Optimal Design of High-Frequency Induction Heating Apparatus for Wafer Cleaning Equipment Using Superheated Steam

energies

Article Optimal Design of High-Frequency Induction Heating Apparatus for Wafer Cleaning Equipment Using Superheated Steam

Sang Min Park 1 , Eunsu Jang 2, Joon Sung Park 1 , Jin-Hong Kim 1, Jun-Hyuk Choi 1 and Byoung Kuk Lee 2,*

1 Intelligent Mechatronics Research Center, Korea Electronics Technology Institute (KETI), Bucheon 14502, Korea; [email protected] (S.M.P.); [email protected] (J.S.P.); [email protected] (J.-H.K.); [email protected] (J.-H.C.) 2 Department of Electrical and Computer Engineering, Sungkyunkwan University (SKKU), Suwon 16419, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-31-299-4581

Received: 19 October 2020; Accepted: 22 November 2020; Published: 25 November 2020

Abstract: In this study, wafer cleaning equipment was designed and fabricated using the induction heating (IH) method and a short-time superheated steam (SHS) generation process. To prevent problems arising from the presence of particulate matter in the fluid flow region, pure grade 2 titanium (Ti) R50400 was used in the wafer cleaning equipment for heating objects via induction. The Ti load was designed and manufactured with a specific shape, along with the resonant network, to efficiently generate high-temperature steam by increasing the residence time of the fluid in the heating object. The IH performance of various shapes of heating objects made of Ti was analyzed and the results were compared. In addition, the heat capacity required to generate SHS was mathematically calculated and analyzed. The SHS heating performance was verified by conducting experiments using the designed 2.2 kW wafer cleaning equipment. The performance of the proposed pure Ti-based SHS generation system was found to be satisfactory, and SHS with a temperature higher than 200 ◦C was generated within 10 s using this system.

Keywords: induction heating (IH); superheated steam (SHS); LCL resonant network; cleaning equipment; titanium (Ti); high frequency; wafer cleaning

1. Introduction The cleaning process is the most important task in improving the yield and reliability of the manufacturing process. With advancements in the fabrication technology of liquid crystal displays (LCDs), the size of the mother glass (MG) is continually being increased, and the cleaning processes employed during fabrication are becoming increasingly vital. In particular, the process of depositing a transistor layer on an MG substrate that is employed to fabricate thin ﬁlm transistors (TFTs) requires a cleaning process for the MG substrate, as shown in Figure1. Contaminants such as residual organic materials and particulates are present on the surface of work pieces such as semiconductor wafers and the MG of LCDs [1,2]. Cleaning processes for removing such organic materials and particles are indispensable because these substances can generate defects in manufactured products; various cleaning methods have been proposed in this regard [3–5]. A wet cleaning method, known as vapor phase cleaning (VPC), is widely used because it is inexpensive and has proven technology. However, wet cleaning methods that involve the use of strong acids are regulated due to environmental pollution and safety issues [6,7]. A dry cleaning method has been developed to compensate for the disadvantages

Energies 2020, 13, 6196; doi:10.3390/en13236196 www.mdpi.com/journal/energies Energies 2020, 13, 6196 2 of 16

Energies 2020, 13, x FOR PEER REVIEW 2 of 16 of the wet cleaning methods. This dry cleaning method includes processes such as ultrasonic cleaning, excimerultrasonic ultraviolet cleaning, (EUV) excimer cleaning, ultraviolet and plasma (EUV) cleaning cleaning, [8– 10and]. However,plasma clea thening disadvantage [8–10]. However, of dry cleaningthe disadvantage equipment of dry is that cleaning it is di equipmentﬃcult to construct is that it and is difficult is large to in construct size [9,11 and]. is large in size [9,11].

