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Mechanism for the Formation of Isolated Poly(p-xylylene) Fibrous Structures under Shadowing Growth Ming He,* Pei-I Wang, and Toh-Ming Lu Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, New York 12180, United States ABSTRACT: Because of the low sticking coefficient, conventional parylene deposition is known to achieve the conformal coating on corrugated or patterned surfaces. However, recently, it has been shown that in contrary to the conformal coating, extremely nonconformal and isolated fibrous parylene structures can be formed on surfaces if it is deposited at an oblique angle using a directional flux. We demonstrate that directional flux can create a high local vapor pressure facing the flux, while the reflection of monomers because of a small sticking coefficient would generate a background vapor pressure. The parylene oblique angle deposition is a combination of the shadowing growth and a much slower conformal coating process, which together give rise to the isolated fibrous structure.
’ INTRODUCTION Recently, there has been considerable interest in the oblique angle deposition (OAD) technique, where a number of interesting nanostructures, including nanorods and nanospirals, can be created on surfaces.13 OAD can be performed using physical vapor deposition techniques, such as thermal evaporation and sputtering. The collimated vapor flux is designed to arrive at the substrate at a large angle with respect to the surface normal. If the starting substrate is flat and diffusion is limited, islands of different heights are initially nucleated at the surface. Subsequently, the incident flux of material that strikes the surface with an oblique angle is preferentially deposited onto the top of surface features with larger height values. The regions with smaller height become shadowed during the growth, and thus, voids are generated. The high surface points keep growing ahead of their local neighbors as long as they are not shadowed by other taller ones. This preferential growth dynamic gives rise to the formation of well-separated nanostructures. Figure 1a shows a schematic of the OAD process for a stationary substrate without rotation. If the substrate is rotating, nanostructures with a wide variety of sizes and shapes can be obtained by controlling the substrate rotation speed. Similarly, if the substrate already contains some structures, such as patterned pillars or trenches, the flux would preferentially deposit over the surface features, as shown in Figure 1b. Again, if the substrate is rotating, different shapes of isolated structures (porous) can be generated depending upon the substrate motion. One important aspect of the isolated structure formation by OAD is that the surface diffusion must be limited during deposition. Otherwise, coalescence of the structures would occur, and shadowing would not be effective in this case. The r 2011 American Chemical Society
voids would become filled by strong surface diffusion, and the isolated structures are destroyed. This has been demonstrated by OAD at different substrate temperatures.4 Another aspect, namely, the sticking coefficient, is also an important factor to define the growth front morphology. For example, in the conventional chemical vapor deposition, if the sticking coefficient of the incident flux is small, the reflection of particles from the substrate surface would redistribute the flux into the shadowed region, and thus, isolated structures cannot be formed during deposition.5 Hence, a small sticking coefficient, which can be realized in chemical vapor deposition, is very often used for conformal coating. In this case, shadowing is not effective to create isolated structures. This is partially the reason why OAD normally uses physical vapor deposition (PVD) instead of chemical vapor deposition techniques. If the initial surface contains pillars or trenches, the deposited layer would be conformed to the surface, as shown schematically in Figure 1c. Conformal coating and OAD of isolated structure formation are two opposite phenomena. It is well-known that the poly(para-xylylene) family of polymeric films, also known as parylene, can be grown by vapor deposition.6 Among the parylene family, poly(chloro-p-xylylene), parylene-C (or PPX-C), is perhaps most frequently used in applications. In its deposition process, the dimer precursor is sublimated and then cleaved into two monomer units in a pyrolysis reactor/furnace. The monomers are then led into a deposition chamber and become adsorbed onto a substrate, but Received: February 8, 2011 Revised: March 18, 2011 Published: March 29, 2011 5107
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detailed analysis of the vapor dynamics during PPX deposition. We shall show that, despite the low sticking coefficient, the microfibrous PPX can still be formed. It is a result of a high growth rate at the tips of the surface structure because of the high vapor pressure that is exerted locally, combined with a much lower growth rate within the shadowed region, whose flux is from the reflected monomers that are not adsorbed on the surrounding structures. The measured tilt angle of the PPX fibers agrees well with the prediction based on a recent semi-empirical model.
’ EXPERIMENTAL SECTION
Figure 1. Schematics showing the OAD process. (a) OAD of material on a flat surface. (Left) Initial nucleation of islands of different height. (Right) Preferential deposition of the material on top of the taller islands to form isolated nanorods that incline toward the incidence flux. (b) OAD of material on a trench structure. (Left) Cross-section view of a trench structure. (Right) Isolated walls of material are grown on top of the trenches. The cross-section appears to be a rod-like structure. (c) Cross-section view of the conformal coating process by chemical vapor deposition, where the flux strikes the surface from all directions toward the trench structure and the sticking coefficient is small.
