Strained semiconductor, devices and systems and methods of formation

In various method embodiments, a device region is defined in a semiconductor substrate and isolation regions are defined adjacent to the device region. The device region has a channel region, and the isolation regions have volumes. The volumes of the isolation regions are adjusted to provide the channel region with a desired strain. In various embodiments, adjusting the volumes of the isolation regions includes transforming the isolation regions from a crystalline region to an amorphous region to expand the volumes of the isolation regions and provide the channel region with a desired compressive strain. In various embodiments, adjusting the volumes of the isolation regions includes transforming the isolation regions from an amorphous region to a crystalline region to contract the volumes of the isolation regions to provide the channel region with a desired tensile strain. Other aspects and embodiments are provided herein.

TECHNICAL FIELD

This disclosure relates generally to semiconductor devices, and more particularly, to strained semiconductor, devices and systems, and methods of forming the strained semiconductor, devices and systems.

BACKGROUND

The semiconductor industry continues to strive for improvements in the speed and performance of semiconductor devices. Strained silicon technology has been shown to enhance carrier mobility in both n-channel and p-channel devices, and thus has been of interest to the semiconductor industry as a means to improve device speed and performance. Currently, strained silicon layers are used to increase electron mobility in n-channel CMOS transistors. There has been research and development activity to increase the hole mobility of p-channel CMOS transistors using strained silicon germanium layers on silicon.

FIG. 1Aillustrates a known device for improved hole mobility with an n-type silicon substrate101, a silicon germanium layer102, a silicon capping layer103, a gate oxide104, a gate105, and N+ source/drain regions106and107.FIG. 1Billustrates a band structure for the device ofFIG. 1A, and indicates that some carriers or holes are at the silicon-oxide interface and some are confined in the silicon germanium layer. Both the silicon germanium and the silicon capping layers will be strained if they are thin. Alternatively, the silicon germanium layer may be graded to a relaxed or unstrained layer resulting in more stress in the silicon cap layer. The crystalline silicon layer is strained by a lattice mismatch between the silicon germanium layer and the crystalline silicon layer.

More recently, strained silicon layers have been fabricated on thicker relaxed silicon germanium layers to improve the mobility of electrons in NMOS transistors. Structures with strained silicon on silicon germanium on insulators have been described as well as structures with strained silicon over a localized oxide insulator region. These structures yield high mobility and high performance transistors on a low capacitance insulating substrate.

Wafer bending has been used to investigate the effect of strain on mobility and distinguish between the effects of biaxial stress and uniaxial stress. Bonding a semiconductor onto bowed or bent substrates has been disclosed to introduce strain in the semiconductor. Stress can also be introduced by wafer bonding. Packaging can introduce mechanical stress by bending. Compressively-strained semiconductor layers have been bonded to a substrate.

FIGS. 2-4illustrate some known techniques to strain channels and improve carrier mobilities in CMOS devices.FIG. 2illustrates a known device design to improve electron mobility in NMOS transistors using a tensile strained silicon layer on silicon germanium. As illustrated, a graded silicon germanium layer208is formed on a p-type silicon substrate209to provide a relaxed silicon germanium region210, upon which a strained silicon layer211is grown. The transistor channel is formed in the strained silicon layer211. There is a large mismatch in the cell structure between the silicon and silicon germanium layers, which biaxially strains the silicon layer. The biaxial strain modifies the band structure and enhances carrier transport in the silicon layer. In an electron inversion layer, the subband splitting is larger in strained silicon because of the strain-induced band splitting in addition to that provided by quantum confinement. As illustrated inFIG. 3, uniaxial compressive stress can be introduced in a channel312of a PMOS transistor to improve hole mobility using silicon germanium source/drain regions313in trenches adjacent to the PMOS transistor. Large improvements in hole mobility, up to 50%, have been made in PMOS devices in silicon technology using strained silicon germanium source/drain regions to compressively strain the transistor channel. Silicon-carbon source/drain regions in trenches adjacent to an NMOS transistor can introduce tensile stress and improve electron mobility.FIG. 4illustrates a known device design to improve mobility for both NMOS and PMOS transistors using silicon nitride capping layers414. These silicon nitride capping layers can be formed to introduce tensile stress for NMOS transistors and can be formed to introduce compressive stress for PMOS transistors.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawings which show, by way of illustration, specific aspects and embodiments in which the present subject matter may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present subject matter. The various embodiments of the present subject matter are not necessarily mutually exclusive as aspects of one embodiment can be combined with aspects of another embodiment. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present subject matter. In the following description, the terms “wafer” and “substrate” are interchangeably used to refer generally to any structure on which integrated circuits are formed, and also to such structures during various stages of integrated circuit fabrication. Both terms include doped and undoped semiconductors, epitaxial layers of a semiconductor on a supporting semiconductor or insulating material, combinations of such layers, as well as other such structures that are known in the art. The term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizontal as defined above. Prepositions, such as “on”, “side”, “higher”, “lower”, “over” and “under” are defined with respect to the conventional plane or surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

