Removal of silicon oxide

Silicon oxide is removed from an article employing a liquid composition containing a fluoride-containing compound, organic solvent, and water. The methods of the invention are especially useful for removal of silicon oxide formed by thermal oxidation of a silicon substrate.

TECHNICAL FIELD 
The present invention is concerned with removing silicon oxide from a 
substrate and particularly removing thermal silicon oxide without 
attacking underlying substrate also exposed to the composition used for 
removing the silicon oxide, the underlying substrate typically being a 
semiconductor substrate and more particularly a silicon substrate. 
BACKGROUND 
In the fabrication of microelectronic components, a number of the steps 
involved, for instance, in preparing integrated circuit chips and the 
packaging for the chips (articles to which the chips are attached for 
electrical interfacing and/or protection), are cleaning and etching 
processes. Accordingly, over the years, a number of vastly different types 
of etching processes to remove material, sometimes in selective areas, 
have been developed and are utilized to varying degrees. Moreover, the 
steps of etching different layers which constitute, for instance, the 
finished integrated circuit chip are among the most critical and crucial 
steps. 
For example, producing an oxide-free silicon surface for rendering it ready 
for subsequent processing is often essential. In many processes, the 
silicon wafer, used in the production of integrated circuit chips, is 
processed to form wells along with desired isolation. Typically, an oxide 
layer may be thermally grown on the silicon substrate for protecting 
previously formed active silicon regions against adverse effects from 
subsequent processing steps, as well as for removing (by oxidation) the 
near or top surface silicon which may have been damaged or contaminated in 
the previous processing. 
However, at some further point in the manufacturing process, this 
sacrificial silicon oxide layer typically must be removed to provide an 
oxide-free silicon surface (e.g. for construction of further features of 
the desired integrated circuit design). Where a gate is to be formed, this 
oxide removal step is commonly referred to as pre-gate cleaning. This 
cleaning is performed in an aqueous medium followed by a deionized water 
rinse or as a vapor-phase process using HF vapor. One of the problems 
related to aqueous processing is the appearance of water stains after the 
drying (water drying marks). These stains are becoming more and more of an 
issue in device manufacture as device dimensions are shrinking. 
The mechanism behind the water stain formation is that water itself is a 
very aggressive chemical towards the bare silicon surface (more aggressive 
than HF for instance) and is a very good solvent for silica (silicon 
dioxide). During the rinsing process, silicon and silica dissolve in very 
small quantities in the deionized water forming mono-molecular soluble or 
colloidally-dispersed silicic acid (non-ionic because of the neutral pH of 
deionized water). Due to its has an extremely low solubility, its 
non-ionic nature and its non-volatility, the silicic acid deposits on the 
silicon wafer surface when the deionized water evaporates in the dryer. 
Other drying residues can originate from any intrinsic contaminants the 
still present in the already highly purified deionized water. 
In addition to staining problems, the processes of the prior art often 
result in undesired roughening of the silicon surface. 
It would therefore be desirable to provide a procedure capable of removing 
silicon oxide layers to render a surface oxide-free while avoiding or 
minimizing the problems associated with conventional deionized water-based 
processes. 
SUMMARY OF INVENTION 
The present invention is concerned with a method for selectively removing 
silicon oxide from a substrate. The process of the present invention 
comprises contacting an article having exposed silicon oxide with a liquid 
composition that contains about 0.05 to about 3 molar of a 
fluoride-containing compound, an organic solvent, and about 0.05 to about 
3.5 molar of water to thereby remove at least a portion (more preferably 
all) of the exposed silicon oxide. The present invention makes possible 
the removal of silicon oxide to render the surface free of silicon oxide 
while minimizing or eliminating the occurrence of stains commonly 
associated with deionized water-based processes. 
The organic solvents employed in the present invention typically have 
relatively high flash point and provide low viscosity compositions. 
Preferred solvents are propylene carbonate, N-methylpyrrolidone and gamma 
butyrolactone, ethylene glycol and propylene glycol with propylene 
carbonate being the most preferred. HF is a preferred fluoride-containing 
compound. 
