Depth profiling of copper thin films by resonant laser ablation

In Resonant Laser Ablation (RLA), material is related and selectively ionized by a low-energy pulse from a tunable laser. The selectivity and efficiency allow detection and quantitation at very low concentrations. We demonstrate that RLA has potential use in profiling thin layer and multilayer structures. Quantitative results are reported on the analysis of 20 and 100 Å copper thin films on Si(110) surfaces. Removal rates range from 10^–3 to 10^–2 Å/shot. Prospects for interrogation of dopants and impurities are also evaluated.     

Electron and ion beam induced heating effects in solids measured by laser interferometry

A sensitive interferometric technique has been applied for studying the thermal displacement of thin film heated by electron or ion beams. The steady state displacement has been measured and we discuss the dependence on material properties and film thickness showing that this method has a potential for in-situ monitoring of thin-film deposition or etching. Transient effects are studied in a thin quartz plate and the propagation velocity of thermal waves is measured.Our colleague Martin Nonnenmacher passed away May 1992. We deplore his early death deeply and acknowledge gratefully the excellent cooperation with him.     

Study of the gas-phase parameters affecting the silicon-oxide film deposition induced by an ArF laser

A study of the gas-phase parameters involved in ArF laser induced chemical vapour deposition of silicon-oxide thin films is presented. A complete set of experiments has been performed showing the influence of the concentration of the precursor gases, N₂O and SiH₄, and their influence on total and partial pressures on film growth and properties. In this paper we demonstrate the ability of this LCVD method to deposit silicon oxide films of different compositions and densities by appropriate control of gas composition and total pressure. Moreover, a material specific calibration plot comprising data obtained using different preparation techniques is presented, allowing determination of the stoichiometry of SiO _x films by using FTIR spectroscopy independently of the deposition method. For the range of processing conditions examined, the experimental results suggest that chemical processes governing deposition take place mainly in the gas phase.     

Photoexcitation of electron-hole pairs during SIMS

During analysis with SIMS (secondary ion mass spectroscopy) a HeNe laser beam was focussed on the sample surface. While sputtering Si with oxygen ions, the laser irradiation produced a strong increase of the target current and the SIMS intensities as well. This was found for lightly p-doped Si only, whereas no effect was observed for highly p-doped or n-doped Si. To explain this we assume that a depletion layer exists under the surface oxide layer and free charged carriers are created therein by laser excitation. The laser induced effects observed in the SIMS intensity or the target current can be used for measuring the profile of an ion beam or for measuring the alignment of an ion beam at a laser marked target. In addition, laser irradiation combined with SIMS allows one to measure qualitatively both the profile of the doping impurity and its electrically active part.     

Experimental and theoretical investigations into the origin of cross-contamination effects observed in a quadrupole-based SIMS instrument

The minimum-detection limits achievable in SIMS analyses are often determined by transport of material from surrounding surfaces to the bombarded sample. This cross-contamination (or memory) effect was studied in great detail, both experimentally and theoretically. The measurements were performed using a quadrupole-based ion microprobe operated at a secondary-ion extraction voltage of less than 200 V (primary ions mostly 8keV O ₂ ⁺ ). It was found that the flux of particles liberated from surrounding surfaces consists of neutrals as well as positive and negative ions. Contaminant species condensing on the bombarded sample could be discriminated from other backsputtered species through differences in their apparent energy spectra and by other means. The apparent concentration due to material deposited on the sample surface was directly proportional to the bombarded area. For an area of 1 mm² the maximum apparent concentration of Si in GaAs amounted to 5 × 10¹⁶atoms/cm³. The rate of contamination decreased strongly with increasing spacing between the bombarded sample and the collector. The intensities of backsputtered ions and neutrals increased strongly with increasing mass of the target atoms (factor of 10 to 50 due to a change from carbon to gold). The effect of the primary ion mass (O ₂ ⁺ , Ne⁺, and Xe⁺) and energy (5–10keV) was comparatively small. During prolonged bombardment of one particular target material, the rate of contamination due to species not contained in the sample decreased exponentially with increasing fluence. In order to explain the experimental results a model is presented in which the backsputtering effect is attributed to bombardment of surrounding walls by high-energy particles reflected or sputtered from the analysed sample. The level of sample contamination is described by a formula which contains only measurable quantities. Cross-contamination efficiencies are worked out in detail using calculated energy spectra of sputtered and reflected particles in combination with the energy dependence of the sputtering yield of the assumed wall material. The experimental findings are shown to be good agreement with the essential predictions of the model.     

Thin films by regular patterns of metal nanoparticles: Tailoring the optical properties by nanodesign

Ultrathin metal films consisting of regular two-dimensional arrays of 44–95 nm sized silver nanoparticles were fabricated by electron-beam lithography. By independent variation of particle shape and interparticle distance, respectively, the film's optical extinction maximum wavelength can be tuned in a range from 460 nm to 520 nm and the bandwidth between 50 and 100 nm. The results suggests the potential of lithographically designed nanoparticle films as thin layer material with tailorable optical properties for applications in microoptics and optoelectronics.     

