# Patent

## Title
Method for treatment of deposition reactor

## Abstract

A method for treating a deposition reactor is disclosed. The method removes or mitigates formation of residue in a gas-phase reactor used to deposit doped metal films, such as aluminum-doped titanium carbide films or aluminum-doped tantalum carbide films. The method includes a step of exposing a reaction chamber to a treatment reactant that mitigates formation of species that lead to residue formation.

## Background
FIELD OF INVENTION
The disclosure generally relates to methods and systems for treating deposition reactors. More particularly, exemplary embodiments of the present disclosure relate to methods and systems for mitigating or removing buildup in gas-phase deposition reactors.
BACKGROUND OF THE DISCLOSURE
Doped metal films, e.g., doped metal carbides, nitrides, borides, and silicides, such as aluminum-doped metal carbides, may be used for a variety of applications. For example, aluminum-doped titanium carbide and similar materials may be used for gate electrodes in metal oxide field effect transistors (MOSFETs) or insulated gated field effect transistors (IGFETs), such as complementary metal oxide semiconductor (CMOS) devices, as a barrier layer or fill material for semiconductor or similar electronic devices, or as coatings in other applications.
When used as a layer of an electronic device or as a coating, the doped metal films are typically deposited using gas-phase deposition techniques, such as chemical vapor deposition techniques, including atomic layer deposition. Precursors for the gas-phase deposition often include an organometallic compound (e.g., including aluminum) and a metal halide compound (e.g., including titanium or tantalum). Unfortunately, a decomposition temperature of the organometallic compound can be much lower (e.g., more than 200° C. lower) than the temperature of formation of the desired doped metal film. As a result, precursor decomposition products or residue may form in the deposition reaction chamber during a deposition process. The residue may, in turn, create particles, which result in defects in layers deposited using the reactor. In addition, some of the decomposition products may undergo polymerization in the presence of the metal halide compound, and the polymerization products may result in additional defects in the deposited layers. A number of defects within a deposited layer generally correlates to an amount of material deposited within the reactor; the number of defects within a layer generally increases as a number of deposition runs or amount of material deposited increases.
To mitigate the number of defects in the deposited layer, the reactor may be purged with an inert gas for an extended period of time, on the order of hours, after a certain amount of material is deposited or a number of substrates have been processed. This extended purge process significantly reduces the throughput of the deposition reactor and increases the cost of operation of the reactor.
Accordingly, improved methods and systems for treating a deposition reactor to reduce or mitigate particle formation—such as particles resulting from buildup of precursor decomposition products of materials used to deposit doped metal films—are desired.
SUMMARY OF THE DISCLOSURE
Various embodiments of the present disclosure provide an improved method and system for removing or mitigating the formation of residue in a deposition reactor or otherwise transforming the residue, such that it generates fewer particles. More particularly, exemplary systems and methods mitigate formation of, transform, or remove residue resulting from the use of one or more precursors used in the deposition of doped metal films, such as metal films including carbon, boron, silicon, nitrogen, aluminum, or any combination thereof, in a gas-phase deposition reactor. While the ways in which the various drawbacks of the prior art are discussed in greater detail below, in general, the method and system use a gas-phase reactant to mitigate the formation of, transform, or remove unwanted residue within a reactor chamber. By mitigating the formation of, transforming, or removing the unwanted residue, fewer particles are formed within the reactor and thus fewer defects are formed within deposited films. In addition, substrate throughput of the reactor is increased and the cost of operating the reactor is decreased.
