Patent Application
20070012574
This invention relates to processes for the fabrication of porous silicon products and in particular to techniques for the fabrication of such products with average equivalent pore diameters in the range of about 40 to 250 nanometers.
Porous silicon parts are produced by electrochemically etching silicon wafers in a hydrofluoric acid bath, which yields porous layers on one of the surfaces of the wafer. The first experiments at producing these layers were more than 40 years ago. In the etching process, silicon atoms are dissolved from the bulk material forming tiny pores and the original crystal structure remains unaffected. Dependent on the experimental conditions (doping density of bulk material, electrolyte concentration, current density, etching voltage, temperature) different modifications of porous silicon can be formed. With the different pore etching techniques it is possible to vary pore sizes in the silicon on a scale from millimeters to nanometers. A very large number of technical applications of porous silicon have been proposed. These include optical components, electronic components and electro-optical components.
In a typical prior art process, a silicon wafer is immersed in an ethanolic hydrofluoric acid solution between a platinum cathode and a platinum anode and a constant electric current is applied to the wafer. The silicon atoms at the silicon/electrolyte interface facing the cathode are polarized, and are subject to attack by the fluoride ions in solution. Silicon atoms are released in the form of silicon hexafluoride. Porous silicon tends to etch as a distribution of approximately cylindrical pores with very small diameters that tend to be much deeper than they are wide. The approximately cylindrical shape of the pores and their depths can be amazingly uniform. The distribution of pore diameters and the depth of the pores are controlled by adjusting the current density and the etching time. Additional details relating to these processes are contained in U.S. Pat. No. 6,248,539 that is incorporated herein by reference.
The International Union of Pure and Applied Chemistry (IUPAC) guidelines define ranges of pore sizes as shown in Table 1 below:
Prior art fabrication techniques to produce porous silicon parts included a single-step, electrochemical dissolution of very highly-doped, p-type porous silicon. The disadvantages of this method are:
In addition, SEM imaging of the etched silicon revealed an upper shallow layer of micropores and mesopores (with equivalent diameters generally much less than 50 nanometers) covering a mixed, macroporous layer (a deeper layer of pores having pores with equivalent diameters generally greater than 50 nanometers). The upper layer appeared to vary in depth from several hundred nanometers to several microns.
What is needed is a better method for making porous silicon products with uniform average pore diameters in the range of about 50 nanometers to about 250 or greater.
This invention provides a process for the fabrication of macroporous silicon from silicon wafers. Preferred embodiments include a two-etch-step process that results in a single, macroporous layer of silicon with pore depths of several microns and relatively uniform equivalent diameters with an operator chosen mean equivalent diameter. The mean diameter determined by the operator is a mean diameter within the range of about 40 nm to about 250 nm and is determined at least in part by a selection of an applied current density. Uniformity of equivalent pore diameter is greatly improved as compared to prior art porous silicon fabrication techniques. In a first electrochemical anodisation step a macroporous layer is created that is covered by a shallower combination microporous-mesoporous layer. The wafer is removed and the top surface of the wafer is dissolved under alkaline conditions, producing “pits”. These “pits” serve as defect sites during a second electrochemical anodisation step, resulting in a single, uniform macroporous layer in the silicon wafer. Preferably, the wafer with its porous silicon surface is quickly rinsed in acetone and then pentane to preserve the structure of the pores and prevent collapsing of pore walls. The process of the present invention eliminates the upper, nanoporous region produced by prior art processes. In a preferred embodiment, utilizing a current density of 181.8 mA/cm2 in the two-etch-step process, the resulting mean equivalent pore diameter is about 100 nm with more than half of the equivalent pores diameters within about ±50 nm of the mean equivalent pore diameter. By increasing or decreasing the current porous layers with mean diameters within the range of about 40 mu to about 250 nm can be created.
In addition to the elimination of the upper, nanoporous layer, the new method also has the advantages of:
In preferred embodiments for producing wafers for use in a molecular sensor, the porous structure is surface modified by molecular vapor deposition of silane compounds to increase wettability, stability and confer functionality.
Described below by reference to the drawings is a preferred process for fabricating porous silicon with an average equivalent pore diameter of about 100 nanometers with more than half of the pores having equivalent pore diameters within about ±20 nm of the average 100 nm equivalent diameter.
In this specification, we will use the phrase, “equivalent pore diameter” De, of a pore to refer to the approximate diameter of a comparable circular cylinder having the same volume as that of the pore. Since the cross sectional area of each pore is typically approximately uniform along the depth of the pore, we can estimate this equivalent pore diameter by measuring the area, A, of the pore at the surface of the wafer and calculating a value for De as follows: De=2√{square root over (A/π)}.
