Dominant cleaning chemistries in semiconductor fabrication

1. I ~ _ _ l_~ _
A PROVEN SUB-MICRON PHOTORESIST STRIPPER SOLUTION
FOR POST METAL AND VIA HOLE PROCESSES
by
WaiMunLee
Vice President, Research & Development
EKC Technology, Inc.
Hayward, California, U.S.A.
ABSTRACT
A wet chemistry process based on hydroxylamine (HDATM)* chemistry has been found to remove
positive photoresist, sidewall polymers and other plasma process residues. The development of new chip
metallization materials, multimetal, and multilevel interconnect schemes for sub-micron processes have
placed new demands on wafer cleaning technology'. The high density connections of ULSI devices
require low resistance contacts2 •3 which in tum require extreme via hole cleanliness. The industry has
turned to combinations of wet and plasma photoresist stripping processes to achieve acceptably clean
,surfaces. Unfortunately, a result of plasma etching is the presence of' 'sidewall polymers" in the via holes
and other etching residues. SEM and surface contact angle evaluations indicate a HDA based solution
leaves surfaces free of resist and etching visual contamination. HDA processes exhibit lower levels of
mobile ionic contamination, as indicated by eN shifts, VT shifts and TOF-SIMS measurements.
'Product EKC265™ from EKC Technology, Inc.,
A ChE!mFirst Company, U.S. Patents 20 •
Other U.S. and Foreign Patents Pending.
>
~EKC Technolo~ Inc.
Additional prints available from:
EKe Technology, Inc.
A ChernFirsl Company
2520 Barrington Court, Hayward, CA 94545
ph: ~'1 510-784-9105 facs: +1 510-784-9181

2. INTRODUCTION
The work reported in this paper is based on evaluations perfonned in the EKC Technology Inc.,
Research and Development Laboratory and from an EKC sponsored study at Edinburgh
University, Scotland (Ref. #24). This paper is published in the pirOceedings of the Symposium on
Interconne.cl:s, Contact Metallization and Multilevel Metallization, Volume 93-25 of The
Electrochemical Society, Inc.
REVIEW OF RESIST STRIPPING CHElVtISTRY
A conventional positive photoresist consists of three components: a Novalak resin (a condensation
reacted product of cresol and formaldehyde - Figure 1), a photo active compound (a substituted
Napthoquinone Diazide - Figure 2) and a solvent (usually a mixture of glycol ethers)4,5,6.
x
Figure 1: Novalak Resin Figure 2: Substituted Di'azide
The stripping of organic photoresists occurs by oxidation, dissolution, or reduction mechanisms.
The mechanisms for the most popular resist stripping methods are shown in Table 1.
TABLE I' PHOTORESIST STRIPPING MECHANISMS
Dry Process Wet Process Mechanism
02 Plasma H202 IH 2S04
(NH4hS20sIH2S04 Oxidation
Fuming Nitric Acid
Solvent Stripper Dissolution
Ih Plasma Reduction
~i Page "2Jh;-19-9-6-EK-C-"j;-e-ch-n-OIO-9-y,-ln-C-'- - - - - - - - - - - - - - - - - - - - - - - - - - -

3. Oxidation The use of an oxidizer, such as hydrogen peroxide or ammonium persulfate in
concentrated sulfuric acid, for photoresist removal and wafer cleaning processes has been widely
reported?,8.9. A wafer cleaning process using hydrogen peroxide and ammonium hydroxide is
commonly referred to as the RCA clean 10. Fuming nitric acid has also been used extensively asa
cleaning and stripping agent, especially in Europe and Japan. Each of these solutions act on the
photoresist through an oxidation mechanism.
Oxygen plasma resist removal processes (also called dry stripping or ashing) carne into wide use
dUring the 1980's to remove the hardened resists created during p.asma etching processes. Plasma
systems have a variety of designs as discussed by Skidmore 11. They are downstream, parallel
plate12, UV/Ozone, etc. In the system the plasma field oxidizes (Equation #1) the resist molecules
(containing carbon, nitrogen, sulfur, hydrogen, and oxygen) into volatile gasses (carbon dioxide,
nitrogen dioxide, sulfur dioxide, and water) which are removed from the system by vacuum.
