Patent Application
20070253668
The present invention relates to a method of producing germanosilicate that includes a refractive index change Δn (within the formed germanosilicate). The invention also relates to the formation of a waveguide, including a waveguide that can be used in optical circuits. The method of the invention includes forming a gel from a sol comprising germaniumoxide and silicate by means of a sol-gel process. The method further includes forming germanosilicate by annealing the gel under elevated temperature and exposing the formed germanosilicate to pulsed UV light of at least 350 mJ/pulse.
Optical circuits such as planar lightwave circuits (PLCs) are used in a variety of applications, for example in the area of communication systems. PLC functional devices such as channel waveguides, array waveguides (AWG), and other waveguide based devices, are for instance essential for the realization of high-speed optical telecommunication network. In photonic integrated circuit devices, a variety of semiconductor optoelectronic devices are monolithically integrated and interconnected with waveguides. The telecommunications industry uses integrated optics for multigigabit bidirectional communication data transmission, signal splitting and loop distribution. In Community Access Television (CATV) for example, modules that include optical circuits are used for external modulation in fiber-optic-based signal distribution systems. The conventional approach for fabricating waveguide-based devices involves depositions of waveguiding core materials, photolithography, etching and deposition of over-cladding materials [Zuo, L. et al., Optics Letters, 28 (2003) 12, 1046-1048; Holmes, A. S., et al., Applied Optics, 32 (1993) 4916-4921]. This approach requires many steps and complex processes.
Micro-scale optical components such as guiding channels can also be formed by illuminating photosensitive materials with ultraviolet (UV) radiation to induce refractive index changes, a method known as the direct writing technique. A laser is focused onto the desired workpiece, optionally by means of a mask, and a desired pattern or shape is written by moving either the beam itself or the respective workpiece. This simple fabrication technique has attracted considerable interest since, compared to traditional lithographic techniques, it involves fewer steps and does not require the use of etchants. Furthermore, by means of direct writing it is possible to form smooth interfaces in buried channel waveguides. The use of lasers in the process also allows for flexibility and for the creation of waveguides with novel shapes such as unique curves and bends.
However, the fabrication of waveguide based devices by the direct writing technique requires a high refractive index change (Δn) by UV illumination, which demands an appropriate choice of photosensitive materials. So far this has been achieved by using polymers such as polymethyl-methacrylate or organic materials. These materials are not very stable, show low performance due to poor mechanical resistance, and need low temperature functioning environment. An alternative is the use of photosensitive inorganic silica based glasses. They have a high reliability and good compatibility with optical fiber, are of low cost and of good performance with low propagation loss (<0.3 dB) in doped silica waveguides [Zhang, Q. Y. et al., Chemical Physics Letters 368 (2003) 183-188; Zhang, Q. Y. et al., Chemical Physics Letters 379 (2003) 534-538]. Silica (SiO2) glasses containing germanium dioxide have attracted considerable interest because germanium dioxide is well established as an iso-structural analogue of SiO2 and is sensitive to UV.
Silica glass materials can be deposited by flame hydrolysis deposition (FHD), plasma enhanced chemical vapor deposition (PECVD), inductive coupled plasma enhanced chemical vapor deposition (ICP-CVD) and the sol-gel method. The latter is a chemical process that has inherent advantages over other small-scale fabrication methods since it allows for flexible chemistry, and the resulting materials are both homogenous and of high purity. It is furthermore a low-cost method, which is flexible with respect to design and material changes such as dopants. Additionally it allows for the fabrication of large-area coatings where required. When combined with a coating technique it is a promising route to synthesize doped silica based materials (e.g. Ho, C. K. F. et al., Proceedings of the 11th European Conference on Integrated Optics, [2003] 305-308). It offers the flexibility to tailor the optical properties and to control the molecular structure of the materials through chemistry and processes.
