At the molecular level, resonant coupling of infrared radiation with oscillations of the electric dipole moment determines the absorption cross section, $σ$. The parameter σ relates the bond density to the total integrated absorption. In this work, $σ$ was measured for the Si–N asymmetric stretch mode in SiNx thin films of varying composition and thickness. Thin films were deposited by low pressure chemical vapor deposition at 850 °C from mixtures of dichlorosilane and ammonia. σ for each film was determined from Fourier transform infrared spectroscopy and ellipsometric measurements. Increasing the silicon content from 0% to 25% volume fraction amorphous silicon led to increased optical absorption and a corresponding systematic increase in σ from 4.77 × 10–20 to 6.95 × 10–20 cm2, which is consistent with literature values. The authors believe that this trend is related to charge transfer induced structural changes in the basal SiNx tetrahedron as the volume fraction of amorphous silicon increases. Furthermore, experimental $σ$ values were used to calculate the effective dipole oscillating charge, q, for four films of varying composition. The authors find that q increases with increasing amorphous silicon content, indicating that compositional factors contribute to modulation of the Si–N dipole moment. Additionally, in the composition range investigated, the authors found that $σ$ agrees favorably with trends observed in films deposited by plasma enhanced chemical vapor deposition.
The creation of microelectromechanical systems (MEMS) that can operate through elevated temperatures would enable systems diagnostics and controls that are not possible with conventional-off-the-shelf components. The integration of silicon carbide (SiC) with aluminum nitride (AlN) has led to the fabrication of devices that can withstand elevated temperature anneals >935 °C. The results from a piezoelectric micromachined ultrasonic transducer (PMUT) and a microresonator are reported as demonstrations of the fabrication process. Testing the PMUT response before and after annealing at 935 °C led to a change in resonant frequency of less than 1%, which is attributable to a shift in film stress. The response of the microresonator was RF tested in situ up to 500 °C and showed no degradation in its electromechanical coupling coefficient. The resonant frequency decreased with temperature due to the temperature coefficient of Young's modulus, and the quality factor decreased with temperature and remained unrecoverable upon cooling. The degradation in the quality factor is suspected to be a result of oxidation of the titatium nitride (TiN) top electrode, which increases the resistivity and leads to an unrecoverable reduction in the quality factor. The robust piezoelectric response of AlN at these temperatures show that AlN is a very promising candidate for elevated temperature applications.
The formation of thin film superlattices consisting of alternating layers of nitrogen-doped SiC (SiC:N) and C is reported. Periodically terminating the SiC:N surface with a graphitic C boundary layer and controlling the SiC:N/C thickness ratio yield nanocrystalline SiC grains ranging in size from 365 to 23 nm. Frequency domain thermo-reflectance is employed to determine the thermal conductivity, which is found to vary from 35.5 W m-1 K-1 for monolithic undoped α-SiC films to 1.6 W m-1 K-1 for a SiC:N/C superlattice with a 47 nm period and a SiC:N/C thickness ratio of 11. A series conductance model is employed to explain the dependence of the thermal conductivity on the superlattice structure. The results indicate that the thermal conductivity is more dependent on the SiC:N/C thickness ratio than the SiC:N grain size, indicative of strong boundary layer phonon scattering.
A number of important energy and defense-related applications would benefit from sensors capable of withstanding extreme temperatures (>300degC). Examples include sensors for automobile engines, gas turbines, nuclear and coal power plants, and petroleum and geothermal well drilling. Military applications, such as hypersonic flight research, would also benefit from sensors capable of 1000deg C. Silicon carbide (SiC) has long been recognized as a promising material for harsh environment sensors and electronics. Yet today, many advanced SiC MEMS are limited to lower temperatures because they are made from SiC films deposited on silicon wafers. Other limitations arise from sensor transduction by measuring changes in capacitance or resistance, which require biasing or modulation schemes that can withstand elevated temperatures. We circumvented these issues by developing sensing structures directly on SiC wafers using SiC and aluminum nitride (A1N), a high temperature capable piezoelectric material, thin films.
