WO2010005831A2 - Grayscale patterning of polymer thin films using direct-write multiphoton photolithography - Google Patents

Abstract

Improved, high resolution, laser ablated grayscale assembles (A) are provided including a substrate (24a) and a polymer layer (24b) having an etched region (R) presenting areas of different, predetermined thicknesses. Preferably, the region (R) exhibits a maximum RMS roughness value of up to about 5 ran. The fabrication method involves providing a sample (24) having a substrate (24a) and polymer layer (24b), and laser ablating the layer (24b) by multiphoton photolithography to give the different thickness areas characteristic of a desired grayscale pattern. Preferably, the fabrication involves transmissivc laser ablation wherein the incident laser beam is transmitted through the substrate (24a) to ablate the layer (24b). Advantageously, the polymer layer (24b) comprises a poly(alkylene dioxythiophene):poly(styrene sulfonate) mixture.

Description

GRAYSCALE PATTERNING OF POLYMER THIN FILMS USING DIRECT-WRITE MULTIPHOTON PHOTOLITHOGRAPHY

GOVERNMENT RIGHTS This invention was made with U.S. Government support under Grant Nos. DMR-

0076169, CHE-970-9034 and CHE-0404578 awarded by NSF. The U.S. Government has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is broadly concerned with improved grayscale patterned assemblies and methods of fabrication thereof. More particularly, the invention concerns such assemblies and methods wherein a substrate-supported polymer layer is subjected to controlled multiphoton laser ablation to yield high resolution, low roughness value etched regions. The resulting assemblies are useful in electronic and optoelectronic devices and in various chemical/biological sensors.

Description of the Prior Art

The ability to pattern polymer films on micron to nanometer length scales is required for the development of many organic electronic and optoelectronic devices, chemical and biological sensors, and nanofluidic chemical separations systems. As described in recent reviews,^1"3 stamping, molding, embossing and printing techniques have been widely used for polymer film patterning. These methods allow for rapid replicate production of binary and grayscale structures over large substrates, but alteration of the structures to be prepared requires fabrication of a new master and stamp. Conventional photolithography using a photomask has also been widely employed to produce patterns over wide areas in photoresist films; but again, fabrication of a new mask is required each time the pattern is altered.³ Lithographic methods based on direct laser writing in photoresist films offer distinct advantages over these methods in that they can be used to produce arbitrary binary and three- dimensional structures by simply changing the preprogrammed illumination pattern.^3"12 However, most resist-based techniques employ chemical development steps to obtain the final structures. Laser ablation methods^13"15 eliminate the need for photomasks and additional chemical development steps. In laser ablation, material is directly removed from the substrate surface by photothermal or photochemical processes triggered by irradiation of the polymer film.'⁴ Such methods are most commonly employed for fabricating micron-scale binary patterns.

Grayscale patterning by laser ablation has been reported in only a very limited number of papers¹⁶ and is difficult to achieve for a number of reasons. Ablation usually involves nonlinear processes that exhibit strong thresholding and can be difficult to control.¹ Ablation by nanosecond lasers is especially hard to control, due to the delivery of significant quantities of energy in a very short period of time (i.e. a single laser pulse). The great majority of prior laser ablation methods also make use of reflective ablation, where the laser beam is incident on the front side of the polymer film. In most such techniques, the substrate is substantially opaque to the laser beam. Such reflective ablation presents a number of challenges, primarily stemming from the creation of "plumes" of ablated material during the course of etching. Such plumes can cause scattering of the laser beam, resulting in a loss of control over the etching process. In practice, prior attempts at achieving grayscale patterning by laser ablation have resulted in very rough etched regions and relatively poor resolution as a result of these difficulties.

Mode-locked femtosecond lasers offer a distinct advantage over nanosecond lasers in that each pulse delivers a much smaller quantity of energy and many pulses are required to deliver the energy required for ablation. The result is frequently a less explosive removal of material and much better control over the ablation process. It has recently been demonstrated that submicron-scale binary patterns can be prepared in a number of common polymer films by ablative multiplioton photolithography, using femtosecond pulses of near IR laser light from a mode-locked Ti:sapphire laser.¹⁵'¹⁷'¹⁸ It was shown that polymers possessing UV- absorbing chromophores and high glass transition temperatures yield the greatest lateral etching resolution, with etched poly(methylmethacrylate) exhibiting edge sharpnesses of ~ 120 nm in films of ~ 80 nm thickness.¹⁷ In contrast, visible-absorbing polythiophene films exhibited significantly worse etching resolution. These differences in etching resolution were attributed to differences in the order of the nonlinear optical processes involved in each case.

