High aspect ratio containers for ultrahigh purity chemicals

Containers for ultrahigh purity chemicals having aspect ratios of greater than 3:1 and methods of making the same from seamless electropolished stainless steel tubing are described. Chemical delivery systems for semiconductor fabrication processes that utilize these high aspect ratio containers also are described.

TECHNICAL FIELD 
The present invention relates to containers for ultrahigh purity chemicals 
having aspect ratios of between 3:1 and 6:1 and methods of making the 
same. The present invention also relates to the use of these high aspect 
ratio containers in chemical delivery systems for semiconductor 
fabrication processes. 
BACKGROUND 
Ultrahigh purity chemicals are used as source chemicals in Chemical Vapor 
Deposition (CVD) processes. Three methods currently are utilized for 
delivering the chemical to the process chamber. One of these methods is 
known as direct liquid injection. 
Direct liquid injections systems typically employ liquid (or mass) flow 
control systems that are sensitive to the presence of dissolved gases in 
the liquid CVD source chemical. These liquid flow controllers are 
necessary to deliver precise quantities of the source chemical to the 
process tool. The sensitivity of these devices is a result of the 
thermodynamic operating principal of typical liquid flow controllers and 
can be better understood by a description of the operation of such a 
device. 
A typical liquid flow controller has a precision power supply that directs 
heat to the midpoint of a sensor tube which is carrying a constant 
percentage of the flow to be measured. On the same sensor tube, 
temperature measuring elements are placed equidistant upstream and 
downstream of the heat input. If no liquid or gas is flowing in the sensor 
tube, the heat reaching each temperature element is equal. However, with 
increasing flow, the flow stream carries heat away from the upstream 
element toward the downstream element. An increasing temperature 
difference develops between the two temperature measuring elements and 
this temperature difference is proportional to the liquid or gas flow 
rate. 
If the liquid flow being measured is devoid of dissolved gases, a 
thermodynamic mass flow controller can accurately detect the amount of 
liquid flowing through the sensor tube. For typical CVD systems, mass flow 
is monitored in the vapor phase and is not sensitive to dissolved gases. 
However, in direct liquid injection applications, inert gases, such as 
helium and nitrogen, are used to pressurize CVD chemical source canisters 
to force the chemical contents out of the container to the process tool. 
Therefore, dissolved gases may be present in chemicals dispensed under 
pressure from ultrahigh chemical containers. 
As a result of this thermodynamic operating principle, liquid flow 
controllers are very sensitive to gas bubbles being present in the liquid 
stream, because the heat capacity of the inert gas is considerably less 
than the liquid being transferred. If gas bubbles are, in fact, present in 
the liquid stream, the liquid flow controller will experience a period of 
instability in which improper amounts of liquid are delivered to the 
process tool. If this instability occurs, film deposition non-uniformities 
on the IC being manufactured can result and may lead to the fabrication of 
defective ICs. Therefore, it would be desirable to design a direct liquid 
injection system that minimizes dissolved gases in the liquid stream. 
Novellus Systems, the first CVD process tool manufacturer to adopt direct 
liquid injection for tetraethylorthosilicate (TEOS) based CVD processes, 
dealt with the dissolved helium pressurizing gas problem by periodically 
"degassing" the source chemical container. The degassing procedure 
consists of evacuating the helium pressurizing gas from the source 
container and maintaining a vacuum in the container for a period of time 
to remove dissolved helium from the source chemical. However, this 
procedure adds additional degassing requirements and process delays while 
the liquid is being degassed. To avoid equipment downtime that results 
from a degassing step, Novellus also has developed an in-line degassing 
apparatus that eliminates the need to pull a vacuum on the source chemical 
container. Thus, this system requires an additional degassing module. 
More recently, Applied Materials, another CVD process tool manufacturer, 
imposed a minimum distance from the surface of the liquid chemical to the 
bottom of the dip tube in a 5 gallon CVD chemical source container, 
because the process tool developed flow instability when the chemical 
level went lower. It was concluded that a concentration gradient of 
dissolved helium pressurizing gas would create more serious flow 
instability problems as the gas liquid interface approached the bottom of 
the container. The minimum distance established in the chemical container 
was at a level equivalent to 40% of the container volume. 
