Patent Publication Number: US-2016237584-A1

Title: Electroplating with reduced air bubble defects

Description:
BACKGROUND OF THE INVENTION 
     Manufacture of semiconductor integrated circuits and other micro-scale devices typically requires formation of multiple metal layers on a wafer or other substrate. By electroplating metals layers in combination with other steps, patterned metal layers forming the micro-scale devices are created. Electroplating is performed in an electroplating processor with the substrate in a bath of liquid electrolyte, and with electrical contacts of a contact ring touching a conductive layer on the substrate surface. Electrical current is passed through the electrolyte and the conductive layer. Metal ions in the electrolyte plate out onto the substrate, creating a metal film on the substrate. In some processors, the contact ring has a seal to keep the electrolyte away from the electrical contacts, to avoid build-up of plated metal on the contacts. 
     To avoid plating defects, it is important that the entire useable surface of the substrate be fully wetted by the electrolyte. Gas bubbles (typically air) on the substrate interfere with wetting the substrate surface. Various techniques have been proposed for avoiding gas bubbles, including controlled substrate entry trajectories into the electrolyte, rotating the substrate during entry into the electrolyte, and use of sonics. However, avoiding gas bubbles remains as a difficult engineering challenge in processing with a contact ring having a seal. 
     SUMMARY OF THE INVENTION 
     In a first aspect, a method for processing a wafer includes holding the wafer in a contact ring having a seal and applying a bead of liquid directly from a liquid outlet onto the seal, with the bead of liquid contacting the wafer and the seal. The wafer is then moved into contact with an electrolyte and electroplated. The bead of liquid reduces defects caused by air bubbles on the wafer surface. The wafer may optionally be rotated while applying the bead of liquid. Generally, the liquid forming the bead is applied only directly from a supply tube to the contact circumference and without the liquid forming the bead contacting other surfaces of the wafer. 
     In a second aspect, a processor includes a vessel for holding an electrolyte and a rotor rotatably supported on a head, with a rotor motor in the head for rotating the rotor. A contact ring on the rotor has electrical contact fingers for making electrical contact with a conductive layer of a wafer held in the rotor. A seal on the contact ring to seals against the wafer. A liquid bead supply tube has an outlet for applying a bead of liquid onto the seal and the wafer. 
     In another aspect, a method for processing a wafer includes tilting the wafer to an angle of 1 to 5 degrees relative to a horizontal surface of electrolyte in a vessel, and then moving the wafer at from 125 to 300 mm/sec into the electrolyte. The wafer is then decelerated to a stop within 0.05 to 1.0 seconds, with a leading edge of the wafer below a meniscus level of the electrolyte sufficient to allow full wetting of an entire wafer down-facing surface. The wafer is returned to the flat horizontal position and then moved further down to a processing position in the electrolyte. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of an electroplating processor in a head-down processing position. 
         FIG. 2  is a schematic drawing of the electroplating processor of  FIG. 1  in a head-up load/unload position. 
         FIG. 3  is an enlarged perspective view of the contact ring as shown in  FIG. 2 . 
         FIG. 4  is a still further enlarged section view of the contact ring shown in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     Turning now in detail to the drawings, as shown in  FIG. 1 , an electroplating processor  20  has a rotor  24  in a head  22 . The head is supported on a lift/rotate mechanism  25 , that can rotate the head  22  into a head-up position for loading and unloading a wafer, and to a head-down position for processing. The lift/rotate mechanism  25  can also lift and lower the head towards and away from a vessel  38  of the processor  20  holding a bath of electrolyte  48 . The rotor  24  includes a backing plate  26  and a contact ring  30  having a seal ring  50 . Contact ring actuators  34  move the contact ring  30  vertically (in the direction Tin  FIG. 1 ), to engage the contact ring  30  and the seal ring  50  onto the down facing surface of a wafer or substrate  100 . A bellows  32  may be used to seal internal components of the head. 
     The contact ring  30  typically has metal fingers  35  that contact a conductive seed layer on the wafer  100 . The head  22  is positioned to place the substrate  50  into a bath of liquid electrolyte  48  held in the vessel  38  in a base  36 , as shown in  FIG. 1 . One or more electrodes are in contact with the liquid electrolyte  48 .  FIG. 1  shows a design having a center electrode  40  surrounded by a single outer electrode  42 , although multiple concentric outer electrodes may be used. An electric field shaping unit  44  made of a di-electric material may be positioned in the vessel between the electrodes and the wafer. 
