Method to prevent backside growth on substrates in a vapor deposition system

Backside growth on substrates in a vapor deposition system has been a problem resulting in cracking of the material deposited on the substrate, making replication in a vapor deposition system difficult to achieve, and requiring post deposition machining to separate the substrate-deposit from the deposition fixture. A solution to the problem is the following: the substrate is mounted on a plurality of graphite pillars, with the pillars being bonded to the substrate as near the periphery thereof as possible. A hollow body open on one side but closed on the other, and fabricated from GRAFOIL with graphite cement used as a bonding agent, is mounted on the pillars with the open end facing the substrate. The open end of the body is pressed against the substrate and sealed with a bonding agent. This completely covers the backside of the substrate and thus prevents any vapor deposition thereon.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of preventing backside growth on 
substrates in an impinging chemical vapor deposition system. 
2. Description of the Prior Art 
Backside growth on substrates in an impinging chemical vapor deposition 
system, if not prevented, often results in cracking of the deposit. Such 
growth also makes it difficult to achieve, in a vapor deposition system, 
replication of the figure and finish of preshaped structures. The presence 
of such growth is further disadvantageous in that post deposition 
machining is required to separate the substrate deposit from the 
deposition fixture. Backside growth on substrates thus is particularly 
detrimental in lightweight rapid optics fabrication. 
A process is disclosed in application Ser. No. 389,248 filed Aug. 3, 1989 
by J. T. Goela, M. A. Pickering and R. L. Taylor and assigned to the 
assignee of the present invention for fabricating, by vapor deposition, 
lightweight structures out of refractory materials. The methods and 
lightweight structures disclosed in that application, which application by 
reference is incorporated herein, involve a core to define the shape and 
size of each structure. The core is coated with an appropriate deposit, 
such as silicon carbide (SiC) or silicon (Si), to give the structure 
strength and stiffness and for bonding thereof to another surface, for 
example, the surface of a substrate comprising the faceplate of a mirror 
being fabricated. 
In the fabrication of mirrors, graphite may be used to form a mandrel for 
replicating on a SiC faceplate. One side of the mandrel is optically 
fabricated, either as flat or as a convex spherical shape. The other side 
of the mandrel is lapped flat. The lapped side of the mandrel is bonded by 
means of pillars and graphite cement to a baffle plate in a vapor 
deposition reactor. The mandrel is then coated with multiple coats of a 
suspension of carbon in solvent, following which the surface of the 
mandrel is buffed or polished to make it as shiny as possible without 
significantly altering its figure. Deposition of SiC on the mandrel is 
then effected. Without separating the faceplate from the mandrel, the 
exposed SiC surface may be etched with hot potassium hydroxide (KOH) to 
improve bonding of graphite to SiC. A lightweight structure core is then 
fabricated from flat or curved graphite ribs, as disclosed in the 
aforementioned application for patent. After being bonded together with 
graphite cement, the lightweight structure core is bonded with graphite 
cement to the etched SiC surface of the mandrel. SiC is then deposited to 
enclose the lightweight structure core following which the baffle plate is 
separated from the baffle pillars. Controlled edging may be performed to 
remove excess SiC deposit. Using a blade, the interface between the 
graphite mandrel and the SiC faceplate may then be opened to recover the 
SiC coated mirror faceplate. The latter is then ready for Si coating. 
Selective deposit to confine Si growth to only the front face of the SiC 
coated faceplate in the fabrication of the mirror surface is very 
important. This is for the reason, mentioned hereinbefore, that Si growth 
on the backside of the SiC faceplate is disadvantageous in that, if not 
prevented, cracking of the deposit on the front side often results, making 
replication in a vapor deposition system difficult to achieve, and 
additionally requiring post deposition machining to separate the 
substrate-deposit from the deposition fixture. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a facile, inexpensive method to 
prevent backside growth on substrates in a vapor deposition system. 
Another object of the invention is to provide such a method that does not 
require any machining to remove excessive material. 
A further object of the invention is to provide a method to selectively 
deposit SiC and Si in a chemical vapor deposition reactor. 
A more specific object of the invention is to provide a method for 
preventing backside growth on a substrate in a vapor deposition reactor 
comprising the steps of: 
(a) mounting the substrate on pillars made of suitable material, the 
pillars being supported by a baffle backplate in the vapor deposition 
reactor and bonded with a suitable bonding agent to the substrate as near 
the periphery of the substrate as possible, 
(b) fabricating from flexible graphite a hollow body that is compatible 
with the substrate and is open at one end but closed at the other end, 
using graphite cement as a bonding agent, 
(c) mounting the hollow body on the pillars with the open end facing the 
backside of the substrate, the interface of the closed end with the 
pillars being sealed, 
(d) pressing and sealing with a suitable bonding agent the open end of the 
hollow body against the substrate adjacent the periphery thereof, whereby 
during the vapor deposition process vapor deposition on the backside of 
the substrate is prevented with vapor deposition occurring on the hollow 
body, and 
(e) upon completion of the vapor deposition process removing the hollow 
body from the substrate by piercing and breaking up the hollow body by 
mechanical means. 
In accomplishing these and other objectives of the invention, an 
appropriate substrate which may be of any shape (hexagonal, square, 
triangular, circular, etc.) and size is mounted on a sufficient number of 
pillars made of graphite or any other suitable material. The pillars are 
bonded with a suitable bonding agent to the substrate as near the 
periphery of the substrate as possible. An appropriate hollow body, open 
on one end but closed on the other and compatible with the substrate, is 
fabricated from flexible graphite, a graphite cloth made under the trade 
name "GRAFOIL" and commercially available from Union Carbide Corporation, 
Old Ridgebury Road, Danbury, Conn. 06817. The hollow body is then mounted 
on the pillars with the open end facing the backside of the substrate, 
with the interface of the closed end with the pillars being sealed. The 
open end of the body is pressed against the substrate and sealed thereto 
with a bonding agent. This arrangement completely covers the backside of 
the substrate and thus prevents any vapor deposition thereon during the 
vapor deposition process. Vapor deposition occurs, however, on the outer 
surface of the hollow body. 
