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Organic airborne contamination on Si wafers and its quantitative characterization (Toni)
===Walter Den, Hsunling Bai, and Yuhao Kang, “[http://jes.ecsdl.org/content/153/2/G149.full Organic Airborne Molecular Contamination in Semiconductor Fabrication Clean Rooms
A Review]” Journal of The Electrochemical Society, 153(2), G149-G159, 2006.===
Volatile or semivolatile organic contaminant have been detected e.g. by subjecting sampling tubes to gas chromatography (GC) coupled with mass spectrometer (MS) => GC-MS.
These provide very low detection limits but are very elaborating and time-consuming
Monolayer contamination has been studied by XPS or TOF-SIMS
Both require high vacuum => loos of volatile components may become significant. Neither is capable of full wafer analysis
The adsorbed organic molecules can be thermally desorbed and swept into a sampling tube with a metered stream of carrier gas => sorption tube is analyzed by TD-GC/MS.
Even commercially manufactured infrared-heated full-wafer vacuum chamber are available for this purpose => this type of analytical technique has become accepted and issued as the industrial standard method for organic analysis of Si wafer surfaces
Contamination on surfaces and those found in cleanroom air are two different things. Species with low concentration in air may tend to adsorb on a surface
[From another reference] Wafers stored in a box. Organic contaminant with –C=O or –S=O functional groups tended to be adsorbed on a wafer surface within 2 days of storage, while those with –O- groups were detected only after nearly 10 days. => Degree of polarity of the contaminants has a critical role in the adsorption of organic molecules. Adsorption behavior depends heavily on the electronegative difference of the surface and the contaminant.
Organic contamination from air has an important role on degradation of electrical properties of H-terminated Si surfaces. The electrical performance was studied by measuring IV characteristics of mercury-Si junctions.
The fundamental behavior of airborne organic contaminants is modelled using the MOSAIC model developed by the authors in earlier publications. In addition, the silicon plate sampling (SPS) method is used experimentally.
The maximum concentration of contaminants and ratio of desorption and adsorption are independently described from each other by the model.
Conclusion: The nature of airborne organic contamination can be comprehensively evaluated by integrating MOSAIC model and SPS method.
A “silicon plate method” is developed to evaluate the amount of organic contamination on a Si wafer surface coming from cleanroom air.
As the amount of some organic species start to decrease after a peak concentration, it seems that organic species compete for the adsorption sites on Si wafer surfaces.
Traditional way to measure organic contamination really complex (WTD-GC-MS or TOF SIMS)
In this developed method, TD-GC-MS (thermal desorption gas chromatography mass spectrometry) is still used, but one heating step is removed. Contamination is evaluated directly from the Si plate, and additional exposure to cleanroom air is prevented.
Time-dependent behavior of contamination adsorption is evaluated by multicomponent organic species adsorption-induced contamination (MOSAIC) model.
The authors study behavior of organic contaminants absorbed onto Si wafer surface by solvent-dissolution GC-MS technique.
Experimental
Wafers precleaned via RCA, wafers covered with native oxide
Wafers expose to a conventional cleanroom for several hours and placed vertically on a quartz carrier
Other wafers were sealed and stored for several days in two polypropylene boxes (commercially finished products): (i) not used prior the experiment (“new box”), (ii) box was left open in clean room for several months before the experiment (“used box”)
Exposure time and storage time varied
Characterization by GC-MS analysis:
Organic contaminants collected by dissolving them in a solvent (pure acetone used: suitable for this kind of study, see details from the paper)
Contaminants then studied from the solvent
Results
Cleanroom air
Organic contaminants found from a wafer exposed to clean room air for only 1 h
Different contaminants are found after 24 h cleanroom air exposure than after 1 h exposure
Contaminants with low boiling point were adsorbed immediately and decreased with exposure time; those with high bp increased with exposure time (does not apply for everything) => Organic contaminants having a low boiling point were replaced by those with higher boiling points
New box
No cleanroom airborne contaminants found but other contaminants resulting from the plastic additives: crosslinking agent (DAB), antioxidant (DBQ) and plasticizer (DBP)
Used box
Airborne contamination peaks found due to contamination already present in the box which was stored open beforehand. Nevertheless, the amount considerable smaller than on wafers stored in cleanroom air directly.
