Patent Grant
6987072
The present invention relates to a method for fabricating a semiconductor crystal, and to a semiconductor crystal substrate. More specifically, the present invention relates to a method for fabricating a semiconductor crystal that is composed of a carbon-containing Group IV element.
Recently, semiconductor crystals composed of Group IV elements for use in high-speed semiconductor devices have attracted widespread public attention. Among these, research and development related to semiconductors composed of Group IV elements containing carbon (SiGeC, Si:C (Si crystal containing a few atomic percent substitutional C atoms). Ge:C (Ge crystal containing a few atomic percent substitutional C atoms), etc.) has been actively conducted in recent years (for example, Japanese Unexamined Patent Publication Nos. 2001-196317, 2001-93900, 1999-284065 (Specification of U.S. Pat. No. 6,251,751). This is because these crystals have the following excellent properties.
(1) First, the advantages of the Si:C crystal will be explained. The Si:C crystal has a smaller lattice constant than the Si crystal because the carbon atom is smaller than the silicon atom. Therefore, when epitaxially grown on an Si substrate, Si:C crystals become strained, because they receive tensile stress toward the in-plane direction of the substrate. In strained Si:C crystals, so-called intervalley scattering, which is one of the scattering mechanisms of conduction electrons, is reduced compared to that of bulk Si crystals. The reduced scattering enables Si:C crystals that have been epitaxially grown on an Si substrate to obtain greater mobility than that of bulk Si crystals. Therefore, by applying a heterostructure composed of Si:C crystals and Si crystals, it is possible to achieve a device that operates at higher speed than is possible using only bulk Si crystals.
(2) Next, the SiGeC crystal, which can be considered as an improvement of the SiGe crystal, will be explained.
The SiGe crystal has a larger lattice constant than the Si crystal. Therefore, when an SiGe crystal has been epitaxially grown on an Si substrate, it is subjected to an extremely large compressive stress, generating strain. This compressive strain limits the film thickness to the so-called critical film thickness (the upper limit of film thickness that can be deposited without dislocation), and may lead to relaxation accompanied by crystal defects (dislocations) when the SiGe crystal is additionally subjected to heat treatment. Furthermore, in a heterostructure composed of Si and SiGe crystals, band offset appears only on the valence band of the SiGe crystal. Therefore, when an MOS transistor with an SiGe crystal channel is fabricated, only a p-channel transistor can be obtained.
However, since carbon has a smaller atomic radius than Si or Ge, adding carbon atoms to an SiGe crystal reduces the lattice constant of the crystal and compensates the strain. The critical film thickness can thus be made thicker than that of the SiGe crystal. Furthermore, it is also possible to reduce the amount of strain accumulated in the crystal, thereby raising the thermal resistance of the crystal. From the viewpoint of its application to devices, in a heterostructure composed of SiGeC and Si crystals, when the concentrations of Ge and C are increased (Ge: dozens of %, C: several % or greater), band offsets appear both on the valence band and conduction band of the SiGeC crystal. In this case, carrier trapping occurs in both the conduction band and valence band. Therefore, the use of SiGeC crystal as a channel material is advantageous in that not only p-channel MOS transistors but also n-channel MOS transistors can be fabricated.
As described above, the Si:C crystal and SiGeC crystal have excellent properties, and the additional effects obtained by adding carbon atoms thereto become more distinctive by increasing the concentration of carbon thereof. For example, if the carbon concentration of the Si:C crystal becomes higher, a significant effect on the prevention of scattering can be expected.
However, it is essentially difficult to dissolve a carbon atom in Si or Ge, and therefore a high-quality crystal with a high carbon concentration cannot be readily fabricated. Furthermore, a high carbon concentration in the crystal leads to the following problems.
Carbon atoms tend to be incorporated not only into substitutional lattice sites but also into interstitial lattice sites. The interstitial carbon atoms tend to form a trap with a positive or negative charge in the crystal. In fabricating transistors, having such traps causes the recombination or scattering of carriers, thereby degrading the properties of the device.
