Optical Fiber Packaging for MEMS interfacing

Optical Fiber Packaging for MEMS interfacing Jose Mireles Jr.*a, Miguel A. Garciaa, Roberto C. Ambrosioa, Ernest J. Garciab; Wilfrido Callejac; Claudia Reyesc a Universidad Autónoma de Ciudad Juárez, Ave. Del Charro 450 N., Cd. Juárez, Chih, MX 32310; b Sandia Natl Laboratories, Electromechanical Engineering Dept.,Albuquerque, NM USA 87185; c Instituto Nacional de Astrofisica Optica y Electronica, Tonantzintla, Cholula, Puebla MX 72840 ABSTRACT An investigation study concerning positioning, alignment, bonding and packaging of optical fibers for interfacing with optical MEMS devices is being reviewed in this paper. The study includes a review of techniques and critical issues for optical fiber positioning, alignment, bonding, optical improvements, and coupling and interfacing through micro-lenses and waveguides. Also, we present a packaging design structure for hermetic sealing of optical MEMS devices requiring interfacing through optical fibers which considers aspects such as processes, assemble schemes and bonding techniques for Optical Fibers, which are briefly reviewed in this work. This packaging design considers the following conditions: hermeticity of the MEMS devices, optical fiber and MEMS die alignment and positioning, assembly process, and Simachined fixturing design for final assembly and positioning. Keywords: MEMS Packaging, Optical Fibers, Micro-Assembly, MOEMS Packaging, Optical Fibers Fixturing.

1. INTRODUCTION The encapsulation of MEMS optical devices is a big concern due that it usually represents around 70% of the total cost of the development / manufacturing of microsystem devices [1]. The quest towards the identification of cheaper and less costly encapsulation solutions are key for the release of new electronic products end services needing optimized and cost effective solutions [1-3]. In addition, a key pitfall in taking a semiconductor or MEMS devices from prototype to manufacturing would be the poor selection of attachment materials of interconnected layers, specially the encapsulation layers of the design [4]. This pitfall is commonly found in research institutions wanting to demonstrate a proof-ofconcept prototype, where teams are not constrained by manufacturing issues. At prototype development stages, the device assembly configuration is often not revised in detail with regard to selections of attachment methods of interface layers of semiconductor and MEMS designs. Usually, the attachment / bonding methods may be determined by the chosen outsource manufacturer's capabilities without assessment of optimal interfacing in designs. This work presents a summary of an investigation study concerning attachment methods, materials, processes and assembly procedures to specifically attach photonic MEMS devices, including optical fibers into complete electronic devices. Many coupling issues exist while developing alignment of optical fibers to micromirrors, starting with the obvious optical dispersion of photons in between them, and ending with the assembly and package design tolerances. Proper understanding of device’s optical design is the only way to perform informed decision making on device package design and assembly process. Different erroneous decision making on assembly and alignment problems are associated if specifications are not understood properly, including poor attachment choices, under-tolerances on attachment methods for components that might require high positional tolerance requirements, over specification of tolerances of attachment methods for components requiring lower tolerance requirements can lead to increased costs and to extra assembly process constraints [4]. Depending on the required tolerances, designers can use low cost solutions (std. pick and place solutions) if requirements for tolerances are 10-20μm, expensive solutions (flip-chip positioning equipment) for 1-5μm required tolerance, or slow and very expensive solutions (usually active alignment required) for tolerances below 1μm [5]. Very important alignment losses should be addressed while coupling optical fibers such as [17]: lateral and angular misalignment, separation between fibers, and convexity of fiber ends. This work was partially supported by Sandia National Laboratories under contract 9602, PO#659783 from January 2007 through July 2008 [5]. *[email protected]; phone +52 656 688-4800, x4571; fax +52 656 688-4813; uacj.mx/mems Micromachining and Microfabrication Process Technology XIV, edited by Mary-Ann Maher, Jung-Chih Chiao, Paul J. Resnick, Proceedings of SPIE Vol. 7204, 720405 · © 2009 SPIE CCC code: 0277-786X/09/$18 · doi: 10.1117/12.815170 Proc. of SPIE Vol. 7204 720405-1

