Abstract:
The invention relates to a method for removing a plurality of raised places of contact made of a meltable metal, such as tin or indium or an alloy, such as tin-containing solder, silver-containing solder or lead-containing solder, the meltable metal being meltable above a first temperature limit, the places of contact being surface-distributed over a substrate. It is also possible to form vaulted domes on a plurality of metallic support segments which are located on one of the surfaces of a substrate. The invention aims at reducing production costs, particularly at removing a soldered layer once applied. If defective contact places occur, a plurality of the raised contact places, particularly substantially all contacts, are at least in substantial portions melted off from the substrate by contacting them with a molten metal. Between the substrate and the support segments distributed over the substrate and a surface of the molten metal an organic fluid may be present, the organic fluid being provided as a covering layer only and evaporating off the substrate surface, after the vaulted domes have been formed.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a method for removing a plurality of raised places (spots, points) of contact of a meltable metal, such as tin or indium or an alloy, such as tin-containing solder, silver-containing solder or lead-containing solder, said meltable metal being meltable above a first temperature limit, said places of contact being surface-distributed over a substrate. The invention also relates to a method for forming vaulted domes on a plurality of metallic support segments which are distributed over one of the surfaces of a substrate. 
   In order to explain the problem solved by the methods of the invention, it is noted that not only flat conductive domes provide raised places of contact—also designated as “menisci” which are applied onto metallic support segments—, but that also stronger raised metallic structures can be applied on metallic support segments, said structures being used as solder balls or contact bumps for a flip-chip contact, compare the publication “Semiautomatic Solder Ball Bumper (SB 2 ) by Pac Tech and the publication “Solder Ball Bumper (SB 2 )—A flexible manufacturing tool for 3-dimensional sensor and microsystem packages”, Kasulke, Schmidt, Titerle, Bohnaker, Oppert, Zakel, published in IEMT Europe 1998,  FIG. 6  and  FIG. 8  in cross-section. Said raised places of contact are manufactured by several different methods, e.g. by laser or by (re-) shaping in an oven, after a preceding placement of a print of smaller solder particles which are melted in said oven, to provide the described raised places of contact in the form of balls. Usually, their height is substantially larger than that of the metallic support segments which are distributed over the substrate. A not completely avoidable probability that metallic bridges are formed by solder balls growing over to other solder balls, caused by too thick layers of flow medium, or that individual solder balls are generally missing, makes the use of the substrate technically impossible, because certain contact places are either not contacted or have a shortcut already prior to being contacted. 
   A further defective contact can occur due to the fact that a too large quantity of the solder forming said contact bump was applied on the substrate, thus causing an asymmetry of the formed raised contact places, said contact places possibly showing asymmetries in height as well as in a lateral extension, similar to the defects at certain, usually individual metallic support segments, as described before. 
   Substrates, particularly for forming wafers, are usually too precious to deny the use of the complete substrate if individual portions are damaged by applying the raised contact places upon preparing the contact. Therefore, an inexpensive solution has to be found to repair these contacted substrates which have been damaged during contact. In this respect, a specific solution has already been proposed according to the prior art, namely individually repairing individual contact places as SBB Repair Station (compare the above mentioned publication “Solder Ball Bumper (SB 2 )”,  FIG. 10  with the associated description). One single laser impulse is used for obtaining a reflow of the defective contact point and for removing the melted and softened solder bump by using a vacuum. Simultaneously, a nitrogen atmosphere is used to avoid an oxidation of the remaining solder cap on a laterally limited support segment. A basic technical problem solved by the invention is to reduce production cost, particularly to allow a removal of a solder once applied on a substrate, when defects occur, large raised places of contact not being (selectively) located at the positions of the metallic support segments, but, by forming bridges, providing a contact structure which no longer allows a regular use of the substrate, for example as a contacted wafer. 
   According to the invention, several contact places can be repaired together, such that at least a substantial part of the individual contact places is removed by melting off, said melting taking place in a molten metal, and no individual places of contact have to be removed individually and successively by local heating. 
