Active Probe Atomic Force Microscopy with Quattro-Parallel Cantilever Arrays for High-Throughput Large-Scale Sample Inspection.

An Atomic Force Microscope (AFM) is a powerful and versatile tool for nanoscale surface studies to capture 3D topography images of samples. However, due to their limited imaging throughput, AFMs have not been widely adopted for large-scale inspection purposes. Researchers have developed high-speed AFM systems to record dynamic process videos in chemical and biological reactions at tens of frames per second, at the cost of a small imaging area of up to several square micrometers. In contrast, inspecting large-scale nanofabricated structures, such as semiconductor wafers, requires nanoscale spatial resolution imaging of a static sample over hundreds of square centimeters with high productivity. Conventional AFMs use a single passive cantilever probe with an optical beam deflection system, which can only collect one pixel at a time during AFM imaging, resulting in low imaging throughput. This work utilizes an array of active cantilevers with embedded piezoresistive sensors and thermomechanical actuators, which allows simultaneous multi-cantilever operation in parallel operation for increased imaging throughput. When combined with large-range nano-positioners and proper control algorithms, each cantilever can be individually controlled to capture multiple AFM images. With data-driven post-processing algorithms, the images can be stitched together, and defect detection can be performed by comparing them to the desired geometry. This paper introduces principles of the custom AFM using the active cantilever arrays, followed by a discussion on practical experiment considerations for inspection applications. Selected example images of silicon calibration grating, highly-oriented pyrolytic graphite, and extreme ultraviolet lithography masks are captured using an array of four active cantilevers ("Quattro") with a 125 µm tip separation distance. With more engineering integration, this high-throughput, large-scale imaging tool can provide 3D metrological data for extreme ultraviolet (EUV) masks, chemical mechanical planarization (CMP) inspection, failure analysis, displays, thin-film step measurements, roughness measurement dies, and laser-engraved dry gas seal grooves.

Novel type of whisker-tip cantilever based on GaN microrods for atomic force microscopy.

High-resolution scanning probe microscopy (SPM) is a fundamental and efficient technology for surface characterization of modern materials at the subnanometre scale. The bottleneck of SPM is the probe and scanning tip. Materials with stable electrical, thermal, and mechanical properties for high-aspect-ratio (AR) tips are continuously being developed to improve their accuracy. Among these, GaN is emerging as a significant contender that serves as a replacement for standard Si probes. In this paper, for the first time, we present an approach that demonstrates the application of GaN microrods (MRs) as high-AR SPM probes. GaN MRs were grown using molecular beam epitaxy, transferred and mounted on a cantilever using focused electron beam-induced deposition and milled in a whisker tip using a focused ion beam in a scanning electron/ion microscope. The presence of a native oxide layer covering the GaN MR surface was confirmed by X-ray photoelectron spectroscopy. Current-voltage map measurements are also presented to indicate the elimination of the native oxide layer from the tip surface. The utility of the designed probes was tested using conductive atomic force microscopy and a 24-hour durability test in contact mode atomic force microscopy. Subsequently, the graphene stacks were imaged.

Advanced Scanning Probe Nanolithography Using GaN Nanowires.

A fundamental understanding and advancement of nanopatterning and nanometrology are essential in the future development of nanotechnology, atomic scale manipulation, and quantum technology industries. Scanning probe-based patterning/imaging techniques have been attractive for many research groups to conduct their research in nanoscale device fabrication and nanotechnology mainly due to its cost-effective process; however, the current tip materials in these techniques suffer from poor durability, limited resolution, and relatively high fabrication costs. Here, we report on employing GaN nanowires as a robust semiconductor material in scanning probe lithography (SPL) and microscopy (SPM) with a relatively low-cost fabrication process and the capability to provide sub-10 nm lithography and atomic scale (<1 nm) patterning resolution in field-emission scanning probe lithography (FE-SPL) and scanning tunneling microscopy (STM), respectively. We demonstrate that GaN NWs are great candidates for advanced SPL and imaging that can provide atomic resolution imaging and sub-10 nm nanopatterning on different materials in both vacuum and ambient operations.

Phase-Modulated Standing Wave Interferometer.

