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Self-sustained bridges of a-SiC:H films obtained by PECVD at low temperatures for MEMS applications
Journal of Non-Crystalline Solids 338–340 (2004) 490–495 www.elsevier.com/locate/jnoncrysol
Self-sustained bridges of a-SiC:H films obtained by PECVD at low temperatures for MEMS applications ~o *, A.T. Lopes M.N.P. Carren Escola Politecnica, Universidade de S~ao Paulo, CP 61548, 5424-970, S~ao Paulo, Brazil
Abstract In this work PECVD obtained a-SiC:H films are utilized to fabricate self sustained microbridges and microtunnels. The deposition conditions for these films, which present optimized properties for MEMS application, were established elsewhere. The utilized a-SiC:H films are 1 lm thick and are obtained at 320 C from mixtures of CH4 and SiH4 , with and without H2 dilution. The microstructures are fabricated by surface micromachining over silicon substrates utilizing 4 lm thick SiOx Ny films as sacrificial layer, also obtained by PECVD. The geometry of the bridges and tunnels is varied from 25 lm up to several hundred microns and was defined by plasma RIE process established after a study also presented here. The final aspect of the microstructures and the evaluation of the fabrication process are investigated by scanning electron microscopy. The results show that bridges fabricated with stoichiometric a-SiC:H grown in ‘silane starving plasma’ condition with H2 dilution of the gaseous mixture can be very flat, smooth and free of residual stress. Since this material was previously optimized to be a real amorphous counterpart of crystalline SiC, can be doped very efficiently n- and p-type and can be crystallized by thermal annealing, the results presented here are the first step toward the development of MEMS all based on PECVD materials. 2004 Elsevier B.V. All rights reserved. PACS: 81.20.)n; 81.05.Dz
1. Introduction Micro-electro-mechanical devices are of a major economical and technological interest for development of different specific applications as pressure and temperature sensors, accelerometers, microfluidic channels, microlens, etc. [1,2]. With these wide range of different possible applications, the development of microelectro-mechanical systems (MEMS) requests a constant development of new materials with specific properties of elasticity, hardness, chemical and thermal resistance and others. Since compatibility with conventional silicon based microelectronics is highly convenient, these new materials are normally required to be obtained at relatively low temperatures and for this reason plasma enhanced chemical vapor deposition
* Corresponding author. Address: Cid. Universitat, Av. Prof Luciano Gualberto 158, Trav. 3, Butanta, S~ao Paulo, SP, Brazil. Tel.: +5511 3091 5256; fax: +55-11 3091 5585. E-mail address: [email protected] (M.N.P. Carre~ no).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.026
(PECVD) technique and materials obtained by this method are gaining more attention. With this goal in the last years we have worked in the utilization of PECVD obtained materials to fabricate basic and generic microstructures with the aim of future development of micro-electro-mechanical devices all based on PECVD materials. So, in recent works [3] we have reported the fabrication of self-sustained grids and membranes (flat and corrugated) of silicon oxinitride films (SiOx Ny ) obtained by PECVD with areas as large as 1 cm2 . These microstructures can be applied in silicon based microphones, microrecipients and microflow channels for chemical and biochemical systems and were fabricated by bulk micromachining [4] of silicon substrates and utilize just one PECVD material and deposition step. In this work we extend these previous studies and explore a more complex surface micromachining process [4] to fabricate microbridges of hydrogenated amorphous silicon carbide (a-SiC:H), where the previously mentioned SiOx Ny films are used as sacrificial layer. Both materials are obtained by PECVD at the same low temperature (320 C).
