Patent Abstract:
A silicon element having macrocavities beneath its exterior surface is fabricated by electrochemical etching of a p-type silicon wafer. Etching at a high current density results in the formation of deep macrocavities overhung by a layer of crystalline silicon. The process works with both aqueous and non-aqueous electrolytes.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit of U.S. Provisional Patent Application 60/242,726, filed Oct. 24, 2000, the disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to processes for etching silicon to form macroscopic cavities within the interior of a silicon wafer, each cavity having at least one opening connecting to the exterior surface of the wafer. 
     A number of references teach the electrolytic etching of silicon in acidic solutions. For example, German Patent No. 3,324,232 to Foll et al. teaches an etching process whereby a number of honeycomb pattern of open cells are formed in the surface of a silicon wafer, thereby increasing its effective surface area. 
     U.S. Pat. No. 5,544,772 to Soave et al. proposes the fabrication of microchannel plate devices by light-assisted electrochemical etching of n-type &lt;100&gt; silicon. The light-assisted electrochemical etching process described in the &#39;772 patent is applied only to n-type silicon, and a light source is required to generate surface charges so that etching may proceed. 
     As described, for example, in Lehmann et al., Formation Mechanism and Properties of Electrochemically Etched Trenches in N-Type Silicon, J. Electrochemical Society, Vol. 1-7, No. 2, pp. 653-659 (1990) and in U.S. Pat. No. 4,874,484, light-assisted electrochemical etching of n-type silicon produces deep channels perpendicular to the surface of the silicon. If the silicon surface is provided with pits at preselected locations, the channels form at the pits and hence at the same preselected locations. As described in these references, and in numerous other references, it has long been believed that the mechanism responsible for such selective etching limited its application to n-type silicon. 
     U.S. Pat. No. 5,997,713 to Beetz, Jr. et al. disclosed the successful application of controlled deep-channel etching to p-type silicon. In processes disclosed in the &#39;713 patent, the silicon surface is provided with pits at preselected locations with the result that channels are etched into the silicon body at the same preselected locations as the pits. 
     Propst et al., The Electrochemical Oxidation of Silicon and Formation of Porous Silicon in Acetonitrile, J. Electrochemical Society, Vol. 141, No. 4, pp. 1006-1013 (1994), discloses the formation of deep channels at random locations in a p-type silicon body using electrochemical etching with a non-aqueous, anhydrous electrolyte. This reference does not disclose processes for etching channels at preselected locations. Moreover, this reference emphasizes that the use of aqueous electrolytes results in formation of highly branched, porous structures rather than trenches, cavities or other non-branched deep structures. Similar teachings are found in Rieger et al., Microfabrication of Silicon by Photo Etching, The Electrochemical Society Proceedings, Vol. 94-361 (1994). U.S. Pat. Nos. 5,348,627 and 5,431,776 to Propst et al. relate to this same work. 
     Despite these and other efforts in the art, substantial needs remain for further improvements in processes for forming buried cavities in silicon elements. It would be desirable to provide a process for forming cavities in p-type silicon at preselected locations. P-type silicon wafers are fabricated in large numbers for use in manufacture of conventional silicon semiconductor devices. Therefore, p-type wafers are readily available at low cost. 
     It would be particularly desirable to produce silicon elements having a number of macroscopic cavities beneath the surface of the silicon element wherein the cavities are covered by a layer of monocrystalline silicon near an exterior surface of the element. Structures having a monocrystalline region overhanging a cavity would be desirable in standard silicon device processing to produce active and passive microelectronic structures. It is not possible to produce such structures using a single-etching process by the state-of-the-art processing methods described above. While it is possible to make such overhung cavities by combinations of standard processing methods and wafer bonding, these approaches are less efficient than a single-etching process would be, because they require additional processing time, equipment and materials. 
     Applications such as high-speed radio frequency (RF) electronics, e.g., those used in cellular communications, require microelectronic devices having silicon layers that are electrically isolated from the bulk wafer substrate. It is desirable, but not possible in current practice, to produce such layers by removing material between the silicon layer and the bulk of the wafer in a single process. Moreover, it is desirable to produce thin layers of silicon oxide, as could be produced by etching macrocavities beneath a layer of crystalline silicon, then subjecting the remaining silicon to thermal treatment in an oxygenated environment. Such layers can be produced, at present, by SIMOX processing where oxygen atoms are injected individually beneath the surface of a silicon body, but the silicon body is subject to radiation damage that is intrinsic to the SIMOX process. 
