Abstract:
A method for forming an antifuse interconnect structure, for a one-time fusible link, with field-programmable gate arrays, has been developed. The process features the use of an amorphous silicon layer, used as the antifuse layer, with the sidewalls of the amorphous silicon layer protected by critical silicon nitride sidewall spacers, during the patterning/etch procedure of the overlying metal layer. The protective sidewall spacers prevent the amorphous Si antifuse from being etched by subsequent processes.

Description:
BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     This invention relates generally to the fabrication of antifuse-based, programmable interconnect links, for field programmable gate arrays, (FPGA) for semiconductor devices and more particularly method for forming sidewall spacers on antifuse to prevent damage during subsequent metal etching. 
     2) Description of the Prior Art 
     Field-programmable gate arrays, (FPGA), have been designed to contain the needed row of arrays, as well as additional rows of spare arrays accessed if needed to replace ineffective counterparts, or to modify a specific design. Recent FPGA designs, feature one-time fusible link structures as possible programmable low resistance interconnect links, if accessed. These additional or spare arrays, or one-time fusible link structures, are sometimes comprised of an antifuse based programmable interconnect structure. The structure consists of an antifuse layer, usually a thin dielectric layer, placed between electrodes or conductive materials. When needed this antifuse material can be ruptured, or converted to a lower resistance layer, via a high voltage electrical pulse, resulting in creation of the replacement array structure. 
     The antifuse layer, used with the one-time fusible link structure, can be a dielectric layer, such as silicon oxide or silicon nitride. However to perform as an antifuse layer, the dielectric layer has to be thin, to allow reasonable programmable voltages to be successfully used. Thus small increases in the thickness of the thin antifuse dielectric layer, due to uniformity&#39;s in the dielectric layer deposition procedure, may result in inadequate programmed links. U.S. Pat. No. 5,807,786 (Chang), (assigned to the same assignee as the invention) describes a simpler process for forming one-time fusible link structures, using an amorphous silicon layer as the antifuse layer. However to avoid contamination, and additional oxide growth, on the amorphous silicon layer, during patterning procedures, a thin conductive barrier layer is used to overlie, and protect, the amorphous silicon antifuse layer, during specific fabrication sequences. 
     A problem the inventors have discovered is that the sidewalls of amorphous Silicon anti-fuses are damaged (and etched laterally) during the etching of the second electrode (the overlaying metal layer). This invention addresses the problem of lateral etching of the Amorphous Si antifuse. 
     The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently mote relevant technical developments in the patent literature can be gleaned by considering the following patents. U.S. Pat. No. 5,508,220 (Eltoukhy et al.) shows a method to form antifuses. U.S. Pat. No. 5,763,299 (McCollum et al.) shows a method to reduce leakage for antifuses. McCollum uses silicon nitride (SiN) layers and a barrier layer over the antifuse. U.S. Pat. No. 5,807,786 (Chang) shows a barrier layer to protect an antifuse. However, none of these patents effectively solves the problem of lateral etching of the Amorphous antifuse during the etching of the overlying metal layer. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a structure offering a one-time fusible link, for field programmable gate array designs. 
     It is another object of this invention to use an antifuse based interconnect, featuring an amorphous silicon, antifuse layer, for the one-time fusible link structure. 
     It is another object of this invention to use an antifuse based interconnect, featuring an amorphous silicon antifuse layer, for the one-time fusible link structure that has a silicon nitride anti-fuse sidewall spacer that protects the amorphous Si antifuse form antifuse from etchants and prevent lateral etching of the amorphous Si antifuse. 
     It is yet another object of this invention to form the amorphous Si antifuse having a SiN sidewall spacer that protects the amorphous Si antifuse form antifuse from etchants and protects the sidewalls of the amorphous silicon layer from subsequent processing procedures. 
     To accomplish the above objectives, the present invention provides a method of manufacturing an antifuse based interconnect structure, to be used for a one-time fusible link. A semiconductor substrate, comprised of overlying metal interconnect patterns, contacting conductive regions in the semiconductor substrate, is provided. A first metal interconnect structure, to be used as the lower electrode of the antifuse based interconnect structure, is formed, contacting conductive regions in the semiconductor substrate. An interlevel dielectric layer is deposited, preferably followed by a chemical mechanical polishing procedure, used for planarization purposes. A via hole opening is created in the interlevel dielectric layer, exposing the top surface of the first metal interconnect structure. A metal layer is deposited filling the via hole forming a metal plug. An amorphous silicon antifuse layer is next deposited. Patterning, using photolithographic and dry etching procedures, is used to form an amorphous Si antifuse, overlying the metal plug. 
     In a key step, protective antifuse sidewalls spacers  20  are formed on the antifuse  7  sidewalls. The protective spacers are formed of an etch resistance material and preferably of silicon nitride. A second metal layer is deposited thereover. A photoresist layer is formed over the second metal layer. The second metal layer is etched preferably using an etch chemistry, such as Cl 2 , or BCl 3 , that also could etch the amorphous Si antifuse if the invention&#39;s spacers where not protecting the antifuse sidewalls. 
     The metal layer is patterned to form a second interconnect structure, to be used as the upper electrode of the antifuse based interconnect structure, overlying the composite antifuse layer. The invention&#39;s protective spacers protect the Silicon amorphous silicon containing antifuse from etch damage in the upper electrode etch process. 
     The invention solves the following problems: 
     1. Prevents the amorphous Si antifuse sidewall from being damaged at the subsequent metal (upper electrode) etch. 
     2. Prevents the amorphous Si antifuse sidewalls from being damaged by stripper in the polymer strip step after metal etch. 
     The present invention achieves these benefits in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of a semiconductor device according to the present invention and further details of a process of fabricating such a semiconductor device in accordance with the present invention will be more clearly understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions and in which: 
     FIGS. 1,  2 ,  3 ,  4 ,  5  and  6  are cross sectional views for illustrating a method for manufacturing an amorphous Si antifuse  7  having silicon nitride sidewall spacers  20  according to the present invention. FIGS. 1 through 6 are taken along the axis  6 / 6 ′ in FIG.  7 . 
     FIG. 7A is a top down view of the antifuse with protective sidewall spacers  20  according to the present invention. 
     FIG. 7B is a top down view of the antifuse with protective sidewall spacers  20  where the top metal layer  10 B is mis-aligned according to the present invention. 
     FIGS. 8A and 8B are cross sectional views of a prior art antifuse showing the lateral etching of the antifuse during the top metal layer to for the top electrode. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The Problem the Invention&#39;s Spacers Solves—Lateral Etching of Antifuse 
     Referring now to the drawings and more particularly to FIGS. 8A and 8B, there is shown an antifuse structure over which the present invention is an improvement. It is understood in this regard that no portion of FIGS. 8A and 8B is admitted to be prior art for the present invention. Rather, this highly simplified diagram is an effort to provide an improved understanding of the problems that are overcome by the invention. 
     FIG. 8A shows a bottom meal layer  101 . An interlevel dielectric layer  102  is formed thereover. A via hole is formed in the interlevel dielectric layer and a W plug  105  is formed filling the via hole. An amorphous Si antifuse  107  is formed over the plug  105  by depositing an amorphous Si layer and patterning using conventional photo and etch processes. 
     Next, a metal top layer (not shown) is formed over the antifuse  107 . As shown in FIG. 8B, a top photoresist layer  111  is formed over the metal layer. The metal layer is etched to form the 2 nd  top interconnect  110 B. 
     However, the inventor has found that serious problems are caused by the top-metal etching. The second metal layer  10 A is etched preferably using an etch chemistry, such as Cl 2 , or BCl 3 , that also could etch the amorphous Si antifuse  107 . 
     The inventor has found that: 
     (1) Amorphous Si antifuse  107  will be damage by metal etching. The etch creates an undercut and weakens the antifuse performance. 
