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Fabrication method for elastomer spatial light modulators for short wavelength maskless lithography
Sensors and Actuators A 114 (2004) 528–535
Fabrication method for elastomer spatial light modulators for short wavelength maskless lithography Jen-Shiang Wang∗ , Il Woong Jung, Olav Solgaard E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA Received 1 July 2003; received in revised form 18 November 2003; accepted 1 December 2003 Available online 3 February 2004
Abstract In this paper we present a process for fabricating elastomer spatial light modulator (SLM) that can be scaled to meet the requirements of extreme ultraviolet (EUV- 13 nm wavelength) maskless lithography. The feasibility of the proposed process was tested in a series of release and injection experiments, which showed that the elastomers are successfully injected by the capillary forces and the surface quality is not adversely affected by the introduction of a soft elastomer in the structure. We fabricated an elastomer SLM with an array of four by four micromirrors and demonstrated localized and sinusoidal responses. Analysis of the experimental results showed that patterning of the reflective multilayer, as well as its supporting nitride shell and electrode, is required for SLMs with pixel sizes of 1 by 1 m or less. © 2003 Elsevier B.V. All rights reserved. Keywords: Spatial light modulator; Maskless lithography; EUV lithography; Deformable viscoelastic layers; Elastomer
1. Introduction Extreme ultraviolet (EUV) lithography is one of the most promising technologies for delineating structures with critical dimension (CD) smaller than 100 nm [1]. Due to the high absorption in the EUV, it is difficult to protect EUV masks by pellicles. Therefore, fabricating defect-free EUV mask becomes quite challenging and expensive. To reduce the cost of masks, maskless lithography using spatial light modulators (SLMs) is seen as a promising alternative [2]. A schematic architecture of a maskless EUVL system is shown in Fig. 1. The image pattern on the wafer is modulated by a two-dimensional micromirror array, in which the image can be controlled electronically. The SLM image only covers a part of the wafer area, so the wafer is mechanically scanned to complete the coverage. Due to high absorption in the EUV, the optics in EUV lithography systems is composed of reflective mirrors coated with Mo/Si multilayer instead of the refractive lenses in traditional optical lithography systems. In addition, EUV lithography systems must be operated in vacuum to reduce the scattering and absorption of the ambient gas or air. The SLM can be implemented as an array of tilting micromirrors, similar to the digital micromirror device (DMD) [3], but with analog control of the angular deflection [4]. The ∗
Corresponding author. Tel.: +1-650-723-6104; fax: +1-650-725-7509. E-mail address: [email protected] (J.-S. Wang). 0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.12.001
intensity of each pixel is controlled by tilting the mirror. A dark pixel is formed when the light is reflected outside the aperture of the imaging system. An alternative SLM architecture is an array of piston-motion micromirrors (Fig. 2), which operates on a principle similar to a grating light modulator [5]. The light from a pixel that is phase-shifted by radians with respect to its surroundings will diffract outside the numerical aperture of the imaging optics and will appear dark in the image pattern. Larger dark areas are created by alternating pixels of 0 and phase shift. Gray scale is created by analog shifting between 0 and . Phase-shifting mirrors can be implemented as elastomer pillars as shown in Fig. 3. Each pixel of the SLM contains a capacitive actuator and a Mo/Si multilayer mirror, so the height and thus the phase shift of a pixel is a function of the applied potential. This structure, which is the focus of this paper, simplifies down scaling to match the size requirements for EUV lithography. In this paper, we will refer to one elastomer pillar as one pixel, although in most maskless lithography systems four such pixels would be used to form an “image pixel” with improved imaging qualities.
2. Fabrication The fabrication of SLMs for EUV lithography poses several technological challenges. The optical surfaces must be very smooth and compatible with Mo/Si multilayer
J.-S. Wang et al. / Sensors and Actuators A 114 (2004) 528–535
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2-D Mirror Array Imaging Optics Fig. 1. The architecture of a maskless EUV lithography system. A collimated EUV beam is modulated by a two-dimensional micromirror array, and then focused on the wafer. The pattern on the wafer is varied according to the configuration on the micromirror SLM, after [2].
technology to achieve high reflectivity. Large arrays are required so the fabrication process must allow direct integration with electronics for multiplexing. The individual pixels must be as small as possible to minimize the need for de-magnifying optics in the EUV. Finally, the process compatibility and contamination of elastomers must be taken into account. To meet these challenges we have developed an SLM-fabrication process as shown in Fig. 4. The process starts with depositing and patterning addressing electrodes, which will be replaced by the integrated circuitry later, on top of an insulation layer composed of thermally growth oxide and LPCVD nitride. A sacrificial oxide layer and a nitride shell are deposited by LPCVD and patterned on top of addressing electrodes to provide a flat surface for the top electrode and Mo/Si multilayer. The reflectivity of the Mo/Si multilayer is affected significantly by the surface roughness of the substrate, so the surface has to be polished in preparation for deposition of the multilayer [6]. Conventional
Fig. 3. The structure of the elastomer spatial light modulator composed of a two-dimensional array of elastomer pillars. Each pillar contains a capacitive actuator with an elastomer as the supporting and dielectric structure. A stack of 81 layers of Mo/Si multilayer mirror is deposited on the surface to achieve a reflectivity around 70% in EUV.
