WO1992016014A2

WO1992016014A2 - Method and apparatus for concave substrates

Info

Publication number: WO1992016014A2
Application number: PCT/US1992/001484
Authority: WO
Inventors: Thomas M. Crook; Theodore J. Podgorski
Original assignee: Honeywell Inc.
Priority date: 1991-03-07
Filing date: 1992-02-27
Publication date: 1992-09-17
Also published as: JPH06503209A; WO1992016014A3

Abstract

A method of fabricating a deposited concave mirror substrate (40) includes adjusting the background pressure in a vacuum system (3) and interposing a mask (50) between a directed flow of deposition material (60) and a flat base (30) so that the deposition material forms a concave substrate (40) on the base (30).

Description

METHOD AND APPARATUS FOR CONCAVE SUBSTRATES BACKGROUND OF THE INVENTION I. Field of the Invention:

The present invention is related to producing deposited concave substrates used for ring laser gyro mirrors and, in particular, is directed to a method which uses a mask for forming a deposited concave substrate for a multilayer mirror assembly.

High performance ring laser gyros (sometimes referred to as RLGs) require optical cavities formed by highly reflecting, multi-layer mirrors. For high performance, all of the mirrors must possess low scattering behavior at the wavelength of laser light employed in such gyros. In addition, one or more of the cavity mirrors must provide a positive focusing of the laser beams in such ring laser gyros.

The current methods of obtaining low scatter, high reflectance ring laser gyro mirrors require depositing multiple quarter wavelength optical thickness layers of alternating refractive index materials onto very low scattering substrates, whose surfaces have been carefully polished to achieve nearly complete planarity. For the focusing mirror or mirrors, the conventional practice is to optically grind or polish an essentially spherical concavity into suitable substrates. Such a substrate 10 is shown in Figure 1. Then, as shown in Figure 2, a multi- layered, high reflectance mirror 20 is deposited onto the substrate, producing a curved mirror appropriate for the RLG cavity. U.S. Patent No. 4,737,946 to Yamashita, et al. , for example, shows a waveguide layer used for processing optical data wherein a concavity is formed by polishing or etching.

U.S. Patent 4,776,868 to Trotter, Jr., et al. shows the use of a mask in a vapor deposition process for preparing a lens or lens array. The mask is so positioned between the vapor source and the substrate that obscuration by the solid portions of the mask around a hole in the mask cause the deposit to assume a curved surface and function as a lens. Trotter, Jr., et al. does not provide any teaching for fabricating a concave mirror substrate as may be used for a ring laser gyroscope mirror. Rather, Trotter, Jr., et al. is directed to depositing smooth convex surfaces to form lenses with a convex face.

U.S. Patent No. 3,846,165 is illustrative of a vacuum deposition process for applying an anti-reflective coating on a semi-conductor laser. U.S. Patent 3,846,165 employs a mask with a substantial open area placed in contact, or nearly in contact, with the surface of the object to be coated to confine the coating deposit to the proper location.

SUMMARY OF THE INVENTION The invention disclosed herein provides a concave mirror substrate, suitable for use in making a mirror for use in a ring laser gyro.

In one aspect of the invention, means for coating a substrate from one or more ion beam sputtering sources is provided, while interposing a mask between the substrate and the deposition source or sources. By means of suitable masks, the concavity of the deposited material layer or layers is determined by parameters including mask aspect ratios, mask-to-substrate separation and the thickness of material deposited.

In yet another aspect of the invention, a method is provided for fabricating a deposited concave mirror substrate comprising interposing a mask having a selected profile and surface dimension between the flow of deposition material and a base so that the deposition material forms a concave mirror substrate deposit.

In yet another aspect of the invention, multiple film layers of different materials are deposited onto the concave mirror substrate formed by the method of the invention.

In yet another aspect of the invention, the multiple layers of different materials deposited onto the concave mirror substrate form a ring laser gyroscope mirror. In yet another aspect of the invention, the flow of deposition material emanates from an ion beam sputtering process.

