WO2008035043A1

WO2008035043A1 - Method for thermally curing thin films on moving substrates

Info

Publication number: WO2008035043A1
Application number: PCT/GB2007/003487
Authority: WO
Inventors: Philip Rumsby
Original assignee: Exitech Ltd; Oerlikon Balzers Coating UK Ltd
Current assignee: Exitech Ltd; Oerlikon Balzers Coating UK Ltd
Priority date: 2006-09-20
Filing date: 2007-09-13
Publication date: 2008-03-27
Anticipated expiration: 2009-03-20
Also published as: GB0618562D0; GB2442016B; TW200823607A; GB2442016A

Abstract

A method for forming a regularly repeating pattern in a thin film on a substrate comprising the steps of: imprinting on the applied film a pattern formed by exposing it by laser radiation, which has been caused to pass through a suitable mask delineating the pattern, so as to cause exposure of the film in a repetitive series of discrete laser exposure steps using a stationary mask that represents only a small area of the total area of the substrate and using a single short pulse of laser radiation at each step to illuminate the mask, the radiation pulse having such an energy density at the substrate that it is below the threshold value for ablation of the film; and (ii) the series of discrete laser exposure steps being repeated over the full area of the surface of a substrate, to give a full pattern comprising a plurality of pixels, by moving the substrate in a direction parallel to one axis of the pattern to be formed on the substrate and jicriyating_the pulsed mask illumination laser source at the instant that the substrate has moved over a distance equivalent to a complete number of periods of the repeating pattern on the substrate. The invention further comprises a laser exposure tool for carrying out the method as aforesaid.

Description

METHOD FOR THERMALLY CURING THIN FILMS ON MOVING SUBSTRATES

TECHNICAL FIELD

This invention relates to a method for using a pulsed laser for curing or exposing a thermally sensitive thin film. It is particularly concerned with the field of laser exposure for the high resolution curing of thin films on large area glass substrates used in the manufacture of flat panel displays.

BACKGROUND ART

The manufacture of the component parts of a flat panel display ('FPD') requires multiple process steps which include lithographic pattern transfer from a mask to form an image in a suitable resist layer which is then used to define a pattern in a film below the resist during a subsequent etching process. Alternatively the image is formed directly in a photo-sensitive film which itself forms the patterned layer.

To create high resolution patterns it is generally necessary to use an optical projection system where the mask pattern is imaged onto the film surface using suitable optics. Projection exposure tools have a lens or mirror optical system between the mask and substrate to relay the image. These can be used to project a part of the full FPD pattern and operated in a step and repeat mode in order to build up a large area image at the substrate but for a large FPD it is more usual to transfer the full pattern from the mask to the substrate by means of a unity (1-times) imaging system operating in a scanning mode

In this case the mask and substrate are either mounted onto the same mechanical structure and moved together or are on separate stages whose motion is accurately linked by the control system. Only a small area of the mask is illuminated at any given time but by performing either a single one-dimensional scan or a repeating two dimensional raster scan of the mask and substrate together the full area of the device is exposed. Such systems usually use lamps operating in the ultra violet region as a source of radiation to illuminate the mask and expose the resist or photosensitive layer.

A characteristic common to these 1 x magnification exposure tools is that they use masks of the same size as the device to be exposed. Such an approach is satisfactory for exposure of smaller FPDs with mask sizes up to 800 x 920 mm being readily available. However as FPD displays get larger e.g. 52 inch (1320 mm) and greater diagonal and especially where sizes over 60 inch (1500 mm) and greater diagonal or more are needed the provision of suitable unity magnification masks is difficult and very costly.

In earlier patent applications (GB0501793 and???) we described a method where a pulsed ultra violet ('UV") laser source is used to replace the lamp source in an FPD exposure tool. This change allows the exposure of large area FPD devices using only a small size mask so reducing dramatically the complexity of the exposure tool and the difficulties and high costs_associated with the use of large masks^ - - -

The present invention extends this small mask, pulsed laser exposure tool technology to the case where the films to be exposed are activated or cured by thermal processes rather than by the UV photochemical processes that operate with conventional photo resists and photo sensitive films.

Thermally curable organic films are well known in the form of adhesive and sealing layers and also appear commonly in the printed circuit board ('PCB') industry as solder mask resists. Such layers are often exposed or cured using CW IR lamps or CW near IR semiconductor diode sources. Inorganic thermal resists have also been developed for use in integrated circuit ('IC) manufacture. Multi layer bi-metallic stacks of Bi and In (Bin) have been thermally exposed using lasers to create fine line structures. Thermally exposed resists and thermally cured films have the advantage that they can be exposed by radiation at a wide range of wavelengths. So far thermally exposed films have not been used for creating the dense, high resolution, large area, repeating structures found in an FPD.

The present invention details a novel laser based method for exposing thermal resists or curing thermally sensitive films on moving large area FPD substrates to create high resolution patterns. The method retains the key advantages of small masks but still allows the formation of complex, repeating, high resolution patterns over large substrate areas. The invention uses pulsed multimode infra red (IR) lasers, appropriate versions of which occur in both low repetition rate and high repetition rate form.

