Patent Application
20130330911
1. Field of the Invention
Technology described herein relates to the manufacture of semiconductor devices. More specifically, methods are described for forming epitaxial group IV semiconductor materials.
2. Description of the Related Art
Germanium was one of the first materials used for semiconductor applications such as CMOS transistors. Due to vast abundance of silicon compared to germanium, however, silicon has been the overwhelming semiconductor material of choice for CMOS manufacture. As device geometries decline according to Moore's Law, the size of transistor components poses challenges to engineers working to make devices that are smaller, faster, use less power, and generate less heat. For example, as the size of a transistor declines, the channel region of the transistor becomes smaller, and the electronic properties of the channel become less viable, with more resistivity and higher threshold voltages. Carrier mobility is increased in the silicon channel area by using silicon-germanium stressors embedded in the source/drain areas, as some manufacturers have done for the 45 nm node. For future nodes, however, still higher mobility devices are needed.
One attempted method of forming higher mobility devices includes forming silicon-germanium-tin alloy, germanium-tin alloy, or germanium epitaxial layers. To improve the quality of the deposited epitaxial layers, a cyclical deposition/treatment process, such as deposition/etch or deposition/anneal may be performed. In the example of deposition/etch, after the deposition of a certain amount of epitaxial material, a brief etch back is performed to remove deposited material from masked areas to facilitate deposition selectivity. In another cyclical process, after deposition, the flow of deposition gases may be halted for a period of time, for example, to perform an anneal, which may improve the crystallinity of the epitaxial layer and/or activate dopants. However, during the non-deposition treatment of the epitaxial layer, the composition of silicon, germanium, and tin can change due to migration. Additionally, other dopants within the epitaxial layer, such as group III or group V elements, may also migrate or outgas, thus degrading film quality. Moreover, in the beginning of deposition in each cycle, the incorporation of a group IV element, for example tin, can lag the incorporation of other group IV elements such as silicon and/or germanium and/or even group III and group IV dopants. These are the potential sources of film degradation and reduced film composition uniformity.
Therefore, there is a need in the art for a method for forming epitaxial layers having uniform composition profiles.
Embodiments of the invention generally relate to methods for forming silicon-germanium-tin alloy epitaxial layers, germanium-tin alloy epitaxial layers, and germanium epitaxial layers that may be doped with boron, phosphorus, arsenic, or other n-type or p-type dopants. The methods generally include positioning a substrate in a processing chamber. A germanium precursor gas, and optionally a silicon precursor gas and a group III or group V gas, is then introduced into the chamber concurrently with an alloying precursor gas, such as a tin precursor gas, to form an epitaxial layer. The flow of the germanium gas is then halted, and an etchant gas is introduced into the chamber. An etch back is then performed while in the presence of the alloying precursor gas used in the formation of the epitaxial film. The flow of the etchant gas is then stopped, and the cycle may then be repeated. In addition to or as an alternative to the etch back process, an annealing processing may be performed in the presence of the tin precursor. When a group III or group V gas is utilized, the group III or group V gas may also be provided to the processing chamber during the etching and/or annealing.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
Embodiments of the invention generally relate to methods for forming silicon-germanium-tin alloy epitaxial layers, germanium-tin alloy epitaxial layers, and germanium epitaxial layers that may be doped with boron, phosphorus, arsenic, or other n-type or p-type dopants. The methods generally include positioning a substrate in a processing chamber. A germanium precursor gas, and optionally a silicon precursor gas and a group III or group V gas, is then introduced into the chamber concurrently with an alloying precursor gas, such as a tin precursor gas, to form an epitaxial layer. The flow of the germanium gas is then halted, and an etchant gas is introduced into the chamber. An etch back is then performed while in the presence of the alloying precursor gas used in the formation of the epitaxial film. The flow of the etchant gas is then stopped, and the cycle may then be repeated. In addition to or as an alternative to the etch back process, an annealing processing may be performed in the presence of the tin precursor. When a group III or group V gas is utilized, the group III or group V gas may also be provided to the processing chamber during the etching and/or annealing.
In some embodiments of the invention, tin may be alloyed with germanium and/or silicon to form a silicon-germanium-tin alloy epitaxial layer or a germanium-tin alloy epitaxial layer. The alloying of tin with silicon and/or germanium increases the compressive stress/strain of the alloyed film, particularly when deposited on a germanium buffer layer. Additionally, the alloying of tin with germanium and/or silicon reduces the bandgap of the silicon or germanium and brings the “gamma valley” in the conduction band closer than the “L valley” to the valence band top. The carriers in the gamma valley have higher mobility than the carrier in the L valley due to the structure of the bandgap. Tin alloying above a certain point, for example about seven percent in germanium, facilitates high carrier mobility due to changes in the bandgap of the germanium which allow carriers of higher mobility to dominate in the conduction of electricity.
