Optimizing the PECVD Process for Stress-Controlled Silicon Nitride Films: Enhancement of Tensile Stress via UV Curing and Layered Deposition

Abstract

Silicon nitride (SiN) films deposited via plasma-enhanced chemical vapor deposition (PECVD) exhibit tunable tensile stress, which is critical for various microelectronic and optoelectronic applications. In this paper, the effects of silane (SiH₄) flow rate during PECVD deposition, ultraviolet (UV) curing, and layered deposition on the tensile stress of SiN films are mainly investigated. The results reveal that increasing the SiH₄ concentration raises hydrogen incorporation, which modifies internal stress dynamics. UV curing significantly increases tensile stress by breaking N-H and Si-H bonds, facilitating hydrogen desorption, and promoting Si-N-Si crosslinking. The optimal UV curing duration stabilizes tensile stress at approximately 1570 MPa, while excessive UV power alters hydrogen content dynamics, reducing stress. Additionally, layered deposition further amplifies stress enhancement, with films subjected to multiple deposition cycles exhibiting increased densification and crosslinking. The combined optimization of PECVD deposition parameters, UV curing, and layered deposition provides a robust strategy for tailoring SiN film stress, offering a versatile approach to engineering mechanical properties for advanced applications.

1. Introduction

Silicon nitride (SiN) is widely used in both semiconductor and nonsemiconductor industries due to its excellent chemical stability, oxidation resistance, and mechanical properties [1,2,3,4,5,6,7,8,9]. In integrated circuits (ICs), SiN serves as a dielectric and passivation layer, providing protection against environmental degradation. The material’s high hardness and wear resistance also render it well-suited for microelectromechanical systems (MEMS) and microelectronics packaging. A critical characteristic of SiN films is the ability to achieve a tunable stress state, ranging from compressive to tensile, depending on deposition conditions [10,11]. Effective stress control is essential for semiconductor devices, as it influences mechanical strength, adhesion, and long-term stability [12]. Plasma-enhanced chemical vapor deposition (PECVD) is widely used for SiN deposition due to its relatively low processing temperature compared to other CVD techniques, which allows integration with a variety of substrates, although compatibility with highly temperature-sensitive or flexible substrates may still be limited. However, PECVD-deposited SiN films typically exhibit high compressive stress, which can lead to cracking, delamination, and mechanical failures in advanced semiconductor applications [1,2,3,7,11].

The formation of internal stress in thin films is closely related to the film fabrication process, deposition conditions, and the microstructure of the films. Studies have shown that there are certain regularities in the mechanisms underlying the formation of internal stress in thin films. E. Chason et al. [13] examined stress evolution during the electrodeposition of Ni on Si substrates, proposing a model where stress is attributed to competing processes at growing grain boundaries, with the stress depending on the rate of grain boundary formation. W. D. Nix et al. [14] studied the stress during crystallite coalescence in polycrystalline films, suggesting that the balance between surface area reduction, grain boundary creation, and film strain leads to large tensile stresses in the films. G. Guisbiers et al. [15] developed a model for intrinsic residual stress in thin films based on nanograin size-dependent phase transitions, and showed that stress arises from volume changes in the nanograins, considering relaxation processes and experimental comparisons with Ta, Mo, Pd, and Al films. Seel [16] suggested that after island-like deposition, atoms diffuse on the island surfaces or enter strain regions at grain boundaries, leading to the relaxation of tensile stress. P. Chaudhari [17], however, argued that grain growth causes the relaxation and reduction of compressive stress. Additionally, it is believed that the formation of internal stress in thin films may result from impurity atoms or capillary stress. Impurity effects refer to the incorporation of substrate atoms or impurity atoms, such as inert gases or oxygen, into the film during growth, which generates compressive stress due to the surface size effect. The capillary stress mechanism suggests that as the film is deposited, the microcrystalline lattice constant increases, but due to the constraints of the substrate, the lattice constant cannot continue to increase, resulting in compressive stress inside the film.

