Comprehensive downstream effluent management
05/01/1998
Comprehensive downstream effluent management
Youfan Gu, Dana Hauschulz, Vacuum Products Group of MKS Instruments, Boulder, Colorado
Inappropriate handling of downstream effluents in semiconductor manufacturing processes can lead to chip yield loss and excessive equipment downtime. Serious safety issues also result from hazadous effluents trapped as solid depositions in vacuum pipelines. Developed solutions for processes include silicon nitride LPCVD and PECVD, tungsten CVD, aluminum etching, and TEOS/ozone CVD of silicon dioxide.
Product yield and tool productivity are two of the main concerns for semiconductor manufacturers. The challenge of meeting ever-tighter process control requirements forces process development engineers to evaluate previously ignored events occurring downstream of the reaction chamber.
A key issue is the sometimes-surprising behavior of process effluents. Inappropriate handling of the downstream effluents in some IC manufacturing processes leads to chip yield losses and excessive equipment downtime due to clogged forelines, damaged pumps and valves, and corrupted transducers. Clogged lines simultaneously reduce flow conductance and raise particle counts.
Effluent problems can also create serious safety issues. For example, plasma enhanced chemical vapor deposition (PECVD) of silicon nitride (Si3N4) can trap pyrophoric silane (SiH4) gas underneath otherwise benign solid depositions in downstream vacuum piping. Upon exposure to oxygen - usually during a maintenance cycle - the trapped SiH4 will combust spontaneously.
Downstream problems are often more complex than anticipated. First, extremely reactive gases can form solid by-products at the relatively low temperatures found in vacuum piping and pumps. Also, by-products of a chemical reaction in the chamber may condense in an unheated pump line. Finally, deposition processes are often mixed with in situ etch processes, particularly in cluster tools, leading to unintended cross-chemical reactions in the pipeline and/or vacuum pump.
There are many effluent management techniques, such as integrated heated forelines, process-specific traps, and a new nitrogen Virtual Wall (patent pending). Fabs that implement downstream solutions can increase tool uptime (5-10 times more runs between scheduled maintenance operations), and achieve greater product yields through reduction in particulate contamination (Fig. 1). However, the right solution must be matched to each specific process problem.
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Figure 1. TEOS pump line after 20 runs without a nitrogen Virtual Wall device.
Process-specific solutions
By-products inevitably form during any chemical reaction. Semiconductor manufacturing processes are generally tuned to produce volatile by-products that can be readily pumped away, but by-products that are in a vapor state in the processing chamber may condense into liquids or solids in the cooler downstream vacuum piping.
Sublimation refers to the phase transition of a material from the solid state directly to the vapor state without the formation of an intermediate liquid. It also (sometimes called desublimation) refers to direct condensation of a vapor to the solid state without going through the liquid phase. Both are temperature-driven events that form the basis of the effluent management technique discussed here. For simplification, we will define by-products as "sublimable" when the transition (both via heating and cooling) between the solid and vapor phase takes place readily.
Downstream effluent problems vary from process to process due to the nature of the reactants used, temperature and pressure of the chamber, and chemical by-products of the process reaction. Problematic materials are formed either through physical sublimation or surface chemical reactions, and commonly create residues in the foreline (between the furnace exit and dry pump) through to the exhaust line (Fig. 2).
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Figure 2. Typical areas of ammonium chloride deposits in a nitride LPCVD system.
Due to the different mechanisms causing solid formation in a downstream pump line, the properties of the deposited solids vary, as do the approaches to downstream solutions. For example, when materials can physically sublime, purely heating or cooling (trapping) can be very effective in solving the problem. Yet, the application of heat can actually aggravate the problem in other processes, necessitating the use of other methods, such as a boundary layer device (Virtual Wall).
Effective downstream effluent management solutions incorporating a combination of heating, trapping, and boundary layer techniques have been developed for several fab processes, including those that produce sublimable by-products, such as silicon nitride low pressure chemical vapor deposition (LPCVD) and PECVD, aluminum etching, tungsten CVD/etch; and nonsublimable by-products, such as those produced in TEOS LPCVD of silicon dioxide (SiO2).
