Background of the Invention

This invention relates to a sputtering device and a target assembly therefor and, more particularly, to a magnetron sputtering device with a target assembly adapted to be cooled by a cooling liquid such as water. The fabrication of very large scale integrated

(VLSI) circuits on semiconductor substrates or wafers generally involves a large number of processing steps including that of depositing a metallization layer, say, of aluminum. One of the most common methods of depositing thin films of aluminum or other layers has been by sputtering whereby a plasma of an inert gas at relatively low pressure is created m an evacuated chamber m the vicinity of a target cathode and energetic ions are caused to strike the target cathode such that atoms of the target material are ejected. These ejected atoms travel through the sputtering chamber, a portion thereof becoming deposited on the semiconductor substrate. One of the most important aspects of a sputtering process in the fabrication of semiconductor devices is the need to deposit a film uniformly with very narrow tolerances over the entire surface of a semiconductor wafer. As the target is consumed m a sputtering process, not only does the separation change between the sputtering surface of the target and the semiconductor wafer to be sputter-coated, but also the surface profile of the target will change. The difficulty in achieving acceptable uniformity has been

increasing steadily as the size of the wafers used for device fabrication has increased, and the film uniformity tolerances have become more stringent due to the steady shrinkage in device geometries. Eight-inch diameter wafers are now commonly used m device fabrication, and the device geometries have shrunk to the sub icron level. A great deal of effort has been directed to the development of sputtering devices which produce highly uniform films, and sputtering devices using rotating magnets have been developed not only because the magnetic field created thereby near the surface of the target serves to confine and intensify the plasma and therefore to increase the efficiency of the sputtering device but also for improving the uniformity and permitting better target utilization.

Another problem related to a sputtering device has been how to keep the target cool in order to avoid melting of components near by, or other ill effects thereon from heat. It has been known to provide a coolant-passing upper chamber above the target, below which is the sputtering chamber under a near vacuum condition. In this configuration, the target is subjected to a large pressure difference between the sputtering chamber there below and the upper chamber thereabove through which water or a liquid coolant of a different kind is caused to flow. Such a large pressure difference tends to cause the target, as well as its backing plate, if any, to bend or be deformed substantially. This problem is particularly severe if the target is adapted to sputter-coat a semiconductor substrate with a large diameter.

Summary of the Invention

It is therefore an object of this invention to provide a target assembly for a sputtering device capable of efficiently cooling the target.

It is another object of this invention to provide a magnetron sputtering device capable of not only efficiently cooling its target but also reducing the pressure difference between two sides of the target.

A target assembly embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a target plate having a sputtering surface and a back surface to which is attached a cooling plate having grooves formed on one of its surfaces. The target plate may include a backing plate behind a planar target, and the grooves are formed in a pattern including a plurality of mutually parallel passages. The cooling plate may be attached to the back surface of the target plate either through this grooved surface or the opposite surface. If the opposite surface of the cooling plate is attached to the back surface of the target plate, a cover plate is attached to the grooved surface to form passages for a liquid coolant from an inlet to an outlet. In order to be able to effectively and uniformly cool the target, the grooves are formed m a pattern having a plurality of mutually parallel straight passages and configured such that the coolant flow rates through these passages will be approximately equal.

A magnetron sputtering device embodying this invention may be characterized as comprising not only a sputtering chamber of a known kind and a target assembly as described above, but also a pressure- controllable chamber adjacent the sputtering chamber. The target assembly is usually flat and is disposed

between these two chambers, sealing them individually such that the pressure inside the pressure-controllable chamber can be independently adjusted. In order to keep a constant distance between the sputtering surface of the target and the object to be sputtered as the sputtering surface is consumed, the target assembly is movable perpendicularly to the sputtering surface while keeping the sputtering chamber and the pressure- controllable chamber individually sealed.

Brief Description of the Drawings

The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles ol the invention. In the drawings:

Fig. 1 is a schematic sectional side view of a system incorporating a sputtering device according to this invention; Fig. 2 is a sectional view of a portion of the target assembly of Fig. 1;

Fig. 3 is a bottom view of the cooling plate n the target assembly of Fig. 2;

Fig. 4 is a sectional view taken along line 4-4 of the cooling plate of Fig. 3 with the target attached thereto as shown m Fig. 2;

Figs. 5, 6 and 7 are sectional views of portions of other target assemblies according to dificrent embodiments of this invention taken in the direction of the portions of their grooves which are mutually parallel; and

Fig. 8 is a sectional view of a portion of still another target assembly embodying this invention taken

perpendicularly to the direction of the portions of its grooves which are mutually parallel.

