Abstract:
Structure and method for providing a programmable anti-fuse in a FET structure. A method of forming the programmable anti-fuse includes: providing a p− substrate with an n+ gate stack; implanting an n+ source region and an n+ drain region in the p− substrate; forming a resist mask over the n+ drain region, while leaving the n+ source region exposed; etching the n+ source region to form a recess in the n+ source region; and growing a p+ epitaxial silicon germanium layer in the recess in the n+ source region to form a pn junction that acts as a programmable diode or anti-fuse.

Description:
This non-provisional application claims the benefit of the provisional application filed with the United States Patent and Trademark Office as Ser. No. 61/149,527 entitled “Programmable PN Anti-Fuse”, filed Feb. 3, 2009. 
    
    
     BACKGROUND 
     1. Technical Field 
     The disclosure relates generally to electrically programmable fuses packaged in semiconductor devices, and more particularly relates to an electrically programmable anti-fuse implemented using a PN diode. 
     2. Background Art 
     In the integrated circuit (IC) fabrication industry, electrically programmable fuses (“eFuses”) are being incorporated in IC chips to provide, e.g., memory redundancy, passive component (e.g., resistors and capacitors) trimming, and post packaging product customization. However, current eFuse devices utilize silicide electromigration, which requires a very high energy to blow the fuse. This required level of energy results in a thermal and mechanical impact to the chip, which can damage surrounding devices. Current eFuses also require a large drive transistor (e.g., 6-10 um wide) to supply sufficient current for programming. 
     SUMMARY 
     Disclosed is a structure and method for forming a programmable anti-fuse in a field effect transistor (FET) structure. The initial state of the anti-fuse is “open.” After programming, the anti-fuse is “shorted.” 
     A first aspect of the disclosure provides a method of forming a programmable anti-fuse in a field effect transistor (FET device), comprising: providing a substrate with a gate stack; implanting a source region and a drain region in the substrate; forming a resist mask over the drain region, while leaving the source region exposed; etching the source region to form a recess in the source region; and growing an epitaxial silicon germanium layer in the recess in the source region to form a pn junction. 
     A second aspect of the disclosure provides a field effect transistor (FET) structure having a programmable anti-fuse, comprising: a substrate of a first type implanted with drain and source regions of a second type; an epitaxial silicon germanium layer positioned above the source region to form a pn junction; a conductive layer placed within each of the drain region, a gate region, and the epitaxial silicon germanium layer; and wherein the pn junction forms a diode that acts as a programmable anti-fuse that initially prevents current from flowing through the epitaxial silicon germanium layer between the source region and the conductive layer over the epitaxial silicon germanium layer. 
     A third aspect of the disclosure provides a method for programming an anti-fuse on an integrated circuit device, comprising: (a) providing a field effect transistor (FET) structure having: a substrate of a first type implanted with drain and source regions of a second type; an epitaxial silicon germanium layer positioned above the source region to form a pn junction; a conductive layer placed within each of the drain region, a gate region, and the epitaxial silicon germanium layer; and wherein the pn junction forms a diode that acts as a programmable anti-fuse that initially prevents current from flowing through the epitaxial silicon germanium layer between the source region and the conductive layer over the epitaxial silicon germanium layer; (b) applying a voltage Vdd to the conductive layer above the drain region; and (c) turning on the FET by applying a voltage Vg to the conductive layer above the gate region. 
     The illustrative aspects of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
         FIG. 1  shows a field effect transistor (FET) structure having an PN diode fabricated within, in accordance with an embodiment of the present invention. 
         FIG. 2  depicts a corresponding circuit diagram for the structure of  FIG. 1  in accordance with an embodiment of the present invention. 
         FIGS. 3-9  show the fabrication steps for forming the structure of  FIG. 1  in accordance with an embodiment of the present invention. 
     