     (2). The amorphous Si antifuse is a little undercut  130  (etched laterally) during the metal etching; and 
     (3) The amorphous Si antifuse will be damaged by the polymer strip after metal etching. The polymer strip is a wet strip process. A stripper can be ACT 690, ACT 933, EKC25 etc. The main component in these strippers would be cathecal, Dimethyl sulfoxide, (DMSO), MEA, DGA, etc. These chemicals damage the Si of the antifuse. 
     Invention&#39;s Sidewall Spacers  20  Process 
     The process for forming an amorphous Si antifuse having sidewall SiN spacers  20 , to be used for a one-time fusible link, will now be described in detail. 
     In the following description numerous specific details are set forth such as flow rates, pressure settings, thicknesses, etc., in order to provide a more thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these details. In other instances, well known processes have not been described in detail in order to not unnecessarily obscure the present invention. Also, the flow rates in the specification can be scaled up or down keeping the same molar % or ratios to accommodate difference sized reactors as is known to those skilled in the art. 
     The structure is formed on a semiconductor substrate, preferably comprised of N type and P type conductive regions, in the semiconductor substrate, with overlying metal and insulator patterns, used for wiring purposes. The antifuse based interconnect structure will be fabricated on the integrated wiring patterns present on the semiconductor substrate. The specific conductive regions in the semiconductor substrate, and the overlying wiring patterns, can be formed to achieve complimentary metal oxide semiconductor field effect transistor (CMOS), designs, or BiCMOS designs, used for desired memory applications. 
     FIG. 1, schematically shows a first interconnect structure  1 , formed over an underlying semiconductor substrate (not shown) with the first interconnect structure  1 , either contacting a conductive region of the underlying semiconductor substrate, or contacting a wiring level, that in turn contacts a conductive region in the semiconductor substrate. For example, first interconnect structure  1  can be a metal line over a dielectric layer or an active area of a silicon substrate. First interconnect structure  1 , is preferably created by initially depositing a layer of an aluminum based metallization, such as pure aluminum, or aluminum, with between about 0 to 3% copper, or aluminum with between about 0 to 3% copper, and with between about 0 to 1% silicon. The aluminum based metallization is preferably deposited using R.F. (radio frequency) sputtering, to a thickness between about 1000 to 10,000 Angstroms. 
     An optional layer of titanium nitride (not shown) is preferably also deposited, via R.F. sputtering, overlying the aluminum based layer. The TiN layer preferably has a thickness between about 100 to 1600 Angstroms. The metallization used for the first interconnect structure can also be tungsten or tungsten silicide. Patterning of the metal layer is performed using conventional photolithographic and anisotropic reactive ion etching procedures, using Cl 2  as an etchant, creating first interconnect structure  1 , used as the lower electrode of the antifuse based interconnect structure. 
     An interlevel dielectric layer (ILD)  2  (e.g., insulator layer), preferably comprised of silicon oxide, is next deposited using plasma enhanced chemical vapor deposition, (PECVD), preferably to a final thickness (after planarization) between about 0.8 to 3.0 μm. A chemical mechanical polishing (CMP) procedure is preferably then employed to create a smooth top surface topography for interlevel dielectric layer  2 . This is shown schematically in FIG.  1 . Conventional photolithographic and anisotropic reactive ion etch (RIE) procedures, preferably using CHF 3  as an etchant, are used to open via hole  3 , in interlevel dielectric layer  2 . Via hole  3 , with a diameter between about 0.4 to 1.5 μm, results in the exposure of the top surface of first interconnect structure  1 . 
     A barrier layer of preferably titanium nitride  4 , is next deposited using R.F. sputtering procedures, preferably to a thickness between 100 to 1200 Angstroms, coating the exposed surfaces of via hole  3 . A tungsten layer  5  is then deposited; via low pressure chemical vapor deposition (LPCVD) procedures, to a thickness between about 4000 to 6000 Angstroms, completely filling via hole  3 . This is schematically shown in FIG.  1 . 
