Design anD Fabrication oF suspenDeD-gate MosFets For MeMs resonator, switch anD MeMory applications

............................................................................................................................... VERSION ABREGEE ............................................................................................................... INTRODUCTION....................................................................................................................1 CHAPTER I MEMS FOR COMMUNICATIONS SYSTEMS, STATE-OF-THE-ART AND CHALLENGES...............................................................................................................5 I.A. MEMS-BASED WIRELESS CIRCUIT ARCHITECTURES: WHY?............................................6 I.A.1. Time reference components .....................................................................................6 I.A.2. Quartz oscillator .......................................................................................................7 I.A.3. MEMS-based integrated oscillator...........................................................................7 I.B. RESONATOR ARCHITECTURES..........................................................................................7 I.B.1. Piezoelectric resonators (Bulk Acoustic Wave BAW)...........................................7 I.B.2. Electrostatic resonators.............................................................................................8 I.B.2.a. Flexural mode architectures..............................................................................9 I.B.2.b. Bulk or Lamé mode..........................................................................................10 I.B.2.c. Coupled resonator architecture.......................................................................11 I.C. ELECTROSTATIC AND PIEZOELECTRIC RESONATORS STATE-OF-THE-ART, CHARACTERISTICS AND PERFORMANCES..............................................................................12 I.D. APPLICATIONS OF MEMS RESONATORS AND SPECIFICATIONS.....................................19 I.D.1. Local oscillator.......................................................................................................19 I.D.2. Voltage Control Oscillator (VCO) .........................................................................19 I.D.3. Filters......................................................................................................................20 I.E. INTEREST OF MOSFET DETECTION FOR MEMS RESONATORS .....................................21 REFERENCES .......................................................................................................................22 CHAPTER II – STATIC AND DYNAMIC MODELING OF THE SG-MOSFET .........25 II.A. STATIC MODELING OF A CLAMPED-CLAMPED SG-MOSFET .......................................27 II.A.1. Suspended-gate MOSFET electrical modeling.....................................................27 II.A.2. Electro-mechanical modeling ...............................................................................28 II.A.2.a. Pull-in and pull-out effects description ..........................................................29 II.A.2.b. Beam shape during mechanical deflection.....................................................31 II.A.2.c. Total SG-MOSFET drain current...................................................................32 II.A.3. Model validation ...................................................................................................34 II.A.4. Electrical air-gap for SG-MOSEFT ......................................................................35 II.A.5. Disturbing forces on nano-gap devices The Casimir effect................................36 II.B. DYNAMIC MODELING OF THE RSG-MOSFET..............................................................38 II.B.1. Mechanical dynamic behavior and mode shape....................................................38 II.B.1.a. Nodes on CC-beam.........................................................................................40 II.B.1.b. Effect of stress on the resonant frequency......................................................41 II.B.2. General study of non-linearities in a capacitive MEMS resonator .......................43 II.B.3. Mechanical and electrical non-linearities for a CC-beam resonator .....................45 II.B.3.a. Mechanical non-linearities.............................................................................46 II.B.3.b. Electrical non-linearities................................................................................46 II.B.3.c. Influence of mechanical and electrical non-linearities on the response ........46 II.B.4. Quality factor ........................................................................................................47 II.B.4.a. Stored energy in mechanical resonators ........................................................47 II.B.4.b. Energy dissipation in resonator .....................................................................48 II.B.4.b.i Structural or thermoelastic damping (TED) ..........................................48 II.B.4.b.ii Dry friction or Coulomb damping ........................................................50 II.B.4.b.iii Gas or viscous damping.......................................................................51 II.B.4.b.iv Support loss damping ..........................................................................53 II.B.4.b.v Surface loss damping............................................................................54 II.B.4.b.vi Other source of losses..........................................................................54 II.B.5. Equivalent electrical circuit model........................................................................55 II.B.5.a. General analysis on pure capacitive resonator..............................................55 II.B.5.b. Influence of coupling capacitance on signal transmission.............................56 II.B.5.c. Influence of coupling capacitance on signal measurement ............................56 II.B.5.d. Resonator with MOSFET detection................................................................60 II.B.5.e. Comparison between capacitive and MOSFET detection ..............................61 II.C. RSG-MOSFET MODEL FOR CIRCUIT DESIGN...............................................................62 II.D. MODELING PERSPECTIVES OF THE RESONANT SG-MOSFET......................................63 REFERENCES .......................................................................................................................64 CHAPTER III – SG-MOSFET FABRICATION PROCESSES........................................67 III.A. METAL-BASED SG-MOSFET PROCESS ......................................................................69 III.A.1. Process description ..............................................................................................69 III.A.1.a. Active zones definition and silicon dry oxidation .........................................69 III.A.1.b. Source and drain implantation .....................................................................70 III.A.1.c. Polysilicon sacrificial layer ..........................................................................70 III.A.1.d. Polyimide sacrificial layer ............................................................................71 III.A.1.e. CMP of polyimide and polysilicon sacrificial layers ....................................71 III.A.1.f. Contact and structural layer..........................................................................72 III.A.1.g. Metal-gate layer............................................................................................73 III.A.1.h. Releasing step ...............................................................................................73 III.A.2. Structural layer stress investigation.....................................................................74 III.A.2.a. Stress measurement technique ......................................................................74 III.A.2.b. Residual stress and characteristic on Al-based film .....................................74 III.A.2.c. Temperature effect.........................................................................................75 III.A.2.d. Stress measurement from devices..................................................................77 III.A.2.e. Stress gradient measurement ........................................................................78 III.A.2.f. Young’s modulus extraction...........................................................................79 III.A.2.g. Summary of sputtered Al-Si1% properties....................................................80 