Many process tools require the use of triethyl phosphate (TEPO), triethyl 
borate (TEB) and other ultrahigh purity chemicals in very low volume. The 
delivery conditions described above create unacceptable conditions for the 
five gallon containers traditionally used, as all of the chemical may not 
be consumed before the shelf life of the chemical is exceeded. 
SUMMARY 
In order to minimize dissolved gases in high purity chemicals being 
delivered by direct liquid injection techniques, high aspect ratio 
containers have been developed. The height-to-diameter ratio of these 
containers is at least about 3:1 to 6:1 and is preferably about 5:1. These 
high aspect ratio containers are designed to minimize the surface area of 
the liquid that is subject to gas contact and provide the advantageous 
feature of reducing the gas dissolution rate into the chemical. 
Preferably, the volume of high purity chemical in the present containers 
is maintained at at least 40% of the container level to further minimize 
the amount of dissolved gas in the chemical. More preferably, the volume 
of chemical is maintained at or above 50% of the container volume. High 
aspect ratio containers also minimize the floorspace necessary to store a 
given volume of high purity chemical. 
The present high aspect ratio containers are fabricated from a section of 
seamless 316L electropolished (EP) stainless steel (SS) tubing. Milled 
316L EP SS end caps are welded to each end of the seamless tubing. The top 
end cap has 316L tube stubs to weld inlet, outlet and refill valves to the 
container. A level sensor fitting also is welded onto the container before 
electropolishing. 
These high aspect ratio containers are preferably used to store and/or 
dispense high purity chemicals that are utilized in semiconductor 
fabrication processes. These high purity chemicals include TEOS, TEB, 
TEPO, TMB and TMP. Preferably, one gallon canisters constructed according 
to the present invention are used to store and/or dispense these 
chemicals, although the present teachings may be used to construct 
canisters of volumes from one liter to ten gallons. 
The invention containers may be used as "bulk" containers, as "process" 
containers or as both. A bulk container typically stores larger volumes of 
high purity chemical and is cleaned and refilled by the chemical 
manufacturer. The bulk container can be located either in a cabinet with a 
process container or in a remote chemical cabinet and dispenses high 
purity chemical to a process container. 
The process container may be located close to the semiconductor fabrication 
equipment (process tool), such as a CVD reactor, and stores the high 
purity chemical for delivery to the process tool. Typically, the process 
container is refilled with chemical from the bulk container when a 
"refill" sensor is activated. Preferably, the process container is 
refilled when the container volume reaches 50%. The present high aspect 
ratio containers can be utilized as bulk containers for chemicals used in 
small volumes, as process containers for most applications or as bulk and 
process containers in the same system. 
Therefore, one object of the present invention is to provide high aspect 
ratio containers for ultrahigh purity chemicals. 
A second object is to provide a direct liquid injection system for high 
purity chemicals that minimizes dissolved gases in the liquid being 
delivered from the container to the process tool. 
A third object is to provide a container for high purity chemicals that 
minimizes floorspace requirements. 
A fourth object is to provide reduced volume process canisters for low 
usage semiconductor fabrication applications. 
Other objects will be apparent to those skilled in the art from the 
following description, figures and claims.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
To meet low volume use requirements for many semiconductor fabrication 
applications, high aspect ratio containers have been designed that 
minimize dissolved gases in chemicals delivered to a semiconductor process 
tool via direct liquid injection techniques. The resulting container 
design possesses a unique height-to-diameter aspect ratio of greater than 
3:1. Such an aspect ratio makes a chemical container inherently unstable 
and prone to being tipped over, but this design is necessary to enable the 
volume of the container to be useful as a low volume CVI) source chemical 
container. For example, a one gallon high aspect ratio source chemical 
container has been constructed which measures 20.6 in. height.times.4.0 
in. diameter. Conventional CVD chemical source container dimensions are 
presented in table 1 for comparison. 
