Circuit board and sealing structure and methods for manufacturing the same

A circuit board is manufactured by a method having the steps of depositing a metal pattern on a ceramic board, depositing a thin layer of a high polymer material on the ceramic board formed with the metal pattern, depositing a protective layer of a high polymer material on the thin layer of the high polymer material, and directing a laser beam toward and onto the protective layer and the thin layer deposited on a plating region of the metal pattern and a cutting region of the ceramic board thereby selectively removing part of those layers. The laser beam has a wavelength range of from 150 nm to 400 nm, an energy density range of from 0.5 J/cm.sup.2 to 5.0 J/cm.sup.2, and a pulse width range of from 100 ps to 1 .mu.s.

BACKGROUND OF THE INVENTION 
This invention relates to a circuit board and a sealing structure, and more 
particularly to a technique which is effectively applicable to selective 
removal and cleaning of a layer of a material such as a high polymer 
material or an organic material in a circuit board or a sealing structure. 
It is known that, in a ceramic circuit board or the like used for mounting 
or sealing a semiconductor integrated circuit element, a thin film or 
layer of, for example, a high polymer material such as a polyimide resin 
is used for inter-layer insulation in a multilayer interconnection wiring 
arrangement provided on the circuit board or used as a protective film or 
layer for such a multilayer interconnection wiring arrangement. 
This thin layer of the high polymer material such as polyimide is formed by 
coating the material in liquid form on the entire surface of the board by 
means such as spin-coating and then thermally setting or hardening the 
coating. Therefore, it becomes necessary to selectively remove, prior to 
the later step of soldering or plating, the polyimide layer where it 
covers, for example, a metallized pattern formed in an external connection 
terminal region of the interconnection wiring arrangement or a metallized 
pattern formed in a region where the package is sealed by a cap. 
Hitherto, etching, for example has been dry etching commonly employed in 
the art as a means for selectively removing such a layer of a high polymer 
material. 
According to this dry etching, plasma of, for example, oxygen gas is 
directed toward and onto the entire surface of a ceramic board where such 
a thin layer of a high polymer material is masked by, for example, a mask 
of a metal material, so as to selectively etch the thin layer of the high 
polymer material by bombardment with ions or electrons. 
Another method is disclosed in, for example, JP-A-62-145797. According to 
this disclosed method, a polyester film covering a copper foil pattern 
deposited on a flexible circuit board is selectively removed by 
irradiating the film with an ultraviolet laser beam so as to externally 
expose a region such as an external terminal region of the copper foil 
pattern. 
SUMMARY OF THE INVENTION 
However, in the former method of dry etching, the surface of the metal mask 
(for example, a mask of aluminum) selectively masking the thin layer of 
the high polymer material is also subjected to the etching action and is 
deteriorated. As a result, products of etching and/or fragments of the 
metal mask subjected to the etching treatment attach as foreign matter to 
the etched region of the thin layer of the high polymer material, and the 
underlying metal pattern exposed from the etched region of the thin layer 
of the high polymer material is inevitably contaminated and oxidized. 
Therefore, the method of dry etching has the problem that the plating 
property of the metal pattern, i.e. its capability of being plated by a 
metal such as gold or nickel, is inevitably degraded. 
Also, to deal with the problem of contamination, oxidization, etc. 
occurring as a result of the dry etching, a method has been proposed in 
which foreign matter, an oxide film, etc. remaining on a metal pattern are 
mechanically or chemically removed. However, this method is also not 
preferable, because the surface of the metal pattern is deteriorated, and 
the capability of being later plated by a metal is also degraded. 
Another cause giving rise to degradation of the plating property will be 
described. The metal pattern is formed by coating a metal paste in a 
predetermined pattern on a green sheet of a ceramic material by means of, 
for example, printing, and then baking both the green sheet and the metal 
paste to thermally set the metal pattern. Therefore, fine concavities and 
convexities are present on the surface of the baked metal pattern. 
A high polymer material is then coated to cover the entire surface of the 
ceramic board having the metal pattern formed thereon and is then 
thermally set to form the thin layer of the high polymer material. 
