Binary optics: A VLSI-based microoptics technology

MICROELECTRONIC ENGINEERING

ELSEVIER

Microelectronic Engineering 32 (1996) 369-388

Binary Optics: A VLSI-based microoptics technology 11argaret B. Stern Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood St., Lexington, MA 02173-9108, USA

Abstract

Binary optics technology applies VLSI and ULSI processing techniques to the field of microoptics to enable the fabrication of unique wavefront engineering devices from x-ray to IR wavelengths. The nested character of these stepped diffractive surface relief structures, coupled with submicron linewidths and precise etch depths, puts stringent demands on process tolerances. We summarize our efforts to produce high-optical-quality diffractive and refractive microoptics with this technology. We also describe novel techniques developed to fabricate the deep structures needed to form high-aspect-ratio gratings and analog refractive lenslet arrays, including thick resist processing and deep anisotropic etching. Keywords: Binary optics; Vl.Sl-based microoptics

1. Introduction

Unique applications for diffractive optics cover the entire light spectrum: from x-ray zone plates and UVaberration correctors; to optics for flat panel displays, laser beam addition and beam steering; to wavefront multiplexers and IR focal plane arrays [1-6]. Surface relief structures, whose dimensions range from a fraction of to many multiples of the design wavelength, modulate the phase and amplitude of an incident optical wavefront. While a number of volume holographic and lithographic techniques have been used to fabricate these quasi continuous structures [7], binary optics technology is applicable over a wide range of design wavelengths and materials. Binary optics technology combines microelectronics and nanoelectronics processing techniques with optical CAD to translate an arbitrary optical design into a stepped surface relief profile in practically any arbitrary optical substrate. Its widespread implementation can be attributed primarily to its flexibility in choice of substrate material (e.g., fused silica, Si, Ge, GaAs, CdTe) and to the inherent reduction in process iterations, due to the binary coding of the phase topography [8], that are needed to achieve high diffraction efficiency. Although the work at MIT Lincoln Laboratory has focused on conventional optical photolithography techniques [1-6,8,9], other workers have utilized this binary coding scheme for both electron beam lithography and laser direct write techniques [10-12]. 0167-9317/96/$15.00 Copyright © 1996 Elsevier Science B.Y. All rights reserved SSDI: 0167-9317(95)00369-X

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This article focuses on the fundamental role of microfabrication and nanofabrication in translating optical designs into high-quality optics, concentrating on three main topics. First, we describe the basic tenets of the binary coded fabrication process and delineate the differences between conventional microlithography and optical fabrication. Next, we examine the effect of processing parameters on the optical efficiency of diffractive fused silica microlenses. Finally, we describe processes developed to form the very deep three-dimensional microstructures required for applications such as color discrimination and broadband (high bandwidth) optics. These include multilayer planarization/masking schemes, and highly anisotropic reactive ion etching (RIE) of deep structures in silicon and fused silica.

2. Binary optics technology Binary optics technology consists of five main elements: optical design of the phase profile; transformation of this design into a set of amplitude patterns (photomasks); substrate fabrication consisting of pattern replication into photoresist, high fidelity pattern transfer into or onto the substrate material to a precise depth, and accurate registration of subsequent mask levels to the patterned substrate; device characterization; and replication of the master, when applicable. To obtain high-quality optics, VLSI techniques must be customized to meet the specific and distinct concerns of microoptics fabrication. The evolution of binary optics components from relatively coarse macroscopic elements to arrayed microelements has placed increasingly stringent tolerances on process control of etch depths, linewidths and overlay registration, while novel processing techniques are required to fabricate deep structures for refractive microoptic arrays. To achieve the maximum optical efficiency in these nested multilevel diffractive structures, fabrication limits must be defined and fabrication-related errors must be avoided. Accurate measurements of the optical performance of micro optics must be conducted and compared with theoretical expectations. Finally, to reduce the cost of microoptic components, accurate replication of submicron features over large areas must be accomplished. In the optical design step the topographical profile of the phase, or blaze, required to perform an arbitrary optical function is defined numerically or algebraically. This continuous phase-only relief profile (see Fig. l(a)) is divided up modulo the design wavelength A, or 27T, and is known as a kinoform (see Fig. l(b) [13,14]. It has a scalar diffraction efficiency of 100% at A. The maximum phase height of a kinoform facet is d = A/(n -1), where n is the material index of refraction. While it is difficult to pattern a substrate with the exact continuous profile either holographically or lithographically, this is an area of active research [15-18]. The variable exposure dose delivered to the resist material by either direct write methods [15-17] or with greytone photomasks [18] must accommodate the nonlinear exposure and development properties of the resist. Moreover, the continuous resist structure must often be etched into the substrate surface. An alternative approach is to build an N-step approximation to the blaze by N sequential lithography steps, where each phase increment has height d = A/[N(n -1)] (see Fig. l(c») [19,20]. The increase in first-order diffraction efficiency with the number of phase steps is tabulated in Table 1 assuming scalar diffraction theory [8,19]. Practical optics require optical efficiencies of at least 90%. To achieve such high

M.B. Stern / Microelectronic Engineering 32 (1996) 369-388

371

n~ ~(a) II I

dM=d max

2M

I

I"

+-~ (e)

Fig. 1. (a) Continuous refractive lens, (b) Diffractive kinoform; (c) N-step approximation to the continuous blaze.

