CONTROLLED COBALT RECESS FOR ADVANCED INTERCONNECT METALLIZATION.

Transcription

1 CONTROLLED COBALT RECESS FOR ADVANCED INTERCONNECT METALLIZATION. Antoine Pacco a *, Y. Akanishi b, Q.T. Le a, E. Kesters a, G. Murdoch a, F. Holsteyns a A IMEC VZW, KAPELDREEF 75, 3001 LEUVEN, BELGIUM B SCREEN, SEMICONDUCTOR SOLUTIONS CO., LTD., 480-1, TAKAMIYA-CHO, HIKONE, SHIGA , JAPAN * ANTOINE.PACCO@IMEC.BE PUBLIC

2 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

3 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

4 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. will cobalt climb up the interconnect stack? taking place... 4

5 COBALT RECESS FOR FSAV (FULLY SELF ALIGNED VIA) M1 recess Target = 10nm metal recess Topography for SAV M2V1 Etching Via confined in y-direction by M2 HM Via-first etch lands selectively on SiCN barrier Topography is maintained by selective removal of SiCN M2V1 Fill FSAV is formed Min-distance M1-V1 is maintained by topography M2 V1 M1 5

6 crystal grain boundary CHALLENGES FOR METAL RECESS 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

7 CHALLENGES FOR METAL RECESS: COBALT non-uniform (crystal-grain/boundary dependent) etch uncontrolled dissolution of cobalt during slightly acidic / neutral rinse alkaline solutions, NH 4 OH/H 2 O 2 preferred (*,**) selective removal of TiN liner/barrier after or simultaneously during Co recess * SPCC 18 Y. Ogawa, [Kurita] ** SPCC 19 H. Iino, [Kurita] SPECIFICATIONS: control of the recess amount within the nanometer range: 10 +/- 1 nm small within-line roughness (R q ~ 1nm) uniform within-wafer recess (center-to-edge NU < 1nm) avoid pattern-density-depending etching (CD-independent) cyclic process (digital etch) = process of choice 7

8 APPROACH FOR CONTROLLED RECESS DIGITAL ETCHING : = self-limiting metal-oxide growth on the metal surface 1 = metal-oxide removal selective towards metal surface CoOx Co Co Co X cycles Co initial 0 oxidation 1 oxide dissolution after X cycles 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 8

9 EFFECT OF DIFFERENT OXIDIZING SOLUTIONS FOR THE SAME OXIDE DISSOLUTION STEP: 10S HF 0.05% increasing the peroxide concentration in both slightly acidic H 2 O 2 /H 2 O and alkaline NH 4 OH/H 2 O 2 /H 2 O solutions does not significantly affect the total cobalt loss removing the oxidizing agent, being it H 2 O 2 or atmospheric O 2 does reduce the total cobalt loss. self-limiting oxide film formation when a sufficiently oxidizing solution is used (H 2 O 2 ). # oxidizing solutions 9

10 Ra (nm) COBALT ROUGHNESS AFTER DIFFERENT OXIDIZING SOLUTIONS AFM (ECD COBALT) 2.5 recess depth approx. 10nm ref CMP SC1/HF H2O2/HF DIW/HF 0 ref CMP SC1/HF H2O2/HF DIW/HF # oxidizing solutions Slight (<1nm) roughness increase after recess. No significant difference between different oxidizing solutions. 10

11 IMPACT OF DISSOLVED [do 2 ] IN DILUTED HF (0.05%) EXAGGERATED TIMES & AGGRAVATED CONDITIONS... non controllable recess + high roughness WHAT ABOUT THE IMPACT OF [O 2, ATM ] IN THE PROCESS AMBIENT? 11

12 EFFECT OF OXIDE DISSOLUTION STEP & RINSE STEP TIMES STD(NON-CONTROLLED) VS. LOW [O 2 ] 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 O 2 content in the ambient. 12

13 TIN LINER REMOVAL TWO APPROACHES: 1-STEP VS. 2-STEPS 1-STEP: combined TiN liner removal & Co recess etch 2-STEPS: Co recess etch followed by TiN liner removal (APM 40 C /HF) x4 (APM RT /HF) x4 + 30s APM 40 C TiN removal most effective by using a final 60 C APM (APM 60 C /HF) x4 (APM RT /HF) x4 + 40s APM 60 C 13

14 ELECTRICAL TESTING TARGET: 10 NM CO RECESS FOR FSAV Line resistance of 10 µm long lines 21 nm 30 nm CD 21 nm Process: digital Co recess etch [APM followed by HF] x times + TiN barrier etch APM 60 C CMP RECESS Uniform distributions > 95% yield for smallest CD Electrically active lines 14

15 ELECTRICAL TESTING TARGET: 10 NM CO RECESS FOR FSAV Line resistance of 10 µm long lines 21 nm 30 nm CD Measured vs. predicted line resistance before and after 10 nm Co recess 60 nm 50 nm Co Data follows the expected trend: line resistance increase for smaller lines due to line X-section reduction (R ~ 1/A) Limited pattern loading 15

16 MORPHOLOGICAL ANALYSIS TEM/EDX CD ~ 30 nm Limited pattern loading ~10 nm Co recess & TiN liner removal CD ~ 21 nm 16

17 MORPHOLOGICAL ANALYSIS TEM/EDX EDGE of the wafer CENTER of the wafer Good within-wafer uniformity TiN removal height ~ Co recess 17

18 CONCLUSIONS Digital etch of cobalt-metal way to go: 1. Promising morphological & electrical results with commodity chemistries 2. Oxidizing step less critical (~ self limiting with H 2 O 2 ) 3. Predominant role of the oxide dissolving agent (acid) step & DO 2 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 18

19 PUBLIC