MODELING OF TRENCH FILLING DURING IONIZED METAL …
Transcript of MODELING OF TRENCH FILLING DURING IONIZED METAL …
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MODELING OF TRENCH FILLINGDURING IONIZED METAL PHYSICAL VAPOR DEPOSITION+
JLU_AVS 00
Junqing Lu* and Mark J. Kushner**
*Department of Mechanical and Industrial Engineering
**Department of Electrical and Computer Engineering
University of Illinois at Urbana-Champaign
October 2000
+Supported by SRC, NSF and DARPA/AFOSR
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
AGENDA
JLU_AVS 01
• Motivation and introduction to Cu IMPVD
• Description of the model and the sputter algorithm
• Metal densities in Cu IMPVD
• Ion and neutral distributions
• Surface diffusion model for profile simulation
• RF coil voltage
• Trench filling
• Radial locations
• Pressures
• Magnetron and ICP power
• Aspect ratio of the trench
• Conclusions
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MOTIVATION FOR Cu IONIZED PVD
JLU_AVS 02
• The resistance of Cu is only half that of Al.
• To increase processor speed, Cu is replacing Al as the metal for interconnect wiring.
• The filling of large aspect ratio trenches benefits from ionized PVD.
• Ions are able to fill deep trenches because their angular distributions are narrowed by an rf bias.
METALATOMS
SiO2
METAL
VOID
METALIONS
SiO2
METAL
RFBIAS
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
COMPUTATIONAL PLATFORM
JLU_AVS 03
Hybrid Plasma Equipment Model(HPEM, including Plasma Chemistry Monte Carlo Simulation*)
Flux to wafer
Monte Carlo Feature Profile Model (MCFPM)
Angular and energy distributions
*PCMCS generates angular and energy distributions for the depositing fluxes, using species sources and time-dependent electric fields obtained by HPEM.
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
FEATURES OF SPUTTER MODEL
JLU_AVS 04
• Ion energy-dependent yield* for sputtered atoms.
• The effective yield of 1 for the reflected neutrals.
• Ion energy-dependent kinetic energy
• Sputtered atoms: Cascade distribution
• Reflected neutrals: TRIM** and MD***
• Cosine distribution in angle for sputtered and reflected atoms emitted from target.
• Momentum and energy transfer from sputtered and reflected atoms to background gas (sputter heating).
• Electron impact ionization for in-flight sputtered and reflected neutrals.
• Source terms for thermalized sputter species and gas heating.
*Masunami et al., At. Data Nucl. Data Tables 31, 1 (1984) . **D. Ruzic, UIUC.***Kress et al., J. Vac. Sci. Technol. A 17, 2819 (1999)
00 200 400 600
0.5
1.0
1.5
Incident Ar + Energy (eV)
2.0
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
Cu IMPVD: METAL DENSITIES
JLU_AVS 05
• Reactor is based on *Cheng et al.
• Operating conditions:
• 40 mTorr Ar
• 1.0 kW ICP
• 20 V rf voltage on coils
• 0.3 kW magnetron
• -25 V dc bias on substrate
• Cu peaks below the target since most of the sputtered Cu atoms are thermalized a few cm below the target.
• Cu+ peaks at the center due to peak in plasma potential.
*Cheng, Rossnagel, and Ruzic, JVST 13(2), 1995, p. 203.
Radius (cm)
Substrate
Target MagnetInlet
Density
(cm-3)
9.0 x 1010
9.0 x 109
1.0 x 109
Cu Density Cu+ Density
Cu Target
Target to Substrate Distance = 13 cm
Substrate
Coils
InletMagnet
Outlet
0 5 10 155101518 18
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
Cu IMPVD: ELECTRON TEMPERATURE AND DENSITY
JLU_AVS 06
• The electron temperature is > 3 eV throughout the reactor.
• The large Te near the coils is due to the large ICP power deposition in this region.
• The electron density peaks off center due to the magnetron effect and off-axis ionization by ICP power.
