Abstract:
A method of forming interconnects on an electronic device that can be bonded to another electronic device at a low processing temperature can be carried out by depositing a first interconnect material on the electronic device forming protrusions and then depositing a second interconnect material to at least partially cover the protrusions, wherein the second interconnect material has a lower flow temperature than the first interconnect material. The method is carried out by flowing a molten solder into a mold having microcavities to fill the cavities and then allowed to solidify. The mold is then aligned with a silicon wafer containing chips deposited with high melting temperatures solder bumps such that each microcavity of the mold is aligned with each high melting temperature solder bump on the chip. The aligned mold/wafer assembly is then passed through a reflow furnace to effect the transfer of the low melting temperature solder in the mold cavities onto the tip of the high melting temperature solder bumps on the wafer. A dual metallurgical composition bump is thereby formed by the two different solder alloys.

Description:
This is a divisional of application Ser. No. 08/710,992 filed on Sep. 25, 1996, now U.S. Pat. No. 6,127,735. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to a method for making interconnect on an electronic structure and the structure made and more particularly, relates to a method for making electronic device made by such method. 
     BACKGROUND OF THE INVENTION 
     In the present semiconductor fabrication process, semiconductor chips are frequently attached to other chips or other electronic structures. The attachment of the chip is frequently accomplished by one of two different techniques. The first is a wire bonding process in which each of a series of I/O bump terminal on a chip built on an aluminum bonding pad is sequentially bonded to the connecting pads on a substrate. The second technique is a flip chip attachment method in which all the I/O bumps on the semiconductor chip are first terminated with a solder material. A frequently used solder material is a 97% lead/3% tin high melting temperature solder alloy. The semiconductor chip is then flipped over and the solder bumps are aligned and reflowed in a reflow furnace to effect all the I/O connections with the bonding pads on the substrate. One advantage achieved by the flip chip process is its applicability to very high density CMOS circuits and its high reliability in the interconnects formed when compared to the wire bonding process. The latter technique also has limitations in the number of I/O interconnections that can be made in high performance devices. One of the disadvantages of the flip chip process is that the process was originally designed for a high temperature bonding process and thus the solder bump material chosen for mechanical and electrical reliability is a high melting temperature lead/tin solder. The high melting temperature solder can only be bonded by reflowing the solder at above 350° C. and therefore, limiting the process of bonding to high temperature substrates such as ceramic and the like. Others have attempted to find solutions to overcome this problem, for instance, by using alternative solder alloys that have lower melting temperatures. Even though most of the lower melting temperature alloys do not meet the reliability requirement for high performance, high density circuits. 
     One solution proposed by others for performing a direct chip attach of a chip to a card with high melting temperature lead/tin solder bumps is to pre-deposit a low temperature solder on the card prior to the chip attach. Others have attempted the process of evaporating additional tin cap onto the high temperature solder bump. Both techniques introduce added costs and cards with more complex structure. For instance, the first technique requires that molten solder be injected through patterned openings in a mask so that they can be deposited at the desired pad locations on a substrate. The injection process is relatively complex which leads to a higher fabrication cost. 
     Still others have attempted electrochemically plating a low temperature solder onto the metal pads of a receiving substrate card or board. The high temperature solder bumps installed on a semiconductor chip are then joined to the low temperature solder on the substrate or the board. The drawback of the process is that the electrochemical process which applies the low temperature solder bumps on the pads of the substrate is a very complicated process which involves various steps of seeding, photo processing, plating, etc. It is difficult to control the uniformity of the solder bump thickness and to prevent bridging problems from occurring between the bumps. The electrochemical plating process is also a high cost process. 
     It is therefore an object of the present invention to provide a method of forming interconnects on a semiconductor chip that does not have the drawbacks and shortcomings of the conventional methods. 
     It is another object of the present invention to provide a method of forming interconnects on a semiconductor chip such that the chip can be joined to a second electronic device in a low temperature bonding process. 
     It is a further object of the present invention to provide a method of forming interconnects on an electronic device by building interconnects with at least two different materials wherein a lower melting temperature material covers a higher melting temperature material. 
     It is another further object of the present invention to provide a method of forming interconnects on an electronic device by first forming a bump with a high melting temperature solder and then capping the bump with a low melting temperature solder. 
     It is yet another object of the present invention to provide a method of forming interconnects on an electronic device by first forming a low melting temperature solder in a mold and then transferring the molded solder onto high melting temperature solder bumps built on a semiconductor chip. 
     It is still another object of the present invention to provide a method of forming interconnects on an electronic device wherein a low melting temperature lead/tin solder is first formed in a mold and then transferred onto a high melting temperature lead/tin solder built on a semiconductor chip. 
