I ~__l_~ _

A PROVEN SUB-MICRON PHOTORESIST STRIPPER SOLUTIONFOR POST METAL AND VIA HOLE PROCESSES

byWaiMunLee

Vice President, Research & DevelopmentEKC Technology, Inc.

Hayward, California, U.S.A.

ABSTRACT

A wet chemistry process based on hydroxylamine (HDATM)* chemistry has been found to removepositive photoresist, sidewall polymers and other plasma process residues. The development of new chipmetallization materials, multimetal, and multilevel interconnect schemes for sub-micron processes haveplaced new demands on wafer cleaning technology'. The high density connections of ULSI devicesrequire 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 holesand other etching residues. SEM and surface contact angle evaluations indicate a HDA based solutionleaves 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. Patents20•

Other U.S. and Foreign Patents Pending.

>

~EKCTechnolo~ Inc.A ChernFirsl Company

Additional prints available from:

EKe Technology, Inc.2520 Barrington Court, Hayward, CA 94545ph: ~'1 510-784-9105 facs: +1 510-784-9181

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 EdinburghUniversity, Scotland (Ref. #24). This paper is published in the pirOceedings of the Symposium onInterconne.cl:s, Contact Metallization and Multilevel Metallization, Volume 93-25 of TheElectrochemical Society, Inc.

REVIEW OF RESIST STRIPPING CHElVtISTRY

A conventional positive photoresist consists of three components: a Novalak resin (a condensationreacted product of cresol and formaldehyde - Figure 1), a photo active compound (a substitutedNapthoquinone Diazide - Figure 2) and a solvent (usually a mixture of glycol ethers)4,5,6.

xFigure 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 H202IH2S04

(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-'---------------------------

Oxidation The use of an oxidizer, such as hydrogen peroxide or ammonium persulfate inconcentrated sulfuric acid, for photoresist removal and wafer cleaning processes has been widelyreported?,8.9. A wafer cleaning process using hydrogen peroxide and ammonium hydroxide iscommonly referred to as the RCA clean 10. Fuming nitric acid has also been used extensively asacleaning and stripping agent, especially in Europe and Japan. Each of these solutions act on thephotoresist through an oxidation mechanism.

Oxygen plasma resist removal processes (also called dry stripping or ashing) carne into wide usedUring the 1980's to remove the hardened resists created during p.asma etching processes. Plasmasystems have a variety of designs as discussed by Skidmore 11. They are downstream, parallelplate12

, 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}

DissolutionWhile wet strippers based on oxidation mechanisms are the most frequently used for chemicalstripping, they are limited to wafer steps where no metals are present. The acid based strippersattack the metallization materials. At the steps with metal on the wafer the preferred wet stripperfor positive resists is a solvent/amine type. A solvent/amine stripper removes resist by a processof penetration, swelling, and dissolutionS. The solvent molecules solvate the polymer moleculeand overcome the attractive forces that hold the polymer together. This mechanism is optimizedin a number of proprietary stripper solutions that mix various aprotic solvents (N-rnethyl-2­pyrrolidone [NMP], dimethyl sulfoxide [DMSO], sulfolane, dimethylformamide [DMF], ordimethylacetamide [DMAC]) with different organic amines. Table #2 summarizes a number ofcommercially available, patented positive photoresist strippers.

TABLE 2' PHOTORESIST STRIPPER PATENT SURVEYSolvent Amine Patent No. Assigned to

1 NMP Aminoethyl JK 60-131535 Allied ChemicalPiperadine

2 'NMP I _ JK 61-6827 Shipley3 NMP/Sulfolane Isopropyl amine JK 63-186243 J. T. Baker

DMF/Sulfolane EP 1026284 NMP Amine US 4617251 Olin Hunt5 NMP Hydroxyl ethyl WO 8705314 Mac Dermid

Morpholine6 DMSO I Amino alcohol JK 64-81950 Asahi Chern.7 LPMSOIBLO Amino alcohol JK 64-42653 Tokyo Ohka8 NMPIDMF Diethylenetriamine US 4824763 EKC Technology9 DMAC Diethanolamine US 4770713 ACT10 DMAC··others Amine JK 63-231343 Hitachi11 DMF Amino alcohol JK 64-81949 Asahi Kasei12 NMP or DMF Ammonium Salt JK 61-292641 Hoechst Japan

--------------------------1. Page 3 tr

Re<l1Jction Hydrogen plasma processes can be used for photoresist removal. In the plasmachamber the resist molecule is reduced to methane, sulfite, nitrogen, and water (Equation 2). Thegeneralized chemical reaction can be described as follows.

