Abstract:
A sputtering apparatus for forming a film by a physical gas-phase growth on a substrate having a irregular or flat shape is provided including three or more axes for independently varying a relative positional relationship between a substrate and a cathode in the course of film formation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sputtering apparatus and a sputtering method for obtaining a uniform film and desired film thickness characteristics in a film formation on a substrate with surface irregularities, mounted on a substrate holder, and more particularly to a film forming apparatus and a film forming method for forming a uniform film with a desired film thickness distribution such as a multi-layered antireflection film or a multi-layered film mirror or an X-ray reflecting multi-layered film, on a substrate such as a large-aperture lens. 
     2. Description of the Related Art 
     A magnetron sputtering apparatus of parallel flat plate type, employed already widely, is an apparatus in which a target constituting a material of a thin film and a substrate mounted on a substrate holder are positioned in a mutually opposed manner in a vacuum chamber, and a plasma is generated to sputter the target whereby particles sputtered out are deposited on the substrate to form a thin film thereon, and has features simpler than other methods and capable of a high-speed film formation, a large-area film formation and a longer target life. 
     In relation to such sputtering apparatus, a sputtering in the field of optical films has been recently investigated, and, particularly in a semiconductor exposure apparatus such as a stepper, together with an improvement toward a higher NA for improving the exposure performance, there are requested an increase in the lens aperture, an improvement in inclined incident characteristics for a ray entering the lens, and, an inclined film of a large diameter and a high precision (for improving the inclined incident characteristics) in an X-ray (EUV) exposure apparatus of next generation. 
     In a multi-layered X-ray mirror of molybdenum (Mo) and silicon (Si) for an X-ray wavelength of 13.4 nm, the reflection characteristics are lowered depending on an incident angle of the incident X-ray entering the mirror surface, because of an extremely narrow bandwidth of the reflection characteristics. As a countermeasure, a method of forming reflecting mirror characteristics, matching the incident X-ray angle within the plane of the X-ray mirror, is being adopted. It is therefore necessary to highly precisely control the film thickness distribution of molybdenum (Mo) and silicon (Si) within the mirror plane. 
     For controlling the film thickness distribution, Japanese Patent Application Laid-open No. H10-30170 discloses a method of employing a shield mask and controlling a shape of such shield mask and a moving speed distribution thereof, thereby controlling the film thickness distribution. 
     Also Japanese Patent Application Laid-open No. H9-213634 discloses a method of film formation under a movement of the substrate in an X or XY direction parallel to an evaporation source. 
     Also U.S. Pat. No. 6,010,600 proposes an inclined film formation by a revolution speed control when a substrate, executing a revolving motion and a rotating motion, passes over a target. 
     However, optical elements employed in an exposure apparatus, etc. are designed for increasingly shorter wavelengths and in various shapes such as an aspherical surface, an arbitrarily curved surface and a parabolic surface. For this reason, a film thickness control of a higher precision than in the prior technology is being requested. 
     For realizing an optical element requiring such highly precise film thickness control, the following film forming apparatus and film forming method are indispensable:
         (1) Ability for film thickness distribution control for various irregular shapes;   (2) Adaptability to plural targets (showing different film thickness distributions by a difference in an emission angle distribution of a material);   (3) A film formed by an inclined entry of sputtering particles has a low film density, thus showing a change in the refractive index and a change in the optical characteristics by a moisture adsorption. Therefore the film forming apparatus should minimize an inclined entry component of the sputtering particles;   (4) A load lock type film formation should be possible in order to minimize particle generation, and influence of residual gas, particularly water;   (5) The film forming apparatus should not introduce a complex moving mechanism into the vacuum chamber.       

