Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications

Materials Science and Engineering B 149 (2008) 292–298

Fabrication of near-field optical probes using advanced functional thin films for MEMS and NEMS applications J.S. Hyun, J.-S. Moon, J.-H. Park, J.W. Kim, Y.D. Kim, J.-H. Boo ∗ Department of Chemistry and Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon 440-746, Republic of Korea

Abstract Highly efficient atomic force microscopy (AFM) cantilevers and an near-field scanning optical microscopy (NSOM) aperture array were fabricated using the functional Si3 N4 and SiO2 thin films for the micro electro mechanical systems (MEMS) and NEMS applications. In order to generate the cantilevers and array, a silicon nitride (Si3 N4 ) thin film with a stress below 100 MPa was deposited using NH3 and SiCl2 H2 gases with a relative ratio of 1:5 at 140 mTorr, and 835 ◦ C by low pressure chemical vapor deposition (LPCVD). Cantilevers of 10–100 ␮m in length were fabricated with Si3 N4 thin films. Photolithography and magnetically enhanced reactive ion etching (MERIE) techniques were employed for patterning the Si3 N4 thin films and selective bulk etching of Si was carried out using a strong alkaline solution of tetramethylammoniumhyroxide (TMAH) to generate the Si3 N4 -based cantilevers with 3D shapes. In addition, we also successfully fabricated an array of SiO2 apertures with sub-wavelength sizes as near-field optical probe in order to examine the possible light resonance-tunneling phenomenon. Initially, a (50 × 50) array with a dimension of (5 mm × 5 mm) was fabricated on a Si wafer followed by the V-groove formation using alkaline solution Si bulk micromachining technology. The size of the aperture on top of the pyramidal array was carefully controlled with an opening rate of ∼27 nm/min using HF solution diluted by a factor of 50 using water. The Al thin film was thermally evaporated on the (50 × 50) array pattern for the fabrication of apertures with sub-wavelength sizes. © 2007 Elsevier B.V. All rights reserved. Keywords: Si3 N4 and SiO2 thin films; MERIE dry etching; NSOM + AFM cantilever; MEMS/NEMS

1. Introduction Nowadays, scanning probe microscopes (SPM) are widely used to investigate various surface properties on a macro- and/or a nanometer scale. SPM techniques often use cantilevers with a tip at one end as a sensing element. The deflection of a cantilever can easily be detected and converted to electrical signals by various methods [1,2]. Another aspect is the use of such tips as tools for the modification of surfaces on a nanometer scale. Well-known members of this family are the scanning tunneling microscope (STM) and the scanning force microscope (SFM) [3,4]. Even though various types of SPM have been recently developed, which allow imaging surface structures on a nanometer scale, determination of both chemical and molecular identifications with a conventional sensing probe still remains as a great challenge. In the last decade, this concept has been expanded for measuring optical (NSOM) [5–7], thermal (SThM) [8], electronic (SEFM) [9], and ion-conducting

∗

Corresponding author. E-mail address: [email protected] (J.-H. Boo).

0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.10.009

(SICM) [10] properties. Among them, near-field scanning optical microscopy (NSOM) is an established technique based on the collection or transmission of light through a sub-wavelength size aperture scanning near a surface [6]. Therefore, developments in the NSOM technique, which can measure both surface morphology and optical property, have been accelerated over the last few years, which already resulted in the development of the industrial area such as wireless telecommunication, Raman spectrometer, chemical etching in nano-scale, etc. [11]. In this paper, we mainly describe the process of fabrication of atomic force microscopy (AFM) cantilevers with both 3Dpyramidal and 3D-rectangular shapes using Si3 N4 and SiO2 functional thin films for MEMS and nano electro mechanical systems (NEMS). For this application, a fine aperture is required at the apex of the tip, working as a point source or detector. Photolithography and MERIE techniques were employed for patterning the small cantilevers on the Si3 N4 thin films and probe tips with a high resolution on the bare small cantilevers were also achieved using electron-beam deposition in carbon atmosphere. In addition, we also successfully fabricated a SiO2 aperture array with sub-wavelength sizes as near-field optical probes in order to examine the possible light resonance-tunneling phenomenon.

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Fig. 1. Cross-sectional SEM image of silicon (1 0 0) wafer (left) using TMAH (20 wt.%) solution at 80 ◦ C for 8 h, and etching rate (right).