Doped silica based materials are typically required when used with a silica optical telecommunications fiber, in order to match the refractive index of the waveguide materials of the planar optical device to that of the fiber. The transmission of light via optical fibers by means of total reflection is achieved by a difference of optical refractive indices between a cladding portion of silica glass and a core portion, in which elements such as germanium (Ge) are added, thereby slightly increasing the refractive index. Thus the refractive index difference for the waveguide materials can likewise be achieved by doping with germanium. Germanium doped silica glasses (germanosilicate) have already been widely used for photosensitive fiber brag gratings (FBG) and waveguide based devices for optical communication [see e.g. Miyake, Y. et al., Journal of Non-Crystalline Solids, 222 (1997) 266-271].
However, the refractive index change induced by UV radiation is mostly in the order of 10−5˜10−4, which is too small to be able to form waveguiding channels.
Accordingly it is an object of the present invention to provide a method of obtaining a refractive index change that is suitable for the formation of waveguiding channels in germanosilicate that has been fabricated by the sol-gel method.
In one aspect the present invention provides a method of producing germanosilicate. The formed germanosilicate possesses a refractive index change Δn.
In a further aspect the invention provides a method of forming a waveguide that can be used in optical circuits.
The methods include forming a gel from a sol comprising germaniumoxide, or a precursor thereof, and silicate, or a precursor thereof, by means of a sol-gel process. The method further includes forming germanosilicate by annealing the gel under elevated temperature. The method also includes exposing the formed germanosilicate to UV light of at least 350 mJ/pulse.
In yet a further aspect the invention relates to germanosilicate comprising a refractive index change Δn, obtainable by a method of the present invention.
In yet another aspect the invention relates to a waveguide obtainable by a method of the present invention.
The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:
The present invention is based on the surprising finding that high refractive index changes Δn can be achieved by radiating germanosilicate formed by a sol-gel process with UV. This finding lead to the development of the method of the invention as explained in the following. As explained above, refractive index changes induced by UV radiation of germanosilicate were so far too small to be able to form waveguiding channels. Only recently, Sakoh et al. reported a refractive index change of about 10−3 in a germanosilicate glass fabricated by plasma enhanced chemical vapor deposition [Sakoh, A. et al., Optics express, 11 (2003) 21]. The method of the present invention, which uses the sol-gel process, is suitable for producing germanosilicate with a refractive index change of more than 10−3, including a refractive index change of more than 5×10−3.
The sol-gel process used in the present invention can be performed according to any protocol. The process may for example include forming two separate sols, which are then combined. One of these sols includes silica, while another one includes germaniumoxide. The silica, the germaniumoxide, or both may be formed from a precursor, for example in situ during the reaction process.
Where such a process is used, in which two separate sols are combined one sol is a silica sol, i.e. a suspension of colloidal silica-based particles, for instance nanoparticles. This sol may for instance be generated by hydrolysis of a precursor such as silicon alkoxide. The hydrolysis of a silicon alkoxide is thought to induce the substitution of OR groups linked to silicon by silanol Si—OH groups, which then lead to the formation of a silica network via condensation polymerisation. Examples of silicon alkoxides include for instance methyl silicate (Si(OMe)4), ethyl silicate (Si(OEt)4), propyl silicate (Si(OPr)4), isopropyl silicate (Si(Oi-Pr)4), pentyl silicate (Si(OCH5H11)4), octyl silicate (Si(OC8H17)4), isobutyl silicate (Si(OCH2iPr)4), tetra(2-ethylhexyl)orthosilicate (Si(OCH2C(Et)n-Bu)4), tetra(2-ethylbutyl)silicate (Si(OCH2CHEt2)4), ethylene silicate ((C2H4O2)2Si), tetrakis(2,2,2-trifluoroethoxy)silane (Si(OCH2CF3)4), tetrakis(methoxyethoxy)silane (Si(OCH2CH2OMe)4), benzyl silicate or cyclopentyl. Typically, but not limited thereto, sol preparation by hydrolysis of a silicon alkoxide can be performed in a mixture of water and an alcohol such as ethanol or isopropanol. Any known catalyst such as hydrochloric acid or ammonia may be added as well. Hence, sol-gel protocols using acid-catalyzed, base-catalyzed and two-step acid-base catalyzed procedures may for instance be followed. In embodiments that employ an acid-catalysed process, the pH value may for instance be in the range of about 1 to about 4, such as for example about pH 3.
