Electroplating apparatus and method

An electroplating apparatus is provided with a metal target and a device for supporting a semiconductor wafer (or other workpiece) in an electroplating solution. The target (anode) may be located relatively far from the wafer surface (cathode) at the beginning of the plating process, until a sufficient amount of metal is plated. When an initial amount of metal is built up on the wafer surface, the target may be moved closer to the wafer for faster processing. The movement of the target may be controlled automatically according to one or more process parameters.

FIELD OF THE INVENTION
 The present invention relates to a system for electroplating the surfaces
 of semiconductor wafers and other workpieces. More particularly, the
 present invention relates to an electroplating apparatus and method that
 achieves improved performance with respect to thickness uniformity and
 rate of metal deposition.
 BACKGROUND OF THE INVENTION
 It is known to electroplate the surfaces of semiconductor wafers. It has
 been difficult, however, to obtain an electroplated layer of uniform
 thickness. It has been especially difficult to achieve the desired
 thickness uniformity at a high rate of metal deposition. Known systems for
 electroplating semiconductor products are described in U.S. Pat. Nos.
 5,833,820 (Dubin), 5,670,034 (Lowery), 5,472,592 (Lowery), and 5,421,987
 (Tzanavaras).
 SUMMARY OF THE INVENTION
 The present invention relates to an apparatus for electroplating a
 semiconductor product. The apparatus includes a support device for
 supporting the product in an electroplating solution, an electrical
 circuit for applying an electrical potential across the electroplating
 solution, and a control device for reducing the current distance to the
 product through the solution after an initial amount of conductive
 material is electroplated on the product surface. The semiconductor
 product may be, for example, a semiconductor wafer or chip. Integrated
 circuits may be formed in the product if desired.
 According to one aspect of the invention, the support device includes
 conductive contacts. The contacts may be used to connect the product to
 the electrical circuit.
 According to another aspect of the invention, the control device includes a
 mechanism for moving a metal target (anode) toward the electroplated
 product. In an alternative embodiment of the invention, the product may be
 moved toward the anode.
 According to another aspect of the invention, a processor is used to
 operate the control device in response to data correlated to the
 electroplating process. The input data may be functionally related or
 correlated to elapsed electroplating time, the resistance of the product
 in the electroplating solution, the optical characteristics of the
 product, the surface capacitance of the product, etc.
 The present invention also relates to a method of electroplating the
 surface of a semiconductor wafer. The method includes the steps of using
 an electrode to electroplate an initial amount of conductive material on
 the wafer surface, then changing the distance between the electrode and
 the wafer surface, and then using the electrode to electroplate an
 additional amount of material on the wafer surface. According to a
 preferred embodiment of the invention, at the start of the process, while
 the resistance of the wafer is significant, thickness uniformity is
 promoted by locating the target far from the wafer. Then, when the wafer
 resistance is reduced by the initial amount of electrodeposited metal,
 higher plating efficiency may be obtained by moving the target closer to
 the wafer.
 According to another aspect of the invention, the wafer may be provided
 with a refractory seed layer. The seed layer contains metal and adheres to
 the semiconductor wafer material. The resistance of the seed layer is
 greater than that of the electrodeposited metal.
 Thus, according to a preferred embodiment of the invention, a metal target
 (anode) is located relatively far from the wafer (cathode) at the
 beginning of the plating process, until a sufficient amount of metal is
 plated on the wafer surface. Once the metal is built up on the wafer
 surface, the target is moved closer to the wafer for faster processing.
 As explained in more detail below, before the metal is built up on the
 wafer surface, the high resistance of the seed layer is a significant
 factor. The electrical potential near the contacts on the edges of the
 wafer is greater than the potential at the center of the wafer.
 Consequently, according to the invention, the target and the wafer are
 separated from each other to increase the resistance of the electroplating
 solution (the bath). A relatively high bath resistance mutes the
 significance of the potential difference in the radial direction of the
 wafer. Metal built up on the wafer surface has less resistance than the
 seed layer, such that the difference in potential across the surface of
 the wafer becomes less significant. Eventually, the target can be moved
 closer to the wafer (to reduce the bath resistance and increase the
 deposition rate) without impairing plating uniformity.
 These and other features and advantages of the invention will become
 apparent from the following detailed description of preferred embodiments
 of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Referring now to the drawings, where like reference numerals designate like
 elements, there is shown in FIG. 1 an electroplating apparatus 10
 constructed in accordance with a preferred embodiment of the present
 invention. The apparatus 10 has a tank 12 containing electroplating
 solution 14, a wafer support 16 for supporting a wafer 18 in the solution
 14, and a metal target (anode) 20. The wafer support 16 may have metal
 clips 22, 24 for holding the wafer 18 in the desired position. An
 electrically conductive seed layer 26 may be formed on the wafer surface
 28. The seed layer 26 may be electrically grounded through the clips 22,
 24 and suitable wires 30.
 In operation, voltage is applied to the target 20 by a control device 32.
 The electrical potential causes current to flow from the target 20,
 through the solution 14, through the seed layer 26, and through the clips
 22, 24 to the grounding wires 30. The electroplating process causes a
 metal layer 34 (FIG. 2) to form on the seed layer 26. The process may be
 continued until the metal layer 34 achieves the desired thickness. The
 electroplated wafer 18 may then be removed from the tank 12 for further
 processing.