DISCLOSURE OF INVENΉON

According to a first aspect of the present invention there is provided a method for Forming a regularly repeating pattern in a thin film on to a substrate by exposing it directly with radiation from a pulsed IR laser which has been caused to pass through a suitable mask delineating the pattern, the image of the mask pattern being formed on the surface of the film by a suitable projection lens and the energy density at the film being insufficient to cause the film to be removed directly by ablation, the imprinting steps being carried out:

(i) in a repetitive series of discrete laser exposure steps using a mask that is stationary with respect to the projection lens and represents only a small area of the total area of the substrate and using a single short pulse of radiation at each step to illuminate the mask; and

(ii) the series of discrete laser exposure steps being repeated over the full area of the surface of a substrate, to give a full pattern comprising a plurality of pixels, by moving the laser beam or substrate in a direction parallel to one axis of the pattern to be formed on the substrate and activating the pulsed laser mask illumination light source at the instant that the substrate or beam has moved over a distance equivalent to a complete number of periods of the repeating pattern on the substrate;

According to a first preferred version of the first aspect of the present invention during the imprinting stage the size of the illuminated area at the substrate in the direction parallel to the direction in which the beam or substrate is moving is sufficient to provide that, after passage of the substrate under the illuminated area or the moving beam across the substrate, each part of the film has received a sufficient number of pulses of radiation to fully expose or cure it. The number of pulses of radiation received by each area can be any value from a single pulse up to several 100 pulses depending on the absorption properties of the film to be exposed, the energy density or the radiation on each shot and the temperature rise required to cure or expose the film.

According to a second preferred version of the first aspect of the present invention or of any preceding preferred version there of the light source is a pulsed flash lamp pumped solid state laser or pulsed diode pumped solid state laser, both operating in the infra-red (IR) region of the spectrum emitting pulses at repetition rates in the range 100Hz to 2kHz.

According to a third preferred version of the first aspect of the present invention or of any preceding preferred version thereof the light source is a CW lamp pumped solid state laser or CW diode pumped solid state laser, both operating in the infra-red (IR) region of the spectrum emitting pulses of radiation at repetition rates in the range 5kHz to 50kHz.

According to a fourth preferred version of the first aspect of the present invention or of any preceding preferred version thereof wherein during the imprinting stage an edge of the area to be exposed on the substrate is defined by means of moveable blades located close to the surface of the mask.

According to a fifth preferred version of the first aspect of the present invention or of any of the preceding preferred first to fourth versions thereof wherein during the imprinting stage the substrate is exposed in a series of parallel bands and the number of laser shots applied to each area in the regions where the bands overlap is controlled by using an image forming mask that has a stepped or randomised transmission profile at each side of the mask pattern, the steps or random features corresponding to one or more complete cells in the FPD array.

According to a sixth preferred version of the first aspect of the present invention the mask is able to move with respect to the projection lens at appropriate times during or after the moving process to allow unique (non-repeating) border patterns to be exposed or cured around the outside of the area that contains the regularly repeating pattern. __ - - - - - —

According to a second aspect of the present invention there is provided a laser exposure tool for carrying out the method of the first aspect or of any preceding preferred versions thereof.

According to a third aspect of the present invention there is provided a product formed by means of the method of the first aspect or of any preceding preferred versions thereof.

The invention is concerned with the use of only small masks to expose the full area of even the largest display and with working on substrates that are in motion. The present invention relates to a laser illuminated mask projection method for exposing thin thermally sensitive films to cure the film in selected areas to create high resolution, dense, regularly repeating patterns over large area FPDs using only small masks.

The invention relies on the use of a pulsed light source such as a high or low repetition rate pulsed infra-red ('IR') laser to create the film exposing radiation. Where regularly repeating patterns are to be created the mask remains stationary with respect to the projection lens during the laser exposure process while the film coated FPD substrate is moving continuously in the image plane of the projection lens or the image is moved across the surface of the substrate by means of a beam scanner system used in conjunction with a special scanning and imaging projection lens. In the case where unique (non-repeating) patterns are required to be exposed in areas adjacent to the repeating pattern the mask may then contain these patterns around the repeating pattern mask area and be caused to move in such a way as to introduce the non repeating pattern area into the beam at a suitable instant during or after the movement of the FPD substrate.

The key to the successful implementation of this process is that the pattern to be exposed has a regular pitch in the direction of relative movement of the substrate and image and that the pulsed laser source is activated at exactly the correct time so that the substrate or image moves by a distance exactly equal to (or to multiples of) the pattern pitch in the time between successive laser exposure pulses. We call this process synchronised image scanning ('SIS') as the triggering of the laser source and hence the creation of the exposure image on the FPD substrate is exactly synchronised with the substrate or beam motion so that successive images are displaced by integral numbers of pitches of the pattern. Several key conditions are necessary for the SIS laser exposure process to be effective in the creation of patterns suitable for FPDs. These are listed below.

Firstly the projection lenses used need to have low distortion and adequate resolution and field size. In general the finest patterns needed in FPDs are of a few microns in size so that optical resolutions in the range of 1 to a few microns are required. The combination of resolution and wavelength leads to the requirement that the lens numerical aperture ('NA') usually needs to be in the range 0.05 to 0.2. Field sizes of such lenses are in the range of lmm to several 10s of mm. Such values are adequate for the SIS laser exposure process discussed here.

The lens magnification factor can be any value that is convenient so long as the energy density at the substrate is sufficient to expose it without direct ablation and the energy density at the mask is insufficient to damage it. In general de-magnifying lenses with de-magnification factors in the range 2 to 10 are used for this invention but it is also possible to use lenses with unity (Ix) magnification and even enlarging lenses in some cases.

For the case where SIS exposure is performed using a high repetition rate pulsed IR laser the lens has to be specially designed so that it can be used for high resolution imaging in conjunction with a beam scanner unit. Such lenses are unusual in that image fidelity needs to be maintained very closely across the full scanned field of the projection lens.

