Polymer Electrooptic Waveguide Fabrication - ACS Symposium Series

Chapter 31

Polymer Electrooptic Waveguide Fabrication 1

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M . Ziari , A. Chen , S. Kalluri , W. H . Steier , Y. Shi , W. Wang , D. Chen , and H. R. Fetterman Downloaded by STANFORD UNIV GREEN LIBR on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch031

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Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089-0483 Tacan Corporation, 2330 Faraday Avenue, Carlsbad, CA 92008 Department of Electrical Engineering, University of California, Los Angeles, CA 90024 2

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Various methods for the fabrication of etched waveguide structures, environmentally stable electrooptic modulators and the integration of polymer waveguides with silicon circuitry is discussed. Dry etching of the active polymer layer by reactive ion etching was used to fabricate millimeter-wave modulators (dc-60 GHz) with long term stability. Further improvement in etch quality was achieved by employing the novel etch method of electron cyclotron resonance etching. Our polymer modulator was tested in a community access television optical-fiber link demonstration and general optical power handling requirements of such devices are outlined. We also discuss the fabrication of polymer waveguides on planarized silicon VLSI circuits. Multiple fiber attachment using v-grooves etched in Si/SiO substrate is demonstrated. 2

Photonic applications of second-order optical nonlinear polymers require a welldeveloped and cost effective device fabrication process. The material advances made over the past few years that addressed problems with environmental and thermal stability of these active polymers have made this technology more attractive for commercial applications. Polymeric integrated optical devices, however, must still overcome the skepticism that exists regarding their performance and be able to compete favorably with mature technologies based on LiNb03 and III-V semiconductor materials. In this article, we will discuss various fabrications processes and demonstrate devices that are long-term environmentally stable, operate at very high frequencies (40 GHz) and perform well in an experimental community access television (CATV) optical link. In addition, we will discuss and demonstrate integration and packaging concepts that can eventually place polymeric devices one step ahead of the competing technologies in both functionality and production cost. This article is organized in the following manner. First we discuss general waveguide and modulator design and fabrication methods. We have extensively used dry etching processes and devote a section to present the details of reactive ion etching and electron cyclotron resonance etching. The next section provides the details of electrooptic device fabrication process, testing, and CATV applications.

0097-6156/95/0601-0420$12.00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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The next section will discuss the subject of integration of polymer devices with electronic circuits. Finally, we present our efforts on device packaging and multiple fiber attachment.

Downloaded by STANFORD UNIV GREEN LIBR on October 14, 2012 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch031

Polymer Waveguides A polymeric optical waveguide structure often consists of multiple polymer layers. A proper choice of the upper cladding, core, and lower cladding materials based on their thicknesses and refractive indices can provide single mode confinement in the vertical direction. In addition, the processing compatibility of these films and their linear and nonlinear optical properties such as loss and electrooptic, dielectric, electrical, and mechanical properties are all very important. Fabrication parameters like ease in spinning, poling, cutting and polishing also influence the choice of materials. The availability of good cladding layer materials is critical for overall device performance and is the subject of current research. The cladding index of refraction affects the mode confinement and mode matching to optical fibers. For vertical poling through the cladding layer, the electrical resistivity of the cladding material must be less than that of the electrooptic layer to assure efficient poling.(7) As an alternative to polymer films, thermally grown S1O2 on Si substrate can also be used for the lower cladding. The high optical quality of Si02 reduces scattering and absorption losses and provides a smooth surface for spinning the guiding layer. The waveguide must be designed however to prevent losses due to the high index Si substrate. Calculations show that an S1O2 layer thickness of 3 to 4 am is adequate to keep this loss below 0.1 dB/cm.(2). It is important, however, to note that the very high resistivity and the high breakdown voltage of SiC>2 lower claddings makes such structures impractical for corona discharge poling. The lateral confinement of guided modes requires additional processing steps whose effectiveness, cost, and performance are the subject of many research efforts. Photobleaching (3), and physical etching or laser ablation have been the primary techniques used for this purpose. Photobleaching is a simple, low cost and highly accurate technique for patterning the refractive indices. The accuracy of UV-induced changes in the refractive indices is advantageous since it allows precise control of the guided mode profile which can improve fiber coupling efficiency. This method relies on photochemical processes such as isomerization, realignment of the optically active moieties, or material decomposition. We have used photobleaching with UV light for the fabrication of birefringent cladding waveguides and directional couplers (4). Although we have not observed any sign of degradation in these devices after many months in a dark environment, their long term stability under operation conditions requires further investigation. Etched structures, on the other hand, are expected to offer greater stability because of their physically structured nature. Lateral confinement in these structures can be provided by one of several methods that involve the etching of either the lower cladding or the active layer. One approach that we have taken is to etch a trench in the polyurethane or S1O2 lower cladding (see Fig. 1). In this case, the active layer is spun after the etching process and an inverted ridge structure is formed. Many of the process compatibility issues are relaxed because lateral confinement does not require any further processing. The other approach relies on forming a ridge structure on the active guiding layer by dry etching and requires compatible lithographic patterning. We have employed this technique in the fabrication of electrooptic modulators. The dry etching is performed by reactive ion etching (RIE) or electron cyclotron resonance (ECR) etching. The next section provides a more detailed description of the various etching methods.

