Abstract:
A package and method for packaging a semiconductor device formed in a surface portion of a semiconductor wafer. The package includes: a dielectric layer disposed on the surface portion of the semiconductor wafer having a device exposing opening to expose one of the devices and an electrical contacts pad opening to expose an electrical contact pad; and a porous material in the device exposing opening over said one of the devices.

Description:
TECHNICAL FIELD 
     This invention relates generally to methods for packaging (i.e., encapsulating) semiconductors and more particularly to methods for packaging semiconductors at a wafer level (i.e., wafer-level packaging). 
     BACKGROUND AND SUMMARY 
     As is known in the art, traditionally in the microelectronics industry, electrical devices are fabricated on wafers and then diced into individual chips. The bare chips would then get assembled with other components into a package for environmental and mechanical protection. In commercial applications, the chips were generally assembled into plastic packages. In military applications, where electronics are generally exposed to harsher environments, the parts are generally housed in a hermetic module. Such packages or modules would then be further assembled unto circuit boards and systems. However, as electronic systems advance, there is a need to increase functionality while decreasing the size and cost of components and sub-systems. 
     One way to reduce size and cost is to create packages at the wafer level and then subsequently dicing the wafer into individual packaged semiconductors (i.e., wafer-level packaging). Many methods have been suggested to create wafer-level packages. One method, call wafer bonding, is to bond a wafer with pre-formed cavities over the device wafer. The bonding can be achieved through thermal bonding, adhesive or solder bonding, see for example, Rainer Pelzer, Herwig Kirchberger, Paul Kettner, “Wafer-to Wafer Bonding Techniques: From MEMS Packaging to IC Integration Applications”, 6 th  IEEE International Conference on Electronic Packaging Technology 2005 and A. Jourdain, P. De Moor, S. Pamidighantam, H. A. C. Tilmans, “Investigation of the Hermeticity of BCB-Sealed Cavities For Housing RF-MEMS Devices”, IEEE Electronic Article, 2002. 
     However, this method introduces a lot of complexity into the process. Thermal bonding is generally achieved at very high temperatures, in excess of 400 C. Adhesive bonding can be achieved at lower temperature, but adhesive outgassing is a concern. Therefore wafer bonding is not a suitable and cost-effective method for some applications. 
     Another approach is to use Liquid crystal polymer (LCP). It has recently become a popular candidate for various packaging approaches, due to its excellent electrical, mechanical and environmental properties. The material comes in rolls and can be laminated unto the wafer as a film. A general method is to use multiple stacks of LCP. Individual holes were created in a layer of LCP and laminated over the wafer so that the device or FETs are exposed through the holes. This first layer of LCP forms the sidewall of the cavity. Then a second layer of LCP is laminated over the entire wafer, thus enclosing the cavity, see Dane. C. Thompson, Manos M. Tentzeris, John Papapolymerou, “Packaging of MMICs in Multilayer of LCP Substrates,”  IEEE Microwave and Wireless Components Letters , vol. 16, No. 7, July 2006. Single stack of LCP can also be used, but cavities still must be formed on the material before lamination unto wafer, see Dane. C. Thompson, Nickolas Kinglsley, Guoan Wang, John Papapolymerou, Manos M. Tentzeris, “RF Characteristics of Thin Film Liquid Crystal Polymer (LCP) Packages for RF MEMS and MMIC Integration”,  Microwave Symposium Digest,  2005  IEEE MTT - S International,  12-17 Jun. 2005 Page(s):4 pp. and Mogan Jikang Chen, Anh-Vu H. Pham, Nicole Andrea Evers, Chris Kapusta, Joseph Iannotti, William Komrumpf, John J. Maciel, Nafiz Karabudak, “Design and Development of a Package Using LCP for RF/Microwave MEMS Switches”,  IEEE Transactions on Microwave Theory and Techniques , vol. 54, No. 11, November 2006. The prior work mentioned above requires very accurate alignment during wafer bonding which limits the flexibility to create small cavities that cover just the active devices and individual passive components. Generally, with a larger cavity, not only is the risk for mechanical failure is greater, environmental protection of the package may also be compromised, see Aaron Dermarderosian, “Behavior of Moisture in Sealed Electronic Enclosures,” International IMAPS conference in San Diego, October of 2006. These issues with traditional methods limit the manufacturability and performance of the package. 