efficiencies requires a large number of processing steps - especially if each phase step is patterned separately, as was done by early researchers [19,20]. However, by auantizing the phase in a binary fashion so that N = 2M , each step now has height d = '\/[2 (n - 1)], and efficient fabrication can be achieved with a greatly reduced number of process steps [8]. Only M process iterations are needed to create N = 2 M phase steps. The name binary optics derives in part from this binary coding of the mask layers and process steps. The topographical cross section associated with each quantized level is encoded into a format compatible with either photomask generation (e.g., anSII or CIF for MEBES machines) or direct write machines. The end result is a translation into a binary amplitude or intensity code (Cr/no Cr on a photomask; beam on/beam off or modulated in a direct write technique). The mask set provides the template for the subsequent substrate fabrication, shown schematically in Fig. 2 [8,20]. In our laboratory, Cr hardmasks are currently replicated by optical photolithography into a thin layer of positive photoresist, using exposure systems at 365 nm (step and repeat, vacuum contact, and full-field scanner), at 248 nm (step and repeat), and at 193 nm (step and scan). The mask with the smallest features is printed first to maximize linewidth control, although the reverse order is also possible. The relief structures can be transferred to the substrate by a variety of methods, including etching, deposition, and ion- or photon-induced changes of the refractive index; however, dry etching techniques offer the greatest amount of flexibility, and are the methods we use most frequently. The processing steps are iterated to achieve the requisite number of phase levels. Subsequent mask layers are Table 1 Scalar diffraction efficiency vs number of phase steps 7]1

= sinc

2(1I2 M

)

No. of phase levels

No. of masks

o 41% 81%

95% 99% 100%

2

4 8

16 2

M

1 2 3 4 M

372

M.B. Stem I Microelectronic Engineering 32 (1996) 369-388 MASK

2 PHASE LEVELS

PATIERN PR ETCH

ALIGN MASK

4 PHASE LEVE LS

PATIERN PR ETCH

8 PHASE LEVELS ALIGN MASK PATIERN PR ETCH

Fig. 2. Fabrication steps for an eight-phase-level binary optics microlens. The binary coding scheme doubles the number of phase steps after each etch.

carefully aligned to the patterned surface. Four-phase-level elements require one photomask alignment; eight phase levels require two alignments, and so on.

2.1. Fresnel phase zoneplate microlens As an example we consider the fabrication of Fresnel phase zoneplate microlenses. Large arrays of diffractive microlenses of arbitrary size, shape, and format can be fabricated by exploiting the flexibility inherent to VLSI processing. Close packed arrays of hexagonal or rectangular lenses can be fabricated with 100% fill factor. Diffraction limited aspheric lenses or astigmatic anamorphic lenslets can be readily designed [4]. The surface relief profile for diffractive lenses is dictated by the design wavelength, the ~-number, and the optical properties of the substrate [8,19,20]. For an aspheric lenslet, the k zone radii rk satisfy the relation (r~ + f2)112 - f = kJl./ N. The chirped zonewidths resemble a slowly varying grating, and can range from 0.5 j.Lm to several hundred micrometers on a single mask layer. For Fresnel-phase lenses, the precision with which we can realize the smallest zone annulus Ar is determined by the $i-number, A, and quantization level M: f!.r """ 2Ao(8if-number)/2 M ; equivalently, f!.r determines the smallest 8if-number (highest numerical aperture) of the lens. Diffractive elements and large arrays of diffractive elements are inherently more difficult to fabricate as the local grating period decreases (small 8if-number and short wavelengths), as seen in Table 2. The minimum zonewidth, or critical dimension (CD), currently available for ring structures from commercial photomask houses is =0.5 urn [21]. The etched depths that correspond to the A/[2 M(n - 1)] phase shifts must be within 5% of the optical design to realize near optimal performance, while the overlay tolerance should be held to 10% of the minimum feature size. An eight-phase-level, ~/ 4.5 microlens at A = 633 nm, with a 0.5 urn CD, has an overlay requirement of 50 nm and a minimum phase step height of 175 nm in Si0 2 •

M.B . Stern / Microelectronic Engineering 32 (1996) 369-388

373

Table 2 Phase zoneplate lens fabrication design parameters • Zonewidths: !ir =

fA

-~-{- ; 0.5 ~ Ar ~ 2 rm

• Minimum zone width (!ir m in o Phase step height: elM

20 J1-m

= (2M!f/#) /2 M ) A

= 2 M(n-

I.)

Fabrication parameters for fiji/ 3 lenses at A == 632.8 nm in Si0 2 , M == 3 M • Minimum zone width flr m in. == (2M!!/#)/2 ) = 0.47 J1-m A

• Etch-depth control: ±5 % elM= 2M(n -1) = 8 om • Regi stration accuracy: ~ ~ ±10%!ir m i n = 0.05 J1-m • Nonpl anar topograph y: dT 0 1• 1 =

Ld M

M

= 1.1 J1-ffi

After fabrication the optic is characterized by both physical and optical measurements. Excellent wavefront quality has been demonstrated for single lenslets [4] and coherent arrays [5]. Arrays can be fabricated with excellent fidelity with regard to lenslet uniformity and spatial registration of the lenslets. Indeed, binary optics is, at present, the only technology that can successfully (and consistently) produce coherent arrays of centimeter size. As it is expensive to generate a prototype optic , a single master is often fabricated for subsequent replication using established embossing or molding technologies.

3. Fabrication and optical performance

Many important applications require microlens arrays of short focal lengths in the visible or near IR, where deviation from fabrication tolerances have the greatest impact. Hence, one goal of our work in bin ary optics is to understand and identify the underlying efficiency limits of the technology , in order to balance demanding fabrication efforts with incremental gain in optical performance [9,22,23]. For example , scalar modeling predicts an efficiency of sinc2(1/ 16) = 0.987 for a lens with sixteen phase levels , but models more rigorous than scalar predict lower efficiency values for fast lenslets [24-26]. Such inefficiencies multiply for applications requiring several layers of diffractive optics. Because the optical efficien cy (defined by the ratio of desired spot intensity to incident intensity) of diffractive phase relief structures depends not only on the optical design parameters but on the quality of the fabricated device, it is of utmost import anc e to discriminate between the efficiency limits imposed by Maxwell's equations and the limits imposed by fabrication capabilities. To assess the effects of process variables, we have begun to quantify the efficiency limits of diffractive binary optic microlenses and to correlate losses in optical efficiency with specific fabricat ion errors. Binary optics structures impose constr aints on fabr ication that are not normally a concern in standard VLSI processing. First, there is the essentially 0% overlay tolerance in mask to

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substrate pattern registration for the nested structures, as opposed to the 0.1-0.25/-lm tolerances allowed in standard VLSI processing. Accurate registration must be achieved over areas large compared to the available field size in steppers and advanced electron beam lithography (EBL) machines. Second, the phase steps must be etched to precise depths into the material, often to better than 1%, for both shallow (=10 nm) and deep (=10 JLm) structures. Because we are not etching a film to completion, common types of optical endpoint detectors are not usable. Finally, vertical sidewall profiles must be etched without lateral undercutting or erosion of the mask edge which would reduce the phase step width. Alternatively, if the materials are deposited by liftoff, angled profiles or film overlap errors would cause similar problems. One must achieve high-fidelity pattern replication of curvilinear linewidths, which can range from 0.5 to 50 JLm on a single Cr photomask level, and smaller still if written by EBL. It is nontrivial to include the correction for the linewidth bias associated with the photolithography process in the calculation of the phase topography prior to encoding the data. Failure to maintain linewidth fidelity and line placement accuracy results in phase-step-width errors, which appear as trenches and ridges in the etched structures. Phase-step-width and phase-step-height errors can significantly reduce the optical efficiency [22-24]. Finally, uniformity of feature sizes and etched depths must be maintained across the entire array area. 3.1. Microlens test set To systematically evaluate both the fabrication requirements and the optical performance of diffractive microlenses, we have designed and fabricated a microlens test set with a distribution of focal lengths - the best efficiency array set (BEAST) [9]. Microlenses having 200 x 200}Lm square apertures and focal lengths f of 170 to 14 mm at A = 633 nm (@P-numbers between @p10.8 and 3'160) were fabricated simultaneously as both isolated lenslets and 10 x 10 microlens arrays. The 0.5 JLm CD limited the two-phase-level lens to gjPIO.8, the four-phaselevel lens to 3'/2, and the eight-phase-level lens to 3'14. Requisite etch depths in fused silica (n" = 1.4572) were 0.175, 0.349, and 0.699 JLm for layers, 3,2, and 1, respectively, resulting in cumulative etch depths of 1.223}Lm (eight phase levels) or 1.048 JLm (four phase levels). Three separate sets of these microlenses were fabricated as two-, four-, and eight-phase-level devices, respectively. The microlenses were fabricated in two-inch-diameter, six-mm-thick Suprasil fused silica substrates that had a ;\/10 surface finish. Substrates were reactive ion etched in a CHF 3 plasma. Typical etching rates were 16.5 nm/min at 180 W rf power and 220 V bias voltage; etch depths were controlled by etch time. Selectivity between the photoresist mask and the fused silica substrate was =2: 1. Actual etch depths of the eightphase-level 3'16 microlens, shown in Fig. 3, were measured by stylus profilometry to be 0.22, 0.33, and 0.70 }Lm, representing a 3% total etch depth error for the worst case. The overlay registration accuracy was evaluated by optical microscopy of a background grid of twodimensional vernier-style alignment marks that spanned a ±0.5 JLm in 0.1 }Lm increments in both x and y. Using these verniers, we achieved overlay registration to better than 0.10 /-Lm over the 1.2 x 1.8 em field.

M.B. Stern / Microelectronic Engineering 32 (1996) 369-388

375

Fig. 3. SEM of an eight-phase-level, 200 J1.m square aperture [Ji/6 quartz microlens. The minimum zone width on level three is 0.8 p.m.