While high-resolution binary structures are best prepared using materials that are etched by high order nonlinear processes, the fabrication of grayscale structures is difficult in such materials because of the strong dependence of the etching rate on incident laser intensity. The associated difficulties were manifested in previous studies as increased film roughness in regions where the polymer was not completely removed to the underlying substrate.¹⁵

There is accordingly a need in the art for improved laser ablation grayscale etching methods and resultant grayscale etched assemblies wherein highly precise, controlled grayscale patterning is obtained with very low roughness values and high resolution.

SUMMARY OF THE INVENTION

The present invention overcomes many of the difficulties outlined above and provides grayscale etched assemblies comprising as self-supporting substrate (which may be rigid or flexible) together with a layer of polymer supported on the substrate. The polymer is laser ablated and has an etched region presenting areas of different, predetermined thicknesses. In one aspect of the invention, the etched assemblies exhibit etched areas commonly having (root mean square) RMS roughness values of up to about 5 run, with roughnesses of about 0.5-3 nm achieved under optimum circumstances. In another aspect, the polymer layer comprised of any of a variety of poly(alkylene dioxythiophene):poly(styrene sulfonate) mixtures, wherein the alkylene moiety typically has from 2-4 carbon atoms, but may also include a number of different side chains to increase the solubility/dispersibility of the polymer, hi terms of properties, the preferred polymers should absorb radiation within the range of from about 400-1100 nm, and with peak absorbance falling in the range of from about 600-900 nm. Moreover, the polymer should be soluble or dispersible in water or organic solvents such as alcohols, toluene and chloroform. Finally, the polymer should be able to form continuous films on transparent glass or synthetic resin substrates with the film exhibiting roughness values no greater than 10% of the film thickness, and preferably less than about 5 nm. The single most preferred polymer is poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOTrPSS) wherein the ratio of PEDOT to PSS is from about 1 :0.5 to 1 :5, and most preferably about 1 :2.5.

The methods of the invention involve first providing a sample including a self- supporting substrate with a layer of polymer supported on the substrate, followed by laser ablation of the polymer layer to form the described etched regions. Advantageously, laser ablation is accomplished in a transmission mode, i.e., the incident ablating laser beam is directed through the substrate for etching of the polymer on the remote face of the substrate. This lessens the problems of ablation plumes, and permits more precise grayscale etching. During laser ablation, the intensity of the incident laser beam is modulated so as to create areas of different thicknesses. Generally, laser beam intensity is modulated by (a) modulating the focus of said incident laser beam, (b) modulating the power of said incident laser beam, or combinations of (a) and (b).

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 depicts a representative range of chemical structures for the preferred poly(alkylene dioxythiophene):poly(styrene sulfonate) mixtures suitable for grayscale laser ablation in accordance with the invention, wherein m is 0, 1, or 2, n and n' are integers individually and respectively being greater than about 20, more preferably from about 20- 10,000, most preferably from about 100-2000, and R and R' are individually and respectively selected from the group consisting of H, or other substituent serving to increase the solubility or dispersibility of the polymer (e.g., Cl-ClO straight or branched chain alkyl or cycloalkyl, Cl-ClO ethers, alcohols, and esters, and substituted or unsubstituted aryl groups such as phenyl); Fig. 2 is a schematic illustration of the sample scanning confocal microscope arrangement used for multiplioton-induced laser ablation in accordance with the invention;

Fig. 3 is an atomic force microscopy (AFM) topographic image of a binary text pattern etched into a PEDOT:PSS film, wherein the dark regions depict areas where the polymer has been at least partially removed and the lighter regions designate unetched areas; Fig. 4 is a topographic image of 5x5 μm² squares etched into PEDOT:PSS at the average powers shown in mW;

Fig. 5 is a line profile graph taken across three of the etched regions (designated by the white line) in Fig. 4;

Fig. 6 is a power dependent etch depth plot of the mean etch depth from each square in Fig. 4;

Fig. 7 is a plot of absorbance (solid line) and fluorescence (dashed line) versus wavelength, illustrating a fluorescence peak near 375 nm with excitation at 210 mi;

Fig. 8 is a plot of fluorescence versus focus position wherein the experimental data is plotted in squares, their fit to a Loreπtzian function is shown in solid line, and the expected profile for one photon fluorescence excitation shown in dashed line (the experimental curve is broadened by bleaching and etching of the polymer film);

Fig. 9 is a series of AFM images showing Hie focus dependence of PEDOT:PSS film etching, wherein each square region (5 x 5 μm²) was etched at 20 mW incident power (40 ms iiτadiation, 100 run pixels) at the focus positions (in μm) designated on the image, and wherein the focus position is given relative to the film center, as determined in Fig. 8;

