Rapid SAW Sensor Development Tools

Transcription

1 Rapid SAW Sensor Development Tools W. (Cy) Wilson NASA Langley Research Center G. M. Atkinson Virginia Commonwealth University

2 Outline Motivation Introduction to Surface Acoustic Wave Devices Approach Impulse Response Method Modeling and Analysis Tool Integration into existing EDA Tool Automated Layout Generation Tool Prototype Results Conclusions

3 Non-Destructive Evaluation (NDE) of Aerospace Vehicles Motivation Exterior Inspection in space Exterior Inspection by hovering (Terrestrial) Interior Inspection by walking (Terrestrial) Interior SHM Wireless Sensors

4 Surface Acoustic Wave (SAW) Devices Inexpensive Small (low mass and low volume) Extremely Low power (RF or Ambient) Versatile Signal Processing Resonators Filters Strain sensors Pressure sensors Chemical sensors Ultrasonic sensors Temperature sensors RF Antenna RF Power & Comm. SAW Device Sensing Area Piezoelectric Substrate Wireless SAW Device. Reflector

5 Approach SAW Modeling Tools Layout Editor Automatic Layout Generation Fabrication 3D Modeling Finite Element Analysis New Tools Existing Tools

6 Impulse Response Model The SAW device was modeled using the Impulse Response method 1. This is a first order model only and does not take into account second order effects such as reflections, and Triple Transit Echoes (TTE). Simulates the mechanical, piezoelectric and electrical behavior. The model calculates the Frequency response Loss of the system Admittance & Conductance Electrical parameters such as the matching inductor. This model is valid only for transducers where at least one of the two IDTs is un-weighted. Assumes a constant metallization ratio of 0.5, and uniform finger overlap or aperture. [1] Hartmann, C.S., Jr.; Bell, D.T.; Rosenfeld, R.C.; Impulse Model Design of Acoustic Surface-Wave Filters, Microwave Theory and Techniques, IEEE Transactions on, Volume 21, Issue 4, Apr 1973, pp

7 Circuit model used in Impulse Response Modeling IDT IDT Z S G G a (f) C B a (f) a (f) V S V 1 T B a (f) C T V 2 Z L Uses the Mason equivalent circuit model. Based on Crossed field model. C T is the total capacitance for an IDT. B a (f) is the acoustic susceptance. G a (f) is the radiation conductance.

8 Example Parameters IDT Surface Acoustic Wave IDT Basic SAW Delay line. Quartz Piezoelectric Substrate SAW delay line that consists of two identical IDTs. The synchronous frequency is MHz. The substrate is ST cut Quartz. Cs = pf/cm (capacitance per finger pair, per cm) k = 0.04 (piezoelectric coefficient) ν = 3158 m/s (acoustic velocity) NBW = 1.5 MHz (null bandwidth, or fractional bandwidth) The delay length between the two IDTs is 5 wavelengths. The source and load resistances are assumed to be 50 Ω.

9 Parameter Optimization Optimal number of finger pairs Zf () Aperture width optimization := 1 G a () f + j ( 2 π f C T + B a () f ) N p := round 2 NBW f H a := R in f 0 2C s N p 4k 2 N p 4k 2 N p 2 + π 2 N p = 70 H a = μm

10 Conductance and Susceptance f f 0 xf ():= N p π f 0 G a () f 8k sin( x() f ) := C s H a f 0 N p B xf () a () f := G a f 0 G n () f 1 G a () f := B G a f n () f := 0 ( ) 0.7 ( ) B a () f ( ) G a f 0 ( sin( 2 x() f )) 2xf () 2xf () Gn(f) 0.5 Gn(f) Frequency (MHz) Radiation conductance of the SAW delay line. Frequency (MHz) Acoustic susceptance of the SAW delay line.

11 IL() f := 10 log Insertion Loss 2G a () f R g ( G a () f R g ) 2 + R g 2 π 2 ( f C T + B a () f ) IL(f) (db) Frequency (MHz) Insertion Loss of the SAW delay line.

12 Frequency Response Loss (db) xf ():= N p π f f 0 f 0 H n () f 20 log 4 k 2 2 := C s H a Z r f 0 N p Frequency (MHz) Z r := v λ sin( x() f ) xf () H a 2 ρ e ( ) j N p + 5 f 0 s

13 MathCad Worksheet for modeling an IDT v λ := λ = 60 μ m f Wavelength 0 λ W f := W 4 f = 15 μm Electrode Finger Width 2 N p round NBW f := 0 N p = 70 Number of Finger pairs k 2 N p H a := R in f 0 2 C s N p 4 k 2 2 H a = 2399 μm Finger Aperture N p + π 2 H t := H a + H b H t = μm Finger Height Total f f 0 x() f := if f f 0, 0, N p π f 0 G a () f 8 k sin ( x() f ) := C s H a f 0 N p Radiation Conductance x() f C T := N p C s H a Total IDT Capacitance ( sin ( 2 x() f )) 2 x() f B a () f := G a ( f 0 ) 2 x() f 2 Acoustic Susceptance j N p + 5 H n () f := 20 log 4 k 2 C s H a Z r f 0 2 N p sin ( x() f ) x() f 2 e ( ) f 0 s Frequency Response

