Patent Application
20200332438
The present invention generally relates to semiconductor process technology and devices. More particularly, the present invention generally relates to an improved method for protecting a silicon substrate during epitaxial deposition of a group III nitride layer on top of the silicon substrate.
Semiconductor devices comprising Gallium Nitride, also referred to as GaN, have the ability to carry large currents and to support high voltages. This makes them increasingly desirable for power semiconductor devices. In the recent years, research efforts focused on developing devices for high power/high frequency applications. In general, the manufactured devices for these types of applications are based on device structures that exhibit high electron mobility and are referred to as heterojunction field effect transistors, also called HFETs, high electron mobility transistors, also called HEMTs, or modulation doped field effect transistors, also called MODFETs. HEMTs are for example useful for analog circuit applications, such as RF/microwave power amplifiers or power switches. Such devices can typically withstand high voltages, e.g. up to 1000 Volts, while operating at high frequencies, e.g. from 100 kHz to 100 GHz.
The semiconductors devices comprising Gallium Nitride are grown on top of substrates comprising silicon, for example silicon substrates also referred to as Si substrates. In order to obtain high-quality GaN layers on top of a Si substrate, a growth method with different steps and optimized processing parameters is used to accommodate for the lattice and the thermal mismatch between GaN and Si. The use of an AlN buffer layer between the silicon substrate and the GaN layer is known from the prior art. However, it is very challenging to grow AlN on top of a Si substrate. Indeed, Al has a low mobility on the surface of the Si substrate and the AlN grows in islands. Lateral growing is then necessary to obtain a layer of AlN on top of a Si substrate. The US patent publication US 2004/0119063 A1 for example describes a device wherein a few monolayers of aluminium are deposited on the silicon substrate to protect the silicon substrate from ammonia and then forming a nucleation layer of AlN and a buffer structure including multiple superlattices of AlGaN semiconductor having different compositions and an intermediate layer of GaN or another Ga-rich nitride semiconductor.
On the other hand, it appears that silicon carbide, also referred to as SiC, is particularly suited for growing a group III nitride layer on top of a silicon substrate, more particularly an AlN layer. Silicon carbide is useful in many high temperature, high power and high frequency semiconductor device applications because of properties such as a wide bandgap, high saturated electron drift velocity, high breakdown electric field, high thermal conductivity and good chemical resistance. The silicon carbide prevents Si diffusion into the layers deposited on top of the silicon carbide, and silicon carbide can also protect the Si substrate from nitridation and can introduce compressive stress in the AlN.
It has been demonstrated that a silicon surface may be carbonized using one or more hydrocarbons. However, this requires heating the silicon substrate up to high temperatures, for example a substrate temperature higher than 1000° C. The U.S. Pat. No. 7,666,765 B2 of Cheng et al. entitled “Method for forming a group III nitride material on a silicon substrate” for example describes a method for protecting the Si substrate from nitridation and for introducing compressive stress in the AlN. The method is referred to as “pre-dosing” and comprises the use of organometallic compounds. In fact, one or more organometallic compounds comprising Al, for example trimethylaluminium also referred to as TMA1, dimethylaluminium hybride also referred to as DMAl-H, or triethylaluminium also referred to as TEAL are provided in a reaction chamber to provide methyl or ethyl groups for reacting with the Si substrate preferably heated up from 1000° C. to 1200° C. Thanks to the use of an ultra-low flow of precursors in U.S. Pat. No. 7,666,765 B2, the silicon surface is carbonized and consequently passivated by the methyl groups. A monolayer of SiC is formed on top of the Si substrate and an AlN layer can then be grown on top of the resulting SiC/Si stack.
The method described in U.S. Pat. No. 7,666,765 B2 is very challenging as it requires accurate switching sequences when introducing the organometallic compounds and other gases in the reaction chamber. At a temperature higher than 1000° C., the TMA1 is already decomposed in the reaction chamber and the methyl groups and the Al atoms bind to the surface of the silicon substrate. Meanwhile, desorption of the Si substrate also occurs at this temperature. Additionally, Al atoms may diffuse into the Si substrate, which is undesirable.
