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Post-release deformation and curvature correction of an electrothermally actuated MEMS bilayer platform
Microelectronic Engineering 221 (2020) 111192
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Post-release deformation and curvature correction of an electrothermally actuated MEMS bilayer platform
T
Amit Kumara,b, , Ashudeepa, Deepak Bansala,b, Prem Kumara, Anuroopa,b, Khushbua,b, Kamaljit Rangrac ⁎
a
CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan 333031, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India c Indian Institute of Technology Jodhpur, Jodhpur, India b
ARTICLE INFO
ABSTRACT
Keywords: MEMS platforms Micromirror Electrothermal actuators Residual stress
Surface micromachined devices are known to have residual stress-induced deformation. This paper presents the effects of residual stress on the flatness of an electrothermally actuated large aperture MEMS bilayer platform. The platform consists of a SiO2–Al composite plate of area 500 × 500 μm2 suspended over a cavity through bimorph actuators. The bilayer platform consists of 0.95 μm thick aluminum film on a thermally grown 0.75 μm thick silicon dioxide on a silicon substrate. Bimorph actuators also consist of laminated layers of silicon dioxide and aluminum of thicknesses the same as on the platform. The ensuing compressive stress in silicon dioxide (240 MPa) & tensile stress in aluminum (35 MPa) manifests itself in significant post-release curling of the bilayer platform. Finite Element Simulation is done in Coventorware® to analyze the room temperature post-release deformation behavior of the platform. In order to correct the platform curvature, two methods of stress counterbalancing i.e. (i) metal reinforcement framing, and (ii) deposition of a stress compensation layer are proposed and their effectiveness is investigated using FEM simulations. The simulation results show that a 1 μm thick gold reinforcement frame results in a 13 μm peak-to-valley height difference between center and corner of the platform, which improves to 6 μm for a gold reinforcement thickness of 3 μm. The maximum height difference reduces to 1 μm for silicon dioxide of thickness 0.75 μm. According to FEM results, the presence of a stress compensation layer at the top is more effective in curvature correction compared to metal reinforcement framing; however, the post-release elevation of the stress-compensated platform is 5 μm below the post-release elevation of the reinforced platform and 25 μm below the zero reference plane. To verify the simulation results, the platform is fabricated with a deposition of 1 μm thick silicon dioxide layer at the top. The fabricated platform exhibit significant improvement in postrelease deformation with a stress compensation layer compared to an unbalanced platform.
1. Introduction High aspect ratio MEMS platforms are useful in numerous applications such as projection displays [1], maskless lithography [2], adaptive optics [3,4], multi-object spectroscopy [5] and optical scanning [6,7]. In most of the applications, a rectangular/circular plate is either tiptilted or actuated in piston mode by electrostatic, electrothermal or other actuation methods to direct incident light into the desired direction or for wavefront shaping. Due to lightweight, small size and low power consumption, MEMS micromirror platforms offer huge commercial potential in biomedical [8] and optical communication applications [9]. In tunable cavity filters/disk resonators, a MEMS platform
⁎
is required to perturb the electromagnetic field inside a micromachined cavity for frequency or bandwidth tuning [10]. While small-displacement (< 10 μm) is mostly achieved using electrostatic actuation, electrothermal actuation is preferred for large displacements at lower operational voltage. Due to slow speed and large power consumption, the use of electrothermal actuation is limited to applications with slow operational speeds. While the static and dynamic response of MEMS platforms is important for structural stability, actuation speed, operational lifetime, surface roughness and flatness are equally important. Rough surfaces lead to unwanted scattering of light and deformed platforms cause the focal aberrations and degraded optical performance. Similarly, non-uniform platform bending leads to erroneous
Corresponding author at: CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan 333031, India E-mail address: [email protected] (A. Kumar).
https://doi.org/10.1016/j.mee.2019.111192 Received 11 November 2019; Accepted 23 November 2019 Available online 26 November 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
frequency or bandwidth tuning in resonators. Deformation of micromachined structures due to the presence of residual stress in structural layers is discussed extensively in the literature [11,12]; however, the majority of these are limited to the deformation of cantilevers and beams with little emphasis on large dimension plates. Major reasons for residual stress in thin films include crystal lattice mismatch [11] and the difference in grain size at the interface of two films [13]. Stoney formula (1) is frequently used to calculate the magnitude of residual stress in an un-patterned thin film by curvature measurement of a thick substrate coated with the film under investigation stress
2 Etsub 6Rtfilm
=
(1)
where tsub and tfilm are the thicknesses of the substrate and film respectively. E is Young's modulus of the substrate and R is the curvature of the bent substrate. However, this formula is not suitable for patterned layers and in cases where film and substrate thicknesses are comparable. In this paper, we present the effect of residual stress on the curvature of a large aperture MEMS bilayer platform fabricated using silicon dioxide and aluminum. The platform has an active area of 500 × 500 μm2 and fabricated using surface micromachining. The thickness of silicon dioxide and aluminum are 0.75 μm and 0.95 μm respectively, making it a high aspect ratio structure. A closed-form analytical equation is presented to find the misfit stress, bending moment and resulting deformation of the platform at room temperature. Finite Element Simulation is done in Coventorware® to predict the post-release behavior of the platform. Simulation results show significant curling of central plate which is also confirmed by fabrication results. Two methods are proposed to alleviate the issue of platform deformation without modification in basic structural design and their effectiveness is compared using FEM simulation. The simulation result shows that the deposition of a stress compensation layer is more effective in counterbalancing the deformation of the platform compared to metal reinforcement framing. The platform is finally fabricated with a stress compensation layer to validate the effectiveness of the proposed method in alleviating the issue of post-release deformation.
Fig. 1. Schematic diagram of a bilayer plate.
