The control of light-matter interaction through the use of subwavelength structures known as metamaterials has facilitated
the ability to control electromagnetic radiation in ways not previously achievable. A plethora of passive metamaterials as
well as examples of active or tunable metamaterials have been realized in recent years. However, the development of
tunable metamaterials is still met with challenges due to lack of materials choices. To this end, materials that exhibit a
metal-insulator transition are being explored as the active element for future metamaterials because of their characteristic
abrupt change in electrical conductivity across their phase transition. The fast switching times (▵t < 100 fs) and a change
in resistivity of four orders or more make vanadium dioxide (VO2) an ideal candidate for active metamaterials. It is known
that the properties associated with thin film metal-insulator transition materials are strongly dependent on the growth
conditions. For this work, we have studied how growth conditions (such as gas partial pressure) influence the metalinsulator
transition in VO2 thin films made by pulsed laser deposition. In addition, strain engineering during the growth
process has been investigated as a method to tune the metal-insulator transition temperature. Examples of both the optical
and electrical transient dynamics facilitating the metal-insulator transition will be presented together with specific
examples of thin film metamaterial devices.
Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 68°C, at which point its electrical conductivity changes by several orders of magnitude. This extremely fast transition (Δt < 100 fs) can be induced thermally, mechanically, electrically, or optically. The combination of fast switching times and response to a broad range of external stimuli make VO2 an ideal material for a variety of novel devices and sensors. While the MIT in VO2 has been exploited for a variety of microwave/terahertz applications (i.e. tunable filters and modulators), very few devices exploiting the fast switching time of VO2 have been reported. The electrical properties of thin film VO2 (conductivity, carrier concentration, switching speed, etc.) are highly dependent on growth and post-processing conditions. The optimization of these conditions is therefore critical to the design and fabrication of VO2 devices. This paper will report the effects of various pulsed laser deposition (PLD) growth conditions on the metal-insulator transition in thin film VO2. In particular, we report the effect of PLD growth conditions on the stress/strain state of the VO2 layer, and the subsequent change in electrical properties. Finally, results from fabricated VO2 devices (THz emitters and THz modulators) will be presented.
Recently, three-dimensional (3D) electrode architectures have attracted great interest for the development of lithium-ion micro-batteries applicable for Micro-Electro-Mechanical Systems (MEMS), sensors, and hearing aids. Since commercial available micro-batteries are mainly limited in overall cell capacity by their electrode footprint, new processing strategies for increasing both capacity and electrochemical performance have to be developed. In case of such standard microbatteries, two-dimensional (2D) electrode arrangements are applied with thicknesses up to 200 μm. These electrode layers are composed of active material, conductive agent, graphite, and polymeric binder. Nevertheless, with respect to the type of active material, the active material to conductive agent ratio, and the film thickness, such thick-films suffer from low ionic and electronic conductivities, poor electrolyte accessibility, and finally, limited electrochemical performance under challenging conditions. In order to overcome these drawbacks, 3D electrode arrangements are under intense investigation since they allow the reduction of lithium-ion diffusion pathways in between inter-digitated electrodes, even for electrodes with enhanced mass loadings. In this paper, we present how to combine laser-printing and femtosecond laser-structuring for the development of advanced 3D electrodes composed of Li(Ni1/3Mn1/3Co1/3)O2 (NMC). In a first step, NMC thick-films were laser-printed and calendered to achieve film thicknesses in the range of 50 μm – 80 μm. In a second step, femtosecond laser-structuring was carried out in order to generate 3D architectures directly into thick-films. Finally, electrochemical cycling of laser-processed films was performed in order to evaluate the most promising 3D electrode designs suitable for application in long life-time 3D micro-batteries.
The use of metamaterials structures has been the subject of extensive discussions given their wide range of applications. However, a large fraction of the work available to date has been limited to simulations and proof-of-principle demonstrations. One reason for the limited success inserting these structures into functioning systems and real-world applications is the high level of complexity involved in their fabrication. Direct-write processes are ideally suited for the fabrication of arbitrary periodic and aperiodic structures found in most metamaterial and plasmonic designs. For these applications, laser-based processes offer numerous advantages since they can be applied to virtually any surface over a wide range of scales. Furthermore, laser direct-write or LDW allows the precise deposition and/or removal of material thus enabling the fabrication of novel metamaterial designs. This presentation will show examples of metamaterial and plasmonic structures developed at the Naval Research Lab using LDW, and discuss the benefits of laser processing for these applications.
This work was sponsored by The Office of Naval Research.
