Multistep immersion of wafer into liquid bath

Abstract

A holder-wafer conjugate, including a wafer in a wafer holder, is tilted so that the planar wafer surface makes a small angle with a plane parallel to the liquid surface of a liquid bath. Then, the holder-wafer conjugate is moved downward and a leading edge of the holder-wafer conjugate makes initial contact with the liquid surface of a liquid bath at a slow piercing speed to minimize generation of shock waves and bubbles. After a small portion of the holder-wafer conjugate is immersed in the liquid bath at a first pause location, the downward movement is stopped for a first pause time. After the first pause time, downward movement of the holder-wafer conjugate is resumed at a faster post-piercing speed until the wafer is fully immersed.

Description

FIELD OF THE INVENTION

The present invention pertains to the field of metal plating of integrated circuit substrate wafers, particularly to multistep immersion of a substrate wafer to reduce bubble formation on the substrate surface.

BACKGROUND OF THE INVENTION

Integrated circuits are formed on wafers by well-known processes and materials. These processes typically include the deposition of thin film layers by sputtering, metal-organic decomposition, chemical vapor deposition, plasma vapor deposition, and other techniques. These layers are processed by a variety of well-known etching technologies and subsequent deposition steps to provide a completed integrated circuit.

A crucial component of integrated circuits is the wiring or metallization layer that interconnects the individual circuits. Conventional metal deposition techniques include physical vapor deposition, e.g., sputtering and evaporation, and chemical vapor deposition techniques. Integrated circuit manufacturers are investigating electrolytic and electroless plating techniques to deposit primary conductor films on semiconductor substrates.

Wiring layers traditionally contained aluminum and a plurality of other metal layers that are compatible with the aluminum. In 1997, IBM introduced technology that facilitated a transition from aluminum to copper wiring layers. This technology has demanded corresponding changes in process architecture towards damascene and dual damascene architecture, as well as new process technologies.

Copper damascene circuits are produced by initially forming trenches and other embedded features in a wafer, as needed for circuit architecture. These trenches and embedded features are formed by conventional photolithographic processes in a nonconductive substrate, such as a silicon oxide. A barrier layer, e.g., of silicon nitride or tantalum, is deposited next. An initial seed or strike layer typically comprising copper and having a thickness of about 20 nanometers (nm) to 200 nm is then deposited by a conventional physical or vapor deposition technique. The seed layer is used as a base layer to conduct current for electroplating thicker films. Thinner seed layers are preferred so as to reduce overhang and closure of very small features with metal from the seed layer. The seed layer functions as the cathode of an electroplating cell. Electrical contacts to the wafer are normally made at its edge.

Generally, in electroplating processes, the thickness profile of the deposited metal is controlled to be as uniform as possible. This uniform profile is advantageous in subsequent etchback or polish removal steps, as well as uniform void-free filling of the trench structures. Prior art electroplating techniques are susceptible to thickness irregularities. Factors contributing to these irregularities include the size and shape of the electroplating cell, electrolyte depletion effects, hot edge effects, and the terminal effect.

The introduction of damascene metallization for copper interconnects led to the development and modification of processes. The implementation of new process flows caused new device-killing defect formation, as well as nuisance defects, which interfere with the ability to identify accurately the device-killing defects. In copper damascene metallization, defects generally arise during the three main process sequences: deposition of barrier and seed layers; electrofill operations, including pre- and post-anneal; and chemical mechanical polishing (CMP). Critical post-plating in-film killer defects in electroplated copper layers include pits, craters, and voids, which typically form during the electroplating process or during the post-plate anneal steps. Another type of defect are single isolated protrusions.

Electroless plating (or electroless deposition) of copper and other metals has received increasing interest in recent years. This interest is due in part because of the relatively low cost of electroless processes compared to other (e.g., vacuum) deposition techniques, and because of generally surface-controlled, selective, conformal deposition properties of electroless processes. Electroless deposition has a number of potential applications, such as repair of marginal seed layers for copper damascene electroplating, creation of seed layers and barrier layers directly on dielectrics that can be plated, and selective deposition of barrier and electromigration capping layers onto damascene metal (e.g., cobalt and cobalt alloys on copper).

Conventional electroless metal deposition is conducted in a system containing one or multiple open baths containing plating solution. In a typical operation, a wafer holder immerses a substrate wafer face down in the plating solution during plating operations.

Wafer immersion into a plating bath generally comprises only a few hundred milliseconds during a plating process, which typically lasts a few tens of seconds up to a few minutes. Yet the defects formed as a result of entry conditions are critical to maximizing yield (the number of chips that function in a full wafer of, e.g., 200 mm or 300 mm diameter). One of the reasons defects form as a result of wafer immersion into a plating bath is because of air trapped during entry. Air bubbles trapped on the plating surface of a wafer can cause many problems. Bubbles shield a region of the plating surface of a wafer from exposure to electrolyte, and thus produce a region where plating does not occur. The resulting plating defect can manifest itself as a region of no plating or of reduced thickness, depending on the time at which the bubble became entrapped on the wafer and the length of time that it stayed entrapped there. In an inverted (face down) configuration, buoyancy forces tend to pull bubbles upwards and onto the wafer's active planar surface. They are difficult to remove from the wafer surface because the plating cell has no intrinsic mechanism for driving the bubbles around the wafer edges, the only path off the wafer surface. Typically, a wafer is rotated about an axis that passes through its center and is perpendicular to its planar plating surface. This also helps to dislodge bubbles through centrifugal force, but many of the smaller bubbles are tenacious in their attachment to the wafer. Another problem associated with immersion of a horizontally oriented wafer is multiple wetting fronts. When a wafer is immersed in this way, the electrolyte contacts the wafer at more than one point, creating multiple wetting fronts as the wafer is submerged in the electrolyte. Where individual wetting fronts converge, bubbles may be trapped. Also, defects in the finished plating layer can be propagated from microscopic unwetted regions formed along convergence lines of multiple wetting fronts.