A further sol generated in a process that includes a first step of forming two sols is a germaniumoxide sol, which is typically prepared by a protocol similar to the preparation of a silica sol above. In case a protocol of hydrolysis of a germanium alkoxide is carried out, generally an alcohol such as isopropanol is used as a solvent. Typically the use of water is avoided where the germanium alkoxide is reactive/sensitive to moisture. Examples of germanium alkoxides include, but are not limited to, tetrapropyloxygerman, tetramethyloxygerman, o-phenylene germinate, ethylene germanate or 2,2′-spirobi[naphtho[1,8-de]-1,3,2-dioxagermin. Further dopants such as boron- or tin-based compounds may be used where desired, for instance in order to increase the photosensitivity of the germanosilicate layer produced.
By combining these two sols in a second step a sol is obtained that contains silica and germaniumoxide. Upon varying the ratio of each sol used for mixing, a desired molar ratio of SiO2 to GeO2 can be generated. Where a one-step method is used to form a sol that includes germaniumoxide and silicate, the respective ratio is accordingly determined by the amounts of silica- and germaniumoxide-compounds applied. Any ratio of SiO2 to GeO2 may be chosen, as long as a refractive index change can be obtained (see below). Depending on the remaining conditions used, it may be advantageous to select a ratio of SiO2 to GeO2 in the range between 8:1 and 2:1 in order to obtain a high refractive index change (see
The sol subsequently undergoes a catalysed transition (cf. above) to form a gel. Before this occurs, the sol may be transferred in order to achieve a desired form. Alternatively the sol may for instance be prepared in a device that already provides the desired final form. In some embodiments forming a gel from a sol includes contacting the sol with a substrate. Contacting the sol with a substrate may for instance include depositing the sol onto a substrate.
As an illustrative example, it may be intended to obtain germanosilicate in form of a layer. In such embodiments the sol may for instance be deposited onto a substrate such as silica glass, by for example casting, molding or in form of a coating. As already indicated above, generally the process of depositing assists the formation of a gel. In yet other embodiments contacting the sol with a substrate may be performed by immersing a respective substrate, which may be of any shape, into the sol that contains silica and germaniumoxide.
It may be desired to clean a selected substrate before generating a respective gel thereon, for instance by means of a solution of a strong base such as potassium hydroxide. While not required when using the method of the present invention, in some embodiments an undercladding may be deposited onto the substrate before depositing the germanosilicate thereon. Where a layer of germanosilicate is formed, it may be of any desired form and thickness. Thick layers may for example be produced by means of repeated coating (cf. below). Where desired, a wetting agent may be used prior to such coating. Examples of such a coating include, but are not limited to, dipping or spin coating.
The method of the present invention further includes forming germanosilicate by annealing the obtained gel under elevated temperature. Any elevated temperature may be chosen that does not prevent a later refractive index change to occur. The increase in temperature may be generated by any means, including irradiation. The exposure to an elevated temperature may be selected to also contribute to or to allow for a later refractive index change of a desired extent. In some embodiments the elevated temperature is within a range of about 500° C. to about 1000° C., for example in the range of about 800° C. to about 1000° C. In some embodiments the elevated temperature is about 900° C. An illustrative example of annealing is rapid thermal annealing (RTA) at a selected temperature. Typically layers annealed below about 900° C. are porous and layers annealed at about 900° C. and above are dense. Any period of time may be selected for annealing the gel. Typical periods of time used in the art may be employed, for example within the range of about 2 sec to about 1 minute, such as e.g. 15 sec or 20 sec. The annealing may furthermore be repeated where desired.
