Methods for curing thermosetting polymers

Abstract

A method for curing a thermosetting polymer is disclosed. A thermosetting polymer precursor is deposited on a substrate. After preheating the thermosetting polymer precursor, the thermosetting polymer precursor is then exposed to light pulses from a flash lamp while simultaneously cooling the thermosetting polymer precursor to maintain its average temperature below its maximum operating temperature. After exposure to the light pulses, byproducts are allowed to dissipate from the thermosetting polymer precursor. The light pulse exposure step and the byproduct dissipation step are repeated multiple times until a thermosetting polymer thin film is formed.

Description

[Technical Field]

[0001] Technical field This application relates, in general terms, to thermosetting polymers, and more specifically, to a method for curing thermosetting polymers. [Background technology]

[0002] background Many functional materials can exhibit better performance after processing at higher temperatures, and therefore, processing thin film materials at higher temperatures is more desirable. However, processing at higher temperatures generally requires substrates with higher temperature specifications, which in most cases means requiring more expensive substrates. For example, polyimide (PI) is a high-temperature polymer and is considered superior to polyethylene terephthalate (PET), the main low-temperature substrate material for printed electronics. However, curing PI requires heat treatment at higher temperatures for longer periods, resulting in three reasons that prevent PI from being widely adopted in more applications. Firstly, maintaining the PI precursor at high temperatures for extended periods consumes a lot of energy, which increases the cost of the final product. Secondly, the temperature and time required for PI curing exceed what most polymers can withstand without damage, meaning that PI cannot be formed adjacent to many other polymers. Thirdly, PI curing produces a small amount of water in the form of vapor, making it essentially impossible to form thick layers. In other words, PI must be formed in thin films.

[0003] Therefore, despite being a highly desirable material for the printed electronics industry due to its high-temperature stability and chemical resistance, PI is rarely used, mainly because its curing process must be carried out at high temperatures over a long period of time.

[0004] As a result, it is desirable to provide an improved method for rapidly forming PI as a thick layer on a low-temperature substrate with minimal input energy. [Overview of the project] [Means for solving the problem]

[0005] Summary of the Invention According to one embodiment, a thermosetting polymer precursor is deposited on a substrate. After preheating the thermosetting polymer precursor, it is then exposed to light pulses from a flash lamp while simultaneously cooling the thermosetting polymer precursor to maintain its average temperature below its maximum operating temperature. After exposure to the light pulses, gaseous byproducts are allowed to be released from the thermosetting polymer precursor. The light pulse exposure step and the byproduct release step are repeated multiple times until a thermosetting polymer thin film is formed.

[0006] All the characteristics and advantages of the present invention will become apparent from the following detailed description.

[0007] The present invention itself, as well as its preferred uses, further purposes, and advantages, will be best understood by referring to the following detailed description of exemplary embodiments, when read in conjunction with the accompanying drawings. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 illustrates a method for curing a thermosetting polymer according to one embodiment.

[0009] [Figure 2] Figure 2 shows an example of curing a thermosetting polymer on a silicon wafer according to one embodiment.

[0010] [Figure 3] Figure 3 shows an example of curing a thermosetting polymer on PET according to one embodiment.

[0011] [Figure 4] Figure 4 shows an example of curing a thermosetting polymer underfill for a flip-chip according to one embodiment.

[0012] [Figure 5] Figure 5 shows an example of curing a thermosetting polymer on a copper-clad laminate according to one embodiment. [Modes for carrying out the invention]

[0013] Detailed description of preferred embodiments The conventional method for curing a polyimide (PI) precursor to form a thin film PI is as follows: First, the PI precursor is deposited on a substrate in liquid form. The substrate is placed in an oven to remove the liquid (or solvent) from the PI precursor at a high temperature, and then the PI precursor is cured. In this conventional curing method, the temperature of the PI precursor is generally maintained at approximately 350°C for about 2 hours to completely crosslink (imide) the PI precursor. At lower temperatures, the curing of the PI is much slower, while at higher temperatures, it can be cured more rapidly. However, if the temperature is too high and crosslinking occurs too quickly, the water vapor (or other volatile substances) generated by the reaction cannot be released from inside the thin film without damaging it.

