JP3644864B2 - Deposition equipment - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a film forming apparatus that forms a thin film based on a catalytic reaction. In particular, the present invention relates to a film formation apparatus based on a catalytic reaction that can perform film formation uniformly and at high speed on a large-area substrate while suppressing an increase in temperature of the substrate due to a heated catalyst body.
[0002]
[Prior art]
As a method for forming a thin film in a device manufacturing process such as LSI, LCD, thin film solar cell, etc., particularly a silicon-based thin film, a plasma CVD method is currently the mainstream because it can be formed at a relatively low temperature.
[0003]
However, in the plasma CVD method, there are some problems such as large damage caused by charged particles in the plasma to the substrate and high concentration of hydrogen taken into the film.
[0004]
For this reason, a technique called a Cat-CVD method has recently attracted attention as a technique capable of forming a thin film (silicon-based thin film) containing low hydrogen without using plasma.
[0005]
In this film forming method, a catalyst body heated to a predetermined temperature is placed in a film forming chamber, a raw material gas supplied into the chamber is brought into contact with the catalyst body, and a substrate is formed based on chemical species generated by the contact reaction. This is a method of forming a predetermined thin film on the top.
[0006]
There are various thin films that can be formed by this method, such as a-Si, poly-Si, SiNx, etc. In particular, an a-Si film produced by this method is a solar cell with high stabilization efficiency with little photodegradation. It is highly expected as a power generation layer.
[0007]
The film forming apparatus based on the Cat-CVD method is described in JP-A-63-40314, JP-A-11-54441, etc. In particular, the apparatus described in JP-A-11-54441 It is also effective for suppressing heat given to the substrate by the catalyst body.
[0008]
Hereinafter, as a conventional example of a film forming apparatus based on the Cat-CVD method, an apparatus described in JP-A-11-54441 will be described with reference to FIG.
[0009]
The film forming apparatus shown in FIG. 5 includes a box-shaped chamber 101, a gas supply pipe 103 inserted into the chamber 101 from the center of the upper surface of the chamber 101, and a horizontally long rectangular parallelepiped attached to the lower end of the gas supply pipe 103. The catalyst container 104 is provided. A coiled catalyst body 105 is provided inside the catalyst container 104. A plurality of gas outlets 106 are provided on the bottom surface portion 104 b of the catalyst container 104.
[0010]
The substrate 109 is held on a substrate holder 110 provided below the catalyst container 104 so as to be parallel to the bottom surface portion 104 b of the catalyst container 104.
[0011]
In the film forming operation, the source gas 107 is supplied from the gas supply source 102 to the gas supply pipe 103. The source gas 107 comes into contact with the catalyst body 105 heated in the catalyst container 104, thereby generating chemical species 108 suitable for film formation. The gas containing the chemical species 108 is supplied to the surface of the substrate 109 from the gas outlet 106 of the bottom surface portion 104b of the catalyst container 104 by a shower method. As a result, a thin film is formed on the surface of the substrate 109, and the remaining chemical species 108 on which the thin film has been formed is discharged out of the chamber 101 from the side of the substrate 109 by the exhaust system 111.
[0012]
According to the film forming apparatus having such a configuration, an extremely high quality film having no plasma damage and having a low hydrogen content can be obtained. Further, since the probability of decomposition of the raw material gas by the catalyst body 105 is high, high-speed film formation is possible. Further, in the film forming apparatus shown in FIG. 5, since the catalyst container 104 separates the catalyst body 105 and the substrate 109, an increase in the temperature of the substrate 109 due to heat radiation from the catalyst body 105 to the substrate 109 is suppressed. .
[0013]
[Problems to be solved by the invention]
However, the above film forming apparatus has the following three problems.
[0014]
The first problem is an increase in the temperature of the substrate. In the film forming apparatus shown in FIG. 5, a source gas containing chemical species is blown out at a substantially right angle with respect to the substrate 109 from the gas outlet 106 of the catalyst container 104, and then flows along the surface of the substrate 109. Exhaust from the exhaust system 111 on the side wall. That is, since the gas flows along the entire surface of the substrate 109, the conductance of the gas flow is not so large. This effect is particularly noticeable when a large-area substrate is used.
