Abstract:
A process for producing a terminal metal pad structure electrically interconnecting a package and other components. More particularly, the invention encompasses a process for producing a plurality of corrosion-resistant terminal metal pads. Each pad includes a base pad containing copper which is encapsulated within a series of successively electroplated metal encapsulating films to produce a corrosion-resistant terminal metal pad.

Description:
This application is a divisional of U.S. patent application Ser. No. 09/184,169, filed on Nov. 2, 1998, now U.S. Pat. No. 6,083,375. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to the high-density, corrosion-resistant, terminal metal pads for semiconductor thin film packages. 
     BACKGROUND OF THE INVENTION 
     The increasing input/output (I/O) interconnection density requirements for thin film packages suggest that the current techniques for producing terminal metal pads, such as evaporation through masks and screening, will not be able to meet the tighter ground rules of the advancing technology. This concern is expected to be especially true for land grid array (LGA) type interconnect applications where the typical ground rules for high end packages are in the range of 0.2 to 1.2 millimeters for pad diameters, with a minimal pitch in the range of 0.25 to 1.3 millimeters. 
     Terminal metal pads require certain characteristics to function as contact points for thin film packages. Typically, a wetting material such as gold (Au) is needed as an outermost metal film to which an electrical connection is provided. Also required is a metal such as nickel (Ni) which serves as a barrier and provides strength. Additionally required as a processing necessity is copper (Cu). Copper is a very ductile material, and acts as a cushion to absorb the residual stresses from a film such as nickel. Unfortunately, the required presence of a film such copper raises corrosion concerns: copper is susceptible to corrosion when it combines with the moisture present in the air. Because copper is typically exposed at least in the sidewalls of a terminal metal pad, the presence of s copper makes the pads susceptible to corrosion. 
     The currently available processes for producing terminal metal pads may include the following limitations: 
     (1) they are applicable only for pads in a low-density pattern and cannot be scaled to high-density patterns; and 
     (2) they produce multilayer patterns only in the vertical direction, thus leaving exposed the sidewalls of the lower, usually corrosion-suspect metals. 
     Currently available processes for producing terminal metal pads such as: (a) evaporation through a is contact mask; and (b) screening through a mask before ceramic sintering, followed by electroless plating, cannot produce pads in the high-density patterns necessary in modern technology. 
     To meet the tight packaging requirements required today and in the future, the process of photolithographically patterning a film is the only conventionally known process for producing the high-density pattern of closely spaced pads needed. Using this process, a photolithographic pattern is formed on top of a metal film structure, which may include multiple metal films formed on top of one another. The photolithograph pattern includes a masked region and an exposed region. Next, an etching procedure or series of procedures may be used to remove the film or films in the exposed region and produce a plurality of discrete terminal pads. In this manner, however, the underlying metal films are exposed laterally on the sidewalls of the terminal pads. Therefore, the terminal metal pad may have a structure whereby the films are layered only in the vertical direction and each film is exposed in the lateral direction along the sidewalls. Using this process, when copper is used as an underlying cushioning film, it is included as one of the films exposed in the sidewall, producing a corrosion susceptibility concern in the pads. 
     The contemporary processes available which can provide sidewall coverage use the electroless plating approach. Traditionally, the process of electroless plating is limited in its versatility; only a limited number of metals can be conveniently deposited using this procedure. Electroless plating is also a relatively expensive, time-consuming process. In addition, the process control for electroless plating is rather complicated due to the relatively narrow process window. Because of poor uniformity characteristics and poor process control, electroless plating is an approach not suited to high-density patterns produce by photolithography. 
     In the current technology for producing metal terminal pads, the elimination of copper from the terminal pads does not loom as a viable alternative. Without copper, the pads would be susceptible to failure due to cracking of the substrate and/or the more brittle metals that are used to fabricate the pad. Contemporary processes for producing pads include the limitations discussed above. A possible alternative process, for producing corrosion-free pads capable of meeting modern packaging density requirements, would be to deposit a passivation layer on top of the metal pads after the metal pads have been formed. Once deposited, the passivation layer must be patterned and openings must be created to expose only the top of the terminal pads. The shortcomings of this process include the addition and patterning of a separate passivation layer. Such passivation layers require additional processing materials and the additional time and expense associated with forming and patterning the passivation layer. Furthermore, a passivation layer may not be an option for some packaging technologies including those applications for which the terminal pads need to be raised and cannot be “buried.” 
