XSIL vFinal DR 30/6/08 16:51 Page 12
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COVER STORY
laser energy stability and could reach
±1% (Fig.2). This compatibility of the
laser Via drilling process with 300mm
wafers stands in contrast to existing
difficulties which some DRIE approaches
experience in maintaining uniformity and
significantly reduces the cost of ownership
(CoO) for large volume industry.
Being maskless, the laser process
eliminates the need for various expensive
lithographic steps such as coating,
exposure, development and photoresist
stripping [4]. Via positions are
programmed by CAD on the tool,
enabling rapid setup times, product
changeover and reduction of CoO. The Fig.5 Cross-sectioned Via of 30µm diameter by SEM: sidewalls tapered at
laser process for TSV is flexible and can optimal 85° angle (left), close-up (centre) and Cu-metallized (right)
be utilised for both Front End and Back
End machining allowing both “Via-first”
and “Via-last” approaches to be adopted. excess ~20µm/shot have been Throughput of laser
In addition the bypassing of the mask attributed to laser induced explosive drilling tool
laser process gives developers a rapid boiling with secondary plasma heating Material removal rates reported are
chip prototyping tool that enables [9]. coupled with very high drill rates of up to
them to iterate designs quickly from However, violent ejection of large ~2- 2000-2500Vias/s for certain Via
design modifications to machined wafer. 10µm micro droplets by phase explosion dimensions and densities. Unlike DRIE
forms unwanted debris [10] and bubble which is a parallel process, the laser
nucleation causes a degradation of the process proceeds sequentially where
Via sidewall quality as well as ~20% typically individual Vias are completed
depth variation [11]. prior to subsequent Vias being processed.
For the first time it is reported here Rapid movement between Vias is achieved
that material removal rates exceeding using optical galvanometers and the laser
20µm per single laser shot, and which process is extremely rapid within their
do not impair the sidewall and entrance scanning field which may cover one or
quality of resulting Via, are several dies. Movement between scanning
experimentally observed (Fig.3) Fluence, fields or die groups is performed by linear
spatial beam quality and shape, focusing stages resulting in a step and scan
geometry, pulse shape, duration and approach.
Fig.3 Single-shot ablation rate of repetition rate have all been optimised to Consequently, drill rates for a
mono-crystalline Si wafers versus achieve this result. particular device depend both on the die
incident laser fluence at 355nm
wavelength
Finally, laser drilling tools have low
cost consumables and little maintenance
requirements comprising primarily factory
supplied DI water for washing and pump
diode replacements in the laser once every
2-3 years (>20,000 hours of average
lifetime).
Material removal rates in
laser ablation
Dual pulse lasers have been shown by
Forsman et al. to enhance Si removal
rates to ~4µm/shot with optimised Fig.6 Optical microscope images of debris on protective coating after laser
temporal separation between the dual drilling (left), coating with debris washed (centre) and resulting Via close-up
pulses [8] while silicon ablation rates in (right)
www.euroasiasemiconductor.com July 2008
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