Abstract:
A method of creating an electrical connection involves providing a pair of contacts each on one of two different chips, the pair of contacts defining a volume therebetween, the volume containing at least two compositions each having melting points, the compositions having been selected such that heating to a first temperature will cause a change in at least one of the at least two compositions such that the change will result in a new composition having a new composition melting point of a second temperature, greater than the first temperature and the melting point of at least a first of the at least two compositions, and heating the pair of contacts and the at least two compositions to the first temperature.

Description:
FIELD OF THE INVENTION 
     The present invention relates to semiconductors and, more particularly, to electrical connections for such devices. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to a U.S. patent application entitled “Mobile Binding In An Electronic Connection” filed concurrently herewith, the entirety of which is incorporated by reference as if fully set forth herein. 
     BACKGROUND 
     When stacking chips, there are essentially two ways that chip to chip connections can be formed. One way, is to form the complete stack and then concurrently subject the entire stack to the conditions, such as heating and cooling, required to cause the electrically conductive bonding materials to establish the conductive path. The other way is to sequentially connect two components together, through heating and cooling of the connection points, and then sequentially attaching the next chip in the same manner, and so on, until the stack is complete. 
     The first, “concurrent” approach, has drawbacks because, in practice, it is difficult to maintain alignment among the stack components. In addition, it is difficult to ensure that all of the individual connections are each subjected to the appropriate conditions necessary to form reliable interconnections among the constituent components of the stack. Thus, there is a risk that any particular connection will not be made because the conditions near the connection were insufficient or detrimentally in excess of what was required. 
     The second, “sequential” approach, does not have the alignment problem. However, it still has drawbacks because the heat used to join each chip after the first can adversely affect the connections of previously joined chips. 
     Thus, there is a need for a better way of joining chips to form a stack that reduces or eliminates the problems that can be encountered by using conventional concurrent or sequential joining techniques. 
     SUMMARY OF THE INVENTION 
     We have devised an approach that allows a stack of chips to be created (on a chip, die or wafer basis) through a sequential approach or a concurrent approach that does not suffer from the problems noted above. 
     One aspect involves a method of creating an electrical connection. The method involves providing a pair of contacts each on one of two different chips, the pair of contacts defining a volume therebetween, the volume containing at least two compositions each having melting points, the compositions having been selected such that heating to a first temperature will cause a change in at least one of the at least two compositions such that the change will result in a new composition having a new composition melting point of a second temperature, greater than the first temperature and the melting point of at least a first of the at least two compositions, and heating the pair of contacts and the at least two compositions to the first temperature. 
     Another aspect involves a stack of chips. The stack of chips has a first set of paired contacts electrically joining two chips together using a first material composition having a first melting point, and a set of contacts on one of the two chips including thereon a bonding constituent composition having a second melting point that is lower than the first melting point, so that when the set of contacts is mated with a corresponding set of contacts on a third chip and the set of contacts and corresponding set of contacts are heated, to a temperature between temperatures corresponding to the first and second melting points, the bonding constituent composition will become the first material composition. 
     A further aspect involves a method performed on an electrical contact. The method involves heating at least two conductive materials located at a connection point to a first temperature, the first temperature being in excess of a temperature needed to cause at least some of a first of the at least two conductive materials to change to a state having a higher melting point than a melting point of the first of the at least two conductive materials, but lower than a liquidus temperature corresponding to a liquidus state on a state diagram for the first of the at least two conductive materials, and allowing the connection point to cool to below the temperature needed to cause the at least some of the first of the at least two conductive materials to change to the state having the higher melting point. 
     The advantages and features described herein are a few of the many advantages and features available from representative embodiments and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. For instance, some of these advantages are mutually contradictory, in that they cannot be simultaneously present in a single embodiment. Similarly, some advantages are applicable to one aspect of the invention, and inapplicable to others. Thus, this summary of features and advantages should not be considered dispositive in determining equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representation of the phase diagram for binary alloys of gold and tin; 
         FIG. 2  is a cross sectional photograph of a contact  200  according to a variant of the approaches described herein; 
         FIG. 3  is a cross sectional photograph of a similar original composition contact, such as shown in  FIG. 2 , after it has been joined to a mating contact; 
         FIG. 4  is a cross sectional photograph of a contact; 
         FIG. 5  is a cross sectional photograph of a contact like from  FIG. 4  after it has been joined to another contact; 
         FIG. 6  is a cross sectional photograph of a contact like from  FIG. 5  after completion of an elevated temperature exposure; 
         FIG. 7  is a representation of the phase diagram for binary alloys of Pb and Sn; and 
         FIG. 8A  through  FIG. 8H , which illustrate stack creation using a variant of the above approaches. 