[0015]
Thus, if the conductance of the gas flow is not large, a large amount of gas cannot be flowed. It should be noted that the gas flow rate can be increased to some extent by significantly increasing the differential pressure between the space in the catalyst container 104 and the space in the chamber 101. When the supply is attempted, the flow is disturbed, and the quality of the film and the uniformity of the film formation are impaired. Thus, if the conductance of the gas flow is not large, the gas flow rate is limited. If the gas flow rate is small, it is difficult to supply (transport) the chemical species 108 to the surface of the substrate 109 by the gas flow. That is, the time required to supply (transport) the chemical species 108 to the substrate 109 by the gas flow becomes longer than the lifetime of the scientific species 108.
[0016]
Therefore, in order to supply (transport) the chemical species 108 to the surface of the substrate 109 in a time shorter than the lifetime of the chemical species 108 while the gas flow rate is small, the catalyst body 105, that is, the medium container 104 is attached to the substrate 109. It is necessary to approach. As a result, despite the use of the catalyst container 104, the temperature of the substrate 109 rises due to heat radiation from the catalyst body 105 (catalyst container 104). Thus, the effect of suppressing the temperature rise of the substrate 109 due to the use of the catalyst container 104 is diminished.
[0017]
The second problem is non-uniform film formation. As described above, the conductance of the gas flow is not large, and the state of the flow is different between a portion close to the exhaust system 111 and a portion far from the exhaust system 111. Further, on the substrate 109, many chemical species 108 reach the portion facing the gas outlet 106, but few chemical species 108 reach other portions. As a result, it becomes difficult to obtain film formation uniformity on the surface of the substrate 109. This effect is particularly noticeable when a large-area substrate 109 is used.
[0018]
A third problem is that a sufficient film formation rate is not ensured. As described above, since the conductance of the gas flow is not large and the gas flow rate is limited, the number of source gas molecules that come into contact with the catalyst body 105 per unit time is limited, which hinders the improvement of the deposition rate.
[0019]
The present invention solves such a conventional problem, and an object of the present invention is to increase the conductance of the gas flow, thereby increasing the temperature of the substrate, the problem of non-uniform film formation, and An object of the present invention is to provide a film forming apparatus that can solve all the problems of insufficient film forming speed.
[0020]
[Means for Solving the Problems]
A film forming apparatus of the present invention is a film forming apparatus that forms a predetermined thin film on a surface of a substrate, and is heated to a predetermined temperature and generates a chemical species by contact with a source gas, and the source gas A gas supply unit that supplies the gas including the chemical species generated by the contact to be in contact with the catalyst body, and supplies the gas to only a part of the surface of the substrate. A gas discharge means provided close to the gas supply means so that the gas containing the chemical species supplied to a part of the surface is discharged in a direction away from the substrate; the gas supply means; and Moving means for moving the substrate relative to the gas discharge means in a direction along the surface thereof, and each tip portion of the gas supply means and the gas discharge means has a U-shape or a circle. Arc-shaped smooth gas The catalyst body is inclined substantially symmetrically with respect to the gas supply position by the gas supply means so that a flow is formed, and the catalyst body is provided in the gas flow path of the gas supply means. It is characterized by being arranged so as not to face the surface directly, whereby the above object is achieved.
[0021]
That is, in the film forming apparatus of the present invention, the gas supplied to a part of the surface of the substrate is discharged in a direction away from the substrate, so that the gas is limitedly supplied to the surface of the substrate to form a film. To do. As a result, the gas flow path on the substrate can be shortened and the conductance of the gas flow can be increased compared to the case where the gas is supplied to the entire surface of the substrate.
[0022]
Therefore, a large amount of gas can flow from the gas supply means to the gas discharge means, and as a result, even if the distance from the catalyst body to the substrate is increased, the chemical species can be accelerated at a shorter time than the lifetime of the chemical species. The gas flow can be supplied to the substrate. Thus, since the distance from the catalyst body to the substrate can be increased, the temperature rise of the substrate due to the radiant heat from the catalyst body can be suppressed.