     Other technologies, such as inkjet printing, slow-release drug delivery systems, and miniature reagent supply/storage reservoirs for “lab-on-a-chip” chemical analysis systems require a plurality of precisely placed reservoirs on a single chip. Under current techniques, such reservoirs are produced by etching wafers of diverse materials and bonding them to each other. It is desirable to produce such structures by a single process. 
     The processes used to etch voids in silicon heretofore have operated at relatively low etch rates, so that the dimensions of the voids increase at less than 1 μm per minute. It would be desirable to form cavities at a faster rate to reduce the cost of the process. 
     Moreover, processes which require anhydrous electrolytes incur additional costs due to the precautions that must be taken to eliminate water from the solvents and to isolate the process from moisture in the environment. These processes incur further costs associated with purchase and disposal of the required organic solvents. It would be desirable to eliminate these costs by providing an aqueous etching method. 
     SUMMARY OF THE INVENTION 
     A method according to a preferred aspect of the invention begins with providing a p-doped silicon element having front and back surfaces. The method further includes the steps of forming a plurality of pits at preselected locations on the front surface of the element and subjecting the pitted silicon element to electrochemical etching. In the electrochemical etching procedure, the front surface of the element and a counter-electrode are maintained in contact with an electrolyte while maintaining the silicon element at a positive potential with respect to the counter-electrode. A patterned electrode provides electrical contact at the back surface of the element within discrete regions which are aligned with the pits of the front surface. The element is etched preferentially at the pits to form cavities beneath the wafer surface. The silicon body is maintained at a positive potential relative to the counter-electrode, and the electrochemical cell is operated at a constant current density throughout the etching process. Preferably, the current density is maintained at a value on the order of twenty times as large as the current densities typically used to etch p-type silicon. More preferably, the current density is maintained at a value between 0.05 and 0.9 amps/cm 2 , most preferably at a value of about 0.4 amps/cm 2 . As cell impedance decreases, the voltage is allowed to decrease so that the current density is maintained near a constant value. 
     The term “p-doped silicon” as used in this disclosure refers to silicon having an appreciable quantity of p-type dopants such as B, Al and Ga, which tend to form positively charged sites, commonly referred to as holes in the silicon crystal lattice. Desirably, the silicon element contains at least about 10 14  and more preferably at least about 10 15  atoms of p-type dopants per cubic centimeter. The silicon element therefore has an appreciable number of holes in the silicon crystal lattice. The p-type material may optionally include some n-type dopants as well as the p-type dopants, and its electrical characteristics may be p-type, n-type or compensated. Most typically, the material is doped only with p-type dopants, or with an excess of p-type dopants over n-type dopants, and hence exhibits p-type electrical conductivity with holes as the majority carriers. 
     This aspect of the present invention incorporates the discovery that when a p-doped silicon body is provided with pits at preselected locations in its exposed surface, electrochemical etching will proceed at these preselected locations. Most preferably, the silicon element is substantially monocrystalline silicon, such as a wafer of the type commonly used in a semi-conductor fabrication or a portion cut from such a wafer. Desirably, the exposed surface of the silicon element is a &lt;100&gt; surface of the crystal. 
     According to a particularly preferred aspect of the invention, the electrolyte is an aqueous electrolyte which includes fluoride ions and a surfactant. The aqueous electrolyte desirably has a pH of about 1 to about 4, more desirably about 2 to about 4 and most desirably about 3 to about 4. Most desirably, the electrolyte includes an acid other than hydrofluouric and a fluoride salt as a source of fluoride ions. Whether the fluoride ions are added as HF or as salt, the resulting electrolyte contains some HF. The aqueous electrolyte desirably has an HF concentration at least 0.25 M and more desirably between about 0.25 M and 10 M. HF concentrations of about 1.5 M to about 2 M are most preferred. Inorganic acids and salts are preferred. For example, the electrolyte may include HCl and NH 4 F. This preferred aspect of the present invention incorporates the realization that the teachings of the art, to the effect that etching of the p-type silicon with an aqueous electrolyte will result only in a branched microporous structure, are incorrect. 
     The most preferred processes do not require either the expense of anhydrous processing or the expense and hazards associated with handling and storing liquid HF as a starting reagent. 