The refractive index change obtainable by the method of the present invention generally increases with the temperature selected for annealing up to a certain limit. An example for this tendency for embodiments where rapid thermal annealing is employed is depicted in
In embodiments where it is desired to generate a thick layer, for instance of a thickness in the range of about 1 μm to about 10 μm, such as 3 μm, 10 μm or 20 μm, a first thin layer may be generated by a selected technique as indicated above and thereafter annealed. Subsequently a second layer may be generated in the same or a different way and annealed as above. Where desired subsequently a third, fourth etc. layer may be deposited and annealed. Such a procedure is for example typically performed in the manufacture of waveguide based devices. Where desired, the outer periphery of a generated layer may be removed before further processing as described in US patent application 2004/0115347. This step may be chosen to address the so called “edge bead” formation in order to prevent the formation of micro-cracks at the edge of the layer.
In some embodiments the method of the present invention further includes a consolidation treatment of the annealed germanosilicate by further exposure to elevated temperature. The term “consolidation treatment” as used herein thus refers to a second exposure of annealed germanosilicate to elevated temperature, regardless of the thickness of the germanosilicate. Thus the term is for instance equally used for a respective treatment of a thin single-layer film and a film obtained by multiple layer depositions. This may for instance be performed where a density of the formed germanosilicate is desired which is higher than the density that is typically obtained during annealing as described above. This may be the case in embodiments where a thick layer of germanosilicate is generated by successive formation, e.g. deposition, of thin layers, for example. In some embodiments, such as in the manufacture of waveguide based devices, this consolidation step also helps assuring that no refractive index gradient across the thickness of a respective layer occurs. In embodiments where a thick film is generated by sequential deposition of multiple layers, the first layer is annealed many more times than e.g. the top layer (depending on the number of layers). As thus each individual layer, which becomes part of one thick layer, has been treated differently, a refractive index gradient is usually created across the thickness of the obtained thick layer.
Any treatment with an elevated temperature may be chosen that yields a desired density and that allows for a later refractive index change to occur. As an illustrative example, the germanosilicate may be heated in a furnace. Any period of time may be selected for consolidating the annealed germanosilicate. Suitable periods of time include the range of about 0.5 to about 4 hours, for example about 1 hour or about 2 hours. An optimal period of time for a certain embodiment can easily be determined experimentally.
In order to obtain a high densification effect it is furthermore advantageous to carry the consolidation treatment out at a higher temperature than the temperature used for annealing. Up to ˜1000-1100° C. this consolidation step shows no or only a very moderate influence on the refractive index obtainable by the method of the present invention (compare e.g. the two values with [▪] and without [▴] a consolidation treatment at 1000° C. in
Where it is for instance desired to generate a thick layer of germanosilicate and to obtain a high refractive index change, a procedure to generate a respective layer may thus be chosen that includes e.g. successive depositing of thin layers and subsequent annealing (supra), and that further includes subsequently consolidating the thick layer at a temperature below 1100° C.
The method of the invention further includes exposing the germanosilicate to UV light of an energy of at least 350 mJ/pulse. Typically the UV light is pulsed. As an illustrative example, pulsed UV light of a UV fluency of about 450 mJ/pulse may be chosen. Any UV light source may be employed to generate a respective radiation. An example of a means for the generation of one or more radiation pulses that may be used is a laser. Typically the germanosilicate is exposed to more than one UV pulse. A convenient frequency of pulses and exposure time may be selected. As an illustrative example, the repetition rate may be chosen in the range of about 5 to about 50 Hz, such as for instance about 10 Hz. The exposure time may for instance be selected within the range of about 0.5 minutes to about 5 hours or about 1 minute to about 1 hour. In typical embodiments the fabricated germanosilicate is first consolidated and thereafter exposed to a respective pulsed UV light.
The germanosilicate may be irradiated by any suitable beam form. The UV light may illuminate a part of selected area at a time or on the entire generated germanosilicate at once. As an illustrative example, the UV light may be provided as a beam, for instance in an embodiment where the UV light source is a laser. Several such beams may be provided and, where desired, overlap. In typical embodiments the UV light is of an energy density of at least 122 mJ/cm2 per pulse. A respective density is for instance obtained with a laser beam of a size of about 24 mm×12 mm, where 350 mJ/pulse are applied. In some embodiments the UV light is of an energy density of at least 156 mJ/cm2 per pulse. Such a density may for instance be provided by using an energy of 450 mJ/pulse with a laser beam of a size of about 24 mm×12 mm.
