Jianing Sun

Determination of pore-size distribution in low-dielectric thin films

Positronium annihilation lifetime spectroscopy is used to determine the pore-size distribution in low-dielectric thin films of mesoporous methylsilsesquioxane. A physical model of positronium trapping and annihilating in isolated pores is presented. The systematic dependence of the deduced pore-size distribution on pore shape/dimensionality and sample temperature is predicted using a simple quantum mechanical calculation of positronium annihilation in a rectangular pore. A comparison with an electron microscope image is presented.

Transfer matrix method for interface optical-phonon modes in multiple-interface heterostructure systems

Interactions of carriers with interface optical phonons dominate over other carrier–phonon scatterings in narrow quantum-well structures. Herein, a transfer matrix method is used to establish a formalism for determining the dispersion relations, electrostatic potentials, and Frohlich interaction Hamiltonians of the interface optical phonons for multiple-interface heterostructure systems within the framework of the macroscopic dielectric continuum model. This method facilitates systematic calculations for complex structures where the conventional method is very difficult to implement. Several specific cases are treated to illustrate the advantages of the general formalism.

Probing diffusion barrier integrity on porous silica low-k thin films using positron annihilation lifetime spectroscopy

The technique of positron annihilation lifetime spectroscopy (PALS) has been used to investigate the continuity and thermal stability of thin barrier layers designed to prevent Cu atom diffusion into porous silica, low-dielectric constant (k) films. Nanoglass™ K2.2-A10C (A10C), a porous organosilicate film, is determined to have interconnected pores with an average tubular-pore diameter of (6.9 ± 0.4) nm. Cu deposited directly on the A10C films is observed to diffuse into the porous structure. The minimum necessary barrier thickness for stable continuity of Ta and TaN layers deposited on A10C is determined by detecting the signal of positronium (Ps) escaping into vacuum. It is found that the 25 nm thick layers do not form continuous barriers. This is confirmed by the presence of holes observed in such films using a transmission electron microscope. Although 35 nm and 45 nm Ta and TaN layers perform effectively at room temperature as Ps barriers, only the Ta-capped samples are able to withstand heat treatm...

Depth-profiling plasma-induced densification of porous low-k thin films using positronium annihilation lifetime spectroscopy

Positronium annihilation lifetime spectroscopy (PALS) has been used to depth profile the densification induced in a porous low-dielectric constant (k) thin film by typical device integration processing, including exposure to plasmas and oxygen ashing. Such “integration damage” has previously been observed as an undesirable increase in k accompanied by shrinkage in the porous film thickness. PALS confirms that the structural damage is confined to a surface layer of collapsed pores with the underlying pores being undamaged. The dense layer thickness determined by PALS increases with plasma exposure time.

Observation of intrinsic bistability in resonant tunneling diode modeling

Intrinsic bistability has been observed experimentally and attributed to the effect on the potential profile from stored charge in the quantum well through Poisson’s equation. This effect leads to two possible current states corresponding to a single voltage within the negative resistance region. In this letter a simulation method is presented which clearly shows bistability in the current‐voltage curve of a resonant tunneling diode. This method self‐consistently combines a Thomas–Fermi equilibrium model for the electron concentrations outside the double‐barrier structure with a quantum calculation for the concentration inside the structure.

C-V and I-V characteristics of quantum well varactors

A theoretical model for quantum well varactors is presented. The model is used to calculate the device C‐V and I‐V characteristics and very good agreement has been found between the calculated and measured results. Based on the model, a triple barrier double well varactor has been designed and fabricated. A very high capacitance ratio within a very small bias range is achieved, as designed. Details of the design calculations and experimental results are presented.

Phonon assisted intersubband transitions in step quantum well structures

We evaluate effects of heterointerfaces on optical phonon modes and phonon assisted electron intersubband transition rates in step quantum well structures for intersubband lasers. Various phonon modes and electron–phonon interaction Hamiltonians, including the interface modes, confined longitudinal-optical modes, and half space modes in the quantum well structures are calculated based on the macroscopic dielectric continuum model and microscopic analysis. The transfer matrix method is used to calculate the interface modes. The intersubband transition rates due to electron–phonon scattering by these phonon modes are evaluated using Fermi’s golden rule, with the electron wave functions obtained by solving the Schrodinger equation for the heterostructures under investigation. Our results show that, compared with the transition rates in the same structures calculated using the bulk phonon modes and the bulk Frohlich interaction Hamiltonian, the electron interface–phonon interactions give significantly larger ...