[2]

It is reported that the addition of carbon tetrafluoride18 in a hydrogen plasma can removefluorocarbon residues.

KEY ISSUES RELATED TO RESIST STRIPPING PROCESSES

Intl:9Jiuctio.nProcess simplicity and control favor the use of one-step processes. However, the wide range ofdevice requirements and materials has resulted in a variety of application and combinationcleaning processes. Most resist stripping processes for advanced circuits are a combination ofwetldry or dry Iwet techniques.

Mobile Ionic ContaminationGenerally oxygen plasma processes are favored for their ability to strip hardened resist films andthe 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 inthe general wafer fabrication environment13• This problem is particularly acute for ULSI leveldevices with their higher component density and thinner junctions and layers12

• Wet stripping ismore effective in the removal of heavy metallic contamination. Also, the high energies associatedwith plasma processing can cause radiation damage in sensitiVE! circuits 1.

Another issue is the presence of surface metallic contaminants, that are suspected of causing lossof selectivity during tungsten deposition into high aspect ratio "plug" holes13

• These contaminantsbecome surface nucleation sites during tungsten deposition that can result in tungsten particlesdepositing on the silicon oxide surface. They can cause electrical short circuits under subsequentlydeposited metal interconnecting layers. Again, the industry has determined that a combinationof oxygen plasma and wet stripping is effective for removal of this type ofmetallic contamination14•

Metal CorrQ.sionParekh and Price studied corrosion associated with Al-Si-Cu and TiW metal systems resultingfrom AIC~ residues1s. They compared! various post etch treatments for their ability to reducechlorine ion (CO levels which in tum inhibits metal line corrosion. They concluded that post etchtreatments in a 01 water rinse or in a N2 bake have a negligible impact on corrosion compared toresist removal by either an oxygen plasma ashing at 180°C followed by a dip in fuming nitric acidor an organic stripper (EKC 830 from EKC Technology, Inc.). The results in Table 3 show that theEKe 830 wet stripper produced the cleanest surface.

TABLE 3: EFFECT OF VARIOUS POST TREATMENTS ON THE RESIDUAL CIAND F LEVELS FOR HPR204 RESIST 15

(~g/Cm2) No post H2O N2 Bake, 02 Ash + Rinse + N2 Bake EKetreatment Rinse 200 0 e/1 Hr. Fuming HN01 I O.,Ash +0, Ash J 830

Ct 1.08,

0.01 1<0.01 ,0.0110.99 0.50 0.01

F 0.47 10.28 0.20 0.18 1~.08 10.13 0.01-"--.J Page4 L~~""",",- _~ , U©1996 EKe Technology, Inc.

Etch Residue ProblemDuring anisotropic plasma etching processes for via contacts, metal patterns, and passivationopenings, "sidewall residues" are frequently deposited on the f(~sist sidewalls. After the oxygenplasma ashing process these deposits become metal oxides. Incomplete removal of these residuesinterfere with the pattern definition and/or complete filling of via-holes. Thus, wet strippingoptions must be available.

Etching Residue Removal MechanismSeveral different chemistries have been identified for removing aluminum etching residues.Alkaline based positive resist developers, such as NaOH, tetram(~thylarnrnoniumhydroxide andchoHne, are known to attack aluminum16• The hydroxyl ions attack the aluminum to form analuminum oxide hydrated anion (Equation 3).

Positive resist developers are limited to removing aluminum residues, but they do not removeresidues associated with multimetal systems such as Al/Si/Cu. They also are ineffective onresidues from polysiHcon plasma etch processes. Stringent process control must be exercised toprevent 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 asidentified in Table 2. The attack mechanism is a two step reaction starting with the formation ofhydroxyl ions, when the amine component in the stripper is hydrolyzed with water17 (Equation 4).