Alternatively, other methods were developed to carbonize the Si substrate before the growth of a group III nitride material, wherein the methods comprise the use of one or more organometallic compounds comprising Ga, for example trimethylgallium also referred to as TMG or TMGa, dimethylgallium also referred to as DMGa, or triethylgallium also referred to as TEGa.
One of the key issues of the use of Ga-based precursors or of Ga pre-dosing is the issue of melt back etching described in the scientific publication of Krost et al. entitled “GaN-based optoelectronics on silicon substrates” published in Materials Science and Engineering: B, Volume 93, Issues 1-3, pages 77-84 on 30 May 2002. The reaction of Ga on Si indeed results in etching of the Si substrate due to the formation of Ga-rich Ga—Si alloy. The results in the formation of a rough interface between the SiC and the Si prior to the formation of AlN, which can result in disadvantageous effects on the performance of processed devices.
It is an objective of the present invention to disclose a device that overcomes the above identified shortcomings of existing solutions. More particularly, it is an objective to disclose a method for protecting a silicon substrate during epitaxial deposition of a group III nitride layer on top of the silicon substrate.
According to a first aspect of the present invention, the above defined objectives are realized by a method for forming silicon carbide onto a silicon substrate by reaction of the silicon substrate and a first precursor comprising indium and a plurality of carbon atoms.
The present invention provides an improved method for the in-situ protection of the surface of a silicon substrate during the epitaxial deposition of a wide bandgap material such as for example a group III nitride layer on top of the silicon substrate. In other words, the method according to the present invention is an improved method for growing silicon carbide or SiC in-situ on top of a silicon substrate. The first precursor of the present invention used for the in-situ formation of SiC onto the silicon substrate comprises Indium, also referred to as In, and further comprises a plurality of carbon atoms. For example, the first precursor according to the present invention comprises trimethylindium, also referred to as TMIn. The Si substrate is loaded into a reaction chamber suitable for epitaxial growth, for example a reaction chamber in a Metal-Organic Chemical Vapour Deposition reactor or MOCVD reactor, or a Molecular Beam Epitaxy reactor or MBE reactor, or an Atomic Layer Deposition reactor or ALD reactor, Chemical Beam Epitaxy reactor, or a Metalorganic Vapour Phase Epitaxy reactor or MOVPE reactor, etc. The Si substrate is heated up in-situ at a temperature comprised between 500° C. and 1400° C., preferably comprised between 800° C. and 1400° C., more preferably comprised between 900° C. and 1300° C., and even more preferably comprised between 1000° C. and 1200° C. The decomposition temperature of the first precursor comprising indium and a plurality of carbon atoms is comprised between 700° C. and 1200° C. The publication of R. E. Honig entitled “Vapor Pressure Data for the More Common Elements” published in 1957 in the RCA Review 18, pages 195-204, lists the vapor pressures In, Al and Ga for different temperatures. At 1000° C. for example, Ga has a vapor pressure close to 5.10−6 atm, while Al has a vapor pressure close to 2.10−7 atm. On the contrary, in has a vapor pressure close to 5.10−5 atm at 1000° C. Indium therefore has a higher vapor pressure than Al and Ga: the indium evaporates from the silicon substrate at this temperature when the first precursor is provided in the reaction chamber onto the silicon substrate. The pressure in the reactor is preferably from about 20 mbar to 100 mbar. Additionally, the publication of S. K. Ghandhi entitled “The Theory and Practice of Microelectronics”, published by John Wiley in 1968 presents the size of impurities atoms relative to silicon. When the size of an atom of silicon is 1.0, the size of an atom of aluminium is 1.07 and the size of an atom of gallium is 1.07. However, the size of an atom of indium is 1.22 when the size of an atom of silicon is 1.0. Indium therefore has a lower solubility than Al and Ga in the silicon substrate. The indium atoms therefore migrate less in the silicon substrate than Al or Ga atoms would do at this temperature. The surface of the silicon substrate in the present invention is therefore carbonized with the carbon atoms of the first precursor, thereby resulting in the formation of silicon carbide on top of the silicon substrate.
