Abstract:
A high tensile stress capping layer on Cu interconnects in order to reduce Cu transport and atomic voiding at the Cu/dielectric interface. The high tensile dielectric film is formed by depositing multiple layers of a thin dielectric material, each layer being under approximately 50 angstroms in thickness. Each dielectric layer is plasma treated prior to depositing each succeeding dielectric layer such that the dielectric cap has an internal tensile stress.

Description:
RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. application Ser. No. 11/564,314 filed Nov. 29, 2006. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to semiconductor integrated circuits (IC&#39;s), and more particularly to back-end-of-the-line (BEOL) interconnects structures. 
         [0003]    The continuous scaling to smaller dimensions and the introduction of low-k dielectrics in copper interconnects have caused reliability issues to become a greater concern in addition to increasing process complexity. In semiconductor interconnect structures Electromigration (EM) has been identified as a metal failure mechanism. EM is a serious reliability concern for very large scale integrated (VLSI) circuits. 
         [0004]    Electromigration failure along the Cu/dielectric interface has been identified as a major reliability concern for VLSI circuit applications. It has been observed that voiding in the metal line with the mass transport in the Cu line occurring along the interface of the dielectric, for example Si3N4, SiC, and SiC(N,H), capping layer and Cu. Reducing Cu transport and atomic voiding at the Cu/dielectric interface is a major objective in the semiconductor industry for reliability enhancement. 
         [0005]      FIG. 1  is a cross sectional view of a conventional interconnect structure. A copper interconnect  10  is formed in an inter level dielectric (ILD) material  20 . A compressive capping layer  30  is then formed over the exposed surface of the interconnect structures  10  and ILD  20 . In the conventional interconnect structure shown in  FIG. 1 , a single compressive capping layer  30  is provided with a single CVD deposition step. The compressive capping layer is deposited with a typical thickness of approximately 250 Å. The compressive capping layer in this case results in a tensile stress within the Cu interconnect. The CVD deposition parameters are typically at a pressure of approximately 2.4 Torr, temperature of approximately 400° C., and HF power of approximately 1,170 Watts, LF power of approximately 130 Watts. The gas composition and flow rates are typically SiH 4  at approximately 250 standard cubic centimeters per minute (sccm), N 2  at approximately 19,000 sccm, and NH 3  at approximately 1,300 sccm. 
         [0006]    Reducing Cu transport and atomic voiding at the Cu/dielectric interface is disclosed in the present invention by creating compressive stresses in the Cu interconnect. In contrast to a Cu interconnect in the prior art with tensile stress, a Cu interconnect with high compressive stress results in better resistance to electromigration effects. It is also disclosed in the present disclosure that depositing a tensile film on top of the Cu interconnect and/or depositing a compressive film around the Cu interconnect results in high compressive stress in the Cu interconnect, which is beneficial for improving EM resistance in the Cu interconnect. 
         [0007]    Therefore, an object of the present invention is to provide a high tensile stress capping layer on top of the Cu interconnects in order to reduce Cu transport and atomic voiding at the Cu/dielectric interface. The high tensile dielectric film is formed by depositing multiple layers of a thin dielectric material, each layer being under approximately 50 angstroms in thickness. Each dielectric layer is plasma treated prior to depositing each succeeding dielectric layer such that the dielectric cap has an internal tensile stress. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a semiconductor device comprising; a dielectric layer with conductive interconnects partially embedded therein; and a tensile capping layer in contact with the conductive interconnects. The present invention further comprises a compressive capping layer in contact with the dielectric layer. The semiconductor device may further comprise a diffusion barrier layer partially surrounding the conductive interconnects. The tensile capping layer is preferably comprised of sequentially deposited layers. 
         [0009]    The tensile capping layer is preferably comprised of a material selected from the group consisting of silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), and compounds of silicon such as SiC x N y H z , where x, y and z are variable percentages. The conductive interconnects are preferably comprised of a material selected from the group consisting of aluminum, copper, tungsten, silver, gold, aluminum-copper and nickel. The diffusion barrier layer is preferably comprised of a material selected from the group consisting of Ta, TaN, Ti, TiN, Ru, RuN, RuTa, RuTaN, W, and WN. 
         [0010]    The present invention also provides a method for forming a semiconductor device, comprising the steps of: providing a dielectric layer with conductive interconnects partially embedded therein; depositing a tensile capping layer on the dielectric layer and conductive interconnects; performing a plasma treatment on the tensile capping layer; removing the portion of the tensile capping layer on the dielectric layer; depositing a compressive capping layer on the tensile capping layer and the dielectric layer; and removing the compressive capping layer from the tensile capping layer. 