1
D1
1x
2x
=
D1 2
D2
+
i
D1 i
D2
z
1
1 2
z+
h1 2 h2 2
1
i
+
D1
h1 = 2 2 D2
1
M1 =
i
bh1
h1 = 2
2 i bh1
M2 =
i
bh2
h2 = 2
2 i bh 2
1
1
(4)
i
i
D2
1 2
h2 2
(5)
Interfacial moment across two layers and radius of curvature are given by the following equations:
The schematic of a bilayer plate is shown in Fig. 1(a). For simplicity, the cross-section of the platform along xz plane is presented and the layer interface is located at the origin as shown in Fig. 1(b). The plate of thickness h consists of two layers of thickness h1 and h2 and residual stress σ1 and σ2 respectively. After high-temperature deposition and cooling, the layers try to contract by different amounts resulting in the development of interfacial stress (σi) between two layers due to constrained motion as shown in Fig. 1(c). The interfacial stress can be replaced by biaxial stress (σi) and interfacial moment (Mi) acting across the centroid of the cross-section of layers as shown in Fig. 1(d) [14]. The biaxial strain in two layers occurs due to three distinct components: (a) Residual stress (σ1 and σ2), (b) Interfacial stress-induced biaxial stress (σi), and (c) Curvature of the plate (ρ1 and ρ2). Since the thickness of the platform is negligible compared to lateral dimensions, strain along the z-axis is insignificant. The strain in layer 1 and layer 2 along the x-axis is expressed as 1
1
Since strains in two layers are equal and the layer interface coincides with the origin, (2) and (3) are equal:
2. Device design
=
Ei
Di =
2
(7)
=
M1 6 i = D1 I1 D1 h1
(8)
=
M2 6 i = D2 I2 D2 h2
(9)
1
1
(6)
2
2
Biaxial interfacial stress is calculated by substituting (8) and (9) in (4).
i
=
4
2
1
D2
D1
1 D2
+
1 D1
(10)
Interfacial bending moments across two layers are calculated by substituting (10) in (6) and (7).
M1 =
(2)
M2 =
(3)
2 i bh1
2
2 i bh 2
2
=
=
8
8
2
1
D2
D1
1 D2
+
1 D1
2
1
D2
D1
1 D2
+
1 D1
(bh12) (11)
(bh 22) (12)
The equivalent bending moment (Meq) and the deformation (Zdeform) of the plate supported at one end are given by (13) and (14) respectively.
where Di is the biaxial modulus and νi is the Poisson ratio of two layers. The relationship is given by the formula: 2
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
Meq = M1 + M2 =
Zdeform =
Meq Dc Ic
.
b 8
2
1
D2
D1
1 D2
1 D1
+
(h12 + h 22 )
x2 2
(13) (14)
where Dc and Ic are biaxial modulus and moment of inertia of the composite plate. Substituting (13) in (14), we get
Zdeform =
3 4Dc
2
1
D2
D1
1 D2
1 D1
+
h12 h2 + 23 x 2 3 h h
(15)
Deformation is a thus a function of plate dimension, residual stress, biaxial modulus and the ratio of layer thicknesses. For a special case, where thickness and biaxial modulus of both layers are equal (h1 = h2 = h/2), (D1 = D2 = D). The equation of bending moment and deformation reduces to.
Meq =
b ( 32
Zdeform =
2 1) h
2
3 16
2
1
D
(16)
1 2 x h
(17)
To investigate post-release deformation due to residual stress, a bilayer structure is fabricated using surface micromachining. The 3D model of the structure is shown in Fig. 2. The central platform of dimension 500 × 500 μm2 is suspended by four serpentine-shaped bimorph thermal actuators whose other ends are rigidly attached to the substrate. Both platform and bimorph actuators are fabricated using thermally grown silicon dioxide and sputtered aluminum of thickness 0.75 μm and 0.95 μm respectively. The platform moves in an upward direction when the bimorph actuator is heated by electrical current passing through it. The platform is electrically isolated from the bimorph actuator to ensure minimum thermal coupling. The unwanted thermal coupling may result in the platform warpage; however, the major concern in the present case is the post-release deformation of the central platform at room temperature i.e. without Joule's heating. Mask design and dimensions of different segments of the structure are shown in Fig. 3. To predict the post-release curvature, the biaxial compressive stress of 100 MPa and tensile stress of 35 MPa is applied to silicon dioxide and aluminum respectively and all simulations are performed at room temperature i.e. 27 °C. A lower value of residual stress in oxide is taken due to the issue of non-converging solution at higher stress values; however, it is found adequate to give a firsthand approximation of post-release behavior. The thickness and material properties of different structural layers taken in the simulation are given in Table 1 and Table 2 respectively. The simulation results show that corners of the platform are curled in z-direction which is in agreement with the direction of the resultant moment (16) and displacement (17). The postrelease deformation of the platform at room temperature is shown in
Fig. 3. Dimension of different segments of MEMS platform. Table 1 Dimension and thickness of different layers. Parameter
Value
Aluminum thickness Silicon dioxide thickness Silicon substrate thickness Oxide segment length Width of aluminum beam Width of silicon dioxide beam
0.95 μm 0.75 μm 280 μm 90 μm 10 μm 12 μm
Table 2 Material property of different structural layers. Material
TCE (1/K)
Thermal conductivity (W/ mK)
Biaxial stress (MPa)
Aluminum Silicon dioxide Silicon
23.1e-6 0.50e-6 2.50e-5
2.37e2 1.42 1.48e2
35 −100 0
Fig. 4. The maximum peak-to-valley height difference between center and corner of the deformed platform is calculated to be 24 μm. 3. Fabrication The process flow of device fabrication is shown in Fig. 5. It begins with the chemical cleaning of 2-in., 〈100〉, P-type, double-sided polished silicon wafers with trichloroethylene (TCE), acetone and methanol to remove oil and greasy materials from the wafer surface followed by a dip in Piranha solution (H2SO4:H2O2::3:1) for 20 min to remove organic contaminations. Wafers are rinsed in deionized (DI) water and dipped in dilute HF solution (1%) for 30 s to remove nascent oxide from the silicon surface. Silicon dioxide of thickness 0.75 μm is
Fig. 2. 3D model of the proposed MEMS platform. 3
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
Fig. 4. Post-release deformation of the central platform
Fig. 6. Optical image of the platform after lift-off.
