Microscopy of 3D interconnect structures is challenged by the opaque nature of silicon. Infrared (IR) microscopy
provides a way of "looking" through silicon where microscopes based on visible wavelengths fail. Perhaps the most
prevalent application of IR microscopes in 3D manufacturing is imaging sub-surface features at the interface of a bonded
wafer pair. The ability to see through silicon using IR microscopes enables a variety of metrology techniques, including
the overlay of circuit layers (e.g., metal 2 to via). IR microscopy is a non-destructive technique and, as such, it is an
ideal candidate for in-line metrology for the bonded wafer pairs required for 3D interconnects.
This paper reviews overlay metrology capability for an IR microscope. The ability to measure the overlay of bonded
wafer pairs according to the 2009 International Technology Roadmap for Semiconductors (ITRS) is demonstrated.
Overlay tolerances for a variety of copper interconnect test structures is predicted based on electrical designs, and overlay results are compared to electrical test results. The use of IR microscopy as an early indicator of electrical yield is clearly demonstrated.
In immersion lithography a fluid with a high refractive index is used to enable increases in the numerical aperture (NA) of the imaging system and therefore decrease the minimum feature size that can be patterned. Water has been used in first generation immersion lithography at 193 nm for the 45 nm node. To generate still smaller features, fluids with a higher index than water are needed. Both saturated hydrocarbons and a new class of salts with incorporated alkane groups have been studied. Both of these types of fluids possess the "adjustable" absorbance edge behavior needed to provide a fluid with a high index and low absorbance at 193 nm. Since alkanes have physical properties that are difficult to integrate into current fluid handling systems, the aqueous solutions are particularly attractive as more semiconductor-friendly fluids. A full characterization of the optical properties of these fluids will be reported, as well as physical property results and confirmation of the feasibility of 32 nm l/s imaging with 1.5 NA using the salt solutions.
Immersion lithography at 193nm has rapidly evolved from a novel technology to the top contender for the 45nm device node. The likelihood of immersion implementation in semiconductor manufacturing has raised interest in expanding its capabilities. Extending resolution requires immersion fluids with higher refractive indices than those currently available. We have therefore sought substances which, when added to water, increase the refractive index at 193nm without increasing the absorbance and viscosity beyond acceptable limits. This work explores the relationship between index of refraction and absorbance, with specific focus on the identification of fluids that have a high index and low absorbance. The majority of the fluids studied either have prohibitively high absorbance values or material properties that would be incompatible with current fluid handling systems. However, a class of methylsulfonate salts was identified with optical and material properties approaching the target values. Fluid testing and imaging is included to confirm the resolution enhancing capability of these new high index fluids.
Photonic crystals are structures which exhibit a band gap in the electromagnetic spectrum as a result of dielectric periodicity. These structures present the potential to control electromagnetic waves in a similar manner to the way electrons are controlled by semiconductors. To obtain a photonic band gap in a specific region of the spectrum, there are two important characteristics of the photonic crystal that must be considered. The first is the length scale of the periodicity of the crystal, which governs the frequency range in which the band gap falls. The second is the dielectric contrast between the two media which comprise the crystal, which controls the size of the bang gap. Therefore, to construct a photonic crystal which could be used as an optical device, such as a waveguide or filter, the features should be on the order of optical wavelengths, or nanometers. The dielectric contrast through the visible region should also be large enough to open a band gap. Lithography techniques are ideally suited to pattern such structures. This work focused on the use of step and flash imprint lithography as an ideal patterning technology for two dimensional photonic crystals because of its capability for sub-50 nm patterning. Another attractive aspect of using step and flash imprint lithography is the potential to pattern a functional polymer as the crystal. The feasibility of printing structures needed for photonic crystals using imprint lithography was first demonstrated. Then, a strategy to raise the index of refraction of imprint compatible polymer formulations for large dielectric contrast using metal oxide nanoparticles was investigated. A maximum index of n = 1.65 was achieved, but at the high nanoparticle concentrations needed to reach this value, the formulations would not photocure. At low concentrations, imprints were obtained and uses for the resulting moderate index polymer composites as partial band gap photonic crystals were suggested.
In order to quickly and cheaply test candidate fluids and coatings for immersion lithography, we have devised a fluid handling scheme that we call drag-a-drop. We have constructed a prototype tool in order to test materials using this fluid scheme, and conducted several experiments with it. From these tests, we have determined that a hydrophobic topcoat with low contact angle hysteresis on the substrate increases the maximum stable scanning velocity by at least a factor of 2 over a standard 193 nm photoresist. We observed that instabilities on the receding contact line are unaffected by height, but the onset of instability on the advancing contact line occurs when the height of the lens is low. We also examined the drag-a-drop technique for possible use in laser mask writing, and found that by means of a hydrophobic topcoat, the lens can be completely removed from the substrate while keeping the immersion droplet affixed to the lens.
