Management of TEOS LPCVD process effluents
07/01/1999
TEOS LPCVD processes tend to clog the vacuum pumping foreline with effluent byproducts that can increase particle levels, impede gas flow, and cause catastrophic pump failure. An integrated effluent management solution involves the correct combination of a nitrogen boundary layer near the inner physical wall of the pump line, optimized pump line temperature, and a turbulent-flow trap. Depending on the installation, the hardware upgrade payback time can be less than three weeks, and in some severe cases less than three days.
High process yield and long tool uptime are always the ultimate goals in semiconductor fabs as they increase tool utilization and wafer throughput, reduce the cost of material, improve productivity, and enhance profitability. Until now, most attention has been paid to improving the process inside reaction chambers, while effluent management is often neglected.
The effect of downstream problems on the process is often significant. Particles deposited in a pump line will backstream and increase particulate levels in the furnace. The accumulation of solid deposition products in the pump line requires frequent cleaning and reduces system uptime. If isolation valves, throttle valves, and downstream vacuum sensors become clogged, the process tool must be shut down for maintenance. In the worst case, there can be loss of expensive product wafers and catastrophic vacuum pump failure.
The method chosen for downstream effluent management depends on the process chemistry, including both the deposition mechanism and the properties of the effluent by-products. The process considered in this study is the low-pressure chemical vapor deposition (LPCVD) of silicon dioxide (SiO2) using tetraethylorthosilicate (TEOS, Si(OC2H5)4) as a precursor. The solid and/or liquid deposits that are found in a TEOS LPCVD pump line are primarily polymerized TEOS formed by chemical reactions between TEOS and water. Water is a major by-product of the chemical reactions occurring inside the deposition chamber. It turns out that water is the major culprit that causes downstream problems associated with the TEOS LPCVD process (see table).
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Heating the vacuum line to prevent condensation and then capturing the material downstream in a cold trap can control reactor effluent deposits formed by physical sublimation in some processes (such as silicon nitride LPCVD and aluminum etching), but this will not work for TEOS effluents. The TEOS LPCVD downstream reaction is typical of such molecular species: the chemical polymerization requires a physical surface (e.g., the walls of the pump line) to initiate the reaction. Thus, one effective solution is to prevent these surface reactions by creating a flowing nitrogen boundary layer between the physical wall and effluent gas flow. MKS' HPS Virtual Wall does this, such that the polymerization processes are moved downstream to a dedicated TEOS trap.
TEOS CVD downstream problems
The problems associated with TEOS CVD processes strongly depend on the process conditions. In a typical TEOS LPCVD process at 100-500 mtorr pressure and 650-750°C temperature, a hard white opaque solid snowflake-like powder forms adjacent to the furnace pump port. Transparent crystalline TEOS polymers form further downstream in the pumping line, and liquid TEOS polymers often collect at lower portions of the line. Even when the rest of the pumping line is quite clean, white solid powders often form inside the pump, where high temperature and pressure promote the chemical reactions between water and unreacted and partially polymerized TEOS.
Because of its low melting point, it is difficult to solidify TEOS in a vacuum line. Though a liquid nitrogen (LN2) cooled trap can solidify TEOS and maintain a very low vapor pressure (<10-4 torr) of TEOS in the pump line, it is not a practical solution for volume manufacturing. A LN2-trap is relatively expensive, periodic service is difficult, and safety concerns may be raised because of possible oxygen condensation.
Water readily reacts with TEOS, even at room temperature. Also, either solid, liquid, or vapor TEOS polymer deposits can form in the pump line depending upon the chain length of the polymer. In general, downstream effluent polymer phases change from solid to liquid to gas as the distance from the furnace increases. Frequent cleaning of the pump line is required, since back transmission of powders can increase the particle level in the furnace and dramatically reduce product yield.
Vacuum pumps can be quickly destroyed by TEOS polymerization buildup inside the pump. The generation of water and multistep polymerization byproducts leads to the formation of polymerized TEOS in the pump line. In effect, the pump line acts as an extension of the reaction furnace. Due to lower temperatures in the pump line, however, neither TEOS oxidization nor decomposition is likely to occur and hydrolysis and polymerization of TEOS are the main reactions.