In yet another aspect of the invention, the flow of deposition material may be produced by an electron beam deposition process or any other vapor deposition process.

Yet another advantage of the invention is that it provides a method for manufacturing a large number of concave substrates simultaneously. Other objects, features and advantages of the present invention will become apparent to those skilled in the art through the Description of the Preferred Embodiment, Claims, and drawings herein wherein like numerals refer to like elements. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view in cross section of a mirror substrate having a surface which includes a concave portion.

Figure 2 is a side view in cross section of a mirror substrate of the type shown in Figure 1 having a multi¬ layer mirror deposited on the substrate.

Figure 3 is a view in . cross section which schematically shows one aspect of the invention whereby a concave deposit is deposited onto a flat substrate. Figure 4 is a view in cross section of a multilayer mirror deposited on a concave mirror substrate made in accordance with the invention.

Figure 5 shows schematically one example apparatus used in the fabrication of a deposited concave mirror substrate.

Figures 6, 7 and 8 are graphs showing the relationships between mask separation and center thickness, radius of curvature and efficiency, respectively.

Figure 9 is a schematic diagram of one embodiment of the invention employing an ion beam deposition process to fabricate a deposited concave mirror substrate. DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to facilitate understanding of the invention, it is discussed herein in terms of its use in fabricating a deposited concave mirror substrate. This example described in detail herein is meant by way of illustration and the invention is not to be considered as so limited. Those skilled in the art will appreciate that the invention can provide a deposited substrate having one of a variety of selected concave shapes, depending upon the mask used and the intended application.

Referring now to Figure 3, a base 30 having a flat surface, deposited concave mirror substrate 40, mask 50 and a directed flow of mirror substrate material 60 is shown. Background gas molecules 61 cause scattering of the deposition material indicated by arrows 60A. In one embodiment of the invention, the mirror substrate material 60 may be any material suitable for use in a directed flow deposition process, such as, for example, the dielectrics, zirconium dioxide, silicon dioxide or titanium dioxide and the metals zirconium or titanium. According to the method provided by the invention, the directed flow of mirror substrate material 60 is deposited onto the flat surface of the base 30 to form a deposited concave mirror substrate. Such deposition techniques are well known in the art and may be produced by, for example, ion beam sputtering as described in US Patent RE 32,849 to Wei, et al. and U.S. Patent 4,424,103 to Cole, which are incorporated herein by reference. Other well known deposition techniques may produce a directed flow of deposition material such as, for example, electron beam deposition techniques or any other vapor deposition process. Such techniques are long well known in the art.

The base 30 may be, for example, a silica based ceramic such as Cer-Vit, Zerodur or other crystalline silica based material. Cer-Vit and Zerodur are trade names for two-phase ceramics containing a high percentage (for example 85%-90%) of crystalline silica based material with the remainder consisting of glass (non crystalline) . Silica based glasses such as fused silica, soda-limes silicate, and the like may also be used. Other materials, well known in the art, may also serve as a base substrate material. Once the deposited curve mirror substrate is completed, multiple layers of optical films suitable for fabricating a ring laser gyroscope mirror may be deposited onto the concave mirror substrate. In order to deposit the mirror material, the mask is first removed and mirror material is deposited onto the concave mirror substrate in a manner as described herein using a suitable deposition process.