Various methods have been suggested for avoiding problems commonly encountered in deposition of metal, particularly electroplating of copper, in integrated circuit fabrication. For example, U.S. Pat. No. 6,551,487, issued Apr. 22, 2003, to Reid et al., which is hereby incorporated by reference, teaches a method and an apparatus for controlling the angle and speed of wafer immersion into a plating solution to reduce multiple wetting fronts and air bubble formation on the substrate surface and, thereby, to reduce electroplating defects. U.S. Pat. No. 6,551,487 teaches that an angle of about 5 degrees to horizontal and a slow immersion speed of about 12 millimeter (mm) per second is effective for minimizing bubble formation.

Nevertheless, as the industry transitions to 90 nm design mode (the smallest openings now being filled are on the order of 100 nm), a new problem has been identified. It has been determined that proper wetting of embedded features (i.e., the displacement of gas from the spaces within features and its replacement with plating solution) requires a relatively high shear force on the wafer. This new requirement, however, seems to contradict the idea of using a relatively slow wafer immersion speed to minimize bubble formation and resulting defects.

SUMMARY OF THE INVENTION

The present invention helps to solve some of the problems outlined above by providing methods and apparati for minimizing bubble formation and attachment to the wafer surface arising from wafer immersion, while also providing sufficient shear force to achieve good wetting of features and good wafer-liquid mass transfer. The invention is described herein mainly with reference to the electroplating of thin copper films in integrated circuits. It is understood, however, that methods and apparati in accordance with the invention are also useful for any process involving the immersion of a wafer into a plating solution, including electrolytic and electroless plating of any metal during integrated circuit fabrication.

Studies by the inventors have determined that bubble formation in a plating bath during wafer immersion is related to wave propagation in the plating bath. Based on the studies, it is believed that wave propagation arises because the surface tension of plating solution a in plating bath is broken by the wafer-clamping hardware. It is further believed that as plating solution is initially displaced while the wafer and wafer-holding apparatus initially enter into the plating bath, the displaced plating solution sets up a linear shock wave that carries bubbles created by the turbulence of this event. Generally, the shock wave impacts the active wafer surface, and the bubbles carried by the shock wave become attached to the wafer surface and cause defects.

The tenacity with which a bubble adheres to a wafer surface is influenced by several factors, such as bubble size and seed-surface condition.

The present invention provides methods and apparatus for segmenting the wafer immersion process to minimize shock wave formation. A wafer is moved toward a liquid bath (e.g., a plating bath), generally along a trajectory substantially normal to the surface of the liquid. Along this trajectory, the wafer is angled before entry into the liquid for angled immersion. Once the wafer or wafer carrier has entered partially into the bath liquid, the movement of the wafer carrier is stopped for a certain time period (pause) at a location at which the wafer carrier has entered the bath and broken the surface tension of the bath liquid. After the pause in the immersion process, movement of the wafer carrier is continued to further immerse the wafer carrier and the wafer in the bath liquid. The pause allows the “shock wave” to dissipate prior to further immersion. One or more pause times at one or more pause locations are implemented, depending on particular requirements.

A wafer is rotated or not while moving to a pause location depending on the particular design. Similarly, the wafer can be rotated or not moving from a pause point to a fully-immersed treatment (e.g., plating) position or to another pause location. Similarly the wafer can be moved at various immersion speeds to and from the intermediate pause points depending on the requirements.

Thus, embodiments in accordance with the invention provide methods and apparati for wafer immersion that comprise two or more processes after an immersion process has begun. In first processes, the wafer holding apparatus pierces the surface of the plating bath liquid. In second processes, a pause is effected prior to further movement to allow a shock wave to dissipate. A first basic embodiment of a method in accordance with the invention for immersing a wafer substrate into a liquid bath includes processes of positioning a wafer above a horizontal liquid surface of a liquid bath, and combining the wafer substrate and a wafer holder to form a holder-wafer conjugate. Generally, the wafer has a substantially planar surface. Also, the wafer holder is positioned so that the planar surface of the wafer is tilted at an angle to a plane parallel to the horizontal liquid surface. Then, while the planar surface is tilted, the holder-wafer conjugate is moved at a piercing speed downward toward the horizontal liquid surface so that an outer edge of the holder-wafer conjugate pierces the liquid surface. After the holder-wafer conjugate pierces the liquid surface moving at a slow piercing speed, downward movement of the holder-wafer conjugate is substantially stopped for a first pause time at a first pause location at which a first portion of the holder-wafer conjugate is immersed in the liquid bath. After the first pause time, downward movement of the holder-wafer conjugate is resumed. Downward movement of the holder-wafer conjugate is resumed at a final post-piercing speed until the wafer is fully immersed in the liquid bath. Some embodiments further include substantially stopping the holder-wafer conjugate for an intermediate pause time at an intermediate pause location before the wafer is fully immersed in the liquid bath. Typically, a pause time, particularly a first pause time, comprises a time period in a range of about from 0.1 seconds to five seconds.

Some embodiments further comprise processes of rotating the wafer about an axis normal to the planar surface prior to the first pause time, typically at a speed in a range of about from 1 rotation per minute (rpm) to 10 rpm. Some embodiments further comprise rotating the wafer about an axis normal to the planar surface during the first pause time, typically at a speed in a range of about from 1 rpm to 10 rpm. Some embodiments further comprise rotating the wafer about an axis normal to the planar surface after resuming moving the holder-wafer conjugate downward after the first pause time, typically at a speed in a range of about from 1 rpm to 100 rpm; for example, at about 30 rpm.

Usually, embodiments in accordance with the invention further comprise rotating the wafer about an axis normal to the planar surface after the wafer is fully immersed. Generally, rotation after full immersion is conducted in a range of about from 30 rpm to 150 rpm; for example, at about 100 rpm.

Generally, moving the holder-wafer conjugate toward the horizontal liquid surface is conducted along a trajectory substantially normal to the horizontal liquid surface. Downward movement as the holder-wafer conjugate makes initial contact with the liquid surface and pierces the liquid surface is done at a relatively slow piercing speed, generally in a range of about from 0.1 mm per second to 15 mm per second; for example, in a range of about from 3 to 8 mm per second. In contrast, downward movement at a final post-piercing speed is typically conducted at a speed in a range of about from 25 to 150 mm per second; for example, at about 100 mm per second.

A method in accordance with the invention for immersing a wafer in a liquid bath is particularly useful for immersing wafers into plating solutions; for example, into an electroless plating solution or into an electrolytic plating solution. Embodiments in accordance with the invention are described below with reference to electroplating copper.

