Papers

AVS1999 Session MS-MoA: Ultra-Clean Society and Contamination Free Manufacturing

Monday, October 25, 1999 2:00 PM in Room 611

Monday Afternoon

Start	Invited?	Item
2:00 PM		MS-MoA-1 Highly Reliable Ultra Thin Gate Oxide Grown using Water Vapor Generator O. Nakamura, T. Ohkawa, M. Nakagawa, Y. Shirai (Tohoku University, Japan); K. Kawada, N. Ikeda, Y. Minami, A. Morimoto (Fujikin Incorporated, Japan); T. Ohmi (Tohoku University, Japan) High-reliability ultra thin oxide film is required for future ULSI manufacturing, where perfect uniformity and very high yield in volume production of large diameter wafers era must be guaranteed. In case of wet oxidation, water vapor is most commonly generated by burning H₂ in O₂ ambient. So, this torch-type has a potential problem of particle contamination of the water due to the micro-powder from the quartz combustion nozzle. In order to obtain wet ambient without contamination, we have developed a new Water Vapor Generator (WVG) using catalytic reactor. Additionally, the WVG can generate H2O from the vacuum to the high-pressure condition. This WVG is expected to be used in various processes where water vapor is needed.In this presentation, we will demonstrate a new gate oxidation process to form high-reliability ultra thin gate oxide at low temperature with highly concentrated H₂O using WVG system. For example, for substrate injection, the 50% Q_BD values of conventional dry oxides (900°C) and advanced wet oxides formed at the low temperature using highly concentrated moisture (90% H₂O/Ar, 750°C) are 9.5 C/cm² and 70 C/cm², respectively. For gate injection, the 50% Q_BD values of conventional oxides and advanced oxides are 6.5 C/cm² and 25 C/cm², respectively. Moreover, we will discuss the influence of oxidation ambiences such as surplus O₂ and H₂ ambience and oxidation temperature for electrical characteristics. In conclusion, ultra thin gate oxide using WVG has high breakdown strength under electrical stress. A newly developed oxidation is effective to grow a tunnel oxide for flash memory, which is operated under high electric field.
2:40 PM		MS-MoA-3 High-integrity Ultra-thin Silicon Nitride Film Grown by Plasma Nitridation of Silicon Surface at Low-temperature for Giga Scale Devices K. Sekine, Y. Saito, M. Hirayama, T. Ohmi (Tohoku University, Japan) The progress of MOSLSI technology has been based on the shrinking of MOSFET's. Along with downsizing MOSFET's for more than 25 years, the gate oxide equivalent thickness of MOSFET's has continued to be reduced. Since the invention of MOS device, thermally grown silicon oxide, the prevailing gate dielectric for Si based MOS devices, processes remarkable electrical properties that are unmatched by other materials. However, transistor scaling is driving gate oxide equivalent thickness to 3 nm and below, when direct tunneling current becomes significant. Ultra thin silicon oxide below 3 nm is not expected to be robust enough for future transistor gate dielectric application. In order to continue downsizing MOSFET's, thermally grown silicon oxide will be replaced by higher dielectric-constant films, for example Ta₂O₅ and Si₃N₄. A radial line slot antenna (RLSA) high-density plasma system can form high-integrity silicon nitride film at a temperature of 400 °C. We focus attention on electrical properties of ultra-thin silicon nitride films grown by radial line slot antenna high-density plasma system at a temperature of 400°C as an advanced gate dielectric film. The results show low density of interface trap and bulk charge, lower leakage current than jet vapor deposition silicon nitride and thermally grown silicon oxide with same equivalent oxide thickness. Furthermore, they represent high breakdown field intensity, almost no stress-induced leakage current, very little trap generation even in high-field stress, and excellent resistance to boron penetration and oxidation.
