Oligomer Orientation in Vapor-Molecular-Layer-Deposited Alkyl-Aromatic Polyamide FilmsQing Peng,*,† Kirill Efimenko, Jan Genzer, and Gregory N. Parsons*
Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, UnitedStates
ABSTRACT: The surface-limited molecular-layer deposition of alkyl-aromaticpolyamide films using sequential doses of 1,4-butane diamine (BDA) andterephthaloyl dichloride (TDC) is characterized using in situ quartz crystalmicrobalance and ex situ spectroscopy analysis. For the first time, near-edge X-rayabsorption fine structure (NEXAFS) spectroscopy is used to offer insight intomolecular orientation in films deposited via molecular-layer deposition (MLD). Theresults show that the oligomer units are lying nearly parallel to the surface, whichdiffers from the linear vertical growth mode often used to illustrate film growth.
■ INTRODUCTIONThere is growing interest in vapor-phase molecular-layerdeposition (MLD) to synthesize oligomers/polymers1−11 andorganic−inorganic thin-film materials.12−21 MLD follows acyclic surface-limited reaction scheme used in atomic-layerdeposition (ALD)22,23 to produce highly conformal coatingswith precision control of film thickness and composition.3−21
Whereas ALD is typically focused on inorganic materials,22−25
MLD processes are designed to use organic reactants toincorporate the organic component intentionally into thedeposited layer.3−21 Binary surface reactions between a metal−ligand reactant and an organic reactant can yield a hybridorganic−inorganic film, whereas the reactions between twoorganic coreactants result in a thin film comprising organicoligomeric or polymeric materials. To date, researchers havestudied MLD processes leading to the formation ofpolyamides2−5 and polyimides7 consisting of linear andaromatic alkyls. In addition, the MLD growth of polyurea,10,13
polythiourea,8 and others26 and alucone,12,15 zincone,12,14,21
and other organic and inorganic hybrid materials has also beenexplored.16,17,19,27−29
In an ideal MLD process, the growth rate (increase in filmthickness per deposition cycle) is expected to be close to thelength of the molecular units involved. This linear verticalgrowth mode is often depicted in reaction schemes thatillustrate the growth sequence and mechanism.2,12,19 However,in reality most MLD experiments exhibit growth rates that aremuch less than the ideal repeat unit length.4,5,7,12 As discussedin previous reports of MLD, the growth of materials on asubstrate surface depends on the density of surface reactivesites, the number of active sites in reactants, the substratetemperature, steric effects, and the existence of doublereactions.12 In most MLD processes, the film growth ratedecreases with increasing temperature,4,5,7,12,14 which is
attributed to the thermally driven desorption of precursors. Ahigher deposition rate implies that the growing film will attain ahigher film density, corresponding to a layer of a more denselypacked oligomer layer, which tends to orient the moleculesparallel to the substrate normal because of van der Waals forces.A few early studies of MLD materials used X-ray diffraction2 orFourier transform infrared spectroscopy30 to infer the structureand orientation of the deposited films. Even so, there is verylittle physical evidence related to the physical structure andmolecular orientation of organic or hybrid organic−inorganicfilms formed through MLD.In this article, the in situ quartz crystal microbalance (QCM)
was used to study the surface-limited behavior of the MLDprocess of alkyl-aromatic polyamides. Importantly, for the firsttime, near-edge X-ray absorption fine structure (NEXAFS)spectroscopy25,26 was employed to analyze the molecularorientation in MLD films. NEXAFS is a powerful tool fordetermining the orientation of molecules on substrates.31,32 InNEXAFS, a resonant soft X-ray excites a K-shell electron to anunoccupied low-lying antibonding σ* or π* state.31,32 The K-shell excitation energy provides elemental specificity, and thefinal-state unoccupied molecular orbital provides chemicalbonding information. (Note that L and M edges for elementswith higher atomic numbers can provide similar information;however, they are not accessible in our experiments owing tothe lower-energy (250−800 eV) X-ray used.) The partialelectron yield (PEY) intensity in the NEXAFS spectra identifiesthe chemical bonds and their relative population density near asample surface (the probe depth is 2 to 3 nm).31,32 Moreimportantly, because the incident X-ray beam is linearly
Received: May 2, 2012Revised: June 19, 2012Published: July 5, 2012
polarized, collecting NEXAFS spectra at various incident X-raybeam angles (θ) between the surface normal and the X-raypolarization vector provides direct information regarding theorientation of molecules on the substrate surface.31,32
Specifically, we varied the incident angle (θ) between 20 and90° and analyzed trends in the PEY NEXAFS spectra toquantify the details of the molecular orientation within the film.