In order to obtain high-quality Si:C or SiGeC crystals having a low interstitial carbon concentration, methods known as CVD (Chemical Vapor Deposition) and MBE (Molecular Beam Epitaxy) have been heretofore employed. In these methods, in order to minimize the number of interstitial carbon atoms, various modifications have been made to the crystal growing method (lowering the growth temperature or increasing the total pressure of the source gas).
However, even when these techniques are employed, if the carbon concentration increases to a certain degree, there is a tendency for carbon atoms to be inserted between the lattice positions. Therefore, it was difficult to fabricate a Group IV element semiconductor wherein all the carbon atoms were incorporated into substitutional lattice sites. As a result, the interstitial carbon atoms cause the recombination or scattering of the carriers as described above. Therefore, applying such crystals to a semiconductor device poses a problem because they have a significant effect on the electrical properties.
The present invention aims to solve the problems described above and provides a method for fabricating semiconductor crystals wherein the substitutional carbon concentration is satisfactorily reduced in Group IV element semiconductor crystals.
The above-described object of the present invention can be achieved by a method for fabricating semiconductor crystals that comprises a first step of forming a semiconductor crystal layer containing carbon and at least one of the Group IV elements other than carbon on a substrate, a second step of adding an impurity that can react with carbon to the semiconductor crystal layer, and a third step of removing the carbon contained in the semiconductor crystal layer by a reaction between the carbon and the impurity.
a–2d are cross-sectional views showing the fabricating steps of the semiconductor crystal according to the first embodiment of the present invention.
a–3b are cross-sectional views illustrating a modified example of a method for fabricating the semiconductor crystal according to the first embodiment of the present invention.
a–4f are cross-sectional views showing the fabricating steps of the semiconductor crystal according to the second embodiment of the present invention.
Hereinafter, embodiments of the present invention will be explained with reference to the accompanying drawings. In order to get a clear picture of the problems of the known methods, the present inventors first conducted the following experiments.
By employing the ultra high vacuum chemical vapor deposition (UHV-CVD) method, which is a known crystal growing method, alloy semiconductor crystals (SiGeC) composed of carbon-containing Group IV elements were obtained. Disilane (Si2H6) was used as the source gas for Si, germane (GeH4) was used as the source gas for Ge, and monomethylsilane (SiH3CH3) was used as the source gas for C. The pressures of the Si2H6 and GeH4 were set in such a manner that the Ge concentration became approximately 25%, and, while keeping these gas pressures constant, only the pressure of the SiH3CH3 gas was gradually raised. Then, changes in the concentration of carbon atoms at the lattice positions were observed.
In
From such experimental results and the like, it became clear that when the substitutional carbon concentration of carbon-containing Group IV element semiconductor crystals falls in the range from 0.5 to 2.0%, carbon atoms exist in interstitial sites, and particularly when the substitutional carbon concentration is in the range from 1.0 to 1.3%, the concentration of interstitial carbon atoms becomes high and the removal of these carbon atoms becomes necessary. A method for fabricating semiconductor crystals, such as SiGeC crystals, in which the interstitial carbon concentration is kept in a low range will be explained below.
(1) First Embodiment
In the present embodiment, after placing the Si wafer 201 in the crystal growth chamber, the chamber is evacuated until the pressure becomes 2×10−9 Torr. Then, the substrate is heated to 850° C. in a hydrogen gas atmosphere so that all foreign matter on the surface of the Si wafer 201 is removed and a clean Si surface is exposed.
Then, the temperature of the Si wafer 201 placed in the crystal growth chamber is lowered to 490° C. and the source gas is supplied therein. In the present embodiment, Si, Ge, and C source gases are used. Examples of Si source gas include SiH4, Si2H6, SiH2Cl2, etc. Examples of Ge source gas include GeH4, C(GeH3)4, etc. Examples of C source gas include CH4, C2H6, SiH3CH3, etc., and alcohols, such as CH3OH, C2H5OH, etc. Note that C(GeH3)4, which is used as a Ge source gas, also serves as a C source gas. Then, SiGeC crystals begin growing on the Si wafer 201. The pressures of the source gases at the time of crystal growth are, for example, 7×10−5 Torr for Si2H6 gas, 2.3×10−4 Torr for GeH4 gas, and 9×10−6 Torr for SiH3CH3 gas.
