Quantitative, high resolution two-and-three dimensional dopant mapping in the Scanning Electron Microscope by Secondary Electron Spectro-Micro

总的来说，移动电话和现代电子设备正变得越来越小、更快、更强大。这是可能的，因为其操作所需的电子电路变得越来越小，从而允许器件缩小或在相同体积中添加更多电路。这些电路的原理是，当施加电压时，某些特定区域的电导率会发生变化。这是通过将几个电子较少或较多的原子(称为掺杂剂原子)放入硅等半导体材料中实现的。至关重要的是，准确地将正确数量的原子放到正确的位置。这本身就具有挑战性，但此外，我们还必须确保我们能够确认我们已经在正确的位置获得了正确数量的掺杂原子，因为否则设备将无法按照其规格工作。这被称为掺杂映射，因为它将掺杂原子的数量与一(1D)、二(2D)或三维(3D)的空间坐标联系起来。对于未来的设备，我们需要知道在3D的3纳米内掺杂剂的数量是如何变化的。这里提出的工作的主要目的是为上述挑战提供解决办法。因为这是一个很难解决的问题，所以到目前为止已经发展了许多技术，但都有缺点。一种这样的技术是使用扫描电子显微镜，在扫描电子显微镜中，一定能量的电子撞击表面，导致其他电子，称为二次电子(SE)S，离开该表面。Se的数量取决于辐照区域中的掺杂原子的数量，但以一种复杂的方式和准确的量化是困难的。此外，这种方法不可能从直径小到3纳米的区域为我们提供所需的信息，因为即使我们的入射电子束如此小，硅中的Se也可以来自表面下12倍的原子。为了解决这个问题，我们建议开发SE的另一个特性，这就是它们的能量。SE有一系列的能量(能谱)，这取决于它们在地表下产生的深度。我们预计，仅通过使用高能Se，我们就能够定位几个纳米分辨率的掺杂原子。我们希望通过测量不同掺杂区的能谱位移来获得准确的量化。为了将2D技术扩展到3D，我们需要以受控的方式去除薄层材料，并对每一层应用2D技术。聚焦离子束(FIB)仪器是为此目的而制造的，它通过向靶表面发射一定能量(通常为30KV)的Ga+离子来实现靶表面原子的去除。这项技术的一个副作用是在表面掺入了镓。我们发现，这种效应在30KV时非常明显，不可能对掺杂剂进行量化。因此，我们建议在现有的FIB上增加一个特殊的低能量模块，使我们能够将Ga离子的能量降低120倍，从而减少Ga的掺入和目标表面的其他损害。拟议的工作解决了目前阻碍扫描电子显微镜中准确、高分辨率(3D)掺杂剂测绘的所有问题。因此，它有可能使我们所有人在未来更接近更小、更好、更强大的半导体设备。

Mobile phones and modern electronic devices in general are becoming increasingly smaller, faster and more powerful. This is possible because electronic circuits required for their operation become smaller and smaller allowing the devices to shrink or to add more circuits in the same volume. These circuits are based on the principle that the conductivity in certain well specified areas changes when a voltage is applied. This is achieved by putting a few atoms with fewer or more electrons, called dopant atoms, into a semiconductor material such as silicon. It is crucial that exactly the correct number of atoms are put into exactly the right place. This is challenging in itself but in addition we have to make sure that we can confirm that we have achieved the right number of dopant atoms in the right place, because otherwise the device will not work to its specification. This is called dopant mapping because it links the number of dopant atoms to spatial coordinates in one (1D), two (2D) or three(3D) dimensions. For future devices we need to know how the number of dopants changes within three nanometers in 3D. The main aim of the work proposed here is to provide a solution to the above challenge. Because it is such a difficult problem to tackle many techniques have been developed so far but all have short comings. One such technique is to use a scanning electron microscope (SEM), where electrons of certain energy impinge on a surface causing other electrons, called secondary electrons (SE)s, to leave that surface. The number of SEs depends on the number of dopant atoms in the irradiated region but in a complex way and accurate quantification is therefore difficult. Also this approach does not have the potential to give us the information we need from regions as small as 3nm in diameter because, even when our impinging electron beam is that small, SEs in silicon can come from atoms12 times further below the surface. To solve this problem we propose to exploit another property of the SEs and this is their energy. SEs have a range of energies (energy spectrum) depending on how deep below the surface they were generated. We anticipate that we will be able to locate dopant atoms with a few nanometer resolutions by using high energy SEs only. We hope to obtain an accurate quantification by measuring the shift of the energy spectra of differently doped regions. To extend the 2D technique to 3D we need to remove thin layers of material in a controlled way and apply the 2D technique for each layer. Focused ion beam (FIB) instruments are made for this purpose and operate by firing Ga+ ions of a certain energy (normally 30kV) at the target surface, which leads to the removal of target surface atoms. A side effect of this technique is the incorporation of Ga in the surface. We have found that this effect is so pronounced at 30kV that a quantification of dopants is not possible. Therefore we propose to add a special low energy module to our existing FIB that allows us to reduce the Ga ion energy by up to 120 times, thus reducing the incorporation of Ga and other damage in the target surface. The proposed work addresses all the issues which currently hamper accurate, high resolution (3D) dopant mapping in the SEM. It therefore has the potential to bring us all one step closer to smaller, better and more powerful semiconductor devices in the future.