FLUID COOLED CONTACT MASK

MASQUE DE CONTACT REFROIDI PAR UN FLUIDE

English Abstract

2106477 9216822 PCTABS00016
A mask (17) for use in forming features in a surface (12) by high
energy pulses of laser radiation is disclosed which comprises a
base plate (19) which is located adjacent said surface and has
apertures (20) in it through which locations of said surface are
exposed to the laser radiation. Channel means (21) formed in a
cover (23) of the mask lie in adjoining regions of the mask exposed
to the radiation and during ablation of said surface having
cooling fluid circulated through there. The mask is described in use
for making nozzles (25) in the nozzle plate (13) of an ink jet
printhead.

Claims

PCT/GB92/00513
- 10 -
Claims
1. A mask (17) for use in forming features (25) on a surface (12) by
laser ablation comprising a baseplate (19) which is located adjacent said
surface and is formed with apertures (20) through which respective
locations of said surface are exposed along respective aperture beam paths
to high energy radiation pulses of said laser to form said features,
characterised in that channel means (21) is provided adjoining regions of
the mask exposed to said high energy pulses and spaced from each aperture
beam path, through which channel means, during ablation of said surface,
fluid is caused to flow to cool the mask.
2. A mask as claimed in Claim 1, characterised in that said channel
means comprise enclosed channels (21) adapted for connection to means for
circulating cooling fluid therethrough.
3. A mask as claimed in Claim 2, characterised in that said channels
include deflectors (29) which impart sinuous flow to fluid passing
therethrough.
4. A mask as claimed in Claim 2 or Claim 3, characterised in that a
heat exchanger is provided through which fluid heated in the channels is
passed for heat extraction therefrom prior to recirculation.
5. A mask as claimed in any preceding claim, characterised in that
the channel means extend through locations adjacent said mask apertures.
6. A mask as claimed in Claim 5, characterised in that said channel
means are spaced from said mask apertures within a distance <IMG> where k is
the thermal diffusivity of the base plate material of the mask and f is the
frequency of the laser pulses.

WO 92/16822 PCT/GB92/00513
- 11 -
7. A mask as claimed in any preceding claim, characterised in
that said mask on the surface thereof on which said laser radiation is
incident is formed with a mirror surface to reflect said radiation.
8. A mask as claimed in Claim 7, characterised in that said
mirror surface comprises a coating of aluminium.
9. A mask as claimed in Claim 7, characterised in that said
mirror surface is a dielectric coating which is of thickness
wavelength matched to the wavelength of the incident radiation.
10. A mask as claimed in any preceding claim, characterised in
that said mask is adapted so that said base plate flatwise contacts
said surface in which said features are to be formed.
11. A mask as claimed in any one of Claims 1 to 9, characterised
in that contact pads (28) are provided on the mask surface which faces
the surface in which said features are to be formed, said pads
extending respectively around said mask apertures.
12. The method of forming nozzles (25) in an ink jet printhead
having parallel ink channels (15) with which said nozzles respectively
communicate, characterised by bonding a polymer nozzle plate to
corresponding ends of said ink channels, applying a contact mask to
said nozzle plate, said mask being formed with apertures at the
spacing of said nozzles circulating cooling fluid through channels
formed in said mask and exposing said mask to high energy pulses of
laser radiation at least in the regions of the mask including said
apertures thereby to form said nozzles by ablation.
13. The method of Claim 12, characterised by rocking said
printhead, nozzle plate and mask relatively to the axis of said
radiation thereby to form said nozzles with an undercut.

WO 92/16822 PCT/GB92/00513
- 12 -
14. The method claimed in Claim 12 or Claim 13, characterised by
circulating water as said cooling fluid.
15. The method claimed in Claim 12 or Claim 13, characterised by
circulating hydrocarbon solvent as said cooling fluid.

