Chiang Mai J. Sci. 2013; 40(6) 947

Chiang Mai J. Sci. 2013; 40(6) : 947-956http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Fabrication of Thin Nanoporous Alumina Templateson Semiconductor SubstratesSomyod Denchitcharoen* and Pichet LimsuwanDepartment of Physics, Faculty of Science, King Mongkut’s University of Technology Thonburi,Bangkok 10140, Thailand.*Author for correspondence; e-mail: somyod.den@kmutt.ac.th

Received: 11 April 2012Accepted: 4 September 2012

ABSTRACTIn this study, thin nanoporous alumina templates were fabricated from provided

pure thin aluminum films deposited on GaN surface by electron beam evaporation aftergrowing GaN on Sapphire substrates by metal organic chemical vapor deposition (MOCVD).The aluminum films were anodized in electrochemical system with the solution of oxalic acid(H2C2O4). The nanoporous alumina template was gradually formed on the substrate with thegrowth rates of between 36 and 39 nm/min. To investigate the template, field emissionscanning electron microscopy (FESEM) was introduced. The structure and the cell size wereinvestigated by transmission electron microscopy (TEM). It was found that the pore diameterand vertical length were about 42 nm and 400 nm, respectively. Moreover, the thicknesses ofthe sidewall and the barrier layer at the bottom of alumina nanopores are similar about26 nm.

Keywords: nanoporous alumina, FESEM, TEM, GaN/Sapphire, thin aluminum film

1. INTRODUCTIONPorous alumina is a self-ordered

hexagonal array of cells with cylindrical hasbeen studied more than three decades [1].While the pore formation mechanismduring anodization is not yet fullyunderstood, numerous groups still haveextensively investigated in different ways byboth theory and experiment to understandthe formation of porous alumina and itsapplication [2-5]. In the literature we find thatmost porous alumina membranes are createdfrom bulk aluminum or from depositedaluminum on desired substrates [6-9].

Surfaces covered by dense arrays ofnanometer-scale pattern have great potentialapplications in diverse areas such asoptoelectronics, information storage, andsensing [10]. So, there are several methodsreported to produce nanopattern arrays onsurfaces such as UV photolithography,electron- and ion-beam lithography, X-raylithography, or lattice-mismatch inducedstrain, and template-based methods [11, 12].They can be categorized into two groups,lithography and self-organization growthfor fabricating ordered nanopattern arrays

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on the template.However, both lithographic techniques

and self-organization methods have their owndrawbacks. The throughput of lithographictechniques is low and typically only small areascan be patterned. The investment for a highresolution lithographic instrument is alsovery high [13]. As for the self-organizationgrowth method, only a few types of materialsare suitable for the fabrication of orderednanopattern arrays, and most of these areorganic materials. This motivates us toinvestigate methods to fabricate highlyordered nanoparticles over large areas usingrelatively inexpensive equipment.

Recent years have seen a renewed interestin porous alumina due to its potential useas a good mask or template in makingmicro- and nanostructures [14]. Moreover, theanodization process is not complicated andwe can create nanohole arrays on the wholearea that depends on the size of the desiredmaterial. When we consider the advantagesof an alumina mask we find that it is

suitable for the applications with its ownfour main advantages. That is nanometersized pored arrays, high aspect ratio, highregularity of the pore arrays and controllablepore size on a larger scale. In addition, it iselectrically insulating (1018 Ω cm resistivity),optically transparent over a wide spectralrange, and chemically robust which makesalumina suitable for being an idealembedding material for optical and electronicdevices [5].

The geometry of anodic porous aluminamay be schematically represented as ahoneycomb structure of fine channelscharacterized by a closed-packed array ofcolumnar hexagonal cells, each containing acylindrical pore at the centre with thin curvelayer called barrier at the bottom as shown inFigure 1. Pore diameters ranging from below30 to 400 nm, oxide thickness from 0.3 to200 mm and pore densities in the range109-1011 cm-2 are obtained. The size andinterval can be easily controlled by changingthe forming conditions [4].

Figure 1. Drawing of porous alumina membrane.

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Aluminum with purity of 99.999 % isused as a source to create porous alumina.Many researchers use porous aluminatemplates made from pure aluminum sheetsfor transferring pattern to obtain nanopillarsand nanoholes [15]. However, by using free-standing template as a nanomask on theconventional devices, it is very difficult toachieve as needed because there are a fewproblems such as low adhesion, high aspectratio and contamination during the process.

In this research, we present the way toobtain porous alumina without usingcomplicated facilities and techniques. Thisnanomask is directly fabricated on the desiredsubstrates such as GaN/sapphire. Purealuminum is first deposited on the substrate.In this way, the process is taken forshorter period of time to fabricate thinporous alumina template. The nanomask ischaracterized by FESEM and TEM to seethe dimension and a close-packed structure.

