H. Sinha

The effects of vacuum ultraviolet radiation on low-k dielectric films

Plasmas, known to emit high levels of vacuum ultraviolet (VUV) radiation, are used in the semiconductor industry for processing of low-k organosilicate glass (SiCOH) dielectric device structures. VUV irradiation induces photoconduction, photoemission, and photoinjection. These effects generate trapped charges within the dielectric film, which can degrade electrical properties of the dielectric. The amount of charge accumulation in low-k dielectrics depends on factors that affect photoconduction, photoemission, and photoinjection. Changes in the photo and intrinsic conductivities of SiCOH are also ascribed to the changes in the numbers of charged traps generated during VUV irradiation. The dielectric-substrate interface controls charge trapping by affecting photoinjection of charged carriers into the dielectric from the substrate. The number of trapped charges increases with increasing porosity of SiCOH because of charge trapping sites in the nanopores. Modifications to these three parameters, i.e., (1) VU...

Time-dependent dielectric breakdown of plasma-exposed porous organosilicate glass

Time-dependent dielectric breakdown (TDDB) is a major concern for low-k organosilicate dielectrics. To examine the effect of plasma exposure on TDDB degradation, time-to-breakdown measurements were made on porous SiCOH before and after exposure to plasma. A capillary-array window was used to separate charged particle and vacuum ultraviolet (VUV) photon bombardment. Samples exposed to VUV photons, and a combination of VUV photons and ion bombardment exhibited significant degradation in breakdown time. The samples exposed to VUV photons and ion bombardment showed more degradation in breakdown time in comparison to samples exposed to VUV photons alone.

Charge Trapping within UV and Vacuum UV Irradiated Low-k Porous Organosilicate Dielectrics

Vacuum ultraviolet (VUV) spectroscopy is used to determine the valence-band structure and location of defect states within the bandgap of porous organosilicate (SiCOH) dielectrics both before and after VUV and UV irradiation. SiCOH dielectrics have bandgap energies of about 9 eV. In addition, positive charge is trapped by defect states located 1 eV above the top of the SiCOH valence-band edge. These defect states can be populated or depopulated with electrons during UV and VUV irradiation, respectively. This is verified by measuring the magnitude and polarity of the trapped charge after VUV irradiation using two techniques: (i) capacitance vs voltage characteristics obtained with a mercury probe and (ii) surface-potential measurements obtained with a Kelvin probe. Both techniques show that the defect states are uncharged when occupied with electrons and positively charged when depleted of electrons.

Effect of vacuum ultraviolet and ultraviolet Irradiation on capacitance-voltage characteristics of low-k-porous organosilicate dielectrics

High frequency capacitance-voltage (C-V) measurements are used to determine the effects of vacuum ultraviolet (VUV) and ultraviolet (UV) irradiation on defect states in porous low-k organosilicate (SiCOH) dielectrics. The characteristics show that VUV photons depopulate trapped electrons from defect states within the dielectric creating trapped positive charge. This is evidenced by a negative shift in the flat-band voltage of the C-V characteristic. UV irradiation reverses this effect by repopulating the defect states with electrons photoinjected from the silicon substrate. Thus, UV reduces the number of trapped positive charges in the dielectric and can effectively repair processing-induced damage.

Effects of vacuum ultraviolet radiation on deposited and ultraviolet-cured low-k porous organosilicate glass

The authors compare the effects of vacuum ultraviolet (VUV) irradiation on pristine and UV-cured low-k porous organosilicate glass (SiCOH). The authors find that during VUV irradiation, more trapped charges are generated in UV-cured SiCOH as compared to pristine SiCOH. VUV is also used as a tool to investigate effects of UV curing. From comparison of VUV spectroscopy and photoinjection current of the two samples, the authors find that UV curing reduces the number of defect states in SiCOH. The authors also find that UV-cured SiCOH has higher photoconductivity and intrinsic conductivity from VUV spectroscopy and trapped-charge decay rate, respectively.

Surface potential due to charge accumulation during vacuum ultraviolet exposure for high-k and low-k dielectrics

The surface potential due to charge accumulation during vacuum ultraviolet irradiation of high-k and low-k thin dielectric films is measured. Measurement of the substrate current, which is the sum of the charge-accumulation and photoinjection currents, allows an in situ monitoring of the charge accumulation during irradiation. The relationship between the substrate current and the calculated in situ surface potential is also found, eliminating the need for a separate surface-potential measurement. With a high photon dose, the surface potential and substrate current reach a steady-state value with no further net charge accumulation.

Effect of vacuum ultraviolet and ultraviolet irradiation on mobile charges in the bandgap of low-k-porous organosilicate dielectrics

Capacitance-voltage (C-V) measurements are used to determine the effect of vacuum ultraviolet (VUV) and ultraviolet irradiation on mobile charges in porous low-k organosilicate (SiCOH) dielectrics. Hysteresis in the C-V characteristics shows that VUV irradiation increases the number of mobile charges in the dielectric. This is because VUV photons excite the trapped electrons from defect states to make them mobile carriers. Conversely UV reverses this effect by reducing the mobile charges through photoemission of free electrons and repopulation of trap states. Thus UV irradiation can be used to improve the electrical properties of plasma-processed dielectrics that are subjected to VUV irradiation.

Numerical simulation of vacuum-ultraviolet irradiation of dielectric layers

Vacuum-ultraviolet irradiation produces trapped charges in dielectrics. The trapped charges often generate self-consistent electric fields. A Monte Carlo simulation coupled with a Poisson equation solver is used to model the relationship between the irradiation photon flux and electrostatic potential. The simulation includes photoconduction, photoemission, photoinjection, and the effects of self-consistent electric fields. Calculations show that photoemission and photoinjection are responsible for changes in the electric potential as photon dose or dielectric thicknesses are varied. Experimental surface-potential measurements were made to compare the results of the simulation.

Reflectance and substrate currents of dielectric layers under vacuum ultraviolet irradiation

The reflectance of low-k porous organosilicate glass (SiCOH) as a function of photon energy under synchrotron vacuum ultraviolet (VUV) radiation was measured using a nickel mesh reflectometer. The authors found that during VUV irradiation, the reflectance of SiCOH and the substrate current were inversely correlated. Thus, reflectance can be inferred from substrate current measurements and vice versa. The authors conclude that reflectance or substrate current measurements can determine the photon energies that are absorbed and, therefore, cause dielectric damage during processing. Thus, reducing the flux of deleterious photon energies in processing systems can minimize dielectric damage.

Plasma and vacuum ultraviolet induced charging of SiO2 and HfO2 patterned structures

The authors compare the effects of plasma charging and vacuum ultraviolet (VUV) irradiation on oxidized patterned Si structures with and without atomic-layer-deposited HfO2. It was found that, unlike planar oxidized Si wafers, oxidized patterned Si wafers charge up significantly after exposure in an electron-cyclotron resonance plasma. The charging is dependent on the aspect ratio of the patterned structures. This is attributed to electron and/or ion shading during plasma exposure. The addition of a 10 nm thick HfO2 layer deposited on top of the oxidized silicon structures increases the photoemission yield during VUV irradiation, resulting in more trapped positive charge compared to patterns without the HfO2 dielectric.