Characterization of negative-type photoresists containing polyhedral oligomeric silsesquioxane methacrylate

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<ul><li><p>es</p><p>,*</p><p>n, 310aiw</p><p>POSS</p><p>hothacformicbotitio</p><p>locally and thus enhances the rate of photo-polymerization and sensitivity.</p><p>been wid crysand opill be</p><p>photosensitive resins have also attracted considerable interest,however, most prior studies have concentrated on the applicationsof positive-type photoresists [1317] and UV curable resins [1821], research on negative-type photoresist has been relatively rare[22,23].</p><p>group is utilized. Characterizations of this blending system of thegeneral negative-type photoresist were carried out by Fouriertransform infrared spectroscopy (FT-IR), 1H nuclear magnetic reso-nance spectroscopy (1H-NMR), gel permeation chromatography(GPC), differential scanning calorimetry (DSC) and ultraviolet/visi-ble spectroscopy (UV/vis). Photoresist performances were investi-gated in terms of sensitivity, contrast and pattern resolution. Thescanning electron microscope (SEM) was used to observe the mor-phology and line pattern of photoresists. The exotherms for photo-polymerization of photoresists were also investigated by usingphoto-differential scanning calorimetry (photo-DSC).</p><p>* Corresponding author. Tel./fax: +886 3 5131512.</p><p>Microelectronic Engineering 85 (2008) 16241628</p><p>Contents lists availab</p><p>Microelectronic</p><p>.eE-mail address: (F.-C. Chang).through new materials and new processes.Combining different materials, such as organic and inorganic</p><p>compounds, to obtain improved performance is an important trendfor developing new materials [8]. Polyhedral oligomeric sils-esquioxanes (POSS) contain inner inorganic cage structure withrigidity and high thermal stability. The outer surface of POSS mol-ecule can be substituted by certain organic groups which are sol-vent soluble or polymer compatible. POSS-containing polymersusually result in improved mechanical strength and thermal stabil-ity [9], better dimensional stability [10], high glass transition tem-perature [11], and low dielectric constant [12]. POSS-based</p><p>tive copolymer solution increases with the increase of POSS con-tent and thus causes handling difculty. The tendency of forminghydroxylsiloxane hydrogen bonding interaction within thePOSS-containing photosensitive copolymer attracts those methac-rylate groups close together surrounding POSS units but tends toretard molecular chain movement.</p><p>In this study, a blending method has been adopted to overcomethe viscosity problem. A series of negative-type photoresists areprepared by blending a general negative-type photoresist with var-ious POSS contents. POSS macromer of isobutylpropylmethacrylpolyhedral oligosilsesquioxane (MI-POSS) with one methacrylateNegative-type photoresistHydrogen bonding</p><p>1. Introduction</p><p>Negative-type photoresists havetronics [1,2], printing plates [3], liqufabrications [5], nanoimprints [6],novel negative-type photoresists w0167-9317/$ - see front matter 2008 Elsevier B.V. Adoi:10.1016/j.mee.2008.03.012 2008 Elsevier B.V. All rights reserved.</p><p>idely adopted in elec-tal displays [4], micro-tics [7]. In the future,continually developed</p><p>In our previous study [23], the POSS moiety was incorporatedinto negative-type photoresists by a copolymerization method.The POSS-containing photosensitive copolymer formulated nega-tive-type photoresist for UV-lithography is able to accelerate thephotopolymerization signicantly even with small amount ofPOSS. However, the viscosity of the POSS-containing photosensi-Keywords:Polyhedral oligomeric silsesquioxanes</p><p>polymerization rate and exothermic heat. Hydrogen bonding interaction between the hydroxyl of photo-resist and the siloxane of POSS tends to attract methacrylate double bonds surrounding POSS particlesCharacterization of negative-type photorsilsesquioxane