Description

WO 92/1682 2 PCI /G B92/005 13
Fluid Cooled Contact Mask
l'his invention relates to masks for use in forming features in a
surface by high energy pulses of laser radiation and in particular to
A method of forming nozzles in an ink jet printhead having parallel
ink channels with which said nozzles respectively communicate.
The use of excimer laser for patterned ablation of surfaces is
~ell known in the literature and the application of laser ablation to
the formation of nozzles for an ink ~et printhead i~ described in
EP-A-0309146 the content of ~hich is incorporated herein by
reference.
In this reference the preferred method of nozzle manufacture is
to place a contact mask having apertures corre~ponding to nozzle
locatlon in contact with a nozzle plate attached to the printhead.
Exposure to successive pul~es of W light of high intensity causes the
nozzles to be ablated. Rocking of the mask and printhead during the
pulses énables the nozzles to be undercut so that the nozzle inlets
are greater in area than the nozzle outlets. Typical incident energy
of the W light pulses is 0.3-lJcm
Practical tests indicate that a contact mask tends to heat up
during exposure to light energy density of this magnitude, which may
result in the thermal expa~sion of the mask. It also causes the mask
to become dished due to thermal stress cycling in the mask surface and
the mask becomes progressively cracked and damaged, limiting its
useful life. -
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P~T/GB n ~ / 0 0 5'1~'
2~6~77 il9 M~Y 1993
- 2 -
One recognised method of avoiding the problems resulting from a high energy
density of radiation incident on the contact mask is to employ a projection
mask at an expanded par~ in the path of tho incider.t opt:c~l beam i.e. at a
location of the beam where the energy density is less than that at the
ablating locations. In the present application, however, a contac~ mas` in
contact with the printhead is to be preferred. Contact against the face of
the mask, e.g. by locating dowels or by optical alignmen,, locates the
printhead relative to the mask and reduces manufacturing tolerances,
particul~rly in a process incorporating rocking. -
It is an object of this invention ts prsvide a mask for use in
forming features in a surface by high energy pulses of radiation wnich is
of extended useful life. ~ further objec- is tO provide an improved meshod
of forming nozzles in an ink jet printhead.
The present invention consists in a mask for use in forming features
on a surface by laser ablation comprising a baseplate which is located
adjacent gaid surface and i9 formed with apertures through which respective
locations of said surface are exposed along respective aperture beam paths
to high energy radiation pulses of said laser to form said features,
characterised in that enclosed channel means are provided adjoining regions
of the mask exposed to said high energy pul~es and spaced from each
aperture beam path, through which channel means, during ablation of said
surface, fluid is caused to flow to cool the mask.
Suitably, said channels means comprise enclosed channels adapted for
conn-ction to means for circulating cooling fluid therethrough.
Advantageously, a heat exchanger i9 provided through which fluid i9
heated in the channels is passed for heat extraction therefrom prior to
recirculation.
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W O 92/16822 PCT/GB92/OOSt3
213~3 6 4 7 7
Prefersbly, s~id maqk on the surface thereof on which said laser
radiation is incident is formed with a mirror surface to reflect said
rsdiation, In one form the mirror surface is a coating of aluminium.
In another form, the mirror surface is a dielectric costing which is
of thickness wavelength matched to the wavelength of the incident
radiation.
The surface of the mask may be such as flatwise to engage the
surface in which features are to be ablated. Alternatively, the mask
may be formed around the apertures therein wlth pads which contact the
surface in which the features are to be formed respectively around
those features.
The invention further consists in the method of forming nozzles
in an ink Jet printhead having parallel ink channels wlth which said
nozzles respectively co _ unicate, characterised by bondir~s a poly er
nozzle plate to corresponding ends of ~aid ink channels, applying a
