Thermal Oxidation

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Thermal Oxidation

The oxide of silicon, or silicon dioxide (SiO2), is one of the most important ingredients in semiconductor manufacturing, having played a crucial role in the development of semiconductor planar processing. The formation of SiO2 on a silicon surface is most often accomplished through a process called thermal oxidation. Thermal oxidation, as its name implies, is a technique that uses extremely high temperatures (usually between 700-1300 deg C) to promote the growth rate of oxide layers.

The thermal oxidation of SiO2 consists of exposing the silicon substrate to an oxidizing environment of O2 or H2O at elevated temperature, producing oxide films whose thicknesses range from 60 to 10000 angstroms. Oxidation of silicon is not difficult, since silicon has a natural inclination to form a stable oxide even at room temperature, as long as an oxidizing ambient is present. The elevated temperature used in thermal oxidation therefore serves primarily as an accelerator of the oxidation process, resulting in thicker oxide layers per unit of time.

Thermal oxidation is accomplished using an oxidation furnace (or diffusion furnace, since oxidation is basically a diffusion process involving oxidant species), which provides the heat needed to elevate the oxidizing ambient temperature. A furnace typically consists of: 1) a cabinet; 2) a heating system; 3) a temperature measurement and control system; 4) fused quartz process tubes where the wafers undergo oxidation; 5) a system for moving process gases into and out of the process tubes; and 6) a loading station used for loading (or unloading) wafers into (or from) the process tubes.

Figure 1. Example of an Oxidation Furnace

The heating system usually consists of several heating coils that control the temperature around the furnace tubes. The wafers are placed in quartz glassware known as boats, which are supported by fused silica paddles inside the process tube. A boat can contain many wafers, typically 50 or more. The oxidizing agent (oxygen or steam) then enters the process tube through its source end, subsequently diffusing to the wafers where the oxidation occurs.

Depending on which oxidant species is used (O2 or H2O), the thermal oxidation of SiO2 may either be in the form of dry oxidation (wherein the oxidant is O2) or wet oxidation (wherein the oxidant is H2O). The reactions for dry and wet oxidation are governed by the following equations:

1) for dry oxidation: Si (solid) + O2 (vapor) --> SiO2 (solid); and

2) for wet oxidation: Si (solid) + 2H2O (vapor) --> SiO2 (solid) + 2H2 (vapor).

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