# Patent

## Title
Unguarded Schottky barrier diodes with dielectric underetch at silicide interface

## Abstract

One embodiment of the invention relates to an unguarded Schottky barrier diode. The diode includes a cathode that has a recessed region and a dielectric interface surface that laterally extends around a perimeter of the recessed region. The diode further includes an anode that conforms to the recessed region. A dielectric layer extends over the dielectric interface surface of the cathode and further extends over a portion of the anode near the perimeter. Other devices and methods are also disclosed.

## Background
RELATED APPLICATIONS
This application is a divisional from U.S. Nonprovisional patent application Ser. No. 11/757,767, filed Jun. 4, 2007, entitled “Unguarded Schottky Barrier Diodes,” which claims priority to U.S. Provisional Application Ser. No. 60/804,192, filed Jun. 8, 2006, entitled “Method for Making an Ideal PtSi Unguarded Schottky Barrier Diode,” and to U.S. Provisional Application Ser. No. 60/804,195, filed Jun. 8, 2006, entitled “Method for Suppression of Hot Carrier Damage in Unguarded Schottky Barrier Diodes”, which are incorporated herein by reference in their entirety.


FIELD OF INVENTION
The present invention relates generally to semiconductor devices and more particularly to unguarded Schottky barrier diodes.
BACKGROUND OF THE INVENTION
A diode is an electrical component that allows current to flow in one direction, but not in the other. FIG. 1 is a circuit symbol of a typical Schottky barrier diode 100, where the diode 100 has an anode 102 (metal) in direct contact with a cathode 104 (n-type semiconductor). In Schottky barrier diodes, conventional current can typically flow from the anode 102 to the cathode 104, but not from the cathode to the anode. Schottky barrier diodes typically result in fast switching times and low forward voltage drop, relative to conventional p-n diodes.
Schottky barrier diodes are available in guarded and unguarded diode configurations. Guarded Schottky barrier diodes include a p-n junction guard ring disposed in the substrate around the perimeter of the diode. Although the p-n junction guard ring virtually eliminates some adverse leakage current effects around the perimeter, at higher forward bias (especially for high barrier height silicides) there is significant injection of minority carriers. This injection leads to very slow diode recovery after the forward bias is removed. This phenomenon cannot be tolerated in many high-speed applications.
Therefore, many high speed applications require integrated unguarded Schottky barrier diodes to be able to operate at high reverse bias voltages while offering very fast recovery speeds. For some time, however, these diodes have suffered from hot carrier damage even after a modest reverse bias of 10-15 volts or less. This hot carrier damage can lead to unguarded Schottky barrier diodes with non-ideal I-V characteristics. Accordingly, there remains a need for improved unguarded Schottky barrier diodes and manufacturing techniques.
SUMMARY OF THE INVENTION
The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary presents one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later and is not an extensive overview of the invention. In this regard, the summary is not intended to identify key or critical elements of the invention, nor does the summary delineate the scope of the invention.
One embodiment of the invention relates to an unguarded Schottky barrier diode. The diode includes a cathode comprising a recessed region and a dielectric interface surface that laterally extends around a perimeter of the recessed region. A silicide layer at least substantially conforms to the recessed region. The diode also includes a dielectric layer having a window with sidewalls that are near the perimeter of the recessed region, and the dielectric layer extends continuously over the dielectric interface surface and at least a portion of the silicide layer near the perimeter of the recessed region.
The following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a circuit symbol for a Schottky barrier diode;
FIG. 2 shows a diode that may suffer from leakage current through a metal insulator diode;
FIGS. 3A-3B show an unguarded Schottky barrier diode that may mitigate formation of a metal insulator diode at its perimeter;
FIG. 4 shows a somewhat general method of forming an unguarded Schottky barrier diode; and
FIGS. 5-12 show cross-sectional views of stages in a more detailed method of forming an unguarded Schottky barrier diode.



