Magnetron sputtering device and method for thin film coating

Abstract: A magnetron sputtering apparatus (10) can be used to achieve high quality coating on substrates if the substrates (50) are positioned adjacent to the sputtering target (38, 138). Furthermore, the coating can be enhanced if the substrate is rotated in a planetary manner about the sputtering target. This device can be used in the production of high quality mirrors if the device allows for alternating coatings of different materials.

Description: FIELD OF THE INVENTION The following invention relates to a magnetron sputtering device and a method for thin film coating. A magnetron sputtering device according to the preamble of claim 1 is e.g. known from US-A-4 437 961. 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 substrate surface. BACKGROUND OF THE INVENTION Sputter deposition of materials has been widely used in the past for a number of applications including semiconductors, superconductors, and optical coatings. This sputter deposition has been accomplished through the use of DC diode sputtering, RF diode sputtering, RF magnetron sputtering and other sputtering methods. Typically, sputtering is accomplished by first establishing a gas discharge within a chamber. This gas discharge causes ions to be accelerated towards the cathode and eventually collide with the cathode surface. This collision with the cathode surface causes ion particles to be released from that surface and deposited on other surfaces in close proximity to the cathode. A substrate is typically placed close to the cathode and thus can be coated with the ion particles released from the cathode. Magnetron sputtering is achieved by placing a magnet assembly behind the cathode. This magnet assembly can contain the plasma that is created by the gas discharge. Control of the plasma is very beneficial since it often maximizes the use of the cathode surface. By varying the design and configuration of the magnet assembly, depletion of the target surface can thus be controlled and maximized. In a typical gas discharge a plasma is created directly in front of the cathode. The plasma typically takes on an overall positive charge. A space is created between the plasma and the cathode surface which is free from electric charge. When electrons collide with the cathode surface, occasionally negatively charged ions are released. These negatively charged ions are accelerated across the dark space toward the plasma, due to the charge of the plasma. Subsequently the ions are accelerated through the plasma and collide with any objects that are situated behind this plasma. Due to the high rate of acceleration and large amount of energy, these ions can cause destruction or disturbance to a coated surface when they collide with that surface. Sputter deposition is often used to produce high quality optical mirrors. When sputtering is used for this purpose, alternating layers of thin films are required to be deposited on a substrate. Therefore, it is necessary to sputter a first material to a certain thickness upon a substrate, and subsequently sputter a second material to a desired thickness on the same substrate. This sequence is then repeated for as many times as desired. The need for alternating layers of sputtered material creates a problem in typical sputtering apparatus. The different materials must be sputtered onto the same substrate without unnecessarily handling the substrates themselves. Also, constant handling and changing of the target material is undesirable since that requires opening and closing of the gas filled chamber. One approach has been to have numerous targets within one chamber and position the substrate in front of the appropriate target. While this method does allow for alternating coatings, the problem of direct, high energy collisions still exists. Additionally, the number of mirrors that can be produced concurrently is limited. SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus for the production of high quality optical mirrors. The present invention provides an apparatus that will produce these optical mirrors through sputter deposition whereby the substrates are placed perpendicular and adjacent to the sputtering targets. Placing the substrates in this off-axis position reduces the destructive effects of the high energy ions that are accelerated across the dark space and through the plasma of the gas discharge. The present invention provides a system suitable for mass production of optical mirrors. This system may include apparatus by which two or more different materials may be sputtered onto a substrate in alternating layers. In the present invention a magnetron sputtering device is used to sputter material from a target surface. However the substrates are placed adjacent and perpendicular to the target surface. Although positioning the substrates in this manner causes the deposition rate to be significantly lower a much higher quality thin film is deposited upon the substrate. Higher quality thin films are obtained because the substrate surface is not bombarded with extremely high energy ions which are ejected from the target surface. As previously