Marleen H. van der Veen

Bandgap opening in oxygen plasma-treated graphene

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.

Modified, semiconducting graphene in contact with a metal: Characterization of the Schottky diode

In this paper, we report the fabrication and characterization of Schottky rectifying junctions between semiconducting, modified single-layer graphene and a metal. The pristine, semimetallic behavior of graphene is altered by controlled exposure to an oxygen plasma, resulting in the opening of an optical band gap as shown by photoluminescence spectroscopy. The occurrence of a Schottky barrier between semiconducting graphene and metals with different work functions (Al, Cr, Pd, and Yb) is investigated by electrically characterizing the as-fabricated junctions. The rectifying properties of our Schottky diodes show the potential of semiconducting, modified graphene as building block of elementary logic circuits.

Cobalt bottom-up contact and via prefill enabling advanced logic and DRAM technologies

This work introduces two new metallization schemes using the electroless deposition (ELD) technique; one based on contact fill and one based on via prefill. One of the key features of the electroless process is its selective deposition, which can be used for bottom-up fill of high aspect ratio features. The feasibility of this Co ELD process is demonstrated on contacts landing on W and vias landing on Cu. Our simulation of the Co via resistance shows that it can serve as alternative to Cu with lower via resistance below 15nm dimension. The results from a planar capacitor study show that there is no degraded reliability in an organo-silicate glass low-k film when Co is in direct contact with this dielectric. Therefore, selective Co ELD process for contact and via prefill has the potential to enable future scaling of advanced logic and DRAM technologies.

Electrical improvement of CNT contacts with Cu damascene top metallization

We discuss the improvement in the electrical characterization and the performance of 150 nm diameter contacts filled with carbon nanotubes (CNT) and a Cu damascene top metal on 200mm wafers. The excellent agreement between the yield curves for the parallel and single contacts shows that a reliable electrical characterization is obtained. We demonstrate that integration changes improved the resistivity of the CNT contact significantly by reducing it from 11.8·10³ μΩ·cm down to 5.1·10³ μΩ·cm. Finally, a length scaling of the CNT contacts was used to find the individual contributors to the lowering of the single CNT contact resistance.

Barrier/liner stacks for scaling the Cu interconnect metallization

Self-forming barriers and advanced liner materials are studied extensively for their Cu gapfill performance and interconnect scaling. In this paper, 22nm1/2 pitch Cu low-k interconnects with barrier (Mn-based, TaN) /liner (Co, Ru) combinations are compared and benchmarked for their resistivity, resistance scaling, and electromigration (EM) performance. Extendibility to 16nm copper width was explored experimentally and a projection towards 12nm width is performed. It is found that the Ru-liner based systems show a higher overall Cu-resistivity. We show that this increase can be compensated by combining Ru with a thinner Mn-based barrier, which increases the effective Cu-area at a particular trench width. The EM performance reveals that the Ru-liner systems have a better EM lifetime compared to the Co-liner based systems. More interestingly, in a comparison of the maximum current density Jmax a significant improvement is found for the scaled Mn-based/Ru system, making it therefore a serious candidate to extend the Cu metallization.

Electrochemical Deposition of Subnanometer Ni Films on TiN

In this paper, we show the electrochemical deposition of a subnanometer film of nickel (Ni) on top of titanium nitride (TiN). We exploit the concept of cluster growth inhibition to enhance the nucleation of new nuclei on the TiN substrate. By deliberately using an unbuffered electrolyte solution, the degree of nucleation is enhanced as growth is inhibited more strongly. This results in a very high particle density and therefore an ultralow coalescence thickness. To prevent the termination of Ni deposition that typically occurs in unbuffered solutions, the concentration of Ni(2+) in solution was increased. We have verified with RBS and ICP-MS that the deposition of Ni on the surface in this case did not terminate. Furthermore, annealing experiments were used to visualize the closed nature of the Ni film. The closure of the deposited film was also confirmed by TOF-SIMS measurements and occurs when the film thickness is still in the subnanometer regime. The ultrathin Ni film was found to be an excellent catalyst for carbon nanotube growth on conductive substrates and can also be applied as a seed layer for bulk deposition of a smooth Ni film on TiN.

Carbon nanotube interconnects: Electrical characterization of 150 nm CNT contacts with Cu damascene top contact

The integration of Carbon Nanotubes (CNT) in contact holes with TiN underlayer using CMOS compatible processes is discussed. Each process step was optimized by evaluating the electrical results obtained with contact test structures. Subsequently, this process was transferred to 150 nm diameter contact holes. We present the first electrical data obtained from automated probing of 150 nm diameter contacts filled with CNT connected by a Cu damascene top contact module. This constitutes a significant step forward towards the realization of CMOS contact modules with CNT interconnects.

Charge transfer effects in graphene-CdSe/ZnS quantum dots composites

Graphene possesses unique physical properties, due to its specific energy bands configuration, substantially different from that of materials traditionally employed in solid-state optoelectronics. Among the variety of remarkable properties, strong field effect, high transparency in the visible-light range and low resistivity of graphene sheets are the most attractive ones for optoelectronic applications. Zero-dimensional colloidal semiconductor nanocrystals, known as quantum dots (QDs), attract immense attention in the field of photonics due to their size-dependent tunable optical properties. By combining these two types of nanomaterials together, we demonstrate the role of graphene as an efficient charge transfer medium from- and to II-VI quantum dots. The optical excitation of II-VI quantum dots dispersed on single layer graphene results in an electron transfer from the nanocrystals to graphene. This is evidenced from photoluminescence imaging and confirmed by the electrical measurements on QDs-decorated single layer graphene field effect transistors (SLG-FET). In the second part of this paper we demonstrate an efficient hole injection from graphene into QDs-layered nanocrystalline structures and the operation of the corresponding graphene-based quantum dot light emitting diodes (QD-LED). We also benchmark graphene vs. indium-tin-oxide (ITO) based QD-LEDs in terms of device electroluminescence intensity performance. Our experimental results show better hole injection efficiency for graphenebased electrode at current densities as high as 200 mA/cm2 and suggest single layer graphene as a strong candidate to replace ITO in QD-LED technology.

Electron mean-free path for CNT in vertical interconnects approaches Cu

A carbon nanotube (CNT) contact length scaling is used to derive the electron mean-free path (λCNT) after full integration. A CNT-to-metal contact resistance of 76 Ω and lower was obtained for 150 nm diameter contacts. By estimating the number of conducting walls in the CNT bundle, a λCNT of 74 nm is found, which is longer than for Cu. We propose a more conservative approach of calculating λCNT solely from electrical data. The result is that our CNT interconnects have ballistic transport over 24 nm, which is 5 times longer than reported so far.

Wafer-Level Electrical Evaluation of Vertical Carbon Nanotube Bundles as a Function of Growth Temperature

We have evaluated the resistance of carbon nanotubes (CNTs) grown at a CMOS-compatible temperature using a realistic integration scheme. The structural analysis of the CNTs by transmission electron microscopy (TEM) showed that the degree of graphitization decreased significantly when the growth temperature was decreased from 540 to 400 °C. The CNTs were integrated to form 150-nm-diameter vertical interconnects between a TiN layer and Cu metal trenches on 200 mm full wafers. Wafers with CNTs grown at low temperature were found to have a lower single-contact resistance than those produced at high temperatures. Thickness measurements showed that the low contact resistance is a result of small contact height. This height dependence is masking the impact of CNT graphitization quality on resistance. When benchmarking our results with data from the literature, a relationship between resistivity and growth temperature cannot be found for CNT-based vertical interconnects.