Abstract:
A high purity, non-toxic, environmentally friendly method for anisotropically etching single crystal silicon and etching polysilicon, suitable for microelectronics, optoelectronics and microelectromechanical (MEMS) device fabrication, using high purity aqueous ammonium hydroxide (NH 4 OH) solution generated at the point of use, is presented. The apparatus of the present invention supports generation of high purity aqueous NH 4 OH solution from ammonia NH 3  gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of NH 3 , within a hermetically enclosed chamber at the optimal temperature between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution for achieving a high anisotropic etching rate. Other liquid anisotropic etching methods for silicon may use tetramethylammonium hydroxide (TMAH). In contrast to carbon containing TMAH, the NH 3  gas and H 2 O precursors of NH 4 OH etchant eliminate risk for solid residues to be deposited on silicon due to being composed entirely of elements having a gaseous form at room temperature.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    For the purposes of fabricating electronic, optoelectronic semiconductor and microelectromechanical (MEMS) devices in silicon, it is often necessary to perform liquid anisotropic etching of the silicon through either silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ) masks, whereby the silicon crystal planes such as (100), (110) and (111) planes are etched at different rates. The structures resulting in the silicon following such liquid anisotropic etching processes can consist of pyramidal mesa frustum shapes, inverted pyramidal cavities as well as other geometries. Wet etchants that have been used for etching silicon preferentially along crystallographic planes include aqueous solutions of sodium hydroxide (NaOH), potassium hydroxide (KOH), rubidium hydroxide (RbOH), cesium hydroxide (CsOH). Aqueous solutions of tetramethylammonium hydroxide (TMAH, (CH 3 ) 4 NOH) and tetraethylammonium hydroxide ((C 2 H 5 ) 4 NOH) have also been used for liquid anisotropic etching of silicon. Other substances that are known to etch silicon anisotropically, include ethylene diamine pyrocatechol (EDP) and hydrazine (N 2 H 4 ). Each of the aforementioned liquid anisotropic etchants have drawbacks for application in the anisotropic etching of silicon. The alkali metal hydroxides cannot be used for fabricating electronic and optoelectronic silicon devices due to the nature of the alkali metal ion that acts to degrade the silicon dioxide dielectric material properties in MOS and CMOS type structures. The alkali metal hydroxides can therefore only be utilized for liquid anisotropic etching of silicon in MEMS applications. The anisotropic liquid etchant TMAH does not contain alkali metal cations and can be prepared with sufficient purity to support liquid anisotropic etching of silicon for electronic and optoelectronic device applications. The drawbacks of using TMAH include its toxicity, making it difficult to handle as well as difficult to dispose of, falling under the category of a hazardous waste. The liquid anisotropic etchant EDP, is a very effective etchant but extremely corrosive and even more toxic and carcinogenic than TMAH. It has to be treated and disposed of as a hazardous waste which makes it difficult and costly to use. Hydrazine (N 2 H 4 ) also functions as a liquid anisotropic etchant for silicon but is extremely corrosive, toxic and carcinogenic, thereby complicating its use and making it costly due to the problem of disposal as a hazardous waste. Moreover, hydrazine is an extremely flammable liquid, having very high energy content, making its principal application as a component in fuels for rocket and jet engines. The existing liquid anisotropic etchants have major drawbacks for application to silicon, with TMAH being the least problematic of the ones described for fabrication of silicon electronic and optoelectronic devices. 
         [0005]    The present invention describes a method for implementing liquid anisotropic etching of silicon for the full range of applications including silicon electronic and optoelectronic devices as well as silicon MEMS device fabrication using ammonia (NH 3 ) gas dissolved in high purity deionized water, to form the aqueous base ammonium hydroxide (NH 4 OH) which acts as the anisotropic etchant. An overpressure of NH 3  gas is maintained within the hermetically enclosed etching apparatus, to prevent the dissolved ammonia gas from evaporating from the solution at the elevated temperatures required to effect a high anisotropic etching rate of the silicon in the aqueous NH 4 OH solution. The principal advantage of using aqueous NH 4 OH over other methods to anisotropically etch silicon includes the capability of preparation in an extremely pure form at the point of use by dissolving ultra high purity (99.9999%) semiconductor grade ammonia (NH 3 ) gas into distilled/deionized water that contains the silicon wafer substrate which must be etched. The ammonium hydroxide anisotropic etching solution, similar to TMAH, EDP and hydrazine, does not contain alkali metal cations and therefore can be used for silicon microelectronic device fabrication as well as for MEMS fabrication. In addition, neither the ammonia gas nor the aqueous ammonium hydroxide (NH 4 OH) solution are as corrosive, toxic or carcinogenic as TMAH, EDP or hydrazine and therefore, require only normal precautions for handling. The spent aqueous ammonium hydroxide solution can be easily neutralized with a weak acid and does not constitute a hazardous waste, making disposal environmentally friendly and therefore, far less costly to use. 
