Precise control of the arrangement and positioning ofmolecules is important in the fabrication of manybirefringent and dichroic optical devices such as dis-plays, filters, and detectors. The uniform alignmentof elongated molecules on a planar surface has beenachieved by usage of a predefined substrate [1,2], anexternal magnetic field [3,4], an external electricfield [5], plasma treatment [6], ion beams [7], or sur-face acoustic waves [8]. In particular, the uniformalignment of liquid crystals, such as those in a dis-play, has traditionally relied on mechanically buffinga film, such as polyimide [9,10]. However this processhas inherent disadvantages that include surface de-fects [11], particle generation, and electrostatic char-ging [12], which all detract from production yields.Second, the mechanical process is only capable ofuniform alignment of an entire surface. For manydisplay and nondisplay applications, it is necessaryto generate arbitrary patterns of aligned moleculesinstead of a uniform film.In this paper, we demonstrate and investigate the
resolution of a new technique to create such a filmusing linearly polarized ultraviolet (LPUV) light.
Linear photopolymerizable polymers (LPPs) are anew class of materials that can align LCs photo-chemically [13–20]. Multiple methods of LPUV-induced surface alignment have been reported usingazo dye doped polymer [21], polyimide [22], and cin-namoyl or coumarin side-chain polymers [23]. Forthis experiment the LPP layer was coated with a di-chroic liquid crystal polymer (LCP). The LCP layeradopts the alignment of the LPP layer, can be coatedto provide an arbitrary amount of retardance, and iscured to form a humidity and temperature stabilizedfilm. This LPP/LCP system can be used to generatehigh resolution patterned retarders, polarizationconverters, and interference filters [24].
Additionally, we investigate the combination ofpatterned LC and an opaque layer, such as chrome,on the same substrate. LC features were added to apreexisting chrome mask. By mixing chrome and LCfeatures, multiple images can be defined on a singlemask, which are dependent on the polarization of theillumination source. This type of hybrid mask wasdemonstrated for use as a variable pitch grating oras a novel double-patterning mask.
2. Recording and Viewing Linear PhotopolymizerablePolymer/Liquid Crystal Polymer Images
Small domains of LCP were patterned using contactlithography with a prefabricated chrome mask. An
arbitrary number of exposures with nonoverlappingmasks can be performed on each substrate. The dia-gram in Fig. 1a shows the pattern transfer processfrom the chrome mask to the LPP layer. A chromecontact mask is applied to a LPP substrate and ex-posed with LPUV light. The mask is then removed,the substrate is rotated 45°, and the substrate isexposed.The LCP layer is applied at a thickness that pro-
vides 180° of retardance, equal to that of a half-waveplate. Half-waveplates rotate linear polariza-tion 0° or 90° when the incident polarization is 0°or 45° to its fast axis, respectively. In the imagingsystem shown in Fig. 1b, linearly polarized light in-cident on the 0° or 45° zones of the patterned half-waveplate will be blocked or transmitted by the ana-lyzer, respectively. The ideal transmittance of awaveplate between crossed polarizers, as determinedby the reduced system Mueller matrix, is
IðθÞ ¼ I0sin2ð2θÞ; ð1Þ
where θ is the angle between the incoming linearlypolarized light and the waveplate.
3. Experimental Methods
A. Patterned Waveplate Fabrication
The LPP material (ROP-103) and polymer LC mate-rial (ROF-5102) are supplied by Rolic Technologies(Switzerland). A custom exposure system was devel-oped to align and cure the ROP-102 and ROF-5102,respectively. A Hamamatsu LC5 UV light source iscollimated and filtered (passband of 280nm to350nm) and then linearly polarized by passingthrough a dichroic UV polarizer manufactured byBoulder Vision Optics (Colorado, USA). The sampleis placed on a rotational stage so that it can be ex-posed with an arbitrary direction of polarized light.The beam intensity at the stage is 12 μW=cm2. Expo-sure times vary based on the number of alignmentdirections and the substrate reflectivity. The processfor coating and alignment of ROP-103 is (1) spincoating at 2500RPM for 60 s, (2) a 5 min bake stepat 175 °C to evaporate the residual solvent, and(3) alignment exposure(s) using the described align-ment system. The resulting film thickness is ap-proximately 50nm and is of negligible retardance.Patterned alignment is achieved by adding a contactmask during the first exposure. The mask is then re-moved, the stage/substrate is rotated 90°, and a sec-ond exposure is performed. For our experiment a1951 USAF resolution target was used for semi-isolated features. A variable spatial frequency targetwas used for dense features.
