US3543717A

US3543717A - Means to adjust collimator and crucible location in a vapor deposition apparatus

Info

Publication number: US3543717A
Application number: US724081A
Authority: US
Inventors: Iwao P Adachi
Original assignee: Itek Corp
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1968-04-25
Filing date: 1968-04-25
Publication date: 1970-12-01
Anticipated expiration: 1987-12-01

Description

United States Patent [72] Inventor lwao P. Adachi [56] References Cited Lexington, Massachusetts UNITED STATES PATENTS P 7242081 2,341,827 2/1944 Sukumlyn 11s/49x F'led 2 435 997 2/1948 Bennett 118/49 Patemed 2'439983 4/1948 M l 1 11s 49 (73] Assignee ltek Corporation Organ eta Lexington Massachusetts 2,736,671 2/1956 Ransburg et al.... 118/631X acorporation Delaware 2,745,773 5/1956 Weimer 1 18/49X 2.767,682 10/1956 Smith 118/49 3,187,715 6/1965 Wellard 118/49 3,393,658 7/1968 Ott 118/7 q 3,437,734 4/1969 Roman et a1 118/49.5X 3,359,945 12/1967 Hastings et a1 1l7/37(1X)X I 54] MEANS o ADJUST COLUMATOR AND Primary Examiner-Morris Kaplan CRUCIBLE LOCATION [N A VAPOR DEPOSITION Attorneys-Homer 0. Blair, Robert L. Nathans, Lester S. APPARATUS Grodberg and John E. Toupal 9 Claims, 4 Drawing Figs.

[52] U.S.Cl .1 118/7, ABSTRACT: A surface generating system in which means 1111/4 1 319/271 move a vapor column over the work surface of an optical Int-Cl B050 blank in an irregular path so to produce relative elevational 1 1 Field Search 118/4,7,8. changes thereon. Means for independent two-dimensional 4149.5, 301, 323: 219/(1nqu1red); 13/31; 117/(Controls Digest). (lnquired 106 107.2

control of the vapor columns movement permits selective, asymmetrical correction of the work surface.

ELECTRICAL CONTROL SYSTEM Patent ed D elc. 1, 1970 1 3,543,717

Sheet 1 of 2 ELECTRICAL CONTROL SYSTEM 1m?! [loan 1? fldaciul,

Patented Dec. 1, 197 0 Sheet MEANS T ADJUST COLLIMATOR AND CRUCIBLE LOQATEON IN A VAPOR DEPOSITION APPARATUS BACKGROUND OF THE INVENTION This invention relates generally to an apparatus for generat ing optical surfaces. More particularly, the invention relates to apparatus for correcting asymmetrical imperfections in the surfaces of optical blanks.

According to known methods, optical surfaces are ground and polished by utilizing completely empirically developed techniques. The practice of optical surface generation in accordance with these techniques suffers from a number of significant disadvantages including requirements for lengthy processing and for highly skilled technicians. Furthermore, since such empirically developed techniques are designed to produce symmetrical alteration of optical surfaces, they are generally inappropriate for eliminating asymmetrical deviations from a desired surface contour. This latter deficiency is particularly troublesome with regard to relatively large optical surfaces of, for example, 50-inch diameter and larger in which rotational asymmetries and random irregularities are more prominent than in smaller surfaces. Also known are so-called template grinding systems wherein a polishing tool is controlled by a guide having a surface configuration conforming to the surface contour desired for the optical blank. The sur face quality produced with such systems is limited obviously by both the precision of the mechanical equipment used and the exactness of the guide itself. Furthermore, template systems are similarly unsuitable for correcting asymmetrical surface errors. Thus, a general need exists for improved optical surface generation systems and especially for generation systems capable of producing asymmetrical surface changes.

The object of this invention, therefore, is to provide an improved optical surface generating system that can be selectively controlled to eliminate asymmetrical imperfections in an optical surface.

CHARACTERIZATION OF THE INVENTION The invention is characterized by the provision of an optical surface generating system including a blank support disposed within a vacuum chamber and adapted to support an optical blank therein, a vapor source for producing vapor suitable for deposition on a work surface ofthe optical blank, a collimator for directing a column of the vapor onto a limited area of the work surface, and a drive mechanism for producing two independently controlled components of relative transverse movement between the vapor column and the work surface. According to this unique system, the desirable characteristics of vapor deposition are made available in an apparatus capable of directing a vapor column onto any discrete portion of an optical blanks work surface thereby permitting selective modification thereof.

A feature of this invention is the provision of an optical surface generating system of the above type wherein the drive mechanism produces independent movement-of the vapor source in orthogonally related directions with respect to the work surface. The capability for producing independently controlled orthogonal movement of the vapor column renders this system particularly well suited for generating asymmetrical surface changes by a selective buildup of deposit.

