Abstract:
The present invention is a field effect transistor having a strained semiconductor substrate and Schottky-barrier source and drain electrodes, and a method for making the transistor. The bulk charge carrier transport characteristic of the Schottky barrier field effect transistor minimizes carrier surface scattering, which enables the strained substrate to provide improved power and speed performance characteristics in this device, as compared to conventional devices.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     The present application claims priority to U.S. provisional patent application 60/351,114, filed Jan. 23, 2002, U.S. provisional patent application 60/319,098, filed Jan. 25, 2002, and U.S. provisional patent application 60/381,320, filed May 16, 2002, all of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to devices for regulating the flow of electric current, and has specific application to the fabrication of these devices in the context of an integrated circuit (“IC”). More particularly, it relates to a transistor for regulating the flow of electric current having a Schottky-barrier source and/or drain.  
         [0003]     One type of field effect transistor (“FET”) known in the art, a metal oxide semiconductor field effect transistor (“MOSFET”), is shown in  FIG. 1 . As shown, the MOSFET device  100 , typically includes a silicon substrate  110 , an impurity doped source  120 , and an impurity doped drain  130 , separated by a channel region  140 . Atop the channel region  140  is a gate insulating layer  150 , which typically consists of silicon dioxide. A gate electrode  160 , made from electrically conductive material, is located on top of the insulating layer  150 . An insulating layer  170  typically surrounds the gate electrode  160 . A field oxide  180  electrically isolates devices  100  from one another. When an appropriate voltage V g  is applied to the gate electrode  160 , current flows between the source  120  and drain  130  through the channel region  140 . This current is referred to as the drive current, or I d .  
         [0004]     One consideration in the design of current regulating devices is the charge carrier mobility or ease with which charge carriers (i.e., electrons or holes) travel through the substrate lattice in the channel region  140 . From conventional MOSFET theory, drive current scales linearly with carrier mobility. Channel regions  140  that have higher charge carrier mobilities allow charge carriers to travel in less time between the source  120  and the drain  130 , and also to dissipate less power in the carrier transport process. This directly results in devices operating at higher speeds and consuming less power. One known technique for increasing the charge carrier mobility of the channel region  140  is to employ a strained substrate. For example, the mobilities of electrons and holes in strained silicon can be enhanced by factors of approximately two and ten respectively, compared to unstrained silicon. (M. V. Fischetti, S. E. Laux, Journal of Applied Physics, vol. 80 no. 4, 15 August 1996, pp. 2234-52.) As a result, MOSFET devices with strained silicon channel regions  140  are expected to demonstrate power and speed performance characteristics superior to conventional, unstrained silicon devices.  
         [0005]     Another known substrate used to fabricate MOSFET devices is a silicon-on-insulator (“SOI”) substrate. This semiconductor substrate includes a buried oxide layer to reduce source-to-drain leakage currents and parasitic capacitances. The prior art includes fabrication of MOSFET devices on a semiconductor substrate having a strained SOI layer. (B. Metzger, “Silicon Takes the Strain for RF Applications,” Compound Semiconductor, vol. 7, no.  7 , August 2001; T. Mizuno, “Design for Scaled Thin Film Strained-SOI CMOS Devices with Higher Carrier Mobility,” IEDM Proceedings, December 2002, p. 31.)  
         [0006]     Experimental results, however, for MOSFETs having impurity doped sources and drains and strained silicon channels, show that the devices do not fully benefit from the improvement in carrier mobility. For example, in one study, a 70% improvement in electron mobility led to only a 35% improvement in drive current. (K. Rim, S. Koester, M. Hargrove, J. Chu, P. M. Mooney, J. Ott, T. Kanarsky, P. Ronsheim, M. Ieong, A. Grill, H.-S. P. Wong, Proceedings of the 2001 IEEE VLSI Symposium, Kyoto, Japan (2001).) Because drive current scales linearly with mobility, the net improvement of 35% in drive current implies that the effective mobility for electrons only improved 35% for this example.  
