Accurate Gas Control Systems

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:

improved deposition 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 controllers (MFC's).

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 eight-hour period.

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 uniformity.

Condensation 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.

Phase Diagrams
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 Te respectively.

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.


Gas Laws
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.

Illustrative Examples
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 than diborane.

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 monitoring HCl

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 [3].

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 lines.

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 problems.

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.


Discussion
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 temperatures.
Insulating gas 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 realized.

Conclusions
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 safely.

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.

References
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, Pennsylvania, 1982.

TABLE 1 - Some Gases Requiring Temperature Control - Click here to view