Anodic alumina as a material for MEMS.Kirill I. Delendik*, Olga L. Voitik*

Vacuum Microelectronic Lab., Institute ofElectronics, Belarus Academy of Science

ABSTRACT

A new promising material for MEMS elements— anodic aluminium oxide is described. There were developed fewmethods of anodic aluminium oxide MIEMS elements production on the base of microelectronic technology. Describedmethods of 3D MEMS elements obtaining can essentially supplement and in some cases substitute existing LIGA-liketechnologies owing to their cheapness and simplicity. It is necessary to point out that the realization of MEMS elementsby described methods requires no complex and expensive equipment.

Keywords: anodic aluminium oxide, porous structure, barrier and porous layer, anodization, local oxidation, precisionetching.

Acronyms: anodic aluminium oxide (AAO), volume factor of AAO growth (VF).

1. INTRODUCTION

Porous AAO has been the subject of investigation for more then 40 years. It is widely studied material that was usedmainly for corrosion protection of aluminum surfaces or as dielectric material in microelectronics applications. RecentlyAAO as ordered nanochannel-array material have attracted increasing attention due to its utilization as templates fornanosize structures, such as magnetic, optoelectronic and electronic devices.1 '

2

In present work we report anodic alumina as a convenient material for MEMS elements production as it hasunequal structural, chemical and mechanical properties. Also methods of its precision treatment are discussed.

2. GENERAL FEATURES OF ANODIC ALUMINA

AAO films with uniform pore structure can be formed by electrochemicaloxidation of aluminium in electrolytes containing agents slightly dissolvingAAO, such as phosphoric, oxalic, sulfuric or some other acids separately or intheir mixture. AAO consists of a compact barrier layer directly attachingmetal and located over it thick porous layer (Fig. 1). The porous layer consistsof regular hexagonal cells, which are parallel to each other and create normalto the initial surface. Each individual cell has a central axial pore reaching upto barrier layer.

It has been established that the steady-state barrier layer thickness,cell diameter (D) (usually the diameter of cell is the "width' of the hexagon)and pore diameter (d) are directly proportional to the formation voltage.3 Themost important characteristics of AAO films are cell and pore diameters. Toprepare oxide with uniform porosity, predictable and uniform pore size, themost suitable method is the potentiostatic method of anodization.

The cell diameter is determined mainly by the formation voltage. Fordifferent electrolytes D is constant and approximately equal to 2.8 nm/V.4Factors influencing on pore diameter are more complex. The pore diameter isdetermined by a field-assisted dissolution process on the bottom of pores anddepends on parameters of electrolyte (nature of anions, concentration, pH andtemperature) and current density. The ratio of cell and pore diameters shouldbe constant for the same electrolyte and the conditions of anodization. As for

*E-mail: voitikinel.bas-net.by; phone 375 017 2653230; Institute of Electronics, Belarus Academy of Science, 22

Logojski Trakt, Minsk, 220841, Belarus

Device and Process Technologies for MEMS and Microelectronics II,Jung-Chih Chiao, Lorenzo Faraone, H. Barry Harrison, Andrei M. Shkel, Editors,Proceedings of SPIE Vol. 4592 (2001) © 2001 SPIE · 0277-786X/01/$15.00

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AAO formed in the oxalic electrolyte these ratio is equal to 3 .01. The ratio of pure and anion-contaminated aluminalayer thicknesses depends on the nature of electrolyte and it is equal to 0. 1 for AAO grown in oxalic acid.

Varying parameters of anodization AAO films can be formed with different characteristics: the cell diameterscan be varied from 0.03 up to 0.7 micrometers, the diameter of pore in the cell from 0.01 up to 0.3 micrometers. Thethickness of AAO films can be 0. 1-180 micrometers. The length-to-diameter ratio can be varied in an extremely widerange and is limited by the minimal AAO thickness due to mechanical stability and can be 1-300 at a 100 micrometersthickness.

AAO amorphous phase turn into polycrystalline y-phase after thermal treatment ( 1000 °C). This phase hasbigger microsociality and elasticity then amorphous one. This is a very important fact for MEMS elements.