In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

One important requirement for successful polymer waveguide device fabrication is to have a reliable design tool that is consistent with the capabilities and tolerances of the fabrication process. The calculation of waveguide parameters such as the layer thicknesses, waveguide width, etch depth or photobleaching exposure dose, and electrode characteristics can be obtained using analytical methods for simple structures but require detailed computer simulations for more complex multilayer structures. We rely on the effective index method to simulate waveguide structures and calculate the propagation constant, the mode profile and, possibly the number of modes. One important parameter obtained from these calculations is the optimum etch depth for single mode operation. We have previously calculated the cut-off etch depth for multi-mode waveguides based on our active and cladding layer indices (4). Recent experimental results and detailed simulations using the effective-index method (5) show that modal characteristics such as the mode shape and the number of modes can be effectively predicted and controlled. Figure 2 shows the out-coupled 1.3 mm patternfroma 4 jum wide and 1 \im thick PU-DR19 waveguide with a 4 ^im lower and a 9 |Lim upper polyurethane cladding layers (n=1.55). The buried ridge height (etch depth) is 0.5 ^m. The patterns show that two horizontal modes and one vertical mode can be excited in this structure which is in very good agreement with the predictions of the simulation. An etch depth of ~ 0.2 jam is requires for single mode operation of a 5 jum width ridge waveguides. This shallow etch depth is attainable with RIE and ECR dry etching methods. Very high frequency (multi-GHz) electrooptic modulators are traveling wave devices both for the optical and the modulationfrequenciesand must be designed such that the whole structure including the polymer optical waveguides, the millimeter wave electrode structure, and the millimeter wave connectors meet both optical and millimeter wave requirements. These can often be conflicting requirements. For example the width and spacing of the metallic layers to achieve a required millimeter wave impedance may not be compatible with a low loss optical waveguide since the electrodes can introduce additional metallic losses for the optical waves. Fiber coupling of the optical beam and coaxial or metallic waveguide coupling of the millimeter wave energy all impose additional constraints on the device structure. It is important that all of these design issues be addressed from the beginning and the modulator be considered as a combination of an optical and a millimeter wave device. The complexity of the design is many-fold and the final structure is often a realistic compromise between optical and electrical requirements and the material and processing capabilities. Etching The large demand for semiconductor devices over the past decades has resulted in the development of various dry and wet etching technologies that can now be employed for the fabrication of optical polymeric devices. Dry plasma etching of organic polymers, polyimides, and photoresist layers is routinely performed in an oxygen plasma environment. We have used two forms of ion-assisted plasma etching methods, RIE and ECR, for patterning polymer films. The important issues here are again, the compatibility of the photolithography and the etch mask with the polymer, etch selectivity, the etch anisotropy required to achieve vertical etched walls, etch rate, and wall smoothness. The isotropic and purely chemical etch in a plasma discharge is the most common form of plasma etching (6-8). Because of its isotropic nature, this method has limited use in polymer waveguide fabrication. It is rather the ion-assisted plasma etching methods such as RIE or ECR that are anisotropic and can result in vertical wall structures (7,8).