[0014] Referring here to the drawings, particularly Figure 1, a method for curing a thermosetting polymer according to one embodiment is shown. Examples of thermosetting polymers include polyimide, epoxy, silicone, and acrylic.

[0015] deposition

[0016] According to one embodiment of the present invention, as shown in block 11, a thermosetting polymer precursor is deposited on a substrate. Examples of thermosetting polymer precursors include polyamic acid and epoxy. The thermosetting polymer precursor can be deposited in a liquid or solid form to a thickness of about 1 to 100 microns. The thermosetting polymer precursor can be deposited on the substrate by various full coating techniques including spin coating, doctor blade, roll coating, spray coating, etc. The thermosetting polymer precursor can also be selectively deposited on the substrate by inkjet printing, flexographic printing, gravure printing, syringe dispense printing, screen printing, stencil printing, etc. The substrate can be metal, glass, ceramic, semiconductor, or polymer. Instead of a single substrate, the thermosetting polymer precursor can also be deposited on a stack having multiple layers.

[0017] Drying

[0018] When the thermosetting polymer precursor (for example, polyamic acid which is a precursor of PI) is deposited in a liquid form, as shown in block 12, the thermosetting polymer precursor is dried to remove the solvent from the thermosetting polymer precursor. Drying can be performed by placing the thermosetting polymer precursor and the substrate in an oven. The environment can be inert to prevent oxidation of the thermosetting polymer precursor. The thermosetting polymer precursor can also be dried using a near-infrared lamp. The thermosetting polymer precursor can also be dried using a plurality of light pulses from a flash lamp. When the flash lamp approach is used for drying, an inert atmosphere is not essential, and the peak temperature in each light pulse may be higher than the boiling point of the solvent with the lowest boiling point in the thermosetting polymer precursor, but the average temperature during the processing time may be lower than the boiling point of the solvent with the lowest boiling point in the thermosetting polymer precursor.

[0019] During the drying process, the thermosetting polymer precursor can be cooled by convection, for example, by an air knife, or / and by conduction, for example, by a relatively large heat capacity or / and actively temperature-controlled block that is in direct contact with the thermosetting polymer precursor or through its substrate or stack. When drying, both the peak and average temperatures that can be reached without damaging the thermosetting polymer precursor are generally lower than the peak and average curing temperatures of the thermoset, because the solvent in the thermosetting polymer precursor boils at a temperature lower than the maximum rated temperature of the thermosetting polymer precursor or the fully cured thermoset. Some thermosetting polymer precursors, for example, epoxies, do not need to be dried.

[0020] Preheating

[0021] After drying (if necessary) and before curing, as shown in block 13, the thermosetting polymer precursor is preheated. Preheating can be done by using various methods, including hot air, near-infrared lamps, infrared lamps, conduction from the lower chuck, rollers, or flash lamps.

[0022] Curing

[0023] Instead of using an oven for curing, a series of light pulses are utilized along with active cooling to modulate the temperature of the thermosetting polymer precursor during curing. First, as shown in block 14, the thermosetting polymer precursor is exposed to light pulses from a flash lamp while being simultaneously cooled to maintain the average temperature of the thermosetting polymer precursor below its maximum use temperature. The thermosetting polymer precursor is heated by the light pulses above its maximum steady-state use temperature, thereby increasing the curing (e.g., cross-linking) rate. At the same time, the thermosetting polymer precursor is also cooled below its maximum use temperature.

[0024] As shown in Block 15, a predetermined period may elapse after the application of the light pulse to allow any gaseous or volatile byproducts to be released from the thermosetting polymer precursor.

[0025] As shown in block 16, a determination is made as to whether the thermosetting polymer precursor has cured. If the thermosetting polymer precursor has not cured, the light pulse exposure step in block 14 and the byproduct emission step in block 15 are repeated. If the thermosetting polymer precursor has cured (i.e., a thermosetting polymer thin film is formed), as shown in block 16 of Figure 1, the process is complete.