     It is still another further object of the present invention to provide an electronic structure that has interconnects built on the structure with at least two different materials wherein one material which has a lower melting temperature substantially covers the other material which has a higher melting temperature. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method of forming interconnects on an electronic device that can be bonded to another electronic device at a low processing temperature and a device made by such method are provided. 
     In a preferred embodiment, the method of forming interconnects on an electronic device can be carried out by depositing a first interconnect material on the electronic device forming protrusions and then depositing a second interconnect material to at least partially cover such protrusions, wherein the second interconnect material has a lower flow temperature than the first interconnect material. The method can be carried out by flowing a molten solder into a mold equipped with microcavities to fill the mold and then allowed to solidify. The mold is then aligned with a wafer consisting of chips deposited with high melting temperature solder bumps such that each microcavity of the mold is aligned with each high melting temperature solder bump on the chip. The aligned mold/wafer assembly is then passed through a reflow furnace to effect the transfer of the low melting temperature solder in the mold cavities onto the tip of the high melting temperature solder bumps on the wafer. A dual metallurgical composition bump formed of the two different solder alloys is therefore formed. The dual metallurgical composition bump allows for the chip to be bonded to a low temperature substrate such as an epoxy board or the like by a process in which only the tip of the bump containing the low melting temperature solder is molten to form a joint with the metal pad on the substrate board. The low melting temperature solder utilized has a melting temperature that is comparable with solders that are normally used on laminated cards or boards. 
     The present invention is further directed to an electronic structure that has interconnects deposited on a structure for making electrical connections, wherein the interconnects are formed of at least two different materials in intimate contact with each other where one material has a lower melting temperature and the other material has a higher melting temperature, the lower melting temperature material at least partially cover the higher melting temperature material in an area on the interconnect where electrical connection will be made. The solder material can be selected from a group of materials containing lead, tin, bismuth, molybdenum, silver, indium, antimony and polymeric based solder materials. It is desirable that the lower melting temperature material has a melting temperature that is at least 10° C. lower than the melting temperature of the higher melting temperature material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will become apparent upon consideration of the specification and the appended drawings, in which: 
     FIG. 1 is an enlarged, cross-sectional view of a mold apparatus for molding the low melting temperature solder. 
     FIG. 2 is an enlarged, cross-sectional view of a composite mold constructed of polyimide and glass. 
     FIG. 3 is an enlarged, cross-sectional view of the mold in FIG. 1 filled with a solder material. 
     FIG. 4 is an enlarged, cross-sectional view of a mold/wafer assembly wherein the mold in FIG. 3 is flipped over on top of a wafer having high melting temperature solder bumps built thereon. 
     FIG. 5 is an enlarged, cross-sectional view of the wafer/chip in FIG. 4 showing the low temperature solder capping the high temperature solder bumps. 
     FIG. 6 is an enlarged, cross-sectional view of the wafer/chip in FIG. 5 bonded to a substrate having bonding pads. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention provides a method of forming an interconnect structure on an electronic device by building solder bumps with two different solder compositions and an electronic structure made by such method. 
     Referring initially to FIG. 1, wherein an enlarged, cross-sectional view of a mold  10  having a mold body  14  and a plurality of cavities  12  as shown. The mold  10  can be constructed of a variety of different materials as long as two key requirements are met. First, the mold is preferred to be made of a transparent material in order to allow inspection of the solder fill quality and the initial alignment of the low melting temperature (or eutectic) solder in the mold cavities with the solder bumps on the wafer (or chip). Secondly, the coefficient of thermal expansion (CTE) should be sufficiently close to that of the silicon wafer, such that the wafer mold alignment can be maintained during reflow of the eutectic solder in the mold cavities onto the solder bumps on the wafer/chip. One example of a suitable material for the mold is a semi-transparent, low CTE polyimide film (of approximately 0.005″ thickness) in which the mold cavities are created by the laser ablation or by the micropunching technique. The mold cavities  12  are subsequently filled with a low melting temperature solder (the eutectic solder) of the present invention. The solder-filled mold is then aligned with the solder bumps on the wafer/chip and pressure is applied by mechanical means to effect the transfer at a reflow temperature of the solder in the mold. After the transfer, the mold/wafer assembly is cooled to ambient temperature followed by the step of removing the mold from the wafer. Alternatively, the mold can be removed prior to the cooling of the assembly as soon as the solder transfer has been completed. 
     An alternate construction of the mold is a glass plate, in which blind holes are created by chemical etching of the glass through openings in a metal or photoresist film that is previously deposited and patterned on the glass plate. Reactive ion etching and laser ablation technique can also be used to create the cavities on the surface of the glass plate. After the cavities are etched in the glass, the thin film is removed from the surface. 
