Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF AND APPARATUS FOR FORMING NOZZLES
The present invention relates to methods and apparatus for forming a
nozzle in a nozzle plate for an ink jet printhead, the nozzle having a nozzle
inlet and a nozzle outlet in respective opposite faces of sard nozzle plate.
W093/15911 concerns methods of forming nozzles in a nozzle plate
for an inkjet printhead utilising a high energy beam, in particular the
ablation
of nozzles in a polymer nozzle plate using an excimer laser. By means of a
mask having a single aperture, a high energy beam is shaped prior to being
directed by a converging lens onto the surface of a nozzle plate where a
nozzle is formed.
W093/15911 recommends increasing the divergence of the beam
incident into the aperture of the mask by passing the beam through a layer
such as a ground or etched surface or a thin film containing a medium
having suitable light-scattering properties such as a colloid or opalescent
material. Such a layer may be placed against a convergent lens which Is
itself located upstream of the mask for the purpose of focusing the beam
into the aperture.
The divergence of the beam will determine the angle of taper of the
nozzle. Furthermore, a second mask can be used to reduce the angle of
divergence in one plane of the beam relative to another (both planes
containing the beam axis), thereby to obtain a nozzle having a greater
nozzle taper in one plane than in another. This will result in a nozzle inlet
that is larger in one direction than in another direction perpendicular
thereto
- W093/15911 points out that this advantageously allows the nozzle ink inlet
to match the (generally rectangular) shape of an ink channel in the printhead
with which the nozzle will communicate, whilst allowing the nozzle outlet to
remain preferably circular.
The present invention has as its objective improvements to the
processes described in the aforementioned W093/15911, in particular to the
manner in which the nozzle taper and the shape of the nozzle inlet and
outlet are controlled.
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According ,=
to a first aspect, the present invention comprises the
method of forming a nozzle in a nozzle plate for an ink jet printhead, the
nozzle having a nozzle inlet and a nozzle outlet in respective opposite faces
of said nozzle plate, the method comprising the steps of: 5 directing a high
energy beam towards said nozzle plate; introducing
divergence into said beam; thereafter directing said beam at a single
aperture of a mask, thereby to shape said beam; and thereafter passing said
beam through beam converging means prior to impingement on the face of
said nozzle plate in which said nozzle outlet is formed, thereby to form a
nozzle, the nozzle outlet being conjugate through said beam converging
means with said single aperture; wherein
the step of introducing divergence into said beam comprises splitting
said beam into a number of sub-beams, each sub-beam having divergence,
the origin of divergence of each sub-beam lying apart from the point at
which the respective sub-beam is created by splitting; the sub-beams
thereafter being passed through further beam converging means prior to
being recombined and directed through said single aperture of a mask,
wherein the dimensions of the section of said recombined beam directly prior
to impinging the plane of said mask are substantially equal to the
dimensions of the aperture in said mask.
The present invention includes the step of introducing divergence into
said beam'by splitting said beam into a number of sub-beams, each sub-
beam having divergence, the origin of divergence.of each sub-beam lying
apart from the point at which the respective sub-beam is created by
splitting, the sub-beams thereafter being passed through further beam
converging means prior to being recombined. This arrangement allows
substantially more accurate control of the angle of divergence of the beam
than has been possible in prior art arrangements: as mentioned,
W093/15911 proposes increasing the divergence of the high energy beam
by scattering the light using a ground or etched surface or a thin film
containing a medium having suitable light-scattering properties. It has been
recognised in the present invention that divergence can be obtained in a
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=~
much more controlled manner by splitting the high energy beam into a
number of sub-beams which are subsequently recombined. Furthermore,
the beam is split such that each sub-beam has divergence having an origin
at a point lying apart from the point at which the respective sub-beam is
created by splitting. It will be appreciated that the divergertce obtained In
this manner - which may be achieved using a lens to create each sub-
beam - will be subject to substantially less variation than is achieved using
prior art methods based on scattering. It follows that less variation in the
angle of divergence of the combined beam will result in less variation in the
angle of taper of the manufactured nozzles - resulting in better Ink ejection
performance of the final inkjet printhead.