Referring now to Figure 4, a deposited concave mirror of the type suitable for use in a ring laser gyro is shown having been made by the process of the invention. The mirror comprises multilayer surfaces comprising multiple firm layers of at least two different materials 20 and 21. The multiple film layers 20, 21 may be comprised of alternating layers of optical materials having different indices of refraction. For example, in the case of a ring laser gyro mirror, the multiple film layers 20, 21 may comprise quarter wavelength thickness optical layers with indices of refraction greater than 2.0 alternating with optical layers having index of refraction less than 1.5. One example of a material having an index of refraction greater than 2.0 is titanium dioxide. One example of a material having an index of refraction of less than 1.5 is silicon dioxide. Zirconium dioxide may also be used as one of the alternating multiple film layers. Referring now to Figure 5, an apparatus for fabricating a deposited curve substrate is shown, including a support 70, mask 50, concave mirror substrate deposit 40 and base 30. The support 70 may be selected to have as little profile as possible. If the dimensions of the support are too large, shadowing will occur under the support. This shadowing may result in a slight groove or channel in the deposited substrate in the area directly under the support indicated generally by reference numeral 52. It may be desirable to rotate the substrate 30 in relation to the support and mask, however, this would result in a mechanically complex holding fixture. Such complex fixtures are undesirable inside of a deposition chamber which is kept under vacuum. Such rotation, however, should eliminate any shadowing effects. In one aspect of the invention, the mask 50 was chosen to have a selected profile and dimension such that the surface 54 which masks the base 30 allowed a deposited concave mirror substrate to form on the base 30. In one example of a deposited curve substrate made by Honeywell Inc. of Minneapolis, Minnesota, a piece of metal rod was used as a mask. Typical rods used in some experimental runs at Honeywell Inc. included rods having .1, .15 and .2 inch diameters. Alternatively, a flat disk having a surface area smaller than the base substrate may be used. Other types of masks that may be used are quartz rods, fused silica or the like. One example of a base which is of a convenient size for use in fabricating a ring laser gyro mirror assembly is a flat disk-like substrate having a diameter of about 1.0 inches and a thickness of about .25 inches. The radius of curvature indicated by R in Figure 5 depends on the specification of the laser cavity into which the mirror is to be mounted. Typically, the radius of curvature may be from 1 to 5 meters but can be larger, for example up to 20 meters, depending upon the application.

In fabricating deposited concave mirror substrates in accordance with the present invention, parameters which are controlled include the background gas pressure, the deposition thickness, separation of the mask from the base substrate (i.e. mask positioning) , the mask diameter and the support shadowing. The background pressure is controlled to control the molecular scattering of the depositing material. Those skilled in the art will appreciate that an increase in background pressure results in a higher rate of molecular scattering. some scattering is useful in reducing shadowing effects and achieving desired curvature. A typical vacuum used in an ion beam deposition process is about 10^*"⁴ Torr, for example. The deposition thickness is a function of deposition rate and duration as is well known in the art. The separation parameter will be discussed immediately below with reference to Figures 6, 7, and 8. The mask diameter is directly related to the amount of surface area which is concave on the base. Making the support as small as possible and as far away from the base as possible while still maintaining rigidity is important in this process in order to eliminate or reduce the shadowing effect as discussed above. Higher background gas pressure will also reduce the shadowing effect.

Referring now to Figure 6, the relationship between the separation of the mask from the base 30 is shown in its relationship to center thickness of the substrate deposit. As shown by plot CT in Figure 6 at 0 distance separation from the substrate, the center of thickness is 0 because the mask is in contact with the substrate at that point substantially blocking all deposition material. As the mask is moved further and further away from the base, the center thickness approaches a maximum thickness, namely the thickness of the unmasked portion 44, as shown in Figure 5 of the base.

Figure 7 shows in curve RC the radius of curvature of the deposited substrate as it relates to separation of the mask from the base 30. In general, the mask is separated from the base by a distance equivalent to the diameter of the mask, in the case of a circular mask, for example. Therefore, in one example having a .2 inch diameter piece of rod serving a mask, the separation from the base typically may be about .2 inches. Using such a mask, the resulting radius of curvature can be advantageously made in the range of 2.5 - 5 meters depending upon the background gas pressure, the rate of deposition and the time of exposure of the base to the directed flow of deposition. As those skilled in the art will appreciate, a longer deposition duration at a predetermined rate will increase the thickness and the radius developed is inversely proportional to the thickness. For example, doubling the thickness will cut the radius in half. The total thickness at the edges in the unmasked portions 44 of the deposited concave substrate depends upon the rate and time that the base is exposed to the directed flow of deposition material. Control of the rate and duration parameters are well-known in the art. In one typical example, the thickness of material deposited at the unmasked edges of a base may be in the range of about 2.5 - 3 microns yielding a concave curvature having a radius of about 2.5 meters.

