Article pubs.acs.org/Langmuir
In Situ Monitoring of Atomic Layer Deposition in Nanoporous Thin Films Using Ellipsometric Porosimetry Jolien Dendooven,*,† Kilian Devloo-Casier,† Elisabeth Levrau,† Robbert Van Hove,† Sreeprasanth Pulinthanathu Sree,‡ Mikhail R. Baklanov,§ Johan A. Martens,‡ and Christophe Detavernier† †
Department of Solid State Sciences, COCOON, Ghent University, Krijgslaan 281/S1, B-9000 Ghent, Belgium Centre for Surface Chemistry and Catalysis, KU Leuven, Kasteelpark Arenberg 23, B-3001 Heverlee, Belgium § IMEC, Kapeldreef 75, B-3001 Leuven, Belgium ‡
ABSTRACT: Ellipsometric porosimetry (EP) is a handy technique to characterize the porosity and pore size distribution of porous thin films with pore diameters in the range from below 1 nm up to 50 nm and for the characterization of porous low-k films especially. Atomic layer deposition (ALD) can be used to functionalize porous films and membranes, e.g., for the development of filtration and sensor devices and catalytic surfaces. In this work we report on the implementation of the EP technique onto an ALD reactor. This combination allowed us to employ EP for monitoring the modification of a porous thin film through ALD without removing the sample from the deposition setup. The potential of in situ EP for providing information about the effect of ALD coating on the accessible porosity, the pore radius distribution, the thickness, and mechanical properties of a porous film is demonstrated in the ALD of TiO2 in a mesoporous silica film.
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INTRODUCTION Mesoporous thin films are of interest to a variety of technologies. Functionalization of the interior surface plays a crucial role in rendering porous films appropriate for applications, e.g., as sensor devices,1,2 filtration membranes,3−6 or supported catalysts.7,8 Methods to incorporate active species in nanoporous films or membranes include grafting,5,9 liquid impregnation,10,11 and atomic layer deposition (ALD).12−17 ALD uses sequential self-terminating gas−solid reactions to deposit ultrathin coatings in a layer-by-layer fashion.18,19 The self-limiting nature of the chemical reactions ensures superb thickness control and excellent conformality, even on highly porous materials. These advantages not only allow for functionalization of porous materials but also for atomic-level tuning of their pore size.20−23 In the field of microelectronics the performance of Cu/low dielectric constant (low-k) interconnects can be improved by introducing porosity in the intermetal dielectrics. The incorporation of pores, typically on the order of angstroms to a few nanometers, poses, however, significant integration challenges. Because the dielectric constant is highly sensitive to physical and chemical damage, a sealing layer is required to shield the pores from moisture, gas precursors, and plasma radicals that are involved in subsequent processing steps. Some authors have investigated the use of ALD to seal the pores of low-k dielectrics.24−27 The implementation of ALD for functionalization or sealing of porous thin films requires characterization techniques that can evaluate the effect of the ALD treatment on the accessibility © 2012 American Chemical Society
and size of the pores. Nondestructive techniques that have been applied to determine the pore radius distribution (PRD) in nanoporous films, especially in low-k dielectrics, include positron annihilation lifetime spectroscopy,28 X-ray reflectivity porosimetry,29 small-angle neutron scattering porosimetry,30 and ellipsometric porosimetry (EP).31−34 Given that ellipsometry is increasingly employed for in situ thickness monitoring during ALD on planar substrates,35 the EP technique has a lot of potential to become a significant characterization technique for ALD coatings in porous substrates. EP is based on spectroscopic ellipsometry (SE) to measure the change in optical properties and thickness of a porous film during adsorption and desorption of a solvent vapor. Provided that the optical properties of the liquid adsorptive are known, the measured optical properties can directly be related to the volume fraction of the liquid in the film. As such, adsorption and desorption isotherms can be obtained by EP from which the porosity and PRD can be calculated. Although some authors, including us, have reported the use of EP to characterize ALD-coated porous materials,23,36,37 the EP technique is still relatively new in ALD research. The aim of this paper is to demonstrate that the EP technique can provide valuable information about the effect of an ALD coating on the porosity, the PRD, the thickness, and mechanical properties of a porous film as well as to give an overview on how this Received: January 4, 2012 Revised: February 4, 2012 Published: February 4, 2012 3852
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information can be obtained. Furthermore, we demonstrate how the EP technique can be implemented on an ALD reactor equipped with in situ SE. This allows for studying ALD in porous thin films without exposing the sample to ambient air. Several advantages as well as difficulties that need to be overcome are addressed. As an example, we will apply our in situ approach to ALD of TiO2 in a mesoporous silica thin film.
