Thin MEMS Chips on SOI Substrate via Etching
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Solution Overview
Problem
Current methods for producing thin MEMS chips on SOI substrates are either costly, cause mechanical vibrations that can destroy sensitive structures, or result in uneven surfaces and inhomogeneous thickness due to grinding or etching processes, making them unsuitable for sensitive MEMS sensors like acceleration and rotation-rate sensors.
Innovation Solution
A method involving an SOI substrate with a silicon layer and an oxide intermediate layer, where a MEMS structure is produced, capped, and the back side is etched down to the oxide layer using a non-mechanical removal process, allowing for precise thinning without compressing or vibrating the cavity, and using a cap wafer or additional SOI substrate for capping and etching to achieve thin, defect-free chips.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If the substrate is ground down from behind to produce very thin wafers, then the chip thickness is reduced, but vibrations are coupled into the substrate causing freely movable MEMS structures to break
Solution Approach 1:
The substrate is divided into multiple thin layers including a first substrate layer, a sacrificial layer, and a second substrate layer. This segmentation allows the sacrificial layer to be selectively removed to create a cavity while maintaining structural support from the remaining substrate layers, preventing vibrations from destroying sensitive MEMS structures during thinning processes
Solution Approach 2:
The sacrificial layer is deposited and structured before the MEMS device is fully formed. This preliminary action creates a pre-defined cavity space that guides subsequent processing steps, allowing the substrate to be thinned without compromising the integrity of freely movable structures since the cavity is already in place
2Length of moving object
If grinding pressure is applied to remove material, then the substrate is thinned, but the cavity is squeezed together and MEMS structures are destroyed
Solution Approach 1:
The mechanical grinding process is replaced with a chemical etching process that removes material from the back side of the substrate. This substitution eliminates the high pressure and mechanical contact that would squeeze and distort the cavity, allowing precise thickness control while maintaining cavity integrity through controlled chemical reactions rather than mechanical force
3Object-affected harmful factors
If wet etching or plasma etching is used to remove substrate material, then mechanical contact is avoided, but the removal rate is highly inhomogeneous across the wafer resulting in uneven surfaces
Solution Approach 1:
The etching parameters are optimized by controlling etch depth, etch rate, and etch uniformity across the wafer surface. By carefully adjusting these parameters and using the sacrificial layer as an etch stop, the process achieves both gentle material removal without mechanical contact and highly uniform thickness reduction across the entire wafer, producing even surfaces suitable for thin MEMS chips
4Length of moving object
If buried cavities are created in the substrate to produce very thin chips, then chip thickness is reduced without back-thinning, but the creation process is painstaking and expensive with limitations in process control
Solution Approach 1:
The sacrificial layer automatically defines the cavity geometry and serves as an etch stop during back-side thinning. This self-service approach eliminates the need for complex, painstaking cavity creation processes while providing excellent process control - the sacrificial layer itself guides the etching process to the correct depth, simplifying manufacturing and reducing costs
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 enables the production of very thin, robust MEMS chips with precise thickness and reduced surface area, minimizing mechanical stress and process fluctuations, allowing for smaller chip spacings and lower production costs while maintaining the integrity of sensitive structures.
Implementation Method 1
etching a back side of the SOI substrate down to the oxide intermediate layer
Data Source
AI summary
A method for producing thin MEMS chips on SOI substrate including: providing an SOI substrate having a silicon layer on a front side and having an oxide intermediate layer, producing a layer structure on the front side of the SOI substrate and producing a MEMS structure from this layer structure, capping the MEMS structure and producing a cavity, and etching a back side of the SOI substrate down to the oxide intermediate layer. Also described is a micromechanical component having a substrate, a MEMS layer structure having a MEMS structure in a cavity and a cap element, the MEMS structure and its cavity being enclosed by the substrate underneath and the cap element above, the substrate being made of polycrystalline silicon.