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Reactive Ion Etching (RIE). This method is now widely used for creating polymer waveguide structures^, 9). A radio frequency (RF) source (30 to 200 W) creates the plasma and the necessary bias to achieve vertical etching. The exact RIE etch rate is pressure, RF power, material and machine dependent.(70) We typically etch at a rate of 2200 ± 250 A/min at 45 W power and 0.15 Torr pressure for PU-DR19 thermoset polymer. Higher etch rates result in wall roughness and depth nonuniformities. We also used RIE for etching thermally grown S1O2 with an etch rate of 550 A/min at 150 W and 0.1 Torr pressure. In RIE, RF power is used to excite the plasma. The sample is placed on the electrode that is connected to the RF source and thus is unavoidably biased relative to the plasma(6, 8). The excitation of the plasma and the necessary bias are both created by the RF source and therefore it is not possible to generate the necessary high plasma density independent of the bias condition. This limits the control of the etch rate and quality of the sidewall structure. Electron Cyclotron Resonance (ECR). In this novel method, a magnetic field is applied to the reaction chamber. A microwave source at approximately 2.54 GHz is then tuned to the electron cyclotron resonance frequency which creates a very high plasma density (11). This electrodeless mechanism of exciting the plasma relieves the biasing problems associated with the RIE method. Nevertheless, an optional RF power and bias is often used to independently energize the ions and achieve anisotropic etch. The etch rate, etch anisotropy, and smoothness can be controlled by adjusting the flow rate and pressure of gases, the microwave power, the RF power, and the bias. Very high selectivity and a broad range of etch characteristics are achieved with this technique. The smoothness of the vertical surfaces is fairly good and high quality mirrors for semiconductor lasers have been obtained by ECR etching of the facets. We have etched polymer films with this novel method. We used a PlasmaQuest model 375 ECR machine to etch PU-DR19 polymer under a gas mixture of oxygen and argon. Our initial results indicate an etching rate of roughly 2000 A/min for a microwave power of 400 W, an RF power of 153 W and -200 V bias, a cyclotron magnet current of 180 Amps at 18 ° C temperature, and O2 and Ar flow rates of 17.6 and 49 s e e m respectively. Further improvement in the etch quality, i.e. rate and smoothness, can be achieved by adjusting the above etching parameters. Figure 3 shows a comparison of an RIE (top) and an ECR (bottom) etched PUDR19 samples. The photographs, taken by a scanning electron microscope (SEM), demonstrate the improved smoothness of ECR-etched structures. The picture also illustrates the excellent vertical profile of the wall for the case of ECR etched sample. Note the ECR sample is still partially covered with patterned AZ 5214-E photoresist. The depth of the photoresist etch suggests that PU-DR19 and this resist have similar etch rates. A decrease in the etch rate of photoresist relative to the nonlinear polymer should be possible under different operating conditions. Etching of SK>2. Patterning of the Si02 lower cladding layer can be performed with dry etching methods such as RIE or with inorganic wet etchants such as buffered hydrofluoric acid (HF). The wet etching of Si02 is isotropic and causes a considerable undercut. However, it is still a useful method to define trench (or groove) patterns in the lower cladding because the required etch depths for single mode operation are typically only about 0.2 to 0.4 jLim. The etch rate of buffered HF is typically 500 A/min. We have also used the RIE and ECR instruments with a CF4 gas source for etching Si02 and have achieved smooth vertical walls. The RIE etch rate is 500 A/min. at 0.15 Torr pressure and 150 W of RF power. Higher pressures increase the etch rate but may also cause a problematic redeposition of S1O2 on the sample. The availability of multiple gas sources and the improved control of ECR


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Figure 1. Schematic drawing of an etched trench in lower cladding (left) and an inverted ridge structure (right).

Figure 2. Out-coupled light from 4 jam wide waveguide showing the excitation of two horizontal modes. The 1.3 jum wavelength light was detected using an infrared camera.

Figure 3. Comparison of ECR (bottom) and RIE (top) etched structures. (Scale is shown on the top).


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also helps to accurately and smcjothly etch the Si02 cladding layer. ECR etching can be considerably faster (~ 2000 A/min) and more selective. Electrooptic Waveguide modulator.


The fabrication and applications of electrooptic modulators are discussed in this section. It will also serve to present the materials currently being used and discuss device related issues in the fabrication process. Material. We used the thermosetting polyurethane Disperse Red-19 (PUDR19) for the active layer. This material is not a guest-host system and the nonlinear chromophore is covalently attached to the polymer backbone with a loading density of 32%. The nonlinear properties, poling characteristics and the thermal stability of this crosslinked polymer system have been reported earlier(72). In summary, the absorption resonance peak is at 470 nm and the indices of refraction at 850, 1.06 and 1.32 urn are 1.685, 1.673 and 1.667 respectively. This material has shown long term stability in its electrooptic coefficient for over two years at room temperature, longer than 3000 hours at 90° C with only 27% drop in nonlinearity, and for short term at 100° C (4). After curing the thermoset films are chemically resistant to the solvents used in the lithography process and subsequent film spinning. Using second harmonic generation at a fundamental wavelength of 1.064 jum and electrooptic measurements of ri3 coefficient at 633 nm and 800 nm, we have extrapolated an r33 value of 12 pm/V at 1.064 nm and 10-11 pm/V at 1.3 urn wavelengths. Waveguide measurements at these wavelengths have now confirmed the r33= 12 pm/V value at 1.064 um wavelength. Commercially available polyurethane (Epoxilite 9651-1) with an index of 1.55 was used for the 9 um thick upper and 4 um thick lower cladding. Fabrication. Electrooptic phase/polarization modulators were fabricated on gold coated quartz substrates. Figure 4 outlines the fabrication process. The Epoxilite cladding material was cured at 60° C for one hour or at room temperature for 24 hours. The active layer of PUR-DR19 was spin cast and left overnight at 60 °C vacuum for drying and precuring. Thermal setting and corona poling was done at 140 °C (Fig 4a). After spin coating with a photoresist layer (Fig 4b), and lithographic patterning (Fig 4c), the cured active layer was etched by RIE to form a ridge structure (Fig 4d). The lithographic patterning required a low dosage of UV illumination and two oven bakes of half hour duration at 90° C. Our waveguide pattern consisted of several sets of 2, 3, 4, 5, 8 and 10 p wide stripes. After the application of the top cladding (Fig 4e), a gold microstrip line was patterned and gold-platted to a total thickness of 5 jam and a width of 36 jum (pig 4f). The end surface preparation of many polymer waveguides can often be achieved by carefully cutting them with a dicing saw. Our triple-stack polymer structure required additional polishing which we believe is due to the mechanical properties of our cladding material. This particular cladding material stretches and tears during cleavage which is an indication of its rubber-like properties. This material was chosen mainly because it is easy to pole through and it is solvent and cure temperature compatible with our active layer. The electrode dimensions and the total polymer layer thickness (15 jam) were chosen to achieve a 50 D, impedance at microwave frequencies. This 50 Q microstrip line electrode was used for high speed traveling wave operation and two Wiltron K connectors were utilized to launch and terminate the mm-wave signal through tapered contact pads. The connector to connector dc resistance of the modulator was 3 Cl. Our time domain microwave reflection measurements on the modulator structure suggest an effective mm-wave dielectric constant of 1.50 and a bandwidthlength product limitation of greater than 100 GHz.cm. A coplanar circuit structure



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Figure 4. Fabrication steps of the inverted ridge electrooptic modulator.


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Polymer Electrooptic Waveguide Fabrication

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that can benefit from this very high bandwidth is now under investigation for frequencies over 60 GHz. Measurement. The electrooptic waveguide was used both as a phase or polarization (birefringence) modulator.(75,14) We used a single frequency Nd:YAG laser with an input power level of 15 mW at 1.064 jum wavelength for dc to 18 GHz measurements (73). Measurements at higher frequencies utilized a Nd:YAG laser at 1.32 jum wavelength with an input power of 30 mW. Full details of the 1.064 p,m measurements are given in Reference 13. In summary, the half-wave voltage, V^, of the amplitude modulator was determined to be 35 V at 1.064 jum, yielding an electrooptic coefficient r33 of 12 pm/V which is consistent with the extrapolations made with a two-level model from second harmonic generation measurements. The half-wave voltage is less than 24 V when the device is configured as a phase modulator. The waveguide loss was estimated to be ~2dB/cm from scattering measurements. This loss is mainly attributed to the surface roughness of the lower cladding layer due to fast drying during spin cast. We believe the etched waveguide walls also contribute to this scattering loss and that this loss can be reduced when the ECR etching process is employed. For measurements under 20 GHz, we used an amplified sweep oscillator(13) with a driving microwave power of 24 dBm. The light output of the modulator was coupled to a fast photo detector via a single mode fiber. The detected signal was amplified and monitored by an electronic spectrum analyzer with 20 GHz bandwidth. Figure 5a shows the measured optical response. For higher frequencies, a combination of an HP8350B sweep oscillator and a Hughes Ka band (26.5-40 GHz) traveling wave tube (TWT) was used as the source (14). These measurements show a 3 dB bandwidth of 26 GHz for this device (14), however, optical modulation at frequencies up to 60 GHz has also been observed in this device. Due to the lack of a direct observation technique at these high frequencies, the modulation signal was down converted electronically using a microwave mixer and a 20 GHz local oscillator. Figure 5b shows the modulated signal at 37 GHz. Note that this spectrum analyzer plot shows an electronically down-converted signal (by 20 GHz) with a 12 dB loss at the mixer. The bandwidth and sensitivity of the detection system was further improved by employing optical heterodyne detection and optically down-converting the mm-wave phase modulation. Using this method, we observed and characterized mm-wave modulation at frequencies up to 60 GHz. The details of the measurement system and 60 GHz results will be reported in future publication. In addition to our current efforts that utilize microstrip line electrodes, we are also evaluating coplanar electrode structures and better microwave coupling schemes for further extending the modulation bandwidth. Our modulator device has so far operated for longer than 2 years under laboratory ambient conditions (room light, room temperature) and with measurements taken periodically at the above stated electrical and optical input power levels without any degradation in responsitivity. This is a strong confirmation of the stability of this polymer electrooptic modulator. Systematic long term measurements at higher input power levels are in progress. Applications. Optical modulators have many potential applications in fiber telecommunication and optical interconnection systems (15). In addition to these, another possible application with a vast emerging market is the use of a polymer electrooptic modulator in the transmitter for the fiber-optic community access television (CATV) networks. Traditional fiber-optic CATV transmitters employ laser diodes for direct modulation or LiNb03 waveguide modulators for external modulation. To demonstrate the applications of polymer modulators in CATV


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Polymer Electrooptic Waveguide Fabrication - ACS Symposium Series

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