   According to the invention, the substrate and all contact places are contacted with a molten metal to remove the complete pre-contacting in an inexpensive and timesaving way, to start from a new status quo and to again coat the substrate with raised places of contact. Said repairing method is a removal of a preceding contact in combination with an option to start the new contact at a state, at which all metallic support segments, which continue to be present, have the same design. This concern, both the contact places not having been contacted by a raised solder ball, and the contact places undergoing a shortcut with at least one further contact place due to an inexactly positioned solder ball. 
   By melting off, the solder of the raised contact point is taken up by the molten metal. Domes of small height which remain on said support segments receive substantially the metal or the metal alloy of which said molten metal consists. At the same time, said option allows also for repair of defectively contacted substrates by exchanging the first solder which has already solidified, and replacing said solder by the solder corresponding to the molten metal, particularly the molten metallic alloy, which is present in the bath. 
   When the two metals, particularly the two metal alloys are identical, i.e. the metal of the raised places of contact and the metal of the melted bath, the substances are only removed and not exchanged. At least substantial parts of the already solidified raised places of contact on the substrate are, however, removed, the described domes remaining as “menisci” of small height. 
   The method is suitable for both de-bumping, i.e. removing of already existing bumps, which bumps are the raised contact places, but it is also suitable for a first time coating of support segments which have not yet been coated. Coating can be performed by applying the molten metal which is present in the bath, but coating can also be performed by removing and newly applying the mentioned molten metal. 
   Additionally, no more than a covering layer of an organic fluid can for example be applied on the substrate, said covering layer as a film having a small thickness with respect to the substrate. Due to the small thickness of said fluid layer (a thin layer), said layer evaporates after having contacted the molten metal due to the vapor pressure, so that a subsequent cleaning step of the support segments coated for the first time or of the support segments coated again after removal of the pre-contacting and of the associated substrate is not required. Production or repair therefore becomes less expensive. A substantially smaller volume of the fluid applied particularly as a polyalcohol, like glycerol, is used which fluid has evaporated after the described process. In general, the invention avoids the use of a bath of considerable volume with respect to the described fluid. 
   A first temperature limit is to be understood such that a meltable metal is to be removed, but the substrate is not to be melted. Raised contact places, which are usually made of solder, are meltable at temperatures of below 200° C. (usually below 250° C.). When using glycerol, the organic fluid used has a boiling temperature of 290° C. Similar or comparable substances should have a boiling temperature of above 250° C. When the molten bath is made of an identical metal (particularly as a metal alloy), said first temperature limit also refers to said metal which should accordingly be at least at or above said temperature limit to allow the raised contact places to be melted off and taken up from the substrate. 
   The distributed arrangement of the described raised contact spots or of the metallic support segments carrying said spots concerns a distribution of the basic contact over a large surface, which are in turn each provided on a metallic basic pad which allows a contact with the interconnecting paths of the substrate when provided as wafers. Said large-surface distribution is a starting point or basis of the invention for proposing a large-area removal by melting off from the substrate. 
   In accordance with the invention, the raised contact places can be removed substantially completely or completely, and at the same time, their material property can be exchanged. 
   Suitable substrates can be the substrates also described in the prior art, for example wafers, printed cards, printed boards or ceramic substrates. 
   As far as a metal is mentioned in the following discussion, a metal alloy is always included. This is valid for both the raised places of contact and the molten metal in the bath. As a bath both a stationary melt, which is contacted by a relative movement with the contact bumps to be removed, and a melt which is at least partly in motion and towards which the substrate with the raised contact places to be removed is moved, can be used. A lowering in a vertical direction corresponds to a practically simultaneous lateral or surface removal of all contact bumps. A relative movement in a horizontal direction in which the substrate approaches a wave-shaped or surge-shaped vertically protruding metal bath corresponds to a band-shaped removal of the raised contact places which is performed successively, but uniformly with regard to the complete surface. 