The progressive miniaturization of electronic devices requires an ever-increasing density of interconnects attached via solder joints. As a consequence, the overall size and spacing (or pitch) of these solder joint interconnects keeps shrinking. When the pitch between interconnects decreases below 200 μm, current technologies, such as stencil printing, find themselves reaching their resolution limit. Laser direct-write (LDW) techniques based on laser-induced forward transfer (LIFT) of functional materials offer unique advantages and capabilities for the printing of solder pastes. At NRL, we have demonstrated the successful transfer, patterning, and subsequent reflow of commercial Pb-free solder pastes using LIFT. Transfers were achieved both with the donor substrate in contact with the receiving substrate and across a 25 μm gap, such that the donor substrate does not make contact with the receiving substrate. We demonstrate the transfer of solder paste features down to 25 μm in diameter and as large as a few hundred microns, although neither represents the ultimate limit of the LIFT process in terms of spatial dimensions. Solder paste was transferred onto circular copper pads as small as 30 μm and subsequently reflowed, in order to demonstrate that the solder and flux were not adversely affected by the LIFT process.
This paper reviews recent work on the fabrication of energy storage and power generation using laser-based processes such as pulsed laser deposition (PLD), laser-induced forward transfer (LIFT), and laser surface processing techniques. PLD is a versatile technique for depositing high-quality layers of materials for cathodes, anodes, and solid electrolytes for thin-film microbatteries. Using sequential PLD processes, solid-state thin-film lithium-ion microbatteries can be successfully fabricated. LIFT is a powerful tool for printing complex materials with highly porous structures for the fabrication of micropower sources such as thick-film batteries and metal oxide-based solar cells. In particular, using the LIFT process it is possible to print thick layers (∼100 μm) while maintaining pattern integrity and low-internal resistance. As a consequence, power sources fabricated in this manner exhibit higher energy densities per unit area than those obtained by traditional thin-film growth techniques. In addition, the printed active materials can be modified by postlaser processes, such as laser sintering and laser structuring, to further improve the device performance by enhancing the electrodes’ three-dimensional networked structure and increasing the overall active surface, respectively. This review will discuss various examples where laser materials’ processing has led to new approaches in the development of micropower sources applications.
Conventional metals with high carrier concentrations have served to date as the materials of choice for plasmonic and metamaterial devices. However, typical metals are not well suited for near IR (NIR) plasmonic applications because their associated plasma frequencies correspond to the visible and ultraviolet regions of the spectrum. On the other hand, materials with lower plasma frequencies such as conducting oxides like ZnO and VO2 are capable of more efficiently coupling the electromagnetic radiation for optical metamaterial and plasmonic applications in the NIR. Furthermore, unlike metals, the electrical transport properties of conductive oxides can be modulated intrinsically by doping or extrinsically by applying heat, light or an electrical bias, thus allowing tuning of their electro-optical behavior. At the Naval Research Laboratory (NRL), we have investigated the use of laser processing techniques for the deposition and processing of various types of conducting oxides, such as Al-doped ZnO and W-doped VO2, which can be optimized over a wide range of optical/electrical properties. This paper will describe the laser deposition of these oxide films and their electrical and optical characterization in the NIR.
Laser induced forward transfer (LIFT) process was used to print thick-film electrodes (LiCoO2 cathode and carbon
anode) and solid-state polymer membranes for Li-ion microbatteries. Their electrochemical behaviors were characterized
by cyclic voltammograms, capacity measurement and cycling performance. Microbatteries based on these laser-printed
thick-film electrodes showed significantly higher discharge capacities than those made by sputter-deposited thin film
techniques. This enhanced performance is attributed to the high surface area porous structure of the laser-printed
electrodes that allows improved diffusion of the Li-ions across the 100 μm-thick electrodes without a significant internal
resistance. In addition, a laser structuring process was used to prepare three-dimensional microstructures on the laserprinted
thick-film electrodes to further improve battery performance by increasing the active surface area. These results
indicate that the laser processing techniques are a viable approach for developing Li-ion microbatteries in
microelectronic devices. This paper will show examples of Li-ion microbatteries fabricated with various polymer
separators and structured electrodes using a combination of LIFT and excimer laser structuring processes.
Lithium manganese oxide composite cathodes are realized by laser-printing. The printed cathode is a composite and
consists of active powder, binder and conductive agents. Laser-printed cathodes are first calendered and then laser
structured using femtosecond-laser radiation in order to form three-dimensional (3D) micro-grids in the cathode material.
