Method of and apparatus for measuring film thickness

Abstract: Light in the ultraviolet region is applied toward a transparent thin film of an object sample to measure energy of light relfected by the object sample. Further, on the basis of the measured energy value, the thickness of the transparent thin film of the object sample. Thus, the thickness thereof can be correctly measured even if the film thickness is not more than 10nm.

Description: The present invention relates to a method which can correctly measure the thickness of a transparent thin film formed on a substrate by optical means. In a film thickness inspection of a semiconductor manufacturing process or the like, it is necessary to measure the thickness of a silicon oxide film which is formed on a silicon substrate, for example, in a non-contact manner. In order to measure such film thickness, generally employed are a method (hereinafter referred to as "polarization analyzing system) of measuring change in a polarized state of reflected light and obtaining the film thickness from such change and another method (hereinafter referred to as "reflected light energy measuring system") of irradiating a sample to be measured with light for measuring energy of light reflected by the sample to thereby obtain the film thickness from the energy. In the polarization analyzing system, light is applied to a sample to be measured obliquely from above for measuring the thickness of a transparent thin film formed on the sample. Thereafter, detected are change in a polarized state of light reflected by the sample to be measured (i.e., change in relative phase shift between the P wave, which is a vibration component parallel to the plane of incidence, and the S wave, which is a vibration component perpendicular to the plane of incidence) and change in the amplitude ratio between the waves. Further, the thickness of the transparent thin film is obtained on the basis of the detected change. This polarization analyzing system is applied to an ellipsometer, for example, so that the thickness of a thin film formed on a sample to be measured can be accurately measured by the ellipsometer in a non-contact manner even if the film thickness is not more than 10 nm. However, it is difficult to measure a region to be measured while limiting the same in a narrow range. Thus, the reflected light energy measuring system, which is described below, is generally employed in a semiconductor manufacturing process. When light is applied to a sample to be measured, light reflected by the surface of a transparent thin film formed on the sample interferes with that reflected by the surface of a substrate of the sample, whereby constant relation holds between the energy of the reflected light and the thickness of the transparent thin film. In reflected light energy measuring system, the thickness of the thin film is obtained on the basis of such relation. In more concrete terms, the thickness of the transparent thin film is obtained as follows: First, samples (hereinafter referred to as "first standard samples") provided with transparent thin films having different thickness values are prepared in order to obtain correlation data of energy of reflected light and thickness values of the transparent thin films. Optical constants of the transparent thin films and substrates of the first standard samples are identical to those of a sample to be measured, and the thickness values of the transparent thin films of the first standard samples are known, respectively. Visible light is applied to the respective standard samples to measure energy of light reflected by the surface of the standard samples, to thereby obtain correlation data of the energy of the reflected light and the thickness values of the transparent thin films. Thus, prepreparation is completed. Then, visible light is applied to the sample to be measured, in order to measure energy of light reflected by the sample to be measured. Finally, the thickness of a thin film formed on this sample is obtained on the basis of the measured values and the correlation data. As understood from the above, the reflected light energy must be extremely changed in response to change in film thickness, so that the film thickness can be correctly measured by the reflected light energy measuring system. In general, most of thin films formed on semiconductor substrates are at least 40 nm in thickness, and the reflected light energy is extremely changed even if the amount Δd of change in film thickness is very small. Thus, it has been possible to measure the thickness of such a thin film in relatively high accuracy by the reflected light energy measuring system. The thickness of a thin film has been reduced with recent improvement in semiconductor manufacturing technique, and it has been increasingly demanded to accurately measure such film thickness also within a range of not more than 10 nm. However, it is impossible to correctly measure film thickness of not more than 10 nm by the reflected light energy measuring system. The reason for this is that energy of light reflected by a thin film of not more than 10 nm in thickness is approximate to energy E₀ of light reflected by a sample (hereinafter referred to as "second standard sample") which is provided with no transparent thin film. In other words, energy of reflected light is substantially constant within a range of 0 to 10 nm in thickness. In more concrete