Cleaning Deposition Cleaning PR Coating

PR Strip Etch Develop Exposure Figure 1.1. OverviewOverview of of the the thin thin film film transistor transistor (TFT)–liquid (TFT)–liquid crystal crystal display displa (LCD)y manufacturing(LCD) manufacturing process. process. Recently, cleaning equipment that is eco-friendly and has excellent washing capability has been developedRecently, using cleaning superheated equipment steam that (SHS) is eco-friendly at a high temperature and has excellent and pressure washing [12 ].capability The SHS has cleaning been equipmentdeveloped using uses asuperheated heater to heat steam water (SHS) contained at a high in temperature a tank to generate and pressure SHS at[12]. temperatures The SHS cleaning above 200equipment◦C; then, uses the a SHS heater is sprayedto heat water onto thecontained MG and in wafer a tank to to clean generate them. SHS The at conventionaltemperatures equipment above 200 utilized°C; then, for the generating SHS is sprayed SHS heats onto water the MG directly and using wafer a heatingto clean wire.them. In The this conventional method, the powerequipment used toutilized generate for generating SHS is significant SHS heats and water the processdirectly isusing time a consuming.heating wire. Furthermore,In this method, in the such power methods, used theto generate preparation SHS timeis significant for the cleaning and the process is ti longme consuming. and the precise Furthermore, temperature in such control methods, capability the requiredpreparation for cleaningtime for maythe cleaning be lacking. process is long and the precise temperature control capability requiredIn this for study, cleaning considering may be lacking. the need for a short-time SHS generation process, cleaning equipment that canIn this uniformly study, considering heat the heating the need object for with a short- hightime efficiency SHS generation was designed proc byess, applying cleaning the equipment induction heatingthat can(IH) uniformly method. heat Common the heating metals, object such with as cast high iron, efficiency cannot bewas used designed as heating by objectsapplying in the TFT–LCDinduction cleaningheating (IH) process method. because Common of particle metals, problems such as in thecast fluid iron, flow cannot area be [13 used,14]. as Thus, heating pure objects grade 2in titanium the TFT–LCD (Ti) R50400 cleaning was usedprocess as thebecause heating of objectparticle for problems IH cleaning in andthe fluid IH joule flow heating area [13,14]. of the Ti Thus, load. Additionally,pure grade 2 titanium the Ti load (Ti) was R50400 designed was used and manufacturedas the heating inobject a specific for IH shape, cleaning along and with IH joule the resonant heating network,of the Ti load. to effi cientlyAdditionally, generate the high-temperature Ti load was designed steam byand increasing manufactured the residence in a specific time ofshape, the fluid along in thewith heating the resonant object. network, Moreover, to efficiently several heating generate objects high-temperature were designed steam and by manufactured increasing the using residence pure Ti,time and of thethe IH fluid performances in the heating of the object. heating Moreover, objects with several respect heating to the objects shape werewere experimentallydesigned and compared.manufactured The using SHS heating pure Ti, performance and the IH performances of the proposed of cleaningthe heating system objects was with verified respect via experimentsto the shape usingwere experimentally the designed 2.2 compared. kW wafer The cleaning SHS heating equipment. performance of the proposed cleaning system was verified via experiments using the designed 2.2 kW wafer cleaning equipment. 2. Analysis of Specific Enthalpy Steam and Titanium Characterization 2. AnalysisIn this of section, Specific the Enthalpy heat capacity Steam forand generating Titanium Characterization SHS is mathematically calculated, and the characteristicsIn this section, of the non-magneticthe heat capacity Ti load for are genera analyzed.ting TheSHS power is mathematically capacity of the calculated, cleaning equipment and the wascharacteristics selected based of the on thenon-magnetic results of this Ti analysis; load are in addition,analyzed. the The cleaning power equipment capacity isof manufacturedthe cleaning byequipment new power was plasma selected (NPP) based Corporation on the results in Korea.of this analysis; in addition, the cleaning equipment is manufactured by new power plasma (NPP) Corporation in Korea. 2.1. Calculation of Steam Heat Capacity 2.1. CalculationSHS is high-temperature of Steam Heat Capacity steam that is further heated using saturated steam generated from boilingSHS water, is high-temperature as shown in Figure steam2. Because that is SHSfurthe isr a heated gas in ausing low-oxygen saturated state steam in which generated only water from (Hboiling2O) molecules water, as shown exist, oxidation in Figure of 2. the Because heated SHS object is a doesgas in not a low-oxygen occur, and the state risk in of which fire or only explosion water is reduced [15,16]. Furthermore, SHS has a strong drying ability because it possesses high thermal (H2O) molecules exist, oxidation of the heated object does not occur, and the risk of fire or explosion conductivity.is reduced [15,16]. However, Furthermore, as the specific SHS has heat a strong capacity drying of water ability is relatively because it higher possesses than high that ofthermal other substances,conductivity. the However, temperature as the change specific is heat slow, capacity as shown of inwater Figure is 2relativelya [ 17–19 ].higher Furthermore, than that itof can other be observedsubstances, from the Figuretemperature2b that change higher temperaturesis slow, as show needn in to Figure be considered 2a [17–19]. because Furthermore, the boiling it can point be observed from Figure 2b that higher temperatures need to be considered because the boiling point varies with the pressure in the cleaning equipment. Thus, it is time consuming to generate SHS above a temperature of 200 °C by rapidly heating water at a room temperature of 25 °C. Therefore, in this Energies 2020, 13, 6196 3 of 16 Energies 2020, 13, x FOR PEER REVIEW 3 of 16

Energies 2020, 13, x FOR PEER REVIEW 3 of 16 variesstudy, with the thesaturated pressure steam in the was cleaning prepared equipment. by preheating Thus, itthe is timewater consuming contained to in generate a water SHSsupply above tank a using a heating wire, as illustrated in Figure 3. By applying this method to the steam cleaning process, study,temperature the saturated of 200 ◦ Csteam by rapidly was pr heatingepared waterby preheating at a room the temperature water contained of 25 ◦C. in Therefore, a water supply in this study,tank usingtheSHS saturated acan heating be generated steam wire, as was illustratedwithin prepared a short in by Figure time, preheating 3.an Byd thus applying the processing water this contained method time can to in the be a water reduced.steam supply cleaning tank process, using SHSa heating can be wire,generated as illustrated within a inshort Figure time,3. an Byd applyingthus processing this method time can to be the reduced. steam cleaning process, SHS can be generated within a short time, and thus processing time can be reduced. ) ℃

