Manuel Hofer

Nanolithography by scanning probes on calixarene molecular glass resist using mix-and-match lithography

Abstract. Going “beyond the CMOS information-processing era,” taking advantage of quantum effects occurring at sub-10-nm level, requires novel device concepts and associated fabrication technologies able to produce promising features at acceptable cost levels. Herein, the challenge affecting the lithographic technologies comprises the marriage of down-scaling the device-relevant feature size towards single-nanometer resolution with a simultaneous increase of the throughput capabilities. Mix-and-match lithographic strategies are one promising path to break through this trade-off. Proof-of-concept combining electron beam lithography (EBL) with the outstanding capabilities of closed-loop electric field current-controlled scanning probe nanolithography (SPL) is demonstrated. This combination, whereby also extreme ultraviolet lithography (EUVL) is possible instead of EBL, enables more: improved patterning resolution and reproducibility in combination with excellent overlay and placement accuracy. Furthermore, the symbiosis between EBL (EUVL) and SPL expands the process window of EBL (EUVL) beyond the state of the art, allowing SPL-based pre- and post-patterning of EBL (EUVL) written features at critical dimension levels with scanning probe microscopy-based pattern overlay alignment capability. Moreover, we are able to modify the EBL (EUVL) pattern even after the development step. The ultra-high resolution mix-and-match lithography experiments are performed on the molecular glass resist calixarene using a Gaussian e-beam lithography system operating at 10 keV and a home-developed SPL setup.

Scanning probes in nanostructure fabrication

Scanning probes have enabled modern nanoscience and are still the backbone of todays nanotechnology. Within the technological development of AFM systems, the cantilever evolved from a simple passive deflection element to a complex microelectromechanical system through integration of functional groups, such as piezoresistive detection sensors and bimaterial based actuators. Herein, the authors show actual trends and developments of miniaturization efforts of both types of cantilevers, passive and active. The results go toward the reduction of dimensions. For example, the authors have fabricated passive cantilever with a width of 4 μm, a length of 6 μm and thickness of 50–100 nm, showing one order of magnitude lower noise levels. By using active cantilevers, direct patterning on calixarene is demonstrated employing a direct, development-less phenomena triggered by tip emitted low energy (<50 eV) electrons. The scanning probes are not only applied for lithography, but also for imaging and probing of the sur...

Scanning probe lithography approach for beyond CMOS devices

As present CMOS devices approach technological and physical limits at the sub-10 nm scale, a ‘beyond CMOS’ information-processing technology is necessary for timescales beyond the semiconductor technology roadmap. This requires new approaches to logic and memory devices, and to associated lithographic processes. At the sub-5 nm scale, a technology platform based on a combination of high-resolution scanning probe lithography (SPL) and nano-imprint lithography (NIL) is regarded as a promising candidate for both resolution and high throughput production. The practical application of quantum-effect devices, such as room temperature single-electron and quantum-dot devices, then becomes feasible. This paper considers lithographic and device approaches to such a ‘single nanometer manufacturing’ technology. We consider the application of scanning probes, capable of imaging, probing of material properties and lithography at the single nanometer scale. Modified scanning probes are used to pattern molecular glass based resist materials, where the small particle size (<1 nm) and mono-disperse nature leads to more uniform and smaller lithographic pixel size. We also review the current status of single-electron and quantum dot devices capable of room-temperature operation, and discuss the requirements for these devices with regards to practical application.

Self-actuated, self-sensing cantilever for fast CD measurement

The conventional optical lever detection technique involves optical components and its precise mechanical alignment. An additional technical limit is the weight of the optical system, in case a top-scanner is used in high speed and high precision metrology. An alternative represents the application of self-actuated AFM cantilevers with integrated 2DEG piezoresistive deflection sensors. A significant improvement in performance of such cantilevers with respect to deflection sensitivity and temperature stability has been achieved by using an integrated Wheatstone bridge configuration. Due to employing effective cross-talk isolation and temperature drift compensation the performance of these cantilevers was significantly improved. In order to enhance the speed of AFM measurements we are presenting a fast cantilever-approach technology, Q-factor-control and novel adaptive scanning speed procedure. Examples of AFM measurements with high scanning speed (up to 200 lines/s) committed to advanced lithography process development are shown.

