Patent Application
20080076191
The inventors have also concluded that recesses in the interconnect layer 115 contribute to problems with sidewall coverage of the capacitor stack 125 by the diffusion barrier 132. The recesses in the plugs 114 create an irregular side wall surface for the capacitor stacks 125. A diffusion barrier 132, such as an Al2O3 barrier for example, which is intended to protect the capacitor 125 from hydrogen during back end processing, has reduced functionality due to these irregularities.
One of the inventors' concepts is to reduce the tungsten recesses 402 by GCIB smoothing of the interconnect layer 115.
GCIB is a process in which a surface is bombarded with relatively large ion clusters, e.g., from about 5,000 to about 50,000 atoms each. The clusters are accelerated to from about 1 to about 30 keV. The resulting energy per atom is typically less than about 10 eV, preferably 1 eV or less. Due to the large cluster size and low velocity in comparison to other plasma etch processes, the clusters impart their energy within a shallow region of the surface and have a low scattering cross-section (large area of impact), which results in a smoothing of the surface.
An appropriate GCIB process for the inventors' purposes has a high removal rate for the dielectric 112 in comparison to the plug material 114 and any diffusion barrier 116. The process also preferably avoids increasing the resistance of the plugs 114, by avoiding oxidation of tungsten, for example. Further, the process also preferably minimizes roughening of the dielectric 112.
In GCIB processing, a beam is scanned over the surface. The extent of material removal depends on the scan rate, the beam chemistry, and the beam energy. One or more of these parameters can vary over the surface to vary the degree of material removal from point to point.
Another of the inventors' concepts is to vary a GCIB process condition in response to a surface condition as the beam is scanned. For example, it has been found that the depth of tungsten recesses often varies over the surface of a wafer. Accordingly, the beam energy can be increased near where the recessed are deeper in order to obtain the required degree of smoothing. Rather than altering the beam energy, the scan rate or the chemistry can be varied.
Where the plugs 114 are tungsten and the sleeve or liner 116 is TiN, appropriate beam chemistries can be, for example, Ar, Ar/O2, fluorine containing species such as CH2F2 , and CF4/Ar. If oxygen containing species are used, they are preferably used in small quantities to prevent oxidation of the tungsten. Preferred chemistries are Ar and CxFyHz, wherein x≧0, y≧1, and z≧0
In one embodiment, the surface of a wafer is uniformly scanned with a gas cluster ion beam. The surface is effectively smoothed due to the selectivity of the ion beam among the surface materials and due to the physical characteristics of the beam, which tends to smooth surfaces regardless of the compositions of the surface structures.
In another embodiment, the surface is mapped for a parameter such composition or average recess depth. Based on the map, the surface is scanned with varying beam intensity, the variation being in relation to the mapped parameter. Instead of a map, a surface parameter can be detected as the gas cluster ion beam is scanned and the intensity of the beam (or other GCIB process parameter) varied based on the detected parameter.
In a further embodiment, the surface is mapped for a parameter such as composition or average recess depth and the surface is scanned with gas cluster ion beams two or more times, each time with a different chemistry. During each scan, the beam intensity can be varied over the surface, the beam shut on and off, or the beam's movement rate varied in relation to the mapped parameter. In this manner, a different chemistry can be used to etch the sleeve 116 than is used to etch the dielectric 112. In another example, a first scan is used to level the surface and a second scan is used to smooth the field oxide. Again, rather than mapping the surface in advance, a surface condition can be detected as the scan is progressing.
GCIB smoothing reduces the average height difference between the plugs 114 and the dielectric 112. Typically, this height difference is initially at least about 100 Angstroms. Preferably, this difference reduced to less than about 50 Angstroms, more preferably to less than about 30 Angstroms, and still more preferable to less than about 15 Angstroms.
Additional smoothing may be desired after initial GCIB processing of the interconnect layer 115. GCIB is not generally used, as here, to remove large amounts of material. Removing large amounts of material may result in roughening of the surface of the dielectric 112 even as the overall contact layer is smoothed on a larger measurement scale. One approach, suggested above, is a second GCIB process scan with conditions tailored to smoothing the dielectric.
A further of the inventors' concepts is, following GCIB smoothing of the contact layer, to perform GCIB smoothing of another layer above the contact layer. In one embodiment, the bottom electrode layer 124 is smoothed by GCIB. In another embodiment, the PZT layer 126 is smoothed by GCIB. Chemistries for these additional GCIB processing steps can be, for example, Ar/O2, N2, or CF4/O2.
The invention as delineated by the following claims has been shown and/or described in terms of certain concepts, components, and features. A particular component or feature may have been disclosed herein with respect to only one of several concepts or examples, and may have been described both broadly and narrowly. Different components or features in their broad or narrow conceptions may be combined with other components or features in their broad or narrow conceptions where such combinations would be recognized as logical by one of ordinary skill in the art. Also, this one specification may describe more than one invention and the following claims do not necessarily encompass every concept, aspect, embodiment, or example described herein.