Disclosed herein, among other things, is a process to adjust a volume of isolation regions to provide device channels regions with a desired strain. Thus, the isolation region volume is expanded to provide a compressive strain for p-channel device, and the isolation region volume is reduced or contracted to provide a tensile strain for n-channel devices. According to various embodiments to improve hole mobility, isolation regions adjacent to the source/drain regions are implanted to provide compressive stress for channels of PMOS transistors. If the isolation regions are crystalline, the implant amorphizes the isolation region, thus expanding the isolation regions. According to various embodiments to improve electron mobility, isolation region trenches are filled with amorphous semiconductor (e.g. amorphous silicon), and then recrystallized to contract the isolation regions and provide tensile strain to an NMOS transistor.

FIGS. 5A-5Cillustrate a process for fabricating a transistor with a channel under compressive strain, according to various embodiments of the present subject matter.FIG. 5Aillustrates a substrate515, and a mask516formed over the substrate to define isolation regions517. The illustrated substrate is a crystalline semiconductor, such as a crystalline silicon wafer. An implant, illustrated by arrows518, is performed. The implant functions to transform the crystalline isolation regions into amorphous isolation regions. This process is sometimes referred to herein as amorphizing the substrate. According to various embodiments, the implant includes silicon ions implanted into a silicon substrate. Some embodiments implant the substrate with inert gas atoms. The amorphous isolation regions have a lower density than the crystalline isolation regions. After the implant, there are more atoms in the original volume or space, which causes the volume of the implanted isolation region517to expand, as illustrated by the arrows519inFIG. 5B. The expanding isolation regions push against the adjacent crystalline regions, in which a device channel is to be formed. Thus, the expanding isolation regions compress the adjacent crystalline isolation regions.

FIG. 5Cillustrates a PMOS transistor520with a compressed p-channel521to improve hole mobility, according to various embodiments. The illustrated device520includes a first source/drain region522and a second source/drain region523that define the channel region521in the crystalline substrate. A gate524is separated from the channel region521by a gate insulator525. The implanted isolation regions517cause the crystalline substrate upon which the PMOS transistor is formed to be under compressive stress, as illustrated by the arrows519.

