Pulsed excimer laser deposition of ferroelectric thin films

Abstract

Pulsed UV excimer laser ablation was employed to deposit multi-axial, bi-axial and uni-axial ferroelectric compositions of PZT, bismuth titanate and lead germanate respectively. In general, a fluence lower than 2 J/cm² caused a preferential evaporation of volatile components, resulting in stoichiometric imbalance. However, the fluences beyond 2 J/cm² enabled the deposition of stoichiometric thin films of multi-component oxide systems. The intrinsic bombardment due to the energetic ablated species during the thin film deposition seemed to influence the composition, structure, orientation and the electrical properties. The electrical characterization of ferroelectric films indicated a dielectric constant of 800–1000, a P_r of 32μC/cm² and E_c of 130KV/cm for polycrystalline PZT films and the corresponding quantities were measured to be 150, 7 μC/cm² and 20 KV/cm for in-situ crystallized c-axis preferred oriented bismuth titanate films. Lead germanate thin films oriented along c-axis (003) showed a dielectric constant of 30, a P_r of 2.5 μC/cm² and E_c of 55 KV/cm.     

Antiferroelectric lead zirconate thin films by excimer laser ablation

Abstract

Utilization of antiferroelectric thin films was proposed for high charge storage capacitors and transducer applications. The volume changes that are associated with the AFE? FE and FE ? AFE phases are high enough to use them in MEMs device technology. Lead zirconate was the first identified antiferroelectric compound with a reported dielectric phase transition temperature of ～ 230°C. In this article, deposition of lead zirconate thin films by a pulsed excimer laser ablation technique is reported. The antiferroelectric nature of the lead zirconate thin films were confirmed by the presence of double hysteresis loop in polarization vs. applied electric field response as well as double butterfly behavior in capacitance vs. voltage characteristics. The variations in the polarization hysteresis with temperature were elucidated in detail. The switching times between the field induced FE and AFE phases (backward switching) were studied at various applied electric fields.     

Optical parametric amplification of femtosecond ultraviolet laser pulses

Broad-band amplification of femtosecond laser pulses using the scheme of noncollinear optical chirped pulse parametric amplification is modeled. The effect of two-photon absorption at the pump wavelength was also taken into account. The signal pulses range from 220 to 410 nm with pump pulses at 267, 248, and 213 nm. The best four crystals chosen among 12 possible ones are BBO, KDP, CLBO, and LB4. In an experiment, 30-fs laser pulses at 400 nm were amplified in a BBO crystal pumped by 267 nm pulses, exhibiting a single pass gain of 3550. The gain was found spectrally flat within the available 17-nm bandwidth of the signal pulse.     

Pulse-extended excimer laser annealing of lead zirconate titanate thin films

Abstract

Excimer laser annealing is a promising method for the crystallisation of ferroelectric layers, such as PZT, in low thermal budget integrated device fabrication processes. A technique is described whereby the problem of very high surface temperatures in PZT is overcome by the use of temporal pulse extension, where the effective laser pulse length is increased from 24ns to 374ns. Modelled temperature profiles through a PZT thin film structure during laser irradiation illustrate the benefit of pulse extension due to enhanced heat propagation into the PZT. The modelling also shows that underlying silicon is not heated significantly even with pulse extension. Initial results show that PZT can be crystallised into the perovskite phase from the top downwards with minimal surface damage.     

Process research on micro-machining diamond microgroove by femtosecond laser

Abstract

Using micro-nano processing system of 120 fs and 800?nm low-frequency femtosecond laser, the microgroove was processed on the CVD diamond. The effects of some processing parameters on the microgroove size were studied. The results showed that the depth and width of microgrooves increase with the increase of laser power, scanning times, while with the decrease of scanning speed. Moreover, the suitable scanning speed of femtosecond laser processing diamond is about 0.1?mm/s, and focusing on diamond can improve material removal efficiency and processing quality. It is indicated that these research work has certain guiding significance for processing diamond microchannels for heat dissipation of electronic chips.     

