Direct write in the past has generated the excitement of possibly replacing photoresist for all electronic applications. Removing the mask would substantially reduce the number of steps required to produce electronic circuits. A reduction in steps represented time and dollar savings. The advantage of being able to direct write a manufacturable device would also save time and money in the design process as well. With all of the obvious advantages, it seemed inevitable that research dollars would continue to mount and thus overcome the obstacles preventing this technology from becoming more than a novel technique used in laboratories. As Moore's law began to settle in, so did photoresist and direct write was little more than a novelty.

That was then, and this is now. Developers have come to terms with the true value direct write can supply to the manufacturers and design engineers. Techniques such as Focused Ion Beam (FIB), Laser Chemical Vapor Deposition (LCVD), ink jetting and ink penning have found real applications that are making a difference in industry. A summary will be presented describing the various direct write techniques, their current applications and the possible or probable applications.

Laser microchemical direct write deposition and etching methods have found an essential niche in debug and design for yield of wire-bonded and flip-chip integrated circuits. Future applications should develop in package-level system modification.

The formation of highly uniform charged molten metal droplets from capillary stream breakup has recently attracted significant industrial and academic interest for applications requiring high-speed and high-precision deposition of molten metal droplets such as direct write technologies. Exploitation of the high droplet production rates intrinsic to the phenomenon of capillary stream break-up and the unparalleled uniformity of droplet sizes and speeds attained with proper applied forcing to the capillary stream make many new applications related to the manufacture of electronic packages, circuit board printing and rapid prototyping of structural components feasible. Recent research results have increased the stream stability with novel acoustic excitation methods and enable ultra-precise charged droplet deflection. Unlike other modes of droplet generation such as Drop-on-Demand, droplets can be generated at rates typically on the order of 10,000 to 20,000 droplets per second (depending on droplet diameter and stream speed) and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ±12.5 µm. Droplets are charged on a drop-to-drop basis, enabling the direct writing of fine details at high speed. New results are presented in which fine detailed patterns are “printed” with individual molten metal solder balls, and issues relevant to the attainment of high quality printed artifacts are investigated.

An emerging selective metallization process utilizes Drop-On-Demand (DOD) inkjet printing, and recent developments in nano-particle fluid suspensions to fabricate fine-line circuit interconnects. The suspensions consist of silver or gold particulates of 1–10 nm in size that are homogeneously suspended in an organic carrier solvent. A piezo-electric droplet generator driven by a bipolar voltage signal is used to dispense 50–70 µm diameter droplets traveling at 1-3 m/s before impacting a compliant substrate. The deposit/substrate composite is subsequently processed at 300°Cfor 15 minutes to allow for evaporation of the solvent carrier and sintering of the nano-particles, thereby yielding a finished circuit product. Test vehicles created using this technique exhibited features as fine as 120–200 µm wide and 1–3 µm thick. The circuitry performed well during environmental conditioning studies. However, repeatability of the results showed sensitivity to the generation of steady, satellite-free droplets. In an effort to generate droplets consistently, it is essential to develop a strong fundamental understanding of the correlation between device excitation parameters and fluid properties, and resolve the microrheological behavior of the conductive ink as it flows through the droplet generator.

Dry powder and sol gel ceramic films were deposited using a Matrix Assisted Pulsed Laser Evaporation Direct Write (MAPLE-DW) technique developed at NRL for optical and electrical device applications. The MAPLE-DW technique uses a high-energy focussed photon source in combination with a “ribbon” to fabricate materials onto a range of substrates at room temperature without material degradation. Two different classes of materials were processed in this research, (1) ceramic dry powder materials and (2) sot gel precursor materials. Cathodoluminescent measurements demonstrated that the efficiencies of the transferred phosphor materials were not degraded during the laser transfer process. Scanning electron microscopy and 3-D surface profilometry of the ribbon after the MAPLE-DW process revealed a 90–95% transfer efficiency for the dry powders. Scanning electron microscopy and energy dispersive spectroscopy revealed that the sol gel materials also transferred with an efficiency in the 90-95% range. Three distinct regions were identified on the so gel ribbon after the laser transfer process. The regions suggest that the matrix absorbing layer and the sol gel materials are completely removed for the areas irradiated by the laser pulse, and in a second so-called “heat affected region,” only the sol gel material is ejected from the ribbon.

