Abstract:
A structure and method of fabrication for PFET devices in a compressively strained Ge layer is disclosed. The fabrication method of such devices is compatible with standard CMOS technology and it is fully scalable. The processing includes selective epitaxial depositions of an over 50% Ge content buffer layer, a pure Ge layer, and a SiGe top layer. Fabricated buried channel PMOS devices hosted in the compressively strained Ge layer show superior device characteristics relative to similar Si devices.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to semiconductor technology and, more specifically, to a PMOS device hosted in strained Ge. This device is fabricated in a manner that it can be effectively scaled and integrated into high performance silicon based CMOS technologies.  
       BACKGROUND OF THE INVENTION  
       [0002]     Today&#39;s integrated circuits include a vast number of devices. Smaller devices are key to enhance performance and to improve reliability. As MOSFET (Metal Oxide Semiconductor Field-Effect-Transistor, a name with historic connotations meaning in general an insulated gate Field-Effect-Transistor) devices are being scaled down, the technology becomes more complex and new methods are needed to maintain the expected performance enhancement from one generation of devices to the next.  
         [0003]     One of the most important indicators of potential device performance is the carrier mobility. There is great difficulty in keeping carrier mobility high in devices of deeply submicron generations. A promising avenue toward better carrier mobility is to modify the semiconductor that serves as raw material for device fabrication. It has been known, and recently further studied, that tensilely or compressively straining semiconductors have intriguing carrier properties. In particular, a 90-95% improvement in the electron mobility has been achieved in a strained silicon (Si) channel NMOS as described in U.S. Pat. No. 6,649,492 B2 to J. O. Chu entitled “Strained Si Based Layer Made By UHV-CVD, and Devices Therein” incorporated herein by reference. Similarly for hole enhancement, compressively-strained buried germanium (Ge) MODFETs have yielded high hole mobilities as described by S. J. Koester, et. al. in “Extremely high transconductance Ge/Si 0.4 Ge 0.6  p-MODFET&#39;s grown by UHV-CVD,” IEEE Elect. Dev. Lett. 21, 110 (2000), and in U.S. patent application “High Speed Ge-Channel SiGe/Ge/SiGe Heterostructure for Field Effect Transistor” by J. O. Chu, Ser. No. 09/936,320 filed Sep. 12, 2000, incorporated herein by reference. Combination of tensilely and compressively strained SiGe regions in the same wafer is described in U.S. patent application “Dual Strain-State SiGe Layers for Microelectronics” by J. O. Chu, Ser. No. 10/389,145, filed Mar. 15, 2003, incorporated herein by reference.  
         [0004]     Because of its enhanced hole mobility there is renewed technological interest in Ge-based MOSFET devices for high performance CMOS logic. In particular, surface channel Ge MOSFET devices, using oxynitride (GeON) as the gate insulator, have been demonstrated by H. Shang et al, IEDM, p. 441, 2002. Or, using high-K as the gate insulator, Ge PMOS is described in the following references: C. Chui et al, IEDM, p. 437, 2002, C. H. Huang et al, VLSI symp. p. 119, 2003 and A. Ritenour et al, IEDM, p. 433, 2003 all three of which are incorporated herein by reference.  
         [0005]     Buried channel strained Ge PMOS have also been reported having hole mobility enhancement as described in the following references of M. Lee et al, IEDM, p. 429 2003 and H. Shang et.al, VLSI symp. 2004 both of which are incorporated herein by reference. Nonetheless, the Ge devices reported have employed simple device structures, such as ring type gate structure lay-out for simplified integration, and usually have relative large dimensions. Such features are not or suitable for integration into advanced high performance CMOS technologies.  
         [0006]     A process compatible with standard CMOS technology, in order to incorporate strained Ge structures for enhanced hole mobility in PMOS devices, is not available.  
       SUMMARY OF THE INVENTION  
       [0007]     This invention describes an integration scheme for advanced CMOS technology which incorporates a high mobility strained Ge buried channel structure, leading to PMOS device improvement. The scheme is readily scalable with decreasing dimensions.  
         [0008]     A layered structure, including a SiGe seed layer, wherein the SiGe seed layer is monocrystalline and has about between 50% and 90% of Ge concentration, and a compressively strained Ge layer covering the SiGe seed layer, wherein the compressively strained Ge layer is monocrystalline and in epitaxial relation with the SiGe seed layer is disclosed. A CMOS circuit, including a PMOS device hosted in a compressively strained Ge layer, wherein the compressively strained Ge layer is covering in epitaxial relation a SiGe seed layer, wherein the SiGe seed layer is monocrystalline and has about between 50% and 90% of Ge concentration is further disclosed.  
