Patent Application
20050274772
It can be difficult to achieve a fine control of the distribution of a fluid on a surface, e.g. in a fine surface pattern of thin lines.
Formation of conductive traces on a substrate surface can be done using photolithographic techniques to etch away all of a conductor layer on the substrate except in the conductor trace pattern. This is a relatively expensive procedure. Photolithographic techniques also tend to be high temperature, which may be a disadvantage for a plastic substrate application, or for an application in which multiple layers are being built up, and lower layers may experience electromigration when heated.
Features and advantages of the disclosure will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein:
In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals.
In an exemplary embodiment, it is desired to distribute a solution of particles on a surface of a substrate in a defined pattern which establishes a conductor trace pattern. The particles can be small particles, and in one exemplary embodiment can be nanoparticles. Nanoparticles may be organic or inorganic particles that are typically smaller than 200 nanometers. These particles may also have an organic shell with a ligand structure to reduce agglomeration. The particles are then suspended in an aqueous or solvent based solution. In other embodiments, the particles may be larger that nanoparticles.
Laser surface treatment can be used to modify a surface to change the surface energy of the substrate so that a solution, e.g. a solution of particles, more evenly distributes on the surface. One laser surface treatment is to decrease the surface energy. Alternatively, a different laser surface treatment can modify a surface to increase the surface energy. A combination of the two treatments can be used to selectively pattern a surface to attract particles in the solution to areas on which conductive traces are to be placed.
The two treatments may differ in fluence, shot count, gas environment. An example of a treatment to increase the attraction for silver nanoparticles in alpha-terpineol solution air-brushed onto a Kapton (TM) substrate is a laser treatment using a 248 nm laser, 200 mJ/cm2 fluence, 100 shots, resulting in a continuous, substantially even coverage when air-dried. The laser treatment increases the surface roughness from RMS roughness of 59 nm to 215 nm, and the contact angle of a test fluid, distilled water, drops from 98 degrees to 65 degrees immediately after deposition and then to 35 degrees after the fluid has been in contact with the surface for about 10 minutes. An exemplary treatment that decreases the attraction is a 248 nm laser on a substrate of PEN (polyethylene naphthalate) with a fluence of 250 mJ/cm2 and 7 shots, resulting in a contact angle increase for a test fluid, distilled water, from 69 degrees to 145 degrees. Other types of surface modifications suitable for the purpose include a film, such as a fluorinated polymer, or a plasma treatment. For example, by applying a 0.1-10 μm thick polymer coating, the surface energy of a material can be changed to either increase or decrease the wettability. By using a plasma treatment, surface roughness and/or chemistry changes can be caused.
After this surface modification, when a solution, e.g. a solution of nanoparticles, is dispensed, e.g. sprayed, jetted or spincoated, onto the surface, the nanoparticles will arrange themselves into a predetermined pattern that is then ready for the next step in processing. This next step may include laser-sintering the nanoparticles together to form conductive traces, or other treatment processes, such as, by way of example only, thermal annealing in an oven and infrared treatment.
Laser surface treatment can increase nanoparticle coverage on the surface. For example, nanoparticle coverage on a Kapton (TM) surface has been increased when the surface is subjected to a laser surface treatment. A substantially even coverage of nanoparticles may be obtained, while reducing clumping together or the formation of striated patterns. This may be useful for making continuous conductors or dielectrics with proper insulating properties. Other suitable substrate materials include polymers and glass, such as Corning 1737.
Depositing nanoparticles on a surface and laser-sintering them into conductive traces can be employed in the fabrication of electronic devices. In the laser-sintering, heat may flow laterally from the focused laser beam, melting a swath of nanoparticles that is wider than the beam. This can make it more difficult to create relatively narrow trace widths. By treating the surface of the substrate before application of the nanoparticles, the nanoparticles are substantially confined to an area that is the width of the trace that is desired. For example, in one embodiment, if a 10 micron wide trace is desired, a mask is used to expose a 10 micron wide area to a laser-surface treatment that increases the affinity of the treated surface to the nanoparticle solution. The surface on either side of the affinity-increasing treated surface can then be optionally exposed to a laser treatment that results in a surface that repels the nanoparticle solution. The result is a 10 micron wide line of nanoparticles that is ready for a subsequent treatment, such as laser sintering. Thus, in this exemplary embodiment, there are two laser treatments steps, the surface treatment to affect the nanoparticle distribution, and the second a subsequent laser-sintering treatment to melt the nanoparticles into a conductive trace. The surface treatment can in turn be two treatments, i.e. the first treatment on the trace area to increase surface affinity to the nanoparticles, and the second treatment on surface regions on either side of the first treatment area to decrease surface affinity to repel the nanoparticles.
