by Michael A. Fury, Techcet Group

April 16, 2010 – The third day (Weds. 4/7) of the MRS Spring 2010 meeting in San Francisco delivered another full regimen of meta-stable seating as attendees resonated among the parallel symposium sessions. Highlights included: Noncontact planarization; nanoimprint lithography; sub-32nm memory; sensors made with field-effect transistors (FETs), Si, and carbon nanotubes (CNTs), solution-phase metal-semiconductor nanowires; patterned organic electronics, inkjet printed sensors, and a flow coating method for highly aligned semiconducting polymer films.

(Underscored codes at the beginning of papers reviewed refer to the symposium, session and paper number; additional presentation details can be found in the MRS Spring 2010 program.)

E5.3. Sum Huan Ng at the Singapore Institute of Manufacturing Technology described an alternative non-contact planarization technique by utilizing the electrokinetic phenomenon based on the movement of suspended particles under fluid flow in an electric field. A DC field directs the number of particles attracted toward the work surface, and the AC field controls the frequency of particle impingement. The magnitude of these fields in combination with the fluid flow rate control the angle of attack on the work surface. Rates for Al and Au removal were 250-300nm/hr, with a resulting surface roughness of <10nm. Planarization data was not presented, so the incumbency of chemical mechanical planarization (CMP) is safe, for now.

E5.4. Duane Boning at MIT discussed the characterization and modeling of pad asperity response in CMP using nanoindentation on the surface asperities, in an effort to develop a ground-up physics model for pad asperity contact. Asperity heights were observed to range from +15μm to -25μm about the mean pad surface, resulting in an actual wafer contact area of 0.02-0.05% of the gross wafer area. This contact percentage is higher if the asperity height distribution is narrow and more uniform, consistent with polishing behavior with finer grade pad conditioners. Shallow nanoindentation (<100nm) on conditioned pad slices indicates that the asperity may be stiffer than the bulk. In addition, pad modulus can vary ±50% within a 10μm × 10μm region, which is an unexpectedly high spatial variation.

F7.1. Christopher Soles of NIST described the direct patterning of organosilicate interconnect materials by nanoimprint lithography (NIL), and several characterization techniques for verifying the resulting patterns and materials. The NIL mold can increase the effective porosity of a low-k dielectric, because the solvent cannot escape during the initial cure. The mold can also induce a skin effect that densifies the material on the surface. They have developed critical dimension small angle X-ray scattering and specular X-ray reflectivity methods to verify the fidelity of the pattern transfer. X-ray porosimetry (XRP) is used quantify the average density, the porosity, and the wall density of the material between the pores of these imprinted patterns. Positron annihilation lifetime spectroscopy (PALS) measurements are used to quantify the pore size distributions and the degree of pore interconnectivity in the patterned material. Finally, the porosity characteristics determined by XRP and PALS are correlated with high resolution transmission electron microscopy (TEM) images of the pattern cross section to obtain a complete picture of how the imprint process affects the porosity of these materials. Examples were shown in which the porosity level was pushed to over 50% by volume, well into the ultralow-k regime where the expected dielectric constants will be <1.8.

F7.2. James Watkins at U Mass. talked about directly patterned ULKs using existing resist platforms and sacrificial adhesion layers for single step Cu metallization. The method employs block copolymer phase separation in a CO₂ dilation process. The resulting material exhibits two classes of pores: 50% mesopores from the copolymer porogens and 50% nanopores from the manner in which the CO₂ dilates the bulk material. Early tests produced templated materials with k=1.8. Processes using commercial 248nm and 193nm photoresist as a base were shown to produce both positive and negative tone patterning, achieving 97nm line/space structures. For metallization from scCO₂ fluids, a thin film of polyacrylic acid (PAA) was found to catalyze Cu deposition and promote adhesion to the dielectric, but the Cu itself depolymerizes the PAA, making it a sacrificial layer that does not remain in the final device structure.

G9.6. Michael Kozicki at ASU described some operational aspects of cation-based NV resistive memory beyond 32nm. Cation-based resistive memory devices have a silver- or copper-containing oxide or higher chalcogenide electrolyte sandwiched between oxidizable and inert (ohmic or rectifying) electrodes. They operate through the growth and dissolution of conductive metal filaments in a glassy solid solution electrolyte such as Ag-Ge-Se or Cu-SiO2. Analogous anion-based devices depend on the formation and destruction of conductive oxide domains. Memory endurance of >10¹⁰ cycles and storage retention of 10 years have been achieved.

K7.1. Ananth Dodabalapur at UT Austin talked about materials and device physics of organic and hybrid organic-inorganic field-effect chemical sensors. One design objective is to leverage the reliability and stability of well-established CMOS devices, while adding new functionality at the gate by providing a mechanism for environmental sensing and response. This particular design depends on grain boundary diffusion, so the channel length must be much greater than the grain size of the channel sensing material. Sensitivity of 100ppb EtOH in N₂ was shown.

