# Patent

## Title
Method of forming alternating regions of Si and SiGe or SiGeC on a buried oxide layer on a substrate

## Abstract

Semiconductor structures and methods of forming semiconductor structures, and more particularly to structures and methods of forming SiGe and/or SiGeC buried layers for SOI/SiGe devices. An integrated structure includes discontinuous, buried layers having alternating Si and SiGe or SiGeC regions. The structure further includes isolation structures at an interface between the Si and SiGe or SiGeC regions to reduce defects between the alternating regions. Devices are associated with the Si and SiGe or SiGeC regions.

## Background
FIELD OF THE INVENTION
The invention relates to semiconductor structures and methods of forming semiconductor structures, and more particularly to structures and methods of forming SiGe and/or SiGeC buried layers for CMOS/BiCMOS and passive devices over Si or SOI substrates.
BACKGROUND DESCRIPTION
Generally, metal-oxide semiconductor transistors include a substrate made of a semiconductor material, such as silicon. The transistors typically include a source region, a channel/well region and a drain region within the substrate. The channel/well region is located between the source and the drain regions. A gate stack, which usually includes a conductive material, a doped poly Si, a gate oxide layer and sidewall spacers, is generally provided above the channel/well region. More particularly, the gate oxide layer is typically provided on the substrate over the channel region, while the gate conductor is usually provided above the gate oxide layer.
The amount of current flowing through a channel is generally directly proportional to the mobility of the carriers in the channel. Thus, by increasing the mobility of the carriers in the channel, the operation speed of the transistor can be increased. Also, mechanical stresses within a semiconductor device can modulate device performance by, for example, increasing the mobility of the carriers in the semiconductor device. Thus, to improve the characteristics of a semiconductor device, tensile and/or compressive stresses are created in the channel of the n-type devices (e.g., NFETs) and/or p-type devices (e.g., PFETs). However, the same stress component, for example tensile stress or compressive stress, improves the device characteristics of one type of device (i.e., n-type device or p-type device) while discriminatively affecting the characteristics of the other type device. Also, some devices do not require tensile and/or compressive stresses in the substrate.
Also with the progress of CMOS scaling, the distance from source to drain is greatly shortened. Therefore the doping of source/drain and the LDD (lightly doped drain) have to be limited to prevent device punch through. This limitation requires source/drain and extension dose reduction, which will impact the device performance. Because of this, there is a need to keep the same doping level in the source/drain and extension but confine the dopant diffusion.
While known methods provide structures that have tensile stresses applied to the NFET device and compressive stresses applied along the longitudinal direction of the PFET device, they may require additional materials and/or more complex processing, and thus, resulting in higher cost. Thus, it is desired to provide more cost-effective and simplified methods for creating large tensile and compressive stresses in the channels NFETs and PFETs, respectively.
SUMMARY OF THE INVENTION
In a first aspect of the invention, an integrated structure comprises discontinuous, buried layers having alternating Si and SiGe or SiGeC regions. The structure further includes isolation structures at an interface between the Si and SiGe or SiGeC regions to reduce defects between the alternating regions. Devices are associated with the Si and SiGe or SiGeC regions.
In another aspect of the invention, an integrated structure comprises a substrate and alternating Si and SiGe or SiGeC regions formed on the substrate. Isolation structures are at an interface between the Si and SiGe or SiGeC regions. A buried oxide (BOX) layer is formed above the substrate. A first type device is associated with the Si regions and a second type device is associated with the SiGe or SiGeC regions.
In yet another aspect of the invention, a method of forming an integrated structure comprises forming SiGe or SiGeC regions alternating with Si regions on a substrate from continuous layer of SiGe or SiGeC material or Si material formed over the substrate. The method further includes forming isolation structures at interfaces between the SiGe or SiGeC regions and the alternating Si regions. A buried oxide layer is formed over the substrate.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-6 show structures according to a first embodiment of the invention;
FIGS. 7-13 show structures according to a second embodiment of the invention;
FIGS. 14-22 show structures according a third embodiment of the invention;
FIG. 23 shows a schematic representation of a band alignment in accordance with an implementation of the invention;
FIG. 24 shows a graph of out diffusion of Boron in a structure implementing aspects of the invention;
FIG. 25 shows a final structure according to an aspect of the invention; and
FIG. 26 shows a final structure according to an aspect of the invention.



DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
This invention is directed to semiconductor structures and methods of manufacturing the semiconductor structures, and more particularly to structures and methods of forming SiGe and SiGeC buried layers for CMOS/BiCMOS and passive devices over Si or SOI substrates. In embodiments, the structures are formed on a silicon substrate which includes discontinuous buried layers having alternating regions of silicon and SiGe or SiGeC. In further embodiments, isolation trenches extend through the buried layer at the Si and SiGe or SiGeC interfaces to reduce defects between the alternating regions. This SiGe(C) film, for example, can be used to form the channel/well so the dopant diffusion can be suppressed.
More specifically, multiple structures and methods are developed in accordance with the invention by burying SiGe and/or SiGeC films, doped and undoped, in conventional Si and SOI wafers to improve device characteristics, e.g., high voltage CMOS, PIN diode, high performance CMOS, circuit body effect, substrate current reduction and noise cross-talk, etc. The SiGe and/or SiGeC profiling has the capabilities to allow for bandgap modulation and formation of built in electric fields.
FIGS. 1-6 show structures according to a first embodiment of the invention. In this implementation, the SiGe or SiGeC film is formed above a BOX. As in each of the embodiments, the SiGe or SiGeC regions may be non-doped, or n-typed doped or p-typed doped, depending on the particular application.
FIG. 1 shows a starting structure comprising an optional BOX layer 12 formed on a silicon wafer 10. In embodiments, the BOX layer 12 is an oxide layer in a Si substrate. The oxide buried in the silicon wafer can be a depth ranging from less than 100 nm to several micrometers from the wafer surface, depending on the application. In embodiments, the thickness of the BOX layer 12 is in the range from about 40 nm to about 100 nm. A silicon (Si) film 14 is formed on the BOX layer 12.
In FIG. 2, an optional buffer layer 16 is formed on the Si film 14. The optional buffer layer 16 may be Si which is grown on the Si film 14. The optional buffer layer 16 forms a smooth, planar upper surface for further processing steps, thereby preventing any defects. As further shown in FIG. 2, a SiGe or SiGeC film 18 is formed on the buffer layer 16. In embodiments, the SiGe or SiGeC film 18 can be anywhere upwards of 4000 Å and in further embodiments can be about 5000 .ANG. The SiGe or SiGeC layer may be formed by conventional deposition methods such as, for example, low temperature epitaxy chemical vapor deposition (LTECVD). A Silicon (Si) epi layer 20 is formed on the SiGe or SiGeC film 18. The Ge and C composition can be in the range from any upwards to 40% and 5%, respectively.
FIG. 3 represents a selective etching process to form troughs in the structure of FIG. 2. In this representative processing, a reactive ion etching (RIE) is undertaken to form troughs 22 to the underlying buffer layer 16. This RIE process is provided so that in the final structure only selective devices are formed on the SiGe or SiGeC film 18.
In FIG. 4, a second Si epi layer 24 is formed over the entire structure. This process includes filling the troughs 22 with the Si epi layer 24.
In FIG. 5, a chemical mechanical polishing (CMP) is performed to planarize the structure. Also, in a conventional manner, isolation structures 26 are formed between the interfaces of the Si epi layer 24 and SiGe or SiGeC film 18. The isolation structures may include, for example, shallow trench isolation structures (STI), deep trench isolation structures (DT) and/or trench isolation structures (TI), any of which will eliminate the defects which may exist between the interfaces of the Si layer and SiGe or SiGeC film. In embodiments, the STI structures may be about 4000 Å in depth, the TI structure may be about 2.5 microns and the DT structures may be about six microns in depth, any of which depend on the specific heights applications.
FIGS. 6-13 show structures according to a second embodiment of the invention. In this implementation, the SiGe or SiGeC film is formed above the BOX.
FIG. 6 shows a starting structure comprising an optional BOX layer 12 formed on a silicon wafer 10. In embodiments, the BOX layer 12 is similar to that discussed with reference to FIG. 1. A silicon film 14 is formed on the BOX layer 12.
In FIG. 7, an Si epi layer 28 is formed on the Si film 14. The Si epi layer 28 is formed to make a smooth upper surface for further processing steps, thereby preventing any defects in the final device.