The lenses used for both low repetition rate pulsed IR laser and high repetition rate pulsed IR laser SIS exposure are generally designed to be telecentric on the image side. This ensures that the size of the image is maintained constant if the substrate is displaced slightly from the exact image plane along the optical axis. Secondly it is important that the light source creating the exposing radiation is of a sufficiently short duration. This is important as the substrate to be exposed or the laser beam are moving continuously and the light pulse needs to be sufficiently short to 'freeze' its motion so that the image created is not blurred. For substrates or beams moving at speeds up to several meters per second, to limit the image blur to less than 1 micron requires that the pulsed source has a duration of a fraction of a micro second (IQ* sec). For this reason pulsed lasers make ideal light sources as the pulses emitted are usually well under 1 micro second in duration so that no image blur effects are seen even when the relative speed between the substrate and the image exceeds many metres per second. Low repetition rate pulsed IR lasers and high repetition rate pulsed IR laser lasers are particularly good light sources as they emit pulses at wavelengths that can readily thermally expose films and have convenient repetition rates (from a few Hz to few tens of kHz) This means that FPDs with pattern pitches in the range of a small fraction of a mm (e.g. 50μm) up to over 1 mm in size can be processed by this SIS laser exposure method at modest beam or stage speeds. As an example, an FPD pattern with a lOOμm pitch in the substrate moving direction can be patterned by a low repetition rate pulsed IR laser firing at 100Hz forming an image that has a width in the moving direction of lmm with the firing synchronised so that the images overlap every second pattern pitch with the substrate moving with respect to the image at a speed of only 20mm/sec. In this case the image contains ten repeat patterns in the full beam width so that after the substrate has moved through the full image area each area will have received five laser shots. If the film requires only one laser shot to expose to it fully then in this case the substrate would be moving at a speed of 100mm per second. If the film is thicker and needs ten shots to expose it fully the substrate speed is only 10mm per second. As another example an FPD pattern with a lOOμm pitch in one direction can be patterned by a high repetition rate pulsed IR laser firing at 2OkHz forming an image that is moved by a beam scanner system in this direction and has a width in the beam movement direction of 0.6mm with the laser firing synchronised so that the images overlap every pattern pitch with the beam moving at 2 meters per second. In this case the image contains six repeat patterns within the full width so that after passage of the full beam over the substrate each area will have received six laser shots.

The third key requirement for the successful implementation of this SIS laser exposure process is that the laser firing has to be timed exactly with respect to the stage or beam motion. For low repetition rate pulsed IR laser based SIS pattern exposure, where either the image is stationary and the substrate is moved in the image plane of the projection lens or the substrate is stationary and the mask and projection lens are moved with respect to the substrate, this means that the stages need to have high resolution encoders fitted and to be highly repeatable. It also means that fast and jitter free control electronics are needed to generate the laser firing pulses from the stage encoder signals so that small changes in stage speeds (due to servo control loop delays) do not affect the exact positioning of the images. Such electronics are readily available in standard CNC stage control systems. For the case where high repetition rate pulsed IR lasers are used and the image is moved across the substrate by means of a beam scanner system the accurate control and synchronisation of the beam scanner system with the laser pulses is critical.

The fourth important condition for successful SIS laser exposure is that the energy density of the radiation created at the image plane by each laser pulse is below the threshold energy density needed to cause direct ablation of the film.

Hence the envisaged method for best using this SIS laser exposure process with low repetition rate pulsed IR lasers is to create an image of a mask, that is held stationary with respect to the projection lens, at the FPD surface which is then moved under the optical projection system to expose a band of film across one axis of the FPD. After one band has been exposed the optical system is stepped sideways and another band adjacent to the first exposed. Clearly the sidestep distance has to be an integral number of pattern pitches in the stepping direction so that the 2nd exposed band pattern is exactly registered to the first band. In general the width of each band exposed should be such that when all scans are complete the full area of the FPD has been exposed. This is desirable but not essential as is discussed later.

It is also possible to perform low repetition rate pulsed IR laser based SIS exposure using different methods for achieving the correct relative motion of the optical system and substrate. In one case the optical system incorporating the projection lens and mask is held stationary at all times and the substrate is moved in two orthogonal directions. In another case the substrate is held stationary at all times and the optical mask projection system is able to move in two orthogonal directions. To maximise the speed of the FPD SIS laser exposure process it is necessary to reduce the total number of parallel bands and to move the FPD at the highest possible speed. The former requirement is met by creating an image that is as wide as possible though this is limited by the availability of suitable lenses. The requirement to scan at the highest possible speed is met in the following way. - — - - - - -

In general FPDs are rectangular in shape and have approximately square pixels each of which is divided up into at least three sub-pixels or cells representing the different colours necessary to form a full colour display. This means that the repeating patterns have different pitches in the two different FPD axes. In general the pixels are divided into sub-pixels or cells along the long axis of the FPD so that there are considerably more (x 5 or x 6) cells in the long axis of the FPD compared to the short axis. The low repetition rate pulsed IR laser SIS laser exposure technique discussed here can be implemented such that the substrate or beam is moved in a direction parallel to either the short or long FPD axes though there is some advantage in moving parallel to the long axis in that the number of passes required to cover the full FPD area is less than when moving parallel to the short axis and hence the number of times the substrate has to be slowed, brought to rest and accelerated in the reverse direction is minimised and the process rate is maximised.

