Nano probing case studies and ‚advanced’ transistor characterization Peter Egger, Markus Grützner, Christian Hollerith, Sebastien Meziere Infineon AG, Failure Analysis Munich
Outline Motivation and introduction Atomic force probing versus SEM based nano probing Case study SEM based nano probing Standard (SRAM) transistor characterization (case study) Advanced transistor characterization Tunneling current 2nd. order transistor effects Outlook and conclusion
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SEM-based Probing Strengths: Samples with high topography (different layers) can easily be measured Shorts between probes can be excluded by optical inspection Electrical measurements at high or low temp (-20°…+120°C) Probing on fast oxidizing metal lines (Cu, Al) Active voltage contrast Applications: Test structures (drawback: beam shift limitation!) Characterization of complex logic gates Measurements of metal interconnects (open Via?) But: risk of e-beam radiation damage!
19.06.2013
Atomic Force Probing Strengths: No influence of electron beam on transistor characteristics Current imaging gives more detailed information than passive voltage contrast
Applications: Preferred method for measurements on contact level Transistor characterization Inspection for leaky gates / diodes 2nd order transistor parameters Characterization of gate oxides (tunneling current)
19.06.2013
Nano probing using SEM based systems – risk of ebeam radiation damage 1,0E-03
1E-4
1,0E-04 1E-5
0min 10min 20min 5min
1E-6
1,0E-05
SRAM NMOS transitor
I/A
1E-7
1E-8
1,0E-06 1,0E-07 1,0E-08 1,0E-09
1E-9
1,0E-10
1E-10
1,0E-11 1E-11 -0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,0E-12
1,4
-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6
U/V
SEM based system 1kV acceleration voltage
AFM based system
-6
2,0x10
0,0E+00
0,0 -6
-2,0x10
0min 3min 10min 15min
-6
-4,0x10
I/A
-6
-6,0x10
-5,0E-06 -1,0E-05
-6
-8,0x10
SRAM PMOS transitor
-5
-1,0x10
-5
-1,2x10
-5
-1,4x10
-1,5E-05 -2,0E-05
-5
-1,6x10
-1,2
-1,0
-0,8
-0,6
Ugate / V
-0,4
-0,2
0,0
-2,5E-05 -1,40
-1,20
-1,00
-0,80
-0,60
-0,40
-0,20
0,00
Influence of ebeam on device characteristic: increase of I_off on NMOS and decrease I_on on PMOS devices Set date
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2,5E-05
1E-08
2,0E-05
1E-09
1,5E-05 1,0E-05
I/A
I/A
Nano probing using SEM based systems – risk of ebeam damage
Ebeam=500eV
5,0E-06
Ebeam=500eV
1E-10 1E-11
0,0E+00
1E-12 0
20
40 t / min
60
80
0
20
40
60
80
100
t/min
2,5E-05 1E-08
Ebeam=1000eV
1E-09
1,5E-05
I/A
I/A
2,0E-05
1,0E-05
Ebeam=1000eV
5,0E-06
1E-10 1E-11 1E-12 0
0,0E+00 0
20
40 t / min
60
20
40
60
80
100
t / min
80
2,5E-05
1E-08 1E-09
1,5E-05
I/A
I/A
2,0E-05
1,0E-05
1E-11
Ebeam=1500eV
5,0E-06 0,0E+00 0
20
40 t / min
60
Ebeam=1500eV
1E-12 80
0
20
40
60
80
100
t / min
Ion vs. time of PMOS with different beam energies Set date
1E-10
Ioff vs. time of NMOS with different beam energies
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Nano probing using SEM based systems – risk of ebeam radiation damage
2,5E-05
2,5E-05
2,0E-05
2,0E-05
1,5E-05
1,5E-05
Thin oxide
e
I/A
PMOS at 1.5kV with different oxide thicknesses
1,0E-05
1,0E-05
Thick oxide
5,0E-06
5,0E-06 0,0E+00
0,0E+00 0
10
20
30 t / min
40
50
60
0
20
40
60
t / min
Ebeam radiation damage due to electrons unlikely (depth of penetration too low) X-ray might damage transistor (depth of penetration approx. 1µm) but does not perfectly fit to measurement (no damage due to x-ray at 500V assumed) Set date
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Outline Motivation and introduction Atomic force probing versus SEM based nano probing Case study SEM based nano probing Standard (SRAM) transistor characterization (case study) Advanced transistor characterization Tunneling current 2nd. order transistor effects Outlook and conclusion
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Case study SEM based probing - Analogue simulation of failing flip-flop Analogue simulation lead to 3 different hypotheses: Resistive drain contact at Ptrans118 (1) Resistive source contact at Ptrans118 (2) Resistive gate at Ptrans122 (3) (1) TRE
Simulation
9
(3)
Case study SEM based probing – verification of failure hypothesis Sample preparation: Deprocessing to Via1 Isolation of transistors with FIB
Results: – No confirmation of high resistance in drain or source contacts – No irregular transistor characteristics – No other defect
High resistive gate in PMOS is the only possible solution
TEM analysis 10
Outline Motivation and introduction Atomic force probing versus SEM based nano probing Case study SEM based nano probing Standard (SRAM) transistor characterization (case study) Advanced transistor characterization Tunneling current 2nd. order transistor effects Outlook and conclusion
