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1、精品论文Application, Theory and Process of Glancing AngleDepositionCAO Yongzhi, WU Chao, HU Zhenjiang, YU Fuli5(Center for Precision Engineering, Harbin Institute of Technology, Harbin 150001) Abstract: Thin film technology has a long history. With the development of nanotechnology, thin film characteri
2、zed by nanostructure has been a new research hotspot. In recent years, glancing angle deposition (GLAD), as a effective way to build nanosturcture thin film, has gained significant attention due to a manifold of possible applications, such as photonic crystals, humidity or pressure sensors and10ener
3、gy. This review summarizes the typical application, theory and method of GLAD.Keywords: Nanotechnology; Glancing angle deposition; nanostructure thin film0IntroductionThin film technology is a classical surface treatment with a long history and usually used for15anti-corrosion and wear resistance. W
4、ith the development of nanotechnology, thin film characterized by particular nanostructure illustrates some new feature, proposing thin film technology a research hotspot.Glancing angle deposition (GLAD) is the most promising method for fabrication of nanostructure thin film due to High efficiency a
5、nd relatively simple process comparing with other20nanofabrication 1. GLAD is an extension to oblique angle deposition which has been of interestto the thin film community for more than a century owing to the enhancement of properties resulting from porous structure. Oblique depositions usually use
6、fixed substrate, but during GLAD processing the substrate position is manipulated. Researchers expect that they could manipulate the columnar structure by actively managing substrate position during deposition and as a result25focus on it more. Especially modern GLAD has been added to real-time feed
7、back and computer control to the substrate position, making structure incresing improvement in structure and quality 2-6.1GLAD Applications1.1 Optical Applications30It is an important application of GLAD for fabricating three-dimensional (3D) photonic crystals (PCs). In a PC, the material is precise
8、ly structured to form a 3D dielectric crystal lattice. Photonic band gap resulting from scattering effects due to nanoscale porous structure is similar to the electronic stopbands in semiconductor crystals. In Joshua and D. Krabbes work, Nanoimprint lithography was combined with GLAD of titanium dio
9、xide to fabricate a square spiral columnar35film with a bandgap in the visible spectral range 7. SEMs of square spiral structures are shown inFigure 1. Summers et al 8. fabricated 2D3D photonic crystal heterostructures based on the silicon and compared the normal incidence reflection properties of t
10、he fabricated 2D3D heterostructures to simulated spectra generated. Reflection peaks are observed, resulting from the presence of a photonic band gap, and defect modes are created by the 2D layer. There are also40much work has been devoted to optimizing the square spiral structure, eliminating probl
11、ems suchas broadening and bifurcation9, 10.Foundations: The Doctoral Foundation of Ministry of Education of China(No.20092302120005);National NaturalScience Foundation of China (No. 51075088)Brief author introduction:曹永智，(1971-)，男，副教授，微纳制造。E-mail: cflying- 7 -Fig.1 Square spiral structure451.2 Senso
12、rsPorous structure, high specific surface area, surface morphology can be controlled, and ability to use any PVD source material makes GLAD films strong candidates for sensing applications. Any fluids moving into the porous film will change the materials optical properties. On this account, GLAD fil
13、ms are inherently sensitive to the ambient environment. This50environmental sensitivity is advantageous for optical sensing applications. Matthew designed, fabricated and evaluated a mesoporous PC sensor optimized to exhibit as large as possible color-shift in response to small changes in relative h
14、umidity (RH) as been seen in figure 2. The PC sensor is shown to be highly sensitive and stable. The PC structural-color changes visibly due to RH changes smaller than 1%, and the response is stable over hundreds of hours of sensor55operation 11. But the film will react to any gas which enters the f
15、ilm is one of the weaknesses ofGLAD sensors. Fu et al12. demonstrated fluorescence detection of Salmonella on SiAu GLAD films. Kesapragada et al13. fabricated Cr nanospring and nanorod with GLAD as pressure sensors. The resistivity changes due to physically touching between nanostructures when loade
