Contamination due to process gases

Contamination due to process gases

Microelectronic Elsevier Engineering 10 (1991) 259-267 Contamination gases 259 due to process Henry Berger The BOC Group Inc. Technical Center, ...

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Microelectronic Elsevier

Engineering

10 (1991) 259-267

Contamination gases

259

due to process

Henry Berger The BOC Group Inc. Technical Center, Murray Hill, New Jersey, U.S.A.

Abstract. A survey is given of The BOC Group technical research programs, whose purpose is to relate purity in gas processing to ULSI device parameters. Results from the following research programs are presented: how inert gas purity affects the Ti silicide for IGFET metallization; the effects of argon versus nitrogen used in silicon gate oxidation process steps; and preliminary work on a new gas analytical tool, Atmospheric Pressure Ionization Mass Spectrometry (APIMS), which allows measurement of sub ppb impurity levels in processing gases.

Keywords. Gas purity APIMS (Atmospheric Pressure Ionization Mass Spectrometry), silicide annealing, trace oxygen levels, sheet resistance, oxidation, defect density, electron trapping, oxidation uniformity, flatband voltage, gas dilution system, contamination.

Henry Berger, Manager,

Microelectronics Process Technology with The BOC Group Inc. Technical Center (supporting Airco Industrial Gases Division in Murray Hill, NY) at the Microelectronics Center of North Carolina, MCNC, in Research Triangle Park, NC has been with BOC since 1985. From October 1988 through 1989, he was on assignment in Sendai, Japan, collaborating with Prof. Tadahiro Ohmi at Tohoku University. As device physicist, Dr. Berger studies the relationship between 13 cleanliness of raw materials and processing (with a focus on gases) and ’ new semiconductor materials for advanced integrated circuit devices. Dr. Berger received his PhD in physics from the University of North Carolina. Prior to joining The BOC Group, Dr. Berger was a development scientist with Energy Conversion Devices, Troy, Michigan. There he was responsible for research and development of amorphous silicon, thin film, photovoltaic solar cells. He also worked at Exxon Research, Linden, New Jersey, on solar energy-related technology. Berger, a member of the Electrochemical Society, has authored numerous articles on his work. 0167-9317/91/$3.50 0

1991, Elsevier Science Publishers B.V.

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1. Introduction

As semiconductor devices are scaled to smaller dimensions and the levels of integration on a chip are increased, it becomes increasingly difficult to achieve high yields [l]. Empirically, a quadrupling of defect density has been seen for every reduction of feature size by a factor of two. The concurrent trend towards larger chip sizes makes the yield problem even more severe. Furthermore, as pattern dimensions scale down, voltages have not historically scaled as rapidly, giving rise to hot electron phenomenon, whereby energetic electrons are injected and subsequently trapped in gate insulators resulting in threshold voltage instability. These higher operating fields also result in reduced dielectric reliability due to dielectric breakdown. Currently, the semiconductor industry is in a position of intense international competitiveness where manufacturing costs, yields, and reliability are critical to economic viability. To meet the yield requirements of current and feature device technologies, BOC researchers serving the semiconductor industry have investigated ways to improve gas processing for a variety of chip fabrication process steps. This paper focuses on three areas: (i) Furnace silicide annealing has been studied for the effects of inert gas purity. For titanium silicide films, trace oxygen levels in nitrogen and argon, used as the annealing atmosphere, are related to sheet resistance values and phase composition. (ii) Argon versus nitrogen gas processing was studied in the oxidation/annealing of Silicon Metal-Oxide-Semiconductor device structures. For both dry and dry-wet-dry oxidation processes, oxidation uniformity, flatband voltage, oxide dielectric strength and defect density, as well as electron trapping characteristics were measured. The inert gases were used during the initial temperature ramp-up cycle prior to oxidation and during the post-oxidation annealing step. (iii) High-sensitivity gas analytical capabilities have been developed using APIMS (Atmospheric Pressure Ionization Spectrometry). Improved APIMS calibration techniques were studied. A newly developed, twostage gas dilution system is described that allowed contamination-free and accurate control of gas mixing for APIMS calibration.

