Selective Hard Mask for Sub-60nm MRAM Ion Beam Etching
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Solution Overview
Problem
The existing etching methods for magnetic tunnel junctions (MTJ) in MRAM devices face challenges in achieving selective physical etching below 60 nm, leading to sidewall damage and re-deposition of metal materials, which results in electrically shorted devices and pattern integrity issues.
Innovation Solution
A method involving a selective hard mask with low etch rate materials like carbon, combined with high-angle ion beam etching, allows for selective etching and over etching to prevent re-deposition on the tunnel barrier layer, while using dielectric-enclosed vias to connect the bottom electrode, ensuring pattern integrity and reducing shorted devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If pure physical etching (ion beam etching) is used to etch the MTJ stack, then sidewall damage is avoided, but conductive materials in the MTJ and bottom electrode are re-deposited into the tunnel barrier, resulting in shorted devices
Solution Approach 1:
A dielectric hard mask layer is introduced as an intermediary between the conductive materials and the tunnel barrier. This dielectric layer acts as a barrier that prevents re-deposited conductive materials from creating shorting paths through the tunnel barrier, while allowing the physical etching process to continue without causing sidewall damage.
Solution Approach 2:
The dielectric hard mask is deposited and patterned before the ion beam etching process. This preliminary action ensures that the protective barrier is in place before any etching occurs, preventing re-deposition issues from the outset rather than attempting to correct them afterward.
2Reliability
If high angle IBE is used to trim the MTJ sidewall to remove re-deposition, then device shorting is reduced, but the MTJ hard mask must be very thick, requiring very thick photoresist that can easily collapse at sub 60 nm dimensions
Solution Approach 1:
The solution moves from modifying the sidewall geometry (horizontal dimension) to adding a protective layer (vertical dimension). Instead of requiring thick hard masks for sidewall trimming, a thin dielectric hard mask layer is deposited on top of the patterned MTJ structure, providing protection in the vertical dimension without affecting the horizontal photoresist thickness requirements.
3Reliability
If great over etch is applied to confine re-deposition below the tunnel barrier, then device shorting is reduced, but the MTJ hard mask must be very thick because physical etching is not selective
Solution Approach 1:
The etch selectivity parameter is changed by introducing the dielectric hard mask material, which has different etch characteristics compared to the original hard mask materials. This dielectric material provides the necessary protection during over-etching without requiring excessive thickness, as it can be precisely controlled and deposited to optimal thicknesses.
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 selectivity of ion beam etching, reduces re-deposition, and maintains device integrity by confining conductive material re-deposition below the MTJ junction, thereby improving the yield and reducing electrical shorts in sub-60 nm MTJ devices.
Implementation Method 1
pure physical etching techniques such as ion beam etching (IBE) have been applied to etch the MTJ stack
Implementation Method 2
The top electrode is trimmed using ion beam etching (IBE) at an angle of 70 to 90 degrees with respect to a normal line of a top surface of the top electrode
Data Source
AI summary
A via connection is provided through a dielectric layer to a bottom electrode. A MTJ stack is deposited on the dielectric layer and via connection. A top electrode is deposited on the MTJ stack. A selective hard mask and then a dielectric hard mask are deposited on the top electrode. The dielectric and selective hard masks are patterned and etched. The dielectric and selective hard masks and the top electrode are etched wherein the dielectric hard mask is removed. The top electrode is trimmed using IBE at an angle of 70 to 90 degrees. The selective hard mask, top electrode, and MTJ stack are etched to form a MTJ device wherein over etching into the dielectric layer surrounding the via connection is performed and re-deposition material is formed on sidewalls of the dielectric layer underlying the MTJ device and not on sidewalls of a barrier layer of the MTJ device.