spin-on deposition of the elastomer has been used to fabricate similar SLMs [7,8], but this process is not suitable for EUV applications because of the difficulties of polishing soft materials. This problem can be solved by chip or wafer bonding with elastomer bonding layers [9], but, unlike the process described in this paper, these fabrication techniques have not been designed for scaling to pixels in the m range and below. After the deposition and patterning of the top electrode and the Mo/Si multilayer, the SLM is completed by sacrificial removal and replacement of the oxide layer with an elastomer to create the phase-shifting pixels of the SLM. If required, the top electrode layer and the nitride shell can be pixilated after the elastomer replacement step. Our process allows the formation of very small pixels (limited by the electrode size) with polished surfaces in a process with few high-temperature steps so it can be vertically integrated with electronics, thus meeting the
Fig. 2. The operation principle of piston-motion micromirrors. The intensity distribution on the wafer can be modulated by the phase distribution on the SLM with selected values of numerical aperture (NA) and incoherence of light source.
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purpose of the first-generation devices is to characterize the mechanical properties of the elastomer SLMs, so the Mo/Si mirror is replaced by a polysilicon layer with a thickness of 250 nm, roughly equal to the total thickness of the Mo/Si multilayer. The thickness of the sacrificial oxide and the nitride shell are 2 and 0.7 m, respectively. The sacrificial oxide is released by concentrated HF (49%) and dried with the critical-point-drying (CPD) method. The elastomer used in the SLMs is Dow Corning Sylgard 527 silicone dielectric gel. The reason of choosing this material will be addressed in the next section. After the components of the elastomer are mixed (1:1), the preform is de-aired in a vacuum chamber to remove air bubbles and is then injected into the SLM by capillary forces. After the injection, the SLM is heated to 100 ◦ C for an hour to solidify the elastomer.
3. Injection tests
Fig. 4. Process flow of elastomer SLMs: the bottom electrodes are deposited on top of an insulation layer. The sacrificial oxide layer is then deposited, patterned and polished. A nitride protection layer and the top electrode are deposited on the flat surface. The mirror material is deposited and patterned, and finally the oxide is released and replaced by the elastomer by capillary forces.
requirements of EUV maskless lithography. Meanwhile, because only one common top electrode is used, the wire connection and multiplexing are simplified. It should also be pointed out here that before filling the cavity defined by the nitride shell, the process does not contain any elastomer materials. This not only circumvents material incompatibilities and contamination, but also allows testing of different elastomers without changing the process. In the current design, the addressing electrodes are a 4×4 array of polysilicon squares, each 20 m on a side. The distance between the centers of adjacent pixels is 40 m. The
The elastomer replacement is a critical step in the fabrication sequence. A test structure was therefore designed to examine the release and injection processes. The test structure is similar to the SLMs shown in Fig. 3 except that the top electrode and multi-layer are not deposited. The reason for removing the reflecting layers is to allow monitoring of the injection progress through the transparent nitride shell. Fig. 5 shows the top view of the structure. Two large square holes with dimensions 200 × 200 m are etched in two ends of the channel. The elastomer is injected into one hole and the air inside the channel escapes through the other one. An array of 2×2 m etching holes on a 30×30 m raster is used to facilitate the oxide release. The length, width, and thickness of the channel are 1000, 400, and 2 m, respectively. Fig. 6 shows the process of the elastomer injection in the test structure. The elastomer is injected at one end and pulled into the cavity by the capillary forces. The injection test is performed with materials with different viscosities and the results are listed in Table 1. The injection time, as expected, is highly related to the viscosity of the material. Nonetheless,
Fig. 5. The top view and the dimension of the test structure. The figure is not drawn to scale. Table 1 Injection time of different elastomers Material
Viscosity (mPa s)
Injection time (min)
Dow Corning Sylgard 527 Dow Corning Sylgard 182 Masterbond Mastersil 773
450 5000 60–70
∼3 ∼27 <1
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Fig. 6. Elastomer injection. The elastomer is pulled in by the capillary forces.
the results show that injection in shallow channels is possible even with relatively high viscosity elastomers. Based on the injection results, Masterbond Mastersil 773 seems like the best candidate because it has the lowest viscosity. However, we were unable to produce working SLMs with this material in spite of several trials. From Table 1, the next candidate is Dow Corning Sylgard 527 with a moderate viscosity. The capability of the material was verified through successful deformation measurements, which are detailed in the next section. This material has also been proven to have repeatable behavior in long-term operation [8]. To examine the influence of the elastomer injection, the surfaces of a SLM before and after the elastomer injection were measured under a white-light interferometer (Fig. 7). The result shows that the surface profile is not adversely affected by the injection.
