Patent Application
20070238267
Crystalline alloys of silicon carbon (alone or possibly with other elements), e.g., Si1−yCy, y<0.1 where carbon is present as a substitutional solid solution (i.e., at lattice positions) is a very useful material for semiconductor device applications. For convenience, these alloys (optionally containing additional elements) will be referred to in this specification as “SC3S” materials. For example, SC3S can be employed for local strain engineering in semiconductor devices. SC3S can be also employed in bandgap engineering. In the strain engineering application, islands and/or layers of SC3S may be integrated (typically epitaxially) into a different crystalline material to beneficially alter the strain of that different crystalline material resulting in performance improvements of various semiconductor devices.
Local strain engineering techniques using lattice-mismatched crystalline stressors are particularly useful for sub-100 nm groundrule (nano-scale) advanced CMOS integrated circuits. There have been a number of proposals for different geometrical arrangements of SC3S structures for improving CMOS performance, e.g., U.S. Pat. No. 6,891,192, US20050082616, US20050104131, US20050130358, and Ernst et al., VLSI Symp., 2002, p 92; the disclosures of these references are incorporated herein by reference. A common geometrical feature of these structures is the localization of SC3S objects, e.g., as islands and/or layers of SC3S. There may be other applications for SC3S outside of CMOS integrated circuits, e.g., other types of integrated circuits and/or other applications outside the field of integrated circuits. These other applications may involve other geometries other than those mentioned above.
Regardless of the application, but especially in the context of integrated circuit devices and more especially in the context of CMOS integrated circuit devices, the challenge of SC3S is in its manufacture. The challenge is especially apparent in the context of epitaxial SC3S. The substitutional solid solubility of C in crystalline Si is extremely low. Therefore, it is very difficult to grow SC3S with high substitutional C concentration with traditional epitaxial (epi) growth techniques. This difficulty of producing SC3S is very fundamental in nature and is due to a large disparity between Si—Si and Si—C bonds in both binding energy and bond length. A substitutional C atom placed into Si crystalline lattice would highly distort the lattice causing a local increase in Gibbs free energy which, in turn, would limit incorporation of substitutional carbon into Si lattice in thermodynamic equilibrium.
Solid phase epitaxy (SPE) of amorphized silicon layers has been employed to “electrically activate” (place substitutionally into silicon lattice) implanted dopants such as B, As, P. Such SPE is carried out by annealing doped amorphous layers in a furnace at temperature of from 500° C. to 1300° C. for from 20 minutes to several hours or in a rapid thermal processor (RTP) from about 600° C. to 1200° C. for from 1 second to 180 seconds. Using these techniques to form SC3S is difficult. For example, attempts to produce SC3S by furnace-based SPE (e.g., 650° C., 30 minutes) have result in poor crystallinity and a low amount of carbon in substitutional lattice positions. In the case of attempts to make SC3S by RTP-based SPE (e.g., 1050° C., 5 sec), only about 0.2% substitutional carbon is incorporated into lattice. It has been generally accepted in the art that at high temperature only limited amount (less than about 1%) of substitutional carbon can be incorporated into silicon lattice.
Conventional low temperature non-localized epitaxial SC3S is extremely difficult in the low temperature range (Tepitaxy<700° C.) where less than 2% of substitutional carbon can be incorporated into the silicon lattice in a nonequilibrium state. Nonselective (ordinary) epitaxy process deposits crystalline, polycrystalline or amorphous material on entire substrate surface substantially impeding creation of localized structures from SC3S.
Accordingly, there is a continuous need for techniques suitable for fabricating localized structures of SC3S. Accordingly, there is a strong need for crystal-growing technique suitable for fabricating localized SC3S structures with substitutional C concentration in excess of 0.5% atomic percent and preferably in 1 to 4 atomic percent range.
The invention utilizes ultra-fast annealing techniques of amorphous silicon and carbon-containing material such that the material is preferably exposed to temperatures at or above the recrystallization temperature, but below the melting point of such material, for a relatively short period of time. In this manner, SC3S materials can be created in a variety of structural configurations as may be desired in electronics manufacture or for other purposes.