cover the central platform, bimorph actuators, and contact pads. A special-purpose Microchem LOR 10A lift-off photoresist is spin-coated and baked at 180 °C for 15 min followed by coating and pre-bake of S1813 photoresist at 90 °C for 30 min. The photoresist is selectively exposed in UV light and developed in Shipley Microposit MF CD-26 developer to get 2.1 μm thick lift-off resist. Aluminum is sputter deposited for 32 mins. at a chamber pressure of 6.8 mtorr and 400 W DC power. Aluminum is patterned by stripping lift-off resist in Dimethyl Sulfoxide (DMSO) at 70 °C and acetone at 60 °C for 3 h with frequent ultrasonic agitations. Wafers are then dipped in methanol and dried using nitrogen. The optical image of the platform after aluminum liftoff is shown in Fig. 6. The platform is finally released using Deep Reactive Ion Etching (DRIE) instead of wet etching due to the incompatibility of aluminum with alkaline silicon etchants e.g. Tetramethylammonium Hydroxide (TMAH). Thermally grown silicon dioxide and photoresist of thickness 0.75 μm and 1.5 μm respectively are used as etch mask during DRIE. The backside of the wafer is spin-coated with Microposit S1813 photoresist and patterned using UV lithography. Silicon dioxide from the exposed area is removed by dipping wafers in BOE for 11 min. Prior to oxide etching, the front side of the wafer is coated with photoresist and baked at 90 °C for 20 min to mask aluminum and silicon dioxide. For DRIE, SF6 and C4F8 are used for etching and surface passivation cycles respectively. The temperature of wafer chuck is maintained at 4 °C by helium at a flow rate of 0.2 sccm and 16 mbar pressure. The flow rate of SF6 and C4F8 are kept at 600 sccm and 200 sccm with 4.8 × 10−2 and 2.5 × 10−2 mbar pressure respectively. A test wafer is taken out after every 10 mins. to measure the etch rate of silicon. With the abovementioned parameters, the etch rate of silicon is found to be 3.5 μm/ min as shown in Fig. 7. A total of 80 min of DRIE is done to ensure complete etching of 280 μm thick silicon underneath the central platform. Photoresist from both sides of the wafer is removed by dipping in acetone followed by methanol and dried in a convection oven at 60 °C for 10 min. SEM images of the platform before and after release are shown in Fig. 8 and Fig. 9 respectively.
Fig. 5. Fabrication process flow.
thermally grown over silicon at 1050 °C. For oxide patterning, Microposit S1813 photoresist is spin-coated and pre-baked at 90 °C in convection oven followed by exposure in UV light and development in Microposit MF 312 developer. Patterned wafers are rinsed in DI water and post-baked at 120 °C for 30 min. Silicon dioxide is selectively etched by dipping wafers in buffered oxide etchant (BOE) for 11 min. Etch time is decided by monitoring the etch rate of silicon dioxide on a test wafer using a stylus surface profiler at regular intervals. Prior to oxide etching, the backside of the wafer is protected using photoresist. The aluminum layer is sputter deposited and patterned using lift-off to
4. Results and discussion Released structures exhibit significant curling of the central 4
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
Fig. 7. Etch rate of silicon in Deep Reactive Ion Etching.
Fig. 9. SEM image of the released platform.
Fig. 10. Schematic diagram of the platform with reinforcement.
Fig. 8. SEM image of the platform before DRIE.
platform due to the presence of compressive stress (240 MPa) in silicon dioxide and tensile stress (35 MPa) in aluminum. The observed curling is larger than the simulation results which is in agreement with the assumptions taken during the simulation. The presence of residual stress in the structural layer makes the proposed platform unsuitable for optical or tuning applications. Residual stress is unwanted in most of the cases and its effect is vividly clear in multilayer processes. Methods like post-process annealing [15] and carefully designed deposition parameters [16] have been used in literature to achieve stress-free monolayers but not much progress is seen in the case of multilayer deposits. Reinforcement of RF MEMS switches using metal frame [17] and deposition of stress compensation sandwich layers [18] are used to reduce buckling of suspended cantilevers. The effectiveness of both the methods in correcting stress-induced curling of platform under investigation is analyzed using finite element simulation. In the first method, a reinforcement frame of electroplated gold is deposited and patterned over the central platform. The schematic diagram and 3D model with the reinforcement frame are shown in Fig. 10 and Fig. 11 respectively. The width of the frame is fixed at 30 μm and
Fig. 11. 3D model of the proposed design with reinforcement.
thickness is increased from 1 μm to 3 μm. The simulation results show improvement in platform curvature with reinforcement thickness. For the reinforcement frame thickness of 1 μm, the maximum height difference between corner and center of the platform is 13 μm which improves to a maximum height difference of 6 μm for the reinforcement frame thickness of 3 μm as shown in Fig. 12. The thick reinforcement frame is expected to degrade the optical performance due to the focal aberration created by frame steps. In the second method, a film with compressive residual stress is deposited over the platform to counterbalance the moment generated by the stress of bottom layers as shown in Fig. 13. To investigate the effectiveness of this method, FEM simulations of the modified design (Fig. 14) are performed with 0.75 μm thick silicon dioxide having compressive stress of 100 MPa. Silicon dioxide is also patterned over bimorph beams in Inverse-Series-Connected (ISC) fashion to eliminate lateral shift and tangential tilt of the central platform [19]. While simulation results show a perfectly flat micromirror platform with almost zero curling, the initial position of the micromirror platform after 5
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
Fig. 15. Simulation result of the platform with stress compensation layer.
Fig. 12. Simulation results showing the effect of reinforcement on platform curvature.
Fig. 13. Deposition of stress compensation layer at the top.
Fig. 16. Comparison of curvature correction of bilayer platform by different methods
5. Implementation A modified process flow is required for the application of stress compensation methodologies. For the reinforcement framing, a seed layer of Cr/Au is deposited over a patterned aluminum layer followed by creation of a high thickness photoresist mold. Gold is deposited to the desired thickness by electroplating and the mold is subsequently removed in acetone and methanol. The seed layer is etched from the unwanted areas by Au/Cr etchants respectively. This is followed by selective etching of silicon in DRIE. For stress compensation using film deposition, a silicon dioxide layer of appropriate thickness is deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) over patterned aluminum followed by lithography and patterning. The critical part is the deposition of a perfectly stress-matched top and bottom silicon dioxide layer. Although it is hard to achieve a perfectly stress-matched layer even using the same process, the thickness of the oxide layer deposited using PECVD can be judiciously optimized [20] to get the desired value of compressive stress so that stress difference at the top interface is similar to the stress difference at bottom interface. To prove the technical feasibility, a similar platform structure with
Fig. 14. 3D model of the platform with stress compensation layer.
release is 23 μm below the zero-reference plane due to the presence of stress in bimorph actuators as shown in Fig. 15. A comparison graph of both methods of curvature correction is shown in Fig. 16. According to simulation results, the second method of stress compensation layer deposition is better than the first method in curvature correction. The peak-to-valley height difference between center and corner of the platform is 13 μm for a reinforcement frame thickness of 1 μm which improves to peak-to-valley height difference of 6 μm for the reinforcement frame thickness of 3 μm. The best result is observed in the case of a stress compensation layer where peak-tovalley height difference between center and corner is 1 μm only. 6
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
thicknesses of different structural layers. Finite Element Simulations of the platform is done in Coventorware® to predict the post-release deformation of the platform at room temperature. To alleviate the issue of stress-induced curling of the platform without significant structural modification, two methods of curvature correction i.e. metal reinforcement framing and stress compensation layer deposition are proposed and their effectiveness in curvature correction is investigated. It is observed that the deposition of a stress compensation layer gives a platform with peak-to-valley height uniformity of 1 μm which is better than the peak-to-valley height uniformity of 6 μm observed in case of metal reinforcement framing. Fabrication process and challenges to obtain a curvature corrected platform are briefly discussed and the technical feasibility of the proposed process is proved by the fabrication of a bilayer platform with a stress compensation layer at the top. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 17. SEM image of the platform with stress compensation layer – Before release.
