Patent Publication Number: US-6210547-B1

Title: Enhanced solder surface and process for chemically and mechanically enhancing solder surface properties

Description:
This application is a divisional of U.S. patent application Ser. No. 09/113,445, filed on Jul. 10, 1998, now U.S. Pat. No. 6,056,831. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a process for preparing a solder surface before joining and, more specifically, to an ion-bombardment-induced sputtering process for enhancing surface properties of solder used in microelectronics. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of microelectronic devices, various microelectronic components are electrically joined together into circuits using metal alloy solders. Such solders are generally alloys of tin (Sn) and lead (Pb), but can comprise other metal components. The metal alloy solder is generally optimized for bulk properties that control the thermal behavior of the solder. For example, the melting temperature of Sn/Pb solder is controlled by adjusting the relative amounts of tin and lead in the solder. The minimum melting temperature of a Sn/Pb alloy is achieved when the ratio by weight of Sn to Pb in the solder is 63:37. This is called the eutectic composition. 
     A solder that is homogenous across its mass and optimized for bulk properties does not take into account the desirable surface properties of the solder. Therefore, solder compositions that are optimized for bulk behavior are not necessarily optimized in terms of surface properties. Such surface properties control wetting to metal surfaces, reactivity with organic materials such as fluxes, and the melting point of the surface layer. Such surface properties depend upon both the chemical composition of the solder and the physical characteristics of the surface. For instance, a rough surface may have more surface area for holding flux than a smooth surface. 
     For some applications, it is desirable to use bulk solder alloys at other than the eutectic composition. For example, Controlled Collapse Chip Connection (C 4 ) solder balls, which are used to join Integrated Circuit (IC) chips to chip carriers, are typically Pb-rich in the range of 90% to 97% Pb. Because Pb-enriched solders have higher melting temperatures, the solder joint between the chip and the chip carrier maintains its shape throughout subsequent assembly of the chip carrier (with chip attached) to another electrical component such as a printed circuit board. The Pb-enriched solders also require higher temperatures, however, to fuse them to metal surfaces such as copper pads on chip carriers. Thus, organic laminate chip carriers having copper joining pads may be exposed to undesirable high temperatures during joining processes. 
     A tradeoff results: without further treatment, the benefit of using a Pb-enriched metal alloy solder that maintains its shape at high temperatures is offset by the need for a high fusing temperature. Such high fusing temperatures can be avoided by applying a eutectic solder to the joining pads on the organic laminate chip carrier. To ensure proper fusing at every joint, however, the height of the applied eutectic solder layer must be controlled within very narrow limits, a requirement that is difficult to satisfy. 
     Another modification that retains high temperature stability while providing low temperature fusing comprises placement of an additional layer or cap of tin or other low melting point metal on the surface of the Pb-enriched solder, as described in U.S. Pat. No. 5,634,268 and U.S. Pat. No. 5,729,896 and assigned to the common assignee of the present invention. Such a cap enables the bulk solder to retain its desirable high-temperature resistance while providing a surface layer capable of fusing at low temperatures. 
     The present invention proposes an alternative process for providing a solder surface layer capable of fusing at low temperatures while maintaining the bulk solder properties. It is an object of the present invention to provide a process for altering the chemical composition and physical characteristics of a metal alloy solder surface without adding an additional metal layer. It is a further object of the present invention to provide a process for altering the surface characteristics of a metal alloy solder simultaneously with roughening the surface of the substrate to which the solder is connected. 
     SUMMARY OF THE INVENTION 
     To achieve these and other objects, and in view of its purposes, the present invention provides a process for altering surface properties of a mass of metal alloy solder comprising a first metal and a second metal, such as lead and tin respectively, and having a bulk ratio of the first metal to the second metal. The process comprises exposing the mass, for a predetermined time period, to energized ions of a sputtering gas. The ions have sufficient energy to remove more atoms of the first metal than atoms of the second metal, thus forming a surface layer having a desired depth on the mass and having a surface layer ratio of the first metal to the second metal that is less than the bulk ratio. 
     The mass of metal alloy solder may be located on the surface of a substrate. If so, the process further comprises placing over the substrate a mask made of a material such as molybdenum, titanium, or zirconium that has a lower sputtering yield than the first metal. The mask shields from the ion beam all of the substrate except the selected area. 
     The sputtering gas may comprise a noble gas, and may further comprise a reactive gas such as oxygen. The substrate may be an organic substrate. The process may further comprise also exposing the organic substrate to energized ions of the reactive gas to roughen the organic substrate surface. 