AAO has unequal properties such as: high mechanical strength and hardness, chemical stability and hightechnological effectiveness.

Amorphous AAO reacts with alkali and its dissolution increases in the row of acids: nitric, sulfuric,hydrochloric, oxalic and chroimc. Speed of dissolving of 'y-AAO decreases in ten times in every etchant in comparisonwith amorphous.

Some physical properties of amorphous and 'y-AAO (thickness 0.5 micrometers) are shown in table 1.

Table 1 : Physical properties of amorphous and y-AAO.Parameters

- .

Amorphous AAO 'y-AAOMicrohardness, GPa Barrier layer 600 850

Porous layer 340 460Coefficient of thermal expansion (300 K), 1/K• 106 45 5.1Resistance, Ohm•m Air 1.2•10' 1.4.108

.4Vacuum(7•10 Pa)

14>1J 14>iuLoss tangent (290 K,1000Hz)

Air 0.003 >0.1

Vacuum(7•104Pa) 0.009 0.005Dielectric constant(290K, 1000 Hz)

Air 7.2 6.9Vacuum (7. i04 Pa) 6. 1 5.3

Modulus of elasticity, GPa (100 micrometers) 137 149

Flexural strength, GPa (100 micrometers) 425 235

3. PRODUCTION OF MEMS ELEMENTS ON THE BASE OF ANODIC ALLUMINA

Nowadays LIGA-technology is wildly used for MEMS production. For the realization of MEMS on its base it isnecessary to apply expensive special photoresists, masks, smcrotronic radiation and special equipment. These processesare rather difficult and not much universal.

We developed few batch methods of AAO MEMS elements production on the base of microelectronicteclmology. These methods are cheaper and simpler, then methods mentioned above. We investigated three maindirections:1 . MEMS elements formation by local oxidation of aluminium;2. MEMS elements formation by precision etching of AAO;3 . MEMS elements formation by combination oflocal oxidation and precision etching.

3.1. MEMS elements formation by local oxidation of aluminiumThe essence of the first method consists in following: in order to obtain through precision openings in AAO,corresponding fragments on aluminium surface are protected by photoresistive mask, which prevents contact betweenaluminium and electrolyte. Backside of aluminium plate is protected by photoresistive layer or by special vanish inorder to prevent its anodization (Fig. 2a). So aluminium oxidation passes locally and AAO achieves necessary thickness(Fig. 2b). After the ending of the anodization process it is necessary to free obtained elements from photoresistive maskand backside protective layer. Aluminium substrate is moved off with the help of etchant indifferent to AAO (Fig. 2c).

It is clear that one can obtain a great number of AAO MEMS elements on the single aluminium substrate.Theoretically their quantity can be infinitely large and is determined by unit sizes, by substrate sizes and quality, bypossibilities of electrochemical equipment. For this purpose it is necessary to contour the every elementphotolythographically.

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C

N1b

/Figure 2: Sceme of the process of MEMS elements formation by single-stage local oxidation of aluminium.

1- aluminium substrate;2- photoresistive mask;3- protective layer;4- anodic alumina.

In order to obtain complex microrelief in anodic alumina (for example, openings and cavities with differentdepths) we used multi-stage local oxidation of aluminium, which bases on the volume factor of AAO growth. AAOgrowth takes place mainly on metal-oxide interface. The volume of formed anodic alumina is bigger then the volume ofreacted aluminium because oxygen reacts with aluminium (theoretically three atoms of oxygen associate with one atom

iiiWhereas photolythography is used it is possible to obtain openings of small sizes and different forms by

mentioned method. The size accuracy of formed contours is determined by the accuracy of photoresistive mask. 15micrometersopenings can be obtained in AAO plates 70-80 micrometersthick. Utilization of thinner plates guaranteeshigh accuracy of openings with sizes less then 15 micrometers.

Some elements formed by this method are shown in f .3, 4 and 5.