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EXPERIMENTAL METHODS
ALD Setup with Integrated EP System. The EP technique has been implemented on a home-built ALD setup consisting of a remotely placed plasma source, a wall heated deposition chamber, and a turbomolecular pump, which are separated by pneumatic gate valves. The setup can accommodate up to 8 ALD precursors and 6 reactants. The gas lines are heated to prevent condensation of the precursors. A combination of needle and pneumatic valves is used to control the gas flow into the chamber. The substrate temperature can be set up to 350 °C with resistive heating elements that are incorporated in a copper block supported by stainless steel tubes. A Woollam M-2000U ellipsometer (245−1000 nm) can be mounted directly onto the ALD chamber using two optical viewports. To prevent deposition on the optical windows, they are sealed off by pneumatic gate valves when precursor gases are present in the reactor. A first requirement to employ EP on an ALD setup is the connection of an appropriate adsorptive to the chamber. During an EP measurement, the ALD chamber must be filled with the adsorptive until the pressure reaches the vapor pressure at the temperature of the nanoporous film (P0). The adsorptive should therefore be a volatile liquid with a sufficiently high vapor pressure at the working temperature for the EP measurements. Ideally, EP should be performed at the temperature that is required for the ALD depositions (150−250 °C). It is, however, important to realize that the maximum pressure of the adsorptive is determined by the coldest point in the ALD chamber. Therefore, in order to allow the pressure of the adsorptive to reach the vapor pressure corresponding to the sample temperature, the complete ALD chamber must be heated to a temperature that equals or is higher than the sample temperature. In our setup, it was not possible to heat all the parts of the ALD chamber to a temperature above the deposition temperature. Especially the stainless steel tubes supporting the sample heater and the large flanges separating the ALD chamber from the turbomolecular pump and the plasma source could not be heated sufficiently. Therefore, we decided to switch off all the heating elements and perform the EP measurements at room temperature (RT), i.e., with both the sample and the complete ALD chamber at RT. Toluene is chosen as adsorptive because it is known to be suitable for RT porosimetry.38 Similar to the ALD precursor lines, the toluene delivery line is equipped with a needle valve and a pneumatic valve. For the desorption, a rotary pump has been added to the setup. To enable gradual pumping of toluene, the connection is made via a quarter inch tubing with a needle valve and a pneumatic valve. A last important issue is the need for accurate pressure readouts (P). Therefore, an additional pressure gauge has been connected to our ALD setup. It concerns a gas-independent capacitance manometer (MKS Baratron) with a pressure range of 1 Pa−13.33 kPa, which can be sealed off from the reactor during ALD. A sketch and photograph of the ALD-EP setup are shown in Figure 1. Note that the plasma source (not used in this work) enables plasma-enhanced ALD39,40 or plasma pretreatment of (porous) samples prior to conventional ALD.36 The described setup thus allows us to sequentially apply ALD depositions and EP measurements on a porous thin film without moving the sample or exposing it to ambient air. However, heating and cooling of the sample are required during the ALD−EP sequence because ALD is usually performed at elevated temperatures, while for EP the whole deposition chamber has to be brought to RT (Figure 2). This substantially extends the duration of an in situ experiment, where the term in situ refers to the vacuum environment, as opposed to operando techniques which are carried out under operating conditions.
Figure 1. (a) Schematic representation of the ALD chamber, equipped with a plasma source and in situ SE, that is adapted for in situ EP measurements by connecting a container with toluene, a pumping system, and a dedicated pressure gauge. (b) Photograph of the ALD chamber used for EP measurements.
