EP1738227A2

EP1738227A2 - Method for producing two-dimensional periodic structures in a polymeric medium

Info

Publication number: EP1738227A2
Application number: EP05762341A
Authority: EP
Inventors: Christophe Hubert; Céline FIORINI-DEBUISSCHERT; Jean-Michel Nunzi
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2004-04-23
Filing date: 2005-04-22
Publication date: 2007-01-03
Also published as: WO2005105662A2; FR2869306A1; US20080257873A1; FR2869306B1; JP2007535397A; WO2005105662A3

Abstract

A method for producing periodic structures at the surface of a sol-gel type, hybrid organic-inorganic or organic material, characterised in that it includes the step of directly illuminating the material with a laser beam having a uniform intensity profile at near-normal incidence, while moving said material and said laser beam relative to each other.

Description

PROCESS FOR MANUFACTURING BI-DEVIENSIONAL PERIODIC STRUCTURES IN POLYMERIC MEDIA

FIELD OF THE INVENTION

The present invention relates to the field of manufacturing periodic structures on the surface of certain organic materials, such as polymers. STATE OF THE ART

The possibility of organizing organic or hybrid organic-inorganic materials on a sub-microscopic (and nanoscopic) scale opens up many interesting perspectives, in particular and not limited to, for example, the realization of given functions or the optimization of properties. optical (such as modulation of absorption / emission, modulation of wave propagation properties ...) or electronic of these materials.

Among the possible applications, we can cite the development of electro-optical modulators for optical signal processing (in the telecommunications field), the production of organic lasers and more generally the whole field of plastic electronics: for example the design and optimization of photovoltaic cells, the optimization of light-emitting diodes ...

More specifically, for optical effects, for example, the structuring of matter at the sub-wavelength scale, makes it possible to envisage the implementation of new effects, such as the possibility of completely controlling the emission. of light in photonic crystals ... Another example of application relates to the realization of functions of coupling and decoupling of light in photonic systems, such as for example organic light-emitting diodes (OLEDs). Indeed, in an OLED, about 80% of the light emitted by the electroluminescent material is lost by guiding effect in the different layers. By structuring the diode, that is to say by inserting for example a one-dimensional network in it, it could be demonstrated that it was possible to reduce the amount of light lost by guidance [1]. This is due to Bragg diffraction on the grating of the waves which are initially guided in the different layers of the diode.

Two types of structuring known from the prior art can be distinguished: the first corresponds to a volume structuring (case for example of photonic crystals), the second corresponds to a surface structuring (case for example of diffraction gratings) . The present invention relates to the second field, namely that of surface structuring.

The known structuring methods can be classified into two categories: the first groups optical (photolithography) and electronic lithography (these techniques are mainly used in the semiconductor industry [2]), the second groups the so-called “contact” methods such as techniques known in English as “embossing” and “stamping”.

Among the many known reproduction techniques, photolithography is one of the most extensively developed techniques. The main steps implemented for a photolithography are the following: exposure of a sensitive material (eg: polymer resin) to a beam of photons with wavelengths which can be located in the UN-visible or the domain of X-rays, according to the devices and according to the desired resolution, this through a mask comprising the motif to be inscribed, ^' revelation of ^' this material and engraving. Although today well mastered lithographic methods have several disadvantages, among which we can cite:

- a complex experimental implementation,

-. the need to use several stages (exposure, revelation, etching) before obtaining the final pattern, - the need for great stability and precise alignment of the different elements (mask and sample) in order to reproduce with the greatest high precision the initial pattern

- the need for a dust-free environment, even a clean room type.

