Magnetically Guided Nano–Micro Shaping and Slicing of Silicon

Letter pubs.acs.org/NanoLett

Magnetically Guided Nano−Micro Shaping and Slicing of Silicon Young 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

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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 the global 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 energy source. Here we report the first successful direction-guided, nano/microshaping of silicon, the intended direction of which is dictated by an applied magnetic field. Micrometer thin, massively parallel silicon sheets, very tall Si microneedles, zigzag bent Si nanowires, and tunnel drilling into Si substrates have all been demonstrated. The technique, utilizing narrow array of Au/Fe/Au trilayer etch lines, is particularly effective in producing only micrometer-thick Si sheets by rapid and inexpensive means with only 5 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 and highly antireflective zigzag nanowire coatings. KEYWORDS: Silicon slicing, magnetic field, zigzag nanowire, solar cell, electroless etching

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shaping method using a magnetically direction-guided etching. The method can produce very thin Si sheets or create zigzag Si nanowires, with a possibility of obtaining Si wafers essentially ignoring the crystallographic preferred etch directions. The application of magnetic force and gradient field accelerates the kinetics of Si etching/slicing. The Si slicing waste during the magnetically guided slicing can be minimized to be as small as 5 μm or less thickness per each Si slicing, as compared to at least an order of magnitude more loss for common mechanical slicing. For experimental demonstration, we used a p-type Si (100) wafer with a thickness of 550 μm as well as thicker Si pieces up to ∼1 cm thickness. The wafer was cut into a 2 × 2 cm2 area for experimental samples. The sample structuring approach for magnetically guided Si etching method is schematically illustrated in Figure 1. For the 2 × 2 cm2 area Si processed, there are ∼1330 (Au/Fe/Au) etch strip lines patterned, which means that 1330 slices of 5 μm wide Si slices, 10 μm spaced apart are obtained simultaneously. While the Si sample is in the bath, a NdFeB permanent magnet was brought to the outside bottom 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 the sandwiched magnetic Fe layer, robust and reliable Si slicing was very difficult to obtain. Without the magnetic material involved,

t least 90% of current photovoltaic (PV) cells are dominated by silicon-based structures. For practical use, silicon needs to be physically processed by slicing into wafers and polished, and micro/nanoscale geometrical changes have to be added for circuits and devices. After the wire sawing procedure, less than 50% of the silicon feedstock ends up as useful wafers, with the remaining Si material lost as sawing slurry (kerf loss). Since approximately one-half of the cost of the high-efficiency, crystalline Si solar cells is the silicon materials cost, it would be highly desirable to reduce the slicing loss 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 nanowire fabrications for potential applications in photonics, solar cells, membranes, and interconnects.4−7 The catalyst metals utilized include gold, silver, and platinum8−11 and the etching solutions include H2O2 as an oxidizing agent and HF to dissolve away silicon dioxide. Such Si etching is initiated from the contact interface between metal particles and silicon substrate in the HF, H2O2, and H2O solution. The metal particles penetrate into the silicon substrates and produce irregular porous silicon or silicon nanowires as a remnant of vertical etching by catalyst particles. The electroless etching for Si generally follows crystallographic preferred directions such as ⟨100⟩. In view of the expanding needs for thin Si for a variety of applications, such as flexible, bendable, or stretchable electronics,12−15 nanowire-shaped Si for advanced PV solar cells7 as well as affordable PV solar cells with substantially reduced Si materials cost and a convenient, direction-controllable and rapid Si shaping technique is highly desirable. We have created here a new and unique Si slicing and patterned © 2012 American Chemical Society

Received: January 12, 2012 Revised: February 26, 2012 Published: March 12, 2012 2045