TABLE 1 
______________________________________ 
Conventional CVD Source Chemical Container Dimensions 
Container 
Container Height 
Container Diameter 
Height-to-Diameter 
Volume (in.) 
(in.) 
Aspect Ratio 
______________________________________ 
1.3 liter 
5.56 5.50 1.11 
2.3 liter 
5.63 
0.866 
1 gal. 0.792 
2 gal. 9.15 
1.13 
5 gal. 2.33 
10 gal. 
12.0 
2.02 
______________________________________ 
Preferably, these high aspect ratio containers are part of a canister for 
pressure delivery of high purity chemicals to a semiconductor fabrication 
process tool. Referring to FIG. 1, the container (10) has an inlet valve 
(11) connected to the top portion of the container which communicates an 
inert gas, such as helium or nitrogen, under pressure to the contents of 
the container (10). The gas pressure inside of the container (10) pushes 
the liquid chemical up through a draw tube (12) that extends nearly to the 
bottom of the container (10). The draw tube (12) is connected to an outlet 
valve (13) which communicates the chemical either to a process canister or 
to a process tool. The container (10) also may have a level sensor (not 
shown) present to detect when the liquid chemical has dropped below a 
predetermined container volume. The designer may set the level sensor to 
trigger at a variety of volumes depending upon the application. In order 
to minimize dissolved gases in the high purity liquid chemical, preferably 
a "refill" signal is initiated at about 40%, or more preferably 50%, of 
the canister volume. This refill signal serves as a indicator that the 
canister is ready to be refilled. A refill valve and port (15, 16) and 
fill tube (17) also may be incorporated. 
The specific height-to-diameter ratio of the invention containers can be 
modified depending upon the specific design constraints of the chemical 
delivery system, such as usage rate, floorspace, cabinet space and the 
particular liquid flow controller being used. Containers having 
height-to-diameter aspect ratios of about 3:1 to about 6:1 are preferred. 
Aspect ratios of about 4.5:1 to about 5.5:1 are more preferred and aspect 
ratios of about 5:1 are most preferred. 
One benefit of the one gallon high aspect ratio container (aspect ratio of 
about 5:1) is that it produces a similar proportionate surface area in 
contact with the pressurizing gas to volume ratio as a conventional five 
gallon canister when each canister is at 40% volume. The gas contact 
surface area to volume ratio of the one gallon high aspect ratio container 
(20.6 in. height v. 4.0 in. diameter) is 31 sq.in./gal. The gas contact 
surface area to volume ratio of the five gallon container described in 
table 1 is 32 sq.in./gal. Preservation of the surface area to volume ratio 
was a key consideration in the design in order to maintain similar 
pressurizing gas dissolution rates and chemical delivery characteristics. 
Another benefit of the high aspect ratio CVD source chemical container is 
the gas contact surface area to volume is much less than a conventional 
container that holds the same volume. For example, the conventional one 
gallon container referenced in table 1 above has a gas contact surface 
area-to-volume ratio of 159 sq. in./gal. at 40% fill volume. The 
comparable gas contact surface area-to-volume ratio of the one gallon high 
aspect ratio container (aspect ratio about 5:1) of the present invention 
is 31.4 sq. in./gal. at 40% fill volume. The lower gas contact 
surface-to-volume ratio is believed to reduce the dissolution rate of 
pressurizing gas into the source chemical as compared to a conventional 
one gallon container. 
Thus, one unexpected result of the present invention is high aspect ratio 
chemical containers improve the performance of direct liquid injection 
delivery systems as compared to the same volume conventional container by 
limiting the dissolution rate of pressurizing gas into the CVD source 
chemical. By minimizing dissolved gas in the liquid being transferred, 
liquid flow controllers have improved stability and the uniformity of 
chemical delivery rates is substantially improved. No degassing steps or 
procedures are therefore necessary. 