Fig. 10 is a line profile of the images of Fig. 9, as designated by the white line therein;

Fig. 11 is a graph of average etch depth (squares) as a function of focus position determined from three replicate experiments, wherein the error bars depict the mean RMS roughness of the etched regions and the solid line shows the curve expected for an etching process defined by the kinetic parameters of Fig. 6;

Fig. 12 is a grayscale template (a pyramid surrounded by a flat region) used in the preferred microscope control software for grayscale patterning; Fig. 13 is an AFM image of a pyramid etched into thin PEDOT:PSS films at 5 mW;

Fig. 14 is an AFM image of a pyramid etched into thin PEDOT:PSS films at 20 mW; Fig. 15 is an AFM image of a pyramid etched into thin PEDOT:PSS films at 30 mW; Fig. 16 is a series of line profiles taken across the center of each pyramid depicted in Figs. 13-15; Fig. 17 is an AFM image of a spiral ramp etched into a PEDOT:PSS film at 20 mW, wherein the mean RMS roughness of the pyramid shown in Fig. 13 was determined to be 1.0 nm (± 0.8 ran), while the spiral ramp yielded a mean roughness of 2 nm (± 2 nm); and

Fig. 18 is a logic flow diagram illustrating the preferred software control algorithm employed in the grayscale etching process of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following examples set forth presently preferred methods for the fabrication and analysis of etched structures in accordance with the invention. It is to be understood, however, that these examples are by way of illustration only and that nothing therein should be taken as a limitation upon the overall scope of the invention.

Materials.

PEDOT:PSS (see Fig. 1, where m is 0, and R and R' are both hydrogen) was obtained from H.C. Starck as an aqueous dispersion (Baytron-P). The solution obtained was diluted 1 :3 (by volume) with methanol to aid in film formation. As received, the dispersion was ~ 1.3% by weight PEDOT:PSS with a PEDOT:PSS weight ratio of 1 :2.5. Aqueous solutions of PEDOTrPSS for use in spectroscopic experiments were prepared by further diluting the methanol containing dispersions 10:1 in deionized water. Glass coverslips (I" x 1 "; 130-170 μm thick; greater tlian 90% transmission from 355 nm to greater than 1100 nm; Fisherfinest Premium) were used as substrates for all film samples. Prior to use, all such substrates were cleaned in a Harrick plasma cleaner (air plasma) for 5 min.

Preparation of Polymer Films.

PEDOT:PSS films were prepared by spin-coating (at 2000 rpm) directly from the solution described above. The films were subsequently cured at 110 ⁰C in an oven for ~ 20 min. Film thickness in each case was measured by removing a portion of the film using a razor blade and imaging the resulting film edge by AFM. Film thicknesses obtained in this manner where found to average 86 ± 5 nm for PEDOT:PSS films prepared as described above.

Direct-Write Multiphoton Photolithography.

The experimental setup used for direct-write multiphoton photolithography has been described in previous publications,¹⁵'¹⁷'¹⁸ and such experimental setup descriptions are incorporated by reference herein.

More specifically, and referring to Fig. 2, the photolithography apparatus 20 includes a moveable stage 22 adapted to hold a film sample 24 comprising substrate 24a and polymer film 24b, as well as an inverted confocal microscope 26 below stage 22. The stage 22 is shiftable via a closed-loop piezoelectric driver 28 controlled by a digital processor and computer 30, the latter being loaded with control software. The microscope 26 includes an oil-immersed IOOX objective lens 32 supported on a closed-loop piezoelectric objective mount (Physik Instrumente), serving as a focus scanner 33, a mode-locked titanium:sapphire laser 34, optical modulator 36 and dicliroic mirror beam splitter 38. A detector 40 is also provided as shown. The computer 30 is also operably coupled with the focus scanner 33, modulator 36 and detector 40.

The apparatus 20 was capable of etching the sample 24 using 170 femtosecond (fs) pulses of light at 870 nm at a repetition rate of 76 MHz. The objective lens 32 had a high numerical aperture (NA=I.3) and was employed to achieve a diffraction-limited focused spot of approximately 550 nm 1/e² diameter on the sample 24, thereby obtaining the intensities necessary for etching of polymer film 24b, and the spot size required to achieve high resolution. The optical modulator 36 included conventional polarization optics to control the incident power, an electronic shutter to control irradiation of the sample 24, and a 630 nm longpass filter to block light in the UV and visible regions.