         [0011]    The tensile capping layer is preferably deposited by chemical vapor deposition at a pressure of approximately 7 Torr, a temperature of approximately 400° C., and a high frequency power of approximately 600 Watts and low frequency power of approximately 0 Watts. The chemical vapor deposition comprises a gas composition and flow rate of SiH 4  at approximately 150 sccm, N 2  at approximately 8,000 sccm, and NH 3  at approximately 2,500 sccm. 
         [0012]    The tensile capping layer is preferably a dielectric material selected from the group consisting of silicon dioxide, silicon nitride and a compound of silicon, carbon, nitrogen and hydrogen in the form of SiC x N y H z  where x, y and z are variable percentages. 
         [0013]    The plasma treatment is preferably at a pressure of approximately 5 Torr, a temperature of approximately 400° C., and a high frequency power of approximately 500 Watts and low frequency power of approximately 0 Watts. The plasma treatment comprises a gas composition and flow rate of N 2  at approximately 8,000 sccm for approximately 40 seconds. The tensile capping layer and the plasma treatment are preferably performed in situ. 
         [0014]    The compressive capping layer is preferably removed from the tensile capping layer by chemical mechanical polishing. The tensile capping layer is preferably a multiple deposition capping layer deposited on the dielectric layer and the conductive interconnects by a least two sequential deposition and plasma treatment steps. 
         [0015]    The present invention also provides a method for forming a semiconductor device, comprising the steps of: providing a dielectric layer with conductive interconnects partially embedded therein; depositing a compressive capping layer on the dielectric layer and the conductive interconnects; removing the portion of the compressive capping layer on the conductive interconnects; depositing a tensile capping layer on the compressive capping layer and the conductive interconnects; and removing the tensile capping layer from the compressive capping layer. 
         [0016]    The tensile capping layer is preferably deposited by chemical vapor deposition at a pressure of approximately 7 Torr, a temperature of approximately 400° C., and a high frequency power of approximately 600 Watts and low frequency power of approximately 0 Watts. The chemical vapor deposition comprises a gas composition and flow rate of SiH 4  at approximately 150 sccm, N 2  at approximately 8,000 sccm, and NH 3  at approximately 2,500 sccm. 
         [0017]    The tensile capping layer is preferably a dielectric material selected from the group consisting of silicon dioxide, silicon nitride and a compound of silicon, carbon, nitrogen and hydrogen in the form of SiC x N y H z  where x, y and z are variable percentages. The plasma treatment is preferably at a pressure of approximately 5 Torr, a temperature of approximately 400° C., and a high frequency power of approximately 500 Watts and low frequency power of approximately 0 Watts. The plasma treatment comprises a gas composition and flow rate of N 2  at approximately 8,000 sccm for approximately 40 seconds. The tensile capping layer and the plasma treatment are preferably performed in situ. 
         [0018]    The compressive capping layer is preferably removed from the tensile capping layer by chemical mechanical polishing. The tensile capping layer is preferably a multiple deposition capping layer deposited on the dielectric layer and the conductive interconnects by a least two sequential deposition and plasma treatment steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]    The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
           [0020]      FIG. 1  is a schematic cross-sectional view illustrating a conventional interconnect structure. 
           [0021]      FIG. 2  is a schematic cross-sectional view illustrating a preferred embodiment of the present invention. 
           [0022]      FIGS. 3 through 12  are schematic cross-sectional views illustrating preferred structures according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    The present invention is now described in greater detail by referring to the following discussion and drawings that accompany the present application. The drawings of the present application, which are referred to in greater detail herein below, are provided for illustrative purposes and, as such, they are not drawn to scale. 
         [0024]    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 a thorough understanding of the present invention. However, it will be appreciated by one of ordinary skill in the art that the invention 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 invention. 
         [0025]    It will be understood that when an element as a layer, region or substrate is referred to as being “on” or “over” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “beneath” or “under” another element, it can be directly beneath or under the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly beneath” or “directly under” another element, there are no intervening elements present. 
         [0026]    A preferred embodiment of the present invention is illustrated in  FIG. 2 . A multiple deposition tensile capping layer  40  is formed only on the copper interconnect  10 . A single deposition compressive capping layer  41  is formed only on the ILD  20  surface. In the present invention specially tailored CVD deposition parameters and used in conjunction with a post deposition surface treatment in between capping layer depositions to provide a tensile stress capping layer on the interconnects. In a preferred embodiment a single deposition will deposit less than approximately 50 Å. In a preferred embodiment the multiple layer tensile capping layer  40  is comprised of at least 3 sequentially deposited and treated layers. 