■ EXPERIMENTSFor molecular-layer deposition, 1,4-butane diamine (BDA) (>98%),terephthaloyl dichloride (TDC, >99%), and anhydrous toluene wereused as received (Sigma-Aldrich). In addition, 3-aminopropyltriethox-ysilane (APTES, >98%) was purchased from Gelest Inc. Alkyl-aromatic polyamide MLD deposition was carried out on a homemadehot wall vacuum reactor as described previously.14 The depositiontemperatures investigated here ranged from 75 to 155 °C. The BDAand TDC precursors were evaporated at 40 and 60 °C, respectively.The processing pressure during the reaction was ∼0.75 Torr. Betweenreactant exposures, the reactor was purged using 100 sccm of argon(99.999%) purified with a filter (Gatekeeper) to remove residual watervapor. Silicon (100) wafers (1 × 1 cm2) with a thin native oxide layerwere used as substrates and were prepared by wet cleaning with asolution of BakerClean JTB-100, followed by rinsing in deionizedwater and then drying with a flow of N2. In some cases, Si wafers withan APTES-terminated surface were used. To form the APTES-terminated substrate, the cleaned Si wafer was exposed to UV-ozonefor 20 min, resulting in a hydrophilic surface with a high density of−OH groups. Then the UV-ozone-treated Si wafer was immersed in a1 vol % APTES solution in toluene for 5 min at 60 °C, followed bythree rinsing steps in toluene, methanol, and deionized water. An insitu QCM14 was used to monitor the adsorption and desorptionbehavior during half cycles of each reactant in MLD. Ellipsometry(Rudolf/Auto EL) provided information about the film thickness afterdeposition. A refractive index of 1.56 was used for polyamide films.33
Transmission Fourier transform infrared (FTIR) spectroscopy ofpolyamide films provided chemical information about the filmcomposition. A ThermoNicolet IR bench with a deuterated triglycinesulfate detector (KBr beam splitter) was used in transmission mode.The typical measuring conditions for FTIR were 2048 scans at 4 cm−1
resolution; the measuring chamber was purged with purified dry air. Abackground spectrum was collected using the same wafer beforedeposition. Contact angle measurements (model 200 Rame-Hartgoniometer) were employed to analyze the surface wettability afterAPTES treatment.NEXAFS spectroscopy was used to probe the chemical and
molecular orientation information of the as-formed polyamide film.NEXAFS analysis was performed at the NIST/Dow Soft X-rayMaterials Characterization Facility of the National Synchrotron LightSource (NSLS) at Brookhaven National Laboratory.31 Experimentaldetails have been described previously.31 The incident photon sourceapplied during analysis has a resolution of 0.2 eV, an intensity of 5 ×1010 photon/s, and an energy of 300 eV. The typical storage current isaround 500 mA. From our past experience, hydrocarbons are not assusceptible to damage as other materials (e.g., fluorocarbons),particularly for short run times (about 8 min for each scan). Eachscan (different geometries) was performed on a fresh spot on thesample.
■ RESULTS AND DISCUSSION
Figure 1 depicts the reaction mechanism scheme for 1,4-butanediamine reacting with terephthaloyl chloride in the MLDreaction sequence. These two reactants can react with oneanother without a catalyst at room temperature with thegeneration of HCl vapor.2,4,30 After the BDA half cycle, thesurface becomes amine-terminated.2−5 The amine groups thenreact during the TDC half cycle to form an amide bond and acarboxylic chloride terminus.2−5 The stepwise repetition of the
above reactions results in the oligomeric amide on thesubstrate.The saturation behavior of the BDA and TDC half-reaction
steps measured by QCM at 75 °C is shown in Figure 2. InFigure 2a, TDC exposure on the BDA-treated surface showsthe reaction and net mass gain. Repeating the TDC/Ar steps(10 s/120 s) (without the BDA half cycle) shows continuedmass uptake and near saturation after approximately three
Figure 1. Mechanism of the MLD reaction between BDA and TDC. *indicates a surface-bonded ligand.