Another feature of this invention is the provision of an optical surface generating system of the characterized type wherein the drive assembly independently produces both circumferential and radial components of relative transverse movement between the vapor column and the work surface. Although also capable of producing asymmetrical changes on a work surface, this relatively simple system is particularly suited for generating radially symmetrical optical surfaces.

Another feature of this invention is the provision of an optical surface generating system of the above-featured type including an area control unit for controlling the area of the vapor column directed onto the work surface. By varying the area of the vapor column, the resolution of the system can be modified as desired for given applications.

Another feature of this invention is the provision of an optical surface generating system of the above-featured type wherein the area control unit automatically varies the spacing between the vapor collimator and the optical blanks work surface. In this very simple system, modification of the spacing between the collimator and the work surface produces a proportional change in the area of the vapor column impinging upon the work surface.

Another feature of this invention is the provision of an opti cal surface generating system of the next above-featured types wherein the vapor collimator includes a vapor discharge aperture and the area control unit automatically controls the size of the discharge aperture. In addition to permitting controlled area variation of the vapor column, this arrangement allows total masking ofthe vapor column which is desirable in certain types of selective surface generation applications.

Another feature of this invention is the provision of an optical surface generating system of the above-featured types including a controller programed to produce a predetermined irregular path of relative transverse movement between the vapor column and the work surface. Programed control of the relative movement between the vapor column and the work surface facilitates the deposition of highly precise and predictable optical surfaces.

The invention is further characterized by the provision of an optical surface generation means for producing a vapor column, directing the vapor column onto the work surface of an optical blank, and producing between the work surface and the vapor column relative transverse movement along an irregular path so as to deposit the vapor on selected discrete portions of the work surface. Accordingly, vapor is deposited successively onto selected discrete areas of a work surface so as to produce a predictable surface contour thereon.

These and other characteristics and features of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic cross-sectional view of a preferred embodiment of the invention;

FIG. 2 is an isometric view of the evaporator assembly shown in FIG. 1;

FIG. 3 is an enlarged view of the vapor collimator shown in FIGS. 1 and 2; and

FIG. 4 is an enlarged cross-sectional view of another vapor collimator embodiment of the invention.

Referring now to FIG. 1, there is shown the vacuum chamber 11 connected by the evacuation line 12 to a suitable vacuum pumping system (not shown). Rotatably supported by the shaft 13 extending into the vacuum chamber 1! is the circular optical blank 14. Also mounted in the vacuum chamber Ell directly opposite the work surface 15 of the optical blank 14 is the evaporator assembly 16.

As shown more clearly in FIG. 2, the evaporator assembly 16 comprises the vapor source unit 17 supported between the end walls 18 and 18'. Included in the vapor source unit 17 is the conventional evaporator 20 mounted on the transport table 19. The ball slides 2i and 22 extend between the traverse plate 23 and the traverse housing 24 and slidably support the transport table 19. Longitudinal movement of the vapor source unit 17 along the ball slides 21 and 2.2 is produced by the rotatable ball screw shaft 25 that operatively engages the transport table 19.

Slidably supporting the traverse plate 23 and the traverse housing 24 between the end walls 18 and 18' are, respectively, the ball slides 27 and 28. The rotatable ball screw shaft 29 is operatively engaged with the traverse housing 24 and is adapted to produce longitudinal movement thereof along the ball slide 28. Also supported for rotation between the end walls 1.8 and 18' and extending through the traverse housing 24 is the spline shaft 31. Keyed for rotation with the spline shaft 31 and for rectilinear movement with the traverse housing 24 is the ball spline 32. The 45 bevel gear 33 is fixed for rotation with the ball spline 32 and is operatively engaged with the mating 45 bevel gear 34 that is keyed for rotation with the ter 20 mounted thereon.

ble drive motor 36 that is operatively coupled to the spline shaft 31. A similar reversible drive motor 37 is mounted on the end wall 18' and is operatively coupled to the'ball screw shaft 29. As shown more clearly in FIG. 3 the evaporator 20 supports the vapor collimator 41 that surrounds the conventional crucible42. The collimator 41 is formed by the upper cylinder 43 slidably mounted on the lower cylinder 44 and having the end wall 45 with the central aperture 46. Axial movement of 'theupper cylinder 43 on the lower cylinder 44 is produced by the reversible drive motor 47 supported from the evaporator housing 20 by the bracket 48. Operatively engaged are the rack gear 51 fixed to the upper cylinder 43 and the bevel gear 52keyed for rotation with the shaft 53 of the drive motor 47.