As the low repetition rate pulsed IR laser SIS laser exposure process requires that the FPD and image move relatively to each other by an integral number (one or more) cells between laser pulses it is possible to increase the relative speed by moving more than one cell pitch between laser pulses. Moves of two, three or more pitches can be used to increase speeds. The consequence of increasing the distance moved between exposing pulses is that the exposing beam at the FPD increases in size in the moving direction. As an example consider an FPD with a pixel size of 0.6 x 0.6mm. Each pixel is divided into three cells each of 0.6 x 0.2mm in size. If a laser firing at 100Hz is used and the FPD or beam is moved in the cell short axis (FPD long axis) direction a speed of 20mm/sec is achieved if the substrate or beam moves just one cell pitch each laser pulse. By moving two cell lengths between laser exposure pulses the speed is increased to 40mm/sec.

The requirement that each area of the FPD receives a certain number of pulses to fully expose it means that the size of the beam in the scan direction is given by the product of the cell pitch, the cell number moved between pulses and the number of exposing pulses required by each area. For the example above with a 0.2mm cell pitch and a movement of two cell lengths between pulses if five pulses are required to achieve the correct dose in the film then the beam size in the moving direction is 2mm.

The envisaged method for best using this SIS laser exposure process with high repetition rate pulsed IR laser lasers is to create an image of a stationary mask at the FPD surface that is moved by means of a beam scanner system to expose a row of pixels across a narrow band of film parallel to one axis of the FPD. After one row of pixels has been exposed the beam scanner reverses the direction in which the beam is moving to expose an adjacent parallel row. This backwards and forwards moving process repeats and at the same time the substrate is moved continuously in the direction perpendicular to the beam scan direction. By this means a continuous band parallel to the substrate moving direction is exposed. We call this beam scanning in conjunction with substrate motion to expose a band of repeating structures "Bow Tie Scanning" ('BTS'). After one band has been exposed the optical system incorporating mask, scanner unit and projection lens is stepped sideways and another band adjacent to the first exposed. Clearly the sidestep distance has to be an integral number of pattern pitches in the stepping direction so that the second exposed band pattern is exactly registered to the first band. In general the width of each band exposed should be such that when all bands are complete the full area of the FPD has been exposed. This is desirable but not essential as is discussed later.

It is also possible to perform high repetition rate pulsed IR laser based SIS exposure using different methods for achieving the correct relative motion of the optical system and substrate. In one case the optical system incorporating the projection lens, beam scanner umt and mask is, held stationary at all times and the substrate is moved in two orthogonal directions. In another case the substrate is held stationary at all times and the optical mask projection and scanner system is able to move in two orthogonal directions.

When exposing films in bands by either low repetition rate pulsed IR lasers or high repetition rate pulsed IR lasers using SIS and BTS techniques great care has to be taken to ensure that no discontinuities exist at the boundary between bands. Such band boundary discontinuities are sometimes referred to as 'stitching errors' or 'stitching Mura effects'. One way to avoid these band boundary Mura effects utilizes the fact that the image area imprinted on the film surface at each laser shot consists of a 2D pattern of repeating identical cells and that the two side edges of the pattern imprinted can be formed to create a stepped cell structure or even have isolated cell patterns. These structures can be shaped such that the side edge of one band exactly interleaves at the scan boundary with the side edge of the adjacent band so that all cells receive the same number of laser shots and the line that joins any two adjacent bands is no longer exactly straight. This technique can be applied to either low repetition rate pulsed IR laser SIS exposure or to high repetition rate pulsed IR laser SIS exposure.

For the case of low repetition rate pulsed IR laser SIS exposure a typical image imprinted on the surface of the film could be 100-200 pixels long in the direction perpendicular to the moving direction and many tens of pixels long in the direction parallel to the moving direction. The multiplicity of cells in the direction parallel to the moving direction allows the possibility of forming a staircase of cells or more complex pattern with isolated cells at the side edges of the pattern to give a staircase or non- straightness to the beam edge. Many stepped or isolated cell patterns are possible so long as both ends of each image are symmetrically patterned in a way that ensures all cells within the band and in the overlap region between bands are subjected to the same number of laser shots.

For the case of high repetition rate pulsed IR laser SIS exposure a typical image imprinted on the surface of the film is much smaller but can still contain multiple cells. As an example for a laser exposure process that needs five laser shots on each area of the substrate to expose the film fully the image would be five cells long in the direction parallel to the moving direction and a similar number in the direction perpendicular to the moving direction. The multiplicity of cells in the direction perpendicular to the moving direction allows the possibility of forming a staircase of cells or more complex pattern with isolated cells at the side edges of the image to give a staircase or non- straightness to the beam edge. Many stepped or isolated cell patterns are possible so long as both sides of each image are symmetrically patterned in a way that ensures all cells within the scan band and in the overlap region between bands are subjected to the same number of laser shots. For the case of low repetition rate pulsed IR laser SIS exposure, control of the number of laser shots each area of the substrate receives right up to the two boundaries of the FPD device in the scan direction is an important issue. This is a potential problem with this SIS laser exposure process as the beam width in the scan direction is such that many patterns are exposed on each laser pulse. If multiple laser pulses are required on each area then the substrate or beam moves only a fraction of the image width between laser pulses and if the triggering of the exposing laser is suddenly stopped at the boundary of the FPD there will be an area extending over part of the image where the number of shots delivered to each area is incomplete. Depending on the number of shots needed on each area this partially exposed band will be up to almost the full width of the image in the scan direction and the number of laser exposure shots received by each area over this distance will change from one to the maximum value. Clearly this is highly undesirable so that a method is needed to prevent this

The same problem exists at the sides of the FPD if the edge of the beam used is stepped or discontinuous to cpritroLMura effects at the band boundary regions: At the outer edges of the extreme bands used to expose the FPD a partially exposed region with width equal to the width of the structured region on the ends of the beam will be created. In this region the number of laser shots received by each area will fall from the full value to one. Clearly this is highly undesirable so that a method is needed to control it.