Heating and trapping sublimable by-products
Processes such as LPCVD of Si3N4, dry etching of aluminum, and tungsten CVD yield sublimable by-products. The majority of nitride furnaces are delivered to customers with unheated forelines, which are susceptible to downstream residue problems. At the chamber connection to the exhaust line in some LPCVD nitride reactors, effluent temperatures drop from 790 to 40?C, causing large build-ups of ammonium chloride [1] and resulting in line cross-sectional areas decreasing from 165 to 40 mm2.
To deal with line-condensation, many fabs stop production for maintenance, typically for about 24 hours every 20 runs, to remove the foreline and clean out the hazardous chemical deposits, test and (if necessary) replace damaged components, and reassemble the foreline. During this procedure, engineers must cool down the furnaces; break the vacuum seals; purge, dry, and then heat the system back to operating conditions; and finally test the system before returning it to production. These procedures consume valuable production time and increase the tool cost of ownership.
For better control of unwanted temperature-driven sublimation of materials, some fabs heat specific components or foreline areas. Initial attempts at heating foreline areas used tape wraps or blankets combined with heaters, or the one-by-one replacement of standard components with heated ones. These post-system-design solutions often result in cold spots that build up deposits. For example, the inside of the tape-wrapped foreline of a nitride furnace often displays a spiral, similar to a barber`s pole, of ammonium chloride deposits.
Overheating of components causes numerous unintended consequences: grease evaporation from the valve actuators, thermal expansion mismatches between mechanical components, and exceeding a Baratron transducer`s temperature operating range. Exceeding its temperature range brings a Baratron out of specification limits and creates condensation during pumpdown, as well as traveling particle problems.
To resolve these temperature-related condensation issues, we have developed a complete downstream management solution. Effluents are heated so that they remain in a vapor phase during transport from the chamber, and/or cooled and collected in a specially designed vapor "sublimation" (vapor-to-solid) trap (patent pending). We integrated all foreline components into a precise temperature-controlled environment that transports the effluents to where they can be safely managed.
The integrated heated foreline approach incorporates heated valves, elbows, tees, pressure control valves, pressure transducers, bypass lines, and bypass valves as needed (Fig. 3). Heaters (patent issued) used are safety-approved; components are available with "touchable" heaters; and all materials are cleanroom-compatible. Traps are used where effluent materials are not too toxic to handle, and are carefully positioned to minimize contamination.
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Figure 3. An MKSheated line. Process effluent enters the line at the lower right, passes through a heated bellows section, up to a HPS two-stage isolation valve, through a MKS pressure control throttle valve, into a HPSVapor Sublimation Trap located at the upper left corner. The trap cools and collects the ammonium chloride byproduct and discharges the remaining gases down the unheated vertical stainless steel tube that is connected to a vacuum pump (not shown).
A sublimation trap draws in and solidifies effluent vapors before they have a chance to backstream to the reaction chamber and contaminate the wafer surface. Traps also prevent gases from entering and damaging the vacuum pump. Collection of the solidifiable vapor occurs in two stages. The first stage at the entrance of the trap catches >95% of all condensable gases. The second stage incorporates a polishing scrubber that maximizes heat transfer between the gas and the cooling surface, while maintaining high flow conductance. A combination of cooling water coils and perforated stainless steel cones solidifies the residual vapors that pass through the first stage. Trapping efficiency, capacity, and flow conductance all affect the trap`s efficacy, as well as its maintenance cycle. Heated lines and valves upstream of a vapor sublimation trap can maximize the benefits and efficiency of the traps.
When implemented correctly, combining these heating and trapping methods significantly increases system uptime by minimizing cleaning requirements, and improves product yield through reduction of particulate contamination. Pumping efficiency also increases due to the maximum flow conductance maintained and the decrease in water vapor. The application of these methods to specific processes with condensable by-products is described below.
Silicon nitride LPCVD
The use of a heated foreline and vapor sublimation trap on an LPCVD reactor resulted in product yield increases of >3%, improved uptimes by 16 hours/week [1], and eliminated backstreamed particle contamination on wafers (the "starring" effect).