Compact geometry for diode-pumped Cr:LiSAF femtosecond laser

A novel geometry for dispersion compensation in femtosecond lasers without specific cut of optical parts in a flat/Brewster angle or Gires-Tournots mirrors is demonstrated for attaining a compact diode-pumped Cr:LiSAF femtosecond pulse laser. In this geometry, the laser medium lies between the dispersion-compensation prism pair. This approach enables the laser to operate at variable repetition rates from 163 to 235 MHz keeping the pulsewidth less than 90 fs, where the value of the time-bandwidth product are not larger than 0.34. An 89 fs pulse duration generated at the 235 MHz repetition rate is, to our knowledge, the shortest pulse achieved in an all-solid-state Kerr-lens mode-locked laser operating above the 200 MHz repetition rate     

Control of SF6 discharge gap switch by XeCl excimer laser

Experimental studies have been carried out on triggering characteristics of the SF₆ discharge gap switch by use of XeCl excimer laser (wavelength = 308 nm). First, laser irradiation characteristics are studied on a pure SF₆ in the pressure range of p = 160 ～ 3,800 torr. Using a lens of f (focal length) = 133 mm, the laser is irradiated into the gas, where the energy absorption is studied. If the laser is injected into the gas with the energy above a certain threshold for the breakdown, the rate of energy absorption is found to be ～ 17 percent of the incident energy at p > 760 torr. Injecting the laser into the SF₆-filled gap switch (gap length = 7 mm, p = 760 torr), we have studied the triggering characteristics. Excellent triggering characteristics were obtained; delay time for the discharge ～ 20 ns, and the jitter ～ 260 ps when the gap voltage is operated at 99 percent of the self-breakdown voltage. In addition, the triggering characteristics are studied by changing the focusing point axially. It is found that both the delay time and the jitter decrease when the focusing point tends to approach the high-voltage electrodes.     

Reconstruction of the time profile of femtosecond laser pulsesthrough cross-phase modulation

Experimental results concerning the determination of the time profile of femtosecond laser pulses are presented. The method is based on the analysis of the changes of the laser spectrum induced at different delays by cross-phase modulation. Pulses from a 30-fs Ti:sapphire laser at 800 nm are characterized over a high dynamic range. Second-harmonic pulses at 400 nm from the same laser are also characterized, exhibiting the expected improvement in contrast ratio     

Generation and spatiotemporal evolution of optical vortices in femtosecond laser pulses

Topology has become one of the key concepts allowing one to understand the intrinsic, qualitative properties of phenomena throughout various scientific fields. To date, this concept has been extended to the field of material science and technology. On the other hand, we can now utilize the spatially controlled light defined by the topology (so‐called “optical vortices”) in order to characterize the topological properties of materials. In particular, optical vortices in femtosecond pulses will be invaluable for advanced topological spectroscopy. In this work, the authors created femtosecond optical vortices using a spatial light modulator. Their spatiotemporal properties were evaluated using interferogram and correlation measurements. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 39–46, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20791     

Waveguide shaping and writing in fused silica using a femtosecond laser

Fused silica waveguides were written and reshaped with an 800-mn amplified femtosecond laser in a lateral writing configuration. The asymmetry of the waveguide shape and the index profile was partially corrected through multiple scans shifted by 1 or 2 /spl mu/m. Following the careful adjustment of both laser pulse energy and writing speed, the resulting waveguides exhibited substantial reduction in propagation and coupling losses by more than 50% in the C-band.     