Set date
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Standard (SRAM) transistor characterization Fixed analysis flow for transistor characterization available Nano probing: check for leakages Transfer characteristic of all cell transistors Output characteristic of all cell transistors Additional measurements? Generate physical failure hypothesis? Physical preparation Set date
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Standard (SRAM) transistor characterization Typical (standard) measurement conditions Check for leakages: ‘current imaging’ using +/- 1V and +/0.5V bias NMOS: Vdrain = Vnom, Vgate = -0.5V … Vnom; Vsource = 0V; Vbulk = 0V PMOS: Vdrain = 0V, Vgate = -0.5V … Vnom; Vsource = Vnom; Vbulk = Vnom To avoid artifacts due to contact resistance: measure transistors in both directions (swap definition of source and drain)
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Standard (SRAM) transistor characterization – case study Voltage dependent SRAM single cell fail (fail at Vmin only) Characterization of all 6 SRAM transistors: NMOS access
PMOS pull-up
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Standard (SRAM) transistor characterization – case study NMOS pull down
-> Significant mismatch NMOS pull down But: Can this explain the electrical fail behavior of the SRAM @ Vmin only? How can we generate a reliable physical failure hypothesis (and visualization of the root cause)? Set date
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Standard (SRAM) transistor characterization – root causes for Vt shift GOX – thickness measure tunneling current (if possible) TEM Gate depletion (looks electrically like too thick GOX) surface parallel TEM – check poly grains
Masking implantation (diffusion / Ldd) measure diffusion – well diodes TEM Ionic contamination try anneal Set date
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Masking due to Particle (asymmetric)
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Standard (SRAM) transistor characterization – case study One approach to get answers to the two open questions is simulation and specific nano probing measurements: Explain the fail behavior of the SRAM macro using circuit simulation Requires additional and more precise nano probing results for model generation Reliable failure hypothesis Additional nano probing to exclude possible failure root causes (measure tunneling current and calculate GOX thickness) Parameter extraction for device simulation
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Outline Motivation and introduction Atomic force probing versus SEM based nano probing Case study SEM based nano probing Standard (SRAM) transistor characterization (case study) Advanced transistor characterization 2nd. order transistor effects tunneling current Outlook and conclusion
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Advanced transistor characterization – add. measurements for model generation Output characteristic with different bulk biasing
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Advanced transistor characterization – add. measurements for model generation Transfer characteristic using different bulk biasing
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Add. measurements for reliable physical failure hypothesis – GOX characterization Electrical characterization of the gate oxide thickness of a single transistor Measurement of tunneling current of the MOS capacitor by nano probing (AFP or SEM-based prober) One probe needle on gate contact, one on bulk Measurement in accumulation mode (positive bias for pfet, negative for nfet) Increase voltage from 0V until current is measurable (pA range) Calculate current density for a given voltage (Itunnel / Achannel) Calculate GOX thickness using a device simulation model (e.g. TCAD), roughly "1nm/Vbreak" Good correlation with physical measurement on TEM cross sections Set date
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Add. measurements for reliable physical failure hypothesis – GOX characterization Typical IV-curves (log scale): thin GOX, 2 different channel areas
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Add. measurements for reliable physical failure hypothesis – GOX characterization Pfets with different GOX thickness (green curve = thin)
linear scale
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log scale
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Outlook and conclusion Nano probing is an indispensable tool for probing
