Photoexcitation method

Abstract

A method and composition for enabling indirect photoexcitation whereby a large energy gap between energy levels in a second material is circumvented by a series of lower energy photoexcitations in a first material.

Claims

1. A method of photolithography, comprising the steps of: providing a translatable substrate having a composite thereon, the composite comprising: a first material having at least a first energy level, a second energy level, and at least one interceding energy level between the first and second energy levels, and a second material having at least a first energy level and a second energy level, wherein the second material is photodegraded when at least one electron is excited into the second energy level of the second material, wherein an energy gap between the first energy level and the at least one interceding energy level of the first material is less than the energy gap between the first and second energy level of the second material, and wherein the first material and second material are in communication with one another; determining an energy flux threshold of a pulse of photons required to be transmitted onto the composite in order to: excite at least one electron from the first energy level of the first material to the at least one interceding energy level; excite the at least one electron from the at least one interceding energy level to the second energy level of the first material; and transfer the at least one electron from the second energy level of the first material to the second energy level of the second material, to thereby form photodegraded areas in the second material; focusing a continuous wave laser with an objective lens including maintaining a focusing accuracy of approximately +/−0.5 percent using focused spot full width at half maximum (FWHM) diameter as a reference; exposing the composite to pulses of photons from the focused continuous wave laser directly modulated in pulse duration and pulse amplitude, wherein the continuous wave laser exhibits temporally a square-like profile, as a result of its direct modulation, and spatially a Gaussian laser beam profile, wherein photons at a center of the Gaussian laser beam profile deliver energy at or greater than the energy flux threshold, and less than the energy gap of the second material, and photons at a periphery of the Gaussian laser beam profile deliver less energy than the energy flux threshold; such that an electron of the first material contacted by the center of the Gaussian laser beam profile is excited from the first energy level of the first material to the second energy level of the first material, and wherein said electron transfers from the second energy level of the first material to the second energy level of the second material; and thereby forming photodegraded areas in the second material comprising holes with dimensions of less than approximately 100 nm; wherein the exposing step is performed while continuously translating the translatable substrate, so as to form a photolithographic pattern on the translatable substrate.

2. The method according to claim 1, wherein the second energy level of the first material is substantially equal to the second energy level of the second material.

3. The method according to claim 1, wherein the laser is a continuous diode laser.

4. The method according to claim 1, wherein the first material is a polyaromatic compound.

5. The method according to claim 1, wherein the first material is a dye.

6. The method according to claim 5, wherein the dye includes at least one of: a perylene, a coumarin, an aminoanthracene and an anthracene.

7. The method according to claim 1, wherein the second material is a polymer.

8. The method according to claim 1, wherein the first material is suspended in the second material.

9. The method according to claim 1, wherein the first material is bonded to the second material.

10. The method according to claim 1, wherein the second material is doped with the first material.

11. The method according claim 1, wherein the composite is a photoresist.

12. The method according to claim 1, wherein the pulse of the photons has an energy less than or equal to about 80 mW, even when assessed within a duration of a single pulse.

13. The method according to claim 1, wherein the substrate comprises at least one of: glass and silicon.

14. The method according to claim 1, wherein the focusing step is performed with an objective lens having a numerical aperture of 0.90 or greater.

15. The method according to claim 1, wherein the dimensions of the holes are less that approximately 50 nm.

16. The method according to claim 15, wherein the dimensions of the holes are less that approximately 20 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a flow chart of the method in accordance with an embodiment of the invention.

(3) FIG. 2 is an energy level schematic of the composite.

(4) FIG. 3 is a schematic drawing of the laser writing setup.

(5) FIG. 4 is an illustration of high resolution structure formation by using only a small part of the laser focused spot in non-linear response materials.

(6) FIGS. 5a and 5b show astigmatism along XY uncorrected diodes.

(7) FIGS. 6a and 6b sow astigmatism along XY correct diodes.

(8) FIG. 7 is a representation of the formation of a copolymer useable in the invention.

(9) FIG. 8 shows UV-Vis spectrum of a random copolymer thin film doped with 5% perylene.

(10) FIGS. 9a and 9b show a thin film of polymers 1 and 2 respectively.

(11) FIG. 10 shows a simulation on irradiance encircling 2×1/e.sup.2 and 30 nm diameters.

