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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 LambMEMC Electronic Materials, Inc.

Abstract: The storage life of bare silicon wafers has been historically defined as theelapsed 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 haveseen 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 highpotential for surface degradation, strict control of moisture inside the wafer package is theprimary key to 18-month storage life. We show that surface organics, ions, oxide

thickness, metals and particles remain very stable in a well-controlled packageenvironment. 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 generalmechanism for tdh formation is proposed.

Introduction

Silicon wafer users expect that the surface of the wafers they receive from wafer makerswill 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 haveobserved wafers with a very large number of >0.12um LPDs that were not present when

the wafers left the manufacturers facility. This phenomenon has come to be called time-dependent 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 willalways suggest to the user that something in the wafer makers process is out of control.

A typical hazed wafer map is shown in Figure 1. It is common to see a localized regionof high LPDs in a random pattern. LPD counts on a hazed wafer are usually at leastseveral 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 thedefects 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 waferwith water or heating it to 200C. Individual defects also tend to disappear very quicklywhen 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 withhigh vapor pressures at


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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 thatformed 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 comparethem to those of freshly cleaned and packaged wafers. The relationship between thesechanges and TDH formation is discussed.

Experimental Procedures

Wafer sampling and storage. For over two years, five prime 200 mm diameter waferswere 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, packagedand analyzed within 30 days to establish a fresh wafer baseline. The warehouse

temperature varied from 40F to 100F depending on seasonal temperatures. Warehousehumidity was not controlled. At the end of the 6 month or 18 month storage time, thewafers 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 inspectedfor 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 notbe 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 ionanalysis was done using a combination of water extraction of the surface followed by

analysis of the extract by ion chromatography or capillary electrophoresis described bySun (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 dropexamined by ICP/MS.A Dimension 5000 Nanoscope III AFM made by Veeco- DigitalInstruments 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 milliliterof water was placed in the box and the box was closed and packaged normally, butwithout the desiccant. The box was staged in a temperature-controlled chamber for 16 hrs

at 17C then for 4 hrs at 50C then for 4hrs at 10C and finally staged for 16 hrs at17C. The relative humidity holds at about 90% for most of this test except the 50Cstep. The wafers were unboxed and re-inspected for LPDs and the change in LPDs wasdetermined.


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Results and Discussion

Trend charts for changes in LPDs and whole wafer averages for surface organics, surfaceions 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 re-inspected 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 andlower control limits. Six-month storage data are shown up till late December 1998 and18-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 ofstorage and the variability around this average primarily reflects the long-termreproducibility (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 post-storage 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 theTDH events up till that time. No TDH events were observed after this organic wasremoved 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 weswitched 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 towafers 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 (5 X 1014 C atoms/cm2 of the cleaning agent. This wafer exhibitedTDH. The organic cleaning agent mentioned earlier was found to be the reason for all the


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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 forsurface organics. It appears that 1012 ions/cm2, placeNH4+ 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 willdiscuss 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 isaffected 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 1packaging during the >6 months of storage. The tiny remote probes that are commerciallyavailable to measure relative humidity and temperature could not be used because they

would contaminate the environment inside the package. However, we realized that anincrease 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 willcontinue to oxidize if further exposed in humid air (8) for extended periods, reaching athickness 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 inFigure 8. It is clear that no significant additional oxidation occurred and therefore themoisture 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 waferssampled at the same time but stored for 18 months. So, the rapid TDH test appears to be

more sensitive to surface changes.


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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 quicklysignal 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 developedfor 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 averagewafers 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 lifesamples. The results are shown in the trend chart in Figure 10. Small increases in LPDchanges 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 storagetest.

Even given the low levels of surface contamination on commercially available siliconwafers, the potential to form millions of 0.12um particles is great. For example, 1012

molecules/cm

2

of NH4Cl could form 1,000 particles/cm

2

or 300,000/wafer under idealconditions. Similarly, 1015 C atoms could form 100,000 0.12um particles/cm2. However,

this usually doesnt 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. Otherorganic 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 inthe packaged cause water to condense on the wafer surface. 3. The surface waterdissolves the water-soluble contaminants. 4. The hydrophobic surface causes the water to

form microscopic droplets. 5. The micro-droplets evaporate and leave individual TDHdefects. The diameter of a micro-droplet of NH

4Cl solution could be as small as 2.5X

that of the particle from which it forms before the salt begins to precipitate from thedroplet. This would inhibit the formation of extended films or residues that would not bedetected as LPDs.

