JP2002346453A - Thin film forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin film forming device capable of forming a uniform thin film by supplying coating liquid mist onto a substrate surface with excellent in-plane uniformity, and forming a thin film of high quality by effectively collecting surplus coating mist which is scattered from the substrate surface of the coating liquid mist. SOLUTION: The range of ejection of a resist mist 52 is narrowed to a coating area 44, the resist mist 52 is uniformly supplied to the substrate surface, and the in-plane uniformity of the thin film is enhanced. Furthermore, an ejection nozzle 40 is arranged in the upper position of the substrate surface of the ejection nozzle 40 and supplies the resist mist 52 to the coating area 44 so as to make an acute angle with respect to a perpendicular 48 of the coating area 44, and on the contrary, a suction nozzle 42 is arranged in the upper position of the substrate surface and on the opposite side of the ejection nozzle 40 sandwiching the perpendicular 48, and surplus resist mist 54 which is scattered from the coating area 44 is effectively sucked by the suction nozzle 42.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor substrate, a glass substrate for a liquid crystal display, a glass substrate for a plasma display, a glass substrate for a photomask, a substrate for an optical disk, and the like (hereinafter referred to as "substrate"). The present invention relates to a thin film forming apparatus for forming a thin film of a coating liquid such as a photoresist liquid, and more particularly to an apparatus for supplying a fine mist of a coating liquid to a substrate surface to form a thin film of the coating liquid on the substrate surface.

[0002]

2. Description of the Related Art As an apparatus for forming a thin film of a coating liquid on a substrate surface, for example, an apparatus for supplying a coating liquid to a substrate from a shower nozzle, or a method in which the coating liquid is dropped from a nozzle to a substantially central portion of the substrate, and There is known an apparatus or the like in which a coating liquid is uniformly diffused over the entire surface of a substrate by rotating the substrate at a high speed. However, these thin film forming apparatuses have a problem that a large amount of the coating liquid is wasted without being coated on the substrate surface, and the consumption of the coating liquid increases. Therefore, instead of supplying the coating liquid to the substrate surface, a fine-particle coating liquid mist is generated, and the coating liquid mist is supplied to the substrate surface to form a thin film of the coating liquid on the substrate surface. Proposed.

As this type of thin film forming apparatus, there is one described in, for example, Japanese Patent Application Laid-Open No. Hei 5-18489.
In this thin film forming apparatus, a substrate is placed on a substrate support in a coating chamber (processing chamber), and a conical mist supply unit is disposed above the substrate. The lower end side of the mist supply unit is opened so as to face the entire surface of the substrate, and the coating liquid mist is transported to the mist supply unit together with the nitrogen gas through a mist transportation pipe communicated with the upper end of the mist supply unit. It is supplied to the substrate surface through the opening. Thus, when the coating liquid mist is supplied on the substrate surface, it is deposited on the substrate surface to form a thin film of the coating liquid. As described above, in this conventional apparatus, the thin film of the coating liquid is formed by depositing the coating liquid mist on the substrate surface, so that the coating liquid itself is wasted compared to the case where the coating liquid itself is supplied to the substrate surface to form the thin film. The consumption of the coating liquid can be reduced, and the amount of the coating liquid consumed is reduced.

[0004]

However, in the above-described conventional apparatus, since the coating liquid mist is simultaneously supplied to the entire surface of the substrate from the opening of the conical mist supply unit, the coating liquid mist is uniformly distributed over the entire surface of the substrate. Difficult to supply. For example, at the center of the substrate surface closest to the connection point (upper end of the mist supply unit) between the mist supply unit and the mist transport pipe, and at the peripheral edge of the substrate farthest from the connection point, the flow rate of the coating liquid mist, The supply amount and density are greatly different. Therefore, it was difficult to form a thin film uniformly on the substrate surface. In particular, in recent years, it has become more difficult to supply the coating liquid mist uniformly over the entire surface of the substrate by the above-described conventional apparatus with the increase in the size of the substrate.