Figure 2. Schematic representation of the EP−heating−ALD−cooling sequence used to monitor the effect of TiO2 ALD on the properties of a mesoporous silica thin film. Performing an EP Measurement. An experiment typically starts with the EP characterization of the porous thin film that will be exposed to ALD. It is important to ensure that any atmospheric content is removed from the sample before the EP measurement is performed. This is achieved either in situ by degassing the sample overnight in high vacuum at RT or ex situ by drying the sample in an oven at 70 °C for a few hours before loading. 3853
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nanoslabs.23 After EP characterization of the porous thin film, the sample is exposed to 10 cycles of TiO2 ALD using tetrakis(dimethylamino)titanium (TDMAT, Ti(N(CH3)2)4) as Ti precursor and H2O as oxygen source.42 The substrate temperature is set to 200 °C, and the chamber walls are heated to 100 °C. During ALD, the chamber is continuously pumped by the turbomolecular pump (base pressure ∼10−4 Pa), and the gate valves to the ellipsometer windows are closed. An ALD cycle consists of 30 s TDMAT exposure at ca. 0.3 Pa, 60 s pumping, 30 s H2O exposure at ca. 0.3 Pa and 60 s pumping. Argon is used as a carrier gas for the TDMAT precursor. It is expected that these exposure times are sufficient to reach saturation in the porous thin film, as based on previous saturation tests (via X-ray fluorescence) in similar silica films.17 Following the ALD, the setup is cooled down again and another EP measurement is performed. This ALD-EP sequence has been repeated three times.
All heating elements are switched off and allowed to equilibrate, usually overnight, to RT before an EP measurement is started. The gate valves to the plasma source and the turbomolecular pump are closed and those to the ellipsometer windows opened. Initially, the pressure is below 1 Pa, and a first SE measurement is performed. Then, toluene is gradually introduced into the ALD chamber by opening and closing the pneumatic valve that is installed in the toluene delivery line. The pressure is typically raised in steps of 50 Pa, and each step is followed by a SE measurement. The rate of pressure rise is controlled by a needle valve, which is regulated to have a pressure increase of about 5 Pa s−1. With increasing pressure in the chamber, the needle valve has to be opened more to maintain this rate. When the pressure in the chamber approaches the equilibrium pressure of toluene at RT, the rate of pressure rise decreases, even though the needle valve is fully opened (Figure 3a). To avoid vapor condensation
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RESULTS EP Characterization of the Mesoporous Silica Thin Film. In this subsection, we aim to demonstrate how valuable information can be obtained from the SE data recorded during adsorption and desorption of toluene in and out of the mesoporous silica thin film. Ellipsometry measures the change in polarization of light upon reflection from a sample. This change is described as an amplitude ratio, psi, and a phase difference, delta, and is probed for a range of photon wavelengths (245−1000 nm). Figure 4a shows the psi and delta values measured at different relative pressures (P/P0) during adsorption of toluene in the (uncoated) mesoporous silica film. From these data, the thickness of the porous film and the refractive index n can be deduced through a fitting-based analysis. To this end, a certain dispersion relationship of the optical constants need to be assumed. For materials that are transparent over the total wavelength range, such as porous SiO2, a Cauchy relationship B C n(λ) = A + 2 + 4 λ λ
(1)
35
is often a good choice. This relation introduces three fitting parameters, A, B, and C in addition to the film thickness. Using the Cauchy equation, excellent fits are obtained to psi and delta measured on the empty porous silica film, P/P0 ∼ 0. The initial refractive index n0 is ∼1.1, and the film thickness is ∼164 nm. Although the quality of the fit becomes less good with more toluene adsorbed, the Cauchy model is found to be appropriate enough to describe the optical constants of a porous silica thin film (partially) filled with toluene. The fitting procedure results in values for the film thickness and refractive index (at 632.8 nm) at a series of relative toluene pressures (Figure 4b). For relative pressures below ∼0.85, the change in refractive index and film thickness is limited. This pressure range corresponds to first monolayer and then multilayer adsorption of toluene on the pore walls of the mesopores. For P/P0 ∼ 0.85, the refractive index quickly increases due to capillary condensation of toluene in the mesopores. Capillary condensation occurs because of the curvature of the pores, and the relative pressure at which this phenomenon takes place highly depends on the size of the pores. Once the pores are completely filled with toluene, the refractive index remains constant with increasing relative pressures. As also reported in previous studies, film shrinkage is observed in the relative pressure range where capillary condensation occurs.33,34,43,44 This effect is induced by microscopic capillary forces.33,43,45−47 After completion of capillary condensation, the film thickness increases due to relaxation of