Therefore, even when the thin layer of the high polymer material on the 
metal pattern is selectively removed by the dry etching, the high polymer 
material trapped in the fine concavities of the surface of the metal 
pattern tends to remain in these concavities. Thus, the surface of the 
metal pattern will not be successfully plated with the metal, and the 
plated metal layer tends to be easily stripped off from the surface of the 
metal pattern. When the dry etching is repeated so as to avoid such a 
trouble, the metal pattern itself will be partly etched away, and the 
metal pattern itself will be damaged. Therefore the metal pattern has 
previously been checked by human eyes for presence/absence of the residue 
of the high polymer material. 
Further, in the case of the method of dry etching, the rate or speed of 
etching is relatively low, and as many as ten extra steps or more are 
required for the metal mask formation, removal, etc., with the result that 
there arises the problem that a long period of time is required for 
completing the process. The steps of the metal mask formation and removal 
have required a period of time as long as several-tens of hours. 
The ceramic board is usually cut from a ceramic sheet by grinding by a 
rotary grinding wheel. However, when a thin layer of, for example, a high 
polymer material is present in a cutting region of the ceramic board, it 
leads to undesirable clogging of the gaps between the grains of the 
grinding wheel, thereby obstructing the cutting operation, and it also 
leads to a shortened useful service life of the grinding wheel itself. The 
above trouble can be avoided by removing beforehand the thin layer of the 
high polymer material from the cutting region by dry etching. However, the 
dry etching requires many steps as described above and is not practical. 
On the other hand, the latter method disclosed in JP-A-62-145797 is 
effective in that, in the course of the step of automatic deposition of a 
cover film in the process for manufacturing a flexible circuit board, a 
region not requiring the cover tape can be accurately removed. However, 
JP-A-62-145797does not refer to any measure for dealing with the foreign 
matter produced during irradiation with the laser beam directed onto a 
region not requiring the cover tape and accumulating on the remaining 
resion. Also, it does not refer to required conditions for irradiation 
with the laser beam for improving the plating property of the exposed 
copper foil pattern. 
In the case of packaging a semiconductor integrated circuit element, 
effective radiation of heat from the package becomes quite important. For 
example, a heat radiating member is brought into contact with the surface 
of the package in which a sealing cap of, for example, a ceramic material 
is bonded to the ceramic board on which the semiconductor integrated 
circuit element is mounted, so that heat can be radiated to the exterior 
by thermal conduction. However, when an organic substance such as an 
organic flux used in the step of sealing the package by, for example, a 
solder remains and attaches to the boundary between the package and the 
heat radiating member, the heat conduction resistance at the boundary 
becomes excessively large, resulting in the problem that the effect of 
heat radiation is greatly decreased. 
Therefore, it is an object of the present invention to provide a circuit 
board in which a thin layer of a high polymer material in a plating region 
and a cutting region of a metal pattern can be easily and accurately 
removed so as to improve both the plating property; and the cutting 
property i.e. to ease in cutting a region to be cut. 
Another object of the present invention is to provide a sealing structure 
in which the heat conduction resistance at the boundary between a heat 
radiating member and a package can be greatly decreased so that the effect 
of heat radiation can be satisfactorily achieved. 
According to one aspect of the present invention, a circuit board is 
manufactured by a method comprising the steps of depositing a metal 
pattern on a surface of a board of a ceramic material, depositing a thin 
layer of a high polymer material on the surface of the ceramic board 
formed with the metal pattern, depositing a protective layer of a high 
polymer material on the surface of the thin layer of the high polymer 
material, and directing a laser beam toward and onto part of the 
protective layer and the thin layer deposited on at least one of a plating 
region of the metal pattern and a cutting region of the ceramic board so 
as to remove that part of the layer. 
According to another aspect of the present invention, an integrated circuit 
sealing structure comprises a package hermetically enclosing therein a 
semiconductor integrated circuit element having a desired function, and a 
heat radiating member brought into contact with the outer surface of the 
package so as to externally radiate heat generated from the semiconductor 
integrated circuit element during operation, no foreign matter including 
an organic substance being substantially present between the contact part 
of the package and that of the heat radiating member. Prior to assembling 
the package and the heat radiating member, a laser beam is directed toward 
and onto at least one of the contact part of the package and that of the 
heat radiating member, thereby cleaning the irradiated contact part. 
