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Epitaxy is the ordered crystalline growth which bears a definite relation to underlying monocrystalline substrate. It is normally used to grow silicon layers in which devices will be fabricated. The main advantage of epitaxy is that the doping of the epitaxial layer is not much affected by the substrate doping. Hence it is straightforward to get abrupt changes in the doping with the epitaxial growth. There are many methods to grow these layers such as gas phase epitaxy, liquid phase epitaxy, and molecular beam epitaxy. Fairly high growth rates at reasonably low temperature are possible. Also, these layers can be grown selectively on crystalline substrate where the layer does not grow on the areas covered with amorphous materials like oxide or nitride.
This is a natural phenomenon due to the tendency for impurities to move from a region of high concentration to a region of low concentration. Hence if some impurities are introduced near silicon surface, they will diffuse deeper into silicon at higher temperature. Diffusion was a very popular way of selectively introducing impurities in silicon before implants came along. Due to slow impurity diffusion in SiO2, oxide was used as a mask for selected introduction of impurities (Fig. 9.16).
Even if this technique is not so much used for introducing dopants in the present implant age, the diffusion is still a part of processing life. The
Fig. 9.16. Oxide as diffusion mask.
Fig. 9.17. Redistribution of implanted impurities after anneal.
Fig. 9.18. Effect of impurity on diffusion.
post-implant anneal makes the implanted impurities redistribute by diffusion as shown in Fig. 9.17.
The diffusion takes place at any time when silicon is heated. The presence of lateral diffusion at about 70% of depth decides some of the critical device features and inherently limits some of the device features and properties.
For very deep diffusions, the profile is a near Gaussian. For very shallow ones, it is a near error function. The characteristic depths are determined by the diffusion coefficient D which is an exponential function of the temperature and the time t of diffusion. VDt is typically the characteristic depth for the distribution. The diffusion coefficient D is a very strong function of concentration of vacancies and interstitial within the crystal. It also depends on the presence of other impurities and the other materials. One such effect we saw in segregation before. The other one is commonly known as emitter push effect because this structure occurs in the fabrication of bipolar junction transistor. Here, phosphorus is diffused into a region where boron was previously doped as shown in Fig. 9.18. Boron is pushed deeper into the region where phosphorus is present. The diffusion also affects the steepness of epitaxial layer — substrate junction.
Simple estimation of 1D depth profile after diffusion with a very thin initial layer of dopants is possible. D in this simple model depends on impurity type
Table 9.2. Diffusion parameters for common dopants.
where Ёa is the activation energy, k is the Boltzmann constant (here, k = 8.62 x 10-5 eV/k). D0 and Ёa are tabulated in Table 9.2 (Sze, 1988). The parameters do not depend on spatial coordinates.
In specific case when a wafer is exposed to a gas source of a dopant so that diffusion with a constant surface concentration Cs occurs, the dopant concentration C(x) at a depth x is given by
where erfc is the complementary error function.
In case the dopants are first introduced in a thin layer with integrated dopant concentration Ф,
If this is done by a very shallow ion implantation, Ф is basically the dose.
If the implant depth is significant, the combined implant and diffusion profile in an inert ambient annealing as a function of depth is given by
This simple profile does not take into account point defect-diffusion linkage, electric field effects, different contributions to D from a variety of D0 and
Ea and effect of the presence of other impurities (Fig. 9.18). Here, boron is pushed deeper under heavy phosphorus doping and is called as dopant push effect. Accurate profiles taking into account all the effects can only be obtained with process simulators (Synopsys; Silvaco).
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