DETAILED DESCRIPTION OF THE INVENTION
The present invention will now be described with reference to the attached drawing figures, wherein like reference numerals are used to refer to like elements throughout, and wherein the illustrated structures and devices are not necessarily drawn to scale.
Aspects of this invention are aimed at preventing formation of a leaky metal insulator semiconductor (MIS) parasitic diode at the Schottky barrier diode perimeter. A more detailed description of this problem is now discussed with reference to FIG. 2, which shows an unguarded Schottky barrier diode 200 having a leaky, low-barrier height MIS diode 202 at its perimeter. In essence, the MIS diode 202 allows current to “leak” through or around the diode, which causes the overall diode characteristics to significantly deviate from those of an ideal diode.
Briefly, the diode 200 includes an n-type lightly doped Silicon cathode 204 under which a buried layer 206 (cathode contact) resides. A dielectric layer 208 is formed over the cathode 204, where a window 210 with tapered sidewalls 212, 214 is formed within the dielectric layer 208. A silicide layer 216 is in direct contact with the cathode 204 and is directly adjacent to the bottom of the tapered sidewalls 212, 214. Although not shown, a Ti-based diffusion barrier metal (or other refractory metal) is typically formed over the silicide layer 216.
The parasitic MIS diode 202 is formed at a perimeter 218 of the Schottky barrier diode (i.e., near the bottom of the tapered sidewalls 212, 214 of the dielectric). More particularly, the MIS diode 202 exists where a thin dielectric tail 220 is sandwiched between the diffusion barrier metal (not shown here) and the cathode 204. Ideally, during operation, all current would flow directly between the cathode 204 and anode 216 of the diode, and not through the MIS diode 202. However, the inventors have appreciated that a sufficiently thin dielectric tail 220 allows the MIS diode 202 to conduct a significant amount of current by tunneling. This leakage current, combined with the current flowing through the anode 216 causes the Schottky barrier diode 200 to have non-ideal I-V characteristics.
Moreover, this parasitic leakage current from the MIS diode is highly variable and difficult to control. This results in poor manufacturing repeatability and poor device to device matching. Thus, diode 200 is not generally usable in high precision circuits unless the problem with the formation of the MIS diode can be prevented. Note that the MIS diode dominates the forward bias characteristics only at lower bias voltages because its high series resistance limits its current at higher forward bias voltages (i.e., the cathode and anode 216 of the Schottky barrier diode have a low series resistance that does not limit the current flow at higher bias voltages).
Other aspects of this invention are aimed at suppressing hot carrier damage at the perimeter 218 of unguarded Schottky barrier diodes. Hot carrier damage can occur when charged carriers (electrons or holes) are accelerated by a high electric field present at the perimeter 218 of the diode 200. These hot carriers are injected into the dielectric 208, and damage the dielectric/silicon interface by generating charged trapping centers. Note that the electric field at the perimeter of the diode is described by the following equation:




  
    
      
        
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where E is the electric field, rw is the depletion layer edge and rj is the radius of the anode/cathode junction at the perimeter. Equation 1 shows that the electric field is inversely proportional to the radius rj.

Another element contributing to hot carrier damage is the oxide/silicon interface at the diode's perimeter, (i.e., the location where hot carriers can inflict damage by creating positively charged trapping centers). The density of these charge centers is a linearly dependent function of the perimeter current and time. This charge enhances the carrier concentration at the perimeter of an n-type substrate and reduces the carrier concentration of a p-type substrate. Increase in the effective doping concentration further increases the electric field, as described in Equation 1, by increasing ND in this example for an n-type silicon substrate. The effective Schottky barrier height is reduced by the barrier lowering terms below,




  
    
      
        
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where ΦB0 is the intrinsic barrier height, and α is the tunneling coefficient (α=3 nm for PtSi). Increase in the electric field, caused by the localized increase in the carrier concentration, results in a drop of the effective Schottky barrier height at the perimeter after the reverse bias stress damage occurs. This explains why there is an increase in the reverse current after subjecting the diode to a significant reverse bias stress. The opposite effect, i.e. decrease in reverse current is observed in diodes with a p-type silicon substrate where the localized carrier concentration decreases after the stress.