FIGS. 6A-6Eillustrate a process for fabricating a transistor with a channel under tensile strain, according to various embodiments of the present subject matter.FIG. 6Aillustrates a substrate626, and trenched isolation regions627. The illustrated substrate is a crystalline semiconductor, such as a crystalline silicon wafer. The trench can be etched using conventional techniques. As illustrated inFIG. 6B, a native oxide628(e.g. silicon oxide on a silicon wafer) forms after the trenches are formed due to air exposure. The oxide is selectively removed from the substrate surface. For example, the native oxide is left on one side of the trench. As will be described in more detail below, the other side of the trench will serve to seed a recrystallization process. However, recrystallization will not be initiated on surfaces with the native oxide. As illustrated inFIG. 6C, an amorphous semiconductor629is deposited to fill the trenches. Where the substrate is a crystalline silicon wafer, for example, an amorphous silicon can be deposited. The resulting structure is planarized, such as by a chemical mechanical polishing (CMP) process, to the level of the original substrate surface, as illustrated inFIG. 6D. The resulting structure is heat treated to recrystallize the amorphous semiconductor629in the isolation trenches. Recrystallization will proceed only from the surfaces without native oxide. The recrystallization process is illustrated by arrows630. The density of amorphous semiconductor is lower than crystalline semiconductor. During recrystallization the crystalline volume is not sufficient to fill the trenches, which places the adjoining crystalline semiconductor regions under tensile stress.

FIG. 6Eillustrates an NMOS transistor631with a tensile strained n-channel632to improve electron mobility, according to various embodiments. The illustrated device631includes a first source/drain region633and a second source/drain region634that define the channel region632in the crystalline substrate. A gate635is separated from the channel region632by a gate insulator636. The contracted, recrystallized isolation regions637cause the crystalline substrate upon which the NMOS transistor is formed to be under tensile stress, as illustrated by the arrows638. With proper modeling there can be a large tensile strain in the direction of the transistor channel of a NMOS transistor.

FIGS. 5C and 6Eillustrate PMOS and NMOS, respectively. One of ordinary skill in the art would understand how to provide strained channels for other p-channel and n-channel devices, including non-volatile memories such as floating gate devices.

Thus, it has been illustrated how the adjusted volume of isolation regions can provide the desired compressive or tensile strain in the channel direction, referred to herein as the x-direction. There are also stresses applied in the direction into the paper (referred to herein as the y-direction) and the vertical direction (referred to herein as the z-direction).FIGS. 7A-7Cillustrate forces associated with an expanding isolation region; andFIGS. 8A-8Cillustrate forces associated with a contracting isolation region.

FIGS. 7A-7Cillustrate an expanding isolation region volume and corresponding stresses, including a compressive stress, in adjacent channel regions, according to various embodiments of the present subject matter. With respect to the expanding isolation regions740, the volume tends to grow in all directions (x, y and z), as illustrated inFIG. 7A. The corresponding compressive forces in the x-direction (the channel direction) are illustrated inFIGS. 7B and 7Cby arrows741. However, as the volume740expands, the volume pulls vertically on the surrounding crystalline regions, resulting in a vertically-oriented tensile strain, illustrated inFIG. 7Bby arrows742. Additionally, the expanding volume pulls on the surrounding crystalline regions in the y-direction too, resulting in a corresponding tensile strain743illustrated inFIG. 7C.

The tensile strain743in the y-direction can be avoided by having the amorphous isolation regions740constrained in the y-direction. One example for constraining the expanding isolation region includes limiting the extent of the isolation trenches in the y-direction. For example, the area to be implanted is masked in both the x-direction and y-direction to delineate the area in which the implant is to take place and therefore delineate the x and y regions to be strained. In the z-direction the implant location is controlled by the implant energy, as the locus of the implanted species in the silicon is deeper with higher implant energies. In the z-direction, the edges of the implanted region are constrained by the un-implanted material. The implanted material wishes to move vertically and the un-implanted material does not. At the interface between the implanted and unimplanted material, the implanted material is under compression and the un-implanted material is in tension742. With proper modeling, there can still be a large compressive strain in the direction of the transistor channel of a PMOS transistor.

The tensile strain in the y-direction and z-direction does not negatively affect hole mobility when the structure is compressively strained in the x-direction to improve hole mobility. The channel area has a x, y and z dimension. The diffusions can be formed before or after the channel is strained. If the diffusions are in place prior to the introduction of the stress in the channel, the structure can be designed to provide as near as possible a uniform strain throughout the channel. The implanting and activating of the diffusions can change the strain locus if the diffusions are implanted and activated after the channel is strained.

