Abstract:
A device for depositing a high quality thin film of material upon a surface is disclosed. The device is particularly adaptable to the construction of mirrors since it allows for coating of alternating layers of material. The quality of the film deposited is greatly improved by placing the substrates adjacent to the target surface and not directly in front of it. Furthermore, the substrates are rotated to improve uniformity of the coating.

Description:
FIELD OF THE INVENTION 
     The following invention teaches a method and apparatus for coating substrates with thin films of material. More specifically, the invention discloses a method and apparatus for the fabrication of high quality mirrors by coating a substrate with a plurality of materials which will affect the reflective properties of the surface. 
     BACKGROUND OF THE INVENTION 
     Diode sputtering is a commonly used method of coating objects with a thin film of material. This method of material deposition has been used in the past for a number of applications including semiconductors, superconductors, and optical coating. 
     Generally diode sputtering first requires the creation of a gas discharge within a chamber. Operation of the gas discharge causes an exchange of electrons and ions between a cathode and an anode. Collision of ions with the cathode, or a target placed in front of the cathode causes material to be dislodged from the cathode and thus deposited on other objects within the chamber. Typically, the substrate or object to be coated is placed directly in front of the cathode. Because of its positioning in front of the cathode, material dislodged from the cathode is deposited on the substrate surface at a very high rate. 
     In a typical gas discharge a plasma exists between the cathode and the anode. The plasma is a collection of ions and electrons that have an overall neutral charge. During operation of the discharge ions are accelerated towards the cathode surface. Their collision with the cathode surface causes material to be released from the cathode surface, and, occasionally, a negatively charged ion is released. Due to the negative charge on the ion and the negative charge of the target, the negatively charged ion is accelerated away from the target at a very high rate. These highly accelerated ions pass through the plasma and collide with any objects that are directly in their path. Secondary electrons are also emitted from the surface being sputtered. These electrons are also accelerated across the dark space and can cause heating of objects in their path. 
     In the past the substrate has been placed directly in front of the cathode surface. Consequently, when negatively charged ions, as well as electrons, are released from the cathode surface and pass through the plasma they collide with the substrate surface. Due to the high energy these ions possess, their collision with the substrate surface is often very destructive, resulting in damage and imperfection of the thin film coating. The electrons can cause excessive heating of the substrate and undesirable film growth. 
     Diode sputtering is often used to fabricate high quality mirrors. These high quality mirrors then are used for such applications as lasers and ring laser gyroscopes. As is well known in the art, the mirrors are fabricated by depositing alternating layers of material upon a substrate. High quality mirrors are achieved by having each of these coatings be very uniform and of high quality. 
     Lastly, it is advantageous to produce a large quantity of mirrors at one time. This becomes very complicated since alternating layers of material are required for fabrication of a mirror. Gas purity and cathode material purity are a requirement for quality sputtering, consequently, it is necessary to provide a method to coat the substrate with alternating layers of material without opening and closing the chamber constantly. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a system which will produce high quality mirrors using diode sputtering. It is a further object of the present invention to provide a system that will produce high quality optical mirrors in mass quantities, and also is more adaptable to a production setting. 
     In the present invention, diode sputtering is used in the production of high quality mirrors. During deposition of the necessary thin films the substrates to be coated are placed adjacent to the cathode and not directly in front of the cathode. This positioning avoids direct collisions with the high energy ions that are occasionally released from the cathode. The coating process is, consequently, less destructive and achieves a uniform, higher quality coating. 
     As expected, the deposition rate is greatly reduced by placing the substrate in the off axis position. However, this problem is offset by a higher quality of thin film coating. Furthermore, the longer processing time is also offset by the ability to automate production. Lastly, by appropriately positioning the substrate fairly close to the cathode, a sufficient deposition rate is achieved. 
     To further enhance the quality and uniformity of the thin film coating the substrates are rotated or moved about within the coating chamber. In one embodiment of the present invention these substrates are rotated about an axis which is parallel to the cathode surface. 