         [0006]    To effectively use aqueous ammonium hydroxide for anisotropic etching of silicon, the solution must be maintained at a temperature between 70-90° C. At these temperatures however, the dissolved ammonia will evaporate rapidly from a solution heated in the open atmosphere, thereby diminishing the concentration of ammonium hydroxide in the aqueous solution and inhibiting the etching action of silicon. To prevent evaporation of ammonia from the aqueous NH 4 OH solution at the optimal etching temperature of 70-90° C. and thereby reducing the NH 4 OH concentration in solution, an apparatus must contain the etching solution within a hermetically sealed chamber with an overpressure of NH 3  gas maintained above the NH 4 OH solution at a pressure level above the normal atmospheric pressure. An overpressure of NH 3  gas above the NH 4 OH liquid anisotropic etching solution that prevents further evaporation of NH 3  from the solution can be created in one example approach by dissolving a predetermined weight of ammonia gas into a set volume of deionized water contained in a polytetrafluoroethylene (PTFE) beaker inside the hermetically enclosed pressure chamber at room temperature, to form a fixed and known concentration solution of NH 4 OH. The temperature of the NH 4 OH solution is subsequently raised between 70-90° C. to increase the etch rate of the silicon. Some ammonia will evaporate from the solution at the elevated temperature required for etching, however, the sealed pressure chamber prevents its escape beyond the volume of the apparatus. The pressure in the hermetic chamber increases as more ammonia evaporates from the NH 4 OH solution, eventually reaching an equilibrium steady state between the rate of NH 3  evaporation from the aqueous NH 4 OH solution into the enclosed chamber and NH 3  dissolving back into the solution. The equilibrium will occur at a NH 3  gas pressure above normal atmospheric pressure, and it is for this reason that special apparatus is required for the liquid anisotropic etching method, that is capable of withstanding the pressure at equilibrium of NH 3  above the aqueous NH 4 OH solution, as well as the corrosive effects of NH 3 . 
         [0007]    Although it has been possible to perform liquid anisotropic etching of single crystal silicon using aqueous solutions of alkali metal hydroxides, TMAH, EDP and hydrazine, where for example the (111) crystallographic plane of silicon is etched at a slower rate compared to the (100) and (110) silicon planes, application of these etchants is very much limited due to the contaminating effects of the alkali metal cations to silicon dioxide in MOS and CMOS electronic device structures. For the case of TMAH, and especially EDP and hydrazine, the corrosive, carcinogenic and environmentally hazardous nature of the chemicals requires special safety precautions, making them costly to use. To date, no effective method with supporting apparatus, exists or has been described, for performing very high purity liquid anisotropic etching of silicon, suitable for electronic, optoelectronic and MEMS device applications while supporting etch rates comparable with the aforementioned existing anisotropic etchants and using instead, environmentally clean and minimally hazardous liquid anisotropic etchants. In contrast to the existing technology for liquid anisotropic etching of silicon using alkali metal hydroxides, TMAH, EDP and hydrazine, the versatile method and apparatus of the present invention supports the use of a very high purity aqueous NH 4 OH solution with elevated NH 3  pressure (overpressure) above the aqueous NH 4 OH solution, prepared at the point of use, from two precursors including distilled/deionized water held in a pure fluoropolymer (PTFE) material beaker and very high purity (99.9999%) semiconductor grade NH 3  reacting together in a specially designed and constructed corrosion resistant nickel alloy hermetic chamber, to form the very high purity aqueous NH 4 OH solution for liquid anisotropic etching of the silicon. The apparatus consisting of the specially designed and constructed corrosion resistant nickel alloy hermetic chamber, allows the aqueous NH 4 OH anisotropic etching solution to be heated to an optimal temperature between 70-90° C. to enable a high etching rate of the single crystal silicon or polycrystalline silicon, by preventing the ammonia from evaporating and escaping from the liquid anisotropic aqueous NH 4 OH etching solution. The byproducts of liquid anisotropic silicon etching according to the method of the present invention, include NH 3  gas and unreacted aqueous NH 4 OH solution containing consumed silicon hydroxides. These substances are environmentally friendly by virtue of being easy to neutralize and are minimally hazardous in contrast to TMAH, EDP, and hydrazine. 