In one aspect, the invention encompasses a method of forming SC3S structures, the method comprising:
(a) providing substrate having an amorphous region containing silicon and carbon atoms, and
(b) ultra-fast annealing the amorphous region to crystallize the region whereby at least a portion of the carbon atoms occupy lattice positions in crystalline material resulting in the region.
Preferably, the amount of substitutional carbon is about 0.5 to 10 atomic percent. Preferably, the annealing comprises heating the amorphous region to an annealing temperature above the recrystallization temperature of the material, but below its melting point for a very short time (e.g., less than 100 milliseconds). Preferred ultrafast annealing techniques are laser annealing and flash annealing, more preferably with a millisecond-scale characteristic anneal time (e.g., from about 5 milliseconds to about 50 microseconds). The amorphous region is preferably created in situ by amorphizing implant of a silicon material followed by an implant of carbon atoms. The sequence of these implantation steps can be accomplished in any order. Alternatively, the amorphous region can also be deposited amorphous Si—C mixture by chemical or physical vapor deposition or deposited amorphous Si followed by C implantation. Other suitable methods of introducing carbon atoms may also be used.
The invention also encompasses methods for forming NFET structures where the methods comprise:
(a) providing a substrate having an NFET gate stack over a semiconductor channel and at least one amorphous source/drain region containing silicon and carbon proximate to the channel, and
(b) ultrafast annealing the amorphous region to crystallize the region whereby at least a portion of the carbon atoms occupy lattice positions in crystalline material resulting in the region.
Preferably, the method involves making both source/drains of the NFET as SC3S regions. The invention also encompasses the formation of CMOS transistor structures and other integrated circuit structures incorporating SC3S.
These and other aspects of the invention are described in further detail below.
FIGS. 1(a)-1(d) show cross section views of examples of some possible SC3S configurations according to the invention.
The invention is characterized in part by the use of ultra-fast annealing techniques to convert an amorphous silicon and carbon-containing material to SC3S. The ultra-fast anneal is preferably such that the amorphous material is quickly heated to a high temperature at which the recrystallization process occurs rapidly. The temperature is preferably at or more preferably well above the recrystallization temperature, but below the melting point of the material, only for a relatively short period of time. In preferred embodiments, the invention is further characterized by the creation of amorphous silicon- and carbon-containing material regions using (i) amorphization of a silicon-containing material on or in the substrate by implantation, followed by or preceded by (ii) implantation of carbon atoms into the amorphous region. These methods enable the obtaining of SC3S structures with high substitutional carbon concentration in a simple process at high anneal temperatures. The integration of SC3S structures into CMOS processes becomes very easy, especially using the preferred embodiments in part because amorphization, implantation, and solid phase epitaxy can be performed locally where desired.
The invention can be used to create SC3S materials in a variety of structural configurations as may be desired in electronics manufacture or for other purposes.
The invention encompasses a method of forming SC3S structures, the method comprising:
(a) providing substrate having an amorphous region containing silicon and carbon atoms, and
(b) ultra-fast annealing the amorphous region to crystallize the region whereby at least a portion of the carbon atoms occupy lattice positions in crystalline material resulting in the region.
The invention is not limited to any particular method for forming the amorphous silicon and carbon containing material. For example, the amorphous silicon and carbon containing material can be formed by chemical or physical vapor deposition or other known technique for forming amorphous silicon layers.
Preferably, however, the amorphous silicon and carbon containing layer is formed by providing a silicon or silicon-containing substrate material target region where the SC3S is desired, rendering the target region amorphous by an amorphizing ion implantation (also known as a pre-amorphizing implant or PAI), and implanting the desired amount of carbon into the target region. The carbon may be implanted before or after the amorphizing step.
