Abstract:
A process for forming multilevel metallization structures that improve semiconductor reliability. Multilevel metallization structures are formed through a two-step etch process which alleviates the problem of conductive etch residue forming between metal layers in multilevel structures. The resulting metallization structure has sidewall insulators on selected layers that prevent conductive etch residue from forming between the metal layers. The integration scheme of the present invention is especially applicable to metal-insulator-metal (MIM) capacitors.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a process for forming multilevel metallization structures and, more particularly, to an integration scheme for metal-insulator-metal (MIM) capacitors that are not compromised by conductive etch residue. 
     BACKGROUND OF THE INVENTION 
     The approach used to manufacture integrated circuits (ICs) on monolithic pieces of silicon substrate involves the fabrication of successive layers of insulating, conducting, and semiconducting materials. Each layer is patterned to form a structure that performs a specific function, usually linked with surrounding areas and subsequent layers. Therefore, the fabrication steps used to manufacture an IC must be executed in a specific sequence, which constitutes an IC process. 
     It is now the practice to fabricate multiple levels of conductive (typically metal) layers above a substrate. The multiple metallization layers accommodate higher densities as device dimensions shrink well below one micron design rules. Likewise, the size of interconnect structures will also need to shrink, in order to accommodate the smaller dimensions. Thus, as IC technology advances into the range below 0.25 microns, more advanced interconnect architecture is required. 
     One such architecture is a dual damascene integration scheme in which a dual damascene structure is used. The dual damascene process offers an advantage in process simplification by reducing the process steps required to form the vias and trenches for a given metallization level. The openings for the wiring of a metallization level and the underlying via connecting the wiring to a lower metallization level are formed at the same time. The procedure provides an advantage in lithography and allows for improved critical dimension control. Subsequently, both the via and the trench can be filled using the same metal-filling step, thereby reducing the number of processing steps required. The simplicity of the dual damascene process permits newer materials to replace cost-effectively the use of the existing aluminum-silicon dioxide scheme. 
     One such newer material is copper. The use of copper metallization improves performance and reliability over aluminum. Copper introduces additional problems, however, which are difficult to overcome when using known techniques for aluminum. For example, in conventional aluminum interconnect structures, a barrier layer is usually not required between the aluminum metal line and a silicon dioxide inter-level dielectric (ILD). When copper is used, copper must be encapsulated from the surrounding ILD, because copper diffuses or drifts easily into the adjoining dielectric. Once the copper reaches the silicon substrate, it will significantly degrade the performance of the device. 
     In the production of ICs upon semiconductor wafers or chips, the back end of production involves connecting all the fabricated semiconductor devices on the chip with electrically conductive materials. This back-end-of-line (BEOL) “wiring” step, which is the electrical connection scheme for connecting semiconductor devices, completes the circuits as designed to function within the total integrated circuit device. Metal lines are used in the metallization process as electrical connections between semiconductor devices. 
     In the fabrication of semiconductor devices, BEOL wiring often determines the function of the device. Therefore, BEOL processes are critical in semiconductor manufacturing. BEOL processes complete semiconductor fabrication in the final manufacturing steps. Errors due to faulty BEOL processes forfeit the entire production investment in the nearly completed device. As a result, device failure due to BEOL errors are very costly, and manufacturers strive to avoid BEOL defects. 
     A common BEOL process involves forming metal-insulator-metal (MIM) capacitors. Typically, these metallization structures are formed as diagrammed in FIGS. 1,  2 , and  3 . A multilevel structure is formed, as shown in FIG. 1, with a bottom metal layer  10 , an interlayer dielectric  12 , and a top metal layer  14  deposited sequentially on top of a substrate  8 . Interlayer dielectric  12  is preferably an oxide and, more preferably, silicon dioxide. A photoresist pattern  16  is formed on the surface of the top metal layer as illustrated in FIG.  2 . 
     Bottom metal layer  10 , interlayer dielectric  12 , and top metal layer  14  are simultaneously etched, as illustrated in FIG. 3, according to photoresist pattern  16  in a single etch step along direction arrow  17 . An etch residue  18 , which can poison the capacitor and render the capacitor useless, may form on interlayer dielectric  12  when bottom metal layer  10 , interlayer dielectric  12 , and top metal layer  14  (i.e., all three layers) are etched in a single etch step. Etch residue  18  on interlayer dielectric  12  offers an alternate electrically conductive path that can short the capacitor. Because the charge has an alternate path along the sidewall residue, the capacitor cannot function. 
     To overcome the shortcomings of conventional processes for forming multilevel metallization structures, a new integration scheme for MIM capacitors, is provided. An object of the present invention is to integrate a MIM capacitor into the dual damascene copper BEOL process without impacting wiring and via yield and parametrics. Other objects are to provide a process that yields reliable products while minimizing process cost and manufacturing steps. A related object is to provide a process that substantially eliminates the etch residue problem while forming a beneficial spacer (which contains contaminants and prevents the contaminants from breaking down the capacitor) around the bottom conductive plate. Still another related object is to provide a process that uses conventional semiconductor tooling. 