methacrylate</p><p>Ho-May Lin a,b, Kuo-Hung Hseih c, Feng-Chih Chang a</p><p>aDepartment of Applied Chemistry, National Chiao Tung University, 300 Hsinchu, TaiwabMaterial and Chemical Research Laboratories, Industrial Technology Research Institutec Institute of Polymer Science and Engineering, National Taiwan University, 106 Taipei, T</p><p>a r t i c l e i n f o</p><p>Article history:Received 27 September 2007Received in revised form 8 January 2008Accepted 19 March 2008Available online 29 March 2008</p><p>a b s t r a c t</p><p>A series of negative-type psilsesquioxane (POSS) metcrystallize or aggregate tomodied photoresist is signDSC analyses indicate thatwith POSS macromer. Add</p><p>journal homepage: wwwll rights reserved.ists containing polyhedral oligomeric</p><p>Chutung, Taiwanan</p><p>oresists made by blending with various contents of polyhedral oligomericrylate were prepared and characterized. These POSS macromers tend totheir own domains within the photoresist matrix. Sensitivity of the POSS</p><p>antly improved with the increase of the POSS content. Results from photo-h induction time and peak maximum of heat ux are reduced by blendingn of the proper amount (</p></li><li><p>5 g of photosensitive copolymer, 2 g of TMPTA, 1 g of Irgacure 907</p><p>3. Results and discussion</p><p>Themethacryloyl-containingphotosensitive copolymerwaspre-pared by free radical copolymerization and glycidylcarboxyl reac-tion. The obtained Mw of the copolymer is 75,600 with apolydispersity ca. 2.32 (Table 1). The infrared characteristic peaksof the photosensitive copolymer are assigned as follows:1620 cm1 as the C=C stretch vibration, 1726 cm1 as the C=Ostretch vibration, 758 cm1 and 700 cm1 as the CH out-of-planebending vibration of the phenyl ring. The peak assignments of the1H-NMR spectrum for the photosensitive copolymer are: C=CH2protons at 5.68 and 6.12 ppm, COOCH3 protons at 3.60 ppm, andCC6H5 protons at 7.20 ppm. Table 1 summarizes compositionsand properties of the copolymer. The glass transition temperatureof the copolymer is 128.6 C.</p><p>Fig. 1 displays the UV/vis spectrum of the photosensitive copoly-mer lmwith 25 lm thickness on a quartz plate. The transmittanceof the copolymer is greater than 90% at the wavelength range of300800 nm as shown in Fig. 1 and Table 1. This copolymer showsacceptable transmittance.</p><p>OHO</p><p>OO</p><p>O</p><p>OH</p><p>O</p><p>OO</p><p>OOm n o p q</p><p>photosensitive copolymer </p><p>O</p><p>O</p><p>O</p><p>OO</p><p>Si</p><p>OO</p><p>SiO</p><p>Si SiOO OR</p><p>R</p><p>R</p><p>Engineering 85 (2008) 16241628 1625and 0.25 g of ITX in 5 g of DEGME. MI-POSS of 0 g, 0.05 g, 0.25 g,0.5 g, 1.25 g, or 2.5 g was added to the general photoresist andcoded from photoresist 1 to photoresist 6. The photoresist wasspin-coated onto 10 10 cm2 copper clad laminate and the solventwas removed at 80 C for 30 min. The lm was exposed to broad-band UV radiation with a collimated exposure (Tamarack 161-OAcontaining a 5 kWmercury short arc lamp and an internal UV inte-grator to control exposure dose), developed by dipping in a 1 wt%Na2CO3 aqueous solution at 25 C for 3 min, and rinsed by water.The image of the photoresist was obtained by a Hitachi S-4200scanning electron microscope. The characteristic curve was deter-mined by normalizing lm thickness against exposure dose in log-arithmic scale. The photopolymerization exotherm curves weredetected by a photo-DSC (Perkin-Elmer DSC 7 with Perkin-ElmerDPA 7 Photocalorimeter) under a nitrogen ow at 30 C. Approxi-2.3. Characterizations</p><p>Infrared spectrum was recorded on a Bio-Rad FTS 3000 FT-IRspectrometer. 