Method for operating a chemical vapor deposition process

Abstract

A method of operating a chemical vapor deposition (CVD) process, comprising the steps of providing a substrate 20 within a reaction zone 10 of a reaction chamber 1, providing a flow of at least one precursor gas containing precursor molecules into the reaction chamber 1, heating the reaction chamber 1 to a temperature higher than the reaction initiation temperature of the precursor molecules, and providing at least one inert diffusion additive gas into the reaction chamber 1, wherein the inert diffusion additive contains inert diffusion additive molecules, the inert diffusion additive molecules have a larger molecular weight than the precursor molecules, and the partial pressure of the inert diffusion additive gas is greater than the partial pressure of the precursor gas. Also disclosed are protective layers 40, 40', 40'' on a substrate 20 having at least one high aspect ratio feature.

Description

Technical Field

[0001] This book relates to a chemical vapor deposition (CVD) process, and more particularly to a method of operating a CVD process to improve the distribution of coating on a flat surface and / or to coat or fill high aspect ratio features such as trenches or vias in a substrate.

Background Art

[0002] Chemical vapor deposition (CVD) processes have found many applications, for example, in the manufacture of semiconductor products. The quality of the deposited layer is very important for achieving good material properties suitable for use in intended applications.

[0003] In addition to the general problem of providing a uniform coating on a substrate, CVD processes have specific problems in providing a coating on the surface of porous structures and high aspect ratio features such as trenches and vias.

[0004] The coating can grow on the substrate in various forms, and by calculating the step coverage of the deposited layer, the state of growth can be determined. The step coverage is obtained by dividing the thickness of the coating layer at the bottom of the trench or hole by the thickness of the deposited layer at the top of the trench or hole. A step coverage less than 1 indicates that sub-conformal growth has occurred. It is desirable for the deposited layer to grow conformally, which is indicated by a step coverage equal to 1.

[0005] However, in some applications, it is desirable to completely fill the trenches or holes in the substrate with the deposited material. In the case of sub-conformal growth, the upper deposited layer grows faster than the growth of the material at the bottom, and thus there is a very high probability that this growth will result in the formation of voids at the bottom of the trench or hole because the deposited material closes the entrance for reaching the bottom. The same can occur for conformal growth.

[0006] It is particularly difficult to completely fill trenches or vias having a high aspect ratio. Therefore, it may be desirable to provide deposition by superconformal growth, i.e., growth with a step coverage greater than 1. When growing a superconformal layer, the thickness of the material layer increases more rapidly at the bottom of the trench or hole, resulting in a V-shaped layer as an intermediate product. When filling a structure by superconformal growth, the bottom of the V-shape moves upward as the layer grows, and the structure will be completely filled.

[0007] However, in a normal CVD process, since the partial pressure of the precursor decreases with the depth of the trench, it is difficult to maintain a uniform incident flow over the entire high aspect ratio feature, so superconformal growth can be difficult. To overcome this, CVD processes have been performed at lower temperatures to reduce the reaction probability, promote more diffusion of the gas phase, and achieve better conformality of the thin film. However, lowering the temperature deteriorates the performance of the density of the material of the deposited layer.

[0008] Therefore, a process for growing a material layer having good material properties is very interesting.

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] An object of the present disclosure is to provide a method for providing a process for depositing a protective layer having good material properties and surface coverage.

MEANS FOR SOLVING THE PROBLEMS

[0010] The present invention is defined by the appended independent claims, and embodiments are described in the appended dependent claims, the following description and the appended drawings.

[0011] According to a first aspect, a method of operating a chemical vapor deposition (CVD) process, comprising the steps of providing a substrate within a reaction zone of a reaction chamber, providing a flow of at least one precursor gas containing precursor molecules into the reaction chamber, heating the reaction chamber to a temperature higher than the reaction initiation temperature of the precursor molecules, and providing at least one inert diffusion additive gas into the reaction chamber, wherein the inert diffusion additive contains inert diffusion additive molecules, the inert diffusion additive molecules have a higher molecular weight than the precursor molecules, and the partial pressure of the inert diffusion additive gas is higher than the partial pressure of the precursor gas.

[0012] The inert diffusion additive gas has a higher partial pressure than the precursor gas, and its molecules have a higher molecular weight, thus having a lower diffusion rate compared to the precursor gas, which promotes more gaseous diffusion of the precursor gas and results in better surface coating properties of the substrate. Accordingly, an improved chemical vapor deposition process is provided.