4. Mechanical tests Several mechanical measurements were performed to further verify the feasibility of the proposed process. First, the effects of the elastomer and the nitride shell on the deflection of the SLM actuators were examined using a simple SLM with only one electrode. The single-electrode devices were fabricated on the same wafers with the same process as the 4 × 4 SLMs, so the thicknesses of each layer are the same. The deflections (at the center) versus voltages of single-electrode SLMs with and without elastomer are plotted in Fig. 8. The equivalent spring constant increases from 7 to 350 N/m after the elastomer injection, demonstrating that the elastomer provides the majority of the restoring force. The increased stiffness of the structure allows a more
precise and localized control of the deflection, as required in EUV lithography systems. The operation principle requires isolated piston motion of each pixel, or localized deformation, in the elastomer SLMs. Fig. 9 shows the localized deformation when one pixel of a 4 × 4 SLM is actuated with 160 V dc. The surface profiles at 40 V to 200 V dc with an interval of 40 V are shown in Fig. 10. Along with a main lobe, two side lobes with significant opposite deflection are evident. The formation of the side lobes is caused by the large ratio of the deformation to the thickness of the elastomer (∼6% when the voltage is 200 V). The area of the deformed region (∼100 m × 100 m) is substantially larger than the pixel size (20 m × 20 m), but the result shows that the restoring force provided by the elastomer allows localized deformations of the membrane. Another possible operation mode of the elastomer SLMs is forming a sinusoidal grating on the surface by applying two different voltages to interleaved pixels [10]. We will use a three-pixel grating as an example. When the center pixel is actuated with one voltage and the pixels on the sides are actuated with another voltage, a sinusoidal shape with one-and-half period can be formed (Fig. 11). We define the average of the two voltages as the bias voltage and one half of the difference between the two voltages as the modulation voltage. Sinusoidal surface profiles were successfully demonstrated (Fig. 11) when three pixels are actuated with a bias voltage of 90 V and modulation voltages of 10, 50 and 90 V. Again, two significant side lobes accompanying the one-and-half-period sine wave were observed, consistent with the results of the one-pixel actuation.
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Fig. 7. Surface profiles of elastomer SLM before (a) and after elastomer injection (b). One-dimensional profiles from (a) and (b) are shown in (c). The membrane is not adversely affected by the injection.
Measurements of the dynamic response of the 4 × 4 SLM show a 3 dB frequency of approximately 350 Hz. Some hysteresis of the response is observed when large electric fields are applied. The extra deformation relaxes to 0 in 20–30 min, and can be removed by electrostatic flattening of the mirror surface.
5. Discussion From the above experiment results, the deformed areas are found to be significantly larger than the area of the actuated pixels. The neighbor pixels are deformed because
of the mechanical connection to the actuated pixels by the top nitride layer and the top electrode. This kind of cross talk can be decreased by applying compensated voltage to the neighbor pixels or reducing the deformed area. The deformed area of the SLM is strongly dependent on the thickness of the top membrane. To reduce the area of deformation, the thickness, and therefore the strength, of the membrane must be reduced. This becomes even more important when the pixel size is scaled down to smaller sizes, because the reaction force of the top membrane becomes the dominant factor that limits the deflection of the SLM when the pixels are small [8]. Based on an analytic model developed for structures similar to ours, the deformation
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Fig. 8. Deflection (m) vs. voltage (V) for single-electrode SLMs with and without elastomer. The membrane is composed of 700 nm nitride and 250 nm polysilicon. The membrane is 200 m × 500 m, and the electrode is 200 m × 480 m.
small pixels, the term E(πhd−1 )3 /24(1 − ν2 ), which corresponds to the reaction force of the membrane, dominates the denominator of Eq. (1). The deflection amplitude can then be approximated as:
amplitude a can be expressed as [8,10]. a=
V0 Vs exp(−πgd−1 ) ε1 −1 g Ge + (E(πhd )3 /24(1 − ν2 )) − (ε1 /2)(V0 /g)2 (1)
when the applied voltage is π V(x) = V0 − Vs cos x . d
a ≈ 24ε1 V0 Vs (1 − ν2 )(gE)−1 (πh)−3 d 3 exp(−πgd−1 ). (2)
(3)
where h is the thickness of the membrane, and d the pixel size. E and ν are the Young’s modulus and the Poisson ratio of the membrane, and Ge , ε1 , and g are the shear modulus, the permittivity, and the thickness of the elastomer. For
This expression shows that the amplitude depends strongly on the pixel size and the thickness of the membrane. The deflection amplitudes at various membrane thicknesses with pixel sizes of 1, 2, 4, and 8 m are shown in Fig. 12. To
Fig. 9. The surface profile of an elastomer SLM when one pixel is actuated with 160 V.