contact zask to said nozzle plate, said ask being forced wlth
apertures at the spacing of said nozzles clrculating cooling fluld
through channels for~ed in said ask and exposing said ask to high
energy pulses of laser radiatlon at least in the regions of the mask
including said apertures thereby to ablate said nozzles.
The invention will now be described, by way of example, wlth
reference to the acconpanying, so ewhat diagrar atic drawlngs, in
which:-
PIoURE 1 is a side elevation partly in section of equipment usedfor laser ablation of features in a surface, in psrticular nozzles in
a nozzle plste of an ink ~et printhead, which includes a mask
according to this invention;
1. .
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W O 92/16822 P ~ /GB92/00513
2~ 7
FIGURES 2a and 2b are respectively a sectional s$de elevation and
a sectional plan view of the mask of Figure 1, the side elevation of
Figure 2a being taken on the line IIa-IIa of Figure 2b and the plan
view of Figure 2b being taken on the line IIb-IIb of Figure 2a; and
FIGURE 3 is a fragmentary sectional vie~ illustrating details of
the mask of the earlier figures.
In the drawings, like parts are accorded the same references.
Referring to Figure ' an excimer laser 10 affords a high energy
optical beam 11 employed for forming features in a surface 12 which,
ln this case, is a surface of a noz~le plate 13 of an inX ~et
printhead 14 to which the plate 13 is bonded at corresponding ends of
parallel channels 15 which extend in the printhead in a plane normal
to that of the drawing. An fexa ple of this process ls described in
EP-A-03D9146 referred to earller.
In this process the surface 12 is ablated by e~posure to pulse~
of high energy W light generated by the l er 10. The wavelengths of
light cho~en are typically 193,248 or 308 nm corresponding to photon
e ission at the excimer line of argon fluoride ~ArF), krypton fluoride
(KrF) or xenon chlorlde (Xe Cl). The pulse period generated by ~uch
lasers is typically 10-30ns, delivered at freguencies of up to 200Hz
or hlgher.
m e energy density of the pulses may be concentrated, by means of
a suitable lens 16, to a level dePending on the ablation threshold of
the surface 12. Typically where the surface is a polymer suitable for
the nozzle plate for an ink Jet printhead, the threshold energy
density for ablation ii8 0.1-0.2Jcm 2. In a process for ablating the
~urface at a suitably high rate, an energy density in the
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W O 92/16822 _ 5 _ PCT/GB92/00513
range 0.3-lJcm 2 will be selected : but for the sblation of surf~ces
having a higher threshold energy density a higher exposure energy
density up to lOJcm 2 may be employed.
In known art a projection mask disposed in the region of the lens
16 is u~ed, but where small precise features are to be ablated, or
rocking of the surface 12 is employed it i5 convenient to use a
contact mask 17 including apertures 20 made in a base plate 19 of the
mask which is located precisely rslatively to the ciurface 12, e.g. by
dowels or optlcal means (not shown). The mask 17 i~ e~posed to the
full energy density of the incident light puls~. -
A problem with the contact mask is that it may absorb energy
during the period of exposure to light pulses and progressively heat
up during the ablation process. A8 a result the ~ask ay expand by
ther al expanslon, which li its the accuracy of anufacture of the
nozzleB. Purther, lt has a tendency to beco e dished, due to thermal
stressi cycling and lts isurface becomes cracked and dauaged 80 that the
i ask has a li d ted llfe. These dlfflculties would generallY be
j avolded with a proJectlon asik where the energy density can be lower,
¦~ and the rate of heating correispondingly less.
~ Another problem wlth a contact mask arises when it is used to
I forc nozzles of an ink Jet printhead when heating of the mask gives
, rl~e to ther al degradation of the non-wetting coating formed on the
! outer face of the polyner nozzle plate. This coating is the sub~ect
¦ of EFi-A-036743O and is formed on the polymer sheet from which the
nozzle plate is made. m e coating as well as being of low surface
energy, i.e. non-wetting, iis rub resistant and tolerant up to 180C
which isi attained during its manufacture. m ere is however evidence
. . . .