Various production techniques are available for many materials for the direct write (DW) applications. Single metals and alloys are prepared from vapor and spray reactors as well as by precipitation techniques. Single and multicomponent oxides such as ferrites are also possible from these techniques in the nanometer to micrometer size ranges. Examples with the use of these techniques for production of metals and ferrites for DW are described with examination of crystallinity and morphology.

In today's electronic industry, manufacturers are continuously improving capital utilization, developing flexible manufacturing processes, reduce changeover time and improving yield and throughput. Interest in rapid prototyping and 3-D fabrication capabilities are rapidly increasing, and a number of candidate direct writing technologies are in development to meet these demands.

This work studies material systems used by data driven materials deposition (DDMD) technologies for potential low temperature reel-to-reel high volume manufacturing on low cost substrates. Characterization results of fabricated discrete and RF devices using commercially available micro dispensing and ink jet systems will be discussed. Material rheological properties, deposition process characterization, deposition repeatability, fabricated device reliability and electrical performance will be presented. The test vehicles contain resistors and capacitors, transmission lines, open and short series stub filters, and half-wavelength resonators. The material/substrate compatibility will be demonstrated through environmental conditioning of the test vehicles. In addition, a cost estimate for using micro dispensing technologies was conducted to compare current manufacturing technologies to DDMD.

We have investigated the direct printing of polymer solutions from a chemically patterned stamp onto a hydrophilic target substrate as a new high-throughput alternative to optical lithography. The patterns on the stamp, which are typically in the micron size range, define regions of alternating wettability. They are produced by patterning a hydrophobic self-assembled monolayer previously deposited onto a hydrophilic surface, typically a glass slide or silicon wafer with a natural oxide coating. Polar liquids or aqueous polymeric solutions are then deposited only onto the hydrophilic surface patterns by dip-coating the stamp in a liquid reservoir. The deposited film thickness depends critically on the speed of withdrawal and the feature size and shape. For vertically oriented hydrophilic stripes dipped in a reservoir containing a polar liquid, we have developed a theoretical model whose prediction for the maximum deposited film thickness agrees exceptionally well with experimental measurements. After deposition, the wetted stamp is pressed against a target substrate by means of a motion controlled press. In this way we have so far printed 5µm wide polyethylene oxide lines onto a silicon wafer.

The use of sol–gel-derived 0–3 composite ceramics for low-temperature direct-write electronics applications was investigated. The 0–3 composite paste materials were prepared using selected metal alkoxides and commercial low- and high-κ' dielectric powders. The composite pastes were deposited onto alumina and polyimide substrates using conventional screening and micro-dispensing techniques. The deposited films were oven-dried at or below 200°C and thermally densified using a CO2 laser. The 0–3 composites exhibited good adhesion and structural density. Electrical characterization of the laser-processed dielectrics revealed κ' values as high as 295 and tan δ as low as 0.02 on polyimide substrates.

We have employed inks containing nanometer-sized particles of Ag and Al (nano-Ag and nano-Al, respectively) as precursor inks for the formation of contacts to n- and p-type Si, respectively. The particles as formed by the electroexplosion process were dispersed in toluene, applied to Si and annealed above the respective eutectic temperatures. In the case of nano-Ag, this directly yields an ohmic contact. However, the nano-Al was found to be coated with an oxide layer that impairs the formation of an ohmic contact. A chelating chemical etch involving treatment with hexafluoroacetylacetone was developed to remove this oxide coat. This treated nano-Al produced a good ohmic contact. Smooth, pure Ag films have also been deposited by spray printing organometallic inks prepared from Ag(hfa)(SEt2) and Ag(hfa)(COD). These films are deposited in one step onto heated glass and Si substrates at one atmosphere pressure. The films show resistivities of ∼2 µΩ·cm. These inks appear to be amenable to ink-jet printing of Ag lines and as a low temperature glue for the Ag nanoparticles for thicker metallizations.

We have successfully printed green ceramic objects from slurries of Al2O3 dispersed in paraffin wax using a commercial ink-jet printer developed for pattern making (Sanders Prototype MM6PRO). Concentrated suspensions are generally more viscous than the fluids normally passed through ink jet heads. This may alter the response of the printing system to its process parameters, e.g. driving voltage and frequency. We have explored the influence of fluid properties on the ink jet behaviour using Computational Fluid Dynamics (CFD) modelling and a parallel experimental study to determine the optimum printing conditions for particulate suspensions.

Functional materials configured as liquid toners are printed on a variety of substrates for various manufacturing processes. The materials include metal toners, resistor toners, high k dielectric toners, phosphors and glass. The substrates printed upon include glass, bare and coated metal, polymeric films and even paper. A fixed configuration electrostatic printing plate is used in most manufacturing applications though traditional photo receptor plates can be used if electronic addressability is desired.

