Coated laser mirror and method of coating

A method of applying an intermediate bond coat on a laser mirror substrate is described comprising surface polishing the mirror substrate followed by depositing a layer of amorphous silicon, amorphous germanium, or mixtures thereof on the mirror surface, and polishing the thus coated mirror surface to a substantially void-free surface finish. Laser mirror substrates such as graphite fiber reinforced glass, molybdenum and silicon carbide coated by such process are also described.

DESCRIPTION 
1. Technical Field 
The field of art to which this invention pertains is composite optical 
elements of the reflecting type, and particularly coating methods for such 
optical elements. 
2. Background Art 
A recent discovery has advanced the state of the laser mirror art 
significantly. Laser mirrors comprising graphite fibers in a glass matrix 
have been found to provide a laser mirror of low density, high elastic 
stiffness, high strength, high fracture toughness, low thermal expansion, 
high thermal conductivity and environmental stability. Note commonly 
assigned U.S. Pat. No. 4,256,378, the disclosure of which is incorporated 
by reference. 
In the process of making such mirrors, great difficulty has been 
encountered in the application of the reflective coatings to these 
mirrors. In preparatory surface polishing of the optical surfaces of the 
graphite fiber reinforced glass matrix composite mirror substrates, 
surface voids up to 15 microns in depth and width have formed. (Note FIG. 
1.) Accordingly, when applying the laser radiation reflective coating to 
the mirror surface, a coating is required which will not only fill these 
voids, but can be polished back to a specular finish of radiant energy 
reflecting quality. 
Other desirable attributes of a coating in this environment include good 
adhesion to the mirror substrate, preferably a low processing temperature, 
coefficients of thermal expansion matched as closely as possible to the 
mirror substrate, and a coating which is polishable to an optical finish, 
preferably utilizing conventional techniques. 
Surprisingly, the very qualities which make the improved laser mirror 
substrates described above superior in this area contribute to the 
difficulties in applying the optically reflective coating. For example, 
conventional chemical vapor deposition which is generally used in this 
art, requires relatively high substrate temperatures for deposition. But 
since the fiber reinforced glass mirror substrates are so dimensionally 
stable over a wide range of temperatures, upon cooling the thus deposited 
films shrink, crack and spall from the mirror substrate surface. 
Accordingly, until now vacuum deposition techniques have been used at low 
temperatures, but these deposition rates are so slow as to be impractical 
for building films up to the several mil thickness required to fill the 
substrate voids which occur upon polishing. Furthermore, additional 
build-up in coating thickness is desirable so as to be tolerant to the 
subsequent removal of the coating material which takes place when the 
mirrors are polished. 
In addition to the recent graphite-glass mirror substrates, improved 
coating methods would also be advantageous for such conventional mirror 
substrates as molybdenum and silicon carbide. To obtain a smooth finish on 
silicon carbide, being a very hard substance, is both very expensive and 
highly labor intensive. And molybdenum is only polishable up to about a 50 
.ANG. rms surface finish. 
Accordingly, what is needed in this art is a relatively inexpensive coating 
process for mirror substrates which can also provide a highly polished 
surface. 
DISCLOSURE OF INVENTION 
The present invention is directed to a laser mirror substrate coated with 
an adherent, void filling, highly polishable intermediate layer, upon 
which the ultimate laser radiation reflecting outer layer can be applied. 
The intermediate layer comprises an amorphous silicon and/or an amorphous 
germanium coating applied by plasma enhanced chemical vapor deposition. 
Another aspect of the invention comprises a method of coating a laser 
mirror substrate by surface polishing the mirror substrate, applying an 
intermediate bond coating of amorphous silicon and/or amorphous germanium 
by plasma enhanced chemical vapor deposition to the laser mirror 
substrate, polishing the thus deposited coating, and applying a laser 
radiation reflecting outer layer to the intermediate coating. 
The foregoing, and other features and advantages of the present invention, 
will become more apparent from the following description and accompanying 
drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Although the process according to the present invention can be used on any 
surface where a highly polishable coating is desired, the process has 
particular applicability in the laser mirror field and those fields where 
high reflectivity is important. And while distinct advantages are imparted 
to conventional mirror substrates such as molybdenum and silicon carbide, 
the process has particular applicability to graphite fiber reinforced 
glass mirror substrates. 
As described in U.S. Pat. No. 4,256,378, any graphite fiber with the 
requisite high strength and good modulus of elasticity can be used in 
these laser mirrors coated according to the present invention. Hercules 
HMS graphite fiber and Celanese GY70 graphite fibers are particularly 
suitable. The fibers are used in the glass matrix at about 40% to 70% by 
volume based on the graphite glass composite and preferably at about 60% 
by volume. 
