Several obstacles currently impede effective laser sintering of materials. One limitation is that current methods inhibit sintering throughout the material. A second problem is that adhesion of the material to a substrate is also inhibited.
Several factors exist that interfere with the propagation of sintering throughout a target material and with the adhesion of the target material to a substrate. A need exists for laser sintering of materials that overcomes these problems.
Existing laser sintering processes damage substrates that are not able to withstand the high temperatures associated with the laser sintering process. Substrates for directly written electronic circuitry are generally some type of plastic. Unfortunately, the highest temperatures known plastics can survive without degradation are on the order of 350° C. Relatively few formulations can even survive at 200° C. In contrast, most materials of utility in constructing electronics (e.g., metal conductors, metal or oxide resistors, and oxide dielectrics) melt at far higher temperatures. When such materials are to be formed into devices, their crystals or grains must have continuity with each other for electrical contact and with the substrate for adhesion. Continuity generally requires that individual particles be sintered into one conjoined structure. In turn, the methods by which continuity may be achieved all require high temperatures approaching the melting point of the bulk material (Tm).
Therefore, the construction of high-Tm electronics components upon a low-Tm substrate presents a difficult materials-science challenge. A need also exists for protecting a substrate from laser damage during the laser sintering process.
The present invention is a method and apparatus for laser sintering of materials that provides complete sintering throughout the material and that enhances adhesion of the material to the substrate. Lasers may be used to sinter materials of interest to electronics applications.
The laser interacts with both the material to be sintered and the substrate upon which the material is positioned. This allows for a more complete heating process. The top of the material is heated via the laser and the bottom of the material is heated via the substrate. As the sintering occurs, the thermal spread throughout the material allows for sintering to occur completely through the material. This also enhances the adhesion significantly since the temperature difference between the substrate and the material are the same. If they are different, the temperature gradient stops the adhesion. This technique “fixes” both of the aforementioned limitations.
The present invention allows the laser to interact with both the target material to be sintered and the substrate upon which it rests with controlled exposure times. This controlled dual interaction provides a more complete heating process. The top of the target material is heated by the laser, the bottom portion via the heated substrate. Diffusion of heat allows sintering to occur throughout the material. This controlled-dual-interaction procedure also significantly enhances adhesion because no temperature gradient exists between the substrate and the sintered material. Temperature gradients may interfere with adhesion. The laser-sintering technique of the present invention solves the aforementioned problems.
The present invention also includes a method and apparatus for protecting a substrate from laser damage during a laser sintering process. The present invention protects a low-Tm substrate with a thermal barrier coating designed to shield it from high temperatures. With such a thermal barrier in place, the electronics materials may be sintered into functioning components without damage to the substrate. This thermal barrier method works especially well with such deposition methods as laser-assisted chemical vapor deposition (LCVD) or laser sintering, in both of which laser irradiation provides a highly localized region of high temperatures.
A protective layer is placed on top of a low temperature substrate to provide a protective thermal barrier. The thermal barrier allows for exposure to much more intense laser irradiation, thereby aiding in the sintering of deposited materials. The thermal barrier may be applied to any material. Several benefits are provided by the use of a thermal barrier on a substrate during a laser sintering process. One benefit is that the substrate is protected from the excessive heat of the laser sintering process. A second benefit is that adhesion of the deposited material to the substrate is enhanced.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.
The laser processing of materials involves consideration of several aspects of the target material. First, the laser-power density (Φ) needed to accomplish laser sintering is strongly dependent upon the light-absorption characteristics of the material, chiefly absorptivity (α), which is in turn dependent upon temperature (T), light wavelength (λ), and light temporal pulse width or duration (τ). Materials are used for which the sintering temperatures (Ts) are much lower than their bulk melting points (Tm). However, the present invention provides a method of laser sintering of any material without damaging the substrates upon which they rest. Typical values for some materials of interest are listed in Table I.
The effects of low α at a particular λ have significant consequences. The initial material dispensed is composed of various compounds and solvents, all of which change the absorption behavior of the composite. The initial composite is “wet” and must be treated appropriately. If not, the laser may “splatter” the paste and destroy the device. A drying process must be used to reduce the solvent concentration; however, even small amounts of remaining solvent often strongly absorb the laser.