[0023]
In addition, since the substrate moves relative to the gas supply means and the gas discharge means in the direction along the surface, the gas is supplied to a part of the surface of the substrate in a limited manner. A film can be formed on the entire surface.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0031]
<Embodiment 1>
FIG. 1 shows Embodiment 1 of the present invention. The film forming apparatus according to the first embodiment includes a rectangular parallelepiped chamber 1, a substantially rectangular tube-shaped gas supply pipe 3, a pair of gas discharge pipes 4 arranged around the gas supply pipe 3, and a gas supply pipe 3. A catalyst body 6 disposed inside and a substrate holder 9 provided in the chamber 1 below the gas supply pipe 3 and the gas discharge pipe 4 are provided.
[0032]
The substrate 8 as a film formation target is horizontally supported on the substrate holder 9, and moves in a specific horizontal direction (X direction) by the horizontal linear drive of the substrate holder 9 by the moving means 10.
[0033]
The substantially square cylindrical gas supply pipe 3 as a gas supply means is opposed to a part of the surface of the substrate 8 supported on the substrate holder 9 at a right angle with a predetermined gap. More specifically, it is opposed to a part of the surface of the substrate 8 in the X direction at right angles over the entire horizontal direction perpendicular to the X direction. As a result, the gas supply pipe 3 allows the source gas 7 supplied from the source gas source 2 to pass through the internal catalyst body 6 to a part of the surface of the substrate 8 supported on the substrate holder 9 in the X direction. , And supplied from directly above over the entire horizontal direction perpendicular to the X direction.
[0034]
A pair of gas discharge pipes 4 serving as gas discharge means are provided vertically and integrally on both sides in the X direction around the gas supply pipe 3 using the side wall portions 5 on both sides. Each gas discharge pipe 4 is formed in a substantially rectangular tube shape that is flat in the X direction, and is arranged vertically in the chamber 1 together with the gas supply pipe 3. The gas supply pipe 3 is opposed to a part of the surface of the substrate 8 in the X direction at a close position in the X direction at right angles over the entire horizontal direction perpendicular to the X direction.
[0035]
The catalyst body 6 arranged inside the gas supply pipe 3 is desired to have a large surface area so that it can easily come into contact with the molecules of the raw material gas flowing inside the gas supply pipe 3. For example, the catalyst body 6 has a coil shape and is heated to a predetermined temperature by energization. As the material of the catalyst body 6, tungsten, molybdenum, or the like is used and heated to about 1500 to 2000 ° C.
[0036]
As described above, the substrate holder 9 supports the substrate 8 horizontally on the substrate holder 9 and is driven in the X direction by the moving means 10. As a result, the substrate holder 9 moves the substrate 8 thereon in the X direction. Further, the substrate 8 is heated or cooled to a predetermined temperature as required by a heater or a cooling mechanism (not shown) provided inside.
[0037]
Next, a method for forming a thin film on the surface of the substrate 8 using the film forming apparatus of Embodiment 1 will be described.
[0038]
The source gas 7 is supplied from the source gas supply source 2 to the gas supply pipe 3 while moving the substrate 8 supported on the substrate holder 9 in the X direction. The source gas 7 supplied from the source gas source 2 to the gas supply pipe 3 comes into contact with the catalyst body 6 in the gas supply pipe 3, and chemical species are generated in the gas by this catalytic reaction. The generated gas containing the chemical species is formed into a vertically downward gas flow in the gas supply pipe 3, and a part of the surface of the substrate 8 in the X direction is directly above the entire horizontal region perpendicular to the X direction. Collide at right angles. Thereby, a thin film based on the chemical species is formed on a part of the surface of the substrate 8 in the X direction.
[0039]
The gas that has collided with a part of the surface of the substrate 8 in the X direction tends to spread to the periphery, in particular, both sides in the X direction, but the gas discharge pipes 4 are provided adjacent to both sides of the gas supply pipe 3 in the X direction. As a result, after dividing into both sides in the X direction, the direction is immediately changed upward to leave the surface of the substrate 8 and flow into each gas discharge pipe 4. Then, a vertical upward exhaust gas flow in each gas exhaust pipe 4 is exhausted to the outside of the chamber 1.
[0040]
Then, by moving the substrate 8 in the X direction, a thin film is sequentially formed from one end in the X direction on the surface of the substrate 8 to the other end in the X direction, and finally a thin film is formed on the entire surface of the substrate 8.
