CA2106477A1

CA2106477A1 - Fluid cooled contact mask

Info

Publication number: CA2106477A1
Application number: CA002106477A
Authority: CA
Inventors: Anthony D. Paton; Stuart P. Speakman; Robert A. Harvey
Original assignee: Individual
Current assignee: Xaar Ltd
Priority date: 1991-03-20
Filing date: 1992-03-20
Publication date: 1992-09-21
Also published as: WO1992016822A3; WO1992016822A2; GB9105870D0; EP0576533A1; JPH06506069A

Abstract

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.

Description

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. -- , . : : ~ :- : . :

P~T/GB n ~ / 0 0 5'1~' 2~6~77 il9 M~Y 1993 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.

' . . .
, . .

. :' :

~ ~ .
,, , . . . ..
. .. , ., . . .; .. .. , .. , .. . - . ,- . ., ., . ~. .. " . ,~ ;. . , . ,, .. ., . , .. . , , ", . . .. . .. . . .. .. .. .

, , - .- . . .. - . - ~ . . . . .. .. .

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. .
~ , .
.

' . - .. ' . ' ' : ' :: ' ' '' , ~ ' . '. . . ,, : . . . .

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 ~ .

~.. , ;. ;, -, . . -. ,, . . , ~ . ... ... : ,, ,,. ,, '' . :

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.

":' ' '` ~. ~ ' ~". . :

: ': . : ' ~-. ' ' -. .' . :: ' ., : .

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 ~,. . .. , _ .. . .. ~ . ... .
~;~ d~e~ 0 SIJ85~ SH~

n~ plicat~on ..
~ . ;

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.

~' ' . '. .. : ' : ', .,. : . :.': ' . . , .' . . . .. , - ~ . , , 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.

'' ~' ~ ' ' ' ' :.- -" -. ~ "' " ,. . .
,~ - ;':'''. ':,.

- . . . , ,: .
. ' ., " .. ..
~- ~ ' .,.'.

'~: - , ;
_1 ' . .. . ...
~', ; ';~, ', , . ', ' .'. ' , ,~,__' ' ' "

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 where k is the thermal diffusivity of the base plate material of the mask and f is the frequency of the laser pulses.

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.

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.