Electrochemical Aspects of Copper Chemical Mechanical Planarization (CMP)Esta Abelev, D. Starosvetsky and Y. Ein-Eli.

Introduction:Copper is used as a replacement of aluminum in integrated circuit interconnections. The advantages of copper interconnectors are based on two important properties of copper; higher electric conductivity and

stronger electromigration resistance.

Copper Metallization Technology:(I) Etching trenches and vias in ILD or low-k dielectric.

(II) Deposition of diffusion barrier layer.

ILD (b) Si

(a) Si

ILD

(III) Copper deposition: Electroplating or Electroless.

(c) ILD

Si

(IV) Global Planarization of the surface.

ILD (d) Si

Research objectives: To study and understand the electrochemical behavior and compatibility of copper CMP slurry solutions. Results:Ammonium hydroxide (NH4OH)

ConcentrationNH4OH

Ecorr

VSCE

Icorr

mA/cm2

Corrosion Rate

nm/min

2.35 g/l 0.315 29.76 1.313

30 g/l 0.509 51.93 2.29

2.35 g/l NH3 30 g/l NH3

1 minIn solution

60 minIn solutionActive Copper Dissolution

Nitric Acid (HNO3)

Concentration pH Ecorr IcorrCorrosion Rate

%wt HNO3 VSCE mA/cm2 mm/min

0.2 1.78 0.02 0.604 13.3

1 1.19 0.04 1.658 36.6

3 0.9 0.052 4.468 100.45

Active Copper Dissolution

Nitric Acid (HNO3) and Inhibitor (benzotriazole)

N

N

N

Cu

N

N

N

Cu

Cu

10-7 10-6 10-5 10-4 10-3 10-2

0,0

0,1

0,2

3 wt% HNO3

3 wt% HNO3 + 0.02M BTA

neat 3% wt HNO3

3% wt HNO3 + 0.02M BTA upon immersion

3% wt HNO3 + 0.02M BTA after 1hr in solution

B D F

Pote

ntia

l ( V

SCE )

Current ( A/cm2 )

With Inhibitor (BTA) Without Inhibitor (BTA)

Hydrogen Peroxide (H2O2)

Hydrogen Peroxide (H2O2) and Inhibitor (benzotriazole)

a) b)

10-6 10-4 10-2

0.4

0.6

0.8 cba

3% wt H2O

2No Pretreatment

Scan Rate 5mV/s: 3% wt H

2O

2 3% wt H

2O

2 + Buffer pH 4

3% wt H2O

2 + Buffer pH 4 + 10g/l Na

2SO

4

Pote

ntia

l (V

SCE)

Current (A/cm2)

B B B

Figure 6: Anodic potentiodynamic curves (Scan rate of 1 mV/s) of copper immersed in 3 vol% peroxide Solutions with and without the addition of buffer and Na2(SO4) additives: (a) without additives; (b) with 5 ml addition of buffer (pH 4); (c) with buffer and 10 g/l Na2SO4 (pH 4).

10-8 10-7 10-6 10-5 10-4 10-3 10-2

0.0

0.2

0.4

0.6

0.81 mV/s

Addition 0.01M BTA plus 3 vol% H2O2

Addition 0.01M BTA

10g/l Na2SO4 (H2SO4 drop) pH 4.2

Cu, Scan rate 1 mV/s, upon immersion

Pote

ntia

l (V

SCE)

Current (A/cm2)

B D F

10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2

0.0

0.2

0.4

0.6

1 mV/s

files:CV_x(V)

Cu, 10g/l Na2SO

4 (H

2SO

4 drop) + 0.01M BTApH 4.3

Upon immersion, scan rate 1 mV/sDifferent reverse potentials

Reverse potential: 0.7V 0.1V 0.2V 0.35V 0.4V 0.5V

Pote

ntia

l (V

SCE)

Current (A/cm2)

B D01 L02 P035 R04 B

0 1000 2000 3000 4000

0.0

0.1

0.2

0.3

0.4

0.5

Cu, 10g/l Na2SO

4 (H

2SO

4 drop) pH 4.3

addition of H2O

2

Ecor

r (V

SCE)

Time (sec)

B

Planarization is an important technological step in copper metallization. This research work is focused on problems associated with copper planarization technique-Chemical Mechanical Planarization (CMP).

