WO1994027187A1

WO1994027187A1 - Fabrication of microcomponents

Info

Publication number: WO1994027187A1
Application number: PCT/GB1994/000974
Authority: WO
Inventors: Donald Richard Purdy
Original assignee: Gec-Marconi Limited
Priority date: 1993-05-13
Filing date: 1994-05-05
Publication date: 1994-11-24
Also published as: AU6683194A; GB9310013D0; GB2277998A

Abstract

Microcomponents such as microlenses or other micro-optic components are fabricated by known exposure of a pattern onto photoresist material such that the intensity on each elemental area of the photoresist material corresponds to the depth of material to be etched away after the photoresist is exposed and developed. The area of the surface is notionally divided into elemental squares each of which in the mask is given an appropriate grey scale value by means of an appropriate number of small, precisely arranged openings. To blur the sharp transitions this would produce in the exposed and developed photoresist shape, the image of the mask on the photoresist is arranged to be out of focus by a predetermined amount such as between lines (12a, 12b) to smooth the sharp transitions and/or for the use of openings in the mask of a size less than the diffraction limit of the imaging optics.

Description

FABRICATION OF MICROCOMPONE TS

This invention relates to the fabrication of components, especially but not exclusively micro-optical components.

Typical of micro-optical components are microlenses. The sizes of such microlenses

may typically range from a diameter of a few micrometres to a few millimetres.

Photoresist has been used as the material for microlenses. Resists are generally applied

to a substrate, and selectively exposed to light, which causes chemical changes in portions of the resist. The exposed layer is then developed to selectively remove either the exposed portions (positive resist) or the unexposed portions (negative resist). As an alternative to using the photoresist as the material for the microlens, the photoresist shape which replicates the lens has been used as a sacrificial image by ion-beam etching the sacrificial image to transfer the shape to the substrate (which is the desired optical material) to produce the microlens (Wada, O., "Ion-beam etching of InP and its

application to the fabrication of high radiance InGaAsP/InP light emitting diodes", J.

Electrochem. Soc. vol. 131 no. 10 1984 p. 2373).

The shaping of the photoresist has been attempted in a variety of ways. Thus, in Wada,

advantage is taken of the effect of surface tension to shape the resist when heated.

Other workers have used multiple masks to create a desired lens shape. Each mask is

first exposed, the resist developed to remove the required areas. The revealed areas of

the substrate are etched to the required depth. The first layer of resist is then removed and the next mask is applied and the process repeated. One problem with such a technique is obtaining the necessary registration between the masks for the various

exposures. Another is that the resulting lens shape has a stepped appearance which will

probably be undesirable from the point of view of its optical properties. Another

technique is the direct write technique reported by Bird ("The computer controlled

generation of microlens arrays", K. Bird et al, I.O.P. short meeting no. 30, Microlens

Arrays 1991). This comprises a focused light spot of varying intensity to provide the

exposure, and movement of the resist layer in the perpendicular plane to achieve the

complete patterning of the resist image. When developed, the variable exposures result in varying thickness in the developed photoresist. In US patent number 4 861 140, a scanner is used to expose a photographic film, and this is used for contact printing the photoresist layer to produce the shaped component in the photoresist after developing. It has also been proposed (Hutley M.C. et al, The Manufacture of Blazed Oblique Zone Plates for use at 10.6 μm National Physical Laboratory Report MOM 91) to contact print photoresist using photographic film, such that the variation of density in the film is recorded as a variation of depth in the developed photoresist. The desired density in the

film was produced by photographing a larger pattern which was produced on paper using

a computer and photo typesetter, the required variations in density being produced by

corresponding areas of half-tone greys. The method was used for the production of

blazed zone plates.

The invention provides a method of fabricating in a material a component having a

surface which varies in height relative to a notional datum plane, which includes the steps of exposing a photoresist material to light projected through a mask having an array of openings, the openings being sized and arranged such that the intensity of the light passed by the openings in each part of the mask corresponds to the height of the respective parts of the surface above the datum plane, developing the exposed

photoresist material to produce a shape in the photoresist material corresponding to that

of the surface of the component, and using the developed photoresist material to produce the component in the material.

The use of the mask, the apertures of which represent varying grey scale densities, to

produce a photoresist replica which is then used to produce the component in a desired

material enables complex components to be produced with improved accuracy compared to the use of photographic film in which the nature of the emulsion inherently restricts the amount of detail which can be recorded in the film. It is to be understood that the

terms light, optical etc used herein are not limited to the visible range of the electromagnetic spectrum but includes other appropriate wavelengths such as ultraviolet.

To avoid discontinuities on the surface of the component resulting from- he digitisation produced by the openings, the optical system is preferably such that the individual

openings cannot be imaged with sharp edges in the image of the mask on the photoresist

material, by choice of the size and spacing of the openings in relation to the aperture of

the optical system and the wavelength the light below the diffraction limit of the optical

system and/or by positioning the photoresist offset from the focal plane of the optical

system.

Fabrication of components, particularly micro-optic components, constructed in accordance with the invention, will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a part of a silicon microlens array fabricated in

accordance with the invention;

Figure 2 is an end view of the array of Figure 1 illustrating the transfer of the exposed

and developed photoresist replica to the substrate;

Figure 3 illustrates the projection exposure of a photoresist through a grey scale mask;

Figure 4 is a perspective view of a part of a single microlens illustrating the procedure for designing the mask;

Figure 5 is a section taken through the remote face of the microlens part in Figure 4 also illustrating the procedure for designing the mask;

Figures 6a to 6d show four possible grey scale densities which could be used for each

of the squares in Figure 4;

Figures 7a and 7b show two possible ways of producing the grey scale densities for each

square of Figure 4;

Figure 8 illustrates characteristic curves relating the amount of photoresist removed against the exposure time;

Figure 9 illustrates the exposure of an initial reticle by an electron beam;

Figure 10 illustrates the preparation of a photo-mask for exposing the photoresist;

Figure 11 illustrates the exposure of the photoresist in accordance with the invention;

and

Figure 12 illustrates the predetermined out of focus positions of the photoresist surface.

Figure 1 illustrates a part of an array of microlenses to be fabricated in accordance with the invention. The microlens could be used, for example, with a corresponding array

of infrared detectors. The lenses are made of silicon, but are first of all made in

photoresist by exposing silicon coated with photoresist (Figure 3). A mask 1 is illuminated with ultraviolet radiation shown by arrows and an optical system 2 (illustrated schematically) projects a reduced size image of the mask onto the photoresist

coated silicon 3. As will be explained below, the photoresist 4 after development and

exposure reproduces the desired shape of the component 5. In order to produce the

shape desired in the silicon substrate 6, known etching techniques may be carried out,

such as ion-beam etching, which removes material at a uniform rate. The rates of

erosion of the chosen photoresist layer and silicon substrate 6 are made identical by

control of the erosion parameters so the effect of a uniform removal of the surface of the developed component shape is to transfer that shape 7 into the surface of the silicon substrate.

As commonly used for the fabrication of integrated circuits, photoresist is either totally

removed or totally unaltered to permit etching of only the desired areas of the integrated

circuits. For the purposes of the present invention, a thicker layer of photoresist is used,

and different depths are exposed, depending on the intensity of illumination. Thus,

referring to Figure 4, each part of the surface to be fabricated is plotted into elemental

squares in the X-Y direction, and the height of the respective squares in the Z-direction

is derived. When the photoresist is exposed through a mask, the intensity of

illumination in each small square of the X-Y grid is matched to the respective Z co¬ ordinate of the square. Assuming that the image of the mask is projected onto the top surface of a photoresist block identical to the block shown in Figure 4, the intensity of illumination for the square of the left-hand edge of the back wall will be very much less than the intensity of illumination for the square of the rear right-hand edge. This is also apparent from Figure 5, which is a section through the rear face of the block of Figure 4. Using the gamma curve for the photoresist (Figure 8), which relates the amount of photoresist exposed (and hence removed upon development) with the time of exposure

for a given intensity of illumination, and relying on test exposures, the transmittance

required for each X-Y square to produce the required exposure in the Z direction was

obtained.

