Process control system

This invention relates to a process control system and method of controlling a chemical vapor deposition (CVD) process where a coating is deposited on a substrate heated by passing a current through the substrate to create a heating zone. The control system relies on detecting a signal induced on the coated substrate outside of the heating zone and using the induced signal to control one or more process parameters.

BRIEF SUMMARY OF THE INVENTION 
This invention is concerned with a process control system for controlling 
chemical vapor deposition (CVD) processes. In particular it is concerned 
with CVD processes for making or coating filaments. 
CVD processes have been widely utilized to make and/or treat high-modulus, 
high-strength filaments, such as boron filaments and silicon carbide 
filaments. In addition, these basic filaments are often treated by 
depositing additional thin coatings on these filaments in order to modify 
or enhance specific properties of these filaments. For example, U.S. Pat. 
No. 3,846,224 discloses a process for depositing a boron carbide coating 
on boron filament. A filament coating in the trade under the name of 
"Borsic" is a boron filament with a thin silicon carbide coating applied 
to the exterior surface of boron filament. 
U.S. Pat. No. 4,068,037 discloses the use of a carbon-rich silicon carbide 
layer on a silicon carbide filament for the purpose of improving the 
strength of the filament. 
It will be noted in patents and other literature describing processes for 
making these very delicate high-modulus, high-strength filaments that CVD 
process parameters are generally closely regulated in order to maintain 
the quality of the filament. This invention deals with a process control 
system wherein the control capability is derived from electrical signals 
induced within the coated substrate. At this time the point must be made 
that the phenomenon which produces such induced voltages is not 
understood. The voltages, however, are capable of being detected and used 
to control process parameters. 
It is an object of the invention to provide a process control system for 
controlling one or more process parameters associated with CVD reactors. 
It is another object of the invention to provide a process control system 
for CVD reactors used to make and/or treat filament substrates. 
It is still another object of the invention to provide a process control 
system for depositing boron carbide (B.sub.4 C) on a boron filament 
substrate. 
It is still another object of the invention to provide a process control 
system for making silicon carbide filaments. 
It is yet another object of the invention to provide a method for 
controlling one or more process parameters in CVD reactors, and in 
particular, CVD reactors for making continuous filaments. 
It is yet another object of the invention to provide a method of 
controlling the CVD process utilized to make and/or treat boron and 
silicon carbide filaments. 
In accordance with the invention, a process control system for a CVD 
reactor system for depositing a coating on a filament substrate is 
provided. In general a continuous filament moves through a reaction 
chamber wherein a heating zone is established by passing an electric 
current through the substrate filament. The process control system also 
includes a pair of electrodes, at least one of which is outside of the 
heating zone. A voltage detector is coupled to the pair of electrodes for 
detecting a voltage induced between the electrodes. A means for generating 
a control voltage in response to the induced voltage is coupled to the 
voltage detection means, and means responsive to the control means is used 
to control metering valves or other such devices for controlling process 
parameters. 
The invention also includes a method for controlling a CVD process whereby 
the induced voltage is coupled to a control means for generating a control 
signal, and the control signal is, in turn, used to control process 
parameters. 
The novel features that are considered characteristic of the invention are 
set forth in the appended claims; the invention itself, however, both as 
to its organization and method of operation, together with additional 
objects and advantages thereof, will best be understood from the following 
description of a specific embodiment when read in conjunction with the 
accompanying drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Classically, CVD processes are used to make and/or treat boron and silicon 
carbide filaments, in particular. Typically, a continuous filamentary core 
is passed through a reactor in which a heated reaction zone is developed. 
Reagents, such as boron halides for boron and methylchlorosilanes for 
silicon carbide, are supplied to the reactor where upon contact with the 
heated filament, they dissociate and deposit on the filament either boron 
or silicon carbide. This basic process includes many variations; for 
example, a buffer layer may be deposited between the core and the boron or 
silicon carbide layer. Additionally, external coatings are often deposited 
on the surface of the boron or silicon carbide layer. The character of the 
buffer layer, the exterior layer, and even the main boron or silicon 
deposit can be altered by varying the speed at which the filament 
traverses the reactor, the deposition temperatures, and the blend of 
materials used to form the essential deposit. 
The aforementioned U.S. Pat. No. 3,846,224 discloses a multi-reactor 
process wherein the filament passes through two or more separate and 
distinct reactors. U.S. Pat. No. 4,068,037 discloses the use of a single 
reactor to form multiple deposits. This invention will be described in 
relation to a single B.sub.4 C reactor configuration, it being assumed 
that a boron filament is the feedstock to the B.sub.4 C reactor. The 
control system may be used at the end of a multi-stage system wherein a 
substrate deposit and a coating are developed in a continuous sequence. 
