Patent Application
20060046010
1. Field of the Invention
The present invention relates generally to semiconductor work pieces or wafers and, more particularly, to the use of stereolithography to create a fixture to hold the work piece in position within a coating system and the sealing barrier or support structure that is built stereolithographically about the periphery of the work piece.
2. Description of the Related Art
A manufacturing technique known as stereolithography, which utilizes layer by layer manufacturing of UV curable photopolymers, has been developed to a sufficient degree to permit its widespread use in multiple industries. Stereolithography, as conventionally practiced, utilizes a computer to generate a three-dimensional mathematical simulation or a model of an object to be fabricated, normally generating the simulation or model by converting computer-aided design (CAD) data into a file format known as stl. The mathematical simulation is mathematically separated or sliced into cross-sections which are used in a vertical assembly of superimposed layers to create the desired three-dimensional object. This process was originally described in U.S. Pat. No. 4,575,330 to Hull and is currently assigned to the assignee of the present invention. The slicing techniques employed in stereolithography are described in U.S. Pat. Nos. 5,059,359; 5,184,307; 5,345,391; and 5,137,662, all issued to Hull et al. and assigned to the assignee of the present invention.
The application of sterelithography has evolved initially from early rapid fabrication of molds and prototypes of objects from CAD files to developing and refining object designs of relatively inexpensive materials and, more recently, to directly manufacturing relatively small quantities of objects where it is economically prohibitive to employ conventional fabrication techniques. Custom fabrication of products has become more widely accepted and explored. Most recently, stereolithography has been used to apply material to substrates with preformed electronic components, resulting in structures with the high degree of precision that stereolithography affords. This is especially helpful in the manufacture of semiconductor wafers or work pieces. This approach has been especially actively pursued in packaging of semiconductor dice and the creation of encapsulated semiconductor wafers or work pieces. Micron Technology, Inc. of Boise, Id. has been particularly active in patenting and publishing applications related to the use of stereolithography in the production of semiconductor devices. The following table lists the relevant publications pursued by Micron Technology, Inc.
As the evolution of the application of stereolithography to semiconductor work pieces or wafers has progressed, there has been an increased desire to create thinner work pieces or wafers. The semiconductor devices are placed on a substrate and it is the excess substrate that is removed, usually by an acid etch process. For example, where the substrate is silicon, the excess silicon on the reverse side from the semiconductor devices is etched away by a particular acid etch mixture. One problem that arises with this process is the relative fragility of the already thin work pieces or wafers. The work pieces are easily fractured. During the etching process chips or cracks also can occur in the edge or periphery of the work piece. The chips can lead to cracks, or the cracks themselves will propagate and run through the work piece or wafer, essentially ruining the semiconductor devices that had been applied to the substrate. There is a need for a way to strengthen these work pieces to reduce their vulnerability to flexural stress and where fracture does occur, to salvage some of the semiconductor devices.
Additionally, where an acid etch process is employed, one approach has used a technique that places the semiconductor work piece on a carrier or chuck device to hold the wafer in place while the unlithographed side free of any semiconductor devices is etched away. This approach requires the use of some sealing means to prevent the highly corrosive acid solution from getting into the area where the circuitry patterns are applied to the working side of the substrate. Any seal or barrier must be precisely and accurately placed, resistant to the acid etch solutions, and be easily removable without damaging the semiconductor devices within the selected area of the wafer upon completion of the acid etch or back-etch process.
These problems are solved by the present invention which supplies a support structure that sealingly emplaces a barrier about the periphery of the semiconductor work piece or wafer to simultaneously strengthen the wafer and prevent the incursion of corrosive acid etch solution into the area containing the semiconductor devices during the acid etch process.
It is an aspect of the present invention that a UV curable stereolithographic seal or barrier and support structure for semiconductor work pieces or wafers is provided by stereolithographically creating a fixture to hold and place the work piece or wafer within the coating system, thereby building a barrier about the periphery of the wafer external to the circuitry pattern embedded on the wafer.
It is another aspect of the present invention that a vision system is used to ensure precise and accurate exposure of the UV curable material to ensure the precise and accurate creation and placement of the support structure.
It is a feature of the present invention that the seal or barrier and support structure is of sufficient thickness to resist an acid etch process and prevent damage from occurring to the semiconductor devices or circuitry patterns on the front or first side of the semiconductor work piece interiorly of the structure in a selected area of the work piece.
It is another feature of the present invention that the support structure is of sufficient height to prevent acid from overflowing into the circuitry pattern area during the etching process in a device or carrier used to accomplish the etching.
It is another feature of the present invention that a UV curable stereolithographic resin is used to provide an acid resistant material that forms the support structure and serves as the seal and barrier to the impingement of acid into the circuitry pattern.
It is an advantage of the present invention that the support structure is a sealing barrier that is built directly onto the semiconductor work piece or wafer.
It is another advantage of the present invention that the support structure increases the strength of the semiconductor work piece or wafer and makes it more resistive to flexural failure and easier to handle.
It is still another advantage of the present invention that the support structure helps prevent cracking and chipping about the periphery of the semiconductor work piece or wafer that destroys semiconductor devices in the wafer.
It is another advantage of the present invention that the support structure serves as a barrier and an effective seal against the corrosive acids used to acid etch excess substrate, such as silicon, from the semiconductor work piece on the opposing second side during a back etch process.