         [0009]     A method for fabricating a PMOS device, including the steps of depositing epitaxially a monocrystalline SiGe seed layer having about between 50% and 90% of Ge concentration over a relaxed SiGe layer having about up to 50% of Ge concentration; depositing epitaxially a compressively strained Ge layer over the SiGe seed layer; and hosting the PMOS device in the compressively strained Ge layer is further disclosed.  
         [0010]     A method for fabricating a CMOS circuit, including the steps of providing an SGOI wafer having a relaxed SiGe layer with about up to 50% of Ge concentration; using a shallow trench, or other, isolation technique to define NMOS and PMOS regions on the SGOI wafer; capping with a dielectric material the NMOS regions; depositing epitaxially and in a selective manner a monocrystalline SiGe seed layer having about between 50% and 90% of Ge concentration over the relaxed SiGe layer; depositing epitaxially and in a selective manner a compressively strained Ge layer over the SiGe seed layer; and hosting a PMOS device in the compressively strained Ge layer is further disclosed. The method for fabricating a CMOS circuit further including the step of depositing epitaxially and selectively a SiGe top layer over the compressively strained Ge layer, wherein the SiGe top layer is chosen to have about up to 10% of Ge concentration is further disclosed.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     These and other features of the present invention will become apparent from the accompanying detailed description and drawings, wherein:  
         [0012]      FIG. 1  shows a schematic cross sectional view of a layered structure used for fabricating devices;  
         [0013]      FIG. 2  shows a schematic top view of isolation and NMOS and PMOS regions;  
         [0014]      FIG. 3  shows a schematic cross sectional view of a buried channel PMOS device hosted in a compressively strained Ge layer;  
         [0015]      FIG. 4  shows a plot of measured mobility values for a buried channel PMOS device hosted in a compressively strained Ge layer;  
         [0016]      FIG. 5  shows a plot of measured transconductance values for a buried channel PMOS device hosted in a compressively strained Ge layer; and  
         [0017]      FIG. 6  shows a measured conduction plot for a buried channel PMOS device hosted in a compressively strained Ge layer. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     In the integration of strained Ge into advanced CMOS technology it is preferable to maintain the overall standard fabrication process of mainline Si CMOS as much as feasible. This invention integrates compressively strained Ge into such a process with changing/adding only a few steps out of the well over a hundred steps of typical advanced Si CMOS processing. In an exemplary embodiment the overall process flow of CMOS fabrication, including the strained Ge hosted PMOS, would follow these steps: Start with a silicon-on-insulator (SOI) or silicon-germanium-on-insulator (SGOI) wafer; go through the standard shallow trench isolation (STI) process, as known to one skilled in the art; cap NMOS region with a mask; open up Si or SiGe islands only in PMOS region; grow selectively Ge on top of exposed Si or SiGe surface; strip masking cap on NFET region; continue with the standard CMOS fabrication as known to one skilled in the art. This invention also teaches steps and structures which are novel outside the framework of CMOS fabrication process.  
         [0019]      FIG. 1  shows a schematic cross sectional view of a layered structure used for fabricating devices.  FIG. 2  shows a schematic top view of isolation and NMOS and PMOS regions. In a representative embodiment the starting point is a SOI or SGOI wafer. A substrate  151 , which is usually Si, has a so called buried oxide (BOX)  150  on over it, as known to one skilled in the art. On top of the BOX there is a relaxed monocrystalline SiGe layer  130  typically with about up to 50% of Ge concentration, with the possibility of being essentially pure Si. At the start this relaxed SiGe is a blanket layer on top of the BOX  150 , but  FIG. 1  shows a state of processing where an isolation dielectric  140  has already been applied and the relaxed SiGe layer  130  is fragmented. The isolation in an exemplary embodiment is so called shallow trench isolation (STI), but it could be of other various kinds as known to one skilled in the art. The isolation  140  separates, or defines, regions designated for PMOS devices  210  from regions designated for NMOS devices  220 . Preferably the strained Ge layer introduced in this invention is used for the PMOS devices in the PMOS regions  210 . The NMOS regions are handled in ways known to one skilled in the art, possibly with material layers  190  over the relaxed SiGe  130 . Layer, or layers,  190  in the NMOS region are shown only symbolically, since layer  190  may not even exist at this stage of the process, or may never be implemented. The present invention uses methods known in the art in the processing of the NFET regions and devices. At this stage, as represented in the figures, the NMOS regions are covered with a mask  160 . Mask  160  preferably is a dielectric, such as SiO 2 , or nitride, or others, as known to one skilled in the art.  