[1}
Dissolution
While wet strippers based on oxidation mechanisms are the most frequently used for chemical
stripping, they are limited to wafer steps where no metals are present. The acid based strippers
attack the metallization materials. At the steps with metal on the wafer the preferred wet stripper
for positive resists is a solvent/amine type. A solvent/amine stripper removes resist by a process
of penetration, swelling, and dissolutionS. The solvent molecules solvate the polymer molecule
and overcome the attractive forces that hold the polymer together. This mechanism is optimized
in a number of proprietary stripper solutions that mix various aprotic solvents (N-rnethyl-2-
pyrrolidone [NMP], dimethyl sulfoxide [DMSO], sulfolane, dimethylformamide [DMF], or
dimethylacetamide [DMAC]) with different organic amines. Table #2 summarizes a number of
commercially available, patented positive photoresist strippers.
TABLE 2' PHOTORESIST STRIPPER PATENT SURVEY
Solvent Amine Patent No. Assigned to
1 NMP Aminoethyl JK 60-131535 Allied Chemical
Piperadine
2 'NMP I _
JK 61-6827 Shipley
3 NMP/Sulfolane Isopropyl amine JK 63-186243 J. T. Baker
DMF/Sulfolane EP 102628
4 NMP Amine US 4617251 Olin Hunt
5 NMP Hydroxyl ethyl WO 8705314 Mac Dermid
Morpholine
6 DMSO I Amino alcohol JK 64-81950 Asahi Chern.
7 LPMSOIBLO Amino alcohol JK 64-42653 Tokyo Ohka
8 NMPIDMF Diethylenetriamine US 4824763 EKC Technology
9 DMAC Diethanolamine US 4770713 ACT
10 DMAC··others Amine JK 63-231343 Hitachi
11 DMF Amino alcohol JK 64-81949 Asahi Kasei
12 NMP or DMF Ammonium Salt JK 61-292641 Hoechst Japan
--------------------------1. Page 3 tr

4. Re<l1Jction Hydrogen plasma processes can be used for photoresist removal. In the plasma
chamber the resist molecule is reduced to methane, sulfite, nitrogen, and water (Equation 2). The
generalized chemical reaction can be described as follows.
[2]
It is reported that the addition of carbon tetrafluoride18 in a hydrogen plasma can remove
fluorocarbon residues.
KEY ISSUES RELATED TO RESIST STRIPPING PROCESSES
Intl:9Jiuctio.n
Process simplicity and control favor the use of one-step processes. However, the wide range of
device requirements and materials has resulted in a variety of application and combination
cleaning processes. Most resist stripping processes for advanced circuits are a combination of wetl
dry or dry Iwet techniques.
Mobile Ionic Contamination
Generally oxygen plasma processes are favored for their ability to strip hardened resist films and
the lack of a need for liquid chemicals and rinsing steps. While effective on organic materials,
oxygen plasma stripping does not remove metal contaminants found in photoresist films and in
the general wafer fabrication environment13• This problem is particularly acute for ULSI level
devices with their higher component density and thinner junctions and layers12 • Wet stripping is
more effective in the removal of heavy metallic contamination. Also, the high energies associated
with plasma processing can cause radiation damage in sensitiVE! circuits 1.
Another issue is the presence of surface metallic contaminants, that are suspected of causing loss
of selectivity during tungsten deposition into high aspect ratio "plug" holes 13• These contaminants
become surface nucleation sites during tungsten deposition that can result in tungsten particles
depositing on the silicon oxide surface. They can cause electrical short circuits under subsequently
deposited metal interconnecting layers. Again, the industry has determined that a combination
of oxygen plasma and wet stripping is effective for removal of this type of metallic contamination14 •
Metal CorrQ.sion
Parekh and Price studied corrosion associated with Al-Si-Cu and TiW metal systems resulting
from AIC~ residues 1s . They compared! various post etch treatments for their ability to reduce
chlorine ion (CO levels which in tum inhibits metal line corrosion. They concluded that post etch
treatments in a 01 water rinse or in a N2 bake have a negligible impact on corrosion compared to
resist removal by either an oxygen plasma ashing at 180°C followed by a dip in fuming nitric acid
or an organic stripper (EKC 830 from EKC Technology, Inc.). The results in Table 3 show that the
EKe 830 wet stripper produced the cleanest surface.
