Moving Atoms: Scanning Tunnelling Microscopy for making devices 搬动原子：用扫描隧道显微镜制作量子设备

Last week I posted about using a scanning tunnelling microscope to see atoms. It turns out that you can also use STM to pick up and move atoms. I am sure you have probably seen this image. This image is made by IBM using STM, where Xenon atoms on a Nickel surface is picked up by a STM tip and arranged to the letters IBM. In this post I will be talking about how that is possible, and one of the applications of this known as STM lithography.

上星期我给大家介绍了用扫描隧道显微镜（STM）看原子的原理和图片。其实STM还可以用来搬动原子。我猜大家肯定见过以下图片。这图片是IBM团队用STM在镍表面移动三十五个氙原子所拼成。在今天的帖子里我会介绍其中的奥妙，和移动原子的应用。

image source: IBM Almaden Research Center

As recall from last week's post, a STM tip is so sharp that there is only a single atom, and it is the interaction of this single atom with surface atoms that allows us to image atoms. It turns out that for some types of atoms, it is possible to apply a certain condition, such as a high voltage or a high current or both, such that it is enough to break the bonds on the surface atom to remove it from the surface or have it stick to the tip. Similarly, one can also deposit atoms stuck on the tip onto the surface. One way that this can be applied is to write atomic scale pattern on a semiconductor surface, thereby creating device that is much smaller than achievable by traditional CMOS lithography techniques. This is called STM lithography.

我上次介绍过，STM的扫描针是异常的尖锐，针头只有一个原子。而这针头的原子和样本表面的相互作用正是我们可以看见原子的原理。其实这个相互作用是可以调整的。在特定的情况下，例如高电压或高电流情况下，针头的原子可以把表面原子间的键拆开，继而把这个原子吸引到针头上。同时，也个有别的情况下，针头原子会把吸在针上的表面原子排斥，把原子放回。这样一收一放，就可以移动原子了！除了看上到酷，移动原子的原理可以用于STM刻蚀法来制作原子大小的电子和量子设备。

In traditional lithography, the semiconductor, most likely silicon, it first coated with a layer of organic material, known as resist. This resist is first harden through heat, and then a quartz mask with device pattern and areas defined using metal on top of it, is placed on the Si wafer. The design pattern is then transferred onto the wafer through by shining UV light or electron beam. The uncovered area is then exposed and chemically changed to be soluble in chemical solutions called developers. So similar to developing a film where image only appear on parts of the film that is exposed, the development process where we put the Si wafer into a developer dissolves the resist that is exposed while leaving the unexposed resist intact, thereby transferring the pattern on to the wafer surface. Further processing such as dopant implantation, etching or metal deposition allows the patterned Si to turn into actual devices.

在传统的光学刻蚀里，半导体晶片(通常是硅,Si) 上会先放一层有机光刻胶。然后晶片上会放一块印有用铬合金画出的电子设备的图案和设计的水晶模板。在紫外线的曝光下，没有被图案遮住的光刻胶会产生化学反应，在显影液中溶解。而没有被曝光的部分则完好无缺，从而把水晶模板上的图案移植到晶片上。

In STM lithography, instead of using an organic resist, we use hydrogen. Hydrogen passivate the surface or Silicon, making it non-reactive and inert. Also, instead of transferring the pattern onto the resist through a mask, the device pattern is directly drawn on the sample. In particular, by applying a high energy condition to the tip allows the tip to break the H Silicon bond wherever it draws, leaving behind holes in the H resist leaving exposed dangling bonds. The patterned wafer can then subsequently exposed to phosphine, which would only react and stick where the dangling bonds are exposed, creating the device. Unlike traditional lithography where the feature size is limited to the wavelength of the exposing light, STM lithography can in theory create features down to the atomic scale.