Three-dimensional micro-grids in calendered/laser structured cathodes exhibit improved discharge capacity retention at a
1 C discharging rate. Calendered but unstructured cathodes indicate the poorest cycling behavior at 1 C discharge. The
improved capacity retention and the reduced degradation of calendered/structured cathodes can be attributed to both the
increased electrical contact through calendering as well as shortened Li-ion pathways due to laser-induced 3D microgrids.
Additive manufacturing techniques such as 3D printing are able to generate reproductions of a part in free space without the use of molds; however, the objects produced lack electrical functionality from an applications perspective. At the same time, techniques such as inkjet and laser direct-write (LDW) can be used to print electronic components and connections onto already existing objects, but are not capable of generating a full object on their own. The approach missing to date is the combination of 3D printing processes with direct-write of electronic circuits. Among the numerous direct write techniques available, LDW offers unique advantages and capabilities given its compatibility with a wide range of materials, surface chemistries and surface morphologies. The Naval Research Laboratory (NRL) has developed various LDW processes ranging from the non-phase transformative direct printing of complex suspensions or inks to lase-and-place for embedding entire semiconductor devices. These processes have been demonstrated in digital manufacturing of a wide variety of microelectronic elements ranging from circuit components such as electrical interconnects and passives to antennas, sensors, actuators and power sources. At NRL we are investigating the combination of LDW with 3D printing to demonstrate the digital fabrication of functional parts, such as 3D circuits. Merging these techniques will make possible the development of a new generation of structures capable of detecting, processing, communicating and interacting with their surroundings in ways never imagined before. This paper shows the latest results achieved at NRL in this area, describing the various approaches developed for generating 3D printed electronics with LDW.
The field of metamaterials has expanded to include more than four orders of magnitude of the electromagnetic spectrum, ranging from the microwave to the optical. While early metamaterials operated in the microwave region of the spectrum, where standard printed circuit board techniques could be applied, modern designs operating at shorter wavelengths require alternative manufacturing methods, including advanced semiconductor processes. Semiconductor manufacturing methods have proven successful for planar 2D geometries of limited scale. However, these methods are limited by material choice and the range of possible feature sizes, thus hindering the development of metamaterials due to manufacturing challenges. Furthermore, it is difficult to achieve the wide range of scales encountered in modern metamaterial designs with these methods alone. Laser direct-write processes can overcome these challenges while enabling new and exciting fabrication techniques. Laser processes such as micromachining and laser transfer are ideally suited for the development and optimization of 2D and 3D metamaterial structures. These laser processes are advantageous in that they have the ability to both transfer and remove material as well as the capacity to pattern non-traditional surfaces. This paper will present recent advances in laser processing of various types of metamaterial designs.
Laser forward transfer of arbitrary and complex configurable structures has recently been demonstrated using a spatial light modulator (SLM). The SLM allows the spatial distribution of the laser pulse, required by the laser transfer process, to be modified for each pulse. The programmable image on the SLM spatially modulates the intensity profile of the laser beam, which is then used to transfer a thin layer of material reproducing the same spatial pattern onto a substrate. The combination of laser direct write (LDW) with a SLM is unique since it enables LDW to operate not only in serial fashion like other direct write techniques but instead reach a level in parallel processing not possible with traditional digital fabrication methods. This paper describes the use of Digital Micromirror Devices or DMDs as SLMs in combination with visible (λ = 532 nm) nanosecond lasers. The parallel laser printing of arrayed structures with a single laser shot is demonstrated together with the full capabilities of SLMs for laser printing reconfigurable patterns of silver nano-inks Finally, an overview of the unique advantages and capabilities of laser forward transfer with SLMs is presented.
The opportunities presented by the use of metamaterials have been the subject of extensive discussions. However, a large fraction of the work available to date has been limited to simulations and proof-of-principle demonstrations. One reason for the limited success inserting these structures into functioning systems and real-world applications is the high level of complexity involved in their fabrication. Most approaches to the realization of metamaterial structures utilize traditional lithographic processing techniques to pattern the required geometries and then rely on separate steps to assemble the final design. Obviously, composite structures with arbitrary and/or 3-D geometries present a challenge for their implementation with these approaches. Non-lithographic processes are ideally suited for the fabrication of arbitrary periodic and aperiodic structures needed to implement many of the metamaterial designs being proposed. Furthermore, non-lithographic techniques are true enablers for the development of conformal or 3-D metamaterial designs. This article will show examples of metamaterial structures developed at the Naval Research Laboratory using non-lithographic processes. These processes have been applied successfully to the fabrication of complex 2-D and 3-D structures comprising different types of materials.