Further, even when a thin film is formed using a coating liquid mist, not all of the coating liquid mist supplied from the mist supply section toward the substrate surface is deposited on the substrate surface. Some of them may be scattered from the substrate surface and remain in the coating chamber as excess mist of the coating liquid. If the excess coating liquid mist remaining in the coating chamber adheres to the substrate surface in this way, there may be problems such as uneven thickness of the thin film and contamination of the substrate. Therefore, in this type of thin film forming apparatus, it is desired to form a high quality thin film by increasing the collection efficiency of the excess coating liquid mist to prevent the above-mentioned problems from occurring.

The present invention has been made in view of the above problems, and provides a coating liquid mist with excellent in-plane uniformity to a substrate surface to form a uniform thin film, and scatters the coating liquid mist from the substrate surface. It is an object of the present invention to provide a thin film forming apparatus capable of efficiently collecting excess coating liquid mist and forming a high quality thin film.

[0007]

SUMMARY OF THE INVENTION The present invention is a thin film forming apparatus for supplying a coating liquid mist to a surface of a substrate to form a thin film of the coating liquid on the surface of the substrate. An ejection nozzle that is disposed at a position above the substrate surface and that ejects a coating liquid mist to the coating area so as to form an acute angle with respect to a perpendicular to a coating area to be coated with the coating liquid on the substrate surface; And a suction nozzle disposed on the opposite side of the ejection nozzle with the perpendicular line interposed therebetween and for sucking excess coating liquid mist scattered from the coating area (claim 1).

[0008] In the invention configured as described above, the coating liquid mist jetted from the jet nozzle is supplied to a part of the substrate surface, that is, the coating area, to form a thin film of the coating liquid. Thus, by narrowing the ejection range of the coating liquid mist, the coating liquid mist is uniformly supplied to the coating area, and the in-plane uniformity of the thin film is improved. In addition, since the coating liquid mist ejected from the ejection nozzle to the application area forms an acute angle with respect to the perpendicular of the application area, most of the excess coating liquid mist scattered from the application area is in the ejection direction across the perpendicular. Splashes on the other side. Since the suction nozzle is arranged in the scattering direction, the excess coating liquid mist is suctioned and collected by the suction nozzle, and the re-adhesion of the excess coating liquid mist to the substrate surface is reliably prevented.

Here, when the ejection nozzle and the suction nozzle are relatively moved with respect to the substrate by the driving means, the application area moves within the surface of the substrate with the relative movement. Thus, when the coating liquid mist is jetted from the jet nozzle while moving the coating area relative to the substrate surface in this way (claim 2), a thin film of the coating liquid can be formed over a wide area on the substrate surface. it can. Further, by sucking the excess coating liquid mist with the suction nozzle in parallel with the formation of the thin film, the excess coating liquid mist scattered at the time of forming the thin film can be collected by suction to prevent re-adhesion.

Further, a temperature adjusting means for controlling the surface temperature of the substrate may be further provided to adjust the surface temperature of the application region to a temperature lower than the temperature of the application liquid mist. That is, when the surface temperature of the application area is lower than the application liquid mist, the deposition of the application liquid mist on the application area is promoted. In addition, the deposition rate can be controlled by adjusting the temperature.

Further, a part of the substrate surface may be an effective area for forming a thin film of the coating liquid, and the remaining may be an ineffective area where no thin film of the coating liquid is formed. It is desirable to set the surface temperature of the effective area higher than the surface temperature of the effective area. Where the surface temperature of the substrate is relatively low, the deposition of the coating liquid is promoted, while where the surface temperature is relatively high, the deposition of the coating liquid is suppressed. Therefore, when the surface temperature of the non-effective area is set higher than the surface temperature of the effective area as described above, the deposition of the coating liquid in the non-effective area is suppressed as compared with the deposition of the coating liquid in the effective area. .

[0012]

FIG. 1 is a view showing one embodiment of a thin film forming apparatus according to the present invention. This thin film forming apparatus is an apparatus for forming a thin film of a coating liquid on the surface of a substrate 4 in a processing chamber 2. In this embodiment, a photoresist liquid is used as a coating liquid.