The transmittance for each small X-Y square is varied by arranging that each small X-Y

square as projected onto the surface of the photoresist is of a form like one of the squares shown as Figure 6a, 6b, 6c or 6d. Each square contains a variable sized window or via defined by chromium on glass. In Figure 6a, the via is small in size, and in Figures 6b, 6c and 6d, it progressively increases in size. Thus the desired transmittance

can be obtained by arranging that each X-Y square of the projected mask image has the

appropriate sized via. A grey scale range of approximately 10,000 discrete via sizes

gives a sufficient range to fabricate most components. An individual via size could be

a variable size eg 0.125 μm square in a fixed square of 0.5 μm, and the exposing light

could be 0.4 μm wavelength. These conditions constitute an object of a size below the

resolution limit of the optical projection equipment. This invention uses average light

intensity at the exposing surface, not resolved sharp image edges, ie. adjacent vias

cannot be resolved as separate images on the photoresist.

Thus, after the photoresist is exposed, the exposed photoresist is developed and removed (since the resist is a positive resist), to leave a replica in photoresist of the component shape it is desired to produce.

It should be noted that a grey scale of transmittance values can be produced in two ways, viz., with black dots of variable size (as in a photograph in a newspaper) as

shown in Figure 7a, or with black dots of constant size but variable spacing, as shown

in Figure 7b.

In an embodiment of the invention described with reference to Figures 9 to 12, the

former arrangement is used. In order to define sufficiently small features of the

component configuration, the image projected onto the photoresist on a silicon substrate

is reduced in a two stage process, although more than two stages of reduction could be employed if desired.

First, referring to Figure 9, a chromium film on glass 8 covered by electron resist is

exposed with an electron beam writer over squares of varying size (the squares being

shown criss-crossed by diagonals in Figure 9) in a pattern in which the grey scale value

of each elemental square of mask (corresponding to the entire area illustrated in Figure

9) corresponds to the vertical amount of photoresist it is necessary to remove to create

a photoresist replica of a micro-optic component. The exposed electron resist is then developed and removed and the exposed chromium etched away. The remainder of the

photoresist is then removed.

The electron beam mask 8 is then reduced to produce a working mask 9 by means of an optical system 10. The electron beam mask 8 (chromium on glass) is illuminated by ultraviolet radiation, and is focused onto a chromium film on glass 9 covered with ultraviolet sensitive photoresist. The area of the electron mask 8 does not fill the whole of the layer 9. Once one exposure is made therefore, the layer 9 is stepped in X and Y

directions (i.e. in the plane of the layer 9), and a separate exposure is made at each

location. In this way, an array of micro-optic components is exposed onto the

photoresist layer. The exposed areas are developed to remove them, and the underlying

chromium etched away. The remainder of the photoresist is then removed. Additional

stages of size reduction may now be undertaken if desired. When developed, the final

photo-mask is able to generate an array of micro-optic components.

Referring to Figure 11, the photo-mask 9 is illuminated with ultraviolet radiation and imaged by means of the optical system 11, again as in Figure 10 on a reduced scale, on a sihcon substrate based layer of photoresist 12. Also again the whole of the area of the photoresist 12 is not illuminated by the imaged photo-mask. Hence the photoresist 12

is stepped in the X and Y directions (i.e. in the plane of its surface), and an exposure

is made at each new position. The exposed photoresist is then developed and removed

as previously described, leaving the multiple arrays of micro-optic components replicated

in photoresist. In the manner described, the multiple arrays are then used for direct

replication or are transformed into the sihcon substrate by ion-beam etching to produce

the components required in the material.

A key feature of the invention is the use of average light intensity at the exposing surface, since the optical system is such that the individual openings cannot be imaged

with sharp edges at the exposing surface. Thus, the size and spacing of adjacent

openings, and the wavelength of the exposing light, in relation to the aperture of the exposing lens 11, is such that the edges of the openings at the exposing surface are not individually resolved. That is, the dimensions and spacing of the openings in the

photomask 9 is below the limit set by the Rayleigh criterion. The angle subtended at the lens 11 by two adjacent vias must be less than the Rayleigh criterion for resolution,

namely,

1.22λ

where λ is the wavelength of the exposing light, and a is the diameter of the lens 11. The resultant intensity level at any point in the final image is then determined by summing the contribution of the central bright peak of the appropriate via for this location plus contributions from the diffraction pattern of all the neighbouring vias that fall on this site. Every location on the resist surface is receiving exposing radiation by

this method and the resultant range of intensity levels across the resist surface smooth

out the digital nature of the mask system used.