Conclusions

Wafers can be consistently stored for up to 18 months without any detectable degradationin >0.12um LPDs, organics, oxide thickness, surface metals and surface ions relative tofreshly-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 siliconwafers already provide a huge potential to form TDH. But this potential is seldomrealized if humidity in the package is controlled since, for normal contamination levels,

condensed water is needed to accelerate the process of TDH formation. Therefore, watersolubility of the impurities seems to be an important parameter as noted by other

investigators (1).


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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 properlycontrolled at current levels. For example in our specific case, an organic cleaning agent

caused TDH formation even though package integrity (and therefore relative humidityinside the package, also) was maintained.

Our proposed mechanism suggests that the formation of micro-droplets of water onpartially hydrophobic surfaces leads to highly localized precipitation of water-soluble

contaminants on the wafer. The density and size of these micro-droplets will be affectedby organic contamination and by the relative humidity in the package. Given that organicmolecules 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 inthe 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 oftime 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 ContaminationMonitoring for Cleanroom Air and Wafer Surfaces inIn-Line CharacterizationTechniques 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 extractionsystem for use with CE and IC,MICRO, April 2000.

6. K. Vepa, K. Baker and L. Shive, A Method for Native Oxide ThicknessMeasurement 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.


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Acknowledgements

Special thanks for the analytical methods development and data collection go to GaryAnderson, 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 fordevelopment of the accelerated storage test methods and to Gianpaolo Mettifogo for hison-line testing of this method.

About the Authors

Larry W. Shive, PhD, is an MEMC Fellow in the Epi Technology Department ofMEMC 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 [email protected].)

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 physicsfrom Michigan State University. (He can be reached by phone at 636.474.7322 or email

at [email protected].)

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 at636.474.5534 or email at [email protected]. )


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A List of Figures

Figure 1. A whole wafer LPD map showing a localized pattern of TDH.


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Figure 2. An AFM image of a 4um X 4um region of a wafer that exhibited TDH. Typicaldefect size is 5- 25 nm height and 100- 500 nm width. The estimated defect density in

this region is 10

8

defects/cm

2

.


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Figure 3. Trend chart of the change in >0.12um LPDs after 6 month and 18 monthstorage. 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-December98 wafers. All points prior to then are for 6-month storage.

-60

-40

-20

0

20

40

60

80

100

1/20/97 8/8/97 2/24/98 9/12/98 3/31/99Manufacturing Date

Changein>=0.1

2mcironLPDs

CL = 2.0 UCL = 48.9 LCL = -44.8

UCL

CL

LCL

18 month

storage

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-2,5-

cyclohexadiene-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-


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Figure 4. Trend chart of total desorbable surface organics after 6-month and 18-monthstorage. 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 C-atoms/cm2. The 18-month storage testing began with the late-December 98 wafers.Freshly packaged wafers (


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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 (8.46min., 12.26 min. and 15.10 min.) that has contaminated the wafer.


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Figure 6. Trend chart for surface ions extracted from wafers after 6

months and 18 months of storage. Each point represents the average oftwo wafers sampled after 6 or 18 months of storage. The original

manufacturing date is shown on the x-axis. The 18-month storagetesting began with the late-December 98 wafers. The trends forammonia 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.

0

1000

2000

3000

4000

5000

6000

6/19/97 1/5/98 7/24/98 2/9/99

Manufacturing date

1010i

ons/cm

2

Cl = 611 UCl = 2300

Cl

UCl

18 month storage

NH4+ trend

0

100

200

300

400

500

600

700

6/19/97 1/5/98 7/24/98 2/9/99

Manufacturing date

1010ions/cm

2 UCl

Cl

Cl = 206 UCl = 618

18 month storage

Cl- trend


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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 ineach box.


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Figure 8. Trend chart for whole-wafer average oxide thickness after 6 month and 18months 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. Freshly packaged wafers

have


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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 onthe x-axis. The samples were taken at the same time as those shown in Figure 3.

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1/20/99 2/9/99 3/1/99 3/21/99 4/10/99 4/30/99 5/20/99 6/9/99

Manufacturing Date

Changein>=0.1

2LPDs

CL

UCL

CL = 25.5 UCL = 102.8

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