Acknowledgment The author acknowledges the financial assistance under RESPOND Project-OGP-155, SAC-ISRO, India. References [1] P.F. Van Kessel, L.J. Hornbeck, R.E. Meier, M.R. Douglass, A MEMS-based projection display, Proc. IEEE 86 (8) (1998) 1687–1704. [2] C. Sun, N. Fang, D.M. Wu, X. Zhang, Projection micro-stereolithography using digital micro-mirror dynamic mask, Sensors Actuators A Phys. 121 (1) (May 2005) 113–120. [3] T.G. Bifano, J. Perreault, R. Krishnamoorthy Mali, M.N. Horenstein, Microelectromechanical deformable mirrors, IEEE J. Sel. Top. Quantum Electron. 5 (1) (1999) 83–89. [4] M. Helmbrecht, et al., “Micromirrors for adaptive-optics arrays,” TRANSDUCERS ‘01, 11th, Int. Conf. Solid-State Sensors Actuators, Munich, Ger. June 10 (2001) 14. [5] S. Waldis, F. Zamkotsian, P.-A. Clerc, W. Noell, M. Zickar, N. de de Rooij, Arrays of high tilt-angle micromirrors for multiobject spectroscopy, IEEE J. Sel. Top. Quantum Electron. 13 (2) (2007) 168–176. [6] H. Toshiyoshi, W. Piyawattanametha, C.-T. Chan, M.C. Wu, Linearization of electrostatically actuated surface micromachined 2-D optical scanner, J. Microelectromech. Syst. 10 (2) (Jun. 2001) 205–214. [7] K.E. Petersen, Silicon torsional scanning Mirror, IBM J. Res. Dev. 24 (5) (Sep. 1980) 631–637. [8] J. Singh, et al., A two axes scanning SOI MEMS micromirror for endoscopic bioimaging, J. Micromech. Microeng. 18 (2) (Feb. 2008) 025001. [9] L.Y. Lin, E.L. Goldstein, R.W. Tkach, Free-space micromachined optical switches with submillisecond switching time for large-scale optical crossconnects - OSA trends in optics and photonics, Wavel. Div. Mult. Components 29 (4) (1999) 152. [10] D.Y. Winter, R.R. Mansour, Tunable dielectric resonator bandpass filter with embedded MEMS tuning elements, IEEE Trans. Microw. Theory Tech. 55 (1) (2007) 154–159. [11] P. Taylor, M.F. Doerner, W.D. Nix, Stresses and deformation processes in thin films on substrates, CRC Crit. Rev. Solid State Mater. Sci. 14 (3) (1988) 225–266. [12] P.J. Withers, H.K.D.H. Bhadeshia, Residual stress part 2 – nature and origins, Mater. Sci. Technol. 17 (2001) 366–375. [13] R.W. Hoffman, Stresses in thin films: the relevance of grain boundaries and impurities, Thin Solid Films 34 (1976) 185–190. [14] A. Jain, H. Qu, S. Todd, H. Xie, A thermal bimorph micromirror with large bidirectional and vertical actuation, Sensors Actuators A Phys. 122 (2005) 9–15 no. 1 SPEC. ISS. [15] X. Zhang, T.Y. Zhang, M. Wong, Y. Zohar, Residual-stress relaxation in polysilicon thin films by high-temperature rapid thermal annealing, Sensors Actuators A Phys. 64 (1) (1998) 109–115. [16] A. Sharma, D. Bansal, A. Kumar, D. Kumar, K. Rangra, Residual stress control during the growth and release process in gold suspended microstructures, Micromach. Microfabr. Process Technol. XIX 8973 (2014) 1–9. [17] D. Bansal, et al., Low voltage driven RF MEMS capacitive switch using reinforcement for reduced buckling, J. Micromech. Microeng. 27 (2) (Feb. 2017) 024001. [18] Z. Cui, L. Wang, A. Jin, J. Hong, Control of Stress in Multilayered MEMS Devices, 2006 IEEE International Conference on Nano/Micro Engineered and Molecular Systems, vol. 1, 2006, pp. 1224–1227. [19] S.T. Todd, A. Jain, H. Qu, H. Xie, A multi-degree-of-freedom micromirror utilizing inverted-series-connected bimorph actuators, J. Opt. A Pure Appl. Opt. 8 (7) (2006). [20] V. Au, C. Charles, D.A.P. Bulla, J.D. Love, R.W. Boswell, Thickness-dependent stress in plasma-deposited silicon dioxide films, J. Appl. Phys. 97 (8) (2005).
Fig. 18. SEM image of the platform after release.