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Fig. 10. The surface profiles at a cross-section of an elastomer SLM when one pixel is actuated with 40 to 200 V. The inlet shows the ideal profile.
achieve the required deflection of one-quarter wavelength in the EUV, the minimum pixel size is around 8 m when the thickness of Mo/Si multilayer mirrors, around 280 nm, is taken into account. To make 1 m × 1 m pixels, the equivalent thickness of the membrane should be below 20 nm. As a consequence, in a practical EUV SLM with pixel sizes of 1 m × 1 m or less, the Mo/Si layer, as well as the nitride shell and the top electrode, must be pixilated as shown in Fig. 2 to reduce the strength of the top membrane. An important issue is the aging of the elastomer. During the period of our experiments (∼1 month) we did not observe any significant long-term imprint effects. The repeatability over time has to be further confirmed with longer periods of testing. Another aging problem is that the behavior of the elastomer may change after long-term EUV exposure. The most promising solution to this problem is to include strong
Fig. 12. Calculated deflection amplitude a vs. thickness of the membrane h for SLMs with pixel sizes d of 1, 2, 4, and 8 m. The following parameters are used in the calculation: V0 = 100 V, Vs = 40 V, g = 1 m, ε1 = 2.7 ε0 , Ge = 10 kPa, E = 200 GPa, ν = 0.4.
absorption layers in the protective shell. The selection of the material and the optimized thickness of the shell have to be studied further by detailed simulations and experiments.
6. Conclusions This paper presented the fabrication process, structure and operation of a spatial light modulator designed to meet the requirements of extreme ultraviolet maskless lithography. Optical characterization of the injection process and the deflection proved the feasibility of the process. Two modulation types, localized piston motion and sinusoidal deflection were successfully demonstrated in an elastomer SLMs composed of a 4 × 4 micromirror array. The scaling possibilities were discussed with the conclusion that pixelating the multilayer mirror as well as the top membrane is necessary for 1 m devices.
Acknowledgements The authors are grateful to the Stanford Nanofabrication Facility where the work was pursued. This research is supported by the Defense Advanced Research Project Agency together with the Semiconductor Research Corporation under the contract MDA972-01-1-0021. The authors would like to thank Dr. Stefan Zappe for assistance and guidance in fabrication.
Fig. 11. The surface profiles at a cross-section of an elastomer SLM when three pixels are actuated with a bias voltage of 90 V and modulation voltages of 10, 50 and 90 V. The distance between adjacent pixels is 40 m from center to center. The inlet shows the ideal profile.
References [1] C.W. Gwyn, R. Stulen, D. Sweeney, D. Attwood, Extreme ultraviolet lithography, J. Vac. Sci. Technol. B 16 (1998) 3142–3149.
J.-S. Wang et al. / Sensors and Actuators A 114 (2004) 528–535 [2] N. Choksi, D.S. Pickard, M. McCord, R.F.W. Pease, Y. Shroff, Y. Chen, W. Oldham, D. Markle, Maskless extreme ultraviolet lithography, J. Vac. Sci. Technol. B 17 (1999) 3047–3051. [3] L.J. Hornbeck, Deformable-mirror spatial light modulators, in: Proceedings of the SPIE 1150 Spatial light Modulators and Applications III, 1989, pp. 86–102. [4] Y. Shroff, Y. Chen, W. Oldham, Fabrication of parallel-plate nanomirror arrays for extreme ultraviolet maskless lithography, J. Vac. Sci. Technol. B 19 (2001) 2412–2415. [5] O. Solgaard, F.S.A. Sandejas, D.M. Bloom, Deformable grating optical modulator, Opt. Lett. 17 (1992) 688–690. [6] D.G. Stearns, R.S. Rosen, S.P. Vernon, Fabrication of high-reflectance Mo-Si multilayer mirrors by planar-magnetron sputtering, J. Vac. Sci. Technol. A 9 (1991) 2662–2669. [7] R. Tepe, R. Gerhard-Multhaupt, W. Brinker, W.-D. Molzow, Viscoelastic spatial light modulator with active matrix addressing, App. Opt. 28 (1989) 4826–4834. [8] W. Brinker, W. Wirges, G. Przyrembel, R. Gerhard-Multhaupt, J. Klemberg-Sapieha, L. Martinu, D. Poitras, M.R. Wertheimer, Metallized viscoelastic control layers for light-valve projection displays, Displays 16 (1995) 13–20. [9] S. Sakarya, G. Vdovin, P.M. Sarro, Technology of reflective membranes for spatial light modulators, Sens. and Actuators A 97–98 (2002) 468–472. [10] R. Tepe, Theoretical analysis of an electrically addressed viscoelastic spatial light modulator, J. Opt. Soc. Am. A 4 (1987) 1273–1282.