A preferred anodization cell 48 is shown in an exploded view in
The porous silicon regions are high surface area regions consisting of nanometer size pores in a crystalline silicon substrate. The pores are produced by anodic electrochemical etches of bulk crystalline silicon. The starting material for porous silicon, for this preferred embodiment, is a heavily doped crystalline silicon wafer, commercially available for semiconductor manufacturing purposes. Wafer specifications for this porous silicon fabrication process include p-type boron doped silicon (0.001-0.0035 Ω-cm resistivity) with a <100> crystal orientation. Four inch diameter, p-type silicon (100) wafers with resistivity ranges between 0.0010 and 0.0035 Ω-cm were purchased from Silicon Quest International, Inc., with offices in Santa Clara, Calif.). The wafers were pre-scribed into 44 individual die sections measuring 10 mm×13 mm by American Precision Dicing (San Jose, Calif.) which section, as indicated above, are referred to as dies, die section or wafers. The actual etch area, defined by the Teflon masks, measures 9.0 mm×5.5 mm and equals 49.5 mm2.
All chemicals used were reagent grade or higher and purchased from Hawaii Chemical & Scientific unless otherwise noted. Ultra pure water was obtained from a Bamstead Nanopure Diamond Analytical Water System (APC Water Services, Inc.).
Immediately prior to anodisation, wafers were pre-cleaned as described in this section. Silicon wafer 56 was placed in 40 ml of concentrated sulfuric acid and heated to about 90 degrees C. Twenty milliliters of hydrogen peroxide (30%) was added to the acid and the wafer was allowed to oxidize for 10 minutes in the heated solution, after which the wafer was rinsed with copious amounts of ultra pure water for 5 minutes. The rinsed silicon wafer was transferred to a clean, glass beaker containing 150 ml of ultra pure water and 30 ml of ammonium hydroxide (30%). The solution was heated and, once it reached 70 degrees C., 30 ml of hydrogen peroxide (30%) was added. The silicon wafer remained in the solution for 15 minutes and again was rinsed with copious amounts of water for 5 minutes. A resulting oxide layer was stripped by soaking the wafer in a 2.5% solution of hydrofluoric acid (diluted with water) for 2 minutes and again rinsed with copious amounts of water for 5 minutes. The silicon wafer was then transferred to a clean, glass beaker containing 120 ml of ultra pure water and 30 ml of hydrochloric acid (37%). The solution was heated to 70 degrees, at which time 30 ml of hydrogen peroxide was added. The silicon wafer remained in the solution for 15 minutes before a final five minute rinse with copious amounts of ultra pure water. The wafer was blown dry under an inert stream of nitrogen gas using a nitrogen source available from GasPro, with offices in Kahului, Hi.
The clean wafer was then assembled into the Teflon etch chamber of anodisation cell 48 and immersed in an ethanolic hydrofluoric acid solution. The solution is a mixture of equal quantities of (1) 50% hydrofluoric acid (equal volumes of hydrofluoric acid and water) and (2) ethanol. Applicants refer to this solution as 25 percent hydrofluoric acid in ethanol. Specifically, 40 milliliters of 25 percent hydrofluoric acid in ethanol is slowly added to the cell reservoir. Conductors from a power supply (not shown) are connected to the platinum wire electrode paddles in the anodisation cell and a constant current density (J=181.8 mA/cm2) is applied for 30 seconds. The total area to be etched, defined by windows 58, is 0.99 cm2.
Therefore, the appropriate anodisation current is 180 mA. The silicon atoms at the silicon/electrolyte interface are attacked by the fluoride ions in solution forming silicon hexafluoride. Silicon atoms are released from the wafer in the form of silicon hexafluoride. The etched silicon wafer is removed from the anodisation cell, rinsed in acetone, then pentane and allowed to air dry. As shown in
The result of the above process is a silicon wafer part with a very uniform, single, macroporous layer as shown in
The distribution of pore diameters and the depth of the pores may be controlled by adjusting current density and anodisation duration, as shown in
In preferred embodiments, the porous silicon surface may be modified for particular applications. In one application the porous silicon is utilized in a molecular sensor to anchor molecules for the purpose of monitoring molecular interactions. For this embodiment, after the porous silicon layer has been produced on the silicon wafer as explained above, a protective layer is applied to prevent or minimize oxidation and contamination with particulates from ambient air. Preferably the wafers are immediately surface modified or stored under a blanket of inert nitrogen gas in a controlled humidity environment to be surface modified later. Surface modifications can be achieved using a variety of techniques including wet chemistry and molecular vapor deposition (MVD). Applicants' first preferred embodiment for surface modification relies on MVD technology. MVD overcomes many limitations associated with wet chemistry including cost, process complexity and surface coverage. The process consists of pre-cleaning using argon or oxygen plasma followed by tunable deposition of a monolayer film under sub-atmospheric pressure.