In the present embodiment, O2 gas is supplied simultaneously with the supply of the source gas. The partial pressure of O2 gas is, for example, 3×10−9 Torr.
When the crystal is grown under the conditions described above for 15 minutes, as shown in
As the next step, the supply of the source gas is completely stopped, the temperature of the Si wafer 201 is raised to 900° C., and the wafer is heated for one minute. Accordingly, as shown in
According to the above-described method for fabricating semiconductor crystals of the present embodiment, by a reaction between the carbon atoms and the oxygen that is added as an impurity and the subsequent heat treatment, it is possible to remove the interstitial carbon atoms that result when inserting carbon atoms into the lattice positions of the semiconductor crystal layer. Thereby, the electrical properties of a device employing this semiconductor crystal can be satisfactorily enhanced.
In the present embodiment, the concentration of the oxygen added to the semiconductor crystal layer 203 is set at 1×1019/cm3; however, the concentration thereof is not limited to this. Nonetheless, adding an unduly large quantity of oxygen is not preferable, because SiO2 may form in the crystals when heat treatment is conducted after the crystal growth. Therefore, it is preferable that the oxygen concentration be set at the same level as the carbon concentration in the semiconductor crystal layer 203, or lower than that. Specifically, it can be in the range from approximately 5×1018 to 1×1020/cm3 (5E18 to 1E20/cm3).
In the present embodiment, oxygen is added to the entire semiconductor crystal layer 203. However, since oxygen and carbon are dispersed in the crystals by heat treatment, adding oxygen to only a portion of the semiconductor crystal layer 203 can achieve almost the same effect as that of the present embodiment. For example, on the Si wafer 201 substrate, a semiconductor crystal layer 202a with oxygen added, a semiconductor crystal layer 202b without oxygen, and a semiconductor crystal layer 202c with oxygen added can be layered in this order, as shown in
Such a layered structure of semiconductor crystal layers can be readily obtained by adjusting the timing for supplying and stopping the O2 gas in the source gas.
As described above, when oxygen is added to only a portion of the semiconductor crystal layer, the concentration of the added oxygen can be lowered. Therefore, the adverse effects resulting from the formation of SiO2 in the crystals can be reduced.
(2) Second Embodiment
Fist of all, as shown in
The temperature of the Si wafer 401 is then lowered to 490° C. and the source gas is supplied, whereupon SiGeC crystal growth begins. The same gases as in the first embodiment can be used as the source gas, and, for example, the pressure of each gas is adjusted such that 7×10−5 Torr for Si2H6 gas, 2.3×10−4 Torr for GeH4 gas, and 9×10−6 Torr for SiH3CH3. Note that, in the present embodiment, O2 gas is not supplied at this stage.
When the crystal is grown for 15 minutes under these conditions, as shown in
Then, the supply of GeH4 gas and SiH3CH3 gas is stopped, only Si2H6 gas is supplied, and the pressure is adjusted to 7×10−5 Torr. Thereafter, the temperature of the Si wafer is raised to 600° C. and maintained for one minute. Thereby, an Si cap 403 having a thickness of 10 nm is formed on the semiconductor crystal layer 402 (see
The resulting sample is taken out from the crystal growth chamber and a portion of the Si cap 403 is subjected to thermal oxidation. The thermal oxidation can be conducted, for example, by pyro-oxidation. In other words, a sample is placed in an electric furnace at 900° C., oxygen and hydrogen are introduced at the flow rate of 9,000 sccm and 8,000 sccm, respectively. With approximately 5 minutes of oxidation, the surface of the Si cap 403 is oxidized to a depth of approximately 5 nm, forming an oxidized film 404 as shown in
Next, the sample is placed in the vacuum CVD chamber and, while maintaining SiH4 in an atmosphere with a pressure of approximately 0.1 Torr at 600° C. for approximately 50 minutes, a poly-Si layer 405 is deposited thereon so that the thickness thereof becomes approximately 300 nm (see
The sample obtained in the above procedure is then placed into an ion implantation chamber and subjected to ion implantation in the manner shown in
After implanting the oxygen ions, as shown in
Then, the resulting sample is placed in a thermal annealing chamber and heated to 900° C. for one minute. The thermal annealing is conducted in an oxygen-free atmosphere so that the sample is not oxidized, for example, under an H2 atmosphere, N2 atmosphere, Ar atmosphere, etc. As shown in
In the present embodiment, the oxidized film 404 and poly-Si layer 405 are formed above the semiconductor crystal layer 402, and the oxidized film 404 and poly-Si layer 405 are removed after oxygen ions are implanted as an impurity. By forming such dummy layers (in the present embodiment, the oxidized film 404 and poly-Si layer 405) before the ion implantation, the concentration peak of the implanted oxygen ions can be readily adjusted to the desired depth in the semiconductor crystal layer 402 (for example, around the center of the thickness direction). In other words, the thickness and the structure of the dummy layers can be appropriately selected depending on the conditions of the oxygen ion implantation so as to achieve effective ion implantation, and therefore the specific structure is not limited to that of the present embodiment.