Active Microcantilevers for High Material Contrast in Harmonic Atomic Force Microscopy

Atomic force microscope (AFM) probes are mechanical beams that can be used to simultaneously map topography and material properties. Upon contact of the tip with the sample surface at each cycle in the intermittent mode, higher harmonics are excited. The harmonics in the vicinity of higher eigenmodes are enhanced and present an amplified response, ultimately carrying information about the material properties. In this paper, active cantilevers with integrated actuation and sensing are used as a basis to create harmonic cantilevers for the signal-to-noise ratio improved measurement of time-varying forces. Focused ion beam milling is used to remove mass from specific areas in the cantilever such that the fundamental and higher eigenmodes are tuned toward each other. Two methods are tested, where the shape and location of mass removal is determined, first by simulation and second through an in situ approach. Higher harmonics of the harmonic cantilevers with piezoresistive deflection sensors indicate a significant response of up to 10% in respect to the first harmonic. The improved material contrast mapping abilities of the modified cantilevers are validated by characterization and AFM images.

Electric field scanning probe lithography on molecular glass resists using self-actuating, self-sensing cantilever

Within last two years, we have shown the positive-tone, development-less patterning of calixarene molecular glass resists using highly confined electric field, current-controlled scanning probe lithography scheme. Herein, we give a more detailed view insight describing the applied Scanning Probe Lithography (SPL) technology platform applying selfactuating, self-sensing cantilever. The experimental results are supported by first preliminary simulation results estimating the local electric field strength, the electron trajectories, and the current density distribution at the sample surface. In addition, the diameter of Fowler-Nordheim electron beam, emitted from SPL-tip, was calculated as function of the bias voltage for different current set-points and tip radii. In experimental part we show the reproducible writing of meander line patterns as well as the patterning of individual features using specially developed pattern generator software tool.

Scanning probe lithography for electronics at the 5nm scale

Electronic devices on silicon chips have been reducing steadily in size for over 40 years. From the very beginning in the 1960s, Gordon Moore predicted that the number of transistors on a silicon chip would double every two years. This trend, the famous Moore’s law, has become a self-fulfilling prophecy, driving industry requirements for each new generation of silicon chips and leading to dramatic increases in the speed and performance of integrated circuits. Here, the basic circuit building blocks are semiconductor transistors, devices that act as valves to control the circuit current. Each transistor contains a ‘channel’ region where current can flow. This current can be switched on or off by a voltage applied to a ‘gate’ region in close proximity to the channel, allowing charge to be controlled and transferred within the circuit. Existing large-scale integrated circuits use CMOS technology based on silicon field-effect transistors (MOSFETs). Each successive generation of CMOS circuits has seen a reduction in the dimensions of the MOSFET, and at present, the minimum dimensions of this device are only 35nm. It is expected that the MOSFET can remain viable down to the 10nm scale. However, below this, difficulty in controlling the device current, and the strong influence of quantum mechanical effects such as electron tunneling, may require new devices. Furthermore, increasing difficulty in fabricating large numbers of highly nanoscale devices using conventional optical lithographic techniques greatly compounds the problem. This implies that a different approach may be essential to create a ‘beyond-CMOS’ generation of electronic devices, perhaps as soon as 2018. Fabricating future devices in nanoelectronics, nanophotonics, and nanoelectromechanical systems requires lithography at the single-nanometer level with high alignment accuracy between patterns, acceptable throughput, cost, and reliability.1 To address Figure 1. (a) Development-less, positive-tone closed-loop scanning probe lithography (SPL) on calixarene-based molecular glass resist, using self-actuating, piezoresistive scanning probes. (b) Scanning electron microscopy image of a corner pattern written in 10nm-thick 4M1AC6 resist, with 40V bias voltage and 30nC/cm line dose.4 (c) Atomic force microscopy image of lithographic test features written with 30V bias voltage and a line dose of 32nC/cm (broad lines) and 20nC/cm (small lines), respectively. The image was taken directly after lithography with the same cantilever.