     The process may further comprise depositing on the substrate a process control mass of solder having a composition similar to the functional mass, but having a surface geometry conducive to performance of a desired surface analytical technique. After exposing the process control mass similar to the exposure of the functional mass and forming a surface layer on the process control mass representative of the surface layer on the functional mass surface layer, the desired surface analytical technique is performed on the process control mass. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     The invention is best understood from the following detailed description when read in connection 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. 1A is an atomic-level, side view cross-section of a portion of a mass of solder, showing a homogenous bulk mixture throughout; 
     FIG. 1B is an atomic-level, side view cross-section of the same portion of the solder mass shown in FIG. 1A, showing the impact of surface layer modification via ion bombardment; 
     FIG. 2 is a side view illustrating a typical apparatus for carrying out the present invention inside a vacuum chamber, showing substrates held upside-down by a holder in the path of an ion beam directed by an ion gun; and 
     FIG. 3 is a side view illustrating an alternate ion gun embodiment of the present invention being used on a substrate shown right-side-up in cross-section. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Referring now to the drawing, wherein like reference numerals refer to like elements throughout, FIGS. 1A and 1B show an atomic-level illustration of a portion of metal alloy solder  32 . This particular metal alloy solder is shown in FIG. 1A having a homogenous composition of Sn atoms  40  and Pb atoms  42 . The Pb atoms comprise roughly 90% of the weight of the solder (a bulk ratio of approximately 1:5.16 Sn atoms to Pb atoms). 
     The process of the present invention comprises altering the surface properties of the metal alloy solder by sputtering. The process comprises exposing the solder for a sufficient, predetermined time period to energize ions of a sputtering gas and form a surface layer having a desired depth on the solder and having a ratio of tin to lead that is less than the bulk ratio. The energized ions may have an energy in the range of about 0.5 to about 10 KeV and the time of exposure may be in the range of about 30 to about 60 minutes, resulting in a surface layer having a depth ranging from about 1,000 Angstroms to about 2 microns. 
     FIG. 1B shows the result of this process when the solder is hit by ion beam  30 . When an ion  44  of the ion beam  30  hits a metal atom, such as Pb atom  42 ′, the metal atom can be sputtered out of the solder as shown in FIG.  1 B. The percentage of metal atoms actually sputtered from the solder per contact with an energized ion is referred to as the “sputtering yield.” The sputtering yield for Pb is typically four to five times greater than the sputtering yield for Sn. Thus, as the ion beam hits the surface layer  46  of metal alloy solder  32 , the Pb atoms are preferentially removed. 
     The resulting mass of solder has a surface layer  46  with a higher ratio of Sn to Pb atoms, while the original homogenous composition remains in the bulk of the mass of solder below the surface layer. Thus, the surface layer becomes Sn-enriched, approaching the same ratio as a eutectic alloy (approximately 3:1 Sn atoms to Pb atoms), while the remainder of the mass of solder  32  is unchanged. 
     Thus, the process of the present invention changes the surface layer  46  of solder  32  in two ways. First, the ratio of Sn to Pb atoms is changed (a chemical change) in surface layer  46 . Second, the roughness of the surface layer is also changed (a physical change) from the smooth surface shown in FIG. 1A to the rough surface shown in FIG.  1 B. 
     The change in chemical composition to a higher Sn concentration promotes a lower melting temperature. Thus, a lower temperature is required to melt the surface layer  46  for fusing. The bulk solder feature still maintains its shape, however, during high temperature processing such as joining of chips to chip carriers and chip carriers to printed circuit boards. The chemical change also promotes better responsiveness to flux because Sn has a higher reactivity to flux than Pb, while the physical change to a rougher surface promotes the ability to hold more flux. The roughened surface may also be beneficial for certain mechanical interconnection schemes that rely on surface roughness to frictionally hold components together. 
     The process of the present invention is also reworkable. The solder surface layer  46  can be redistributed into the bulk by a melting operation, regenerating a homogenous mixture with a smooth surface, if required. 
     Referring now to FIG. 2, there is shown a sputtering apparatus  10  typical of an apparatus that may be used with the process of the present invention to modify the surface properties of a metal alloy solder by ion bombardment. Vacuum chamber  12  contains a holder  14 , for holding substrates  16 , and an ion gun  18 . As shown in FIG. 2, ion gun  18  directs a stationary ion beam  30  at substrates  16  mounted upside-down on holder  14 , while the holder moves the substrates in and out of the beam. Holder  14  as shown in FIG. 2 is a planetary motion device such as that commonly used in chemical vapor deposition (CVD) systems and well known in the art. 