In the circuit board manufacturing method according to the present 
invention, the region where the thin layer of the high polymer material is 
to be removed, is irradiated with the laser beam for removing the thin 
layer of the high polymer material from that region. When the high polymer 
material is irradiated with the laser beam, the molecular bond between C 
(carbon) and H (hydrogen) and that between C and O (oxygen) forming the 
high polymer are broken by the phenomenon known as "photo ablation," 
thereby liberating carbon. However, this liberated carbon accumulates on 
the protective layer overlaid on the thin layer of the high polymer 
material and does not accumulate on the metal pattern. The carbon 
accumulating on the protective layer is removed, together with foreign 
matter accumulating also on the protective layer, by the later step of 
removing the protective layer. Because the later etching is achieved by 
projection of the laser beam, the mask of aluminum required hitherto in 
the case of the dry etching is unnecessary. Instead aperture having a 
desired pattern is provided on the side of the laser projection system. 
The particulars of the laser beam irradiating the region where the thin 
layer of the high polymer material is to be removed are preferably such 
that its wavelength ranges from 150 nm to 400 nm, its energy density 
ranges from 0.5 J/cm.sup.2 to 5.0 J/cm.sup.2, and its pulse width ranges 
from 100 ps to 1 .mu.s. When the particulars of the laser beam are 
selected as described above, the thin layer of the high polymer material 
can be easily and accurately removed without giving rise to undesirable 
degradation of the plating property or the cutting property attributable 
to damage or oxidization of the exposed underlying metal pattern. The 
circuit board thus obtained is highly satisfactory in both the plating 
property and the cutting property. 
Further, in the sealing structure according to the present invention, a 
flux and other organic substances attaching to the contact surfaces of 
both the package and the heat radiating member can be completely 
decomposed and removed by the irradiation with the laser beam, and these 
contact surfaces can be satisfactorily cleaned. Therefore, an undesirable 
increase in the heat conduction resistance attributable to the presence of 
the organic substances at the boundary between the package and the heat 
radiating member can be avoided, so that the heat radiating member can 
satisfactorily exhibit the desired effect of heat radiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the circuit board according to the present invention will 
now be described in detail with reference to FIGS. 1 to 4. 
Referring first to FIGS. 2 and 3, a conductive metal paste containing a 
metal having a high melting temperature such as tungsten (W) or molybdenum 
(Mo) is deposited in a desired pattern by means of, for example, printing 
on a green sheet of a ceramic material in step 101, and, then, in step 
102, the green sheet having the metal paste printed thereon is baked. 
As a result of the baking, both the green sheet and the metal paste are 
thermally set to provide a metal pattern 2 deposited on a ceramic board 1 
having a high hardness and a high mechanical strength, as shown in FIG. 3. 
Then, in step 103, a high polymer material such as a polyimide resin in 
liquid form is coated by a method such as spin-coating on the ceramic 
board 1 so as to cover the entire surface of the ceramic board 1 formed 
with the metal pattern 2. Then, in step 104, the above structure is heated 
at a required temperature to thermally set the high polymer resin thereby 
forming a thin film or layer 3 of the high polymer material as shown in 
FIG. 3. 
It is apparent that a plurality of such metal patterns and a plurality of 
such high polymer material layers may be further alternately deposited on 
the high polymer material layer 3 thus formed so as to form a multilayer 
interconnection wiring arrangement. However, such a multilayer 
interconnection wiring arrangement is not specifically illustrated for 
simplicity of the description. 
Then, in step 105, a solution of a photo resist material is coated by, for 
example, the method of spin-coating on the high polymer material layer 3 
covering the entire surface of the ceramic board 1 in the state shown in 
FIG. 3. Then, in step 106, the ceramic board 1 having the resist solution 
coated on the high polymer material layer 3 is heated at a predetermined 
temperature so as to thermally set the resist solution thereby forming a 
resist film or layer 4. FIG. 3 shows that the resist layer 4 is formed on 
the high polymer material layer 3. 
In a later step of selective removal of the high polymer material layer 3, 
this resist layer 4 acts to protect the remaining region of the high 
polymer material layer 3 from being adversely affected by foreign matter 
including carbon produced as a result of the selective removal of the high 
polymer material layer 3. This resist layer 4 acts also to protect the 
remaining region of the high polymer material layer 3 from fragments 
produced during a later step of cutting the ceramic board 1. 