2. Effects of inert gas purity on titanium silicide films Results of a study of how thin silicide film growth depends on the purity of the ambient inert atmosphere used in processing are presented. The influence of the processing gas in titanium silicide film growth has received attention [24]. Impurity distributions in the titanium-based film have been shown to be influential with respect to parameters such as the electrical resistivity and etching control. The extent that the gas processing affects these material properties

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determines the applicability of the silicide for self-aligned metallization schemes [5] in advanced IC designs. Silicide films were grown in 800°C furnace anneals on substrates of Ti deposited on (lOO), lightly doped, 4” silicon wafers. Typically, the Ti was evaporated to thicknesses of 30 nm. Furnace processing was carried out at pressures slightly above atmosphere in a quartz tube designed to minimize contamination of the processing gas from back-diffusion of room atmosphere from the loading zone. Nitrogen and argon, used as the furnace processing gas, were monitored for oxygen and moisture levels inside the furnace tube. Purposeful doping of the inert gases with oxygen could be controlled to below 0.1 ppmv using a prototype gas purifier which operates at room temperature [6]. Silicide samples were analyzed for solid phase composition using a small spot ESCA from Surface Science Laboratories. Sheet resistance was measured with a Magne-tron M-700 four point probe. For nitrogen processing, doping levels of oxygen in the range of 5-10 ppmv gave rise to sharp increases in film sheet resistance values. In the surface region of the film, above the titanium silicide layer, the relative amount of oxygen, as measured by ESCA, increased as the oxygen level was increased in the process gas [3]. For argon atmospheres, increases in the amount of oxygen incorporated in this top region of the film lead to decreases in the silicide film thickness. This caused the measured sheet resistance value to increase. The same behavior was seen for nitrogen processing up until a critical level of oxygen doping in the gas, at which point the sheet resistance increased discontinuously. This behavior is identified with phase transitions above the surface of the silicide layer which involve replacement of titanium nitride phases by oxide formation.

3. Gas processing effects in silicon gate oxidation For silicon gate oxidations, the effects of using argon vs. nitrogen were compared [7] in temperature ramp-up, ramp-down and in annealing steps under the conditions most commonly used in device processing, namely at moderate temperatures (850-1000°C) for relatively short periods (less than 20 min). Oxidation runs using argon for all these steps were compared to runs using nitrogen. To discern differences between these conditions, a large statistical data base was needed. This evaluation was designed to encompass most of the oxide characteristics that must be considered in optimizing an MOS manufacturing process including oxidation uniformity, C-V characteristics, oxide breakdown, and neutral electron trapping. Metal-oxide-semiconductor capacitors were used in this study. Starting wafers were cleaned, oxidized, metallized, and annealed to form Al/Si02 capacitors. These simple capacitor structures were chosen as a starting point rather than full devices to reduce sample fabrication turn-around time and complexity. VLSI grade nitrogen or argon was used as the inert gas in the oxidation cycle.

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These MOS structures were then characterized for a number of different properties, and comparisons were made between the two different gases. Three types of oxidation processes were used: a 1000°C dry oxidation with 20.5 minutes oxidation time; an 850°C dry-wet-dry (dwd) with 22 minutes wet oxidation time; and a 1000°C dwd cycle. All oxides were expected to be 35 nm. The percentage of HCl introduced during the oxidation step was 4.5 for the dry and 2.0 for the dry-wetdry.

3.1. Results - Capacitance vs. voltage Capacitance-voltage measurements were used to monitor flatband voltage (V,,), and flatband voltage shift (AVfb). All of the oxides grown in this study had very low levels of mobile charge (less than 50 mV), so that the focus of the measurements could be on the flatband voltage. The flatband voltage is determined by both the metal-semiconductor work function difference, and by the charge in the oxide. For inert gas comparisons, C-V measurements on a total of 124 capacitors (combined runs) were taken for the data on flatband voltage and doping (N,). Ar-annealed oxides had an 80 mV lower flatband voltage (indicative of less net positive oxide charge) than N,-annealed oxides. This difference was twice as large as the statistical variation of either population alone.

3.2. Oxide results-Dielectric

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Dielectric breakdown measurements were taken on the MOS capacitor dots to quantify the oxide defect density. Breakdown was measured at a 2 MV/cms ramp rate and was sensed at 10 (IA in a 5~10~~ cm2 capacitor. Breakdown field statistics were used to calculate a “defect density”. Defect density, p, is calculated [S, 91 from the Poisson equation: p = - ln(1 - &)/A, where A is the area, t is the total number of capacitors measured, and II is the number of poor capacitors, having a low breakdown field. While different criteria of failure can be applied, this study considered a capacitor poor if it had a dielectric strength below 80% of the intrinsic dielectric strength of SiO* which was 11.3 MV/cm for the 35 nm oxides used here. Defect density is closely related to the sharpness of the peak of the breakdown field. Any low field breakdown event was assumed to be caused by a defect at some point within the area of the dot. The average breakdown voltage and the standard deviation, as well as the defect density for each wafer, were measured for different process parameters. The average breakdown voltage did not show a significant difference between

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the types of processing. On the other hand, the defect density, a parameter related to the number of breakdowns occuring for low applied fields, shows differences between several of the process groups. The peak in breakdown field for the Ar-annealed runs was seen to be more sharply defined than that for N2anneals. The Ar-annealed oxides had only l/4 the number of defects as NZannealed oxides. Also, dry oxides exhibited fewer defects than wet oxides.

