LPCVD Components trend toward SiC
06/01/1997
LPCVD components trend toward SiC
John A. Tomanovich, Norton Electronics, Worcester, Massachusetts
More and more IC process engineers are replacing quartz furnace components with SiC. The relatively higher thermal expansion of SiC reduces thermal stress in deposited films that accumulate in the furnace, allowing SiC components to remain in the furnaces for longer periods than quartz. SiC`s resistance to HF and HNO3 allows repeated cleaning and reuse.
Low pressure chemical vapor deposition (LPCVD) plays a critical role in the fabrication of integrated circuits (ICs). LPCVD processes are used to deposit thin layers of SiO2, Si3N4, and polycrystalline silicon (poly) on silicon wafers. Commercially available batch furnaces, the workhorses of the industry, typically operate at pressures near 1 torr, with temperatures ranging from 550-900?C, depending on the film.
The quality of deposited films has improved greatly over the years, but the furnaces remain expensive to operate, due in part to the high cost of replacing components inside the reaction chamber (Figs. 1 and 2). Films are deposited not only on the wafers, but also on all the components inside the furnace chamber, including the wafer boats and the inside wall of the process tube. As the films thicken, they begin to crack and flake, sometimes after only a few cycles. The resulting airborne particles settle on the wafers and cause yield loss. Components are usually removed and cleaned before particles are detected, but flaking is surprisingly unpredictable. Furthermore, frequent cleaning leads to other inefficiencies, such as furnace downtime and increased cleaning and inventory costs.
In the last several years process engineers have discovered that silicon carbide (SiC) furnace components can reduce or eliminate cracking and flaking of thin films. This paper explains why thin-film adherence improves for SiC vs. quartz and provides examples of productivity gains and cost reductions experienced by manufacturers who have switched from quartz to SiC.
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Figure 1. LPCVD horizontal furnace components.
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Figure 2. LPCVD vertical furnace components.
Materials for furnace components
Quartz technically refers to SiO2 in its crystalline form. Fused quartz, or vitreous silica, is SiO2 glass, which results when quartz is melted and cools without crystallizing. The IC industry uses SiO2 glass for its furnace components, and commonly refers to these products as "quartz." Though this term is technically incorrect, we will use it here to avoid confusion. Crystalline quartz is melted by either flame-fusion or electric-fusion into plate or tube stock, and then sold to "quartz shops," where it is cut and refused into specific component designs. Quartz components can be either opaque or transparent.
Most of the SiC used in furnace components is actually a composite material containing roughly 85% recrystallized SiC and 15% Si. Man-made SiC powder is mixed with water to create a slurry, and then molded by a technique called slip casting. To improve densification, Si is impregnated into the pore structure of the SiC by a high temperature firing process. According to a recent survey of IC manufacturers, 90% of furnace components in all processes are quartz and 10% are SiC. Other studies, however, show that many fabs are using 100% SiC components, and place the total usage of SiC closer to 25-30%. SiC furnace components are more expensive than quartz, and have traditionally been limited to higher temperature furnace processes, such as diffusion and oxidation, where quartz is known to deform.
Thin-film stress
Most IC linewidths are <0.8 ?m thick, and the latest are 0.18 ?m, so most manufacturers will tolerate no more than 100 particles, 0.3 ?m or larger, on a 200-mm wafer. Manufacturers reluctantly accept flaking and particle generation from thick LPCVD films on quartz furnace components as a necessary evil. In Fig. 3, the polysilicon flaking off the quartz vertical boat is only 80,000 ? thick, and is already visible to the naked eye. This photo clearly shows that the film has fractured. If fracture can be prevented or delayed, particles will be greatly reduced, and occur later.
The specific fracture strength of a material is the amount of force per area (stress) that the material is able to withstand. Fracture of LPCVD films may originate in one of three areas: the thin film, the bond that exists between the thin film and its substrate, and the substrate (i.e. the quartz or SiC). If any one of these materials is subjected to stress greater than its fracture strength, fracture will occur.
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Figure 3. Polysilicon deposition flaking off a quartz vertical boat.
