Patent Publication Number: US-9413063-B1

Title: Antenna-coupled metal-insulator-metal rectifier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/013,175, which was filed on Jun. 17, 2014, by John T. Apostolos et al. for a ANTENNA-COUPLED METAL-INSULATOR-METAL RECTIFIER and is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This patent application relates to the fabrication of antenna-coupled metal-insulator-metal (MIM) rectennas and, specifically to formation of a native oxide insulating layer, providing an ability to be fabricated with various metals. 
     BACKGROUND 
     There are a variety of applications for rectennas and tunnel diodes, including energy harvesting, rectification, and harmonic mixers for down and up conversion. Dipole, bowtie and spiral antenna shapes are some of the geometries of interest. A planar, thin film MIM diode is typically formed in a small overlap region between two (2) “arms” of an antenna. Several such devices have been developed for Radio Frequency (RF) and infrared (IR) frequency ranges. The asymmetry of the work functions of the different metals results in devices having asymmetric current flow, which allows them to be used as rectifiers and similar applications. 
     See the following for further background information: 
     Jeffrey A. Bean, et. al, “Performance Optimization of Antenna-Coupled Al/AlOx/Pt Tunnel Diode Infrared Detectors,”  IEEE J. of Quantum Electronics , Vol 47, No. 1, January 2011. 
     Philip C. D. Hobbs, et. al., “Ni—NiO—Ni tunnel junctions for terahertz and infrared detection,”  Applied Optics , Vol. 44, No. 32, Nov. 10, 2005. 
     U.S. Pat. No. 8,115,683 B1, “Rectenna Solar Energy Harvester.” 
     SUMMARY OF THE INVENTION 
     As the desired operating frequency range increases to the tens or even hundreds of terahertz, device sizes become submicron. At such small dimensions, metal properties become less conductive, making these miniscule devices very difficult to fabricate and also less efficient. Higher frequency operation also requires very small geometries in order to provide a small enough Resistor-Capacitor (RC) time constant. 
     Several attempts at achieving the required asymmetry and small size have resorted to using geometric point-like diodes to favor directional current flow. But these devices are essentially planar whisker diodes that often do not have a reliable fabrication process to produce repeatable metal-insulator interfaces that consistently provide the same geometry and surface contact features when an array of similar devices is required. 
     In particular embodiments, the metal deposition of many MIM devices utilize a shadow mask double angle electron beam (e-beam) write process, where an oxide is typically formed by either Atomic Layer Deposition (ALD) or by forming a native oxide of the bottom metal. The latter native oxidation process has been found to produce a more reliable and controllable oxide that has good surface roughness (e.g. &lt;0.2 nm RMS) and thin enough for field emission (Fowler-Nordheim) tunneling. 
     Different metals oxidize at different rates. Aluminum for example, oxidizes very well, but rapidly. This constrains the design of a MIM rectifier to certain metals, as it is thought that chosen oxidation process must be tailored to the metal. 
     However, preferred herein is a specific native nickel oxide process for MIM devices that can be used with many different metals, to provide a consistent, reliable oxidation layer to optimize field emission tunneling. The approach also provides design flexibility to achieve high efficiency performance. 
     In one implementation, a thin layer (5-20 nm) of nickel is added on top of a bottom layer metal to form a native oxide after Reactive Ion Etching (RIE). 
     The slow native oxidation rate allows for a more controlled, yet flexible process, where the entire thin Ni layer is oxidized after oxygen Reactive Ion Etching (RIE). 
     Several devices have been fabricated using a bowtie antenna and metals consisting of nickel (Ni), platinum (Pt) and gold (Au) at a design wavelength of 10.6 μm. However, devices can be fabricated for other wavelengths including millimeter wave, infrared, near infraread, and visible using the same techniques. 
     The elements may be individually formed as bowties or other shapes and/or fabricated in regularly spaced arrays on a common substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are side and top views, respectfully, of a bottom metal layer with thin Ni layer towards an initial step in forming a bowtie MIM rectenna. 
         FIG. 2  is a top view after RIE oxidation. 
         FIG. 3  illustrates the MIM diode formed in an overlap region after deposition of a top metal layer. 
         FIG. 4  shows the bowtie MIM rectenna after removal of excess NiOx. 
         FIG. 5  shows the final bowtie MIM rectenna fabrication with contact pads. 
         FIG. 6  is a process flow diagram for fabricating the bowtie MIM rectenna. 
         FIG. 7  is a photograph of a single bowtie antenna-coupled MIM. 
         FIG. 8  is a photograph of an antenna-coupled MIM array. 
     
