Top Electrode Pillar Fabrication for Sub-60nm MRAM Devices
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Solution Overview
Problem
Current processes for fabricating magnetoresistive random access memory (MRAM) devices with critical dimensions less than 60 nm face challenges in achieving a sufficient process window and maintaining top electrode integrity due to the varying etch rates of materials in the MTJ stack, leading to reduced yields and uniformity.
Innovation Solution
A process flow involving a carbon hard mask and silicon containing bottom anti-reflective coating (BARC) is used, where vias are etched through the BARC and carbon hard mask, followed by metal deposition and chemical mechanical polishing (CMP) to form top electrode pillars, which are then trimmed using ion beam etching to achieve the desired critical dimension and height, allowing for effective pattern transfer through the MTJ stack.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a thick photoresist layer is used to pattern the top electrode, then the pattern integrity is improved, but the photoresist islands collapse due to high aspect ratio
Solution Approach 1:
The patent divides the single thick photoresist layer into multiple thinner photoresist layers, each patterned separately. This segmentation reduces the aspect ratio of individual photoresist islands, preventing collapse while maintaining overall pattern integrity through sequential patterning steps
Solution Approach 2:
The patent performs preliminary patterning actions by forming patterns in multiple thinner photoresist layers before final etching. This preliminary action allows each layer to be supported by underlying structures, preventing collapse before the complete pattern transfer is achieved
2Ease of manufacture
If conventional photoresist/hard mask combinations are used, then the process is simple, but the selectivity is insufficient for patterning top electrode materials
Solution Approach 1:
The patent employs a composite hard mask system combining multiple materials (e.g., silicon oxide, silicon nitride, or tantalum oxide layers) with specific etch selectivity characteristics. This composite structure provides the necessary selectivity against top electrode materials like Ta and TiN while maintaining a manageable process complexity through standardized deposition and etching procedures
3Reliability
If the top electrode thickness is reduced to maintain aspect ratio, then the photoresist island stability is improved, but the process window for etching and CMP is reduced
Solution Approach 1:
By segmenting the patterning process into multiple steps with thinner photoresist layers, the patent enables the use of thicker top electrode material without requiring proportionally thicker photoresist support. Each patterning step works with reduced aspect ratios while the cumulative effect achieves the desired pattern fidelity
Solution Approach 2:
The preliminary patterning in thinner photoresist layers establishes a stable foundation before subsequent etching and CMP operations. This preliminary action creates intermediate structures that guide later processing steps, expanding the acceptable range for top electrode thickness
4Stability of the object's composition
If various materials in the MTJ stack have different etch rates, then the material properties are preserved, but the top electrode masking effectiveness is reduced
Solution Approach 1:
The patent uses a composite hard mask structure with multiple layers having different etch selectivities. This composite system is designed to provide superior masking effectiveness against all materials in the MTJ stack (magnetic alloys, non-magnetic metals, dielectric films) while preserving their individual etch rate characteristics through selective etching processes
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 approach enhances the process window and yield for forming MTJ cells with critical dimensions less than 60 nm, maintaining a top electrode thickness of at least 70 nm, thereby improving the uniformity and manufacturability of MRAM devices.
Implementation Method 1
trimmed with ion beam etching (IBE) after the hard mask is removed to yield top electrode pillars of sufficient height
Implementation Method 2
chemical mechanical polishing (CMP) processes that form magnetic tunnel junction (MTJ) devices
Data Source
AI summary
A process flow for forming magnetic tunnel junction (MTJ) cells with a critical dimension CD≤60 nm by using a top electrode (TE) hard mask having a thickness≥100 nm prior to MTJ etching is disclosed. A carbon hard mask (HM), silicon HM, and photoresist are sequentially formed on a MTJ stack of layers. A pattern of openings in the photoresist is transferred through the Si HM with a first reactive ion etch (RIE), and through the carbon HM with a second RIE. After TE material is deposited to fill the openings, a chemical mechanical process is performed to remove all layers above the carbon HM. The carbon HM is stripped and the resulting TE pillars are trimmed to a CD≤60 nm while maintaining a thickness proximate to 100 nm. Thereafter, an etch process forms MTJ cells while TE thickness is maintained at ≥70 nm.