Confocal Microscopy of Electronic Devices

James Saczuk

Consumer Optical Electronics

EE594

02/22/2000

Introduction

! Review of confocal principles

! Why is CM used to examine electronics?

! Several methods of CM are used, what are they?

! What kind of data is gathered and what analysis

can be performed on it?

! What is new in the field of CM?

! Conclusions

Confocal Microscopy Basics

! CM is used to produce high resolution, three

dimensional images of specimens of various

thickness.

! How does it work, what resolution can we

achieve?

More

Confocal Microscopy Optics Patent Diagram

Minsky (US Patent 03013467)

Design Rules for Target Resolution

! Confocal lateral resolution is related to the lateral

resolution of the objective lens, which is a

function of a NA, the confocal pinhole size and

the wavelength of the projected light by the

equation:

! As the pinhole size approaches ? , the resolution

improves by a factor of ?2.

Why use C.M. to Characterize Electronics ?

! Non-intrusive, thus non-destructive.

! Provide a method to correlate electrical and

tomographical data.

! Viewing of switching events at the

materials level.

! Establish fault conditions with great

precision for failure analysis.

Methods of Confocal Microscopy

! LASER Scanning

! Fluorescence Mapping

! Single and Multi-Photon Optical Beam Induced

Current Mapping

! Near Field OBIC Imaging

! Future Technologies

LASER Scanning Confocal Microscopy

! LASER sources:

– Ar+ 488/514nm

– Solid State 523nm

– HeNe 633nm

! Source is selected based on the specimen’s

properties to be examined.

! Long pass filter in detector used to protect

operator.

LASER Scanning C.M. Optical Schematic

LASER Scanning C.M. System Diagram

CM Scanning versus Wide Field Microscopy

Simplified Scan of 2D Specimen with S.M.

Data from 3D S.M. Compiled for Viewing

LSCM Image of LEPSi Wafer

· 200x200µm field of view

· Imaged at 488nm, emission above 550nm

Fluorescence Mapping

! Light source is selected to support fluorescence of

the material being observed, and/or the

fluorophore added to the sample.

! The use of ultrafast lasers, optical parametric

oscillators and optical parametric amplifiers

provide for fluorescence imaging without the

addition of dyes, stains or other fluorophores.

! Typically a mode locked Ti:Sapphire LASER is

used at IR wavelengths.

Fluorescence Process

Photons excite the electrons of a

fluorophore to a higher energy level.

As these electrons descend to a low

level they emit photons of a shorter

wavelength than the excitation

Emission spectra and

excitation spectra for

common fluorescence

Fluorescence Mapping Analysis

! Fluorophore Emission Table

LSCM Fluorescence Image of LEPSi Wafer

Light-Emitting Porous

Silicon wafer excited

at 488 nm, emission

above 550 nm

7.5x7.5mm field of

view

Optical Beam Induced Current

! Uses apparatus similar to conventional LSCM

! 1, 2 and 3 photon OBIC

! LASER sources are 633 and 1152nm HeNe

! New techniques use 120fs pulse, 80MHz

repetition rate, tunable, mode locked Ti:Sapphire

lasers (Spectra-Physics, Opal)

! Imaging through the substrate of electronic

devices allows examination for failure analysis

without possible damage from decapsulation.

Optical Beam Induced Current Apparatus

Optical Beam Induced Current Capabilities

! Very low power, 1-10mW @ the focal point

! Excellent response in highly doped

semiconductors where one photon absorption

occurs below the bandgap of Si due to excess

charge carriers.

! Long dwell time of the sample image, ~10µS,

compared to ~1nS for others.

! Out of focus background current eliminated as

compared to the one photon method

Optical Beam Induced Current Samples

Optical Beam Induced Current Samples

Optical Beam Induced Current Samples

Optical Beam Induced Current Samples

Optical Beam Induced Current Samples

Vcc = 5 VDC

Vcc = 0 VDC

Optical Beam Induced Current Samples

Near Field OBIC Imaging

! Use a near field optical “probe” to sense and

excite the specimen

! Tapered optical fibers are used as the probes

! Resolution can approach 10nm

! A tunable Ar+ pumped Ti:Sapphire LASER is

used to provide optical excitation through the fiber

! A scanning microscopy method that has evolved

from LSCM OBIC

Near Field OBIC Imaging

Near Field OBIC Imaging

The Near Future...

! Confocal imaging of transparent objects

! Second harmonic generation

– limited to crystals & structured media

! Third harmonic generation

– all materials generate 3rd harmonic light in the beam

waste when an excitation source (LASER) is tightly

focused at a point in the material

– requires ultrafast pulses with ?>1µm

– 3rd harmonic detection at 400nm for 1.2µm excitation

Third Harmonic Generation

THG in a homogenous sample

shows the beam waste summing

to zero because the 3rd harmonics

on each side of the beam waste are

exactly out of phase

Out of phase beam waste occurs near the interface between

materials with different 3rd order susceptibilities or indices

of refraction. The interference patterns can be mapped to

produce a stereoscopic image of the specimen.

Conclusions

! Confocal microscopy applications are developing

steadily with the availability of cheaper, more user

friendly, ultrafast laser systems.

! The technology and expertise to use these systems

for electronic component characterization and

failure analysis will be in demand as electronics

continue to proliferate in the consumer market.

References

! http://glu.ls.utsa.edu/CLSM/chpt2.html dzf

! http://www.zeiss.com/micro/products/

! http://www.science.uwaterloo.ca/physics/research/confocal/scapp.html#sc

! http://photon.bu.edu/selim/papers/jqe-95/node1.html

! http://www.cs.ubc.ca/spider/ladic/overview.html

! http://www.chips.ibm.com:80/services/asg/capabilities/asweb07.html#obic