TABLE 3: EFFECT OF VARIOUS POST TREATMENTS ON THE RESIDUAL CI
AND F LEVELS FOR HPR204 RESIST 15
(~g/Cm2) No post H2O N 2 Bake, 02 Ash + Rinse + N 2 Bake EKe
treatment Rinse 200 0 e/1 Hr. Fuming HN01 I
,
O.,Ash + 0, Ash J 830
Ct 1.08 0 .99
1
0.50 0.01 0.01 1<0.01 ,0.01
F 0.47 10.28 0.20 0.18 1~.08 10.13 0.01
-"
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5. Etch Residue Problem
During anisotropic plasma etching processes for via contacts, metal patterns, and passivation
openings, "sidewall residues" are frequently deposited on the f(~sist sidewalls. After the oxygen
plasma ashing process these deposits become metal oxides. Incomplete removal of these residues
interfere with the pattern definition and/or complete filling of via-holes. Thus, wet stripping
options must be available.
Etching Residue Removal Mechanism
Several different chemistries have been identified for removing aluminum etching residues.
Alkaline based positive resist developers, such as NaOH, tetram(~thylarnrnoniumhydroxide and
16
choHne, are known to attack aluminum • The hydroxyl ions attack the aluminum to form an
aluminum oxide hydrated anion (Equation 3).
Positive resist developers are limited to removing aluminum residues, but they do not remove
residues associated with multimetal systems such as Al/Si/Cu. They also are ineffective on
residues from polysiHcon plasma etch processes. Stringent process control must be exercised to
prevent resist attack and maintain critical dimension control.
One of the first, for feature sizes down to 1.0 micron, is the solvent/amine type strippers such as
identified in Table 2. The attack mechanism is a two step reaction starting with the formation of
hydroxyl ions, when the amine component in the stripper is hydrolyzed with water17 (Equation 4).
RNH 2 + H 20 -> RNH 3 + + OR [4]
<-
The aluminum residues are removed by the same reaction as shown in Equation 3.
OthE!r alternatives for the removal of the aluminum etching resid.ues after metal and via etch are
(1) a mixture of HF or BOE and ethylene glycol ether or (2) a mixture of nitric acid, acetic acid and
hydrofluoric acid. The active species in these mixtures are hydrogen ions [H+], fluoride ions [F],
and acetate ions [CH3COOl The hydrogen ion non-selectively attacks metal residues and the
fluoride ion non-selectively attacks silicon. The acetic acid reacts with aluminum to form a more
soluble aluminum acetate. The reactions are shown in Equations #5 to #8.
NH4F + ~O -> NH40H + HF [5]
HF + H:P -> H 3 0+ + F- [6]
HN03 + H 20 -> H+ + N03- ,[7]
CH3COOH + H 20 -> CH3 COo- + H+ f8]
These solutions require extreme process control to prevent excessive attack of critical metal and
oxide layers. In some device structures these solutions are not usable due to their non-selective
attack mechanisms.
OthE!r Etching Residues
Sub-micron devices have led to the use of multilevel interconnecting metals, such as Al/Si/Cu,
TiN, TiW, W, and WSi. These metal stacks produce different types of etching residues not removed
by the conventional solvent/amine stripper chemistry. Etching type chemistries, if used to remove
residues associated with advanced metal structures, require tight control and expensive automated
equipment.
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Page tt-

6. A combination of wet cleaning with HF, followed by a NF/hydrogen plasma step has been
described to effectively remove via hole etch residues19 as required for good electrical contacts.
Again, this approach is a multiple step process.
Solvent/amLne chemistryis also incapable of removing the residues produced during the anisotropic
etching of polysiIicon or other silicides, particularly with HBr and HCl etching chemistries.
Oxygen plasma processes and the Piranha (~O/I-4S04) stripping solution are also ineffective in
the removal of polysilicon etching residues. The current process used to remove such etch residues
is based on multiple steps. First the etch residue is removed by immersion of the etched wafer in
a dilute HF (200:1) solution. The second step is the stripping of the photoresist layer with an oxygen
plasma ashing, and followed by a Piranha solution. A major concern with this process is the HF
mixture attack of the gate oxides and the resultant shift in the gate oxide threshold voltage (VT
Shift).
