The present invention generally relates to a wafer packaging method and package formed and more particularly, relates to a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks as stress buffers without the coefficient of thermal expansion mismatch problem and package formed by the method.
In recent years, wafer level packages or wafer level chip scale packages have been developed as a new low cost packaging technique for high volume production of IC chips. One of the chip scale packaging techniques has been developed by the Tessera Company for making a so-called micro-BGA package. The micro-BGA package can be utilized in an environment where several of the packages are arranged in close proximity on a circuit board or a substrate much like the arrangement of individual tiles. Major benefits achieved by a micro-BGA package are the combined advantages of a flip chip assembly and a surface mount package. The chip scale packages can be formed in a physical size comparable to that of an IC chip even though, unlike a conventional IC chip such as a flip chip, the chip scale package does not require a special bonding process for forming solder balls. Furthermore, a chip scale package may provide larger number of input/output terminals than that possible from a conventional quad flat package, even though a typical quad flat package is better protected mechanically from the environment.
In a typical micro-BGA package, a flexible interposer layer (which may contain circuit) is used to interconnect bond pads on an IC chip to an array of solder bump connections located on a flexible circuit. The flexible circuit, normally of a thickness of approximately 25 nm, is formed of a polymeric material such as polyimide which is laminated to a silicon elastomer layer of approximately 150 nm thick. The silicon elastomeric layer provides flexibility and compliance in all three directions for relief of stresses and thermal expansion mismatches. To further reduce the fabrication cost of IC devices, it is desirable that if a whole wafer can be passivated to seal the IC dies on the wafer, and then be severed into individual IC dies from the wafer such that not only the benefits of a chip scale package can be realized, the packaging cost for the IC dies may further be reduced.
The conventional flip-chip bonding process requires multiple preparation steps for IC chips, i.e. the formation of aluminum bond pads on the chip, the under-bump-metallurgy process on the bond pads and the deposition of solder required in the bumping process. The substrate that the IC chip is bonded to requires a flux coating in order to ensure an acceptable bond strength is formed between the solder bumps and the conductive elements on the substrate surface. The flip chip bonding process further requires a reflow process for the bumps, a flux cleaning process to eliminate excess flux material from the surface of the bump, a drying process after the cleaning process, an underfill process for dispensing an underfill material, and an underfill curing process to minimize thermal stresses in the underfill and in the joint formed.
The conventional method for depositing solder bumps described above presents a number of processing difficulties. For instance, in modern high-density semiconductor devices, the distance between I/O pads in a peripheral array continuously being reduced. In order to maintain a minimal required distance between the I/O pads, an I/O pad redistribution process must be conducted such that the pads can be transformed from a peripheral array to an area array. During the pad redistribution process, a plurality of metal traces must be formed to extend the I/O pads from the periphery of an IC die to the center of the IC die. It is desirable that, in order to assure the reliability of the die, a stress buffer layer is provided under the plurality of metal traces to buffer, or absorb, the stress incurred during the fabrication processes and to avoid stress cracking or fracture of the metal traces. The application of the stress buffering layers has been difficult in that if too thin a layer is applied, the stress buffering effect is insufficient to ensure the reliability of the IC die. However, when too thicker a layer of the stress buffering material is applied, numerous processing difficulties are incurred in the application process. Even though commercial stress buffering materials have been available in the marketplace, the fabrication technology for applying such materials to a satisfactory thickness has not been developed.
One of the difficulties incurred in utilizing the stress buffering material is that, since the material must have a relatively low Young""s modulus, it also comes with a coefficient of thermal expansion (CTE) that is significantly larger than other materials normally used in semiconductor processing. The thermal stresses produced by the CTE mismatch may be large enough to cause damages to the solder bumps formed on top of the wafer. The CTE mismatch problem between the stress buffer material and the insulating or metal conductive layers must be resolved before an elastomeric material may be used as a stress buffering layer. Since it is extremely difficult, or impossible to slice or sever the elastomeric material layer after deposited onto the top surface of a wafer, i.e. either mechanically or by a laser, a method for separating the elastomeric material layer into smaller blocks is very desirable in order to fully utilize the technology.
It is therefore an object of the present invention to provide a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks as stress buffering layer that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming a wafer level package incorporating multiplicity of elastomeric blocks as a stress buffering layer that does not require any additional fabrication steps.
It is a further object of the present invention for forming a wafer level package incorporating a multiplicity of elastomeric blocks by first forming metal lines between the IC dies prior to the deposition of elastomeric material on the wafer.
It is still another object of the present invention to provide a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks by first depositing a thick photoresist layer for forming a plurality of metal lines and a plurality of trench openings between the IC dies.
