Abstract:
A semiconductor structure includes a substrate, a dielectric layer disposed on the substrate, a layer of undoped silicate glass disposed on the dielectric layer, a layer of borophosphorous silicate glass on the layer of undoped silicate glass, and a planar dielectric layer disposed on the layer of borophosphorous silicate glass, the layers of undoped silicate glass, borophosphorous silicate glass, and planar dielectric together forming a pre-metal dielectric stack. The planar dielectric may include plasma-enhanced tetraethyl orthosilicate.

Description:
TECHNICAL FIELD 
   The invention relates generally to a semiconductor structure that includes an improved pre-metal dielectric (PMD) stack, and more specifically to a structure that includes an undoped silicate glass/borophosphorous silicate glass/plasma-enhanced tetraethyl orthosilicate (USG/BPSG/PETEOS) PMD stack and a method for forming the same. 
   BACKGROUND OF THE INVENTION 
   Semiconductor processes for manufacturing integrated circuits often require forming a protective layer, or layers, to, e.g., reduce contamination by mobile ions, prevent unwanted dopant diffusion between different layers, and isolate elements of an integrated circuit. Typically, such a protective layer is formed with silicon-based dielectrics, such as silicon dioxide, which may take the form of undoped silicate glass (USG), borosilicate glass (BSG), or borophosphorous silicate glass (BPSG). BSG is typically formed by doping USG with Boron. Likewise, BPSG is typically formed by doping USG with both Boron and Phosphorous. If these dielectrics are disposed beneath the first metal layer of the integrated circuit, they are often referred to as pre-metal dielectrics (PMD). 
   A conventional PMD stack often includes a USG layer disposed on a semiconductor substrate, and a BPSG layer disposed on the USG layer. Typically, a BPSG film layer has a number of advantages over a USG layer. For example, a BPSG layer is often a better moisture barrier than a USG layer. Also, the phosphorous ions in a BPSG layer trap mobile sodium (Na) or other ions. This phenomenon is called “gettering,” and it is sometimes used to reduce or eliminate an unwanted shift in a transistor&#39;s threshold voltage V T  caused by mobile ions trapped in the gate oxide of the transistor. Additionally, the Boron ions decrease the viscous flow temperature of the BPSG layer. This facilitates reflow of the BPSG layer, and thus improves the reflow planarization of the BPSG layer surface and allows for the filling of gaps with fewer voids. A downside of BPSG is that Boron and Phosphorous ions may diffuse from the BPSG into an underlying silicon region such as a silicon substrate. Such diffusion may cause undesirable changes in the doping profile of the silicon region. Therefore, to prevent such diffusion, the PMD stack includes a USG layer beneath the BPSG layer. The USG layer acts as a barrier to such diffusion. 
     FIG. 1  is a cross-section of a portion of a conventional semiconductor structure  1 . The structure  1  has a patterned field-oxide layer  2  that defines active regions  8  in a semiconductor substrate  10 . In one embodiment, the field oxide  2  has a thickness between approximately 3 k to 6 k angstroms. A gate structure  12  is disposed over the active region  8 , and a layer  14  is disposed over a segment of the field oxide  2 . A USG layer  4  having a thickness between approximately 1 k-2 k angstroms is conventionally formed over the surface of the structure  1 . A relatively thick BPSG layer  6  having a thickness between approximately 12 k-25 k angstroms is subsequently formed over the USG layer  4 . In one embodiment, the thickness of the BPSG layer  6  is at about 18 k angstroms. 
   Referring to  FIG. 2 , after the BPSG layer  6  has been formed, the structure  1  is planarized using a conventional chemical mechanical polishing (CMP) process. The CMP process reduces the combined thickness of all layers above the substrate  10 , i.e., from the bottom of the field oxide  2  all the way to the top of the BPSG layer  6 , to a value yk. In one embodiment, yk is approximately 12 k angstroms. The layer  4  and the polished layer  6  form a PMD stack  17 . 
   Referring to  FIG. 3 , an optional plasma-enhanced tetraethyl orthosilicate (PE-TEOS) redeposition dielectric layer  16  having a thickness between approximately 2 k-4 k angstroms may be conventionally formed over the polished BPSG layer  6 . Therefore, when present, the layer  16  composes part of the PMD stack  17 . 
   As the semiconductor industry increasingly relies on the CMP technique for planarization in today&#39;s high-density integrated circuits, drawbacks of the USG/BPSG PMD stack in connection with the CMP technique become more prominent. First, Boron and Phosphorous dopants in the BPSG layer  6  often are not uniformly distributed across and against the depth of the semiconductor wafer. This non-uniform distribution of the boron and/or phosphorous dopants often causes uneven thickness of the BPSG layer  6  even after CMP. Second, forming the BPSG layer  6 , which is typically accomplished by a atmospheric pressure chemical vapor deposition (APCVD) process, is often a low-throughput and high maintenance process. As a result, using a relatively thick sacrificial BPSG layer  6  for the PMD  17  is often burdensome for manufacturing today&#39;s high-density integrated circuits. Additionally, the BPSG layer  6  often is too soft for a CMP process to adequately smoothen or planarize the surface of the layer  6 . 
   SUMMARY OF THE INVENTION 
   In one embodiment of the invention, a semiconductor structure includes a substrate, a dielectric layer disposed over the substrate, an undoped silicate glass layer disposed over the insulator layer, a borophosphorous silicate glass layer disposed over the undoped silicate glass layer, and a planar dielectric layer disposed over the borophosphorous silicate glass, the undoped silicate glass layer, the borophosphorous silicate glass layer, and the planar dielectric layer together forming a pre-metal dielectric stack layer. In one embodiment, the planar dielectric layer includes plasma-enhanced tetraethyl orthosilicate. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a cross-section of a conventional semiconductor structure. 