3.2. Efficiency measurements and discussion of results

The diffraction efficiency is measured at A = 632.8 nm using a dual-beam, auto-referencing apparatus, which exhibits low-noise performance, long-term stability, and excellent repeatability [28]. The measured first-order efficiency is normalized to the relative focal spot power of a perfect lens (0.815 for square apertures). Relative efficiency can be measured with 0.001 precision and, with calibration, absolute efficiency measurements are accurate to 0.005. Efficiencies for four- and eight-phase-level microlenses are plotted as a function of lens speed (or focal length) in Fig. 4 and Fig. 5(a) and compared with the efficiencies predicted from a first-principles diffraction code [25]. Simple scalar calculations, also shown, are strictly valid only for large features (grating period/wavelength ;;::10) [24] and predict efficiencies of 95% for eight-phase-level and 81% for four-phase-level microlenses regardless of .'¥-number (or feature size). For an eight-phase-level ff/4.5 microlens having less than 0.1 /km misalignment, we have measured an absolute efficiency of 0.85, corresponding to 96% of the predicted efficiency (with overall throughput of 0.84); this is, we believe, the highest efficiency reported to date for such a fast, binary optics lens in the visible. Furthermore, this result implies that net fabrication errors contributed at most a 4% efficiency loss. Scalar theory calculations of efficiency losses caused by fabrication-related errors predict losses of 10% due to 0.1 }.L linewidth errors and 0.3 }.LID misalignment (zone position) errors for an @p/3 microlens [8,22-24]. This implies that alignment tolerances better than or equal to 0.05 ,urn are critical to achieving maximum diffraction efficiencies. We have optimized our RIE process to maintain good linewidth fidelity; however, bias errors during photolithography can reduce linewidths by up to 0.1 fLID. Variations in etch depths (phase step heights) up to ±2.5% of the total reset height, will reduce the first-order efficiency by about 1% [8,22-24].

376

M.B. Stern / Microelectronic Engineering 32 (1996) 369-388 1.00

r---------------......,

0.95

>c Z

- - - - - - - - - - -

_-_-_----t--

0.90

UJ

(j

u: LL

0.85

t

- - - SCALAR

--EMTHEORY • 082 0081

UJ

0.80

0.75 2

4

8

16

32

64

F-NUMBER

Fig. 4. Optical efficiency of eight-phase-level microlenses from two samples as a function of lines: scalar calculation; solid line: EM calculation.

~-n u m b er.

Dashed

We generally achieve better control in our RIE process. For example, we have fabricated optical elements in fused silica that required etch depth control of ±5 nm for a 1.454 Mm depth (0.3% precision) [3]. To quantify the effect that pattern misalignment has on optical efficiency, we intentionally introduced a 0.35 fLm translational error between mask layers 1 and 2 in one set of four-phase-level lenslets. [29] Alignments results are shown in Fig. 5(a) for the misaligned optic. Absolute optical efficiencies measured as a function of the lens speed for this "misaligned" set of microlenses exhibit significant differences, when compared with the nominally identical, four-phase level, "well-aligned" set of microlenses Fig. 5(b). The intentional registration error represents a substantial fraction of the zonewidth for the fast lenslets and results in sizable efficiency losses. For example, 0.35 fLm is only 3% of the minimum full zone for the :Ji/60 lenslet, but is 25% of the minimum zonewidth for the :!fr/2 microlens. Note that the performance of the misaligned optic trails the well-aligned lenses by ~/15; at :Ji/6 the efficiency of the misaligned microlenses is 5%Jless than the well-aligned optic; and at :Ji/2 the misaligned lenslets show a 10% decrease in fficiency below that of the well-aligned microlenses. Even the nominally well-aligned optic e hibits decreased efficiency for the fastest lenslets (:!frI2), possibly due to linewidth errors in the nominally 0.5 Mm outer zones which may contribute to the efficiency losses in both sets of microlenses. Etch depth errors of 1-2% of the total phase step height measured on these microlenses have negligible impact on the efficiency.

4. Deep three-dimensional microstructure fabrication for binary optics

Fast broadband microoptics, color discrimination optics, and focal-plane microlens arrays call for the fabrication of deep , high-resolution, three-dimensional structures [6,29-34]. Binary optics processing techniques, which result in uniform arrays of diffractive optics with high spatial coherence, high fill factor, and good optical quality, can be expected to yield

M.B. Stern / Microelectronic Engineering 32 (1996) 369-388

377

Fig . 5. (a) Alignment results for BEAST sample QB3 " misaligned" at each of the 16 vernier sites. The average and standard deviation of these 16 values, weighted equally , are for QB3: -0.01 ± 0.17 (x) , 0.38 ± 0.04 (y), and for the " well-aligned" sample (now shown) QB2 : 0.06 ± 0.03 (x), 0.1 ± 0.04 (y) ; (b) Optical efficiency of four-phase-level microlenses as a function of ~- nu m be r. The open diamonds are the data from QB2 (the " well-aligned" sample); the solid diamonds are the data from QB3 (the intentionally misaligned sample). The dotted line is the scalar theory efficiency for four phase levels (81%) . The solid line is the predicted value from the full electromagnetic calculation [13,14].

similar results for refractive microoptics. However, maintaining tight tolerances over multilevel alignments, submicron feature widths, and multimicron etch depths - critical for high optical efficiencies - will increase in difficulty with the depth of the topography. The refractive designs considered here for the 8-12 p..m bandwidth regime - a prototype sensor microlens

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M.B. Stern / Microelectronic Engineering 32 (1996) 369-388

with 14/Lm sag and a' color dispersive microoptic array with 7.S/Lm sag - require deep anisotropic etching with precisely controlled phase-step heights [30,31,34]. An aspheric profile can be approximated in a stepwise manner by iterative steps of photolithography and RIE (binary optics technology) [30]; alternatively a preshaped refractive polymer micro lens [35-37] can be directly etched into a substrate [34,38,39]. Binary optics technology requires anisotropic etching with high selectivity between mask and substrate and accurate etch depth control to achieve correct phase step heights. Direct replication of a preshaped analog photoresist mask into a substrate requires a low-selectivity etch with high anisotropy,Le., one with no lateral etch component and the substrate and mask are etched to completion, that is, until the mask is completely eroded.