Furthermore, by directing the recombined beam through a single
aperture of a mask, with the dimensions of the section of the recombined
beam directly prior to impinging the plane of said mask being substantially
equal to the dimensions of the aperture in said mask, the high energy beam
that finally impinges on the nozzle plate to form a nozzle does not have its
divergence reduced by any significant amount by the mask. Consequently;
the full range of beam divergence is available to form nozzle bores having a
correspondingly high taper angle from outlet to inlet.
According to a second aspect, the invention comprises the method of
forming a nozzle in a nozzle plate for an ink jet printhead, the nozzle having
a nozzle inlet and a nozzle outlet in respective opposite faces of said nozzle
plate, the method comprising the steps of:
directing a high energy beam towards said nozzle plate; introducing
divergence into said beam; and thereafter passing said beam through beam
converging means prior to impingement on said nozzle plate, thereby to form
a nozzle; wherein
the step of introducing divergence into said beam comprises passing
said beam through an array of optical elements to create an array of sub-
beams, each sub-beam having divergence, the origin of the divergence of
each sub-beam lying apart from the respective optical element; said array of
sub-beams being thereafter directed towards first reflecting means for
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towards second r.eflecting means, said second reflecting means
reflecting
reflecting towards said nozzle plate; the positional relationship of said
first
and second reflecting means being such that a parallel beam impinging on
said first reflecting means is reflected from said second reflecting means as
5 a converging beam; the arrangement of said optical elemehts being such .
that all incoming sub-beams are directed by said first reflecting means
towards said second reflecting means, thereafter to impinge on said nozzle
plate.
This second aspect of the invention also utilises the concept of
splitting (by means. of an array of optical elements) a high energy beam Into
sub-beams having an origin of divergence lying apart from the plane of
beam splitting and thereafter recombining the sub-beams through beam
converging means. It therefore shares with the first aspect of the invention
the advantage that the resulting angle of the beam can be accurately
controlled.
In addition, the high energy beam is directed at the nozzle plate by
means of first and second reflecting means and the optical elements in said
array - e.g. lenses - are arranged such that all sub-beams impinging on the
first reflecting means are directed towards the second reflecting means and
not elsewhere e.g. back towards the array of.lenses. This measure results
in less wastage of the beam and furthermore avoids damage to other
elements in the system by stray laser light. Such system elements may
include lenses, turning mirrors and even the laser itself - located "upstream
of the first and second reflecting means.
A third aspect of the present invention comprises the method of
forming a nozzle in a nozzle plate for an ink jet printhead, the nozzle having
a nozzle inlet and a nozzle outlet in respective opposite faces of said nozzle
plate and a nozzle bore having an axis; the method comprising the steps of:
directing a high energy beam towards said nozzle plate; introducing
divergence into said beam; and thereafter passing said beam through beam
converging means prior to impingement on said nozzle plate, thereby to form
a nozzle; wherein
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the- step of introducing divergence into said beam comprises passing
said beam through an array of optical elements to create an array of sub-
beams, each sub-beam having divergence, the origin of divergence of each
sub-beam lying apart from the respective optical element; said array of sub-
beams having a greater width in a first direction than in a 3econd direction
orthogonal to said first direction, said first and second directions lying
perpendicular to the direction of impingement of said beam on said array;
thereafter passing said array of sub-beams through beam converging
means prior to their impingement on the nozzle plate, thereby to form said
nozzle; the angle of taper of the nozzle bore relative to the nozzle axis in a
direction corresponding to said first direction being greater than the angle
of
taper of the nozzle bore in a direction corresponding to said second
direction.
The third aspect of the invention again shares the concept of splitting
a high energy beam into sub-beams having an origin of divergence lying
apart from the plane of beam splitting and thereafter recombining the sub-
beams through beam converging means. This third aspect also comprises
an array of optical elements having a greater width in a first direction than
in
a second direction orthogonal to said first direction, which allows the
production in a simple and accurate manner of nozzles having a greater
taper angle in one direction than in another. This in turn yields a nozzle
inlet
having a greater dimension in one direction than in the direction orthogonal
thereto - such a configuration may be particularly desirable where the ink
supply channel to which the nozzle is attached is also non-axi-symmetric.