Figure 3. (a) Typical change in relative pressure as a function of time during an automated EP measurement. This figure proves that the applied stopping criterion results in a value of the equilibrium pressure very close to the expected value of P0, as calculated based on the temperature. (b) Toluene vapor pressure as a function of temperature.41 on top of the film and on the reactor walls, the adsorption measurement is stopped when the rate of pressure rise drops below 2.5 Pa s−1.31 This stopping criterion is found to result in an end pressure, which is used as P0 in the analysis, that is close enough to the real vapor pressure to give reliable and reproducible results (Figure 3a). The temperature dependence of the vapor pressure of toluene is plotted in Figure 3b.41 Next, toluene is gradually removed from the reactor using the pneumatic valve that is installed in the pumping line. The valve is closed every 50 Pa to allow for a SE measurement. The needle valve in the pumping line is used to set the pumping rate at 5 Pa s−1. An EP measurement thus results in a set of SE data recorded at different relative toluene pressures (P/P0) in the ALD chamber. How these data are analyzed is described in the Results section. It should be mentioned that the EP measurements presented in this work were performed manually. Automation of the technique was later accomplished by means of a computer controllable needle valve. Performing ALD of TiO2. In this work the ALD−EP setup has been used to study the effect of TiO2 ALD on the properties of a mesoporous silica thin film with a tridimensional pore network built of 3854
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The result of the EMA approach is plotted in Figure 4c and is the adsorption branch of the sorption isotherm. The porosity of the mesoporous silica film is ∼77%. Figure 5a shows that the adsorption and desorption branch of the isotherm do not coincide. A hysteresis loop is formed
Figure 5. From sorption isotherm to pore radius distribution. (a) Adsorbed volume of toluene as a function of relative pressure (bottom axis) and Kelvin radius (top axis). The schematics illustrate the difference in shape of the meniscus that is formed at the liquid−vapor interface during capillary condensation (left) and evaporation (right) in cylindrical pores. (b) Pore radius distributions derived from the adsorption and desorption branches in (a) using the Kelvin equation with t-correction. Dashed lines are Gaussian fits to the data points.
Figure 4. From ellipsometry data to adsorption branch of the sorption isotherm. (a) SE data measured for a series of relative toluene pressures between ∼0 and 1: psi and delta as a function of wavelength. (b) Refractive index and film thickness as a function of relative toluene pressure obtained from fitting the SE data in (a) using the Cauchy model. (c) Relative adsorbed volume of toluene as a function of relative toluene pressure obtained from the fitted refractive index in (b) using the EMA model. The schematics illustrate three stages of toluene adsorption: (1−2) multilayer adsorption on the pore walls, (3) capillary condensation, and (4) complete filling of the mesopore.
due to differences in the mechanisms for capillary condensation and evaporation. In particular, the shape of the meniscus that is formed at the liquid−vapor interface is different. The Kelvin equation relates the mean radius of curvature of the meniscus, also called the Kelvin radius, rk, to the relative pressure ⎛P⎞ 2 1 1 RT = + =− ln⎜ ⎟ γVL cos(θ) ⎝ P0 ⎠ rk r1 r2
where r1 and r2 are the main radii of curvature of the meniscus, R is the gas constant, T is the temperature, γ is the surface tension of toluene, VL is the molar volume of toluene, and θ is the contact angle of toluene.31,48 Assuming that bulk properties are valid for the adsorbed toluene, γ is 0.0284 N m−1, VL is 1.06 × 10−4 m3 mol−1, and θ is ∼0°. The top axis in Figure 5a is derived from the bottom axis by applying eq 3. Meniscus formation depends on the geometry of the pores. Therefore, to relate the Kelvin radius to the pore radius, rpore, assumptions have to be made on the pore shape. For cylindrical pores (with open ends), the meniscus is cylindrical shaped (r1 = rpore, r2 = ∞) during condensation (adsorption) and hemispherical shaped (r1 = r2 = rpore) during evaporation (desorption) (Figure 5a). According to eq 3, this means that rpore = rk,ads/2 = rk,des. In Figure 5a, it can be seen that the mean radius of curvature of the meniscus formed during adsorption, rk,ads, is close to 20 nm, while rk,des is close to 10 nm. This result