The first precursor is introduced in its gaseous form or in a gaseous composition. The first precursor can be diluted in N2 or in any noble gas such as He or Ar, and preferably in H2 gas. The gaseous composition can further comprise traces of nitrogen which can react with the silicon substrate, thereby forming SiN, resulting in a silicon substrate partially covered with SiC and partially covered with SiN, with mutually adjacent regions of SiC and SiN. Also, it is possible that SiN is already present as a contaminant on parts of the silicon substrate before the pre-dosing step. In that case, the pre-dosing step deposits SiC in between the regions of SiN and the silicon substrate is covered in adjacent regions of SiC and SiN. It is understood by the skilled person that the expression SiN is not limited to the compound that is strictly defined by the stochiometric ration of the chemical formula for silicon nitride, i.e. Si3N4. In a layer of SiN, a deviation from the stochiometric ratio may be present.
The silicon carbide may be a layer comprising SiC over the whole surface of the silicon substrate. Alternatively, the silicon carbide may be a layer comprising one or more regions of SiC and one or more regions of silicon nitride, also referred to as SiN. The SiC regions may have a thickness of for example maximum 3 nm. Alternatively, the SiC regions may have a thickness of a few nanometers, preferably smaller than 5 nm. The SiC regions may comprise one SiC monolayer, or a stack of SiC monolayers on top of each other. Alternatively, the silicon carbide may be for example monolayers of SiC, more particularly atomic monolayers of SiC. The silicon carbide in this case comprises a stack of several SiC monolayers, for example 2 or 3 monolayers, on top of each other, over the whole surface of the silicon substrate. This number of monolayers can be uniform over the whole surface of the silicon substrate or can vary over the surface of the silicon substrate. The number of monolayers may depend on the period during which the first precursor is provided in the reaction chamber.
SiC shows a crystalline structure. SiC has a stoichiometric composition and the crystalline structure of the SiC improves the quality of group III nitride layers later possibly grown on top of the resulting SiC/Si stack.
According to an optional aspect of the invention, the method comprising the steps of:
In the context of the present invention, providing the first precursor comprising indium and a plurality of carbon atoms can be referred to as the “pre-dosing step”. The method comprises providing at least 10 μmol of the first precursor, in a flow of less than 50 μmol·min−1, preferably less than 10 μmol·min−1, or less than 2 μmol·min−1, but not null.
According to an optional aspect of the invention, the first precursor comprises indium and a plurality of organic molecules comprising carbon atoms.
According to an optional aspect of the invention, the first precursor comprises indium and a plurality of methyl groups.
At a high temperature, both the precursor decomposition rate and the desorption rate are enhanced. The first precursor is decomposed to methyl groups and indium atoms at high temperature, more precisely to a mono-metal indium and two metal groups in gas phase, as follows:
In(CH3)3+H2↔InCH3+2CH4 (1)
Then, methyl groups and indium atoms are bound to the surface of the silicon substrate. Meanwhile, desorption of the silicon substrate also occurs. By providing a low flow of the first precursor, equilibrium between decomposition of the first precursor and desorption is achieved and thus no indium atoms are accumulated on the surface of the silicon substrate. Indeed, the bond between methyl groups and silicon atoms is stronger. The silicon substrate is therefore carbonized and passivated by methyl groups.
According to an optional aspect of the invention, the temperature threshold is comprised between 650° C. and 1000° C.
According to an optional aspect of the invention, the partial pressure threshold is larger than 1.10−6 mBar.