In addition to the different lithographic methods, other methods have been developed based on the replication of masks through physical contact. These techniques have the advantage of being of low financial investment, as well as simple to implement. These methods are based on the use of a mask or mold, the patterns of which have just been transferred to a substrate by contact or pressure. However, the use of such techniques is often limited by the availability of suitable masks which are mainly produced themselves by lithographic techniques having the drawbacks indicated above. In addition, it should be noted that the average resolution of these contact techniques is still lower than that obtained by lithographic techniques. In this context, it therefore appears useful to arrive at developing new micro and nanostructuring techniques not photo-lithographic in addition to those already existing. The industrial world is in particular demanding techniques requiring in particular a smaller number of implementation steps, not requiring a clean room type environment, and therefore less expensive.

The manufacture of unidirectional or multidirectional structures by laser irradiation of certain materials in thin layers on small surfaces (of the order of the diameter of a laser beam, or of the order of a few mm ² ) is known. Recently, it has been demonstrated that the irradiation of azo polymeric films by an intensity modulation coming from one or more beams leads directly to a controlled topographic modification of the surface of the film and to the formation of a network. surface [3,4]. This technique has the advantage of being low cost by the use of all optical means of structuring. Compared to lithographic processes, this method, based on a photoinduced material transport phenomenon, is direct and does not require any post-treatment of the "revelation / dissolution" type.

However, this method only makes it possible to obtain only one-dimensional networks. The realization of two-dimensional structures is delicate because it requires the realization of interference figures more complex and difficult to implement. In addition, several constraints are to be observed during the production of these structures, among which we can cite the fact that:

- the difference in optical path between each beam interfering on the surface of the material must be less than the coherence length of the laser, - precise adjustments must be made in order to obtain a spatial overlap of the two beams on the surface of the polymer film, the latter must also have the same intensity, and

- the sample must not move during the experiment in order not to blur the interference pattern. Another way of making structures is to illuminate a material with a single laser beam of sufficient intensity, pulsed or continuous. This process, which has several properties in common with Wood's anomalies present in diffraction gratings [5], has been used in a process called LLPS (Laser Induced Periodic Structures) [6]. This structuring process has been highlighted on the surface of materials (inorganic and organic) irradiated in oblique incidence by a polarized laser beam. However, in the various examples of LIPS described in the literature, only the observation of fringes on the surface of the material is described, ^"that is to say of one-dimensional structures.

Similarly, it has been shown that it is also possible using a single laser beam to directly create regular size sub micron structures no longer one-dimensional but two-dimensional, the surface of organic materials _^ [7 , 8]. This method, different from the previous one due to the physical processes involved, requires a normal incidence of the laser beam on the material. However, the surface of the area that can be structured is limited to the diameter of the laser beam used, that is to say a few mm ² , and the geometry of the induced structures is still poorly controlled.

Indeed, a lateral offset of the laser beam to irradiate successively adjacent areas of the material does not ensure continuity of the patterns of the structures at the beam overlap areas. These discontinuities are likely to create defects for optical coupling / decoupling applications in particular.

OBJECT OF THE INVENTION The present invention has the main purpose to provide a novel method for improving the manufacture of periodic structures on the surface of certain ^'materials, such as polymers or organic-inorganic hybrid materials sol-gel type.

The object of the present invention is in particular to propose a simple method of implementation allowing the manufacture of such structures over large areas.

OBJECT OF THE INVENTION ^' • The abovementioned object is achieved within the framework of the present invention by means of a method comprising the step which consists in directly illuminating an organic or hybrid organic-inorganic material of the sol-gel type, by a laser beam. having a uniform intensity profile in quasi-normal incidence, while operating a relative displacement between said material and the laser beam, preferably in the form of a relative rotation. After lengthy research and experimentation, the inventors have in fact discovered, a priori surprisingly and unpredictably, that the aforementioned process in accordance with the present invention makes it possible to create in a single step structures in one or two dimensions on surfaces of organic materials of up to several cm, while using only one and the same laser beam. They have in fact found that the relative mechanical movement between the laser beam and the irradiated material, instead of blurring possible interference effects and reducing the modulation of the structures, surprisingly makes it possible to obtain periodic structures, covering continuously the entire irradiated surface during the. displacement, i.e. for example several cm ² . DESCRIPTION OF THE FIGURES

