. o Unusual Materials n Gold, various other metals. n Piezoelectrics (AlN mostly). n Plastics (e.g. SU-8). n And then just about anything else. Aaron Partridge Gett
Slide 6: Processes
Processes o Early in MEMS many unusual etches were common. o Now standard fab process are preferred when possible. o A few special processes Tuning fork resonator, Bosch 2003 n Bosch etch. n HF vapor etch. n Oxide plasma release. n Xenon difluoride (XeF2) release. o Deep etches are common. S. Pourka
Slide 7: Example Applications
Example Applications o MEMS will find its way into practically every application. o Right now, it is strong in n Automotive (pressure, acceleration, rotation). n Consumer (acceleration, rotation, time). n Industrial and Military (pressure, acceleration). n Medicinal (pressure, biological sensors). o
Slide 8: Pressure Sensors
Pressure Sensors Sensimed intraocular pressure sensor in contact lens Aaron Partridge Getting In Touch with MEMS: 8 of 70 The Electromechanical Interface
Slide 9: Accelerometers & Gyroscopes
Accelerometers & Gyroscopes Freescale accelerometer Aaron Partridge Getting In Touch with MEMS: 9 of 70 The Electromechanical Interface
Slide 10: Microphones
Microphones Akustica microphone Aaron Partridge Getting In Touch with MEMS: 10 of 70 The Electromechanical Interface
Slide 11: Light Modulators & Projectors
Light Modulators & Projectors Two pixels in a TI DLP mirror array Aaron Partridge Getting In Touch with MEMS: 11 of 70 The Electromechanical Interface
Slide 12: Resonators & Oscillators
Resonators & Oscillators SiTime oscillator Aaron Partridge Getting In Touch with MEMS: 12 of 70 The Electromechanical Interface
Slide 13: RF Switches
RF Switches G. Rebeiz UCSD, RF switch Aaron Partridge Getting In Touch with MEMS: 13 of 70 The Electromechanical Interface
Slide 14: Outline: Electrical Interfaces
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 14 of 70 The Electromechanical Interface
Slide 15: Capacitive Overview
Capacitive Overview o Capacitive transduction is used in 90% of MEMS interfaces. o Good Points: n Easy to build, no need for special materials. n With the Bosch etch we can make beautiful cap structures. n Can move small to large distances. n Can move in-plane and out-of-plane. n Can sense tiny disp
Slide 16: Capacitive Drive
Capacitive Drive o How does capacitive drive work? Take a parallel plate example: g ε c wh 2 F= 2 V 2g w o Were F=force, 0=permittivity, w=width, h=height, g=capacitive gap, V=voltage. F h o The voltage squared gives attractive forces and drive nonlinearity. V o The gap squared gives displacement no
Slide 17: Capacitive Drive
Capacitive Drive o Want bipolar force? g o We can offset the attraction (pull more and pull less) with DC bias and AC drive. w ε c wh F= (Vbias + Vdrive ) 2 2g 2 o Set Vbias >> Vdrive and we get a F h bipolar offset drive. ε c wh Foffset ≈ 2 VbiasVdrive g o As bias is increased and drive is Vbias+Vd
Slide 18: Capacitive Drive
Capacitive Drive o Second option for bipolar is differential (pull one way, g g pull the other), ε c wh 2 Fdif = 2g 2 (Vleft − Vright ) 2 w o Offset and differential can be combined, Fdif h ε c wh Fdif ≈ 2 Vbias (Vright − Vleft ) g o Typical bias and drive are 5V and 0.5V. Vleft Vright Aaron Partrid
Slide 19: Capacitive Drive
Capacitive Drive o Interdigitated fingers g (combs) can move further and are more linear. p ε ch 2 Fdif = N V g Fc o N is the number of fingers. h o Since p does not effect Fc it is linear in displacement. o Pairs of fingers can be V V used differentially to linearize V and push-pull. Aaron Partridg
Slide 20: Capacitive Sense
Capacitive Sense o For capacitive sensing, we cap C need to think about charge, Q = CV , i = dQ / dt o We all learned that, i = C ( dV / dt ) o But for MEMS sensing we sometimes care more about, i = Vdc (dC / dt ) i o And we use a bias Vdc, often Any structures with dC/dx about 5V but can be 100’ s!