The partial pressure of the TMIn is equal to:
The total flow comprises a mixture of hydrogen and nitrogen and the total flow is for example about 1 mole/min. The flow of the first precursor is for example between 2 and 10 μmole/min. For example, the flow of TMIn is for example between 2 and 10 μmole/min. The pressure in the reactor is for example set to 40 mBar, preferably being comprised between 20 to 100 mBar. The partial pressure of TMIn is then for example minimum at 40.10−6 mBar, and the partial pressure of TMIn is preferably larger than 1.10−6 mBar.
According to an optional aspect of the invention, the method for forming silicon carbide onto a silicon substrate, by reaction of the silicon substrate and a first precursor comprising indium and a plurality of carbon atoms, comprises the step of subjecting the silicon substrate in the reaction chamber to a reaction cycle, the reaction cycle comprising supplying to said reaction chamber the first precursor at a temperature exceeding a temperature threshold comprised between 650° C. and 1000° C. and at a flow comprised between 2 and 10 μmole/min and at a partial pressure larger than 1.10−6 mbar, thereby forming the silicon carbide onto the silicon substrate.
According to an optional aspect of the invention, subjecting the silicon substrate in the reaction chamber to a reaction cycle further comprises supplying to the reaction chamber a purge gas, or a reducing gas, or combinations thereof.
According to an optional aspect of the invention, the reaction chamber is suitable for epitaxial growth in a Metal-Organic Chemical Vapour Deposition reactor, or a Molecular Beam Epitaxy reactor, or an Atomic Layer Deposition reactor, or a Chemical Beam Epitaxy reactor, or a Metalorganic Vapour Phase Epitaxy reactor.
According to an optional aspect of the invention, the silicon carbide comprises a silicon carbide film.
According to an optional aspect of the invention, when a thickness of the silicon carbide film exceeds a predetermined thickness threshold, the method further comprises the step of supplying ammonia and a second precursor comprising aluminium to the reaction chamber, thereby growing an aluminium nitride layer on top of the silicon carbide film.
A SiC/Si template obtained by carrying out the method according to the present invention appears to be very suitable for the further growth of AlN with very high epitaxial quality and low stress. The second precursor comprises ammonia and all compounds which contain a metal from group three in the periodic system of the elements with as ligand an alkyl, so a group containing carbon and hydrogen. At least one of the ligands should be methyl, ethyl or propyl in order to be able to donate carbon to the silicon substrate.
According to an optional aspect of the invention, the predetermined thickness threshold is comprised between a thickness of a monolayer of silicon carbide and 3 nm of silicon carbide.
Alternatively, the predetermined thickness threshold is comprised between a thickness of a monolayer of silicon carbide and a few nanometers of silicon carbide, for example between a thickness of a monolayer of silicon carbide and less than 5 nm of silicon carbide.
According to an optional aspect of the invention, supplying the ammonia and the second precursor comprising aluminium comprises supplying first the ammonia and then supplying the second precursor.
This way, the first reactor is purged from the first precursor before the second precursor is provided into the reaction chamber.
According to an optional aspect of the invention, supplying the ammonia and the second precursor comprising aluminium comprises supplying first the second precursor and then supplying the ammonia.
This way, the growth of SiN on top of the silicon substrate is minimized. Providing the second precursor allows to grow the aluminium nitride layer on top of the silicon substrate, more particularly on top of the silicon carbide.
According to an optional aspect of the invention, the method further comprises suspending the supply of the first precursor to the reaction chamber prior to supplying the ammonia and the second precursor to the reaction chamber.
This way, no reaction can take place between the first precursor and the ammonia and/or between the first precursor and the second precursor in the reaction chamber.
Optionally, when a thickness of the silicon carbide film exceeds a predetermined thickness threshold comprised between a thickness of a monolayer of silicon carbide and 3 nm of silicon carbide, the method further comprises the step of suspending the supply of the first precursor to the reaction chamber prior to supplying ammonia and a second precursor comprising aluminium to the reaction chamber, thereby growing an aluminium nitride layer on top of the silicon carbide film.