Other characteristics, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of nonlimiting examples and in which:. FIG. 1 shows diagrammatically the assembly in accordance with the present invention allowing the recording of photo-induced structures on the surface of organic or hybrid films,

. FIG. 2 represents an alternative implementation in accordance with the present invention,. FIG. 3 schematizes the structure of molecules capable of being used preferentially in the context of the present invention, and

. FIGS. 4, 5 and 6 represent images taken with an atomic force microscope (AFM) of examples of structures obtained within the framework of the present invention, the images of FIGS. 4 and 5 being obtained using the copolymer DOPPJvlA / MMA, while the image of Figure 6 was obtained using the DR1MA / MMA copolymer.

DETAILED DESCRIPTION OF THE INVENTION

The structuring method according to the present invention consists • essentially in illuminating in quasi-normal incidence, by a laser beam whose intensity distribution is uniform, a polymer film or a hybrid film in relative displacement relative to the laser beam, very preferably in rotation.

In the context of the present invention, the term “quasi-normal” means an angle of incidence less than 5 ° relative to the normal to the material. Obviously, such a rotational movement could be replaced by any equivalent relative displacement between the laser beam and the _. material to be irradiated. Furthermore, as a variant, it is possible to envisage displacing the laser beam, or else operating a movement of both the laser beam and the polymer material. In the appended FIG. 1, an incident laser beam has been shown schematically in 10 and in

20 a support plate for material irradiated by the laser beam 10. The polymeric material can be, for example, in the form of a polymer film carried by a glass substrate. The laser beam 10 is directed perpendicular to the surface of the polymeric material. The support 20 is provided with a shaft 22 capable of being driven in rotation by a suitable motor.

More precisely according to the embodiment illustrated in FIG. 1, the laser beam 10 is centered on the axis of rotation of the support 20.

The registration process typically takes place at room temperature.

However, it can also take place at higher temperatures, in particular for materials having high glass transition temperatures.

The intensity of the laser beam 10 can typically be varied between 0.2 and 2 Watts / cm ² . .

The polymer materials used in the context of the present invention are composed of a polymer skeleton to which are absorbed absorbent molecules. Several types of copolymers can be used, different from each other by the nature of the polymer backbone but also by _. the dye ^molecules' used. In the case of hybrid materials, the skeleton is generally formed on the basis of silicon atoms.

The wavelength of the laser must be within the absorption band of the molecule used or close to this absorption band. In the context of the present invention, the term "close to the absorption band" means a wavelength whose deviation from the lower limit of the band does not exceed 100 nm.

The polymer materials used can be in the form of films deposited on a substrate. The deposits can be produced for example by eentrifugation from a solution consisting of a copolymer dissolved in a

-solvent. The present invention also extends to the use of materials.

"Massifs" of various shapes (cylinders, cubes ...) that can be obtained by all means, for example and without limitation by molding and then polishing of a mass copolymerized mixture.

There is shown diagrammatically in FIG. 2 an alternative embodiment according to which the laser beam 10 of quasi-normal incidence is eccentric with respect to the axis of rotation of the irradiated polymer material, while remaining parallel to this axis of rotation.

EXAMPLES OF REALIZATION

We will specify with reference to Figures 4, 5 and 6 three examples of results obtained by the practical implementation of the structuring technique described above in accordance with the present invention. The copolymers used in the context of these examples are composed of azo molecules of (N-ethyl-N-hydroxyethyl-4- (4'-cyanophenylazo) phenylamine) (DOPR) and 4- (N- (2-hydroxyethyl) -N -ethyl-) amino-4'- nitroazobenzene (DRl) grafted to a polymer backbone, polymethyl methacrylate (PMMA, transparent in the visible range), with a rate of 35% by mole (DOPRMA MMA 35/65, DR1MA / MMA 35/65).