Slide 21: Capacitive Sense
Capacitive Sense o What you will need to do as an engineer n Because capacitances are small, sense currents are small. n Design the lowest noise sense amps possible. n If noise is not critical then shrink the MEMS. n Always push the circuits, always simplify the MEMS. o Drive Circuits n For AC syste
Slide 22: Capacitive Sense
Capacitive Sense sensor diff osc A OUT phase trim o Differential lock-in sense amp for accelerometers. Aaron Partridge Getting In Touch with MEMS: 22 of 70 The Electromechanical Interface
Slide 23: M. Dugger, Sandia Labs
Aaron Partridge Getting In Touch with MEMS: 23 of 70 The Electromechanical Interface M. Dugger, Sandia Labs
Slide 24: Piezoresistive Overview
Piezoresistive Overview o Transduces strain to resistance. o One of the earliest MEMS interfaces and still important. o Good points: n There is mechanical gain, typically about 30x. n The common sensor structure is a Wheatstone bridge. n Silicon-friendly fabrication, doped resistors work well. o Bad
Slide 25: Piezoresistive Sense
Piezoresistive Sense o A simple idea with may coefficients –strain changes resistivity. R ∆ρω 6 = ∑ π ωλσ λ ρ λ =1 o =change in resistivity, are piezo coefficients, are stress. The form a sparse 6x6 matrix. o For specific cases the equation can be simplified to, ∆R = π effectiveσ axial R Aaron Partr
Slide 26: Piezoresistive Sense
Piezoresistive Sense o Typical sense circuits are bridges. n This minimizes the temperature sensitivity that can swamp signals. n Input offset often vital –use switched caps, diversity, etc. n Often must minimize 1/f noise –use switched topologies. n Temperature compensation of offset and gain varia
Slide 27: Piezoresistive Sense
Piezoresistive Sense Vbias offset gain OUT A trim trim OUT temp sense sensor o Bridge amp with temperature offset and gain correction Aaron Partridge Getting In Touch with MEMS: 27 of 70 The Electromechanical Interface
Slide 28: A. Partridge, Stanford
Aaron Partridge Getting In Touch with MEMS: 28 of 70 The Electromechanical Interface A. Partridge, Stanford
Slide 29: Piezoelectric Overview
Piezoelectric Overview o Transduces force to voltage. o Aluminum Nitride (AlN) is the most common material. o Good points: n Moderately easy to fabricate, available in MEMS foundries. n Can provide low impedance and tremendous power handing for RF. n Works well at high frequencies. o Bad points: n D
Slide 30: Piezoelectric Drive and Sense
Piezoelectric Drive and Sense o For sense and drive, think in t terms of S21 and S12. o In the simplest form, w t V =d F F ewh h o Where t=thickness, e=dielectric permittivity, and d-bar is the piezoelectric charge coefficient (a function of material and orientation). V Aaron Partridge Getting In To
Slide 31: G. Vigevani, UC Berkeley
Aaron Partridge Getting In Touch with MEMS: 31 of 70 The Electromechanical Interface G. Vigevani, UC Berkeley
Slide 32: Other Transducers
Other Transducers o Thermal sensors, particularly thermistors, are often used for IR imaging bolometers. o Chemical sensors of all kinds are used in biology. o There are lots of optical transducers, and this is an important area for displays. Optical forces can even be used to drive MEMS, but that i
Slide 33: Remember This:
Remember This: You will usually need to design the lowest noise circuits possible. If the MEMS is producing extra signal then it should be simplified. Aaron Partridge Getting In Touch with MEMS: 33 of 70 The Electromechanical Interface
Slide 34: Outline: Scaling Laws
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 34 of 70 The Electromechanical Interface
Slide 35: Scaling Laws
Scaling Laws o Most things scale against us, not for us. o Mechanical structures n Volume and mass: x3 (simple) n Mechanical stiffness: x (extensional) n Resonant frequency: 1/x (extensional) o Transducers n Piezoresistance: (no scale) n Capacitance: x (voltage to force) n Piezoelectrics: x (voltage
Slide 36: Why Does Scaling Matter?