It is not preferable to add an unduly large quantity of oxygen because SiO2 may form in the crystals when heat treatment is conducted. Therefore, it is preferable that the oxygen concentration be set at the same level as the carbon concentration in the semiconductor crystal layer 403, or lower than that. Specifically, the oxygen concentration added to the semiconductor crystal layer can be in the range from approximately 5E18 to 1E20/cm3.
(3) Preferable Temperature for Heat Treatment
Next, the following experiment was carried out in order to examine the preferable temperature range for the heat treatment, which removes the excess carbon atoms from the SiGeC layer, in the first and second embodiments.
The structure of the sample used in the experiment was a crystal (concentration of Ge: 25%, concentration of C: 1%, and concentration of O: 1×1019/cm3) having a 10 nm of Si layer and 100 nm of SiGeC layer deposited on an Si substrate. Note that this SiGeC layer was formed in the same manner as explained in the first embodiment. In other words, the substrate was maintained at 490° C. in the UHV-CVD chamber, and, while simultaneously supplying source gases at the pressures approximately 7×10−5 Torr for Si2H6 gas, approximately 2.3×10−4 Torr for GeH4 gas, approximately 9×10−6 Torr for SiH3CH3 gas, and approximately 3×10−9 Torr for O2, the crystal growth was conducted for approximately 15 minutes.
In
Hereunder, the thermal desorption spectrum will be briefly explained. The thermal desorption spectrum is used for analyzing the kinds of substances that evaporated from a sample when the sample was heated under the ultra high vacuum atmosphere by conducting the mass spectroscopy. In the thermal desorption spectrum shown in
As is clear from
However, when the temperature reached 700° C. or higher, the ion current of the SiGe layer became almost 0, on the other hand, the ion current of the SiGeC layer turned to increase from approximately 700° C., and therefore the two layers greatly differ in their tendencies. It is believed the reason that the ion current of the SiGe layer became almost 0 at 700° C. and higher is that all of the absorbed CO2 has released and evaporated therefrom. Therefore, in respect to the SiGeC layer, it is believed that the substance having the mass number of 44 observed at 700° C. or higher is not the CO2 that has been absorbed on the surface of the sample but the CO2 that has been generated in the crystals of the SiGeC layer, dispersed in the crystals, and evaporated from the surface.
From the experimental results as shown in
Furthermore, as is clear from the results shown in
In the SiGeC crystal, it is believed that not only CO2 but also CO is generated; however, in the thermal desorption spectrum obtained in this experiment, generation of CO was not clearly confirmed. This is probably because the CO that has been generated in the crystal was formed into CO2 by bonding to oxygen while dispersing in the crystal and, or the CO that has evaporated form the surface of the crystal was formed into CO2 by bonding to the oxygen slightly left in the vacuum atmosphere.
It is also believed that the CO and CO2 formed in the crystal were not only released from the surface of the SiGeC crystal as described above but also partly dispersed in the crystal. However, CO and CO2 are comparatively stable molecules as they are and do not strongly bond to the elements that compose the SiGeC crystal. Therefore, as long as the interstitial carbon atoms exist in the crystal in the form of CO or CO2, it is believed that they will not adversely affect the electrical properties of the device.