[0013] The CVD process may be a continuous process.

[0014] The CVD process may be a thermal CVD process.

[0015] The CVD process may be a plasma CVD process.

[0016] The precursor gas may contain a metal, specifically boron.

[0017] The precursor gas may be triethylboron (B(C2H5)3) or boron trichloride (BCl3).

[0018] The inert diffusion additive gas may be a noble gas, preferably xenon (Xe).

[0019] The inert diffusion additive gas preferably may consist of an element having a higher molecular weight than the precursor gas.

[0020] The inert diffusion addition gas may be combined with the precursor gas upstream of the reaction zone, preferably within the gas manifold.

[0021] The above method may further include the step of providing at least one carrier gas, which is combined with the inert diffusion addition gas and the precursor gas upstream of the reaction zone, preferably within the gas manifold.

[0022] The carrier gas may be hydrogen (H2) or argon (Ar).

[0023] The temperature at which the reaction chamber is heated is higher than about 400 °C, preferably about 400 °C - 410 °C, or about 410 - 420 °C, or about 420 °C - 430 °C, or about 430 °C - 440 °C, or about 440 °C - 450 °C, or about 450 °C - 460 °C, or about 460 °C - 470 °C, or about 470 °C - 480 °C, or about 480 °C - 490 °C, or about 490 °C - 500 °C, or about 500 °C - 510 °C, or about 510 °C - 520 °C, or about 520 °C - 530 °C, or about 530 °C - 540 °C, or about 540 °C - 550 °C, or about 550 °C - 560 °C, or about 560 °C - 570 °C, or about 570 °C - 580 °C, or about 580 °C - 590 °C, or about 590 °C - 600 °C, or about 600 °C - 610 °C, or about 610 °C - 620 °C, or about 620 °C - 630 °C, or about 630 °C - 640 °C, or about 640 °C - 650 °C, or about 650 °C - 660 °C, or about 660 °C - 670 °C, or about 670 °C - 680 °C, or about 680 °C - 690 °C, or about 690 °C - 700 °C.

[0024] The substrate may be flat or substantially flat.

[0025] The substrate may be a wafer. The material of the wafer may be any type of material used in semiconductor manufacturing, such as, but not limited to, silicon (Si). The substrate may have at least one high aspect ratio feature.

[0026] The substrate may have high aspect ratio features that are structures within the substrate. The structures can be made as parts integral with the substrate when forming the substrate, for example, by sintering. Alternatively, the structures within the substrate can be made within the substrate at a later stage, for example, by etching the substrate. As another option, structures with a large aspect ratio can be formed by depositing one or more materials on the substrate. The pattern formed by the high aspect ratio features may be a regular pattern in the form of holes or trenches or grooves that have a certain size and are present at a certain distance from each other to form the pattern.

[0027] The high aspect ratio features may be holes, recesses, trenches, or grooves.

[0028] The inert diffusion additive gas has a higher partial pressure than the precursor gas, such that the concentration of the inert gas is higher at the top of the high aspect ratio features. The concentration of the precursor gas is higher at the bottom than at the top of the high aspect ratio features. The heavier molecules of the diffusion additive gas push the lighter molecules of the precursor into the recesses and small holes of the substrate, causing the surface layer of the precursor molecules to be deposited on the surface of the high aspect ratio features, and the surface layer grows at a faster rate at the bottom of the high aspect ratio features than at the openings of the high aspect ratio features, promoting superconformal deposition.

[0029] The aspect ratio of the at least one high aspect ratio feature may be at least about 5:1, at least about 10:1, at least about 20:1, or at least about 40:1.

[0030] The aspect ratio is defined as the ratio between the depth and the width of the trench for a trench. The aspect ratio is defined as the ratio between the depth and the diameter of the hole for a hole.

[0031] The substrate may be a porous body of material.

[0032] The porous body of the material may have high aspect ratio features constituted by small holes in the material. For example, the porous substrate may be a material having a coarse structure that can be formed by a woven fabric, a non-woven fabric, a fibrous material, or a foaming agent, or through a sintering process.

[0033] The substrate may be a bulk material.