The processes of the invention are especially useful for the removal of 
oxide layers from silicon substrates where the oxide was formed by thermal 
oxidation. 
Other objects and advantages of the present invention will become readily 
apparent to those skilled in this art from the following detailed 
description, wherein it is shown and described only the preferred 
embodiments of the invention, simply by way of illustration of the best 
mode contemplated of carrying out the invention. As will be realized, the 
invention is capable of other and different embodiments, and its several 
details are capable of modifications in various obvious respects, without 
departing from the invention. Accordingly, the description is to be 
regarded as illustrative in nature and not as restrictive. 
BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION 
The present invention is concerned with removing silicon oxide, especially 
silicon oxide formed by thermal oxidation of underlying silicon substrate. 
The present invention enables the removal the silicon oxide without the 
creation of significant residual stains on the substrate. The processes of 
the invention are preferably used to remove high density silicon oxide, 
such as silicon dioxide formed by heating a silicon substrate typically at 
temperatures of about 800 to about 1100.degree. C. in the presence of dry 
or wet O.sub.2. The processes of the invention are not limited to the 
removal of any particular oxide thickness, however, the processes of the 
invention are especially useful for treating articles containing a layer 
of about 30 to 200 .ANG. thick of silicon oxide (especially thermal oxide 
layer) on a silicon substrate. Such oxide layers are often used as 
sacrificial oxide layers in semiconductor processing. 
The liquid compositions employed pursuant to the present invention contain 
a fluoride-containing compound, an organic solvent and water. The amount 
of the fluoride-containing compound in the composition is preferably about 
0.05 to about 3 molar, more preferably about 0.2 to about 2.5 molar, most 
preferably about 0.25 to about 1.5 molar. The amount of water in the 
composition is preferably about 0.05 to about 3.5 molar, more preferably 
about 0.2 to about 2.9 molar, most preferably about 0.25 to about 1.7 
molar. The molarities indicated above are based on the total volume of the 
liquid composition. 
The fluoride-containing compound may be any compound capable of providing 
fluorine ion activity for removal of the oxide. Preferably, the 
fluoride-containing compound is chosen to avoid adverse effects with the 
underlying substrate and the other components of the liquid composition. 
Examples of suitable fluoride-containing compounds for use according to 
the present invention are hydrofluoric acid, ammonium fluoride, 
fluoroborates, fluoroboric acid, tin bifluoride, antimony fluoride, 
tetrabutylammonium tetrafluoroborate, and aluminum hexafluoride. Also, 
fluoride salts of an aliphatic primary, secondary or tertiary alkyl amines 
can be used (the alkyl groups preferably being C.sub.1 -C.sub.12 alkyl 
groups). The preferred fluoride-containing compounds are hydrogen fluoride 
and ammonium fluoride, with hydrogen fluoride being most preferred. If a 
fluoride-containing compound other than HF is used, the molarity of that 
compound is preferably adjusted to provide an fluorine ion activity (F 
concentration) in the liquid composition equivalent to that provided by an 
amount of HF in the above molarity ranges. 
Controlling the amount of water in the liquid compositions used in the 
processes of the invention is important in the prevention of the problems 
associated with conventional aqueous processing. Where the 
fluoride-containing compound source of choice contains a significant 
amount of water (e.g., a typical commercial 49 weight percent aqueous HF 
solution), the overall water content of the liquid composition may be 
controlled by addition of a dehydrating agent, by evaporation of the 
excess water or by other suitable water removal technique. The selection 
of a specific water removal technique may depend on the nature of the 
components making up the liquid composition. For example, use of a 
specific dehydrating agent may not be desirable if the agent would be have 
an adverse interaction with the fluoride component or the use of 
evaporation may not be efficient where a desired component already present 
in the liquid composition has a high volatility. Such techniques may be 
generally employed to reduce the water content of the liquid composition. 