     It is still another object of the present invention to achieve a MIM structure for which the dielectric is scaleable in thickness, can be reworked (easily stripped and redeposited) without impacting wiring yield parameters, and can be one of several alternative materials. An additional object is to provide a modular process, i.e., a process having an set of steps independent of prior or subsequent processing steps. Yet another object of this invention is to provide a process that yields a planar or substantially planar capacitor structure, to avoid dielectric thinning over topography, and permits design of the capacitor structure in parallel or serial layouts. 
     SUMMARY OF THE INVENTION 
     To achieve these and other objects, and in view of its purposes, the present invention provides a process that avoids the problems of conventional capacitors including a dielectric insulator along the etched surfaces of metal-insulator-metal (MIM) capacitors. By forming the first metal layer independently from the insulator and second metal layers, the present invention uses sidewall spacers of insulator formed during conformal deposition. Conformal deposition of the insulator layer after the first conductive layer has been patterned provides dielectric along the conductive sidewalls to insulate the first metal layer from forming conductive etch residue to the second metal layer. 
     The disadvantages associated with the prior art processes of fabricating multilevel metallization and interconnect structures are overcome using the present invention. The present invention encompasses a process for forming a patterned structure comprising first and second stacked conductive layer areas where the stacked conductive layer areas are separated by an insulating layer area. Thus, this invention comprises forming, in a first etching step, the first conductive area having a first perimeter, and forming in a single second etching step the insulating layer and the second conductive layer areas. The insulating layer and the second conductive layer areas extend beyond the first perimeter, so that no etch residue can form between the first and second conductive layer areas. 
     The present invention also encompasses a MIM capacitor structure comprising first and second stacked conductive layer areas separated by an insulating layer area. The first conductive area has a first perimeter, and the insulating layer and second conductive layer areas both extend beyond the first perimeter. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 is a schematic cross-sectional view of a metallization structure of the prior art; 
     FIG. 2 is a schematic cross-sectional view of the metallization structure of the prior art shown in FIG. 1 with a photoresist pattern; 
     FIG. 3 is a schematic cross-sectional view of the metallization structure of the prior art shown in FIG. 2 after etching; 
     FIG. 4 is a schematic cross-sectional view showing a first metal layer on a substrate in accordance with the present invention; 
     FIG. 5 is a schematic cross-sectional view showing a pattern etched into the structure of FIG. 4; 
     FIG. 6 is a schematic top view showing the structure of FIG. 5; 
     FIG. 7 is a schematic cross-sectional view showing the structure of FIG. 5 with a conformal insulator layer; 
     FIG. 7A is a schematic cross-sectional view showing the structure of FIG. 7 with optional barrier layers; 
     FIG. 8 is a schematic cross-sectional view showing the structure of FIG. 7 with a second conformal metal layer on the insulator layer; 
     FIG. 9 is a schematic cross-sectional view showing a pattern etched into the structure of FIG. 8; and 
     FIG. 10 is a schematic top view showing the structure of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing, wherein like reference numbers refer to like elements throughout, FIG. 4 shows a substrate  30  as a supporting layer, which may be a silicon wafer with a dielectric etch-stop layer (not shown) deposited on the wafer, and a first conductive layer  20  on substrate  30 . First conductive layer  20  may be aluminum, nickel, cobalt, silver, gold, palladium, platinum, rhodium, or copper; combinations or alloys of those elements; or any other conductive layer capable of electron transport. The most preferred metal is aluminum and alloys of aluminum. The first conductive layer is deposited to a thickness of 100 to 10,000 Å by physical vapor deposition (PVD), chemical vapor deposition (CVD), or by any technique known in the art. 
     Although not shown, semiconductor substrate  30  is intended to include numerous active electronic devices and passive electronic components. The particular design of the underlying integrated circuit has not been shown in order to more clearly describe and show the aspects of the present invention. In addition, because the present invention uses back-end-of-line (BEOL) processing techniques and complies with very-large-scale-integration (VLSI) design patterning rules, the devices of the present invention may be fabricated above or between other metallization levels. Thus, semiconductor substrate  30  may include additional metallization and wiring levels within itself. 
     FIG. 5 is a schematic of the first conductive layer  20 ′ after having been patterned according to a first photoresist mask. The photoresist mask may be patterned by photolithography or any other technique known in the art. “Photolithography” is a process in which a light source illuminates a circuit pattern and projects the image through a lens assembly onto a semiconductor wafer or substrate. Ultimately, the circuit pattern is etched into the wafer. 
     First conductive layer  20  is etched according to the photoresist pattern. In a preferred embodiment, an etch-stop layer (not shown) of silicon nitride exists between substrate  30  and first conductive layer  20 , which is aluminum. First conductive layer  20  can be etched by any dry or wet etching process, or combinations of such processes, and the etch chemistry is selected according to the composition of first conductive layer  20 . After the first etch, the photoresist is removed to reveal the structure represented in FIG.  5 . 