2. Experimental The etching of the Si using alkaline solutions such as KOH, ethylenediamine/pyrocatechol (EDP), and tetra methyl ammonium hydroxide (TMAH), is anisotropic due to the different atomic densities of various Si crystal surfaces. The ratio of the etching rate of the Si(1 0 0) and that of the Si(1 1 1) surface is of the order of a few hundreds. The intersection of the Si(1 1 1) surfaces will eventually form V-type groove or pyramidal shape at the bottom Si(1 0 0) surface [12]. The oxidation rate is dependent upon both the crystal plane and the angle of the plane intersection. Because of the difference in atomic packing density, the Si(1 1 1) surface will have a higher oxidation rate than the Si(1 0 0) surface. Due to either the stress-induced retarded oxidation or the volume expansion of oxide at a concave surface during thermal oxidation [13,14], inner surfaces of V-groove or hollow pyramid will be non-uniformly oxidized during an appropriate thermal oxidation procedure. That is, the oxide layer at the bottom or apex of hollow pyramid is thinner than the oxide at side surfaces. An isotropic oxide etching technique using water-

diluted HF solution has been employed to fabricate apertures with nano-sizes at the apexes of oxide pyramids [15]. Before the fabrication process, the silicon wafer was cleaned using a standard method. Scanning electron microscopy (SEM) was mainly used for checking the whole fabrication processes. 3. Results and discussion 3.1. Fabrication of NSOM aperture arrays using Si3 N4 and SiO2 thin films As mentioned in Section 2, the Si etching using alkaline solutions such as KOH, EDP, and TMAH is anisotropic. Among those solutions, we have obtained the best result when the 20 wt.% TMAH solution was used. Fig. 1 shows a crosssectional SEM image of silicon (1 0 0) wafer (left) treated using the TMAH (20 wt.%) solution at 80 ◦ C for 8 h. With the TMAH solution, the etching rate (right) to the Si{1 0 0} crystal direction has been estimated to be 0.45 ␮m/min. We could observe that anisotropic Si etching using the 20 wt.% TMAH at 80 ◦ C had

Fig. 2. Fabrication procedure of pyramid-shaped cantilever and nano-aperture array.

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generated hollow pyramids as shown in Figs. 3 and 4(a). The angle is the well-known 54.75◦ due to the anisotropy in atomic packing density. Fig. 2 shows a schematic diagram of the basic fabrication process that was applied in this study for making pyramidshaped cantilevers and nano-aperture array using Si3 N4 and SiO2 thin films grown on Si(1 0 0) wafers with a thickness of 500 ␮m. To produce cantilevers with integrated tips, we need seven steps as following: (a) growth of Si3 N4 and SiO2 thin films on silicon wafers, (b) formation of dot array pattern by photolithography, (c) formation of concave pyramid grooves by chemical etching using the TMAH (20 wt.%) solution at 80 ◦ C for 8 h, (d) re-oxidation (stress-induced retarded oxidation) process, (e) etching process of residual Si atoms on the backside by chemical etching using the TMAH (20 wt.%) solution, (f) formation of the concave pyramidal oxide tip and (g) formation of nano-size holes utilizing both plasma etching (MERIE: magnetically enhanced reactive ion etching) and HF chemical etching. In order to make the V-grooved hollow pyramids, we carried out dry oxidation process to grow SiO2 thin films on silicon wafers and then made a dot array pattern using PR patterning by UV photolithography. AZ 1512 was utilized as photo-resist (PR) and hexamethyldisilane (HMDS) coating was used (created by

spin coating method) for improving the adhesion between PR and substrate. After completing the PR patterning, the oxide layer was etched off by MERIE method using P-5000 dry etcher ˚ with an etching rate of 3600 A/min, and then the residual PR was completely removed using PR remover (AZ Remover 700) and D.I water rinsing. With optical spectrometer and ellipsometry, the etched surface was checked. As a next step, thermal steam oxidation was performed at 1000 ◦ C for 72 min, which generated thin silicon oxide (SiO2 ) layer at inner surfaces of the hollow pyramids as shown in Fig. 3. Fig. 3 is typical cross-sectional SEM images of a hollow pyramid with a (a) 2 ␮m × 2 ␮m and a (b) 5 ␮m × 5 ␮m dot pattern, respectively, obtained after the stress-induced retarded oxidation process. The bottom pictures are the magnified images of the circled regions in Fig. 3(a) and (b), respectively. It reveals non-uniform oxide thicknesses near the bottom of a V-groove, i.e. the oxide layers on Si(1 1 1) surface becomes thinner with approaching to the apex. Using field emission SEM, the oxide thicknesses at Si(1 1 1) and the bottom surface were found to be ∼350 and 100 nm for the 2 ␮m × 2 ␮m (Fig. 3a) and ∼583 and 180 nm for the 5 ␮m × 5 ␮m (Fig. 3b) dot pattern, respectively. The thicker oxide film at the Si(1 1 1) surface will act as an etch mask during the nano-aperture opening process at apexes of pyramids. Regardless of the size of the dot pattern, the non-uniform oxide growth was observed and

Fig. 3. SEM images of concaved pyramids obtained after the stress-induced retarded oxidation process: (a) 2 ␮m × 2 ␮m dot pattern and (b) 5 ␮m × 5 ␮m dot pattern. The bottom images are enlarged pictures of concave pyramidal area of top images, respectively.

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Fig. 4. SEM images obtained before (a) and after (b) bulk Si etching. Fig. (c) shows SEM image of silicon oxide pyramidal-tip cleaned by sulfuric acid and TMAH solution and Fig. (d) shows SEM Image of fabricated silicon nitride cantilever.

Fig. 5. (a) Completed (50 × 50) aperture probes array with nano-aperture opening, and (b) variations of diameters of aperture as a function of HF etch time. Fig. 5(c) and (d) show SEM images that present the controllability of hole diameter using both HF etching (c) and Al deposition thickness (d). With increasing of etch time from 10 to 30 min, the hole diameters are increased up to 400 nm. Also, the diameters of the holes were reduced from 340 nm (c) of diameter after one time of 50 nm of Al deposition to 80 nm (d) finally.

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the boundary between vacuum and inner oxide surfaces looked similar for all the samples. Etching of the backside of Si bulk using the TMAH (20 wt.%) solution at 80 ◦ C was followed to release the pyramid arrays as shown in Fig. 2(f) and Fig. 4(b and c), and the resulting tip-size of the oxide pyramid was observed to be slightly bigger than the size of the original pattern due to both the oxide expansion into Si during the thermal oxidation and the overetching beneath the etch mask during etching the backside. Fig. 4(c) shows an SEM image of a silicon oxide pyramidaltip cleaned by sulfuric acid and TMAH solution. To avoid the tip-size expansion problem and obtain homogeneity in thickness and width of cantilevers, silicon nitride (Si3 N4 ) thin film with a stress below 100 MPa was deposited on the cleaned silicon oxide pyramid tip using NH3 and SiCl2 H2 gases with a relative ratio of 1:5 at 140 mTorr, 835 ◦ C by low pressure chemical vapor deposition (LPCVD). Cantilevers of 10–100 ␮m in length were fabricated with Si3 N4 thin films as shown in the Fig. 4(d). Photolithography and MERIE technique were employed for patterning the small cantilevers on the Si3 N4 thin films and selective bulk etching of Si was carried out using a strong alkaline

solution of TMAH to get the Si3 N4 -based cantilevers with 3D shapes. Next, isotropic oxide etching using a 10:1 or a 50:1 diluted HF solution was employed to create oxide apertures at pyramid apexes as shown in Fig. 2(g). We have successfully brought the procedure under control for the tested array by adapting 50:1 water-diluted HF solution rather than dry etching. It is worth mentioning that one should be very careful for this treatment due to the thin oxide films at the edges of the oxide tetrahedron. From a series of repeated experiments, the progression of the aperture opening as a function of etching time has been observed, and the finally completed probes with an (50 × 50) array of oxide nanoapertures are presented in the Fig. 5(a). The oxide nano-aperture array samples were further treated with diluted HF etching solution. Briefly, after we immersed the pyramidal-tip array samples in the 50:1 water-diluted HF solution, it was dumped in D.I water for 30 min and dried at room temperature. We inspected 15 apertures for each corner, which are 60 apertures in total. When we inspected the hole of an aperture, we measured lengths of four different directions, which are width, length and two of diagonal lines of a hole. Through these inspections, an opening rate

Fig. 6. Schematic diagram for the fabrication of the small cantilevers.

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of 23.6 nm/min was obtained as shown in the inset of Fig. 5(b). Additional experiments prove that the mean opening rate of the hole is 20.2–24.0 nm/min. Fig. 5(c) and (d) show SEM images that present the controllability of the hole diameter using both HF etching and Al deposition thickness. With increasing of etch time from 10 to 30 min, the hole diameters are increased up to ∼400 nm. The Al thin film was then thermally evaporated on the (50 × 50) array pattern for fabricating sub-wavelength size aperture. The diameters of the holes were reduced from 340 (c) to 80 nm (d) upon deposition of 50 nm of Al. In general, the initial diameter of the aperture greater than 300 nm was reduced down to ∼100 nm. 3.2. Fabrication of highly efficient AFM cantilevers It is well known that the resonant frequency of a cantilever can increase with decreasing the dimension of the cantilever [16–19]. However, if there is a decisive increment of the dimension, we can sometimes obtain an extremely high spring constant of a cantilever compared with its resonant frequency. This indicates that it is highly desirable for us to make a rectangular shape of the cantilever with the same thickness and width for a reasonable resonant frequency. We firstly tried to make the new type small cantilevers with the same width and thickness but different