[0026] Each time the thermosetting polymer precursor is exposed to a light pulse, it is heated to a temperature higher than the maximum operating temperature of the thermosetting polymer precursor. In the case of polyimide formation, this temperature is approximately 350°C at the start of curing and approximately 400°C when the thermosetting polymer precursor is completely imidized. At the peak of the light pulse, the temperature that can be reached in the thermosetting polymer precursor film is higher than the maximum allowable steady-state temperature (approximately 200°C higher) for a short time (approximately milliseconds). During the process, the thermosetting polymer precursor is also cooled so that the average surface temperature of the thermosetting polymer precursor is lower than the maximum rated steady-state temperature of the thermosetting polymer material. Cooling is applied continuously and takes place during (1 millisecond) and in between (approximately tens to hundreds of milliseconds) of the light pulse. This light exposure / cooling is repeated multiple times until the thermosetting polymer precursor is completely crosslinked. The total processing time is approximately 5 seconds to 1 minute.

[0027] The purpose is to cool during curing.

[0028] In all cases, the total energy imparted to the thermosetting polymer precursor by the light pulse train is far greater than what is needed to thermally damage the thermosetting polymer precursor. Therefore, in all cases, it is necessary to remove heat from the thermosetting polymer precursor by either conduction or convection during processing to prevent the average temperature of the thermosetting polymer precursor from exceeding its maximum rated equilibrium temperature.

[0029] In addition, by-products of the crosslinking process typically have lower boiling points than the cured polymer, and if they remain in high concentrations within the thermosetting polymer precursor, they will lead to bubble formation and / or combustion upon subsequent light pulses. Cooling is necessary to maintain a suitable operating temperature for removing by-products, as well as to provide suitable access to the peak temperature required for further curing of the thermosetting polymer precursor. Cooling effects due to the release of gases from the crosslinking process are also present. The thermosetting polymer precursor can be cooled by conduction, convection, or both.

[0030] Cooling by conduction

[0031] The thermosetting polymer precursor may be in physical contact with a heat sink during exposure to a flash lamp. The heat sink may be a chuck or roller with a high heat capacity made from metal, ceramic, graphite, glass, etc. The heat sink may be a substrate to which the thermosetting polymer precursor is applied, provided that the substrate is sufficiently thick. Alternatively, the thermosetting polymer precursor may be located on a thin substrate that is in physical contact with the aforementioned heat-capacity heat sink. In other words, the thermosetting polymer precursor is in physical contact with a large thermomass. The surface density of the thermomass is preferably 50 to 500 times higher than the surface density of the thermosetting polymer precursor film. In other words, the thermomass, which may include the substrate, chuck or roller, and any layers between them, is thicker (50 to 500 times) than the thickness of the thermosetting polymer precursor. The chuck or roller may also be temperature-controlled.

[0032] Cooling by convection

[0033] Thermosetting polymer precursors can be cooled by convection, for example, by using an air knife. In addition to providing cooling, air knives have the added advantage of removing any gaseous byproducts from the drying or curing process. Air knives can also be used in addition to the conduction-based cooling described above.

[0034] Flash lamp emission absorption

[0035] The emission spectrum from the flash lamp described above is broadband, ranging from 200 nm to 1,500 nm. In order to heat the thermosetting polymer precursor, a portion of the emission band from the flash lamp must be absorbed. Absorption can be performed directly by the thermosetting polymer precursor, by any of the layers of the composite substrate, by a chuck or roller, or a combination thereof. Preferably, absorption should be performed by the thermosetting polymer precursor or the substrate adjacent to it. In the case of forming polyimide, the precursor (polyamic acid) has a very sharp absorption transition at about 500 nm, which means it is highly absorbent for wavelengths shorter than 500 nm but generally transmittance for wavelengths longer than 500 nm. If the layer directly beneath the thermosetting polymer precursor absorbs wavelengths above 500 nm, an optical filter that removes wavelengths below 500 nm can be placed between the flash lamp and the polyimide precursor, thereby allowing the polyimide polymer precursor to be easily heated by conduction.

[0036] In addition, an absorber (light-absorbing material) can be placed on a thermosetting polymer precursor to enhance its absorption of emission. If it is desirable to maintain its transmittance in the visible spectrum, a near-infrared absorber may be placed on a thermosetting polymer precursor to increase its absorption of emission.