Average incident laser powers given below are estimates of the power at the objective focus, and were measured outside the microscope 26, just prior to the dichroic beam splitter 38. Etching of binary patterns was accomplished by raster scanning the polymer film over the focused laser spot (pixel size: 100 nm; pixel time; 40 ms; scan rate: 2.5 μm/s). Under the conditions employed, 1 m W of incident power corresponds to an average intensity at the laser focus of ~ 4x10⁵ W/cm² and individual sample regions are exposed for a total time of ~ 1 s during raster scanning. In most such experiments, the laser focus was determined as described below. Irradiation of the sample was controlled by a pattern fed into the microscope control software that controlled sample scanning and the state of the electronic shutter.

Etching of grayscale patterns was accomplished by modulating the laser focus in a controlled fashion. In all such experiments, the laser focus was initially determined by detecting fluorescence excited in the sample as the focus position was scanned. Grayscale patterns with 64 bit resolution were then produced by varying the laser focus, using the mathematical model described below. All power-dependent and focus-dependent experiments were perfoπned at least three times, and were observed to yield similar results within experimental and curve fitting errors. Fig. 18 sets forth the preferred software control algorithm 41 for the apparatus 20.

The first overall step 42 involves location of optimum focus, followed by the second general step 44 of etching a grayscale pattern on film 24b.

In the focus optimization step 42, the sample 24 is positioned on stage 22, and a starting focus search range is inputted to the software (step 46). Next, the objective 32 is shifted (step 48) using focus scanner 33, and query is made of whether the focus is within the range(step 50). If yes, the fluorescence is determined and recorded (step 52), and the stage 32 is again moved (step 48). This subroutine is continued until the query of step 50 determines that the focus is no longer in range, whereupon the software fits the fluorescence as a function of focus position and determines the zero point of the range (step 54). Next, the desired predetermined grayscale pattern for film 24b etching is loaded in the computer 30 (step 56), and the appropriate objective positions are calculated (step 58). At this point the overall step 44 is carried out, by a subroutine involving movement of the stage 22 in the X, Y plane (step 60), movement of objective 32 using focus scanner 33 (step 62), and operation of laser 34 in order to transmissively ablate film 24b through substrate 24a (step 64). After each ablation step, a query is made to determine if the stage 22 remains in the range to be patterned (step 66). If Yes, the subroutine continues until the step 66 query provides a No answer, which establishes that the film 24b has been fully grayscale-etched and the process is complete.

Atomic Force Microscopy Measurements.

Detailed characterization of the surface topography of both etched and unetched samples was performed using contact-mode atomic force microscopy (AFM) in air. A Digital Instruments Multimode AFM with Nanoscope IFIa control electronics was employed. RMS surface roughness values were directly determined from the topographic images obtained.

These images were recorded using pyramidal Si₃N₄ probes (Model NP) obtained from Veeco

Metrology. The probes employed have a nominal tip radius of curvature of 20 nrn and a pyramidal half-angle at the tip apex of 35°. RMS roughness was obtained from the etched regions by two methods. In both, RMS roughness in each region was determined as defined in the following equation

(D where t_ave represents the average topographic height in the selected region and t; represents the topographic height at each of the N pixels within that region, hi the case of squares etched at fixed focus or fixed power (i.e., Figs. 4 and 9) the RMS roughness was determined from approximately the full etched area in each case. The mean RMS roughness and standard deviation derived from the data in Fig. 4 was determined from eight different regions etched at intermediate powers. The mean RMS roughness values reported for the data in Figs. 9 and 11 were obtained from three replicate experiments, performed at the same power and focus positions. The roughness values for the pyramids and spiral ramp were obtained by extracting at least three line profiles from each, line fitting these data, and calculating the RMS roughness after leveling by subtraction of the line. Etching Mechanism ofPEDOT:PSS Films. Etching of PEDOT:PSS was readily accomplished by focusing only a few milliwatts of pulsed near IR light from the Ti:sapphire laser into the film. Fig. 3 demonstrates that arbitrary patterns can readily be produced. PEDOT:PSS etching is believed to involve multiphoton-induced depolymerization and vaporization of the polymer fragments from the film surface. In order to determine the effective order of the nonlinear process(es) involved in PEDOT:PSS etching, the dependence of film etch depth on incident laser power was determined. In these experiments, a series of 5 x 5 μm" regions were etched (at fixed focus) using different laser powers. Fig. 4 presents representative AFM data from such experiments, while Fig. 5 shows a line profile taken across these data. The etch depth at each power was determined by measuring the change in film height for each etched region. Fig. 6 plots the results obtained from the data in Fig. 4. As is readily apparent from these data, etching of the film begins at very low incident powers (< 1 mW or average intensities of < 4x10⁵ W/cm²), indicative of etching by a low order process. However, no etching was observed when the laser was operated in continuous wave mode (up to 10 mW), proving that pulsed laser light is required to etch the polymer.