RNH2+ 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 andhydrofluoric 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 thefluoride ion non-selectively attacks silicon. The acetic acid reacts with aluminum to form a moresoluble aluminum acetate. The reactions are shown in Equations #5 to #8.

NH4F + ~O -> NH40H + HFHF + H:P -> H30+ + F-HN0

3+ H

20 -> H+ + N0

3­

CH3COOH + H20 -> CH3COo- + H+

[5][6],[7]f8]

These solutions require extreme process control to prevent excessive attack of critical metal andoxide layers. In some device structures these solutions are not usable due to their non-selectiveattack mechanisms.

OthE!r Etching ResiduesSub-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 removedby the conventional solvent/amine stripperchemistry. Etching type chemistries, ifused to removeresidues associated with advanced metal structures, require tight control and expensive automatedequipment.

~-------------------------liPage 5 tt-

THEORY OF POSITIVE RESIST AND PLASMA ETCH RESIDUE REMOVAL WITHHYDROXYLAMINE

x

xFigure 3a: UV Stabilized Cross Linking Resist

+

oII

~~R

A combination of wet cleaning with HF, followed by a NF/hydrogen plasma step has beendescribed 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 incapableof removing the residues producedduring theanisotropicetching 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 inthe removal of polysilicon etching residues. The current process used to remove such etch residuesis based on multiple steps. First the etch residue is removed by immersion of the etched wafer ina dilute HF (200:1) solution. Thesecond step is the stripping of the photoresist layer with an oxygenplasma ashing, and followed by a Piranha solution. A major concern with this process is the HFmixture attack of the gate oxides and the resultant shift in the gate oxide threshold voltage (VT

Shift).

A buffered hydroxylamine (HDA) solution was investigated as a cleaning and stripping solutionfor its ability to strip positive photoresist, remove a number of plasma etch residues, and meetstringent ULSI wafer cleanliness standards2o

• HDA is a strong nucleophile and a redUcing agentat high pH values. This allows for resist stripping and wafer cleaning through a combination ofreaction mechanisms involving reduction, chelation, and nucleophilic attack.

Positive Resist RemovalTwo techniques used to establish image stability in positive resists are post exposure baking (hardbake) and/or UV stabilization. Both of these techniques are intended to minimize resist flowduring plasma etching. The processes cause cross linking. The photo-active compound reacts withthe Novalak resin4

• Figure 3a and 3b illustrate some of the possible reactions.

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o xim.

Figure 3b: Coupled Diazide

Hydroxylamine is an extremely strong nucleophile that can attack the carbonyl groups. The resultis 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

\OH

Figure 4: Nucleophilic Attack of Carbonyl Group

Polyimide RemovalThe same unique nucleophilic attack mechanism of HDA takes place with cured polyimidepolymer structures, as illustrated in Figure 5. Laboratory tests have demonstrated the removal ofcured polyimide films with a HDA solution.

Figure 5: Nucloephilic Attack of Polyimide

Metal Halides Residue RemovalDuring plasma etching processes, such as metal etching, silicon oxide etching, and polysiliconetching, hydroxyl groups in the photoresist react with metal halide gasses generated in the etchingchamber to form undesirable residues of organometallic compounds. The organometalliccompounds are shown in Figure 6. The compounds cause cross linking of the Novalak resin at themetal centers, greatly reducing its solubility. On any subsequent oxygen plasma treatment, metaloxides, 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-

Other stable metal halides, such as AIF3' WF5' WF6' WOF3' or TiF3also remain on the wafer surface.These salts and oxides are insoluble in water, dilute acids, or bases22, but they are removed in HDAsolutions. Reduction of these metallic species and subsequent formations of chelating complexesplaya role in the removal of these residues. Based on the oxidation/reduction potentials, themetallic species that can be reduced by hydroxylamine are listed in Table 4.