(4) Other Embodiments
In the embodiments described above, oxygen was used as an impurity added to a semiconductor crystal layer. However, there is no limitation as long as the impurity can be removed by the reaction with carbon atoms, and, for example, hydrogen or fluorine can be added as the impurity.
Furthermore, in the above embodiments, an example wherein the semiconductor crystal layer is formed as a SiGeC layer was explained; however, as long as it is a carbon-containing Group IV element semiconductor crystal layer (for example, SiGeC crystal, Si:C crystal, Ge:C crystal, etc.), it is possible to obtain a semiconductor crystal substrate by following the same process as in the embodiments described above and achieve the same effects.
Regarding the crystal growing method in the above embodiments, various known methods other than the UHV-CVD method, including the low-pressure CVD method, MBE method, and the like can be employed.
The temperature and time period of the heat treatment are also not limited to the examples explained in the above embodiments and can be appropriately selected in such a manner that the excess carbon atoms incorporated into interstitial lattice sites can be effectively removed.
Furthermore, in the first embodiment, crystal growth of the SiGeC crystals and the subsequent heat treatment were performed in the same crystal growth chamber; however, for example, it is also possible to once take out the substrate that has been subjected to crystal growth from the crystal growth chamber and conduct the heat treatment using a separate heat treatment chamber. Furthermore, other than the vacuum atmosphere, the heat treatment can be conducted under an H2 atmosphere, N2 atmosphere, Ar atmosphere, etc. It is also possible to conduct the heat treatment after thinly depositing, for example, Si crystals and the like on the semiconductor crystal layer.
In the embodiments described above, explained were the methods for removing the interstitial carbon atoms that were generated during the epitaxial growth of an SiGeC crystal (namely, the interstitial carbon atoms intrinsically generated). However, the methods for removing interstitial carbon atoms of the present invention can also be employed to remove the interstitial carbon atoms generated in the process of fabricating a device after the SiGeC crystal has been epitaxially grown (i.e., the interstitial carbon atoms extrinsically generated). For example, when the SiGeC crystal is used as the base region of a bipolar transistor and the poly-Si that has phosphorus (P) doped thereon is used as the emitter region, it becomes necessary to conduct heat treatment after depositing the phosphorus-doped poly-Si on the SiGeC crystal. In this case, interstitial silicon atoms are dispersed from the phosphorus-doped poly-Si into the SiGeC crystal region and displace carbon atoms at the lattice positions in the SiGeC crystal, generating interstitial carbon atoms. In this case, it is also possible to efficiently remove interstitial carbon atoms by adding an impurity, such as oxygen that is capable of reacting with carbon, to the SiGeC crystal.
As described above, in respect to the Group IV element semiconductor crystal, the present invention provides a method for fabricating a semiconductor crystal in which the concentration of the interstitial carbon is satisfactorily reduced. When this crystal is applied to a semiconductor device, excellent electrical properties can be obtained.
This application is a division of Ser. No. 10/330,080 Dec. 30, 2002 U.S. Pat. No. 6,838,395.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4292083 | Rauch, Sr. | Sep 1981 | A |
| 4368083 | Bruel et al. | Jan 1983 | A |
| 4655849 | Schachameyer et al. | Apr 1987 | A |
| 5024968 | Engelsberg | Jun 1991 | A |
| 5151135 | Magee et al. | Sep 1992 | A |
| 5286658 | Shirakawa et al. | Feb 1994 | A |
| 5445679 | Hansen et al. | Aug 1995 | A |
| 6242368 | Holmer et al. | Jun 2001 | B1 |
| 6251751 | Chu et al. | Jun 2001 | B1 |
| 6348369 | Kusumoto et al. | Feb 2002 | B1 |
| 20020025626 | Hattangady et al. | Feb 2002 | A1 |
| 20020052072 | Hirose | May 2002 | A1 |
| 20020179003 | Lida et al. | Dec 2002 | A1 |
| 20030042480 | Hirose | Mar 2003 | A1 |
| Number | Date | Country |
|---|---|---|
| 11-284065 | Oct 1999 | JP |
| 2001-93900 | Apr 2001 | JP |
| 2001-196317 | Jul 2001 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20050092230 A1 | May 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10330080 | Dec 2002 | US |
| Child | 11009020 | US |