   The residence time for an at least contacting immersion or for an approaching movement is determined, to have sufficient time available for melting off the raised contact places and for maintaining or actually providing the remaining dome coating on the support segments. 
   The thin layer of the medium selected in accordance with the invention evaporates from the substrate surface substantially without residue during contacting with the molten metal. Said film can be applied by spraying or by a vapor atmosphere in the sense of sputtering. When the support segments are provided with raised contact places of a meltable metal already prior to being contacted with the molten metal, said film comes to rest as a thin fluid layer between the substrate and the support segments distributed over said substrate. When raised contact places of a meltable metal have not yet been provided on the support segments, said fluid film also covers the support segments themselves. 
   A vapor coating can be applied on the substrate simultaneously with the contacting of the metal bath; however, the atmosphere can extend only so far in front of the metal melt that the thin, film-like application of the organic fluid is obtained by a passage or a movement of the substrate with the support segments through said atmosphere. 
   Said organic fluid can satisfy the same conditions and can be made of the same materials as described in EP 781 186 B1. 
   The fluid covers the substrate as a covering layer in the form of a film, the film surface which faces away from said substrate directing towards the atmosphere and contacting it. Before being contacted with the molten metal, said support segments can either be covered by said organic fluid film or by said raised contact bumps to be removed; in both cases, the covering layer is formed between the substrate as well as the support segments distributed over said substrate and the surface of the bath. Said formation can also be achieved by spraying before contacting the metal melt. 
   The use of the fluid layer provided as a covering layer only, which due to its film shape cares for a minimum volume of the fluid to be used, can not only be applied on the substrate by spraying or sputtering, it can also be present as a thin layer on the surface of the metallic bath, or it can be formed during the metallic contact, when the evaporated organic fluid is present in the atmosphere above said bath. All embodiments used achieve a minimum volume quantity of the auxiliary processing fluid and dispensability of an additional cleaning step. Further effects of said fluid in connection with the solder to be applied or removed and with the support segments on the substrate showing a high wettability are described in detail in the prior art (uniformity of the surface structure, reducing effect on the surface, and increase of the wettability of the solder). Moreover, when removing (melting off) said raised solder spots, there is an additional advantageous aspect, namely of supporting the pinch of a reduced surface tension when melting the metal to be removed from the substrate into a molten metal which has a substantially larger volume, so that a substantially uniform formation of menisci on all limited support segments is achieved. 
   An example may explain the relative proportions: When a wafer has for example a thickness of 600 μm, and the support segments on the substrate have a height of between 5 μm and 10 μm, the covering layer of auxiliary fluid, which is provided as a film, has a film thickness of 50 μm to 200 μm, particularly in a range of 100 μm, said thickness also depending on the height of the raised contact places which, in a range of between 10 μm and 100 μm, can occur as ball-shaped contact bumps. This explains the definition of de-bumping according to which said contact bumps are removed from a surface of a substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view of a substrate with regularly and correctly applied raised places of contact, said substrate comprising a first metallization  9  and a substantially ball-shaped, raised contact structure  25  provided on said metallization. 
       FIG. 2  is a sectional view of a defectively formed substrate  10  comprising irregular contact structures corresponding to contact bumps  11 ,  12 ,  13 ,  14 , which have neither regular, nor ball shape. 
       FIG. 3  is a sectional view of a substrate comprising metallic support segments  9  according to FIGS.  1 , 2  and dome-shaped menisci  60  provided on said segments, such menisci being formed after a melting-off process or after a first coating of still uncoated support segments  9 . 
       FIG. 4  is a first illustration of a melting-off process wherein a substrate  10  with defectively formed contact places  17 ,  18  is lowered down into a liquid bath of a molten metal  32 . 
       FIG. 5  is an illustration of a second metallic bath, wherein a surge  31  is adapted to have band shape. 
       FIG. 6  is an upside-down illustration of a substrate  10 , wherein defectively formed contact bumps  11 , 12  are approached to the flow bath of  FIG. 5  along a horizontal movement v 1 ,  FIGS. 5 and 6  being spatially coordinated with respect to each other. 
