Mask orbiting for laser ablated feature formation

Details Mask orbiting for laser ablated feature formation Mask orbiting for laser ablated feature formation

1998-11-20

2001-11-06

Evans, Geoffrey S. (Department: 1725)

Electric heating

Metal heating

By arc

C219S121710, C219S121730

Reexamination Certificate

active

06313435

ABSTRACT:

BACKGROUND OF INVENTION
1. Field of Invention
This invention pertains to a method and an apparatus that forms ablated features in substrates such as by laser ablation of polymer substrates for inkjet print head applications.
2. Description of the Related Art
The laser ablation of features on polymer materials using a mask and imaging lens system is well known. In this process, features on the mask are illuminated with laser light. The laser light that passes through the transparent features of the mask is then imaged onto the substrate such as a polymeric film where the ablation process occurs.
FIG. 1
illustrates a basic layout of a conventional excimer laser machining system
10
. Typically, the system
10
is controlled by a computer
12
with an interface to the operator of the system. The computer
12
controls the firing of the pulsed laser system
24
and a low speed, low resolution servo system
14
. The function of the servo system
14
is to position the mask
16
and substrate chuck
18
for proper registration of the laser milled pattern with respect to other features on the substrate
19
prior to ablation of substrate
19
. For this purpose, a vision system (not shown) is often interfaced to the computer system. The servo system
14
or computer
12
may control an attenuator module
20
, to vary the amount of UV radiation entering the system. Alternatively, the laser pulse energy may be varied by adjusting the laser high voltage or a control set point for energy, maintained by the laser's internal pulse energy control loop.
The UV beam path is indicated in this figure with arrows
22
(not intended to be actual ray paths, which are not typically parallel) which show the flow of UV energy within the system. The UV power originates at the pulsed excimer laser
24
. The laser
24
typically fires at 100-300 Hz for economical machining with pulses that have a duration of about 20-40 nanoseconds each. The typical industrial excimer laser is 100-150 watts of time average power, but peak powers may reach megawatts due to the short duration of the pulse. These high peak powers are important in machining many materials.
From the output end of the laser, the UV energy typically traverses attenuator
20
; however, this is an optional component not present in all laser machining systems. The attenuator
20
performs either or both of two possible functions. In the first function, the attenuator
20
compensates for the degradation of the optical train. The attenuator
20
thus used, allows the laser to run in a narrow band of pulse energies (and hence a restricted range of high voltage levels), allowing for more stable operation over the long term. With new optics in the system, the attenuator
20
is set to dissipate some of the power of the laser. As the optics degrade and begin to absorb energy themselves, the attenuator
20
is adjusted to provide additional light energy. For this function, a simple manual attenuator plate or plates can be used. The attenuator plates are typically quartz or fused silica plates with special dielectric coatings on them to redirect some of the laser energy toward an absorbing beam dump within the attenuator housing.
The other possible function of the attenuator
20
is for short term control of laser power. In this case, the attenuator
20
is motorized with either stepper motors or servo system, and the attenuator is adjusted to provide the correct fluence (energy per unit area) at the substrate for proper process control.
From the attenuator
20
, the UV energy propagates to a beam expansion telescope
26
(optional). The beam expansion telescope
26
serves the function of adjusting the cross sectional area of the beam to properly fill the entrance to the beam homogenizer
28
. This has an important effect on the overall system resolution by creating the correct numerical aperture of illumination upon exit from the homogenizer. Typical excimer laser beams are not symmetric in horizontal vs. vertical directions. Typically, the excimer beam is described as “top hat-gaussian,” meaning that between the laser discharge direction (usually vertical), the beam profile is “top hat” (initially relatively flat and dropping off sharply at the edges). In the transverse direction, the beam has a typical intensity profile that looks qualitatively gaussian, like a normal probability curve.
The expansion telescope
26
allows some level of relative adjustment in the distribution of power in these directions, which reduces (but does not completely eliminate) distortion of the pattern being imaged onto the substrate
19
due to the resolution differences in these two axes.
Between the expansion telescope
26
and homogenizer
28
is shown a flat beam folding mirror
30
. Most systems, due to space limitations, will contain a few such mirrors
30
to fold the system into the available space. Generally, the mirrors may be placed between components, but in some areas, the energy density or fluence can be quite high. Therefore, mirror locations are carefully chosen to avoid such areas of high energy density. In general, the designer of such a system will try to limit the number of folding mirrors
30
in order to minimize optics replacement cost and alignment difficulty.
The UV light next enters the beam homogenizer
28
. The purpose of the homogenizer
28
is to create a uniformly intense illumination field at the mask plane. It also determines the numerical aperture of the illumination field (the sine of the half angle of the cone of light impinging on the mask), which as stated above, has an impact on overall system resolution. Since certain parts of the excimer beam are hotter than others, uniform illumination requires that the beam be parsed into smaller segments which are stretched and overlaid at the mask plane. Several methods for this are known in the art, with some methods being based on traditional refractive optics, e.g., as disclosed in U.S. Pat. Nos. 4,733,944 and 5,414,559, both of which are incorporated herein by reference. The method may also be based on diffractive or holographic optics, as in U.S. Pat. No. 5,610,733, both of which patents are incorporated by reference, or on continuous relief microlens arrays (described in “Diffractive microlenses replicated in fused silica for excimer laser-beam homogenizing”, Nikoladjeff, et. al, Applied Optics, Vol 36, No. 32, pp. 8481-8489, 1997).
From the beam homogenizer
28
the light propagates to a field lens
32
, which serves to collect the light from the homogenizer
28
and properly couple it into the imaging lens
34
. The field lenses
32
may be simple spherical lenses, cylindrical lenses, anamorphic or a combination thereof, depending on the application. Careful design and placement of field lenses
32
are important in achieving telecentric imaging on the substrate side of the lens
32
.
The mask
16
is typically placed in close proximity to the field lens
32
. The mask
16
carries a pattern that is to be replicated on the substrate
19
. The pattern is typically larger (2 to 5 times) than the size of the pattern desired on the substrate
19
. The imaging lens
34
is designed to de-magnify the mask
16
in the course of imaging it onto the substrate
19
. This has the desired property of keeping the UV energy density low at the mask plane and high at the substrate plane. High de-magnification usually imposes a limit on the field size available at the substrate plane.
The mask
16
may be formed from chromium or aluminum coated on a quartz or fused silica substrate with the pattern being etched into the metallic layer by photolithography or other known means. Alternatively, the reflecting and/or absorbing layer on the fused silica mask substrate
16
may comprise a sequence of dielectrics layers, such as those disclosed in U.S. Pat. Nos. 4,923,772 and 5,298,351, both of which are incorporated herein by reference.
The purpose of the imaging lens
34
is to de-magnify and relay the mask pattern onto the substrate
19
. If the pattern is reduced by a factor of M in each dimension, the

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