Silicon Oxidation Techniques
One of the properties that makes silicon the elephant in the zoo of semiconducting materials used in microelectronic manufacturing is the superior dielectric properties of its oxide [So1]. An SiO 2 layer can be formed by a simple thermal oxidation process in a wet (water vapor) or dry (oxygen) atmosphere. The growth process is found to be diffusion limited and can be described by a parabolic growth law [De1]. By rapid thermal annealing (RTA), oxidation times in the order of tens of seconds become feasible [Fu1, Go2]. However, independent of the kind of furnace used, the two drawbacks of thermal oxidation remain, which are the high thermal budget that inevitably leads to a smear-out of steep doping profiles and the restriction that thermal oxides can only be formed on a bulk or a polysilicon substrate. Therefore alternative methods of oxide formation such as CVD, LPD and anodic oxidation have been developed.
Anodic oxidation is a very common process in the electrochemical industry, used for example in the manufacture of aluminum and tantalum capacitors. The anodic oxidation of silicon is not of comparable importance, because the electrical properties of anodic oxides are inferior to those of thermal oxides.
To understand the electrochemical behavior of silicon, however, the formation and the properties of anodic oxides are important. The formation of an anodic oxide on silicon electrodes in HF and HF-free electrolytes will therefore be discussed in detail in this chapter. The formation of native and chemical oxides is closely related to the electrochemical formation process and will be reviewed briefly. The anodic oxidation of porous silicon layers is closely related to the morphology and the luminescent properties of this material and is therefore discussed in Section 7.6.