Advanced Technologies
in Laser Micromachining
Laser Micromachining: Definition
Laser micromachining is defined
as laser cutting, drilling, etching, stripping, skiving materials
such as plastics, glass, ceramic and thin metals with dimensions
from 1 micron to 1mm (0.040"). Maximum machining thickness
is 1mm. When laser drilling holes, the entrance diameter is larger
than the exit diameter. The typical half angle side wall taper
is 3 to 5 degrees, material dependent.
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Methods of Laser Micromachining:
Laser micromachining is performed by
two different methods.
Excimer lasers
utilize the mask projection
technique that is akin to laser lithography, the same leading
edge technology that manufactures next generation microcomputer
chips. The excimer laser illuminates a mask (that is not in contact
with the part) that contains a pattern such as a circle, rectangle
or any complex shape. This pattern is imaged by downstream optics
to produce an identical yet significantly smaller pattern.
Most often, the mask is fabricated inexpensively
by chemically etching stainless steel masks whose pattern is
typically 150 microns (0.006") or greater. Since the mask
projection technique optically reduces or "demagnifies"
the mask pattern by an integral amount (typically 5 to 30 times),
simple low cost methods can be used to fabricate the mask. In
some specific applications, with dimensional requirements less
than 5 microns, glass mask (metal deposited on quartz) are used.
Mask Projection has a
number of advantages:
- i) Complex pattern: Any complex pattern
such as an "8" or "S" can be produced flawlessly
without any stitching or scalloping issues (caused by overlapping
a circular laser spot) because the entire pattern is machined
at one time.
- ii) Edge quality: When laser micromachining
blind channels, the excimer laser can image a long thin rectangular
image with perfectly straight line edges. When drilling holes,
perfectly circular holes with "ink jet-type" precision
is achieved.
- iii) Throughput: A large process area
can be laser micromachined at one time, maximizing process throughput
and lowering costs.
- iv) The business case for using excimer
lasers in a mask projection method usually comes down to process
throughput. Having UV laser sources up to 100W in average power,
as opposed to 2.5W to 10W with DPSS, allows excimer lasers to
be a cost-effective manufacturing solution.
- v) This is especially the case at 193nm
laser wavelength that is not currently attainable by DPSS lasers
at reasonable average powers (ie; 30W). The 193nm laser wavelength
is used for laser micromachining specific materials such as pebax,
nylon, glass and bioabsorbable materials.
To automate the mask projection technique,
programmable mask changers are deployed where multiple mask pattern
are mounted on a high speed linear mask stage, permitting different
mask patterns to be shuttled in on-the-fly.
DPSS (Diode-pumped solid-state lasers),
Femtosecond and CO2 lasers utilize a direct
write approach.
The laser
beam is focused to a small spot size (typically 10-25 microns)
and the beam traces the pattern to be cut. If the laser is drilling
a hole larger than the spot size, a method called trepanning
is used where the beam is directed in a series of concentric
circles to "fill" the larger hole. Often, to accelerate
processing times, the beam is directed by galvanometer or spinning
mirrors to direct the beam to a specific location (typically
within a 2 inch diameter field).
Direct Write has a number
of advantages:
- i) Maskless: There is no mask pattern.
Just point and shoot the laser beam.
- ii) Ease of programming: The drawing
is converted by a CAD/CAM program to the machine code, used to
drive the motion controller of the laser system.
Laser Sources:
Excimer
lasers
An excimer laser is gas powered
laser, automatically filled with a specific gas mix and run for
a period of time before the existing gas fill is evacuated and
replenished with new gas. The excimer laser achieves the highest
average power (up to 100 Watts) in the ultraviolet spectrum,
with laser wavelengths ranging from 157nm, 193nm, 248nm, 308nm
and 351nm.
Excimer lasers became industrial workhorses
when the semiconductor industry gravitated from UV lamp sources
to excimer lasers to produce next generation computer chips.
Today's lithographic tools, scanner and steppers, deploy excimer
lasers. In the case of laser micromachining applications, excimer
lasers are utilized as lithographic tools in the range of 1 micron
features (or higher).
DPSS (Diode-pumped
Solid State Lasers)
Diode-pumped solid-state lasers are solid-state
lasers that are pumped by a series of diode bars. There has been
significant development over the years, propelled by the microelectronics
industry (micro vias in cell phones and other hand-held portable
devices) to reach the ultraviolet spectrum at 355nm and 266nm.
The fundamental laser wavelength is 1.06 microns and non-linear
crystals are used to double (532nm), triple (355nm) and quadruple
(266nm) the laser wavelength with the penalty of lower average
power and higher pulse-pulse variation.