Figure 5: Al203 micromechanicalFigure 3: Part of ceramics matrix for Figure 4: Opening in Al203 ceramics. constructional component (3 5x5 mm;galvanization process. thickness 70 micrometers).

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of aluminium). Formed anodic alumina thickness to reacted aluminium thickness ratio evaluates volume factor of AAOgrowth. Our investigations showed that VF has the value 1,4 - 1,6 for different electrolytes. Also VF slightly decreaseswhen AAO thickness is more then 1 50micrometers because of AAO surface etching speed increasing.

Multi-stage local oxithtion of aluminium with using volume factor of AAO growth is shown schematically infig. 6. Although this example is rather particular, but proper process has general utilization and give an opportunity toobtain a wide range of microrelief shapes for different purposes.

Multi-layer protective mask is formed on aluminium surface. n-layer mask corresponds to n+1-layermicrorelief or to n-layer microrelief with opening. The main problem of multi-layer protective mask forming is anattainment of their mutual selectivity — possibility of upper mask layer deliverance without breaking of the next layer.The simplest decision is the utilization of different types of photoresist (Fig. 6a). After coatication of protective maskand backside protective layer unprotected sites of aluminium substrate are exposed to anodization. Because of VFobtained oxide layer raises above aluminium surface (Fig. 6b). Then upper protective layer must be removed to permitthe anodization of the next opened sites of aluminium substrate and the second oxide layer appears. At the same timethickness of the first oxide layer increases as its anodization continues. Thickness increment of the first oxide layer isequal to the thickness of the second one (Fig. 6c). Thus such process can be repeated iteratively. After the achievementof the ground protective layer the last must by moved off. Obtained piece is freed from backside protective layer andaluminium substrate (Fig. 6d).

Multilevel microrelief forming with the help of photolythography is remarkable for its high accuracy,reproducibility and productiveness.

Figure 6: Scheme of the process of MEMS elements formation by multy-stage local oxidation of aluminium1- aluminium substrate;2- upper layer of photoresistive mask;3- ground layer of photoresistive mask;4- protective layer;5- anodic alumina;6- cavities in anodic alumina;7- opening in cavity.

a b

dC

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3.2 MEMS elements formation by precision etching of anodic aluminium oxideIn some cases it is necessary to increase AAO porosity at the specffied value of cell size, so a process of artificialincreasing of AAO pores up to maximal value was investigated.

We used AAO plates grown in the oxalic electrolyte.Initial average plates characteristics are thickness - 98.3micrometers, pore diameter - 0.05 micrometers and cell size -0.487 micrometers (Fig. 7a). Plates were cyclically etched indouble-component etchant Mass, thickness and gas-permeability changmgs were tested after each cycle of etching(Fig. 8). It was founded that pores reach maximal averagediameter 0.212 micrometers after 6 cycles of etching. So wecan increase pore diameter in more than 4 times withoutdestruction of cell walls. During etching the plate thickness isalmost constant and mass decrease is determined by irritationof AAO cell walls that results in pore increasing. This fact isalso confirmed by scanning electron microscopy (Fig. 7 b). Itwas found that in order to destroy cells of AAO completely by

11115 method it is necessary 8-10 cycles of etching (in dependence on initial cell parameters). Analyzing the results of theexperiment one can conclude that the irritation of AAO cells spreads in the direction normal to their axes.

98,5 _______

98a97 L

95.5w 8Number ofetching cycle

Figure 8: Dependence of average plate thickness (a), mass (b) and gas-permeability (c).

Cyclical etching of AAO pores was taken as a base for the development of the method of AAO cyclicalprecision etching. We used AAO 50, 80 and 100 micrometers thick plates grown in the oxalic electrolyte. The barrierlayer was removed. We used nickel mask (matrix of 5 micrometers channels with 15 micrometers step). After theopening of windows in the metal layer plates were treated as it was described above.

It was found that 50 micrometers plate has 5micrometers uniform cylindrical channels (Fig. 9a). 80 and100 micrometers plates had no through channels, which hadcylindrical form only up to 50-60 micrometers depth (Fig. 9b).Apparently this depth is a critical one for this method ofetching because of air corks and problems with etchant andreaction products diffusion on this depth. So this methodpermits to obtain 50 micrometers plates with microprofiles,which have size error equal to two cell diameters.