The thin film forming apparatus includes a resist mist generating section 6 for generating a mist of a photoresist liquid (hereinafter referred to as “resist mist”). The resist mist generated by the resist mist generating section 6 is supplied to a pipe 8. To the processing chamber 2 via a pressure. To elaborate,
The resist mist generating section 6 is provided with a mist generating nozzle 10 for generating a resist mist. The resist mist is generated by spraying a photoresist liquid pressure-fed from a resist pressing section 12 to generate a resist mist. The resist mist is sent to the processing chamber 2 side by an inert gas such as nitrogen gas sent from the pressurizing unit 14 under pressure. The configuration for generating the resist mist is not limited to this, and is arbitrary. For example, the resist mist may be generated by applying ultrasonic vibration to a photoresist liquid.

The resist mist generated in the resist mist generating section 6 is conveyed to the processing chamber 2 by a pipe 8. In order to prevent the resist mist from aggregating into droplets during the conveyance, the pipe 8 is connected. A heater 16 is provided so as to surround it.

In the processing chamber 2, a substrate holding table 18 is provided, and the substrate 4 can be placed and held on the upper surface of the substrate holding table 18 in a substantially horizontal state. Further, a temperature adjustment unit 22 including a plurality of Peltier elements 20 is incorporated in the substrate holder 18, and the substrate 4 mounted on the substrate holder 18 is controlled by individually controlling the Peltier elements 20. Can be partially controlled. For example, it is possible to control the temperature of the peripheral portion of the surface of the substrate 4 to be higher than that of the central portion of the surface, or to control the temperature of the central portion of the surface of the substrate 4 to be lower than that of the resist mist. .

A nozzle head 24 is provided at a position above the substrate holding table 18 so as to be movable in a horizontal plane (XY plane). This nozzle head 24 is an XY stage 26
The substrate 4, which is mechanically connected to the nozzle holder 24 and is held by the substrate holder 18 by the XY stage 26.
It is configured to be able to relatively move with respect to. The nozzle head 24 is connected to the resist mist generation unit 6 by the pipe 8, and moves the resist mist sent to the processing chamber 2 through the pipe 8 toward the substrate surface while relatively moving with respect to the substrate 4. By jetting, a thin film of a photoresist liquid is sequentially formed on the substrate surface. Thus, in this embodiment, the XY stage 26 functions as the “driving unit” of the present invention.

The nozzle head 24 is connected to a suction pump 28 so that the excess resist mist scattered from the substrate surface can be sucked and collected from the processing chamber 2 to the gas-liquid separation unit 30 of the resist mist generation unit 6. Has become. In addition,
For the detailed configuration and operation of the nozzle head 24,
Details will be described later.

Further, in the processing chamber 2, a cup 32 is provided so as to surround the periphery of the substrate holding table 18. The cup 32 is connected to a collection pump 36 by a pipe 34. The cup 32 collects excess resist mist that could not be collected by the nozzle head 24, and the excess resist mist is collected by the collection pump 36. The liquid is collected in the liquid separation unit 30.

The gas-liquid separating section 30, which has collected the excess resist mist, returns the excess resist mist to the photoresist liquid and sends it to the resist pressurizing section 12 for reuse of the photoresist liquid. As described above, in this embodiment, the photoresist solution can be efficiently used for forming a thin film by circulating the photoresist solution, and the consumption of the photoresist solution can be reduced.

In this thin film forming apparatus, a control unit 38 for controlling the entire apparatus is provided.
Resist pressurizing unit 12, inert gas pressurizing unit 14, heater 16, Peltier device 20, X
Y stage 26, suction pump 28 and recovery pump 36
Is drive-controlled.

Next, the configuration and operation of the nozzle head 24 will be described in detail with reference to FIG. 2A and 2B are views showing the nozzle head, FIG. 2A is an enlarged sectional view of the nozzle head, and FIG. 2B is a plan view of the nozzle head viewed from the substrate surface side. Note that reference numeral Z in the figure indicates a vertical direction orthogonal to a horizontal plane (XY plane).