-Referring again to FIG. 1, there is shown the electrical control system 55 that provides selective control of the evaporator assembly 16. Electrical power is supplied from control system 55 to the

drive motors

36 and 37 by, respectively, the

electrical cables

56 and 57. Also connected to the control system 55 is the electrical cable 58 that supplies both the power required by the evaporator 20 and energizing current for the drive motor 47.

During operation of the evaporator assembly 16 power is supplied from the control system 55 to the evaporator 20 which is preferably of the electron beam type. The vapor produced by evaporation of a suitable material in the crucible 42.is formed into a column 61 (FIG. 1) by the collimator 41 and directed onto a given area A of the work surface 15. At the same time, selective energization of the reversible drive motor 37 produces longitudinal movement of the integrally connected traverse housing 24 and traverse plate 23 along the ball slides 27 and 28 in a direction determined by-the direction of motor rotation. Corresponding movement isexperienced by the directly supported transport table 19 and the evaporalndependently controlled, orthogonally related movement of the evaporator 20 is produced by selective energization of the reversible drive motor 36. This energization produces coordinated rotation of the operatively coupled spline shaft 31, the bevel gears 33 and 34, and the ball screw shaft 25 to effect translation of the transport table 19 along the ball slides 21'and 22 in a direction determined by the direction of motor rotation. Thus, by appropriate energization of the

reversible drive motors

36 and 37, the'vapor column 61 can be selectively directed onto any discrete portion of the stationary work surface 15.

According to a preferred method of operation, the work surface of the optical blank 14 is first measured with suitable test apparatus. The measurements can be made mechanically, for example, with dial indicators or traversing probes. However, the measurements preferably are made in the conventional manner by an interferometer which produces an interference picture indicating the contour characteristics of the surface 15. The interference picture is then utilized to determine, with respect to a desired surface contour, the relative elevational errors existing in adjoining discrete portions of the work surface 15. Preferably, these surface portions have areas substantially equal to the area A of the impinging vapor column 61. After derivation of this relative elevational error information, it is used to program the electrical control system 55 which produces selective orthogonal movement of the evaporator as described above. The motion of the evapora-- tor 20 is controlled so as to produce movement of the vapor column 61 in a predetermined, irregular path along the work surface 15. This path is determined so as to produce on the work surface 15 a selective irregular buildup of deposit that corrects the relative 'elevational errors initially existing thereon.

A more complete description of this method of surface correction is presented in the commonly assigned, copending US. Pat. application Ser. No. 719,657 of Ronald Aspden entitled Optical Surface Generating Method and Apparatus" and filed Apr. 8, 1968. However, in that application there is specifically described a material removal process wherein an preference for areas ofmaximum relative elevation. It will be appreciated that in the presently described material depositing system, the movement of the vapor column 61 will have in'- stead a preference for areas of minimum relative elevation.

Referring again to FIG. 3, it will be noted that energization of the reversible drive motor 47 produces movement of the upper cylinder 43. The direction of this axial movement on the lower cylinder 44 is determined by the direction of motor rotation. As a result of such movement, the spacing between the vapor discharge aperture 46 and the work surface 15 is altered to in turn modify the surface area A contacted by the vapor column 61. For example, upward movement of the cylinder 43 reduces the size of the area A while downward movement increases its size. Therefore, during a surface-correcting operation, the resolution of the system can be controlled by selective changes in the area of the vapor column directed against the work surface 15. For example, a relatively large vapor column area can be utilized initially to correct spatial periods of relatively large magnitude and vapor column areas of reduced size used to subsequently correct spatial periods of lesser magnitude.

The adjustable collimator 41 also permits selective control of the vapor columns area density characteristics which are dependent upon the spacing between the discharge aperture 46 and the work surface 15. Typically, the density distribution of a normal cross section through a vapor column decreases radially from a maximum value at its center to a minimum value at its periphery. Therefore, deposit will accumulate in varying depths on the surface area A with a maximum depth at its center and a minimum depth at its periphery. However, the slope of the deposit buildup can be controlled by varying the spacing between the vapor discharge aperture 46 and the work surface 15. For example, increasing this spacing reduces the slope while a decreased spacing produces an increase in slope. Thus, the collimator 41 can be automatically regulated to produce a deposited depth distribution desired for a particular application. In this regard it should be noted that a collimator work surface spacing substantially equal to the diameter of .the work surface is preferred for most applications.

Referring now to FIG. 4, there is shown another collimator embodiment having the cylinder 62'surrounding the vapor source crucible 63 mounted on the evaporator housing 20'.

Closing the upper end of the cylinder 62 is the variable aperture assembly .64 formed by the plurality of slidably engaging leaves 65. The assembly 64 functions like the well-known iris diaphragm used in conventional photographic cameras. Supported from the cylinder 62 by the bracket 68 is the reversible drive motor 67 that is coupled to the drive gear 66. Rotation of the gear 66 operates the aperture assembly 64 in the conventional manner to either enlarge or reduce the size of the central aperture 71 depending upon the direction of motor rotation.

