MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEMISTRY

Transcription

1 MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEMISTRY R. Osgood, Jr To cite this version: R. Osgood, Jr. MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEMISTRY. Journal de Physique Colloques, 1983, 44 (C5), pp.c5-133-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1983 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C5, supplement au nd1o, Tome 44, octobre 1983 page C5-133 MATERIAL DEPOSITION AND REMOVAL USING LASER-INITIATED CHEMISTRY R.M. Osgood, Jr. Departments of EZectricaZ Engineering and Applied Physics, CoZwnbia University, NY, NY 10027, U.S.A. ~6sum; - L'utilisation du rayon laser pour contraler des r6actions chimiques 5 grande et petite 6chelle est discutke. Trois applications spdcifiques sont donne'es comme exemple. Abstract - The use of laser light to control large and small scale chemical reactions is discussed. Three specific applications are given as examples. I - INTRODUCTION In the last few years, it has become increasingly apparent that rapid heating with laser light is a powerful technique for forming new and unexpected materials or materials properties. Laser transient annealing has been used to produce semiconductors with doping levels exceeding the solid solubilities at the annealing temperature /I/. New metal-glasses have been formed by transient heating of mixed films of metal and semiconductors. Picosecond laser pulses have been used to switch a semiconductor repeatedly between its amorphous and crystalline states. The results have given new insight into materials properties and suggested a host of new processing options for preparing electronic materials and components. Laser annealing has achieved these results by controlling only one parameter of materials preparation, namely, the rate of energy input or output. However, it is also possible to control the flow of materials to and from a surface by using laser radiation to initiate and drive a specific chemical reaction 121. Laser chemistry is a research field which antedates any extensive research in laser annealing; however, it is only recently that lasers have been used to control interface reactions and achieve results analogous to laser annealing. Laser chemistry can be used to demonstrate a much wider range of processing options than simple heating /2,3,4/; materials cannot only be modified, but metals, insulators, and semiconductors can be deposited or removed (etched). Small amounts of atoms can be incorporated into a solid surface by single-step photochemical doping. Laser chemistry can be based on resonant or non-thermal phenomena and in these cases, the processing has the advantage of keeping the substrate at room temperature. Because the extent of the reaction zone can be varied by changing the size of the laser-beam, both local and large-area reactions can be obtained. Since the mechanisms of laser interface reactions can be considerably different from the usual homogeneous or inhomogeneous reactions, new "nonequilibrium" materials can be grown. Laser chemical processing can be subdivided according to the phase of the medium being chemically activated. Table I shows this division. Notice that laser photochemical reactions have been demonstrated in virtually all forms of chemically active medium. Each material phase has its particular advantages. A gaseous medium is dry and can be flowed simply over the substrate. Thus, many of the capabilities of gas processing are similar to those for CVD or plasma processing. A liquid medium is useful because ion-chemistry provides a particularly powerful approach to producing selective dissolution effects for etching or carrying noble metals such as gold and platinum. Molecular surface layers provide a Article published online by EDP Sciences and available at

3 C5-134 JOURNAL DE PHYSIQUE high-density chemical medium using a gas ambient. such as a liquid or solid, but may be readily formed TABLE I CHEMICAL MEDIUM FOR LASER CHEMICAL PROCESSING - PHASE Gas EXAMPLES Thermochemical deposition of silicona Etching of silicon by clzb and siozd Deposition Liquid Solid Adsorbed Layers Photoetching of 111-lve compounds Deposition of Au f ~tching of polymersg Surface Nucleation of Cd Atoms h Photodeposition of A1 i a) ALLEN S.D., BASS M., Appl. Phys. Lett. 76 (1981) 431. EWICH D.J., OSGOOD R.M., DEUTSCH T.F., Appl. Phys. Lett. 39 (1981) 957; BAUERLE P., Appl. Phys. 28 (1982) 267. b) EHRLICH D.J., OSGOOD R.M~ DEUTSCH T.F., Appl. Phys. Lett (1982) c) CHUANG T.J., 3. Vac. Sci. Technol. 21 (1982) 800. d) SOLANK1 R., BOYES P.V., COLLINS C.J., Appl. Phys. Lett. K(1982) e) See, for example, OSGOOD R.M., SANCHEZ A., EHRLICH D.J., and DANEU V., Appl. Phys. Lett. 40 (1982) 391 and references cited therein. f) VON GUTFELD R.J., ACOSTA R.E., ROMANKIN L.T., IBM 3. Res Develop. 26 (1982) 136. g) SHRINAVASON R. and WAYNE-BANTOR V., Appl. Phys. Lett. 61 (1982) 576. h) EHRLIGH D.J., OSGOOD R.M., DEUTSCH T.F., Appl. Phys. Lett. 2 (1981) 946. i) D.J. Ehrlich, R.M. Osgood, T.F. Deutsch, J. Vac. Sci. Technol. 2, (1982) 23. Laser chemical processing may also be classified according to the size of the reaction zone. For example, excimer lasers /5/ have recently been used to deposit insulators or to etch GaAs and silicon over an area comparable to that of a semiconductor wafer. This type of laser chemistry provides techniques which become an alternative to the usual planar processing techniques for semiconductor fabrication; e.g., vacuum deposition and plasma etching. The relatively high average power available from excimer lasers is sufficient to process an entire wafer. In general, this type of processing requires a mask for pattern delineation. On the other hand, by using a focussed, cw laser it is possible to form submicrometer reaction zones, and thereby produce a highly local, laser chemistry. This type of laser processing has been called direct writing /3,6/, since deposited or etched

