The SK hynix is Leading the Way in the HKMG Revolution

Just as performance requirements for DRAM increase due to limitations in traditional scaling technology systems, HKMG has emerged as a breakthrough solution. By using this new technology, SK hynix has dramatically improved transistor performance and has applied it to its new 1anm LPDDR5X DRAM, effective even at low power settings. This article will examine what HKMG is and look into the benefits of its use.

TOX PARADOX: NEED FOR A NEW SOLUTION

DRAM is composed of the following transistors: the cell transistor, which stores data; the core transistor, which restores data; and the peripheral transistor, which relates to control logic and data inputs and outputs. As technology has progressed, cell capacitors and transistors have gone through a number of technological breakthroughs to increase the memory capacity of DRAM. For peripheral transistors, on the other hand, the focus has been on technology scaling to improve performance.

An insulating film (gate oxide) and an electrode (gate electrode) make up the gate, which is at the core of a transistor’s on/off functionality. The gate oxide consists of a SiON oxide insulator and a poly silicon-based electrode. As transistor scaling continues to progress, the distance between the source¹ and drain² gets closer and the current moves faster, but the voltage applied to the gate also decreases to reduce power consumption.

There is a problem, however: to improve performance at a lower voltage, the thickness (Tox) of the gate oxide material (SiON) must be reduced. But as the thickness continues to decrease, the gate oxide’s reliability also deteriorates, resulting in power loss and limiting further reductions in thickness.

HKMG: A BREAKTHROUGH IN SCALING AND PERFORMANCE

In the mid-2000s, traditional scaling based on polySi Gate/SiON Oxide (poly/SiON) in logic semiconductors³⁾ started exhibiting limitations in performance improvement since it was no longer possible to reduce the thickness of the SiON gate oxide. To overcome these limitations, a number of disruptive technological innovations have been developed in accordance with the industry roadmap for logic transistors.

It is also evident that peripheral/core transistor characteristics are becoming bottlenecked in DRAM, especially in premium products where rapid performance improvement is needed. Consequently, a new solution was required to overcome the fundamental limit for scaling poly-Si Gate/SiON Oxide-based transistors, and the adoption of High-k/Metal Gate (HKMG) technology in DRAM, which led to the most significant innovation in logic transistor technology, has continued to be necessary.

With HKMGs, a thin High-k film replaces the existing SiON gate oxide in the gate of the transistor to prevent leakage currents and degradation of reliability. Furthermore, by reducing thickness, continuous scaling can be achieved, resulting in a significant reduction in leakage, as well as improved speed characteristics over poly-Si/SiON based transistors.

In both academia and industry, researchers have studied a variety of High-k thin film materials. Generally, Hf-based gate oxides are used in high-temperature semiconductor manufacturing processes because they ensure thermal stability with themselves and with silicon. In order to prevent the interaction between the existing poly-Si electrode material and the High-k gate oxide, a metal electrode must be introduced to replace the poly-Si. This led to the development of an integrated solution called High-k/Metal Gate, which combines a high dielectric constant gate oxide with a metal electrode.

Several sections of the relevant processes were changed to convert poly/SiON gates into HKMG gates, including the gate material where the peripheral circuit (peripheral transistor) is formed during the DRAM process flow (poly/SiON gate plug-out → HKMG electrode plug-in). However, the HKMG materials, processes, and integration flow must be optimized to be suitable for the building blocks of the new material and process. And so, a complex development process needs to be utilized to address the following challenges.

1. Compatibility: Compared to poly/SiON gates, HKMG has relatively weak thermal stability. Specifically, DRAM requires additional processes that take place at high temperatures in order to implement the cell array structure, as opposed to general logic semiconductors in the subsequent process flow. Due to this, HKMG’s own reliability deteriorates, resulting in an interaction not encountered in conventional logic semiconductors. The HKMG process itself and the existing DRAM integration process must therefore be optimized to understand the new problems arising from the new interaction and find solutions.
2. New Materials Control: Introducing process control measures, such as measurement solutions for new substances, is required to prevent existing equipment and products from being affected by new substances and processes.
3. Design & Test Optimization: With a change in the gate material, the transistor characteristics and reliability behavior differ significantly from traditional poly/SiON gates, and in order to maximize the advantages of HKMG and to enhance the reliability characteristics different from those of poly/SiON gates, it is necessary to apply a new design and design scheme and optimize such a test.
4. Cost Effective Process Solution: Finally, a cost-effective solution must be provided that minimizes the increased costs resulting from the introduction of new materials and processes through process integration optimization. By doing so, cost increases associated with new processes, equipment, and process steps can be controlled.

LOW POWER, LEADING SOLUTION

SK hynix pursued its platform development by differentiating the HKMG process integration into a form that is suitable for DRAM processes. In spite of the extreme technological challenges, the company succeeded in developing and mass-producing HKMG by identifying any potential risks associated with DRAM flow interactions and securing a solution through a pre-verification process that included pilot operations. The company’s goal is to bring greater technological innovation to next-generation tech nodes and products by advancing the transition from poly/SiON gates to the upgraded building blocks, HKMG.

SK hynix’s LPDDR5X DRAM, which was successfully mass-produced for the first time using HKMGs in a low power application, dramatically improves transistor performance through aggressive scaling while utilizing the advantages of the new HKMG transistor building blocks. With HKMG’s intrinsic characteristics in mind and a design scheme optimized for HKMG, the leakage current could be effectively controlled, resulting in a 33% improvement in speed and a 25% reduction in power over poly/SiON. SK hynix’s technology not only meets the industry’s target specifications but also maximizes ESG values with minimal power consumption.

Consequently, SK hynix has further developed its HKMG Technology Platform to support both Low Power (LP) and High Performance (HP) products and to also strengthen its technological competitiveness in the next generation of HKMG technology.

As a final note, it is worth noting that the application of the HKMG process to DRAM is in line with the recent technological innovation trend of converging logic and memory semiconductor architectures such as HBM, PIM, AiM, etc. This suggests that the convergence between logic semiconductors’ advanced technology solutions and DRAM process technology in the semiconductor manufacturing process is in full swing.

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