1. Field of the Invention
This invention relates generally to a device and a method for measuring optically the physical properties of solids, liquids and gases, and more particularly to a device and a method for measuring optically the characteristics of a thin film spread on a liquid surface. The present invention is utilized for measurement of the light absorption characteristics which are the basis for various characteristics analysis of thin films, for example, for characteristic analysis of a monomolecular film spread on a liquid surface to be built up during formation of a monomolecular built-up film.
2. Related Background Art
As a device for measuring light absorption characteristics of a material, there is a device for determining light absorption characteristics from transmittance and reflectance. However, when light is irradiated on a material to be measured, light scattering occurs in addition to transmission and reflection of light, and thus direct measurement of the absorbed light component is important in evaluation of light absorption characteristics to achieve a higher precision of measurement.
As a device for measuring directly the light absorbing components, there is Photoacousatic Spectroscopy (PAS) or Photothermal Radiometry (PTR), which is a measuring device utilizing intermittent conversion of light energy to thermal energy according to a radiationless relaxation process of the light energy absorbed by the material to be measured when light is intermittently irradiated thereon.
There is also a device called Photothermal Deflection Spectroscopy (PDS) for measuring directly the light absorbing components. This PDS device works based on the phenomenon that refractive index changes by temperature distribution caused within the material to be measured or in the vicinity thereof by heat generated by light absorption in the material, whereby the light introduced thereto is deflected. That is, on the measuring site of a material, exciting light which changes the refractive index by creating a temperature distribution by heat generation when the light is absorbed and a probe light for measuring the degree of deflection in terms of the change in a refractive index are irradiated, and the light absorption characteristics of the material to be measured are measured from the wavelength of the exciting light and the deflection of the probe light. The device allows to provide a material to be measured and a detecting system independently of each other and is suitable for measurement at the site where the material is prepared and for remote measurement apart from the site. The basic principle of the present invention is common to this PDS device.
The above-mentioned PDS device is inclusive of the two types of the transverse type and the collinear type according to arrangement of the exciting light and the probe light. Both of the types measure the deflection degree of the probe light corresponding to the amount of exciting light absorbed by the material to be measured as mentioned above, and a position sensitive detector (PSD) is frequently used as the detector.
FIG. 37A shows an example of the collinear type, in which the exciting light 2 emitting from the exciting light source 3 is made intermittent or accentuated with the light intensity modulator 4, condensed at the lens 7b to irradiate the material to be measured 1. The probe light 5 emitting from the probe light source 6 transmits through the measuring site of the material 1 on which the exciting light 2 is irradiated by means of an optical path controller such as the lens 7a or a mirror, etc. to reach the detector 8, where the deflection degree when deflected as shown by the dotted line is measured. FIG. 37B is an example of the transverse type, and the probe light 5 is irradiated in parallel to the surface of the material to be measured 1, as different from the collinear type, otherwise being the same as the collinear type.
The PDS device can be theoretically dealt with by solving the thermal conduction equation within the material to be measured. The deflection degree measured in terms of the deflection angle .phi. is proportional to the exciting light intensity, the temperature coefficient of the refractive index (.differential.n/.differential.n), and the temperature gradient (.differential.T/.differential.x) in the region where the probe light passes, etc. The item proportional to the light absorption coefficient of the material to be measured is included in (.differential.T/.differential.x). Also, the (.differential.n/.differential.T) can take either a positive or negative value depending on the material to be measured and this means that the deflection angle can be either positive or negative.
FIG. 38 is a longitudinal sectional view showing a construction example of one-dimensional PSD. In FIG. 38, the one-dimensional PSD constitutes a uniform resistance layer 74 of P layer on the surface of a flat plate silicon 73, and has electrodes X.sub.1 and X.sub.2 provided on both sides thereof, having also a common electrode 76 at the N layer 75 on the back.
FIG. 39 is a schematic illustration showing its actuation principle. The light-forming electric charges made correspondent to the incident position of the light Q reaches the above resistance layer 74 as the photocurrent corresponding to its energy to be divided in inverse proportion to the distances from its position Q to the take-out electrodes X.sub.1 and X.sub.2 at the both ends and outputted from the respective electrodes. If the photocurrent is defined as I.sub.L, the photocurrent I.sub.x1 and I.sub.x2 outputted from the electrodes X.sub.1 and X.sub.2 are represented as follows: ##EQU1## and further, since the resistance between X.sub.1 and X.sub.2 is maintained at uniform distribution, the following respective equations are valid between the resistance between X.sub.1 and X.sub.2 and the length L: EQU R.sub.x1 +R.sub.x2 =L EQU R.sub.x1 =X EQU R.sub.x2 =L-X
Accordingly, the signals taken out from the respective electrodes can be represented by L and x as follows: ##EQU2## Thus, the informations of the incident position of light and light intensity can be obtained at the electrodes of X.sub.1 and X.sub.2.
