CA2296569A1

CA2296569A1 - Process for the fabrication of active and passive polymer-based components for integrated optics

Info

Publication number: CA2296569A1
Application number: CA002296569A
Authority: CA
Inventors: Hans Wilfried Peter Koops
Original assignee: Individual
Current assignee: Deutsche Telekom AG
Priority date: 1997-07-05
Filing date: 1997-07-05
Publication date: 1999-01-21
Also published as: JP2002510408A; ATE414932T1; EP0995148B1; KR20010014400A; EP0995148A1; NO20000013L; WO1999003021A1; KR100477338B1; DE59712982D1; NO20000013D0; NO323741B1

Abstract

The invention relates to a method aimed at producing in an economical manner high-quality active and passive optoelectronic components presenting a high degree of integration and great packing density. According to the invention a high-quality structurable layer of coating polymer is applied to an optoelectronic component. A structure is produced by means of an etching mask in conjunction with highly anisotropic deep etching and the resulting structure filled with monomers by means of gaseous or liquid phase diffusion. Depending on the type of monomer used for diffusion and both temperature and reaction time the optical characteristics of the optical component can be modified in a targeted manner. The method provided for in the invention makes it possible to raise the packing density of future integrated monomode optical devices and allows for the cost-efficient production of large numbers of such devices.

Description

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Process for the fabrication of active and passive polymer-based components for integrated optics The solution according to the invention relates to the fabrication of active and passive polymer-based optoelectronic components. The technical problem to be solved consists in the development of a process geared to the fabrication of passive and active optoelectronic components with a high integration level and high packing density. The fabrication process is to make it possible to control the parameters and properties of the optoelectronic component to be produced, it being intended in particular that the refractive index, nonlinear optical property, polarizability, double refraction and amplification properties should be selectively influenced during the fabrication process.
As described in 1. ] R. Kashyap, in "Photosensitive Optical Fibers: Devices and Applications", Opt.
Fibres Techn. l, pp. 17-34 (1994), present-day fabrication processes for components and circuits of integrated optics are based on optical fiber technology which strives for an "all-fiber" solution for the circuits required in telecommunications.
Integrated optical waveguide circuits are constructed together with active and passive components on expensive semiconductor substrates using even more expensive molecular beam epitaxy or metal-organic deposition from the vapor phase, in order to realize the optical circuits required in telecommunications. A description of such processes can be found in the following sources:

2.] C. Cremer, H. Heise, R. Marz, M. Schienle, G. Schulte-Roth, H. Unzeitig, "Bragg Gratings on InGaAsP/InP-Waveguides as Polarization Independent Optical Filters" J.
of Lightwave Techn., 7, 11, 1641 (1989) 3.] R. C. Alferness, L. L. Buhl, U. Koren, B. I. Miller, M. G. Young, T. L.
Koch, C.
A. Burrus, G. Raybon, "Broadly tunable InGaAsP/InP buried rib waveguide vertical coupler filter", Appl. Phys. Lett., 60, 8, 980 (1992) 99T4081 A.DOC