The amorphizing implantation species may be selected from those known in the art. Preferably, the amorphizing implantation species is selected from the group consisting of Si, Ge, As, Xe, Ar, Sb, P or other ions to amorphize the target silicon substrate location(s) to appropriate depth. PAI can be accomplished with the aid of a mask. In the NFET embodiment, the mask can be a part of the existing structure such as gate spacer or can be formed immediately prior to the implantation step using known photolithography and etching techniques. Examples of some possible PAI conditions, where Ge or As are used as amorphizing atoms, are implant energy of about 10-60 KeV with a dose of about 3E13-4E15 cm−2. In some circumstances, the carbon to be implanted can create sufficient amorphization, especially if the implant temperature is reduced.
The carbon implant is preferably conducted with a dose of from 5E14 cm−2 to 5E16 cm−2 into the regions to achieve the desired carbon concentration. Preferably, the amount of carbon is sufficient to provide about 0.5 to 10 atomic percent carbon in the SC3S material, more preferably about 1 to 5 atomic percent carbon, most preferably about 1.2 to 4 atomic percent carbon. Dopants can also be implanted as needed or desired. If desired, the carbon may be implanted in only a part of amorphized region using an additional mask. Alternatively, if desired, the carbon concentration can be graded both vertically and/or laterally using well known implantation methods: such as by implanting a portion of carbon dose at higher energy and another portion of carbon dose at a lower energy; or implanting a portion of carbon dose at one tilt/twist implant angle set and another portion of carbon dose at another tilt/twist implant angle set.
In general, the SC3S lattice preferably contains at least about 80 atomic percent silicon in the lattice sites, more preferably at least about 90 atomic percent, most preferably about 95 to 99.5 atomic percent. The amount of dopant atoms (other than carbon or silicon) at lattice sites is preferably about 0 to 3 atomic percent. In addition, some of the lattice sites can be occupied by other elements such as germanium. If the silicon-containing material is an SiGe alloy, it preferably has a germanium content of about 50 atomic % or less, more preferably less than about 30 atomic %.
In general, it is preferable to avoid slow temperature ramp-up rate at or near the threshold temperature of recrystallization. Slow ramp-up rates will typically lead to recrystallization at lower temperature and generally to a product having little or no substitutional carbon remaining. Thus, the ramp-up from below the recrystallization temperature to the peak anneal temperature is preferably on the order of 50 nanoseconds to 10 milliseconds. The peak anneal temperature preferably is from 50° C. above the recrystallization temperature to just below the melting point for the material. The peak annealing temperature is preferably at least 900° C., more preferably at least 1100° C., most preferably about 1200-1350° C. The ultrafast anneal preferably has a limited duration in the target temperature regime i.e., the regime within about 100° C. below the peak temperature. Preferably, the duration is about 500 nanoseconds to 10 milliseconds, more preferably from about 0.5 microseconds to 1 millisecond, most preferably from about 5 microseconds to about 5 milliseconds.
Alternatively, the anneal duration can be measured at about full width half maximum (FWHM) of the heating energy pulse. For instance, a preferred anneal duration at measured FWHM of energy pulse is about 5 microseconds to 100 milliseconds, more preferably about 50 microseconds to 50 milliseconds, most preferably about 100 microseconds to 5 milliseconds. Preferably, the annealing is conducted in such a way that the amorphized regions are substantially fully recrystallized at or above about 900° C., more preferably above 1100° C., most preferably about 1100° C. to about 1300° C. Upon full recrystallization of the SC3S regions, the anneal can continue to a higher temperature, but preferably not above the Si melting point (1417° C.), more preferably not above 1390° C.
The energy in ultrafast anneal can be provided using any suitable method as long as the above annealing parameters are achieved. One useful example, the energy is delivered in the form of coherent optical radiation (i.e., laser radiation or a laser anneal). The laser source can operate in a pulsed or continuous wave (CW) mode. The laser beam can be shaped and polarized to allow for more uniform heating of the substrate. The lasing medium can be of different type (e.g. gas laser, solid state laser, dye laser, diode laser) yielding different wavelength of radiation. Further, if desired, one can add and shape extra layers on the wafer surface to aid energy coupling into the substrate. These auxiliary energy-coupling structures can be sacrificial or can be part of the substrate (e.g. printed in the substrate as part of the circuit layout). The invention is not limited to the type of laser, its mode of operation, its wavelength, the use of auxiliary energy-coupling structures, the laser beam shape, its polarization state, number of coherent sources used, coherence or absence thereof between multiple coherent laser source, and or other parameters of the laser anneal process as long as the amorphized regions are heated according to the time and temperature parameter values described above.