         [0007]     There is a need in the art for a FET having a strained substrate, demonstrating an improvement in effective mobility, and therefore improvement in drive current closer to that of the improvement in carrier mobility.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The present invention, in one embodiment, is a FET having a Schottky-barrier source and/or drain and a strained semiconductor substrate. In this embodiment, the device includes a strained semiconductor substrate. A source electrode and a drain electrode are in contact with the strained substrate, and at least one of the electrodes forms a Schottky or Schottky-like contact with the substrate. The source and drain electrodes are separated by a channel. An insulating layer is disposed on the strained substrate above the channel. A gate electrode is disposed on the insulating layer.  
         [0009]     The present invention, in another embodiment, is a method of fabricating a Schottky-barrier FET on a strained semiconductor substrate. In this embodiment, the method includes providing a strained semiconductor substrate. It further includes providing an electrically insulating layer in contact with the strained substrate. The method further includes providing a gate electrode on the insulating layer such that the substrate on one or more areas proximal to the gate electrode is exposed. The method further includes depositing a thin film of metal and reacting the metal with the exposed strained substrate, such that Schottky or Schottky-like source and drain electrodes are formed on the substrate.  
         [0010]     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. As will be realized, the invention is capable of modifications in various obvious aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a sectional view of a metal oxide semiconductor field effect transistor (“MOSFET”), as known in the prior art.  
         [0012]      FIG. 2  is a sectional view of a Schottky-barrier field effect transistor (“FET”), with a strained substrate, according to one embodiment of the present invention.  
         [0013]      FIG. 3  is a flowchart showing a strained Schottky-barrier FET fabrication method, according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 2  shows a Schottky-barrier FET device  200 . The device  200  includes a semiconductor substrate  210  in which a source  220  and drain  225  are formed, separated by a channel  240 . The substrate  210  is strained. In one embodiment, the substrate consists of strained silicon. In other embodiments, other strained semiconducting materials are used. For example, in one embodiment, the device is fabricated on a strained SOI substrate. This embodiment provides both the advantage of improved carrier mobility and reduced source to drain leakage and parasitic capacitive coupling.  
         [0015]     In one embodiment, the source  220  or the drain  225  (or both) are composed partially or fully of a metal silicide. Because the source  220  and/or the drain  225  are composed in part of a metal, they form Schottky contacts or Schottky-like regions  230 ,  235  with the substrate  210 . A “Schottky contact” is defined by the contact between a metal and a semiconductor, and a “Schottky-like region” is a region formed by the close proximity of a semiconductor and a metal. The Schottky contacts or Schottky-like regions  230 ,  235  can be formed by forming the source  220  or the drain  225  from a metal silicide. In one embodiment of the present invention, the source  220  or the drain  225  (or both) are composed partially or fully of Platinum Silicide, Palladium Silicide, Iridium Silicide, or a rare earth silicide.  
         [0016]     The Schottky contacts or Schottky-like regions  230 ,  235  are in an area adjacent to the channel region  240  formed between the source  220  and drain  225 . In one embodiment of the present invention, the channel region  240  is impurity doped, using a conventional non-uniform doping profile, such as a halo implant for example. In another embodiment, the doping profile varies significantly in the vertical direction and is generally constant in the lateral direction, as described in commonly-assigned, U.S. Pat. No. 6,303,479 B1 (“the &#39;479 patent”), and U.S. Pat. No. 6,495,882 (“the &#39;882 patent”), which are hereby incorporated by reference in their entirety. The particular doping profile used in the device is not critical to the present invention.  
