Книги2 / 1993 Dutton , Yu -Technology CAD_Computer Simulation

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o EXPERIMENTS

Figure 2.19: Semi-logarithmic plot of concentration versus depth comparing an ideal Gaussian diffusion profile with a numerical calculation including segregation (m = 0.1) to account for boundary removal of the dopant [2.41].

silicon atoms. Since Ie processing technology routinely uses oxidation during the diffusion step to create a masking layer for subsequent patterning, such oxidation effects are of major concern. The generation

of silicon interstitials during oxidation is closely linked to the oxidation rate, dXo/dt, as indicated by Figure 2.9. An empirical relationship to

describe the total point defect concentration is [2.19]

The "." indicates an excess value above that at equilibrium; in this case, it is used to show the value for the concentration of excess interstitial silicon atoms. ]([ is an empirically determined constant of proportionality, Xo is the oxide thickness, and q is the power-law coefficient, presently thought to be less than unity. The total diffusivity

Figure 2.20: Oxidation-enhanced diffusivities for boron, and phosphorus (dashed lines) compared with those for inert ambient conditions (solid lines) [2.42].

sequences used in both MOS and bipolar technologies. The ensuing subsections give an overview of selected models used in SUPREMo Ion implantation is an essential component of modern IC fabrication and the default Pearson IV model gives excellent agreement with experiments. Also, by modifying the Pearson coefficients a broad range of conditions can be modeled. The model for oxidation includes many effects related to ambient conditions as well as substrate orientation. While the abundance of data and equations may seem overwhelming, the characterization of oxide thickness for these conditions is extensive. Also, since oxide thickness is critical to MOS device performance, it is essential that all such process variations be accounted for. Diffusion modeling is the final topic discussed in Chapter 2. Dopant diffusion is coupled to the oxidation growth in two important ways - segregation and enhanced (or retarded, which is not discussed) diffusion due to point defects generated during oxidation. SUPREM III accurately

models both ofthese effects. However, it does not model the retardation effects. For high-concentration diffusion, SUPREM includes features to account for concentration dependence of diffusivity and even for clustering of atoms as the concentrations approach solid solubility. These models show good agreement for arsenic data and similar effects are observed for other dopants. Research is still in progress to fully characterize these dependencies for phosphorus and boron, especially as processing temperatures are reduced and rapid thermal processing (RTP) is used. In the future we can expect to see significant improvements in high-concentration modeling - especially with the growing importance of bipolar and BiCMOS technologies.

2.6Exercises

2-1 Using the results of Exercise 1-4 and the details given in this chapter, generate and execute the SUPREM files for:

a)nfield, nch.ch

b)pfield, pch.ch

Note that this means that you also have to create and execute the intermediate files as well. Use the "print" command to determine the following quantities:

i)Bi and (B/A)i during the wet field oxidation step (nfield)

ii)boron diffusion and segregation coefficients (excluding the OED effects) during gate oxidation (nch.ch)

iii)phosphorus diffusion and segregation coefficients (excluding OED) during the wet field oxidation step (pfield)

iv)the junction depth for the n-well (pfield)

v)the oxide thickness for the p-channel device (pch.ch)

Add this information as annotations on the plots of log concentration versus depth for parts a) and b).

2-2 Consider the n-well process described in Figure 2.1. Execute the sequence and check to see if the oxide thickness matches that specified

in the process flow described in Chapter 1. Adjust the partial pressure coefficient "pres" to achieve agreement.

2-3 Again consider the n-well steps considered in Exercise 2-1. Modify the fraction interstitialcy coefficient "fii. 0" for phosphorus to a value of 0 and determine the effects of OED on the junction depth. By what percentage has it changed?

2-4 In masking step # 2 shown in Figure 2.1 (a), the nitride layer can be used to mask the p+ channel-stop implant. Using the details given in Chapter 1, determine the minimum thickness required for the nitride layer to mask 90% of the p+ implant as shown.

2-5 Using the results of Exercise 1-4 and the details given in this chapter, generate and execute the SUPREM files for

a)nch.sd

b)pch.sd

Note: this means that you also have to create and execute the intermediate files as well. Use the parameters on the "arsenic" and "boron" commands to simulate and quantify the following:

i)set the arsenic pre-exponential factor for D- to zero and compare the concentration profile (nch.sd) and junction depths.

ii)Set the boron pre-exponential factor for D+ to zero and compare concentration profile and junction depth with the nominal condition (pch.sd).

2-6 Using the default coefficients for arsenic diffusion and the process flow given in Figure 3.12, add a boron implant and diffusion steps to create a base-like profile. That is, use the thermal cycle used for the n-channel threshold adjustment/gate oxidation (before the arsenic implant step, of course), but alter the boron implant parameters to achieve a boron profile with integrated doping in the base region of 1012 cm-2. (Use an n-type substrate of uniform 1015 cm-3 concentration.)

2.7References

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[2.20] D. W. Hess and B. E. Deal, "Kinetics of the thermal oxidation of silicon in 02HCl mixtures," J. Electrochem. Soc., 124, pp. 735-739, May 1977.

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R. Ghez and Y. J. van der Meulen, "Kinetics of thermal growth of ultra-thin layers of Si02 on silicon. Part II. Theory," J. Electrochem. Soc., 119, pp. 1100-1106, 1972.

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