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Influence of C4F8/Ar-based etching and H2-based remote plasma ashing processes on ultralow k materials modifications

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7 Author(s)
Kuo, Ming-Shu ; Department of Materials Science and Engineering and Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742 ; Hua, Xuefeng ; Oehrlein, G.S. ; Ali, A.
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The authors evaluated photoresist (PR) stripping processes that are compatible with ultralow dielectric constant (ULK) materials using H2-based remote plasmas generated in an inductively coupled plasma reactor. The materials used were 193 nm PR and nanoporous SiCOH-based ULK (JSR LKD 5109). PR ashing rates and ULK damage (carbon depletion) were measured for H2, H2/N2, and H2/Ar discharges as a function of substrate temperature over the range of 200–275 °C. They employed ellipsometry, x-ray photoelectron spectroscopy (XPS), optical emission spectroscopy, and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) for analysis. For their H2 remote plasmas and a substrate temperature in the range of 200–275 °C, the PR ashing rate varied from 270 to 880 nm/min, whereas 3–5 nm of ULK damage was measured for 20 s remote plasma exposure. As a useful process metric, they defined ashing efficiency as the thickness of PR removed over the thickness of ULK simultaneously damaged. PR stripping processes can be optimized to an ashing efficiency of ∼60 for substrate temperatures above 250 °C, if pure H2 discharges are employed. The addition of N2 or Ar to H2 did not improve the ashing rate and, especially for N2, such additions dramatically increased ULK damage. This resulted in reduced ashing efficiency for these cases. To clarify the impact of etch- - ing/ashing process interactions on ULK modification, they exposed blanket ULK film to C4F8/Ar-based etching plasmas employing a dual frequency (40.68/4 MHz) capacitively coupled plasma (CCP) reactor. Plasma exposures of the ULK were performed utilizing a silicon roof, which shielded the ULK film located underneath from direct ion bombardment. Since the aspect ratio of the small gap structure was selected to be equal to that of an actual trench structure, trench sidewall-like surface modifications induced by etching processes along with their impact on ashing damage that were introduced during a subsequent PR stripping process can be simulated and studied on blanket films with appropriate size. XPS revealed fluorocarbon (FC) deposition together with ∼3 nm of ULK damage on the ULK film surface after the FC plasma etching process. Most of the deposited FC material was removed during a subsequent H2-based remote plasma treatment at 275 °C. The influence of surface modifications introduced by the prior C4F8/Ar-based etching exposure on hydrogen permeation of the ULK material during a subsequent H2 remote plasma ashing process was studied by substituting deuterium (D2) for H2 in the remote plasma process and performing ToF-SIMS analysis. ToF-SIMS depth profiling of ULK films exposed to D2 plasma showed reduced D permeation in the ULK films with C4F8/Ar etching plasma exposure relative to that without

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Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures  (Volume:28 ,  Issue: 2 )