Anhydrous HF etch reduces processing steps for DRAM capacitors
05/01/1998
COVER ARTICLE
Anhydrous HF etch reduces processing steps for DRAM capacitors
S.-L. Luke Chang, Ron Hanestad, Jeffery W. Butterbaugh, FSI International, Chaska, Minnesota
A new anhydrous hydrogen fluoride (AHF) vapor-phase etch process, at the heart of a commercial single-wafer system, dramatically reduces the number of process steps for fabrication of complex three-dimensional device structures, including cylindrical capacitors for 256-Mbit and 1-Gbit DRAMs. Controlling process temperature, HF concentration, and vapor flow, the process enhances natural etch-selectivity differences of doped and undoped oxides. These natural differences stem from adsorbed water content in oxides. The process is capable of routine doped-to-undoped oxide etch selectivities >1000:1.
Moving new three-dimensional cylindrical DRAM capacitor process requirements into production would be difficult without significant changes and advances in process technology. These three-dimensional capacitors, which include vertical and horizontal layers and are crucial for 256-Mbit and 1-Gbit DRAM designs, require more complex processing than previous-generation stacked capacitors used on 4-Mbit and 16-Mbit DRAMs. With conventional low-selectivity etch processes, the formation of three-dimensional capacitors can take 20 to 30 process steps to deposit, pattern and etch the required layers.
Fabrication of a cylindrical capacitor involves, in part, depositing polysilicon around a core of patterned doped-oxide structures built on a dense dielectric oxide layer [1]. The key process step removes the doped oxide, to leave the polysilicon structure, without etching the underlying oxide, which is not protected by a mask layer [2].
The needed advance has come in the form of an oxide-etch process that selectively removes the doped oxide in one process step. This reduces the total number of process steps used to form cylindrical capacitors to five or six, far fewer than with conventional processes. Selectivity between doped and undoped oxides, which is the key to such processing, must be at least 400:1. Even higher selectivities will be required for future applications. Selectivity >1000:1 is possible with AHF vapor processing [3].
AHF etch chemistry
The AHF oxide-etch reaction takes place only in the presence of adsorbed water on an oxide film [4, 5]; the water is a catalyst. The overall reaction is:
SiO2 + 4HF ---> SiF4 + 2H2O
The AHF process exploits the difference in water content inherent in different types of silicon oxides. Its selectivity stems from the fact that doped oxides with higher water content have faster initiation, acceleration, and overall etch rates than undoped oxides with less water content.
The water for the reaction can come from within less dense doped oxides that are hygroscopic or as a reaction byproduct. In gas-phase etching with AHF, the reagents contain no water vapor. Thus, the water available on oxide surfaces and generated during the reaction plays a crucial role in the AHF process.
By comparison, in wet-chemistry etch processing, water is present in chemical reagents or is intentionally added. Because wet-etch processes have an excess of water, the wet etch reaction rate is similar for all types of oxides. Differences in wet-chemical doped-to-undoped oxide etch rates typically do not exceed 5:1.
Initially, different types of oxide have different amounts of absorbed water. Deposited low-density doped oxides that are hygroscopic, such as BPSG, PSG and BPTEOS [6], react with atmospheric moisture to form internal hydroxyl groups resulting in high levels of absorbed water. Thermal undoped, dense oxides, such as thermally densified TEOS (dTEOS) and high temperature oxide (HTO), do not react significantly with atmospheric moisture, so water absorption is limited to the surface of the oxide layer.
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Figure 1. Oxide etch rates for doped and undoped oxides as a function of time.
The amount of water in each oxide determines its AHF-etch initiation characteristics. Because water is readily available on the surface of doped oxides, the etch reaction begins immediately upon exposure to HF vapor. With undoped oxides, the lack of available water delays etch initiation.
Once the etch reaction is initiated, water is generated as a by-product. The reaction accelerates rapidly until water is in excess, at which point the reaction rate stabilizes.
Figure 1 illustrates differences in etch rates between dense and doped oxides. These data show that the etching of doped oxides initiates almost instantaneously and accelerates rapidly until it stabilizes at a nearly constant rate. On the other hand, the etch of the undoped oxides is slow to initiate, accelerates slowly, and stabilizes at a relatively low etch rate.