( Superheated Supercritical

) Steam Water Temperature Total Heat to Generate Saturated Steam Temperature ℃

( Superheated Supercritical 374 Steam Water Temperature Total Heat to Generate Saturated Steam Temperature superheated steam Saturated at atmospheric Water Boiling to Steam 374 Steam Curve pressure superheated steam Point

Boiling Saturated at atmospheric Water Boiling to Steam Steam Curve 100 pressure Point Boiling Water 100 Vacuum (Liquid) Steam Water Vacuum (Liquid) Steam 0.1 10 22.1 Atmosphere Sensible Latent Heat of Sensible Heat Atmospheric Critical (MPa) (abs) Heat Vaporization Heat Energy 0.1Pressure 10 22.1Pressure Atmosphere Sensible Latent Heat of Sensible Heat Atmospheric Critical (MPa) (abs) Heat Vaporization Heat Energy Pressure Pressure (a) Graphical representation of steam formation (b) Pressure and temperature relationship of steam

(a) Graphical representationFigureFigure of steam 2.2. TemperatureTemperature formation relationship relationship(b) Pressure of of water water and and and temperature steam. steam. relationship of steam Figure 2. Temperature relationship of water and steam. Flow Induction Valve 2 Heating Flow Induction Valve 2 Heating

Superheated Steam Generation System Chamber Superheated Steam Generation System Chamber

Pre-heating System MG Pre-heating Flow System Valve 1 MG Flow Valve 1

Water (Liquid) Water (Liquid) Water Supply Saturated Steam Tank Feed Pump Water Supply SaturatedSuperheated Steam Steam Tank Feed Pump

Superheated Steam FigureFigure 3. 3.Steam Steam heatingheating structurestructure of of the the cleaning cleaning equipment. equipment. MG,MG, mothermother glass.glass. Figure 3. Steam heating structure of the cleaning equipment. MG, mother glass. TheThe heatheat capacitycapacity requiredrequired toto heatheat thethe saturatedsaturated steamsteam enteringentering thethe TiTi load load for for generating generating SHSSHS should be calculated to determine the power capacity of IH-type cleaning equipment. The expression shouldThe beheat calculated capacity torequired determine to heat the powerthe saturate capacidty steam of IH-type entering cleaning the Ti equipment. load for generating The expression SHS for the heat capacity required to heat saturated steam to generate SHS is given as follows: shouldfor the be heat calculated capacity to required determine to theheat power satura capacited steamty of toIH-type generate cleaning SHS is equipment. given as follows: The expression for the heat capacity required to heat saturated steam=⋅⋅Δ to generate SHS is given as follows: QQmcTheat= m c ∆T (1)(1) heat · · QmcT=⋅⋅Δ where m, c, and ΔT are the mass of the steam,heat specific heat capacity, and temperature variation,(1) whererespectively m, c, and [20]. Δ Tm arecan thebe calculatedmass of the as follows:steam, specific heat capacity, and temperature variation, respectively [20]. m can be calculated asmCMH follows:=⋅γγρ ,( =g ) (2) mCMH=⋅γγρ,( =g ) (2) Energies 2020, 13, 6196 4 of 16 where m, c, and ∆T are the mass of the steam, speciﬁc heat capacity, and temperature variation, respectively [20]. m can be calculated as follows:

m = γ CMH, (γ = ρg) (2) · where γ and CMH are the specific weight and cubic meter per hour, respectively. In Equation (2), CMH is equal to the flow rate, and γ can be calculated by multiplying ρ by g (acceleration due to gravity) [20,21]. To generate SHS using the IH process, the pressure and volume of the gas should be considered. These can be calculated using an ideal gas equation (IGE), based on the Boyle–Charles’s law, as follows: PV = nRT (3) t C + K V = 22.414 ◦ (4) × K M ρ = P (5) × V Using Equation (4), the volume of 1 mole of gas at 1 atm can be derived from the absolute temperature and instantaneous temperature of the gas. The density is calculated using volume, molecular mass, and pressure, as expressed in Equation (5). Cp, the specific heat capacity of steam at constant pressure, is calculated using the steam table [22,23]. Therefore, the heat quantity Qheat calculated using Equation (1) is the absolute heat quantity required for heating, and it can be converted into electric power energy for heating the steam by selecting the target temperature and heating time.

2.2. Design and Implementation of the Heating Object for Rapid Induction Heating In the TFT–LCD and wafer cleaning equipment that uses the IH method, SHS is generated by heating the heating object after building up a ﬂow path of the ﬂuid inside the shape of the heating object. However, as shown in Table1, pure grade 2 Ti R50400, which was used as the heating object in this study, has a low relative permeability and electrical resistivity compared to other magnetic metal materials [23–25]. Thus, a high switching frequency and high current were needed to increase the IH joule heat of Ti.

Table 1. Comparison of titanium and stainless steel.

