Abstract:
A method for fabricating and back-end-of-line (BEOL) metalization structures includes simultaneous high-k and low-k dielectric regions. An interconnect structure includes a first inter-level dielectric (ILD) layer and a second ILD layer with the first ILD layer underlying the second ILD layer. A plurality of columnar air gaps is formed in the first ILD. The columnar air gap structure is created using a two-phase photoresist material for providing different etching selectivity during subsequent processing.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to the field of manufacturing semiconductor devices, and more particularly, relates to a method of fabricating and back-end-of-line (BEOL) metalization structures for semiconductor devices including selective formation of simultaneous high dielectric constant (high-k) and low-k dielectric regions. 
   DESCRIPTION OF THE RELATED ART 
   In order to reduce the BEOL interconnect circuit delays resulting from parasitic capacitance between interconnect lines, conventional silicon dioxide dielectric, having a dielectric constant value k of approximately 4.0 forming the inter-level dielectric (ILD) and gap fill has been replaced with dense lower-k films, having dielectric constant values k of approximately 3.0. For further performance improvements for advanced devices, additional dielectric capacitance reduction is required, for example a dielectric constant value k of less than 2.5. 
   On the other hand, for applications requiring high capacitance, for example charge storage, and decoupling, high-k material with a dielectric constant value k of greater than 7 is preferred. 
   From a high-volume manufacturability and low cost point of view, a need exists for a single BEOL integration scheme for fabricating both low-k and high-k contained interconnects. 
   Capacitance reduction can be achieved with new porous low-k dielectrics; however, most of the porous materials have relatively weak mechanical properties as compared to dense dielectrics. It is also a significant challenge for the current BEOL process to integrate these materials with other module processes. 
   For example, conventional polishing processes, such as a chemical mechanical polishing (CMP) process conventionally used in a damascene metalization process have difficulty polishing low mechanical strength-porous dielectric. Also, conventional physical vapor deposition (PVD) diffusion barrier deposition technology does not offer reasonable coverage on surfaces of porous dielectrics. 
   Various techniques have been proposed to form air gaps with a plasma assisted etching process, for example with a reactive ion etching (RIE) process. 
   A need exists for an effective mechanism for implementing BEOL metalization structures for semiconductor devices including simultaneous high dielectric constant (high-k) and low-k dielectric materials. 
   SUMMARY OF THE INVENTION 
   Principal aspects of the present invention are to provide a method and back-end-of-line (BEOL) metalization structures for semiconductor devices including simultaneous high-k and low-k dielectric materials. Other important aspects of the present invention are to provide such method and back-end-of-line (BEOL) metalization structures for semiconductor devices including simultaneous high-k and low-k dielectric materials substantially without negative effect and that overcome many of the disadvantages of prior art arrangements. 
   In brief, a method for fabricating and back-end-of-line (BEOL) metalization structures are provided for semiconductor devices including simultaneous high-k and low-k dielectric materials. An interconnect structure includes a first inter-level dielectric (ILD) layer and a second ILD layer with the first ILD layer underlying the second ILD layer. A plurality of columnar air gaps is formed in the first ILD. 
   In accordance with features of the invention, the columnar air gap structure is created using a two-phase photoresist material for providing different etching selectivity during subsequent processing, such as subsequent reactive ion etching (RIE) processing. To enhance the etching selectivity one phase of the two-phase photoresist material optionally is removed before RIE processing. The two-phase photoresist material includes, for example, two different polymers, or a combination of a polymer and silicon oxide. 
   In accordance with features of the invention, selective cap formation is used to create local topography, and create two different surfaces including a metallic cap surface and dielectric hard mask (HM) surface. A material such as a dielectric hard mask (HM) layer extends over the first ILD layer and an exposed surface of an interconnect conductor is selectively capped with a metallic layer. The two-phase photoresist material is deposited over a surface of the dielectric HM layer and metallic cap. A two-phase separated photoresist material pattern is transferred to the first ILD layer to create the columnar air gap structure. Then a second insulator is deposited on the patterned wafer surface. 
   In accordance with features of the invention, the deposited second insulator material optionally fills the columnar air gap structure in the first ILD layer for applications requiring high capacitance. For other high-speed applications, the deposited second insulator material does not fill the columnar air gap structure, leaving air gaps in the ILD layer in the final interconnect structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the preferred embodiments of the invention illustrated in the drawings, wherein: 
       FIGS. 1 ,  2 ,  3 A,  3 B,  4 , and  5  illustrate exemplary process steps for fabricating interconnect structures in accordance with the preferred embodiments; 
       FIGS. 6 and 7  are schematic side views not to scale illustrating alternative exemplary interconnect structures in accordance with the preferred embodiments; 
       FIGS. 8A and 8B  illustrate exemplary phase separation features in accordance with the preferred embodiments; and 
       FIGS. 9A ,  9 B, and  9 C and  10 A,  10 B, and  10 C illustrate exemplary interconnect structures fabricated in accordance with the preferred embodiments. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In accordance with features of the preferred embodiments, a capacitance modification method is provided for modern semiconductor devices in both high-k and low-k interconnect applications. The capacitance modification method is quickly and easily integrated with present BEOL processes. Neither exotic nor new materials are required. Either an air gap structure or a high-k material can be embedded inside an original inter-level dielectric material. The method is compatible with the current BEOL process flow, and does not require new module development for optimizing etch profiles, improving barrier coverage, or handling CMP processes. 