ROF-5102 is applied at the desired thickness forhalf-waveplate operation at the desired wavelength.Figure 2a shows ROF-5102 thickness and retardanceat 532nm as a function of spin speed. The processused was (1) spin coating at 850 rpm for 2 min,(2) annealing at 52 °C in an oven for 3 min, and(3) broadband 50mWUV cure for 5 min in a nitrogenatmosphere.
Optimum transmittance through the analyzer oc-curs when the incident polarization is rotated a full90° by the LCP half-waveplate therefore the LCPcoating speed was optimized to provide 180° of retar-dance. The material system was characterized bymeasuring retardance and film thickness of the LCPat a variety of spin speeds. Retardance wasmeasuredusing an Axolite Muller matrix polarimeter with a2mm beam diameter. Film thickness was measuredwith a Veeco Wyco NT9800 white light interfer-ometer. Figure 2a shows both retardance and filmthickness at five spin speeds. For half-waveplate op-eration at 532nm, ROF-5102 is coated at 850 rpmand results in a film thickness of 2:2 μm. At this spinspeed the film retardance is controlled to within�5%across the radius of the 1.5 in. wafers used.
Figure 2b shows transmission data for the sub-strate, ROP-103, and a ROP-103/ROF-5102 filmstack. Surface reflection accounts for about 8% ofthe total losses, showing that the LC films are almostcompletely transparent from 350nm to 2500nm.Between 300nm and 350nm the materials absorbheavily, and below 300nm they are entirely opaque.
Fig. 1. (Color online) Recording and viewing of a patterned wa-veplate using linearly polarized light. a, Patterned 0° features arefirst exposed and then an unmasked 45° exposure is performed.The direction of the lines corresponds to the alignment directionof the LC. b, LCP layer is coated to half-waveplate thickness andadopts the alignment of the LPP layer. When linearly polarizedlight is incident on the LCP layer at 0° to the director it has norotational effect, while at 45° the LCP layer rotates the polariza-tion 90°. Therefore maximum transmission occurs in the LPP zonealigned at 45° [18,38].
In order to reconstruct the image in a pattern wave-plate, it must be placed between crossed polarizers. A10mW 532nm diode laser is spatially filtered using a10× objective and a 20 μm pinhole. Following thespatial filter the beam line elements include a(1) 100mm collimation lens, (2) first linear polarizer,(3) patterned waveplate mounted to 3-axis stage,(4) second linear polarizer, (5) 1 in. 0.50 NA aspheri-cal objective lens from Edmond optics, and (6) a 5Mpixel TCA complimentary metal oxide semiconduc-tor CCD. Images were captured using TSView soft-ware. Image analysis and cross-sectional intensityprofiles were done using Gwyddion [25]. The visibi-lity of a feature was quantitatively determined byusing Gwyddion to take an intensity cross sectionof a feature set. The average intensity of the crosssection was set as the threshold intensity value.Everything above or below the threshold value con-tributed to the intensity maximum (Imax) or mini-mum (Imin), respectively. The visibility (V) of thefeature was then determined by Eq. (2):
V ¼ Imax − Imin
Imax þ Imin: ð2Þ
4. Linear Photopolymizerable Polymer/Liquid CrystalPolymer Imaging Results
A. Semi-Isolated Feature Comparison
The resolution capability of the LPP/LC materialwas analyzed by comparing the resolution capabilityof patterned LCP waveplates to the original chromemask. The 532nm imaging system is fully describedin Section 3. Figure 3a shows two magnifications ofthe chrome mask. Figure 3b shows the recordedimages of a patterned waveplate rotated 45° to theincident linear polarization. Here the dark back-ground corresponds to LC orientation of 45°, and thebright feature corresponds to LC orientation of 0°.Figure 3c shows the transmittance of the two LCorientations as a function of the angle between the
incident polarization and the primary axis of thewaveplate. When the waveplate is at 45° to the inci-dent linear polarization, the 0° LC will rotate thelinear polarization 90° and the 45° LC will not ro-tate the polarization. Therefore, viewed through acrossed polarizer, there will be perfect contrast be-tween light that passed through 0° and 45° LC.
