Acoustic GHz-Microscopy: Potential, Challenges and Applications

Acoustic GHz-Microscopy: Potential, Challenges and Applications A Joint Development of PVA TePLa Analytical Systems GmbH and Fraunhofer IWM-Halle Dr. Sebastian Brand (Ph.D.) Fraunhofer CAM Fraunhofer Institute for Mechanics of Materials Department of Microelectronics and Microsystems Characterization of Microsystems Acoustic Microscopy Walter-Huelse-Str. 1 06120 Halle/S. Germany ( + 49 (0) 345 5589-193 + 49 (0) 345 5589-101 * Sebastian.Brand@iwmh.fraunhofer.de www www.iwmh.fraunhofer.de

Agenda The Acoustic GHz-Microscope - Features and Requirements - Microscope Hardware and Technical Background Potential and Challenges - Tray & Stitch Mode - V(z) and Time-integrated Mode - Time-resolved Acquisition - Wave Modes and their Use - Challenges Selected Applications Summary and Outlook

The GHz-SAM and the Acoustic High-Resolution Lens A Joint Development of PVA TePLa Analytical Systems GmbH and Fraunhofer IWM-Halle

Parameter Value Resolution > 0.65 µm Frequency Range Penetration Depth Line Frequency 0.4 2 GHz approx. 1.5 λ < 50 Hz Focussing f#: 0.57 1 Refractive Index approx. 7.5 acoustic GHz-lens

The Acoustic GHz-Lens Purpose : - Active element (piezo) excites elastic wave - Plane wave spherically focussed (cavity) - Wavelength and geometry of cavity define shape of sound field - Achievable lateral resolution dependent on sound field SF in 2D lateral cut through SF in focus axial cut through SF

GHz-SAM Technical Background Communication TRIGGER Gate Pulser Unit Motion Controller Receiver Unit PULSE Fast High-Precision Scanner x - STAGE y - STAGE z - STAGE rf-signal video signal RECEIVE SIGNAL sample Control Center

GHz-SAM Technical Background Device developed by and CAM

The GHz - SAM - Scanner mounted on optical microscope - Scan line repetition frequency (LRF) up to 50 Hz - Extremely fast frame acquisition (15s 30s) - Scan range can be defined freely 50 µm 2 mm - Scan resolution can be defined freely down to 100 nm - Combined with precise X-/Y-stage for sample positioning - Automated scan sequencies

Additional Features / Options of the GHz-SAM Alternative excitation Signal by Arbitrary Waveform Generation - Spike excitation - Chirp signals for high axial resolution - Customized broad-band pluses Acquisition of the unprocessed RF-data - Signal analysis using SAMNALYSIS - Extraction / Identification of Wavemodes - Estimation of wave velocities

Potential and Challenges

Tray - Scan - Max. Lateral Range of Scanner : 2 mm @ 30 Hz LRF - Sample re-positioning by precise X/Y- stage - Fully automated scan sequence - Scan field extension by TRAY Scan - Resulting data stitched to form global image

V(z) - Scan - 2D-Scan repeated at multiple z-positions - Max. z-range of Scanner : 2 mm - Advantage of defocus sequence - Acquisition of optimum image - Excitation of SAW - Fully automated scan sequence

Wave Modes in GHz-SAM longitudinal mode transverse mode direction of density/wave component change direction of wave propagation direction of density/wave component change direction of wave propagation Materials that support both modes also support Rayleigh waves Rayleigh Wave: Surface Acoustic Wave with longitudinal and transverse components Animation courtesy of Dr. Dan Russell, Kettering University

GHz-SAM V(x, y, z) Scan Sequence

amplitude [db] frequency [MHz] Use of V(z) Approach V(z) curve at a single frequency V(z) curve at a broad band excitation 10 20 10 0 Δz 30 40 50-10 60-20 70-30 80-40 -7-6.5-6 -5.5 rel. defocus position [µm] 0 z 2(1 cos( )) v sin( R ) v 0 R V 0 = sound velocity of coupling v R = Rayleigh wave velocity θ R = Rayleigh angle R 90-6.8-6.6-6.4-6.2-6 -5.8-5.6 rel. defocus position [µm]