     In the aforementioned method of employing a shield mask and controlling the shape of the shield mask and a moving speed distribution thereof, thereby controlling the film thickness distribution (Japanese Patent Application Laid-Open No. H10-30170), it is difficult to meet the above requirements (1) and (2) with a mask of a single shape, there is required an operation of replacing the mask by opening the vacuum chamber to the air. The opening of the vacuum chamber to the air causes water in the air to be adsorbed in a film deposited in the vacuum chamber, thereby increasing a stress in the film and inducing a peeling thereof, thus resulting in an influence corresponding to (4). Also the shield mask, positioned in the vicinity of the substrate between the target and the substrate, undesirably becomes a source of particles. 
     The method of employing a shield mask and controlling the shape of the shield mask and a moving speed distribution thereof, thereby controlling the film thickness distribution, as disclosed in Japanese Patent Application Laid-open No. H10-30170, can only deposit a rotationally symmetrical film but cannot be adapted to a film of other configurations. 
     The inclined film formation by a revolution speed control when a revolving and rotating substrate passes over a target, as proposed in U.S. Pat. No. 6,010,600, can only deposit a rotationally symmetrical film. 
     The method of film formation under a movement of the substrate in an X or XY direction parallel to an evaporation source, disclosed in Japanese Patent Application Laid-open No. H9-213634, in a film formation of a peripheral portion of a substrate having a large curvature in an irregularity and/or a complex shape, brings about a large inclined entry component of the sputtering particles entering the substrate, thereby resulting in a film of a low film density and causing a shift in the characteristics. Further, a complex moving mechanism is undesirably required in the vacuum apparatus. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to obtain a uniform and highly precise optical thin film with a desired film thickness distribution on a substrate of various shapes. 
     The aforementioned object can be attained by a sputtering apparatus for forming a film by a physical gas-phase growth on a substrate having a irregular or flat shape, including three or more axes for independently varying a relative positional relationship between a substrate and a cathode in the course of film formation. 
     According to the present invention, there is also provided a film forming method by sputtering, characterized in that, in the aforementioned sputtering apparatus, a film formation is executed by a scan or plural scans with a moving speed under plural controlled conditions of at least two axes among the cathode rotating axis, the substrate rotating axis, the scan axis and the TS control axis. 
     According to the present invention, there is also provided a film forming method by sputtering, characterized in that, in the aforementioned sputtering apparatus, a film formation is executed by a scan or plural scans with a moving speed under plural controlled conditions of at least two axes among the cathode rotating axis, the substrate rotating axis, the scan axis and the TS control axis. 
     According to the present invention, there is also provided a sputtering apparatus for forming a film by a physical gas-phase growth on a substrate having an arbitrary curved surface shape, comprising three or more axes for independently varying a relative positional relationship between a substrate and a cathode in the course of film formation, and at least a function, independently of the control axis for the positional relationship, of varying a rate and/or a film forming area in the course of film formation. 
     In a configuration of the present invention (represented in an XYZ coordinate system), a cathode rotary axis for one or plural cathodes has a scanning axis having a rotation center on an X′-axis offset to a Z-axis on a Y-axis parallel to a target surface and executing a scanning motion on an XZ plane, a rotary axis for rotating a substrate incorporated in a unit moving on the XZ plane by the scanning axis, and a TS control axis for varying a target-substrate distance. When taking as an example a situation where a substrate with a irregular surface is set on a substrate holder, for example a convex lens of an external diameter of 300 mm and a radius of curvature of R=300 mm is set on the rotating substrate holder, in any film forming area of a radius of the rotating lens on the XZ plane and at the side of the target, a normal to the lens is made substantially perpendicular to a film forming face of the target by angles of two axes, namely a cathode rotation angle (Tθ) and an angle (Sθ) of the scanning axis having the rotation center on the X′-axis offset to the Z-axis and executing a scanning motion on the XZ plane. 
     In this manner it is possible to obtain a film rich in a perpendicular entry component of the sputtering particles and having a desired film thickness distribution, by setting plural conditions in which the target and the film forming area in the lens are mutually opposed and by executing a film formation in a single scan or plural scans under a control of a stay time in such conditions. A similar effect can also be obtained by continuously varying such plural conditions and executing a single scan or plural scans under a control of varying speeds of such two axes. 