The AHF etch process uses temperature, HF concentration, and total flow rate (i.e., control of carrier gas flow) to amplify the natural selectivity variation driven by water content in different oxide films.
Temperature. Typically, reaction rates increase with increasing temperatures. However, with AHF vapor-etch processes, the amount of oxide removed does not increase for undoped oxides [7]. In fact, the removal of dense, undoped oxides decreases with increasing temperature because of the desorption of water and HF from the oxide surface. Desorption of water from the surface of doped oxides has minimal effect on reaction rate because water is present throughout the film; there is sufficient water to initiate and sustain the etch reaction. Thus, increasing temperature forces a greater difference in adsorbed water on doped and undoped oxides, thereby increasing AHF reaction selectivity.
HF concentration. Doped oxides etch faster with higher HF concentration. HF concentration has minimal effect on the undoped, dense oxide etch rate at elevated temperatures, however, because the availability of water, not HF, is the limiting factor. Therefore, increasing the mass flow rate of HF, by increasing the HF flow rate into the reaction`s constant nitrogen flow, increases etch selectivity between these two oxide types.
Total flow. High total flow rate increases selectivity in two ways. First, high nitrogen carrier flow removes water created by the HF-oxide reaction; this keeps water formed by the doped oxide etch from building up in the reaction chamber and adsorbing onto undoped oxide. Second - for heated processes - high total gas flow aids in the uniform, rapid transfer of heat to the wafer.
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Figure 2. Cross section of the EXCALIBUR ISR Vapor HF cleaning system.
Process optimization
Precise control of water content, HF flow, and temperature makes the AHF process repeatable from wafer to wafer and lot to lot. This can be done with the single-wafer EXCALIBUR ISR (in situ rinse) Vapor HF cleaning system (Fig. 2); this commercial single-wafer system performs an AHF etch, deionized water rinse, and spin dry in the same process chamber.
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Figure 3. Etch rates of doped and undoped oxides in an AHF vapor-etch process.
Figure 3 shows the etching of blanket PSG, TEOS, and dTEOS oxide films with the AHF vapor-etch process. The data are from experiments with the films etched at room temperature for 40 to 60 sec, with 125 sccm AHF in a carrier stream of 27 slm nitrogen gas.
The process cleared 4900 ? of the PSG oxide after 60 sec, indicating that the PSG removal rate shown is conservative. The same 60-sec process removed only 25 ? of TEOS and 12 ? of dTEOS. Thus, in these experiments, the selectivity ratio between the PSG and dTEOS was >400:1.
Experiments with BPSG and TEOS, each etched for 20 sec at ambient temperature, provided data on the effects of HF concentration on doped and undoped oxides. These tests varied the flow rate of HF gas from 175 to 275 sccm injected into a constant 27 slm nitrogen carrier gas flow.
The data in Table 1 indicate that increasing the HF concentration increases the etch rate of BPSG. This confirms that HF is a rate-limiting reagent in the etch of doped oxides, which contain sufficient adsorbed water to support the reaction. On the other hand, TEOS etching is not consistent with HF flow, since it is strongly controlled by water adsorption.
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Experiments with doped PSG and undoped TEOS oxides provided the relationship between temperature and selectivity. These tests varied the temperature of the nitrogen carrier gas from 22 to 59?C, heating the carrier gas prior to mixing it with AHF before entering the reaction chamber. An increase in nitrogen temperature, thus an increase in wafer temperature, produced a dramatic increase in PSG to TEOS selectivity (Fig. 4).
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Figure 4. The effect of nitrogen carrier gas temperature on the selectivity of PSG to TEOS.
The combination of high nitrogen temperature and high total gas flow rate has a powerful effect on AHF vapor-etch process selectivity. One group of experiments compared nitrogen temperatures of 22 and 60?C and HF flow rates of 450 sccm and 1500 sccm on BPSG and TEOS oxide films. These tests etched the wafers for 10 sec with the nitrogen carrier flow rate constant at 60 slm. Table 2 shows that the combination of high gas flow rate and elevated temperature increased etch selectivity by a factor of ten.
