high quality 3d shapes by silicon anodization

Phys. Status Solidi A 208, No. 6, 1383–1388 (2011) / DOI 10.1002/pssa.201000163 p s sa

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applications and materials science

High quality 3D shapes bysilicon anodization

Alexey Ivanov*, Andras Kovacs, and Ulrich Mescheder

Institute for Applied Research and Faculty of Computer & Electrical Engineering, Hochschule Furtwangen University,

Robert-Gerwig-Platz 1, 78120 Furtwangen, Germany

Received 29 April 2010, revised 22 October 2010, accepted 10 December 2010

Published online 22 March 2011

Keywords anodization, electropolishing, etch shape control, injection moulding, micromoulds, silicon, silicon lens

*Corresponding author: e-mail [email protected], Phone: þ49-7723-9202103, Fax: þ49-7723-9202633

In this paper some process considerations and optimizations of

anodization for three-dimensional (3D)-structuring of silicon

are discussed. For the shape controlling of etched form different

approaches, such as frontside masking design, local backside

doping and surface pre-structuring are presented. Influences of

the opening size and etch depth on the shape of the etching form

are investigated. The surface quality of the resulting 3D

structures is critically dependent on the specific process

parameters and process flow. Best surface quality was obtained

for electropolishing in 7wt.% hydrofluoric acid (HF) at applied

current densities of 100–300mA/cm2. Application of 3D

silicon forms for injection moulding is demonstrated and

further implementations of the process for optical and fluidic

devices are discussed.

3D silicon shapes fabricated using anodization process with

local backside doping design.

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Electrochemical etching of silicon(anodization) in hydrofluoric acid (HF) is a flexible processthat can be applied for etching well-controlled three-dimensional (3D) structures in silicon. The basic principleof shape control – adjusting the local etch rate by changinglocal current flow [1] (Fig. 1) – provides many advantagesover conventional etching techniques, such as smooth andreal 3D shapes (in comparison to 2.5D forDRIE andmultiplephotolithography technique), flexibility (with single- ordouble-side masking and/or local doping regions variousshapes can be produced [1]), high surface quality achievedthrough electropolishing, i.e. within the structuring process(in contrast to rough surfaces after dry etching like DRIE orlaser processing). The proposed technique provides apowerful process for the fabrication of real 3D structures insilicon with form shaping capability and therefore aninteresting alternative to grey-scale lithography or dryetching processes with low selectivity between maskingand substrate material [2]. An additional advantage of theproposed anodization process is the possibility to remove

porous silicon (PS) later in weakKOH solution selectively tobulk silicon. Therefore, PS can be used as sacrificial materialfor fabrication of MEMS [3, 4].

2 Experimental results and discussion2.1 Shape control by frontside masking The

current flow through a p-type silicon (p-Si) sample withfrontside stress-free silicon nitride (SiN) masking layer isschematically shown in Fig. 2a. Backside of low dopedsilicon wafer has reverse-biased Schottky contact to theelectrolyte. In order to provide electrical contact, backside ofwhole wafer should be highly p-doped. The p-Si samples(10–20V cm (100)-Si, thickness ca. 520mm) have beenanodized in double-tank cell configuration in 30wt.% HF atcurrent densities j¼ 1.0–3.5 A/cm2 (ranging from PSformation to electropolishing) and in 7wt.% HF at200mA/cm2 (electropolishing).

Using frontside masks (insulators) the shape of thestructure converts from the so-called edge-effect convexshape to concave during anodization as shown in Fig. 2b.


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Figure 1 (online colour at: www.pss-a.com) Schematic cross-sec-tions of different principles of shape control during anodizationwithadjustment of current flow through Si wafer by front- and back-sidestructuring of the wafer.

