SOI Technology-Introduction to Basics

 Hello Dear Readers,

Today, I will explain SOI Technology in VLSI and how much it influences the VLSI Industry.

Silicon-on-insulator (SOI), has been used for various applications, such as radiation-hardened circuits, since the 1970s. Recent developments have brought CMOS SOI into the limelight again, this time because of the potential for high-speed design operations.

Silicon-on-insulator (SOI) is a "non-bulk" technology that builds transistors on top of an insulating layer instead of in a semiconductor substrate. This reduces parasitic capacitance levels and yields higher-speed operation, but it introduces a class of problems of its own.

Early SOI technologies used crystals, such as sapphire (silicon-on-sapphire or SOS), as an insulating substrate. Sapphire was used because its lattice constant (the distance between neighboring atoms) is close to that of silicon, and thin layers of device-grade crystalline silicon can be grown on it. Unfortunately, Sapphire and other like crystals are expensive to grow and work with. SOS  fell into the realm of specialized circuit families, and attempts to use it for high-speed VLSI designs were never fully realized.

Modern CMOS SOI is based on a more tractable process flow. The objective is still the same: create an insulating layer and build FETs on top. However, SOI starts with a silicon wafer, not a crystal, making it more attractive economically.

An early fabrication sequence for CMOS SOI starts with a wafer that has a thick oxide layer grown on it, as shown in Fig.1(a). An oxide etches Fig.1(b) bares the paths to the underlying silicon. A carefully controlled epitaxial growth of silicon creates a "pillar" that has an important crystal structure. This is shown in Fig.1(c). 

Fig.1

The epitaxial process is continued, giving a thin layer of crystal silicon on top of the amorphous silicon dioxide layer, as portrayed in Fig.2. This provides the crystal-silicon background needed for transistors.

Fig.2

A more modern and elegant technique for creating the SOI structure is to implant heavy doses of oxygen directly into the silicon substrate. This is shown in Fig.3(a). The wafer is then annealed at very high temperatures, which induces oxide growth below the wafer surface and "pushes" a top layer of silicon to the top, resulting in the SOI layering shown in Fig.3(b). This approach was pioneered by IBM and is called SIMOX for "Separation by Implantation of Oxygen".

Fig.3

Once the SOI film is made, transistors can be fabricated. Self-aligned MOSFETs are built by defining active-area regions and then ion-implanting dopants are added for polarity and threshold voltage adjustments.

Fig.4

Fig.4 illustrates the effect of this sequence. Note that the devices are isolated by etching away the silicon in between active areas and depositing oxide. After planarization, the next steps are to grow gate oxide, pattern the poly gate, form the spacers, and implant both drain and source dopants. The transistors at this point have the cross-section is shown in Fig.5.

Fig.5

A refractory metal coating is used to form the polycide and reduce the contact resistance. These represent the final device structures.

From this point, the process is similar to that in standard bulk CMOS. Oxide is deposited, and contact cuts are formed and then filled to create a plug. The next step is a Metal1 deposition and patterning, which yields the structure illustrated in Fig.6. Additional vias and metal patterns are formed in the same manner as in standard processing.

Fig.6

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