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
1.1. a-SiC:H material Hydrogenated amorphous SiC alloys (a-SiC:H) grown by PECVD have been extensively investigated and applied in a variety of electronic and opto-electronic devices, as solar cells [5], thin film transistors [6] and light emitting diodes [7]. Part of the interest in a-SiC:H derives from crystalline SiC and its extraordinary properties, such as high thermal conductivity and breakdown voltage, mechanical hardness and resistance to chemical etching [8], which are of special importance for MEMS development [9]. However, most of the works on a-SiC:H mentioned before dealt with a low carbon content material where it is possible to modulate the band gap by the control of the carbon content in the films [10]. This means that these a-SiC:H films are nonstoichiometric alloys and in fact their properties are not necessarily similar to crystalline SiC ones, specially those strongly related to the intrinsic structural disorder, as the electrical transport properties. Aiming to improve the semiconductor properties of PECVD obtained a-SiC:H films, in previous works [11,12] we have studied the conditions to obtain close to stoichiometry samples of a-Si1 x Cx :H (x 0:5) with a chemical and structural order similar to crystalline SiC thus, preserving some of the notorious properties of this material. This has been achieved through the so-called ‘silane starving plasma’ condition, which promotes the growth of a-SiC:H films where silicon atoms are preferentially bonded to carbon, exhibiting a silicon local order very similar to crystalline SiC. In subsequent works [13–15] we showed that the chemical and structural order of a-SiC:H films grown in ‘silane starving plasma’ condition can be improved even more by adding hydrogen to the precursor gaseous mixture and by a moderate increase of the power supplied to the plasma. These conditions lead to a material with lower H content and where not only the Si atoms are preferentially bonded to C, but where also the C atoms are mainly bonded to Si ones. In other words, the material presents a chemical and structural order very similar to crystalline SiC, being a real amorphous counterpart of crystalline SiC. This material has proved to have very promising properties. In fact, the films present high mechanical hardness, high chemical and mechanical stability, low microvoids density and can be efficiently doped n- and p-type, basic requirement for developing a-SiC:H based devices and MEMS [16–18]. The films are
491
also very resistant to wet and dry etching, although, as we will see here, it can be properly and selectively etched by plasma RIE. All these properties make these optimized a-SiC:H films very promising for application in micro-electro-mechanical devices and motivated this work.
2. Experimental details Since until the moment this PECVD a-SiC:H films have not been used for mechanical purposes, here we utilize the two types of a-SiC:H mentioned before, both obtained in ‘silane starving plasma’ but with (type ‘c3690H’) and without (type ‘c3680’) H2 dilution. These two material types differ just in the structural and chemical order and both are close to stoichiometry. The option for materials with different structural order was made because this property can affect the internal mechanical stress and the resistance to chemical etching of the a-SiC:H films. All the films utilized in this work were obtained at 320 C by rf PECVD technique in a standard capacitively coupled reactor (rf signal of 13.56 MHz) from appropriated gaseous mixtures of (SiH4 + CH4 ) and (SiH4 + N2 O), respectively. For the a-SiC:H films the deposition conditions are shown in Table 1. For the SiOx Ny the specific growth conditions were extracted from previous works to obtain 4 lm thick low stress material [3]. The SiH4 and N2 O flows were 15 and 37.5 sccm, respectively (giving an N2 O/SiH4 flow ratio equal to 2.5). The rf power density was kept at 500 mW/cm2 in order to obtain high deposition rates. The self-sustained bridges of a-SiC:H were fabricated by surface micromachining technique over 25 · 25 mm2 crystalline silicon substrates following the sequence of fabrication steps schematized in Fig. 1. The process starts with the deposition of a 4 lm thick SiOx Ny film that will be used as sacrificial layer (Fig. 1(a)). The second step (Fig. 1(b)) is the lithography and the etching of the SiOx Ny in HF solution (BOE) to define the sacrificial geometries. The third step is the deposition of the a-SiC:H film used as microbridges structural material (Fig. 1(c)). The fourth stage involves lithography and plasma etching of the structural layer of a-SiC:H (Fig. 1(d)). This corrosion is carried out at room temperature in a RIE system and utilizing mixtures of CHF3 and O2 (50%). The final stage (Fig. 1(d)) is the total corrosion of
Table 1 Deposition conditions of the a-SiC:H samples Sample type
SiH4 Flow (sccm)
[CH4 ] (%)
H2 flow (sccm)
RF (mW/cm2 )
Tsub (C)
X
c3680 c3690H
3.6 3.6
80 90
– 200
50 250
320 320
0.53 0.52
Lower order Higher order
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M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
Fig. 1. Fabrication sequence of the self-sustained a-SiC:H bridges: (a) deposition and corrosion of the 4 lm thick SiOx Ny sacrificial layer; (b) deposition of the a-SiC:H structural layer; (c) corrosion (by reactive ion etching) of the a-SiC:H structural layer and (d) corrosion of the SiOx Ny sacrificial layer.
3. Results The etching rate for both types of a-SiC:H films as a function of the O2 concentration in the gas mixture are shown in Fig. 2, which summarize the results of the corrosion study. The curves in this figure were obtained for 100 and 200 W and keeping constant the total process pressure at 100 mTorr. As we can see the etching rate of the a-SiC:H films increases linearly with the O2 concentration in the gas-
Pressure : 100 mTorr
60
Etch Rate (nm/min)
the SiOx Ny sacrificial layer in BOE solution, resulting in the final self-sustained a-SiC:H bridges. The dimensions of the self-sustained areas of the bridges are defined by the geometry of the sacrificial regions, which were varied between 25 · 25 and 200 · 200 lm. The height of the bridges was determined by the thickness of the sacrificial layer that is 4 lm. As mentioned before, the a-SiC:H films utilized here are very resistant to wet etching in substances as HF, HNO3 , H2 SO4 and KOH. So, previous to bridge fabrication a series of dry etching experiments were carried out to determine the best conditions to make the selective corrosion necessary to define the geometry of the bridges. Reactive ion etching (RIE) technique in gaseous mixtures of (CHF3 + O2 ) was utilized, since they have presented good results in crystalline SiC corrosion [19]. The corrosions were performed at room temperature and varying parameters as the O2 concentration in the gas mixture, rf power applied to the plasma and the total pressure. To check the selectivity of the corrosion process relative to other involved materials as SiOx Ny films, crystalline Si, corning-glass (7059) and photoresist, etching experiments on these materials were also performed.