     As is well known, the production of mirrors requires alternating layers of materials. A thin film coating of a first material must first be deposited on a substrate and then a thin film coating of a second material is deposited on top of the first material. This process is then repeated a number of times until the optical quality required is achieved. The present invention accommodates this need for deposition of different materials by providing two different cathodes within the sputtering chamber. Deposition of one material can be achieved through energizing the appropriate cathode and shielding the second cathode. Alternatively, deposition of the second material can be achieved by energizing the second cathode and shielding the first cathode. By having both cathodes within the chamber and allowing the capability of sputtering from either cathode at different times provides a system capable of manufacturing mirrors without operator intervention. Furthermore, this system can be automated to provide for mass production capabilities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects and advantages of the present invention may be seen by reading the following detailed description of the invention in conjunction with the following drawings in which: 
     FIG. 1 is a cross-sectional diagram of the diode sputtering device; and 
     FIG. 2 is an exploded view of the diode sputtering device showing the cathodes, the substrate handling means, and the shielding apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the diode sputter deposition process occurs within a sputtering chamber 10. Sputtering chamber 10 is an airtight chamber capable of maintaining gases at a specified pressure. The configuration of sputtering chamber 10 is somewhat irrelevant; however, it must be large enough to accommodate all of the necessary elements. These necessary elements are further outlined within this specification. 
     In the present embodiment sputtering chamber 10 takes on a cylindrical configuration. The chamber has a cylindrical outer wall 12 with an upper cover 14 and a lower cover 16. The means for evacuating the chamber is not shown, but is understood and well established in the art. 
     Within sputtering chamber 10, there are two different cathode assemblies, a first cathode assembly 20 and a second cathode assembly 22. Each of these cathodes operates similarly, therefore, operation will first refer to first cathode assembly 20 with the understanding that second cathode assembly 22 operates similarly. 
     In the present embodiment first cathode assembly 20 is located in an upper portion of sputtering chamber 10. First cathode assembly 20 has a cathode 21 constructed of an electrically conductive material having a first target surface 32 exposed towards the interior of sputtering chamber 10. First target surface 32 is coated with the material that is desired to be sputtered. In the present embodiment this material is silicon dioxide (SiO 2 ). It will be understood, by those skilled in the art, that a separate target (not shown) could be attached to first cathode 21, so as to allow sputtering therefrom. 
     First cathode 21 is electrically connected to a first terminal 34 of a switch 36. Switch 36 is a single pole, double throw switch having its pole 38 connected to a first terminal 40 of an RF source 42. RF source 42 has a second terminal 44 electrically connected to cylindrical outer wall 12 of sputtering chamber 10. 
     As previously mentioned, within sputtering chamber 10 is substrate handling means 24. Substrate handling means 24 is capable of carrying a number of substrates 50 and moving them to promote uniformity of coating. Substrate handling means 24 is positioned to hold substrates 50 adjacent to first target surface 32. 
     In order to deposit a thin film of material on the surface of substrates 50, a gas discharge is established within sputtering chamber 10. This discharge is established by energizing RF source 42 to apply an electrical potential between first cathode 21 and cylindrical outer wall 12. The establishment of this electrical potential causes electrons and ions to move about within sputtering chamber 10. Typically, a plasma 52 is created between the first cathode assembly 20 and the anode. Plasma 52 is a collection of ionic particles having an overall neutral charge. Between plasma 52 and first cathode 21 is a dark space 54. During operation of the discharge, ions are attracted towards target surface 32 of first cathode assembly 20. As these ions collide with first target surface 32, particles of first target surface 32 are released. These particles are then deposited on other objects within the vicinity of first target surface 32. One of the places these particles are deposited is on substrates 50. 