     What is needed is a process which overcomes the shortcomings of contemporary processing technology options, and provides a process for producing tightly packed terminal metal pads capable of meeting the increasing input/output interconnect density requirements. The individual pads produced by this process must be corrosion resistant and must include the strength, wettability, ductility, and cushioning characteristics required to enable the terminal metal pad to provide a reliable connection to an outside component. The process used to form the contact pads will most desirably be inexpensive, fast, reliable, and versatile. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the shortcomings of the prior art and provides a tightly packed terminal metal pads which are corrosion resistant and include the strength, wettability, ductility, and cushioning requirements for modern packaging needs. The present invention also provides other metal structures such as interconnect lines, which exhibit those properties. The process used to form the terminal metal structures of the present invention provides an inherent passivation scheme; the copper used in forming the terminal metal pads is not expose to the environment. A separate passivation layer is not needed, because the copper film is encapsulated within the other metal films which combine to form the terminal metal pad. 
     The terminal metal structures of present invention are formed by a process which includes an electroplating process to complete the formation of the terminal pads after a photolithographic process, able to meet modern packaging density needs, has been used to form a pattern of base pads from a seed layer. The electroplating process provides terminal metal structures having superior uniformity and is be used to deposit a succession of metal films onto the base metal pad. 
     The terminal metal pads of the present invention each include a base metal pad and several encapsulating films which provide corrosion protection by covering the base metal pad in both the vertical and horizontal directions. The structure thus produced has a multi-layered pattern in the vertical as well as the horizontal plane (in the plane of the pads). The corrosion-suspect, lower-level metals are completely buried under the corrosion-resistant, higher-level metal materials which are formed as an inherent part of the process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 is a cross-sectional view of a terminal metal pad formed on a substrate connected to a power supply; 
     FIG. 2 is a cross-sectional view of a terminal metal pad formed on a substrate; 
     FIGS. 3 through 8 depict the processing sequence used to form a terminal metal pad of the present invention and, more specifically, FIG. 3 is a cross-sectional view of a seed layer formed on a substrate; 
     FIG. 4 is a cross-sectional view of an alternate embodiment of a seed layer formed on a substrate; 
     FIG. 5 is a cross-sectional view showing a base metal pad; 
     FIG. 6 is a cross-sectional view showing the base metal pad and one encapsulating film; 
     FIG. 7 is a cross-sectional view showing the base metal pad and two encapsulating films; 
     FIG. 8 is a cross-sectional view showing a base metal pad with three encapsulating films; 
     FIG. 9 is a cross-sectional view of an alternate embodiment of the present invention having five encapsulating films; 
     FIG. 10 is a plan view of two exemplary embodiments of the present invention formed on a substrate; and 
     FIG. 11 is a cross-sectional view taken along line  11 — 11  of FIG. 10 after the electroplated encapsulating films have been formed showing two exemplary embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a cross-sectional view showing an exemplary embodiment of a terminal metal pad  7  of the present invention formed on bottom surface  12  of substrate  2 . The substrate  2  may be of ceramic or polymeric materials, and includes two opposed surfaces such as bottom surface  12  and top surface  6 . In a preferred embodiment, the substrate  2  may form part of a semiconductor chip package, and the terminal metal pad  7  may be one of a plurality of terminal metal pads formed on the bottom surface  12  of the substrate  2 . 
     FIG. 1 shows an apparatus in which a base metal pad  14  is electrically connected to power supply  10  and conditioned to have films electroplated onto the base metal pad  14 . The process of formation of base metal pad  14  will be discussed in conjunction with FIGS. 3-8. On the top surface  6  of substrate  2 , a blanket shorting metal film  4  is disposed. The blanket shorting metal film  4  may be formed onto the substrate  2  using any process suitable in the art, and may be composed of any metal film suitable for providing electrical contact. Blanket shorting metal film  4  is connected to power source  10  by a contact  8 . A “through via”  22  extends through substrate  2  and provides an electrical connection between the blanket shorting metal film  4  and the base metal pad  14 . With the electrical connection provided as shown, a succession of electroplated films ( 16 ,  18 , and  20 ) may be formed onto base metal pad  14  by an electroplating process common to the art. 
