- Email: [email protected]
PECVD-SiOxNy films for large area self-sustained grids applications
Sensors and Actuators A 100 (2002) 295–300
PECVD-SiOxNy films for large area self-sustained grids applications M.N.P. Carren˜o*, M.I. Alayo, I. Pereyra, A.T. Lopes Polytechnic School, Sa˜o Paulo University, CEP 5424-970, CP 61548, Sa˜o Paulo, SP, Brazil Received 6 August 2001; accepted 22 January 2002
Abstract In this work we study the structural properties and mechanical stress of silicon oxynitride (SiOxNy) films obtained by plasma enhanced chemical vapor deposition (PECVD) technique at low temperatures (320 8C) and report the feasibility of using this material for the fabrication of large area self-sustained grids. The films were obtained at different deposition conditions, varying the gas flow ratio between the precursor gases (N2O and SiH4) and maintaining all the other deposition parameters constant. The films were characterized by ellipsometry, by Fourier transform infrared (FT-IR) spectroscopy and by optically levered laser technique to measure the total mechanical stress. The results demonstrate that for appropriated deposition conditions, it is possible to obtain SiOxNy with very low mechanical stress, a necessary condition for the fabrication of mechanically stable thick films (up to 10 mm). Since this material (SiOxNy) is very resistant to KOH wet chemical etching it can be utilized to fabricate, by silicon substrate bulk micromachining, very large self-sustained grids and membranes, with areas up to 1 cm2 and with thickness in the 2–6 mm range. These results allied with the compatibility of the PECVD SiOxNy films deposition with the standard silicon based microelectronic processing technology makes this material promising for micro electro mechanical system (MEMS) fabrication. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Silicon oxynitride; Plasma CVD; Micro electro mechanical system; Thick films
1. Introduction In the last years the plasma enhanced chemical vapor deposition (PECVD) technique has been extensively utilized to obtain silicon based dielectric materials, specially silicon dioxide (SiO2) and silicon nitride (Si3N4) films, for application as gate insulator in MOS structures, as passivating layer and as protective mask [1–7]. With a chemical composition between them, silicon oxynitride films are particularly interesting due to the possibility of controlling their chemical composition and their structural, optical and mechanical properties by the appropriate choice of the deposition parameters [8–11]. In this way, it is possible to explore and tune properties as the refractive index (n) and dielectric constant (k) of the films, which are fundamental requirements for waveguiding in integrated optics [12,13] and for the development of the high k materials, one of the major current challenges in microelectronics due to the increasing scale integration of the circuits [14]. In the same way, it is expected a composition dependent internal stress, possibility which turns SiOxNy films a promising material for micro electro * Corresponding author. Present address: Cid. Universitat, Av. Prof. Luciano Gualberto 158, Trav. 3, Butanta, Sa˜o Paulo, SP, Brazil. Tel.: þ55-11-3818-5256; fax: þ55-11-3818-5585. E-mail address: [email protected] (M.N.P. Carren˜o).
mechanical systems (MEMS) technology and can be an advantage when compared to SiO2 or Si3N4. In fact, some authors have demonstrated that SiO2 and Si3N4 grown by PECVD present high intrinsic stress, being compressive for SiO2 [15,16] and tensile for Si3N4 [15,17]. In both cases this stress prevents the growth of films thicker than 1 mm. On the other hand, we have had an indication of the low stress of our SiOxNy films since in previous works we have identified deposition conditions which permit to obtain very thick films, up to 10 mm, exhibiting excellent mechanical stability [18]. This is a remarkable result since even with his lower stress, films of SiOxNy PECVD tend to scratch for thickness higher than 5 mm [19,20]. The overall mechanical stress of thin films, when deposited onto Si substrates, can be related to material properties such as density and intrinsic stress as well as to interface mechanical stress originated from the different lattice constants and thermal expansion coefficient between the Si substrate and the dielectric material itself. Therefore it is of uppermost importance to correlate the deposition conditions with the structural properties, composition and stress of the material. Besides, their low stress, high mechanical stability and large final thickness, the SiOxNy films also satisfy requirements of selective chemical etching relative to silicon wafers onto which they are normally deposited, which makes this material an excellent candidate for MEMS fabrication and
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 5 4 - 7
296
M.N.P. Carren˜ o et al. / Sensors and Actuators A 100 (2002) 295–300
developing. On the other hand, the possibility of tuning its refractive index makes SiOxNy a promising material for the integration of MEMS with optical devices, i.e. for micro opto electro mechanical systems (MOEMS) fabrication. Certainly this question is one of the main appeals of this material since MOEMS technology require the junction of surface and bulk micromachining techniques together with an accurate control of the refractive index of materials which at the same time must be selectively etched to define beams, cantilevers, membranes, lens, self-sustained waveguides and so on. Motivated by all these questions, in this work we study the mechanical stress of PECVD SiOxNy films with different composition and correlate them with the material structural properties. Also we demonstrate the feasibility of utilizing these films for the fabrication of large area self-sustained grids that can be generically utilized in MEMS developing.
