Method of making electro-optic window by sputtering material to fill channels of a grid

Abstract

An electro-optic window is provided, together with a method of manufacturing the window. The window (3) is made of a material substantially transparent to at least one of infra-red, visible and UV radiation and treated to have reduced RF/MICROWAVE transmission characteristics by the provision of a grid (1) set into at least one surface (2) thereof. The grid (1) is formed of a material selected to be either reflective or absorptive to RF/MICROWAVE radiation.

Claims

1. A method of making an electro-optic window having reduced RF and microwave transmission characteristics, the method comprising: providing a window that is formed from a material that is substantially transparent to at least one of infra-red, visible and UV radiation; forming on a surface of the window a grid of channels; sputtering over the surface of the window a layer of a material having one of electrically conductive and dielectric properties to substantially fill the channels of the grid; selectively etching the surface of the window, thereby removing the sputtered material from the surface while leaving the channels of the grid substantially filled with the material, creating a corresponding grid pattern of the material within the channels, and thereby rendering the window non transmissive to RF/MICROWAVE radiation; and treating the material as required to render the grid usable to reduce the RF/MICROWAVE transmission characteristics of the window.

2. The method according to claim 1, wherein the material is a metal.

3. The method according to claim 1, wherein the step of forming on a surface of the window a grid of channels comprises forming the grid of channels by laser etching the window material.

4. The method according to claim 1, wherein the step of forming on a surface of the window a grid of channels comprises forming the grid of channels by chemically etching the window material.

5. The method according to claim 1, wherein the step of forming on a surface of the window a grid of channels comprises: forming a mould in the shape of an EO window, the mould defining a positive grid formation whereby to impart to a moulded window a negative grid formation on one surface of the window; forming a sol of a material suitable for sintering and pouring the sol into the mould; converting the sol to a gel by the application of heat; drying the gel whereby to impart to the gel a permanent shape corresponding to that of the mould; and vitrifying the gel by sintering whereby to form a sintered EO window having the grid of channels formed on one surface thereof.

6. The method according to claim 1, further comprising a step of forming a capping layer configured to cover the grid and attach to the window surface by: forming a mould; forming a sol and pouring the sol into the mould; converting the sol to a gel by the application of heat; drying the gel whereby to impart to the gel a permanent shape reflecting that of the mould; and vitrifying the gel by sintering whereby to form a said capping layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and further features of the invention are set forth in the appended claims and will be explained in the following by reference to various exemplary embodiments and the specific examples which are illustrated in the accompanying drawings in which:

(2) FIG. 1 is a sectional schematic view of a portion of EO window according to the invention having a grid embedded therein;

(3) FIG. 2 is a sectional schematic view of a similar window with embedded grid and covered by a capping layer;

(4) FIG. 3 is a flow diagram of the manufacturing steps for making an EO window with an embedded grid and a capping layer, according to the invention;

(5) FIG. 4 is a photograph of a glass wafer, laser etched with channels 2 m wide and deep;

(6) FIG. 5a is a photograph of a chemically etched grid of channels on a glass wafer;

(7) FIG. 5b is a photograph, to a larger scale, of the grid of FIG. 5a;

(8) FIG. 5c is a schematic cross sectional view of a channel;

(9) FIG. 6a is a photograph of a glass sample having a grid of channels, the sample having been selectively etched to leave metal within the channels of the grid;

(10) FIG. 6b is an enlarged photograph of the grid of FIG. 6a;

(11) FIG. 6c is an optical micrograph of a cross section of the channels of FIG. 6a;

(12) FIG. 7 is a graph showing microwave transmission of metallized surfaces and metallized grids;

(13) FIG. 8a is an Image of a free standing mesh pressed into the surface of a pre-sintered ceramic compact;

(14) FIG. 8b is an image of the sintered ceramic of FIG. 8a with residual grid impression;

(15) FIGS. 9a and 9b are images of carbon based fillers being introduced into pre-sintered compressed ceramic;

(16) FIG. 10a is an image of a copper foil upon which to cast a silica gel;

(17) FIG. 10b is a micrograph of a resulting sintered body with the features of the copper foil;

