Metal-Containg and Metallosupramilecular Polymers and Materials

Chapter 22

Lithographic Patterning and Reactive Ion Etching of a Highly Metallized Polyferrocenylsilane *

Downloaded by CORNELL UNIV on October 6, 2016 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch022

Scott B. Clendenning, Alison Y. Cheng, and Ian Manners

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada Corresponding author: [email protected] *

A highly metallized cobalt-clusterized polyferrocenylsilane (Co-PFS) has been patterned using electron-beam lithography, UV-photolithography and a selective dewetting soft lithographic process. Subsequent pyrolysis and/or plasma reactive ion etching to fabricate metal-containing magnetic ceramic films has been demonstrated.

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© 2006 American Chemical Society

Schubert et al.; Metal-Containing and Metallosupramolecular Polymers and Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction New approaches to the patterning of metallic structures using a minimum number of processing steps are of intense current interest. Direct patterning of processible, high resolution lithographic resists with high metal content is one very promising avenue. To date the majority of direct-write resists for metallic structures consist of surfactant-stabilized metal colloids, molecular organometallic species, metal-containing composites or purely inorganic materials. These resists operate in negative-tone mode such that exposed regions undergo a chemical change rendering them insoluble in the developing medium. Studies of transition metal-containing polymers are extremely rare. One recent example is the use of thin films of the organometallic cluster polymer [Ru C(CO)i5Ph2PC PPh2] as a negative-tone electron-beam resist to direct-write conducting wires of metal nanoparticles as small as 100 nm in width. As a metal source, metallopolymers offer the advantages of ease of processibility, atomic level mixing and stoichiometric control over composition. The use of a metallopolymer precursor to form metal containing ceramics has also been shown. Plasma reactive ion etching (RIE) provides a rapid and convenient means to chemically and physically modify surfaces in a uniform manner over square centimeter areas. In the case of metallopolymers, their generally higher etch resistance in a variety of plasmas in comparison to organic polymers, coupled with the possibility of depositing metal-containing ceramic materials with interesting physical properties, is highly intriguing. In this paper we review the patterning of thin films of a highly metallized polyferrocene polymer by electron-beam lithography (EBL), UV-photolithography and a selective dewetting soft lithographic process as well as its processing by pyrolysis and RIE to fabricate metal-containing magnetic ceramics. 1

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The ring-opening polymerization of sila[l]ferrocenophanes (1) by thermal, anionic and transition metal-catalyzed routes yields high molecular weight, soluble polyferrocenylsilanes (2, PFS) containing coordinatively bonded iron atoms in the main chain (Scheme 1). The incorporation of PFS into patterned surfaces has already yielded materials with tunable magnetic properties that may find applications as protective coatings, magnetic recording media and in anti static shielding. Furthermore, the low plasma etch rates of polymers containing organometallic moieties in comparison to their purely organic counterparts suggest their use as etch masks which can also deposit interesting materials. The introduction of additional metals into the PFS chain can increase metal loadings and allow access to binary or higher metallic alloy species. We have 11

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308 R

ROP

Fe

R'

I Fe

'R' 1


Scheme 1. Ring-opening polymerization of a silafljferrocenophane to afford a polyferrocenylsilane β).

(I)

1

found in our research that PFS with acetylenic substituents at silicon (Scheme 2, 2a) can be clusterized by treatment with dicobalt octacarbonyl to yield the highly metallized, soluble, air-stable cobalt-clusterized polyferrocenylsilane (CoPFS) (3) which contains three metal atoms per repeat unit. 15

Ph

Ph

(OC),CoCo(CO)

if,

N

3

-SU

I Fe φ >

C02(CO)e

Φ> Fe

-2(CO)

pyrolysis

Magnetic Ceramics

2a Scheme 2. Cobalt clusterization of the acetylenic substituents of a polyferrocenylsilane β&) to yield the highly metallized Co-PFS β), a precursor to magnetic ceramics.