Both of the edge problems described are solved by the same method, which involves the use of blades positioned close to the mask that move into the beam to obscure the image in the boundary regions. The blades are motor driven and controlled from the stage control system so can be driven into the beam at the correct time during the process. The blades are oriented with their flat faces parallel to the surface of the mask, and are located very close to the mask surface such that the blade edge is accurately imaged on to the substrate surface. Four blades are required in total, one to deal with each of the four substrate boundaries. In practice blades are sensibly mounted in pairs on a two axis CNC stage system and are designed so that the blade edges are exactly parallel to the FPD (and mask) pattern.

To solve the moving direction edge problem a blade is moved into the beam at the mask to reduce the beam width progressively as the FDP boundary is approached. This means that the motion of the blade has to be accurately synchronized in position to the motion of the main FPD stage. This is exactly the method used in standard lithographic exposure tools to link the mask stage to the wafer stage so is readily implemented in the control system. The blade clearly has to move a distance and at a speed that is related to the main stage speed by the lens magnification.

Side boundary blades are used to eliminate the narrow incompletely exposed bands at each side edge of the FPD and can also be used to control the overall width of the area exposed on the FPD surface. It is possible to set the width of each band such when all bands have been cpmpleted the width of the FPD device has been covered exactly:^" Such an arrangement maximises the process rate but is complex to set up. In practice it is preferable to work with bands that are a very small amount (e.g. 1 cell width) wider than the size that fits exactly into the full FPD width. In this case the beam obscuring blades used to obscure the incompletely exposed bands on each outer side of the outer bands are then advanced further into the beam to 'trim' the outer bands to the required width to expose an area of exactly the correct size.

For the case of high repetition rate pulsed IR laser SIS exposure using BTS mode processing the beam is scanned in the direction perpendicular to the direction of relative movement of the substrate and optical system to create a patterned band on the FPD and hence there is generally not an edge issue at the beginning or end of each band as the moving image on the FPD surface moves parallel to the end of each band. There may however be an issue about forming exactly the correct number of pixels along the length of the band since the finite number of pixels in the image in the direction perpendicular to the beam moving direction may not divide exactly into the number of pixels required by the FPD design. If this is the case then the final scan of the beam across each band is adjusted in position along the band by adjustment of the beam scanner controls such that the exact number of pixels along the length of the band is created. Such a procedure leads to some of the lines of cells in the last scan of the beam across the band receiving twice the number of laser shots compared to the rest of the band but since this is a thermal exposure or curing process and is generally used to expose or cure a thin film of material on a lower substrate an excess of laser shots leading to an excessive exposure dose leads only to a change in the widths of the developed structures and since this is at the very edge of the FPD is usually not a problem.

In SIS exposure using BTS mode processing with a high repetition rate pulsed IR laser the join line between adjacent bands must be controlled so that all cells in the boundary region receive the same number of laser shotsrThis is achieved by carefur overlap of the last image laid down by a beam scan in one band with the first image laid down by a corresponding beam scan in the adjacent band. As an example in the case where the moving image contains an array of four cells in the scanning direction and four cells in the perpendicular direction (sixteen cells in total) and the beam scanning speed and laser firing rate are adjusted so that the laser fires each cell pitch in the scanning direction then in the main part of each scanned line each part of the substrate will receive a total of four laser shots but when the laser ceases to fire at the end of the scanned line the last image will contain cells that are incompletely exposed as they contain progressively less than the full number of laser shots. In the case given here the last image contains columns that are four cells wide where the number of shots per unit area reduces from 4 to 3 to 2 to 1 across the image. Full exposure of this incompletely exposed region at each band edge is achieved by overlapping it with the corresponding incompletely exposed region on the adjacent band. In the case given here this means that the images on adjacent bands are caused to overlap by three cells so that the cells that received only three shots in one band receive an additional single shot from the adjacent band, the cells that received only two shots in one band receive an additional two shots from the adjacent band and the cells that received only one shot in one band receive an additional three shots from the adjacent band. In such a way the band boundaries are merged together to form a continuous pattern where all cells receive exactly the same number of laser shots.

Since the process we are considering here is a thermal exposure process it is unlikely that laser shots added to a particular area on an extended timescale add linearly to the required dose. If six successive laser shots are required to thermally expose the film it is unlikely that the film will be fully exposed by the application of two sets of three shots applied with some time interval between the sets. In this case more than six shots in total are likely to be needed. The edge overlap technique discussed here can be readily modified to cause the bands to overlap further in order to cause an excess number of laser shots to be applied to the band overlap region to compensate for the^" time delay between exposure times.

This process is effective for all the boundaries between bands in the body of the FPD but it is clear there is still a problem with incompletely exposed cells at the outer edges of the first and last bands. If the requirement exists to clear all cells completely at these side edges then this is achieved by carrying out an additional process step where a narrow band at each side edge of the FPD is patterned with the same area scanned multiple times and the extreme cell position corresponding to the very outermost edge of the FPD cell pattern so that these outermost cells receive the correct number of laser shots. In the case discussed above with a beam containing an array of four by four cells in order to cause the extreme outermost cell to be subjected to four laser shots during this subsidiary process the beam has to scan up to this cell a further three times to completely expose it. This solves the problem of incomplete exposure at the extreme sides of the FPD but in the process causes a band of cells to receive many more laser shots than the minimum needed for full exposure. For the case considered here over the width of the narrow band that is processed in order to expose each side edge cells receive as many as sixteen shots where four shots are applied during the standard band exposure process and the further twelve shots are imparted during the three extra scans needed to apply four shots to the extreme edge cells.

