Patent Publication Number: US-9899338-B1

Title: Structure and fabrication method for enhanced mechanical strength crack stop

Description:
BACKGROUND 
     The present application relates to an integrated circuit (IC) chip, and more particularly, to methods for improving mechanical strength of back-end-of-line (BEOL) dielectrics to prevent crack propagation within interconnect stacks. 
     Integrated circuits are generally created by forming an array of electronic devices (i.e., transistors, diodes, resistors, capacitors, etc.) and interconnect wiring structures on a semiconductor substrate. Generally, semiconductor devices and gates are formed in a first layer during front-end-of-line (FEOL) processing, followed by formation of interconnect wiring structures in a second layer by BEOL processes. These first and second layers can each contain multiple layers of dielectric material which electrically isolate the devices and interconnect structures. Advanced semiconductor processes utilize dielectric materials with low dielectric constants (low-k) to minimize interconnect parasitic capacitances. 
     After a plurality of integrated circuits (ICs) are formed on a semiconductor wafer, the semiconductor wafer is subjected to a wafer cutting process so as to divide the semiconductor wafer into a plurality of semiconductor chips. The semiconductor chip is then bonded to a substrate package. Due to poor mechanical strength of the low-k dielectric materials during the wafer cutting and bonding processes, cracks can form and propagate through the BEOL dielectrics toward the active area of the IC chip, causing chip failure. There thus exists a need to enhance mechanical strength of the BEOL dielectrics. 
     SUMMARY 
     The present application provides methods for enhancing mechanical strength of BEOL dielectrics to prevent crack propagation within interconnect stacks. After forming interconnect structures in a dielectric material layer, a pore filling material is introduced into pores of a portion of the dielectric material layer that is located in a crack stop region present around a periphery of a chip region. By filling the pores of the portion of the dielectric material layer located in the crack stop region, the mechanical strength of the dielectric material layer is selectively enhanced in the crack stop region. The crack propagation can thus be effectively prevented in the crack stop region. 
     In one aspect of the present application, a semiconductor structure is provided. The semiconductor structure includes a plurality of interconnect structures embedded in a dielectric material layer located over a substrate. The plurality of interconnect structures includes a first set of interconnect structures located in a chip region and a second set of interconnect structures located in a crack stop region around a periphery of the chip region. A pore filling material fills at least some pores in a portion of the dielectric material layer located in the crack stop region. 
     In another aspect of the present application, a method for forming a semiconductor structure is provided. The method includes forming a plurality of interconnect structures embedded in a dielectric material layer located over a substrate. The plurality of the interconnect structures includes a first set of interconnect structures located in a chip region and a second set of interconnect structures located in a crack stop region around a periphery of the chip region. At least some pores in a portion of the dielectric material layer located in the crack stop region is then filled with a pore filling material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a first exemplary semiconductor structure including a dielectric material layer and a plurality of interconnect structures located therein that can be employed in a first embodiment of the application. 
         FIG. 2  is a cross-sectional view of the first exemplary semiconductor structure of  FIG. 1  after forming a first mask layer portion covering a portion of the dielectric material layer and a first set of interconnect structures in a chip region, while exposing another portion of the dielectric material layer and a second set of interconnect structures in a crack stop region around a periphery of the chip region. 
         FIG. 3  is a cross-sectional view of the first exemplary semiconductor structure of  FIG. 2  after filling pores of the exposed portion of the dielectric material layer in the crack stop region with a pore filling material. 
         FIG. 4  is a cross-sectional view of a second exemplary semiconductor structure that can be derived from the first exemplary semiconductor structure of  FIG. 1  after filling pores of an entire dielectric material layer with a pore filling material in accordance with a second embodiment of the present application. 
         FIG. 5  is a cross-sectional view of the second exemplary semiconductor structure of  FIG. 4  after forming a second mask layer portion covering a portion of the dielectric material layer and a second set of interconnect structures in a crack stop region, while exposing another portion of the dielectric material layer and a first set of interconnect structures in a chip region that is surrounded by the crack stop region. 