2. Experimental The SiOxNy films were deposited by the PECVD technique from appropriate gaseous mixtures, silane and nitrous oxide, both of electronic grade (99.999%), in a conventional capacitively coupled reactor. The plasma is activated by a 13.56 MHz RF generator applied to parallel grids near the substrate holder. All the studied samples were deposited at 320 8C since our previous results have shown that this is the optimum deposition temperature [8]. The RF power density was kept at 500 mW/cm2 in order to obtain high deposition rates. The deposition pressure was kept as low as possible, in order, to increase the mean free path of the reactant molecules and thus minimize the gaseous phase reactions in the deposition chamber in order to prevent the precipitate of undesirable solid particles directly from the plasma. In this way for all the samples, we utilized the minimum process pressure reached by our system in order to keep constant the gas residence time. All the studied films were deposited onto p-type single crystalline silicon substrate, (1 0 0) orientation, with resistivity in the 7–13 O cm range. It is known from previous works [8,18] that the crucial parameter controlling the deposition rate is the silane flow. In fact, for higher silane flows, higher is the deposition rate. However, for the highest utilized SiH4 flow the production of a foam like material on the substrate heater, the RF grids and on the walls of the deposition chamber was observed. This undesirable effect is related with the gas phase reactions being that for these high silane flows the pressure in the deposition chamber becomes relatively high. This effect can be so pronounced as to prevent the fabrication of even thicker films, since it limits significantly the deposition rate. So, in this work the samples were deposited with a fixed SiH4 flow, chosen equal to 15 sccm which guaranties a high deposition rate but at the same time is low enough to prevent the gas phase reactions. The N2O/SiH4 flow ratio (R) was varied between 2 and 5 varying the N2O flow in order to obtain different nitrogen incorporation in the films
Table 1 Deposition conditions of the samples studied in this work and RBS resultsa Name
Ox-A Ox-B Ox-C Ox-D Ox-E a
N2O/SiH4 flow ratio (R)
Pressure (mTorr)
Atomic concentration (%) Oxygen
Nitrogen
Silicon
2 2.5 3 4 5
23 26 28 34 40
50.6 54.4 57.5 64.7 65.3
13.7 9.8 6.7 0 0
35.7 35.8 35.8 35.3 34.7
Substrate temperature was 320 8C.
(see Table 1). This range was chosen because for higher R values the material tends to be a SiO2 of great quality but lower deposition rate, and for lower R the obtained material is silicon rich [13]. The structural characterization was made by Fourier transform infrared (FT-IR) spectroscopy, in order to analyze the chemical bonding within the material and to obtain insight about its composition. The total mechanical stress was measured by optically levered laser technique, comparing the silicon substrate curvature before and after the
Fig. 1. Schematic diagram of self-sustained structure: (a) protective SiOxNy mask on the back sideP; (b) SiOxNy membrane itself on the front side; (c) back view of the final structure after the wet corrosion with KOH. The larger geometry is 10 mm 7 mm in size.
M.N.P. Carren˜ o et al. / Sensors and Actuators A 3350 (2002) 295–300
SiOxNy film deposition [16]. Furthermore, an ellipsometer (Rudolph Research Auto El), having a He–Ne laser at wavelength 632.8 nm as light source, was employed to obtain the thickness and thus to determine the deposition rate of the samples. Finally, the amount of Si, N and O per unit area (atoms/cm2) were obtained by RBS experiments using a Heþ beam with an energy E ¼ 1:7 MeV, which are described in detail elsewhere [21]. The self-sustained grids were fabricated on crystalline silicon substrates polished on both sides, square in shape and 25 mm 25 mm in size. Thick (4 mm) SiOxNy films were grown, with the same conditions than Ox-B in Table 1, on both sides of the silicon substrate. In the back side, the SiOxNy film works as a protective mask for the selective silicon etching (Fig. 1a). The membrane itself it defined in the front side, the geometry is shown in Fig. 1b. As it is observed, the whole structure is an array of four grids with square holes of four different sizes (side of 50, 100 and 200 mm), separated by a spacing of 100 mm between them and defined by conventional photolithographic processes. The silicon substrate was etched in 40% KOH solution at high temperature (80 8C) to accelerate the corrosion process (the total etching time, was 7 h). A back view of the final structure is shown in Fig. 1c. In order to evaluate precisely the surface and topography of the self-sustained grids scanning electron microscopy (SEM) analysis was performed in a Philips model 515 equipment.
3. Results and discussion In Fig. 2 the deposition rate as a function of the N2O/SiH4 flow ratio is shown. Previous studies showed that the dependence of the deposition rate with the SiH4 flow is linear [18], so since in this work we kept the silane flow constant for the
Fig. 2. Deposition rate as function of the N2O/SiH4 flow ratio at which the studied samples were grown.
297
Fig. 3. FT-IR spectra for the studied samples and for thermally grown SiO2 (for comparison). Note that in the 2500 and 4000 cm1 region the scale was amplified five times.
different N2O/SiH4 flow ratios, a constant deposition rate should also be expected. However the deposition rate showed a slight decrease for increasing N2O/SiH4 flow especially for the highest utilized ratio (0.76 mm/h). This result is mainly related to the increase in deposition pressure and thus with the increase in the gas phase reactions. In our experiments we keep the residence time constant, so that increasing gas flow increases the deposition pressure. Nevertheless the gas phase reactions the production of white particulate was considerable low for all the deposition conditions studied [18]. The infrared spectra for the set of samples shown in Table 1 is depicted in Fig. 3. We can see that for higher R, the spectra are similar to thermally grown SiO2, with the characteristic stretching, bending and rocking Si–O vibration absorption bands. Also, these samples do not show
Fig. 4. Total stress of oxynitride film in function of the N2O/SiH4 flow ratio (R).