AU of the above discussions relate to the case where the pattern to be imprinted is repeating in a regular way over the full area. There may however be cases where special non repeating patterns occur immediately adjacent to the repeating area. Examples of this would be the complete exposure of a BM resin film in a border area a few mm wide around the edge of the BM matrix on an LCD colour filter assembly or the forming of alignment and reference marks around the edge of the FPD pixel matrix. In these cases it is necessary to incorporate these non regular features adjacent to the regular ones on the mask and mount the mask on a stage system of some_type so thatthese non regular features can be moved into the beamr and hence^" transferred to the substrate as the moving laser exposure process proceeds to the edges of the FPD device.

When using low repetition rate pulsed IR laser SIS processing one method of readily exposing these non-repeating areas is to imprint them in a step and repeat process mode where the mask and substrate are both stationary during each laser exposure process. In this case the edge features can be incorporated into the mask at known positions and the mask is mounted on a two axis stage system so that the appropriate areas on the mask can be moved into the beam at the same time that the substrate or optical system is moved to the corresponding position on the FPD so that the correct edge feature is imprinted at exactly the correct position on the substrate. Such a process is effective but can be slow as several separate steps are required and hence the overall time to expose the full FPD area is extended. In some low repetition rate pulsed IR laser cases a much faster method can be used to expose these edge features. This method requires both mask and substrate to be in relative motion with respect to the projection lens. In this case the edge feature patterns have to be situated on the mask immediately adjacent to the regular feature pattern and the mask and substrate have to move exactly together in exact register at relative speeds set by the lens magnification. This is the type of moving process used in advanced high throughput IC semiconductor exposure tools and Ix FPD exposure tools. Of course if the mask has to be moved during the laser exposure process then its movement (and that of the substrate) has to always obey the requirement that it is in the correct position when the laser is fired to overlay the regular substrate FPD pattern correctly. Since the substrate and its associated chuck and stages are massive and hence cannot change speed rapidly it is important that the mask and associated stages are able to accelerate rapidly to an appropriate speed.

Since the non repeating features always occur around the edge of the regular pattern on the FPD,Jhe substrate stage is generally in the process of slowing at the end of its pass across the FPD in order to turn around and reverse direction. Hence the substrate is likely to be moving slowly at the time the mask stage needs to move and hence the speed that the mask needs to achieve in order to become synchronised with the substrate stage is modest.

For SIS exposure with a high repetition rate pulsed IR laser the repetition rate of the laser and speed of the beam are too high to allow movement of the mask while the laser is firing. In this case in order to create special non repeating features around the main repeating FPD structure appropriate masks are moved into the beam to form a small suitably shaped image on the FPD surface and the beam is then moved over the surface of the FPD using the beam scanner controls and the stage motion if required to expose the film in the desired areas. Such a 2D scanning process is very well known in the areas of laser marking and engraving systems. All of the above discussion has referred to the case where an optical projection system is used to transfer the pattern from a mask to the FPD. Such a method is convenient and straightforward as has been outlined. For some FPD exposure situations however it is possible to use a proximity mask in the SIS exposure mode disclosed. Such a technique is possible as the energy density on each laser pulse at the FPD is low (e.g. 5- 10mJ/cm²) so that a standard chrome on quartz mask can be used without risk of damage. A proximity mask SIS exposure process utilises a Ix mask of modest size (e.g. less than 100 x 100mm). This mask is held stationary at a small distance (e.g. 50μm) above the FPD surface and illuminated with IR radiation from a pulsed radiation source. The radiation source (or laser) is pulsed at the correct instants while the FPD is moved below the mask to achieve an SIS process as discussed above for the image projection case.

Clearly the most difficult aspect of this proximity mask SIS technique is that of controlling the distance between the FPD surface and the mask as the FPD is moved. This is in fact achieved rather easily by incorporating the mask into an air floating optical puck unit of the type described in Patents No PCT WO 2004/087363A1 and GB 0427104.5. Such an arrangement has been shown to be able to maintain a 50μm gap between the resist and mask to typically lOμm accuracy even over large area FPDs.

As with projection SIS exposure, for proximity SIS exposure it is also necessary to use moving blades to define the FPD boundaries correctly. Such devices can be incorporated into the floating optics head immediately above the mask without significant difficulty.

Illumination of the proximity mask must be performed with some care to avoid loss of resolution. If the gap between mask and film to be exposed is maintained at about 50μm to 100 μm it is necessary that the illuminating radiation is collimated to within a range of angles less than 10 milli radians to avoid significant loss of resolution. It is also necessary to ensure that the illumination is uniform across the mask in the direction perpendicular to the scan direction to ensure uniformity of exposure across the scan bands. Illumination uniformity in the beam direction parallel to the scan direction is not critical since any non-uniformities in this direction are averaged by the scan motion.

Illumination of the mask within the correct angular range and with the required uniformity is readily achieved by the type of conventional beam homogenisation systems used in other lithography tools and in laser ablation tools. A pair of cylinder lens arrays together with an output condenser lens can readily form a top hat one dimensionally uniform beam with low (less than 10 milli radians) beam angles if correctly designed, having the correct aperture and by being placed at the correct distance from the mask. Beam homogenisation in the scan direction can also be applied with another pair of cylinder lens arrays if desired but this is not essential.

A key issue for exposure of complex patterns in the film for FPD manufacture is the exact registration of the image pattern onto the existing display pattern on the substrate. For large Ix masks this is particularly problematic. It is readily possible to compare alignment marks on the display with alignment marks on the mask to correct for angular and spatial displacements but correction for distortion or size change of the display with respect to the mask is extremely difficult.