The nozzle head 24 is provided with two through holes 40 and 42 as shown in FIG. And one of the through holes 40 is communicated with the pipe 8,
The resist mist pressure-fed from the resist mist generating section 6 is jetted toward the coating area 44 on the surface of the substrate 4 where the thin film of the photoresist liquid is to be formed.
4, a thin film 46 of a photoresist solution is formed. Thus, the through-hole 40 provided in the nozzle head 24 functions as the “ejection nozzle” of the present invention. In this embodiment, the ejection nozzle 40 extends inside the nozzle head 24 so as to form an acute angle with respect to a perpendicular 48 of the coating region 44, and the resist mist 52 ejected from the ejection port 50 also has a perpendicular 48. At an acute angle to

The other through-hole 42 is disposed on the opposite side of the ejection nozzle 40 with respect to the perpendicular 48, and the upper end thereof is connected to the suction pump 28. For this reason, when the suction pump 28 is operated in response to a control command from the control unit 38, the excess resist mist 54 scattered from the application area 44 is sucked from the suction port 56 into the through-hole 42, and the gas-liquid separation unit 30 Will be collected. Thus, the through-hole 42 provided in the nozzle head 24 functions as the “suction nozzle” of the present invention.

Next, the operation of the thin film forming apparatus configured as described above will be described. In this thin film forming apparatus, when the substrate 4 on which a thin film is to be formed is transferred to the substrate holding table 18 by a transfer robot (not shown) and then held by the substrate holding table 18, the control unit 38 controls each Peltier device 20.
Are individually controlled to adjust the surface temperature of the substrate 4. For example, as shown in FIG. 3, a central portion (hatched region) of the substrate surface is an effective region 58 where a thin film is to be formed, and a peripheral portion surrounding the effective region 58 is a non-effective region 60 where no thin film is formed. If so, the Peltier element 20 corresponding to the effective area 58 cools the substrate 4 to cool the surface temperature of the effective area 58 to the temperature T2. By doing so, on the substrate surface, the surface temperature T1 of the non-effective area 60 becomes higher than the surface temperature T2 of the effective area 58. The lower graph in the figure shows the distribution of the surface temperature of the substrate 4 along the line AA in the upper configuration diagram.

Here, assuming that the temperature of the resist mist 52 ejected from the ejection nozzle 40 is Tr, the surface temperature distribution of the substrate 4 is controlled so as to satisfy the following inequality: T1>Tr> T2. Is preferred. This is because, by setting the surface temperature of the effective region 58 to a temperature lower than that of the resist mist 52, the deposition of the resist on the substrate surface can be promoted. Further, since the temperature of the non-effective area 60 is set to be higher than that of the resist mist 52, even if excess resist mist scatters to the non-effective area 60 as described later, the non-effective area 60 This is because it is possible to suppress the deposition on the substrate.

When the substrate surface has the above-mentioned temperature distribution (FIG. 3), the controller 38 drives the XY stage 26 to move the nozzle head 24 to the first coating area 44 in the effective area 58.
After being moved to a position corresponding to
By controlling the inert gas pressurizing unit 14, the suction pump 28, the recovery pump 36, and the like, the resist mist 52 is supplied toward the application region 44 to form the thin film 46 of the photoresist liquid. Excess resist mist 54 scattered from the application area 44 is collected in the gas-liquid separation unit 30 via the suction nozzle 42 and the suction pump 28.

When the formation of the thin film is started in this manner, the control section 38 controls the resist press section 12 and the inert gas press section 1.
4. While the suction pump 28 and the recovery pump 36 are operated, the XY stage 26 is driven to drive the nozzle head 2
4 in the XY plane along a predetermined movement path.
The application area 44 is moved corresponding to the movement path by moving the application area 44 in a three-dimensional manner. As a result, the thin film formation region can be extended to the entire effective region 58 on the substrate surface.

When the thin film of the photoresist liquid is formed on all of the effective areas 58, each part of the apparatus is returned to a predetermined standby state, and the processed substrate 4 is transferred from the substrate holding table 18 by the transfer robot. It is carried out of the device.
Thereafter, the above-described series of operations is repeated until the next substrate 4 is carried in.

As described above, according to the thin film forming apparatus of this embodiment, since the ejection range of the resist mist 52 is narrowed to the coating region 44, the resist mist 52 can be uniformly supplied to the substrate surface. In addition, the in-plane uniformity of the thin film 46 can be improved.

