broadband form birefringent quarter-wave plate for the mid-infrared wavelength region

Broadband form birefringent quarter-wave plate for the mid-infrared wavelength region

Gregory P. Nordin and Panfilo C. Deguzman

The University of Alabama in HuntsvilleElectrical & Computer Engineering Department, Huntsville, Alabama 35899

[email protected]

Abstract:

We discuss the design, fabrication and optical performance of abroadband form-birefringent quarter-wave plate for the 3.5 to 5 µm wave-length region. Rigorous coupled wave analysis (RCWA) was used to designthe requisite subwavelength grating for silicon substrates in ambient air. Fab-ricated samples yield a measured phase retardation of 89° to 102° over thedesired wavelength range.

© 1999 Optical Society of America

OCIS codes:

(050.1950) Diffraction gratings, (260.1440) Birefringence, (260.5430) Polariza-tion, (260.3060) Infrared, (230.5440) Polarization-sensitive devices, (230.3990) Microstructuredevices

References and links

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42

, 492-49 (1983).2. F. Xu, R. C. Tyan, P. C. Sun, and Y. Fainman, “Fabrication, modeling, and characterization of form-

birefringent nanostructures,” Opt. Lett.

24

, 2457-2459 (1995).3. T. J. Kim, G. Campbell, and R. K. Kostuk, “Volume holographic phase-retardation elements,” Opt. Lett.

20

, 2030-2032 (1995).4. R. C. Tyan, P. C. Sun, A. Scherer, and Y. Fainman, “Polarizing beam splitter based on the anisotropic

spectral reflectivity characteristic of form-birefringent multilayer gratings”, Opt. Lett.

21

, 82-89 (1996).5. A. G. Lopez and H. G. Craighead, “Wave-plate polarizing beam splitter based on a form-birefringent

multilayer grating,” Opt. Lett.

23

, 1627-1629 (1998).6. S. Y. Chou, S. J. Schablitsky, and L. Zhuang, “Application of amorphous silicon gratings in polarization

switching vertical-cavity surface-emitting lasers,” J. Vac. Sci. Technol. B

15

, 2864-2867 (1997).7. H. Kikuta, Y. Ohira, and K. Iwata, “Achromatic quarter-waveplates using the dispersion of form

birefringence,” Appl. Opt.

36

, 1566-1572 (1997).8. D. B. Chenault and R. A. Chipman, “Infrared birefringence spectra for cadmium sulfide and cadmium

selenide”, Appl. Opt.

32

, 4223-4227 (1993).9. William L. Wolfe and George J. Zissis, ed.,

Infrared Handbook

(Environmental Research Institute of Michigan, Ann Arbor, Michigan, 1985), pp. 7-76.

10.S. Grigoropoulos, E. Gogolides, A. D. Tserepi, and A. G. Nassiopoulos, “Highly anisotropic silicon reactive ion etching for nanofabrication using mixtures of SF

6

/CHF

3

gases,” J. Vac. Sci. Technol. B

15

, 640-645 (1997).

1. Introduction

Subwavelength gratings are attractive for compact implementations of polarization-sensitivedevices such as wave plates [1-3] and polarizing beam splitters [4,5]. In particular, wave platesfor single wavelength operation have been previously fabricated as surface relief structures insilicon nitride [1], GaAs [2,4], and amorphous silicon [6]. Recently, Kikuta

et al

. [7] proposeda method of creating achromatic form birefringent wave plates by compensating for the usual1/

λ

dependence of the phase retardation with the strong dispersion exhibited by form birefrin-gence when the grating period is on the order of the optical wavelength. For example, in one of

(C) 1999 OSA 11 October 1999 / Vol. 5, No. 8 / OPTICS EXPRESS 163#13698 - $15.00 US Received August 02, 1999; Revised September 29, 1999

their design studies a retardation error of only 3° is predicted for a ±10% change in wavelength[7]. In this paper, we report the first physical implementation of a broadband form birefringentwave plate that makes use of form birefringence-based dispersion. In this case we havedesigned, fabricated, and tested a quarter-wave plate for the 3.5 to 5 µm wavelength region.The resultant device represents a potentially attractive alternative to the only other mid-infraredachromatic wave plate currently available, which involves the use of two uniaxial materials [8].

