Hydroxylamine Cleaning Chemistries


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Hydroxylamine Cleaning Chemistries

  1. 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. 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. 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. 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 -" --.J Page4 L~~""",",- _ ~, U©1996 EKe Technology, Inc.
  5. 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. ~-------------------------li 5 Page tt-
  6. 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 --.J Page 6 h~~~~~~ . _ ~i U©1996 EKe Technology, Inc.
  7. 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. 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. -----l PagtTl1~---------------_._------------ -----'' c==~ @996EKeTechnology,Jnc. l
  9. 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. 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. 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. 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. 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. 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. 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. REFERENCES 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 Publishing Co. 5. S. K Ghandi, "VLSI Fabrication Principles - SiliCOr· and Gallium Arsenide", John Wiley & Sons. 6. L. F. Thompson, C. G. Wilson and M. J. Bowden, "Introduction to Microlithography" ACS Symposium Series 219, American Chemical SOd~ty, 1989. 7. L. H. Kaplan and B. K. Bergin, "Residue from IWet Processing of Positive Resists," J. 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, 1979. 9. W. Kern, "The Evolution of Silicon Wafer Cleaning Technology," J. Electrochem. Soc., Vol. 197, No.6, p. 1887, 1990. 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. 11. K. Skidmore, "Use the Right Plasma to Strip AWJly Resist," Semiconductor International, 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--
  16. 16. 13. S. Fujimura and H. Yano, "Heavy Metal Contamination from Resists During Plasma Stripping," 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 Deposition be 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 Substrate Exposed to Photoresist Developers," Journal of SPIE, Advances in Resist Technology and Processing,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-Ion Etched 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 ofPositive Photoresist Mask 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 LaserInduced Melting forSub Micron 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. -.---J Page 16 U©1996 EKe Technology,- - - - - - - - - - - - - - - - - - - - - ---, i h - - - Inc.