Fabrication of Wafer-level Nano-optics
BY GEORGE A. RILEY
"Embossing" is an ancient technical term in the English language, traceable to the period 1350 to 1400, and used by Chaucer in its present sense. The technical meaning, creating a raised design or pattern by pressing a die having the raised negative image of the design against the material to be patterned, has not changed during the ensuing 650 years. What has changed are the applications, materials used, and the scale of the design. Today's embossed patterns may have features measured in nanometers, bringing new capabilities to integrated optical systems.
Nano-embossing, often referred to as nano-imprint lithography (NIL), can rapidly pattern relatively large area substrates with feature sizes below 100 nm. Creating patterns at this scale previously required far slower and more costly techniques such as electron-beam writing. NIL, used in combination with related technologies, can create an entire wafer of integrated optical structures with nanometer features in minutes. Figure 1 shows an array of replicated lenses formed on the surface of a 4-in. silicon wafer.
Figure 1. Replicated lens array on a silicon wafer.
Wafer-level NIL promises reductions in the size, cost and complexity of integrated optics, while improving performance. Integrating embossed components on a wafer offers major size reductions. Producing truly "monolithic" optical systems could reduce centimeter-sized systems to millimeter-sized die. Optical performance is improved by locating components physically closer together, reducing the optical path length and the attendant losses. For example, optical elements such as lenses can be fabricated directly onto optical sources, such as lasers. Figure 2 shows diffractive lenses formed directly on a VCSEL laser wafer.1
Figure 2. Portion of a diffractive lens array on a VCSEL wafer, 200x.
Surprising to the non-expert, the small physical size of the nano-structures also creates optical elements that differ significantly in some characteristics from normal macro-optics. Optical elements with feature sizes smaller than the wavelength of the incident light, sometimes referred to as sub-wavelength elements, interact differently with the light than optical elements with larger feature sizes. More specifically, light interacts in different ways with periodic structures having spacing smaller than its wavelength.2 The altered interactions create a variety of devices with characteristics that differ from similar but larger structures.3
The sub-wavelength optical elements offer some new, desirable characteristics. Sub-wavelength grating structures, for example, may have a wider acceptance angle than conventional gratings. Wider acceptance angles relieve manufacturing tolerances and may permit automated assembly, further reducing costs. Sub-wavelength gratings also perform more consistently over a broader range of wavelengths than conventional gratings.
NIL has demonstrated a growing variety of nano-optical components, including gratings, filters, anti-reflective coatings, refractive and diffractive components, fresnel lenses, waveguides, and lens arrays. With all of these potential products and proven benefits, why has nano-optics languished so long in the laboratories? Because formidable difficulties must be surmounted to replicate these tiny dimensions in volume manufacturing processes.
To understand the manufacturing challenges that are now being overcome, look at the basic processing techniques that have been developed. NIL in its various forms has five processing steps:4
- Design & fabricate a master stamp with features patterned by high-resolution techniques.
- Impress the master stamp on a layer of suitably prepared material.
- Maintain the master stamp pressure while the material first flows to duplicate the stamp, and then solidifies to preserve the duplicate pattern.
- Separate the patterned replica from the master stamp, without damaging nanoscale features.
- Post-process the replica further for its desired optical function.
The starting point for NIL is the creation of a master stamp or mold. The ultimate resolution capability of the embossing depends solely on the master stamp. The design and fabrication of the master stamp is critical to minimum feature size, uniformity, fluid flow, and ease of separation. Designing a stamp to allow quick and complete fluid flow of the softened material is challenging. Once the design is satisfactory, it must be transferred to the master. Fine-line approaches, such as electron beam, are used to create nanoscale features on the master stamp. Electron beams have fine-line capability, but direct e-beam writing has a working area only a few millimeters square, and is relatively slow. Stamps used for hot embossing or for injection molding are commonly made from nickel or steel. Stamps for UV embossing must be of UV-transparent materials.
While the completed master stamp could be directly used to produce many replicas, wear or deterioration through repeated use would require creating a new master stamp. In practice, to preserve its pristine quality, the completed master stamp generally is cloned, using the replication techniques described here. Cloned sub-masters are used to produce the nanoscale features in volume production, either simultaneously over a whole substrate, or repetitively, at chip size, onto a substrate.
The substrates may be silicon, quartz, glass, or other materials. Replication layers are generally coatings of heat-curable or UV curable polymers, with carefully controlled coating uniformity and thickness. While polymers are common materials for embossing, they may not offer the dimensional precision or the long-term stability under challenging environmental conditions demanded of some system components. This need has been filled by the development of special materials, such as of sol-gel organic/inorganic composite materials. These composites can be UV cured or thermally cured like polymers, and after curing display optical and mechanical properties close to those of glass.5
Embossing and curing the pattern on the substrate varies with different processes. Once the coating is cured, the stamp must be separated from the cured material, without damaging the replicated pattern. As feature size becomes smaller, clean separations are more difficult. A special coating may be applied to the stamp before molding, to facilitate separation.