         [0017]     An insulating layer  250  is located on top of the channel region  240 . The insulating layer  250  is composed of a material such as silicon dioxide, or any other electrically insulating material. In one embodiment of the present invention, a material having a high dielectric constant (i.e., high K) is used as the insulating layer  250 . Examples of high K materials are those materials having dielectric constants greater than that of silicon dioxide, including for example metal oxides such as TiO 2 . The use of a high K gate insulating layer in combination with a Schottky-barrier device results in additional improvements in drive current, as explained in copending U.S. patent application Ser. Nos. 09/928,124 and 09/928,163, both filed on Aug. 10, 2001, both of which are hereby incorporated by reference in their entirety. Another embodiment is a Schottky-barrier FET fabricated on a strained SOI substrate and including a high K gate insulating layer. Another embodiment of the present invention includes both a high K gate insulating layer and a generally constant lateral doping profile (as set forth in the &#39;479 patent), in combination with a strained silicon, Schottky-barrier device. Yet another embodiment is a Schottky-barrier device fabricated on a strained SOI substrate, including a high K gate insulating layer, and the generally constant lateral doping profile as set forth in the &#39;479 patent.  
         [0018]     A gate electrode  260  is positioned on top of the insulating layer  250 , and a thin insulating layer  270  is provided on one or more gate sidewalls the gate electrode  260 . The gate electrode  260  may be doped poly silicon, a metal, or any electrically conductive material. A field oxide  280  electrically isolates devices from one another.  
         [0019]     The principles of the present invention are applicable to a device constructed on any variety of strained semiconductor substrates known in the art. By way of example, however, according to one embodiment of the present invention, the strained semiconductor is constructed as follows. A first strained semiconductor layer  210  such as silicon, is deposited on top of a second layer  205  such as silicon germanium, such that the lattice mismatch between the first strained semiconductor layer  210  and the second layer  205  causes strain in the first layer  210 . In one embodiment, the second layer  205  is a thin film. In another embodiment, the thin film  205  is deposited on a substrate  215  such as silicon. Other embodiments of the present invention are directed to Schottky-barrier devices constructed on other known strained silicon substrates.  
         [0020]     A conventional FET is, necessarily, a surface conduction device. As shown in  FIG. 1 , carriers  165  leave the source  120  and traverse the channel region  140 , during which time the carriers  165  experience a strong attraction to an interface  155  defined by the contact plane between the substrate  110  and the gate insulating layer  150 . The carriers  165  typically experience many surface scattering events due to roughness of the interface  155 . The surface scattering phenomena substantially degrades the carriers&#39; mobility in the channel region  140 , resulting in a lower effective carrier mobility. The significant shortfall in performance enhancement for conventional FETs using a strained silicon substrate is caused by surface scattering of charge carriers  165  at the interface  155 .  
         [0021]     On the other hand, in the Schottky-barrier FET device  200 , carriers  290  are field emitted from the source  220  in an initial direction normal to the surface of the metallic source  220 . They traverse the channel  240  largely in bulk silicon, not along an interface  255  defined by the contact plane between the strained substrate  210  and the insulating oxide  250 . Accordingly, carriers  290  experience far fewer scattering events caused by surface roughness at the interface  255 , resulting in an effective carrier mobility improvement closer to the two-fold and ten-fold improvement observed in bulk silicon for electrons and holes respectively. The distance between the source  220  and drain  225  is denoted as channel length  245 . The improvement in effective carrier mobility increases as the channel length  245  of the device  200  is reduced.  
         [0022]      FIG. 3  shows a fabrication method for a Schottky-barrier FET according to one embodiment of the present invention. As shown in  FIG. 3  the method begins with a silicon substrate, which is strained (block  302 ). A thin screen oxide is grown (in one embodiment, approximately 200 Å) to act as an implant mask (block  304 ). The appropriate channel dopant species (for example Arsenic and Boron for P-type and N-type devices, respectively) is then ion-implanted through the screen oxide to a pre-determined depth in the silicon (block  306 ).  