16、d.60Fig.2 Mesoporous PC sensor1.3 EnergyThe growing demand for energy and concerns about global warming has encouraged scientists to develop low-cost and convenient renewable energy in recent years and Solar cell65research is a hot research on clean energy. Using high surface area electrodes to impr
17、ove collection efficiency is a common approach in solar cell research. Hsu et al14. report an effective way to produce nanoporous Pt counter electrodes of dye-sensitized solar cells by the GLAD technique as been seen in figure 3, and as a result, the quantum efficiency, short-circuit current, and po
18、wer conversion efficiency of the DSSC can be enhanced by up to 12-13% with using the70nanoporous GLAD Pt counter electrodes. Another important topic of energy research is developing new and efficient charge devices. Electrochemical capacitors have an advantage in high energy density, which have broa
19、d prospects in the respect of battery miniaturization and7580859095100hybrid vehicles. Because electrochemical capacitors benefit from high surface areas, Broughton and Brett examined GLAD Mn films for application as electrochemical capacitors. They report a measured specific capacitance of 256 F/g,
20、 which is only a moderate value. However, they emphasize the suitability of the process for high-throughput fabrication 15. Charge store is also used in microbattery fabrication. GLAD has been used to produce high surface area, structuredelectrodes for Li-ion rechargeable batteries. Au et al. invest
21、igate the anode performance of Si-based nanorods by tuning its composition using an oblique (co)deposition technique. The results show that pure Si nanorods have a higher initial anodic capacity of 1500 mAh g1 16.Fig.3 Nanoporous Pt counter electrodes of dye-sensitized solar cells2GLAD Theory2.1 Bas
22、ic theoryAlthough essential theory of GLAD has not been revealed, atom self-shadowing effect is the foundation of GLAD, which is just realized when the incoming vapor flux is well collimated. If there is a large angular spread in incoming vapor flux, shadowing will be poorly defined. There are two m
23、ain approaches achieving collimated vapor flux including increasing the distance between vapor source and substrate and setting physical obstacles which select a subset of arandom vapor flux according with the orientation 17.Since GLAD originates from oblique deposition, traditional theory is approp
24、riate. During simple oblique deposition process at an incident angle , defined in Figure 4(a), a nominally planar substrate will roughen through VolmerWeber mode growth 18. This is initial stage of GLAD film growth. The arrival of vapor flux and formation of film nuclei is a random process atthis st
25、age and defects in the substrate will accelerate roughening. With the nuclei growing into columns, seen in Figure 4(b), shadowing effect will develop. The columns and shadows they cast will have a size distribution. As a result, some nuclei will screen neighboring nuclei from incoming vapor flux, su
26、ppressing their growth as been seen in Figure 4(c). With deposition processing, smaller nuclei and columns can become completely shadowed and stop growing. This process is illustrated in Figure 4(d). As the nuclei grow, more incoming vapor flux will deposit on them. Eventually, only the top of nucle
27、i are able to grow, developing into columns tilted towardsthe vapor source 19.105110115120Fig.4 Schematic view of GLAD growth(a)Nucleation; (b)Shadowing effect; (c)Dominant growth; (d)Final structureColumn tilt angle is described by a single angle, , shown in Figure 4(d). When a column is parallel w
28、ith the substrate normal, =0, and would be 90 for a column parallel with the substrate surface. Column tilt angle, , in oblique depositions does not fully follow the incident angle of vapor flux, . Various rules have been proposed to describe this behavior, but experimental data reveals that it cann
29、ot be described by a single relationship. The growth of GLAD columns depends on substrate temperature, deposition rate, deposition pressure, vacuum composition, substrate type, substrate preparation, preferred crystallinity of the deposited material and even other factorsunknown 20, 21. Under these
30、conditions, the general relationships known as the tangent rule andTaits rule must be understood as guidelines because these rules should be verified for any material system that has been studied in the literature. The tangent rule was one of the earliest attempts to describe the relationship betwee