The deposition process of PPX-C films/columns is shown schematically in Figure 2a. The dimer precursor dichloro-[2.2]paracyclophane is placed in a vacuum ampule and heated to around 200 °C. After the sublimated dimers are converted to reactive monomers in the pyrolysis reactor at a temperature of 680 °C, the monomer vapor is led into the deposition chamber through a nozzle and deposited onto the substrate at room temperature. The nozzle has a half-inch diameter opening. For the conformal film-coating experiment, a disperser is placed in front of the nozzle and the substrate would be placed 1 ft away from the nozzle, close to the capacitance gauge inlet. For OAD, the substrate would be placed right in front of the nozzle (without the disperser) and the substrate normal is tilted R = 87° relative to the incoming vapor flux. The vapor pressure in the deposition chamber was measured by the capacitance gauge. Thin film thickness was characterized using a VASE ellipsometer and cross-section scanning electron microscopy (Carl Zeiss Supra SEM). Two types of substrates were used for deposition: one was a blank Si substrate, and the other was the Si substrate with a trench pattern. The trench width and height are both 20 μm. The separation between the trenches is 5 μm. The patterned substrate was fabricated on a Si wafer using the conventional photolithography method, followed by a Bosch process to etch the deep trench.
’ RESULTS AND DISCUSSION Figure 2. (a) Schematic of the parylene deposition system, which includes the evaporation source, pyrolysis furnace, and deposition chamber. For conventional conformal coating of parylene, the substrate is place near the capacitance pressure gauge and a disperser would be place at the exit of the nozzle. For the OAD of PPX, the substrate is place near the exit of the nozzle without the disperser. (b) Measured conformal coating deposition rate on the substrate placed near the capacitance gauge is plotted as a function of the vapor pressure recorded at the capacitance gauge.
with a very small sticking coefficient. The monomers are spontaneously polymerized, yielding high-molecular-weight, linear molecular chain PPX films on the substrate. Because of the small sticking coefficient for the monomers, the process is therefore very conformal7,8 and has been used to uniformly coat objects of odd shapes, for example, protective coatings for surgical implants and printed circuit boards. Parylene is also useful in microelectromechanical system (MEMS) applications.9 However, recently, it has been shown that isolated microfibrous PPX can be formed under a specific deposition geometry using a directional flux,1012 similar to the conventional OAD shadowing process despite its low sticking coefficient. This is, of course, counterintuitive. How is it possible that PPX deposition behaves like a conformal coating on one hand but an OAD process on the other hand? In this paper, we shall explain the fundamental mechanism involved in these behaviors through a
Sticking Coefficient and Parylene Conformal Coating. First, we estimate the sticking coefficient of the parylene vapor on the substrate surface in the range of vapor pressure under consideration. It is known that the deposition rate R is given by13
R ¼
SPNa Vm 1010 ð2πmr Ry T0 Þ0:5
ð1Þ
where PNa/(2πmrRyT0)0.5 is the flux of reactant particles to the substrate surface in units of collision per square meter per second, P is the pressure above the substrate, Na is Avogadro’s number, mr is the molecular mass of the reactants, Ry is the Rydberg gas constant, and T0 is the temperature of the gas. Vm is the volume of 1 molecule in cubic meters, which converts the flux into deposition rate units of meters per second. The value of Vm is 1.78 1028 m3 and is calculated from the density and molecular mass of the film. The multiplication factor of 109 converts the unit to nanometers per second. S is the fraction of molecules that stick and react after hitting the surface, also called the sticking coefficient. To estimate the sticking coefficient, a blank Si substrate was placed close to the capacitance gauge during the deposition. This ensured that the measured vapor pressure in the gauge was the actual pressure above the substrate. A disperser was placed in front of the nozzle to ensure the uniform distribution of vapor inside the chamber. The deposition rates (R) of PPX films under different vapor pressures were obtained by measuring the 5108
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Figure 5. (a) Plot of the deposition rate near the nozzle without the disperser as a function of the chamber pressure measured at the gauge. The deposition rate of PPX near the gauge during conformal coating is also plotted for comparison. (b) Effective pressure near the nozzle is plotted as a function of the chamber pressure measured near the gauge.
Figure 3. SEM cross-section image of PPX film deposited on a trench pattern substrate placed near the capacitance gauge. The trench width and height are both 20 μm. The deposition was for 100 min at 5.5 mTorr chamber vapor pressure. During the deposition, a disperser was placed in front of the nozzle. The PPX film is seen to be conformally coated on the trench.
