Abstract:
A method for mounting a dielectric substrate to a semiconductor substrate, such as mounting a dielectric antenna substrate to an MMIC semiconductor substrate. The method includes providing a thin dielectric antenna substrate having metallized layers on opposing sides. In one embodiment, carrier wafers are used to handle and maintain the dielectric substrate in a flat configuration as the metallized layers are patterned. The dielectric substrate is sealed to the semiconductor substrate using a low temperature bonding process. In an alternate embodiment, the metallized layers on the dielectric substrate are patterned simultaneously so as to prevent the substrate from curling.

Description:
GOVERNMENT CONTRACT 
     The U.S. Government may have a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. FA8650-06-C-7600 awarded by the United States Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for attaching a dielectric substrate to a semiconductor substrate and, more particularly, to a method for attaching a dielectric antenna substrate to a monolithic millimeter-wave integrated circuit (MMIC) substrate using carrier wafers or other processes for maintaining the antenna substrate substantially flat. 
     2. Discussion of the Related Art 
     Transceiver modules are known in the art that include an array of antennas each separately mounted to an antenna substrate and coupled to a transceiver front end including a plurality of antenna channels. Typically, the various amplifiers, filters, phase shifters, mixers, analog-to-digital converters, switches, etc. that are part of the front end of the transceiver module are separately fabricated as integrated circuits onto several semiconductor wafers, and then later assembled into the transceiver module using well known semiconductor assembly techniques. 
     It is typically difficult to mount a dielectric antenna substrate to a semiconductor substrate because the antenna substrate is made of a dielectric material and can be very thin for high frequency applications. Particularly, because the antenna substrate can be very thin and its usually flexible, it tends to curl when metal layers on both side of the substrate are patterned into the antenna patches and electrical connections. 
     U.S. Pat. No. 7,067,397, titled Method of Fabricating High Yield Wafer Level Packages Integrating MMIC and MEMS Components, issued Jun. 27, 2006, to Chang-Chien et al., assigned to the Assignee of this application and herein incorporated by reference, discloses a low temperature bonding process for bonding two semiconductor wafers to each other in a wafer-level packaging process. 
       FIG. 1  is a cross-sectional view of a wafer-level package  10  of the type disclosed in the &#39;397 patent. The package  10  includes a semiconductor substrate wafer  12 , such as an InP, GaAs, silicon, etc., and a semiconductor cover wafer  14  bonded to the substrate wafer  12  by a bonding ring  16 . The bonding ring  16  defines a hermetically sealed cavity  18  in which an integrated circuit  20  fabricated on the substrate wafer  12  is sealed when the cover wafer  14  is bonded to the substrate wafer  12 . The &#39;397 patent discloses a process for sealing the cover wafer  14  to the substrate wafer  12  by bonding separate bonding rings together using a low-temperature process. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method for mounting a dielectric substrate to a semiconductor substrate is disclosed. In one non-limiting embodiment, the dielectric substrate is an antenna substrate and the semiconductor substrate is an MMIC substrate. The method includes providing a thin dielectric antenna substrate including metallized layers on opposing sides. In one non-limiting embodiment, carrier wafers are used to handle and maintain the antenna substrate in a flat configuration as the metallized layers are patterned. A carrier wafer is also used to transfer the dielectric substrate to the semiconductor substrate. The dielectric substrate is attached to the semiconductor substrate using a low temperature bonding process. In an alternate embodiment, the metallized layers on the dielectric substrate are patterned simultaneously so as to prevent the substrate from curling, which eliminates the need for the carrier wafers. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a wafer-level package showing two semiconductor wafers attached together by a bonding ring; 
         FIG. 2  is a cross-sectional view of an antenna substrate; 
         FIG. 3  is a cross-sectional view of a dielectric antenna substrate shown in  FIG. 2  mounted to a first carrier wafer, and including a patterned antenna on one surface; 
         FIG. 4  is a cross-sectional view of the antenna substrate shown in  FIG. 3  including a carrier wafer mounted to an opposite side of the antenna substrate; 