(12) FIG. 11 shows a calculation of irradiance and power differences between continuous wave and pulsed laser operation.

(13) FIG. 12 shows a thin film of polymer 1 written by LBR after nickel metallisation.

(14) FIG. 13 shows a thin film of polymer 2 written by LBR.

(15) FIG. 14 shows a thin film of polymer 5 written by LBR.

(16) FIG. 15 also shows a thin film of polymer 5 written by LBR.

(17) FIG. 16 shows another example of a thin film of polymer 5 written by LBR.

(18) FIG. 17 shows a cross-section of a laser beam and corresponding intensity which follows a Gaussian distribution across the beam diameter wherein the photon-flux at the centre of beam provides a threshold level of photon flux.

DETAILED DESCRIPTION

(19) In order to provide for indirect photoexcitation whereby a large energy gap between energy levels in a second material is circumvented by a series of lower energy photoexcitations in a first material, there is provided a method of photoexcitation in accordance with the present invention.

(20) FIG. 1 shows a flowchart of the method steps performed to achieve photoexcitation according to an embodiment of the invention.

(21) The process commences at step S100 wherein a composite 10 is provided as a photoresist 20 spin coated onto a substrate 30. The composite 10 is formed of perylene molecules 40 in a polymer 50 matrix.

(22) At step S200, targeted areas of the composite 10 are exposed to low energy photons 60 provided by a laser 70. The Laser 70 is a 375 nm diode laser operating in continuous wave mode. The laser 70 is focused to provide a diffraction limit spot size on the surface of the composite 10, defining the target area. The perylene molecules 40 within this area absorb electrons having an energy related to in the exposing laser wavelength, whereas the polymer 50 is transparent.

(23) At step S300, the polymer 50 matrix around the sites of the perylene molecules exposed to the low energy photons 60 from the laser 70 undergoes polymer degradation leading to ablation of the photoresist 20, exposing the underlying substrate 30 to the laser 70 and allowing the substrate 30 to be patterned directly. The resulting features created on the substrate 30 have dimensions below the 50 nm domain, well below the laser wavelength.

(24) Without being bound by theory, it is believed that the perylene molecules 40 absorb the low energy photons 60 and undergo a multi-stage photoexcitation. In this multi-stage photoexcitation, an electron occupying an energy level in a perylene molecule 40 absorbs an incident low energy photon 60 and is promoted to a higher energy level (and therefore gaining energy). The continuous wave nature of the laser 70 allows for this electron to undergo further photoexcitations before it is able to re-emit the low energy photon 60 and return to its original energy level. This is enabled by the photon flux being above a threshold necessary for continuous electron promotion. Therefore the electron continues to advance upward through energy levels (and energy) until it occupies an energy level of the perylene molecule 40 that is substantially equivalent in energy to an energy level of the surrounding polymer 50. At this stage, the electron transfers from perylene molecules 40 into the surrounding polymer 50 which then degrades. Thus the polymer 50 is photosensitized by the perylene molecule 40, which acts as a photosensitizer. Accordingly the minimum feature size is not set by the laser 70 wavelength, but by the size and distribution of the absorbing perylene molecules 40 and the characteristic distance over which the high energy electron can transfer between the perylene molecule 40 and the surrounding polymer 50.

(25) This process is illustrated in FIG. 2a-2d which shows an energy level schematic of the perylene 40 and polymer 50. FIG. 2a show the first stage of the process, with an electron in the perylene 40 undergoing photoexcitation to the energy level above. This is repeated in FIG. 2b, with the electron continuing up the energy levels. FIG. 2c shows the now high energy electron being transferred between an energy level of the perylene that is substantially equal in energy to an energy level of the polymer 50, thus the polymer 50 absorbs the high energy electron.

(26) The composite 10 is formed by doping the polymer 50 with 5% perylene (with respect to the polymer weight).

(27) FIG. 4 shows the laser ablation setup. The direct-write system used to selectively expose the photoresist incorporated an air-bearing/precision mechatronic stage. A rotary spindle carried the coated spin-coated substrate by means of a vacuum clamp, a linear stage translated the laser 70 and optics above and across the substrate, and a focusing actuator held the high numerical aperture (NA) objective lens 75 properly positioned.

