Abstract:
The present invention discloses a capacitor in an integrated circuit which comprises a first and second conductive lines substantially parallel to each other and having a thickness equals substantially to a sum of a via thickness and an interconnect thickness, the first and second conductive lines, the via and the interconnect being formed by a single deposition step, and at least one dielectric material in a space horizontally across the first and second conductive lines, wherein the first and second conductive lines serve as two conductive plates of the capacitor, respectively, and the dielectric material serves as an insulator of the capacitor.

Description:
BACKGROUND 
     The present invention relates generally to semiconductor device structures, and more particularly to forming capacitors in an integrated circuit. 
     Capacitors are critical components in an integrated circuit (IC), de-coupling power supply lines or forming series inductance-capacitor circuit for noise immunization and high speed radio frequency (RF) applications. For example, in an IC having analog circuits, capacitors are often placed near the analog circuits to de-coupling or stabilizing a power supply to the analog circuits. 
     A semiconductor capacitor is typically constructed with an insulator or dielectric material sandwiched between two parallel conductive plates. When a voltage difference is applied across the plates, a certain electrical charge is stored in the insulator. The amount of electrical charge stored in the insulator is defined by the capacitance of the capacitor. Both polysilicon and metal are used to form the conductive plates. However, metal-oxide-metal (MOM) capacitors have seen their increased popularity because of their minimal capacitive loss to the substrate and compatibility with logic processes. 
     Conventional MOM capacitors are constructed in such structures as a vertical stack, U-shape plates or horizontal stacks. The vertical stack structure is a bottom conductive plate first covered by a thin dielectric layer, then covered by a top conductive plate on top of the dielectric layer. A capacitor is formed by the vertically stacked conductor/dielectric/conductor. This kind of vertical stack structure may occupy large areas. Forming such capacitor and making connection thereto may require at least two to four layers of material depositions as well as additional two to three masking steps. The U-shape plate structure employs damascene like processes that include oxide deposition, U-shape opening, bottom plate deposition, thin dielectric deposition, top plate deposition, U-shape fill and chemical-mechanical-planarization (CMP) or etch back process steps. Both the vertical stack and U-shape plate structures require complex additional process steps and additional cost on lithograph, etching and deposition. 
     The horizontal stack is the most simple and lowest costly (no additional process steps or complexity) structure among the three conventional structures for forming a semiconductor capacitor. The horizontal stack structure employs coupling capacitances between sidewalls of two adjacent metal lines. The most popular layout style is to place multiple cathode and anode metal lines of a capacitor alternatively next to each other in a cross-finger style. Between these metal lines is an inter-metal-dielectric (IMD) material serving as capacitor&#39;s insulator. A major drawback of the horizontal stack structure is its low unit length capacitance due to limited height and large space of the metal lines. However, as process technologies shrink down to 100 nm generation and beyond, both metal line width and space become small enough to allow the horizontal stack structure to be competitive area wise as well, when the cross-finger layout style is employed. 
       FIG. 1  is a cross-sectional view of an improved conventional horizontal stack capacitor structure  100  which employs not only the sidewalls of two metal lines  115  and  125 , but also the sidewalls of two via lines  110  and  120  as capacitor conductors. The capacitor structure  100  is formed on a dielectric layer  150 . The via lines  110  and  120  are formed in an inter-layer dielectric (ILD)  130 . The metal lines  115  and  125  are formed in an inter-metal dielectric (IMD)  140 . Both the ILD and IMD serves as insulators for the capacitor structure  100 , therefore, its capacitor area becomes larger. However, a design rule may require that the metal lines  115  and  125  to overlay the via lines  110  and  120 , respectively, and at the same time, the metal lines  115  and  125  have to maintain a minimum spacing D. As a result, the insulator of the capacitor structure  100  may not be very thin to achieve a large unit length capacitance value. 
     As such, what is desired is a simple, reliable and low cost semiconductor capacitor with high capacitance per unit area for advanced process technologies. 
     SUMMARY 
     In view of the foregoing, the present invention discloses a horizontal capacitor structure produced by a dual damascene process. In one aspect of the present invention, the horizontal stack capacitor structure comprises a first and second conductive lines substantially parallel to each other and having a thickness equals substantially to a sum of a via thickness and an interconnect thickness, the first and second conductive lines, the via and the interconnect being formed by a single deposition step, and at least one dielectric material in a space horizontally across the first and second conductive lines, wherein the first and second conductive lines serve as two conductive plates of the capacitor, respectively, and the dielectric material serves as an insulator of the capacitor. 