(18) FIG. 11a is an image of coated and uncoated Si wafers;

(19) FIG. 11b is an image of a coated wafer after high temperature processing;

(20) FIG. 12 is an image of coated and uncoated sapphire windows;

(21) FIGS. 13a, 13b and 13c are optical microscope images (20 mag.) of films prepared from precursors with various polymers;

(22) FIG. 14a is a graph showing XPS analysis of the coating on a Si wafer;

(23) FIG. 14b is a graph showing XPS analysis of the coating on a sapphire window;

(24) FIG. 15 is a graph showing UV/Vis spectra of an uncoated sapphire window and a spinel coated sapphire window;

(25) FIG. 16 is a graph comparing the transmission or IR through an uncoated sapphire window, a dried film of precursor material on a sapphire window and the same window following thermal treatment;

(26) FIG. 17a is an image showing light reflected from a section of Si wafer spray coated with spinel precursor & dried at 70 C.;

(27) FIG. 17b is an image of such a sample after thermal treatment;

(28) FIG. 18 is an image of a pair of sapphire windows fusion bonded together, showing Newton's rings;

(29) FIG. 19 is an image of two fusion bonded glass wafers, one having a grid on the surface thereof;

(30) FIG. 20a shows the optical transmission through the two fusion bonded glass wafers of FIG. 19;

(31) FIG. 20b shows the optical transmission through a pair of adhesively bonded wafers, and

(32) FIG. 21 is a graph comparing the optical transmission spectra of various samples.

DETAILED DESCRIPTION

(33) Optically suitable materials for an EO window according to the invention are well-known. Work has been done by the inventors on sapphire (aluminium oxide, Al2O3) and spinel (magnesium aluminium oxide, MgAl2O4).

(34) Synthetic sapphire can be grown in several of its crystal orientations including the A, C, R and M plane. For EO window applications the C or A plane axes may be used.

(35) Sapphire crystals are grown using a variety of crystal growth techniques and then machined and polished into the finished window geometry. Sapphire can be processed to a very high optical specification of flatness and surface quality. For example, scratch/dig (S/D) of 20/10 can be achieved for flatness of /4, which is suitable for almost all optical applications.

(36) Magnesium aluminate or magnesium aluminium oxide, or spinel, is a durable polycrystalline transparent ceramic. Spinel blanks may be made using conventional ceramic processing techniques. A powder of the raw constituent materials is prepared (usually to a proprietary formulation), compacted and can be dry-isostatically pressed, slip cast or injection moulded into the required shape. This is followed by a heat treatment to densify the material. The blanks may then be ground and polished to specification.

(37) Spinel can also be produced by crystal growth methods, chemical vapour deposition and chemical synthesis routes, including sol gel synthesis, as described below.

(38) Sapphire is currently available as follows:

(39) Max. planar dimension: 300 mm500 mm or 225 mm660 mm;

(40) Ratio of thickness to planar length required for polishing: 1:70;

(41) Min. thickness required for optical polishing & processing: 7 mm;

(42) Max. thickness available: 7.7 mm.

(43) Spinel is currently available as follows:

(44) Max. planar dimension: 300 mm460 mm;

(45) Ratio of thickness to planar length required for polishing: 1:35-1:10;

(46) Min. thickness required for optical polishing & processing: 13-40 mm;

(47) Max. thickness available: 25 mm.

(48) For reasons of commercial availability only, the EO window was made of sapphire or spinel, between 5-20 mm thick, with a planar edge of 300-500 mm.

(49) Because a grid that is on the surface of the window will be exposed to abrasion and erosion, it will need protection. Protection may be imparted by adding protective coatings. Referring to FIG. 1, an embodiment of the invention is shown. Here, a sub-surface grid 1 is shown flush with a surface 2 of a window 3; the grid 1 is positioned within the window 3 rather than on it. This reduces the exposure of the grid to impacts and abrasion and still renders electrical connection to the grid very accessible.