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lengths, and their resonant frequencies were compared. Since the 3D-pyramidal shaped cantilevers are relatively heavy and bulky, we designed plat type cantilevers with 3D-rectangular shapes and carbon tips were grown on the 3D-rectangular shaped cantilevers using e-beam deposition method. We expect that this fabrication technique will be very effective for reducing the number of masks and fabrication processes for the industries of MEMS and NEMS application. Fig. 6 shows a schematic diagram for the fabrication of a small cantilever with a carbon tip. The small cantilever could be fabricated by five different steps as following: (a) growth of silicon oxide layer, which are 300–400 nm thick and deposition of low stress silicon nitride thin films with a thickness of 1000 nm, (b) formation of small size cantilevers by patterning only silicon nitride layers using a plasma etching method, (c) patterning of backside bulk etch window, (d) etching of the backside bulk with the TMAH solution at 80 ◦ C for 21 h, (e) etching of sacrificial layers by dry etching technique. After the fabrication of small cantilevers, we have grown carbon tips by e-beam deposition. We fabricated the Si3 N4 -based small cantilever with a width of 10 ␮m, thickness of 1 ␮m, and length of 10–100 ␮m on a 4-in. sized Si(1 0 0) wafer. The total dimension of this small cantilever is similar to that of a triangular-shaped cantilever. It is important to note that there is no pyramidal-tip formation process and the carbon tip (f)

Fig. 7. SEM images of small rectangular cantilevers: (a) 10 ␮m and (b) 100 ␮m in length. Fig. (c) shows a carbon tip image on silicon nitride-based small cantilevers with fixed acceleration voltage at 25 kV and variable beam probe currents; tips from left to right: 5 pA, 10 pA, 15 pA, 20 pA. Fig. (d) shows a carbon tip image with fixed beam probe current at 5 pA and variable acceleration voltages; tips from left to right: 25 kV, 20 kV, 15 kV, 10 kV.

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with a much smaller dimension than the pyramidal-tip has been grown on a cantilever. Fig. 7(a) and (b) show the SEM images of small rectangular cantilevers with different lengths of 10 ␮m (a) and 100 ␮m (c). Fig. 7(c) and (d) show images of the carbon tips grown on small cantilevers displayed in Fig. 7(a) and (b). E-beam deposition was chosen as a method of growing carbon tips in order to gain sharp tips without formation of a pyramidal structure. Cantilever fabrication using CNT has been investigated for improving the measurements of deep trench structures. However, the existence of defects makes the measurements too complicated and less productive. Even though fine cantilevers like CNT are used, it is hard to acquire information of nearly vertical trench walls. Advantages of the growth of the carbon tips using e-beam deposition may be that the aspect ratio is not lower than in the case of CNT attached and ball type tip can be made to measure trench wall more easily. It is possible to grow tips of various aspect ratios by controlling experimental parameters of e-beam deposition such as induction time, acceleration voltage, and e-beam probe current etc, regardless of geometrical morphology of Si cantilevers as substrate. Especially, the technique enables us to grow tips with a high aspect ratio. Carbon tips can reduce the troubles of the fabrication of PZT film drivers, in which a high-speed measurement AFM cantilever is integrated. In the EDX data (not shown), the carbon signal was remarkably increased after e-beam induction than before, indicating growth of carbon materials. The deposition of carbohydrates, oxygen, carbon monoxide and water etc. are predicted, which are known as residual gas in chamber after thermal decomposition by ebeam. In Fig. 7(c) and (d), SEM images are displayed showing the growth behavior of e-beam deposited tips at diffusion pump oil atmosphere as a function of electron-beam acceleration voltages (c) and probe currents (d). The optimal conditions for the growth of high aspect ratio tips are low probe current, high acceleration voltage and proper exposure time. Maximum height of tips is less than 1 ␮m. 4. Conclusions The sub-wavelength size silicon oxide aperture array as a near-field optical probe was fabricated in order to examine the possible light resonance-tunneling phenomenon. We used various semiconductor fabrication processes including anisotropic Si etching using the 20 wt.% TMAH alkaline solution and isotropic HF etching technique to open the nano-size pyramidal oxide apertures. By varying HF etching time, aperture opening could be controlled. In the present work, we found that etching of oxide is not constant but depends on etching time. Highly efficient AFM cantilevers with both 3D-pyramidal and 3D-rectangular shapes with 10–100 ␮m length were fabricated using the Si3 N4 and SiO2 thin films by combination of photolithography and MERIE techniques. Moreover, probe tips with a high resolution on the small naked cantilevers were also

