CMOS device with raised source and drain regions

A method of forming a semiconductor structure includes forming a PMOS device and an NMOS device. The step of forming the PMOS device includes forming a first gate stack on a semiconductor substrate; forming a first offset spacer on a sidewall of the first gate stack; forming a stressor in the semiconductor substrate using the first offset spacer as a mask; and epitaxially growing a first raised source/drain extension (LDD) region on the stressor. The step of forming the NMOS device includes forming a second gate stack on the semiconductor substrate; forming a second offset spacer on a sidewall of the second gate stack; epitaxially growing a second raised LDD region on the semiconductor substrate using the second offset spacer as a mask; and forming a deep source/drain region adjoining the second raised LDD region.

TECHNICAL FIELD

This invention relates generally to semiconductor devices, and more particularly to metal-oxide-semiconductor (MOS) devices with raised source and drain regions.

BACKGROUND

With the scaling of integrated circuits, metal-oxide-semiconductor (MOS) devices become increasingly smaller. The junction depths of the MOS devices are also reduced accordingly. This reduction causes technical difficulties during the formation processes. For example, small MOS devices demand higher dopant concentrations in source and drain regions in order to reduce resistivity in the source and drain regions. Controlling implantation depth for forming shallow junction in source and drain extension regions of small-scale devices is also difficult.

To solve the above-discussed problems, raised source and drain regions and/or raised lightly doped source and drain (LDD) regions have been formed.FIG. 1illustrates a commonly formed MOS device having raised source/drain regions. In its formation, a gate stack including a gate dielectric4and a gate electrode6are formed on substrate2. LDD regions8are then formed by implantation. Gate spacers10are then formed. An epitaxial growth is then performed to grow a crystalline silicon layer12on substrate2. Source and drain regions14are then formed by an implantation.

FIG. 2illustrates a MOS device with raised source and drain regions and raised LDD regions. A typical formation process includes forming offset spacers16on sidewalls of a gate stack including gate dielectric4and gate electrode6, epitaxially growing a first silicon layer18on substrate2, implanting impurities to form LDD regions8, forming main spacers10, epitaxially growing a second silicon layer20on first silicon layer18, and implanting impurities to form source and drain regions14.

In the conventional formation processes as shown inFIGS. 1 and 2, raised regions for PMOS and NMOS are typically formed simultaneously, and thus comprise the same materials. This process incurs several problems. First, since LDD regions are formed prior to the epitaxial growth, the epitaxial layers in PMOS and NMOS devices may have different thicknesses resulting from the different impurities in PMOS and NMOS devices. Second, epitaxial growth of silicon typically requires high temperatures, and thus excessive diffusion of dopant degrades short channel performance of the MOS devices. Further drawbacks include low activation rates and low solubilities (since impurities are implanted), and high silicide contact resistance, which results from the low activation rates and low solubilities of impurities.

What is needed in the art is a MOS device that may incorporate raised source and drain regions and/or LDD regions in order to take advantage of the benefits associated with improved MOS device performance while at the same time overcoming the deficiencies of the prior art.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method of forming a semiconductor structure includes forming a PMOS device in a PMOS region and forming an NMOS device in an NMOS region. The steps for forming the PMOS device include forming a first gate stack on a semiconductor substrate; forming a first offset spacer on a sidewall of the first gate stack; forming a stressor in the semiconductor substrate using the first offset spacer as a mask; and epitaxially growing a first raised source/drain extension region on the stressor, wherein the first raised source/drain extension region is in-situ doped with a first p-type dopant. The steps for forming the NMOS device include forming a second gate stack on the semiconductor substrate; forming a second offset spacer on a sidewall of the second gate stack; epitaxially growing a second raised source/drain extension region on the semiconductor substrate using the second offset spacer as a mask, wherein the second raised source/drain extension region is in-situ doped with a first n-type dopant; and forming a deep source/drain region adjoining the second raised source/drain extension region

