Liquid level detector and method for a vapor deposition container

An optical probe is sealingly mounted in an access port of a pressurized container. The probe comprises a glass rod of a predetermined thickness having a conical end tip. An optical transmitter and receiver is connected to the glass rod. The fiber optic transmitter and receiver sends visible red light down the glass rod. When the liquid level inside the container drops below the end tip of the glass rod, reflected light will be received from the glass rod and detected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to liquid level detectors and more 
specifically to liquid level detectors for use with volatile liquids in 
pressurized environments. 
2. Description of the Prior Art 
Many chemical processes today involve hazardous or volatile chemicals. 
These chemicals must sometimes be used at elevated pressures and in sealed 
containers. The sealed containers make it difficult for a human operator 
to determine when the supply of chemicals in the container has been 
depleted. 
The use of volatile liquid chemicals in pressurized containers is quite 
common in the semiconductor industry. One process for making wafers is 
known as metal organic chemical vapor deposition (MOCVD). Volatile 
chemicals such as trimethylgallium are used. These types of chemicals must 
be sealed in a container to exclude air because they burst into flame upon 
contact with air. In addition, some of these liquids are quite toxic. A 
carrier gas, such as hydrogen, is introduced into the container through a 
dip tube and bubbled through the chemical liquid producing a vapor. This 
vapor is evacuated from the container and used for vapor deposition on the 
wafer. All of the chemical liquid in the container is eventually converted 
to vapor and evacuated. 
In order to maintain the constant production of wafers during the process, 
the liquid chemical must be replaced before it is exhausted. However, the 
pressurized sealed containers cannot be opened and inspected in an air 
atmosphere. The human operators must wait until the flow of carrier gas 
ceases to contain any of the liquid chemical, possibly fouling a whole 
process run, before replacing the container. 
Liquid level detectors are known in the prior art. One such example is U.S. 
Pat. No. 4,201,914, issued to Benno Perron. However, these prior art 
liquid detector systems are made for use in open air systems and are not 
adapted for use in sealed, pressurized environments, especially with 
volatile liquids. 
SUMMARY OF THE PRESENT INVENTION 
It is therefore an object of the present invention to provide a liquid 
level detector for use with volatile liquids in sealed containers. 
It is another object of the present invention to provide a liquid level 
detector which minimizes process time and product losses. 
Briefly, in a preferred embodiment, the present invention includes a 
pressurized container for holding chemical liquids. The container has a 
plug member which sealingly fits over an access port. A tube passes 
through the plug and into the container. A glass rod is sealingly mounted 
inside the tube and extends into the container. The glass rod has a 
conical end section. 
An optical transmitter and receiver is connected to the glass rod by a 
fiber optic bundle. The transmitter and receiver send visible red light 
down the glass rod. If liquid is in contact with the conical end section, 
then the light is dispersed. If liquid is not in contact with the conical 
end section, then the light is reflected back up the glass rod and is 
detected by the optical transmitter and receiver. 
An advantage of the present invention is that it provides a liquid level 
detector for use with volatile liquids in sealed containers operating 
under pressure. 
It is another advantage of the present invention in that it provides a 
liquid level detector which minimizes process time and product losses. 
These and other objects and advantages of the present invention will no 
doubt become obvious to those or ordinary skill in the art after having 
read the following detailed description of the preferred embodiment which 
is illustrated in the various drawing figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a liquid level detection device of the present invention and 
is designated by the general reference number 10. A pipe connector 12 is 
connected to a pressure isolation valve 14. Valve 14 is connected to an 
inlet pipe 16. Pipe 16 is connected to a semiconductor grade metal organic 
(SGMO) container 20. Such containers are typically made of stainless 
steel. 
Container 20 has an access port tube 22 through which an optic probe may be 
inserted. A connector nut 24 is attached to the end of tube 22. Nut 24 may 
be a Cajon VCR type nut. An optical probe 30 is inserted through and is 
sealingly attached to tube 22 by means of nut 24. A flexible fiber optic 
bundle 32 connects probe 30 with a fiber optic transmitter and receiver 
(FOTR) 34. FOTR 34 produces light in the visible red range. Although the 
preferred embodiment uses an FOTR which utilizes visible red light other 
FOTRs which utilize other types of visible light or infrared light may 
also be used. Such FOTRs are manufactured by Banner Engineering 
Corporation. A light emitting diode (LED) 36 is electrically connected to 
FOTR 34. 
An outlet pipe 40 is connected to container 20. A pressure isolation valve 
42 is connected to pipe 40. Valves 14 and 42 may be Parker Bellows Valves. 
A pipe connector 44 is attached to valve 42. 
