Abstract:
The invention describes a method for gettering silicon on insulator wafers without forming regions of heavy doping. Silicon germanium layers ( 201, 304 ) are formed beneath silicon layers ( 200, 305 ) such that dislocations will form in the silicon germanium layers. These dislocations will serve to getter impurities.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is generally related to the field of semiconductor devices and fabrication and more specifically to a method for forming silicon on insulator wafers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Silicon on insulator (SOI) is finding increasing usage as a substrate on which integrated circuits are fabricated. A SOI substrate comprises a silicon wafer with a buried insulator layer. A typical SOI substrate is shown in FIG. 1. A buried insulator layer  101  is formed on a silicon wafer  100 . This buried insulator layer  101  usually comprises silicon oxide or other suitable dielectric material. Silicon layers  102  and  103  are formed on the buried insulator layer  101  to complete the formation of the SOI substrate. A number of different techniques such as high energy oxygen implantation (SIMOX) and wafer bonding can be used to form the SOI substrates. The active devices are usually fabricated in a silicon epitaxial layer  103  which is grown on the silicon layer  102  which is adjacent to the buried insulator layer  101 . In many instances the silicon layer  102  which is adjacent to the buried insulator layer  101  is fairly heavily doped with boron, arsenic, and/or phosphorous and acts as a gettering layer for impurities which may be introduced into the wafer during epitaxial layer growth and subsequent device fabrication. Without this gettering of impurities the electrical properties of the devices fabricated in the epitaxial layer will deteriorate. For example the voltage required for dielectric breakdown of the gate dielectric in the MOS devices will decrease. In addition the breakdown voltage and reverse leakage current of diodes fabricated in the epitaxial layers will decrease and increase respectively.  
           [0003]    The introduction of the heavily doped layer  102  in SOI substrates is therefore required for the fabrication of high performance, reliable, electronic devices in the epitaxial layer  103 . For some applications diodes with high breakdown voltages are required. A high diode breakdown voltage depend on a number of properties including the doping concentration of the epitaxial layer. A high diode breakdown voltage will typically require a fairly lightly doped epitaxial layer. During the processing of the integrated circuit the SOI substrate will be exposed to a number of high temperature cycles. High temperature cycling will result in the out-diffusion of dopants from the heavily doped silicon layer  102  into the epitaxial layer  103 . This dopant diffusion will serve to limit the minimum doping level which can be achieved in the epitaxial layer  103  and therefore limit the breakdown voltage obtainable. There is therefore a need for a method that will allow for the gettering of impurities in SOI substrates without limiting the dopant levels obtainable in the epitaxial layers.  
         SUMMARY OF THE INVENTION  
         [0004]    The present invention describes a method for gettering SOI wafers without regions of heavy doping. In the first embodiment a silicon germanium layer is formed on a silicon substrate and a silicon layer is formed on the silicon germanium layer. The silicon layer is oxidized and bonded to a second silicon oxide layer on a second silicon substrate. The silicon substrate is polished to form a silicon on insulator substrate with a silicon germanium layer to getter impurities.  
           [0005]    In a further embodiment of the instant invention, a silicon germanium layer is formed on a silicon on insulator substrate. A silicon layer is formed on the silicon germanium layer in which electronic devices can be fabricated. The underlying silicon germanium layer will act to getter impurities.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    In the drawings:  
         [0007]    [0007]FIG. 1 is a cross-sectional diagrams showing the prior art.  
         [0008]    FIGS.  2 ( a )-( d ) are cross-sectional diagrams illustrating an embodiment of the instant invention.  
         [0009]    FIGS.  3 ( a )-( b ) are cross-sectional diagrams illustrating an embodiment of the instant invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0010]    The invention will now be described with reference to FIGS. 2 and 3. It will be apparent to those of ordinary skill in the art that the benefits of the invention can be applied to other structures where a silicon on insulator substrate is utilized.  