The content of this review paper is presented as follows. The first section considers general methods and processes for aligning and attaching optical fibers to a substrate, which considers direct aligning into silicon substrate, alignment through microlenses, connectors, optical waveguides, and hybrid approaches. Secondly, we address critical issues concerning the development capability to align and package optical fibers with small elements. Issues such as tolerances, lateral and angular misalignments, and attachment methods are considered in this section. The critical issues related to insuring the reliability and stability of packaging over time and temperature are highlighted. The second report (Part II) of this investigation provides a development of alignment and assembly procedures of fiber optic assembly into Silicon chips.

2. METHODS AND PROCESSES FOR ALIGNING OPTICAL FIBERS In this section we present a summary review of several methods used for aligning optical fibers with microstructures. Generally, there exist techniques for holding optical fibers to the substrate, through the use of connectors, microlenses, optical waveguides, and hybrid solutions (combining several of the earlier.) We next present several techniques proposed by several research groups, and which techniques are sub-divided for each of the general categories described.

2.1 Direct alignment and attachment of optical fibers to a silicon substrate N. Hori in his chapter “Optical Fiber Jisso Technology” from the book [6], provides a review of techniques for aligning and fixing parallel optical fibers (OF) to substrates. The standard way to hold and align optical fibers to the substrate is by using V-grooves micromachined mostly over (100) substrates. To fix optical fiber (including arrays of OF) to substrates, adhesives are generally used and such are applied in between the V-grooves, enclosing the OF and holding also the top and bottom substrates together. However, adhesives can not be used in applications requiring hermetic packaging; neither in high-temperature applications due to reliability issues [4]. If hermetic packaging is required, metal deposition and soldering (including laser soldering) is often used in conjunction to V-grooves. Reference [7] shows an study of different metallization options considering low temperature lead Pb-free soldering in V-grooves, which study is summarized in section 3.3 (solder attachment.) The use of V-grooves is very practical for optical MEMS interfacing, Lee et al in [8] and Huang et al in [9] present the alignment of four optical fibers in a 2x2 MEMS fiber optical switch arrangement. The switch is used in conjunction with vertical torsion mirror devices that are fabricated using silicon surface micromachining processes in a die that is dice cut and attached into a bigger Bulk micromachined substrate which contains V-groves. The V-grooves hold micro lenses and optical fibers to align them to the micromirrors of the central surface micromachined die. This packaging method is used to minimize active optical alignment between the mirror chip and the optical fibers. Figure 1 shows the proposed approach from [8]. However, their approach does not offers proper hermetic sealing of the MEMS devices (since air can pass through the V-grooves), neither discusses the issues while aligning the fibers to the substrate. Mirror chip

Micro Mirror

Figure 1. a) and b) 2x2 fiber arrangement and mirror die submount, approach used by [8,9].

In both works, [8] and [9], their approach to align the fibers to the mirrors is by using 300 µm-diameter ball lenses which are dropped in the center of a lower wafer containing V-grooves, and an upper wafer which has V-groves on both sides of it. One side servers to align the lower wafer through the V-grooves using dummy fibers, and the other side is used to fix the optical fibers, as shown in figure 1.3-b. However, in their work no further results are reported regarding tolerances, efficiency, or assembly problems. The 300 µm-diameter spherical microlenses is also used to diminish power and coupling losses. This is due that the light is collimated through the lenses. These ball lenses are self assembled in a

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bulk micromachined micropit as shown in figure 2. To fabricate these micropits, mask layout design compensations were used to reduce the convexity of corners due to the extra etching in a (100) silicon die. Also, due to the 12 µm dicing cut error of the central chip, tolerances and dimensions of the mirror should be considered, as well as dimension Wc shown in figure 2. >1

Fthei

Ba]J lens

Figure 2. 2x2 fiber arrangement and mirror die submount with fiber and ball lens assembly (from [9]).