298
M.N.P. Carren˜ o et al. / Sensors and Actuators A 100 (2002) 295–300
evidences of hydrogen neither of nitrogen incorporation. On the other hand, for lower R the Si–O stretching band decreases and the shoulder in the 1100–1250 cm1 region (corresponding to Si–O stretching vibration out of phase [22]) becomes less defined. At the same time the absorption band at 3400 cm1, corresponding to the stretching vibration for N–H bonds increases, which indicates hydrogen and nitrogen incorporation in the films. In other words, we can conclude that for samples grown with high N2O/SiH4 flow ratio, the material is turning close to stoichiometric SiO2 while for lower N2O/SiH4 flow ratio nitrogen and hydrogen incorporation increase. These results are confirmed by the RBS measurements shown in Table 1, where it is observed that nitrogen incorporation increases for diminishing N2O/ SiH4 flow ratio. As mentioned before, in our previous works [8,18], we had observed that while stable SiOxNy films can be grown up to 10 mm thick, close to stoichiometry SiO2 films tend to crash for thickness larger than 5 mm. Data on the total mechanical stress at room temperature, depicted in Fig. 4, give insight about why this occurs. In this figure it can observed that for a ratio R < 2:5 the films have a positive stress (tensile) while for R > 2:5 they exhibit an increasing negative stress (compressive). In this way, films grown with higher N2O/SiH4 flow ratio not only tend to be stoichiometric SiO2 but also have a higher compressive mechanical stress. On the other hand, films grown with lower N2O/SiH4 flow ratio, which are silicon oxynitride alloys, present very low tensile mechanical stress. It is also observed that the frontier between these two type of materials occurs for N2O/ SiH4 flow ratio around 2.5, where the absolute value of the stress is minimum. These results explain why we were able to fabricate stable 10 mm thick SiOxNy films but the same was not possible for SiO2 material. The explanation for this is probably related with the hydrogen and nitrogen content in the SiOxNy material, which tend to relax the atomic network strain [23]. In fact, hydrogen atoms have a similar role in
amorphous silicon hydrogenated, a-Si:H, and other amorphous silicon alloys. Based on this result, we utilized the Ox-B (R ¼ 2:5) deposition conditions on Table 1 to fabricate the self-sustained grids in order to obtain a material with the lowest mechanical stress. Etching experiments showed that these SiOxNy films are very resistant to KOH, even at 80 8C. From SEM inspection before and after the substrate corrosion, we estimated an etching rate of 0.1 mm/h. SEM photographs for a self-sustained grid with 4 mm thickness are shown in Fig. 5 (front side view) and Fig. 6 (back side view). Although the view angle in Fig. 5 is not enough to appreciate the total size of the grid (grid area of 4 mm 7 mm), the portion showed is totally self-sustained and permits to appreciate its resistance to the KOH solution, since even after 7 h immersion (80 8C) the surface remains very smooth. Fig. 6a shows approximately the same portion of the grid, but from a back side view. Now we can see that the grid
Fig. 5. SEM photograph of part of the front side of the self-sustained membrane.
Fig. 6. SEM photograph of part of the self-sustained membrane (a) and a detail (b).
M.N.P. Carren˜ o et al. / Sensors and Actuators A 3350 (2002) 295–300
is supporting a big piece of silicon, demonstrating the mechanical resistance of the film, since the membrane is extending, as mentioned before, for a large area. The piece of silicon was partially protected by the back side SiOxNy mask, which also originated a smaller self-sustained membrane. In Fig. 6b we show a detail of previous photograph where it can be appreciated that the SiOxNy surface as well as the silicon walls remain really smooth, disregarding near edge effects. All these results show the feasibility of utilizing PECVD SiOxNy films with appropriate composition and stress, to fabricate large area self-sustained membranes and grids (up to 1 cm2 in size) for different kind of applications. As an example we have utilized SiOxNy microbeams to obtain the thermo-mechanical properties of the SiOxNy itself (Fig. 7). The self-sustained microbeams with 5 mm thickness, 20 mm width and up to 1000 mm in length, were electrically polarized to produce a beam displacement caused by thermal heating from Joule effect. More details about these structures can be find elsewhere [24]. We also utilized SiOxNy self-sustained grids for the definition of mechanical masks of small dimensions for ion implantation processes. In this case there was special interest in preserving the wafer surface from any modification induced by other masking process, as photolithography. In Fig. 8a and b, a SiOxNy mechanical mask developed for this application is shown [25]. The grid in Fig. 8a does not appear so flat as the other grids in previous figures and we attribute this behavior to stress induced by the geometry of the grid, since the holes and the spacing between them are rectangles instead off squares.
299
Fig. 8. Phosphorous ion implantation in Cu substrates. The figure shows: (a) the diagram of the mechanical mask used for ion implantation; (b) the photograph of the final state of the Cu surface after implantation.
4. Conclusions
Fig. 7. A back view of a self-sustained SiOxNy microbeam.
It was demonstrated the possibility of, controlling the composition of SiOxNy films grown by PECVD by proper adjustment of the N2O/SiH4 flow ratios, to transit between SiOxNy alloys to almost stoichiometric SiO2 material (for R > 4), the growth of films with very low total mechanical stress was also asserted. In fact, the stress in SiOxNy films is lower than in stoichiometric SiO2 films, which permits the fabrication of very thick SiOxNy films (up to 10 mm [18]) which is not possible with SiO2 material. In this way, we established the appropriate conditions to obtain high deposition rate and low mechanical stress self-sustained grids, with 4 mm thick and 1 cm2 total area. Finally, the results show that SiOxNy films obtained by PECVD are a suitable material for MEMS applications. Among their advantages we can mention, the low deposition temperature, the high deposition rate, the large final thickness, and an excellent mechanical stability allied with a high resistance to KOH corrosion and a good selective chemical etching relative to silicon wafers, which permits to develop relatively simple fabrication processes for these type of
300
M.N.P. Carren˜ o et al. / Sensors and Actuators A 100 (2002) 295–300
microstructures. Even more, the possibility of tuning the refractive index of the SiOxNy films, makes this material a promising one for the integration of MEMS with optical devices, i.e. for MOEMS. Certainly this question is one of the main appeals of this material since MOEMS technology require the junction of surface and bulk micromachining techniques allied with an accurate control of the refractive index for materials that must be selectively etched to define beams, cantilevers, membranes, lens, self-sustained waveguides and so on.
Acknowledgements The authors are grateful to Fa´ bio G. Araes from, LSI-USP, by SEM analysis, to Dr. Carlos Domı´nguez and Andreu Llobera from the Microelectronic NationalCenter (Barcelona) by the helpful in the stress measurements and to Dr. M. Tabacniks, from Institute of Physics at University of Sa˜ o Paulo, Brazil, by the RBS measurements. The acknowledge are also due to FAPESP for financial support.