The energy for the ultrafast anneal can be also delivered in the form of incoherent radiation (lamp radiation). Such an anneal is referred to as the “flash anneal”. In another alternative, the energy in ultrafast anneal can be supplied via extremely hot gas jet (i.e., a jet anneal or torch anneal). Again, the exact method of coupling energy into the substrate is not so important to the instant invention as long as the amorphized regions are heated according to the time and temperature parameter values described above.
Once the recrystallization to for SC3S has occurred, it is preferred to quickly quench the system to freeze out the carbon atoms in the substitutional sites. In general, it is desirable to avoid excessive heating of the SC3S material and the entire wafer. If only a limited portion of the wafer is heated to the target temperature regime, the other portions of the wafer can dissipate the heat very quickly upon removal of the anneal energy. Thus, the desired quenching can be achieved in conventional tooling using this effect. Alternatively, it may be possible to introduce additional cooling measures to provide quenching. The cool down time should be as short as possible. The cool down time from the peak temperature (e.g., 1200-1350° C.) to 500° C. is preferably between 500 nanoseconds to 100 milliseconds.
A preferred ultra-fast anneal process and associated solid phase epitaxy (SPE) can be specified in terms of ramp-up and ramp-down rates above and below certain temperatures. A preferred ramp-up rate above about 500° C. is larger than about 10,000° C./second, more preferably from about 100,000° C./second to about 100,000,000° C./second, and most preferably from about to about 300,000° C./second to about 10,000,000° C./second. The preferred ramp-down rate from peak or target temperature to below about 500° C. is larger than about 5,000° C./second, more preferably from about 50,000° C./second to about 50,000,000° C./second, and most preferably from about to about 100,000° C./second to about 5,000,000° C./second.
The inventors conducted a series of experiments where SC3S was formed by laser annealing with temperature ramp-up rate of about million degrees per second and anneal duration at above about 1100° C. of less than about 300 microseconds. The anneal peak temperature was varied from about 1200° C. to about 1350° C. The SC3S samples were then analyzed using X-ray diffraction_(XRD) which showed that over 80% of the implanted carbon dose (1.8 atomic percent) was present as substitutional carbon (i.e., at lattice sites). XRD provides an accurate measure of the atomic spacing in the studied crystal. The amount of substitutional carbon was then inferred from the lattice spacing of the SC3S crystal. In this particular set of experiments, increasing peak anneal temperature from 1200° C. to 1350° C. did not result in any reduction of substitutional carbon because the anneal time was short enough preventing any “deactivation” of carbon atoms (i.e., migration from lattice to interstitial positions or forming silicon carbide compounds and clusters). In general, the invention preferably achieves an SC3S material where at least 60%, more preferably more than 80% of the implanted carbon dose is present as substitutional carbon.
Much shorter anneals with ramp-up rates higher than billion (1 e9) degrees per second and duration shorter than about 1 microsecond push silicon recrystallization threshold above silicon melt point leading to melting of amorphized areas. These conditions are generally not desired in the present invention.
The invention also encompasses methods for forming NFET structures where the methods comprise:
(a) providing a substrate having an NFET gate stack over a semiconductor channel and at least one amorphous source/drain region containing silicon and carbon proximate to the channel, and
(b) ultrafast annealing the amorphous region to crystallize the region whereby at least a portion of the carbon atoms occupy lattice positions in crystalline material resulting in the region.
The general discussion above applies to the formation of SC3S in the context of NFETs. Referring to
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The invention also encompasses the formation of CMOS transistor structures and other integrated circuit structures incorporating SC3S. In a simple example of CMOS process flow, a conventional CMOS process can be used to form gate stack, spacers and extension, halo and source/drain doping for both PFET and NFET.
While the invention has been illustrated in the context of NFETs and CMOS devices, it should be understood that the SC3S formation techniques of the invention may be used in other contexts where SC3S structures are desired.
