Some cautions concerning UPW degasification
08/01/2002
by Robert P. Donovan
Contemporary semiconductor fabs universally employ some type of degasification stage in their ultra-pure water (UPW) plants to reduce the concentration of dissolved gases.
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The preference now is for an oxide-free surface prior to initiating gate oxide growth. Any rinsing prior to this crucial step in metal-oxide semiconductor (MOS) technology must be carried out with minimal chemical oxidation formed on the surface during the rinse.
Dissolved oxygen in UPW has also been implicated in the formation of the dreaded "water spots" that are sometimes observed following rinse/dry cycles. Here again, dissolved oxygen is thought to oxidize silicon to silica, which, in subsequent drying cycles, can leave non-volatile silica depo sits on the surface that constitute "water spots." 1
By using UPW of low-dissolved oxygen concentration, this mech anism of water-spot formation is eliminated.
American Society of Test and Measurement (ASTM) D-5127-99 (Stan dard Guide for Ultra Pure Water Used in the Electronics and Semiconductor Industry) recommends that dissolved oxygen concentration in UPW be no greater than 1 ppb to avoid the production problems just cited. UPW plants now typically meet that specification.
However, there is another side to the issue.
Verhaverbeke cites a number of undesirable consequences that can result from eliminating the oxidizing species from UPW used in chemical cleans and rinses. 2 For example, in solutions used to remove chemisorbed metallic surface contaminants, oxidizing species are needed to prevent the subsequent reduction of metallic ions such as Cu2+ that have just been removed from a surface by ion exchange with H+ or other appropriate cation.
Thus, while HCl alone removes most metallic impurities (gold is an exception), standard clean (SC)-2 solutions include H2O2 as an oxidizer to prevent metallic precipitation. It also enables the solution to remove gold.
Oxidizing species are also needed in the UPW subsequently to rinse wafers after removal from the chemical bath and to prevent metallic ion precipitation following the cleaning step. Such species may not be present in today's degasified UPW, meeting D-5127 specifications.
Also, in degasified water containing no dissolved oxygen, the corrosion potential of bare silicon (the standard reduction potential of silicon [-0.857 volts] plus that of the oxidizer in the water) can be no higher than 0 (the standard reduction potential of H2) and will generally be somewhere between -0.857 and 0 volt. Thus, electrochemical deposition of species having higher standard reduction potential than that of the bare silicon, even though negative, can electroplate on bare silicon. Pb2+, Sn2+, Ni2+, Fe2+ and others fall into the class of metal ions that could conceivably electrochemically deposit on bare silicon in degasified UPW.
While minimizing dissolved oxygen concentration in UPW is clearly desirable for many process steps, Verhaverbeke's message is that some process steps require oxidizers to be present to achieve or preserve desirable results. For such processes, degasified UPW should have a little H2O2 or other oxidizing species added. O3 can also serve this function (it's actually a stronger oxidizer than H2O2 and costs less), as can saturating the solutions with O2 at 1 atm.
Blanket acceptance of UPW of minimal dissolved oxygen concentration can have undesirable consequences in some wet processes. A user must understand the chemistry and modify certain aqueous solutions, as appropriate, to avoid unpleasant surprises.
This precaution is easy to implement at the process bench so that the water people can continue to degasify UPW per ASTM D-5127 and proudly claim their state-of-the-art water quality. Wet processing people, however, need to be aware of the consequences of using degasified UPW and add oxidizers to baths and rinses used in specific processing steps, as appropriate.
References
- MacKinnon, S., "Water-spot Formation on Hydrophobic Surfaces," MICRO 94 Proceedings, pp 174 - 184 (Canon Communications LLC, 11444 W. Olympic Boulevard, Ste 900, Los Angeles, CA 90064)
- Verhaverbeke, S., "Deposition of Metallic Contaminants from Liquids and Their Removal," Chapter 10 in Contamination-Free Manufacturing for Semiconductors and Other Precision Products, Robert P. Donovan, Editor, Marcel Dekker, Inc. (2001)
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM.