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Micro/Nano Lithography

Sticking together

An inorganic, low-temperature bonding process offers advantages for applications in space sciences, microlithography, and telecommunications.

From oemagazine May 2003
30 May 2003, SPIE Newsroom. DOI: 10.1117/2.5200305.0005

Joining two or more glass or glass ceramic parts is a task that comes up regularly in optical systems fabrication. The process is more challenging than it would appear on first examination. Joints can lack strength or dimensional stability in changing environments, and/or they can lack transparency at the wavelengths of interest, introducing unacceptable optical loss into the system. Conventional joining methods require high temperature exposure or the use of organic materials. These methods often impact system performance by degrading the components being bonded.

Our group has developed a novel low-temperature bonding (LTB) technology that yields mechanically stable, optically clear joints between two or more glass or glass-ceramic parts at room temperature without the application of high temperature or pressure. This technology has been used to bond a variety of materials in applications areas such as space science, microlithography, and general photonics.

assembling the pieces

Light-weighted optics are commonly used in many land- and space-based optical systems in which optical and mechanical stability are important. In some cases, structures are mechanically honeycombed to form open-back mirrors. In others, thin faceplates and backplates are bonded to ribbed support structures, in which the support units are made by the assembly of small rib parts or by bulk-material removal using abrasive water-jet machining. For most applications, the strength and stiffness requirements of the light-weighted mirrors are nearly identical to those of a monolithic piece of material.

The choice of materials used to make such optics varies depending on the application. Large, space-based mirrors, exposed to extreme environments, may be constructed of thermally stable materials like ULE (Corning Inc.; Corning, NY) or Zerodur (Schott Glass Technologies; Duryea, PA), while other types of mirrors requiring high stiffness may be fabricated from material such as beryllium, aluminum, or silicon carbide.

LTB offers a method to lightweight typical glass and ceramic for optical applications. Conventional technologies such as fusion bonding or frit bonding used in the past to join low-expansion materials sometimes can be problematic when there is an excessive variability in the coefficients of thermal expansion (CTEs) between the joined materials.1-5 In addition, transformation-induced volume changes resulting from ceramization of glass ceramics can introduce large joint stresses during the heating and cooling process, which can result in catastrophic failure. Low-temperature adhesive bonding techniques may provide alternatives, but assemblies produced can suffer from outgassing issues in ultra-high vacuum environments or from poor environmental durability. On the other hand, techniques such as optical contacting can require extremely stringent surface requirements that may not be feasible for large assemblies.

LTB technology uses an inorganic, aqueous-based bonding fluid to chemically join two similar or dissimilar materials. In general, the bonding surfaces of the two materials are cleaned and chemically primed for joining using standard wet chemical or plasma processes. Following chemical activation, the materials are brought into a class 100 clean box where joining is initiated by sandwiching a small volume of bonding solution between the two mating surfaces. The bonding solution reacts with the substrate surfaces to form a bond that can become rigid within a few hours or even a few minutes depending on the process conditions. The bond can reach full mechanical strength within a few weeks at room temperature. In some cases, the bonded assembly can be subjected to a heat treatment in which the joint properties can be further tailored to fit the needs of the application.

To demonstrate the technology, we fabricated a 200-mm-diameter mirror by bonding a faceplate and backplate onto a core ribbed structure produced by abrasive water-jet cutting. First, we contacted to the core structure; subsequently, the process was repeated on the backplate. After curing, the assembly was heat-treated to a temperature below 150°C.

Figure 1. An FIB lift-out image shows the cross section of a sub-100-nm LTB bond (dark line).6

The range of bond-thickness values achieved using our standard LTB technology varied from less than 100 nm up to 2.5 µm, where the thickness of the final bond was dependent on the flatness of the substrate materials as well as the bonding solution conditions (see figure 1). Depending on the application, we have demonstrated the ability to control the final bond thickness, which can translate to an ability to design final bond properties.

Figure 2. The strength of LTB-bonded rods (cured and uncured) compares well with that of monolithic rods both before and after exposure to liquid nitrogen and the Telcordia 85/85 test.

To characterize the bond performance, we used LTB to fabricate samples from two Zerodur disks 50.8-mm in diameter and 50.8-mm tall, with polished λ/3 surfaces bonded together. We used four-point bend flexure testing with rods of circular cross-section per the ASTM 1161-02C standard. We also compared monolithic Zerodur with that of both cured and uncured LTB joints, as well as the strength of these joints after exposure to liquid nitrogen for three minutes or after exposure to the standard Telcordia 85/85 environmental durability test for one week (see figure 2).

We achieved similar cured and uncured bond strength results using fine ground surfaces, with the only difference being strength degradation after exposure to 85/85 conditions. Joints made with the LTB process were thermally stable, resistant to common solvents, vacuum compatible, relatively stress free, machinable, and transparent to UV, visible, and IR radiation.