According to an optional aspect of the invention, the method comprises keeping supplying to the reaction chamber the first precursor at the temperature exceeding a temperature threshold and at the partial pressure exceeding a partial pressure threshold when supplying the ammonia and the second precursor to the reaction chamber.
Optionally, the method comprises keeping supplying to the reaction chamber the first precursor at the temperature exceeding a temperature threshold comprised between 650° C. and 1000° C. and at a flow comprised between 2 and 10 μmole/min and at a partial pressure larger than 1.10−6 mbar when supplying the ammonia and the second precursor to the reaction chamber.
This way, the surface of the silicon substrate is being carbonized and thereby passivated by growing silicon carbide and/or silicon nitride.
According to an optional aspect of the invention, the purge gas is nitrogen and wherein the reducing gas is hydrogen.
This way, the surface of the silicon substrate is cleaned before the growth of the silicon carbide. Native oxide is removed in-situ by the reducing gas, thereby resulting in a pristine surface of the silicon substrate ready for the growth of silicon carbide.
According to an optional aspect of the invention, the method further comprises supplying silane to the reaction chamber.
This way, the growth of silicon carbide on top of the silicon substrate is enhanced. Indeed, the decomposition temperature of silane is such that silane provides silicon atoms in the reaction chamber for example when the first precursor is provided in the reaction chamber. Additionally, silane could be used prior to the growth of silicon carbide to deposit a film of silicon oxide on top of the silicon substrate. The resulting SiO2/Si stack would then be subjected to the flow of the first precursor to grow the silicon carbide.
According to a second aspect of the invention, there is provided a semiconductor structure comprising:
According to an optional aspect of the invention, the silicon carbide is formed when supplying onto the silicon substrate the first precursor at a temperature exceeding a temperature threshold and a partial pressure exceeding a partial pressure threshold; and wherein the silicon carbide acts as a diffusion barrier for Group III elements into the silicon substrate.
According to an optional aspect of the invention, the silicon carbide is formed when supplying onto the silicon substrate the first precursor at a temperature exceeding a temperature threshold comprised between 650° C. and 1000° C. and at a flow comprised between 2 and 10 μmole/min and at a partial pressure larger than 1.10−6 mbar.
The present invention provides a semiconductor structure comprising a surface of a silicon substrate which is protected in-situ during the epitaxial deposition of a wide bandgap material such as for example a group III nitride layer on top of the silicon substrate. In other words, the semiconductor structure according to the present invention is comprises silicon carbide or SiC in-situ epitaxially grown on top of a silicon substrate. The first precursor of the present invention used for the in-situ formation of SiC onto the silicon substrate comprises Indium, also referred to as In, and further comprises a plurality of carbon atoms. The Si substrate is loaded into a reaction chamber suitable for epitaxial growth, for example a reaction chamber in a Metal-Organic Chemical Vapour Deposition reactor or MOCVD reactor, or a Molecular Beam Epitaxy reactor or MBE reactor, or an Atomic Layer Deposition reactor or ALD reactor, Chemical Beam Epitaxy reactor, or a Metalorganic Vapour Phase Epitaxy reactor or MOVPE reactor, etc. The Si substrate is heated up in-situ at a temperature comprised between 500° C. and 1400° C., preferably comprised between 800° C. and 1400° C., more preferably comprised between 900° C. and 1300° C., and even more preferably comprised between 1000° C. and 1200° C. The decomposition temperature of the first precursor comprising indium and a plurality of carbon atoms is comprised between 700° C. and 1200° C. The publication of R. E. Honig entitled “Vapor Pressure Data for the More Common Elements” published in 1957 in the RCA Review 18, pages 195-204, lists the vapor pressures In, Al and Ga for different temperatures. At 1000° C. for example, Ga has a vapor pressure close to 5.10−6 atm, while Al has a vapor pressure close to 2.10−7 atm. On the contrary, in has a vapor pressure close to 5.10−5 atm at 1000° C. Indium therefore has a higher vapor pressure than Al and Ga: the indium evaporates from the silicon substrate at this temperature when the first precursor is provided in the reaction chamber onto the silicon substrate. The pressure in the reactor is preferably from about 20 mbar to 100 mbar. Additionally, the publication of S. K. Ghandhi entitled “The Theory and Practice of Microelectronics”, published by John Wiley in 1968 presents the size of impurities atoms relative to silicon. When the size of an atom of silicon is 1.0, the size of an atom of aluminium is 1.07 and the size of an atom of gallium is 1.07. However, the size of an atom of indium is 1.22 when the size of an atom of silicon is 1.0. Indium therefore has a lower solubility than Al and Ga in the silicon substrate. The indium atoms therefore migrate less in the silicon substrate than Al or Ga atoms would do at this temperature. The surface of the silicon substrate in the present invention is therefore carbonized with the carbon atoms of the first precursor, thereby resulting in the formation of silicon carbide on top of the silicon substrate.