different bimorph length is fabricated with a stress compensation layer on the top of the aluminum layer. Since the silicon dioxide deposited using PECVD has a lower value of residual stress compared to thermal oxide, a thick layer of oxide is deposited. Silicon dioxide of thickness 1 μm is deposited using Silane at 300 °C followed by patterning using Reactive Ion Etching (RIE). The SEM images of the stress compensated platform before and after release are shown in Fig. 17 and Fig. 18 respectively. A section of bimorph beams got damaged during the release process, but significant improvement in post-release curvature is observed which confirms the feasibility of the proposed methodology. The thickness of PECVD oxide needs further optimization to get a perfect stress-match layer as discussed earlier. 6. Conclusion In this paper, the effect of residual stress on the flatness of a large aperture electrothermally actuated MEMS bilayer platform is presented. A micromachined platform of dimension 500 × 500 μm2 is fabricated using laminated layers of silicon dioxide and aluminum. The fabricated device exhibits post-release curling of the central platform due to the presence of compressive stress in silicon dioxide (240 MPa) and tensile stress in aluminum (35 MPa). An analytical model is presented to find the relationship between residual stress, bending moment and 7
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Post-release deformation and curvature correction of an electrothermally actuated MEMS bilayer platform
T
Amit Kumara,b, , Ashudeepa, Deepak Bansala,b, Prem Kumara, Anuroopa,b, Khushbua,b, Kamaljit Rangrac ⁎
a
CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan 333031, India Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India c Indian Institute of Technology Jodhpur, Jodhpur, India b
ARTICLE INFO
ABSTRACT
Keywords: MEMS platforms Micromirror Electrothermal actuators Residual stress
Surface micromachined devices are known to have residual stress-induced deformation. This paper presents the effects of residual stress on the flatness of an electrothermally actuated large aperture MEMS bilayer platform. The platform consists of a SiO2–Al composite plate of area 500 × 500 μm2 suspended over a cavity through bimorph actuators. The bilayer platform consists of 0.95 μm thick aluminum film on a thermally grown 0.75 μm thick silicon dioxide on a silicon substrate. Bimorph actuators also consist of laminated layers of silicon dioxide and aluminum of thicknesses the same as on the platform. The ensuing compressive stress in silicon dioxide (240 MPa) & tensile stress in aluminum (35 MPa) manifests itself in significant post-release curling of the bilayer platform. Finite Element Simulation is done in Coventorware® to analyze the room temperature post-release deformation behavior of the platform. In order to correct the platform curvature, two methods of stress counterbalancing i.e. (i) metal reinforcement framing, and (ii) deposition of a stress compensation layer are proposed and their effectiveness is investigated using FEM simulations. The simulation results show that a 1 μm thick gold reinforcement frame results in a 13 μm peak-to-valley height difference between center and corner of the platform, which improves to 6 μm for a gold reinforcement thickness of 3 μm. The maximum height difference reduces to 1 μm for silicon dioxide of thickness 0.75 μm. According to FEM results, the presence of a stress compensation layer at the top is more effective in curvature correction compared to metal reinforcement framing; however, the post-release elevation of the stress-compensated platform is 5 μm below the post-release elevation of the reinforced platform and 25 μm below the zero reference plane. To verify the simulation results, the platform is fabricated with a deposition of 1 μm thick silicon dioxide layer at the top. The fabricated platform exhibit significant improvement in postrelease deformation with a stress compensation layer compared to an unbalanced platform.
1. Introduction High aspect ratio MEMS platforms are useful in numerous applications such as projection displays [1], maskless lithography [2], adaptive optics [3,4], multi-object spectroscopy [5] and optical scanning [6,7]. In most of the applications, a rectangular/circular plate is either tiptilted or actuated in piston mode by electrostatic, electrothermal or other actuation methods to direct incident light into the desired direction or for wavefront shaping. Due to lightweight, small size and low power consumption, MEMS micromirror platforms offer huge commercial potential in biomedical [8] and optical communication applications [9]. In tunable cavity filters/disk resonators, a MEMS platform
⁎
is required to perturb the electromagnetic field inside a micromachined cavity for frequency or bandwidth tuning [10]. While small-displacement (< 10 μm) is mostly achieved using electrostatic actuation, electrothermal actuation is preferred for large displacements at lower operational voltage. Due to slow speed and large power consumption, the use of electrothermal actuation is limited to applications with slow operational speeds. While the static and dynamic response of MEMS platforms is important for structural stability, actuation speed, operational lifetime, surface roughness and flatness are equally important. Rough surfaces lead to unwanted scattering of light and deformed platforms cause the focal aberrations and degraded optical performance. Similarly, non-uniform platform bending leads to erroneous
Corresponding author at: CSIR-Central Electronics Engineering Research Institute, Pilani, Rajasthan 333031, India E-mail address: [email protected] (A. Kumar).
https://doi.org/10.1016/j.mee.2019.111192 Received 11 November 2019; Accepted 23 November 2019 Available online 26 November 2019 0167-9317/ © 2019 Elsevier B.V. All rights reserved.
Microelectronic Engineering 221 (2020) 111192
A. Kumar, et al.
frequency or bandwidth tuning in resonators. Deformation of micromachined structures due to the presence of residual stress in structural layers is discussed extensively in the literature [11,12]; however, the majority of these are limited to the deformation of cantilevers and beams with little emphasis on large dimension plates. Major reasons for residual stress in thin films include crystal lattice mismatch [11] and the difference in grain size at the interface of two films [13]. Stoney formula (1) is frequently used to calculate the magnitude of residual stress in an un-patterned thin film by curvature measurement of a thick substrate coated with the film under investigation stress
2 Etsub 6Rtfilm
=
(1)
where tsub and tfilm are the thicknesses of the substrate and film respectively. E is Young's modulus of the substrate and R is the curvature of the bent substrate. However, this formula is not suitable for patterned layers and in cases where film and substrate thicknesses are comparable. In this paper, we present the effect of residual stress on the curvature of a large aperture MEMS bilayer platform fabricated using silicon dioxide and aluminum. The platform has an active area of 500 × 500 μm2 and fabricated using surface micromachining. The thickness of silicon dioxide and aluminum are 0.75 μm and 0.95 μm respectively, making it a high aspect ratio structure. A closed-form analytical equation is presented to find the misfit stress, bending moment and resulting deformation of the platform at room temperature. Finite Element Simulation is done in Coventorware® to predict the post-release behavior of the platform. Simulation results show significant curling of central plate which is also confirmed by fabrication results. Two methods are proposed to alleviate the issue of platform deformation without modification in basic structural design and their effectiveness is compared using FEM simulation. The simulation result shows that the deposition of a stress compensation layer is more effective in counterbalancing the deformation of the platform compared to metal reinforcement framing. The platform is finally fabricated with a stress compensation layer to validate the effectiveness of the proposed method in alleviating the issue of post-release deformation.
Fig. 1. Schematic diagram of a bilayer plate.