[0041]
As described above, in the film forming apparatus according to the first embodiment, the gas containing the chemical species is limitedly supplied to a part of the surface of the substrate 8 in the X direction. Further, the substrate holder 9 continuously moves horizontally in the X direction, whereby film formation is performed on the entire surface of the substrate 8. A gas containing a chemical species is supplied to a part of the surface of the substrate 8 in a limited manner, and the gas supplied to a part of the surface of the substrate 8 is quickly discharged from the substrate 8 in the vicinity of the supply position. As a result, the conductance of the gas flow is increased and a large flow rate of gas can be supplied.
[0042]
As a raw material gas used, when an amorphous or polycrystalline silicon thin film is formed, a silane-based gas such as SiH ₄ is used alone or mixed with another gas such as hydrogen. Further, in order to stabilize the decomposition of the gas, an inert gas such as He or Ar may be added. The chemical species generated when such a source gas is brought into contact with the catalyst body 6 is not clear, but is expected to be a desirable chemical species for forming a silicon thin film, such as SiH ₃ .
[0043]
The pressure in the chamber 1 may be set as appropriate. However, in order to increase the film formation rate, it is desirable that the pressure be relatively high, for example, about 0.1 Torr to 100 Torr. In the present embodiment, the gas flow rate can be set to a very large value. For example, the gas flow rate is set to several SLM (Standard Liter / Min) to several tens SLM.
[0044]
The temperature of the substrate 8 is set to about 200 ° C., for example, when an amorphous silicon thin film is formed.
[0045]
In the first embodiment, the distance L from the catalyst body 6 to the substrate 8 can be set large as will be described later. The distance L varies depending on the gas pressure and flow rate, but in the first embodiment, for reasons described later, the distance L can be set to 10 cm or more in the case of 0.1 Torr or more and several SLMs. The influence of heat on 8 can be suppressed. Thus, since the influence of the heat from the catalyst body 6 can be suppressed, the substrate temperature can be managed at an appropriate temperature as described above.
[0046]
Next, the effect of the first embodiment will be described.
[0047]
(1) According to the film forming apparatus of the first embodiment, the gas containing the chemical species is supplied from the gas supply pipe 3 only to a part of the surface of the substrate 8 and is provided close to the gas supply position. The gas exhaust pipe 4 is immediately exhausted. That is, as in the film forming apparatus shown in FIG. 5, since the gas does not flow along the entire surface of the substrate 8, the conductance of the gas flow can be increased, and the gas supply pipe is not disturbed. A large amount of gas can be flowed at a high speed from 3 to the gas exhaust pipe 4. As a result, even if the distance L from the catalyst body 6 to the substrate 8 is increased, the chemical species can be supplied to the substrate 8 by the high-speed gas flow in a time shorter than the lifetime of the chemical species. Thus, since the distance L from the catalyst body 6 to the substrate 8 can be increased, an increase in the temperature of the substrate 8 due to radiant heat from the catalyst body 6 can be suppressed.
[0048]
The gas supply pipe 3 causes a gas containing a chemical species to collide with the surface of the substrate 8 at a right angle, but the gas discharge pipe 4 is disposed around the gas supply pipe 3, here on both sides in the X direction. It is arranged close to the supply pipe 3. For this reason, the gas colliding with the surface of the substrate 8 is efficiently discharged from the substrate 8 and contributes to an increase in conductance of the gas flow.
[0049]
(2) Further, the thin film is formed only in a very small region facing the gas supply pipe 3 based on the chemical species transported by the high-speed gas flow without disturbing the flow. A good quality film is obtained. Then, by moving the substrate 8 by the moving means 10, this uniform high quality film can be formed over the entire surface of the substrate 8.
[0050]
(3) Since a large flow rate of gas can flow from the gas supply pipe 3 to the gas discharge pipe 4, the number of raw material gas molecules in contact with the catalyst body 6 per unit time inside the gas supply pipe 3 is increased. Thus, the number of chemical species can be increased, and the film formation rate can be improved.
[0051]
Since these effects, particularly the effect of (1) above, are obtained by the transport effect of the chemical species by the high-speed gas flow, in the case where the transport of the chemical species by the gas flow is the main, that is, It becomes more prominent when the mean free path is short and high pressure.