         [0008]    As illustrated in U.S. Pat. No. 6,787,052, the method proposed for deep etching of single crystal silicon wafers for fabrication of microstructures within the silicon relies on a first etching step using dry reactive ion etching (RIE) followed by a liquid anisotropic etching step using the well known in the art liquid anisotropic etchants, alkali metal hydroxides, tetramethylammonium hydroxide (TMAH), ethylene diamine pyrocatechol (EDP), gallic acid or hydrazine. The liquid anisotropic etching step of the described method for deep etching of single crystal silicon wafers, does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution for achieving a high anisotropic etching rate. 
         [0009]    As illustrated in U.S. Pat. No. 5,976,767, the method proposed for selectively etching polysilicon using ammonia solution or aqueous NH 4 OH is described, that is selective to silicon dioxide and photoresist. The exposed polysilicon gate which is usually deposited in thin layers of a few tens of nanometers on various substrates, is etched by the aqueous NH 4 OH solution having a 1-5% concentration by volume in water, and maintained at a low temperature between 20-30° C. The described etching method, although using aqueous NH 4 OH solution as the etchant, applies the technique to etching isotropically, only thin layers of polysilicon and is not appropriate for anisotropic etching of single crystal silicon having a thickness of several thousand nanometers. Moreover, the method, does not describe a solution or apparatus that enables increasing the anisotropic etch rate of single crystal silicon or polysilicon using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution for achieving a high anisotropic etching rate. 
         [0010]    As illustrated in U.S. Pat. No. 5,431,777, the method for crystallographically selective etching or anisotropic etching of silicon is presented in the presence of p-doped silicon where part of the silicon is dissolved, while a p-doped pattern in the surface remains largely undissolved. The anisotropic etchant composition of the described method that leaves p-doped silicon largely unetched consists of an aqueous solution of alkali metal hydroxide or tetraalkylammonium hydroxide and a high flashpoint alcohol, phenol, polymeric alcohol or polymeric phenol. The described anisotropic etching method for silicon, does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0011]    As illustrated in U.S. Pat. No. 5,296,093, the method for anisotropically etching a masked polysilicon layer formed over a step on an integrated circuit structure and having oxide portions is presented. The invention describes treating the integrated circuit structure after the polysilicon etch, with an aqueous ammonium-containing base mixed with peroxide solution to selectively remove the polymeric silicon/oxide-containing residues remaining after anisotropic etching of the polysilicon layer. The described anisotropic etching method for polysilicon does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0012]    As illustrated in U.S. Pat. No. 5,207,866, the method for anisotropically etching single crystal silicon is described by placing it in an etching solution consisting of R 4 NOH and solvent wherein R is an alkyl group having between 0 and 4 carbon atoms. The solution will preferentially etch &lt;100&gt; or &lt;110&gt; oriented single crystal silicon, additionally, electrochemical etching may be employed to preferentially etch p-type single crystal silicon. The described anisotropic etching method for single crystal silicon does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0013]    As illustrated in U.S. Pat. No. 5,071,510, the method for electrochemical etching of silicon wafers or plates is described whereby the wafer front-side has a monocrystalline epitaxial layer having a doping type opposite to the remainder of the silicon wafer thereby forming a p/n junction. An organic photoresist film protects the epitaxial layer on the wafer front-side or epitaxy side so that the etchant composed of tetraalkylammonium hydroxide in water solution or in water-free form will etch the wafer back-side and a small voltage bias applied to the junction from the front-side assures an etch-stop at the p/n junction. The described anisotropic etching method for silicon wafers does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0014]    As illustrated in U.S. Pat. No. 4,765,865, the method for increasing the etch rate of a single crystal silicon wafer in anisotropic etching solution by applying a masking layer to part of one face of the wafer and a metal coating to the other face of the wafer making the wafer more anodic than that of only a masked single crystal silicon wafer. Furthermore, an external potential can be applied to the masked and metalized wafer to increase the etching rate on the masked side as long as the potential is less than that which will cause the potential to exceed the passivation potential of a masked single crystal silicon wafer. The described anisotropic etching method for silicon wafers does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0015]    As illustrated in U.S. Pat. No. 4,172,005, the method of etching a semiconductor substrate is described which comprises the steps of mounting a mask for etching on the semiconductor substrate and effecting crystallographically selective etching using an anisotropic