         FIG. 6  is a cross-sectional view of the second exemplary semiconductor structure of  FIG. 5  after removing the pore filling material from the exposed portion of the dielectric material layer in the chip region. 
     
    
    
     DETAILED DESCRIPTION 
     The present application will now be described in greater detail by referring to the following discussion and drawings that accompany the present application. It is noted that the drawings of the present application are provided for illustrative purposes only and, as such, the drawings are not drawn to scale. It is also noted that like and corresponding elements are referred to by like reference numerals. 
     In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps and techniques, in order to provide an understanding of the various embodiments of the present application. However, it will be appreciated by one of ordinary skill in the art that the various embodiments of the present application may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the present application. 
     Referring to  FIG. 1 , a first exemplary semiconductor structure according to a first embodiment of the present application includes interconnect structures formed within a dielectric material layer  30  that is located over a substrate  10 . Optionally, a dielectric cap layer  20  may be formed between the substrate  10  and the dielectric material layer  30 . 
     The semiconductor structure includes a chip region  100  and a crack stop region  200  formed around a periphery of the chip region  100 . Active and passive devices such as, for example, transistors, diodes, resistors, inductors or other components that are part of integrated circuits can be fabricated in the chip region  100 . The crack stop region  200  functions to reduce or prevent the propagation of cracks, which are initiated by chipping and cracking formed along peripheral edges of the chip during subsequent dicing and/or bonding operations, into the chip region  100 . 
     The substrate  10  can include a semiconductor material, an insulator material, a conductive material, or a combination thereof. The semiconductor material can be an elemental semiconductor material such as silicon, germanium, carbon, or an alloy thereof, a III-V compound semiconductor material, a II-VI compound semiconductor material, or any combination or stack thereof. The semiconductor material can be doped with electrical dopants such as B, Ga, In, P, As, and Sb. Multiple semiconductor materials can be present in the substrate  10 . The insulator material can be doped or undoped silicon oxide, doped derivatives of silicon oxide, silicon nitride, silicon oxynitride, or a combination or stack thereof. Multiple insulator materials can be present in the substrate  10 . The conductive material can include a metallic material such as Cu, W, Ti, Ta, Al, WN, TiN, TaN, WC, TiC, or alloys thereof. Further, the substrate  10  can include a contact-level dielectric material layer and contact via structures embedded therein. 
     When present, the dielectric cap layer  20  can protect the underlying substrate  10  from impurities that may diffuse down from upper levels, and can function as a diffusion barrier layer that prevents vertical diffusion of metallic impurities, moisture, or other gaseous impurities. The dielectric cap layer  20  may include, for example, silicon nitride, silicon oxynitride, silicon carbide, nitrogen and hydrogen doped silicon carbide (SiCNH) or a combination thereof. The dielectric cap layer  20  may be formed, for example, by chemical vapor deposition (CVD) or atomic layer deposition (ALD). The thickness of the dielectric cap layer  20  can be from 25 nm to 50 nm, although lesser and greater thicknesses can also be employed. The dielectric cap layer  20  is optional and can be omitted in some embodiments of the present application. 
     The dielectric material layer  30  typically includes a porous low-k dielectric material. By “low-k” it is meant a dielectric material having a dielectric constant that is about 4.0 or less. In one embodiment, the dielectric material layer  30  includes organosilicates, silsequioxanes, undoped silicate glass (USG), fluorosilicate glass (FSG), SiCOH or borophosphosilicate glass (BPSG). The dielectric material layer  30  may be formed by CVD, plasma enhanced chemical vapor deposition (PECVD) or spin coating. The thickness of the dielectric material layer  30  may be from 100 nm to 1,000 nm, although lesser and greater thicknesses can also be employed. 
     The interconnect structures include a first set of interconnect structures (herein referred to as first interconnect structures  40 A) formed in the chip region  100  and a second set of interconnect structures (herein referred to as second interconnect structures  40 B) formed in the crack stop region  200 . The first interconnect structures  40 A provide electric connections for electric components of the substrate  10  located in the chip region  100 . The second interconnect structures  40 B are dummy structures and function as crack stops due to the higher mechanical strength and configuration of the metals within the second interconnect structures  40 B compared to the dielectric material that provides mechanical strength within the dielectric material layer  30 . As used herein, “dummy structures” are elements that are not part of the active circuity, thus they have no function in the active circuitry. Each of the first and second interconnect structures  40 A,  40 B includes a metal liner  42  and a metal portion  44  surrounded by the metal liner  42 . 