2. Design

In designing a broadband form birefringent quarter-wave plate, we used rigorous cou-pled wave analysis (RCWA) to examine the effects of several parameters on the phase retarda-tion and transmission characteristics of a subwavelength grating. As illustrated in Fig. 1, these

parameters include the grating period,

Λ

, fill factor, F=a/

Λ

, and thickness, t. The substrate issilicon with air as the ambient medium. Normally incident illumination is assumed, and TE(TM) polarization is defined as the electric field parallel (perpendicular) to the grating ridges.The refractive index of Si as found in Ref. 9 was used in the RCWA calculations reported inthis paper. All simulations included dispersion.

Based on our original application, we desired to achieve a phase retardation of 0.5

π

±0.1

π

over the 3.5 to 5 µm wavelength range. By systematically varying the values of the grat-ing period, fill factor, and thickness, we found that a grating period of 1 µm, a fill factor of 66%(i.e., trench width of 340 nm), and an etch depth of 1.25 µm yield a satisfactory dispersion inthe effective refractive index with which to compensate the 1/

λ

dependence of the phase retar-dation. We then determined the sensitivity of the retardation to small fabrication errors in thegrating fill factor and thickness. For example, as shown in Fig. 2, only a narrow range of fillfactors (64-68%, which corresponds to a trench width tolerance of +/- 20 nm) satisfies thedesired phase retardation criteria while the etch depth tolerance is less severe. Clearly, tightcontrol of the fill factor is essential for the successful fabrication of a broadband structure.

3. Fabrication

Our fabrication process began with 75 mm diameter p-type <100> silicon wafers witha resistivity of 1-20 ohm-cm. Photolithography was done with a contact mask aligner (made byAB-Manufacturing) that had a mercury arc lamp source. We used a 5" x 5" dark field photo-mask with a grating region that consisted of 500 nm wide chrome lines and spaces within a 1.3cm x 1.3 cm area in the center of the mask. Wafers were prepared for photolithography by firstspin coating an aqueous-based adhesion promoter (Surpass 1000 from DisChem, Inc.) fol-

Fig. 1. Schematic diagram of a form birefringent wave plate and a normally incident beam showing TE and TM polarization definitions.


lowed by a 500 nm thick layer of Shipley 1805 photoresist. Samples were then softbaked at115°C for one minute on a vacuum hotplate.

Vacuum contact printing of small feature sizes, such as the 500 nm lines and spaces ofthe photomask used here, poses significant challenges. We have, however, been able to achieveuniform and well-defined grating patterns in photoresist over nearly the whole 1.3 cm x 1.3 cmgrating area defined on our mask. A critical factor is turning off the substrate vacuum prior toexposure, which allows better vacuum contact between the mask and the wafer. Furthermore,by carefully controlling the exposure, we are able to tightly control the photoresist fill factorwhich is a critical precursor to achieving the desired silicon grating fill factor. For example, asshown in Fig. 3(a), we found that an exposure of 3.3 seconds (for measured intensities at 365nm and 405 nm of 16 mW/cm

2

and 9.9 mW/cm

2

, respectively) and a 1 minute developmenttime in Microposit 352 developer resulted in a grating pattern with an approximately 30% pho-toresist fill factor.

After photoresist patterning, a lift-off method was used to create a chrome etch maskon the silicon substrate. To assure good adhesion between the chrome and the substrate, a short

(a)

(b)

Fig. 2. Phase retardation as a function of wavelength for 1.0 µm period gratings parameterized by (a) fill factor (for a thickness of 1.25 µm) and (b) thickness (for a fill factor of 66%).

1.0

0.8

0.6

0.4

0.2

0.0

(Pha

se R

etar

datio

n)/π

5.04.84.64.44.24.03.83.6

Wavelength (µm)

Thickness 1.20 µm 1.25 µm 1.30 µm

1.0

0.8

0.6

0.4

0.2

0.0

(Pha

se R

etar

datio

n)/π

5.04.84.64.44.24.03.83.6

Wavelength (µm)

Fill Factor 64% 66% 68%


oxygen plasma etch is employed to remove any residual photoresist present after development.A 250 nm thick chrome layer is deposited over the patterned photoresist, following which thewafer is immersed in acetone and gently wiped with a foam-tip applicator to aid in achievinglift-off. Typical results are shown in Fig. 3(b).