Why Does Scaling Matter? o Most MEMS things get worse when made smaller n Mass goes down cubically, so inertial sensing gets tougher. n Capacitive and piezoelectric transducers get worse linearly. n Magnetic transducers scale terribly. o A few things get better n Circuits can be mounted closer, so C
Slide 37: Remember This:
Remember This: Know how your system scales and leverage things that work for you and not against you. Aaron Partridge Getting In Touch with MEMS: 37 of 70 The Electromechanical Interface
Slide 38: Outline: Packaging is Critical
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 38 of 70 The Electromechanical Interface
Slide 39: Packaging is Critical
Packaging is Critical o In circuits we don’ t think about packaging, except for n Size. n Lead inductance and resistance. n Heat dissipation. o In MEMS, packaging is the single most important thing after the transducer selection n How do we protect the parts in operation? n How do we handle the part
Slide 40: Back-end Packaging
Back-end Packaging o Sometimes just put the MEMS and CMOS into a package. o Don’ t touch it! o Production complications n How to dice? n How to pick & place? n Need a clean room? o For some apps, like chemical detectors, it can work well. ADI ADXL-50 circa 1995 Aaron Partridge Getting In Touch with
Slide 41: Bonded Covers
Bonded Covers o Wafer bonded covers protect the MEMS elements n Can use frit glass to glue the wafers together. n These covers can take 80% of the die area. n Building the covers can be expensive. o Much less expensive then Bosch Gyroscope circa 1999 handling naked MEMS wafers. o The dominant techno
Slide 42: Deposited Thin Film Covers
Deposited Thin Film Covers o Save space by depositing rather than bonding covers. o Harder than it looks n How to empty it out inside? n Thermal mismatches. n Contamination. B.H. Stark, JMEMS 2004 n Need strength for plastic. n Limits MEMS designs. o Development is expensive n Only makes sense for h
Slide 43: Deposited Encapsulation Example
Deposited Encapsulation Example SiTime Resonator o SiTime (my company) buries resonators under the wafer surface. Aaron Partridge Getting In Touch with MEMS: 43 of 70 The Electromechanical Interface
Slide 44: Stacked Die in Plastic
Stacked Die in Plastic o When the MEMS is covered or encapsulated n It can be diced like a CMOS wafer. n Pick-place, wire bond, mold, etc, all normal. o A low cost option when it works for the app. o Applications like optical switches can’ t use this. SiTime Oscillator Construction Aaron Partridge G
Slide 45: Remember This:
Remember This: Design from the outside to the inside, and do the packaging first. Aaron Partridge Getting In Touch with MEMS: 45 of 70 The Electromechanical Interface
Slide 46: Outline: CMOS Integration
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 46 of 70 The Electromechanical Interface
Slide 47: Homogeneous Integration
Homogeneous Integration o Only three ways n MEMS on CMOS. n MEMS in CMOS. n MEMS under CMOS. o Usually not the best approach n Do you have the time and money? n Do you feel lucky? o Just a few of the problems… n The MEMS process can change the CMOS behavior. n The CMOS process may need to be adjuste
Slide 48: Homogeneous Example
Homogeneous Example 3-Axis Accelerometer, Sandia Aaron Partridge Getting In Touch with MEMS: 48 of 70 The Electromechanical Interface
Slide 49: Heterogeneous Integration
Heterogeneous Integration o Usually a better approach n Build the MEMS and CMOS die into one package. n Can be wire bonded or bump bonded. n Requires the MEMS be pre-covered for most apps. o Usually cheaper, faster, and more versatile n MEMS+CMOS can be developed in parallel. n Fewer material and te
Slide 50: Heterogeneous Example
Heterogeneous Example Oscillator, SiTime Aaron Partridge Getting In Touch with MEMS: 50 of 70 The Electromechanical Interface
Slide 51: Wafer-scale Integration
Wafer-scale Integration o MEMS wafer bonded to CMOS wafer n The wafers are bonded face-face at the MEMS fab. n Various proprietary metallurgies at MEMS fabs. n Common are gold compression and eutectics. o Has some good points n Electrically close –low capacitance. n Many interconnects possible. n Us
Slide 52: Wafer-scale Example
Wafer-scale Example MEMS Wafer Circuits Wafer Rich Ruby, Avago, IFCS 2011 Aaron Partridge Getting In Touch with MEMS: 52 of 70 The Electromechanical Interface
Slide 53: Remember This:
Remember This: Never integrate MEMS on, in, or under CMOS without a very compelling reason. Aaron Partridge Getting In Touch with MEMS: 53 of 70 The Electromechanical Interface
Slide 54: Outline: How to Succeed
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 54 of 70 The Electromechanical Interface
Slide 55: Problems to Manage
Problems to Manage o You will need to build something early n The MEMS components will not be finished when you start your IC design. n You will not have solid data for your analysis. n If prototypes have been tested, the new parts (the real parts!) will be different. n When the MEMS is tested it wi
Slide 56: Problems to Manage
Problems to Manage o Specs will change in the middle of development n There are three, maybe four things in flux. n Circuits, MEMS design, MEMS process, and software. n On the applications side, marketing and sales learn too. n Rates of flux are higher than you may be used to. o To improve the chanc