Femtosecond
lasers
Femtosecond or ultrafast lasers
are solid state lasers that produce a temporal pulse in the femtosecond
regime (10-15 sec). The laser consists of an oscillator, regenerative
amplifier, amplifier pump laser and stretcher/compressor unit.
In some ways, this technology is viewed as the "holy grail"
because the extremely short pulse duration removes material as
a multiphoton ablation process, ideal for any material type with
little or no heat affected zone. The technology is becoming more
industrialized, packaged in a single unit, with average power
of a few watts and repetition rates up to 5 khz.
CO2 lasers
(Carbon Dioxide)
CO2 lasers are gas powered lasers
with a sealed laser tube (no gas filling required) that have
been the mainstay of laser machining for decades. These lasers
operate in the infrared spectrum (10.6 microns) with average
powers in the kilowatts with high repetition rates. In the field
of laser micromachining, the applications are limited because
the smallest achievable spot size is 50-75 microns diameter.
Still, the laser is applicable for certain "through hole"
applications where high throughput and low operating costs are
required.
Laser Wavelengths:
Machining dimensions are proportional
to approximately twice the laser wavelength. The shorter the
laser wavelength, the smaller the feature size is realizable.
Resonetics has the largest number of
excimer lasers in the United States for contract manufacturing.
With + 50 excimer lasers operating in the ultra-violet wavelength
(193nm, 248nm, 308nm) with average power approaching 100
Watts, these lasers are ideal for processing of plastics, glass,
ceramics and thin metals with tolerances approaching 1 micron.
By operating in the ultra-violet region,
the excimer laser ablate or vaporize the material, minimizing
debris or heat affected zone. This photo chemical ablation process
works by the laser breaking the molecular bonds within the material
and ablated material is ejected upward and away from the material
surface at supersonic speed.
The excimer laser has a typical pulse
duration of 20 nsec. The temporal pulse is so short that there
isn't time to generate heat, minimizing any thermal effects such
as melting. This is especially the case as the wavelength gets
shorter, such as at 193nm.
The laser source operates at a repetition
rate up to 200hz with an etch rate of approx. 0.1 to 0.5 microns
per pulse. The laser can drill through or blind holes. By counting
the number of pulses, the depth of machining can be accurately
controlled. As an example, the laser is ideal for selective removal
of a top coating or layer without damaging the underlying layer.
The excimer laser removes material precisely, like peeling an
onion layer-by-layer.
By using short wavelength such as excimer
lasers, machining dimension as small as 1 micron is realizable.
Resonetics operates Diode-pumped solid
state lasers (DPSS) which are relatively low power laser
sources (10W or less) operating at high repetition rates (50
khz), suited for direct write applications such as laser cutting
of plastics and thin metal foils or laser drilling of non-repeatable
hole patterns in polymers and ceramics. These lasers operate
in the fundamental wavelength (1.06 microns) but can be doubled
(532nm), tripled (355nm) or quadrupled (266nm) to handle a variety
of materials and machining patterns.
PHOTO of DPSS System
The use of non-linear harmonic modules
such as tripler (355nm) and quadrupler (266nm) allow Diode-pumped
solid state lasers (DPSS) to reach the ultraviolet spectrum,
opening up new laser micromachining applications.
 Resonetics
utilizes a Femtosecond laser operating at a wavelength
of 800nm at an average power of 1W and a repetition rate of 1khz.
The attractiveness of the femtosecond laser is the ultra short
pulse duration that offers the possibility of machining any material
with minimal recast material and no heat affected zone.
Photo of Femtosecond laser
system
The femtosecond laser source (or ultra
fast laser) in the Resonetics laser system has a pulse
duration of 120 femtoseconds (10-15 sec) that is six orders of
magnitude shorter than excimer or DPSS lasers that operate in
the nano second regime (10-9 sec). Due to the low average power,
the femtosecond laser has limited industrial applications but
does show promise in machining hard materials such as metals
with reduced slag. The material can be processed in ambient air
or a vacuum chamber.
Resonetics utilizes short pulsed CO2
lasers operating at 10.6 micron wavelength. The CO2 (Carbon
Dioxide) lasers operate at average power of 150W with repetition
rates of 10khz. The CO2 laser has the highest processing speed,
two orders of magnitude faster than excimer lasers and one order
of magnitude faster than DPSS lasers.
Of course, there are consequences to
this processing speed. They include thermal effects such as potential
melting or cracking, limited machining dimensions (0.003")
and tolerances (0.0015") and not suitable for blind hole
drilling or precise selective material removal.