stated, charged ions which are ejected from the target surface are accelerated across a dark space toward the plasma which is situated directly above the target surface. These ions are then accelerated through the plasma and collide with articles placed directly behind the plasma. Since the substrate is not placed directly behind the plasma, as has traditionally been done, none of these high energy ions contact the substrate surface. The substrates are mounted to substrate handling means which causes the substrates to be rotated in a planetary manner around the targets. This rotation and movement of the substrates promotes uniformity in the coatings and avoids any irregularities caused by the configuration of the discharge. Furthermore, the substrate handling means has facilities to carry a large number of substrates at one time. This allows for mass fabrication of high quality optical mirrors. As stated, to produce optical mirrors requires alternating layers of differing materials. One embodiment of the present invention provides a device whereby substrates may be coated with material from a plurality of different targets without the handling of the substrates and/or the substitution of different target materials. This embodiment has two alternatively energizable cathodes, each having different target materials on the surface thereof. Therefore, by simply adjusting the substrate handling means to be positioned adjacent to the appropriate target surface, and energizing the attached cathode, sputtering from that target surface can be achieved. Repositioning of the substrate handling means and again energizing the appropriate target can accomplish sputtering from a different target surface, thereby allowing alternating layers of material to be sputtered upon the substrates. By placing shields between the different sputtering targets, purity of the target surface is maintained, thereby maintaining purity in the thin films sputtered onto the substrates. The shields also protect other portions of the sputtering device from the high energy ions being ejected in a direction perpendicular to the target surface. 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 magnetron sputtering device; Fig. 2 is an exploded diagram of the magnetron sputtering device showing the separate parts thereof and the relationship between these parts; Fig. 3 is a perspective view of the substrate handling means; Fig. 4 is a top view of the magnet assembly used in the present invention; and Fig. 5 is a prospective view of an operational target showing the magnetic lines created by the magnetic assembly and the depletion of the target surface. DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1, to maintain a gas discharge, gas is pressurized within a pressure chamber 10. In the present embodiment, pressure chamber 10 comprises a cylindrical outer wall 12 with a first cover 14, and a second cover 114. In the present invention, material can be sputtered by two separate sputtering devices, 5 & 6. Operation of a first sputtering device 5 will be described with the understanding that the second sputtering device 6 operates similarly. Specific differences in the two sputtering devices will be further explored later in this detailed description. To apply thin film coatings using magnetron sputter deposition, it is first required to establish a gas discharge within gas chamber 10. This gas discharge is established by applying an electrical potential between a first cathode assembly 19 and an anode 22. In the present embodiment cylindrical outer wall 12 is utilized as an anode 22 by electrically connecting cylindrical outer wall 12 to a positive contact 24 of a radio frequency source 26. A negative contact 28 of radio frequency source 26 is electrically connected to first cathode 20 via a target switch 32. Electrical contact between first cathode assembly 19, which is inside pressure chamber 10, and target switch 32 is made through a conducting post 34 which extends through cover 14. It is important that first cathode assembly be electrically isolated from cover 14, therefore insulators 17 are used to provide this isolation. First cathode assembly 19 has a first cathode 20 with a first target 38 on a surface 36. Surface 36 is positioned to be exposed to the interior of sputtering chamber 10. Through the magnetron sputter deposition process, particles of first target 38 will be deposited upon a plurality of substrates 50. In the present embodiment first target 38 is made of titanium dioxide (TiO2). Placed directly behind first cathode 20 and on the exterior side 15 of cover 14 is a magnet assembly 40. Referring now to Fig. 4, magnet assembly 40 comprises an inner ferromagnetic ring 42, an outer ferromagnetic ring 44 and a plurality of radial magnetic members 46. The polarity of magnetic members 46 causes lines of magnetic force to leave magnetic member 46 from a north pole 48 and return to magnetic member 46 at a south pole 49. With magnetic members 46 configured in a spoke like manner, the lines of magnetic force are also configured in a spoke like manner. Magnetron sputtering devices are widely used for thin film coatings. Typically a gas discharge