Parameter Grade 2 Ti Stainless Steel (SUS304) Resistivity [ρ(µΩcm)] 55 72 Relative Permeability [µs] 1.00018 2.291 1 1 Thermal Conductivity [Wm− K− ] 0.041 0.039

The length of the heating object through which the fluid flowed was limited to less than 10 cm to minimize the volume of the cleaning equipment and the transfer path of the steam. In addition, because foreign matter affects fluid purity, the inner surface of the heating object, which constituted the fluid path, was not welded. Therefore, a heating object with a structure in which the inner fluid path was made of pure Ti was designed such that the fluid could stay within a limited length for a long time. A typical heating object, shown in Figure4, has advantages such as ease of manufacture and simplicity of structure. However, it requires sufficient preheating before the fluid flows due to the fact that the outer surface is heated first in IH and the fluid stay time is short [26–28]. Figure5 shows the IH principle and eddy current distribution when heating a cylindrical heating object. As described above, only the outline was heated based on the results of the simulation of the thermal distribution of a cylindrical heating object. Therefore, considering the characteristics of IH, a shape was proposed in which the fluid path was located within the spiral shape on the outer surface of the heating object, as depicted in Figure6. In particular, the proposed heating object could secure a much longer fluid movement path in the same length compared to a conventional heating object. Energies 2020, 13, x FOR PEER REVIEW 5 of 16

Steam Output

Energies 2020, 13, 6196 5 of 16 Steam Input

Figure7a shows a heating cylinder made of Ti that was manufactured based on the proposed geometry. The Ti cylinder was designed, with a spiral groove on the outer surface, to be surrounded by a Ti pipe, which constituted the fluid movement path. The inside surface was safe from particulate matter because only the exterior of the cylinder, which was outside the fluid path, was welded. Moreover, whileFigure the total 4. Structure length of theof shapeheating was objects 10 cm, thetypically length ofemployed the fluid path in the and cl theeaning stay time process. could be increased by changing the shape of the structure. Another advantage of this structure was that the outer surface was heated first in IH. Figure7b shows the results of a Ti load wound on a 1 /4-in copper Figure tubing5 shows coil. the A general IH principle litz wire could and noteddy be usedcurrent because distribution a large current when with a heating high frequency a cylindrical flows heating object. As describedthrough the above, coil [29]. only Therefore, the wateroutline cooling was was heat performeded based by flowingon the distilled results water of the into simulation the coil of the Energies 2020, 13, x FOR PEER REVIEW 5 of 16 thermal distributionusing a copper of a tubing cylindrical coil. heating object. Therefore, considering the characteristics of IH, a shape was proposed in which the fluid path was located within the spiral shape on the outer surface of the heating object, as depicted in Figure 6. In particular, the proposed heating object could secure a much longer fluid movement path in the same length compared to a conventional heating object. Figure 7a shows a heating cylinder made of Ti that was manufacturedSteam based Output on the proposed geometry. The Ti cylinder was designed, with a spiral groove on the outer surface, to be surrounded by a Ti pipe, which constituted the fluid movement path. The inside surface was safe from particulate matter because only the exterior of the cylinder, which was outside the fluid path, was welded. Moreover, while the total length of the shape was 10 cm, the length of the fluid path and the stay time could be increasedSteam by changing Input the shape of the structure. Another advantage of this structure was that the outer surface was heated first in IH. Figure 7b shows the results of a Ti load wound on a 1/4- in copper tubing coil. A general litz wire could not be used because a large current with a high frequency flows through the coil [29]. Therefore, water cooling was performed by flowing distilled water into the coil usingFigureFigure a copper 4. 4. StructureStructure tubing of of heating heating coil. objects objects typically typically employed employed in in the the cl cleaningeaning process. process.

Figure 5 shows the IH principle and eddy current distribution when heating a cylindrical heating Magnetic Flux object. As described above, only the outline was heated based on the results of the simulation of the thermal distribution of a cylindrical heating object. Therefore, considering the characteristics of IH, a AC current shape was proposed in which the fluid pathEddy was current located within the spiral shape on the outer surface of the heating object, as depicted in Figure 6. In particular, the proposed heating object could secure a much longer fluid movement path in the same length compared to a conventional heating object. Figure 7a shows a heating cylinder made of Ti that was manufactured based on the proposed geometry. The Ti cylinder was designed, with a spiral groove on the outer surface, to be surrounded by a Ti pipe, which constituted the fluid movement path. The inside surface was safe from particulate matter because only the exterior of the cylinder, which was outside the fluid path, was welded. Moreover, while the total length of the shape was 10 cm, the length of the fluid path and the stay time could be increased by changing the shape of the structure. Another advantage of this structure was that the outer surface was heated first in IH. Figure 7b shows the results of a Ti load wound on a 1/4- in copper tubing coil. A general litz wire could not be used because a large current with a high frequency flows through the coil [29]. Therefore, water cooling was performed by flowing distilled water into the coil using a copper tubing coil.