The present invention, where a small mask is used, allows not only the regular alignment of the mask image with respect to the display pattern to correct the angular and spatial offsets but also allows the possibility of dynamically correcting for size differences between the image and the display and also for display distortion. Correction for size changes in the direction parallel to the scanning direction are made by adjusting the firing time of the laser to exactly match the display pattern pitch while corrections for size changes in the direction perpendicular to the scan direction are made by making fine corrections to the sidesteps defining the distance between adjacent scan bands. Typical maximum size changes of displays are at the few microns level so with multiple scan bands this level of size correction can be made by adjusting the side step distance by a fraction of a micron at each step. The combined effect of multiple submicron step corrections allows overall size corrections up to several microns to be readily achieved.

It is also possible to correct for distortion of the display with the present invention. Since the image area projected is small, at each firing position very small movements can be made to the mask which gives rise to small corrections at the image plane. Both angular and spatial movements can be made to the mask during the scanning process or at the end of each scan band so that full corrections can be readily made to compensate for any: shape distortions up to a maximum size of a few microns over each display area. Such techniques can be used for both projected images and for proximity mask exposure.

The radiation used for illuminating the mask on an SIS laser thermal exposure or film curing tool can be from a range of sources. The primary requirements are that the wavelength of the radiation is such that it is absorbed sufficiently by the film to expose it effectively and that the sources must emit pulses that are sufficiently short to avoid image blur on moving substrates.

Examples of possible laser sources that can be used with this invention are the following: a) Low repetition rate pulsed semiconductor diode pumped or pulsed lamp pumped solid state lasers based on Neodymium as the active medium operating at (or close to) a wavelength of 1064nm. b) High repetition rate CW semiconductor diode pumped or CW lamp pumped solid state lasers based on Neodymium as the active medium operating at (or close to) a wavelength of 1064nm. c) Any other pulsed diode, lamp or other light source that emits radiation in pulses of duration less than one microsecond at a wavelength that is absorbed fully by the film and is effective in terms of thermally exposing it.

Clearly in all cases optical systems have to be used to create a uniform radiation field at the mask to ensure a uniform exposure dose at the film within the image area.

DESCRIPTION OF DRAWINGS

Exemplary embodiments of SIS laser exposure tool architectures will now be briefly discussed with reference to the accompanying diagrams; Figures \^~, 2, 3 and 47

Figure 1

This shows the principle of the SIS laser exposure method. A substrate 1 coated with a film layer 2 is moved progressively with respect to the exposing pulsed radiation beam 3 in direction Y. The beam creates an image on the film that corresponds to the required pixel or cell pattern of the FPD. In the figure the image is shown to contain 6 pixel cells in the direction in which the substrate is moving. Each pulse of radiation hence exposes a band of film that is 6 cells wide. Between laser pulses the substrate moves exactly 1 cell pitch so that the next pulse creates a pattern that exactly overlaps the first but is displaced by 1 cell pitch. In the figure shown where the beam is 6 cells wide each area of film receives 6 pulses of radiation and then moves from the beam. Figure 2

This shows a possible geometry for a low repetition rate pulsed IR laser SIS projection exposure tool. A glass substrate 5, coated with the film to be thermally exposed is supported on a two-axis table 6 able to move in orthogonal Xi and Yi directions. The mask 7 with the pattern to be transferred is mounted in the beam above the projection lens 8. The beam obscuring blades 11 are supported on another two-axis table 9 able to move in orthogonal X2 and Y2 directions. The mask may be mounted on a third two axis moving stage assembly for the imprinting of non-repeating patterns around the edge of the regularly patterned area if required. The two directions Yi and Y2 (and also Xi and X2) of the tables 6, 9 must be set up to be accurately parallel to each other.

A beam 10 from a low repetition rate pulsed IR laser operating at 1064nm is shaped and processed to create a uniform field at the mask 7. The illuminated area 12 provided by way of the mask 7 is imaged onto the film surface on the substrate 5 using a projection lens 8 with de-magnification factor of (for example) 2.

In operation the system works as follows. The substrate is aligned rotationally and spatially using alignment cameras that are not shown in Fig 2. The substrate is then moved to one edge and a band of film 13 exposed by movement of the FDP in direction Yi. Clearly as this is an edge band the structured edge on one side of the image needs to be obscured to prevent partial laser exposure so that the blade with its edge parallel to the Y direction is moved into the beam by moving the blade stage the correct amount in the X direction. At the start and end of each of the Y movements the blades attached to the table 9 progressively move into the beam 10 in direction Y to obscure the beam in a controlled way to define the edge of the exposed band accurately.

After completing this band the blade obscuring the image edge structure is removed from the beam and the substrate is stepped sideways (in direction Xi) by some appropriate distance that corresponds to the mean size of the image. Further substrate movements in Yi are then repeated. For the last band the appropriate side blade needs to be moved into the beam to obscure the structured image edge and set the exact pattern width. After complete coverage of the substrate the process is completed.

Figure 2 shows the case where the FPD is moved in the direction parallel to short axis and 10 bands are required to cover the full FDP area. Depending on the display and lens field sizes as well as the moving direction chosen the number of scans may be greater or less than 10. A typical lens field could be up to 50mm in diameter but smaller is more usual. Allowing for a structured image edge shape means that the side step distance may typically be in the range 20 to 45mm so that up to 50 or even more bands would be used to cover the full area of a 52" FPD when moved in the short axis direction whereas only 20 bands might be needed to complete the laser exposure of a 42" FPD when scanned in the long axis direction.

