LOCOS Thickness Control Using Maskless Differential Oxidation
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Solution Overview
Problem
Existing methods for forming local oxidation of silicon (LOCOS) structures with different thicknesses on a semiconductor substrate require a mask level, increasing production costs and cycle time.
Innovation Solution
A blanket etch of a sacrificial layer is used to protect a first LOCOS structure while exposing a second LOCOS structure, allowing for differential thickness growth without a mask, facilitated by an oxygen diffusion barrier layer and a test database for process optimization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a mask level is used to form LOCOS structures with different thicknesses, then manufacturing precision is improved, but device complexity and production cost increase
Solution Approach 1:
The patent removes the mask layer from the conventional LOCOS fabrication process. Instead of using a mask to define the second LOCOS structure, the process relies on selective sacrificial layer removal through blanket etching, extracting the masking step entirely and simplifying the overall process complexity while maintaining thickness control precision
Solution Approach 2:
The patent performs preliminary oxidation to form initial LOCOS structures with uniform thickness before selectively removing the sacrificial layer. This preliminary action establishes the base structure, and subsequent selective etching and re-oxidation then create the differential thickness pattern without requiring masks
2Manufacturing precision
If a mask level is used to form LOCOS structures with different thicknesses, then manufacturing precision is improved, but production cycle time increases
Solution Approach 1:
By removing the mask deposition, patterning, and removal steps from the process flow, the patent eliminates multiple time-consuming operations. The blanket etching approach processes the entire wafer simultaneously without the sequential steps required for mask handling, directly reducing production cycle time while preserving thickness control through the sacrificial layer mechanism
Solution Approach 2:
The patent implements continuous processing where the sacrificial layer remains in place during initial oxidation, then is selectively removed in a continuous blanket etch operation followed by continuous re-oxidation. This eliminates the discontinuities introduced by mask deposition and removal, maintaining continuous useful action throughout the LOCOS structure formation process
3Device complexity
If a blanket etch of sacrificial layer is used without mask, then device complexity is reduced, but manufacturing precision may worsen
Solution Approach 1:
The patent applies local quality by creating openings of different widths in the sacrificial layer at different locations. The narrower opening preserves more sacrificial layer material during blanket etching, while the wider opening removes more material. This spatial variation in opening width enables precise local control of LOCOS thickness without masks, maintaining manufacturing precision while simplifying the overall process
Solution Approach 2:
The patent controls LOCOS thickness by changing the width parameter of openings in the sacrificial layer. By varying the opening width, the amount of sacrificial layer remaining after blanket etching is controlled, which in turn controls the final LOCOS thickness. This parameter-based control maintains manufacturing precision while eliminating the need for mask patterns
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method reduces production costs and cycle time by eliminating the need for a mask, enabling efficient fabrication of LOCOS structures with different thicknesses for varied operating voltages in MOS transistors.
Implementation Method 1
the first LOCOS structure can be covered by an oxygen diffusion barrier (ODB) layer, while a portion of the second LOCOS structure can be exposed
Implementation Method 2
A wafer on which the LOCOS structures are formed is then subjected to an oxidation process, allowing the second LOCOS structure to grow to a greater thickness
Data Source
AI summary
A method of fabricating an integrated circuit includes forming a first opening having a first width and a second opening having a second width in a first dielectric layer over a silicon substrate. The openings expose the silicon substrate and the exposed silicon substrate is oxidized to form first and second LOCOS structures having a first thickness. A polysilicon layer is formed over the silicon substrate, so that the polysilicon layer fills the first and second openings. A blanket etch of the polysilicon layer is performed to remove at least a portion of the polysilicon layer over the second LOCOS structure while leaving the first LOCOS structure protected by the polysilicon layer. The silicon substrate under the second LOCOS structure is further oxidized such that the second LOCOS structure has a second thickness greater than the first thickness.