1.  The Field of the Invention
The present invention relates to substrates that include an array of sockets for receiving a ball grid array chip package.  More particularly, the present invention relates to methods for forming an array of sockets and associated electrical traces wherein a relatively thick photoresist layer is used to construct the sockets and traces. 
2.  The Relevant Technology
Frequently, after an integrated circuit is manufactured, a testing process is conducted on the integrated circuit by subjecting it to preselected set of input conditions in order to measure its response or other parameters.  Such testing is often conducted after a semiconductor die has been packaged.  As used herein, the terms xe2x80x9cpackaged chipxe2x80x9d and xe2x80x9cchip packagexe2x80x9d refer to an integrated circuit or another semiconductor structure that has been combined with external and additional structure.  The term xe2x80x9csemiconductor structurexe2x80x9d extends to any device or assembly that includes circuitry defined in a semiconductive material, and further extends to a chip package that includes semiconductive material.  The external and additional structure may be used, for example, for mounting the semiconductor structure to a printed circuit board or other external circuitry, for establishing electrical connection between the semiconductor structure and external circuitry, for improving the ease of handling or transporting the semiconductor structure, or for protecting the semiconductor structure from environment al conditions. 
A common chip package design is a ball grid array package (BGA), in which an array of solder balls are arranged over at least one surface of the chip package in a position and with dimensions that are selected so as to easily establish electrical connection with external circuitry. 
Testing a packaged chip is conventionally accomplished by connecting electrical leads on the packaged chip to testing circuitry in order to determine the reliability and accuracy of the integrated circuit""s response to a predetermined set of input conditions.  Of course, testing is best conducted in a manner such that the tested packaged chips remain in a condition for use without any additional processing.  Likewise, it is important to conduct testing such that the testing device can be easily and quickly reused for testing a subsequent integrated circuit. 
In order to ensure the reusability of both the tested packaged chip and the testing device, there have been developed mounting surfaces on testing devices that are adapted to receive and make electrical connection with a packaged chip.  Typically, such mounting surfaces include an array of electrical contact points that correspond to the pattern of an array of solder balls on a surface of a BGA package. 
An example of a substrate to which a ball grid array package may be temporarily mounted is seen in FIG.  1.  The assembly includes a substrate 10 which may be any one of a wide number of dielectric materials in which a pit or depression 12 is formed.  A via 14 is formed through substrate 10 so as to have an opening at opposite sides of substrate 10.  A conformal metal layer 16 is disposed over selected portions of the surfaces of substrate 10 as seen in FIG.  1.  In particular, conformal metal layer 16 coats the surfaces of pit 12, the inner surfaces of via 14, and provides an electrical trace 18 therebetween.  In this manner, pit 12 is electrically connected with via 14 such that electrical connection may be established with external testing circuitry. 
A ball grid array package 20 is disposed over substrate 10 such that solder ball 22 is aligned with pit 12.  In practice, of course, substrate 10 typically includes a plurality of pits 12 while BGA package 20 includes a corresponding plurality of solder balls 22.  BGA package 20 is pressed down onto substrate 10 such that solder ball 22 partially enters pit 12.  In so doing, solder ball 22 makes electrical contact with conformal metal layer 16. 
Because solder is significantly more malleable than the metal of conformal metal layer 16, solder ball 22 deforms upon being partially inserted into pit 12.  When BGA package 20 is mounted on substrate 10, solder balls 22 are typically not subjected to heat that is sufficient to cause melting or other significant deformation thereof.  Instead, BGA package 20 is ordinarily clamped onto substrate 10 to secure it in place.  After testing is complete, the clamping pressure is removed and solder ball 22 may be retracted from pit 12.  If the method of mounting BGA package 20 to substrate 10 is successful, a tested BGA package 20 typically remains in a condition to be used in the same manner as an untested BGA package. 
Despite the advantages of the assembly seen in FIG.  1, certain problems have been presented during the manufacturing of substrate 10 and the use thereof in testing an integrated circuit.  For example, the formation of pits 12 and vias 14 require a number of individual manufacturing steps.  For example, a drilling, punching, or etching operation must be used to form via 14 and an etching step or other suitable process must be used to form pit 12 in substrate 10 before conformal metal layer 16 may be deposited thereon. 
Another common problem in the industry is that individual solder balls arrayed on a BGA package may vary in size one from another by 20% or more.  This variation may be in the vertical dimension of the solder ball, in its lateral diameter dimension, or in both.  When such variation is experienced, it may be impossible to cause each solder ball 22 to simultaneously contact the corresponding pit 12 in substrate 10.  For example, if one solder ball is significantly shorter than the others, such a solder ball may fail to penetrate pit 12.  Likewise if a solder ball has an exceptionally small diameter, the solder ball may penetrate the pit without making contact with the conformal metal layer.  When this occurs, the testing operation cannot be conducted because electrical signals and power are not delivered to each solder ball. 
