Laser processing method to form an ink jet nozzle plate

A laser processing method irradiates an excimer laser beam at a surface of an object composed of a polymer material that absorbs the excimer laser beam. The polymeric object is substantially free of lubricants that reflect wavelengths within the range of operation of the excimer laser beam. The invention also relates to a nozzle plate of an ink jet device, which does not contain such lubricants, produced with an excimer laser beam.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a laser processing method which conducts 
processing by irradiating an excimer laser beam at an object. 
2. Description of the Related Art 
Conventionally, the use of an excimer laser beam in processing is proposed 
in Japanese Unexamined Patent Publication No. 4-9291, which describes the 
method of processing by irradiating an excimer laser beam at a sheet 
composed of polymer materials such as polyimide or polysulfone. 
In the field of ink jet printers, it is known to form a nozzle plate of the 
ink jet device by forming orifices in the nozzle plate material using a 
laser. See for example U.S. Pat. No. 5,208,604, which is totally 
incorporated herein by reference, which proposes a method of fabricating a 
nozzle plate for an ink jet printer comprising steps of forming a 
water-repellent film over the surface of a plate and forming a discharge 
hole through both the plate and the water-repellent film with an excimer 
laser beam or the like. The plate material may be made of a polyimide 
material such as SE-320 manufactured by Tokyo Ohka Kogyo Co., Ltd. 
Processing of a polymeric workpiece with an excimer laser beam is a laser 
ablation process. According to Chemical Reviews, vol. 89, no. 6, pgs. 
1303-1316 (1989), incorporated herein by reference, the mechanism of 
ablation has three steps as seen in FIG. 3 of the reference. A polymeric 
workpiece absorbs an excimer laser beam transmitted through a mask, high 
molecules of the polymeric workpiece are disintegrated, and the molecules 
are decomposed and atoms are scattered in the process. 
In general, SiO.sub.2 or similar materials have been added as a lubricant 
to polymer materials which form the object, for example a nozzle plate for 
an ink jet device, to be treated by an excimer laser beam. Manufacturers 
conventionally add lubricants to such polymer materials because many end 
uses of the polymeric plates require slipping of the surfaces. For 
example, the polymeric plates are also used in the formation of flexible 
circuit plates which in use are stacked on top of each other, requiring 
that the surfaces have slipperiness to avoid scratching and breaking. 
Other uses for such polymer plates, such as insulation materials, are not 
adversely affected by the presence of lubricants, so that manufacturers 
find it efficient to include lubricants in all polymer plate materials 
because of the ease of having one manufacturing process. 
However, within the ultraviolet ray region, which is the region of excimer 
laser beam wavelengths, Si has a high reflectivity for excimer laser beams 
(Laser Thermal Processing Research Institute documentation, 1990, No. 23). 
Consequently, the problem arises that when particles of Si are at the 
location being processed, the shape to be formed in the polymer plate 
during processing becomes poor, as shown in FIG. 3. 
SUMMARY OF THE INVENTION 
The present invention was designed in order to resolve the above and other 
problems, and it is an object of the present invention to provide a laser 
processing method which results in a favorable shape. 
In order to achieve the above and other objects, the invention is a laser 
processing method, wherein processing is conducted by irradiating an 
excimer laser beam. The method comprises irradiating an excimer laser beam 
at an object composed of a polymer material that absorbs the excimer laser 
beam, while excluding from the object a highly reflective lubricant that 
makes the surface slippery. 
In a preferred embodiment, the excluded lubricant is a silicon oxide or 
silicon oxide compound. In another preferred embodiment, the irradiated 
object is the nozzle plate of an ink jet device which composes images by 
jetting ink from a nozzle. 
With the laser processing method of the present invention, having the 
above-described configuration, the irradiated object can be processed into 
a favorable shape by irradiating an excimer laser beam at the object which 
is composed of a polymer material that absorbs the excimer laser beam, 
while not containing a highly reflective lubricant that makes the surface 
slippery.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 is a drawing showing the configuration of a complete laser 
processing device. With the laser processing device of the present 
embodiment, an excimer laser beam 2 emitted from an oscillator 1 forms an 
optical path that reaches the processing table 8 via mirrors 3a, 3b and 
3c. A beam expander 4 which enlarges the excimer laser beam 2 to the 
desired size is provided on the optical path between mirror 3a and mirror 
3b. An aperture member (unrepresented) which cuts off excess portions of 
the beam is provided on the side of this beam expander from which the 
laser beam is incident. Between mirror 3b and mirror 3c is provided a mask 
5 in order to confer the shape corresponding to the hole that is to be 
made by the excimer laser beam 2. In addition, following the mask 5 is a 
field lens 6 used to guide the mask image, which has passed through the 
mask 5, to the imaging optical system 7. This imaging optical system 7 is 
provided between mirror 3c and the processing table 8, and is a system 
used to restrict the excimer laser beam 2 which has passed through the 
mask 5 to the desired size on the object 9 of processing, which is placed 
on the processing table 8. 