Te
Radius (cm)
Substrate
Target MagnetInlet
1.5 x 1012
1.6 x 1011
2.0 x 1010
0 5 10 155101518 18
3.7
3.4
3.0
e Density
eV cm-3
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
ION AND NEUTRAL SPECIES DISTRIBUTIONS
JLU_AVS 07
• Neither the ion energy nor the neutral energy are mono-energetic.
• The spread in ion energy is due to the rf voltage on the coil and collisional broadening.
• The neutral distributions in angle are broader than that for ions.
• Operating conditions: 40 mTorr Ar, 1.0 kW ICP, 20 V rf voltage on coils, 0.3 kW magnetron, -25 V dc bias on substrate
• Cu+
0 30-30Angle (deg.)
0
20
40
60
80
High
Low
• Cu*
0 50 90-50-900
0.5
1.0
1.5
High
Low
Angle (deg.)
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
MONTE CARLO FEATURE PROFILE MODEL (MCFPM)
JLU_AVS 08
• The MCFPM obtains the etch and deposition profile using ion and neutral distributions from the HPEM.
• Surface processes are implemented using a chemical reaction mechanism:
• Deposition: Cu(g) + Si(s) Cu(s) + Si(s)
• Resputtering: Ar+(g) + Cu(s) Ar(g) + Cu(g)
• The model takes account of angular and energy dependent etch and deposition rates.
• The model is able to simulate many different chemistries and materials.
SiO2
METAL
RFBIAS
Ar+ Cu+ Cu
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING WITH AND WITHOUT DIFFUSION
JLU_AVS 09
• A diffusion algorithm was incorporated into MCFPM to reduce unphysical dendric growth.
• The diffusion probability of the depositing metal depends on the activation energy for each possible diffusion site.
• Without diffusion, the Cu films are unphysically porous and non-conformal.
• With diffusion, Cu species deposit compactly and conformally.
With Diffusion
Trench Aspect Ratio = 1.1, Trench Width = 600 nmCu+ : Cu Neutrals = 4:1
SiO2
SiO2
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2
SiO2
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2 Without Diffusion
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING VS COIL VOLTAGE
JLU_AVS 10
• For ICP power of 500 W and 2 mTorr Ar, measured plasma-potential oscillation* ranges from 10 to 30 V, depending on the termination capacitance of the coil. • The oscillation in plasma potential extends the range of ion energies, thereby regulating the degree of sputtering.
• Operating conditions: 40 mTorr Ar, 1 kW ICP, 0.3 kW magnetron, -30 V dc bias on wafer.
Width (µm)0 0.4 0.8 1.2
0.8
0.4
0
1.2
rf coil voltage = 10 V 30 V20 V
Width (µm)0 0.4 0.8 1.2
Width (µm)0 0.4 0.8 1.2
ExcessiveSputtering
*Suzuki, Plasma Sources Sci. Technol. 9, 199 (2000).
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING AT DIFFERENT RADIAL LOCATIONS
JLU_AVS 11
• Electric field perturbation at the edge of the wafer generates asymmetry in Cu+ distribution, which causes asymmetry in deposition profile.
• Operating conditions:
• 40 mTorr • 1 kW ICP • 20 V rf voltage on coils • 0.3 kW magnetron • -25 V dc bias on wafer
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2
Cu+ : Cu Neutrals = 4:1 2:12.3:1
Width (µm)
0 0.4 0.8 1.2
Width (µm)
0 0.4 0.8 1.2
0 40-40Angle (deg.)
0
40
80
R = 0.5 cm R=5.2 cm R = 9.9 cm Low
High
0 40-40Angle (deg.)
0 40-40Angle (deg.)
Radial Location = 0.5 cm 5.3 cm
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING VS PRESSURE
JLU_AVS 12
• Voids form at low pressure and fill with increasing pressure.
• The ionization fraction increases with increasing pressure, due to slowing of Cu atoms which allows more ionization. • Reasons for pinch-off:
• Diffuse angular distribution of the neutrals
• Less sputtering of over-hanging deposits
• Operating conditions*:
• 1 kW ICP • 0.3 kW magnetron• -25 V dc bias on wafer
*Cheng, Rossnagel and Ruzic, JVST B 13, 203 (1995).