14 Automated Report <<< SAW Delay Line Summary Report >>> Finger Height = e-003 m Aperture Height = e-003 m Finger Width/Spacing = e-005 m Number of Finger Pairs = e+001 Bus Bar Height = e-004 m Delay Length (in wavelengths) = e+000 X Offset = e+000 m Y Offset = e+000 m Wavelength = e-005 m Synchronous Frequency = e+007 Hz Acoustic Velocity = e+003 m/s 2 Series Matching Inductor = e-006 H Minimum Insertion Loss (@f0) = e+001 db Total Capacitance (single IDT) = e-012 F

15 Integration of the models into existing EDA Tools Interface to existing layout tools. Integrate MathCad models with layout software. Parameterizable libraries of SAW devices using scripting language Tcl. We integrated the tools into commercial Electronic Design Automation (EDA) Layout software for MEMS devices. The tools allow us to rapidly create, model, analyze and automatically generate device layouts (fabrication netlist). Enables a higher level of abstraction in the design process thus reducing the development time while increasing productivity. Allows access to existing tools 3D Modeling Finite Element Analysis (FEA)

16 Parameterizable Library Layout Generator dialogue box. This dialogue box is where the parameters for the basic SAW delay line are input. Layout of a basic SAW delay line. - Finger heights of 1000 µm. - Finger widths of 15 µm finger pairs - aperture of 980 µm. - Bus bar heights are 50 µm. - Delay between the two IDTs is 7 λ.

17 Example Tcl code for a simple IDT structure proc Basic_IDT { obj sname llayer dfinger_l dfinger_o dfinger_w inum_fingers dbus_bar_h} { global error_count set Length [expr ($inum_fingers*4*$dfinger_w-$dfinger_w)] set Gap [expr ($dfinger_l-$dfinger_o)] # Top comb fingers #===================== # Finger origin of the first finger set FOx [expr 0] # Loops over all movable fingers for {set i 1} {$i <= $inum_fingers} {incr i} { # Draws the movable comb finger set finger [cat:rectangle -layer $llayer $FOx [expr ($Gap+$dbus_bar_h)] [expr ($FOx+$dfinger_W)] [expr ($Gap+$dbus_bar_h+$dfinger_L)]] $obj addobject $finger # Next finger origin set FOx [expr $FOx + 4*$dfinger_W] } # Bottom comb fingers #=================== # Finger origin of the first finger set FOx [expr (2*$dfinger_W)] # Loops over all fixed fingers for {set i 1} {$i <= [expr $inum_fingers]} {incr i} { # Draws the fixed comb finger set rect [cat:rectangle -layer $llayer $FOx $dbus_bar_h [expr $FOx+$dfinger_W] [expr ($dbus_bar_h+$dfinger_l)] ] $obj addobject $rect # Next finger origin set FOx [expr $FOx + 4*$dfinger_W] } # Top Electrode #================ set rect [cat:rectangle -layer $llayer 0 [expr ($dbus_bar_h+$gap+$dfinger_l)] $Length [expr ((2*$dbus_bar_h)+$Gap+$dfinger_L)] ] $obj addobject $rect # Bottom electrode #================= set rect [cat:rectangle -layer $llayer 0 0 $Length $dbus_bar_h] $obj addobject $rect return }

18 IDT Layout from Tcl Automatically generated layout of the SAW delay line from the example.

19 3-D Modeling Exaggerated 3-D Model of a SAW delay line. The model has been meshed using Manhattan bricks.

20 Finite Element Analysis available from existing tools. FEA Analysis Potential Displacement in X 8 Strain YY Principal Stress 1

21 FEA Analysis

22 Device Results Measured Calculated 40 Hn (db) Frequency (MHz) Frequency response of the SAW delay line device.

23 Conclusion Developed models for a SAW delay line. First order models only which use the Impulse Response Method. Integrated the Models into existing EDA tools. Parameterizable Library components using Tcl scripting language. Automatic Layout Generation w/annotation. Future work will include second order effects such as reflections, triple transit echoes, and temperature effects.

24 Auxiliary Slides

25 Example VHDL-AMS code for modeling an IDT library IEEE; use IEEE.electrical_systems.all; library ieee; use ieee.math_real.all; use ieee.math_complex.all; use work.saw_pack1.all; entity Freq_Response2 is generic (f01,np1,cs4,k4:real); port ( terminal input1 : electrical; terminal output1 : electrical ); end entity Freq_Response2; architecture behavior of Freq_Response2 is quantity vin across input1 to electrical_ref; quantity vout across iout through output1 to electrical_ref; begin vout == 20.0*log10(abs((hh((vin),f01,Np1,Cs4,k4)**2.0)/hh(f01,f01,Np1,Cs4,k4)**2.0)); end architecture behavior;

26 2 nd Order Model λ 0 λ Metal Finger Piezoelectric Substrate λ 8 θ θ u θ m θ u m jz u tan jz u tan jz m tan jz m tan I 1 θu jz u tan 2 θu jz u tan 2 I 2 jz csc( θ ) u u jz m csc( θ ) m jz u csc( θ ) u V 1 V 2 1:1 1:1 1:1 C 0 I 3 V 3

27 Rapid Integrated Prototyping of SAW Sensors (RIPSAWS )