Abstract:
A circuit including at least one strained channel, enhancement mode FET, and at least one strained channel, depletion mode FET. The depletion mode FET includes an ion implanted dopant supply. In exemplary embodiments, the FETs are surface channel or buried channel MOSFETS. In another exemplary embodiment, the FETs are interconnected to form an inverter.

Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 60/207,382 filed May 26, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to the field of buried channel strained-Si FETs, and in particular to these FETs using a supply layer created through ion implantation. 
     The advent of relaxed SiGe alloys on Si substrates introduces a platform for the construction of new Si-based devices. These devices have the potential for wide application due to the low cost of using a Si-based technology, as well as the increased carrier mobility in strained layers deposited on the relaxed SiGe. 
     As with most new technologies, implementing these advances in a Si CMOS fabrication facility requires additional innovation. For example, some of the potential new devices are more easily integrated into current Si processes than other devices. Since process technology is directly relevant to architecture, particular innovations in process technology can allow the economic fabrication of new applications/architectures. 
     FIGS. 1A and 1B are schematic block diagrams showing the variety of strained Si devices that are possible to fabricate given the advent of relaxed SiGe buffer layers. FIG. 1A shows a surface channel strained Si MOSFET  100 . In this configuration, a tensile, strained Si channel  102  is deposited on relaxed SiGe layer  104  with a Ge concentration in the range of 10-50%. This relaxed SiGe layer is formed on a Si substrate  108  through the use of a compositionally graded SiGe buffer layer  106 . A conventional MOS gate stack  110  is on the strained silicon channel and consists of an oxide layer  112 , a poly-Si electrode  114 , and a metal contact layer  116 . Doped source  118  and drain  120  regions are also formed on either side of the gate stack to produce the MOSFET device structure. 
     A buried channel strained Si high electron mobility transistor (HEMT)  130  is shown in FIG.  1 B. In this configuration, the strained Si  102  atop the relaxed SiGe  104  has been capped with a thin SiGe cap layer  132 . The strained Si layer generally has a thickness between 2-30 nm, while the SiGe cap layer has a thickness between 2-20 nm. A metal Schottky gate  134  on the SiGe cap layer is commonly used on the HEMT, and, as in the MOSFET structure, doped source  118  and drain  120  regions are formed on each side of this gate. 
     FIG. 1C shows a buried channel strained Si MOSFET  140 . This device has the same Si/SiGe layer structure as the HEMT configuration, but with a full MOS gate stack  142 , consisting of oxide  144 , poly-Si  146 , and metal  148  layers, rather than the metal Schottky gate. 
     It is important to separate these devices into two categories, surface channel devices, of which an embodiment is shown in FIG. 1A, and buried channel devices, of which embodiments are shown in FIGS. 1B and 1C. In the case of the surface channel device, a light background doping in the SiGe during epitaxial growth or by implantation is sufficient to position the Fermi level such that a MOSFET constructed from the strained surface channel has reasonably large threshold values. Thus, the surface can be inverted for either p or n channel operation. 
     FIGS. 2A and 2B are the energy band diagram for the case of the surface channel FET for an NMOS device, (A) at zero bias, and (B) at a bias to turn on the transistor, respectively. When the transistor is turned on, a relatively large electric field exists in the normal direction to the surface plane, and the electrons are attracted to the surface and operate in the strained Si surface channel. The speed of the transistor is increased due to the fact that the electrons reside in the high mobility, strained Si surface channel. However, the device has noise performance similar to a conventional Si MOSFET since the carriers scatter off the SiO 2 /Si interface, and the device, although it possesses a mobility larger than that of a conventional Si device, still has a mobility that is limited by the SiO 2 /Si interface. 
     However, it is known from III-V materials that a buried channel device should possess a much higher electron mobility and lower noise performance. For example, the structures shown in FIG. 1B and C should have higher channel mobility and lower noise performance than the device in FIG. 1A since the electron scatters off a semiconductor interface instead of an oxide interface. 
     A crucial flaw in the device shown in FIG. 1C that leads to processing difficulties and limitations in circuit layout and architectures is that when the device is biased to invert the channel and turn the device on, the band structure is such that many of the carriers leave the buried channel. FIG. 3 is an energy band diagram showing schematically the problem with a buried channel device in which there is no dopant supply layer. The field required to turn on the device empties the buried channel. This effectively creates a surface channel device even though the buried channel layer is present in the heterostructure. 
     The applied gate bias of FIG. 3 has bent the bands such that many of the electrons from the well escape confinement and create an inversion layer at the oxide/semiconductor interface. Since transconductance of a field effect device is high if the mobility and the number of carriers is high, a high performance FET, i.e., even higher performance than the surface channel device, is difficult to achieve. At low vertical fields, the electrons are in the high mobility buried channel, but there are few in number. If the device is turned on and inverted as shown in FIG. 3, the carrier density in the surface channel is high but the mobility is reduced since the carriers are now at the rough oxide interface. 
     One way to solve this problem is to insert a dopant supply layer into the structure, as shown in FIG.  4 A. FIG. 4A is a schematic block diagram of a structure  400  in which the buried channel can be occupied with a high density of electrons via the insertion of a layer of donor atoms. It will be appreciated that an equivalent schematic can be constructed for a buried hole channel with a layer of acceptor atoms. 
     The structure  400  includes a strained Si channel  402  positioned between two SiGe layers, a relaxed SiGe layer  404  and a thin SiGe cap layer  406 . Although FIG. 4A shows a dopant supply layer  408  in the SiGe cap, the dopants can be introduced into either SiGe layer. As has been shown in the III-V buried channel devices, this layer configuration creates a band structure where now the buried channel is occupied, as shown in FIG.  4 B. In this figure, the supply layer leads to localized band bending and carrier population of the buried strained Si. In the strained Si, the conduction band has been lowered beneath the Fermi level, resulting in a high carrier density in the high mobility channel. One disadvantage of this structure is that now the transistor is on without any applied voltage, and a voltage is supplied to the gate to turn off the transistor. Thus, this transistor is normally on or depletion-mode. As a result, the device is useful in analog and logic applications, but is not easily implemented in a conventional CMOS architecture. 
     Common accepted practice in the buried channel heterostructure FETs is to use a dopant supply layer that is introduced in an epitaxial step, i.e., deposited during the epitaxial process that creates the Si/SiGe device structure. This dominant process originates from the III-V research device community (AlGaAs/GaAs materials system). However, this epitaxial dopant supply layer is undesirable since it reduces thermal budget and limits the variety of devices available in the circuit. For example, if the dopant supply layer is introduced in the epitaxial step, when processing begins, the thermal budget is already constrained due to diffusion of the supply layer dopants. All devices in the circuit must also now be buried channel devices with similar thresholds, since any removal of the dopant layer in a particular region would require complete etching of the local area and removal of critical device regions. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a device structure that allows not only the creation of a low-noise, high frequency device, but also a structure that can be fabricated using conventional processes such as ion implantation. The use of ion implantation to create a carrier supply layer also allows great flexibility in creating different types of strained Si devices within the same circuit. 
     Accordingly, the invention provides a circuit including at least one strained channel, enhancement mode FET, and at least one strained channel, depletion mode FET. The depletion mode FET includes an ion implanted dopant supply. In exemplary embodiments, the FETs are surface channel or buried channel MOSFETS. In another exemplary embodiment, the FETs are interconnected to form an inverter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C are schematic block diagrams showing a variety of strained Si devices fabricated with relaxed SiGe buffer layers; 
     FIGS. 2A and 2B are the energy band diagram for the case of the surface channel FET for an NMOS device, at zero bias, and at a bias to turn on the transistor, respectively; 
     FIG. 3 is an energy band diagram showing schematically the problem with a buried channel device in which there is no dopant supply layer; 
     FIG. 4A is a schematic block diagram of a structure in which the buried channel can be occupied with a high density of electrons via the insertion of a layer of donor atoms; 
     FIG. 4B is the energy band diagram for the structure of FIG. 4A; 
     FIGS. 5A-5I show a process flow in which ion implantation is used to create a buried channel device with an ion implanted dopant supply layer; 
     FIG. 6 is a schematic block diagram of a structure in which both a surface channel device and buried channel device are configured next to each other on a processed Si/SiGe heterostructure on a Si substrate; 
     FIG. 7 is a schematic diagram of an inverter utilizing enhancement mode and depletion mode devices as shown in FIG. 6; 
     FIG. 8A is a schematic block diagram of a structure utilizing the implanted dopant supply layer on buried oxide technology; 
     FIG. 8B is a schematic block diagram of a structure utilizing the implanted dopant supply layer without the use of a buried SiO 2  layer; and 
     FIG. 9 is a schematic block diagram of a buried Ge channel MOSFET. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Fortunately, there is a solution to the problems described heretofore if one resists following the traditional path for dopant introduction in III-V buried channel devices. In the III-V materials, the dopant supply layer is introduced in the epitaxial step since there is no other known method. 
