FlexVia: A flexible via enabling novel chip designs for memory and optical storage applications
Silicon microchips consist of a base layer of silicon with active devices (transistors) in the surface plane. Above it, a complex three dimensional network of horizontal copper interconnects and vertical tungsten plugs connect the active devices together. Antifuses are widely used to permanently program integrated circuits (ICs). ICs that use antifuse technology employ a thin barrier of non-conducting amorphous silicon between two metal conductors. When a sufficiently high voltage is applied across the amorphous silicon it is turned into a polycrystalline silicon-metal alloy with a low resistance, which is conductive, defining a custom circuit after manufacture. Antifuse programmable logic devices are one-time programmable.
The FlexVia allows for movable vias (nanomechanical relays) with non-volatile properties. Silicon transistors are thus replaced by active high-speed vias. This way, switches and memory elements can be present in all locations of the 3D circuit, decreasing interconnect path lengths, decreasing dissipation and power, and increasing processor speed. Manufacturing cost is reduced significantly by removing the silicon processing while substrate material choice can be optimized for cost or thermal conductance (e.g. glass or thermally conductive AlN).
Electrostatic attraction is used to move the flexible via. Non-volatility is provided by Van der Waals attraction. Low power is achieved by using complementary switches, such that no continuous current flows (similar to CMOS logic).
To summarize, key advantages include:
• High speed: Low ON switch-resistance leads to low RC delays, no ohmic contacts, shorter interconnect paths
• Low power: Through electrostatics and complementary switching
• Radiation hard: No charge storage/leakage
• Wide temperature: operates in a wide temperature range
• All metal: Ultra Large Scale Integration with metal lines only
• Cost effective: fewer lithography steps
• 3D integrated: Reduced interconnect complexity due to 3D local presence of logic/memory
Intellectual Property Status: Patent(s) Pending
The technology is broadly applicable in the MEMS/NEMS and semiconductor industry. Chips produced with the flexible via can be used in place of current technology for CMOS digital logic, solid state memory (DRAM, SRAM, FLASH), hard disks, optical disks (for data storage) and more. The flexible via addresses several limitations inherent in integrated circuit architectures. Semiconductor scaling is compromised by the presence of both metallic interconnects and semiconducting elements. Problems include:
• High local doping is required, and as devices get smaller the risk is increased that the dopants will diffuse over the active devices.
• A fast non-volatile memory element is lacking.
• The different and often incompatible materials (semiconductors, metals) require intricate barrier layers and a large number of mask steps
• Active elements can be placed only in the bottom layer, resulting in long (and slower) interconnect paths.
The FlexVia can address these issues by replacing a vertical via with a nano-electromechanically movable tungsten via. This movable via can form both a logic switching element and a memory element. This distributes logic and memory elements in 3D over an integrated circuit, decreasing interconnect path length, increasing IC speed and eliminating complex and costly manufacturing issues by enabling an all metal IC.