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Parametric study and residual gas analysis of large-area silicon-nitride thin-film deposition by plasma-enhanced chemical vapor deposition
Vacuum 165 (2019) 172–178
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Parametric study and residual gas analysis of large-area silicon-nitride thinfilm deposition by plasma-enhanced chemical vapor deposition
T
Dong Xianga, Huanxiong Xiab,∗, Wang Yanga, Peng Moua a b
Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing, 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: PECVD Silicon-nitride Thin film Process parameter Residual gas
Large-area silicon-nitride thin films deposited from silane and ammonia by plasma-enhanced chemical vapor deposition are investigated experimentally in a 300 mm apparatus with a vertical showerhead. The responses of deposition rate and refractive index to the process parameters are found and discussed. The effects of showerhead configuration on the full-wafer deposition rate and refractive index are further examined, and the inherent non-uniformity is improved by using a proper convex showerhead. The residual gases are analyzed online, and a good correlation between the partial pressure of hydrogen and the deposition rate is found.
1. Introduction Silicon-nitride thin film has been widely used in semiconductor device, solar cell, MEMS, and surface coating, etc. due to its excellent performances, such as chemical passivation and thermal stability, electrical properties, anti-reflection, and good mechanical properties. Silicon-nitride thin film deposited from the silane-ammonia system by plasma-enhanced chemical vapor deposition (PECVD) is a widely employed technology, which has been developed for several decades. However, understanding and controlling a PECVD thin-film process precisely are still not easy due to the complex mechanism involving in complicate species and reactions in the plasmas as well as the complicated dependencies of film's qualities on process parameters. Many works on silicon-nitride thin films deposited from the silane-ammonia system by PECVD have been done [1–6], and how the deposition rate (DR) and the refractive index (RI) depend on the process parameters has been reported. However, the reported responses of the DR and RI to the process parameters are different more or less due to the differences in process windows and apparatuses. A qualitative conclusion that DR is sensitive to process pressure, electrode gap, RF frequency and power, substrate temperature as well as the gas ratio of silane to ammonia is supported by most reports [7–11], but the response trends are not exactly the same and even opposite to some process parameters. It was also validated qualitatively by most work that RI is sensitive to the substrate temperature and the gas ratio of silane to ammonia. Bierner et al. [12] explained a mechanism that the ratio of Si-H to N-H in the film accounts for the RI intensity. The densities of Si-H and N-H further
∗
strongly depend on the ratio of the mass-flow rate SiH4 to NH3 (SiH4/ NH3) [13]. However, a regular response of RI to the other process parameters, such as pressure and RF power, is hardly supported. The uniformity of a specific thin-film quality can be evaluated from the corresponding full-wafer quality, which is governed by the spatial profiles representing the fluid flow, the temperature, the plasmas as well as the concentrations of radicals in a reactor. Dollet et al. [14] investigated the concentration profiles of precursors in the silicon-nitride plasma process along the gas-flow direction in a longitudinal reactor by a simple bidimensional model and then compared with experimental measurements in their following work [15]. Caquineau and Despax [16] summarized a few regular dependencies between the precursor profiles and the full-wafer DR and ratio of silicon to nitrogen (Si/N) of the film in reactors with different mass-transportation types. Bavafa et al. [17] developed a comprehensive model for the siliconnitride process and examined the effects of the process parameters on the percentages of the precursors and then on the DR profile. A multimodel simulation for a large-area silicon-nitride thin film was done by Xia et al. [18], and the dependencies of the profiles of the precursors on the full-wafer DR and Si/N were experimentally validated. These studies further suggest that the full-wafer DR and Si/N can be finely controlled by adjusting those spatial profiles. Xiang et al. [19] showed an idea via simulations, where the distributions of the fluid, thermal and plasma profiles can be adjusted as expectation respectively by designing a sophisticated showerhead configuration. This idea was validated in Ref. [20], where the profiles of the full-wafer DR and Si/N were adjusted by using different showerhead configurations. This
Corresponding author. E-mail address: [email protected] (H. Xia).
https://doi.org/10.1016/j.vacuum.2019.04.017 Received 13 February 2019; Received in revised form 11 April 2019; Accepted 11 April 2019 Available online 17 April 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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approach presented a potentially feasible way to control the full-wafer qualities for a large-area thin film. Lee et al. [21] also adjusted the deposition of amorphous carbon layer by changing the hole density of the showerhead, and the profile responses of the fluid, thermal transfer and residence time were further examined. In addition, to avoid involving in the complex physical and chemical mechanisms, an intelligent neural-network algorithm trained by using process data was applied to model the DR for the silicon-nitride PECVD process [22], and both DR and RI [23]. Real-time process state is needed in feedback control. Spectroscopy and mass spectrometry are usually used as in-situ sensors to monitor process state and explore process mechanism during thin film deposition and etching [24,25]. Finding an indicator that is sensitive to the process state as well as showing a regular response is significant for those online analyses. There is a fact that the density of hydrogen bond in a silicon-nitride thin film does great impact on deposition performances and film's properties. The density of hydrogen bond is close to the hydrogen generation in the plasma chemical reactions, whose content can be obtained qualitatively from residual gases by using mass spectrometry. Chowdhury et al. [25] analyzed the residual gases of the silicon-nitride deposition and found a good linear correlation between SiH4 consumption and H2 generation. We believe that H2 generation should tell us more about the fact of the thin film deposited from the silane-ammonia system. This work is done for a high-quality large-area silicon-nitride thin film deposition with 300 mm wafer, focusing on the studies of process “parameters” and residual gases. We investigate the effects of two kinds of “parameters” on the film deposition, including control parameters and geometry structures, which is aiming at improving the film quality by parameter control and structure design. We also show that the mass spectrum of residual gases can be valuable information to monitor realtime process state, which could be useful for controlling the process. In the first section of this work, the dependencies of the DR and RI on the process parameters for the large-area silicon-nitride thin film deposited by a vertical capacitively-coupled-plasmas apparatus are examined, and the inherent non-uniformity of the full-wafer quality is then optimized by a profile-adjustment approach. In the final section, the residual gases are analyzed and an indicator of the silicon-nitride process is found.