   In accordance with features of the preferred embodiments, an interconnect structure containing air gaps inside a dielectric material is provided for overall BEOL capacitance reduction. Also an interconnect structure containing a high-k material embedded in the original dielectric is provided for overall BEOL capacitance increase. Methods of forming the low-k BEOL interconnect structure are provided. 
   Having reference now to the drawings, in  FIGS. 1 ,  2 ,  3 A,  4 ,  5 ,  6 , and  7 , there are shown exemplary process steps for fabricating interconnect structures in accordance with the preferred embodiments. 
   In  FIG. 1 , a first processing step generally designated by the reference character  100  begins with selective cap formation in accordance with the preferred embodiment. In the first processing step  100 , selective cap formation is used to create local topography, and create two different surfaces including a metallic cap surface and dielectric hard mask (HM) surface. 
   As shown an initial structure for the first processing step  100  includes a substrate layer  102  underlying a first inter-level dielectric (ILD) layer  104  and a plurality of conductors  108 . An interface material  110  or dielectric hard mask (HM) layer  110  is provided over the exposed first ILD layer  104 . An exposed surface of each interconnect conductor  108  extending through the dielectric HM layer  110  is selectively capped with a metallic layer or metal cap  112 . 
   The first ILD layer  104  is a low-k material and has preferably a thickness between 50 nm and 1000 nm. For example, the first ILD layer  104  is a material selected from the group consisting of silicon containing materials such as SiOF (FSG), SiCOH, HSQ (hydrogen silsesquioxane polymer), and MSQ (methyl silsesquioxane polymer), or organics such as parylene, BCB, polyphenylene oligomer, fluorocarbons, and combinations thereof. 
   The conductors  108  are formed of any suitable electrically conductive material, for example, of Cu, Al, Al (Cu), and W. The HM layer  110  provides, for example, a hydrophilic surface for subsequent local selective phase separation. The thickness of the HM layer  110  is between 2 nm and 80 nm. The HM layer  110  can be conductor, insulator, or semiconductor. 
   For example, the exposed surface of the copper conductors  108  is selectively capped with the cap layer  112  formed of COWP, which serves both as a passivation and Cu diffusion barrier layer. Preferably, the thickness of the CoWP layer  112  is between 5 nm and 30 nm. In addition to CoWP, other materials such as CoSnP, CoP, CoB, CoSnB, COWB, Pd, and Ru are also good candidates to form the cap layer  112 . It is preferred that the material forming the cap layer  112  has a hydrophobic surface for later random phase formation with the hydrophilic surface provided by the dielectric HM layer  110 . 
   Referring to  FIG. 2 , in a next processing step generally designated by the reference character  200  a two-phase photoresist  202  is deposited on the wafer surface. The photoresist layer  202  has preferably a thickness between 20 nm and 500 nm. The two-phase photoresist  202  can be formed, for example, of two different polymers, or by a combination of a polymer and silicon oxide. 
   Referring to  FIGS. 3A and 3B , in a next processing step generally designated by the reference character  300 , phase separation of the two-phase photoresist material  202  is performed. The phase separation of the two-phase photoresist material  202  only occurs locally in a region generally designated by the reference character  302  on top of the dielectric HM layer  110  into a phase A,  304  and a phase B,  306 .  FIG. 3B  is a fragmentary detail view illustrating resulting phases A, B of the phase-separated region  302  from the phase separation step  300 . 
   Two different phase materials have different etching selectivity, for example phase A  304  has higher etching-resistance than phase B,  306 . The phase separation only happens locally on top of dielectric HM layer  110 . The topography features on the existing wafer or different interface properties, between the two-phase photoresist  202  and the HM layer  110  and between the two-phase photoresist  202  and the metallic cap layer  112 , advantageously initiate this local phase separation. 
   For enhanced subsequent etching selectivity, phase B,  306  optionally may be removed from the wafer surface before the next process. The removal step can be achieved by wet, plasma, or other chemical related processes. 
   Referring to  FIG. 4 , a next processing step generally designated by the reference character  400  provides a pattern transfer from the photoresist phase-separated regions  302  into the underlying first ILD layer  104  through a RIE process. A resulting columnar air gap structure  402  is shown. 
