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Enhanced Wet-Chemical Etching To Prepare Patterned Silicon Mask with Controlled Depths by Combining Photolithography with Galvanic Reaction Nannan Sun, Jianming Chen, Chao Jiang, Yajun Zhang,* and Feng Shi* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, P. R. China ABSTRACT: We have developed an enhanced wet-chemical method to prepare patterned silicon templates with controlled depths at microscale by combining photolithography with electroless metal etching. The silicon masks are obtained in the following procedures: patterned silicon wafers selectively etch through galvanic reactions and result in patterned surfaces with silicon nanoarrays in exposed areas during photolithography; the as-etched silicon wafers are corroded in a mixture etching solution to remove silicon nanoarrays, leading to patterned silicon templates.
1. INTRODUCTION The preparation of patterned silicon templates14 with controlled depths at microscale is one of the key elements to microcontact imprinting5,6 and nanoimprinting lithography.711 In those two methods, patterned transfer from premade masks to targeted polymer is carried out under a high and horizontal pressure between the mask and the polymer, which may damage the silicon templates with improper manipulation and makes masks act as consumable materials. Generally, there are two methods to fabricate patterned templates with controlled depths at microscale. One approach is physical lithography such as ion beam lithography12,13 and electron beam lithography,14 which need equipment with high energetic consumption, a long amount of time for lithography, and are always very costly; the other approach is a chemical-involved procedure including a vapor liquidsolid phase,15 chemical vapor deposition,16 and dry chemical etching,17,18 which contains lots of complex physical and chemical treatment. The above methods need expensive equipments, take complex steps and are time-consuming, resulting in a high cost for the preparation of patterned templates. Therefore, it is important to develop a facile and low-cost method to prepare patterned silicon templates with controlled depths for potential applications in microcontact imprinting and nanoimprinting lithography.1922 Wet-chemical etching of a silicon wafer with hydrofluoric acid (HF) is a well-known facile method to obtain a patterned surface with a controlled depth at nanoscale. We fabricated a patterned multilayer of photosensitive diazoresin and poly(acrylic acid) by photolithography and employed the above patterned surface as a protective layer to selectively etch the bare silicon substrate in HF solutions and remove the surface groups, followed by further surface modification for selective adsorption of nanoparticles.23 Although this kind of method has advantages in ease of preparation and low-cost, when the etching depth reaches the microscale from the surface, normal wet-chemical etching of the silicon wafer with HF cannot obtain a patterned mask with high resolution, and even erase the pattern from the substrate because of uncontrollable etching in the horizontal directions. This r 2011 American Chemical Society
phenomenon can be attributable to isotropic etching of silicon with HF solutions. One possible strategy to solve this problem is to develop an enhanced wet-chemical method with selective etching properties to crystal faces.24 Zhu et al.25 obtained large-area silicon nanowire pn junction diode arrays by selective etching of planar silicon wafers in an aqueous solution of HF that contained appropriate amounts of silver nitrate (AgNO3). Our previous result showed that silver aggregates with micro/nano-structures could be prepared by immersing a p-silicon (100) wafer in the solution of HF and AgNO3 based on galvanic reaction; at the same time, a p-silicon (100) wafer could be selectively etched to silicon nanoarrays with lengths corresponding to different etching time.2628 By combining nanoparticles etching technology with electroless chemical etching, Zhu’s group obtained depthcontrolled micro/nano-silicon arrays which had drawn considerable attention upon widespread applications.29,30 Peng et al.31 obtained aligned arrays of silicon nanowires with desirable diameter and density due to selective and anisotropic etching of silicon induced by the silver pattern, and made use of them to get high performance of solar cells.3234 Electroless metal deposition exhibits good selectivity toward crystal faces in etching silicon and could be applied to fabricate silicon nanoarrays with controlled depth.35,36 Considering that wet-chemical etching has an isotropic property and can easily remove silicon nanoarrays for their high specific surface area, we wonder whether it is possible to obtain patterned silicon templates by combining photolithography, electroless metal deposition, and wet-chemical etching. In this paper, we have developed an enhanced wet-chemical method to prepare patterned silicon templates with controlled depths from nanoscale to microscale by combining photolithography with electroless metal etching. The silicon masks are obtained in the following procedures: patterned silicon wafers selectively etch through galvanic reactions Received: September 2, 2011 Accepted: December 5, 2011 Revised: November 28, 2011 Published: December 05, 2011 788
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and result in patterned surfaces with silicon nanoarrays in exposed areas during photolithography; the as-etched silicon wafers were corroded in a mixture solution of hydrofluoric acid, nitric acid, and acetic acid to remove silicon nanoarrays in exposed areas during photolithography, leading to patterned silicon templates with controlled depths. The key parameters to the fabrication of silicon masks include crystal faces of the silicon wafer, volume ratios of the etching solution, and etching temperatures and time. Results show that the patterned silicon masks with depths of several or several tenths of micrometers could be obtained on p-silicon (100) wafers etched in the mixed solution of HF (40%)/HNO3 (70%)/CH3COOH with a volume ratio of 4:5:1 at the room temperature for 30 s. Moreover, the fabricating process is characterized with atomic force microscopy and scanning electronic microscopy.
