Deepak A. Ramappa

Detection of copper contamination in silicon by surface photovoltage diffusion length measurements

Surface photovoltage minority carrier lifetime/diffusion length analysis of copper contaminated silicon was performed. It was observed that copper and copper associated defects degrade minority carrier lifetime more in n-type than in p-type silicon. This finding is explained by analysis of copper related defect levels identified by other deep level transient spectroscopy studies. In copper contaminated p-type silicon, an optical or thermal activation procedure significantly degrades the diffusion length. A process similar to that of Fe–B in p-type silicon is proposed. The activation process dissociates the Cu–Cu pairs, a weak recombination center in p-type silicon, and the copper forms extended substitutional defects in silicon, which have much greater recombination activity. No recovery of diffusion length was observed following such an activation procedure. The difference in copper and iron diffusion length recovery properties after activation can be used to differentiate iron contamination from copper ...

Diffusion of Iron in Silicon Dioxide

A quantitative analysis of diffusion of iron in silicon dioxide is presented. A source of iron deposited on the surface of thermally oxidized silicon wafers was diffused at temperatures ranging from 700-1100°C in an inert (nitrogen) ambient. The iron concentration in SiO 2 and Si was measured using total reflection X-ray fluorescence, deep level transient spectroscopy, and surface photovoltage techniques. A two-boundary diffusion model was applied to the experimental data to determine the diffusivity and segregation coefficient of iron in SiO 2 . It is observed that iron diffusivity in SiO 2 follows the Arrhenius relationship and has a thermal activation energy of 1.51 eV. Iron exhibits a strong tendency to segregate into silicon dioxide and has a value of k = 1.1 × 10 -7 at 1000°C, where k = N Si /N oxide .

Effects of Copper Contamination in Silicon on Thin Oxide Breakdown

Effects of copper contamination on the breakdown and reliability characteristics of thin silicon gate oxides are discussed. Gate oxide integrity is measured for thermal oxides of 45, 75, 120, and 200 A grown on silicon wafers intentionally contaminated with 10 10 -10 15 cm -3 of copper. Copper doping of silicon was performed according to solubility data considerations. The oxide breakdown voltages as a function of copper concentration for the various oxide thicknesses are reported. For 45 A oxide, copper concentration cannot exceed 10 13 cm -3 without severe degradation in oxide quality. The threshold contamination level for 75 A oxides is ten times higher. Premature oxide breakdown is proposed to occur due to copper silicide precipitation, which locally enhances the electrical field. It is concluded that the impact of copper contamination on oxide breakdown is not as severe as that of iron contamination on oxide breakdown. This is due to the difference in segregation properties of the two metals at the Si/SiO 2 interface.

IRON PRECIPITATION IN FLOAT ZONE GROWN SILICON

Temperature dependent iron precipitation in float zone grown silicon wafers has been experimentally investigated. Results of iron precipitation experiments over a wide thermal process temperature range and time are presented. Precipitation of iron in silicon was analyzed by a quantitative assessment of change in interstitial iron using a surface photovoltage minority carrier lifetime analysis technique. Contamination levels of iron in the range 1011–1013 atoms/cm3 are investigated. It is concluded that maximum iron precipitation occurs in the temperature range of 500–600 °C. Iron precipitation is rapid in this region where more than 90% of the interstitial iron precipitates in a period of 30 min.

Surface photovoltage analysis of phase transformation of copper in p-type silicon

Surface photovoltage minority carrier lifetime/diffusion length analysis of copper contaminated p-type silicon was performed. It was observed that an optical or low-temperature thermal activation procedure on Cu-doped silicon significantly degrades the diffusion length. Unlike iron doped p-type silicon no recovery of diffusion length was observed following such an activation procedure. It is proposed that the activation procedure dissociates interstitial copper agglomerations and forms extended substitutional defects in silicon, which have much greater recombination efficiency. The change in phase of copper and the formation of associated defects is a function of activation light intensity, annealing time, and temperature. An activation energy of 0.419 eV is obtained for the process, which is in good agreement with copper diffusivity value in silicon. It is thus concluded that the change in phase of copper and the formation of extended defects with activation is a diffusion limited process.