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Biographies Jen-Shiang Wang received the BS degree in physics and MS degree in applied mechanics from National Taiwan University, Taiwan, in 1996 and 1998, respectively. He is currently pursuing the PhD degree in the department of electrical engineering at Stanford University. His research interests include designing and fabricating micromirrors for maskless lithography system and adaptive optics system. Il Woong Jung received the BS and MS degree in physics from Yonsei University, Seoul, Korea in 1997 and 2001, respectively. He is currently pursuing the PhD degree in the department of electrical engineering at Stanford University. His research interests are in microelectromechanical deformable mirrors for free space communication and adaptive optics. Olav Solgaard received the BS degree in Electrical engineering from the Norwegian Institute of Technology and the MS and PhD degrees in Electrical Engineering from Stanford University, California. He held a postdoctoral position at the University of California at Berkeley, and an assistant professorship at the University of California at Davis, before joining the faculty of the department of Electrical Engineering at Stanford University in July, 1999. His research areas are optical communication and sensing applications of MEMS. He has authored more than 90 technical publications, and holds 13 patents. He is a co-founder of Silicon Light Machines, Sunnyvale, CA, and an active consultant in the MEMS industry.
Fabrication method for elastomer spatial light modulators for short wavelength maskless lithography Jen-Shiang Wang∗ , Il Woong Jung, Olav Solgaard E. L. Ginzton Laboratory, Stanford University, Stanford, CA 94305, USA Received 1 July 2003; received in revised form 18 November 2003; accepted 1 December 2003 Available online 3 February 2004
Abstract In this paper we present a process for fabricating elastomer spatial light modulator (SLM) that can be scaled to meet the requirements of extreme ultraviolet (EUV- 13 nm wavelength) maskless lithography. The feasibility of the proposed process was tested in a series of release and injection experiments, which showed that the elastomers are successfully injected by the capillary forces and the surface quality is not adversely affected by the introduction of a soft elastomer in the structure. We fabricated an elastomer SLM with an array of four by four micromirrors and demonstrated localized and sinusoidal responses. Analysis of the experimental results showed that patterning of the reflective multilayer, as well as its supporting nitride shell and electrode, is required for SLMs with pixel sizes of 1 by 1 m or less. © 2003 Elsevier B.V. All rights reserved. Keywords: Spatial light modulator; Maskless lithography; EUV lithography; Deformable viscoelastic layers; Elastomer
1. Introduction Extreme ultraviolet (EUV) lithography is one of the most promising technologies for delineating structures with critical dimension (CD) smaller than 100 nm [1]. Due to the high absorption in the EUV, it is difficult to protect EUV masks by pellicles. Therefore, fabricating defect-free EUV mask becomes quite challenging and expensive. To reduce the cost of masks, maskless lithography using spatial light modulators (SLMs) is seen as a promising alternative [2]. A schematic architecture of a maskless EUVL system is shown in Fig. 1. The image pattern on the wafer is modulated by a two-dimensional micromirror array, in which the image can be controlled electronically. The SLM image only covers a part of the wafer area, so the wafer is mechanically scanned to complete the coverage. Due to high absorption in the EUV, the optics in EUV lithography systems is composed of reflective mirrors coated with Mo/Si multilayer instead of the refractive lenses in traditional optical lithography systems. In addition, EUV lithography systems must be operated in vacuum to reduce the scattering and absorption of the ambient gas or air. The SLM can be implemented as an array of tilting micromirrors, similar to the digital micromirror device (DMD) [3], but with analog control of the angular deflection [4]. The ∗
Corresponding author. Tel.: +1-650-723-6104; fax: +1-650-725-7509. E-mail address: [email protected] (J.-S. Wang). 0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2003.12.001
intensity of each pixel is controlled by tilting the mirror. A dark pixel is formed when the light is reflected outside the aperture of the imaging system. An alternative SLM architecture is an array of piston-motion micromirrors (Fig. 2), which operates on a principle similar to a grating light modulator [5]. The light from a pixel that is phase-shifted by radians with respect to its surroundings will diffract outside the numerical aperture of the imaging optics and will appear dark in the image pattern. Larger dark areas are created by alternating pixels of 0 and phase shift. Gray scale is created by analog shifting between 0 and . Phase-shifting mirrors can be implemented as elastomer pillars as shown in Fig. 3. Each pixel of the SLM contains a capacitive actuator and a Mo/Si multilayer mirror, so the height and thus the phase shift of a pixel is a function of the applied potential. This structure, which is the focus of this paper, simplifies down scaling to match the size requirements for EUV lithography. In this paper, we will refer to one elastomer pillar as one pixel, although in most maskless lithography systems four such pixels would be used to form an “image pixel” with improved imaging qualities.