A fourth aspect of the present invention comprises a method of
forming a nozzle in a nozzle plate for an ink jet printhead, the nozzle having
a nozzle inlet and a nozzle outlet in respective opposite faces of said nozzle
plate, characterised by the steps of:
directing a high energy beam having a first axis extending in a first
direction towards said nozzle plate; directing said beam at a first reflecting
surface lying at an angle to said first direction, said surface being arranged
so as to reflect said beam towards a second reflecting surface so arranged
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as to both invert said beam and direct said beam along an
axis collinear with said first axis extending in a first
direction; said first and second surfaces being fixedly
located relative to one another, thereby to form an
assembly, and rotating said assembly about said first axis;
said beam thereafter impinging on said nozzle plate, thereby
to form a nozzle.
As explained in greater detail in the description
that follows, this technique results in a high energy beam
having a uniform intensity at a given radius and, when
applied to the manufacture of nozzles, yields nozzle
dimensions lying within tighter tolerance bands and
consequently a better quality nozzle.
A method of forming a nozzle in a nozzle plate for
an inkjet printhead, the nozzle having a nozzle inlet and a
nozzle outlet in respective opposite faces of the nozzle
plate according to a fifth aspect of the invention includes
the step of directing a high energy beam at the face of the
nozzle plate in which said nozzle outlet is to be formed,
whereby the power of said high energy beam is initially held
low and is increased with increasing depth of the nozzle
formed in said nozzle plate. As is also explained in
greater detail in the description hereafter, this technique
gives a higher quality nozzle outlet, better internal finish
and a more accurate nozzle shape.
According to another aspect of the present
invention, there is provided method of forming a nozzle in a
nozzle plate for an ink jet printhead, the nozzle having a
nozzle inlet and a nozzle outlet in respective opposite
faces of said nozzle plate, the method comprising the steps
of: directing a high energy beam having a first axis
extending in a first direction towards said nozzle plate;
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directing said beam at a first reflecting surface lying at
an angle to said first direction, said surface being
arranged so as to reflect said beam towards a second
reflecting surface so arranged as to both invert said beam
and direct said beam along an axis collinear with said first
axis extending in a first direction; said first and second
surfaces being fixedly located relative to one another,
thereby to form an assembly, and rotating said assembly
about said first axis; directing said high energy beam
towards said nozzle plate; introducing divergence into said
beam; thereafter directing said beam at a single aperture of
a mask, thereby to shape said beam; and thereafter passing
said beam through beam converging means prior to impingement
on the face of said nozzle plate in which said nozzle outlet
is formed, thereby to form a nozzle, the nozzle outlet being
conjugate through said beam converging means with said
single aperture; wherein the step of introducing divergence
into said beam comprises splitting said beam into a number
of sub-beams, each sub-beam having divergence, the origin of
divergence of each sub-beam lying apart from the point at
which the respective sub-beam is created by splitting; the
sub-beams thereafter being passed through further beam
converging means prior to being recombined and directed
through said single aperture of a mask, wherein the
dimensions of the section of said recombined beam directly
prior to impinging the plane of said mask are substantially
equal to the dimensions of the aperture in said mask.
According to still another aspect of the present
invention, there is provided method of forming a nozzle in a
nozzle plate for an ink jet printhead, the nozzle having a
nozzle inlet and a nozzle outlet in respective opposite
faces of said nozzle plate and a nozzle bore having an axis;
the method comprising the steps of: directing a high energy
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beam having a first axis extending in a first direction
towards said nozzle plate; directing said beam at a first
reflecting surface lying at an angle to said first
direction, said surface being arranged so as to reflect said
beam towards a second reflecting surface so arranged as to
both invert said beam and direct said beam along an axis
collinear with said first axis extending in a first
direction; said first and second surfaces being fixedly
located relative to one another, thereby to form an
assembly, and rotating said assembly about said first axis;
said beam thereafter impinging on said nozzle plate, thereby
to form a nozzle directing said high energy beam towards
said nozzle plate; introducing divergence into said beam;
and thereafter passing said beam through beam converging
means prior to impingement on said nozzle plate, thereby to
form a nozzle; wherein the step of introducing divergence
into said beam comprises passing said beam through an array
of optical elements to create an array of sub-beams, each
sub-beam having divergence, the origin of the divergence of
each sub-beam lying apart from the respective optical
element; said array of sub-beams having a greater width in a
first direction than in a second direction orthogonal to
said first direction, said first and second directions lying
perpendicular to the direction of impingement of said beam
on said array; thereafter passing said array of sub-beams
through beam converging means prior to their impingement on
the nozzle plate, thereby to form said nozzle; the angle of
taper of the nozzle bore relative to the nozzle axis in a
direction corresponding to said first direction being
greater than the angle of taper of the nozzle bore in a
direction corresponding to said second direction.