The actual technical implementation of conventional interferometers is quite complex and requires manual manufacturing. In combination with the required construction space defined by the optical setup, their applications are limited to selected measuring tasks. In contrast, Standing Wave Interferometers (SWIs) offer an enormous potential for miniaturisation because of their simple linear optical setup, consisting only of a laser source, a measuring mirror and two transparent standing wave sensors for obtaining quadrature signals. The two sensors are located inside the measuring beam and therefore directly influence the length measurement. To reduce optical influences on the standing wave and avoid the need for an exact and long-term stable sensor-to-sensor-distance, a single sensor configuration was developed. There, a phase modulation is superimposed to the sensor signal by a forced oscillation of the measuring mirror. When the correct modulation stroke is applied, the resulting harmonics in the sensor signal are 90° phase-shifted to each other and can hence be used for obtaining quadrature signals for phase demodulation and direction discrimination by an arctan-algorithm.

Silk as a biodegradable resist for field-emission scanning probe lithography.

The patterning of silk allows for manufacturing various structures with advanced functionalities for optical and tissue engineering and drug delivery applications. Here, we propose a high-resolution nanoscale patterning method based on field-emission scanning probe lithography (FE-SPL) that crosslinks the biomaterial silk on conductive indium tin oxide (ITO) promoting the use of a biodegradable material as resist and water as a developer. During the lithographic process, Fowler-Nordheim electron emission from a sharp tip was used to manipulate the structure of silk fibroin from random coil to beta sheet and the emission formed nanoscale latent patterns with a critical dimension (CD) of ∼50 nm. To demonstrate the versatility of the method, we patterned standard and complex shapes. This method is particularly attractive due to its ease of operation without relying on a vacuum or a special gaseous environment and without any need for complex electronics or optics. Therefore, this study paves a practical and cost-effective way toward patterning biopolymers at ultra-high level resolution.

Lights Out! Nano-Scale Topography Imaging of Sample Surface in Opaque Liquid Environments with Coated Active Cantilever Probes.

Atomic force microscopy is a powerful topography imaging method used widely in nanoscale metrology and manipulation. A conventional Atomic Force Microscope (AFM) utilizes an optical lever system typically composed of a laser source, lenses and a four quadrant photodetector to amplify and measure the deflection of the cantilever probe. This optical method for deflection sensing limits the capability of AFM to obtaining images in transparent environments only. In addition, tapping mode imaging in liquid environments with transparent sample chamber can be difficult for laser-probe alignment due to multiple different refraction indices of materials. Spurious structure resonance can be excited from piezo actuator excitation. Photothermal actuation resolves the resonance confusion but makes optical setup more complicated. In this paper, we present the design and fabrication method of coated active scanning probes with piezoresistive deflection sensing, thermomechanical actuation and thin photoresist polymer surface coating. The newly developed probes are capable of conducting topography imaging in opaque liquids without the need of an optical system. The selected coating can withstand harsh chemical environments with high acidity (e.g., 35% sulfuric acid). The probes are operated in various opaque liquid environments with a custom designed AFM system to demonstrate the imaging performance. The development of coated active probes opens up possibilities for observing samples in their native environments.

Charged particle single nanometre manufacturing.

Following a brief historical summary of the way in which electron beam lithography developed out of the scanning electron microscope, three state-of-the-art charged-particle beam nanopatterning technologies are considered. All three have been the subject of a recently completed European Union Project entitled "Single Nanometre Manufacturing: Beyond CMOS". Scanning helium ion beam lithography has the advantages of virtually zero proximity effect, nanoscale patterning capability and high sensitivity in combination with a novel fullerene resist based on the sub-nanometre C60 molecule. The shot noise-limited minimum linewidth achieved to date is 6 nm. The second technology, focused electron induced processing (FEBIP), uses a nozzle-dispensed precursor gas either to etch or to deposit patterns on the nanometre scale without the need for resist. The process has potential for high throughput enhancement using multiple electron beams and a system employing up to 196 beams is under development based on a commercial SEM platform. Among its potential applications is the manufacture of templates for nanoimprint lithography, NIL. This is also a target application for the third and final charged particle technology, viz. field emission electron scanning probe lithography, FE-eSPL. This has been developed out of scanning tunneling microscopy using lower-energy electrons (tens of electronvolts rather than the tens of kiloelectronvolts of the other techniques). It has the considerable advantage of being employed without the need for a vacuum system, in ambient air and is capable of sub-10 nm patterning using either developable resists or a self-developing mode applicable for many polymeric resists, which is preferred. Like FEBIP it is potentially capable of massive parallelization for applications requiring high throughput.

Polymer-metal coating for high contrast SEM cross sections at the deep nanoscale.