THEORY OF POSITIVE RESIST AND PLASMA ETCH RESIDUE REMOVAL WITH
HYDROXYLAMINE
A buffered hydroxylamine (HDA) solution was investigated as a cleaning and stripping solution
for its ability to strip positive photoresist, remove a number of plasma etch residues, and meet
stringent ULSI wafer cleanliness standards2o • HDA is a strong nucleophile and a redUcing agent
at high pH values. This allows for resist stripping and wafer cleaning through a combination of
reaction mechanisms involving reduction, chelation, and nucleophilic attack.
Positive Resist Removal
Two techniques used to establish image stability in positive resists are post exposure baking (hard
bake) and/ or UV stabilization. Both of these techniques are intended to minimize resist flow
during plasma etching. The processes cause cross linking. The photo-active compound reacts with
the Novalak resin 4• Figure 3a and 3b illustrate some of the possible reactions.
o
II
~~
R
+
x
x
Figure 3a: UV Stabilized Cross Linking Resist
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~i U©1996 EKe Technology, Inc.

7. Figure 3b: Coupled Diazide
Hydroxylamine is an extremely strong nucleophile that can attack the carbonyl groups. The result
is an increased solubility of the reacted product (oxime) in an alkaline medium, as illustrated in
Figure 4. ( "'"
c = 0 --- C --OH C --OH C === N -OH
/ /'
HO
/'~~ /(
~N
/
o xim.
OH
Figure 4: Nucleophilic Attack of Carbonyl Group
Polyimide Removal
The same unique nucleophilic attack mechanism of HDA takes place with cured polyimide
polymer structures, as illustrated in Figure 5. Laboratory tests have demonstrated the removal of
cured polyimide films with a HDA solution.
Figure 5: Nucloephilic Attack of Polyimide
Metal Halides Residue Removal
During plasma etching processes, such as metal etching, silicon oxide etching, and polysilicon
etching, hydroxyl groups in the photoresist react with metal halide gasses generated in the etching
chamber to form undesirable residues of organometallic compounds. The organometallic
compounds are shown in Figure 6. The compounds cause cross linking of the Novalak resin at the
metal centers, greatly reducing its solubility. On any subsequent oxygen plasma treatment, metal
oxides, e.g. Ti02 , TiO, A!;!03, and W02 would be formed and left behind.
o
~N2
~
~~~~?J=
o >=< c'V
I~O 0rY
~~ y
M=AI;Ti;W;Si
Figure 6: Chemical Reaction of Metal Halide
------------------------1, Page 7 tt-

8. Other stable metal halides, such as AIF3' WF5' WF6' WOF3' or TiF3 also remain on the wafer surface.
These salts and oxides are insoluble in water, dilute acids, or bases22, but they are removed in HDA
solutions. Reduction of these metallic species and subsequent formations of chelating complexes
playa role in the removal of these residues. Based on the oxidation/reduction potentials, the
metallic species that can be reduced by hydroxylamine are listed in Table 4.
The combination of HDA and an organic amine
T.ABLE 4: METALLIC REDUCTION BY form a strong reducing and complexing (ligating)
HYDROXYLAMINE22 solution. The insoluble metal oxide could be
Ag(I) _ Ag(O) reduced to a lower oxidation state and
Au(I1)_ Au(I) subsequently dtelated with the ligand to form a
Co(III) _ Co(I1) more soluble metal complex which could
ultimately end up in the solution.
Cr(VI)_ Cr(IV)
Cu(U) Cu(I) Hydroxylamine and organic amines can form
Fe(I1I) _ Fe(I1) coordination complexes through their nitrogen.
Pd(I1) Pd(1) atoms (e.g. Zn(NH20H)2CI2)' The proposed
Ti(lll) _Ti(l) mechan~sm of reduction, chelation, and
W(V) _ W(I1I) solublization results in removal of a number of
plasma generated etching residues without
attacking the pure metal surfaces.
EXPERIMENTAL CONDITION~
Four different stripping and cleaning solutions were chosen for t:he evaluation. They are listed in
Table #5. The first three were commercially available producl:s in general use. The fourth, a
buffered hydroxylamine solution developed by EKC Technology Inc. was a mixture of
hydroxylamine (e.g. NH2 0H) and 2 (2 aminoethoxy) ethanol (e.g. ~NCH2CH2OCH2CH20H).