According to yet another aspect of the present
invention, there is provided method of forming a nozzle in a
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nozzle plate for an ink jet printhead, the nozzle having a
nozzle inlet and a nozzle outlet in respective opposite
faces of said nozzle plate, the method comprising the steps
of: directing a high energy beam having a first axis
extending in a first direction towards said nozzle plate;
directing said beam at a first reflecting surface lying at
an angle to said first direction, said surface being
arranged so as to reflect said beam towards a second
reflecting surface so arranged as to both invert said beam
and direct said beam along an axis collinear with said first
axis extending in a first direction; said first and second
surfaces being fixedly located relative to one another,
thereby to form an assembly, and rotating said assembly
about said first axis; said beam thereafter impinging on
said nozzle plate, thereby to form a nozzle directing said
high energy beam towards said nozzle plate; introducing
divergence into said beam; and thereafter passing said beam
through beam converging means prior to impingement on said
nozzle plate, thereby to form a nozzle; wherein the step of
introducing divergence into said beam comprises passing said
beam through an array of optical elements to create an array
of sub-beams, each sub-beam having divergence, the origin of
the divergence of each sub-beam lying apart from the
respective optical element; said array of sub-beams being
thereafter directed towards first reflecting means for
reflecting towards second reflecting means, said second
reflecting means reflecting towards said nozzle plate; the
positional relationship of said first and second reflecting
means being such that a parallel beam impinging on said
first reflecting means is reflected from said second
reflecting means as a converging beam; the arrangement of
said optical elements being such that all incoming sub-beams
are directed by said first reflecting means towards said
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second reflecting means, thereafter to impinge on said
nozzle plate.
According to a further aspect of the present
invention, there is provided method of forming a nozzle in a
nozzle plate for an ink jet printhead, the nozzle having a
nozzle inlet and a nozzle outlet in respective opposite
faces of said nozzle plate, characterised by the steps of:
directing a high energy beam having a first axis extending
in a first direction towards said nozzle plate; directing
said beam at a first reflecting surface lying at an angle to
said first direction, said surface being arranged so as to
reflect, said beam towards a second reflecting surface so
arranged as to both invert said beam and direct said beam
along an axis collinear with said first axis extending in a
first direction; said first and second surfaces being
fixedly located relative to one another, thereby to form an
assembly, and rotating said assembly about said first axis;
said beam thereafter impinging on said nozzle plate, thereby
to form a nozzle.
The present invention also comprises apparatus for
carrying out the methods outlined above.
The invention will now be described by way of
example by reference to the following diagrams, of which:
Figure 1 is a schematic illustration of a first
embodiment of the present invention when viewed in a
direction X;
Figure 2 is a view of the apparatus of Figure 1 in
a direction Y orthogonal to direction X;
Figure 3 is a further embodiment of the present
invention;
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Figure 4a is a perspective view of yet another
embodiment of the present invention; Figure 4b is a
sectional view through the mirror arrangement 82, 84 of
Figure 4a;
Figure 5a is a sectional view through a beam
conditioning device
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according to the present invention; Figure 5b is a schematic diagram of the
beam section following conditioning;
Figures 6a and 6b illustrate the functioning of the device of Figure 5a
at rotation angles of 0 and 900 respectively.