3:00 PM		MS-MoA-4 Generation of Positively Charged Particles at an Anode and Transport to Device-wafers in a Real rf-plasma Etching Chamber for Tungsten Etch-back Process T. Moriya, N. Ito, F. Uesugi (NEC Corporation, Japan); Y. Hayashi, K. Okamura (NEC Kyushu, Ltd., Japan) In this paper, it is clarified that the particles, flaked off from a grounded anode of parallel-plate rf plasma etching equipment, have positive charges. Moreover, the particles transport from the anode to the device-wafer on the cathode with keeping away from bulk plasma. In previous papers, we have reported that, in the middle space between the two electrodes, many particles were observed at the timing of the rf power off, and seemed to be drawn to the wafer with the residual negative self-bias voltage.^1,2 To clarify the polarity of charge and the transport path of particles, the appearances and the trajectories in relation to the workings of the etching equipment are studied in detail both near the anode and the device-wafer on the cathode. Surprising results are obtained. Near the grounded anode, a few particles appear constantly and have parabolic trajectories with open upward in the duration of rf power, and many particles appear and have sharply curved trajectories from the anode to the chamber wall at the rf power off. On the other hand, near the wafer on the cathode, almost all particles appear at the rf power off and are drawn from the chamber wall to the wafer. These results mean the particles are reflected by the plasma potential, and they transport from the anode to the wafer with keeping away from the residual bulk plasma under the existence of attractive force between the particles' positive charge and the residual negative charge of the wafer. ¹ ¹F. Uesugi, et al., J. Vac. Sci. Technol. A 16, 1189 (1998). ²N. Ito, et al., J. Vac. Sci. Technol. B 16, 3339 (1998).
3:20 PM		MS-MoA-5 Standardization of the Method to a Moisture Concentration in Hydrogen Chloride Gas with Diode Laser Absorption Spectrometry Y. Ishihara (UC Standardization Committee, Japan); Y. Sakakibara (NTT Advance Technology Corporation, Japan); Y. Kunii (Kokusai Electric Co., Ltd., Japan); K. Hasumi (Hitachi Tokyo Electronics Co., Ltd., Japan); I. Matsuda (Showa Denko K.K., Japan); N. Miki (Ultraclean Technology Research Institute, Japan); A. Ohki (Oosaka Sanso Kogyo Ltd., Japan); Y. Shirai (Tohoku University, Japan) A standard method is proposed, using diode LASER absorption spectroscopy, to measure the moisture (H₂O) in hydrogen chloride (HCl) gas at concentrations between 100 ppb to 0.1%. This standard is laid down to measure trace H₂O in HCl at point of use. In this standard, HCl with H₂O of unknown concentration (sample gas) is introduced into a laser absorption spectrometer which is kept at reduced pressure. Measurement is performed in the range of 1370 nm to 1389 nm in wavelength, and the second-derivative absorption intensity of H₂O is calculated. Using the second-derivative absorption intensity and pre-defined calibration curve, the H₂O concentration is determined. The determination limit, which was defined as 3 times of the standard deviation of the second-derivative intensity, was found to be 100 ppb when a program for noise cancellation was employed. For verification of calibration curve, calibration curves which were prepared at different timings at different places by different people showed good agreement of over 95%. Moreover, it is proved that calibration curve of H₂O in HCl can be substituted by that of H₂O in N₂ which is corrected with a correction coefficient.
3:40 PM		MS-MoA-6 Gas Distribution System Using an Advanced Flow Controller M. Nagase, O. Nakamura, M. Kitano, Y. Shirai, T. Ohmi (Tohoku University, Japan) In a single wafer treatment, an individual process is carried out within 30-40 sec. And an accurate control of the working pressure and composition ratio of all source-gases in the process chamber through the entire process period is essentially required for establishing high quality processes. We have developed a total gas system combining a distribution system and a pumping system in order to satisfy this requirement and the system evaluated using FT-IR method. The gas distribution system consists of an advanced flow controller(FCS) and an electrically controlled valve(ECV). The FCS is introduced into the principal that the flow rate is directly proportional to the upstream pressure when the upstream pressure of a orifice is two times higher than the downstream pressure. The advanced distribution system using the FCS and the ECV does not observe overshoot phenomena and so stable gas flow rate can be distributed in the chamber after valve operation. However, in the case of combination chamber volume and gas flow rate, it occurred time lag to become stable gas concentration in chamber. To solve this problem, we developed the multi-step flow rate control. Consequentially, the working pressure rises momentarily because more gas distributes than steady state gas flow rate. This problem is solved to control the pumping property by changing the purge gas flow rate which supplying into the drug screw pump with the FCS. Combining the advanced gas distribution system using the FCS and the ECV and the pumping system, we can perfectly control process parameters such as gas composition and the working pressure on the moment.
