Molecular hydrogen implantation method for forming a relaxed silicon germanium layer with high germanium content

1. Field of the Invention

This invention generally relates to semiconductor fabrication and, more particularly, to a method for forming a relaxed silicon germanium film with high germanium content on a silicon substrate.

2. Description of the Related Art

In enhanced mobility MOSFET device applications, thick relaxed silicon (Si) germanium (Ge) layers have been used as substrates for thin strained Si layers, to increase carrier mobility for both NMOS and PMOS devices. Compared with bulk Si devices, enhancements in electron mobility of up to 70%, for devices with a channel length of less than 70 nanometers (nm), have been reported. Enhancements of up to 40% in high-field hole mobility for long-channel devices have also been found.

Conventionally, a high quality relaxed Si 1−x Ge x buffer layer is formed by growing a several microns (μm) thick compositionally graded layer, where Si 1−x Ge x represents a silicon germanium film with a varying content of Ge. However, the density of threading dislocations is still high (typically >10 6 /cm 2 ). In addition, the integration of Si 1−x Ge x film several microns thick into device fabrication has not been practical.

Alternative methods have been developed to efficiently relax strained SiGe layers on Si. The methods involving the implantation of hydrogen for the relaxation of strained SiGe layers have all utilized ionized atomic hydrogen (H + ). However, this implantation process is expensive due to the long time required. Helium implantation followed by an anneal step has also been explored to promote relaxation in SiGe films.

It would be advantageous if singly ionized molecular hydrogen (H 2 + ) could be used in the relaxation of SiGe films to reduce the process time and cost, since this implantation process can be done at double the energy and half the current.

It would be advantageous if boron, He, Si, or other species could be co-implanted with singly ionized molecular hydrogen (H 2 + ), as they have been shown to be effective for silicon on insulator (SOI) fabrication.

It would be advantageous if the implantation of H 2 + alone, or with a species such as boron, could be used for relaxing strained SiGe films deposited epitaxially on Si substrates.

The present invention method produces a thick (100-500 nm) relaxed, smooth SiGe film with high Ge content of greater than 20-30% as a substrate layer for a tensile strained Si film to be used for high speed MOSFET applications. As mentioned above, atomic hydrogen (H + ) implantation has been shown to be effective for producing such films, however, this implantation process is very expensive due to the long process times required. The use of singly ionized molecular hydrogen (H 2 + ) reduces the time and cost of the process, since the implant can be done at double the energy and half the current. Further, the H 2 + can be implanted alone, or with boron, He, Si, or other species for the purpose of relaxing strained SiGe films deposited epitaxially on Si substrates.

Accordingly, a method is provided for forming a relaxed silicon germanium layer with a high germanium content on a silicon substrate. The method comprises: depositing a single-crystal silicon (Si) buffer layer overlying the silicon substrate; depositing a layer of single-crystal silicon germanium (Si 1−x Ge x ) overlying the Si buffer layer having a thickness of 1000 to 5000 Å; implanting the Si 1−x Ge x layer with ionized molecular hydrogen (H 2 + ) a projected range of approximately 100 to 300 Å into the underlying Si buffer layer; optionally, implanting the Si 1−x Ge x layer with a species selected such as boron, helium (He), or Si; annealing; and, in response to the annealing, converting the Si 1−x Ge x layer to a relaxed Si 1−x Ge x layer. Optionally, after annealing, an additional layer of single-crystal Si 1−x Ge x having a thickness of greater than 1000 Å can be deposited overlying the relaxed layer of Si 1−x Ge x .

Some aspects of the method include depositing a layer of Si 1−x Ge x , where x is greater than 0.2. Alternately, a layer of graded Si 1−x Ge x can be deposited, where x varies in the range from 0.03 to 0.5. Further, the Si 1−x Ge x deposition process includes epitaxially growing the layer of Si 1−x Ge x at a temperature in the range of 400 to 600 degrees C. to form a strained layer of Si 1−x Ge x film, having a lattice structure that matches the underlying single-crystal Si buffer layer lattice structure.

The annealing process is conducted at a temperature in the range of 650 to 1000 degrees C. for a period of time in the range of 0.1 to 30 minutes. Alternately, the annealing process comprises: a low-temperature annealing at a temperature of approximately 250 degrees C. for a period of approximately 10 minutes; and, a high-temperature annealing at a temperature in the range of 650 to 1000 degrees C. for a period of time in the range of 0.1 to 30 minutes.

Additional details of the above-described method are provided below.

FIG. 1 is a flowchart illustrating the present invention method for forming a relaxed silicon germanium layer with a high germanium content.

FIG. 2 is a cross-sectional view showing an initial step in the completion of a relaxed SiGe film produced in accordance with the present invention method.

FIG. 3 illustrates the SiGe film of FIG. 2 during implantation.

