GB2442016A

GB2442016A - Substrate exposure method and tool

Info

Publication number: GB2442016A
Application number: GB0618562A
Authority: GB
Inventors: Philip Rumsby
Original assignee: Exitech Ltd
Current assignee: Exitech Ltd
Priority date: 2006-09-20
Filing date: 2006-09-20
Publication date: 2008-03-26
Anticipated expiration: 2026-09-20
Also published as: TW200823607A; GB0618562D0; WO2008035043A1; GB2442016B

Abstract

A method for forming a regularly repeating pattern in a thin film on a substrate (5) comprises: imprinting on the applied film a pattern formed by exposing it by infra red 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 (7) 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 series of discrete laser exposure steps is repeated over the frill area of the surface of the substrate, to give a full pattern comprising a plurality of pixels, by moving the substrate or the mask in a direction parallel to one axis of the pattern to be formed on the substrate and activating 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.

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 him 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 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 ('1C') 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 INVENTION

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 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 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 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. 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 nonstraightness 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 nonstraightness 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. All 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 that these non regular features can be moved into the beam 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. Since the non repeating features always occur around the edge of the regular pattern on the FPD, the 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. 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 50pm gap between the resist and mask to typically 10pm 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 50 μm to 100 pm 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 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.

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.