Abstract:
A method of providing a CMOS-MEMS structure is disclosed. The method comprises patterning a first top metal on a MEMS actuator substrate and a second top metal on a CMOS substrate. Each of the MEMS actuator substrate and the CMOS substrate include an oxide layer thereon. The method includes etching each of the oxide layers on the MEMS actuator substrate and the base substrate, utilizing a first bonding step to bond the first patterned top metal of the MEMS actuator substrate to the second patterned top metal of the base substrate. Finally the method includes etching an actuator layer into the MEMS actuator substrate and utilizing a second bonding step to bond the MEMS actuator substrate to a MEMS handle substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefit under 35 USC 119(e) of U.S. Provisional Patent Application No. 62/021,626, filed on Jul. 7, 2014, entitled “INTEGRATED CMOS AND MEMS SENSOR FABRICATION METHOD AND STRUCTURE,” which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to CMOS-MEMS integrated devices and more particularly to method of fabrication for CMOS-MEMS integrated devices. 
     BACKGROUND 
     Traditionally to provide a CMOS-MEMS structure with at least one cavity therein a high bonding force (300 psi or greater) is required at a high temperature (above 400 degrees) to effectively bond the CMOS substrate to a MEMS substrate. The high temperature causes high stresses on the bonded structure. In addition, a timed etch to form a standoffs is required and therefore to control a gap height in the structure can be difficult to achieve. Accordingly, what is needed is a system and method to address the above identified issues. The present invention addresses such a need. 
     SUMMARY 
     A method of providing a CMOS-MEMS structure is disclosed. The method comprises patterning a first top metal on a MEMS actuator substrate and a second top metal on a CMOS substrate. Each of the MEMS actuator substrate and the CMOS substrate include an oxide layer thereon. The method includes etching each of the oxide layers on the MEMS actuator substrate and the base substrate, utilizing a first bonding step to bond the first patterned top metal of the MEMS actuator substrate to the second patterned top metal of the base substrate. Finally the method includes etching an actuator layer into the MEMS actuator substrate and utilizing a second bonding step to bond the MEMS actuator substrate to a MEMS handle substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a CMOS-MEMS structure in accordance with an embodiment. 
         FIG. 2  is a flow chart of the process flow of a fabrication of a CMOS-MEMS structure in accordance with an embodiment. 
         FIGS. 3A-3F  are diagrams that illustrate fabrication of a CMOS-MEMS structure in accordance with the process flow of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to CMOS-MEMS integrated devices and more particularly to method of fabrication for CMOS-MEMS integrated devices. 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 embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, a method and system in accordance with the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     In the described embodiments Micro-Electro-Mechanical Systems (MEMS) refers to a class of structures or devices fabricated using semiconductor-like processes and exhibiting mechanical characteristics such as the ability to move or deform. MEMS often, but not always, interact with electrical signals. MEMS devices include but are not limited to gyroscopes, accelerometers, magnetometers, pressure sensors, and radio-frequency components. Silicon wafers containing MEMS structures are referred to as MEMS wafers. 
     In the described embodiments, MEMS device may refer to a semiconductor device implemented as a micro-electro-mechanical system. MEMS structure may refer to any feature that may be part of a larger MEMS device. An engineered silicon-on-insulator (ESOI) wafer may refer to a SOI wafer with cavities beneath the silicon device layer or substrate. Handle wafer typically refers to a thicker substrate used as a carrier for the thinner silicon device substrate in a silicon-on-insulator wafer. Handle substrate and handle wafer can be interchanged. 
     In the described embodiments, a cavity may refer to an opening or recession in a substrate wafer and enclosure may refer to a fully enclosed space. Bond chamber may be an enclosure in a piece of bonding equipment where the wafer bonding process takes place. The atmosphere in the bond chamber determines the atmosphere sealed in the bonded wafers. 
     Additionally, a system and method in accordance with the present invention describes a class of RF MEMS devices, sensors, and actuators including but not limited to switches, resonators and tunable capacitors that are hermetically sealed and bonded to integrated circuits that may use capacitive sensing and electrostatic, magnetic, or piezo-electric actuation. 
     In order to bond a CMOS substrate with a MEMS substrate to CMOS substrate to form a CMOS-MEMS integrated device a process is utilized which provides for two steps. A first bonding step bonds a top metal layer of the MEMS substrate to a top metal layer of the CMOS substrate and a second bonding step bonds a MEMS handle layer to the MEMS actuator layer. Both of these bonding steps can be performed at low temperature (150-400 degrees C.) at a reduced pressure. Both of the bonding steps also can be utilized to provide a hermetic seal for the device. 
     Accordingly, this process overcomes some of the issues associated with high temperature bonding processes. Namely a process in accordance with the present invention eliminates the high bonding force requirement associated with the traditional eutectic bond between the CMOS substrate and the MEMS substrate and therefore reduces stresses and minimizes warping of the bonded structure since a high temperature is not required. 
