Publication Abstracts

Here are the abstracts of our recent publications introducing ellipsometry, reflectometry and their applications. All these articles are available upon request.

1. Characterization of Thin Films and Stack in MOEMS Structure with Ellipsometry and Reflectometry Techniques. Proceedings of SPIE Volume: 5715, Micromachining and Microfabrication Process Technology X , Editor(s): Mary-Ann Maher, Harold D. Stewart, 2005, pp. 176-183
   
   Abstract: Nondestructive characterization on thin films and their stack in MOEMS device is highly desirable. But, it is often a challenging task because the area is usually small. During processing of thin films, the deposition rates, optical properties, and mechanical properties must be fully understood to fabricate a device with desired performance. With the patterned surface, deposition rate of a typical physical vapor deposition (PVD) technique, such as electron-beam evaporation and sputtering, varies at different location due to shadow effect. In this study, spectroscopic ellipsometry and reflectometry were used to characterize the optical properties of electron-evaporation thin films on a flat substrate. On the other hand, microreflectometer was used to monitor the spectrum of deposited multi-stack of optical thin films inside via-holes. Combination of these two techniques provides a practical way to qualify the processing and ensure the device performance.
   
   
2. Spectroellipsometric characterization of Au-Y₂O₃-stabilized ZrO₂ nanocomposite films, Journal of Materials Research (JMR), December 2005 (with George Sirinakis, Rezina Siddique, Kathleen A. Dunn, Harry Efstathiadis, Michael A. Carpenter, Alain E. Kaloyeros)

Abstract: Nanocomposite thin films consisting of Au nanoparticles embedded in yttria-stabilized zirconia (YSZ) were synthesized at room temperature by radio frequency magnetron co-sputtering from YSZ and Au targets and subsequently annealed in an argon atmosphere. Au microstructure and particle size were characterized as a function of annealing temperature from 600 to 1000 °C by x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and Rutherford backscattering spectroscopy. Spectroscopic ellipsometry was also used to determine the optical constants of the resulting films. In particular, the refractive index of the nanocomposites was found to undergo an anomalous dispersion in the spectral region where the extinction coefficient achieves its maximum. Additionally, the incorporation of Au in the YSZ matrix was found to increase the refractive index in comparison to that of YSZ. At annealing temperatures higher than 800 °C, a good agreement was found between experimental findings and theoretical models using bulk dielectric functions for Au, as modified to account for a reduced mean free path for scattering than that for free electrons. However, for annealing temperatures below 800 °C, an additional offset was required for the optical constants of Au to obtain good agreement between theory and experiment. This behavior was attributed to a relatively high atomic Au concentration in the YSZ matrix.

3. Synthesis and Spectroellipsometric Characterization of Y₂O₃-stabilized ZrO₂-Au Nanocomposite Films for Smart Sensor Applications. MRS Fall Meeting Proceedings, Volume 846, 2004
   
   Abstract: Noble metal nanoparticles exhibit significant potential in all-optical, smart-sensing applications due to their unique optical properties. In particular, gold (Au) nanoparticles exhibit a strong surface plasmon resonance (SPR) band, the spectral position and shape of which depends on the size, shape, and density of the nanoparticles and the physical and chemical properties of surrounding environment. Embedding the nanoparticles in an yttria-stabilized zirconia (YSZ) matrix is believed to expand their range of operation to temperatures above 500°C. YSZ is a material that has been proven suitable for optical applications due to its high refractive index, low absorption coefficient and high transparency in the visible and infrared regions. Thus, its use as a base platform for nanocomposite thin films is expected to provide significant benefits in the development of harsh environment multifunctional sensors. In this work YSZ-Au nanocomposite films were synthesized from a YSZ and a Au target by the radio frequency magnetron co-sputtering technique in combination with a post-deposition annealing treatment in an argon atmosphere, with the annealing temperature being varied from 500-1000°C in steps of 100°C. The microstructure and the optical properties of the resulting films were characterized by x-ray diffraction spectroscopy, scanning electron microscopy and spectroscopic ellipsometry. Results on the effect of the Au particle size on the real and the imaginary part of the refractive index of the nanostructured composites are presented. Future smart sensor systems utilizing these multifunctional material sets for harsh environment sensing applications will likewise be outlined.
   