Conclusions

• All the proposed slurries (NH4OH, HNO3 and H2O2) do not provide the conditions required for conventional CMP:

Copper is actively dissolved with a relatively high dissolution rate.

• The active dissolution of Cu proceeds non-uniformly, with deep intergranular penetration. This may lead to a damage of the thin Cu layer, resulting in severe dents and fractures in the copper interconnects.

• Copper protection with the use of inhibitors is not effective for CMP processes, [which continue only for a period of 2 minutes], under rapid surface abrading.

• The use of oxidizers such as peroxide is not effective in conjugation with inhibitors.

-0.6

-0.4

-0.2

0.0

0.2

0.4

10-5 10-4 10-3

30 g/l NH3

2.35 g/l NH3

1 mV/s

2.35 g/l NH3

30 g/l NH3

No pretreatment Upon immertionScan rate 1 mV/s

Current (A/cm2)

Pote

ntia

l (V

SCE)

E C A C

0 1000 2000 3000-0.6

-0.5

-0.4

-0.3

30 g/l NH3

2.35 g/l NH3 2.35 g/l NH

3 30 g/l NH

3

Ecor

r (V

SCE)

Exposure Time (sec)

B B

Potential 0.2V

500 550 600 650 700 7500

10

20

30

40

50

Curre

nt (m

A/c

m2 )

Time (sec)

B

Addison of BTA

Potential 0.2V

500 550 600 650 700 7500

10

20

30

40

50

Curre

nt (m

A/c

m2 )

Time (sec)

B

Addison of BTA

500 550 600 650 700 7500

10

20

30

40

50

Curre

nt (m

A/c

m2 )

Time (sec)

B

Addison of BTAAddition of BTA

Potential 0.1VPotential 0.1V

Addition of BTA

Corrosion & Applied Electrochemistry Laboratory (CAEL)Department of Materials Engineering, Technion, Haifa 32000, Israel.

Figure 1: a) Corrosion potential transient of copper in 2.35 g/l (●) and 30 g/l NH3 g/l (○) solutions at 25 °C, b) Polarization curves of copper electrodes obtained in 2.35 g/l (●) and 30 g/l (○) NH3 at scan rate of 1 mV/s.

Figure 2: a), b) SEM micrographs obtained after one hour exposure at OCP in 3 vol.% nitric acid solution.

a) b)

a) b)

Figure 3: a) Anodic potentiodynamic curves (scan rate 1 mV/sec) of copper in 3 vol.% nitric acid without (●) and with (○) 0.02 M BTA,b) Anodic current transient of copper measured in 3 vol.% nitric acid containing 0.02 M BTA (at applied voltage of 0.1 V).

10-7 10-6 10-5

0.3

0.4

0.5

0.6

15 vol.%

3

1

H2O

2 concentration : 1% wt 3% wt 15% wt

B D F

Pote

ntia

l (V

SCE)

Current (A/cm2)Figure 4: Anodic potentiodynamic curves of copper obtained immediately upon immersion in 1, 3, and 15 vol % peroxide solutions at a scan rate of 1 mV/s.

Figure 5: Two fragments of copper surface after one hour exposure at the OCP in 3 vol.% peroxide solution.

Figure 8: Potentiodynamic profiles (scan rate of 1 mV/s) of copper electrode immersed in three solutions; [a] solutions of Na2SO4 peroxide-free; [b] Na2SO4 with the addition of 0.01M BTA; [c] Na2SO4 solution containing both BTA (0.01M) and peroxide 3% (vol).

Figure 7: Corrosion potential transient of copper in 10 g/l Na2SO4 and 0.01M BTA solution with addition of 3 vol.% H2O2.

ab

c

Figure 10: Potentiodynamic profiles (scan rate of 1 mV/s) of copper electrode immersed in solution containing Na2SO4 and 0.01M BTA. Copper electrode potential was swept back at potentials ranging between 0.1-0.7 V.

Active Copper Dissolution

(a) 0.1V 5min Polished

(b) (c)

(d) (e)

0.3V 5min 0.3V 5min

0.4V 5min 0.4V 5min