In accordance with various embodiments of the disclosure, a method of treating a reactor includes the steps of providing a metal halide chemistry to a reaction chamber of the deposition reactor, providing a metal CVD precursor selected from the group consisting of organometallic compound chemistry and aluminum CVD compound chemistry to the reaction chamber, forming a doped metal film, providing a treatment reactant chemistry to the reaction chamber, exposing the reaction chamber to the treatment reactant chemistry to mitigate particle formation of particles comprising decomposition products of the metal CVD precursor (e.g., by mitigating residue buildup or by transforming the residue to material that is less likely to form particles within the reactor), and purging the reaction chamber. Deposition steps of the method may be repeated to deposit a desired amount of doped metal film or process a desired number of substrates and then treat then reactor with the treatment reactant. In accordance with exemplary aspects of these embodiments, the treatment reactant source comprises a compound selected from the group consisting of compounds comprising one or more hydrogen atoms and compounds comprising a halogen (e.g., chlorine, HCl). In accordance with various aspects, the treatment reactant source comprises a compound selected from the group consisting of ammonia, hydrogen, silane, methane, silicon hydrides, boron hydrides, halosilanes, haloboranes, alkenes (e.g., ethylene), alkynes, and hydrazine and its derivatives, such as alkyl hydrazines etc. And, in accordance with yet further aspects, the treatment reactant source comprises a decomposition product of the metal CVD source. The treatment reactant may be exposed to remote or direct thermal or plasma activation to form activated species.
In accordance with further exemplary embodiments of the disclosure, a system for treating a deposition reactor includes a reactor comprising a reaction chamber, a metal halide source fluidly coupled to the reactor, a metal CVD source selected from the group consisting of one or more of organometallic compounds and aluminum CVD compounds fluidly coupled to the reactor, a treatment reactant source coupled to the reactor, and a vacuum pump coupled to the reactor. The system may include direct or remote plasma and/or thermal excitation devices to provide activated reactant species to the reaction chamber. In accordance with exemplary aspects of these embodiments, the treatment reactant source comprises a compound selected from the group consisting of compounds comprising one or more hydrogen atoms and compounds comprising a halogen (e.g., chlorine, HCl). In accordance with various aspects, the treatment reactant source comprises a compound selected from the group consisting of ammonia, hydrogen, silane, methane, silicon hydrides, boron hydrides, halosilanes, haloboranes, alkenes (e.g., ethylene), alkynes, and hydrazine and its derivatives, such as alkyl hydrazines etc. And, in accordance with yet further aspects, the treatment reactant source comprises a decomposition product of the metal CVD source.
In accordance with yet additional embodiments of the invention, a method of treating a deposition reactor includes the steps of providing a metal halide chemistry to a reaction chamber for a period of time, after the step of providing a metal halide chemistry to a reaction chamber for a period of time, providing a treatment reactant chemistry to the reaction chamber for a period of time, and during or after providing a treatment reactant chemistry to the reaction chamber for a period of time, providing a metal CVD precursor chemistry to the reaction chamber to form a layer of doped metal. In this case, particle formation is mitigated (e.g., via mitigation of residue formation or via densification of the residue) during the deposition step and any residue that forms may be removed during and optionally after the deposition process. The treatment reactant may be introduced with the metal CVD precursor chemistry or before the introduction of the metal CVD precursor. In accordance with exemplary aspects of these embodiments, a treatment reactant chemistry comprises one or more of hydrogen compounds including one or more hydrogen atoms (e.g., hydrogen, HCl, silane, methane, ethylene, and the like) and compounds including a halogen (e.g., chlorine, HCl). The treatment reactant may be exposed to remote or direct thermal or plasma activation to form activated species. In accordance with additional aspects of these embodiments, the step of providing a treatment reactant chemistry to the reaction chamber includes providing a source of a decomposition product of the organometallic compounds or the aluminum CVD compounds.
Both the foregoing summary and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure or the claimed invention.