     In Si, it is well known that ion implantation can be used to create source/drain regions, and that annealing cycles can be used to remove the damage of such an implantation. FIGS. 5A-5I show a process flow in which ion implantation is used to create a buried channel device with an ion implanted dopant supply layer. The implanted layer can be an n-type dopant, such as phosphorus (P), arsenic (As), or antimony (Sb), or a p-type dopant, such as boron (B), gallium (Ga), or indium (In). The main features of the process depicted in FIG. 5 are described below. Note that this process flow is only an example of how the dopant supply layer can be used in combination with a conventional Si process flow to yield new devices and device combinations. This particular process flow was chosen since it is simple, and produces a depletion-mode buried strained channel device that has use in analog applications. 
     The process flow in FIG. 5A starts with a field oxidation process. Although this type of isolation can be convenient for larger gate sizes, it should be realized that at shorter gate lengths, trench isolation is preferable. FIG. 5A shows the starting substrate  500  after deposition of the SiO 2    502  and a SiN x  hardmask  504 , and definition of the active area  508  and field areas  510  with a photoresist  506  and etch. In order to prevent biasing from creating of conduction paths below the field oxide, a channel-stop implant  512  is performed before the field oxidation using the photoresist, SiO 2  and SiN x  as a mask, as shown in FIG.  5 B. 
     Subsequently, the photoresist is removed and a field oxide  514  is grown. FIG. 5C shows the device structure after completion of the field oxidation step. The field area has been oxidized, and the SiO 2 /SiN x  hardmask is still present above the device active area. After stripping the field oxide hardmask materials and creating a sacrificial oxide  516 , as shown in FIG. 5D, the sacrificial oxide is stripped and gate oxidation is performed. In the heterostructures described, the strained Si channel in the surface channel MOSFET can be oxidized directly. For buried channel structures, a thin sacrificial Si layer must be present on the surface for oxidation since oxidizing SiGe directly tends to create a high interface state density. Polysilicon  520  deposition atop the gate oxide  518  completes the deposition of the gate stack of the MOSFET. For reduced gate resistance, a titanium silicide  522  can be formed before the gate etch, to reduce the resistance to the gate for RF and other high-speed applications. FIG. 5E depicts the formation of this silicided gate stack after deposition of polysilicon, deposition of titanium, and reaction of the titanium to form the silicide. 
     The key dopant supply layer implant can be done before or after the gate oxidation step. A shallow implant is performed in order to place the dopants near the strained Si channel layer. In the exemplary sequence, the dopant supply layer is implanted through the sacrificial oxide indicated in FIG.  5 D. In that way, the sacrificial oxide can be stripped after implant, allowing a re-oxidation for achieving the highest gate oxide quality. FIGS. 5F-5I show the remainder of the process, which is standard Si CMOS processing. FIG. 5F shows the device structure after ion implantation of source-drain extensions  524 . Next, SiO 2 /SiN x  spacers  526  are formed by deposition and an anisotropic etch, resulting in the structure pictured in FIG.  5 G. Afterward, the deep source-drain ion implants  528  are performed, and the source-drain regions are silicided, as shown in FIG.  5 H. The source-drain silicide  530  is typically formed via metal deposition, annealing, and removal of unreacted metal.. Finally the interlayer dielectric, in this case SiO 2    532  is deposited over the entire device structure. Contact cuts to the source, drain, and gate are etched away, and the first metallization layer  534  is deposited. FIG. 5I shows the device after the completion of all of the process steps. 