         [0011]    Shown in FIG. 2( a ) is a silicon wafer  200  on which a layer of silicon germanium  201  is formed. The thickness of the silicon germanium layer  201  is such that dislocations will form in the silicon germanium layer  201  during subsequent processing. The germanium concentration in the silicon germanium layer can vary from 0 to 100 atomic percent but in a first embodiment of the instant invention will have a lower limit of 10 atomic percent. Therefore in the first embodiment of the instant invention the concentration of germanium in the silicon germanium layer will be between 10 and 100 atomic percent. The silicon germanium layer  201  can be formed using known semiconductor processing technology. Following the formation of the silicon germanium layer  201  a silicon layer  202  is formed on the silicon germanium layer. The silicon layer  202  can be formed using known semiconductor processing technology.  
         [0012]    Following the formation of the silicon layer  202 , the structure is exposed to an oxidizing ambient sufficient to oxidize the silicon layer  202  to form a layer of silicon oxide  203 . This oxidation process can comprise heating the structure to a temperature above 600° C. and exposing the silicon layer  202  to oxygen. In a further embodiment of the instant invention the entire silicon layer  202  is converted to silicon oxide  203 . This thickness of the silicon layer should be such that at the end of the oxidation process dislocations will form in the silicon germanium layer.  
         [0013]    Following the formation of the silicon oxide layer  203  illustrated in FIG. 2( c ), the silicon oxide layer  203  is bonded to a second silicon oxide layer  204  which was formed on a second silicon wafer  205 . The bonding of the silicon oxide layers  203  and  204  is performed using known silicon wafer bonding technology. The silicon oxide layer  204  can be formed on the silicon wafer  205  by heating the silicon wafer  205  to temperatures above 600° C. and exposing the surface of the wafer to an oxidizing ambient. The bonded structure of FIG. 2( c ) therefore comprises a silicon substrate  200 , a silicon germanium layer  201 , a silicon oxide layer  203  bonded to a second silicon oxide layer  204 , and a second silicon substrate  205 .  
         [0014]    Shown in FIG. 2( d ) is the completed structure. The structure of FIG. 2( c ) is inverted and the thickness of the silicon wafer  200  is reduced by polishing, chemical etching of some other suitable technique. The electronic devices that will comprise the integrated circuit will now be fabricated in the silicon wafer  200  and the second silicon wafer  205  will serve as the substrate. If necessary an addition silicon epitaxial layer can be formed on the surface of silicon wafer  200 . If this additional silicon epitaxial layer is formed the electronic devices will be formed in the additional epitaxial layer. The silicon germanium layer  201  which contains the dislocations will now be beneath the electronic devices and will serve to getter impurities from these electronic devices.  
         [0015]    Shown in FIGS.  3 ( a ) and  3 ( b ) are further embodiments of the instant invention. As illustrated in FIG. 3( a ) a silicon on insulator substrate  300  is provided. This silicon on insulator substrate can be formed by any number of known methods such as oxygen implantation (SIMOX), and wafer bonding. The silicon on insulator substrate will comprise a silicon substrate  301 , a silicon oxide layer  302 , and a silicon layer  303 . A silicon germanium layer  304  is formed on the surface of the silicon layer  303 . The germanium concentration in the silicon germanium layer  304  can vary between 0 to 100 atomic percent but is most preferably between 10 to 100 atomic percent. The thickness of the silicon germanium layer must be such that dislocations will form in silicon germanium layer  304  when a second silicon layer  305  is subsequently formed on the silicon germanium layer  304 . The completed structure is shown in FIG. 3( b ) where the second silicon layer  305  is shown on the silicon germanium layer  304 . The thickness of the second silicon layer  305  must be such that dislocations will form in the silicon germanium layer  304 . The second silicon layer  305  can be formed using known methods for forming silicon layers in semiconductor technology. If necessary an addition silicon epitaxial layer can be formed on the second silicon layer  305 . If this additional silicon epitaxial layer is formed the electronic devices will be formed in the additional epitaxial layer. Electronic devices can therefore be formed in the second silicon layer  305  or in an additional silicon epitaxial layer (if present) where the underlying silicon germanium layer  304  will act to getter impurities.  
         [0016]    While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.