     In the exemplary embodiment, the terminal metal pad  7  includes the base metal pad  14  and three successive encapsulating films formed by electroplating. First electroplated encapsulating film  16  covers the metal base pad  14 . Second electroplated encapsulating film  18  covers the first encapsulating film  16 . A third electroplating encapsulating film  20  covers the second electroplated encapsulating film  18 , to form the three encapsulating film structure of the exemplary embodiment. 
     Although the exemplary embodiment is shown having three successively electroplated films encapsulating the base metal pad  14 , it can be seen that additional electroplated films may be used to provide subsequent metal films covering the base metal pad  14  to form the terminal metal pad  7 . Alternatively, less than three electroplated films may be used to form the terminal metal pad  7 . Any electroplating process suitable in the art may be used to form the succession of encapsulating films onto the terminal metal pad  7 . 
     FIG. 2 is a cross-section showing the structure as in FIG. 1 after the blanket shorting metal film (film  4  in FIG. 1) has been removed from the top surface  6  of substrate  2 . In this manner, FIG. 2 shows an exemplary embodiment of terminal metal pad  7  after the electrical connection, required for electroplating, has been removed. Thus, FIG. 2 shows an exemplary embodiment of a terminal metal pad  7  disposed on bottom surface  12  of substrate  2  in its final form. In the preferred embodiment, the individual pad shown would represent one of a plurality of pads formed simultaneously in a high-density pattern on bottom surface  12 . 
     FIG. 3 shows a cross-section of the substrate  2  with a seed layer  13  formed on the bottom surface  12 . In the exemplary embodiment of FIG. 3, seed layer  13  may be a singular, unitary, monolithic film  24 . In an exemplary embodiment, seed layer film  24  may be a metal, such as aluminum (Al), titanium (Ti), chromium (Cr), tungsten (W), molybdenum (Mo), or copper (Cu) and alloys thereof, or any other film suitable in the art. Seed layer film  24  may be formed on bottom surface  12  by any process suitable to the art, such as sputter deposition or evaporation. The thickness  38  of seed layer  13  may be any appropriate thickness required by the chosen application. Through via  22  provides electrical connection as shown in FIG.  3 . 
     FIG. 4 once again shows the substrate  2  and through via  22  which provides electrical connection as in FIG.  1 . FIG. 4 represents a preferred embodiment of the present invention, in which seed layer  13  is formed from two separate films  26  and  27 , which combine to form the seed layer  13 . The first deposited metal film  26  of the seed layer  13  may be chromium (Cr), but any suitable film may be used. The thickness  41  of film  26  may be on the order of 50-2000 Angstroms. 
     In the preferred embodiment, as shown in FIG. 4, the second deposited metal film  27  also forms part of the seed layer  13 . Second deposited metal film  27  may be copper, but other suitable film materials may be used. The thickness  44  of film  27  may be on the order of 500-50,000 Angstroms. In an alternate embodiment, first deposited metal film  26  may be titanium and second deposited metal film  27  may be tungsten. The total thickness  38  of seed layer  13  is the sum of the thicknesses  44  and  41  of the individual films which combine to form the seed layer. As with the singular film embodiment, the first and second deposited metal films  26  and  27 , respectively, may be formed on bottom surface  12  by any process suitable to the art. 
     Now turning to FIG. 5, a base metal pad  14  is shown as being formed from a seed layer, such as seed layer  13  of FIG.  3 . Base metal pad  14  may be formed by any process capable of providing the high density of pads required in the art. In the preferred embodiment, a photolithographic process may be used to produce tightly spaced pads of small diameter. Using a photolithographic process, a photosensitive film (not shown) is formed on top of the metal seed layer, such as seed layer  13  of FIG. 3, and parts of the film are exposed to light through a photomask. A pattern is formed in the photolithographic film by a developing mechanism, the pattern including an exposed region and a masked area. Next, the exposed portion of the seed layer (such as film  13  in FIG. 3) may be selectively removed by any process suitable to the art such as wet chemical etching, reactive ion etching or physical ion bombardment. In the preferred embodiment, wet chemical etching may be used. 