     In another aspect of the present invention, the horizontal capacitor structure comprises a first, second and third conductive lines substantially parallel to each other and having a thickness equals substantially to a sum of a via thickness and an interconnect thickness, the first, second and third conductive lines, the via and the interconnect being formed by a single deposition step, the third conductive line being connected to the first conductive line, and the second conductive line being flanked by the first and third conductive lines, and at least one dielectric material in a first space horizontally across the first and second conductive lines and in a second space horizontally across the second and third conductive lines, wherein the first and third conductive lines serve as a conductive plate of the capacitor, while the second conductive line serves as another conductive plate of the capacitor, and the dielectric material serves as an insulator of the capacitor. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an improved conventional horizontal stack capacitor structure. 
         FIG. 2A  is cross-sectional view of recesses for vias in a dual damascene process. 
         FIG. 2B  is cross-sectional view of recesses for both vias and interconnect metals in a dual damascene process. 
         FIG. 3A  is a cross-sectional view of a horizontal stack capacitor structure according to an embodiment of the present invention. 
         FIG. 3B  is a cross-sectional view of a connection area for the capacitor structure shown in  FIG. 3A . 
         FIG. 4  is a top view of the capacitor structure shown in  FIG. 1  being layout in a cross-finger style. 
         FIG. 5  is a cross-sectional view of a vertically stacked capacitor structure according to another embodiment of the present invention. 
     
    
    
     The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, wherein like reference numbers (if they occur in more than one view) designate the same elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. 
     DESCRIPTION 
     The following will provide a detailed description of a horizontal capacitor structure that employs sidewalls of adjacent vias, the height of which equals to traditional metal line height plus via line height. The horizontal capacitor structure results in higher capacitance per unit chip area as well as lower sheet resistance than conventional capacitor structures. Lower sheet resistance allows it to operate at very high switching speeds. Besides, the horizontal capacitor structure has a cost advantage as it does not require additional processing steps. 
     The present invention is described by an embodiment using a dual damascene process for making vias which serves as metal plates for the horizontal capacitor. Damascene process basically involves forming a pattern of openings in a dielectric layer and filling the openings with a metal to form a conductive pattern or channel. In a dual damascene process, a first channel of conductive material and a second channel of conductive material below the first channel are positioned in vertically separate planes perpendicular to each other and interconnected by a vertical via at the closet point. The first channel opening and via are filled with metal at the same time, thereby reducing the number of metallization and planarization steps in the manufacturing process. 
       FIG. 2A  is cross-sectional view of via recesses  220  for vias in a dual damascene process, in which the via recesses  220  are defined by a single lithography step. A photo-resist  210  is deposited on a dielectric layer  230 , which has a thickness roughly equals to a sum of the inter-layer dielectric (ILD)  130  and the inter-metal dielectric (IMD)  140  of  FIG. 1  is a conventional process. The via recesses  220  are etched through openings in the photo-resist  210 . Since it is a single etch process, sidewalls of the via recesses  220  are relatively smooth without jogs. 
       FIG. 2B  is cross-sectional view of recesses for both vias and interconnect metals in the dual damascene process. After forming the via recesses  220  in the dielectric layer  230 , another photo-resist layer  260  is deposited on the dielectric layer  230 . Openings in the photo-resist layer  260  define channel areas, typically wider than the via recesses  220 . The channel recesses  270  are formed by another etch process. A time duration of the etch process determines a depth of the channel recesses  270 . 
     After the etching, the via recesses  220  and trench recesses  270  are filled in a single metal-deposition step. After the filling, excess metal that is deposited outside the trench is removed by a CMP process, and a planar structure with metal inlays is achieved. In a modern dual damascene process, a height-to-width aspect ration of the via-plus-trench may be larger than 3. A width of the via recesses  220  or trench recesses  270  as well as a space between two recesses  220  or  270  may be smaller than 0.3 um. Although the dual damascene process mostly uses Cu as the deposited metal, one having skills in the art would appreciate that other metals, such as TaN, SiC, W, TiN, Ag, Au or a combination, may also be deposited in the via-and-trench recesses. 