4.] Wu, C. Rolland, F. Sheperd, C. Larocque, N. Puetz, K. D. Chik, J. M. Xu, "InGaAsP/Inp Vertical Filter with Optimally Designed Wavelength Tunability", IEEE
Photonics Technol. Lett., 4, 4, 457 (1993) 5.] Z. M. Chuang, L. A. Coldren "Enhanced wavelength tuning in grating assisted codirectional coupler filter", IEEE Photonics Technology Lett., 5, 10, 1219 (1993) Alos known is a process for the fabrication of waveguide circuits from polymeric waveguides by mask-assisted exposure processes, as described in source 6.] by L-H.
Losch, P. Kersten and W. Wischmann in "Optical Waveguide Materials" (M. M.
Broer, G. H. Sigel Jr., R. Th. Kersten, H. Kawazoe ed) Mat. Res. Soc. 244, Pittsburg, PA 1992, pp 253-262.
A fizrther known solution is based on the definition of the waveguides through the etching of a step into optically thinner layers. Such a process was described by 7.] K.J.
Ebeling in "Integrierte Optoelektronik" (Springer Verlag 1989) 81 1. A further known process is based on silylation. With the silylation process, waveguides have already been defined in NOVOLAK and investigated for their usability in integrated optics, as described in source 8.] by T. Kerber, H. W.
P. Koops in "Surface imaging with HMCTS on SAL resists, a dry developable electron beam process with high sensitivity and good resolution", Microelectronic Engineering 21 ((1993) 275-278.
2. The therefor required processes for accurate process monitoring were described in source 9.] by H. W. P Koops, B. Fischer, T. Kerber, in "Endpoint detection for silylation processes with waveguide modes", Microelectronic Engineering 21 (1993) 235-238 and in source 10.] by J. Vac, SCI Technol. B 6 (1) (1988) 477.
High refractive index differences can be produced by the implantation of ions with high energies and high doses in PMMA. Such processes were described in source 11.]
by R.
Kallweit, J. P- Biersack in "Ion Beam Induced Changes of the Refractive Index of PMMA", Radiation Erects and Defects in Solids, 1991, Vol. 116, pp 29-36 and in source 12.] by R. Kallweit, U. Roll, J. Kuppe, H. Strack "Long-Term Studies on the 99T4081A.DOC

Optical Performance of Ion Implanted PMMA Under the Influence of Different Media", Mat. Res. Soc. Symp. Proc. Vol. 338 (1994) 619-624. Refractive index differences in solid PMMA material of up to 20% are obtained. However, masking processes must be used for patterning. Owing to the high ion energy and the required absorber layer thickness in the mask, the resolution is limited by the edge roughness achievable with mask production technologies. Electrically switchable regions installed in waveguides can be produced by the diffusion of poled nonlinearly optical materials in polymers. In this manner it is possible to achieve the combination for electrical adjustability of optical paths or the influencing of optical phenomena.
13.] M. Eich, H. Looser, D. Y. Yoon, R. Twieg, G. C. Bjorklund, "Second harmonic generation in poled organic monomeric glasses", J. Opt. Soc. Am. B, 6, 8, (1989) 14.] M. Eich, A. Sen, H. Looser, G. C. Bjorklund, J. D. Swalen, R. Twieg, D.
Y.
Yoon, "Corona Poling and Real Time Second Harmonic Generation Study of a Novel Covalently Functionalized Amorphous Nonlinear Optical Polymer", J. Appl.
Phys., 66, 6, ( 1989)R. Birenheide, 15.] M. Eich, D. A. Jungbauer, O. Herrmann-Schonherr, K. Stoll, J. H.
Wendorif, "Analysis of Reorientational Processes in Liquid Crystalline Side Chain Polymers Using Dielectric Relaxation, Electro-Optical Relaxation and Switching Studies", Mol.
Cryst. Liq. Cryst., 177, 13 (1989) 16.] M. Eich, G. C. Bjorklond, D. Y. Yoon, "Poled Amorphous Polymers of Second Order Nonlinear Optics", Polymers for Advanced Technologies, 1, 189 (1990)M.
Stalder, P. Ehbets, "Electrically switchable diifractive optical element for image processing", Optics Letters 19, 1 (1994) Free configurability of the pattern is achieved if, using the new process of additive lithography, 3-dimensional patterns and periodic arrangements are constructed on any cheap substrates and if the refractive index of the deposited material is adapted to the task through material selection of the precursors. Named as sources with regard to the aforementioned subject area are [sources 8-16] as well as the below-listed sources.
99T4081A.DOC

