Abstract:
A method for integrating a thin film resistor into an interconnect process flow where one of the metal layers is used as a hardmask. After a via ( 42 ) etch and fill, the thin film resistor material ( 62 ) is deposited. The metal interconnect layer ( 76 ) is then deposited, including any barrier layers desired. The metal leads ( 70 ) are then etched together with the shape of the thin film resistor ( 60 ). The metal ( 76 ) over the thin film resistor ( 60 ) is then removed.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is generally related to the field of thin film resistors in integrated circuits and more specifically to a solution for integrating a thin film resistor into a multi-level metal tungsten-plug interconnect.  
         BACKGROUND OF THE INVENTION  
         [0002]    Thin film resistors are utilized in electronic circuits in many important technological applications. The resistors may be part of an individual device, or may be part of a complex hybrid circuit or integrated circuit. Some specific examples of thin film resistors in integrated circuits are the resistive ladder network in an analog-to-digital converter, and current limiting and load resistors in emitter follower amplifiers.  
           [0003]    Film resistors can comprise a variety of materials including tantalum nitride (TaN), silicon chromium (SiCr), and nickel chromium (NiCr). These resistor materials are generally evaporated or sputtered onto a substrate wafer at a metal interconnect level and subsequently patterned and etched. The thin film resistors require an electrical connection to be made to them. Thus, two mask layers are required. One, TFRES, is to form the resistor itself and the other, TFHEAD, is used to form the resistor “heads” or contact points of the resistor. Connection is made from an overlying metal interconnect layer to the resistor heads. The resistor heads are required to protect the resistor during the via etch needed to make contact between the overlying metal interconnect layer and the resistor. In addition to two masks, multiple deposition and dry/wet etch steps are required to incorporate the resistor.  
           [0004]    Morris (U.S. Pat. No.  5 , 485 , 138 , issued Jan. 16, 1996) describes a method of forming an inverted thin film resistor. The resistor structure is deposited directly on top of the metallic interconnects. The metallic interconnects are formed. An interlevel dielectric layer is deposited over the metallic interconnects and polished back to expose the top surface of the metallic interconnects. The resistor is then formed on a portion of the interlevel dielectric and a portion of the metallic interconnect.  
         SUMMARY OF THE INVENTION  
         [0005]    The invention is a method for integrating a thin film resistor into an interconnect process flow where one of the metal layers is used as a hardmask. After a via etch and fill, the thin film resistor material is deposited. The metal interconnect layer is then deposited, including any barrier layers desired. The metal leads are then etched together with the shape of the thin film resistor. The metal over the thin film resistor is then removed.  
           [0006]    An advantage of the invention is providing a method of incorporating an thin film resistor in which only one additional mask step is required to incorporate the resistor.  
           [0007]    This and other advantages will be apparent to those of ordinary skill in the art having reference to the specification in conjunction with the drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    In the drawings:  
         [0009]    [0009]FIG. 1 is a cross-sectional diagram of an integrated circuit having a thin film resistor according to the invention;  
         [0010]    FIGS.  2 A- 2 F are cross-sectional diagrams of the integrated circuit of FIG. 1 at various stages of fabrication;  
         [0011]    FIGS.  3 A- 3 C are cross-sectional diagrams of a second embodiment of the invention at various stages of fabrication.  
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0012]    The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention or its application or uses. The present invention discloses a process for manufacturing a thin film resistor in an integrated circuit using a single additional mask.  
         [0013]    A thin film resistor  60  according to the invention is shown in FIG. 1. A first dielectric layer  30  is formed over a semiconductor body  10 . Semiconductor body  10  may, for example, comprise a silicon substrate with transistors and other devices formed thereon. Semiconductor body  10  may also include an isolation structure such as field oxide or shallow trench isolation. Thin film resistors are typically formed over the isolation regions of a semiconductor body in order to allow laser trimming of the resistor.  
         [0014]    Metal interconnect lines  40  are located over first dielectric layer  30 . Metal interconnect lines  40  is shown as the second metal interconnect level, but may be part of the first or any subsequent metal interconnect layer except the upper most metal interconnect layer. Metal interconnect lines  40  may, for example, comprise aluminum with appropriate barrier layers. However, other suitable metals are known in the art.  
         [0015]    Metal interconnect lines  40  are located within an interlevel dielectric (ILD)  50 . ILD  50  may, for example, comprise a spin-on-glass. Other suitable dielectrics, such as HSQ (hydrogen silsesquioxane) or FSG (fluorine doped silicate glass), as well as combinations of dielectrics (e.g., combinations of TEOS, HDP oxide, and/or PSG), are known in the art.  
         [0016]    Vias  42  are also embedded within ILD  50 . Vias  42  extend from the upper surface of ILD  50  to the upper surface of a metal interconnect line  40 . Tungsten may be used to fill vias  42 . Other suitable materials for filling vias  42  will be apparent to those of ordinary skill in the art.  
         [0017]    Thin film resistor  60  is located on ILD  50  and preferably comprises nickel-chromium (NiCr). Other suitable thin film resistor materials are known in the art. For example, tantalum-nitride (TaN) or silicon chromium (SiCr) may alternatively be used. The thin film resistor material  62  is located not only in the thin film resistor  60  area, but under the metal lines  70  as well.  