Figure 3 (online colour at: www.pss-a.com) Investigation of theedge-effect and theconversionconvex–concaveasa functionof etch

Edge-effect shapes have been characterized in terms of aso-called edge effect factor (EEF) (Fig. 2b), which is definedas the ratio of the depth in the structure deepest part D1

(which is near edge for edge-effect shapes) and the depth inthe centre of the structureD2. For concave and flat formsEEFis therefore equal 1. The evolution of EEF is represented as afunction of etch depth and opening diameter (Fig. 3a). Powerfunction data fit has been applied to the data. The value ofetch depth on the data fit curve, at which EEF¼ 1, is calledthreshold depth Dth. The meaning of Dth is that this is thedepth, at which convex shape converts to concave. Theconversion takes place at smaller etch depth for structureswith smaller opening in the SiNmasking layer than for wideropenings (Fig. 3b) [5].

For the same diameter of opening (1mm) in the SiNfrontside masking layer, the conversion occurs at smallerdepth for the structures produced in electropolishingmode in7wt.% HF electrolyte at 200mA/cm2 as compared tostructures anodized via PS formation mode in 30wt.% HF.

Two mechanisms are proposed to have effect on suchshape development. First, the specific current densitydistribution in silicon provides formation of convex shape

Figure 2 (online colour at: www.pss-a.com) (a) Schematic cross-section of silicon sample, arrows represent current flow; (b) struc-tures anodized through 600mm circular opening of SiN maskinglayer in 30wt.% HF at 2.5A/cm2 for (left) tetch¼ 1min and (right)tetch¼ 10min (straight regions on both sides of the concave profileare measurement artifacts); measurements performed with stylusprofiler.

depth, anodization current density and the diameter of the circularopening in the SiN mask: (a) examples of the measured data for thestructures anodized through the openings of diameter 400 and800mm; (b) dependence of Dth on the diameter of the opening inSiN-mask.


at the beginning of the process, because current flow atstructural edges (to masking layer) is considerably largerthan flow in the centre of the opening (Fig. 4a, left). Whenstructures get deeper (the distance to the backside of thewafer reduces), the flowof current frombeneath the structureincreases because of the lower path resistance and leads to amore concave shape formation (Fig. 4a, right). Thismechanism has demonstrated to be feasible in a FEMsimulation of the anodization process done in cooperationwith University Freiburg (IMTEK). In this case a purelyelectrical model without consideration of diffusion inelectrolyte was simulated. The simulated conversion of etchshape convex–concave is in agreement with our experimen-tal results [6].

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Figure 4 (online colour at: www.pss-a.com) Simplified mecha-nism of shape conversion convex–concave: (a) effect of currentdistribution (arrows represent current flow); (b) effect of diffusion-controlled etching process (arrows represent flow of F-ions to thereaction site); darker arrows mean stronger flow. Left: At thebeginning of the process, right: after deep etching into siliconsubstrate.

Figure 5 (online colour at: www.pss-a.com) (a) Schematic cross-section of Si sample with local backside contact, arrows representcurrent flow; (b) simulated current profile on the frontside for thisdesign; (c)measuredprofilesof structurewith local backside contactetched in two-step process: first, PS formation in 30wt.% HF for15min (current 1.7A for the whole 4 in. wafer), then additionalelectropolishing in7wt.%HFfor2.5min(current10Afor thewhole4 in. wafer); measurements performed with stylus profiler.

Figure 6 (online colour at: www.pss-a.com) Shape developmentfor backside local contact layout (circular contact with diameter1800mm): (a) profiler scans of the anodized structures (30wt.%HFwith ethanol; anodization duration varied; porous silicon removed);(b) dependence of radius of curvature of anodized structures on theetch depth.

The second mechanism which can explain the shapeconversion is based on consideration of ions transport in theelectrolyte in case of diffusion-controlled etching process:the resulting concentration of reacting ions is then criticallydepending on geometry (mask thickness, opening dimen-sions) and thus will change during the etching, which willchange the etch shape from convex to concave (isotropic) asknown for wet chemical etching [7] (Fig. 4b).