Table 1 Values of the selected parameters for the single-variable experiment. Process parameters
Value
Pressure [Torr] Gap [mm] RF power [W] Temperature [°C] SiH4 [sccm]
1.5 13 400 200 220
Benchmark 1.8 14 500 250 250
2.0 15 600 300 280
2.2 16 700 350 310
2.5 17 800 400 340
2.0 15 600 400 280
region between the two electrodes. A single-variable experiment was carried out to examine the effects of the process parameters on the deposition rate and the refractive index of the thin films. Firstly, we obtained a benchmark recipe, where the pressure is 2 Torr, electrode gap 15 mm, RF (radio frequency) power 600 W and frequency 13.56 MHz, substrate temperature 400 °C, massflow rate of SiH4 280 sccm and NH3 2340 sccm, respectively. The RF is powered after 30 s when the working gases are fed, lasting 60 s. The process pressure, electrode gap, RF power, substrate temperature and mass-flow rate of SiH4 are selected as the variables, and only one process variable in the benchmark recipe is changed in each experiment. Table 1 lists the values of the parameters for the single-variable experiment. In addition, the geometric effects were also examined and the DR uniformity was optimized by varying the drilling depth of the arrayed stepped holes on the showerhead faceplate. Here, three comparative showerheads with different drilling depths were prepared, where their profiles are flat, concave and convex, respectively. Through this “special parameter” experiment, the effects of the showerhead configurations on the DR and RI of the thin films were found, and the DR uniformity was then optimized by using a properly designed showerhead. The original data of the full-wafer DR and RI were measured by GES-5E ellipsometer (SEMILAB SOPRA) from 129 points on each wafer. The residual gases were analyzed online by RGA (V2000-C supplied by MSK INSTRUMENTS) when the single-variable experiment was running. All the gaseous species whose molecular weights are in the range of 1–100 amu (atomic mass unit) were scanned, and their partial pressures were then obtained. The relationships between the partial pressures and the corresponding thin films were examined.
2. Experiment 3. Results and discussions The silicon-nitride thin films were prepared by a PECVD reactor and deposited from SiH4 and NH3. Fig. 1 shows the schematic of the cylindrical reactor with a residual gas analyzer (RGA). The apparatus is mainly composed of a vertical showerhead, pedestal, focus ring, and ceramic lining. The showerhead, including a perforated plate and a porous faceplate, serves as both a gas-homogenized chamber and the powered electrode. The pedestal is also versatile, both as a heater and the grounded electrode. The RGA sensor is connected to the chamber through a hole on the lateral and located lower than the discharge
3.1. Effects of process parameters The DR and the RI obtained from the single-variable experiment versus the process parameters are plotted in Fig. 2, where the values for each recipe are the averages of 129 original data on the whole wafer, and the bilateral error bars are evaluated by using the one standard deviation. Fig. 2(a)-(c) shows the responses of the DR and RI to the process pressure, the electrode gap, and the RF power, respectively. The DRs increase almost linearly from 1280 to 2500 Å/min, 1580–2200 Å/ min, and 1610–2150 Å/min as the pressure increases from 1.5 to 2.5 Torr, the gap from 13 to 17 mm and the power from 400 to 800 W, respectively, while the RIs present an irregular and insensitive response and vary in a range of 2.00–2.08. Both the DR and the RI show a monotonous response to the process temperature and the mass-flow rate of SiH4 as shown in Fig. 2(d) and (e). The DR shows a negative response, decreasing from 3180 to 1900 Å/min, as the temperature increases from 200 to 400 °C, and the rate of decrease somewhat becomes slower, while the RI shows a positive response, increasing from 1.83 to 2.07, and the rate of increase somewhat becomes faster. For the mass-flow rate of SiH4, both the DR and the RI show a positive response and increase from 1760 to 2210 Å/min and 1.93–2.16, respectively. The rate of increase of the DR becomes faster, while it becomes slower for the RI. As the total mass-flow rate of the working gases keeps constant, the
Fig. 1. A schematic of a 300 mm PECVD reactor with a vertical showerhead. 173
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Fig. 2. Deposition rate and refractive index versus (a) process pressure, (b) electrode gap, (c) RF power, (d) process temperature and (e) mass-flow rate of SiH4.