Electromagnetic radiation of any wavelength within the ultraviolet range may be used in the method of the present invention. In some embodiments the wavelength is for instance selected within the near UV (380 to 200 nm), while in other embodiments it is for instance selected within the far UV (200 to 10 nM). As an example, the wavelength may be selected to be 248 nm or shorter. An illustrative example of a means of providing UV light of a respective wavelength is a KrF laser. A further illustrative example is an ArF laser, which provides UV light of a wavelength of 193 nm.
The irradiation by UV light induces a change in the refractive index of the germanosilicate. Without the wish to be bound by theory it is believed that this is due to an induction of an oxygen-deficiency in the germanosilicate. The refractive index obtained was found to decrease upon a subsequent exposure to an elevated temperature in an oxygen atmosphere, while it remaining unchanged in an inert atmosphere (see
The value of Δn can be adjusted by varying the material composition, the annealing temperature, the UV radiation intensity and the exposure time. The value of Δn generally increases with increasing GeO2 content, the radiation intensity per pulse, the number of pulse per second and the exposure time. The method of the present invention allows for a refractive index change of higher than 10−3, including a refractive index change of more than 5×10−3, after UV exposure. The influence of the GeO2 content on the change in the refractive index is illustrated in
In some embodiments a certain area of the germanosilicate is selected which is exposed to UV-light as described above, while the remaining area of the germanosilicate is not exposed to UV light. An exemplary means to achieve radiation of a selected area is the use of a mask (cf.
Where desired, the UV-radiated germanosilicate may furthermore be exposed to a subsequent exposure to an elevated temperature. In some embodiments such a further exposure to an elevated temperature may provide additional stability to the generated germanosilicate, including the obtained refractive index change. Any desired temperature and time period may be selected for such treatment with an elevated temperature, as long as a desired refractive index is maintained. As an example, a postbake may be performed at a temperature in the range of about 100 to about 300° C. and for a time period of about 0.5 hours to about two days. In this regard it should be noted that a prolonged exposure to temperatures in the dimension of the annealing or consolidation temperature may reverse the obtained refractive index change in some atmospheres (cf.
The method of the present invention may be used to generate photonics components on a single substrate. Examples of optical components that may be obtained by the method of the present invention include for instance power splitters, couplers, Y branches etc. Accordingly, the method may be used to generate, e.g. fabricate, a light wave circuit, such as planar light wave circuit. An illustrative example of a planar light wave circuit is a waveguide, for instance a channel waveguide. A respective light wave circuit may for instance be included in a photonic integrated circuit device and/or combined with an optical fiber. Thus the present invention also provides a method of fabricating a waveguide. As explained above, for forming a respective waveguide a sol may for example be deposited on a substrate, turned into a gel, annealed and radiated with UV light of at least 350 mJ/pulse. In embodiments where the waveguide is a channel waveguide, an area of the germanosilicate may be irradiated in order to form the respective channel region of the waveguide (supra).
Where desired, a subsequent further UV illumination may be applied in order to generate additional areas with a refractive index change, for example in order to generate a waveguide with gratings. As an example, this may again be achieved by means of a respective mask (cf.
The present invention thus provides a method that avoids techniques which involve multi-step processes for defining waveguide patterns in the films obtained. The devices are thus easy to fabricate as complex photolithography and etching steps are avoided. An additional advantage of using the direct UV writing technique is its ability to form smooth interfaces in the buried channel waveguides.
Furthermore, the generation of groves and/or channels by means of etching—when using conventional methods of forming waveguides such as flame hydrolysis deposition and etching—leads to surface roughness and hence optical loss, once the respective waveguide is in use. The method of the present invention overcomes these difficulties since it does not require the formation of a physical interface.
