Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Investigating the Formation of Time-Dependent Haze on
Stored Wafers
Larry W. Shive, Richard Blank and Karen Lamb
MEMC Electronic Materials, Inc.
Abstract: The storage life of bare silicon wafers has been historically defined as the
elapsed time after packaging before small particles begin to form on the surface. Many
silicon wafer users have observed this time-dependent haze formation on wafers that have
seen extended storage. We have investigated the surface changes of silicon wafers during
6 months and 18 months of storage and show that although most wafers have a very high
potential for surface degradation, strict control of moisture inside the wafer package is the
primary key to 18-month storage life. We show that surface organics, ions, oxide
thickness, metals and particles remain very stable in a well-controlled package
environment. However, we also show that the typical levels of organics and ions that are
present on commercially available silicon wafers have the potential to form over one
million particles >0.12um diameter if the wrong storage conditions exist. A general
mechanism for tdh formation is proposed.
Introduction
Silicon wafer users expect that the surface of the wafers they receive from wafer makers
will meet all requirements for light point defects (LPDs), metals, grown-on film quality
and cleanability regardless of the storage history of the wafers. Yet, many users have
observed wafers with a very large number of >0.12um LPDs that were not present when
the wafers left the manufacturer’s facility. This phenomenon has come to be called “timedependent haze” (TDH). Its appearance may or may not cause a LPD rejection at
incoming Quality Assurance or a performance problem in the device line but it will
always suggest to the user that something in the wafer maker’s process is out of control.
A typical “hazed” wafer map is shown in Figure 1. It is common to see a localized region
of high LPDs in a random pattern. LPD counts on a hazed wafer are usually at least
several hundred more than on non-hazed wafers. It is usually the presence of a pattern
and/or unexpectedly high counts that alerts one to TDH. An AFM image (Figure 2) of the
defects in this pattern may reveal up to 108 /cm2 defects that are typically 5- 25 nm high
and < 500 nm wide. We have found that TDH is usually removed by rinsing the wafer
with water or heating it to 200C. Individual defects also tend to disappear very quickly
when exposed x-ray beams so their composition is not easily determined by XPS. These
observations suggest that TDH is composed of ionic or polar organic compounds with
high vapor pressures at <200C.
An observation of TDH brings up the question of what actually changed on the wafer
surface during shipping and storage and why it only appeared on a small portion of
wafers. It has been clearly shown that organic and inorganic contamination can form
particles on the surface during storage to form a “haze”. Munter et al. (1) have artificially
created TDH by intentionally contaminating wafers with inorganic ions and organic
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
solvents and storing the wafers. Vepa et al. (2) artificially created TDH by exposing
wafers to organic compounds that are commonly observed on wafers. The TDH that
formed was composed of small particles rather than a continuous film or large residues.
In this paper, we look at some key surface characteristics of silicon wafers after 6 & 18
month storage in Grade 1 packaging in a typical warehouse environment and compare
them to those of freshly cleaned and packaged wafers. The relationship between these
changes and TDH formation is discussed.
Experimental Procedures
Wafer sampling and storage. For over two years, five prime 200 mm diameter wafers
were sampled weekly from production after final cleaning, inspected, packaged in Grade
1 packages and immediately stored in a local warehouse for 6 months or 18 months.
Grade 1 packaging consisted of a clean, dry polypropylene box, a polyethylene inner bag,
a package of desiccant and an outer laminated polyethylene bag that includes an
aluminum layer. Other prime samples were pulled during the same time period, packaged
and analyzed within 30 days to establish a “fresh wafer” baseline. The warehouse
temperature varied from 40ºF to 100ºF depending on seasonal temperatures. Warehouse
humidity was not controlled. At the end of the 6 month or 18 month storage time, the
wafers were re-inspected for LPDs and then distributed to the analytical laboratory to
measure surface organics, ions, metals, and oxide thickness. The outer bag was inspected
for tears or pinholes. The “before” and “after” LPDs were measured using an ADE
Corporation CR-80 surface inspection tool.
Analytical methods. The five wafers/week recovered from storage were distributed for
the following analyses. Since only five wafers were available, all the analyses could not
be done every week. Desorbable surface organics were measured by the thermal
desorption/gas chromatographic method with mass selective and NPD (before June 1998)
or atomic emission (after May 1998) detection described by Sun (3-4). Surface ion
analysis was done using a combination of water extraction of the surface followed by
analysis of the extract by ion chromatography or capillary electrophoresis described by
Sun (5). Average oxide thickness was determined using a variation of the method of
Vepa (6). Surface metals were determined using acid drop extraction with the acid drop
examined by ICP/MS. A Dimension 5000 Nanoscope III AFM made by Veeco- Digital
Instruments was used to image the TDH defects.
