3D Nor Flash, coming soon

Latest update time：2024-07-30

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NOR, as a type of non-volatile memory that has a poor relationship with NAND, is expected to achieve a capacity leap with the help of Macronix Electronics' 3D technology.

Taiwan-based Macronix offers niche NAND, NOR and ROM products and leads the NOR market with a 24% market share in 2020, ahead of Infineon (15%) and Micron (5%). The NOR market grew at a compound annual growth rate of 15% last year. Unlike NAND, where the 3D revolution has significantly increased chip capacity using more than 200 layers, NOR has been stuck in the planar 2D era.

NOR 闪存的读取速度比 NAND 更快，但写入速度较慢（约 100 纳秒，而 TLC NAND 约为 100 微秒），且单元尺寸较大。它通常用于在需要快速应用加载时间的系统中存储应用程序代码，例如智能手机。旺宏电子表示，早在 2022 年，它就在研发一款32 层 3D NOR 产品。这样的产品可以显著增加 NOR 芯片的容量。

Anthony Le, vice president of marketing at Macronix, told EE Journal that Macronix is developing a 32-layer 3D NOR product that is seven times denser than planar 2D NOR. Its target market is non-volatile memory products in the embedded, industrial and automotive fields.

We learned that the starting point of this 3D NOR is a semiconductor AND circuit with bit (cell) level access and full gate design. It makes a multilayer structure of oxide and nitride layers, etches holes between them, and then fills the holes with three so-called plugs; two N+ doped and one silicon nitride (SiN) insulating pillar between them, and the separation channel occupies the remaining space of the hole. The two N+ doped plugs act as bit line channels for the source and drain connections of the stacked transistors in the structure. They are orthogonal to the word lines.

It provides a 3D AND graph showing the situation at the end of 2020:

The company demonstrated an updated version of this 3D NVM diagram at the 2023 FMS event.

Why do we need 3D NOR?

About a decade ago, many manufacturers decided to exit the NOR Flash space. Unfortunately, they acted too early, as more and more applications and markets began to take advantage of the advantages that NOR Flash offered.

So, is NOR flash better than NAND flash? Well, the answer is yes, no, or maybe, depending on what you want to do with it.

In short, NAND flash has a smaller cell size, faster write and erase speeds, and higher capacity/density components than NOR flash. In contrast, NOR flash provides true random access and faster read speeds, making it ideal for executing code. In addition, NOR flash is more reliable and has better data retention than NAND flash.

As I wrote in my column, "NAND and NOR flash each have advantages, disadvantages and preferred application areas. In particular, NAND flash is favored in bulk data storage deployments, while NOR flash shines for code storage and execution tasks."

The key points here are that users like NOR Flash’s fast access time (~100ns for NOR Flash, ~100us for TLC NAND Flash), they like its simplicity (simple SPI interface, while NAND Flash relies on complex controllers), and they like the fact that automotive-grade NOR Flash is available.

So, with all the enthusiasm, what's the problem? Well, as always, users are not happy with what they currently have (bless their little cotton socks), and they want higher capacity NOR Flash devices. The problem here is that existing 2D Flash technology is limited in scalability.

To this end, Macronix is leveraging their proven experience with 3D NAND to expand their 2D NOR Flash products to create a suite of next-generation 3D NOR devices.

It is important to note that when we say 3D, we are referring to 3D process stacking technology, not 3D chip stacking technology. Macronix is currently developing a 32-layer 3D NOR flash production line, which will have a memory density seven times that of 2D NOR flash. In the future, Macronix may further increase the density by using hundreds of layers. Like existing 2D NOR flash products, these 3D NOR flash devices will provide industrial and automotive (AEC) grade products.

What does all this mean from a practical perspective? The idea here is to provide a cost-effectiveness comparison between 2D and 3D NOR Flash. Currently 1x for 2D NOR Flash is standardized. It doesn't matter which vendor you are talking about (Macronix, Micron, Infineon).

If you want to get 1Gb NOR Flash using existing 2D technology, you have to stack two 512Mb 2D dies, which means a 2x increase in cost. The table says “>2x” because die stacking also incurs costs, but we’re focusing on the cost of the silicon here. Similarly, 2Gb NOR Flash requires stacking four 512Mb 2D dies (4x the cost of 512Mb), and 4Gb NOR Flash requires stacking eight 512Mb 2D dies (8x the cost of 512Mb).

In contrast, with 3D NOR, everything is implemented on a single die. This is shown below. A 1Gb die costs 1.8x the price of a 512Mb die, a 2Gb die costs 1.8x the price of a 1Gb die, and a 4Gb die costs 1.8x the price of a 2Gb die. Also, there is no additional die stacking fee because… well… we don’t stack dies. Assuming a 512Mb 3D die costs $1, then a 4Gb die costs $1 x 1.8 x 1.8 x 1.8 = $5.80 (versus $8 for 8 512Mb 2D die stacked). Recalling how car manufacturers love to be cost-conscious; I can only imagine how their eyes light up when they hear this news.

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