         [0027]    Referring to  FIG. 3  a preferred embodiment of the present invention is described in more detail.  FIG. 3  is a cross sectional view of a portion of a microelectronic element or chip in which a plurality of metal interconnects are provided by a conductive interconnect material  101  formed in patterned openings in accordance with an embodiment of the invention.  FIG. 3  illustrates a stage in fabrication in which an interlevel dielectric layer or “ILD”  102  has been formed. 
         [0028]    The ILD  102  material is preferably a low-k material such as SiCOH or SiLK. The ILD  102  material typically has a dielectric constant that is approximately 4.0 or less, with a dielectric constant of approximately 2.8 or less being even more typical. All dielectric constants mentioned herein are relative to a vacuum, unless otherwise noted. These dielectrics generally have a lower parasitic cross talk as compared with dielectric materials that have a higher dielectric constant than 4.0. 
         [0029]    Within the ILD  102 , a plurality of patterned openings are provided, typically in form of line patterns which extend horizontally in a direction of a major surface  105  which is defined by the upper surface of the dielectric layer  102 , or which extend in a direction parallel to such surface. Within each patterned opening, a diffusion barrier material  103  is deposited to line the walls and bottom of each opening, after which a conductive interconnect material  101  is deposited to fill the remaining space within each opening. These steps are followed typically by a planarization process, such as a chemical mechanical polishing (“CMP”) process, which removes any excess metal that contacts the exposed upper surface  105  of the ILD  102  outside of the patterned openings. 
         [0030]    The conductive interconnect material  101  that fills each opening in the dielectric layer  102  is preferably a metal, for example, a noble metal, that is not prone to destructive corrosion and has good conductive properties. However, a few metals and alloys of metals stand out as being especially suited for forming conductive interconnect lines in the BEOL fabrication of integrated circuits or chips. Such metals include aluminum, copper, tungsten, silver, gold, aluminum-copper and nickel. In a particular embodiment, the conductive interconnect material  101  which fills the patterned opening in the dielectric layer  102  consists essentially of copper. 
         [0031]    When the conductive interconnect material  101  includes copper or other metal prone to diffusing through dielectric materials, i.e., when the metal has a “high coefficient of diffusion”, the conductive interconnect material  101  is preferably formed over a layer of metal or compound of a metal which functions as a diffusion barrier. The diffusion barrier material  103  prevents the copper from diffusing from the conductive interconnect material  101  into the ILD  102  adjacent to the walls and bottom of the metal line. 
         [0032]    The diffusion barrier material  103  is preferably Ta, TaN, Ti, TiN, Ru, RuN, RuTa, RuTaN, W, WN or any other material that can serve as a barrier to prevent conductive material from diffusing there through. The diffusion barrier material  103  does not interact with the conductive interconnect material  101 , such that the barrier layer  103  does not affect the conductive properties of the conductive interconnect material  101  or interact with the dielectric material of the ILD  102  to affect its dielectric properties. Deposition is formed by a deposition process such as, for example, atomic layer deposition (ALD), chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering, chemical solution deposition, or plating. 
         [0033]    The substrate, which is not shown, may comprise a semiconducting material, an insulating material, a conductive material or any combination thereof. When the substrate is comprised of a semiconducting material, any semiconductor such as Si, SiGe, SiGeC, SiC, Ge alloys, GaAs, InAs, InP and other III/V or II/VI compound semiconductors may be used. In additional to these listed types of semiconducting materials, the present invention also contemplates cases in which the semiconductor substrate is a layered semiconductor such as, for example, Si/SiGe, Si/SiC, silicon-on-insulators (SOIs) or silicon germanium-on-insulators (SGOIs). 
         [0034]    When the substrate is an insulating material, the insulating material can be an organic insulator, an inorganic insulator or a combinations including multilayers thereof. When the substrate is a conducting material, the substrate may include, for example, polySi, an elemental metal, alloys of elemental metals, a metal silicide, a metal nitride or combinations including multilayers thereof. When the substrate comprises a semiconducting material, one or more semiconductor devices such as, for example, complementary metal oxide semiconductor (CMOS) devices can be fabricated thereon. When the substrate comprises a combination of an insulating material and a conductive material, the substrate may represent a lower interconnect level of a multilayered interconnect structure. 