     A vacuum pump  20  is provided to evacuate vacuum chamber  12  via vacuum hose  22 . A vacuum gauge  24  may indicate the amount of vacuum inside the chamber. Sputtering gas is supplied to the ion gun via gas tubing  26 . 
     In operation of apparatus  10 , vacuum pump  20  evacuates the vacuum chamber  12  and sputtering gas is supplied to ion gun  18  through gas tubing  26 . A plasma (not shown) is generated within the ion gun, wherein a voltage grid (not shown) extracts ions from the plasma and accelerates them out of the gun in a directed ion beam  30  that bombards substrates  16  with ions. The amount of exposure time necessary to modify the solder surface depends on the size and strength of the beam, the desired depth of modification, and the surface area of the sample to be treated, and can vary from a fraction of a minute to approximately an hour. 
     Referring now to FIG. 3, there is shown a close-up, cross-sectional side view of a right-side-up substrate  16  and an alternate type of ion gun  18 ′. Substrate  16  typically is made of a polyamide-, polyimide-, polyester-, or polyethylene-based material or some other organic composition, but may also be made of an inorganic material such as SiO 2 , Si 3 N 4 , and the like. Substrate  16  has a plurality of metal joining pads  31  on its surface, preferably made of copper, on each of which is deposited a mass of functional solder  32  or process control solder  32 ′. Process control solder  32 ′ is a test feature deposited on the substrate solely for the purpose of process control. Solder  32 ′ has an area sufficiently large and flat enough to be analyzed by Auger Electron Spectrometry (AES), X-ray Photoelectron Spectroscopy (XPS), or other conventional surface analytical techniques. Such analysis is used to evaluate the solder surface to assess the results of the ion bombardment process. 
     A mask  34  is placed over substrate  16  to shield from ion beam  30 ′ those portions of the substrate not intended for bombardment with ions. The use of a mask is especially important when exposure to ion beam irradiation is detrimental to the performance of the material surrounding the solder-coated areas. A mask may be omitted if it is desired to simultaneously modify the substrate surface by ion bombardment while the solder surface is modified. When no substrate modification is desired, the mask may cover all of the substrate except the solder masses. As shown in FIG. 3, mask  34  exposes only solder masses  32  and  32 ′ through apertures  33 . Alternatively, the mask may cover only the portions of the substrate or attached features sensitive to and not intended for bombardment. 
     Ion gun  18 ′ shown in FIG. 3 has a head  19  capable of directing a rastered ion beam that can be directed across stationary substrate  16 . A rastered beam essentially moves in a line from side-to-side of the substrate starting from one edge, and indexing the width of the beam in a direction perpendicular to the side-to-side motion between each side-to-side pass, so that at the completion of rastering, the beam has covered every portion of the substrate. The ion gun  18 ′ shown in FIG. 3 is also placed within a vacuum chamber (not shown) and supplied with inert gas (not shown) such as the embodiment shown in FIG.  2 . 
     An ion gun apparatus producing a rastered ion beam may also be used in conjunction with a moving sample such as on a conveyor, where a plurality of holders in series each brings one substrate into a position where it is contacted by the ion beam. In such a configuration, the ion beam may merely travel from side-to-side in one direction, while the conveyor provides the indexing motion of the substrates in the perpendicular direction to provide total coverage of the substrate. 
     Other ion generators may be used instead of ion guns. Ions can be produced in an electrical discharge or plasma, such as an argon plasma. The naturally occurring space charge sheath that develops at all surfaces in contact with the plasma may provide the necessary acceleration of these ions toward the solder surface. Because ions having the highest kinetic energy will generally strike the powered electrode of the plasma generator, the surface to be bombarded may benefit from being placed on the powered electrode. Placement elsewhere in the plasma may be sufficient, however, to produce the desired effect. Thus, a stationary or moving substrate with its associated mask may merely be placed in a plasma for surface preparation of the solder. Nevertheless, to ensure that the ions attain sufficient energy for sputtering metal atoms such as Pb, an ion gun for acceleration and propagation of a beam of ions is preferred. 
     For small substrates, the entire area to be treated can be irradiated by ions with a stationary substrate and stationary ion beam. For larger substrates, however, it may be necessary to either move the ion irradiation over the area to be treated, such as by rastering a beam of ions as shown in FIG. 3, or to move the substrates through a stationary zone of treatment such as shown in FIG.  2 . 
