Transcript Effect of VUV and UV Irradiation - University of Wisconsin–Madison

Effect of VUV and UV Irradiation on low-k Dielectrics
H. Sinhaa, J.L. Lauera, M.T. Nicholsa , G.A. Antonellib, Y. Nishic and J.L. Shoheta
aUniversity
of Wisconsin-Madison, Madison, WI 53706
bNovellus Systems, Tualatin, OR 97062
cStanford University, Stanford, CA 94305
Introduction
Effect of VUV irradiation on SiCOH
• Processing plasmas emit vacuum ultraviolet (VUV) radiation which can
cause damage to microelectronic devices by affecting the properties of
dielectrics.
231 nm SiCOH (k = 2.55) on p-type Si substrate
VUV Spectroscopy
– Dielectrics can become charged due to the processes of photoemission and
photocoduction.1,2
– Energetic electrons can be generated within dielectrics which may generate defects.
• Band gap is found to be 8.5eV
• Defect states are found to be
located 0.5eV above valence band.
The peak at 8eV disappears with
VUV irradiation.
• The defect states are depopulated
of electrons after irradiation with 8eV
VUV photons.
• However, VUV radiation can also be beneficial:
– Discharge patterned structures3 and devices
• We show the effects of ultraviolet (UV) and VUV on the generation and/or
depletion of trapped charges with SiCOH dielectric as detected by:
– VUV spectroscopy
– Surface potential measurements
– C-V measurements
C-V Characteristics
Experiment
VUV Irradiation System
Aluminum Plate
Dielectric Layer
Position adjustment knob
Linear translator
Wafer bias and
current measurements
e
e
Compensating
Charge
Wafer
VB
Electrometer
e
e
e
h
h
h
h
VUV Photons
e
e
A
Pico-ammeter
Electrical Ports
Hole
Charge
Photo-emitted
Electrons
48 Volts
VUV Photons
Electrical connection
to back of wafer
(from synchrotron beam)
Ion Pump
• After VUV irradiation, negative
shifts in flat-band voltage are
observed.
• The negative shift indicates
positive trapped charges in the
dielectric.
• The shift in flat-band voltage is
proportional to the number of
defect states in the dielectric.
Substrate
Current
Photoemission
Current
A
Pico-Ammeter
• A mercury probe was used to measure
the C-V characteristics before and after
UV and VUV irradiation.
• Mercury drop contact forms a MetalOxide-Semiconductor structure.
• LCR meter measured the differential
capacitance at stepped DC voltages
C-V Characteristics
VUV Spectroscopy
The peak at 8.2eV decays after
VUV irradiation, but reappears
after UV irradiation.
UV irradiation causes a decrease in the
number of trapped positive charges
Surface Potential
500 nm SiOCH (k = 2.55)
Surface Potential
• Surface potential increases with
increasing VUV dose and saturates
at higher dose.
• Surface potential saturates at ~4V for
both 8eV and 9eV VUV irradiation.
Kelvin Probe System
Mercury Probe System
• HgAr lamp was used as source of UV irradiation
(photon energy peak at 4.9eV).
• SiCOH-Si interface energy barrier is at 4±0.5eV2. Thus, photoinjection of
electrons across SiCOH-Si interface is feasible under UV irradition
Silicon Substrate
• The University of Wisconsin Synchrotron was used as a VUV photons source.
• VUV spectroscopy was performed by measuring the substrate current while
scanning photon energies from 5-15eV.
• Substrate and photoemission currents were measured as a function of time
during irradiation for fixed photon energies.
•A Kelvin probe was used to measure
the surface potential after UV and VUV
irradiation.
•The surface potential is proportional to
the amount of trapped charge within the
dielectric layer.
•Current is zero when
Bias voltage(Vb) = Surface potential(VSP)
i(t) =( VSP + Vb ) dC/dt
Effect of UV irradiation on SiCOH
Photoemission Current
C
• Electrons depopulated from the
defect sates are the major
component of the photoemission
current. Thus it is proportional to
the generation of trapped positive
charges.
• We observe a saturation in
photoemission current with
increasing dose.
VUV spectroscopy, C-V characteristics, surface potential and
photoemission current measurements indicate electron depopulation
caused by the presence of trapped positive charges in defect states.
These quantities show a correlated saturation as photon dose increases.
• The surface potential becomes positive after VUV
irradiation, but returns to its original background potential
after UV irradiation.
Summary
• We determined the valence-band structure of low-k porous-SiCOH (k
= 2.55) dielectrics
– Electronic states absorb photons with energies of 8.2eV are responsible for the
accumulation of positive charge after VUV irradiation.
– These defect states are depopulated of electrons with VUV irradiation.
• The trapped positive charge due to VUV irradiation can be reduced
with UV radiation.
– Photoinjection of electrons from Si into the dielectric can repopulate the defects
with electrons
• Plasmas generate both UV and VUV, thus there is a tradeoff between
charging and discharging of trap states.
– By suitably optimizing or supplementing the spectrum of the emitted radiation, it
is possible to significantly reduce the amount of trapped charge.
References:
1J.L. Lauer, A. Antonelli, Y.Nishi and J.L. Shohet, "Charge
Trapping within UV and VUV Irradiated low-k porous-SiCOH",
Applied Physics Letters (submitted for publication)
2 H. Sinhaa, J.L. Lauera, M.T. Nicholsa , G.A. Antonellib, Y. Nishic
and J.L. Shoheta , “Effect of VUV and UV Irradiation on C-V
characteristics of low-k-porous SiCOH dielectric”, Applied
Physics Letters (submitted for publication)
3 G. S. Upadhyaya, J.B. Kruger and J.L. Shohet, "Vacuumultraviolet-induced charge depletion in plasma-charged
patterned-dielectric wafers", Journal of Applied Physics 105,
053308 (2009).
Work supported by the Semiconductor
Research Corporation under contract
2008-KJ-1781,Task no 1781.001. The
UW Synchrotron is supported by NSF
Grant DMR-0084402.