Design and fabrication of CMOS optical modulator

Design and fabrication of CMOS optical modulator

Ching-Liang Dai

_{, Hung-Lin Chen}

_{, Liang-Bin Yu}

_{, Chun-Hui Lin}

_{, Pei-Zen Chang}

a_{Department of Mechanical Engineering, Oriental Institute of Technology, No. 58, Sze-Chuan Road, Sec. 2, Pan-Chiao, Taipei Hsien 22064, Taiwan, ROC} b_{Institute of Applied Mechanics, National Taiwan University, Taipei 107, Taiwan, ROC}

Accepted 11 September 2001

Abstract

This study describes a micromachined optical modulator with electrostatic actuation, fabricated by the conventional CMOS process. The optical modulator is operated by the interaction of ®xed stationary gratings and movable sliding gratings. The period of the gratings is determined by the slide of the movable part, allowing different diffraction patterns of re¯ected light. In addition, 100% modulation in the ®rst order can act as an optical switch. All procedures following the CMOS process require a simple post-process. Maskless dry etching was the only requirement of the post-process to obtain a high-aspect-ratio microstructure, and only a low voltage of 20 V was necessary to drive the actuator of the optical modulator. The micromachined optical modulator proposed herein is smaller and weighs less than commercially available acoustic optical modulators. # 2001 Published by Elsevier Science B.V.

Keywords: Micromachine; Modulator; Gratings; CMOS

1. Introduction

Advances in semiconductor fabrication technology have inspired a new generation of micro-electro-mechanical sys-tems (MEMS) [1,2] designed by using microfabrication equipment and processing methods. Optical devices are one among the most important applications of MEMS because miniaturization of the system is useful for small light spots, in supporting quick responses, low power con-sumption and speci®c work that can not be done with traditional optical elements. Accordingly, optical switches and modulators have also received considerable interest.

A modulator situated between the source and the observa-tion point in¯uences physical variables. An optical system includes variables such as amplitude, frequency, phase, polar-ization or intensity. Therefore, the optical modulator is useful in industrial applications. For instance, an acoustic optical modulator can be used as an optical switch, a scanner, a ®lter or an isolator [3]. Micromachined optical modulators have also attracted much attention [4]. The light is modulated by three types of mechanical deformation: vertical [5], horizontal [6] and rotatory [7±9]. The optical modulator proposed herein is of the horizontal type that operates in-plane. The stationary part in the new design is strongly ®xed to the anchor, and

the precision etching control is unnecessary, in contrast to the type of micromachined optical modulator reported in Transducers'99 [10]. The con®guration of the modulator and a simulation of the optical modulation are described. 2. Device design

2.1. Operational principles

A periodic shape designed to variably transmit or re¯ect light is called a diffraction grating. Patterns differ according to the wavelength, incident angle, depth and period of the grating. The concept can be described by the grating Eq. (1), which is established by considering the path difference for the optical waves. For light incident at an angle, yi, and

diffracted at angle, ym, the relation can be expressed as

sin ym sin yiˆml_a ; m ˆ 0; 1; 2; . . . (1)

where m is the order number, l the wavelength of the incident light, and a the period of the grating.

The intensity of different orders is also determined by Eq. (1). Therefore, the micromachined optical modulator proposed here simply alters the period of the gratings to modulate the incident light. The modulator with actuation as illustrated in Fig. 1 moves the position of the gratings. Consequently, the optical intensity is redistributed among the orders as the period of the grating changes. Furthermore,

*_{Corresponding author. Tel.: ‡886-2-29610145x331;}

fax: ‡886-2-29543648.

E-mail addresses: [email protected], [email protected] (C.-L. Dai).

0924-4247/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 4 5 - 2

in some particular orders, light is fully turned on or off and thus, the device can also serve as an optical switch. 2.2. Requirements and characteristics

The micromachined optical modulator must satisfy sev-eral performance and structural requirements. First, it should ef®ciently modulate the optical intensity, according to the shape of the gratings. Computer software is used to assess the intensity performance and the design is, thus, modi®ed to obtain a higher ef®ciency.

The second requirement is the stiffness. The structure must have a high z-directional stiffness to avert unwanted vibra-tions, and low in-plane stiffness to require a lower driving voltage. This work adopts a laminated high-aspect-ratio microstructure consisting of metal and oxide layers involving the CMOS process to satisfy these requirements [11].

Easy fabrication is also important. Considerable time can be saved in developing related technologies since the CMOS process is already a mature technology and foundry services for it already exist. The post-process proposed herein requires only maskless etching to release the microstructure, facilitating actualization of the full device.

Integration is further required. The optical modulator can be integrated with operating circuits in monolithic chips since the proposed design corresponds to the CMOS process. This feature not only reduces to induce the noise of interface, but also simply constructs the ®nal package of chips without the need for wire bonding.

The ®nal requirement is low cost. As mentioned above, a relatively short development time effectively responds to ¯uctuating market demands. A monolithic chip to achieve a high performance, a batch processing and a lower manu-facturing cost.