Applications of electrostatic printing for electronic packaging products (printed wiring boards and flex circuits) and of passive electronic components themselves will be shown. Results with a pure silver toner printed on both glass and paper will be reported. Examples of passive electronic components like resistors, capacitors, and even inductors that have been electrostatically printed with liquid toners will be shown. Possible applications of toners to the manufacture of flat panel displays will be discussed.

Photostructurable glass-ceramics are a promising class of materials for MEMS devices. Previous work micromachining these materials used conventional photolithography equipment and masking techniques; however, we use direct-write CAM tools and a pulsed UV laser micromachining station for rapid prototyping and enhanced depth control. We have already used this class of materials to build components for MEMS thrusters, including fuel tanks and nozzles: structures that would prove difficult to build by standard microfabrication techniques.

A series of experiments was performed to characterize process parameters and establish the processing trade-offs in the laser exposure step. The hypothesis that there exists a critical dose of UV light for the growth of an etchable crystalline phase was tested by exposing the material to a fluence gradient for a variety of pulse train lengths, and then processing as usual. By measuring the dimensions of the etched region, we were able to determine the dose. We found that the dose is proportional to the square of the per-pulse fluence. This has allowed us to create not only embedded structures, but also stacked embedded structures. This also implies that we can embed tubes and tunnels with a single exposure inside a monolithic glass sample. We feel that this technique has promise for a number of applications, including microfluidics.

Practical methods for directly patterning hydrogenated amorphous silicon (a-Si:H) films have been developed. Direct patterning involves selectively oxidizing the hydrogen passivated aSi:H surface, with the oxide then serving as an etch mask for subsequent hydrogen plasma removal of the unoxidized regions. Photo induced oxidation has been extensively studied using both far field projected patterns and near field scanning optical microscopy (NSOM) for direct write patterning. Examination of the threshold dose for pattern generation for excitation wavelengths from 248 to 633nm provides indirect evidence for involvement of electron-hole recombination in optically induced oxidation. Optical exposure of a-Si:H in vacuum demonstrated that oxygen must be present in the ambient atmosphere during exposure for successful pattern generation. This suggests that oxidation of the surface may not involve removal of hydrogen, but rather breaking of Si-Si backbonds and insertion of oxygen. An additional mechanism for oxide generation was observed whereby pattern generation resulted from simple proximity of an NSOM probe within ∼30nm from the sample surface. The probe dither amplitude was found to greatly affect the line width and height of patterns generated without light. Line widths of approximately 100nm, comparable to the probe diameter, were obtained.

We demonstrate a laser direct-write method, a maskless process that transfers material directly from a ribbon to a substrate. This process offers the promise of fabricating passive electronic micro-components at a high speed with high spatial resolution. We are developing a workstation implementing this direct-write method, which integrates deposition, direct laser sintering, and micromachining capability on a single machine. Using this workstation we have deposited micro-patterns of conducting lines and resistors on alumina and polyimide substrates under ambient conditions that exhibit good electrical properties and substrate adhesion. From preliminary studies of laser sintering it was found that a wide range of sintering conditions may be used to arrive at silver conducting lines (∼60 µm × 10 µm) on alumina substrate with resistivity in the range of 5 to 10 times the resistivity of bulk silver. Preliminary results also indicate direct laser sintering of cermet resistor material can yield reproducible resistance values.

Laser-induced optical forces are used to guide and deposit 100 nm - 10 µm diameter particles onto solid surfaces in a process called laser-guided direct-writing. Nearly any particulate material, including both biological and electronic materials, can be manipulated and deposited with micrometer accuracy. Potential applications include three-dimensional cell patterning for tissue engineering, hybrid biological and electronic device construction, and biochip array fabrication