References [1] B. Gorowitz, T.B. Gorczyca, R.J. Saia, Applications of plasma enhanced chemical vapor-deposition in VLSI, Solid State Technol. 28 (1985) 197–203. [2] M. Estrada, A. Cerdeira, Y. Matsumoto, F. Melendez, Influence of low-temperature annealing on the dielectric characteristics and final parameters of SiO2 MIS thin film transistors, Thin Solid Films 298 (1997) 241–244. [3] M. Vogt, M. Kachel, K. Melzer, K. Drescher, PECVD of silicon oxynitride for copper metallization systems, Microelectron. Eng. 33 (1997) 349–356. [4] E.F. Schipper, A.M. Brugman, C. Domı´nguez, L.M. Lechuga, R.P.H. Kooyman, J. Greve, The realization of an integrated Mach-Zehnder waveguide immunosensor in silicon technology, Sens. Actuators B: Chem. 40 (1997) 147–153. [5] A.J. Kenyon, P.F. Trwoga, M. Federighi, C.W. Pitt, Opticalproperties of PECVD erbium-doped silicon-rich silica-evidence for energy-transfer between silicon microclusters and erbium ions, J. Phys.-Condens. Mater. 6 (1994) L319–L324. [6] I. Garce´ s, F. Villuendas, J. Subı´as, J. Alonso, M. del Valle, C. Domı´nguez, E. Bartolome´ , Bidimensional planar micro-optics for optochemical absorbance sensing, Opt. Lett. 23 (1998) 225–227. [7] D. Das, S.M. Iftiquar, D. Das, A.K. Barua, Improvement in the optoelectronic properties of a-SiO:H films, J. Mater. Sci. 34 (1999) 1051–1054. [8] I. Pereyra, M.I. Alayo, High quality low temperature DPECVD silicon dioxide, J. Non-Cryst. Solids 212 (1997) 225–231.
[9] J. Viard, R. Berjoan, J. Durand, Composition-density and refractive index relations in PECVD silicon oxynitrides thin films, J. Eur. Ceram. Soc. 17 (1997) 2001–2005. [10] A. Sassella, A. Borghesi, F. Corni, A. Monelli, G. Ottaviani, R. Tonini, B. Pivac, M. Bacchetta, L. Zanotti, Infrared study of Si-rich silicon oxide films deposited by plasma-enhanced chemical vapor deposition, J. Vac. Sci. Technol. A15 (1997) 377–389. [11] C.F. Lin, W.T. Tseng, M.S. Feng, Process optimization of plasmaenhanced chemical vapor deposited passivation thin films for improving non-volatile memory IC performance, Jpn. J. Appl. Phys. Pt. Part 1 37 (1998) 6364–6368. [12] K. Wo¨ rhoff, A. Driessen, P.V. Lambeck, L.T.H. Hilderink, P.W.C. Linders, Th.J.A. Popma, Plasma enhanced chemical vapor deposition silicon oxynitride optimized for application in integrated optics, Sens. Actuators A: Phys. 74 (1999) 9–12. [13] M.I. Alayo, I. Pereyra, W.L. Scopel, M.C.A. Fantini, On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiOxNy films, Thin Solid Films 402 (2002) 154–161. [14] A.I. Kingon, J.-P. Maria, S.K. Streiffer, Alternative dielectrics to silicon dioxide for memory and logic devices, Nature 406 (2000) 1032–1038. [15] D.L. Smith, Controlling the plasma chemistry of silicon nitride and oxide deposition from silane, J. Vac. Sci. Technol. A11 (1993) 1843– 1850. [16] C. Domı´nguez, J.A. Rodrı´guez, F.J. Mun˜ oz, N. Zine, Plasma enhanced CVD silicon oxide films for integrated optic applications, Vacuum 52 (1999) 395–400. [17] L.N. He, T. Inokuma, S. Hasegawa, Properties of stoichiometric silicon oxynitride films, Jpn. J. Appl. Phys. 35 (1996) 1503–1508. [18] M.I. Alayo, I. Pereyra, M.N.P. Carren˜ o, Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures, Thin Solid Films 332 (1998) 40–45. [19] M.S. Haque, H.A. Naseem, W.D. Brown, Characterization of highrate deposited PECVD silicon dioxide films for MCM applications, J. Electrochem. Soc. 142 (1995) 3864–3869. [20] D.A.P. Bulla, N.I. Morimoto, Deposition of thick TEOS PECVD silicon oxide layers for integrated optical waveguide, Thin Solid Films 334 (1998) 60–64. [21] W.L. Scopel, R.R. Cuzinatto, M.H. Tabacniks, M.C.A. Fantini, M.I. Alayo, I. Pereyra, Chemical and morphological properties of amorphous silicon oxynitride films deposited by plasma enhanced chemical vapor deposition, J. Non-Cryst. Solids 288 (2001) 88–95. [22] P.G. Pai, S.S. Chao, Y. Takagi, Infrared spectroscopic study of SiOx films produced by plasma enhanced chemical vapor-deposition, J. Vac. Sci. Technol. A4 (1986) 689–694. [23] C. Domı´nguez, J.A. Rodrı´guez, F.J. Mun˜ oz, N. Zine, The effect of rapid thermal annealing on properties of plasma enhanced CVD silicon oxide films, Thin Solid Films 346 (1999) 202–206. [24] M.S. Guimara˜ es, A. Sinatora, M.I. Alayo, I. Pereyra, M.N.P. Carren˜ o, Mechanical and termophysical properties of PECVD oxynitride films measured by MEMS, Thin Solid Films 398/399 (2001) 626–631. [25] I.R. Jankov, R.N. Szente, I.D. Goldman, M.N.P. Carren˜ o, J.W. Swart, R. Landers, Sample preparation method for scanning force microscopy, Braz. J. Phys. 31 (2001) 552–561.