CO2 lasers
are the industrial workhorse of laser machining utilizing a spot
size of .003" or .004'. Machining tolerances approach one-half
the laser spot size. CO2 laser machining is a thermal
process, liquefying
the material to allow coaxial gas to push the machined material
out the back side. Unlike a UV laser (excimer or DPSS at 355nm
or 266nm), the CO2 laser vibrates the molecular bonds of the
material, generating heat that is used to liquefy the material.
Similar to DPSS and Femtosecond lasers,
CO2 lasers deploy a direct write approach where the focused
beam traces the pattern to be machined.
Two direct write methods are commonly
used: The fixed beam approach involves moving the part on a 300mm
x 300mm XY motion stage; alternatively the CO2 beam can be moved
very quickly by a pair of galvonometers, programmable spinning
mirrors to direct the beam in the X and Y axis within a defined
field.
Photo of CO2 Galvo System
Laser Micromachining: Techniques:
9 Axes Technology (9AT):
Resonetics employs multiple 9 axes laser
systems in house.
Laser micromachining of complex three-dimensional
devices that involves cutting, drilling or trimming of non-planar
geometries cannot be accomplished with traditional XY or rotary
motion stages. If the surface profile of the device is changing
relative to the longitudinal and rotation axes, then 9 axes laser
micromachining technology is implemented.
The 9 axes of motion includes 2 sets
of X,Y theta, Z, tilt and yaw plus the synchronization of firing
the laser beam at the precise time.
PHOTO of 9 axes laser
system
The laser beam is scanned across a large
mask stage. At the same time, the part is motioned in 6 axes
simultaneously to ensure the laser beam remains in focus despite
the ever-changing surface profile. Maintaining focus despite
surface profile undulations is known as Autofocus Technology
(AFT). This technology adjusts the Z axis automatically to
maintain the laser beam is within the depth of field.
Automatic Mapping Technology (AMT) takes into account the variability in the 3D
geometry such as the OD (outer diameter) on a part-by-part basis.
This is accomplished by mapping the part geometry in real-time
and then employing proprietary software algorithms to automatically
adjust the 9 axes to compensate automatically. The additional
benefit is that the in-situ part mapping allows Resonetics to
monitor the variability in part geometry and report back to the
manufacturer to ensure manufacturing consistency. Vital statistics
such as CpK's of every key metric can be provided for every part.
Techniques
List
Beam Homogenization:
Beam Homogenizer Optics are used with
excimer lasers to increase process throughput and ensure process
repeatability. Beam homogenizers transform the raw excimer laser
beam (typically 25mm x 10mm) into a beam of uniform energy distribution
within +/- 5%). For example, the typical beam size on target
to machine plastics is 3mm2. By incorporating beam homogenizers,
the effective machining area can be increased by a factor of
5 to 20 times, translating into significantly faster throughput
with high process yield at lower cost.
Resonetics deploys a number of proprietary
beam homogenizer technologies (such as fly eye and others) to
shape the beam into a square, rectangle or long thin line. The
shape of the beam is dictated by the pattern to be laser micromachined.
PHOTO of Beam Homogenizer
Techniques
List
Machine
Vision Technology (MVT):
Machine Vision Technology positions a
laser beam automatically relative to a distinct feature such
as a previously laser-drilled feature, contact pad or fiducial
mark. MVT (Machine Vision Technology) deploys a camera vision
system, illumination, frame grabber electronics and software
to eliminate operator intervention in a multi-step laser machining
operation; hence increasing process throughput and reducing costs.
Photo of MVT
Techniques
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In-Situ
Metrology Technology (IMT):
As a by product of Machine Vision
Technology (MVT), metrics such as hole diameter or dimension
of ablated areas can be measured on-the-fly. The data is uploaded
to the network, collected in a spreadsheet and the CpK values
are calculated automatically. This automated In-situ Metrology
Technology (IMT) serves several functions: It eliminates or reduces
off line inspection to reduce costs; provides in-situ process
control to verify the manufacturing process is within established
boundaries; and maximizes process yield.
PHOTO of IMT
Techniques
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Laser Lathe Technology:
Traditional mechanical lathes are used
to fashion stock material into specific tubular geometries. As
the overall feature sizes drop below 0.006" and machining
tolerances approach 0.0002" (5 microns or less), then alternative
manufacturing technologies are required. The Resonetics Laser
Lathe Technology (LLT) fills this gap when it comes to machining
plastic medical device components such as catheters, balloons,
membranes and other devices.
The traditional lathe used in woodturning,
metalworking and glass working, spins a block of material that
is shaped by a tool to produce a device with rotational symmetry.
The material is held in place by one or two centers that can
be adjusted horizontally to accommodate the correct part length.