the capillary forces and multilayer formation on top of the filled mesoporous film. Next, the change in refractive index n is converted to the relative volume of toluene adsorbed by the film using the Lorentz−Lorenz effective medium approximation (EMA) ⎛ n2 − 1 ⎞ ⎛ n02 − 1 ⎞ ⎟ ⎜ 2 ⎟−⎜ ⎝ n + 2 ⎠ ⎝ n02 + 2 ⎠ Vtol = ⎛ n 2 − 1⎞ Vfilm ⎟ ⎜ tol2 ⎝ ntol + 2 ⎠
(3)
(2)
where ntol and n0 are the refractive indices (at 632.8 nm) of toluene and the empty mesoporous silica film, respectively.31 Assuming that the adsorbed toluene retains its bulk properties, ntol is 1.496. The main advantage of this method is that it does not require knowledge of the silica skeleton refractive index. 3855
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the silica pore walls. However, for simplicity reasons, the contact angle is assumed to be ∼0 in eq 3 for both cases. Figure 6a shows the toluene sorption isotherms measured after 10, 20, and 30 ALD cycles together with the isotherm
suggests that the cylindrical pore model is an acceptable approach for deriving the PRD of the mesoporous silica film (having a far more complex pore system in reality). The actual pore radius, however, also consists of the multilayer present on the pore walls prior to capillary condensation or following capillary evaporation. Therefore, a so-called tcorrection must be applied, i.e., rpore = rk,ads/2 + t = rk,des + t, where t is the thickness of the preadsorbed multilayer.31,48 This t-value can be estimated, at each P/P0, by measuring toluene sorption isotherms on a nonporous sample having a chemically similar surface.31−34 Here, a thermally grown SiO2 film is considered to have a top surface that is representative for the porous silica. The SE data are analyzed using a model consisting of a toluene layer with a wavelength independent refractive index (ntol = 1.496) on top of a SiO2 substrate. With the thickness of the toluene layer as the only fit parameter, the fitting procedure results in the t-value at each P/P0.34 Finally, the PRD is derived by plotting the derivative of the sorption isotherms with respect to rpore against rpore. Figure 5b displays the PRDs derived from the adsorption branch and desorption branch, respectively. The dashed lines are Gaussian fits to the data points and indicate a mean pore radius of ∼8.9 and ∼9.4 nm for adsorption and desorption, respectively. Note that the width of the PRD obtained from the adsorption branch is larger due to the branched distribution of pores inside the film.31 This subsection is closed with a remark on the applicability range of the Kelvin equation. The lower limit is set to a pore diameter 2 nm due to the fact that capillary condensation does not occur in micropores. In the microporous regime, pore radii can be determined using the Dubinin and Radushkevitch theory.49,50 The upper pore diameter limit of the Kelvin equation is ca. 50 nm due to the nature of the Kelvin equation. When the relative pressure approaches unity, the Kelvin radius rises very quickly (Figure 5a). Therefore, for radii above 25 nm, the isotherm would become far too steep to measure. In Situ EP Characterization of TiO2 ALD in the Mesoporous Silica Film. Following the EP characterization of the original porous film, providing a porosity of ∼77%, a film thickness of ∼164 nm, and a mean pore radius of ∼9.4 nm, the sample is exposed to 30 cycles of TiO2 ALD. To study the effect of the coating on the porous thin film, an EP measurement is performed every 10 cycles. First, we discuss the changes that are made to the earlier described data analysis procedure to make it valid for the TiO2coated porous film. While the uncoated mesoporous silica film is transparent over the whole wavelength range covered by the ellipsometer (245−1000 nm), this is no longer the case after deposition of TiO2 in the porous film. The optical band gap of amorphous TiO2 grown by ALD is typically in the range 3.2− 3.4 eV.35 Therefore, the wavelength range over which the Cauchy relation (eq 3) is fitted to the SE data is reduced to 400−1000 nm. Although the pore walls now exist of silica and titania material, the Lorentz−Lorenz EMA model (eq 2) can still be applied because it does not require the knowledge of the refractive index of the skeleton material. Finally, the tcorrection (needed for the derivation of the PRD) requires the thickness of the toluene multilayer adsorbed on the pore walls coated with TiO2 at each P/P0. These values are determined from the isotherm measured on a ∼30 nm thick TiO2 layer deposited by ALD on a planar SiO2 substrate. Note that the wetting behavior of toluene, i.e., the contact angle of toluene, on the TiO2 ALD coating might be slightly different than on
Figure 6. (a) Toluene sorption isotherms measured in situ on the original silica thin film and after 10, 20, and 30 ALD cycles. (b) Thickness variation during adsorption and desorption of toluene in and out of the original film and after 10, 20, and 30 TiO2 ALD cycles. (c) Pore radius distributions derived from the desorption branches in (a) using the Kelvin equation with t-correction. Dashed lines are Gaussian fits to the data points.