Hydrogen incorporation during PECVD plays a major role in compressive stress formation. Hydrogen from precursor gases such as silane (SiH₄) and ammonia (NH₃) forms Si-H and N-H bonds, increasing internal stress and reducing film stability [1,11,18]. Studies have demonstrated that tuning deposition parameters, including RF power, gas ratios, and temperature, can modify stress states [10,19,20,21,22,23,24]. For instance, a silane-to-ammonia ratio of 1:4 results in compressive stress (−500 MPa), whereas a 1:1 ratio leads to tensile stress (~700 MPa) [22,25,26]. Increasing RF power also enhances tensile stress, reaching up to ~900 MPa at 400 W [21,22]. Furthermore, temperature influences stress characteristics, with lower deposition temperatures (~300 °C) favoring tensile stress, while higher temperatures (~550 °C) induce compressive stress due to film densification [10,20,24]. Post-treatments such as ultraviolet (UV) curing have emerged as effective methods for stress modulation by promoting hydrogen desorption and increasing Si-N bond formation. Despite its potential, the exact mechanisms underlying UV curing-induced tensile stress remain unclear. Research indicates that optimizing UV curing parameters, including lamp power, curing time, and pressure, has a significant impact on stress states. UV treatment at 2.7 mW/cm² for 600 s has been reported to increase tensile stress by ~70% (from 420 MPa to 600 MPa), while a curing pressure of 100 mTorr enhances tensile stress by 30% compared to a 10% increase at 50 mTorr [25,27,28,29,30,31]. Additionally, multilayer PECVD deposition combined with UV curing further improves tensile stress, with optimized treatments yielding a 25% increase over single-layer films [32]. Despite significant advancements, the fundamental mechanisms governing UV curing-induced stress enhancement and its relationship with deposition parameters remain insufficiently understood.

This study aims to address these gaps by systematically investigating the effects of UV curing on PECVD-deposited SiN films. By optimizing process conditions and exploring multilayer deposition strategies, we seek to develop an approach for achieving high-tensile-stress SiN films for advanced semiconductor applications.

2. Experimental Methods

2.1. Film Deposition

SiN:H films were deposited using a PECVD system equipped with a parallel-plate radio frequency (RF) electrode configuration operating at 13.56 MHz, as illustrated in Figure 1. SiH₄ and NH₃ were employed as reactive gases, while high-purity N₂ served as the carrier gas. All process gases used in the deposition exhibited a purity level of 99.999%. The chamber pressure was maintained at 5.3 Torr, with an electrode gap fixed at 14.5 mm. The substrate temperature was precisely regulated at 400 °C using ceramic heaters coupled with thermocouple feedback control. The total film thickness was maintained at 60 nm across all deposition conditions. A summary of the process parameters is provided in

Table 1. Experimental conditions for SiN:H film deposition.

Item	RF Power (W)	RF Frequency (MHz)	T (°C)	Electrode Gap (mm)	SiH4 Flow (sccm)	NH3 Flow (sccm)	N2 Flow (sccm)	Pressure (Torr)
asic	175	13.56	400	14.5	92.5	880	2000	5.3
SiH4 flow rate	175	13.56	400	14.5	85.0	880	2000	5.3
					87.0
					89.5
					92.5
					97.0
					101.0
					107.0

. Deposition parameters were precisely controlled to achieve SiN:H films with varying tensile stress levels, as referenced in a previous study [26]. The key parameter investigated was the SiH₄/NH₃ ratio, which was varied to assess its effect on the film’s stress and composition.

To prevent vapor condensation and ensure thermal stability, the chamber walls were continuously heated using a water-circulation system set to 75 °C. Prior to deposition, the chamber was evacuated to a base pressure below 3 mTorr and subsequently subjected to a one-hour He purge to eliminate residual oxygen and moisture. Additionally, wafers underwent multiple cycles of N₂ purging and vacuum pumping prior to loading into the chamber. These procedures were implemented to minimize oxygen incorporation and suppress the formation of silicon oxynitride (SiON) during film growth. Furthermore, a transfer chamber was employed to connect the deposition and UV curing chambers, allowing UV treatment to be conducted without breaking the vacuum and thereby preventing oxidative contamination.