In this case, the adsorption time did not depend on the boiling points of the contaminants but instead on their structure: Contaminants containing C=O or S=O groups were adsorbed immediately; those having -O- groups only after several days. This is due to different polarities of the molecules.
SUMMARY: TD-GC/MS is used to study organic contaminants originated form plastic boxes supplied by major Si wafer vendors. Also outgassing from the box materials is studied to identify the source of contaminants. Various wet cleaning solutions are evaluated based on their ability to remove such contaminants.
Dilute HF or ozonized ultrapure water can be used to entirely remove the organic contaminants.
After cleaning, organic contaminants adsorb more easily on the ozonized water-treated surface than on the dilute HF-treated surface.
Adsorption of organic contaminants can be reduced by preventing native oxide growth in N2 atmosphere after HF dip.
In the past, organic contaminants have been detected by XPS, TOF-SIMS, FTIR or thermal desorption spectroscopy (TDS). More suitable options are TD-GC/MS or TD-IMS/MS.
Experimental
Wafers stored in four different boxes from different suppliers for more than one month. No wafer cleaning beforehand.
All boxes were polypropylene: some parts were made of several kind of plastics (including polycarbonate) or thermoplastic polyester elastomers.
TD-GC/MS used to analyze the contaminants on wafer surfaces. Desorption by heating up to 400 C in an inert gas.
HS-GC/MS used to analyze outgassing of the box materials.
Different wet cleanings applied to wafers prior to storage: NH4OH/H2O2, H2SO4/H2O2, HNO3, dilute HF, O3/H2O
Storage for 1 month in a box, interior continuously purged with N2 or O2 (5 l/min)
Results
The lower the vapor pressure and/or smaller the molecular weight of the contaminant compounds, the more easily they will adsorb onto the surface.
H2SO4/H2O2 and HNO3 could not completely remove a monolayer of organic contaminants. Dilute HF and O3/H2O succeeded in this. Reasons for these discussed in the paper, relates to the polarity of the molecules.
Oxygen purging resulted in the same amount of organic contaminants than with no purging, whereas purging with N2 reduced the amount of organic additives adsorbing on the wafer surfaces significantly. Reason for this: native oxide did not grown in N2 ambient, and the surface hence remained nonpolar. O2 oxidized the surface, which thus became polar and attracted contaminants.
Conclusion: boxes should be filled with N2 if organic contamination should be prevented.
Outgassing characteristics of PP (natural, antistatic, and blue), PC, PFA, PVDF, ABS, and PTFE
Polymers heated up from 60 C to 20 C below their softening temperatures. Amount of outgassing continuously monitored, and outgassing compounds separately identified.
PTFE and PFA showed lowest amount of outgassing over the entire temperature range; PVDF and PC almost as good.
Separate tables for outgassing of each material (all list various outgassing compounds)
For analytical determination of the volatile contaminants at typical temperatures (25-100 C), only ion mobility spectrometry (IMS) has the required sensitivity and selectivity. However, it may still take even with this technique 320 days at RT to accumulate enough substance to distinguish different products, when advanced polymers are studied.
Polymer materials and coating on polymers to improve mechanical strength and resistance to abrasion
The purpose of this literature review is to:
provide a list of problems encountered with 3D-printing devices regarding their comptability with a cleanroom environment.
Find state-of-the-art solutions to avoid air contamination by 3D-printed devices --> can polymer coatings be used, other coatings?
Find suitable polymers/layers that prevent the wear of 3D-printed mechanical parts and prevent particle dissemination.
Toni searched how to analyze organic contamination on silicon wafers and particle contamination.
Keywords:
“3D printing cleanroom contamination”
“3D-printed cleanroom parts”
see also 3D printing for MEMS (possible coating materials that are compatible with 3D printing and could prevent particle formation).