       FIG. 2  shows the semiconductor structure of  FIG. 1  after it has been subjected to a CMP process. 
       FIG. 3  shows the semiconductor structure of  FIG. 2  after the formation of an optional PE-TEOS redeposition layer. 
       FIG. 4  is a cross-section of a semiconductor structure formed according to an embodiment of the invention. 
       FIG. 5  shows the semiconductor structure of  FIG. 4  after it has been subjected to a CMP process. 
       FIG. 6  shows the semiconductor structure of  FIG. 5  after the formation of an optional PE-TEOS redeposition layer over the polished USG/BPSG/PE-TEOS stack. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 4-6  shows a process for forming a PMD stack  20  according to one embodiment of the invention. Like reference numerals are used to reference elements in common with  FIGS. 1-3 . 
   Referring to  FIG. 4 , a layer  38  is formed over the BPSG layer  6  before CMP. In one embodiment, the layer  38  is conventionally formed from plasma-enhanced tetraethyl orthosilicate (PE-TEOS). The layer  38  acts as a sacrificial layer for the CMP process. In one embodiment, the thickness of the layer  38  is between approximately 8 k-20 k angstrom, for example about 12 k angstroms. The thickness of the BPSG layer  6  is between approximately 1.5 k-8 k angstroms, for example 6 k angstroms. The thickness of the USG layer  4  is between approximately 1 k-3 k angstroms, and the thickness of the field oxide  2  is between approximately 3 k-6 k angstroms. 
   Referring to  FIG. 5 , after the layer  38  is formed, the structure  1  then goes through the CMP process. In one embodiment, the layer  38  is polished until the BPSG layer  6  is exposed. In another embodiment, the polishing stops before the BPSG layer  38  is exposed. In yet another embodiment, the polishing endpoint is empirically determined using a test wafer (not shown). Alternatively, endpoint data provided by the manufacturer of the CMP equipment may be used. After the CMP, the combined thickness yk of all layers, i.e., measured from the bottom of the field oxide  2  to the top of the layer  38 , is between approximately 8 k to 15 k angstroms. In one embodiment, yk is approximately 13 k angstroms. Thus, the layers  4  and  6  and the polished layer  38  together compose the PMD stack  20 . 
   Referring to  FIG. 6 , subsequent to the CMP, an optional redeposition layer  46  may be formed on the layer  38 . In one embodiment, the layer  46  is formed from PE-TEOS, has a thickness of approximately 2 k angstroms, and composes part of the PMD  20 . In another embodiment, the layer  46  is formed from TEOS. 
   All subsequent process steps, such as, e.g., opening windows for tungsten plugs and metallization and finishing manufacturing of the semiconductor structure  1 , are conventional and are thus not discussed in detail. 
   In embodiments where the layer  38  is formed from PE-TEOS, the layer  38  is often harder than the BPSG layer  6 , and thus allows the PMD stack  20  to maintain the advantages of the conventional USG/BPSG stack while eliminating the disadvantages of polishing the BPSG layer  6 . Furthermore, PE-TEOS has a higher throughput rate during processing than BPSG. Therefore, when formed from PE-TEOS, the layer  38  aids in reducing the processing time by allowing the BPSG layer  6  to have a reduced thickness. Because the thickness of the BPSG layer  6  is reduced, the layer  6  processing time is significantly reduced. 
   Specifically, in embodiments where the layer  38  is formed from PE-TEOS, the process for PE-TEOS deposition is up to about twice the throughput rate as compared with that of the BPSG deposition process. Furthermore, the PE-TEOS layer  38  is often deposited by using the plasma-enhanced chemical vapor deposition (PECVD) process. Thus, unlike the BPSG process which often requires substantial efforts of equipment cleaning after deposition to avoid defects, equipment for PE-TEOS deposition often require less efforts to clean after deposition. This will cause less down time for the PE-TEOS deposition process than for the BPSG deposition process. In addition, the CMP process normally polishes the PE-TEOS layer  38  more uniformly than it would polish the BPSG layer  6  since there is no problem of non-uniform distribution of boron and phosphorous ions in the PE-TEOS layer  38 . Another advantage of the present invention is that the same PE-TEOS deposition recipe can be used in the CMP process for both the PMD stack  20  and later inter-level dielectric (ILD) layers (not shown), which are dielectric layers conventionally formed after first metallization of the semiconductor structure  1 . This often simplifies the manufacturing process of the semiconductor structure  1 . 
   Although the detailed description discloses various embodiments of the invention, other embodiments may be made without deviating form the scope of the invention. In particular, the layer  38  may be formed using a High-Density Plasma Chemical Vapor Deposition (HDPCVD or HDP) technique or even a traditional CVD technique. In alternative embodiments, the respective thicknesses of the layer  38 , the USG and BPSG layers  4  and  6 , and the field oxide  2  can be fine-tuned to obtain optimal desired characteristics, e.g., gettering effect of mobile ions or to improve the yield of the manufacturing process. 
   All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, including but not limited to U.S. patent application Ser. No. 09/632,388, filed Aug. 3, 2000, and entitled “SEMICONDUCTOR STRUCTURE HAVING AN IMPROVED PRE-METAL DIELECTRIC STACK AND METHOD FOR FORMING THE SAME” and U.S. patent application Ser. No. 09/061,667, filed Apr. 16, 1998, and entitled “SEMICONDUCTOR STRUCTURE HAVING AN IMPROVED PRE-METAL DIELECTRIC STACK AND METHOD FOR FORMING THE SAME” are incorporated herein by reference, in their entirety. 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.