     Further, it is more preferable to add a TS control axis capable of a target-substrate distance and to execute the aforementioned scanned film formation under a control to maintain a constant distance between the target and the lens, whereby a control of the film thickness distribution is rendered possible while obtaining a uniform film under a constant film forming condition. 
     (These actions have been explained in the case of a convex lens, but a similar effect can be obtained also in case of a change in the emission angle distribution of the sputtering particles, for example; in a concave lens or when the target is changed to a different material, by optimizing the cathode rotation angle (Tθ), the scan axis angle (Sθ) and the distance between the target and the substrate.) 
     The above-described configuration necessitates somewhat complex motions in a vacuum environment of a film forming chamber in a sputtering apparatus provided with a load lock chamber, but, in the present invention, a highly reliable mechanism with little possibility of leakage can be relatively easily realized by constituting the target rotating axis and the substrate rotating axis by rotary axes utilizing a magnetic seal, also the scan axis by a system of covering a central rotating portion with bellows and the TS axis for varying the target-substrate distance by a moving mechanism utilizing bellows. 
     Therefore, in the prior technologies of a mask moving method or a method of moving the substrate parallel to the X, Y-axes, a film formation on a substrate with surface irregularities results in a formation of a low density film, having a large inclined entry component of the sputtering particles in a peripheral portion of the lens, but the sputtering apparatus of the present invention can obtain a uniform and highly precise film on the entire lens with a desired film thickness distribution, even in the presence of a change in the emission angle distribution by the target material or in the profile (irregularities) of the substrate, by optimizing the cathode rotation angle (Tθ), the scan axis angle (Sθ) and the target-substrate distance condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional elevation view of a sputtering apparatus embodying the present invention; 
         FIG. 2  is a cross-sectional plan view of a sputtering apparatus embodying the present invention; 
         FIG. 3  is a chart showing a film thickness distribution (a convex lens, ø300, radius of curvature 300) in each step of a prior method; 
         FIG. 4  is a chart showing a film thickness distribution (a convex lens, ø300, radius of curvature 300) when a staying time in each step is optimized in a prior method; 
         FIG. 5  is a chart showing a film thickness distribution (a convex lens, ø300, radius of curvature 300) in each step of a method of the present invention; 
         FIG. 6  is a chart showing a film thickness distribution (a convex lens, ø300, radius of curvature 300) when a staying time in each step is optimized in a method of the present invention; 
         FIG. 7  is a table showing optimized staying times; 
         FIG. 8  is a schematic view showing a sputtering apparatus in another embodiment of the present invention; 
         FIG. 9  is a schematic view showing a sputtering apparatus in an embodiment 3 of the present invention; and 
         FIG. 10  is a schematic view showing a sputtering apparatus in an embodiment 4 of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be explained with reference to accompanying drawings. 
       FIG. 1  is a cross-sectional elevation view of a sputtering apparatus embodying the present invention, and  FIG. 2  is a cross-sectional plan view. 
     A sputtering apparatus of the embodiment shown in  FIGS. 1 and 2  is basically constituted of an evacuation system  2  for evacuating a vacuum chamber  1 , a gas supply system  3  for supplying sputtering process gases, and a power supply  4  for supplying plural cathodes with a sputtering electric power. The power supply  4  may be provided for each cathode, or may be of a type in which a single power supply is so switched as to supply a cathode to be used with the electric power. In the vacuum changer, there are provided a cathode unit  5  on which plural targets are mounted, and a scan unit  7  for executing a scan operation by rotationally supporting a substrate  6 . 