     The interconnect structures  40 A,  40 B can be formed by first patterning the dielectric material layer  30 , utilizing lithography and etching processes known in the art to provide interconnect openings (not shown) in the dielectric material layer  30 . The interconnect openings thus may be line openings and/or via openings. Line openings are formed in the dielectric material layer  30 , while via opening are formed through both the dielectric material layer  30  and the dielectric cap layer  20 . For example, a photoresist layer (not shown) can be applied over the dielectric material layer  30  and lithographically patterned to form a pattern of openings therein. The openings overlie areas in which formation of the interconnect openings are desired. The pattern in the photoresist layer is transferred into the dielectric material layer  30 , and in some cases, the dielectric cap layer  20  by an anisotropic etch to form the interconnect openings. The anisotropic etch can be a dry etch such as reactive ion etch (RIE), or a wet etch. After forming the interconnect openings, the remaining portions of the photoresist layer can be removed, for example, by ashing. 
     Next, a metal liner layer (not shown) is conformally deposited along sidewalls and bottom surfaces of the interconnect openings and over the top surface of the dielectric material layer  30 . The metal liner layer may include Ti, Ta, Ni, Co, Pt, W, Ru, TiN, TaN, WN, WC, an alloy thereof or a stack thereof such as Ti/TiN and Ta/TaN. The metal liner layer may be formed by a conventional deposition process including, for example, CVD, PECVD, ALD, physical vapor deposition (PVD) or chemical solution deposition. The metal liner layer that is formed may have a thickness from 2 nm to 40 nm, although lesser and greater thicknesses can also be employed. 
     After lining the interconnect openings with the metal liner layer, a metal layer (not shown) is deposited over the metal liner layer to completely fill the interconnect openings. The metal layer may include, for example, W, Cu, Al, Co, Ru, Mo, Os, Ir, Rh or an alloy thereof. 
     The metal layer may be formed by any suitable deposition method such as, for example, CVD, PVD or plating. 
     After deposition, portions of the metal layer and the metal liner layer that are located above the top surface of the dielectric material layer  30  are removed by employing a planarization process, such as, for example, chemical mechanical planarization (CMP). Each remaining portion of the metal liner layer located along the sidewalls and a bottom of each interconnect opening constitutes the metal liner  42 , while each remaining portion of the metal layer located within each interconnect opening constitutes the metal portion  44 . The top surfaces of the metal liners  42  and the metal portions are coplanar with the top surface of the dielectric material layer  30 . 
     Referring to  FIG. 2 , a first mask layer portion  50  is formed over the dielectric material layer  30  to cover the chip region  100 , while exposing the crack stop region  200 . The first mask layer portion  50  can be formed by first applying a first mask layer (not shown) as a blanket layer over the entire top surface of the dielectric material layer  30 . The first mask layer can be a photoresist layer or a photoresist layer in conjunction with a hardmask layer(s). The first mask layer is subsequently patterned by an anisotropic etch to provide the first mask layer portion  50  which is a remaining portion of the first mask layer. The anisotropic etch can be a dry etch such as, for example, RIE, or a wet etch that removes material(s) that provide the first mask layer selective to the dielectric material that provides the dielectric material layer  30  and metals that provide the metal liners  42  and the metal portions  44 . 
     Referring to  FIG. 3 , a pore filling material  60  is introduced into the physically exposed portion of the dielectric material layer  30  to fill pores in the exposed portion of the dielectric material layer  30 . A filled dielectric material portion  30 A with enhanced mechanical strength is thus formed in the crack stop region  200  to surround the second interconnect structures  40 B. The portion of the dielectric material layer that is covered by the first mask layer portion  50  in the chip region  100  is herein referred to as the dielectric material portion  30   b . It should be understood that with respect to this “filling” operation, not every pore is necessarily filled, nor are pores that are filled necessarily completely filled. That is, some pores may be completely filled, some pores may be partially filled, and some pores may not be filled at all so long as there is sufficient “filling” of the pores in the dielectric material layer  30  within the crack stop region  200  to provide the mechanical strength needed to prevent crack propagation. In one example, sufficient filling of the pores can be achieved when approximately 75% of the pores are filled with the pore filling material  60 . 
