Ion tracks - a new route to nanotechnology
Ion tracks - a new route to nanotechnology
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离子径迹——纳米技术的新途径
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发表时间：
2004
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通讯作者：
A. Weidinger
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作者：
A. Weidinger
当能量约为1 MeV/核子的高能重离子(例如140 MeV Xe离子)穿过物质时，就会产生离子径迹。沿路径的极高的局域能量沉积导致了在约10 nm宽的窄圆柱内的材料相变。与更传统的基于离子或电子束照射的光刻技术不同，单个重离子足以改变材料。因此，不会出现像散布或扩散加宽特征这样的问题。具有所需性质的离子束可在例如柏林HMI、德国达姆施塔特和甘尼尔卡昂的国家加速器中心获得。文中介绍了这一领域的一些最新进展。离子径迹在科学和技术方面有着悠久的传统。它们在地质学中发挥着作用，在地质学中，地质形成的年代测定在某些情况下是基于裂变碎片轨迹。工业上，离子径迹用于生产多孔介质，例如用于颗粒过滤器。在这里，用重离子照射聚合物箔，然后对其进行蚀刻，以将材料从磁道区域移除。这种离子束方法的一个独特变体是单孔过滤器，它对粒子过滤达到了极高的选择性[3]。利用现代微束设备，轨道可以排列成有序的阵列[4](见图1)。这对于电子应用很重要，因为它促进了寻址，而寻址对于统计分布的轨道是有问题的。最近，人们清楚地看到，重离子束也可以用于纳米技术[5，6]，因为离子径迹的大小正好适合纳米结构：径迹直径约为10微米，通过选择适当的样品厚度，径迹长度可以从几纳米到几微米不等。这样就可以得到准零维的纳米点或准一维的纳米线。使用离子径迹进行纳米结构基本上有两种方法。第一种是基于滤光片生产中使用的轨迹蚀刻，即照射聚合物箔并蚀刻轨迹以在箔上形成细孔。随后，用适当的材料填充这些孔，以制造纳米结构。在这一过程中，聚合物箔用作模板，并可根据需要移除(溶解)。第二种方法直接使用离子径迹，不需要额外的刻蚀和再填充步骤。这种方法比模板技术更简单，因为不需要填充气孔，但它在材料和结构的选择上当然受到很大的限制。在从晶态到非晶态的轨道上经常发生的材料转变在应用中大多不是很有用。然而，最近发现离子辐照类金刚石碳(DLC)的电导率显著增加[6]，这种材料在轨道上从绝缘(类钻石)转变为导电(类石墨)碳。通过这种方式，形成了绝缘基质中的细导线。另一种具有潜在离子辐照效应的材料是锌铁氧体(ZnFe2O4)，它在初始状态下是顺磁性的，但通过离子辐照转变为亚铁磁性[7]。在YC02[8]中也可以诱导类似的转化。这类例子的数量当然是有限的，但两个好的例子，一个用于电子设备，一个用于磁性(自旋电子)设备，原则上足以进一步探索这一领域。下面将介绍一些已经实现的纳米结构的例子，以及一些建议的实现离子径迹诱导结构的器件
Ion tracks are created when high-energetic heavy ions with ener gy of about 1 MeV/nucleon (e.g. 140 MeV Xe ions) pass through matter. The extremely high local energy deposition along the path leads to a material transformation within a narrow cylin der of about 10 nm width. Unlike in the more conventional lithographic techniques based on ion or electron beam irradia tion, a single heavy ion suffices to transform the material. Thus, problems like straggling or diffusively broadened features do not occur. Ion beams with the required properties are available e.g. at the national accelerator centres HMI Berlin, GSI Darmstadt and GANIL Caen. Some recent developments of this field are described in Ref. 1 and 2. Ion tracks have a long tradition in science and technology. They play a role, e.g., in geology where the dating of geological forma tions is based in some cases on fission fragment tracks. Industrially, ion tracks are used for the production of porous media, e.g. for particle filters. Here, polymer foils are irradiated with heavy ions and subsequently etched to remove the material from the track region. A unique variant of this ion beam method is the single-hole filter which reaches an extremely high selectivi ty for particle filtering [3]. With modern microbeam facilities, the tracks can be placed in an ordered array [4] (see Fig.l). This is important for electronic applications since it facilitates the addressing, which is problematic for statistically distributed tracks. Recently it became clear that heavy ion beams can also be used in nanotechnology [5,6] since ion tracks have just the right size for nanostructuring: the track diameter is of the order of 10 run and the track length can be varied from a few nanometers up to sev eral micrometers by choosing the appropriate sample thickness. In this way, quasi zero-dimensional nanodots or quasi one-dimen sional nanowires can be created. There are essentially two ways to use ion tracks for nanostruc turing. The first is based on track etching as used in the filter production, i.e. one irradiates a polymer foil and etches the tracks to create thin pores in the foil. These pores are subsequently filled with an appropriate material to make nanostructures. In this process, the polymer foil serves as a template and can be removed (dissolved) if required. The second method uses the ion tracks directly without addi tional etching and refilling steps. This method is simpler than the template technique since no filling of the pores is required but it is of course strongly limited in the choice of materials and struc tures. The often occurring material transformation in the track from crystalline to amorphous is mostly not very useful for applications. Recently however, a dramatic increase of the electri cal conductivity in ion irradiated diamond-like carbon (DLC) was found [6], the material changing from insulating (diamond-like) to conducting (graphite-like) carbon in the track. In this way, thin conducting wires in an insulating matrix are created. Another material with a potentially usefid ion irradiation effect is zinc-ferrite (ZnFe2O4) which is paramagnetic in its original state but converted to ferrimagnetic by ion irradiation [7]. A similar conversion can be induced in YC02 [8]. The number of such examples is certainly limited but two good ones, one for electronic and one for magnetic (spintronic) devices is in principle suffi cient to further pursue this field. In the following, some examples of already realized nanostructures will be pre sented and some proposed devices implementing ion track-induced structures will