Summary…

To analyze the optical properties with etch-holes array, the simply microreflectors are qualified. If the reflectivity of poly-Si is not high enough to measure, we can coat aluminum over the devices after all the processes. However, the reflectivity, transmissivity, surface roughness, or other factors which effect optical properties can be neglected in this thesis. The 45^o-fixed microreflector is demonstrated, and the 90^o-fixed micro polarization beam splitter (PBS) with Si3N4 thin film is further developed by this advanced process.

Chapter 5

Experimental Results

5.1 Introduction

According to the simulation and fabrication presented in the previous chapters, the experimental results will be shown in this chapter. The measurement system used to evaluate the irradiance distributions will be described first. Then the fabricated pop-up devices will be shown. After that, the measurement of the diffraction fields will be performed to make sure if the fabricated etch-holes array match the design goal.

5.2 Measurement System

The configuration used for measuring optical properties of the microreflectors is depicted. The experimental setup is shown schematically in Fig. 5.1. A He­Ne laser (λ

= 633 nm) is directed toward the perforated film after passing through a variable attenuator, a spatial filter, and a collimating lens to emerge as a collimated beam. The beam uniformly fills the entrance pupil of the telescope system and scale the diameter of beam size down to 200 µm. The sample is mounted on a plate riding on a translation stage. An etch­holes array acts as a 2D diffraction grating that diffracts the laser beam into several diffraction orders. The diffraction pattern is captured and analyzed by means of a CCD camera and a personal computer, respectively.

Fig. 5.1 The measurement systems

5.3 Fabrication Results

The fabrication results of the 45^o-fixed micromirror shows in Fig. 5.3 (a)-(c). A micro polarization beam splitter (PBS) with single Si3N4 thin film is developed in the same self-assembly process, and shown in Fig. 5.2. Because of Babinet’s principle, the diffraction fields reflected or transmitted from the PBS with etch-holes array have the same results, which are in agreement in the mocroreflector model we calculated before.

Fig. 5.2 A micro PBS with single Si3N4 thin film

(a)

(b) (c)

Fig. 5.3 SEM photographs of (a) Top view of two micromirrors, (b) Side view of a micromirror, and (c) Zoom in on a micromirror

5.4 Measurement Results

To verify the relationship between diffraction and etch­holes layout, we develop a 2D micromirror through MUMPS (Multi­User MEMS Processes). The micromirror contains various etch­holes layouts compatible with MUMPS deign rules. Since we aim to study the dependence of diffraction behavior on various etch­holes array, the MUMPS mirror is not released from the substrate.

The measured irradiance distributions of various etch­holes features are shown.

Fig. 5.4 shows the measured diffraction image of etch-holes array with regular spacing. Fig. 5.5 (a) (b) show the images compared with the calculated results for the array in random transition and additional rotation with random angles, respectively.

The measured results make a close agreement with the previous simulation and qualitatively confirm our theoretical analysis based on Fraunhofer diffraction theory.

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Fig. 5.4 The measured image of etch-holes array with regular spacing compared with its calculated result

x y

x y x

y

(a)

x y

x y x

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(b)

Fig. 5.5 The images compared with their calculated results for(a) the array in random transition and (b) additional rotation with random angles.

Because of the reactive ion etching (RIE), the form of the square hole in geometry is close to octangle. Fig. 5.6 shows the close-up of the etch holes. Such octangle holes cause the envelope of the diffraction pattern in Fig. 5.4 looks close to octangle in geometry. Besides, the dimples by the side of etch holes in Fig. 5.6 are necessary in our MUMPs-like process. They cause some phase shifts of the wavefront, but they are too shallow to cause no effects. In this thesis, we ignore the effects caused by dimples.

Dimples RIE

Octangle Square

Fig. 5.6 the close-up of the etch holes on a devices

Fig. 5.7 shows the measured results of the concentric array. The high diffraction beams can be certainly reduced. The concentric layouts can be used as a beam shaper for centralizing the beam.

5.5 Summary

The self-assembly devices, microreflector and micro PBS, are demonstrated. The microreflectors with different etch-holes layouts are measured to verify the calculated results. In fact, the measured results show the fabricated etch-holes array match our purpose.

Fig. 5.7 The images of the two cases used the concentric array compared with the regular case.

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∆θ

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Chapter 6

Conclusions

6.1 Optical Analyses

To summarize, optical MEMS typically require etch holes to reduce the time required to release the micromechanical structure during the sacrificial undercutting.