Digital microfabrication processes are non-lithographic techniques ideally capable of directly generating patterns and
structures of functional materials for the rapid prototyping of electronic, optical and sensor devices. Laser Direct-Write
is an example of digital microfabrication that offers unique advantages and capabilities. A key advantage of laser directwrite
techniques is their compatibility with a wide range of materials, surface chemistries and surface morphologies.
These processes have been demonstrated in the fabrication of a wide variety of microelectronic elements such as
interconnects, passives, antennas, sensors, power sources and embedded circuits. Recently, a novel laser direct-write
technique able to digitally microfabricate thin film-like structures has been developed at the Naval Research Laboratory.
This technique, known as Laser Decal Transfer, is capable of generating patterns with excellent lateral resolution and
thickness uniformity using high viscosity metallic nano-inks. The high degree of control in size and shape achievable has
been applied to the digital microfabrication of 3-dimensional stacked assemblies, MEMS-like structures and freestanding
interconnects. Overall, laser forward transfer is perhaps the most flexible digital microfabrication process
available in terms of materials versatility, substrate compatibility and range of speed, scale and resolution. This paper
will describe the unique advantages and capabilities of laser decal transfer, discuss its applications and explore its role in
the future of digital microfabrication.
We describe a novel technique, called laser decal transfer, for the laser forward transfer of electronic inks that allows the
non-contact direct writing of thin film-like patterns and structures on glass and plastic substrates. This technique allows
the direct printing of materials such as metallic nano-inks from a donor substrate to the receiving substrate while
maintaining the size and shape of the area illuminated by the laser transfer pulse. That is, the area of the donor substrate
or ribbon exposed to the laser pulse releases an identical area of nano-ink material which retains its shape while it
travels across the gap between the ribbon and the receiving substrate forming a deposited pattern of the same
dimensions. As a result, this technique does not exhibit the limited resolution, non-uniform thickness, irregular edge
features and surrounding debris associated with earlier laser forward transfer techniques. Continuous and uniform
metallic lines typically 5 micrometers or less in width, and a few hundred nanometers in thickness were fabricated by
laser decal transfer. These lines are of similar scale as patterns generated by lithographic techniques. Once transferred,
the lines are laser-cured in-situ using a CW laser beam, becoming electrically conductive with resistivities as low as 3.4
μΩ cm. This novel laser direct-write technique is a significant improvement in terms of quality and fidelity for directwrite
processes and offers great promise for electronic applications such as in the development, customization,
modification, and/or repair of microelectronic circuits.
The use of direct-write techniques might revolutionize the way microelectronic devices such as interconnects, passives,
IC's, antennas, sensors and power sources are designed and fabricated. The Naval Research Laboratory has developed a
laser-based microfabrication process for direct-writing the materials and components required for the assembly and
interconnection of the above devices. This laser direct-write (LDW) technique is capable of operating in subtractive,
additive, and transfer mode. In subtractive mode, the system operates as a laser micromachining workstation capable of
achieving precise depth and surface roughness control. In additive mode, the system utilizes a laser-forward transfer
process for the deposition of metals, oxides, polymers and composites under ambient conditions onto virtually any type
of surface, thus functioning as a laser printer for patterns of electronic materials. Furthermore, in transfer mode, the
system is capable of transferring individual devices, such as semiconductor bare die or surface mount devices, inside a
trench or recess in a substrate, thus performing the same function of the pick-and-place machines used in circuit board
manufacture. The use of this technique is ideally suited for the rapid prototyping of embedded microelectronic
components and systems while allowing the overall circuit design and layout to be easily modified or adapted to any
specific application or form factor. This paper describes the laser direct-write process as applied to the forward transfer
of microelectronic devices.
Laser-based direct-write (LDW) processes offer unique advantages for the transfer of unpackaged semiconductor bare die for microelectronics assembly applications. Using LDW it is possible to release individual devices from a carrier substrate and transfer them inside a pocket or recess in a receiving substrate using a single UV laser pulse, thus per-forming the same function as pick-and-place machines currently employed in microelectronics assembly. However, conventional pick-and-place systems have difficulty handling small (< 1mm2) and thin (< 100 μm) components. At the Naval Research Laboratory, we have demonstrated the laser release and transfer of intact 1 mm2 wafers with thicknesses down to 10 microns and with high placement accuracy using LDW techniques. Furthermore, given the gentle nature of the laser forward transfer process it is possible to transfer semiconductor bare die of sizes ranging from 0.5 to 10 mm2 without causing any damage to their circuits. Once the devices have been transferred, the same LDW system can then be used to print the metal patterns required to interconnect each device. The implementation of this technique is ideally suited for the assembly of microelectronic components and systems while allowing the overall circuit design and layout to be easily modified or adapted to any specific application or form factor including 3-D architectures. This paper describes how the LDW process can be used as an effective laser die transfer tool and will present analysis of the laser-driven release process as applied to various types of silicon bare dies.