Figure 1 shows an embodiment of apparatus for caMying out the
method according to one aspect of the present invention. Reference Figure
20 designates a nozzle plate in which a nozzle is to be formed. The
apparatus 10 comprises a source of a high energy beam such as a UV
excimer laser (not shown) which generates a high energy beam 30 which,
after having undergone various beam conditioning processes (e.g.
collimating, shaping of the beam to fit further optical devices located
"downstream"), is directed at an array 40 of optical elements which, in the
present example, are cylindrical lenses 45. Such an array of lenses is
commonly known as a flyseye lens.
The array 40 splits the beam into a corresponding array of sub-
beams 50, each sub-beam having a focal point 52. As will be clear from
the figure, after passing through the focal point 52, each sub-beam will be
divergent with a divergence angle (Aa, Ab in Figure 1) and an origin of
divergence at the focal point 52 of the respective lens 45 (note that for the
sake of clarity, only outlines of those beams issuing from the outermost
lenses of the array 40 have been shown; the beams from lenses nearer the
centre of the array will fall within these extremes). It will be appreciated
that
range Aa, Ab of angles of divergence of each sub-beam emanating from the
lenses 45 will be much narrower than the range that would be expected from
prior art techniques utilising scattering. As shown in Figure 1, the array of
sub-beams issuing from the array 40 is passed through a converging lens
60, thereby to recombine the sub-beams at 56.
The recombined beam is directed at the aperture 72 of a mask 70,
and to this end, the mask is preferably located at a distance from the lens
60 equal to the focal length of the lens.
Although in the example shown the focal point of the sub-beams 52
is located downstream of the array 40, any arrangement where the focal
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point of the sub-beams is iocated before the subsequent mask 70 will
suffice: the lenses in the array 40 may for example diverge the incoming
beam such that the origin of divergence is located upstream" of the array
40, for exampie. The strength of the subsequent converging lens 60 may be 5
chosen such that the sub-beams still recombine.
As mentioned above and shown in Figures 1-3, the dimensions.of the
section of the recombined beam directly prior to impinging the plane of the
mask are substantially equal to the dimensions of the aperture in said mask.
The recombined beam passing through the aperture (and indicated by 74 in
Figure 1) is subsequently guided by means of a further convergent lens 80
onto the surface 22 of the nozzle plate 20 where It ablates the material of
the nozzle plate, thereby forming a nozzle. The strength of the lens 80 and
the relative positions of the nozzle plate 20 and mask 70 are chosen such
that an image of the mask aperture 72, illuminated by the beam 56, is
projected onto the surface 22 of the nozzle plate. The nozzle section at the
surface 22 and the mask apertUre 72 can be seen to be conjugate through
the lens 80 and consequently, by changing the size of the aperture 72 the
size of the hole formed in the surface 22 (which forms the outlet orifice of
the resulting nozzle) can be altered.
As is evident from the figure, the sub-beams 74a, 74b making up
beam 74 strike the surface 22 of the nozzle plate at an angle, with the result
that the section of the hole ablated by the beams increases with the depth of
the ablated hole. The resulting nozzle is therefore tapered, with the nozzle
section at the "front" surface 22 of the nozzle plate 20 being determined by
the mask aperture 72 and the section at the "rear" surface 24 being
determined by both the aperture 72 and the angle of the incident beams.
The angle of the incident beams is determined both by the strength of
the lens 80 and by the angles of divergence present in the beam 74 passing
through the aperture 72. The former preferably lies in the range: 0.4<
numerical aperture <0.65 (corresponding to magnification of x25 and x52
respectively). The latter is determined by the strength of the lenses in the
array 40 and also the geometry of the array. As already mentioned, the
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features whereby divergence is introduced into the beam by splitting it into a
number of sub-beams, each sub-beam having divergence, allows the angle
of divergence of the nozzle forming beam to be controlled that much more
accurately. This in turn allows accurate control of the three-dimensional
shape of the resulting nozzle, in particular its taper angle and the sections
at
the nozzle outlet and inlet.