The accelerated TDH test was done on fresh prime wafers as follows. Test wafers are
inspected for LPDs and then placed in slots 11, 13 and 15 of the box insert. One milliliter
of water was placed in the box and the box was closed and packaged normally, but
without the desiccant. The box was staged in a temperature-controlled chamber for 16 hrs
at 17ºC then for 4 hrs at 50ºC then for 4hrs at –10ºC and finally staged for 16 hrs at
17ºC. The relative humidity holds at about 90% for most of this test except the 50ºC
step. The wafers were unboxed and re-inspected for LPDs and the change in LPDs was
determined.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Results and Discussion
Trend charts for changes in LPDs and whole wafer averages for surface organics, surface
ions and oxide thickness after 6 month and 18 month storage are shown in Figures 3, 4, 6
and 8. These data will be discussed one at a time.
The trend data for the change in >0.12um LPDs after 6 months and 18 months are shown
in Figure 3. The manufacturing date of the wafers is shown on the x-axis. They were reinspected 6 months or 18 months later. In the chart, each data point is the average LPD
change for a five-wafer sample. The whole database was used to calculate the upper and
lower control limits. Six-month storage data are shown up till late December 1998 and
18-month storage data begin in late December 1998. Data points that exceed the upper
control limit are defined as TDH events.
The average increase of >0.12um LPDs is only 2.0 LPDs/wafer after 6-18 months of
storage and the variability around this average primarily reflects the long-term
reproducibility (six to eighteen months) of the inspection tool. These LPD inspection
tools were serviced and re-calibrated at least once between the pre-storage and poststorage inspections and long term reproducibility was certainly affected. Prior to mid-July
1998, an organic cleaning agent was used in the process and it was the source of all the
TDH events up till that time. No TDH events were observed after this organic was
removed from the process.
The increase in storage time from 6 months to 18 months had no impact on the change in
surface LPDs. There was no shift in the average adders in December 1998 when we
switched to 18 months of storage.
Surface metals do not increase with storage time.
One might expect that surface organic levels would increase very significantly, relative to
wafers stored for only a few days, when wafers have been stored for 6 months or more.
Some common organic contaminants found on stored wafers are listed in Table 1 and
include common plasticizers and antioxidants. Freshly packaged wafers (<30 days)
already typically have 0.5- 1.5 X 1014 C atoms/cm2 of these compounds. Sugimoto (7) has
investigated organic contamination on wafers stored in new boxes for 30 days and found
similar plasticizers and antioxidants. The trend chart for average total desorbable organics
on wafers stored for 6 months and 18 months is in Figure 4 and shows that the average,
1.2 X 1014 C atoms/cm2 , is no different that that of freshly packaged wafers. Two typical
chromatograms of wafers that were stored for six months are shown in Figure 5. Both
show detectable levels of antioxidants, plasticizers and siloxanes but the chromatogram
on the right shows >5 X 1014 C atoms/cm2 of the cleaning agent. This wafer exhibited
TDH. The organic cleaning agent mentioned earlier was found to be the reason for all the
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
TDH events before mid-July 1998 in Figure 3. We used NPD organic detection till June
1998 then switched to AE detection to obtain more stable and quantifiable results for
surface organics. It appears that <5 X 1014 C-atoms/cm2 of these common plasticizers
and antioxidants that are usually deposited during 6- 18 months of storage do not affect
TDH if package integrity is maintained.
It should be noted that our method is only detecting desorbable organics. High molecular
weight, low-volatility organics could easily be present at high levels but not observed on
the wafers. These would more likely be left on the wafer by the wafer-making processes
than transferred from the package materials.
Surface ion contamination, specifically sulfur-containing ions, have been known to cause
severe TDH (1). Inorganic ions commonly found on silicon wafers are listed in Table 1.
In our study, sulfur-containing compounds and ions were below the detection limits of
the GC/AES and CE methods, 109 S-atoms/cm2 and 5 X 1010 SO4 2--ions/cm2 respectively.
Fluoride and nitrate ions were also below the detection limits. However, ammonium and
chloride ions were found in abundance. Freshly packaged wafers typically have as much
as 3 X 1013 NH4 + ions/cm2 and 1.9 X 1013 Cl- ions/cm2 . The trend data after 6 & 18
month storage are shown in Figure 6. The levels of these ions, >1012 ions/cm2 , place
NH4 + and Cl- among the most abundant contaminants on hydrophilic silicon wafers.