Furthermore, when electrical conductive paths, such as electrical trace 18, are formed with relatively small width and thickness dimensions, the resulting resistance of the conductive paths may be greater than ideal values, particularly when using materials with less than optimum conductivity characteristics.  However, current practices for forming mounting substrates for testing devices involve inherent limitations as to the maximum thickness of the electrical conductive paths that may be formed.  Moreover, increasing the width of electrical paths in order to reduce resistance values may not be a suitable solution.  In particular, wide electrical traces may have correspondingly high capacitance characteristics, which may induce noise in the testing operation.  In addition, the physical dimensions of the chip package and the mounting substrate may further constrain the width dimensions of the electrical traces. 
In view of the foregoing, there is a need in the art for a socket that can reliably receive a solder ball of a BGA package such that the BGA package remains reusable.  It would be an advancement in the art to provide such a socket that is also capable of making electrical contact with solder balls of varying sizes.  It would be a further advantage to provide methods of manufacturing such sockets in a cost-effect manner.  There is also a need in the art for a socket and associated structure that may be formed with dimensions that produce relatively low electrical resistance values. 
The present invention is directed to socket assemblies that are configured to receive a solder ball of a ball grid array packet and methods for forming the same.  A socket assembly is defined herein as a structure that includes at least a socket and a ball contact structure at least partially surrounding the socket.  The socket assemblies of the invention are typically formed by using a relatively thick photoresist layer to form a pattern that corresponds to the desired shape of the socket ball contact structure and electrical trace.  An array of socket assemblies are ordinarily arranged over a surface of an interposer which may be used to electrically connect a ball grid array package with external circuitry contained in a testing device. 
In all embodiments of the invention, the socket assemblies preferably include a ball contact structure having an inner sidewall, an outer sidewall, a base disposed on a substrate, and a top surface opposite the base.  The socket assembly also includes a socket defined by the inner sidewall of the ball contact structure and by the substrate.  The socket is at least partially surrounded by the ball contact structure and may be completely circumscribed thereby.  In general, the ball contact structure comprises a raised construction that is disposed on the substrate while the socket includes an opening or a void that is substantially defined by the ball contact structure.  The socket has dimensions selected such that a solder ball of a ball grid array package may be partially inserted therein.  In addition, the socket assemblies generally include an electrical trace extending away from the ball contact structure and disposed on the substrate. 
In a first embodiment of the invention, the ball contact structure and the electrical trace are substantially composed of one or more conductive materials, which are preferably metals.  The socket assembly is formed by first providing a substrate having a substantially planar surface.  Next, a seed metal layer is formed over the substantially planar surface and is patterned so as to correspond to the ball contact structure and the electrical trace that are to be subsequently formed thereover.  The patterned seed metal layer is formed by first coating the substrate with a thin layer of metal.  Next, a masking structure is formed on the conductive layer from a photoresist material or another suitable material in a pattern that corresponds to the socket assembly that is to be formed.  In particular, the masking structure remains on the thin layer of metal at the region over which the ball contact structure and the electrical trace will later be formed.  The exposed portion of the thin layer of metal is etched and the masking structure is removed, thereby forming the patterned seed metal layer. 
A photoresist layer having a thickness preferably in a range from about 20 microns to about 450 microns is then spun onto the substrate and the patterned seed metal layer.  Alternatively, successive layers of photoresist material may be spun on to achieve the desired thickness in a process known as xe2x80x9cresist stackingxe2x80x9d.  The photoresist layer is exposed and patterned, whereby photoresist material is removed from a region generally aligned with the remaining portion of the underlying patterned seed metal layer. 
After patterning of the photoresist layer, the substrate is placed in an electrolytic bath wherein an electroplating process is conducted to form a metal layer over the seed metal layer and within the patterned opening.  This electroplated metal layer is to constitute the bulk of the ball contact structure and the electrical trace.  The electroplating process continues until the electroplated metal layer has a desired thickness.  Finally, the photoresist layer is stripped from the substrate, including from the socket that is adjacent to the ball contact structure. 
In a second embodiment of the invention, a substrate having a substantially planar surface is provided and a photoresist layer is spun onto the surface.  Preferably, the photoresist layer has a thickness in a range from about 20 microns to about 450 microns.  Alternatively, two or more successive layers of photoresist material may be spun on to achieve the desired thickness.  A conductive layer, which preferably includes at least one metal, is then formed on the photoresist layer.  The conductive layer preferably has a thickness that is significantly smaller than the thickness of the photoresist layer.  A masking structure is formed on the conductive layer from a photoresist material or another suitable material in a pattern that corresponds to the socket assembly that is to be formed.  In particular, the masking structure remains on the conductive layer over the region that will later constitute the ball contact structure and the electrical trace.  Portions of the conductive layer and the photoresist layer are consecutively removed such that essentially the only material remaining on the substrate is that which had been positioned under the masking structure.  In this manner, a ball contact structure and an electrical trace are formed, each of which comprise a dual layer structure having an underlying photoresist layer and an overlying and relatively thin conductive layer. 