Magnetic Flux

AC currentFigure 5. Principle and characteristics of induction heating. Figure 5. Principle andEddy characteristics current of induction heating.

Figure 5. Principle and characteristics of induction heating. Energies 2020, 13, x FOR PEER REVIEW 6 of 16 Energies 2020, 13, 6196 6 of 16 Energies 2020, 13, x FOR PEER REVIEW 6 of 16

Steam Output Steam Output

Steam Input Steam Input Figure 6. Shape of the proposed heating object. Figure 6. ShapeShape of of the the proposed proposed heating object.

Fluid Path

Fluid Coil Path Fluid Coil Path (a) Heating object fabricated using Ti (b) Ti load (a) Heating object fabricated using Ti (b) Ti load FigureFigure 7. 7. ProposedProposed titanium titanium (Ti) (Ti) load load stru structurecture of of the the cleaning cleaning equipment. equipment. Figure 7. Proposed titanium (Ti) load structure of the cleaning equipment. 2.3.2.3. Parameter Parameter Analysis Analysis of of the the Titanium Titanium Load Load 2.3. Parameter Analysis of the Titanium Load InIn this this section, section, the the parameter parameter values values of of the the implemented implemented Ti Ti load load are are analyzed. analyzed. The The operating operating frequencyfrequencyIn this and andsection, power power the capacity capacityparameter of of thevalues the cleaning cleaning of the eqimplemented equipmentuipment were were Ti loadselected selected are analyzed.based based on on Thethe the analyzedoperating analyzed parameters.frequencyparameters. and The power principleprinciple capacity of of the the IHof IH technologythe technology cleaning is basedeq isuipment based on Faraday’s onwere Faraday’s selected law of electromagnetic lawbased of onelectromagnetic the induction,analyzed induction,parameters.which states which The that states aprinciple magnetic that of a flux themagnetic isIH generated technology flux is when generated is abased high-frequency whenon Faraday’s a high-frequency current law flows of electromagnetic current through flows a coil. throughinduction,This magnetic a coil. which This flux states inducesmagnetic that an fluxa eddy magnetic induces current fluxan on eddy is the generated current surface ofon when thethe heatingsurface a high-frequency objectof the andheating generates current object flows jouleand generatesthroughheat, owing a joule coil. to heat, theThis skin magneticowing effect to [theflux30, 31skin induces]. Mosteffect an of [30,31]. thiseddy joule currentMost heat of onthis is distributedthe joule surface heat withinofis distributedthe heating the skin withinobject depth andthe (δ), skingenerateswhich depth is a joule significant(δ), which heat, owing factoris a significant into determiningthe skin factor effect thein [30,31]. determining inverter Most operating of the this inverter joule frequency. heat operating is The distributedδ frequency.can be within calculated The the δ as follows: canskin be depth calculated (δ), which as follows: is a significant factorr in determiningr the inverter operating frequency. The δ 1 ρ can be calculated as follows: δ = (6) 2 7 ρ δ =×4π 1 10− × µr f × − (6) 410π 27×1 μ ρf where ρ, µr, and f are the resistivity ofδ the=× material, relative permeability,r and frequency of the current π 27× − μ (6) flowing in the coil, respectively. This δ is410 determined fromr thef point when the skin depth of the where ρ, μr, and f are the resistivity of the material, relative permeability, and frequency of the current high-frequency current becomes 1/e (approximately 0.368) times the current density of the surface, flowingwhere ρ in, μ ther, and coil, f are respectively. the resistivity This of δ the is determined material, relative from the permeability, point when and the frequency skin depth of of the the current high- and most of the current and power distributions belong to the skin depth from the surface to δ, as shown frequencyflowing in current the coil, becomes respectively. 1/e (approximately This δ is determined 0.368) from times the the point current when density the skin of depth the surface, of the high- and in Figure8; therefore, in the IH system, it is advantageous to heat the surface of a heating object under mostfrequency of the currentcurrent becomesand power 1/ edistributions (approximately belong 0.368) to the times skin the depth current from density the surface of the to surface,δ, as shown and high-frequency operating conditions, since all eddy currents are concentrated on the δ of the surface. inmost Figure of the 8; therefore, current and in thepower IH system, distributions it is adva belongntageous to the to skin heat depth the surface from the of a surface heating to object δ, as shownunder Therefore, to increase the low resistivity value of the Ti load, the inverter switching frequency should high-frequencyin Figure 8; therefore, operating in the cond IH itions,system, since it is alladva eddyntageous current tos heatare concentrated the surface of on a heatingthe δ of objectthe surface. under be increased. Generally, the IH system can be represented using a transformer equivalent model in Therefore,high-frequency to increase operating the low cond resistivityitions, since value all of eddy the Ticurrent load,s the are inverter concentrated switching on the frequency δ of the surface.should Therefore, to increase the low resistivity value of the Ti load, the inverter switching frequency should EnergiesEnergies 2020 2020, 13, 13, x, xFOR FOR PEER PEER REVIEW REVIEW 7 7of of 16 16 Energies 2020, 13, 6196 7 of 16 bebe increased. increased. Generally, Generally, the the IH IH system system can can be be represented represented using using a a transformer transformer equivalent equivalent model model in in which the coil and load are the primary and secondary sides, respectively [32–34]. This transformer whichwhich the the coil coil and and load load are are the the primary primary and and secondary secondary sides, sides, respectively respectively [32 [32–34].–34]. This This transformer transformer equivalent model can be simplified using a series connection circuit of the equivalent inductance Leq equivalentequivalent model model cancan bebe simplifiedsimplified using a series series connection connection circuit circuit of of the the equivalent equivalent inductance inductance Leq and equivalent resistance Req, as shown in Figure 9. The mathematically analyzed Req is given as Leqandand equivalent equivalent resistance resistance ReqR, eqas, asshown shown in in Figure Figure 9.9 .The The mathematically mathematically analyzed analyzed ReqR eqis isgiven given as follows: asfollows: follows: 2 (()ωM) ⋅RL 2 = + () ⋅= + Req =+r · =+r A RL (7)(7) =+2 2 =+ (7) R++( (ωL )) L +(2 ) and Leq is calculated as follows: andandL eqLeqis is calculated calculated as as follows: follows: 2 ω 2⋅ ()ωML2 ⋅ 2 2 =−(()ωMML) L2 2 =−2 LLeq =−11222 =− LAL2 (8) LLLeqeq= L1 11222+ ω· = L1 LALA L2 (8)(8) RL2L +()ω 2 2 − RLL+ ( ())2 − RL ωL2