   
4. Atmospheric Stability of E-Beam Deposited Optical Thin Film Stacks. MRS Proceedings, Fall Meeting, Volume 854, 2004
   
   Abstract: Optical thin films with SiO₂-TiO₂ stack were prepared by the technology of electron beam (e-beam) evaporation with ion beam assistant deposition (IBAD). The mechanical (stress) and optical properties of as-deposited thin films were studied as a function of exposure time in the atmospheric environment. Exposing to the air at the ambient temperature causes incremental compressive stress and spectrum profile changes, which is related to the absorption of water moisture into the films. Making a dense film is, therefore, a practical approach to improve structural stability of thin films and then the performance of optical devices.
   
   
5. Advanced Metrology Tool for SiGe Characterization: Infrared Spectroscopic Ellipsometer (IRSE). The 15th Annual IEEE/SEMI Advanced Semiconductor Manufacturing Conference and Workshop, ASMC 2004 Proceedings, Boston, 2004, pp. 425-432
   
   Abstract: The ability to precisely determine epilayer thickness and Ge concentration in Si_1-xGe_x is essential for calibrating growth processes and thus control film quality. Spectroscopic ellipsometry is a non-destructive optical technique and has advantages for in-line process monitoring over SIMS, TEM and other destructive techniques. With integrated UV-Vis-IR spectroscopic ellipsometer, not only epilayer thickness and Ge concentration can be obtained with the UV-Vis channel, but also dopant concentration with the IR channel. Therefore, an advanced IRSE metrology tool will be used to characterize Si_1-xGe_x epilayers. The sub 50-nm Si_1-xGe_x films were processed with LPCVD in an AMAT 5200 Centura platform. The precursors for Si_1-xGe_x growth are SiH₄ and GeH₄. B₂H₆ was used as the p-type dopant source (B) for certain samples. CH₃SIH₃ was also utilized as a carbon source to minimize boron out diffusion. Five types of samples were prepared and characterized, which are (a) single Si_1-xGe_x layer (Box), (b) Si_1-xGe_x layer with Si cap, (c) Si_1-xGe_x layer with graded Ge concentration, (d) Boron doped Si_1-xGe_x layer and (e) carbon effect on both undoped and doped Si_1-xGe_x epilayers. Uniformity for these samples was also examined, which is an essential measure for improving process performance and device yield. Finally, results from spectroscopic ellipsometry were compared with that from other technique such as Xrd for thickness and %Ge.
   
   
6. Spectroscopic Ellipsometry (SE) and Grazing X-Ray Reflectometry (GXR) Analyses on Tungsten Carbide (WC_x) Films for Diffusion Barrier in Copper Metallization Schemes. Thin Solid Films, Vol. 455-456, pp. 519-524
   
   Abstract: Tungsten carbide (WC_x x~0.5) films, for diffusion barrier applications in copper metallization schemes, were prepared by a pulsed chemical vapor deposition (PCVD). Spectroscopic ellipsometry (SE) and grazing x-ray reflectometry (GXR) were used to characterize WC_x films for thickness, surface roughness, and their optical properties. It is found that Drude law with two Lorentz absorption bands could be used to describe the optical dispersion behaviors of WC_x films. Structural and chemical characterization of the films was performed with Auger electron spectroscopy (AES), x-ray diffraction spectroscopy (XRD), and atomic force microscopy (AFM). Surface roughness obtained from GXR and SE is consistent with AFM measurements.
   