FIG. 4 illustrates the SiGe film of FIG. 3 during annealing.

FIG. 5 illustrates the SiGe film of FIG. 4 with the optional deposition of an additional relaxed SiGe film overlying the initial relaxed SiGe film.

FIG. 6 illustrates the SiGe film of FIG. 3 or 4 with the optional deposition of a tensile strained Si film overlying the relaxed SiGe film.

FIG. 1 is a flowchart illustrating the present invention method for forming a relaxed silicon germanium layer with a high germanium content. Although this method is depicted as a sequence of numbered steps for clarity, no order should be inferred from the numbering unless explicitly stated. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. The method starts at Step 100 . Step 102 provides a silicon substrate. Step 104 deposits a single-crystal silicon (Si) buffer layer overlying the silicon substrate. In some aspects of the method, Step 104 includes depositing a Si layer having a thickness in the range of 50 to 1000 Å. Step 106 deposits a layer of single-crystal silicon germanium (Si 1−x Ge x ) overlying the Si buffer layer. Step 108 implants the Si 1−x Ge x layer with ionized molecular hydrogen (H 2 + ). Step 110 anneals. Step 112 , in response to the annealing, converts the Si 1−x Ge x layer to a relaxed Si 1−x Ge x layer.

In some aspects of the method, depositing a layer of Si 1−x Ge x overlying the Si buffer layer in Step 106 includes depositing a layer of Si 1−x Ge x ) where x is greater than 0.2. In other aspects, depositing a layer of Si 1−x Ge x overlying the Si buffer layer includes depositing a layer of graded Si 1−x Ge x , where x varies in the range from 0.03 to 0.5. More specifically, x varies in the range from 0.03 to 0.2 at the Si layer interface, and x varies from 0.2 to 0.5 at the Si 1−x Ge x layer top surface. The value of x increases with the layer thickness, with an increase that can be a linear or step-wise increase.

In some aspects, depositing a layer of Si 1−x Ge x overlying the Si buffer layer in Step 106 includes depositing a layer of Si 1−x Ge x having a thickness in the range of 1000 to 5000 Å. In other aspects, depositing a layer of Si 1−x Ge x overlying the Si buffer layer includes epitaxially growing the layer of Si 1−x Ge x at a temperature in the range of 400 to 600 degrees C. In some aspects of Step 106 a strained layer of Si 1−x Ge x film is formed, having a lattice structure that matches the underlying single-crystal Si buffer layer lattice structure.

In some aspects of the method, implanting the Si 1−x Ge x layer with ionized molecular hydrogen (H 2 + ) in Step 108 includes implanting H 2 + a projected range of approximately 100 to 300 Å into the underlying Si buffer layer. More specifically, Step 108 includes implanting a dosage of H 2 + in the range of 2×10 14 /cm 2 to 2×10 16 /cm 2 and with an energy of in the range of 10 kilo electron volts (keV) to 100 keV.

In some aspects of the invention an optional step is performed. Step 109 a , prior to annealing (Step 110 ), implants the Si 1−x Ge x layer with a species selected from the group including boron, He, and Si. Step 109 a can be performed either prior to the implanting of ionized molecular hydrogen (Step 108 ), or subsequent to Step 108 .

Implanting the Si 1−x Ge x layer with a species selected from the group including boron, He, and Si in Step 109 a includes implanting a dosage in the range of 1×10 12 /cm 2 to 1×10 15 /cm 2 . A relationship exists between Steps 108 and 109 a . The dosage of H 2 + implanted in Step 108 can be reduced in response to increasing the dosage of the implanted species selected from the group including boron, He, and Si (Step 109 a ). Likewise, the dosage of the species in Step 109 a can be reduced by increasing the dosage of H 2 + in Step 108 .

Some aspects of the method include further steps. Step 107 forms a sacrificial layer of silicon oxide overlying the Si 1−x Ge x layer, prior to implanting (Steps 108 and 109 a ). Step 109 b removes the sacrificial silicon oxide layer after implantation, before the annealing in Step 110 .

In some aspects, annealing in Step 110 includes annealing at a temperature in the range of 650 to 1000 degrees C. for a period of time in the range of 0.1 to 30 minutes. Alternately, annealing includes substeps. Step 110 a is a low-temperature annealing at a temperature of approximately 250 degrees C. for a period of approximately 10 minutes. Step 110 b is a high-temperature annealing at a temperature in the range of 650 to 1000 degrees C. for a period of time in the range of 0.1 to 30 minutes. Typically, Step 110 is performed in an atmosphere such as a vacuum, nitrogen, argon, or other inert gases.