         [0023]     The screen oxide is then removed, and a thin gate oxide (in one embodiment, approximately 35 Å) is grown (block  308 ). The gate oxide growth is immediately followed by an in-situ doped silicon film (block  310 ). The film is heavily doped with, for example, Phosphorous for an N-type device and Boron for a P-type device. Using lithographic techniques and a silicon etch that is highly selective to oxide, the gate electrode is patterned (block  312 ). Then, a thin oxide (in one embodiment, approximately 100 Å) is thermally grown on the top surface and sidewalls of the silicon gate electrode (block  314 ). An anisotropic etch is then used to remove the oxide layers on the horizontal surfaces (and thus expose the silicon), while preserving them on the vertical surfaces (block  316 ). Following these steps, a sidewall oxide is formed, and the dopants both in the gate electrode and in the channel region of the device are electrically activated.  
         [0024]     Next, an appropriate metal (for example, Platinum for the P-type device and Erbium for the N-type device) is deposited as a blanket film (in one embodiment, approximately 400 Å) on all exposed surfaces (block  318 ). The wafer is then annealed for a specified time at a specified temperature so that, at all places where the metal is in direct contact with the silicon, a chemical reaction takes place that converts the metal to a metal silicide (block  320 ). For example, in one embodiment, platinum silicide is formed at a maximum temperature between about 400° C. and about 500° C. for less than about 60 minutes. In another embodiment, erbium silicide is formed at a maximum temperature between about 400° C. and about 600° C. for less than about one minute. The metal that was in direct contact with a non-silicon surface is left unaffected. A wet chemical etch (aqua regia for Platinum, HNO 3  for Erbium) is then used to remove the unreacted metal while leaving the metal-silicide untouched (block  322 ). The strained Schottky-barrier FET device is now complete and ready for electrical contacting to gate, source, and drain.  
         [0025]     The source and drain electrodes of a conventional FET are, necessarily, formed by processes having temperatures exceeding 800C. It is known in the art that high temperature manufacturing steps—that is, steps using temperatures greater than 800° C.—may modify and/or degrade the properties of new materials introduced for improving the performance of FET devices. Examples of new materials include strained semiconductor substrates and high K gate insulators. For instance, processing a strained semiconductor substrate at a high temperature may relax the strain layer, thereby decreasing the improvement to charge carrier mobility in the strained semiconductor substrate.  
         [0026]     On the other hand, during the Schottky-barrier FET fabrication process, the source and drain electrodes are formed by a silicide reaction process having temperatures significantly less than those used during a conventional impurity doped source and drain MOSFET device fabrication process, as explained in U.S. provisional patent application 60/381,320, filed May 16, 2002. More specifically, the silicide reacting step used to form the Schottky or Schottky-like source and drain regions of the present invention may be less than 800° C., as detailed above. Accordingly, strained silicon substrates and high K gate insulators can be integrated with a Schottky barrier FET manufacturing process without degradation of the strained silicon and/or high K gate insulator properties.  
         [0027]     This process is only one possible way to achieve strained, metal source/drain Schottky FET devices. One skilled in the art will recognize that many other variants and alternatives exist. For example, various steps in the described process could be replaced by equivalent steps known to those in the art. Likewise, one or more of the various steps could be omitted from the fabrication process. In one embodiment of the present invention, the fabrication method includes fabrication of the strained silicon substrate. As further described above, in one exemplary embodiment, this is accomplished by depositing a layer of silicon on top of a layer of material having a larger lattice structure (such as silicon germanium). This strained silicon substrate is then processed in the manner set forth above. Many other techniques for fabricating a strained silicon substrate are known in the art and could be used in combination with the teachings of the present invention. For example, in one embodiment, a strained silicon substrate is fabricated on an oxide insulator, resulting in a strained SOI substrate, as described in the Compound Semiconductor article.  
         [0028]     By using the techniques of the present invention, the power and speed performance of a transistor, such as a FET, can be substantially improved. Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, while many of the embodiment have been described with reference to a FET device, other transistor-type devices could also employ the techniques of the present invention.  
         [0029]     All references cited above are hereby incorporated by reference in their entirety. Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.