4.1. Refractive binary optics fabrication Fabrication of an aspheric lens profile with binary optics technology is illustrated in Fig. 6. In this case a stepwise approximation is made to the continuous aspheric phase structure, Le. , there are no diffractive resets. The minimum phase step heights are calculated for an efficient modulo 271" phase structure to minimize scattering from step edges. Thus, to obtain 99% optical efficiency the minimum step height is d = A([16(n - 1)r 1 ) . Trilayer resist techniques are used to planarize deep stepped topographies in order to maintain linewidth fidelity during the photolithography and edge acuity during the subsequent etching step. Depth of focus is an issue for mask registration through thick planarizing layers. Each iterative process sequence requires better than 0.25/-Lm alignment precision between layers, etch depth control of 10-100 nm, and vertically etched sidewall profiles. To fabricate IR binary optics in silicon, we have developed multilayer resist processes and anisotropic RIE processes for deep Si structures (>8 /-Lm deep) [30]. Using a O.S-l/Lm thick photoresist imaging layer, a thin transfer layer (nominally 100 nm) of plasma-deposited Si0 2, and a 4-/Lm-thick planarization layer, we were able to align and etch 1.0 p.tx: geometries with high fidelity in both RIE and helicon reactors. To achieve anisotropic etching without undercutting of the thick photoresist planarization layer, we added cyclopentene to the O 2 feed gas to create a sidewall passivation layer in the RIE system or lowered the substrate temperature to -lOoDe in the helicon system [39]. To reduce the 02-RIE time of the thick planarization layer, we use the minimum material that adequately planarized the surface microstructure. To prevent lateral undercutting and to enhance the control over the sidewall profile [30,41], we designed a Si RIB process to induce the in situ formation of a sidewall inhibition layer. We achieved vertical sidewalls without mask undercutting or surface texturing with a >5:1 etch selectivity (ratio of Si:photoresist etching rates) by using a (74%/26%) SF 6/ O 2 gas mixture in a commercial RIE system. We observed no undercutting of the photoresist mask, which is still present on the high-aspect-ratio vertical bars (Lurn lines etched 8!-Lm deep) shown in Fig. 7. A prototype of a fully refractive Si micro lens for an advanced satellite sensor with a total sag or etched depth of 14 urn is shown in Fig. 8. In an actual device, each of these 2 J.Lm high steps would themselves be subdivided into quantized steps to produce an efficient microlens. In this demonstration, however, our motive was to establish the anisotropic deep etching and planarization capabilities required for broadband microoptics.