Abeysinghe et. al in [10] propose to use anodic bonding to hold and maintain the optical fibers to V-grooves in the substrate. As a difference of using metallized fibers, or epoxies to hold the fibers, the Silica fibers are coated with a highNa+ glass (through sputtering of Pyrex, or coating and curing with liquid sodium silicate solution) are placed in a metallized v-groove on the Si submount and anodically bonded, as seen in figure 1.5. This bond has shear strength comparable to solder-bonded components (600 g) and the V-groove autoaligns the fiber to the component in two of the three dimensions. This novel combination of using anodic bonding and coated fiber on precision fabricated V-grooves can be combined to a proper assembly method, but still lacks a solution for hermetic packaging. Hauffe et al. in [15] present a passive alignment coupling of single mode optical fibers (similar alignment methods to those used by [8,9]) with reported coupling losses of 0.7dB. A lower die used in their design use V-grooved structures for optical fiber alignment micromachined using KOH on Silicon (100) following the crystal planes in the Si wafer. Better alignment accuracies in assembles are obtained if alignment ribs are used in the upper die (which have negative profile alignment with respect to the ones located in the lower die,) and the use of flip-chip bonding automation assembly. However, no further assembly problems and hermeticity issues are highlighted in [15].

2.2 Alignment of optical fibers through microlenses While considering coupling an array of fibers into an opposite array through open space, the light going out of the fibers should be collimated through microlenses [6]. Different types of microlenses are available in the market already, and the most standard types are spherical, diffractive, and GRIN microlenses. Details about different fabrication processes (additive, etching, injection, or even ion implantation techniques), applications, wafer assembly, and evaluation processes of microlenses are presented in [6].

2.3 Alignment and coupling of optical fibers through waveguides Several researchers suggest the use of fiber-to-waveguide coupling. Development of waveguides is a very active research today, and the coupling requires small tolerances in assembly and alignment of optical fibers with mirrors, some mirrors embedded in waveguides. One example of utilizing waveguides is presented by Glebov et al. in [12], who propose the use of optical waveguides as a coupling path in between photonic devices for high speed board-to-board optical interconnects. An optical layer consisting on polymer waveguides and 45º reflector micromirrors are embedded in a backplane. The waveguides have propagation losses as low as 0.05 dB/cm at 850 nm, and they are fabricated by direct lithographic patterning. Hotembossing was also evaluated for the waveguide fabrication resulting in waveguide propagation losses in the range of 0.06-0.1 dB/cm but with poor channel-to-channel uniformity. For the fabrication of the 45º reflector micromirrors, wedge dicing technology was utilized resulting in 0.5 dB losses. Glebov et al. present two general low-cost fabrication methods for waveguides, using direct lithographic patterning and hot embossing. The photolithographic patterning technologies have better reproducibility and reliability for waveguides manufacturing. For multimode optical waveguides, which core dimensions vary from 20x20 to 50x50µm2.


2.4 Hybrid approaches for alignment and coupling of optical fibers The coupling lighting through optical fibers in [12] was accomplished by combining waveguides, MTP connectors and optical microlenses. The light from the fiber is focused on a 45º micromirror embedded in waveguides by a microlens mounted in the lower subassembly of the adaptor. The microlens adaptor provides possibilities for an easy light mode size reduction and improved alignment tolerances as the light can be focused in a small spot with the diffraction limited size. The microlens adaptors used in [12] have bi-convex silica microlenses with 230 and 120µm radii of curvature (ROC) formed on a 950µm wide silica plate. The microlens lateral dimensions are less than 250µm so that they can accommodate 250µm pitch of the fiber array. The height of the adaptor subassembly is optimized to focus the light on the embedded 45º mirrors and is typically about 200-300µm. Figure 1.11 shows the adaptor subassembly in a cross section drawing.

Figure 3. Connector assembly shown in a cross section format, from [12].