Ensuring that the dimensions of the section of the recombined beam
directly prior to impinging the plane of the mask are substantially equal to
the dimensions of the aperture in the mask, as mentioned above, ensures
that the high energy beam that finally impinges on the nozzle plate to form a
nozzle does not suffer any significant reduction in its divergence - which
might result in a corresponding reduction in nozzle taper. In practice, the
section of the recombined beam will have slightly greater dimensions than
the mask aperture: were the recombined beam to be smaller than the mask
aperture, then the mask would no longer play any masking function and the
image projected onto the front of the nozzle plate being not that of the
aperture but that of the flyseye lens. It will also be evident from Figure 1
that the matching between the dimensions of the aperture and the
recombined beam also means that the divergence angles (Ba, Bb in Figure
1) of the sub-beams 74a, 74b making up the recombined beam 74 at a
position downstream of the mask 70 correspond to the divergence angles Aa
and Ab of the sub-beams 50 upstream of the mask.
Figure 2 is a view in a direction Y orthogonal to the direction of
viewing X of Figure 1 and illustrates the case where the array 40 has a
rectangular geometry, being wider in the X direction than in the Y direction.
It can be seen that the angle of divergence of the beams leaving the
aperture 72 is correspondingly greater than that shown in Figure 1, as is the
angle of taper of the nozzle in this direction and thus the dimension of the
nozzle at the Nrear" surface of the nozzle plate (indicated by x2 in Figure 2
and greater than distance xl in Figure 1). The overall shape of the nozzle
at the rear surface will be rectangular, in correspondence with the geometry
of the array 40.
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it should be noted that geometry of the array 40 can be altered either
by rearranging the location of the lenses in the array or by blocking out
some of the lenses of an existing array e.g. by means of a mask placed
directly upstream of the array.
The individual lenses making up the array 40 each Contribute a
bundle of diverging beams, each bundle having a section which may be
circular or some other shape depending whether the optical elements
making up the array are lenses, prisms or otherwise having axi-symmetric
or some other shape respectively. Whilst this feature is instrumental in
obtaining many of the advantages described in the present application, It
nevertheless results in the aforementioned section of the nozzle at the "rear"
surface 24 having a corrugated outline. However, where this "rear" section
is circular, the corrugations can be avoided by rotating the flyseye lens
about
its polar axis during the course of the nozzle forming process.
An alternative method of influencing the angle of the incident beams
to control nozzle taper Is to interpose a further mask between the mask 70
and the lens 80. Such an arrangement is illustrated in Figure 3, the further
mask being designated by reference figure 110, the corresponding aperture
by 112. It is evident that the mask 110 blocks out those beams passing
through the aperture 72 which have divergence greater than a certain angle,
resulting in a nozzle with reduced inlet size x3. The dimensions and shape
of the further aperture can be varied to control the dimensions and shape of
the shape of the nozzle at the rear surface, as is known from the
aforementioned WO-A-93/15911.
Advantageously, a further converging ("field") lens can be located
directly upstream of the mask aperture 72, as indicated by reference figure
76 in Figure 3. Movement of this lens In its own plane, i.e. parallel to the
mask 70, allows the combined diverging sub-beams to be aligned with the
mask aperture. Nonalignment results in one side of the beam being
obscured more than another which in turn results in one side of the nozzle
having a lesser taper than the other. Such asymmetry is undesirable in a
nozzle.
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According to another preferred embodiment of the invention, there is
located upstream of the flyseye lens a variable beam attenuator (not shown
in the figures). Such devices are generally known in the art and for this
reason their construction will not be discussed here in any detail. In the
present invention, however, such a device is advantageougly employed to
control the power of the high energy beam during the nozzle formation
process: at the beginning of the nozzle formation process, laser power Is
held low to minimise damage to the nozzle outlet from exhaust products of
the ablation process. Power is then increased as the depth (and section) of
the forming nozzle increases. Towards the end of nozzle formation, high
laser power is employed to give the nozzle a good internal finish and to
ensure faithful reproduction of the shape of the nozzle forming beam. The
initial rate of increase of laser power is preferably low, even zero,
increasing
once the forming nozzle has attained a certain depth. Measurement of the
depth of the forming nozzle is not necessary: the power of the laser may be
controlled as a function of time, the time necessary for a given process to
reach a certain depth being readily determinable by experiment.
