GB2325634A

GB2325634A - Image projection process

Info

Publication number: GB2325634A
Application number: GB9711037A
Authority: GB
Inventors: Paul Joseph Marsden
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1997-05-29
Filing date: 1997-05-29
Publication date: 1998-12-02
Also published as: GB9711037D0

Abstract

A plurality of images (3) are produced on a substrate (4) by passing a beam of collimated light through two or more spaced apart apertures (5) in a mask (1) and through a multi lens array (6) onto the substrate (4). The pitch of the multi lens array (6) is substantially equal to the spacing (P 1 ) between the mask apertures (5) so that the distance (P 2 ) between the images (3) produced on the substrate (4) is substantially equal to the mask aperture spacing (P 1 ).

Description

IMAGE PROJECTION PROCESS This invention relates to an image projection process for projecting a plurality of images onto a substrate which is particularly, but not exclusively, suitable for the production of a series of holes in a substrate.

A conventional imaging process in the field of laser processing is shown in Figure 1 of the accompanying drawings.

A mask 1 is illuminate by a light source such as a laser in the direction A for an exposure period which is dependent upon the requirements of the process. The light transmitted through the mask 1 forms the object which is optically demagnified through a lens 2 onto the surface of material or a substrate to be patterned. The feature size of the image 3 on the substrate is given by the following formula: 1 1 1 U + y - . . . (1) Where u is the object distance from the lens 2, v is the image distance from the lens 2 and f is the focal length of the lens. The value of the demagnification and related image size is given by the equation: v f I Magnification = u = u f . . . (2) Thus for a mask aperture of size S1 the resulting image size S2 on the material or substrate is given by the equation: S2 = M.S1 . . . . (3) Thus if the focal length f is lO0mm and the object distance u is 300mm the magnification will be halved and the image size Swill be half the size of the aperture S1. This for many purposes is unsatisfactory. Additionally, as can be seen from Figure 2, if it is required to image a plurality of apertured shapes from the mask 1 onto a substrate 4, not only are the images 3 reduced in size they are also reduced in pitch P2 from the pitch or separation P1 on the mask 1. This change in pitch is given by the equation: P2 = M. P1 . . (4) Hence if the focal length f is lOOmm and the object distance u is 300mm P2 = P2. Thus the pitch also is demagnified by a factor of 50 percent. This can be overcome by indexing the workpiece past the lens 2 as shown in Figure 1 to any desired distance so that images 3 are produced at any desired pitch P on the workpiece. However this is time consuming as it requires not only separate exposures for each image production sequence but also the indexing of the workpiece by a precise distance between each image exposure.

However, this conventional process also suffers from an undesirably low level of light transmission through the mask 1. The final image size S2 uniquely determines the size of the mask aperture S1. The size of the uniform light source which can be called A is a fixed property of the light source itself. The transmission efficiency T is therefore given by the equation: T- Sl2 . . . (5) A In other words all the light energy incident on the opaque areas of the mask 1 is wasted. Increasing the open area of the mask 1 by including more apertures as shown in Figure 2 increases the efficiency but the images 3 are not at the desired pitch P1 as the separation P2 between adjacent images 3 is demagnified.

There is thus a need for a generally improved imaging process which is capable of producing multiple images at the same pitch or spacing as apertures on the mask.

According to one aspect of the present invention there is provided a process for projecting a plurality of images onto a substrate, in which a beam of collimated light is passed through two or more spaced apart apertures in a mask, which apertures have a desired shape to be imaged onto the substrate, through a multi lens array onto the substrate, with the lens of the multi lens array having a pitch substantially equal to the spacing between the mask apertures, so that the distance between the images produced on the substrate is equal to the mask aperture spacing.

Preferably the beam of collimated light is produced by a laser.

Conveniently the laser light is produced and imaged onto the substrate so as to ablate the substrate to the shape of the mask apertures imaged onto the substrate.

Advantageously the distance between the substrate and the multi lens array and the diameter of the mask apertures is/are varied to vary the size of image produced on the substrate.