4:00 PM		MS-MoA-7 Investigating Molecular Contamination in Cleanrooms P.H. Schnabel, G. Goodman (Charles Evans & Associates); D. Nehrkorn, M. Kendall (Surface Science Laboratories); G. Strossman, P. Lindley (Charles Evans & Associates) As the line widths of microelectronic devices approach 0.1 micron, the presence of airborne molecular contamination (AMC) in fabs and cleanrooms has become a major concern for the semiconductor industry. In order to achieve low defect rates in these next generation devices the technical ability to identify, isolate and eliminate AMC is a substantial challenge. AMC can potentially result from every material within a cleanroom or a fab but the main sources for AMC are process chemicals, construction materials and the local environment. AMC defects can cause changes in the wafer's electrical properties, uncontrolled boron or phosphorous doping, etch rate shifts, threshold voltage shifts, wafer and stepper optics hazing and high contact resistance. In this study we demonstrate that TOF-SIMS can be utilized for identifying different types of condensable airborne contaminants and for monitoring those contaminants in cleanrooms. For this purpose witness wafers were placed in a newly constructed class 10 cleanroom and analyzed periodically over a time of 6 months. In order to identify potential sources of AMC the ougassing of individual materials that are typically present in cleanrooms was studied by TOF-SIMS, GCMS and FTIR. The materials under investigation include cleanroom construction materials (e.g. floor tiles, filters, sealant etc.), cleanroom furniture, cleanroom garments and cleanroom utensils. In these experiments each of the materials 'delivers' a fingerprint which can be used to identify potential sources of cleanroom contamination. The transfer of contaminants onto silicon wafers that are brought in contact with these materials was studied as well. The long term objective of this part of our studies is to generate an extensive database which allows us to link the observed contaminants on wafers with potential sources within the cleanroom environment. Both, transfer through the gas phase and by contact will be evaluated.
4:20 PM		MS-MoA-8 Minimizing Particle Generating Contamination in Polysilicon LPCVD J. Krueger (Texas Instruments Incorporated); J. Snow, J. Hardin, J. Gratz (Millipore Corporation) Increasing wafer size and decreasing critical dimensions exacerbate the chances of a particle coming to rest in a die-killing location. In this study, the effect of point-of-use (POU) purification was evaluated by processing two sets of split lot wafers on an LPCVD polysilicon horizontal furnace. The gas quality downstream of the furnace was evaluated using a closed ion source (CIS) residual gas analyzer (RGA) and an in-situ particle counter (ISPM). Wafer particle counts, film contamination levels by secondary ion mass spectroscopy (SIMS), surface roughness and grain size measurements by atomic force microscopy (AFM) were all used to compare the split lot halves processed with and without purification. SIMS analysis showed that oxygen levels in the film were lower for wafers run with the purifiers. Also, secondary ion counts of silicon were slightly higher in the film of the wafers processed with the purifiers. Wafer particle data showed that wafers run with the purifiers had 72% fewer added defects. The ISPM sensor showed that there were 37% fewer particles with the purifiers in place. RGA results revealed lower moisture levels with the purified silane deposition step compared to unpurified.
