Wafer Thinning for Advanced Packaging Applications

Driven largely by the growing need for more data, increased functionality, and faster speeds, consumer electronic devices have sparked a revolution in IC design. As it becomes increasingly more expensive and technically challenging to scale down semiconductor devices, Moore's law is yielding to the concept of “More than Moore”, which is driving integrated functionality in smaller and thinner packages. Packaging for 2.5D and 3D has become critical to new products requiring higher performance and increased functionality in a smaller package. The use of a Through Silicon Via (TSV) has been discussed as a method for stacking die to achieve a vertical interconnect. The high costs associated with this technology have limited TSV use to a few applications such as high-bandwidth memory and logic, slowing its adoption within the industry. Lower-cost advanced packaging concepts have been developed and are now in high-volume production. Recently, alternative methods for exploiting the z-direction have turned to variations of Fan-Out Wafer Level Packaging (FOWLP), which do not include TSVs. In many of these concepts there is a need to thin the wafer to remove all of the silicon while being selective and not etching a variety of other films that include oxides, nitrides, and metals. In addition, there can be temporary bonding adhesives and mold compounds encapsulating the chips; these must remain undamaged. Another critical element of a successful process is the ability to control the profile of the silicon etch to provide uniform removal. The single wafer wet etching techniques and advanced process control developed for TSV Reveal are applicable to these structures and provide a low-cost alternative to CMP and Plasma processes. To successfully execute the process, several characteristics must be met: the silicon overburden depth and profile need to be determined, the overburden thinning etch needs a fast sculpting etchant, and the finishing etchant needs to be selective to materials that will be exposed at the completion of the etch. In addition, the tool used to perform this sequence needs to have the correct metrology capability, along with properly chosen etchants. Similarly, it is not sufficient to know the required etch profile, the software must be able to execute a unique etch profile for each wafer. In this fashion, the finishing etch time can be kept to a minimum. This is important, as many of the selective etchants have a slow etch rate, and adhesives used do not always hold up to exposure to the chemistries involved for long periods. This paper discusses the use of wet etch wafer thinning processes for new FOWLP applications.