Therefore, aspects of the present invention aim to prevent the formation of the MIS diode and to suppress hot carrier damage at the perimeter of the Schottky barrier diode. FIGS. 3A-3B show one unguarded Schottky barrier diode 300 that may achieve these ends. The diode 300 includes a cathode 302 having a recessed region 304 over which a silicide layer 306 is formed. A dielectric layer 308 having a window 310 therein overlies the cathode 302. To prevent the formation of an MIS diode, the silicide layer 306 undercuts a portion of the dielectric layer 308 near a perimeter 312 of the recessed region 304. A diffusion barrier metal 314 (e.g., a Ti-based diffusion barrier metal or other similar refractory metal) could be formed over the silicide layer. In effect, by providing a silicide layer 306 that undercuts the dielectric layer 308, the diode 300 provides significantly thick isolation between the barrier diffusion metal 314 and the cathode 302, such that current tunneling and thus unwanted parasitic leakage will not occur.
In addition, because the silicide layer 306 undercuts the dielectric 308, the diode 300 has an increased anode/cathode junction radius rj′ (relative to radius rj of diode 220), which reduces the electric field carriers experience, thereby limiting hot carrier damage.
As shown, the cathode 302 typically overlies a highly conductive buried layer 316, which could be an n-type highly doped Si substrate, that serves as a cathode contact.
The cathode 302, which could be an n-type lightly doped Silicon substrate, includes a dielectric interface surface 318 that laterally extends around the perimeter 312 of the recessed region 304. The recessed region 304 may have a recessed surface 320 and concave sidewalls 322, 324.
The silicide layer 306 may substantially conform to the recessed region 304. Depending on the implementation, the silicide layer 306 may have various thicknesses. For example, in the illustrated embodiment, the silicide thickness, T, is such that a top silicide surface 326 is positioned between the dielectric interface surface 318 and the recessed surface 320. However, depending on the silicide layer thickness, T, and the depth, d, of the recessed region, the top silicide surface 326 could also be above the dielectric interface surface 318.
The dielectric layer 308 can be characterized by window 310 or aperture in the dielectric layer, which has sidewalls 328, 330 that are near the perimeter 312 of the recessed region. In the illustrated embodiment, the sidewalls are generally concave, although they could be convex or substantially vertical in other embodiments. These sidewalls may cause the dielectric layer to include a tail 332, which is a thin region of the dielectric sandwiched between the barrier diffusion metal 314 and the silicide layer 306. The extent by which the recessed region must undercut the tail 332 (length L) may be a function of the reverse bias the diode is to withstand. As the reverse bias the diode is to withstand increases, a designer may increase the length L, depth d, and/or the thickness T of the silicide layer. In effect, by increasing the length L, depth d, and/or thickness T, the designer is providing greater isolation between the barrier diffusion metal 314 and the cathode 302, such that unwanted parasitic leakage will not occur. The same is also increasing the radius rj and decreasing the electric field as described by Equation 1. For example, in one embodiment where the diode is to withstand a reverse bias of approximately 20 V, the recessed region undercuts the dielectric by a length L of approximately 10-100 nm. In various embodiments, the Silicide layer may have a thickness, T, of approximately 10-100 nm.
While the potential advantages of the above described Schottky barrier diodes are apparent, successful fabrication of such Schottky barrier diodes are needed. In order to aid in the successful fabrication of these Schottky barrier diodes, aspects of the present invention relate to methods of manufacturing Schottky barrier diodes. For illustrative purposes, a general method 400 is illustrated in FIG. 4, while a more detailed method 500 is described in FIGS. 5-12. While these methods are illustrated and described below as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement a methodology in accordance with one or more aspects or embodiments of the present invention. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.
Referring now to FIG. 4, method 400 starts with a dielectric layer overlying a cathode in a semiconductor substrate. At 402, a window is etched into the dielectric layer, thereby exposing a portion of the cathode. In 404, an etch is performed to form a recessed region in the cathode. In various embodiments, blocks 402 and 404 may be performed in a single etch that results in a significant isotropic overetch at the perimeter of the diode. This overetch creates a silicon profile with a junction radius much larger than created by prior art processes. Thus, this etch leads to an unguarded Schottky barrier diode with significantly reduced electric field (relative to prior art processes), which can suppress the onset of the reverse bias induced hot carrier damage until much higher bias voltages are applied. After the recessed region is formed, in 406 a metal that is used in silicide formation is conformally deposited over the recessed region. In 408, the metal layer in the recessed region is converted to a metal silicide using a high temperature treatment. In 410, the non-silicide metal is removed. Lastly, in 412, a metal layer contact for the anode is formed.
Now that the somewhat general method 400 has been discussed, a more detailed method 500 is illustrated with reference to FIGS. 5-12, which show cross sectional views of various stages in the manufacture of one embodiment of an unguarded Schottky barrier diode.
Referring to FIG. 5, one can see the method 500 starts when a lightly doped Si cathode 502 is formed over a highly doped buried layer 504. The highly conductive buried layer 504 could be formed, for example, by implanting a high-dopant concentration of n-type impurities into the substrate, which is than followed by epitaxial growth of lightly doped silicon layer 502 forming the cathode region.
In FIG. 6, a dielectric layer 600 has been formed over the lightly doped Silicon cathode 502. In various embodiments, the dielectric layer 600 could comprise a single layer or some combination of both, the thermally grown SiO2 and/or deposited SiO2 or silicon nitride or another suitable dielectric. In some embodiments the dielectric layer may include spacers, although in various embodiments the dielectric layer may not include spacers and/or may comprise a single solid material (e.g., field oxide or other continuous material).
After the dielectric layer 600 has been formed, method 500 proceeds to FIGS. 7-8 where a mask (not shown), such as photoresist or a hard mask, has been formed over the dielectric layer. In FIG. 7, an etch 700 is performed to form a window 702 in the dielectric layer 600 through which a portion of the cathode is exposed. In some embodiments, this etch 700 could be an isotropic (multi-directional) etch, such as a wet or plasma etch, and would result in concave sidewalls. In other embodiments, this etch 700 could be an anisotropic etch (uni-directional) etch, such as a plasma (RIE) etch, and would result in relatively vertical sidewalls. In FIG. 8, an etch 800 is performed to form a recessed region 802 within the cathode 502. This etch 800 is typically an isotropic etch that is selective between the Si cathode and the dielectric layer, thereby removing a portion of the cathode and undercutting the dielectric layer by length L.
In some embodiments, etches 700 and 800 could be carried out as separate etches. In other embodiments, etches 700 and 800 may be performed as a single etch, where the semiconductor structure is kept within the etch tool and not exposed to the laboratory environment. Many different single etches, having different material selectivities and etch rates, or sequential etches may accomplish similar end results. The selection of a particular etch technique depends upon fabrication capability, cost or just convenience.
In FIG. 9, a silicidation metal 900 has been conformally deposited over the dielectric layer 600 and the recessed region 702. This silicidation metal 900 may be deposited, for example by a sputtering process, evaporation, CVD or any other suitable technique. In one embodiment, the silicidation metal could be Platinum (Pt), Palladium (Pd) or Cobalt (Co), although any other suitable metal could be used.
In FIG. 10, the structure is exposed to a high temperature treatment(s) to convert the silicidation metal that contacts the cathode to a silicide. As shown, FIG. 9's deposited silicidation metal 900 has now been converted to a silicide layer 1000 over the silicon cathode region with a non-silicided metal 1002 remaining over the regions not having silicon exposed. Typically, the silicide layer 1000 could be a high barrier height near-noble metal silicide, such as PtSi or Pd2Si, for example. However, the invention may be applicable to any silicide. The high temperature treatment can be done in conventional furnace or in rapid thermal processors or flash and or spike anneal processors. The temperature treatment temperature and time are adjusted as required for different silicidation metals. The ambient used during heat treatment can be neutral, oxidazing or reducing, depending up selection of a particular silicidation metal.
In FIG. 11, an etch 1100 is performed to remove the non-silicided metal 1002, where the etch chemistry depends on the chemical composition of the silicide layer.
In FIG. 12, a metal 1200, typically comprised of diffusion barrier metal and/or adhesion metal and some form of high conductivity metal such as aluminum (Al) or copper (Cu), for example, is deposited to form a contact for the anode. A Ti-based metal or TiN compound or their combination are examples of metals used in the industry for this purpose.
Although the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.