(28) An exposing laser beam from the 375 nm diode laser was collimated using a lens 71, passing through a partial mirror 72, before being precisely focused onto the surface of the photoresist 20 through the high NA objective lens 75 and the beam was rastered over the desired areas of the photoresist and the underlying substrate.

(29) Light reflected off of the partial mirror 72 is directed by a second mirror 80 through a second lens 85 into a photo detector 90 to allow for feedback driven autofocusing of the laser 70.

EXAMPLES

2. Experimental

(30) 2.1 Materials

(31) All solvents and reagents were purchased from Sigma or Alfa Aesar and were of analytical or HPLC grade.

(32) Tetrahydrofuran (THF) was distilled three times from potassium. The monomers were passed over a column of basic alumina to remove the inhibitors and protonic impurities. Two distillations over calcium hydride and 2,2-Diphenyl-1-picrylhydrazyl (Aldrich) provided monomers of sufficient purity.

(33) 2.2 Polymerizations

(34) Polymerization reactions were carried out in 250 ml one neck flasks fitted with a rubber septum and a magnetic stirrer bar. The monomers and the solvent (typically tetrahydrofuran (THF)) were transferred in the reaction flask via syringes. Then, the radical initiator (typically AIBN Azobisisobutyronitrile) were added and the flask were heated at 60° C. The polymerizations were being carried out for 8 hours. The final polymer was obtained by precipitation of the reaction mixture in cold hexane and the product was dried under reduced pressure.

(35) 2.3 Preparation of Thin Films

(36) Glass substrates were cleaned by sonication in acetone and isopropanol for 5 min each, dried under a N.sub.2 gas flow and baked for 2 min at 120° C. in an ambient atmosphere to remove any residual IPA. The random copolymers were dissolved in ethyl lactate and PGMEA at room temperature to yield 2.0, 3.0 and 5.0 wt % polymer solutions. Random copolymer thin films were fabricated by spin coating a polymer solution at 1000-2000 rpm for 120 seconds. Prior to spin coating, the silicon substrates were primed 2 times by spin casting ethyl lactate or PGMEA (2000, 120 seconds). After spin coating, the block copolymers thin films were baked for 30 min at 100° C. on an oven to remove any residual solvent.

(37) 2.4 Laser Ablation Writings

(38) The direct-write system used to selectively expose the thin films was an air-bearing/precision mechatronic stage, developed in-house for the purposes of the project, in r-theta-z setup. A rotary spindle carried the coated substrate under test (theta)—by the means of a vacuum clamp—, a linear stage (r) translated the laser and optics above theta and across the substrate radii, and a focusing actuator (z) held the final/high-NA objective properly positioned.

(39) The film exposing laser beam (375 nm diode laser) was precisely focused, through a high NA objective, resulting in the formation of a diffraction limited spot on the surface of the substrate. Throughout the experimentation process a better than 1% (±0.5%) focusing error was maintained, using the focused spot FWHM diameter as a reference, and measuring error in the back-reflected (from the test substrate) beam divergence variation.

(40) The optical arrangement used to setup and focus the exposing laser beam was individually tested for beam astigmatism and Gaussian beam profile uniformity along X and Y axes, and corrected, as depicted in FIGS. 5a and 5b and 6a and 6b. The setup used for testing involved focusing the exposing beam on a two (10× and 100×) infinity corrected microscope objective system, effectively magnifying it by three orders of magnitude, and projecting the up-scaled beam to a beam profiling camera (Neutral Density filters were additionally used to adjust laser power to profiling camera acceptable levels).

(41) The combined rotary-linear motion of the exposure tool stages allowed the focused beam to scan the substrate surface with a constant linear velocity of 5.00 m/s in a spiral mode, while the exposing laser was directly modulated both in pulse duration and in amplitude, forming holes with adjustable dimensions on the film.

(42) Various pulse modulation patterns were tested. The pulse duration/amplitude resolution of the exposure tool used was 10 ns/0.01 mW respectively.

(43) The laser-processed film subsequently underwent thermal evaporation and electroplating treatments for topography inverting the originally made holes on the film into Ni pillars.