The collimator embodiment illustrated in FIG. 4 also permits selective variation in the area A of the vapor column 61 directed against the optical blanks'work surface. For example, enlargement of the aperture 71 causes a corresponding enlargement in the area of the vapor column while contraction of the aperture 71 produces a corresponding reduction in the area of the vapor column. Thus, the collimator shown in F 10. 4can be used in the manner described above with regard to the collimator 41 of H6. '3.

In addition, the aperture 71 can be entirely closed to completely mask the vapor produced in the crucible 63. This is a desirable feature if the control system 55 is of the type disclosed and shown in the applicants copending US. Pat. application Ser. No. 72 l ,47 l, entitled Optical Surface Generating Method and Apparatus and filed Apr. 15, 1968. In that application there is disclosed an electrolytic polishing assembly that is moved with respect to the work surface of an optical blank in a path having both radial and circumferential components. During this movement the units electrodes are periodically energized and deenergized to produce selective polishing of the work surface. it will be obvious that a similar control system could be used in the present invention by rotating the optical blank 14 and reciprocating the evaporator along either of its orthagonal directions of movement. During this movement the aperture 71 would be periodically opened and closed in the selective manner disclosed in the abovenoted application.

Because of a unique combination of properties including hardness, high adherence for glass and an ability to be deposited in extremely smooth layers, silicon oxides including silicon monoxide and silicon dioxide are highly preferred deposition materials for production of high quality optical mirrors. A desired surface is first generated on a glass blank by selective deposition of the silicon oxide in the manner described above. Subsequently, the completed surface is overcoated with a deposited layer of aluminum to provide the necessary reflectance.

However, because silicon oxides absorb light of certain frequencies they are not preferred materials for the production of optical lenses except those intended for use with infrared light. Magnesium fluoride, which also exhibits hardness and a strong adherence for glass in addition to a low refractive index, is a highly preferred material for use in the production of optical lenses. in such applications, enhanced quality is attained by matching the refractive indices of the glass substrate and the vapor deposit. According to the present invention, this desirable result is obtained by utilizing a deposit material comprising a mixture of magnesium fluoride which has a relatively low refractive index and cerium oxide which possesses a relatively high refractive index. By suitable proportioning of these materials, a deposit with a refractive index equal to that of the glass substrate can be obtained.

Obviously, many modifications and variations of the present invention are possible in light of theabove teachings. For example only, control systems other than those mentioned above can be used to control the relative transverse movement between the evaporator unit and the optical blanks work surface. Another example of an applicable control system is disclosed in the concurrently filed, commonly assigned US. Pat. application Ser. No. 724,082 of Ronald Aspden, entitled Optical Surface Generating Method and Apparatus" and filed Apr. 25, 1968. That system is especially suited for generating radially symmetrical optical surfaces. it is to be understood, therefore, that within the scope of the appended claims the invention can be practiced otherwise than -as specifically described.

lclaim:

l. An optical surface generating apparatus comprising an evacuable housing, blank support means disposed within said housing and adapted to support an optical blank, a vapor source disposed within said housing and adapted to produce a source of vapor for deposition on a work surface of the supported optical blank, an automatically controlled adjustable collimator means for directing a vapor column onto a limited area of the work surface and for controlling the area of the vapor column, and drive means for producing two indepen dently controllable components of relative transverse movement between said vapor column and the work surface so as to permit selective direction of the vapor column onto any discrete portion of the work surface. 2. An optical surface generating apparatus according to claim 1 wherein said area control means comprises automatically controlled means for varying the spacing between said collimator means and the work surface.

3. An optical surface generating system according to claim 1 wherein-said collimator means comprises a vapor discharge aperture and said area control means comprises automatically controlled means for varying the area of said aperture.

4. An optical surface generating system according to claim 1 wherein said drive means is adapted to independently move said vapor column in orthogonally related directions.

5. An optical surface generating system according to claim 1 wherein said drive means is adapted to inde endently produce both circumferential and radial componen s of relative transverse movement between said vapor column and the work surface.

6. An optical surface generating system according to claim 1 including movement control means for controlling said drive means so as to produce a predetermined irregular path of relative transverse between said vapor column and the work surface.

7. An optical surface generating system according to claim 6 wherein said drive means is adapted to independently move said vapor column in orthogonally related directions.

8. An optical surface generating system according to claim 6 wherein said drive means is adapted to independently produce both circumferential and radial components of relative transverse movement between said vapor column and the work surface. I

9. An optical surface generating system according to claim 1 wherein the spacing between said collimator means and the work surface is substantially equal to the diameter of said work surface.