It will be apparent that many kinds of lens may be used for the
convergent lenses 60, 74 and 80 referred to above. However, it has been
found particularly advantageous to use for the lens 80 a lens comprising two
mirrors of the type generally known as a Cassegrain reflective lens. An
example is shown schematically in Figure 4a, the mask 70 and convergent
lens 60 having been omitted for the sake of clarity. Figure 4b shows the
mirrors 82, 84 in section, from which is clear that the mirrors are axi-
symmetric, having reflective surfaces that are surfaces of revolution. Such a
lens arrangement has a high magnification (equivalent to a high numerical
aperture value), allowing a high degree of angling of the incident beams
relative to the axis of the lens (equivalent to a lower angle of incidence
between beam and the surface 22 of the nozzle plate 20) and the formation
of nozzles of significant taper. Such lenses also exhibit low aberration since
the beam does not pass through any lens material but is simply reflected
from one surface to another. Finally, it will be appreciated from Figure 4
that
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the reflecting surfaces of such an arrangement are generally located away
from the surface of the nozzle plate and are thus less likely to be
contaminated by debris generated during the nozzle formation process.
The flyseye iens may advantageously be adapted for use with a lens
of the type described above by rendering the centrai lenses of the array
inoperative e.g. by removing the lenses or blocking them out as shown in
Figure 4. Blocking may be achieved by means of a mask located directly
upstream or downstream. The sub-beams from these central lenses might
otherwise reflect back into and damage optical elements (even the laser)
located upstream. In the embodiment shown, utilising an 6 x 6 array,of
lenses, the centre four ienses of the array are masked out.
Figure 5a shows apparatus that is particularly suited for use in the
manufacture of nozzles for inkjet printheads and in particular for use with
arrangements described above. Located upstream of the flyseye lens, the
device 120 comprises an assembly of three reflecting surfaces 121, 122,
123 held fixed relative to one another by means of a housing 124, the
assembly being rotatable together about an axis 125, for example in
bearings 126 by means of a motor (not shown). The incoming beam 30 is
directed along the axis 125, strikes surface 121 and is reflected to surface
122 and back to surface 123 whence it ieaves the device, again along the
axis 125. In the example shown, the reflecting surfaces 121,122,123 are
high reflectance dielectric mirrors.
The paths of top and bottom sections (30u, 301) of the beam at
different rotational angles of the device 120 are illustrated in Figures 6a
and
6b. When the device is at 00 rotation, as shown in Figure 6a, sections 30u
and 301 of the beam strike the reflecting surface 121 at different locations
along the axis 125 with the result that, following further reflection by
surfaces
122 and 123, the initially top and bottom sections 30u and 301 exit the
device at the bottom and top of the beam respectively. However, with the
device oriented at 90 as shown in Figure 6b, both bottom and top sections
of the beam strike the surface 121 at the same axial location and no
inversion of beam sections 30u and 301 occurs. At 180 rotation of the
CA 02507234 1997-01-16
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device (not shown), sections 30u and 301 will again strike surface 121 at
different locations along the beam axis with the result that inversion will
take
place.
It will therefore be evident that apparatus located downstream of the
rotating device 120 described above will be exposed to a 13eam 30' having
an intensity at a given point P at a radius r from the beam axis that varies
at
a frequency corresponding to twice the angular velocity of the housing 124
(see Figure 5b). Were the incoming beam 30 to be totally homogeneous, at
least at a given radius r from the beam axis, the point P would experience
no change in beam intensity. In practice, however, the beam 30 generated
by the laser is not homogeneous, even at a given radius, with the result that
the point P will experience a periodically varying beam intensity. Such a
varying intensity does nevertheless have the virtue of having the same
average value for all points irradiated by the beam which are located at a
radius r from the beam axis. Since beam intensity at a point translates Into
rate of material removal at the nozzle plate, use of the device described
above results in nozzles that are more uniform (at least at a given nozzle
radius) than would be obtained using a beam not subject to such
conditioning.
