Bipolar Transistor Base Air Gap Fabrication
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Solution Overview
Problem
Existing methods for manufacturing bipolar transistors are complex and prone to short-circuits due to difficulties in controlling the dimensions of the air gap between the extrinsic and intrinsic parts of the base, which complicates electrical insulation of the collector.
Innovation Solution
A method involving a stack of selectively etchable layers on a semiconductor substrate, where an intrinsic part of the base is formed by epitaxy with an air gap surrounding the collector, and an extrinsic part is formed by selective deposition of a semiconductor layer, using silicon oxide and silicon nitride layers for insulation, reducing the number of steps and enhancing robustness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an air gap is used to electrically insulate the collector from the extrinsic part of the base, then electrical insulation is achieved, but the manufacturing process becomes complex and difficult to control
Solution Approach 1:
The patent introduces an intermediary sacrificial layer (third layer) made of a material that can be selectively etched. This layer temporarily occupies the space where the air gap will eventually be, allowing the intrinsic base to be formed with proper alignment. The sacrificial layer is then removed to create the air gap, eliminating the complexity of directly forming and controlling air gaps while maintaining reliable electrical insulation.
Solution Approach 2:
The method performs preliminary actions by first forming the complete stack including the sacrificial third layer before creating the air gap. The intrinsic base is formed in relation to this sacrificial layer, and only after all other structures are in place is the third layer removed. This preliminary positioning simplifies the overall process by providing a temporary reference structure that guides subsequent fabrication steps.
2Reliability
If traditional methods are used to form the air gap, then electrical insulation is achieved, but the dimensions of the air gap are difficult to control leading to short-circuit risks
Solution Approach 1:
The sacrificial third layer acts as a mediator that defines the air gap dimensions indirectly. Instead of attempting to directly control the air gap size, the process controls the thickness and position of the solid third layer, which is then removed to create the air gap. This transfers the dimension control problem from an air interface (difficult to control) to a solid layer (easily controlled through standard deposition processes).
Solution Approach 2:
The patent replaces the mechanical challenge of directly forming and measuring air gaps with a material-based approach. The third layer's thickness, which determines the future air gap size, is controlled through deposition process parameters rather than mechanical measurement or adjustment. This substitution of direct air gap formation with sacrificial layer formation enables precise dimensional control.
3Productivity
If the number of steps is reduced in the manufacturing process, then productivity increases, but the robustness of electrical insulation may be compromised
Solution Approach 1:
The patent merges multiple functions into the third layer: it serves as a sacrificial structure for air gap formation, a alignment reference for forming the intrinsic base, and a temporary mechanical support during fabrication. By combining these functions into a single integrated layer rather than using separate structures for each purpose, the process achieves robust electrical insulation with fewer discrete steps.
Solution Approach 2:
The third layer is designed as a multi-functional element that performs multiple roles throughout the fabrication process. It provides the structural template for the air gap, serves as an alignment guide for subsequent etching and deposition steps, and maintains mechanical integrity during processing. This universal design allows a single layer to accomplish what would otherwise require multiple separate components and steps.
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 ensures robust electrical insulation between the base and collector, improves crystalline quality, and simplifies the manufacturing process by reducing the complexity of air gap control, making the bipolar transistor easier to produce on an industrial scale.
Implementation Method 1
each of the first to fourth layers is able to be etched selectively compared to each adjacent layer
Implementation Method 2
forming by epitaxy on the substrate of the collector of the bipolar transistor and the formation by selective etching of an annular opening in the third layer; forming an intrinsic part of the base by epitaxy on the collector
Data Source
AI summary
A method of making a bipolar transistor includes forming a stack of a first, second, third and fourth insulating layers on a substrate. An opening is formed in the stack to reach the substrate. An epitaxial process forms the collector of the transistor on the substrate and selectively etches an annular opening in the third layer. The intrinsic part of the base is then formed by epitaxy on the collector, with the intrinsic part being separated from the third layer by the annular opening. The junction between the collector and the intrinsic part of the base is surrounded by the second layer. The emitter is formed on the intrinsic part and the third layer is removed. A selective deposition of a semiconductor layer on the second layer and in direct contact with the intrinsic part forms the extrinsic part of the base.