The interaction of the laser light and matter causes the sintering process to begin. In the example shown in
With a laser, it is possible to inject a tremendous amount of energy, which translates to heat, into a material. Once the absorption behavior is known (more is better), then the effects of pulse duration (τ) must be determined. Peak powers (Pmax) in the gigawatt range are obtainable using lasers with low energy per pulse but very short pulses. Tradeoffs must be made to optimize τ. Shorter τ yields higher Pmax but this works adversely with penetration depth (δ) in that shorter τ yields shorter δ. Therefore, if τ is too short, the interaction is confined to the surface 5 of the target material 7, as occurred with the sample shown in
If τ 9 is extended out to infinity (τ=∞), i.e., CW mode, then the interaction area extends completely through the paste, into the substrate, and even through the substrate. Therefore, it should be possible to control δ 11 (penetration depth plotted on the vertical axis) by controlling τ 9 (pulse duration plotted on the horizontal axis), as illustrated in
The propagation behavior of the thermal wave throughout the sample material was verified with a thermal-imaging camera. The longer the pulse, the farther the thermal wave traveled. Controlling τ enables δ to be made the same as the thickness of the material (θ). Several nontrivial factors must be considered when implementing this into a CAD/CAM program. They must even be considered in a laboratory setting if reproducibility is a requirement. The best way to ensure reproducibility is through a feedback control system. Such a system has been implemented by using a pyrometer with a relatively small (25 μm) spot size. While many pyrometers are available in the market today, the combination of small spot size and low temperature range is unique.
The output of the pyrometer was sent to the same computer that controls the output of the laser. The effectiveness of this method was demonstrated by setting the laser power to a constant value, then scanning it across a substrate 15 containing metal lines 17 parallel to each other and perpendicular to the laser scanning direction, as shown in
The present invention also includes a machine tool that implements the materials and the laser processes. The present invention allows its end user to interface to CAD/CAM, allowing for a fully automated machine needing very little interaction with or expertise by the user. The apparatus is capable of depositing and processing the desired materials over “any” surface with resolutions as small as 10 μm.
The present invention is capable of depositing lines as small as 75 μm. With the right paste, the shape of the line may be held. The apparatus may write on flat, slightly angled, or dipped surfaces. Preferably, the apparatus has a vertical travel of approximately 1 mm with good precision. In another embodiment, the apparatus is capable of writing over much larger vertical changes.
The present invention also provides a protective layer that is placed on top of a low temperature substrate to provide a protective thermal barrier. The thermal barrier allows for exposure to much more intense laser irradiation, thereby aiding in the sintering of deposited materials. The thermal barrier may be applied to any material. Several benefits are provided by the use of a thermal barrier on a substrate during a laser sintering process. One benefit is that the substrate is protected from the excessive heat of the laser sintering process. A second benefit is that adhesion of the deposited material to the substrate is enhanced.
One preferred thermal barrier material is an aerogel. An aerogel coating was placed onto some typical low-Tm circuit board laminate samples. A simple device was constructed and laser-sintered on thermal-barrier-coated and uncoated substrates. The coated substrate suffered significantly less damage than did the uncoated substrate.
A series of one-dimensional rapid thermal processing (RTP) simulations were performed for the geometry shown in
In the simulations, a stack-up 113 of a silicon substrate 101, an aerogel thermal barrier 103, and a silver deposition material 105 was pulsed once with a uniform distribution of power density (in W/m2) 107. The intensity and duration of the pulse was varied. The sides 109 and bottom 111 of the stack 113 are assumed adiabatic. As such, all the energy of the pulse remains in the stack 113. The results of interest are the maximum temperatures that occur in each layer as a function of pulse length and intensity.
When the energy was added in a short burst, it was fully absorbed by the top layer of silver 105 before it had time to diffuse through the aerogel 103 into the substrate 101. Conversely, adding the same energy over an extended period allowed the energy time to conduct to the substrate 101, thus evenly heating all layers 101, 103 and 105. The bounding, straight lines 115 and 117 on
In between these two bounds 115 and 117, the actual energy required to bring the silver to melting depends on the combination of pulse duration and intensity used. Furthermore, the transition from one limit to the other depends on the thickness of the insulating layer 103 between the substrate 101 and the silver 105.
The combination of pulse duration and intensity used to bring the silver to its melting point becomes critical when the peak temperatures of other layers are of concern. For example,
After an aerogel layer put on a substrate to protect its surface was tested in simulation, the aerogel layer was then tested on simple electronic components. In a trial study illustrated in
While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.
This application claims the benefit of U.S. Provisional Application No. 60/198,377 filed Apr. 19, 2000.
Number | Date | Country | |
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60198377 | Apr 2000 | US |
Number | Date | Country | |
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Parent | 09837265 | Apr 2001 | US |
Child | 11092283 | Mar 2005 | US |