terms, the energy of reflected light is theoretically provided as E in the case of film thickness of 10 nm, for example, as shown in Fig. 1, while actually measured energy of the reflected light includes a measurement error, to thereby be not identical to the theoretical value E. As understood from Fig. 1, variation in energy with respect to change in film thickness is small around film thickness of 10 nm, particularly that of not more than 10 nm. Therefore, film thickness obtained on the basis of the energy value including the aforementioned measurement error may significantly differ from the true film thickness. From WO-A-86/01591 an interferometric method of measuring the thickness of thin films on semiconductor substrates using ultraviolet light is known. A disclosure similiar thereto is also found in IBM TECHNICAL DISCLOSURE BULLETIN vol. 20, no. 3, August 1977, New York, USA; pages 1242-1246: P.S.HAUGE: "Optical measurement of polysilicon film thickness"; and idem, pages 1239-1241: F.H. DILL et al.: "Optical thickness measurement analysis applicable to multilayer films"; see further PATENT ABSTRACTS OF JAPAN : JP-A-62 04 3503 (vol.11, no. 230 (P-599)(2677) 28 July '87) dealing with thickness determination of transparent paint films on steel plate using UV light. As in the present application these prior art documents are based on the insight that the underlying substrate is opaque to light of the particular UV- wavelength used, so that the reflected light effectively consists of two interfering components reflected from the respective boundaries (interfaces) of the thin film overlying the substrate. The method according to the invention is defined in claim 1. In the drawings : Fig. 1 illustrates relations between energy of light reflected by an object sample and the thickness of a transparent thin film of the object sample; Fig. 2 illustrates relations between reflected light energy percentage P(λ) and wavelength λ; Fig. 3 is a schematic block diagram showing a film thickness measuring an apparatus according to the present invention; and Fig. 4 is a flow chart showing the film thickness measuring method according to the invention. A. Principle of the Invention In order to solve the aforementioned technical problem, various researches have been made to obtain the results shown in Fig. 2. Fig. 2 illustrates relations between reflected light energy percentage P(λ) and wavelength λ. In more concrete terms, the relations have been obtained in the following manner: Light was applied to a silicon substrate to obtain energy E₀(λ) of light reflected by the silicon substrate, while light was also applied to samples comprising silicon substrates and SiO₂ films of prescribed thickness values (2 nm, 5 nm and 10 nm) provided on the silicon substrates, respectively to obtain values of energy E(λ) of light reflected by the respective samples. Then, the reflected light energy percentage P(λ) was obtained on the basis of the following expression: Solid lines L₂, L₅ and L₁₀ in Fig. 2 show relations between the reflected light energy percentage P(λ) and the wavelength λ in the case that the SiO₂ films of 2 nm, 5 nm and 10 nm in thickness are formed on the silicon substrates, respectively. Table 1 lists some of values of the reflected light energy percentage P(λ) with respect to the wavelength λ obtained in the aforementioned manner. In order to obtain the aforementioned relations, optical constants of silicon and SiO₂ were determined on the basis of "Handbook of Optical Constants of Solids". Analyzing Fig. 2 and Table 1, the following first to third phenomena have been ascertained: First Phenomenon In the visible and infrared regions (λ= 400 to 900 nm), the reflected light energy percentage P(λ) is reduced as the wavelength λ is shortened (hereinafter referred to as "first phenomenon") under the condition that the film thickness is constant. Noting the solid line L₁₀ in Fig. 2, for example, the reflected light energy percentage P(596) with respect to the wavelength of 596 nm is 98.72 %, while the reflected light energy percentage P(λ) is reduced as the wavelength λ is shortened such that the reflected light energy percentage P(407) with respect to the wavelength of 407 nm is 97.66 %, as shown in Fig. 2 and Table 1. As understood from Fig. 2, detection accuracy for the film thickness is advantageously improved as the wavelength of light is reduced within such a wavelength range. However, since variation in reflected light energy percentage P(λ) with respect to that in wavelength λ is small, it is difficult to attain the object of the present invention through the first phenomenon in practice. Second Phenomenon In the ultraviolet region (λ= 200 to 400 nm), the reflected light energy percentage P(λ) is extremely reduced as the wavelength λ is shortened under the condition that the film thickness is constant. It means that the energy E(λ) is remarkably reduced as compared with the energy E₀(λ) (this phenomenon is hereinafter referred to as "second phenomenon"), as understood from the expression (1). Noting the solid line L₁₀ in Fig. 2, for example, the reflected light energy percentage P(407) with respect to the wavelength of 407 nm is 97.66 %, while the reflected light energy percentage P(207) with respect to the wavelength of 207 nm is 85.30 %, as shown in Fig. 2 and Table 1. In the ultraviolet region (λ= 200 to 400 nm), therefore, the energy E(λ) of the light reflected by the samples