rate keeps constant since the ratio of Si in the reaction chains will be oversaturated. There are two explanations to account for the variants of the RI. One is discussed in Refs. [12,13] that RI mainly depends on the ratio of the chemical bonds Si-H to N-H in a silicon-nitride thin film, and RI is higher in a silicon-rich film and lower in a nitrogen-rich film; the other is in Ref. [10] that RI is higher in a denser film. In fact, there is relevance connecting these two explanations, that is, more combinations among Si-free and N-free bonds result in tighter chemical networks then denser film [5]. Increasing the mass-flow rate of SiH4 contributes more Si-free to the thin film, and the RI thus increases. Increasing the substrate temperature results in more hydrogen released from the film, and the Si-free and N-free bonds then increase, as a consequent, the film is densified and the RI increases. Compared with the responses to the mass-flow rate of SiH4 and the substrate temperature, the responses of RI to the pressure, gap, and RF power seem insensitive and their variants are much smaller. The insensitive response of RI to RF power is supported by Ref. [1], where additionally shows a slightly positive response to pressure but negative in Ref. [5]. The explanation is likely
densities of radicals in the plasmas become larger if the process pressure increases, and the mass diffusion flux adsorbing the wafer surface then increases, producing more mass contributing to the thin film. The residence time of the radicals becomes longer since the effective volume for the reaction region becomes larger as the electrode gap is raised, as a consequence, the radicals have more time to adsorb the wafer surface, yielding more mass for the thin film. As the RF power increases, the concentrations of the radicals increase, resulting in more radicals reacting with the wafer surface and yielding more thin-film mass as well. The effect of the substrate temperature on the DR does not follow a temperature-dependent Arrhenius relation, which results from increased desorption of precursors with higher temperature. This indicates that the effect of adsorption-limited growth is stronger than surface-reaction limited growth in the silicon-nitride PECVD process. In the case increasing the mass-flow rate of SiH4, the positive response of the DR indicates that the SiH4 is not saturated in those recipes. Here, we have been aware of the results reported by El Amrani et al. [3] and Osenbach et al. [6] that the DR, in fact, increases first and then decreases if SiH4/NH3 continuously increases while the total mass-flow 174
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Fig. 3. Dimensions and configurations of the showerhead: (a) a close-up of the cross-section and (b) a top view of the showerhead faceplate, (c) flat showerhead, (d) convex showerhead and (d) concave showerhead.
more convex DR profile and a concave RI profile, while the convex showerhead gives a concave DR profile and a more convex RI profile. The results indicate that where the drilling depth is deeper the rightbelow DR is suppressed but the RI upraised. The mechanism accounting for the effects of showerhead on the deposition phenomena is related to the behaviors and characteristics of the precursors in SiH4-NH3 system. Varying the stepped holes on a showerhead faceplate directly affects the mass-transportation phenomena [26] and then the chemical reaction chains [18], finally the DR and RI. The precursors in SiH4-NH3 system can be classified into two categories: one is early-stage precursor, such as SiHx < 4 and NHx < 3, generated by electron impact and the other is late-stage precursor, such as Hx < 3Si(NH2)y < 4, produced by subsequent bulk chemical reactions. The late-stage precursors have more mass due to their larger formula weight as well as more N-free bond due to the amino in the aminosilane. We can conclude that increasing the late-stage precursors can obtain more thin film with more N- bond. According to the above explanations for deposition rate and reflective index, that will result in higher DR and lower RI. A stepped hole with smaller drilling depth has larger flow resistance and slower right-below gas velocity and thus leads to longer residence time. Consequently, the radicals have more time to go deeper in the chemical reaction chains, generating more latestage precursors Hx < 3Si(NH2)y < 4, contributing more mass with more N- bonds to the thin film, resulting in higher deposition rate but lower refractive index. The effects of the showerhead configuration on the full-wafer DR and RI imply that a showerhead with a properly designed convex profile can appropriately suppress the DR in the center region and upraise in the edge region and does the opposite effects on the RI profile, therefore the uniformity can be improved. We prepared a new convex showerhead, shown in Fig. 5(a), with a smaller slope due to the over adjustment by the above-mentioned convex showerhead. Fig. 5(b)
that the ratio of Si-free and N-free bonds does not change too much with the pressure and power changes since no one changes much larger than the other.
3.2. Effects of geometric parameters Full-wafer property of a thin film implies its local non-uniformity, which points out the direction where the local property should be upraised or suppressed. Here, we continue our previous work [20] to examine the effects of the showerhead configuration on the full-wafer DR and RI, and then show an approach experimentally to improve the uniformity by changing the showerhead configuration. The faceplate of the showerhead used in the experiment is shown in Fig. 3, where stepped holes are arrayed as an ortho-hexagonal pattern with 5 mm length of each side. The total length, that is the thickness of the faceplate, is 8 mm, and the drilling-depth is variable, which allows us to fabricate a flat, convex and concave showerhead, as shown in Fig. 3(c), (d) and (e), by changing the drilling-depth profile. The full-wafer DR and RI obtained from the above-mentioned recipe, pressure 2 Torr, electrode gap 15 mm, RF power 600 W, substrate temperature 200 °C, mass-flow rate of SiH4 280 sccm and NH3 2340 sccm for example, using a flat showerhead are plotted in Fig. 4(b), and the corresponding profile of the drilling-depth of the stepped holes along the radial direction on the showerhead faceplate is shown in Fig. 4(a). The results of the DR and the RI both show obvious convex full-wafer profiles, where the values in the central region are higher than those in the edge region. As the comparative cases, two experiments using the same recipe but different showerhead configurations, shown in Fig. 4(a), are carried out, where one showerhead is convex and the other is concave, and the drilling depths of the stepped holes vary from 3 mm to 6 mm linearly according to their radial positions. The data in Fig. 4(b) show that the concave showerhead produces a 175
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Fig. 4. Comparative experiments with different showerhead configurations: (a) the drilling-depth profiles of the stepped holes along the radial direction on the showerhead faceplate, and (b) the deposition-rate and refractive-index profiles obtained by using the corresponding showerhead configurations. The flat profile indicates the depths are uniform, the convex shows that the depths in the center are deeper than those at the edge, and the concave is opposite.