Figure 3d shows the transmittance of 0°, 30°, and60° LC orientations as the waveplate is rotatedbetween the polarizers. This configuration is demon-strated in Fig. 3e, with two patterns aligned at 0° and30° and the background at 60°. At the peak trans-mittance of the 0° pattern, the 30° pattern and back-ground (60°) both have a transmittance of 25%. Thislimits the maximum visibility to 0.6. Figure 3f showsthe background and second pattern at equal trans-mittance; however, the discontinuity between LC re-gimes causes the pattern to be clearly outlined. Thetransition between regimes is measured to be ap-proximately 1 μm.
Figure 3(g) shows an intensity cross section for themarked features in Figs. 3a, 3b, and 3e. Cutline datafor each feature were used to calculate image visibi-lity. Figure 3h shows visibility as a function of featuresize. The gray bar marks the visibility of the featuresshown in Fig. 3g.
Although the single pattern waveplate has ideallyperfect contrast between LC domains, the featuresize becomes limited by the finite length of the tran-sition between LC orientations. As the transitionlength approaches half the size of the feature, the vis-ibility begins to drop dramatically. A visibility of 0.5is defined in this paper as the resolution limit. Theimaging system with a chrome mask, a 0°–45° singlepattern waveplate, and a 0°–30°–60° double patternwaveplate can image semi-isolated features as smallas 2:5 μm, 4 μm, and 14 μm, respectively.
B. Dense Feature Comparison
Feature visibility was examined for dense featuresusing the same process. Figures 4a and 4b show adense 2:9 μm half-pitch 1∶1 duty ratio feature set inchrome and in 0°–45° aligned LC. The variation inintensity in Fig. 4b is due to both slight variations
Fig. 2. (Color online) a, Spin speed versus both film thickness and retardance measurements were taken in order to determine coatingspeed for half-waveplate thickness. The error barsmark one standard deviation. b, A spectrometer was used to determine the transmissionprofiles of each material. Both Rolic materials absorb strongly below 350nm.
in the LC alignment and intensity variation in theillumination system. Figure 4c shows measured vis-ibility for each pitch of the variable grating. The graymarks the visibility of the images shown in Figs. 4aand 4b. The chrome pattern has a relatively flatvisibility of about 0.75. System flare, uneven illumi-nation, and diffraction limits are some of the contri-butors to loss of mask visibility. Compared to thesemi-isolated features, the flat visibility profile ofthe dense features can be attributed to the decreasein focus sensitivity for dense features compared tosemi-isolated features of the same size.In comparison with the chrome mask, the 0°–45°
aligned LC has a kinked profile that is relatively flatfor half-pitches greater than 5 μm and then drops offmuch faster. The LC features have a very similar pro-file to the semi-isolated features, with a maximumresolution again of roughly 4 μm. The resolutionagain appears to be limited by the finite length ofthe transition between LC orientations.
C. Mixed Chrome and Liquid Crystal Features
Combining chrome and LC features on a substrateallows two separate images to be defined with no
theoretical loss in visibility. In this experiment, wecoated LCP/LPP on top of patterned chrome onquartz. Using 0°–45° aligned LC, two images are de-fined with a transmittance between cross polarizersas shown in Fig. 5a. The complete image defined bythe chrome is shown in Fig. 5b and was taken withthe waveplate at 22:5°, marked b in Fig. 5a. The leftand right halves of the mask are aligned at 45° and0°, respectively, and therefore transmit equally at22:5°. The dark line in the middle of the image isdue to the transition region between LC orientations.When the LC/chrome mask is viewed at 0° or 45°,only half of the chrome masks pattern is visible asshown in Figs. 5c and 5d, respectively. A diagramof the assembled LC/chrome mask is shown inFig. 5e.
Figure 5f shows an example of the chrome/LCmask for double patterning. The dark features areopaque and are made of chrome. The dashed featuresare patterned LC and line directions denote the LCorientation. Current double-patterning methods de-compose a mask with a subresolution pitch into twomasks that have twice the effective pitch, thereforeallowing them to be printed. These two masks are
Fig. 3. (Color online) Visibility comparison of semi-isolated features between original chrome mask, a 0°–90° single pattern waveplate,and a 0°–60°–120° double pattern waveplate. a, Chromemask imaged onto a CCD. b, Patterned waveplate with a single pattern aligned at0° and the background at 45°. c, Plot shows transmittance of 0° and 45° LC as a function of the angle between the crossed polarizers and thewaveplate. The angle at which b was taken is marked. d, Plot shows transmittance of two patterns aligned at 0° and 30°, respectively. Thebackground LC is aligned at 60°. The angle at which e and f were recorded is marked. e, 0° pattern with a transmittance of 1 with thebackground aligned at 60° that has a transmittance of 0.25. f, Second pattern on the same substrate is aligned at 30° and has a trans-mittance equal to the background, which is aligned at 60°. g, Example cutline for the features marked (7-1) in a, b, and e. h, Measuredvisibility is shown as a function of half-pitch for the mask, single-image waveplate, and double-image waveplate.