Thin film and surface characterization by specular X-ray reflectivity

We review the use of specular X-ray reflectivity (XRR) for the characterization of thin-film and surface structures. Specular X-ray scattering at small scattering vectors allows characterization of electron density profiles perpendicular to the surface on the length scale of 0.1 to 100 nm. This allows measurement of surface morphology, thin films, multilayer structures, and buried interfaces. The technique is nondestructive and can be applied in situ in a variety of processing environments. In the first half of the article, we review the theory and methods of XRR, including analysis of XRR spectra by a multilayer optical approach and a discussion of surface roughness measurements by XRR and other techniques. In the second half, we present a wide range of examples of XRR applications in thin-film structures, dynamic processes, liquid surfaces, and macromolecular structures.     

Detection of neutral products formed during excimer laser ablation of polyimide by UV and VUV laser photoionization/mass spectrometry

Some of the neutral species which are produced in the laser ablation of polyimide have been characterized using multiphoton ionization/time of flight mass spectrometry. Three different wavelengths (193 nm, 157 nm, and 118 nm) have been used in an attempt to effect soft ionization of the products formed during or after the initial laser ablation of the polymer. Neutral photo-ablation products detected using this scheme range from atomic to high molecular weight species which, depending on the probe wavelength, include pure carbon clusters as well as a broad distribution of heteroatom containing clusters. However, there is virtually no overlap in the mass spectra recorded at each probe wavelength. When probing with 193 nm, marked changes are observed in the mass spectra as a function of the probe flux used. At moderate fluxes, pure carbon clusters (fullerenes) are observed. The identification of a large distribution of species other than pure carbon clusters is in dramatic contrast to the recent observation [W.R. Creasy, J.T. Brenna: Chem. Phys. 126, 453 (1988)] of the positively charged ionic species produced, which are solely carbon clusters. These results suggest that the neutral and ionic products observed after ablation of the polymer are due to both condensation of the atomic and molecular fragments which form during the ablation laser pulse and nascent polymer fragments. Various implications of this result for the unambiguous determination of the true ablation product distribution are discussed.     

Silicone polymer deposition by CO2 laser induced decomposition of silane in the presence of methyl methacrylate

Infrared multiphoton decomposition of monosilane in the presence of methyl methacrylate results in the formation of gaseous methane, acetylene, butenes and carbon monoxide along with a solid polymer whose chemical mechanism of formation is discussed in line of the poly(dimethylsiloxane) structure inferred from ESCA and IR spectral analysis.     

基于激光外差干涉术的薄膜厚度测量方法

针对光学薄膜厚度测量困难问题，提出了一种基于激光外差干涉术的薄膜厚度测量方法。采用经典迈克尔逊干涉光路，利用外差干涉原理将薄膜厚度差转换为光程差，以精密位移平台为扫描机构实现薄膜厚度的逐行扫描测量。测量系统在恒温实验条件下20 min内的漂移不超过8 nm，测量结果平均差小于1 nm，通过与椭圆偏振仪的测量结果比较，测量差值为12.97 nm，表明了该方法的可行性。     

Influence of substrate in photothermal measurements of thin film absorption

In photothermal measurements of low optical absorption coefficients, as in the case of thin films of amorphous semiconductors, the substrate absorption can play a non-negligible role. In this paper, the substrate influence is discussed and generalized on the basis of a theoretical model of a two-layer absorbing sample, and the interaction of the thermal and optical parameters which control the photothermal process in the sample is shown. Some experimental results obtained on thin films of amorphous silicon carbide deposited on different substrates are presented and discussed.     

Melting threshold of crystalline and amorphized Si irradiated with a pulsed ArF (193 nm) excimer laser

Modifications induced by a pulsed ArF excimer laser at surface of implanted silicon were investigated by a new and simple optical method which consists to follow the evolution of solid reflectivity, at 633 nm wavelength, resulting from the amorphouspolycrystalline (or monocrystalline) transition during the laser melting process. These results, which have been compared to those obtained using time resolved reflectivity experiments have demonstrated the capability of this simple technique to determine the melting threshold of implanted silicon.     