In accordance with another aspect of the present invention, a method of forming a semiconductor structure includes providing a semiconductor substrate comprising a PMOS region and an NMOS region; forming a first gate stack in the PMOS region and a second gate stack in the NMOS region; forming a first offset spacer on a sidewall of the first gate stack; forming a second offset spacer on a sidewall of the second gate stack; epitaxially growing a first epitaxy region comprising silicon and substantially free from germanium on the semiconductor substrate, wherein the first epitaxy region comprises a first portion adjoining the first offset spacer; and a second portion adjoining the second offset spacer, and wherein the first epitaxy region is in-situ doped with a first n-type dopant; forming a recess adjacent the first offset spacer by removing the first epitaxy region in the PMOS region and etching into the semiconductor substrate; epitaxially growing a silicon germanium stressor in the recess; and epitaxially growing a second epitaxy region on the silicon germanium stressor, wherein the second epitaxy region has at least a portion higher than a top surface of the semiconductor substrate, and wherein the second epitaxy region is in-situ doped with a first p-type dopant.

In accordance with yet another aspect of the present invention, a semiconductor structure includes a semiconductor substrate comprising a PMOS region and an NMOS region; a PMOS device in the PMOS region; and an NMOS device in the NMOS region. The PMOS device includes a first gate stack on the semiconductor substrate; a first offset spacer on a sidewall of the first gate stack; a stressor in the semiconductor substrate and adjacent to the first offset spacer; and a first raised source/drain extension region on the stressor and adjoining the first offset spacer, wherein the first raised source/drain extension region has a higher p-type dopant concentration than the stressor. The NMOS device in the NMOS region includes a second gate stack on the semiconductor substrate; a second offset spacer on a sidewall of the second gate stack; a second raised source/drain extension region on the semiconductor substrate and adjoining the second offset spacer; and a deep source/drain region adjoining the second raised source/drain extension region, wherein the deep source/drain region is free from stressors formed in the semiconductor substrate.

In accordance with yet another aspect of the present invention, a semiconductor structure includes a semiconductor substrate comprising a PMOS device in a PMOS region and an NMOS device in an NMOS region. The PMOS device includes a first gate stack on the semiconductor substrate; a first offset spacer on a sidewall of the first gate stack; a stressor in the semiconductor substrate and having a portion under the first offset spacer; a first raised source/drain extension region on the stressor and adjoining the first offset spacer, wherein the first raised source/drain extension region has at least a portion higher than a top surface of the semiconductor substrate; and a first main spacer on a sidewall of the first offset spacer, wherein the first main spacer has at least a portion on a top surface of the first raised source/drain extension region. The NMOS device includes a second gate stack on the semiconductor substrate; a second offset spacer on a sidewall of the second gate stack; a second raised source/drain extension region on the semiconductor substrate and adjoining the second offset spacer; a second main spacer on a sidewall of the second offset spacer, wherein the second main spacer has at least a portion on a top surface of the second raised source/drain extension region; and a deep source/drain region adjoining the second raised source/drain extension region, wherein a portion of the deep source/drain region, that is lower than a top surface of the semiconductor substrate, is free from stressors.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Research results have revealed that solubilities of impurities in source and drain regions of metal-oxide-semiconductor (MOS) devices are related to the strain in the source and drain regions. Typically, p-type impurities, such as boron, have improved solubility under compressive strains. N-type impurities, such as arsenic, have improved solubility under tensile strains. However, further research results have revealed that the improvement in solubility of arsenic under tensile strain is significantly less than the improvement in solubility of boron under compressive strain.

Based on this finding, a method for forming MOS devices is provided. The intermediate stages of manufacturing an embodiment of the present invention, which combines the formation of a PMOS device and an NMOS device, are illustrated. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements.

Referring toFIG. 3, a substrate30, which includes an NMOS region100and a PMOS region200, is provided. Substrate30may comprise bulk silicon, although other commonly used structures and materials, such as silicon-on-insulator (SOI) structure and silicon alloys, can be used. Substrate30is preferably lightly doped.

