Increased Yield and Reliability of Packaged MEMS Resonator Devices. Carl Arft, Ph.D. Director of Test Dev. Engineering MEPTEC 2011

Transcription

1 Increased Yield and Reliability of Packaged MEMS Resonator Devices Carl Arft, Ph.D. Director of Test Dev. Engineering MEPTEC 2011

2 SiTime Summary $5B Timing Market 3 Segments Resonators, Oscillators, and Clock Generators SiTime's Advantage Siliconizing, Integration More Features, Higher Performance, Lower Cost Category Creator, 85% Share Customers in Production, Pipeline of 5000 >75MU Shipped through Sept 2011 Accelerating Growth 2

3 Applications That Use SiTime Products Storage server Xilinx Reference Design EPON/GPON Gateway Computer server 10G Switch Ethernet Switch Tablet / Ebook Notebook Pico projector SSD Set-top box Base-station MFP Infotainment System VoIP phone DVR / CCTV / IPCam DSC Intercom Automotive blackbox Smart meter Video Phone 3

4 SiTime Technology

5 SiTime s Silicon MEMS Oscillator VDD Charge Pump GND Resonator Sustaining Circuit Fractional-N PLL Output Dividers And Drivers CLK MEMS Die GND Temperature Sensor A/D Digital Temperature Compensation OE STBY CMOS Die OTP Memory I/O 5

6 SiTime s Silicon MEMS Resonator Vacuum sealed in silicon Can be integrated in plastic packages, unlike quartz >50,000 g shock resistance, >70 g vibration resistance 2 FIT reliability (0 MEMS failures) Chip surface with metal traces and bond pads Wafer-level encapsulation SOI MEMS resonator 6

7 A SiTime MEMS Resonator in Action 7

8 ppm offset SiTime Architecture Enables First MEMS TCXO Temperature ( o C) 8

9 MEMS: Considerations for Increased Yield & Reliability

10 Yield (%) Yield (%) What is the Issue? Common myths about MEMS: They re not reliable! You can t get high yield! Performance is not consistent! Yield Trend How to turn this... Yield Trend into this! 10

11 Three Key Considerations for Yield and Reliability of MEMS Devices 1. Robustness of Package/Encapsulation 2. Avoiding Shock or Vibration Damage 3. Avoiding ESD or Over-Voltage Damage Questions to answer for each Why is it an issue for MEMS devices? How can we avoid/mitigate the issue? 11

12 Three Key Considerations for Yield and Reliability of MEMS Devices 1. Robustness of Package/Encapsulation 12

13 Packaging/Encapsulation Is Key For MEMS Ensure zero contamination Small (<1 µm) gaps must remain clean Conductive contamination can cause shorts Ensure zero gas exchange Air damping decreases Q Frequency depends upon mass of resonator Small mass changes or leaks will shift resonant frequency Ensure robustness Dicing/molding/wirebond, etc. Robust against chemicals/solvents All this at the lowest possible cost! 13

14 One Method: Wafer-Level Packaging using Glass Frit Sealing is done after MEMS fabrication using silicon cap wafer Frit = low melting point glass paste Some drawbacks: Added process step + cap wafer + sealing material Added expense Need a getter to absorb gases Reliability concerns 14

15 SiTime s Solution: EpiSeal TM Encapsulation The package is created during the wafer fabrication process! Chip surface with metal traces and bond pads Wafer-level encapsulation SOI MEMS resonator 15

16 SiTime s EpiSeal TM Process How does the process work? Play Advantages Standard CMOS processes and materials High temperature in-process encapsulation to protect MEMS structure Standard IC backend (packaging and test) High-stability vacuum cavity 16

17 EpiSeal TM Allows Low-Cost QFN Packaging Encapsulated MEMS die, stacked and wirebonded Inexpensive plastic QFN package 17

18 Three Key Considerations for Yield and Reliability of MEMS Devices 2. Avoiding Shock or Vibration Damage 18

19 Deflection Shock/Vibration in MEMS Devices Any mechanical MEMS device subject to resonant modes Not limited to resonators Example: TI DLP mirror Excitation near resonant peaks may cause deflection leading to Stiction Breakage Impact damage Impact issue worsens as gap widths decrease MEMS pushing toward smaller gaps to decrease actuation voltages Frequency 19

20 What is Stiction? Adjacent structures stick after making contact Occurs when surface adhesion forces are higher than the mechanical restoring force of the micro-structure Free-standing beams Beams exhibiting vertical & horizontal stiction) 20

21 Shock/Vibration: Possible Sources Wafer Fab Ultrasonic/Megasonic cleaning Back End Wafer backgrind Die attach Wirebond Mold Saw/singulation Pick & place Test Bowl feeders Hard stops along handler track 21

22 Shock/Vibration: Ultra/Megasonic Cleaning Definitions: Ultrasonic cleaning: <100 khz Megasonic cleaning: ~1MHz Industrial wafer cleaning stations may use ultra/megasonic cleaning In addition to the fundamental frequency, harmonics are generated which can excite resonant modes of a MEMS structure Small bench unit Example spectrum of 40 khz ultrasonic bath Industrial wafer cleaning equipment 22

23 Shock/Vibration Mitigation Avoidance Line audits to determine sources of shock/vibration Wafer fab Back end Test Process changes to avoid shock/vibration Different or modified handling equipment Modified process to avoid ultrasound, etc... Altered recipes to change frequencies or intensities (wirebond type, for example) Design for robustness Simulation & design (e.g. FEM) to avoid certain vibrational modes Include all critical structures that may resonate Design-in features to mitigate shock/vibration Increase gaps or spacing in critical areas Design springs or stops for controlled impact (think bumpers on leaf springs) Surface treatments or surface topology to avoid stiction 23

24 Three Key Considerations for Yield and Reliability of MEMS Devices 3. Avoiding ESD or Over-Voltage Damage 24

25 ESD or Over-Voltage in MEMS Devices The usual CMOS-type faults due to ESD or over-voltage Blown metal traces Ruptured passivation Etc... MEMS-specific failures Pull-in (or Snap-down ) Silicon welding Susceptibility increases as gaps decrease

26 Pull-in Due to Over-Voltage/ESD An attractive force is generated when a voltage exists between two conductors Model this as two parallel plates, with one plate attached to a spring V Model as spring/plates V d At equilibrium, net force F =

27 Displacement, x Pull-in Due to Over-Voltage/ESD d d Actuation Voltage, V V p Past the pull-in voltage (V p ), the plates snap together Results in high current flow (if no insulator) After pull-in, structure may stick due to Stiction Welding (melting silicon!) 27

28 Possible Sources of ESD/Over-Voltage Wafer Fab Back End ESD susceptibility at every step where electrical pads are exposed! Test 28

29 ESD/Over-Voltage Mitigation Avoidance Follow proper ESD protocol throughout process Heel straps, lab coats, gloves Wafer wands / tweezers ESD ionizers Etc... Line audits to determine areas with poor ESD control Proper power-up/down procedures during test Design for robustness ESD protection on I/O Design-in features to mitigate over-voltage Increase gaps or spacing in critical areas Design insulated stops Surface treatments or topology designed to avoid stiction 29

30 Three Key Considerations for Yield and Reliability of MEMS Devices 1. Robustness of Package/Encapsulation 2. Avoiding Shock or Vibration Damage 3. Avoiding ESD or Over-Voltage Damage Questions to answer for each Why is it an issue for MEMS devices? How have we worked to avoid/mitigate the issue? 30

31 Thank You! - Questions?