is established within a gas chamber thus causing positively charged ions to bombard the target surface. The collision of these positively charged ions with the target surface causes particles of the target material to be released from the target material. These particles from the target are then deposited on other structures within the vicinity of the target. Specifically, substrates are placed in close proximity to the target surface thus causing the substrate to be coated with the target material. The magnetic field created by magnet assembly 40 acts to concentrate the majority of target sputtering. Most sputtering will occur in regions similar to the magnetic geometries. With magnet assembly 40 configured as shown in Fig. 4, a "race track" 96 is formed around the periphery of the target. Referring to Fig. 5, a majority of target sputtering will occur in this "race track" region 96. By configuring magnet assembly differently any desired pattern can be achieved where a majority of the sputtering will occur. Referring again to Fig. 1, as the result of the gas discharge a plasma 60 is formed within pressure chamber 10. Plasma 60 is made up of a collection of electrons and ions and generally has a overall positive charge. Plasma 60 is located a slight distance above first target 38. The magnetic forces from magnet assembly 40 causes the plasma to stay within a certain defined region. Between the surface of first target 38 and plasma 60 is a dark space 62. During sputtering negatively charged ions can be released from first target 38. Due to the positively charged plasma 60 positioned a slight distances from the surface of target 38, these negatively charged ions are caused to be accelerated across dark space 62 and through the plasma. These negatively charged ions thus collide with any objects placed directly behind plasma 60. Due to the very large amount of energy the negatively charged ions have after they have been accelerated through plasma 60, they can cause destruction to any surface with which they may collide. In particular, many oxide target compounds form negative oxygen ions during sputtering. In the present embodiment a shield assembly 70 is placed directly behind plasma 60, in parallel with first target 38. This shield is positioned to receive the high energy ions that are accelerated across dark space 62 and through plasma 60. Shield assembly 70 is composed of target material to avoid any substrate contamination during sputtering. As previously mentioned, should the substrates 50 be placed directly behind the plasma the high energy negatively charged ions, released from the target surface, would cause destruction to the uniformity of the thin film surface deposited upon the substrates. When sputtering is used to make mirrors, this destruction can result in mirrors with high scatter and high absorption. In addition to the negatively charged ions a large number of neutral ions are released from the target surface when a gas discharge is established. These neutral ions are deposited on any elements in proximity to the target surface. Typically, the distribution of these neutral ions is in a sine squared pattern. Referring again to Fig. 1, substrates 50 are mounted adjacent to first target 36 on a substrate handling means 80. Substrates 50 are then positioned within this sine squared area while not being directly behind the plasma and not being exposed to the previously mentioned highly destructive ions. As would be expected, the deposition rate at which material is deposited on the substrates 50 is much lower in this position than if the substrates where positioned directly opposite first target 38. However, since substrates 50 are not exposed to high energy ions from first target 38 the uniformity and the quality of the coating on substrates 50 is vastly improved. The already low deposition rate can be changed by altering pressure at which sputtering is done. As previously mentioned, sputtering occurs within pressure chamber 10. The pressure at which sputtering is performed will effect the rate of deposition on substrates 50 because the gas within pressure chamber 10 provides resistance to particles ejected from targets 38. If the pressure within the chamber is reduced, there are fewer gas particles within the chamber, thus less resistance to particles sputtered from target 38. Therefore a lower pressure can help to increase the deposition rate. During operation cathode 20 is typically heated to a very high temperature. To help reduce the temperature of cathode 20 a cooling coil 52 is placed directly behind first cathode 20. This cooling coil has an inlet 54 and a cooling outlet 56 to allow coolant to flow there through. Referring now to Fig. 3, substrate handling means 80 is capable of handling a plurality of substrates and rotating them in a planetary manner to promote uniformity of the coatings thereon. Substrate handling means 80 has a holding ring 82 and a plurality of substrate holders 84. Substrates 50 are mounted on an inwardly facing surface 86 of substrate holders 84. Holding ring 82 is caused to be rotated about a central axis 90. While holding ring 82 is rotated about central axis 90, each