A third embodiment of the invention involves forming a ball contact structure and an associated socket in the same manner as in the second embodiment.  However, the electrical trace is not formed at the same time as the ball contact structure, but is instead formed afterwards.  Accordingly, the third embodiment proceeds after a ball contact structure of the second embodiment has been formed, at which point a first conformal metal layer is formed over the exposed surfaces.  An anisotropic etching process, known in the art as a spacer etch, is used to remove part of the first conformal layer, while leaving a portion of the first conformal metal layer on the sidewalls of the ball contact structure.  Next, a second conformal metal layer is formed over the substrate, the ball contact structure, and the remaining portion of the first conformal layer. 
A patterned photoresist layer is provided over selected portions of the second conformal metal layer in a position that corresponds to the electrical trace that is to be formed.  During a subsequent anisotropic etch, the photoresist layer acts as an etch mask such that a part of the second conformal metal layer is removed, while a portion of the second conformal metal layer remains over the substrate and forms an electrical trace.  This electrical trace is electrically connected to the ball contact structure and has a thickness that is significantly less than the thickness of the ball contact structure. 
In view of the foregoing, it can be appreciated that the present invention provides methods for forming a ball contact structure and an associated socket without requiring the formation of pits, depressions, or vias into the substrate.  Instead, a substrate having a substantially planar surface may be used, thereby eliminating the additional substrate patterning steps that have been common in the prior art.  In particular, the present invention uses a relatively thick photoresist layer to form the socket assemblies so as to improve the cost-effectiveness of the manufacturing process over that which has been experienced in the past.  In addition, the electrical traces may be formed with thicknesses much greater than has been previously possible, thereby allowing the electrical traces to exhibit relatively low electrical resistance values. 
In addition, the present invention includes certain features that permit the sockets of the invention to adequately make electrical contact with solder balls of varying sizes.  For example the invention optionally includes a plurality of ball penetration structure integrally formed on the ball contact structure.  Preferred ball penetration structures include ribs, fins, blades, and the like that are integrally formed on the inner sidewall of the ball contact structure.  These ball penetration structures protrude radially inward from the inner sidewall into the socket.  Typically, a ball penetration structure according to the invention may be described as having a longitudinal axis that is substantially radially aligned with respect to the socket and that is generally perpendicular to a tangent of the inner sidewall at the junction of the inner sidewall and the ball penetration structure.  The width of the ball penetration structure, measured in a direction perpendicular to the longitudinal axis thereof and parallel to the plane defined by the substrate, is selected such that the ball penetration structure may easily cut into a solder ball and be embedded therein. 
These optional ball penetration structures may be included in the socket assemblies so as to facilitate electrical connection between a solder ball and a socket assembly without significantly deforming the solder ball in the vertical direction.  In particular, as the solder ball is pressed into the socket, the ball penetration structures first make contact with the solder ball and become at least partially embedded therein.  Because the ball penetration structures have a width that is significantly smaller than the diameter of the solder ball, they are designed or sized to penetrate the solder ball without causing significant deformation of the solder ball, especially in the vertical direction.  Preferably, the ball penetration structures cut into the lateral portions of the solder ball and not into the spherical xe2x80x9ccrownxe2x80x9d, or lower portion.  Such ball penetration structures permit an array of sockets to make contact with an array of solder balls on a BGA package even if there is variation in the sizes of the individual solder balls. 
The socket assemblies of the invention allow a ball grid array package to be easily placed in electrical connection with external circuitry while preserving the ability of the BGA package to be reused in its final intended product.  Moreover, the socket assemblies of the present invention are sufficiently raised above the surface of the substrate such that a clearance remains between the bottom of the socket and the solder ball.  This clearance ensures that the solder ball is not vertically deformed by making contact with the substrate, with the result that the crown of the solder ball is not flattened. 
In some circumstances, for manufacturing efficiency reasons, it may be advantageous to modify the first embodiment in order to reduce the thickness to which the electroplated metal layer is formed.  Accordingly, in a fourth embodiment of the invention, a pit or a depression is formed in a substrate prior to conducting the remainder of the processing steps of the first embodiment.  Subsequently, the socket assembly is formed over the substrate in a position such that the socket is opened over the depression.  This configuration allows a reduction of the thickness of the electroplated metal layer by an amount substantially equal to the depth of the depression.  In particular, the socket maintains an adequate aspect ratio such that a clearance is maintained between a solder ball and the substrate, while the depth of the electroplated metal layer is reduced. 
A fifth embodiment of the invention is similar to the fourth embodiment, with the variation that the depression is replaced with a via extending through the substrate.  This allows the electroplated metal layer to be significantly thinner than may be possible without the via.  In the fifth embodiment, the via is optionally plated with a conductive layer, which may then replace the electrical trace that otherwise extends across the surface of the substrate. 
Thus, it will be appreciated that the present invention provides a socket assembly that preserves the reusability of solder balls, is capable of being used with solder balls of varying dimensions, and may be formed by cost-effective manufacturing processes.