Abstract:
The surface of a zinc selenide substrate is ground to curve in the opposite direction from that which occurs due to the bimetallic effect when zinc sulfide is deposited on a flat substrate by the chemical vapor deposition process. The bowing of the interface that occurs upon cooling of the hot laminate when the surface of the substrate is flat before deposition is compensated for by the pre-figured bowing. A distortion free window for the transmission of infra-red radiation is provided by this invention.

Description:
This is a continuation of application Ser. No. 07/960,756 filed Oct. 14, 1992, now abandoned, which was a division of application Ser. No. 07/729,980, filed Jul. 15, 1991, now U.S. Pat. No. 5,183,689. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for preparing an infra-red transmissive window that is substantially free of optical distortion. More particularly, it relates to a method for preparing a laminate of zinc selenide and zinc sulfide which overcomes the bimetallic effect arising from the different coefficients of thermal expansion of the two layers. The IR window made by this invention meets the demands of certain applications for which the prior art windows were either unsuitable or prohibitively expensive. 
     An improved method for the chemical vapor deposition of zinc sulfide on a zinc selenide substrate is taught in U.S. Pat. No. 4,978,577, which is incorporated herein by reference. There, the substrate is heated in the presence of H 2  S and the absence of zinc vapor for a certain period of time before the zinc vapor is introduced into the heating chamber. The adhesion of the zinc sulfide to the substrate is greatly improved by that treatment. A problem inherent in the high temperature lamination of materials having different coefficients of thermal expansion is the bowing of the laminate in response to the stress sometimes referred to as &#34;the bimetallic effect.&#34; The deposition of the zinc sulfide occurs at a temperature of about 700° C. As the laminate is cooled to room temperature, the initially flat upper surface of the zinc selenide substrate becomes convex and the contiguous surface of the zinc sulfide is, perforce, concave. If the substrate is mounted in the heating chamber in such a way that it can not expand and contract freely, the bowing will not be symmetrical and will be extremely difficult to correct by optical grinding methods. 
     Infra-red transmissive windows made from such bowed laminates are difficult to use in wide-angle &#34;forward looking infra-red imager&#34; systems because of the optical path differences (OPD) for the light rays that pass through the window at different angles to the surface. As the angle of incidence grows larger, the OPD becomes greater. An optician can reduce the OPD by introducing some curvature into the outer surfaces of the laminate to compensate for the interfacial curvature. This is a lengthy and costly process, especially when the curvature is unsymmetrical. Thus, the bowing phenomenon is a deterrent to the widespread application of the ZnSe/ZnS laminate as an infra-red window. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention, therefore, to provide a method for the production of a zinc selenide/zinc sulfide laminate having substantially planar interface and substantially no interfacial bowing. 
     It is a related object to provide a laminate of those materials that needs no optical correction even though the laminate was made by chemical vapor deposition of the sulfide onto the selenide. 
     It is another related object to provide a &#34;diffraction limited&#34; optical system comprising a chemical vapor deposition laminate of zinc selenide and zinc sulfide. 
     These and other objects of the invention which will become apparent from the following disclosure and the accompanying drawings are achieved by establishing a heating and cooling regimen for the chemical vapor deposition, determining the curvature of the interface between the substrate and the deposit which will occur during the cooling phase of said regimen, imparting the negative of that curvature to the surface of the substrate upon which the zinc sulfide is to be deposited, heating the substrate in a chemical vapor deposition chamber, coating that surface with the sulfide by chemical vapor deposition, and cooling the resulting laminate. 
     Another aspect of the invention is the provision of a mounting frame which allows free lateral, longitudinal, and transverse movement of the substrate so that it may expand and contract freely during and after the chemical vapor deposition of the zinc sulfide. 
     The amount and shape of the bowing that will occur may be calculated using modern finite-element modeling techniques since the mechanical and thermal properties of ZnSe and ZnS are well known. The calculation of stress induced by the bimetallic effect is taught by Roark, Raymond J. and Young, Warren C., &#34;Formulas for Stress and Strain&#34;, 5th edn. McGraw-Hill, Inc., 1938, Section 10-4, pp 337-381, &#34;Bimetallic Circular Plates&#34;, particularly in case 15a and also at page 366. The bowing may also be measured by conventional optical means, e.g., interferometry. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     Turning now to the drawings: 
     FIG. 1 is a cross section of a laminate of the prior art. 
     FIG. 2 is a graph of the coefficient of thermal expansion of ZnSe and ZnS at increasing temperatures. 
     FIG. 3 is a cross section of a substrate mounted in a frame inside a chemical vapor deposition furnace according to this invention. 
     FIG. 4 is a cross-section of a broken away portion of a hot vapor deposition furnace wherein ZnS is deposited on the ZnSe substrate. 
     FIG. 5 is a sectional view taken along line 5--5 of FIG. 4. 
     FIG. 6 is graph of the sag of a ZnSe/ZnS laminate of the prior art. 
