Effect of VUV and UV Irradiation on low-k DielectricsH. 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• Processing plasmas emit vacuum ultraviolet (VUV) radiation which can

cause damage to microelectronic devices by affecting the properties of dielectrics.– 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.

• 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

Experiment

• 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.

Electrometer

h

e

h

hh

48 Volts

e

ee

Photo-emittedElectrons

Hole Charge

Dielectric Layer

Silicon Substrate

VUV Photons

Aluminum Plate

A

A

eee

Compensating Charge

Pico-Ammeter

Substrate Current

Photoemission Current

Wafer bias andcurrent measurements

VUV Photons(from synchrotron beam)

Ion Pump

Linear translator

Wafer

Electrical connection to back of wafer

Electrical Ports

Position adjustment knob

VB

Pico-ammeter

VUV Irradiation System

• A mercury probe was used to measure the C-V characteristics before and after UV and VUV irradiation.

• Mercury drop contact forms a Metal-Oxide-Semiconductor structure.

• LCR meter measured the differential capacitance at stepped DC voltages

Mercury Probe System

Kelvin Probe System

• 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 whenBias voltage(Vb) = Surface potential(VSP)

i(t) =( VSP + Vb ) dC/dt

Effect of VUV irradiation on SiCOH231 nm SiCOH (k = 2.55) on p-type Si substrate

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.

C-V Characteristics

VUV Spectroscopy

Effect of UV irradiation on SiCOH• 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

The peak at 8.2eV decays after VUV irradiation, but reappears after UV irradiation.

VUV Spectroscopy C-V Characteristics

UV irradiation causes a decrease in the number of trapped positive charges

500 nm SiOCH (k = 2.55)Surface Potential

• The surface potential becomes positive after VUV irradiation, but returns to its original background potential after UV irradiation.

• 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.

Summary

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.

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, "Vacuum-ultraviolet-induced charge depletion in plasma-charged patterned-dielectric wafers", Journal of Applied Physics 105, 053308 (2009).

C

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.

• 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.

Photoemission Current

• 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.

• 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.