[0052]
In addition, the shape of the catalyst body 6 is not limited to a coil shape, and any shape may be used as long as the surface area in contact with the raw material gas is designed to be large. In order to further reduce the influence of radiant heat on the substrate 8, it is desirable to simultaneously reduce the area of the portion facing the substrate 8. For this reason, the shape of the catalyst body 6 is preferably a plate-like body having a large surface area along the direction of gas flow, for example, as shown in FIG. By setting it as such a shape, the area of the part facing the board | substrate 8 can be made small, without making the contact area with gas small, and the influence of the radiant heat to the board | substrate 8 can be reduced efficiently.
[0053]
Moreover, the catalyst body 6 arrange | positioned inside the gas supply pipe 3 is not restricted to one piece, You may provide two or more. For example, as shown in FIG. 3, if a plurality of catalyst bodies 6 are provided along the direction of the gas flow, the raw material gas that has not been decomposed by the upstream catalyst body 6 in the flow can be converted into the downstream catalyst body 6. Can be decomposed by. Therefore, the gas decomposition efficiency is improved, and the film formation rate can be further improved.
[0054]
In addition, since this invention is an improvement invention regarding a Cat-CVD apparatus, it is natural that a low hydrogen content thin film without damage can be formed.
[0055]
<Embodiment 2>
FIG. 4 shows a film forming apparatus according to Embodiment 2 of the present invention. This film forming apparatus is different from the film forming apparatus shown in FIG. 1 only in the structure of the gas supply pipe 3 as a gas supply means and the gas discharge pipe 4 as a gas discharge means. Since the other parts have the same structure, the common parts are denoted by the same reference numerals and description thereof is omitted.
[0056]
In the film forming apparatus shown in FIG. 4, the gas supply pipe 3 has an upper vertical straight portion 3a and a curved portion 3b formed therebelow. The curved portion 3b is curved so as to approach the gas exhaust pipe 4 as the substrate 8 is approached, and the cross-sectional area gradually decreases. On the other hand, the gas discharge pipe 4 also has an upper vertical straight part 4 a and a curved part 4 b formed below the upper part, and the curved part 4 b is symmetrical to the curved part 3 b of the gas supply pipe 3. As it approaches the substrate 8, it is curved so as to approach the gas supply pipe 3, and its cross-sectional area is gradually reduced.
[0057]
With such a configuration, the gas flowing out from the gas outlet of the gas supply pipe 3 draws an arc projecting downward and is supplied to the surface of the substrate 8 at a small inclination angle with respect to the substrate 8. Thereafter, the gas is discharged to the gas inlet of the gas discharge pipe 4 at a small inclination angle with respect to the substrate 8. Thereby, a smooth continuous U-shaped or arc-shaped gas flow is formed from the gas outlet of the gas supply pipe 3 toward the gas inlet of the gas discharge pipe 4.
[0058]
As described above, the gas supply pipe 3 and the gas discharge pipe 4 are provided symmetrically on both sides of the gas collision position on the substrate 8 in the X direction, and the lower portions thereof are inclined so as to approach each other. Since the curved portions 3b and 4b are respectively curved, there is no sudden change in the flow line of the gas flow, and the conductance of the gas flow is larger than in the case of the film forming apparatus shown in FIG. As a result, a larger flow rate of gas can be allowed to flow. Therefore, the effect described in the first embodiment becomes more remarkable.
[0059]
In the film forming apparatus shown in FIG. 4, the gas supply pipe 3 has a structure having a curved portion 3b below the straight portion 3a. The medium 6 is shielded from the substrate 8 by the wall portion 3b ′ constituting the lower curved portion 3b, and the catalyst body 6 is prevented from directly facing the substrate 8. Thereby, the radiant heat from the catalyst body 6 to the board | substrate 8 can be suppressed more effectively.
[0060]
【The invention's effect】
The film forming apparatus of the present invention thus forms a predetermined thin film on the surface of the substrate based on chemical species generated by bringing the source gas into contact with the catalyst body heated to a predetermined temperature. The gas containing the chemical species is supplied only to a part of the surface of the substrate, and the gas supplied to a part of the surface of the substrate is exhausted in a direction away from the substrate. Conductance can be increased. That is, a large amount of gas can be flowed on the substrate, and chemical species mainly composed of gas flow can be transported. As a result, even if the distance from the catalyst body to the substrate is increased, the chemical species can be supplied to the substrate in a period shorter than the lifetime of the chemical species. Thus, since the distance from the catalyst body to the substrate can be increased, an increase in the temperature of the substrate due to radiant heat from the catalyst body can be suppressed.