provided with SiO₂ films of not more than 10 nm in thickness is remarkably reduced as compared with the energy E₀(λ) of the light reflected by the silicon substrates. Third Phenomenon In the ultraviolet region (λ= 200 to 400 nm), the reflected light energy percentage P(λ) is extremely changed as the SiO₂ films are increased in thickness (this phenomenon is hereinafter referred to as "third phenomenon") under the condition that the wavelength λ is constant as one-dot chain line in Fig. 2 shows. This can be easily understood also from Table 1. Assuming that the wavelength λ is 596 nm, for example, reflected light energy percentage P (at 2 nm) is 99.95 % when the film thickness is 2 nm, while reflected light energy percentage P (at 10 nm) is 98.72 % when the film thickness is 10 nm. Therefore, the reflected light energy percentage P(λ) is changed by only about 1 % under the aforementioned condition. Assuming that the wavelength λ is 207 nm, on the other hand, the reflected light energy percentage P (at 2 nm) is 97.93 % and the reflected light energy percentage P (at 10 nm) is 85.30 %. Thus, the third phenomenon is clearly understood also from Table 1. While the above description has been made on the relation between the reflected light energy percentage P(λ) and the film thickness, phenomonons similar to the above are recognized also between the energy E(λ) of the reflected light and the film thickness when the wavelength λ is constant. That is, values P₁(λ₁), P₂(λ₁), ... of reflected light energy percentage with respect to values d₁, d₂, ... of film thickness under the condition that the wavelength is constant ( λ=λ₁ ) are respectively obtained as follows: As understood from the expressions (2), energy E₀(λ₁) of the light reflected by the silicon substrates is constant, and hence the energy E(λ) of the reflected light is extremely changed as the SiO₂ films are increased in thickness, similarly to the third phenomenon. The aforementioned second and third phenomena were also observed when the thickness values of the SiO₂ films were at least 10 nm. While the above description has been made with respect to the case where the SiO₂ films were formed on the silicon substrates, similar phenomena were also recognized in other case. Table 2 shows relations between reflected light energy percentage P(λ) and wavelength λ with respect to silicon substrates provided with Si₃N₄ films. Table 3 shows relations between reflected light energy percentage P(λ) and wavelength λ with respect to GaAs substrates provided with SiO₂ films. Table 4 shows relations between reflected light energy percentage P(λ) and wavelength λ with respect to GaAs substrates provided with Si₃N₄ films. Table 5 shows relations between reflected light energy percentage P(λ) and wavelength λ with respect to Cr substrates provided with SiO₂ films. Table 6 shows relations between reflected light energy percentage P(λ) and wavelength λ with respect to Cr substrates provided with Si₃N₄ films. Procedures for deriving the values shown in these Tables are identical to that of Table 1. Optical constants of GaAs and Si₃N₄ are determined on the basis of "Handbook of Optical Constants of Solids" and that of Al is determined on the basis of "Physics Daten Physics Data, Optical Properties of Metals 1981 Nr. 18-2", while that of Cr is determined on the basis of "Physics Daten Physics Data, Optical Properties of Metals 1981 Nr. 18-1". As hereinabove described, the energy E(λ) of light reflected by a sample to be measured is remarkably reduced as compared with the energy E₀(λ) when light of the ultraviolet region is applied to the sample even if the film thickness is not more than 10 nm, while the energy E(λ) of the reflected light is extremely changed in response to slight change in thickness within the range of the film thickness of not more than 10 nm. Therefore, the thickness of a transparent thin film can be correctly measured through the light of the ultraviolet region even if the film thickness is not more than 10 nm, in addition to the case of film thickness of at least 10 nm. Fig. 3 is a schematic block diagram showing a film thickness measuring apparatus 1 according to the present invention. As shown in Fig. 3, the film thickness measuring apparatus 1 is formed by an illumination optical system 2, a microscopic optical system 3, a reflecting mirror 20, a spectroscopic unit 32, a monitoring optical system 4, an arithmetic unit 34 and a control unit (not shown) for controlling the entire apparatus. The illumination optical system 2 comprises a light source 10 such as a halogen lamp, for example, emitting visible light L VS , a lens 12 and a shutter mirror 14 which is vertically reciprocable with respect to the plane of the figure, so that visible light L VS outgoing from the light source 10 is guided to the microscopic optical system 3 through the lens 12 and the shutter mirror 14. The illumination optical system 2 also comprises a light source 28 emitting ultraviolet light and a lens 30, so that ultraviolet light L UV can be guided to the microscopic optical system 3 in place of the visible light L VS . The light source 28 may apply light of a specific ultraviolet region, and can be prepared by a deuterium lamp, a mercury lamp, an ultraviolet laser irradiator, a xenon lamp or the like. The microscopic optical system 3 is formed by a half mirror 16 and a mirror type image formation element 18. Therefore, when the shutter mirror 14 