Abstract:
A quantum well transistor or high electron mobility transistor may be formed using a replacement metal gate process. A dummy gate electrode may be used to define sidewall spacers and source drain contact metallizations. The dummy gate electrode may be removed and the remaining structure used as a mask to etch a doped layer to form sources and drains self-aligned to said opening. A high dielectric constant material may coat the sides of said opening and then a metal gate electrode may be deposited. As a result, the sources and drains are self-aligned to the metal gate electrode. In addition, the metal gate electrode is isolated from an underlying barrier layer by the high dielectric constant material.

Description:
BACKGROUND  
       [0001]     This invention relates generally to the formation of quantum well transistors.  
         [0002]     A quantum well is a potential well that confines particles in a dimension forcing them to occupy a planar region. A first material, sandwiched between two layers of a material with a wider band gap than the first material, may form a quantum well. Quantum well or high electron mobility transistors (HEMTs) are field effect transistors with a junction between two materials with different band gaps as the channel. The junction may exhibit very low resistance or high electron mobility. A voltage applied to the gate may alter the conductivity of the junction.  
         [0003]     Quantum well transistors may be prone to high gate leakage and parasitic series resistance. Particularly, quantum well transistors using elements from columns III through V of the periodic table may be prone to such problems. Examples of such materials include indium gallium arsenide/indium aluminum arsenide and indium antimony/aluminum indium antimony.  
         [0004]     In current state of the art quantum well transistors, a direct Schottky metal gate may be deposited on a barrier layer to form the Schottky junction which may be prone to high gate leakage. Also, the source and drain regions may be patterned and source and drain contact metallization completed before gate patterning. The gate patterning is done as the last step in the process, which may result in non-self-aligned source drain regions. Such non-self-aligned source drain regions may be prone to parasitic series resistance. Devices with parasitic series resistance may exhibit poor performance.  
         [0005]     Thus, there is a need for better ways to make quantum well transistors. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is an enlarged, cross-sectional view of one embodiment of the present invention;  
         [0007]      FIG. 2  is an enlarged, cross-sectional view of the embodiment shown in  FIG. 1  at an early stage of manufacture in accordance with one embodiment of the present invention;  
         [0008]      FIG. 3  is an enlarged, cross-sectional view of the embodiment shown in  FIG. 2  after subsequent processing in accordance with one embodiment of the present invention;  
         [0009]      FIG. 4  is an enlarged, cross-sectional view corresponding to  FIG. 3  after subsequent processing in accordance with one embodiment of the present invention;  
         [0010]      FIG. 5  is an enlarged, cross-sectional view corresponding to  FIG. 4  after subsequent processing in accordance with one embodiment of the present invention;  
         [0011]      FIG. 6  is an enlarged, cross-sectional view corresponding to  FIG. 5  after subsequent processing in accordance with one embodiment of the present invention;  
         [0012]      FIG. 7  is an enlarged, cross-sectional view corresponding to  FIG. 6  after subsequent processing in accordance with one embodiment of the present invention;  
         [0013]      FIG. 8  is an enlarged, cross-sectional view corresponding to  FIG. 7  after subsequent processing in accordance with another embodiment of the present invention;  
         [0014]      FIG. 9  is an enlarged, cross-sectional view corresponding to  FIG. 8  after subsequent processing in accordance with a depletion mode embodiment of the present invention; and  
         [0015]      FIG. 10  is an enlarged, cross-sectional view corresponding to  FIG. 7  after subsequent processing in accordance with an enhancement mode embodiment of the present invention; 
     
    
     DETAILED DESCRIPTION  
       [0016]     Referring to  FIGS. 1 and 10 , a depletion ( FIG. 1 ) or enhancement mode ( FIG. 10 ) self-aligned source drain quantum well transistor may be formed with a high dielectric constant dielectric layer  24  and a metal gate electrode  38  that acts as a Schottky gate metal. As used herein “high dielectric constant” refers to dielectrics having dielectric constants of 10 or greater.  
         [0017]     Over a silicon substrate  10  may be an accommodation layer  12 . The accommodation layer  12  may be AlInSb with 15% aluminum in one embodiment. Over a silicon substrate  10 , a germanium layer (not shown) may be included under the layer  12  as well. The accommodation layer  12  functions to accommodate for the lattice mismatch problem and to confine dislocations or defects in that layer  12 .  
         [0018]     Over the accommodation layer  12  may be formed a lower barrier layer  14  in accordance with one embodiment of the present invention. The lower barrier layer  14  may, for example, be formed of aluminum indium antimonide or indium aluminum arsenide, as two examples. The lower barrier layer  14  may be formed of a higher band gap material than the overlying quantum well  16 .  
         [0019]     Over the lower barrier layer  14  is formed the undoped quantum well  16 . In one embodiment, the undoped quantum well  16  may be formed of indium antimonide or indium gallium arsenide, as two examples.  
         [0020]     Next, the upper barrier layer  20  may be formed. The upper barrier layer  20  may be formed of the same or different materials as the lower barrier layer  14 . The upper barrier layer  20  may include a delta doped donor layer  18 . The delta doping may be done using silicon or tellurium, as two examples. The doped donor layer  18  supplies carriers to the quantum well  16  for transport. The doped donor layer  18  is formed by allowing Te or Si dopants to flow into the MBE (Molecular Beam Epitaxy) chamber in a controlled fashion from a solid source.  
