Formation techniques for upper active channel in monolithic 3D integration: an overview.

The concept of three-dimensional stacking of device layers has attracted significant attention with the increasing difficulty in scaling down devices. Monolithic 3D (M3D) integration provides a notable benefit in achieving a higher connection density between upper and lower device layers than through-via-silicon. Nevertheless, the practical implementation of M3D integration into commercial production faces several technological challenges. Developing an upper active channel layer for device fabrication is the primary challenge in M3D integration. The difficulty arises from the thermal budget limitation for the upper channel process because a high thermal budget process may degrade the device layers below. This paper provides an overview of the potential technologies for forming active channel layers in the upper device layers of M3D integration, particularly for complementary metal-oxide-semiconductor devices and digital circuits. Techniques are for polysilicon, single crystal silicon, and alternative channels, which can solve the temperature issue for the top layer process.

Impact of Pt grain size on ferroelectric properties of zirconium hafnium oxide by chemical solution deposition.

The effects of the grain size of Pt bottom electrodes on the ferroelectricity of hafnium zirconium oxide (HZO) were studied in terms of the orthorhombic phase transformation. HZO thin films were deposited by chemical solution deposition on the Pt bottom electrodes with various grain sizes which had been deposited by direct current sputtering. All the samples were crystallized by rapid thermal annealing at 700 °C to allow a phase transformation. The crystallographic phases were determined by grazing incidence X-ray diffraction, which showed that the bottom electrode with smaller Pt grains resulted in a larger orthorhombic phase composition in the HZO film. As a result, capacitors with smaller Pt grains for the bottom electrode showed greater ferroelectric polarization. The smaller grains produced larger in-plane stress which led to more orthorhombic phase transformation and higher ferroelectric polarization.

In situ TEM observation of void formation and migration in phase change memory devices with confined nanoscale Ge2Sb2Te5.

The reliability of Ge-Sb-Te phase-change memory (PCM) devices has been limited by failure due to void formation and this still remains one of the critical issues affecting their use in storage-class memory applications. To directly observe the void formation processes in real-time, we implemented in situ switching of PCM devices by applying set and reset voltage pulses to a Ge2Sb2Te5 (GST) cell inside a transmission electron microscope (TEM). The in situ TEM observations directly show that a void nucleates preferentially near the TiN bottom electrode in the GST cell, where the temperature is the highest. The nucleated void grows gradually until it reaches a certain size while migrating slowly toward the positively biased electrode. The fully grown void then continues migrating toward the positively biased electrode in subsequent set pulses. The observed polarity-dependent void migration can be explained by the field-induced redistribution of the constituent elements, especially by the electromigration of under-coordinated Te- ions which have vacancies around them. When the reset pulse with the same voltage polarity is applied, the voids exhibit a slight volume shrinkage but are not completely eliminated, resulting in a reset-stuck failure. The present in situ TEM observations revealing the nucleation, growth, and polarity-dependent migration of voids will contribute to the fundamental understanding of the failure by void formation in nanoscale GST-based PCM devices and help improving the design of reliable PCM devices.

Polarization-angle-scanning 2DIR spectroscopy of coupled anharmonic oscillators: a polarization null angle method.

Two-dimensional (2D) optical spectroscopy based on stimulated photon echo geometry requires multiple ultrashort pulses of which spatiotemporal properties and optical phases can be precisely controlled. Also, it is possible to change the incident beam polarization directions defined in a laboratory frame. Here, we introduce the polarization-angle-scanning (PAS) 2D spectroscopy and show that the diagonal and cross-peak amplitudes in the 2D spectrum can be arbitrarily modulated by spatially controlling the beam polarization directions. For a coupled anharmonic oscillator system, we specifically demonstrate that either diagonal or cross-peaks in the measured 2DIR spectra can be selectively eliminated and show that such polarization angles provide direct information on the relative angles between coupled transition dipoles and thereby on intricate details of molecular structures. We thus anticipate that the present PAS 2D optical spectroscopy can be a useful experimental method enabling us to probe structural evolutions of nonequilibrium state molecules by monitoring the time-dependent changes of the relative transition dipole directions.

Ultrafast vibrational spectroscopy of cyanophenols.