Dichlorosilane (DCS, SiCl2H2) and ammonia (NH3) are common source gases for the LPCVD of Si3N4 thin films in batch furnaces. The stoichiometric chemical reaction for the LPCVD nitride process in the furnace is:
3SiCl2H2+10NH3 ~ 700?C, 150 mtorrSi3N4O +6NH4Cl?O +6H2?
Typically, the ratio between NH3 and DCS varies from 3:1 to 10:1. A higher NH3/DCS ratio may enhance the uniformity of the deposited films, but it may also reduce the rate of deposition.
As a result of the LPCVD process, ammonium chloride (NH4Cl) generated in the deposition chamber can solidify and deposit in an unheated pump line - clogging the line and generating particles. Solid NH4Cl, a relatively hard material, can also damage a vacuum pump if condensation occurs within a line.
To minimize downstream NH4Cl deposits, a heated pump line is installed in combination with a high capacity vapor sublimation trap. A heated pump line keeps the NH4Cl in the vapor phase to avoid solidification and particle generation in the pump line close to the reaction furnace, while a trap prevents the NH4Cl from entering the pump. A temperature of ~150?C in the heated line prevents the unwanted NH4Cl vapor-to-solid sublimation (Fig. 4).
In a heated pumping line, the NH4Cl vapor moves further downstream, keeping the furnace clean. However, excess amounts of NH4Cl vapor entering a dry pump may still cause damage due to the compression of the gas in the pump. Even if a certain amount of NH4Cl vapor can pass through the pump, it will solidify rapidly in the exhaust line and require additional maintenance. Incorporating a well-designed vapor sublimation trap collects the NH4Cl vapor and protects the pump. High trap flow conductance enables the system to attain lower base pressures, which leads to higher product yield due to less residual gas in the chamber.
Results show that the particle level in the furnace and foreline can be reduced more than fourfold because of the heated pump line [2]. The use of an integrated heated foreline, including valves and sensors, also significantly increases the system`s uptime because of its ability to eliminate harmful solid deposits.
Silicon nitride PECVD
High-quality silicon nitride thin films can be obtained with LPCVD at temperatures above 700?C. This is not suitable for final passivation deposition on ICs and FPDs, as this process temperature is above the melting point of interlayer metal and glass substrates. PECVD is used as an alternative method for Si3N4 thin film deposition.
SiH4 and NH3 are commonly used process gases for the PECVD of silicon nitride. A pure PECVD silicon nitride deposition process is relatively clean, but deposits of Si3N4 on the chamber wall are inevitable due to the reaction inside the furnace, and formation of small amounts of powder, due to the plasma`s moving further downstream may be observed in the pump line near the exit.
Downstream problems are further complicated when an in-situ etch process takes place after a sequence of deposition runs to etch the Si3N4 deposited on the reaction chamber wall. While this leads to a cleaner reaction chamber as the fluorine-based etch process converts Si3N4 powder into gas phase SiF4, it also leads to a complex cross-chemical reaction inside the dry pump. The higher temperature and pressure produce both ammonium fluoride (NH4F) and diammoniosiliconhexafluoride [(NH4)2SiF6] in the pump.
The formation of NH4F and [(NH4)2SiF6] in the pump leads not only to a clogged exhaust line, but also causes very serious safety concerns. Some pyrophoric SiH4 molecules will be trapped underneath the NH4F/(NH4)2SiF6 deposition layer in the exhaust line. Once the clogged exhaust pump line is opened for maintenance, SiH4 reacts violently with oxygen in the air. Explosions may result from careless maintenance, and cleaning of the clogged exhaust line may release large amounts of SiH4 while NH4F/(NH4)2SiF6 dissolves in water.
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Figure 4. a) The case for unheated downstream piping leading to the unwanted situation of sublimation of ammonium chloride as soon as the effluents drop below about 130?C. b) Unwanted sublimation can be avoided by heating the vacuum piping to 150?C, thereby maintaining the ammonium chloride in the vapor phase.
Careful heating can greatly increase the safety and performance of vacuum forelines and pump exhaust lines for PECVD silicon nitride systems. For example, the exhaust line is heated to minimize the adsorption of SiH4 on the inner wall. This sublimes the majority of NH4F because of its higher vapor pressure and a small amount of NH4F/(NH4)2SiF6. However, it will not completely stop the formation of NH4F/(NH4)2SiF6 inside the pump, and regular cleaning (bimonthly cleaning without exhaust line heating) of the exhaust line is still necessary.