2. Fabrication The fabrication of SLMs for EUV lithography poses several technological challenges. The optical surfaces must be very smooth and compatible with Mo/Si multilayer
J.-S. Wang et al. / Sensors and Actuators A 114 (2004) 528–535
EUV Source
Condenser Optics
529
Mechanical Scan Wafer
2-D Mirror Array Imaging Optics Fig. 1. The architecture of a maskless EUV lithography system. A collimated EUV beam is modulated by a two-dimensional micromirror array, and then focused on the wafer. The pattern on the wafer is varied according to the configuration on the micromirror SLM, after [2].
technology to achieve high reflectivity. Large arrays are required so the fabrication process must allow direct integration with electronics for multiplexing. The individual pixels must be as small as possible to minimize the need for de-magnifying optics in the EUV. Finally, the process compatibility and contamination of elastomers must be taken into account. To meet these challenges we have developed an SLM-fabrication process as shown in Fig. 4. The process starts with depositing and patterning addressing electrodes, which will be replaced by the integrated circuitry later, on top of an insulation layer composed of thermally growth oxide and LPCVD nitride. A sacrificial oxide layer and a nitride shell are deposited by LPCVD and patterned on top of addressing electrodes to provide a flat surface for the top electrode and Mo/Si multilayer. The reflectivity of the Mo/Si multilayer is affected significantly by the surface roughness of the substrate, so the surface has to be polished in preparation for deposition of the multilayer [6]. Conventional
Fig. 3. The structure of the elastomer spatial light modulator composed of a two-dimensional array of elastomer pillars. Each pillar contains a capacitive actuator with an elastomer as the supporting and dielectric structure. A stack of 81 layers of Mo/Si multilayer mirror is deposited on the surface to achieve a reflectivity around 70% in EUV.
spin-on deposition of the elastomer has been used to fabricate similar SLMs [7,8], but this process is not suitable for EUV applications because of the difficulties of polishing soft materials. This problem can be solved by chip or wafer bonding with elastomer bonding layers [9], but, unlike the process described in this paper, these fabrication techniques have not been designed for scaling to pixels in the m range and below. After the deposition and patterning of the top electrode and the Mo/Si multilayer, the SLM is completed by sacrificial removal and replacement of the oxide layer with an elastomer to create the phase-shifting pixels of the SLM. If required, the top electrode layer and the nitride shell can be pixilated after the elastomer replacement step. Our process allows the formation of very small pixels (limited by the electrode size) with polished surfaces in a process with few high-temperature steps so it can be vertically integrated with electronics, thus meeting the
Fig. 2. The operation principle of piston-motion micromirrors. The intensity distribution on the wafer can be modulated by the phase distribution on the SLM with selected values of numerical aperture (NA) and incoherence of light source.
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purpose of the first-generation devices is to characterize the mechanical properties of the elastomer SLMs, so the Mo/Si mirror is replaced by a polysilicon layer with a thickness of 250 nm, roughly equal to the total thickness of the Mo/Si multilayer. The thickness of the sacrificial oxide and the nitride shell are 2 and 0.7 m, respectively. The sacrificial oxide is released by concentrated HF (49%) and dried with the critical-point-drying (CPD) method. The elastomer used in the SLMs is Dow Corning Sylgard 527 silicone dielectric gel. The reason of choosing this material will be addressed in the next section. After the components of the elastomer are mixed (1:1), the preform is de-aired in a vacuum chamber to remove air bubbles and is then injected into the SLM by capillary forces. After the injection, the SLM is heated to 100 ◦ C for an hour to solidify the elastomer.
3. Injection tests
Fig. 4. Process flow of elastomer SLMs: the bottom electrodes are deposited on top of an insulation layer. The sacrificial oxide layer is then deposited, patterned and polished. A nitride protection layer and the top electrode are deposited on the flat surface. The mirror material is deposited and patterned, and finally the oxide is released and replaced by the elastomer by capillary forces.
requirements of EUV maskless lithography. Meanwhile, because only one common top electrode is used, the wire connection and multiplexing are simplified. It should also be pointed out here that before filling the cavity defined by the nitride shell, the process does not contain any elastomer materials. This not only circumvents material incompatibilities and contamination, but also allows testing of different elastomers without changing the process. In the current design, the addressing electrodes are a 4×4 array of polysilicon squares, each 20 m on a side. The distance between the centers of adjacent pixels is 40 m. The
The elastomer replacement is a critical step in the fabrication sequence. A test structure was therefore designed to examine the release and injection processes. The test structure is similar to the SLMs shown in Fig. 3 except that the top electrode and multi-layer are not deposited. The reason for removing the reflecting layers is to allow monitoring of the injection progress through the transparent nitride shell. Fig. 5 shows the top view of the structure. Two large square holes with dimensions 200 × 200 m are etched in two ends of the channel. The elastomer is injected into one hole and the air inside the channel escapes through the other one. An array of 2×2 m etching holes on a 30×30 m raster is used to facilitate the oxide release. The length, width, and thickness of the channel are 1000, 400, and 2 m, respectively. Fig. 6 shows the process of the elastomer injection in the test structure. The elastomer is injected at one end and pulled into the cavity by the capillary forces. The injection test is performed with materials with different viscosities and the results are listed in Table 1. The injection time, as expected, is highly related to the viscosity of the material. Nonetheless,
Fig. 5. The top view and the dimension of the test structure. The figure is not drawn to scale. Table 1 Injection time of different elastomers Material
Viscosity (mPa s)
Injection time (min)
Dow Corning Sylgard 527 Dow Corning Sylgard 182 Masterbond Mastersil 773
450 5000 60–70
∼3 ∼27 <1
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Fig. 6. Elastomer injection. The elastomer is pulled in by the capillary forces.