shows that the DR is upraised in the center region and suppressed in the edge region in a certain degree, and the uniformity is significantly improved. The RI presents a much more inert response, however, a slight decrement in the center region and increment in the edge region still can be seen. 3.3. Residual gases analysis In the single-variable experiment, the working gases keep feeding in 0–90 s, the RF is powered in 30–90 s, and the mass spectrums in the range of 1–100 amu/charge of the residual gases are scanned in the whole process. Fig. 6 shows the partial pressures of the residual gases in the benchmark recipe with the flat showerhead. The spectrums for the other recipes are similar except the exact values of the partial pressures. The spectrums in 51–100 amu/charge are not shown due to their extremely low signals. The flags in Fig. 6 point out the species, where the secondary or more charged species are ignored due to their much lower concentrations. Three significant peaks, located at 1–2 (hydrogen peak), 15–17 (ammonia peak) and 28–32 (silane peak), representing the species H, H2, NH, NH2, NH3, Si, SiH, SiH2, SiH3, SiH4 produced from the silane-ammonia system, can be clearly seen. Here, we notice that the species except for NH3 and SiH4 can also be detected in the first 30 s even though the RF is not powered. Those species are generated from charging the working gases in the RGA as their ratios of mass to
Fig. 6. The mass spectrums of the residual gases obtained from the benchmark recipe, where the pressure is 2 Torr, electrode gap 15 mm, RF power 600 W and frequency 13.56 MHz, substrate temperature 400 °C, mass-flow rate of SiH4 280 sccm and NH3 2340 sccm, respectively. The working gases keep feeding in the whole time 0–90 s, and the RF is powered in 30–90 s. The species of the residual gases are scanned in the range of 1–100 amu/charge, and the spectrums in 51–100 amu/charge are not shown due to their extremely low signals.
Fig. 5. Improving the deposition-rate uniformity by properly designing the slop of the drilling-depth profiles: (a) two convex showerhead, where the slope of the “convex−” one is smaller than that of the “convex” one, and (b) the deposition-rate and refractive-index profiles obtained by using the corresponding showerhead configurations. 176
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deposition rate is very good. The positive correlation between H2 and DR is likely related to H radicals, which could become growth sites on the substrate or deposited film and promote film growth, and recombination of H radicals to H2 thus decreases. 4. Conclusion The responses of the deposition rate and the refractive index to the process parameters for the large-area silicon-nitride thin film deposited from the silane-ammonia system are investigated. The deposition rate shows a positive linear response to the process pressure, electrode gap, RF power and mass-flow rate of silane, and a negative linear response to the substrate temperature. The refractive index shows a positive response to the substrate temperature and the mass-flow rate of silane, while an insensitive response to the process pressure, electrode gap, and RF power. The showerhead configuration has a significant effect on the full-wafer deposition rate and refractive index, and the inherent nonuniformity of the deposition rate can be optimized by using a proper convex showerhead. We believe that this approach can also be applied to optimize the full-wafer refractive index and extended to other properties. The partial pressure of hydrogen in the residual gases has a good correlation to the average deposition rate, which allows us to choose a valuable indicator from the residual gases to the silicon-nitride plasmas process. This finding could be useful for monitoring process state and real-time process control.
Fig. 7. The deposition rate (solid line) and the time-average partial pressure of H2+ (dash line) in the deposition time 30–90 s versus the normalized process parameters obtained from the single-variable experiment, where the range of the process pressure is 1.5–2.5 Torr, gap 13–17 mm, RF power 400–800 W, substrate temperature 200–400 °C and mass-flow rate of SiH4 220–340 sccm.
charge are scanned. The aminosilane species, Hx < 3Si(NH2)y < 4, detected by Smith et al. [27] using mass spectrometry and defined as precursors in the chemical-reaction chains by Kushner [28], are not distinct here when the RF is on. This probably results from their low concentrations and further dissipation leaving the deposition region, such as dissociated in the RGA device and adsorbing surfaces due to their high activity. In addition, the species H has very low partial pressure in the whole process time, which likely results from the most H adsorbing a surface or combining as H2 due to their high activity. The responses of the peaks to the RF status are obvious. The silane peak rapidly increases and keeps at the highest level before the RF is powered and then goes down as the RF is on. The decrements of SiH3 and SiH2 are much more than the other species, while SiH4 is always in low partial pressure during the processing time. These indicate that SiH4 is of high usage ratio in the deposition. The ammonia peak, however, shows a different characteristic, where NH3 is of the highest partial pressure and the change of NH is slight. Hence, the usage ratio of NH3 is much lower than SiH4, and NH2 is one of the important precursors. The hydrogen peak is not obvious in 0–30 s while dramatically increases once the RF is powered and keeps steady essentially. Here, the hydrogen peak presents a very clean signal: RF sensitive, stable response, and no any other mixed species due to the low content of H. The response characteristics of the hydrogen peak and the silane peak observed here are agreed with those reported in Ref. [25]. It is distinct that the silane related signal shows a considerable decrease and the hydrogen-related signal shows a sharp increase when the plasma is on and both present opposite responses if the plasma is off. To further examine the characteristics of the hydrogen peak, we compare the steady partial pressures of H2 during the deposition time and the corresponding DR obtained from all the recipes listed in Table 1. The DR and the partial pressure of H2 are plotted together versus the normalized process parameters in Fig. 7. It is clear that the partial pressure of H2 presents a similar response to the process parameters as the DR does. We further compute their correlation coefficients (ρ(X,Y) = cov(X,Y)/(σXσY), where X, Y are two samples, ρ is the correlation coefficient, σ is the standard variance, and cov is computing the covariance) for each recipe and get 0.9876, 0.9831, 0.9852, 0.9925 and 0.9973 for the process pressure, electrode gap, RF power, substrate temperature and mass-flow rate of SiH4, respectively. These results indicate that the correlation between the partial pressure of H2 and the
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Parametric study and residual gas analysis of large-area silicon-nitride thinfilm deposition by plasma-enhanced chemical vapor deposition
T
Dong Xianga, Huanxiong Xiab,∗, Wang Yanga, Peng Moua a b
Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China School of Mechanical Engineering, Beijing Institute of Technology, Beijing, 100081, China
A R T I C LE I N FO
A B S T R A C T
Keywords: PECVD Silicon-nitride Thin film Process parameter Residual gas
Large-area silicon-nitride thin films deposited from silane and ammonia by plasma-enhanced chemical vapor deposition are investigated experimentally in a 300 mm apparatus with a vertical showerhead. The responses of deposition rate and refractive index to the process parameters are found and discussed. The effects of showerhead configuration on the full-wafer deposition rate and refractive index are further examined, and the inherent non-uniformity is improved by using a proper convex showerhead. The residual gases are analyzed online, and a good correlation between the partial pressure of hydrogen and the deposition rate is found.