An apparatus in accordance with the invention is characterized in that downward movement of a wafer holder into a liquid bath can be gradually stopped at a selected pause location for a selected pause time, and then gradually resumed at one or more other locations until a wafer in the wafer holder is fully immersed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts in schematic form a cross-sectional view of a generalized electroplating apparatus suitable for performing a multistep immersion method in accordance with the invention;

FIG. 2 contains a process flow sheet of a generalized method in accordance with the invention for immersing a wafer substrate into a plating solution;

FIG. 3 depicts a substrate wafer positioned in accordance with the invention above a liquid bath;

FIG. 4 depicts the substrate wafer of FIG. 3 in a tilted position so that the planar surface of the wafer forms an angle with a plane parallel to liquid bath surface;

FIG. 5 depicts rotation of the wafer of FIG. 4 before initial immersion of the wafer into the liquid bath;

FIG. 6 depicts the outside edge of the wafer of FIG. 5 piercing the liquid bath surface in accordance with the invention;

FIG. 7 depicts a small portion of the wafer of FIG. 6 at its outside edge immersed in the liquid bath at a first pause location in accordance with the invention;

FIG. 8 depicts a wafer at an angle fully-immersed in the liquid bath in accordance with the invention;

FIG. 9 depicts the fully-immersed wafer of FIGS. 7 and 8 being rotated in a horizontal orientation;

FIG. 10 depicts a wafer moving upward during post-plating processes;

FIG. 11 depicts a side-section of a wafer holder in accordance with the invention holding a substrate wafer above a liquid bath containing plating solution;

FIG. 12 depicts a front section of the wafer-holder and wafer (holder-wafer conjugate) of FIG. 11, including a dashed outline of the holder-wafer conjugate in a tilted position so that the planar surface of the wafer forms an angle with a plane parallel to the liquid bath surface in accordance with the invention;

FIG. 13 depicts a front view of a movement scenario of the wafer positioning apparatus depicted in FIGS. 11 and 12, in which the distal end of the wafer positioning apparatus is moved along an arced track by an actuator in accordance with the invention;

FIG. 14 shows a small portion of the holder-wafer conjugate at its outside edge immersed in a liquid bath at a first pause location in accordance with the invention;

FIG. 15 depicts the holder-wafer conjugate of FIG. 14 at a second, intermediate pause location in accordance with the invention;

FIG. 16 depicts the fully-immersed wafer of FIGS. 14 and 15 in a horizontal orientation;

FIG. 17 depicts schematically defects in electroplated copper on a wafer that was immersed using a slow, single-step immersion technique known in the art;

FIG. 18 depicts schematically defects in electroplated copper on a wafer that was immersed using a fast, single-step immersion technique known in the art; and

FIG. 19 depicts schematically the small number of defects in electroplated copper on a wafer that was immersed using a multistep immersion method in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described herein with reference to FIGS. 1-19. It should be understood that the structures and systems depicted in schematic form in FIGS. 1, 3-16 are used to explain the invention and are not precise depictions of actual structures and systems in accordance with the invention. Furthermore, the preferred embodiments described herein are exemplary and are not intended to limit the scope of the invention, which is defined in the claims below.

In this specification, the terms “anode” and “cathode” refer to structures at which an oxidation and reduction process occur, respectively. In descriptions of electroplating systems and methods, the term “cathode” refers to the workpiece, typically an integrated circuit wafer, and the term “anode” refers to the counter-electrode.

FIG. 1 depicts in schematic form a cross-sectional view of a generalized electroplating apparatus 100 suitable for performing a method in accordance with the invention. Apparatus 100 includes a clamshell wafer holder 110 mounted on a rotatable spindle 112, which allows rotation of clamshell 110 at a controllably variable rotation rate. A clamshell wafer holder 110 has been described in commonly-owned U.S. Pat. No. 6,156,167, issued Dec. 5, 2002 to Patton et al., which is hereby incorporated by reference. Clamshell 110 comprises a cone 114, a cup 116, and a flange 118. Flange 118 is known in the art, and is implemented in various forms to control the DC current density at the edge of a wafer and thereby reduce the “terminal effect” during initial stages of electroplating. For electroplating, a wafer 120 having plating surface 121 is mounted in cup 116. Clamshell 110 and wafer 120 are immersed in a plating bath 122 containing plating solution 123 and located in bath container 124. As indicated by arrow 126, plating solution is continually delivered into bath container 124 by a pump 128. Generally, plating solution flows upwards towards the center of wafer 120 and then radially outwards and across wafer plating surface 121, as indicated by arrows 130. Plating solution 123 then overflows bath container 124 and flows into overflow reservoir 132, as indicated by arrows 134, 135. The plating solution is then filtered (not shown) and returned to pump 128, as indicated by arrow 136.

A DC power supply 140 and a negative output lead are electrically connected to wafer 120 through one or more rings, brushes, and contacts (not shown) mounted in wafer holder 110. The positive output lead of power supply 140 is electrically connected to an anode 142 located in plating bath 122 at the bottom of bath container 124. Shields 144, 146 are provided to shape the electric field between anode 142 and cathode wafer 120. Preferably, flange 118 and shields 144, 146 are dynamically variable, as disclosed in U.S. Pat. No. 6,402,923 B1, issued Jun. 11, 1002 to Mayer et al., which is hereby incorporated by reference. An apparatus suitable for conducting a method in accordance with the invention is disclosed in U.S. Pat. No. 6,755,954, issued Jun. 29, 2004, to Mayer et al., which is hereby incorporated by reference.

FIG. 2 contains a process flow sheet of a generalized method 200 in accordance with the invention for immersing a wafer substrate into a plating solution. Method 200 is described herein initially with reference to electroplating apparatus 100 of FIG. 1 and to the diagrams of FIGS. 3-16. Although method 200 is described with reference to electroplating, it is understood that a method in accordance with the invention is applicable to any wet wafer processing wherein a wafer is immersed into a bath of liquid treating solution.