III.A.3. Conclusion and limitations of the metal gate SG-MOSFET process ..................80 III.B. SILICON RESONATOR PROCESS....................................................................................81 III.B.1. Wafer pre-processing...........................................................................................82 III.B.2. Sacrificial layer etching .......................................................................................82 III.B.2.a. HF vapor etching ..........................................................................................82 III.B.2.b. BHF etching combined with CPD.................................................................82 III.B.3. Effect of gate oxidation on resonance frequency.................................................83 III.B.4. Resonator encapsulation ......................................................................................83 III.B.5. Contact metallization and release ........................................................................84 III.B.6. Improvements of the silicon gate SG-MOSFET process.....................................85 III.C. PACKAGING PROCESS..................................................................................................86 III.C.1. Process flow.........................................................................................................86 III.C.2. Effect of opening size on releasing rate and clogging effect ...............................88 III.C.3. Packaging issues for production environment .....................................................89 III.C.4. Perspectives of the 0-level thin film packaging process ......................................90 III.D. FRONT-END PROCESS ..................................................................................................91 III.D.1. Lateral-gate Vertical-MOSFET architecture [104]..............................................91 III.D.2. Lateral-gate Horizontal-MOSFET architecture [105] .........................................92 III.D.3. Process flow and technological blocks validation ...............................................92 III.D.4. Perspectives of the Front-end process .................................................................94 III.E. CONCLUSION ...............................................................................................................95 REFERENCES .......................................................................................................................96 CHAPTER IV – RESONANT SG-MOSFET CHARACTERIZATIONS ........................99 IV.A. CHARACTERIZATION METHODOLOGY ......................................................................101 IV.A.1. Capacitive and MOSFET detection characterizations.......................................101 IV.A.1.a. Capacitive response of a RSG-MOSFET ....................................................102 IV.A.1.b. Transistor response and gain of the RSG-MOSFET ...................................103 IV.A.2. Comparison of capacitive and MOSFET detection on transmission response..105 IV.A.3. Characterization setup for bulk-mode RSG-MOSFET .....................................106 IV.A.4. Insertion loss and impedance matching.............................................................106 IV.A.5. Practical characterization methodology ............................................................107 IV.A.5.a. Impedance measurement setup....................................................................107 IV.A.5.b. Transmission measurement setup................................................................109 IV.B. ALSI-BASED RESONANT SG-MOSFET CHARACTERIZATIONS.................................109 IV.B.1. Quasi-Static characterizations ...........................................................................110 IV.B.2. Influence of the gate and drain voltages on the RSG-MOSFET response ........110 IV.B.2.a. Effect of gate voltage variation ...................................................................111 IV.B.2.b. Effect of drain voltage variation .................................................................112 IV.B.3. Non-linearities ...................................................................................................113 IV.B.4. Effect of air damping on quality factor..............................................................113 IV.B.5. Temperature effect.............................................................................................114 IV.B.6. Impact of resonator scaling on voltage-frequency dependency.........................115 IV.C. SILICON-BASED RESONANT SG-MOSFET CHARACTERIZATIONS............................115 IV.D. OSCILLATOR DESIGN ................................................................................................118 IV.D.1. Capacitive MEMS-based oscillator...................................................................118 IV.D.2. RSG-MOSFET based oscillator ........................................................................119 IV.E. CONCLUSIONS AND PERSPECTIVES...........................................................................120 REFERENCES .....................................................................................................................122 CHAPTER V – ABRUPT SWITCH AND MEMORY APPLICATIONS OF THE SGMOSFET...............................................................................................................................123 V.A. ABRUPT CURRENT SWITCH.........................................................................................125 V.A.1. SG-MOSFET switch...........................................................................................126 V.A.2. SG-MOSFET sub-threshold swing performance................................................128 V.A.3. Switch isolation ..................................................................................................129 V.A.4. Scaling perspectives............................................................................................130 V.A.5. Scaling of the switching current amplitude ........................................................132 V.A.6. SG-MOSFET switching speed ...........................................................................133 V.A.7. Conclusion and perspectives...............................................................................134 V.B. MEMS-BASED MEMORY ............................................................................................135 V.B.1. MEMS memory: State of the art .........................................................................135 V.B.2. Mechanical hysteresis-based memory ................................................................136 V.B.3. SG-MOSFET Memory: combining mechanical and electrical hysteresis ..........137 V.B.3.a. Operating principle ......................................................................................137 V.B.3.b. Amplitude of the memory effect ....................................................................139 V.B.3.c. Retention .......................................................................................................140 V.B.3.d. Cycling..........................................................................................................141 V.B.3.e. Effect of drain voltage on pull-in and pull-out effects ..................................142 V.B.3.f. Addressing .....................................................................................................143 V.B.3.g. Scalability .....................................................................................................144 V.B.4. Conclusion and perspectives...............................................................................144 REFERENCES .....................................................................................................................145 CHAPTER VI – SUMMARY AND OUTLOOK ..............................................................147 VI.A. SUMMARY OF MAIN ACHIEVEMENTS ........................................................................148 VI.B. OUTLOOK..................................................................................................................149 APPENDIX ...........................................................................................................................150 ACKNOWLEDGMENTS....................................................................................................151 CURRICULUM VITAE ......................................................................................................153 PUBLICATIONS..................................................................................................................154