The structures of the copolymers thus used are presented below

The dye molecules used in the context of these examples are azo molecules of the push / pull type, that is to say having electron acceptor and donor groups separated by two benzene rings linked together by a nitrogen double bond (N = N). These molecules are highly absorbent in the visible domain. They also have the advantage of being isomerizable (Cis-Trans isomerization), the repeated passages of the molecule from one form to another inducing photoinduced molecular movements (rotation ^' and translation) inside the ^• polymer matrix.

The present invention is not however limited to this particular type of molecule. More generally, the present invention can be implemented

, work with molecules of the type illustrated in the attached FIG. 3 or any other molecule having photoinduced isomerizations or having photoinduced molecular movements.

This FIG. 3 shows molecules having an electron donor group chosen from the group comprising CH ₃ , OCH ₃ , NH ₂ , NRιR ₂ where RI and R2 are aliphatic chains (for example N (CH ₃ ) ₂ ) and an electron acceptor group chosen from the group comprising CN, CHO, COCH ₃ , NO ₂ , separated by two benzene rings linked together by a nitrogen-nitrogen double bond. As a variant, the electron transmitter assembly constituted in FIG. 3 of two benzene rings linked together by a nitrogen-nitrogen double bond, can be replaced by any other group having a sufficiently rapid reversible isomerization, typically less than 1 ms.

Within the framework of the experiments carried out, the thickness of the films was 500 nm. The experiments were carried out with the line at 514nm of an Argon laser.

The intensity of the incident laser beam was 1 W / cm ² , the irradiation time of 90 minutes and the polarization of the laser beam was linear. The motor rotation frequency was 5 hertz.

The three images reproduced in FIGS. 4, 5 and 6 annexed were obtained using an atomic force microscope (AFM) under the conditions indicated above, that is to say using the copolymer DOPRMA / MMA for Figures 4 and 5 and using the DR1MA / MMA copolymer for Figure 6. They represent photo-induced structures obtainable with the technique according to the present invention.

The amplitude of modulation of the structures can reach lOOnm, the structures having amplitudes of modulation the higher the higher the quantity of energy absorbed. However, experience shows that in terms of power density, a threshold exists below which no structure develops. Furthermore, beyond a certain dose of energy absorbed, the modulation amplitudes saturate.

The period of the structures observed is of the order of the irradiation wavelength and does not vary depending on the material used.

The structuring method according to the present invention allows a coupling in the plane of the polymer film, of a light beam of normal incidence, and offers interesting perspectives in particular as regards the optimization of the efficiency of photovoltaic solar cells. In this context, in fact, it will be noted that, for example, if we want to couple a given wavelength in the film plane, it suffices to directly apply this wavelength during structuring (the absence of a mask or other process intermediate eliminates any need for special adjustment).

The geometry of the induced structures varies, as a function of various parameters, and in particular: the irradiation wavelength, the periodicity of the structures obtained being of the same order of magnitude as the irradiation wavelength,

- the power of the laser beam and the duration of exposure which act on the amplitude of the modulation, - the relative position of the irradiation wavelength with respect to the absorption band of the material,

- the frequency of rotation of the sample,

- the type of copolymer used,

- the polarization of the laser beam, - the position of the laser beam incident on the sample with respect to the axis of rotation of the motor ("off-axis" of rotation or "on the axis" of rotation). For illustration : the image of FIG. 4 (hexagonal organization) was obtained following the irradiation of a DOPRMA / MMA sample using a laser beam centered on the axis of rotation, the polarization of the laser being linear.

the image of FIG. 5 (fringes) was obtained following an irradiation of a DOPRMA / MMA sample using a laser beam eccentric relative to the axis of rotation, the polarization of the laser being linear. The orientation of the fringes varies continuously depending on the position of the zone analyzed in relation to the axis of the support (position on the "illumination crown").