As previously reported,¹⁵ fitting of the power dependent etch depth data to a simple kinetic model can be used to determine the order of the nonlinear optical process(es) involved in etching. The data are specifically fit to the following equation:

D(τ,P)-D(0)lϊ-expAkτP^π)l (2)

Here, D(O) is the unetched firm thickness and D(τ,P) is the thickness of a region etched for a time τ at a given photon flux, P. The parameter k is a kinetic rate constant associated with the removal of the polymer and n is the order of the nonlinear process involved in etching. Fitting of the data shown in Fig. 6 (depicted by the solid line plotted through the experimental data) yields n = 1.5 ± 0.1, indicating PEDOT:PSS is likely etched by a combination of linear and low order nonlinear processes when 870 nm light is employed. The k value obtained (0.08) is difficult to determine with reasonable precision, and therefore is not easily interpreted, as noted previously.¹⁵

The etching of PEDOT:PSS by low order nonlinear optical processes is understandable, given its absoiption spectrum in the visible and near-IR. Fig. 7 (solid line) plots the absorption spectrum obtained from a dilute aqueous PEDOT:PSS solution. As is characteristic of the conducting form of PEDOT:PSS, the absorption spectrum is broad and ddeevvooiidd ooff rreessoollvveedd ffeeaattuurreess iinn tthhee vviissiibbllee aanndd nneeaarr--IIRR rreeggii<ons.²³ The rise in the absorption spectrum in the UV is due primarily to the presence of PSS.

Fluorescence-Based Positioning of Laser Focus - General Step 42 of Control Algorithm.

Reproducible etching of patterns into polymer films by laser ablation methods requires precise positioning of the laser focus within the film, This was accomplished in these experiments by collecting and detecting the weak blue fluorescence emitted by PEDOT.'PSS films following multiphoton excitation. Fig. 7 (dashed line) plots the fluorescence spectrum obtained from a dilute PEDOTiPSS solution. The fluorescence spectrum shown was excited at 210 nm and was found to be insensitive to excitation wavelength up to at least 250 nm. Fluorescence from the PEDOT:PSS solution is peaked near 375 nm and extends well into the visible.

Fluorescence in this same spectral range was obtained by multiphoton excitation of the polymer during film etcl^ήng. Fig. 8 plots the fluorescence obtained at low laser power (0.75 mW) as a function of focus position. Maximum fluorescence is obtained with the laser focused in the center of the film (defined as 0.0 μm in Fig. 8). These data exhibit the Lorentzian functional form expected for polymer films that are much thinner than the depth of focus of the objective lens. However, the focus dependence of the fluorescence peak is wider than expected, even for a linear excitation process (the expected curve is plotted as a dashed line in Fig. 8). Deviation from theory is partly due to the etching and bleaching of the polymer that occurs even at very low incident powers (i.e., 0.75 mW). Since bleaching and etching are most efficient at the focus, the fluorescence peak is somewhat suppressed, leading to broadening of the curve. The observed broadening may also reflect imperfections in the optical system. Nevertheless, the data shown allow for the optimum focus position to be determined very precisely (i.e., to within ±8 nm, as deteπnined from the curve fit).

Etching of Grayscale Patterns in PEDOT: PSS Films - General Step 44 of Control Algorithm.

Etching of grayscale structures in PEDOT:PSS films was readily accomplished by varying the power of the incident 870 nm light, as evidenced by the data shown in Figs. 4-6.

While complete removal of the film requires average incident powers greater than 10 mW

(see Fig. 6), this film could be etched to ~ 50% of its depth using 5 mW and ~ 20% of its depth at 2 raW. hi contrast to previous results using UV-absorbiπg polymers,¹⁵ partially etched PEDOT:PSS films are remarkably smoofh. From regions etched at powers between 4 and 9 inW (Fig. 4), mean RMS roughnesses of 1 ± 1 nm were obtained. This roughness corresponds favorably to the roughness of the original unetched films (1.0 ± 0.1 nm RMS). It also suggests that grayscale patterns can be produced by ablative multiphoton methods with remarkable precision in etch depth. However, as noted in previous work,¹⁵ the lateral resolution afforded by polymers that absorb in the visible is somewhat reduced compared to those that absorb in the UV. Lateral resolution is defined in this case as the slope of the edge for etched regions (i.e. the etch depth divided by the lateral distance from fully etched to unetched polymer).¹⁵ A mean value of 0.13 ± 0.03 was obtained from several measurements of the slope at incident powers ranging from 7-30 mW, yielding resolution consistent with our previous results on other polythiophene films.¹⁵