In addition to bonding glass and glass ceramic materials, we have demonstrated LTB with other crystalline and non-crystalline materials, both organic and inorganic. We have commonly worked with silicate-based materials for a host of applications that can involve anywhere from one to 20 bonds per piece.

photonic applications

Besides structural applications, LTB has also shown the potential for joining silicate materials to form etalons and bond optical fibers. We have also investigated bonding phosphate glasses in photonic applications. Phosphate-based glasses are used widely in photonic amplification applications due to the enhanced laser properties of the material, which stem from the increased cross section for absorption and emission, as well as the reduced nonlinear refractive index. In particular, phosphate glass (both doped and undoped) shows promise for use in hybrid integrated optical components based on planar waveguide technology. In such devices, the hybrid planar substrate would contain two or more discrete components and or compositions of glass bonded together to act as a single substrate with increased functionality and integration. Applications for this type of technology include lossless splitters in which an incoming beam is split into multiple output beams, each amplified by passing through the rare-earth-doped glass.7

Planar hybrid substrate applications require glass compositions that are compatible with standard masking and ion-exchange technologies and possess good amplification and laser properties while still maintaining high chemical durability. In addition, the same properties required in the glass are also required in the bond that joins the pieces of glass together. The stringent requirements on this interface often make the joining of materials for hybrid substrate manufacture a difficult, if not unachievable, task for most conventional joining technologies. Again, high-temperature joining techniques have been problematic, as the difference in CTE values of the materials being bonded can cause stress at the interface, resulting in a refractive index change or even catastrophic failure during device manufacture. Although LTB techniques exist, the issues that arise with stringent surface preparation and/or durability of the bonds in multiple environments have often precluded their use for various applications.

We investigated the use of LTB bonds for this application. Not only could devices be fabricated at room temperature, but the properties of the resulting joints were favorable. Initial studies were done by bonding "active" Schott IOG-I glass doped with erbium, ytterbium, and lanthanum (La) to IOG-1 glass that contained only La. This preform was made by bonding two glasses with λ/3 (P-V) surfaces and joining them as described previously, subsequently heat-treating the hybrid preforms at temperatures from 150°C to 375°C. Following the heat treatments, the preforms were sliced into wafers, polished, masked, and subjected to ion exchange in a potassium nitrate (KNO3) bath.

Atomic-force-microscopy studies showed that the LTB joint interfaces were barely attacked during the corrosive, high-temperature lithographic and ion-exchange treatment in the KNO3 batch. Even though we observed some degradation, the performance of the bonded hybrid was still exceptional as insertion losses through hybrid phosphate bonds were typically below 0.005 dB at 1300 nm, and the back reflectance of light from the LTB interface was measured as -34 ±2 dB2.8

LTB shows the ability to be tailored to fit a host of applications for numerous materials that require joints with unique properties. As more opportunities arise we hope to work with new materials and applications to expand our knowledge base and provide the capability to make large, complex bonded assemblies for advanced applications. oe


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freeform fabrication

In the past, optical designers primarily worked with spherical, cylindrical, or planar components because of the inherent ease of manufacturing and associated cost benefits. In principle, aspheric and optical freeform components often hold the promise of enhanced performance, expanded functionality, reduced component counts, reduced size and weight, and simplified assembly. In practice, however, cylinders, spheres, and planar surfaces still dominate most optical systems. Now, advanced computer-numerical-manufacturing capabilities allow optical designers to think about using noncylindrically or spherically symmetric optical elements.

The fabrication process is divided into preshaping and polishing. Using standard computer-numerical-control technology. In the preshaping step, producing high-quality surfaces can dramatically reduce the subsequent polishing time, which ultimately reduces cost. Software to evaluate principal machine errors can improve accuracy by providing feedback, enabling the manufacture of freeform contours to within ±1 µm. The feedback is not real time; it is determined after evaluating the manufactured contour by a 3-D coordinate measurement machine. This knowledge is applied in an additional shaping step.

For polishing, Swiss Optic AG developed a six-axis robotic system that is essentially unlimited by specific geometries. This design allows the manufacturing of aspheres, off-axis aspheres, and optical freeform shapes to optical tolerances. A simulation software tool determines the ideal polishing strategy based on the surface metrology and system parameters (temperature, concentration of polishing slurry, and material properties). This approach was successfully used to polish large aspheres (up to 800 mm in diameter) to better than ?/10 P-V and optical freeform shapes.

Shaping a specific illumination distribution is an inherent challenge in photonics. Applications include condenser optics for lithography, slide projectors, video projectors, and printers; microscopy optics for high-numerical-aperture illumination and phase contrast illumination; technical illumination for optical sensors and medical optics; automotive systems such as headlamps, signal lamps, brake lights, and reading lights; and general illumination for applications such as street lamps, security lighting, and spotlights.

So far, most designs rely on the use of apertures and multiple facets and are characterized by poor efficiency and limitations in design. To overcome these limitations, special algorithms have been developed to calculate a freeform shape that redirects the light of a small point source to the desired distribution on a target surface. Based on the theory of geometric optics, the tilt of the refracting or reflecting surface hit by a ray determines its deflection. The curvature of the surface determines the concentration of light in this direction, used to create lighter or darker regions on the target surface. This enables the design of efficient systems to create any illumination distribution?for example, uniform distributions, sharp contrasts?using only a single smooth optical freeform element.

To prove the design and manufacturing capabilities, we fabricated an element to create a bright uniform rectangle with a darker circle in the center. These elements have been built as single components for high-end applications, but for high-volume, standard-illumination elements, mass production technologies are desired. Our group is evaluating the possibility of building molding tools or master forms to meet this demand. -Andreas Schwarzhans, SwissOptic AG

Mary Strzelecki, Nate Wyckoff, Roxane O'Malley, Leo Gilroy, David Schimmel

Mary Strzelecki is technical project leader for LTB, Nate Wyckoff and Roxane O'Malley are lead engineers, Leo Gilroy is business development manager, and David Schimmel is marketing services manager at Schott Glass Technologies, Duryea, PA.