Materials: A low-temperature solution for silicon nitride deposition
04/01/2000
Overview
BTBAS (bis(tertiary-butylamino)silane) is a nonchlorine precursor able to deposit device-quality silicon nitride at a temperature 200°C below that of the conventional LPCVD process with dichlorosilane and ammonia. The low-temperature BTBAS process is well suited for temperature-sensitive front-end applications.
Ravi K. Laxman, Timothy D. Anderson, John A. Mestemacher, Schumacher, Carlsbad, California
The 1999 International Technology Roadmap for Semiconductors identifies several challenges for front-end-of-line (FEOL) applications in the near-term years (through 2005 at a 100nm technology node). Two of these challenging areas are advanced gate dielectrics and sidewall spacers [1]. For advanced gate dielectrics, thin films of higher dielectric constant material (>3.8) with good interface characteristics are critical. In the case of sidewall spacers, resistance to dopant permeability between the gate and source/drain contact is extremely important. In both of these applications, one of the demanding requirements is a lower thermal budget due to shrinking device geometries. Silicon nitride, silicon oxynitride, and silicon dioxide films are being evaluated as near-term solutions for these applications.
As the lateral and vertical dimensions are scaled down in ultra-large-scale integration applications, self-aligned metal silicide processes are used to lower the sheet resistance of the gate electrode and to lower the source/drain series resistance, thus increasing device performance and reducing RC delays. Low-temperature deposition of silicon nitride provides a number of benefits for this type of structure. Silicon nitride deposition below 600°C is compatible with the metal silicide applications, and silicon nitride films have superior performance as sidewall spacers in reducing junction leakage between gate and source/drain [2].
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Figure 1. Structure of bis(tertiary-butylamino)silane, BTBAS. Boiling point: 167°C; vapor pressure: 6.5torr at 45°C; density: 0.81g/ml; viscosity: <1.5cPs at 22°C.
Silicon nitride films used in the FEOL are currently deposited by LPCVD in a hot wall reactor at >750°C using dichlorosilane and ammonia [3]. The high deposition temperatures are typically employed to get the best film characteristics and higher deposition rates (20-25Å/min). High deposition temperatures also enhance the evaporation of ammonium chloride, a byproduct formed in the reaction of dichlorosilane and ammonia. The drawbacks of this process are the impact of the high process temperatures on thermal budgets, and the formation of ammonium chloride, which can cause particulate contamination. The ammonium chloride accumulates at the exhaust of the furnace system, in the plumbing lines, and in the pumping system. These deposits require frequent cleaning and result in significant down time for the reactors.
Alternate methods of silicon nitride deposition involve the reaction of silane and ammonia by LPCVD, plasma-enhanced chemical vapor deposition (PECVD), and jet vapor deposition [4]. An LPCVD batch process using silane results in poor uniformity, and the deposition is usually carried out at very high temperatures. PECVD processes for silicon nitride are typically used in passivation layers and are not preferred in FEOL applications due to plasma damage to the active regions of the device [4-6]. Jet vapor deposition and other processes result in extremely low deposition rates and throughputs.
An organosilicon alternative—BTBAS
The low-temperature (<600°C) LPCVD of high-purity silicon nitride films can be achieved using a chlorine-free organosilicon precursor, BTBAS. Attributes such as deposition uniformity, oxidation resistance, film density, electrical performance, and etch rates chemistry have been evaluated and optimized without sacrificing film quality [7, 8].
BTBAS (Fig.1) is a chlorine-free, nonpyrophoric, stable liquid with a vapor pressure of 6.5torr at 40-45°C (Fig. 2). Silicon nitride deposition using BTBAS offers the following advantages:
- a lower deposition temperature (550-600°C);
- a chlorine-free process that avoids ammonium chloride formation;
- the ability to deposit a variety of film compositions, (silicon nitride, silicon oxynitride, and silicon dioxide) under similar conditions by simply changing the reactant gases; and
- a safe liquid precursor with vapor delivery similar to that of TEOS.
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Figure 2. Vapor pressure of BTBAS and TEOS.