Throughout herein, components which are equivalent or similarly structured are indicated by the same numerals even if they may belong to devices according to different embodiments of this invention.

Detailed Description of the Invention

Fig. 1 shows a sputtering device 5 embodying this invention, as comprising a vacuum chamber 10, which can be evacuated by means of a vacuum pumping system 13 of a kind known in the art and may also be referred to as the sputtering chamber. Within the vacuum chamber lυ are a target 40 and a substrate holder 30 which holds thereon a substrate, such as a semiconductor wafer 33, the upper surface of which is intended to be sputter- coated. A vacuum chamber door 15 is used to transfer wafers into and out of the vacuum chamber 10 by means of a transfer arm 18 which is located inside a vacuum lock 17. An arm-driving motor 19 serves to drive the transfer arm 18 for transferring wafers to and from the holder 30 and also into and out of the load lock 17 through a load lock door 16.

The target 40 has its lower surface 45, or its sputtering surface from which material is sputtered, inside the vacuum chamber 10 and is incorporated as d part of a target assembly 20, which is only schematically shown m Fig. 1. During a sputtering process, the target 40 serves as a cathode, as is well known m the art, although no voltage source therefor is shown in Fig. 1.

A pressure-controllable upper chamber 2 ^r > is provided above the vacuum chamber 10, the target assembly 20 being disposed therebetween and serving to

individually seal the vacuum and upper chambers 10 and 25. Numeral 29 symbolically indicates pressure- controlling means which may include a gas pump and a pressure gauge for controlling the pressure inside the upper chamber 25. A rotatable magnet array 50 (of a suitable configuration as described, for example, m U.S. patent 4,995,958, assigned to the assignee herein) is positioned inside this upper chamber 25 closely behind (or above) the target 40 and is adapted to be driven by a magnet-driving motor 60 through its vertically oriented drive shaft 65. For better efficiency, the shape of the magnet array 50 is approximately the same as that of the target 40 such that the magnet array 50, when rotated around the drive shaft 65, will sweep a zone (shown by dashed lines) which will remain close to the back surface of the target 40 and hence to the vacuum chamber 10 where sputtering takes place .

As the sputtering process progresses, the sputtering surface 45 of the target 40 is eroded. As the sputtering surface 45 is consumed, not only does the surface profile change, but the spacing increases between the sputtering surface 45 and the wafer 33, while the distance between the sputtering surface 45 and the magnet array 50 decreases correspondingly. An increase in the spacing between the sputtering surface 45 and the wafer 33 can have dramatic effects on the uniformity of the deposited film, while a decrease in the distance between the sputtering surface 45 and the magnet array 50 has the effect of increasing the intensity of the magnetic field in the vicinity of the sputtering surface 45. An intensified magnetic field implies a changed sputtering rate. In view of the above, the target assembly 20 is supported such that

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the target 40 can be moved selectably downward toward or upward away from the substrate 33, as indicated by double headed arrows 47. For this purpose, shafts 81, which may be threaded, are provided, connected to a driving means such as a motor-driven lead' screw (not shown) and separated from the pressure-controlled interior of the upper chamber 25 through bellows 52 and from the vacuum environment of the vacuum chamber 10 through another bellows 51 supported by an annular insulator 90.

After the vacuum pumping system 13 pumps the vacuum chamber 10 down to a suitably high vacuum, and the pressure inside the upper chamber 25 is accordingly reduced, a small amount of argon or other suitable gas is introduced into the vacuum chamber 10 from a gas supply 14. A high negative voltage is applied to the target 40 which is electrically insulated from the other walls of the vacuum enclosure. The remaining portions of the vacuum envelope are held at ground potential and serve as the anode of the sputtering system. The negative high voltage of the cathode 40 creates a plasma discharge inside the vacuum chamber 10, which is confined to a region near the surface of the target 40 by the magnetic field associated with the magnet array 50. Attractive forces cause positive ions in the plasma to strike the target 40 at a negative voltage with sufficient energy to cause atoms to be ejected from the sputtering surface 45 of the target 40. Some of these ejected atoms land on the substrate 33 and form a film. The above-described sputtering process is well known in the art and, accordingly, will not be described in any greater detail.