Their levels did not increase during storage and they did not cause TDH formation when
organics were controlled and package integrity was maintained. However, as we will
discuss later, the potential for particle formation is significant for this level of
contamination.
Water is the well-recognized accelerating agent for TDH formation. For example, the
quantity of water added to a package can be related to the total area of a wafer that is
affected by haze, all other variables being kept constant (Figure 7). Therefore we began
to look for a test to indicate whether traces of moisture had leaked into the Grade 1
packaging during the >6 months of storage. The tiny remote probes that are commercially
available to measure relative humidity and temperature could not be used because they
would contaminate the environment inside the package. However, we realized that an
increase in oxide thickness would be a very sensitive indicator of a leak. This is because
the chemical oxide from a freshly cleaned wafer is only 0.8- 1.1 nm and the wafer will
continue to oxidize if further exposed in humid air (8) for extended periods, reaching a
thickness of 1.5 nm. Therefore, if the oxide thickness of the wafer reached 1.5 nm, it
would indicate that the moisture barrier had failed.
A trend chart for average oxide thickness after 6 & 18 months of storage is shown in
Figure 8. It is clear that no significant additional oxidation occurred and therefore the
moisture barrier was completely effective. However, the data suggest a trend for which
we currently have no explanation. That trend is not echoed in the LPD changes on wafers
sampled at the same time but stored for 18 months. So, the rapid TDH test appears to be
more sensitive to surface changes.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Although 6 & 18 month tests clearly demonstrate the satisfactory storage life of silicon
wafers, they do not provide rapid feedback to process engineering that could quickly
signal a potential change in wafer storage life and allow corrective action to be taken.
Therefore, a rapid TDH test that uses water and temperature as accelerants was developed
for this purpose. Rapid TDH tests can be designed to produce intense TDH on almost any
wafer, but this test was designed to create a weak but significant LPD increase on average
wafers but extremely high counts on contaminated wafers. For example, the effect of 1015
C atoms/cm2 of the aforementioned cleaning agent was easily observed by the rapid TDH
test (Figure 9). The rapid TDH samples were pulled at the same time as the storage life
samples. The results are shown in the trend chart in Figure 10. Small increases in LPD
changes that were observed by this test were not detected on the parallel sample set used
for 6 & 18 month storage, so the rapid test may be slightly more sensitive than the storage
test.
Even given the low levels of surface contamination on commercially available silicon
wafers, the potential to form millions of 0.12um particles is great. For example, 1012
molecules/cm2 of NH4 Cl could form 1,000 particles/cm2 or 300,000/wafer under ideal
conditions. Similarly, 1015 C atoms could form 100,000 0.12um particles/cm2 . However,
this usually doesn’t occur without accelerants such as condensed water.
We propose the following general mechanism for TDH formation. 1. The wafer is
contaminated with water-soluble ions or water-soluble organic molecules. 2. Other
organic molecules also deposit on the wafer, making it more hydrophobic. 3. Changes in
relative humidity caused by temperature changes and/or changes in total water content in
the packaged cause water to condense on the wafer surface. 3. The surface water
dissolves the water-soluble contaminants. 4. The hydrophobic surface causes the water to
form microscopic droplets. 5. The micro-droplets evaporate and leave individual TDH
defects. The diameter of a micro-droplet of NH4 Cl solution could be as small as 2.5X
that of the particle from which it forms before the salt begins to precipitate from the
droplet. This would inhibit the formation of extended films or residues that would not be
detected as LPDs.
Conclusions
Wafers can be consistently stored for up to 18 months without any detectable degradation
in >0.12um LPDs, organics, oxide thickness, surface metals and surface ions relative to
freshly-packaged wafers. If proper precautions are taken to minimize humidity in the
package and maintain package integrity, TDH will not form.
The typical average levels of water-soluble inorganic ions on freshly packaged silicon
wafers already provide a huge potential to form TDH. But this potential is seldom
realized if humidity in the package is controlled since, for normal contamination levels,
condensed water is needed to accelerate the process of TDH formation. Therefore, water
solubility of the impurities seems to be an important parameter as noted by other
investigators (1).
