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FEATURE ARTICLE Area-Selective ALD with Soft Lithographic Methods: Using Self-Assembled Monolayers to Direct Film Deposition Xirong Jiang† and Stacey F. Bent*,‡ Department of Physics, and Department of Chemical Engineering, Stanford UniVersity, Stanford, California 94305 ReceiVed: June 5, 2009; ReVised Manuscript ReceiVed: July 8, 2009
Area-selective atomic layer deposition (ALD) is a technique that can be used for fabricating 3D structures with dimensions down to the nanoscale. A patterned resist, typically a self-assembled monolayer (SAM), directs film deposition through area-selective ALD, leading to lateral patterning. This article will describe the overall approach to area-selective ALD, introduce the process of atomic layer deposition, and discuss the development of monolayer ALD resists. We will describe the results of studies which show that monolayers with a high degree of packing and hydrophobicity perform best in blocking ALD, and that resistance against ALD can be used as a sensitive probe of SAM quality. We further describe patterning of the SAM through soft lithography, in particular microcontact printing (µCP), for the area-selective ALD process, and compare the area-selective ALD processes for HfO2 and Pt ALD. The powerful patterning capability of µCP and the flexibility of area-selective ALD for various materials can impact many potential applications, and studies applying this process to fuel cell devices and integrated circuits will be described. 1. Introduction Three-dimensional (3D) nanostructures play a central role in many areas of technology such as fuel cells,1,2 catalyst supports,3,4 data storage,5,6 sensors,7 photonics,8 and microfluidics.9 Moreover, the role of 3D nanostructures is expected to become even more important in future technologies. The most prevalent method for developing such structures is based on a standard fabrication process, borrowed from the integrated circuit (IC) industry, that relies on sequential applications of two-dimensional (2D) patterning techniques, such as electron beam lithography and photolithography, followed by selective removal of sacrificial resist materials. However, its application to 3D structures is difficult because of the highly sophisticated engineering processes required. Some alternative techniques have emerged to complement the established lithographic methods and to form 3D nanostructures more conveniently and effectively. These include controlled chemical etching,10 colloidal self-assembly,4,11 phase separation of polymers,12 templatecontrolled growth,13 holographic based lithography,14 direct-write techniques based on multiphoton exposures,6,15 filamentary inks,8,16 self-assembly in fluids,17 and proximity field nanopattering.18 Nevertheless, none provides a complete solution to the challenge of 3D nanofabrication, due to various limitations such as slow speeds, inadequate flexibility in the structure geometries, applicability only to relatively small areas, complex experimental setups, and uncertain yields and defect densities. Hence, there is still a need for new methods of forming 3D patterns. * Corresponding author. Tel.: 001 650 723 0385. Fax: 001 650 723 9780. E-mail address:
[email protected]. † Department of Physics. ‡ Department of Chemical Engineering.
Area-selective atomic layer deposition (ALD) has been developed over the past several years as a new approach for fabricating 3D nanostructures. ALD is a vapor phase method for depositing thin films,19-23 and area-selective ALD refers to the case where spatial control is exerted over the deposited film. Area-selective ALD differs from conventional (i.e., subtractive) lithographic patterning in that it is an additive process in which material is deposited only where needed.2,24-26 In area-selective ALD, a “resist”, which can be a self-assembled monolayer2,24-32 or polymer,33-36 is patterned onto the substrate prior to ALD to direct the subsequent atomic layer deposition. In the case of a blocking resist, ALD will not occur wherever the resist is present, but will deposit only in unblocked regions of the surface, leading to negative pattern transfer. Various strategies, including photolithographically patterned SiO2/Si substrates, direct writing by electron and photon beams, and soft lithography have been explored to create the resist pattern for area-selective ALD. Area-selective ALD has been demonstrated in the selective deposition of a range of materials, such as TiO2, ZnO, HfO2, Ir, Pt, and Ru.2,25-42 This article will summarize our group’s recent work on areaselective atomic layer deposition based on microcontact printing (µCP), a form of soft-lithography, where the µCP step is used to transfer to the substrate a resist pattern to direct the subsequent ALD growth. The focus will be on ALD resists consisting of self-assembled monolayers (SAMs). We will start with an overview of the microcontact printed, area-selective ALD process, followed by an introduction to the key aspects of the ALD process. We will then discuss studies on the organic interfacial layers, which provide insight into the chemical and physical properties of the SAMs. Our studies are aimed at developing an understanding of these organic SAM-based resists at the molecular level, including the impacts of their chain
10.1021/jp905317n CCC: $40.75 2009 American Chemical Society Published on Web 09/03/2009
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Xirong Jiang received her B.S. in Material Science and Engineering from Beijing Normal University in 2003 and M.S. in Management Science and Engineering from Stanford University in 2008. She is completing her Ph.D. in Physics at Stanford University. She is currently a research associate at Stanford University with Prof. Stacey F. Bent’s group. Research interests include soft lithography, 3D nanostructures, novel fabrication techniques, fuel cell devices, and solar cell devices.
