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Electromigration
Vacancy
Al
Self Diffusion
Black’s Equation
Lifetime = A (1/J)n e-εa/KT
Where: J = Current Density
n = Current Density Exponent
εa = Thermal Activation Energy
K = Boltzmann’s Constant
T = Absoulte Temperature
Grain Boundary Diffusion
Grain
Grain Boundary
Electromigration - Effect of Temp
Extracting fundamental physical parameters for use in reliability & TCAD models from wafer level results – e.g., bulk and grain diffusion constants.
Bamboo Effect
Log Time
50%
1%
10%
32%
68%
90%
99%Normal
Probability
Narrow Line
Wide Line
Phase Diagrams of Binary Alloy Systems
AI-Cu Phase Diagram
G. Borelius et. al. Handlinger NR 169, 1943
Al/Si Phase Diagram
Al
Al + Si
0.5% Si
450 C
100% Al
Interface Diffusion
Ti/TiNINTERMETALLIC
INTERMETALLIC
Al/Cu(0.5%)
Ti/TiN/Ti
INTERMETALLICS
Ti/TiNINTERMETALLIC
INTERMETALLIC
Al/Cu(0.5%)
Ti/TiN/Ti
Ti/TiN
INTERMETALLIC
Al
Ti/TiN/Ti
INTERMETALLIC
Electromigration Test Structure 1
Electromigration Test Structure 2
T
Lower Average Temperature
Steep Temperature Gradient
Electromigration Lifetime Distribution vs. Log Time
Log Time
50%
1%
10%
32%
68%
90%
99%Normal
Probability
Structure 1
Structure 2
Log-Normal Distribution
• Lifetimes of a Line Follow a Gausian Distribution
• Distributions are a function of Line Length
Accelerated Electromigration
• SWEAT Test - Drives Constant Acceleration Factor - Recommended for Processes with Poor Line Width Control (Wet Etched Metal)
• Isothermal Test - Forces Constant “Temperature” - Recommended for Dry Etched Metal Processes
• Constant Current Test - Most Easily Extrapolated Test Results - Not Recommended for Highly Accelerated Tests Which Have high Joule Heating
Highly Accelerated Tests
• Joule Heating Produces Temperature Gradients
• Temperature Gradients Produce Flux Divergancies
• Flux Divergancies Reduce Test Lifetime
• Black’s Equation Assumes Consistent Temperatures and Current Densities - Can not be applied to Joule Heated Tests
Cu Electromigration
• Higher Recrystalization Temperature
• Intermetallics on Three Sides
• Adhesion to Top Silicon Nitride Layer Not Good (surface migration)
• Line Thickness Effect
• Sidewall Short Issue
Electromigration Sidewall Fracture
I
Expansion of Wide Cu Line Causes Fracture of Sidewall Oxide Allowing Shorts
Expansion Test for Sidewall Strength
• Test Structure: Wide Metal Line Between 2 other Wide Metal Lines
• Ramp up Current Through Line to Heat the Line
• Measure Temperature by the Change in Resistance and the TCR
• Calculate Stress based on Thermal Expansion and Measured Metal Space
Electromigration Interactions
• Line Width and Thickness Impacts Current Density- Photolithography Effects-Etch Effects-CMP Effects
METAL LINE WIDTH MEASUREMENT
Narrow Line
Wide Line
Force Low
Wide Measure Low
Wide Measure High
Wide Force High
Narrow Measure Low Narrow Measure High
Narrow Force High
WIDE AND NARROW LINES ARE EACH ATLEAST 800u LONG
2 Resister Line Width
Cross Bridge Line Width
Joule Heating Effects
• Joule Heating Effects are Generally Seen at About 0.022 degrees C/watt/sq. u of area/u of Oxide Thickness Under the Conductor
• All Conductors Have a TCR (Temperature Coefficient of Resistance)
• Heating Causes Resistance Increase Which Causes Higher Power Dissipation (Constant Current) and Thus More Heating
• Heating Causes Inaccurate and Unstable Resistance Measurements Leading to Inaccurate and Unstable Line Width, Contact Resistance...