 Figure 1. Effect of temperature on the formation of solid buildup in a TEOS pump line. |
Polymerized TEOS solid deposits that form inside the pump line do not readily sublime, so heating the pump line is not an effective way to remove them. However, heating the pump line can significantly reduce the initial solid formation. In a typical pump line, the residence time for a non-adsorbing gas molecule is short (less than a few seconds) due to high gas velocity under vacuum conditions. The residence times for water and TEOS molecules are much longer, due to adsorption within the line, increasing the time during which low-temperature polymerization can occur. Heating inhibits the physical adsorption of reactants (and residence time), while simultaneously increasing the rate of the polymerization reactions (Fig. 1). Minimum deposition occurs at 100-110°C. Although a 50% reduction of pump line solid deposits was achieved by reducing the temperature from 150 to 105°C, it is not possible to eliminate the solid buildup in the TEOS LPCVD pump line by heating alone.
Virtual Wall The deposition and accumulation of polymerized TEOS can be significantly reduced if the chemical reaction on the inner surface of the pump foreline can be minimized. A Virtual Wall isolates the effluent gas from the physical wall by forming a thin boundary layer of an inert gas (such as nitrogen) adjacent to the inner wall (Fig. 2). The Virtual Wall assembly consists of an upstream collar, a downstream spacer ring, and several identical modular middle sections composed of annular nozzle assemblies. The modular construction allows the length of the assembly to be matched to the application.
 Figure 2. Nitrogen Virtual Wall design. |
The streaming nitrogen gas boundary layer inhibits the TEOS and water molecules from reaching and adsorbing on the inner surface of the annular nozzle assembly, preventing the surface chemical reactions and the buildup of polymer. In addition to minimizing the surface chemical reaction, the nitrogen flow in the gas boundary moves particles downstream to reduce potential contamination in the reactor.
It is not practical to install an extremely long Virtual Wall; the amount of gas flow required to maintain the boundary could exceed the capacity of the pumping system. Since it is difficult to create and maintain a stable boundary layer inside a curved region or elbow, installation must be in a straight line. Typically, up to a 25cm-long Virtual Wall is located immediately after the furnace exit where most solid buildup occurs. For a vertical LPCVD reactor, this is inside the spool piece that is welded on the furnace base. For a horizontal furnace, this location is at the water-cooled flange downstream of the quartz tube reaction chamber pump port.
TEOS traps The Virtual Wall can be very effective if it is installed at the furnace exit, but it cannot protect the entire pumping line. Therefore, an efficient trap is still required to protect downstream devices such as valves, sensors, and the vacuum pump system. An effective TEOS trap will prevent polymerization reactions from occurring downstream of the trap. If water is removed from the effluent, the partially polymerized TEOS vapor will be stable, and no solid buildup will occur in the pump line or the vacuum pump (the pump temperature is not high enough to initiate the TEOS oxidization reaction). There are several ways to eliminate the water from the TEOS LPCVD effluent, but the most efficient and inexpensive solution is to promote the water-TEOS reaction inside the trap.
 Figure 3. Schematic showing the installation of an integrated TEOS downstream solution. |
The low-temperature polymerization reaction inside the trap consumes water and generates ethyl alcohol and TEOS polymers as by-products. The HPS TEOS trap provides a large surface area to promote the surface reaction between water and TEOS. The trap design also creates turbulence that increases the probability of contact between the reactant molecules and the surface, while maintaining high flow conductance in the pumping line. High conductance lowers the attainable base pressure and extends the range of pressure control within the furnace. The trap replacement element is inexpensive, and can be discarded rather than cleaned (allowing safer handling and eliminating the cost of cleaning chemicals).
Inside the trap, complex polymerization reactions form by-products that can be in vapor, liquid, or solid phases. To enhance the trapping capacity, liquid (medium-chain-length) TEOS is collected in the first stage located at the bottom of the trap. Solid (long-chain-length) TEOS will stay inside the trap, which can provide even more surface area for polymerization. The majority of the polymerization reaction by-products are vapor TEOS polymers (short-chain-length TEOS) that can be safely pumped away.
Integrated solution for TEOS LPCVD
The recommended downstream TEOS effluent control solution integrates a Virtual Wall with a downstream TEOS trap as shown in Fig. 3. The nitrogen flow to the Virtual Wall is started a few minutes before the deposition process begins.
