Abstract:
The invention discloses a method of electroplating a material onto a semiconductor substrate. A substrate is placed in a cylindrical processing chamber enclosure. A nozzle for spraying a liquid electroplating solution opposes the top surface of the substrate. The electroplating solution flows through the nozzle and outward angularly from the tip of the nozzle, so that the solution flows rotationally on the surface of the substrate.

Description:
FIELD OF THE INVENTION 
     This invention is in the field of semiconductor processing. More specifically, the invention discloses a method for electroplating a material onto a semiconductor substrate. 
     BACKGROUND OF THE INVENTION 
     A step in the fabrication of semiconductor integrated circuits is to apply a thin film of material onto the surface of a semiconductor substrate. The thin film is then patterned to form openings within the film. A second thin film of material is applied to the patterned thin film such that the second thin film fills the openings of the pattern in the first film. The second thin film is then patterned, and a third thin film is applied to the second thin film and is patterned, and the sequence is repeated until the desired integrated circuit structure is created. 
     The sequence for building an integrated circuit begins with a transistor structure formed on the semiconductor substrate. Alternating layers of electrically conducting and insulating thin film materials are formed over the transistor structure, and the electrically conducting film layers are interconnected to one another to form electrically conducting pathways throughout the integrated circuit. The conducting film material is commonly aluminum or an alloy of aluminum. Vapor deposition is the preferred method for applying the conducting film to the surface of the substrate. 
     As technology advances toward faster speeds for integrated circuits, the widths of individual lines of the circuitry decrease in size. Although vapor deposition continues to be widely used for depositing films, new methods such as electroplating are being developed for depositing conductor films within tight spacings in a patterned film layer. Additionally, it becomes necessary to use conductors with reduced resistance such as copper due to speed limitations posed by aluminum and alloys of aluminum. 
     A viable technique for forming a copper thin film layer on a patterned substrate surface is electroplating. A patterned semiconductor substrate is prepared for electroplating. The patterned film on the semiconductor substrate may be an insulating film such as silicon dioxide. The patterned silicon dioxide contains openings to the underlying conductor film material. The underlying conductor film material may be copper or it may be another conductor material. To prepare for electroplating, a seed layer of material may be formed on the underlying conductor film material using vapor deposition. 
     Once prepared with the necessary seed layer, if any, the patterned semiconductor substrate is placed face down at the top end of an electroplating cup. A cathode contact is created on the edge of the substrate by coupling the substrate to the negative terminal of a power source. There is an anode at the bottom of the electroplating cup. The anode is coupled to the positive terminal of a power source. The substrate is clamped against an O-ring to form a watertight seal around the substrate perimeter. An inlet pipe is inserted into the cup through the bottom of the cup, so that a nozzle at the end of the pipe is inside the cup and faces the substrate surface. A liquid electroplating solution flows through the inlet pipe and out of the nozzle, spraying a liquid jet of fluid directed perpendicularly toward the substrate surface. The electroplating solution contacts the substrate surface, the power supply is turned on, and a circuit is formed between the anode and cathode through the electroplating solution. The desired material is electroplated onto the surface of the substrate. 
     One aspect of making electroplating a viable process for semiconductor fabrication is to form a uniformly deposited electroplated film layer. Utilizing the standard technique described above, film thickness uniformities of 6% can be achieved. However, for achieving desired process yields an even better uniformity is needed. Moreover, as the substrate size increases from 200 millimeters in diameter to 300 millimeters and beyond, it will be more difficult to attain electroplated film thickness uniformity using the currently known electroplating techniques. There are several factors causing non-uniformity in electroplating. One of the factors is lack of continuity of the cathode contact. This can be corrected by utilizing a cathodic contact ring at the edge of the substrate to form a continuous cathode contact. Another factor causing non-uniform electroplated film thickness is an accumulation of electrolytic solution on surface points on the substrate due to the perpendicular transport of liquid to the substrate surface. 