Stacey F. Bent is a Professor of Chemical Engineering and Professor, by courtesy, of Chemistry, Electrical Engineering, and Materials Science and Engineering, at Stanford University. She earned her B.S. in Chemical Engineering from U.C. Berkeley and her Ph.D. in Chemistry from Stanford University. Following postdoctoral work at AT&T Bell Laboratories, she joined the faculty at New York University before moving to Stanford University in 1998. Her research interests are in understanding surface and interfacial chemistry and materials processing and applying this knowledge to a range of problems in semiconductor processing, nanotechnology, and sustainable energy.
length, tailgroup structure, and reactive headgroup, as well as the role of SAM formation time on its ability to block ALD. Finally, we will describe the patterning capability of areaselective ALD for various materials and its application to integrated circuits and fuel cell devices. 2. Overview of Approach The area-selective ALD procedure based on patterning by microcontact printing is shown in Figure 1. The general scheme was first demonstrated by Yan et al.,26 and several other groups have since investigated this process.2,24-32 The process begins with casting and curing of an elastomeric stamp on a Si master developed through standard lithographic methods.43,44 The structure of surface relief on the Si master is transferred onto the elastomeric stamp, with its spatial geometry defined by the layout and depth of the relief. Next, the elastomeric stamp is “inked” with an organic interfacial layer which can be either a nucleation site blocker or multiplier, depending on the end group
Jiang and Bent
Figure 1. Schematic outline of the procedure used to fabricate patterned thin films using microcontact printing and selective atomic layer deposition.45
and the chemistry of the ALD process. This inked stamp is placed onto a substrate, forming conformal contact due to the action of generalized adhesion force (primarily van der Waals interactions). The pattern encoded in the elastomeric stamp is transferred onto the substrate with the aid of applied pressure to improve the fidelity of the pattern. The resulting organic interfacial layer that is patterned on the substrate will direct the subsequent ALD growth, with material selectively deposited onto areas free of the organic layer if it acts as an ALD nucleation resist, a case demonstrated in Figure 1e. Alternatively, more growth will occur on the organic-coated regions if the organic layer is a nucleation multiplier. Consequently, the atomic layer deposited material will form a structure which possesses relief features that are the negative (or positive) of the elastomeric stamp. The whole process involves three essential components: (i) an elastomeric stamp created through lithography, (ii) an organic interfacial layer, which either blocks or seeds ALD nucleation, and (iii) an ALD process whose chemistry can be manipulated by the organic interfacial layer. Step (i) may be carried out under different variations of soft lithography, as long as the result is a patterned organic layer. Investigation and understanding of each component is important to the design of an area-selective ALD process. Moreover, the study of steps (ii) and (iii) yields fundamental insight into SAM properties and ALD nucleation mechanisms. 3. Atomic Layer Deposition Atomic layer deposition (ALD) is a promising, ultrathin film deposition method which is a modified form of chemical vapor deposition. ALD is a layer-by-layer process for the deposition of thin films with atomic scale accuracy, where each atomic layer formed in the sequential process is a result of saturative, surface-controlled chemical reactions. A schematic illustration of the atomic layer deposition process is shown in Figure 2. Precursor A is introduced and reacts with the substrate until saturation, and then, an inert gas is introduced to ensure removal of any unreacted precursor and physisorbed molecules. At this point, the surface is covered by ligands of precursor A. Then precursor B is introduced into the reactor where it reacts with the surface until saturation. Again, an inert gas is introduced to remove unreacted precursors, and the system goes back to the
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J. Phys. Chem. C, Vol. 113, No. 41, 2009 17615 in many ALD processes the actual film coverage per cycle is less than 1 monolayer due to effects such as steric hindrance of the ligands and the limited number of active bonding sites at the surface.48,55-58 The precursors used for Pt ALD are (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) and dry air, which acts as a source of oxygen. The growth of the Pt film has been proposed to occur by the following two self-limiting chemical reactions, repeated in alternating ABAB sequences:
Figure 2. Schematic illustration of the atomic layer deposition process. One ALD cycle consists of a sequential pulse of precursor A, purge by an inert gas, pulse of precursor B, and purge by an inert gas again. The ALD cycle will be repeated a number of times until the desired film thickness is achieved.