In scanning electron microscopy (SEM), imaging nanoscale features by means of the cross-sectioning method becomes increasingly challenging with shrinking feature sizes. However, obtaining high quality images, at high magnification, is crucial for critical dimension and patterned feature evaluation. Therefore, in this work, we present a new sample preparation method for high performance cross-sectional secondary electron (SE) imaging, targeting features at the deep nanoscale and into the sub-10 nm regime. Different coating architectures including conductive and non-conductive polymer, carbon and metal are compared on their ability to discern etching feature profiles and materials interfaces of densely packed nano-patterned features. A stacked coating of polymer and metal produced better visibility mainly due to enhancement of contrast between feature and background. Contrast was evaluated by using histograms of intensity of gray levels directly derived from SE images, obtained by the SE in-lens detector. In polymer-metal coatings (PMC), optimization of contrast is explored by varying the thickness of the metal layer and results are discussed in terms of the effectiveness of the metal layer in reducing the escape of secondary electrons (SE) generated in the polymer layer and feature. Other advantages of PMCs are their cleanroom compatibility and ease of coating removal.

Atomic layer deposition for spacer defined double patterning of sub-10 nm titanium dioxide features.

The next generation of hard disk drive technology for data storage densities beyond 5 Tb/in2 will require single-bit patterning of features with sub-10 nm dimensions by nanoimprint lithography. To address this challenge master templates are fabricated using pattern multiplication with atomic layer deposition (ALD). Sub-10 nm lithography requires a solid understanding of materials and their interactions. In this work we study two important oxide materials, silicon dioxide and titanium dioxide, as the pattern spacer and look at their interactions with carbon, chromium and silicon dioxide. We found that thermal titanium dioxide ALD allows for the conformal deposition of a spacer layer without damaging the carbon mandrel and eliminates the surface modification due to the reactivity of the metal-organic precursor. Finally, using self-assembled block copolymer lithography and thermal titanium dioxide spacer fabrication, we demonstrate pattern doubling with 7.5 nm half-pitch spacer features.

Low temperature dry etching of chromium towards control at sub-5 nm dimensions.

Patterned chromium and its compounds are crucial materials for nanoscale patterning and chromium based devices. Here we investigate how temperature can be used to control chromium etching using chlorine/oxygen gas mixtures. Oxygen/chlorine ratios between 0% and 100% and temperatures between -100 °C and +40 °C are studied. Spectroscopic ellipsometry is used to precisely measure rates, chlorination, and the thickness dependence of n and k. Working in the extremes of oxygen content (very high or very low) and lower temperatures, we find rates can be controlled to nanometers per minute. Activation energies are measured and show that etch mechanisms are both temperature and oxygen level dependent. Furthermore, we find that etching temperature can manipulate the surface chemistry. One surprising consequence is that at low oxygen levels, Etching rates increase with decreasing temperature. Preliminary feature-profile studies show the extremes of temperature and oxygen provide advantages over commonly used room temperature processing conditions. One example is with higher ion energies at -100 °C, where etching products deposit.

Fabrication Process for an Optomechanical Transducer Platform with Integrated Actuation.

This article reports a process for batch fabrication of a fiber pigtailed optomechanical transducer platform with overhanging. The platform enables a new class of high bandwidth, high sensitivity, and highly integrated sensors that are, compact, robust, and small, with the potential potential for low cost batch fabrication inherent in Micro-Opto-Electro-Mechanical-Systems technology. This article provides a guide to the whole fabrication process and explains critical steps and process choices in detail. Possible alternative fabrication techniques and problems are discussed. The fabrication process consists of electron beam lithography, i-line stepper lithography, and back- and frontside mask aligner lithography. The goal of this article is to provide a comprehensive description of the fabrication process, presenting context and details which are highly relevant to the rational implementation and reliable repetition of the process. Moreover, this process makes use of equipment commonly found in nanofabrication facilities and research laboratories, facilitating the broad adaptation and application of the process. Therefore, while this article specifically informs users of the Center for Nanoscale Science and Technology (CNST) at the National Institute of Standards and Technology (NIST), we anticipate that this information will be generally useful for the nano- and microfabrication research communities at large.

Multi-eigenmode control for high material contrast in bimodal and higher harmonic atomic force microscopy.

High speed imaging and mapping of nanomechanical properties in atomic force microscopy (AFM) allows the observation and characterization of dynamic sample processes. Recent developments involve several cantilever frequencies in a multifrequency approach. One method actuates the first eigenmode for topography imaging and records the excited higher harmonics to map nanomechanical properties of the sample. To enhance the higher frequencies' response two or more eigenmodes are actuated simultaneously, where the higher eigenmode(s) are used to quantify the nanomechanics. In this paper, we combine each imaging methodology with a novel control approach. It modifies the Q factor and resonance frequency of each eigenmode independently to enhance the force sensitivity and imaging bandwidth. It allows us to satisfy the different requirements for the first and higher eigenmode. The presented compensator is compatible with existing AFMs and can be simply attached with minimal modifications. Different samples are used to demonstrate the improvement in nanomechanical contrast mapping and imaging speed of tapping mode AFM in air. The experiments indicate most enhanced nanomechanical contrast with low Q factors of the first and high Q factors of the higher eigenmode. In this scenario, the cantilever topography imaging rate can also be easily improved by a factor of 10.