The instrumentation employed did not allow for rapid modulation of the incident laser power for the purposes of grayscale patterning. In the present study, grayscale structures were instead prepared by modulating the laser focus. Laser focus modulation provides an effective means to vary the incident energy flux and hence, the etch depth. hi all grayscale patterning studies, the optimum initial focus of the laser was determined by finding the peak in film fluorescence, as described above. Variation of the incident photon flux and etch rate was then accomplished by moving the focus position off the optimum focus in a controlled fashion. The dependence of etching on the focal position can be predicted using Eqn. 2, along with the experimental parameters determined by fitting the power-dependent etch depth data depicted in Fig. 6. In the case of focus-dependent etching, the photon flux, P, becomes P(z) and is dependent on the focus position (z). P(z), follows a Lorentzian profile:^"

where wo = 0.41 /NA is the beam waist (i.e., the 1/e² radius), λ is the wavelength of the incident light, NA is the numerical aperture of the microscope objective and P_raax is the maximum energy flux when the laser is focused within the film. To test the dependence of etch depth on the laser focus, 5 μm square regions were etched into PEDOT:PSS films at constant average power (20 mW) by varying the laser focus position using a computer-controlled piezo-eleetric objective mount. Fig. 9 depicts the results obtained. Fig. 10 plots a line profile taken from these data. As expected, the etch depth clearly varies with focus position. The lateral etching resolution also depends on focus position, with the edges of etched regions yielding slopes ranging from 0.13 ± 0.03 to 0.005 ± 0.004 for focused and defocused incident light. The average etch depth in each region was determined from Fig. 9; the values obtained are plotted as a function of focus position in Fig. 11. The error bars shown depict the RMS roughness measured in each etched region. The solid line plotted through the data in Fig. 11 is not a fit to the data shown. Rather, it depicts the normalized etch depth expected based on the model described by Eqn. 2 and 3, using n = 1.5 and k = 0.08. The experimental etching data follow the predicted trend reasonably well, indicating that the dominant etching process is explained well by this model, which was subsequently used to position the laser focus for each gray level during etching at powers of 20 mW or higher. At lower powers, the etching mechanism is slightly different, and likely involves greater contributions from photothermal processes. A broader focus- dependent etch depth curve is obtained under low power conditions and is used to properly position the laser focus during grayscale patterning in these situations.

Fig. 12 depicts a grayscale template (a pyramid) used to create pyramidal features in PEDOT:PSS films. Figs. 13-15 depict AFM images of grayscale assembly A having etched regions R obtained from pyramids etched at 5 mW, 20 mW and 30 mW. In all three cases, the AFM images clearly show pyramidal structures that closely approximate the template. Fig. 16 plots line profiles taken horizontally across central portions of the pyramids shown in Figs. 13-15. These line profiles are plotted on the same scale to depict differences in the etch depths. The pyramids etched at 20 and 30 mW encompass approximately the full depth of the film with the outer portions extending all the way to the underlying glass surface. These pyramids exhibit somewhat greater topographic roughness than does the pyramid etched at 5 mW, due to the presence of etching debris in the former. Such debris is routinely observed when films are etched at powers near or just above saturation (where almost all of the film has been removed) and is attributed here to a preference for etching of PEDOT over PSS.

Nevertheless, the pyramid etched at 5 mW clearly demonstrates that three- dimensional surface relief structures can be prepared with remarkable precision in the etch depth for materials etched to a fraction of their full thickness. The RMS roughness of the sloped portions of the pyramid shown in Fig. 13 is 1.0 ± 0.8 run, indicating that ~ 20 nm tall structures can be etched with ~ 1 nm precision. The maximum feature height that can be prepared by this method has not yet been determined. As a final demonstration, Fig. 17 depicts a grayscale assembly A having a spiral ramp etched region R into a PEDOT:PSS layer at 20 nϊW incident power. The film height within this latter structure varies linearly over a height range of 45 nm. Significantly less debris is formed because the laser was moved off the optimal focus relatively rapidly during etching of this pattern. As a result, 2 ± 2 nm RMS roughness is observed within the etched region R.

More generally, the grayscale etched assemblies of the invention preferably exhibit a maximum RMS roughness value of up to about 5 nm, across the etched regions having areas of different, predetermined polymer thicknesses. Roughnesses of about 0.5-3 nm in etched regions are readily obtained under more optimum conditions, as demonstrated above.