the results show that injection in shallow channels is possible even with relatively high viscosity elastomers. Based on the injection results, Masterbond Mastersil 773 seems like the best candidate because it has the lowest viscosity. However, we were unable to produce working SLMs with this material in spite of several trials. From Table 1, the next candidate is Dow Corning Sylgard 527 with a moderate viscosity. The capability of the material was verified through successful deformation measurements, which are detailed in the next section. This material has also been proven to have repeatable behavior in long-term operation [8]. To examine the influence of the elastomer injection, the surfaces of a SLM before and after the elastomer injection were measured under a white-light interferometer (Fig. 7). The result shows that the surface profile is not adversely affected by the injection.
4. Mechanical tests Several mechanical measurements were performed to further verify the feasibility of the proposed process. First, the effects of the elastomer and the nitride shell on the deflection of the SLM actuators were examined using a simple SLM with only one electrode. The single-electrode devices were fabricated on the same wafers with the same process as the 4 × 4 SLMs, so the thicknesses of each layer are the same. The deflections (at the center) versus voltages of single-electrode SLMs with and without elastomer are plotted in Fig. 8. The equivalent spring constant increases from 7 to 350 N/m after the elastomer injection, demonstrating that the elastomer provides the majority of the restoring force. The increased stiffness of the structure allows a more
precise and localized control of the deflection, as required in EUV lithography systems. The operation principle requires isolated piston motion of each pixel, or localized deformation, in the elastomer SLMs. Fig. 9 shows the localized deformation when one pixel of a 4 × 4 SLM is actuated with 160 V dc. The surface profiles at 40 V to 200 V dc with an interval of 40 V are shown in Fig. 10. Along with a main lobe, two side lobes with significant opposite deflection are evident. The formation of the side lobes is caused by the large ratio of the deformation to the thickness of the elastomer (∼6% when the voltage is 200 V). The area of the deformed region (∼100 m × 100 m) is substantially larger than the pixel size (20 m × 20 m), but the result shows that the restoring force provided by the elastomer allows localized deformations of the membrane. Another possible operation mode of the elastomer SLMs is forming a sinusoidal grating on the surface by applying two different voltages to interleaved pixels [10]. We will use a three-pixel grating as an example. When the center pixel is actuated with one voltage and the pixels on the sides are actuated with another voltage, a sinusoidal shape with one-and-half period can be formed (Fig. 11). We define the average of the two voltages as the bias voltage and one half of the difference between the two voltages as the modulation voltage. Sinusoidal surface profiles were successfully demonstrated (Fig. 11) when three pixels are actuated with a bias voltage of 90 V and modulation voltages of 10, 50 and 90 V. Again, two significant side lobes accompanying the one-and-half-period sine wave were observed, consistent with the results of the one-pixel actuation.
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Fig. 7. Surface profiles of elastomer SLM before (a) and after elastomer injection (b). One-dimensional profiles from (a) and (b) are shown in (c). The membrane is not adversely affected by the injection.
Measurements of the dynamic response of the 4 × 4 SLM show a 3 dB frequency of approximately 350 Hz. Some hysteresis of the response is observed when large electric fields are applied. The extra deformation relaxes to 0 in 20–30 min, and can be removed by electrostatic flattening of the mirror surface.
5. Discussion From the above experiment results, the deformed areas are found to be significantly larger than the area of the actuated pixels. The neighbor pixels are deformed because
of the mechanical connection to the actuated pixels by the top nitride layer and the top electrode. This kind of cross talk can be decreased by applying compensated voltage to the neighbor pixels or reducing the deformed area. The deformed area of the SLM is strongly dependent on the thickness of the top membrane. To reduce the area of deformation, the thickness, and therefore the strength, of the membrane must be reduced. This becomes even more important when the pixel size is scaled down to smaller sizes, because the reaction force of the top membrane becomes the dominant factor that limits the deflection of the SLM when the pixels are small [8]. Based on an analytic model developed for structures similar to ours, the deformation
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Fig. 8. Deflection (m) vs. voltage (V) for single-electrode SLMs with and without elastomer. The membrane is composed of 700 nm nitride and 250 nm polysilicon. The membrane is 200 m × 500 m, and the electrode is 200 m × 480 m.