Mix & match electron beam & scanning probe lithography for high throughput sub-10 nm lithography

The prosperous demonstration of a technique able to produce features with single nanometer (SN) resolution could guide the semiconductor industry into the desired beyond CMOS era. In the lithographic community immense efforts are being made to develop extreme ultra-violet lithography (EUVL) and multiple-e-beam direct-write systems as possible successor for next generation lithography (NGL). However, patterning below 20 nm resolution and sub-10 nm overlay alignment accuracy becomes an extremely challenging quest. Herein, the combination of electron beam lithography (EBL) or EUVL with the outstanding capabilities of closed-loop scanning proximal probe nanolithography (SPL) reveals a promising way to improve both patterning resolution and reproducibility in combination with excellent overlay and placement accuracy. In particular, the imaging and lithographic resolution capabilities provided by scanning probe microscopy (SPM) methods touches the atomic level, which expresses the theoretical limit of constructing nanoelectronic devices. Furthermore, the symbiosis between EBL (EUVL) and SPL expands the process window of EBL (EUVL) far beyond state-of-the-art allowing SPL-based pre- and post-patterning of EBL (EUVL) written features at critical dimension level with theoretically nanometer precise pattern overlay alignment. Moreover, we can modify the EBL (EUVL) pattern before as well as after the development step. In this paper we demonstrate proof of concept using the ultra-high resolution molecular glass resist calixarene. Therefor we applied Gaussian E-beam lithography system operating at 10 keV and a home-developed SPL set-up. The introduced Mix and Match lithography strategy enables a powerful use of our SPL set-up especially as post-patterning tool for inspection and repair functions below the sub-10 nm critical dimension level.

Advanced electric-field scanning probe lithography on molecular resist using active cantilever

The routine “on demand” fabrication of features smaller than 10 nm opens up new possibilities for the realization of many novel nanoelectronic, NEMS, optical and bio-nanotechnology-based devices. Based on the thermally actuated, piezoresistive cantilever technology we have developed a first prototype of a scanning probe lithography (SPL) platform able to image, inspect, align and pattern features down to single digit nano regime. The direct, mask-less patterning of molecular resists using active scanning probes represents a promising path circumventing the problems in today’s radiation-based lithography. Here, we present examples of practical applications of the previously published electric field based, current-controlled scanning probe lithography on molecular glass resist calixarene by using the developed tabletop SPL system. We demonstrate the application of a step-and-repeat scanning probe lithography scheme including optical as well as AFM based alignment and navigation. In addition, sequential read-write cycle patterning combining positive and negative tone lithography is shown. We are presenting patterning over larger areas (80 x 80 μm) and feature the practical applicability of the lithographic processes.

Cu(OH)2 and CuO Nanorod Synthesis on Piezoresistive Cantilevers for the Selective Detection of Nitrogen Dioxide

Self-controlled active oscillating microcantilevers with a piezoresistive readout are very promising sensitive sensors, despite their small surface. In order to increase this surface and consequently their sensitivity, we nanostructured them with copper hydroxide (Cu(OH)2) or with copper oxide (CuO) nanorods. The Cu(OH)2 rods were grown, on a homogeneous copper layer previously evaporated on the top of the cantilever. The CuO nanorods were further obtained by the annealing of the copper hydroxide nanostructures. Then, these copper based nanorods were used to detect several molecules vapors. The results showed no chemical affinity (no formation of a chemical bond) between the CuO cantilevers and the tested molecules. The cantilever with Cu(OH)2 nanorods is selective to nitrogen dioxide (NO2) in presence of humidity. Indeed, among all the tested analytes, copper hydroxide has only an affinity with NO2. Despite the absence of affinity, the cantilevers could even so condensate explosives (1,3,5-trinitro-1,3,5-triazinane (RDX) and pentaerythritol tetranitrate (PETN) on their surface when the cantilever temperature was lower than the explosives source, allowing their detection. We proved that in condensation conditions, the cantilever surface material has no importance and that the nanostructuration is useless because a raw silicon cantilever detects as well as the nanostructured ones.