         [0018]    Metal lines  70  are shown as the third interconnect level and may comprise aluminum with appropriate upper and lower barrier layers  72  and  74 , respectively. For example, Ti, TiN, Ta, TaN, and combinations thereof may be used for the upper and lower barrier layers  72  and  74 . Portions of metal lines  70  serve as the resistor contacts  64  and  66 .  
         [0019]    Thin film resistor  60  and metal lines  70  are embedded within ILD  80 . ILD  80  may be comprised of a spin on glass. Other suitable materials for ILD  80  are well known in the art. For example, FSG, HSQ, or a combination of dielectrics (e.g., combinations of TEOS, HDP oxide, and/or PSG) may be used.  
         [0020]    A method for forming thin film resistor  60  according to the invention will now be discussed with reference to FIGS.  2 A- 2 E. Referring to FIG. 2A, a semiconductor body  10 . Semiconductor body  10  is typically a silicon substrate processed through the formation of isolation structures, transistors, and other devices (not shown). Deposited over semiconductor body  10  is a dielectric layer  30 . Dielectric layer  30  may be a PMD (poly-metal dielectric) layer if lower metal interconnect lines  40  are part of the first metal interconnect layer, sometimes referred to as Metal-1. Alternatively, dielectric layer  30  may be an interlevel dielectric layer located between interconnect levels as shown in FIG. 2A. In this case, metal interconnect lines  40  are part of the second metal interconnect layer, sometimes referred to as METAL-2.  
         [0021]    After the deposition, pattern, and etch to form metal interconnect lines  40 , ILD  50  is formed. ILD  50  is preferably a planarized layer and may be formed in any of a number of ways. Some examples include: deposition followed by CMP (chemical-mechanical-polishing), resist etch back, deposition of a flowable oxide such as HSQ, dep-etch-dep, deposition of a spin-on-glass (SOG) and etchback. Dielectric  50  may be any planarized dielectric suitable for interlevel dielectric layers, such as SOG, BPSG (boron and phosphorous doped silicate glass), PSG (phosphorous doped silicate glass), USG (undoped silicate glass) and HSQ.  
         [0022]    Still referring to FIG. 2A, vias  42  are etched in ILD  50  and filled with a conductive material such as tungsten. Vias  42  provide connection to various metal interconnect lines  40 .The surface of vias  42  is planar with the surface of ILD  50 .  
         [0023]    Referring to FIG. 2B, resistor material  62  is deposited over ILD  50  and vias  42 . Resistor material  62  preferably comprises NiCr. Other suitable materials such as TaN and SiCr are known in the art. As an example, sputter deposition may be used. Resistor material  60  may be, for example, 50-2000Å thick.  
         [0024]    Metal stack  76  is deposited over resistor material  60 , as shown in FIG. 2C. Metal stack  76  preferably includes upper and lower barrier layers  72  and  74 . Barrier layers  72  and  74  may, for example, comprise Ti, TiN, Ta TaN, or combinations thereof. Metal layer  70  typically comprises aluminum.  
         [0025]    Next, a photoresist mask  78  is formed over metal stack  76 . The photoresist mask  78  covers those portions of metal stack  76  where metal lines  70  are desired as well as the areas that will become the thin film resistor. The exposed portions of metal stack  76  and resistor material  62  are then removed using a dry etch, for examples BCl 3 , Cl 2 . The photoresist mask  78  is then removed, resulting in the structure shown in FIG. 2D.  
         [0026]    A second photoresist mask  82  is formed over ILD  50  and metal lines  70 , as shown in FIG. 2E. Second photoresist mask  82  exposes the portion of metal stack  76  where thin film resistor  60  is desired. The exposed portion of metal stack  76  is then wet etched leaving thin film resistor  60 . Photoresist mask  82  is removed. The resulting structure is shown in FIG. 2F.  
         [0027]    The process then continues with the formation of ILD  80  and any desired subsequent metal interconnect levels.  
         [0028]    A resistor is accordingly added to the interconnect process by adding only one deposition steps, one pattern step, and one etch step. The process is significantly simplified versus prior art approaches. Formation of resistor “heads”, special vias, and split interlevel dielectrics are not required. Special resistor “heads” are not required as the same metal stack used to form a metal interconnect level is used to form connections to the thin film resistor. A special polishing step is not required.  
         [0029]    A second embodiment for forming thin film resistor  60  will now be discussed with reference to FIGS.  3 A- 3 C. The semiconductor body  10  is processed as shown in the first embodiment through FIG. 2D. A hardmask  86  is deposited over the structure as shown in FIG. 3A. Hardmask  86  may, for example comprise silicon dioxide or silicon nitride. Other suitable examples will be apparent to those of ordinary skill in the art. The second photoresist mask  82  is then formed over hardmask  86 . Referring to FIG. 3B, the exposed portion of hardmask  86  is removed using a selective wet etch. This is followed by the removal of photoresist mask  82 .  
         [0030]    Using hardmask  86  as a mask, the exposed portions of metal stack  76  are removed as shown in FIG. 3C also using a wet etch. The second embodiment is useful for cases in which it is difficult to wet etch the metal stack in the presence of photoresist.  
         [0031]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.