Abstract:
In one embodiment, a device includes a substrate having a top surface and cavity that defines generally vertical walls, a thin film of material that has been deposited on the walls of the cavity, and a further material that fills the cavity, wherein a top edge of the thin film is exposed and forms a trace that is flush with the top surface of the substrate and has substrate material on one side and the further material on the other side.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of co-pending U.S. application Ser. No. 14/032,614, filed Sep. 20, 2013, which claims priority to U.S. Provisional Application Ser. No. 61/703,537, filed Sep. 20, 2012, both of which being hereby incorporated by reference into the present disclosure. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support 1029067 awarded by the National Science Foundation. The government has certain rights to the invention. 
    
    
     BACKGROUND 
     A tunneling diode is a type of semiconductor diode that is capable of femtosecond operation. This fast operation is made possible by the use of the quantum mechanical effect called tunneling. Metal-insulator-metal (MIM) tunneling diodes are micro-scale or nano-scale devices that include a very thin layer comprising one or more dielectric materials that is sandwiched between two metal electrodes having different work functions. 
     MIM tunneling diodes having a response time of less than a picosecond hold great promise to outperform their semiconductor counterparts (e.g., Schottky diodes) in mixing and detection of terahertz and infrared radiation. The most significant limitation on the speed of operation of an MIM tunneling diode is its junction capacitance. To a large extent, the junction capacitance of such a diode is determined by the size of the tunnel junction. Therefore, one method of reducing the capacitance of an MIM tunneling diode is to reduce the size of the diode&#39;s junction. 
     To achieve a response time in the femotosecond range, the junction needs to have a nanoscale size, in which case the area of the junction must be on the order of 0.01 μm 2 . While such small junction sizes have been achieved using e-beam lithography, e-beam lithography is a slow “direct-write” nanolithography method that is not suitable for low-cost, high-volume, production. In order to be able to manufacture MIM tunneling diodes in a commercial context, an alternative fabrication method is required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood with reference to the following figures. Matching reference numerals designate corresponding parts throughout the figures, which are not necessarily drawn to scale. 
         FIGS. 1A-1H  are sequential cross-sectional drawings illustrating steps in an embodiment of a method for fabricating a metal-insulator-metal (MIM) tunneling diode. 
         FIGS. 2A-2C  are sequential perspective drawings at different stages of the fabrication shown in  FIGS. 1A-1H . 
         FIG. 3  is a top detail view of the diode of  FIG. 2C  that identifies the area of a junction of the diode. 
         FIG. 4  is a flow diagram of an embodiment of a method for fabricating a device having nanoscale structures, which can be used to fabricate an MIM. 
         FIGS. 5A and 5B  are I-V responses for two MIM tunneling diodes that were fabricated. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, in order for metal-insulator-metal (MIM) tunneling diodes to have response times of less than a picosecond, the junctions of the diodes need to have a nanoscale size. While such small sizes can be achieved using relatively slow techniques, such as e-beam lithography, alternative fabrication methods are required if such diodes are to be mass-produced in a commercial setting. Disclosed herein are methods with which an MIM tunneling diode capable of very short response times can be fabricated using conventional lithography techniques that are suitable for mass-production. In some embodiments, a bottom electrode of the diode can be created by forming a cavity in a substrate, conformally depositing metal on the substrate and in the cavity to form a thin film of metal on the walls of the cavity, depositing an isolation layer of material that covers the metal and fills the cavity, and planarizing the isolation layer to expose the top edges of the thin film formed on the cavity walls. The exposed lengths of the thin film can then be used as the bottom electrode of one or more MIM tunneling diodes. Notably, a similar process can be used to form a high aspect ratio vertical structure made of any thin-film depositable material for use in fabricating substantially any other device. Accordingly, the disclosed fabrication method is not limited to the fabrication of tunneling diodes. For instance, it could be used to form a nanoscale capacitive gap in a parallel plate capacitor that is filled with a chosen high-k insulator. 
     In the following disclosure, various specific embodiments are described. It is to be understood that those embodiments are example implementations of the disclosed inventions and that alternative embodiments are possible. All such embodiments are intended to fall within the scope of this disclosure. 
     As described above, an MIM tunneling diode comprises two metal electrodes with different work functions, which are separated by a very thin dielectric layer. In some cases, the dielectric layer can include a stack of multiple layers of insulator materials. For example, the dielectric layer can comprise two insulator layers, in which case the device can be referred to as a metal-insulator-insulator-metal (MIIM) tunneling diode. As used herein, the term “MIM tunneling diode” is an inclusive term that refers to any tunneling diode having metal electrodes and a dielectric layer that includes one or more layers of insulator materials. Therefore, as used herein, the term “MIM tunneling diode” includes MIIM tunneling diodes, MIIIM tunneling diodes, and so forth. 
     The MIM tunneling diode is a quantum device that relies on direct electron tunneling through the dielectric layer. The main design aspects for THz MIM tunneling diodes are the thickness and area of the tunnel junction. 
