In Situ Analysis of the Growth and Dielectric ... - ACS Publications

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In Situ Analysis of the Growth and Dielectric Properties of Organic Self-Assembled Monolayers: A Way To Tailor Organic Layers for Electronic Applications Aleksandr Markov,* Kyrylo Greben, Dirk Mayer, Andreas Offenhaü sser, and Roger Wördenweber Peter Grünberg Institute (PGI-8), Forschungszentrum Jülich, Jülich 52425, Germany

ABSTRACT: Organic nanoscale science and technology relies on the control of phenomena occurring at the molecular level. This is of particular importance for the self-assembly of molecular monolayers (SAM) that can be used in various applications ranging from organic electronics to bioelectronic applications. However, the understanding of the elementary nanoscopic processes in molecular film growth is still in its infancy. Here, we developed a novel in situ and extremely sensitive detection method for the analysis of the electronic properties of molecular layer during molecular layer deposition. This low-frequency sensor (1 kHz) is employed to analyze the standard vapor deposition process of SAMs of molecules and, subsequently, it is used to optimize the growth process itself. By combining this method with an ex situ determination of the effective thickness of the resulting layers via ellipsometry, we observe a large difference of the permittivity (1 kHz) of the examined aminosilanes in the liquid state (εliquid = 5.5−8.8) and in SAMs (εSAM = 22−52, electric field in the plane of the layer). We ascribe this difference to either the different orientation and order of the molecules, the different density of molecules, or a combination of both effects. Our novel in situ analyses not only allows monitoring and optimizing the deposition of organic layers but also demonstrates the high potential of organic SAMs as organic high-k layers in electronic devices. KEYWORDS: self-assembled monolayer, APTES, molecular layer deposition, in situ characterization, permittivity

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INTRODUCTION Organosilane reagents have attracted significant attention in recent years due to the production of functionalized thin films on silicon oxide and other oxide substrates that can be used for a variety of technological applications. For example, aminosilanes may serve as the foundation layer for the fabrication of various biosensor and bioarrays.1−5 They can be used for the production of arrays of metal nanoparticles,6−8 energy storage devices,9 and probe protein and cell adhesion,10−12 and, furthermore, it is known to be compatible with graphene.13 Due to the fact that the morphology and surface concentration of the aminosilane film is crucial for these applications, layers prepared via several deposition techniques like self-polymerization in solution,14 vapor coating,15 or chemical vapor deposition16 have been investigated and different types of growth (e.g., with respect to the density or angle of orientation of the molecules) have been reported for the growth of thin films of aminosilanes on silicon wafers, glass, or other oxide substrates.1,15−21 However, the analytical techniques which © 2016 American Chemical Society

have been used in those studies represent either ex situ techniques and thus provide information about layers after deposition (e.g., wetting angle measurements or ellipsometry) or they are very complicated in situ techniques (e.g., in situ Xray diffraction or X-ray photoelectron spectroscopy). Furthermore, among the in situ controlled parameters the significant ones are missing, that is, the electronic properties (resistive and dielectric properties) of the molecules, which are especially important with respect to the possible applications in organic electronics. In this work we introduce an in situ method to precisely control the growth of molecular layers during molecular layer deposition (MLD), which can provide exquisite control of the film thickness22 and electronic properties of the layer. To do that, we use interdigitated capacitive electrodes on glass Received: April 5, 2016 Accepted: June 6, 2016 Published: June 6, 2016 16451

DOI: 10.1021/acsami.6b04021 ACS Appl. Mater. Interfaces 2016, 8, 16451−16456

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ACS Applied Materials & Interfaces

Table 1. Molecules and Their Properties Used in This Work Including Structure, Experimentally Determined Permittivity εliquid in the Liquid State, and Experimentally Determined Thickness hmol of the SAMa