TABLE 5: STRlPPING SOLUTIONS
-
Stripping Composition Temp.oC Time (Min.) Patent Number
,
NMP.I Alkanofamine 95 30 US 4617251
DMSOlMonoethanolamine 95 30 JK-64-42653
- I
DMAClDiethanolamine 100 30 US 4770713
Hydroxylamine Buffered 65 30 Patent Pending in U. S, I
So'lution Japan, Europe, Taiwan and
Korea
Sample wafers from various process steps were supplied by wenfer fabrication production lines.
The wafers were cleaned in different solutions heated according to the recommended process
tempE!ratures and for 30 minutes (Table 5). The cleaning took plaCie in either quartz or stainless steel
baths inside a standard wet bench. The wafer boats received intermittent manual agitation during
the snip cycle. After stripping, the wafers were transferred to a deionized cascade rinser for iniual
rinsing and finished with a cycle in a commercial spin/rinse dryer.
After cleaning the experimental wafers were compared for removal of surface contaminants. The
effect of the cleaning process on specific electrical parameters was also investigated.
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-----'' c==~ @996EKeTechnology,Jnc.
l

9. EXPERIMENTAL RESULTS
Introduction
The '"hernical compositions of etch residues vary with wafer conditions and process parameters.
Laser Ionization Mass Absorption (LIMA) analysis confirmed tha,t the residue after via etching of
wafE'rs with TiN anti-reflective coating contained TiD as shown in Figure 7.
3 ~sn 92 11: 23: 37
Ii IIO
K
Na
,.'
>--"~-""'-=fe!o""""""-""'-""'''''''''-s'-!:o~o ..... ---"--="~"..'--:-:!s~:-=o~~~---'-~ilO~O-'---"---"-""""'~il"O
'
ION MASS (m/z)
Figure 7': Analysis of residue using LIMA showed the "sidewall
polymer" in Figure 8a contained oxide of titanium and some
organic residual.
Etch Residues
The following series of SEMs show the removal of etch residues after specific processes. In each
evaluation the HDA process successfully removed the particular etch residue.
Figure 8a - Etching residue after plasma Figure 8b - ThE' metal oxide residue is
ashing. removed by the Hydroxylamine buffered
solution at 65°C for 10 minutes.
-------------------------1. Page 9 tr

10. Figure 9a - Residue remained on the via Figure 9b - The via residue is completely
after cleaning in DMAC/DEA mixtures, removed after processing in the
such as in USP 4770713 at 90'C for 30 Hydroxylamine buffered solution at 65'C
minutes. for 30 minutes.
Figure lOa - Residue remained at the Figure lOb - The via residue is completely
bottom of a 0.61l via after the wafer was removed after processing the wafer in the
cleaned in solvent/amine, USP 4617251, Hydroxylamine buffered solution at 65'C
stripper at 95 'C for 30 minutes. for 30 minutes.
I =----=
Figun! 11a - Etching residue left behind Figure llb - The Hydroxylamine buffered
on the metal line after ashing. solution removed aU the etching residue
which was present after plasma metal etch.
--1 I
D
Page 1 ©1996 EKe Tec nology, Inc.

11. Figure 12a - The via residue is decorated Figure 12b - The via residue is completely
on the wafer surface after the wafer was removed after being processed through
cleaned in a mixture of DMSO/MEA, Hydroxylamine buffered solution at 6S'C
JK-64-426S3 at 9S'C for 30 minutes and for 30 minutes.
followed with an isotropic etch.
Figure 13a - Polysilicon etching residue Figure 13b - No residue or gate oxide
after plasma ashing. undercut after processing with the
Hydroxylamine buffered solution.
i
. Figure 14a - Polysilicon etching residue Figure 14b - The polysilicon residue is
on the polysilicon line (with structure of completely removed after the wafer was
Nitride/TiSi/Polysilicon) after 02 plasma cleaned in a Hydroxylamine buffered
ashing. solution at 65'C for 30 minutes.