2. MATERIALS AND METHODS2.1 The Preparation of Substrates

The substrates used in this research wereGaN/sapphire (Al2O3). GaN was grown onsapphire by metal organic chemical vapordeposition (MOCVD) at the University ofBath. Substrates were cleaned in acetoneusing an ultrasonic cleaner for 10 min toremove organics and other contaminations.The samples were rinsed in DI water and thendried with N2.

For this research, pure aluminum is usedfor the fabrication of the thin porous aluminamembranes on undoped GaN templates.Base pressure for e-beam evaporator is about2-3 × 10-6 Torr to deposit Al (99.999%) withthe different thicknesses of about 800 nm(samples SA1,SA2 and SA3), 1200 nm(samples SA4 and SA5), 1145 nm (sampleSA6), 1285 nm (sample SA7) and 1576 nm(sample SA8) measured by Dektak 6M Stylus

Profilometer. In order to examine the grainsize, aluminum was also deposited on rocksalts which are standard practice for TEManalysis.

2.2 The Anodization ProcessPorous alumina membranes are gradually

grown in the aqueous solution whichcontinuously anodizes aluminum under theelectric field between two electrodes. Variouselectrolytes can be employed for theanodization process such as sulphuricacid (H2SO4), oxalic acid (H2C2O4), andphosphoric acid (H3PO4) [4,16,17]. Differentpore diameter and interpore distance can beobtained by using different electrolytes.

For this research, oxalic acid is chosenbecause the average pore size about 40 nmis suitable for low dimensional structure.The other advantage is the moderate appliedvoltage (40 V) for anodizing. It is easier toavoid the local heat occurring on the surfaceof the aluminum which tends to be serious athigher applied voltage (> 100V). Small partsof aluminum surface affected by heat aredifficult to form nanopores because theseareas are burnt while the anodization is stillproceeded [18].

To fabricate a densely packed hexagonalpore structure, because the pores initiate atalmost random positions (Figure 2(a)) andorder thereafter by self-adjusting duringa long-time anodization process, porearrangements on the surface are disorderedand have a broad size distribution (Figure 2(b)).Ordered pore domains can only be obtainedat the bottom of the layers. Therefore thepores produced in the first anodization stepare not parallel to one another. To fabricateordered nanopore arrays the first porousoxide film which contains the barrier layer atthe bottom has to be removed by wetchemical etching (Figure 2(c)). The barrier layeris not flat but consists of periodically arranged

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crests and troughs. The fluctuation changes thesurface of the aluminum substrate into adimpled and undulating landscape that has thesame spatial ordering as the barrier layer.After the removal of the porous film (Figure2(d)), the periodic concave patterns thatremain act as a self-assembled template for

the second anodization process. An orderednanopore array is obtained after the secondanodization if the same parameters are usedas in the first anodization step. During theanodizing process the current was recordedand plotted against time in the case of apotentiostatic (constant-voltage) mode.

Figure 2. Diagram of porous aluminum oxide growth.

The parameters of the anodizationprocess consist of temperature, appliedvoltage, and electrolyte concentration.The temperature is set up at 5°C to cool thesample during forming the membrane andanother temperature is used to heat thesolution at 60°C during etching the firstoxide layer of two step anodization processes.The solution for etching is a mixture of0.4 M H3PO4 and 0.2 M H2CrO4. The appliedvoltage and the concentration of electrolyteare 40 V and 0.3 M, respectively.

3. RESULTS AND DISCUSSION3.1 Grain Size of Thin Aluminum Film

Prior to the fabrication of thin aluminamembrane, thin Al film deposited on the

substrate was investigated.Figure 3(a) presents examples of rock-

salts used for this study. Figure 3 (b) showsthe aluminum film on the rock salts afterthermal evaporation. The aluminum filmwas separated from the rock-salts bydissolving in water. Since the aluminumfilm can float on the water surface it iseasy to transfer the film on to TEMcopper grids, as included in Figure 3 (b).

Figure 4 shows the grain sizes of Alfilm. The thin material consists of differentgrain sizes and orientations, whichrange around 50 nm in size. Moreover,the film which was deposited bye-beam evaporation appears to bepolycrystalline.

Figure 3. Pieces of rock-salts (a) to prepare thin Al film for TEM (b).

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Figure 4. TEM image of pure Al film.

3.2 The Characteristic Curve of FormingAlumina Membranes

Samples SA1, SA2 and SA3 are undopedGaN quarter wafers, all with the same Althickness of about 800nm. The purpose ofthis part is to investigate the etch time forremoving the oxide after the first anodization.The etching times allocated to samplesSA1, SA2 and SA3 are 6, 7 and 8 minutes,respectively.

