Method of making a semiconductor device having a strained semiconductor layer

[0001] This invention relates to semiconductor devices, and more particularly, to a semiconductor device structure using a strained semiconductor layer to enhance performance of transistors formed in that strained semiconductor layer.

[0002] One of the ways that is being used for improving transistor performance is by straining the semiconductor layer where the transistor is formed. In the case of P channel transistors, a desirable semiconductor layer is compressive silicon germanium (SiGe). In one case, this SiGe layer is formed by epitaxial growth on a silicon seed layer. The silicon seed layer ensures that the crystal growth is monocrystalline. With the germanium in the silicon, the SiGe is ensured of being compressive if the SiGe layer is substantially defect free. Several difficulties have been found with the typical epitaxial growth methods. One problem is controlling the thickness. Another is a reduction in surface quality. Another is that surface faceting, the unwanted <112> surface, is often seen when the epitaxial growth is done selectively. This faceting adversely affects device characteristics.

[0003] Another technique is to implant germanium into silicon to dope it with germanium and to amorphize the silicon. The amphorized germanium-doped silicon is then recrystalized to form the semiconductor layer used for transistor formation. One of the problems with this approach is that the germanium profile is graded due to channeling. The tail of the profile diffuses further during a subsequent anneal. Another problem is that the interface at the end of the implant and the silicon is the region with high defects but also the high strain. The defects have the effect of degrading the strain and thus reducing the performance enhancement of the transistors.

[0004] Thus, there is a need for a method for reducing or overcoming one or more of the problems stated above.

[0010] Skilled artisans appreciate 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 improve the understanding of the embodiments of the present invention.

[0011] In one aspect, an implant is performed in the P channel regions, while masking the N channel regions, to deeply amorphize a layer at the surface of a semiconductor layer. After this amphorization step, germanium is implanted into the amorphized layer. The germanium is implanted to a depth that is less than the amorphization depth. This germanium-doped layer that is amorphous is heated so that it is recrystallized. The recrystallization results in a semiconductor layer that is silicon germanium (SiGe) and compressive. P channel transistors are then formed in this recrystallized semiconductor layer. This is better understood by reference to the figures and the following description.

[0012] Shown in FIG. 1 is a semiconductor device 10 comprising semiconductor layer 12, a trench isolation region 11, an implant mask 14 over an N channel portion of layer 12, and an amorphous region 16 in a P channel portion of layer 12. Amorphous region 16 is formed by an implant 18. Semiconductor layer 12 is preferably silicon and may be, for example, bulk silicon or a top semiconductor layer of a semiconductor on insulator (SOI) substrate. The depth of amorphous region 16 is preferably about 250 Angstroms in this example, but could be a different depth. Preferably it should not exceed 400 Angstroms. This depth has been found to be achievable with an implant of xenon at an implant energy of 35 keV and a dose of 1.2×10¹⁴ (1.2e14) ions per centimeter squared. Other depths could also be effective and be achieved using different energies and doses as well as a different implant species. The energy is preferably greater than 18 keV and the dose is preferably in the range of 4e13 to 4e14. Other implant species, for example, could be carbon, silicon, germanium, and argon. The dose is preferably as low as possible to still achieve the amorphization. An option is to form a sacrificial layer of oxide prior to performing implant 18 as well as before forming mask 14. This can be beneficial in protecting the surface of semiconductor layer 12 from the impact of the implant and moving the concentration peak nearer the surface, but it has the disadvantage of driving oxygen atoms into semiconductor layer 12.

[0013] Shown in FIG. 2 is semiconductor device 10 after performing a germanium implant 20 that forms germanium-doped silicon region 22 in amorphous region 16. Germanium-doped silicon region 22 in this example is at a depth of about half that of amorphous region 16 and is also amorphous at this stage in processing. This implant 20 is at a much higher dose than that of implant 18. An effective implant for implant 20 has been found to be 9 keV and a dose of 1e16. The energy and dose can be adjusted to achieve the desired depth and percent of germanium. The energy is preferably less than 15 keV and the dose is preferably between 1e15 and 3e16. Preferably the bottom of layer 22 is at least 80 Angstroms above the bottom of layer 16. The region of high concentration of germanium needs to be far enough above the amorphous-monocrystalline interface to avoid a relatively high number of defects at that interface. The amount of xenon is so small that it has only a very small effect in creating defects upon crystallization.

[0014] Shown in FIG. 3 is semiconductor device 10 after an anneal step that causes amorphous region 16 to recrystallize to form a SiGe region 24 that is monocrystalline. This is in effect is a recrystallization so that the portion of region 16 below region 22 returns to be part of semiconductor layer 12. Region 22 changes from being amorphous to monocrystalline SiGe and shown as region 24. The anneal is preferably a rapid thermal anneal (RTA) spike of about 1070 degrees Celsius. Lower temperatures can also be effective in recrystalling, but preferably the temperature should be at least 900 degrees Celsius. A somewhat slower anneal has also been found to be effective. For example an RTA of 1022 degrees Celsius for ten seconds has been found to be effective in recrystallizing amorphous layer 16. If an oxide layer is formed over semiconductor layer 12 prior to implant 18 of FIG. 1, such oxide layer should be removed after the anneal. Semiconductor device 10 is now ready for transistor formation, which is preferably by conventional means, but could be by a later developed technique.

[0015] Shown in FIG. 4 is a P channel transistor 25 formed in region 24 and an N channel transistor 26 formed in semiconductor substrate 12. Transistor 25 comprises a gate dielectric 27 over region 24, a gate 28 formed over gate dielectric 27, a sidewall spacer 30 around gate 28, a source/drain region 26 on one side of gate 28 and formed in region 24, and a source/drain 28 formed on the other side of gate 28 and formed in region 24. Thus, it is seen that P channel transistor 25 is formed in a compressively stressed SiGe layer. Transistor 26 comprises a gate dielectric 32 over semiconductor layer 12, a gate 34 formed over gate dielectric 32, a sidewall spacer 36 around gate 34, a source/drain region 38 on one side of gate 34 and formed in semiconductor layer 12, and a source/drain 40 formed on the other side of gate 34 and formed in semiconductor layer 12.

[0016] This same basic approach can be used to achieve alternative strains. For example, the semiconductor layer 12 can itself be strained either as tensile or relaxed. In the example described for FIGS. 1–4, it would be preferably for semiconductor layer 12 to be tensile in order to enhance the performance of the N channel transistors. The amount of compressive stress for the P channel transistor can be adjusted by adjusting the dose of germanium that is applied.

[0017] As an alternative, the implanting steps can be applied to the N channel side while masking the P channel side. Semiconductor layer 12 in such case would preferably be compressive and perhaps SiGe suitable for forming enhanced-performance P channel transistors. The first implanting step 18 would not need to change to form the relatively deep amorphous region 16. The second implant 20 would in such case preferably be a carbon implant to form a carbon-doped region in amorphous region 16. The recrystallization of such carbon-doped region would result in a tensile stressed semiconductor layer suitable for forming performance-enhanced N channel transistors.

[0018] In all these examples, the result from the two implants and the recrystallization is a region that is strained in relation to semiconductor layer 12. This recrystallized region can even be relaxed as a result of the two implants and recrystallization if semiconductor layer 12 is under compressive or tensile stress. This approach can also be used on both the P and N channel side. After masking one side and performing the two implants and recrystallization, the other side can be masked and two implants and a recrystallization can also be performed. In such case the second implants would be of a different species.

[0019] In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, other implant species than those listed may be used. Certain thicknesses and materials were described and these may be varied. The optional sacrificial layer is described as being oxide but it could be a different material. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention.

[0020] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.