M.B . Stern I Microelectronic Engineering 32 (1996) 369-388

-

2lt PHASE STRUCTURE

-

379

ALIGN MASK PAlTEAN TRILAYER PR AlE SiOxAND PLANARIZATION LAYERS

AlESi

4lt PHASE STRUCTURE

ALIGN MASK

PAlTERN TRILAYER PA

AIESiOxAND PLANARIZATION LAYERS

RIESI

~~~~I ~

SUBSTR ATE PHOTORE SIST SiOx PLANARIZING LA YEA

I I vlll??/1 Fig. 6. Sequence for fabricating refractive binary optics, consisting of iterated steps of phot olith ography , multilayer resist pr ocessing, and RIE. RIB is used to pattern both the intermediate SiO., transfer layer and the planarization level. This forms the mask for the subsequent RIE of the Si substrate . Each Si RIE step doubles the previous etch depth.

4.2 . Continuous profile microlenses: preshaped micro/ens etch mask

We have investigated two processes to fabricate refractive polymer micro lenses: a polymer reflow technique that relies on surface tension to reshape cylinders of photoresist [35,36]; and a laser direct write technique, that varies the exposure energy to directly define an analog lens

380

M .B. Stern I Microelectronic Engineering 32 (1996) 369- 388

(8)

(b)

Fig . 7. SEM of anisotropic features etched 8 fJ.ill deep in Si: (a) 3000x, (b) 9000x .

profile in the photoresist [37,42]. We can fabri cate these polymer micro lenses with less than 0.1 wav e devi ation from sphere [36,37]. Other direct write methods , such as electron beam lithography or focused ion-beam lithography could also be used to form the analog polymer etch mask. All of these techniques rely on the ability of the researcher to map out and control the complex phase space that relates process parameters to optic al pe rformance . While the photoresist lenslets can be used directly in the visible region , it is desirable to transfer this profile into a substrate such as fused silica or silicon for robustness and op er ation bandwidth. In our experiments , all etching masks are positive photor esist (Shipley 1800S and AZ 4000S) . Patterns are defined either by UV contact photolithography or by a 442 nm He-Cd polar coordinate laser writer [36,37,42]. Once the photoresist master is generated it is transferred into the substrate by dry etching , as illustrated in Fig. 9 [34]. The desired refractive profile can be dir ectly encoded in the mask for an equal-rate etching process or it can be compressed in x an d/ or y to accommodate the selectivity and anisotropy of the etching process. The complexity of the pattern transfer step is reduced if later al etching of both the substrate and

M .B. Stern / Microelectronic Engineering 32 (1996) 369-388

381

1 00~m

Fig. 8. Eight-phase-level 200-p.m-diameter refractive silicon microlens with sag height of 14 JLrn.

the mask is suppressed or eliminated. RIB in SF 6/ CzF6/ O, gas mixtures is used to etch the refractive contours into p-type, (100) silicon substrates at pressures between 5 and 200 mTorr and RF powers between 10 and 500 W. Fig . 10(a) shows a scanning electron micrograph (SEM) of a photoresist master array composed of 200-p,m-diameter lenslets with 12.5 p.m sag . Fig. 10(b) shows a SEM of a similar array etched into Si. The resultant microlenses have 200 p.m diameter and 13.5 p,m sag. For this example, the vertical etch rate is 0.11 ,urn/min, the lateral etch rate is ::::::0, and the selectivity is 1.1:1.

4.3. Color discrimination optics (visible and infrared) Color discrimination can be achieved by taking advantage of the dispersion inherent to a diffraction grating, i.e., dispersion among the first order. Separation of color in the visible has been investigated with both conventional gratings and "echelon"-type gratings [31-33]. The first type of grating, when combined with a lens for focusing the individual orders, has been used for entertainment [43] and for IR separation and imaging of thermal radiation [31]. The color dispersive Si microoptic, pictured in Fig. 11, is a combination 5'/2 refractive microlens and 17-p.m-period diffraction grating for color separation and focusing in the 8-12 ,urn band. Four mask levels were used to create 16 phase steps with a total phase height or sag of 7.5 p.m. To maintain the high-fidelity pattern replication necessary for efficient binary optics structures , we used the deep anisotropic Si etching process described above. There are 64 x 64 identical pixel elements , each a 100 x 100 p.m square , in this array . The design and performance of this device are detailed elsewhere [31]. Besides the dispersive grating, a unique design based on an echelon-type grating can prove useful when greater control over the placement and intensity of the color is required [32,33] . The echelon-like grating can be thought of as the superposition of several blazed gratings operating at different wavelengths with each grating blazed efficiently for a different order. This allows the zero order of one wavelength to be transmitted, while the shorter and longer

382

M.B. Stern J Microelectronic Engineering 32 (1996) 369-388 CHROME PHOTOMASK

EZ2Zl PHOTORESIST f~ ;(~ I SILICON SUBSTRATE

A

B PATIERN PR

C THER MAL REFLOW

D RIE SUBSTRATE

Fig. 9. Schematic illustration of fabrication of analog refractive microlenses by polymer reflow and etching.

wavelengths are diffracted into the -1 and + 1 orders, respectively (Fig. 12). While the phase step height of a conventional N-step blazed grating is A/N(n -1), for the echelon-type grating it is A/(n -1) or N times deeper! Visible echelon-type gratings designed for use in fused silica required etch depths of 1.14 and 2.28}.Lm for a total etch depth of 3.42 }.Lill. Fig. 12 shows the theoretical and measured spectral composition of a four-step echelon with a 43 mill diagonal fabricated in fused silica [33].

M.B . Stern / Microelectronic Engineering 32 (1996) 369-388

383

H l00~m

(a)

H 100 urn