   It should be understood that the above resist-deposition, phase-separation, and RIE processes optionally may be repeated in different orientations for creating a structure with a net of air gaps in order to further reduce the final dielectric capacitance. 
   Referring to  FIG. 5 , in a next processing step generally designated by the reference character  500  a fill insulator material  502  is then deposited on the wafer surface for further processing. The deposition technique can be chemical vapor deposition (CVD), atomic level deposition (ALD), or spin-on related processes. Preferably, the second insulator  502  is a high-k material. The fill insulator material  502  is formed for example, of a high-k material, metal oxide, Ta2O5, BaTiO3, HfO2, ZrO2, Al2O3, metal silicates, HfSixOy, HfSixOyNz and combinations thereof. The fill insulator material  502  is formed for example, of a material selected from the group consisting of silicon dioxide, silicon nitride, silicon carbide, silicon oxy nitride, silicon oxy carbide, hydrogen doped silica glass, and plasma-enhanced silicon nitride. As shown in  FIG. 5 , the deposited fill insulator material  502  fills the air gaps inside previously formed columnar air gap structure  402  containing ILD  104 . 
   Preferably a blocking layer is deposited and patterned prior to the deposition of the fill insulator material  502 . The purpose of the blocking layer is to selectively protect those air gap regions  402  intended to remain unfilled, while providing openings which allow fill insulating material  502  to fill the air gaps  402 , as described above. The blocking layer material may comprise silicon nitride or other suitable material, which is deposited such that the openings to the columnar air gaps are sealed. Following filling of the exposed air gaps  402  with second insulator material  502 , as described above, the blocking layer may be removed by selective etching. Optionally, the removal of the blocking layer may be masked to provide regions where the blocking layer remains. Thus at this stage of processing, selective regions of ILD  104  have been converted to high-K dielectric, while other portions of the surface contain open air gaps  402 . 
     FIG. 6  illustrates a next processing step generally designated by the reference character  600  following the deposition of the fill insulator material  502  and removal of the high-k insulator material  502  from the wafer surface. In step  600 , a second insulator material  602  is then deposited on the wafer surface for further processing. As shown in  FIG. 6 , an interconnect conductor  604  is embedded in the second insulator material  602 . The deposition technique can be CVD, ALD, or spin-on related processes. Preferably, the second insulator  602  is a low-k material forming a second ILD layer. The second ILD layer  602  is formed for example, of a low-k material, silicon containing materials such as SiOF (FSG), SiCOH, HSQ (hydrogen silsesquioxane polymer), and MSQ (methyl silsesquioxane polymer), or organics such as parylene, BCB, polyphenylene oligomer, fluorocarbons, and combinations thereof.  FIG. 6  illustrates a preferred final structure for capacitor or high-capacitance required devices. 
   Referring to  FIG. 7 , in an alternative next processing step generally designated by the reference character  700  following step  400  in  FIG. 4 . In processing step  700 , a second insulator material  702  is then deposited on the wafer surface for further processing. As shown in  FIG. 7 , an interconnect conductor  704  is embedded in the second insulator material  702 .  FIG. 7  illustrates a preferred final structure for high-speed device applications. The deposition technique can be CVD, ALD, or spin-on related processes. Preferably, the second insulator  702  is a low-k material forming a second ILD layer. The second ILD layer  702  is formed for example, of a low-k material, silicon containing materials such as SiOF (FSG), SiCOH, HSQ (hydrogen silsesquioxane polymer), and MSQ (methyl silsesquioxane polymer), or organics such as parylene, BCB, polyphenylene oligomer, fluorocarbons, and combinations thereof. As shown in  FIG. 7 , the deposited second insulator  702  does not fill the air gaps inside the columnar air gap structure  402 , and leaves air gaps in the final structure and the second insulator material  702  seals the openings to air gaps  402 . 
   In accordance with features of the preferred embodiments, an advantage of the present invention as compared to the prior art is that the air gap structure is formed compatibly with current Cu dual damascene processing, without the need for additional critical masking. When both low-k and high-k regions are to be formed in the same BEOL, only a non-critical block mask is needed. Prior art techniques require additional critical masks. The process cost imposed by the present invention is lower than prior art techniques. 
   Referring also to  FIGS. 8A and 8B , there is shown an exemplary structure generally designated by the reference character  800  illustrating other exemplary phase separation features  802  fabricated in accordance with the preferred embodiments. 
   Referring now to  FIGS. 9A ,  9 B, and  9 C and  10 A,  10 B, and  10 C respectively illustrate an exemplary interconnect structure generally designated by the reference character  900  and  1000  fabricated in accordance with the preferred embodiments. 
   While the present invention has been described with reference to the details of the embodiments of the invention shown in the drawing, these details are not intended to limit the scope of the invention as claimed in the appended claims.