Phosphorus doped silicon oxide

It has been seen in the previous section that the ratio of the onsite electron-electron Coulomb repulsion and the one-electron bandwidth is a critical parameter. The Mott-Hubbard insulating state is observed when U > W, that is, with narrow-band systems like transition metal compounds. Disorder is another condition that localizes charge carriers. In crystalline solids, there are several possible types of disorder. One kind arises from the random placement of impurity atoms in lattice sites or interstitial sites. The term Anderson localization is applied to systems in which the charge carriers are localized by this type of disorder. Anderson localization is important in a wide range of materials, from phosphorus-doped silicon to the perovskite oxide strontium-doped lanthanum vanadate, Lai cSr t V03. 

Silicon nitride has received the most attention, but only a few manufacturers are using it commercially because it is difficult to control some of the properties which can affect the operation of an IC (such as hydrogen content and stress). The main attribute of silicon nitride is that it is impervious to moisture and this makes it the ideal passivation when highly phosphorus-doped silicon dioxide layers are used to separate two levels of interconnection, as, for example, in gate arrays. (One requirement of an inter-layer dielectric is that it should smooth out underlying steps, and 6-10% phosphorus-doped silicon dioxide has the appropriate flow properties to do this.) In such a case silicon nitride prevents the uniform cathodic corrosion which is associated with the use of such oxide layers as passivations. 

For example, during oxidation, enhanced diffusion of phosphorus, boron, and arsenic are observed, as well as retarded diffusion of antimony. However, if direct nitridization of the silicon surface occurs, then the inverse effects are observed, that is, enhanced antimony diffusion and retarded phosphorus diffusion. Also, oxidation-enhanced diffusion is significantly affected by doping. As either p- or n-type doping concentration increases above nh oxidation-enhanced diffusion diminishes. If chlorine is introduced into the oxidizing ambient, oxidation-enhanced diffusion is likewise diminished. 

Effects of Silicon Doping. Silicon heavily doped with donor or acceptor impurities can exhibit oxidation rates that are considerably enhanced relative to lightly doped silicon (84,112). For example, the dependencies of the rate constants on substrate phosphorus doping level are shown in Figure 31 for oxidations of <111> silicon at 900 °C. B/A increases sharply by more than an order of magnitude as the phosphorus level increases beyond lO /cm3. 

The anodic treatment process was carried out in a combined regime first, galvanostatically at a current density of 1 mA/cm up to the formation voltages (Uform) of 10-50 V, and then, when the given Uform was reached, potentiostatically, up to the current of 0.01-0.02 mA. To measure characteristics of the anodically oxidized silicon (AOS) (thickness, dielectric properties, and their surface distribution) the anodic treatment of the silicon wafers was done in a cell with the anodization area of 5 cm. Single-crystal boron-doped (100) silicon wafers of resistivity 0.3 Ohm cm, phosphorus-doped (100) and (111) silicon substrates of resistivities 0.1 and 4.5 Ohm-cm, respectively, and boron-doped (100) and( 111) silicon wafers of resistivity 4.5 Ohm-cm were used as silicon anodes. 

As an example of the use of this technique, a silicon wafer lightly doped with phosphorus is doped with additional phosphorus by ion implantation (dose of 3.5 x 10ncm"2). A thermal oxide film of 857 A thickness was initially grown on the wafer. The variation of dopant concentration with depth from the oxide-silicon interface is shown in Figure 16. The rise in dopant close... 

Doping. Electron-acceptor atoms such as boron or electron-donors such as phosphorus are introduced into the area exposed by the etch process to alter the electrical character of the pure silicon, which is an intrinsic semiconductor. These areas are called p-type (e.g., with boron) or n-type (e.g., with phosphorus) to reflect their particular charge carrier in the conduction process. Repeating the previous steps, i.e., thermal oxidation, masking, etching, and doping operations are repeated several times until the last front-end layer is completed, i.e., all active devices have been formed. 

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