Joule Heating Effects
Van der Pauw Cross forSheet Resistivity Measurement
R @ 25 C = 10 ohms
R @ 43 C = 10.82 ohms(assume 0.5%/degree C)8.2% error
Voltage Drop@10uA = 100uV@1mA = 10mV
Temperature @10uA = 25 C@1mA = 43.1 C
Required Voltage Resolution
Voltage Resolution Required to Measure a Metal Line Width with 0.5% Instrument Resolution
0.1
1
10
0.1 0.12 0.14 0.16 0.18 0.2
Line Width (u)
Vo
ltag
e R
es
olu
tio
n (
uV
)
Line Width and Resistivity Measurements on Al Lines
SMU 1 Force I
SMU 2 Measure V1
SMU 3 Measure V2
Length of Test Line Drawn so that V1 - V2 = 100mV
Accuracy of Voltage Measure = 0.04% range +
0.04% Reading + (Rmat X Io) = +/- 870uV
Box Cross Resistivity Requires a Differential Voltage Measurement
Conductor
I in
I out
V2
R = 4.53 (V/I)
Box Cross
Current Limited to prevent Joule Heating of Connecting Lines
V1
V1 - V2 = 7µV
870 µV Inst. Accuracy is Now Very Significant
PHOTOLITHOGRAPHY ISSUES
PHOTORESIST EROSION
FIELD OXIDE
PHOTORESIST
METAL 2
METAL 1
POLY
FIELD OXIDE
METAL 2
METAL 1
POLY
PHOTORESIST
Reflective Notching
REFLECTIVE NOTCHING
LIGHT
MASK
PHOTORESIST
METAL
POLY POLY
CMP Dishing
Wide Lines are Worst Case
Cu Line Width and Resistivity Measurements
Sidewall Barrier Layers Complicate Resistivity and Line Width Measurements
Barrier Metal
Grain Boundary
Cu Cu
CuBarrier Metal
Cu
Cu Grain Size is a Function of Line Width, this impacts Resistivity
CMP Dishing Can Increase Resistivity for Wide Lines
Cu
Sidewall Barrier is thinned for Narrow Lines
Cu Cu
Barrier Metal
Barrier Metal
Barrier Metal
Barrier Metal Barrier Metal
Cu Resistivity Changes with Line Width
A B C D
Al
Cu
Sheet Resistivity
Metal Line Width (µ)
Section A: Barrier Sidewall Thinning Section B: Cu Grain Size Reduction Section C: Increasing Effect of Sidewall Section D: CMP Dishing
0.2 2.0 5.0
Cu Sheet Resistivity Measurement
II
VCu Sheet Resistivity = 4.53 V/I
Measures Cu/Barrier Sheet Resistivity.
Sidewall Barrier Has Negligible Effect.
Dishing Effect Detected with 2 Different Sizes Measurement Noise Levels Required to Measure Sheet Resistivity
of Cu Metal Lines (Nominal 0.02 ohms/square)
ConnectingLine Width
Box SideLength
Max. I forMeasurement
Max.Noise
Min. Box Width (100X BarrierLayer Thickness of 0.05)
1 5 4.77mA 1.1V
Typical Structure 3 15 14.3mA 3.2V
Max. Box Width (Limited by80 Scribe Lane)
7.2 36 34mA 7.5V
Cu Line Width Measurement by Capacitance
A
PolyPoly
Metal 2 Metal 2
A
Scribe Lane Fingered Capacitors of Two Sizes Used to Measure Line Width
Capacitance Noise and Resolution < 2.7fF Required to make this measurement on a typical Scribe Lane Capacitor
Electromigration Process Control
• Highly Accelerated Test to Measure Material Effects (Isothermal with Wide/Narrow Structure)- Grain Size and Texture (Wide Lines) - Intermetallic Growth (Narrow Lines)
• Cu Lines Need Sidewall Strength Test
• Multiple Metal Line and Thickness Measurement Structures to Look for Process Interactions that will Impact Current Density