     In multichip-module packaging approaches, the chips are packaged by spinning or laminating the dielectric film over the entire chip. Prior work have been done using various combination of Kapton E, BCB, SPIE, etc., see Vikram B. Krishnamurthy, H. S. Cole, T. Sitnik-Nieters, “Use of BCB in High Frequency MCM Interconnects”,  IEEE Transactions on Components, Packaging, and Manufacturing Technology —Part B, vol. 19, No. 1, February 1996. A dielectric film deposited directly on top of transistors generally degrades its performance due to the increased parasitic capacitance. The multichip-module packaging is a chip-level rather than a wafer-level approach. 
     In another wafer-level packaging approach, caps made from different material, such as LCP, glass, etc. were dropped unto the wafer to cover individual chips. The caps were sealed in place using adhesives. Again, this is a complex process that picks and places the caps on individual chips; see George Riley, “Wafer Level Hermetic Cavity Packaging”, http://www.flipchips.com/tutorial43.html. 
     Another method and package is described in U.S. Pat. No. 8,035,219 entitled, “Method for Packing Semiconductors at a Wafer Level”, inventor William J. Davis, Ward G. Fillmore, and Scott MacDonald, issued Oct. 11, 2011 assigned to the same assignee as the present invention. 
     In accordance with the present embodiment, a method is provided for packaging a plurality of semiconductor devices formed in a surface portion of a semiconductor wafer. The method includes: forming device exposing openings and electrical contact exposing opening in a dielectric layer disposed on the surface portion of the semiconductor wafer to expose the devices and electrical contacts pads, respectively; and depositing a porous material in the device exposing openings over the devices. 
     In one embodiment, a package is provided for a semiconductor device formed in a surface portion of a semiconductor wafer. The package includes: a dielectric layer disposed on the surface portion of the semiconductor wafer having a device exposing opening to expose one of the devices and an electrical contacts pad opening to expose an electrical contact pad; and a porous material in the device exposing opening over said one of the devices. 
     In one embodiment, the dielectric layer is a lithographically processable material. 
     In one embodiment, the package includes a second dielectric layer deposed over the first-mentioned dielectric layer and over the porous material. 
     In embodiment, the second dielectric has openings in registration with the electrical contact exposing opening. 
     In one embodiment, the second dielectric layer is a lithographically processable material. 
     In one embodiment, a package is provided for a semiconductor device formed in a surface portion of a semiconductor wafer. The package includes: a first dielectric layer comprising a first lithographically processable material disposed on the surface portion of the semiconductor wafer having a device exposing opening to expose one of the devices and an electrical contacts pad opening to expose an electrical contact pad such one of the devices; a porous material in the device exposing opening over said one of the devices; and a second dielectric layer comprising a second lithographically processable material disposed on the porous material and on selected regions of the first dielectric layer and absent from the electrical contacts pad opening of the first dielectric layer. 
     In one embodiment, the porous material is an aerogel. 
     In one embodiment, the porous material is an aerogel-polymer. 
     In one embodiment, the porous material is greater than 95 percent air by solid. 
     In one embodiment, the porous material has a porosity in the range of 60-95 percent air. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIGS. 1 through 12  show a semiconductor wafer having devices therein packaged in accordance with the invention at various steps in such packaging. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , a semiconductor wafer  10 , somewhat simplified, is shown having a plurality of semiconductor integrated circuits  11 . Devices  12  are formed in a surface portion of each of the integrated circuits  11 , here the upper surface portion thereof, is shown. An exemplary one of the devices  12  within one of the integrated circuits  11  is shown in  FIG. 2 . Thus, here the wafer is for example, a GaAs or SiC wafer  10 , and the devices are, for example, field effect transistors (FETs) here shown for example connected to bond pads  16 ,  18  through transmission lines  20 ,  22  respectively, as shown. 
     Next, a lithographically processable, etchable material  30  is deposited over the upper surface portion of the semiconductor wafer  10 , as shown in  FIG. 3 . Here, for example, lithographically processable, etchable material  30  can be an organic or inorganic material, that can be easily patterned on a wafer using conventional lithographic and etch process to form the sidewall of a cavity to be described. In one embodiment, Benzocyclobutane (BCB) is used being a dielectric material with excellent electrical properties. It has been used in many applications for dielectric coating, 3D interconnect and packaging, see for example, Kenjiro Nishikawa, Suehiro Sugitani, Koh Inoue, Kenji Kamogawa, Tsuneo Tokumitsu, Ichihiko Toyoda, Masayoshi Tanaka, “A Compact V-Band 3-D MMIC Single-Chip Down-Converter Using Photosensitive BCB Dielectric Film”,  IEEE Transactions on Microwave Theory and Techniques , vol. 47, No. 12, December 1999, and Rainer Pelzer, Viorel Dragoi, Bart Swinnen, Philippe Soussan, Thorsten Matthias, “Wafer-Scale BCB Resist-Processing Technologies for High Density Integration and Electronic Packaging”, 2005 International Symposium on Electronics Materials and Packaging, December 11-14. 
