Blisters formation mechanism during High Dose Implanted Resist Stripping Marion Croisy a,b,c*, Cécile Jenny a, Claire Richard a, Denis Guiheux a, Sylvain Joblot a, Alain Campo b, Erwine Pargon c, Nicolas Possémé b a STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France b CEA-Leti, 17 avenue des Martyrs, F-38054 Cedex 09, Grenoble, France c CNRS-LTM, Université Grenoble Alpes, 17 rue des Martyrs, F-38054 Grenoble Cedex, France * marion.croisy@st.com
Outline 2 Context Observation and characterization of blisters Blisters characterization (STEM, STEM-EDX and ToF-SIMS) Influence of implantation parameters on blisters formation Dopants species (As vs P) Implantation energy and dose Solutions to avoid blisters formation Optimization of dry strip process Integration modification
Context /High Dose Implantation Stripping (HDIS) 3 Dry strip process after implantation step Dry strip challenges No remaining residues Crust Resist No added defectivity (blisters) Minimize substrate loss and oxidation No impact on dopant profile N 2 H 2 is the most suitable chemistry for implanted resist stripping in terms of residues removal and low substrate consumption Resist Limitation blisters formation after several N 2 H 2 stripping processes on implanted area
Context /Integration 4 Example of CMOS integration from Spacer Z to SD formation Gate patterning SpacerZ After LDD and NSD the dry strip chemistry used is forming gas (N2H2 with 3% of H2) All lithographic rework are based on forming gas Si epitaxy LDD x5 Formation of blisters after several N2H2 processes Example with P implantation (4keV 1.5 E 15 at/cm²) After 4 N 2 H 2 dry strip 5 N 2 H 2 dry strip 6 N 2 H Resist 2 dry strip Spacer2 SD x4
Experimental protocol 5 GAMMA tool (LAM Research) Downstream plasma 6 platens 1 preheat station 5 RF sources High temperature (285 C) Wafer preparation Bulk Si substrate NSD lithography As or P implant N2/H2 dry strip cumul Characterizations In-line : Top view SEM-CD observations Off-line : Cross section STEM/EDX observation, chemical analysis by ToF-SIMS
Outline 6 Context Observation and characterization of blisters Blisters characterization (STEM, STEM-EDX and ToF-SIMS) Influence of implantation parameters on blisters formation Dopants species (As vs P) Implantation energy and dose Solutions to avoid blisters formation Optimization of dry strip process Integration modification
STEM cross section views Blisters Characterization / STEM and EDX analysis 7 P 4keV 1.5E15 at/cm² after 2 N2H2 processes 16 nm Pt Encapsulation a-si c-si 70-100nm As 4keV 2E15 at/cm² after 6 N2H2 processes 14 nm EDX analysis P not detectable Pt O As Si Bubbles are formed near the Si surface in the implanted area corresponding to amorphous Si The surface is lifted by the bubbles No dopants are detected on the voids left by the bubbles
no implant 2 N2H2 processes P 4keV 2N2H2 processes P 4keV post implantation Blisters Characterization / ToF-SIMS analysis Evaluation of plasma diffusion in the Si substrate thanks to H - and SiN - profiles 8 Relative intensity (a.u.) 10 5 10 4 0 200 400 600 800 Erosion time (s) Relative intensity (a.u.) 10 6 H SiN 10 5 10 4 10 3 0 200 400 600 800 Erosion time (s) No implantation Diffusion of N but not H atoms P implantation Similar N diffusion as for the non implanted substrate But H diffusion is greatly enhanced and an accumulation is observed H diffusion and accumulation in the amorphized Si created during the implant are responsible for blister formation
Photoresist coating and lithography Implant P 4keV 1.5 E 15 at/cm² or Implant As 4keV 2 E 15 at/cm² Dry strip N 2 H 2 Blisters Observation / Influence of implanted specie (As vs P) 9 P 4keV 70-100nm Blisters size 70-100nm 1 2 3 As 4keV 50 60nm 1 6 7 8 Influence of dopant specie blisters appear later with As larger blisters with P P 4keV 1.5 E 15 As 4keV 2 E 15 Number of dry strip 1 2 3 4 5 6 7 8
Blisters Characterization / Influence of implanted specie (As vs P) 10 What can explain the difference observed between As and P? SRIM simulations Atoms/cm3 4x10 21 3x10 21 2x10 21 1x10 21 Native Oxide Si As 2 P Dopant profile P 4keV 1.5E15 at/cm² As 4keV 2E15 at/cm² Si recoil (Atoms/cm3) 4x10 23 3x10 23 2x10 23 70-100nm c-si 1x10 23 Native Oxide Si Si recoil profile Si recoil (P 4keV 1.5E15 at/cm²) Si recoil (As 4keV 2E15 at/cm²) 0 0 5 10 15 20 25 Simulated (SRIM) (nm) Depth (nm) Amorphized Si thickness Measured (STEM) (nm) P 18 16 As 14 14 0 5 10 15 20 25 Different dopants profiles but same Rp Higher concentration of Si recoils with As higher amorphization rate Thinner amorphized layer for As Effect of amorphization rate or amorphized layer thickness? 0 Depth (nm) Si is considered amorphous when Si recoils > 3.6atoms/cm 3