[0034] The bulk material may contain a certain amount of fine particles or granules, which may be disposed on a carrier or allowed to flow through a reaction chamber. The high aspect ratio features can be formed by voids and / or recesses between the fine particles or granules, or in the form of recesses or through-holes within the fine particles or granules.

[0035] According to a second aspect, there is provided a product having a coating grown by the method described above.

Brief Description of the Drawings

[0036]

Figure 1

Figure 2

Figure 3a

Figure 3b

Figure 3c

Figure 4a

Figure 4b

Figure 5a

Figure 5b

Figure 6a

Figure 6b

Figure 7a

Figure 7b

Figure 7c

Figure 7d

Figure 7e

Figure 7f

Figure 8a

Figure 8b

Figure 8c

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention relates to a method of operating a chemical vapor deposition (CVD) process. This method has been found applicable to all kinds of substrates including flat substrates, three-dimensional patterned substrates, and substrates having one or more high aspect ratio features. This method has been found applicable to substrates in the form of solid substrates, as well as porous substrates and bulk materials.

[0038] As a non-limiting example, the porous substrate may be a woven fabric, non-woven fabric, fibrous material, or a material having a bubble structure that can be formed, for example, by a foaming agent or through a sintering process.

[0039] The substrate in the form of a bulk material may contain a certain amount of fine particles or granules. The high aspect ratio features may be formed by the voids or recesses between the fine particles or granules. Alternatively, or in addition, the high aspect ratio features may be formed inside or on the surface of at least some of the fine particles or granules themselves.

[0040] FIG. 1 schematically shows an apparatus for chemically vapor-depositing a substrate 20.

[0041] The reaction chamber 1 houses the reaction zone 10, in which the substrate 20 to be processed is positioned. The reaction chamber 1 may be, as a non-limiting example, a horizontal hot-wall type CVD reactor.

[0042] The control unit 30 controls the flow f of the precursor gas from the precursor gas supply unit 14 and the inert additive gas from the inert additive gas supply unit 15. The precursor gas and the inert additive gas are mixed in the gas manifold 13 and introduced into the reaction zone 10 of the reaction chamber 1 through the gas inlet 11. The gas reacts on the surface of the substrate 20 in the reaction zone 10, and then the by-products and the excess gas are discharged through the gas outlet 12.

[0043] The precursor gas contains precursor molecules, the inert diffusion additive contains inert diffusion additive molecules, and the inert diffusion additive molecules have a larger molecular weight than the precursor molecules.

[0044] Compared with the flow of the precursor gas from the precursor gas supply unit 14, the flow of the inert additive gas from the inert additive gas supply unit 15 may produce a higher partial pressure of the inert additive gas compared to the partial pressure of the precursor gas when mixed in the gas manifold 13 and introduced into the reaction chamber 1.

[0045] As mentioned in the introduction section, the substrate 20 may be a flat substrate or a substrate having one or more high aspect ratio features 21. The substrate may be a porous substrate or a bulk material.

[0046] The present invention was developed mainly with the intention of providing a coating of boron carbide on a silicon substrate. Accordingly, the examples and experiments disclosed herein are provided with respect to boron carbide systems. However, the principles of the present invention are considered applicable to other coating systems as well.

[0047] The precursor gas may be triethylboron (B(C2H5)3), or boron trichloride (BCl3), or any other single-source precursor gas having the desired material for the deposition layer.

[0048] The inert additive gas may be a noble gas such as xenon (Xe).

[0049] In some embodiments, a carrier gas is used. The control unit 30 controls the flow of the carrier gas from the carrier gas supply unit 16 for mixing with the precursor gas and the inert additive gas in the gas manifold 13 upstream of the reaction chamber 1. The carrier gas may be hydrogen (H2).

[0050] FIG. 2 schematically shows a substrate 20 having high aspect ratio features 21. The high aspect ratio features 21 can be holes, trenches, and various porous structures. The structure has a depth d and a width w. The depth d is greater than the width w, and the aspect ratio may be at least 5:1 or more. The high aspect ratio features 21 of the experiments described and illustrated in FIGS. 7a-7f below have a depth d of 60.97 μm and a width w of 5.982 μm. In some embodiments, the width w may be about 100 nm or less.

[0051] The opening 211 of the high aspect ratio feature 21 defines an inlet for the gaseous material to be deposited within the structure and is located at one end of the structure, and at the opposite end of the structure, the bottom 212 of the high aspect ratio feature 21 is located.