In sintering a nanoparticle distribution, porosity of the sintered trace may decrease conductivity. In some embodiments, the surface treatment can result in increased density of the nanoparticles in the trace area. This increase in density can correspond to an increase in conductivity, so that the conductivity of the sintered trace is closer to the conductivity of the bulk conductor material.
The laser mask 130 is designed to pattern certain areas on the substrate 10. One mask is used to pattern areas with the laser treatment that is designed to attract nanoparticles.
The laser beam is scanned across the masks in order to pattern the area on which nanoparticles are to be deposited on the substrate.
Excimer lasers of a type selected from the following non-limiting alternatives can be employed in the laser ablation system: F2, ArF, KrCl, KrF, and XeCl. An exemplary laser ablation system is described, for example, in U.S. Pat. No. 5,305,015. In an exemplary embodiment, the mask may be highly reflective at the laser wavelength, such as a multi-layer dielectric or a metal such as aluminum or chrome. Exemplary pulse energies and durations include 20-750 mJ/cm2, 1-1000 pulses, 0.3-250 ns. Simplified block diagrams of exemplary laser surface treatment systems are shown in
System 100-2 also includes an excimer laser 110, optional collimating and beam expansion optics 120, optional turning mirror 160 and an X-Y stage 106. This alternate embodiment employs an aperture 124 in an opaque plate. For certain types of materials, where intensity uniformity may be less, the raw Excimer beam is good enough to perform the surface modification without beam modification optics. A simple aperture will clip the edges of the beam to obtain edge definition.
Solid state lasers such as, but not limited to Nd:YAG or Nd:YVO4, can be used for the surface treatment. These lasers have a smaller beam diameter (1-3 mm beam diameter) and thus cover a smaller area than that of an Excimer laser. At times, these solid state lasers may provide the capability of being able to direct write a treated pattern onto the substrate by using its point source beam and using a galvanometer or X-Y stage to draw the pattern. Also for harder materials, the solid state lasers can produce energy densities greater than 200 J/cm2. Solid state lasers can be focused to a smaller spot and the better beam quality allows all the laser beam's energy to be focused. An exemplary system 100-3 is shown in the simplified block diagram of
In an exemplary embodiment, the fluence of the laser may be adjusted to cause ablation of the substrate surface. Fluence, as used herein, refers to the number of photons per unit area, per unit time. Ablation, as used herein, refers to the removal of material through the interaction of the laser with the surface. Through this interaction, the surface is activated such that the surface bonds are broken and surface material is displaced away from the surface, thereby changing the surface texture.
The fluence of the laser typically is adjusted based on the characteristics of the substrate material to be ablated as well as the desired surface texture. In one embodiment, the laser light is directed to areas of the substrate that are intended to receive the laser surface treatment, while areas that are not to receive the surface treatment may be masked off, or otherwise not exposed to laser light, so that these areas remain unaltered.
The actual texture of the surface obtained via laser ablation may depend on the number of pulses, pulse width, pulse intensity, frequency, wavelength and energy density, and/or the type of surface material. In one embodiment, the fluence typically should exceed a predetermined threshold before ablation of the surface occurs. If the fluence is below this threshold, then there will be little or no ablation and no removal of the surface material. The ablation threshold is dependent on the characteristics of the material being ablated and the light source. In laser ablation, short pulses of intense laser light are absorbed in a thin surface layer of material within about 1 micrometer or less of the surface. In an exemplary embodiment, the laser pulse energies are on the order of 50-700 mJ, with pulse durations on the order of 10-100 ns.
