1. Thin Films and Diffusion

4. Diffusion is not constant across cross section, and continues with every subsequent high-temperature step; hence, we use charts as below to calculate surface concentrations, Cs, from average conductivity,

5. Effective diffusivity is: D A eff =D o +D - (n/n i )+D = )n/n i ) 2 for N-type D eff A =D o +D + (p/n i )+D ++ (p/n i ) 2 for P-type Values are tabulated, as in table 7.5 Effective diffusion-time, (Dt) eff, is the sum of the diffusivity and time at each step: (Dt) eff = D 1 t 1 +D 1 t 2 (D 2 /D 1 )=D 1 t 1 +D 2 t 2

6. Diffusion Data

7. Example Figure 7-17 Dopant surface concentration vs. effective conductivity for various substrate concentrations, C B

9. Where do defects come from? Naturally occurring vacancies and interstitials Thermal stresses inducing dislocations and stacking faults Precipitation of second phases inducing dislocations and stacking faults Impurities Process damage (e.g, ion implantation)

12. The oval shaped area with a lighter contrast is the emitter of a bipolar transistor. In preferential etching this would look similar to what was shown as an illustration for etching.illustration for etching Some of these small stacking faults have a peculiar, "sailing-boat" like shape (marked by "S" in the picture above). Below, a detailed view of a "sailing boat stacking fault":

13. Oxidation of Silicon produces interstitials in supersaturation. These surplus interstitials tend to agglomerate in discs - i.e. stacking fault loops. The difficult part is the nucleation; it determines what will happen. We have to consider two ways of oxidizing Si, we first consider Surface oxidation: The surface oxidizes homogeneously by exposing it to an oxidizing atmosphere at high temperatures. This is the normal oxidation process. The emission of interstitials occurs at the interface; the interstitials diffuse into the bulk; the supersaturation decreases with the distance from the surface. There is no easy nucleation for an interstitial type dislocation loop as long as the interface is defect free. If defects are present, most prominent small precipitates of metal impurities (Fe, Ni, Cu) may serve as nucleation centers for the interstitials; a stacking fault penetrating in a semicircular fashion into the bulk is formed. If many precipitates are available, a large density of small stacking faults may be observed: precipitates of metal impurities

14. The name "swirl" comes from the spiral "swirl-like" pattern observed in many cases by preferential etching as shown on the right. Close inspection revealed two types of etch features which must have been caused by different kinds of defects. Lacking any information about the precise nature of the defects (which etching can not give), they were termed "A-" and "B-swirl defects".