Magnetically Guided Nano−Micro Shaping and Slicing of SiliconYoung Oh,† Chulmin Choi,† Daehoon Hong,‡ Seong Deok Kong,† and Sungho Jin*,†

†Materials Science and Engineering, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093, United States‡Western Digital Corporation, 5863 Rue Ferrari, San Jose, California 95138, United States

*S Supporting Information

ABSTRACT: Silicon is one of the most important materials for modern electronics,telecom, and photovoltaic (PV) solar cells. With the rapidly expanding use of Si in theglobal economy, it would be highly desirable to reduce the overall use of Si material,especially to make the PVs more affordable and widely used as a renewable energysource. Here we report the first successful direction-guided, nano/microshaping ofsilicon, the intended direction of which is dictated by an applied magnetic field.Micrometer thin, massively parallel silicon sheets, very tall Si microneedles, zigzag bentSi nanowires, and tunnel drilling into Si substrates have all been demonstrated. Thetechnique, utilizing narrow array of Au/Fe/Au trilayer etch lines, is particularly effectivein producing only micrometer-thick Si sheets by rapid and inexpensive means with only5 μm level slicing loss of Si material, thus practically eliminating the waste (and also the use) of Si material compared to the∼200 μm kerf loss per slicing and ∼200 μm thick wafer in the typical saw-cut Si solar cell preparation. We expect that such nano/microshaping will enable a whole new family of novel Si geometries and exciting applications, including flexible Si circuits andhighly antireflective zigzag nanowire coatings.

KEYWORDS: Silicon slicing, magnetic field, zigzag nanowire, solar cell, electroless etching

At least 90% of current photovoltaic (PV) cells aredominated by silicon-based structures. For practical use,

silicon needs to be physically processed by slicing into wafersand polished, and micro/nanoscale geometrical changes have tobe added for circuits and devices. After the wire sawingprocedure, less than 50% of the silicon feedstock ends up asuseful wafers, with the remaining Si material lost as sawingslurry (kerf loss). Since approximately one-half of the cost ofthe high-efficiency, crystalline Si solar cells is the siliconmaterials cost, it would be highly desirable to reduce the slicingloss and usage of Si in the solar cells.Electroless etching of Si using noble metal particles as a

catalyst1−3 has been proposed for Si nanopore or nanowirefabrications for potential applications in photonics, solar cells,membranes, and interconnects.4−7 The catalyst metals utilizedinclude gold, silver, and platinum8−11 and the etching solutionsinclude H2O2 as an oxidizing agent and HF to dissolve awaysilicon dioxide. Such Si etching is initiated from the contactinterface between metal particles and silicon substrate in theHF, H2O2, and H2O solution. The metal particles penetrateinto the silicon substrates and produce irregular porous siliconor silicon nanowires as a remnant of vertical etching by catalystparticles. The electroless etching for Si generally followscrystallographic preferred directions such as ⟨100⟩.In view of the expanding needs for thin Si for a variety of

applications, such as flexible, bendable, or stretchableelectronics,12−15 nanowire-shaped Si for advanced PV solarcells7 as well as affordable PV solar cells with substantiallyreduced Si materials cost and a convenient, direction-control-lable and rapid Si shaping technique is highly desirable. Wehave created here a new and unique Si slicing and patterned

shaping method using a magnetically direction-guided etching.The method can produce very thin Si sheets or create zigzag Sinanowires, with a possibility of obtaining Si wafers essentiallyignoring the crystallographic preferred etch directions. Theapplication of magnetic force and gradient field accelerates thekinetics of Si etching/slicing. The Si slicing waste during themagnetically guided slicing can be minimized to be as small as5 μm or less thickness per each Si slicing, as compared to atleast an order of magnitude more loss for common mechanicalslicing.For experimental demonstration, we used a p-type Si (100)

