Method for detecting structural defect of film

Abstract: Provide is a method for detecting defects in a film structure of a thin film formed on a substrate, comprising projecting a light flux in a direction precisely parallel or nearly parallel to the face of the substrate; separating a reflected light from the thin film and the substrate from a scattered light toward the direction precisely perpendicular or nearly perpendicular to the face of the substrate; and reading information only of the scattered light.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a use of a method for detecting imperfections of thin film formed on a substrate. Related Background Art Recently, the ordered structure of organic materials (or organic molecules) has come to be noticed of connection with developement and improvement in electronic functions. In particular, electronic devices employing organic molecules are attracting attention. As one of the techniques of constructing molecular electronic devices, many reports have been disclosed regarding Langmuir-Blodgett films (LB films). An LB film is constituted of a regular lamination of monomolecular layers of an organic compound. The LB film has enabled formation of a uniform ultra-thin film by controlling the film thickness thereof in the order of the unit of a molecular length, that is in the order of magnitude of nm (nanometer, 10 -9 m). Many attempts have been made to use the LB film as an insulator. The examples of the attempts include a tunnel junction element having a structure of metal/insulator/metal (MIM) [G.L. Larkins, et al.: "Thin Solid Films", Vol. 99 (1983)]; a light-emitting element having a structure of metal/insulator/semiconductor (MIS) [G.C. Roberts, et al.: Electronics Letters, Vol. 20, p. 489 (1984)]; and a switching element [N.J. Thomas, et al.: Electronics Letters, Vol. 20, p. 838 (1984)]. Even after the investigations of characteristics of the elements in these studies, the problems still remain unsolved such as variation of the properties of respective elements and deterioration by aging thereof which are considered to be ascribable to structural defects in LB films, causing lack of reproducibility and of stability. As one of the problems in detection of a structural defect of an LB film, the structural defect of the LB film is required to be visualized and to be numerically evaluated within a limited time. In conventional evaluation, the results are shown as indirect and average data such as in a copper metallizing process [I.R. Peterson: Aust. J. Chem., Vol. 33, p. 173 (1980)]; and measurement of electroconductivity for an MIM structure [P. Lesieur, et al.: Thin Solid Films, Vol. 152, p. 155 (1987), and I.R. Peterson: J. Mol. Electron., Vol. 2, p. 95 (1986)] Further, the evaluation by electron microscopy is complicated for the evaluation of structural defects of an LB film because many parameters have to be controlled to obtain significant results. Therefore, the electron microscopy requires technical skill as well as much time for the measurement. The US-A-4 966 457 discloses a method that requires two light beams of different wavelength to detect particles adherent to a thin film, as for instance reticles and pellicles. These films are several µm thick, typically 3 to 10 wavelengths and the incident angle of the two light beams is varied for detecting the particles. The JP-A-0 118 5934 discloses a method of detecting a foreign body on a periodically structured surface with known structure. The setup is done in a way that the image of a proper, clean object is cancelled out at the point of observation and only particles or defects, that are not part of the desired pattern, result in a high contrast image. In the GB-A-2 112 00 a method is described using physical contact between a light guide and a film covered substrate, like a soft plastic film on paper, to detect defects, namely non coated areas in the film. This known method is not suited for sensitive films because those films will be destructed thereby. For the thin films, to which this known method is directed, are very thick compared to the films in question, as for instance Langmuir-Blodgett films. SUMMARY OF THE INVENTION It is an object of the present invention to provide a use of a method of the general kind referred to in claim 1, for detecting imperfections of an organic thin film quickly and with high precision by a simple apparatus in the air. This object is attained by the features of claim 1. Advantageous developments of the invention are defined in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates schematically the principle of the detection of defects in the structure of a thin film in the present invention. Fig. 2 illustrates schematically a apparatus for detecting a defect in the structure of an organic thin film by the method of the present invention. Fig. 3 is an photographic image of defects of a fatty acid LB film obtained by use of an apparatus employing the method of the present invention. Fig. 4 is a polarizing microscopic image of the same part of a fatty acid LB film as in Fig.3. Fig. 5 illustrates schematically the principle of the detection of the structure at and around an interface of a thin film in the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 illustrates schematically the principle of the detection of defects in structure of a thin film in the present invention. A flat thin film 1 in the drawing is made of an organic material. The