PECVD-SiOxNy films for large area self-sustained grids applications M.N.P. Carren˜o*, M.I. Alayo, I. Pereyra, A.T. Lopes Polytechnic School, Sa˜o Paulo University, CEP 5424-970, CP 61548, Sa˜o Paulo, SP, Brazil Received 6 August 2001; accepted 22 January 2002
Abstract In this work we study the structural properties and mechanical stress of silicon oxynitride (SiOxNy) films obtained by plasma enhanced chemical vapor deposition (PECVD) technique at low temperatures (320 8C) and report the feasibility of using this material for the fabrication of large area self-sustained grids. The films were obtained at different deposition conditions, varying the gas flow ratio between the precursor gases (N2O and SiH4) and maintaining all the other deposition parameters constant. The films were characterized by ellipsometry, by Fourier transform infrared (FT-IR) spectroscopy and by optically levered laser technique to measure the total mechanical stress. The results demonstrate that for appropriated deposition conditions, it is possible to obtain SiOxNy with very low mechanical stress, a necessary condition for the fabrication of mechanically stable thick films (up to 10 mm). Since this material (SiOxNy) is very resistant to KOH wet chemical etching it can be utilized to fabricate, by silicon substrate bulk micromachining, very large self-sustained grids and membranes, with areas up to 1 cm2 and with thickness in the 2–6 mm range. These results allied with the compatibility of the PECVD SiOxNy films deposition with the standard silicon based microelectronic processing technology makes this material promising for micro electro mechanical system (MEMS) fabrication. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Silicon oxynitride; Plasma CVD; Micro electro mechanical system; Thick films
1. Introduction In the last years the plasma enhanced chemical vapor deposition (PECVD) technique has been extensively utilized to obtain silicon based dielectric materials, specially silicon dioxide (SiO2) and silicon nitride (Si3N4) films, for application as gate insulator in MOS structures, as passivating layer and as protective mask [1–7]. With a chemical composition between them, silicon oxynitride films are particularly interesting due to the possibility of controlling their chemical composition and their structural, optical and mechanical properties by the appropriate choice of the deposition parameters [8–11]. In this way, it is possible to explore and tune properties as the refractive index (n) and dielectric constant (k) of the films, which are fundamental requirements for waveguiding in integrated optics [12,13] and for the development of the high k materials, one of the major current challenges in microelectronics due to the increasing scale integration of the circuits [14]. In the same way, it is expected a composition dependent internal stress, possibility which turns SiOxNy films a promising material for micro electro * Corresponding author. Present address: Cid. Universitat, Av. Prof. Luciano Gualberto 158, Trav. 3, Butanta, Sa˜o Paulo, SP, Brazil. Tel.: þ55-11-3818-5256; fax: þ55-11-3818-5585. E-mail address: [email protected] (M.N.P. Carren˜o).
mechanical systems (MEMS) technology and can be an advantage when compared to SiO2 or Si3N4. In fact, some authors have demonstrated that SiO2 and Si3N4 grown by PECVD present high intrinsic stress, being compressive for SiO2 [15,16] and tensile for Si3N4 [15,17]. In both cases this stress prevents the growth of films thicker than 1 mm. On the other hand, we have had an indication of the low stress of our SiOxNy films since in previous works we have identified deposition conditions which permit to obtain very thick films, up to 10 mm, exhibiting excellent mechanical stability [18]. This is a remarkable result since even with his lower stress, films of SiOxNy PECVD tend to scratch for thickness higher than 5 mm [19,20]. The overall mechanical stress of thin films, when deposited onto Si substrates, can be related to material properties such as density and intrinsic stress as well as to interface mechanical stress originated from the different lattice constants and thermal expansion coefficient between the Si substrate and the dielectric material itself. Therefore it is of uppermost importance to correlate the deposition conditions with the structural properties, composition and stress of the material. Besides, their low stress, high mechanical stability and large final thickness, the SiOxNy films also satisfy requirements of selective chemical etching relative to silicon wafers onto which they are normally deposited, which makes this material an excellent candidate for MEMS fabrication and
0924-4247/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 2 ) 0 0 0 5 4 - 7
296
M.N.P. Carren˜ o et al. / Sensors and Actuators A 100 (2002) 295–300
developing. On the other hand, the possibility of tuning its refractive index makes SiOxNy a promising material for the integration of MEMS with optical devices, i.e. for micro opto electro mechanical systems (MOEMS) fabrication. Certainly this question is one of the main appeals of this material since MOEMS technology require the junction of surface and bulk micromachining techniques together with an accurate control of the refractive index of materials which at the same time must be selectively etched to define beams, cantilevers, membranes, lens, self-sustained waveguides and so on. Motivated by all these questions, in this work we study the mechanical stress of PECVD SiOxNy films with different composition and correlate them with the material structural properties. Also we demonstrate the feasibility of utilizing these films for the fabrication of large area self-sustained grids that can be generically utilized in MEMS developing.