FIGS. 8A-8Cillustrate a contracting isolation region volume and corresponding stresses, including a tensile stress, in adjacent channel regions, according to various embodiments of the present subject matter. With respect to the contracting isolation regions845, the volume tends to contract in all directions (x, y and z), as illustrated inFIG. 8A. The corresponding tensile forces in the x-direction (the channel direction) are illustrated inFIGS. 7B and 7Cby arrows846. However, as the volume845contracts, the volume pulls vertically on the surrounding crystalline regions, resulting in a vertically-oriented compressive strain, illustrated inFIG. 8Bby arrows847. Additionally, the contracting volume pulls on the surrounding crystalline regions in the y-direction too, resulting in a corresponding compressive strain848illustrated inFIG. 8C.

There is some residual compressive stress in the y-direction and z-direction if the isolation regions are recrystallized to provide tensile strain in the x-direction, but this does not negatively affect electron mobility enhancement. The expanding or contracting volume imposes an equal and opposite strain on each side of the interface in all three directions. If an isotropic material is implanted, such that there is an equal distribution of the implanted species in all areas of a cube, then the cube will wish to grow an equal amount in all directions (x, y, z). However, if the cube is constrained in one direction, the stress in each direction will be the same but the growth (strain) in the constrained direction will be less.

FIG. 9illustrates a top view of device regions under compressive stress due to expanded isolation regions. Device regions950, including device channel regions, are illustrated in a substrate. Isolation regions951define the device regions950. As illustrated inFIG. 9, as the isolation regions expand, due to an implant in a crystalline semiconductor for example, the device regions are compressively strained. The expanding isolation regions can be defined to provide uniaxial compressive strain or biaxial compressive strain.

FIG. 10illustrates a top view of device regions under tensile stress due to contracted isolation regions. Device regions1052, including device channel regions, are illustrated in a substrate. Isolation regions1053define the device regions1052. As illustrated inFIG. 10, as the isolation regions contract, due to recrystallization of an amorphous semiconductor for example, the device regions have a tensile strain. The contracting isolation regions can be defined to provide uniaxial tensile strain or biaxial tensile strain.

The isolation regions can be appropriately defined to provide a desired strain when the volume of the isolation regions are adjusted. Thus, for example, various embodiments adjust the volumes of isolation regions on a first side and on an opposing second side of the device region to provide a predominantly uniaxial strain. Various embodiments adjust volumes of isolation regions surrounding the device region to provide a predominantly biaxial strain.

FIG. 11illustrates a method for forming a device with a strained channel, according to various embodiments of the present subject matter. At1154, device channel regions and isolation regions are defined. The volume of the isolation regions are adjusted at1155. For example, the isolation region can be expanded by implanting ions into a crystalline region. Various embodiments implant silicon ions or inert gases into a crystalline silicon substrate to transform the crystalline isolation regions into an expanded, amorphous isolation region. In another example, the isolation region can be retracted by recrystallizing an amorphous semiconductor. The adjusted volume induces a desired strain in the adjacent device channel regions. A compressive strain to improve hole mobility for a PMOS transistor can be induced by expanding the adjacent isolation regions. A tensile strain to improve electron mobility for an NMOS transistor can be induced by contracting the adjacent isolation regions. At1156, devices are formed using the strained channel regions. P-channel devices, such as a PMOS transistor, are formed using the compressive strained channel regions, and N-channel devices, such as a NMOS transistor, are formed using the tensile strained channel regions. In various embodiments, the devices are formed before the volumes of the isolation regions are adjusted to induce the strain in the channel regions.