etchant comprising an aqueous solution of 0.1-20% by weight of trihydrocarbon-substituted and tetrahydrocarbon-substituted ammonium hydroxide. Preferred are tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide. The described anisotropic etching method for semiconductor substrates does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0016]    As illustrated in U.S. Pat. No. 4,137,123, the method of etching a textured surface into silicon is described using anisotropic etchant. The etchant provides a textured surface of randomly spaced and sized pyramids on a silicon surface and is composed of 0.5-10% by weight silicon and aqueous solutions of alkali metal hydroxides or ammonium hydroxide which optionally contains monohydric, dihydric or polyhydric alcohol where preferably solutions of potassium hydroxide containing isopropyl alcohol or ethylene glycol are employed. The described anisotropic etching method for silicon does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0017]    As illustrated in U.S. Pat. No. 4,113,551, the method of etching polycrystalline silicon with aqueous solution of NR 4 OH, where R is an alkyl group is described. Alternate etching solutions for the polycrystalline silicon may consist of aqueous solutions of N(R m )(H) 4-m OH where R is an alkyl group and m is an integer from zero to four, having a molar concentration in the range from 0.0001 moles to the solubility limit or also, aqueous solution of a monoamine selected from the group consisting of R—NH 2 , R 2 NH, R 3 N, R a R b NH and (R a ) 2 R b N, where R, R a  and R b  are alkyl functional groups and R a ≠R b . The described anisotropic etching method for polycrystalline silicon does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0018]    As illustrated in U.S. Pat. No. 3,738,881, the method of anisotropically etching silicon and germanium is described using a novel etchant comprised of a strongly alkaline aqueous solution, an oxidizing agent, and a passivating alcohol. The etchant will etch germanium at a high rate with the same degree of geometry control as for silicon. The alkaline etchants proposed in the invention include alkali metal hydroxides such as sodium, potassium, rubidium and cesium hydroxide, as well as quarternary ammonium hydroxides. The described anisotropic etching method for silicon and germanium does not propose using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
         [0019]    Note that the above methods for anisotropically etching single crystal silicon or etching polycrystalline silicon do not envision, nor describe a method of using high purity aqueous NH 4 OH solution generated at the point of use from ammonia gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. 
       BRIEF SUMMARY OF THE INVENTION 
       [0020]    The challenges associated with realizing a method of etching single crystal silicon anisotropically using a very clean, high purity etching process suitable for microelectronics, optoelectronics and microelectromechanical (MEMS) device fabrication in a microelectronics clean room setting, can be overcome by using high purity aqueous ammonium hydroxide (NH 4 OH) solution generated at the point of use from high purity (99.9999%) semiconductor grade NH 3  gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of ammonia, within a hermetically enclosed chamber at the optimal temperature required for etching between 70-90° C., to prevent evaporation of NH 3  gas from aqueous NH 4 OH solution, for achieving a high anisotropic etching rate. Etching silicon anisotropically with aqueous NH 4 OH solution at a high rate, requires the solution to be heated to between 70-90° C. Heating aqueous NH 4 OH anisotropic etching solution to the optimal 70-90° C. temperature required for high rate anisotropic etching of silicon, cannot be achieved in the open atmosphere due to the resulting increase in the vapor pressure and evaporation rate of NH 3  gas from the aqueous NH 4 OH solution, thereby, quickly reducing the concentration of NH 4 OH in aqueous solution to very low levels, which slows and eventually stops altogether the etching action. To prevent the loss through evaporation of dissolved NH 3  gas from NH 4 OH solution at the etching temperature, the aqueous NH 4 OH anisotropic etching solution must be enclosed hermetically inside a pressure vessel having fixed volume and fabricated from corrosion resistant material capable of withstanding the effects of hot NH 3  gas, mixed with water vapor. Most stainless steel materials such as grade 302, 303 and 304 will become corroded, therefore, nickel based alloys Inconel 600, C-276 and Nickel 200 or 201 should be used in the construction of apparatus that will support such etching method of the present invention. By maintaining an overpressure of NH 3  gas above the aqueous NH 4 OH solution at the etching temperature, a favorable equilibrium steady state condition can be attained at the etching temperature (70-90° C.) with the rate of evaporation of NH 3  gas from aqueous NH 4 OH solution equal to the rate of NH 3  gas dissolving back into the aqueous NH 4 OH solution described by Henry&#39;s law. The NH 3  gas overpressure above aqueous NH 4 OH anisotropic etching solution will depend on the concentration of NH 3  required in solution for the anisotropic etching application and will usually not exceed between 1-4 atmospheres at the etching temperature of 70-90° C. 