BRIEF DESCRIPTION OF THE DRAWING FIGURES
A more complete understanding of the embodiments of the present disclosure may be derived by referring to the detailed description and claims when considered in connection with the following illustrative figures.
FIG. 1 illustrates a system in accordance with various exemplary embodiments of the disclosure.
FIG. 2 illustrates a method in accordance with exemplary embodiments of the disclosure.
FIG. 3 illustrates another method in accordance with exemplary embodiments of the disclosure.
FIG. 4 illustrates a number of defects on a substrate based on a number of substrates processed with no treatment.
FIG. 5 illustrates a number of defects on a substrate based on a number of substrates processed using the treatment described herein.



It will be appreciated that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of illustrated embodiments of the present disclosure.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
The description of exemplary embodiments of methods and systems provided below is merely exemplary and is intended for purposes of illustration only; the following description is not intended to limit the scope of the disclosure or the claims. Moreover, recitation of multiple embodiments having stated features is not intended to exclude other embodiments having additional features or other embodiments incorporating different combinations of the stated features.
The method and system described herein can be used to mitigate formation of, remove, and/or transform residue in a reactor used to deposit doped metal films (e.g., films including carbon, boron, silicon, and/or nitrogen) that otherwise buildups and/or generates particles during a deposition process. Use of the methods and systems described herein results in a reduction of particle formation from residue and therefore results in higher throughput and in a lower cost of operation of deposition reactors, compared to reactors that are merely purged after similar deposition processes.
Turning now to FIG. 1, a system 100, for mitigating deposition residue buildup, as described herein, is illustrated. System 100 includes a reactor 102, including a reaction chamber 104, a substrate holder 106, and a gas distribution system 108; a metal halide source 110; a metal chemical vapor deposition (CVD) source 112; a treatment reactant source 114; lines 116, 118, 120 connecting sources 110-114 to reactor 102; valves 122, 124 and 126 interposed between the sources 110-114 and reactor 102; a vacuum pump 128, and optionally a carrier and/or purge gas source 130 coupled to reactor 102 via line 132 and valve 134.
Reactor 102 may be a standalone reactor or part of a cluster tool. Further, reactor 102 may be dedicated to doped metal deposition and treatment processes as described herein, or reactor 102 may be used for other processes—e.g., for other layer deposition and/or etch processing. For example, reactor 102 may include a reactor typically used for physical vapor deposition (PVD), chemical vapor deposition (CVD), and/or atomic layer deposition (ALD) processing, and may include remote or direct thermal excitation, direct plasma, and/or remote plasma apparatus. Using thermal or plasma activation apparatus during a deposition or treatment process creates excited molecules or species from one or more of sources 110-114 to enhance the reactivity of the reactants from sources 110-114. By way of one example, reactor 102 includes a reactor suitable for ALD deposition. An exemplary ALD reactor suitable for system 100 is described in U.S. Pat. No. 8,152,922, the contents of which are hereby incorporated herein by reference, to the extent such contents do not conflict with the present disclosure.
Substrate holder 106 is designed to hold a substrate or workpiece 136 in place during processing. In accordance with various exemplary embodiments, holder 106 may form part of a direct plasma circuit. Additionally or alternatively, holder 106 may be heated, cooled, or be at ambient process temperature during processing.
Although gas distribution system 108 is illustrated in block form, gas distribution system 108 may be relatively complex and be designed to mix vapor (gas) from sources 110 and/or 112 and carrier/purge gas from one or more sources, such as gas source 130, prior to distributing the gas mixture to remainder of reactor 102. Further, system 108 may be configured to provide vertical (as illustrated) or horizontal flow of gasses to the chamber 104. An exemplary gas distribution system is described in U.S. Pat. No. 8,152,922. By way of example, distribution system 108 may include a showerhead.
Metal halide source 110 includes one or more gases, or materials that become gaseous, that include a metal and a halide. Exemplary metals include titanium, tantalum, and niobium. Exemplary halides include chlorine and bromine. Source 110 may include, for example, titanium chloride (e.g., TiCl4), tantalum chloride (e.g., TaCl5), and niobium chloride (e.g.,NbCl5). Gas from source 110 may be exposed to a thermal and/or remote plasma and/or direct plasma source to form activated or excited species, such as ions and/or radicals including one or more of chlorine, titanium, tantalum, and niobium. The term “activated species” includes the precursor and any ions and/or radicals than may form during exposure of the precursor to any thermal and/or plasma process. Further, the term “chemistry,” when used in connection with a compound, includes the compound and any activated specie(s), whether or not the compound (e.g., a reactant) has been exposed to thermal or plasma activation.