Figure 5 shows the characteristic curvesof the three samples. The curves of (a),(b)and (c) are similar, although the secondanodization is slightly longer for (c). It can beseen from the curves that all Al films were

anodized completely in the second anodizingstep, where the current density reaches zero.

SA4 and SA5 have Al thickness of about1200 nm. Figure 6 shows the curves ofsamples SA4 and SA5, which are slightlydifferent from each other. The firstanodization of SA5 shows that the initial Alsurface is smoother without fine features,which is favorable for anodization. Moreover,the red curve is lower than the blue curve,as shown in Figure 6 (b), which confirmssmoothness. In the case of Figure 6 (a), thecurves are similar to that of the previoussamples, which were not ideal due to surfaceroughness.

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Figure 5. Characteristic curves of samples (a) SA1 (b) SA2 and (c) SA3.

Figure 6. Characteristic curves of samples (a) SA4 and (b) SA5.

3.3 Thin Porous Alumina MembranesFigure 7 (a) and (b) show the membrane

of sample SA1 after anodization, whileFigure 7 (c) presents the membrane afterwidening the pores in 5% H3PO4 at 35°Cfor 15 minutes. Images (a) and (b) show that

the first oxide layer is not completelyremoved within the 6 minute etching time.However, the oxide layer can be removedaway from the surface of the lower porousalumina layer by widening the pores, as shownin Figure 7 (c).

Figure 7. Sample SA1 after: second anodization (a) and (b) and widening the pores (c).

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Figure 8 presents the membrane ofsample SA3 which is similar to that of SA2(figure not included). The first oxide layer ofSA3 was removed in 8 minutes. Analysis ofthe cross section shows one single layer afterthe second anodization. Hence, the optimizedetching time is estimated to be between 7 and8 minutes. However, a longer etching time of9 minutes is performed for samples SA4and SA5, which will be discussed below.

The initial surface of the Al layer consistsof many small grains after evaporation.These grains can affect the fabrication ofporous alumina, if it is performed with onlyone anodizing step, because the concaveswill be too undulating and too random.Hence, a two-step anodization process is

essential to fabricate alumina. The anodizingrate of sample SA4 is estimated to be ~ 38.17nm/min.

The results of both SA4 and SA5 showthat the etching time of 9 minutes is stillsuitable for removing the first oxide layer,although care must be taken not to over etchthe lower layer. Figure 9 shows the aluminamembrane of sample SA5, after wideningthe pores in 5% H3PO4 at 35°C for 18 minutes.The pore diameter is estimated to be ~ 65nm. The anodizing rate of sample SA5 isestimated to be ~ 39.17 nm/min. From TEMimage as shown in Figure 10, it shows thatthe geometry of the closed-pack arrays isobserved to be hexagonal and the porediameter is about 42 nm.

Figure 8. Sample SA3 after second anodization: (a) top view and (b) cross section.

Figure 9. Sample SA5 after second anodization and widening the pores: (a) lower and (b)higher magnifications.

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Figure 10. TEM image of self-ordered porous alumina membrane.

Figure 11 shows a plot of porousalumina (PA) thicknesses versus anodizing timefor samples SA6, SA7 and SA8.The growthrates of each sample to form porous aluminaare determined to be between 36.75 and38.0 nm/min which are slightly less than

that of SA4 and SA5.All samples after anodization are found

that the pore diameter and the vertical lengthare about 42 nm and 400 nm, respectively.Moreover, the thicknesses of the sidewalland the barrier layer are similar about 26 nm.

Figure 11. Plot of porous alumina (PA) thicknesses versus anodizing time.

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4. CONCLUSIONSIn this study, porous alumina on GaN

grown on sapphire substrate is a very simpleand a good technique to fabricate a templatefor nanofabrication. The experiments weresuccessful to form thin PA membranes onthe substrate. The relationship between theoxidation behavior and the current variationfor the anodization of Al film on the GaN/sapphire substrates was presented. Theetching time to remove first oxide layer oftwo step anodization has been optimized.It should be about 8 min. The growth ratesof converting pure aluminum to be aluminaare between 36 and 39 nm/min. Moreover,we achieved to obtain thin membrane withthickness of about 400 nm. The pore diameterand the thickness of barrier layer are about42 nm and 26 nm, respectively.

ACKNOWLEDGEMENTSThe authors are grateful to the research

group in electrical department at Universityof Bath for their samples and facilities to thiswork. This research was supported byDepartment of Physics, Faculty of science,King Mongkut’s University of TechnologyThonburi, Bangkok.

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