achieved using electron-beam deposition in carbon atmosphere. In addition, we also successfully fabricated the sub-wavelength size SiO2 aperture array as a near-field optical probe in order to examine the possible light resonance-tunneling phenomenon. The nano-size apertures on top of the pyramidal array were carefully controlled with the (50:1) water-diluted HF acid solution with an opening rate of ∼27 nm/min. The Al thin film was thermally evaporated on the (50 × 50) array pattern and for subwavelength size aperture fabrication. The initial diameter greater than 300 nm of the aperture was reduced down to ∼100 nm. Acknowledgements Supports of this research by the BK21 program of the Ministry of Education and by the Center for Advanced Plasma Surface Technology of the Sungkyunkwan University are gratefully acknowledged. This work was also supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, KRF-2005-005-J11902). References [1] G. Meyer, N.M. Amer, Appl. Phys. Lett. 53 (1991) 1045. [2] R. Linnemann, T. Gotszalk, L. Hadjiiski, I. Rangelow, Thin Solid Films 264 (1995) 159. [3] G. Binnig, H. Rohrer, Helv. Phys. Acta 55 (1982) 726. [4] G. Binnig, C.F. Quate, C. Gerber, Phys. Rev. Lett. 56 (1986) 930. [5] E.A. Ash, G. Nicholls, Nature 237 (1972) 510. [6] (a) E. Betzig, M. Isaacson, A. Lewis, Appl. Phys. Lett. 51 (1987) 2088; (b) E. Betzig, J.K. Trautman, Science 257 (1992) 189; (c) E. Betzig, R.J. Chichester, Science 262 (1993) 1422. [7] (a) U.Ch. Fischer, U.T. Duerig, D.W. Pohl, Appl. Phys. Lett. 52 (1988) 249; (b) U.T. Duerig, D.W. Pohl, F. Rohner, J. Appl. Phys. 59 (10) (1986) 3318; (c) D.W. Pohl, Adv. Opt. Electron. Microsc. 12 (1991) 243. [8] (a) C.C. Williams, H.K. Wickramasinghe, Appl. Phys. Lett. 49 (1986) 1587; (b) G.M. Credo, D.L. Winn, S.K. Buratto, Chem. Mater. 13 (4) (2001) 1258. [9] Y. Martin, D.W. Abraham, H.K. Wickramasinghe, Appl. Phys. Lett. 52 (1987) 1103. [10] C.B. Prater, P.K. Hansma, Rev. Sci. Instrum. 62 (1991) 2634. [11] R.D. Schaller, J. Ziegelbauer, L.F. Lee, L.H. Haber, R.J. Saykally, J. Phys. Chem. B 106 (34) (2002) 8489. [12] G.T.A. Kovacs, N.I. Maluf, K.E. Peterson, Proc. IEEE 86 (1998) 1536. [13] D.B. Kao, J.P. McVittie, W.D. Nix, K.C. Saraswat, IEEE Trans. ED-34 (1987) 1008; see also D.B. Kao, J.P. McVittie, W.D. Nix, K.C. Saraswat, IEEE Trans. ED-35 (1988) 25. [14] H.L. Liu, D.K. Biegelson, F.A. Ponse, N.M. Johnson, R.F.W. Pease, Appl. Phys. Lett. 64 (1994) 1383. [15] D.W. Kim, J.T. Ok, S.S. Choi, C.K. Kim, J.W. Kim, J.-H. Boo, Microelectronic Eng. 73 (2004) 656. [16] M.B. Viani, T.E. Schaffer, A. Chand, M. Rief, H.E. Gaub, P.K. Hansma, J. Appl. Phys. 86 (4) (1999) 2258. [17] G.T. Paloczi, B.L. Smith, P.K. Hansma, D.A. Walters, M.A. Wendman, Appl. Phys. Lett. 73 (12) (1998) 1658. [18] M.J. Cunningham, D.F.L. Jenkins, W.W. Clegg, M.M. Bakush, Sens. Actuators, A Phys. A50 (1–2) (1995) 147. [19] D.A. Walters, J.P. Cleveland, N.H. Thomson, P.K. Hansma, M.A. Wendman, G. Gurley, V. Elings, Rev. Sci. Instrum. 67 (10) (1996) 3583.