Reprint - Control of Gas Delivery System Temperatures For Improved
|Accurate Gas Control Systems
Larry Anderson, Engineering Section Head, National Semiconductor
Corp., Santa Clara, California, and Michael Shepherd, President,
Accurate Gas Control Systems, Sequel, California
This article addresses the long-standing gap in
available information on the benefits of accurately controlling
temperatures of compressed gas cylinders such as B2H6, BCl3, and
SiH2Cl2.. Temperatures can be controlled from the cylinder to the
system, resulting in:
and percent boron rates in BPSG
better etch rate
uniformity in aluminum plasma etching
a 99 percent reduction
in the condensing of corrosive gases in gas lines and mass flow
The problems long associated with gas bottle delivery
temperature and pressure variations are established factors affecting
semiconductor processes such as LPCVD silicon nitride, epitaxial
deposition, BPSG deposition, aluminum plasma etching, and silicide
deposition. Controlling the temperature of gas cylinders and delivery
lines is more necessary than ever before because of new safety regulations
requiring gas cylinders to be moved outside the fabrication building.
In addition to longer gas delivery lines, ambient temperature fluctuation
in the hazardous process material (HPM) storage areas can cause
changes in gas pressure delivery of as much as 15 lb/in2 over an
Common gases affected are those with boiling points
close to room temperature, so the liquid can form, thereby clogging
both gas delivery lines and mass flow controllers (MFCs). Common
examples include dichlorosilane, SiH2Cl2, and boron trichloride,
BCl3. These fluctuations can cause changes in deposition rates with
associated film thickness uniformity and dopant distribution problems
throughout a load of wafers. With boron trichloride in aluminum
plasma etching, these variations in gas temperature and pressure
can cause the pressure to drop too low and destroy radial etch rate
of low vapor pressure gas in process plumbing is a significant cause
of equipment downtime and reworking or scrapping discrepant product
wafers. The cost of quality is high due to the poor process control
and damaged product reputation. Many of these problems can be eliminated
by controlling the temperature of the gas cylinders, gas lines,
and MFC's, thus resulting in uniform gas delivery pressure despite
ambient temperature changes. This consideration is especially important
in systems which use condensable gases with boiling points near
the ambient temperature. Many gases in pressurized cylinders do
not require temperature control. How then do we determine which
ones will benefit from temperature control? The best way is to understand
how to read phase diagrams in gas tables, and look up the chemical
and physical properties in the references listed for the gases.
The three-dimensional phase diagram defines, for a unit weight of
any chemically stable substance, the zones in which the different
phases (solid, liquid, and gas), exist as a function of temperature,
pressure, and volume [l]
Figure 1 illustrates the two-dimensional pressure vs temperature
plane. Four major zones are defined in the drawing. In Zone 1 the
substance exists in the solid state. This zone is bounded by the
sublimation Curve which distinguishes the direct passage from the
solid state to the gaseous state, and by the liquefaction or solidification
curve. An example would be nitrous oxide (N2O) coming out of a gas
cylinder and causing an ordinary regulator to freeze up. The solid
zone may extend to very high pressures.
Zone 2, characteristic of the liquid state, is bounded on
the right by the saturation vapor pressure curve (sometimes called
the vapor tension curve), which itself terminates at a point C,
the critical point, beyond which liquid-vapor equilibrium ceases
to exist. It is not possible to liquify a gas at any temperature
above the critical temperature, regardless of pressure. In effect,
each boundary curve defines the temperatures and pressures at which
two phases coexist: solid-gas, solid-liquid, and liquid-gas. All
three gases exist at point T, the Triple Point. The intersection
of the isobar Pa, relative to normal atmospheric pressure, with
the sublimation, liquifaction, and vapor pressure curves, defines
the normal sublimation point Ts, melting point Tf, and boiling point
Zone 3 represents the gaseous or vapor
state. The term "gas" is generally applied to any pure substance
existing in this state in standard conditions of temperature and
pressure, while the term "vapor" is applied to the gas phase of
a substance which normally exists in the solid or liquid state in
similar conditions (STP).
