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WAFER EDGE EFFECTS CONSIDERING ION INERTIA IN CAPACITIVELY COUPLED
DISCHARGES*
Natalia Yu. Babaeva and Mark J. Kushner
Iowa State UniversityDepartment of Electrical and Computer Engineering
Ames, IA 50011, USA [email protected] [email protected]
http://uigelz.ece.iastate.edu
June 2006
* Work supported by Semiconductor Research Corp. and NSF
ICOPS2006_Natalie_01
Iowa State UniversityOptical and Discharge Physics
AGENDA
Wafer Edge effects and their origin.
Description of the model:
Improvement of nonPDPSIM to include ion momentum equation
Effect of wafer-focus ring gaps on Ar and Ar/Cl2 CCPs
Plasma penetration
Ion focusing
Concluding remarks
ICOPS2006_Natalie_02
Iowa State University
Optical and Discharge Physics
WAFER EDGE EFFECTS
It is desirable to use wafer area to the edge of the wafer to maximize utilization.
Perturbation of fluxes may occur by method of terminating wafer and matching to tool material
Wafer is beveled at edge with small gap (< 1 mm) between wafer and focus ring.
Penetration of plasma into gap is bad due to formation of particles and deposition of contaminating films.
ICOPS2006_Natalie_03
Iowa State University
Optical and Discharge Physics
ION MOMENTUM EQUATION IN nonPDPSIM
Goal is to computationally investigate edge effects and penetration of plasma into wafer-focus ring gap.
Large dynamic range (> 100) requires unstructured mesh.
Large Knudson number in gap requires accounting for inertia.
nonPDPSIM, a 2-dimensional plasma hydrodynamics model, was improved by adding ion momentum equations on unstructured mesh.
The coupling between the dynamics of charged and neutral transport is through the species resolved collision terms in momenta equations.
ICOPS2006_Natalie_04
Iowa State UniversityOptical and Discharge Physics
nonPDPSIM CHARGE PARTICLE TRANSPORT• Poisson equation for the electric potential
• Transport equations for conservation of the charged species j
• Surface charge balance
• Full momentum for ion fluxes of species j
• Equations are simultaneously solved using a Newton’s iterations.
)( j
jj Nq
St
N j
materialj
j Sqt
)(1
)(1
,,,,,
,,,,,
iyi
jyjjj
yjjjy
jjjy
jy
ixi
jxjjj
xjjjx
jjjx
jx
VVNM
ENqP
MV
t
VVNM
ENqP
MV
t
ICOPS2006_Natalie_05
Iowa State University
Optical and Discharge Physics
2-D GEOMETRY AND CONDITIONS
Conditions:
Ar, 90 mTorr, 300 sccm, 500 V
Ar/Cl2 = 70/30, 90 mTorr, 300 sccm, 500 V
Biased substrate, grounded housing
Showerhead to wafer distance = 4 cm
Transport of energetic secondary electrons from biased substrate is addressed with a Monte Carlo simulation.
ICOPS2006_Natalie_06
MESHING TO RESOLVE WAFER-FOCUS RING GAP
Iowa State UniversityOptical and Discharge Physics
Unstructured mesh with multiple refinement zones was used to resolve wafer-focus ring gap.
Gaps of < 1 mm were investigated.
ICOPS2006_Natalie_07
Iowa State University
Optical and Discharge PhysicsMIN MAX
Log scale
Electron penetration into the gaps is nominal due to surface charging and sheath formation.
Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V
Animation slide
ELECTRON DENSITY NEAR THE GAPS
0.9 mm Gap
106 –108 cm-3 106 –108 cm-3
Electrons (106 – 3 x109 cm-3)
ICOPS2006_Natalie_08
0.3 mm Gap
Iowa State University
Optical and Discharge Physics
EDGE REGION: NEGATIVE CHARGE
Negative charging of wafer surface (and focus ring) extends beyond edge of bevel in large gap.
Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V
ICOPS2006_Natalie_09
0.9 mm Gap 0.3 mm Gap
MIN MAX Log scale
EDGE REGION: IONS
Iowa State UniversityOptical and Discharge Physics
Animation slide
106 – 3x108 cm-3108 –3 x108 cm-3
ICOPS2006_Natalie_10
MIN MAX Log scale
0.9 mm Gap 0.3 mm Gap
Ions are modulated by 10 MHz e-field variation.
Ions penetrate into the large gap reaching the biased substrate.
Ions do not penetrate into the small gap but do respond to “sentinal” surface charge.
Ar, 90 mTorr, 10 MHz,300 sccm, 500 V
Iowa State University
Optical and Discharge Physics
EDGE REGION: ELECTRON TEMPERATURE
MIN MAX
Te is higher near the small gap due to overlapping os sheaths and higher local electric fields.
Electron temperature (and electron density) is negligibly small inside the gaps.
Ar, 90 mTorr, 10 MHz, 300 sccm, 500 V
ICOPS2006_Natalie_11
0.9 mm Gap 0.3 mm Gap
Ar/Cl2 DISCHARGE
Iowa State UniversityOptical and Discharge Physics
Maximum electron density shifts towards the focus ring.
Negative ion density comparable to electron density, though are trapped in the plasma bulk and do not reach the wafer
Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V
ICOPS2006_Natalie_12
MIN MAX Log scale
[e] [Cl2+]
[Ar+] [Cl-]
Iowa State University
Optical and Discharge Physics
EDGE REGION: Ar+ AND Cl2+ FLUXES
Cl2+ flux is larger and less collisional than Ar+ due to lower rate of
charge exchange. There is some focusing of flux to the corner of the bevel that
could lead to excessive heating and sputtering. Some ion trajectories terminate on the lower bevel. Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V
ICOPS2006_Natalie_13
0.9 mm Gap 0.9 mm Gap
Iowa State University
Optical and Discharge Physics
EDGE REGION: Ar+ AND Cl2+ FLUXES
Less focusing of ion fluxes to corner of bevel occurs with the smaller gap due to lack of charging of wafer into wafer-focus ring cavity.
Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V
ICOPS2006_Natalie_14
0.3 mm Gap 0.3 mm Gap
Iowa State University
Optical and Discharge Physics
EDGE REGION: Ar+ FLUX STREAMTRACES
Streamlines penetrate into large gap throughout rf cycle.
In small gap, momentary penetration occurs at peak of cathode cycle. Slightly conductive wafer is able to dissipate that charge.
Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V
Animation slideICOPS2006_Natalie_15
0.3 mm Gap 0.3 mm Gap
Iowa State University
Optical and Discharge Physics
EDGE REGION: Cl2+ FLUX STREAMTRACES
Focusing of ion flux streamlines to edge of wafer is more severe for Cl2
+ than Ar+ due to lower collisionality.
Periodic flux into gap is also larger.
Ar/Cl2 = 85/15, 90 mTorr, 300 sccm, 500 V
Animation slideICOPS2006_Natalie_16
0.9 mm Gap 0.3 mm Gap
CONCLUDING REMARKS
Penetration of plasma into narrow wafer-focus ring gap of a capacitively coupled discharge was computationally investigated.
Gap sizes > 0.5 mm allow significant penetration of the plasma.
Charging and ion fluxes may penetrate to bottom side of bevel.
Focusing of ion flux to the corner of the bevel depends on the ion species and collisionality: chemically enhanced sputtering is problematic.
Iowa State UniversityOptical and Discharge Physics
ICOPS2006_Natalie_17