The preferred laser ablation method of the invention is transmissive ablation, meaning that the incident ablating laser beam passes through the substrate 24a in order to ablate the polymer layer 24b. Such transmissive ablation largely eliminates the problems heretofore encountered in reflective laser ablation, and especially the ejection of "plumes" of ablated material from the polymer films, which can interfere with laser ablation and lead to imprecise binary and grayscale etching. Transmissive ablation requires that the substrate 24a be at least partially transparent at the wavelengths of the incident laser beams. Generally, the substrate 24a should be capable of transmitting at least 50%, more preferably from about 60-90%, of the incident laser beam. Typically, etch resolution is inversely related to the thickness of the substrate. Therefore, in preferred forms the substrate should be up to about 200 microns, more preferably from about 130-170 microns in thickness. Likewise, the thickness of the polymer layer can have an effect on the quality of the final etch, and thus the polymer layer should have a thickness of up to about 1 micron, more preferably from about 20 nm-1 micron.

As described above, one technique of grayscale laser ablation involves modulation of the focus of the incident laser beam across the etched region, hi the example, such focus modulation was achieved using the focus scanner 33 to move the microscope objective. The same effect could be achieved by modulating the sample position relative to a fixed objective, using a three-dimensional (X, Y, Z) sample positioning stage. Those skilled in the art will appreciate mat other methods may also be used to obtain grayscale patterns, such as by modulation of the incident laser power, while maintaining a constant laser focus in the sample. Laser power modulation can readily be achieved by use of eleotrooptic or acoustooptic modulators, for example. Additionally, it would be possible to combine these two techniques, i.e., by altering both incident laser power and focus during etching.

Patterning of the polymer films as described above involved scanning the sample in the X and Y directions, while modulating the laser focus. Sample patterning can also be accomplished by instead holding the sample fixed while scanning the laser beam position in

X and Y. This may be accomplished using mirrors mounted to computer-controlled galvanometers, as is routinely done in beam-scanning optical microscopic methods. Such methods have the potential to dramatically increase in the rate of polymer film patterning, allowing for much more rapid fabrication of structures. Laser focus and laser power modulation methods described above both work with beam-scanning methods to produce grayscale patterns.

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Claims

We claim:

1. A grayscale etched assembly, comprising: a self-supporting substrate; and a layer of polymer supported on said substrate, said polymer layer having an etched region presenting areas of different, predetermined thicknesses, said region having a maximum RMS roughness of up to about 5 nm.

2. The assembly of claim 1, said polymer layer being laser ablated to provide said different thickness areas.

3. The assembly of claim 1, said substrate being at least partially light transmissible to near-infrared light.

4. The assembly of claim 3, said substrate being at least 50% light transmissible to near-infrared light.

5. The assembly of claim 1, said substrate having a thickness of up to about 200 microns.

6. The assembly of claim 1, said polymeric layer having a thickness of up to about 1 micron.

7. The assembly of claim 1, said polymeric layer formed of a poly(alkylene dioxythiophene):poly(styrene sulfonate) mixture.

8. The assembly of claim 7, wherein said layer is formed of PEDOT:PSS.

9. The assembly of claim 8, wherein said PEDOT:PSS has a PEDOT to PSS ratio of from about 1 :0.5 to 1 :5.

10. The assembly of claim 1, said roughness is from about 0.5-3 nm.

11. A grayscale etched assembly, comprising: a self-supporting substrate; and a layer of polymer supported on said substrate, said polymer formed of a poly(alkylene dioxythiophene):poly(styrene sulfonate) mixture, and said polymer layer having areas of different, predetermined thicknesses at respective areas of the layer.

12. The assembly of claim 11, said polymer layer being laser ablated to provide said different thickness areas.

13. The assembly of claim 11, said substrate being at least partially light transmissible to near-infrared light.

14. The assembly of claim 13, said substrate being at least 50% light transmissible to near-infrared light.

15. The assembly of claim 11, said substrate having a thickness of up to about 200 microns.

16. The assembly of claim 11, said polymeric layer having a thickness of up to about 1 micron.

17. The assembly of claim 11, wherein said layer is formed of PEDOT:PSS.

18. The assembly of claim 17, wherein said PEDOT:PSS has a PEDOT to PSS ratio of from about 1 :0.5 to 1:5.

19. The assembly of claim 11, said polymer being of the type depicted in Fig. 1.

20. A method of fabricating a grayscale etched assembly, comprising the steps of: providing a sample including a self-supporting substrate with a layer of polymer supported on the substrate; laser ablating said polymer layer to form an etched region presenting areas of different, predetermined thicknesses, said region having a maximum RMS roughness of up to about 5 nm.

21. The method of claim 20, said laser ablating step being a transmissive ablation by passage of incident laser beams through said substrate in order to ablate said polymer layer.

22. The method of claim 21, said substrate being at least partially light transmissible at the wavelengths of said incident laser beams.

