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I ~ _ _ l_~ _
A PROVEN SUB-MICRON PHOTORESIST STRIPPER SOLUTION
FOR POST METAL AND VIA HOLE PROCESSES
Vice President, Research & Development
EKC Technology, Inc.
Hayward, California, U.S.A.
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
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.
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
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-'- - - - - - - - - - - - - - - - - - - - - - - - - - -
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.
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
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
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
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
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 
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 
HF + H:P -> H 3 0+ + F- 
HN03 + H 20 -> H+ + N03- ,
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
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
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
Figure 4: Nucleophilic Attack of Carbonyl Group
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 >=< c'V
Figure 6: Chemical Reaction of Metal Halide
------------------------1, Page 7 tt-
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.
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.
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
DMAClDiethanolamine 100 30 US 4770713
Hydroxylamine Buffered 65 30 Patent Pending in U. S, I
So'lution Japan, Europe, Taiwan and
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.
-----'' c==~ @996EKeTechnology,Jnc.
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
>--"~-""'-=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
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
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.
. 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--
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
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
'fABLE 7: INCREASE IN MOBILE ION CONCENTRAnON FROM PLASMA ASHING
Substrate Ions/Cm 2 ,
Damaged Oxide 4.4E+1O
Damaged BPSG 1.2E+ 13 ,
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.
O.OOE+OO.jool,----I:::Z.------r----L.~-----r-- ..... - - ,
Damaged Damaged BPSG
Figure 15: TOF-SIMS Analysis of Mobile Ions
----------------------------ll, Page 13 tt--
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.
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,
• 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.
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.
3. N. Yoshida, "Multilevel Interconnect Technology for ASIC," Technical Proceeding Semicon
Japan, p. 127, 1991.
4. David J. Elliot, "Integrated Circuit Fabrication T~hnology", 2nd Edition, McGraw Hill
5. S. K Ghandi, "VLSI Fabrication Principles - SiliCOr· and Gallium Arsenide", John Wiley &
6. L. F. Thompson, C. G. Wilson and M. J. Bowden, "Introduction to Microlithography" ACS
Symposium Series 219, American Chemical SOd~ty, 1989.
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Electrochem. Soc., Vol. 127, No.2, p. 986, 1980.
8. Pintchovski, J. B. Price, P. ]. Tobin, J. Pavey and IK Kobold, "Thermal Characteristics of
H 2S04 -H 20 2 Silicon Wafer Cleaning Solution," J. Electrochem. Soc., Vol. 126, No.8, p. 1428,
9. W. Kern, "The Evolution of Silicon Wafer Cleaning Technology," J. Electrochem. Soc., Vol.
197, No.6, p. 1887, 1990.
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during Thennal Oxidation of Silicon," J. Electrocltem. Soc., Vol. 197, No. 10, p. 9287, 1990.
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p. 54, August, 1988.
12. R. L. Maddox and H. L. Parker, "Application of Reactive Plasma Practical Microelectronics
Processing SystE~ms," Solid State Technology, April. 1978.
--------------------------1, Page 15 n--