Scheme 1. The Schematic Representation of Fabricating a Patterned Silicon Mask
2. EXPERIMENTS 2.1. Materials and Substrate Preparation. p-Silicon (100) and p-silicon (111) wafers were purchased from GRINM semiconductor Materials Co., Ltd., Beijing, China. Tackifier (HMDS, RZN-6200), photoresist (RZJ-30425 cP) and developer (RZX-3038) were purchased from Suzhou Ruihong Electronic Chemicals Co., Ltd. AgNO3 and HF (aq, 40%) were obtained commercially from Beijing Chemical Reagents Company (Beijing, China). Deionized water was prepared with Hitech-K Flow Water Purification System (Hitech Instruments Co. Ltd., Shanghai, China). The silicon wafers used in all experiments were strictly washed by immersing them in a piranha solution (a mixture solution of 98% H2SO4 and 30% H2O2 with a volume ratio of 7:3) for 1 h, flushed with deionized water, and dried with N2. 2.2. Formation of Patterned Surface with Photoresist by Photolithography on Silicon Wafers. First HMDS was spincoated on the polished silicon wafers at the speed of 8000 rpm for 30 s and dried at 100 °C for 10 min. Second, the dried substrate was spin-coated with photoresist at 8000 rpm for 30 s, followed by drying at 100 °C for 10 min. Third, the as-prepared silicon wafers were exposed to UV radiation of 10 mw cm2 for 8 s covered by a photomask, whose transparent areas are quadrates with sizes of 90 μm 90 μm and whose opaque zones are spaces between quadrates with widths of 20 μm. Finally the patterned surface with photoresist was obtained after immersing the wafers in the developer solution for 1 min, followed by a rinse with deionized water, drying with nitrogen, and heating at 120 °C for 15 min. 2.3. Fabrication of Silicon Nanoarrays on the Silicon Wafer with Patterned Photoresist. The silicon wafers with patterned photoresist were immersed in a mixture solution of HF (5 mol/L)/ AgNO3 (20 mmol/L) for different etching times at the constant temperature of 50 °C. After the galvanic reaction between silicon wafers and AgNO3 in the presence of HF, silicon nanoarrays with tuned depth on the as-prepared silicon wafers were obtained; at the same time, silver aggregates with coralloid structures were generated. The silver aggregates were removed by dipping in HNO3 solution quickly for 30 min, followed by rinsing with deionized water and drying with nitrogen. 2.4. Preparation of Patterned Templates with Controlled Depth on the As-Etched Silicon Wafers. Patterned templates with controlled depth were obtained by immersing the silicon wafers with patterned nanoarrays in different solutions to etch the silicon with specific times to remove the silicon nanoarrays.