 The rate at which metal 34 is deposited on the wafer surface 28 is
 proportional to the combined resistance of the solution 14 and the seed
 layer 26, as follows:
EQU I=A/(R.sub.1 +R.sub.2),
 where I is the metal deposition rate, A is a constant, R.sub.1 is the
 resistance of the solution 14, and R.sub.2 is the resistance of the wafer
 18. The solution resistance R.sub.1 depends on (1) the distance D between
 the target 20 and the wafer surface 28 and (2) the conductivity of the
 solution 14. For any particular point on the wafer surface 28, the wafer
 resistance R.sub.2 depends on (1) the distance from that point to the
 electrical contacts 22, 24 and (2) the conductivity of the wafer 18.
 At the start of the electroplating process (that is, before any metal 34 is
 formed on the seed layer 26), the wafer resistance R.sub.2 is a
 significant factor with respect to the deposition rate I. The resistance
 of the seed layer 26 may be substantial. Consequently, at the start of the
 process, the value of R.sub.2 may vary substantially as a function of
 radial position on the wafer 18. That is, the value of R.sub.2 would tend
 to increase as distance increases from the clips 22, 24. To mute the
 significance of the wafer resistance R.sub.2 and to thereby improve the
 thickness uniformity of the initially deposited metal 34, the target 20
 initially may be located relatively far from the wafer 18 (FIG. 1). As the
 conductive metal 34 is formed on the seed layer 26, the wafer resistance
 R.sub.2 becomes much less significant relative to the solution resistance
 R.sub.1. After the initial amount of metal 34 is formed on the wafer 18,
 the target may be moved closer to the wafer 18 to reduce the solution
 resistance R.sub.1 and to increase the deposition rate I.
 The target 20 may be moved by a suitable mechanism 36 controlled by the
 control device 32. In an alternative embodiment of the invention, shown in
 FIG. 3, the wafer 18 may be moved closer to the target 20. In another
 alternative embodiment, (not shown) more than one anode may be
 employed--one relatively far away from the wafer 18 to form the initial
 amount of metal on the wafer 18 and the other located relatively close to
 the wafer 18 to form the rest of the metal layer 34 at a relatively high
 deposition rate.
 The control device 32 (FIG. 2) may be operated by a suitable microprocessor
 38 (FIG. 1) or the like. Signals 40 may be input to the processor 38
 representative of elapsed electroplating time, the measured resistance of
 the wafer 18, the optical characteristics (e.g., reflectivity) of the
 wafer 18, and/or the surface capacitance of the wafer 18. The input
 signals 40 may be generated by a suitable input device 42, such as a clock
 or a suitable measuring device. The resistance of the wafer 18 may be
 determined by measuring the voltage between the contacts 22, 24. The bulk
 resistance of the wafer 18 also may be determined off-line, for example,
 by a four-point probe device (not shown).
 The processor 38 may have a look-up table and/or an algorithm that
 correlates elapsed electroplating time to metal thickness and/or
 deposition rate for known solutions 14 and target positions. Feedback
 signals 46 representative of the position of the target 20 (and/or the
 distance D between the target 20 and the wafer 18) may be provided to the
 processor 38 by the controller 32. The processor 38 may be programmed to
 send operating signals 44 to the controller 32 to automatically move the
 target 20 closer to the wafer 18 when a predetermined amount of metal 34
 is formed on the seed layer 26.
 The motion of the target 20 toward the wafer 18 may be continuous or
 gradual, and, the motion may be programmed to optimize plating efficiency
 while achieving the desired uniformity. In an alternative embodiment of
 the invention, the target 20 may be moved in a stepwise fashion toward the
 wafer 18 at a predetermined time in the process or when a predetermined
 amount of metal 34 is determined to have been formed on the wafer 18.
 In a preferred embodiment of the invention, the target 20 may be located
 about five centimeters from the wafer surface 28 in the start position,
 (FIG. 1), and about one to two centimeters in the high efficiency plating
 position (FIG. 2). The present invention should not be limited, however,
 to the preferred embodiments described and illustrated in detail herein.
 The solution 14 may be arranged to deposit copper, platinum, gold or
 another suitable material on the wafer 18. The seed layer 26 may be formed
 by a known chemical vapor deposition (CVD) process. The seed layer 26 may
 be, for example, a refractory and metal composite material that adheres to
 the wafer surface 28. The metal component of the seed layer 26 may be the
 same as or different than the plated metal material 34.
 If desired, the tank 12 may be provided with a cascade structure (not
 shown) to ensure that fresh solution 14 is made available to the wafer
 (cathode) 18. Other suitable means, such as a diffuser or baffle plate,
 for agitating and flowing the solution 14 against the wafer 18 may be
 employed, if desired. Although the tank 12 is shown with only one support
 device 16, the invention may be employed with more than one support device
 16 per tank 12. If desired, a number of wafers 18 may be electroplated in
 the same solution 14 simultaneously. Suitable electrodes 20, 22, 24 may be
 provided for each wafer 18.
 The above descriptions and drawings are only illustrative of preferred
 embodiments which achieve the features and advantages of the present
 invention, and it is not intended that the present invention be limited
 thereto. Any modification of the present invention which comes within the
 spirit and scope of the following claims is considered part of the present
 invention.