100% 100% 90% 90% 80% 80% 70% 70% 60% 60% 50% 50% PercentagePercentage Current Current Density Density 40% 40% 30% 30% PercentagePercentage Power Power Density Density 20% 20% Percentage Power & Current Densities Current & Power Percentage Percentage Power & Current Densities Current & Power Percentage 10% 10% 0 0 0 0 Strip Depth ( ) Strip Depth ( ) Figure 8. Parameter values of the Ti load based on frequency variation. FigureFigure 8. 8.Parameter Parameter values values of of the the Ti Ti load load based based on on frequency frequency variation. variation.

r RL Leq r RL Leq

I1 I2 I1 I2 V1 L1 L2 V1 L1 L2 Req Req

IH Load N:1 IH Load N:1 FigureFigure 9. 9. Equivalent Equivalent circuit circuit transformation transformation of of the the induction induction heating heating (IH) (IH) system. system.

These equivalent parameters, parameters, RReq andand LeqL, are, are dependent dependent on onthe the size size of the of theheating heating object, object, the These equivalent parameters, Reqeq and Leqeq, are dependent on the size of the heating object, the theconductivityconductivity conductivity andand and permeabilitypermeability permeability ofof of thethe the materialmaterial material,, , andand and the the operating operatingoperating frequency. frequency.frequency. The The transformer transformertransformer secondary resistance RRL, ,which which can can be be regarded regarded as as the the resistance resistance component component of ofthe the Ti Ticylinder, cylinder, is secondary resistance RLL, which can be regarded as the resistance component of the Ti cylinder, is isdetermineddetermined determined using using using the the the δ δ valueδ valuevalue of of ofthe the the eddy eddy eddy current current current and and and is is iscalculated calculated calculated as as as follows: follows: follows: ==qρμ π = RRkLTi==ρμ r2, π fINI21 = (9) RRRkLLTi= RTi = k ρµr2 r2,π f , fINII2 =21NI1 (9)(9)

The graph in Figure 10 shows changes in the Req and Leq of the proposed Ti load with frequency TheThe graph graph in in Figure Figure 10 10 shows shows changes changes in in the theR eqReqand andL Leqeqof of the the proposed proposed Ti Ti load load with with frequency frequency variation. Therefore, the output power generated by the actual IH cleaning system is given by the variation.variation. Therefore,Therefore, thethe output output power power generated generated byby the the actual actual IH IH cleaning cleaning system system is is given given by by the the relationship between the resistance and current, as follows: relationshiprelationship between between the the resistance resistance and and current, current, as as follows: follows: 2 =⋅2 ⋅ρμ π PkNI=⋅()1 ⋅q ρμr 2π f (10) PkNI()12 r 2 f (10) P = k (NI ) ρµr2π f (10) · 1 · Energies 2020, 13, 6196 8 of 16 EnergiesEnergies 20202020,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1616 where kk andandand N N are areareconstants constantsconstants that thatthat are areare related relatedrelated to toto the thethe permeability permeapermeability and and number number of turnsof turns of the of the Ti load Ti load coil, coil,respectively.coil, respectively.respectively. Finally, Finally,Finally, the load thethe current loadload currentcurrent required requiredrequired to generate toto generategenerate SHS is calculatedSHSSHS isis calculatedcalculated using the usingusing heat the capacitythe heatheat heat capacityQcapacityheat expressed Qheat expressedexpressed in Equation inin EquationEquation (1). (1).(1).