As shown more in detail in Fig. 2, the target assembly 20 includes a cooling plate 41 clamped to a

planar back surface (away from the sputtering surface 4b) Of the target 40 and connected to coolant conduits 26 and 27 (the latter being shown in Fig. 1 but not in Fig. 2) . Rubber gaskets or O-rings (not shown) may be in connection with the clamping. The cooling plate 41 may be made of aluminum, stainless steel or any such appropriate material and, as shown m Figs. 3 and 4, is characterized as having a network of grooves 42 formed on its bottom surface. The network of grooves 42 is connected to the two coolant conduits 26 and 27 at diametrically opposite peripheral positions and includes mutually parallel straight passages 42a mutually separated by partitioning walls 43. Each of these straight passages of the grooves 42a is connected at both ends to circumferential passages 42b which are curved along the periphery of the cooling plate 41 such that, when the cooling plate 41 thus structured is clamped onto the planar back surface of the target 40, the coolant conduits 26 and 27 are connected through mutually parallel passages through the grooves 42 formed between the target 40 and the cooling plate 41 and hence a cooling liquid can be caused to flow from one to the other of the conduits 26 and ?7 through these passages while cooling the target 40. The sectional dimensions (widths and depths) of the individual passages of the grooves 42 are determined such that hydrodynamic impedance w ll be about the same between the two coolant conduits 2t> and 27 independently of whichever of the parallel grooves 42 the liquid coolant may flow through. Those of the parallel straight passages 42a away from the one at the center are shorter but the coolant has to travel a longer distance between the conduits 26 and ? ~ although the circumferential passages 42b may be made deeper

than the straight passages 42a so as to have a smaller impedance per distance and hence the widths of the parallel straight passage 42a need not necessarily be sequentially varied for this purpose. Cross channels 44 connecting mutually adjacent ones of the straight passages 42a may be provided, as a precautionary measure, for allowing the cooling liquid to travel through alternate routes m the case where any of the straight passages 42a becomes clogged. The invention has been described above by way of only one example, but this example is not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of the invention. For example, the mechanism for allowing the target assembly 20 to move vertically (that is, perpendicularly to the sputtering surface 45), including the shafts 81 and the bellows 51 and 52 as shown in Fig. 1, is not an essential element of this invention. Although Figs. 2 and 4 showed an embodiment wherein the grooved surface of the cooling plate 41 was directly attached to the back surface of the target 40, a backing plate 48 (say, of copper) may be inserted therebetween as shown m Fig. 5. Fig. 6 shows another target assembly characterized as having a cooling plate 41 formed with grooves 42 on its top surface, that is, the surface away from the target 40 to which its bottom surface is intimately contacted, and a cover plate 49 attached to this grooved top surface of the cooling plate 41. The cover sheet 49 may be of any material having no unwanted effect due to its contact with the cooling plate 41. The cover sheet 49 and the cooling plate 41 may be brazed together, screwed together or attached together by any other appropriate means. Fig. 7 shows still another target assembly characterized as

having grooves 42, of a kind and according to a pattern described above, formed on the upper surface (opposite the sputtering surface 45) of the target 40 with a backing plate 48 attached to this upper surface to cover the grooves 42 and to thereby form mutually independent passages for a liquid coolant, the backing plate 48 being provided with openings through which the conduits (only 27 being shown in Fig. 7) for the liquid coolant are connected to the grooves 42. Although planar targets 40 with a flat sputtering surface 45 have been described above, this is not intended to limit the scope of the invention. Fig. 8 shows still another target assembly embodying t lie invention, characterized as having a concave target 40' with a concave sputtering surface 45' One of the advantages of a concave target is its ability tc focus the sputtered atoms towards the wafer 33, and another advantage is its structural strength which becomes a significant advantage especially for a device adapted to handle large wafers.

In summary, target assemblies according to this invention may be characterized not only as having on one side a generally planar target with a sputtering surface but also as containing passages for a liquid coolant serving as heat exchange means for cooling the target. Grooves for providing such passages for a liquid coolant may be formed either on the surface of the target opposite the sputtering surface or on a cooling plate on the other side of the target and may be on the side facing the target or on the opposite side with a cover plate covering the grooves. In order to uniformly and efficiently cool the target sucn that a generally uniform temperature can be maintained across the back surface of the target, the grooves are

formed according to a pattern including a plurality of mutually parallel, preferably straight passages.

Sputtering devices according to this invention may likewise be characterized as having not only a sputtering chamber which can be evacuated to create a low pressure environment adapted for a sputtering process therein but also an upper chamber, the pressure inside which can be controlled, and a target assembly, as described above, sealing the opening which connects these two chambers such that the pressure difference on both sides of the target assembly can be significantly reduced.

The disclosure above is intended to be broadly construed, and it must be remembered that the drawings, which are intended to be schematic, do not fastidiously show every detail of the illustrated examples such as insulators between a target assembly adapted to have a high negative voltage applied thereon and components adapted to serve as anode, as described above. Planar targets are intended to include concave targets and mutually parallel portions of the grooves need not be straight but may be curved. All modifications and variations of the examples illustrated above, that may be apparent to a person skilled in the art, are intended to be within the scope of this invention.