wafer with a thickness of 550 μm as well as thicker Si pieces upto ∼1 cm thickness. The wafer was cut into a 2 × 2 cm2 area forexperimental samples. The sample structuring approach formagnetically guided Si etching method is schematicallyillustrated in Figure 1. For the 2 × 2 cm2 area Si processed,there are ∼1330 (Au/Fe/Au) etch strip lines patterned, whichmeans that 1330 slices of 5 μm wide Si slices, 10 μm spacedapart are obtained simultaneously. While the Si sample is in thebath, a NdFeB permanent magnet was brought to the outsidebottom of the Teflon beaker (Figure 1c) for direction-guided,accelerated Si slicing.If only the Au layer stripes are used (10 or 15 nm thick) with

identical line dimensions and processing but without thesandwiched magnetic Fe layer, robust and reliable Si slicing wasvery difficult to obtain. Without the magnetic material involved,

Received: January 12, 2012Revised: February 26, 2012Published: March 12, 2012

Letter

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it is observed that Si is etched in a more wire-like fashion ratherthan the desired microsheet configuration, as shown in Figure S1a,Supporting Information. The exact reason for this behavior is

not clearly understood, but the presence of the sandwichedmagnetic layer, under applied field, plays an important role inpreventing the nonuniform etching into wire-type geometry,perhaps due to the magnetic force pressing on the lower Aufilm enhancing the intimate physical and chemical contact ofthe Au layer onto the Si surface to be catalytically etched. If themagnetic trilayer is used but no magnetic field is applied, thevertical etching still tends to be irregular with wire-like or crookedshaped geometry, with an example shown in Figure S1b,Supporting Information. The presence of magnetic field alsoaccelerates the etching speed, e.g., by ∼300% when the appliedfield is increased from ∼300 to ∼1500 Oe (see Figure S2,Supporting Information).The magnetic slicing technique described here also offers a

promise of Si slicing at any desired direction ignoring thecrystallographic orientations, as indicated in Figure 2. As is well-known, Si exhibits much different electronic and photonicproperties along different crystallographic orientations. How-ever, Si has a preferential chemical etching orientations, e.g.,⟨100⟩ direction.8,9 Being able to slice and prepare Si wafersalong any crystal orientation is likely to open up a newdimension in Si electronics and potential applications. Since weemployed an array of ferromagnetic lines (Fe layer sandwichedbetween protective and catalytic Au layer), the magnetic fieldorientation influences the Si etching direction. It is interestingto note that the magnetic catalytic trilayer surface is rotatedfrom the original horizontal orientation to a tilted positionwhen the magnetic field is applied at an angle (Figure 2a,c).Since the deposited Au/Fe/Au catalytic layer is attached ontothe Si surface and since some portion of the silicon material

Figure 1. Magnetically direction-guided silicon slicing process. (a)Photoresist (PR) line prepattern on Si surface by photolithography ornanoimprinting lithography (NIL), with the photoresist (SU-8photoresist, ∼2 μm thick) patterned into polymer resist lines of10 μm wide × 20 μm spacing apart or a narrower pattern of 5 μm wide ×10 μm spacing. (b) Thin film catalytic trilayer (10 nm Au/10 nm Fe/10 nm Au thick) deposited on the resist-patterned silicon groovesusing sputtering or thermal evaporation. Instead of such lithographicpatterning, a shadow mask may conveniently used. (c) Field-accelerated, guided etching/slicing of Si in a Teflon beaker etchingbath containing a mixture solution of diluted hydrofluoric acid andhydrogen peroxide at room temperature (∼18 °C). (d) Resist lift-offand Au catalyst film removal to obtain thin Si sheets. The 550 μmthick wafer was vertically etched, with the slicing completed in ∼12 hunder these particular experimental conditions. It is anticipated andactually demonstrated that the etching time will be substantiallyreduced by adjusting various process parameters such as bathconcentration and temperature, magnetic field gradient pullingstrength, amount of Au and Fe utilized, and so forth.