The surface texture can be defined and quantified by a “contact angle” value, which is the angle of intersection between the surface and a fluid drop, i.e. the angle defined by the surface and a tangent to the drop where it contacts the surface. A high contact angle, for example, corresponds with a non-wetting surface, while a low contact angle corresponds with a wetting surface. In one embodiment, a contact angle of 10 degrees or less corresponds with a “highly wettable” surface that causes a fluid to spread extensively, or “wets out” over the surface. A contact angle between 10 and 90 degrees corresponds with a wetting surface. A contact angle of 90 degrees or greater corresponds with a non-wetting surface.
After the substrate surface has been treated as described above, the nanoparticles are deposited on the trace region. In an exemplary embodiment, the nanoparticles are in a fluid, and the fluid is jetted onto the surface using a thermal fluid drop generator, e.g. of a type used in inkjet printing.
Exemplary nanoparticle materials suitable for the purpose include gold, silver, copper, nickel and palladium, as well as alloys of these materials. Suitable nanoparticles are commercially available, but can also be fabricated by a number of methods, including physical vapor synthesis. Nanoparticles may include organic or inorganic particles that are typically smaller than 200 nanometers. These particles also have an organic shell with a legend structure to reduce. agglomeration. The particles are then suspended in an aqueous or organic media. Examples of organic media suitable for the purpose include Toluene, alcohols (including ethanol and iso-propanol)and n-methyl pyrrolidone (NMP). Other types of nanoparticles and fluids can alternatively be used.
Laser sintering is the use of laser light to thermally decompose the particles into a continuous film. This can be done using laser energy in a continuous wave (cw) or pulsed format. Typically a cw format is preferred. The wavelength of the laser is selected to match the absorption of the nanoparticles and/or its organic shell. In the case of high temperature substrates, the substrate may be locally heated by the laser beam to sinter the nanoparticles.
Thermal annealing can be done in an oven. The process is to heat up the substrate and nanoparticle solution until the melt temperature of the nanoparticles is exceeded. The component is then baked for a specific time.
Infrared heating is another technique for transforming the nanoparticle solution into a solid trace. This can be done, e.g., by industrial ovens utilizing infrared heaters or tubular quartz heaters, which allow top coats and powders to absorb the energy directly, curing with the substrate interface, without heating the entire thickness of the substrate to peak substrate temperature. These systems can provide a precise temperature control by incorporating zones and closed loop feedback for product quality. Air can be introduced to aid in drying water and solvent based coatings, primers and pretreatments.
An exemplary embodiment of a process 350 for forming conductive traces on a substrate is depicted in the general flow diagram of
The substrate with the conductive traces can be utilized in electronic devices. For example, the substrate can be used as a wiring board or substrate to conduct electrical signals along the trace pattern. The substrate can be populated with passive and active electronic devices.
In another embodiment, a flexible substrate material, e.g. PET or PEN, (polyethylene terephthalate and polyethylene naphthalate) has a polymer layer attached to it or coated on a surface of the substrate. The polymer layer is partially linked, and is designed so that the partially linked surface area is non-wetting. Non-wetting polymers can be formed with hexylacrylates and dodexylacrylates. Wetting polymers can be created with polyethylene glycols, particularly graft polymers or carboxylates. Suitable layer thickness range from about 0.5 um to 50 um, in an exemplary embodiment. A laser is then used to finish the polymerization of certain areas of the polymer layer to define wetting surface areas.
The polymer layer 10-2 also acts as a thermal layer for the flexible substrate 10-1, such that the heat generated by the laser light in the polymer layer does not affect the dimensional stability of the substrate. In some embodiment, a polymer layer may be coated on both sides of the substrate to reduce deformation caused by film layer stresses. Other methods include liquid spin coating and pressed-on films.
Another technique for creating non-wetting and wetting regions is to place a second very thin layer 10-3 of a non-wetting material, such as PTFE (poly tetra fluro ethylene), on top of the first polymer layer 10-2, as illustrated in
Referring to the embodiments of
Although the foregoing has been a description and illustration of specific embodiments, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the claimed subject matter as defined by the following claims.