Challenges in Acoustic GHz-Microscopy - acoustic attenuation - penetration depth (lens aperture, focussing) - resolution (wavelength depending on sound velocity) - requires scanning - requires coupling fluid (impedance matching) acoustic attenuation x1; yn x2; yn x3; yn xn; yn x1; y1 x2; y1 x3; y1 xn; y1 2D-scan required

Selected Applications

GHz-SAM Inspection of a microbolometer device 40 µm leg Potential problem area (void?) contact window leg contact window

GHz-SAM Inspection of a microbolometer device microbolometer decive cut - A - transfer bonded thermistor onto read-out-integrated-circuit - Imaging through layer of 2 µm SiO 2 /Si/TiAl/SiO 2 cut - A cut - B - voids 2.5 µm beneath surface cut - B Bonding interface reflector pixel leg thermistor

GHz-SAM Inspection of Induced µ-crack - artificial defects induced by nano-indentation - GHz-SAM inspection in V(x,y,z) mode optical micrograph GHz-SAM micrograph (in focus) GHz-SAM micrograph (de-focussed) 40 µm 40 µm

Defocus Sequence in GHz-SAM V(x,y,z) minor subsurface crack large, obvious crack features way beneath surface (10 µm)

GHz-SAM Inspection of sub-surface µ-crack surface GHz-SAM - micrograph SE- micrograph 50 µm defocussed GHz-SAM - micrograph PFIB - cut 40 µm clear indication for crack defocussed GHz-SAM - micrograph

Void Inspection on TSV s GHz-SAM near surface image V(x, z) of TSV A V(x, z) of TSV B - contrast in defocussed GHz-SAM image correlates With voids found in TSV s - dimensions of TSV larger than theoretical penetration depth - likely interface wave excited between TSV and Si - further research required for clarification sample courtesy by J. Wolf, Fraunhofer ASSID

Transient Simulation of Wave Propagation in GHz-SAM - 1 GHz -> enormous computational effort - Mode conversion, reflection, diffraction, interference - Computation of received acoustic signal @ 1GHz - Identification of individual modes TSV with edge delamination Lens Lens H 2 O Air couplant Lens Si void H 2 O Si Si TSV with void Sound intensity in Si substrate with void

GHz-SAM visualization of sub-surface features µ-bumps BCB (5 µm) Silicon (800 µm) interconnect wiring 20 µm

z [µm] GHz-SAM visualization of sub-surface features 30 µm -30 20 µm -40-50 -60-70 -80-90 20 40 60 80 100 120 140 x [µm]

GHz-SAM in Life Science

GHz-SAM in Life Science Cell Thickness Estimation Cell Thickness Sound Velocity

Summary and Outlook Summary Developed an acoustic GHz microscope fast an precise scanner; stable RF-chain combined with optical microscope successfully employed in first applications Outlook Quantitative parameter estimation (Z, E, G, ν) Signal Analysis for parametric imaging Application specific and Coded Excitation Design of application specific GHz-lenses Optimized Matching Layers improve wave transmission Numerical simulation for investigating the propagation behaviour in specific structures and geometries

Acoustic GHz-Microscopy: Potential, Challenges and Applications A Joint Development of PVA TePLa Analytical Systems GmbH and Fraunhofer IWM-Halle Dr. Sebastian Brand (Ph.D.) Fraunhofer CAM Fraunhofer Institute for Mechanics of Materials Department of Microelectronics and Microsystems Characterization of Microsystems Acoustic Microscopy Walter-Huelse-Str. 1 06120 Halle/S. Germany ( + 49 (0) 345 5589-193 + 49 (0) 345 5589-101 * Sebastian.Brand@iwmh.fraunhofer.de www www.iwmh.fraunhofer.de