6.4.8.4 Buried layer diffusion:
Buried layer diffusion is analogous to pre-deposition step. Here solid diffusant source As 2 O 3 is used and a high surface concentration of the order of 10 20 /cc on the Si wafer but shallow diffusion (1 µm) is achieved. Arsenic is chosen as the dopant because it has a diffusion coefficient an order of magnitude lower that of P. Hence buried layer region remains localized during subsequent thermal cycles. Also ‘As’ has a solid solubility of 10 21 /cc in Si. Therefore surface concentration of the order of 10 20 /cc , giving rise to low R sh = 1 ohm per square (sheet resistance), is easily achievable.
6.4.9 .Growth of N type epitaxial film over P type substrate:
Fourth important step is the growth of 10 µm thick N type epitaxial film of resistivity 0.5 ohm-cm over a p type substrate of 10 ohm-cm. The epitaxial film has donor concentration of 10 16 donor impurity atom/cc and substrate has acceptor concentration of 10 15 acceptor impurity atom/cc. Components are fabricated in epitaxial film hence for isolation from one another and for isolation from the substrate, epitaxial film and substrate are kept of opposite type so that they can be reversed biased and provide isolation.
Epitaxial growth is a process whereby a thin film of single crystal Si is grown from a vapour phase upon on existing single crystal Si wafer with the same crystallographic orientation so that the lattice structure of the resulting layer is an exact extension of the substrate crystal structure.
6.4.9.1 Epitaxial System:
Epitaxial growth system consists of a reaction chamber in which there is a long cylindrical quartz tube surrounded by R.F. induction coil as shown in Figure 6.15. Wafers are put on a graphite boat encased by a quartz sleeve and the boat is pushed in a reaction chamber where localized heating of graphite takes place by RF Induction method. A control console permits the introduction of the wide variety of gases needed to produce epitaxial film with the requisite properties within wide limits.
Silicon tetrachloride with hydrogen is introduced in the reaction chamber. SiCl 3 is easy to purify, nontoxic and inexpensive hence utilized generally for epitaxy.
To achieve the desired impurity concentration in the epitaxial film, Phosphine in correct proportion for N type doping and di-borane in correct proportion for P type, are introduced in SiCl 4 : H 2 mixture. At the surface of the Si wafer where temperature is almost 1200 0 c, hydrogen reduction of SiCl 4 takes place releasing free Si which skid about on the surface of the growing film until they find a correct position in the lattice and get fastened to the growing film. The reaction, taking place on Si surface only, is known as heterogenous reaction and is given in Eq.(6.4).
1200°C
SiCl 4 + 2H 2 →Si + 4HCl ; (6.4)
Equation 6.4 is reversible. In forward direction epitaxial growth of single crystal Si film takes place. In reverse direction Si surface is etched by HCl vapour.
Similar reduction takes place with respect to phosphine or di-boranes. This results in the incorporation of phosphorous or boron atoms in the growing silicon lattice. By this process a variety of controlled resistivity and complex impurity profiles can be achieved. There are certain precautions which are imperative for epitaxial growth. No matter how well the wafers are cleaned, there are mechanical imperfections, silicon dioxide, residual dust and other contaminants on the surface of the wafer. These surface defects nucleate crystal imperfections which manifest themselves as bumps, pits and stacking faults. A scratch on the substrate would show up in the film also. All these faults and imperfections affect the reproducibility and yield of I.C. chips. (Yield means percentage of I.C. chips which has normal performance). It has been found that if vapor phase HCl is passed over the wafers before epitaxial growth, to etch away a few microns at the surface of the wafers, then scratches, dust and contaminants are removed to a great extent, resulting in epitaxial films which are completely free of bumps and pits and where stacking faults are reduced several orders of magnitude.