     In addition the gap height control is improved over conventional bonding processes for CMOS-MEMS integrated devices. Finally using the process in accordance with the present invention a timed etch to form standoffs on the CMOS-MEMS integrated device is no longer required. The processes described below provide for the fabrication of CMOS-MEMS integrated devices using first and second low temperature bonding steps to create a sealed enclosure between the MEMS and CMOS wafers. The first bonding step comprises a metal to metal bond that can provide electrical connection between a MEMS substrate and a CMOS substrate. The second bonding step comprises a fusion bond that coupled a handle layer of the MEMS substrate to an actuator layer of the MEMS substrate and does not provide for any electrical interconnection. 
     Below is provided an approach available with a method and system in accordance with the present invention, in one or more embodiments, providing for the integration of such devices to create a CMOS-MEMS integrated device. In the described embodiments, the CMOS wafer may be replaced by any suitable capping wafer or substrate. 
       FIG. 1  is a diagram of a CMOS-MEMS structure in accordance with an embodiment. For the embodiment, it will be appreciated that a CMOS-MEMS integrated device  100  comprises a MEMS substrate  102  and a CMOS substrate  104 . The CMOS substrate  104  includes a bump stop  119  that can in an embodiment composed of metal  120  such as Copper or Nickel surrounded by an oxide layer  122 . The bump stop  119  can be electrically connected to the underlying metal or can be electrically isolated. The MEMS substrate  102  includes a MEMS actuator layer  106  and a MEMS handle layer  108  with at least one cavity  110  bonded to the MEMS actuator layer  106  through a dielectric layer  112  disposed between the MEM handle layer  108  and the MEMS actuator layer  106 . The MEMS actuator layer  106  also includes a moveable portion  114 . 
     A top metal  118  of the MEMS actuator layer  106  and a top metal  120  of the CMOS substrate  104  are used to first bond the CMOS substrate  104  to the MEMS actuator layer  106 . The top metal  118  of the MEMS actuator layer  106  includes a contact layer  124 , which is composed of, for example, Titanium Nitride (TIN). In an embodiment, the top metals  118  and  120  can be made of materials that bond at temperatures between 150-400 degrees Celsius that include, but are not limited to any of copper (Cu) and nickel (Ni). The standoffs  130  are formed via an etch of the oxide layers  122  on the CMOS substrate  104  and the MEMS actuator layer  106 . 
     The MEMS actuator layer  106  is coupled to the MEMS handle layer  108  and the dielectric layer  112  via a second bond. In an embodiment, the first bond comprises a compression bond for a metal to metal connection that is provided at a temperature in the range of 150-400 degrees C. and the second bond comprises a fusion bond which is also provided at a temperature in the range of 150-400 degrees C. 
     In an embodiment, first and second bonds are implemented utilizing the Direct Bond Interconnect (DBI) process which has been developed by Ziptronix Inc. To describe the features of the present invention in more detail refer now to following discussion in conjunction with the accompanying Figures. 
       FIG. 2  is a flow chart of the process flow of a fabrication of a CMOS-MEMS structure in accordance with an embodiment.  FIGS. 3A-3F  are diagrams that illustrate fabrication of a CMOS-MEMS structure in accordance with the process flow of  FIG. 2 . Referring to  FIGS. 2 and 3A-3F  together, first, top metals  118  and  120  are patterned on the CMOS substrate  104  and the MEMS actuator layer  106  as shown in  FIG. 3A , via step  202 . Thereafter an oxide layer is etched on the CMOS substrate  104  and the MEMS actuator layer  106  as shown in  FIG. 3B  to form the standoffs  130  and the bump stop  119 , via step  204 . 
     Thereafter the top metals  118  and  120  of the CMOS substrate  104  and the MEMS actuator layer  106  are bonded using a low temperature bond as shown in  FIG. 3C , via step  206 . As before mentioned, in an embodiment the low temperature bond is in a temperature range of 150-400 degrees C. In an embodiment, MEMS actuator layer  106  is ground down to a desired thickness. The desired thickness in some embodiments is between 10-100 microns. Through the first bond an electrical or conductive connection is made between the CMOS substrate  104  and the MEMS actuator layer  106 . 
     Thereafter, the MEMS actuator layer  106  is etched to provide a movable portion  114  as shown in  FIG. 3D , via step  208 . Then a cavity  110  is formed and a MEMS handle layer  108  is oxidized as shown in  FIG. 3E , via step  210 . Thereafter the MEMS handle layer  108  is bonded to the MEMS actuator layer  106  as shown in  FIG. 3F , via step  212 . 
     A process in accordance with the present invention provides the following features: 
     1. Utilizes a low temperature process that reduces stresses on the device while still having a high bond energy. 
     2. Provides bonded electrical interconnections between MEMS substrate and CMOS substrate. 
     3. Provides a well controlled gap between the CMOS substrate and the MEMS substrate 
     4. Does not require a top anchor for the MEMS substrate because the Moveable MEMS structure is only anchored to the CMOS substrate, making it less sensitive to external stresses placed on the MEMS handle substrate. 
     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 present invention.