2.2. UV Curing Approach and Layered Deposition

Following PECVD deposition, SiN films underwent UV curing using a high-intensity mercury vapor lamp. UV curing was performed using a microwave-excited mercury (Hg) lamp system designed to deliver high-intensity ultraviolet radiation. The UV light is generated by exciting high-purity mercury vapor and inert gases inside an electrodeless quartz lamp using microwave energy at 2.45 GHz, which is supplied by a magnetron source. The resulting UV radiation, primarily in the wavelength range of 200–400 nm, is concentrated using a coated reflector and transmitted into the reaction chamber through a quartz window. The chamber pressure during curing was controlled at 6.5 Torr, and the temperature was maintained at 400 °C using a heated chuck. An He flow of 16,000 sccm was introduced to assist heat transfer and maintain uniform thermal conditions. During the curing process, the UV lamp module rotated horizontally within a range of 0–180° to ensure uniform illumination across the wafer surface. The entire curing process was conducted without breaking the vacuum following film deposition, using a transfer chamber that connects the PECVD and UV curing chambers. This setup effectively prevents oxidation and ensures the chemical integrity of the SiN:H films. After the curing cycle, the system was evacuated to a vacuum before wafer unloading to avoid further air exposure. The lamp power was varied from 55 W to 90 W to investigate its effect on the tensile stress of the films, while the curing duration ranged from 100 s to 600 s.

To further enhance the tensile stress, a layered deposition approach was employed. This method involved multiple cycles of PECVD deposition followed by UV curing. For each cycle, a thin SiN layer was deposited, followed by UV curing to modify the film’s structure further and enhance its mechanical properties, as shown in Figure 2. The number of cycles varied from 1 to 6, with each cycle contributing to the densification and crosslinking of the SiN film. The effect of multiple deposition-curing cycles on the tensile stress was systematically investigated to determine the optimal number of cycles for maximizing the film’s mechanical performance.

2.3. Characterization Analysis

Fourier-transform infrared spectroscopy (FT-IR) spectra were collected with a Nicolet 6700 Flex (Thermo Fisher, Waltham, MA, USA), which provides the structural properties and bonding configuration of SiN films before and after UV curing. The FT-IR spectra were measured within the wavelength range from 4000 cm⁻¹ to 400 cm⁻¹ with a step width of 2 cm⁻¹. The presence of Si-H, N-H, and Si-N bonds was determined with the location of the characteristic peaks. Meanwhile, the atomic percentage of the H atom ($H_{Si\left(N\right)-H}$) in Si-H or N-H was calculated with Equation (1) based on the bond density (${\rho }_{Si\left(N\right)-H}$) of Si-H or N-H (Equation (2)). And the total atomic percentage of H atom in Si-H and N-H ($H_{total}$) was obtained with Equation (3), and the bond density ratio of N-H and Si-H (H_ratio) was measured by Equation (4) accordingly [26].

$$H_{Si\left(N\right)-H}={\displaystyle \frac{{\rho }_{Si\left(N\right)-H}}{6.0108\times {10}^{22}+{\rho }_{Si-H}+{\rho }_{N-H}}}\times 100\%$$

(1)

$${\rho }_{Si\left(N\right)-H}={\displaystyle \frac{S_{Si\left(N\right)-H}\times {10}^8}{{\sigma }_{Si\left(N\right)-H}\times t_f}}$$

(2)

$$H_{total}=H_{Si-H}+H_{N-H}$$

(3)

$$H_{ratio}={\displaystyle \frac{{\rho }_{N-H}}{{\rho }_{Si-H}}}$$

(4)

where $S_{Si\left(N\right)-H}$ is the absorption peak area of the Si-H or N-H in the SiN film, ${\sigma }_{Si\left(N\right)-H}$ is the Si-H or N-H bond cross section, ${\sigma }_{Si-H}$ and ${\sigma }_{N-H}$ are 7.4 $\times$ 10⁻¹⁸ and 5.3 $\times$ 10⁻¹⁸ cm², and $t_f$ is the SiN flim thickness. The constant 6.1 × 10²² in the denominator is a proportionality factor derived from the Lanford–Rand method, which correlates FTIR absorbance with hydrogen atomic concentration. It has been validated against RBS/NRA measurements, as referenced in Thermo Fisher Application Note AN53081 (2019).