[0052] Superconformal growth of the deposition layer exhibits a step coverage greater than 1. The step coverage is defined as the ratio of the thickness of the layer at the bottom 212 of the high aspect ratio feature 21 to the thickness of the layer at the opening 211 of the high aspect ratio feature 21.

[0053] When superconformal growth occurs in the CVD process, the layer at the bottom 212 grows faster than the layer at the opening 211, resulting in a step coverage greater than 1. This can be schematically illustrated by FIGS. 3a - 3c, in which, in the figures, it can be seen in cross - section that the substrate 20 has surface layers 40, 40', 40'' deposited by superconformal growth.

[0054] The growth process may be a continuous process, but the deposition molecules are present in the reaction zone 10. In FIG. 3a, it can be seen that the surface layer 40 grows faster as it approaches the bottom 212 of the high aspect ratio feature. In FIG. 3b, the surface layer 40' at the bottom 212 grows and forms a V - shape.

[0055] As the thickness of the deposition layer grows on the wall surface of the hole or trench, the bottom of the V - shape continues to grow upward towards the opening 211 in the depth d direction, and the high aspect ratio feature 21 may be filled by the surface layer 40'' as seen in FIG. 3c.

[0056] Examples Hereinafter, examples of processes for coating a substrate that embody the concept of the present invention are provided.

[0057] In a first series of experiments whose results are shown in FIGS. 4a - 4b, 5a - 5b and 6a - 6b, a flat and polished silicon substrate 20 was coated.

[0058] In a second series of experiments whose results are shown in FIGS. 7a - 7f and 8a - 8c, a substrate 20 having high aspect ratio features 21 was coated.

[0059] Triethylboron (B(C2H5)3 or TEB (triethyl boron)) was used as a single-source precursor gas, and xenon (Xe) was used as an inert diffusion additive gas. Hydrogen gas (H2) for purifying the palladium film was used as the carrier gas and co-reactant during the process.

[0060] Deposition was carried out in reaction chamber 1, in the reaction zone 10 of a horizontal hot-wall CVD reactor in this example. As a result of holding the single-source precursor gas in a stainless-steel bubbler in a thermostatic bath at 0 °C, the vapor pressure was about 1.65 kPa. Substrate 20 was a silicon (Si) substrate, which was washed in acetone for 3 minutes and in ethanol for 3 minutes using an ultrasonic bath, then rinsed with deionized water, and finally blown dry with nitrogen (N2) gas. The washed Si substrate 20 was placed in reaction chamber 1, then pumped down, filled with the carrier gas, and heated to the deposition temperature. The pressure in reaction chamber 1 was maintained at about 5 kPa, controlled by the throttle valve in the pump of the process, and H2 with a flow rate of about 2000 sccm was sent to the carrier gas supply section 16. The temperature of the process was set at about 500 °C to 600 °C and monitored with a thermometer. TEB gas was sent by the precursor gas supply section 14 at a flow rate of about 1 sccm to a partial pressure of about 1.8 Pa, and Xe gas was sent by the inert additive gas supply section 15 at a flow rate that varied from about 10 sccm to about 100 sccm. Thereby, the partial pressure of the Xe gas was one to two orders of magnitude higher than that of the TEB gas. The gases were mixed in the gas manifold 13 and introduced into reaction chamber 1 while keeping the partial pressure of the Xe gas higher than that of the TEB gas.

[0061] The reaction occurs on the surface of substrate 20 where the protective layers 40, 40’, 40’’ grow in reaction zone 10. The by-products and unreacted gases are transported out of reaction chamber 1 through the gas outlet 12.

[0062] In the first series of experiments, a polished Si substrate with an area of 1 cm × 10 cm was used. Substrate 20 was placed in reaction chamber 1 in reaction zone 10 and aligned so that the longer side was in the direction f of the gas flow. Figures 4a - 4b and Figures 5a - 5b show the B deposited on the polished Si substrateX It shows C.

[0063] In the first part of the first series of experiments, the substrate was coated at a temperature of 450 °C, an H2 flow rate of 2000 sccm, a TEB gas flow rate of 1 sccm, and a pressure of 50 hPa (mbar).

[0064] One sample was coated without Xe as the diffusion additive gas (Figure 4a), and the other sample was coated with Xe gas at a flow rate of 100 sccm (Figure 4b).