       FIG. 7  is an alternative illustration with respect to  FIG. 5 , showing the movement of a substrate towards a flow bath as a molten metal  30  kept in motion, an atmosphere  50  being provided as from a vertical level  51 , said atmosphere favouring a uniform melting-off. 
       FIG. 8  represents an alternative method with respect to  FIG. 4 , wherein a film-like coating of an organic fluid is sprayed onto the raised contact places and onto the substrate prior to lowering said substrate down into an immobile or stationary bath  32  by a combination of a horizontal movement v 1  and a vertical movement v 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  illustrate in a direct comparison a substrate  10 , which may be a wafer or a printed board. The illustration according to  FIG. 1  shows that the contact places  25 , usually called contact bumps, which are made of a meltable solder material melting at a melting temperature of 183° C., are adapted to have a regular or uniform shape. They are arranged on a UBM metallization (under bump metallization) which is a segmented intermediate metallization  9  made of nickel with for example a gold cover, for obtaining a diffusion barrier with respect to the actual bond pads on the substrate  10 , said bond pads usually being made of aluminium. The proportions are not illustrated to correct scale, the illustrations being only schematic, for showing the method and for providing an enlarged view of asymmetries in the contact geometries as shown in  FIG. 2 . 
     FIG. 1  shows that a surface of the substrate  10  is designated as  10   a . The thickness of the substrate is s. For wafers, said thickness is in a range of above 500 μm, whereas the height of the intermediate nickel supports  9 , which are provided as UBM, is in a range of between 5 μm and 10 μm. Said metallic support segments have a substantially smaller surface than the surface  10   a  of the substrate  10 . The distance to be provided between two such limited support segments  9  corresponds to at least the double height of regularly applied contact structures  25  according to  FIG. 1 . 
   The defects according to  FIG. 2  can be explained as follows: Between the left two intermediate metallizations, an enlarged contact bump  11  is visible which has been formed as an excessively raised contact bump by two neighbouring contact bumps melting together. Such an inner shortcut between two contact bumps to be insulated sometimes occurs during the production of contact bumps, when too large quantities of a solder are applied, for example by screen-printing, or when a too large layer thickness of a flow medium is applied which is required for a reflow (a reflow process) in an oven, to reshape smaller grain structures of an applied preliminary contact bump to a ball-shaped contact bump with a smooth surface. A similar contact between two contact bumps  12  and  13  is illustrated at the following two contact segments  9 , the height of the contact bumps, however, being smaller. Only a displacement and a bridging take place. A contact bump  14  of a slightly too small height is arranged on the following intermediate metallization  9 . A non-illustrated variant of a defect is the absence of a contact bump on an intermediate support  9 , which in this case is empty. A variant, which is also not shown, is an unintentional application of a wrong solder as a metallic bump on the complete surface of the substrate  10 , which would be undesirable even if a ball-shaped structure  25  according to  FIG. 1  would have been obtained for all contact bumps of said substrate  10 . 
   When the substrate  10  is a wafer, its value is higher than the value which has to be expended for an inexpensive method of repairing said wafer with respect to its contact structures, thus removing the defective contact bumps and replacing them by regular contact bumps  25 . 
   The remaining figures show how such unification on the surface of a substrate  10  can be achieved. In this respect, reference is first made to  FIG. 4 . In  FIG. 4 , a substrate  10  is provided with asymmetric contact bumps  17 ,  18  as shown. A still tolerable contact bump beside said contact bump  17  could in fact be left unchanged, but the embodiment of the method is based on completely melting off all contact bumps, when lowering said substrate down in a vertical direction v 2 , a tub  28  containing a metallic bath  32  being provided below the substrate  10 , a surface  32   a  of said bath being oriented towards the contact bumps  17 ,  18  to be melted off. 