How Pore Size and Surface Roughness Affect Diffusion Barrier Continuity on Porous Low-k Films

The structural features that affect continuity of ultrathin diffusion barriers on porous low-dielectric-constant (k) thin films has been investigated. The dimensions of interconnected nanopores in a series of Dow Corning XLK films are found to increase as the dielectric constant becomes smaller. The minimum thickness required for tantalum (Tal to form a continuous barrier layer is affected by the pore size and surface roughness of the porous low-k films according to positronium annihilation lifetime spectroscopy analysis. The films with large pores require thick barrier layers to form effective diffusion harriers, The surface roughness of the porous films as observed by atomic force microscopy has a significant influence on the continuity of diffusion barriers.

Effect of material properties on integration damage in organosilicate glass films

Most organosilicate glass1 (OSG), low dielectric constant (low-k) films contain Si–R groups, where R is an organic moiety such as –CH3. The organic component is susceptible to the chemically reactive plasmas used to deposit cap layers, etch patterns, and ash photoresist. This study compares a spin-on, mesoporous OSG film with a completely connected pore structure to both its nonmesoporous counterpart and to another low-density OSG film deposited by plasma-enhanced chemical vapor deposition. The results show that the film with connected pores was much more susceptible to integration damage than were the nonmesoporous OSG films. As integrated circuit device and interconnect dimensions continue to scale smaller, low dielectric constant (k) interlayer dielectric (ILD) materials will become necessary to mitigate RC (product of resistance and capacitance) propagation delay and reduce power consumption and crosstalk. Lowering the k-value of a material requires either altering the chemical bonding to reduce the bond polarizability or decreasing the number of bonds (density) in a material. To reduce the k-value below 2.2, most dielectric materials will require a density decrease by introducing free volume (micropores < 2 nm in diameter) or mesoporosity (2–50 nm diameter pores). Unfortunately, lowering the density also compromises the mechanical strength and other properties of the material. The material properties of mesoporous dielectric films, such as connected pores and low mechanical strength, create a host of integration problems including integration damage to the film. The Si–R groups make organosilicate glass (OSG) films hydrophobic and they lower the density by breaking up the tetrahedral Si–O bonding. However, the carbon component is susceptibleto degradation when exposed to the reactive plasmas used for capping, etching, and ashing processes, especially oxidizing plasmas that induce silanol formation. Such plasma-induced chemical modifications can cause film densification, dangling bonds and defects, and moisture uptake. Recently, International SEMATECH monitored several OSG films for change in k caused by integration damage (ID) while integrating the films into SEMATECH’s Cu/Damascene test chip using their standard processing flow (Table I). Mesoporous OSG films with connected pores exhibited a large increase in k due to ID during integration. In contrast, nonmesoporous OSG films showed much smaller changes in k. Thus, connected mesoporosity appears to facilitate film damage during processing by allowing reactive species to more easily penetrate the film. Various plasma pretreatments (PPT) have been reported to form densified and chemically modified interface layers on OSG films, and these skin layers can prevent film damage by photoresist ash processes. This study reports the effects of oxygen and nitrogenbased plasmas on one mesoporous and two nonmesoporous blanket films. To our knowledge, this is the first direct demonstration that a mesoporous film is more susceptible to ID than its nonmesoporous counterpart. OSG-1 is a proprietary spin-on mesoporous methylsilsesquioxane (MSQ)-based film where porosity is created by thermal decomposition of a porogen that is incorporated into the MSQ-based film. The mesoporosity of OSG-1 is about 58%. Positronium annihilation lifetime spectroscopy (PALS) determined that the average pore diameter is 2.5 nm and that the pores are 80–100% connected to the surface of the film. OSG-1 was deposited using a TEL Mark-8 spin-coater and oven-cured at 425 °C. OSG-2 is the nonmesoporous version of OSG-1 deposited with the same MSQ-based precursor but without the porogen. OSG-2 was spin-deposited and cured for less time at 450 °C by the supplier. The MSQ-based precursor for both OSG-1 and OSG-2 contains approximately 12% carbon. OSG-3 is a trimethylsilane-based, plasma-enhanced chemical vapor deposited (PECVD) Address all correspondence to this author. e-mail: r6758c@email.sps.mot.com

Inhomogeneous broadening of intersubband transitions due to nonscreening roughness of heterointerfaces

The shape of the terahertz absorption peak in quantum wells with rough heterointerfaces is studied. Although the long-range variations of the ground level are screened in heavily doped structures, the intersubband in-plane energy remains nonuniform due to the second-level variations. The equation for intersubband polarization is considered in the resonant approximation, taking into account the depolarization shift. The line shape of the intersubband absorption peak is described for the case with long-range variations of heterointerfaces.