The silicon carbide formed on top of the silicon substrate acts as a diffusion barrier for Group III elements into the silicon substrate. For example, the silicon carbide acts as a diffusion barrier for aluminium atoms and/or gallium atoms of layers formed onto the silicon carbide into the silicon substrate. In other words, when ammonia and a second precursor comprising aluminium are supplied to the reaction chamber to grow an aluminium nitride layer on top of the silicon carbide on top of the silicon substrate, the silicon carbide acts as a diffusion barrier for the aluminium atoms of this aluminium nitride layer. In other words, when for example an aluminium nitride layer is present on top of the silicon carbide on top of the silicon substrate, the silicon carbide acts as a diffusion barrier for the aluminium atoms of this aluminium nitride layer. Silicon carbide further acts as a diffusion barrier for Group III elements of additional layers formed onto this aluminium nitride layer into the silicon substrate. For example, the silicon carbide further acts as a diffusion barrier for aluminium atoms and/or gallium atoms of additional layers formed onto this aluminium nitride layer into the silicon substrate.
According to a third aspect of the invention, there is provided the use of a first precursor comprising indium and a plurality of carbon atoms with a silicon substrate for forming silicon carbide onto a silicon substrate.
Optionally, there is provided the use of a first precursor comprising indium and a plurality of carbon atoms for forming silicon carbide onto a silicon substrate in a reaction chamber, wherein forming the silicon carbide comprises:
Optionally, there is provided the use of a first precursor comprising indium and a plurality of carbon atoms for forming silicon carbide onto a silicon substrate in a reaction chamber, wherein forming the silicon carbide comprises:
According to an embodiment shown in
The method according to the present invention comprises forming silicon carbide 2 onto a silicon substrate 1 by reaction of the silicon substrate 1 and a first precursor 200 comprising indium and a plurality of carbon atoms. According to an example of an embodiment of the steps of the method shown in
According to an embodiment shown in
Although the present invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various changes and modifications without departing from the scope thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. In other words, it is contemplated to cover any and all modifications, variations or equivalents that fall within the scope of the basic underlying principles and whose essential attributes are claimed in this patent application. It will furthermore be understood by the reader of this patent application that the words “comprising” or “comprise” do not exclude other elements or steps, that the words “a” or “an” do not exclude a plurality, and that a single element, such as a computer system, a processor, or another integrated unit may fulfil the functions of several means recited in the claims. Any reference signs in the claims shall not be construed as limiting the respective claims concerned. The terms “first”, “second”, third”, “a”, “b”, “c”, and the like, when used in the description or in the claims are introduced to distinguish between similar elements or steps and are not necessarily describing a sequential or chronological order. Similarly, the terms “top”, “bottom”, “over”, “under”, and the like are introduced for descriptive purposes and not necessarily to denote relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and embodiments of the invention are capable of operating according to the present invention in other sequences, or in orientations different from the one(s) described or illustrated above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 17209740.4 | Dec 2017 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/086194 | 12/20/2018 | WO | 00 |