    
    
     It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings,  FIG. 1  depicts an NFET structure  10  having a pn junction  12  that forms a programmable diode or anti-fuse. NFET structure  10  generally includes a drain  14  coupled to power source Vdd, a gate  16  formed above a channel region  22  coupled to a gate voltage source Vg, a source  18 , and a p+ region  24  formed between source  18  and a conductive region  20 . Source  18  and conductive region  20  comprise the input and output of the diode formed by pn junction  12 . As described in detail below, p+ region  24  is formed from an epitaxial silicon germanium, which is initially reverse biased to prevent current from flowing through. However, p+ region  24  can be altered using Vdd and Vg to create a short and allow current to flow. 
       FIG. 2  depicts an electrical schematic  26  of the NFET structure  10  shown in  FIG. 1 . As can be seen, NFET  28  has a drain  14  connected to Vdd, a gate  16  connected to Vg, and a source  18  connected to a first node of diode  36 . A second node of diode  36  is connected to ground voltage Vgnd. As constructed, diode  36  initially prevents current from passing between source  18  and ground (Vgnd). In a programming mode, diode  36  can be placed in a permanent “short” mode, after which current can pass between source  18  and Vgnd (i.e., a blown anti-fuse). In order to “blow the anti-fuse” in the programming mode, a voltage Vdd is applied to drain  14  and gate  16  is turned on with a voltage Vg. This results in a current flow out the source  18  and into the p+ region  24  ( FIG. 1 ), causing the pn junction  12  to become disable. A permanent low resistance between source  18  and Vgnd results, allowing current to flow. 
       FIGS. 3-9  depict an illustrative embodiment for forming the NFET structure  10  of  FIG. 1 .  FIG. 3  depicts a standard FET gate stack process  40  in which an n+ region  44  (gate stack region) is formed above a p− region  42  (substrate), in which a gate dielectric  46  separates the two regions  42 ,  44 . In  FIG. 4 , n+ junctions  48  for the source region and drain region are formed, e.g., with an implant process. In  FIG. 5 , a resist mask  50  is placed over half of the structure containing the drain region. In  FIG. 6 , a recessed region  52  is etched from the source region. In  FIG. 7 , epitaxial silicon germanium Epi-SiGe is grown in the recessed region  52  over the n+ source region on the structure. An Epi-SiGe growth (in-situ p+ doped) region  54  is thus formed, which creates a PNP structure including p+ region  54 , doped n+ region  56 , and p− region  58 . The pn junction between the p+ region  54  and doped n+ region  56  forms a closed path that initially prevents current from flowing. As noted above, the anti-fuse can thereafter be enabled by applying Vdd to the drain and Vg to the gate. In  FIG. 8 , the cap  60  is removed from the gate region, and a salicidation process is utilized to form conductive regions  62 ,  64 ,  66  in  FIG. 9 . 
     In an illustrative embodiment for a 45 nm device, the Epi-SiGe dopant may be between e −19  and e −20  cm −3 . Doping levels for the p−, n+, and p+ regions may for instance be approximately e −17 , e −20 , and e −20  cm −3 . A 4.25 volt application to Vdd in the programming mode will create a reverse bias in the diode that will result in an approximate 500 ohm resistance on a 3 mA compliant device. A 5 volt application to Vdd in the programming mode will create a reverse bias in the diode that will result in an approximate 600 ohm resistance on a 5 mA compliant device. 
     Note that while the illustrative embodiments described herein are implemented in an NFET device, the inventive feature of a programmable diode could likewise be implemented in a PFET structure. In such a case, the PFET device would be fabricated with n+ doped SiGe source region. 
     The method as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     The foregoing drawings show some of the processing associated according to several embodiments of this disclosure. In this regard, each drawing or block within a flow diagram of the drawings represents a process associated with embodiments of the method described. It should also be noted that in some alternative implementations, the acts noted in the drawings or blocks may occur out of the order noted in the figure or, for example, may in fact be executed substantially concurrently or in the reverse order, depending upon the act involved. Also, one of ordinary skill in the art will recognize that additional blocks that describe the processing may be added. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.