     Methods according to the foregoing aspect of the present invention can be used to fabricate structures with numerous macrocavities buried within p-type silicon elements. The cavities can be placed at any desired locations on the silicon element. Etching p-doped monocrystalline silicon according to the preferred processes of the invention produces macroscopic cavities buried within the body with a uniform layer of crystalline silicon overlying the macrocavity. The thickness of the layer can be controlled by the initial diameter of the pits etched into the front surface of the silicon element. The use of particularly preferred etching processes produces monocrystalline silicon bodies wherein at least a major portion of the volume below the layer of crystalline silicon has been etched away, effectively isolating the silicon layer from the rest of the silicon body. An etching process according to the present invention may also be used to fabricate a silicon body having a single macrocavity with an overhanging layer of crystalline silicon, with the macrocavity occupying a major portion of the planar area beneath the surface of the silicon body. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-4 are fragmentary diagrammatic sectional views of a silicon element at progressively later stages of treatment in a manufacturing process according to an embodiment of the present invention. 
     FIG. 5 is a microphotographic sectional view at 125× magnification of a silicon element etched according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     A preferred process, according to one aspect of the present invention, for fabricating a microchannel plate from a p-type silicon wafer begins with providing a p-type silicon element such as a substantially monocrystalline p-doped silicon wafer  10  having a front surface  12  and a rear surface  22 . Front surface  12  is oxidized or nitrided to form a front surface layer  16 . A pattern is transferred into front surface layer  16  using standard photolithographic techniques. The pattern may consist of any desired arrangement of circular or other shaped holes or apertures  18 . The pattern of holes, for example, may be a square array of 30 μm diameter circular holes arranged on 300 μm centers. The number and location of holes may be determined by the number and size of the desired macrocavities. Silicon elements with as few as one cavity may be produced. The pattern of holes is transferred to the silicon oxide/nitride surface by coating the surface with a photoresist (not shown), properly curing the photoresist, and then exposing the photoresist-covered surface with an appropriate light source that has passed through a photolithographic mask containing the desired pattern of openings. The photoresist is then developed, and the oxide or nitride layer is then etched using either wet or dry etching techniques to expose the underlying silicon substrate. The photoresist mask may then be removed. The silicon substrate is then etched in a separate step to form depressions or pits  20  in the silicon exposed by the opening  18  in layer  16 . Pits  20  serve as preferential etch sites during the electrochemical etching process. A preferred method for making these depressions is to anisotropically etch the silicon in a solution of potassium hydroxide to produce an array of pyramidal pits in the &lt;100&gt; silicon surface having the same periodicity as the pattern on the photolithographic mask. The silicon oxide/nitride layer  16  may then be removed. Preferably, the resulting pits have relatively large, open ends at the front surface  12  and relatively small ends pointing into the silicon element toward the rear surface  22 . 
     A patterned electrode is provided to establish electrical contact with discrete regions of back surface  22 . Preferably, at least some of these regions are aligned with at least some of the pits  20  in front surface  12 . Preferably, patterned electrode  32  is formed on the back surface of the wafer  10 . More preferably, back surface  22  is oxidized or nitrided to form back surface layer  26 , and openings  28  are created within the oxide/nitride layer, exposing back surface  22  within discrete regions of back surface layer  26 . The same photolithographic techniques may be used to create a pattern of openings in back surface layer  26  as were used to create openings in front surface layer  16 . Preferably, the pattern consists of openings  28  which are aligned with openings  18  in front surface layer  16 . The pattern may be transferred and the exposed oxide/nitride areas etched as described above. In contrast to the method of preparing front surface layer  16 , it is preferred that back surface layer  16  not be subjected to an anisotropic etch. The back side of the wafer  10  is then implanted with boron to produce a heavily doped region near back surface  22  of wafer  10 . Following boron implantation, metal is deposited onto back surface layer  26  to form the patterned electrode  32 . This metallization step may comprise evaporating aluminum metal onto back surface layer  26  and openings  28  and providing a consolidating heat treatment at temperatures of about 400° C. to about 480° C. to form a good low-resistance contact with wafer  10  at regions  28 .  