     This first etch step forms a first conductive layer area defined by two dimensions: the width “X” shown in FIGS. 5 and 6, and the length “L” shown in FIG.  6 . FIG. 6 is the schematic top view of the structure shown in FIG.  5 . The X and L dimensions define the first conductive area and the perimeter of the first conductive area (2L+2X). Although the conductive layer areas are shown as rectangular shapes in the figures, the conductive and insulating layer areas may be any shape including, but not limited to, square, rectangular, triangular, cylindrical, or circular. 
     Referring next to FIG. 7, a dielectric insulator  22  is deposited conformally over first conductive layer  20 ′. Dielectric insulator  22  is a continuous layer fully covering first conductive layer  20 ′. Dielectric insulator  22  may be silicon oxide, barium-strontium titanate, aluminum oxide, tantalum pentoxide, or other dielectric materials suitable for MIM functionality, including organic polymers. The capacitor dielectric may range in thickness from 20 to 8,000 Å, as appropriate for the dielectric constant and deposition process. The deposition may be done by physical vapor deposition (PVD), chemical vapor deposition (CVD), a spin-on process, or a combination of these techniques. 
     The conformal deposition of the dielectric material over first conductive layer area  20 ′ forms dielectric insulator  22 . Dielectric insulator  22  has a thickness “Z” along the top surface of the first conductive area, and a thickness “W” along the sidewall of the first conductive area. The ideal W:Z ratio is 1. In practice, however, ratios may vary from 0 to 1. 
     As shown in FIG. 7A, a first barrier layer  32  may be deposited over first conductive layer  20 ′ before dielectric insulator  22  is deposited and a second barrier layer  34  may be deposited after the deposition of dielectric insulator  22 . First barrier layer  32  second barrier layer  34  prevent diffusion of material from first conductive layer  20 ′ and from a second conductive layer  24  (see FIG. 8) into dielectric insulator  22 . First barrier layer  32  and second barrier layer  34  are typically tantalum, titanium, tungsten, tantalum nitride, titanium nitride, tungsten nitride, silicon nitride, other refractory metals, or combinations of such materials. A preferred barrier layer material is titanium nitride, TiN. First barrier layer  32  and second barrier layer  34  may be applied as PVD or CVD layers having a thickness of between about 50 and about 1,000 Å. Barrier layers of this type are known in the art and their deposition techniques are also well known. First barrier layer  32  and second barrier layer  34  are also conformally deposited, and the W and Z dimensions of dielectric insulator  22  include each barrier layer. 
     Next, referring to FIG. 8, the second conductive layer  24  is conformally deposited over dielectric insulator  22 . Second conductive layer  24  is also deposited by PVD, CVD, or by any technique known in the art to a thickness of 100 to 5,000 Å. Second conductive layer  24  may consist entirely of a barrier metal layer, the same material as first conductive layer  20 , any conductive material, or a combination of barrier layer and another conductive layer. 
     A second photoresist pattern is exposed and developed on second conductive layer  24 . Second conductive layer  24  and dielectric insulator  22  are then etched according to the second pattern. The second pattern prescribes a structure analogous to the first pattern. The dimensions of each feature in the second pattern are such, however, that the areas of second conductive layer  24  and dielectric insulator  22  are larger than the area of first conductive layer  20 ′. As shown in FIGS. 9 and 10, the resulting second conductive layer  24 ′ and dielectric insulator  22 ′ extend beyond the perimeter of first conductive layer  20 ′. FIG. 9 is a schematic cross-sectional view showing the MIM structure and FIG. 10 is a schematic top-view of the same structure. The area of first conductive layer  20 ′ with dimensions X and L is shown along with the area of second conductive layer  24 ′ with dimensions Y and H. 
     The width and length dimensions, Y and H respectively, define the area and the perimeter of both second conductive layer  24 ′ and dielectric insulator  22 ′. The second etch process of the present invention is designed such that the sidewall thickness W of dielectric insulator  22 ′ prevents the formation of conductive etch residue between first conductive layer  20 ′ and second conductive layer  24 ′. Accordingly, the dimension Y of second conductive layer  24 ′ satisfies the relationship X&lt;Y, and preferably satisfies the relationship X&lt;Y&lt;X+2W. The dimension H satisfies the relationship L&lt;H, and preferably satisfies the relationship L&lt;Y&lt;L+2W. The dimensions of second conductive layer  24 ′ extend beyond the dimensions of first conductive layer  20 ′ by 50 to 8,000 Å, depending on the thickness of dielectric insulator  22  and the conformality of the layer. When barrier layers exist between the conductive layers and the insulator layer, the thickness of the barrier layers are included in the thicknesses Z and W. 
     The restrictions on X and Y provide that there always exists an insulator layer along the sidewalls of first conductive layer  20 ′. The insulated sidewalls prevent any conductive etch residue, which contacts second conductive layer  24 ′, from contacting first conductive layer  20 ′. This improves the durability and reliability of the metallization structure. 
     Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.