then printed sequentially to recreate the original[26]. However, misalignment between the two expo-sures leads to negative effects such as nonlinearcritical dimension and overlay errors. The mixedchrome and LC mask shown in Fig. 5f defines twoseparate images in the same way demonstrated inFigs. 5a, 5b, 5c, 5d, and 5e. Therefore in a negativeresist system, the mask does not have to be changedbetween exposures, which can significantly increaselithographic tool throughput and decrease overlay er-ror. The main difficulty is that the LC and chrome
patterns must be aligned to a tolerance equal thechrome half-pitch during photoalignment of theLPP layer.
5. Discussion and Conclusion
In this paper, we demonstrate the ability to recordmultiple patterns on the same substrate using pat-terned LCP. Two important parameters investigatedwere the maximum resolution and the transmissionof the LPP/LCP material system.
Fig. 4. (Color online) Visibility comparison of dense features between a chrome mask and a 0°–90° patterned waveplate. a 2:9 μm half-pitch 1∶1 duty ratio chrome pattern imaged onto a CCD. b The same 2:9 μm pattern with LC in the bright areas aligned at 0° and the darkareas at 45°. The waveplate is imaged onto a CCD with its primary axis at 45° to the incident linear polarization. c Visibility data for avariety of half pitches. The gray bar marks the pitch shown in a and b.
Fig. 5. (Color online) Patterned LC coated on top of dense chrome features. The left and right sides of each photo are aligned at 45° and 0°,respectively. The 5 μmhalf-pitch features are defined by the chromemask. a, Transmittance of the two orientations of LC as the waveplateis rotated with respect to the incident polarization. Themarked locations show the angle at which b, c, and d were each taken at. b Both LCorientations transmit equally. c Only the 45° LC allows light to be transmitted. d Only the 0° LC allows light to be transmitted. e Diagramof current chrome/LC hybrid mask. f Diagram of next generation chrome/LC hybrid mask for double patterning. The dashed lines indicatethe LC alignment.
The maximum resolution of the LPP/LCP featureswas a half-pitch of 4 μm for both semi-isolated anddense features. Themaximum resolution was limitedby the LC reorientation length between alignmentzones, which was determined to be 1 μm in ourexperiments. As the feature size approaches the re-orientation length, the visibility of the feature goes tozero. Therefore, decreasing the reorientation lengthis critical to improving the maximum resolution ofthe system. The approximate limit of the reorienta-tion length is the persistence length of the polymer inthe material, defined as the distance over which thelocal direction of the molecules persist. It has pre-viously been shown that the persistence length canbe as short as 10–100nm depending on the lengthof the molecule, the thickness, and the temperature[27–29]. In our case, the exact value of the LCP/LPPreorientation length is limited by the resolution limitof the coarse contact lithography, which has a resolu-tion of roughly a micrometer [30]. By using a higherresolution lithography system, reduction of the re-orientation length to the persistence length of themolecule is possible, leading to submicrometer LPP/LCP features.The Rolic ROF-5102 LCP material was found to be
highly transmissive from 300 to 2500nm and is anexcellent candidate for near-UV, visible, and infraredapplication including patterned filters, polarizationcontrol, infrared communications, multidomain LCdisplays [31], rotational media memory density, andtransflective displays [32]. A variety of fluoropolymerbased LC has been shown to be transmissive down to200nm and could be substituted for the current LCPlayer [33–37]. This UV-LC can potentially be ex-tended to applications in deep UV projection litho-graphy where a single mask could replace multiplemasks in double-patterning technology.
This research is partially funded by the Technol-ogy Research Infrastructure Fund (TRIF) and theUnited States Air Force Office of Scientific Research(AFOSR) Multi-University Research Initiative(MURI) Program. The authors thank Prof. ThomasMilster’s and Prof. Nasser Peyghambarian’s researchgroups for allowing us to utilize their equipment. G.Myhre thanks J. Wohltmann and M. Jungwirth forhelpful comments on the manuscript.
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