Figure 4. (a) Schematic of the setup for OAD of PPX. The substrate is placed right outside the nozzle, without the disperser. The substrate normal is tilted 87° with respect to the flux direction. (b) SEM crosssection image of the PPX film deposited on a flat Si substrate. The arrows point to the PPX film that was stretched apart during sample dicing in liquid N2. (c) SEM cross-section image of the PPX film deposited on the trench pattern. The thickness of the film at the top of the trench is dramatically different compared to that of the sidewalls (see the inset). The arrow points to the PPX film that was stretched apart during sample dicing in liquid N2. β is the tilt angle of the rod, with respect to the surface normal.
thickness of the films deposited at a fixed time. Then, the deposition rate was plotted as a function of the vapor pressure, as shown in Figure 2b. On the basis of eq 1, the sticking coefficient was extracted from the slope of the curve to be 7.3 104. This value is within the same order of magnitude reported in the literature for PPX8 and is much smaller than the sticking coefficient for the PVD processes, which is usually close to 1 for many inorganic materials.14 Normally, a low sticking coefficient is beneficial for the conformal coating of patterned surfaces.15 As an example, we deposited PPX film on a trench pattern at a chamber vapor pressure of 5.5 mTorr for 100 min. The deposition rate was estimated to be around 0.40.5 nm/s from Figure 2b. From the cross-section SEM image shown in Figure 3, it is clear that the deposition is very conformal, with nearly the same thickness of
PPX film (∼2.5 μm) on the top, sidewalls, and bottom of the trench. Also, there is no overhang observed after the deposition. An isotropic distribution of monomer vapors within the chamber is created with the help of the disperser in front of the precursor inlet. Meanwhile, the low sticking coefficient of the precursor results in more collisions of the monomers with the trench wall. The monomers would thus have a higher likelihood to bounce to the bottom of the trench before they eventually become adsorbed onto the surface. Therefore, the vapor pressures inside and outside the trench would be similar, generating a conformal coating. Parylene Directional Flux and OAD. For OAD, the substrate was placed right in front of the nozzle (without the disperser) and the substrate normal was tilted R = 87° relative to the incoming vapor flux, as shown in the cross-section schematic in Figure 4a. During the deposition, the chamber vapor pressure was again 5.5 mTorr, as measured by the capacitance gauge. In Figure 4b, we show the SEM cross-section of the films on a blank Si substrate. A fibrous structure was grown after a 5 min deposition. This result is consistent with the PPX OAD results published earlier.10 To investigate the origin of such distinct behaviors from the same precursor, OAD was performed on a Si trench pattern under the same conditions as that for the blank Si substrate. From Figure 4c, it is clear that, besides the oblique nanorod structure on the top of the trench, there is also a thin PPX-C film deposited on the sidewalls of the trench. Some of these films were stretched apart during sample dicing in liquid N2. The thickness of the film is around 350 nm, suggesting a deposition rate of 1.2 nm/s. Apparently, the conformal coating process within the trench continued to occur even during the OAD process. Such deposition on the sidewall does not occur for the common PVD OAD processes, which are based on the line-of-sight principle. Therefore, the monomers inside the trench are present because of the reflections after they strike against the top of the trench and the chamber walls. With a sticking coefficient of 7.3 104, the vapor pressure within the trench is estimated to be about 8.5 mTorr based on eq 1, ∼55% higher than the chamber pressure measured by the pressure gauge. The rod-like PPX structure grown on top of the trenches is around 29 μm in thickness (vertical height). To examine the vapor pressure directly striking against the trench top, we estimated the deposition rate of films (normal incidence to the Si samples) near the nozzle as a function of the chamber pressure. Again, the deposition rate was determined by measuring the thickness of the films deposited for a fixed time. Figure 5a is a plot of the deposition rate near the nozzle as a function of the chamber pressure measured at the gauge. For comparison, we 5109
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Figure 6. (a) Schematic of the setup for the PPX fan growth. The pattern substrate is placed right outside the nozzle, without the disperser. The flux is directed normal to the substrate surface. (b) SEM crosssection image of the PPX fan deposited on the trench pattern. The fan angle is ∼81°.