Embodiments of the invention may be practiced in a Centura® RP EPi chamber available from Applied Materials, Inc. in Santa Clara, Calif. It is contemplated, however, that other equipment, including equipment available from other manufacturers, may be utilized to practice embodiments of the invention.
In operation 214, the substrate is elevated to a desired processing temperature such as about 150° C. to about 500° C., for example between about 200° C. and about 400° C. In operation 216, a germanium-tin alloy epitaxial layer is formed on the substrate, for example, in a thermal chemical vapor deposition (CVD) process. The germanium-tin alloy epitaxial layer is formed on the substrate by introducing a germanium precursor gas and a tin precursor gas into the chamber. A carrier gas may optionally be introduced into the chamber as well. It is contemplated that the germanium precursor gas and the tin precursor gas may be thermally or chemically decomposed onto the substrate to form the germanium-tin alloy epitaxial layer.
Suitable germainum precursors include germanium hydrides such as germane (GeH4), digermane (Ge2H6), or higher hydrides (GexH2x+2), or a combination thereof. The germainum precursor may be mixed with a carrier gas, which may be a non-reactive gas such as nitrogen gas, hydrogen gas, or a noble gas such as helium or argon, or a combination thereof. The ratio of germanium precursor volumetric flow rate to carrier gas volumetric flow rate may be used to control gas flow velocity through the chamber. The ratio may be any proportion from about 1% to about 99%, depending on the flow velocity desired. In some embodiments, a relatively high velocity may improve uniformity of the deposited layer. Pressure in the processing chamber is maintained between about 5 Torr and about 200 Torr, such as between about 20 Torr and about 80 Torr, for example about 40 Torr.
A tin precursor gas is introduced into the chamber concurrently with the germanium precursor gas to deposit a germanium-tin alloy epitaxial layer on the surface of the substrate. The tin precursor gas may include a tin halide gas. For example the dopant gas may be SnCl4, SnCl2, or an organometallic chloride having the formula RxMCly, where R is methyl or t-butyl, x is 1 or 2, M is Sn, and y is 2 or 3. The tin precursor gas is provided to the processing chamber at a flow rate between about 0.1 sccm and about 300 sccm, such as between about 50 sccm and about 100 sccm, for example about 5 sccm. The tin precursor gas may also be mixed with a carrier gas to achieve a desired space velocity and/or mixing performance in the processing chamber. The tin precursor gas may be sourced from a solid source of crystals sublimed into a flowing carrier gas stream such as N2, H2, Ar, or He, or the tin precursor gas may be generated by passing a halogen gas, optionally with one of the above carrier gases, over a solid metal in a contacting chamber to perform the reaction M+2Cl2→MCl4, where M is Sn. The contacting chamber may be adjacent to the processing chamber, coupled thereto by a conduit which is preferably short to reduce the possibility of metal halide particles depositing in the conduit.
The germanium-tin alloy epitaxial layer may be deposited to a thickness between about 100 Å and about 800 Å. In one example, the concentration of tin atoms in a germanium matrix may be between about one percent and about 12 percent, such as between about seven percent and about nine percent.
The tin precursor gas and the germanium precursor gas are usually provided to the processing chamber through different pathways. The germanium precursor gas is provided through a first pathway, and the tin precursor gas is provided through a second pathway. The two pathways are generally different and kept separate up to the point of entry into the processing chamber. In one embodiment, both streams enter through a side wall of the chamber proximate an edge of the substrate support, travel across the substrate support from one side to an opposite side thereof and into an exhaust system. The substrate support may rotate during formation of the germanium-tin alloy epitaxial film to improve uniformity. The first pathway generally communicates with a first entry point into the processing chamber, which may comprise one or more openings in a wall of the chamber or a gas distributor, such as a showerhead, coupled to a wall of the chamber. The one or more openings may be proximate an edge of the substrate support or may be portals in a dual or multi path gas distributor. The second pathway likewise communicates with a second entry point similar to the first entry point. The first and second entry points are disposed such that the two streams mix and provide a deposition or layer growth mixture in a region above the substrate support. Use of a gas distributor may reduce or eliminate the need to rotate the substrate during processing in some embodiments.
In operation 218, the flow of the germanium precursor gas is halted. Subsequently, in operation 220, an etchant is introduced into the processing chamber. The etchant gas may be, for example, Cl2 or HCl. In operation 222 an etch back of the deposited material is performed in the presence of the tin precursor gas. It is contemplated that the flow of the tin precursor gas may be continuous throughout the deposition and etch, or the flow of the tin precursor gas may be stopped after the deposition process and then resumed for the etch back process.
During the etch back process, the tin precursor gas continues to be introduced into the processing chamber, for example, at substantially the same flow rate as during the deposition process described with respect to operation 216. The presence of the tin precursor gas within the chamber during the etch back process reduces the migration of the tin within the germanium-tin alloy epitaxial film, resulting in a film of uniform tin composition. It is believed that the reduced migration of the tin can at least be partly contributed to the partial pressure of tin in the process chamber atmosphere. Because the migration of the tin is reduced, each deposition/etch of the cyclical process can be repeatedly performed to form a germanium-tin alloy epitaxial layer of uniform tin composition. In operation 224, the flow of etching gas is halted. The deposition/etch process may then be repeated.
