XSIL vFinal DR 30/6/08 16:51 Page 14
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COVER STORY
which results from the subsequent [14]. However, HAZ is significantly
condensation of material ablated by the minimised in optimised laser process Preliminary results for 5µm
laser. It is necessary to carefully deal with described here. Induced stress and HAZ diameter Via
this material which can adhere to the of cross sectioned Via samples have been Via diameters ranging from 10-80µm are
wafer surface in the vicinity of the Via. In measured by Micro-Raman Spectroscopy covered in a rapid percussion drilling
certain configurations and setups a lip or on a Renishaw-1000 system coupled to a regime by tuning the optical set up in a
raised rim which extends for a few CCD camera and a Leica optical standard Via drilling tool. R&D work is
microns around laser drilled Vias results. microscope. Single spot Raman spectra currently ongoing for Vias of 4-6µm
An optimised laser process eliminates the were recorded from different areas of the diameter addressing targeted roadmaps of
lip surrounding Vias by utilising the sample in a backscattering geometry with DRAM [15] and logic [16] manufacturers
higher efficiency of plume ejection by an Ar+ ion laser at 514nm and 457nm until 2012.
recoil pressure while a vacuum fume wavelength used as an excitation source. The main impediment to attaining
extraction arm collects ejected particles All peaks observed had a symmetrical large Via diameters is imposed by the
during micromachining reducing the shape and could be fitted with a single maximum pulse energy available from
incidence and degree of potential Lorentzian function. The LO Si-Si phonon industrial UV high repetition rate Q-
contamination. peak for unstressed Si is located at switched DPSS lasers. Therefore, Via
To satisfy the most demanding clean 520cm
-1
. Under the compressive stress diameters >80µm are machined on the
room requirements an efficient cleaning this peak is shifted to the higher same tool in trepanning regime with a
process is incorporated into production frequency side, while for tensile stress, to consequent loss of throughput. However,
tools, eliminating residual Si debris the low frequency side. Induced stress was large Vias in Silicon are not of significant
(Fig.6). A built in washing station, calculated from these shifts and scanning interest for interconnect applications.
comprising an integrated spin coater was performed in the area of maximal Limitations for small diameter Vias
allows wafers to have a water soluble expected stress within 7 microns of the arise from opto mechanical restrictions:
sacrificial layer deposited on them prior sidewall as shown in Figure (7). spherical aberrations in commercially
to laser machining which is them washed The results of the analysis demonstrate available scan lenses, inevitable trade off
off subsequent to machining. Any debris that the degree of induced stress is low between focal length, separation between
produced during the laser drilling process attaining a maximum value of 100MPa the scanning mirrors in galvanometer and
that is not captured by the vacuum system and rapidly relaxes within 1µm of the Via its aperture; maintenance considerations:
is trapped on the sacrificial layer and also sidewall. This analysis suggests that the hazard of scan lens contamination by
removed during the wash process. Figure wafers being drilled do not suffer adverse ejected plume and effective throughput
6 illustrates the appearance of typical temperature variations, that little heat is reduction with smaller scan field; strict
wafers before and after wash. absorbed into the wafer bulk and that the requirements for the chuck flatness.
heat affected zone surrounding the Vias is Nevertheless, optimisation of opto
Induced Stress and Heat less than 1µm, and as such exhibits mechanical design allows Via diameters
Affected Zone (HAZ) negligible induced stress. No amorphous as small as 4-6µm to be achieved at the
Induced stress impacts on yield, rendering Si layer was detected. Stress relief at speeds approaching 3000Vias/s
devices more susceptible to fracture high fluencies may be associated with (Fig.8)while maintaining an optimal
during packaging processes, while the annealing mechanism by plasma heating sidewall taper angle of 85°. Restrictions
Heat Affected Zone (HAZ) dictates the [14]. for smaller Via diameters are also
interconnect density and hence defines the It is known that magnetically enhanced imposed by wavelength related diffraction
device form factor. A substantial HAZ is dense plasma causes physical and latent limit and spatial quality of laser beam.
thought to exist when machining with Q- device damage in RIE reaction chamber. Nevertheless, substantial diameters
switched lasers. Induced stress exceeding Laser drilling does not escalate charging reduction <2-3µm is envisioned sub
1GPa has been observed by Amer et al. and is safe for devices. wavelength focusing by precise control of
instantaneous power with ps lasers and
other emerging techniques [17].
Backside wafer drilling.
Machining through metal
Laser process is less selective than DRIE
to a diversity of microelectronics
materials and is suitable even for
patterned and structured wafers
comprising oxide and nitride layers, Al,
Ni, Ti and Cu pads (Fig.9), unpolished
contaminated surfaces (Fig.10) and even
epoxy compounds.
Metals are more susceptible to thermal
damage in laser ablation than Si due to
Fig.9 SEM images of Vias machined through the copper pad higher thermal conductivity. Nevertheless
www.euroasiasemiconductor.com July 2008
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