After being rinsed with deionized water and dried with nitrogen, the morphologies of patterned silicon masks were characterized by SEM to investigate the effective parameters of the fabrication of silicon masks, including the crystal faces of the silicon wafer, volume ratios of the etching solution, etching temperatures, and time. First the influence of the crystal faces was studied by using p-silicon (100) and p-silicon (111) wafers: the patterned silicon wafers of p-silicon (100) and p-silicon (111) were both etched in the solution of KOH (30%) at 60 °C for 1 min; the patterned silicon nanoarrays were etched in a mixture solution of HF/ HNO3/CH3COOH = 4:5:1 (volume ratio) at room temperature for 15 s on p-silicon (111) wafers and for 30 s on p-silicon (100) wafers. Second the effects of composites of the etching solutions were analyzed on p-silicon (100) wafers: the nanoarrays were removed respectively in the mixture solutions of HF (40%)/ HNO3 (70%)/H2O = 8:11:1 (volume ratio) and HF (40%)/HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) for 30 s at room tempreture. Third, the affects of the etching time were studied on p-silicon (100) wafers: nanoarrays were immersed in the mixture solution of HF (40%)/HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) at room tempreture for 10, 30, and 90 s, respectively. 2.5. The Fabricating Procedure of Patterned Silicon Templates with Controlled Depth. The fabrication of patterned silicon templates with controlled depth is illustrated in Scheme 1: (i) the cleaned p-silicon (100) wafers were spin-coated with photoresist; (ii) the precoated substrates were exposed to UV radiation of 10 mw cm2 for 8 s covered by a photomask; (iii) the patterned surface with photoresist was obtained after immersion in the developer solution for 1 min, followed by rinsing with deionized water and drying with nitrogen; (iv) the silicon wafers with patterned photoresist were immersed in a mixture solution of HF (5 mol/L)/AgNO3 (20 mmol/L) for different etching times at the constant temperature of 50 °C; (v) silicon nanoarrays on the p-silicon (100) wafers were etched in the mixed solution of HF (40%)/HNO3 (70%)/CH3COOH at the volume ratio of 4:5:1 for 30 s at room temperature and then the patterned silicon templates which have controlled depths of several or several tenths of micrometers were obtained. 789
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Figure 1. AFM images and their corresponding section analyses of (a) the photomask, and (b) the patterned surface with photoresist.
2.6. Analysis. The following instruments were used: a spincoater (KW-4A) from Institute of Microelectronics of Chinese Academy of Sciences; photolithography equipment (G-25) from Chengdu Xinnanguang Mechanical Equipment Co., Ltd., China; atomic force microscope (AFM, Dimension 3100) from Veeco, U.S.A.; scanning electron microscope (SEM, JSM-6360LV) from JEOL Inc., Japan.
areas of silicon wafer were regarded as anodes, which led to a quick anisotropic etching. After the galvanic reaction between silicon wafers and AgNO3 in the presence of HF, silicon nanoarrays with tuned depth on the as-prepared silicon wafers were obtained; at the same time, silver aggregates with coralloid structures were generated. The silver aggregates and the residual photoresist were removed by dipping the wafers in HNO3 solution for 30 min, followed by rinsing with deionized water and drying with nitrogen. After the above treatments, the morphologies of silicon nanoarrays were observed by SEM. From the top view of the silicon nanoarrays in Figure 2e, we can see clearly that when the etching time was 30 min, there were massive silicon nanoarrays on the surface and they formed microscale aggregates due to the surface tension during the process of drying. When the etching time was prolonged to 1 h in Figure 2f, the aggregates formed by silicon nanoarrays became sparser and more discrete than those in Figure 2e owing to enhanced agglomeration of silicon nanoarrays with enlarged aspect ratio. The cross-sectional views showed that many separate silicon nanoarrays with diameters of about 100 nm dispersed on the bare silicon surface after electroless chemical etching and lengths of silicon nanoarrays increased gradually with increasing etching time. When the etching time was 5 min, the average etching depth was 3.2 μm as shown in Figure 2a. With an increase in the reacting time in 5 min intervals, the etching depths reached 5.4 μm (Figure 2b), 7.1 μm (Figure 2c), 12.5 μm and 14.9 μm (Figure 2d), respectively. Moreover, the depth reached more than 20 μm when the reacting time was prolonged to 1 h. We have the histogram of the relation between electroless chemical etching time and the resulted etching depths in the process of fabricating patterned silicon nanoarrays as shown in Figure 3. It is clearly observed that expected silicon nanoarrays were prepared by regulating the reaction time to control the etching depth
3. RESULTS AND DISCUSSION 3.1. AFM Analysis of Patterned Surface with Photoresist by Photolithography on Silicon Wafers. The surface morphol-
ogies of the photomask and the patterned surface with photoresist were characterized by AFM. From the height image of the photomask in Figure 1a, we can observe clearly that the width of the stripe was around 20 μm. After photolithography, the width value of the patterned surface with photoresist was also about 20 μm from the section analysis in Figure 1b, which fitted that of the photomask well. Moreover, the vertical distance of the stripe was 1750 nm, which indicated the thickness of the photoresist preserved on the silicon wafer. The phenomenon meant that the photolithography process had been carried out successfully. 3.2. Controlled Depth of Silicon Nanoarrays Depending on the Etching Time on the Silicon Wafer with Patterned Photoresist. When the silicon wafers with patterned photoresist were immersed in a mixture solution of HF/AgNO3, the silver ions were reduced on the surface of silicon wafers according to the following two half-cell reactions:30 Agþ þ e f Ag
ð1Þ
Si þ 6F f SiF6 2 þ 4e
ð2Þ
During the process of etching silicon by HF solution, the deposited silver nanoclusters acted as cathodes and the surrounding 790
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Figure 2. SEM images of Si nanowire arrays in different etching time: (a) 5, (b) 10, (c) 15, (d) 25, (e) 30, and (f) 60 min.