In this paper, UV-induced large area growth of high dielectric constant (Ta2O5, TiO2and PZT) and low dielectric constant (polyimide and porous silica) thin films by photo-CVD and sol-gel processing using excimer lamps, as well as the effect of low temperature UV annealing, are discussed. Ellipsometry, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), UV spectrophotometry, atomic force microscope (AFM), capacitance-voltage (C-V) and current-voltage (I-V) measurements have been employed to characterize oxide films grown and indicate them to be high quality layers. Leakage current densities as low as 9.0×10−8 A·cm−2and 1.95×10−7 A·cm−2at 0.5 MV/cm have been obtained for the as-grown Ta2O5 films formed by photo-induced sol-gel processing and photo-CVD, respectively – several orders of magnitude lower than for any other as-grown films prepared by any other technique. A subsequent low temperature (400°C) UV annealing step improves these to 2.0×10−9A·cm−2and 6.4×10−9A·cm−2, respectively. These values are essentially identical to those only previously formed for films annealed at temperatures between 600 and 1000°C. PZT thin films have also been deposited at low temperatures by photo-assisted decomposition of a PZT metal-organic sol-gel polymer using the 172 nm excimer lamp. Very low leakage current densities (10−7A·cm2) can be achieved, which compared with layers grown by conventional thermal processing. Photo-induced deposition of low dielectric constant organic polymers for interlayer dielectrics has highlighted a significant role of photo effects on the curing of polyamic acid films. I-V measurements showed the leakage current density of the irradiated polymer films was over an order of magnitude smaller than has been obtained in the films prepared by thermal processing. Compared with conventional furnace processing, the photo-induced curing of the polyimide provided both reduced processing time and temperature. A new technique of low temperature photo-induced sol-gel process for the growth of low dielectric constant porous silicon dioxide thin films from TEOS sol-gel solutions with a 172 nm excimer lamp has also been successfully demonstrated. The dielectric constant values as low as 1.7 can be achieved at room temperature. The applications investigated so far clearly demonstrate that low cost high power excimer lamp systems can provide an interesting alternative to conventional UV lamps and excimer lasers for industrial large-scale low temperature materials processing

Conventional direct write processes are multi-step requiring at least one additional process to change conductive properties. A direct conversion technique that uses lasers to irradiate silicon carbide, providing tracks which are highly conductive has been demonstrated. It was found that laser irradiation of insulating silicon carbide films could cause a drop from 1011 to 10−4 ohm-cm in a 4-point resistance test. However, in the presence of pure oxygen, laser-irradiated silicon carbide conductor and semiconductor samples exhibit insulating characteristics. Pattern formation was achieved by a computer program controlled galvo-mirror. The pads, 0.4 cm × 0.7 cm were formed by beam rastering with an overlap of 30% of the 0.025 cm beam diameter. This computer assisted processing allows the design of patterns using conventional CAD/CAE technologies and smart material behavior via selective and controlled electrical property transitions by laser irradiation

Parmod™ is a family of materials that can be printed and thermally cured to create metallic conductors on printed wiring boards. This additive process provides a way to produce circuitry direct from CAD files without the necessity for intermediate tooling of any kind. The printed images are converted to pure metallic traces in seconds at a temperature low enough to be compatible with commonly used rigid and flexible polymer-based substrates. This simple, two-step process eliminates the hazardous wastes and employee health and safety issues associated with conventional plate-and-etch photolithographic technology

Recently the Parmod™ technology has been extended from metals to oxides to enable printing passive electronic components such as resistors, capacitors and inductors, as well as the metallic interconnects. While thermal curing of the oxides provides useable electronic properties, particularly of resistors and capacitors, the performance of all these novel materials could be improved by laser processing. This paper discusses preliminary results on laser processing of Parmod™ conductors and components in two different systems

We demonstrate a novel laser-based approach to perform rapid prototyping of active and passive circuit elements called MAPLE DW. This technique is similar in its implementation to laser induced forward transfer (LIFT), but different in terms of the fundamental transfer mechanism and materials used. In MAPLE DW, a focused pulsed laser beam interacts with a composite material on a laser transparent support transferring the composite material to the acceptor substrate. This process enables the formation of adherent and uniform coatings at room temperature and atmospheric pressure with minimal post-deposition modification required, i.e., ≤400°C thermal processing. The firing of the laser and the work piece (substrate) motion is computer automated and synchronized using software designs from an electromagnetic modeling program validating that this technique is fully CAD/CAM compatible. The final properties of the deposited materials depend on the deposition conditions and the materials used, but when optimized, the properties are competitive with other thick film techniques such as screen-printing. Specific electrical results for conductors are < 5X the resistivity of bulk Ag, for BaTiO3/TiO2composite capacitors the k can be tuned between 4 and 100 and losses are < 1-4%, and for polymer thick film resistors the compositions cover 4 orders of magnitude in sheet resistivity. The surface profiles and fracture cross-section micrographs of the materials and devices deposited show that they are very uniform, densely packed and have minimum resolutions of -10 jtm. A discussion of how these results were obtained, the materials used, and methods to improve them will be given