         [0021]    The specialized method and supporting apparatus is described herein that was developed specifically to allow high purity aqueous NH 4 OH anisotropic etching solution for silicon, to be prepared at the point of use, having any required concentration, spanning the full range of possible concentrations from dilute to fully saturated aqueous NH 4 OH anisotropic etching solution for silicon. The preferred embodiment of the apparatus consists of a corrosion resistant Inconel 600, nickel alloy hermetic chamber having fixed volume ranging between 2-4 Liters or larger, depending on the diameter of the silicon wafers that must be processed, and having walls sufficiently thick to withstand the pressure of 1-4 atmospheres at the etching temperature of 70-90° C. Such hermetic vessel has a polytetrafluoroethylene (PTFE) liner to contain the liquid form, aqueous NH 4 OH solution used to anisotropically etch the silicon. The vessel possesses an external electric ring heater at the base of the unit that allows the correct temperature for the aqueous NH 4 OH solution to be set. A mechanical arm with built in thermocouple well containing a thermocouple or RTD sensor and supported by a flexible bellows, provides a means to lower and raise the silicon wafer into the aqueous NH 4 OH etching solution. A gas inlet/outlet port allows the vessel to be evacuated as well as allows NH 3  gas to be delivered at the appropriate pressure into the vessel to be dissolved into distilled/deionized H 2 O contained in the PTFE liner, to form the aqueous NH 4 OH anisotropic etching solution having the required concentration. 
         [0022]    A normal operating procedure for the apparatus supporting the anisotropic etching method of the present invention for silicon, begins when distilled/deionized water is introduced to the PTFE liner within the Inconel 600 etch reactor vessel. The wafer is loaded into the etching reactor vessel and lowered into the distilled/deionized water held in the PTFE liner. The vessel is sealed and first evacuated by removing the air with a pump. Subsequently, the water temperature is raised to between 70-90° C. The high purity (99.9999%) semiconductor grade NH 3  gas is introduced to the chamber either using a pressure regulator or alternatively, with a mass flow controller to form the precise concentration aqueous NH 4 OH solution needed for anisotropically etching the silicon at the required rate. After the correct amount of time for the anisotropic etching has elapsed, the silicon wafer is raised from the etching solution, thereby stopping the etch. The vessel is vented to release the NH 3  gas overpressure and the wafer can be extracted from the vessel and rinsed in preparation for its next fabrication processing step. 
         [0023]    In summary, the principal advantages of the anisotropic etching method of the present invention include first and foremost a cleanliness level of the process that surpasses the other existing methods using other types of liquid etchants for performing high rate anisotropic etching of silicon. For example, etching silicon anisotropically with tetramethylammonium hydroxide (TMAH), currently regarded as one of the cleanest and least contaminating anisotropic etching methods for silicon, still poses a risk of contamination to silicon due to its carbon content. By contrast, the anisotropic etching method described in the present invention uses aqueous NH 4 OH solution generated directly at the point of use from the a gaseous precursor NH 3  and distilled/deionized H 2 O. In contrast to TMAH, the NH 3  gas and H 2 O precursors of NH 4 OH etchant eliminate risk for solid residues to be deposited on the silicon due to being composed entirely of elements having a gaseous form at room temperature. Both NH 3  gas and H 2 O are available at reasonable cost in very high purity, exceeding the highest purity grades available for TMAH. For sensitive, high performance microelectronic and optoelectronic device fabrication in silicon foundry cleanrooms, purity of etchants and chemicals used to process silicon is paramount, together with ease of use. The anisotropic etching method for silicon described in the present invention, using aqueous NH 4 OH, generated at the point of use, provides the very high degree of purity required, together with ease of use which includes zero generation of chemical byproducts hazardous to humans or the environment. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0024]    These and other features of the subject of the invention will be better understood with connection with the Detailed Description of the Invention in conjunction with the Drawings, of which: 
           [0025]      FIG. 1  illustrates the method and apparatus for etching silicon anisotropically using aqueous NH 4 OH solution generated at the point of use inside a hermetically enclosed nickel alloy pressure vessel by dissolving NH 3  gas in distilled/deionized water. 