4:40 PM		MS-MoA-9 Highly Concentrated Ozone Gas Supplied at Atmospheric Pressure Condition as a New Oxidizing Reagent for the Formation of SiO₂ Thin Film on Si K. Koike (Iwatani International Corporation, Japan); S. Ichimura, A. Kurokawa, K. Nakamura (Electrotechnical Laboratory, Japan) Ozone is expected to be one of promising oxidizing reagents for the fabrication of future ULSI device. We have investigated ozone oxidation on Si(100) substrate with high purity (about 80 vol%) ozone gas, and have revealed various merits of ozone oxidation; e.g., ozone can form dense SiO₂ film on a Si substrate at lower substrate temperature than that used at a conventional thermal oxidation process, ozone can oxidize hydrogen-terminated silicon surface which oxygen molecules cannot, etc.^1,2 One major problem which has to be solved before the ozone oxidation is applied to a practical process is low oxidation rate. Since the pressure of the high purity ozone gas was low (typically; <10^-4 Pa), only 2 nm thick SiO₂ film could grow on a Si(100) substrate by 2 hours ozone gas exposure at a substrate temperature of 973K. In the present study, we report that the problem could be solved by fabricating another type of ozone generator. The generator can supply highly concentrated ozone gas at atmospheric pressure condition, by desorbing ozone from silica-gel on which ozone/oxygen mixture gas had been adsorbed at lower temperature. Ozone concentration in the gas from the generator can be changed between 0 and 70 vol%, by controlling the ozone adsorption and desorption condition. Even with 25 vol% ozone gas, it was confirmed that SiO₂ film as thick as 3.3 nm grew on a Si (100) wafer kept at 648 K by 30 min exposure. The wafer had chemical oxide film (thickness; 1.2 nm) before the ozone oxidation. So SiO₂ film with thickness of 2.1 nm could be additionally formed with the ozone gas, while under the same experimental condition only 0.6 nm thick SiO₂ film could be formed on the same wafer with pure oxygen. It should be emphasized that the density of the SiO₂ film formed with 25 vol% ozone gas was equivalent to the density of a film formed by a thermal oxidation process at 1023K, judging from their etching rates with dilute HF solution. The result suggests that the present ozone oxidation process has high possibility to be adopted as a new process for Si oxidation. The details and performance of the new ozone generator are presented together with the dependence of the oxidation of Si(100) on ozone concentration and on Si substrate temperature. ¹ A. Kurokawa, S. Ichimura, and D.W. Moon, Mat. Res. Soc. Symp. Proc. 477, 359 (1997). ² A. Kurokawa, K. Nakamura, and S.Ichimura, Mat. Res. Soc. Symp. Proc. 513, 37 (1998).
5:00 PM		MS-MoA-10 A Comparison of VPD, TXRF, and Surface SIMS to Detect Fe, Ni, Cu, and Al on Silicon Wafers V.K.F. Chia, J. Metz, M.J. Edgell (Charles Evans & Associates) The emphasis on contamination free manufacturing (CFM) continues within the manufacturing environment through the use of cleanrooms and the practice of contamination-free procedures. Stringent contamination limits for polished and epitxial substrates and surface preparation (i.e. before gate oxide growth) are suggested by the National Technology Roadmap for Semiconductors (NTRS). Today's requirement for surface metals is typically in the range of 10¹⁰ at/cm². Future needs are anticipated to be in the mid-10⁹ at/cm². The transition metals Fe, Ni, and Cu are considered to be very damaging at the gate oxide level, and therefore requirements for these are becoming more stringent. Al is important to monitor because at concentrations below 10¹¹ at/cm² it can increase the oxide growth in the very thin gate oxide regime. This is a different effect compared to higher levels of Al (e.g. >10¹² at/cm²), which decreases gate oxide thickness for thicker gate oxides. VPD, TXRF and SurfaceSIMS are commonly used in surface clean technology. VPD is a collection/scanning procedure that concentrates the metal contaminants on a wafer surface into a droplet. VPD procedure is popular because it improves the detection limit of the final analytical measurement technique. TXRF is well established as a surface sensitive technique. It can detect medium- and high-Z elements (sulfur to uranium) on silicon wafers at very low concentration levels. Routine detection limit is approximately 10¹⁰ at/cm² or better. SurfaceSIMS is a powerful analytical technique for substrate engineering. Typical detection limit of this technique is 10⁸ to 10¹⁰ at/cm². SurfaceSIMS complements TXRF by detecting low-Z elements, such as Li, Na, K, and Al. This presentation provides an overview of VPD, TXRF, and SurfaceSIMS and their application to detect Fe, Ni, Cu, and Al.