A wide variety of chemicals can be deposited on the surface depending upon the ultimate application. For a preferred embodiment in which the porous silicon dies are to be used as a molecular sensor for measuring binding interactions, Applicants describe below the deposition of 10-(carbomethoxy)decydimethylchlorosilane (Gelest, Inc.) using a molecular vapor deposition unit Model MVD-100 available from Applied Microstructures Inc. with offices in San Jose, Calif. Post-etching, samples were placed in the MVD-100 and cleaned of any organic contamination by an oxygen plasma treatment, in this case, for 90 seconds with a chamber pressure of 0.5 Torr and RF power in the range of 100-300 watts. The plasma treatment serves a dual purpose, not only eliminating the etched surface of contaminants, but also uniformly hydroxylating the silicon surface with OH-groups for subsequent silanization. The organic linker [10-(carbomethoxy)decyldimethylchlorosilane] (Gelest, Inc.) was vaporized before metered delivery of approximately 2.0-3.0 microliters to the reaction chamber where it reacted with the hydroxylated silicon surface in the presence of trace amounts of water, resulting in the release of a negligible amount of HCL gas and the functionalized silicon surface. In this case, the vapor was allowed to react for 25-30 minutes. The dies can be used, as is, to couple proteins via standard amine coupling techniques or further modified with different bioconjugates to increase hydrophilicity and/or create specific functionalized surfaces. Using this preferred embodiment, Applicants and thier fellow workers have coupled Amino-dPEG12™-t-butyl ester (Quanta Biodesign) to the surface by first activating the carbomethoxy group of the silicon surface with 200 mM EDC [1-Ethyl-3-(3-Dimethylaminopropyl)carbodiimide Hydrochloride] (Pierce Biotechnology) and 50 mM NHS [N-Hydroxysuccinimide] (Pierce Biotechnology) in water for 10 minutes. The activated surface is then allowed to react with 1 mg/ml of Amino-dPEG12™-t-butyl ester for 30 minutes and any remaining NHS esters are capped with 1M ethanolamine, pH 8.0 for 10 minutes. The surface is rinsed in ultra pure water, pure ethanol and dried under a stream of inert nitrogen gas. The final product is a pegylated, porous silicon surface with a protected carboxylic acid functional group. The functional group may be deprotected by exposure to 25% trifluoroacetic acid (TFA) in ice cold methylene chloride (CH2Cl2) for 5 hrs and used for immobilization with standard amine coupling techniques. Alternatively, the deprotection step may be avoided by coupling the Amino-dPEG12 ™ acid (Quanta Biodesign) instead of the Amino-dPEG12™-t-butyl ester. In this case, the end user can proceed with activation and immobilization of the target using EDC/NHS and standard amine coupling. The end product is a functionalized, hydrophilic porous silicon die with cylindrical, straw-like pores of 100 nm diameters and 2 μ depths and two optically flat, parallel surfaces resulting from the top (air/porous silicon) and bottom (porous silicon/bulk silicon) surfaces of the porous silicon matrix. The structural morphology of the dies provides a convenient two-beam interferometer while the high surface area and adaptable surface chemistry provide the platform for numerous protein and DNA sensing applications.
While the present invention is described in terms of preferred embodiments, the reader should understand that these are merely examples and that many other embodiments are changes to the above embodiments will be obvious to persons skilled in this art. For example, the size, shape and number of pores in the porous silicon regions could vary greatly depending on the particular application of the present invention. Applicants and thier fellow workers have been able to achieve reproducible pore sizes with diameters as small as 20 nm and as large as several microns. The porosity of the regions may vary greatly with the application and many other porosity values could be utilized. Also, the self assembled monolayer and secondary linkers can take limitless forms, depending upon the end user's ultimate application. For instance, a short pegylated molecule with a hydroxyl group on one end and a protected acid on the other could be deposited directly onto the porous silicon surface after plasma treatment using the MVD-100. Linker with free thiol groups (versus the acid described in the preferred embodiment) can be utilized if the ultimate goal is to immobilize targets via the sulfhydryl group. Therefore, the scope of the invention should be determined by the claims and their legal equivalents.