Aromatic compounds with electron-donating or -accepting substituents exhibit interesting resonance effects on a variety of chemical reactivities and optical properties. To understand such effects and the possible relationship between vibrational energy dissipation pathways and resonance structures of aromatic compounds, we studied ortho-, meta-, and para-substituted cyanophenols and their anionic forms in methanol by using time- and frequency-resolved pump-probe and two-dimensional IR spectroscopy, where the nitrile group acts as an IR probe. From the measured transient spectra and singular-value decomposition analyses, we found that there is a combination band whose frequency is very close to that of the nitrile stretch mode. Due to the difference in the lifetimes of these two mode excited states, the transient pump-probe spectra commonly show notable blue-shifting behaviors in time. Comparing the vibrational lifetimes of neutral cyanophenols and cyanophenoxide anions in methanol and carrying out quantum mechanical/molecular mechanical molecular dynamics simulations to study hydrogen-bonding dynamics, we found that the vibrational energy of the nitrile stretch mode initially relaxes to intramolecular degrees of freedom instead of solvent modes. Also, the vibrational anharmonic frequency shifts, intrinsic lifetimes, and bandwidths of the nitrile stretch mode and the combination mode in these molecular systems are fully characterized, and their relationships with resonance structures are discussed. It is believed that the present work sheds light on the intrinsic vibrational relaxation process of the nitrile stretch mode in cyanophenols, even in the case when their IR spectra are congested by the spectrally overlapping combination bands, and the resonance effects of aromatic compounds on vibrational dynamics and relaxation processes.

Two-photon absorption properties of hexa-substituted benzene derivatives. Comparison between dipolar and octupolar molecules.

Hexa-substituted benzene derivatives show larger two-photon cross sections than the corresponding dipolar counterparts and the largest values of delta(max) = 2000 GM and delta(max)/MW = 2.25 GM g(-1) have been obtained with a compound having C[triple bond]C and C=C bonds in the conjugation bridge.

Integrated and dispersed photon echo studies of nitrile stretching vibration of 4-cyanophenol in methanol.

By means of integrated and dispersed IR photon echo measurement methods, the vibrational dynamics of C-N stretch modes in 4-cyanophenol and 4-cyanophenoxide in methanol is investigated. The vibrational frequency-frequency correlation function (FFCF) is retrieved from the integrated photon echo signals by assuming that the FFCF is described by two exponential functions with about 400 fs and a few picosecond components. The excited state lifetimes of the C-N stretch modes of neutral and anionic 4-cyanophenols are 1.45 and 0.91 ps, respectively, and the overtone anharmonic frequency shifts are 25 and 28 cm(-1). At short waiting times, a notable underdamped oscillation, which is attributed to a low-frequency intramolecular vibration coupled to the CN stretch, in the integrated and dispersed vibrational echo as well as transient grating signals was observed. The spectral bandwidths of IR absorption and dispersed vibrational echo spectra of the 4-cyanophenoxide are significantly larger than those of its neutral form, indicating that the strong interaction between phenoxide and methanol causes large frequency fluctuation and rapid population relaxation. The resonance effects in a paradisubstituted aromatic compound would be of interest in understanding the conjugation effects and their influences on chemical reactivity of various aromatic compounds in organic solvents.

Femtosecond characterization of vibrational optical activity of chiral molecules.

Optical activity is the result of chiral molecules interacting differently with left versus right circularly polarized light. Because of this intrinsic link to molecular structure, the determination of optical activity through circular dichroism (CD) spectroscopy has long served as a routine method for obtaining structural information about chemical and biological systems in condensed phases. A recent development is time-resolved CD spectroscopy, which can in principle map the structural changes associated with biomolecular function and thus lead to mechanistic insights into fundamental biological processes. But implementing time-resolved CD measurements is experimentally challenging because CD is a notoriously weak effect (a factor of 10(-4)-10(-6) smaller than absorption). In fact, this problem has so far prevented time-resolved vibrational CD experiments. Here we show that vibrational CD spectroscopy with femtosecond time resolution can be realized when using heterodyned spectral interferometry to detect the phase and amplitude of the infrared optical activity free-induction-decay field in time (much like in a pulsed NMR experiment). We show that we can detect extremely weak signals in the presence of large achiral background contributions, by simultaneously measuring with a femtosecond laser pulse the vibrational CD and optical rotatory dispersion spectra of dissolved chiral limonene molecules. We have so far only targeted molecules in equilibrium, but it would be straightforward to extend the method for the observation of ultrafast structural changes such as those occurring during protein folding or asymmetric chemical reactions. That is, we should now be in a position to produce 'molecular motion pictures' of fundamental molecular processes from a chiral perspective.