--------------------------1, Page 11 tJ--

12. Contact Angle After HMOS Layer RemovaP4
Hexamethyl-disilazane (HMOS) is the standard photoresist adhesion promoter. It is applied to the
wafer before the photoresist layer is spun on the wafer. The HMOS is applied by spinning or vapor
priming. There is an interaction between the HMOS and the resist that results in mobile ions in the
resist migrating through the HMOS to the wafer surface. Clean surfaces require the complete
removal of the HMOS layer.
An industry standard test for clean surfaces is the contact angle measurement of water droplets on
a wafer surface. Low contact angles indicate a cleaner surface. Wafers were processed through
three cleaning solutions. They were compared through contact angle measurement to a bare silicon
surface and one that had been HMOS primed24 . The results showed that the buffered HDA solution
(EKe 265) produced a surface quantitatively as clean as the starting bare silicon wafer. In contrast,
the standard JPositive resist stripper, an NMP/ amine solution, exhibited a two fold increase in
contact angle over the starting wafer, despite the fact that its surface tension is much lower than
the I-IDA solution 24.
60
C
o 50 I
n
t
a 40 +--------11111,111------.,----.. .[" j,,,,,,--
t 30
A 20
n
g
10
I
e
0
Si HMOS Fuming Nitric EKC 265 NMP/Amine
Primed
Figure 14: Contact Angle Study ofHMDS Primed Wafers
Cleaned by Various Methods
Metal. Via-Hole Contact Resistance
The results of a successful via-hole etch residue removal process can ultimately be demonstrated
by low via-hole contact resistance. The via contact resistance of a 0.6 micron via-hole chain (aspect
ratio 2:1) was measured after a standard cleaning cycle in the buffered hydroxylamine solution.
The average contact resistance was measured to be 0.90 K-Chms. This result is within the
theoretical calculation of electrical resistance for contacts23, indicating that the buffered HDA
solution significantly removed any films or residues that could have increased the contact
resistance above unacceptable levels.
~ Page 12 h~ ,"",,- _
---,.. U©1996 EKe Technology, Inc.

13. C/V Shift, VT Shift And TOF-SIMS Analysis
Table 5 lists the results of cleaning wafers in an NMP solvent solution compared with cleaning in
buffered HDA solution.
TABLE 6' COMPARISON OF NMP AND EKC·265 PROCESSES
Chemical Sodium lTemp jfime Avg. Vfb ~TLo-VTHi Damaged
Purity I
I/Min.) (+ to -bias) BPSG
N-Methyl-Pyrrolidone f ppb ~O°C ,20 2.04 1.28-2.92 3.5E+ 13
(NMP) . .ons/Cm 2
!EKC 265 100 ppb k>5°C ~O 0.21 3.08-3.13 1.2E+13
,
I ons/Cm 2
The results indicate that the wafers cleaned in HDA solution measured an order of magnitude
lower Vfb (flat band voltage shift), 0.21 to 2.04 volts. The range (difference between VT 10 and VT
Hi) of VT shift was 0.05 compared to 0.64 for the NMP. The boron-phosphorus silicate glass layer
(BPSG) damage evaluation showed a lower level of damage for the HDA cleaned wafers. Overall,
the incidence of mobile ionic contamination was lower with the HDA, despite the fact that the HDA
solution initially contained a higher level ofsodium. The conclusion is that the HDA buffer solution
is better at holding the mobile ions in the liquid phase as compared to the solvent/amine stripper
solution. .
'fABLE 7: INCREASE IN MOBILE ION CONCENTRAnON FROM PLASMA ASHING
Substrate Ions/Cm 2 ,
Damaged Oxide 4.4E+1O
Damaged BPSG 1.2E+ 13 ,
~PSG 5.4E+ll
Post Plasma Mobile Ion Contamination
The search for the cause of mobile ionic contamination in processes tends to focus on the
contamination level of the process chemicals. In a separate study wafers were measured for ion
density immediately after a plasma ashing step (no cleaning step). TOF-SIMS analyses were
performed to detennine the ion densities. The results presented in Table 7 and Figure IS
demonstrate that there are high concentration levels of ions found on the wafers that are directly
associated with a plasma process.