(50) FIG. 2 shows a further embodiment with a grid 1 completely protected from the elements. The grid 1 is located, as in FIG. 1, within the window 3. In addition, both the grid 1 and the upper surface 2 of the window are covered with a capping layer 4. This design will completely protect the grid from a harsh aerospace environment. It will be noted that electrical connection to the grid 1, in this embodiment, is through the exposed cross-section of the embedded grid. However, electrical connection can be made through vias from the back of the window or from top or edge surfaces, with bus bars or electrical connections surrounding the edge. For use on aircraft, it may be also be important to connect electrically to airframe window surrounds, for metal grids at least.

(51) The flow diagram of FIG. 3 shows the manufacturing steps required to make a window according to the invention, by two alternative routes. On the left side of the diagram, is shown one route, on the right, another. Looking at the left side, channels 5 of a grid are etched into a surface 2 of a sapphire or spinel window 3 by any suitable etching process. The channels 5 are then metallized, again, by any suitable process. Following this step, capping layer 4 which may also be of spinel or of another suitably hard material having transparent properties, is attached to the upper surface 2 of the window 3 by one of several methods described below.

(52) Following the right side of the diagram, an alternative method is shown where, following etching of the channels 5, the capping layer 4 is immediately attached to the window, followed by metallisation, again, as further described below.

(53) In an example, according to the invention, a grid has been etched in a window using a laser system suitable for both glass and sapphire substrates. The laser etching system uses a 200 KHz pulsed excimer laser with a 193 nm lens and a chrome on quartz mask of the required grid. A glass wafer etched by this method is shown in FIG. 4. This technique is also applicable to etching spinel.

(54) This method of etching has potential to be scaled up, but this may be at significant cost. The throughput of such a process may be of the order of 10 mm/s. Vacuum processing techniques have been developed for achieving mirror finishes on laser etched arrays.

(55) Chemical etching has also been used to etch glass wafers, see FIGS. 5a, b and c, and may be used according to the invention to etch sapphire. The channels 5 are 3 m deep, 7 m wide and with a 100 m separation.

(56) Work has been carried out to research the deposition of metal into surface channels. Many materials are suitable for use as RF/MICROWAVE reflector fillers for the grid channels. Most metals and many alloys are suitable. Gold, silver, aluminium, platinum and the refractory metals are suitable, as are iron, cobalt, nickel and many fluids.

(57) In an example, aluminium was sputtered over a gridded sample 6 and then selectively etched to leave metal 7 within the channels 5 of the grid 1, see FIGS. 6a and 6b.

(58) Cross sectional image analysis of this sample was used to evaluate the channel metallisation, see FIG. 6c. A channel 5 in the glass 8 is filled with aluminium 9. The initial channel 5 was 7 m wide and 3 m in depth. Variability across the grid can lead to over etched patches of metal. To mitigate this, the grid channels 5 were deepened to ensure the grid could be etched back from the surface 10 without affecting the continuity of the grid.

(59) The microwave reflectivity of the samples were characterised and the results are shown in FIG. 7. The results indicate that sputtered layers of metal just a few microns thick and electrolessly plated metal layers of the order of a micron thick have a high enough conductivity to provide significant radar reflection. The use of electroless plating to generate a grid is described in the following section.

(60) An electroless gold plating process occurs in the liquid phase at elevated temperatures (50 C). Therefore it is important that the electroless solution is contained, to avoid evaporation during heating. In order to achieve this, a section of pre-cavitated glass wafer was placed face down (cavities side down) on a glass slide. The electroless gold plating solution was introduced to the edge of the wafer section by pipette and was observed to be drawn into the channels by capillary action. Once the sample was fully wetted with plating solution it was placed in an oven at 50 C for 15 minutes to activate the plating process. The wafer section was removed from the carrier slide and examined. A thin layer of gold was seen to be plated across the entire top surface of the wafer (visible as a transparent purple film) in addition to the metal filling the trenches. The surface gold film was wiped off leaving the metal in the trenches intact.

(61) The present invention is partly concerned with methods of forming metal coatings within channels embedded within window structures.

(62) Optically transparent spinel is manufactured using ceramic processing techniques.

