PUBLIC

CONTROLLED COBALT RECESS FOR ADVANCED

INTERCONNECT METALLIZATION.

Antoine Paccoa*, Y. Akanishib, Q.T. Lea, E. Kestersa, G. Murdocha, F. Holsteynsa

A IMEC VZW, KAPELDREEF 75, 3001 LEUVEN, BELGIUMBSCREEN, SEMICONDUCTOR SOLUTIONS CO., LTD., 480-1, TAKAMIYA-CHO, HIKONE, SHIGA 522-0292, JAPAN

*EMAIL: ANTOINE.PACCO@IMEC.BE

OVERVIEW

copper cobalt transition

challenges for metal recess & specifically for cobalt recess

specifications for wet recess

approach & results: Co recess by digital wet etch

effect of different oxidizing solutions

effect of oxide dissolution step & rinse step times

effect of the ambient

wet recess of cobalt for FSAV

Electrical testing

Morphological results

conclusions

2

WHEN & WHY COBALT OFFERS AN ADVANTAGE OVER COPPER

AS AN INTERCONNECT METAL

1. The electron mean free path of cobalt is

considerably lower than copper, reducing

surface scattering in small trenches

lower line resistivities compared to

copper.

2. A thinner combined barrier-nucleation

layer can be used for cobalt lower

cross-sectional resistance.

3. Cobalt can be annealed at reasonable

thermal budgets thereby enabling grain

growth which reduces resistivity.

3

Ref.: Applied Materials Internal Resistivity

Benchmark

WHEN & WHY COBALT OFFERS AN ADVANTAGE OVER COPPER

AS AN INTERCONNECT METAL

1. The electron mean free path of cobalt is

considerably lower than copper, reducing

surface scattering in small trenches

lower line resistivities compared to

copper.

2. A thinner combined barrier-nucleation

layer can be used for cobalt lower

cross-sectional resistance.

3. Cobalt can be annealed at reasonable

thermal budgets thereby enabling grain

growth which reduces resistivity.

4

will cobalt climb up the interconnect stack?

taking place....

COBALT RECESS FOR FSAV (FULLY SELF ALIGNED VIA)

M1 recess M2V1 FillM2V1 Etching

Target = 10nm

metal recess

Topography for SAV

Via confined in y-direction by M2 HM

Via-first etch lands selectively on SiCN barrier

Topography is maintained by selective removal

of SiCN

FSAV is formed

Min-distance M1-V1

is maintained by

topography

5

M2

V1

M1

CHALLENGES FOR METAL RECESS

6

NON-IDEAL (NON-UNIFORM RECESS) CASES

wet metal recess can be affected by:

1. Crystal orientations

• different crystallographic planes can have

different etching rates (anisotropy)

2. Crystal grain boundaries

• preferential etching can occur at grain

boundaries

• smaller grain size grain boundary density

increases

3. Galvanic corrosion

• Local increase in oxidation / etch rate due to

galvanic contact of the fill metal with the liner/

barrier metals

6

crys

tal

grai

n b

oundar

y

CHALLENGES FOR METAL RECESS: COBALT

7

• non-uniform (crystal-grain/boundary dependent) etch

• uncontrolled dissolution of cobalt during slightly acidic / neutral rinse alkaline solutions, NH4OH/H2O2 preferred (*,**)

• selective removal of TiN liner/barrier after or simultaneously during Co recess

SPECIFICATIONS: • control of the recess amount within the nanometer range: 10 +/- 1 nm

• small within-line roughness (Rq ~ 1nm)

• uniform within-wafer recess (center-to-edge NU < 1nm)

• avoid pattern-density-depending etching (CD-independent)

cyclic process (digital etch) = process of choice

* SPCC ’18 Y. Ogawa, [Kurita]

** SPCC ‘19 H. Iino, [Kurita]

APPROACH FOR CONTROLLED RECESS

8

“DIGITAL ETCHING”: 0-1-0-1-0-1-0-1-....

