SOI Substrate Thinning via Thermal Oxidation and Etching
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Solution Overview
Problem
Thinning of the active silicon layer in silicon-on-insulator (SOI) substrates to below 50 nanometers is challenging due to edge fragility and particle redeposition, which damages the encapsulation layer and leads to substrate pollution, making it unfit for further component fabrication.
Innovation Solution
A method involving thermal oxidation, followed by two cycles of etching and cleaning using hydrofluoric acid, to thin the silicon surface layer and remove polluting particles, while maintaining the protective encapsulation layer integrity, even after rapid thermal annealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the active silicon surface layer is thinned by etching to below 50 nanometers, then the substrate becomes suitable for fully depleted transistor fabrication, but the edges become fragile and break forming particles that pollute the active layer
Solution Approach 1:
The method performs preliminary actions by conducting thermal oxidation to form a protective oxide layer on the silicon surface before etching, and performing multiple cleaning cycles between etching steps to remove particles before they can cause pollution. The first etching cycle is followed by a cleaning step, then a second etching cycle with another cleaning step, ensuring particles are removed at each stage rather than allowing accumulation that would pollute the final active layer.
Solution Approach 2:
The etching process is segmented into multiple cycles rather than a single continuous etching step. The method divides the thinning process into: first etching cycle to remove thermal oxide and thin the layer, followed by cleaning, then second etching cycle to achieve final thickness, with cleaning steps in between. This segmentation allows particle removal at intermediate stages, preventing pollution of the final active layer.
2Manufacturing precision
If Rapid Thermal Annealing is performed to smooth the silicon surface layer, then the surface roughness is reduced, but the encapsulation layer becomes damaged and loses its protective role
Solution Approach 1:
The method performs preliminary thermal oxidation to form a robust oxide layer on the silicon surface before performing any annealing or etching operations. This pre-formed oxide layer serves as a protective barrier that maintains encapsulation layer integrity during subsequent processing steps, preventing the damage that would otherwise occur during RTA treatment.
Solution Approach 2:
The method changes the oxidation parameters by performing thermal oxidation at controlled temperatures to form an oxide layer with specific properties that protect the encapsulation layer. By controlling the oxidation temperature and duration, the method creates an oxide layer that is thick enough to protect during annealing but can be subsequently removed to achieve the desired final thickness.
3Manufacturing precision
If substantial material is removed from the initial SOI substrate to achieve thin active layers, then fully depleted transistor fabrication becomes possible, but the quantity of material removal increases process complexity and time
Solution Approach 1:
The method merges multiple functions into the thermal oxidation step: it forms a protective oxide layer on the silicon surface, creates a defined etch stop layer, and prepares the surface for subsequent etching. This single oxidation step accomplishes what would otherwise require multiple separate preparation steps, reducing overall process complexity despite the multiple etching cycles needed.
Solution Approach 2:
The method maintains continuous useful action by performing etching and cleaning cycles that progressively thin the layer while continuously removing particles. Rather than stopping and restarting, the process flows continuously through oxidation, first etching, cleaning, second etching, and final cleaning, with each step building on the previous to achieve the final thin layer without interruption or rework.
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
The method effectively thins the active silicon layer to less than 50 nanometers, ensuring a clean and suitable substrate for component fabrication by removing particles and maintaining the protective encapsulation layer, thus overcoming the limitations of existing techniques.
Implementation Method 1
thermal oxidation treatment of the initial substrate to cause oxidation of part of the silicon surface layer
Implementation Method 2
a first cycle of etching then cleaning; and a second cycle of etching then cleaning, the etching of the first cycle being performed so as fully to remove the thermal oxide layer formed
Data Source
AI summary
A method to thin an initial silicon-on-insulator substrate that has a layer of silicon oxide buried between a silicon carrier substrate and a silicon surface layer. This method is noteworthy in that it includes conducting a thermal oxidation treatment of the initial substrate to cause oxidation of part of the silicon surface layer and form a thermal oxide thereon; conducting a first cycle of etching followed by cleaning of the silicon surface layer after the thermal oxidation treatment, wherein the etching of the first cycle is conducted so as to fully remove the thermal oxide from the silicon surface layer to thin it and lift off all unstable parts of the initial substrate at edges thereof to form a thinned substrate; conducting, after the first cycle, a second cycle of etching followed by cleaning of the silicon surface layer, wherein the etching of the second cycle is conducted to remove from the surface of the thinned substrate, polluting particles formed during the first etching cycle and that have deposited thereupon, in order to obtain a final SOI substrate having a thinned surface layer that forms an active layer for the substrate.