Etching a substrate: processes – Forming or treating optical article – Lens
Reexamination Certificate
2001-08-21
2003-10-14
Powell, William A. (Department: 1765)
Etching a substrate: processes
Forming or treating optical article
Lens
C216S055000, C216S038000, C216S088000
Reexamination Certificate
active
06632375
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for the formation of a concave micro-relief in a substrate and use of this process for making different types of components or optical systems.
The invention is used for applications in micro-optics and particularly for the manufacture of concave mirrors, and also plane-concave, convex-convex or concave-concave type lenses. These components may be in individual form or may be integrated in optical systems. Micro-relief is also useful in the biotechnology field.
Note that the process according to the invention may also be used to make classical, cylindrical or spherical components with a given constant radius of curvature, or non-spherical components with a variable radius of curvature.
One particular application of the invention is the manufacture of a cavity and/or a mirror with a concave cavity for a microlaser with an unstable cavity, or for the production of a microdish that can be used in biotechnology equipment.
STATE OF PRIOR ART
For the purposes of this invention, the terms convex and concave are used to qualify the type of curved surface of an optical component, a layer of material or a substrate. These terms are applicable to the curved surface as seen from the outside of the said component, layer or substrate.
At the present time, there are several known techniques for manufacturing curved surfaces. Most of these techniques have been developed for the fabrication of spherical or cylindrical lenses.
FIGS. 1
to
3
in the appendix diagrammatically show the main steps of a process designed to apply a convex relief to the surface of a substrate.
A first step illustrated in
FIG. 1
comprises the definition of a resin embossment
10
on a substrate
12
. In this case the embossment is a disk. For example, the embossment
10
may be formed by the deposition of a photosensitive resin layer and by a photolithography treatment to eliminate the layer away from the embossment. The embossment
10
is centered on the part of the substrate on which the surface is to be curved.
A second step illustrated in
FIG. 2
consists of heating the resin embossment to make it melt. During this heat treatment, surface tensions will vary the shape of the resin embossment to change it to the form of a spherical drop (convex).
As the heat treatment is continued, the resin becomes cross-linked and solidifies while keeping its spherical shape.
A third step consists of transferring the shape of the resin drop into the substrate. This transfer takes place by applying a vertical anisotropic etching to the substrate and the resin. This etching eliminates a thickness of the substrate, that is thinner when the resin is thicker, at all points on the surface of the substrate. The resin is also eliminated as the etching progresses.
After the resin has been completely eliminated, the etching is stopped. This gives a substrate
12
conform with
FIG. 3
with a convex relief locally corresponding to the shape of the resin drop (that has disappeared).
The production of concave relief is more complex and more expensive. It requires an additional number of steps in the process.
According to a first technique, a substrate according to the substrate
12
in
FIG. 3
can be used as a stamping die to make a concave relief with a complementary shape in another substrate.
In particular, the stamping technique may be used to form the complementary concave relief in a layer of photosensitive resin. This resin layer may then be used for transfer by etching of the concave shape in a substrate, in accordance with the third step in the process described above.
Obviously in this case, the melting heat treatment is not performed on the resin, to avoid eliminating the relief formed by stamping.
Examples of this technique are described in documents (1) and (2), for which the references are given at the end of this description.
Other techniques for producing a concave relief have been developed in order to avoid the need to make a stamping die in advance and the corresponding costs.
These techniques also make use of a resin layer, and more particularly a photosensitive resin layer that can be formed according to photolithographic processes. A concave relief is applied to the resin by insolation for which the intensity is controlled locally.
The insolation intensity may be controlled (as shown in document (3)), using a lithography mask. The mask is used to create different “gray shades” and thus modulate the depth of the insolated resin.
This technique is advantageous in that it enables the development and structuring of a resin layer with a given relief in a single insolation cycle. However, lithography machines for embossment are expensive. The development and use of machines for the above process are also long and complex operations.
Insolation may also be controlled by controlling a laser beam or an electron beam used as the insolation source.
According to one possibility, the power of the source may be varied to correspond with the region of the insolated resin layer.
According to another possibility, the power of the source can also be kept constant while scanning the resin layer. In this case, the insolation is controlled by adjusting the speed and the relative displacement directions between the insolation source and the resin layer.
These techniques make use of opto-mechanical treatment benches that are also expensive and complex. They also introduce difficulties related to making joints at the center of patterns with a symmetry of revolution. These difficulties can reduce the quality of the relief obtained and therefore make the technique unsuitable for the manufacture of some optical components.
For example, an illustration of the techniques involving control of the insolation source is given in documents (4) and (5). The references of these documents, and the references of other documents mentioned, are given at the end of the description.
The resin layer may be scanned by an insolation beam combined with the use of a mask, as described in document (6). Therefore the technique described in this document is similar to the techniques described above and has more or less the same disadvantages.
Document (7) describes a method for making concave relief that does not make use of an intermediate layer of photosensitive resin that will define the shape of the relief.
The substrate in which it is required to form the relief is etched directly using a reactive ionic etching process. The etching anisotropy is controlled by adjusting the dimensions of an etching mask and the reactive etching parameters.
This technique is apparently advantageous, and has a scope limited to materials that may be etched using a reactive plasma. Furthermore, it cannot be used to obtain a good optical surface quality with a wide range of radii of curvature, or a good manufacturing efficiency.
A better manufacturing efficiency may be obtained by collective treatment of optical component preforms. According to this method, the preforms are embedded in a coating material and are then abraded and polished with the coating material. The surface of components may be made convex by selecting a coating material with a lower resistance to abrasion than the resistance of the preform materials, or it may be made concave if the abrasion resistance of the coating material is higher.
For example, this abrasion treatment may be applied to the production of plane-convex type microlaser cavities.
These cavities, also called unstable cavities, are used to increase the size of the beam and therefore the output power of the microlasers on which they are used. The concave radius of curvature of the surface of the microcavities is within a range from a few millimeters to a few hundreds of millimeter, for diameters ranging from a few tens of micrometers to a few hundred micrometers.
Documents (8) and (9) provide illustrations of these cavities.
As described above, the technique for the formation of concave (or convex) relief by abrasion is suitable for the collective and economic manufacture of optical components such
Marty Jean
Rabarot Marc
Commissariat a l'Energie Atomique
Krebs Robert E.
Powell William A.
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