The curvature radius of the wafer was measured via T910 (Skyverse Technology, Beijing, China). The test mode was a line scan with 51 points. The mechanical stress of the SiN film was measured by substrate bow methods, in which the stress of the SiN film (${\sigma }_f$) is determined by the Stoney Equation [26].

$${\sigma }_f={\displaystyle \frac{E_s{t}_s^2}{\left(1-v_s\right)t_f}}\left({\displaystyle \frac{1}{R_f}}-{\displaystyle \frac{1}{R_0}}\right)$$

(5)

where $R_0$ is the radius of curvature of the bare substrate and $R_f$ is the radius of curvature with the film on the substrate. $E_s$ and $v_s$ are the Young’s modulus and Poisson ratio of the substrate, respectively, and $t_s$ and $t_f$ are the thicknesses of the substrate and the film, respectively. The refractive index (RI) of the SiNx:H film was measured via the film metrology system with a wavenumber range of 200 to 800 cm⁻¹ (Tensor 8500, KLA, Milpitas, CA, USA). The RI data were collected from five wafers, and 49 points were measured for each wafer. A systematic study of the optical properties of PECVD-prepared silicon nitride films was conducted using the Tauc-Lorentz fitting model. The film thickness, refractive index, and fitting errors were calculated. Nonlinear regression fitting was performed using the Levenberg–Marquardt algorithm, with the minimum square error (MSE) used to evaluate the fitting quality. The Tauc–Lorentz model, specifically designed for silicon nitride and amorphous semiconductor films, was applied to fit the silicon nitride films, and the fitting values showed good agreement with the measured data.

3. Results and Discussion

The PECVD process parameters had a significant impact on the tensile stress of the SiN films, such as RF power, silane-to-ammonia ratio, and deposition temperature and the relationship among these parameters is complex and interdependent, which collectively influence deposition characteristics together. In particular, the silane-to-ammonia ratio plays a crucial role in determining the chemical composition of the SiN:H films, as the ratio directly influences the Si-N bond formation and the hydrogen content in the film. Higher NH₃ concentrations tend to promote the formation of stronger Si-N bonds, whereas higher SiH₄ concentrations can lead to an excess of hydrogen in the film, which may negatively impact the tensile stress [33,34], which introduces additional internal stress and reduces the overall tensile strength of the SiN films [22,23,26]. Therefore, the effect of SiH₄ flow rate on tensile stress is mainly discussed in this section. For clarity, the experimental results are discussed in two distinct regimes. The first focuses on low-tensile-stress conditions derived from standard PECVD deposition (Figure 2, upper process), and the second emphasizes enhanced tensile stress achieved through UV curing and layered deposition (Figure 2, lower process).

3.1. Low-Tensile-Stress Regime: Effect of PECVD Process Parameters

Figure 3a presents the initial stress and refractive index (RI) of SiN:H films as the SiH₄ flow rate increases from 85 sccm to 107 sccm. The results indicate that the prestress remains constant at approximately 715 MPa, while the RI steadily increases from 1.80 to 1.835. The rise in RI is primarily attributed to an increase in Si-H bonds within the SiN:H films. As shown in Figure 3b, after UV curing, when the SiH₄ flow rate increases from 85 sccm to 97 sccm, the tensile stress of the film rises to approximately 1520 MPa. However, upon further increasing the SiH₄ flow rate to 100 sccm, the stress decreases slightly to 1470 MPa. The trend of loss-stress (Δ stress) follows a similar pattern to that of post-stress. It is noteworthy that after UV curing, the tensile stress of the SiN:H films exhibits a significant increase.

