KEYWORDS: Dielectrics, Electron beam lithography, Lithography, Photomasks, Silicon, Scanning electron microscopy, Scanners, Copper, Back end of line, Optical lithography
Increasing complexity and manufacturing costs, along with the fundamental limits of planar CMOS devices, threaten to
slow down the historical pace of progress in the semiconductor industry. We have proposed and demonstrated proof-of-concept
of a simple and low-cost way to fabricate dual-damascene copper (Cu) on-chip interconnect or Back-End-Of-the-
Line (BEOL) structures using a novel multifunctional on-chip electrical insulator called a photo-patternable low
dielectric constant (low-κ) dielectric (PPLK) material [Q. Lin, et al, Proc. SPIE, 2010]. This demonstration was
accomplished with a silsesquioxane-based (SiCOH), κ=2.7 material which is compatible with 248 nm optical
lithography.
In this paper, we report on the extension of the photo-patternable low-κ concept to the ultra-low-κ (κ<2.5) regime and
resolution down well below 100 nm with 193 nm lithography as well as e-beam lithography. We have accomplished this
demonstration using the same silsesquioxane-based (SiCOH) material platform as that of the 248 nm photo-patternable
low-κ materials. The 193 nm photo-patternable low-κ materials possess dielectric constants below 2.5 and are able to
resolve 100 nm half-pith line/space features with dry 193 nm single exposure lithography. The resolution of photopatternable
low-κ materials can be pushed down to 40 nm half-pith line and space features with a line-edge-roughness
less than 3.0 nm with e-beam lithography.
Qinghuang Lin, S. Chen, A. Nelson, P. Brock, S. Cohen, B. Davis, N. Fuller, R. Kaplan, R. Kwong, E. Liniger, D. Neumayer, J. Patel, H. Shobha, R. Sooriyakumaran, S. Purushothaman, T. Spooner, R. Miller, R. Allen, R. Wisnieff
KEYWORDS: Back end of line, Copper, Dielectrics, Optical lithography, Scanning electron microscopy, Silicon, Manufacturing, Semiconducting wafers, Ultraviolet radiation, Polymers
In this paper, we wish to report, for the first time, on a simple, low-cost, novel way to form dual-damascene copper (Cu)
on-chip interconnect or Back-End-Of-the-Line (BEOL) structures using a patternable low dielectric constant (low-κ)
dielectric material concept. A patternable low-κ dielectric material combines the functions of a traditional resist and a
dielectric material into one single material. It acts as a traditional resist during patterning and is subsequently converted
to a low-κ dielectric material during a post-patterning curing process. No sacrificial materials (separate resists or
hardmasks) and their related deposition, pattern transfer (etch) and removal (strip) are required to form dual-damascene
BEOL patterns. We have successfully demonstrated multi-level dual-damascene integration of a novel patternable low-κ
dielectric material into advanced Cu BEOL. This κ=2.7 patternable low-κ material is based on the industry standard
SiCOH-based (silsesquioxane polymer) material platform and is compatible with 248 nm optical lithography. Multilevel
integration of this patternable low-κ material at 45 nm node Cu BEOL fatwire levels has been demonstrated with
very high electrical yields using the current manufacturing infrastructure.
We report the formation of robust organosilicate line patterns of ~20nm half-pitch on surfaces from the self-assembled
lamellar phase of a diblock copolymer of polystyrene and poly(ethylene oxide), PS-b-PEO, and an oligomeric
organosilicate precursor mixtures. We could control the orientation and alignment of microdomains of this hybrid to the
same degree of the thin films of organic block copolymers. By controlling the surface energy of substrates using dense
organosilicate, the perpendicular orientation of lamellae to the surface was achieved. Topographic prepatterns were
generated by E-beam lithography and used for alignment of the line patterns from lamellar phase. Upon removing the
organic component (i.e. PS-b-PEO) by thermal treatment, the organosilicate microdomains remain as periodic line
patterns with global alignment on surfaces. This method gives well-aligned silicon-containing line patterns with sublithographic
length scales on surface. The self-assembled organosilicate line patterns were successfully transferred into
underlying silicon substrate using anisotropic plasma etching.