The use of discrete reflecting surfaces 121, 122 and 123 is
particularly appropriate in a device employing a high energy beam: these
have the advantage of low aberration when used with high energy beams,
as well having lower losses and being more robust than conventional
lenses/prisms. In the example shown above, high reflectance dielectric
mirrors are used.
It should be noted that other types of beam homogeniser, as are well
known in the art, may be used in place ofrn addition to the beam
conditioning device just described.
A further imperfection in real-life optic systems is the presence of
stray beams caused by imperfections in the optical elements making up the
system: such stray beams, if allowed to hit the nozzle plate, may result in a
nozzle that deviates from the ideal. This can be avoided by the use of a
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spatial filter, shown by way. of example in Figure 3, and comprising a mask
130 placed just in front of the nozzle plate at the point where the beams
cross prior to impinging on the nozzle plate. The aperture in the mask is
chosen to pass the nozzle-forming beatn yet exclude any stray beams
failing outside of the nozzle-forming beam. The accuracybf the aperture is
therefore crucial. Advantageously, the aperture can be formed by the insitu
ablation of a mask blank using the same beam and optics subsequently
used for nozzle ablation. The material of the mask blank should of course
be chosen such that, unlike the nozzle plate material, it does not ablate
significantly under the action of stray beams.
A further process step for in&easing the quality of the manufactured
nozzles is to carry out the ablation process in an atmosphere of Helium or
Oxygen. Accordingly, the nozzle plate is placed in a chamber supplied with
the appropriate gas and having a window through which the beam is
transmitted. Components such as the spatial filter which lie very close to the
nozzle plate may also be accommodated in the chamber. Helium used in
the chamber acts as a cooling medium, condensing the ablation products
before they have the opportunity to damage any other part of the nozzle
plate, whilst oxygen used in the chamber reacts with the ablation products,
turning them to gas. Both methods result in a cleaner end product
The present application is directed in the main to methods of
manufacturing nozzles in a nozzle plate of an inkjet printhead. Although only
a single nozzle is shown in the figures, most designs of printhead will have a
substantial number of nozzles e.g. 64 or 128. Manufacturfng time can
obviously be reduced by forming more that one nozzle at a time, these being
either nozzles in the same printhead or nozzles belonging to separate
printheads. However, full optical systems of,the type shown in Figures 1
and 2 are not necessarily required for each nozzle to be formed
simultaneously: for example, the beam from a single high energy beam
source may be used to feed a number of individual optical systems.
Furthermore, only a single variable beam attenuator is necessary if it used to
control the power of the single beam prior to splitting. Altematively, the
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beam splitting optics may be inserted between the mask 70 and the
convergent lens 80, thus reducing duplication to the convergent lens 80 and
any other elements (spatial filter etc.) that might be required downstream
thereof.
As regards the printhead itself, the nozzle plate 22 isnnade of a
material, e.g. polyimide, polycarbonate, polyester, polyetheretherketone or
acrylic, that will ablate when irradiated by light from a UV excimer laser.
Whilst the process of ablation - which is well known in the context of inkjet
printheads as being capable of forming accurate nozzles - is to be
preferred, the present invention is not intended to be restricted to this type
of
high energy beam. Radiation from-other types of laser or other sources may
be employed as a high energy beam.
It will be appreciated from the foregoing description that the present
invention is particularly suited to forming tapered nozzles. In use, the broad
section of the tapered nozzle serves as the nozzle ink inlet and is connected
to an ink channel of the printhead whilst the narrow section of the nozzle
serves as the droplet ejection outlet. The Ofront" surface of the nozzle plate
in which the outlet is formed may have a low energy, non-wetting coating to
prevent ink build-up around the nozzles.. In the case where this coating is
applied to the nozzle plate before nozzle formation, the beam must break
through this coating as well as the nozzle plate material.
Nozzles may be formed in the nozzle plate either before or after
attachment of the nozzle plate to the printhead (as is known in the art, see
for example the aforementioned W093/15911). In both cases, the location of
the nozzle relative to the respective channel is important and is facilitated
by
means for manipulating the nozzle plate/printhead relative to the optical
system prior to nozzle formation.