Figure 2. Behavior of the adsorption/desorption of monomers onto aAu-coated quartz crystal surface at a reaction temperature of 75 °C. (a)Adsorption behavior of TDC evaporated at 60 °C. After the dosing ofBDA (at t = 7 min, duration = 7 s, solid arrow), TDC was dosed every2 min, with a duration of 10 s of exposure (dotted arrow). Argon gaswas purged in between doses. This procedure was repeated 10 times.The adsorption of TDC is almost saturated after 60 s of dosing. Twominutes of Ar purging is enough to remove physically absorbed TDC.(b) Saturation curve of BDA evaporated at 40 °C, which shows a pulseat 40 °C with 7 s of saturation at a reaction temperature of 75 °C.
subpulses. The net mass gain from the TDC half cyclecorresponds to the condensation reaction between carboxylicacid chloride groups in TDC and surface amino groups, asshown in Figure 1. For the case of BDA exposure on a TDC-treated surface, the QCM results in Figure 2b exhibit differentbehavior. A large mass gain occurs during BDA exposure owingto the physisorption and chemisorption of BDA. In thefollowing purging step, there is significant mass loss owing tothe desorption of physically absorbed BDA. The net mass gainafter the half cycle of the BDA dose/purge step is ascribed tocovalently grafted BDA during the reaction steps. Repeating theBDA dose/purge steps (without the TDC half cycle) leads tosignificant physisorption but decreased chemically adsorbedBDA. However, it takes a long time (∼240 s) to remove mostof the physisorbed BDA molecules. The QCM data shown inFigure 2 provide direct evidence of the surface-limited growthbehavior in the alkyl-aromatic polyamide MLD process.The thickness per cycle during MLD of the alkyl-aromatic
polyamide film was studied at different reaction temperatures.On the basis of the QCM analysis (cf. Figure 2), for realgrowth, pulsing times 60 and 7 s were used for TDC and BDA,respectively. A purging time of 300 s was employed for bothreactants. The growth rate of MLD film decreases withincreasing growth temperature. At 85 and 125 °C, the growthrates are ∼2 and 0.5 Å/cycle, respectively, whereas at 155 °Cthe growth rate is almost zero. This trend is consistent with thetemperature-dependent growth rates reported in the liter-ature.4,7,12 Pretreatment of the silicon substrate with amonolayer of APTES was investigated as a means to modifyMLD nucleation. The as-formed APTES monolayer had athickness of ∼8−10 Å (based on ellipsometry) with a staticwater contact angle of ∼45−50°. These values are consistentwith those reported previously.34 Interestingly, the presence ofAPTES did not significantly affect the nucleation and growth ofthe oligoamide. The growth rate of the MLD film thickness onthe −NH2-terminated silicon substrate at 85 °C was ∼2 Å/cycle, which is indistinguishable from the growth rate for MLDdirectly on silicon coated with native oxide. This result indicatesthat polyamide MLD growth can nucleate on both the −OH-and −NH2-terminated surfaces.4,7
Figure 3 shows a transmission FTIR spectrum for a filmprepared at 85 °C on native oxidized silicon. The spectrumfeatures peaks for (CO)−N−H bending and C−N stretchingat 1543 cm−1, CO stretching (amide I) at 1638 cm−1, andN−H stretching at 3310 cm−1, all consistent with thecharacteristic IR spectrum of polyamide.2,4,5,30 Additionally,C−H stretching modes are detected at 2800−3000 cm−1. Theabsence of any −COCl mode near ∼1786 cm−1 suggests thatno carboxylic acid chloride monomer remains inside thedeposited film.4 The small absorption peak centered at ∼2100cm−1 may result from nitriles formed from the dehydration ofamide during deposition or air exposure.4
The NEXAFS spectra of the as-deposited alkyl-aromaticpolyamide on a Si wafer coated with native oxide collected atvarious incident angles of the X-ray beam relative to the samplesurface (θ) are shown in Figure 4. The film was deposited at 85°C for 100 cycles, producing a ∼20-nm-thick film.All NEXAFS spectra exhibit a sharp peak at 284.6 eV, with
the signature of the 1s→ π*CC transition originating from thephenyl group.35−37 The peak at 287.6 eV is attributed to 1s →σ*C−H transitions.35 The peaks centered at 289.4 eV areascribed to the 1s → π*CO transition.36,38 Peaks at 296 and302 eV are associated with the 1s → σ*C−C transitions. The
presence of the various peaks confirms the chemicalcomposition of the film. The change in peak intensity withvarying the angle between the incident X-ray beam and thesample provides evidence that the oligomer is oriented on thesurface. The positions of the peaks in the PEY NEXAFS spectrawere determined using the method outlined below.To gain more quantitative insight into the molecular
orientation, the NEXAFS spectra shown in Figure 4 wereanalyzed according to the procedure developed by Outka andco-workers.32,39,40 Specifically, the presence of the NEXAFSsignals corresponding to the various 1s → σ* and 1s → π*
Figure 3. Typical FTIR spectrum of an alkyl-aromatic polyamidedeposited at 85 °C for 150 cycles with a thickness of ∼28 nm. Thespectrum shows the characteristic peaks of the polyamide (a N−Hstretching mode at 3310 cm−1, a −CO stretching mode at 1638cm−1, and a CO−N−H bending mode and a C−N stretching mode at∼1543 cm−1).