     More specifically, in the interior of the vacuum chamber, the cathode unit  5  is fixed by cathode rotating axes  10 ,  11  on upper and lower faces of the chamber  1  across magnetic seals  8 ,  9 , and a cathode drive system  12  is provided under the magnetic seal  9 . On lateral faces of the cathode unit  5  of a polygonal pillar shape, there are provided electrically insulated plural cathodes  13   a ,  13   b ,  13   c ,  13   d  on which targets of different materials  14   a ,  14   b ,  14   c ,  14   d  are mounted. The cathodes are provided with shutters  15   a ,  15   b ,  15   c ,  15   d  which can be opened or closed independently. The rotary axes  10 ,  11  are made hollow, of which interior serves for accommodating a cathode cooling water system, a cable for supplying the sputtering electric power, sputtering gases, a shutter driving air system etc. Such configuration enables a rotational displacement while a sputtering is executed with a desired target. Also by employing a servo motor for the drive motor of the cathode drive system  12 , a film formation is made possible under a highly precise positional control. 
     In the scan unit  7 , a substrate holder  16  for supporting the substrate  6  is fixed at an end of a substrate rotary axis  17 . The rotary axis  17  is constituted of a substrate rotary drive system  19  for rotating the substrate across a magnetic seal  18 , a T-S drive system  21  for varying the target-substrate distance under isolation from the air by T-S bellows  20 , and a scan drive system  23  having a rotary center at a center of Sθ bellows  22  capable of a bending motion under isolation from the air and causing a swinging scan motion of the entire unit parallel to the bottom face of the chamber. Such configuration enables a scanned film formation under a substrate rotation during the sputtering and with a variable target-substrate distance. Also, as in the aforementioned cathode drive system, by employing a servo motor for the drive motor of each control axis, a film formation is made possible under a highly precise positional control. 
     The positional relationship of the units in the chamber, taking the rotary axis of the cathode unit  7  as a Y-axis and axes passing the center of the targets  14  and parallel to the bottom face of the chamber as X and Z axes, is such that the substrate rotary axis  17  has a rotating axis on an X′-axis offset in the direction of the Z-axis from the X-axis, and the scan axis has a rotary center on the X′-axis and executes a scanning operation of the entire scan unit on the X-Z plane. 
       FIG. 8  is a cross-sectional view of a sputtering apparatus in another embodiment of the present invention. 
     A sputtering apparatus of the embodiment shown in  FIG. 8  is basically constituted of an evacuation system  32  for evacuating a vacuum chamber  31 , and a rotating sputtering target  4 . A scan unit  35  for rotationally supporting a substrate  3  for executing a scanning thereof is also provided. Between the substrate and the target, there are provided a movable mask  36  and a fixed mask  37  for changing a film forming area, and a collimator  38  for determining a direction and a rate of sputtering particles. The collimator is rendered rotatable, and a change in the angle thereof allows a film forming rate to vary. 
     The sputtering may be achieved either by an ion beam sputtering utilizing an ion source, or by a magnetron sputtering by supplying an electric power to the target. 
     EXAMPLES 
     Example 1 
     An actual multi-layered film formation by the sputtering apparatus of the present invention is executed in the following procedures:
         setting of scan conditions; and   optimization of staying time or moving speed in the scan conditions;       

     Film formation step: 
     (1) The setting of the scan conditions can be executed either by a method, based on a CAD plan drawing of the present sputtering apparatus, of determining a target angle Tθ, a T-S distance and a scan axis angle Sθ of each step conditions, or by a method of a programmed determination of Tθ and Sθ conditions by an optimizing calculation of maintaining a constant T-S distance and minimizing an angle sin(tθ) 2 +sin(sθ) 2 , 
     between the center of the target and a normal at each crossing point of lines connecting each step on the substrate. In the following, there will be explained a method based on the CAD drawing. 
     At first, on the CAD drawing, a radial direction of a dimension larger by about 20% than the radius of the lens mounted on the substrate holder is divided into 10 to 20 divisions, and such divided points are taken as P 0 , P 1 , P 2 , . . . , P 20  from the lens center. Then, while the T-S distance between the center of the target and P 0 , P 1 , P 2 , . . . , P 20  on the lens surface is kept constant at a distance of 100 mm, each target angle Tθ and each scan axis angle Sθ are determined in such a manner that the normals to the lens surface at P 0 , P 1 , P 2 , . . . , P 20  and the normal at the target center substantially coincide. 