     Unwanted regions of tungsten layer  5 , as well as unwanted regions of titanium nitride layer  4 , are next removed preferably by an anisotropic RIE etch back procedure, using either CF 4 —O 2 , NF 3 —O 2 , or SF 6 —O 2 , as an etchant. The etch back procedure removes unwanted material from the top surface of interlevel dielectric layer  2 , creating tungsten plug  5 , and tungsten nitride spacers  4 , in via hole  3 . Alternately, the metal can be chemical-mechanical polished. 
     Antifuse  7   
     As shown in FIG. 3, a layer of amorphous silicon  7 , is deposited preferably using PECVD procedures, at a temperature between about 300 to 500° C. and more preferably between 350° to 450° C., to a thickness between about 600 to 1400 Å and more preferably between 800 to 1200 Angstroms. Amorphous silicon layer  7  will be the layer ruptured, via high voltage pulsing, to form a conductive link between underlying first interconnect structure  1 , and a subsequent, overlying second interconnect structure, if desired. Therefore the thickness of the antifuse layer is critical in choosing the correct pulsing voltage. 
     Also, an optional barrier layer (not shown) composed of TiN can be formed over the amorphous silicon layer  7 . The barrier layer preferably has a thickness of between about 200 and 300 Å. It is a major advantage of the present invention that this barrier layer is not required because of the invention&#39;s SiN antifuse spacers  20 . 
     FIG. 3 shows the patterning of the amorphous silicon layer is accomplished via anisotropic RIE procedures, preferably using Cl 2  as an etchant, and using a photoresist shape as a mask resulting in the antifuse shape, with a diameter between about 0.7 to 0.9 uM, directly overlying tungsten plug  5 . This is schematically shown in FIG.  3 . 
     Removal of photoresist shape, via oxygen ashing and wet clean procedures, is next performed exposing the amorphous Si antifuse  7 . 
     Protective Sidewall Spacers  7   
     Next, as shown on FIG. 4, antifuse sidewall spacers  7  are formed on the sidewalls for the amorphous Si antifuse  7 . The spacers are preferably formed of an etch resistance material such as SiO 2 , Silicon oxynitride (SiON) or silicon nitride (SiN) and are most preferably formed of SiN. The inventors have found that spacers formed of SiN have performance advantages over spacers formed of SiO 2 . SiO 2  spacers are inferior to SiN spacers. SiN spacers are the most preferred composition. The spacers preferably have a width of between about 100 and 2000 Å. 
     The invention&#39;s spacers  20  are formed by the well known process of forming a SiN layer over the surface and then anisotropically etching the SiN layer to form spacers  7 . It is critical that the antifuse spacers are used as an endpoint detection during the SiN anisotropic. This is a major point of the invention. Because the invention can use the SiN layer for endpoint detection, it reduces the oxide layer that reduces the topology (e.g., invention has a lower module height because of less oxide loss. 
     The invention&#39;s SiN spacers have major advantages over SiO 2  spacers. The invention&#39;s SiN spacers can be used for an endpoint detect whereas SiO 2  spacers can not. The spacers can solve the topography issues. 
     Top Electrode  10 B 
     A metal layer  10 A, needed for a second interconnect structure, or the upper electrode  10 B of the antifuse interconnect structure, is next deposited. Metal layer  10 A, is preferably deposited using R.F. sputtering procedures, is comprised of an aluminum based layer, that can contain copper and silicon, at a thickness between about 4000 to 10000 Angstroms. 
     In a preferred embodiment, the aluminum based layer is placed between an underlying layer of titanium nitride (not shown), and an overlying layer of titanium nitride (not shown). Photoresist shape  11 , to be used as a mask for the creation of the second interconnect structure, is also shown schematically in FIG.  5 . 
     RIE Etch Step—Spacers  20  Protect the Antifuse  7   
     As shown in FIG. 6, The metal layer  10 A is anisotropic RIE etched form a second interconnect structure  10 B. The etch preferably uses Cl 2  or BCl 3 , Ar, CF 4  or SF 6 . 