     The pore filling material  60  may include a polymer that can be easily diffuse into pores in the dielectric material layer  30 . In one embodiment, the pore filling material  60  may include a low molecular weight material, e.g., a polymer having a molecular weight between about 100 g/mol and about 5,000 g/mol. In another embodiment, the pore filling material  60  may include a polymer with a broader molecular weight range. For example, a polymer may be used whose molecular weight is in the range of 100 g/mol to 20,000,000 g/mol or more. The polymer may be a linear or branched polymer selected from the group of polyimides, polybenzoxazoles, polybenzimidazoles, poly(aniline), poly(phenylene sulfide), phenol-formaldehyde/cresol-formaldehyde resins, polystyrenes, polylactic acid, polyesters, poly(bisbenzocyclobutene), poly(divinylsiloxanebis-benzocyclobutene), poly(aromatics) such as SiLK™, polyamides, polyamide-imides, polyetherimides, polyphenylquinoxalines, poly(perfluoroethers), soluble, fluorinated polyalkanes, poly(acrylonitrile), polyetherketones, poly(vinylalcohol), poly(styrene-co-styrene sulfonic acid), poly(acrylic acid), poly(hydroxyethylmethacrylate), poly(vinyl imidizolium sulfobetaine), poly(vinyl pyridinium sulfobetaine), poly(amido amines), poly(styrene-butadiene-styrene) block copolymers, poly(butadiene), poly(isoprene), polysilazanes, polyureasilazane, poly(perhydrosilazane), poly(metal oxides) including metal from one or more of groups IV, XIII and XIV of the Periodic Table, silicates, titanates, aluminosilicates, aluminates, and zirconates. The polymer is preferably a linear polymer, a linear di- or tri-block copolymer, or a hyperbranched or a polymeric unimolecular amphiphile. 
     A layer of the pore filling material  60  is first applied over the top surface of the first mask layer portion  50 , the top surface of the exposed portion of the dielectric material layer  30  and the top surfaces of the second interconnect structures  40 B by, for example, spin coating. The pore filling material  60  is then allowed to diffuse into the pores of the exposed portion of the dielectric material layer  30 . Depending on the pore filling material  60  used, the pore filling material  60  may diffuse into the pores at room temperature, or it may be necessary to heat the pore filling material to promote its flow into the pores. For example, the pore filling material  60  (e.g. a polymer) may be heated above its glass transition temperature (T g ) to allow it to flow into the pores. If heating is necessary, the temperature must remain below the decomposition temperature of the pore filling material  60 . In one embodiment, the pore filling material  60  may be heated to a temperature in a range from 150° C. to 400° C. The temperature for the filling may depend, at least in part, on the nature (composition) of the porous material that provides the dielectric material layer  30 . For example, if the surface of the porous material has a good affinity for the pore filling material  60 , the penetration temperature will be lower since less energy is needed to drive the pore filling material  60  into the pores. 
     After diffusion, the excess pore filling material  60  that remains on top of the filled dielectric material portion  30 A may be removed by a suitable method, such as, for example, plasma etch, RIE strip, wet dissolution or gentle polishing. Care should be exercised not to remove the pore filling material  60  from the filled pores in the structure. If not removed during the process employed to remove the excess pore filling material  60 , the first mask layer portion  50  may be removed by oxygen-based or N 2 /H 2 -based plasma etch or it may be retained. 
     Subsequently, upper level metallization layers of a BEOL structure may be constructed by performing the processing steps described above in  FIGS. 1-3 . 