           [0026]      FIG. 2  illustrates the apparatus that is used to raise and lower the silicon wafer substrate into the aqueous NH 4 OH etching solution together with plumbing fittings including pressure gauge, overpressure safety check valve and NH 3  gas inlet valve. 
           [0027]      FIG. 3  illustrates the drainage system detail in the base flange of the nickel alloy pressure vessel, for recondensed ammonia water vapor (NH 4 OH) outside the PTFE liner, between the exterior walls of the PTFE liner and the walls of the pressure vessel. 
           [0028]      FIG. 4  illustrates the electrical system of the etching apparatus including the ring heater mounted on the base flange of the nickel alloy pressure vessel, temperature controller and RTD input block. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    Referring to  FIG. 1 , a depiction of the hermetically sealed, nickel alloy pressure vessel is shown in a non-scale rendering to be used for liquid anisotropic etching of silicon or etching of polycrystalline silicon using high purity aqueous ammonium hydroxide (NH 4 OH) solution generated at the point of use or in advance of use, from high purity (99.9999%), semiconductor grade ammonia NH 3  gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of NH 3 , within a hermetically enclosed chamber at the optimal temperature between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution for achieving a high anisotropic etching rate. The nickel alloy pressure vessel consists of a flange base plate  10 , manufactured from corrosion resistant nickel alloy such as Inconel 600 or Nickel 200, having a knife edge  11 , that allows the base plate flange to be sealed hermetically using an inert fluoropolymer o-ring for example, to the main pressure vessel body  12 , also manufactured from corrosion resistant nickel alloy such as Inconel 600 or Nickel 200. The main body  12 , of the nickel alloy pressure vessel is characterized by having two end flanges  13  &amp;  14  with bolt holes  15  arranged in a circular pattern, connected by a seamless pipe extension body  12 , where both end flanges are characterized by having knife edges  16  &amp;  17 , that allow sealing the pressure vessel hermetically using a base flange  10 , and a top cover flange  18  using fluoropolymer o-rings  19  &amp;  20  respectively. The main pressure vessel body  12 , with two end flanges  13  &amp;  14 , can be manufactured from a single section of drawn seamless Inconel 600 or Nickel 200 pipe, which is machined to proper form through a succession of lathe and milling operations, thereby providing unparalleled strength and seal integrity as opposed to for example, welding the two end flanges  13  &amp;  14  to the pipe shaped body  12 . 
         [0030]    A liner  21  manufactured from the chemically inert material, polytetrafluoroethylene (PTFE) which tolerates temperatures of up to 250° C., is used to contain the high purity liquid anisotropic etching solution of NH 4 OH which is generated either at the point of use for etching of silicon or in advance of the etching procedure. The NH 4 OH anisotropic etching solution is heated using a heating element in the shape of a ring  22 , affixed to the exterior wall of the base plate flange  10  of the etching reactor pressure vessel. Mounting the ring heater on the exterior wall of the base plate flange  10  of the pressure vessel as opposed to using an immersion type heater suspended directly in the anisotropic etching solution, helps preserve the purity of the NH 4 OH etching solution. The base flange  10 , design is also characterized by having a drain hole  23 , in the interior center of the plate, drilled to a depth half way down into the thickness of the flange  10 , together with a cross-drilled channel with a female pipe thread connection  24 , to facilitate collection and removal of any recondensed ammonia water vapor (NH 4 OH) outside the PTFE liner  21 , between the exterior walls of the PTFE liner  21  and the walls of the pressure vessel  12 . 
         [0031]    A mechanism for controlling the precise start and stop times of the anisotropic etching process for the silicon is shown, whereby, a bellows assembly  25  fabricated either from the corrosion resistant Inconel 600 or Nickel 200 materials together with a supporting mechanical assembly  26 , is used to lower and raise  27  the silicon wafer substrate  28  into and out of the aqueous NH 4 OH anisotropic etching solution. The flexible bellows assembly  25  is welded in between a lower flange plate  29  with knife edge and upper flange plate  30  also with knife edge. The lower flange plate  29  is hermetically sealed to the top flange plate  18  of the etching reactor pressure vessel. A plumbing assembly  31 , also fabricated from corrosion resistant Inconel 600 or Nickel 200 materials supports a thermocouple or RTD temperature sensor well  32 , which has been modified to enable mounting of a susceptor or plate  33  of equal or slightly larger diameter than the wafer substrate that allows the silicon wafer substrate  28  to be mounted loosely to its face using clips  34 . The plumbing assembly  31  contains an inlet valve having a positive shut off capability  35  and fabricated from corrosion resistant Inconel 600 or Nickel 200 material, for evacuation of the etch reactor pressure vessel  12  volume as well as for admitting compressed, high purity (99.9999%) semiconductor grade NH 3  gas into the interior of the etch reactor pressure vessel. 