In order to obtain 3D microstructures with thicknessmore than 100 micrometers first of all it is necessary to obtainAAO with regular structure at most. As it is pointed aboveduring AAO etching the irritation of its cells spreads in thedirection normal to their axes and this allows to producedifferent microprofiles with minimal size disperse bychemical etching. AAO pores are very long in comparisonwith their diameters and because of this the main limiting

Figure 7: AAO plates:a - initial AAO; b- after 6 cycles of etching.

E97,5

96.5

i 96

2 4 6 2 4 6Number of etching cycles Number of etching cycles

Figure 9: Scanning electron microscopy images of50 micrometers (a) and 80 micrometers (b) etchedAAO plates

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factor of AAO etching is the etchant penetration deep into pores. So AAO etching is not a process of the simpleimmersion of AAO plates into the etchant but the etching process with forced etchant delivery deep into pores.

We used AAO plates (100 micrometersthick) grown in the oxalic acid. Photoresist mask (5micrometers channel matrix with step - 8micrometers) was applied on the barrier layer, theporous side of plates was protected. In order toincrease diffusion etchant was pumped into pores bya turbine-type set. In the result AAO plates withetched 5 micrometers channel matrix were obtained.Scanning electron microscopy allows to concludethat plates have through and cylindrical channelswith ratio 20 (Fig. 10).

So method of MEMS elements formationby precision etching consists in following:- coatication of photoresistive mask withnecessary configures on AAO plate;- protection of back side of AAO plate by

chemically firm vanish or by metalization;- etchingof AAO plates and following deliverance of obtained pieces from protective masks.

Sizes of openings exactly correspond to sizes of photoresistive mask elements when the thickness of the AAOplate is near 150 because the structure of AAO thick layers permits deep etching without considerable sidedivergence. Minimal size of opening in AAO plate 100-120 thick is 5-8 . Some elements (metalized ceramic modulatordevice - 13 mm in diameter, width of modulator unit 35 micrometers, thickness 40 micrometers) formed by this methodareshowninfig. 11.

3.3 MEMS elements formation by combination of local oxidation and precision etching.Combination of local c ion and precision etching permits to obtain more complicate MEMS elements. For example,

AAO elements with different kinds of hollows can beformed by the local many step oxidation of aluminimnand then their forms complicated by deep etching (Fig.12).

Besides it is possible to form ceramic matrixesby local oxidation of aluminium (Fig. 1) for thefollowing electrochemical deposition of the metals.This gives a possibility to obtain MEMS elements frommetals (Fig. 13).

So described methods of 3D MEMS elementsobtaining can essentially supplement and in some casessubstitute existing LIGA-like technologies owing totheir cheapness and simplicity. It is necessary to pointout that the realization of MEMS elements by describedmethods requires no complex and expensive equipment.

4. CONCLUSION

Anodic alumina is promising material for obtaining of MEMS elements as it has unequal properties such as: regularporous structure, high mechanical strength and hardness, chemical stability and high technological effectiveness; alsosimplicity and low prices for production. It is necessary to point out that MEMS elements can be obtained by AAOgrowing or by its chemical treatment. For these purposes conventional microelectronic processes were utilised. Thisresults in high economy of the suggested methods. Precision microrelief ceramic elements (from 0.5 up to 200micrometers thick) with minimal linear size of single (individual) element about 5 micrometers and less can beobtained. It is necessary to emphasize that such structures can be fabricated from polycrystal or amorphous ceramics.

Figure 10: 100 micrometers matrixwith 5 micrometers microchannels.

Figure 11: Metalized ceramicmodulator device.

Figure 12: Part of aperture forelectron microscopes.

Figure 13: Thick metal grid.

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To our opinion they will be a good addition to well-known LIGA-process and silicon technology and in some casesthese methods can substitute them.

REFERENCES

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