     A way of improving electroplated film thickness uniformity was identified in U.S. Pat. No. 4,304,641 “Rotary Electroplating Cell with Controlled Current Distribution”. There the method was to use a flow-through jet plate having nozzles of increasing size and uniformly spaced radially therethrough or the same sized nozzles with varying radial spacing to provide a differential flow distribution of the plating solution. Additionally, the patent disclosed the technique of rotating the substrate by connecting the cathode to a spindle, which in turn is rotated by a motor. Alternatively, the cathode and anode can be rotated at the same time. Rotating the substrate relative to the anode helps to create a more uniform distribution of electroplating solution over the surface of the substrate by preventing accumulation on contact from the liquid spray. 
     One problem with using a jet plate is that a jet plate must be specially fabricated with exact hole distribution and dimensions. This can significantly add to the cost of fabricating the electroplating equipment. A problem with rotating the cathode and possibly the anode, of course, is the increased complexity of the equipment. Whenever there are moving parts in equipment, equipment maintenance becomes more complex and chances of mechanical failure are greater. Using a motor in the equipment drives the cost of the equipment up. Higher costs are desirably avoided because of the generally increasing costs of producing integrated circuits while prices of manufactured integrated circuit parts continue to decrease. 
     It is therefore advantageous to use an electroplating technique that can improve the film thickness uniformity while at the same time avoiding increased cost and complexity to the electroplating equipment. 
     SUMMARY OF THE INVENTION 
     The present invention discloses a method of applying a liquid material onto a substrate surface. The liquid material is applied by placing the substrate surface within an enclosure, and introducing the liquid material into the enclosure. The liquid material is directed angularly toward the substrate surface so that the liquid material flows rotationally upon contact with the substrate surface. 
     In conjunction with the method, there is also described an apparatus for coating a substrate with a liquid material. There is a chamber having cylindrical interior walls, where the chamber has a first end and an opposing second end. An opening in the first end holds the substrate. An inlet pipe having an end that is directed within the chamber is coupled to the second end of the chamber. A nozzle is coupled to the end of the inlet pipe, through which the liquid material is sprayed generally toward the substrate surface, wherein the liquid material flows rotationally upon contact with the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings are provided to enable the reader to help understand the workings of the invention through example only, and are not set forth as visual limitations of the present invention. The drawings are described briefly as: 
         FIG. 1A  is a cross-sectional view of a substrate held in an enclosure with a liquid spraying onto the substrate. 
         FIG. 1B  is a top view of the substrate surface, representing lines of liquid flow as the liquid contacts the substrate surface. 
         FIG. 1C  is a top view of the nozzle through which liquid spray emanates. 
         FIG. 1D  is a side view of a single liquid spray outlet to demonstrate an angle θ by which the liquid may be sprayed. 
         FIG. 1E  is a side view of a single liquid spray outlet to demonstrate an alternative way of achieving the angle θ shown in  FIG. 1D . 
         FIG. 1F  is a side view of a single liquid spray outlet to demonstrate an alternative angle φ by which the liquid may be sprayed. 
         FIG. 2  is a cross-sectional view of an application of the present invention in an electrochemical cell. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a technique for coating a substrate with a liquid material in such a way that the liquid material forms a coating with acceptable thickness uniformity across the substrate while avoiding the use of rotary or other mechanical motion to create a rotational liquid material flow. The invention is useful for electroplating or for other manufacturing processes where a liquid is applied to a substrate surface. The inventive method includes directing the liquid material angularly toward the substrate surface. The invention will be described in more detail below. To facilitate a description, the context for description is set forth in the field of semiconductor manufacturing, more specifically an electrochemical cell for electroplating metal, in particular, copper, on a semiconductor substrate. Any reference to “approximate” dimensions should be construed as the designated dimension plus or minus variation within tolerances that would be reasonable according to the judgment of a person of ordinary skill in the art, in the context of using a given embodiment of the invention. 
