Laurent Couraud

Electron beam lithography: resolution limits and applications

Abstract We report on the resolution limits of Electron Beam Lithography (EBL) in the conventional polymethylmethacrylate (PMMA) organic resist. We show that resolution can be pushed below 10 nm for isolated features and how dense arrays of periodic structures can be fabricated at a pitch of 30 nm, leading to a density close to 700 Gbit/in2. We show that intrinsic resolution of the writing in the resist is as small as 3 to 5 nm at high incident electron energy, and that practical resolution is limited by the development of the resist after exposure and by pattern transfer. We present the results of our optimized process for reproducible fabrication of sub-10 nm lines by lift-off and 30-nm pitch pillar arrays by lift-off and reactive ion etching (RIE). We also present some applications of these nanostructures for the fabrication of very high density molds for nano-imprint lithography (NIL) and for the fabrication of Multiple Tunnel Junction devices that can be used for single electron device applications or for the connection of small molecules.

Nanoimprint lithography for a large area pattern replication

We report on replication of high resolution patterns over a 4 in. wafer area by imprint lithography with a commercial hydraulic press and a pair of hot plates. The experiments confirm that the imprint lithography can be used for large area patterning. As a result, sub-100 nm features were obtained by the imprint lithography and lift-off with a good uniformity and an accurate pattern placement over the 4 in. wafer area.

Electro-osmotic propulsion of helical nanobelt swimmers

Micro and nanoscale mobile agents capable of self-propulsion in low Reynolds number fluids would have a great technological impact in many fields. Few known mechanisms are able to propel such devices. Here we describe helical nanobelt (HNB) swimmers actuated by an electric field-generated electro-osmotic force. These HNB swimmers are designed with a head and a tail, similar to natural micro-organisms such as bacteria and their flagella. We show that these electro-osmotic propulsion of HNB swimmers achieve speeds (24 body lengths per second), force (1.3 nN), and pressure (375.5 Pa) above those demonstrated by other artificial swimmers based on physical energy conversion. Although nature’s bacteria are still more dynamic, this paper reports that the demonstrated electro-osmotic HNB microswimmers made a big step toward getting closer to their performances. Moreover, an unusual swimming behavior with discontinuous pumping propulsion, similar to jellyfish, was revealed at or above the speculated marginal limit of linear propulsion. These electro-osmosis propelled HNB swimmers might be used as biomedical carriers, wireless manipulators, and as local probes for rheological measurements.

Tri-layer systems for nanoimprint lithography with an improved process latitude

We present two tri-layer systems which can be used to improve the process latitude of nanoimprint lithography. By hot empossing, the top layer polymer can easily deformed while the bottom layer, separated by a 10 nm thick germanium from the top layer, remains to be thermally stable. With a sequential reactive ion etching, the top layer image can be transferred into the bottom layer with a large thickness contrast, thereby providing a way to generate dense features with high aspect ratio. Consequently, commonly used pattern-transfer techniques such as lift-off, reactive ion etching and electrodeposition can be employed.

Temperature evolution of multiple tunnel junction devices made with disordered two-dimensional arrays of metallic islands

The temperature behavior of multiple tunnel junction (MTJ) devices made with sub-5-nm gold islands is investigated. A smooth decrease of the Coulomb gap with increasing temperatures is observed. The critical temperature beyond which the Coulomb blockade effect is suppressed is found to change as a function both of the average size of the islands and of the size of the two-dimensional (2D) array of islands forming the MTJ. This latter property is attributed to the role of disorder in the 2D array. Results are compared with Monte Carlo simulations of current transport through highly disordered 2D arrays which reproduce the experimental evolution of the Coulomb gap with temperature.

Ultra-low noise high electron mobility transistors for high-impedance and low-frequency deep cryogenic readout electronics

We report on the results obtained from specially designed high electron mobility transistors at 4.2 K: the gate leakage current can be limited lower than 1 aA, and the equivalent input noise-voltage and noise-current at 1 Hz can reach 6.3 nV/Hz1∕2 and 20 aA/Hz1∕2, respectively. These results open the way to realize high performance low-frequency readout electronics under very low-temperature conditions.

Electrothermally driven high-frequency piezoresistive SiC cantilevers for dynamic atomic force microscopy

Cantilevers with resonance frequency ranging from 1 MHz to 100 MHz have been developed for dynamic atomic force microscopy. These sensors are fabricated from 3C-SiC epilayers grown on Si(100) substrates by low pressure chemical vapor deposition. They use an on-chip method both for driving and sensing the displacement of the cantilever. A first gold metallic loop deposited on top of the cantilever is used to drive its oscillation by electrothermal actuation. The sensing of this oscillation is performed by monitoring the resistance of a second Au loop. This metallic piezoresistive detection method has distinct advantages relative to more common semiconductor-based schemes. The optimization, design, fabrication, and characteristics of these cantilevers are discussed.

Swimming property characterizations of Magnetic Polarizable microrobots

The development of new mobile microrobotic swimmers, limited by low Reynolds dynamics and stiction effects in confined environments, should aim for efficient propulsion mechanisms in wet confined environments and more elaborated manipulation strategies. We first introduced a novel type of microrobots called MagPol (Magnetic Polarizable) integrated in a microfluidic chip controlled through magnetic waves. We developed a simple method analyzing rotational dynamics to characterize swimming performances inside microfluidic environments and provide experimental criteria to improve hydrodynamics of future designs. Magpols demonstrated their in-plane mobilities, with a maximum speed up-to 566 millimeters per second (1390 body lengths per second), and the completions of sophisticated trajectory through the microchannels, forward or backward. In addition to their enhanced planar mobilities, we newly demonstrated a backward towing technique by reversing intrinsic magnetic moment.

High Frequency 3C-SiC AFM Cantilever Using Thermal Actuation and Metallic Piezoresistive Detection

One way to improve the force sensitivity of Atomic Force Microscopy (AFM) cantilevers is to increase their resonance frequency. SiC is an excellent material for that purpose due to its high Young’s modulus and low mass density. This size reduction makes conventional optical motion detection methods inappropriate. Here, we introduce self-sensing, self-excited high frequency AFM cantilevers. The motion detection is based on the measurement of a metallic piezoresistor incorporated in the cantilever. The motion excitation is performed by electrothermal actuation using another metallic circuit. Cantilevers with sizes as low as 4 μm in length, 1.2 μm in width and 0.5 μm in thickness were realized by using different steps of e-beam lithography, deposition of thin gold films to pattern the piezoresistor and the electrothermal actuation electrode. Dry etching SF6 plasma was used for etching the SiC cantilever and TMAH solution heated to 80°C to release the cantilever. In this case, a thigh control of underetching, which reduces the cantilever resonance frequency was required.

Multi-flagella helical microswimmers for multiscale cargo transport and reversible targeted binding

In-vivo and in-vitro micro-interventions by mobile robotic agents require their precise control. Numerous types of microrobotic swimmers have been developed, assuming different peculiar applications, but often suffering from a lack of mobility and robustness, due to their unique propulsion mode. Hence we present in this paper the multi-flagella helical microswimmers that combine complementary propulsion modes for covering different motions and applications. The numerous possibilities of motions make the robots capable of moving rapidly, on long distances and in hard conditions, as expected in biological organisms. Besides, our microswimmers prove to be able of precise targeted binding, reversible binding, and multi-scale cargo transport moving particles from 5 to 30 μm large. Thus the demonstrated skillful multi-flagella helical microswimmers are very promising for future interventions in microfluidic chips and biological organisms, such as cell manipulation, precise drug delivery, minimally invasive surgery.