a

Additionally, literature values for the length lmol (long axis) of the molecules are given. molecules on the different types of substrates has been checked among others via ellipsometry, atomic force microscopy, and contact angle measurements. According to these measurements the layers turn out to be identical within the measurement accuracy.23 Cleaning Procedure. Since the formation of the SAMs on the substrate depends strongly on the surface quality,24 the cleaning of the sample surface is very important. The substrates are cleaned first in acetone for 5 min in an ultrasonic bath (25 °C at 320 W power and 37 kHz frequency), then in isopropyl alcohol (2-propanol, > 99.8%, KMF), and then in an ultrasonic bath (5 min at 25 °C, 320 W, and 37 kHz) and, finally, dried with nitrogen. For the ex situ thickness measurement of the SAMs we perform reference measurements on the SiO2-terminated Si samples via ellipsometry (SE 800 PV) directly after the cleaning procedure. Molecules. The different molecules (APTES, 99%), (3-glycidyloxypropyl)-trimethoxysilane (GLYMO, ≥98%), (3-mercaptopropyl)trimethoxysilane (MPTES, 95%), and 3-(ethoxydimethylsilyl)-propylamine (APDMES, 97%) are obtained from the company SigmaAldrich (St. Louis). For the test of the sensor, ethanol (99%) is used. Molecular formulas and properties of the different molecules are listed in Table 1.16,25−27 The molecules are kept in a glovebox. Before deposition an amount of 0.2 mL of the molecule is filled in a small, vacuum-tight glass flask (the molecule source) which is then connected to the deposition chamber via a valve. Before deposition the valve is closed; for the deposition of the molecules the valve is opened. Surface Functionalization. After the cleaning process and the ellipsometry, the Si substrate is placed in the deposition chamber next to the sensor for in situ capacitive measurement (the capacitive sensor is described below). The chamber is evacuated and then filled with pure oxygen gas (99.9%) with a pressure of 1 mbar and a radio frequency discharge is applied for 3 min, generating activated oxygen. The oxygen plasma activation represents an important step in the process of surface modification for silanization processes. It takes place at the sample and at the sensor surface. First of all, organic residues are removed. Moreover, the oxygen plasma treatment results in an activation of the surfaces of the sample and the sensor. The activation of the silicon surface represents a reconstruction of the silanol surface bonds. Freshly prepared SiO2 exhibits silanol groups Si−OH on the surface. When exposed to a humid environment, the silanol groups undergo a condensation reaction, forming more stable siloxane groups Si−O−Si which reduces the reaction with the molecules during

substrates, which allow a direct determination of the resistance, capacitance, and loss tangent of the growing organic layer. We demonstrate the performance of the sensor for a test molecule (ethanol) that adsorbs but does not form a stable film and for a number of silanes that form a self-assembled monolayer (SAM). Using this in situ control, we can not only optimize the deposition process for SAMs but also, using an ex situ determination of the thickness of the SAM (via ellipsometry), determine the permittivity of the SAM. The permittivity of the SAMs turns out to be much larger than the permittivity of the molecules in the nonoriented liquid state. We ascribe this difference to either the different orientation and order of the molecules, the different density of molecules, or a combination of both effects. We demonstrate that this new method not only allows monitoring and optimizing of the molecular deposition process but also demonstrates that we can achieve molecular layers with very high permittivity [e.g., εSAM ≃ 52 for (3aminopropyl)-triethoxysilane (APTES)] which might be of interest for a number of applications that require high-k layers.

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EXPERIMENTAL SECTION

In this section we describe the basic steps of the SAM deposition and the new in situ sensor technique, the capacitive sensor. The data obtained via the capacitive sensor are compared to and analyzed using ellipsometry and wetting-angle measurements (the latter is not shown here). It should be noted that all experiments represent mean-field methods and, thus, provide integral properties of the layer. Nevertheless, especially ellipsometry is well established for the characterization of molecular layers (including SAMs), our ellipsometry data on monolayers agree perfectly with literature values, and images recorded via atomic force microscopy (a local analysis) support our results. Substrates. In this work we used borosilicate glass (Präzisions Glas & Optik, Iserlohn, Germany) as substrates for the in situ capacitive sensor and p-doped silicon (Si(111)) with a 100 nm thick SiO2 termination layer for the ex situ ellipsometry measurements, respectively. Both types of substrates are well suitable for biological applications and compatible for electronic circuity. Furthermore, molecules with silane head group such as APTES can be chemisorbed to these substrates. The quality of the SAM layers obtained for these 16452


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where ε0 is the vacuum permittivity, εmol and hmol represent the permittivity and thickness of the molecular layer, respectively, s represents the gap between the electrodes, and l represents the length of the electrodes. Since normally the gap s between the electrodes is significantly larger than the thickness hmol of the molecular layer, we can simplify eq 2 to

deposition.28 In this state, the Si surface has fewer open bonds and the quality of the silane deposition would be poor. Activation with an oxygen plasma recovers the silanol groups and leads to a homogeneous and dense coverage of the surface with molecules. Deposition Process. After the oxygen plasma treatment the oxygen is removed from the chamber, and the process parameters are established and stabilized. This involves the stabilization of the substrate temperature (all depositions take place at room temperature) and the gas pressure. As the working gas N2 is used, different pressure set points and pressure time profiles are used and will be described with the experiment below. Most important is the in situ electronic characterization using the capacitive sensor which is described in detail in the following section. Capacitive Sensor for the In Situ Monitoring of the Deposition. The novel method of in situ control of the deposition process is based on a capacitive sensor which can be used to record the dielectric permittivity, the dielectric losses, or even the conductivity of the growing molecular layer. An SEM image of the interdigitated structure is shown in the inset of Figure 1 A. The interdigitated