         [0032]    The outer diameter  36  as well as the inner diameter  37  of the pressure vessel is determined according to the size or diameter of the silicon wafer substrate  28 . To allow for example 2″, 3″ and 4″ diameter silicon wafer substrates to be anisotropically etched in the said etch reactor, a flange outer diameter  36  of 6.75″ will be adequate with an inner diameter  37  of 5″. The height of the PTFE liner  38  can be on the order  6 ″ while the bottom flange thickness  39  and top flange thickness  40  is on the order of 1″. The height of the main pressure vessel body  41 , can be on the order of 8-12″. If the silicon wafer diameter will be 6″ or larger, then all the relevant dimensions which primarily include the etch reactor vessel outer and inner diameters  36  &amp;  37  respectively, must be enlarged and sized appropriately to accommodate the increased diameter of the silicon wafer substrates  28 . 
         [0033]    The preferred embodiment of the anisotropic etching reactor for silicon shown in  FIG. 1  is meant to be operated by generating the high purity aqueous NH 4 OH anisotropic etching solution at the point of use, to be maintained in equilibrium with an overpressure of NH 3 , within the hermetically enclosed chamber at the optimal temperature between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution for achieving a high anisotropic etching rate. Although the aqueous NH 4 OH anisotropic etching solution can also be generated well in advance of the silicon etching operation using the apparatus of the present invention shown in  FIG. 1 , it is advantageous to generate it at the time and point of use by dissolving high purity (99.9999%) semiconductor grade NH 3  gas into distilled/deionized water contained in the PTFE liner  21  in the pressure vessel, in order to reduce the possibility of contamination of the liquid NH 4 OH anisotropic etching solution and consequently the silicon material being etched, due to prolonged storage of the etchant before being used. The etching reactor is meant to be operated by first opening and removing the top flange cover  18  of etching reactor pressure vessel. The PTFE liner  21  is filled with distilled/deionized water and a silicon wafer substrate  28  is attached with clips  34 , to the susceptor or plate  33 . The top cover flange  18 , is sealed back hermetically to the main pressure vessel body  12 . The silicon wafer  28  is lowered  27  into the distilled/deionized water using the mechanical apparatus  26  of the flexible bellows assembly  25  thereby also allowing the RTD contained in the thermowell  32  to sense the temperature of the distilled/deionized water. The electric heater  22  is turned on and the temperature of the distilled/deionized water is set between 70-90° C. using a temperature controller. Once a stable operating temperature between 70-90° C. has been reached, the air from the pressure vessel of the etching reactor  12  is evacuated using a vacuum pump via the valve  35 , followed by the introduction of high purity (99.9999%) semiconductor grade NH 3  gas using either a pressure regulator or mass flow controller. The NH 3  gas metered into the etch reactor pressure vessel partially dissolves into the destilled/deionized water to produce the NH 4 OH anisotropic etching solution of known concentration and fixed 70-90° C. temperature, maintained in equilibrium with an overpressure of NH 3 , within the hermetically enclosed chamber, for achieving a known rate of silicon removal. After the predetermined etching time for the silicon substrate  28 , has elapsed, the etching action can be stopped rapidly by raising  27  the silicon substrate  28  from the aqueous NH 4 OH anisotropic etching solution using the mechanical apparatus  26  with flexible bellows  25 , followed by turning off the power to the heater  22 . The NH 3  gas overpressure can subsequently be vented via the valve  35 , and the top flange plate  18 , removed to recover the etched silicon substrate  28 . 