200 W
"c3680" "c3690H"
50 40 30 20
100 W
10 0
0
10
20
30
40
50
60
70
80
90
% O2 Fig. 2. Etch rate of both studied a-SiC:H types as a function of the concentration of O2 .
eous mixture. This occurs for both type of samples but the etch rates for a-SiC:H grown without H2 dilution (‘c3680’) are systematically higher than the obtained for the material grown with H2 dilution (‘c3690H’), result consistent with the fact that materials grown with H2 dilution are more similar to crystalline SiC. The rf power also shows an important effect in the reactivity of the plasma. So, the higher etch rates were obtained for 200 W and 70% of O2 :60 nm/min for type ‘c3680’ a-SiC:H samples and 50 nm/min for ‘c3690H’ type. These values are two times higher than the obtained for 100 W maintaining the other parameters. Results on the selectivity of the etching process will be described in detail elsewhere. Here it is just important to mention that relative to 7059 corning glass the selectivity is very good, in the range of 16–30:1. Relative to SiOx Ny and Si the selectivity is 2–4:1 and 2–1:1, respectively. Relative to photoresist, which its utilized as a mask material, the selectivity is very poor since the etching
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
rates of this material are higher that the obtained for aSiC:H films. Even more, this is more pronounced for oxygen rich gaseous mixtures, where the corrosion of aSiC:H is more effective. Anyways, for 50% of O2 a compromise is attained and 2–3 lm thick photorresist films can be utilized to totally etch 1 lm of a-SiC:H of both types. These conditions were utilized to fabricate the a-SiC:H microbridges. In Figs. 3 and 4 a local view of the final aspect of the self-sustained bridges for both a-SiC:H types is shown. The images reveal that while a-SiC:H type c3680 (grown without H2 dilution) presents a residual stress that warps and even breaks the self-sustained films, preventing the fabrication of microbridges (Fig. 3), the results for c3690H type a-SiC:H (grown with H2 dilution) are very satisfactory. In fact, the microbridges made with this material clearly show the self-sustained areas and that the films are flat, smooth and free of residual stress (Fig. 4(a) and (b)). Besides the stress problem, microstructures fabricated with c3680 type a-SiC:H also present a rougher surface when compared with c3690H type a-SiC:H (see Fig. 5). This can be appreciated in the micrographs of Fig. 5, where it can be also observed that the roughness is more pronounced in the anchored area. In these images it is also noticed that the thickness of the 3680 type a-SiC:H film in self-sustained area is clearly thinner than in the anchored area. Note that this does not occur with type 3690H material. These features are still not well understood and more studies are necessary to find a satisfactory explanation. However, the roughness can be related to substrate effects on film growth and on its structure and deposition rate, which can also explain the lower thickness in the self-sustained areas. Since different microbridge geometries were fabricated, some of them are so short and so wide that are
493
Fig. 4. Details of the final aspect of bridges of a-SiC:H 3690H type after removing the sacrificial layer of SiOx Ny : (a) closer view of the the same geometry showed in Fig. 2 and (b) detail of a bridge with ‘L’ geometry.
more precisely described if named as ‘microtunnels’. This is shown in Fig. 6, where we can also see an unexpected feature around the region where the sacrificial SiOx Ny layer was removed. This feature is a ‘wall’ of a-SiC:H formed during the deposition of the structural a-SiC:H film, that grows conformly over the sacrificial layer regions, including their lateral faces. These ‘lateral’ a-SiC:H should be removed during the plasma etching corrosion to define the bridge geometries but this does not occur due to the vertical preferential corrosion direction, of the RIE process. This can be corrected by turning inclined the vertical sides of the sacrificial regions, which probably should involve and additional step in the fabrication process. 4. Discussion
Fig. 3. Local view of the final aspect of the self-sustained bridges of type ‘c3680’ a-SiC:H. The images were obtained after total etching of the 4 lm thick sacrificial layer of SiOx Ny (in BOE solution for 240 min).
From these results we assess the possibility of fabricating 1 lm self-sustained films of a-SiC:H that are very flat, smooth and free of residual stress. This result by itself is very interesting but gains more significance in view of some considerations.