Further, by taking a ratio of the difference between I.sub.x1 and I.sub.x2 to the sum thereof and representing it as P, the following equation can be obtained: ##EQU3## and the positional signals irrelevant with light intensity change can be obtained continuously corresponding to x=0 to L as follows: ##EQU4##
Having described about the one-dimensional case, the two-dimensional case may also be considered similarly, and the positional signals can be determined from the block diagram of the actuation circuit as shown in FIG. 40.
Here, from the actuation principle of PSD, when light is introduced at two or more points, the positional signal weighted in proportion to the respective light intensity is obtained. Also, in the case when the light flux is expanded, the positional signal corresponding to the gravitational center of light intensity can be obtained.
On the other hand, there has been known in the art a device for forming monomolecular built-up films in which monomelecular films are transferred onto a substrate by laminating them one by one according to the monomolecular layer film built-up method called the Langmuir-Blodgett method (hereinafter called LB method) named after the inventors (see Shin Jikken Kagaku Koza (New Course of Experimental Chemistry), Vol. 18, pp. 498-507, Maruzen).
The above device is constituted schematically by a liquid tank containing a liquid, a film-forming frame which is floated so as to divide the liquid surface into two and is capable of two-dimensional piston movement, a driving means for moving the film-forming frame, a surface-pressure-measuring instrument for measuring the surface pressure of the monomolecular film spread on the liquid surface and a substrate holder for moving the substrate held vertically relative to the liquid surface. Formation of a monomolecular film and transfer thereof onto a substrate are practiced as described below.
First, with the film-forming frame being kept at one side of the liquid tank, a solution of a film-forming substance dissolved into a volatile solvent such as benzene, chloroform, etc. at a concentration of, for example, ca. 5.times.10.sup.-3 mol/liter is added in several drops onto the liquid surface by means of a fountain pen filler, etc. When the solution is spread over the liquid surface and the solvent is evaporated, the monomolecular film remains on the liquid surface.
The above monomolecular film exhibits the behaviour of a two-dimensional system on the liquid surface. When the surface density of molecular is small, it is called as a gas film of two-dimensional gas, and the state equation of two-dimensional ideal gas is valid between the occupied area per molecule and the surface pressure.
Subsequently, by increasing the surface molecular density by narrowing the region of the liquid surface carrying the spread molecular film by moving gradually the film-forming frame, the interaction between molecules is intensified, whereby the gas film changes via a liquid film of two-dimensional liquid to a solid film of two-dimensional solid. When such a solid film is formed, the molecules become oriented fairly regularly and the film has highly ordered characteristic and uniform ultra-thin film characteristic. And, by moving vertically the substrate with the substrate holder, the monomolecular film which has become said solid film can be attached and transferred thereon. Also, by transferring the monomolecular film for plural times on the same substrate, a built-up monomolecular film can be obtained. As the substrate, there may be employed, for example, a glass, a synthetic resin, a ceramic, a metal, etc.
In order to practice the transfer operation under the preferable state of the monomolecular film for transfer onto the above substrate, the surface pressure of the monomolecular film is measured. The surface pressure preferable for transfer is generally accepted to be 15 to 30 dyn/cm. Outside this range, the alignment or orientation of the molecules may be disturbed, or otherwise peel-off of the film will readily occur. However, in a special case, for example, depending of the chemical structure of the film-forming substance, the temperature conditions, etc., preferable values of the surface pressure may be outside the above range, and therefore the above range may be a tentative measure.
The surface pressure of the above monomolecular film is measured automatically and continuously by means of a surface-pressure-measuring instrument. As the surface-pressure-measuring instrument, there is one in which the method for determining the difference in surface tension between the liquid surface covered with no monomolecular film and the liquid surface covered with monomolecular film is applied, or one in which the two-dimensional pressure applied on the film-forming frame which will float by partitioning the liquid surface into the liquid surface covered with no monomolecular film and the liquid surface covered with monomolecular film, etc., each having specific features. The occupied area per molecule constituting monomolecular film and the degree of the change are also generally measured together with the surface pressure. The occupied area and its change can be determined from the movement of the film-forming frame.