     In another alternate construction of the mold, the mold can be constructed of a combination of materials such as a polyimide film deposited on a glass plate. The deposition process can be carried out by spin coating, lamination or any other methods that is capable of applying a polyimide film which has a thickness comparable with the thickness of the molded solder. The polyimide film can be subsequently laser ablated through its thickness to the glass surface to form cavities. After a solder injection process, the low melting temperature solder can be transferred to the solder bumps on the wafer/chip in the previously described manner. 
     As shown in FIG. 1, the mold body  14  can be a glass plate having a thickness between about {fraction (1/16″)} and about {fraction (1/8″)}. The glass composition of the mold is selected such that it has a coefficient of thermal expansion (CTE) similar to that of a silicon wafer to which the molded solder will be transferred. A suitable glass material to be used can be a borosilicate glass or a pyrex glass. For wafers of gallium arsenide (GaAs) instead of silicon, a suitable glass material to be used is a sodium lime glass which has a CTE close to that of gallium arsenide. The cavities  20 , in a microscopic scale of approximately 80˜100 μm in diameter are created in the top surface  16  of the glass body  14  by a chemical or physical etching process. In a chemical etching process, a metallic film or a photoresist is applied to a top surface  16  of the glass body  14  and then patterned by a standard photolithography process. The chemical etching process of the glass can be carried out in a hot KOH or NH 4 F solution. The microcavities  12  are thus created. Subsequently, the base metal or the photoresist film is removed and the mold fabrication is completed. In a physical etching process, a reactive ion etching method can be used by utilizing a metal mask in close contact with the surface  16  of the glass body  14 . The mask can be made of a chromium metal or silicon having holes in locations corresponding to the locations of the cavities  12  shown in FIG.  1 . The cavities  12  are created by exposing the glass body  14  to a reactive ion etching plasma through the mask. 
     An alternate construction of a composite mold of polyimide/glass is shown in FIG. 2. A polyimide film  18  is first applied to the glass surface  16  by spin coating a polyimide fluid precursor on the surface  16  of the glass body  14  and then cured in an annealing process. Alternatively, a polyimide or other similar type of film can be laminated onto the glass body  14 . A frequently used mold construction is a glass plate having a thickness between about {fraction (1/16″)} and about {fraction (1/8″)} thick made of a material described previously. The overall dimension for the glass body  14  matches the size of a silicon wafer that is usually of 5″ or 8″ in diameter. The polyimide film  18  deposited on the glass body  14  has a thickness between about 30 and about 50 μm. A preferred method of creating cavities  12  is by laser ablation through a metal mask directly onto the mold surface or by using a projection mask and the ablation procedure. 
     Referring now to FIG. 3, wherein the mold  10  in FIG. 1 is shown with the mold cavities  12  filled with a injection moldable solder  22  of a low melting temperature alloy. A typical low melting temperature alloy that can be suitably used is a lead/tin solder of 63 wt. % tin and 37 wt. % lead. In an injection molded solder process, a molten solder can be injected into tiny cavities by means of a injection tool which has a head containing solder in a liquid form and a vacuum/pressure system. When the injection head glides over the mold surface  16 , the cavities  12  are filled with the molten solder. After the injection molded solder filling process, mold  10  is cooled to a temperature to allow the solder  22  to solidify in the mold cavities  12 . 
     After the solder is solidified, the mold  10  is turned upside down and positioned in alignment with a silicon wafer (or chip)  30 . This is shown in FIG.  4 . The mold/wafer assembly  50  is shown in its aligned, pre-transfer state with the eutectic solder filled cavities  12  of mold body  14  aligned to the high melting temperature solder bumps  40  on the silicon wafer  30 . The alignment procedure can be greatly facilitated by the transparency of the mold and by a visual contrast between the glass and the solidified solder in the cavities  12 . The optical contrast allows for a quality inspection of all solder filled cavities in the mold to verify the fill completion and quality prior to the alignment procedure. After the verification, mold body  14  is flipped over and aligned with the silicon wafer  30 . Before the alignment procedure, a small amount of a flux may optionally be applied to the mold surface  16  and the eutectic solder  22  in the cavities in order to improve the wetting characteristics during the solder transfer process and to prevent the oxidation of the transferred solder. The flux may be applied by selectively spraying onto the eutectic solder in the mold cavities by a mask. The flux can be one of a number of known flux compositions commercially available and, preferably those of a water soluble nature or of the “no clean” classification. The mold  10  and the silicon wafer  30  can be precisely aligned such that the solder bumps  40  are positioned against the eutectic solders  22  of mold  10 , when the mold is transparent in the case of a glass mold or semi-transparent in the case of a polyimide/glass mold. 
     After the alignment procedure is completed, and during the transfer process, the mold  10  and the wafer  30  can be locked in a fixed position by using any mechanical means. A differential pressure can be achieved by including a compliant base material in a clamp assembly such that an improved contact is achieved across the entire wafer/mold interface. A typical compliant material can be used is a high temperature polymer foam which compresses under pressure and has sufficient elasticity to return to its original dimension after the pressure is removed. 