While other processes may be contemplated or used, it should be apparent from above that there is presently a need to exploit the advantages of the sol-gel process. The present invention thus allows for the use of silica (SiO2) glasses containing germanium dioxide as a photosensitive material for direct UV writing instead of the presently used polymers or organic materials. The invention thereby also provides a means of forming waveguiding channels using germanosilicate. It should be understood that the method of the present invention is not restricted to germanosilicate or material that includes germanosilicate, but that it is of a general applicability and also suitable for other materials.
In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.
Photosensitive germanosilicate layers were prepared using the sol-gel technique shown schematically in
TEOS and EtOH (ethanol) were mixed in 1:1 ratio by volume. For hydrolysis, acid catalyst HNO3 (nitric acid) was added to maintain pH equal to 3 and water to alkoxide molar ratio (R) of 2. The pH level was measured using a Cyberscan pH2000 pH meter supplied by Eurotech with the combination of glass electrodes from Orion Research Inc. The solution was stirred continuously. This sol is called sol S. For the dopant, a sol called sol G was prepared by mixing TPOG with isopropanol (IPA) in 1:1 volume ratio. As TPOG is very reactive no catalyst was required. Furthermore the reactions were performed in a dry glove box maintained at relative humidity (RH) about 15% by continuous flushing of dry nitrogen. Sol G and sol S were then mixed to obtain a 4 SiO2: 1 GeO2 composition (sol SG) and vigorously stirred.
Films were prepared by spin coating sol SG on a substrate (e.g. Si wafer or silica plate). The spin coated films were heat treated using the rapid thermal processing (RTP) technique in a JIPELEC rapid thermal processor (RTP) for 15 s in the presence of an O2 atmosphere. In order to obtain dense films for the fabrication of PLC, films were annealed at 900° C. and above. Some of the as-deposited films annealed at 900° C. in RTP were further heat treated in a furnace for 1 hour in at various temperatures ranging from 1000° C. to 1200° C. (consolidation). Thick films (e.g. of 3 or 5 μm) were generated by repetitive spin coating and annealing. Each spin coated layer was annealed in RTP. Thick films were usually subsequently exposed to a consolidation heat treatment. For a thick film, the refractive index change (after UV exposure, cf. below) was found to be 10−3 or larger (e.g. 5×10−3 or larger).
The films were then radiated by KrF excimer laser (λ=248 nm) operating at 10 Hz repetition rate at a UV fluency of 450 mJ/pulse with a beam size of about 24 mm×12 mm. Different exposure times were selected as indicated in the figures, usually the exposure time was varied from 1 minute to 60 minutes, since the refractive index changes can be easily adjusted by varying the exposure time. Films (e.g. thick films) may be irradiated for several hours. To stabilize the samples, they were post baked at 140° C. for periods of 1 hour to 24 hours under vacuum.
Using the highly photosensitive materials obtainable by the method of the invention, gratings on waveguides can be easily fabricated as follows:
The highly photosensitive layer (2) is deposited and fabricated on a substrate (5) as described above. To fabricate a waveguide, the highly photosensitive film is then radiated by UV light (10) through a mask (1) as shown in
Having formed the waveguide, the gratings can be fabricated by further radiating the waveguide by UV light, for the second time, through a mask (3) defining the gratings (see
A cladding layer may be deposited. Using the technique described above, the deposition of the cladding layer is very easy and leads to better performance devices since there are no steps in the waveguides and gratings like those fabricated using the conventional etching technique.
Using the above indicated photosensitive materials, graded index waveguides can be easily fabricated as follows:
The highly photosensitive layer (2) is deposited and fabricated on a substrate (5) as described above. To fabricate a graded index waveguide, the highly photosensitive film is radiated by a UV light (10) through a grey scale mask (6) as shown in
A cladding layer may be deposited. As above, the deposition of the cladding layer is easy and leads to better performance devices since there are no steps in the waveguides.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SG05/00414 | 12/7/2005 | WO | 6/1/2007 |
| Number | Date | Country | |
|---|---|---|---|
| 60634499 | Dec 2004 | US |