0.8
0.4
0
1.2
Cu+ : Cu Neutrals = 1:3 4:12:1
5 mTorr 20 mTorr 40 mTorr
600 nm
5 mTorr 20 mTorr 40 mTorr
1 µm
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING WITH AND WITHOUT RESPUTTERING
JLU_AVS 13
• Without resputtering, the overhang deposits grow faster than the bottom deposits, leading to pinch-off at the top.
• Resputtering reduces the overhang deposits, opens up the top of the trench, and enables more fluxes to arrive at the bottom.
• Note that Ar+ contributes significantly to resputtering.
Cu+ : Cu Neutrals = 4 : 1
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2 No Resputtering With Resputtering
Ar+ : Cu Total = 7 : 1
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING VS MAGNETRON POWER
JLU_AVS 14
• As magnetron power increases, the incident ion flux and the target bias increase, and more Cu atoms are sputtered into the plasma.
• The ionization fraction of the Cu atoms decreases since more ICP power would be required to maintain the same ionization fraction.
• The small void at low magnetron power was caused by microtrenching.
• Operating conditions: 30 mTorr Ar, 1 kW ICP, -30 V dc bias on wafer.
Cu+ : Cu Neutrals = 3:1
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2
1.6:1
Magnetron Power = 0.3 kW 2 kW 0.3 kW3:1
Microtrenching
Width (µm)
0 0.4 0.8 1.2
Width (µm)
0 0.4 0.8 1.2
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING VS ICP POWER
JLU_AVS 15
• As ICP power decreases, the power available for Cu ionization decreases, and the Cu ionization fraction decreases.
• The pinch-off at low ICP power is caused by low ionization fraction.
• Operating conditions: 30 mTorr Ar, 0.3 kW magnetron, -30 V dc bias on wafer.
Cu+ : Cu Neutrals = 3:1
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2
1.4:1
ICP Power = 1.0 kW 0.3 kW
Width (µm)
0 0.4 0.8 1.2
Width (µm)
0 0.4 0.8 1.2
2.5:10.6 kW
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
TRENCH FILLING AT DIFFERENT ASPECT RATIOS
JLU_AVS 16
• As the aspect ratio increases, trench filling becomes more difficult.
• The fluxes that are able to completely fill shallow trenches may left voids in deeper trenches.
• Operating conditions:
• 40 mTorr • 1 kW ICP • 0.3 kW magnetron • -25 V dc bias on wafer
Width (µm)
0 0.4 0.8 1.2
0.8
0.4
0
1.2
Width (µm)
0 0.4 0.8 1.2
Aspect Ratio 1.1
Width (µm)
0 0.4 0.8 1.2
Aspect Ratio 2
Aspect Ratio 3
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
COMPLETE TRENCH FILLING ATDIFFERENT ASPECT RATIOS
JLU_AVS 17
• The ionization fraction required for complete filling increases with the aspect ratio.
• The highest possible ionization fraction is about 90%, due to gas heating.
• The simulated results indicate the largest aspect ratio for a complete filling is 3, the consensus in literature for highest aspect ratio filling is 4.
• For aspect ratio > 4, experimental results suggest that tapered trench walls are needed for seed layer deposition at the bottom.
• Operating conditions:
• 1 kW ICP • 0.3 kW magnetron • -25 V dc bias on wafer • Radius = 0.5 cm
Tapered Trench
Aspect Ratio0 1 2 3
0
0.2
0.4
0.6
0.8
1.0
Void
Complete Filling
Increasing Pressure
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UNIVERSITY OF ILLINOISOPTICAL AND DISCHARGE PHYSICS
CONCLUDING REMARKS
JLU_AVS 18
• An integrated plasma equipment-feature scale model has been developed and applied to IMPVD modeling.
• The depositing ions have a broadened energy distribution due to oscillation of the plasma potential.
• Surface diffusion is an important process in metal deposition.
• Electric field enhancement at the wafer edge may cause asymmetry in trench filling.
• Formation of voids in trench filling occurs when the ionization fraction of the depositing metal flux is low.
• As aspect ratio of the trench increases, the ionization fraction for complete filling also increases.
• The desirable conditions for complete trench filling are high pressure, low magnetron power, and high ICP power.