Both mechanisms appear to play a role in the observedconversion of etch shape in different extent depending on theapplied current, electrolyte concentration, etc.

2.2 Shape control by backside doping Currentflow and anodization shape for low doped silicon substratecan also be adjusted by local electrical backside contactsbetween sample and electrolyte. As described in Section 2.1,backside contacts to low doped silicon wafer are formed bylocal pþ doping. Localization can be achieved with isolatingmasking layer (Fig. 1) or, as used in this experiment, by localformation of the pþ-doped regions (Fig. 5).

The time development of the etch shape has beeninvestigated for structures generated using a backside localcontact (pþ-Si circular region of diameter 1800mm) withoutfrontside masking (Fig. 6a). The structures have beenanodized in 30wt.% HF at current of 13mA/structure.Formed PS was subsequently removed in weak KOHsolution.

It was observed, that the shape of the structures in thecentre fitswell to aGaussian function.With increase of depthsome over-structures appear in the central region of thestructure, which can be eliminated by subsequent electro-polishing step.

The etched structures have been characterized in termsof radius of curvature in the centre of the structures. Strongdependence of curvature radius on etch depth has beenobserved (Fig. 6b), which might be useful in fabrication ofoptical lenses for adjustment of the lens focus.

2.3 Shape control by combination of frontsidemasking and backside doping As shown in Sections2.1 and 2.2, frontside masking leads to formation of convex/

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Figure 7 (online colour at: www.pss-a.com) (a) Profiles of thestructures obtained by application of a frontside SiN mask withslit-opening and local backside doping region, as shown in theschematic wafer cross-sections (b); anodization performed via PSformation mode (30wt.% HF, 5V, 20min); profile measurementsperformed with stylus profiler.

Figure 8 (online colour at: www.pss-a.com) (a) Schematic cross-section of Si sample with pre-structured V-grooves etched in KOH.The pre-structured sample was subsequently anodized, resultingprofile is shown in (b) in comparison to the initial V-formand profileof the structure produced only with anodization through frontsideopening of same size in SiN without KOH-pre-structuring.

Figure 9 Surface roughness measurements with AFM for (a) sur-face after anodization, (b) not anodized surface after etching in 1%KOH and in photoresist developer solutions.

concave shape whereas local backside doping to formationof funnel-like structures during the anodization process. Incase both frontside masking and local backside doping areused, the resulting etched form is a combination of thesespecific shapes. Application of the frontside insulating masklimits the anodized area and increases the current density,which results in pronounced vertical etching for the smallerfrontside opening and brings convex/concave shape devel-opment as shown for two different widths of frontsideopenings in Fig. 7. Anodization through a 260mm wideopening provided formation of concave shape (dominationof frontside shape development), whereas for a 4160mmwide opening both effects are observable (enhanced etchingin the centre due to backside local contact and the edge effectof frontside mask).

2.4 Shape control by combination of etchingtechniques Different anodization processes and otheretching techniques can be combined in order to producestructures with specific properties. For example, formationof PS can be followed by electropolishing (in same ordifferent electrolyte) to form structures with high surfacequality (Fig. 5c).

Combination of conventional etching techniques andanodization was demonstrated using pre-structured surfacesprovided by anisotropic KOH etching (Fig. 8a) followed byanodization in 30wt.% HF at 1.5 A/cm2 (Fig. 8b). It can beseen from the measured profiles in Fig. 8b, that the two-stepprocess using KOH-pre-structuring and subsequent anodiza-tion provides different shape formation in comparison to theshape using single step anodization and results in moreflexible shape control.