   
7. Spectroscopic Ellipsometry Study on E-Beam Deposited Titanium Dioxide (TiO₂) Films. Thin Solid Films, Vol. 455-456, pp. 525-529
   
   Abstract: Spectroscopic ellipsometry (SE) was applied to study the optical inhomogeneity of TiO₂ films deposited by the technique of ion-beam assisted e-beam evaporation. A three-sub-layer model of ellipsometry analysis can successfully simulate the structure variation of TiO₂ films, which consists of a dense amorphous layer near the substrate, a crystal columnar layer, and a very thin roughness layer on top. Such a theoretical model is further confirmed by microscopic observations with SEM, TEM, and AFM. The structure variation of TiO₂ films is related to the increase of chamber temperature during the e-beam evaporation process. Annealing evolution of as-deposited TiO₂ films in atmospheric environment at elevated temperatures of 150°C and 250°C was also investigated. The ellipsometry analysis shows that the total film thickness of TiO₂ film increases with the annealing time at both temperatures. A more detailed analysis further reveals that thickness of the top sub-layer increased; whereas the region of the bottom amorphous sub-layer shrunk when the film was annealed at 250°C.
   
   
8. Precise Characterization of Silicon on Insulator (SOI) and Strained Silicon on Si_1-xGe_x on Insulator (SSOI) Stacks With Spectroscopic Ellipsometry. Fundamentals of Novel Oxide/Semiconductor Interfaces, Boston, 2003, MRS Proceedings Vol. 786, pp. 103-108
   
   Abstract: Further improvements in CMOS circuit performance such as switching speed and power reduction will rely on the use of silicon on insulator (SOI) substrates with decreased functional layer thicknesses. According to the International Technology Roadmap for Semiconductors (ITRS), the silicon and buried SiO₂ (BOX) layer thicknesses for a fully depleted device should be in the ranges of 10 - 16nm and 24 - 40nm by 2005, respectively. A key issue for fully-depleted CMOS transistors is control of such ultra-thin layer thicknesses and their uniformity along with other parameters such as surface and interface roughness. This poses a challenge to metrology, especially to conventional reflectometry technique because the layer thicknesses must be determined with angstrom precision for both silicon cap and SiO₂ box layer. Spectroscopic ellipsometry (SE) is an optical and non destructive technique for determining thin film thickness and material optical properties. Because ellipsometry measures the change in the polarization state for both the amplitude ratio of the p to s polarizations, and in the phase retardation, it provides a precise way to characterize such ultra thin SOI stacks. Comprehensive characterization results for a number of thin and ultra thin SOI stacks with different thickness ranges will be presented together with measurement repeatability results relevant to the film thickness process tolerances. In addition, characterization results for advanced device applications such as strained silicon-on- Si_1-xGe_x -on-insulator (SSOI) will also be shown, demonstrating the use and capability of spectroscopic ellipsometry for precise determination of layer thickness, material composition, interfacial layers, etc. Principles and advantages of the technique will also be discussed in the presentation.
   
   
9. Evaluation on Stress and Optical Property of Thin Films Used in Optical MEMS Device. Thin Films - Stresses and Mechanical Properties X MRS Proceedings Vol. 795, pp. 479-484, 2003
   
   Abstract: Control of the film stress and optical property has long been considered as an issue in the tunable optical MEMS devices. In this paper, the atmospheric evolution of TiO₂ and SiO₂ thin films for the optical MEMS devices were studied. These films were prepared by the way of ion-assisted e-beam evaporation. It is found that as-deposited SiO₂ films exhibit compressive stress; whereas, it is tensile in the TiO₂ films under present processing conditions. When annealed at 150°C, both SiO₂ and TiO₂ films show slight changes in stress with annealing time. However, increasing the anneal temperature to 250°C caused an apparent change of film stresses with time, in which SiO2 film turns into less compressive and TiO₂ film appears to be more tensile. The optical properties after annealing were also investigated by measuring the thickness and the refractive index changes using the technique of spectroscopic ellipsometry analysis. At both experimental temperatures, the film thickness increases slightly and the refractive index at 1550 nm decreases a little at the initial annealing stage for SiO₂ films. For TiO₂ films, it is found that the refractive index increases after annealed at 250°C. This might be caused by the TiO₂ film densification process of amorphous-to-crystalline phase transformation. Because most of the significant film evolutions occur during the initial 12 hours of annealing, a practical way of stabilizing the film properties in a MEMS device is to pre-anneal the as-deposited thin films.
   