1
D1
1x
2x
=
D1 2
D2
+
i
D1 i
D2
z
1
1 2
z+
h1 2 h2 2
1
i
+
D1
h1 = 2 2 D2
1
M1 =
i
bh1
h1 = 2
2 i bh1
M2 =
i
bh2
h2 = 2
2 i bh 2
1
1
(4)
i
i
D2
1 2
h2 2
(5)
Interfacial moment across two layers and radius of curvature are given by the following equations:
The schematic of a bilayer plate is shown in Fig. 1(a). For simplicity, the cross-section of the platform along xz plane is presented and the layer interface is located at the origin as shown in Fig. 1(b). The plate of thickness h consists of two layers of thickness h1 and h2 and residual stress σ1 and σ2 respectively. After high-temperature deposition and cooling, the layers try to contract by different amounts resulting in the development of interfacial stress (σi) between two layers due to constrained motion as shown in Fig. 1(c). The interfacial stress can be replaced by biaxial stress (σi) and interfacial moment (Mi) acting across the centroid of the cross-section of layers as shown in Fig. 1(d) [14]. The biaxial strain in two layers occurs due to three distinct components: (a) Residual stress (σ1 and σ2), (b) Interfacial stress-induced biaxial stress (σi), and (c) Curvature of the plate (ρ1 and ρ2). Since the thickness of the platform is negligible compared to lateral dimensions, strain along the z-axis is insignificant. The strain in layer 1 and layer 2 along the x-axis is expressed as 1
1
Since strains in two layers are equal and the layer interface coincides with the origin, (2) and (3) are equal:
2. Device design
=
Ei
Di =
2
(7)
=
M1 6 i = D1 I1 D1 h1
(8)
=
M2 6 i = D2 I2 D2 h2
(9)
1
1
(6)
2
2
Biaxial interfacial stress is calculated by substituting (8) and (9) in (4).
i
=
4
2
1
D2
D1
1 D2
+
1 D1
(10)
Interfacial bending moments across two layers are calculated by substituting (10) in (6) and (7).
M1 =
(2)
M2 =
(3)
2 i bh1
2
2 i bh 2
2
=
=
8
8
2
1
D2
D1
1 D2
+
1 D1
2
1
D2
D1
1 D2
+
1 D1
(bh12) (11)
(bh 22) (12)
The equivalent bending moment (Meq) and the deformation (Zdeform) of the plate supported at one end are given by (13) and (14) respectively.
where Di is the biaxial modulus and νi is the Poisson ratio of two layers. The relationship is given by the formula: 2
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A. Kumar, et al.
Meq = M1 + M2 =
Zdeform =
Meq Dc Ic
.
b 8
2
1
D2
D1
1 D2
1 D1
+
(h12 + h 22 )
x2 2
(13) (14)
where Dc and Ic are biaxial modulus and moment of inertia of the composite plate. Substituting (13) in (14), we get
Zdeform =
3 4Dc
2
1
D2
D1
1 D2
1 D1
+
h12 h2 + 23 x 2 3 h h
(15)
Deformation is a thus a function of plate dimension, residual stress, biaxial modulus and the ratio of layer thicknesses. For a special case, where thickness and biaxial modulus of both layers are equal (h1 = h2 = h/2), (D1 = D2 = D). The equation of bending moment and deformation reduces to.
Meq =
b ( 32
Zdeform =
2 1) h
2
3 16
2
1
D
(16)
1 2 x h
(17)
To investigate post-release deformation due to residual stress, a bilayer structure is fabricated using surface micromachining. The 3D model of the structure is shown in Fig. 2. The central platform of dimension 500 × 500 μm2 is suspended by four serpentine-shaped bimorph thermal actuators whose other ends are rigidly attached to the substrate. Both platform and bimorph actuators are fabricated using thermally grown silicon dioxide and sputtered aluminum of thickness 0.75 μm and 0.95 μm respectively. The platform moves in an upward direction when the bimorph actuator is heated by electrical current passing through it. The platform is electrically isolated from the bimorph actuator to ensure minimum thermal coupling. The unwanted thermal coupling may result in the platform warpage; however, the major concern in the present case is the post-release deformation of the central platform at room temperature i.e. without Joule's heating. Mask design and dimensions of different segments of the structure are shown in Fig. 3. To predict the post-release curvature, the biaxial compressive stress of 100 MPa and tensile stress of 35 MPa is applied to silicon dioxide and aluminum respectively and all simulations are performed at room temperature i.e. 27 °C. A lower value of residual stress in oxide is taken due to the issue of non-converging solution at higher stress values; however, it is found adequate to give a firsthand approximation of post-release behavior. The thickness and material properties of different structural layers taken in the simulation are given in Table 1 and Table 2 respectively. The simulation results show that corners of the platform are curled in z-direction which is in agreement with the direction of the resultant moment (16) and displacement (17). The postrelease deformation of the platform at room temperature is shown in
Fig. 3. Dimension of different segments of MEMS platform. Table 1 Dimension and thickness of different layers. Parameter
Value
Aluminum thickness Silicon dioxide thickness Silicon substrate thickness Oxide segment length Width of aluminum beam Width of silicon dioxide beam
0.95 μm 0.75 μm 280 μm 90 μm 10 μm 12 μm
Table 2 Material property of different structural layers. Material
TCE (1/K)
Thermal conductivity (W/ mK)
Biaxial stress (MPa)
Aluminum Silicon dioxide Silicon
23.1e-6 0.50e-6 2.50e-5
2.37e2 1.42 1.48e2
35 −100 0
Fig. 4. The maximum peak-to-valley height difference between center and corner of the deformed platform is calculated to be 24 μm. 3. Fabrication The process flow of device fabrication is shown in Fig. 5. It begins with the chemical cleaning of 2-in., 〈100〉, P-type, double-sided polished silicon wafers with trichloroethylene (TCE), acetone and methanol to remove oil and greasy materials from the wafer surface followed by a dip in Piranha solution (H2SO4:H2O2::3:1) for 20 min to remove organic contaminations. Wafers are rinsed in deionized (DI) water and dipped in dilute HF solution (1%) for 30 s to remove nascent oxide from the silicon surface. Silicon dioxide of thickness 0.75 μm is
Fig. 2. 3D model of the proposed MEMS platform. 3
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Fig. 4. Post-release deformation of the central platform
Fig. 6. Optical image of the platform after lift-off.
