Method of forming a bump contact structure on a semiconductor device

Abstract: A bump structure for bonding leads to a semi-conductor, in which the bump has a thin lower portion (32) which overlaps and seals the edges of the passivation layer (18), and a thicker upper or stem portion (38) of smaller lateral dimensions to fit within the margins of the opening in the passivation layer. Thus, during bonding, downward compressive forces are applied primarily through the bump stem directly to the metal termination pad (14) beneath the bump, and very little force is applied to the edges of the passivation layer. This reduces the likelihood of passivation layer cracking, increasing device reliability. Because the bump stem is formed within the margins of the passivation layer opening, it has a flat top, resulting in better lead bonding.

Description: This invention relates to a method for forming a bump structure for thermo-compression or other bonding of leads to a semi-conductor device. Bonding of leads to integrated circuits has been known since prior to 1972, when U.S. Patent 3,689,991 entitled "Method of Manufacturing a Semi-Conductor Device Utilizing a Flexible Carrier" was issued. This patent deals with tape automated bonding, in which sets of leads carried by a tape are bonded by the application of heat and pressure to small raised "bumps" on the semi-conductor devices. Typically both the leads and the bumps formed on the circuits are made of gold, solder, copper, or other metal or alloy. A persistent problem with typical bonding of leads to bumps is that since high temperatures and considerable pressures are usually used to form a sound joint, the process tends to create cracks in the layers under the bump. The cracks permit air and moisture to reach the very thin underlying aluminum metallization layer over which the bumps are formed. These layers then oxidize and effectively disappear, allowing the bump to lift off the device and destroying the utility of the device. The above problem has been known for some time, and various attempts have been made to alleviate it. For example, in a paper entitled "Failure Mechanisms in Tab Inner Lead Bonding and the Relationship Between Design and Reliability" by James D. Hayward, published in IEEE/CHMT '91 IEMT Symposium, page 6, the reliability problems were discussed, and it was suggested that reduction in cracking could be achieved by various design approaches. It was noted that the degree of cracking correlated with the degree of passivation overlap but that neither a reduction in passivation overlap nor a change to make the passivation layer less brittle completely solved the problem. It was suggested that a reduction in the degree of overlap and the use of less brittle passivation layers, as well as the use of appropriate organic die coatings for further sealing, would improve mechanical integrity and reliability. However, it is found that these approaches do not in fact solve the problem. EP-A-0068091 discloses a joint for a solder mound. The joint includes a chromium film providing a glass-to-metal seal and thin films of copper and gold to assist adherence of the solder mound to the chromium film. Another bump structure according to the precharacterising parts of claims 1 and 6 of the accompanying claims is known from US-A-4,263,606. According to the invention there is provided a method of forming a bump structure on an integrated semiconductor device having a surface, a laterally extending conductive termination pad on said surface and a protective passivation layer covering said surface and overlapping the edges of said pad, said passivation layer having an opening therein having edges, said opening being of lateral dimensions between said edges of said opening which are less than the lateral dimensions of said pad to expose a portion of said pad through said opening, the method including forming a lower bump portion over said opening to overlap the edges of said opening, the lower bump portion being formed with a lower layer of a barrier material, being located over said opening and having side margins extending laterally beyond said edges of said opening to cooperate with said passivation layer to seal said pad, said lower layer having lateral dimensions greater than the lateral dimensions of said opening, and forming an upper bump portion having a top and a stem of gold, said stem having side margins and being of lateral dimensions less than those of said opening, said stem being located over said lower bump portion and over said opening with said side margins of said stem being entirely within said edges of said opening, said stem being of thickness substantially greater than the thickness of said lower bump portion characterised in that said lower bump portion is formed by depositing a first layer of gold over said barrier layer and extending over said barrier layer, and then by depositing a second layer of gold over said first layer over said pad, said second layer being thicker than said first layer, and then forming said stem over said second layer, and etching all exposed gold layers to remove all of said first layer of gold which is not covered by said second layer of gold while removing some but not all of said second layer of gold and said stem, so that when a lead is compressed downwardly onto the top of the stem, the compressive forces transmitted through said stem will be primarily applied directly through said lower bump portion to the portion of said pad immediately below said stem and will not tend to crack said passivation layer, and said side margins of said lower bump portion form a protective sealing skirt extending laterally beyond said edges of said opening to seal said pad. Further objects and advantages of the invention will appear from the following description, taken together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Fig.1 is a plan view of a prior art assembly of integrated circuits, before being sawn into individual circuits; Fig.2 is a sectional view of a typical