494
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
Fig. 5. Comparison among the surface roughness of a-SiC:H selfsustained bridges of type 3680 (a) and 3690H (b). We can also observe that the 3680 type a-SiC:H films are clearly thinner in the self-sustained regions than in the anchored areas. On the other hand, the thickness of 3690H type materials remains uniform in both regions.
Fig. 6. Self-sustained bridges very short and very wide defining ‘micro tunnels’ of a-SiC:H. Above, ‘microtunnel’ of a-SiC:H type 3690H. Note the ‘wall’ of a-SiC:H formed around the region where the sacrificial SiOx Ny layer was removed.
The results show that the structural properties of the a-SiC:H films are determinant to obtain satisfactory results and this occurs for the material that was previously optimized to be the real amorphous counterpart of crystalline SiC. In previous works we have shown that this material can be electrically doped, so that n and ptype doped samples with high electrical conductivity can be obtained. This is a basic requirement to develop any semiconductor based electronic device to be implemented in a-SiC:H based MEMS. Also, this a-SiC:H films exhibit high mechanical stability and can be annealed without breaking or cracking up to temperatures in the 1000–1200 C range, where a polycrystalline material is obtained. In this way, the possibility of developing polycrystalline SiC based MEMS is opened. Besides this is important to consider that since the studied a-SiC:H are very resistant to wet etching in KOH, a more easy approach to fabricate microbridges and other similar microstructures could be the bulk micromachining of Si substrates. However we prefered the more complex surface micromachining method uti-
lizing SiOx Ny as sacrificial layer due to the ‘all PECVD’ character of this process. As a consequence of this approach we observed undesirable effects, as the discontinuity of the a-SiC:H films in the transition between anchored to self-sustained regions, that of course need to be corrected for the future development of practical devices. However, we had the opportunity of studying a relatively complex fabrication process, involving two different PECVD materials in two different PECVD deposition steps. From this considerations, the obtained results are quite promising and represent a necessary well succeeded first step for the development of all PECVD based MEMS.
5. Conclusions The aim of this work was to take advantage of the versatility of PECVD technique to produce materials with appropriate properties for application in microstructure development. In particular we utilize amor-
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
phous silicon carbide (a-SiC:H) and silicon oxinitride films (SiOx Ny ) to fabricate self-sustained microbridges utilizing surface micromachining technique. In theses structures 1 lm thick films of a-SiC:H were utilized as the bridges structural material and 4 lm thick SiOx Ny films as sacrificial material. The results allow one to identify between the two type of a-SiC:H studied, the material with the better properties for microstructure development. In fact, films of aSiC:H with optimized structural order (referred in the papers as type c3690H) proved to be very promising for microstructures fabrication since it does not present appreciable residual stress and produces flat and smooth self sustained films which can be efficiently etched by RIE plasma etching. These materials also proved to be very resistant to HF solutions, which is another important result since this material is also insensitive to KOH, other traditional corrosion solution in silicon MEMS technology. In other words, the results reported here demonstrate the feasibility of developing more complex micro-electro-mechanical devices based in PECVD a-SiC:H that even could be integrated with other silicon based devices or circuits.
Acknowledgements The authors are grateful to Dr Ines Pereyra for helpful discussion of the results. This work was financially supported by FAPESP (Process No. 00/10027-3) and CNPq Brazilian agencies.