In processes 210, a wafer 120, 320, 420 is positioned substantially above liquid bath 122, 322, 422 containing plating solution 123, 323,423, as depicted in FIGS. 3 and 11, respectively. This is typically performed using conventional robotic tools known in the art. In processes 220, typically substantially concurrently with processes 210, wafer 120, 320, 420 is located in wafer holder 110, 410, thereby forming a holder-wafer conjugate 111, 411, comprising wafer holder 110, 410 and wafer 120, 320, 420 in which a wafer substrate 120, 320, 420 is held firmly. As depicted in FIGS. 3 and 11, planar plating surface 121, 321, 421 is held facedown, facing liquid surface 325, 425.

In processes 230, wafer holder 110, 410 is positioned to tilt planar surface 121, 321, 421 of wafer 120, 320, 420, forming an angle with a plane parallel to liquid surface 325, 425, as depicted in FIGS. 4, 12 and 13. Generally, a relatively small angle less than 10° is formed, typically in a range of about from 2° to 5°, as depicted in FIG. 13.

Optionally, in processes 234, rotation of wafer 120, 320, 420 is started before initial contact of the wafer with the liquid surface, as indicated by arrows 326 in FIG. 5. A typical rotation speed, if the wafer is rotated before and/or during initial wafer-liquid contact, is in a range of about from 1 to 10 rpm.

In processes 240, wafer holder 110, 410 is moved downward so that an outer edge 431 of holder-wafer conjugate 411 pierces liquid surface 325, 425, as depicted in FIGS. 6,14. Movement proceeds along a downward trajectory towards liquid surface 325, 425, that is, downward along the Z-axis as depicted by arrow 454 in FIG. 12. In some embodiments, downward movement in processes 240 is conducted substantially at a relatively slow immersion speed, selected to minimize shock waves upon initial contact and initial partial immersion. In preferred embodiments, however, the speed along the downward trajectory towards liquid surface 325, 425 is dynamically varied during processes 240 so that the vertical translation speed is a relatively fast pre-piercing speed during initial movement of holder-wafer conjugate 411 from its original position above the liquid bath, and then the vertical translation speed is a relatively slow piercing speed as holder-wafer conjugate 411 pierces liquid surface 325,425 and moves to its first pause location. In this manner, total immersion time and thereby total throughput time is minimized, thereby maximizing tool efficiency, while achieving benefits of a slow piercing speed. Typically, a pre-piercing speed from the original wafer position above the liquid bath to a position just above (e.g., 1 cm) the liquid bath surface is in a range of about from 50 mm/sec to 150 mm/sec. Typically, initial contact of holder-wafer conjugate 411 with liquid surface 425 and movement to the first pause location is conducted at a piercing speed in a range of about from 1 mm/sec to 15 mm/sec, preferably in a range of about from 3 mm/sec to 8 mm/sec.

Numerous embodiments of a holder-wafer conjugate are suitable for use in accordance with the invention. In some embodiments, such as in a clamshell wafer holder disclosed in U.S. Pat. No. 6,156,167, issued Dec. 5, 2002 to Patton et al., and U.S. Pat. No. 6,551,487, issued Apr. 22, 2003 to Reid et al., which are incorporated by reference, the cup 116 completely encloses the peripheral outside edge of wafer 120, as depicted in FIG. 1. As a result, the leading edge of holder-wafer conjugate 111 comprises an outside edge of wafer holder 110. In other embodiments, a large portion of the peripheral outside edge of a wafer is not enclosed by the wafer holder, so that the leading edge of the holder-wafer conjugate initially contacting and piercing the liquid surface of the treating bath comprises the outside wafer edge. For example, in FIGS. 3-10, a wafer holder is not depicted, and the outside wafer edge 331 is shown piercing liquid surface 325 in FIG. 6. In other words, in various embodiments in accordance with the invention, the part of holder-wafer conjugate 111, 411 that initially pierces liquid surface 325,425 of treating bath 322,422 in processes 240 comprises an outside portion of the wafer, an outside portion of the wafer holder, or a combination of portions of both wafer and wafer holder.

In processes 250, the motion of holder-wafer conjugate 111, 411 in the downward direction, and thereby the motion of wafer 120, 320, 420 in the downward direction, is substantially stopped at a first pause location so that just a small portion of holder-wafer conjugate 111, 411 (e.g., a small portion of wafer 320) is immersed in liquid bath 322, 422. FIG. 7 depicts a sectional view in which a small portion of wafer 320 at edge 331 is immersed in liquid bath 322 containing liquid solution 323. Similarly, FIG. 14 shows a small portion 470 of holder-wafer conjugate 411 at edge 431 immersed in liquid bath 422. In processes 250, the motion of holder-wafer conjugate 411 (wafer 320 in FIGS. 6 and 7) is stopped for a first pause time. A first pause time typically comprises a time in a range of about from 0.1 seconds to 5 seconds, more commonly in a range of about from 0.5 seconds to 2 seconds, which is usually enough time for shock waves and resulting bubbles generated by the initial immersion of edge 331, 431 to dissipate. Typically, the first pause location is designed so that a portion of holder-wafer conjugate is immersed about 1 mm to 5 mm below the surface 325, 425. Preferably, stopping of downward movement is conducted gradually and continuously to avoid sudden and jerky deceleration. For example, in an exemplary embodiment, downward movement was slowed from a piercing speed of about 5 mm/sec to zero speed at a first pause location during a deceleration time of about 0.1 second.

In processes 260, movement of wafer 320, 420 in a holder-wafer conjugate 411 downward into treating bath 322, 422 is resumed. Preferably, downward movement is resumed by accelerating from zero to a post-piercing speed quickly enough to achieve steady-state conditions rapidly at the entire plating surface 121, 321, 421. In an exemplary embodiment, downward movement accelerated from zero at the first pause location up to a post-piercing speed of about 100 mm/sec in about 0.1 second. It is believed that rapid downward movement of the holder-wafer conjugate in the downward Z-direction from the first pause point achieves full immersion and complete wetting of the wafer plating surface before waves and bubbles generated by the resumed downward movement have a chance to impact the wafer surface.

Optionally, as indicated by the dashed arrows leading to and from processes 262 of FIG. 2, movement of wafer 320, 420 in holder-wafer conjugate 411 is stopped one or more times for an intermediate pause time at an intermediate pause location, as depicted in FIG. 15, and then continued in a downward direction in processes 270. In processes 270, movement of wafer 120, 320, 420 in holder-wafer conjugate 111, 411 is continued downward until wafer 120, 320, 420 is fully immersed in liquid bath 122, 322, 422, as depicted in FIGS. 1, 8, and 16. Downward movement is stopped at a final plating (or treating) location. Movement along the downward trajectory downward along the Z-axis in processes 260, 270, as depicted by arrow 454 in FIG. 16, is typically conducted at a speed in a range of about from 50 mm/sec to 150 mm/sec, preferably in a range of about from 80 mm/sec to 120 mm/sec.