T.ABLE 4: METALLIC REDUCTION BYHYDROXYLAMINE22

Ag(I) _ Ag(O)Au(I1)_ Au(I)Co(III) _ Co(I1)Cr(VI)_ Cr(IV)Cu(U) Cu(I)Fe(I1I) _ Fe(I1)Pd(I1) Pd(1)Ti(lll) _Ti(l)

W(V) _ W(I1I)

The combination of HDA and an organic amineform a strong reducing and complexing (ligating)solution. The insoluble metal oxide could bereduced to a lower oxidation state andsubsequently dtelated with the ligand to form amore soluble metal complex which couldultimately end up in the solution.

Hydroxylamine and organic amines can formcoordination complexes through their nitrogen.atoms (e.g. Zn(NH20H)2CI2)' The proposedmechan~sm of reduction, chelation, andsolublization results in removal of a number ofplasma generated etching residues withoutattacking the pure metal surfaces.

EXPERIMENTAL CONDITION~

Four different stripping and cleaning solutions were chosen for t:he evaluation. They are listed inTable #5. The first three were commercially available producl:s in general use. The fourth, abuffered hydroxylamine solution developed by EKC Technology Inc. was a mixture ofhydroxylamine (e.g. NH20H) and 2 (2 aminoethoxy) ethanol (e.g. ~NCH2CH2OCH2CH20H).

TABLE 5: STRlPPING SOLUTIONS

-Stripping Composition Temp.oC Time (Min.) Patent NumberNMP.IAlkanofamine 95 30 US 4617251

,

DMSOlMonoethanolamine 95 30 JK-64-42653- IDMAClDiethanolamine 100 30 US 4770713Hydroxylamine Buffered 65 30 Patent Pending in U. S, I

So'lution Japan, Europe, Taiwan andKorea

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 processtempE!ratures and for 30 minutes (Table 5). The cleaning took plaCie in eitherquartz or stainless steelbaths inside a standard wet bench. The wafer boats received intermittent manual agitation duringthe snip cycle. After stripping, the wafers were transferred to a deionized cascade rinser for iniualrinsing and finished with a cycle in a commercial spin/rinse dryer.

After cleaning the experimental wafers were compared for removal of surface contaminants. Theeffect of the cleaning process on specific electrical parameters was also investigated.

-----l PagtTl1~---------------_._-----------------'''cl==~ @996EKeTechnology,Jnc.

EXPERIMENTAL RESULTS

IntroductionThe '"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 ofwafE'rs with TiN anti-reflective coating contained TiD as shown in Figure 7.

3 ~sn 92 11: 23: 37

Na

K

Ii IIO

,.'

>--"~-""'-=fe!o""""""-""'-""'''''''''-s'-!:o~o .....'---"--="~"..'--:-:!s~:-=o~~~---'-~ilO~O-'---"---"-""""'~il"O

ION MASS (m/z)

Figure 7': Analysis of residue using LIMA showed the "sidewallpolymer" in Figure 8a contained oxide of titanium and someorganic residual.

Etch ResiduesThe following series of SEMs show the removal of etch residues after specific processes. In eachevaluation the HDA process successfully removed the particular etch residue.

Figure 8a - Etching residue after plasmaashing.

Figure 8b - ThE' metal oxide residue isremoved by the Hydroxylamine bufferedsolution at 65°C for 10 minutes.

-------------------------1. Page 9 tr

Figure lOa - Residue remained at thebottom of a 0.61l via after the wafer wascleaned in solvent/amine, USP 4617251,stripper at 95 'C for 30 minutes.

--1 I

Figure 9a - Residue remained on the viaafter cleaning in DMAC/DEA mixtures,such as in USP 4770713 at 90'C for 30minutes.

Figun! 11a - Etching residue left behindon the metal line after ashing.

Page 1D ©1996 EKe Tec nology, Inc.

Figure 9b - The via residue is completelyremoved after processing in theHydroxylamine buffered solution at 65'Cfor 30 minutes.

Figure lOb - The via residue is completelyremoved after processing the wafer in theHydroxylamine buffered solution at 65'Cfor 30 minutes.I =----=

Figure llb - The Hydroxylamine bufferedsolution removed aU the etching residuewhich was present after plasma metal etch.

Figure 13b - No residue or gate oxideundercut after processing with theHydroxylamine buffered solution.

Figure 12b - The via residue is completelyremoved after being processed throughHydroxylamine buffered solution at 6S'Cfor 30 minutes.