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not clearly understood, but the presence of the sandwiched magnetic layer, under applied field, plays an important role in preventing the nonuniform etching into wire-type geometry, perhaps due to the magnetic force pressing on the lower Au film enhancing the intimate physical and chemical contact of the Au layer onto the Si surface to be catalytically etched. If the magnetic trilayer is used but no magnetic field is applied, the vertical etching still tends to be irregular with wire-like or crooked shaped geometry, with an example shown in Figure S1b, Supporting Information. The presence of magnetic field also accelerates the etching speed, e.g., by ∼300% when the applied field 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 the crystallographic orientations, as indicated in Figure 2. As is wellknown, Si exhibits much different electronic and photonic properties along different crystallographic orientations. However, Si has a preferential chemical etching orientations, e.g., ⟨100⟩ direction.8,9 Being able to slice and prepare Si wafers along any crystal orientation is likely to open up a new dimension in Si electronics and potential applications. Since we employed an array of ferromagnetic lines (Fe layer sandwiched between protective and catalytic Au layer), the magnetic field orientation influences the Si etching direction. It is interesting to note that the magnetic catalytic trilayer surface is rotated from the original horizontal orientation to a tilted position when the magnetic field is applied at an angle (Figure 2a,c). Since the deposited Au/Fe/Au catalytic layer is attached onto the 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 or nanoimprinting lithography (NIL), with the photoresist (SU-8 photoresist, ∼2 μm thick) patterned into polymer resist lines of 10 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 grooves using sputtering or thermal evaporation. Instead of such lithographic patterning, a shadow mask may conveniently used. (c) Fieldaccelerated, guided etching/slicing of Si in a Teflon beaker etching bath containing a mixture solution of diluted hydrofluoric acid and hydrogen peroxide at room temperature (∼18 °C). (d) Resist lift-off and Au catalyst film removal to obtain thin Si sheets. The 550 μm thick wafer was vertically etched, with the slicing completed in ∼12 h under these particular experimental conditions. It is anticipated and actually demonstrated that the etching time will be substantially reduced by adjusting various process parameters such as bath concentration and temperature, magnetic field gradient pulling strength, amount of Au and Fe utilized, and so forth.

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

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 is about 1 cm, which provides a magnetic field of ∼800 Oe, a relatively modest field strength. While this represents a rather preliminary experiment, the SEM 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 etch strips are tilted perpendicular to the applied magnetic field direction. Further research is needed to expand on this tilted direction Si slicing. Such magnetic field-guided Si etching/shaping will is also evident with the zigzag Si nanowire formation and Si tunnel drilling by magnetic orientation control discussed later. 2046

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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) a magnified 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: no solution 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 in maintaining the etching rate, as indicated in Figure 3d. The catalyst metal Au donates electrons to the H2O2 solution and accepts in electrons from the silicon. The positive holes injected into silicon are more liable to induce oxidative dissolution of silicon in mixed solutions that include a HF solution.

For transformative industrial applications of thin Si, a highspeed etching/slicing is an important parameter as it relates to manufacturing throughput and ultimate materials/devices cost. Indeed, simple changes in the processing conditions, such as an increase of HF concentration and bath temperature, resulted in a striking 10-fold increase in the Si slicing rate to ∼500 μm/h, as shown in Figure 4. Additional optimization of process parameters is likely to further increase the etch rate. An ability to slice thick Si ingot is also an important aspect. Au serves as a catalyst, so it is not consumed during the etching/slicing process, as we were able to carry out magnetically guided, vertical Si etching for at least up to 0.8 cm depth (using thick 0.8 −1.0 cm thick Si pieces) with the same, thin Au/Fe/Au etch line (data not shown). The presence of Fe or Au in a silicon based semiconductor may pose a significant issue. These metals, if dissolved into the etchant, will be in an ionic state and are not likely to diffuse into Si material at or near room temperature. Nevertheless, a thorough rinsing of the sliced Si is desirable 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 Si geometry, which can lead to broader scientific research, new device phenomena, and technological applications. For example, being able to drill microscale or nanoscale tunnels and grooves or create three-dimensional Si micro/nano arrays in a

