Diffusion Process in Semiconductor Fabrication: Theories and Practical Applications - Prof, Study notes of Electrical and Electronics Engineering

Download Diffusion Process in Semiconductor Fabrication: Theories and Practical Applications - Prof and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 Chapter 7: Dopant Diffusion • High temperature diffusion has historically been one of the most important processing steps used in fabrication of monolithic integrated circuits (IC). • Today, diffusion is used in the formation of “deep” layers exceeding few tenths of micron in depth. Therefore we must study diffusion process in order to understand its limitation and various problems associated with redistribution of impurities. • In this chapter we will discuss and explore theoretical and practical aspects of the diffusion process. Diffusion: Basic Concepts • Diffusion is the redistribution of atoms from regions of high concentration of mobile species to regions of low concentration. • It occurs at all temperatures, but the diffusivity has an exponential dependence on T. • Diffusivity, a property that describe the “ease” with which they move through the medium. • The “driving force” of diffusion is the concentration gradient. Diffusion: Basic Concepts • Predeposition : doping often proceeds by an initial predep step to introduce the required dose of dopant into the substrate. • Drive-In : a subsequent drive-in anneal then redistributes the dopant giving the required junction depth and surface concentration. Throughout the 1960s the dominant predeposition methods were solid phase Diffusion and high temperature gas phase depositions. Diffusion: Methods Ion implantation became the dominant doping method by the mid 1970s and Continues to be so today. The new emerging method for dopant diffusion is rapid thermal processing 2 Tip or extension (LDD) formation Dopant Solid Solubility • Dopants are soluble in bulk silicon up to a maximum value before they precipitate into another phase. Discrepancy in dopant concentration: an example Dopants may have an “electrical” solubility that is different than the solid solubility. Two electrons form a dangling bond And do not contribute free electrons to the crystal 5 3.2 Atomic Scale Diffusion: Fair’s vacancy model • Many effects that are very important experimentally, cannot be explained by the macroscopic models discussed so far. Thus we need to look deeper at atomic scale effects. • In the vacancy model, vacancy can be neutral (Vo), positively charged by donating an electron (V+), double positively charged by donating two electrons (V++), negatively charged by accepting an electron (V-), double negatively charged by accepting two electrons (V=). However, the probability for high level charged is very low. Due to these probabilities, the most general expression for the diffusion coefficient in the vacancy model is given by, ........... 2 2 3 3 2 2 D n pD n pD n nD n nD n nDD iiiii o       +++      +      ++= +−−− For substrate with excess free Electron (n-type), positive charge term can be neglected and for substrate with excess free holes(p-type) the negati charge terms can be neglected. Concentration Dependent Diffusivity ........... 2 2 3 3 2 2 D n pD n pD n nD n nD n nDD iiiii o       +++      +      ++= +−−− The dash line show the erfc profiles. The solid lines are numerical simulation which agree with experimental results At high doping concentrations, the diffusivity appears to increase. Fick's equation must then be solved numerically since D ≠ constant. 6 Example ........... 2 2 3 3 2 2 D n pD n pD n nD n nD n nDD iiiii o       +++      +      ++= +−−− kTEo o o oaeDD /−−= Possible mechanism for diffusion: self interstitials diffusion • In this method an interstitial silicon atom displace the impurity, driving it into a interstitial site. Interstitially is not believed to occur unless vacancy diffusion does as well. • In order to accurately find D one must add the contribution of all possible mechanism. Diffusion mechanism: Kick-out and Frank-Turnbull mechanisms • In kick-out mechanism the impurity replaces a lattice atom. • In Frank mechanism the interstitial impurity is capture by vacancy. Correction to simple theory: Electric field effect • When the doping is higher than ni at the diffusion temperature, ε-field effects become important. • The origin of the field comes from the higher mobility of the electrons and holes compared to dopant atoms. x ChDJ ∂ ∂ +−= )1( h is the field enhancement term. When C>>ni h→2 7 Diffusion Coefficient for common dopants: boron and arsenic Boron diffusion is dominant by neutral vacancy mechanism Arsenic diffusion is believed to be controlled by neutral and single negatively Charged vacancies and is relatively low. At high concentration field enhancement is also dominant: Asi i As Dn nD )(2≈ The effect of this enhancement is to produce a very steep profile Interstitial mechanism is dominant (P M. Fahey et al.) Vacancy mechanism is dominant (P. M. Fahey et al) Diffusion Coefficient for common dopants: Phosphorus diffusion • Phosphorus diffuses much more rapidly than arsenic. • The diffusivity near the surface is believed due to the neutral vacancy exchange and single negatively charged pair of phosphorus ions and double negatively charged vacancies. 2 2       += − i iiPh n nDDD The increase in the diffusivity in the tail region is due to the dissociation of the (PV)- pair which cause an excess vacancy concentration. kink tail unpaired (PV-1) Its application to VLSI is limited primarily to wells and isolation. A silicon wafer was doped in a 1000oC predeposition diffusion with phosphorus to its solid solubility limit. The process time was 20 min. After the predeposition, the surface of the silicon was sealed and 1100oC drive in was done. Find the drive in time necessary to obtained a junction depth of 4.0 micron. Assume a substrate concentration of 1017cm-3. What is the surface concentration after the drive in? (ref table 3.2) (3.28) (predeposition flux)