Isolation structures for semiconductor integrated circuit substrates and methods of forming the same

Abstract

Isolation regions for semiconductor substrates include dielectric-filled trenches and field oxide regions. Protective caps of dielectric materials dissimilar from the dielectric materials in the main portions of the trenches and field oxide regions may be used to protect the structures from erosion during later process steps. The top surfaces of the isolation structures are coplanar with the surface of the substrate. Field doping regions may be formed beneath the field oxide regions. To meet the demands of different devices, the isolation structures may have varying widths and depths.

Description

FIELD OF THE INVENTION [0001] This invention relates to semiconductor chip fabrication and in particular to methods of fabricating structures to isolate electrically the active or passive devices formed on a semiconductor chip. BACKGROUND OF THE INVENTION [0002] In the fabrication of semiconductor integrated circuit (IC) chips, it is frequently necessary to electrically isolate devices that are formed on the surface of the chip. There are various ways of doing this. A way is by using the well-known LOCOS (Local Oxidation Of Silicon) process, wherein the surface of the chip is masked with a relatively hard material such as silicon nitride and a thick oxide layer is grown thermally in an opening in the mask. Another way is to etch a trench in the silicon and then fill the trench with a dielectric material such as silicon oxide. [0003] It is desirable to form these isolation structures early in the process because they can also act as barriers or stops to the lateral diffusion of dopan...

AI Technical Summary

Benefits of technology

[0008] According to this invention, an isolation structure is formed by filling a trench in a semiconductor substrate with a “dielectric fill.” The dielectric fill includes a first dielectric material and a second dielectric material. The first dielectric material is located in a lower portion of the trench; the second dielectric material is located in an upper portion of the trench, the lower portion typically being larger in the vertical dimension than the upper portion. The surface of the second dielectric material is substantially coplanar with a surface of the substrate. The first and second dielectric materials are dissimilar in the sense that the second dielectric material is not etched by a chemical which etches the first dielectric material. Thus in subsequent processing the second dielectric material forms a protective cap over the first dielectric material. Typically, the first dielectric material is a relatively soft, low-stress material and the second dielectric material is a relatively hard, etch-resistant material. Cracking problems can be avoided by limiting the thickness of the second dielectric layer to a value that provides protection during later etching processes but does not create stress problems.

[0010] The sidewalls of the trench may be lined with an oxide layer to prevent dopants from the dielectric fill from migrating into the semiconductor substrate.

[0017] The methods of this invention are highly flexible and can be used to form isolation regions necessary meet the varying demands of different regions and devices in a semiconductor substrate. The topography of the substrate is maintained extremely flat, or at least sufficiently flat as not to interfere with or complicate the formation of fine line widths and submicron features or the interconnection thereof during subsequent processing. Protective caps can be used to protect the dielectric materials from erosion during subsequent processing.

Problems solved by technology

The problem with forming a dielectric-filled trench early in the process it that subsequent process steps, which frequently include etching and cleaning, can etch or erode the dielectric material in the trench.

This can impair the value of the trench as an isolation structure and can create depressions in the top surface of the chip, rendering further processing more difficult.

Dielectric materials that are resistant to etching in normal semiconductor processes (e.g., silicon nitride) tend to be hard, brittle, high-stress materials.

When these materials are deposited in a trench they tend to crack.

A second problem stems from the fact that chips are generally divided into two general areas: broad or wide “field” areas and more densely-packed device areas, sometimes referred to as “active” areas.

This creates a problem in filling the trenches.

The narrow trenches may be filled while the wide trenches are difficult to fill.

Alternatively, using numerous narrow trenches to cover large distances in the field areas can complicate the topography of the chip.

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