Electric resistance heating devices – Heating devices – Radiant heater
Reexamination Certificate
2000-02-17
2001-01-09
Walberg, Teresa (Department: 3742)
Electric resistance heating devices
Heating devices
Radiant heater
C219S390000, C118S724000, C359S273000
Reexamination Certificate
active
06173116
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a furnace for Rapid Thermal Processing of wafers, comprising a housing in which there are provided:
means for supporting the wafer,
heat control means capable of reflecting heat radiation to the wafer,
means for generating heat radiation situated between the wafer and the heat control means.
The invention also relates to a method for Rapid Thermal Processing of wafers, especially semiconductor wafers.
In the manufacture of integrated circuits from semiconductor wafers (such as silicon), Rapid Thermal Processing (RTP) is gaining ground. The main advantages of RTP are the reduced temperature load of the wafers, and the reduction of the process cycle time. In an RTP furnace wafers are heated by infrared lamps. The inner walls of the furnace are provided with highly heat (infrared) reflective coatings in order to obtain a maximum heat rate and an optimum temperature uniformity during the steady state cycle. In an ideal process a heating rate of 300° C./s to 1100° C., an annealing time of 5 s at 1100° C., and a cooling rate of 100° C./s are required. A disadvantage is that during the cooling stage the heat content of the wafer cannot be dissipated quickly. A more rapid cooling rate is prevented by the highly reflective walls of the furnace. A large part of the heat emitted by the wafer will be reflected back towards the wafer. Consequently, the throughput of the wafers is reduced. By making the walls absorb heat during the cooling stage and reflect heat during the heating stage, the cooling rate will be enhanced too and hence the process cycle time reduced.
In German patent application DE-A-4141466, a description is given of a process room for RTP which has been provided with an array of rotatable lamellae between the lower and upper walls and the heat sources. These lamellae are mechanically rotatable around their longitudinal axes and function as heat control means. Each lamella has been provided on one side with a reflective gold coating, whereas the other side has been provided with a black surface. During heating, the reflective sides of the lamellae are exposed to the wafer. For rapid cooling of the wafer, the rear sides of the lamellae, which are black, are turned towards the wafer in order to absorb the heat radiation emitted by the wafer during the cooling stage. In this way, rapid cooling of the wafer could be obtained. A disadvantage is that in the known furnace the reflectors are mechanically driven, which, in most cases, is rather complicated and increases the risk of particle formation. Moreover, the heat dissipation is limited, because the lamellae have a limited heat capacity. Also the control of an individual reflector, to increase the uniformity of the temperature profile, is cumbrous.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a furnace for RTP of wafers in which the alternate reflection and absorption of heat radiation is obtained in an alternative way. Moreover, in the furnace of the present invention the above mentioned disadvantages are prevented.
According to the invention, these objects are achieved by a furnace as described in the opening paragraph, which is characterized in that the heat control means comprise an optical switching device having a switching film containing a trivalent metal capable of forming hydrides, which film can be reversibly switched by an exchange of hydrogen from a first state, which is a heat-reflecting state, to a second state, which is a black heat-absorbing state or a transparent state.
The switching film as used in the present invention is based on the phenomenon that some trivalent metals, such as the transition metals Y, Sc and La, and rare earth metals, such as Gd and Sm, can be reversibly converted from a low hydrogen content (approx. dihydride) composition into a hydrogen-rich (approx. trihydride) composition by an exchange of hydrogen, which phenomenon has been described in the non-prepublished international patent application WO 96/38758, filed by Applicants. Both compositions have different optical properties. At a low hydrogen content, the switching film prepared from such a trivalent metal has a metallic character: it is non-transparent, and reflective or mirror-like, at least in the infrared region of the spectrum. At a high hydrogen content, the switching film is transparent. During heating-up and annealing of the wafer, the switching film is switched to the reflective state, whereas during cooling the switching film is switched to the transparent state. In this embodiment, the walls of the furnace have been made heat absorbing, for example, by providing the walls with a black coating, so that in the transparent state of the switching film, the heat of the wafer can be easily dissipated away into the (water-)cooled walls. In this way, the reflectivity of the heat control means is regulated in a non-mechanical way, thereby preventing the risk of particle formation.
Switching of the switching film takes place with hydrogen. The reflection of the switching film is governed by the hydrogen content: the reflection decreases as the hydrogen content increases. If molecular hydrogen gas is supplied to the switching film, the reflection decreases as the hydrogen pressure increases. The hydrogen must be dissociated to atomic H. The rate of dissociation can be increased by providing the surface of the switching film with a thin cap layer of palladium having a thickness, for example, of 5 nm. At said thickness, the palladium layer is discontinuous. The layer thickness is not critical and is chosen to be in the range between 2 and 25 nm. Thin layers of 2 to 10 nm are preferred, however, because the thickness of the palladium layer determines the maximum transmission of the switching device. In addition, the palladium cap layer protects the underlying switching film against oxidation.
Apart from palladium, other catalytically active metals which promote hydrogen dissociation, such as platinum, nickel and cobalt, or alloys with these metals, can be used as the cap layer.
The molecular hydrogen can be passed from a gas cylinder filled with H
2
into the furnace in a simple manner. A low-hydrogen switching film in the reflective state will change into a transparent high-hydrogen state. This conversion is reversible: the transparent film is converted to the reflective state by heating and/or evacuation of hydrogen.
The switching film in accordance with the invention is thin, i.e. its film thickness is less than 2 &mgr;m. The film thickness of the switching film preferably ranges between 100 and 1,000 nm. As hydrogen must diffuse in the switching film, the film thickness determines the rate of full conversion from the reflective to the transparent state, and conversely.
Preferably, in addition to the trivalent metal, the switching film also comprises magnesium. The addition of Mg to the trivalent metal increases the transmission of the switching film in the transparent state and the reflection in the non-transparent state. The addition of more than 60 at. % Mg to a switching film containing, e.g. Gd results in three stable states which are dependent on the hydrogen content of the switching film, i.e. as the hydrogen content increases: a reflective state, a black absorbing state, and a transparent state. Up to 95 at. % Mg may be added to the switching film. In this connection, atomic percentages are expressed as a percentage of the total metal content, i.e. excluding the hydrogen content. The presence of a trivalent metal, even in a small amount, is essential. Apart from Gd, other trivalent transition and rare earth metals, and alloys of these metals, exhibit similar phenomena. Examples of these metals are e.g. lutetium (Lu), yttrium (Y) and lanthanum (La). In this embodiment, switching of the switching film is performed between the reflective state, during the heating stage of the wafer, and the black absorbing state, during rapid cooling of the wafer. In this embodiment, there is no need to provide the furnace with black absorbing walls. Preferab
Duine Peter A.
Roozeboom Freddy
Van Der Sluis Paul
Biren Steven R.
Fuqua Shawntina
U.S. Philips Corporation
Walberg Teresa
LandOfFree
Furnace for rapid thermal processing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Furnace for rapid thermal processing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Furnace for rapid thermal processing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2467673