Then, in step 107, the high polymer material layer 3 covering the entire 
surface of the metal pattern 2 deposited on the ceramic board 1 is 
selectively removed by a laser irradiation device 20 having an arrangement 
as shown in FIG. 1. 
Referring to FIG. 1, one form of the laser irradiation device 20 used in 
the illustrated embodiment includes a laser source (not shown) emitting a 
laser beam 21, an optical system including a reflector 22 guiding the 
laser beam 21 so that the laser beam 21 is incident in substantially 
perpendicular relation on the surface of the ceramic board 1, a mask 23 of 
a metal material passing the laser beam 21 so that the laser beam 21 
irradiating the surface of the ceramic board 1 has a desired sectional 
shape, and a positioning stage 24 on which the ceramic board 1 is placed 
and which is movable relative to the laser source so that the laser beam 
21 of the desired shape passed through the mask 23 irradiates the desired 
region of the ceramic board 1. 
By a suitable combination of the movement of the stage 24 relative to the 
laser source and on-off control of emission of the laser beam 21 from the 
laser source, the high polymer material layer 3 covering the desired 
region of the metal pattern 2 is selectively removed by the energy of the 
laser beam 21, so that soldering can be applied later for the purpose of 
mounting an electronic part (not shown) or desired plating can be applied 
later for the purpose of securing a sealing cap by soldering. 
That is, in the region of the high polymer material layer 3 irradiated with 
the laser beam 21, the bond between the molecules of the high polymer 
material is broken by, for example, the photochemical reaction, and since 
the liberated individual molecules have a large amount of energy, the part 
of the high polymer material layer 3 in the region irradiated with the 
laser beam 21 is instantaneously decomposed into carbon, etc. which are 
scattered, with the result that this part of the high polymer material 
layer 3 disappears from the laser irradiated region. 
At this time, the foreign matter including carbon scattered in the 
selective removal step attaches to the resist layer 4 covering the high 
polymer material layer 3. Thus, there is no possibility that the foreign 
matter contaminates the high polymer material layer 3 by directly adhering 
to the remaining part of the high polymer material layer 3. 
Further, because that part of the high polymer material layer 3 is removed 
by the instantaneous application of the high energy, the part of the high 
polymer material layer 3 in the region irradiated with the laser beam 21 
can be completely removed without leaving any residue of the high polymer 
material, and the surface of the underlying metal pattern 2 is not 
subjected to undesirable oxidization or damage. 
The photo ablation of a high polymer material by irradiation with a laser 
beam is described in detail in, for example, "EXCIMER LASERS: AN EMERGING 
TECHNOLOGY IN MATERIALS PROCESSING", LASER FOCUS/ELECTRO-OPTICS, page 54, 
56, 58, 60 and 63, May, 1987. 
In the illustrated embodiment, an excimer laser is used as the laser source 
emitting the laser beam 21 having a wavelength of, for example, 308 nm, so 
that the energy of the laser beam 21 is sufficiently high for breaking the 
bond between the molecules of the high polymer material layer 3. 
Optimum values of the wavelength and power (the energy density) of the 
laser beam 21 are determined depending on, for example, the material and 
thickness of the metal pattern 2, those of the ceramic board 1 and the 
shape of the metal pattern 2. According to the results of research 
conducted by the inventors, the wavelength range of from 150 nm to 400 nm 
and the energy density range of from 0.5 J/cm.sup.2 to 5.0 J/cm.sup.2 were 
found most preferable. The pulse width of the laser beam 21 preferably 
lies within the range of from 100 ps to 1 .mu.s. 
That is, when the energy of the laser beam 21 is excessively high, both the 
metal pattern 2 and the ceramic board 1 will be damaged, and the surface 
plating property will be degraded. On the other hand, when the energy of 
the laser beam 21 is excessively low, the high polymer material will leave 
a residue. 
After selectively removing the high polymer material layer 3 thereby 
exposing the desired region of the underlying metal pattern 2 in step 107, 
the resist layer 4 protecting the high polymer material layer 3 from the 
foreign matter produced as a result of irradiation with the laser beam 21 
as described above is removed in step 108, and then, in step 109, a layer 
5 of, for example, gold (Au) is plated on the exposed metal pattern 2, as 
shown in FIG. 4, for the purpose of improving the wettability for or 
affinity with a solder which is applied later. 