The glass matrix used is particularly selected to have a very low 
coefficient of thermal expansion, preferably matched closely but not equal 
to that of the graphite fibers used since the graphite has a highly 
negative axial coefficient of thermal expansion, and the glass has a 
positive but small coefficient of thermal expansion. Particularly suitable 
for the purpose of the present invention is a borosilicate glass such as 
Corning Glass Works CGW 7740. 
And while a variety of methods may be used to produce these laser mirror 
substrates, to be coated according to the present invention, the preferred 
method is that described in the above cited patent and comprises passing 
the graphite fibers through a slip of the powdered glass and solvent to 
impregnate the fibers. The fibers are then wound on a drum and dried, 
removed from the drum and cut into strips up to the diameter of the mirror 
to be fabricated. While typical test samples made were about ten 
centimeters in diameter, mirrors up to twenty centimeters in diameter have 
also been treated by the processes according to the present invention. 
However, mirrors of even larger diameters can be made according to the 
present invention. The fibers are then laid in alternating ply stacks of 
any orientation desired such as 0.degree. and 45.degree.; 0.degree., 
45.degree. and 90.degree.; 0.degree., 30.degree. and 90.degree.; 0.degree. 
and 60.degree., etc., with 0.degree. and 90.degree. being preferred. The 
assembled composite is then hot pressed either under vacuum or inert gas 
such as argon on metal dies coated with colloidal boron nitride or 
graphite dies sprayed with boron nitride at pressures of 6.9-13.8 MPa 
(1,000-2,000 psi) at temperatures of 1050.degree.-1450.degree. C. 
Initial polishing is then done with conventional polishing equipment and 
grinding apparatus. Generally at this time, under such surface polishing, 
surface voids in excess of 15 microns in depth and width develop. Note 
FIG. 1. This is a consequence of the breaking and pulling away of the 
graphite fibers at the surface during this polishing operation. The 
intermediate bond coating operation according to the present invention, as 
discussed above, in addition to having the advantages of good adhesion of 
coating material to the mirror substrate, relatively low processing 
temperature, coating coefficients of thermal expansion compatible with the 
mirror substrate and easy polishability with conventional polishing 
techniques after application, also has the ability to fill these voids 
securely. This is done by plasma enhanced chemical vapor deposition of 
amorphous silicon, amorphous germanium, or mixtures thereof in any 
relative proportions desired. 
By plasma enhanced chemical vapor deposition is meant the plasma excitation 
of a low pressure chemical vapor deposition system. This provides for 
effectively high gas temperature which results in the pyrolysis of the 
reactant gas while still being able to maintain a low substrate 
temperature for deposition of the coating. Deposition conditions (gas 
pressure, flow rate and substrate temperature) can be adjusted to provide 
for the deposition of a stress-free film. This is a key factor 
contributing to the highly polishable nature and adherence of the 
intermediate bond layer deposited according to the present invention. In 
order to accomplish this stress-free coating, a narrow range of deposition 
conditions should be utilized. For example, utilizing an about 1% to about 
5% by volume reactive gas content and a 13.5 Mhz induction coil at power 
levels of approximately 100 watts for excitation, a pressure of about 1 
torr at a temperature of about 200.degree. C. and total system flow rate 
of about 100-200 cc per minute should be used. 
An apparatus useful for applying the plasma enhanced chemical vapor 
deposition coatings is shown schematically in FIG. 2. In this Fig., the 
initially polished mirror substrate 11 is placed on support 10 in quartz 
reactor 1. The substrate can be rotated during deposition to ensure a 
uniformly deposited layer. The reactant gas enters the system as indicated 
by arrow 2. The high frequency coil 3 contained on the metal shield 4 is 
used for plasma excitation. Pressure guage 5 is used to monitor the 
reaction conditions. Excess reactants are trapped in cracking furnace 6 
and liquid nitrogen trap 7 and molecular sieve 8. A vacuum is maintained 
by means of vacuum pump 9. In this embodiment, the mirror substrate 
supported upstream of the gas by a flat quartz holder 10 is placed 3 
inches (7.62 cm) from the center of a six-turn rf coil, and held at 
200.degree. C. by an externally controlled resistance heater during 
deposition. The 1.0 meter long horizontal quartz reactor tube, having a 42 
mm inner diameter, was surrounded with a grounded metal shield to ensure a 
uniform silane-helium plasma. Pressure conditions of about a 1 to 10 torr 
may be used, but best results are obtained with a total pressure of about 
1 torr for deposition. And while any thickness of coating may be 
deposited, in general, coating thicknesses of at least 15 microns are 
deposited, and preferably in the range of 3-5 mils. 
