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Kilopass Unveiled Vertical Layered Thyristor (VLT) Technology for DRAMs

Gabe Moretti, Senior Editor

Kilopass Technology, Inc., is a leader in embedded non-volatile memory (NVM) intellectual property (IP).  Its patented technologies of one-time programmable (OTP) NVM solutions scale to advanced CMOS process geometries. They are portable to every major foundry and integrated device manufacturer (IDM), and meet market demands for increased integration, higher densities, lower cost, low-power management, better reliability and improved security.  The company has just announced a new device that potentially allows it to diversify into new markets.

According to Charlie Cheng, Kilopass’ CEO, VLT eliminates the need for DRAM refresh, is compatible with existing process technologies and offers significant other benefits including lower power, better area efficiency and compatibility.  When asked the reason for this additional corporate direction Charlie replied: “Kilopass built its reputation as the leader in one-time programmable memories,” says Charlie Cheng, Kilopass’ chief executive officer. “As the next step on our roadmap, we examined many possible devices that would not need new materials or complex process flows and found this vertical thyristor to be very compelling.  We look forward to commercializing VLT DRAM in early 2018.”

VLT Overview

Kilopass’ VLT is based on thyristor technology, a structure that is electrically equivalent to a cross-coupled pair of bipolar transistors that form a latch. The latch lends itself to memory applications since it stores values and, as opposed to current capacitor-based DRAM technology, does not require refresh. The thyristor was first invented in the 1950s and several attempts have been made to use it for the SRAM market without success.  Kilopass’ VLT is the realization of DRAM requirements based on implementing the thyristor structure vertically.

Since VLT does not require complex performance- and power-consuming refresh cycles, a VLT-based DDR4 DRAM lowers standby power by 10X when compared to conventional DRAM at the same process node. Furthermore, VLT requires fewer processing steps and is designed to be built using existing processing equipment, materials and flows.

The VLT bitcell operations and silicon measurement were completed in 2015 and shown to have excellent correlation to Kilopass’ proprietary ultra-fast TCAD simulator that is one hundred thousand times faster than a traditional TCAD simulator. The TCAD simulator enables Kilopass to predict the manufacturing windows for key process parameters, and optimize the design for any given manufacturing process.  A full macro level test chip was taped-out in May and initial silicon testing is underway.
Industry Perspective

The $50B DRAM market is being driven by strong demand in the server/cloud computing market as mobile phone and tablet market growth are slowing down and computing is moving increasingly to the cloud. The outlook for DRAM growth remains strong. In a report published in 2015, IC Insights forecasts DRAM CAGR of 9% over the period from 2014 – 2019. This growth rate shows DRAM growing faster than the total IC market.

Servers and server farms consume a tremendous amount of energy with memory being a major contributor. In an ideal world, the current generation of 20 nanometer (nm) DRAM would migrate to sub-20nm processes to deliver even lower power.

Current DRAM technology is based on the 1 transistor, 1 capacitor. The (1T1C) bitcell is difficult to scale since the smaller transistors exhibit more leakage and the smaller capacitor structure has less capacitance, resulting in the need to reduce the time between refresh intervals. Up to 20% of a 16Gb DDR DRAM’s raw bandwidth will be lost due to the increased frequency of refresh cycles, a negative for multi-core/multi-thread server CPUs that must squeeze out every bit of performance to remain competitive. The DRAM industry is in a quandary trying to increase memory performance while reducing power consumption, a tough challenge given the physics at play with the current 1T1C technology. In order to address the need for lower power consumption a new DRAM technology and architecture is needed.

Kilopass stated that its initial target markets include “PCs” and servers. I am of the old school and associate the term “PC” to personal computers.  But Kilopass uses the term to mean Portable Computing Devices so it is talking about a different market.  Kilopass expects to have test silicon by early 2017 that will confirm performance of the new VLT DRAM technology and manufacturability.   Kilopass has two primary reasons to announce the new technology over one year in advance of product delivery.  First the company is in the IP business, so it is giving itself time to look for licensees.  Secondly it thinks that the DRAM market has been stuck at 20nm. Adoption of new technology takes time, though VLT has been shown to be manufacturable. This is the right time to alert the market that there are alternative solutions, allowing time for investigation of this new technology.   Market penetration of new technology is not always assured.  Wide acceptance almost always requires a second source, especially with something so new as the VLT device.  Memories play a critical role cloud computing but a far smaller one in PC since power consumption in PC is not a widespread issue.

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