Extraction of small molecule components into water from photoresist materials designed for 193 nm immersion lithography has been observed. Leaching of photoacid generator (PAG) has been
monitored using three techniques: liquid scintillation counting (LSC); liquid chromatography mass spectrometry (LCMS); and scanning electrochemical microscopy (SECM). LSC was also used to detect leaching of residual casting solvent (RCS) and base. The amount of PAG leaching from the resist films, 30 - 50 ng/cm2, was quantified using LSC. Both LSC and LCMS results suggest that PAG and photoacid leach from the film only upon initial contact with water (within 10 seconds) and minimal leaching occurs thereafter for immersion times up to 30 minutes. Exposed films show an increase in the amount of photoacid anion leaching by upwards of 20% relative to unexposed films. Films pre-rinsed with water for 30 seconds showed no further PAG leaching as determined by LSC. No statistically significant amount of residual casting solvent was extracted after 30 minutes of immersion. Base extraction was quantified at 2 ng/cm2 after 30 seconds. The leaching process is qualitatively described by a model based on the stratigraphy of
Immersion lithography at 193 nm has rapidly changed status from a novel technology to the top contender for the 45 nm device node. The likelihood of implementation has raised interest in extending its capabilities. One way to extend immersion lithography would be to develop immersion fluids and resists with higher refractive indices than those currently available (n193 nm = 1.44 for water and n193 nm = 1.7 for typical resists). This work explores methods by which the index of refraction of immersion fluid could be increased to that of calcium fluoride (n193 nm = 1.56) or higher. A survey of the optical properties of various aqueous solutions was performed using spectroscopic ellipsometry. The refractive index of the solutions is measured to identify additives that might increase index while maintaining suitable pH, viscosity and contact angle. Also, ways to increase the index of model resist systems were explored. Higher index resists would help improve contrast in hyper-NA exposure tools.
Three modes of scanning electrochemical microscopy (SECM) - voltammetry, pH, and conductivity - have been used to better understand the chemistry at, and diffusion through, the solid/liquid interface formed between a resist film and water in 193 nm immersion lithography. Emphasis has been placed on investigating the photoacid generator (PAG), triphenylsulfonium perfluorobutanesulfonate, and the corresponding photoacid. The reduction of triphenylsulfonium at a hemispherical Hg microelectrode was monitored using square wave voltammetry to detect trace amounts of the PAG leaching from the surface. pH measurements at a 100 μm diameter Sb microelectrode show the formation of acid in the water layer above a resist upon exposure with UV irradiation. Bipolar conductance measurements at a 100 μm Pt tip positioned 100 μm from the surface indicate that the conductivity of the solution during illumination is dependent upon the percentage of PAG in the film. Liquid chromatography mass spectrometric analysis of water samples in contact with resist films has been used to quantify the amounts (< 10 ng/cm2) of PAG leaching from the film in the dark which occurs within the first 30 seconds of contact time. Washing the film removes approximately 80% of the total leachable PAG.
Immersion lithography has been proposed as a technique to print sub-100nm features using 193nm lithography. The process involves filling the space between the lens fixture of an exposure tool and the photoresist-coated silicon wafer with a liquid. In the case of immersion 193nm lithography, water can serve as that liquid. The immersion option raises questions about how photoresists and water interact. Components of the photoresist could be leached into the water, thus modifying the refractive index of the medium, depositing material on the lens, or altering the solubility switching process of the photoresist. Several phenomena could affect the optical properties of the resist and water and, ultimately, the resolution of the process. To better understand the impact that immersion lithography would have on photoresist performance, a study has been undertaken to measure the amount of resist components that are leached by water from model 193nm photoresists. The components studied were residual casting solvent (propylene glycol methyl ether acetate), the photoacid generator (triphenylsulfonium nonaflate), and the base quencher (triethanolamine). Since it was expected that only a small amount of each material would be leached into the water, 14C-labeled samples of each resist component were synthesized and added to the 193nm resists. Films of the labeled resists were coated onto a silicon wafer and immersed in water. The water was collected and the film was dissolved in casting solvent and collected. The amount of material leached into the water was determined by radiochemical analysis. Spectroscopic ellipsometry was also used to quantify changes in the optical constants of the resists and the water.
The use of in situ spectroscopic ellipsometry (SE) is demonstrated as a technique for studying photoresist dissolution. Experiments carried out using a J.A.Woollam M-2000 ellipsometer and a custom built cell designed for in situ film measurements show that bulk dissolution rate measurements using the SE technique agree with dissolution rate data obtained using multiwavelength interferometry. SE is also demonstrated as a method for measuring thin film dissolution rates, water sorption, and films that swell. An additional focus of this work was the topic of interfacial “gel” layer formation during photoresist dissolution. Ellipsometry and interferometry were used to test several photoresist resins, with an emphasis on phenolic polymers. Single and multiple layer models were used to analyze the data, and were compared to model calculations predicting formation of a gel layer. For the materials studied, interfacial gel layer formation in low molecular weight phenolic polymers was not detected, within the resolution of the experimental techniques (< 15 nm).