Patent Publication Number: US-2017355597-A1

Title: METHOD OF FABRICATION OF Al/GE BONDING IN A WAFER PACKAGING ENVIRONMENT AND A PRODUCT PRODUCED THEREFROM

Description:
PRIORITY CLAIM 
     Under 35 U.S.C. 120, this application is a Continuation Application and claims priority to U.S. application Ser. No. 15/364,478, filed Nov. 30, 2016, which is a Continuation Application of U.S. application Ser. No. 14/853,873, filed Sep. 14, 2015, now U.S. Pat. No. 9,533,880, which is a Continuation Application of U.S. application Ser. No. 14/157,456, filed Jan. 16, 2014, now U.S. Pat. No. 9,139,428, which is a Continuation Application of U.S. application Ser. No. 13/333,580, now U.S. Pat. No. 8,633,049, which is a Continuation Application of U.S. application Ser. No. 12/184,231, now U.S. Pat. No. 8,084,332, which is a Continuation Application of U.S. application Ser. No. 11/084,296, now U.S. Pat. No. 7,442,570, which is related to U.S. Pat. No. 6,892,575, and related to U.S. Pat. No. 6,939,473, and related to U.S. Pat. No. 7,104,129, and related to U.S. Pat. No. 7,247,246, all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to wafer bonding and more particularly to a method and system of bonding in a wafer packaging environment. 
     BACKGROUND 
     MEMS technology has been under steady development for some time, and as a result various MEMS devices have been considered and demonstrated for several applications. MEMS technology is an attractive approach for providing inertial sensors, such as accelerometers for measuring linear acceleration and gyroscopes for measuring angular velocity. A MEMS inertial sensor typically includes a proof mass which is flexibly attached to the rest of the device. Relative motion between the proof mass and the rest of the device is driven by actuators and/or sensed by sensors in various ways, depending on the detailed device design. Other MEMS applications include optical applications such as movable mirrors, and RF applications such as RF switches and resonators. 
     Since MEMS fabrication technology is typically based on processing planar silicon wafers, it is useful to classify MEMS devices according to whether the actuation and/or sensing performed in an inertial sensor (or other application) is in-plane or out of plane (i.e., vertical). More specifically, a device is “in-plane” if all of its sensing and/or actuation is in-plane, otherwise it is “vertical”. Thus MEMS devices are undergoing steady development, despite fabrication difficulties that tend to increase. 
     One approach which has been used to fabricate vertical MEMS devices is hybrid integration, where elements of a MEMS assembly are individually assembled to form the desired vertical structure. For example, attachment of a spacer to a substrate, followed by attachment of a deformable diaphragm to the spacer, provides a vertical MEMS structure having a spacing between diaphragm and substrate controlled by the spacer. U.S. Pat. No. 6,426,687 provides further information on this approach. Although hybrid integration can provide vertical MEMS devices, the cost tends to be high, since manual processing steps are usually required, and because hybrid integration is typically performed on single devices. Therefore, there is a need for reduced cost integrated MEMS devices that is unmet in the prior art. 
     CMOS compatible wafer-wafer bonding is very desirable for wafer-level-packaging. Its use has been demonstrated in a variety of different technologies. However, most of these processes have been limited to providing protection of a sensitive feature from post process handling, such as sawing, die bonding, testing, package, etc. 
     The need for a robust wafer level integration that can allow for simultaneous wafer-level-packaging and electrical interconnect is very high and can open up a multitude of new smaller, low-cost and feature rich MEMS products. The following describes conventional methods for bonding and their problems. Organic or Adhesive Based Methods 
     Materials such as Benzocyclobutene (BCB), polyamide, photo resists, patternable RTV, and others have been spun on and used to form permanent bonds between wafers. These materials have disadvantages in that because they are organic, they tend to outgas and so are unsuitable for forming hermetic enclosures, and also they are susceptible to solvents, or moistures which can lead to problems with long term reliability and drift of a device&#39;s performance. Additionally, they are insulating materials and so are incapable of forming a conductive path between two substrates. 
     One popular method of making wafer-wafer bonding is by use of frit glass. Frit glass is typically screen printed on the cover wafers and reflowed to form a patterned glass interface for subsequent wafer-wafer bonding. Frit glass has a typical melting point near 500° C. and can be remelted post wafer-wafer aligned bond in a special temperature chamber with a controlled environment. The primary use of glass frit is to provide for the cover substrate and a hermetic sealed cavity for the MEMS. Frit glass technology has been utilized in the MEMS industry for many decades. Several major drawbacks are that frit glass does not provide for electrical interconnection between the MEMS and cover, to achieve a hermetic seal interface, minimum of 400 micron seal ring width is required which makes small MEMS devices, such as resonators and optical devices, much larger than otherwise. Also, frit glass screen printed is inherently a thick film process with tens of microns in thickness and several microns of nonuniformity. 
     Metal-Metal Bonding 
     CMOS compatible eutectic bonding has been demonstrated with indium-gold, solder-gold, gold-gold, etc. In order to bond a CMOS wafer, all of these prior art systems require the addition of non-standard layers, such as plating of lead, indium, gold, etc., to be added to the CMOS wafer. Although these processes are capable of hermetic seals and electrical interface, achieving fine features, small gaps and wafer uniformity is very challenging and will result in yield losses. 