Cmol = ε0εmol

lhmol s

(3)

A demonstration of the sensor is shown in Figure 1, where we deposited a molecule that does not form a stable layer, that is, ethanol. Since ethanol possesses a large vapor pressure, we choose a large N2 work pressure of 30 mbar for the deposition. Figure 1A shows the development of the thickness hmol of the molecular layer in time using eqs 1 and (3); the sketches in Figure 1A schematically illustrate the situation of the deposition. After stabilization of the N2 pressure, the valve that separates the molecule source from the deposition chamber is opened and the deposition starts. After a brief instability due to the opening of the valve, the thickness of the layer increases nearly linearly (see dashed line Figure 1A). Ethanol molecules adhere with a constant rate and gradually fill the gap between the electrodes. At a certain point, the thickness of the adsorbed layer exceeds the thickness of the electrodes (15 nm) and ethanol will start to form a continuous layer covering also the Pt electrodes. This obviously leads to a stronger increase of the capacitance, which is visible in the figure (steeper slope in Figure 1A). Actually, for the latter case eq 3 is not strictly valid anymore; moreover, the contribution Cgas has to be modified. Nevertheless, also for this regime the evaluation provides a good estimation of the thickness of the layer. For the deposition of silanes that is discussed later, we do not use this regime. After closing the valve, deposition of additional ethanol molecules is stopped (there might be a small amount of redeposition of molecules) and desorption of molecules dominate the growth process. Consequently, the thickness of the layer decreases steadily and, finally, the layer thickness approaches zero, that is, no continuous layer of molecules in the gap. This example nicely illustrates the potential of this sensor.

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Figure 1. (A) Molecular thickness evaluated from the capacitance of the capacitive sensor for the deposition of ethanol at a pressure of 30 mbar using εliquid = 24.3 (see Table 1). The shaded (red) area represents the time when the molecule source is open and molecules can be deposited, the large inset shows a SEM image of part of the interdigitated structure of the capacitive sensor (gap size 1 μm), and the small sketches illustrate the situation of ethanol deposition at different times. Furthermore, a schematic illustration of the partial capacitance model (B) and the capacitive structure with molecules between the electrodes (C) is given.

RESULTS AND DISCUSSION Figure 2A shows the molecular contribution Cmol to the capacitance for “standard” APTES deposition processes that are performed at different N2 gas pressures (7, 5, 1, and 0.1 mbar). In these standard processes32 a given process pressure is established and stabilized before the molecular source is opened and deposition sets in. Generally, the valve is opened at time defined as t = 0 (see Figure 2). The deposition rate strongly

electrodes (IDE) are prepared via e-beam lithography and lift-off technology. The electrodes consist of a combination of Ti (5 nm thick) and Pt (10 nm thick) layers. The IDEs form a capacitor with a gap s = 1 μm and an effective length of the capacitor of 10.8 mm (73 fingers with an overlapping gap length of 150 μm). The large gap size of 1 μm is chosen to simplify the analysis and ensure a homogeneous deposition of molecules between the electrodes, whereas the large effective length of 10.8 mm leads to a high sensitivity of the sensor. The total capacitance of the sensor is given by the partial capacitance model, C total = Cair + Csub + Cmol = Cref + Cmol

(1)

where the experimentally determined capacitance Ctotal consists of different contributions of the gas (Cgas), the substrate (Csub), and the molecular layer (Cmol) as illustrated in Figure 1. By measuring the total capacitance before deposition, we obtain Cref. The change of the capacitance during deposition provides the resulting capacitive contribution of the molecular layer. This contribution can be evaluated for planar capacitors according to29−31 Cmol = ε0εmol 4 π

l ln 2 +

s hmol

Figure 2. (A) Capacitance of molecular layers of APTES as a function of time for 7, 5, 1, and 0.1 mbar N2 work pressure and (B) comparison of the capacitive contribution Cmol and ex situ measured (ellipsometry) effective thickness of the APTES layer for the deposition at 1 mbar. Additionally, the literature value for a monolayer of APTES is indicated in (B).