240.0240.0 2.22.2

210.0210.0 2.12.1

180.0180.0 22

150.0150.0 1.91.9

120.0 1.8 120.0 Ti Load ReqReq 1.8 [μH] [mΩ] Ti Load ReqReq [μH] [mΩ] Ti Load LeqLeq eq eq Ti Load LeqLeq eq eq 90.0 1.7

90.0 1.7 L R L R

60.060.0 1.61.6

30.030.0 1.51.5

0.00.0 1.41.4 1010 50 50 90 90 130 130 170 170 210 210 250 250 290 290 330 330 370 370 410 410 450 450 490 490 Frequency [kHz]

FigureFigure 10. 10. ParameterParameter values values of of Ti Ti load load based based on on frequency frequency variation.

2.4. Design Design of of Electric Electric Power Power Converter Converter Specifications Specifications and Simulation Verification Verification Because thethe electric electric power power converter converter for thefor IH-typethe IH-type-type cleaning cleaningcleaning device operatesdevicedevice operatesoperates at a high atat frequency, aa highhigh frequency,itfrequency, must be designeditit mustmust bebe to designeddesigned operate toto with operateoperate zero-voltage withwith zero-voltagezero-voltage switching switchingswitching (ZVS) to minimize(ZVS)(ZVS) toto minimizeminimize switching switchingswitching loss [35]. lossFurthermore,loss [35].[35]. Furthermore,Furthermore, the large current thethe largelarge flowing currentcurrent through flowingflowing the coilththroughrough causes thethe aburden coilcoil causescauses on the aa electric burdenburden power onon thethe converter. electricelectric powerTherefore, converter. LCL topology Therefore, was LCL selected, topology as shownwas selected in Figure,, asas shownshown 11, to reduceinin FigureFigure the 11,11, inverter toto reducereduce current thethe inverterinverter on the currentprimarycurrent onon side thethe [ 36 primaryprimary,37]. sideside [36,37].[36,37]. Full-Bridge Full-Bridge LCL Resonant Network Resonant Inverter

L Lmat.

L Leqeq Cp R Cp Reqeq

Ti Load

FigureFigure 11. 11. TiTi load load with with LCL LCL topology topology structure. structure.

mat. As describeddescribed above, above, the the LCL LCL network network topology topologytopology consists consistsconsists of an inductor ofof anan (inductorinductorLmat.) added ((Lmat. for)) impedanceaddedadded forfor p eq impedancematching,impedancea matching,matching, parallel capacitor aa parallel (C capacitorp), and a Ti(Cp load),), andand (L eqaa )TiTi at loadload the output((Leq)) atat stage.thethe outputoutput Therefore, stage.stage. itTherefore,Therefore, is possible itit toisis mat. p possibleﬁlter the to harmonic filter the components harmonic components through the through added L themat. addedand C pL.mat. In and addition,and Cp.. InIn there addition,addition, is an therethere advantage isis anan advantageof increasing of increasing the current the in current the coil in ofthe the coil load of thethe while loadload reducing whilewhile reducingreducing the current thethe currentcurrent ﬂowing flowingflowing through throughthroughLmat. mat. Lmat. basedbased onon thethe resonanceresonance networknetwork designdesign [38].[38]. ThThee third-orderthird-order filterfilter networknetwork ofof thethe LCLLCL topologytopology Energies 2020, 13, x FOR PEER REVIEW 9 of 16 has characteristics of constant current (resonant frequency = ωr1) and constant voltage (resonant frequency = ωr2) [39,40]. Each resonant frequency and quality (Q) factor can be calculated as follows:

Energies 2020, 13, 6196 ω = 1 9 of 16 r1 (11) LCmat. p based on the resonance network design [38]. The third-order ﬁlter network of the LCL topology has characteristics of constant current (resonant frequency1= ωr1) and constant voltage (resonant frequency = ω )[39,40]. Each resonant frequencyω and= quality (Q) factor can be calculated as follows: r2 r 2 (12) (||)LLCmat. eq p 1 ωr1 = p (11) Lmat.Cp ω LZ Q ==on 1 (13) ωr2 = qRRLL (12)