     It will be appreciated that one objective of the invention, and the process in general, is to inject the advantages of strained-Si technology into the current Si manufacturing infrastructure. The further one deviates from these typical Si processes, the less impact the strained-Si will have. Thus, by utilizing the implanted dopant supply layer described herein, the device design capability is increased, and manufacturability is improved. If the dopant supply layer were created by the conventional method of doping during epitaxial growth, the flexibility would be less, leading to non-typical architectures, different manufacturing processes, and procedures that differ much more significantly from typical process flows. The flow described in FIGS. 5A-5I is compatible with current Si VLSI processing and thus is more likely to have widespread impact. 
     As one can see with the above process, the goals of creating a new Si-based device are achieved by producing a highly populated buried channel, yet the dopants were not inserted at the very beginning of the process through epitaxy. Although ion implantation may not produce a dopant profile that is as abrupt as a profile created through epitaxy, and thus the electron mobility in the buried channel may decrease slightly, the manufacturability of this process is far superior. In addition, the combination of buried channel devices and surface channel devices on the same wafer is enabled, since the local presence or absence of the implantation process will create a buried channel or surface channel device, respectively. Furthermore, buried channel devices can be created on the same wafer and within the same circuit with different thresholds by choosing the implant dose and type. 
     An example is shown in FIG. 6 that shows a structure  600  in which both a surface channel device  650  and buried channel device  660  are configured next to each other on a processed Si/SiGe heterostructure on a Si substrate  608 . The elements of the buried channel device are the same as shown in FIG. 1C while the elements of the surface channel device are the same as shown in FIG.  1 A. The depletion mode, buried channel device results from the incorporation of a dopant supply implant  670 . Other devices on the wafer, like the enhancement mode device  650 , can be masked off and not receive the supply implant. The SiGe cap layer can be removed  632 , if desired, forming surface channel enhancement mode strained Si devices in these regions. In the case where the dopant supply layer is grown epitaxially and embedded in the wafer from the beginning, integration of conventional MOS devices with the buried channel device is difficult, since the MOS devices must not contain the dopant supply layer. 
     The ability to mix these devices on a common chip area is a great advantage when creating system-on-chip applications. For example, the low noise performance and high frequency performance of the buried channel devices suggest that ideal applications are first circuit stages that receive the electromagnetic wave in a wireless system. The ability to form such devices and integrate them with surface channel MOS devices shows an evolutionary path to system-on-chip designs in which the entire system from electromagnetic wave reception to digital processing is captured on a single Si-based chip. 
     In such a system, there is a trade-off in circuit design in passing from the very front-end that receives the electromagnetic signal to the digital-end that processes the information. In general, the front-end requires a lower level of complexity (lower transistor count), but a higher performance per transistor. Just behind this front-end, it may be advantageous (depending on the application) to design higher performance digital circuits to further translate the signal received by the front end. Finally, when the signal has been moved down to lower frequencies, high complexity MOS circuits can be used to process the information. Thus, the buried channel MOSFET has an excellent application in the very front-end of analog/digital systems. The buried channel MOSFET will offer low noise performance and a higher frequency of operation than conventional Si devices. 
     For just behind the front-end, in some applications it may be desirable to have high-performance logic. In FIG. 6 the surface channel device  650  is an enhancement-mode device (turned off without applied gate bias) and the buried channel device  660  can be a depletion-mode device (turned on without applied gate voltage) or an enhancement mode device, depending on the implant conditions. Thus, the device combination shown in FIG. 6 can be used to create enhancement-depletion logic, or E/D logic. An example of an inverter  700  using this combination of devices is shown in FIG.  7 . The E/D inverter  700  is virtually identical to a typical CMOS inverter, but utilizes enhancement mode  702  and depletion mode  704  devices rather than NMOS and PMOS devices. This fundamental unit of digital design shows that the process described herein is critical in creating high performance circuits for analog applications such as wireless applications and high-speed electronic circuitry. 
