Use of air gap structures to lower intralevel capacitance Page: 3 of 9
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increasingly complex, evolving from thermal CVD to plasma enhanced CVD, CVD/SOG
(spin-on glass) sandwich structures, deposition/etch/deposition systems, ozone enhanced
processes, and more recently to high density plasma deposition systems. This trend has
increased process complexity and cost per wafer. The process we propose exploits the
trend towards higher aspect ratio trenches between closely spaced metal lines and uses
simple techniques to lower intralevel capacitance and fill gaps between the metal lines.
This is done by intentionally creating an air gap between closely spaced metal lines. In
this paper we introduce the process and report on its integration into a standard CMOS
back end process flow. We will consider integration issues such as the landing of vias and
the overall intralevel dielectric thickness. We also present a simple model to assess the
potential of the approach to lower the intralevel dielectric constant.
THE AIR GAP PROCESS
The process is schematically shown in Fig. 1. It starts with the intentional formation of the
air gap between closely spaced metal lines using a simple, high throughput, high sticking
coefficient, poor step coverage, CVD silicon dioxide process. It is important to realize
that this step of the process becomes easier as the aspect ratio between metal lines
increases. Figure 2 shows examples of test structures coated with oxide deposited by
plasma enhanced CVD using SiH4 and N20.
oxide /Reentrant features 1. Deposit oxide
a using a process
- with poor step
Reentrant features Open void filled
filled with HSQ with HSQ
HSQ 2. Fill open gaps
A and reentrant
Capping oxide deposited and planarized by CMP
3. Deposit capping
.oxide and planarize.
Schematic of the air gap process flow.
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Fleming, J.G. & Roherty-Osmun, E. Use of air gap structures to lower intralevel capacitance, article, March 1, 1997; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc674146/m1/3/?rotate=270: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.