2. Experimental The SiOxNy films were deposited by the PECVD technique from appropriate gaseous mixtures, silane and nitrous oxide, both of electronic grade (99.999%), in a conventional capacitively coupled reactor. The plasma is activated by a 13.56 MHz RF generator applied to parallel grids near the substrate holder. All the studied samples were deposited at 320 8C since our previous results have shown that this is the optimum deposition temperature [8]. The RF power density was kept at 500 mW/cm2 in order to obtain high deposition rates. The deposition pressure was kept as low as possible, in order, to increase the mean free path of the reactant molecules and thus minimize the gaseous phase reactions in the deposition chamber in order to prevent the precipitate of undesirable solid particles directly from the plasma. In this way for all the samples, we utilized the minimum process pressure reached by our system in order to keep constant the gas residence time. All the studied films were deposited onto p-type single crystalline silicon substrate, (1 0 0) orientation, with resistivity in the 7–13 O cm range. It is known from previous works [8,18] that the crucial parameter controlling the deposition rate is the silane flow. In fact, for higher silane flows, higher is the deposition rate. However, for the highest utilized SiH4 flow the production of a foam like material on the substrate heater, the RF grids and on the walls of the deposition chamber was observed. This undesirable effect is related with the gas phase reactions being that for these high silane flows the pressure in the deposition chamber becomes relatively high. This effect can be so pronounced as to prevent the fabrication of even thicker films, since it limits significantly the deposition rate. So, in this work the samples were deposited with a fixed SiH4 flow, chosen equal to 15 sccm which guaranties a high deposition rate but at the same time is low enough to prevent the gas phase reactions. The N2O/SiH4 flow ratio (R) was varied between 2 and 5 varying the N2O flow in order to obtain different nitrogen incorporation in the films
Table 1 Deposition conditions of the samples studied in this work and RBS resultsa Name
Ox-A Ox-B Ox-C Ox-D Ox-E a
N2O/SiH4 flow ratio (R)
Pressure (mTorr)
Atomic concentration (%) Oxygen
Nitrogen
Silicon
2 2.5 3 4 5
23 26 28 34 40
50.6 54.4 57.5 64.7 65.3
13.7 9.8 6.7 0 0
35.7 35.8 35.8 35.3 34.7
Substrate temperature was 320 8C.
(see Table 1). This range was chosen because for higher R values the material tends to be a SiO2 of great quality but lower deposition rate, and for lower R the obtained material is silicon rich [13]. The structural characterization was made by Fourier transform infrared (FT-IR) spectroscopy, in order to analyze the chemical bonding within the material and to obtain insight about its composition. The total mechanical stress was measured by optically levered laser technique, comparing the silicon substrate curvature before and after the
Fig. 1. Schematic diagram of self-sustained structure: (a) protective SiOxNy mask on the back sideP; (b) SiOxNy membrane itself on the front side; (c) back view of the final structure after the wet corrosion with KOH. The larger geometry is 10 mm 7 mm in size.
M.N.P. Carren˜ o et al. / Sensors and Actuators A 3350 (2002) 295–300
SiOxNy film deposition [16]. Furthermore, an ellipsometer (Rudolph Research Auto El), having a He–Ne laser at wavelength 632.8 nm as light source, was employed to obtain the thickness and thus to determine the deposition rate of the samples. Finally, the amount of Si, N and O per unit area (atoms/cm2) were obtained by RBS experiments using a Heþ beam with an energy E ¼ 1:7 MeV, which are described in detail elsewhere [21]. The self-sustained grids were fabricated on crystalline silicon substrates polished on both sides, square in shape and 25 mm 25 mm in size. Thick (4 mm) SiOxNy films were grown, with the same conditions than Ox-B in Table 1, on both sides of the silicon substrate. In the back side, the SiOxNy film works as a protective mask for the selective silicon etching (Fig. 1a). The membrane itself it defined in the front side, the geometry is shown in Fig. 1b. As it is observed, the whole structure is an array of four grids with square holes of four different sizes (side of 50, 100 and 200 mm), separated by a spacing of 100 mm between them and defined by conventional photolithographic processes. The silicon substrate was etched in 40% KOH solution at high temperature (80 8C) to accelerate the corrosion process (the total etching time, was 7 h). A back view of the final structure is shown in Fig. 1c. In order to evaluate precisely the surface and topography of the self-sustained grids scanning electron microscopy (SEM) analysis was performed in a Philips model 515 equipment.
3. Results and discussion In Fig. 2 the deposition rate as a function of the N2O/SiH4 flow ratio is shown. Previous studies showed that the dependence of the deposition rate with the SiH4 flow is linear [18], so since in this work we kept the silane flow constant for the
Fig. 2. Deposition rate as function of the N2O/SiH4 flow ratio at which the studied samples were grown.
297
Fig. 3. FT-IR spectra for the studied samples and for thermally grown SiO2 (for comparison). Note that in the 2500 and 4000 cm1 region the scale was amplified five times.
different N2O/SiH4 flow ratios, a constant deposition rate should also be expected. However the deposition rate showed a slight decrease for increasing N2O/SiH4 flow especially for the highest utilized ratio (0.76 mm/h). This result is mainly related to the increase in deposition pressure and thus with the increase in the gas phase reactions. In our experiments we keep the residence time constant, so that increasing gas flow increases the deposition pressure. Nevertheless the gas phase reactions the production of white particulate was considerable low for all the deposition conditions studied [18]. The infrared spectra for the set of samples shown in Table 1 is depicted in Fig. 3. We can see that for higher R, the spectra are similar to thermally grown SiO2, with the characteristic stretching, bending and rocking Si–O vibration absorption bands. Also, these samples do not show
Fig. 4. Total stress of oxynitride film in function of the N2O/SiH4 flow ratio (R).