Application of HfO2 high-k gate insulator for excimer laser annealed poly-Si TFT

The electrical characteristics of polycrystalline silicon (poly-Si) thin film transistor (TFT) crystallized by excimer laser annealing (ELA) method with high-k gate dielectrics were evaluated. Because a high thermal budget is inevitable for conventional fabricating process of poly-Si TFTs, an amorphous silicon film on a buried oxide was crystallized by annealing with a KrF excimer laser (248) to fabricate a poly-Si film at low temperature. Furthermore, the high permittivity HfO₂ film with a thickness of 20 nm as the gate-insulator was deposited by atomic layer deposition (ALD) to low temperature process. In addition, the solid phase crystallization (SPC) was compared to the ELA method as a crystallization technique of amorphous-silicon film. As a result, the crystallinity and surface roughness of poly-Si crystallized by ELA method was superior to the SPC method. Also, we obtained excellent device characteristics from the poly-Si TFT fabricated by the ELA crystallization method.     

Experimental investigation of femtosecond laser micro/nano processing of quartz material

Abstract

Quartz material is widely used in the automotive industry, aerospace, optical and electronic fields and so on. But quartz is a hard and brittle material, which makes it very difficult to process. Femtosecond laser is applied to micro/nano processing of quartz material due to its excellent performance. This paper uses different energy density femtosecond lasers to process quartz materials and analyzes the surface morphology. The experimental results show that the etching threshold of quartz material is 4.63?J/cm². The etching depth of the single pulse and the femtosecond laser beam waist radius are 0.005?μm and 8.7?μm, respectively. Besides, there are some granular characteristics on the side wall of etching groove that affect surface quality. The larger the laser power, the more deposits there are.     

Mechanism for self-synchronization of femtosecond pulses in atwo-color Ti:sapphire laser

An experimental and theoretical analysis of the nonlinear coupling mechanism between the two solitary pulses circulating in a two-color femtosecond laser is presented. Two operation regimes; synchronized; and nonsynchronized; and a hysteresis of the transition between the two regimes are clearly observed; while independent modelocking and tunability of the output pulse trains is found in both regimes. Pulses in the range from 15 to 100 fs are synchronized with a timing jitter below 2 fs. The combined effects of cross-phase modulation and negative group velocity dispersion are shown to be responsible for the strong pulse correlation in the synchronized regime. Our experimental observations are in agreement with numerical simulations, thus confirming the theoretical model     

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variat...     

Development of dicing technique for thin semiconductor substrate using temporally shaped femtosecond laser

Recently, semiconductor substrates for integrated circuits (ICs) have been required to be as thin as 50 µm, because many electronic devices must be miniaturized and light in weight. Machining of such thin substrates with conventional dicing techniques is very difficult. Therefore, we have proposed processing them using femtosecond laser ablation. In this work, we investigate the influence of conditions of a double pulsed laser such as the delay time and fluence on the depth and diameter in order to develop a new dicing technique for very thin ICs. A double pulsed laser (λ = 780 nm, τ = 150 fs, f = 10 Hz, Δt = 0 to 100 ps, E₁ + E₂ = 100 µJ) was focused on the Si substrate with a plano‐convex lens having a nominal focal length of 100 mm. At a delay time of 10 ps, singularly shallow and flat‐bottomed holes were obtained. When the substrates were diced under these conditions, the bottom of the processing groove was flat and very smooth, whereas many microcracks starting from the bottom of the groove formed by the conventional method have been observed. From these results, we were able to identify femtosecond laser processing conditions that were applicable to dicing of thin Si substrates. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(3): 43–48, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20028     

Orthogonal control of femtosecond mode-locked laser having zero carrier-offset frequency with three-axis PZT