(b) Fig. 10 . (a) SEM o f a section of an array of 200-/L m-diameter 3'"/1.3 photoresist microlenses with 12.5/Lm sag heights ; (b) microlenses et ched in Si with 13.5 /Lm sag heights.

5. The future of binary optics: integration Binary op tics is primarily an enabl ing technology. Implementation of this technology can reduce the size, cost, and weight of optical or mechanooptical systems while providing increased performance and increased functionality . Future progress in binary optics technology is dependent on the integration of high-quality microoptics into systems. Such multilayered optics and integrated systems of optics and electronics, optics and MEMs, or optics and OEICs, will form the initial building blocks of powerful miniaturized systems [44]. Moreover,

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M. B. Stern ! Microelectronic Engineering 32 (1996) 369-388

Fig. 11. A 16-phase-level color dispersing microlens array etched 7.5 !LID deep. This is one pixel element of a 64 x 64 array.

the monolithic integration of active and passive devices on the same substrate will eliminate difficult and time-consuming alignments between discrete planes of devices and/or optics. For example, by exploiting the fill factor enhancement available from microlens arrays, detectors on focal plane arrays can be made smaller or spaced further apart and the newly available space can be filled with preprocessing circuitry. Arrays of microlenses monolithically integrated with epitaxially grown HgCdTe photodetectors on opposite sides of a single CdTe substrate result in increased gamma radiation hardening and improved quantum efficiency [6]. Arrays of Si-concentrators have been used to enhance focal plane efficiency [45] and are also available in high end video cameras [46]. Areas that can benefit from binary optics include displays, medical instrumentation, telecommunications, satellite imaging systems, military applications, and entertainment. New fabrication capabilities will develop in tandem as needed to realize these new structures. Major research and development efforts are required to realize smart or amacronic [44] focal planes that will combine detection with preprocessing capabilities.

6. Conclusion

Fabrication capability is fundamental to the achievement of high-quality microoptics, We have reviewed our effort to identify and customize VLSI processes. This has led to significant improvement in optical performance and to new micro optics capabilities. In particular, we have fabricated benchmark-quality, diffractive binary optics microlenses and measured the absolute efficiency as a function of gp-number for microlenses with two-, four -, and eight-phase steps at A = 633 nm. A clear correlation between misalignment and efficiency loss has been demonstrated. High-resolution Si RIE and trilevel resist capabilities have been developed to fabricate accurate, high-aspect-ratio, deep three-dimensional optical structures. Fully refractive IR microoptics for applications in advanced sensors and color dispersive microoptics have been

M.B. Stern / Microelectronic Engineering 32 (1996) 369-388

385

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386

M.B. Stern / Microelectronic Engineering 32 (1996) 369-388

demonstrated. All of these achievements are traceable to careful attention to the fabrication requirements of the particular microoptic device. Acknowledgements

We thank our coworkers over the years: Wilfrid Veldkamp, Mordechai Rothschild, Gary Swanson, Jim Leger, Mike Holz, Bob Knowlden, Mike Faro, Thereas Jay, and Woody Goltsos. This work was sponsored by the Advanced Research Projects Agency. Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Air Force. References [1] 1. Kirz, Frontiers of x-ray microscopy, presented at International Symposium on Electron, Ion and Photon Beams, 1991; D.M. Tennant, J.E. Gregus, C. Jacobsen and E.L. Raab, Construction and test of phase zone plates for x-ray microscopy, Opt. Lett., 16 (1991) 621-623. [2] W.B. Veldkamp and G.J. Swanson, Developments in fabrication of binary optical elements, SPIE Proc. 437 (1983) 54-59. [3] VV. Wong and G.J. Swanson, Binary optic interconnects: Design, fabrication and limits on implementation, SPIE Proc. 1544 (1991) 123-133. [4] J.R. Leger, M. Holz, G.J. Swanson and W Veldkamp, Coherent laser beam addition: An application of binary-optics technology, Lincoln Lab. J. 1 (1988) 225-246; J.R. Leger, M.L. Scott, P. Bundman and M.P. Griswold, Astigmatic wavefront correction of a gain-guided laser diode array using anamorphic diffractive microlenses, SPIE Proc. 884 (1988) 82-89; J. Leger, App!. Phys. Lett. 56 (1990) 4-6. [5] W. Goltsos and M. Holz, Agile beam steering using binary optics microlens arrays, Opt. Eng. 29 (1990) 1392-1397. [6] M.B. Stern, WF. Delaney, M. Holz, KP. Kunz, K.R. Maschhoff and J. Welsch, Binary optics microlens arrays in CdTe, Mater. Res. Soc. Symp. Proc. 216 (1991) 107-112. [7] Selected Papers on Holographic and Diffractive Lenses and Mirrors, SPIE Milestone Series MS34, T.W. Stone and B.J. Thommpson, eds, (1991). [8] G.J. Swanson, Binary optics technology: The theory and design of multi-level diffractive optical elements, MIT Lincoln Laboratory Tech. Rept. 854, DTIC #AD-A-213404, MIT Lincoln Laboratory, 1989; G.J. Swanson and W.B. Veldkamp, Infrared applications of diffractive optical elements, SPIE Proc. 833 (1988) 1151-1162. [9] M.B. Stern, M. Holz, S. Medeiros and R.E. Knowlden, Fabricating binary optics: Process variables critical to optical efficiency, J. Vac. Sci. Techno!. B 9 (1991) 3117-3121. [10] B. Kress, D. Zaleta, W Daschner, K. Urquart, R. Stein and S.H. Lee, Diffractive optics fabricated by direct write methods with an electron beam, NASA Conf. Pub!. 3227 (1993) 195-205. [11] V'P, Koronkevich, v.P. Kiriyanov, F.I. Kokoulin, LG. Palchikova, A.G. Poleshchuk, A.G. Sedukhin, E.G. Churin, A.M. Shcherbachenko and Yu.L Yurlov, Fabrication of kinoform optical elements, Optik 67 (1984) 257-266. [12] C, Dix, P.F. McKee, A.R. Thurlow, J.R. Towers, D.C. Wood, N.J. Dawes and J.T. Whitney, Electron-beam fabrication and focused-ion-beam inspection of submicron structured diffractive optical elements, J. Vac. Sci. Techno!. B 12 (1994) 3708-3711. [13] K Miyamoto, The phase fresne11ens, J. Opt. Soc. Amer. 51 (1961) 17-20. [14] L.B. Lesem, P.M. Hirsch and J.A. Jordan Jr., The kinoform: A new wavefront reconstruction device, IBM J. Res. Dev. 13 (1969) 150-155.