Some aspects of the method include further steps. Step 114 , after annealing (Step 110 ), deposits an additional layer of single-crystal Si 1−x Ge x overlying the relaxed layer of Si 1−x Ge x . In some aspects, Step 116 , after annealing, deposits a layer of tensile strained single-crystal Si overlying the relaxed Si 1−x Ge x layer. Note that Step 114 can be performed without Step 116 . Likewise, Step 116 can be preformed without Step 114 , or both steps can be preformed.

In some aspects, depositing an additional layer of single-crystal Si 1−x Ge x overlying the relaxed layer of Si 1−x Ge x in Step 114 includes depositing an additional layer having a thickness of greater than 1000 Å. In some aspects, Step 114 includes epitaxially growing a layer of tensile strained Si, having a lattice structure matching the lattice structure of the underlying Si 1−x Ge x layer. Depositing a layer of Si overlying the relaxed Si 1−x Ge x layer in Step 116 includes depositing a Si layer having a thickness in the range of 50 to 300 Å.

FIG. 2 is a cross-sectional view showing an initial step in the completion of a relaxed SiGe film produced in accordance with the present invention method. A strained SiGe film 200 is shown deposited on a Si substrate layer 202 . In some aspects, a silicon buffer layer 203 of 50 to 1000 Å is deposited on the silicon substrate 202 underlying the SiGe layer 200 . The SiGe film has a thickness 204 of 1000 to 5000 Å. The Ge content in the SiGe film 200 can be up to 30% or greater. Alternatively, a graded Ge profile can be used. The growth conditions and source gases are chosen to minimize surface roughness while ensuring good crystallinity. A low temperature growth (400-600° C.) can be used to produce a metastable strained SiGe film 200 .

FIG. 3 illustrates the SiGe film of FIG. 2 during implantation. The Si buffer layer is implanted with H 2 + , with or without other species, such as boron, He, or Si. The H 2 + can be implanted either before or after the other species. The dose of H 2 + is in the range of 2e14/cm 2 (2×10 14 /cm 2 ) to 2e16/cm 2 , but depends on the dose of the co-implanted species. The dose of the other species (e.g. boron, He, or Si) varies over a wide range, such as from 1e12/cm 2 to 1e15/cm 2 . Generally, the higher the dose of the co-implanted species, the more the H 2 + dose can be reduced. The implant energies depend on SiGe thickness, and are chosen so that the implantation ranges are similar. To avoid contamination in the implantation steps, a thin sacrificial silicon oxide (not shown) in the range of 50 Å to 300 Å thick can be deposited on the first SiGe layer 200 .

FIG. 4 illustrates the SiGe film of FIG. 3 during annealing. The thermal annealing step converts the strained SiGe film into relaxed SiGe. Annealing is performed in an inert ambient, such as Ar, at a temperature in the range of 650° C. to 1000° C. Alternatively, a low temperature pre-anneal can be used, for example 250° C. for 10 minutes, followed by the higher temperature anneal. This step relaxes the strained SiGe layer while maintaining a smooth surface.

FIG. 5 illustrates the SiGe film of FIG. 4 with the optional deposition of an additional relaxed SiGe film 500 overlying the initial relaxed SiGe film 200 . The additional SiGe film can have a thickness 502 of greater than 1000 Å.

FIG. 6 illustrates the SiGe film of FIG. 3 or 4 with the optional deposition of a tensile strained Si film 600 overlying the relaxed SiGe film 200 . The Si film 600 typically has a thickness 602 of 50 to 300 Å.

The use of singly ionized molecular hydrogen (H 2 + ) to relax strained SiGe films was demonstrated in a series of experiments. SiGe films of approximately 300 nm thick were epitaxially grown on six-inch Si( 001 ) wafers. The films had a graded Ge profile, varying linearly from approximately 20% at the SiGe/Si interface to approximately 30% at the wafer surface. The as-deposited films were strained to be lattice-matched to the Si substrates. These wafers were then implanted with 1e16/cm 2 H 2 + ions at energies from 58 keV to 66 keV. Finally, the wafers were annealed at 800° C. for 9 minutes in an argon ambient.

Nomarski microscopy images of the wafer surfaces as well as x-ray diffraction (XRD) reciprocal space maps near the Si( 224 ) substrate peak were observed. If the implant is too shallow, the SiGe film has a high degree of lattice relaxation (R), but the lattice planes are not well-oriented (giving a broad XRD peak) and the surface becomes rough. With higher implant energy the SiGe film still has R>80%, but the surface roughness decreases considerably. At even higher implant energies the film relaxation decreases somewhat, but the surface is very smooth. By adjusting implant energy and dose, a compromise was found, giving sufficient lattice relaxation, while maintaining good crystalline quality and a smooth surface.

A method of using ionized molecular hydrogen (H 2 + ), with or without additional species, has been provided for forming a relaxed SiGe film overlying a Si substrate. Some process specifics have been given as examples. However, the present invention is not limited to merely these examples. Other variations and embodiments will occur to those skilled in the art.