     The enhanced performance is directly related to the mobility of the carriers in the strained Si and the low noise figure of the buried channel device. The enhanced mobility will increase the transconductance of the field effect transistor. Since transconductance in the FET is directly related to power-delay product, logic created with this E/D coupling of the strained devices described herein can have a fundamentally different power-delay product than conventional Si CMOS logic. Although the architecture itself may not be as low power as conventional CMOS, the lower power-delay product due to strained Si and/or buried channels can be used either to increase performance through higher frequency operation, or to operate at lower frequencies while consuming less power than competing GaAs-based technologies. Moreover, since the devices are based on a Si platform, it is expected that complex system-on-chip designs can be accommodated at low cost. 
     To achieve an even lower power-delay product in the devices, it is possible to employ this process on strained-Si/relaxed SiGe on alternative substrates, such as SiO 2 /Si or insulating substrates. FIG. 8A is a schematic block diagram of a structure  800  utilizing the implanted dopant supply layer on buried oxide technology. FIG. 8A shows the same types of devices and elements depicted in FIG. 6 processed on a slightly different substrate. This substrate, a hybrid of relaxed SiGe and SOI substrates, incorporates a buried SiO 2  layer  880  beneath a thin layer of relaxed SiGe  804 . Just as with the relaxed SiGe platform illustrated FIG. 6, strained Si devices can be formed atop this new substrate. The buried oxide layer provides the advantages of a SOI-like substrate, including lower power consumption and decreased junction leakage. 
     If the substrate shown in FIG. 8A does not have a buried SiO 2  layer, then the structure  890  shown in FIG. 8B is produced. This embodiment is useful in high power applications where the low thermal conduction of a SiGe graded buffer (FIG. 6) or an oxide layer (FIG. 8A) leads to the accumulation of heat in the resulting circuit. 
     Since the mobility in the buried channel can be in the range of 1000-2900 cm 2 /V-sec, and the mobility of the surface channel can be as high as 400-600 cm 2 /V-sec, the power-delay product in a conventional Si E/D design will be much larger than the power-delay product for the strained-Si E/D design. Thus, analog chips containing high performance strained Si devices using the ion implant methodology will have a significantly lower power-delay product, which means the chips can have higher performance in a wide-range of applications. 
     The exemplary embodiments described have focused on the use of ion implantation in strained Si devices; however, the benefits of ion implantation can also be realized in surface and buried channel strained Ge devices. FIG. 9 is a schematic block diagram of a buried Ge channel MOSFET  900 . In this embodiment, a relaxed SiGe layer  904  has a Ge concentration in the range of 50-90% Ge. The higher Ge concentration in the relaxed SiGe layer is necessary to ensure that the thickness of the Ge channel  902 , which is compressively strained, is not limited by critical thickness constraints. In FIG. 9, the relaxed SiGe layer is shown on a SiGe graded buffer layer  904  on a Si substrate  908 . However, the layer can be directly on a Si substrate or a Si substrate coated with SiO 2 . Like the Si buried channel device, the MOSFET contains a SiGe cap layer  932 , usually with a similar Ge concentration as the relaxed SiGe layer, a gate stack  942  containing oxide  944 , poly-Si  946  and metal  948  layers, and doped source  918  and drain  920  drain regions at each end of the gate. The ion implanted dopant supply layer can be introduced into either the SiGe cap layer or the relaxed SiGe layer. 
     In summary, the ion-implantation methodology of forming the dopant supply layer allows the creation of a manufacturable buried channel MOSFET or MODFET. The methodology also has the advantage that process flows can be created in which depletion-mode transistors can be fabricated by local implantation, but other nearby devices can be shielded from the implant or implanted with different doses/impurities, leading to enhancement-mode devices. Co-located enhancement and depletion mode devices can further be utilized to create simple digital building blocks such as E/D-based logic. Thus, the invention also leads to additional novel high-performance Si-based circuits that can be fabricated in a Si manufacturing environment. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.