     The composition of the mask  34  is critical. To extend the useful life of the mask and to avoid generation of potential contamination of the substrate  16  by sputtered fragments from the mask, mask erosion must be avoided. This is best accomplished by use of a mask material having a sputtering yield far lower than that of the element to be preferentially sputtered from the alloy being modified. For example, for enrichment of Sn on the surface of Pb-Sn alloys, molybdenum, titanium, and zirconium are desirable mask materials. For ease of mask fabrication, molybdenum is preferred. 
     The vacuum chamber  12  is fabricated from suitable materials of construction such as aluminum, stainless steel, quartz, chemical and temperature resistant glass, or the like. 
     Noble gases such as argon are typically used for sputtering applications, and thus are also suitable for Sn enrichment of solder surfaces according to this invention. If simultaneous Sn enrichment and substrate surface chemical treatment are desired, however, high energy ions generated from a reactive gas, such as oxygen, can be used. When oxygen gas is incorporated into the sputtering gas mixture, a significant amount of substrate modification (roughening or incorporation of chemical functional groups of organic material) can be achieved simultaneously with the solder surface modification. Such roughening may be used to improve adhesion between an organic substrate, such as a laminate, and any encapsulating layer, such as a polymer, deposited on the substrate to reinforce the solder joints and to reduce fatigue damage. 
     Substrate surface roughening simultaneous with solder surface preparation provides an advantage over processes using low-melting point capping of C4 solder. To obtain a roughened, low-melt surface, the capping technologies require both a step to provide the cap and a step to plasma-roughen the substrate, whereas the process of the present invention requires only the plasma step that can both roughen the substrate and prepare the solder surface. 
     To provide such a reactive plasma, a reactive gas such as oxygen may be introduced to the sputtering application directly into the ion gun  18  or other plasma generator through gas tubing  26  along with inert gas. In an alternate embodiment, a plasma generator external to vacuum chamber  12  may generate the reactive gas ions and pipe the energized ions into the chamber through a dedicated line. In yet another alternate embodiment, a reactive gas plasma generator may be inside the vacuum chamber  12  with its own gas supply piping to provide reactive gas for generating reactive ions from the gas. 
     In addition to use with Sn—Pb alloys, the present invention may also be used with ternary alloys that contain Sn, Pb, and a third component such as silver (Ag). The process and resulting structure of the present invention may also be extended to alloys comprising metals other than Sn and Pb. 
     EXAMPLES 
     The following examples are included to more clearly demonstrate the overall nature of the invention. These examples are exemplary, not restrictive, of the invention. 
     Example 1 
     A Pb/Sn metal alloy solder sample having a composition of 40% Pb by weight was placed in a vacuum chamber evacuated to 1×10 −7  to 1×10 −8  Torr. Using an ion gun and argon as a sputtering gas, an ion beam of 4 KeV argon ions having a raster size of 3×3 mm was directed at the solder. The solder was sequentially sampled over time starting at a fraction of a minute to an hour of exposure to determine the impact of the ion bombardment over time. Sampling was performed by X-ray Photoelectron Spectroscopy (XPS), which can typically sample the surface layer at a depth of 50-100 Angstroms. Ion bombardment was shown to be capable of modifying the surface layer of the 60:40 Sn:Pb alloy to a 77:23 Sn:Pb composition. 
     Example 2 
     A eutectic Sn/Pb solder sample was placed in a chamber at 0.13 Torr. A plasma was generated having a total applied power of 6,500-7,000 Watts at 40 KHz, resulting in 0.1 Watts/cm 2  of powered electrode area, using a gas having a composition of 93% argon and 7% oxygen. The sample was subjected to the plasma treatment for one hour. A significant increase in roughness of the solder balls was recognized in SEM micrographs, and the solder surface composition changed from the original level of 65:35 Sn:Pb by weight to 85:15 Sn:Pb by weight. The testing showed that, for this example, the oxygen content needed to be below 15%, with less than 10% oxygen being preferred. Higher oxygen levels formed thicker oxide layers that lessened the probability of sputtering. Also, pressures above 0.225 Torr tended to lessen the sputtering effect. 
     Example 3 
     A Pb/Sn metal alloy solder sample having a composition of 90% Pb by weight was placed in a vacuum chamber evacuated to 0.085 Torr. A plasma was generated using argon at a flow rate of 14.3 standard cubic centimeters per minute and a 300 Watt power source with 13.56 MHz frequency applied at the cathode, resulting in 0.45 Watts/cm 2  coverage of the sample. The resulting plasma had an average peak-to-peak (V pp ) energy of 1240 Volts and a self-bias or DC-offset (V dc ) of 200 Volts. The sample was treated for 45 minutes with continuous sampling and analysis to determine the degree of surface change. Over the course of treatment, the Sn:Pb atomic concentration increased. 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.