     The transfer of solder  22  from mold body  10  to the high melting temperature solder bumps  40  on wafer  34 , as shown in FIG. 4, can be performed by passing the mold/wafer assembly  50  after the fluxing, alignment and clamping process through a reflow furnace. It is recommended that hydrogen, nitrogen or a forming gas to be used to purge through the furnace at a temperature between about 200° C. and about 300° C. The temperature ramp-up rate and dwell times are typical of those used in conventional reflow process. Under these processing conditions, the eutectic solder  22  in the cavities  12  melt and leaves the mold body  14  and transfer onto the top of solder bumps  40  and then solidifies to form a low melting temperature cap  42  on top of the solder bumps  40 . This is shown in FIG.  5 . The solder bumps  40  shown in FIG.  4  and have a significantly higher melting temperature (i.e., about 320° C.) and therefore their structure and shape are not affected by the solder transfer process. This permits the semiconductor chips obtained by the present invention procedure to retain its inherent reliability of the original fabricated device. 
     FIG. 5 shows an enlarged, cross-sectional view of a wafer  30  having a number of solder bumps  40  deposited on top and a cap layer  42  of low melting temperature solder deposited by a transfer process. The resulting bump configuration achieved by the present invention shown in FIG. 5 consists of a dual metallurgical structure which on one hand maintains the high reliability and electrical/mechanical properties of a high melting temperature solder and on the other hand, allows wafer/chip  30  to be bonded at a lower bonding temperature by joining to a substrate  60 . This is shown in FIG.  6 . 
     An advantage made possible by the present invention method is that the amount of the eutectic solder and the volume required are limited for forming a cap (or layer) on top of the high melting temperature solder. It is made possible by the present invention method by providing eutectic solder volume of the cap  42  (shown in FIG. 5) to be only approximately ¼ of the total volume of the solder bump  40 . The conditions for the transfer process can be suitably adjusted such that the molten eutectic solder does not run down to the bottom of the solder bump  40  to avoid an undesirable reaction between the eutectic solder which has a high tin content with the interface  62  and consequently affecting the electrical reliability of the wafer/chip  30 . 
     A chip/substrate assembly  80  prepared by the present invention low temperature chip attachment method is shown in FIG.  6 . An semiconductor chip  70  which is diced from a wafer on which the eutectic cap transfer process of the present invention was carried out is joined to a substrate  60 . The substrate  60  can be of any polymeric substrate utilized in the printed circuit board technology, for instance, epoxy boards or the like. The joining process can be carried out by fluxing the bonding pads on the boards or cards using conventional flux procedures to prevent oxidation and to enhance the wetting of the metal pads by the low melting temperature solder. As shown in FIG. 6, the joint of the bump/card interface  62  has a unique configuration which is formed by a layer of molten eutectic solder between the substrate bonding pads  32  (usually of copper or gold) and the high melting temperature solder bump  40 . The high melting temperature solder bump  40  does not normally involve in the joining process. 
     The present invention method can be applied to substrates other than silicon wafers, i.e., to GaAs substrates for the packing of electronic chips such as CMOS and other logic and memory products which utilize high melting temperature bump termination and require a low temperature chip attachment method. The present invention method can also be applied in a flip chip attachment to other substrates such as ceramic and the like. 
     The present invention low melting temperature solder (or the eutectic solder) composition can be suitably selected from a number of alloy compositions. For instance, a lead/tin alloy can be suitably used with a lead content between about 22% and about 52% by wt, and a tin content of between about 48% and about 78% by wt. A more preferred composition is between about 27% by wt. and about 47% by wt. lead, and between about 53% by wt. and about 73% by wt. tin. One suitable example is about 37% by wt. lead and 63% by wt. tin. The eutectic solder composition can also be made of a two component tin alloy. For instance, a tin/bismuth, tin/silver, tin/indium and tin/antimony alloy. A proper percentage of tin in these alloys can be from about 85% to about 98% by wt, and the percentage of the alloying element of bismuth, silver, indium, and antimony can be between about 3 and about 15% by wt. A three component alloy composition of tin/bismuth/indium can also be used which contains approximately between about 85 and about 95% by wt tin, between about 2 and about 7% by wt bismuth, and between about 2 and about 8% by wt indium. The composition of the high melting temperature solder bump can be a lead/tin alloy containing between about 90% and about 97% by wt. lead and between about 3% and about 10% by wt. tin. 
     While the present invention has been described in an illustrative manner, it should be understood that the terminology used is intended to be in a nature of words of description rather than of limitation. 
     Furthermore, while the present invention has been described in terms of a preferred and several alternate embodiments, it is to be appreciated that those skilled in the art will readily apply these teachings to other possible variations of the inventions.