24  The wafer  10  is then placed into an electrochemical cell with front surface  12  facing into the cell cavity. The ratio of the exposed surface area of the silicon wafer  10  to the exposed surface area of the counter-electrode  34  may be from about 0.2 to about 100. The cell has a platinum cathode or counter-electrode  34 , and silicon wafer  10  serves as the anode. The cell is filled with an aqueous electrolyte  36  containing fluoride and desirably having a pH of about 1 to about 7, more desirably between about 3 and about 4. The fluoride concentration desirably is about 0.25 to about 5 M. The electrolyte may consist essentially of HF and water, and a surfactant. More preferably, the electrolyte includes an acid other than HF and a fluoride salt, with or without a surfactant. Inorganic acids and salts are preferred. The preferred inorganic acids include HCl, H 2 SO 4  and H 3 PO 4 , whereas the preferred inorganic fluoride salts include NH 4 F and flouroborate salts such as NH 4 BF 4 , and HBF 4 . The surfactant may be anionic, cationic or nonionic. Suitable surfactants include ethanol, formaldehyde and the material sold under the trademark Triton X-100. The surfactant is added in an amount effective to promote wetting of the silicon surface by the electrolyte. Aqueous electrolytes and etchants disclosed in commonly assigned U.S. Pat. No. 5,997,713, the disclosure of which is incorporated by reference, are suitable for use in preferred processes of the present invention. 
     The wafer  10  is biased to a positive voltage relative to counter-electrode  34 . The cell is operated at initial voltages in excess of 5 volts up to as much as 25 volts. The cell is operated in a current-controlled mode so that as the cell impedance decreases, the voltage also decreases so as to maintain the electrochemical current density near a constant value. Preferably, the cell is initially biased to produce an electrochemical current density on the order of 20 times larger than the current densities typically employed in ordinary anodic etching of p-type silicon as practiced, for example, in commonly owned U.S. Pat. No. 5,997,713. More preferably, the current density is maintained at a value between 0.05 and 0.9 amps/cm 2  based on the area of the exposed silicon surface without considering any increase in surface area due to the presence of pits or cavities. Most preferably, the current density is maintained at a value of about 0.4 amps/cm 2 . Under the preferred conditions, the electrochemical cell operates at higher voltages than are normally employed to etch p-type silicon. The largest voltage drop occurs at the silicon-electrolyte interface, so that electrons entering the silicon during removal of a silicon atom from the surface of the cavity are injected into the body of the silicon element with an excess kinetic energy. These energized electrons then produce impact ionization that locally accelerates the etching process. 
     Under the preferred operating conditions, a nearly isotropic etching proceeds from the tip of etch pit  20  in contact with the etchant  36 . The etch front propagates parallel to the front surface  12 , causing lateral expansion of the cavity, e.g., from sidewall location  46   a  to sidewall location  46 , and towards the back surface  22  causing extension of the cavity, e.g., from back wall location  44   a  to back wall location  44 . Expansion toward front surface  12  is negligible, resulting in formation of an overhanging layer  42  of monocrystalline silicon. These movements of the etch front appear to occur because the regions from which current can originate are limited to those regions where electrode  32  contacts back surface  22 . The electrons are swept to these contact points by the applied electric field, preventing the etch front from propagating toward front surface  12 . The use of a patterned electrode  32  also induces a higher operating voltage for a given current density relative to the voltage required for a wafer having an electrode in contact with the entire back surface  22 . As the cavity enlarges, the voltage on the cell decreases due to the increasing surface area being etched. 
     Etching processes according to preferred embodiments of the invention lead to the etching effects shown in FIG. 5 which shows the front surface  52  of the silicon element, the overhanging layer of monocrystalline silicon  62 , macroscopic cavities  60  and wafer back side  54 . Back wall  64  and side walls  66  of cavities  60  are also shown. The openings in front surface  52 , similar to openings  20  in front surface  12  shown in FIG. 4, are not visible in the cross-section of FIG.  5 . 
     The thickness of overhanging silicon layer  62  is substantially uniform across the planar area occupied by macrocavities  60 . The thickness of overhanging layer  62  may be controlled by the initial depth of pit  20 , which will be approximately the same as the diameter of the opening  18 . This effect appears to be determined by the geometry of the silicon crystal. When the exposed surface of the silicon element is a &lt;100&gt; surface of the crystal, the caustic anisotropic etch produces a pit that has a sloping wall along the &lt;111&gt; plane, i.e., 54 degrees off the vertical plane relative to the exposed surface, and is, therefore, approximately as deep as opening  18  is wide. Since etching proceeds most rapidly along the &lt;100&gt; plane, the etching front moves parallel to front surface  12 , resulting in an overhanging layer  62  that has a thickness roughly equivalent to the initial depth of pit  20 . 
     Under preferred embodiments of the etching process, the extent of the lateral expansion of the macrocavities is self-limiting. The final thickness of sidewall  66  between adjacent cavities  60  is about two to three times the diameter of the opening for the illustrative sample presented herein. The ratio of wall thickness to the diameter of the opening in the front surface of the silicon body can be controlled by altering the resistivity of the silicon element, higher resistivity resulting in a greater final thickness of sidewall  66 . The back wall  64  of cavity  60  continues to move toward back surface  54  after lateral expansion ceases. The lateral extent of the cavities is limited by the spacing of the openings in front surface  52  and the thickness of sidewalls  66 . 