Photoresist coating and lithography Implant As 3keV Dry strip N 2 H 2 Blisters Post Dry Strip / Influence of implantation energy 11 As 5keV 2E15 As 4keV 2E15 As 3keV 2E15 Nb of dry strip processes 1 2 3 4 5 6 7 8 Amorphized Si thickness (SRIM) As 5keV 16.4 nm As 4keV 14 nm As 3keV 11.4 nm Si recoil (At/cm3) SRIM simulations 4x10 23 3x10 23 2x10 23 1x10 23 0 Native Oxide Si 0 5 10 15 20 Depth (nm) 5keV 4keV 3keV 2 Influence of implantation energy Blisters are formed a little earlier with low implantation energies At lower energy, the amorphization rate and the amorphized thickness are lower Influence of implantation energy on blisters is limited
Photoresist coating and lithography Implant As 3keV Dry strip N 2 H 2 Blisters Post Dry Strip / Influence of implantation dose 12 As 3keV 2E15 As 3keV 1E15 Nb of dry strip processes 1 2 3 4 5 6 7 8 Amorphized Si thickness (SRIM) As 3keV 2E15 11.4 nm As 3keV 1E15 10.4 nm Si recoil (At/cm3) 3,5x10 23 3,0x10 23 2,5x10 23 2,0x10 23 1,5x10 23 1,0x10 23 5,0x10 22 Native Oxide Si 2E15 at/cm² 1E15 at/cm² Influence of implantation dose Lower dose favors the blisters formation At lower dose, the amorphization rate is more than twice lower Influence of amorphization rate on H diffusion and blisters formation 2 Depth (nm) 0 5 10 15
Outline 13 Context Observation and characterization of blisters Blisters characterization (STEM, STEM-EDX and ToF-SIMS) Influence of implantation parameters on blisters formation Dopants species (As vs P) Implantation energy and dose Solutions to avoid blisters formation Optimization of dry strip process Integration modification
Photoresist coating and lithography Implant As 3keV 1 E 15 at/cm² Dry strip N 2 H 2 Blisters Post Dry Strip / Influence of stripping recipe 14 Reference (30s/platen) 30-50nm T ramp + Low P (45s/platen) Low P (30s/platen) Number of dry strip processes Recipe 1 2 3 4 5 Low process pressure and temperature ramp the blisters formation is delayed compared to reference recipe Limitation remaining residues after dry and wet processes The stripping is less efficient because of temperature ramping Post dry stripping Post wet cleaning With low process pressure only the blisters formation is delayed and no residues remain
Thermal oxide growth (55A) Implantation P Dry strip N 2 H 2 Blisters Post Dry Strip / Influence of oxide layer 15 Modification of the stack Implantation is made through an thermal oxide layer (55A) Implantation step is adapted to have similar dopant profiles 30-50nm P 4kev P 6kev Si bulk SiO2 (55A) Si bulk SRIM simulations Dopant profile Si recoil profile Atoms/cm3 1.4x10 21 1.2x10 21 1.0x10 21 8.0x10 20 6.0x10 20 4.0x10 20 2.0x10 20 0.0 Thermal Oxide Si -5 0 5 10 15 20 25 Depth (nm) P 4keV (Si) P 6keV (Ox) Si recoil (At/cm3) 2x10 23 1x10 23 0 Thermal Oxide Si -5 0 5 10 15 20 25 30 Depth (nm)
Thermal oxide growth (55A) Implantation P 1.5 E 15 at/cm² 4keV or 6keV Dry strip N 2 H 2 Blisters Post Dry Strip / Influence of oxide layer 16 Si P 4keV Oxide 30-50nm P 6keV Nb of dry strip 1 2 3 4 5 The oxide layer allows to delay the formation of blisters What is the effect of the oxide layer? Oxide Oxide removed before strip P 6keV P 6keV Nb of dry strip 1 2 3 4 5 Effect on H diffusion The blisters are formed sooner if the oxide is removed before stripping The growth of an oxide layer before implantation allows to delay blisters formation by limiting H diffusion in Si substrate
Conclusions 17 Blisters characterization o During N2H2 stripping, H atoms diffuse though the amorphous Si layer and accumulate to form a bubble o N2 also diffuses in the substrate but is not responsible for blistering Influence of implantation parameters o Blisters are favored by low implantation dose corresponding to less amorphized Si conditions Solutions against blistering effect o Dry stripping process optimization low pressure allows to delay blisters formation o Integration modification oxide layer growth before the implantation is beneficial in particular because it decreases H diffusion in the substrate during the plasma process
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