The development of embedded surface mount devices, IC's, interconnects and power source elements offers the ability to achieve levels of miniaturization beyond the capabilities of current manufacturing techniques. By burying or embedding the whole circuit under the surface, significant reduction in weight and volume can be achieved for a given circuit board design. In addition, embedded structures allow for improved electrical performance and enhanced function integration within traditional circuit board substrates. Laser-based direct-write (LDW) techniques offer an alternative for the fabrication of such embedded structures at a fraction of the cost and in less time that it would take to develop system-on-chip designs such as ASIC’s. Laser micromachining has been used in the past to machine vias and trenches on circuit board substrates with great precision, while laser forward transfer has been used to deposit patterns and multilayers of various electronic materials. At NRL, we have been exploring the use of these LDW techniques to both machine and deposit the various materials required to embed and connect individual components inside a given surface. This paper describes the materials and processes being developed for the fabrication of embedded microelectronic circuit structures using direct-write techniques alongside with an example of a totally embedded circuit demonstrated to date.
Significant reduction in weight and volume for a given circuit design can be obtained by embedding the required surface mount devices, bare die and power source elements into the circuit board. In addition, embedded structures allow for improved electrical performance and enhanced function integration within traditional circuit board substrates and non-traditional surfaces such as the external case. Laser-based direct-write techniques can be used for developing such embedded structures at a fraction of the cost and in less time that it would take to develop system-on-chip alternatives such as ASIC's. Laser micromachining has been used in the past to machine vias and trenches on circuit board substrates with great precision, while laser forward transfer has been used to deposit patterns and multilayers of various electronic materials. This paper describes recent work performed at the Naval Research Laboratory using the above laser direct-write techniques to machine the surface and deposit the materials required to embed, connect and encapsulate individual electronic components and microbatteries inside a plastic substrate.
The development of micro power systems, on the mm size scale, is necessary for emerging technologies in small, portable micro-electronic device applications. Direct-write processes are used to produce the high-power, low-power and recharging elements of such mesoscale micro power systems. Successful fabrication of alkaline and lithium based micro-batteries, micro-ultracapacitors, and dye-sensitized micro-solar cells are possible on various low processing temperature and flexible substrates using laser direct-write approaches that are ideally suited for the many different structurally complex electrochemical materials used in these systems. Increased areal energy density is realized by depositing thick layers > 10 μm, while maintaining patterns as small as 2 mm2. Micro-ultracapacitors exhibit high power densities > 250 mW/cm2, while primary alkaline microbatteries exhibit open circuit potentials of 1.5 V with high capacities and discharge currents up to 1 mA. Secondary LiCoO2 and LiMn2O4 based microbatteries employing a novel nanocomposite solid-state electrolyte exhibit open circuit potentials > 4 V and have shown multiple recharging cycles without loss of capacity. Results of the different systems will be discussed with particular emphasis on the combination of elements to produce hybrid micro power systems.
Laser processing techniques, such as laser direct-write (LDW) and laser sintering, have been used to deposit mesoporous nanocrystalline TiO2 (nc-TiO2) films for use in dye-sensitized solar cells. LDW enables the fabrication of conformal structures containing metals, ceramics, polymers and composites on rigid and flexible substrates without the use of masks or additional patterning techniques. The transferred material maintains a porous, high surface area structure that is ideally suited for dye-sensitized solar cells. In this experiment, a pulsed UV laser (355nm) is used to forward transfer a paste of commercial TiO2 nanopowder (P25) onto transparent conducting electrodes on flexible polyethyleneterephthalate (PET) and rigid glass substrates. For the cells based on flexible PET substrates, the transferred TiO2 layers were sintered using an in-situ laser to improve electron paths without damaging PET substrates. In this paper, we demonstrate the use of laser processing techniques to produce nc-TiO2 films (~10 μm thickness) on glass for use in dye-sensitized solar cells (Voc = 690 mV, Jsc = 8.7 mA/cm2, ff = 0.67, η = 4.0 % at 100 mW/cm2).
This work was supported by the Office of Naval Research.