The performance of the entire PECVD silicon nitride system further improves through heating of the entire foreline. Physical adsorption of reactive gases on the inner wall of the pump line leads to chemical cross-reactions between the etch and deposition processes, significantly increasing their resident time within the line. The effluent generated during the deposition process can stay in the pump line (mostly adsorbed on the wall) until the next etch process, or vice versa, and the mixture of these effluents forms unexpected material inside the pump. Heating the pump line reduces the effluent residence time significantly, minimizing the cross-reactions and eliminating/minimizing the formation of (NH4)2SiF6 within the pump.
Aluminum dry etch
Deposition of aluminum often takes place via PVD technology, and can be easily dry etched using fluorine-based gases such as Cl2, BCl3 and SiCl4. A major problem is the formation of aluminum chloride (AlCl3) during the etch process and its subsequent solidification in both the foreline and exhaust lines.
Adding heat to the pump line will keep the AlCl3 in vapor phase and prevent depostion on inside walls (Fig. 5a, b). To minimize the corrosion inside the pump line, the foreline should be maintained at a slightly higher temperature (105?C) to reduce the adsorption of water vapor on the inner wall of the pump line, and to give some margin of safety over the sublimation threshold temperatures of 70-80?C.
Unlike LPCVD silicon nitride, it is unwise to use condensation to trap the aluminum chloride. AlCl3 is hydroscopic and reacts violently with water, so it should be passed through the pump for delivery to the effluent scrubber. Fortunately, dry pumps can easily handle AlCl3 because it is in the vapor phase at normal dry pump operating temperature. However, one must dilute the effluent with an inert gas, such as nitrogen (N2), to keep the AlCl3 salt moving to its destination and to continue heating the pump exhaust lines all the way to the scrubber. Cautious heating can completely solve the problem of unwanted AlCl3 condensation during aluminum dry etch.
Tungsten CVD/etch
While clean forelines are often observed in tungsten CVD and etch systems, exhaust lines are usually clogged and require frequent maintenance. Tungsten hexafluoride (WF6) is a common process gas for the CVD process. In tungsten etch, a reverse chemical reaction process occurs inside the etch chamber, converting tungsten into WF6. This is a very toxic material and should be handled by the scrubber. A clean exhaust line would be necessary.
Our studies show that solid depositions in tungsten CVD/etch exhaust lines are primarily WOF4, formed by chemical reactions between the process gas and water or oxygen. Scrubbers are the most likely sources of water (wet scrubber) or oxygen (dry scrubber) in the exhaust line. The N2 gas used for purging the dry pump is another possible water/oxygen source because of the lower-grade N2 often used. A heated pump line can easily sublime the WOF4 formed in the exhaust line.
Managing effluents with a virtual wall of nitrogen
Unfortunately, not all semiconductor processes yield sublimable by-products. When this is the case, heating and cooling methods often do not completely correct a downstream problem, and may actually worsen it. A new approach using a "Virtual Wall" boundary layer device was developed to manage these by-products downstream.
The device forms a N2 gas boundary to separate the reactive gas from the physical wall of the foreline at the chamber exit (Figs. 6a, b). Locating the Virtual Wall immediately after the exit minimizes solid deposition and moves potential deposits farther away from the process chamber, thus reducing particle levels in the furnace. Typical wall lengths are 6-10 in. and are adjustable. A mass flow controller controls the N2 gas flow rate at approximately 100-200 sccm, depending on the system pressure, process gas flow rate, pump line size, and pumping speed. Because of this Virtual Wall, adsorption of water and nonsublimable substances such as TEOS on the physical wall (inner wall of the pump line) decreases dramatically, resulting in a clean line.
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Figure 5. a) Unheated downstream piping leads to the unwanted sublimation of aluminum chloride as soon as the effluents drop below 80?C.
b) Unwanted sublimation can be avoided by heating the vacuum piping to 105?C, thereby maintaining the aluminum chloride in the vapor phase.