     
    
    
     DETAILED DESCRIPTION 
     U.S. patent applications, Ser. Nos. 11/329,481, 11/329,506, 11/329,539, 11/329,540, 11/329,556, 11/329,557, 11/329,558, 11/329,574, 11/329,575, 11/329,576, 11/329,873, 11/329,874, 11/329,875, 11/329,883, 11/329,885, 11/329,886, 11/329,887, 11/329,952, 11/329,953, 11/329,955, 11/330,011 and 11/422,551, incorporated herein by reference describe various techniques for forming small, deep vias in, and electrical contacts for, semiconductor wafers. These techniques allow for via densities and placement that was previously unachievable and can be performed on a chip, die or wafer scale. 
     The techniques described therein are, as described, particularly well suited for use in creating very compact stacks of chips. Moreover, those techniques can involve use of a “tack &amp; fuse” process that provides further advantages independent of many of the techniques described therein. 
     In addition, U.S. patent application entitled “Mobile Binding In An Electronic Connection” filed concurrently herewith and incorporated herein by reference, describes ways to reduce or eliminate the problem of phase segregation in connections. As will become apparent, that approach is completely compatible with the techniques described herein and, in doing so, further advantages can be achieved that are not obtained with either one by themselves. 
     Specifically, we have devised a further improvement to those techniques that allows a stack of chips to be created (on a chip, die or wafer basis) through a sequential approach or a concurrent approach that does not suffer from the problems noted above. In addition, in some implementations, we take advantage of the use of specific barriers with specific bonding materials to prevent mobile atoms from creating phase segregation or voids. This is accomplished by specifically selecting bonding materials used, based upon the temperature at which the bonding materials flow, to allow multiple high-temperature processing steps to occur without detrimentally affecting any previously formed connections. 
     This can be accomplished in different ways. However, in general the approach involves selecting the particular materials that will be part of the electrical connection formation (whether they are pure, substantially pure or alloyed metals) such that the initial formation of an electrically conductive connection will occur at about a particular temperature “T 1 ”. However, as a result of that formation, the materials that form the conductive connection will change into a composition that has a higher melting point than that of the original materials. As a result, a subsequent connection that is created using the same materials will not cause the original connection to begin to liquify (or if the new melting point is higher than T 1 , but very close to T 1 , such that no minimally present original materials appreciably liquify). 
     Optionally, the materials can have been selected so that subsequent elevation to a higher temperature “T 2 ” will cause a further change into a composition with yet a higher melting point. In this manner, subsequent elevations in temperature to T 1  will have no effect at all. 
     Advantageously, this approach is particularly useful in conjunction with a tack &amp; fuse connection process, because it already involves a lower temperature “tack” phase and a subsequent higher temperature “fuse” phase. 
     One example of this approach is illustrated with respect to  FIG. 1  through  FIG. 3 . 
     In this example, an alloy is deposited on a contact. In this example, the deposited alloy is an alloy of gold and tin, specifically, 80% Au and 20% Sn.  FIG. 1  is a representation of the phase diagram  100  for binary alloys of gold and tin (although not precisely shown to scale). As can be seen this 80/20 AuSn alloy is at a eutectic point  102  of the phase diagram  100 . This eutectic point  102  represents the lowest melting point of this composition alloy. However, as the alloy is taken to a higher temperature the alloy interacts with some other material that is present, for example, the contact material or, preferably, a material that was deposited either i) between the contact and the alloy, or ii) on top of the alloy, to cause a compositional change of the alloy. Depending upon the interaction and materials, this compositional change will cause movement to the right or left on the phase diagram based upon which of the weight percentage component concentrations decrease or increase. As long as the composition does not cause a shift on the phase diagram to a composition of lower melting point, the result is a composition that has a higher melting point than the starting composition. 
     Depending upon the particular implementation, this left or right movement on the phase diagram can be the result of addition of one of the constituent components, removal of one of the constituent components, or addition of one or more other components. 
     By way of one example, a pure tin or high tin concentration gold-tin layer could have been deposited on top of the alloy. Thus, when the melting temperature of the main gold-tin alloy was reached, the additional tin or gold-tin alloy will also have melted and combined with the 80/20 alloy to form a new concentration that was higher in tin, for example a 70/30 gold-tin alloy. This concentration is to the right of the original composition and thus, has a higher melting point than the 80/20 alloy. 
     Alternatively, a material could have been used that would draw tin out of the gold-tin alloy and itself alloy with the tin leaving a higher concentration of gold, for example. This would result in leftward movement on the phase diagram due to the higher concentration of gold and, hence, a higher melting point for the resultant alloy. 