Delivery of BTBAS can be done by vapor draw or direct liquid injection. Since its vapor pressure is similar to that of TEOS (Fig. 2), standard TEOS delivery systems can be used. The process emissions consist primarily of ammonia and tert-butylamine and also include unreacted BTBAS and isobutylene. A point-of-use pH-controlled wet scrubber would be a suitable method for abatement of these emissions. Other factors to consider when making abatement decisions include ammonia efficiency, ammonia flow rates, number of processes run/unit time, and emissions from other processes.
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Figure 3. Deposition rate and activation energy of silicon nitride using BTBAS.
LPCVD silicon nitride films using BTBAS have been successfully developed by SVG Thermal Systems and Kokusai Semiconductor Equipment Corp. Other furnace manufacturers, including Tokyo Electron Ltd., have BTBAS programs currently under evaluation. The deposition of silicon nitride using BTBAS has also been implemented in device fabrication for production.
BTBAS deposition of silicon nitride
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In our experiments, 100-200nm-thick silicon nitride films were deposited in a 100mm horizontal LPCVD batch process. In this process, a 75-100 wafer load was monitored to evaluate the uniformity of films deposited with BTBAS. BTBAS and ammonia were metered through standard thermal mass flow controllers or pressure control flow meters. (Silicon nitride films have also been successfully deposited by others in 200mm vertical hot wall LPCVD batch furnaces, low-pressure CVD systems, and single-wafer deposition systems with similar film properties.) The deposited silicon nitride films (>1000Å) were analyzed by ellipsometry, Fourier transform infrared (FTIR), energy-dispersive x-ray, Auger electron spectroscopy, and Rutherford backscattering spectrometry (RBS). The properties for the films deposited by the LPCVD process are given in the table. The deposited films were compared with reference samples of Si3N4 deposited at 790°C in a 5:1 mixture of NH3 and SiH2Cl2.
In all of our experiments, BTBAS was delivered by vapor draw. Typical deposition conditions consisted of BTBAS flows in the range of 35-100sccm, ammonia:BTBAS ratios of 2:1 to 8:1, and a total pressure of 300-500mtorr.
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Figure 4. Conformal behavior of BTBAS-deposited silicon nitride films, with an aspect ratio of 3:1 and 0.30µm structures.
The silicon nitride films were deposited in the temperature range of 525-650°C, and the Arrhenius plot for the results shows an activation energy of 55.4kcal/mole (Fig. 3). A similar analysis of dichlorosilane:ammonia depositions gives an activation energy of 36kcal/mole [9]. A film thickness uniformity of <2% (3Sigma) can be obtained using a temperature ramp (as in the case of dichlorosilane:ammonia processes). The silicon nitride films that are deposited from BTBAS:ammonia are free of ammonium chloride and chlorine contamination.
The deposition rates, refractive indices, and carbon incorporation are correlated with the ammonia:BTBAS ratio. At very high ammonia ratios (>6:1), the deposition rates drop due to the dilution of BTBAS.
In this case, the refractive index falls below 1.96, which is indicative of nitrogen-rich films. A stoichiometric silicon nitride film with a refractive index of 1.96-2.00 was obtained at a 3:1 ammonia:BTBAS ratio. FTIR of the silicon nitride film deposited at 575°C shows a band at 823cm-1 indicative of Si-N stretch, and a small band at 2170cm-1 indicative of Si-H stretch. The hydrogen concentration in the films was <10 atomic percent as measured by hydrogen forward scattering. Wet etch rates using 10:1 H2O:(49%)HF indicate that the BTBAS silicon nitride films etch at 14Å/min, which is slower than standard silicon nitride obtained using dichlorosilane and ammonia (26Å/min) [10]. The etch resistance of BTBAS-derived films increases with deposition temperature and with increasing ammonia ratios. The films are also oxidation resistant as observed by infrared spectroscopy, even when subjected to oxygen at 1000°C. Silicon nitride films deposited using BTBAS contain low carbon content as indicated by secondary mass ion spectrometry (SIMS) and Auger electron spectroscopy (AES). The remaining low carbon levels were found to have no detrimental effect on the electrical properties of the devices. Residual gas mass spectral analysis and FTIR spectral analysis of reactor effluent confirm that isobutylene and tertiary-butylamine are the decomposition products. Silicon nitride films deposited using BTBAS have excellent conformal behavior (Fig. 4), a significant improvement over silane and dichlorosilane processes.
