Semiconductor device manufacturing: process – Chemical etching – Liquid phase etching
Reexamination Certificate
2001-03-29
2003-07-08
Ghyka, Alexander (Department: 2812)
Semiconductor device manufacturing: process
Chemical etching
Liquid phase etching
C438S960000
Reexamination Certificate
active
06589883
ABSTRACT:
TECHNICAL FIELD
The present invention is generally related to porous silicon substrates and, more particularly, is related to a method for photoluminescence enhancement, photoluminescence stabilization, and the metallization of porous silicon substrates.
BACKGROUND OF THE INVENTION
High surface area porous silicon (PS) substrates formed in wafer scale through electrochemical (EC) etching fall into two groups. PS substrates fabricated from aqueous electrolytes consists of highly branched nonporous substrates while PS substrates fabricated from a nanoqueous electrolyte is comprised of open and accessible macroporous substrates with deep, wide, well-ordered channels.
High-surface area substrates formed in wafer scale through etching display a visible photoluminescence (PL) upon excitation (PLE) with a variety of visible and ultraviolet light sources. This room-temperature luminescence has attracted considerable attention primarily because of its potential use in the development of silicon-based optoelectronics, displays, and sensors.
Although the PL is thought to emanate from regions near the PS substrate surface, the origin of the PL is the source of some controversy as the efficiency and wavelength range of the emitted light can be affected by the physical and electronic properties of the surface, the nature of the etching solution, and the nature of the environment into which the etched sample is placed. Given this range of parameters, it is surprising that, with few exceptions, PL spectra are reported for PS substrates formed in dilute aqueous HF solutions, that have already been dried in air or more inert environments following etch and rinse treatments. These ex situ samples, while providing spectral information, do not indicate the evolution of the PS substrates, and thus, they do not indicate means by which it might be modified and enhanced during or following the etch treatment.
An existing problem in fabricating PS devices rests with establishing electrical contact to the PS substrates. Another problem with PS includes the relatively long excited-state lifetime associated with the PS substrate PL. A further problem includes the relatively low PL quantum yield and the instability of the PL from PS substrates.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
SUMMARY OF THE INVENTION
An embodiment of the present invention provides for a post-etch treatment method of enhancing and stabilizing the PL from a PS substrate. The method includes treating the PS substrate with an aqueous hydrochloric acid (HCl(H
2
O)) solution and then treating the PS substrate with an alcohol. Another exemplary embodiment provides a post-etch method of enhancing and stabilizing the PL from a PS substrate, which includes treating the PS substrate with an HCl(H
2
O) and alcohol solution.
Still another embodiment provides a post-etch method for metallizing a PS substrate in an electroless environment. The method includes treating the PS substrate with an HCl(H
2
O) solution and then treating the PS substrate with an alcohol, or alternatively, treating the PS substrate with a hydrochloric acid/alcohol solution. Subsequently, the PS substrate is treated a hydrazine solution to remove fluorides from the PS substrate. Next, a metal-containing electroless solution is introduced to the PS substrate. Thereafter, the PS substrate is illuminated with a light source at wavelengths less than about 750 nanometers to cause PL of the PS substrate. Then, the metal from the metal-containing solution is induced by the PL to reduce onto the PS substrate (e.g. metallization).
A further embodiment provides for a post-etch method of enhancing PL from a PS substrate by treating the PS substrate with a dye. The dye can be selected from the group including, but not limited to 3,3-diethyloxadicarbacyamine iodide; Rhodamine dye compounds; Fluorocein; and dicyanomethylene (DCM) dye compounds.
Still a further embodiment provides for a metallized PS substrate. The PS substrate can be metallized with copper, silver, or other appropriate metal and can have a resistance of about 20-100 ohm.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
One of a number of embodiments of the present invention includes the treatment of PS substrates generated in an aqueous and nonaqueous etch with an HCl(H
2
O) solution, which results in the stabilization and enhancement of the in situ PL of the PS substrates. More specifically, in an exemplary embodiment, in a post-etch treatment method, an HCl(H
2
O) solution can be used to enhance and stabilize the PL (in situ) from a PS substrate. In addition, in an other exemplary embodiment, a method of treating the PS substrate with HCl(H
2
O) followed by an alcohol solution (e.g. methanol or ethanol) further enhances and stabilizes the PL (in situ and ex situ) of the substrate. A non-limiting illustrative example includes PS substrates that are treated in an aqueous hydrochloric acid and water (HCl/H
2
O) solution and display a strongly enhanced in-situ luminescence; however, the PL decays rapidly in an ex-situ environment without treatment in alcohol, preferably a high purity alcohol. An exemplary embodiment includes treating the PS with methanol (MeOH). Further, PS substrates treated in an HCl (H
2
O)/ alcohol solution (of at least 0.2 molar (M)) maintain their enhancement for extended periods of time. The PS substrate may be stabilized and enhanced by the presence of a chloride ion (Cl). The treatment appears to be independent of the method of preparing the PS substrate, implying that the chloride salt treatment largely stabilizes the surface states of the photoluminescent PS substrate. This stabilization may be demonstrated by various techniques including, but not limited to the following: scanning electron micrographs (SEM), which show the profound change which accompanies the HCl treatment of the PS surface; Energy Dispersive Spectroscopy (EDS) which, reveals chloride incorporation into the PS surface at strongly photoluminescent regions; and Raman scattering, which demonstrates that the PL is correlated with the creation of amorphous structural regions. All of these testing methods indicate the manner in which the chloride salt stabilizes the PS substrate.
Another exemplary embodiment of the present invention includes treating PS substrates with a dye (e.g., 3,3′-diethyloxadicarbocyanine iodide (DODCI) and Rhodamine 700). In general, the dye should have negligible absorption at the wavelengths of maximum absorption for the PS substrate. After a period of aging in darkness these dye-treated PS substrates can be pumped at about 337.1 nanometers (nm) (nitrogen laser) near the maximum in the PS absorption spectrum (far from the major absorption regions of the impregnating dye). Time-dependent PL histograms indicate that the resulting PL emission rate is enhanced. The enhancement in the PL emission rate may be attributed to an interaction between the surface-bound fluorophors, which characterize PS substrates and the dye. This interaction results in the creation of a distribution of PS-dye complexes, which enhance the nominal PL emission rate from the untreated PS surface. In a preferred embodiment, the DODCI treated samples display PL that exceeds that of nominally prepared PS by a factor of five or more.
A further exemplary embodiment of the present invention includes the metallization of a PS substrate. One of the existing challenges in fabricating PS devices rests with establishing electrical contact to the PS substrate. In an exemplary embodiment, PS substrates are capable of being metallized in a controlled manner using electroless
Gole James L.
Seals Lenward T.
Georgia Tech Research Corporation
Ghyka Alexander
Thomas Kayden Horstemeyer & Risley LLP
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