[0065] In the second part of the first series of experiments, the substrate was coated at a temperature of 550 °C, an H2 flow rate of 2000 sccm, a TEB gas flow rate of 1 sccm, and a pressure of 50 hPa (mbar).

[0066] One sample was coated without Xe as the diffusion additive gas (Figure 5a), and the other sample was coated with 100 sccm of Xe (Figure 5b).

[0067] In the second series of experiments, protective surface layers 40, 40’, 40’’ of silicon carbide (B X C) were provided on the high aspect ratio features 21 of the Si substrate 20 by a superconformal growth process. The growth conditions were the same as those described above, and Xe was present as an inert additive gas.

[0068] Figures 7a - 7f show the results of deposition at 450 °C, and Figures 8a - 8c show the results of deposition at 550 °C.

[0069] Explanation of Results When an inert additive gas is introduced into the mixed gas entering the reaction chamber 1, this gas does not participate in the reaction itself, but due to its presence, it further diffuses the precursor gas into the reaction zone 10. When the inert additive gas consists of molecules heavier than the precursor gas, it means that in thermal equilibrium, the diffusion rate of the precursor gas is greater than that of the inert additive gas. While depositing, the gas pressure decreases across the entire substrate 20. Since the diffusion of the precursor gas is faster and the partial pressure of the heavier inert additive gas is higher, the precursor gas is spread farther from the gas inlet by the inert additive gas, and thus the partial pressure of the precursor gas is higher at the bottom 212 of the high aspect ratio feature than at the opening 211 of the high aspect ratio feature. This promotes faster growth at the bottom 212 than at the opening 211. For the same reason, because of the high partial pressure of the inert additive gas, the molecules of the precursor gas are spread and can cover a wider area of the substrate 20, and the coating area of the deposited layer can be made larger. This does not depend on whether the substrate 20 has high aspect ratio features or not.

[0070] In FIGS. 4a - 4b and FIGS. 5a - 5b, a 10 - cm long Si substrate is divided into five sections to facilitate comparison between a sample deposited in an environment without Xe and a sample deposited in an environment containing Xe.

[0071] In FIGS. 4a - 4b where the deposition temperature is 450 °C, it can be seen that the sample (FIG. 4b) deposited in the presence of Xe as an inert additive gas has a wider surface coating area than the sample without Xe shown in FIG. 4a. When Xe is present as an inert additive gas, the diffusion of the TEB gas is promoted.

[0072] In FIGS. 5a - 5b where the deposition temperature is 550 °C, since the diffusion rate is higher at a higher temperature, the surface coating is better for both samples. In FIG. 5a without using Xe, a thickness gradient pattern like that in FIGS. 4a - 4b can be seen. However, in FIG. 5b using Xe as an inert additive gas, the surface coating increases and the thickness non - uniformity is reduced.

[0073] Figures 6a - 6b show the X - ray photoelectron spectroscopy spectra of samples for boron in Figure 6a and for carbon in Figure 6b, showing the peaks of 1s electrons in both an environment without Xe and an environment with added Xe gas. From the plots, it can be seen that no chemical shift occurs due to the addition of Xe gas. This result suggests that the chemical environment does not change even when Xe gas is added. Xe gas acts as a diffusion additive while not affecting the chemical properties of the deposition of the deposited thin film.

[0074] Figures 7a - 7f show the results of the deposition of TEB at a temperature of 450 °C on an Si substrate 20 having high - aspect - ratio features 21.

[0075] Figure 7a shows an Si substrate 20 with an aspect ratio of 8:1. Figure 7b shows the bottom of the trench in the substrate in Figure 7a, and Figure 7c shows the top surface of the trench in Figure 7a.

[0076] Figure 7d shows an enlarged view of the upper part of the trench in Figure 7a. Figure 7e shows an enlarged view of the bottom surface of the trench in Figure 7a. Figure 7f shows a further enlarged view of the bottom surface of the trench in Figure 7a.

[0077] By measuring in Figure 7c, it is concluded that the thickness of the layer at the upper part of the trench is 436 nm, and by measuring in Figure 7f, it is concluded that the thickness of the layer is 437 nm. Therefore, the experiments disclosed in Figures 7a - 7f show perfect conformality, which approaches super - conformal behavior.

[0078] Figures 8a - 8c show the results of the deposition of TEB at a temperature of 550 °C on an Si substrate 20 having high - aspect - ratio features 21.