Femtosecond spectral interferometry of optical activity: theory.

Optical activities such as circular dichroism (CD) and optical rotatory dispersion (ORD) are manifested by almost all natural products. However, the CD is an extremely weak effect so that time-resolved CD spectroscopy has been found to be experimentally difficult and even impossible for vibrational CD with current technology. Here, we show that the weak-signal and nonzero background problems can be overcome by heterodyned spectral interferometric detection of the phase and amplitude of optical activity free-induction-decay (OA FID) field. A detailed theoretical description and a cross-polarization scheme for selectively measuring the OA FID are presented and discussed. It is shown that the parallel and perpendicular electric fields when the solution sample contains chiral molecules are coupled to each other. Therefore, simultaneous spectral interferometric measurements of the parallel and perpendicular FID fields can provide the complex susceptibility, which is associated with the circular dichroism and optical rotatory dispersion as its imaginary and real parts, respectively. On the basis of the theoretical results, to examine its experimental possibility, we present numerical simulations for a model system. We anticipate the method discussed here to be a valuable tool for detecting electronic or vibrational optical activity in femtosecond time scale.

Two-photon fluorescent probes for biomembrane imaging: effect of chain length.

Two-photon fluorescent probes for the cellular membrane, derived from 6-acyl-2-aminonaphthalene as the fluorophore and hexanoyl (CH), lauryl (CL), and stearyl (CS) groups as the receptor, have been synthesized. Their photophysical properties and utility as membrane probes were also studied. Whereas CH cannot be used as a membrane probe due to its high water solubility, CL and CS are useful two-photon probes for membrane lateral heterogeneity, as they can easily stain cells, emit fluorescence with high sensitivity to the environment polarity, and are capable of imaging the membrane lateral heterogeneity in live cells. Moreover, CS is more likely to be located in the plasma membrane due to its negligible water solubility. Our results show that the liquid ordered-like domain covers 31-35% of the cellular surface.

Two-photon absorption properties of 9,10-disubstituted 2,6-bis(p-dihexylaminostyryl)anthracene derivatives. Effect of 9,10-substituents.

A series of 2,6-bis(p-dihexylaminostyryl)anthracence derivatives having phenyl, styryl, and phenylethynyl groups at 9,10-positions (1-4) have been synthesized and their two-photon cross-sections were determined. Overall, the wavelengths of the longest wavelength absorption band and emission spectra increase with increase in the conjugation length and the electron withdrawing ability of the 9,10-substituents. All compounds show two-photon cross sections in the range of 740-3940 GM at 780-960 nm, which increase significantly by the donor and acceptor groups at 9,10-positions. In addition, Ph and phenylethynyl groups are better when compared to the styryl group at the 9,10-positions in terms of the two-photon action cross section. From a practical perspective, 1a, 2a-c, and 4b showed significant two-photon action cross-section and are most useful for applications that use two-photon excited fluorescence.

Magnesium ion selective two-photon fluorescent probe based on a benzo[h]chromene derivative for in vivo imaging.

A novel, two-photon probe for the detection of free Mg2+ ions in living cells and live tissues has been developed. The probe can be excited by 880 nm laser photons, emits strong two-photon excited fluorescence in response to Mg2+ ions, can be easily loaded into the cell and tissue, shows high photostability, and can measure the Mg2+ ion concentration without interference by Ca2+ ions in living cells. The intracellular dissociation constant (Kdi) for Mg2+ determined by the two-photon process is 2.5 mM, which is suitable for dynamic Mg2+ concentration measurement. In addition, the probe is capable of imaging endogenous stores of free Mg2+ at a few hundred micrometers depth in live tissues using two-photon microscopy (TPM).

A two-photon fluorescent probe for lipid raft imaging: C-laurdan.