Figure 2.Magnetically guided Si etching direction. (a) Magnetic field orientation varying along the sample width. (b) Magnetic field vs gap distance,and (c) SEM for resultant altered Si slicing directions (after 20 min etch). The actual magnet distance from the Si work piece for this experiment isabout 1 cm, which provides a magnetic field of ∼800 Oe, a relatively modest field strength. While this represents a rather preliminary experiment, theSEM data in Figure 2c indeed indicates that the early stage Si etching direction is dictated by the magnetic force. It appears that the Au/Fe/Au etchstrips are tilted perpendicular to the applied magnetic field direction. Further research is needed to expand on this tilted direction Si slicing. Suchmagnetic field-guided Si etching/shaping will is also evident with the zigzag Si nanowire formation and Si tunnel drilling by magnetic orientationcontrol discussed later.

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underneath the originally horizontal Au/Fe/Au trilayer has tobe etched away for the trilayer to change the angle, it is likelythat this shift of the angle occurs gradually as the etchingproceeds, possibly with more accelerated etching on the localregions of Si where the magnetic force pressure is higher. Moredetailed microstructural analysis as a function of etching timewould be useful for understanding the process and mechanismof tilted Si slicing by an applied magnetic field.We have characterized the morphology and dimension of the

magnetically guided, vertically and parallely sliced Si micro-sheets. Figure 3a represents a cross-sectional view of Si beingsliced using the Au/Fe/Au triple-layer lines having ∼10 μmwidth, which produce Si slices with corresponding ∼10 μmthickness. Shown in Figure 3b is an SEM top-view image after12 h etching (also see Figure S1c,d, Supporting Information).From Figure 3c, it is apparent that the vertical wall of themagnetically etched slots is quite straight and relatively smooth.The slicing depth is dependent on the etching time as well asthe concentration of the electroless etching solution, asillustrated in Figure 3d. The slicing speed is also dependenton the magnitude of the applied magnetic field (Figure S2,Supporting Information), since the magnetic attractive forceexerts a vertical pull-down force and more intimate contacts ofthe Au/Fe/Au triple layer on Si, which apparently causes en-hanced catalytic Si etching.

For transformative industrial applications of thin Si, a high-speed etching/slicing is an important parameter as it relates tomanufacturing throughput and ultimate materials/devices cost.Indeed, simple changes in the processing conditions, such as anincrease of HF concentration and bath temperature, resulted ina striking 10-fold increase in the Si slicing rate to ∼500 μm/h,as shown in Figure 4. Additional optimization of processparameters is likely to further increase the etch rate. An abilityto slice thick Si ingot is also an important aspect. Au serves as acatalyst, so it is not consumed during the etching/slicingprocess, as we were able to carry out magnetically guided,vertical Si etching for at least up to 0.8 cm depth (using thick0.8 −1.0 cm thick Si pieces) with the same, thin Au/Fe/Au etchline (data not shown). The presence of Fe or Au in a siliconbased semiconductor may pose a significant issue. Thesemetals, if dissolved into the etchant, will be in an ionic state andare not likely to diffuse into Si material at or near room temper-ature. Nevertheless, a thorough rinsing of the sliced Si isdesirable to prevent any left-over contamination with Fe or Au.The magnetically guided Si slicing technique described here

can also be employed for creating useful nonconventional Sigeometry, which can lead to broader scientific research, newdevice phenomena, and technological applications. Forexample, being able to drill microscale or nanoscale tunnels andgrooves or create three-dimensional Si micro/nano arrays in a

Figure 3. SEM pictures of vertically and magnetically guided Si slicing to ∼10 μm thickness using Au/Fe/Au = 10/10/10 nm trilayer line arrays. (a)A partially sliced array of thin Si sheets after 2 h etching, (b) top view image of near complete slicing after 12 h (the inset = higher mag image), (c) amagnified image of the etched slot, and (d) a plot of the Si etched depth vs etch time (red: etching solution was changed every 6 h, black: nosolution change). The etching rate can be changed according to solution replenishing time, solution temperature, ratio of the mixed etchant solution,crystal direction of the silicon, strength of magnetic force, and thickness of metal film. The solution replenishing plays an important role inmaintaining the etching rate, as indicated in Figure 3d. The catalyst metal Au donates electrons to the H2O2 solution and accepts in electrons fromthe silicon. The positive holes injected into silicon are more liable to induce oxidative dissolution of silicon in mixed solutions that include a HFsolution.