As further shown, in FIG. 8 a silicon dioxide or silicon nitride layer 30 is formed over the Si epi layer 28. The silicon dioxide or silicon nitride layer 30 is a hardmask, which is used for subsequent formation of troughs 22, The trough depth is preferably to the Si film 14. By way of one illustrative example, a conventional photolithographic process is then performed to form the trough etch openings 22, the hardmask is then removed from the opening as shown in the structure of FIG. 8. Then photoresist is stripped and troughs etch is conducted using remaining silicon dioxide or silicon nitride layer as the hardmask. The etch depth may be anywhere upwards of 4000 Å and in further embodiments can be about 5000 Å.
As shown in FIG. 9, an SiGe or SiGeC film 18 is grown in the troughs 22. A silicon film 32 is formed on the SiGe or SiGeC film 18 and the remaining structure. In embodiments, the silicon film 32 may be formed by chemical vapor deposition (CVD) processes well known to those of skill in the art. As one of skill in the art should further understand, the silicon film 32 will form a polysilicon 32a over the silicon dioxide or silicon nitride 30 since there is no seed to nucleate to form single crystal silicon. That is, the silicon will grow randomly forming the polysilicon 32a. In contrast, the silicon film 32 will remain in a single crystal form over the SiGe or SiGeC film 18.
As represented in FIG. 10, a chemical mechanical polishing (CMP) process is performed on the structure formed in FIG. 9. The CMP will remove the damaged areas of the polysilicon 32a and silicon film 32.
In FIG. 11, an oxidation process is performed on the structure. In this manner, an oxide layer (dielectric layer) 34 is formed over the structure. The oxidation process may be performed by any conventional oxidation growing process.
In FIG. 12, the oxide layer (dielectric layer) 34 is wet stripped using any well known stripping process. For example, the oxide layer (dielectric layer) 34 may be stripped using wet chemical processing.
In FIG. 13, after the wet strip process, in a conventional manner, isolation structures 26 are formed between the interfaces of the Silicon layer 28 and SiGe or SiGeC film 18. As discussed above, the isolation structures may include, for example, shallow trench isolation structures (STI), deep trench isolation structures (DT) and/or trench isolation structures (TI), any of which will be eliminate the defects which may exist between the interfaces of the Si layer 28 and SiGe or SiGeC film 18. In embodiments, the STI structures may be about 4000 Å, the TI structure may be about 2.5 microns and the DT structure may be about six microns, as discussed above. As further shown in FIG. 13, an Si epi layer 38 may be formed over the structure. In embodiments, the Si epi layer 38 may be anywhere upwards of about 200 to 300 Å, and more preferably about 50 to 200 nm.
FIGS. 14-22 show structures according a third embodiment of the invention. In this implementation, the SiGe or SiGeC film is formed below the BOX. FIG. 14 shows a starting structure comprising a silicon wafer 10.
In FIG. 15, a SiGe or SiGeC film 18 is formed on the silicon wafer 10. In embodiments, the SiGe or SiGeC film 18 is grown on the silicon wafer 10. A silicon layer 40 is grown on the SiGe or SiGeC film 18. The silicon layer 40 may be about 500 Å; although the silicon layer 40 may comprise different thicknesses as contemplated by the invention.
In FIG. 16, an oxide layer (dielectric layer) 42 is formed via an oxygen ion implantation process. The oxidation process may be performed by any conventional oxidation implantation process. For example, a conventional SIMOX activation process is contemplated by the invention. In this manner, oxygen ions are implanted into the Si layer to form a buried oxide layer. The ion implantation energy is in a range of about 100 Kev to 250 kev. The oxygen dose is in a range of about 5E16 to 1E18 cm−2. The operating temperature is in a range of about 500 to 600° C.
As shown in FIG. 17, a silicon dioxide or silicon nitride layer 44 is formed over the Si epi layer 40, which is worked as a hardmask. In a conventional manner, the hardmask is used for subsequent formation of troughs 22, preferably to the underlying substrate. By way of one illustrative example, a conventional photolithographic process is then performed over the hardmask layer to form the troughs 22. The dielectric is removed at the trough region to form the structure of FIG. 17. The etch depth may be anywhere upwards of 4000 Å and in further embodiments can be about 5000 Å.
In FIG. 18, a non-doped silicon film 50 is grown over the structure of FIG. 17, including within the troughs 22. In embodiments, the silicon film 50 may be formed by chemical vapor deposition (CVD) processes well known to those of skill in the art. As one of skill in the art should understand, the silicon film 50 will form a polysilicon 50a over the silicon dioxide or silicon nitride 44 since there is no seed to nucleate. That is, the silicon will grow randomly forming the polysilicon 50a. In contrast, the silicon film 50 will remain in a single crystal form over the SiGe or SiGeC film 18.