4 structures can be produced without a mask and, in most cases, in a single step. The applications for direct writing are generally those which require a discretionary or custom type of serial processing over a selected region of a surface; e.g. mask-metallization repair or discretionary interconnections. Finally, laser chemical processing can also accomplish material growth and etching in ways which are unique to a coherent optical source. Recently ultrahigh-resolution GaAs gratings have been made via a maskless technique which relies on the interference of two laser beams in a chemical medium. Similarly, a series of experiments has shown that materials grown with polarized light have a coherently arranged microstructure. The former technique has direct applications to fabricating various electrooptical components. The latter offers the eventual promise of producing materials whose electrical and structural properties are controlled by laser light. In the remainder of this review, I would like to describe briefly three examples of laser chemical processing. The examples are chosen so that they each illustrate one of the generic approaches to laser processing given in Table 11. More extensive reviews of laser chemical effects have been given in the reviews listed in the references /2,3,4,5,7/. TABLE I1 GENERIC APPLICATIONS OF LASER CHEMICAL PROCESSING TYPE - CHARACTERISTICS EXAMPLES Planar Processing Large-area, i.e. -10cm 2 Deposition of insulators Requires masking or and metals patterned laser beam High average power laser Etching of GaAs and Si Etching of Polymers Serial or Direct- Processing of a Writing of conducting Write Process micrometer-scale pixel links Low-average-power laser Maskless Etching of vias in One-s tep Silicon and GaAs wafers Modulated doping Coherent Processing Relies on the coherence Etching of high of laser light resolution gratings in GaAs I1 - PHOTON-ASSISTED PLANAR PROCESSING-ETCHING OF GaAs Dry-etching is a general term for fabrication of high-resolution structures without using aqueous solutions. Dry-etching is now done by using charged particles from a gas discharge or an ion source. In the case of the compound semiconductors, this form of etching is sometimes unsatisfactory because of surface damage due to charge-particle incorporation. We have recently developed an approach to dry-etching GaAs which uses photodissociation of CH3Br, CF3Br, etc., to produce free radicals which remove the surface metalloids 181. The approach used is to place the GaAs in a sample cell containing the undissociated etching gas. The gas is then irradiated with the pulsed output from an ArF excimer laser. The etching parameters and the etching rates are summarized in

5 C5-136 JOURNAL DE PHYSIQUE Fig. 1. To obtain the rapid etching rates shown in the figure, it is necessary also to illuminate the surface with laser radiation. We believe that this surface exposure removes the reaction products, such as Ga(CF3)3, which form a condensed film on the semiconductor surface. The removal mechanism is attributable to a combination of thermal and photochemical desorption. EXPOSURE TIME Imbn.1 Fig. 1 - Etch depth in GaAs as a function of exposure time. The reaction rate may be monitored by physically measuring the etch depth via a mechanical stylus. An additional approach relies on the fact that the uv light photodissociates the Ga molecular products and thereby produces excited Ga atoms; these atoms then emit at the green gallium resonance line. Thus, one can probe the etching by simply observing the fluorescing gallium atoms. We are currently comparing the etching rate using various, standard and perfluorinated methyl-halides. III - DIRECT WRITING - LASER WRITING OF CONDUCTING LINES In many areas of I.C. fabrication, it would be desirable to write,on adiscrrtionary basis,micrometer-scale conducting lines. During the last few years. we have shown that metal lines can be written by using ultraviolet photodissociation of metalalkyl vapor. Thus far, most of this work has been directed toward understanding the limits on the resolution of the process. In this connection, the author and D. Ehrlich /9/ have recently shown that lines as narrow as 0.5 pm can be produced by using a 257-nm laser beam focused with a lens with a numerical aperture of 0.5. An important criteria for direct writing to be practical is that the writing rate must be sufficiently fast that usefully large linear distances can be written. We have recently begun a systematic measurement of these rates at sufficiently high gas pressures that rapid growth rates can be expected. These measurements show that for even very low laser powers, e.g ~IW, nm/sec rates can be obtained. Use of higher laser powers should produce rates several orders of magnitude faster. Andther important recent result has been to achieve sufficient control of laser processing that several types of laser writing can be used to fabricate more complex structures than a single conducting line. Fig. 2 shows a microstructure consisting of an Si02 line written on an Si-substrate /9/; the line is then crossed by a metal line (Cd). The SiO2 line is written using a spin-on silicate paterial obtained from Allied Corporation. The unexposed silicate is removed using an alcohol rinse before the metal deposition. The three materials shown in Fig. 2allow