A third benefit of the present invention is the high aspect ratio chemical 
container provides a unique advantage over conventional containers, 
because higher canister packing densities are made possible with high 
aspect ratio containers. For example, using a conventional container in a 
two canister continuous chemical delivery system, a single chemical can be 
delivered from one system with a typical footprint of 28 in..times.24 in. 
However, using high aspect ratio containers, two continuous delivery 
systems can be put into the same footprint. Thus, the present invention 
minimizes costly semiconductor cleanroom floorspace that is necessary to 
support the subject chemical delivery systems. 
High aspect ratio chemical containers of the present invention are 
preferably all-welded containers fabricated from 316L Electropolished (EP) 
Stainless Steel (SS). Currently, many conventional all-welded containers 
also are fabricated from 316L SS. However, these conventional 316L SS 
containers are electropolished after fabrication. Electropolishing the 
containers requires large diameter openings in the container to insert the 
electrodes that are necessary for the electropolishing process. 
Post-electropolish inspection also is made difficult, because after 
fabrication, the interior surfaces cannot be observed directly. Optical 
inspection tools, such as a baroscope, must be used for visual inspection. 
Therefore, to minimize these problems, the high aspect ratio chemical 
containers are fabricated from a section of seamless 316L SS EP tubing. 
Milled 316L SS EP end caps are welded to each end of the tubing. All 
components preferably have a low surface roughness which is desirable for 
ultrahigh purity CVD source chemicals. The top end cap has 316L tube stubs 
for the welding of inlet, outlet and refill valves to the container and a 
level sensor fitting also is welded inside the container before 
electropolishing. The end caps are machined to provide a means for welding 
the end caps to the tube section. 
The welds joining the two end caps to the tube section locally destroy the 
EP finish. Therefore, the bottom end cap is welded to the tube section 
first and the weld is electropolished prior to the top end cap being 
added. This welding and electropolishing order allows the bottom end cap 
weld to be accessed with a large diameter concentric electrode. 
After electropolishing the bottom weld, the top end cap is welded in place 
and a second electropolish step is completed to provide passivation to the 
final weld on the container. A specially designed electrode (&lt;0.75" in 
diameter and &gt;25" in length) is inserted into the container through the 
small diameter level sensor fitting and a local electropolish finish is 
applied to the weld zone. Surface analysis performed on the weld zone of 
the container has shown that the finish on the weld is equivalent to the 
end caps and tube sections. 
The use of seamless tubing for the present high aspect ratio containers 
provides an additional advantage over conventional SS containers. 
Currently, all of the conventional SS containers supplied by U.S. 
producers of ultrahigh purity CVD source chemicals are believed to be 
fabricated by one of two methods: 
1. The container body is machined from a solid rod of 316L SS (3.5 in., 5.5 
in. or 6.5 in. diameters). 
2. The container shell is formed by rolling 316L SS sheet stock into a 
cylinder and the seam is welded. The end caps are then welded to the 
cylinder. 
Especially compared to the second method, the use of seamless tubing 
according to the teachings of this disclosure eliminates degradation of 
the surface finish caused by the cylinder forming operation and eliminates 
the weld seam to form the cylinder. While post-forming work can be done to 
remove tooling marks and surface roughness of the seam weld, this work 
adds time and expense to the container manufacturing process. Thus, the 
use of seamless tubing improves the quality of the final product and 
simplifies the manufacturing method at the same time. 
The high aspect ratio containers described herein can be used in a variety 
of chemical delivery systems and the inlet, outlet and refill valves of 
the canisters can be modified to fit any system. Preferably, the present 
containers are used in high purity chemical delivery systems, such as the 
systems described in U.S. Pat. No. 5,562,132, which commonly owned patent 
is incorporated herein by reference. Those skilled in the art will 
recognize that the present high aspect ratio containers can be used as (a) 
a "bulk" canister, (b) a refillable ampule (process canister) to supply 
semiconductor fabrication equipment, such as a CVD reactor, or (c) both a 
bulk canister and a process canister in the same chemical delivery 
systems. 