A different hybrid scheme was used by Karppinen et al. in [13] to couple optical fibers to waveguides. In their work, two sets of microlenses were used to collimate the optical light coming from VCSELs through micromirrors and into optical waveguides. The required tolerances for their approach were ±10µm, and even if “big” BGA bumps (compared to most MEMS devices) were used for assembly, the microlenses were able to get down to such tolerances of alignment. The emitted wavelength of VCSELs were 850nm. Another hybrid approach was proposed by Bakir et. al in [14], who fabricated and tested microscopic polymer pillars which are used as flexible optical bridge between optical devices and waveguides located in the substrate. Using Avatrel polymer, pillars are machined using photo-imaging to a height up to 350 µm. The photodefinable polymer Avatrel was used for the fabrication of the optical pillars due to its ease of processing and its unique material properties that include high Tg and low modulus. An evaluation performance of these polymer pillars was accomplished in [14], where the optical coupling efficiency measurements were compared using light source to an optical aperture with and without an optical pillar. A 30×150 µm polymer pillar improves the coupling efficiency by 3 to 4.5 dB compared to pillar-free (freespace) optical coupling.

3. METHODS FOR HOLDING AND FASTENING OPTICAL FIBERS 3.1 Submount attachment In order to maintain optical alignment and device performance, stability of submounts and main components is necessary. The stability can be jeopardized under thermal loads of elevated package temperature environment, TEC operation, and laser diode operation. Design solutions to these thermo-mechanical packaging issues include [16]: • choice of submount thickness for resistance to thermal gradient warpage; • proper TEC sizing; • material selection of submounts for thermal conduction; and • submount ability to provide temperature uniformity across its surfaces. Gengenbach presented in [19] a submount to hold and align a single mode optical fiber, two spherical lenses (φ 900µm ±5µm,) a wavelength filter (3mm ±50µm x 3mm ±50µm x 1mm ±0.5µm,) a laser diode and a photodiode. The spherical lenses are placed down to ±2µm relative to the bottom of the optical bench, this can be accomplished thanks to the LIGA process submount, as shown in figure 2.3. Small amount of UV-cured epoxy has to be used to fix these spherical optical


lenses. The optical fibers are placed in the submount cavity having 1µm wider and deeper shaft. The positioning tolerance is ±50µm close to the spherical lenses. The important result from this work to our investigation is that even having big tolerances and rough submounts, spherical lenses were able to align optical coupling of 9µm core optical fibers to detectors being several milimeters apart. One procedure to create a submount with high numerical aperture (NA) lenses such as that from [20], but using smaller dimensions is through constructing an array using a three-lens surface scheme [6]. Figure 2.5 shows the structure applied for an optical disk pickup objective lens. The lens diameter was 200 μm, and it had an NA of 0.85. Overall length is 708 μm. The high-NA microlens consists of three lens surfaces. L1 and L2 are convex lenses. L3 is a concave lens filled with a resin with a high refractive index. Substrates and cover glass materials are fused quartz. The refractive index of the resin between L3 and the cover glass was 1.620. The designed tolerances of each surface spacing and optical axis displacement were ±3 μm and ±1 μm, respectively. This structured lens achieved high NA with the 650-nm wavelength design. We conclude from this reference that it is possible to fabricate high-NA microlenses if necessary for demanding applications.

3.2 Epoxy Attachment The lowest cost attachment material for building photonic devices is epoxy. Also, epoxy is commonly used for developments of proof-of-concept of photonic devices. However, while epoxy is an excellent choice for building up prototypes, its use in manufacturing should be assessed by experienced engineers on processes and reliability. Issues that should be reviewed include [16]: Outgassing properties, cure profiles (UV snap cure and thermal cure), shrinkage upon cure, thermal expansion (CTE), mechanical stability, adhesion strength, refractive index and optical transmission in some applications, ability to act as a reversible gas getter within the device package, ability to maintain device optical coupling over Telecordia requirements, and processing costs relative to solder or laser welding attachment in a production environment. All these issues are summarized in [5]. Positional shifts in epoxy attachments can degrade optical coupling in the device [16]. These shifts can be caused by thermal cycling, due to material expansion, loss of adhesion, and changes in material properties due to environmental exposures such as humidity. The optical misalignment shifting can be a process-dependent function of epoxy volume, cure profile, and shape after cure. To diminish this problem, the use of UV snap cured or thermally cured epoxy is often required for precision alignment. The UV cured epoxy normally require the development of cure profiles that minimize both epoxy outgassing potential and component alignment shift. One more advantage of UV cured epoxies is its curing room temperature (fast none UC “snap cures” epoxy materials require 130˚ C to 160˚ C, [16].) Normally, the use of both thermally and UV snap cured epoxies minimize alignment shift of components when initially they are attached by UV snap cure. The outgassing and stability of UV cured epoxies should be reviewed in detail to assess their compatibility with the photonic device in question (see reliability issues.) Table 1 (from [16]) shows general requirements of epoxy for optical fiber bonding. TABLE 1. General requirements of epoxy for optical fiber bonding Optimal Viscosity Several 1,000 ~ 10,000 cps Low Shinkage < 2 ~ 3% Low CTE (by TMA) ~ 10-5 / ºC Low absorption and excellent Durability Fast curing and good adhesiveness

3.3 Solder Attachment A common method used in holding photonic devices is solder attachment of submounts [Here, a distinction should be made between the use of solder attachment for mechanical structures such as submounts and the use of solder for attachment of pre-aligned optical components.] Often soldering processes for attachments require the use of flux. However, the use of flux requires special attention in the manufacture of photonic devices due to reliability concerns including contamination and cleaning process restrictions of many optoelectronic components [16]. Therefore, a fluxless soldering process should be considered, since the removal of flux has shown significant thermal cycling life improvement when flux is thoroughly removed. Figure 4 shows a sample process that utilizes soldering for hermetic sealing from [6]. However, this work lacks details on the final assembly requirement for optical fiber end tip, requiring low resolution positioning down to the 1~2 micron.


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Figure 4. Hermetical sealing using soldering technique, from [6].

Ou et. al in [7] present a study of soldering optical fibers into V-grooves using a metallization technique including the use of Pb-free soldering. Electron-beam evaporation was used to deposit a multi-layered metallic coating on the surfaces of fibers and V-grooved chips to overcome the problem that solder does not wet the SiO2 surface. Three types of coatings were investigated: Ti/Au, Ti/Cu/Au, and Ti/Ni/Au. These metallic film coatings have good adhesion on fibers and the V-grooves. Low melting point Pb-free solder was used, eutectic 43Sn-57Bi (in wt.%), with a melting point of 139 ◦C to bond an array of fibers to V-grooved chips. Figure 5 shows the coating layers used in their soldering technique, where precision alignment of fibers and the bonded structures were stable at room temperature. L. Si-chip V-groove

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Figure 5. Cross-section of the bonding structure, from [7].

In order to use proper solder attachment, components require compatible metal interfaces. Nickel/gold plating is often used to assure a good repeatable solder joint. Optical fibers, lenses and thin film filters require costly metallization processes, either require thin film metal evaporation deposition or by using electroless plating for metal adhesion to glass. Optical components and fiber pigtails requiring precision attachment using solder requires special attention in setting up a process that achieves proper attachment position repeatability. Some process controls that allow solder attachment with high positional tolerances and repeatability include [16]: solder volume control; placement of solder volume (frequently resolved by employing solder performs); conditioning the solder preforms for optimal solder surface wetting by etching oxides off of preforms prior to use; solder solidification shape; application of a repeatable and controllable solder reflow temperature profile; and finally, the use of pulsed laser energy, resistance heating, and inductive heating are methods that can deliver such repeatable temperature profiles for solder attachment. The use of laser soldering for attachment within photonic packages has an advantage because it is a contact-less heating method that can by far access the small components to be soldered that are hidden within small device packages. These recessed components are hard to access for resistance heating contacts and inductive heating coils. A good discussion on this matter is shown in [16].


4. ASSEMBLY EXPERIMENTS FOR OPTICAL FIBERS As a reminder to reader, our purpose in this work is to look for important aspects of processes, assemble schemes and bonding techniques for Optical Fibers, which are briefly reviewed in the last sections, to design a packaging scheme for hermetic sealing of optical MEMS devices requiring interfacing through optical fibers. Based on the previous review sections, we decided that the requirements for the proposed design were to create a design that would enclose a MEMS die and with the following conditions: 1) Hermeticity.- The Optical fiber should fit into a V-grooved channel (v-channel) and this channel should be covered by a metal filing (soldered) that should surround the MEMS die to hermetically seal and protect the MEMS devices. This is, the MEMS die should be in the center of a MEMS packaging chip which contain V-grooves for optical fiber interfacing; 2) Optical Fiber Alignment.- An external V-grooved channel will help in the assembly process while aligning the fiber(s) with the MEMS device(s) inside for initial bonding before the (final bonding) metallization process. These features shall be designed to overcome lateral and angular missalignmnets of fibers and might use a UV-curable, thermally-curable epoxy, or anodic bonding for holding the positioning of fibers in the desired location; 3) MEMS Die Alignment.- The central section of the packaging substrate should be DRIE-machined to create mechanical datums to permit precision positioning of the MEMS die to a central desired location in the MEMS package. This is a key feature for proper alignment of the fiber(s) through the V-grooved channels in the MEMS package; 4) Assembly.- The design should consider also positioning datums that will help final Optical Fiber positioning and separation with respect to MEMS die inside the package; 5) Optical Window.- The design should consider also an optical window for the possible interfacing through optical fibers through connectors;

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a) MEMS die alignment datums, DRIE etched A

MEMS die with Optical devices

b) V-grooves for alignment with Substrate #2 c) V-grooved pair channel for alignment of fibers A’

d) Internal ring for final bonding (metal) e) External ring for prebonding (epoxy or anodic)

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B’ Figure 6. Layout design representation showing the upper view of Si substrate #1, including assembled optical fibers (in blue) and an optical MEMS die (orange), and the following features: alignment datums, V-grooves for alignment and positioning, internal and external bonding features (denominated rings).

4.1 MEMS Packaging Design This section presents the details of the suggested design of the MEMS packaging scheme following the directions just presented above. The proposed MEMS packaging design consist of three main parts that enclose an optical MEMS die consisting of two Si substrates and a glass section which serves as a window for upper interfacing with the internal optical MEMS device. A Si substrate, refer as #1 substrate, is shown in figure 6 showing a representation of the upper view of this Si (100) MEMS substrate (in red) showing two ring regions with V-grooved channels (aligned with the planes) and in the center a cavity for placing the MEMS die (in orange color). The following features are found in


figure 6: a) alignment datums for positioning the optical MEMS die on its cavity (no through hole), design showing four corner datums; b) V-grooves for alignment with the upper substrate, design showing four x-axis alignment grooves (one showing a fiber placed on it); c) V-grooves channels for lateral and angular positioning of fibers to optical MEMS die, design showing seven channels with a fiber on it, and one without fiber; d) internal sealing ring for final bonding; and e) external ring for pre-bonding. The drawing from this figure also shows in blue the representation of the optical fibers (in blue) already assembled and aligned with respect to the MEMS die. The second Si substrate is similar to this first substrate with the only difference that instead of having a cavity for holding the optical MEMS die, it has a through hole for the optical window. Figure 7 shows two cross sections in two regions of assembled substrates #1, #2 and upper glass window. We defined “contact rings” as the area where the silicon micro-machined substrates make contact (are bonded), we can from figures 6 and 7 that this package design has two contact rings. This is, the red layout corresponds to the contact/bonded area of the lower and upper silicon substrates. The orange layout in the same figure 1.6 can be the center substrate. Two features are included in this new design, the substrate- and the side-alignment features. From figure 6, notice the rounding the edges in the “entrance” of the layout of the substrate #1, one can diminish the possibilities damaging the optical fibers while trying to assemble the fibers into the v-channels, as suggested by [7]. Obviously, if one lets the bulk micromachining process to run for longer amount of time to machine the V-grooves, the edges will not be that smooth [7]. Also, dummy optical fibers placed on the groves located in the external ring are used to facilitate aligning substrates #1 and #2. The use of dummy fibers was followed from [8]. Notice that dummy fibers might not be required if lateral groove channels are fabricated on the vertical features in both rings. Substrate #1 use datums (corner sections and side 2D positioning) to align final positioning of the MEMS die with respect to an internal corner of the opening in substrate.