[0079] Measurements at the upper part of the trench (Figure 8b) and at the bottom of the trench (Figure 8c) show that the step coverage is 0.78 when xenon as a diffusion - additive gas is absent and 0.97 when xenon as a diffusion - additive gas is present.

[0080] The process of superconformally growing the surface layers 40, 40', 40'', which can be seen schematically in FIGS. 3a - 3c, is performed at a higher temperature than conventionally known, and thus the material properties of the surface layers 40, 40', 40'' can be better.

[0081] The precursor gas has molecules with a lower molecular weight than the inert additive gas molecules, and thus can diffuse faster into the high aspect ratio feature 21 than the heavier inert additive gas molecules, and accordingly, the deposition rate at the bottom 212 of the high aspect ratio feature 21 is improved.

Claims

1. A method for operating a chemical vapor deposition (CVD) process, - The step of providing a substrate (20) in the reaction area (10) of the reaction chamber (1), - A step of providing at least one precursor gas stream (f) containing precursor molecules into the reaction chamber (1), - The step of heating the reaction chamber (1) to a temperature higher than the reaction initiation temperature of the precursor molecule, - A step of providing at least one inert diffusion additive gas into the reaction chamber (1), wherein the inert diffusion additive comprises an inert diffusion additive molecule. Includes, The inert diffusion additive molecule has a larger molecular weight than the precursor molecule. A method in which the partial pressure of the inert diffusion-added gas is greater than the partial pressure of the precursor gas.

2. The method according to claim 1, wherein the CVD process is a continuous process.

3. The method according to claim 1, wherein the CVD process is a thermal CVD process.

4. The method according to claim 1, wherein the CVD process is a plasma CVD process.

5. The method according to claim 1, wherein the precursor gas comprises a metal, more particularly boron.

6. The aforementioned precursor gas is triethylboron (B(C) ２ H ５ ) ３ ) or boron trichloride (BCl ３ The method according to claim 5, which is as follows.

7. The method according to claim 1, wherein the inert diffusion additive gas is a noble gas, preferably xenon (Xe).

8. The method according to claim 1, wherein the inert diffusion-added gas is combined with the precursor gas upstream of the reaction area (10), preferably in a gas manifold (13).

9. The further step includes providing at least one carrier gas, The method according to claim 1, wherein the carrier gas is combined with the inert diffusion additive gas and the precursor gas upstream of the reaction area (10), preferably in a gas manifold (13).

10. The carrier gas is hydrogen (H ２ The method according to claim 9, wherein the material is ) or argon (Ar).

11. The temperature at which the reaction chamber (1) is heated is higher than approximately 400°C, preferably approximately 400°C to 410°C, or approximately 410°C to 420°C, or approximately 420°C to 430°C, or approximately 430°C to 440°C, or approximately 440°C to 450°C, or approximately 450°C to 460°C, or approximately 460°C to 470°C, or approximately 470°C to 480°C, or approximately 480°C to 490°C, or approximately 490°C to 500°C, or approximately 500°C to 510°C, or approximately 510°C to 520°C, or approximately 520°C to 530°C, or approximately 530°C to 540°C, or approximately 5 The method according to claim 1, wherein the temperature is 40°C to 550°C, or about 550°C to 560°C, or about 560°C to 570°C, or about 570°C to 580°C, or about 580°C to 590°C, or about 590°C to 600°C, or about 600°C to 610°C, or about 610°C to 620°C, or about 620°C to 630°C, or about 630°C to 640°C, or about 640°C to 650°C, or about 650°C to 660°C, or about 660°C to 670°C, or about 670°C to 680°C, or about 680°C to 690°C, or about 690°C to 700°C.

12. The method according to claim 1, wherein the substrate is a flat or substantially flat member.

13. The method according to claim 1, wherein the substrate (20) is a porous material.

14. The method according to claim 1, wherein the substrate (20) is a bulk material containing a plurality of fine particles or granules.

15. The method according to claim 1, wherein the substrate (20) has at least one high aspect ratio feature (21).

16. The method according to claim 15, wherein the aspect ratio of the at least one high aspect ratio feature (21) is at least about 5:1, at least about 10:1, at least about 20:1, or at least about 40:

1.

17. A product comprising a substrate having a coating grown by the method claimed in any one of claims 1 to 16.

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