2.3. Diffraction efficiency

The micromachined optical modulator, consisting of slid-ing and stationary gratslid-ings, allows the period of the gratslid-ings

to be changed by electrostatic actuation to modulate the incident light. Different diffraction patterns of re¯ected light are observed for the same incident light. The diffraction pattern and ef®ciency can be theoretically determined using the approximate physical model of the idealized pitch-varying grating. Suppose, the grating vector is in the x-direction, and the optical axis is in the z-x-direction, then some functions and operators can be de®ned [12]:

comb…sx† ˆ X‡1

nˆ 1

d…sx n† ˆ1_s X‡1

nˆ 1

d x n_s (2) where n is an integer, s a scale constant, and d(x) the delta function. rect…x† ˆ 1; as 12 x 12 0; otherwise  (3) sinc…x† ˆsin…px†_px (4)

Consider the diffracted ®eld distribution near the grating. The complete grating consists of a stationary and a sliding grating. If a, b and d are the physical parameters of the pitch-varying grating shown in Fig. 2, then the ®eld distribution on a speci®c plane can be simply described by,

W…x† ˆ1_acomb _ax  rect x_b ‡1_acomb x ‡ …a=2† ‡ d_a

 

 rect _bx (5) where the asterisk () stands for ``convolution''. From physical optics, the far field diffraction pattern of the above field distribution is just its ``Fourier transform'':

W…x†  F…fx† /  comb _1=afx   sinc _1=bfx   comb _1=afx    sinc _1=bfx   e i2p a=2…… †‡d†fx  (6) where  stands for ``Fourier transform'' and fxis the spatial

frequency projected on the x-axis. The grating's special

property of splitting the incident light beam into specific directions is easily understood by considering the character-istics of the comb function. The variation of the diffraction efficiency with the displacement is given by Eq. (6) because parameter a represents the grating pitch and d the displace-ment between the stationary and sliding gratings. For exam-ple, if fxˆ 1=a and d ˆ 0 initially, then the phase shift term

2p‰…a=2† ‡ dŠfxˆ p, and the diffraction efficiency is zero.

All the odd orders actually disappear when the displacement is zero, corresponding to the grating with double spatial frequency. Furthermore, since the intensity of the diffracted rays is proportional to jF…fx†j2, substituting in Eq. (6) yields,

sinc2 fx 1=b   1 ‡ cos 2pa₂‡ dfx h i n o ; fxˆm_a; where m is an integer: (7)

The diffraction ef®ciency for every order is proportional to Eq. (7). According to the speci®c dimensions of the grat-ing in the modulator, substitutgrat-ing a ˆ 12:2 mm, b ˆ 2:5 mm, d ˆ 0 (initial position, non-shifted) and d ˆ 1:2 mm (shifted) into Eq. (7), yields the theoretical diffraction ef®ciency of the modulator as illustrated in Fig. 3. The derivation shows that 100% modulation ef®ciency is achiev-able for the odd orders. Hence, the micromachined optical

modulator can serve as an optical switch in a direction corresponding to the odd orders.

3. Fabrication

The proposed design follows the 0.6 mm single poly and triple metals (SPTM) CMOS process. The micromachined optical modulator is developed according to the foundry service and design rules. All post-process procedures require only maskless dry etching and thus avoid the sticking problem.

Fig. 2. The model of the gratings and its physical parameters: (a) the initial position of each gratings; (b) displacement for the movable part.

SF6RIE to release the whole microstructure as illustrated in

Fig. 4(c). Fig. 5 shows a scanning electron micrograph (SEM) of the laminated-suspension microstructures.

4. Experimental results

The structure of the micromachined optical modulator enhances the stiffness, which is proportional to the square of the aspect ratio. A 5:1 aspect ratio laminated structure, which theoretically enables a 25:1 ¯exure stiffness ratio, is shown in Fig. 6. Constructing the optical modulator from the high-aspect-ratio laminated structure can eliminate the gradient deformation caused by a lower z-directional stiffness.

The ®xed gratings connect to large anchors, in contrast, the free gratings are connected to a small mass with etching holes. The design ensures not only that the movable part is fully released, but also that the ®xed part remains attached to the substrate. The bending effect in the micromachined optical modulator could pose a serious problem to the optical ®eld, which requires an accuracy of less than a half-wave-length. Therefore, the gradient deformation and stiffness may limit the working range. Furthermore, metal is depos-ited at a high temperature, implying the occurrence of residual stress at room temperature. After the sacri®cial layer is removed, the residual strains occur in the structure. Modulator with active areas of 100 mm  100 mm is tested; they exhibit in z-directional maximum deformation of about 1 mm, respectively. This phenomenon causes an additional voltage demand for displacement in the vertical direction (z-direction).