Pyrolysis of bulk 3 under a N atmosphere at either 600°C or 900°C affords black magnetic ceramics in relatively high yield (72% and 59%, respectively). Powder X-ray diffraction (PXRD) studies revealed the ceramic residual was composed of Fe/Co alloy particles embedded in an amorphous C/SiC matrix. Figure 1 shows the cross-sectional transmission electron micrographs of the ceramics prepared at 600 °C and 900 °C. The presence of electron-rich metal nanoparticles can clearly be seen. The chemical compositions of these metal particles were determined by element mapping electron spectroscopic imaging (ESI) experiments. The results indicated that both iron and cobalt were localized 2


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in the same nanoparticles. Superconducting quantum interference device (SQUID) magnetometry showed that ceramics formed at 900°C were ferromagnetic with no blocking temperature up to 355 K .

Figure 1. Cross-sectional TEM images of ceramics resultingfrom pyrolysis of Co-PFS β) at (a) 600°C and (b) 900°C The scale bars are 50 nm. (Reproduced with permission from Reference 15.)

One of the advantages of polymers is the ease with which they can form films. We have recently demonstrated the patterning of thin films of Co-PFS (3) by electron beam lithography ( E B L ) , UV-photolithography, and a soft lithography/selective dewetting technique as well as the formation of magnetic ceramic films via pyrolysis and/or reactive ion etching (RIE) in a secondary magnetic field. 1 6

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Cobalt-Clusterized Polyferrocenylsilane as an Electron-Beam Lithography Resist Electron-beam lithography (EBL) is a maskless lithography in which a resist material is exposed to an electron-beam with controlled dose in a predetermined pattern. Areas exposed to the beam in positive-tone resists become more soluble in a developing medium whereas exposed areas in a negative-tone resist become less soluble. In order to determine whether Co-PFS could function as an electron beam resist, uniform thin films (ca. 200 nm thick) of 3 were spin-coated on to silicon substrates and electron beam lithography was carried out at varying doses and current in a modified scanning electron microscope. The treated films were then developed in T H F prior to characterization. Resists of 3 were found to operate in a negative-tone fashion. Shapes including dots and bars were successfully fabricated (Figure 2). 20

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Figure 2. SEM images of (a) dots and (b) bars fashioned by EBL using a Co-PFS β) resist. (Reproduced with permission from Reference 16)

The elemental composition of these patterns was investigated using time-offlight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS). Iron and cobalt elemental maps of the arrays of bars on the Si substrate were obtained using TOF-SIMS. The mapping clearly revealed that Fe and Co were concentrated within the bars. Information regarding the chemical environment and distribution of Fe and Co throughout the bars was obtained via X P S and compared to data obtained from an untreated film of Co-PFS. The atomic ratio of Fe.Co for the bars was in agreement with the theoretical value of 1:2. Detailed scans for iron and cobalt revealed no change in binding energy for the elements at 3 nm and 12 nm depths indicating uniformity in average chemical environment. Overall, X P S revealed little change in the chemical composition of the polymer resist after E B L . Finally, magnetic force microscopy ( M F M ) indicated no appreciable magnetic field gradient above the bars following E B L . In order to enhance the magnetism of the patterned bars, an array of bars on a Si substrate was pyrolyzed at 900 °C under a N atmosphere to promote the formation metallic nanoclusters ' The same array of bars was characterized before and after pyrolysis by tapping mode atomic force microscopy ( A F M ) . Comparison of the images and cross-sectional profiles suggests there is excellent shape retention accompanying a decrease in the dimensions of the bars. M F M studies of the bars indicated that they consist of heterogeneous ferromagnetic clusters which magnetic dipoles appear to be randomly oriented. The magnetooptic Kerr effect ( M O K E ) was also used to independently confirm the ferromagnetic behavior of the ceramic bars. 2

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Although the aforementioned proof-of-concept experiments dealt with the formation of micron-scale objects, E B L can be used to routinely fabricate structures down to 30-50 nm. Indeed, this process has already been extended to the patterning of sub 500 nm bars and dots from Co-PFS. 22