Photoreflectance characterization of CdTe thin films grown by Molecular Beam Epitaxy on GaAs(100)

CdTe films have been grown on top of GaAs(100) by means of Molecular Beam Epitaxy (MBE) at 300 °C substrate temperature. Different procedures for the CdTe growth and for the preparation of the GaAs substrates resulted in diverse crystalline qualities of the CdTe films. We present the results obtained from PhotoReflectance (PR) measurements of these films employing HeNe and Ar-ion lasers as modulating excitation. For Ar excitation, the ratio of CdTe to GaAs signal strength for the E ₀ transition is enhanced, allowing a differentiation of the contributions from film and substrate. Both the PR line shape and intensity are correlated to the structural quality of the CdTe films. One of the samples presented a below-band-gap transition of the GaAs substrate around 30±5 meV from E ₀ which is attributed to donor states produced by Te atoms diffused in the interface; this result demonstrates the high sensitivity of the photoreflectance technique to the structural properties of interfaces.     

Reactive pulsed laser deposition and laser induced crystallization of SnO2 transparent conducting thin films

Transparent conducting SnO₂ thin films with a thickness between 1000–2000 Å were deposited on glass, quartz and silicon substrates using standard pulsed laser deposition techniques with two different targets (Sri and SnO₂) and with three different laser wavelengths (1.06, 0.532 and 0.266 ) from a Q-switched Nd: YAG laser. Tin dioxide films with optical transmission over most of the visible spectrum exceeding 80% were obtained using a Sn target and a background oxygen pressure of 20 Pa. The electrical resistivity () depended strongly on the substrate temperature during deposition, with the lowest values of of about 10^–2 -cm obtained when the substrate was maintained at 400°C during deposition. Using SnO₂ targets, predominantly amorphous phase SnO₂ films were deposited on Si substrates and then transformed into polycrystalline Sn₃O₄ by laser induced crystallization ( = 1.06 m). Whereas these later films were essentially non-conducting as deposited ( > 400 -cm), the electrical resistivity was permanently reduced after laser induced crystallization by a factor greater than 1000 to a value of approximately 4 × 10^–1 -cm.     

Dopant-induced excimer laser ablation of Poly(tetrafluoroethylene)

Poly(tetrafluoroethylene) (PTFE) does not exhibit excimer laser etching behavior at conventional, e.g., single photon absorption, emissions of 193, 248, and 308 nm, due to the lack of polymer/photon interaction. This is not surprising since the electronic transitions available to the PTFE molecule are high energy and thus require short wavelength the radiation However, by incorporating a small quantity of material into the non-absorbing fluoropolymer matrix that interacts strongly with the emitted laser energy, e.g., a dopant, successful ablation, both in terms of etch rate and structuring quality occurs. Specifically, excimer laser ablation of PTFE films containing 5, 10, and 15% polyimide (wt/wt) as a dopant was achieved at 308 nm in a fluence range of 1 to 12 J/cm². Ablation rates for the materials increased with increasing fluence and, at the polyimide levels investigated, varied inversely with dopant concentration. All compositions exhibited excellent structuring quality.     

光电极值法结合外差干涉法监控膜厚的研究

光电极值法是光学薄膜厚度监测的常用方法,该方法在镀膜前引用块状材料的折射率设计膜系。而在实际镀制过程中,用于镀制光学薄膜的材料折射率会发生改变,从而给膜厚的监控带来误差。为了避免折射率变化的影响,采用外差干涉法测量折射率,将实际测得的薄膜折射率应用光电极值法监控薄膜的设计,从而减少了因材料折射率的变化引起的误差。以750nm截止滤光片的镀制为被测对象进行了实验,对制备的滤光片透射率光谱曲线进行了比较。结果表明,实际的透射率曲线与设计的透射率曲线吻合较好,两次实验曲线平均吻合度均在98%以上,系统稳定性很好,从而说明结合外差干涉法的光电极值监控法可以很好地克服折射率变化引起的误差。     

Infrasound detection by laser diode self-mixing interferometry

An alternative technique for infrasound detection based on the self-mixing (SM) interference of a laser diode is described. The principle involved is the dependence of the power emitted by the laser diode on infrasound-induced membrane vibration. The Fourier transform and fringe-counting methods are used to analyze the self-mixing signal. Infrasound signals are experimentally measured from 2 to 20 Hz with a resolution of 0.25, and the results well agree with the theoretical ones.     

Absorption and saturation mechanisms in aluminium laser ablated plasmas

-2 ). The interpretation of the ion TOF distributions in terms of theoretical shifted Maxwell–Boltzmann distributions produces a good agreement with the experimental data. This has allowed us to infer the ion flow velocity and temperature associated with the measured TOF distributions, as well as the ion kinetic energies as a function of the laser fluence. We have also studied the total ion yield at different laser fluences. Our results show that all the plume parameters investigated are increasing functions of the laser fluence until a saturation plateau is reached at high fluences (>20 Jcm^-2). We ascribe this saturation behav iour to strong absorption and partial, or total, reflection of the laser light by the hot plasma produced by the leading edge of the intense laser pulse. This interpretation is supported by a semi-quantitative analysis of the laser photon absorption and ionization mechanisms in Al plasma, at both laser wavelengths. Received: 6 January 1997/Accepted: 14 March 1997