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Figure 6. a) Schematic of a TEOS nitrogen Virtual Wall design embedded in a larger diagram showing location of the Virtual Wall in a system;b) cross section of a Virtual Wall.
TEOS LPCVD
SiO2 is a good insulator for semiconductor devices, but silane-based oxide films have poor step coverage. Tetraethylorthosilicate [TEOS, Si(OC2H5)4]-based processes have better film quality, safer handling, and excellent conformal step coverage over interconnect metal lines.
Polymerized TEOS residues found downstream of TEOS SiO2 LPCVD processes differ from downstream contaminants previously discussed: the residue is the result of a surface-dominated chemical reaction. The pump foreline is an extension of the reaction furnace where the formation and deposition of lower-quality SiO2-rich polymers occurs continuously. Since the formation of these solid depositions inside the pump line involves chemical reactions, heating may make the problem worse by increasing the reaction rate.
The persistence of effluent depositions is primarily due to the fact that water (one of the major by-products of the TEOS LPCVD process) is involved in the chemical reaction that forms polymers. Lower temperatures lead to more adsorption of water and TEOS-related species on the pump line wall, thus involving more reactants in the chemical reaction and creating more depositions in the pump line. This reaction between TEOS and water can occur even at ambient temperatures.
In general, an optimized heating temperature can alleviate the TEOS downstream problems. However, unlike the processes discussed earlier, the application of heat alone does not completely eliminate deposition in the pump line. The nitrogen Virtual Wall prevents the TEOS from contacting a physical surface, thereby preventing the surface-driven chemical reaction from occurring on the inside of the stainless steel tubing. The combination of an optimized heating temperature and a boundary layer device in a TEOS LPCVD reactor increases the uptime fivefold from original systems, prevents backstreaming of particles, lowers particle levels up to 30%, and decreases both maintenance time and costs.
Conclusion
With new materials needed for barrier layers, metal interconnects, interlevel low-k dielectrics, and high-k dielectrics, new processes will continue to cause condensation problems in downstream effluents. The implementation of downstream effluent management techniques, such as integrated heated forelines, trapping, and the nitrogen Virtual Wall, demonstrate dramatic particle, yield, and throughput improvements in silicon nitride LPCVD and PECVD, aluminum etch, tungsten CVD/etch, and TEOS SiO2 systems. By implementing effective by-product solutions for each process, the industry will be able to move closer toward its future productivity and product yield goals.n
Acknowledgments
Reference for the heater patent: U.S. Patent No. 5,714,738, Apparatus and methods of making and using heater apparatus for the heating an object having two dimensional or three dimensional curvature, Feb. 3, 1998.
The trap and Virtual Wall patents are pending.
References
1. F. Lee, E.M. Howard, D.A. DeMuynck, "Detecting and Reducing Particles for LPCVD Silicon Nitride Deposition," Motorola, Microcontamination, March 1994.
2. Perry`s Chemical Engineers` Handbook, 4th Edition, McGraw-Hill Book Co., pp. 17-23, 1969.
YOUFAN GU received his MSc degree in power machinery engineering in 1985 from Xian Jiaotong University, and his PhD degree in chemical engineering in 1993 from University of Colorado at Boulder. He is the manager of research at MKS Instruments Vacuum Products Group. Before joining MKS, he worked for 10 years in cryogenics. In the last five years, he has been involved in developing downstream solutions and prod-
ucts associated with a variety of semiconductor manufacturing processes. MKS Instruments, 5330 Sterling Dr., Boulder, CO 80301; ph 303/449-9861, e-mail [email protected]
DANA HAUSCHULZ received his BS degree with honors in mechanical engineering from the University of Colorado in Boulder. He is the manager of heater technologies for the Vacuum Products Group of MKS Instruments Inc. and has been directly involved in new product development for the past 18 years, covering plane nutation mechanisms, infrared hygrometry, high vacuum valves, ionization gauging, and heaters. For the last eight years, he has been developing downstream process solutions and products associated with a variety of semiconductor manufacturing processes. He has written papers on vacuum gauging and vacuum standards, holds two patents, and is a member of the American Vacuum Society. MKS Instruments, ph 303/449-9861, e-mail [email protected]