This work demonstrates a lateral overtone bulk acoustic resonator (LOBAR), which consists of an aluminum nitride (AlN) transducer coupled to a suspended thin silicon carbide (SiC) film fabricated using standard CMOS-compatible processes. The LOBAR design allows for high transduction efficiency and quality factors, by decoupling the transduction and energy storage schemes in the resonator. The frequency and bandwidth of the resonator were lithographically defined and controlled. A LOBAR operating at 2.93GHz with a Q greater than 100,000 in air was fabricated and characterized, having the highest reported f×Q product of any acoustic resonator to date.
Charge transport and dielectric breakdown is studied in silicon oxynitride films with optical index of refraction varying from 1.77 to 2.01, and thickness ranging from 20 to 50 nm. Assuming Poole-Frenkel emission as the dominant charge transport mechanism, a compositionally dependent ionization potential ranging from 1.22 to 1.51 eV is observed. Over the same composition range, the barrier lowering energy at the point of dielectric breakdown is independently determined to vary between 1.24 and 1.56 eV. The correlation between these energies suggests a causal relationship between field saturation-induced trap ionization and dielectric breakdown. It is concluded that in the vicinity of the field saturation point a diminished capacity for regulating hot electron injection via the action of charge trapping results in an increased probability for impact ionization and subsequent dielectric breakdown.
Optical waveguide propagation loss due to sidewall roughness, material impurity and inhomogeneity has been the focus of many studies in fabricating planar lightwave circuits (PLC's)1,2,3 In this work, experiments were carried out to identify the best fabrication process for reducing propagation loss in single mode waveguides comprised of silicon nitride core and silicon dioxide cladding material. Sidewall roughness measurements were taken during the fabrication of waveguide devices for various processing conditions. Several fabrication techniques were explored to reduce the sidewall roughness and absorption in the waveguides. Improvements in waveguide quality were established by direct measurement of waveguide propagation loss. The lowest linear waveguide loss measured in these buried channel waveguides was 0.1 dB/cm at a wavelength of 1550 nm. This low propagation loss along with the large refractive index contrast between silicon nitride and silicon dioxide enables high density integration of photonic devices and small PLC's for a variety of applications in photonic sensing and communications.
Poole-Frenkel emission in Si-rich nitride and silicon oxynitride thin films is studied in conjunction with compositional aspects of their elastic properties. For Si-rich nitrides varying in composition from SiN{sub 1.33} to SiN{sub 0.54}, the Poole-Frenkel trap depth ({Phi}{sub B}) decreases from 1.08 to 0.52 eV as the intrinsic film strain ({Epsilon}{sub i}) decreases from 0.0036 to -0.0016. For oxynitrides varying in composition from SiN{sub 1.33} to SiO{sub 1.49}N{sub 0.35}, {Phi}{sub B} increases from 1.08 to 1.53 eV as {Epsilon}{sub i} decreases from 0.0036 to 0.0006. In both material systems, a direct correlation is observed between {Phi}{sub B} and {Epsilon}{sub i}. Compositionally induced strain relief as a mechanism for regulating {Phi}{sub B} is discussed.
Low residual stress silicon oxynitride thin films are investigated for use as a replacement for silicon dioxide (SiO{sub 2}) as sacrificial layer in surface micromachined microelectrical-mechanical systems (MEMS). It is observed that the level of residual stress in oxynitrides is a function of the nitrogen content in the film. MEMS film stacks are prepared using both SiO{sub 2} and oxynitride sacrificial layers. Wafer bow measurements indicate that wafers processed with oxynitride release layers are significantly flatter. Polycrystalline Si (poly-Si) cantilevers fabricated under the same conditions are observed to be flatter when processed with oxynitride rather than SiO{sub 2} sacrificial layers. These results are attributed to the lower post-processing residual stress of oxynitride compared to SiO{sub 2} and reduced thermal mismatch to poly-Si.