     In the case of the 80/20 gold-tin alloy, looking at the phase diagram, to achieve the desired result, the compositional change could be one that added tin or removed gold to cause rightward compositional movement on the phase diagram or removed tin or added gold to cause leftward compositional movement on the phase diagram. 
     In another similar alternative, a material could be used that will alloy with both the gold and tin to itself create a small amount of ternary alloy and, as long as the composition of the main alloy changed such that it had a higher melting point, the same effect would be achieved. 
     By way of a similar example with reference to  FIG. 2  and  FIG. 3  presume that the same approach and materials as above were used (i.e. 80/20 Au—Sn), but a sufficient volume of nickel was deposited, for example, under the alloy as a barrier between the alloy and the contact, or on top of the alloy as a cap.  FIG. 2  is a cross sectional photograph of a contact  200  according to a variant of the approaches described herein having a connection material  202  (the 80/20 Au—Sn alloy) deposited on top of the barrier material  204  (the Ni barrier). In such a case, at an elevated temperature of or above about 300° C., the materials will diffuse to some extent. The result will be that tin atoms will be absorbed by, or alloy with, the Ni layer leaving behind a Au—Sn alloy composition with a high percentage of the lower mobility species (in this case gold) while the nickel has trapped some of the higher mobility tin and absorbed the remainder of it. 
       FIG. 3  is a cross sectional photograph of a similar original composition contact  302 , such as shown in  FIG. 2 , after it has been joined to a mating contact  304  at an elevated temperature, as discussed above, to form an electrically conductive connection between the two contacts  302 ,  304 . In  FIG. 3 , as a result of the above approach, the new composition within the volume between the contact pair  302 ,  304  is far from the original 80/20 percent ratio. Some areas  306  have 97% gold while some areas  308  have 25% tin. As a result, from the phase diagram  100  of  FIG. 1 , it should be evident that the melting point of the various resulting materials has changed dramatically. Moreover, due to the fact that a substantial part of the contact is now more than 95% gold, the majority of the contact will have a melting point closer to 1000° C. than to 300° C. Even those regions with higher tin concentrations will have melting points closer to 400° C. than 300° C. 
     Still further, and advantageously, the addition of nickel into the compositional mix makes the tin less likely to be mobile. This means that the likelihood of a phase segregation problem is a reduced. 
     Advantageously, in addition, the absorption or alloying of the tin prevents the entire mix from becoming self-centering, what we and the above-incorporated applications call a “liquidous” or “liquidus” state which is different from the liquidus state referred to in a phase. 
     Note, however, that it is possible for the connections of some implementations to soften to something less that a liquidus state on a phase diagram (i.e. the melting point) for the material or some component thereof, for example, with a post and penetration connection approach, the “malleable” material may need to be at an elevated temperature to allow the material to soften sufficiently to allow the post to penetrate but not soften to the “liquidus” state as we have defined it, let alone to a molten state. 
     As a result, the contact can be raised up to the original melting point over and over again without having any effect on this connection. Thus, it should be appreciated that this approach could be used over and over to add chips to a stack without concern that the forming of the new connections will disrupt the prior connections. 
     Of course, it should now be appreciated that the starting composition need not be one at the lowest melting point, so long as the compositional change results in an upwards shift in melting point. This can be seen from the following example where the materials start off segregated and then combine into a final state. 
       FIG. 4  is a cross sectional photograph of a contact  400  according to one variant that is made up of a barrier  402 , in this case nickel, on top of a device pad  404  covered by a layer  406  of 100% gold which, in turn, is covered by a layer  408  of 100% tin. 
     A contact like that of  FIG. 4  will be joined to another contact (not shown in  FIG. 4 ) to form an electrical connection. In this example, the other contact will be a post that is covered by a layer of nickel. 
       FIG. 5  is a cross sectional photograph of a contact  400  like from  FIG. 4  after it has been joined to the other contact  500  in a tack phase of a tack &amp; fuse process. 
     Heating of the contact has caused the layer  408  of tin, with its lower melting point, to diffuses into the layer  406  of gold and through to the barrier  402  of nickel. This changes the compositions so that there is still a layer  406  with a substantial amount of about 100% gold, but there is also now an alloy layer  502  of about 80% gold and 20% tin instead of the original 100% tin layer. Thus, whereas the original compositions were located at points “C 0 ” and “C 1 ” on the phase diagram of  FIG. 1 , the gold layer is still at about point “C 0 ”, but the new composition is now located at point “C 2 ” on the phase diagram of  FIG. 1  so that, in the case of the tin to tin-alloy change, the melting point has gone up from about 232° C. to about 282° C. and the melting point of the gold has essentially not changed. (Of course in actuality, the composition of the gold might now be slightly less than 100%, thereby slightly lowering the melting point, but it would be an insubstantial change because the meaningful temperature would be between about 232° C. and about 282° C.). Referring back to  FIG. 5 , if that contact pair  400 ,  500  was subjected to the appropriate higher temperature, for example through use of a fuse phase of a tack &amp; fuse process, a further compositional change would occur. 