The embossed wafer may be post-processed by etching, deposition, or by repeating further lithographic processes. The cured patterned material can be the etch resist, for reactive ion etching of the underlying substrate. Alternatively, the patterned material may be used as a resist for metal deposition and lift-off. Post-processing might also include additional lithography, further depositions, and the addition of antireflective or protective coatings.
Hot embossing of wafers uses the cloned master die as the wafer stamp. Wafers are first coated with a thermoplastic polymer, that is, a polymer that softens when heated. The wafer is positioned in a controlled-atmosphere stamping chamber. After heating the wafer, the stamp is brought down with pre-determined pressure and temperature to soften the polymer and simultaneously form the image on the whole wafer. After allowing time for polymer flow during heating, the stamp and wafer are cooled to harden the image. To maintain the pattern, pressure must continue to be applied until both stamp and substrate are cooled below the thermoplastic softening point.
The hot-embossed wafer may be used for its embossed features, or it may be post-processed as a resist for etching, material deposition, and lift-off patterning. For resist use, curing may be followed by a light etch to remove the remaining traces of materials from the clear areas.
Step and Stamp
An alternative hot embossing approach is step-and-repeat stamping, using a smaller than wafer size master stamp.6 Step-and-stamp imitates the action of step-and-repeat optical steppers used in semiconductor fabrication. The smaller master stamp is simpler to design, produce, and control. It is also less costly than a full wafer stamp. The step-and-repeat approach mitigates the dimensional, planarity, flow, and separation anxieties inherent in wafer embossing, by literally reducing the problem. The tradeoffs include the added time required for multiple repetitions of the stamping process, the need to confine heating to the stamp area, and the cost and complexity of a precision step-and-repeat apparatus.
Cold embossing uses UV light to polymerize and solidify materials after forming by the stamp. The starting materials are thin films of UV-curable materials. Typical materials may be monomers, or inorganic-organic hybrid polymers, which are polymerized by UV to polymers with desirable optical properties. The films may be deposited onto silicon, gallium arsenide, or similar wafers. The UV-transmissive patterned master template is pressed against the film, imprinting the pattern, which is UV cured before pressure is released.
UV embossing allows accurately aligning and overlaying multiple levels of features. UV embossing has a clear advantage over hot embossing in allowing multi-level applications, since the first level is polymerized after exposure, making it irrevocably formed and dimensionally stable, regardless of subsequent layer applications and exposures.
UV embossing is a low pressure, room temperature process, with little shrinkage of the pattern. It also allows partial UV casting, by limiting the illumination to maintain clear areas on the wafer. Cold embossing may be done on one or both sides of the wafer. Double-sided embossing allows micromolding. The laser lens array shown in Figure 1 was formed by cold embossing.
As with hot embossing, UV embossing may be used with a single wafer size stamp, or with a smaller stamp in a "step and flash" procedure similar to the "step and stamp" described above.7
Injection molding of optical components offers throughput and cost advantages for high-volume production. Injection molding can produce 3-D objects and integrated multicomponent systems. To move beyond conventional injection molding and capture nanofeatures, electroformed metal inserts are nanopatterned and included as part of the master mold.
Double-sided injection molding at wafer scale has been used to create optical structures on both sides of a supporting substrate.8 These structures may be the same on both sides of the substrate, or they may be different; for example, combining a diffractive lens on one side with a refractive lens on the other. The two sides may be processed separately, or simultaneously. In either case, precise alignment must be made between the features through the intervening substrate.
Injection molding has been used to produce optical components in hundreds of thousands. However, the high tooling cost can only be justified for these large quantities. Injection molding draws upon a wider range of materials than direct embossing. Acrylics or polycarbonates may be used for all-plastic systems in low-cost, high-volume injection molding, producing inexpensive commercial products in the hundreds of thousands.
Comparison of NIL techniques
Table 1 provides a summary comparison of the three embossing processes. Hot embossing, at wafer level or by step-and-stamp, lends itself to single-side patterning on thermoplastic materials. Process temperatures are typically 90°C above the glassivation temperature. Throughput is limited by the time required to heat, hold, and cool the die and substrate. Step and stamp avoids the high cost and tolerances of a wafer-size stamp, at a penalty in multiplying the time delays.
Table 1. Comparison of embossing production processes.
Hot embossing has some well established and perhaps more profitable commercial applications. Music CDs are commonly replicated in volume by embossing. Roll-to-roll hot embossing has been used to add interesting color effects to wrapping paper, and to produce material with difficult to counterfeit optical patterns for security documents, such as identification cards.