Figure 4a,b show the variation in the Si-H and N-H peaks in the FTIR spectra as a function of SiH₄ flow rate, while Figure 4d illustrates the changes in $H_{N-H}$ and $H_{Si-H}$ with varying SiH₄ flow. Figure 4c presents the full-range FTIR spectra, which exhibit the characteristic vibrational modes of Si–N, Si–H, and N–H bonds, confirming the chemical stability of the deposited SiN:H films. A comparison between Figure 4a,b reveal a significant decrease in both Si-H and N-H concentrations following UV curing treatment. As the SiH₄ flow rate increases, the Si-H bond concentration gradually increases, while the N-H bond concentration decreases. Specifically, as shown in Figure 4d, before UV treatment, as the SiH₄ flow rate increases from 85 sccm to 107 sccm, the N-H bond concentration decreases from ~15% to 11%, while the Si-H bond concentration in the SiN:H film increases from 11.5% to 17%. After UV curing treatment, with increasing SiH₄ flow, the N-H bond concentration in the SiN film decreases from 9.5% to 5.2%, while the Si-H bond concentration increases from 5% to 12.5%. Based on Equations (1)–(4), the change in the N-H/Si-H ratio ($H_{ratio}$) was subsequently calculated. Figure 5a illustrates the variation of the $H_{ratio}$ as a function of SiH₄ flow rate before and after UV curing. It can be seen that when the SiH₄ flow rate is less than approximately 93 sccm, the N–H/Si–H ratio is greater than 1:1. As the SiH₄ flow rate increases, the N–H/Si–H ratio gradually approaches 1:1. Furthermore, after UV curing, the N–H/Si–H ratio shifts further away from 1:1 compared to the SiN films without UV treatment. It is observed that when the initial N-H/Si-H ratio is close to 1, it remains nearly unchanged after UV curing, indicating that the release rates of H atoms from N-H and Si-H bonds are nearly equivalent. However, when the N-H bond concentration in the SiN films exceeds that of the Si-H bonds, UV curing leads to a further reduction of Si-H bonds, significantly increasing the N-H/Si-H ratio. Studies have shown that in the ground state, the bond energies of N-H and Si-H bonds are 4.8 eV and 4.0 eV [28], respectively. The silane-to-ammonia ratio is a critical parameter that affects the mechanical properties of SiN films. A higher ammonia concentration promotes the formation of Si-N bonds, which are stronger and more stable than Si-H bonds [18,29,31]. Our results corroborate this, as films deposited with a ~1:1 silane-to-ammonia ratio exhibited the highest tensile stress. This is because ammonia is the primary source of nitrogen in the film, and higher ammonia flow rates increase the likelihood of forming Si-N bonds, which are responsible for the tensile stress in the SiN films. However, in the excited state, N-H bonds are more susceptible to H emission, and these thermally activated interstitial H atoms can collide with Si-H bonds, leading to their dissociation [28,29]. Moreover, the results indicate that when the concentration of Si-H bonds in the film is significantly higher than that of N-H bonds, as shown in Figure 5a, the N-H/Si-H ratio continues to decrease after UV curing. This suggests that the thermally generated solute H atoms may also interact with dangling Si bonds, thereby forming additional Si-H bonds. Furthermore, the total H retention within the film decreases significantly after UV curing. This reduction in hydrogen content is directly responsible for the increase in tensile stress in SiN films, as demonstrated in Figure 5b. With a further increase in the SiH₄ flow rate, the total H content continues to decrease, suggesting that a higher Si-H bond concentration inhibits the UV-induced hydrogen desorption process, ultimately leading to a decline in tensile stress.

It should also be noted that, as the hydrogen content was measured ex situ, there exists a possibility of limited surface oxidation occurring upon air exposure prior to characterization. While the short ambient exposure time in this study minimizes such effects, minor oxidation at the film surface could potentially contribute to slight stress relaxation or variations in hydrogen bond concentrations. However, given the consistent measurement trends and the dominant influence of UV curing observed, the contribution of air oxidation is considered to be secondary. For further modulate the stress state of the as-deposited low-tensile-stress SiN films, UV curing was introduced as a post-treatment method. The following section explores how variations in UV curing parameters, particularly curing timed and UV power, influence hydrogen desorption behavior and tensile stress development.