prior art bump structure; Fig.3 is a sectional view of a bump structure part way through a method of manufacture according to the invention; Fig.4 is a sectional view of the bump structure of Fig. 3, but taken further in the process of manufacture; Fig.5 is a sectional view of the completed bump structure; Fig.6 is a sectional view of another partly completed bump structure; and Fig.7 is a sectional view of the bump structure of Fig. 6 after completion Reference is first made to Fig. 1, which shows at 10 a silicon semi-conductor wafer 10 containing a large number of integrated circuits 12. (Wafer 10 may be 75 to 100 mm in diameter). Each of the circuits 12 contains a number of circuit termination points (not shown in Fig. 1) to which leads must be attached. As shown in Fig. 2 (which is prior art), lead attachment points are formed by thin aluminum pads 14 on the surface 16 of the silicon dioxide substrate 17 of the wafer 10. After the thin pads 14 are deposited on surface 16, the entire surface of the circuit (except for pads 14) is covered with a thin passivation layer 18, typically of glass or ceramic material, to protect the circuit and to act as an insulator. The passivation layer 18 overlaps the edges of the aluminum pads 14, leaving openings 19 which expose the aluminum pads 14. The openings 19 are always of dimensions smaller than those of pads 14, to seal the edges of pads 14. (The term "dimensions" refers to lateral dimensions.) Next, a barrier layer 20 of titanium tungsten (TiW) is applied over the aluminum pads 14, and then a gold bump 22 (other suitable metals and alloys can also be used) is electroplated over the barrier layer 20. The stem of the gold bump 22 is wide enough to overlap the edges of the passivation layer 18, i.e. the stem of the gold bump 22 has dimensions which are at least as large as those of the aluminum pad 14. This ensures that the aluminum pad 14 is completely sealed. Typically the bump 22 is square in plan view and is about 100 microns wide on each side. The difficulty with the Fig. 2 structure, as mentioned is that when a lead (indicated in dotted lines at 24) is compressed on to the bump 22 by a heated bonding tip 26 (also shown in dotted lines), the compression forces transmitted through the stem of the bump 22 tend to crack the passivation layer 18. This allows moisture and oxygen to reach the aluminum layer 14, causing the aluminum to oxidize and disappear. Reference is next made to Fig. 3 which illustrates the first stages of a process for making a bump which has a reduced tendency to cause cracking during thermo-compression bonding. In this process, the wafer 10, after the passivation layer 18 has been applied leaving the aluminum pads 14 partly exposed, is placed in a chamber where the surfaces of the exposed pads 14 are bombarded with energetic particles from a plasma to remove a very thin (submicron) oxide layer from the aluminum. Then, a very thin layer 30 (typically .15µm (1,500 angstroms))of barrier material (TiW) is sputtered on to the entire surface of the wafer. This is conventional. Next, in the same chamber, a further thin layer 32 of gold (typically .15µm (1,500 angstroms)) is sputtered onto the layer 30 to prevent the layer 30 from oxidizing. Layer 32 also covers the entire wafer. Again, this is conventional. The wafer 10 is next coated with a photo-resist layer 34 and is then masked and exposed to light. Only the areas over the pads 14 are exposed. The exposed photo-resist is then washed away, leaving openings 36 which are of the same dimensions as the pads 14 and are located over the pads 14, aligned with the pads 14. This is conventional. Next, the wafer is placed in a gold solution, and gold is electroplated into the openings 36, forming a layer 38 of gold. In the past, the layer 38 would have been built up to about 25 microns in thickness, forming the bump 22. (One micron = 10 -6 meters = 10,000 angstroms.) However in the present process, the layer 38 is very thin, although it is thicker than the sputtered layer 32. Typically the layer 38 is about three times as thick as the sputtered layer 32, i.e. it is typically about .45µm (4,500 angstroms) thick. As will be seen, a portion of layer 38 will form the lower portion of the final gold bump. Next, the photo-resist layer 34 is removed, leaving the thin layer 32 of gold covering the entire wafer, and also leaving partly formed bumps 38. Then, as shown in Fig. 4, a new photo-resist layer 40 is applied, masked, exposed to light, and the exposed portions are removed, leaving openings 42 in the remaining photo-resist layer 40. Each opening 42 is centered over the opening 19 in the passivation layer 18 and is of dimensions equal to or slightly smaller than those of opening 19. Thus, the margins of opening 42 are entirely within the margins of opening 19. Next, the wafer is again electroplated with gold, to build an upper gold bump portion 44 typically 25 microns in thickness. After the upper gold bump portion 44 has been formed, the resist layer 40 is removed, and then the entire layer 32 of gold overlying the wafer is etched away. This is accomplished by etching away about .15µm (1500 angstroms) of gold. The etching also reduces the thickness of the exposed margins of bump lower portion 38 from .45µm (4,500 angstroms) to .3µm (3,000 angstroms), and reduces the height of the upper gold bump portion 44 by .15µm (1,500 angstroms). Next, the barrier layer 30 of TiW is etched away (the etchant for TiW does not attack gold), leaving a final bump 50 as shown in Fig. 5. As shown, bump 50 has an upper or stem portion 44 which overlies opening 19 in the passivation layer 18 and does not overlap the passivation layer. In other words, the margins of stem 44 are entirely within those of opening 19. The bump 50 may have a top cap 52 which has larger dimensions (resulting from the electroplating overflowing the photo-resist layer) but the cap 52 does not create a problem, for the reasons to be described. Also, because step 44 is entirely within the margins of opening 