     After the formation of the pattern within the seed layer is complete, the photolithographic film is removed from the structure, to produce a plurality of discrete metal base pads, such as base metal pad  14  as in FIG.  5 . Base metal pad  14  is formed on bottom surface  12  of substrate  2 . Base metal pad  14  includes a surface  30  and sidewalls  32 . The height of the base metal pad  14  is substantially the same height as the thickness  38  of the seed layer  13  from which it was formed. The base metal pad  14  is positioned in a pre-determined location on the bottom surface  12  which is over through via  22 . Through via  22  provides electrical connection from base metal pad  14 , through the substrate  2 , and to a power supply  10 , as shown in FIG.  1 . 
     FIG. 6 shows an exemplary embodiment of the present invention after an electroplated first encapsulating film  16  has been formed on the base metal pad  14 . The electroplated first encapsulating film  16  may be formed by any electroplating process suitable in the art. Electrical connection to a power supply is provided by through via  22  as shown in FIG.  1 . Electroplated first encapsulating film  16  is formed to cover surface  30  of base metal pad  14  as well as sidewalls  32 , encapsulating the base metal pad  14 . First encapsulating film  16  has an outer surface  17 . In the preferred embodiment, first encapsulating film  16  may be a metal of a material determined by product application. 
     FIG. 7 shows the exemplary embodiment of the present invention after the next step in the process sequence. Second encapsulating film  18  is formed over outer surface  17  of first encapsulating film  16 . Second encapsulating film  18  has an outer surface  19 . As in the previous process step, any process of electroplating suitable in the art may be used to form second encapsulating film  18 . As with the first encapsulating film  16 , the second encapsulating film  18  may be a metal of a material determined by product application. 
     FIG. 8 shows the exemplary embodiment of the terminal metal pad  7  of the present invention after a third encapsulating film  20  has been formed over the outer surface  19  of second encapsulating film  18 . Third encapsulating film  20  has an outer surface  21 . As with the previous electroplated films, third encapsulating film  20  may be formed by any electroplating procedure suitable to the art. FIG. 8 shows a completed structure of an exemplary embodiment of the present invention having three encapsulating films. In the exemplary embodiment formed of three encapsulating films, the third, outermost film may be gold. It can be seen that the structure includes three layers of encapsulating films in both the vertical direction  50  and the horizontal direction  51 . It can also be seen that base metal pad  14  is completely encapsulated by the encapsulating films. It can be further seen that each encapsulating film is completely encapsulated by the subsequently deposited encapsulating film which covers it. 
     In the embodiment of the three encapsulating film structure as shown, a preferred embodiment may include a first encapsulating film being copper, a second encapsulating film being nickel, and a third and outermost encapsulating film being gold. In another embodiment, the outermost film may be palladium (Pd). It can be seen by one skilled in the art that the three encapsulating film embodiment of the present invention may use different materials in different combinations to form the three-film structure. 
     The present invention also contemplates alternate embodiments which incorporate additional electroplated films, or less than three electroplated films. The terminal metal pad  7 , which includes a base metal pad  14 , may have any number of subsequently electroplated films covering it. In addition, the films used for each encapsulating layer may be different films as determined by the application. In the preferred embodiment of any structure, the outermost film may be gold or palladium, but other films may be used depending on the connection required. 
     FIG. 9 shows a cross-section of an alternate embodiment of the present invention having five encapsulating films. In this alternate embodiment, base metal pad  14  is formed on bottom surface  12  of substrate  2 . Through via  22  provides electrical connection as shown in FIG. 1, to an electrical power supply which enables the electroplated deposition of subsequent films to cover base pad  14 . In this alternate embodiment, first encapsulating film  16  covers base metal pad  14 , and second encapsulating film  18  covers first encapsulating film  16 . Likewise, third encapsulating film  20  covers second encapsulating film  18 , and fourth encapsulating film  33  covers third encapsulating film  20 . The outer encapsulating film  35  covers the fourth encapsulating film  33 . 
     In the embodiment using a five encapsulating film structure, the preferred sequence of deposited films may be copper, nickel, gold, nickel, and gold. It can be seen by one skilled in the art that other metals including chromium (Cr) and cobalt (Co) may be used, and that they may be used in various sequences. In the preferred embodiment, the outermost encapsulating film  35  may be gold or palladium. 
     The terminal metal pad  7  shown in FIG. 9 is typically formed as one of a plurality of similarly formed discrete terminal metal pads (not shown), all formed simultaneously on bottom surface  12  of substrate  2  by the process described in conjunction with FIGS. 3 through 8. 