3.2. Low-Tensile-Stress Regime: Effect of UV Curing Parameters

Figure 6a shows the variation of the Si-H and N-H peaks in the FTIR spectra as a function of UV treatment time, while Figure 6b presents the changes in $H_{Si-H}$ and $H_{N-H}$ with increasing UV treatment time. As shown in Figure 6, before UV treatment, both N-H and Si-H bond concentrations are approximately 13.8%, with an $H_{ratio}$ of 1:1. During the increase in UV treatment time from 100 s to 400 s, both N-H and Si-H bond concentrations exhibit a slight decrease. However, when the UV treatment time reaches 600 s, the concentrations of N-H and Si-H bonds in the SiN film stabilize at approximately 8% and 9%, respectively. Figure 7a illustrates the variations in post-stress and loss-stress as a function of UV curing time, with the UV curing power maintained at 85 W. The results indicate that as the UV curing time increases from 0 s to 200 s, the tensile stress rapidly rises from 738 MPa to 1450 MPa. Further increasing the UV curing time to 500 s results in a tensile stress increase to 1550 MPa. When the UV curing time reaches 600 s, the tensile stress stabilizes at approximately 1570 MPa. These results demonstrate that UV curing time significantly influences the enhancement of tensile stress in SiN films. During UV curing, the increase in tensile stress is attributed to the breaking of N-H and Si-H bonds and the subsequent formation of crosslinked Si-N-Si structures [29,30,31,32,33]. Shioya et al. [25,33,34] reported that under UV irradiation, the enhancement of tensile stress occurs due to a reduction in N-H bond absorption and a slight decrease in Si-H bond absorption, which collectively promote the formation of Si-N bonds. Our study reveals similar results, as shown in Figure 7b. Before UV curing, the N-H/Si-H ratio in SiN films is approximately 1:1. As the UV curing time increases from 100 s to 600 s, the N-H/Si-H ratio remains nearly constant at 0.9:1, excluding measurement errors. This suggests that UV curing preferentially disrupts N-H bonds, as illustrated in the inset of Figure 7b. Studies have shown that in the ground state, the bond energies of N–H and Si–H bonds are approximately 4.8 eV and 4.0 eV, respectively [28], suggesting that Si–H bonds should be more easily dissociated thermally. However, under ultraviolet irradiation, the dissociation dynamics are influenced by photo-excitation and the resulting nonequilibrium processes. Overall, UV curing time has minimal impact on the N-H/Si-H ratio.

The effect of UV curing power on the stress of SiN films and the N-H/Si-H ratio has been investigated. Figure 8a shows the variation of the Si-H and N-H peaks in the FTIR spectra as a function of UV curing power, while Figure 8b illustrates the changes in $H_{Si-H}$ and $H_{N-H}$ with increasing UV power. It can be observed that as the UV curing power increases from 55 W to 70 W, both the N-H and Si-H bond concentrations remain relatively stable, at approximately 8% and 9%, respectively. When the UV power increases to 75 W, the N-H bond concentration decreases significantly while the Si-H bond concentration increases. Based on the results averaged over three measurements, when the UV power exceeds 75 W, the concentrations of Si-H and N-H stabilize, and no further significant changes are observed. As shown in Figure 9a, with an increase in UV power from 55 W to 90 W, post-stress initially decreases slightly, followed by a gradual increase. Meanwhile, loss-stress remains nearly unchanged at first, then decreases slightly, and eventually increases. Overall, increasing UV power does not effectively enhance the tensile stress of SiN films. Conversely, excessive UV power leads to a reduction in tensile stress. Figure 9b presents the variation of the N-H/Si-H ratio as a function of UV curing power. As shown in Figure 9b, the N–H/Si–H ratio remains approximately 1:1 in the absence of UV treatment. When the UV power increases from 55 W to 70 W, the H ratio remains relatively constant. Beyond 70 W, the N-H/Si-H ratio decreases significantly. Previous studies have suggested that the probability of solute H atom collisions within the bulk of SiN films is relatively low [28,31]. During UV curing, most H atoms become activated and diffuse into interstitial sites. These interstitial H atoms are unstable and tend to recombine with isolated N or Si atoms possessing dangling bonds. The results indicate that increasing UV power significantly promotes the transition of H from the ground state to an excited state, forming interstitial H atoms. However, these H atoms are not readily released from SiN films. The tensile stress of SiN films is not significantly influenced by UV power.