Optionally, during one or several of processes 260, 262 and 270, substrate wafer 320, 420 is rotated at a rotational speed in a range of about from 1 rpm to 100 rpm, more typically at 30 rpm.

In processes 280, wafer 120, 320, 420 is rotated at a plating rotation rate. Typically, a rotation rate during plating (or other treatment) processes is in a range of about from 10 rpm to 150 rpm.

In processes 290, plating or other liquid treatment processes are conducted. In some embodiments, as depicted in FIGS. 1, 9 and 16, wafer holder 110, 410 is positioned during processes 290 so that wafer 120, 320, 420 is substantially parallel to horizontal liquid surface 325, 425. Alternatively, wafer holder 110, 410 is positioned during processes 290 so that wafer 120, 320, 420 is tilted with respect to liquid surface 325, 425.

In processes 294, post-plating (or other post-treatment) processes are conducted. For example, typical post-plating processes include moving wafer 320 upward out of plating bath 322, as depicted in FIG. 10. Processes 294 also typically include spinning a wafer during its extraction from the plating bath to remove excess liquid. Rotation of a wafer during extraction and spinning is typically conducted at a speed in a range of about from 2 rpm to 100 rpm. Processes 294 also typically include rinsing plating solution from the wafer using deionized water or a rinsing solution.

Slowing the rotational speed during post-plating processes 294 when the wafer is extracted from the bath helps minimize splashing and/or frothing. It has been found that splashing or frothing can leave surface bubbles in the bath, which can become attached to the subsequently plated wafer. Slowing rotation also minimizes vortexing. If a significant vortex is formed in an electrolyte from the rotational drag on the electrolyte surface during wafer extraction, then the resultant turbulence on the electrolyte surface can detrimentally effect wetting of the next wafer. Preferably, the rotational speed during extraction is in a range of about from 5 rpm to 75 rpm. More preferably, the extraction rotational speed is in a range of about from 10 rpm to 50 rpm.

After its rotation is slowed, the wafer is extracted in processes 294 from liquid bath 422 along an upward trajectory (Z-axis), as indicated by arrow 455 in FIG. 16. Also, in conjunction with slowing the rotational speed, wafer extraction may be done at a vertical linear speed (z-speed) that is the same as or less than that used for immersion. Preferably, extracting the wafer from the electrolyte is performed at a speed in a range of about from 5 mm/sec to 50 mm/sec. More preferably, extracting the wafer from the electrolyte is performed at a speed of about 12 mm/sec. Typically, the wafer is elevated to a second height 342, below first height 340, about 30 millimeters above liquid surface 325. Typically, processes 294 include liquid reclaim techniques, as described in U.S. Pat. No. 6,551,487, which is incorporated by reference.

Loading of a wafer 120, 320, 420 into a wafer holder 110, 410 in processes 220 is generally conducted in a horizontal orientation at a first height 340, as depicted in FIG. 3. A horizontal orientation is commonly employed in wafer handling apparatus including robots for transferring wafers from one piece of apparatus to the next. The wafer can be loaded and unloaded from associated plating hardware in a horizontal orientation at first height 340. First height 340 can vary, depending on the hardware used to implement the method. Typically, a wafer's initial position (shown in FIGS. 3, 11) is above liquid bath 322, 422 so that electroplating (or other treatment) takes place directly under this starting position. Since many wafer processing tools require horizontal orientation of wafers, this facilitates wafer throughput via integration with other tools; that is, wafers need not be re-oriented before loading into a tool utilizing this method or before loading into another wafer processing tool after unloading from a tool utilizing this method. In a broader sense, however, the invention embodies methods and apparati wherein the wafer's starting position is not horizontal, but rather tilted, as depicted in FIGS. 4, 12. Also, handling a wafer along a vertical trajectory allows associated apparati implementing the method to be designed with a smaller relative footprint than those requiring delivery of a wafer to a plating bath from positions not above the electrolyte.

Multistep angled immersion reduces the problems of bubble entrapment on the treatment surface and of multiple wetting fronts. Depending on the treatment (e.g., electroplating or electroless plating) conditions and the details of the wafer-holding apparatus (clamshell), optimally, different angles are used. For example, electroplating at an angle helps prevent entrapment of bubbles on the plating surface during electroplating. Defects in the plated film are reduced when angled plating is employed. In some embodiments, tilting of holder-wafer conjugate 111, 411 is dynamically varied from one angle to another angle one or several times during processes 230-294 to optimize process conditions. During processes 240 through 270, the angle of the wafer is generally in a range of about from 1 degree to 10 degrees, preferably about 1 degree to 3 degrees.

As described above, different rotational speeds are usually employed during different processes of a method 200. For a 200 mm diameter wafer, the rotational speed during downward movement in processes 260, 270 after the first pause time is generally in a range of about from 30 rpm to 100 rpm. For a 300 mm diameter wafer, the rotational speed during downward movement in processes 260, 270 after the first pause time is generally in a range of about from 20 rpm to 70 rpm.

Allowing the wafer orientation to be returned to horizontal during plating (or other treatment) processes 290 allows flexibility for plating environments that do not accommodate a tilted wafer for plating or in cases where horizontal plating is preferred. In such embodiments, the angular speed or “swing speed” of the wafer may be important. Since the wafer is rotating relatively fast, if the wafer is tilted back to horizontal too quickly, then too much turbulence may result and create bubbles or splashing (and possibly contamination of equipment with splashed electrolyte). As with all events in a high throughput environment, if the swing speed is too slow, throughput suffers. Preferably, the swing speed of the wafer is in a range of about from 0.25 degrees per second to 3 degrees per second. More preferably, the swing speed is in a range of about from 0.25 to 1.5 degrees per second. Most preferably, the swing speed is in a range of about from 0.5 degrees per second to 1 degree per second. Once the wafer is oriented horizontally, as in FIG. 16, then it is plated in that orientation. If the wafer is to be treated in a tilted orientation, then the treating (e.g., plating) is conducted with the wafer fully-immersed at an angle, as depicted in FIG. 8.