         FIG. 5  is a cross-sectional view of the antenna substrate mounted to a plurality of MMIC wafers in a transceiver array, according to an embodiment of the present invention; 
         FIG. 6  is a cross-sectional view of another dielectric antenna substrate; 
         FIG. 7  is a cross-sectional view of the antenna substrate shown in  FIG. 6  including a patterned photoresist on one side; 
         FIG. 8  is a cross-sectional view of the antenna substrate shown in  FIG. 7  including a patterned photoresist layer on an opposite side; 
         FIG. 9  is a cross-sectional view of the antenna substrate shown in  FIG. 6  where metallized layers have been patterned using the photoresist layers; and 
         FIG. 10  is a cross-sectional view of the antenna substrate mounted to a plurality of MMIC wafers in a transceiver array, according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a method for mounting a dielectric substrate to a semiconductor substrate is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the discussion below describes mounting a dielectric antenna substrate to a stack of MMIC wafers in a transceiver array module. However, it will be appreciated by those skilled in the art that the method for mounting a dielectric substrate to a semiconductor substrate will have application for other devices. 
     As will be discussed below, the present invention proposes a process for mounting a dielectric substrate to a semiconductor substrate, and has a particular non-limiting application for mounting a dielectric antenna substrate to an MMIC substrate for each channel of a transceiver array. The present invention eliminates or minimizes the antenna sub-assembly processes, and can provide batch integration of antenna substrates and MMICs, which are done in parallel at the wafer level by wafer bonding. Because antenna substrates are typically very thin and are made of a dielectric material, it is difficult to attach the antenna substrate to the MMIC substrate in an integrated manner to reduce the size and cost of fabricating the system. The process of the invention allows a direct low loss contact for direct feed antenna configurations for very close proximity attachments for electro-magnetic coupling of antenna configurations. Further, the invention allows direct integration of MMICs to reduce losses from interconnections. The present invention can be provided for common antenna substrates that are commercially available, and can accommodate pre-fabricated antenna substrates. 
       FIG. 2  is a cross-sectional view of a commercially available antenna substrate  30  including a dielectric layer  32 , a top copper layer  34  and a bottom copper layer  36 . In one non-limiting embodiment, the antenna substrate  30  can have a thickness as thin as tens of microns or a few hundred microns or less, and includes a suitable dielectric material, such as the commercially available Duroid 5880. However, the antenna substrate  30  can be made of other dielectric materials, such as Rexalite, and can also be an assembly of layers formed as a laminate. Other antenna substrate thicknesses are also available commercially. The copper layers  34  and  36  will be patterned using a suitable photoresist and etching process to define a patch antenna, bonding metal and signal and ground traces. 
       FIG. 3  is a cross-sectional view of the antenna substrate  30  attached to a first carrier wafer  38  by a low temperature thermal release tape  40 . Other attachment methods can also be used to attach the antenna substrate  30  to a carrier wafer including using wax or polymer based bonding agents. The carrier wafer  38  can be made of any suitable material, such as silicon, sapphire, glass or III-V compound semiconductor substrates, and can have any suitable thickness, such as 20 mils. The copper layer  34  has been patterned by a suitable patterning process using a photoresist to define a patch antenna  42 . 
     Once the copper layer  34  has been patterned to define the antenna  42 , a second carrier wafer  44 , shown in  FIG. 4 , is attached to an opposite surface of the antenna substrate  30  from the carrier wafer  38  using a low temperature thermal release tape  46  or other suitable attachment methods. The first carrier wafer  38  is then removed by heating the release tape  40  or by releasing the bonding agents, and the copper layer  36  is patterned using a photoresist process to form a wafer-level packaging bonding ring  48  and signal and/or ground traces  50 . By using the carrier wafer  44 , dicing and separation of individual elements, including the antenna  42 , from the wafer can be provided without damaging the fragile and thin antenna substrates. The carrier wafer  38  is shown being attached to the antenna substrate  34  for a single antenna. In a practical fabrication process, a single carrier wafer will be used to handle an antenna substrate from which many antennas will be patterned in a batch integration processes. 