The deposition parameters, such as filament speed, deposition temperature, 
reagent blends, etc. do not deviate in this invention from the conditions 
described in one or more of the patents referred to above. This invention 
deals with ways of controlling these parameters. 
Referring to FIG. 1, there is shown at 10 a CVD reactor 12, together with a 
process control system 14. The CVD reactor 12 includes a closed reactor 
vessel 16. A substrate filament 18, such as tungsten, carbon monofilament, 
or boron etc. is obtained from a supply reel 20 and is fed into the 
reactor core 16 through mercury electrode 22. The filament 18 traverses 
the length of the reactor vessel 16 and exits by means of a mercury 
electrode 24. 
It then passes through a third mercury electrode 26 before it is wound up 
on a take-up roll 28. The filament enters the reactor as a core and leaves 
the reactor with one or more coatings. The mercury electrodes 22 and 24 
are coupled to a current supply means 30. The current supply means 30 
couples current to the electrode 22 through the length of the filament 
between the electrodes 22 and 24 back to the current supply means 30. The 
terminal 24 is depicted as a ground terminal in FIG. 1. 
The length of the filament 18 between the electrodes 22 and 24 represents a 
reaction zone 19. The current flowing through this zone is adjusted until 
the filament is heated to the desired deposition temperatures. Electrode 
26 is outside of the reaction zone. 
The reactor vessel 16 also includes a gas inlet 32 for supplying a blend of 
reactants to the reaction zone. Typically, such reactants will include 
boron trichloride (BCl.sub.3) or other boron halides in combination with 
hydrogen and a hydrocarbon where a B.sub.4 C plating is to be developed on 
a boron filament. Somewhat analagously, a blend of dimethylchlorosilanes 
is supplied to a conventional reactor to form silicon carbide filaments. A 
gas outlet 36 for withdrawing reactants from the reactor is also provided. 
The process control system embodied in this invention includes a voltmeter 
38 coupled to the third electrode 26 for detecting a voltage induced 
within the filament between electrode 26 and the current electrode 24. An 
alternative construction would be to provide a fourth electrode positioned 
between electrodes 24 and 26, if desired. 
The voltmeter 38 is coupled to a signal conditioning means 40. The induced 
voltage signal is modified and formed into a control signal. The control 
signal leaves the signal conditioning means 40 and is coupled to a flow 
control valve 42 via a valve control circuit 41. The flow control valve 
regulates the flow of hydrocarbon to the gas inlet 32. 
It is obvious that the control signal and similar control means can be used 
to control the flow of reactants into gas inlet 32 or to control the 
magnitude of the current from the control supply means to the heating zone 
or to control the speed of the filament through the reactor or any 
combination of these. For the purpose of this discussion, it will be 
assumed that a B.sub.4 C coating is to be deposited on a boron filament. 
To achieve this, methane is supplied through the flow control valve 42 
into gas inlet 32 and into the reactor vessel 16. It has been determined 
that the most critical parameter relative to controlling filament quality 
is the percent of carbon in the plating gas. Some carbon is supplied via 
the BCl.sub.3 because it mixes the recycled BCl.sub.3 generally in the 
form of methylboranes. The flow of CH.sub.4 was chosen to adjust the 
carbon content. Clearly, a similar setup can be provided for controlling 
the flow of hydrogen to the reaction zone either alone or in combination 
with controlled quantities of a hydrocarbon. 
In the process of providing such a B.sub.4 C coating, it was observed that 
an electrical signal of up to 2 megahertz in frequency, which, for 
purposes of this discussion we will call an induced voltage, occurs when 
an electrode, such as electrode 26, is coupled to the filament outside of 
the reaction zone. It is not known what causes this voltage. What is 
known, however, is that as one or more of the process parameters are 
varied, the voltage varies when controlling the flow of methane. It was 
observed that where the magnitude of the voltage was below a lower 
threshold, the filament was very weak and unsatisfactory. On the other 
hand, when the induced voltage was greater than an upper threshold level, 
the filament did not withstand the debilitating action of molten aluminum. 
In fact, the upper and lower thresholds represented a very narrow window. 
So long as the system was operated within this narrow window, an excellent 
B.sub.4 C coated boron filament was produced. The variations of these 
properties as a function of these parameters are illustrated in FIG. 2. 