organic thin film as the object of the present invention has generally a thickness of from several ten Å to several thousand Å. In particular in the case where the thin film is an LB film, the thickness is in the range of from several Å to thousand Å, preferably from 4 Å to 1000 Å, more preferably from 50 Å to 100 Å as the object of-evaluation of the film. Minute defects 1a like pinholes inside the organic thin film are also shown schematically in Fig. 1. A specimen 16 is constituted of a flat substrate 2 and an organic thin film 1 formed thereon. A light flux (a projected light) L is projected from a light source (not shown in the drawing) such as a laser source to the substrate 2 in a direction nearly parallel to the substrate 2. If the organic thin film 1 is uniform and has no defect in the internal structure the introduced light flux L is simply reflected by the organic thin film 1, or transmitted through the thin film 1 and then reflected by the substrate 2, thus the reflected light is ejected from the other end. On the contrary, if the organic thin film involves a defect 1a like a pinhole inside, the light flux L introduced to the defect 1a is scattered by the defect 1a. That is, the light flux L is scattered by the change of the refractive index resulting from the defect 1a, and the scattered light is ejected in various directions. Among various directions of a scattered light caused by the defect 1a, the scattered light 17 directed precisely or nearly perpendicular to the substrate 2 is collected by a condensor lens (or an objective) 8, and an image of the thin film structure of the organic thin film 1 caused by the scattered light 17 is formed on a plane of an imaging means such as a CCD (not shown in the drawing). As described above in the present invention, the light flux L is projected in a direction parallel or nearly parallel to the organic thin film 1, whereby the light which is transmitted simply through the thin film 1 and the regular reflection light 18 which is reflected regularly by the substrate 2 is not introduced to the condensor lens 8. The image of the scattered light caused by defects in the thin film structure and formed by an imaging means is subjected to image analysis with an image-processing means (not shown in the drawing), thereby the structural defects in organic thin films being detected. Fig. 2 illustrates schematically an apparatus for detecting a defect in structure of an organic thin film by the method of the present invention. In this example, an optical microscope system is applied to the present invention. The construction of this example does not interfere with the normal use of the optical microscope system such as a light source for observation, a polarizer plate, an analyzer plate, a camera, and so forth. In Fig. 2, the flat organic thin film 1 is formed on an optically flat substrate 2. A support 3 is constructed of an XYZ three-axis stage which is movable in an XY plane and in a Z axis direction. A substrate 2 having an organic thin film 1 is placed on the support. A light source means 4 comprises an LED or a laser source. A light-projecting means 5 converges the light flux from the light source means 4, and projects the light as fine flux to the organic thin film 1 in a direction parallel or nearly parallel to the substrate 2. The light source means 4 and the light-projecting means 5 are integrated into one body, and are provided with a position control device for projecting light flux in a direction nearly parallel to the organic thin film. The light source means 4 and the light-projecting means 5 are constructed in one body, for example, so as to be rotatable around the centor of the support 3. A photosensor 6 monitors the variation of light output from the light source means 4 by detecting the light flux having passed through the organic thin film 1, or the light flux of both having passed through the organic thin film and having been regularly reflected by the substrate 2. That is, the light output from the light source means 4 is stabilized by inputting the output signal from the photosensor 6 to the stabilized power source 7. Thereby image information obtained by an optical area sensor 10, which is an imaging means described later, is standardized to facilitate comparison of organic thin films to be tested. An objective 8 concentrates the scattered light caused by a structural defect in a direction perpendicular or nearly perpendicular to the organic thin film 1, and introduces the light through a half mirror 9 to an optical area sensor 10 at a predetermined magnification to form image information thereon regarding the structural defects of the organic thin film 1. In the optical area sensor 10, an image is formed as a bright spot caused by scattered light from the defect in the organic thin film 1 as shown in drawings shown later. The optical area sensor 10 converts image information regarding the organic thin film 1 into an electric image data. The sensitivity of the optical area sensor 10 is optimized by an optimizing circuit 15 so as to carry out digital treatment effectively. The image data from the optical area sensor 10 is digitalized by an A/D converter 11, and is inputted to an image-processing means 12. The image signal, after being processed by the image-processing means 12, is inputted to a computer 13. The computer 13 analyses the image signal from the image-processing means 12, thereby characterizing the structural defect of the organic thin film 1. The computer 13 controls the movement of the support 3 three-dimensionally (in XYZ directions), thereby enabling automatic observation such as automatic focusing, and automatic positional measurement of density of structural defects. With an ocular 14, the state of the structural defects is observed by utilizing the scattered light, which is caused by structural defects of the organic thin film 1, through the objective 8 and the half mirror 9. In this example constituted as above, defects in the organic thin film 1 like a pinhole are detected as image information, and the presence of structural defects of organic thin films is inspected with high accuracy. Fig. 3 is a photographic image of defects in a fatty acid LB film obtained by use of an apparatus employing the method of the present invention. Fig. 4 is a polarizing microscopic image of the same fatty acid LB film taken by means of a conventional optical microscope, for reference. In Fig. 3, the white spots show structural defects such as pinholes. The present invention enables clear detection of structural defects of an organic thin film as shown in Fig. 3, and allows detection of structural defect and evaluation of an organic thin film with high accuracy in comparison with the photograph obtained with a conventional optical microscope as shown in Fig. 4. Fig. 5 illustrates another evaluation method employing a method of detecting defects at the interface of a thin film with a second material with higher refractive index. A first medium 19 has a higher refractive index than a second medium 20, the film under investigation. The light flux that is introduced at an angle of incidence corresponding to the critical angle or smaller which depends on the refractive indexes of the both mediums 19 and 20 is totally reflected at the interface 19/20 without leaking into the film 20 more than a small fraction of the wavelength. If the refractive index changes at or around the interface, the light is not totally reflected at the interface any more but is scattered by the imperfection. Accordingly, in the same manner as described by reference to Fig. 1, distribution of refractive index around the interface of the medium 20 (or the medium 19) can be shown imagewise by collecting the component of light in a perpendicular or nearly perpendicular direction. Moreover, use of polarized light as the projecting light enables detailed analysis of information on anisotropy at the interface. In other words, the method of the present invention, when applied to the construction as shown in Fig. 5, enables detecting structural defects scattered around the interface with high sensitivity, even if an area of each structural defect is minute. As described above, the present invention provides a method for detecting a structural defect such as a film-lacking portion (like a pinhole) in a simple manner. The method of the present invention is applicable not only to a defect which pierces through a film like the defect la at the left side in Fig. 1, but also to a defect which is localized near an interface like the defect la at the right side of Fig. 1. The present invention enables visualization of a structural defect of an organic thin film (structural change which causes change of refractive index to incident light) in a simple manner as with a conventional optical microscope in the air. Further the present invention enables image analysis to analyse a density of defect and the distribution thereof, a size of defect and the distribution thereof, further a correlation of a distribution of defect with that of domain, and so forth. The present invention provides a method for testing and evaluating the quality of a thin film structure, especially of an organic thin film quickly with high precision in a simple manner in the air by setting the elements as described above. The present invention also provides a method for testing the interfacial structure of a thin film. The testing method and the testing apparatus of the present invention are applicable widely to evaluation of the state of a film, such as an LB film and a liquid crystal film or similar organic films with molecular dimensions.

Claims: Use of a method for detecting imperfections of a thin film formed on a substrate, said method comprising projecting a visible light flux in a direction nearly parallel to the face of the substrate, separating light reflected from the thin film and the substrate from a light scattered in a direction perpendicular or nearly perpendicular to the face of the substrate, and reading information only of the scattered light, to detect defects in the film structure of an organic thin film (1, 20) on top of a substrate (2), said film having a thickness in the range of from several 0.1 nm to 100 nm. The use according to claim 1, wherein the thickness of said film is in the range of from 0.4 nm to 100 nm. The use according to claim 2, wherein the thickness of said film is in the range of from 5nm to 100 nm. The use according to claim 1, 2 or 3, wherein the film (20) is a Langmuir-Blodgett film (20). The use according to claim 4, wherein the Langmuir-Blodgett film (20) is covered by a second film (19) having a higher refractive index than the Langmuir-Blodgett film.

Inventors: ALBRECHT OTTO, EGUCHI KEN, ALBRECHT, OTTO, EGUCHI, KEN, Albrecht, Otto, c/o Canon Kabushiki Kaisha, EGUCHI, KEN, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: G01N21
IPC codes: G01N21, H01L21, G06T1