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curvatured, zigzag, or spiral configuration along any intendeddirections could lead to a new paradyme of Si devices forphotonic, electronic, microelectromechanical (MEMS), nano-electromechanical (NEMS), metamaterials, and energy harvest-ing devices and structures.Flexible Si Devices. Our slicing technique can easily

produce very thin Si wafers (300 nm or smaller dimension Sietching demonstrated) from which flexible or conformable Sidevices and circuits could be constructed. As suggested byrecent research, there are many exciting potential applica-tions using thin Si, for example, for flexible displays, sensors,actuators, and integration of compliant semiconductor chips forin vivo biomedical applications on a curvatured surface.12−15

Since the magnetic field guides the direction of Si slicing in theproposed research, there is an intriguing possibility that off-axisorientation Si wafers (e.g., non-(100), (110), (111)) could becreated. Such different crystallographic orientations of Siwafers, if properly developed, could lead to interesting andnew semiconducting, photonic, optoelectronic, and mechanicalproperties which might be exploited to create new devices withexciting characteristics.Microtunnel Formation Within Si Crystal. The magneti-

cally guided Si etching technique reported here can drillcurvatured hole paths within Si overcoming the inherenttendency of Si etching along selected crystallographicorientations. This may enable unique applications, such asmicrofluidic channel devices, microfuel cells, and micro-combustion channels. Shown in Figure 5a−c are experimental

results showing a curved 3-D tunnel in Si produced by usingour magnetically direction-guided chemical etching. The Aucoating on ∼1 μm diameter magnetic beads (containing Fe2O3,Fe3O4 oxide, or other metallic particles) reacts with Si andcatalytically etches it at the contact interface pulled by magneticforce, thus forming direction-guided, curved or straight micro-tunnels. While we used ∼1 μm diameter magnetic beads toform a tunnel with a corresponding diameter, it is anticipatedthat much smaller capsuled ferromagnetic nanoparticles (e.g.,with ∼100 nm diameter)16 can be utilized for nanotunnel arrayformation using similar magnetic guidance.

Very Tall Si Microneedles. By utilizing swiss cheesepatterned catalyst trilayer deposition (Au/Fe/Au) for verticaldownward etching, instead of parallel stripe array patterns formicrosheet slicing of Figures 1−3, our magnetic-guided Sietching allows to easily fabricate “very tall” high-aspect ratio Simicroneedles. These needles are as tall as ∼200 μm tall withonly a few μm in diameter (see Figure 5d and Figure S5c,Supporting Information).

Zig-Zag Si Nanowires for Antireflective Coating. Theswiss-cheeze patterned catalyst trilayer can also be utilized tocreate zigzag Si nanowire arrays by periodically altering themagnetic field orientation at specific desired time and at certaindesired etched pattern lengths, as demonstrated in Figure 5e,f.The zigzag bending can be repeated many times by chang-ing the magnetic field direction through stepwise or continualmovement of permanent magnet direction or by selectiveactivation of multipole electromagnets. The zigzag Si or SiO2nanowires can be useful for antireflection (AR) coatings forenhanced light transmission and imaging and reduced sunlightloss in solar cells and other energy devices or glint-free opticallens systems. Zig-zag Si nanowires, such as made by glancingangle deposition by vacuum evaporation, were shown tosuppress reflection in the visible and near-IR.17−19