Adaptive AFM scan speed control for high aspect ratio fast structure tracking.

Improved imaging rates in Atomic Force Microscopes (AFM) are of high interest for disciplines such as life sciences and failure analysis of semiconductor wafers, where the sample topology shows high aspect ratios. Also, fast imaging is necessary to cover a large surface under investigation in reasonable times. Since AFMs are composed of mechanical components, they are associated with comparably low resonance frequencies that undermine the effort to increase the acquisition rates. In particular, high and steep structures are difficult to follow, which causes the cantilever to temporarily loose contact to or crash into the sample. Here, we report on a novel approach that does not affect the scanner dynamics, but adapts the lateral scanning speed of the scanner. The controller monitors the control error signal and, only when necessary, decreases the scan speed to allow the z-piezo more time to react to changes in the sample's topography. In this case, the overall imaging rate can be significantly increased, because a general scan speed trade-off decision is not needed and smooth areas are scanned fast. In contrast to methods trying to increase the z-piezo bandwidth, our method is a comparably simple approach that can be easily adapted to standard systems.

Experimental setup for characterization of self-actuated microcantilevers with piezoresistive readout for chemical recognition of volatile substances.

This paper summarizes our achievements in the development of an advanced microcantilever-based platform for the detection and recognition of various volatile analytes. The implemented microcantilevers include integrated piezoresistive readout, integrated thermally driven bimorph actuator, and a gold pad at the cantilever apex for functionalization toward the detection of specific substances. Up to eight single microcantilevers can be installed and investigated quasisimultaneously in either gas flow or gas/vapor single injection mode. The experimental setup enables the detection of the microcantilever bending via surface stress changes, characterization of either amplitude or phase spectra of the microcantilever, and also calibration of its sensitivity.

On total internal reflection investigation of nanoparticles by integrated micro-fluidic system.

We report on a novel sensor for characterization of nanoparticles colloidal suspensions. We employ a diffraction grating under total internal reflection for investigation of nanodisperse fluids passing through an integrated microfluidic channel. Dispersions containing polymeric, metallic, and ferromagnetic nanoparticles are studied. Using this device, we can accurately determine in real-time the specific refractive index for the nanoparticle suspension and the nanoparticle concentration. The nanoparticle concentrations can be calculated with a resolution of 0.3-0.5 wt% for polymeric nanoparticles, 0.03-0.05 wt% for metallic nanoparticles, and 0.05-0.1 wt% for ferromagnetic nanoparticles. This translates to an effective refractive index that can be determined with an accuracy of 7 x 10(-4) for the polymeric and 2 x 10(-4) for the metallic and ferromagnetic dispersions.

Components for high speed atomic force microscopy.

Many applications in materials science, life science and process control would benefit from atomic force microscopes (AFM) with higher scan speeds. To achieve this, the performance of many of the AFM components has to be increased. In this work, we focus on the cantilever sensor, the scanning unit and the data acquisition. We manufactured 10 microm wide cantilevers which combine high resonance frequencies with low spring constants (160-360 kHz with spring constants of 1-5 pN/nm). For the scanning unit, we developed a new scanner principle, based on stack piezos, which allows the construction of a scanner with 15 microm scan range while retaining high resonance frequencies (>10 kHz). To drive the AFM at high scan speeds and record the height and error signal, we implemented a fast Data Acquisition (DAQ) system based on a commercial DAQ card and a LabView user interface capable of recording 30 frames per second at 150 x 150 pixels.

Calibration and examination of piezoresistive Wheatstone bridge cantilevers for scanning probe microscopy.

This paper describes the method of determining the force constant and displacement sensitivity of piezoresistive Wheatstone bridge cantilevers applied in scanning probe microscopy (SPM). In the procedure presented here, the force constant for beams with various geometry is determined based on resonance frequency measurement. The displacement sensitivity is measured by the deflection of the cantilever with the calibrated piezoactuator stage. Preliminary results show that our method is capable of measuring the force constant of Wheatstone bridge cantilevers with an accuracy of better than 5% and this is used as feedback for improvement of sensor micromachining process.