Zone 4 is sometimes called the "dense gas" zone. This designates
an intermediate between gas and liquid. The pressure-temperature
phase diagrams shown in the gas tables usually show the results
for the temperatures and pressures of interest. In addition, many
of the physical properties of the gas are listed.
The condensable gases like BCl3 and SiH2Cl2; can condense, according
to the proper conditions in their respective phase diagrams, if
the temperature is too low or the pressure is too high. Usually,
the pressure is not too high for a low vapor pressure condensable
gas. However, purging a gas line with high pressure nitrogen (N2)
gas can cause the process gas to liquefy. Therefore, to avoid clogging,
these gases should only be purged with low pressure nitrogen gas.
For an ideal gas,
PV = nRT
where P is the absolute pressure, V is the volume
occupied by the gas, n is the number of moles contained in the volume
V, R is the ideal gas molar constant, and T is the absolute temperature
in degrees Kelvin.
This pertains to "ideal" gases only; an ideal gas being one in which
the molecules are far apart and the density of the gas is low. Low
density corresponds to conditions of high temperature or low pressure,
well below the saturation vapor pressure curve. The ideal gas law
is a satisfactory (within 5 percent) approximation for most gases
discussed in this work. The "R" in the equation is a constant of
proportionality whose value depends on the units used to express
the four quantities. The table below gives different values of the
constant R for different units of pressure and volume:
The ideal gas law is not a good approximation
at high pressure and low temperature near the saturated vapor curve.
We must use the equation for a so-called "real gas" conforming to
the following equation,
PV = Z(T,P)nRT
where Z = the compressibility factor which depends simultaneously
on temperature and pressure. Hence, an ideal gas is by definition
a gas with a compressibility factor of 1. Values of Z, typically
between 0.90 and 0.99, are often listed in the gas tables.
The following three examples illustrate some of the process control
applications of accurate control of gas cylinder pressures For safe,
reliable operation of gases knowledge of the chemical and physical
characteristics are crucial, as shown by these examples.
Example No. 1: Diborane Cylinder for Borophosphosilicate
Glass Deposition (BPSG)
Diborane (B2H6) is a colorless, unstable flammable, and highly toxic
gas with a characteristic "sickly sweet" odor. It is shipped as
a pressurized gas, and it is usually cooled to about 0°C during
ship meant to prevent decomposition. At room temperature, diborane
decomposes slowly to produce hydrogen and the higher boranes, especially
tetraborane (B4H10). The higher boranes are in general more stable
Final results of the BPSG deposition process depend upon control
of film thickness, percent boron, percent phosphorus, etc. Measurement
of percent boron is difficult at best but very essential. If the
percent boron in the film drops, the BPSG film will not flow at
the specified flow temperature Tf. Thus, the temperature required
to flow the BPSG increases. As the diborane in the cylinder slowly
thermally decomposes to the less reactive tetra-borane, the deposition
rate and percent B of the BPSG slowly drops. When kept dry at ambient
temperature and initial pressures of 200 lb/in2 atm., diborane suffers
about a 2-3 percent decomposition per month with an expected pressure
increase of 25 lb/in2 per month [2,4]. With contamination by water
vapor, it hydrolyzes rapidly to boric acid and hydrogen. The easiest
solution is to accurately control the temperature of the diborane
cylinder with a constant temperature cooling jacket kept at about
5°C. A refrigeration unit should be considered to preclude normal
temperature fluctuations associated with plant or building cooling
water systems. A thermal insulation blanket provides additional
thermal efficiency, and eliminates condensation problems often associated
with cooling gas cylinders more than 10°C below ambient temperature.
In addition, the diborane gas line and MFC can be thermally insulated
from the gas cylinder to the system by electrically heat tracing
techniques. The return on investment calculation depends upon eliminating
down time associated with the gas system, reduced number of test
wafers, yield improvement, measurement techniques, etc., but is
about 90 days in most cases.