FIG. 2 shows a front view of the optic probe 30. Probe 30 comprises a 
ferrule 50 for receiving the fiber optic bundle 32. Ferrule 50 is fitted 
to a plug member 52. Plug member 52 is shaped to mate and seal with nut 24 
and tube 22. An optic tube 54 is welded to plug 52. A Pyrex glass rod 56 
is shaped to fit inside tube 54. Glass rod 56 has an outer diameter of 
greater than 0.200 inches and in the preferred embodiment, has an outer 
diameter of 0.235 inches. Rod 56 has a conical tip 58 which has an angle 
60 of approximately forty-five degrees relative to the horizontal. 
FIG. 3 shows a cross-sectional view of a portion of the device 10 in FIG. 
1. The fiber bundle 32 is held in place inside ferrule 50 by means of an 
Allen head screw 102. Plug 52 and tube 54 are typically made of stainless 
steel and are welded together. Glass rod 56 is then inserted inside tube 
54 until it abuts a countersunk portion 104 of ferrule 50. Rod 56 is 
sealed to tube 54 by means of a glass-to-metal bond 110. The bond 110 is 
formed by means of a glass-to-metal high vacuum seal at 1000.degree. C. 
temperature. This seal actually attaches the glass to the outer oxide 
layer of the metal of tube 54. Other types of high pressure glass seals 
could also be used. 
Probe 30 is inserted into tube 22 and is attached by means of nut 24. A 
metal gasket 112 is placed between plug 52 and tube 22 to form a helium 
leak tight seal. A dip tube 120 is connected to inlet pipe 16 and extends 
to the bottom of container 20 and has a nozzle 122 at its end. 
Device 10 may be detached from a processing system by disconnecting 
connectors 12 and 44. The system 10 may then be isolated in an air free 
environment for purposes of filling the container 20. Probe 30 is removed 
from the tube 22 and liquid chemical may be added to container 20 through 
tube 22 or can be filled through valves 12 or 44. System 10 may then be 
connected back into a processing system. 
In operation, valves 14 and 42 are opened and a carrier gas enters 
container 20 through inlet pipe 16. The gas goes through dip tube 120 and 
bubbles up through the chemical liquid in the container. See FIG. 3. A 
vapor is formed which flows out of the container 20 through outlet pipe 
40. As the carrier gas transports the liquid out of the container, the 
liquid level 130 begins to fall. 
FOTR 34 constantly sends a visible red beam of light down the transmitting 
fibers of bundle 32. The bundles 32 abut glass rod 56. The light is 
conducted down rod 56 by being bounced off the side walls of the rod until 
the light reaches the conical tip 58. 
If conical tip 58 is covered by liquid, then the light is dispersed into 
the liquid. However, as the liquid level 130 drops below tip 58, the light 
is reflected back up rod 56. This reflective light is conducted to FOTR 34 
by the receiving fibers of bundle 32. FOTR 34 detects the reflected light 
and triggers a warning device such as LED 36 to let the operator know that 
the liquid chemical is nearing depletion. 
Gas bubbling systems have some problems due to the unstable surface of the 
liquid. The bubbling action creates a wide zone where there is a frothy 
combination of both liquid and gas. This makes the detection of the liquid 
level difficult. In order to prevent this, the FOTR 34 of the present 
invention is equipped with a delay, e.g. one second. This ensures that the 
LED 36 is turned on only when liquid level 130 actually passes tip 58. 
Another advantage of the present invention is that it avoids contamination 
of the system. Typical light conducting rods have a cladded outer shell in 
order to reflect the light internally and keep the light from passing out 
of the sidewall of the rod. However, elements in the cladded material, 
such as lead, can enter the system and become a source of contamination. 
The present invention uses a Pyrex glass rod having an outer diameter of 
at least 0.200 inches. It has been found that this diameter allows the 
visible light to be reflected internally without the need for cladding. 
The Pyrex glass is a noncontaminating material and because the probe rod 
is made completely of this material there is no danger of contaminating 
the system. The present invention utilizes visible red light in the 
preferred embodiment because this light provides the present invention 
with greater sensitivity than infrared when used with the noncladded 
probe. 
Another advantage of the present invention is that it provides a very 
accurate measure of the level of the liquid. By varying the length of 
glass rod 56 it is possible to set the liquid level detector to indicate 
the liquid level at any desired elevation from the top of the container 
20. 
Although the present invention has been described in terms of the presently 
preferred embodiment, it is to be understood that such disclosure is not 
to be interpreted as limiting. Various alterations and modifications will 
no doubt become apparent to those skilled in the art after having read the 
above disclosure. Accordingly, it is intended that the appended claims be 
interpreted as covering all alterations and modifications as fall within 
the true spirit and scope of the invention.