This invention also pertains to an apparatus that facilitates angled immersion of a wafer into a liquid bath at variable speeds along a downward trajectory (Z-axis) and which can be stopped and started at 1 or more pause locations. Preferably, an apparatus allows the active planar surface of a wafer to assume multiple angles with respect to the liquid surface of the liquid bath. These angles represent deviations from a horizontal wafer orientation, in which its active planar surface is parallel to the plane of the liquid bath surface. In other words, an apparatus in accordance with the invention allows the wafer to tilt about a pivot location on or near the wafer. Preferably, the apparatus accomplishes this without significantly varying the wafer's overall position in three-dimensional Cartesian space. In other words, an apparatus preferably is able to tilt the wafer's angle with respect to a plane parallel to the liquid bath surface without significantly translating the wafer. More specifically, some point or line on or near the wafer remains stationary during such pivoting. For example, the wafer's pivot point or its center point may remain fixed during pivoting.

Obviously, an apparatus in accordance with the invention permits the full range of operations associated with electroplating or other liquid bath treatment. Thus, for example, the apparatus permits and/or drives movement of the wafer into and out of a liquid bath. Preferably, though not necessarily, this is accomplished along a linear trajectory, that is, along a path substantially normal to the surface of the electrolyte. In addition, the apparatus allows and/or drives rotation of the wafer about an axis through the center of a wafer's active planar surface. Parameters that are controllable and variable in an apparatus in accordance with the invention include, among others: the speeds at which the wafer is rotated; the swing speed at which the wafer is tilted over a range of angles; the total range of angles over which the wafer's planar surface is tilted; the speed at which the holder-wafer conjugate is translated into and out of the electrolyte; the pause locations at which downward and upward movement of the holder-wafer conjugate is stopped and started; and the rates of acceleration and deceleration of movement at one or more pause locations.

An apparatus suitable for use with this invention can take on many different forms. It may include a variety of drive mechanisms, holders, pivot devices, and structural members. Generally, there is a drive mechanism for controlling the rotation of the wafer. There are one or one or more other drive mechanisms that control tilting of the wafer and translation of the wafer. Suitable drive mechanisms include many different types, such as hydraulic actuators, electric motors, screw drives, and the like. Various wafer holders and tracks for moving the wafer holders may also be employed.

In some embodiments, wafer tilting is accomplished by an apparatus that holds the wafer at a proximate end of a longitudinal member. The apparatus maintains this end of the longitudinal member at a substantially constant position in three— dimensional Cartesian space. The distal end of the longitudinal member is allowed to move over an arced path. This causes the wafer to tilt as described above. As described in U.S. Pat. No. 6,551,487, which is incorporated by reference, and as illustrated in FIGS. 11-16, described further below, the longitudinal member may take the form of a wafer holder in an “inverted pendulum” orientation.

A simplified block diagram depicting a front view of a wafer positioning apparatus 400 using this approach is depicted in FIG. 12. Apparatus 400 has a wafer holder 410 at its proximal end that engages a wafer 420, and a distal end 441 movably connected to an arced track 440 via an actuator 442. This arrangement is more easily visualized in conjunction with FIG. 11, which depicts a block diagram of a side view of wafer positioning apparatus 400. Wafer holder 410 is immovably coupled to actuator 442. Also, wafer holder 410 rotates wafer 420 about an axis normal to the wafer planar plating surface 421. The components of 410 responsible for driving wafer rotation are not depicted.

Arced track 440 can be integral to plate 444 or attached to plate 444. For example, arced track 440 can be a groove, channel, or race formed in plate 444, a curved track or support along which actuator 442 travels, or a curved surface upon which actuator 442 rolls or slides, etc. Arced track 440 could be simply a curved rail, without a support structure 444 that is directly attached to actuator 442. Thus, an “arced track” is any assembly that provides an arced trajectory to the distal end 410 of the wafer positioning apparatus 400 having the inverted pendulum design, thus tilting the holder-wafer conjugate 411 upon a virtual pivot, as shown by the dashed outline in FIG. 12.

Actuator 442 moves the distal end 441 along arced track 440. Plate 444 is immovably coupled to a second actuator 450. Actuator 450 is movably coupled to a shaft 452, and moves bi-directionally along fixed shaft 452 as indicated by the Z-axis arrows 454, 455, thereby moving plate 444, actuator 442, and wafer positioning apparatus 400 along with it. In this embodiment, there is component movement along the Z-axis and the X-axis, but none along the Y-axis. Thus, the assembly comprising positioning apparatus 400, actuator 442, plate 444, and actuator 450 moves along a vertical path (Z-axis) above liquid bath 422 and by this action, wafer 420 is moved into and out of liquid solution 423 of bath 422. In accordance with the invention, translation of holder-wafer conjugate 411 along fixed shaft 452 into liquid solution 423 of bath 422 is “stoppable” (able to be stopped) and “startable” (able to be resumed) precisely and controllably at one or more selected pause locations with minimal vibrations. Therefore, movement by actuator 450 preferably is controllably accelerated and decelerated to avoid shaking of wafer holder 410 that would result from sudden starting and stopping of movement along fixed shaft 452.

Depicted in FIG. 13 is cross-sectional block diagram depicting a front view of a movement scenario of wafer positioning apparatus 400 depicted in FIGS. 11 and 12. For this discussion, a point 460 on the distal end 441 of wafer positioning apparatus 400 traces an arced path as the distal end is moved along arced track 440 by actuator 442. As indicated by the dashed arrows 462, 463, as the distal end of the wafer holder is moved along the arced path, the holder-wafer conjugate 411 is tilted from horizontal. Thus, wafer 420 is tilted on a virtual pivot 466. Pivot 466 is “virtual” in that it lacks an associated physical pivot joint or hinge. As a result of the tilting movement about pivot 466, half of wafer 420 is tilted 5 degrees above horizontal and half is tilted 5 degrees below horizontal. Thus, with respect to a plane defined by liquid surface 425 of plating bath 422, wafer 420 is tilted 5 degrees from horizontal.