     FIG. 7 is a cross section of the laminate of this invention after deposition of the ZnS and cooling. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, the ZnSe substrate 10 is bowed upward and is seen to have a convex upper surface 12 and a concave lower surface 14. The ZnS layer 16 is bowed in like manner. The amount of sag is shown at 18 by the dimension lines depicting the highest and lowest points of the curved surface. 
     The relation between the coefficient of thermal expansion and the temperature for both the ZnSe and the ZnS is shown in FIG. 2. 
     In FIG. 3 H 2  S is introduced into the mandrel box 20 through the pipe 21 to contact the ZnSe substrate 22 in the absence of zinc vapor while the zinc metal 23 is being heated in the pots 24 and when the zinc reaches its vaporization temperature, the vapor flows through the ports 26 into the deposition zone 27. The mandrel box 20 is mounted upright in the furnace 28 between the heating elements 29. The box is made up of graphite plates, including the plates 30 and 31 shown here. The substrate holding frame 32 is mounted on the mandrel plate 31 in defining relation to the passage 33 through the plate 31 to place the substrate within the passage where it may be exposed to the vapors in the zone 27. As shown in larger detail in FIGS. 4 and 5, the opening 34 in the frame 32 is larger in each dimension than the substrate and the distance between a lip 35 and the back plate 36 is greater than the thickness of the substrate. A restraint-free assembly is thus made a part of the furnace 28 so that the bowing of the laminate upon cooling will be symmetrical and can be remedied by the method of this invention. The lips 35, which project toward each other from opposite boundaries of the opening 34 and are integral parts of the holding frame 32, are adapted to the support of the substrate in an upright position while blocking off only a minimal portion of the passage 33 for vapors traveling toward the substrate. 
     In like manner, the shields 37 are integral parts of the mandrel plate 31 which project toward each into the deposition zone from opposite boundaries of the passage 33. They are superposed over the outer periphery of the frame 32 to provide a shadowing effect whereby the deposition of ZnS in the recess 38 of the mandrel plate and in the corners between the frame and the plate is limited. The propagation of cracks from a ZnS coating 40 on the mandrel plate 31 and the shield 37 into the desired overcoat 39 on the substrate 22 is thereby minimized by isolation of the overcoat from the deposits 40. 
     The amount and location of the bowing that would occur during the deposition of ZnS onto a ZnSe substrate are shown in FIG. 6. The substrate is 7.2 inches long, 5.5 inches wide, and 0.70 inch thick; the thickness of the ZnS is to be 0.125 inch. Using the ANSYS® finite element analysis program of Swanson Analysis Systems, Inc. on an ARIES ConceptStation® (Aries Technology, Inc.), the sag is calculated to be 78 μm. and symmetrical about the center of the laminate. From the results of that calculation and in accordance with this invention, a ZnSe substrate is ground and lapped to produce a blank having a concave spherical surface with a total sag of 78 μm. The blank is then mounted in the box 20 of FIG. 3 and that assembly is placed in the vapor deposition zone 27 and ZnS is deposited on the substrate at about 700° C. as described in U.S. Pat. No. 4,978,577. Upon cooling to room temperature, the laminate has a flat interface as shown at 41 in FIG. 7. The curvature that had been imparted to the substrate mechanically is cancelled by the bimetallic effect being put to work for us instead of against us. The following computer program may also be used to calculate the center sag of a laminate to be made by chemical vapor deposition of ZnS on a ZnSe substrate: 
     
         ______________________________________30   REM: This is a program to calculate maximum deflection andstress in a ZnS/ZnSe40   REM: Window subjected to a change in temperature50   REM: The equations used are from Roark55   REM: ZnS is material 1, (top layer); ZnSe is material 2(bottom layer)60   REM: Following are the variables used70   REM: A - radius of plate in inches80   REM: A1 - thermal expansion coef. of material 1 ininch/inch C90   REM: A2 - Thermal expansion coef. of material 2 ininch/inch C100  REM: DT - Temperature gradient in degrees C.110  REM: T1 - Thickness of material 1 in inches120  REM: T2 - Thickness of material 2 in inches130  REM: E1 - Modulus of material 1 in psi140  REM: E2 - Modulus of material 2 in psi150  REM: K1P - Derived constant160  REM: V1 - Poisson&#39;s ratio of material 1170  REM: V2 - Poisson&#39;s ratio