Based on these findings, this study further explores the intrinsic mechanism by which UV curing enhances the tensile stress of SiN films. In SiN films, Si-H and N-H bonds are particularly susceptible to UV-induced bond dissociation. High-energy UV photons provide enough energy to break these bonds, leading to the release of H₂ from the film [18,25,34]. This H outgassing is a crucial step in the stress enhancement process, as it reduces internal H content and decreases the free volume within the film. As H atoms are expelled, the material density increases and the SiN films become more ordered. This densification is accompanied by a reduction in the film’s internal stress, which counteracts the intrinsic compressive stress often found in PECVD-deposited films. The removal of H atoms from the Si-H and N-H bonds also promotes the formation of Si-N-Si crosslinks, which further strengthen the SiN films and contribute to the observed increase in tensile stress. The mechanism of H outgassing and bond rearrangement is confirmed by FTIR spectra, which show a significant reduction in the intensities of the Si-H and N-H peaks after UV curing [28,29]. This indicates that H is effectively removed from the film, and the remaining bonds are reorganized to form Si-N-Si crosslinks. The formation of Si-N-Si crosslinks is a critical factor in the enhancement of tensile stress in SiN films. Crosslinking occurs when Si-N bonds in the film reorganize into a three-dimensional network, increasing stability and mechanical strength. This process not only improves tensile stress but also reduces overall film thickness, as densification leads to a more compact structure. As observed in this study, UV curing results in a decrease in film thickness and an increase in tensile stress, suggesting that the UV-induced crosslinking process effectively densifies the film structure. The results indicate that longer curing times lead to greater increases in tensile stress, as extended curing allows for more complete hydrogen removal and enhanced crosslinking. The formation of Si-N-Si crosslinks can be viewed as a form of chemical strain engineering, in which the internal structure of the film is modified to achieve a more favorable stress state. This process is analogous to strain engineering in silicon devices, where external mechanical strain is applied to modify the electronic properties of the material.

3.3. High-Tensile-Stress Regime: Layered Deposition and UV Curing

Figure 10 illustrates the effect of the number of deposition layers on the stress and RI of SiN films before and after UV curing. In this study, the total thickness of the SiN films is maintained at 60 nm for both single and multiple deposition layers. Figure 10a presents the relationship between initial stress and the number of deposition layers before UV curing. The results indicate that as the layer number increases from 1 to 6, the initial stress rises from 725 MPa to 920 MPa, representing an increase of approximately 30%. Similarly, the RI increases from 1.828 to 1.853, suggesting that multiple deposition cycles promote the densification of the SiN films. Figure 10b shows the variations in post-stress and loss-stress as a function of the number of deposition layers. Notably, after three PECVD deposition layers followed by UV curing, the SiN films exhibit the highest tensile stress, reaching approximately 1630 MPa—nearly twice that of the as-deposited film without UV curing. This increase in tensile stress is attributed to film densification and the formation of additional Si-N-Si crosslinking structures during each deposition-curing cycle. After four deposition cycles, the tensile stress stabilizes at approximately 1620 MPa, demonstrating a significant improvement compared to the single-layer film. These findings confirm the beneficial effect of multiple deposition and curing cycles on the mechanical properties of SiN films. Additionally, Figure 10b illustrates the trend of loss-stress with the increasing number of deposition layers. The variation in stress gradually decreases as the number of deposition layers increases. With more deposition-curing cycles, the cumulative effect leads to further densification and structural reinforcement of the film, thereby enhancing overall stress. This observation highlights the importance of the layered deposition approach in improving the mechanical performance of SiN films.

Figure 11a,b show the variation in the Si-H and N-H peaks in the FTIR spectra as a function of deposition cycles, while Figure 11c illustrates the changes in $H_{N-H}$ and $H_{Si-H}$ with increasing deposition cycles. As shown in Figure 11a, before UV treatment, the concentrations of Si-H and N-H bonds in the SiN film remain largely unchanged with an increase in the number of deposition cycles, exhibiting only a slight decreasing trend. As illustrated in Figure 11c, when the number of deposition cycles increases from 1 to 6, both Si-H and N-H bond concentrations decrease from ~13.5% to 12.5%. When both layer-by-layer deposition and UV curing are applied, the concentrations of Si-H and N-H bonds in the SiN film continue to decrease. As shown in Figure 11c, when the number of deposition cycles is less than 3, the concentration of N-H bonds is slightly lower than that of Si-H bonds. However, when the number of deposition cycles exceeds three, the concentration of N-H bonds becomes slightly higher than that of Si-H bonds. This suggests that, even with a thinner film, further UV curing does not promote the release of hydrogen from the film. It is worth noting that, as observed in Figure 4a,b, Figure 6a, Figure 8a and Figure 11a,b, different forms of noise signals appear in the ~3600–3800 cm⁻¹ range. This is primarily attributed to variations in the background atmosphere during each FTIR measurement. However, since no characteristic species are formed in this wavenumber range, the impact on the results is considered negligible. Figure 12a illustrates the variation of the N-H/Si-H ratio as a function of the number of deposition layers. As shown in Figure 12a, the N-H/Si-H ratio increases with increasing deposition layers, with a more pronounced increase observed after UV curing. Notably, when the number of deposition layers is less than two, the $H_{N-H}$ in SiN films is lower than the $H_{Si-H}$. However, as the number of deposition layers increases, the N-H/Si-H ratio undergoes a reversal. With further deposition cycles, the N–H/Si–H ratio deviates increasingly from 1:1, leading to a noticeable reduction in tensile stress. This trend is consistent with the experimental results shown in Figure 3b and Figure 5a, which indicate that maximum tensile stress occurs when the N–H/Si–H ratio is close to 1:1, while stress decreases significantly as the ratio exceeds this threshold. Figure 12b presents the variation in hydrogen content within the SiN films as a function of the number of layered deposition cycles. It can be observed that, in the absence of UV treatment, the hydrogen concentration slightly decreases with an increase in the number of deposition layers. However, when UV curing is combined with the layered deposition process, a substantial reduction in hydrogen content is evident, reaching a minimum at three deposition-curing cycles. Further increasing the number of cycles does not lead to additional hydrogen removal, which is consistent with the observed trend in tensile stress.