3.3. Oxide results- Electron trapping Electron trapping was measured by using avalanche injection [lo] of electrons into the oxides on (100) 0.1 ohm-cm P-type wafers. For measurement details see [7]. The shift in flatband voltage, giving the first moment of charge in the oxide, can be approximated by: AV = do,qNi,tlE* Here Nint (sometimes called Nequivarent ) is the area1 electron density at the interface, E is the dielectric constant of SiOZ and q is the density of trapped electrons in the oxide. The approximation is based on the assumptions that all of the trapped charge resides at the Si/Si02 interface and that all traps are singly charged (for trapped electrons which are assumed to be singly charged, linearly distributed over the bulk of the oxide, AV = d&qNbulk/2E, where Nbulk is the volume density of electron traps). Oxides contain different types of traps each of which might have its own spatial distribution. Traps in insulators are conveniently characterized by their capture cross section, u. Traps with the largest cross section (a = lo-l3 cm”) are initially positively charged and believed to be all at the Si/Si02 interface. Injection of 1013 to 1Ol4 electrons/cm2 are sufficient to fill all of these traps. Traps with (T= lo-l6 are initially neutral and fill up with 1016 injected electrons/cm2; these traps are associated with radiation damage, and their spatial distribution is largely unknown. Smaller neutral traps (a = lo-“) are associated with water in the oxide and are often taken as being distributed throughout the oxide. In this work we characterized the charge trapped after injections from 1.2 X 1015 to 101’ electrons/cm2, in order to roughly quantify the amounts of these types of traps. Electron trapping measurements as a function of inert annealing gas were compared. For 1015injected electrons, both dry and dwd oxidations showed less than 50 mV of shift. Differences in this test between different inert gases could not be considered statistically significant. This result demonstrates our ability to grow high-quality, charge-free oxides. The advantages of argon are most pronounced at 101’ electrons/cm2 injections where N2-annealed capacitors average more than 100 mV more neutral electron traps and exhibit a greater sampleto-sample variation. The wet oxides, as expected, showed much greater electron trapping, particularly at high injections, because of water-related traps.

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APIMS studies

The increasing gas purity requirements for electronics have resulted in inert gas impurity concentrations decreasing from the ppm range to ppb and ppt levels. Concurrently, high-sensitivity gas analytical capabilities have been developed using Atmospheric Pressure Ionization Spectrometry (APIMS) [ 11, 121. However, for these low impurity concentrations APIMS measurement results have uncertainty factors which are not trivially assessed. Accuracy here is dependent on the interpretation of, sometimes, complex gas ionization chemistry. Similary, this uncertainty pertains to establishing APIMS calibration settings. This has motivated the APIMS calibration study, reported here [13]. At the Super Cleanroom Facility (SCR) at Tohoku University, Prof. T. Ohmi has reported [14-161 APIMS measurements on gas purity that represents next-generation detection levels in gas analysis. Over conventional gas analysis (near or above atmosphere) APIMS provides increased sensitivity (by a factor of about 100-1000) to selected trace impurities. At the Tohoku University SCR, APIMS contributes to reasearch in the following areas: l Contamination in the form of outgassing from gas tubing, gas components and new semiconductor materials. This is accomplished by measuring the impurities in an inert carrier gas after it has flowed past test samples subjected to temperature cycling. The following Tohoku University projects [17] all make use of the APIMS instrument for analysis of outgassing chemistry: gas particle filter study, DI water tubing study, epi reactor susceptor study, gas valve study, gas cylinder study, stainless steel passivation study and photoresist on wafer study. Using this same methodology, APIMS has been used to evaluate the integrity of semiconductor processing steps by measuring the impurities at the gas outlet of specific process tools. l Monitor day-to-day, ultra-high purity levels in facility gas supply (nitrogen, argon, oxygen and hydrogen). This also serves to monitor the operation of gas purifiers. l R & D effort to increase gas analysis sensitivity by improving APIMS operation. APIMS uses a selective, two-stage ionization process that increases impurityto-host gas ratios for mass spectrometry sampling. APIMS works by first ionizing sample gas in a corona discharge (in the kV range), using a tungsten needle electrode. The primary ionization, which occurs at atmospheric pressure, is then followed by secondary ionization, at reduced pressures (about 0.5 torr), via ion-molecule collisions. The APIMS measurement sensitivity, for a specific set of sample conditions (such as species of carrier gas and contaminant, the approximate level of gas contamination) can be optimized by adjusting the primary discharge voltage, V,,. VP determines the percentage of the gas sample, Nr, undergoing primary ionization (at atmospheric pressure). As an example, with VP approximately adjusted, 1% of a gas sample, containing, say 1 ppm of impurity, can be ionized.