temperatures and time. First of all, the influence of crystal faces was studied by using p-silicon (100) and p-silicon (111) wafers and employing HF/HNO3/CH3COOH mixed solution as isotropy etching solution and KOH solution as anisotropy etching solution. From Figure 4a, we can observe that by immersing the wafers in an aqueous solution of KOH (30%) at 60 °C for 1 min, the surface pattern on the p-silicon (111) wafers almost had been erased from the substrate. Under the same condition, the surface pattern on the p-silicon (100) wafers remained but the surface became very rough, which decreased the resolution of the patterned templates as shown in Figure 4b. Those experiment data suggested that although the resolution of p-silicon (100) wafers was better than that of p-silicon (111) wafers using an aqueous solution of KOH (30%) as etching medium, this resolution still could not meet the standards for silicon templates. Since anisotropy etching solution could not attain silicon templates with high qualities, we wonder whether isotropy etching solution would be beneficial to gain patterned silicon mask. Therefore, we chose the mixed solution of HF (40%)/HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) as isotropy etching solution, and immersed the p-silicon (111) and p-silicon (100) wafers with silicon nanoarrays into etching solution for 15 and 30 s, respectively. As shown in Figure 4c, the surface pattern on the psilicon (111) wafers were distorted and nearly removed from the surface. Under the condition of the p-silicon (100) wafers, the silicon nanoarrays were wiped off and silicon templates with
Figure 3. Histogram of the relations of etching time and etching depth.
effectively. Therefore, we could select appropriate etching time to obtain patterned silicon nanoarrays with controlled depth, and then remove silicon nanoarrays on the silicon substrate to accomplish the fabrication of patterned silicon mask using wet etching. (Note: all of the samples used in the following experiments were prepared by immersing the wafers in the mixture solution of HF/AgNO3 for 60 min at 50 °C.) 3.3. Effects of Selective Property of Crystal Faces on the Formation of Patterned Silicon Templates. The key parameters to the fabrication of silicon masks include crystal faces of silicon wafer, composites of the etching solution, and etching 791
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Figure 4. SEM images of the silicon templates prepared with different crystal faces. The silicon substrates with nanoarrays etched in KOH solution for 1 min with the crystal faces of (a) p-silicon (111) and (b) p-silicon (100); etched in a mixture solution of HF/HNO3/CH3COOH at room temperature for (c) 15 s on p-silicon (111) wafer and (d) 30 s on p-silicon (100) wafer.
Figure 5. SEM images of the silicon templates: (a) prepared in the mixture solution of HF (40%)/HNO3 (70%)/H2O = 8:11:1 (volume ratio) for 30 s at room temperature; (b) prepared in the mixture solution of HF (40%)/HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) for 10 s at room tempreture; (c) the mixture solution of HF (40%)/HNO3 (70%)/CH3COOH=4:5:1 (volume ratio) for 90 s at room tempreture.