of material 2180  REM: KYC - Loading constant from Roark310  LET A1=7.700001E-06320  LET A2=8.300001E-06330  INPUT &#34;Temperature gradient&#34;; DT340  INPUT &#34;Thickness of ZnS (material 1) in inches&#34;; T1350  INPUT &#34;Thickness of ZnSe (material 2) in inches&#34;; T2360  LET E1=1.08E+07370  LET E2=9750000|380  LET V1=.29390  LET V2=.28400  LET KYC=.5410  LET K1P=4+(6*T1/T2)+(4*(T1/T2) 2)+(E1*T1 3*(1-V2))/(E2*T2 3*(1-V1))+(E2*T2*(1-V1))/(E1*T1*(1-V2))413  LET K2P=(1+((E2*T2 3*(1-V1 2))/(E1*T1 3*(1-V2 2)))+((3*(1-V1 2)*(1+T2/T1) 2*(1+(E1*T1)/(E2*T2)))/((1+E1*T1/(E2*T2)) 2-(V1+(V2*E1*T1)/(E2*T2)) 2)))416  LET K3P=(1+((V2*E2*T2 3*(1-V2 2))/(V1*E1*T1 3*(1-V1 2)))+((3*(1-V1 2)*(1+T2/T1) 2*(1+(V2*E1*T1/(V1*T2*E2))))/((1+(E1*T1/E2*T2))) 2-(V1+(V2*E1*T1/(E2*T2))) 2)))420  LET X = (6*(A2-A1)*DT*(T1+T2))/(K1P*T2 2)425  INPUT &#34;Enter plate radius in inches&#34;; A427  REM: Calculate center deflection430  LET Y = KYC*X*A 2500  PRINT510  PRINT520  PRINT &#34;ZnSe Thickness&#34;;T2;&#34; inches&#34;,&#34;ZnS thickness&#34;;T1;&#34;inches&#34;521  PRINT522  PRINT &#34;Plate radius&#34;;A;&#34; inches&#34;, &#34;temperature gradient&#34;;DT;&#34; degrees C.&#34;523  PRINT530  PRINT &#34;The maximum deflection is &#34;;Y;&#34; inches&#34;532  REM: Calculate stresses associated with deflection535  LET VE = V1*(K3P/K2P)540  LET DE = (E1*K2P*T1 3)/(1-V1 2))545  LET L8 = .5*(1+VE)550  LET M = DE*X*(1-VE)*(1-L8)560  LET S1 = ((-1)*6*M/(T1 2*K2P))*(1+(((1-V1 2)*(1+T2/T1)*(1+(E1*T1/(E2*T2)))) ((1+(E1*T1/(E2*T2))) 2-(V1+(V2*E1*T1)/(E2*T2))) 2)))-((A2-A1)*DT*E1/((1-V1)*K1P*(3*T1/T2+2*(T1/T2) 2-((E2*T2*(1-V1))/(E1*T1*(1-V2)))))563  LET S2A = E2*T2*(1-V1 2)/(E1*T1*(1-V2 2))565  LET S2N = (1-V1 2)*(1+T2/T1)*(1+E1*T1/(E2*T2))567  LET S2D = (1+E1*T1/(E2*T2)) 2-(V1+V2*E1*T1/(E2*T2)) 2568  LET S2S = ((A2-A1)*DT*E2)/((1-V2)*K1P)*(3*(T1/T2)+2-((E1*T1 3*(1-V2))/(E2*T2 3*(1-V1))))570  LET S2 = (6*M/(T1 2*K2P))*(S2A+(T1/T2)*S2N/S2D)+S2S575  PRINT576  PRINT580  PRINT &#34;The stress of top surface of ZnS is &#34;;S1;&#34;PSI&#34;590  PRINT &#34;The stress on bottom surface of SnSe is &#34;;S2;&#34;PSI&#34;750  END______________________________________ NOTE: An asterisk is a multiplication sign here;   indicates superscript follows 
    
     A particular example of the process of this invention is as follows: 
     The potential center sag of the interface of a laminate window measuring 11.75×7.25 inches and having a ZnSe thickness of 0.72 inch and a ZnS thickness of 0.06 inch was calculated, using the above program, to be 22.9 μm over a 6 inch chord spanning the bowed interface. A ZnSe substrate having the recited dimensions was then fabricated to have a concave surface with a radius of curvature equal to -5000±500 inches and the calculated center sag on one side. The substrate was loaded into the restraint-free holding frame 32 and the assembly was mounted over the passage 33 of the plate 31 so that the concave surface would be exposed to the zinc and H 2  S vapors in the zone 27. Ater the substrate was coated with the ZnS to the desired thickness of 0.06 inch and the laminate was cooled, the window was ground flat on the top and bottom surfaces to the proper thicknesses. The radius of curvature of the interface was measured as -347,640 inches and the center sag was -0.3 μm over the 6 inch span. Thus, the bowing of the interface was reduced more than 98%. 
     The method of this invention places no limits on the dimensions of the window that may be produced--one measuring 18×12 inches having been fabricated by it--but as a practical matter the larger the window the thicker it must be for rigidity. Because its transmission in the infra-red is poorer than that of the ZnSe, the thickness of the ZnS layer on the final window should be no greater than what is required for its function as a protector of the substrate against erosion by rain and dust particles at high speeds. The minimal thickness for that protection is about 0.030 inch. The actual thickness deposited, however, will be considerably greater, i.e., about 0.10 inch, to allow for the optical grinding and polishing that is necessary to provide the desired window. 
     The window prepared by the method of this invention is essentially &#34;diffraction limited&#34;, the optimal condition when one is considering the distortion of an image by an optical system. The transmitted wave front aberration for rays that are not normal to the surface of the window prepared according to this invention is less than 0.1 of a wavelength for rays 40°-60° from normal. In contrast, the window having the curved interface caused by the bimetallic effect gave aberrations in the range of 0.3-0.7 wavelength in the infra-red (i.e., one wavelength=10.6 μm).