     The self-limiting nature of the lateral expansion creates sidewalls that effectively isolate adjacent macrocavities from each other. The resulting macrocavity is physically isolated from the adjacent cavities and communicates with the exterior of the silicon element only through the initial opening etched in the front surface  52 . The pyramidal shape of back walls  64  apparently is controlled by the geometry of the crystal as discussed with regard to pits  20 . 
     In another preferred aspect of the invention, a single pit is etched into front surface  12  of silicon body  10 . Electrode  32  is provided to establish electrical contact within a discrete region of back surface  22 , preferably aligned with the pit formed in front surface  12 . The etching process proceeds as described herein, resulting in a silicon body  10  having a single macrocavity with an overhanging layer of crystalline silicon. Operating conditions of current density and duration, and the resistivity of silicon body  10 , are selected to control the extent of etching. These conditions may be adjusted to produce a silicon body in which the macrocavity occupies a major portion of the planar area beneath the front surface  12 . 
     According to other preferred aspects of the invention, the electrolyte may be a non-aqueous electrolyte such as anhydrous acetonitrile with tetrabutylammonium perchlorate and hydrogen fluoride. Other non-aqueous electrolytes such as dimethylformamide, dimethylsulfoxide, diethylene glycol, propylene carbonate, methylene chloride and the like may be employed. Sources of fluoride ions other than HF may also be employed in the non-aqueous electrolyte. Tetrabutyl ammonium perchlorite may be added to increase the electrical conductivity of the electrolyte. Other additives may be employed for the same purpose. Several non-limiting examples of non-aqueous electrolytes that are suitable for use with the present invention are disclosed in commonly assigned U.S. Pat. No. 5,997,713. When using a non-aqueous electrolyte, it is important to keep the residual amount of water in the solution low, i.e., at less than 100 ppm. Preferably, initial current densities in accordance with the present invention will be on the order of 20 times as great as those disclosed in the examples of U.S. Pat. No. 5,987,713. 
     The etching processes discussed above are light-insensitive. The processes do not depend upon the presence of light for operation, and can be conducted in essentially any lighting conditions, including the complete absence of light or normal room lighting. Although some holes may be formed in the p-doped silicon by incident light, such holes are insignificant in comparison to the number of holes present as a result of p-doping. 
     The following non-non-limiting example describes the conditions under which the etching effects shown in FIG. 5 were produced: 
     A 200 mm diameter silicon wafer was patterned with 30 micron openings on 300 micron centers using conventional semiconductor processing techniques. 
     The patterned silicon was then subjected to an isotropic KOH etch to produce a pyramidally-shaped depression in the silicon surface. The masking layer on the front surface of the wafer was then removed. A pattern of 30 micron openings on 300 micron centers was patterned on the back surface of the wafer, also using conventional semiconductor processing techniques, with the pattern array aligned to the pattern on the front side of the wafer. The back side of the wafer was then implanted with boron in the patterned area, and the wafer back side was metallized and given a consolidating heat treatment at about 400° C. The wafer was placed in an electrochemical cell containing the following electrolytes and operating at the specified voltages and current density. The silicon wafer was biased to a positive potential relative to a platinum wire cathode. 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 1. 
                 Electrolyte 
                   
               
               
                   
                   
                 NH 4 F (40 wt % aqueous solution) 
                 700 ml 
               
               
                   
                   
                 HCl (36.5 wt % aqueous solution) 
                 300 ml 
               
               
                   
                   
                 H 2 O 
                 2100 ml  
               
               
                   
                 2. 
                 Cell Operating Conditions 
               
               
                   
                   
                 Initial: 18 mA at 25 V 
               
               
                   
                   
                 Final: 18 mA, 1 V 
               
               
                   
                   
                 Duration: 4 hours 
               
               
                   
                   
               
             
          
         
       
     
     Typical etch rates under these conditions are 100 to 150 microns per hour. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

Technology Classification (CPC): 7