The emission color stability of poly(fluorene) derivatives upon thermal annealing or passage of current in an electroluminescent device is affected by the polymer molecular wight, the structure of the main chain polymer unit and particularly by the polymer chain end substituents. Proper attention to these features leads to colorfast blue emission in both photo and electroluminescence. Furthermore, the spectral emission characteristics can be tuned by the incorporation of various comonomers. Preliminary single layer device studies validate the potential utility of poly(fluorene) homo and copolymers for OLED applications.
The formation and decay dynamics of photogenerated excitons in polyfluorene statistical co-polymers in solutions and in thin films have been studied using femtosecond transient absorption spectroscopy. In solution photoexcitation of the polymer generates primarily intrachain singlet excitons which are initially hot and then relax quickly (< 200 fs) towards the equilibrium position in the excited state. The exciton subsequently decays following a double exponential with time constants of 30 ps and 330 ps in toluene. The fast decay is attributable to vibrational relaxation, spectral diffusion, or internal conversion (recombination) of the exciton from the excited to the ground electronic state through tunneling or thermal-activated barrier crossing before thermalization. The slow decay is assigned to conversion of the thermalized exciton to the ground state through both radiative and non-radiative pathways. In films the exciton dynamics are found to depend strongly on excitation intensity. At low intensity, the dynamics are similar to that in solutions, with a double exponential decay with time constants of 15 ps and 300 ps. At high intensities, a fast decay component with a time constant of 0.8 ps appears, which becomes more dominant at higher intensities. This fast decay is attributed to exciton- exciton annihilation due to high density of excitons created. The signal in films at both low and high excitation intensities is attributable to intrachain singlet excitons, as in solution. There is no evidence for formation of interchain bound polaron pairs in films at low intensities. At high intensities, the possibility cannot be ruled out completely, especially in relation to the fast decay. If bound polaron pairs are formed as indicated by the fast decay, they must be generated as a result of interaction between excitons on different chains since they are absent at low power, an they must be created and then decay within about 1 ps.
We review our recent research in the field of photochromic polymeric structures for optical data storage and nonlinear optics. We unify some of the sub-themes of azopolymer structures in the light of photo-induced movement of azobenzene molecules. In particular, we discuss photo-induced effects in supramolecular assemblies containing azobenzene molecules (e.g. Langmuir-Blodgett-Kuhn structures and ultrathin silane layers). Reorientation of azobenzenes in these structures will be compared to that observed in spin-cast films. Photoisomerization and photo-induced orientation of azobenzene molecules is also studied at the molecular level by means of azosilane molecules chemisorbed on Silicon Oxide substrates. We establish a correlation between polymer architecture and sub-glass transition temperature (Tg) light-induced molecular movement in high Tg nonlinear optical azo-polyimide polymers. We show that the isomerization reaction itself depends on the polymer molecular structure, and we present evidence of light-induced molecular movement 325 °C below Tg of a rigid NLO azopolyimide polymer containing no flexible connector or tether.
Materials to be used for optoelectronic, photorefractive, or frequency doubling applications must have high nonlinearities, good thermal stabilities, and low optical loss (high transparency). Organic compounds synthesized for incorporation into poled nonlinear polymers typically exhibit tradeoffs between nonlinearity and each of the other two qualities. By judicious use of arylamino donor groups and cyano-containing acceptor groups, a small set of chromophores that are both highly nonlinear and stable at high temperatures has been prepared. By selecting delocalized bridging moieties that are either tuned for optimum hyperpolarizability or exhibit two charge-transfer excited states, highly transparent chromopohores with excellent nonlinearities can be prepared. The results suggest that thermal stability and nonlinearity are jointly achieved by modifying donor and acceptor groups, while transparency and nonlinearity are jointly achieved by modifying bridging groups.
A new class of photorefractive polymers is described in which an inert polymer binder is doped with a great molecule called a dual-function dopant, which has the dual functions of providing optical nonlinearity as well as charge transport as required for the photorefractive effect. These materials are the first photorefractive polymers to show both nondestructive readout as sufficiently low reading power and optical trap activation, in which pre-irradiation by a uniform light beam increases the concentration of deep trapping sites in the material. In the area of potential applications, a new sample configuration is described in which photorefractive polymers are fabricated into an electric field switchable stratified volume holographic structure. Individual layers may be activated by applied electric field leading to improved diffraction efficiency and angular selectivity.