The surface of the magnetically sliced Si is surprisinglysmooth with the average roughness of Ra ∼ 7.3 nm (see Figure 5gand Figure S3, Supporting Information). Due to the chemicaletching nature, it is also expected to be essentially free ofmechanically induced stresses, as compared to the case oftraditional mechanical saw-cut Si wafers that produce a strained,chipped, and rather rough Si surface, which should be helpfulfor producing, handling, and utilizing very thin and fragile Siwaters for device applications. For Si solar cell applications,antireflective coating is often added, which can easily beproduced by branched Si nanowires (Figure S4, SupportingInformation).The antireflection property of the magnetically sliced Si

wafer is in fact slightly enhanced by the etch process. Anaddition of bent or zigzag Si nanowires substantially improvesthe AR properties as shown in Figure 5h. Reflectance spectrameasurement data (350−750 nm range) in Figure 5h also showthat the zigzag Si nanowire array exhibits improved and strongantireflection characteristics, with the total reflection signifi-cantly reduced to 2−6% regime up to the 650 nm wavelength,as compared to the typical ∼40% reflection value for the bare Sisurface.20−22

In summary, we anticipate that a whole new family of novelSi geometries and exciting applications will be enabled due tothese versatile shaping techniques. Not only is the science ofunique Si shaping processes interesting but also its potentialimpact to help enable affordable Si photovoltaics by virtue ofdrastically reduced material loss and usage can be exploited forglobal clean energy technology.

Figure 4. Magnetically guided Si slicing etch rate altered by acidconcentration and bath temperature employed. The etch rateincreased from ∼50 μm/h for HF solution (1.87 M) at roomtemperature (18 °C) to ∼120 μm/h for HF solution (3.73 M) at thesame room temperature and to ∼500 μm/h for HF solution (3.73 M)at an elevated bath temperature of 50 °C. The Si slicing is conductedin a massively parallel way. For the case of 5 μm thick Si slicing with5 μm spacing, assuming a starting Si ingot of 20 × 20 cm2 area in eachbath, there would be 20 000 lines simultaneously being etched, withthe slicing time per cut (through an assumed Si ingot thickness of1 cm starting material) estimated to be ∼104 μm thickness divided by[(500 μm/h) × 20 000 slices] = ∼3.6 s/slicing, a rather fast slicingrate. Since a larger-area or longer-ingot Si can be used as a startingmaterial, or hundreds of multiple baths could be operatedsimultaneously without sophisticated or costly sawing equipmentinvolved, the average time and cost for Si slicing could be furtherreduced. Such a convenient and manufacturable Si slicing withminimal loss of Si material (only ∼5 μm thickness of Si wasted percut) could have significant implications for enabling practicalphotovoltaics applications.

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■ ASSOCIATED CONTENT

*S Supporting InformationExperimental methods (the photoresist patterning on Sisurface, magnetically guided Si etching, and magnetic tunneldrilling particle preparation), supporting data on the Si etchingusing a trilayer catalyst and a magnet, and optical measure-ments. This material is available free of charge via the Internetat http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: jin@ucsd.edu

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was partially supported by NSF-CMMI award no.0856674.

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Figure 5. Nonconventional Si geometry fabrication. (a) SEM of ferromagnetic bead microsphere with catalytic Au surface coat. (b) Sectional SEMmicrograph showing microscale curved tunnel drilling into Si using magnetically guided etching. (c) Schematics showing the principle of guidedtunneling into Si. (d) Example of very tall Si microwire array (∼1 μm diameter and 100−200 μm tall) on large area Si surface, prepared bymagnetically guided chemical etch directions. (e) Bent Si nanowaires (∼700 nm dia) by magnetic etching. (f) Dense forest of zigzag Si nanowirearray (∼300 nm diameter) by ∼30° direction-changing magnetic etching steps (total etch time = 2 min). (g) Surface roughness of magnetic sliced Si(inset = atomic force microscopy data). (h) Comparative light reflectivity of processed Si showing a dramatic increase in light absorption bymagnetic nanoshaping.

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