19, and since the topography of each layer tends to follow that of the layer beneath it, the top 53 of cap 52 is essentially flat. Bump 50 also has a lower and very thin portion 38 which overlaps the passivation layer 18 to ensure that the aluminum pad 14 is properly sealed. When a lead is bonded to bump 50, the downward compressive forces will be applied through stem 44 directly to the aluminum pad 14. Because the margins of stem 44 are within those of opening 19, very little compressive force is applied to the overlapped edges of the passivation layer 18. While some force is applied to the overlapped edges of the passivation layer because of the presence of lower bump portion 38 and barrier layer 30, these layers are very thin and transmit little force. The force transmitted by the margins of lower bump portion 38 is also small because gold is soft and has low shear strength. The margins of bump portion 38 may range between .1µm (1,000 angstroms) and 1µm (10,000 angstroms) in thickness, but preferably they should be as thin as possible to transmit as little force as possible. Thus, it will be seen that since little force is transmitted to the passivation layer 18, therefore the likelihood of cracking is greatly reduced. The main compressive forces are applied directly to the aluminum pad 14 which is better able to withstand these forces. At the same time, the overlap of the passivation layer 18 provided by the lower bump portion 38 ensures that the passivation layer 18 is adequately sealed. In addition, since top 53 is now flat, the lead 24 will now be more likely to bond to the entire top surface of the bump, rather than only to the raised edges of the bump top surface, as had previously been the case. Figures 6 and 7 illustrate a different method for forming the bump in a device. However, the device and method described with reference to Figures 6 and 7 do not form part of the present invention. Referring to Figure 6, the upper bump portion 44 can be formed directly over opening 19, on gold layer 32. The margins of bump portion 44 will, as before, be within the margins of opening 19. Then, before gold layer 32 is etched away, its margins out to the edges of the aluminum pad 14 (and also the upper bump portion 44) are masked by a small area of photo-resist as shown at 60 in Fig. 6. The gold layer 32 is then etched away, leaving a remnant or skirt 32' (Fig. 7) which forms the lower portion of the bump 50' and seals the opening 19. A difficulty with this second method is that it requires very accurate tolerances which can be difficult to achieve. As mentioned, any suitable materials can be used, e.g. solder or copper, or other metals or alloys.

Claims: A method of forming a bump structure (50) on an integrated semiconductor device (12) having a surface (16), a laterally extending conductive termination pad (14) on said surface (16) and a protective passivation layer (18) covering said surface (16) and overlapping the edges of said pad (14), said passivation layer (18) having an opening (19) therein having edges, said opening (19) being of lateral dimensions between said edges of said opening (19) which are less than the lateral dimensions of said pad (14) to expose a portion of said pad (14) through said opening (19), the method including forming a lower bump portion (38,32,30) over said opening (19) to overlap the edges of said opening (19), the lower bump portion (38,32,30) being formed with a lower layer (30) of a barrier material, being located over said opening (19) and having side margins extending laterally beyond said edges of said opening (19) to cooperate with said passivation layer (18) to seal said pad (14), said lower layer (30) having lateral dimensions greater than the lateral dimensions of said opening (19), and forming an upper bump portion (44,52) having a top (53) and a stem (44) of gold, said stem (44) having side margins and being of lateral dimensions less than those of said opening (19), said stem being located over said lower bump portion (38,32,30) and over said opening (19) with said side margins of said stem (44) being entirely within said edges of said opening (19), said stem (44) being of thickness substantially greater than the thickness of said lower bump portion (38,32,30)characterised in that said lower bump portion (38,32,30) is formed by depositing a first layer of gold (32) over said barrier layer (30) and extending over said barrier layer (30), and then by depositing a second layer of gold (38) over said first layer (32) over said pad (14), said second layer (38) being thicker than said first layer (32), and then forming said stem (44) over said second layer (38), and etching all exposed gold layers to remove all of said first layer of gold (32) which is not covered by said second layer of gold (38) while removing some but not all of said second layer of gold (38) and said stem (44), so that when a lead (24) is compressed downwardly onto the top (53) of the stem (44,52), the compressive forces transmitted through said stem (44) will be primarily applied directly through said lower bump portion (38,32,30) to the portion of said pad (14) immediately below said stem (44) and will not tend to crack said passivation layer (18), and said side margins of said lower bump portion (38,32,30) form a protective sealing skirt extending laterally beyond said edges of said opening (19) to seal said pad (14). A method according to claim 1 wherein the side margins of said first and second gold layers (32,38) extending beyond said side margins of said stem (44) are in the range between 0.1µm (1,000 Angstroms) to 1µm (10,000 Angstroms) in thickness. A method according to claim 2 wherein said side margins of said first and second gold layers (32,38) extending beyond said side margins of said stem (44) have a total thickness of 0.45µm (4500 Angstroms) following said etching step.

Inventors: ERDOS GEORGE, ERDOS, GEORGE
Applicants: GENNUM CORP, GENNUM CORPORATION
Application year: 1993
Main classification: H01L23
IPC codes: H01L21, H01L23