     (2) A simulation for the film thickness distribution is executed with the conditions of the target angle Tθ, the T-S distance and the scan axis angle Sθ for each point determined in (1) to calculate a film thickness distribution of each target material. Then a staying time is optimized by a least square method in such a manner that a sum of the calculated film thickness distribution for each point multiplied by the staying time becomes a desired film thickness distribution. 
     (3) In the film forming step, based on the film forming rate of each target material under the optimized condition, a staying time in each point is inputted into the sputtering apparatus so as to obtain the desired film thickness. Then a lens of an optimized shape is mounted on the substrate holder and is set in the scan unit of the vacuum chamber, through the load lock chamber. 
     After the interior of the vacuum chamber is sufficiently evacuated with the evacuating unit, the film formation is initiated with the target angle Tθ, the T-S distance, the scan axis angle Sθ and the staying time, optimized for each point. 
     After the film formation, the substrate is taken out and is subjected to a film thickness measurement, and in case a desired film thickness distribution is not obtained, a desired film thickness distribution can be easily obtained by optimizing the staying time corresponding to an observed error. 
     Example 2 
     An effect similar to that in the example 1 can also be obtained, in a configuration similar to that of the example 1, under a condition setting by continuously varying the changing speed of the target angle Tθ, the T-S distance and the scan axis angle Sθ based on optimum conditions providing a desired film thickness distribution, obtained by the result of a film thickness simulation for each step. 
     Then, for comparing with the film thickness distribution control on an irregular substrate, a comparative simulation was made on the method of film formation under a movement of the substrate in X or XY direction parallel to the evaporation source (Japanese Patent Application Laid-Open No. H9-213634) and the method of the present invention under the control of the target angle Tθ, the T-S distance and the scan axis angle Sθ. 
     There were assumed conditions of a target of a diameter of 5 inches, a T-S distance of 100, 120 or 150 mm, movements of 16 steps with a pitch of 15 mm in the radial direction from the center of the substrate, a convex-shaped substrate of an external diameter of 300 mm and a radius of curvature of 300 mm in a rotating motion, and a film forming pressure of a discharge maintaining limit of about 0.1 Pa where the influence of scattering becomes low, and the scattering effect was disregarded. Also the emission angle distribution was calculated according to a cosine rule. 
       FIG. 3  shows a film thickness distribution in each step of the prior method, in case the aforementioned convex lens is moved by 16 steps with a pitch of 15 mm in the X-direction parallel to the evaporation source.  FIG. 4  shows film thickness distributions, obtained by optimizing the staying time in such a manner that a sum of a product of a film thickness distribution and a film forming rate multiplied by a staying time in each step becomes a desired value of 100% (an entirely uniform film thickness distribution) or 115% (film thickness linearly increasing to 115% from the lens center to the periphery). 
       FIG. 5  shows a film thickness distribution of each step simulated in the configuration of the present invention, and, though the basic conditions are same as before, the target angle Tθ, the scan axis angle Sθ and the T-S distance were determined from a CAD drawing. A radial direction of the lens was divided into 16 divisions, and such divided points are taken as P 0 , P 1 , P 2 , . . . , P 15  from the lens center. Then, the T-S distance was kept constant at a distance of 100, 120 or 150 mm, and each target angle Tθ and each scan axis angle Sθ were determined in such a manner that the normals to the lens surface at P 0 , P 1 , P 2 , . . . , P 10  and the normal at the target center substantially coincide. 
       FIG. 6  shows film thickness distributions, obtained by optimizing the staying time in such a manner that a sum of a product of a film thickness distribution and a film forming rate multiplied by a staying time in each step in  FIG. 5  becomes a desired value of 100% (an entirely uniform film thickness distribution) or 115% (film thickness linearly increasing to 115% from the lens center to the periphery). 
