1.  Field of the Invention
The present invention relates to a fabrication method for a capacitor.  More particularly, the present invention relates to a fabrication method for a self-aligned metal-insulator-metal capacitor for integrated circuits. 
2.  Description of the Related Art
Capacitors are often used in integrated circuits for storing an electric charge.  Capacitors basically comprise two conductive plates separated by an insulator.  The capacitance or the amount of charge held by the capacitor per applied voltage depends upon the area of the plates, the distance between them, and the dielectric value of the insulator.  Capacitors in integrated circuits are usually fabricated from polysilicon, metal to polysilicon or metal to polycide structures. 
In many applications, as in analog-to-digital converters, it is desirable for capacitance not to vary with charges in voltage.  A measure of the variation of capacitance with applied voltage is called the voltage coefficient of capacitance (VOC).  Generally, VOC of capacitors used on integrated circuits is not zero and needs to be nulled.  Various null circuit techniques have been employed to increase the precision of VOC""s, these techniques, however, consume chip area and thus increase the cost. 
Capacitors formed with metal-insulator-metal (MIM) layers have been shown to provide a capacitance that does not vary with voltage.  The MIM capacitors are desirable because they provide depletion-free, high conductance electrodes suitable for high speed applications at the lowest cost.  A conventional method for manufacturing a semiconductor device including a capacitor that is formed with metal-insulator-metal layers is first disclosed by Radosevich et a. in U.S. Pat.  No.  5,576,240 and is described herein with reference to FIGS.  1A-1D.
As shown in FIG.  1A, a field oxide layer 102 is formed, for example, by local oxidation or deposition, on a silicon substrate 100.  An optional polysilicon layer (not shown in Figure) is formed on the field oxide layer 102 as lead to conduct electric charge to and from the bottom plate to facilitate the incorporation of the capacitor into an integrated circuit.  An interlevel dielectric layer 104 is formed on the field oxide layer 102.  Openings are formed in the interlevel dielectric layer 104 in which a capacitor and a contact via are going to be formed. 
Continuing to FIG.  1B, a layer of titanium (Ti) 106a and a layer of titanium nitride (TiN) 106b are deposited, preferably by sputtering deposition, in the openings in the interlevel dielectric layer 104, to form a bottom plate for the capacitor.  A capacitor dielectric layer 108 is then deposited in the openings.  The capacitor dielectric layer 108 can form from silicon nitride or a ferroelectric material. 
As shown in FIG.  1C, the capacitor dielectric layer 108 is then patterned using a photoresist layer 110a.  The capacitor dielectric layer 108 in the contact via 109 is removed. 
Referring to FIG.  1D, the photoresist layer 110a (as in FIG.  1C) is stripped off and a layer of aluminum 112 deposited.  Photoresist layers 110b and 110c are then formed on the aluminum layer 112 to define gaps to expose portions of the integrated circuit to an etchant. 
The aluminum layer 112 is then over etched sufficiently to remove portions of the dielectric layer 108 and the titanium-titanium nitride layer 106 positioned at the gaps as illustrated in FIG.  1E. Structure 120, formed according to the conventional approach, is a contact via comprising conductive layers of aluminum 112 and Ti/TiN 106, and the structure 140 is a capacitor comprising the titanium-titanium nitride layer 106 as the low electrode, the capacitor dielectric layer 108 and the aluminum layer 112 as top electrode. 
Accordingly, the method of manufacturing a conventional semiconductor device including a capacitor as described above uses multiple masking and etching steps for the patterning of capacitor, greatly increase the complexity of the manufacturing process.  The probabilities of misalignment, leading to the formation of a defective device also increases.  Additionally, each masking and etching step is performed using lithography, which is a relatively expensive step in the manufacturing of semiconductor devices.  The MIM capacitor formed according to the conventional approach, moreover, does not provide a smooth topography, greatly increasing the processing difficulties of the subsequent layers. 
Based on the foregoing, a fabrication method for a self-aligned metal-insulator-metal capacitor using a metal interconnect as the bottom electrode is provided, wherein a single self-aligned masking is required for the patterning of capacitor. 
According to a preferred embodiment of the present invention for fabricating a self-aligned metal-insulator-metal capacitor, a plurality of damascene metal interconnects are formed in an inter-metal dielectric layer.  A metal interconnect is etched back to form a recess in the metal interconnect.  A conformal insulation layer is then formed on the inter-metal dielectric layer, covering other metal interconnects and partially filling the recess in the metal interconnect.  A metal layer is further deposited on the insulation layer, completely filling the recess in the metal interconnect.  The excess metal layer formed on the surface of the insulating layer above the recess of the metal interconnect is subsequently removed. 
Accordingly, the present invention is compatible with the damascene backend processing, wherein the MIM capacitor uses the damascene metal layer as the bottom electrode.  Additionally, only a single mask is used for the patterning of the capacitor.  The single capacitor masking, namely the mask that is used for the etching back of the metal interconnect, is also self-aligned.  Moreover, the MIM capacitor of the present invention is formed in the recess of the metal interconnect, the topography of the resulting structure is thus relatively smooth, reducing the difficulties in the manufacturing of the subsequent layers. 
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.