underneath the originally horizontal Au/Fe/Au trilayer has to be etched away for the trilayer to change the angle, it is likely that this shift of the angle occurs gradually as the etching proceeds, possibly with more accelerated etching on the local regions of Si where the magnetic force pressure is higher. More detailed microstructural analysis as a function of etching time would be useful for understanding the process and mechanism of tilted Si slicing by an applied magnetic field. We have characterized the morphology and dimension of the magnetically guided, vertically and parallely sliced Si microsheets. Figure 3a represents a cross-sectional view of Si being sliced using the Au/Fe/Au triple-layer lines having ∼10 μm width, which produce Si slices with corresponding ∼10 μm thickness. Shown in Figure 3b is an SEM top-view image after 12 h etching (also see Figure S1c,d, Supporting Information). From Figure 3c, it is apparent that the vertical wall of the magnetically etched slots is quite straight and relatively smooth. The slicing depth is dependent on the etching time as well as the concentration of the electroless etching solution, as illustrated in Figure 3d. The slicing speed is also dependent on the magnitude of the applied magnetic field (Figure S2, Supporting Information), since the magnetic attractive force exerts a vertical pull-down force and more intimate contacts of the Au/Fe/Au triple layer on Si, which apparently causes enhanced catalytic Si etching. 2047

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results showing a curved 3-D tunnel in Si produced by using our magnetically direction-guided chemical etching. The Au coating on ∼1 μm diameter magnetic beads (containing Fe2O3, Fe3O4 oxide, or other metallic particles) reacts with Si and catalytically etches it at the contact interface pulled by magnetic force, thus forming direction-guided, curved or straight microtunnels. While we used ∼1 μm diameter magnetic beads to form a tunnel with a corresponding diameter, it is anticipated that much smaller capsuled ferromagnetic nanoparticles (e.g., with ∼100 nm diameter)16 can be utilized for nanotunnel array formation using similar magnetic guidance. Very Tall Si Microneedles. By utilizing swiss cheese patterned catalyst trilayer deposition (Au/Fe/Au) for vertical downward etching, instead of parallel stripe array patterns for microsheet slicing of Figures 1−3, our magnetic-guided Si etching allows to easily fabricate “very tall” high-aspect ratio Si microneedles. These needles are as tall as ∼200 μm tall with only a few μm in diameter (see Figure 5d and Figure S5c, Supporting Information). Zig-Zag Si Nanowires for Antireflective Coating. The swiss-cheeze patterned catalyst trilayer can also be utilized to create zigzag Si nanowire arrays by periodically altering the magnetic field orientation at specific desired time and at certain desired etched pattern lengths, as demonstrated in Figure 5e,f. The zigzag bending can be repeated many times by changing the magnetic field direction through stepwise or continual movement of permanent magnet direction or by selective activation of multipole electromagnets. The zigzag Si or SiO2 nanowires can be useful for antireflection (AR) coatings for enhanced light transmission and imaging and reduced sunlight loss in solar cells and other energy devices or glint-free optical lens systems. Zig-zag Si nanowires, such as made by glancing angle deposition by vacuum evaporation, were shown to suppress reflection in the visible and near-IR.17−19 The surface of the magnetically sliced Si is surprisingly smooth with the average roughness of Ra ∼ 7.3 nm (see Figure 5g and Figure S3, Supporting Information). Due to the chemical etching nature, it is also expected to be essentially free of mechanically induced stresses, as compared to the case of traditional mechanical saw-cut Si wafers that produce a strained, chipped, and rather rough Si surface, which should be helpful for producing, handling, and utilizing very thin and fragile Si waters for device applications. For Si solar cell applications, antireflective coating is often added, which can easily be produced by branched Si nanowires (Figure S4, Supporting Information). The antireflection property of the magnetically sliced Si wafer is in fact slightly enhanced by the etch process. An addition of bent or zigzag Si nanowires substantially improves the AR properties as shown in Figure 5h. Reflectance spectra measurement data (350−750 nm range) in Figure 5h also show that the zigzag Si nanowire array exhibits improved and strong antireflection characteristics, with the total reflection significantly reduced to 2−6% regime up to the 650 nm wavelength, as compared to the typical ∼40% reflection value for the bare Si surface.20−22 In summary, we anticipate that a whole new family of novel Si geometries and exciting applications will be enabled due to these versatile shaping techniques. Not only is the science of unique Si shaping processes interesting but also its potential impact to help enable affordable Si photovoltaics by virtue of drastically reduced material loss and usage can be exploited for global clean energy technology.