The layered deposition approach introduced in this study provides an alternative strategy for enhancing the tensile stress of SiN films. By repeatedly depositing thin SiN layers followed by UV curing, a cumulative stress effect is observed in the films. Each deposition-curing cycle contributes to SiN film densification and crosslinking, resulting in a progressive increase in tensile stress [32]. The fundamental mechanism of the layered deposition approach lies in the controlled formation of SiN films, where each layer undergoes UV curing to improve its mechanical properties. This method also focuses on the impact of the N-H/Si-H ratio on tensile stress, a key factor that differentiates this study from previous research. It is observed that when the N-H/Si-H ratio approaches 1:1, tensile stress reaches its maximum, demonstrating a clear link between bonding configuration and mechanical properties.

This method yields films with greater uniformity and reduced defect density compared to single-layer films. Additionally, we systematically optimized the parameters for UV curing and layered deposition to achieve the highest tensile stress. The cumulative effect of multiple deposition-curing cycles facilitates the formation of highly crosslinked, dense SiN films, ultimately enhancing tensile stress. The results demonstrate that tensile stress increases significantly with the number of deposition-curing cycles, reaching optimal stress levels after three to five cycles. Notably, the optimized combination of layer thickness (~20 nm) and deposition cycles ensures the highest tensile stress without compromising film quality. The thickness of individual deposition layers plays a crucial role in determining the overall mechanical properties of the film. During the deposition process, the accumulation of hydrogen and other defects often leads to higher intrinsic stress in thicker layers. In contrast, thinner layers provide more opportunities for effective curing and densification, thereby improving mechanical performance. In this study, the use of thin layers (~20 nm) in the layered deposition approach resulted in films with better uniformity and significantly higher tensile stress. The optimized combination of layer thickness and deposition cycle number provides a means to tailor the mechanical properties of SiN films for specific applications. By adjusting these parameters, it is possible to achieve films with a wide range of tensile stresses, from moderate to highly stressed films, depending on device requirements.

The enhancement of tensile stress in SiN films via UV curing and layered deposition, as demonstrated in this study, offers distinct advantages over conventional stress engineering methods. Traditional approaches for stress modulation in PECVD-deposited SiN films include thermal annealing, doping, deposition parameter optimization, and plasma post-treatment. This study contextualizes the findings within the broader landscape of stress engineering techniques and highlights the unique benefits of the UV curing-based approach, especially in terms of achieving high tensile stress through controlled hydrogen desorption and crosslinking, a strategy not commonly explored in previous works.

4. Conclusions

This study demonstrates that PECVD-deposited SiN films can be significantly enhanced in terms of tensile stress through UV curing and a layered deposition approach. The UV curing process facilitates the removal of hydrogen, leading to the formation of Si-N-Si crosslinks, which increase the tensile stress of the films. The layered deposition technique amplifies this effect by allowing for the sequential buildup of tensile stress through multiple deposition-curing cycles. These findings provide valuable insights into the optimization of PECVD and UV curing processes for the fabrication of SiN films with high tensile stress, which are essential for advanced semiconductor applications, particularly in the development of high-performance MOSFETs and other electronic devices.