BTBAS deposition of silicon oxynitride and SiO2
Deposited silicon dioxide and silicon oxynitride films are desirable in a variety of advanced applications such as shallow trench isolation (STI), antireflective coatings (ARCs), and DRAM capacitors. STI involves the deposition of dielectrics into silicon trenches separating active areas [11]. Deposition of defect-free conformal silicon dioxide at low temperatures reduces stress and warpage in tight geometries. For ARCs, silicon oxynitride films (SixOyNz) have been used. Absorption and reflection of light depends on the refractive index and extinction coefficient of the film. In this regard, silicon oxynitride is a versatile material, since the refractive index and extinction coefficient can be tailored by changing the composition of the film. It is advantageous if the desired film composition can be obtained without the need for subsequent thermal or plasma treatment.
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Figure 5. Deposition rate of silicon dioxide using BTBAS with an oxygen:BTBAS ratio of 2:1 and pressure of 250mtorr.
For conventional LPCVD of silicon dioxide, a variety of processes are used, including silane and oxygen above 400°C, dichlorosilane and oxygen above 800°C, and TEOS above 650°C. Deposition of silicon oxynitride has been obtained using dichlorosilane, N2O, and NH3 above 750°C [4-6].
BTBAS can be used for low-temperature LPCVD of both silicon dioxide and silicon oxynitride by varying the reactant gases. These films can be deposited in horizontal/vertical hot wall LPCVD batch furnaces, subatmospheric and atmospheric pressure CVD systems, or single-wafer deposition systems.
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Figure 6. Refractive index changes in silicon oxynitride films deposited with BTBAS.
The deposition of silicon dioxide and silicon oxynitride using BTBAS was carried out at pressures in the range of 250-400mtorr. BTBAS and mixtures of reactant gases (such as O2, N2O, or NH3) were metered into the reaction chamber using standard flow controllers. Silicon dioxide films using BTBAS and O2 can deposit in reactor systems at 500-800°C. For our low-temperature studies, the deposition was carried out at 525-625°C. A typical process was carried out in a 150mm hot wall LPCVD horizontal tube furnace with a 90 silicon wafer load. Films were analyzed with oxygen:silicon ratios from 1.5:1 to 9:1. Refractive indices for these films were measured by ellipsometry at 632.4nm, and the refractive indices ranged from 1.47 to 1.51. The silicon dioxide films were also characterized by refractive index, wet etch rate, SIMS, and AES. Figure 5 shows the temperature dependence of the silicon dioxide deposition rate.
Silicon oxynitride films were fabricated by reacting BTBAS with controlled mixtures of N2O, O2, and NH3. Films with the desired O/N ratios can be easily obtained by changing the ratios of the reactant gas mixtures (Fig. 6). Surprisingly, silicon dioxide is not formed by adding NO or N2O to BTBAS at these low temperatures, which is in contrast to oxide formation using dichlorosilane and N2O.
Conclusion
BTBAS (bis(tertiary-butylamino)silane) is a nonchlorine precursor for the deposition of device-quality silicon nitride at temperatures 200°C below that of the conventional dichlorosilane:ammonia LPCVD process. BTBAS can also be used for deposition of silicon dioxide and silicon oxynitride. The results show good conformal behavior, and in the case of the oxynitride, the ability to control the oxygen and nitrogen contents. The low-temperature BTBAS process is well suited for temperature sensitive front-end applications.
Acknowledgments
The authors thank Arthur K. Hochberg and Kirk S. Cuthill for technical discussions and support in developing the process.
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Ravi K. Laxman received his PhD in chemistry from the University of Missouri, St. Louis. He was a senior principal research chemist developing new dielectric materials at Schumacher. Laxman is currently a technologist in PECVD dielectrics at Novellus Systems Inc., San Jose.
Timothy D. Anderson received his BS in business and economics from Westmont College, Santa Barbara, CA. He is a senior product specialist at Schumacher, responsible for the marketing and management of Schumacher's BTBAS program. Schumacher, 1969 Palomar Oaks Way, Carlsbad, CA 92009; ph 760/929-6278, fax 760/929-6276, e-mail [email protected].
John A. Mestemacher received his BS in physics from San Jose State University. He has been a diffusion process engineer at Monolithic Memories Inc. and Precision Monolithics Inc. He is marketing manager at Schumacher.