These and other aspects, features and advantages are obtained by the use of stereolithography to create directly on the work piece a barrier and seal that serves as a support structure for the semiconductor work piece and prevents the incursion of corrosive acids into the area containing the semiconductor devices. The support structure is applied directly to a first side of a work piece on which in a selected area there is a desired circuitry pattern. The support structure is applied layer by layer in a stereolithographic process completely about and interiorly of the periphery of the work piece, but external to the selected area; the support structure being of a desired height and of a material resistive to an acid etch process effective to seal the circuitry pattern in the selected area from acid when the work piece is subjected to an acid etch on its opposing second side and about the periphery. The support structure further strengthens the work piece against flexural failure.
These and other aspects, features and advantages of the invention will become apparent upon consideration of the following detailed disclosure of the invention, especially when it is taken in conjunction with the accompanying drawings wherein:
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After completion of the now raised fixture 11, the work piece 10 is snapped into place inside of the abutments 18 and aligned by the alignment nib 19. The abutments 18 retain the wafer or work piece 10 in position so that the stereolithographic vision system ensures accurate exposure of the photocurable resin material used to form the support structure 16 during the subsequent build process. The support structure 16 is built, again in a layerwise fashion, about the periphery 15 of the work piece 10, ensuring that the support structure 16 does not extend or intrude into the selected area on which the circuitry pattern forming the semiconductor devices 12 has been placed. The semiconductor devices 12 are created by a separate lithographic or other appropriate process to form them on the semiconductor work piece or wafer 10. The photocurable or photopolymer resin adheres directly to the work piece 10 to form an effective seal against the top or first side 14 of the semiconductor work piece 10.
Upon completion of the building of the support structure 16 and optionally the application of an encapsulating layer of a photopolymer resin, the semiconductor work piece 10 is removed from the coating system and its fixture 11 and exposed to a post-curing apparatus wherein UV light is applied to fully cure the material. U.S. Pat. No. 5,164,128 to Modrek et al. and assigned to the assignee of the present invention describes an appropriate post-curing process that can be employed. Once fully cured, the work piece 10 with the support structure 16 has added strength against flexural stress and can be then processed by back-etching to remove excess substrate from the opposing second side of the work piece 10.
The support structure 16, whether in its “green” state or after additional post-curing, adds strength to the thin and flexible work piece 10. The back-etching process is performed to obtain a thinner wafer that has several inherent advantages. First, the thinner wafer or work piece 10 retains less heat and can have improved heat dissipation properties in a working device since more heat can be conducted out through an appropriate heat sink. Additionally, the thinner microchips can be stacked one atop of another and thinner microchips may be employed in a wider range of applications, including “smart cards.” The support structure 16 also protects the edge or periphery 15 of the work piece 10, keeping it from cracking or chipping. The cracking, if unchecked, can propagate across the entire work piece 10, making it worthless. The primary value, however, of the support structure 16 lies in its ability to protect the semiconductor devices on its first side 14 from attack by the highly corrosive acids used in the acid etching process, as well as strengthening the wafer during handling especially after the wafer has been thinned by etching. The support structure 16 can also reduce the loss of valuable semiconductor devices on wafers where cracking may have occurred by containing the crack propagation and supplying support to the thin wafer so some semiconductor devices can be salvaged.
This acid etching process employs the use of a suitable carrier or chuck (not shown) which holds the work piece 10 with the semiconductor devices 12 facing upwardly. A vacuum can be used to draw the work piece 10 down onto the carrier. An acid mixture comprised of 49% concentrated hydrofluoric acid, 70% concentrated nitric acid, and 85% concentrated phosphoric acid, is employed in two different mixtures. One mixture can have a ratio of approximately 1:3:3 and the other can have a mixture ratio of approximately 1:7:7. This is used to back-etch, for example when silicon is used, the excess silicon from the second opposing side 17 of the work piece until a desired thickness is obtained. The original work piece thickness can be from about 700 to 750 microns or more in thickness but can be reduced to a thickness of something less than one-seventh that thickness. The thinner the final thickness, the wider the potential application of the finished wafer. The bath chemistry balance is observed in the acid etch to accurately control the time of exposure, such as by using electrical resistance measurements, to obtain the desired thickness. At the appropriate time, the acid etch is removed from the carrier device and the reduced thickness work piece is ready for removal of the support structure 10, such as by cutting with a diamond bladed saw.
The thickness of the support structure 16 that serves as the barrier and the seal to the acid mixture can range from about 0.1 inches to about 0.8 inches in thickness and have a height of from about 0.060 to about 0.2 inches, depending upon the particular carrier structure and other design attributes. The support structure 16 not only is an effective barrier and seal, but it also is resistive to the acid etch mixture.
A suitable stereolithography resin is that sold commercially by 3D Systems, Inc. as Accura® si 40 resin. This resin contains at least one polymerizing organic substance, having at least one alicyclic epoxide with two epoxide groups, at least one free-radical polymerizing substance including at least one aromatic di(meth)acrylate compound and optionally a tri-or higher functional methacrylate compound, at least one hydroxyl functional aromatic compound and appropriate cationic and free-radical polymerization initiators. Upon polymerization the resin has proven resistant to acid etch compositions. Other stereolithography resins may also be employed dependent upon their resistiveness to the particular acid etch compositions employed.
While the invention has been described above with reference to specific embodiments thereof, it is apparent that many changes, modifications and variations can be made without departing from the inventive concept disclosed herein. For example, a suitable coating system may include a three-dimensional ink jet printing system that utilizes photocurable resins as the jetting material to create the support structure or to apply an encapsulating layer of resin on the work piece or wafer. Accordingly it is intended to embrace all such changes, modifications and variations that fall within the spirit and broad scope of the accompanying. All patents and patent applications referenced herein are hereby specifically incorporated by reference in pertinent part.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10440624 | May 2003 | US |
| Child | 11255628 | Oct 2005 | US |