298
M.N.P. Carren˜ o et al. / Sensors and Actuators A 100 (2002) 295–300
evidences of hydrogen neither of nitrogen incorporation. On the other hand, for lower R the Si–O stretching band decreases and the shoulder in the 1100–1250 cm1 region (corresponding to Si–O stretching vibration out of phase [22]) becomes less defined. At the same time the absorption band at 3400 cm1, corresponding to the stretching vibration for N–H bonds increases, which indicates hydrogen and nitrogen incorporation in the films. In other words, we can conclude that for samples grown with high N2O/SiH4 flow ratio, the material is turning close to stoichiometric SiO2 while for lower N2O/SiH4 flow ratio nitrogen and hydrogen incorporation increase. These results are confirmed by the RBS measurements shown in Table 1, where it is observed that nitrogen incorporation increases for diminishing N2O/ SiH4 flow ratio. As mentioned before, in our previous works [8,18], we had observed that while stable SiOxNy films can be grown up to 10 mm thick, close to stoichiometry SiO2 films tend to crash for thickness larger than 5 mm. Data on the total mechanical stress at room temperature, depicted in Fig. 4, give insight about why this occurs. In this figure it can observed that for a ratio R < 2:5 the films have a positive stress (tensile) while for R > 2:5 they exhibit an increasing negative stress (compressive). In this way, films grown with higher N2O/SiH4 flow ratio not only tend to be stoichiometric SiO2 but also have a higher compressive mechanical stress. On the other hand, films grown with lower N2O/SiH4 flow ratio, which are silicon oxynitride alloys, present very low tensile mechanical stress. It is also observed that the frontier between these two type of materials occurs for N2O/ SiH4 flow ratio around 2.5, where the absolute value of the stress is minimum. These results explain why we were able to fabricate stable 10 mm thick SiOxNy films but the same was not possible for SiO2 material. The explanation for this is probably related with the hydrogen and nitrogen content in the SiOxNy material, which tend to relax the atomic network strain [23]. In fact, hydrogen atoms have a similar role in
amorphous silicon hydrogenated, a-Si:H, and other amorphous silicon alloys. Based on this result, we utilized the Ox-B (R ¼ 2:5) deposition conditions on Table 1 to fabricate the self-sustained grids in order to obtain a material with the lowest mechanical stress. Etching experiments showed that these SiOxNy films are very resistant to KOH, even at 80 8C. From SEM inspection before and after the substrate corrosion, we estimated an etching rate of 0.1 mm/h. SEM photographs for a self-sustained grid with 4 mm thickness are shown in Fig. 5 (front side view) and Fig. 6 (back side view). Although the view angle in Fig. 5 is not enough to appreciate the total size of the grid (grid area of 4 mm 7 mm), the portion showed is totally self-sustained and permits to appreciate its resistance to the KOH solution, since even after 7 h immersion (80 8C) the surface remains very smooth. Fig. 6a shows approximately the same portion of the grid, but from a back side view. Now we can see that the grid
Fig. 5. SEM photograph of part of the front side of the self-sustained membrane.
Fig. 6. SEM photograph of part of the self-sustained membrane (a) and a detail (b).
M.N.P. Carren˜ o et al. / Sensors and Actuators A 3350 (2002) 295–300
is supporting a big piece of silicon, demonstrating the mechanical resistance of the film, since the membrane is extending, as mentioned before, for a large area. The piece of silicon was partially protected by the back side SiOxNy mask, which also originated a smaller self-sustained membrane. In Fig. 6b we show a detail of previous photograph where it can be appreciated that the SiOxNy surface as well as the silicon walls remain really smooth, disregarding near edge effects. All these results show the feasibility of utilizing PECVD SiOxNy films with appropriate composition and stress, to fabricate large area self-sustained membranes and grids (up to 1 cm2 in size) for different kind of applications. As an example we have utilized SiOxNy microbeams to obtain the thermo-mechanical properties of the SiOxNy itself (Fig. 7). The self-sustained microbeams with 5 mm thickness, 20 mm width and up to 1000 mm in length, were electrically polarized to produce a beam displacement caused by thermal heating from Joule effect. More details about these structures can be find elsewhere [24]. We also utilized SiOxNy self-sustained grids for the definition of mechanical masks of small dimensions for ion implantation processes. In this case there was special interest in preserving the wafer surface from any modification induced by other masking process, as photolithography. In Fig. 8a and b, a SiOxNy mechanical mask developed for this application is shown [25]. The grid in Fig. 8a does not appear so flat as the other grids in previous figures and we attribute this behavior to stress induced by the geometry of the grid, since the holes and the spacing between them are rectangles instead off squares.
299
Fig. 8. Phosphorous ion implantation in Cu substrates. The figure shows: (a) the diagram of the mechanical mask used for ion implantation; (b) the photograph of the final state of the Cu surface after implantation.
4. Conclusions
Fig. 7. A back view of a self-sustained SiOxNy microbeam.
It was demonstrated the possibility of, controlling the composition of SiOxNy films grown by PECVD by proper adjustment of the N2O/SiH4 flow ratios, to transit between SiOxNy alloys to almost stoichiometric SiO2 material (for R > 4), the growth of films with very low total mechanical stress was also asserted. In fact, the stress in SiOxNy films is lower than in stoichiometric SiO2 films, which permits the fabrication of very thick SiOxNy films (up to 10 mm [18]) which is not possible with SiO2 material. In this way, we established the appropriate conditions to obtain high deposition rate and low mechanical stress self-sustained grids, with 4 mm thick and 1 cm2 total area. Finally, the results show that SiOxNy films obtained by PECVD are a suitable material for MEMS applications. Among their advantages we can mention, the low deposition temperature, the high deposition rate, the large final thickness, and an excellent mechanical stability allied with a high resistance to KOH corrosion and a good selective chemical etching relative to silicon wafers, which permits to develop relatively simple fabrication processes for these type of
300
M.N.P. Carren˜ o et al. / Sensors and Actuators A 100 (2002) 295–300
microstructures. Even more, the possibility of tuning the refractive index of the SiOxNy films, makes this material a promising one for the integration of MEMS with optical devices, i.e. for MOEMS. Certainly this question is one of the main appeals of this material since MOEMS technology require the junction of surface and bulk micromachining techniques allied with an accurate control of the refractive index for materials that must be selectively etched to define beams, cantilevers, membranes, lens, self-sustained waveguides and so on.
Acknowledgements The authors are grateful to Fa´ bio G. Araes from, LSI-USP, by SEM analysis, to Dr. Carlos Domı´nguez and Andreu Llobera from the Microelectronic NationalCenter (Barcelona) by the helpful in the stress measurements and to Dr. M. Tabacniks, from Institute of Physics at University of Sa˜ o Paulo, Brazil, by the RBS measurements. The acknowledge are also due to FAPESP for financial support.