     There are many MEMS device applications that require an electro-mechanical interface between the CMOS substrate and the MEMS substrates that are in micron gaps and require submicron uniformity. Most plating processes require under-layer barrier metalization with tens of microns thickness, and uniformity across the wafer is measured in microns. Hence it is not possible to specify one or two micron gap controls between the MEMS and CMOS substrates using this bonding methodology. 
     The ability to make high density and reliable electrical contacts between the MEMS and CMOS substrates can be very beneficial and provide for an all new generation of MEMS devices with much added functionality, smart electronics, smaller size, and lower cost. Finally, it is important to provide a lead free alloy based upon environmental considerations. 
     Accordingly, what is needed is a system and method for providing wafer bonding that overcomes the above-identified problems. The system and method should be easily implemented, cost effective and adaptable to existing bonding processes. The present invention addresses such a need. 
     SUMMARY 
     A method of bonding two substrates to create a robust electrical and mechanical contact by using aluminum and germanium eutectic alloys is disclosed. An aluminum-germanium bond has the following unique combination of attributes: (1) it can form a hermetic seal; (2) it can be used to create an electrically conductive path between two substrates; (3) it can be patterned so that this conduction path is localized; (4) the bond can be made with the aluminum that is available as a standard foundry CMOS process; (5) this process is compatible with completely fabricated CMOS wafers as post process; (6) this process can provide for high density electrical interconnect; and (7) this process is highly controllable and provides for the smallest gap between two substrates. This has the significant advantage of allowing for wafer-level bonding or packaging without the addition of any additional process layers to the CMOS wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow chart of a method for fabrication of a wafer level package in accordance with the present invention. 
         FIGS. 2A and 2B  are cross sectional and top views of a first embodiment of an assembly in accordance with the present invention. 
         FIGS. 3A and 3B  are cross sectional and top views of a second embodiment of an assembly in accordance with the present invention. 
         FIG. 4  illustrates an exemplary bonding profile to achieve a proper Al/Ge bond. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to wafer bonding and more particularly to a method and system of bonding utilizing aluminum and germanium in a wafer-level packaging of MEMS devices with electrical substrate interconnect environment. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
       FIG. 1  is a flow chart of a method of fabrication of a wafer level package in accordance with the present invention. The method comprises providing a MEMS structure including a substantially germanium top layer, via step  12 , and providing a CMOS structure including a substantially aluminum top layer, via step  14 . Finally, the method comprises bonding the top layer of the MEMS structure with the top layer of the CMOS structure, via step  16 . 
     The following describes a preferred embodiment in accordance with the present invention.  FIGS. 2A and 2B  are cross sectional and top views of a first embodiment of an assembly  100  in accordance with the present invention. Referring to the embodiment shown in  FIG. 2A , a standard foundry CMOS wafer  104  which includes aluminum is bonded to a MEMS substrate  102  which includes germanium to provide an aluminum/germanium (Al/Ge) bond  110 . In this embodiment, a cavity  106  is within the substrate  104 . The CMOS substrate wafer  104  can be any substrate with patterned aluminum shown in  FIG. 2B  that is designed to interface with the MEMS substrate  102  to make for a complete functioning product. In addition, a plurality of aluminum contacts  116  are on the top of the CMOS substrate  104  which are coupled to bond pads  105  by interconnect  107 . Vias  107  are provided in both the bond pads  105  and the aluminum contacts  116  to allow for electrical connection thereto. As an example, the substrate  104  can comprise only a collection of metal layers and interconnects for providing for an electrical interconnection to the MEMS layers. Furthermore, the MEMS substrate  102  includes a MEMS feature  108  to complement the MEMS layers on the MEMS substrates  104 , such as corresponding cavity  106 . A gap control standoff  111  is provided to provide a precise separation of the MEMS substrate  102  from the CMOS substrate  104 . The gap control standoff  111  provides a seal ring  112  for the device. 
       FIGS. 3A and 3D  are cross sectional and top views of a second embodiment of an assembly in accordance with the present invention. The assembly  200  includes many of the same elements as assembly  100  of  FIG. 2A and 2B  and those elements have the same reference numerals. Additionally, the assembly  200  has via contacts  202  through the MEMS substrate  102 ′ and the gap control standoff  111  to provide electric feedthrough of signals. 
     Another important feature of the substrate  104  is the availability of the multilayer metallization standard in CMOS foundries with chemical-mechanical-polishing of the oxide to make for a very planar metallized layer suitable for forming Al/Ge eutectic alloy with a germanium presence on the MEMS layer. The MEMS substrate  102  can be a silicon wafer or combination of silicon wafers assembled with all the MEMS features and functionalities including any type of preprocessed features. 
     In the preferred embodiment, the MEMS substrate on which the germanium has been patterned is a silicon substrate doped with boron to a conductivity of 0.006-0.020 cm. This p+ doping forms an ohmic contact with the aluminum-germanium eutectic mix following the bond. 