     One application for a THz MIM tunneling diode is to create a THz/infrared detector by coupling the diode with an antenna. The cutoff frequency (f c ) for the antenna-coupled MIM diode detector is given by: 
                     f   c     =     1     2   ⁢   π   ⁢           ⁢     R   A     ⁢     C   D                 [     Equation   ⁢           ⁢   1     ]               
where C D  is the junction capacitance and R A  is the resistance of the antenna. Reducing the MIM diode&#39;s junction area minimizes the capacitance while still maintaining the diode&#39;s electron tunneling probability. To have a cut-off frequency higher than 28 THz, a junction area of less than 0.01 μm 2  (e.g., 100 nm×100 nm) is required. In such a case, the resulting junction capacitance is on the order of 0.1 fF.
 
     Inside the tunnel junction, there are three major electron tunneling mechanisms: (1) photon-assisted, (2) thermal-assisted, and (3) field-assisted tunneling. Both photon- and thermal-assisted tunneling are bias voltage dependent, while photon-assisted tunneling is junction area dependent as well. Field-assisted tunneling is the primary tunneling mechanism for THz MIM diodes. The other two tunneling effects are regarded as noise, which deteriorates the diode&#39;s performance. By reducing the size of the junction area, the negative impact of thermal- and photon-assisted tunneling is reduced, while field-assisted tunneling is improved. The utilization of a very small junction area also enhances the nonlinearity and asymmetry of the diode&#39;s response. The nonlinearity and asymmetry of an MIM tunneling diode with a junction area of 100 nm×100 nm closely resembles the behavior of an ideal diode (as shown in  FIG. 6 ). 
     As described above, most of prior efforts in this area have relied upon e-beam lithography to achieve nanoscale junction sizes. However, e-beam lithography is fairly limited from the fabrication perspective because it is slow and non-repeatable. In general, e-beam lithography is not ideal for mass production, especially when the design of the tunneling diode is very material-centric. Disclosed herein are alternative fabrication methods that can be used in a mass-production context. These methods do not require e-beam lithography and instead can be performed using conventional lithographic processes. 
       FIGS. 1 and 2  illustrate various stages of an example fabrication method for an MIM tunneling diode. Beginning with  FIG. 1A , illustrated is a highly insulating substrate  10  having a top surface  12 . By way of example, the substrate  10  comprises a highly resistive silicon wafer or any insulating or semi-insulating substrate (e.g., quartz, glass, GaAs). 
     Referring next to  FIG. 1B , a cavity  14  is formed in the substrate  10 . The size of the cavity  14  is not critical for the fabrication of the MIM tunneling diode. By way of example, however, the cavity  14  can be on the order of approximately 10 to 100 μm long, 10 to 100 μm wide, and 1 to 10 μm deep. The cavity  14  is illustrated in perspective view in  FIG. 2A . As shown in that figure, the cavity  14  is rectangular and includes a generally horizontal bottom surface  16  and multiple generally vertical walls  18 . Irrespective of the specific configuration of the cavity  14 , it can be formed using a conventional lithographic process followed by reactive ion etch process (also known as dry etching). 
     With reference to  FIG. 1C , a thin film  20  of metal can be conformally deposited on the top surface  12  of the substrate  10  and in the cavity  14 . The thickness of the thin film  20  will dictate the thickness of the bottom electrode of the MIM tunneling diode, so the thin film is preferably very thin. In some embodiments, the thin film  20  is approximately 1 to 100 nm thick. In other embodiments, the thin film  20  is approximately 1 to 10 nm thick. Again, any conventional conformal thin-film deposition process, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), or atomic layer deposition processes (ALD), can be used to deposit the thin film  20 . Because the metal is conformally deposited, the thickness of the thin film  20  is generally uniform on the bottom surface  16  and walls  18  of the cavity  14 . 
     The metal that is used to form the thin film  20  can be selected based upon the desired configuration. For an MIM tunneling diode, the metal that is selected for one of the electrodes must have a different work function than the metal selected for the other electrode. Therefore, the metal used to form the thin film  20  will have a different work function than the metal used to form the other electrode (described below). By way of example, the metal used to form the thin film  20  can be platinum, titanium, or nickel. 
     Next, an insulating material can be deposited that covers the thin film  20  and fills the cavity  14  to serve as an isolation layer  22 , as shown in  FIG. 1D . In some embodiments, the isolation layer  22  can comprise a layer of amorphous silicon, silicon dioxide or silicon nitride, or parylene or polyimide. By way of example, the isolation layer  22  can be deposited using CVD. 
     Referring next to  FIG. 1E , the isolation layer  22  can be planarized using a polishing process to expose the top edges of the thin film  20  deposited on the walls  18  of the cavity  14 . The result of such planarization is shown in the perspective view of  FIG. 2B . As shown in this figure, the cavity  14  is filled with the insulating material of the isolation layer  22  and a continuous trace  24  of metal is formed at the surface of the device, flush with the top surface  12  of the substrate  10 . The trace  24  comprises four connected metal lines  26 , which result from the thin film  20  being applied to the four walls  18  of the cavity  14  ( FIG. 2A ). Each line  26  can be used to form one or more bottom electrodes of a device, such as an MIM tunneling diode, and each has a width that is equal to the thickness, t, of the thin film  20  that can be repeatedly controlled down to 1 to 10 nm scale. 