FIG. 14 depicts angled immersion of the wafer 420 using apparatus 400. Apparatus 400 moves a wafer along an axis normal to the surface 425 of liquid bath 422 and tilts the wafer from horizontal, allowing for angled immersion and angled plating. The use of a virtual pivot 466 is one embodiment of the tilting capability of the invention. Other embodiments may use actual pivot joints located in the vicinity of the wafer. Importantly, the apparatus provides two distinct movement capabilities: vertical movement along a vertical trajectory (Z-axis) normal to the liquid surface 425 and a tilting movement of the wafer.

An apparatus in accordance with the invention provides wafer movement at speeds appropriate for embodiments of the invention. Preferably, rotational drive components of apparatus 400 provide a wide range of rotational speeds for wafer holder 410. In some embodiments, an apparatus 400 rotates a wafer at a speed of between about 1 and 600 rpm. Actuator 450 provides linear bi-directional movement at a speed in a range of from 0 up to preferably at least about 150 mm/sec. In some embodiments, movement of distal end 441 of apparatus 400 along arced track 440 is provided by a hydraulic cylinder, although other suitable means can be used such as gears, lead screws, and the like. As explained, movement of distal end 441 of wafer apparatus 400 along arced track 440 provides tilting of the planar plating surface 421 of wafer 420 from a horizontal position, parallel to the plane defined by liquid surface 425 of liquid bath 422, at an angle such that planar plating surface 421 of wafer 420 is no longer parallel to the plane defined by liquid surface 425 of liquid bath 422. Preferably, apparatus 400 provides tilt at angles of between 0 degrees and at least about 5 degrees, preferably in a range of about from 0 degrees to 10 degrees. Preferably, the angle can be actively (dynamically) adjusted during any electroplating or other liquid treating operation. As mentioned, planar plating surface 421 of wafer 420 is tilted at a specific swing speed when a wafer is returned to a horizontal position after full immersion. Preferably, apparatus 400 provides a swing speed of the wafer in a range of about from 0.25 to 3 degrees per second.

Although other wafer-holder components are suitable for the invention, a good example of a wafer holder is the clamshell apparatus described in U.S. Pat. Nos. 6,156,167 and 6,139,712. When a clamshell is used as the wafer-holder component of the apparatus, the other components essentially comprise positioning elements for the clamshell, since the clamshell has necessary electrical contacts, holding and rotational components, and other appropriate components.

EXAMPLE 1

A Novellus Model Sabre xT apparatus equipped with a clamshell-type wafer holder was used to electroplate copper on integrated circuit substrate wafers using a typical electroplating solution. Copper was electroplated on a series of 300 mm silicon wafers having a PVD copper seed layer with a thickness of approximately 100 nm. Process specifications of a standard Sabre xT copper DC electrofill process are known in the art.

The electroplating solution contained: 40 grams per liter (g/l) of dissolved copper metal, added as copper sulfate pentahydrate (CuSO₄.5H₂O); 10 g/l H₂SO₄; 50 milligram per liter (mg/l) chloride ion, added as HCl; 6 milliliter per liter (ml/l) Viaform accelerator; 2.5 ml/l Viaform leveler; and 2 ml/l Viaform suppressor. The liquid plating bath had a volume of approximately 150 liters.

Copper plating was conducted at a bath temperature of about 25° C. The plating solution was pumped into the liquid bath at a volumetric flow rate of about 12 liters per minute. The distance between the cathodic plating surface of the wafer and the top surface of the anode was about 7 mm.

Each of three groups of five wafers was immersed in the plating bath differently from the other groups. Wafers in a first group were fully immersed using known techniques at an angle of 2 degrees, at a relatively slow downward (Z-axis) immersion speed of approximately 12 mm/sec, with 30 rpm rotation during immersion, but with no pause in accordance with the invention. Wafers in a second group were fully immersed using known techniques at an angle of 2 degrees, at a relatively fast downward (Z-axis) immersion speed of approximately 100 mm/sec, with 30 rpm rotation during immersion, and also with no pause in accordance with the invention. In contrast, wafers in the third group were immersed using a multistep immersion method in accordance with the invention. Each wafer of the third-group wafers, tilted at an angle of 2 degrees and having zero rotational speed, was moved downward at a downward piercing speed (Z-axis) of approximately 5 mm/sec and then stopped at a pause location at which an outside edge of the clamshell wafer holder piercing the liquid surface was immersed about 3 mm past the first point of contact with the electroplating solution. Downward movement slowed from 5 mm/sec to zero at the pause location over a deceleration time of about 0.1 seconds. The wafer holder was stopped at the pause location for a pause time of approximately 1 second. After the pause time, movement of the wafer holder was resumed downward. Downward movement resumed from zero at the pause location to a post-piercing speed of approximately 100 mm/sec. Downward movement proceeded for about 80 milliseconds until the wafer surface was fully immersed in the liquid bath, and then downward movement was stopped again at a final plating location. During movement of the holder-wafer conjugate from the pause location until stopping full immersed at the final plating location, the wafer was rotated at about 30 rpm.

After reaching a final plating location, each of the tilted wafer substrates was returned to a horizontal orientation at a swing speed corresponding to a linear movement of the wafer outside edge of about 50 mm/sec, requiring less than a second. Wafer substrates were plated in the horizontal orientation. During plating of each wafer, the wafer cathode was rotated at 90 rpm and negatively biased during a first plating time of 5.5 seconds to generate a DC current of 3 amp between the anode and the cathode, which corresponds to a DC current density of approximately 4.3 mA/cm² at the deposition surface of the 300 mm wafer. During an intermediate plating time of 30 seconds, the wafer was rotated at 12 rpm and a negative bias was applied to generate a DC current 6.75 amps, corresponding to a current density of approximately 9.5 mA/cm² at the deposition surface of the wafer. Then, during a final plating time, the wafer was rotated at 12 rpm and a negative bias was applied for 32 seconds, thereby generating a DC current of 33.75 amps, corresponding to a current density of approximately 48 mA/cm². The resulting layer deposited on each wafer had a thickness of approximately 1.0 μm, or 1000 nm.