     The BCB material  30  can be dispensed as a liquid, spun on, exposed, developed and cured, all using conventional semiconductor fabrication equipment. Because BCB can be patterned by conventional photolithographic technique, it can achieve alignment tolerances and critical dimensions similar to that of photoresist (limited by film thickness). A spin-on process is preferable to a lamination process (such as that for LCP) from a mechanical and process simplicity standpoint. The spin-on process introduces less stress to the wafer, especially for the mechanical fragile structures such as air bridges and is more capable of self leveling over complex circuit topologies. 
     Next, the material  30  is photolithographically processed, as shown in  FIG. 4 , using a mask  31  having windows  35  disposed over the devices  12  and contact pads  16 ,  18 . After unexposed portions of the BCB material  30  are developed away, device openings  32  are formed therein to expose the devices  12  and electrical contacts pads openings  34  are formed therein to expose electrical contact pads  16 ,  18  as shown in  FIG. 5 . 
     Next, a gelling precursor solution of dielectric material  33  is deposited in the device openings  32  covering the devices  12  and on adjacent portions of the transmission lines  20 ,  22 , as shown in  FIG. 6 . Here the porous, dielectric material  33  is for example, a gelling precursor solution  33  of the aerogel-polyimide in liquid form which can be subsequently processed to form an aerogel-polyimide having greater than 95 percent air by solid, preferable having a range between 60 to 95 percent air. 
     The process includes depositing a gelling precursor solution  33  of the aerogel-polyimide in liquid form, on a portion of the circuit including the FET  12 . The gelling precursor solution  33  may be deposited by any suitable process such as, for example, spin coating, stencil printing or dot dispensing. In one embodiment, the gelling precursor solution  33  is deposited by a dot dispensing  53  in  FIG. 6 . The gelling precursor solution is then processed on the portion of the circuit including the FET to form an aerogel dielectric layer  33  on the FET  12 . The processing may include drying by any suitable process, for example, supercritical CO 2  drying at 55° C. and 1600 psi  54  in  FIG. 7  to thereby solidify the deposited precuror into the final aerogel-polyimide dielectric  33  to provide the porous dielectric described above. Here, for example, the aerogel has a low dielectric constant. The aerogel dielectric layer may have a loss tangent of approximately 0.002 to 0.007. A low loss tangent is desirable because it reduces the rate at which electromagnetic energy is dissipated. 
     After openings or cavities  32 ,  34  in the BCB material  30  are filled with the porous dielectric material  33 , the openings or cavities  32 ,  34  are enclosed using a spin coated, low dielectric layer  40 . Layer  40  is mechanically stronger than the porous dielectric material  33  and has good adhesion to BCB material  30 . More particularly, a dielectric layer  40  of a second lithographically processable material  40  is provided over the formed first lithographically processable material, such material  40  being coated over the device filled openings  32 ,  24  (i.e., cavities) in the material  30 ,  FIG. 8 . 
     Here, the first lithographically processable material  30  and the second lithographically processable material  40  comprise the same type of material. In this embodiment, the first lithographically processable material  30  and the second lithographically processable material  40  comprise BCB, here for example having thicknesses of about 17 micrometers. 
     Next, referring to  FIG. 9 , the second lithographically processable material i.e., layer  40 , is photolithographically processed by exposing selected unmasked region  45 ′ of the second lithographically processable material  40  to UV radiation as indicated passing through openings  45  in a photo mask  51 , as shown. Thus, after immersing the structure in a developer solution as shown in  FIG. 10 , the unexposed portions  40 ″ of the second photolithographically processable material  40  are removed forming areas for interconnection to bond pads  16  and  18 . 
     Next, the protected device  12  is ready for wire bonds  50  and final assembly, as shown in  FIG. 11 . Or, the structure shown in  FIG. 11  is provided with solder balls  52  as indicated in  FIG. 12  for flipped assembly. 
     A number of embodiments of the invention have been described. For example, materials other than BCB may be used such as SU — 8. Further, while the porous material is shown disposed over a transistor, it may be disposed over other devices such as capacitors, resistors, for example. Still further, it should be understood that positive or negative tone photolithographically processable materials may be used. Still further, the contact pad and device exposing openings may be formed using lasers without the use of photolithographically processable materials. It should therefore be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.