CN10-pn fabrication

CN10-pn fabrication - 1 1 Fabrication of p-n junctions and...

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Unformatted text preview: 1 1 Fabrication of p-n junctions and devices in general 2 Outline ¡ P-n junctions and semiconductor devices ¡ Semiconductor devices and interconnects: fabrication issues ¡ Doping depth profile control ¢ during epitaxial growth ¢ via diffusion ¢ via ion implantation ¡ Insulation/dielectrics/oxide layers ¡ Contacts and metallization ¡ Lithography for layout control ¡ Summary 2 3 Device Fabrication issues Devices (including their internal p-n junctions) ― “ front-end- processing ” ― and their interconnects ― “ back-end processing ” ― are created in a multistep process. Device and interconnect fabrication requires control of ¡ semiconductor material in multi-semiconductor devices ¡ doping profiles including junction location , that is where the transition from p to n-type carrier concentrations occurs ¡ electrical insulation layers (e.g., oxides) for current blocking within and among devices ¡ device contacts, metal and/or polysilicon ¡ “metallization” or metal layers for connecting devices (essentially wiring) 4 Control of the above ¡ normal to the plane of the wafer ¡ in the plane of the wafer during fabrication is achieved by what can be quite different respective approaches. 3 5 Doping profile/junction depth control Can control doping profile and p-n junction depth via ¡ doping of the original ingot during growth (as previously discussed) ¡ doping during Epitaxial growth ¡ diffusion of dopants ¡ ion implantation of dopants Can be used in combination. 6 Doping during epitaxial growth: Simply add desired dopants during epitaxial growth. Provides most precise control of dopant profiles in growth direction. Figure 1—16 Crystal growth by molecular beam epitaxy (MBE): (a) evaporation cells inside a high -vacuum chamber directing beams of Al, Ga, As, and dopants onto a GaAs substrate; (b) scanning electron micrograph of the cross section of an MBE-grown crystal having alternating layers of GaAs (dark lines) and AlGaAs (light lines). Each layer is four monolayers (4 3 a/2 5 11.3Å) thick. (Photograph courtesy of Bell Laboratories.) 4 7 Diffusion of dopants: Dopants are introduced (into a probably already oppositely doped semiconductor ― compensation doping) from the surface via diffusion (random motion of dopants) at high temperatures. If all dopants supplied are at surface at time t = 0, a half- Gaussian distribution results where D is the “ diffusivity ” (Appendix VIII where the vertical axis should be “ diffusivity ”) … … or if the Surface concentration is held constant, a “ complimentary error function ” (just another function that can be looked up) of x and Dt is obtained. (See Figure P5-2 in problems at the end of the chapter, pg. 242.) ) ; form (of ) ( 2 2 2 2 2 2 Dt e e x N x x Dt x dopant x = ∝ − − σ σ 8 E.g., qualitatively, for compensation doping of acceptors into an initially n-type material by diffusion (by either process; only the details differ)....
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This note was uploaded on 02/03/2009 for the course EE 339 taught by Professor Banjeree during the Spring '08 term at University of Texas.

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CN10-pn fabrication - 1 1 Fabrication of p-n junctions and...

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