As described already, prior to the deposition of the plated layer 5, the 
surface of the plating region of the metal pattern 2 is neither oxidized 
nor damaged during the step of selective removal of the high polymer 
material layer 3, and no residue of the high polymer material is present 
on the surface of the plating region of the metal pattern 2. Therefore, 
the surface of the plating region of the metal pattern 2 is sufficiently 
flat and shows a satisfactory wettability for the plated layer 5, and 
there is no possibility of occurrence of a defect or the like on the 
plated layer 5 due to the presence of contaminating foreign matter and 
also due to surface irregularity. 
Thus, in a later step of soldering an electronic part (not shown) on the 
metal pattern 2, a bond having sufficient mechanical strength can be 
achieved. 
Further, because the foreign matter that may be produced during irradiation 
with the laser beam 21 and attach to the surface of the resist layer 4 is 
removed when the resist layer 4 is removed, the metal pattern 2 is not 
contaminated and is kept clean. 
FIG. 5 is a schematic perspective view showing a cutting step in a method 
for manufacturing another embodiment of the circuit board of the present 
invention, and, in FIG. 5, like reference numerals are used to designate 
like parts appearing in FIG. 1. 
In the second embodiment of the circuit board, a high polymer material 
layer 3 is similarly deposited to cover a ceramic board 1, and part of the 
high polymer material layer 3 covering a cutting region 1b of the ceramic 
board 1 is selectively removed by laser beam irradiation as in the case of 
the first embodiment. Then, the ceramic board 1 is cut at the cutting 
region 1b by the grinding action of a dicer blade 30 rotating at a high 
speed. 
Thus, the dicer blade 30 is not subjected to clogging of the gaps between 
its grains due to the presence of fragments of the high polymer material 
layer 3 left in the cutting region of the ceramic board 1. Therefore, the 
dicer blade 30 can efficiently cut the circuit board 1 without being 
obstructed in its cutting operation and without being shortened in its 
useful service life. 
FIGS. 6A, 6B and 6C are schematic sectional views showing in sequential 
order the steps of a method for manufacturing an embodiment of the 
integrated circuit sealing structure according to the present invention. 
In this embodiment, a package base 201 is formed of, for example, a ceramic 
material similar to that forming the aforementioned circuit board, and a 
plurality of high polymer material layers 203 and a plurality of metal 
patterns (not shown) are alternately laminated on the package base 201 as 
shown in FIG. 6A so as to form a multilayer interconnection wiring 
arrangement (not shown). Also, as shown in FIG. 6A, a metallized pattern 
202 in the form of a desired metal pattern is formed so as to surround the 
multilayer interconnection wiring arrangement. The wiring on the uppermost 
high polymer material layer 203 is formed with electrodes to be bonded to 
a semiconductor integrated circuit element 206. 
Then, as shown in FIG. 6B, the laser beam 21 from the aforementioned laser 
irradiation device 20 is directed so as to selectively remove the high 
polymer material layers 203 covering the metallized pattern 202 as well as 
connection electrodes (not shown) disposed inside relative to the 
metallized pattern 202 and also to selectively remove, when required, the 
high polymer material layer 203 covering the outside relative to the 
metallized pattern 202. A plurality of such package bases 201 are usually 
obtained by cutting a single ceramic board. In this case too, the high 
polymer material layers 203 covering the cutting regions between the 
individual package bases 201 ar also selectively removed by the 
irradiation with the laser beam 21, as shown in FIG. 6B. 
Then, the semiconductor integrated circuit element 206 is bonded to the 
connection electrodes (not shown) in the multilayer interconnection wiring 
arrangement (not shown) formed on the central area of the package base 
201, and, after plating a layer 205 of a metal such as gold (Au) on the 
metallized pattern 202, a sealing cap 207 of, for example, a ceramic 
material is secured to the package base 201 through a solder layer 208. 
On the sealing portion of the cap 207, too, a metallized pattern and a 
plating layer similar to those provided on the package base 201 are also 
provided. However, they are not illustrated for simplicity of the 
description. 
The semiconductor integrated circuit element 206 disposed inside the 
package is connected at its entire back surface to the inner wall of the 
cap 207 through a solder layer 209, so that heat generated during 
operation of the semiconductor integrated circuit element 206 can be 
radiated toward the cap 207. 