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[15] M. Tanigami, S. Ogata, S. Aoyama, T. Yamashita and K. lmanaka, Low wave front aberration and high temperature stability molded micro Fresnel lens, IEEE Photon. Techno\. Lett. 1 (1989) 384-385; S. Ogata, M. Yoneda, T. Maeda and K. Imanaka, Low cost and compact fibre-to-laser coupling with micro Fresnel lens, SPIE Proc. 1544 (1991) 92-100. [16] P.D. Maker and R.E. Muller, Phase holograms in PMMA with proximity effect correction, NASA Conf. Pub\. 3227. (1993) 207-221. [17] M.T. Gale, G.K. Lang, J.M. Raynor and Helmut Schutz, Fabrication of microoptical components by laser beam writing in photoresist, SPlE Proc. 1506 (1991) 65-70; E.J. Gratix and C.B. Zarowin, Fabrication of microlenses by laser assisted chemical etching (LACE), SPIE Proc. 1544 (1991) 238-243. [18] E.J. Gratix, Laser figuring for the generation of analog microoptics and kinorform surfaces, NASA Conf. Publ. 3227 (1993) 187-193. [19] Dammann, H., Blazed synthetic phase-only holograms, Optik 31. (1970) 95-104.; Spectral characteristics of stepped-phase gratings, Optik 53 (1979) 409-417. [20] L. d'Auria, J.P. Huignard, A.M. Roy and E. Spitz, Photolithographic fabrication of thin film lenses, Opt. Commun. 5 (1972) 232-235. [21] Micro Mask, Inc., Sunnyvale, CA. [22] J.A. Cox, T. Werner, J. Lee, S. Nelson, B. Fritz and J. Bergstrom, Diffraction efficiency of binary optical elements, SPIE Proc. 1211, (1990) 116-124. [23] MW. Farn and J.W. Goodman, Effect of VLSI fabrication errors on kinoform efficiency, SPIE Proc. 1211 (1990) 125-136. [24] G.J. Swanson, Binary optics technology: Theoretical limits on the diffraction efficiency of multilevel diffractive optical elements, MIT Lincoln Laboratory Tech. Rept. 914, DTIC #AD-A-235404, March 1991. [25] R.E. Knowlden adapted the code by S.A. Gaither, Coupled Wave Theory of Crossed Gratings, Masters Thesis, MIT, 1987. [26] G.J. Swanson and R.E. Knowlden, presented at the 1990 OSA Annual Meeting, Boston, MA, 1990. [27] W.M. Moreau, Semiconductor Lithography, Chapter 15, Plenum Press, New York, 1989. [28] M. Holz, M.B. Stern, S.S. Medeiros and R.E. Knowlden, Testing binary optics: Accurate high-precision efficiency measurements of microlens arrays in the visible, SPIE Proc. 1544 (1991) 75-89. [29] M.B. Stern, M. Holz and T.R. Jay, Fabricating binary optics in infrared and visible materials, SPIE Proc. 1751 (1992) 85-95. [30] M.B. Stern and S.S. Medeiros, Deep three-dimensional microstructure fabrication for IR binary optics, J. Vac. Sci. Technol. B 10 (1992) 2520-2525. [31] MW. Farn, M.B. Stern, W.B. Veldkamp and S.S. Medeiros, Color separation by use of binary optics, Opt. Lett. 18 (1993) 1214-1216. [32] H. Dammann, Color separation gratings, Appl. Opt. 17 (1979) 2273. (33) MW. Farn, R.E. Knowlden, M.B. Stern and WB. Veldkamp, Color separation gratings, NASA Conf. Pub!. 3227 (1993) 409. [34] M.B. Stern and T.R. Jay, Dry etching for coherent refractive microlens arrays, Opt. Eng. 33, (1994) 3547-3551; also SPIE Proc. 1992 (1993) 283. [35] Z.D. Popovic, R.A. Sprague and G.A. Neville Connell, Technique for monolithic fabrication of microlens arrays, Appl. Opt. 27 (1988) 1281-1284; Microlens Arrays, edited by M. Hutley, lOP Short Meeting Series 30, lOP Publishing Ltd., UK 1991. [36] T.R. Jay, M.B. Stern and R.E. Knowlden, Refractive microlens array fabrication parameters and their effect on optical performance, SPIE Proc. 1751 (1992) 236-245. [37] T.R. Jay and M.B. Stern, Preshaping photoresist for refractive microlens fabrication, SPIE Proc. 1992 (1993) 275-282; also Opt. Eng. 33 (1994) 3552-3555. [38] E. Gratix, Evolution of a microlens surface under etching conditions, SPIE Proc. 1992 (1993) 266-274. (39) G. Gal, Microoptics technology for advanced sensors, presented at the SPIE Conference on Miniature and MicroOptics CR49, San Diego, CA, 1993. [40] M.B. Stern, S.c. Palmateer, M.W Horn, M. Rothschild, B. Maxwell and J.E. Curtin, Profile control in dry-development of high aspect ratio resist structures, J. Vac, Sci. Techno\. B (Nov./Dec. 1995), to appear.

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[41] D.L. Flamm and V.M. Donnelly, The design of plasma etchants, Plasma Chern. Plasma Processing 1, (1981) 317-363. [42] W. Goltsos and S. Liu, Polar coordinate laser writer for binary optics fabrication, SPIE Proc. 1211 (1990) 137-147. [43] Chromadepth-3D, Chromatek, Inc., Alpharetta, GA. [44] W.B. Veldkamp, Overview of micro-optics: Past, present and future, SPIE Proc. 1544, (1991) 287-299. [45] BY. Tsaur, C.K. Cheng and S.A. Marino, Heterojunction GexSi1_)Si infrared detectors and focal plane arrays, Opt. Eng. 33, (1994) 72-78. [46] SONY Corp., Japan.

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Margaret B. Stern graduated with a B.A. in Physics from the University of Pennsylvania in 1974, and received her PhD in solid state physics from S.U.NY. at Stony Brook in 1981. She is presently a member of the technical staff at MIT Lincoln Laboratory in the submicron lithography group. Her research activities have focused on the physics and fabrication of micro- and nanoelectronic, optoelectronic and microoptic devices and she has over 50 publications and several patents in this area. Her current research includes the development of VLSI and ULSI processing techniques for binary optics fabrication.