         [0034]    Referring to  FIG. 2 , a detailed depiction of the mechanical assembly  26 , used to lower and raise  27  the silicon wafer substrate  28  into and out of the aqueous NH 4 OH anisotropic etching solution is shown. A flexible and variable length bellows assembly  25  fabricated either from the corrosion resistant Inconel 600 or Nickel 200 materials together with a supporting mechanical assembly  26 , is used to lower and raise  27  the silicon wafer substrate  28  into and out of the aqueous NH 4 OH anisotropic etching solution. The flexible bellows assembly  25  is welded in between a lower flange plate  29  with knife edge and upper flange plate  30  also with knife edge. A nickel alloy flange with knife edge for hermetic sealing  42 , caps the top flange  30  of the bellows assembly and supports a female pipe thread in the center which supports a plumbing type Inconel 600 or Nickel 200 street-T fitting  31 , into which is threaded a modified thermocouple/RTD well  32 , that has a hollow interior channel  43 , for installing the thermocouple or RTD. A compression fitting  44  mounts to the bottom of the thermocouple well  32  to support a susceptor or plate  33  having the same or slightly larger diameter than the silicon wafer substrate  28  which is held loosely to the plate with clips  34 . Further plumbing hardware is attached to the street-T fitting  31 , near the top of the mechanical assembly including a pressure gauge  45  that indicates the pressure inside the etching reactor chamber of NH 3  gas in equilibrium, above the aqueous NH 4 OH liquid anisotropic etching solution. The safety check valve  46  provides an emergency relief to vent excess NH 3  gas should the NH 3  gas pressure inside the vessel somehow begin to exceed safe limits beyond 4-5 atmospheres. The manual valve  35 , serves to allow the etch reactor pressure vessel to be evacuated prior to introducing high purity (99.9999%) semiconductor grade NH 3  gas for dissolving into the distilled/deionized water to form the high purity aqueous NH 4 OH anisotropic etching solution for silicon. 
         [0035]    Referring to  FIG. 3 , a detailed depiction of the interior or knife edge face of the base or bottom flange  10  of the etching reactor pressure vessel is shown. The flange  10  is manufactured from Inconel 600 or Nickel 200 corrosion resistant material. The bolt holes  15  are arranged in a circular pattern around a machined knife edge  11 . The center of the flange  10  has a machined drain hole  23 , drilled to a depth half way down into the flange with a drilled cross channel  47  that intersects the drain hole  23 . The cross channel  47  has a machined female pipe thread at the outlet  24 , for attaching a drain valve to facilitate collection and removal from the pressure vessel of any recondensed ammonia water vapor (NH 4 OH) outside the PTFE liner, between the exterior walls of the PTFE liner and the walls of the pressure vessel. The angle  48  of the drilled cross-channel  47  relative to the horizontal reference line is shown to be 40 degrees. 
         [0036]    Referring to  FIG. 4 , a depiction of the silicon etch reactor temperature control apparatus with power distribution circuitry for the electric heater is shown. The electric ring shaped heater  22  has a diameter  49  slightly smaller than the diameter of the bottom flange of the etch reactor pressure vessel. The ring heater  22  has two power terminals  50  for supplying electric current to the heater element that warms the aqueous NH 4 OH etching solution. A solid-state relay  51  having two output terminals  52  and two input terminals  53  is mounted on a heat sink  54 , and controls the current delivered at 120 Volts AC to the ring heater  22 . A fuse  55  mounted on a fuse block  56  provides protection from overcurrents or other types of faults that may develop in the electric heater circuit. A standard computer receptacle  57 , allows a standard computer power cord to be used to connect to a 120 VAC power outlet. Two circuit breakers  58  and  59  provide on/off switching capability for the line and neutral return respectively. A temperature controller  60  takes an input from an RTD or thermocouple element that senses the temperature of the aqueous NH 4 OH anisotropic etching solution, and is plugged into the RTD receptacle  61  shown which is connected to the temperature controller  60 . The standard RTD receptacle  61  has three terminals for red wire (positive)  62 , black wire (negative)  63 , and a second black wire for ground  64 . The temperature controller  60  receives its power from a 120 VAC circuit provided from the receptacle  57  and in turn provides electronic control signals  53  to the solid-state relay  51 , in order to tune the flow of current to the ring heater  22 . 
         [0037]    In summary, a novel method and apparatus for implementing very high purity, anisotropic etching of silicon wafer substrates and etching of polycrystalline silicon has been described for application to microelectronics, optoelectronics and microelectromechanical (MEMS) device fabrication, using high purity, aqueous ammonium hydroxide (NH 4 OH) solution generated at the point of use, from high purity (99.9999%), semiconductor grade ammonia NH 3  gas dissolved into distilled/deionized water and maintained in equilibrium with an overpressure of NH 3 , within a hermetically enclosed chamber at the optimal temperature between 70-90° C., preventing evaporation of NH 3  gas from aqueous NH 4 OH solution for achieving a high anisotropic etching rate.