311 Cobalt-Clusterized Polyferrocenylsilane (Co-PFS) as a UV-Photoresist The serial nature of E B L can result in long processing times especially i f large doses are required. In UV-photolithography, square centimeter areas of the resist can be exposed through a mask at the same time normally leading to much faster processing. Organic polymers incorporating acetylenic moieties have been shown to be thermally crosslinkable. Upon heating, the acetylene groups from adjacent chains undergo cyclotrimerization and coupling reactions, creating crosslinks in the polymer. In addition, photo-induced polymerization of alkyland aryl-substituted acetylenes are known to be catalyzed by metal carbonyls such as Cr(CO) , Mo(CO) and W(CO) . Recently, Bardarau et al. demonstrated that polyacrylates with pendent acetylenic side groups could be photocrosslinked in the presence of W(CO) as catalyst. In the case of Co-PFS (3), which inherently contains both the acetylenic unit and the metal carbonyl catalyst, is a promising candidate as a resist material for UV-photolithography. To study this possibility, a thin film (ca. 200 nm) of 3 on Si substrate was exposed to near U V radiation (λ=350-400 nm, 450 W) for 5 minutes. The exposed film was developed in T H F before characterization. Co-PFS was found to be a negative-tone photoresist. This appears to be consistent with photoinitiated crosslinking mechanism of acetylenes in the presence of metal carbonyls. However, it is also possible to have crosslinking in Co-PFS as a result of decarbonylation of the Co-cluster. The thickness of the film before and after U V treatment was determined by ellipsometry. A 200 nm thick film of Co-PFS had a thickness of ca. 170 nm after exposure to U V radiation and solvent development. The decrease in thickness is probably a reflection of the decreased volume of the polymer upon crosslinking. Patterning of Co-PFS films was accomplished using a metal foil shadow mask fabricated by micromachining with features ca. 50-300 μηι. Figure 3a is an optical micrograph of a straight line of Co-PFS patterned with the shadow mask. Smaller features (ca. 10-20 pm) were obtained using a chrome contact mask (Figure 3b). In both cases the unexposed polymer was completely removed during development with THF, leaving behind patterns with well-defined edges. Patterned Co-PFS was pyrolyzed at 900 °C under a N atmosphere in an attempt to fabricate magnetic ceramic lines. The resulting ceramics lines have the same dimensions as the polymer precursor showing excellent shape retention in the lateral directions. Inspection of the ceramic line at higher magnifications revealed the formation of what appeared to be Co/ Fe nanoparticles throughout the line. UV-photolithography using Co-PFS (3) as a resist provides a convenient route to deposit patterned polymer and magnetic ceramic onto flat substrates. Due to the excellent RIE resistance of the polymer, Co-PFS patterned by UV-photolithography could also potentially be utilized for pattern transfer onto 23

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the underlying substrate using conventional plasma etching Investigations are underway to improve the resolution of this resist.

techniques.

Figure 3. Optical micrographs of Co-PFS (3) lines fabricated by UVphotolithography using (a) a shadow mask and (b) a chrome contact mask. (Reproduced with permission from Reference 17.)

Ordered 2D Ring Arrays of Cobalt-Ciusterized Polyferrocenylsilane Arrays of nanoparticle rings are a desirable target in terms of expanding nanofabrication techniques as well as potential applications in high density magnetic data storage devices. Recently, long range order and excellent size control in 2D arrays of polymer rings were achieved by Yang et al using selective dewetting of a solution of a polymer-containing film on a substrate patterned with micron-sized hydrophilic and hydrophobic areas using soft lithography. In collaboration with the group of Prof. G . A . Ozin at the University of Toronto, we used a modified version of this technique to pattern arrays of rings of Co-PFS (3) with diameters of ca. 2, 5 and 12 pm onto a thin gold film (ca. 100 nm) sputtered onto a silicon wafer that had been primed with a 5 nm layer of titanium. Figure 4a shows a scanning electron micrograph of an array of ca. 5 pm diameter rings. The rings had a volcano-like morphology with the light interior corresponding to the hydrophilic area while the area outside the rings corresponded to the hydrophobic area. The rings were then treated with a hydrogen plasma (20 mTorr, 100 W, 12 min) in order to remove carbonaceous material, thereby consolidating the structure, along with the excess polymer around the rings (Figure 4b). Traces of gold were removed using a K I etch prior to pyrolysis under a dinitrogen atmosphere at 900 °C for 5 h. Visualization of the pyrolyzed rings using S E M (Figure 4c) revealed a highly ordered array of nanoparticle rings with inner and outer diameters of 5.4 ± 0.2 pm and 6.3 ± 0.2 pm, respectively. The presence of Fe and Co in the bright nanoparticles was confirmed by E D X . Interestingly, the nanoparticles appeared to self-assemble into concentric rings to give an overall annular width 26