about 11 μm. However, in contrast to the condition where the nanoarrays on the p-silicon (100) wafer were etched in a mixture solution of HF (40%)/HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) for 30 s at room temperature, the as-prepared surface pattern on the silicon mask became irregular and its resolution decreased. Comparing the above three etching solutions, we can draw a conclusion that the mixture solution of HF (40%)/ HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) is the best of these three etching solutions. 3.5. Effects of Etching Time on the Formation of Patterned Silicon Templates. To investigate the influence of the reaction time to the fabrication of silicon masks, three different etching times, 10, 30, and 90 s, were studied. When the as-prepared silicon substrates were immersed in the mixture solution of HF (40%)/HNO3 (70%)/CH3COOH = 4:5:1 (volume ratio) for 10 s at room tempreture, the surface of patterned silicon template was very rough and irregular, and some of the silicon nanoarrays still remained, as shown in Figure 5b. However, with an increase
good resolution were prepared successfully. The above experimental results demonstrated that the crystal face of the silicon wafer had important effects on the formation of patterned silicon masks and p-silicon (100) wafers hold outstanding advantages in this fabrication. Therefore, we finally selected isotropy etching solution to etch P (100) type silicon at room temperature. (Note: the silicon wafer used in this paper is p-silicon (100) wafer, unless otherwise noted.) 3.4. Effects of Composites for the Etching Solution on the Formation of Patterned Silicon Templates. In the previous experiment, isotropy etching solution showed advantages in the formation of patterned silicon templates over anisotropy etching solution. To clarify the importance of the composites of the etching solutions, two kinds of isotropy etching solutions were studied. Figure 5a showed that after immersion in a mixture solution of HF (40%)/HNO3 (70%)/H2O = 8:11:1 (volume ratio) for 30 s at room tempreture, the nanoarrays on the silicon wafers were wiped off, resulting in a quadrate pattern with an average depth of 792
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in the etching time to 90 s, the corrosion extent of patterned silicon templates was so strong that the altitude difference between the strip and the bottom of the pattern almost disappeared. Herein, we come to the conclusion that control over the etching time is significant to obtain patterned silicon templates, and the optimized condition for etching is 30 s.
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4. CONCLUSIONS In summary, we have developed an enhanced wet-chemical method to prepare patterned silicon templates with controlled depths from nanoscale to microscale by combining photolithography with electroless metal etching. The depth is controlled by the reaction time of electroless metal deposition. Moreover, the key parameters to the fabrication of silicon masks, crystal faces of silicon wafer, composites of the etching solution, and etching time, are investigated. Results show that the patterned silicon masks with depths of several or several tenths of micrometers could be obtained on a p-silicon (100) wafer etched in the mixed solution of HF (40%)/HNO3 (70%)/CH3COOH with a volume ratio of 4:5:1 at room temperature for 30 s. Herein, we provide a facile wet-chemical method with low cost to prepare patterned silicon templates with controlled depths at microscale, which might not only be further applied in microcontact imprinting and nanoimprinting, but also have potential applications in the fields of solar cells, nanoreactors, and biocompatible surfaces. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-10-64423889. Fax: +86-10-64434784. E-mail address: shi@ mail.buct.edu.cn (F. Shi). Tel.: +86-10-64423316. Fax: +86-1064434784. E-mail address:
[email protected] (Y. J. Zhang).
’ ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant No. 50903005), Beijing Nova Program of China (Grant No. 2009B011), Open Project of State Key Laboratory of Supramolecular Structure and Materials (Grant No. SKLSSM201101), the Fundamental Research Funds for the Central Universities. ’ REFERENCES (1) Xia, Y. N.; Matthias, G. Patterning: principles and some new developments. Adv. Mater. 2004, 16, 1249–1499. (2) Zeballos, L. J.; Castro, P. M.; Mendez, C. A. Integrated constraint programming scheduling approach for automated wet-etch stations in semiconductor manufacturing. Ind. Eng. Chem. Res. 2011, 50, 1705– 1715. (3) Gates, B. D.; Q., Xu; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. New approaches to nanofabrication: Molding, printing, and other techniques. Chem. Rev. 2005, 105, 1171–1196. (4) Diaz-Quijada, G. A.; Maynard, C.; Comas, T.; Monette, R.; Mealing, G. Surface patterning with chemisorbed chemical cues for advancing neurochip applications. Ind. Eng. Chem. Res. 2011, 50, 10029– 10035. (5) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Unconventional methods for fabricating and patterning nanostructures. Chem. Rev. 1997, 99, 1823–1848. (6) Auletta, T.; Dordi, B.; Mulder, A.; Sartori, A.; Onclin, S.; Bruinink, C. M.; Peter, M.; Nijhuis, C. A.; Beijleveld, H.; Reinhoudt, 793
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