17. ] M. Stalder, P. Ehbets, "Electrically switchable diffractive optical element for image processing", Optics Letters 19, 1 ( 1994) 18.] H. W. P. Koops, R. Weiel, D. P. Kern, T. H. Baum, "High Resolution Electron Beam Induced Deposition", Proc. 31. Int. Symp. On Electron, Ion, and Photon Beams, J. Vac. Sci. Technol. B 6( 1 ) ( 1988) 477 19.) H. W. P. Koops, J. Kretz, M. Rudolph, M. Weber "Constructive 3-dimensional Lithography with Electron Beam Induced Deposition for Quantum Effect Devices", J.
Vac. Sci. Technol. B 10(6) Nov., Dec. (1993) 2386-2389 20.] H. W. P. Koops, J. Kretz, M. Rudolph, M. Weber, G. Dahm, K. L. Lee "Characterization and application of materials grown by electron beam induced deposition", Invited lecture Micro Process 1994, Jpn. J. Appl. Vol. 33 (1994) 7107, Part. 1 No. 12B, December 1994 21.] Hans W. P. Koops, Shawn-Yu Lin, "3-Dimensional Photon Crystals Generated Using Additive Corpuscular-Beam-Lithography" patent specification filed on 20.08.1995 It is thus possible to construct narrow-band, geometrical and permanently adjustable filters and highly reflective mirrors on a miniaturized scale from photon crystals. If the photon crystals produced using deposition techniques are combined with nonlinear-optical materials in the interstices of the deposited materials, it is possible to obtain miniaturized adjustable optical components [source 21].
Present-day surface-imaging processes make it possible, using optical phase masks and steppers and with the use of dry etching processes, to achieve the resolution and height conditions required for optical gratings and other optical elements. This can be accomplished by the lithography and process equipment of the manufacturers of electronic storage devices of a size of 1 G-bit and with corresponding resolution. High-throughput production processes are used in corpuscular-beam optical miniaturization techniques, as stated in the following sources:
99T4081 A.DOC

23.] H. Koops, 1974, DE-PS 2446 789.8-33 "Corpuscular-Beam Optical Device for Corpuscular Irradiation of a Preparation", 24.] H. Koops, 1974, DE-PS 2460 716.7 "Corpuscular-Beam Optical Device for Corpuscular Irradiation of a Preparation", 25.] H. Koops, 1974, DE-PS 2460 715.6 "Corpuscular-Beam Optical Device for Corpuscular Irradiation of a Preparation in the Form of a 2-Dimensional Pattern with a Plurality of Identical 2-Dimensional Elements", 26.] H. Koops, 1975, DE-PS 2515 550.4 "Corpuscular-Beam Optical Device for Imaging a Mask onto a Preparation to be Irradiated", 27.] H. W. P. Koops, "Capacities of Electron Beam Reducing Image Projection Systems with Dynamically Compensated Field Aberrations" Microelectronic Engineering 9 ( 1989) 217-220 A further known miniaturization technique is based on die techniques with small mask templates as described in the following sources:
28.] H. Elsner, P. Hahmann, G. Dahm, H. W. P. Koops "Multiple Beam-shaping Diaphragm for Efficient Exposure of Gratings" J. Vac. Sci. Technol. B 0(6) Nov, Dec.
(1993) 2373-2376 29.] H. Elsner, H.-J. Doring, H. Schacke, G. Dahm, H. W. P. Koops "Advanced Multiple Beam-shaping Diaphragm for Efficient Exposure", Microelectronic Engineering 23 (1994) 85-88 Miniaturization can also be achieved through the use of electron-beam-induced deposition in projectors.
99T4081 A.DOC

30.] M. Rub, H. W. P. Koops, T. Tschudi " Electron-beam-induced deposition in a reducing image projector", Microelectronic Engineering 9 (1989) 251-254 Integrated optical patterns in which the process of refractive index modulation through the diffusion of nonlinear optical, high-refractive-index or liquid-crystal monomers into existing polymers in connection with free-standing polymer patterns is employed and in which the refractive index difference with respect to vacuum is used as the essential step of the refractive index increases, are presently not known.
The process according to the invention for the fabrication of active and passive optical components is based on the known processes of surface imaging in order to produce an oxygen-resistant etch mask in unexposed regions and the diffusion of molecules into patterned polymer layers.
According to the invention, at least one patterned polymer resist layer of high sensitivity is applied to an optoelectronic component, consisting of glass and conductor or of substrate. Subsequently, defined regions of the polymer resist layer are exposed, thereby producing an etch mask. By highly anisotropic deep etching of the non-protected regions, the etch mask is transferred to the polymer resist layer below the etch mask. The exposed regions of the polymer resist layer are removed in the vertical direction, with the result that the non-exposed side surfaces of the regions protected by the etch mask are uncovered.
In the ensuing process of gas-phase or liquid-phase diffusion, the unexposed polymer resist layer is, from its surface through the mask of the surface masking and from its side surfaces uncovered by the oxygen deep etching, filled with monomers under application of heat. Use is made of monomers which are suitable for filling the already existing pattern of the polymer, for breaking it up and repatterning it, it being possible for the optical properties of the optoelectronic component to be selectively changed as a function of the type of monomers used and also as a fiznction of the temperature and application time. In the diffusion process, the polymer then swells on all sides and, therefore, the previously lost edge region can be selectively smoothed out in controlled manner by the swollen material through the diffusion time and temperature. In 99T4081A.DOC