Preferably the mask apertures utilised are shaped, the collimated light utilises selected and the multi lens array utilised is positioned and the lens pitch selected so as to be operable to drill holes in the substrate.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings; in which Figure 1 is a schematic view of a conventional imaging process using a single aperture mask, Figure 2 is a schematic illustration of a further conventional imaging process using a multi apertured mask, Figure 3 is a schematic view of an imaging process and apparatus according to the present invention, Figure 4 is a diagrammatic illustration of the use of the conventional process of Figure 1 for producing an aperture in a substrate and, Figure 5 is a diagrammatic illustration of the use of the process of the present invention of Figure 3 for producing an aperture in a substrate.

A process according to the present invention for projecting a plurality of images 3 onto a substrate is shown in Figure 3. A beam of collimated light produced in any convenient manner is shone in the direction A on the mask 1 and passed through the apertures 5 therein. The apertures 5 have any desired shape such as the shape of aircraft as illustrated in Figure 3. The apertures 5 are spaced apart by a distance P1. Each aperture 5 has a size S1. The light passing through the mask 1 passes through a multi lens array 6 onto the substrate 4. The lens of the array 6 have a pitch P3 substantially equal to the spacing P1 between the mask apertures 5 so that the distance or pitch P2 between the images 3 produced on the substrate 4 is equal to the mask aperture spacing or pitch P1.

Thus for the process of the present invention: P2 = P3 = P1 (6) The magnification is such that applying equation (3) the image size S2 is half the image size S1. Hence the pitch remains fixed and is equal to the pitch of the array 6 but the size of the image 3 is demagnified.

With a conventional simple lens system as shown in Figure 4 the size of the image or hole produced in the substrate 4 if the collimated light beam size is a laser and the system is being used for drilling the substrate 4 is fixed for a given lens and laser beam. Thus this relationship is given by the equation: hole diameter = focal spot diameter = fi . . . (6) 0 where 8 is the beam diameter, f is the focal length of the lens 2 and 1 is the wavelength of the collimated light.

With the process of the present invention, particularly when used for hole drilling by ablation, the size of the image and thereby of the hole produced on the workpiece 4 is as shown in Figure 5 of the accompanying drawings in which a single lens element of the multi lens array 6 is shown for convenience. In this process of the invention the size of the image 3 produced on the substrate 4 and thus the diameter of the hole formed through the substrate 4 if a laser beam 7 is used for ablation is given by the following relationship: Image size = spot diameter = (U6fh . . . (7) Where the letters in the equation (7) are as previously described.

By varying the object distance u and the mask aperture size 6 the image 3 and hence any hole produced therewith can be made variable for a given lens. The shape of the hole or image produced is dependent solely on the shape of the mask aperture 5 chosen.

The process of the present invention can be used to improve the efficiency of any optical projection patterning system where an image is to be repeated at a fixed pitch such as in photolithography.

Claims

1. A process for projecting a plurality of images onto a substrate, in which a beam of collimated light is passed through two or more spaced apart apertures in a mask, which apertures have a desired shape to be imaged onto the substrate, through a multi lens array onto the substrate, with the lens of the multi lens array having a pitch substantially equal to the spacing between the mask apertures, so that the distance between the images produced on the substrate is substantially equal to the mask aperture spacing.

2. A process according to claim 1, in which the beam of collimated light is produced by a laser.

3. A process according to claim 2, in which the laser light is produced and imaged onto the substrate so as to ablate the substrate to the shape of the mask apertures imaged onto the substrate.

4. A process according to any one of claims 1 to 3, in which the distance between the substrate and the multi lens array and the diameter of the mask apertures is/are varied to vary the size of image produced on the substrate.

5. A process according to any one of claims 1 to 4, in which the mask apertures utilised are shaped, the collimated light utilises selected and the multi lens array utilised is positioned and the lens pitch selected so as to be operable to drill holes in the substrate.

6. A process for projecting a plurality of images onto a substrate, substantially as hereinbefore described and as illustrated in Figure 3 as modified or not by Figure 5 of the accompanying drawings.