Cold (UV) embossing may be preferred for critically dimensioned materials. It can be a room temperature process, eliminating the heating and cooling times and thermal expansions/contractions of hot embossing. It allows single or repeated multiple patterns to be layered on one or both sides of a substrate with accurate alignments. Both sides of the substrate may be embossed with identical or different patterns. For applications with more critical planarity, dimensional tolerances, or environmental requirements, cold embossing of photosensitive organic-inorganic sol-gel materials gives results close to those of glass.
Injection molding produces 3-D molding, allowing entire systems to be molded as a unit. It is not a room temperature process; processing time and temperature depend on the materials used. These may be inexpensive commercial plastics, or highly accurate specialty materials. Patterning can be single- or double-sided, in 3-D, although not generally multilayered as in cold embossing. The high tooling cost makes injection molding practical for high-volume (e.g. 100,000 units and up) products.
Figure 3. Replicated pattern generator forming cross-hair pattern from a single laser beam.
A mixture of techniques and equipment from the semiconductor and micropackaging industries has been adapted to nano-optics fabrication. Much of the development work reported above used one-of-a-kind laboratory equipment, built for that purpose, or modified equipment, originally developed for other purposes. One report referred to a modified semiconductor mask aligner. Another recent paper reported demonstrating hot embossing using a standard flip chip aligner-bonder.9 One nano-optics manufacturer has developed customized robotic embossing systems specifically for volume production. Only recently have major equipment manufacturers announced production equipment intended to meet the challenges of nanometer optics. Dedicated aligner-steppers with sub-20-nm embossing resolution are now available.
Figure 4. Nano-optical building blocks, 1.4-mm square, 0.5-mm thick.
Product applications based upon NIL include wavelength monitors, beam splitters, pattern generators, polarization filters, switches, and many others. Figure 3 shows a nano-optic pattern generator for forming a cross-hair pattern from a single laser beam. Nano-optics allows generating essentially an arbitrary pattern of dots in this manner.
Also commercially available now are replicated components that are high performance building blocks for fabricating optical assemblies.10 These offer manufacturing advantages in comprising a common family of similar products, allowing standardization of manufacturing operations and equipment. Figure 4 shows a typical component size.
Embossing has come a long way since Chaucer. After decades of laboratory development, nano-embossing promises to open new optical frontiers with truly monolithic production systems. The growing commercialization of nano-optic systems will subject them to the scrutiny and judgment of the marketplace. Success in surmounting the manufacturing challenges will determine whether nano-embossed optical systems remain a technology niche, or become a broad new technology opportunity.
- Chr. Gimkiewicz et al.,"Wafer-scale replication and testing of micro-optical components for VCSELs," SPIE Vol. 5433, Micro-Optics, VCSELs, and Photonic Interconnects, Strasbourg, France, April, 2004.
- J. Seekamp, "Optical Applications of Nanoimprint lithography," In: Alternative Lithography, Sotomayor Torres (Ed.), Kluer Academic/Plenum Publishers, 2003, p. 290 ff.
- M. Schnieper et al., "Fabrication and applications of subwavelength gratings," Diffraction Optics 2003, Oxford, U.K. Sept 2003.
- M. T. Gale, "Replication technology for micro-optics and optical microsystems," SPIE Vol. 5177, Gradient Index, Miniature, and Diffractive Optical Systems III, San Diego, California, August 2003.
- Chr. Gimkiewicz et al., "Cost-effective fabrication of waveguides for PLCs by replication in UV-curable sol-gel material," SPIE Vol. 5451, Integrated optics and photonic integrated circuits, Strasbourg, France, April 2004.
- T. Haatainen et al., "Pattern transfer using step & stamp imprint lithography" Physica Scripta Vol. 67, pp. 357-360, 2003.
- T.C. Bailey et al., "Step and flash imprint lithography," In: Alternative Lithography, Sotomayor Torres (Ed.), Kluer Academic/Plenum Publishers, 2003, pp. 117-137.
- M. Rossi, "Micro-optical modules fabricated by high-precision replication processes," OSA Diffractive Optics and Micro-optics meeting, June, 2002.
- T. Haatainen et al., "Step & stamp imprint lithography using a commercial flip chip bonder," SPIE 25th Annual Symposium of Microlithograpy, Emerging Lithographic Technologies IV, Santa Clara, California, 2002.
- H. Kostal, J. Wang, "Nano-optic devices enable integrated fabrication," Laser Focus World, Vol. 40, # 6, June 2004.
GEORGE A. RILEY, founder and owner, may be contacted at FlipChips Dot Com, 210 Park Avenue #300, Worcester, MA 01609; (508) 753-3572; e-mail: email@example.com.