     Next, as shown in  FIG. 1F , a very thin dielectric layer  28  of insulating material can be deposited. In some embodiments, the dielectric layer  28  is approximately 1 nm to 10 nm. As noted above, the dielectric layer  28  can itself comprise multiple layers of insulating material, which can, in some cases, be layers of different materials. As with the other deposited layers, the dielectric layer  28  can be deposited using conventional thin film deposition processes, such as PVD, CVD, ALD, or polymer spinning. By way of example, the dielectric layer  28  can comprise one or more oxide layers made of a suitable oxide material (e.g., silicon oxide, Al 2 O 3 , HfO 2 , TiO 2 ) or one or more polymer layers made of a suitable polymer material (e.g., parylene or polyimide). 
     Referring next to  FIG. 1G , the dielectric layer  28  can be etched to again expose the edges of the thin film  20  to facilitate simple connection with the MIM tunneling diode. An example result of such etching is shown in  FIG. 2C . As shown in this figure, the etching can be performed so as to leave a pad  30  of dielectric material that overlies one of the lines  26  of the trace  24  and serves as a tunnel junction area. 
     With reference next to  FIG. 1H , a top electrode  32  can then be formed on the dielectric pad  30  to complete the MIM tunneling diode. As before, conventional lithographic processes can be used to form the electrode  32 . For example, the top electrode  32  can be patterned using conventional ultraviolet (UV) photolithography. When such a technique is used, the top electrode  32  can have a width, w, that is approximately 500 nm to 1 μm. As mentioned above, the metal that is selected for one electrode must be one that has a different work function than the metal selected for the other electrode. For example, if the metal used to form the thin film  20  is platinum, the metal used to form the top electrode  32  may be titanium. 
     The completed MIM tunneling diode  34  is shown in  FIG. 2C . As can be appreciated from this figure, the line  26  that is positioned beneath the dielectric pad  30  forms the bottom electrode of the diode  34  and is separated from the top electrode  32  by the dielectric pad  30 . The size of the diode&#39;s junction is dictated by the overlap of the top and bottom electrodes. In particular, the size is dictated by the area of this overlap. This area, A, is illustrated in the detail view of  FIG. 3 . As can be appreciated from  FIG. 3 , the top electrode  32  is relatively wide as compared to the bottom electrode, whose width is dictated by the thickness, t, of the thin film  20 . However, because the thin film  20  is so thin (narrow), the area, A, is still small enough to achieve the desired functionality. In some embodiments, the area, A, is approximately 0.01 to 0.1 μm 2 . By way of example, the top electrode  32  can have a width, w, of approximately 1 μm and the thin film  20  can have a thickness, t, of approximately 10 nm. In such a case, the junction will have an area, A, of approximately 0.01 μm 2 . Because the trace  24  is flush with the surface  12  of the substrate  10 , it is easy to connect to the line  26 . Moreover, as is apparent from  FIG. 2C , because the trace  24  includes multiple elongated lines  26 , multiple devices can be formed on the surface of the substrate  10 , if desired. 
     As expressed above, the disclosed fabrication method can be used to manufacture electrical devices other than MIM tunneling diodes. Indeed, the fabrication method can more generally be described as a method for making devices having nanoscale structures with a high aspect ratio.  FIG. 4  describes an example of such a method. Beginning with block  40 , a cavity is formed in a substrate. Next, a thin film material is conformally deposited on the substrate and in the cavity, as indicated in block  42 . In some embodiments, the thin film has a thickness of no more than 100 nm. In other embodiments, the thin film has a thickness of no more than 10 nm. An isolation layer of insulating material that covers the thin film and fills the cavity is then deposited, as indicated in block  44 . The isolation layer is then planarized to expose the top edges of the thin film material deposited on the cavity walls, as indicated in block  46 . Finally, with reference to block  48 , other structures can be added to the device, as desired. Significantly, each of the above-described actions can be performed using conventional lithography processes such that devices can be quickly and inexpensively manufactured in a mass-production context. 
     MIM tunneling diodes were fabricated for testing purposes. In particular, two diodes were fabricated, both having a junction area of 200 nm×200 nm. A first diode comprised platinum and titanium electrodes and a 3 nm thick Al 2 O 3  tunnel junction. The second diode comprised platinum and titanium electrodes and a dual-layer dielectric layer including a 1.5 nm Al 2 O 3  layer and 1.5 nm TiO 2  layer. The I-V responses for the two diodes are shown in  FIGS. 5A and 5B , respectively. As can be appreciated from these figures, the dual-layer approach changes the I-V characteristics by making the response much more asymmetric in forward- and reverse-biasing conditions.