measured on the original porous silica film. The porosity decreases from ∼77% to ∼67%, ∼53%, and ∼39%, respectively. This result proves that TiO2 is deposited on the pore walls without clogging the pore entrances. The hysteresis loop clearly shifts to lower relative pressures, indicating a decrease in pore radius with more ALD cycles deposited. After ALD, a different behavior is observed for low relative pressures (P/P0 ∼ 0). This can be attributed to the presence of micropores.51 Note that our previous ex situ EP study of TiO2-coated mesoporous silica films did not reveal this microporosity,23 probably due to adsorption of water during transfer from the ALD setup to the EP system. The variation in thickness of the porous film upon adsorption and desorption of toluene vapor is displayed in Figure 6b for the original film and after 10, 20, and 30 ALD cycles. Two things are observed in this figure. First, the film 3856
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saturation pressure P0 is taken as the highest pressure at which a SE measurement is performed. However, the presented EP data are recorded manually, and therefore, an error of about 50 Pa on the value of P0 cannot be excluded. For pore radii in the range of 9−10 nm, such an error would cause a shift of ∼0.5 nm on the calculated pore radius. This result shows the crucial importance of an accurate and reproducible stopping criterion for EP measurements, and this requires automation of the setup. Note that the error on the derived pore radius introduced by the inaccuracy of the P0 value becomes smaller for smaller pore radii due to the nature of the Kelvin equation (eq 3 and Figure 5a). When the P0 value is determined accurately and no film compression occurs, a good correspondence is found between the decrease in pore radius and the expected GPC of the applied ALD process. A previous automated, ex situ EP measurement showed, for example, that the pore diameter of a mesoporous silica film with ∼4 nm pores decreased to ∼3.6 nm after 10 ALD cycles of TiO2, indicating a pore radius reduction of ∼0.07 nm per cycle.17 This result was in agreement with in situ X-ray fluorescence data, indicating a GPC of ∼0.07 nm for the TiO2 process in the mesopores. One of the main advantages of the EP technique is that it also measures the variation in film thickness upon adsorption and desorption of toluene. From this, information can be obtained on the mechanical properties of the film.33,43 As seen in Figure 6b, the extent of shrinkage upon capillary condensation of toluene decreased with more TiO2 deposited, suggesting a thickening and stiffening of the pore walls by the coating. This result is in agreement with earlier nanoindentation measurements which indicated an increase of the Young modulus with increased TiO2 deposition.23
thickness is affected by the ALD treatment. When 10 cycles are applied, the thickness increases from ∼164 to ∼166 nm. Depositing more cycles results in a significant decrease of the film thickness, namely, to ∼154 nm after 20 cycles and to ∼140 nm after 30 cycles. Second, capillary shrinkage becomes less pronounced with more ALD cycles deposited. Finally, the PRDs (obtained from the desorption branches) are shown in Figure 6c. As expected, the pore radius gradually decreases with progressing TiO2 ALD. The mean pore radius decreases from ∼9.4 nm to ∼7.7, ∼6.7, and ∼5.2 nm after 10, 20, and 30 ALD cycles, respectively.