[0061]
In addition, since the thin film is formed in a very small region facing the gas supply pipe, a uniform high quality film can be obtained in this portion. Then, by moving the substrate by the moving means, this homogeneous high-quality film can be formed on the surface of the large-area substrate.
[0062]
In addition, since a large amount of gas can flow from the gas supply pipe to the gas exhaust pipe, the number of raw material gas molecules in contact with the catalyst body inside the gas supply pipe can be increased, thereby reducing the number of chemical species. The film formation rate can be improved.
[0063]
Further, the gas flow conductance is provided in the vicinity of the gas supply means so that the gas supplied to a part of the surface of the substrate is quickly discharged from the substrate in the vicinity of the supply position. The above three effects, namely, the suppression of the temperature rise of the substrate, the uniform film formation on the large area substrate, and the higher film formation speed can be obtained.
[0064]
Further, the gas supply means supplies a gas containing chemical species at a substantially right angle to the surface of the substrate so that the gas supplied to the surface of the substrate is discharged from the substrate around the gas supply position. By providing the gas discharge means around the gas supply means, the conductance of the gas flow can be increased, and the above three effects, namely, suppression of the temperature rise of the substrate, and uniform film formation on the large area substrate In addition, a great effect can be obtained with respect to increasing the film forming speed.
[0065]
Further, the gas supply means and the gas discharge means are inclined substantially symmetrically with respect to the gas supply position so that a smooth gas flow having a substantially U shape or a substantially arc shape is formed from the gas supply means to the gas discharge means. Therefore, the conductance of the gas flow can be increased, and the above three effects can be achieved, namely, suppression of the temperature rise of the substrate, uniformity of film formation on a large area substrate, and increase of the film formation speed. Greater effects can be obtained.
[0066]
In this case, the radiant heat from the catalyst body to the substrate is effectively suppressed by disposing the catalyst body at a position not directly facing the surface of the substrate in the gas supply means.
[0067]
Further, by forming the catalyst body so as to have a large surface area along the gas flow flowing inside the gas supply means and having a surface area perpendicular to the gas flow, Radiant heat is effectively suppressed.
[0068]
Furthermore, the production efficiency of chemical species can be improved by arranging a plurality of catalyst bodies at intervals in the direction of the gas flow flowing inside the gas supply means.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an example of an embodiment of a film forming apparatus of the present invention.
FIG. 2 is a schematic configuration diagram of a main part showing another example of the film forming apparatus.
FIG. 3 is a schematic configuration diagram of a main part showing still another example of the film forming apparatus.
FIG. 4 is a schematic configuration diagram showing another example of the embodiment of the film forming apparatus of the present invention.
FIG. 5 is an explanatory diagram of a conventional film forming apparatus.
[Explanation of symbols]
1 Chamber 2 Source gas source 3 Gas supply pipe (gas supply means)
3a Straight line part 3b Curved part 4 Gas discharge pipe (gas discharge means)
4a Linear part 4b Curved part 5 Side wall part 6 Catalytic body 7 Raw material gas 8 Substrate 9 Substrate holder 10 Moving means

Claims (1)

A film forming apparatus for forming a predetermined thin film on a surface of a substrate,
A catalyst body that is heated to a predetermined temperature and generates chemical species by contact with a raw material gas;
A gas supply means for supplying the source gas so as to be in contact with the catalyst body, and supplying a gas containing the chemical species generated by the contact only to a part of the surface of the substrate;
Gas discharge means provided close to the gas supply means so that the gas containing the chemical species supplied to a part of the surface of the substrate by the gas supply means is discharged in a direction away from the substrate. When,
Moving means for moving the substrate relative to the gas supply means and the gas discharge means in a direction along its surface;
Comprising
The tip of each of the gas supply means and the gas discharge means is inclined almost symmetrically across the gas supply position by the gas supply means so that a smooth U-shaped or arc-shaped gas flow is formed. Is provided,
The film forming apparatus, wherein the catalyst body is disposed in the gas flow path of the gas supply means so as not to directly face the surface of the substrate.

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