Optec Laser Micromachining!"!Technote!RA27!www.optec.be!
HY120 Beam Shaper – principle, adjustments & selection
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HY120 was principally designed for excimer lasers though has been successfully use with some other
laser types. With the standard uncoated UVFS optics it is wavelength tolerant from the UV though to
near IR, special versions with ZnSe optics have been built for CO2-TEA lasers at 9-11µm.
A well-behaved excimer laser has a typical beam profile often
described as quasi-Gaussian in the short, low divergence
axis, whereas the long axis generally has a flattish top >>
However, most applications call for a uniform intensity beam.
Theory
Optec HY120 Beam Shaper works by employing
specially cut cylindrical lens segments to recover the
‘lost’ energy from the low energy ‘wings’ of the raw
beam and using it cleverly to compensate for the fall in
energy density away from the centre of the
distribution. Schematically, we have:-
In a ray tracing,- here of just half the beam,- the light blue rays (coming from the left) are those which
pass through no optics, the darker blue ones are those intercepted by the lens element, brought to a
line focus and then added to the main beam. Profiles in different locations are shown in red, beam
spots with single axis HY in blue:-
Practice
Here we compare directly, beam profiles at a mask with Optec logo, and ablation results on PI using a
standard Optec excimer workstation.
First without the Beam Shaper:-
… showing incomplete machining in the area corresponding to the reduction in e.d. towards the half
power point of the beam.
Simply inserting the HY120 at the correct position upstream of the mask leads to a dramatic
improvement in both profile & results; there was no other change on the system.
We should make no apology for these results & pictures being over 25 years old at the time of writing ;
that is for how long Optec HY120 Beam Shapers have been providing a low cost pulse energy upgrade
to any excimer laser !
A Beam Shaper can be used to maximize useable energy in new or old lasers, and with periodic
adjustment to maintain top performance from ageing lasers as profile deteriorates due to electrode
erosion.
A tribute to its performance, the HY120 has been copied, but never bettered ;- Use Your Photons !
Fully Adjustable
In principle the elements are inserted to the nominal 50% power point of each edge of the beam,
though the exact optimum insertion depends on the raw beam profile, and can be precisely controlled
individually for each of the 4 elements, using the micrometer screws. The profile also evolves with
distance downstream from the device, leading to the concept of working distance.
Overleaf are shown the typical range of (short axis) profiles generated with different insertion degrees and
at different distance from the HY120. The centre plot (red contour) is at the optimal insertion & working
distance. Rows are at w.d. -20%, -10%, 0%, +10%, +20% w.r.t. the optimum, to illustrate the range of
profiles generally observed.
Inserted 10% too much
50% power point
Inserted 10% too little
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N.B. In general, navigating in the lower left area of this parameter field can still
produce acceptable profiles, over-insertion of the elements leads to smaller beams
with higher energy density, obtained at greater than nominal working distance.
This applies only to the short quasi-Guassian contour, is much less true of the
more elongated distribution of a typical long axis profile.
With careful control over the degree of insertion of
the lens elements, and exact distance of the
working plane, one can trim the final profile very
finely, and cater for a very wide variety of beam
shapes, including quite highly asymmetric. It is also
possible to generate unusual beam profiles which
can be useful in specific processing applications >
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Key Points
HY120 is extremely cost effective in using the laser photons to their full, as has been demonstrated in
many installations over 25 years.
Since only a small proportion of the beam passes through any optics at all the HY120 is highly
efficient; typically >98% in each axis.
Unlike integrating homogenizers using microlens arrays or engineered diffusers, HY120 does not
produce unwanted interference effects in the beam.
HY120 introduces minimal unwanted divergence into the beam, an important point where there are
downstream optics with limited n.a.
All four elements of the HY120 are independently adjustable to suit asymmetric beams. For e.g. the
idealised long axis profile at left, or a quite severaly deformed profile at right.
Minor Limitations
It is clear that the input beam wings must not be truncated as
at left upstream from the HY, the otherwise the compensation
will be incomplete (right) and show a defect in the shaped
distribution.
HY120 is a beam shaper, not an integrating homogenizer, &
unlike the latter it can do nothing about a ‘hole’ or hot spot in
the beam. (N.B. Optec also supplies integrating homogenizers
which are useful in such cases)
Depending on the exact beam profile, it can happen that the optimum w.d. is not the same in the two
axes. In that case, the (identical) modules should be separated (2 screws), and each placed at the
appropriate distance to obtain optimum uniformity in both axes in the target plane.
Beam Sizes/Shapes
A typical excimer beam has an aspect ratio of 1,5-2,5 between long & short axes.
When used as intended the shaped beam has the size & shape of the 50% power
limits in the two axes. Where other beam shapes are required then cylindrical
lenses and/or BET/BCT can be used in conjunction with the HY120.
For e.g. in this simple example, a cylindrical BET has been used to expand the
short axis upstream of the HY120.
N.B. Singlet lenses can also be used in conjunction with the HY120 to modify the beam size,
Depending on location, such lenses may also affect working distance in that axis. In Optec machines
quite commonly different nominal w.d. is used in the two axes, for that reason. The situation becomes
too complex to discuss generally, an optical simulation is required for any particular case.
Working Distance
HY120 is available in a range of working distances., so which one should you choose?
a)
The nominal working distance given in the product code is twice the focal length of the
elements at 248nm, though for most laser beams the optimum top-hat profile is usually found
at a distance about 10% greater than this (see for e.g. the ray tracing on P1), so for the
HY120/300 around 330mm at 248nm. 308nm & 193nm will be respectively about 10% more
& 10% less than this latter figure.
b)
Choice might be dictated by machine layout or other constraints
c)
Shorter w.d. generally produces more uniform
profiles with steeper sides, because the divergence
of the laser has less effect.
c)
As for any illumination device, where there are downstream optics, numerical apertures have
to be matched to avoid over-filling of those optics. For e.g. a typical situation is where the
HY120 is used to illuminate a mask whose image is project onto the part using a projection
lens.
In the diagram overleaf, showing half the beam on the long axis:EW is the edge ray, i.e. the ray furthest removed from the beam axis.
HP is the half power point, to where the HY element is normally inserted.
WD is the nominal working distance of the HY unit, twice the focal length of the elements.,-;
there is a line focus at FP
X is the 10% excess to the nominal WD, noted above
OD is the object distance to the imaging optic
CA is the clear aperture of that optic
For given values of ER, HP, OD & CA, an appropriate WD can be deduced, or necessary CA for a
given WD.
For e.g. in the example used for the simulation, ER=18mm, HP 13mm. A WD of 300mm was used
with OD 1000mm, resulting, with X=30, in a CA of 52mm. Optec can supply a small Excel file to aid
such calculations.
HY!
FP!
Lens!
Mask!
ER!
HP!
OD!
WD!
X!
CA/2!
The most popular working distances, suiting typical mask projection layouts, are 450mm & 650mm.
Specific Applications
HY units have been successfully used in PLD set-ups to provide uniform energy density on the target
which helps in preserving correct stoichiometry of deposited films. Typically the constraints of large
OD & small CA of those systems leads to an optical path on the order of 3m, with long w.d. HY120
Simulations
Detailed beam delivery design is best carried out by optical simulation using Beam4 from
https://www.stellarsoftware.com , now a free open source ray tracing program.
Optec will be happy to carry out simulations for particular cases or help the user make his own
simulations. Details of how to do that are outside the scope of this note, but can be supplied on
request.
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