FIG. 12illustrates a method for forming p-channel and n-channel devices with appropriately strained channels, according to various embodiments of the present subject matter. Those of ordinary skill in the art, upon reading and comprehending this disclosure, will understand that the disclosed methods for straining semiconductor can be used in CMOS technology. Appropriate masking of isolation regions adjacent to NMOS devices and isolation regions adjacent to PMOS devices can be used to selectively expand the isolation regions adjacent to the PMOS channels to compressively strain the PMOS channels and improve hole mobility, and to selectively contract the isolation regions adjacent to the NMOS channels to tensile strain the NMOS channels and improve electron mobility. At1257, device channel regions and isolation regions are defined, and devices are formed using the strained channel regions. At1258, the volume of isolation regions is adjusted to provide the desired strain for adjacent channel regions. Devices are formed using the strained channel regions. For p-channel transistors, the volume of isolation regions are expanded at1259to provide the desired compressive strain for adjacent p-channel regions. For n-channel transistors, the volume of the isolation regions are contracted at1260to provide the desired tensile strain for adjacent n-channel regions. P-channel transistors are formed using the compressed channel regions at1261, and n-channel transistors are formed using the tensile strained channel regions at1262.

According to various embodiments, the process to provide a desired compressive strain for a p-channel device includes engineering the process to induce a compressive strain within a range of approximately 0.2% and 1.0%. According to various embodiments, the process to provide a desired tensile strain for an n-channel device includes engineering the process to induce a tensile strain greater than approximately 0.5%. For example, various embodiments provide a tensile strain within a range of approximately 0.75% to approximately 1.5%. It is also desirable to reduce unnecessary strain and provide a margin for error without unduly affecting the mobility enhancement. Thus, it is desirable to provide a tensile strain in the range of approximately 1% to approximately 1.2%.

The strain level is controlled by the size of the implanted region and the implant dose and the implant species. Larger implant regions correspond to larger strains, higher implant doses correspond to larger strains, and larger implant species correspond to larger strains. Other factors, such as the minimum photo dimension, are likely to determine the volume of the implanted region; thus, the implant dose and species are likely to be used to engineer the strain. The implant energy is controlled to determine the depth of the maximum stressed area. Various embodiments use multiple implant energies to increase the vertical dimension in which the maximum strain is achieved. The implant dose and species also increase the strain. The volume of the recrystallized material controls the level of tensile strain.

FIG. 13illustrates a method for expanding isolation regions to provide compressive stress in adjacent semiconductor regions. The illustrated method1359is an embodiment of a process for expanding the volume of isolation regions, such as is illustrated at1259inFIG. 12. At1363, the substrate (e.g. crystalline silicon substrate) is appropriately masked to expose semiconductor isolation regions. Ions are implanted at1364to amorphize and expand the semiconductor isolation regions. Various embodiments implant silicon ions into a crystalline silicon substrate to expand the isolation regions. Various embodiments implant an inert gas to expand the isolation regions. At1365, the structure is planarized in preparation to form devices with compressed p-channels

FIG. 14illustrates a method for contracting isolation regions to provide tensile stress in adjacent semiconductor regions. The illustrated method1460is an embodiment of a process for reducing or contracting the volume of isolation regions, such as is illustrated at1260inFIG. 12. At1466, trenches are formed in semiconductor isolation regions. For example, a mask can be formed to expose the semiconductor isolation regions, and these regions can be etched to form the trenches. At1467, native oxide (e.g. silicon oxide on a silicon substrate) that forms from air exposure is selectively removed to expose a side of the trench for use in seeding a recrystallization process. At1468, the trenches are formed with an amorphous semiconductor. In various embodiments, the amorphous semiconductor is of the same type as the semiconductor substrate, such that an amorphous silicon is deposited to fill trenches formed in a silicon wafer, for example. The resulting structure is planarized at1469, and heat treated at1470to recrystallize the amorphous semiconductor. The crystal growth is seeded by the crystalline semiconductor where the native oxide has been removed. This transformation of this volume from an amorphous volume into a crystalline volume reduces or contracts the volume.