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Organic or ionic wafer contamination on the wafer surface that exceeds the typical levels
shown in this report may cause TDH to form even if package humidity is properly
controlled at current levels. For example in our specific case, an organic cleaning agent
caused TDH formation even though package integrity (and therefore relative humidity
inside the package, also) was maintained.
Our proposed mechanism suggests that the formation of micro-droplets of water on
partially hydrophobic surfaces leads to highly localized precipitation of water-soluble
contaminants on the wafer. The density and size of these micro-droplets will be affected
by organic contamination and by the relative humidity in the package. Given that organic
molecules from plastic packages and some inorganic ions such as NH4 + and Cl- are
ubiquitous, it may be necessary to take further measures to control relative humidity in
the shipping package in order to avoid TDH formation as customers begin to inspect for
40- 90 nm LPDs.
References
1. N. Munter, B. Kolbesen, W. Storm and T. Muller, “Preparation and characterization of
time dependent haze on silicon surfaces”, Proceedings of UCPSS 2000, Oostende,
Belgium, pp. 91- 29.
2. K. Vepa, J. D. Dowdy, E. J. Mori and L. W. Shive, Proceedings of ECS Spring
Meeting, The Electrochemical Society, 93(1), 1141 (1993).
3. P. Sun, M. Adams and T. Bridges, “Monitoring organics on wafers surfaces using
thermal desorption GC-MSD/AED,” MICRO, March 2000.
4. P. Sun, M. Adams, L. Shive and S. Pirooz, “Molecular and Ionic Contamination
Monitoring for Cleanroom Air and Wafer Surfaces” in In-Line Characterization
Techniques for Performance and Yield Enhancement in Microelectronic Manufacturing,
SPIE Vol. 3215, D. K. DeBusk and S. Ajuria, eds., October 1997, pp. 118- 127.
5. P. Sun and M. Adams, “Demonstrating a contamination-free wafer surface extraction
system for use with CE and IC”, MICRO, April 2000.
6. K. Vepa, K. Baker and L. Shive, “A Method for Native Oxide Thickness
Measurement” in Cleaning Technology in Semiconductor Device Manufacturing IV, The
Electrochemical Society, Inc., R. Novak and J. Ruzyllo (Eds.), 95-20, pp. 358- 365
(1995).
7. F. Sugimoto and S. Okamura, J. Electrochem. Soc., 146(7), 2725 (1999).
8. L. Shive, C. Frey and C. Vitus, “A Probe of Chemical Oxide Growth Conditions”,
Proceedings of UCPSS 2000, Oostende, Belgium, pp. 127- 128.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Acknowledgements
Special thanks for the analytical methods development and data collection go to Gary
Anderson, Marty Adams, Kenny Ruth, Phil Schmidt, Dr. Andrei Stefanescu, Dr. Peng
Sun and Dr. Hao Zhang. Additional thanks go to Mike Tyler and Lillian Rose for
development of the accelerated storage test methods and to Gianpaolo Mettifogo for his
on-line testing of this method.
About the Authors
Larry W. Shive, PhD, is an MEMC Fellow in the Epi Technology Department of
MEMC Electronic Materials (St. Peters, MO). He has a PhD in chemistry from Texas
A&M University. (Shive can be reached at (636)474-5370 or lshive@memc.com. )
Richard E. Blank, Ph.D., is a Senior Engineer in the EPI Technology Department of
MEMC Electronic Materials Corporation (St. Peters, MO). He has a Ph.D. in physics
from Michigan State University. (He can be reached by phone at 636.474.7322 or email
at rblank@memc.com. )
Karen Lamb is a Research Technician in the EPI Technology Department of MEMC
Electronic Materials Corporation (St. Peters, MO). (She can be reached by phone at
636.474.5534 or email at klamb@memc.com. )
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
A List of Figures
Figure 1. A whole wafer LPD map showing a localized pattern of TDH.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 2. An AFM image of a 4um X 4um region of a wafer that exhibited TDH. Typical
defect size is 5- 25 nm height and 100- 500 nm width. The estimated defect density in
this region is 108 defects/cm2 .
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 3. Trend chart of the change in >0.12um LPDs after 6 month and 18 month
storage. Each point represents the average of five wafers sampled on the manufacturing
date shown on the x-axis. The 18-month storage testing began with the late-December
’98 wafers. All points prior to then are for 6-month storage.
100
Change in >= 0.12 mciron LPDs
80
18 month
storage
60
UCL
40
20
CL
0
-20
-40
LCL
-60
1/20/97
8/8/97
Manufacturing Date
2/24/98
9/12/98
CL = 2.0 UCL = 48.9
3/31/99
LCL = -44.8
Table 1. A list of common surface contaminants found on cleaned & packaged wafers.
