Abstract:
Varying the porosity through the thickness of a porous silicon layer allows conflicting needs to be met by the same layer: a low porosity surface layer allows a high-quality epitaxial layer of silicon to be grown, or can provide structural support, while greater porosity in other portions of the layer increases circuit isolation and provides stress relief between layers.

Description:
This application claim priority under 35 USC §119(e)(1) of provisional application number 60/094,503 filed Jul. 29, 1998. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to integrated circuit structures and fabrication methods, and especially to techniques for creating silicon-on-insulator substrates. 
     Background: Porous Silicon 
     Porous silicon is formed by anodic oxidation of a silicon wafer in a solution of hydrofluoric acid and a surfactant, such as ethanol. The size of the pores, the porosity, and the thickness of the porous silicon can be controlled; pore size is typically in the range of 10-20 nm, while porosity is typically in the range of 30-70 percent. 
     Porous silicon has a low dielectric constant and high resistivity, which make it useful for isolation. Since porous silicon has a high surface area for its volume (a few hundred square meters per cubic centimeter), it can be oxidized much more quickly than bulk silicon. 
     One prior use of porous silicon is in FIPOS—full isolation by porous oxidized silicon, which uses an epitaxial layer grown on a heavily-doped silicon surface. The heavily-doped layer is selectively converted to porous silicon by anodization through holes in the epitaxial layer, then oxidized to form a buried layer of oxide. 
     Variable Porosity Porous Silicon 
     The present application discloses varying the porosity throughout a layer of porous silicon. For example, low porosity is advantageous to provide a good surface for epitaxial silicon growth and to provide material strength, while high porosity layers provide better isolation and a reduction of stresses between the porous silicon and the substrate. Variable porosity allows these different requirements to be met by varied regions within the layer. 
     Advantages of the disclosed methods include the ability to optimize a layer of porous silicon to meet the opposing requirements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein: 
     FIG. 1A shows a variable porosity porous silicon layer which will be used in an SOI substrate, while FIG. 1B shows a variable porosity porous silicon layer to be used in RF isolation. 
     FIGS. 2A-E are cross-sections of a partially fabricated integrated circuit structure, showing formation of the disclosed SOI structure, as well as formation of transistors. 
     FIGS. 3A-C are cross-sections of a partially fabricated integrated circuit structure, showing formation of the disclosed RF isolation structures. 
     FIGS. 4A-B are cross-sections of a partially fabricated integrated circuit structure, showing formation of the disclosed mixed signal isolation structures. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. 
     Overview 
     In general, the layer of porous silicon is formed by anodic oxidation of the top layer of a silicon wafer. The electrolyte is generally hydrofluoric acid (HF) with water and a surfactant, e.g., a HF/C2H5OH/H2O solution of approximately 1:2:1. The formation of pores begins at the exposed surface of the silicon wafer and proceeds isotropically. Preferably (but not necessarily) the exposed surface is oxidized with a low current density to create a low porosity surface portion. The current density is then increased to give a higher porosity which is optimized for the particular process and provides better isolation. Preferably (but not necessarily) the oxidation ends with formation of a high porosity region at the interface to the silicon substrate to provide stress relief between the two layers. 
     First Embodiment: Use in SOI 
     Provisional patent application No. 60/086,959, filed May 28, 1998, which is hereby incorporated by reference, discloses a layer of porous silicon to be used in the formation of an SOI substrate. That process, which can use the disclosed variable porosity porous silicon, will now be discussed with reference to FIGS. 2A-E, which show a cross-section of the wafer at various points in the disclosed process. 
     A boron-doped silicon substrate  200  is anodically oxidized to form a layer of porous silicon  210 , seen in FIG.  2 A. FIG. 1A is a simplification of FIG. 2A, showing only porous silicon layer  210  and substrate  200 . For the outer layer  210 A, where an epitaxial layer is to be grown, a current density of approximately 1-10 mA/cm2 is used to create a region of low porosity (approximately 15-35 percent) which is approximately 10-50 nm thick. The current density is then increased to approximately 10-100 mA/cm2 to create a porosity of about 30-70 percent in region  210 B, which is approximately 5 micron thick. For region  210 C, which is nearest the substrate, a current density of approximately 35-100 mA/cm2 is used to create a region approximately 50-500 nm thick and having a porosity of about 60-80 percent. This high porosity layer provides stress relief between the porous silicon layer and the underlying substrate. 
     As seen in FIG. 2B, partial oxidation of the porous silicon is performed, forming regions of SiO2  215  on the surface of the pores  205 . A short dip in HF, or a plasma etch, is used to remove the oxide from the top surface of the wafer. Once the surface oxide is removed, the wafer is heated in an atmosphere of H2 at 900-1100 degrees C. for approximately 5-10 minutes to allow the crystalline silicon to reflow and seal the pores on the surface, making the wafer surface once again single crystal silicon. Epitaxial layer  220  is then grown on the porous silicon  210  and a capping layer  240  is formed on the epitaxial layer, such as chemical-vapor-deposited (CVD) Si3N4. The capping layer provides structural support to the thin epitaxial layer during the anneal to follow. 