“pla surface modification 3D”
“3D-printed abs abrasion”
Review:
Surface functionalization --> treatment of the surface with a material that prevents wear of the parts.
Lifton, V., Lifton, G., & Simon, S. (2014). Options for additive rapid prototyping methods (3D printing) in MEMS technology. Rapid Prototyping Journal, 20(5), 403-412
See part “surface treatments”.
-> List possible surface treatments that are used in MEMS 3D printing in order to improve the surface with respect to quality or to functionalize it. For instance, mention electroless plating to deposit metal on a plastic.
McCullough, E. J., & Yadavalli, V. K. (2013). Surface modification of fused deposition modeling ABS to enable rapid prototyping of biomedical microdevices. Journal of Materials Processing Technology, 213(6), 947-954
-> Propose methods to modify ABS. They make ABS water-impermeable, hydrophilic and biocompatible. This could be useful maybe to prevent degradation of 3D-printed objects with time and thus limit particle formation?
Rasal, Rahul M., Amol V. Janorkar, and Douglas E. Hirt. "Poly (lactic acid) modifications." Progress in polymer science 35, no. 3 (2010): 338-356
-> Discuss the advantages and drawbacks of PLA as a polymer and discuss possible modifications of this material. Focusses on biological applications of PLA.
PLA is a particularly brittle material (so maybe not best choice for 3D printing of cleanroom tools?).
Plasma treatments can improve wettability (this can be useful if there is a need for ALD layer deposition, as this requires preferably hydrophilic surfaces). NH3 and O2 plasma have been used for this purpose.
Photografting can be used to modify the PLA surface.
Olivera, Sharon, Handanahally Basavarajaiah Muralidhara, Krishna Venkatesh, Keshavanarayana Gopalakrishna, and Chinnaganahalli Suryaprakash Vivek. "Plating on acrylonitrile–butadiene–styrene (ABS) plastic: a review." Journal of materials science 51, no. 8 (2016): 3657-3674
-> Review the mechanical and chemical properties of ABS and discuss possibilities to improve them, especially by plating techniques.
Mention that plating improves resistance to abrasion. ABS is a good choice for plating because of its chemical properties.
Properties of ABS: possesses high stress resistance due to its butadiene component (ABS made of two phases, one styrene-acrylonitrile = SAN, and one butadiene). Rigidity, thermal stability, resistance to cracking and chemical reactions. Easily molded. Its properties are determined by the distribution and the size of the rubber particles (the two plastic phases). For instance, higher toughness if proportion of butadiene is increased. SAN phase determines chemical properties.
Plating: mention that “ABS is the most electroplated plastic”. Process for plating on ABS:
Etching to roughen the surface to enhance adhesion of the metal
Activation --> some kind of seed layer deposited
Rocha, Carmen R., Angel R. Torrado Perez, David A. Roberson, Corey M. Shemelya, Eric MacDonald, and Ryan B. Wicker. "Novel ABS-based binary and ternary polymer blends for material extrusion 3D printing." Journal of Materials Research 29, no. 17 (2014): 1859-1866. (search “3D-printed abs abrasion”)
-> Propose a new ABS-based polymer combining the properties of several polymers.
State that ultrahigh molecular weight polyethylene (UHMWPE) possesses high resistance to abrasion (see references) and also high toughness and strength. It cannot be extruded, that is why they study its combination with ABS to allow 3D-printing.
Cleanrooms and associated controlled environments. Part 1: Classification of air cleanliness by particle concentration (ISO 14644-1:2015), p.14
Recommend light scattering particle counter able to discriminate size and number of particles.
Also time-of-flight measurement can be used to determine particle size (two laser beams measure the time needed for a particle to pass, from that the aerodynamic diameter can be calculated).
Cleanroom technology: fundamentals of design, testing and operation, Whyte, W. 2001
p. 179 cleanroom particle measurements
p. 265 cleanroom hardware
F. Bürger, U. Ringe, G. Heyder, M. Hirt, Test Report -- Determination of the cleanroom suitability of the cleanroom chair Axia Flex cleanroom manufactured by BMA ergonomics B.V., Fraunhofer IPA, 2011
