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Effect of VUV and UV Irradiation on low- k Dielectrics. H. Sinha a , J.L. Lauer a , M.T. Nichols a , G.A. Antonelli b , Y. Nishi c and J.L. Shohet a a University of Wisconsin-Madison, Madison, WI 53706 b Novellus Systems, Tualatin, OR 97062 c Stanford University, Stanford, CA 94305.

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Effect of VUV and UV Irradiation on low-kDielectrics

  • 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

Effect of UV irradiation on SiCOH

231 nm SiCOH (k = 2.55) on p-type Si substrate

  • 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

  • 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 asdetected by:

    • VUV spectroscopy

    • Surface potential measurements

    • C-V measurements

VUV Spectroscopy

  • 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.

C-V Characteristics

VUV Spectroscopy

C-V Characteristics

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

Experiment

  • 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.

VUV Irradiation System

Surface Potential

500 nm SiOCH (k = 2.55)

Surface Potential

  • 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.

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

  • Surface potential increases with increasing VUV dose and saturates at higher dose.

  • Surface potential saturates at ~4V for both 8eV and 9eV VUV irradiation.

Summary

Kelvin Probe System

  • 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.

  • 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)

Photoemission Current

  • 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.

C

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

Mercury Probe 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

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).

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.


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