31、n incident angle and column tilt angle according toexperimental data by Nieuwenhuizen and Haanstra 22：tan = 2 tan (1)As the incident angle becomes increasingly oblique, usually larger than 70 degree, the tangent rule fails to describe experimental data. Using a ballistic model for columnar growth, T
32、aitderived the following relationship 23.1251 cos = )arcsin(2However, the difference resulting from materials was not considered in equation (2).2.2 Substrate seed(2)130135140Advanced GLAD process is added to designing of substrate seed 24, 25. The atom self-shadowing for producing GLAD structures b
33、egins with the initial nucleation of deposited material. However, the nucleation process is random and leads to nonuniform column size and random locations on the substrate surface. To produce a film ordered in a substrate, it is necessary to circumvent the randomness of nucleation. Introduction of
34、substrate seed is an effective method. Substrate seed acts as artificial nuclei for column growth on the substrate plane. With correct design of the shape, size, and spacing of these seeds, a single GLAD column will even grow at each lattice point.There are some rules for designing substrate seed. T
35、he first objective of substrate seed is tocontrol the location of GLAD columns. This requires that growth from substrate seed could dominate growth from other sites. Thus, it is desirable to maximize the incident vapor flux that deposits on the seeds, rather than allowing flux to freely impinge on t
36、he other sites and nucleate at random. This consideration produces the first rule for designing seeds. As shown in figure 5, to prevent condensation between seeds for a given incident angle , the following relationship shouldbe satisfied: h tan + d(3)145150is the center-to-center seed spacing, h is
37、seed height, is the incident angle, and d is seed width as be seen in figure 5. When equation (3) is exactly satisfied, a shadow cast from one seed will just touch the base of a nearest neighboring seed.a bFig.5 Schematic view for substrate seed designinga. Geometric parameters of substrate seed; b.
38、 Multiple growing from one seed155The second objective of substrate seed is to minimize the competitive growth from one seed. Neglecting column broadening, the plane density would be fit to bulk density. The bulk density is determined by the macroscopic geometry of the deposition. Taits equation for
39、 GLAD thin filmdensity is described as 23: = 2 cos 0 1 + cos (4)A GLAD film substrate seed could be designed according to this relationship. Take circularseeds for example:d 242= 2 cos1 + cos(5)160165, d and are defined in equation (3). The left-hand side of equation (5) is the fractional area cover
40、ed by the substrate seeds, and the right side comes from equation (4). If the fractional area covered by the seeds does not approximately match the density determined by the incident angle , the order initially imposed by substrate seed will be lost as the GLAD film returns to its equilibrium densit
41、y.3GLAD process1703.1 GLAD ApparatusA schematic of a GLAD system is shown in Figure 6. The incident angle is defined as the angle between the substrate normal and the incident vapor flux. Azimuthal rotation about the substrate normal is measured by the angle . In general, a line connecting the cente
42、rs of the substrate and vapor source goes through the maximum of vapor flux plume. Substrate rotation alters the location of the vapor source from the perspective of the growing columns. This changes the shadowing dynamics and the column growth will follow the source location. The manipulation of su
43、bstrate orientation can therefore sculpt column growth, leading to the nanostructure thin film19, 26.175180185190195200205Fig.6 GLAD apparatus and characteristic angles3.2 Key point of GLAD ProcessIn GLAD isolated columns develop from the self-reinforcing behavior of nuclei, provided that surface di
44、ffusion, influenced directly by substrate temperature, is low. So Tsubstrate0.3Tmelting with the purpose of limiting surface diffusion is usually proposed to build high quality nanostructure.As distance from the source increases, incident vapor collimation improves. However, the number of collisions
45、 also increases the farther the vapor flux travels before reaching the substrate, leading to disorder of incident particles. To fabricate high-quality structures, the vapor mean free path should be larger than the sourcesubstrate distance 19. Electron-beam evaporation system is preferred for GLAD process. Sputtering system is used for some materials not suited toevaporation methods at the cost of a wider angular distribution due to large t