The authors demonstrate an optical frequency synthesizer based on a femtosecond (fs) mode-locked Ti:sapphire (Ti:s) laser by simultaneously stabilizing the carrier-offset frequency f/sub ceo/ and repetition rate f/sub rep/, referenced to the Cs atomic frequency standard. By using two wide-band digital phase-detectors they realize a phase-coherent link between f/sub rep/ and f/sub ceo/ with the relation f/sub ceo/=f/sub AOM/-5/6f/sub rep//spl equiv/0, where f/sub AOM/=5/6f/sub rep/ is the phase-locked driving frequency of an acousto-optic modulator in a self-referencing interferometer and f/sub rep/=100 MHz. For the simultaneous control of f/sub rep/ and f/sub ceo/, an orthogonalization scheme using a three-axis piezoelectric transducer is introduced by which the end mirror of fs laser is controlled. As a result, the authors could stabilize all components of the fs laser comb at once with an equal frequency separation f/sub rep/=100 MHz with f/sub ceo/=0. In their optical frequency synthesizer, the frequency of the nth component (f/sub n/) is given exactly by the simple relation f/sub n/=nf/sub rep/, enabling them to use the fs laser comb as a frequency ruler in the optical frequency metrology.     

Waveguide lasers in the C-band fabricated by laser inscription with a compact femtosecond oscillator

Femtosecond laser writing of high-quality optical waveguides by means of a compact diode-pumped cavity-dumped Yb:glass laser oscillator is reported. Waveguides have been written on an erbium-ytterbium co-doped phosphate glass for active device production. A detailed analysis of the optical characteristics of waveguides written with different repetition rates, pulse energies, and translation speeds is presented, and an optimum set of writing parameters is established. Coupling losses as low as 0.1 dB/facet with standard telecom fibers and propagation losses lower than 0.4 dB/cm have been obtained. Characterization of the active properties of the waveguides is also presented, together with the demonstration of waveguide laser action in the whole C-band with peak output power of more than 30 mW.     

Surface texturing of aluminum alloy 2024-T3 via femto- and nanosecond pulse excimer laser irradiation

Surface texturing effects of aluminum alloy 2024-T3 using femtosecond and nanosecond pulse laser irradiation were studied. The micrographs of the scanning electron microscopy (SEM) were characterized as a function of incident laser fluence. Results indicated that the surface features, ranging from nano to microdimension, can be developed through variation in laser fluence intensities. Two ablation regimes in the logarithmic fluence dependence of the ablated depth for the 500-fs-pulse irradiation were observed. The theoretical analysis for ablation processes is in good agreement with the experimental results.     

Analysis of noise mechanisms limiting the frequency stability of microwave signals generated with a femtosecond laser

Excess phase noise is observed in the spectrum of the microwave signal extracted from a photodetector illuminated by a train of ultrashort light pulses from the femtosecond laser. This noise affects the stability of frequency transfer from optical to microwave domains with the femtosecond laser. Some contributions to the excess phase noise are related to intrinsic beam-pointing fluctuations of the femtosecond laser and optical power fluctuations of the detected light. These factors contribute to excess phase noise at the harmonics of the pulse repetition rate due to power-to-phase conversion in the photodetector, spatially dependent time delays, and photodiode nonlinearities that distort the pulse shape. With spatial filtering of the laser beam and active control of its power, the additional fractional frequency fluctuations of pulse repetition rate associated with the excess noise of the photodetection process were reduced from 6/spl middot/10/sup -14/ to approximately 3/spl middot/10/sup -15/ over 1 s of averaging. The effects of other noise mechanisms, such as laser shot noise and phase noise introduced by a microwave amplifier, were also examined but were found to be at a less significant level.     

Characterization of Pb(Zr,Ti)O3 thin films on SOI prepared by excimer laser deposition

Abstract

Ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films were prepared by pulsed excimer laser deposition on Silicon-on-Insulator (SOI) substrates with and without an electrode. Their properties can be improved by rapid thermal annealing, based on the structural and interfacial characteristics analysis by X-ray diffraction, Rutherford backscattering spectroscopy and automatic spreading resistance measurements. The thin films were revealed of to be polycrystalline perovskite structure with mainly ?100? and ?110? orientations; the crystallite size and the structure are dependent on the annealing time. The PZT thin films did not interact with the top silicon layers of SOI, and the composition was on the tetragonal side of the morphotropic phase boundary in the PbTiO₃-PbZrO₃ phase diagram.