(2) 16453


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complete formation of the layer. In detail, after opening the valve (no. 1 in Figure 3, time t = 0) there is only a small pressure peak due to the pressure difference between the molecule source and recipient, but no deposition as indicated by the stable capacitive signal of the sensor (Cmol = 0). After ∼2 min of stabilization, the N2 pressure is continuously decreased. At ∼8 mbar (no. 2 in Figure 3) evaporation sets in and is detected in the form of an increase of the capacitance of the sensor. With decreasing pressure the molecule vapor pressure increases. The increase of the capacitance indicates that the film thickness also increases. After 30 min a N2 pressure of ∼0.5 mbar is reached and according to the sensor signal the molecular layer is already quite thick. Here, we start to increase the N2 pressure again (no. 4 in Figure 3). Simultaneously, the capacitance starts to decrease, which indicates that with increasing pressure (decreasing molecular vapor pressure) the desorption dominates the adsorption of the molecules at the sensor. At high pressure (no. 5 in Figure 3) the valve of the molecule source is closed and N2 gas is continuously pumped out. Finally, the N2 flow is stopped (no. 6 in Figure 3) and the pressure drops to the background pressure of ∼1.5 × 10−3 mbar. The capacitance decreases continuously during this process of pressure reduction and finally saturation at ∼3.7 fF (no. 7 in Figure 3). The modified deposition process recorded in Figure 3 shows a number of interesting features: (i) The deposition (no. 2 in Figure 3) starts at ∼8 mbar, which agrees with the maximum pressure at which APTES molecules vaporize, given in the literature.15,16,20 (ii) With decreasing pressure the thickness of the molecular layer increases. It might be accidental, however; there are two different behaviors visible. In the first part (no. 2−3 in Figure 3) the increase is more “noise” and shows a different slope compared to the second part (no. 3−4 in Figure 3). The transition from the first part to the second part (no. 3 of Figure 3) occurs at a value Cmol that seems to agree with the final values obtained after deposition (no. 7 of Figure 3) which represents the value obtained for a single molecular layer. Therefore, it might be that the first part (no. 2−3 of Figure 3) represents the formation of the SAM, whereas the second part (no. 3−4 of Figure 3) represents the deposition of additional molecules onto the SAM. These molecules are removed after deposition. Further work on this is in progress and will be published in a forthcoming publication. (iii) Once the SAM is formed (no. 3 in Figure 3), adsorption dominates desorption for decreasing pressure and vice versa for increasing pressure. (iv) After deposition (no. 4 in Figure 3) additional molecules are removed from the SAM. This process can take a long time (here about 1.5 h). (v) Finally (no. 7 in Figure 3), the capacitance contribution Cmol saturates, indicating that a stable molecular layer is formed. The thickness of the layer measured via ellipsometry is hmol ≃ 0.7 nm; that is, it agrees with the literature value for SAMs of APTES. From this we conclude that we deposited a SAM of APTES. However, the capacitive signal leads to a permittivity of the SAM of εSAM ≃ 51, which is much larger than the values εliquid reported for APTES in the liquid state; see Table 1. This will be discussed in detail in the following. For comparison, we performed similar experiments for the other molecules given in Table 1. Similar to those of APTES, we obtained quite large values for the permittivity εSAM of the resulting SAM. The data are given in Table 1 and Figure 4. To compare the permittivity εSAM with the permittivity εliquid of the molecules in the liquid state, we constructed a simple parallel-

depends on the vapor pressure of the molecules which is related to the process pressure. For “standard” deposition of APTES we typically use a process pressure of 0.1−7 mbar. After deposition, the valve of the molecule source is closed and the recipient is pumped down to 10−4 mbar to remove excess molecules from the sample and the recipient. Finally, the layer thickness of all films is measured via ellipsometry. In all cases the layer thickness agrees with the value reported for SAMs of APTES in the literature;25 that is, hmol ≃ 0.7 nm. In a next step we analyze the layer thickness during the deposition in a series of experiments in which the deposition is stopped at different times and the thickness of the molecular layer on the reference sample is measured via ellipsometry after each of these depositions. Figure 2B suggests that the thickness measured via ellipsometry seems to agree with the in situ measured capacitance changes. This indicates that in this standard process the SAM is gradually formed with the increase of the thickness of the molecular layer and that the SAM is finally accomplished when the thickness of the molecular layer saturates. This already tells us a lot about the SAM formation; however, it demonstrates two problems: (i) Why does the capacitance Cmol saturate at different values for different pressures, although we finally obtain monolayers with ∼0.7 nm thickness in all cases, and (ii) inserting hmol = 0.7 nm into eq 3 would yield different and unrealistically large values for the permittivity of the molecule (e.g., εmol ≃ 190 and εmol ≃ 120 for 0.1 and 5 mbar, respectively). Therefore, we examined the deposition process and the permittivity of the molecules in more detail. For this we modified the deposition procedure by introducing a pressure profile (see Figure 3). To avoid the