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of ca. 800 nm. The particles, which likely have a metal oxide shell, exhibited a bimodal size distribution with the larger particles 111 ± 20 nm and the smaller ones less than 70 nm. The majority of particles were larger than the estimated magnetic single-domain size for spherical particles of iron (14 nm) and Co (70 nm). Magnetic force microscopy confirmed that the nanoparticles in the rings were magnetic. While it is also possible to image superparamagnetic metallic nanoparticles using M F M , the patterned Fe/Co alloy nanoparticles resulting from the pyrolysis of microbars of 3 (vide supra) under similar conditions proved to be ferromagnetic by magneto-optic Kerr effect ( M O K E ) measurements thus supporting the presence of ferromagnetic nanoparticles in the case of the patterned rings. Therefore, by employing a consolidating hydrogen plasma treatment followed by pyrolysis, we have demonstrated the transformation of metallopolymer rings into a highly ordered array of ferromagnetic nanoparticles. Through judicious choice of the metallopolymer precursor and its concentration in solution, ordered arrays of many different metallic nanoparticle rings with a range of dimensions should be accessible. 28

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Figure 4. Field Emission SEM images ofSpm Co-PFS β) rings (a) as prepared, (b) following H -RIE (20 mTorr, 100 W 12 min) and (c) after subsequent pyrolysis (N , 900 °C, 5 h). Insets show a representative section of rim at 40k magnification. (Reproduced with permission from Reference 18.) 2

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Polyferrocenylsilanes as Reactive Ion Etch Resists Pyrolytic routes to ceramics generally involve the input of large amounts of thermal energy for prolonged periods of time in order to cause the decomposition of the ceramic precursor, removal of volatiles and the formation of the desired ceramic material. We wished to explore the possibility of carrying out such a transformation using plasma reactive ion etching (RIE). It has been demonstrated that polyferrocenylsilanes possess low plasma etch rates which can be attributed to the formation of a protective layer of involatile iron and silicon compounds. This property has been exploited with PFS block copolymers selfassembled into micelles or phase-separated thin films to deposit patterned ceramics as well as for pattern transfer to the substrate. In our research, we are interested in the direct formation of high metal content magnetic ceramic films by RIE treatment of Co-PFS in the presence of a secondary magnetic field. 20

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314 Thin films (ca. 100-200 nm thick) of Co-PFS (3) on a Si substrate were treated with either H or 0 plasma (10-20 mTorr, 100 W). The effects of the plasmas on the chemical composition of the treated films were studied by TOFSIMS depth profiling. In both cases, by plotting the intensity of the S i , Fe , FeO , C o and CoO* signals as a function of depth, it could be seen that the plasma affected only the top 10 nm of the films, leaving the underlying polymer untouched. The chemical composition of the modified films was analyzed by X P S . In both cases a Co: Fe ratio of approximately 1:1 was found at 3 nm depth. In comparison to the 2:1 ratio of Co: Fe ratio found in the untreated polymer, this deficiency of Co in the film surface is most likely a reflection of the volatility of the cobalt carbonyl clusters during the high vacuum processing required for RIE. Formation of iron oxides (FeO and F e 0 ) as well as cobalt oxides (CoO and C o 0 ) was observed at 3 nm depth in the O pIasma treated films. In the case of H -RJE, small amounts of oxides were also observed at 3 nm depth, presumably due to the oxidation of reduced metal upon exposure to atmospheric oxygen. 2