[0037] In some cases, it is desirable to process the thermosetting polymer precursor without using ultraviolet (UV) light. A UV-blocking filter can be placed in the beam, and curing can be performed in the visible and near-infrared portions of the emission. An absorber can be placed in the thermosetting polymer precursor to increase absorption in the near-infrared portion of the spectrum without affecting the absorption of the thermosetting polymer precursor in the visible spectrum. Any light that is not absorbed or reflected by the thermosetting polymer precursor may be absorbed by the substrate below it. In a preferred embodiment, a filter is inserted to remove all absorption of the flash lamp emission by the thermosetting polymer precursor, so that absorption occurs only directly beneath the thermosetting polymer precursor.

[0038] temperature measurement

[0039] The temperature of the surface of a thermosetting polymer precursor, as well as the temperature of the substrate beneath the thermosetting polymer precursor, and any conductive cooling means, or / or convection means, such as an air knife, can be simulated using SimPulse® (available from PulseForge, Inc., Austin, TX). This simulation is performed using pulsed light bolometer measurements (J / cm²). 2 The simulation can be verified as long as the measurement of radiation exposure in units, as well as the temporal variation of the radiation output from the flash lamp, are measured. Additional inputs to the simulation are the thermophysical properties of the stack, which include the absorption of the beam from the flash lamp, heat capacity, thermal conductivity, and the thickness of each layer of the stack. Finally, the heat transfer coefficient and the temperatures of the top and bottom of the stack must be determined. In this way, the heat input due to the absorption of pulsed light in the stack (which can be absorbed by the top layer of thermosetting polymer precursor, the thermosetting polymer precursor, the substrate underneath, the cooling chuck, or anything placed between them) can be determined. From this information, the peak temperature at any point in the depth direction of the stack, including the thermosetting precursor film, as well as the average temperature at any point in the stack, can be determined.

[0040] The surface temperature of the thermosetting polymer precursor may be measured directly using a pyrometer. Since pyrometers are generally sensitive only in the 10-micron range (10,000 nm), and the emission from a flash lamp is 200 nm to 1,500 nm, the pyrometer is not sensitive to the emission from the flash lamp, and can be used while the flash lamp is emitting light pulses. In addition, pyrometers also have a low-frequency response and cannot detect a rapid temperature increase in the thermosetting material due to absorption from pulsed light from the flash lamp. Effectively, this means that the pyrometer measures the average surface temperature of the thermosetting polymer precursor during processing. In this embodiment, the average thermosetting polymer precursor temperature does not exceed the average temperature limit of the thermosetting material precursor when the thermosetting polymer precursor is processed in an oven.

[0041] Theory of thermosetting polymer curing methods

[0042] The following is the theory explaining why the above-described thermosetting polymer curing method works. Maintaining the thermosetting polymer precursor at a higher temperature allows the crosslinking reaction to proceed much faster. However, for many thermosetting polymer precursors, such as polyimide, curing is generally limited to a processing temperature of approximately 350°C in an oven. At this temperature, the crosslinking, or imidation process in the case of PI, occurs for about two hours, and the resulting thermosetting polymer film can withstand approximately 400°C without any damage after it has fully cured. This temperature limitation can be circumvented by cycling the temperature to a much higher temperature (around 550°C) for a short time, followed by rapid cooling to well below 350°C, and repeating this many times until the imidation reaction is complete. The average temperature during processing is maintained below the maximum operating temperature of the precursor. This is achieved by using a broadband light source emitting rapid pulses, such as a flash lamp, in combination with means for continuously cooling the thermosetting polymer precursor during processing. Light from a flash lamp is absorbed by the thermosetting polymer precursor, the substrate beneath it, or an absorber above it. Upon light absorption, the thermosetting polymer precursor is heated, either directly or combined with conduction from the absorber above or below it. Once the thermosetting polymer precursor is heated, a crosslinking reaction follows. This generates gaseous products, but before these products can accumulate enough to damage the film, the film is rapidly cooled to a temperature below the maximum rated temperature of the thermosetting polymer precursor, allowing for gas evacuation. The average temperature of the thermosetting polymer precursor during processing can be monitored by a pyrometer.