In yet another embodiment, it is contemplated that a germanium epitaxial layer which includes a group III or group V dopant may be deposited without the incorporation of tin. In such an embodiment, the presence of the dopant in the chamber atmosphere during processing reduces out gassing and migration of the group III or group V dopant. In another embodiment, it is contemplated that the etch process may be replaced with an annealing process which may occur in an annealing gas atmosphere. For example, a boron-doped germanium epitaxial film may be formed, and then the film may be annealed to activate the dopant. In such an embodiment, the dopant gas is provided to the processing chamber during both the deposition process and the annealing process. Because the dopant gas is provided to the chamber during the annealing process, out gassing and migration of the dopant within the germanium epitaxial layer is reduced.
In yet another embodiment, it is contemplated that the tin precursor gas or dopant gas used during the formation of the epitaxial layer and the tin precursor gas or dopant gas used during the etch process may be different gases. In such an embodiment, the two different gases generally include the same dopants species (e.g., tin). Thus, it is not necessary that the same gas be present in the chamber atmosphere during non-deposition processes; rather the presence of the same species is generally sufficient to reduce undesired migration and out gassing.
In yet another embodiment, it is contemplated that the tin precursor gas may be optionally introduced to the processing chamber prior to operation 216 to pre-condition the substrate and/or the processing chamber. Preconditioning of the substrate and/or the processing chamber mitigates the “lag” of incorporation of tin into an alloyed epitaxial film. Additionally or alternatively, a group III dopant or a group V dopant may be utilized to precondition the chamber in a similar manner. In such an embodiment, preconditioning of the chamber with a group III dopant or a group V dopant may further reduce migration or out gassing of dopants within the deposited epitaxial film. In one example, preconditioning may begin about 1 second to about 60 seconds before deposition. It is contemplated that the preconditioning may constitute the introduction of precursor during that etching and/or annealing. That is, a single flow of tin precursor could be utilized to reduce tin migration during annealing/etching, and simultaneously precondition the processing chamber for the next deposition.
In addition to facilitating dopant concentration uniformity, the flow of dopant gases, such as group III or group V dopant gas, during non-deposition phases of processing also reduces film surface roughness. In one example, a boron-doped germanium epitaxial film is deposited and then annealed. During deposition, a germanium hydride precursor gas and diborane are flowed into a chamber, and a boron-doped germanium epitaxial layer is formed. The boron-doped germanium epitaxial layer is deposited to a thickness of about 140 angstroms. At the conclusion of the deposition process, the boron-doped germanium epitaxial layer has a surface roughness of about 2.5 angstroms (arithmetic mean). After deposition, the boron-doped germanium epitaxial layer is annealed at 590° C. in a hydrogen atmosphere for 90 seconds. After annealing, the surface roughness of the boron-doped germanium epitaxial layer was 32.6 angstroms (arithmetic mean). The increased surface roughness is believed to be due to the migration of boron through the germanium epitaxial film because of the elevated annealing temperature.
In contrast, a similar layer deposited on a different substrate under the same conditions and having the same surface roughness was annealed in an atmosphere of hydrogen gas and diborane for 90 seconds at 590° C. The surface roughness of the layer after annealing in the presence of diborane was about 2.6 angstroms (arithmetic mean). Thus, by providing a dopant-containing gas to the process chamber atmosphere during non-deposition phases of processing, dopant migration can be reduced, film surface roughness can be improved, and overall film quality is maintained.
The anneal methods of the example described above may utilize thermal or laser anneals. Additionally, the annealing may take place in the same chamber as deposition, or in a different chamber. The migration of dopants is generally minimal during idle times, such as transportation of the substrate form one chamber to another. However, during processing, such as annealing, the migration of dopants is increased because of elevated processing temperatures. Therefore, it is desirable that a dopant-containing gas be provided to the process chamber during periods of elevated temperature in order to mitigate or reduce undesired dopant migration, as discussed above.
Benefits of the invention include the formation of epitaxial layers having uniform concentrations and improved surface roughness. The methods described herein are particularly beneficial for cyclical process which include a deposition/etch process or a deposition/anneal process. However, it is contemplated that embodiments described herein may be useful in any process in which it is desirable to reduce migration of elements within a film or to reduce out gassing of dopants from the film, including processes in which the deposition is not cyclical or repeated (e.g., only a single deposition operation is performed).
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/657,494, filed Jun. 8, 2012, and U.S. Provisional Patent Application Ser. No. 61/660,382, filed Jun. 15, 2012. The aforementioned applications are herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61657494 | Jun 2012 | US | |
| 61660382 | Jun 2012 | US |