23. The method of claim 22, said substrate being at least 50% light transmissible at the wavelengths of said incident laser beams,

24. The method of claim 20, said substrate having a thickness of up to about 200 microns.

25. The method of claim 20, said polymeric layer having a thickness of up to about 1 micron.

26. The method of claim 20, said polymeric layer formed of a poly(alkylene dioxythiophene):poly(styrene sulfonate) mixture.

27. The method of claim 26, wherein said layer is formed of PEDOT:PSS.

28. The method of claim 27, wherein said PEDOTrPSS has a PEDOT to

PSS ratio of from about 1:0.5 to 1:5.

29. The method of claim 20, said roughness is from about 0.5-3 nm.

30. The method of claim 20, said laser ablating step comprising the step of modulating the intensity of incident laser beams at said areas.

31. The method of claim 30, said intensity modulating step being carried out by (a) modulating the focus of said incident laser beams, (b) modulating the power of said incident laser beams, or combinations of (a) and (b).

32. The method of claim 31 , said intensity modulating step comprising the step of modulating the focus of said incident laser beams.

33. The method of claim 32, said focus modulating step comprising the step of moving said sample during said laser ablation.

34. The method of claim 20, said polymer being of the type depicted in

Fig. 1.

35. A method of fabricating a grayscale etched assembly, comprising the steps of: providing a sample including a self-supporting substrate with a layer of polymer supported on the substrate, said polymer formed of a poly(alkylene dioxythiophene):poly(styrene sulfonate) mixture; and laser ablating said polymer layer to form areas of different, predetermined thicknesses at respective areas of the layer.

36. The method of claim 35, said laser ablating step being a transmissive ablation by passage of incident laser beams through said substrate in order to ablate said polymer layer.

37. The method of claim 36, said substrate being at least partially light transmissible at the wavelengths of said incident laser beams.

38. The method of claim 37, said substrate being at least 50% light transmissible at the wavelengths of said incident laser beams.

39. The method of claim 35, said substrate having a thickness of up to about 200 microns.

40. The method of claim 35, said polymeric layer having a thickness of up to about 1 micron.

41. The method of claim 35, wherein said layer is formed of PEDOT:PSS.

42. The method of claim 41, wherein said PEDOT:PSS has a PEDOT to PSS ratio of from about 1 :0.5 to 1:5.

43. The method of claim 35, said laser ablating step comprising the step of modulating the intensity of incident laser beams at said areas.

44. The method of claim 43, said intensity modulating step being carried out by (a) modulating the focus of said incident laser beams, (b) modulating the power of said incident laser beams, or combinations of (a) and (b).

45, The method of claim 44, said intensity modulating step comprising the step of modulating the focus of said incident laser beams.

46. The method of claim 45, said focus modulating step comprising the step of moving said sample during said laser ablation.

47. The method of claim 35, said polymer being of the type depicted in Fig. 1.

48. A grayscale etched assembly, comprising: a self-supporting substrate; and a layer of polymer supported on said substrate, said polymer absorbing radiation of from about 400-1100 nm with a peak absorbance of from about 600-900 nm, and forming a continuous film on a substrate with a roughness value up to about 10% of the thickness of the continuous film; and said polymer layer having areas of different, predetermined thicknesses at respective areas of the layer.

49. The assembly of claim 48, said polymer film having a roughness value of up to about 5 nm.

50. The assembly of claim 48, said polymer being of the type depicted in Fig. 1

51. The assembly of claim 48, said polymer layer being laser ablated to provide said different thickness areas.

52. The assembly of claim 48, said substrate being at least partially light transmissible to near-infrared light.

53. The assembly of claim 52, said substrate being at least 50% light transmissible to near-infrared light.

54. The assembly of claim 48, said substrate having a thickness of up to about 200 microns.

55. The assembly of claim 48, said polymeric layer having a thickness of up to about 1 micron.

56. A method of fabricating a grayscale etched assembly, comprising the steps of: providing a sample including a self-supporting substrate with a layer of polymer supported on the substrate, said polymer absorbing radiation of from about 400-1100 nm with a peak absorbance of from about 600-900 nm, and forming a continuous film on a substrate with a roughness value up to about 10% of the thickness of the continuous film; and laser ablating said polymer layer to form areas of different, predetermined tiiicknesses at respective areas of the layer.

57. The method of claim 56, said polymer film having a roughness value of up to about 5 nm.

58. The method of claim 56, said polymer being of the type depicted in Fig. 1

59. The method of claim 56, said polymer layer being laser ablated to provide said different thickness areas.

60. The method of claim 56, said substrate being at least partially light transmissible to near-infrared light.

61. The method of claim 60, said substrate being at least 50% light transmissible to near-infrared light.

62. The method of claim 56, said substrate having a thickness of up to about 200 microns.

63. The method of claim 56, said polymeric layer having a thickness of up to about 1 micron.