[0043] In exemplary cases, there is little to no absorption of the light pulse by the thermosetting polymer precursor film, but absorption exists directly beneath the thermosetting polymer precursor film. This can be achieved by placing an optical filter in the path of the light pulse to remove the portion of the spectrum that would be absorbed by the thermosetting polymer precursor film. If the substrate beneath the thermosetting polymer precursor film does not absorb the light pulse, an absorber (i.e., a light-absorbing layer) may be placed there. When this occurs, the curing of the thermosetting polymer precursor film proceeds from the bottom of the film up to the top surface. This is extremely unusual when curing a thin film. Typically, the film is maintained at a constant temperature to crosslink it. As the thermosetting polymer precursor film crosslinks, volatile substances are slowly released and diffuse into the film boundaries. One of the limitations of the prior art is that the temperature is uniform throughout the film, and therefore the curing across the entire thickness of the thermosetting polymer precursor film is also uniform as a result. Therefore, as crosslinking progresses, the film itself acts as a barrier to the release of volatile substances. This increases the time required to cure the thermosetting polymer precursor film and further limits the substantially achievable thickness of the thermosetting polymer precursor film. In contrast, the short light pulses in this embodiment ensure that the temperature at the precursor-absorber boundary is higher than that of any other part of the thermosetting polymer precursor film. In other words, the length of each light pulse is shorter than the thermal equilibrium time across the entire thermosetting polymer precursor film. This creates a temperature gradient across the entire thermosetting polymer precursor film, meaning that the crosslinking rate is highest at the boundary and decreases as it moves away from the boundary. As the thermosetting polymer precursor film is cured by subsequent light pulses, crosslinking progresses from the precursor-absorber boundary towards the top of the precursor film. A major advantage of the method of the present invention is that the volatile gases generated by the crosslinking reaction can easily escape to the top surface of the thermosetting polymer precursor film because the top surface portion of the film is not yet cured and does not form a barrier to the volatile gases.In a sense, the curing method of the present invention is similar to the preference for wound healing from bottom to top. Because this process is directional, it can be called "zipper curing." As with wounds, when the thickness of the thermosetting polymer precursor film becomes very thin (a few microns), the direction of curing becomes less important, and consequently, the site of absorption also becomes less important, whether it is above or below the beam precursor, or whether it is directly caused by the beam precursor. [Examples]

[0044] Example 1 PI curing on a silicon wafer [Figure 2]

[0045] A PI precursor (PI-2525 polyamic acid manufactured by HD Microsystems) was deposited onto a 6'' diameter, 800 micron thick silicon wafer 22 using a #10 Meyer Rod (film thickness in wet state = 25 microns), and dried in a 120°C oven for 20 minutes to form a (dried) PI precursor film 21 with a thickness of approximately 15 microns. The wafer 22 was placed on a 6 mm thick graphite chuck 23 maintained at 130°C and processed with a PulseForge Invent (Model IX2-93-15, PulseForge, Inc.) photocuring tool using the following pulse profile: voltage = 380 V, pulse length = 1 millisecond, pulse frequency = 10 Hz, total exposure time = 20 seconds. The radiation exposure for each pulse was 2.31 J / cm². 2 The total radiation exposure was 462 J / cm². 2The imidization of the PI precursor film 21 was observed by a characteristic increase in darkness relative to the PI precursor film 21. If necessary, an optical filter may be placed between the flash lamp head and the PI precursor to eliminate any absorption by the PI precursor. By eliminating absorption, the curing process can be made even faster without damaging the PI precursor film. Ideally, since typical PI precursors are relatively transparent above 500 nm, this would be a 500 nm long pass filter. However, even eliminating the UV component of the light pulse would dramatically reduce the time required for curing. This embodiment was carried out using silicon, but other semiconductors, such as gallium arsenide, can be used in a similar manner. Preferably, the PI layer should have a coefficient of thermal expansion (CTE) that closely matches that of the semiconductor, especially when the semiconductor is used as an interposer.

[0046] Using this method, PI can also be cured on low-temperature substrates, such as PET. Unlike cases where PI is cured on high-temperature substrates, such as metals, silicon, or glass, PET has a maximum steady-state rated temperature of only 150°C. This is much lower than that of either PI or PI precursors. Thin-film PI can be cured on PET using the same method of the present invention, provided that the maximum temperature on the surface of the PET does not exceed its maximum transition temperature, which is approximately 400°C. In the following embodiments, the back side of the PET substrate is thermally connected to a relatively low-temperature and thermally conductive thermomass so that the back side of the PET does not exceed its maximum steady-state temperature, which is approximately 150°C, during processing. This ensures the dimensional stability of the PET substrate during processing. Furthermore, an air knife is used to cool the top of the PI precursor and exhaust any gases from the imidization process.