     The invention&#39;s SiN spacer can be used as an endpoint detect. This is a key advantage of the invention over the prior art. In contrast, oxide spacers can not. Oxide spacers can not be used for the endpoint detection because the layer  2  is formed of an oxide containing material. 
     If the invention&#39;s spacers  7  where not present, the etchants would attach and laterally etch (undercut  130 ) the amorphous Si antifuse  7  as shown in FIG.  8 B. 
     The width of second interconnect structure  10 B, is preferably between about 4000 Å to 1000 Å. Photoresist shape  11 , is again removed via oxygen plasma ashing and wet cleans, resulting in the antifuse interconnect structure featuring an amorphous silicon antifuse  7 , placed between conductive electrodes  5  and  10 B. This is schematically shown in FIG. 6. A top view of the antifuse interconnect structure, cross-sectional shown in FIG. 6, is presented in FIG.  7 . FIG. 7 shows a top down view of the structure. 
     Photoresist Removal and Polymer Removal—SiN Spacer Protect the Antifuse  20   
     Next the photoresist layer is removed preferably using photoresist stripper. 
     Often during the metal etch, a polymer forms on the antifuse sidewall and spacer sidewalls. An additional stripper or wet acid etch is used to remove the polymer. Without the invention&#39;s SiN spacers, the amorphous Si antifuse will be damaged by the polymer strip after metal etching. The polymer strip is a wet strip process. A stripper can be ACT 690, ACT 933, FKC25 etc. The main component in these strippers would be cathecal, Dimethyl sulfoxide, (DMSO), MEA, DGA, etc. These chemicals damage the Si of the antifuse. 
     The invention&#39;s SiN sidewalls spacers prevents antifuse damage from the photoresist and polymer strippers. In contrast, if SiO 2  spacers (not the invention&#39;s SiN spacers) were used, the wet etch and strippers (used to remover the polymer) would damage the SiO 2  spacers and antifuse  7 . This is a key advantage of the invention&#39;s SiN spacer over SiO 2  spacers. 
     FIG. 6 shows a dielectric layer  30  formed over the structure. Other conductive and dielectric layers and interconnects can be formed thereover the complete the device. 
     FIG. 7A is a top down view of the antifuse with protective sidewall spacers  20  where the top metal layer  10 B is aligned properly according to the present invention. FIG. 6 is taken along axis  6 / 6 ′ in FIG.  7 . 
     FIG. 7B is a top down view of the antifuse with protective sidewall spacers  20  where the top metal layer  10 B is mis-aligned according to the present invention. 
     Advantages of the Invention over the Prior Art 
     The invention&#39;s SiN spacers and amorphous antifuse have the following advantages over the prior art. 
     1) The antifuse can be made with out a barrier layer therefore a different and simpler/less costly antifuse  7  can be made comprising only of amorphous Si. 
     2) SiN spacers can be used for endpoint detection during the SiN etch that defines the spacers (anisotropic spacer etch), whereas oxide spacer can not. This improves the critical 2 nd  interconnect  10 B layer etch. 
     3) The SIN spacers improve the topography issue. 
     In contrast with U.S. Pat. No. 5,763,299 (McCollum) the invention does not use barrier layer (McCollum&#39;s layers  16  and  18 ) below the amorphous Si antifuse. The invention&#39;s lack of a McCollum&#39;s barrier layer allows the invention&#39;s module height to be reduced. Also, it increases the process window during the top electrode etch. Because the invention can use the SiN layer for endpoint detection, it reduces the oxide layer that reduces the topology (e.g., invention has a lower module height because of less oxide loss.) 
     It should be recognized that many publications describe the details of common techniques used in the fabrication process of integrated circuit components. Those techniques can be generally employed in the fabrication of the structure of the present invention. Moreover, the individual steps of such a process can be performed using commercially available integrated circuit fabrication machines. As specifically necessary to an understanding of the present invention, exemplary technical data are set forth based upon current technology. Future developments in the art may call for appropriate adjustments as would be obvious to one skilled in the art. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.