substrate holder 84 is rotated about a radial axis 92. This planetary motion of substrates 50 promotes uniformity in the coating thereof. In order to increase the efficiency and quality of sputtering, the substrates could be electrically biased so as to attract particles from first target 38. The bias level should be relatively low however to avoid the generation of high energy collisions between substrates 50 and sputtered particles. As previously mentioned the present invention is capable of sputtering from two different targets. Referring again to Fig. 1, a second cathode assembly 119 is situated close to second cover 114. Again, second cover 114 is insulated from cylindrical wall 12 by an insulator 117. Second cathode assembly 119 is electrically connected to RF source 26 through target switch 32. It will be noted that target switch 32 is a single pole double throw switch, capable of energizing either first cathode assembly 19 or second cathode assembly 119. As with first cathode assembly 19, second cathode assembly 119 has a second cathode 120 with a second target 138 attached thereto. Mounted on the interior surface 136 of second cathode 120 is second target 138. In this embodiment, second target 138 is made of silicon dioxide (SiO2). Located directly behind second cathode 120 is a second cooling coil 152. Also, located behind second cathode 120 and on an exterior surface 115 of second cover 114 is a second magnet assembly 140. Second magnet assembly 140 is configured identical to first magnet assembly 40. It should also be noticed that second target 138 and second cathode 120 are positioned such that second target 138 is parallel to shield assembly 70 as was first target 38 and first cathode 20. It should be understood that first cathode 20 and first target 38 could be constructed as a single piece. Also, first target 38 may simply be a thin layer of material upon the surface 36 of first cathode 20. Alternatively, first cathode 20 and first target 38 could be separate pieces. The same is true for second cathode 120 and second target 138. To sputter material onto substrate 50 from second target 138 substrate handling means 80 must be repositioned to be adjacent to second target 138. This is accomplished by a translating means (not shown) capable of moving substrate handling means 80 from a first position, adjacent to first target 38, to a second position, adjacent to second target 138. To sputter from second target 138 the target switch 32 must be moved to a position that allows second cathode 120 to be energized. Sputtering from second target 138 is identical in operation to sputtering from first target 38. It will be understood that only a single target assembly would be needed if it was desired that only one material be coated upon the substrates. Uniformity of coating will still be enhanced by rotation of substrates 50 about the target as previously described.

Claims: Magnetron sputtering device for use in the deposition of a thin film of material on a substrate, comprising: an air tight sputtering chamber (10) capable of maintaining gas at a pressure required to establish a discharge and having a substantially cylindrical outer wall (12) and a first (14) and a second cover (114) perpendicular to a central axis of said cylindrical outer wall; a cathode assembly (19) disposed within said chamber in close proximity to said first cover (14) of said chamber, said cathode assembly having a target material (38) on a planar target surface of said cathode assembly opposite said first cover; a magnet assembly (40) situated behind said cathode assembly and opposite said target surface, said magnet assembly having lines of magnetic force that leave said target surface and return to said target surface; a radio frequency source (26) of electric potential having a pair of contacts, a first contact electrically connected to said chamber and a second contact electrically connected to said cathode assembly (19); and substrate handling means (80) positioned within said chamber for holding a plurality of substrates (50); characterized in that said substrate handling means (80) is positioned in an orientation such that the substrate surface to be coated is perpendicular and adjacent to said target surface, said substrate handling means causing said substrates to be rotated about a first axis (90) normal to said target surface and also causing said substrates to be rotated about a second axis (92) parallel with said target surface. Magnetron sputtering device according to claim 1, in particular for use in the construction of mirrors by sputtering a combination of alternating layers of material on a substrate, characterized in that : a) said cathode assembly comprises: a1) a first cathode assembly (19) disposed within said chamber (10) in close proximity to said first cover (14), said first cathode assembly having a first target material (38) on a first target surface of said first cathode assembly, said first target surface being opposite said first cover and perpendicular to said central axis; and a2) a second cathode assembly (119) disposed within said chamber in close proximity to said second cover (114) of said chamber (10) having a second target material (138) on a second target surface of said second cathode assembly, said second target surface being opposite said second cover and parallel to said first target surface; b) a sputter shield (70) is disposed between said first cathode assembly (19) and said second cathode assembly (119), said sputter shield having said first target material on a surface facing said first cathode assembly and said second target material on a surface facing said second cathode assembly; c) said magnet assembly comprises: c1) a first magnet assembly (40) situated behind said first cathode assembly (19) and opposite said first target surface, said first magnet assembly having lines of magnetic force that leave said first target surface and return to said first target surface; c2) a second magnet assembly (140) situated behind said second cathode assembly (119) and opposite said second target surface, said second magnet assembly having lines of magnetic force that leave said second target surface and return to said second target surface; and d) said radio frequency source (26) is switchable by having one contact connected to said chamber and a second contact switchably connected to either said first cathode assembly (19) or said second cathode assembly (119); and e) said substrate handling means (80) being arranged for positioning said substrates in one of a plurality of positions, a first position being such that a radial vector normal to said substrate surface perpendicularly intersects said central axis between said first cathode assembly (19) and said sputtering shield (70), and a second position being such that a radial vector normal to said substrate surface perpendicularly intersects said central axis between said second cathode assembly (119) and said sputtering shield (70). Sputtering device according to claim 1 or 2, characterized in that said substrate handling means comprises an annular member (80) having a plurality of substrate holders (86) attached thereto on the inner side of said annular member, said substrate holders capable of being rotated about a radial axis (92) of said annular member. Sputtering device according to claim 3, characterized in that the substrate holders (86) are electrically biased at a predefined bias voltage. Sputtering device according to claim 2, characterized in that said substrate handling means has first rotation means for causing said substrates to be rotated about said central axis and second rotation means for causing said substrates to be rotated about an axis perpendicular to said substrate surface. Sputtering device according to claim 2, characterized in that said first and second target surface is circular. Sputtering device according to claim 6, characterized in that said first magnetic means (40) cause said lines of magnetic force to leave said first target (38) surface from a center portion and extend radially outward until said lines return to said first target surface at an outer portion and wherein said second magnetic means (140) causes said lines of magnetic force to leave said second target (138) surface from a center portion and extend radially outward until said lines return to said second target surface at an outer portion. Sputtering device according to claim 2, characterized in that said sputtering shield (70) has said first target material on a surface parallel to and facing said first target surface, and said sputtering shield has said second target material on a surface parallel to and facing said second target surface. A method of coating a substrate with a plurality of thin films, comprising the steps of: a. positioning a substrate (50) adjacent to a first cathode (19) such that said substrate is perpendicular to a first target surface of said first cathode, said first cathode and said substrate both being located within a gas filled chamber; b. applying an electrical potential between said first cathode and said gas filled chamber creating a gas discharge within said gas filled chamber and thereby causing sputtering of material from said first target surface onto said substrate, and rotating said substrate about an axis which is perpendicular to said first target surface and concurrently rotating said substrate about an axis which is normal to the surface of said substrate; c. repositioning said substrate (50) adjacent to a second cathode (119) such that said substrate is perpendicular to a second target surface of said second cathode, said second cathode and said substrate both being located within a gas filled chamber; d. applying an electrical potential between said second cathode and said gas filled chamber creating a gas discharge within said gas filled chamber and thereby causing sputtering of material from said second target surface onto said substrate, and concurrently rotating said substrate about an axis which is perpendicular to said second target surface and while also rotating said substrate about an axis which is normal to the surface of said substrate.

Inventors: COLE BARRETT E, COLE, BARRETT E.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1993
Main classification: H01J37
IPC codes: C23C14, H01L21, H01J37