         [0021]     Thus, the quantum well  16  is sandwiched between the upper and lower barrier layers  20  and  14 . The upper barrier layer  20  may be an electron supplying layer whose thickness will determine the threshold voltage of the transistor, along with the workfunction of the Schottky metal layer forming the gate electrode  38 .  
         [0022]     The metal gate electrode  38  may be formed over a high dielectric constant dielectric material  26 . The material  26  brackets the metal gate electrode  38  on three sides. The high dielectric constant layer  26  may, in turn, be bracketed by a self-aligned source drain contact metallization  22  and a spacer layer  28 .  
         [0023]     Fabrication of the depletion mode transistor, shown in  FIG. 1 , and the enhancement mode transistor of  FIG. 10  may begin, as shown in  FIG. 2 , by forming the structure up to and including an n+ doped layer  30 . The layer  30  may include an indium antimonide or indium gallium arsenide doped with Te and Si impurities. The layer  30  may be highly doped to later form the source drain regions in the finished transistor.  
         [0024]     The multilayer epitaxial substrate  10  may be grown using molecular beam epitaxy or metal organic chemical vapor deposition, as two examples.  
         [0025]     Referring to  FIG. 3 , a dummy gate  32  may be formed over the n+ doped layer  30  in accordance with one embodiment of the present invention. It may be formed after patterning and etch out of nitride, carbide, or oxide films (not shown). Advantageously, these films may be formed by low temperature deposition to preserve the integrity of the epitaxial layer structure. The dummy gate  32  may, for example, be formed of silicon nitride or metal. The dummy gate  32  may be formed by patterning through either lithography and etching, in the case of a silicon nitride dummy gate  32 , or through evaporation and liftoff in the case of a metal gate  32 , such as an aluminum metal dummy gate.  
         [0026]     Referring next to  FIG. 4 , low temperature silicon oxide, nitride or carbide spacers  28  may be formed that bracket the dummy gate  32 . The spacers  28  may be formed by a low temperature deposition technique, followed by anisotropic etching.  
         [0027]     Turning next to  FIG. 5 , the self-aligned source drain contact metallizations  22  may be formed by electron beam evaporation or reactive sputtering, either followed by a chemical mechanical planarization process, to create self-aligned contacts to the yet to be formed source drain regions in the layer  30 . The source drain contact metallization  22  may, for example, be formed of titanium or gold.  
         [0028]     Then, as shown in  FIG. 6 , the dummy gate  32  may be selectively etched out using a wet etch. As a result, an opening  34  is formed. A metal dummy gate removal process may, for example, include a wet etch using phosphoric acid etch. For a nitride dummy gate, hydrochloric acid may be used. For a silicon dioxide dummy gate a hydrofluoric acid etch can be used. The wet etch process is selective to the n+ doped layer  30 .  
         [0029]     Then, as shown in  FIG. 7  for a depletion mode device, a selective etch out of the n+ doped layer  30  may be achieved to form an inverted T-shaped opening having wings  36  and a base  34 . Dry or wet etching may be utilized to form the wings  36 . For example, the n+ doped layer  30  is selectively removed using a wet etch process such as citric acid plus peroxide.  
         [0030]     Atomic layer deposition of the high dielectric constant material  26  may be followed by electron beam evaporation or sputtering of a metal gate electrode  38 . The gate electrode  38  may, for example, be platinum, tungsten, palladium, or molybdenum, to mention a few examples. The high dielectric constant dielectric  26  may, for example, be hafnium dioxide or zirconium dioxide, as two examples. A low temperature deposition process may be utilized with an organic precursor (such as alkoxide precursor for hafnium dioxide deposition).  
         [0031]     The structure shown in  FIG. 8  may then be subjected to a chemical mechanical polish of the metal gate electrode  38  and the high dielectric constant dielectric  26  to achieve the depletion mode structure shown in  FIG. 9 .  
         [0032]     Right after the n+ doped layer  30  etch out to form the opening  34  including wings  36  and base  34 , as shown in  FIG. 7 , a further recess etch may be done through the electron supplying barrier layer  20 , stopping just above the delta doped layer  18  to make an enhancement mode device as shown in  FIG. 10 . A time drive etch (not shown in  FIG. 7 ) may partially recess into the electron supplying barrier layer  20  in  FIG. 7  and under the spacers  28 , to increase the threshold voltage of the transistor and to form an enhancement mode device.  
         [0033]     The device layer structure survives the high dielectric constant deposition process. This may be followed by sputter deposition or electron beam deposition of the Schottky gate electrode  38 . The gate electrode  38  workfunction may be chosen to be as high as possible to create an enhancement mode device.  
         [0034]     Some embodiments of the present invention may achieve lower gate leakage from the incorporation of a high dielectric constant dielectric  20  in between the Schottky gate metal of the electrode  38  and the semiconductor barrier layer  20 . Lower parasitic series resistance may result, in some embodiments, from the highly doped source drain region self-aligned to the gate. In some embodiments, the recess etch of the electron supplying barrier layer  20  to the desired thickness forms an enhancement mode quantum well field effect transistor.  
         [0035]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.