Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-07-24
2003-03-18
Meeks, Timothy (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S255370
Reexamination Certificate
active
06534125
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to the field of semiconductors and more particularly, to adhesion of silica to diamond.
BACKGROUND OF INVENTION
Electronic devices based on storage of electrons at an interface are well known. Devices containing interfaces formed by a silica (SiO
2
) layer disposed on a silicon (Si) substrate are presently in predominant use where the silica layer is provided by controlled oxidation of the silicon substrate. Examples of other substrates in addition to silicon include gallium arsenide (GaAs) and indium phosphide (InP). The charge carrier, i.e., electrons or holes, in such devices are stored at the interfaces and it is desirable to keep leakage current in such devices to a minimum in order to minimize loss of the charge. The electronic devices of silica disposed over a silicon substrate have low leakage current. Unfortunately, the present electronic devices of silica disposed over a diamond substrate have an undesirably high leakage current which drains the charge in such devices over a period of time.
Energy band gap is important especially in optical applications for the electronic devices. The width of the band gap is the lowest energy to which the device is responsive. This means that a device with a large or a wide band gap will be responsive to a limited energy range. Since the band gap of diamond is about five times that of silicon, electronic devices with diamond substrates are responsive to a more limited energy range than electronic devices with silicon substrates. Specifically, electronic devices with diamond substrates don't respond to visible light whereas electronic devices with silicon substrate are responsive to visible light.
Diamond has many desirable properties which make it especially suitable for electronic, optical and medical applications. Diamond has an energy band gap of 5.5 ev and it generates a limited number of electrons when bombarded with alpha particles, which is indicative of high radiation resistance. Attempts have been made in the past to make semiconductor electronic devices with a diamond substrate and a thin layer of silica on the diamond substrate. Electronic devices having a diamond substrate with a layer of silica thereon have many desirable attributes including reduced leakage current and reduced response to visible light. However, electronic devices of a diamond substrate with a silica coating thereon have not proven to be feasible because of poor adhesion of the silica coating to the diamond substrate and the weak or fragile coating itself, as evidenced by the fact that the silica coating on the diamond substrate can be easily scratched with a tungsten probe.
OBJECTS OF INVENTION
An object of this invention is an electronic device having a low dark current and a low response to visible light;
Another object of this invention is an electronic device comprising a diamond substrate with a robust silica coating and having a low dark current and low responsiveness to visible light; and
Another object of this invention is to provide a robust coating of silica tenaciously adhering to the diamond surface.
SUMMARY OF INVENTION
These and other objects of this invention are accomplished by removing the non-diamond layer inherently formed on diamond substrates. The non-diamond carbon is removed by electrochemical cleaning and a silica layer is deposited thereon. This as-deposited layer, which is fragile, is annealed at an elevated temperature to strengthen the silica layer and to obtain a tenacious bond between the diamond surface and the silica layer.
DETAILED DESCRIPTION OF INVENTION
The diamond substrate contemplated herein can be polycrystalline or single crystal. Preferably, however, the diamond substrate is single crystal diamond. The dimensions of the diamond substrate can vary in response to what is required. It is contemplated that surface area of the diamond substrate will generally vary from about 1 to about 10,000 mm
2
, but is typically in the approximate range of about 5 to about 1,000 mm
2
. Its thickness is generally in the approximate range of about 5 to about 500 microns and preferably 10 to 100 microns.
As grown diamond substrates inherently have a surface layer of non-diamond carbon. The non-diamond carbon layer typically has a thickness of about 100 to about 10,000 angstroms, and more often in the approximate range of about 200-5000 angstroms. This non-diamond carbon layer is removed during the cleaning operation described hereinafter. The non-diamond carbon layer results from the methods, such as ion implantation, CVD, etc., used to grow and stabilize diamond substrates to graphitization.
The term non-diamond carbon includes amorphous carbon. A layer of non-diamond carbon may contain, in addition to metal carbides, small amounts of other atoms such as nitrogen, argon, helium, iron, and the like.
Silicon oxides (silica) with various impurities can be used as the layer on the diamond substrate. Impurities such as nitrogen, hydroxyl ions, phosphorus and boron, can be present in the oxide of silicon. Silicon dioxide is the preferred layer on the diamond substrate. Silicon dioxide is preferred because it is a familiar material in the silicon device field and because it has a number of beneficial properties such as a large band gap, facile deposition in thin layers on the order of a few hundred angstroms, and a good dielectric strength of about 4×10
6
v/cm. It is also easily made, has good performance, is easy to pattern, and has the largest energy gap (8.8 ev) of any known stable material.
In the device of this invention, the planar dimensions of the silicon dioxide layer may or may not be coextensive with the diamond substrate. The thickness of the layer on the diamond substrate should be in the range of about 0.001-1 micron, preferably 0.04-0.1 micron. If the silica layer is too thick, it will crack and if it is too thin, it will be ineffective for purposes herein.
The cleaning step is the electrochemical removal of non-diamond carbon from the diamond surface on which silica is deposited. Electrochemical removal of the non-diamond carbon from the diamond surface results in a stable, terminated surface.
Electrochemical removal of non-diamond carbon from a diamond surface can be accomplished by immersing the diamond surface covered by the non-diamond carbon in a suitable electrolyte solution between electrodes, impressing a voltage between the electrodes to provide a sufficient electric field in the electrolyte solution to remove the non-diamond carbon, keeping the diamond surface submerged in the electrolyte solution for a sufficient duration to remove the non-diamond carbon, and removing from the electrolyte solution the diamond surface devoid of the non-diamond carbon. The electrodes may be immersed in the electrolyte solution, or positioned outside of it, provided that the diamond surface is subjected to an electric field of sufficient strength to remove the non-diamond carbon. The non-diamond carbon may be completely removed in a single electrochemical step, or over a series of electrochemical steps, each of which removes a portion of the non-diamond carbon.
Although the electric field strength required to obtain the electrochemical removal of non-diamond carbon depends on the particular electrolyte employed, electrode spacing, electrode material and its shape, thickness of the non-diamond carbon to be removed, and other considerations, the electric field in the electrolyte is typically above 1 v/cm, preferably 10 to 100 v/cm. To produce the necessary electric field in the electrolyte for a small separation of the electrodes, the impressed voltage is typically in the approximate range of 5-5000 volts, preferably 10-1000 volts.
The distance between the electrodes should be sufficient to at least accommodate the substrate(s) and obtain the required electric field strength. Electrochemical removal rates or etching rates for the non-diamond carbon are controlled by the electric field between the electrodes, increasing with either applied voltage or a decrease in electrode
Fletcher, III William Phillip
Kap George A.
Karasek John J.
Meeks Timothy
The United States of America as represented by the Secretary of
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