Figure 4. Typical near-edge X-ray absorption fine structure(NEXAFS) spectrum of an alkyl-aromatic polyamide on a Si waferfrom different incident angles. The film was deposited at 85 °C for 100cycles with a film thickness of around 20 nm. The spectrum provideschemical information about CC, CO bonding. The relativepartial-electron-yield intensities of 1s → π*CC, 1s → σ*C−H, and 1s→ σ*C−C at different incident angles indicate that the polyamidemolecules tilted quite a bit to the substrate normal.
excitations was established from a difference spectrum obtainedby subtracting the NEXAFS spectra collected at θ = 90° (i.e.,normal incidence) and θ = 20° (i.e., glancing incidence). Thepositions and widths of the Gaussian peaks used to fit thedifference spectra were then employed to establish a back-ground signal from the NEXAFS spectrum collected at θ = 50°.NEXAFS spectra collected at various values of θ were thenfitted to the series of Gaussian peaks established earlier and thecarbon excitation step edge (placed at 290.2 eV, a typical valuefor hydrocarbons) by varying the intensities of the Gaussianpeaks through an in-house simulated annealing fitting routine.In Figure 5 we plot the experimental PEY NEXAFS spectra
(thick red line), which are summarized in Figure 4, and thecalculated NEXAFS spectra (thick black line). Gaussians andthe excitation edge lines (thin black lines) are also shown inFigure 5.Stohr and Samant41 define molecular orientation factors f x,
f y, and fz for an X−Y bond as follows
∫ α α= Ωf f ( ) cos ( ) dz2
(1)
= =−
f ff1
2x yz
(2)
where z is an axis normal to the film surface and x and y areorthogonal axes in the plane of the surface whereas α is theangle between the z axis and the direction of the σ* orbital(along the σ bond) or the π* orbital (orthogonal to the πbond), and dΩ is the differential solid angle. The molecular axisdistribution function f(α) is normalized so that ∫ f(α) dΩ = 1(i.e., f x + f y + fz = 1), as apparent from eq 2.The PEY NEXAFS intensity IXY(θ) originating from an X−Y
bond adopts the following form
θ θ= +I A B( ) sin ( )XY XY XY2
(3)
where AXY and BXY are constants. Further following Stohr andSamant, one can write
= =+
f fA B
Ix XY y XYXY XY
, ,tot (4)
=+ −( )
fA B
I
1z XY
XY XY P,
1
tot (5)
where P is the polarization factor of the X-ray beam and Itot,XY isthe total integrated intensity originating from the X−Y bondgiven by
= + −⎜ ⎟⎛⎝
⎞⎠I A
PB3 3
1XY XYtot (6)
as apparent from eqs 2, 4, and 5.42 Hence
=+ −
+ −
( )( )
fA B
A B
1
3 3z XY
XY XY P
XY P XY,
1
1(7)
Stohr and Samant have shown that a uniaxial orientationorder parameter S can be defined as
≡ −S f12
(3 1)z (8)
where S ranges from +1 (σ* or π* aligned perfectly along z) to−1/2 (σ* or π* lying in the plane of the surface). The abovetreatment assumes that there is molecular symmetry normal tothe plane of the surface (i.e., there is no preferred direction inthe plane). This condition is very likely fulfilled in our samplesgiven that no preferential in-plane orientation was establishedduring deposition. The order parameter defined in this way isanalogous to the Herman orientation parameter of X-raydiffraction.41 Orientation order parameter SXY can bedetermined from the values of AXY and BXY by combining eqs7 and 8 as follows:
= −+ −( )
SA B3 3
XY
BP
XY P XY1
XY
(9)
We have obtained the values of AXY and BXY as the interceptand slope in the experimental data corresponding to each bonddetected in the NEXAFS spectra (cf. Figures 4 and 5). By usingP = 0.85 (relevant for the setup at BNL), we have calculated thecorresponding values of SXY; these are listed in Table 1.Using the values of the orientation parameters listed in Table
1, we construct the conformation of the molecule on thesurface. Let us consider first the orientation of the phenylgroups. The value of orientation parameter (0.274) suggeststhat the transition dipole moment is oriented toward thesample normal but not aligned perfectly with it. Considering
Figure 5. PEY NEXAFS spectra collected from the sample in Figure 4(thick red line) and the corresponding model fits (thick black lines)using the Gaussian peaks and the ionization edge (thin black line) andbackground (not shown) with sample/beam angles of (a) 20, (b) 30,(c) 40, (d) 50, (e) 60, (f) 70, (g) 80, and (h) 90°.