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Morphological changes in the surface of Co-PFS (3) films following RIE were investigated by transmission electron microscopy (TEM) and A F M . Thin films of the polymer (ca. 50 nm thick) on a carbon-coated copper T E M grid were exposed to oxygen and hydrogen plasma. In both cases, analysis by T E M revealed the presence of electron rich nanoworms with widths ranging from 4 12 nm which remained on the supporting carbon film. Untreated samples were featureless, supporting plasma-induced nanoworm formation. Electron energy-loss spectroscopic (EELS) elemental mapping and energy-dispersive X ray (EDX) indicated that there was a high concentration of Fe, Co and Si in the nanoworms. In contrast to the essentially flat and featureless surface of the untreated film, A F M images of both plasma treated films exhibited a pervasive interconnected reticulated structure which would appear as nanoworms in projection normal to the plane of the sample. These features are reminiscent of surface reticulations observed by Thomas et al. by A F M in organosilicon polymers following ambient temperature 0 - R I E , which were hypothesized to originate due to a polarity difference between the overlying inorganic layer and the native polymer causing dewetting or spinodal decomposition of the strained inorganic layer. In order to access films with useful magnetic properties, plasma-induced crystallization of metallic nanoclusters was attempted by carrying out H - R I E on a thin film of Co-PFS in a secondary magnetic field. The Co-PFS films (ca. 200 nm thick) on Si substrates were placed between two samarium-cobalt (SmCo) magnets aligned with opposite magnetic poles facing each other during H - or O R I E . The magnets caused the formation of an intense plasma plume around the sample which in turn resulted in intense etching conditions. Nanostructures obtained under these conditions were proven to be magnetic by M F M . A tapping mode A F M micrograph and the corresponding magnetic force micrograph of a H plasma-treated film are shown in Figure 5. The reticulations were much 2

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larger than those found for samples treated under similar plasma conditions in the absence of the samarium-cobalt magnets. We postulate that the secondary magnetic field concentrated the plasma thereby accelerating nanoworm formation through more efficient removal of carbonaceous material and silicon while the additional thermal energy present in the plasma plume promoted metal crystallization. To the best of the authors' knowledge this is the first example of the formation of a ferromagnetic film directly from the plasma treatment of a metallopolymer.

Figure 5. Tapping mode AFM images of thin film of Co-PFS β) following H RIE in a secondary magnetic field (a) and the corresponding MFM image (b). (Reproduced with permission from Reference 19.) 2

Conclusions High molecular weight Co-PFS (3) is readily accessible via transition metal-catalyzed ROP of sila[l]ferrocenophane (2a) followed by clusterization of the acetylenic substituents with Co (CO) . The intrinsic high metal content, airstability and solution processability make Co-PFS an excellent candidate for lithographic patterning. Fabrication of patterned arrays of polymer and magnetic ceramics on the sub-micron scale via E B L was demonstrated while U V photolithography gave micron scale features. Furthermore, the use of a template-assisted selective dewetting technique involving soft lithography led to large 2D arrays of Co-PFS rings which could be transformed into magnetic nanoparticle ring arrays. We have also demonstrated the use of this highly metallized polymer as a RIE resist. Treatment of the polymer films with either H or 0 plasma in the presence of a secondary magnetic field afforded magnetic ceramics. We are currently studying the use of these patterned ceramics in spintronics as an isolating, magnetic layer in a nano-granular in-gap structure 2

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as well as catalysis. Concurrently, we are developing new highly metallized polymers containing Fe, Co, M o and/or N i while working on improving resist sensitivity and resolution. 3 5


Acknowledgments I.M. would like to thank the Canadian Government for a Canadian Research Chair. S.B.C. is grateful to N S E R C for a PDF. The authors would also like to acknowledge the excellent work of our collaborators: EBL: Prof. Harry E . Ruda; Dr. Stéphane Aouba (Center of Advanced Nanotechnology, University of Toronto); AFM/ MFM: Prof. Christopher M . Y i p , Mandeep S. Rayat, Patrick Yang (Department of Chemical Engineering and Applied Chemistry, University of Toronto); TEM: Dr. Neil Coombs, Ellipsometry: Chantai Paquet (Department of Chemistry, University of Toronto); RIE UV Photolithography, XPS: Prof. Zheng-Hong L u , Dr. Sijin Han, Dr. Dan Grozea (Department of Materials Science and Engineering, University o f Toronto); TOF-SIMS: Dr. Rana N . S. Sodhi, Dr. Peter M . Brodersen, (Surface Interface Ontario, Department of Chemical Engineering and Applied Chemistry, University of Toronto); MOKE: Prof. Mark R. Freeman, Jason B . Sorge (Department of Physics, University of Alberta). t

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Metal-Containg and Metallosupramilecular Polymers and Materials

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