Figure 8 is a graph showing curve E which is a graph of the efficiency of providing a minimum radius of curvature for a fixed thickness as related to separation of the mask from the base. That is, there is an optimal separation distance for each mask which will produce a minimum radius of curvature for a fixed thickness of deposited material. This separation will vary with the rate of deposition, duration of deposition, and mask dimensions. It is useful in practicing the invention to make the mask thin and to locate the mask close to the base. Some examples have been discussed herein above with reference to Figure 7. The mask support should be as far away from the base as possible while maintaining rigidity and minimizing shadow effects. Figure 9 illustrates a vacuum chamber 3 coupled to an ion beam source 1, a stream of ions 5, an electron emitter 7, a target 2, a flow of deposition material 60, apparatus as shown in Figure 3 including support 70, mask 50, mirror substrate 40, and base 30. Gas molecules 61 cause some scattering of the deposition material as indicated by paths 60A. The base 30 and support 70 are mounted conventionally to a fixture 11 in the vacuum chamber 3. Fixture 11 may be, for example, a planetary fixture of the type typically used in ion beam deposition processes. The ion beam source may comprise a linear ion source such as is available from Ion Tech, Inc. , Ft. Collins, Colorado 80522. The electron emitter 7 is used to neutralize charged ion beam 5 when non-conductive materials such as dielectrics or when semi-conductor materials are sputtered.

The ion beam source 1 emits an ion 5 directed at the target 2. The atmosphere within the ion beam source is controlled to provide sufficient gas to sustain a discharge which generates ion beam 5. Ion beam 5 impacts target 2 which may be a rotatable multi-target assembly for use in applying more than one material in the same vacuum cycle. That is, each face of the target 2 may be a different material. A multilayer concave mirror substrate may be fabricated by rotating the target in a well known manner and using appropriate, alternating target materials on the faces of target 2. The impacted target area provides a source of sputtered target material which sputters in all directions forward of the target. The sputtered target material bombards base 30. However, a portion of base 30 is partially blocked by mask 50, although some sputter material is still deposited under the area covered by mask 50 because of molecular scattering of sputtered material by the background gas 61 and divergence of the sputtered beam. By controlling the size and shape of the mask 50 as explained herein, a concave mirror substrate 40 is formed on base 30. The target materials may be comprised of materials as indicated above such as zirconium dioxide, silicon dioxide, titanium dioxide or any other materials or combination of target materials suitable for use as a concave substrate. Once the concave substrate 40 is formed, the mask 50 may be removed from in front of the substrate and multilayer optical films may be deposited for form, for example, ring laser gyro mirrors, on the concave substrate 40 using the same sputtering method and apparatus.

Those skilled in the art will recognize that large numbers of concave substrates can be manufactured simultaneously in the same deposition chamber limited only by the size of the chamber and holding fixtures. In this way the invention provides a method for manufacturing a large number of concave substrates simultaneously. In one embodiment of the invention, it may be advantageous to bevel or round off the mask edges 58 as shown in Figure 3. Such edge treatment allows the mask to be placed closer to the base substrate in order to achieve a higher radius with lower deposition thickness. However, there are added costs for profiling the mask.

In yet another alternative embodiment, the lateral aspect ratio-to-mask may be altered in order to achieve other concave deposited surfaces. For example, the mask may be an ellipse. In yet another aspect, the mask may comprise a rod which extends across the entire base substrate. This would result in a deposition having a cylindrical contour which may be used, for example, for a cylindrical lens arrangement. In another example of the invention, the base may be comprised of titanium and silica in alternating layers. Such a composite, layered deposition may achieve low stress, low defects and other surface characteristics such as cleanability which are desirable on a mirror substrate. This invention has been described herein in considerable detail in order to comply with the Patent Statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment details and operating procedures, can be accomplished without departing from the scope of the invention itself.