1. Introduction Silicon-nitride thin film has been widely used in semiconductor device, solar cell, MEMS, and surface coating, etc. due to its excellent performances, such as chemical passivation and thermal stability, electrical properties, anti-reflection, and good mechanical properties. Silicon-nitride thin film deposited from the silane-ammonia system by plasma-enhanced chemical vapor deposition (PECVD) is a widely employed technology, which has been developed for several decades. However, understanding and controlling a PECVD thin-film process precisely are still not easy due to the complex mechanism involving in complicate species and reactions in the plasmas as well as the complicated dependencies of film's qualities on process parameters. Many works on silicon-nitride thin films deposited from the silane-ammonia system by PECVD have been done [1–6], and how the deposition rate (DR) and the refractive index (RI) depend on the process parameters has been reported. However, the reported responses of the DR and RI to the process parameters are different more or less due to the differences in process windows and apparatuses. A qualitative conclusion that DR is sensitive to process pressure, electrode gap, RF frequency and power, substrate temperature as well as the gas ratio of silane to ammonia is supported by most reports [7–11], but the response trends are not exactly the same and even opposite to some process parameters. It was also validated qualitatively by most work that RI is sensitive to the substrate temperature and the gas ratio of silane to ammonia. Bierner et al. [12] explained a mechanism that the ratio of Si-H to N-H in the film accounts for the RI intensity. The densities of Si-H and N-H further
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strongly depend on the ratio of the mass-flow rate SiH4 to NH3 (SiH4/ NH3) [13]. However, a regular response of RI to the other process parameters, such as pressure and RF power, is hardly supported. The uniformity of a specific thin-film quality can be evaluated from the corresponding full-wafer quality, which is governed by the spatial profiles representing the fluid flow, the temperature, the plasmas as well as the concentrations of radicals in a reactor. Dollet et al. [14] investigated the concentration profiles of precursors in the silicon-nitride plasma process along the gas-flow direction in a longitudinal reactor by a simple bidimensional model and then compared with experimental measurements in their following work [15]. Caquineau and Despax [16] summarized a few regular dependencies between the precursor profiles and the full-wafer DR and ratio of silicon to nitrogen (Si/N) of the film in reactors with different mass-transportation types. Bavafa et al. [17] developed a comprehensive model for the siliconnitride process and examined the effects of the process parameters on the percentages of the precursors and then on the DR profile. A multimodel simulation for a large-area silicon-nitride thin film was done by Xia et al. [18], and the dependencies of the profiles of the precursors on the full-wafer DR and Si/N were experimentally validated. These studies further suggest that the full-wafer DR and Si/N can be finely controlled by adjusting those spatial profiles. Xiang et al. [19] showed an idea via simulations, where the distributions of the fluid, thermal and plasma profiles can be adjusted as expectation respectively by designing a sophisticated showerhead configuration. This idea was validated in Ref. [20], where the profiles of the full-wafer DR and Si/N were adjusted by using different showerhead configurations. This
Corresponding author. E-mail address: [email protected] (H. Xia).
https://doi.org/10.1016/j.vacuum.2019.04.017 Received 13 February 2019; Received in revised form 11 April 2019; Accepted 11 April 2019 Available online 17 April 2019 0042-207X/ © 2019 Elsevier Ltd. All rights reserved.
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approach presented a potentially feasible way to control the full-wafer qualities for a large-area thin film. Lee et al. [21] also adjusted the deposition of amorphous carbon layer by changing the hole density of the showerhead, and the profile responses of the fluid, thermal transfer and residence time were further examined. In addition, to avoid involving in the complex physical and chemical mechanisms, an intelligent neural-network algorithm trained by using process data was applied to model the DR for the silicon-nitride PECVD process [22], and both DR and RI [23]. Real-time process state is needed in feedback control. Spectroscopy and mass spectrometry are usually used as in-situ sensors to monitor process state and explore process mechanism during thin film deposition and etching [24,25]. Finding an indicator that is sensitive to the process state as well as showing a regular response is significant for those online analyses. There is a fact that the density of hydrogen bond in a silicon-nitride thin film does great impact on deposition performances and film's properties. The density of hydrogen bond is close to the hydrogen generation in the plasma chemical reactions, whose content can be obtained qualitatively from residual gases by using mass spectrometry. Chowdhury et al. [25] analyzed the residual gases of the silicon-nitride deposition and found a good linear correlation between SiH4 consumption and H2 generation. We believe that H2 generation should tell us more about the fact of the thin film deposited from the silane-ammonia system. This work is done for a high-quality large-area silicon-nitride thin film deposition with 300 mm wafer, focusing on the studies of process “parameters” and residual gases. We investigate the effects of two kinds of “parameters” on the film deposition, including control parameters and geometry structures, which is aiming at improving the film quality by parameter control and structure design. We also show that the mass spectrum of residual gases can be valuable information to monitor realtime process state, which could be useful for controlling the process. In the first section of this work, the dependencies of the DR and RI on the process parameters for the large-area silicon-nitride thin film deposited by a vertical capacitively-coupled-plasmas apparatus are examined, and the inherent non-uniformity of the full-wafer quality is then optimized by a profile-adjustment approach. In the final section, the residual gases are analyzed and an indicator of the silicon-nitride process is found.