The use of average light intensity is facilitated as well by the arrangement of Figure 12.

It will be noted that rays are drawn from two dots on the photo-mask 9 and traced

through onto the photoresist 12 in Figure 11. Whatever separate features these dots

represent on the photo-mask, they will be recorded as individual features at the photoresist. Figure 12 shows this more clearly. However, as has been explained before, each individual micro-optic component is built up of adjacent small squares of appropriate grey level, which is digital in nature representing as it does one of a discrete number of values. While the individual openings are not resolved at the image plane, mere may still be abrupt edges in the finished component. This could be a disadvantage.

It will also be noted that in Figure 12, the cones of ultraviolet light defining those

respective features overlap the dotted lines 12a and 12b.

The photoresist is positioned so that the image of mask 9 on it is not m focus as it

would be at the image plane 12, but is defocussed to a predetermined extent by being

positioned at the dotted line 12b. This blurs the digitisation of the image of each

component and consequently smooths sharp transitions in the component. The positions

of dotted lines 12a, 12b depend on the closeness of the features of the mask which it is

desired to blur. The photoresist may be of course positioned between the lines 12a and 12b on either side of the plane of focus 12. As an alternative, the desired blurring of the image on the photoresist which negates the effect of digitisation could be effected

wholly by the offset positioning of the photoresist as in Figure 12, in an arrangement where it was desired for some reason for the individual openings to be sized and spaced

above the resolution limit of the imaging optics.

It should be noted that projection printing permits exposure of the photoresist in a

manner not possible with contact printing e.g. it would not be possible to form undercuts

if contact printing (as hitherto) was used.

While the dimensions and size of the openings will normally be below the Rayleigh criterion in the photomask 9 as far as exposure in the optical system of Figure 11 is concerned, via size and spacing must be designed to remain above this limit in the intermediate stage shown in Figure 10, and for the production of the electron mask in Figure 9 i.e. individual edges of the vias exposed in the electron mask, and of the vias

in the photomask 9, must be sharp, so there must be resolution of adjacent vias as separate images and no defocussing. All the detail of the component to be produced must be recorded in the photomask, which was not possible when a photographic film

was used hitherto.

A typical size for the electron beam written reticle 8 is 100mm x 100mm. It may have

a grey scale with approximately 10,000 equally spaced grey tones. A typical size for

the photo-mask 9 is also 100mm x 100mm. Typically the reticle pattern will be reduced by 1/5 or 1/10 of its actual size during the exposure with ultraviolet light. An individual via size could (like those in Figure 3) be a variable size eg 0.125μm square in a fixed square of 0.5μm in a rectangular grid as shown in Figure 4, and the exposing light could

be 0.4μm wavelength. The photo-mask may be stepped in the X and Y directions to

record 100 x 100 images. Typically the reduction in exposing the photoresist is also 1/5

or 1/10. The photoresist may be stepped to produce 500 x 500 the image on the photo¬ mask. This could fill an area of up to 100mm square. The type of photoresist needed

for work of this nature is one formulated to give thick layers in excess of 10 μm. All

samples of resist are coated onto substrates, and the pre-bake performed in a free- flowing air oven. The thickness is adjusted during application to achieve a layer which

just exceeds the depth range required by the design. It has been found possible to etch within a range of from 0.5 μm to the maximum thickness below the original resist surface using the grey scale master.

Of course variations may be made without departing from the scope of the invention.

Other erosion techniques than ion-beam milling may be used to transfer the photoresist replica to the substrate. Also, substrates with different etch rates to the photoresist

replica may be used. In such a case the shape in the photoresist which corresponds to

the desired component shape may need to be internally elongated or shortened in the

direction of etch. Instead of transferring the shape of the developed photoresist to a

substrate, the developed photoresist may be used to produce the component in another

material (or for that matter in photoresist itself) by using the developed photoresist as

a master. Thus, the resist image could be electroplated, and the electro-form could be

used to produce components by moulding or embossing. Alternatively, an image transferred onto a substrate by erosion could be used in this way. Different reductions or 1:1 or enlargements are possible for the processes of Figures 10 and 11.