By securing or sealing the sealing cap 207 to the package base 201 in the 
manner described above, an integrated circuit sealing structure 200 is 
formed in which the semiconductor integrated circuit element 206 having a 
required function is hermetically enclosed as shown in FIG. 6C. 
A heat radiating member 210 is then brought into contact with the back 
surface of the cap 207 in the integrated circuit sealing structure 200, so 
that heat generated from the semiconductor integrated circuit element 206 
enclosed in the sealing structure 200 is radiated toward the exterior by 
conducting through the solder layer 209, the cap 207 and the heat 
radiating member 210. 
After the cap 207 is secured to the package base 201 in the embodiment of 
the sealing structure according to the present invention, the laser beam 
21 is directed toward and onto the contact surfaces of both the package 
base 201 and the heat radiating member 210 so as to remove foreign matter 
including any organic substance remaining on the contact surfaces thereby 
cleaning such surfaces. 
When an organic substance such as part of a flux used for securing the cap 
207 to the package base 201 by the solder layers 208 and 209 remains on 
the contact surface of the cap 207 in contact with the heat radiating 
member 210, the heat conduction resistance between the cap 207 and the 
heat radiating member 210 will increase, and the desired effect of heat 
radiation by the heat radiating member 210 will not be achieved. In the 
illustrated embodiment, the laser beam 21 is directed toward the contact 
surfaces of both the cap 207 and the package base 201 so as to completely 
remove such an organic substance existing between them, so that the 
desired effect of heat radiation through the cap 207 and the heat 
radiating member 210 can be reliably achieved. 
Consequently, the reliability of operation of the semiconductor integrated 
circuit element 206 can be improved. 
The particulars of the laser beam 21 used in the embodiment shown in FIGS. 
6A to 6C are the same as those of the laser beam 21 used in the embodiment 
shown in FIG. 1. Any suitable laser source can be used, provided that 
these particulars are satisfied. For example, any one of an excimer laser, 
a YAG laser and other suitable lasers can be used. 
While preferred embodiments of the present invention have been described in 
detail, it is apparent that the present invention is in no way limited to 
such specific embodiments, and various changes and modifications may be 
made without departing from the subject matter of the present invention. 
Some of the advantages of the present invention will now be summarized as 
follows. 
The circuit board according to the present invention is manufactured by a 
method comprising at least the steps of depositing a metal pattern on a 
board of a ceramic material, depositing a thin layer of a high polymer 
material on the ceramic board formed with the metal pattern, depositing a 
protective layer of a high polymer material on the thin layer of the high 
polymer material, and directing a laser beam toward and onto the 
protective layer and the thin layer deposited on at least one of a plating 
region of the metal pattern and a cutting region of the ceramic board to 
remove that part of the layer. The part of the thin layer of the high 
polymer material to be selectively removed is irradiated with a laser beam 
whose particulars are preferably such that the wavelength ranges from 150 
nm to 400 nm, the energy density ranges from 0.5 J/cm.sup.2 to 5.0 
J/cm.sup.2, and the pulse width ranges from 100 ps to 1 .mu.s. By so 
selecting the particulars of the irradiating laser beam, the thin layer of 
the high polymer material can be easily and accurately selectively removed 
without degradation of the plating property attributable to, for example, 
damage and oxidization of the exposed underlying metal pattern, so that 
the circuit board thus obtained is satisfactory in both the plating 
property and the cutting property. 
Also, the integrated circuit sealing structure according to the present 
invention comprises a package hermetically enclosing therein a 
semiconductor integrated circuit element having a desired function, and a 
heat radiating member brought into contact with the outer surface of the 
package so as to externally radiate heat generated from the semiconductor 
integrated circuit element during operation. Prior to assembling the 
package and the heat radiating member, a laser beam is directed toward and 
onto at least one of the contact part of the package and that of the heat 
radiating member thereby cleaning the contact part. Therefore, foreign 
matter including an organic substance such as a flux attaching to the 
boundary between the package and the heat radiating member can be 
completely removed because the foreign matter can be completely decomposed 
when irradiated with the laser beam. Therefore, an undesirable increase in 
the heat conduction resistance at the boundary between the package and the 
heat radiating member due to the presence of such an organic substance can 
be avoided, so that the heat radiating member can satisfactorily exhibit 
the desired effect of heat radiation.