The hollow body with a vapor deposited coating thereon is readily removed 
from the substrate with no resulting damage to the substrate. 
Specifically, since GRAFOIL is quite flexible, the Si coated hollow body 
is readily broken into small pieces by using a piercing tool and a pair of 
pliers. Cracks produced in the coated GRAFOIL, while it is being removed 
from the substrate, do not propagate to the substrate. This is for the 
reason that the substrate is perpendicular to the GRAFOIL body, and 
GRAFOIL is flexible, and therefore, deforms as stresses develop due to 
material growth and any thermal expansion mismatch. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the specification. For a better understanding of the invention, its 
operating advantages, and specific objects attained by its use, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates the method used to prevent growth on the backside of a 
circular SiC substrate 10 in an impinging chemical vapor deposition system 
which may be of a conventional type such as that designated 12 in FIG. 4. 
Substrate 10 may comprise the faceplate of a mirror being fabricated. As 
best seen in FIGS. 2 and 3, there is provided, in accordance with the 
invention, a hollow cylindrical body 14 which preferably is made of 
GRAFOIL, a flexible graphite, that is compatible with substrate 10. The 
hollow body 14 has substantially the same diameter as the substrate 10 and 
is open at one end but closed at the other. The hollow body 14 is mounted 
in cooperative relation with the substrate 10 on a plurality of pillars 16 
of which, for convenience of illustration, two only are shown in the 
drawings. The pillars 16 are bonded with a suitable bonding agent to the 
substrate 10 as near the periphery thereof as possible. The open end of 
the hollow body 14 is positioned facing the backside of the substrate 10, 
with the pillars 16 extending through the closed end thereof in sealing 
relation therewith, being sealed with graphite cement. The pillars 16, in 
turn, are mounted on and supported by a baffle backplate 18 having a 
centrally located opening 20. The open end of the hollow body 14 is 
pressed against the substrate 10 and sealed there against with a suitable 
bonding agent (not shown). In FIG. 2 the arrows indicate the direction of 
flow relatively to the substrate 10, of the deposition vapor during the 
vapor deposition process. 
With this arrangement, the backside of the substrate 10 is completely 
covered whereby during a Si vapor deposition process, vapor deposition on 
the backside of the substrate 10 is prevented. Vapor deposition of Si 
occurs, however, on the outer surface of the hollow body 14. 
FIG. 4 schematically illustrates a chemical vapor deposition system that 
may be used for the selective deposit of SiC and Si on the substrate of 
FIGS. 1-3. As shown in FIG. 4, Argon enters a bubbler chamber 24 from a 
source (not shown) by way of a valve 26 and line 28. Chamber 24 may 
contain methyltrichlorosilane (CH.sub.3 SiCl.sub.3) or trichlorosilane 
(SiHCl.sub.3). Methyltrichlorosilane is preferred to produce a silicon 
carbide (SiC) deposit. Trichlorosilane is preferred to produce a silicon 
(Si) deposit. As those skilled in the art understand, however, other 
hydrocarbon and silane sources can be used to produce SiC and Si deposits. 
Both of these materials have been fabricated over a wide range of 
deposition temperature and reactor pressure, as described more fully in 
the aforementioned application for patent. 
Argon bubbles carrying the reagent CH.sub.3 SiCl.sub.3 or SiHCl.sub.3 enter 
a line 30 under the control of a valve 32. Hydrogen enters the line 30 
from a source (not shown) under the control of a valve 34. The reagents 
may be introduced into the reactor 22 through a central injector 31. 
Reactor 22 includes a conventional ceramic deposition chamber 23 contained 
within a reaction zone tube 25, which chamber 23 may be heated between 
about 830.degree. and 1350.degree. C. by suitable heating elements (not 
shown). 
Gaseous products are removed from the reactor 22 through a filter 36 
through a line 38 to a vacuum pump 40. From the vacuum pump 40, the gases 
are conveyed through a line 42 to a scrubber 44. The gases are then vented 
to the atmosphere. 
In FIG. 5 there is illustrated a perspective view of the substrate 10 and 
GRAFOIL body 14 after the completion of Si deposition. Since GRAFOIL is 
quite flexible, the Si coated GRAFOIL is readily broken into small pieces 
by using a piercing tool and a pair of pliers. While removing the coated 
GRAFOIL, cracks do not propagate to the substrate 10 because the substrate 
10 is perpendicular to the GRAFOIL body 14, and because GRAFOIL is 
flexible and therefore deforms as stresses develop due to Si growth and 
thermal expansion mismatch. 
Thus, in accordance with the invention, there has been provided a method to 
selectively deposit SiC and Si in a chemical vapor deposition system. The 
method is unique in providing a facile and inexpensive way to prevent 
backside growth in a vapor deposition system and thus facilitate 
replication of the figure and finish of preshaped structures. The method 
is further characterized in that machining is not required to remove 
excessive material. 
With this description of the invention in detail, those skilled in the art 
will appreciate that modifications may be made to the invention without 
departing from its spirit. Therefore, it is not intended that the scope of 
the invention be limited to the specific embodiment illustrated and 
described. Rather, it is intended that the scope of the invention be 
determined by the appended claims and their equivalents.