     In the present application, since no pore filling material  60  is introduced into the dielectric material portion  30 B in the chip region  100 , the first interconnect structures  40 A in the chip region  100  remain surrounded by the pristine low-k dielectric material which is desirable for reducing the capacitance of the first interconnect structures  40 A. By “pristine” it is meant, that the low k dielectric material does not contain any pore filing material. In the meanwhile, by selectively filling pores of a portion of the dielectric material layer  30  that is located in the crack stop region  200 , the mechanical strength of the portion of the dielectric material layer  30  in the crack stop region  200  is greatly improved. The crack prorogation can thus be stopped within the crack stop region  200  and prevented from propagating into the chip region  100 . 
     Referring to  FIG. 4 , a second exemplary semiconductor structure according to a second embodiment of the present application can be derived from the first exemplary semiconductor structure of  FIG. 1  by introducing a pore filling material  60  into the entire dielectric material layer  30 . The pore filling material  60  can be introduced into the dielectric material layer  30  by performing the processing steps of  FIG. 3  to provide a filled dielectric material layer  30 F. In the second embodiment, pores of the dielectric material layer  30  in both the chip region  100  and the crack stop region  200  are filled by the pore filling material  60 . The pore filling material  60  may include a decomposable polymer that can be completely decomposed and/or extracted under irradiation, plasma, microwave, increased temperature or a combination thereof. In one embodiment, the pore filling material  60  includes a radiation decomposable polymer that decomposes upon exposure to radiation e.g., ultraviolet (UV), x-ray, electron beam or the like. Exemplary decomposable polymers include, but are not limited to, polyethers such as polypropylene oxide, polyacrylates such as poly(methyl methacrylate), aliphatic polycarbonates such as polypropylene carbonate and polyethylene carbonate, polyesters, polysulfones and polystyrenes such as poly (α-methyl styrene). 
     Referring to  FIG. 5 , a second mask layer portion  70  is formed covering the crack stop region  200 , while exposing the chip region  100 . The second mask layer portion  70  can be formed by performing the processing steps of  FIG. 2 . For example, a second mask layer (not shown) is first formed as a blanket layer over the top surface of the dielectric material layer  30  and the top surfaces of the first and second interconnects  40 A,  40 B. The second mask layer can be a photoresist layer or a photoresist layer in conjunction with a hardmask layer(s). The second mask layer is subsequently patterned by an anisotropic etch to provide the second mask layer portion  70  which is a remaining portion of the second mask layer. The anisotropic etch can be a dry etch such as, for example, RIE or a wet etch that removes materials that provide the second mask layer selective to the dielectric material that provides the dielectric material layer  30 , the metals that provide the metal liners  42  and the metal portions  44  and the pore filling material  60 . 
     Referring to  FIG. 6 , the pore filling material  60  in the chip region  100  that is not covered by the second mask layer portion  70  is removed from pores of the filled dielectric material layer  30 F. In one embodiment, the pore filling material  60  is removed, for example, by exposing the structure to an UV irradiation. The pore filling material  60  in the chip region  100  is decomposed to volatile fragments that diffuse out of the filled dielectric material layer  30 F. The exposed portion of the filled dielectric material layer  30 F in the chip region  100  is thus reverted back to its porous form. After selective removal of the pore filling material  60  from the chip region  100 , the pristine dielectric material portion thus formed in the chip region  100  is herein referred to as a dielectric material portion  30 B. The low-k characteristics of the dielectric material portion  30 B provides desirable capacitance between the first interconnect structures  40 A in the chip region  100 . The remaining portion of the filled dielectric material layer  30 F in the crack stop region  200  is herein referred to as the filled dielectric material portion  30 A. The filled dielectric material portion  30 B is mechanically stronger than the dielectric material portion  30 A, thus is more effective in preventing or reducing the crack propagation. 
     After removing the pore filling material  60  selectively from the chip region  100 , the second mask layer portion  70  may be removed by oxygen-based or N 2 /H 2 -based plasma etch or it may be retained. 
     Subsequently, upper level metallization layers of a BEOL structure may be constructed by performing the processing steps described above in  FIGS. 4-6 . 
     While the present application has been particularly shown and described with respect to various embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present application. It is therefore intended that the present application not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.