Example No. 2: Boron Trichloride Cylinder
for Plasma Metal Etch
Boron trichloride (BCl3) is a liquid inside the gas cylinder. It
has a very narrow operating temperature range. Like most low vapor
pressure liquefied gases, the inlet pressure does not vary greatly,
but the cylinder must be kept above the boiling point. Boron trichloride
has a cylinder pressure of 4.4 lb/in2 gauge at 21.1°C and a boiling
point of 12.5°C. The upper limit of the temperature is determined
by the ambient temperature in the fab. The temperature should increase
downstream from the cylinder to the processing chamber by at least
2°C. For example, for a 23° C fab, the cylinder can be kept at 21°C
by using an accurate temperature control jacket on the cylinder.
For fluctuations in fab temperature, the gas delivery lines can
be thermally insulated and heated by heat tracing techniques to
23°C or greater. In summary, this example shows that the boron trichloride
has a small operating temperature range from about 13° C to 21°C
at the gas cylinder.
Another problem with boron trichloride is that it condenses so readily.
It condenses if the temperature drops below its boiling point or
if the pressure rises so that BCl3 is in the liquid state in the
phase diagram. The origin of the high pressure is usually nitrogen
purging. Therefore, a dedicated, low pressure nitrogen purge gas
source should be used to prevent condensation and cross-contamination.
Boron trichloride is a colorless, toxic gas, which produces thick
hydrochloric fumes when the gas cylinder or delivery lines are contaminated
with moist air. The HCl acid corrodes the hardware and causes severe
particulate problems. Boron trichloride is difficult to monitor
with gas detectors - usually the BCl3 is detected indirectly by
When boron tricholoride liquifies in the gas lines it can cause
the MFC to malfunction (see Figure 2), and can pass liquid into
the processing chamber. The problem with frequent MFC malfunctions
is the safety hazards to the person changing the MFC. Some safety
engineers require the evacuation of all production personnel from
the fab if the gas line has to be opened. Boron trichloride contains
phosgene (COCl2), in concentrations of 15 to 1000 ppm, and the permissible
exposure limit is 5 ppm with an "immediately dangerous to life or
health" level of 100 ppm .
Installing constant temperature control
to the boron trichloride cylinder at about 21 °C, heating and thermally
insulating the gas line, and thermally insulating the boron trichloride
MFC can dramatically reduce the MFC failures. About 99 percent of
all condensation problems with boron trichloride can be eliminated
with accurate temperature control.
There are other problems with handling boron trichloride. These
problems include leaks, inadequate purge procedures, and no vacuum
source for evacuating the lines. In addition to the solutions described
above, other solutions include: (1) long evacuation cycles at low
pressures; (2) high purity nitrogen purge gas; (3) no gas regulator;
(4) installation of an excess flow switch and in-line filter (to
remove particulate contamination) at the gas cylinder; and (5) a
customized source gas cabinet manifold for purging and evacuating
The 21 °C temperature also increases the vapor pressure of the gas
in the bottle to about 20 lb/in2 atm. Aluminum plasma etch manufacturers
usually require the installation of temperature control when boron
trichloride is used, because a pressure differential of about 5
lb/in2 is needed across the MFC for proper operation. Ideally the
temperature control should include a heater/cooler unit, not just
a water-cooled cylinder jacket. If the cooling water drops below
15.5°C, boron trichloride in the cylinder drops below the boiling
point and the cylinder has no vapor pressure (14.7 lb/in2 atm, which
is the same as 0 lb/in2 gauge).
In some applications, like using boron trichloride in a BPSG diffusion
tube, a thermostatically controlled heating system will suffice
in maintaining the gas bottle and gas delivery lines at a temperatue
sufficiently above liquidus to avoid condensation.
How does one measure the return on investment for this pressure
and temperature control from the boron trichloride cylinder to the
system? First, minimizing the safety hazards to our fellow workers
is a price we must be willing to pay. Second, avoiding fluctuations
in pressure below MFC standards improves radial etch rate uniformity
in aluminum plasma etchers. The resultant reduced downtime of the
plasma etch system and the reduced metal etch rework rate will pay
for itself in a very short time.
Example No. 3: Dichlorosilane for LPCVD Silicon Nitride
This example is similar to the last one, from the aspect of MFC
problems. It is included because this is a common problem in fabrication
areas, and it complements the information in Example No. 2.