is on an optical axis as shown in Fig. 3, the visible light L VS outgoing from the light source 10 is applied to the surface of an object sample S to be measured through the lens 12, the shutter mirror 14, the half mirror 16 and the mirror type image formation element 18. When the shutter mirror 14 is evacuated from the optical axis, on the other hand, the ultraviolet light L UV outgoing from the light source 28 is applied to the surface of the object sample S through the lens 30, the half mirror 16 and the mirror type image formation element 18. A beam reflected by the surface of the object sample S is enlargedly imaged in a position close to the reflecting mirror 20 through the image formation element 18 and the half mirror 16. The reflecting mirror 20 is provided with a pinhole 20a. Within the reflected beam, therefore, reflected light L S passing through the pinhole 20a is incorporated in the spectroscopic unit 32. The spectroscopic unit 32 is formed by a diffraction grating (not shown) for separating the reflected light L S into its spectral components and an optical detector (not shown) for detecting the spectrum. Therefore, the spectroscopic unit 32 detects the spectrum of the light L S incorporated therein, and supplies a signal corresponding to the spectrum of the light L S to the arithmetic unit 34. The arithmetic unit 34 is formed by a memory 35 and a film thickness calculating unit 36, to calculate the film thickness of the object sample S on the basis of the signal outputted from the spectroscopic unit 32. Within the reflected beam, on the other hand, light reflected by the reflecting mirror 20 is incident upon the monitoring optical system 4, and imaged in a camera unit 26 through a mirror 22 and the image formation lens 24. Thus, the camera unit 26 picks up an enlarged image of the surface of the object sample S and projects the same on a CRT (not shown). Confirmation of the position of measurement and focusing of optical system of the apparatus 1 can be performed on the basis of the enlarged image. Although visible light is employed in order to confirm the position for measuring the film thickness of the object sample S and to focus the optical system in the aforementioned apparatus, the surface of the object sample S can also be observed through ultraviolet light. Thus, the light source 10, the lens 12 and the shutter mirror 14 are not requisite elements of the present invention. The absolute reflectance R S of an object sample S to be measured is univocally determined upon provision of a refractive index n₀ and an absorption coefficient k₀ of an incident-side medium (air in general, since film thickness is measured in the atmospheric air), a refractive index n₁ and an absorption coefficient k₁ of a transparent thin film, a refractive index n₂ and an absorption coefficient k₂ of the substrate, wavelength λ and an incident angle  of illumination light and thickness d x of the transparent thin film. Therefore, when these values n₀, n₁, n₂, k₀, k₁, k₂, λ and  are specified, the thickness d x can be obtained on the basis of the value of the absolute reflectance R S . Although it is not easy to actually measure the absolute reflectance R S of the object sample S, the absolute reflectance R S can be calculated from the energy of light reflected by the object sample S. Thus, the thickness of the thin film formed on the object sample S can be measured on the basis of the aforementioned principle of film thickness measurement. The film thickness measuring apparatus carries out film thickness measurement while controlling the overall apparatus as hereinafter described by a control unit. Fig. 4 is a flow chart showing the film thickness measuring method according to the embodiment. First, as prepreparation for measurement of thickness of a transparent thin film provided in the object sample, an operator inputs correlation data of the absolute reflectance R S and the film thickness d x on the basis of literature such as Handbook of Optical Constants of Solids, Academic Press, for example, through a keyboard (not shown) to store the same in a memory 35. Similarly to the first embodiment, the correlation data are so stored that corresponding film thickness is outputted upon addressing with the absolute reflectance R S . As the prepreparation, further, the operator sets a sample P (hereinafter referred to as "reflectance-known sample"), at least the absolute reflectance of which is known, in the measuring apparatus (step S12). Preferably the absolute reflectance of the reflectance-known sample P, which may be arbitrarily prepared so far as its absolute reflectance is clear, is not extremely different from that of the object sample S. Thus, the reflectance-known sample P may be prepared from the same material as the substrate of the object sample S. Ultraviolet light L UV is applied to the sample S (step S13), to measure energy E P of a specific wavelength of light reflected by the reflectance-known sample P in the ultraviolet region (step S14). The measured energy E P is stored in the memory 35 (step S15). The operator further inputs the absolute reflectance R P of the reflectance-known sample P through the keyboard to store the same in the memory 35 (step S16). Then the operator sets the object sample S in the measuring apparatus (step S17). At this time, the position of measurement can be correctly specified. Thereafter the operator supplies a measurement command