     Occasionally when particles are ejected from first target surface 32 these particles will have a negative charge. Due to the negative charge of these ionic particles and the overall negative charge of the target 21 these particles are accelerated across dark space 54 and into the plasma 52. These ionic particles then either collide with particles in plasma 52 or travel directly through plasma 52 and collide with any objects in their path. It is these particles that can cause destruction and irregularities in thin films. The primary direction that these high energy particles can be transmitted is directly away from first target surface 32. Secondary electrons are also emitted from the surface being sputtered. These electrons are also accelerated across the dark space and can cause heating of objects in their path. 
     Since substrates 50 are positioned adjacent to and not directly in front of first target surface 32, few of the high energy ions which are transmitted through the plasma 52 can collide with the surface of substrates 50. Therefore a coating of high quality material is deposited upon the surface of substrates 50. 
     Second cathode assembly 22, also positioned within sputtering chamber 10, is also capable of establishing a similar discharge which in turn allows for coating substrates 50 with a thin film of a second material. Second cathode assembly 22 also has a second cathode 23 with a second target surface 62 which is coated with a second material. Alternatively, it will be understood that a second target (not shown) could be positioned directly in front of second target surface 62. In the present embodiment the second material is titanium dioxide (TiO 2 ). Second cathode assembly 22 is positioned directly across from first cathode assembly 20. Therefore substrate handling means 24 is also adjacent second cathode assembly 22. Second cathode 23 is electrically connected to RF source 42 through switch 36. A second switch terminal 35 is electrically connected to second cathode 23. 
     When the RF source 42 is electrically connected to second cathode 23 a similar gas discharge is established within sputtering chamber 10. Now particles are ejected from second target surface 62 and deposited upon substrates 50. This deposition occurs in a manner identical to that previously described, thus achieving high quality, uniform thin films. 
     Within sputtering chamber 10 there is also a first shutter plate 26 and a second shutter plate 28. Referring to FIG. 2, these shutters are each disc shaped with a hole 66, 68 therein. Furthermore, each shutter is capable of being rotated about a shutter rotation axis 70. First shutter plate 26 has a first shutter extension 72 extending therefrom. Similarly, second shutter 28 has a second shutter extension 74 extending therefrom. 
     First shutter plate 26 and second shutter plate 28 provide a number of important functions. When first shutter plate 26 is positioned such that hole 66 is positioned directly in front of first cathode assembly 20 this allows for sputtering from first cathode assembly 20. When material is being sputtered from first cathode assembly 20 second shutter plate 28 is positioned such that it is blocking second cathode assembly 22. This protects second cathode assembly 22 from having undesired material deposited upon it. Similarly second shutter plate 28 can be positioned that hole 68 is positioned directly in front of second cathode assembly 22. This allows for material to be sputtered from second cathode assembly 22. As would be expected, when material is sputtered from second cathode assembly 22, first shutter plate 26 is positioned to cover first cathode assembly 20. 
     Prior to coating substrates 50 it is often necessary to &#34;clean&#34; the target surface before deposition occurs. Therefore, when first shutter plate 26 is positioned to allow sputtering from first cathode assembly 20, second shutter plate 28 can be positioned so that second shutter extension 74 protects the substrates 50 from having material deposited thereon. First shutter extension 72 performs a similar function when material is being sputtered from second cathode assembly 22. 
     Substrates 50 are mounted upon substrate handling means 24. Substrate handling means 24 is capable of holding a large number of substrates and is also capable of rotating the substrates about a substrate handling axis 78. This rotation promotes uniformity and quality of the thin film coating. Also once a large number of substrates 50 are mounted upon substrate handling means 24 the substrates do not have to be repositioned or remounted. Furthermore, substrate handling means 24 could be configured so as to cause substrates 50 to be moved in a planetary fashion, further promoting uniformity of coating. 
     As has been demonstrated, the production of mirrors can be achieved by properly energizing the appropriate cathode and properly positioning the shutters so that alternating layers of material can be deposited upon substrates 50. This device is easily adaptable to a production setting and is capable of producing very high quality mirrors. 
     Having illustrated and described the principles of the invention in the preferred embodiment it should be apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the following claims.