       FIG. 7  shows a table in which the staying time in each step is represented by a percentage with respect to the entire staying time. As being apparent from  FIGS. 3 ,  5  and  7 , in the prior method, the film in a peripheral portion of the lens becomes thinner because of an increase in an inclined entry component of the sputtering particles into the substrate and an increase in the T-S distance, whereby a staying ratio increases in the film formation of the peripheral portion and the film thickness uniformity is inferior to the present invention because the film formation takes place simultaneously also in a central portion of a smaller area. In the present invention, it is further preferable to provide a hollow cylindrical chimney in front of the target, thereby further removing an inclined entry component and achieving a further improvement in the film thickness distribution and in the uniformity of film quality. 
     Example 3 
       FIG. 9  shows an apparatus of the present embodiment. 
     The sputtering apparatus shown in  FIG. 9  is basically constituted of an evacuation system  32  for evacuating a vacuum chamber  31 , and a rotating sputtering target  34 . A scan unit  35  for rotationally supporting a substrate  33  for executing a scanning thereof is also provided. Between the substrate and the target, there are provided a movable mask  36  and a fixed mask  37  for changing a film forming area, and a collimator  38  for determining a direction and a rate of sputtering particles. The collimator is rendered rotatable, and a change in the angle thereof allows a film forming rate to vary. The sputtering is executed by an ion beam sputtering utilizing an ion source  39 . 
     A film forming procedure with this apparatus is as follows. 
     The substrate  33  is introduced into the vacuum chamber  31 , and waits until a pressure equal to or lower than 10 −4  Pa is reached, then the film formation is initiated by rotating the substrate  33  and setting the ion beam  39  and the target  34  at desired angles. In this operation, the scan unit and the movable mask  36  are moved to control the film thickness distribution on the substrate  33 . Also the film forming rate is controlled by the collimator  38  to improve the uniformity. 
     With this apparatus, a desired film thickness could be obtained with a precision within ±0.2%. 
     Example 4 
       FIG. 10  shows an apparatus of the present embodiment. 
     The sputtering apparatus shown in  FIG. 10  is basically constituted of an evacuation system  32  for evacuating a vacuum chamber  31 , and a rotating sputtering target  34 . A scan unit  35  for rotationally supporting a substrate  33  having a rotational parabolic surface and for executing a scanning thereof is also provided. Between the substrate and the target, there are provided movable masks  36 ,  40  and a fixed mask  37  for changing a film forming area, and a collimator  38  for determining a direction and a rate of sputtering particles. The collimator is rendered rotatable, and a change in the angle thereof allows a film forming rate to vary. The sputtering is executed by an ion beam sputtering utilizing an ion source  39 . 
     A film forming procedure with this apparatus is as follows. 
     The substrate  33  is introduced into the vacuum chamber  31 , and waits until a pressure equal to or lower than 10 −4  Pa is reached. Then the film formation is initiated by rotating the substrate  33  and setting the ion beam  39  and the target  34  at desired angles. In this operation, the scan unit and the movable masks  36 ,  40  are moved to control the film thickness distribution on the substrate  33 . Also the film forming rate is controlled by the collimator  38  to improve the uniformity. 
     With this apparatus, a desired film thickness could be obtained with a precision within ±0.2%. 
     In a film forming process employing, as a substrate, a high-performance optical component having an irregular shape such as a lens, a step-scan operation under a continuous control or a staying-time control with a scan speed control on a partial film forming area, principally subjected to a vertical entry component of sputtering particles obtained by optimizing a target angle Tθ and a scan axis angle Sθ in each step, allows a uniform film of a high film density to be formed on the lens surface, achieving an efficient film formation of an antireflection film and the like with a desired film thickness distribution. 
     Also in a film forming process employing, as a substrate, a high-performance optical component having various shapes such as a lens or a mirror, the addition of a function capable of varying a rate and/or a film forming area in the course of film formation allows an efficient film formation with a desired film thickness distribution to be realized.