As an example of the use of the present containers, FIG. 2 shows a chemical 
delivery system that utilizes a bulk canister (20) and a process canister 
(30). Either or both of the canisters may comprise a high aspect ratio 
container constructed according to the present invention. The designation 
of "bulk" and "process" is simply to define the order of the canisters in 
the described chemical delivery system and is not intended to define the 
particular physical attributes of each canister. 
The operation of the embodiment in FIG. 2 can be described as follows: The 
monitor/control unit (40) initiates the refill protocol by opening valve 
(42), which is preferably a pneumatically activated valve. Since bulk 
container (20) is continuously pressurized with an inert gas via inlet 
valve (64), when valve (42) is opened, high purity chemical in the 
container (20) is forced up the draw tube (60) through the outlet value 
(66) and is delivered to the process canister (30) via delivery line (44). 
The bulk canister (20) preferably contains a level sensor system (21) for 
detecting the level of high purity chemical in the bulk container. The 
level sensor is typically incorporated on a separate structure (21). 
Preferably, a metallic level sensor that includes at least one two-pole 
read switch and at least one metallic float is used. However, depending 
upon the application, other level sensors such as capacitance probes or 
optical level sensors also may be used. The reader is directed to U.S. 
Pat. No. 5,562,132 for additional description of appropriate level 
sensors. 
A level sensor system (21) also is included in the process canister (30) to 
generate signals that indicate the level of high purity chemical in the 
process canister (30). When the chemical level is low ("needs refill"), a 
refill signal is sent to a monitor/control unit (40) and the refill 
procedure is initiated. When the process canister (30) is filled, a high 
level signal is sent to the monitor/control unit (40) to stop the refill 
procedure. Those skilled in the art will recognize that the specific steps 
in the refill procedure can be performed manually or automatically. 
Additional specific refill procedures and steps are described in U.S. Pat. 
No. 5,562,132. 
The process canister (30) also has a refill inlet valve (38) and a chemical 
outlet valve (36) which are connected to the top of the canister. 
Vacuum/pressurization valve (37) permits the process canister (30) to be 
pressurized with an inert gas during normal process operation. Outlet 
valve (36) connects the process canister (30) to a delivery line (32) that 
supplies the high purity chemical to the process tool. A separate level 
sensor (34) is shown in FIG. 2, although this level sensor is not 
necessary to practice the present invention. 
The present high aspect ratio containers also are appropriate for use in a 
two canister "bulk" delivery system. Referring to FIG. 3, a second 
chemical delivery system using two "bulk" canisters is shown. Either or 
both of these canisters may be constructed according to the teachings of 
the present invention. 
The operation of the chemical delivery system of FIG. 3 can be described as 
follows: a bulk manifold (71) supplies inert gas under pressure to the 
first bulk canister (70) via a gas inlet (72). High purity chemical is 
forced out of the canister (70) via the liquid outlet (73) through a 
refill tube (74) and refill port (75) into the second bulk canister (80). 
Level sensors (76) are disposed within the bulk canisters to signal when 
the canisters require refilling. Further discussion of such level sensors 
can be found in the description of the previous example and in U.S. Pat. 
No. 5,562,132, which is incorporated herein by reference. 
The second bulk canister (80) is operated in a similar manner as the first 
bulk canister (70). To dispense the high purity chemical to the process 
tool, the process manifold (81) communicates an inert gas under pressure 
to the second bulk canister (80) via the gas inlet (82). The high purity 
chemical is forced out of the second bulk canister (80) through the liquid 
outlet (83) and the process manifold (81) to the process tool. 
By using the high aspect ratio containers of the present invention and 
maintaining at least about 40%, or more preferably 50%, fill volume in the 
canister, the liquid flow controller can accurately dispense the high 
purity chemical to the desired destination. Since dissolved gases in the 
liquid being transferred are minimized by these techniques, the need for 
degassing steps or apparatus is eliminated. 
Although the invention has been described in connection with reference to 
preferred embodiments and specific examples, it will be readily understood 
by those skilled in the art that many modifications and adaptions of the 
inventions described herein are possible without departure from the spirit 
and scope of the invention as claimed hereafter.