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References [1] D.J. Nagel, Surf. Coat. Technol. 103&104 (1998) 138. [2] M. Bao, W. Wang, Sens. Actuat. A 56 (1996) 135. [3] M.N.P. Carre~ no, M.I. Alayo, I. Pereyra, A.T. Lopes, Sens. Actuat. A 100 (2002) 295. [4] K.E. Petersen, Proc. IEEE 70 (5) (1982). [5] C. Mukherjee, U. Weber, H. Seitz, B. Schroder, Thin Solid Films 395 (2001) 310. [6] I. Pereyra, M.N.P. Carre~ no, A.M. Andrade, in: Springer Proceedings in Physics, vol. 62, Springer, Berlin, 1990, p. 387. [7] Y.A. Chen, Y.H. Wu, W.C. Tsay, L.H. Laih, J.W. Hong, C.Y. Chang, Electron Lett. 34 (1998) 1433. [8] H. Morkocß, S. Strite, G.B. Gao, M.E. Lin, B. Sverdlov, M. Burns, J. Appl. Phys. 76 (3) (1994) 1364. [9] P.M. Sarro, Sens. Actuat. 82 (2000) 210. [10] B.J. Bulot, M. Gauthier, M. Schidt, Philos. Mag. B. 49 (5) (1984) 489. [11] I. Pereyra, M.N.P. Carre~ no, J. Non-Cryst. Solids 201 (1996) 110. [12] V. Mastelaro, A.M. Flanck, M.C.A. Fantini, D.R.S. Bittencourt, M.N.P. Carre~ no, I. Pereyra, J. Appl. Phys. 79 (1996) 1324. [13] R.J. Prado, M.C.A. Fantini, M.H. Tabacniks, I. Pereyra, A.M. Flank, Mater. Sci. Forum 338 (2000) 329. [14] R.J. Prado, M.C.A. Fantini, M.H. Tabacniks, C.A. Villacorta Cardoso, I. Pereyra, A.M. Flank, J. Non-Cryst. Solids 283 (2001) 1. [15] R.J. Prado, M.C.A. Fantini, I. Pereyra, G.Y. Odo, C.M. Lepienski, J. Appl. Crystallogr. 34 (2001) 465. [16] M.N.P. Carre~ no, I. Pereyra, H.E.M. Peres, J. Non-Cryst. Solids 227–230 (1998) 483. [17] M.N.P. Carre~ no, I. Pereyra, J. Non-Cryst. Solids 266–269 (2000) 699. [18] A.R. Oliveira, M.N.P. Carr~eno, Electrochem. Soc. Proc. 8 (2002) 381. [19] P.H. Yih, V. Saxena, A.J. Steckl, Phys. Stat. Sol. (b) 202 (1997) 605.
Self-sustained bridges of a-SiC:H films obtained by PECVD at low temperatures for MEMS applications ~o *, A.T. Lopes M.N.P. Carren Escola Politecnica, Universidade de S~ao Paulo, CP 61548, 5424-970, S~ao Paulo, Brazil
Abstract In this work PECVD obtained a-SiC:H films are utilized to fabricate self sustained microbridges and microtunnels. The deposition conditions for these films, which present optimized properties for MEMS application, were established elsewhere. The utilized a-SiC:H films are 1 lm thick and are obtained at 320 C from mixtures of CH4 and SiH4 , with and without H2 dilution. The microstructures are fabricated by surface micromachining over silicon substrates utilizing 4 lm thick SiOx Ny films as sacrificial layer, also obtained by PECVD. The geometry of the bridges and tunnels is varied from 25 lm up to several hundred microns and was defined by plasma RIE process established after a study also presented here. The final aspect of the microstructures and the evaluation of the fabrication process are investigated by scanning electron microscopy. The results show that bridges fabricated with stoichiometric a-SiC:H grown in ‘silane starving plasma’ condition with H2 dilution of the gaseous mixture can be very flat, smooth and free of residual stress. Since this material was previously optimized to be a real amorphous counterpart of crystalline SiC, can be doped very efficiently n- and p-type and can be crystallized by thermal annealing, the results presented here are the first step toward the development of MEMS all based on PECVD materials. 2004 Elsevier B.V. All rights reserved. PACS: 81.20.)n; 81.05.Dz
1. Introduction Micro-electro-mechanical devices are of a major economical and technological interest for development of different specific applications as pressure and temperature sensors, accelerometers, microfluidic channels, microlens, etc. [1,2]. With these wide range of different possible applications, the development of microelectro-mechanical systems (MEMS) requests a constant development of new materials with specific properties of elasticity, hardness, chemical and thermal resistance and others. Since compatibility with conventional silicon based microelectronics is highly convenient, these new materials are normally required to be obtained at relatively low temperatures and for this reason plasma enhanced chemical vapor deposition
* Corresponding author. Address: Cid. Universitat, Av. Prof Luciano Gualberto 158, Trav. 3, Butanta, S~ao Paulo, SP, Brazil. Tel.: +5511 3091 5256; fax: +55-11 3091 5585. E-mail address: [email protected] (M.N.P. Carre~ no).
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.03.026
(PECVD) technique and materials obtained by this method are gaining more attention. With this goal in the last years we have worked in the utilization of PECVD obtained materials to fabricate basic and generic microstructures with the aim of future development of micro-electro-mechanical devices all based on PECVD materials. So, in recent works [3] we have reported the fabrication of self-sustained grids and membranes (flat and corrugated) of silicon oxinitride films (SiOx Ny ) obtained by PECVD with areas as large as 1 cm2 . These microstructures can be applied in silicon based microphones, microrecipients and microflow channels for chemical and biochemical systems and were fabricated by bulk micromachining [4] of silicon substrates and utilize just one PECVD material and deposition step. In this work we extend these previous studies and explore a more complex surface micromachining process [4] to fabricate microbridges of hydrogenated amorphous silicon carbide (a-SiC:H), where the previously mentioned SiOx Ny films are used as sacrificial layer. Both materials are obtained by PECVD at the same low temperature (320 C).
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
1.1. a-SiC:H material Hydrogenated amorphous SiC alloys (a-SiC:H) grown by PECVD have been extensively investigated and applied in a variety of electronic and opto-electronic devices, as solar cells [5], thin film transistors [6] and light emitting diodes [7]. Part of the interest in a-SiC:H derives from crystalline SiC and its extraordinary properties, such as high thermal conductivity and breakdown voltage, mechanical hardness and resistance to chemical etching [8], which are of special importance for MEMS development [9]. However, most of the works on a-SiC:H mentioned before dealt with a low carbon content material where it is possible to modulate the band gap by the control of the carbon content in the films [10]. This means that these a-SiC:H films are nonstoichiometric alloys and in fact their properties are not necessarily similar to crystalline SiC ones, specially those strongly related to the intrinsic structural disorder, as the electrical transport properties. Aiming to improve the semiconductor properties of PECVD obtained a-SiC:H films, in previous works [11,12] we have studied the conditions to obtain close to stoichiometry samples of a-Si1 x Cx :H (x 0:5) with a chemical and structural order similar to crystalline SiC thus, preserving some of the notorious properties of this material. This has been achieved through the so-called ‘silane starving plasma’ condition, which promotes the growth of a-SiC:H films where silicon atoms are preferentially bonded to carbon, exhibiting a silicon local order very similar to crystalline SiC. In subsequent works [13–15] we showed that the chemical and structural order of a-SiC:H films grown in ‘silane starving plasma’ condition can be improved even more by adding hydrogen to the precursor gaseous mixture and by a moderate increase of the power supplied to the plasma. These conditions lead to a material with lower H content and where not only the Si atoms are preferentially bonded to C, but where also the C atoms are mainly bonded to Si ones. In other words, the material presents a chemical and structural order very similar to crystalline SiC, being a real amorphous counterpart of crystalline SiC. This material has proved to have very promising properties. In fact, the films present high mechanical hardness, high chemical and mechanical stability, low microvoids density and can be efficiently doped n- and p-type, basic requirement for developing a-SiC:H based devices and MEMS [16–18]. The films are
491
also very resistant to wet and dry etching, although, as we will see here, it can be properly and selectively etched by plasma RIE. All these properties make these optimized a-SiC:H films very promising for application in micro-electro-mechanical devices and motivated this work.
2. Experimental details Since until the moment this PECVD a-SiC:H films have not been used for mechanical purposes, here we utilize the two types of a-SiC:H mentioned before, both obtained in ‘silane starving plasma’ but with (type ‘c3690H’) and without (type ‘c3680’) H2 dilution. These two material types differ just in the structural and chemical order and both are close to stoichiometry. The option for materials with different structural order was made because this property can affect the internal mechanical stress and the resistance to chemical etching of the a-SiC:H films. All the films utilized in this work were obtained at 320 C by rf PECVD technique in a standard capacitively coupled reactor (rf signal of 13.56 MHz) from appropriated gaseous mixtures of (SiH4 + CH4 ) and (SiH4 + N2 O), respectively. For the a-SiC:H films the deposition conditions are shown in Table 1. For the SiOx Ny the specific growth conditions were extracted from previous works to obtain 4 lm thick low stress material [3]. The SiH4 and N2 O flows were 15 and 37.5 sccm, respectively (giving an N2 O/SiH4 flow ratio equal to 2.5). The rf power density was kept at 500 mW/cm2 in order to obtain high deposition rates. The self-sustained bridges of a-SiC:H were fabricated by surface micromachining technique over 25 · 25 mm2 crystalline silicon substrates following the sequence of fabrication steps schematized in Fig. 1. The process starts with the deposition of a 4 lm thick SiOx Ny film that will be used as sacrificial layer (Fig. 1(a)). The second step (Fig. 1(b)) is the lithography and the etching of the SiOx Ny in HF solution (BOE) to define the sacrificial geometries. The third step is the deposition of the a-SiC:H film used as microbridges structural material (Fig. 1(c)). The fourth stage involves lithography and plasma etching of the structural layer of a-SiC:H (Fig. 1(d)). This corrosion is carried out at room temperature in a RIE system and utilizing mixtures of CHF3 and O2 (50%). The final stage (Fig. 1(d)) is the total corrosion of
Table 1 Deposition conditions of the a-SiC:H samples Sample type
SiH4 Flow (sccm)
[CH4 ] (%)
H2 flow (sccm)
RF (mW/cm2 )
Tsub (C)
X
c3680 c3690H
3.6 3.6
80 90
– 200
50 250
320 320
0.53 0.52
Lower order Higher order
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M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
Fig. 1. Fabrication sequence of the self-sustained a-SiC:H bridges: (a) deposition and corrosion of the 4 lm thick SiOx Ny sacrificial layer; (b) deposition of the a-SiC:H structural layer; (c) corrosion (by reactive ion etching) of the a-SiC:H structural layer and (d) corrosion of the SiOx Ny sacrificial layer.
3. Results The etching rate for both types of a-SiC:H films as a function of the O2 concentration in the gas mixture are shown in Fig. 2, which summarize the results of the corrosion study. The curves in this figure were obtained for 100 and 200 W and keeping constant the total process pressure at 100 mTorr. As we can see the etching rate of the a-SiC:H films increases linearly with the O2 concentration in the gas-
Pressure : 100 mTorr
60
Etch Rate (nm/min)
the SiOx Ny sacrificial layer in BOE solution, resulting in the final self-sustained a-SiC:H bridges. The dimensions of the self-sustained areas of the bridges are defined by the geometry of the sacrificial regions, which were varied between 25 · 25 and 200 · 200 lm. The height of the bridges was determined by the thickness of the sacrificial layer that is 4 lm. As mentioned before, the a-SiC:H films utilized here are very resistant to wet etching in substances as HF, HNO3 , H2 SO4 and KOH. So, previous to bridge fabrication a series of dry etching experiments were carried out to determine the best conditions to make the selective corrosion necessary to define the geometry of the bridges. Reactive ion etching (RIE) technique in gaseous mixtures of (CHF3 + O2 ) was utilized, since they have presented good results in crystalline SiC corrosion [19]. The corrosions were performed at room temperature and varying parameters as the O2 concentration in the gas mixture, rf power applied to the plasma and the total pressure. To check the selectivity of the corrosion process relative to other involved materials as SiOx Ny films, crystalline Si, corning-glass (7059) and photoresist, etching experiments on these materials were also performed.
200 W
"c3680" "c3690H"
50 40 30 20
100 W
10 0
0
10
20
30
40
50
60
70
80
90
% O2 Fig. 2. Etch rate of both studied a-SiC:H types as a function of the concentration of O2 .
eous mixture. This occurs for both type of samples but the etch rates for a-SiC:H grown without H2 dilution (‘c3680’) are systematically higher than the obtained for the material grown with H2 dilution (‘c3690H’), result consistent with the fact that materials grown with H2 dilution are more similar to crystalline SiC. The rf power also shows an important effect in the reactivity of the plasma. So, the higher etch rates were obtained for 200 W and 70% of O2 :60 nm/min for type ‘c3680’ a-SiC:H samples and 50 nm/min for ‘c3690H’ type. These values are two times higher than the obtained for 100 W maintaining the other parameters. Results on the selectivity of the etching process will be described in detail elsewhere. Here it is just important to mention that relative to 7059 corning glass the selectivity is very good, in the range of 16–30:1. Relative to SiOx Ny and Si the selectivity is 2–4:1 and 2–1:1, respectively. Relative to photoresist, which its utilized as a mask material, the selectivity is very poor since the etching
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
rates of this material are higher that the obtained for aSiC:H films. Even more, this is more pronounced for oxygen rich gaseous mixtures, where the corrosion of aSiC:H is more effective. Anyways, for 50% of O2 a compromise is attained and 2–3 lm thick photorresist films can be utilized to totally etch 1 lm of a-SiC:H of both types. These conditions were utilized to fabricate the a-SiC:H microbridges. In Figs. 3 and 4 a local view of the final aspect of the self-sustained bridges for both a-SiC:H types is shown. The images reveal that while a-SiC:H type c3680 (grown without H2 dilution) presents a residual stress that warps and even breaks the self-sustained films, preventing the fabrication of microbridges (Fig. 3), the results for c3690H type a-SiC:H (grown with H2 dilution) are very satisfactory. In fact, the microbridges made with this material clearly show the self-sustained areas and that the films are flat, smooth and free of residual stress (Fig. 4(a) and (b)). Besides the stress problem, microstructures fabricated with c3680 type a-SiC:H also present a rougher surface when compared with c3690H type a-SiC:H (see Fig. 5). This can be appreciated in the micrographs of Fig. 5, where it can be also observed that the roughness is more pronounced in the anchored area. In these images it is also noticed that the thickness of the 3680 type a-SiC:H film in self-sustained area is clearly thinner than in the anchored area. Note that this does not occur with type 3690H material. These features are still not well understood and more studies are necessary to find a satisfactory explanation. However, the roughness can be related to substrate effects on film growth and on its structure and deposition rate, which can also explain the lower thickness in the self-sustained areas. Since different microbridge geometries were fabricated, some of them are so short and so wide that are
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Fig. 4. Details of the final aspect of bridges of a-SiC:H 3690H type after removing the sacrificial layer of SiOx Ny : (a) closer view of the the same geometry showed in Fig. 2 and (b) detail of a bridge with ‘L’ geometry.