     To describe the bonding layers in more detail refer now to the following. 
     Bonding Layers 
     In a preferred embodiment, the top metal layer of the foundry CMOS wafer is a ratio mix of 97.5:2:0.5 Al:Si:Cu and is 700 nm thick and is on a planarized oxide layer using CMP which is the standard processing step for most CMOS processes of 0.5 um or lower geometries. In a preferred embodiment, the bonding layers on the MEMS are 500 nm of germanium deposited in a standard vacuum sputter deposition system which is properly patterned to match the corresponding aluminum patterned for bonding. 
     Below is an example of the preferred embodiment of the equipment and process in accordance with the present invention. 
     Required Equipment 
     The bonding is performed in a commercially available wafer bonder such as that supplied by Electronic Visions Group, Inc., or Suss Microtec, Inc. The equipment should meet the following standards and have the following capabilities: (1) temperature control of both the top and the bottom chuck to nominally 450 degrees Celsius (C); (2) ambient pressure control to sub-tor; (3) ambient gas control (via a purge line); (4) plumbed with 4-3-5 percent forming gas; and (5) the capability of applying a uniform force across the wafer pair of a minimum of 3000 N. 
     In the preferred embodiment, the wafers are pre cleaned and then aligned prior to bonding in a compatible alignment tool. 
     Pre-bond Cleaning 
     In the preferred embodiment, both the CMOS wafer and the MEMS wafer are cleaned prior to bonding. Both wafers are assumed to be free of any photoresist or other extraneous materials from previous processing steps. The wafers are cleaned by: (1) a 1.30 second dip in deionized water, (2) a 1.30 second dip in 50:1 HF; (3) a dump rinse; and (4) a standard spin-rinse-dry process. 
     Alignment 
     The bonding pair is aligned in an Electronic Visions 620 wafer-wafer aligner. Separation flags are inserted to maintain separation of the bonded pair prior to bonding. 
     Bonding 
     The aligned pair is transferred to an Electronic Visions 501 bonder. The purge line of this machine has been plumbed with forming gas. Following the cool down period of the bonding recipe, the bonding is complete and requires no further processing. An example of an exemplary bonding temperature profile for achieving the proper Al/Ge bond is shown in  FIG. 4 . 
     DESCRIPTION OF VARIOUS AND ALTERNATE EMBODIMENTS 
     Alternative embodiments include, for example, (1) the utilization of different materials on top of the germanium to protect it during subsequent MEMS processing; (2) the employment of different pre-bond cleaning methods; (3) the bond may be performed unaligned; (4) the bond may be performed without patterning the aluminum and/or the germanium; (5) the CMOS wafer may be bonded without any additional processing other than the pre-bond clean; (6) the aluminum-germanium bond may be configured so as not to create a hermetic seal; (7) utilization of a substrate other than a MEMS wafer (such as a simple cover wafer); (8) the MEMS substrate may comprise something other than a gyroscope (such as a pressure sensor or accelerometer); (9) the aluminum of the standard CMOS wafer may comprise different formulations of standard aluminum (2% silicon, 2% silicon/1% copper, etc.) 
     In addition, (10) a specific temperature profile can be utilized; (11) a forming gas can be used to deoxidize the contact surfaces; (12) aluminum can be utilized as the standard metallization used for IC fabrication; (13) aluminum substrate can be kept below a predetermined temperature to prevent complete leaching of aluminum and germanium alloy from the oxide on the substrate; (14) the bonding can be performed using controlled ambient such as forming gas; (15) the bonding can be performed using low pressure bonding force or high pressure bonding force as assistance for/in order to assist in breaking of the aluminum oxide to initiate the interaction; (16) two wafers can be prealigned prior to bonding process; (17) a special cleaning solution can be utilized to clean the oxide from both surfaces; (18) the bonding surfaces can be cleaned by sputter etching; (19) a thin layer of TiW can be utilized to protect the bonding surface during MEMS processing; (20) including pre bond cleaning using plasma and/or other insitu cleaning techniques, in addition to a more concentrated forming gas and higher force bond; (21) the germanium can be deposited on a non-conductive layer (such as silicon dioxide) to create an insulating contact. 
     The germanium can be deposited on a semiconductor substrate that has been doped such that the resulting contact between the aluminum of the substrate and the MEMS is rectifying. This substrate can be a silicon substrate with n-type doping to 0.020.05 Q-cm. 
     The germanium can be deposited on a semiconductor substrate that has been doped such that the resulting contact between the aluminum of the substrate and the MEMS is ohmic. 
     A method and structure of bonding of germanium to aluminum between two substrates to create a robust electrical and mechanical contact is disclosed. An aluminum-germanium bond has the following unique combination of attributes: (1) it can form a hermetic seal; (2) it can be used to create an electrically conductive path between two substrates; (3) it can be patterned so that this conduction path is localized; (4) the bond can be made with the aluminum that is available as a standard foundry CMOS process. This has the significant advantage of allowing for wafer-level bonding or packaging without the addition of any additional process layers to the CMOS wafer. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.