PHOTO of Laser Lathe System
The Laser Lathe borrows the same
principles as a traditional mechanical lathe except the shaping
or cutting tool is replaced with a laser beam. One could call
the laser beam a virtual tool bit. A pair of programmable collets
holds the device in place and a direct drive system rotates the
device around its rotational axis. This tooling fixture is mounted
on a XY translation stage to allow three degrees of freedom,
letting the laser beam to be positioned anywhere on the device.
Unlike a mechanical cutting tool where
the cutting tool is of a fixed length, the laser beam continues
to propagate towards the central axis of the device. When machining
plastics, an excimer laser, operating in the ultra-violet region,
has unique properties that make it ideal for medical devices.
The laser etches the plastic material such as pebax, polyurethane,
nylon, PET and bioabsorbable materials, as low as 0.1 micron
per laser pulse. This means that the laser beam acts like a surgeon's
scalpel, fashioning the material layer by layer, much like peeling
an onion. Unlike a metalworking lathe, the tool post cannot control
the depth of cutting to such precision.
To control the shape of the cutting tool,
the excimer laser beam is projected on to a mask whose pattern
follows the shape of the tool. The pattern of the mask is imaged
by an optical lens onto the device. Multiple laser beams can
be created. This virtual tool, created by the laser, can be configured
to any shape such as a circle, square or hexagon. The Laser
Lathe employs an automatic mask changer that allows any pattern
to be shuttled into the beam path, permitting on-the-fly machining.
To micromachine plastic tubular devices,
the Resonetics Laser Lathe incorporates a number of novel technologies:
Long Line
Homogenization Optical Technology (LLO)
can greatly lengthen the tool path, affording much higher throughputs.
An untreated excimer laser can produce a virtual tool with a
length of 3 mm. Using Resonetics proprietary LLO, beam lengths
up to 300mm can be produces for specific applications, drastically
reducing cost.
Photo of LLO
Often plastic
catheters are required to navigate small tortuous vessels. These
catheters have small wall diameters. Holding the catheter taut
during the laser process is important but what is equally important
is Dynamic Tension Control (DTC). Why? Imagine the laser
beam is drilling small holes in a catheter. If the catheter is
under slight tension (or expansion), then upon release, the hole
diameter will change and may go out of specification. By monitoring
the tension, the Laser Lathe can drill repeatable holes, taking
into account the elasticity of the tubular device.
Direct
Drive Technology (DDT) uses encoded
rotary stages to rotate the tubular devices, not belt-driven
modules, to ensure the rotational motion is precise and repeatable.
Belt-driven systems can slip, resulting in gross errors.
PHOTO of DDT
A byproduct
of plastic tubular devices is that, despite using Dynamic Tension
Control (DTC), the device may not be perfectly straight. Too
much tension may cause other issues. Resonetics deploys Automatic
Runout Compensation (ARC) where the Laser Lathe dynamically
senses the longitudinal consistency and compensates on-the-fly,
using high speed real-time machine vision and an integrated motion
system. This technology allows the laser to drill small holes
along the center line of the catheter even if it's meandering
like the Colorado River.
To laser drill
tiny holes in angioplasty balloons, stepped hypo tubes are inserted
into the balloon to inflate the balloon prior to laser drilling.
The ARC technology (Automatic Runout compensation) and
AMT (Automatic Mapping Technology) may be used to account
for balloon symmetry that may affect run out.
To account
for a device with significant surface profile undulations that
might result in the laser being out of focus, then AFT (Auto
focus Technology) is deployed. The surface profile is monitored
and the information is fed back to the process computer to dynamically
control the Z axis.
In some cases,
whether laser stripping or etching a material such as plastic
or ceramic from an underlying layer, the coating thickness can
vary from part-to-part. Therefore it is not possible to preprogram
the laser with a fixed number of pulses since the coating thickness
varies. Resonetics deploys proprietary End Point Detection
Technology (EPD) where the laser system detects the underlying
layer before the laser reaches it and automatically compensates
the laser drilling accordingly, ensuring that the underlying
layer is not damaged or compromised.
Techniques
List
Glovebox Technology (GBT):
When laser drilling or cutting plastic
materials, they can shift or become mechanically unstable. Resonetics
employs Glovebox Technology (GBT) where the device is environmentally
sealed in a nitrogen-filled glovebox to "freeze" the
material. In this manner, the plastic behaves like a metal, allowing
precise laser machining to take place.
Other process gases can be fed into the
glovebox system to change the processing environment. This can
help minimize debris, discoloration or oxidation
PHOTO of Glove Box system
Techniques
List
Holographic Optics:
To laser drill a high density of holes,
holographic optics can produce a dense array of laser beams (potentially
thousands of beamlets) at one time to accelerate process times.
This method is generally reserved for high volume manufacturing.
Techniques
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