The lipid-rafts hypothesis proposes that naturally occurring lipid aggregates exist in the plane of membrane that are involved in signal transduction, protein sorting, and membrane transport. To understand their roles in cell biology, a direct visualization of such domains in living cells is essential. For this purpose, 6-dodecanoyl-2-(dimethylamino)naphthalene (laurdan), a membrane probe that is sensitive to the polarity of the membrane, has often been used. We have synthesized and characterized 6-dodecanoyl-2-[N-methyl-N-(carboxymethyl)amino]naphthalene (C-laurdan), which has the advantages of greater sensitivity to the membrane polarity, a brighter two-photon fluorescence image, and reflecting the cell environment more accurately than laurdan. Lipid rafts can be visualized by two-photon microscopy by using C-laurdan as a probe. Our results show that the lipid rafts cover 38 % of the cell surface.

First hyperpolarizabilities of 1,3,5-tricyanobenzene derivatives: origin of larger beta values for the octupoles than for the dipoles.

A series of donor-acceptor substituted stilbene and diphenylacetylene derivatives and their octupolar analogues have been synthesized and the linear and nonlinear optical properties (beta) studied by both experiments and theoretical calculation. The lambda(max) of the dipoles increases with the conjugation length and is always larger when the C=C bond is used, instead of the C[triple bond]C bond, as the conjugation bridge. Although the lambda(max) values of the octupoles show no clear trend, they are much larger than those of the dipoles. The beta(0) values of the dipoles increase with conjugation length and as the conjugation bridge is changed from the C[triple bond]C to C=C bond. This increase is accompanied by an increase in either lambda(max) or the oscillator strength. Similarly, the beta(0) values of the octupoles increase with the conjugation length and with a change in the donor in the order: NEt2 < N(i-amyl)Ph < NPh2. Moreover, beta(yyy)/beta(zzz) ratios are in the range of 1.6-3.9 and decrease with the conjugation length. Beta values calculated by the finite-field and sum-over-states methods are in good agreement with the experimental data. Also, there is a parallel relationship between the calculated beta values and bond length alternation (BLA). From these results, the origin of the larger beta values for octupoles than for dipoles is assessed.

Two-photon dyes containing heterocyclic rings with enhanced photostability.

A series of donor-pi-donor derivatives containing phenyl, naphthyl, and anthryl groups as the pi-center and heterocyclic rings as the conjugation bridge have been synthesized and their one- and two-photon spectroscopic properties and photostability were determined. These compounds show bathochromic shifts in the absorption and emission spectra, larger two-photon cross-section, and enhanced photostability in comparison to their open-chain analogues. In addition, a convenient method for the qualitative photostability measurement is proposed.

Two-photon absorption properties of 2,6-bis(styryl)anthracene derivatives: effects of donor-acceptor substituents and the pi center.

A series of 2,6- and 2,7-bis(styryl)anthracene derivatives with the donors at the styryl group and acceptors at the 9,10-positions have been synthesized, and their two-photon cross sections (Phidelta(max)) were determined. These compounds exhibit a peak two-photon absorptivity (delta(max)) in the range of 700-2500 GM at 780-1030 nm. Values of lambda(max) and Stokes shifts increase as the acceptor is changed to a stronger one. There is also a parallel increase in lambda(2)max and delta(max) with the same variation of the chromophore structure. Both lambda (2)(max)and Phidelta(max) have been optimized by introducing donor-substituted styryl groups at the 2,6-positions and p-cyanophenyl groups at the 9,10-positions, respectively. The effect of a pi center on the two-photon absorption properties has been assessed by comparing the existing data for a variety of D-pi-D derivatives.

2,6-Bis[4-(p-dihexylaminostyryl)styryl]anthracene derivatives with large two-photon cross sections.

[Structure: see text] Anthracene derivatives with a variety of donor-acceptor substituents have been synthesized and shown to exhibit large two-photon cross sections over a wide range of wavelengths.

Two-photon sensor for metal ions derived from azacrown ether.

A two-photon sensor for the metal ions derived from azacrown ether as the receptor is reported. The sensor emits strong two-photon fluorescence when excited by 800 nm laser photons. Moreover, the binding constants measured by the one- and two-photon fluorescence are similar. This result may be useful for the design of efficient two-photon fluorescence probes for biological substrates.