Inside 1α — the world’s most advanced DRAM process technology

Micron recently announced 我们的存储芯片采用了世界上最先进的DRAM工艺技术. That process is, cryptically, called “1α” (1-alpha). What does that mean and just how amazing is it?

芯片制造的历史就是缩小电路以在芯片上容纳更多的晶体管或存储单元. Sixty years ago, 第一批芯片有元件——晶体管之类的——你可以用肉眼看到. Now those same components are just a few nanometers across. That’s a billion times smaller!

Smaller transistors switch faster, use less energy and, through pure economy of scale, are cheaper to make. The jump to our latest technology node — which, by the way, is the most advanced in the world today — is no different. 它在性能、能效和制造成本方面都有重大改进.

Imagine if cars had improved at the same rate. 它们可以在一眨眼的时间内从每小时0英里加速到60英里，只用几滴燃料就能环绕地球飞行.

Now, making chips is, to put it mildly, complicated. 制造一个现代芯片需要超过一千个独立的过程和测量步骤——所有这些都必须几乎是完美的. Those steps are performed on machines, known as tools, made by hundreds of specialized companies, using ultra-pure materials, 在巨大的洁净室里，空气中的颗粒比月球上的少.

由于这种复杂性，行业倾向于从一个节点到另一个节点遵循类似的节奏. We call each of these “nodes” and refer to them by the smallest feature on the chip. 例如，在本世纪初，我们正处于180纳米(nm)节点. About ten years ago, we were at the 22nm node.

But a funny thing happened a few years ago in the memory world. We stopped talking about exact numbers and started to use terms like 1x, 1y and 1z. For DRAM particularly, 节点的名称通常对应于记忆单元阵列中活动区域的一半音高(half-pitch). As for 1α, 您可以将其视为第四代10nm级，其半间距范围为10至19nm. 当我们从1x纳米到1y, 1z和1α，这个维度变得越来越小. We started with 1x, but as we continued to shrink and name the next nodes, we hit the end of the roman alphabet. That’s why we switched to the Greek alphabet alpha, beta, gamma and so on.

Putting the size into perspective

Just how small are we talking here?

Chips are fabricated, hundreds at a time, on 300mm diameter wafers of silicon. Each chip, or “die” is about the size of a fingernail.

Now imagine one die, blown up to the size of a football field. Reach down and pull out one blade of grass. Snip it in half, in half and in half again.

That's one transistor, one bit of storage out of 8 billion on a typical memory chip.

Limitations to lithography

Amazing though this is, the semiconductor industry has been doing this kind of thing, shrinking devices every year or two, for decades. We’re pretty good at it. Indeed, we know how to lay down films of material just one atom thick, and our ability to etch — selectively remove — material isn’t that far behind. So, what’s different now?

Perhaps the most difficult challenge is defining the circuit patterns on the wafer. The first part of this is called photolithography (writing on stone with light!). It’s similar to the predigital photography process, where light is shone through a small, transparent version of the photograph onto light-sensitive paper. 在我们的例子中，我们用深紫外光照射在透明方形石英上的图案，这种图案被称为光掩膜, using a bus-sized machine. But the principle is the same.

The problem is one of physics. Thanks to something called the Rayleigh criterion, or the diffraction limit, 人们认为，要投射出小于所用光波长一半的特征图像是不可能的. 只是不可能制造出足够锐利的光束来做出精确的图案. 在我们的例子中，波长是193nm，所以我们在衍射极限以下工作. Simplified to the point where physicists will twitch involuntarily, it’s like trying to write 10-point text using a 4-inch paintbrush.

There is a new kind of litho tool that uses smaller, 13.5nm wavelength extreme ultraviolet light (EUV), but for a number of complicated reasons, we don’t think it’s ready for prime time. 其中一个原因是波长太短，以至于光不能穿过玻璃, so conventional optical lenses don’t work. Fifteen years ago, people thought EUV litho would be ready for the 32nm node. EUV’s time will come, but it’s not the right solution for 1α at Micron.

Cheating the Rayleigh criterion

We use a number of techniques to get around the diffraction limit. 首先是修改掩模上的图案，“愚弄”光线，使其变得清晰, small features. The current state of the art is called computational lithography 并使用大量的处理能力，有效地从晶圆上所需的图案逆向工程掩模图案.

第二种方法是利用水对光线的衍射比空气小的事实，将晶圆片暴露在水下! This is less dramatic than it sounds. 我们实际上用一滴水代替了最后透镜和晶圆表面之间的气隙. 这种方法使我们的生产工艺降至40纳米以下——这是一个巨大的进步，也是巨大的工程合作努力的成果, but not all the way home.

The magic of multiple patterning

解决方案是添加一系列非光刻步骤，将一个“大”特征神奇地变成两个，然后是四个, each a quarter of the size of the original. This is, frankly, brilliant. Lots of different methods to do this were worked on concurrently, 但如果我不指出美光是第一个使用双模式开发闪存的公司，那就太失职了, back in 2007, thanks to the pioneering work of our own Gurtej Singh Sandhu, now a senior fellow (one of only four; it’s an exclusive club) in Micron’s pathfinding group.

Oversimplifying quite a bit, the basic idea is to create sacrificial features using the stepper, 在这些特征的侧面涂上不同的材料，然后去除原始的牺牲特征. Voilà — two half-size features! Repeat the process and we have four features of the size we need for 1α. See the diagram for more detail.

Rinse and repeat

Now we know we can accurately pattern the tiny features we need, but we’re still a long way from one complete die, let alone high-volume production. 我们刚刚画出了一个层的特征轮廓，每个芯片有几十个层. 我们非常自豪的一件事是，我们可以精确地将每个新图层与前一个图层对齐, which we call overlay. Getting that exactly right is key to making the whole thing work.

然后，我们必须将该模式转化为功能电路器件，如控制数据读写的晶体管和传感器, skinny capacitors that can store the charge representing 1’s and 0’s. 这个过程意味着精确地控制材料的组成和这些材料的机械和电气性能, and doing it exactly the same every time.

我们不仅结合了我们自己的创新，而且利用了我们供应商合作伙伴的进步. 我们到处都在采用最新最好的东西:新材料(比如更好的导体和更好的绝缘体)和新的沉积机器, modify or selectively remove, or etch, those materials. The list is long, and all those things have to work together.

We’ve developed out fabrication plants, called fabs, into artificial intelligence-driven, highly automated marvels. As I mentioned earlier, 制造一个现代芯片需要在晶圆厂内跨越上千个步骤和数百英里. And every one of those steps must be perfect.

Semiconductor fabrication isn’t like making a car. You can’t go back and fix a defect introduced earlier in the process. Any defects are literally buried under later layers. The key to success is data — and insight from that data. 来自成千上万个传感器的数据涌入我们10拍字节的制造执行系统. 我们每天通过我们的检测系统提供超过一百万张图像，并使用深度学习在问题发生之前发现问题. Chip fabrication is perhaps the most complicated human undertaking on the planet.

How did we do it?

It’s worth asking how Micron’s engineering teams were able to pull the 1α node off, and in record time, to put us at the forefront of the industry. 美光拥有数以万计的工程师和科学家，但这只是故事的一部分.

It’s a testament to the spirit of collaboration among all the disciplines involved, from our technology development, design, and product and test engineering folks to manufacturing and quality. 这也证明了我们团队成员的激情和坚韧，以永久的“全体员工”模式运作，使美光走在DRAM技术的最前沿.

I’m proud of this team — and certainly proud to be a part of this team.