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
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
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
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.
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
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
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
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.)
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
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
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).
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
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
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
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
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
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
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
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.