Lightly positively doped silicon wafer anodization process

Details Lightly positively doped silicon wafer anodization process Lightly positively doped silicon wafer anodization process

1998-08-21

2001-03-06

Elms, Richard (Department: 2824)

Semiconductor device manufacturing: process

Formation of electrically isolated lateral semiconductive...

Total dielectric isolation

C438S408000, C438S960000, C438S795000, C205S124000, C205S106000

Reexamination Certificate

active

06197654

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method of anodizing lightly doped silicon semiconductor wafers to provide porosity therein as applied to fabrication of such wafers one at a time and in batches of plural such wafers.
BRIEF DESCRIPTION OF THE PRIOR ART
Anodization of semiconductor wafers involves the application of a current through an electrolyte disposed over both major surfaces of the wafer via a pair of platinum electrodes disposed within the electrolyte, on opposite sides of the major surfaces of the wafer and spaced from the wafer. The electrolyte is generally a mixture of 30 to 50 percent by volume of HF (49%), an alcohol and optionally water. The preferred alcohols are methanol, ethanol, propanol and isopropanol. The result of application of this current is a porous formation on the surface of the wafer more closely adjacent the negative electrode. The amount of porosity is a function of the current density, the electrolyte used and the time length of current application through the wafer and within the electrolyte. The electrolyte is preferably constantly being recycled to maintain electrolyte component ratios.
Anodizing heavily doped wafers to produce porous silicon in the above described manner generally does not present any substantial problems with the electrochemical process itself. However, lightly doped wafers present the added difficulty of not forming a good ohmic contact to the electrolyte. A lightly doped wafer is defined within the context of this disclosure as having from about 1×10
14
to about 5×10
17
atoms of dopant/cc, this being non-degeneratively doped. This problem is most acute at the backside of the wafer, which faces the positive electrode. Negative charges accumulate on the backside of the wafer to the level such that the surface is inverted, forming a p-n junction. This p-n junction is reverse biased and restricts current flow through the wafer from the positive electrode of the current source, through the electrolyte on both sides of the wafer and the wafer to the other negative electrode. Anodization cannot proceed until this junction is broken down, however, when such breakdown occurs, the anodization process is uncontrolled.
An attempt to overcome the above described problem has been to deposit aluminum or some other metal on the backside of the wafer with subsequent sintering of the wafer to form an ohmic contact. This approach presents a problem in the semiconductor fabrication process due to metal contamination caused by the addition of the metal to the wafer backside. Furthermore, this approach is not amenable to immersion of both sides of the wafer into the electrolyte since the side of the wafer containing the metal and the metal must be kept out of the electrolyte to avoid metal contamination of the wafer and electrolyte by the metal, making it very difficult to perform the anodization as a batch process on many wafers concurrently. Making good, uniform metal contact to the coated backside also presents a problem.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described problem of the prior art is overcome and there is provided a process whereby porous silicon can be provided in lightly doped semiconductor wafers. This is accomplished by heavily doping the backside or side of the wafer which will not be rendered porous with a heavy p-type dopant, preferably boron in the case of a p-type substrate. The amount of dopant is in sufficient quantity such that the backside surface will not be inverted to n-type when the voltage from the current source is applied across the wafer through the electrolyte. The depth of the dopant is about equal to or greater than 100 nanometers. This step can be accomplished either through ion implantation or by spin coating with a boron containing slurry and diffusing the boron into the wafer. The high doping level prevents inversion of the backside of the wafer and an ohmic contact is present to allow anodization. In this way, a lightly doped silicon wafer can be provided with a porous surface either along the entire frontside surface (surface facing the negative electrode of the current source) or selectively along the frontside surface by first masking the frontside surface and then placing the wafer in the electrolyte and applying the current through the wafer and the electrolyte. The depth of the pores is generally from about 1 to about 100 microns, depending upon process requirements. The mask, when used, is preferably a silicon carbide or nitride, though other masking materials which are well known can be used.
A frontside surface of a plurality of wafers can be made porous concurrently by placing the wafers within the electrolyte spaced apart from each other and with their major surfaces in parallel. The current source electrodes are placed in the electrolyte spaced from the extreme wafers such that the current passes through all of the wafers in passing from the positive electrode to the negative electrode.
It is preferred that the wafers provide a liquid seal between the major surfaces thereof, thereby causing the totality or as much of the totality of the current flow as possible to pass through the wafers rather then around the wafers.
The current density of the current passing through the wafers is from about 1 to about 100 milliamperes per square centimeter with a preferred value depending upon process requirements.


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Murate, M.; Iwate, H.; Itoigawa, K.; Micromechatronics and Human Science, 1998. MHS '98. Proceedings of the 1998 International Symposium on , 1998; pp. 57-63.

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