Heating the Virtual Wall increases its efficiency. HPS Series 45 heater jackets will keep the nitrogen gas at an optimum temperature to reduce the physical adsorption of water and TEOS. No additional nitrogen gas heater is required. No heater is needed downstream of the Virtual Wall; in fact, heat ing of the downstream pump line may actually adversely affect the trap performance by reducing the residence time for water.
TEOS polymers collect as solids at the exit of the Virtual Wall (Fig. 4). These solid polymers move into the first stage of the trap where they are glued together by the liquid polymer formed in the second stage. This mixing in the first stage minimizes the volume of trapped polymers, effectively increasing trap capacity. Other advantages of this first trap stage include: collected deposits do not reduce pump line conductance; trap life is extended, since partially polymerized reactants adsorbed in these deposits are less likely to carry further downstream; andparticle generation and backstreaming is reduced.
 Figure 4. Accumulation of polymerized TEOS at the inlet of the trap's first stage. |
The fully integrated HPS TEOS Effluent Management Subsystem has been tested on a vertical LPCVD reactor at the AMD Submicron Development Center. Before the integration was implemented, frequent mandatory cleaning of the pump line was required. The benefits of the vertical LPCVD tool upgrade were significant:
- the preventative maintenance interval increases at least fourfold,
- defects are reduced by at least 20%,
- roots vacuum pump life is significantly extended,
- quartz tube life is doubled, due to less particle generation on its wall, and
- downtime is reduced (easy trap maintenance, and clean downstream valves).
Figure 5 shows deposits at the exit port of a vertical LPCVD reactor. In this installation, a weld seam prevented the initial installation of the Virtual Wall at the optimum location immediately adjacent to the furnace. This led to the unexpected discovery that solid deposition in the pump line upstream of the Virtual Wall was reduced more than fivefold. The upstream improvement may be due to the fact that reduced polymer deposition in the pump line reduces sur face roughness, retards the reaction rate, and discourages deposition further upstream toward the furnace. Thus, reducing deposition downstream should also reduce deposition upstream.
Conclusion
 Figure 5. Exit port on the vertical LPCVD reaction furnace base, showing effluent buildup. |
The integration of a Virtual Wall and a TEOS trap has successfully solved the downstream effluent management problems associated with a TEOS LPCVD silicon dioxide process. Tool uptime was dramatically increased, since the PM interval was increased 3-5 times. Unexpectedly, the quartz reactor tube life was doubled because of lower particle contamination. An efficient and compact TEOS trap allows less pump line maintenance and much longer pump life.
Savings from this hardware upgrade are significant, and the payback time can be as short as three days. Based on a 20% particle reduction, yield is expected to increase 2-5%, for total annual savings of $500,000-$1.5 million. The savings from less frequent maintenance
ebuilds can be as high as $50,000/year, and the savings from the extended quartz tube life can be around $20,000/year.
Acknowledgments
HPS, Virtual Wall, and Effluent Management Subsystem are trademarks of MKS Instruments. The HPS TEOS trap is patent pending for MKS Instruments.
References
1. US Patent 5,827,370.
2. W. Noll, Chemistry and Technology of Silicones, Academic Press, 1968.
3. CRC Handbook of Chemistry and Physics, 74th edition.
Youfan Gu received his MS in power machinery engineering in 1985 from Xian Jiaotong University and his PhD in chemical engineering in 1993 from the University of Colorado in Boulder. He is the manager of research at MKS Instruments Vacuum Products Group. MKS Instruments Vacuum Products Group, 5330 Sterling Drive, Boulder, CO 80301; ph 303/449-9861, e-mail [email protected].
Paul Dozoretz received his BS degree, with a double major in aerospace engineering and mechanical engineering, and his MS degree in mechanical engineering with an emphasis in materials, from the University of Colorado. Dozoretz is the manager of the mechanical engineering department at the Vacuum Products Group of MKS Instruments.
Jay Bhakta received his BS in chemistry and MS in chemical engineering at UC Davis. He is currently working at Advanced Micro Devices' submicron facility, specializing in LPCVD processes. AMD, One AMD Place, PO Box 3453, Sunnyvale, CA 94088; ph 408/749-5536, e-mail [email protected].
Fuodoor Gologhlan received his AA degree and electric specialist degree from IIAF and USAF. He has been working in the semiconductor industry for more than 25 years. Gologhlan has recently worked at the submicron facility of Advanced Micro Devices as a maintenance technician.