Potential benefits of this approach:

• recess amount can be controlled within the nanometer range

• uniform within-wafer recess (single wafer processing)

• uniform within-structures recess = non pattern-density-depending etching

Co Co CoCo

CoOx

initial after X cycles“0” oxidation “1” oxide dissolution

X cycles

0 = self-limiting metal-oxide growth on the metal surface

1 = metal-oxide removal selective towards metal surface

EFFECT OF DIFFERENT OXIDIZING SOLUTIONS

9

FOR THE SAME OXIDE DISSOLUTION STEP: 10S HF 0.05%

• increasing the peroxide concentration in

both slightly acidic H2O2/H2O and alkaline

NH4OH/H2O2/H2O solutions does not

significantly affect the total cobalt loss

• removing the oxidizing agent, being it H2O2

or atmospheric O2 does reduce the total

cobalt loss.

• self-limiting oxide film formation when a

sufficiently oxidizing solution is used (H2O2).

# oxidizing

solutions

0

0.5

1

1.5

2

2.5

ref CMP SC1/HF H2O2/HF DIW/HF

Ra

(nm

)COBALT ROUGHNESS AFTER DIFFERENT OXIDIZING SOLUTIONS

10

AFM (ECD COBALT)

ref CMP SC1/HF

H2O2/HF DIW/HF

recess depth approx. 10nm

• Slight (<1nm) roughness increase after recess.

• No significant difference between different oxidizing solutions.

# oxidizing solutions

IMPACT OF DISSOLVED [dO2] IN DILUTED HF (0.05%)

11

EXAGGERATED TIMES & AGGRAVATED CONDITIONS ...

WHAT ABOUT THE IMPACT OF [O2, ATM] IN THE PROCESS AMBIENT ?

+ high roughness

non controllable

recess

EFFECT OF OXIDE DISSOLUTION STEP & RINSE STEP TIMES

12

STD(NON-CONTROLLED) VS. LOW [O2] AMBIENT

• DIW time has a small impact on cobalt loss.

• Increasing the HF time has a more noticeable

impact on the total cobalt loss.

• The parameter that has highest impact is the O2

content in the ambient.

TIN LINER REMOVAL

13

TWO APPROACHES: 1-STEP VS. 2-STEPS

TiN removal most effective

by using a final 60°C APM

1-STEP: combined TiN liner removal & Co

recess etch

2-STEPS: Co recess etch followed by

TiN liner removal

(APM RT /HF) x4 + 30s APM 40°C

(APM RT /HF) x4 + 40s APM 60°C

(APM 40°C /HF) x4

(APM 60°C /HF) x4

ELECTRICAL TESTING

14

TARGET: 10 NM CO RECESS FOR FSAV

Line resistance of 10 µm long lines

21 nm 30 nm CD

Process: “digital” Co recess etch [APM followed by HF] x times

+ TiN barrier etch APM 60°C

Uniform distributions

> 95% yield for smallest CD

21 nm

Electrically active lines

CMP

RECESS

ELECTRICAL TESTING

15

TARGET: 10 NM CO RECESS FOR FSAV

Line resistance of 10 µm long lines

21 nm 30 nm CD

Data follows the expected trend: line resistance increase for

smaller lines due to line X-section reduction (R ~ 1/A)

Limited pattern loading

Measured vs. predicted line resistance before and after 10 nm Co recess

60 nm 50 nm

Co

MORPHOLOGICAL ANALYSIS

16

TEM/EDX

CD ~ 21 nm

CD ~ 30 nm

Limited pattern loading ~10 nm Co recess & TiN liner removal

MORPHOLOGICAL ANALYSIS

17

TEM/EDX

EDGE of the wafer CENTER of the wafer

• Good within-wafer uniformity

• TiN removal height ~ Co recess

CONCLUSIONS

18

Digital etch of cobalt-metal way to go:

1. Promising morphological & electrical results with commodity chemistries

2. Oxidizing step less critical (~ self limiting with H2O2)

3. Predominant role of the oxide dissolving agent (acid) step & DO2

• The local line roughness was relatively high for the ECD cobalt post-CMP & post-recess

etch

• Limited pattern loading observed for CVD cobalt

• Cobalt recess was controlled in the nanometer range (10 +/- 1 nm)

• Good WIW uniformity and yield obtained

PUBLIC