Method of reducing the formation of watermarks on semiconductor wafers

A method for reducing the formation of watermarks includes providing a semiconductor wafer and contacting the semiconductor wafer with a solution containing a watermark reducing amount of at least one cationic surfactant.

TECHNICAL FIELD 
Methods for reducing the formation of watermarks on semiconductor wafers 
are described herein. More particularly, the formation of watermarks on 
semiconductor wafers is reduced by contacting the wafer with a solution 
containing cationic surfactants. 
BACKGROUND OF RELATED ART 
During semiconductor chip manufacture, unintentional formation of surface 
defects on the semiconductor wafers is undesirable. Watermarks are one 
such defect unintentionally formed during wafer processing. 
Generally, a semiconductor wafer has a silicon surface with a thin layer of 
oxide on the silicon. The oxide layer can be removed by subjecting the 
semiconductor wafer to a wet chemical processing step, e.g., a dilute 
hydrofluoric acid dip. After such processing, the semiconductor wafer is 
normally rinsed with deionized water to remove the chemicals used in the 
chemical processing step. However, silicon at the surface of the wafer can 
undergo dissolution upon contact with the deionized water to form the 
reaction product Si(OH).sub.4 in solution. When the rinsed semiconductor 
wafer is subsequently dried the silicon in solution precipitates back onto 
the surface of the semiconductor wafer, producing watermarks on the 
surface of the wafer. 
The etching of silicon in the presence of dissolved oxygen in water is 
explained in Eisenberg et al., Surface Chemical Cleaning and Passivation 
for Semiconductor Processing, Materials Research Soc. Symp. Proc. No. 315, 
Pittsburgh, pp. 485-490 (1993) as follows: First, the Si--Si backbond is 
reacted with oxygen. Second, the silicon atoms attain a positive charge 
due to the electronegativity of the oxygen which initiates the reaction of 
OH.sup.- with the Si--Si backbond. The third step involves the release of 
Si.sub.x O.sub.y H.sub.z into the solution and the termination of the 
silicon bond with hydrogen. Unintentional etching of silicon and the 
formation of watermarks is a potential problem whenever an aqueous 
solution comes into contact with a bare or unprotected silicon surface. 
It would be desirable to provide an easily implemented method of inhibiting 
silicon dissolution to reduce the formation of watermarks during wafer 
processing and thereby increase production yield of semiconductor chips. 
SUMMARY OF THE INVENTION 
Novel methods for reducing the formation of watermarks on semiconductor 
wafers have been discovered which include the steps of providing a 
semiconductor wafer and contacting the semiconductor wafer with a solution 
containing a watermark reducing amount of a cationic surfactant. The 
cationic surfactants, which adsorb on the wafer surface (particularly on 
any bare silicon surfaces), minimize the extent of water interaction with 
the wafer surface and help in reducing silicon dissolution, thereby 
minimizing the formation of watermarks. 
In a particularly useful embodiment, the method involves providing a 
semiconductor wafer having a surface including oxide and bare silicon 
regions and increasing the hydrophobic properties to the surface of the 
wafer by contacting the surface of the semiconductor wafer with a fluid 
containing a watermark reducing amount of at least one cationic 
surfactant. The cationic surfactant(s) not only render the oxide regions 
hydrophobic, but also adsorb on the silicon regions and reduce the extent 
of water interaction with the wafer surfaces. The watermark reducing 
amount of cationic surfactant can then be removed by contacting the 
surface of the semiconductor wafer with an organic solvent.