References [1] B. Gorowitz, T.B. Gorczyca, R.J. Saia, Applications of plasma enhanced chemical vapor-deposition in VLSI, Solid State Technol. 28 (1985) 197–203. [2] M. Estrada, A. Cerdeira, Y. Matsumoto, F. Melendez, Influence of low-temperature annealing on the dielectric characteristics and final parameters of SiO2 MIS thin film transistors, Thin Solid Films 298 (1997) 241–244. [3] M. Vogt, M. Kachel, K. Melzer, K. Drescher, PECVD of silicon oxynitride for copper metallization systems, Microelectron. Eng. 33 (1997) 349–356. [4] E.F. Schipper, A.M. Brugman, C. Domı´nguez, L.M. Lechuga, R.P.H. Kooyman, J. Greve, The realization of an integrated Mach-Zehnder waveguide immunosensor in silicon technology, Sens. Actuators B: Chem. 40 (1997) 147–153. [5] A.J. Kenyon, P.F. Trwoga, M. Federighi, C.W. Pitt, Opticalproperties of PECVD erbium-doped silicon-rich silica-evidence for energy-transfer between silicon microclusters and erbium ions, J. Phys.-Condens. Mater. 6 (1994) L319–L324. [6] I. Garce´ s, F. Villuendas, J. Subı´as, J. Alonso, M. del Valle, C. Domı´nguez, E. Bartolome´ , Bidimensional planar micro-optics for optochemical absorbance sensing, Opt. Lett. 23 (1998) 225–227. [7] D. Das, S.M. Iftiquar, D. Das, A.K. Barua, Improvement in the optoelectronic properties of a-SiO:H films, J. Mater. Sci. 34 (1999) 1051–1054. [8] I. Pereyra, M.I. Alayo, High quality low temperature DPECVD silicon dioxide, J. Non-Cryst. Solids 212 (1997) 225–231.
[9] J. Viard, R. Berjoan, J. Durand, Composition-density and refractive index relations in PECVD silicon oxynitrides thin films, J. Eur. Ceram. Soc. 17 (1997) 2001–2005. [10] A. Sassella, A. Borghesi, F. Corni, A. Monelli, G. Ottaviani, R. Tonini, B. Pivac, M. Bacchetta, L. Zanotti, Infrared study of Si-rich silicon oxide films deposited by plasma-enhanced chemical vapor deposition, J. Vac. Sci. Technol. A15 (1997) 377–389. [11] C.F. Lin, W.T. Tseng, M.S. Feng, Process optimization of plasmaenhanced chemical vapor deposited passivation thin films for improving non-volatile memory IC performance, Jpn. J. Appl. Phys. Pt. Part 1 37 (1998) 6364–6368. [12] K. Wo¨ rhoff, A. Driessen, P.V. Lambeck, L.T.H. Hilderink, P.W.C. Linders, Th.J.A. Popma, Plasma enhanced chemical vapor deposition silicon oxynitride optimized for application in integrated optics, Sens. Actuators A: Phys. 74 (1999) 9–12. [13] M.I. Alayo, I. Pereyra, W.L. Scopel, M.C.A. Fantini, On the nitrogen and oxygen incorporation in plasma-enhanced chemical vapor deposition (PECVD) SiOxNy films, Thin Solid Films 402 (2002) 154–161. [14] A.I. Kingon, J.-P. Maria, S.K. Streiffer, Alternative dielectrics to silicon dioxide for memory and logic devices, Nature 406 (2000) 1032–1038. [15] D.L. Smith, Controlling the plasma chemistry of silicon nitride and oxide deposition from silane, J. Vac. Sci. Technol. A11 (1993) 1843– 1850. [16] C. Domı´nguez, J.A. Rodrı´guez, F.J. Mun˜ oz, N. Zine, Plasma enhanced CVD silicon oxide films for integrated optic applications, Vacuum 52 (1999) 395–400. [17] L.N. He, T. Inokuma, S. Hasegawa, Properties of stoichiometric silicon oxynitride films, Jpn. J. Appl. Phys. 35 (1996) 1503–1508. [18] M.I. Alayo, I. Pereyra, M.N.P. Carren˜ o, Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures, Thin Solid Films 332 (1998) 40–45. [19] M.S. Haque, H.A. Naseem, W.D. Brown, Characterization of highrate deposited PECVD silicon dioxide films for MCM applications, J. Electrochem. Soc. 142 (1995) 3864–3869. [20] D.A.P. Bulla, N.I. Morimoto, Deposition of thick TEOS PECVD silicon oxide layers for integrated optical waveguide, Thin Solid Films 334 (1998) 60–64. [21] W.L. Scopel, R.R. Cuzinatto, M.H. Tabacniks, M.C.A. Fantini, M.I. Alayo, I. Pereyra, Chemical and morphological properties of amorphous silicon oxynitride films deposited by plasma enhanced chemical vapor deposition, J. Non-Cryst. Solids 288 (2001) 88–95. [22] P.G. Pai, S.S. Chao, Y. Takagi, Infrared spectroscopic study of SiOx films produced by plasma enhanced chemical vapor-deposition, J. Vac. Sci. Technol. A4 (1986) 689–694. [23] C. Domı´nguez, J.A. Rodrı´guez, F.J. Mun˜ oz, N. Zine, The effect of rapid thermal annealing on properties of plasma enhanced CVD silicon oxide films, Thin Solid Films 346 (1999) 202–206. [24] M.S. Guimara˜ es, A. Sinatora, M.I. Alayo, I. Pereyra, M.N.P. Carren˜ o, Mechanical and termophysical properties of PECVD oxynitride films measured by MEMS, Thin Solid Films 398/399 (2001) 626–631. [25] I.R. Jankov, R.N. Szente, I.D. Goldman, M.N.P. Carren˜ o, J.W. Swart, R. Landers, Sample preparation method for scanning force microscopy, Braz. J. Phys. 31 (2001) 552–561.