cover the central platform, bimorph actuators, and contact pads. A special-purpose Microchem LOR 10A lift-off photoresist is spin-coated and baked at 180 °C for 15 min followed by coating and pre-bake of S1813 photoresist at 90 °C for 30 min. The photoresist is selectively exposed in UV light and developed in Shipley Microposit MF CD-26 developer to get 2.1 μm thick lift-off resist. Aluminum is sputter deposited for 32 mins. at a chamber pressure of 6.8 mtorr and 400 W DC power. Aluminum is patterned by stripping lift-off resist in Dimethyl Sulfoxide (DMSO) at 70 °C and acetone at 60 °C for 3 h with frequent ultrasonic agitations. Wafers are then dipped in methanol and dried using nitrogen. The optical image of the platform after aluminum liftoff is shown in Fig. 6. The platform is finally released using Deep Reactive Ion Etching (DRIE) instead of wet etching due to the incompatibility of aluminum with alkaline silicon etchants e.g. Tetramethylammonium Hydroxide (TMAH). Thermally grown silicon dioxide and photoresist of thickness 0.75 μm and 1.5 μm respectively are used as etch mask during DRIE. The backside of the wafer is spin-coated with Microposit S1813 photoresist and patterned using UV lithography. Silicon dioxide from the exposed area is removed by dipping wafers in BOE for 11 min. Prior to oxide etching, the front side of the wafer is coated with photoresist and baked at 90 °C for 20 min to mask aluminum and silicon dioxide. For DRIE, SF6 and C4F8 are used for etching and surface passivation cycles respectively. The temperature of wafer chuck is maintained at 4 °C by helium at a flow rate of 0.2 sccm and 16 mbar pressure. The flow rate of SF6 and C4F8 are kept at 600 sccm and 200 sccm with 4.8 × 10−2 and 2.5 × 10−2 mbar pressure respectively. A test wafer is taken out after every 10 mins. to measure the etch rate of silicon. With the abovementioned parameters, the etch rate of silicon is found to be 3.5 μm/ min as shown in Fig. 7. A total of 80 min of DRIE is done to ensure complete etching of 280 μm thick silicon underneath the central platform. Photoresist from both sides of the wafer is removed by dipping in acetone followed by methanol and dried in a convection oven at 60 °C for 10 min. SEM images of the platform before and after release are shown in Fig. 8 and Fig. 9 respectively.
Fig. 5. Fabrication process flow.
thermally grown over silicon at 1050 °C. For oxide patterning, Microposit S1813 photoresist is spin-coated and pre-baked at 90 °C in convection oven followed by exposure in UV light and development in Microposit MF 312 developer. Patterned wafers are rinsed in DI water and post-baked at 120 °C for 30 min. Silicon dioxide is selectively etched by dipping wafers in buffered oxide etchant (BOE) for 11 min. Etch time is decided by monitoring the etch rate of silicon dioxide on a test wafer using a stylus surface profiler at regular intervals. Prior to oxide etching, the backside of the wafer is protected using photoresist. The aluminum layer is sputter deposited and patterned using lift-off to
4. Results and discussion Released structures exhibit significant curling of the central 4
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Fig. 7. Etch rate of silicon in Deep Reactive Ion Etching.
Fig. 9. SEM image of the released platform.
Fig. 10. Schematic diagram of the platform with reinforcement.
Fig. 8. SEM image of the platform before DRIE.
platform due to the presence of compressive stress (240 MPa) in silicon dioxide and tensile stress (35 MPa) in aluminum. The observed curling is larger than the simulation results which is in agreement with the assumptions taken during the simulation. The presence of residual stress in the structural layer makes the proposed platform unsuitable for optical or tuning applications. Residual stress is unwanted in most of the cases and its effect is vividly clear in multilayer processes. Methods like post-process annealing [15] and carefully designed deposition parameters [16] have been used in literature to achieve stress-free monolayers but not much progress is seen in the case of multilayer deposits. Reinforcement of RF MEMS switches using metal frame [17] and deposition of stress compensation sandwich layers [18] are used to reduce buckling of suspended cantilevers. The effectiveness of both the methods in correcting stress-induced curling of platform under investigation is analyzed using finite element simulation. In the first method, a reinforcement frame of electroplated gold is deposited and patterned over the central platform. The schematic diagram and 3D model with the reinforcement frame are shown in Fig. 10 and Fig. 11 respectively. The width of the frame is fixed at 30 μm and
Fig. 11. 3D model of the proposed design with reinforcement.
thickness is increased from 1 μm to 3 μm. The simulation results show improvement in platform curvature with reinforcement thickness. For the reinforcement frame thickness of 1 μm, the maximum height difference between corner and center of the platform is 13 μm which improves to a maximum height difference of 6 μm for the reinforcement frame thickness of 3 μm as shown in Fig. 12. The thick reinforcement frame is expected to degrade the optical performance due to the focal aberration created by frame steps. In the second method, a film with compressive residual stress is deposited over the platform to counterbalance the moment generated by the stress of bottom layers as shown in Fig. 13. To investigate the effectiveness of this method, FEM simulations of the modified design (Fig. 14) are performed with 0.75 μm thick silicon dioxide having compressive stress of 100 MPa. Silicon dioxide is also patterned over bimorph beams in Inverse-Series-Connected (ISC) fashion to eliminate lateral shift and tangential tilt of the central platform [19]. While simulation results show a perfectly flat micromirror platform with almost zero curling, the initial position of the micromirror platform after 5
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A. Kumar, et al.
Fig. 15. Simulation result of the platform with stress compensation layer.
Fig. 12. Simulation results showing the effect of reinforcement on platform curvature.
Fig. 13. Deposition of stress compensation layer at the top.
Fig. 16. Comparison of curvature correction of bilayer platform by different methods
5. Implementation A modified process flow is required for the application of stress compensation methodologies. For the reinforcement framing, a seed layer of Cr/Au is deposited over a patterned aluminum layer followed by creation of a high thickness photoresist mold. Gold is deposited to the desired thickness by electroplating and the mold is subsequently removed in acetone and methanol. The seed layer is etched from the unwanted areas by Au/Cr etchants respectively. This is followed by selective etching of silicon in DRIE. For stress compensation using film deposition, a silicon dioxide layer of appropriate thickness is deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) over patterned aluminum followed by lithography and patterning. The critical part is the deposition of a perfectly stress-matched top and bottom silicon dioxide layer. Although it is hard to achieve a perfectly stress-matched layer even using the same process, the thickness of the oxide layer deposited using PECVD can be judiciously optimized [20] to get the desired value of compressive stress so that stress difference at the top interface is similar to the stress difference at bottom interface. To prove the technical feasibility, a similar platform structure with
Fig. 14. 3D model of the platform with stress compensation layer.
release is 23 μm below the zero-reference plane due to the presence of stress in bimorph actuators as shown in Fig. 15. A comparison graph of both methods of curvature correction is shown in Fig. 16. According to simulation results, the second method of stress compensation layer deposition is better than the first method in curvature correction. The peak-to-valley height difference between center and corner of the platform is 13 μm for a reinforcement frame thickness of 1 μm which improves to peak-to-valley height difference of 6 μm for the reinforcement frame thickness of 3 μm. The best result is observed in the case of a stress compensation layer where peak-tovalley height difference between center and corner is 1 μm only. 6
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thicknesses of different structural layers. Finite Element Simulations of the platform is done in Coventorware® to predict the post-release deformation of the platform at room temperature. To alleviate the issue of stress-induced curling of the platform without significant structural modification, two methods of curvature correction i.e. metal reinforcement framing and stress compensation layer deposition are proposed and their effectiveness in curvature correction is investigated. It is observed that the deposition of a stress compensation layer gives a platform with peak-to-valley height uniformity of 1 μm which is better than the peak-to-valley height uniformity of 6 μm observed in case of metal reinforcement framing. Fabrication process and challenges to obtain a curvature corrected platform are briefly discussed and the technical feasibility of the proposed process is proved by the fabrication of a bilayer platform with a stress compensation layer at the top. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 17. SEM image of the platform with stress compensation layer – Before release.