## Claims

1. A method for manufacturing comprising:
forming a dielectric layer over a silicon cathode;
defining a window opening in the dielectric layer through which a portion of the silicon cathode is exposed;
forming a recessed region in the silicon cathode through the window opening, the recessed region having a central portion overlapping the window opening, and the recessed region having a peripheral etched undercut portion spaced below the window opening and extending laterally undercutting the dielectric layer;
forming a silicide layer within the recessed region, the silicide layer positioned within the peripheral etched undercut portion undercutting the dielectric layer; and
forming an anode contact in electrical communication with the silicide layer, the anode contact overlapping the central portion of the recessed region and separated by an undercut part of the dielectric layer from the silicide layer within the peripheral etched undercut portion of the recessed region,
wherein the window opening is defined by sidewalls that are concave,
wherein the undercut part of the dielectric layer is concave and extends beyond an interface of the dielectric layer and the silicon cathode, and wherein the undercut part is sandwiched between the silicide layer and a diffusion metal layer to separate the diffusion metal layer from silicide layer within the peripheral etched undercut portion of the recessed region, thereby preventing a metal-insulator-semiconductor diode from being formed,
wherein the silicon cathode and the dielectric layer interfaced directly and free of any interposed layer.

2. The method of claim 1, wherein the sidewalls of the window opening are substantially vertical.

3. The method of claim 1, further comprising a filler metal that overlies the diffusion metal layer and fills the window opening.

4. The method of claim 1, further comprising a highly conductive buried layer under the silicon cathode.

5. The method of claim 1, wherein the silicide layer undercuts the dielectric layer by approximately 10-100 nm.

6. The method of claim 1, wherein the diffusion metal layer overlies the undercut part of the dielectric layer and the sidewalls of the window opening.