     The wafer is then annealed at approximately 1325 degrees C. for approximately six hours. At this temperature, the silicon and silicon dioxide will begin to flow and separate. Over time, the partially-oxidized porous silicon layer separates and coalesces into an oxide layer  230  between slightly thickened epitaxial and substrate layers. FIG. 2C shows the layers when this process is partially complete, showing an irregular boundary between the substrate  200 , oxide layer  230 , and epitaxial region  220 . After the anneal is complete (i.e., interfaces are smooth), the wafer is cooled, then the nitride capping layer can either be stripped, or left on to facilitate formation of further isolation structures, such as shallow trench isolation (STI). FIG. 2D shows the wafer after stripping of the nitride layer. Formation of transistors can then proceed. FIG. 2E shows partially fabricated transistors, with trench isolation  260 , gate oxide  270 , gate  280 , sidewall spacers  290 , and implanted regions  292  and  294 , all overlying the buried oxide layer  230 . 
     Second Embodiment: Use in RF 
     Provisional patent application No. 60/069,522, filed Dec. 12, 1997, which is hereby incorporated by reference, discloses a method for forming vertical substrate isolation in radio frequency (RF) circuits by a patterned anodic oxidation of the substrate to selectively form porous silicon in desired areas. A variable porosity porous silicon can be used in this application, which will now be discussed with reference to FIGS. 3A-D. 
     In this embodiment, as seen in FIG. 3A, field oxide  108  is grown and transistors  102  are formed, including silicide  110 . 
     A resist layer  112  is deposited and patterned to expose the field oxide in those areas in which porous silicon isolation is desired. Exposed portions of field oxide  108  are removed, giving the structure of FIG. 3B, followed by ashing of the resist. 
     A hard mask layer  104 , e.g. of SiC, is deposited and patterned, by means of a resist layer, to provide opening  118 , which is approximately in the center of the area where the porous silicon is desired. Anodic oxidation is then performed to create region  106  of porous silicon. 
     A simplified version of the isolation areas is seen in FIG. 1B, region  106 , which overlies substrate  100  has four regions of different porosities. In the uppermost region  106 A, a current density of approximately 1-40 mA/cm2 is used to create a region about 50-500 nm thick, having a porosity of approximately 25-50 percent. This low porosity provides desirable material strength in the region of porous silicon. 
     The current density is then changed to about 35-100 mA/cm2 to create a porosity of approximately 60-80 percent in region  106 B. The thickness of this region will be about 30 percent of the total thickness of the porous silicon region and provides the major part of the isolation. Next, a current density of approximately 1-40 mA/cm2 forms region  106 C with a porosity of about 25-50 percent to provide further material strength. This region has a thickness about 20 percent of the total thickness of the layer. 
     Finally, a current density of about 35-100 mA/cm2 forms final region  106 D, which is about  1  micron thick. A porosity of about 60-80 percent provides stress reduction between the porous silicon and the underlying substrate. 
     After formation of the porous silicon regions, processing of the wafer continues with deposition of the interlevel dielectric and formation of metallization layers. 
     Third Embodiment: Use in Mixed Signal Applications 
     Patent application Ser. No. 09/120,215, filed Jul. 21, 1998, which is hereby incorporated by reference, discloses a method for forming lateral porous silicon isolation in high frequency and mixed signal circuits. The use of a variable porosity porous silicon in this application will now be discussed with reference to FIGS. 4A-B. 
     In FIG. 4A, silicon substrate  1  is overlain by a mask layer  3 , e.g. of silicon carbide, except in windows  2  where the formation of porous silicon is desired. Regions of porous silicon  5  are formed through windows  2  using, e.g., a process such as described above for the RF embodiment. 
     After stripping of the SiC layer, digital circuitry  7  and analog circuitry  9  are formed in different regions of the wafer, separated by the porous silicon. A dielectric can be formed over the surface of the wafer, with interconnects  13  connecting the digital and analog circuitry, at least some of the interconnects crossing the region of porous silicon. 
     Alternate Solutions for Anodic Oxidation 
     In the anodic oxidation process, other surfactants can be used, such as methanol, isopropanol, and TritonX-100, the latter a wetting agent well known in the industry. It appears in preliminary tests that TritonX-100 can be used in much smaller quantities, e.g. 1-3 percent of the total solution. Additionally, the literature on anodic oxidation includes references to numerous organics which are being investigated. Thus, the wetting agents mentioned do not constitute an exhaustive list, but merely a sample of the possibilities. 
     In addition to changing the surfactant used, the proportions of the materials can be changed to produce porous silicon with the desired characteristics. 
     Alternate Embodiment: Varying Electrolyte 
     In a less preferred embodiment, the porosity can also be modified by changing the electrolyte concentrations, with lower HF concentrations yielding higher porosity. It is anticipated, however, that it will be important not to expose the wafer to air, e.g., by pulling the wafer from one electrolyte bath and immersing in another bath during the anodization process. This embodiment is considerably slower than the preferred embodiment, since once the electrolyte were altered, it would be necessary to allow time for the concentration change to wick back into the pores before anodization could be continued. For example, the electrolyte concentration can be changed by pumping in more water to reduce HF levels and increase porosity, but a waiting period is necessary to allow the electrolyte concentrations to approach equilibrium within the pores before the current is turned on. 
     Modifications and Variations 
     As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given, but is only defined by the issued claims. 
     It should be obvious to one of ordinary skill in the art that the disclosed embodiments are not the only instances in which variable porosity can be created in a porous silicon structure. The disclosed process can be used in many other applications which utilize porous silicon. 
     In alternate embodiments, the variable porosity can be produced in porous silicon layers used in other isolation structures, with appropriate modification of the parameters. 
     In a further alternate embodiment, the isolation for RF applications can be performed prior to the formation of transistors.