     Referring to  FIG. 1A , there is provided a substrate  10  within an enclosure  20 . Enclosure  20  contains cylindrical walls  30 . Substrate  10  is a semiconductor wafer with a surface  40  containing a pattern (pattern not shown). Substrate  10  is secured to the top end  50  of enclosure  20 . Substrate surface  40  faces a nozzle  60 . Nozzle  60  directs liquid solution  70  out of nozzle  60  toward surface  40 . Nozzle  60  contains four spray outlets  90 ,  92 ,  100 ,  102  which direct liquid  70  at an angle from perpendicular. The resulting flow of liquid  70  on surface  40  is a rotational pattern. 
       FIG. 1B  sketches the theoretical shape of liquid  70  as it flows on surface  40 . Flow lines are shown to extend in a curve and radially outward toward the perimeter  110  of surface  40 . 
       FIG. 1C  is a top view of nozzle  60  containing four spray outlets  90 ,  92 ,  100 ,  102  where each pair of spray outlets  90 ,  92 ,  100 ,  102  are at opposite sides of a cross shape. Each spray outlet  90 ,  92 ,  100 ,  102  sprays liquid out of nozzle  60  in the general direction of substrate  10  ( FIG. 1A ). 
       FIG. 1D  is a side view of nozzle  60  with spray outlet  100  emanating therefrom. Spray outlet  100  is preferably a cylindrical pipe having an elbow joint  125  and a shoulder joint  145 . Spray outlet  100  is angled away from vertical by angle θ  115 . Angle θ is preferably in a range of 20 to 60 degrees from vertical. Nozzle  60  is held in the vertical direction. Angle θ  115  can be formed by bending elbow joint  125  outward radially away from nozzle  60 . Spray outlet  100  is attached to nozzle  60  at the shoulder joint  145 . Shoulder joint  145  is held at an approximately 90-degree angle with respect to the vertically-shown nozzle  60 . 
     Alternatively,  FIG. 1E  is a side view of nozzle  60  containing one spray outlet  100 . Spray outlet  100  is angled outward from nozzle  60  in a similar radial angular direction θ  115  as in  FIG. 1D . Although the angle shown herein is about 45 degrees, the angle is preferably in a range of approximately 20 to 60 degrees from vertical. Nozzle  60  is held in the vertical direction. Spray outlet  100  is preferably a cylindrical pipe having an elbow joint  120  and a shoulder joint  140 . Elbow joint  120  is shown here to be angled at approximately 90 degrees. Instead of bending spray outlet  100  outward by angling the elbow joint as in  FIG. 1D , angle θ 115 is formed by angling shoulder joint  140  relative to nozzle  60 . Liquid flow  70  emanating from elbow joint  125 ,  120  in either  FIGS. 1D  or  1 E is directed radially outward from nozzle  60 , so that liquid contacts cylindrical walls  30  of enclosure  20  ( FIG. 1A ). When in contact with cylindrical walls  30 , liquid flow  70  rotates according to the shape of cylindrical walls  30 . 
     Still alternatively, as shown in  FIG. 1F , spray outlet  150  is shown to be angled at approximately 45 degrees from vertical, in an angular direction φ160. Preferably, angular direction φ is directed such that liquid spray emanates from spray outlet  150  at approximately 20 to 60 degrees from vertical. However, unlike  FIGS. 1D and 1E , angular direction φ is formed by twisting spray outlet  150  at shoulder joint  170 . Elbow joint  165  and shoulder joint are both at approximately 90 degrees. Spray outlet  150  is therefore angled sideways if one is facing nozzle  60  and spray outlet  150 . Liquid spray emanating from spray outlet  150  would thus be directed circumferentially toward the substrate surface. 