Metal CVD source 112 includes one or more gases, or materials that become gaseous, that react with or form reactive species that react with compounds or species from metal halide source 110 to form a deposited layer of metal-doped film, such as a layer of aluminum-doped titanium carbide or aluminum-doped tantalum carbide, other carbines, nitride, silicides, or borides. Metal CVD source 112 may include, for example, organometallic compounds and/or aluminum CVD compounds, such as alane compounds. Exemplary suitable organometallic compounds include trimethylaluminum (TMA), triethylaluminum (TEA), triisobutylaluminum (TIBA), diethylaluminum chloride (DEACL), diethylaluminum hydride (DMAH), and tritertiarybutylaluminum (TTBA). Exemplary aluminum CVD alane compounds include trimethylamine alane (TMAA), triethylamine alane (TEAA), dimethyl ethylamine alane (DMEAA), trimethylaminealane borane (TMAAB), and methylpyrrolidine alane (MPA).
Use of organometallic compounds and alane compounds may be advantageous, because such compounds allow for atomic layer deposition, which allows, precise, conformal, self-limiting deposition of layers of desired material. However, the organic precursors are susceptible to decomposition at or below film deposition temperatures. Indeed, some of the precursors decompose at temperatures 200° C. (or more) less than the temperature of formation of the film. As a result, the compounds may decompose into undesired products prior to reaching substrate 136, resulting in residue formation within chamber 104—for example at or near gas distribution system 108, such as a showerhead. As noted above, the residue formation may, in turn lead to particle formation, which causes defects in the deposited metal films.
For example, many of the organometallic compounds may undergo a beta-hydride elimination reaction, in which an alkyl group bonded to a metal center is converted into a corresponding meta(hydride and an alkene compound. The formation of the alkene compound, particularly at or near gas distribution system 108, can result in residue buildup, which includes organic and inorganic materials. In addition, the decomposition products can polymerize, e.g., in the presence of species from metal halide source 110, Which may result in additional or alternative residue formation.
Gas from source 112 may be exposed to a thermal and/or a direct plasma source and/or a remote plasma source to form activated species, such as ions and/or radicals.
Treatment reactant source 114 includes one or more gases, or materials that become gaseous, that include a compound or species that mitigates formation of residue within a reactor and/or that transforms the residue in a manner that generates less particles—e.g., by densifying the residue. Exemplary compounds and species can react with a halogen on a halogen (e.g., Cl)-terminated molecule (e.g., on a deposited film) to mitigate formation of undesired decomposition products. Treatment reactant source 114 may include, for example, a compound selected from the group consisting of compounds comprising one or more hydrogen atoms and compounds comprising a halogen (e.g., chlorine, HCl). In accordance with various aspects, the treatment reactant source comprises a compound selected from the group consisting of ammonia, hydrogen, silane, methane, silicon hydrides, boron hydrides, halosilanes, haloboranes, alkenes (e.g., ethylene), alkynes, and hydrazine and its derivatives, such as alkyl hydrazines etc. And, in accordance with yet further aspects, the treatment reactant source comprises a decomposition product of the metal CVD source, e.g., a beta hydride elimination product of the metal CVD source.
Gas from source 114 may be exposed to a thermal and/or a remote plasma and/or a direct plasma source to form activated or excited species, such as ions and/or radicals including one or more of hydrogen and/or chlorine and/or other activated species.
Carrier or inert source 130 includes one or more gases, or materials that become gaseous, that are relatively unreactive in reactor 102. Exemplary carrier and inert gasses include nitrogen, argon, helium, and any combinations thereof.
FIG. 2 illustrates a method 200 of treating a reactor in accordance with exemplary embodiments of the disclosure. Method 200 includes the steps of: providing a metal halide chemistry (step 202), providing a metal CVD precursor chemistry (step 204), forming a doped metal film (step 206), providing a treatment reactant chemistry (step 208), exposing the reaction chamber to the treatment reactant chemistry (step 210), optionally purging the reactor (step 212), and, if the desired amount of material has not been deposited (step 214), repeating steps 202-212, and if the desired amount of material has been deposited (step 214), the process is complete (step 216). Although not separately illustrated, a substrate or workpiece may be removed from the reaction chamber before treatment step 210, such that the film on the workpiece is not exposed to the treatment reactant chemistry. Alternatively, the substrate may be exposed to the treatment reactant chemistry.
Step 202 includes providing metal halide chemistry to a reaction chamber and step 204 includes providing a metal CVD precursor chemistry to a reaction chamber. Steps 202 and 204 may be performed in any order or be performed simultaneously. Further, although illustrated with only two reactant sources, exemplary methods may include the use of more than two reactants.
The metal halide chemistry may include any of the compounds described above in connection with metal halide source 110. During step 202, the metal halide source may be exposed to a thermal activation process and/or a remote and/or direct plasma source to create metal halide source chemistry that includes activate species. Similarly, the metal CVD precursor may include any compound noted above in connection with metal CVD source 112. And, during step 204, the metal CVD precursor may be exposed to a thermal activation process and/or a remote and/or direct plasma source to create metal CVD source chemistry including activated species.