       FIG. 5  is a cross-sectional view of a transceiver module  60  including the patterned antenna substrate  30 . The antenna substrate  30  is mounted to a semiconductor wafer  62 , such as an MMIC wafer, while the carrier wafer  44  is still bonded thereto, and the carrier wafer  44  is thereafter removed. Particularly, the bonding ring  48  is bonded to a bonding ring  64  formed on a top surface of the wafer  62 , and the trace  50  is bonded to a trace  66  on the top surface of the wafer  62  to provide signal connection, as shown. According to the invention, the low-temperature bonding process that is used to bond the semiconductor wafers  12  and  14  together can be used to bond a semiconductor wafer and a dielectric wafer, particularly the substrate  30  and the wafer  62 . Other suitable wafer bonding processes can also be used for this purpose. The bonding ring  64  will include the required bonding layers, as discussed above with reference to  FIG. 1 . In one non-limiting embodiment, the wafer  62  is a group III-V semiconductor wafer acting as an interposer, and is about 4 μm thick, but can have any suitable thickness. 
     The semiconductor wafer  62  is part of a semiconductor wafer assembly including wafer-level packages provided by stacked wafers including a substrate wafer  70 , a first intermediate semiconductor wafer  72 , a second intermediate semiconductor wafer  74  and a third intermediate semiconductor wafer  76 . More or fewer wafers can be provided in other embodiments. In this non-limiting embodiment, the wafers  62 ,  70 ,  72 ,  74  and  76  are mounted together using bonding rings  80  that define wafer-level packaging and hermetically sealed cavities  82  in which the various circuit components for the transceiver module  60  are fabricated by the bonding process discussed above. For example, the substrate wafer  70  may include RF distribution and control circuits  88 , the wafer  72  may include phase shifters  90 , the wafer  74  may include gain and power amplifiers  92 , etc. 
     In an alternate embodiment, the carrier wafers  38  and  44  are not used to handle the dielectric antenna substrate  30 . The carrier wafers  38  and  44  are mounted to the antenna substrate  30  to prevent it from curling as copper is removed from the patterning process on the substrate  30 . In the alternate embodiment, the copper layers  34  and  36  are patterned simultaneously to prevent the antenna substrate  30  from curling. 
       FIGS. 6-10  show this embodiment of the invention. Particularly,  FIG. 6  shows a thin dielectric antenna substrate  100  including a dielectric layer  102  and opposing copper layers  104  and  106 , and is similar to the antenna substrate  30 . 
     In  FIG. 7 , a photoresist layer  108  is deposited on the copper layer  104  and patterned to define the antenna. Before the antenna is formed, a photoresist layer  110  is deposited on the copper layer  106  and patterned as shown in  FIG. 8  to define the signal traces and the bonding ring.  FIG. 9  shows the antenna substrate  100  after the copper layers have been etched to define a patch antenna  112 , signal and/or ground traces  114  and a bonding ring  116 , which is the same as the antenna substrate  30  shown in  FIG. 4 . Thus, by merely using a wafer-handling fixture (not shown), well known to those skilled in the art, the antenna substrate  100  can be patterned in this manner where the dielectric layer  102  will not curl as a result of copper being removed from one of the surfaces. 
       FIG. 10  is a cross-sectional view of a transceiver module  120  similar to the transceiver module  60 , where like elements are identified by the same reference numeral. In this embodiment, the antenna substrate  100  is mounted to the semiconductor wafer  62  without the need of the carrier wafer  44 . The antenna substrate  100  is bonded to the semiconductor wafer  62  in the same manner using the low-temperature bonding process or other suitable wafer bonding processes. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.