that the π* orbitals are oriented orthogonal to the plane of thephenyl rings, the phenyls are lying in the plane of the samplesurface but are not perfectly aligned with it. We note that anyother orientation of the phenyl ring (i.e., edge-on or standingup) would result in a negative value of S. We now turn to theorientation of the oligo(ethylene) spacer. The large negativevalues of the orientation factor at peak energies of 296.0 and302.0 eV imply that the transition dipole moment associatedwith the 1s → σ*C−C transition lies within (or close to) theplane of the surface. Taking into account that the σ*C−Cantibonding for a C−C bond is oriented along the C−Cbond, this would suggest that the planar zig-zag of theoligo(ethylene) spacer is oriented close to parallel with respectto the surface plane. Overall, the conformation of the topmost 2to 3 nm of sample, as sensed by PEY NEXAFS, is nearly parallelto the sample surface.The NEXAFS analysis therefore provides clear evidence that
the molecular chain of the MLD oligomer is heavily slanted onthe substrate surface.43 A similar result is also observed for thepolyamide film deposited on the APTES-modified Si wafer asillustrated in Figure 7. This demonstrates that surfacemodification of the Si wafer with a dense array of −NH2groups does not change the orientation of molecular chains inthe MLD film.The tilted geometry of the polyamide oligomer units on the
deposited surface is consistent with the measured growth rateand in situ QCM results. The geometry also suggests that underthe MLD reaction conditions investigated the van der Waalsforces between molecules are not sufficient to induce a vertical
orientation and high packing density within the molecular film.The low oligomer density likely results from double reactionsduring the MLD sequence. In a double reaction, both reactiveend groups on the TDC or BDA monomers react with availablesurface sites, consuming and/or blocking surface reactive sitesthat would otherwise be available during the following surfacereaction step.12 In addition, whereas a higher temperature maybe expected to help increase the packing density, we find thatincreasing temperature acts to reduce the growth rate. Thereduced growth rate is ascribed to a smaller sticking coefficientof adsorbed precursors at higher temperature, which thereforedecreases the molecular density on the surface. With a shortersurface residence time at higher temperature, it is more difficultfor reactants to find a favorable reaction site to react.1,20
Moreover, even when the starting surface has a high density ofreactive sites, the size of the molecular chain introduces a sterichindrance effect that will decrease the number of availablereaction sites and the oligomer packing density as the filmreaches steady-state growth.
■ CONCLUSIONSSurface-limited MLD growth of alkyl-aromatic polyamide filmswas confirmed by using in situ QCM analysis. For the first time,the alignment of the molecular chain in the MLD films wascharacterized using NEXAFS spectroscopy. The results revealsignificant tilting of the grown oligomer units with respect tothe substrate normal. As the MLD field continues to advance, amore detailed understanding of the molecular structure insidethe material is essential to adjusting the film properties to meetthe desired function. NEXAFS, as demonstrated here, is apowerful technique for this purpose.
Table 1. Values of Orientation Order Parameter SXY for EachBond Detected in the PEY NEXAFS Spectra
peak energy (eV) S description
284.6 0.274 1s → π*CC35−37
285.3 0.267 1s → π*CC35−37
287.6 0.315 1s → σ*C−H35
288.5 0.071 1s → π*CO35,36,38
289.5 0.137 1s → π*CO35,36,38
296.0 −0.285 1s → σ*C−C37
302.0 −0.177 1s → σ*C−C
Figure 6. PEY NEXAFS intensities for the various bonds found in thesample as a function of the sample/beam angle (θ).
Figure 7. Near-edge X-ray absorption fine structure (NEXAFS)spectrum of an alkyl-aromatic polyamide on an APTES self-assembled-monolayer-modified Si wafer from incident angles of 20 and 90°. Thefilm was deposited at 85 °C for 100 cycles with a film thickness of ∼20nm. The spectrum shows chemical information from CC and CObonding. The relative partial-electron-yield intensity of 1s → π*CC,1s → σ*C−H, and 1s → σ*C−C at different incident angles, indicatingthat the polyamide molecules tilted quite a bit toward the substratenormal.
Present Address†Department of Electrical and Computer Engineering, DukeUniversity, Durham, North Carolina 27708, United States.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
NEXAFS spectroscopy experiments were carried out at theNational Synchrotron Light Source, Brookhaven NationalLaboratory, which is supported by the U.S. Department ofEnergy, Division of Materials Sciences and Division ofChemical Sciences. We thank Dr. Daniel A. Fischer (NIST/BNL) for his assistance during the NEXAFS experiments.
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