Abstract:
Methods of fabricating highly conductive regions in semiconductor substrates for radio frequency applications are used to fabricate two structures: (1) a first structure includes porous Si (silicon) regions extending throughout the thickness of an Si substrate that allows for the subsequent formation of metallized posts and metallized moats in the porous regions; and (2) a second structure includes staggered deep V-grooves or trenches etched into an Si substrate, or some other semiconductor substrate, from the front and/or the back of the substrate, wherein these V-grooves and trenches are filled or coated with metal to form the metallized moats.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit under 35 U.S.C. §119(e) of the following co-pending and commonly-assigned U.S. Provisional Patent Application Ser. No. 60/331,854, entitled “METHODS OF FABRICATING HIGHLY CONDUCTIVE REGIONS IN SEMICONDUCTOR SUBSTRATES FOR RADIO FREQUENCY APPLICATIONS,” filed on Nov. 20, 2002, by KingNing Tu, Ya-Hong Xie and Chang-Ching Yeh, which application is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to fabricating semiconductor devices, and more particularly, to methods of fabricating highly conductive regions in semiconductor substrates for radio frequency applications. 
   2. Description of the Related Art 
   It has been a recent trend in the Si (silicon) integrated circuit industry to integrate radio transmitters and other radio frequency (RF) devices onto digital integrated circuits. Such integration requires RF shielding to prevent interference with other noise sensitive portions of the integrated circuits. 
   Oxidized porous Si has been used to provide effective DC (direct current) isolation. However, oxidized porous Si cannot be made too thick because of the thermal expansion coefficient mismatch between oxidized porous Si and Si. Therefore, it cannot be used for effective RF shielding, similar to silicon nitride and silicon dioxide films. On the other hand, the use of unoxidized porous Si as an insulating material has been successful in reducing RF crosstalk to a level identical to that across a vacuum. 
   However, there is a need in the art to further reduce crosstalk for high-end RF applications. The present invention satisfies that need. 
   SUMMARY OF THE INVENTION 
   The present invention describes methods of fabricating highly conductive regions in semiconductor substrates for radio frequency applications. These methods are used to fabricate two structures: (1) a first structure includes porous Si regions extending throughout the thickness of an Si substrate that allows for the subsequent formation of metallized posts and metallized moats in the porous regions; and (2) a second structure includes staggered deep V-grooves or trenches etched into an Si substrate, or some other semiconductor substrate, from the front and/or the back of the substrate, wherein these V-grooves and trenches are filled or coated with metal to form the metallized moats. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
       FIG. 1  illustrates a structure for incorporating highly conductive metallic regions into semiconductor substrates according to the preferred embodiment of the present invention; 
       FIG. 2  illustrates an alternative embodiment of  FIG. 1 , wherein a highly conductive metallized moat is formed by staggered deep V-grooves etched into the substrate; 
       FIG. 3  illustrates an alternative embodiment of  FIG. 1 , wherein a highly conductive metallized moat is formed by a deep trench etched into the substrate; 
       FIGS. 4A ,  4 B and  4 C are cross-sectional side views of the structures of  FIGS. 1 ,  2  and  3 , respectively; 
       FIG. 5  is a flowchart illustrating the process steps used in creating metallized porous Si regions according to the preferred embodiment of the present invention; 
       FIG. 6  is a flowchart illustrating the process steps used in creating V-grooves for a metallized moat according to the preferred embodiment of the present invention; and 
       FIG. 7  is a flowchart illustrating the process steps used in creating trenches for a metallized moat according to the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     FIG. 1  illustrates a structure for incorporating highly conductive metallic regions into semiconductor substrates according to the preferred embodiment of the present invention. A Si substrate  10  is divided into a noisy circuit area  12  and a noise sensitive circuit area  14 , which are separated by a metallized moat  16 . The highly conductive metallized moat  16  is formed from metallized porous Si regions extending through the thickness of the substrate  10 . The noisy circuit area  12  also includes metallized posts  18 , created from metallized, localized porous Si regions and extending through the thickness of the substrate  10 , wherein the metallized posts  18  act as “true ground” points. 
     FIG. 2  illustrates an alternative embodiment of  FIG. 1 , wherein the highly conductive metallized moat  16  is formed by staggered deep V-grooves etched into the Si substrate  10 , from both the front and the back of the substrate  10 , wherein these V-grooves are filled with metal. 
     FIG. 3  illustrates an alternative embodiment of  FIG. 1 , wherein the highly conductive metallized moat  16  is formed by a deep trench etched into one side (either front or back side) of the Si substrate  10 , wherein the trench is also filled with metal. 
     FIGS. 4A ,  4 B and  4 C are cross-sectional side views of the structures of  FIGS. 1 ,  2  and  3 , respectively.  FIG. 4A  illustrates the metallized porous Si regions  16  or  18  extending through the substrate  10 ,  FIG. 4B  illustrates the staggered deep V-grooves  16  etched into the Si substrate  10  from both the front and the back of the substrate  10 , and  FIG. 4C  illustrates the deep trench  16  etched into one side of the Si substrate  10 . 