Table 1 Values of the selected parameters for the single-variable experiment. Process parameters
Value
Pressure [Torr] Gap [mm] RF power [W] Temperature [°C] SiH4 [sccm]
1.5 13 400 200 220
Benchmark 1.8 14 500 250 250
2.0 15 600 300 280
2.2 16 700 350 310
2.5 17 800 400 340
2.0 15 600 400 280
region between the two electrodes. A single-variable experiment was carried out to examine the effects of the process parameters on the deposition rate and the refractive index of the thin films. Firstly, we obtained a benchmark recipe, where the pressure is 2 Torr, electrode gap 15 mm, RF (radio frequency) power 600 W and frequency 13.56 MHz, substrate temperature 400 °C, massflow rate of SiH4 280 sccm and NH3 2340 sccm, respectively. The RF is powered after 30 s when the working gases are fed, lasting 60 s. The process pressure, electrode gap, RF power, substrate temperature and mass-flow rate of SiH4 are selected as the variables, and only one process variable in the benchmark recipe is changed in each experiment. Table 1 lists the values of the parameters for the single-variable experiment. In addition, the geometric effects were also examined and the DR uniformity was optimized by varying the drilling depth of the arrayed stepped holes on the showerhead faceplate. Here, three comparative showerheads with different drilling depths were prepared, where their profiles are flat, concave and convex, respectively. Through this “special parameter” experiment, the effects of the showerhead configurations on the DR and RI of the thin films were found, and the DR uniformity was then optimized by using a properly designed showerhead. The original data of the full-wafer DR and RI were measured by GES-5E ellipsometer (SEMILAB SOPRA) from 129 points on each wafer. The residual gases were analyzed online by RGA (V2000-C supplied by MSK INSTRUMENTS) when the single-variable experiment was running. All the gaseous species whose molecular weights are in the range of 1–100 amu (atomic mass unit) were scanned, and their partial pressures were then obtained. The relationships between the partial pressures and the corresponding thin films were examined.
2. Experiment 3. Results and discussions The silicon-nitride thin films were prepared by a PECVD reactor and deposited from SiH4 and NH3. Fig. 1 shows the schematic of the cylindrical reactor with a residual gas analyzer (RGA). The apparatus is mainly composed of a vertical showerhead, pedestal, focus ring, and ceramic lining. The showerhead, including a perforated plate and a porous faceplate, serves as both a gas-homogenized chamber and the powered electrode. The pedestal is also versatile, both as a heater and the grounded electrode. The RGA sensor is connected to the chamber through a hole on the lateral and located lower than the discharge
3.1. Effects of process parameters The DR and the RI obtained from the single-variable experiment versus the process parameters are plotted in Fig. 2, where the values for each recipe are the averages of 129 original data on the whole wafer, and the bilateral error bars are evaluated by using the one standard deviation. Fig. 2(a)-(c) shows the responses of the DR and RI to the process pressure, the electrode gap, and the RF power, respectively. The DRs increase almost linearly from 1280 to 2500 Å/min, 1580–2200 Å/ min, and 1610–2150 Å/min as the pressure increases from 1.5 to 2.5 Torr, the gap from 13 to 17 mm and the power from 400 to 800 W, respectively, while the RIs present an irregular and insensitive response and vary in a range of 2.00–2.08. Both the DR and the RI show a monotonous response to the process temperature and the mass-flow rate of SiH4 as shown in Fig. 2(d) and (e). The DR shows a negative response, decreasing from 3180 to 1900 Å/min, as the temperature increases from 200 to 400 °C, and the rate of decrease somewhat becomes slower, while the RI shows a positive response, increasing from 1.83 to 2.07, and the rate of increase somewhat becomes faster. For the mass-flow rate of SiH4, both the DR and the RI show a positive response and increase from 1760 to 2210 Å/min and 1.93–2.16, respectively. The rate of increase of the DR becomes faster, while it becomes slower for the RI. As the total mass-flow rate of the working gases keeps constant, the
Fig. 1. A schematic of a 300 mm PECVD reactor with a vertical showerhead. 173
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Fig. 2. Deposition rate and refractive index versus (a) process pressure, (b) electrode gap, (c) RF power, (d) process temperature and (e) mass-flow rate of SiH4.