The operation of the system shown in FIG. 1 is as follows. A boron filament 
18 is drawn through the reactor 12 where it is heated to deposition 
temperatures by the passage of a DC electrical current from current supply 
30. Within the reaction zone of the reactor is a plating gas comprised 
primarily of BCl.sub.3, H.sub.2, and CH.sub.4. Minor constituents in the 
gas may include diborane and alkyl boranes. Under these conditions a boron 
carbide coating is deposited on the filament. Induced electrical impulses 
are produced during the deposition process and are sensed and detected by 
the sensing electrode 26 and the voltmeter 38. The induced signals are 
appropriately conditioned in signal conditioning means 40, and a resulting 
control signal is coupled to a logic circuit 41 which determines whether 
the amplitude is below, within, or above the preferred range. If the 
amplitude is outside the preferred range, a logic module activates a flow 
control valve 42 which changes the carbon-to-hydrogen ratio in the gas 
entering the gas inlet 32 in a direction which will bring the reactor back 
to within the specified operating parameters. Many types of control 
circuits are possible. In the present case we use a motorized 
micrometering valve on the CH.sub.4 inlet. The logic circuit 41, in 
combination with the flow control valve 42, makes an incremental change in 
the gas flow, then pauses until the effects of this change appear at the 
sensing electrode 26. If further correction is needed, the cycle is 
repeated until the proper flow of CH.sub.4 is reached. 
The induced signal detected at the sensing electrode 26 can also be used to 
provide a permanent, continuous record of the filament characteristics. 
For example, appropriately conditioned signals could be directed to the 
strip chart recorder 44. Because of the one-to-one correspondence between 
the amplitude of the induced signal and filament strength and resistance 
to metallic matrix materials, such a recording provides a continuous 
evaluation of the quality of the coated filament, eliminating the need for 
extensive testing after the production process. 
The invention is not limited to the production of boron carbide coatings on 
boron. A similar electrical phenomenon occurs during the chemical vapor 
deposition of silicon carbide on a carbon substrate and is related to the 
concentration of nitrogen and oxygen in the plating gas mixture. Further, 
a similar phenomenon occurs during the deposition of boron carbide on 
silicon carbide. 
The advantages of the invention can be seen from the following example. 
FIG. 3 shows a portion of a strip chart recording of the electrical 
impulses produced during the operation of a reactor producing boron 
carbide coatings on boron. The preferred range of operating conditions 
corresponds to the range of electrical impulses shown on the Figure. At 
time A, for example, the filament produced possessed the preferred 
characteristics as shown in Table 1 below. At time B, a fluctuation in 
operating conditions caused the reactor to deviate from the preferred 
conditions; the largest deviation occured at time C. By time D, the 
automatic control system had increased the methane flow so that the 
filament again had the preferred qualities. Operation continued within the 
preferred range, including time E. (It should be noted that the 
fluctuation shown in the example is large in comparison with what is 
normally observed during operation of a coating reactor). Table 1 shows 
that the preferred range of operating conditions were resumed 
automatically by the system, and further, that the recording of the 
electrical impulses provides the information necessary to determine the 
quality of the filament. 
TABLE 1 
______________________________________ 
Percent Retention of 
Properties after Molten 
As-Produced Properties (Ksi) 
Aluminum Exposure 
Tensile Surface 
Sample 
Strength Strength Tensile 
Surface 
______________________________________ 
A 554 806 87% 88% 
B 483 800 94% 92% 
C 227 410 94% 93% 
D 571 806 79% 88% 
E 546 800 76% 87% 
______________________________________ 
Table 1 indicates the properties of the coated boron filament as produced 
and the percent of these properties retained after exposure to molten 
aluminum for filament made under the conditions illustrated in FIG. 3 
(specifically for the values of the induced voltage observed in FIG. 3). 
In the embodiment of the invention discussed above, we find that when the 
amplitude of the induced electrical impulses is less than 80 millivolts, 
the filament produced has relatively low tensile strength, but shows no 
degradation in contact with molten aluminum. When the amplitude of the 
impulses is greater than 100 millivolts, the filament produced is 
exceptionally strong, but degrades in contact with molten aluminum. When 
the amplitude of the induced electrical impulse is between the thresholds 
of 80 and 100 millivolts, the filament produced has both high tensile 
strength and resistance to adverse interactions with matrix materials. 
Measured induced electrical amplitudes are sensitive to many factors, 
including the temperature of the reactor electrode, the geometry of the 
sensing electrode, and the nature of the sensing circuit. Therefore, the 
values of voltage given are for illustration only, though the generic 
concept of using induced electrical impulses for control purposes can be 
generally applied. 
The various features and advantages of the invention are thought to be 
clear from the foregoing description. Various other features and 
advantages not specifically enumerated will undoubtedly occur to those 
versed in the art, as likewise will many variations and modifications of 
the preferred embodiment illustrated, all of which may be achieved without 
departing from the spirit and scope of the invention as defined by the 
following claims: