Wafer bumping process with solder balls bonded to under bump metallurgy layer formed over active surface by forming flux on solder ball surfaces and reflowing the solder

A bumping process, which is a method of forming a plurality of bumps over a wafer, is provided. The wafer has an active surface having a passivation layer and a plurality of bonding pads thereon. The passivation layer exposes the bonding pads on the active surface. An adhesion layer is formed over the active surface of the wafer covering both the bonding pads and the passivation layer. A metallic layer is formed over the adhesion layer. The adhesion layer and the metallic layer are patterned, so that the adhesion layer and the metallic layer remain on top of the bonding pads. A photoresist layer is formed on the active surface of the wafer. The photoresist layer has a plurality of openings that exposes the metallic layer. Next, solder balls with a solidified material on the surface of each solder ball are disposed into each opening. Then, a reflow process is carried out, so that the solder balls bond with the metallic layer. Finally, the photoresist layer is removed.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a bumping process. More particularly, the present invention is related to a method of forming high-quality bumps for a high-density package.

2. Related Art

In this information explosion age, integrated circuit products are used almost everywhere in our daily life. As fabricating technique continue to improve, electronic products having powerful functions, personalized performance and a higher degree of complexity are produced. Nowadays, most electronic products are relatively light and have a compact body. Hence, in semiconductor production, various types of high-density semiconductor packages have been developed. Flip chip is one of the most commonly used techniques for forming an integrated circuit package. In a flip-chip package, the bonding pads on a die and the contacts on a substrate are connected together through a plurality of bumps. Hence, compared with a wire-bonding package or a tape automated bonding (TAB) package, a flip-chip package uses a shorter electrical path on average and has a better overall electrical performance. Moreover, the back surface of a flip-chip die may be exposed to the outside to increase the performance of the heat dissipation of said flip chip package. Due to the above and other reasons, flip-chip packages are produced in large volumes in the semiconductor industry.

FIG. 1toFIG. 4are partially enlarged cross-sectional views showing the progression of steps in a conventional method of forming a bump on the surface of a metallurgy layer120includes an adhesion layer and one or a stack of metallic layers, for example a barrier layer and a wetting layer. To form the under bump metallurgy layer120, a sputtering process is first conducted to form an adhesion layer on the active surface112of the wafer110. Next, a sputtering or plating process is conducted to form one or more metallic layers over the adhesion layer. Thereafter, photolithography and etching processes are used to pattern the under bump metallurgy layer120so that a residual portion of the under bump metallurgy layer120remains on top of the bonding pad116.

As shown inFIG. 2, a spin-coating process is conducted to form a photoresist layer130over the active surface112of the wafer110, wherein the photoresist layer130can be a dry film. Through photolithography and etching processes, a plurality of openings132(only one opening is shown) are formed in the photoresist layer130. The openings132expose the under bump metallurgy layer120. Next, as shown inFIG. 3, a flux material160is dispensed in the openings132and above the surface of the photoresist layer130. Afterwards, a solder ball mounting process is performed to place the solder balls140in the openings132as shown in FIG.3. Then a reflow process is performed to dispose the solder balls above the bonding pads116more securely as shown inFIG. 4, wherein the solder balls140are directly mounted onto the under bump metallurgy layers120and the flux material160flows on the surfaces of the solder balls140and vaporized. Thereafter, a liquid cleaner is applied to remove the residual flux material from the surface of the solder balls140. Finally, the photoresist layer130is removed from the active surface112of the wafer110as shown inFIG. 4so that a bump150is produced. Therein, the bump150actually comprise the solder ball140and the under bump metallurgy layer120.

In the aforementioned fabrication process, the solder ball140is disposed into the openings132of the photoresist layer130by a solder ball placer. However, the flux material160is easily flowed into the solder ball placer so as to contaminate said solder ball placer. In addition, the flux material160will enhance the connection between the solder balls so as to lower the performance and operation efficiency of said solder ball placer.

Therefore, providing another method for forming bumps to solve the mentioned-above disadvantages is the most important task in this invention.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, an objective of this invention is to provide a method of forming bumps, and more particularly, a method of forming high-quality bumps inside a high-density package.

To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of forming a plurality of bumps over a silicon wafer. The wafer has an active surface having a passivation layer and a plurality of bonding pads formed thereon. The passivation layer exposes the bonding pads on the active surface. To form the bumps, an under bump metallurgy layer is formed over the active surface of the wafer covering both the bonding pads and the passivation layer. Next, the under bump metallurgy layer are patterned so that a residual portion of the under bump metallurgy layer remains on top of the bonding pads. Thereafter, a photoresist layer is formed on the active surface of the wafer. The photoresist layer has a plurality of openings that exposes the residual portion of the under bump metallurgy layer. Then, a plurality of solder balls are provided to dispose in the openings of the photoresist layers by a solder balls placer, wherein each solder ball has a solid-like flux material or a solid flux material on the surface of said solder ball. Afterwards, a reflow process is carried out so that the solder balls will connect to the residual portion of the under bump metallurgy layer. Finally, the flux material is cleared and the photoresist layer is removed.

According to one preferred embodiment of this invention, the material constituting the under bump metallurgy layer includes aluminum, titanium, titanium-tungsten alloy, chromium, chromium-copper alloy, copper or tantalum. In addition, the material constituting the under bump metallurgy layer includes nickel-vanadium alloy, titanium nitride compound, tantalum nitride compound, nickel, chromium-copper alloy, chromium, copper, palladium or gold. The material constituting the solder ball includes lead-tin alloy or lead-free alloy, and the lead-free alloy includes tin, gold, copper, magnesium, bismuth, antimony, indium, zinc or an alloy made up from any combination of the elements in the above list. Moreover, the material constituting the bonding pad includes copper or aluminum.

As mentioned above, the surface of each solder ball has a solid-like flux material or a solid flux material. Accordingly, when the solder balls are disposed in the openings, it will not cause the flux material contaminate the solder ball placer. Moreover said flux material can not make the solder balls connect with each other more securely. Thus it cannot lower the operation efficiency of the solder ball placer.

DETAILED DESCRIPTION OF THE INVENTION

The method of forming bumps according to the preferred embodiment of this invention will be described herein below with reference to the accompanying drawings, wherein the same reference numbers are used in the drawings and the description to refer to the same or like parts.

FIGS. 5to9are partially enlarged cross-sectional views showing the progression of steps for forming a bump on the surface of a chip according to one preferred embodiment of this invention.

As shown inFIG. 5, a silicon wafer210having an active surface212thereon is provided. The active surface212of the wafer210further includes a passivation layer214and a plurality of bonding pads216(only one is shown). The passivation layer214exposes the bonding pads216. The bonding pads216are aluminum or copper pads, for example. A process to form an under bump metallurgy layer220over the bonding pad216is performed. First, an adhesion layer is formed over the active surface212of the wafer210by sputtering. Thereafter, one or more metallic layers are formed over the adhesion surface by sputtering or electroplating. Hence, the under bump metallurgy layer220has a structure that includes a single adhesion layer and a single or a stack of metallic layers, such as a barrier layer or a wetting layer. Photolithography and etching processes are carried out to pattern the under bump metallurgy layer220so that only a residual portion of the under bump metallurgy layer220remains on the top of the bonding pads216. The adhesion layer is made from a material including, for example, aluminum, titanium, titanium-tungsten alloy, chromium, chromium-copper alloy, copper or tantalum. The metallic layer is made from a material including nickel-vanadium alloy, titanium nitride compound, tantalum nitride compound, nickel, chromium-copper alloy, chromium, copper, palladium or gold. Details of their structures can be found in U.S. Pat. App. No. 20030189249 and 20030164552.

Afterwards, a polymer layer230, for example a photoresist layer and a dry film, is provided over the active surface212of the wafer210as shown in FIG.6. Therein, a spin-coating process is performed to form a photoresist layer230over the active surface212of the wafer210and the dry film is directly attached on the active surface212of the wafer210. Next, a photolithography and etching process are carried out to form a plurality of openings232(only one opening is shown) in the polymer layer230. The openings232expose the under bump metallurgy layer220.

As mentioned above, there is further provided solder balls250with a solid-like or solid flux material252formed on the surfaces and said solder balls250are disposed in the openings232by the solder ball placer (not shown) as shown in FIG.7. Therein, the solder balls250can be in the form of exact balls or ball-like shape, and the solder balls250are made from a material including lead-tin alloy, tin, gold or other lead-free alloys. The lead-free solder balls250are mainly binary, tertiary, quaternary alloy consisting of some of the following metals: tin, gold, silver, copper, magnesium, bismuth, antimony, indium and zinc. Since the metals can be combined in different proportions, there are virtually countless types of lead-free blocks. In general, each type of lead-free solder ball has a unique set of physical and electrical properties.

The material constituting the solder balls may include a lead-tin alloy. More particularly, the material constituting the solder balls is selected from the group consisting of lead, gold, silver, copper, magnesium, bismuth, antimony, indium and zinc. This obviously includes mixtures of these materials.

Then, through the flux material252on the surface of said solder ball250, the lower portion of the solder ball250melts in a reflow process so that the solder ball250and the under bump metallurgy layer220are bonded together as shown in FIG.8. In the reflow process, foaming gas, for example nitrogen gas or hydrogen gas, can be filled in the reflow chamber so as to make the oxygen gas to activate with hydrogen gas or to lower the capability of the oxygen gas for activating by the nitrogen gas. Thus the surfaces of the solder balls250can be prevented from oxidation. Next, a liquid cleaning agent is used to remove the residual flux material240from the surface of the solder balls250. Finally, the polymer layer230is removed from the active surface212of the wafer210and the process for forming bumps260is completed as shown in FIG.9. Thus, the bump260is a composite structure comprising the solder ball250and the under bump metallurgy layer220.

In the aforementioned process, the solder balls have a solid-like or solid material formed on the surfaces of the solder balls. Thus when the solder balls are disposed in the openings of the polymer layers, the flux material will not contaminate the solder ball placer. Moreover, said solid-like flux material and said solid flux material will not make the solder balls connecting and joining with each other, so the operation efficiency of the solder ball placer will be upgraded.

Although the invention has been described in considerable detail with reference to certain preferred embodiments, it will be appreciated and understood that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.