For minimal problems with dichlorosilane (SiH2Cl2), the cylinder
needs to be kept at about 25 °C (gives a 15 lb/in2 gauge pressure
gradient across the MFC). Some dichlorosilane condenses back to
the liquid state below 18°C, even though the boiling point in 8.4
°C. Problems without temperature control relate to dichlorosilane
in the summer, when the gas cylinder cabinets area may be warmer
than the air-conditioned fab and the gas delivery lines. An additional
problem in some fab areas is the temperature can fluctuate by 10°C
along the gas delivery lines, allowing condensation problems in
the lines and MFC's. A general rule of thumb is there should be
a positive temperature gradient of at least 2°C from the cylinder
to the LPCVD system. Controlling the temperature of the dichlorosilane
cylinder so it is about 2°C cooler than the gas delivery lines,
and insulating and thermally heating low spots and constrictions
("cold traps") in the dichlorosilane lines will eliminate many MFC
MFC's should be used for dichlorosilane and not rotameter type flowmeters.
This is because the MFC is designed so the pressure of the gas in
vacuum systems does not change the flow readings. Flow fluctuations
with rotameters can cause changes in deposition rates and film thickness
along a load of wafers.
Accurate temperature control of gas cylinders and gas feeder lines
has many applications. These include:
Cooling the gas
cylinder to prevent thermal degradation of the substance.
Cooling the gas
cylinder to maintain a positive temperature gradient from the bottle
to the system.
Heating the gas
cylinder to increase the vapor pressure of the substance. Gas cylinders
should not be heated above 55°C.
Cooling the gas
cylinder and heating the lines to eliminate condensation in the
lines, especially for substances having boiling points near ambient
lines and MFC's from ambient temperature fluctuations.
Heating the gas
to prevent regulators and constriction valves from freezing shut.
Some of the most common gases using temperature control in semiconductor
manufacturing plants are listed in Table 2.
In a nutshell, the temperature at the gas cylinder inside the gas
cabinet, along the gas delivery lines, and at the machine level
can change. "Ideal" design of a system might have the gas cabinets
in the basement with short, straight gas feeder lines right up to
the system. But many of us do not have these conditions. Another
fairly good design is having the gas lines go from the gas cabinet
to the system with a positive angle slope of 2° or greater, so any
gases that condense will flow back downhill toward the supply source.
A positive temperature gradient of 2°C or more should be maintained
with condensable gases.
However, many fab areas have the gas lines running horizontally
and vertically, with occasional elbows and fittings at walls, corners,
etc. These may be low spots in the lines that can act as cold traps
(i.e., where the substance condenses as a liquid). Using the techniques
in the examples above, significant improvements in process control,
machine uptime, a safer workplace, and reduce product cost can be
As engineers and technologists, we need to understand the process
gases with which we are working. We need to know chemical, physical,
and safety characteristics, which are available in published gas
tables and their references. Armed with this knowledge, we can "bullet
proof" our process so it will run efficiently, economically, and
Methods for temperature control of gas delivery systems all the
way from the gas cylinder to the system are available. The choice
of temperature and methodology depends on the requirements of the
process and the physical and chemical characteristics of the gas.
The implementation and use of temperature control techniques on
specific systems will result in improved process control, higher
yields, reduced downtime, and greater productivity.
Acknowledgments: The authors would like to thank Ronald Shoenholtzer
of National Semiconductor for proofreading this article and offering
many valuable inputs.
1. Neel, L.,"Foreward to Gas Encyclopedia," Gas Encyclopedia, Elsevier/
North-Holland Inc., New York, 1976.
2. Heslop, R.B., and Robinson, P.L, Inorganic Chemistry, 3rd Edition,
Elsevier, New York, pp 270, 1976.
3. Herb, G.K., et. al., "Plasma Processing: Some Safety, Health
and Engineering Considerations,"5o/i'd5tate Technology, August 1983.
4. "Diborane Handling Bulletin," Callery Chemical Company, Callery,
TABLE 1 - Some Gases Requiring
Temperature Control -