a) Bonding region, internal ring

Cross section A-A’

b) Pre-bonding region, external ring Glass substrate Substrate #2 Cross section B-B’ Substrate #1 Figure 7. Cross section views A-A’and B-B’ from figure 6 showing datums for optical MEMS die alignment features, optical fiber alignment features, and the internal and external bonding regions of substrates #1 and #2.

4.2 Fabricated Fixtures and Structures for Assembling Optical Fibers to MEMS package To perform the assembly procedures of this work, we had to fabricate silicon substrates and fixtures, as well as to use one assembly setup. The designed fixtures were used for handling and assembling optical fibers consist of Si micromachined v-grooved channels. These fixtures are manipulated through a micromanipulator and a nanopositioner. The fixtures were fixed and assembled over a PI’s nanopositioner called Nanocube system. Figure 8 shows four views of a fixture for holding two optical fibers in v-grooved channels, and in order to maintain the fibers mounted over the vgrooves in the fixtures, a magnetic holder is placed over the fibers to maintain them aligned and attached to the V section of the grooves. Figure 9 shows the use of two channel fixtures in a schematic representation. This design has several advantages. One advantage is that it can hold several fibers and assemble them at once. One more advantage is that even if the initial length of hold fibers in the fixture is not the same, i.e., the initial Δ misalignment shown in figure 9 is allowable in the suggested fixtures, since the magnet holding the fibers permits sliding the longer fiber(s) while pushing the fixture towards the substrate after initial contact. Finally, after further pushing the fixture towards the substrate datum, all the fibers will meet the substrate datum making Δ=0.


2' View A

View B View A View B

Figure 8. Fixture designs for holding optical fibers during assembly procedures.

Fixture Optical fiber

substrate

Magnetic holder Optical fiber

Figure 9. Alignment problem while considering holding more then one fiber at a time.

Figure 10. Assembly setup type B, including a Signatone 1160 probe station, a Newmark micro-manipulator, a PI nano-positioner, and stereo zoom optical systems with 2.0 USB color cameras and monitors.


The assembly setup shown in figure 10 was used for the assembly of optical fibers to the designed MEMs packaged discussed earlier. This setup consisted in one Newmark XYZ-θ micro-manipulator with resolutions down to 1-2 micron resolution on each axis, and 0.05 degree resolution in the rotational axis, a PI XYZ nano-positioner with resolutions down to 1-2 nanometers on each axis (with 100μm travel on each axis), two stereo zoom optical systems with video color cameras, two monitors, and a computer with system interfaces for controlling the micropositioner and the nanopositioner.

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a) b) Figure 11. Images showing: a) two substrates showing datums making contact, and b) resulting fiber end tip coplanar alignment to one of these substrates’ contact wall.