         [0020]     Introduction of the layers for the Ge hole-conduction type device, such as a PMOS, or P-MODFET, commences with depositing epitaxially a monocrystalline SiGe seed layer  101  having about between 50% and 90% of Ge concentration over the relaxed SiGe layer  130 . Preferably the epitaxial growth of the SiGe seed layer  101  is done in a selective manner, and with a Ge concentration of around 70%. Selectivity in deposition is with regard to dielectric materials, such as the STI dielectric  140 , or the NMOS mask  160 . The Ge concentration in this SiGe seed layer  101  is not necessarily uniform, it can have various Ge concentration gradients depending on requirements of a specific embodiment. Non-uniform concentration typically serves the purpose of improving material quality. The preferred thickness range for this SiGe seed layer  101  is about between 0.3 nm and 3 nm. Some of the preference for this SiGe seed layer  101  is to improve the surface quality of the relaxed SiGe  130  of the SGOI wafer. The relatively high Ge concentration of the SiGe seed layer is an enabling aspect for the introduction of the compressively strained device quality Ge layer  100 .  
         [0021]     The compressively strained monocrystalline Ge layer  100  is deposited epitaxially over the SiGe seed layer  101 . Preferably the epitaxial growth of the compressively strained Ge layer  100  is done in a selective manner. Selectivity in deposition is with regard to dielectric materials, such as the STI dielectric  140 , or the NMOS mask  160 . The preferred thickness range for this compressively strained Ge layer  100  is about between 5 nm and 20 nm. The Ge layer  100  is compressively strained because the relaxed lattice constant of Ge is larger than that of SiGe, and the epitaxial relationship through the layers forces the lattice of Ge layer  100  to comply with the layers underneath, all of which have smaller relaxed lattice constants than Ge. The compressively strained Ge layer is the one hosting a hole-type conduction device, such as a PMOS. The term of hosting a device in a certain material, or layer, means that the critical part of the device, that which is mainly sensitive to carrier properties, such as for instance, the channel of a MOS device, is residing in, composed of, housed in, in that certain material, or layer.  
         [0022]     If one would prefer a surface channel PMOS device the material depositions could stop with the compressively strained Ge layer  100 . For having a buried channel PMOS, as well as to preferably improve the interface qualities of the p-channel, a monocrystalline SiGe top layer  120  is epitaxially deposited covering the compressively strained Ge layer  100 . The SiGe top layer  120  has about up to 10% of Ge concentration. In an exemplary embodiment this SiGe top layer is essentially pure Si, in which case a thickness of about between 0.3 nm and 10 nm is preferred. Preferably the epitaxial growth of the SiGe top layer  120  is done in a selective manner. Selectivity in deposition is with regard to dielectric materials, such as the STI dielectric  140 , or the NMOS mask  160 . Either the SiGe top layer  120 , or the compressively strained Ge layer  100  is the last layer to be deposited, this layered structure has a well defined top surface  121 . It is preferable for easy of processing that this surface  121  be coplanar with the top surface  141  of the isolating dielectric. However, lack of such coplanarity would not be a limiting factor.  
         [0023]     The localized formation or growth of the buried Ge channel heterostucture within the selected confines of the PMOS regions entails using a selective CVD growth process whereby the growth of the device layers are selective to the known dielectric materials of SiO 2 , Si 3 N 4 , SiON etc. Typical, or available selective growth processes for Si, SiGe and Ge films can be found in various growth techniques such as RT-CVD, UHV-CVD, LP-CVD, AP-CVD, etc. as known to one skilled in the art. In the preferred selective growth process of ultra-high-vacuum-chemical-vapor-deposition (UHV-CVD), the growth temperature for the SiGe seed layer  101  and the compressively strained Ge layer is the range of 250-350° C.  