An ejection nozzle 40 is arranged above the substrate surface to supply the resist mist 52 to the coating area 44 at an acute angle with respect to a perpendicular 48 of the coating area 44, while the suction nozzle 42 Are located above the substrate surface and on the opposite side of the ejection nozzle 40 with the perpendicular 48 interposed therebetween, so that the excess resist mist 54 reflected and scattered by the coating area 44 of the resist mist 52 can be efficiently suctioned and collected. can do. That is, as shown in FIG.
2 is supplied to the coating area 44, most of the excess resist mist 54 scattered from the coating area 44 scatters on the side opposite to the ejection direction across the perpendicular 48, but the suction port 56 of the suction nozzle 42 in the scattering direction. Is opened, the excess resist mist 54 is sucked and collected by the suction nozzle 42. As a result, re-adhesion of the excess resist mist 54 to the substrate surface can be reliably prevented, and a high-quality thin film 46 can be formed.

Further, in this embodiment, a temperature adjusting unit 22 for controlling the surface temperature of the substrate is provided.
As described above, the temperature distribution on the substrate surface is controlled to promote the deposition of the resist on the effective area 58 to efficiently form the thin film, and the deposition of the resist on the non-effective area 60 can be suppressed. Can be performed favorably. Further, the resist deposition rate can be controlled by controlling the temperature of the substrate surface.

The present invention is not limited to the above-described embodiment, and various modifications other than those described above can be made without departing from the gist of the present invention. For example, in the above embodiment, the ejection nozzle 40 and the suction nozzle 42 are symmetrical about the perpendicular 48,
Needless to say, the suction port 62 may be asymmetrically configured. For example, as illustrated in FIG. 4, the suction port 62 may be expanded in order to more efficiently suction and collect the excess resist mist 54.

Further, in the above embodiment, the shape of the coating area 44 is circular as shown in FIG. 2, and the nozzle head 24 is moved two-dimensionally in the XY plane to form the coating area 44 on the effective area 58 of the substrate 4. Although a thin film of a photoresist liquid is formed, it may be configured as in the following embodiments.

FIG. 5 is a partial plan view showing another embodiment of the thin film forming apparatus according to the present invention. 6 is a view showing a nozzle head in the thin film forming apparatus of FIG. 5, FIG. 6 (a) is an enlarged sectional view of the nozzle head, and FIG. 6 (b) is a view of the nozzle head from the substrate surface side. It is a partial plan view. This embodiment is significantly different from the previous embodiment in the configuration and operation of the nozzle head, and the other configurations and operations are the same. Therefore, the following description will focus on the differences, and the same components will be denoted by the same reference numerals and description thereof will be omitted.

In this embodiment, as shown in FIG. 5, the nozzle head 64 extends in the X direction so as to cross both ends of the substrate 4 in the X direction, and is movable in the Y direction.
In the nozzle head 64 as well, the ejection nozzle 40 and the suction nozzle 42 are symmetrically arranged with the perpendicular 48 of the application area 44 interposed therebetween as in the previous embodiment, but in this embodiment, as shown in FIG. The ejection port 66 of the nozzle 40 and the suction port 68 of the suction nozzle 42 are slit-shaped openings extending in the X direction, and the coating region 44 is also elongated in the X direction corresponding to the shape of the ejection port 66. Area. Therefore, in this embodiment, when the resist mist 52 is ejected from the ejection nozzle 40, the slit-like thin film 46 extending in the X direction is formed.

Therefore, in this embodiment, the XY stage 2 is operated while the resist pressurizing section 12, the inert gas pressurizing section 14, the suction pump 28 and the recovery pump 36 are operated.
6 to drive the nozzle head 24 in the Y direction, the coating area 44 is moved in the Y direction with the movement in the Y direction, and the thin film 46 of the photoresist liquid is formed on the entire effective area 58. are doing. In FIGS. 5 and 6,
This shows a state in which the nozzle head 64 is being moved in the Y direction.

In the embodiment configured as described above, the same operation and effect as those of the previous embodiment can be obtained, and a thin film can be formed by simply moving the nozzle head 64 in the Y direction. This is advantageous in improving the throughput as compared with the previous embodiment.

In the above embodiment, the temperature control unit 22 functioning as the "temperature control means" of the present invention is constituted by the plurality of Peltier elements 20, but a heating element may be further provided in addition to the Peltier element 20. Often, in this case, the temperature of the substrate surface can be controlled over a wider range.

In the above embodiment, the ejection nozzle and the suction nozzle are formed by extending the through holes 40 and 42 in the block-shaped nozzle head 24. However, the nozzles 40 and 42 are separately formed. It may be configured so that these nozzles are integrally moved.