W O 92/l6822 PCT/GB92/00513
4~ - 6 -
that when an uncooled mask overheats during nozzle ablation, the
coatin~ degrades as a result of which ink in the nozzles, instead of
being confined to the nozzle, then spreads over the outer surface of
the nozzle plate. For this reason, therefore, cooling of the mask is
desirable.
The degree of heating of a contact mask depends upon the optical
absorption or reflection coefficlent of the mask at the wavelength of
the incident light energy. For example, if the mask is formed of
silicon, whose absorption coefficient is about 0.4, and of thickness
100~, under incident energy of 0.5Jc~ 2, the mask will heat about
10C per pulse. When the mask is metallised with Aluminium, which
has an absorption coefficlent of about 0.1 (i.e. approxlmates to a
rirror of 90~ refleotlon efficiency) the tenperature rise is still
about 2.5 & per pulse. m us at a typical pulse rate of 200Hz the
contact mask will be found to rise in te perature at about 500C per
second absorbing heat at a rate of lOWcn 2. In an ablation process
requiring several thousand pulses, it has hitherto been practical only
to ablate at lower freguencies i.e. 1-2Hz, 80 that the mask cools
between pulses and ls limited to an acceptable peak temperature.
It will thus be seen that only an extremely high guality mirror
coating having an absorptlon coefficient less than 0.001 will be
suitable for a passive contact mask - one in which nothlng is done
about absorption of energy - wlthout overheatlng ln llmlted perlods at
hlghest laser pulse rates. A high gu~lity mirror coating although it
absorbs less heat, also loses less heat by infra-red radiation. One
does not accordingly want to rely on heat conduction into the
printhead to cool the mask, therefore cooling by other means is
desirable to keep the mask temperature within reasonable range.
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21 ~ ~ ~ 7 7 PCTtGB9~/OOSt3
7 ~ i12 JUlY l9g3
To prevent overhesting, the ccntact mask in Figure 1 incorporates
flu_d ch&~nels 21, The cooling f'uid which i9 caused to flow throush
~he c-&~21s 2' b; -ear.s c, a ?u=?. ma~ be gas but in view of the
~ ited space available fo^ the ch&~nels is, preferably, a 'i~uid such
as water inclucin$ i~hibitants to 1 ~- oxidation or solubility of the
ch&~nel walls or a hydroca~~on solvent.
'ne ch&~nels 21 a~e I O ~ed in a co~er _3 of t;.e mask ~hich s
bo-.ces to the base plate 1~. ~oth the cove- and base plate may be
made o~ metal or s '-con cr a `^._~h ~e-?era:u~e pol;ies bon~ed o slued
tm~et~er. Advanta~eo~s1;, the bond is a low vapour pressure bond such
as a diffusion or solder bond. In tne mas~ 'llustraeed, the apertures
20 comprise a line of apertur~s at the spac'n3 of chanr.el nozzles 25 ~ -
which are ablated into the nozzle plate 13 by the beam 11 and
respectively communicate with the channels 15 of the printhead. As
shown ln Figure 3, apertureis 27 in the cover which overlie the
apertures 20 in the base plate 19 may have a larger diameter than the -;
apertures 20 to facilitate ablation of the -.ozzles 25 by relative
; rocklng between the incident lig~t beam 11 and the printhead 14
without shading or occluding the exit of the nozzles 2~
The cooling channels 21 for2ed in the cover include deflectors 29
~ which inpart ~uo~S fiow to the cooling li5uid to ensure optimum heat -
'i absorption therein. The channels 21 are pl~ced so as to cool, as much
~ as practical, the area of the mas~ exposed :o the incident light pulse
`~ on the mask.
- ~ Ihe surface of the cover 23 ay be c^o-ed by a mirror surfoce
, ~
(for example, an aluminium coat~ns). T~.is ~ s the heat absorbed
.- during the pulse period of t~picsll- 10-3~ .. s ~nd thus reduces the
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WO 92/16822 . PCI/GB92/00513
peak temperature attained by the surface layer of the cover to
typ~cally 1-200C temperature rlse. Without the coating, the layer
may reach 500-1000C or more during the pulse causing the mask cover
to deteriorate and distort, as well as increase the rate of heating of
the mask.
The material of the base plate 19 round the apertures 20 (as
illustrated in Figure 3) may similarly be coated with a dielectric
mirror coating. The choice of coating, i.e. metallised coating or
dielectric costing, is made primarily to ensure that the life of the
cooled mask when exposed to W laser pulses is adequate for the
manufacturing duty specified. For aluminium the threshold energy of
ablation is linited by surface segre6ation of lnpurities in ths
deposited etal and wlll not exceed O.ô - 1.1 Jc 2. For hlgher
energy density a dielectric coating is required.
In order to ensure that the aterial round the apertures ls
effectlvely cooled between llght pulses, lf the pulse frequency is f
and the thermisl diffuslvlty la k, the distsnce between the apertures
20 and the coolin6 channel 18 preferably, less than ~ . As shown in
Figure 3, the heat in the material ln the b e plate 19 in this region
then has time to dlffuse towards the cooling channels and to become
easentlally uniform before the next pulse. ~he thermal dlff w lvlty k
K/~c where K ~ the thermal conductlvlty, p ~ the denslty and c the
specific heat of the mask material.
A contact pad 28 may be placed round each of the apertures 20 of
the mask on the side of the base plate facing the nozzle plate 13
which ensures a good contact between the mask and surface 12 of the
nozzle plate. Alternatively, the base plate may lle flush a6ainst ~he
surface 12.
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WO 92/16822 - PCI'/G1~92/OOS13
:2 I ~ rl
The cooling channels are filled with cooling fluid, preferably
liquid, which is circulated through an inlet 22 and an outlet (not
shown) formed in the cover suitably at respective ends of the
channels, so that the heat is continuously removed during ablation. ~ -
The fluid is then passed prior to recirculation through a heat
exchanger (not shown), which dissipates the heat keeping the mask at a
steady temperature, preferably less than 20-40C above ambient or
similar, when the thermal expansion of the cooled mask 17 is kept
within acceptable limits.
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Representative Drawing