During step 206, a metal film is formed. The metal film may include, for example, aluminum, silicon, and/or boron doped titanium carbide, aluminum, silicon, and/or boron doped tantalum carbide, and/or aluminum, silicon, and/or boron doped niobium carbide, or other metal films including one or more of C, Si, B, or N.
During step 208, a treatment reactant chemistry to mitigate formation of residue and/or to densify the residue and/or to transform the residue to form fewer particles within a reaction chamber is introduced into the reaction chamber. The reactant chemistry may include any of the compounds noted above in connection with treatment reactant source 114, and the reactant from a source may be exposed to thermal and/or plasma activation as described herein to form treatment reactant chemistry including activated species.
By way of examples, the treatment reactant chemistry may include hydrogen gas, and the hydrogen gas may be introduced to a reaction chamber (e.g., chamber 104) via a gas distribution system (e.g., system 108). Additionally or alternatively, hydrogen gas may be exposed to a remote plasma to form treatment reactant chemistry including activated species, such as hydrogen radicals. In accordance with exemplary aspects, the remote plasma is configured, such that the activated species can reach and react with material on the surface of the gas distribution system (e.g., a showerhead), as well as within holes of the system near the surface. In addition or as an alternative, step 208 may include providing a halogen, such as chlorine, or a halogen activate species, such as chlorine radicals, to the reaction chamber to mitigate formation of residue within the reaction chamber.
In accordance with other embodiments, the treatment reactant chemistry includes ammonia, which may or may not by subjected to thermal and/or direct and/or remote plasma activation as described herein. The ammonia is thought to react with a halogen (e.g., chlorine)-terminated surface of deposited material and mitigate formation of decomposition products within a reaction chamber.
Exemplary conditions for an ammonium residue reactant process include depositing about 1250 Å carbide, followed by a 10 min exposure of NH3, followed by a 20 min purge (remove residue NH3), followed by depositing about 1250 Å carbide, followed by about 10 min exposure of NH3, followed by about 20 min purge (remove residue NH3). The 1250 Å carbide may be deposited onto, for example, 25 wafers at 50 Å each (one lot of wafers).
It is thought that this process transforms the residue in the reactor to provide better adhesion, lowering stress or even making it less susceptible to oxidation to prevent this residual film from breaking off the reactor surface and landing on the wafer-thus reducing on wafer defect levels.
Although illustrated as including a decision or determination step 214, method 200 may be configured to automatically run a predetermined number of cycles of steps 202-212. For example, method 200 may be configured to run 1, 2, 3, 4, 5, or 50 number of cycles of steps 202-212 and complete (step 216) upon the conclusion of step 208 of the last cycle. Alternatively, steps 202-212 may be repeated based until a predetermined amount of doped metal film is deposited. For example, the steps 202-212 may be run until an accumulated film thickness of about 20 Å to about 1250 Å or about 5 Å to about 5000 Å is reached.
FIGS. 4 and 5 illustrate a number of defects counted on a particle meter on a surface of a substrate when a reactor is not treated (FIG. 4) and when the reactor is treated (FIG. 5) in accordance with method 200, with ammonia as the treatment reactant chemistry, under the conditions noted above.
FIG. 3 illustrates another method 300 in accordance with additional exemplary embodiments of the disclosure. Method 300 includes the steps of providing a metal halide chemistry (step 302), providing a metal CVD precursor chemistry (step 304), providing treatment reactant chemistry (step 306), forming a doped metal film (step 308), determining whether a desired amount of material has been deposited (step 310) and completing the process (step 312) if a desired amount of material has been deposited and repeating steps 302-310 if a desired amount of material has not been deposited. Although not illustrated, method 300 may include a purge step (similar to step 212) prior to step 312.
Steps 302 and 304 may be the same as steps 202 and 204, except, in accordance with exemplary aspects of these embodiments, step 302 is performed prior to step 304. And, in accordance with further aspects, step 306 is performed after step 302 and prior to or simultaneous with step 304. By way of example, a metal halide chemistry from a metal halide source may be introduced to a reaction chamber for a period of time (e.g., a pulse of about 800 ms) during step 302. Then, during step 306, a treatment reactant, such as hydrogen, activated hydrogen, silane, activated silane, ethylene, and/or activated ethylene is introduced to the reaction chamber for a period of time. After or during step 306, the metal CVD reactant chemistry is introduced to the reaction chamber—e.g., for about 3.5 sec. exposure—to form a doped metal film. Using a pulse of the treatment reactant chemistry during step 306, prior to or during step 304, is thought to reduce a number of halide-terminated species on a surface of deposited material and therefore reduce or eliminate residue formation.
Although exemplary embodiments of the present disclosure are set forth herein, it should be appreciated that the disclosure is not so limited. For example, although the system and method are described in connection with various specific chemistries, the disclosure is not necessarily limited to these chemistries. Various modifications, variations, and enhancements of the system and method set forth herein may be made without departing from the spirit and scope of the present disclosure.