       FIG. 3A  is a cross-sectional view of a horizontal capacitor structure  300  produced by the dual-damascene process according to an embodiment of the present invention. Metal lines  310  and  320  are a result of filling the via recesses  220  shown in  FIG. 2A  in the single metal-deposition step. The metal lines  310  and  320  serve as a cathode and anode of the capacitor structure  300 , respectively. Then height H of the capacitor structure  300  equals a sum of the metal thickness and via depth in a conventional non damascene process as shown in  FIG. 1 . Both the inter-layer-dielectric (ILD)  130  and inter-metal-dielectric (IMD)  140  serves as dielectric for the capacitor structure  300 . Here, the ILD  130  refers to a dielectric between two, often conductive layers, such as metal or polysilicon layers. The IMD  140  refers to a dielectric sharing the same layer as a metal, filling gaps between the metal. 
     Referring again to  FIG. 3A , the sidewalls of the metal lines  310  and  320  are substantially straight due to the single etching process described in the above paragraphs. Therefore, a space E between the metal lines  310  and  320  is determined solely by a via spacing design rule. As processing technologies progresses, the space E is shrinking to a level comparable to oxide thicknesses in conventional vertical stack capacitor structures. With improved height H and minimized space E, the capacitor structure  300  achieves a greater capacitance per unit length of the metal lines  310  and  320 . Besides, since the metal used to form the metal lines  310  and  320  is Cu in the dual damascene process, sheet resistance of the capacitor structure  300  is relatively low, which enables the capacitor structure  300  to be used in high speed switching applications. On the other hand, since forming the capacitor structure  300  does not require any addition processing step, hence the cost associated with manufacturing such capacitor structure  300  is low as well. 
       FIG. 3B  is a cross-sectional view of a connection area  350  for the capacitor structure  300  shown in  FIG. 3A . Metal lines  360  are formed by depositing the metal in the via recesses  220  and channel recesses  270  in the same metal deposition step that forms the metal lines  310  and  320  of  FIG. 3A . Therefore, a metal width at a channel portion  360 A is wider than that at a via portion  360 B. Metal lines  360  may serve as connections between the capacitor structure  300  and other devices in an integrated circuit. 
       FIG. 4  is a top view of a capacitor structure  400  with conductive plates placed in a cross-finger style according to another embodiment of the present invention. Essentially the capacitor structure  400  is obtained by placing multiples of the capacitor structure  300  shown in  FIG. 3A  next to each other. Referring to  FIG. 4 , fingers  415  are connected to one terminal of the capacitor structure  400  through a connection area  410  and via  430 . Fingers  425  are connected to another terminal of the capacitor structure  400  through a connection area  420  and vias  440 . Cross-sections of the fingers  415  and  425  may be identical, and at a location M-M′ the finger cross-section is the same as the metal line  310  or  320  shown in  FIG. 3A . Cross-sections of the connection areas  410  and  420  may also be identical. At a location N-N′ the connection area cross-section is the same as the connection area  350  shown in  FIG. 3B . Each finger of the fingers  415  is flanked by two fingers of the fingers  425 , except the ones on the very edge of the capacitor structure  400 , therefore for a given length of the fingers  415  or  425 , capacitor area is essentially doubled. Only the horizontal stack structure can utilize this cross-finger style layout, which offers another advantage to the capacitor structure  400 . 
       FIG. 5  is a cross-sectional view of a vertically stacked capacitor structure  500  according to yet another embodiment of the present invention. Sections  502  and  506  are capacitor structures individually similar to the capacitor structure  300  shown in  FIG. 3A . They are so designed and processed that a conductor  530  of the section  506  is stacked on and in contact with a conductor  510  of the section  502 , and similarly, a conductor  540  of the section  506  is stacked on and in contact with a conductor  510  of the section  502 . A sum of the conductors  510  and  530  serves as one conductive plate while a sum of the conductors  520  and  540  serves as another conductive plate of the capacitor structure  500 . Therefore, the capacitance area per unit length of the capacitor structure  500  is essentially doubled than that of the capacitor structure  300  shown in  FIG. 3A . In a modern dual damascene process, widths of the vias  510  and  530 , and widths of the vias  520  and  540 , are drawn equal in a layout, because aligning processes are done through the transparent dielectric film, which allows tighter design-rule tolerances to be used when performing this alignment. A slight misalignment between the vias  510  and  530  or between the vias  520  and  540  is often negligible in terms of performance and reliability. 
     Although only the two-stack capacitor structure  500  is shown in  FIG. 5 , one skilled in the art would recognize that more than two stacks can be formed just in the same way as the two stacks described above. For practical reasons, a total number of stacks should be no more than ten. 
     The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.