Dielectric

Aside from the passivation and glass layer deposited over the surface of the die to protect it from mechanical damage and corrosion, dielectric layers are also used for isolating components or structures in the active circuit from each other, and as dielectric structures for MOS transistors, capacitors, and the like.

Other uses for dielectric layers include: 1) masking for diffusion and ion implant processes; 2) diffusion from doped oxides; 3) overcoating of doped films to prevent dopant loss; 4) gettering of impurities; and 5) mechanical and chemical protection.

Silicon dioxide (SiO2), the oxide of silicon, is the most widely used dielectric in wafer fabrication. There are many ways to grow silicon dioxide on the surface of silicon, but it is most often done through a process known as thermal oxidation. Thermal oxidation consists of exposing the silicon to oxidizing agents such as water and oxygen at elevated temperatures. This process has good control over the thickness and properties of the SiO2 layer.

Fig. 1. Example of a 4-Stack Diffusion Furnace that can be used for Thermal Oxidation of SiO₂

The mechanism by which SiO2 is formed from silicon has been fully understood over the years. A newly exposed silicon surface quickly oxidizes to form an SiO2 film on its surface. As oxidation progresses, silicon is consumed and the SiO2 layer thickens, moving the Si-SiO2 interface deeper into the silicon substrate.

The process of thermal oxidation can be classified as either dry or wet oxidation. In dry oxidation, the moxidizing agent is oxygen, and is governed by the following reaction: Si (solid) + O₂ (vapor) = SiO₂ (solid). In wet oxidation, the main oxidizing agent is water, and is governed by the following reaction: Si (solid) + H₂O (vapor) = SiO₂(solid) + 2H₂.

There are other commonly-used dielectric materials aside from SiO₂. Silicon dioxide doped with phosphorus (commonly referred to as P-glass, phospho-silicate glass, or PSG) is used in many applications because it inhibits diffusion of sodium impurities and exhibits a smooth topography. Adding boron to PSG results in boro-phospho-silicate glass (BPSG), which flows at lower temperatures than PSG (850C-950C for BPSG versus 950C-1100C for PSG).

Polysilicon with enough oxygen content is also semi-insulating and has actually been used in circuit passivation. Silicon nitride is an excellent moisture barrier while stoichiometric silicon nitride is used in oxidation masks and for MOS gate dielectric.

These dielectric layers are usually deposited by sputtering or chemical vapor deposition (CVD). The layer material deposited depends on the reactants used during processing.

Fig. 2. A CVD system (left) and a sputtering system (right) which may

be used for depositing various dielectric layers

Wafer Fab Links: Incoming Wafers; Epitaxy; Diffusion; Ion Implant; Polysilicon; Dielectric; Lithography/Etch; Thin Films; Metallization; Glassivation; Probe/Trim

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