What is claimed is:

Claims

1. A method of fabricating a concave deposited substrate comprising:

(a) interposing a mask between a source of deposition material and a base;

(b) controlling the background gas pressure between the source and the base to help form the concave substrate; and

(c) bombarding the base with deposition material wherein the mask is located to block a portion of the deposition material so that the deposition material forms a concave substrate on the base.

2. The method of Claim 1 further comprising the steps of:

(a) removing the mask; and

(b) depositing multiple film layers of at least two different materials onto the concave substrate to form the mirror.

3. The method of Claim 2 wherein the multiple film layers comprise a plurality of optical film layers having at least two different indices of refraction.

4. The method of Claim 3 wherein the plurality of optical film layers comprise a first group of optical film layers having an index of refraction greater than 2.0, and a second group of optical film layers having an index of refraction less than 1.5, wherein the first and second groups are deposited in alternating layers and each layer is a quarter wavelength optical thickness.

5. The method of Claim 4 wherein the first group of optical layers are substantially comprised of titanium dioxide.

6. The method of Claim 4 wherein the second group of optical layers are substantially comprised of silicon dioxide.

7. The method of Claim 3 wherein at least one of the plurality of optical layers is substantially comprised of zirconium dioxide.

8. The method of Claim 1 wherein the base is substantially comprised of low expansion ceramic material.

9. The method described in Claim 2 wherein the base, the concave substrate and the multiple film layers form a ring laser gyroscope mirror.

10. A method of fabricating a deposited concave mirror substrate on a flat base comprising:

(a) operating an ion beam source to sputter a target material so as to produce a directed flow of sputtered target material;

(c) interposing a mask between the flow of sputtered target material and the flat base wherein the mask partially blocks the sputtered target material from bombarding a central portion of the base so as to form a concave mirror substrate on the central portion.

11. The method of Claim 1 wherein the concave substrate includes a radius of curvature in the range of about 1 to 5 meters.

12. A substrate apparatus comprising:

(a) a base, and

(b) deposited concave mirror substrate wherein the deposited concave mirror substrate is formed on the base by a directed flow deposition process wherein a mask is interposed between a flow of deposition material and the base.

13. The substrate apparatus of Claim 12 wherein the directed flow deposition process comprises ion beam sputtering.

14. the substrate apparatus of Claim 12 wherein the base is substantially comprised of a low expansion ceramic.

15. The apparatus of Claim 14 further including multiple film layers of at least two different materials formed on the deposited substrate layer.

16. The apparatus of Claim 15 wherein the base deposited concave substrate layer and multiple film layers form a mirror.

17. The apparatus of Claim 16 wherein the multiple film layers comprise alternating quarter wavelength optical layers having at least two different indices of refraction.

18. A method of fabricating a multilayer deposited concave mirror substrate on a flat base comprising:

(a) operating an ion beam source to alternately sputter a first target material and a second target material so as to alternately produce directed flows of sputtered first target material and sputtered second target material so as to form alternating layers of first and second target materials on the flat base;

(b) controlling the background gas pressure between the source and the base to help form the multilayer concave substrate; and

(c) interposing a mask between the flow of alternately sputtered first and second target materials and the flat base wherein the mask partially blocks the first and second sputtered target materials from directly bombarding a central portion of the base so as to form a multilayer concave mirror substrate on the central portion.

19. The method of Claim 18 further comprising the steps of:

(d) removing the mask; and (e) operating the ion beam source to alternately sputter a third and fourth material so as to alternately produce directed flows of sputtered third target material and sputtered fourth target material onto the multilayer concave mirror substrate wherein the third and fourth target materials form a multilayer mirror covering the multilayer concave mirror substrate.