more precisely described if named as ‘microtunnels’. This is shown in Fig. 6, where we can also see an unexpected feature around the region where the sacrificial SiOx Ny layer was removed. This feature is a ‘wall’ of a-SiC:H formed during the deposition of the structural a-SiC:H film, that grows conformly over the sacrificial layer regions, including their lateral faces. These ‘lateral’ a-SiC:H should be removed during the plasma etching corrosion to define the bridge geometries but this does not occur due to the vertical preferential corrosion direction, of the RIE process. This can be corrected by turning inclined the vertical sides of the sacrificial regions, which probably should involve and additional step in the fabrication process. 4. Discussion
Fig. 3. Local view of the final aspect of the self-sustained bridges of type ‘c3680’ a-SiC:H. The images were obtained after total etching of the 4 lm thick sacrificial layer of SiOx Ny (in BOE solution for 240 min).
From these results we assess the possibility of fabricating 1 lm self-sustained films of a-SiC:H that are very flat, smooth and free of residual stress. This result by itself is very interesting but gains more significance in view of some considerations.
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Fig. 5. Comparison among the surface roughness of a-SiC:H selfsustained bridges of type 3680 (a) and 3690H (b). We can also observe that the 3680 type a-SiC:H films are clearly thinner in the self-sustained regions than in the anchored areas. On the other hand, the thickness of 3690H type materials remains uniform in both regions.
Fig. 6. Self-sustained bridges very short and very wide defining ‘micro tunnels’ of a-SiC:H. Above, ‘microtunnel’ of a-SiC:H type 3690H. Note the ‘wall’ of a-SiC:H formed around the region where the sacrificial SiOx Ny layer was removed.
The results show that the structural properties of the a-SiC:H films are determinant to obtain satisfactory results and this occurs for the material that was previously optimized to be the real amorphous counterpart of crystalline SiC. In previous works we have shown that this material can be electrically doped, so that n and ptype doped samples with high electrical conductivity can be obtained. This is a basic requirement to develop any semiconductor based electronic device to be implemented in a-SiC:H based MEMS. Also, this a-SiC:H films exhibit high mechanical stability and can be annealed without breaking or cracking up to temperatures in the 1000–1200 C range, where a polycrystalline material is obtained. In this way, the possibility of developing polycrystalline SiC based MEMS is opened. Besides this is important to consider that since the studied a-SiC:H are very resistant to wet etching in KOH, a more easy approach to fabricate microbridges and other similar microstructures could be the bulk micromachining of Si substrates. However we prefered the more complex surface micromachining method uti-
lizing SiOx Ny as sacrificial layer due to the ‘all PECVD’ character of this process. As a consequence of this approach we observed undesirable effects, as the discontinuity of the a-SiC:H films in the transition between anchored to self-sustained regions, that of course need to be corrected for the future development of practical devices. However, we had the opportunity of studying a relatively complex fabrication process, involving two different PECVD materials in two different PECVD deposition steps. From this considerations, the obtained results are quite promising and represent a necessary well succeeded first step for the development of all PECVD based MEMS.
5. Conclusions The aim of this work was to take advantage of the versatility of PECVD technique to produce materials with appropriate properties for application in microstructure development. In particular we utilize amor-
M.N.P. Carre~no, A.T. Lopes / Journal of Non-Crystalline Solids 338–340 (2004) 490–495
phous silicon carbide (a-SiC:H) and silicon oxinitride films (SiOx Ny ) to fabricate self-sustained microbridges utilizing surface micromachining technique. In theses structures 1 lm thick films of a-SiC:H were utilized as the bridges structural material and 4 lm thick SiOx Ny films as sacrificial material. The results allow one to identify between the two type of a-SiC:H studied, the material with the better properties for microstructure development. In fact, films of aSiC:H with optimized structural order (referred in the papers as type c3690H) proved to be very promising for microstructures fabrication since it does not present appreciable residual stress and produces flat and smooth self sustained films which can be efficiently etched by RIE plasma etching. These materials also proved to be very resistant to HF solutions, which is another important result since this material is also insensitive to KOH, other traditional corrosion solution in silicon MEMS technology. In other words, the results reported here demonstrate the feasibility of developing more complex micro-electro-mechanical devices based in PECVD a-SiC:H that even could be integrated with other silicon based devices or circuits.
Acknowledgements The authors are grateful to Dr Ines Pereyra for helpful discussion of the results. This work was financially supported by FAPESP (Process No. 00/10027-3) and CNPq Brazilian agencies.
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