       FIG. 6  is a cross sectional photograph of a contact like from  FIG. 5  after completion of a process of elevated temperature exposure, by way of example, a fuse process. As can be seen in  FIG. 6 , the final composition of the connection is now a mostly gold (about 98%) center area  602  flanked by a ternary alloy  604  of gold, tin and nickel, in this example, of approximately 45% Au, 35% Sn and 20% Ni. Since this alloy  604  has a higher melting point than the 80%/20% gold-tin alloy, the same effect described above has been obtained. Thus, this contact could be subjected to heat cycling up to the temperature that caused this ternary alloy  604  to form, without the risk of this connection being disrupted because the component melting points are all above that temperature. 
     At this point it is worth noting that the above approach is not limited to use with pure metals or alloys. The same approach can be used for solders (whether they include lead or are lead free). 
       FIG. 7  is a representation of the phase diagram  700  for binary alloys of lead and tin (although not precisely shown to scale). Bearing the above description and examples in mind, one can now see that if an initial composition coinciding with the eutectic point  702  is used and an additional composition is used to either add lead or remove tin, the melting point will go up as the composition moves to the left side of the phase diagram. Alternatively, with that same starting point  702 , if an additional composition can is used to either remove lead or add tin, the melting point will still go up by virtue of the composition moving to the right side of the phase diagram  700 . 
     It should now be appreciated that the same approach can advantageously even be used with conventional C4 solder which, as indicated on the phase diagram, has approximately a 60% Pb/40% Sn composition. To do so, as it should now be evident, an additional composition is used that will remove tin or add lead to the C4 solder upon heating. In this manner, the compositional change will result in a point to the left of the composition of C4 solder and, consequently a higher melting point. If one presumes that this is done for a connection and the resulting composition is 25% tin and 75% lead, the melting point will have gone up from the temperature TC 4  to the temperature corresponding to the point on the phase diagram labeled “TN”. Thereafter, the temperature of the connection can go up to and above the C4 melting point and, as long as it does not exceed the TN point temperature, the connection will not be disturbed. 
     Now, it can be appreciated that, by using a variant of this approach, a stack  800  of chips can be easily created in a sequential manner as follows. This process is illustrated, in overly simplified form, in  FIG. 8A  through  FIG. 8H , which illustrate stack creation using a variant of the above approaches. 
     Using a connection such as described above, two chips  802 ,  804  for the stack (whether still part of a wafer or after separation) ( FIG. 8A ) are brought together ( FIG. 8B ) and the temperature at the connection points  806  are raised to above the melting point of one of the two components  808 ,  810  of the connections which contain one variant of materials suitable for the approach described herein. This will cause an electrical connection to be formed at each of the connection points  806  and also change the composition of the electrically conductive bonding material to a new composition  812  that will have a higher melting point than the temperature used to create the composition. Then, the joined chips  802 ,  804  can be cooled to a temperature below the connection forming temperature to cause the connection to “set.”Thereafter, a further chip  814  can be brought to the now formed stack ( FIG. 8C ), its contacts (which also use a variant as described herein, and ideally the same variant used for joining the first two chips) can be brought into contact with mating contacts on the stack at their respective connection points. A subsequent raising of the temperature at these connection points to the connection forming temperature will cause the third chip to be connected to the stack ( FIG. 8D ) but, owing to the higher melting point of the composition created by the joining process, the connections between the first two chips will not be affected. This process can be repeated again ( FIG. 8E ,  FIG. 8F ) and again ( FIG. 8G ,  FIG. 8H ), as many times as necessary to create the stack. 
     It should thus be understood that this description (including the figures) is only representative of some illustrative embodiments. For the convenience of the reader, the above description has focused on a representative sample of all possible embodiments, a sample that teaches the principles of the invention. The description has not attempted to exhaustively enumerate all possible variations. That alternate embodiments may not have been presented for a specific portion of the invention, or that further undescribed alternate embodiments may be available for a portion, is not to be considered a disclaimer of those alternate embodiments. One of ordinary skill will appreciate that many of those undescribed embodiments incorporate the same principles of the invention and others are equivalent.