Following the post-CMOS process, a voltage is applied to the actuator of the optical modulator. With electrostatic actuation, the suspended parts slide to the side of the ®xed parts to modulate the light. The actuator with 20 V moves it by 1.2 mm. Therefore, the comb structure for the actuator is successfully used in this study.

Fig. 7(a) and (b) illustrate the optical measurement devices for the modulator. The measurement involves a trade-off of two factors. First, the area of the grating is very small. Increasing the signal-to-noise ratio requires a light beam in the order of tens of microns in width, reducing the depth of focus and causing an earlier divergence of the diffracted light beam. However, the grating pitch is almost

Fig. 5. SEM of the micromachined optical modulator.

10 times the wavelength, so that the difference in the diffracted angle for the neighboring orders is only about several degrees. The most important trick is to control the effective numerical aperture of the focusing lens to prevent the light diverging too seriously. After adjusting controlling the numeric aperture value to optimize the diffraction pattern shown in Fig. 7(c), a power meter can be used to measure the intensity of the separate orders and then compute their diffraction ef®ciencies.

Fig. 8 presents the experimentally obtained, normalized diffraction ef®ciency of the pitch-varying grating. The

experimental results concerning the fourth and higher orders of the normalized diffraction ef®ciency apparently deviate from the theoretical values shown in Fig. 3 owing to the residual strains in the modulator after the post-process is completed. However, the experimental results concerning the normalized diffraction ef®ciency under the fourth order, match closely the theoretical values. The results support the feasibility of the design.

5. Conclusion

This study has proposed a micromachined optical mod-ulator with small volume, quick response and fully compa-tible with the CMOS process. The modulation functions with and without actuation were simulated. Maskless dry etching was the only requirement of the post-process, and only a low voltage of 20 V was necessary to drive the actuator of the optical modulator. A high-aspect-ratio micro-structure was developed to obtain high stiffness in the z-direction. Beam widths as small as the minimum allowable metallization line-width given by the CMOS foundry service were generated. The experimental results concerning the normalized diffraction ef®ciency under the fourth order match closely the theoretical values. The results prove the feasibility of the design.

Acknowledgements

This work is accomplished with much needed support, and the authors wish to thank the following people: Jennyi Chen, Chiyuan Lee, Hunghsuan Lin, Fuyuan Xiao, Tsungwei Huang, Shihchen Chang and Chunyuan Chi of the Institute of Applied Mechanics, Nation Taiwan University, for their valuable advise and assistance in experiment. References

[1] J. Bryzek, Impact of MEMS technology on sociality, Sens. Actuators A 56 (1996) 1±9.

Fig. 7. (a) Schematic optical measurement devices for the modulator; (b) photograph of the actual optical measurement devices and; (c) diffraction pattern from gratings of the modulator.

approach to optical interconnect, IEEE (1995) 780±785.

[8] D.M. Burns, V.M. Bright, Micromachined thermally based CMOS microsensors, MEMS 97 (1997) 55±60.

[9] J. Bryzek, Development of microelectromechanical variable blaze gratings, Sens. Actuators A 64 (1998) 7±15.

[10] H. Chen, C. Chang, J. Chen, J. Chio, K. Yen, F. Xiao, et al., Fabrication of a micromachined optical modulator using the CMOS process, Transducers 99 (1999) 808±811.

[11] G.K. Fedder, S. Santhanam, M.L. Reed, S.C. Eagle, D.F. Guillou, M.S.-C. Lu, et al., Laminated high-aspect-ratio microstuctures in a conventional CMOS process, Sens. Actuators A 57 (1996) 103±110. [12] J.W. Goodman, Introduction to Fourier Optics, McGraw-Hill, San

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Biographies

Ching-Liang Dai was born in Chia-Yi, Taiwan, ROC, in 1965. He received the MS degree in the applied mechanics from National Taiwan University, Taiwan, in 1993, and the PhD degree in mechanical engineering from National Taiwan University, in 1997. His PhD dissertation was about the

of Applied Mechanics, National Taiwan University, Taiwan. His PhD dissertation is about the mechanical and optical properties of subwave-length grating and its applications. His research interests are optomecha-nical system design and nanostructure analysis.

Chun-Hui Lin was born in Taipei, Taiwan, ROC, in 1976. He received the dual BS degrees both in mechanical engineering and civil engineering from the National Taiwan University, Taiwan, in 1999. Now, he is a graduated student in the Institute of Applied Mechanics of National Taiwan University. His research interests are in the area of optomechanical and optoeletronics system design.

Pei-Zen Chang was born in Chia-Yi, Taiwan, ROC, in 1962. He received the BS degree in civil engineering from National Taiwan University, Taipei, Taiwan, in 1984, and the PhD degree in theoretical and applied mechanics from Cornell University, Ithaca, NY, in 1991. His PhD dissertation was about the mechanics of superconducting magnetic bearings. He joined as a faculty of Institute of Applied Mechanics, National Taiwan University in 1991 and became a Professor in 1999. His current research interests are in the area of micromachined sensors and actuators.