As represented in FIG. 19, a chemical mechanical polishing (CMP) process is performed on the structure. The CMP will remove the damaged areas of the polysilicon 50a and silicon film 50 down to the level of layer 44.
In FIG. 20, a reoxidation process forms an oxide layer (dielectric layer) 52 on the structure. The reoxidation process is a conventional process, well known to those of ordinary skill in the art such as discussed above. The oxide layer 52 is wet cleaned using conventional processes such as, for example, a DHF process using dilute HF or buffered HF (BHF). In embodiments, the wet cleaning process removes contaminants from the wafer surface in the liquid-phase. As is known to those of skill in the art, wet cleaning chemistries are selected to form soluble compounds of surface contaminants; often enhanced by megasonic agitation and followed by deionized water rinse and dry cycle, for example.
FIG. 21 shows a structure with smooth starting surface after wet clean process. In FIG. 22, in a conventional manner, isolation structures 26 (as discussed above) are formed between the interfaces of the Si and SiGe or SiGeC film. As discussed above, the isolation structures may include, for example, shallow trench isolation structures (STI), deep trench isolation structures (DT) and/or trench isolation structures (TI), any of which will be eliminate the defects which may exist between the interfaces of the Si layer and SiGe or SiGeC film.
FIG. 23 shows a schematic representation of a band alignment in accordance with an implementation of the invention. As schematically shown, due to the fact of the band gap barrier, electrons, for example, in a FET, find it more difficult to flow to the substrate due to the barrier provided by the SiGe (or SiGeC) doped areas of the structure. In this manner, the SiGe (or SiGeC) doped areas will direct the electrons into the source of the device which is normally grounded, blocking the electrons from flowing into the substrate. This will substantially decrease device leakage to the substrate or prevent the body current generated from other parts flow to this device.
FIG. 24 shows a graph depicting out diffusion of Boron in a structure implementing aspects of the invention. In this graph, the diffusion of Boron is significantly decreased over SiGe (or SiGeC) doped areas of the structure. More specifically, out diffusion of Boron significantly decreases at SiGe (and SiGeC) doped areas. In this manner, in scaled devices implementing the structures of the invention, where the source and drain are very close together (e.g., 0.1 micron or less), the boron will not out diffuse and cause shorting of the device.
The method as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.
By way of illustration, referring to FIGS. 25 and 26, final devices can be formed on the structures of the invention. For example, as shown in FIG. 25, a CMOS device may be formed on the above BOX structures described with reference to FIGS. 1-13. In FIG. 26, an HB NPN HBT structure 100 may be formed on the structures of the invention, as well as passive devices such as varactors and PIN diodes 200.
While the invention has been described in terms of exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modifications and in the spirit and scope of the appended claims.

## Claims

1. A method of forming an integrated structure, comprising:
etching troughs in a continuous layer of Si material formed over a substrate and hardmask that was formed over the Si material;
depositing in the troughs SiGe or SiGeC material to form alternating Si regions with SiGe or SiGeC regions;
forming a Si film over the Si material and the SiGe or SiGeC material, wherein the Si film forms polysilicon over the hardmask;
removing damaged portions of the Si layer and the polysilicon layer;
forming isolation structures at interfaces between the SiGe or SiGeC regions and the alternating Si regions;
forming a buried oxide layer over the substrate; and
forming an epitaxial layer over isolation structures, SiGe or SiGeC regions and Si material after an oxidation process.

2. The method of claim 1, wherein the buried oxide layer is formed one of under the SiGe or SiGeC regions and over the SiGe or SiGeC regions.

3. The method of claim 2, wherein the SiGe or SiGeC regions are non-doped, or n-typed doped or p-typed doped.

4. The method of claim 3, further comprising forming a first type device over the Si regions and a second type device over the SiGe or SiGeC regions.

5. The method of claim 1, wherein the isolation structures extend into the buried oxide layer.