Acknowledgment The author acknowledges the financial assistance under RESPOND Project-OGP-155, SAC-ISRO, India. References [1] P.F. Van Kessel, L.J. Hornbeck, R.E. Meier, M.R. Douglass, A MEMS-based projection display, Proc. IEEE 86 (8) (1998) 1687–1704. [2] C. Sun, N. Fang, D.M. Wu, X. Zhang, Projection micro-stereolithography using digital micro-mirror dynamic mask, Sensors Actuators A Phys. 121 (1) (May 2005) 113–120. [3] T.G. Bifano, J. Perreault, R. Krishnamoorthy Mali, M.N. Horenstein, Microelectromechanical deformable mirrors, IEEE J. Sel. Top. Quantum Electron. 5 (1) (1999) 83–89. [4] M. Helmbrecht, et al., “Micromirrors for adaptive-optics arrays,” TRANSDUCERS ‘01, 11th, Int. Conf. Solid-State Sensors Actuators, Munich, Ger. June 10 (2001) 14. [5] S. Waldis, F. Zamkotsian, P.-A. Clerc, W. Noell, M. Zickar, N. de de Rooij, Arrays of high tilt-angle micromirrors for multiobject spectroscopy, IEEE J. Sel. Top. Quantum Electron. 13 (2) (2007) 168–176. [6] H. Toshiyoshi, W. Piyawattanametha, C.-T. Chan, M.C. Wu, Linearization of electrostatically actuated surface micromachined 2-D optical scanner, J. Microelectromech. Syst. 10 (2) (Jun. 2001) 205–214. [7] K.E. Petersen, Silicon torsional scanning Mirror, IBM J. Res. Dev. 24 (5) (Sep. 1980) 631–637. [8] J. Singh, et al., A two axes scanning SOI MEMS micromirror for endoscopic bioimaging, J. Micromech. Microeng. 18 (2) (Feb. 2008) 025001. [9] L.Y. Lin, E.L. Goldstein, R.W. Tkach, Free-space micromachined optical switches with submillisecond switching time for large-scale optical crossconnects - OSA trends in optics and photonics, Wavel. Div. Mult. Components 29 (4) (1999) 152. [10] D.Y. Winter, R.R. Mansour, Tunable dielectric resonator bandpass filter with embedded MEMS tuning elements, IEEE Trans. Microw. Theory Tech. 55 (1) (2007) 154–159. [11] P. Taylor, M.F. Doerner, W.D. Nix, Stresses and deformation processes in thin films on substrates, CRC Crit. Rev. Solid State Mater. Sci. 14 (3) (1988) 225–266. [12] P.J. Withers, H.K.D.H. Bhadeshia, Residual stress part 2 – nature and origins, Mater. Sci. Technol. 17 (2001) 366–375. [13] R.W. Hoffman, Stresses in thin films: the relevance of grain boundaries and impurities, Thin Solid Films 34 (1976) 185–190. [14] A. Jain, H. Qu, S. Todd, H. Xie, A thermal bimorph micromirror with large bidirectional and vertical actuation, Sensors Actuators A Phys. 122 (2005) 9–15 no. 1 SPEC. ISS. [15] X. Zhang, T.Y. Zhang, M. Wong, Y. Zohar, Residual-stress relaxation in polysilicon thin films by high-temperature rapid thermal annealing, Sensors Actuators A Phys. 64 (1) (1998) 109–115. [16] A. Sharma, D. Bansal, A. Kumar, D. Kumar, K. Rangra, Residual stress control during the growth and release process in gold suspended microstructures, Micromach. Microfabr. Process Technol. XIX 8973 (2014) 1–9. [17] D. Bansal, et al., Low voltage driven RF MEMS capacitive switch using reinforcement for reduced buckling, J. Micromech. Microeng. 27 (2) (Feb. 2017) 024001. [18] Z. Cui, L. Wang, A. Jin, J. Hong, Control of Stress in Multilayered MEMS Devices, 2006 IEEE International Conference on Nano/Micro Engineered and Molecular Systems, vol. 1, 2006, pp. 1224–1227. [19] S.T. Todd, A. Jain, H. Qu, H. Xie, A multi-degree-of-freedom micromirror utilizing inverted-series-connected bimorph actuators, J. Opt. A Pure Appl. Opt. 8 (7) (2006). [20] V. Au, C. Charles, D.A.P. Bulla, J.D. Love, R.W. Boswell, Thickness-dependent stress in plasma-deposited silicon dioxide films, J. Appl. Phys. 97 (8) (2005).
Fig. 18. SEM image of the platform after release.
different bimorph length is fabricated with a stress compensation layer on the top of the aluminum layer. Since the silicon dioxide deposited using PECVD has a lower value of residual stress compared to thermal oxide, a thick layer of oxide is deposited. Silicon dioxide of thickness 1 μm is deposited using Silane at 300 °C followed by patterning using Reactive Ion Etching (RIE). The SEM images of the stress compensated platform before and after release are shown in Fig. 17 and Fig. 18 respectively. A section of bimorph beams got damaged during the release process, but significant improvement in post-release curvature is observed which confirms the feasibility of the proposed methodology. The thickness of PECVD oxide needs further optimization to get a perfect stress-match layer as discussed earlier. 6. Conclusion In this paper, the effect of residual stress on the flatness of a large aperture electrothermally actuated MEMS bilayer platform is presented. A micromachined platform of dimension 500 × 500 μm2 is fabricated using laminated layers of silicon dioxide and aluminum. The fabricated device exhibits post-release curling of the central platform due to the presence of compressive stress in silicon dioxide (240 MPa) and tensile stress in aluminum (35 MPa). An analytical model is presented to find the relationship between residual stress, bending moment and 7