Figure 3 _ __ — - - -

This shows another possible laser exposure tool arrangement. Here the substrate stages are much larger so that glass sheets 14 with multiple FPDs can be exposed. Because of the larger size of the substrate it is convenient to restrict the motion of this stage to one axis (Yi). In this case the motion of the beam with respect to the substrate in the X direction is achieved by mounting the mask and lens assembly on a carriage that moves on a stage in the X direction on a gantry over the substrate. Such an arrangement using split axes is convenient for large substrates as the footprint of the tool is reduced.

Figure 3 also shows the use of two parallel identical optical projection channels creating two exposing areas (A, A') on the FPD substrate 15 at the same time. Such an arrangement reduces the total laser exposure time without having to increase stage speed. It is certainly technically possible to have more than two parallel projection channels operating at the same time. If the sheet to be processed is sufficiently large, systems having 8 or even more optics heads fed by either a single laser or multiple lasers can be envisaged. The practical limit is set by the proximity of the masks and blade stages on the optics heads as well as the increasing complexity of the tool.

Different tool architectures to those shown in Figures 2 and 3 are also feasible. For the case where the substrate is very large it is possible to maintain it stationary during laser exposure and have the optical mask projection system move in 2 axes. In this case the mask and projection optics are carried on a carriage that can move in two axes on gantries over the top of the substrate.

An alternative arrangement is to operate with the substrate held in the vertical plane. Such an arrangement could apply to both of the architectures shown in Figures 2 and 3 but is likely to be more easily realised for the split axis system shown in Figure 3. In this case the (large) substrate to be exposed would be held on its edge and move horizontally in the Yl direction while the mask stages move in the parallel Y2 direction. Movement of the laser exposure pattern along the length of each FPD is achieved by stepping the mask carriage vertically in the Xl direction with corresponding mask position correction by movement in the parallel X2 direction.

Figure 4

This shows an arrangement similar to that shown in figure 2 but where a beam scanner unit 16 is included in the optical projection system to allow SIS exposure with the beam from a high repetition rate pulsed IR laser 10 to be carried out. In this case the image is moved with respect to the substrate in BTS mode by the beam scanner unit in the Xl direction perpendicular to the direction Yl in which the substrate is moving and at the end of each band the substrate is stepped sideways in the Xl direction by the width of the band. As with the low repetition rate pulsed IR laser case other tool geometries are also possible for high repetition rate pulsed IR laser SIS exposure. The substrate can remain stationary at all times and the optical system consisting of projection lens, scanner unit and mask moved in two orthogonal axes or alternatively the substrate can move in only one direction and the optical system moves in the other. Vertical orientation of the substrate is also possible.

Claims

1 A method for forming a high resolution, regularly repeating pattern in a thermally sensitive thin film on a substrate by exposing it directly with radiation from a pulsed IR laser which has been caused to pass through a suitable mask delineating the pattern, the image of the mask pattern being formed on the surface of the film by a suitable optical system so that the energy density at the film is not sufficiently high so as to cause the film to be removed directly by ablation, the imprinting steps being carried out : (i) in a repetitive series of discrete laser exposure steps using a mask that represents only a small area of the total area of the substrate and using a single short pulse of radiation at each step to illuminate the mask; and (ii) the series of discrete laser exposure steps being repeated over the full area of the surface of a substrate, to give a full pattern comprising a plurality of pixels, by moving the laser beam or substrate in a direction parallel to one axis of the pattern to be formed on the substrate and activating the pulsed laser mask illumination light source at the instant that the substrate or beam has moved over a distance equivalent to a complete number of periods of the repeating pattern on the substrate;

2 A method as claimed in Claim 1 wherein during the imprinting stage the size of the illuminated area at the substrate in the direction parallel to the direction in which the substrate or beam is moving is sufficient to provide that, after passage of the substrate under the illuminated area, each part of the film has received a sufficient number of pulses of radiation to fully expose it.

3 A method as claimed in any preceding claim wherein the imprinting stage makes use of an optical projection system to transfer the mask pattern on to the substrate. A method as claimed in any preceding claim wherein the mask is the same size as a small area of the full pattern on the substrate and during the imprinting stage the mask is held in close proximity to the substrate by attaching it to a puck that is caused to float on the surface of the moving substrate by means of an air flow emitted from the puck.

A method as claimed in any preceding claim wherein the pulsed light source is a pulsed lamp or pulsed diode pumped solid state laser operating in the infrared (IR) region of the spectrum emitting pulses at repetition rates in the range 50Hz to 2kHz.

A method as claimed in any preceding claim wherein the pulsed light source is a CW lamp or CW diode pumped solid state laser operating in the infra-red (IR) region of the spectrum emitting pulses of radiation at repetition rates in the range 5kHz to 5OkHz.

A method as claimed in any preceding claim wherein during the imprinting stage an edge of the area to be exposed on the substrate is defined by means of moveable blades located close to the surface of the mask.

A method as claimed in any preceding claim wherein the mask moves at an appropriate time during or after the moving laser exposure process to allow non-repeating border regions of the device to be imprinted

A method as claimed in any preceding claim where the substrate is exposed in a series of parallel bands and the dose of illuminating radiation at the regions where the bands overlap is controlled by using an image forming mask that has a stepped or randomised transmission profile at each side of the mask pattern, the steps or random features corresponding to one or more complete cells in the FPD array.

A method of forming a regularly repeating pattern on to a substrate as hereinbefore described with reference to the accompanying Figures 1 to 4.

A laser exposure tool for carrying out the method of any preceding claim.

A product formed by means of a method as claimed according to any of preceding claims 1 to 10.