         [0024]     For a detailed description of the UHV-CVD technique for growing epitaxial layers, reference is made to U.S. Pat. No. 5,259,918 “Heteroepitaxial Growth of Germanium on Silicon by UHV/CVD” to S. Akbar et al, issued Nov. 09, 1993 assigned to the assignee herein and incorporated herein by reference. More UHV-CVD growth techniques are discussed in U.S. Pat. No. 6,350,993 B1 “High Speed Composite p-Channel Si/SiGe Heterostructure for Field Effect Devices” to J. O. Chu et al, issued on Feb. 26, 2003 assigned to the assignee herein and incorporated herein by reference. Epitaxial depositions for the layered structure of the SiGe seed layer  101 , the compressively strained Ge layer  100 , and the SiGe top layer  120  are done with ultrahigh vacuum integrity: in the range of about 10 −9  Torr prior to the epitaxial deposition. In particular, a hot wall isothermal CVD apparatus is utilized whereby essentially no homogeneous gas phase pyrolysis of the silicon and/or germanium precursor such as silene (SiH 4 ) or germane (GeH 4 ) source gas occurs during the residence time, which is less than 1 second, within the selected temperature and pressure regime where the preferred growth process is operated. Typically, a batch of pre-patterned SGOI wafers are loaded in the UHV-CVD reactor and then heated in the range from 300° C. to 480° C. The growth pressure typically is in the in range from 1-5 millitorr. The SiGe seed layer  101  in a representative embodiment is grown over the SGOI region using a flow combination of SiH 4  at 25 sccm and of GeH 4  at 95 sccm. To grow the compressively strained Ge layer  100 , the growth temperature is lowered close to 300° C. and then the GeH 4  is turned on at a flow of 50 sccm. After the completion of layer  100 , the growth temperature is raised higher, and the SiH 4  is turned on at a flow of 30 sccm, and the GeH 4  is turned on at a flow of 0 to 15 sccm to form the thin SiGe top layer  120  over the compressively strained Ge layer  100 . The thin top layer  120  in an exemplary embodiment is essentially pure Si.  
         [0025]     Typically, but not necessarily, as known to one skilled in the art, selective CVD growth techniques prefer additional use of a chlorine based precursor or gas source, such as HCl, Cl 2 , SiCl 4 , SiHCl 3 , SiH 2 Cl 2  to induce the selective growth by removing any film growth over the standard masking materials.  
         [0026]      FIG. 3  shows a schematic cross sectional view of a buried channel PMOS device hosted in a compressively strained Ge layer  100 . Following the building up of the layered structure in  FIG. 1 , the device fabrication follows steps known to one skilled in the art. At one point source/drain junctions  380  are being created. The source/drain  380  as shown in the figure extend downward touching the BOX layer  150 . This is only an illustration, in an exemplary embodiment the source/drain  380  may, or may not, reach down to the BOX layer interface  150 , or may even happen that it penetrates into the BOX layer  150 . At another point in the process a gate insulator  310  is employed. Preferred gate insulators are, but not limited to, a deposited oxide, typically by a plasma low temperature process, and so called high-K (high dielectric) materials, such as HfO 2 , HfSiO, and others known to one skilled in the art. Similarly, a large variety of materials can be employed for the gate  390 , as known to one skilled in the art. In further fabrication of the devices, such as during processing of the gate insulator  310 , one may, or may not, consume from the Si/SiGe top layer  120 . For illustration, the figure shows schematically a slight consumption of layer  120 . Holes are conducted in a channel on the top surface  301  of the compressively strained Ge layer  100 , which is the interface with the Si or SiGe top layer  120 . The channel forms here due to the well known bandgap alignment between Ge and Si.  
         [0027]      FIG. 4  shows a plot of measured mobility values for a buried channel PMOS device hosted in a compressively strained Ge layer as function of inversion charge. The maximum hole mobility in the disclosed device is over six times the Si universal hole mobility, which is also shown for comparison.  
         [0028]      FIG. 5  shows a plot of measured transconductance values for a buried channel PMOS device hosted in a compressively strained Ge layer. The displayed characteristics is from a device with a deposited oxide gate insulator. As shown, the transconductance of the disclosed device is enhanced over that of a Si device by over a factor of two.  
         [0029]      FIG. 6  shows a measured conduction plot for a buried channel PMOS device hosted in a compressively strained Ge layer. Subthreshold and conduction characteristics of buried channel PMOS with high-K, specifically HfO 2  gate insulator are shown for low and high drain voltage. A Si control device with the same HfO 2  as gate insulator is shown for comparison. The s-Ge (strained-Ge) device shows over two times enhancement in drive current.  
         [0030]     Many modifications and variations of the present invention are possible in light of the above teachings, and could be apparent for those skilled in the art. The scope of the invention is defined by the appended claims.