The pitch of adjacent openings is 0.5μm with an exposing wavelength of 0.4μm in the

examples of Figures 1 to 12 but, more generally, the pitch of adjacent openings may lie

within a range of from one tenth to twice the wavelength of the exposing light,

preferably within a range of from one half to twice the wavelength of the exposing light,

and most desirably, within a range of from one half to one and a half times the

wavelength of the exposing light. This applies whether the photoresist is exposed at a

plane offset from the focal plane of the optical system, or whether it is exposed in the

focal plane. The possibility also exists of incorporating areas of different or varying pitch, or continuous lines (as in a phase shift mask) of bright or dark areas at varying pitch, in the mask image in order to achieve different diffraction effects at the resist surface to aid the formation of abrupt changes in height of the resist image when these are required by the design. Different photo-mask and final image areas are possible, as are different numbers of step and repeat operations.

As described with reference to Figure 9, the reticle 8 from which the photomask 9 is produced, is produced using an electron beam writer. Nevertheless, other methods of

producing the reticle are possible. For example, a laser fibre optic exposive system may

be employed, in which an ultraviolet pulse or spot exposes photoresist. While the

resolution expected from an electron beam writer might be expected to be superior to

that from a system using ultraviolet light, it must be remembered that optical means can

be more accurate for focussing than coils for focussing electron beams, and faster

exposure is possible with optical means. The components fabricated have been described as microlenses and micro-optic components. However, more generally, any microcomponent may be made whether for optical or other purposes.

Claims

1. A method of fabricating in a material a component having a surface which varies

in height relative to a notional datum plane, which includes the steps of exposing a

photoresist material to light projected through a mask having an array of openings, the openings being sized and arranged such that the intensity of the light passed by the

openings in each part of the mask corresponds to the height of the respective parts of

the surface above the datum plane, developing the exposed photoresist material to

produce a shape in the photoresist material corresponding to that of the surface of the

component, and using the developed photoresist material to produce the component in the material.

2. A method as claimed in claim 1, in which the optical system is such that the

individual openings are not imaged with sharp edges in the image of the mask on the photoresist material.

3. A method as claimed in claim 2, in which the spacing of adjacent openings and

the wavelength of the exposing light is such that adjacent openings are not resolvable as individual images on the photoresist material.

4. A method as claimed in claim 3, in which the pitch of adjacent openings lies

within a range of from one tenth to twice the wavelength of the exposing light.

5. A method as claimed in claim 4, in which the pitch of adjacent openings lies within a range of from one half to twice the wavelength of the exposing light.

6. A method as claimed in any one of claims 2 to 5, in which the photoresist

material is positioned at a plane offset from the focal plane of the optical system.

7. A method as claimed in any one of claims 1 to 6, in which the developed

photoresist material is used to produce a mould or die for production of the components.

8. A method as claimed in any one of claims 1 to 6, in which the photoresist material is on a substrate, and the shape of the developed photoresist is transferred into the substrate.

9. A method as claimed in claim 8, in which the shape of the photoresist is transferred into the substrate by ion-beam milling.

10. A method as claimed in any one of claims 1 to 9, in which the mask is of chromium on a transparent carrier.

11. A component fabricated using the method of any one of claims 1 to 10.

12. Apparatus for fabricating in a material a component having a surface which

varies in height relative to a notional datum plane, which comprises means for exposing

a photoresist material to light projected through a mask having an array of openings, the openings being sized and arranged such that the intensity of the hght passed by the openings in each part of the mask corresponds to the height of the respective parts of the surface above the datum plane and means for developing the exposed photoresist

material to produce a shape corresponding to that of the surface of the component in the

photoresist material.

13. Apparatus as claimed in claim 12, in which the optical system is such that the

individual openings cannot be imaged with sharp edges in the image of the mask on the

photoresist material.