[0047] Composite substrates composed of thin layers of high-temperature polymers (e.g., PI on top of a low-temperature substrate (e.g., PET)) form a highly desirable structure that retains many of the advantages of both pure PI and pure PET substrates, without many of their respective disadvantages. Specifically, it possesses the excellent chemical resistance, barrier, and adhesive properties of pure PI. Like pure PET substrates, it retains most of the permeability of PET because the PI is very thin. Like PET substrates, it is thermoformable. Like PET, it is inexpensive to manufacture because it is mostly PET. When photocuring a film, e.g., printed metal ink, on a composite substrate, higher temperatures can be achieved compared to PET alone, and higher temperature materials can be processed on the composite substrate without damaging either the film or the substrate. The main reason is that the coefficient of thermal expansion (CTE) of the upper layer of the composite film, i.e., PI, is much lower, resulting in a better match with many of the functional materials desired to be processed on it, such as metals, ceramics, and semiconductors.

[0048] Example 2 Curing of PI on PET [Figure 3]

[0049] A PI precursor (PI-2525 polyamic acid manufactured by HD Microsystems) was deposited onto an ST505PET polyester (PET) film 32 (manufactured by DuPont, Inc.) using a #3 Meyer rod. The sample was then placed in a 140°C oven for 15 minutes to dry the PI precursor, forming a PI precursor film 31 approximately 4-5 microns thick. The sample was then placed with the PI precursor film 31 side facing upwards on a 6 mm thick, room-temperature graphite vacuum chuck 33 maintained at 25°C, and the PI precursor film 31 was imidized by exposure to a flash lamp (PulseForge #IX2-93-15). The processing conditions were 600 V, 120 μs pulse length, 20 Hz for 30 seconds. Radiation exposure was 1.2 J / cm² per pulse. 2 720 J / cm² 2 This was the total radiation exposure. Complete imidization was observed without damage to either the PI precursor film 31 or the PET film 32.

[0050] Example 3 Curing of PI Underfill for Flip Chip [Fig. 4]

[0051] The semiconductor chip 44 was soldered to the upper part of the FR4 interposer 42 with solder bumps 45. The low-viscosity PI precursor 41 was injected through a syringe from the edge of the stack to fill the gap between the semiconductor chip 44, the interposer 42, and the solder bumps 45. The stack was then placed (with the FR4 side down) on a vacuum chuck 43 maintained at 120 °C. The PI precursor 41 was first dried by applying, from the semiconductor chip 44 (i.e., the upper) side, a flash lamp model #IX4-52-30 (PulseForge, Inc) with a pulse length of 1 J / cm 2 , 300 V, and 3,000 microseconds in length at 5 Hz for a period of 30 seconds. The total radiation exposure during this period was 150 J / cm 2 .

[0052] The dried PI precursor 41 was then cured using the same tool at 450 V, a pulse length of 1 millisecond, and 20 Hz for 10 seconds. The total radiation exposure was 200 J / cm 2 . The quality of the final underfill was examined by evaluating cross-sectional SEM images to verify the absence of gaps.

[0053] Since the semiconductor chip 44 was on top, during both the drying and curing of the PI precursor 41. The heating of the PI precursor 41 was due to the absorption of the flash lamp light pulses by the semiconductor chip 44 and the subsequent heat conduction to the PI precursor 44.

[0054] A similar test was performed on the same stack using an alternative underfill material, epoxy (LOCTITE ECCOBOND E 1172 A). This was dispensed identically by syringe into the same structure as described above, with a similar 1 J / cm per pulse 2The final product was obtained by curing the mixture on a vacuum chuck 43 maintained at a temperature of 60°C for a period of 30 seconds at 5 Hz. This alternative formulation cured in a single curing step, requiring no drying. The curing level of the final coating was checked by TGA analysis, and no weight loss was observed at temperatures below 250°C, indicating the absence of unreacted monomers, e.g., complete crosslinking.