A gate stack, including gate dielectric132and gate electrode134, is formed in NMOS region100. Another gate stack, including gate dielectric232and gate electrode234, is formed in PMOS region200. Each of the gate stacks may further include a mask layer (not shown) on respective gate electrodes134and234, wherein the mask layers may be formed of silicon nitride. Alternatively, gate electrodes134and234are formed of other commonly used conductive materials such as metals, metal silicides, metal nitrides, and combinations thereof. Gate dielectrics132and232preferably include commonly used dielectric materials such as oxides, nitrides, oxynitrides, carbides, and combinations thereof. Gate electrodes134and234may be formed of polysilicon. As is known in the art, gate dielectrics132and232and gate electrodes134and234may be formed by stacking a gate electrode layer on a gate dielectric layer, and then patterning the stacked layers.

FIG. 4illustrates the formation of offset spacers136and236and epitaxy regions138and238. Preferably, offset spacers136and236are thin spacers, with preferred thicknesses less than about 100 Å. The preferred materials include commonly used spacer material such as oxides including silicon oxide, silicon nitride, and combinations thereof. As is known in the art, the formation of offset spacers136and236may include forming a spacer layer, and then patterning the spacer layer to remove its horizontal portions.

Epitaxy regions138and238are formed on exposed surfaces of substrate30, preferably by selective epitaxial growth (SEG). Preferably, epitaxy regions138and238are formed of silicon. N-type impurities, such as arsenic and/or phosphorous, are preferably in-situ doped with the formation of epitaxy regions138and238. In an exemplary embodiment, the thickness of epitaxy regions138and238is between about 50 Å and about 200 Å. N-type impurities are preferably doped to a concentration of between about 5*1019/cm3and about 1021/cm3. Preferably, the temperature for the epitaxial growth is about 650° C. and about 850° C.

FIG. 5illustrates the formation of hard mask layer40, which includes a first portion in NMOS region100and a second portion in PMOS regions200. Hard mask layer40is preferably blanket formed. A photoresist142is then applied and patterned to cover NMOS region100. The second portion of hard mask40is then removed, followed by the removal of photoresist142.

Referring toFIG. 6, recesses244are formed along the edges of offset spacers236, preferably by etching anisotropically. In an exemplary embodiment formed using 90 nm technology, the preferred depth of recesses244is between about 500 Å and about 1000 Å, and more preferably between about 700 Å and 900 Å. It is appreciated, however, that the dimensions recited throughout the description are merely examples, and will scale accordingly with the scaling of the technology used in forming the integrated circuits.

After the formation of recesses244, an isotropic etching may be performed to extend recesses244under offset spacers236. In an embodiment, the isotropic etching uses HCI as a reaction gas, and is preferably performed at an elevated temperature, for example, higher than about 700° C. After the isotropic etching, recesses244preferably extend under offset spacers236for a distance D substantially equal to the thickness of offset spacers236.

FIG. 7illustrates the formation of epitaxy regions (often referred to as SiGe stressors), for example, by SEG. Preferably, SiGe stressors include SiGe regions246and overlying SiGe regions248. In an exemplary embodiment, SiGe regions246and248are formed using plasma enhanced chemical vapor deposition (PECVD) in a chamber. The preferred temperature is between about 500° C. and about 700° C., which is lower than the temperature for forming epitaxial silicon regions138and238. The precursors include Si-containing gases and Ge-containing gases, such as SiH4and GeH4, respectively, and the partial pressures of the Si-containing gases and Ge-containing gases are adjusted to modify the atomic ratio of germanium to silicon. The resulting SiGe regions246have a germanium atomic percentage of between about 10 atomic percent and about 50 atomic percent. In one embodiment, no p-type dopant is doped during the epitaxial growth of SiGe regions246. In alternative embodiments, p-type impurities, such as boron and/or indium, are in-situ doped to a low concentration, such as less than about 1018/cm3. A top surface of SiGe regions246is preferably level with a top surface of substrate30, and thus the subsequently formed SiGe regions248are raised regions. Alternatively, the top surfaces of SiGe regions246are higher than the top surface of substrate30.