After electroplating, each wafer was extracted at an extraction speed in the upward Z-direction of approximately 50 mm/sec while being rotated at about 12 rpm. The plated wafers were examined for defects using a laser surface scattering measurement tool (Model AIT2 made by KLA). The results are depicted in FIGS. 17-19. The incidence of defects on wafers in the second group, which were immersed at a fast, commercially-acceptable single-step immersion speed was unacceptably high, numbering in a range of about from 800-1500, as depicted in FIG. 18. As depicted in FIG. 17, the incidence of defects on wafers in the first wafer group, which were immersed at a slow, commercially-unacceptable single step immersion speed numbered about 4000, higher than on the first-group wafers immersed at a fast speed. As depicted in FIG. 1-9, the incidence of defects on wafers of the third wafer group, which were immersed using a multistep method in accordance with the invention, was less than 100.

The results show that a multistep method in accordance with the invention significantly decreases the incidence of plating defects arising as a result of immersion processes. Furthermore, a multistep method in accordance with the invention provides a relatively fast wafer rotation speed and a relatively fast downward translation speed after a pause time to generate a high shear rate, which is important for satisfactory filling of embedded features with plating (or other treatment) solution. A multistep method in accordance with the invention also provides commercially acceptable total immersion times so that fabrication throughput is not adversely affected.

Methods in accordance with the invention are useful in a wide variety of circumstances and applications. It is evident that those skilled in the art may now make numerous uses and modifications of the specific embodiments described, without departing from the inventive concepts. It is also evident that the steps recited may, in some instances, be performed in a different order; or equivalent structures and processes may be substituted for the structures and processes described. Since certain changes may be made in the above systems and methods without departing from the scope of the invention, it is intended that all subject matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or inherently possessed by the methods and structures described in the claims below and by their equivalents.

Claims

1. A method of immersing a wafer substrate into a liquid bath, comprising processes of:

positioning a wafer substrate above a horizontal liquid surface of a liquid bath, said wafer substrate having a planar surface;

combining said wafer substrate and a wafer holder to form a holder-wafer conjugate;

positioning said wafer holder so that said planar surface of said wafer substrate is tilted at an angle to a plane parallel to said horizontal liquid surface;

while said planar surface is tilted, moving said holder-wafer conjugate at a piercing speed downward toward said horizontal liquid surface so that an outer edge of said holder-wafer conjugate pierces said horizontal liquid surface;

then substantially stopping said holder-wafer conjugate for a first pause time at a first pause location at which a first portion of said holder-wafer conjugate is immersed in said liquid bath to a depth in a range of about from 1 mm to 5 mm below said horizontal liquid surface;

after said first pause time, resuming moving said holder-wafer conjugate downward; and

continuing moving said holder-wafer conjugate downward at a final post-piercing speed until said wafer substrate is fully immersed in said liquid bath.

2. A method of immersing a wafer substrate as in claim 1, further comprising:

substantially stopping said holder-wafer conjugate for an intermediate pause time at an intermediate pause location before said wafer substrate is fully immersed in said liquid bath.

3. A method of immersing a wafer substrate as in claim 1 wherein:

said first pause time comprises a time period in a range of about from 0.1 seconds to five seconds.

4. A method of immersing a wafer substrate as in claim 1, further comprising processes of:

rotating said wafer substrate about an axis normal to said planar surface prior to said first pause time.

5. A method of immersing a wafer substrate as in claim 4 wherein:

said rotating is conducted in a range of about from 1 rpm to 10 rpm.

6. A method of immersing a wafer substrate as in claim 1, further comprising:

rotating said wafer substrate about an axis normal to said planar surface during said first pause time.

7. A method of immersing a wafer substrate as in claim 6 wherein:

said rotating during said first pause time is conducted in a range of about from 1 rpm to 10 rpm.

8. A method of immersing a wafer substrate as in claim 1, further comprising:

rotating said wafer substrate about an axis normal to said planar surface after resuming moving said holder-wafer conjugate downward after said first pause time.

9. A method of immersing a wafer substrate as in claim 8 wherein:

said rotating is conducted in a range of about from 1 rpm to 100 rpm.

10. A method of immersing a wafer substrate as in claim 9 wherein:

said rotating is conducted at about 30 rpm.

11. A method of immersing a wafer substrate as in claim 1, further comprising:

rotating said wafer about an axis normal to said planar surface after said wafer substrate is fully immersed.

12. A method of immersing a wafer substrate as in claim 11 wherein:

said rotating after said wafer substrate is fully immersed is conducted in a range of about from 30 rpm to 150 rpm.

13. A method of immersing a wafer substrate as in claim 12 wherein:

said rotating after said wafer substrate is fully immersed is conducted at about 100 rpm.

14. A method of immersing a wafer substrate as in claim 1 wherein:

moving said holder-wafer conjugate toward said horizontal liquid surface is conducted along a trajectory substantially normal to said horizontal liquid surface.

15. A method of immersing a wafer substrate as in claim 1 wherein:

said piercing speed is slower than said final post-piercing speed.

16. A method of immersing a wafer substrate as in claim 1 wherein:

said moving downward at said piercing speed is conducted at a speed in a range of about from 0.1 mm per second to 15 mm per second.

17. A method of immersing a wafer substrate as in claim 16 wherein:

said moving downward at said piercing speed is conducted at a speed in a range of about from 3 mm per second to 8 mm per second.

18. A method of immersing a wafer substrate as in claim 1 wherein:

said moving downward at said final post-piercing speed is conducted at a speed in a range of about from 25 mm per second to 150 mm per second.

19. A method of immersing a wafer substrate as in claim 18 wherein:

said moving downward at said final post-piercing speed is conducted at about 100 mm per second.

20. A method of immersing a wafer substrate as in claim 1 wherein said liquid bath comprises a plating solution.

21. A method of immersing a wafer substrate as in claim 20 wherein said plating solution comprises an electroless plating solution.

22. A method of immersing a wafer substrate as in claim 20 wherein said liquid bath comprises an electrolytic plating solution.

23. A method of immersing a wafer substrate as in claim 22 wherein said electrolytic plating solution comprises copper ions.

24. In an apparatus comprising a wafer holder for moving a wafer downward into a liquid bath so that a planar surface of said wafer forms an angle with a plane parallel to a liquid surface of the liquid bath, the improvement characterized in that:

said wafer holder is stopable at a selected pause location for a selected pause time; and

said wafer holder is startable at said pause location for moving until a wafer in the wafer holder is fully immersed.