The total stress in a thin-film system is the sum of the external stress imposed on the system, the intrinsic stress of the material itself, and the thermal stress [1]:
The external stress is negligible, since the wafers are not heavy, but the intrinsic stresses and thermal stresses are significant.
The intrinsic stress results from the structure of the growing film, and is thought to be affected by lattice mismatches between the substrate and the film, lattice defects, and impurities [2]. Intrinsic stresses, in general, increase with increased film thickness. They also depend on deposition rate, deposition temperature, and other processing conditions. In SiO2 deposition, for example, deposition at 400?C developed large intrinsic stresses, but deposition at 800-900?C left the films essentially stress free [3]. During Si3N4 deposition, decreasing the percent of ammonia among the reactive gases produces a more silicon-rich film, thereby decreasing the intrinsic stress in the film [1]. In most cases, however, process conditions are already optimized for thin films on Si wafers, not for thicker films on quartz.
Thermal stress results when the film and the substrate expand and contract at different rates during thermal cycling. The equation for thermal stress is [3]:
where
af and as = the average coefficients of thermal expansion of the film and the substrate
DT = the difference between the temperature of deposition and the temperature of measurement
Ef = the Young`s modulus of the film
Materials shrink during furnace cooldown. Materials with higher CTEs (coefficients of thermal expansion) shrink more than materials with low CTEs. If the CTE of the film is greater than that of the substrate, then the substrate will prevent the film from shrinking, leaving the film in tension at room temperature (RT). If the CTE of the substrate is greater than that of the film, then the substrate pulls on the film, leaving the film in compression at RT. Tensile stress is always quantified as positive, while compressive stress is negative.
Table 1 and Fig. 4 compare the thermal stress states for each possible combination of SiO2, Si3N4, and poly films on quartz, SiC, and silicon to the fracture strength of the films, assuming common deposition temperatures. In each film, the thermal stress of the film on SiC is more compressive than on quartz, because SiC has a significantly higher CTE than quartz.
As a general rule, these materials can withstand five times more compressive stress than tensile stress. Of course, the ideal state is no stress at all, but it is more practical to target a stress state in which the actual stress of the film is centered between the compressive fracture strength and the tensile fracture strength of the film, to allow for process variation.
Adhesive strength
The bond layer between the thin film and the substrate is another potential location for fracture. Most of the factors that determine the actual adhesive strength of LPCVD films on furnace components are process-dependent. However, the cleanliness and roughness of the substrate surface prior to coating can significantly affect adhesive strength. These factors are within the process engineer`s control. Cleanliness is not normally an issue. Quartz cleaners are very effective in removing contaminants from quartz and SiC furnace components alike. The surface roughness for SiC is typically much greater than quartz, which gives the coating something to anchor to, and improves mechanical adherence. Poor adherence will often result in delamination rather than cracking.
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Figure 4. Comparison of thermal stresses for various LPCVD films and furnace component materials.
Case histories
Si3N4. Of all the LPCVD processes, engineers complain the most about Si3N4. For Si3N4 on quartz, the calculated thermal stress is greater than the fracture strength of the film. Si3N4 films on silicon typically crack at thicknesses of ~2000 ? [2].
Table 2 compares the cost of ownership for a quartz vertical wafer boat (for 200-mm wafers) with one made of SiC. Quartz boats required cleaning after every 40,000 ?, whereas twice as much total deposition was possible with SiC boats. The cleaning process is also detrimental to the quartz. HF acid is used to etch Si3N4, but when the acid etches through the film, it attacks the quartz. The quartz boats thus last for only six cleaning cycles. SiC, on the other hand, is resistant to HF, and is not affected or damaged by repeated cleanings. Similar results were observed for horizontal wafer boats in a 150-mm process, except that the SiC boats were able to remain in the LPCVD chamber six times longer than quartz before particles were detected.
Polysilicon. IC manufacturers can typically deposit between 30,000 and 100,000 ? of poly on quartz wafer boats and cantilever paddles before process monitors begin to detect flaking particles. Components that remain in the furnace at a relatively constant temperature, such as liners and process tubes, can tolerate more total deposition.