Stability of Iron‐Silicide Precipitates in Silicon

A quantitative analysis of iron-silicide precipitate stability with respect to time and temperature is presented. Iron precipitation and dissolution in silicon was analyzed by a quantitative assessment of change in interstitial iron using a surface photovoltage minority carrier lifetime/diffusion length analysis technique. Interstitial iron is shown to rapidly precipitate to the silicide phase between 500 and 600°C. Iron-silicide precipitates were found to dissolve above a temperature of 760°C. Dissolution of FeSi 2 precipitates releases iron back to an interstitial position in the silicon matrix. The amount of precipitate dissolved was found to be a function of dissolution process temperature and time. It is concluded that the precipitate phase of iron, FeSi 2 , is thermally unstable above a temperature of 760°C.

Physical understanding of cryogenic implant benefits for electrical junction stability

We investigate the effect of cryogenic temperature implants on electrical junction stability for ultra shallow junction applications for sub-32 nm technology nodes and beyond. A comprehensive study was conducted to gain physical understanding of the impact of cryogenic temperature implants on dopant-defect interactions. Carborane (C2B10H12) molecule, a potential alternative to monomer boron was implanted in carbon preamorphized silicon substrates at cryogenic implant temperatures. Results indicate implants at cryogenic temperatures increase dopant activation with reduced diffusion, resulting in lower sheet resistance for a lower junction depth. Further, this study emphasizes the benefits of co-implants performed at cryogenic temperatures as alternative to traditional preamorphizing implants.

Surface photovoltage analysis of copper in p-type silicon

Surface photovoltage minority carrier lifetime/diffusion length analysis of copper-doped p-type silicon was performed. Minority carrier recombination behavior of the deep level induced by copper and its thermal stability in the temperature range of 25–300 °C was investigated. This defect is attributed to interstitial copper pairs in p-type silicon. Isochronal annealing to a temperature of 150 °C results in improvement in diffusion length. This change in recombination behavior is attributed to precipitation of copper. At temperatures higher than 150 °C, copper forms extended substitutional defects in silicon, which results in a decrease in diffusion length. Interstitial copper concentration is estimated using the change in recombination behavior with isochronal annealing. Since copper is known to precipitate rapidly during cooling from high temperatures, it is observed that less than 0.01% of the total copper dissolved in silicon occupies an interstitial position.

Dominant iron gettering mechanism in p/p+ silicon wafers

Fe gettering mechanisms in p/p+ epitaxial Si were investigated under controlled contamination and annealing cycles. The dominant Fe gettering mechanism is the Fermi level controlled coulomb attraction between Fe+ and B− in the p+ substrate of the p/p+ wafers. Oxygen precipitates do not appear to contribute when using normal cooling rates following heat treatments. The epi-substrate interfacial strain plays no role in Fe gettering.

Quantitative analysis of copper contamination in silicon by surface photovoltage minority carrier lifetime analysis

Surface photovoltage minority carrier lifetime/diffusion length analysis of copper contaminated silicon was performed. It was observed that copper and copper associated defects degrade minority carrier lifetime more in n-type than in p-type silicon. In n-type silicon the copper associated defect exhibits strong hole capture and thus minority carrier lifetimes are greatly reduced. There are few electron traps reported in p-type silicon and the Cus-Cui pair is only a weak electron recombination center. Further in copper contaminated p-type silicon, an optical or thermal activation procedure is shown to significantly degrade the minority carrier diffusion length. A process similar to that of Fe-B in p-type silicon is proposed. The activation process dissociates the Cu-Cu pairs and forms extended substitutional defects in silicon, which have much greater recombination activity. No recovery of diffusion length was observed following such an activation procedure. The change in phase of copper with low temperatu...