[0055] Example 4 Curing of PI on copper-clad laminate [Figure 5]

[0056] UBE material (UPIA-ST-1001), a polyimide precursor 51, was deposited on a flexible copper-clad laminate (CCL) using a roll-to-roll method. The flexible CCL consisted of two copper layers 52 (17.5 μm thick) sandwiching a hardened polyimide layer 53 (75 μm thick). Deposition was carried out using a syringe pump system 55 operating at 1 m / min. The coating yielded a wet film thickness of approximately 30 μm, which was then heated using a near-infrared heater connected to a printer to obtain a dry film thickness of approximately 20 μm. The support solvent was removed by exhaust. The dried product was subjected to 1 J / cm² per pulse at 800 microseconds in pulse length using a 300 mm wide PulseForge tool model #IX4-52-30. 2 This results in radiation exposure, with a flash at 12 Hz and overlap of 50, for a total radiation exposure of 50 J / cm². 2 This resulted in imidation. The substrate was suspended by tension between idlers during the process, and heat was retained in the film by the absorption of light from a flash lamp. An air knife 54 was applied to the top of the film during the process to cool the film, balance the heat input from the flash lamp, and acted as an aid in the removal of solvent and gaseous byproducts. Imidation of the product was carried out at wavenumbers 1,840–1,770 cm⁻¹. -1 This was observed through the near-absence of the FTIR peak during the interval, which corresponds to the near-absence of most of the C=O variants in the coating.

[0057] Self-supporting PI films can be fabricated using the same technique. This process may also be carried out using a temperature-controlled roller, which has the effect of both preheating the PI precursor after each pulse and increasing its cooling rate. An air knife may also be used to cool the precursor and remove volatile byproducts from the solvent and imidization.

[0058] As described, the present invention provides a method for curing thermosetting polymer precursors. Examples of precursors include polyamic acids for forming thin film PI or bipartite epoxy resins for forming thin film epoxy. By using the method of the present invention, thermosetting polymers can be constructed layer by layer to any thickness. The method is not limited to the formation of thin film polyimides or epoxys, but also includes the formation of general thermosetting polymers including acrylics, polyesters, silicones, polyurethanes, phenols, melamines, henzooxazines, bismaleimides, cyanate esters, thiolites, vinyl esters, and the like.

[0059] While the present invention is specifically illustrated and described with reference to preferred embodiments, those skilled in the art will understand that various formal and detailed modifications can be made without departing from the spirit and scope of the invention.

Claims

1. A method for curing a thermosetting polymer, wherein the method is A step of depositing a thermosetting polymer precursor onto a substrate, The steps include preheating the thermosetting polymer precursor, The steps include exposing the thermosetting polymer precursor to light pulses from a flash lamp while simultaneously cooling the thermosetting polymer precursor in order to maintain its average temperature below its maximum operating temperature, The following steps are taken after the exposure step, to release any gaseous by-products from the thermosetting polymer precursor: The steps of exposure and radiation are repeated until a thermosetting polymer thin film is formed. Methods that include...

2. The method according to claim 1, wherein the depositing step further comprises the step of depositing the thermosetting polymer precursor on the substrate in liquid form.

3. The method according to claim 2, further comprising the step of removing a fluid from the thermosetting polymer precursor.

4. The method according to claim 3, wherein the removal step is performed by an oven.

5. The method according to claim 3, wherein the removal step is performed by a flash lamp.

6. The method according to claim 1, further comprising the step of filtering light pulses from the flash lamp so as to remove light in a frequency band absorbed by the thermosetting polymer precursor.

7. The method according to claim 1, wherein the thermosetting polymer precursor is a polyamic acid.

8. The method according to claim 1, wherein the thermosetting polymer precursor is epoxy.

9. The method according to claim 1, wherein the substrate is made of a semiconductor.

10. The method according to claim 1, wherein the substrate is made of a polymer.

11. The method according to claim 1, wherein the substrate is made of metal.

12. The method according to claim 1, wherein the substrate is made of glass.

13. The method according to claim 1, wherein the cooling is performed by a temperature-controlled heat sink.

14. The method according to claim 1, wherein the cooling is performed by an air knife.

15. The method according to claim 1, wherein the thermosetting polymer precursor is moved together with the substrate with respect to the flash lamp while the thermosetting polymer precursor is being cured.

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