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Figure 5. Polysilicon deposition on a SiC cantilever paddle.
Figure 5 shows a cutoff section from a SiC cantilever paddle that had been in service for several years. The polysilicon coating on this part is 3.5-mm thick, or 35,000,000 ?. When this paddle was removed from the system, there was no sign of particles, but the buildup was so great that the boats would no longer fit properly in the paddle. This is an extreme case - typically a total deposition of 2,000,000-5,000,000 ? is recommended.
Table 3 outlines the cost of ownership for a SiC internal liner used in poly deposition. Since the total deposition layer becomes very thick, and since removal of poly by HF:HNO3 acid etch can pose a safety risk, internal liners are often discarded instead of being cleaned. The quartz liners last ~22 days and cost $3200 each. The SiC liner costs $10,000. Dozens of SiC liners in poly processes around the world have already reached over 3,000,000 ? total deposition, with no signs of flaking particles.
Sometimes, the problem with poly is not the flaking of thin films, but breakage of the quartz components themselves. When quartz process tubes are used in poly deposition, up to 1,000,000 ? of poly can be deposited because the tube stays at 600?C and is not exposed to temperature cycles. When the furnace is shut down to pull the tube for cleaning, however, the thermal stress of the thick layer of poly on the inside of the quartz tube can exceed the fracture strength of the quartz and implode the tube. The particles caused by this type of failure can be catastrophic. There is no danger of implosion with SiC, since SiC is 50% stronger than quartz and generates less thermal stress.
In one customer`s case, a quartz injector was breaking for the same reason. When the furnace was shut down to replace the injector, the tube broke. The injector lifetime was unpredictable, sometimes failing after as little as 90,000 ? of total deposition, sometimes after as much as 800,000 ?. The quartz injector was replaced with SiC, and the furnace set up now lasts to 1,000,000 ? consistently.
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Figure 6. Polysilicon film flaking off a 200-mm silicon wafer.
SiO2. The thermal stress of TEOS deposited SiO2 on quartz is very small, since the film and substrate are basically the same material. Customers see very little difference between SiC and quartz in terms of the amount of total deposition possible before flaking is detected. However, customers still maintain that SiC offers a cost benefit, since it can withstand many cleanings in HF.
SiC wafers. For decades, furnaces have used Si dummy wafers at the ends of the wafer load to maintain temperature and gas flow uniformity. These dummy wafers cause particles after relatively few furnace cycles (Fig. 6). Table 4 shows the cost benefit of Si vs. SiC dummy wafers in a polysilicon deposition process.
Conclusion
A clear trend has emerged in the industry. SiC components offer significant productivity and cost advantages over quartz components in LPCVD batch furnaces. Over the next several years, more and more quartz designs will be converted to SiC, especially for Si3N4 and poly. IC manufacturers will require the best LPCVD processes that modern technology can offer for the next generation of 300-mm machines.n
Acknowledgments
The author wishes to thank Werner Kern of Kern Associates; Yuan Zhang of ADE Corp.; Robert Clary of Rose Associates; Bryan, Kim, Frank, Karen, and Dick at Norton Electronics; and Paula, Shannon, Joe, Kevin, and Julia at home.
References
1. Stanley Wolf, Silicon Processing for the VLSI Era, Lattice Press, pp.114-116, 1986.
2. K. Ramkumar, et al. "Stress Variations in TEOS-Based SiO2 Films During Ex-Situ Thermal Cycling," Journal of the Electrochemical Society, Vol. 140, No. 9, September 1993.
3. S.M. Sze, Semiconductor Devices: Physics and Technology, John Wiley and Sons, p. 362, 1985.
JOHN TOMANOVICH received his BS degree in ceramic engineering from the NYS College of Ceramics at Alfred University in 1986. He is the product manager of horizontal SiC CRYSTAR products and SmartWafer products at Norton Electronics. Norton Electronics, 1 New Bond st., P.O. Box 15136, Worcester, MA 01615; ph 508/795-2289, fax 508/795-5976.