However, the size and density of the etch-holes array has a strong effect on the diffraction patterns, which plays a key role in most optical characteristics. In this thesis, we examined the dependence of diffracted pattern on the etch-holes configuration based on an analytical Fourier study. The diffracted irradiance caused by etch-holes array is contributed by two factors, namely, array factor and shape factor. Array factor determined by the period of the etch holes can cause high-order diffraction beams, which can be averaged out by use of random translation layout. On the other hand, the shape factor determined by the form factor of each element provides the envelope of the overall diffraction pattern. The envelope can be centrally symmetrized by the randomly rotation of each element. These diffraction fields are similar to the light through single circle and square aperture. The analysis is suitable on all transmission or reflection plate-type micromachined devices.

Otherwise, concentric etch-holes array has been proposed without varying the arrangement density and the well-etching interval between holes. The diffracted high order beams are reduced by destructive interference between 0th and m-th ring. The diffraction efficiency is at least 50% compared with 80% in traditional case. Moreover, the diffraction efficiency can be reduced as 18% by rotated the 4^th ring with θ4/2. To

summarize the analyses, Table-6.1 lists the optical qualities of several etch-holes layouts.

Table 6.1 the compare of different etch-holes arrangement Arrangement

Type Impulse Response

Diffraction

Field is formed by sinc functions with sinc-type beams are reduced by destructive interference.

6.2 Micro Fabrication

In experiment, we successfully produced the devices, microreflectors and micro PBS, with designed etch-holes layouts to verify the calculation results by self-assembly technology. Fig. 6.1 shows the pictures of three kinds of etch-holes layouts captured from the devices. Optical experiments quantitatively confirmed our analyses. The proposed design of random distributed etch-holes array is expected to provide a useful consideration for future surface micromachining design rule.

20 μ m 20 μ m

Random Array Circular Array Regular Array

Fig. 6.1 Pictures of three kinds of etch-holes layouts captured from the devices

6.3 Future Works

According to our experimental results, the optical properties of the micro devices produced by the self-assembly technology are analyzed. However, the discussions in the thesis are all analyzed by Fraunhofer approximation. There are many interesting effects, e.g. Telbot effect, in the Fresnel region. Therefore, the diffraction caused by etch-holes array in Fresnel approximation will be discussed. Moreover, we will combine the self-assembly devices with movable mechanisms. For example, an adjustable polarization beam splitter with micro actuator will be demonstrated.

Reference

[1] Peter F. Van Kessel, Larry J. Hornbeck, Robert E. Meier, and Michael R.

Douglass, “A MEMS-Based Projection Display,” Proceedings of the IEEE, vol.

86, no. 8, August 1998.

[2] Paul M. Hagelin, Student, and Olav Solgaard, “Optical Raster-Scanning Displays Based on Surface-Micromachined Polysilicon Mirrors,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 5, no. 1, January/February 1999.

[3] R. Apte, F. Sandejas, W. Banyai and D. Bloom, " Grating Light Valves for High Resolution Displays,"Solid State Sensors and Actuators Workshop, June 1994.

[4] M. C. Wu, L. Y. Lin, S. S. Lee, and K. S. J. Pister, “Micromachined free-space integrated micro-optics,” Sens. Actuators: A (Physical), Vol. 50, pp. 127-134, 1995.

[5] R. T. Howe and R. S. Muller, “Polycrystalline silicon micromechanical beams,”

J. Electrochem. Soc., Vol. 130, pp. 1420–1423, 1983.

[6] MEMS Technology Applications Center, Microelectronics Center at North Carolina (MCNC), Research Triangle Park, NC, and Integrated Micro Electro Mechanical Systems, offered at Analog Devices, Cambridge, MA.

[7] D. Koester, A. Cowen, R. Mahadevan, and B. Hardy, PolyMUMPs Design Handbook Revision 10.0.

[8] W. N. Sharpe, Jr., R. Vaidyanathan, B. Yuan, G. Bao and R. L. Edwards, “Effect of etch holes on the mechanical properties of polysilicon,” J. Vac. Sci. Technol.

Vol. B 15, pp. 1599-1603, 1997.

[9] X. Fang, N. Myung, K. Nobe, and J. W. Judy, “Modeling the effect of etch holes on Ferromagnetic MEMS,” IEEE Tran. on Magnet. Vol. 37, pp. 2637-2639,

2001.

[10] J. Zou, M. Balberg, C. Byrne, C. Liu, and D. J. Brady, “Optical properties of surface micromachined mirrors with etch holes,” IEEE J. Microelectromech.

Syst., Vol. 8, pp. 506-513, 1999.

[11] J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill, Singapore), pp.

35-36.

[12] Eugene Hecht, Optics (Addison Wesley, San Francisco), pp. 508-509.

[13] K. Iizuka, Elements of Photonics (Wiley-Interscience, New York, USA), pp.

32-40.

[14] W. H. Beyer, CRC Standard Mathematical Tables, 28^th ed., (CRC Press, FL., 1987).

[15] J. A. Walraven, “Failure mechanisms in MEMS,” IEEE ITC International Test Conference Paper 33.1, pp. 828-833, 2003.