After the formation of SiGe regions246, process conditions are changed to form SiGe regions248. Preferably, SiGe regions248are in-situ doped to a p-type dopant concentration of about 5*1019/cm3or greater. In an exemplary embodiment, in-situ doped p-type impurities in SiGe regions248are at least about two orders higher than in-situ doped p-type impurities in SiGe regions246, if SiGe regions246are in-situ doped. SiGe regions248preferably have a germanium atomic percentage of between about 10 atomic percent and about 50 atomic percent. After the formation of epitaxy regions, the remaining portion of mask layer40is removed.

FIG. 8illustrates the formation of main spacers150and250, which are preferably formed by blanket depositing gate spacer layer(s), and then removing horizontal portions of the gate spacer layer(s). The deposition may be performed using commonly used techniques, such as plasma enhanced chemical vapor deposition (PECVD), low-pressure chemical vapor deposition (LPCVD), sub-atmospheric chemical vapor deposition (SACVD), and the like. The patterning may be performed by either wet etching or dry etching. In the preferred embodiment, main spacers150and250include liner oxide portions and overlying nitride portions. In alternative embodiments, main spacers150and250include one or more layers, each comprising oxide, silicon nitride, silicon oxynitride (SiON) and/or other dielectric materials.

Deep implantations are then performed to form deep source and drain regions152and252(herein after referred to as source/drain regions). As is known in the art, to form deep source/drain regions, a photoresist (not shown) is formed to cover NMOS region100. An implantation is then preformed to introduce p-type impurities to form deep source/drain regions252. The photoresist is then removed. An additional photoresist (not shown) is formed to cover PMOS region200, and an implantation is preformed to introduce n-type impurities to form deep source/drain regions152. The additional photoresist is then removed.

It is noted that raised epitaxy regions138and248form portions of source and drain extension regions (also referred to as lightly doped source and drain regions, or LDD regions). In subsequent annealing processes, the impurities in raised epitaxy regions138and248are driven into underlying substrate30, hence extending LDD regions under respective offset spacers136and236.

FIG. 9illustrates the formation of silicide regions154and254. Throughout the description, germano-silicide regions254are also referred to as silicide regions254. As is known in the art, silicide regions154and254are preferably formed by blanket depositing a thin layer of metal, such as nickel, platinum, palladium, titanium, cobalt, and combinations thereof. The substrate is then heated, which causes silicon and germanium to react with the metal where contacted. After the reaction, a layer of metal silicide is formed between silicon (or silicon germanium) and metal. The un-reacted metal is selectively removed through the use of an etchant that attacks metal but does not attack silicide and germano-silicide.

In the embodiments discussed in preceding paragraphs, stressors are only formed for the PMOS device, but not for the NMOS device. This is due to the fact that the solubility improvement of n-type impurities from strain is relatively small, and thus may not justify the cost for forming stressors of NMOS devices. Silicon germanium stressors, however, are formed to maximize performance gain of PMOS devices.

The embodiments of the present invention have several advantageous features. First, the epitaxial growth of raised silicon regions, which needs high temperatures, is performed before the formation of LDD regions, including raised SiGe regions of PMOS devices. Therefore, the adverse effect to the LDD regions by high temperatures in the epitaxial growth of raised regions is reduced. The epitaxial growth of SiGe regions246and248, on the other hand, needs lower temperatures. Therefore, it can be performed after the formation of LDD regions. Second, LDD regions are formed by in-situ doping impurities. As is known in the art, in-situ doped impurities have higher solubilities and activation rates than implanted impurities. Therefore, higher solubilities and activation rates are achieved. Third, higher solubilities and activation rates of impurities also reduce the resistivity of subsequently formed silicide regions.