Figure 3. Example of the deposition process for APTES using a modified N2 pressure profile (right scale). The molecular contribution Cmol to the capacitance of the sensor is recorded even after deposition. The different numbers represent the opening of the valve of the molecular source (1), the onset of deposition (2), the change of the deposition characteristic (most likely due to completion of the SAM) (3), the increase of the N2 pressure profile (4), the closing of the molecular source (5), the closing of the N2 flow (6), and the saturation of the capacitive signal (7).

instability of the process when opening the valve of the molecule source, we started and terminated the actual deposition by opening and closing the valve of the molecular source at a high pressure (here, 18 mbar N2) at which no evaporation of molecules is possible. For the actual deposition the N2 pressure is reduced, and furthermore, the development of the layer is monitored even after deposition to observe the 16454


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molecules, and (iii) the desorption of these disordered additional molecules after deposition until (iv) the final SAM is left over. The resulting permittivity of the SAM (electric field in the plane of the layer) turns out to be much larger (4−9 times) than the permittivity of the molecules in the liquid state. This discrepancy can be explained by the anisotropy of the permittivity of the molecule, the difference in the molecular density in SAMs and liquids, or a combination of both explanations. The permittivity of the SAM might be an indication of the quality (density, order, and orientation) of the SAM. Values of εSAM ≃ 51 obtained for our SAMs of APTES demonstrate that molecular layers have a high potential for a number of electronic applications that require high permittivity.

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Figure 4. Comparison or the permittivity of molecules in SAMs (upper symbols) and in the liquid state (lower symbols) as a function of the thickness of the SAM layer. The permittivities εSAM and εliquid are evaluated from the capacitive measurements on SAMs (electric field in the plane of the layer) using the layer thickness hmol determined via ellipsometry and a parallel plate capacitor, respectively. The difference measurements are sketched; the inset shows the ratio of the different permittivities.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.M.). Notes

The authors declare no competing financial interest.

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plate capacitor setup with electrodes of 10 × 10 mm2 and a gap s = 1 mm (see sketch in Figure 4), and determined the permittivity, losses, and conductance of the different molecules in the liquid state. The resulting permittivity εliquid ranges between 5.5 and 8.8 (see Table 1 and Figure 4). The comparison of the permittivity obtained for the molecules in the liquid state and the SAMs (electric field in the plane of the layer) shows that the values for εSAM are 4−9 times larger than εliquid for the same type of molecule (see inset of Figure 4). There are mainly two reasonable explanations for the large difference of the permittivity of the molecules in the different phases. First, complex molecules are known to be birefringent,33 which automatically implies that their permittivity is anisotropic. Because of their structure, the molecules which are used in this work should also possess anisotropic permittivity. Consequently, they should automatically show different permittivity in SAMs and in the liquid state. In SAMs molecules are orientated, in the ideal case, normal to the substrate surface or slightly tilted, whereas in the liquid state there exist no well-defined orientation of the molecules. Second, it is known that SAMs with different density of molecules can be obtained via different deposition techniques.21 Consequently, not only the orientation but also the density of molecules in SAM and liquid state differs appreciably. Both effects will have an impact on the electronic characteristics.23 From this we can conclude that the permittivity of the dense state (SAM) can be larger than the permittivity of the less dense state (liquid). This is what we observe in the experiments for all molecules that were examined. Most likely, both of these explanations (i.e., anisotropic permittivity and different molecular density) are responsible for the large difference in the permittivities εliquid and εSAM.

ACKNOWLEDGMENTS Authors thanks Rolf Kutzner, Tino Ehlig, and Stephan Trellenkamp for productive cooperation and discussions.

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REFERENCES

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CONCLUSIONS In this work we introduced a relatively simple but very effective analytical tool (i.e., a capacitive structure) which allows in situ analysis of the electronic properties of organic layers during their growth and, thus, monitor and optimize the growth of molecular layers especially SAMs. Using this sensor, we monitored (i) the growth of SAMs of molecules, (ii) the subsequent deposition of additional (typically nonoriented) 16455


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