addition, owing to the acting surface tension, the surfaces produced by swelling are very smooth, i.e. peak-to-valley heights in the 2 nm range are obtained. The obtained refractive index profile is assured in the long term by UV hardening and deep cross-linking of the diffused molecules carried out after diffusion.
Through the diffusion into the uncovered deep polymer patterns of heavy-metal-oxide-containing, nonlinearly optical or liquid-crystal monomers or molecules containing "rare earths", it is possible, in addition to passive materials, also to produce nonlinearly optical active materials in selected regions. It is therefore possible to produce diffused refractive index profiles in regions defined by optical and corpuscular beam lithography.
The solution according to the invention is to be described in greater detail with reference to an example embodiment.
Fig hows the scheme of the production of refractive index profile patterns by means of chemical diffusion in the extended silylation process A patternable polymer layer of high sensitivity is applied to the substrate composed of glass and conductor. Novolak was used in the example embodiment. The etch mask is produced by the exposure of defined regions of the polymer resist layer, corresponding to the later component, in conjunction with a silylation process of the unexposed regions. Through the combination of the silylation process for high-resolution pattern definition with the dry etching of the cross-linked polymers to produce the great height to width conditions of the patterns, it is ensured that the non-cross-linked/unexposed material is available for further chemical diffusion of monomers for the various desired effects. In the exposure of negative working novolak, this part of the material is normally removed in the development process. Through silylation, it is retained after dry etching. If the silylation process is started with a short isotropic process attacking the silicon oxide of the silylation mask, the pattern broadens, but the rough edge structure of the silylated region, obtained by the shot noise of the electron exposure in the edge region of the mask, is smoothed. Consequently, in the following anisotropic dry etching process, which employs an etchant which attacks the silicon oxide of the etch mask, it is possible with directional oxygen ions to achieve smooth side walls of 99T4081A.DOC

the polymer. This solves the shot-noise edge roughness problem inevitable with corpuscular beam optics. This also minimizes the scattering losses to be expected at the rough surfaces.
In the subsequent diffusion process, the polymer then swells on all sides, with the result that the previously lost edge region can be evened out in controlled manner by the swollen material and by the diffusion time and temperature. Through the diffusion into the uncovered deep polymer patterns of heavy-metal-oxide-containing, nonlinearly optical compounds or other similar compounds or through the diffusion of molecules contained in "rare earths", it is possible, in addition to passive materials, also to produce nonlinearly optical active materials in selected regions. It is therefore possible to produce diffused refractive index profiles in regions defined by optical and corpuscular beam lithography. Such diffusion can take place, as in conventional manner, into non-etched polymer layers, this resulting in refractive index differences of up to 10%. If diffusion is carried out in polymer layers already patterned by wet-chemical development or by dry etching, then refractive index differences of between 1.5 and 3 can be produced.
With this process, the refractive index difference of 10-3 to 10-4 in the case of UV- and electron-exposed plexiglass can be increased to 0.06 as the refractive index difference between silylated and unsilylated novolak. The achieved refractive index differences can be further increased in that the resist regions negatively polymerized by the exposure process are removed by high-resolution oxygen dry etching from the optically active and passive pattern, this leading to refractive index differences with respect to vacuum of n = 1. In the case of the free-standing silylated region, the refractive index difference increases to 1.57, while it is 1.63 for the unsilylated material. Consequently, the finished component consists of chemically inert saturated materials of glass-like composition and good durability. The diffused regions can be cross-linked with long-term stability by UV deep cross-linking, this permitting a long life of the components. The combination of electrical and integrated optical components in the layers of the component is readily possible, because the process involves processes which have been in use for a number of years in lithography.
Fabrication is accelerated because the novolak resist systems are characterized by approx. 20 times higher sensitivity in comparison with PMMA (plexiglass). The 99T4081A.DOC