     Referring to  FIG. 2 , an application for the present invention is shown. An electrochemical cell  200  for electroplating a copper film on a semiconductor substrate  230  is provided in cross-sectional view. Electrochemical cell  200  includes a cup  210  inside which the electrochemical reactions occur. An opening  220  in the top end of cup  210  holds semiconductor substrate  230  face down on the rim  240  of cup  210 . The surface of substrate  230  to be plated faces the interior of cup  210 . An O-ring  250  provides a seal to prevent liquid from leaking out from the perimeter of substrate  230 . Substrate  230  is in contact with a cathode  260 , which in the present embodiment is preferably a continuous ring cathode. Cathode  260  is secured within rim  240  of cup  210  to prevent cathode  260  from moving about. Centering ring  270  positions substrate  230  in place. Exit slots  275  allow liquid solution to exit cup  210 . Cylindrical walls  280  are electrically non-conductive. The anode  290  is coupled to positive terminal of power supply (not shown). Cathode  260  is coupled to the negative terminal of the power supply. 
     A pipe  300  is introduced through the bottom end of cup  210  through a hole  310 . A gasket  320  forms a plug to prevent liquid from leaking down through the bottom of cup  210 . Pipe  300  is capped on the pipe top end  330 . Four spray outlets  340 ,  342 ,  344 ,  346  and a fifth spray outlet  348  extend outward from top end  330  to form a nozzle. Four spray outlets  340 ,  342 ,  344 ,  346  form elbow joints  350 , and are twisted at their shoulder joints  360  to form a sideways angle φ365. Angle φ365 is preferably approximately 20 to 60 degrees from vertical. Fifth spray outlet  348  is shown as a straight extension outward from the center of pipe top end  330  so that liquid spray emanating therefrom is directed at an angle perpendicular to substrate surface  230  being held directly above. The purpose of fifth spray outlet  348  is to prevent a void from forming in the vortex of the electroplating liquid as it sprays against substrate  230  in a rotational flow manner. Spray outlets  340 ,  342 ,  344 ,  346  direct liquid at a sideways angle with respect to pipe  300 . Alternatively, and not shown in  FIG. 2 , spray outlets  340 ,  342 ,  344 ,  346  may be angled radially outward from pipe  300 . Liquid spray emanating from four spray outlets  340 ,  342 ,  344 ,  346 , respectively, contacts substrate surface  230  at an angle to create a rotational liquid flow effect on substrate surface  230 . 
     The electrochemical cell described in reference to  FIG. 2  can be used to form a copper film on a silicon wafer. An electrical current applied to cathode  260  creates a current density of approximately 0.5 to 5 amps per square decimeter on substrate  230 . Substrate  230  in this example is a silicon wafer of a diameter of 200 millimeters. Dimensions within the electrochemical cell depend on the shape and size of the substrate being treated. Substrate  230  is held at a distance of approximately 3 to 5 centimeters from the tips of spray outlets  340 ,  342 ,  344 ,  346 . Cup  210  is approximately 190 millimeters in inner diameter and approximately 10 centimeters in depth. Inlet pipe  300  extends inward approximately 2 to 4 centimeters from the bottom of cup  210  interior. Liquid solution consisting of commercially available acid-copper plating bath is introduced to cup  210  at a flow rate of approximately 10 to 40 liters per minute. The liquid solution is directed at an angle φ365 of approximately 20 to 60 degrees away from vertical, to cause liquid to contact substrate  230  in a rotational direction. A copper film forms on substrate  230 . 
     The embodiments of the present invention demonstrate the advantages of the invention in its simplicity for achieving an acceptably uniform coating on the surface of a substrate. Moving parts are not needed for directing the liquid spray, nor are there complex hole patterns in the anode. Note that the invention is not at all limited to electroplating, to a semiconductor substrate, or for that matter, to semiconductor processes. A person of ordinary skill in the art can experiment with angles, the number of spray outlets and other factors such as liquid flow rate and distance from the spray outlet to the substrate surface, to achieve the desired uniformity of thickness of material coated on the substrate. The nozzle may be different from that described here. For example, there could be a pipe inlet into the cup, where the pipe has a ribbon cutout instead of spray outlets. Nothing herein should be interpreted to be reducing the spirit or scope of the invention. To the extent details are described, such details are provided for facilitation of understanding the invention, the invention being limited only by the claims below.