   The described relative movement v 2  can also be performed such that the substrate is stationary or immobile and the tub  28  is moved towards the substrate. In a relative lowering movement, the substrate  10  is lowered with respect to the surface  32   a  so far that at least the laterally limited support segments  9  contact said surface  32   a  and the complete area of defectively applied contact bumps, which area extends over said support segments in a downward direction, and the area of correctly applied contact bumps—in the figure, only the defective contact bumps are illustrated with otherwise regular contact bumps according to FIG.  1 —is removed completely and taken up by said bath  32 . 
   The period of time during which said substrate  10  remains approached to said surface  32   a  should be dimensioned such that sufficient time remains for melting off the contact bumps  18 , 17  and for heating the support segments  9  to have continuously throughout their surface at least the melting temperature of the solder of the contact bumps. In this case, a uniform melting off of the contact bumps is achieved and, after removal in a direction v 2 ′ and turning over of said substrate  10 , a structure according to  FIG. 3  is obtained. 
   In  FIG. 3 , a known structure is illustrated in which each individual intermediate support  9  carries a dome-shaped meniscus  60  which forms a vault at an acute angle starting from the edge of the respective laterally limited support  9 , said vault being caused by the surface tension of the solder remaining on the support. The height of the domes  60  is designated as h, said height being substantially smaller than the regular height of the contact structures  25  of  FIG. 1 , but substantially in a range of the height of the intermediate metallization  9 , which is also known in the prior art as a UBM metallization. The substrate  10  and also the metallic intermediate supports  9  are not modified in structure, their melting temperature being above the temperature of the metallic bath  32  of molten metal. 
   It is to be understood that the molten metal will usually be an alloy which is suitable to a “soldering” of nickel and which, with regard to its resistance, is suitable for contact structures. Tin or alloys of tin are used, as well as silver alloys or indium alloys. 
   The bath  32  can have the same material property as the contact bumps to be melted off. When the contact bumps have unintentionally been prepared from a false material, said bath  32  serves not only for melting off the contact bumps, but also for exchanging the menisci  60  according to  FIG. 3 , which, after having been treated according to  FIG. 4 , have the material property determined by said bath  32 . 
   The difference between the volume quantity of a meniscus  60  and the volume quantity of a contact bump either regularly or defectively applied according to  FIG. 1  or  FIG. 2  is the quantity which is removed when the molten metal  32  has a material property identical with that of the contact bumps. When said molten metal has a different material property, the total volume of the contact bumps  25  of  FIG. 1  is removed by melting. 
   A further embodiment of providing an alternative molten metal is illustrated in  FIG. 5 . Said figure shows a flow bath  31  which is in a continuous movement along a portion which in a top plan view has a band shape, to which movement a metallic bath  30  provided inside a receptacle  29  is subjected. The wave crest raised with respect to the remaining surface of the metallic bath serves as a melting-off portion for a substrate  10  which is moved in a horizontal direction above said wave crest, said substrate, according to  FIG. 6 , being moved towards said wave  31  along a movement v 1 . An auxiliary layer  40  on said wave  31  will be explained later and shall at present not be considered for the description of the melting-off process. 
   Said movement v 1  is so slow that the width of the wave is sufficient for obtaining a uniform melting off of the defective contact bumps  11 ,  12  and for achieving a surface which is identical to the surface shown in  FIG. 3  as a result of the method of  FIG. 4 , after moving said substrate  10  over. 
   In contrast to  FIG. 4 , when using the method according to  FIGS. 5 and 6 , the total number of contact bumps is removed successively, independent of having been formed defectively or regularly. In the embodiment of  FIG. 4 , all contact bumps were melted off practically simultaneously by contacting the surface  32 , the only difference in time being due to a different height of the contact bumps, which results in a slightly later time of contact with said surface  32   a . Accordingly, this is designated as simultaneous, whereas a horizontal movement provides a band-shaped melting off of the contact bumps along a surface  10   a  of said substrate  10 , maintaining the meniscus-like domes  60  on the support segments  9  of said substrate  10 . 