    
    
     DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT 
     One embodiment of a process specific for fabricating an Al—NiOx-Pt “bowtie” MIM rectenna is illustrated in  FIGS. 1-6 . These steps are a specific example of fabricating a bowtie MIM rectenna using an arbitrary set of bottom and top metal layers while still using a NiOx insulator. This arrangement is favorable because its slow native oxidation rate allows for a more controlled process. In general, a thin Ni layer (of between 5-20 nm in thickness) is deposited over a bottom metal layer to form a first radiating element of a bowtie antenna. The entire thin Ni layer can then be oxidized using RIE such that a metal-NiOx bottom layer is obtained. Note that partial oxidation of the Ni layer results in a metal-Ni—NiOx bottom layer. Afterwards, an arbitrary top metal layer may be deposited without any additional complexity. The top metal layer forms a second radiating element of the bowtie. A contact pad deposition step or other fabrication steps may then follow. 
     Process Steps for Al—NiO—Pt MIM Diode 
     A high level flow diagram of process steps for fabricating the MIM rectenna is provided in  FIG. 6 . This figure can be referred to with the more detailed text explanation that follows. Please note certain steps, such as cleaning steps, are not shown in the high level diagram of  FIG. 6  but are described below.
 
1. Targets Deposition
 
     Alignment targets for subsequent e-beam and optical lithography steps are deposited on a Si wafer with SiO 2  buffer layer in step  104 , after initial optical lithography patterning in step  102 . 
     2. Bottom Aluminum (Al) Deposition with Nickel (N) Top Layer 
     A 60 nm bottom Al layer is deposited (step  108 ) after e-beam patterning (step  106 ).  FIGS. 1A and 1B  are side and top views of the result. Note that an Si/SiO 2  substrate is not shown in the side view of  FIG. 1A . In  FIG. 1A , reference numeral  10  indicates the Al layer and  12  indicates the Ni layer.  FIG. 1B  shows the first bowtie element  14  (shaped as a triangle in this example) and associated arm  16 . 
     Process (Steps  106 ,  108  and  110 ): 
     2.1. Wafer solvent clean (Acetone, Methanol, IPA)+N 2  dry+dehydration bake 2.2. Double-layer e-beam photoresist (PR) spin coating (double-layer process for easy liftoff) 
     a. PMMA EL13 
     b. PMMA A2 950K 
     2.3. Pattern Definition (e-beam write and O2 descum) 
     a. E-beam writing using JEOL e-beam 
     b. Develop in 1:3 MIBK:IPA with IPA rinse and N 2  dry 
     c. O 2  plasma descum using TI Planar RIE (to remove PR residues) 
     2.4. Evaporate Al 60 nm 
     2.5 Evaporate Ni 10 nm 
     2.6. Bottom Al/Ni Liftoff 
     a. Methylene Chloride soak until liftoff 
     b. 1:1 Methylene Chloride:Methanol soak 
     c. Methanol rinse and IPA rinse and N2 dry 
     3. Bottom Ni Oxidation (Step  112 ) 
     The Ni layer is then oxidized using Slave RIE to form a NiOx layer that acts as the insulator in the MIM diode.  FIG. 2  illustrates the result. O 2  plasma oxidation using RIE was found to yield the best results compared to atomic layer deposition and dionized water oxidation. 
     4. Top Pt Deposition+MIM Diode Formation (Step  114 ) 
     A 60 nm top Platinum (Pt) layer is then deposited over the oxidized bottom layer after e-beam patterning (see step  2  for detailed process), with a controlled overlap region. The overlap forms a Al-NiOx-Pt MIM structure (See  FIG. 3 ). Care should be taken in selecting developers that will not etch away the NiOx. 
     After this step, the second bowtie element  18  and associated CRM  20  are formed. 
     5. Contact Pad Deposition (Step  116 ) 
     Next, 0.02 μm/0.15 μm Ti/Au contact pads ( 22 ,  24 ) may be deposited over the contact arms for measurement purposes. Before deposition, a Hydrogen Chloride (HCl) etch is performed to remove NiOx from the MIM contact arms in order to provide good contact with the Ti/Au contact pads. 
     6. Process: 
     6.1. Wafer solvent clean (Acetone, Methanol, IPA)+N 2  dry+dehydration bake 
     6.2. Double-layer optical PR Coating (Step  118 ) 
     a. PMGI SF11 b. BPRS-100 
     6.3. Pattern Definition (Step  120 ) 
     a. Contact exposure and+develop in 1:4 PLSI:H 2 O 
     b. Deep UV Exposure and develop in  101 A 
     c. Deep UV Exposure and develop in  101 A (repeated to form undercut for easy liftoff) 
     d. O 2  plasma descum using RIE (to remove PR residues) 
     6.4. 40% HCl etch (to remove NiOx from contact arms) (Step  122 ) 
       FIG. 4  shows the result at this stage, after excess NiOx is removed from the bottom contact arm  16 . This step is expose the bottom contact arm  16  to a contact pad. 
     6.5. Evaporate Ti/Au 20 nm/150 nm 
     6.6. Contact Pad Liftoff 
     a. Acetone soak until liftoff and Methanol Rinse 
     b. 1165 Microposit Remover soak and Methanol Rinse 
     c. Methanol rinse and IPA rinse and N2 dry 
       FIG. 5  sows the final bowtie MIM rectenna with contact pads ( 22 ,  24 ). 
       FIG. 7  is a photograph of a single bowtie antenna-coupled MIM fabricated as described above. 
       FIG. 8  is a photograph of a 2×7 bowtie antenna-coupled MIM array fabricated as described above.