1.State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 2.Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 51772087) and the Strategic Priority Research Program (B) of Chinese Academy of Sciences (Grant No. XDB30000000)
Received Date:31 August 2020
Accepted Date:20 September 2020
Available Online:15 January 2021
Published Online:20 January 2021
Abstract:With the development of future information devices towards smaller size, lower power consumption and higher performance, the size of materials used to build devices will be further reduced. Traditional “top-down” technology has encountered a bottleneck in the development of information devices on a nanoscale, while the vapor deposition technology has attracted great attention due to its ability to construct nanostructures on an atomic scale, and is considered to have the most potential to break through the existing manufacturing limits and build nano-structures directly with atoms as a “bottom-up” method. During molecular beam epitaxy, atoms and molecules of materials are deposited on the surface in an “atomic spray painting” way. By such a method, some graphene-like two-dimensional materials (e.g., silicene, germanene, stanene, borophene) have been fabricated with high quality and show many novel electronic properties, and the ultrathin films (several atomic layers) of other materials have been grown to achieve certain purposes, such as NaCl ultrathin layers for decoupling the interaction of metal substrate with the adsorbate. In an atomic layer deposition process, which can be regarded as a special modification of chemical vapor deposition, the film growth takes place in a cyclic manner. The self- limited chemical reactions are employed to insure that only one monolayer of precursor (A) molecules is adsorbed on the surface, and the subsequent self- limited reaction with the other precursor (B) allows only one monolayer of AB materials to be built. And the self- assembled monolayers composed of usually long- chain molecules can be introduced as the active or inactive layer for area- selective atomic layer deposition growth, which is very useful in fabricating nano- patterned structures. As the reverse process of atomic layer deposition, atomic-layer etching processes can remove certain materials in atomic precision. In this paper we briefly introduce the principles of the related technologies and their applications in the field of nano- electronic device processing and manufacturing, and find how to realize the precise control of the thickness and microstructure of functional materials on an atomic scale. Keywords:vapor deposition/ atomic manufacturing/ molecular beam epitaxy/ atomic layer deposition
图 9 Al2O3层的ALD制备过程[76] Figure9. The ALD process of Al2O3[76].
相比其他沉积方式(如图10(a)所示), 例如传统化学气相沉积、物理气相沉积、溶胶凝胶(sol-gel)等薄膜沉积技术, ALD具有优异的三维贴合性(conformality)和大面积均匀性(uniformity), 特别适合复杂表面形貌及高深宽(high aspect ratio)结构的填隙生长. 在具有窄纳米深槽的硅基底进行Cu2S薄膜生长实验[120], 结果表明薄膜具有近100%的覆盖率和良好的贴合性(图10(b)), 保形性是ALD最突出的优点, 可很好地解决目前功能器件中的缺陷和均匀性的问题. 图 10 (a) ALD与其他方式镀膜效果比较; (b) 在深高宽比Si结构上原子沉积Cu2S薄膜的SEM照片[120] Figure10. (a) The coating effects of ALD and other methods; (b) cross-sectional SEM images of ALD Cu2S film on silicon trench structure[120].
在沉积过程中, 影响原子层沉积速率的因素包括气体暴露量和沉积温度. 对大多数前驱体而言, ALD过程中完全占据基底表面的过程大约会在2 s内完成, 延长吸附时间只会造成前驱体浪费, 并不能提高吸附量; 而吹扫过程一般会在10 s左右, 否则会造成上一个前驱体残留于下一个前驱体在到达表面前就发生反应, 影响表面共构型和均一性. 化学吸附是一个热力学过程, 会受反应温度的影响. 如图11所示, 一般来说原子层沉积速率有一个温度窗口, 若低于窗口温度, 前驱体会产生物理冷凝吸附; 而温度过高的话, 前驱体会受热分解, 甚至已经沉积好的原子层会解吸附. ALD过程中需要通过控制使整个基板不同区域的温度处于原子层沉积温度窗口, 沉积速率接近恒定值. 图 11 气体前驱体暴露量和沉积温度对原子层沉积镀膜速率的影响 Figure11. Effects of gaseous precursor exposure and deposition temperature on deposition rate of atomic layers.
石墨烯等二维材料由于具有高载流子迁移率且有大开关比等优异的电子特性, 理论上非常适用于制造更快更节能的纳米电子器件, 因此, 对于它的原子精度加工尤为重要. 近年来, 很多研究人员致力于石墨烯材料的纳米形貌和电子性质调控[66-74,131], 而ALE技术则是非常有效的原子级精度加工手段. 2017年, 韩国Kim等[136]将铜箔衬底上CVD生长的单层和双层石墨烯, 经PMMA转移到SiO2/Si基底上, 然后通过ALE技术对该石墨烯实现了可控的原子级刻蚀. 图15(a)和图15(b)为双层石墨烯刻蚀前后的光学显微图像, 仅通过一个循环的ALE过程, 就实现了单层石墨烯的均匀刻蚀. 为了确认刻蚀效果, 对该样品进行了原子力显微镜(AFM)成像, 如图15(c)和图15(d)所示, 图中黑色线是石墨烯层厚的高度轮廓线, 刻蚀后石墨烯厚度由1.45 nm变为0.72 nm, 而在石墨烯薄膜的不同区域(见图15(a)和图15(b)中点1—12)测得的拉曼光谱(图15(e))可以看出, 石墨烯在这些位置已经由双层变为单层, 说明已经被均匀的刻蚀掉一个单原子层. 而对于单层石墨烯, 拉曼光谱数据表明一个ALE刻蚀循环就可以实现石墨烯的完全去除, 如图16所示, 所有单层石墨烯的特征峰都在一个ALE循环后消失. 图 15 双层石墨烯ALE刻蚀前后的光学显微图像(a), (b)以及相应的AFM图像(c), (d)和在各位点的拉曼谱(e)[136] Figure15. Optical microscopic images (a), (b) and AFM images (c), (d) of bilayer graphene before and after one cycle of ALE etching. (e) Raman spectrum of graphene taken at twelve points indicated in (a), (b) before and after etching[136].
图 16 单层石墨烯经过一个循环的ALE刻蚀前后的拉曼光谱[136] Figure16. Raman spectrum of monolayer graphene before and after one cycle of ALE[136].
当然, 只是无差别的去除整层石墨烯是无法实现对纳米器件制备的, 需要结合纳米掩膜技术. 研究人员利用聚苯乙烯(PS)纳米球自组装密排列结构作为掩膜, 通过CF4和O2的等离子体进行ALE, 成功在SiO2表面制备了石墨烯纳米掩膜[142], 如图17所示. 研究人员先在SiO2/Cu箔表面自组装PS纳米球, 它们会自发排列成六角密堆结构, 然后通过CF4等离子体刻蚀掉没有覆盖PS纳米球的区域, 然后溶剂清洗去除PS纳米球, 露出图形化的SiO2区域, 再通过CVD沉积石墨烯, 因为CVD过程中铜箔具有催化作用, 而SiO2没有催化性, 所以成型的石墨烯具有孔洞形状, 最后用具有选择性的ALE对表面SiO2区域进行刻蚀, 最后得到在铜箔表面的具有纳米模板的石墨烯(图17(g)). 图 17 (a)?(f) 经过PS纳米球掩膜的石墨烯加工过程; (g) 铜箔表面具有纳米模板的石墨烯[142] Figure17. (a)?(f) The growth and etching processes of graphene via PS nanoparticle mask; (g) the nano-patterned template graphene on Cu foil[142].