(63) A metallic mesh or conductive grid may be embedded within the window during manufacture by embedding a mesh of a sacrificial material in any suitable window bulk material. Examples of suitable sacrificial materials are: polymers, some low melting-point metals, eutectics, carbon nanotubes, and wax. The mesh or grid is then removed by, for example, melting the sacrificial material to leave a grid of channels in the manufactured window for receiving a conductive or dielectric grid, as desired, for use in operation.

(64) As illustrated on an opaque ceramic 14 in FIGS. 8a and 8b, it is also possible to use free standing meshes to create a grid image 13 within the pre-sintered compressed ceramic 14 and to fire the ceramic to form a grid 13 in the surface of the fired ceramic 14 that may then be filled with conductive or resistive materials. In principle, this is applicable to spinel using a similar process.

(65) It is also possible to use this technique to form a resistive or absorbing grid from carbon based materials such as carbon powders or nanotubes. FIGS. 9a and 9b illustrate forming, with an expanded metal foil, see FIG. 9a, subsurface carbon grids in a pre-sintered ceramic compact. Carbon particles or nanotubes can then be distributed in the grid channels as required and the ceramic sintered. FIG. 9b illustrates the result, with carbon residing in the channels.

(66) Examples of absorptive materials include ferrites such as nickel zinc, manganese zinc and cobalt ferrites; magnetites; ceramics, and carbon based materials as above.

(67) Fluids may also be used to form the grid material, in use. Examples are: electrolyte solutions, such as potassium ferrocyanate; ethylene glycol; methanol, and acids. Colloids such as magnetic colloids like ferro-fluids are also suitable to act as the grid material. Spinel can be made using sol gel techniques, allowing for optically transparent thin films to be synthesized. These may be used to protect surface or sub-surface grids.

(68) Conventionally, spinel films are deposited using chemical vapour deposition methods but that method is presently limited to relatively small areas (a few cm.sup.2). According to the present invention, the use of sol gel methods for manufacturing large area capping layers of spinel is proposed.

(69) It is known that mixtures of salts of magnesium and aluminium in the appropriate ratio decompose at high temperatures to produce spinel and the method is often used to manufacture powders of spinel.
Mg(NO3)2+2Al(NO3)3.fwdarw.MgAl2O4

(70) Studies were initially undertaken using silica, rather than spinel because of the simpler chemistry involved. A thin film of a silica gel was cast onto a perforated copper foil, see FIG. 10a. When the gel was partially dry, and shrinkage was minimal, the film was removed from the foil and then dried and sintered. The final body, although dimensionally smaller, showed the features of the original green body, see FIG. 10b. Thus, this technique may be adopted for the manufacture of EO windows with grid channels set into one surface thereof.

(71) For spin coating of spinel, methanolic solutions of the mixed metal nitrates were spun onto substrates (glass slides, silicon wafer and sapphire windows). It was found that the addition of a very small amount of a suitable polymer led to excellent film formation after spin coating, and the films remained intact and continuous after low temperature drying to remove solvent and subsequent high temperature thermal treatment. The choice of polymer was found to be very important and certain polymers were more suitable than others for ensuring good quality film formation. FIGS. 11a and 11b show examples of good continuous solvent free films of mixed metal nitrates on silicon wafer substrates produced by spin coating. In FIG. 11a, an uncoated Si wafer 18 is shown on the left and a spin coated and dried wafer 19, on the right. FIG. 11b shows a coated wafer 20, after high temperature processing. FIG. 12 shows sapphire windows before and after coating and subsequent heat treatment, with 21 being an uncoated sapphire window and 22 being a sapphire window having a spin coating of mixed Mg and Al nitrates, with polymer. FIGS. 13a, b and c show the importance of the choice of polymer for aiding film formation. Here, FIGS. 13a and 13b show films prepared from a precursor with a good choice of polymer and FIG. 13c shows film prepared from a poor choice of polymer where the nitrates have crystallised from solution.

(72) XPS (x-ray photoelectron spectroscopy) analysis of coatings deposited on silicon wafers and sapphire windows confirms the presence of magnesium oxide and aluminium oxide, see FIG. 14a and FIG. 14b.