1.60E+ 13
1.40E+13
1.20E+ 13
E I.OOE+13
~ 8.00E+12
<::
oS 6.00E+12
4.00E+12
2.00E+12
O.OOE+OO.jool,----I:::Z.------r----L.~-----r-- ..... - - ,
--
Damaged Damaged BPSG
Oxide BPSG
Figure 15: TOF-SIMS Analysis of Mobile Ions
----------------------------ll, Page 13 tt--

14. CONCLUSION
A chemical stripping and cleaning solution based on hydroxylamine (HDA) chemistry has been
proven to effectively remove positive photoresists, polyimides, "sidewall polymers", or etching
residu(~s from oxide, metal, and polysilicon plasma etches. The solution contains hydroxylamine,
a metal ion free reducing agent, mixed in an alkaline buffer. The cleaning mechanism is postulated
to be a two step reducing and complexing reaction process.
SEM anaylsis reveals that a HDA based solution removes a number of plasma etch residues.
Compared to solvent type strippers, HDA shows a lower level of mobile ionic contamination as
measured by surface contact angle, C/V Shift, VT Shift, and TOP-SIMS analyses. The resistance of
metal via-hole chain structures was measured after cleaning with HDA. The results fell within the
theoretical range for the materials involved, indicating the remova[ of residues and other
contamination likely to cause electrical resistance in the contacts.
In addition, the standard HDA cleaning temperature is typically 30· C lower than solvent and acid
bath process temperatures. Lower bath temperatures generally result in greater process control
and lower operating costs.
One of the approaches to metallic ion free wafer processing has beE'n the development of ultra high
purity chemicals. Results using HDA chemistry indicate that the propensity to hold mobile
metallic ions in these stripper solutions can produce similar and/ or superior cleaning without the
need to start with ultra high purity stripper solutions.
---i, Page 14 ~ ©'1996 EKe Technology, Inc.

15. Mr. Wai Mun Lee is the Vice President of Research & Developmemt for EKC Technology, Inc. and
is responsible for thE! development and characterization of new photoresist strippers and wafer
cleaning products. He holds several patents on novel chemical compositions for removing posi-
tive and negative resists from wafer surfaces. Mr. Lee earned a BS. in Chemical Engineering at
the University of California, Berkeley, California.
He joined EKC Technology, Inc. in 1981. Previous to that he held research chemist positions with
the Specialty Coating Department of Hercules, Inc., Wilmington, DE and Pigments and Additives
Division of Ciba Geigy Corp., Ardsley, NY.
ACKNOWLEDGEMENTS
The author would like to thank the following individuals and their companies for their
encouragement, advice and wafer samples. I
• Dr. M. Haslam and Dr. Spinner, Advanced Technology Development, S.G.S. Thompson,
Carrolton, TX.
• Mr. J. Kava, Mr. J. Hamilton and Ms. W. M. Chu, LSI Logic Corp., Milpitas, CA.
• Dr. P. Koch, Mr. L. Wilson and Mr. M. Nghyen, Submicron Development Center,
Advanced Micro Devices, Sunnyvale, CA.
The author expresses special thanks to Mr. Russ Kuroda nd Mr. Chiu Tse for their indispensable
help taking the SEM pictures. He also thanks Peter Van Zant for assistance with editing the
manuscript and graphic design.
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1. K Yoneda, ''Wafer Clean Technology for Sub Mitron Processing," Technical Proceeding
Semicon Japan, p. 162, 1991. I
2. Vic CornelIo, "Semiconductor International," p. ~6, March 1991.
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4. David J. Elliot, "Integrated Circuit Fabrication T~hnology", 2nd Edition, McGraw Hill
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6. L. F. Thompson, C. G. Wilson and M. J. Bowden, "Introduction to Microlithography" ACS
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9. W. Kern, "The Evolution of Silicon Wafer Cleaning Technology," J. Electrochem. Soc., Vol.
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10. S. D. Hossain, C. G. Pantano and J. Ruzyllo, "Removal olf Surface Organic Contaminants
during Thennal Oxidation of Silicon," J. Electrocltem. Soc., Vol. 197, No. 10, p. 9287, 1990.
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12. R. L. Maddox and H. L. Parker, "Application of Reactive Plasma Practical Microelectronics
Processing SystE~ms," Solid State Technology, April. 1978.
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16. 13. S. Fujimura and H. Yano, "Heavy Metal Contamination from Resists During Plasma
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