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After the secondary step (assuming 100% yield during the secondary ionization of the contaminant), the initial 1 ppm impurity level in the gas will be concentrated (in the ionized gas) to: 1 ppmlN, = 10-2%. It follows that maximized APIMS sensitivity is achieved for VP adjusted so that Nr, is minimized. If in the above example, the fraction of primary ionized gas could be lowered to: Np = 10-4%,

the 1 ppm impurity level of the initial gas sample would be boosted to an 100% concentration in the analyzed gas. In order to use APIMS to measure ppb and ppt impurity levels in gases, calibration gases with impurity levels controlled in this range must be prepared. Since it is extremely difficult to prepare cylinder gas with accurate control of impurities in the sub-ppm range, a gas dilution system is used. To achieve the desired low levels of impurities the gas dilution system mixes ppm level span gas, from high pressure cylinders, with purified, inert (zero) gas. Gas dilution systems of conventional design adjust gas flow volume by using any number of float-type flow meters or mass flow controllers (MFC’s). The dilution ratio here (span gas/zero + span gas flow) is limited to about 0.003. In addition, using currently available manufacturing processes, the individual components used in conventional gas dilution systems can themselves be sources of uncontrolled impurities to the gas. To accurately and repeatedly prepare subppm calibration gas, impurity adsorption and desorption from the inner surfaces of gas cylinders, gas tubing, gas valves, MFC’s and pressure regulators must all be minimized. The high-purity gas dilution system used in this work consists of: a gas cylinder with compound-electropolished stainless steel inner surface [18]; a gas cylinder valve [18] and gas regulator [19] with high-purge capabilities; a highprecision, fast-response, high-purge capability MFC [20] with three-column dynamic range; and electropolished stainless steel tubing [ 161. The response time and reproducibility of the high-purity gas dilution system was evaluated by switching through various gas dilution ratios in the ppb range. With APIMS measurement delay taken into account, the estimated response time of the system is of the order of a few seconds. APIMS calibration curves were then generated which relate APIMS data output with absolute impurity concentrations, as calculated from the gas dilution system flow settings. APIMS output is in the form of ion intensity (volts) for each specific contaminant’s mass number (M/Z). For example, CO2 in nitrogen is identified by M/Z = 44 (CO,‘). Data was generated relating APIMS ion intensity voltages to the concentration levels of CO2 in nitrogen, CO2 in argon and CO in argon, respecitvely.

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5. Conclusions

In terms of sheet resistance and film surface composition, the sensitivity of titanium silicide film growth to process gas purity depends on the inert gas used in the process. For argon, changes in oxygen contamination levels in the range 0.1-5 ppm, during the process, result in relatively small changes in sheet resistance and film morphology. However for nitrogen, fluctuations in the oxygen level over a similar range can result in large changes in film composition and sheet resistance. These are important considerations in device applications that require tightly controlled etching steps for self-aligning processes. In terms of the MOS flatband voltage, the dielectric breakdown defect density and electron trap levels, VLSI grade argon leads to superior results over VLSI grade nitrogen when used for ramp-up, annealing and ramp-down steps in gate oxidation processing. It is speculated that this is due to the nitrogen reacting with silicon. Lower and tighter flatband voltages result from the use of argon. The dielectric breakdown defect density is reduced by a factor of 4 when argon replaces nitrogen. Even electron trapping is slightly reduced (15% at 1017/cm2 injection) when argon is used. In view of these results, it is surprising that there is not a more widespread use of argon rather than nitrogen in silicon oxidation processing. Using a newly developed, two-stage gas dilution system, reproducible and controllable calibration gases were prepared with impurity levels in the 10 ppb to 10 ppt range (from cylinder gas containing about 1 ppm impurity concentrations) for APIMS calibration. Calibration curves were generated at the ppb and ppt levels for CO2 in nitrogen and in argon and for CO in argon. This data, plotted logarithmically, fit reasonably well to straight lines. Using these calibration curves, APIMS ion voltage measurements can be related to a wide range of gas impurity concentrations. Acknowledgements

