Trench Effects in Copper Electroplating

Student: Oleg Gurinovich Industry: Novellus SystemsAdvisor: Dr. Stacy Gleixner John Kelly, Jim Stimmell, Chiu ChiInstructor: Dr. Richard Chung TFI: Bill Gadson

Mate 198B Spring 2001

Research Objectives & Motivations

• Study variations in critical dimensions (CDs) of trenches during damascene process (lithography and etching steps)

• Study how variations in CDs affect electroplating performance by measuring electrical resistance of copper interconnects

• More companies switch to copper as a material choice for interconnects

• Strong correlation between to be filled feature size and electroplating bath chemistry

Why Use Copper and Electroplating?

• Interconnect lines density increase

• Need for more conductive and reliable material

• Relatively low cost of the process

• High speed of deposition

• High quality of the deposited film for well-optimized process

• Electroplated copper has good electromigration characteristics

Steps in Damascene Process

• Lithography

• Etching

• Diffusion Barrier Deposition (Ta, TiN, TaN)

• Seed Layer Deposition (Cu)

• Electroplating

• Chemical-mechanical Polishing

Electroplating Scheme

CuSO4+H

2O+H

2SO

4 + additives

Cathode (Wafer):

Cu2+ + e- = Cu+

Cu+ + e- = Cu0

Anode:

4OH-=O2+ 2H

2O + 4e-

2H 2O=O

2+ 2H+ +2e-

Trench Cross Section

Electroplating Bath Additives

Brighteners

• accelerate Cu deposition at sites where present 4,5-dithiaoctane-1,8-disulphonic acid (SCH

2CH

2CH

2SO

3H)

2

Levelers

• adsorb at high-charge density sites organic compounds containing nitrogen Suppressing agents

• create uniform diffusion layer Example: polyethylene glycol, (C

2H

4O)

n

Trench Fillings

Trenches with voids

Experimental Procedure

• Measure width of 0.25 m trenches after lithography and etch steps to find variance in the trench width introduced by these two processes

• Measure electrical resistance of copper interconnects to find how actual trench size variations affect electroplated copper characteristics

Trench Width MeasurementsEquipment used: Top-down SEM, Hitachi-S6780

• Precision measurements were performed -random trench was selected -same trench was measured 25 times -SD was found 0.00273 m

Lithography defects

Normal trenches Underdeveloped trenches Overdeveloped trenches

Measurements Map

After-lithography measurements After-etch measurements

Trench width distribution, lithography

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0 2 4 6 8 10 12 14 16 1820 22 24 26 28 3032 34 36

Measurement point

Tre

nch

wid

th,

mic

ron

s

Series1

Trench width distribution, etching

0.15

0.16

0.17

0.18

0.19

0.2

0.21

0.22

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Measurement point

Tre

nch

wid

th,

mic

ron

s

Series1

Measurements Data

Width is distributed around mean value of 0.257 mNo significant trend

Trend is observed, central dies on the wafer have wider trenchesafter etching step

After-etch Measurements MapFour 8-inch wafers were measured

Trench Width After Etch Step

0.2380.240.2420.2440.2460.2480.250.2520.2540.256

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Measurement point

Tren

ch w

idth

, mic

rons

W1

W2

W3

W4

After-etching measurements

Discussions

• Lithography step doesn’t introduce special cause variation -data distribution does not show significant trend -mean value 0.257 m, SD is 0.0114 m

• Etching step does have special cause variation

-central area of the wafer is etched more than edges -account for this effect during electrical resistance measurements -mean trench width after etching 0.2454 m, SD is 0.00574

• Need to be aware of lithography defects while analyzing resistance data

Conclusions

• Etch step may affect electrical resistance of copper interconnects

• Based on the literature research, bath additives concentrations will affect trench fill capability

• Lithography step do not affect overall trench width distribution

• Measuring procedure (map) is developed

Future Work

• Measure electrical resistance of copper interconnects

• Anneal wafers at different temperatures after electroplating to study effect of annealing on copper grain “re-growth”

• Study effects of electroplating bath chemistry on copper interconnects electrical characteristics