The grating pattern is transferred into the silicon substrate by reactive ion etching in aPlasma-Therm 790 system with 9.5" diameter electrodes. We used a graphite electrode coverplate. CHF

3

and SF

6

were used to achieve an anisotropic etch based on polymer sidewall passi-vation [10]. The process parameters include gas flow rates of 34 and 6 sccm for the CHF

3

andSF

6

, an RF power of 125W (0.27W/cm

2

), and a pressure of 10 mTorr. This results in a Si etchrate of 16 nm/min. A typical grating cross section is shown in Fig. 4. The etch depth is approx-imately 1.23 µm and the sidewalls are slightly sloped. There is also some evidence of a smallamount of redeposited silicon on the upper sidewalls.

Fig. 3. (a) Scanning electron microscope (SEM) cross section image of a photoresist grating on Si. (b) SEM top view image of Cr etch mask (on Si) with a Cr fill factor of ~70%.

(a) (b)

Fig. 4. SEM cross section image of etched Si grating.


4. Results

The fabricated subwavelength grating was optically tested using an FTIR-based spectropola-rimeter. The phase retardation and the transmission coefficients of the TE and TM modes weremeasured at normal incidence. A comparison between these measurements and RCWA simula-tions of the etched grating profile, which was approximated by a 6-layer binary grating stack, isshown in Fig. 5. The grating stack parameters used in the simulation are listed in Table 1. Asillustrated in Fig. 5(a), the measured phase retardation of the quarter-wave plate over the 3.5 to

Fig. 5. (a) Measured and simulated phase retardation as a function of wavelength and (b) the corresponding TE and TM transmission coefficients.

(a)

(b)

1.0

0.8

0.6

0.4

0.2

0.0

TE

& T

M T

rans

mis

sion

5.04.84.64.44.24.03.83.6

Wavelength (µm)

RCWA Measured TTM TTM TTE TTE

1.0

0.8

0.6

0.4

0.2

0.0

(Pha

se R

etar

datio

n)/π

5.04.84.64.44.24.03.83.6

Wavelength (µm)

Measured RCWA Simulation


5 µm wavelength range varies from 0.49

π

to 0.57

π

(89° to 102°), and the RCWA simulationresult compares well with experimental measurement. Note that the measured phase retarda-tion is flatter over the left-hand third of the retardation curve than the simulation results shownin Fig. 2(a) would suggest. Our simulation experience indicates that this beneficial effect islikely due to the slightly sloped sidewalls of the etched grating profile.

The transmission measurements and corresponding RCWA results are shown in Fig.5(b). Since the backside of the silicon wafer was not anti-reflection coated, the RCWA trans-mission values are adjusted to include a 30% Fresnel loss at the backside of the wafer to facili-tate direct comparison with measurement. The measured transmission coefficients of the TEand TM modes vary between 52% and 60%. In comparison with RCWA simulation, one cansee that RCWA agrees qualitatively with the measured values, although there is some quantita-tive difference at shorter wavelengths.

5. Summary

We have designed, fabricated, and measured the optical performance of a subwavelength grat-ing for use as a broadband quarter-wave plate over the 3.5 to 5 µm wavelength region. Disper-sion of the effective refractive indices when the grating period is on the order of the wavelengthis clearly an effective method of compensating for the usual 1/

λ

dependence of the phase retar-dation as long as the grating parameters are carefully controlled in the fabrication process.Such grating structures offer an attractive and flexible method of implementing broadbandwave plates in the infrared.

Acknowledgements

G. P. Nordin acknowledges support by National Science Foundation CAREER Award ECS-9625040 and grant EPS-9720653. P. Deguzman acknowledges support by a National ScienceFoundation Traineeship (grant GER-9553475). The authors also wish to thank Lynn Deiblerand Dr. Matthew H. Smith for performing the spectropolarimeter measurements.

Table 1: RCWA binary grating layer parameters. The layers are numbered from top to bottom.

LayerThickness

(µm)Width (µm) Layer

Thickness (µm)

Width (µm)

1 0.11 0.55 4 0.22 0.60

2 0.24 0.63 5 0.27 0.62

3 0.10 0.62 6 0.28 0.67


broadband form birefringent quarter-wave plate for the mid-infrared wavelength region

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