- the image of figure 6 (organization having no privileged direction) was obtained following an irradiation of a sample of DR1MA / MMA using a laser beam centered on the axis of rotation, the polarization of the laser being linear. Depending on the frequency of rotation of the sample, structures identical to those of FIG. 6 can also be obtained in the case of the irradiation of an identical sample using a laser beam eccentric to the axis of rotation, as illustrated in figure 2.

When a circular polarization is used, whatever the frequency of rotation and the type of irradiation ("on axis" as illustrated in figure 1 or "off axis" as illustrated in figure 2), the experiments led to Induced structures identical to those of FIG. 6. The structuring technique proposed in the context of the present invention has the advantage of taking advantage of the properties of polymeric or hybrid materials: a low manufacturing cost coupled with the possibility of depositing films on surfaces larger than several square centimeters. In addition, the use of a single laser beam involves a low cost of installation. Compared to the already existing methods known from the state of the art, the all-optical structuring method in accordance with the present invention has in particular the following advantages:

- great ease of implementation: no mask manufacturing is required, no precise alignment is to be carried out (only the quasi-normal incidence of the laser beam on the polymer or hybrid film is necessary) due to the '' use of a single laser beam, - the possibility of structuring the material over large areas (several cm ² ), simply by increasing the size of the beam by a system of lenses or else by performing “off-axis” irradiation of the polymer film,

- the diversity of structures: the geometry of the induced structures and their amplitudes can be controlled by varying. the experimental parameters: frequency of rotation of the sample, quantity of energy absorbed by the sample, polarization of the laser beam, position of the incident laser beam on the sample with respect to the axis of rotation of the engine ("except - axis "of rotation or" on the axis "of rotation), the type of molecule used '- the possibility of working in a free atmosphere, without the need for a clean room.

The present invention can find application in particular in the field of organic optoelectronics, for example for: optimization of electroluminescent devices (by decoupling on structures of initially guided light), - optimization of photovoltaic cells (by optimization of absorption of the incident solar spectrum and coupling in the film plane).

In general, the present invention can give rise to numerous applications.

The structures obtained in the context of the present invention can also serve as a substrate for the conformal deposition of layers of other materials, with different optical, electronic or mechanical properties, but which will retain the same structural properties.

The structures obtained in the context of the present invention can also be used to serve as a replica mask using various techniques known per se to those skilled in the art, such as contact techniques (embossing, stamping ) or optical (photolithography type).

In the examples illustrated above, the optical polarization of the laser beam was linear, or circular, but could have been elliptical.

BIBLIOGRAPHICAL REFERENCES

[1] L. Rocha, C. Fiorini-Debuisschert, C. Denis, P. Maisse, P. Raimond, B. Geffroy, JM Nunzi, Organic nanophotonics, F. Charra et al. (eds.), Klu er Académie Publishers, 405, 2003. [2] Y. Xia, JA Rogers, KE Paul, GM Whitesides, Unconventionnal methods for fabricating and patteming nanostructures, Chern. Rev., 1999, 99, 1823 and references cited.

[3] P. Rochon, E. Batalla, A. Natansohn, Optically induced surface gratings on azoaromatic polymer films, Appl. Phys. Lett., 1995, 66, 2, 136

[4] D. Y. Kim, S. K. Tripathy, L. Li, J. Kumar, Laser induced holographis surface relief gratings on nonlinear optical polymer films, Appl. Phys. Lett., 1995, 66, 10,

1166

[5] A. E. Siegman, P. M. Fauchet, Stimulated Wood's anomalies on laser-illuminated surfaces, IEEE J. Quantum Elec, 1986, 22, 1384

[6] M. Bolle, S. Lazare, M. Le Blanc, A. Wilmes, Submicron periodic structures produced on polymer surfaces with polarized excimer laser ultraviolet radiation,

Appl Phys. Lett., 1992, 60, 6, 674

[7] C. Hubert, C. Fiorini-Debuisschert, P. Raimond, J. M. Nunzi, Adv. Mat., 14, 729, 2002.