starting point. Two half reactions form a complete ALD cycle, and the same cycle is repeated to achieve the desired film thickness. In an ideal ALD process, each surface reaction is self-limiting so that additional exposure to precursors beyond saturation in a given pulse does not lead to further deposition. The selflimiting surface reaction characteristic enables some of the most desirable properties of ALD, including Angstrom-level control of film thickness in the vertical direction, high conformality, and uniformity over large areas.19-23,46-48 Currently, ALD is being actively investigated to deposit a wide range of materials,19-23 including semiconductors, nitrides, metals, metal oxides, and metal sulfides. It can be used to deposit polycrystalline and amorphous layers49 in addition to epitaxial thin films.50 In our group, ALD has been used to deposit metal oxides such as HfO2 and TiO2; metals such as Cu, Pt, and Ru; and sulfides such as ZnS, for use in different applications such as high-κ dielectrics, gate metals for next generation electronic devices, as well as catalysts for fuel cell electrodes.2,24,25,38-42,51-53 Our studies on area-selective ALD using microcontact printing have focused primarily on the materials HfO2 and Pt. Therefore, we will review the chemistry of these two ALD processes here. For ALD of HfO2 thin films, two different Hf precursors, hafnium(IV)-tetrachloride (HfCl4) and tetrakis(dimethylamido)hafnium(IV) (Hf[N(CH3)2]4), have been employed in our studies, together with H2O as the counter-reactant. The HfO2 ALD process includes two self-limiting chemical reactions, repeated in alternating ABAB sequences as shown below:
A:Hf - OH* + HfCl4 f Hf - O - HfCl3* + HClv B:HF - Cl* + H2O f Hf - OH* + HClv for the HfCl4 precursor, and
A:Hf - OH* + Hf[N(CH3)2]4 f Hf - O Hf[N(CH3)2]3* + NH(CH3)2v B:Hf - N(CH3)2* + H2O f Hf - OH* + NH(CH3)2v for the Hf[N(CH3)2]4 precursor, where the asterisks represent the surface species.54-56 In both reactions, a gas-phase molecular precursor reacts with the surface functional groups and saturates the surface in a self-limiting manner.20,55,56 Ideally, each AB reaction cycle produces a HfO2 monolayer terminated by hydroxyl groups, with the byproduct pumped away. However,
Pt(S) + O2 f Pt - O*
(A)
CH3C5H4Pt(CH3)3 + Pt - O* f Pt*(s) + CO2(g) v + H2(g) v other byproductsv (B) where the asterisks again represent the surface species.59-61 Each AB reaction cycle produces a Pt layer terminated by oxygen species, with the byproduct pumped away. These processes have been shown to lead to the deposition of good quality films. For example, pure films of Pt having low resistivity and bulk-like density have been deposited in our laboratory with the above process. 4. Organic Interfacial Layer The materials that have been most commonly studied for areaselective ALD resists are organic-both organic monolayers and organic thin films. Organic monolayers and films provide the ability to tune the reactivity between the ALD precursors and the surface by changing key functional groups in the organic layer. In particular, organic layers that block (or remove) active sites at the underlying substrate, such as hydroxyl or oxide groups, without adding new moieties that react with the ALD precursors, have been used successfully as ALD resists. Examples include poly (methyl methacrylate) (PMMA) polymer films33-35 and self-assembled monolayers that terminate with methyl groups.26,27,29-32,62 The majority of the area-selective ALD studies in the literature have used SAMs. SAMs are monolayer organic films that form spontaneously on solid surfaces, and they have been used to modify the physical, chemical, and electrical properties of semiconducting, insulating, and metallic surfaces.62-64 Their potential applications include the control of wetting and adhesion, tribology, chemical sensing, ultrafine scale lithography, and protection of metals against corrosion.62-64 Furthermore, SAMs have been used previously as ultrathin resists for microcontact printing at the subnanometer scale,43,44 in photolithography,65,66 for electron beam lithography,67-69 and for scanning probe lithography.70,71 In the area-selective ALD process that we will describe, the SAM is used to modify the chemical properties of the substrate surface such that it resists the atomic layer deposition chemistry. Traditional SAMs, including organosilanes on oxide substrates, as well as other monolayers such as alkenes attached directly to Si and Ge surfaces, have been examined by our group as ALD resists. In addition to the monolayers, we have also investigated polymers as ALD resists. This article will focus primarily on the organosilane SAMs. For more information on other monolayer resists, the reader is referred to ref 53. 