It is yet another object of the present invention to provide a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks as stress buffering layer by first depositing photoresist layer of at least 20 xcexcm thickness for forming metal lines between the IC dies.
It is still another further object of the present invention to provide a wafer level package that has a multiplicity of elastomeric blocks formed on top as stress buffering layer without causing any CTE mismatch problem with other layers in the IC dies.
It is yet another further object of the present invention to provide a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks formed by first forming a plurality of metal lines between the IC dies and then removing the metal lines during a die sawing process for severing the IC chips.
In accordance with the present inventions, a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks as stress buffering layer in the package is disclosed.
In a preferred embodiment, a method for forming a wafer level package incorporating a multiplicity of elastomeric blocks can be carried out by the operating steps of first providing a wafer that is pre-processed with a plurality of IC dies formed on a top surface, each of the plurality of IC dies being spaced-apart from its immediately adjacent IC dies by a distance that is at least a width of a scribe line, each on the plurality of IC dies further having a multiplicity of input/output pads formed on top; depositing a first material layer on the top surface of the wafer; depositing a photoresist layer of at least 20 xcexcm thickness on the first metal layer; forming a plurality of trench openings separating the plurality of IC dies and a plurality of via openings each on top of one of the multiplicity of input/output pads exposing the first metal layer; depositing a second metal into and filling the plurality of trench openings forming a plurality of metal lines and the plurality of via openings forming a plurality of metal vias; removing the photoresist layer in the first metal layer that is not covered by the plurality of metal lines and the plurality of metal vias; and depositing an elastomeric material on top of the wafer filling cavities formed between the plurality of metal lines and metal vias forming the multiplicity of elastomeric blocks.
The method for forming a wafer level package incorporating a multiplicity of elastomeric blocks may further include the steps after the deposition of the elastomeric material of depositing a first insulating material layer on top of the plurality of elastomeric blocks; depositing a third metal layer on top of the first insulating material layer and forming into conductive leads for input/output redistribution; and depositing a second insulating material layer on top of the conductive leads and the first insulating material layer. The method may further include the steps of after the deposition of the second insulating material layer, forming a plurality of openings in the second insulation material layer exposing the third metal layer forming a plurality of solder bumps on the plurality of input/output redistribution pads.
The method may further include the step of forming under-bump-metallurgy (UBM) layers on the plurality of input/output redistribution pads before the formation of the plurality of solder bumps, or the step of depositing the first metal layer with a metal of Cu, Al, or alloys thereof, or the step of depositing the photoresist layer to a thickness between about 20 xcexcm and about 200 xcexcm. The method may further include the step of depositing the second metal with a metal of Cu, Al or alloys thereof. The formation step for the plurality of trench openings and via openings may further include the steps of photolithography and dry etching.
The method may further include the step of depositing the elastomeric material by a printing technique or by a spin-coating technique. The method may further include the step of forming the plurality of solder bumps by a technique selected from the group consisting of printing, electroplating and electroless plating, or the step of forming the plurality of solder bumps with a solder material containing Pb and Sn. The method may further include the step of severing the plurality of IC dies by a mechanical means of by wet etching.
The present invention further discloses a wafer level package that has a multiplicity of elastomeric blocks formed on top including a wafer that is pre-processed with a plurality of IC dies formed on a top surface. Each of the plurality of IC dies has a multiplicity of input/output pads formed on top; a plurality of metal lines separating the plurality of IC dies formed on a plurality of scribe line on the top surface of the wafer; a plurality of metal vias each in electrical communication with one of the plurality of input/output pads; and an elastomeric material filling cavities formed in-between the plurality of metal lines forming the multiplicity of elastomeric blocks.
The wafer level package that has a multiplicity of elastomeric blocks formed on top may further include a first insulating material layer on top of the multiplicity of elastomeric blocks; a plurality of conductive leads for input/output redistribution wherein one end of each of the plurality of conductive leads being in electrical communication with one of the plurality of metal vias; and a second insulating material layer on top of the plurality of conductive leads and the first insulating material layer. The wafer level package that has a multiplicity of elastomeric blocks on top may further include a plurality of openings in the second insulating materials layer exposing the third metal layer forming a plurality of input/output redistribution pads; and a plurality of solder bumps with each formed on one of the plurality of input/output redistribution pads. The wafer level package may further include UBM layer in-between the plurality of input/output redistribution pads and the plurality of solder bumps. The plurality of solder bumps may be reflown into a plurality of solder balls. The plurality of metal lines may be formed of Cu, Al or alloys thereof, while the plurality of metal vias may be formed of Cu, Al or alloys thereof, the elastomeric material has a Young""s modulus of not higher than 10 MPa.