Recently, substantial progress has been made in molecular design of nonlinear optical chromophores toward the goal of large nonlinearities while retaining thermal stability. Modifications of donor groups, delocalized (pi) -systems, and acceptor groups have all been made, and examples of each are described. The utility of each modification for preparation of electro-optic polymers is discussed. Resistance to thermal decomposition is not sufficient for practical applications; NLO polymers must also be resistant to thermal depoling and to sublimation of chromophores out of the polymers. It is therefore concluded that chemical tethering is required. Synthetic strategies for tethering our better chromophores to thermally stable polyimides are described, and the results of materials prepared to date are given.
Among the many physical properties of organic nonlinear optical (NLO) chromophores that require optimization, their generally inadequate thermal stability has recently attracted significant attention. Here we describe protocols for the evaluation of the thermal stability of chromophores that may be employed prior to their incorporation into a device environment. These protocols involve a variety of conventional analytical techniques including thermal analysis (TGA, DSC) and electrochemistry (CV) that have led to the identification of the most facile thermal decomposition process of the widely utilized family of 4-amino-4'- nitroazobenzene NLO dyes. Knowledge of the decomposition mechanism has facilitated implementation of structural modifications in the amine donor of these dyes that deliver enhanced thermal stability. It is anticipated that these protocols and structure modifications are of general relevance.
Due to the requirements for thermal stability in the fabrication and operation of electro-optic devices, the quest for suitable NLO chromophores and polymers involves more than the search for highly nonlinear materials. We describe here the preparation and characterization of aryl substituted azole derivatives as a class of thermally stable nonlinear chromophores and their incorporation into high temperature thermoplastics as a host-guest array. Criteria for predicting the thermal stability of NLO chromophores are discussed as are the polar order decay properties of various poled polymer systems as a function of temperature.
The fluorescence decay kinetics in the copolymer, poly(phenylmethyl-co-p- dimethylanilinemethyl silane) have been studied by steady state and time correlated fluorescence spectroscopy and three absorbing and emitting species are proposed to explain the complex fluorescence decay behavior. In the case of poly(phenyl methyl silane) homopolymer, excitations have been shown to delocalize over several tens of monomer units, however, in the copolymers discussed here the extent of the delocalization has been shown to be dependent on the degree of substitution of the dimethylaniline side group. In the more highly substituted copolymers the excitations are shown to be confined to almost a single monomer unit by the electron donating nature of the dimethylaniline side group. A simple geometric distribution of chain lengths is used to predict the delocalization length of the excitations.
The realization of practical electro-optical devices based on the poled polymer waveguide approach is contingent on the simultaneous solution of a number of outstanding materials and fabrication issues. We have prepared and studied the molecular properties of a number of heterocycle containing chromophores with structures derived from high temperature polymer materials and further evaluated many of them as guests in polyimide polymer hosts. Correlations between the structure of these chromophores and their physical properties including optical nonlinearity, absorption, dipole moment and thermal stability have been made. Furthermore, a correlation between thermal stability and oxidation potential for some of these heterocyclic chromophores has been found and extension of this correlation to other types of nonlinear chromophores is suggested.
Experimental measurements of molecular hyperpolarizabilities can be useful in two ways. First, they allow comparison between different chromophores, so that structure-function relationships can be understood and increasingly nonlinear compounds synthesized. This application requires only good relative measurements. The second and ultimately more important application is prediction of the macroscopic nonlinear optical properties of custom materials. The latter application requires accurate absolute values for the molecular parameters. Examples of both types of comparisons are discussed below. We describe experimental measurements on several types of nonlinear optical chromophores, and the choice of conventions and reference standards that leads to accurate predictions of electro- optic coefficients in poled polymers. The degree to which nonlinearity and thermal stability are correlated is also discussed.
The current status of electric field poled polymeric materials for second harmonic generation (SHG) and optoelectronic devices is discussed. For SHG applications polymeric materials have still not significantly overcome the trade-off between high nonlinearity and absorption of the frequency doubled light. On the other hand, the nonlinear properties of polymeric systems appear to be large enough to consider the use of these materials in electrooptic devices such as amplitude modulators and switches. Here the problem is maintaining the poled orientational ordering of the chromophores within the polymer throughout the device fabrication process, which can involve brief excursions to high temperatures. An empirical relationship is described relating the decay of the poled order to the guest-host system's glass transition temperature. This relationship's extension to side chain polymer systems is also examined.