to the control unit, whereby steps S18 to S22 are carried out in response to a command from the control unit. That is, the ultraviolet light L UV is applied to the surface of the object sample S (step S18), and energy E S of the light reflected by the said surface is measured by a spectroscopic unit 32 (step S19). A signal relating to the energy E S of the reflected light is supplied from to the spectroscopic unit 32 the arithmetic unit 34, while the values of the energy E P and the absolute reflectance R P are read from the memory 35, so that absolute reflectance R S of the object sample S is operated on the basis of the following expression (step S20): R S = (R P /E P ) · E S (3) The reason why the absolute reflectance R S is obtained from the expression (3) is as follows: It is generally known that the following relation holds between the energy E P of the light reflected by the reflectance-known sample P and the absolute reflectance R P of the reflectance-known sample: E P = K · R P (4) Symbol K represents a value specific to the apparatus measuring the said energy E P . In the aforementioned film thickness measuring method, energy values of the reflected light of the reflectance-known sample P and the object sample S are measured by the same apparatus, and hence the following relation holds: E S = K · R S (5) The expression (3) is derived from the expressions (4) and (5). On the basis of the absolute reflectance R S obtained at the step S20 and the correlation data stored in the memory 35, the thickness of the transparent thin film formed on the object sample S is obtained (step S21), and the thickness value is displayed on a display unit (not shown) (step S22). Since the energy of reflected light is measured through the light of the ultraviolet region similarly to the first embodiment, the thickness of the transparent thin film can be correctly measured even if the thickness is no more than 10 nm. Although an exemplary method of obtaining film thickness through the relation holding between absolute reflectance and film thickness in relation to the embodiment, the method of deriving the film thickness is not restricted to this. For example, the ratio of absolute reflectance R S obtained from reference etc. to absolute reflectance R P of a reflectance-known sample P may be operated every film thickness d x to store correlation between the film thickness d x and the ratio (R S /R P ) in the memory 35 and to thereafter perform division E S /E P with respect to energy E S of light reflected by an object sample S in the film thickness calculating unit 36 for retrieving the memory 35 with an address of the value E S /E P , to thereby regard film thickness stored with an address of R S /R P being coincident with or approximate to E S /E P as that of the transparent thin film. In the aforementioned embodiment, at a constant wavelength in the ultraviolet region, the correlation data of absolute reflectance and film thickness and energy of reflected light from the reflectance-known sample and the object sample S must be obtained. Although the wavelength may be arbitrarily set so far as the same is within the ultraviolet region, the same is preferably selected within a range of 180 to 240 nm.

Claims: A method of measuring the thickness of a transparent thin film having a thickness of not more than 10 nm on a substrate of Si, GaAs or Cr, said method including: preparing known correlation data showing a correlation between absolute reflectance and thickness for different thickness values of said transparent thin film on said substrate, determining the absolute reflectance of an object sample comprising said thin film on said substrate by applying light of a wavelength in the UV-range to said object sample and measuring the reflected energy Es; obtaining the thickness of said transparent thin film of said object sample on the basis of absolute reflectance R P thereof and said known correlation data, characterized in that the UV-light used has a wavelength of not more than 240 nm and by the steps of preparing a standard sample whose absolute reflectance R P is known, applying light of said wavelength toward said standard sample and measuring the energy E P of of said light reflected by said standard sample, and subsequently calculating the absolute reflectance value Rs of said object sample in accordance with: Rs = (R P / E P )Es where Es denotes the energy of said light reflected by said object sample. A method in accordance with claim 1, characterized by selecting an absolute reflectance value most approximate to said absolute reflectance value Rs out of said known correlation data, thereby to obtain the thickness value of said transparent thin film of said object sample corresponding to the selected absolute reflectance value. A method in accordance with claim 1, characterized by selecting a plurality of absolute reflectance values approximate to said absolute reflectance value Rs out of said known correlation data, thereby to obtain the thickness value of said transparent thin film of said object sample on the basis of the selected absolute reflectance values. A method in accordance with claim 3, characterized by calculating the thickness value of said object sample by interpolation.

Inventors: KONDO NORIYUKI DAINIPPON SCREE, KONDO, NORIYUKI DAINIPPON SCREEN MFG. CO., LTD.
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1989
Main classification: G01B11
IPC codes: G01B11