[Chip Talk] DRAM Faces Unprecedented Demand... '3D Evolution' Opens a New Chapter

DRAM Market on the Brink of 3D Transformation
Cell Structure Shifts from Horizontal to Vertical
Devised as Conventional DRAM Hits Miniaturization Limits
Advantages in Space Utilization, Density, Speed, and Power Efficiency
Direct Transition Challenging Due to Major Structural Changes
Shift to Transitional 4F² Structure Required
Leading Semiconductor Companies Aim to Release Prototypes This Year
Samsung and SK Hynix Pursue Stepwise Changes
Micron Targets Direct Development of 3D DRAM
China Expected to Join, Intensifying Market Competition

DRAM, widely regarded as the flagship of memory semiconductors, is seeking to evolve amid a surge in demand brought about by the era of artificial intelligence (AI). With a three-dimensional (3D) and groundbreaking transformation on the horizon, there are signs that the industry could enter a new phase this year. As Samsung Electronics, SK Hynix, and Micron-who together divide the global memory market into thirds-accelerate their research and development efforts, analysts expect that the arrival of tangible results will come sooner than anticipated. Some even suggest that significant progress could be achieved within this year. Both Samsung Electronics and SK Hynix are reportedly focusing their development efforts on producing prototypes of 4F square (4F²) DRAM, a transitional step before moving to 3D DRAM, with plans to unveil them within the year.

Once the prototypes are released, the path toward 3D DRAM is expected to become smoother. By next year, the DRAM market could undergo a full-scale restructuring, led by 3D technology. Industry experts believe this could be an event that fundamentally overturns the entire landscape of the memory market. This is because DRAM serves as the foundation for producing high-performance memory, such as high bandwidth memory (HBM), which is gaining prominence in the AI era. Even with a simple assumption-considering that HBM is made by stacking DRAM chips-it is anticipated that HBM, after DRAM evolves into 3D, will overcome current technological limitations and usher in a new era. This rosy outlook and its significance have become even more pronounced in the market recently. As a result, competition among companies to take the lead has intensified more than ever.

Changing Cell Structure from 'Horizontal' to 'Vertical'

Transforming DRAM into 3D means changing the arrangement of cells from 'horizontal' to 'vertical.' Cells are the spaces where data is stored. 3D DRAM was conceived as traditional DRAM, which arranges cells horizontally, began to reach its limits in miniaturization. As AI performance continues to advance, DRAM is being required to store increasingly larger amounts of data, but conventional DRAM can no longer accommodate all of these demands. The greater the data storage, the more pressing the challenges of power consumption, energy efficiency, and heat generation become.

For this reason, the industry believes that DRAM must change starting from the sub-10-nanometer (nm, one-billionth of a meter) scale. In searching for ways to maximize and subdivide the limited space within DRAM, the concept of 'flipping' emerged. The idea was to arrange the cells vertically rather than horizontally. If cells could be placed vertically, it was expected that the utilization of space within DRAM would increase, improving data density, speed, and power efficiency simultaneously. With cells standing upright, it is also believed that issues such as interference from transistors-which act as switches for the cells-could be avoided. However, since this is a major structural shift from horizontal to vertical, making a direct leap to 3D DRAM is challenging. Companies are therefore taking a step-by-step approach in their development. Currently, they are attempting to transition DRAM from the existing 6F² structure to 4F². F² refers to the unit area occupied by a cell. Thus, 4F² means that a single cell occupies an area of 2F x 2F, a high-density technology aimed at fitting more cells onto a single chip.

Samsung and SK Take a Stepwise Approach, Micron Focuses on Patents

Although all three major players in the memory industry are striving to realize 3D DRAM, their approaches differ. The Korean companies, Samsung Electronics and SK Hynix, are advancing toward 3D DRAM step by step. While there are minor differences, their overall strategy is to first convert DRAM from 6F² to 4F², and then move on to 3D DRAM.

In this context, Samsung Electronics is reportedly seeking to differentiate itself by introducing innovations in detailed areas such as transistors. The company is exploring various technologies that could enable 3D DRAM, such as vertical stacked cell array transistors (VS-CAT) and vertical channel transistors (VCT), and is carefully considering the best solutions. In this process, it was reported that Samsung succeeded in stacking 3D DRAM up to 16 layers as of May 2024, drawing significant attention from the industry.

SK Hynix plans to enhance the completeness of its 3D DRAM using the 'bonding' technology previously applied in manufacturing HBM. In June of last year, SK Hynix CTO Cha Sunyong presented the company's DRAM technology development roadmap at the IEEE VLSI Symposium held in Tokyo, Japan, where he explained this approach. He stated, "Although there are concerns that the manufacturing cost of 3D DRAM may increase in proportion to the number of layers, the company will overcome this and secure competitiveness through technological innovation." As a preliminary step, the company is also applying wafer bonding technology-which places the circuit part below the cell area along with 4F² cells-to its currently developing '4F² VG platform.'

Micron, the American competitor, is striving to leapfrog 4F² and develop 3D DRAM directly. To this end, the company has recently been focusing on securing related patents. In fact, it began these efforts five years ago. According to data compiled in 2022 by market research firm TechInsights, Micron already held more than 30 patents related to 3D DRAM at that time.

China Joins the Race, Intensifying the 'First-Mover' Competition

There is a view that the competition for 3D DRAM will become even fiercer as China enters the fray. Leading Chinese memory companies such as ChangXin Memory Technologies (CXMT), which manufacture DRAM, are aiming to seize the initiative in 3D DRAM and leapfrog Samsung Electronics and SK Hynix, who currently lead in technological capabilities. Some believe that the likelihood of China overtaking its competitors is relatively high. This is because, once DRAM becomes 3D, extreme ultraviolet (EUV) lithography equipment no longer needs to be high-performance. If the traditional horizontal structure were maintained, the patterning of DRAM cells would need to become even finer, requiring more advanced EUV equipment. However, with the transition to a vertical 3D structure, this is no longer necessary. This newfound freedom from EUV constraints could provide China with an opportunity to catch up. Aware of this, Chinese companies like CXMT and Yangtze Memory Technologies (YMTC) are making large investments in the development of 3D DRAM technologies and are taking an aggressive stance.