Beam shaping: four main fabrication techniques
INO’s expertise in this field draws on numerous design tools as well as a highly diverse array of fabrication techniques. These techniques can be divided into four main categories: optical thin films, photolithography, holography, and etching. In addition, INO disposes of numerous characterization tools as well as plastic injection molding replication capabilities. Interaction with INO's optical design department is a major asset, because conventional refractive elements (lenses, windows, etc.) often play a key role in resolving beam shaping issues. INO does not fabricate these elements, but acquires them from external suppliers as needed. However, the assembling and testing of individual optical components into the final optical system is often carried out by INO's specialists.
A few basics…
Beam shaping can be defined as the science of maintaining or redistributing the energy profile of a beam in a controlled manner. The most common example is the application of antireflective coatings on various components; other well-known examples include wavelength filters, laser beam homogenizers for machining applications, and computer-generated holograms used to divide incident beams into sub-beams for machining and vision applications. Applications for beam shaping exist in virtually all optics-related fields, since the manipulation of light rays is the basis for all optical systems.
At INO, beam shaping involves the use of refractive and reflective conventional optical elements (lenses, mirrors, prisms, etc.) on the one hand, and the use of microstructured elements whose properties are based on interference and diffraction, on the other. An example of such a component is a substrate covered with several layers of optical thin films: the numerous reflections from the various layers interfere with each other to achieve the desired spectral response. Examples of less conventional components include surface-relief diffractive optical elements etched with predetermined patterns. Components like these can perform functions similar to those of conventional optical elements, but can also produce transformations with no equivalent in classical optics. This is the case with computer-generated holograms used to divide a single incident beam into a number of secondary beams.
Our fabrication technologies
INO's activities in beam shaping are supported by four main fabrication technologies carried out in class 100 and 1000 clean rooms, depending on the stringency of the component requirements: optical thin films (OTF), photolithography, holography, and etching.
Optical thin films (OTF)
INO's first sustained commercial activity was related to OTF. Sales of our graded reflectivity mirrors (GRMs) earned INO an international reputation in this area. That was in 1987. Since then, other components have been developed, including apodized phase masks for side lobe elimination in fiber Bragg gratings, as well as a variety of filters based on dielectric and/or metallic thin film stacks for use in a wide range of applications, notably cryogenic and astronomic. In fact, we are currently collaborating with the company COMDEV to develop a custom filter for the James Webb Space Telescope (JWST). Scheduled for launch in 2013, the JWST will replace the Hubble telescope and is one of the most ambitious international projects in existence.
A process involving two technologies
The optical thin film deposition processes available at INO involve two technologies. The first of these, ion plating, uses a plasma to densify the optical thin films, enhancing their optical and mechanical properties. Dense layers are resistant to scratching, delamination, and hostile environments and can also withstand very high optical powers. The second technique, ion-beam assisted deposition, is a physical vapor evaporation technique assisted with an ion gun to densify the optical thin films and generate the same benefits as mentioned above. INO has two ion guns that use oxygen or argon as ionization gas. Neither ion plating nor ion-beam assisted deposition raises the substrate temperature during the deposition process, a definite advantage for keeping stress levels to a minimum.
The deposited materials available cover a spectral range from the UV (0.23 µm) to the far-IR (14 µm) and include oxide-based dielectrics, metals (Au, Ag, Al), and amorphous silicon. Dielectric materials deposited by ion plating have demonstrated excellent stability at cryogenic temperatures of 77°K, a crucial property for filtering applications in astronomy. Indeed, the development of thin film stacks of amorphous silicon and silicon dioxide allowed the realization of a uniform reflector exhibiting 90% reflectance without phase jumps. The use of these reflectors in a Fabry-Perot cavity enabled the creation of a filter capable of operating at cryogenic temperatures and tunable over a wide spectral range. This is the filter being developed for the JWST space telescope project mentioned earlier.