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DISCUSSION At 200 °C, the TiO2 ALD process is characterized by a growth per cycle (GPC) of ∼0.05 nm for deposition on a planar SiO2 substrate.42 Assuming a similar GPC on the pore walls of the porous silica thin film, in each 10 ALD cycles deposition of a ca. 0.5 nm thick coating is expected to occur, thus reducing the pore radius by ∼0.5 nm. Our results, however, indicate a larger reduction of the pore radius per 10 cycles, namely ∼1.7, ∼1, and ∼1.5 nm after a total of 10, 20, and 30 ALD cycles, respectively. For a cylindrical pore system with an initial pore radius of 9.4 nm and a porosity of 77%, a pore radius reduction of 0.5 nm per 10 ALD cycles would result in a porosity of ∼69%, ∼61%, and ∼54%, after 10, 20, and 30 cycles, respectively, whereas we find values of ∼67%, ∼53%, and ∼39%. In the above reasoning it is assumed that the reduction in pore radius and porosity are due to the ALD treatment only and that the volume of the film remains constant. The data in Figure 6b indicate that this assumption is questionable since the film thickness significantly decreased after 20 and 30 ALD cycles. A decrease of film thickness must give rise to a reduction in pore radius and porosity. The decrease in film thickness with progressing deposition was also observed in our previous ex situ EP study.23 The porous silica film used in this work was synthesized using a procedure that is known to result in highly porous and mechanically flexible thin films, as indicated by low values for the Young modulus.23 In this type of film, the stress generated by the conformal TiO2 deposition into the interconnected porous network may cause a relatively uniform shrinkage of the pore network, resulting in a decrease of the film thickness. According to our results, the internal stresses in the TiO2 layer are only built up after 10 ALD cycles. This might be related to a delayed coalescence of the TiO2 layer, as often observed in ALD processes. Typically, several ALD cycles are needed before a continuous and dense layer is formed.18 After 10 ALD cycles, the film thickness is still close to the film thickness of the uncoated porous silica film. Consequently, the decrease in porosity and pore radius are expected to be caused by the ALD coating only. The measured porosity after 10 ALD cycles, ∼67%, is indeed very close to the expected value, ∼69%, but the pore radius reduction, ∼1.7 nm, is larger than expected, ∼0.5 nm. This contradiction might be related to (1) a different contact angle of toluene on the TiO2 ALD layer than on the silica pore walls and (2) measurement errors. While the porosity is known as soon as capillary condensation is completed, the derivation of the pore radius requires an accurate determination of the vapor pressure of toluene, P0. As explained in the Experimental Methods section, the adsorption of toluene inside the pores is considered terminated when the rate of pressure rise drops below 2.5 Pa s−1. The value of the
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CONCLUSIONS ALD becomes increasingly important for functionalization or sealing of porous thin films. Here, it is demonstrated that EP, known to be a powerful technique to obtain information on the porosity and PRD of porous low-k films, has the potential to become an important in situ characterization technique in this field of ALD-related research. It is shown that the EP technique is compatible with ALD and can directly be implemented onto the ALD growth chamber. This allows investigation of the evolution of the porosity and the pore radius with progressing ALD growth in porous thin films without moving of the sample or uncontrolled exposure to ambient air. In our setup, a technical drawback is that the duration of an experiment is significantly increased by the required heating and cooling in between subsequent ALD depositions and EP measurements, as EP has to be performed at RT. In an ALD setup that can be heated completely to a temperature that equals or is higher than the deposition temperature of the sample, it would be possible to perform EP in shorter time. Naphthalene could be used as an adsorptive in that case because it has a suitable high vapor pressure in the temperature range 100−200 °C.52 By using appropriate models for the optical properties of an ALD-coated porous film, the EP technique can provide information about the effect of the ALD coating on the porosity, the PRD, the thickness, and mechanical properties of the porous film. From the EP data obtained during ALD of TiO2 on a mesoporous silica thin film, we obtained the following information. Each 10 ALD cycles of TiO2, the porosity decreased, but was still accessible, indicating that TiO2 was 3857
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deposited on the pore walls without clogging the pore mouth. The derived PRDs indicated a gradual decrease in pore radius with more TiO2 deposited. The capillary shrinkage effect became less pronounced with more ALD cycles applied, indicating that the pore walls became stiffer due to thickening by ALD of TiO2. The thickness measurements also revealed that the porous film shrunk upon ALD of TiO2, probably caused by internal stresses in the deposited TiO2 coating. This contraction was accompanied by a decrease in porosity and pore radius, as indicated by the sorption isotherms and the derived PRDs. These results confirm the great suitability of the in situ EP technique for the characterization of ALD coatings in porous thin films.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the European Research Council through an ERC Starting Grant awarded to C.D. (Grant 239865). J.D. acknowledges the Flemish FWO for a Ph.D. research grant. J.A.M. acknowledges the Flemish Government for long-term structural funding (Methusalem). C.D. and J.A.M. acknowledge the Flemish FWO for funding a common research project.
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