rate keeps constant since the ratio of Si in the reaction chains will be oversaturated. There are two explanations to account for the variants of the RI. One is discussed in Refs. [12,13] that RI mainly depends on the ratio of the chemical bonds Si-H to N-H in a silicon-nitride thin film, and RI is higher in a silicon-rich film and lower in a nitrogen-rich film; the other is in Ref. [10] that RI is higher in a denser film. In fact, there is relevance connecting these two explanations, that is, more combinations among Si-free and N-free bonds result in tighter chemical networks then denser film [5]. Increasing the mass-flow rate of SiH4 contributes more Si-free to the thin film, and the RI thus increases. Increasing the substrate temperature results in more hydrogen released from the film, and the Si-free and N-free bonds then increase, as a consequent, the film is densified and the RI increases. Compared with the responses to the mass-flow rate of SiH4 and the substrate temperature, the responses of RI to the pressure, gap, and RF power seem insensitive and their variants are much smaller. The insensitive response of RI to RF power is supported by Ref. [1], where additionally shows a slightly positive response to pressure but negative in Ref. [5]. The explanation is likely
densities of radicals in the plasmas become larger if the process pressure increases, and the mass diffusion flux adsorbing the wafer surface then increases, producing more mass contributing to the thin film. The residence time of the radicals becomes longer since the effective volume for the reaction region becomes larger as the electrode gap is raised, as a consequence, the radicals have more time to adsorb the wafer surface, yielding more mass for the thin film. As the RF power increases, the concentrations of the radicals increase, resulting in more radicals reacting with the wafer surface and yielding more thin-film mass as well. The effect of the substrate temperature on the DR does not follow a temperature-dependent Arrhenius relation, which results from increased desorption of precursors with higher temperature. This indicates that the effect of adsorption-limited growth is stronger than surface-reaction limited growth in the silicon-nitride PECVD process. In the case increasing the mass-flow rate of SiH4, the positive response of the DR indicates that the SiH4 is not saturated in those recipes. Here, we have been aware of the results reported by El Amrani et al. [3] and Osenbach et al. [6] that the DR, in fact, increases first and then decreases if SiH4/NH3 continuously increases while the total mass-flow 174
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Fig. 3. Dimensions and configurations of the showerhead: (a) a close-up of the cross-section and (b) a top view of the showerhead faceplate, (c) flat showerhead, (d) convex showerhead and (d) concave showerhead.
more convex DR profile and a concave RI profile, while the convex showerhead gives a concave DR profile and a more convex RI profile. The results indicate that where the drilling depth is deeper the rightbelow DR is suppressed but the RI upraised. The mechanism accounting for the effects of showerhead on the deposition phenomena is related to the behaviors and characteristics of the precursors in SiH4-NH3 system. Varying the stepped holes on a showerhead faceplate directly affects the mass-transportation phenomena [26] and then the chemical reaction chains [18], finally the DR and RI. The precursors in SiH4-NH3 system can be classified into two categories: one is early-stage precursor, such as SiHx < 4 and NHx < 3, generated by electron impact and the other is late-stage precursor, such as Hx < 3Si(NH2)y < 4, produced by subsequent bulk chemical reactions. The late-stage precursors have more mass due to their larger formula weight as well as more N-free bond due to the amino in the aminosilane. We can conclude that increasing the late-stage precursors can obtain more thin film with more N- bond. According to the above explanations for deposition rate and reflective index, that will result in higher DR and lower RI. A stepped hole with smaller drilling depth has larger flow resistance and slower right-below gas velocity and thus leads to longer residence time. Consequently, the radicals have more time to go deeper in the chemical reaction chains, generating more latestage precursors Hx < 3Si(NH2)y < 4, contributing more mass with more N- bonds to the thin film, resulting in higher deposition rate but lower refractive index. The effects of the showerhead configuration on the full-wafer DR and RI imply that a showerhead with a properly designed convex profile can appropriately suppress the DR in the center region and upraise in the edge region and does the opposite effects on the RI profile, therefore the uniformity can be improved. We prepared a new convex showerhead, shown in Fig. 5(a), with a smaller slope due to the over adjustment by the above-mentioned convex showerhead. Fig. 5(b)
that the ratio of Si-free and N-free bonds does not change too much with the pressure and power changes since no one changes much larger than the other.
3.2. Effects of geometric parameters Full-wafer property of a thin film implies its local non-uniformity, which points out the direction where the local property should be upraised or suppressed. Here, we continue our previous work [20] to examine the effects of the showerhead configuration on the full-wafer DR and RI, and then show an approach experimentally to improve the uniformity by changing the showerhead configuration. The faceplate of the showerhead used in the experiment is shown in Fig. 3, where stepped holes are arrayed as an ortho-hexagonal pattern with 5 mm length of each side. The total length, that is the thickness of the faceplate, is 8 mm, and the drilling-depth is variable, which allows us to fabricate a flat, convex and concave showerhead, as shown in Fig. 3(c), (d) and (e), by changing the drilling-depth profile. The full-wafer DR and RI obtained from the above-mentioned recipe, pressure 2 Torr, electrode gap 15 mm, RF power 600 W, substrate temperature 200 °C, mass-flow rate of SiH4 280 sccm and NH3 2340 sccm for example, using a flat showerhead are plotted in Fig. 4(b), and the corresponding profile of the drilling-depth of the stepped holes along the radial direction on the showerhead faceplate is shown in Fig. 4(a). The results of the DR and the RI both show obvious convex full-wafer profiles, where the values in the central region are higher than those in the edge region. As the comparative cases, two experiments using the same recipe but different showerhead configurations, shown in Fig. 4(a), are carried out, where one showerhead is convex and the other is concave, and the drilling depths of the stepped holes vary from 3 mm to 6 mm linearly according to their radial positions. The data in Fig. 4(b) show that the concave showerhead produces a 175
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Fig. 4. Comparative experiments with different showerhead configurations: (a) the drilling-depth profiles of the stepped holes along the radial direction on the showerhead faceplate, and (b) the deposition-rate and refractive-index profiles obtained by using the corresponding showerhead configurations. The flat profile indicates the depths are uniform, the convex shows that the depths in the center are deeper than those at the edge, and the concave is opposite.