Three nonlinear optical polymeric systems based on nonlinear optical chromophores attached as side chains to poly(methyl methacrylate) were synthesized and their nonlinear optical properties measured in thin film form by second harmonic generation and by the electro-optic effect. Also, attenuation of light propagation in optical waveguides and photothermal deflection infrared spectra were measured on these films to aid in selecting the best system for device development. Based on these results, a phase modulator was constructed and with sub- carrier multiplexing the signals from five TV channels were placed on an infrared laser beam as sidebands. After a short transmission through space, these signals were then heterodyne detected and displayed on a TV monitor with good signal to noise and with no crosstalk.
A negative-tone surface-imaged resist process based upon the area-selective oxidation of silicon-backbone polymers is described. A bromine-based plasma is the resist developer, where the oxidized polymer inhibits the bromine-initiated etching to yield a negative-tone image. Using either polysilanes or polysilynes, resist sensitivities in the range of 50 mJ/cm2 have been obtained and resolutions to 0.2 micrometers achieved. Photosensitizers can be added to further accelerate the photo-oxidation, resulting in sensitivities less than 20 mJ/cm2. The latent image formation is reciprocal with respect to fluence in the range 0.05 to 1.5 mJ/cm2 per pulse and with respect to repetition rate. The photo-oxidation contrast is one, whereas the bromine-based etch step can have a contrast as high as 5. In addition, the exposure, focus, and development latitudes have all been characterized and compared to other surface-imaged 193 nm resist systems.
We demonstrate resonant third harmonic generation as a powerful tool for analyzing the quasi 2-D molecular orientational order in a Langmuir-Blodgett film of poly(bis-m- butoxyphenylsilane). By recording Maker-fringes at different angles between the polarization of the fundamental beam and the dipping direction, we were able to determine the order parameters *2> and *4> simultaneously. We found *2> equals 0.466 +/- 0.030 and *4> equal to -0.12 +/- 0.10.
We have recently reported the formation of scum-free micron images in a bilayer composed of a thin (1500 angstrom) imaging layer of poly(cyclohexylmethylsilane) coated over a thick planarizing layer of a hardbaked diazoquinone-novalac type photoresist using deep UV exposure. Although excellent resolution was achieved (< 0.5 micrometers ) in this process, the exposure doses required (125-150 mJ/cm2) were higher than desirable for commercial DUV imaging tools. We have addressed this problem using two approaches: (i) the synthesis of new polysilanes which are intrinsically more sensitive to photodegradation in the solid state and (ii) the incorporation of additives which enhance the photosensitivity. Regarding the former, a number of aryl substituted polysilane homopolymers which are significantly more photolabile than the standard poly (methylphenylsilane), as assayed by the rate of spectral photobleaching upon exposure to DUV radiation, have been prepared and tested. In addition, a number of small molecule additives which quantitatively quench the polymer fluorescence in the solid state have been identified. Some of the additives which efficiently quench the polymer fluorescence in the solid state also accelerate the rate of photodegradation. The combination of the new polysilane materials with sensitizing additives has allowed submicron DUV imaging at exposure doses as low as 20 mJ/cm2 or less in a bilayer configuration employing O2-RIE for image transfer of the wet-developed images.
A novel non-contact method of forming groove servo patterns on optical disk substrates has been demonstrated. The servo forming layer consists of a copolymer of (p-t-butyloxycarbonyloxy)styrene and styrene formulated with an onium salt which photogenerates acidic species upon exposure to deep UV radiation. If the film is heated after exposure to light the photogenerated acid cleaves the p-t-butyloxycarbonyl ("t-BOC") group resulting in a thickness change in the regions exposed to radiation. Glass substrates coated with this formulation were exposed with an appropriate mask to deep CV light and the image developed by a post exposure bake of 2 minutes at 90°C. A magneto-optic trilayer consisting of 710 A Zr02/800 A TbFeCo/710 A Zr02 was deposited onto the polymer surface. The complete structure has sufficient track error signal level to be trackable on a precision optical disk test stand with the TES servo loop closed and has a CNR of 53 dB, comparable to the same MO composition deposited on injection molded grooved polycarbonate substrates.
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