FIG. 15is a simplified block diagram of a high-level organization of various embodiments of a memory device according to various embodiments of the present subject matter. The illustrated memory device1571includes a memory array1572and read/write control circuitry1573to perform operations on the memory array via communication line(s) or channel(s)1574. The illustrated memory device1571may be a memory card or a memory module such as a single inline memory module (SIMM) and dual inline memory module (DIMM). One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that semiconductor components in the memory array and/or the control circuitry are able to be fabricated using the strained semiconductor, as described above. For example, in various embodiments, the memory array and/or the control circuitry include p-channel transistors with compressively-strained channels for improved hole mobility and/or n-channel transistors with tensile-strained channels for improved electron mobility. The structure and fabrication methods for these devices have been described above.

The illustrated memory array1572includes a number of memory cells1575arranged in rows and columns, where word lines1576connect the memory cells in the rows and bit lines1577connect the memory cells in the columns. The read/write control circuitry1573includes word line select circuitry1578, which functions to select a desired row. The read/write control circuitry1573further includes bit line select circuitry1579, which functions to select a desired column. The read/write control circuitry1573further includes read circuitry1580, which functions to detect a memory state for a selected memory cell in the memory array1572.

FIG. 16illustrates a diagram for an electronic system having one or more transistors with strained channels for improved mobility, according to various embodiments of the present subject matter. Electronic system1681includes a controller1682, a bus1683, and an electronic device1684, where the bus1683provides communication channels between the controller1682and the electronic device1684. In various embodiments, the controller and/or electronic device include p-channel transistors with compressively-strained channels and/or n-channel transistors with tensile-strained channels as previously discussed herein. The illustrated electronic system1681may include, but is not limited to, information handling devices, wireless systems, telecommunication systems, fiber optic systems, electro-optic systems, and computers.

FIG. 17illustrates an embodiment of a system1785having a controller1786and a memory1787, according to various embodiments of the present subject matter. The controller1786and/or memory1787may include p-channel transistors with compressively-strained channels and/or n-channel transistors with tensile-strained channels fabricated according to various embodiments. The illustrated system1785also includes an electronic apparatus1788and a bus1789to provide communication channel(s) between the controller and the electronic apparatus, and between the controller and the memory. The bus may include an address, a data bus, and a control bus, each independently configured; or may use common communication channels to provide address, data, and/or control, the use of which is regulated by the controller. In an embodiment, the electronic apparatus1788may be additional memory configured similar to memory1787. An embodiment may include a peripheral device or devices1790coupled to the bus1789. Peripheral devices may include displays, additional storage memory, or other control devices that may operate in conjunction with the controller and/or the memory. In an embodiment, the controller is a processor. Any of the controller1786, the memory1787, the electronic apparatus1788, and the peripheral devices1790may include p-channel transistors with compressively-strained channels and/or n-channel transistors with tensile-strained channels formed according to various embodiments. The system1785may include, but is not limited to, information handling devices, telecommunication systems, and computers. Applications containing strained semiconductor films, such as p-channel transistors with compressively-strained channels, as described in this disclosure include electronic systems for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems, such as cameras, video recorders and players, televisions, displays, games, phones, clocks, personal computers, wireless devices, automobiles, aircrafts, industrial control systems, and others.

The memory may be realized as a memory device containing p-channel transistors with compressively-strained channels formed according to various embodiments. It will be understood that embodiments are equally applicable to any size and type of memory circuit and are not intended to be limited to a particular type of memory device. Memory types include a DRAM, SRAM (Static Random Access Memory) or Flash memories. Additionally, the DRAM could be a synchronous DRAM commonly referred to as SGRAM (Synchronous Graphics Random Access Memory), SDRAM (Synchronous Dynamic Random Access Memory), SDRAM II, and DDR SDRAM (Double Data Rate SDRAM).

This disclosure includes several processes, circuit diagrams, and semiconductor structures. The present subject matter is not limited to a particular process order or logical arrangement. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover adaptations or variations of the present subject matter. It is to be understood that the above description is intended to be illustrative, and not restrictive. Combinations of the above embodiments, and other embodiments, will be apparent to those of skill in the art upon reviewing the above description. The scope of the present subject matter should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.