shows that the DR is upraised in the center region and suppressed in the edge region in a certain degree, and the uniformity is significantly improved. The RI presents a much more inert response, however, a slight decrement in the center region and increment in the edge region still can be seen. 3.3. Residual gases analysis In the single-variable experiment, the working gases keep feeding in 0–90 s, the RF is powered in 30–90 s, and the mass spectrums in the range of 1–100 amu/charge of the residual gases are scanned in the whole process. Fig. 6 shows the partial pressures of the residual gases in the benchmark recipe with the flat showerhead. The spectrums for the other recipes are similar except the exact values of the partial pressures. The spectrums in 51–100 amu/charge are not shown due to their extremely low signals. The flags in Fig. 6 point out the species, where the secondary or more charged species are ignored due to their much lower concentrations. Three significant peaks, located at 1–2 (hydrogen peak), 15–17 (ammonia peak) and 28–32 (silane peak), representing the species H, H2, NH, NH2, NH3, Si, SiH, SiH2, SiH3, SiH4 produced from the silane-ammonia system, can be clearly seen. Here, we notice that the species except for NH3 and SiH4 can also be detected in the first 30 s even though the RF is not powered. Those species are generated from charging the working gases in the RGA as their ratios of mass to
Fig. 6. The mass spectrums of the residual gases obtained from the benchmark recipe, where the pressure is 2 Torr, electrode gap 15 mm, RF power 600 W and frequency 13.56 MHz, substrate temperature 400 °C, mass-flow rate of SiH4 280 sccm and NH3 2340 sccm, respectively. The working gases keep feeding in the whole time 0–90 s, and the RF is powered in 30–90 s. The species of the residual gases are scanned in the range of 1–100 amu/charge, and the spectrums in 51–100 amu/charge are not shown due to their extremely low signals.
Fig. 5. Improving the deposition-rate uniformity by properly designing the slop of the drilling-depth profiles: (a) two convex showerhead, where the slope of the “convex−” one is smaller than that of the “convex” one, and (b) the deposition-rate and refractive-index profiles obtained by using the corresponding showerhead configurations. 176
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deposition rate is very good. The positive correlation between H2 and DR is likely related to H radicals, which could become growth sites on the substrate or deposited film and promote film growth, and recombination of H radicals to H2 thus decreases. 4. Conclusion The responses of the deposition rate and the refractive index to the process parameters for the large-area silicon-nitride thin film deposited from the silane-ammonia system are investigated. The deposition rate shows a positive linear response to the process pressure, electrode gap, RF power and mass-flow rate of silane, and a negative linear response to the substrate temperature. The refractive index shows a positive response to the substrate temperature and the mass-flow rate of silane, while an insensitive response to the process pressure, electrode gap, and RF power. The showerhead configuration has a significant effect on the full-wafer deposition rate and refractive index, and the inherent nonuniformity of the deposition rate can be optimized by using a proper convex showerhead. We believe that this approach can also be applied to optimize the full-wafer refractive index and extended to other properties. The partial pressure of hydrogen in the residual gases has a good correlation to the average deposition rate, which allows us to choose a valuable indicator from the residual gases to the silicon-nitride plasmas process. This finding could be useful for monitoring process state and real-time process control.
Fig. 7. The deposition rate (solid line) and the time-average partial pressure of H2+ (dash line) in the deposition time 30–90 s versus the normalized process parameters obtained from the single-variable experiment, where the range of the process pressure is 1.5–2.5 Torr, gap 13–17 mm, RF power 400–800 W, substrate temperature 200–400 °C and mass-flow rate of SiH4 220–340 sccm.
charge are scanned. The aminosilane species, Hx < 3Si(NH2)y < 4, detected by Smith et al. [27] using mass spectrometry and defined as precursors in the chemical-reaction chains by Kushner [28], are not distinct here when the RF is on. This probably results from their low concentrations and further dissipation leaving the deposition region, such as dissociated in the RGA device and adsorbing surfaces due to their high activity. In addition, the species H has very low partial pressure in the whole process time, which likely results from the most H adsorbing a surface or combining as H2 due to their high activity. The responses of the peaks to the RF status are obvious. The silane peak rapidly increases and keeps at the highest level before the RF is powered and then goes down as the RF is on. The decrements of SiH3 and SiH2 are much more than the other species, while SiH4 is always in low partial pressure during the processing time. These indicate that SiH4 is of high usage ratio in the deposition. The ammonia peak, however, shows a different characteristic, where NH3 is of the highest partial pressure and the change of NH is slight. Hence, the usage ratio of NH3 is much lower than SiH4, and NH2 is one of the important precursors. The hydrogen peak is not obvious in 0–30 s while dramatically increases once the RF is powered and keeps steady essentially. Here, the hydrogen peak presents a very clean signal: RF sensitive, stable response, and no any other mixed species due to the low content of H. The response characteristics of the hydrogen peak and the silane peak observed here are agreed with those reported in Ref. [25]. It is distinct that the silane related signal shows a considerable decrease and the hydrogen-related signal shows a sharp increase when the plasma is on and both present opposite responses if the plasma is off. To further examine the characteristics of the hydrogen peak, we compare the steady partial pressures of H2 during the deposition time and the corresponding DR obtained from all the recipes listed in Table 1. The DR and the partial pressure of H2 are plotted together versus the normalized process parameters in Fig. 7. It is clear that the partial pressure of H2 presents a similar response to the process parameters as the DR does. We further compute their correlation coefficients (ρ(X,Y) = cov(X,Y)/(σXσY), where X, Y are two samples, ρ is the correlation coefficient, σ is the standard variance, and cov is computing the covariance) for each recipe and get 0.9876, 0.9831, 0.9852, 0.9925 and 0.9973 for the process pressure, electrode gap, RF power, substrate temperature and mass-flow rate of SiH4, respectively. These results indicate that the correlation between the partial pressure of H2 and the
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