The movement is controlled based on the surface pressure of the monomolecular film measured by means of the above measuring instrument. More specifically, the driving means for moving the film-forming frame is controlled based on the surface pressure of monomolecular film measured by the surface pressure measuring instrument so that the monomolecular film can constantly maintain a predetermined surface pressure selected within the preferable range for transfer operation. The movement control of the film-forming frame is continuously made not only until initiation of the transfer operation of monomolecular film after dropwise addition of the film-forming substance, but also during the transfer operation continuously. For example, in the transfer operation, as the monomolecular film is transferred onto the substrate, the surface density of the monomolecular-film-forming molecules on the liquid surface will be lowered, whereby the surface density will be also lowered. Accordingly, by moving the film-forming frame, the spreading area of the monomolecular film is narrowed thereby to maintain the constant surface pressure by correction corresponding to the lowered surface pressure.
However, when a PAS device, PTR device or PDS device itself is to be used for measurement of an extremely thin material under a specific environment such as a thin film of monomolecular film being spread on liquid surface, problems arise such as difficulty of measurement, liability of declining precision, sensitivity of measurement because of the location on a liquid surface of the extreme thinness of the measured material.
PAS devices can be classified into the microphone system and the piezoelectric element system depending on the kind of the detector. In the case of the microphone system, a sample is required to be placed in a hermetically sealed sample chamber, while in the case of the piezoelectric element system, arrangement of the detector and the sample is restricted. Thus, either one of them is not suitable for measuring a thin film as spread on a liquid surface.
In the case of the collinear type PDS device, since the probe light passes through the measuring site of the thin film where the greatest refractive index change occurs by irradiation of exciting light, there is the advantage that a relatively greater positional change of probe light can be obtained on the detector. However, in the collinear type PDS device, since the probe light transmits through the thin film which the material to be measured, it receives at the same time the influences from both the liquid phase and the gas phase of which refractive index varies based on exciting light absorption of the thin film. Accordingly, no correct measurement is possible unless the variation in refractive index between the liquid phase and the gas phase is considered, whereby there is the problem that measurement of high precision becomes extremely difficult.
Also, when the material to be measured is a thin film as in the present invention, the refractive index change of the surrounding caused by absorption of exciting light is small. Accordingly, in the case of a transfer type PDS device, it is required that the probe light be brought close to the thin film on the liquid surface so that it can pass through the region where refractive index change as great as possible occurs. Particularly, since the detector of the PDS device has the property of detecting the position of the "gravitational center" of the light-receiving intensity, and it is preferred that the flux center of the probe light with strong light intensity be nearer to the thin film. However, it is not possible to access the center of the probe flux nearer than its radius. Therefore the probe flux center with strong light intensity is liable to pass through the region apart from the thin film, resulting in little refractive index change. This makes a high precision and high sensitivity measurement difficult.
In PDS measurement, the convergence state of the fluxes of exciting light and probe light have influences on sensitivity, and the relative positional relationship between exciting light and probe light will affect sensitivity greatly. Accordingly, it is necessary to monitor the irradiation positions of exciting light and probe light and their relative positions and, in prior art, scattered light from excitation light and probe light have been monitored by visual observation with a microscope, but such a method will give undesired external disturbance such as vibration of liquid surface. Also, in the case of a very thin or homogeneous film, the scattered light may be sometimes undetectable. Further, in carrying out measurement of light absorption characteristics, the wavelength of the light used is not limited to a visible light region and, for example, in the case of using IR-ray, there has been the drawback that the irradiation position cannot be confirmed by visual observation.
Further, concerning measurement of a material spread on the liquid surface such as monomolecular film, noise by wobbling of the liquid surface brings about lowering in measurement precision.
On the other hand, as described above, various fine controls are required for obtaining a monomolecular built-up film. However, many experiments must be conducted in order to search the optimum condition, and it can be confirmed only indirectly by the surface pressure, etc. whether the monomolecular film on the liquid surface is in the suitable condition for built-up or not, and thus precision is insufficient. This can be improved by grasping directly the physical properties of the monomolecular film on the liquid surface by means of PAS, PTR or PDS device, but due to the problems as mentioned above, such a demand cannot be met under the present situation.