Bumpless bonding process having multilayer metallization
DESCRIPTION 
     1. Technical Field 
     The present invention relates generally to electrically connecting a bond lead to an input/output pad of an integrated circuit and relates more particularly to bumpless attachment of a tape automated bonding frame to an integrated circuit chip. 
     2. Background Art 
     The conventional method of attaching bond leads of a tape automated bonding frame to contact pads of an integrated circuit die requires forming conductive bumps on the contact pads. The contact pads are typically aluminum. Gold is electroplated at the contact pads to a height of approximately 25 μm. Lead ends of the bonding frame are then brought into contact with the electroplated gold bumps and the temperature is elevated to electrically and mechanically bond the lead ends to the gold bumps. 
     U.S. Pat. No. 4,842,662 to Jacobi identifies a number of difficulties with this conventional process. One identified difficulty is the complexity of fabricating gold bumps. Furthermore, there are certain spatial and mechanical drawbacks resulting from the fact that the gold bumps often have a rounded top contour. The contour causes mating problems. For example, when a flat lead end is forced onto a rounded top contour, the lead end may slide somewhat, potentially causing contact between adjacent leads. 
     Jacobi describes an alternative method of attaching bonding frame leads to the aluminum input/output pads of an integrated circuit chip. The lead ends are brought into direct contact with the aluminum input/output pads. The lead ends are ultrasonically vibrated to cause a bonding of the materials. Typically, the frame leads are copper members having a coating of gold. The ultrasonic vibration bonds the three materials. This operation is also described in U.S. Pat. No. 4,917,286 to Pollacek, and is referred to as a &#34;bumpless&#34; tape automated bonding (TAB) method. Jacobi and Pollacek teach that because the method allows direct connection between the lead ends and the aluminum input/output pads, the manufacturing cost is reduced and manufacturing yield is improved. 
     It is an object of the present invention to provide a method which further improves the interconnection of a lead frame to input/output pads of an integrated circuit chip. 
     SUMMARY OF THE INVENTION 
     The above object has been met by a method which is able to achieve the advantages of both the conventional gold bump process and the bumpless bonding method. The present method utilizes ultrasonic bonding and is bumpless, but a film of gold is formed on an aluminum input/output pad prior to attachment to a lead end of a bond lead. The gold film provides a more reliable electrical and mechanical connection. 
     An adhesive layer is sandwiched between the aluminum input/output pad and the gold film. In a preferred embodiment, a diffusion barrier layer is formed between the adhesive layer and the gold film. Each of the three layers that are atop the input/output pad can be formed using sputtering techniques. 
     The gold film retards oxidation of the aluminum input/output pad, thereby improving stability of the contact area. Moreover, gold has a lower contact resistance than aluminum. The diffusion barrier layer below the gold film prevents the gold from diffusing into the aluminum. The diffusion barrier layer may be palladium, nickel, chromium or platinum, but this is not critical. 
     The adhesive layer has desirable characteristics with respect to adhering to the aluminum input/output pad. Possible materials include commercially available titanium-tungsten (TiW), titanium and chromium. Optionally, the adhesive layer and the diffusion barrier layer may be combined, as for example by nitrifying a TiW or Ti layer to achieve the desired adhesion and barrier characteristics. 
     An advantage of the present invention is that improved mechanical and electrical results are possible without requiring steps having the complexity of forming gold bumps. The three layers can be sputtered onto the integrated circuit, whereafter the combination of vibrational energy and pressure is used to connect lead ends to input/output pads. Another advantage is that the multi-layer scheme forms a hermetic seal for the aluminum input/output pad. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of an integrated circuit chip having an input/output pad on an upper surface in accordance with the present invention. 
     FIG. 2 is a side sectional view of the chip of FIG. 1 having an adhesive layer. 
     FIG. 3 is a side sectional view of the chip of FIG. 2 having a diffusion barrier layer. 
     FIG. 4 is a side sectional view of the chip of FIG. 3 having a noble metal layer. 
     FIG. 5 is a side sectional view of the chip of FIG. 4 having photoresist material patterned thereon. 
     FIG. 6 is a side sectional view of the chip of FIG. 5 following etching of metallic layers exposed at the sides of the photoresist material. 
     FIG. 7 is a side sectional view of the chip of FIG. 6 following removal of the photoresist material. 
     FIG. 8 is a side sectional view of a second embodiment of the chip of FIG. 7. 