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Cylindrical capacitors with AHF processing
The AHF vapor-etch process characterized with the experiments above successfully produced cylindrical capacitors for DRAMs (Fig. 5). This SEM shows that the BPTEOS oxide core in place before polysilicon deposition has been completely etched away, leaving the polysilicon cylinder. In addition, the SEM shows no evidence of damage to the underlying TEOS layer or undercutting of the cylinder.
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Figure 5. A cylindrical polysilicon structure formed by selective etching with a vapor-etch AHF process.
While the AHF vapor etch process has initially targeted cylindrical capacitors, data also show that the selectivity of this processing makes it suitable for forming trench and fin capacitors [1, 4] and other three-dimensional IC structures.
Conclusion
Emerging memory cell designs call for complex three-dimensional structures. In particular, the cylindrical capacitor is becoming the structure of choice for advanced DRAMs. The fabrication of this structure requires selective removal of sacrificial oxide films from a cylinder core. AHF vapor-etch processing has the ability to remove a doped oxide completely without significantly etching the underlying oxide; this process takes advantage of natural differences in oxide water content. Selectivities of 1000:1, achieved with the AHF vapor-etch process, enable the straightforward fabrication of these three-dimensional structures.n
Acknowledgments
EXCALIBUR ISR is a registered trademark of FSI International.
References
1. D.E. Kotecki, "High-k Dielectric Materials for DRAM Capacitors," Semiconductor International, p. 109, Nov. 1996.
2. H. Watanabe, et al., "A New Cylindrical Capacitor Using Hemispherical Grained Si (HSG-Si) for 256 Mbit DRAMs," Technical Digest IEDM, IEEE, pp. 259-262, Dec. 13, 1992.
3. J.R. Mehta, T. Rogers, S. Kikuchi, "Selective Etching for Making Cylindrical Capacitors Using Anhydrous HF Vapor Phase Chemistry," Proc of the 4th Intl Symp on Cleaning Technology in Semiconductor Manufacturing IV, The Electrical Chemical Society, pp. 194-200, 1996.
4. B. Blackwood, R. Biggerstaff, L.D. Clements, R. Cleavelin, U.S. Patent No. 4,749,440.
5. R. Bersin, R.F. Reichelderfer, "The DryOx Process for Etching Silicon Dioxide," Solid State Technology, Vol. 20, No. 4, pp. 78-80, 1977.
6. VLSI Technology, S.M. Sze, ed., McGraw-Hill, pp. 115-119, 1983.
7. M. Wong, M-M. Moslehi, R.A. Bowling, "Wafer Temperature Dependence of the Vapor-Phase HF Oxide Etch," Journal of the Electrochemical Society, Vol. 140, No. 1, pp. 205-208, 1993.
LUKE CHANG received his PhD degree in physical chemistry from Iowa State University in 1988. From 1988 to 1995, he conducted research in epitaxial growth of III-V compounds, using molecular beam epitaxy and metal thin-film growth under ultrahigh-vacuum conditions. He joined FSI International in 1995 to develop processes using anhydrous HF, HCl and ozone. Chang is currently a section manager for the company`s Surface Conditioning Division. FSI International, 3455 Lyman Blvd., Chaska, MN 55318; ph 612/361-7900, fax 612/361-7964.
RON HANESTAD received his BS degree in physics from the University of Wisconsin at River Falls in 1993. He worked at Cray Research for three years on semiconductor industry applications of chemical-mechanical planarization. In 1996, he joined FSI International`s Surface Conditioning Division as an applications engineer to work on process development of single-wafer applications.
JEFFERY W. BUTTERBAUGH received his BS degree in chemical engineering from the University of Minnesota and his PhD in chemical engineering from MIT. Previously, he was involved with wafer process engineering at IBM and Seagate Technology. At FSI, Butterbaugh is applications engineering manager for SCD Single Wafer Systems, responsible for applications development on all single-wafer products. He has written several journal articles on plasma processing and dry-wafer cleaning, and holds four US patents.