Scott A. Mathews

Laser direct-write and its application in low temperature Co-fired ceramic (LTCC) technology

Seeking better efficiency and faster turn-around in microelectronics manufacturing, a maskless laser direct-write process has been developed and applied to fabricate practical devices. The process uses a short pulse UV laser to transfer material from a source layer, or ribbon, to a substrate. Patterning is achieved by scanning the laser beam, translating the receiving substrate, or both. No wet chemistry is required. This technique has a minimum resolution of 10 µn. Using low temperature Co-fired ceramic materials and the laser direct-write process, a multi-layer laminated band pass filter was fabricated. The inductors, resistors, and capacitors were fabricated on separate layers. The connections between the layers were made through interlayer vias. Test results show fully functional band pass filters with center frequency around 1.15 GHz. Simulation results are also discussed.

Laser forward transfer using structured light.

A digital micromirror device (DMD) is used to spatially structure a 532 nm laser beam to print features spatially congruent to the laser spot in a laser-induced forward transfer (LIFT) process known as laser decal transfer (LDT). The DMD is a binary (on/off) spatial light modulator and its resolution, half-toning and beam shaping properties are studied using LDT of silver nanopaste layers. Edge-enhanced checkerboard beam profiles led to a ~30% decrease in the laser transfer fluence threshold (compared to a reference checkerboard profile) for a 20-pixel bitmap pattern and its resulting 10-μm square feature.

Analysis and characterization of the laser decal transfer process

We have studied the kinetics of a congruent, pixilated laser forward transfer process known as laser decal transfer (LDT). This process allows the transfer and patterning of silver nanoparticle inks such that the transferred pixels or voxels maintain the shape of the laser illumination. This process is capable of creating freestanding and bridging structures with near thin-film like properties.

Laser forward transfer of solder paste for microelectronics fabrication

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.

Laser embedding electronics on 3D printed objects

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.

Laser transfer of reconfigurable patterns with a spatial light modulator

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.

Laser origami: a new technique for assembling 3D microstructures

The ability to manufacture and assemble complex three-dimensional (3D) systems via traditional photolithographic techniques has attracted increasing attention. However, most of the work to date still utilizes the traditional patterning and etching processes designed for the semiconductor industry where 2D structures are first fabricated, followed by some alternative technique for releasing these structures out-of-plane. Here we present a novel technique called Laser Origami, which has demonstrated the ability to generate 3D microstructures through the controlled out-of-plane folding of 2D patterns. This non-lithographic, and non silicon-based process is capable of microfabricating 3D structures of arbitrary shape and geometric complexity on a variety of substrates. The Laser Origami technique allows for the design and fabrication of arrays of 3D microstructures, where each microstructure can be made to fold independently of the others. Application of these folded micro-assemblies might make possible the development of highly complex and interconnected electrical, optical and mechanical 3D systems. This article will describe the unique advantages and capabilities of Laser Origami, discuss its applications and explore its role for the assembly and generation of 3D microstructures.

Spatially modulated laser pulses for printing electronics.

The use of a digital micromirror device (DMD) in laser-induced forward transfer (LIFT) is reviewed. Combining this technique with high-viscosity donor ink (silver nanopaste) results in laser-printed features that are highly congruent in shape and size to the incident laser beam spatial profile. The DMD empowers LIFT to become a highly parallel, rapidly reconfigurable direct-write technology. By adapting half-toning techniques to the DMD bitmap image, the laser transfer threshold fluence for 10 μm features can be reduced using an edge-enhanced beam profile. The integration of LIFT with this beam-shaping technique allows the printing of complex large-area patterns with a single laser pulse.

Laser processing of 2D and 3D metamaterial structures

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 for digital microfabrication

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.