Effect of Laser Shock Wave Cleaning Direction on Particle Removal

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					  Effect of Laser Shock Wave Cleaning Direction on                                         REFERENCES
        Particle Removal Behavior at Trenchs
                                                               1. J. M. Lee and K. G. Watkins, J. Appl. Phys. 89, 6496
 Jin-Su Kim a, Ahmed A. Busnaina b and Jin-Goo Park a          (2001)
                                                               2. S. H. Lee, et al, Jap. J. Applied Physics, Part 1 44,
    Department of Materials Engineering and Bionano            5560 (2005)
Technology, Hanyang University, Ansan 426-791, Korea           3. T. G. Kim et al, Microelectronic Engineering 83,
   Center for Microcontamination Control, Northeastern         688(2006)
              University, Boston, MA 02115                     4. H. Lim, et al, J. Appl. Phys. 97, 054903 (2005)


    Dry cleaning has been studied to control the removal
force on pattern wafers as device feature sizes decrease to
below 100 nm. Laser shock wave cleaning (LSC) has
been introduced to remove particles on patterned wafers
[1]. In this method particles are removed by shock wave        Fig. 1. Schematic diagram of a laser shock wave system.
which generated by a focused laser beam having biconvex
lens as shown in Fig. 1. LSC can be applied to BEOL,
local cleaning of wafer and extreme ultraviolet (EUV)
lithography mask [2]. For the application of LSC to EUV
mask cleaning or any BEOL cleaning process, the
questions have been raised how the particles behave when
patterns exist. For example, EUV lithography mask
contains various layers with their topographies as shown       Fig. 2. Cross sectional pattern topography of a EUVL
in Fig. 2. This topography might create “shadow effect”        mask.
during cleaning process [3]. In this paper, the effect of
pattern topography on the particle removal was
investigated on trench patterns using LSC.
    A laser shock wave was generated by a Q-switched
Nd:YAG laser (IMT, Korea) with a maximum pulse
energy of 1800 mJ at a wavelength of 1064 nm. To
characterize the topographical effect on LSC, samples
with trench patterns were aligned for and against the
shockwave propagation direction as shown in Figure 3.
The pattern width and aspect ratio were 5 µm and 0.25,
respectively. PSL microspheres (500 nm red fluorescence,                     (a)                         (b)
Duke Scientific, USA) were used as the particulate             Fig. 3. LSC application in (a) horizontal and (b) vertical
contaminant source and were deposited on the STI               direction .
patterned wafers by dipping them into the IPA solution
which      contained    PSL      microspheres.      Cleaning
performances such as PRE and cleaning area were                              80

analyzed by a fluorescence microscope (LV-150, Nikon,                        70

Japan).                                                                      60
    The topographical effect was characterized as a                          50
function of laser shock wave propagation direction. Fig. 4

shows the comparison of particle removal efficiency
(PRE) in between horizontal and vertical direction of laser                  30

shock wave to the trench patterns. PRE was lower in case                     20

of vertical direction when compared to horizontal                            10

direction. It means that the topography of a trench is                       0
critical in removing particles. Fig. 5 (a) and (b) shows the                      Horizontality

optical microscopic images of the particles remained after
cleaning by LSC in vertical and horizontal direction,
                                                               Fig. 4. Comparison of PRE in between horizontal and
respectively. It could be observed that LSC in vertical
                                                               vertical direction of laser shock wave.
direction left more particles than horizontal and also in
both cases the remained particles were present at the
trench side wall. Based on these results, it could be
concluded that LSC in horizontal direction has more PRE
than in vertical direction and hence topography is an
important consideration during LSC.


This work was supported by the Korea Science and
Engineering Foundation (KOSEF) grant funded by the                           (a)                             (b)
                                                               Fig. 5. Optical microscope images after LSC in (a)
Korea government (MEST) (No. R11-2008-044-00000-0).            vertical direction and (b) horizontal direction.

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