1H-NMR spectrum was obtained using a Varian INO-VA500 (500 MHz) spectrometer in acetone-d6. UV/vis spectra wererecorded on lms (ca. 25 lm) on quartz plates with a Perkin-ElmerLambda 900 spectrophotometer. The molecular weight and poly-dispersity were determined in THF by a Waters gel permeationchromatography (GPC) system using a calibration curve of PS stan-dards. The calorimetric measurement was performed on a Perkin-Elmer DSC 7 instrument. Approximately 10 mg of sample was heldfor 15 min at 150 C to evaporate residual solvent. The sample wasthen quickly cooled and then scanned between 25 C and 250 Cwith a scan rate of 20 C min1 to obtain the glass transition tem-perature (Tg). The general photoresist was formulated by dissolving2. Experimental</p><p>2.1. Chemicals</p><p>N,N0-azobisisobutyronitrile (AIBN, TCI) was recrystallized inmethanol before use. Isobutylpropylmethacryl polyhedral oli-gosilsesquioxane (MI-POSS, Hybrid Plastics), methyl methacrylate(MMA, Lancaster), methacrylic acid (MAA, Showa), 2-ethylhexylacrylate (2-EHA, TCI), glycidyl methacrylate (GMA, TCI), styrene(SM, TCI), diethylene glycol monomethyl ether (DEGME, TEDIA),triphenylphosphine (TPP, Lancaster), hydroquinone monomethylether (HQME, Lancaster), trimethylolpropane triacrylate (TMPTA,SARTOMER), 2-methyl-1-[4-(methylthio)phenyl] -2-(4-morpholi-nyl)-1-propanone (Irgacure 907, Ciba) and isopropylthioxanthone(ITX, Lamberti) were used as received.</p><p>2.2. Syntheses of the photosensitive copolymer</p><p>The MAA/MMA/2-EHA/styrene copolymers were prepared inDEGME using AIBN (1 wt% of monomers) as initiator at 70 C for5 h under nitrogen atmosphere. After adding TPP (1 wt% of mono-mers) as catalyst and HQME (0.1 wt% of monomers) as free radicalpolymerization inhibitor to the above copolymer solution, GMAwas added and reacted for 3 h at 90 C. The acid value of thecopolymer solution was monitored to ensure that all the GMAunderwent the ring-opening reaction of epoxide group. The photo-sensitive copolymer was isolated in excess hexane, ltered andwashed thoroughly by hexane. The product was puried by repre-cipitation from THF/hexane. Finally, the sample was dried undervacuum at 60 C for one day.</p><p>H.-M. Lin et al. /Microelectronicmately 1.5 mg photoresist was placed in the aluminum DSC pan.The heat of photopolymerization was calculated by integrated thepeak area over the curve using the horizontal baseline (Scheme 1).OO</p><p>OR = i-butyl</p><p>Si OSiO</p><p>Si O Si</p><p>OO</p><p>O R</p><p>RR</p><p>R</p><p>TMPTA MI-POSS </p><p>N OO</p><p>SS</p><p>O</p><p>Irgacure 907 ITX </p><p>Scheme 1. Chemical structures of chemicals studied.</p><p>Table 1Characterizations of the photosensitive copolymer</p><p>MMA/MAA/styrene/2-EHA/GMAloading (in copolymer; wt%)</p><p>Mw PDI Tg (C) Transmittance* (%)18.9/32.9/18.0/22.1/8.1(19.0/34.8/12.5/24.6/9.1)</p><p>75,600 2.32 128.6 98.2</p><p>* Transmittance was observed at 365 nm wavelength.</p></li><li><p>Fig. 2 gives the UV/vis spectra of various unexposed dry photore-sist lms on quartz plates. The lm of photoresist 1, free ofMI-POSS,</p><p>has high transmittance (99%) in the wavelength region from 425 to800 nm, but low transmittance at 300425 nmdue to the strong UV</p><p>300 400 500 600 700 800</p><p>0</p><p>20</p><p>40</p><p>60</p><p>80</p><p>100</p><p>Tran</p><p>smitt</p><p>ance</p><p> % </p><p>Wavelength (nm)Fig. 1. UV/vis spectrum of the photosensitive copolymer.</p><p>300 350 400 450 500 550 600 650 700 750 800</p><p>0</p><p>20</p><p>40</p><p>60</p><p>80</p><p>100 MI-POSS (wt%)</p><p> 0 0.6 2.9 5.7 13 23</p><p>Tran</p><p>smitt</p><p>ance</p><p> %</p><p>Wavelength (nm)Fig. 2. UV/vis spectra of the unexpose photoresist lms.</p><p>1626 H.-M. Lin et al. /Microelectronic Engineering 85 (2008) 16241628Fig. 3. SEM images of the photoresist sample sections with variant MI-POSS contents.</p></li><li><p>absorption of photoinitiators (Irgacure 907 and ITX). As theMI-POSSloading is increased, the photoresist lm gradually becomes opaquebecause the MI-POSS tends to aggregate or crystallize [24,25] toform its own domain in the photoresist before exposure.