4.1. SAM ALD Resists and the Mechanism of Blocking. In order to work as a blocking resist, a SAM must prevent the ALD precursors from nucleating. To understand the factors that determine the SAM’s capacity to prevent ALD, we have studied a series of SAMs with different properties, including the chain length, tail group structure, reactive headgroup, and formation
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Figure 3. Chain-length dependence of alkyltrichlorosilane blocking efficiency. Water contact angle before ALD (b), film thickness before ALD (2), hafnium atomic percentage after ALD (9). Data from ref 51.
time, and tested each against HfO2 ALD.51,72 HfO2 provides a good test case because it involves highly reactive precursors which are difficult to block. In ALD of HfO2, the initial growth depends on the presence of surface hydroxyl groups and Si-O-Si bonds.39,54-56,73 The goal of the resist is therefore to block these sites without introducing any new nucleation sites. In this section, we will discuss the impact of chain length, tail group structure, and reactive headgroup of the SAMs on their deactivating efficiency against ALD of HfO2. The formation time dependence will be discussed in the following section. The chain length dependence of the SAM was probed by examining a series of linear aliphatic alkyltrichlorosilanes against HfO2 growth. The properties (film thickness and hydrophobicity) of the SAM were characterized using ellipsometry and water contact angle measurement, and the deactivating effectiveness was measured by applying XPS to determine the Hf at % after a fixed ALD process of 50 cycles. The results are summarized in Figure 3. As seen, the film thickness of the SAMs increases linearly with the number of carbon atoms, and the water contact angle increases rapidly upon adding the first two methyl units to the alkyl chain and then gradually approaches a plateau at a value of 110°. Such behavior has been previously reported for alkylsilane SAMs.62 The observed Hf at % after the ALD process on the SAM-coated substrates decreases from the value obtained for the SAM-free substrates, and the magnitude of the decrease depends upon the chain length of the SAM. Complete blockage of the HfO2 growth can be achieved only by molecules with alkyl chain lengths greater than 12 units of carbon.51 For the chain length dependence, in addition to linear aliphatic alkyltrichlorosilanes which require oxides for attachment, we have also studied monolayers formed from 1-alkenes and 1-alkynes, which can directly attach onto hydrogen-terminated Si or Ge. Attachment occurs via hydrosilylation (hydrogermylation) chemistry.37 The results for Ge are summarized in Table 1. For both the alkenes and the alkynes, the hydrophobicity, the thickness, and the deactivating efficiency of the organic films are found to increase with an increase in the alkyl chain length. Although the attachment mechanism is quite different, it is again found that the alkyl chain length must exceed 12-carbons before the Hf at % after ALD drops below the XPS detection limit (0.1%). To further investigate the behavior of the SAMs toward ALD, cross-sectional, high-resolution (HR) TEM images were obtained following ALD on an octyltrichlorosilane (OTS) deactivated silicon wafer and an octadecyltrichlorosilane (ODTS)-coated silicon wafer. For comparison, a chemical-oxide-covered silicon wafer is also included.37,51 Figure 4 shows that a uniform HfO2