         [0035]    Referring now to  FIG. 4  there is illustrated the deposition of a first dielectric capping layer  215 . In a preferred embodiment the CVD deposition parameters are at a pressure of approximately 7 Torr, temperature of approximately 400° C., and HF power of approximately 600 Watts, LF power of approximately 0 Watts. The gas composition and flow rates are SiH 4  at approximately 150 sccm, N 2  at approximately 8,000 sccm, and NH 3  at approximately 2,500 sccm. 
         [0036]    The first dielectric capping layer  215  can include any dielectric material or combination of dielectric materials which is capable of maintaining a stress relative to the metal filled line patterns. Preferably, the first dielectric capping layer  215  consists essentially of one or more dielectric materials or combinations of dielectric materials selected from the group consisting of silicon dioxide (SiO 2 ), silicon nitride (Si 3 N 4 ), and or other dielectric compound of silicon such as SiC x N y H z , where x, y and z are variable percentages. 
         [0037]    After deposition, the first dielectric capping layer  215  is subjected to an extended time, high temperature and high frequency power plasma treatment, preferably using a combination of ammonia and nitrogen species (NH 3  and N 2 ) or, alternatively, hydrogen (H 2 ). In a preferred embodiment the plasma treatment parameters are at a pressure of approximately 5 Torr, temperature of approximately 400° C., and HF power of approximately 500 Watts, LF power of approximately 0 Watts. The gas composition and flow rates are N 2  at approximately 8,000 sccm. The plasma treatment is for approximately 40 seconds. 
         [0038]    The dielectric cap deposition and subsequent plasma treatment are preferably performed in situ, i.e., in the same process chamber or in the same tool having one or more connected chambers. In that way, it is performed without having to manually remove the substrate from the chamber to deposit the dielectric cap after plasma treating the surface of the ILD. In a preferred embodiment, a first dielectric cap layer  215  is deposited to cover the exposed upper surface  105  of the ILD  102  and the conductive interconnect material  101  to a thickness of less than approximately 50 angstroms. After deposition, the exposed upper surface  225  of the first dielectric cap layer  215  is subjected to a second plasma treatment. The second plasma treatment has the effect of imparting a tensile stress to the first dielectric cap layer  215 . 
         [0039]    Referring now to  FIG. 5(   a ) and  FIG. 5(   b ), a second dielectric capping layer  226  is deposited to cover the exposed upper surface  225  of the first dielectric capping layer  215 . Like the first dielectric capping layer  215 , the second dielectric capping layer  226  preferably is formed to a thickness of less than about 50 angstroms. Like the first dielectric capping layer  215 , the second dielectric capping layer  226  can include any dielectric material or combination of dielectric materials which is capable of maintaining a stress. Preferably, the second dielectric capping layer  226  consists essentially of one or more dielectric materials or combinations of materials selected from the group consisting of silicon dioxide, silicon nitride and a compound of silicon, carbon, nitrogen and hydrogen in the form of SiC x N y H z  where x, y and z are variable percentages. 
         [0040]    Preferably, the second dielectric capping layer  226  consists essentially of the same dielectric material as the first dielectric cap layer. Like the first dielectric capping layer  215 , the exposed upper surface  230  of the second dielectric capping layer  226  also undergoes plasma treatment after deposition to control the tensile stress of the deposited second dielectric capping layer  226 , preferably using the same parameters and species that are used during the plasma treatment of the first dielectric capping layer  215 . For example, the plasma treatment can include a mixture of ammonia and nitrogen, or alternatively, hydrogen as the reactive species. As in the case of the first dielectric capping layer  215 , these deposition and plasma treatment processes are preferably performed in situ. 
         [0041]    Further regarding  FIG. 5(   a ) and  FIG. 5(   b ), following deposition and plasma treatment of the second dielectric capping layer  226 , a third dielectric capping layer  236  is deposited to cover the exposed upper surface  230  of the second dielectric capping layer  226 . Like the first dielectric capping layer  215  and second dielectric capping layer  226 , the third dielectric capping layer  236  is formed to a thickness of less than about 50 angstroms. Like the first and second dielectric capping layers  215 ,  226 , the third dielectric capping layer  236  can include any dielectric material or combination of dielectric materials which is capable of maintaining a stress. Preferably, the third dielectric capping layer  236  consists essentially of one or more dielectric materials or combinations of dielectric materials selected from the group consisting of silicon dioxide, silicon nitride and a compound of silicon, carbon, nitrogen and hydrogen in the form of SiC x N y H z . 
