Conductive Shielding Plates for MEMS Cross-Talk Mitigation
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Solution Overview
Problem
MEMS devices encapsulated by active circuitry face issues with time-varying parasitic capacitance and impedance cross-talk due to their close proximity to ASIC circuitry, leading to potential electrostatic attraction and interference.
Innovation Solution
Formation of conductive shielding plates on the ASIC wafer using a TiN layer, which is selectively etched to create standoffs and electrodes, allowing for electrical isolation and potential biasing to mitigate cross-talk and maintain zero electrostatic attraction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If MEMS device structures are placed in close proximity to ASIC circuitry to achieve compact wafer-level packaging, then packaging density and integration are improved, but parasitic capacitance and impedance cross-talk increase causing circuit interference
Solution Approach 1:
A conductive shielding plate is introduced as an intermediary element between the MEMS device structure and ASIC circuitry. The shielding plate is positioned in the cavity above the MEMS structure and connected to ground through a standoff, creating an electrostatic shield that blocks parasitic capacitance and impedance cross-talk while allowing the compact wafer-level packaging to be maintained
Solution Approach 2:
The cavity space is segmented into distinct functional zones by introducing the shielding plate structure. This segmentation separates the MEMS active area from the ASIC circuitry, creating an electrostatic boundary that reduces interference while preserving the integrated package architecture
2Object-affected harmful factors
If conductive shielding plates are added to block cross-talk, then interference is reduced, but device complexity and manufacturing steps increase
Solution Approach 1:
The TiN layer serves multiple functions: it provides the conductive material for the shielding plate, forms the standoff bases, and creates electrodes for signal transmission to the MEMS device. This multi-functionality reduces the need for separate structures and simplifies the overall device architecture despite adding shielding capability
Solution Approach 2:
The shielding plate structure is merged with the existing passivation layer and TiN layer architecture. The TiN layer is patterned to simultaneously create the shielding plate, standoffs, and electrodes, combining multiple functional elements into a single integrated fabrication process
3Object-affected harmful factors
If TiN layer and selective etching are used to form shielding plates, then shielding effectiveness is improved, but manufacturing process complexity increases
Solution Approach 1:
The TiN layer is deposited with specific thickness parameters (50-100 nm) that optimize both the shielding effectiveness and the etching selectivity. The passivation layer is designed with specific layer compositions (oxide-nitride-oxide) that provide the necessary etch selectivity, allowing the TiN to be selectively removed in defined patterns without affecting underlying structures
Solution Approach 2:
The passivation layer and TiN layer are deposited and patterned in advance during the ASIC fabrication process, before wafer bonding. This preliminary action prepares the shielding structures in advance, allowing them to be formed as part of the standard ASIC manufacturing flow rather than as separate post-bonding 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
The conductive shielding plates effectively reduce cross-talk and interference by maintaining a controlled electrical potential, ensuring the integrity of MEMS device operation within the wafer-level chip scale package.
Implementation Method 1
forming at least one conductive shielding plate... effectively blocks cross-talk between MEMS device structures and ASIC circuitry, maintaining zero electrostatic attraction
Data Source
AI summary
One or more conductive shielding plates are formed in a standard ASIC wafer top metal layer, e.g., for blocking cross-talk from MEMS device structure(s) on the MEMS wafer to circuitry on the ASIC wafer when the MEMS device is capped directly by the ASIC wafer in a wafer-level chip scale package. Generally speaking, a shielding plate should be at least slightly larger than the MEMS device structure it is shielding (e.g., a movable MEMS structure such as an accelerometer proof mass or a gyroscope resonator), and the shielding plate cannot be in contact with the MEMS device structure during or after wafer bonding. Thus, a recess is formed to ensure that there is sufficient cavity space away from the top surface of the MEMS device structure. The shielding plate is electrically conductive and can be biased, e.g., to the same voltage as the opposing MEMS device structure in order to maintain zero electrostatic attraction force between the MEMS device structure and the shielding plate.