## Claims

1. A method of treating a deposition reactor, the method comprising the steps of:
providing a metal halide chemistry, selected from the group consisting of titanium halides, tantalum halides, and niobium halides, to a reaction chamber of the deposition reactor;
providing a metal CVD precursor selected from the group consisting of organometallic compound chemistry and aluminum CVD compound chemistry to the reaction chamber;
forming a deposited doped metal film comprising one or more of B, C, Si N overlying a substrate;
optionally removing the substrate;
providing a treatment reactant chemistry to the reaction chamber;
exposing the reaction chamber to the treatment reactant chemistry to mitigate formation of particles comprising decomposition products of the metal CVD precursor; and
purging the reaction chamber.

2. The method of claim 1, wherein the step of providing a treatment reactant chemistry to the reaction chamber includes providing a reactant selected from one or more of the group consisting of hydrogen compounds including one or more hydrogen atoms and compounds including a halogen.

3. The method of claim 1, wherein the step of providing a treatment reactant chemistry comprises providing a compound selected from the group comprising ammonia, hydrogen, silane, methane, silicon hydrides, boron hydrides, halosilanes, haloboranes, alkenes, alkynes, and hydrazine and its derivatives.

4. The method of claim 1, wherein the step of providing a treatment reactant chemistry comprises providing ammonia to the reaction chamber.

5. The method of claim 1, wherein the step of providing a treatment reactant chemistry comprises providing a source of a decomposition product of a compound selected from the group consisting of organometallic compounds and aluminum CVD compounds.

6. The method of claim 1, further comprising the step of exposing a treatment reactant to a plasma.

7. The method of claim 1, further comprising the step of exposing a treatment reactant to thermal excitation.

8. A method of treating a deposition reactor, the method comprising the steps of:
providing a metal halide chemistry, selected from the group consisting of titanium halides, tantalum halides, and niobium halides, to a reaction chamber for a period of time;
after the step of providing a metal halide chemistry to a reaction chamber for a period of time, providing a treatment reactant chemistry to the reaction chamber for a period of time; and
during or after providing a treatment reactant chemistry to the reaction chamber for a period of time, providing a metal CVD precursor chemistry to the reaction chamber to form a layer of doped metal,
wherein use of the treatment reactant chemistry mitigates particle formation in the reaction chamber.

9. The method of claim 8, wherein the step of providing a treatment reactant chemistry to the reaction chamber includes providing a reactant chemistry from a source selected from one or more of the group consisting of hydrogen compounds including one or more hydrogen atoms and compounds including a halogen.

10. The method of claim 8, wherein the step of providing a treatment reactant chemistry comprises providing a compound from a source selected from the group comprising ammonia, hydrogen, silane, methane, silicon hydrides, boron hydrides, halosilanes, haloboranes, alkenes, alkynes, and hydrazine and its derivatives.

11. The method of claim 8, wherein the step of providing a treatment reactant chemistry comprises providing activated hydrogen to the reaction chamber.

12. The method of claim 8, wherein the step of providing a treatment reactant chemistry comprises providing a source of a decomposition product of a compound selected from the group consisting of organometallic compounds and aluminum CVD compounds.

13. The method of claim 8, further comprising the step of exposing a treatment reactant to a plasma.

14. The method of claim 8, further comprising the step of exposing a treatment reactant to thermal excitation.