而STM刻蚀法则用氢原子来做刻胶。氢原子把硅表面上的悬挂键锁住，抑制了硅表面的活性。然后，用STM 针直接把图案和设计在表面上画。针头把划过的氢原子释放，曝露了下面的悬挂键，从而把图案转移到硅表面上。有图案的样本在磷化氢下，只有悬挂键曝露了的地方与磷反应，形成可通电的设备。与光刻不同，STM刻蚀不受紫外线波长的限制，理论上可以制造出一个原子大小的图案。

The actual feature size created using STM lithography is dependent on the condition used. In particular, there are two different writing modes that the STM can operate at. In the first mode, multi-vibrational excitation (ME), a small voltage and high current is used, producing a low energy condition which vibrationally break the H-Si bond. This low energy approach produces pattern that are about 2 nm wide, and is the model used to create small scale devices. The second mode is the field emission mode (FE), where a high voltage and high current result in high energy electrons emitting from the tip that directly break the H-Si bond. Because the high energy electrons tends to spill everywhere, the features created by this model is generally much larger. The field emission mode is crucial for the making large contact pads that is required to interface between the small atomic scale devices and the standard CMOS process.

实际上，图案的大小取决于针上的电压和电流。在低电压，高电流下，针上的原子和电子以共振的放式把氢-硅键拆开。在这种共振模式(ME) 下出来的图案可以只有几纳米宽，可以用来制造原子设备。而在高电压，低电流下，针会发射出高能量电子直接摧毁氢-硅键。因为高能电子数量众多，在这发射模式FE下出来的图案会宽得多。FE通常用于原子级设备和传统微米级设备的衔接。

Here are some images illustrating STM lithography. In the first set of images, we first have a Si surface covered with hydrogen. It looks normal, apart from little white spots covering the surface. These are single dangling bonds that has not been covered by hydrogen. The image of the left shows the same surface, with three lines drawn on it using the STM tip and different conditions. The bright lines are essentially openings on the hydrogen resist, where the dangling bonds of the Si underneath has been exposed. Depending on the voltage and current applied, the widths of the line can be as thin as 1.3 nm.

这第一组图片里的，左手边的是有氢刻胶的硅表面。在图片里的白色点点是没有被氢刻胶覆盖的悬挂键。在右手边，我们在氢刻胶表面画了三条线。这三条亮线就是被移走了氢的洞，曝露出底下的硅。可以看到在低电压高电流时，线的宽度可以达到1.3nm,而高电压低电流所画的则有6nm宽。

This next set of images shows the difference between FE and ME is more detail. It can be seen that a quantum dot drawn using the FE mode is larger, and results in a lot of unwanted dangling bonds. This means that the dot is not very well defined. On the other hand, using the ME mode allows us to make quantum dots that are very well define, and we can also make well define source and drain leads, even gates, that can be used to control electron transport in and out of the dot.

这下一组照片演示了ME和FE的区别。在左手边是用FE画的量子点。可以看到量子点比较大，而且点的界限比较模糊。这样的量子点做出来会有很多噪音。而用ME做的量子点界限分明，可以很好地做出所想的设计。所以ME才是STM刻蚀的主力。

Finally, this last set of images shows the use of FE model in the STM lithography process. Here, a large area of opening, of the order of 1 – 3 micrometer long and 1 micrometer wide, is written using the FE mode. The doping of this large area allows the STM defined device to be interfaced to the macroscopic world using traditional CMOS processing techniques.

可是FE也有它的用处。在这最后一组图里，我们可以看到一大片用FE在氢刻胶画出的开口。这片长方形有1-3微米长，是用于STM所做的量子设备以传统CMOS工艺和宏观世界的衔接。

I hoped that you enjoy this little exploration of how to make atomic devices using a STM. Next week, I will talk about how these atomic scale devices can be use in the quantum regime, especially towards quantum computers.

希望你们喜欢这对用STM来设计原子设备的简短介绍。下次我会带大家来了解一下这些原子设备的用途，特别是在量子计算机的应用。

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All images, unless otherwise stated (eg the IBM atoms image here) was prepared by me. All STM images were taken by me at UNSW while I was working there. Most of the STM images here have been published (by me as the first author) in Nanotechnology 25 (2014) 145302

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