     Each terminal metal pad  7  may include a base metal pad thickness  38  and a diameter  37 . In the preferred embodiment, the diameter  37  may be on the order of 0.5 millimeters. The spacing (not shown) between adjacent terminal metal pads may be on the order of 0.2 to 0.3 millimeters in the preferred embodiment, forming a tightly packed, dense packaging structure. Also in the preferred embodiment, the film thickness  40  of an electroplated film may be on the order of 100-50,000 Angstroms. 
     After the succession of electroplated films have been formed in over the base metal pad  14 , the blanket shorting layer metal film (film  4  as in FIG. 1) may be removed. Electrical connection is no longer required because subsequent electroplating is not needed. Any process for removing such a metal film suitable in the art may be used. Alternatively, the shorting layer metal film may be left on the top surface  6  of the substrate  2 , and could be used for subsequent thin film processing. 
     FIG. 10 is a plan view showing exemplary embodiments of two different structures formed by the present invention. On surface  60  of substrate  68 , both a substantially circular terminal metal pad  65  and an interconnect metal line  66  are formed. In an alternate embodiment, the terminal metal pad  65  may be shaped differently. For example, the terminal metal pad  65  may be substantially rectangular. Each structure is connected through the substrate  68  by means of a via. Terminal metal pad  65  is connected through the substrate  68  by via  61  and interconnect metal line  66  is connected through the substrate  68  by via  62 . The two different exemplary embodiments (structures  65  and  66 ) of the present invention are formed simultaneously, both through the formation of the metal seed structure, and the subsequent electroplating steps. In the embodiment shown in FIG. 10, the two different structures  65  and  66  are each defined during the patterning step which forms both the terminal metal pad structure  65  and the interconnect metal line structure  66 , simultaneously, using a single photomask. With respect to substantially circular terminal metal pad  65 , the electroplated encapsulating metal films (not shown) will be formed to cover the sidewall  78 , which extends circumferentially around the terminal metal pad  65 . With respect to interconnect metal line  66 , the electroplated encapsulating metal films (not shown) will be formed to cover the two opposed sidewalls  79  as shown. 
     FIG. 11 is a cross-sectional view taken along line  11 — 11  of FIG.  10 . and after the electroplated encapsulating films have been formed FIG. 11 shows that the cross-sections of structures  65  and  66 , are substantially similar. Both structures  65  and  66  are formed on surface  60  of substrate  68 . Each is connected through the substrate  68  through a via: structure  65  through via  61  and structure  66  through via  62 . Each structure includes a metal seed structure  74  and a plurality of electroplated encapsulating metal films  75  formed over the metal seed structure  74 . For each structure, the cross-section shows two opposed sidewalls (sidewalls  78  for structure  65  and sidewalls  79  for structure  66 ). In each case, the plurality of electroplated encapsulating metal films  75  substantially covers the sidewalls shown in the cross-section. 
     In alternate embodiments not shown, the substrate  68  may include a plurality of similar or dissimilar structures, including, but not limited to the pad structure and line structure shown in FIGS. 10 and 11. The structures are formed simultaneously according to the process of the present invention. The structures differ because the patterns formed within the metal seed layer differ, but have similar cross-sections, each including a plurality of electroplated encapsulating metal films formed over a metal seed structure, as described in conjunction with FIGS. 10 and 11 above. 
     It should be understood that the foregoing description of preferred embodiments has been presented for the purpose of illustrating and describing the main points and concepts of the present invention. The present invention is not limited, however, to these embodiments. The geometry of the individual terminal metal pads and the spacing between the terminal metal pads may be varied according to packaging need. The number of electroplated films may also vary depending on the application. Likewise, the films used to form the terminal metal films may vary according to the application. The geometry of an individual terminal metal pad, with a fixed base metal pad, will vary depending on how many electroplated films are used to encapsulate the base metal pad. The pattern formed of terminal metal pads and the density within the pattern may also vary. Alternate embodiments may include a different number of films to form the terminal metal pad. The thicknesses of the films which combine to form the terminal pad may also vary. 
     As described above, the process description and structures produced are very suitable for LGA-type interconnects. Other applications where the proposed invention may be beneficial include: providing capped connections with differential heights for special interconnects; producing elastically compliant capping for semiconductor and other industries (such as for self-lubricated soft coatings on gears); and producing hard, wear-resistant cappings for tribological applications.