DESCRIPTION OF THE INVENTION 
The invention relates to substantially reducing formation of watermarks on 
a substrate during the fabrication of integrated circuits (ICs), including 
random access memories (RAMs), dynamic RAMs (DRAMs), synchronous DRAMs 
(SDRAMs), and logic devices. It is noted that typically, ICs are 
fabricated in parallel on a semiconductor substrate such as a wafer. Once 
processing is finished, the wafer is diced, separating the ICs into 
individual chips for packaging. 
In accordance with the invention, a cationic surfactant is employed to 
increase the hydrophobic properties of the surface of the wafer. The 
cationic surfactant reduces or minimizes the amount of silicon dissolution 
in the deionized water used to rinse the wafer to remove chemicals used in 
a previous process. As a result, the likelihood that watermarks will be 
produced upon drying is reduced. The cationic surfactant can subsequently 
be rinsed using an organic solvent which does not dissolve silicon to any 
significant extent and therefore poses no threat of forming watermarks. 
In one embodiment, the semiconductor wafer comprises, for example, silicon. 
Other types of semiconductor wafers, such as silicon-germanium and silicon 
on insulator (SOI), are also useful. The wafer includes a plurality of ICs 
fabricated thereon. The ICs can be in any stage of processing and, as 
such, the wafer comprises various different types of region. For example, 
the wafer comprises a plurality of oxide and bare silicon regions during 
processing to form the ICs. The oxide regions have hydrophilic properties 
and the bare silicon regions have generally hydrophobic properties. 
The fabrication of semiconductor devices or ICs is typically conducted in 
successive stages, one or more of which can involve the forming of a 
pattern on the surface by lithographic techniques. Forming a pattern 
typically involves coating at least a portion of the substrate with a 
resist material, exposing in the appropriate pattern and developing the 
resist layer. Once the resist is developed, subsequent processing steps, 
e.g., etching, then can be performed to locally impart physical 
characteristics to the semiconductor material to yield a desired 
structure. 
Etching can be achieved, for example, by a wet chemical processing. 
Typically, during such wet chemical processing, the surface of the 
semiconductor wafer is contacted with an acid, e.g., dilute hydrofluoric 
acid. Suitable parameters for patterning and wet chemical processing 
(e.g., choice of etchant, concentration of etchant, time, temperature, 
etc.) are well known. 
Following the chemical processing step, the etched surface of the wafer is 
rinsed to stop the chemical processing by removing, diluting and/or 
neutralizing the etchant. One suitable rinsing fluid is deionized water. 
However, when deionized water contacts the wafer, it causes dissolution of 
silicon, particularly at the bare silicon regions. Upon drying, watermarks 
will result. 
In accordance with one embodiment of the invention, an aqueous solution 
comprising a cationic surfactant is contacted with the wafer to reduce or 
prevent watermarks from forming. The solution, for example, can include 
organics to facilitate the preparation of the surfactants therein. The 
amount of cationic surfactant in the solution is sufficient to effectively 
reduce the formation of watermarks during subsequent drying. The solution 
can be applied to the wafer during etching as part of the etching 
solution. The solution can also be applied as a separate step prior to the 
rinsing of the wafer with deionized water or simultaneously with the 
application of the rinsing fluid to the wafer. 
Cationic surfactants useful herein include any conventional cationic 
surfactants. In one embodiment, the cationic surfactants comprises alkyl 
amines. Suitable alkyl amines include, but are not limited to, those 
having alkyl chains comprising from 4 to 20 carbon atoms, preferably from 
6 to 18 carbon atoms, and more preferably from 8 to 16 carbon atoms. 
Examples of such alkyl amines include butyl amine, pentyl amine, hexyl 
amine, heptyl amine, octyl amine, nonyl amine, decyl amine, dodecyl amine 
and the like. Preferred alkyl amines include those having alkyl chains 
containing 10, 12 or 14 carbon atoms such as, for example, decyl amine and 
dodecyl amine. Combinations of cationic surfactants are also useful. 
In one embodiment, The amount of cationic surfactant that contacts with the 
wafer is sufficient to effectively reduce the formation of watermarks. 
What constitutes a sufficient amount depends on a number of factors. Such 
factors include, for example, the specific surfactant used, the size of 
the wafer, and the surface characteristics of the wafer. Typically, a 
watermark reducing amount ordinarily ranges from about 0.002 mM to about 
40 mM, preferably from about 0.05 mM to about 20 mM, and more preferably 
from about 0.5 mM to about 2 mM. Generally, for alkylamines, the longer 
the chain length of the alkyl group(s), the lower the concentration of 
surfactant needed. Though not critical, the wafer should be contacted with 
the cationic surfactant solution for a time ranging from about 2 seconds 
to about 60 seconds, preferably from about 5 seconds to about 45 seconds, 
and more preferably from about 10 seconds to about 30 seconds. The 
temperature during contact with the surfactant solution need not be 
precisely controlled, but normally can range from about 5C. to about 
50.degree. C. and more preferably from about 15.degree. C. to about 
30.degree. C. 
After contacting the wafer with the cationic surfactant, any subsequent 
rinsing can be performed with deionized water alone, provided the cationic 
surfactant has not been removed from the wafer. 
The cationic surfactant can be subsequently removed from the rinsed surface 
of the wafer, if desired. This can be achieved by contacting an organic 
solvent to the surface of the semiconductor wafer. Useful organic solvents 
include alcohols such as methanol, ethanol, propanol, isopropyl alcohol, 
butanol, isobutyl alcohol, and the like. Preferably, ethanol and isopropyl 
are used for removing the cationic surfactant. The use of alcohol as the 
organic solvent is advantageous to the removal process due to the complete 
solubility of the surfactant in the alcohol solution, which will limit any 
carbon residue or contamination problems. 
The oxide regions of the wafer reportedly exhibit a negative charge at the 
pH level of the rinsing fluid (i.e., at a pH level greater than two) 
providing hydrophilic properties at those regions. The cationic 
surfactant, on the other hand, typically exhibits a positively charged 
head group and a hydrophobic tail. 
While not wishing to be bound to any theory, it is believed that once the 
surfactant is contacted with the oxide region of the wafer, the surfactant 
will advantageously be adsorbed by the oxide. Specifically, it is believed 
that the positively charged head group of the cationic surfactant adsorbs 
to the negatively charged oxide surface. In this manner, the oxide region 
will now appear hydrophobic. On the bare silicon regions of the 
semiconductor wafer surface, it is believed that the surfactant will be 
adsorbed which will render the bare silicon region of the semiconductor 
wafer surface more hydrophobic and less susceptible to attack by water. 
Accordingly, it is postulated that the addition of the cationic surfactant 
will cause the entire surface of the semiconductor wafer to appear 
hydrophobic. In this manner water will be repelled from the surface of the 
wafer, thereby minimizing dissolution of silicon. This will inhibit the 
formation of watermarks caused by redeposition of dissolved silicon that 
might occur upon drying of the wafer. 
While the invention has been described with reference to various 
illustrative embodiments, it will be apparent to those skilled in the art 
that many changes and variations are possible without departing from the 
scope and spirit thereof. The invention should therefore be determined not 
with reference to the above description but with reference to the appended 
claims along with the full scope of equivalents.