Figure 14b - The polysilicon residue iscompletely removed after the wafer wascleaned in a Hydroxylamine bufferedsolution at 65'C for 30 minutes.

Figure 12a - The via residue is decoratedon the wafer surface after the wafer wascleaned in a mixture of DMSO/MEA,JK-64-426S3 at 9S'C for 30 minutes andfollowed with an isotropic etch.

Figure 13a - Polysilicon etching residueafter plasma ashing.

i. Figure 14a - Polysilicon etching residueon the polysilicon line (with structure ofNitride/TiSi/Polysilicon) after 02 plasmaashing.

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Contact Angle After HMOS Layer RemovaP4Hexamethyl-disilazane (HMOS) is the standard photoresist adhesion promoter. It is applied to thewafer before the photoresist layer is spun on the wafer. The HMOS is applied by spinning or vaporpriming. There is an interaction between the HMOS and the resist that results in mobile ions in theresist migrating through the HMOS to the wafer surface. Clean surfaces require the completeremoval of the HMOS layer.

An industry standard test for clean surfaces is the contact angle measurement of water droplets ona wafer surface. Low contact angles indicate a cleaner surface. Wafers were processed throughthree cleaning solutions. They were compared through contact angle measurement to a bare siliconsurface 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 incontact angle over the starting wafer, despite the fact that its surface tension is much lower thanthe I-IDA solution24.

+--------11111,111------.,----.....[" j,,,,,,--

60C

o 50 Inta 40

t 30

A 20n

g10

Ie

0

Si HMOS Fuming Nitric EKC 265 NMP/AminePrimed

Figure 14: Contact Angle Study ofHMDS Primed WafersCleaned by Various Methods

Metal. Via-Hole Contact ResistanceThe results of a successful via-hole etch residue removal process can ultimately be demonstratedby low via-hole contact resistance. The via contact resistance of a 0.6 micron via-hole chain (aspectratio 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 thetheoretical calculation of electrical resistance for contacts23, indicating that the buffered HDAsolution significantly removed any films or residues that could have increased the contactresistance above unacceptable levels.

~ Page 12 h~ ,"",,- _---,.. U©1996 EKe Technology, Inc.

C/V Shift, VT Shift And TOF-SIMS AnalysisTable 5 lists the results of cleaning wafers in an NMP solvent solution compared with cleaning inbuffered HDA solution.

TABLE 6' COMPARISON OF NMP AND EKC·265 PROCESSESChemical Sodium lTemp jfime Avg. Vfb ~TLo-VTHi Damaged

PurityII/Min.) (+ to -bias) BPSG

N-Methyl-Pyrrolidone f ppb ~O°C ,20 2.04 1.28-2.92 3.5E+ 13(NMP) . .ons/Cm2

!EKC 265 100 ppb k>5°C ~O 0.21 3.08-3.13 1.2E+13I

,ons/Cm2

The results indicate that the wafers cleaned in HDA solution measured an order of magnitudelower 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 ofmobile ionic contamination was lower with the HDA, despite the fact that the HDAsolution initially contained a higher level ofsodium. The conclusion is that the HDA buffer solutionis better at holding the mobile ions in the liquid phase as compared to the solvent/amine strippersolution. .

'fABLE 7: INCREASE IN MOBILE ION CONCENTRAnON FROM PLASMA ASHING

Substrate Ions/Cm2

Damaged Oxide 4.4E+1O,

Damaged BPSG 1.2E+ 13 ,

~PSG 5.4E+ll

Post Plasma Mobile Ion ContaminationThe search for the cause of mobile ionic contamination in processes tends to focus on thecontamination level of the process chemicals. In a separate study wafers were measured for iondensity immediately after a plasma ashing step (no cleaning step). TOF-SIMS analyses wereperformed to detennine the ion densities. The results presented in Table 7 and Figure ISdemonstrate that there are high concentration levels of ions found on the wafers that are directlyassociated 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+12O.OOE+OO.jool,----I:::Z.------r----L.~-----r-- .....----,

Damaged Damaged BPSGOxide BPSG

Figure 15: TOF-SIMS Analysis ofMobile Ions

----------------------------ll, Page 13 tt--

CONCLUSION

A chemical stripping and cleaning solution based on hydroxylamine (HDA) chemistry has beenproven to effectively remove positive photoresists, polyimides, "sidewall polymers", or etchingresidu(~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 postulatedto 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 asmeasured by surface contact angle, C/V Shift, VT Shift, and TOP-SIMS analyses. The resistance ofmetal via-hole chain structures was measured after cleaning with HDA. The results fell within thetheoretical range for the materials involved, indicating the remova[ of residues and othercontamination likely to cause electrical resistance in the contacts.