This (FIG. 2) represents a conventional plasma enhanced vapor deposition 
apparatus particularly adapted for the use described herein. However, best 
results have been obtained with a novel reactor geometry developed 
specifically for the deposition of amorphous silicon and germanium at 
rates which are nearly an order of magnitude greater than that which can 
be obtained with the best commercial systems. Note FIG. 3. For example, 
with the best commercial systems depositions of 180-200 .ANG. per minute 
at 200.degree. C. have been obtained. However, employing the reactor 
geometry described herein, a deposition rate of 1700 .ANG. per minute at 
200.degree. C. has been obtained. Furthermore, an increase in temperature 
in the reactor results in a corresponding increase in deposition rate. The 
key to obtaining such rates is the unique manner in which inductive 
coupling is employed for plasma excitation, whereas typical commercial 
systems primarily employ capacitive coupling for plasma excitation. The 
plasma excitation is performed by employing the plasma excitation 
downstream (31) of the piece to be coated (32). Secondly, inserts (33 and 
34) which reduce the cross sectional area of the reactor tube are also 
used. The inserts are generally identical in shape with an outer diameter 
substantially the same as the inner diameter of the reaction tube. For 
example, in the apparatus described above, the inserts had an outer 
diameter of about 45 mm, doughnut shaped with a rectangular opening in the 
center with dimensions of 0.5 by 1.5 inches (1.27 by 3.81 cm). Other 
materials such as quartz or metals stable in this environment may also be 
used. The insertion of these units results in nearly a four-fold increase 
in deposition rate. The upstream insert (33) is believed to introduce 
turbulence to the gas flow and the downstream insert (34) provides not 
only turbulence, but functions as a barrier for controlling back diffusion 
of excited species in the deposition area. The remainder of the apparatus 
as shown in FIG. 3 is indicated by a silane or germane source tank 35 
controlled by valve 36, which results in gas flow in the direction shown 
by arrow 37. The rf generator 38 is a 250 to 400 kilocycle commercial rf 
induction heating generator which provides the source power for induction 
heating coil 39. Although an inductive rf coupler is shown as the heat 
source in this apparatus, other conventional heat sources such as 
resistance heating and radiant heating can be used. 
The temperature ranges used in the process according to the present 
invention will depend on the heat source used. For resistance heating, 
temperatures of room temperature to 500.degree. C. can be used. For rf 
coupling (inductive heating), room temperature to the melting temperature 
of the substrate can be used; and for radiant heating, room temperature to 
650.degree. C. can be used. A conventional drive motor 40 is used to 
rotate the substrate 32 which is preferably placed on the base 44 in an 
inverted position to prevent any extraneous materials from inadvertently 
dropping onto the substrate during the coating operation. The essential 
feature of the apparatus according to the present invention is the use of 
the inductive coil 31 downstream of the gas flow and substrate being 
coated. An rf generator 42 in conjunction with 31 provides the necessary 
plasma excitation. A trap 43 is used to collect any extraneous conductive 
particulate material. The remainder of the system can be as shown by 
characters 5-9 in FIG. 2. 
Generally, the systems are run at reduced pressures of about 1 torr, 
utilizing gas mixtures of about 1% to about 5% SiH.sub.4 or GeH.sub.4 in 
an inert gas such as helium, nitrogen, argon, etc. While silane and 
germane are the preferred reactant gases, any volatilizable silicon or 
germanium compound may be employed. The power requirements are matched to 
the size of the system with smaller volume systems requiring lesser power 
inputs and larger volume systems having greater power requirements. 
By this process, not only does the silicon film deposited adequately fill 
the mirror voids, but a surface roughness is produced which is no more 
than approximately 3 microns. At this point, conventional mechanical or 
chemical polishing techniques can be employed producing a highly specular, 
void-free surface suitable for mirror fabrication. For example, 
conventional molybdenum laser mirror substrates generally polish up to a 
50 .ANG. rms or greater surface finish. However, with the amorphous 
silicon and germanium coatings according to the present invention, surface 
finishes of about 10 to about 15 .ANG. rms can be obtained. 
FIG. 3 shows the surface of the mirror after deposition, and FIG. 4 shows, 
left to right, the mirror before and after the final conventional 
polishing step. As the outermost reflective coating, any conventional 
reflective coating, in the infra-red, visible, ultraviolet, laser, etc. 
range may be deposited once the highly polished intermediate layer is 
formed. This can include such things as gold; gold, silver or aluminum 
overcoated with thorium tetrafluoride; gold, silver, or aluminum 
overcoated with silicon oxide or magnesium fluoride; and multi-layered 
dielectric coatings such as alternating layers of one quarter optical 
wavelength thickness of zinc sulfide and thorium tetrafluoride. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and scope of the claimed invention.