Patent Application
20080083993
The apparatus “A” of
The second workpiece 103, for instance an insulating substrate, has a protective layer 213 on its surface. A window of width 211 in layer 213 exposes the second contact pad 212 and defines the lateral dimension of the pad. As an example, contact pad 212 may be a layer of copper or copper alloy. The lateral dimension 211 may be the side of a rectangle for a rectangular window, or the diameter for a circular window. It is preferred that the second contact pad is circular with 211 as its diameter, because, as a deformed sphere, gold stud 110 also has an approximately circular contour. For controls in the fabrication method (see below) of the interconnection 102, it is preferred that diameter 211 is smaller than diameter 110a of the deformed gold sphere.
In addition to deformed gold sphere 110, metal interconnection 102 includes the 278° C. eutectic structure 111. The structure contacts the second pad 211 and the gold stud 110. The eutectic structure 111 has the outline of an approximate pillar or column with a diameter about equal to the lateral dimension of the window in layer 213. Eutectic structure 111 consists of about 80 weight percent gold and about 20 weight percent tin, formed in a eutectic mixture at 278° C. Eutectic structure 111 has a lamellar microstructure of the phases Au5Sn and AuSn, but does practically not include brittle AuSn4 crystals. Scallop-like Au5Sn is also at the gold/solder interface, which is helpful to improve the joint reliability. At the copper/solder interface towards copper layer 212 is the compound (Au,Cu)5Sn, also beneficial to high reliability.
Eutectic structure 111 has a Young's modulus of 59.2 GPa, as compared to 71.1 GPa of the brittle AuSn4. The experience of applicants has shown that it is more difficult for potential microcracks to propagate in the eutectic structure 111 than in conventional gold/tin solders with typical large, brittle AuSn4 crystals. The interconnection reliability is thus greatly improved.
In contrast, conventional gold/tin solders operate at the (second) eutectic point 303, which is characterized by 93.7 atomic percent tin, or 90 weight percent tin. Clearly, the amount of tin needed for the eutectic mixture is large. The eutectic point employed conventionally has a melting temperature of 217° C. The contributing phase for the (second) eutectic point is AuSn4, which is brittle and harmful for the solder joint reliability.
Another embodiment of the invention is a method for fabricating a metallic connection between two workpieces.
In the next process step, a wire bonder with a capillary is selected suitable for round gold wires in the diameter range between 15 and 33 μm, preferably 20 to 25 μm. For subsequent bonding to aluminum pads and controlling the heat-affected zone in ball formation, the wire may include small contents of beryllium, copper, palladium, iron, silver, calcium or magnesium. From the length of the gold wire protruding from the capillary, a free air ball is formed using either a flame or a spark technique. The ball has a size with a preferred diameter from about 1.2 to 1.6 wire diameters, for example, between about 20 and 30 μm.
For bonding gold to aluminum, the semiconductor chip is positioned on a heated pedestal heated to a temperature between 150 and 300° C. The free air bail is placed on the pad 402 and pressed against the metallization of the pad. For pads of aluminum, a combination of compression force and ultrasonic energy creates the formation of gold-aluminum intermetallics and a strong metallurgical bond. The compression force is between about 17 and 75 g; the ultrasonic time between about 10 and 20 ms, the ultrasonic power between about 20 and 50 mW. At time of bonding, the temperature usually ranges from 150 to 270° C. As depicted in
The capillary is lifted and the wire is broken off from the squeezed sphere in the heat-affected mechanically weak zone. Dependent on the shape of the capillary orifice, stud 403 may have an additional truncated cone with a fiat top as shown in
Next, a second workpiece 410 is provided, for example an insulating substrate integral with at least one layer of patterned conductive lines. Preferably the metal of the layer is copper. Workpiece 410 is covered with an insulating layer 411; an example is a “soldermask”, an epoxy-based material such as a brominated novolac resin. Layer 411 has a height 411a. For many semiconductor devices, the preferred height 411a is between about 10 and 30 μm. A window in the layer exposes the metal of a patterned conductive line to become the second contact pad. The window has a lateral dimension 413. As depicted in
In the next process step, tin is deposited in window 413 on top of the metal of the second pad. The preferred deposition method is plating, either by electroplating using a buss bar connection or by immersion plating without the buss bar. The deposited tin, indicated by shading 412 in
As indicated by line 420 in
A calculation delivers the volume portion Vgold of the gold stud volume Vstud (with Vgold<Vstud) to be dissolved in order to consume all tin of volume Vtin.
In the eutectic composition, the weight Wdisgold of the dissolved gold is 4 times the weight Wtin of the dissolved tin. With the specific weights for gold 19.32 and for tin 7.285, the eutectic composition requires
V
disgold·19.32=4·Vtin·7.285;
Vdisgold=1.5 Vtin,
or Vtin=0.667 Vdisgold
with Vtin<0.667 Vstud.
Considering the relatively small volume Vstud of gold balls in contemporary semiconductor devices, the needed volume Vtin of tin in the second contact pad is small indeed, especially when, for practical reasons, only the gold cone 404 of the scud should be consumed for forming the eutectic structure, while the more massive deformed sphere 403 should be preserved for purposes of absorbing thermo-mechanical stress.
Vtin can numerically be determined by selecting window diameter 413 and tin height 412a (see
As
In the next process step, the first and the second workpiece together with the metal interconnect of the eutectic structure and the leftover gold are cooled to ambient temperature. As stated above, the relatively high temperature of 278° C. for forming the eutectic composition prevents a re-melting of the eutectic structure in subsequent assembly steps, since these subsequent steps operate at reflow temperatures lower than the 278° C. eutectic temperature. Consequently, the gold content of the eutectic structure is stabilized (except for the negligible solid state diffusion).
As an example for subsequent assembly, many semiconductor devices require three more reflows: Attachment of the ball grid array solder bodies; attachment of a memory component package; and assembly on a mother board.
While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description.
As an example, the embodiments are effective in semiconductor devices and any other device with contact pads, which have to undergo assembly on a substrate or printed circuit board followed by underfilling the gap between device and substrate. As another example, the semiconductor devices may include products based on silicon, silicon germanium, gallium arsenide and other semiconductor materials employed in manufacturing.
As yet another example, the concept of the invention is effective for many semiconductor device technology nodes and not restricted to a particular one. Especially, the invention provides a method for reliable chip-on-chip devices.
It is therefore intended that the appended claims encompass any such modifications or embodiments.
| Number | Date | Country | |
|---|---|---|---|
| 60828155 | Oct 2006 | US |