3. Results and Discussion

(44) 3.1 Polymeric Materials Design and Properties

(45) FIG. 7 shows the structure of the designed random copolymers synthesized by free radical polymerization. The monomers selected in the direction to improve specific properties of the final random copolymers. Random copolymers consisted of about 85% of the monomer R1, about 10% of the monomer R2 and about 5% of the monomer R3. Monomer R1, which is the main component of the random copolymer, gives properties such as glass transition temperature control, improved surface adhesion, solubility and compatibility with other substances. Monomer R2 was selected to improve the compatibility between the random copolymer and the dispersed dye molecules. Finally, the monomer R3 is a functional monomer that can be crosslinked during the post-apply-bake step increasing the Tg of the random copolymer thin film in order to eliminate the berm formation around the created structures.

(46) Five random copolymers were synthesized with different amounts of the monomers R1, R2 and R3. Table 1 contains the composition of these random copolymers as well as the molecular characteristics. The molecular weights of the random copolymers were about 50 KDa and the polydispersities were up to 2.35.

(47) TABLE-US-00001 TABLE 1 compositions and molecular characteristics of the synthesized random copolymers. R1 R2 R3 Polymers (% moles) (% moles) (% moles) Mn PD Polymer 1 100 0 0 45000 2.25 Polymer 2 90 10 0 44000 2.15 Polymer 3 85 10 5 49000 2.30 Polymer 4 88 10 2 51000 2.25 Polymer 5 89 10 1 48000 2.35
3.2 Polymeric Thin Films Preparation

(48) The dye used for doping the random copolymers was perylene. Perylene is a polyaromatic molecule which does not photodegrade during the laser ablation process. The random copolymers were doped with Perylene in 3, 4 and 5% with respect to the random co-polymer weight. The polymeric material doped with 5% Perylene had the proper amount of the dye to trigger the ablation process. FIG. 8 contains a UV-Vis spectrum of a polymeric material doped with 5% Perylene. Absorbance of the doped random polymer at 375 nm, which is the wavelength of the laser used for ablation, does not exceed 0.02.

(49) Polymer 1 thin films were made by spin coating, the concentration of Perylene molecules was 5% with respect to polymer's weight. As it is shown in FIG. 9a, the thin films contained regions with dye aggregations, caused by the chemical incompatibility between the polymeric matrix and the dye molecule. The perylene molecules tend to crystallize, due to their aromatic nature, and if the concentration of the dye exceeds 3% with respect to the polymer's weight, regions with crystallized perylene aggregates appear. By introducing groups that improve the compatibility between the polymer matrix and the perylene molecules into the polymer's backbone, this behavior can be eliminated. Polymer 2 contains 10% of monomer R2, with respect to the polymer, which is selected to have chemical similarities with perylene molecules, delivering better dye distribution into the polymeric matrix and eliminating the dye aggregation problem. A thin film of polymer 2 doped with 5% perylene, with respect to the polymer's weight, appears in FIG. 9b. The film does not contain any dye aggregation defects and can be used for laser processing.

(50) 3.3 Laser Characteristics

(51) In order to improve our understanding on the Laser characteristics effecting the process, a simple MATLAB simulation was developed to quickly estimate Laser spatial-temporal parameters at the focal point during tests, as shown in FIG. 10. The simulation was capable of accepting laser beam power set-point (mW) and focused spot diameter (nm at 1/e2), delivering: irradiance (MW/cm2) across a circle with diameter 2×1/e2 diameter, and along a circle with a diameter equal to a diameter of interest (ROI). 2×1/e2 irradiance was used for comparing direct measurements of the focused beam (after ×1000 magnification) on the beam profiler, irradiance across ROI was helpful to estimate the energy flux threshold that was triggering the ablation process on the test films of various chemical compositions. As discussed above, the Gaussian laser beam profile enable a setup in which only the very centre of the beam is above this threshold, as depicted above FIG. 17.

(52) Focused laser beam profile, as measured by the beam profiler, matched the MATLAB/theoretical calculated profile (for the given laser source wavelength, and the 0.90 NA objective lens used) within the measurement uncertainty range:

(53) TABLE-US-00002 Calculated FWHM (XY) 247.3 nm FWE2 (XY) 420.0 nm Measured FWHM (X) 255 μm FWHM (Y) 262 μm (scaled ×1000) Measured FWE2 (X) 419 μm FWE2 (Y) 427 μm (scaled ×1000)

(54) By feeding actual feature dimensions (measured by SEM) of experiment results back to MATLAB, we were able to get detailed graphical representations of irradiance, power (the later by integrating irradiance over ROI) and energy around areas equal to the ablated ones during exposure time.

