Patent Application
20120094418
The present invention relates to packaged electrical devices and in particular to wafer level packaged acoustic wave devices.
Typically, a wafer level package device includes a device wafer, comprising a Si, GaAs, LiTaO3, LiNbO3, or glass material, and the like, with an active area on a surface of the material that needs to be protected. To realize a clear cavity on an active area, a common technique is to use a sacrificial layer on carrier wafer that will support the pattern to transfer. The carrier wafer is the bonded to the device wafer and released either by etching, thermal decomposition or viscosity change of the sacrificial material.
By way of example and as described in U.S. Pat. No. 7,288,435 to Aigner et al., one method for producing a cover for a region of a substrate includes first producing a frame structure in a defined region of the substrate, and then attaching a cap structure to the frame structure so that the region under the cap structure is covered. As a result, sensitive devices may be protected from external influences and particularly from a casting material for casting the entire packaged device, which typically results when a diced chip is cast. One method described for producing such covers for a plurality of regions on a system wafer, each region having a device, wherein contact pads for the devices are provided outside each region, comprises producing a frame structure for each region of the system wafer. The step of producing the frame structure includes spinning a photostructurable epoxy resin material onto the system wafer, exposing the photostructurable epoxy resin material, developing the photostructurable epoxy resin material, and removing the epoxy resin material defined by the exposure to obtain the frame structure for each region of the system wafer. A support wafer having a sacrificial layer is provided for attaching a cap structure for attaching the cap structure to the frame structure so that the region between the cap structure and the system wafer is covered. The cap producing step includes spinning a photostructurable epoxy resin material onto the sacrificial layer on the support wafer and structuring the photostructurable epoxy resin material to produce a cap structure. The method further includes connecting the cap structure with the frame structure and removing the sacrificial layer from the support wafer to separate the cap structure from the support wafer, wherein the structuring occurs when producing the cap structure prior to removing the sacrificial layer or after the step of removing the sacrificial layer.
Typically, the cavity created by the frame and cap structures need to be cleared of any residue coming from the sacrificial layer. Further, the carrier wafer is typically recycled for it is not left in a reusable condition. It is desirable to be able to manufacture the above described cover for the devices without the need for a sacrificial layer and the costly steps associated with its use.
In view of the foregoing background, one embodiment of the invention is directed to a method based on creating a clear cavity over an active area by using a carrier wafer holding either a cap pattern or the cavity itself that will be bonded and then transferred (debonding) on the device wafer without using any sacrificial material or temporary material. The invention teaches a technique using a poor interface adhesion between the carrier wafer and the negative photo sensitive epoxy.
One embodiment of the invention is realized in a method for producing a wafer level package. The method may comprise positioning a plurality of devices on a surface of a device wafer and forming an electrically non-conducting frame structure on the device wafer around each of the plurality of devices. A surface of a carrier wafer is coated with an adhesive material having low surface adhesion strength and a cap structure is placed onto the coated surface of the carrier wafer for temporarily attaching the cap structure to the carrier wafer. A bonding material is placed on the frame structure or the cap structure, with the bonding material being such that it has greater adhesion strength than the adhesive material. The cap structure is attached to the frame structure via the bonding material. The carrier wafer may then be separated from the cap structure and thus the device wafer with a force sufficient for separating the carrier wafer from the cap structure while maintaining the cap structure attachment to the frame structure. The carrier wafer may be reused after cleaning as desired.
The method may further include the step of coating a surface of the carrier wafer with an adhesive material comprising the step coating the surface of the carrier wafer with a photosensitive polymer. Yet further, the step of coating the surface of the carrier wafer with a photosensitive polymer may comprise coating the surface with an SUB epoxy. As desired, the carrier wafer separating step may comprise applying heat.
The method may further comprise a metallization layer on the surface of the carrier wafer, wherein the adhesive material is coated onto the metallization layer. The metallization layer may be applied by sputtering or evaporating a metallic material onto the surface of the carrier wafer. Yet further, the metallization layer may result from coating the carrier wafer with Ti/Ni, Ti/Au, or Ti/AICu (99/1).
For a fuller understanding of the invention, reference is made to the following detailed description, taken in connection with the accompanying drawings illustrating various embodiments of the present invention, in which:
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
With reference initially to
With continued reference to
As illustrated with reference to
By way of example, epoxy resins used as photo definable polymers are well known to be classified in three stages, A, B and C. It is also generally well known that the A-Stage for an epoxy resin refers to an early stage in a reaction of certain thermosetting resins in which the material is fusible and still soluble in certain liquids. The B-Stage refers to an intermediate stage in which the material generally softens when heated and swells when in contact with certain liquids, but may not entirely fuse or dissolve. Uncured resins are usually in this B-Stage. The C-Stage is a final stage in the reaction in which the material is relatively insoluble and infusible. Certain thermosetting resins in a fully cured state are in this C-Stage.
While the polymers 22, 28 are in the partially cured state (state A or B, by way of example), the cap structure 30 is attached to the frame structure 20 so as to place the devices 16 within a cavity 32, wherein sufficient pressure, force, or heat 34 is applied to the cap structure, frame structure, or both to bond the frame structure to the cap structure via a bonding of the partially cured photo definable polymers 22, 28. The bonding is characterized by second adhesion strength greater than the first adhesion strength securing the cap structure 30 to the carrier wafer 24.
As illustrated with reference to
For multiple device structures as earlier described with reference to
One embodiment of the invention makes use of a relatively poor adhesion interface between an SU8 photo sensitive epoxy used for both the first and second polymers 22, 28, where the SU8 photo sensitive epoxy is used to from the frame and cap structures 20, 30.
By way of further example, and with reference again to
Using an SU8 photo sensitive epoxy for the polymers 22, 28, the polymers may be coated directly onto the surface 26 of the carrier wafer 24 if the carrier wafer is made of LiNbO3. Alternatively, the SU8 photo sensitive epoxy may be coated onto a metallization layer 40 that has been sputtered or evaporated onto the surface 24 of the carrier wafer 22. If used, the metallization layer 40 illustrated with reference to
Using the SU8 polymer, permanent structures, such as the cap 30 and the frame 20, can be applied on the carrier wafer 24 and the device wafer 14, respectively, by various techniques such as spin coating, lamination for dry films, or spray coating. By way of example, the first polymer may be formed into the frame structure around each of the plurality of devices by spinning an epoxy resin material onto the device wafer, exposing the epoxy resin material, developing the epoxy resin material, and removing the epoxy resin material defined by the exposure to obtain the frame structure. Forming the second polymer into a cap structure may likewise include spinning an epoxy resin material onto the carrier wafer and structuring the epoxy resin material to produce the cap structure.
As above provided and with reference to
In an alternate embodiment, and as illustrated with reference again to
The process of the present invention above described desirably avoids using a sacrificial layer to transfer patterns from the carrier to the device wafer. When using a sacrificial layer in the transfer steps, cavity pattern or structure then needs to be cleaned of any residues coming from the sacrificial layer used and the carrier wafer if recycled. Therefore, one clear benefit desirable by those skilled in the art includes no need for a sacrificial material and no need for cleaning the transferred structure.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.