Further, in the above embodiment, the nozzle heads 24 and 64 are moved with respect to the substrate 4. However, the nozzle nozzles are fixed by moving the substrate, or by moving the nozzle head and the substrate. Needless to say, the suction nozzle may be integrally moved relative to the substrate.

[0041]

As described above, according to the present invention, the coating liquid mist ejected from the ejection nozzle is supplied to the coating area in a limited manner to form a thin film of the coating liquid. The coating liquid mist can be uniformly supplied to the substrate surface, and the in-plane uniformity of the thin film can be improved. Moreover, while the ejection nozzle is arranged at a position above the substrate surface and the application liquid mist is supplied to the application region so as to form an acute angle with respect to the perpendicular of the application region, the suction nozzle is arranged at a position above the substrate surface. And since it is located on the opposite side of the ejection nozzle across the perpendicular, the excess coating liquid mist scattered from the coating area can be efficiently suctioned and collected, and the excess coating liquid mist can be effectively re-attached to the substrate. Can be prevented. As a result, a high-quality thin film can be formed.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a thin film forming apparatus according to the present invention.

FIG. 2 is a view showing a nozzle head in the thin film forming apparatus of FIG.

FIG. 3 is a diagram showing a temperature distribution on a substrate surface in the thin film forming apparatus of FIG.

FIG. 4 is a view showing another embodiment of the thin film forming apparatus according to the present invention.

FIG. 5 is a partial plan view showing another embodiment of the thin film forming apparatus according to the present invention.

6 is a diagram showing a nozzle head in the thin film forming apparatus of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 4 ... Substrate 20 ... Peltier element (temperature adjustment means) 22 ... Temperature adjustment unit (temperature adjustment means) 26 ... XY stage (drive means) 40 ... Jet nozzle 42 ... Suction nozzle 44 ... Coating area 46 ... Thin film 48 ... (Coating area) ) Perpendicular line 52: Resist mist 54: Excess resist mist 58: Effective area (of substrate) 60: Non-effective area (of substrate)

Continuation of the front page (72) Inventor Yoshiyuki Nakagawa 4-chome, Horikawa-dori-Terauchi, Kamigyo-ku, Kyoto-shi, Kyoto 1F, Tenjin Kita-cho 1 Dainippon Screen Mfg. Co., Ltd. F-term (reference) 2H025 AB08 AB16 AB17 EA04 4F035 AA03 CA01 CA05 CD05 CD11 CD16 CD18 CD19 4F041 AA02 AA06 BA05 BA12 BA22 BA38 BA59 4F042 AA02 AA07 AA08 AA10 BA08 BA19 CB10 CB26 CC03 CC07 CC09 5F046 JA02 JA03 JA20 JA24 JA27

Claims (4)

[Claims] 1. A thin film forming apparatus for supplying a coating liquid mist to a surface of a substrate to form a thin film of the coating liquid on the surface of the substrate. A jet nozzle for jetting a coating liquid mist to the coating area so as to form an acute angle with respect to a vertical line of the coating area to be coated, and a position above the substrate surface and opposite to the jet nozzle with the vertical line interposed therebetween. And a suction nozzle for suctioning excess coating liquid mist scattered from the coating area. 2. A driving unit for integrally moving the ejection nozzle and the suction nozzle relative to the substrate, wherein the driving unit moves the ejection nozzle and the suction nozzle relative to the substrate. 2. The thin film forming apparatus according to claim 1, wherein the application area is moved within the surface of the substrate while the application area is moved from the ejection nozzle, and the excess application liquid mist is sucked by the suction nozzle. 3. 3. The apparatus according to claim 1, further comprising a temperature adjusting unit configured to control a surface temperature of the substrate, wherein the temperature adjusting unit adjusts a surface temperature of the application region to a temperature lower than a temperature of the application liquid mist. The thin film forming apparatus as described in the above. 4. The substrate according to claim 1, wherein a part of the substrate surface is an effective area where a thin film of the coating liquid is formed, and the remaining part is an ineffective area where a thin film of the coating liquid is not formed. The thin film forming apparatus according to claim 1, wherein a surface temperature of the non-effective area is set higher than a surface temperature of the effective area.