Slide 57: Important Points
Important Points o Be sure you have a MEMS expert on the team n Hire one or consult with one, but get one. n Amateurs who think they are smart enough are wrong. o Understand the application drivers n How does the scaling work? n Does it require integration? o Consider the packaging carefully n Desig
Slide 58: Important Points
Important Points o Build flexibility in your IC n The MEMS will not work as expected, so roll with it. n The MEMS will be improved later, and the earlier IC should support that. o Insist on many small learning cycles n Use test chips to prove out the concepts. n Don’t go into product design too earl
Slide 59: Aaron's Complexity Paradox
Aaron’ s Complexity Paradox o The “Simple”solutions Easy are difficult. Software o The “Complex”solutions Firmware are easy. Microcode Complex Simple o Move toward easy Digital solutions whenever possible, this means Analog increasing complexity. Device o Embrace complexity to Material succeed. Diff
Slide 60: Aaron's Complexity Paradox
Aaron’ s Complexity Paradox o For system designers this means n Push the problem out of MEMS whenever possible. n Don’t ever make the MEMS designers do something that one can possibly do elsewhere. o For IC designers this means n You will be (or should be!) asked to push your circuits to the limit t
Slide 61: Complexity Management Example
Complexity Management Example o MEMS oscillator researchers (including me!) tried to build frequency-accurate temperature-flat resonators. n That didn’ t work. o Then my colleagues and I moved the complexity into CMOS and managed the frequency with frac-N PLLs. n That worked! o Then all sorts of gre
Slide 62: Complexity Management Example
Complexity Management Example o SiTime Architecture. MEMS Resonator o Will step through this block by block. CMOS Oscillator Prog Configuration Sustaining OE I/O Regs Vcc PROM Amp /ST Temperature Frequency Frac-N GND Drive Clk Sensor Control PLL Aaron Partridge Getting In Touch with MEMS: 62 of 70 T
Slide 63: Choose the Right Products
Choose the Right Products o Must have a Starving Market. (Kurt Petersen) o Must have an Unfair Advantage. (Arno Penzias) o Why discuss this in a circuits tutorial? n Because we engineers and scientists can almost always make our stuff work. n But failure happens when people don’ t want our stuff. n
Slide 64: Remember This:
Remember This: We are valued not by what we do, but by what we do that makes a difference. Aaron Partridge Getting In Touch with MEMS: 64 of 70 The Electromechanical Interface
Slide 65: Your Work Does Make a Difference
Your Work Does Make a Difference o Technology has fundamentally helped Humanity. o Engineers make small contributions that are multiplied countless times. o What you do matters to Billions of people! Dorothea Lang, “ Migrant Mother”1936 Aaron Partridge Getting In Touch with MEMS: 65 of 70 The Electr
Slide 66: Thank You! Questions?
Thank You! Questions? Aaron Partridge Getting In Touch with MEMS: 66 of 70 The Electromechanical Interface
Slide 67: Outline: References
o MEMS Materials, Processes, and Example Applications o Electrical Interfaces o Scaling Laws o Packaging is Critical o CMOS Integration o How to Succeed o References Aaron Partridge Getting In Touch with MEMS: 67 of 70 The Electromechanical Interface
Slide 68: References
References o A complete background to MEMS and thorough basic references n Gregory Kovacs, “ Micromachined Transducers Sourcebook” , McGraw- Hill Science/Engineering/Math, ISBN 0-0729-0722-3 , 1998. o A good general introduction of scaling and technology options n Marc Madou, “ Fundamentals of Micro
Slide 69: References
References o A solid theoretical underpinning of common MEMS devices n Ville Kaajakari, “Practical MEMS: Design of microsystems, accelerometers, gyroscopes, RF MEMS, optical MEMS, and microfluidic systems” , Small Gear Publishing, 098-2299109, 2009. o The primary conference proceedings and journals
Slide 70: Additional References
Additional References Capacitive transduction o W.C. Tang, T-C.H. Nguyen, M.W. Judy, R.T. Howe, “ Electrostatic-Comb Drive of Lateral Polysilicon Resonators” , Sensors and Actuators A: Physical, v.21, pp.328- 331, 1990. o A. Selvakumar, K. Najafi, “Vertical Comb Array Microactuators” , J. Microelect
Slide 71: Additional References
Additional References o F.N. Hooge “ 1/f Noise Sources,”IEEE Transactions of Electron Devices, v.41, n.11, pp.1926-1935, 1994. o J.A. Harley, T.W. Kenny, “ 1/F Noise Considerations for the Design and Process Optimization of Piezoresistive Cantilevers”, J. Microelectromechanical Systems, v.9, pp.226-
Slide 72: Additional References
Additional References Examples of less common drive and sense technologies o C.H. Liu, A.M. Barzilai, J.K. Reynolds, A. Partridge, T.W. Kenny, J.D. Grade, H.K. Rockstad, “ Characterization of a High-Sensitivity Micromachined Tunneling Accelerometer with Micro-g Resolution,”IEEE Journal of Microelect
Slide 73: Additional References
Additional References MEMS wafer-level packaging o Y.T. Cheng, L. Lin, and K. Najafi, "Localized Bonding with PSG or Indium Solder as Intermediate Layer," Twelfth IEEE International Conference on Micro Electro Mechanical Systems, pp. 285-289, 1999. o C.H. Tsau, S.M. Spearing, and M.A. Schmidt, "Fabr
Slide 74: End
End Aaron Partridge Getting In Touch with MEMS: 74 of 70 The Electromechanical Interface