Photolithography is the process of imparting a surface-relief pattern onto a photosensitive layer deposited on a substrate, a technique first perfected by the microelectronics industry. In simple terms, the process involves the use of a photomask, an ultraviolet (UV) light source, and a UV-sensitive medium called photoresist. The photoresist is deposited on the substrate using a spinner to obtain a layer about 1 µm thick. As for the mask, it is made up of a glass plate covered with a complex chromium pattern (making the plate transparent or opaque) that is put in contact with the photoresist layer. After UV illumination and development of the photoresist using an appropriate chemical agent, the mask pattern is revealed as a two-level surface-relief onto the photoresist, i.e., the photoresist thickness is modulated according to the pattern of the mask. This process does not damage the mask, which can be reused numerous times for cost-effective fabrication of identical components. In principle, production of multi-level components is possible by running several successive photolithographies on a single substrate, but experimental limitations severely restrict the range of components that can be produced in this way.
There are two key aspects in the photolithographic process: the design of the pattern required to obtain the desired optical function, and the fabrication of the corresponding photomask. Most design work is conducted using a versatile, high-performance software suite developed at INO. Some commercial software is also used. Photomasks are not fabricated at INO, but are generated by external firms using electron beam writing systems. These systems provide very high resolution if required (< 1 µm) and make it possible to create quasi-arbitrary patterns, which explains the extreme versatility of photolithography.
Components produced using photolithography are often called “diffractive optical elements” or “computer-generated holograms.” Their optical behaviors are generally based on diffraction rather than on refraction/reflection, as is the case in conventional optics. They are usually used to divide beams into numerous sub-beams, a function with applications primarily in computer vision and metrology.
The holographic fabrication process shares certain similarities with photolithography in that both involve the recording of a surface-relief pattern on a photoresist layer. However, whereas a photomask is used in photolithography, holography records the interference pattern created by superposing at least two coherent laser beams. If the two laser beams are collimated, the interference pattern is made up of equidistant parallel lines. By modifying the collimation state of one or both beams, it is possible to alter the interference pattern to obtain variable-spaced lines. Such a pattern is said to be "chirped."
Photolithography vs. holography
Compared to photolithography, holography is a difficult recording process. Numerous precautions must be taken to maintain optical path stability to within a fraction of micron during the recording process. Furthermore, holography is much less versatile than photolithography in terms of the variety of relief patterns that can be produced. So what is the utility of this process? Holography is recognized for its ability to generate very high spatial frequency gratings (5,000 lines/mm) that are difficult to achieve with photolithography. In addition, the spatial frequencies that can be produced are continuous, whereas photolithography is limited to discrete frequencies. These characteristics played a key role in the success of the holographic phase masks jointly developed with StockerYale for telecommunication applications.
Another INO achievement pertaining to holographic recording are the holographic beam samplers (HBS) developed in collaboration with Gentec-EO. These components take advantage of the intrinsic sinusoidal form of holographic interference patterns, along with INO’s etching capabilities, to exhibit very high damage thresholds.
Although photoresist is the basic recording material for photolithography and holography, it features low mechanical, chemical, thermal, and optical resistance. This is why INO has acquired reactive ion etching (RIE) systems to transfer the photoresist surface-relief patterns to their underlying substrates made of high quality optical materials. Etched components can withstand significant temperature variations as well as high optical power densities. They are also easy to clean and take on the same resistance to mechanical and chemical agents, and to UV rays, as the optical material used. Phase masks and holographic beam samplers are good examples.
INO currently has two RIE systems capable of processing substrates up to 20 cm in diameter. These systems and the associated etching recipes are perfectly adapted for the transfer of structures to fused silica (SiO2), one of the best materials for applications from the UV to the near IR. These recipes even make it possible to introduce controlled deformation of the photoresist profile during its transfer to the substrate. Lastly, the use of certain gases in RIE systems allows for the etching of other materials such as zinc selenide (ZnSe) and silicon (Si).
Take a look at our various achievements and discover the range of possible applications or contact us to find out how we can meet your specific needs.