   
10. Characterization of High-k Dielectrics by Combined Spectroscopic Ellipsometry (SE) and X-Ray Reflectometry (XRR). Fundamentals of Novel Oxide/Semiconductor Interfaces, MRS Proceedings, Vol. 786, pp. 95-102
    
    Abstract: At present, new high-k dielectric materials are being intensively investigated to replace the silicon dioxide as gate dielectric for the next generation of electronic devices. Several candidate materials (such as ZrO₂, HfO₂, Al₂O₃) and deposition processes are currently under investigation. Because the layer thickness which is required in the next generations of devices is of the order of few nanometers, a precise determination and control of layer thickness will be mandatory. Although spectroscopic ellipsometry (SE) is well established non-contact, non-destructive and precise technique for determining thickness and optical properties of thin films, it becomes more difficult to obtain this information unambiguously and simultaneously for ultra-thin films with traditional SE alone because of possible high correlations between film structure and optical properties. In this study, a complementary non-destructive Grazing X-ray reflectometry (GXR) was first used to extract the thickness of such ultra thin films. Then, the optical properties of the films were determined with Vacuum Ultra-Violet Spectroscopic Ellipsometry (VUV-SE), which covers a wide spectral range down to 140nm wavelength. Within the VUV range, all the high-k dielectrics under evaluation are no longer transparent. This enhances the capability of using SE for such high-k dielectric film characterization. HfO₂, Al₂O₃ and HfAlO_x films were deposited on silicon substrates with Atomic Layer Deposition (ALD) technique. HfCl4 and Al(CH₃)₃ mixing with H₂O were used as precursors for HfO₂ and Al₂O₃ respectively. Al concentration in HfAlOx was varied by changing the relative number of HfCl₄ and Al(CH₃)₃ cycles (2:1, 1:1 and 1:2). The characterization results show a strong dependency between aluminum concentration and optical constants (refractive index and extinction coefficient) in VUV range. This proposes a way to quantatively determine Al concentration in HfAlO_x films and their thickness.
    
    
11. Application of Spectroscopic Ellipsometry Techniques in OLED Film Development and Processing. SID/MAC OLED Research & Technology Conference, Princeton, NJ 2002 SID - Society for Information Display
    
    
12. Precise Characterization of Optical Thin Films and Compound Semiconductor Stacks Using Spectroscopic Ellipsometry. Opto-Canada: SPIE Regional Meeting on Optoelectronics, Photonics, and Imaging, Ottawa, Canada, Vol. 01, pp. 512-514, 2002
    
    Abstract: Spectroscopic ellipsometry is a technique that is powerful for characterizing thin films and materials. The technique is now routinely used in R&D laboratories for characterizing a very wide diversity of thin films and materials, and within manufacturing facilities for monitoring deposition processes. Spectroscopic ellipsometry relies on the determination of the polarization state of an incident beam upon reflection on the sample under characterization. When performing spectroscopic ellipsometry, the polarization state is determined at many discrete wavelengths over a broad spectral range. The change in the polarization state can be traced to the physical properties of the thin film by means of a model and through regression analysis. Physical characteristics such as layer thickness, surface roughness, index of refraction and coefficient of absorption of the materials can be determined with excellent precision.
    
    
13. Properties of Titanium Oxide Thin Film Prepared With E-Beam Evaporation. MRS Fall Meeting Proceedings: Morphological and Compositional Evolution of Thin Films, Vol. 749, pp. 101-106
    
    Abstract: The optical and mechanical properties of TiO₂ film prepared by ion-assisted e-beam evaporation have been examined in this research. Spectroscopic ellipsometry analysis revealed an inhomogeneous behavior in both optical property and growth structure, vertically from substrate to the top surface of the film. This phenomenon was further confirmed with the electron microscopic analyses. The effects of deposition rate, chamber pressure, anode voltage and current on the stress of TiO₂ films were also investigated and reported. Further study showed that a structural homogeneous film could be obtained through TiO₂-SiO₂ co-evaporation.
    
    

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