Examples of suitable dehydrating agents useful in the invention are alkyl 
anhydrides and aryl anhydrides. The alkyl group preferably contains 1-8 
carbon atoms and the aryl group preferably contains 6-10 carbon atoms in 
the ring. The aryl groups can be substituted on the ring with an alkyl 
group containing 1-8 carbon atoms. Acetic anhydride and benzoic anhydride 
are generally preferred. As noted above, the selected dehydrating agent 
preferably does not adversely affect to an undesirable extent the 
performance characteristics of the composition. Other suitable dehydrating 
agents include triethylorthoformate and 2,2' dimethoxypropane. The amount 
of the anhydride is preferably less than one anhydride group 
(O.dbd.COC.dbd.O) per H.sub.2 O molecule, so that the composition retains 
the desired amount of water. 
In the alternative, the cleaning composition can be produced by adding the 
fluoride-containing compound as a non-aqueous component such as by 
bubbling anhydrous HF gas into the organic solvent or by adding an organic 
fluoride. 
Examples of suitable organic solvent compounds for use in the invention 
include oxolanes, sulfolanes, esters, ketones, aldehydes, lactones, 
halogenated hydrocarbons, amines, imides and alcohols (including mono- and 
polyhydric alcohols). Examples of suitable esters are esters of carbonic 
acids, benzoic acid, phthalic acid, isophthalic acid and terephthalic 
acid, and especially the C.sub.1 -C.sub.6 alkyl esters. Preferred organic 
solvents are propylene carbonate, N-methyl pyrrolidone, gamma 
butyrolactone, methylene chloride, benzyl alcohol, N-formyl morpholine, 
N-formyl piperidine, cyclohexanone, cyclopentanone, methyl benzoate, 
diglyme, 2-methyl tetrahydrofuran, and methyl and ethyl esters of 
phthalic, isophthalic or terephthalic acids. 
The more preferred solvents employed pursuant to the present invention are 
propylene carbonate, N-methylpyrrolidone and gamma butyrolactone, ethylene 
glycol and propylene glycol with propylene carbonate being the most 
preferred. 
Organic solvents employed in the present invention preferably enable the 
overall liquid composition to have a low viscosity. Low viscosity is 
desirable for improving overall workability of the composition as used in 
the processes of the invention and for providing better oxide removal 
uniformity. The organic solvent used in the compositions of the invention 
preferably have an absolute viscosity less than that of glycerol at 
20.degree. C. Thus, while glycerol can be used as the organic solvent in 
embodiments of the invention, it is generally not preferred. 
The oxide removal processes of the present invention are preferably carried 
out at temperatures of about 20.degree. C. to about 90.degree. C., more 
preferably about 30.degree. C. to about 70.degree. C. Employing increased 
temperature generally results in increasing the removal rates of the 
silicon oxide. For example, a solution providing a thermal oxide etch rate 
of about 20 .ANG./min. at room temperature may provide an etch rate to 
about 60 .ANG./min. at about 70.degree. C. The oxide removal processes of 
the invention generally avoid removal of any measurable amount of silicon 
or other material making up the underlying substrate. 
If desired, the oxide removal step of the present invention can be followed 
by non-aqueous rinsing step (e.g. isopropyl alcohol or other low 
viscosity, low boiling point solvent) and a drying step (such as isopropyl 
alcohol vapor drying). 
The following non-limiting examples are presented to further illustrate the 
present invention.

EXAMPLE 1 
A cleaning composition is prepared by admixing one part by volume of a 49 
percent by weight aqueous solution of HF with about 14 parts by volume of 
propylene carbonate to provide a 2 molar HF solution in propylene 
carbonate. Acetic anhydride is added to the solution in an amount to 
provide about 2.5 molar (prior to reaction with the water from the initial 
HF solution) concentration of the acetic anhydride. A silicon wafer 
containing a layer of about 30-60 .ANG. of thermally grown silicon oxide 
thereon is immersed in the liquid cleaning composition. The cleaning 
(oxide removal step) is carried out at temperature of about 35.degree. C. 
for about 45 seconds. The cleaning solution removes the silicon oxide 
without removing any measurable amount of the underlying silicon wafer. 
In this disclosure, there are shown and described only the preferred 
embodiments of the invention, but as aforementioned, it is to be 
understood that the invention is capable of use in various other 
combinations and environments and is capable of changes or modifications 
within the scope of the inventive concept as expressed herein.