2.5 Surface quality High surface quality of thefabricated structures plays an important role especially foroptical applications. However, for some conventionalapplications surface quality just after removal of PS mightbe sufficient.After removal of 10mmthick PS layer (30wt.%HF at 30mA/cm2) in 1%KOH (10min) surface roughness of41.5� 0.1 nm has been measured with AFM (Fig. 9a). The


subsequent removal of PS, for example in weak KOHsolution or photoresist developer, will itself increase surfaceroughness for longer etching duration aswas shown for blankpolished (not anodized) silicon surface (Fig. 9b).Electropolishing of the rough surface, or direct structuringby electropolishing, will provide very high surface quality,as was shown in our study for different electrolytes andcurrent densities (Fig. 9a). Optimized anodization processusing 7wt.% HF electrolyte concentration and currentdensity of 100–300mA/cm2 yields shapes with high surfacequality comparable with the surface quality of not anodizedsilicon surfaces.

3 Applications Important applications of real 3Dforms are in MOEMS and fluidics. Various structures inmicrometer range with high surface qualities can be

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Figure 10 Application of the optimized anodization process: (a)silicon master mould and (b) polycarbonate lens replicated from it;left: profiler 3D scans, right: microscope pictures; injection mould-ing done in cooperation with HS Heilbronn [8].

Figure 12 (online colour at: www.pss-a.com) Examples of struc-tureswithmodulation of the surface topography, pictures takenwith(a) confocal microscope and (b) profiler.

produced, such as microlenses or fluidic channels. Theapplicability of the developed process using optimizedprocess conditions has been demonstrated with the fabrica-tion of optical lens-structures (Fig. 10a) with high surfacequality (Fig. 11a). These lenses have been produced withapplication of non-conductive masking (Fig. 2a) andanodization in electropolishing mode (7wt.% HF,j¼ 200mA/cm2).

The realized silicon structures can be used as mastermoulds for injection of polymer microstructures (Fig. 10b).

As was shown in Fig. 9a, surface roughness dependsstrongly on anodization process conditions. This effectallows the application of the fabricated forms in fluidicsystems with adjusted surface quality for variation andcontrol of the flow conditions in fluidic channels.

Figure 11 (online colour at: www.pss-a.com) AFM topographymeasured in the middle of (a) the silicon mould (average roughness15 nm) and (b) the polycarbonate structure (average roughness10.6 nm).

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Even though most applications for 3D shapes will needlateral resolution in the mm-scale, it is also possible to createstructures in mm scale within mm-scale profiles (Fig. 12).Here, additional local modulation of the surface topographyin the anodized structure can be achieved by special frontsidemasking (Fresnel-like structures).

4 Conclusions Anodization of silicon is a powerful3D structuring technique. Surfaces with optical quality(some nm roughness) can be obtained by process optimiz-ation. However, the precise control of the process needsdeeper understanding of the limiting factors during 3Detching,which restricts wide application till now. In theworkbasic design rules for 3D structuring of silicon have beenestablished and the influence of masking layer and processconditions has been analyzed. The applicability of theoptimized process has been demonstrated with the fabrica-tion of optical lenses with high surface quality.

Acknowledgements The authors would like to thank theresearch teamat the Institute forAppliedSciences and the staff of theTechnologic Laboratory for Micro- and Nanosystems at theHochschule Furtwangen University for their support. Prof. A.Burr, S. Kuhn and M. Kubler from the Hochschule Heilbronn areregarded for the injection moulding process. Prof. P. Woias, Dr. F.Goldschmidtboing and M. Kroner from the University Freiburghave conducted the FEMsimulation of the anodization process. Theproject has been financed by the German Ministry of Economy andTechnology (BMWi) under FKZ 16IN0465 (MiSS3D).



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References

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[5] A. Ivanov, U. Mescheder, M. Kroner, and P. Woias, in:Proceedings of the 19th MicroMechanics Europe Workshop,28–30 September, Aachen, Germany, 2008.

[6] M. Kroener, A. Ivanov, F. Goldschmidtboeing, U. Mescheder,and P. Woias, in: Proceedings of the 215th ECS Meeting, SanFrancisco, USA, 2009, ECS Trans. 19(26) 93–102 (2009).

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high quality 3d shapes by silicon anodization

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