[8] C. Hubert, C. Fiorini-Debuisschert, P. Raimond, J. M. Nunzi, Organic

Nanophotonics, 317-325, F. Charra et al (Eds), Kluwer Académie Publishing, 2003.

Claims

DREAM ND ICATIONS

1. A method of manufacturing periodic structures on the surface of an organic or inorganic organic-inorganic material of the sol-gel type, characterized in that it comprises the step which consists in directly illuminating the material, by a laser beam having a uniform intensity profile in quasi-normal incidence, while operating a relative displacement between said material and the laser beam.

2. Method according to claim 1, characterized in that the relative displacement between the material and the laser beam corresponds to a relative rotation.

3. Method according to one of claims 1 or 2, characterized in that the relative displacement between the material and the laser beam corresponds to a rotation of the material.

4. Method according to one of claims 1 to 3, characterized in that the surface covered by the laser beam during the irradiation corresponds to several cm ² of the material. •

5. Method according to one of claims 1 to 4, taken in combination with claim 2, characterized in that the laser beam (10) is centered on the axis of rotation (22).

6. Method according to one of claims 1 to 5, characterized in that the optical polarization of the laser beam is linear, or circular, or elliptical.

7. Method according to one of claims 1 to 6, characterized in that a lens system is interposed on the path of the laser beam to increase and control the size of the impact of the laser beam on the material.

8. Method according to one of claims 1 to 4, taken in combination with claim 2, characterized in that the laser beam (10) is eccentric relative to the axis of rotation (22) and at least substantially parallel to this one.

9. Method according to one of claims 1 to 8 characterized in that the irradiated material is composed of a polymer skeleton or sol-gel to which are absorbed molecules.

10. Method according to one of claims 1 to 9, characterized in that the irradiated material is formed of molecules having an electron donor group and an electron acceptor group.

11. Method according to one of claims 1 to 10, characterized in that the irradiated material is formed of molecules having a donor group of electrons and an electron accepting group separated by an electron transmitting group having photoinduced isomerization or having ^• photoinduced molecular movements.

12. Method according to one of claims 1 to 11, characterized in that the irradiated material is formed of azo molecules

13. Method according to one of claims 1 to 12, characterized in that the irradiated material is formed of molecules having an electron donor group and an electron acceptor group separated by two benzene rings linked together by a double bond nitrogen-nitrogen.

14. Method according to one of claims 1 to 13, characterized in that the irradiated material is formed of molecules having an electron donor group chosen from the group comprising CH ₃ , OCH ₃ , NH ₂ , NR [R where RI and R2 are aliphatic chains, and an electron acceptor group chosen from the group comprising CN, CHO, COCH ₃ , NO ₂ . .

15. Method according to one of claims 1 to 14, characterized in that the irradiated material is chosen from the group comprising azo molecules of (N- ethyl-N-hydroxyethyl-4- (4'-cyanophenylazo) phenylamine) ( DOPR) and 4- (N- (2-hydroxyethyl) -N-ethyl-) amino-4'-nitroazobenzene (DRl) grafted to a polymer backbone.

16. The method of claim 15 ,. characterized in that the skeleton

, polymer is polymethyl methacrylate.

17. Method according to one of claims 1 to 16, characterized in that the wavelength of the laser beam is included in or close to the absorption band of the irradiated material.

18. Method according to one of claims 1 to 17, characterized in that it implements means capable of controlling at least one of the parameters chosen from the group comprising: the irradiation wavelength, the power of the laser beam and the duration of exposure, - the relative position of the irradiation wavelength relative to the absorption band of the material, the frequency of rotation of the material, the polarization of the laser beam, the position of the incident laser beam on the material relative to the axis of rotation of the motor, the type of molecule selected.