         [0042]    Preferably, the third dielectric capping layer  236  consists essentially of the same dielectric material as the first and second dielectric capping layers. As in the plasma treatments performed to the first and second dielectric capping layers, the exposed upper surface  240  of the third dielectric capping layer  236  also undergoes plasma treatment after deposition to control or produce a tensile stress in the third dielectric capping layer  236 , preferably using the same parameters and species that are used during the plasma treatment of the first and second dielectric capping layers. For example, the plasma treatment can include a mixture of ammonia and nitrogen, or alternatively, hydrogen as the reactive species. As in the case of the first and second dielectric capping layers, these deposition and plasma treatment processes are preferably performed in situ. 
         [0043]    The purpose of successively depositing the dielectric capping layers and the plasma treatments is to achieve a tensile dielectric capping layer  200  which has an internal tensile stress. The internally tensile stressed dielectric capping layer  200  will exert a compressive stress at the surface of the conductive interconnect material  101  which it contacts. As a result, the conductive interconnect material  101 , e.g., copper, is less likely to exert a tensile stress on the metal features it contacts, which could then result in the problems described above. 
         [0044]    The number of dielectric capping layers that are used and other parameters associated with their fabrication are not of primary importance. Rather, the magnitude and stability of the internal tensile stress of the dielectric capping layer are of greater importance. Therefore, if a single layer of the dielectric capping material is deposited to have the required internal tensile stress of the desired magnitude and properties, then a single such dielectric capping layer suffices. Alternatively, if more than three such dielectric capping layers are needed to achieve these properties, then more than three such dielectric capping layers need to be deposited and plasma treated in accordance with the method described above. In the preferred embodiment illustrated in  FIG. 5(   a ) and  FIG. 5(   b ), the tensile layer  200  is comprised of three sequentially deposited layers and accompanying plasma treatments. 
         [0045]    Referring now to  FIG. 6  there is illustrated the removal of the portion of the tensile capping layer  200  on the ILD  102  surface by conventional lithography and etching processes. The lithographic process includes applying a photoresist, exposing the photoresist to a desired pattern of radiation, and developing the exposed resist utilizing a conventional resist developer. The etching step may comprise a dry etching process, a wet chemical etching process or a combination thereof. The term “dry etching” is used herein to denote an etching technique such as reactive-ion etching, ion beam etching, plasma etching or laser ablation. 
         [0046]    Referring now to  FIG. 7  there is illustrated the deposition of a single compressive dielectric layer  416  by conventional processes. The blanket deposition will deposit the compressive dielectric layer  416  over the tensile dielectric capping layer  200  and re-exposed upper surface  105  of the ILD  102 . 
         [0047]    Referring now to  FIG. 8  the structure is illustrated after CMP. Removing the compressive layer material deposited on the tensile layer  200  results in a multiple deposition tensile capping layer  200  formed only on the conductive interconnect material  101 . A single deposition compressive dielectric layer  416  is formed only on the ILD  102  surface. In this preferred embodiment the multiple deposition tensile capping layer  200  is comprised of three sequentially deposited and treated layers. 
         [0048]    Referring now to  FIG. 9  there is illustrated another embodiment of the present invention. As described above a conductive interconnect material  101  is formed in an inter level dielectric (ILD) material  102 . There will typically also be a diffusion barrier material  103  between the conductive interconnect material  101  and ILD  102 . There is now deposited a single compressive dielectric layer  416 . 
         [0049]    Referring now to  FIG. 10  there is illustrated the removal of the portion of the single compressive dielectric layer  416  on the conductive interconnect material  101  surface by conventional lithography and etching processes. 
         [0050]    Referring now to  FIG. 11  there is illustrated the deposition of a tensile dielectric capping layer  200 . The blanket deposition will deposit the tensile dielectric layer  200  over the compressive dielectric layer  416  and the conductive interconnect material  101 . In this preferred embodiment the tensile layer  200  is comprised of multiple sequentially deposited layers and accompanying plasma treatments as described in the prior embodiment and illustrated in  FIGS. 4 and 5 . 
         [0051]    Referring now to  FIG. 12  the structure is illustrated after CMP. Removing the tensile layer material deposited on the compressive layer  416  results in a multiple deposition tensile capping layer  200  formed only on the conductive interconnect material  101 . A single deposition compressive dielectric layer  416  is formed only on the ILD  102  surface. 
         [0052]    It will be apparent to those skilled in the art having regard to this disclosure that other modifications of this invention beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.