     FIG. 6   a  illustrates an advantageous supplementation of said substrate  10  according to  FIG. 6 . According to  FIG. 6   a , an additional fluid layer  41  is provided as a film to cover the contact structures  11 ,  12 ,  13 , the support segments  9  and the surface  10   a  of the substrate. The thickness of said film  41  as a thin layer is designated as d. Said thickness substantially corresponds to the height of the contact bumps  11 , but said layer  41  can be adapted to fill up only the portions between the contact bumps. 
   The consistency of said fluid layer  41  corresponds to that of the fluid layer  40  of  FIG. 5  which is provided alternatively or cumulatively. It consists of a polyalcohol, particularly glycerol, which has a boiling point of 290° C. and is above a first temperature limit which is required for melting off the contact bumps from the support segments  9 . 
   As the provided layer is thin, only a small volume of such a fluid is required as an auxiliary processing substance. Above all, when using a temperature in the range of the melting temperature of the solder and a thin film, an evaporation of said film after contacting the molten metal  31  is obtained. Due to said evaporation, an additional cleaning step is not required. The surface profile and also the surface structure shown in  FIG. 3  already form a surface suitable for being again provided with metallic bumps, without a cleaning step having to first remove the auxiliary processing substance. Said auxiliary substance also has a favourable effect on the detachment, resulting in the pinching during melting off and also unifying or homogenizing the formation of menisci according to  FIG. 3 . Alternative substances of glycerol are paraffin wax or other substances mentioned in the prior art, as cited above. 
   Said substrate  10  with said film-like covering layer according to  FIG. 6   a  can also be used in connection with  FIG. 4 . 
   The formation of the film shall now be described according to  FIGS. 7 and 8 : Said figures schematically illustrate a substrate  10  entering an atmosphere  50  as from a vertical is barrier level  51 , said atmosphere causing a film to be formed on a side  10   a  of said substrate  10  on which side raised contact bumps  11 ,  25  (as examples of the contact bumps of  FIGS. 1 ,  2 ) are schematically illustrated. In fact, the atmosphere also cares for the back side of the substrate to be covered with such a film layer which, however, is not critical for a melting off in a flow bath  31  with a molten metal  30  provided according to  FIG. 5 . 
   Said barrier  51  can be shifted to the left or to the right. The substrate with its surface  10   a  is coated on the side of the contact structures either prior to or simultaneously with contacting the molten metal  31 . Despite a small volume of the described fluid, a uniform covering of the illustrated structures is obtained by using a gaseous phase  50 . 
     FIG. 8  shows an alternative coating method, by spray-coating the contact structures  25 ,  11  on said substrate  1 . By providing a fountain  70  from an ascent pipe  60  and a fluid swamp  61  in a tub  62 , a formation of the film-like coating  41  of the surface of the contact structures is obtained. According to this embodiment, said contact structures are introduced into a metallic bath according to  FIG. 4  by a combined horizontal and vertical movement v 1 , v 2 , all contact bumps being melted off simultaneously. Said thin layer  41  is also melted off in the sense of an evaporation. The variants of  FIGS. 7 and 8  as well as of  FIGS. 6 and 4  can be supplemented and extended by introducing uncoated support segments  9  for the first time into the metallic flow bath  31  or the metallic bath  32  with a surface  32   a . In this respect, the film-like coating according to  FIG. 6   a  at a reduced thickness d can illustrate that only a very small quantity of the fluid is required for obtaining a first coating of the metallic support segments  9 . The resulting structure also corresponds to that of  FIG. 3 , by forming dome-shaped menisci  60  on the free surfaces of the metallic support segments  9 . The film-like layer  41  evaporates as described before, so that no additional cleaning step is required. 
   When using a substrate not yet provided with contact bumps, the flow bath of  FIG. 5 , by a film-like covering  40  of at least the wave portion  31  provide a uniform first coating by first menisci  60  on the metallic support segments  9  and cause the remaining layers formed on the surface  10   a  of the substrate to evaporate. 
   Consequently, the method is equally suitable for both a melting off of under bump metallizations already provided with raised contact structures and a first coating of said under bump metallizations  9 , which are usually made of nickel on an aluminium bond pad.