E E 339 - CN10-device fabrication

E E 339 - CN10-device fabrication - 1 Fabrication basics...

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Unformatted text preview: 1 Fabrication basics Fabrication basics 1 Semiconductor devices and interconnects: fabrication issues Doping depth profile control Outline during epitaxial growth via diffusion via ion implantation Insulation/dielectrics/oxide layers Contacts and metallization Lithography for layout control Summary 2 2 Devices (including doping profiles) front-end processing and their interconnects back-end processing are created in a multistep process. Device and interconnect fabrication requires control o Device fabrication issues Device and interconnect fabrication requires control of Device materials and their locations (crystalline and polycrystalline semiconductors, insulators) doping profiles (including junction locations where the dominant doping type, donor or acceptor, changes) metallization or metal layers used as device contacts and for inter-connecting devices (essentially wiring) 3 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. 4 3 Can control doping profile and p-n junction depth where the predominate doping type, acceptor or donor changes via doping of the original ingot during growth (as previously discussed Doping profile/junction depth control discussed) doping during Epitaxial growth diffusion of dopants ion implantation of dopants Can be used in combination (not an either/or situation). 5 Simply add desired dopants during epitaxial growth. Provides most precise control of dopant profiles in growth direction. Doping during epitaxial growth Figure 116 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 11.3) thick. (Photograph courtesy of Bell Laboratories.) 6 4 Dopants are introduced (into a probably already oppositely doped semiconductor compensation doping) from the surface via diffusion (random motion of dopants) at high temperatures. Two basic profiles Diffusion of Dopants Two basic profiles: All dopants supplied at surface at time t = 0 produces half- Gaussian distribution: where D is the diffusivity (See appendix VIII where the e tical axis sho ld be diff si it ) ) ; form (of ) ( 2 2 2 2 2 Dt e e x N x x Dt x dopant x vertical axis should be diffusivity ) Surface concentration held constant produces a complimentary error function of x and Dt (qualitatively similar if quantitatively different function; See Figure P5-2 in problems at the end of the chapter, pg. 242.) 7 E.g., qualitatively, for compensation doping of acceptors into an initially n-type...
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E E 339 - CN10-device fabrication - 1 Fabrication basics...

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