K7.4. Daniel Smetaniuk at U Alberta described photocatalytic TiO₂ nanostructures for self-regenerating relative humidity (RH) sensors and PV, using self-cleaning anatase TiO₂ layers to reduce device fouling. Superhydrophilicity is photo-induced in TiO₂ using UV wavelengths shorter than 387nm, which increases surface hydroxylation on the films and is important to the physics of water sensing. TiO₂ is also an excellent photocatalyst that is able to break down organic contaminants on its surface. Glancing angle deposition (GLAD) was used to fabricate high-porosity nanostructures with very fine control over parameters such as porosity, pitch, and film thickness. This device structure will be adapted for sensing other organic and inorganic species.

K7.5. James Gole at GA Tech presented work on highly sensitive, selective, rapid response, room-temperature conductometric gas sensors fabricated with porous Si. The concept calls for nanoporous Si surfaces with a sub-monolayer deposit of a sensing element such as Au, Sn, Cu, Ni, Al₂O₃, MgO, TiO₂, or ZrO₂ for the detection of gases including NO, NO₂, CO, NH₃, PH₃, SO₂, H₂S, HCl, and toluene. The desired sensing behavior (fast response, reversible, high sensitivity) depends on a miss-match of the Lewis acid and base characteristics between the detector element and the compound sensed. The sub-monolayer coating favors physisorption over chemisorption, desirable for sensing. A complete coating would favor chemisorption, which would be preferred for micro-reactors.

K8.2. Louis Gorintin at École Polytechnique spoke on large-scale production of selective gas sensors based on CNT mat transistors, in which the 5-15μm channel is covered with a non-woven mat of SWNT. Airbrush deposition of a CNT suspension gives <10μm droplet size and homogeneous deposition at the CNT density necessary to form a mat. At a 15μm channel length, the device shows I_ON/I_OFF > 105. Devices with different metal electrode doping (Ti, Pd, Au, Pt) are used as a set to provide differential responses to the gas analyte (CO, NH₃ and NO₂ were tested) resulting in a unique fingerprint response for each, with sub-ppm sensitivity. Subsequent work will focus on deconvoluting data from a mixture of analyte gases.

Q5.8. David Smith at U Southampton described a body of work on supercritical fluid electrodeposition and supercritical chemical fluid deposition (SCFD) for filling nanoscale templates. The technique does not suffer from pore blocking, which has often been a problem with CVD methods. One example was 10nm diameter Cu pipes with a uniform wall thickness of 2-3nm. Other systems discussed included deposition of Cu, Ag, Au, Co, Ge and CdS from fluids such as CO₂, MeCN and CH₂F₂. Cu can be grown at rates of several hundred nm/hr, but other materials grow only at rates of nm/day.

Q5.9. Rawiwan Laocharoensuk of LANL discussed a versatile solution-phase approach for fabrication of metal-semiconductor heterostructured nanowires. The method takes advantage of the well-known low surface tension of scCO₂ to give fast growth of free-standing nanowires, but the addition of flow control during deposition results in vertical structures and greater control over growth density. The technique was able to produce vertical CdTe nanowires with a CdSe tip.

II3.1. George Malliaris at Cornell sorted through the adaptations needed for patterning of organic electronics with material-compatible processes. Liftoff is invoked for pentacene gates, resulting in low mobility but high-resolution structures. Several examples of orthogonal process chemistry were shown, wherein non-standard solvents are employed to ensure the right combinations of solubility and etch resistance at each step. Fluorinated resists have both fluorinated and non-fluorinated components to promote adhesion to non-fluorinated surfaces.

II3.2. Marcus Halik of University Erlangen-Nürnberg provided insight to organic thin-film electronics at the molecular scale. Simple n-alkyl molecules as monomolecular dielectric in organic transistors are suitable for a wide range of semiconductor materials (p- and n-type, nanoparticles, polymers) and enables CMOS like organic electronics with reduced power consumption even on flexible substrates. With specially designed molecules the realization of p- and n-type self-assembled monolayer field effect transistors (SAMFETs) is possible. The SAM design is assisted by Π-Π bond overlap, and uses C₆₀ buckyball doping for n-type and thiophene or pentacene doping for p-type, both with phosphonic acid SAM anchors. The resulting devices can operate as low as 1nW per logic gate in a process with no lithography and process temperatures less than 90°C.

II3.3 (was II9.28). Ana Arias at PARC described a flexible, all-inkjet-printed sensor tape based on organic solution chemistry, to monitor a soldier’s environment and assist with diagnosis of traumatic head injury. The flex strip includes functional sensor modules for acceleration, pressure, intense light and sound, NVRAM storage up to 1 week, event triggers, clock and battery, all for the price of $1. This compares favorably — ridiculously so — with commercial sports head injury monitoring equipment at $1,000.

II3.4. R. Joseph Kline of NIST described the merits of highly aligned semiconducting polymer films, using a method called flow coating. This method results in biaxially oriented films with the backbone of the polymer molecules preferentially oriented in-plane, but not until after recrystallization of the films from the melt. The transformation was characterized by in situ grazing-incidence x-ray diffraction (GIXD), and provides a nice understanding of how and why ordering occurs in this class of materials. The method is likely to prove useful for accelerating commercialization of organic electronics.

Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; e-mail [email protected].