The author acknowledges the assistance of Dr. Carl Osburn, Dr. A. Reisman and the staff of the Microelectronics Center of North Carolina. Kinney Williams, of North Carolina State University provided the avalanche injection measurement apparatus and assistance in interpreting results. Also the support of Mr. W. Weltmer and Dr. R. Sherman, of The BOC Group Technical Center, is appreciated. The author would like to also express his thanks for the persistent support from the staff of the Tohoku University Super Cleanroom, including Prof. T. Ohmi, Prof. T. Shibata and Mr. K. Sugiyama. Also, the help of Osaka Sanso Kogyo Ltd. staff was indispensable. References [l] C. M. Osburn, H. Berger, semiconductor manufacturing

R. Donovan and G. Jones, The effect yield, J. Environ. Sci. 31(2) (1988) 45.

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[2] H. Berger and S-Y. Lin, Effects of inert gas purity on Ti silicide films, Proc. First Znt. Symp. on ULSZ Science and Technology, The Electrochemical Society, 87-11 (1987) 434.

[3] R. Sherman and H. Berger, ESCA results relating titanium silicide formation to gas purity, J. Vat. Sci. Technol. A5(4) (1987) 1418. [4] H. Berger and C. Tollin, A high purity inert gas system that insures Ti silicide reproducibility, SEMZCONIEast ‘87 Tech. Proc., 1987, p. 31. [S] C. Y. Ting, S. S. Iyer, C. M. Osburn, G. J. Hu and A. M. Schweighart, Proc. Zst Znt. Symp. on VLSI Science and Technology, 1982, p. 213. [6] W. Weltmerand W. Whitlock, U.S. Patent #4579723. [7] H. Berger, C. M. Osburn, A. Reisman, M. A. Graham and L. Lipkin, The effects of argon versus nitrogen in silicon gate oxidation processing, Airco Technology Report, RE150, Vol. 1, 1988. [8] C. M. Osburn and D. Ormond, Dielectric breakdown in SiOZ films on Si. (I) Measurement and interpretation, J. Electrochem. Sot. 119 (1972) 591. [9] C. M. Osburn and D. Ormond, Dielectric breakdown in Si02 films on Si. (II) Influence of processing and materials, J. Electrochem. Sot. 119 (1972) 597. [lo] E. H. Nicollian and J. R. Brews, MOS (Metal Oxide Semiconductor) Physics and Technology, Wiley, New York, 1982, p. 637. [ll] H. Kambara and I. Kanomata, Determination of impurities in gases by atmospheric pressure ionization mass spectrometry, Anal. Chem. 49(2) (1977) 270. [12] Y. Mitsui, H. Kambara, M. Kojima, H. Tomita, K. Katoh and K. Satah, Determination of trace impurities in highly purified nitrogen gas by atmospheric ionization mass spectrometry, Anal. Chem. 55(3) (1988) 477. [13] Data was first presented at Proc. 8th Symp. on ULSZ Ultra Clean Technology,

[14]

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[19]

[20]

Institute of Basic Semiconductor Technology Development (Ultra Clean Society), Tokyo, January 1989, p. 49. S. Mizogami, Y. Kunimoto and T. Ohmi, Ultra clean gas transport from manufacture to users by newly developed tank lorries and gas storage tanks, Proc. 9th Znt. Symp. on Contamination Control, September 1988, p. 352. K. Sugiyama and T. Ohmi, Part I: ULSI fab must begin with ultraclean nitrogen system, Microcontamination 6(11) (November 1988) 49. K. Sugiyama, T. Ohmi, T. Okumura and F. Nakahara, Electropolished, moisture-free piping surface essential for ultrapure gas system, Microcontamination 7(l), (January 1989) 37. H. Berger and J. Davidson, Advanced semiconductor processing at Tohoku University’s Laboratory for Microelectronics, Sol. State Technol. 12 (1989) 69. Y. Kanno and T. Ohmi, Development of contamination-free gas components and ultra clean gas supply system for ULSI manufacturing, Proc. 9th Znt. Symp. on Contamination Control, September 1988, p. 345. T. Muruyama, An ultraclean regulator, pressure gauge and pressure switch, in: T. Ohmi and T. Nitta (eds.), Ultra-High-Purity Gas Delivery Systems, Realize Inc., Tokyo, 1986, p. 40. H. Mihara, High quality mass flow controller, in: T. Ohmi and T. Nitta (eds.), Ultra Clean Technology Symposium No. 7, (Semiconductor Basic Technology Series), Realize Inc., Tokyo, July 1988, p. 137.