small pixels, the term E(πhd−1 )3 /24(1 − ν2 ), which corresponds to the reaction force of the membrane, dominates the denominator of Eq. (1). The deflection amplitude can then be approximated as:
amplitude a can be expressed as [8,10]. a=
V0 Vs exp(−πgd−1 ) ε1 −1 g Ge + (E(πhd )3 /24(1 − ν2 )) − (ε1 /2)(V0 /g)2 (1)
when the applied voltage is π V(x) = V0 − Vs cos x . d
a ≈ 24ε1 V0 Vs (1 − ν2 )(gE)−1 (πh)−3 d 3 exp(−πgd−1 ). (2)
(3)
where h is the thickness of the membrane, and d the pixel size. E and ν are the Young’s modulus and the Poisson ratio of the membrane, and Ge , ε1 , and g are the shear modulus, the permittivity, and the thickness of the elastomer. For
This expression shows that the amplitude depends strongly on the pixel size and the thickness of the membrane. The deflection amplitudes at various membrane thicknesses with pixel sizes of 1, 2, 4, and 8 m are shown in Fig. 12. To
Fig. 9. The surface profile of an elastomer SLM when one pixel is actuated with 160 V.
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Fig. 10. The surface profiles at a cross-section of an elastomer SLM when one pixel is actuated with 40 to 200 V. The inlet shows the ideal profile.
achieve the required deflection of one-quarter wavelength in the EUV, the minimum pixel size is around 8 m when the thickness of Mo/Si multilayer mirrors, around 280 nm, is taken into account. To make 1 m × 1 m pixels, the equivalent thickness of the membrane should be below 20 nm. As a consequence, in a practical EUV SLM with pixel sizes of 1 m × 1 m or less, the Mo/Si layer, as well as the nitride shell and the top electrode, must be pixilated as shown in Fig. 2 to reduce the strength of the top membrane. An important issue is the aging of the elastomer. During the period of our experiments (∼1 month) we did not observe any significant long-term imprint effects. The repeatability over time has to be further confirmed with longer periods of testing. Another aging problem is that the behavior of the elastomer may change after long-term EUV exposure. The most promising solution to this problem is to include strong
Fig. 12. Calculated deflection amplitude a vs. thickness of the membrane h for SLMs with pixel sizes d of 1, 2, 4, and 8 m. The following parameters are used in the calculation: V0 = 100 V, Vs = 40 V, g = 1 m, ε1 = 2.7 ε0 , Ge = 10 kPa, E = 200 GPa, ν = 0.4.
absorption layers in the protective shell. The selection of the material and the optimized thickness of the shell have to be studied further by detailed simulations and experiments.
6. Conclusions This paper presented the fabrication process, structure and operation of a spatial light modulator designed to meet the requirements of extreme ultraviolet maskless lithography. Optical characterization of the injection process and the deflection proved the feasibility of the process. Two modulation types, localized piston motion and sinusoidal deflection were successfully demonstrated in an elastomer SLMs composed of a 4 × 4 micromirror array. The scaling possibilities were discussed with the conclusion that pixelating the multilayer mirror as well as the top membrane is necessary for 1 m devices.
Acknowledgements The authors are grateful to the Stanford Nanofabrication Facility where the work was pursued. This research is supported by the Defense Advanced Research Project Agency together with the Semiconductor Research Corporation under the contract MDA972-01-1-0021. The authors would like to thank Dr. Stefan Zappe for assistance and guidance in fabrication.
Fig. 11. The surface profiles at a cross-section of an elastomer SLM when three pixels are actuated with a bias voltage of 90 V and modulation voltages of 10, 50 and 90 V. The distance between adjacent pixels is 40 m from center to center. The inlet shows the ideal profile.
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Biographies Jen-Shiang Wang received the BS degree in physics and MS degree in applied mechanics from National Taiwan University, Taiwan, in 1996 and 1998, respectively. He is currently pursuing the PhD degree in the department of electrical engineering at Stanford University. His research interests include designing and fabricating micromirrors for maskless lithography system and adaptive optics system. Il Woong Jung received the BS and MS degree in physics from Yonsei University, Seoul, Korea in 1997 and 2001, respectively. He is currently pursuing the PhD degree in the department of electrical engineering at Stanford University. His research interests are in microelectromechanical deformable mirrors for free space communication and adaptive optics. Olav Solgaard received the BS degree in Electrical engineering from the Norwegian Institute of Technology and the MS and PhD degrees in Electrical Engineering from Stanford University, California. He held a postdoctoral position at the University of California at Berkeley, and an assistant professorship at the University of California at Davis, before joining the faculty of the department of Electrical Engineering at Stanford University in July, 1999. His research areas are optical communication and sensing applications of MEMS. He has authored more than 90 technical publications, and holds 13 patents. He is a co-founder of Silicon Light Machines, Sunnyvale, CA, and an active consultant in the MEMS industry.