Jiang and Bent layer is formed on the silicon oxide substrate, as expected for this ALD process.39,54-56,73 However, some aggregation of the HfO2 film is observed on the sample coated with OTS, and no HfO2 is detectable on the ODTS deactivated sample. The TEM analysis is consistent with the XPS results, where the blocking efficiency of HfO2 was found to increase with the longer-chain SAMs. The 8-carbon chain SAM is not sufficient to block HfO2 ALD, according to both XPS and TEM analysis, whereas the 18-carbon chain is sufficient. Moreover, the results of the crosssectional TEM images indicate that for the partially deactivated case using OTS the HfO2 film grows directly at the SiO2 interface instead of on top of the organic film. Based on the observations above, a possible deactivation mechanism for the SAMs is proposed. This model is sketched schematically in Figure 5 using aliphatic alkyltrichlorosilanes as an example. The aliphatic alkyltrichlorosilanes are known to react with surface hydroxyl groups, leaving the alkyltrichlorosilanes bonded to the surface through covalent Si-O bonds. Hence, the reactive Si-OH groups at the SiO2 surface are removed by reaction with the silane molecules. However, studies have shown that, due to local bonding mismatch between the organic layer and the Si-O-Si interface, together with the complex surface structure of the SiO2 surface, there are always some unreacted surface hydroxyl groups.62,74 In addition, Si-O-Si bridging oxygen atoms and other defects remain at the SAM/SiO2 interface.62,74 These can all serve as potential nucleation sites for ALD. For example, if HfCl4 molecules reach those sites, they can react and nucleate the ALD process.38,75 Consequently, the SAM resist must serve a second purpose, which is to act as a physical mask to prevent the precursors from reaching the SiO2 interface. This is why the long chain alkylsilane SAMs work best: their tail groups pack well, forming a barrier against precursor diffusion. For shorter chain SAMs, the interchain van der Waals attraction, which is proportional to number of CH2 groups for linear alkanes, is relatively weak, resulting in loosely packed SAMs where the precursors can penetrate the organic layer to reach the organic-SiO2 interface. As the chain length of the organic layer increases, the interchain van der Waals attraction becomes stronger and the SAMs become better packed and more hydrophobic as illustrated in Figure 5a. Under current ALD conditions, once the chain length increases to a certain point, indicated by the plateau in the value of the contact angle in Figure 3, the SAM is able to completely block the penetration of precursors to the reactive organic-SiO2 interface. At the same time, the long purging times between each precursor pulse is also important to ensure the removal of any physisorbed ALD precursors, which may act as nucleation sites, from the substrates and to achieve complete (or nearcomplete) inhibition of the ALD reactions. In addition to linear alkylsilanes, we also examined other SAMs to investigate the effect of the tail group structure on SAM quality and resist performance. These included phenyl groups, branched chains, and fluorocarbons. The properties of the SAMs, including water contact angle and thickness, and deactivating efficiency represented by the resulting Hf at %, are listed in Table 2. The results show that the tail group structure has a substantial effect on the blocking efficiency. The deactivation capability becomes worse as the tail groups become more complex and bulky. This is most likely because of the degraded packing density of such SAMs, as shown in Figure 5b. The lower packing density allows penetration of the precursors through the SAM to reach the surface active sites. In contrast to the hydrocarbon alkyltrichlosilanes, fluoroalkyltrichlorosilanes demonstrate better inhibition of ALD growth.
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TABLE 1: Water Contact Angle and Film Thickness for Monolayers Formed from Reaction of 1-Alkenes and 1-Alkynes on Ge(100)-H Sample before ALD and XPS Elemental Analysis after HfO2 ALDa sample description (organic monolayer on Ge(100)-H)
1-alkenes CnH2n
1-alkynes CnH2n-2 none
static water contact angle (degree)
film thickness of organic film (Å)
Hf at % after HfO2 ALD
1-octadecene 1-hexadecene 1-tetradecene 1-dodecene 1-decene 1-octene
109.2 ( 0.7 108.9 ( 1.1 108.4 ( 0.9 107.3 ( 0.7 105.1 ( 1.3 103.6 ( 1.2
24.8 ( 0.6 22.5 ( 1.3 20.1 ( 1.1 18.3 ( 0.8 15.5 ( 0.7 13.8 ( 1.1