Figure 4. Magnetically guided Si slicing etch rate altered by acid concentration and bath temperature employed. The etch rate increased from ∼50 μm/h for HF solution (1.87 M) at room temperature (18 °C) to ∼120 μm/h for HF solution (3.73 M) at the same 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 conducted in a massively parallel way. For the case of 5 μm thick Si slicing with 5 μm spacing, assuming a starting Si ingot of 20 × 20 cm2 area in each bath, there would be 20 000 lines simultaneously being etched, with the slicing time per cut (through an assumed Si ingot thickness of 1 cm starting material) estimated to be ∼104 μm thickness divided by [(500 μm/h) × 20 000 slices] = ∼3.6 s/slicing, a rather fast slicing rate. Since a larger-area or longer-ingot Si can be used as a starting material, or hundreds of multiple baths could be operated simultaneously without sophisticated or costly sawing equipment involved, the average time and cost for Si slicing could be further reduced. Such a convenient and manufacturable Si slicing with minimal loss of Si material (only ∼5 μm thickness of Si wasted per cut) could have significant implications for enabling practical photovoltaics applications.

curvatured, zigzag, or spiral configuration along any intended directions could lead to a new paradyme of Si devices for photonic, electronic, microelectromechanical (MEMS), nanoelectromechanical (NEMS), metamaterials, and energy harvesting devices and structures. Flexible Si Devices. Our slicing technique can easily produce very thin Si wafers (300 nm or smaller dimension Si etching demonstrated) from which flexible or conformable Si devices and circuits could be constructed. As suggested by recent research, there are many exciting potential applications using thin Si, for example, for flexible displays, sensors, actuators, and integration of compliant semiconductor chips for in vivo biomedical applications on a curvatured surface.12−15 Since the magnetic field guides the direction of Si slicing in the proposed research, there is an intriguing possibility that off-axis orientation Si wafers (e.g., non-(100), (110), (111)) could be created. Such different crystallographic orientations of Si wafers, if properly developed, could lead to interesting and new semiconducting, photonic, optoelectronic, and mechanical properties which might be exploited to create new devices with exciting characteristics. Microtunnel Formation Within Si Crystal. The magnetically guided Si etching technique reported here can drill curvatured hole paths within Si overcoming the inherent tendency of Si etching along selected crystallographic orientations. This may enable unique applications, such as microfluidic channel devices, microfuel cells, and microcombustion channels. Shown in Figure 5a−c are experimental 2048

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Figure 5. Nonconventional Si geometry fabrication. (a) SEM of ferromagnetic bead microsphere with catalytic Au surface coat. (b) Sectional SEM micrograph showing microscale curved tunnel drilling into Si using magnetically guided etching. (c) Schematics showing the principle of guided tunneling into Si. (d) Example of very tall Si microwire array (∼1 μm diameter and 100−200 μm tall) on large area Si surface, prepared by magnetically guided chemical etch directions. (e) Bent Si nanowaires (∼700 nm dia) by magnetic etching. (f) Dense forest of zigzag Si nanowire array (∼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 by magnetic nanoshaping.

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ASSOCIATED CONTENT

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ACKNOWLEDGMENTS

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REFERENCES

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

Experimental methods (the photoresist patterning on Si surface, magnetically guided Si etching, and magnetic tunnel drilling particle preparation), supporting data on the Si etching using a trilayer catalyst and a magnet, and optical measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest. 2049

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