     A membrane  46  may optionally be included, with anolyte in a lower chamber below the membrane and with catholyte in an upper chamber above the membrane  46 . Electric current passes from the electrodes through the electrolyte  48  to a conductive surface on the wafer. A motor  28  in the head may be used to rotate the wafer during electroplating. 
     Turning to  FIGS. 3-4 , the seal ring  50  typically has an elastomer tip  52  which contacts and forms a seal against the wafer  100 , with the tip  52  supported on, or part of, a rim  86  having a beam-like or cantilever structure. The annular surface of the wafer  100  that the seal ring  50  seals against is referred to as a seal contact circumference. The contact fingers  35 , which are typically flexible metal elements, touch the wafer to the outside of the seal, so that they are not exposed to the electrolyte  48 . 
     In use, the wafer  100  is clamped into the rotor  24 , between the backing plate  26  and the contact ring  30 , as shown in  FIGS. 2-4 . The head  22  is then inverted and moved down with the wafer  100  lowered into contact with the electrolyte  48 . As this occurs, bubbles can be trapped along the interface between the contact ring  30  and wafer  100 . The bubbles can migrate inwardly into the usable region of the wafer resulting in voids during the subsequent electroplating steps. 
     To reduce or prevent formation of bubbles, a small bead of liquid  64  may be applied along the circumferential interface between the seal ring  50  and the wafer  100 , before the wafer  100  is moved into contact with the electrolyte  48 . As shown in  FIGS. 2-4 , the bead of liquid  64  may be applied with the head  22  and the wafer  100  in the face up position, via a liquid supply tube  60  on a moving arm  62 , such as a swing arm  62  supported on the processor  20 . The moving arm  62  may retract after applying the bead of liquid  64 , and before the head  22  is rotated into the head down position shown in  FIG. 1 . Using this technique, subsequent plating steps may be performed with bubble related defects on the wafer surface reduced or eliminated. 
     Alternatively, the moving arm  62  may be omitted, and the supply tube  60  is fixed in place on the processor. In this case, the lift/rotate mechanism  25  performs movements to properly position the contact ring  30  relative to the supply tube, to allow formation of the bead of liquid, and performs a flip or rotate movement from the head-up position of  FIG. 2  into the head-down position of  FIG. 1 , while also clearing the supply tube. 
     Water, or in some cases electrolyte  48 , may be used to form the bead of liquid  64 . The bead of liquid  64  may be formed by slowly rotating the rotor  24  with the head  22  in the head-up position shown in  FIG. 2 . The bead of liquid  64  may have height generally matching the height of the vertical tip  52  of the seal ring, generally 1 to 3 mm. In most applications, the bead of liquid  64  has a largest characteristic dimension (i.e., dimension in any direction) of 1-5 mm. 
     The outlet or nozzle of the supply tube  60  may be positioned close to the seal ring  50 , e.g., within 0.5 to 3 or 4 mm. The bead of liquid  64  is formed via direct application of the liquid in a laminar flow, without spraying or forming discrete droplets. The supply tube  60  may have an inside diameter or a round nozzle opening of 0.5 to 2.0 mm. As shown in  FIG. 4 , the supply tube  60  may be oriented at an acute angle to the wafer surface, and with the supply tube angled outwardly away from a center of the wafer. 
     When the head  22  is moved into the head-down position shown in  FIG. 1 , the bead of liquid  64  remains in place as the surface tension forces are greater than the gravitational forces acting on the bead of liquid. This method may be used with any plating application where the wafer  100  is not pre-wetted before plating. It is particularly useful for copper interconnect applications to eliminate bubbles on the leading edge of the wafer as the wafer is moved into the electrolyte  48 . The dimensions of the liquid bead  64  may be controlled by the seal geometry near the wafer interface, the wetting angle of the seal material, and the surface tension of the applied liquid forming the bead 
     The processor  20  may be modified to allow formation of the bead of liquid with the head in the head-down position shown in  FIG. 1 . In this case the supply tube  60  applies the bead of liquid from below the wafer  100 , rather than from above the wafer as shown in  FIGS. 2-4 . Hence, the bead of liquid may be used in processors where the head  22  or rotor  24  holding the wafer  100  does not rotate between head-up and head-down positions. 