</p><p>Fig. 3 displays SEM images of the exposed photoresist samplesections containing 023 wt% MI-POSS. The photoresist 1 did notcontain MI-POSS, showing a smooth surface while the surface ofphotoresist 2 (0.6 wt% MI-POSS) becomes rough. Micron-sizedparticles appear in the SEM images of photoresists 36. As theMI-POSS content is increased, the size and number of particles alsoincrease. The methacrylate functional group of MI-POSS can reactwith photosensitive copolymer and TMPTA monomer. The organ-icinorganic compatibility is enhanced by the i-butyl substituentsof the MI-POSS, or the hydrogen bonding between MI-POSS andphotosensitive copolymers. Nevertheless, MI-POSS molecules pre-fer to crystallize or aggregate to form their own domain particles[25] in this series of organicinorganic hybrid photoresists.</p><p>The characteristic curves of the photoresists for exposure dosesranging from 2 to 100 mJ cm2 are shown in Fig. 4. The parametersof sensitivity (D0:5n ) and contrast (c) were determined from thesecharacteristic curves to evaluate the photoresist performance andthe results are summarized in Table 2. The sensitivity (D0:5n ) of thephotoresist is dened as the minimum energy dose required toform resist pattern to retain 50% of the original lm thickness afterdevelopment [26,27]. The sensitivity is inversely proportional toD0:5n (1/D</p><p>0:5n ) in the strict sense of notation [28]. The sensitivity for</p><p>the POSS-free photoresist 1 is 71.5 mJ cm2 while sensitivities ofthe POSS-containing photoresists, photoresists 26, are signi-</p><p>presence of MI-POSS unit does play an important role in improvingthe negative type photoresist properties. The bulky MI-POSS struc-ture and the hydrogen bonding between MI-POSS particles and thephotosensitive copolymer tend to restrict polymer chain motionand result in changing the characteristics of photoresists [3033].Table 2 shows that sensitivities (D0:5n ) for photoresists 5 and 6 arenearly constant at 25 mJ cm2, implying that the photoresist 5(13 wt% MI-POSS) has already reached its minimum induction en-ergy for this series of POSS-containing photoresists. In Fig. 4, thenormalized thickness of photoresists 5 and 6 are less than 0.8 evenat higher exposure doses. This MI-POSS macromer has a largemolecular structure (Mw = 943.64 g mol1) with only one methac-rylate functional group, increasingMI-POSS content in a blend leadsto a decrease in the crosslinking density of the photoresist andresults in reduction of normalized thickness after development.</p><p>The inuence of physically cross-linked MI-POSS structure onphotopolymerization of photoresists was studied by photo-DSC.Fig. 5 shows the exothermal curves of photo-polymerizations forthese photoresists. It has been reported [34] that the heat ux isrelated to the rate and extent of photopolymerization. Accordingto the photo-DSC results, the induction time and the peak maxi-mum of heat ux shift to shorter time (Table 3) with the increaseof MI-POSS content, implying that the photo-polymerization rateof the photoresist is accelerated by the increase of theMI-POSS con-tent. Competitive hydrogen bonding interactions are present inthese MI-POSS-containing blends in terms of hydroxylsiloxaneand carboxylic acid-siloxane. However, the hydrogen bondinginteraction of hydroxylsiloxane is dominant as amajor contributorto inuence the methacrylate double density. Therefore, it is rea-</p><p>H.-M. Lin et al. /Microelectronic Engineering 85 (2008) 16241628 1627cantly lower ranging from 32.0 to 24.9 mJ cm2. It implies thatthe photoresist becomes more sensitive to radiation energy and alower dose is needed to get pattern formation by incorporatingthe MI-POSS macromer. The contrast (c) is obtained as the slopeof the initial tangent of the characteristic curve with logarithmicabscissa [28,29]. The contrast for photoresist 1 is 23.1 and photore-sists 26 are ranging from 35.1 to 17.3. These results imply that the</p><p>10 100</p><p>0.0</p><p>0.2</p><p>0.4</p><p>0.6</p><p>0.8</p><p>1.0 MI-POSS (wt%)</p><p> 0 0.6 2.9 5.7 13 23</p><p>Nor</p><p>mal</p><p>ized</p><p> thick</p><p>ness</p><p>Dose (mJ/cm2)Fig. 4. Characteristic curves of photoresists.</p><p>Table 2Characteriza...</p></li></ul>