6 one to write MOS structure with a laser, a result recently reported by McWilliams, et al. /lo/. -- Fig. 2 - Cd line directly written over Si02 pattern on a silicon wafer. IV - COHERENT PROCESSING - LASER INTERFERROMETRIC PRODUCTION OF DIFFRACTION GRATINGS Visible light can be used to enhance the dissolution rate of semiconductors immersed in certain aquaeous mixtures, containing, typically, agents for oxidation and oxide dissolution. This technique, which is a form of anodic oxidation, has recently been used in conjunction with laser radiation to produce highly localized etching of a semiconductor surface. In an effort to determine the ultimate limitations in the process resolution, we have investigated the etching of very-small-period gratings Ill/. These gratings are made by interfering two collimated laser beams in an etching solution. By paying careful attention to the etching solution chemistry and to the stability of the optical train, we have produced gratings with 130-nm periods on n-type GaAs. We believe that 100-nm gratings have been produced; however, detection of this fine a grating period is extremely difficult. A wide variety of unusual grating profiles can be obtained using this technique. For a shallow but fully developed grating, the profile is scalloped; an example is shown in Fig. 3 for the case of a 200-nm grating. With other solutions the grating profile resembles a series of narrow peaks; a tilted substrate will yield a grating with a blazed profile. Fig. 3 - Profile of GaAs grating produced by interfering two visible laser beams. V - CONCLUSION The above presents a brief introduction to the use of laser interface chemistry for materials processing. Unlike the case of laser annealing, laser chemistry

7 JOURNAL DE PHYSIQUE cannot only change the composition of a solid surface, but it can also cause addition and deletion of material to the substrate. Laser processing is interesting not only for its immediate application such as photolithographic mask repair, but also for its long term impact on surface science and the preparation of novel materials. VI - ACKNOWLEDGMENT I would like to acknowledge members of the Columbia Microelectronics Science Laboratory for their contribution to and support of this work. In addition, special thanks go to my former colleagues, Tom Deutsch, Dan Ehrlich, and Don Silversmith at Lincoln Laboratory for their collaboration over the previous several years. REFERENCES (1) See, for example, APPLETON B.R. and CELLER G.K. (eds.) Laser and Electron- Beam Interactions with Solids, North Holland: New York (1982). (2) OSGOOD R.M., Jr. in Ann. Rev. Phys. Chem. (1983) in Press. (3) EHRLICH D.J., OSGOOD R.M., DEUTSCH T.F., J. Quantum Electron. QE-16 (1980) (4) OSGOOD R.M., Jr., BRUECK S.R.J., and SCHLOSSBERG H., Ed. Laser Diagnostics and Photochemical Processing for Semiconductor Electronics. North Holland: New York (1983). (5) For a brief review see OSGOOD R.M., Jr. in Proceedings of Excimer Laser Meeting, Lake Tahoe (1983) AIP Press. (6) OSGOOD R.M., Jr., EHRLICH D.J., DEUTSCH T.F., SILVERSMITH D., and SANCHEZ A., "Direct-Write Laser Fabrication Customization, Correction and Repair", to be published in Proceedings of Les Deux Alpes Meeting on Microelectronics (1983). (7) CHUANG T.J., J. Vac. Sci. Technol. 21 (1982) 800. (8) BREWER P., HALLE S., OSGOOD R.M., Jr. Paper presented at North-East Regional Chemical Society, Hartford (June 1983). (9) EHRLICH D.J., OSGOOD R.M., and DEUTSCH T.F., J. Vac. Sci. Technol. z, (1982) 23. (10) MCWILLIAMS B., HERMAN I.P., MITLITZKY F., and WOOD L. paper presented at CLEO '83, Baltimore, Md (1983). (11) PODLESNIK D., GILGEN H.H., OSGOOD R.M., Jr. to be published. See also related article in Ref. 4.