(73) TABLE-US-00001 TABLE 1 eV Assignment On wafer On sapphire disc Carbon 284.9 Hydrocarbon 22.9% 16.8% Oxygen 531.2 Inorganic oxide 47.3% 51.57% Aluminium 74.6 Al oxide 20.2% 22.9% Magnesium 50.5 Mg oxide 9.7% 8.7%

(74) Quantitative analysis of the composition of the coating on silicon shows that the ratio of magnesium to aluminium (as oxide) is the expected 1:2, see Table 1, above. The ratio of the coating on the sapphire window shows a higher amount of aluminium but this is to be expected because of contributions from the aluminium oxide present in the structure of the sapphire substrate, see FIG. 15.

(75) Comparison of an uncoated sapphire window and sapphire window that has been spin coated with the precursor mixture followed by thermal treatment reveals only small differences in the transmission window. The uncoated window transmits 85% of light from 1100 nm to 270 nm at which point the transmission falls rapidly to 55% at 190 nm. Transmission through a spinel coated sample has similar transmission from 100 nm to 270 nm but thereafter the fall in transmission is faster than in the control sample and the final transmission is 35% at 190 nm, see also FIG. 15.

(76) FIG. 16 shows the IR transmission spectra of an uncoated sapphire window, a sapphire window coated with the dried precursor and the same window after full thermal treatment. The transmission window for all samples is 3 m to 5 m and at 5 m and higher the transmission is essentially zero. The OH stretch arising from the presence of a polymer can clearly be seen. This absorption band disappears following thermal treatment. The presence of the final coating does not significantly attenuate transmission in the 3 to 5 m band. In all samples transmission falls below 50% at 4 m. Thus, there is little degradation in the optical and IR transmission spectra of a capped window.

(77) For large substrates spin coating may not be appropriate, thus deposition of magnesium aluminate films using spray techniques was investigated. Modification of the spin coating precursor for spinel, used above, produced a mixture that could be sprayed onto substrates (glass, and silicon wafers). Visual inspection by eye and under an optical microscope showed the wet, (as deposited) film and the dried film to be fairly uniform. FIG. 17a shows an example of a coated Si wafer 23, a portion 24 of which was masked off prior to spray coating and FIG. 17b shows a sample having a masked portion 25 and an unmasked portion 26 that has subsequently undergone full thermal treatment.

(78) Fusion bonding is a method of joining materials including ceramics to each other through the application of pressure and heat without the use of adhesives. It has been successfully used by the inventors on several materials such as silicon and glass to form strong bonds. If fusion bonding is possible with gridded windows, then grids could potentially be protected by a layer of the substrate material without a glue line. Such glue lines can severely compromise the optical and mechanical properties of the structure. Fusion bonding can create optically transparent bonds under the right conditions.

(79) Fusion bonding requires flat, clean surfaces. The surfaces are mated under pressure and at elevated temperatures. The surfaces of the material are prepared using a proprietary process to degrease, clean and chemically activate the surface of a wafer. The surfaces are then bonded using wafer bonding equipment and post treated in a vacuum oven.

(80) The bonding process has been demonstrated on a pair of sapphire windows, as shown in FIG. 18. Interferences fringes 27 (known as Newton's rings) indicate there is a bond gap; the gap can be estimated using the separation between the interference fringes, as indicated by the tips of the arrows.

(81) Analysis of the fringes indicated a gradual separation between the windows from a successful fusion bond at a clear part 28 of the sample to 1 m separation between the windows at the edge 29 of the sample. The specification of these windows has a flatness of 2 over the 20 mm window. Generally, for fusion bonding, a flatness of /10 over 50 mm would be necessary for a good fusion bond. The defect in the bond is probably due to a variation in flatness across the window samples.

(82) From the above it is concluded that fusion bonding techniques may be used to create a window according to the invention with an embedded grid by fusion bonding a capping layer onto the gridded window.

(83) A gridded pattern was etched into a glass wafer and a second glass wafer was fusion bonded onto the surface. An image of the resulting structure is shown in FIG. 19. The interference fringes 30 indicate the central area has not properly bonded.

(84) The optical transmission through this sample, see FIG. 20a, is compared to an adhesively bonded wafer, see FIG. 20b, and the IR transmission of the samples is compared to an untreated glass wafer in FIG. 21.