Lubricants that are to be excluded from the present invention are those 
lubricants that have high reflectivities for excimer laser beams, i.e., 
reflect ultraviolet ray wavelengths of 10 to 400 nm, particularly 50 to 
300 nm. Examples include, without limitation, silicon oxides such as 
SiO.sub.2 and silicon oxide compounds, calcium hydrogenphosphate and 
calcium hydrogencarbonate. In a preferred embodiment, the silicon oxide 
lubricant is excluded. Lubricants that do not reflect wavelengths within 
the range of operation of excimer laser beams may be present in the 
polymer object to be irradiated, if desired. 
The wavelength of reflectivity of a lubricant can be determined in a 
simple, conventionally known manner. The method of the present invention 
includes determining the wavelength of reflectivity of a desired 
lubricant, and selecting polymer materials, which are to form the sheet to 
be irradiated with the excimer laser beam, which are free of or 
substantially free of the lubricant if it reflects wavelengths within the 
range of operation of the excimer laser beam, i.e., within the range of 10 
to 400 nm. 
In a preferred embodiment, the irradiated object is the nozzle plate of an 
ink jet device which composes images by jetting ink from a nozzle. 
The object 9 to be processed is a polyimide sheet which does not include 
SiO.sub.2 particles that would make the surface more slippery. The 
thickness of this sheet is 100 .mu.m. With the present invention, 60 
nozzles that jet ink are made in a single row on the object 9. The intake 
diameter of the nozzles is 80 .mu.m, while the jet spray diameter is 40 
.mu.m. The pattern diameter of the mask 5 is 400 .mu.m, so that the 
process compression ratio is one-fifth. The excimer laser beam 2 used in 
the present embodiment is a KrF excimer laser beam with a wavelength of 
248 nm. 
The shape of a nozzle that has been processed by an excimer laser beam 
according to the above described process, conditions and materials on the 
polyimide sheet containing no SiO.sub.2 particles is shown in FIG. 2. As 
is clear from FIG. 2, the shape of the nozzle is an extremely smooth 
circle. Accordingly, the precision of the dimensions of the nozzle is 
good, so that there is no dispersion in the direction of flight of ink 
droplets. Printing quality is very good with an ink jetting device that 
uses this nozzle. 
The shape of a nozzle that has been processed by an excimer laser beam 
identically to the process, conditions and materials used in producing the 
nozzle shown in FIG. 2, with the exception that the polyimide sheet 
contains 1500 ppm SiO.sub.2, is shown in FIG. 3. As can be seen in FIG. 3, 
the presence of the SiO.sub.2 lubricant resulted in a poor shape for the 
nozzle formed by the excimer laser. 
As is clear from the above explanation, with the laser processing method of 
the present invention, an excimer laser beam is irradiated at an object to 
be processed which is composed of a polymer material that absorbs the 
excimer laser beam, while not containing a lubricant that makes the 
surface slippery. Consequently, it is possible for the excimer laser beam 
to be absorbed well by the object of processing without reflection, so 
that the result is a favorable shape. 
While a polyimide sheet is described in the present embodiment, other 
preferred polymer materials can also be used, such as for example 
polysulfone, polyether sulfone, polyphenylene oxide or polypropylene. 
Other polymer materials that are processable by an excimer laser include 
polyesters, epoxies, polycarbonates and polyurethanes, although these 
materials are less preferred as nozzle plates in ink jet devices. 
FIG. 4 shows another apparatus for forming a nozzle of an ink jet device 
using an excimer laser. An excimer laser beam 11 emitted by an excimer 
laser 14 is projected through a mask 12 having an aperture similar to a 
desired shape in which the nozzle orifice is to be formed toward the 
polymer plate 15 of the nozzle plate 30. The excimer laser beam 11 is 
focused on the polymer plate 15 for laser machining. The excimer laser is 
preferably a KrF excimer laser that emits an excimer laser beam of 248 nm 
in wavelength. The mask 12 and lens 13 are designed properly taking into 
consideration the shape of the nozzle orifice and laser machining 
conditions. The lens 13 is preferably a reducing lens having a reduction 
ratio of 1/5, for example. 
The present invention is not limited to the above-described embodiments, 
for various changes may be made without departing from the spirit and 
scope of the invention. For example, the thickness of the object 9 is not 
limited to 100 .mu.m, but can range for example from 1 to 1,000 .mu.m, 
preferably 5 to 200 .mu.m, most preferably 50 to 100 .mu.m. Furthermore, 
the shape of the nozzle is not limited to a circle, but also may be an 
ellipse or a rectangle. 
In addition, the excimer laser beam 2 in the present embodiment was a KrF 
excimer laser beam, but the excimer laser beam could be an ArF excimer 
laser beam or a XeCl excimer laser beam or the like.