     The bead of liquid  64  may be precisely placed along the seal interface with the wafer such that the corner or gap between the wafer  100  and the seal tip  52  is filled in. The bead of liquid  64  forces air out of the region between the seal tip  52  and the wafer surface, before the wafer  100  enters into the electrolyte  48 . The bead of liquid  64  creates a hydraulic interface between the seal ring  50 , or the seal tip  52  and the wafer  100 . The bead of liquid  64  may also prevent unwanted de-ionized water or other liquid from filling the features across the entire wafer  100 . 
     The bead of liquid  64  may advantageously be located outside of the useable die area of the wafer  100 , or at least to a minimal distance such that exchange of the bead liquid  64  and the electrolyte  48  is minimized. Inward intrusion of the bead of liquid  64  onto the wafer may be minimal, as the bead  64  requires only a small volume of liquid and is confined and retained near the seal ring  50  via rotational motion. For example, a bead of liquid on a 300 mm diameter wafer  100  may be formed with only 2-8 or 3-5 ml of liquid. 
     The entry motion profile (i.e. speed, acceleration, angle, timing) of the wafer into the electrolyte  48  may be designed to minimize the amount of bubbles that are retained beneath the wafer  100  during entry. Computer simulations and high speed camera video of the wafer  100  entering into the electrolyte  48  (referred to as wafer entry) show air is trapped at the interface between the seal ring tip  52  and the wafer  100  during the initial phase of entry. If the trapped bubbles are not removed immediately, then as the wafer vertical entry motion stops, the bubbles may migrate upward along the tilted wafer surface towards the center of the wafer. Subsequent wafer motions (tilting flat and moving to the process position shown in  FIG. 1 ) can further spread out the trapped bubbles. 
     Increasing the wafer entry speed may assist in quickly remove trapped bubbles at the interface where the seal ring tip  52  contacts the surface of the wafer  100 . An entry speed of 177 mm/sec is generally sufficient to remove the initial trapped air. However, this entry speed, along with a deep plunge of the wafer  100  into the electrolyte  48  can cause significant electrolyte  48  displacement or splashing. The displaced electrolyte  48  may subsequently fall back into the vessel  38 , entraining bubbles under the wafer  100 . 
     This potential drawback may be reduced via a more shallow plunge, which reduces the amount of electrolyte  48  displacement. On the other hand, generally wafer entry is performed with the wafer at a slight tilt (e.g., 3° to the horizontal electrolyte surface). Using a wafer entry profile with a shallow plunge exposes the trailing edge of the wafer  100  (the high edge due to tilt) to de-wetting because it momentarily stays above the current meniscus level. The potential for de-wetting may be reduced by using a fast, shallow initial plunge to lessen the splash, but then quickly followed by a tilt to flat and a downward move to protect the trailing edge from de-wetting. 
     In summary, bubble defects may be reduced using a wafer entry motion as follows: 
     1. A fast initial wafer entry to shed the bubbles trapped at the seal/wafer interface at first entry. The fast entry may range from 125 or 177 mm/sec to 300 mm/sec. The wafer is tilted at an angle of 2-4°, and may optionally be rotated at 1-30 rpm. 
     2. A rapid deceleration to a stop with leading edge of the wafer  100  just far enough below meniscus level of the electrolyte  48  to allow full wetting of the entire wafer down-facing surface. The deceleration step may be performed within 0.05 to 1.0 seconds. 
     3. Start tilting the wafer  100  to the flat horizontal position and moving wafer downward soon enough to protect trailing edge of the wafer from de-wetting and/or air entrainment due to splashing electrolyte  48 . 
     4. The wafer entry movement from initial contact with the electrolyte  48  until the wafer  100  is in the final processing position may be performed over an elapsed time of 1 or 0.5 to 0.1 seconds. The total move time from initial contact to the final processing position is typically 2 or 1 to 0.2 seconds. The wafer is typically moved down to a position where the bottom surface of the wafer is 3-10 mm below the surface of the electrolyte. 
     As used here, wafer means a substrate, for example a silicon wafer, on which microelectronic, micro-mechanical and/or micro-optical devices are formed. 
     Thus, novel apparatus and methods have been shown and described. Various changes and substitutions may of course be made, without departing from the spirit and scope of the invention. The invention, therefore, should not be limited except by the following claims and their equivalents.