     FIG. 9 is a side sectional view of the chip of FIG. 7 having a bond lead ultrasonically bonded thereto. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     With reference to FIG. 1, an aluminum input/output pad 10 is shown on a semiconductor substrate 12. &#34;Aluminum&#34; is defined as including aluminum alloys and other such highly conductive materials used to form input/output pads on an integrated circuit. Also shown is a passivation layer 14 that extends onto the input/output pad. The passivation layer may be formed by chemical vapor deposition of a nonconductive material such as silicon oxynitride. The passivation layer may have a thickness in the range of 0.5 micron to 3 microns, but this is not critical. 
     The aluminum input/output pad 10 is used to supply a signal or a utility to the semiconductor substrate 12. Alternatively, the pad is of the type to channel a signal from the semiconductor substrate to another component, not shown. 
     In FIG. 2, the upper surface of the semiconductor substrate 12 has been coated with an adhesive layer 16. While not critical, the preferred material is titanium-tungsten. The titanium-tungsten is sputtered onto the passivation layer 14 and the input/output pad 10. A typical thickness is 1000 to 2000Å. The titanium-tungsten exhibits the desired characteristics with regard to step coverage and adhesion to the passivation layer and the input/output pad. Other possible materials are titanium and chromium. As will be described below, the use of a nitrided titanium layer or a nitrided titanium-tungsten layer allows the material to also serve the function of retarding diffusion of atoms from subsequently deposited layers into the aluminum of the input/output pad 10. 
     Following formation of the adhesive layer 16, a diffusion barrier layer 18 is deposited onto the upper surface of the substrate. As shown in FIG. 3, the diffusion barrier layer coats the adhesive layer 16. Again, a material having high electrical conductance is required. In a preferred embodiment, palladium is selected, but other materials, such as nickel, palladium, chromium or platinum, can be employed. The diffusion barrier layer is sputtered to a thickness of 1000 to 2000Å. The layer has a sufficiently low porosity to retard diffusion of atoms between layers on opposite sides of the diffusion barrier layer. 
     In FIG. 4, another conductive layer has been added to the semiconductor substrate 12. A gold layer 20 coats the diffusion barrier layer 18. While the thickness of the gold layer is not critical, the thickness is preferably in the range of 2000 to 15,000Å. The function of the gold layer is to provide a surface for ultrasonic bonding of a lead frame to the semiconductor substrate. Moreover, the gold layer acts to inhibit oxidation of the aluminum input/output pad 10, thereby extending the life and the reliability of the device. While other noble metals may be used, gold is the preferred material. 
     Referring now to FIGS. 5 and 6, photolithographic techniques are utilized to pattern the gold layer 20, the diffusion barrier layer 18, and the adhesive layer 16. A photoresist layer 22 is formed and patterned to cover only that portion of the gold layer 20 that is to be used for contact to a bond lead. An etchant removes the exposed metal of the gold layer 20, the diffusion barrier layer 18, and the adhesive layer 16. 
     The photoresist layer 22 is then removed. As shown in FIG. 7, the result is a tri-metal structure that functions as a hermetic seal for the aluminum input/output pad 10 and that provides a desirable surface for ultrasonic bonding. As compared to gold bump contacts in which the gold bumps have a height of approximately 25 μm, the thin film gold layer 20 does not add significantly to fabrication complexity. Rather, the tri-metal structure is manufactured using known sputtering and photolithographic techniques. While electroplating the gold layer is an alternative embodiment, the sputtering technique increases manufacturing yield and decreases fabrication complexity. 
     As noted above, the adhesive layer may undergo treatment to incorporate nitride so as to increase the diffusion-barrier characteristics of the layer. Where titanium or titanium-tungsten has undergone such treatment, the resulting TiN or TiWN layer 24 shown in FIG. 8 substitutes for the separate adhesive and barrier layers described above. However, the remainder of the structure is unchanged. 
     FIG. 9 illustrates the use of a bonding tool 26 to simultaneously apply pressure and vibrational energy to the end of a lead 28 of a tape automated bonding frame. Ultrasonic energy at a frequency of 60 kHz having a movement of approximately 75 microinches may be employed, but this is not critical. Heating the structure during the bonding process offers advantages. However, the bonding process does not require the application of heat. A proper bond is achieved in less than one second. 
     For best bonding results, the copper leads should have a hardness in the range of 100 KH 50  ±30 as tested per ASTM 8-578-80.