   Within the realm of integrated circuit technology, there are two potential applications using the present invention. Both applications address important issues associated with mixed-signal integrated circuits, which comprise a family of newly emerged type of integrated circuits that is used for cellular telephones, portable electronics, high speed modems, and data storage devices, such as computer hard drives. 
   A first application provides one or more metallized posts  18  on the substrate as low impedance paths to ground which is typically located at the backside of the chip. These posts  18  can be used as “true ground” points, i.e. points with very low impedance contact to the ground potential outside the chip, across the substrate  10 . Such posts  18  have very short paths to ground points, as compared to typical ground lines in conventional Si integrated circuit technology. As a result, these posts  18  have much lower impedance to ground, especially for high frequency signals. 
   A second application creates the metallized moat  16  from a metallized porous Si region. Alternatively, the second application creates the metallized moat  16  by etching deep V-grooves or trenches in the Si substrate  10  and then depositing metals in the V-grooves or electroplating the trenches. In this second application, the metallized moat  16  shields the noise sensitive circuits  14  from high frequency noise generated by the noisy circuits  12 . The metallized moat  16 , in essence, creates a conducting cage, which is an electromagnetic shield that reduces RF crosstalk between the circuits  12  and  14 . 
   As noted above, metallized porous Si regions can be used for both the first and second applications, whereas the metal-filled deep V-grooves and trenches are more suited for the noise isolation of the second application. Moreover, while the preferred embodiment uses Si substrates, other semiconductor substrates, such as GaAs (gallium arsenide) and InP (indium phosphide), may be used in alternative embodiments, especially in applications involving the V-grooves and trenches. 
     FIG. 5  is a flowchart illustrating the process steps used in creating the metallized porous Si regions according to the preferred embodiment of the present invention. 
   Block  20  represents the formation of porous Si regions being performed by anodization, which is a well-known art that was first invented about half a century ago. In this step, the surface of the Si substrate is exposed to HF (hydrogen fluoride) containing an electrolyte. Porous Si forms into the Si substrate when an electrical current is passed through the Si-electrolyte interface with the Si substrate acting as the anode. Adjusting the HF concentration in the electrolyte and the current density during anodization alters the microstructure of the porous Si region so formed. 
   Block  22  represents the metallization of the porous Si regions being performed. The porous Si regions, with their interconnected pores, provide an excellent skeleton for metal deposition. Metals can be introduced into the porous Si regions in a number of different ways: by vapor deposition, solid state interdiffusion and reaction, and liquid state penetration. Because the porous Si regions each have a very large interconnected internal surface area, a capillary effect can be used to facilitate the penetration of any low melting point molten metal that wets the surface of the Si substrate throughout the entire porous Si region. 
   Two metals that fit the low melting point requirement are Au (gold) and Al (aluminum). The eutectic point of Au—Si is 370 degrees C. and that of Al—Si is 577 degrees C. Both metals are acceptable with respect to processing of integrated circuit devices on Si substrates. 
   Since Au is a deep trap impurity in Si, the Au penetration is followed with a penetration by molten Sn (tin) or an Sn-based solder. Sn will fill up the pores of the porous Si regions. Moreover, Sn serves the important function of retaining Au from outdiffusion, because of Au—Sn intermetallic compound formation. Sn also provides mechanical strength to the otherwise porous structure. 
     FIG. 6  is a flowchart illustrating the process steps used in creating the V-grooves for the metallized moat  16  according to the preferred embodiment of the present invention. 
   Block  24  represents the V-grooves being created along a [110] direction on an (001) surface of the Si substrate can be created using standard lithography techniques, followed by an anisotropic wet etching in solutions such as KOH (potassium hydroxide). The step may prepare V-grooves on both surfaces of the Si substrate for isolation purposes. The width of the V-groove is selected to give a depth of the V-groove that is about half the thickness of the substrate. 
   Block  26  represents the metallization of the V-grooves being performed. A lift-off process is used to deposit a multilayer metallic thin film (the total thickness of which is preferably on the order of a few hundred nanometers) into the V-grooves. For example, a trilayer of Cr/Cu/Au (chromium/copper/gold) or Ti/Ni/Pd (titanium/nickel/palladium) can be used. The resulting structure can be strengthened by a flow of molten solder into the V-grooves using a horizontal capillary effect. The solder may be Pb-free (lead-free) alloys such eutectic SnAg (tin-silver) or SnAgCu (tin-silver-copper) with a melting point around 220 degrees C. 
     FIG. 7  is a flowchart illustrating the process steps used in creating the trenches for the metallized moat  16  according to the preferred embodiment of the present invention. 
   Block  28  represents the trenches being created along a [110] direction on an (001) surface of the Si substrate can be created using standard lithography techniques, followed by an anisotropic wet etching in solutions such as KOH. 
   Block  30  represents the metallization of the trenches being performed. Preferably, the Cu is deposited into the trenches by electro-plating or chemical vapor deposition (CVD). The entire trench can be filled with Cu, or a layer of Cu can be deposited and the rest of the trench filled with molten solder. 
   Conclusion 
   This concludes the description of the preferred embodiment of the invention. The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.