oxygen etching process additionally tempers the regions diffused with chemicals and thus ensures the durability of the components.
The process according to the invention makes it possible to produce diffractive patterns of high quality and effectiveness with few grating planes or lines and thus to fabricate integrated optical components such as couplers, gratings, selectors and reflectors with few grating periods. If such high refractive index differences are employed in the optical patterns and gratings, the same optical qualities can be achieved with much shorter components than is possible using polymer-plexiglass techniques. This greatly increases the packing density of the integrated optical elements in miniaturized integrated optics. There are the following possibilities for the large-scale realization of the optical components according to the invention:
1. Through beam-guiding or die-mask-projecting lithography tools with variably shaped beam, fast development steps in the technology could be carried out in short times for small quantities 2. The mass production of the optoelectronic components according to the invention can be preferably realized at low cost using the conventional lithography processes known from optical memory device construction, such as corpuscular beam and optical template projection techniques and optical mask projection techniques including X-ray lithography processes.
The process permits an increase of the packing density of future integrated monomode optics together with the simultaneous low-cost production of large quantities.
99T4081 A.DOC

Claims

Process for the fabrication of active and passive polymer-based components for integrated optics (6) What is claimed is:

1. Process for the fabrication of active and passive polymer-based components for integrated optics employing the principle of gas-phase or liquid-phase diffusion, characterized in that - at least one patternable polymer resist layer of high sensitivity is applied to one optoelectronic component;
- an etch mask is produced through the exposure of defined regions of the polymer resist layer;
- the etch mask is transferred, by highly anisotropic deep etching of the non-protected regions, to the polymer resist layer below the etch mask, the exposed regions of the polymer resist layer being removed in the vertical direction, with the result that the non-exposed side surfaces of the regions protected by the etch mask are uncovered;
- the unexposed polymer resist layer is, from its surface through the mask of the surface masking and from its unexposed side surfaces uncovered by the deep etching, filled with monomers by gas-phase or liquid-phase diffusion under application of heat, said monomers being suitable for filling the already existing pattern of the polymer, for breaking it up and repatterning it, it being possible for - the optical properties of the optoelectronic component to be selectively changed as a function of the type of monomers used for the doping and also as a function of the temperature and application time.

2. Process according to claim 1, characterized in that the material swelling occurring of necessity during the diffusion process is selectively controlled through the diffusion time and the process temperature until the inaccuracies of the pattern have again been evened out, there simultaneously being a smoothing of the surface roughness caused by the effectiveness of the surface tension in the material.

3. Process according to claim 1, characterized in that, through the use of vacuum or air at standard pressure in the interstices of the patterned polymer, a refractive index difference >1.5 is set with respect to the patterns in the filled polymer, with the result that optical elements of extremely high quality with few periods and therefore with few refracting surfaces are created.

4. Process according to claim 1, characterized in that the polymer pattern filled with nonlinear material is surrounded with electric electrodes and the polymer pattern is influenced in its optical properties through the control of the electric field applied between the electric electrodes.

5. Process according to claim 1, characterized in that the polymer pattern filled with nonlinearly optical material is connected to waveguides through which light is injected into the polymer pattern and the polymer pattern is influenced in its optical properties through a change of the injected light.

6. Process according to claim 1, characterized in that the etch mask is produced through the exposure of defined regions of the polymer resist layer in conjunction with the silylation of the unexposed regions of the polymer resist layer and, after silylation, the etch mask is smoothed at its edges with an isotropic etching attack using an agent which attacks the silicon oxide of the etch mask.