It is extremely time consuming to place individual fibers to a desired position if no contact datums are used. Only through the use of expensive solutions such those that use setups with optical feedback control alignment, one can get down to 1~2 μms resolutions in the placing of optical fibers [4-6]. However, the suggested design which uses fixtures with grooves and datums can improve the positioning resolution below 2μm without the need of using expensive setups. For instance, if a simple requirement such as placing the end tip of a fiber by the end of an edge of a substrate with a groove, as shown in figure 11, datums are very helpful. This figure shows the fiber already bonded with epoxy, after placed to the desired location (edge of v-grooved substrate, in this case). In this figure the edge of the substrate was cut using a Disco dicing machine, and the optical fiber was previously cleaved.

a) b) Figure 12. Image showing: a) a fixture showing Vgrooves in the closer substrate, and wall datum in the farther substrate; and b) assembly process showing three fibers alignment, two of them already making contact to the datum wall.

Figure 12 shows a) a fixture showing its Vgrooves in the closer substrate, and a wall datum in the farther substrate; and part b) shows the alignment process of three fibers, two of them already making contact to the datum wall in the right. In this case, the left substrare from the b) part of figure 12, and which holds the optical fibers, is a fixture moving towards the right substrate with the datum wall. We have also developed alignment through Vgrooves not perpendicular with respect to the mating datum in the target substrate, as shown in figure 13. The Vgrooves from this fixture were bulk etched over a (110) Si substrate, getting a 70.6° degree groove etching with respect to the axis.

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'IIT a) b) Figure 13. Image showing: a) a fixture showing Vgrooves in the closer substrate, and wall datum in the farther substrate; and b) assembly process showing three fibers alignment, two of them already making contact to the datum wall.

4.3 Optical Fiber bonding to the MEMS package The further steps remaining in this work to finalize the MEMS package concept presented in this section is to perform the final bonding process in the internal ring. This is, the previous section covers the positioning and alignment of optical fibers using the vgrooved channels passing through the first and second rings, as defined in section 4.1. Once aligned the fibers to a mating datum, UV cured epoxy was used to prebond the fibers to maintain its desired position while the final bonding process is developed. We foresee the use of two techniques for the final bonding of fibers: a wafer bonding technique, and a flip-chip bonder technique. The use of the wafer bonder will require opening the cover of our Suss Microtec SB6a bonder, to place in the cavity the optical fiber patch cables surrounding the wafer bonding area to finally bond with metal melting the inner ring of our MEMS package design. More promising, a flip chip bonding technique using our FC-150 flip chip bonder, we will perform the final bonding step required in the most crucial step for further hermitic sealing, after closing the top section with a window glass through Anodic Bonding.

5. CONCLUSIONS The purpose of this work is to look for important aspects of processes, assemble schemes and bonding techniques for Optical Fibers to design a packaging scheme for hermetic sealing of optical MEMS devices requiring interfacing through optical fibers. This work presents: 1) a summary of a review [5] of general methods and processes for aligning and attaching optical fibers to a substrate, which considers direct aligning into silicon substrate, alignment through microlenses, connectors, optical waveguides, and hybrid approaches; 2) we also addressed briefly critical issues concerning the development capability to align and package optical fibers with small elements. Issues such as tolerances, lateral and angular misalignments, and attachment methods were investigated in [5], including the critical issues related to insuring the reliability and stability of packaging over time and temperature. The last section of this work provides a suggested scheme of a MEMs Package design for optical fiber interfacing with an optical MEMs device chip, which device shall be aligned, positioned, bonded and hermetically sealed. Initial development for ensuring alignment and positioning in the assembly process of optical fibers is presented. We presented design and development of fixtures for optical fiber holding, positioning and alignment to mechanical datums. We showed that through the use of the designed fixtures and mechanical datums, cheap assembly processes can reach the 1~2 μm resolution requirements by optical fiber interfacing for the lowest loss requirements. The further steps remaining in this work to finalize the MEMS package concept presented in section 4 is to perform the final bonding process in the internal ring suggested in the MEMS package concept. We foresee the use a flip chip bonder to perform the final bonding step required in the most crucial step for further hermitic sealing (the bonding in the second ring). Finally, a window glass bonding will be performed through Anodic Bonding for hermetically sealing the MOEMS chip.

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