(55) Additionally we calculated irradiance, power and energy for a pulsed laser equivalent of the CW laser used, as shown in FIG. 11, delivering the power of our laser over a single pulse, keeping the rest of the operational parameters the same. Calculations covered various repetition rates and typical temporal pulse profiles (Gaussian, exponential and squared hyperbolic secant), to confirm the unique exposure conditions of the test, compared to the commonly used on ablation experiments, pulsed lasers.

(56) Formulas used to calculate Power, integrated over time for calculating each pulse energy, and multiplied by the repetition rate to calculate total energy:

(57) $P_P=P\times \mathrm{FWHM}\times f,P_{\mathrm{EXP}}\left(t\right)=P_P.Math.4.Math.\ln \left(2\right){\left(\frac{t}{\mathrm{FWHM}}\right)}^2.Math.\exp \left(-\frac{t}{\mathrm{FWHM}}\right)$$P_G\left(t\right)=P_P.Math.\exp \left[-4.Math.\ln \left(2\right).Math.{\left(\frac{t}{\mathrm{FWHM}}\right)}^2\right]$$P_{\mathrm{SECH}}\left(t\right)=P_P.Math.{\mathrm{sech}}^2\left[2\ln \left(1+\sqrt{2}\right)\frac{t}{\tau }\right]$$E_G\left(t\right)=\frac{P_P.Math.\mathrm{FWHM}}{4.Math.\sqrt{\ln \left(2\right)/\pi }}\left[1+\mathrm{erf}\left(\frac{t}{\mathrm{FWHM}}\right)\right].fwdarw.E_G\left(\infty \right)=\frac{P_P.Math.\mathrm{FWHM}}{2.Math.\sqrt{\ln \left(2\right)/\pi }}$$E_{\mathrm{EXP}}\left(t\right)=P_P.Math.4.Math.\ln \left(2\right).Math.\mathrm{FWHM}\left[\frac{\sqrt{\pi }}{4}\mathrm{erf}\left(\frac{t}{\mathrm{FWHM}}\right)-\frac{t}{2\mathrm{FWHM}}{e}^{-{\left(\frac{t}{\mathrm{FWHM}}\right)}^2}\right].fwdarw.E_{\mathrm{EXP}}\left(\infty \right)=P_P.Math.\sqrt{\pi }.Math.\ln \left(2\right).Math.\mathrm{FWHM}$$E_{\mathrm{SECH}}\left(t\right)=\frac{P_P.Math.\mathrm{FWHM}}{2.Math.\ln \left(1+\sqrt{2}\right)}\left[1+\tanh \left(\frac{t}{\mathrm{FWHM}}\right)\right].fwdarw.E_{\mathrm{SECH}}\left(\infty \right)=\frac{P_P.Math.\mathrm{FWHM}}{\ln \left(1+\sqrt{2}\right)}$
3.4 Materials Evaluation

(58) As shown in the FIG. 12, in the regions of the polymeric thin film (polymer 1) where the dye molecules were aggregated, the film remained untouched after exposing it with the laser, due to the lack of dye molecules in these areas. Furthermore, the big aggregates strongly effected/downgraded the quality of inverting the originally made holes on the film into Ni pillars.

(59) Thin film coatings (polymer 2), without defects produced by perylene molecule aggregation, appear in FIG. 13. The power of the exposing laser varies from 40 to 80 mW, which attributes to several MW/cm2 light intensity. The length of the created structures is adjusted by the duration and the modulation of the laser pulses, taking values from 10 to several hundred nanoseconds. Structures shown have several dimensions in response to different power setting of the laser firing each pulse. The smallest created structure is in the sub 20 nm regime. Unfortunately, berm formation took place along to the nanopillars as it noticed with red circles.

(60) Polymer 3 and polymer 4 cannot be ablated in this range of laser power due to excessive crosslinking in the polymeric films. As it is shown in FIGS. 14, 15 and 16, well defined structures were possible to create by using Polymer 5, containing only 1% of crosslinkable groups. In this polymer structures, berm formation was not observed due to finely crosslinking the polymer.