In addition, the standard HDA cleaning temperature is typically 30· C lower than solventand acidbath process temperatures. Lower bath temperatures generally result in greater process controland lower operating costs.

One of the approaches to metallic ion free wafer processing has beE'n the development of ultra highpurity chemicals. Results using HDA chemistry indicate that the propensity to hold mobilemetallic ions in these stripper solutions can produce similar and/or superior cleaning without theneed to start with ultra high purity stripper solutions.

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Mr. Wai Mun Lee is the Vice President of Research & Developmemt for EKC Technology, Inc. andis responsible for thE! development and characterization of new photoresist strippers and wafercleaning products. He holds several patents on novel chemical compositions for removing posi­tive and negative resists from wafer surfaces. Mr. Lee earned a BS. in Chemical Engineering atthe University of California, Berkeley, California.

He joined EKC Technology, Inc. in 1981. Previous to that he held research chemist positions withthe Specialty Coating Department of Hercules, Inc., Wilmington, DE and Pigments and AdditivesDivision of Ciba Geigy Corp., Ardsley, NY.

ACKNOWLEDGEMENTS

The author would like to thank the following individuals and their companies for theirencouragement, 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 indispensablehelp taking the SEM pictures. He also thanks Peter Van Zant for assistance with editing themanuscript and graphic design.

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2.3.

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13. S. Fujimura and H. Yano, "Heavy Metal Contamination from Resists During PlasmaStripping," J. Electrochem. Soc., Vol. 195, No.5, p. 1195, il988.

14. A. SE~llers and R. J. Mattox, "Can the Selectivity Loss Assodated with Tungsten Depositionbe Minimized? "Microcontamination," p. 29, January, 19~1.

15. Parekh and J. Price, "Cl Level Effects on Corrosion for V2lriOUS Metallization Systems," J.Electrochem. SOC., Vol. 197, No.7, p. 2199, 1990.

16. K. Graziano, C. Allen and H. Y. Liu, "Surface Characterization of Aluminum SubstrateExposed to Photoresist Developers," Journal of SPIE, Advances in Resist Technology andProcessing,1989.

17. P. L. Pai, C. H. Ting, R. Kuroda and W. M. Lee, "Metal Corrosion in Wet Stripping Process,"K. T. I. Interface, Technical Proceeding, p. 197, 1989.

18. P. Simko and G. S. Oehrlein, "Removal fo Fluorocarbon Residues on CF/~Reactive-IonEtched Silicon Surfaces Using a Hydrogen Plasma, 'J. Electrochem. Soc., Vol. 198, No. I, p.277, 1991.

19. T. Hara, T. Ohba and Y. Furumura, "Tungsten Contacts and Interconnection Techniques,"Technical Proceeding, Semicon Japan, p. 119, 1991.

20. U.S. Patents 5,279,771, 5,334,332, 5,381,807, and 5,482,566, EKC Technology, [nco Other U.S.and foreign patents pending.

21. Ch. C.ardinaud, M. C. Peignon, and G. Turban, "SurfaceModification ofPositivePhotoresistMask During Reactive Ion Etching ofSiand WinSF6 Plasma," J. Electrochem. Soc., Vol. 198,No.1, p. 284, 1991.

22. "Handbook of Chemistry and Physics," 49th Ed., CRC Press, Boca Raton, FL. 1989.23. R. Mukai, "PlanarizationTechnologyby LaserInducedMelting forSubMicron Interconnect,"

Technical Proceeding, Semicon Japan, p. 909, 1991.24. J. Ne'wby and N. Porfiris, "Private Communication," EKC Technology sponsored study at

Edinburgh University, Scotland, United Kingdom.

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