also plotted together the deposition rates from Figure 2b. Using eq 1 with the calculated sticking coefficient, the effective pressure near the nozzle can be estimated and is plotted in Figure 5b as a function of the chamber pressure. The local pressure right outside the nozzle is orders of magnitude higher than the averaged chamber pressure. For the PPX coating shown in Figure 4c, the chamber pressure is 5.5 mTorr. It corresponds to an effective vapor pressure of around 400 mTorr near the nozzle, with a deposition rate of 90 nm/s. For a deposition time of 5 min, we estimate the rod vertical thickness to be 27 μm, which is close to the actual vertical height value shown in Figure 4c. During OAD, the monomer vapor pressure on the top of trench is 400 mTorr, while the vapor pressure inside the trench is only 8.5 mTorr. The high vapor pressure on the top of the trench is due to the collimated flux concentrated onto the trench top, while the low vapor pressure within the trench is mainly from the reflected monomers, limited by the shadowing effect. In other words, in PPX OAD, shadowing growth is accompanied with a background conformal coating. The conformal coating part is much slower and less significant. Meanwhile, the fast growth rate of the PPX rods on the top of the trench increases the aspect ratio of the effective trench very quickly and further limits the reflected monomers from diffusing into the trench. On the basis of these findings from the substrate with a trench pattern, the growth dynamics of OAD on the flat Si surface can be understood with a similar mechanism. At the beginning of the deposition, the monomer flux is not shadowed by the smooth surface. A PPX film, instead of isolated rods, is grown on the flat Si surface. However, this initial PPX film surface is not smooth, despite its low sticking coefficient. The growth front of the PPX film will have certain roughness, even under a conformal deposition setup.16 The surface roughness becomes more severe under an oblique angle (directional) flux with such a high deposition rate. Once the valleys and peaks are formed because of the surface roughness, the shadowing effect begins to take place and would affect the growth dynamics. The top of surface features with larger height values will receive more flux and would preferentially grow with a higher rate compared to the shadowed region. Meanwhile, the redistributed flux can penetrate into the shadowed space and form a background deposition at the bottom and sidewalls of the fibers. As demonstrated in the case of the pattern substrate, the redistributed flux pressure is much smaller than the directional flux. This preferential growth dynamic gives
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rise to the formation of a fibrous structure, as shown in Figure 4b. The figure also shows the evidence of the background film coating. At the bottom of the fibrous structure, there is a thin PPX film, which is partially a result of the background coating. This film was stretched apart during the sample dicing in liquid N2 because the film was too thin, similar to that in the trench pattern. Overall, the behavior of OAD on the flat Si substrate is similar to that on the trench pattern, except that the background coating was deposited to the sidewalls and bottom of the fiber film. This is different from the conventional PVD OAD process, which generates completely discrete rod structures without the sidewall coating. Figure 4 also shows that the PPX rods are leaning toward the incident flux, similar to the PVD OAD. However, the tilt angle β (relative to the substrate normal) of the PVD OAD, taking Si and Ge as examples,17,18 is larger than that of the PPX rods under the same flux incident angle R = 87°. The tilt angle for the PPX rod is measured to be 42.0 ( 0.5°, while the angles for Si and Ge are above 55°. On the basis of a recent semi-empirical model, the angle β is determined by the flux incident angle R and the fan angle φ.19 The fan angle is obtained by placing a pattern substrate in front of the nozzle with a normal incident flux, as shown in Figure 6a. Under 5.5 mT chamber pressure for 4 min deposition, a fanlike structure grows from the trench top with a fan angle φ = 81.1 ( 1.8°, shown by the cross-section in Figure 6b. Because the fan angle φ is smaller than R, the tilt angle β can be calculated simply using the relationship β = R (φ/2). From the value of R and φ, we obtain β ∼ 46.5°. This result is very close to the actual measured value of β, which is about 42°. The fan angle, which dictates the tilt angle, depends upon the material type and the deposition process. A likely origin of the larger PPX fan angle is the limited diffusion of the PPX monomers on the surface. A larger fan would subsequently lead to a smaller tilt angle.
’ CONCLUSION We have demonstrated that, during PPX deposition, the vapor dynamics in the chamber can alter the film morphology in a dramatic manner. With the help of a trench pattern, we show that the PPX OAD process is actually a combination of shadowing growth and conformal coating. The growth rate of the conformal coating part is orders of magnitude smaller. As a consequence, the growth rate on the top of the surface structure is much faster than that at the sidewalls. This explains the origin of the fibrous structures observed in recent reports on the deposition of parylene using OAD. The limited diffusion of the parylene monomers on the surface also create a larger fan angle, which causes the PPX rods to tilt more toward the substrate normal compared to Si and Ge OAD. These fibrous structures may have potential applications, such as antifouling coatings, biosensors, and tissue-culture polymers.79,20,21 ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported in part by National Science Foundation (NSF) Nanoscale Interdisciplinary Research Team (NIRT) 5110
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Grant 0506738. We thank Rahul Krishnan for proofreading the manuscript.
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