Organics
• TXIB, 2,2,4-trimethyl-1,3-pentane diol
Diisobutyrate (plasticizer)
• DBP, dibutyl phthalate (plasticizer)
• DOP , dioctyl pathalate (plasticizer)
• BHT (antioxidant)
• 2,6-di-t-butyl-4-methylene-2,5cyclohexadiene-1-one (oxidized BHT)
• 2,6-di-t-butyl-1,4-benzoquinone (oxidized
form of antioxidant)
• cyclic polydimethylsiloxanes ((Si(CH3)2O)n-), n=5-10
Inorganic
Ions
• NH4+
• Cl• SO4=
• NO3• NO2• F-
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 4. Trend chart of total desorbable surface organics after 6-month and 18-month
storage. Each point represents the average for two wafers sampled after 6 or 18 months of
storage. The original manufacturing date is shown on the x-axis. The y-axis is 1014 Catoms/cm2 . The 18-month storage testing began with the late-December ’98 wafers.
Freshly packaged wafers (<30 days) typically have 0.5- 1.5 X 1014 C atoms/cm2.
14
E14 C atoms/cm
2
12
18 mo storage
10
8
UCL
6
4
2
Cl
0
06/19/97
01/05/98
Manufacturing date
07/24/98
Cl = 1.6
02/09/99
UCL = 6.8
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 5. Chromatograms of organics desorbed from wafers after six months of storage.
The retention times are on the x-axis and the signal intensity is on the y-axis. The
dominant signals in the left chromatogram are a quinone(15.87 minutes), an isobutyrate
(24.12 min) and a cyclic methylsiloxane (10.3 min). These are also present in the right
chromatogram but are overshadowed by signals from an organic cleaning agent (8.46
min., 12.26 min. and 15.10 min.) that has contaminated the wafer.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 6. Trend chart for surface ions extracted from wafers after 6
months and 18 months of storage. Each point represents the average of
two wafers sampled after 6 or 18 months of storage. The original
manufacturing date is shown on the x-axis. The 18-month storage
testing began with the late-December ’98 wafers. The trends for
ammonia and chloride are shown on the left and right, respectively.
Freshly packaged wafers have up to 2.9 X 1013 NH4 + ions/cm2 and 0.8 X
1013 Cl- ions/cm2 .
6000
10 10 ions/cm 2
5000
NH4+ trend
18 month storage
4000
3000
UCl
2000
1000
Cl
0
6/19/97
1/5/98
Manufacturing date
700
7/24/98
2/9/99
Cl = 611 UCl = 2300
Cl- trend
600
10
10
ions/cm
2
18 month storage
UCl
500
400
300
200
Cl
100
0
6/19/97
1/5/98
Manufacturing date
7/24/98
2/9/99
Cl = 206 UCl = 618
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 7. The area of a wafer affected by TDH as a function of water volume added to the
wafer package in a rapid TDH test. Five wafers were equally spaced in slots 1- 25 in
each box.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 8. Trend chart for whole-wafer average oxide thickness after 6 month and 18
months of storage. Each point represents the average of two wafers sampled after 6 or 18
months of storage. The original manufacturing date is shown on the x-axis. The 18month storage testing began with the late-December ’98 wafers. Freshly packaged wafers
have <10 Angstroms of oxide.
Oxide thickness, Angstroms
14
13
UCl
12
11
10
Cl
9
18 month storage
8
7
6
1/5/98
4/15/98 7/24/98
Manufacturing Date
11/1/98
2/9/99
5/20/99
Cl = 9.6 UCL = 12.9
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 9. Wafer map of >0.12um LPDs after an accelerated TDH test on
a wafer contaminated with 1015 C atoms/cm2 of an organic cleaning
agent.
Originally appeared in the March 2001 issue of Micro Magazine. © Canon Communications 2001
Figure 10. Trend chart for the change in LPDs after an accelerated TDH test. Each points
the average of three wafers sampled immediately after the manufacturing date shown on
the x-axis. The samples were taken at the same time as those shown in Figure 3.
Change in >= 0.12 LPDs
120.0
UCL
100.0
80.0
60.0
40.0
CL
20.0
0.0
1/20/99
2/9/99
3/1/99
Manufacturing Date
3/21/99
4/10/99
4/30/99
CL = 25.5 UCL = 102.8
5/20/99
6/9/99