(Lmat. Leq)Cp In addition, the input/output voltage gain equation according to γ, which is the ratio of Lmat. and Leq, is given by: ωoL Zn Q = = (13) R R LL L In addition, the input/output voltage gainγ = equationeq according to γ, which is the ratio of L and mat(14). Leq, is given by: Lmat. Leq γ = (14) V Lmat. out = 1 V 231 (15) V out =()(1(1)−+ωγωγωjQ + − ) (15) in nnn2 3 Vin (1 ωn ) + jQ((1 + γ)ωn γωn ) − − DependingDepending on on the the design design of of the the inductance inductance ratio ratio γγ, ,the the LCL LCL resonant resonant network network can can be be set set to to step step upup or or step step down down output output linearly. linearly. In In addition, addition, when when designing designing Lmat.mat. forfor impedance impedance matching matching in in high high power,power, air air core core coils coils are are mainly mainly used used to to prevent prevent saturation saturation and and fluctuations fluctuations in in inductance inductance values, values, dependingdepending on on temperature. Moreover, operationoperation atat high high frequencies frequencies requires requires film film capacitors capacitors rated rated for forhigh high voltage voltage and and high high currents currents of tens of tens of hundreds of hundreds of nanofarads. of nanofarads. In addition, In addition, more accuratemore accurate results resultscan be can obtained be obtained by considering by considering the parasitic the parasi capacitancetic capacitance of the switch of the when switch designing when designing the resonant the resonantnetwork network in operation in operation at high frequencies at high frequencies and high and voltages high [voltages41,42]. In [41,42]. general, In thegeneral, power the control power of controlthe IH applicationof the IH application with the LCL with structure the LCL allows structur frequencye allows controlfrequency in the control region in higherthe region than higher the ωr2 thanresonance the ωr2 frequency, resonance and frequency, the inverter and switch the inverter operates swit inch the operates region where in the ZVS region is possible. where TheZVS DCis possible.voltage gainThe curveDC voltage of the gain LCL curve resonant of the network LCL re accordingsonant network to the variation according of to the theQ -factorvariation value of the can Qbe-factor expressed value ascan shown be expressed in Figure as shown12. Table in 2Figure shows 12. the Table amount 2 shows of power the amount required of power to heat required SHS at totemperatures heat SHS at abovetemperatures 200 ◦C withinabove 200 10 s °C when within saturated 10 s when steam saturated at 133.25 steam◦C flows at 133.25 intothe °C Tiflows load into at a theflow Ti rateload of at6 aL flow/min rate and of a 6 pressure L/min and of 3 aatm. pressure Table of2 also3 atm. lists Table the 2 cleaning also lists equipment the cleaning specifications equipment specificationsand resonant and network resonant parameters. network parameters.

8 ωr2

6 Q=0.3

4 DC Gain

2 Q=1 Q=3 Q=6 0 00.51 1.5 ωn=ωs/ωo FigureFigure 12. 12. DCDC gain gain characteristics characteristics of of the the LCL LCL resonant resonant network. network. Energies 2020, 13, 6196 10 of 16

Table 2. Speciﬁcations of the cleaning equipment and resonant network parameters.

Wafer Cleaning Equipment Spec. Resonant Network Parameters Value [Unit] Parameters Value [Unit]

DC-link Voltage 280 [V] Capacitance, Cp 100 [nF] Input Steam Temp. 133.25 [◦C] Primary Inductance, Lmat. 7.5 [µH] Output Steam Temp. >200 [◦C] Secondary Inductance, Leq 1.556 [µH] Active Power 2.2 [kW] Coil Resistance, Req 209.1 [mΩ] Coil Current 100 [Arms] Coil Conduit Size 1/4 [inch] Switching Frequency 465 [kHz] Coil Turns 9 [turn]

Figure 13 shows the simulation waveforms of the current and phase of the inverter and coil based on the variations in frequency and considering the parameter values. Figure 13a shows the inverter current and phase angle of the voltage and current. The inverter can operate with ZVS at a switching frequency of 465 kHz and an inverter current of approximately 35 Arms. Figure 13b shows the coil current and phase angle of the voltage and current. The current of the coil is approximately 100 Arms atEnergies the operating 2020, 13, x FOR frequency, PEER REVIEW and it can be conﬁrmed that it is similar to the calculated active power.10 of 16

(a) Inverter current and phase (b) Coil current and phase

Figure 13.13. Current and phase of inverter and coil with respect to frequency variation.variation.

195 3.inverter Experimental current and Verification phase angle of the voltage and current. The inverter can operate with ZVS at a 196 switching frequency of 465 kHz and an inverter current of approximately 35 Arms. Figure 13(b) shows 197 the coilExperiments current and were phase performed angle of using the voltage a 2.2 kW and prototype current. to The validate current the of proposed the coil waferis approximately cleaning IH system. Preliminary experiments were conducted to compare the heating performance of the proposed 198 100 Arms at the operating frequency, and it can be confirmed that it is similar to the calculated active 199 Tipower. load with the various Ti heating objects. Finally, an SHS generation experiment was performed on the cleaning equipment with the proposed Ti load. 200 3. Experimental Verification 3.1. Preliminary Experiment Using a Water Chiller System 201 Experiments were performed using a 2.2 kW prototype to validate the proposed wafer cleaning Preliminary IH experiments were conducted using a water chiller system before generating the 202 IH system. Preliminary experiments were conducted to compare the heating performance of the SHS, as shown in Figure 14. The experiment was carried out using a heated object made of Ti for 203 proposed Ti load with the various Ti heating objects. Finally, the SHS generation experiment of the four cases. The detailed experimental specifications and heating object conditions are presented in 204 cleaning equipment with the proposed Ti load was performed. Table3. The water to be passed through the Ti loads was cooled to 25 ◦C and supplied, using the 205 chiller3.1. Preliminary system, atExperiment a flow rate Using of 9 a L Water/min. Chiller Ti loads System in all four cases were fitted with a load current of approximately 100 Arms. Figure 15 shows the preliminary experimental results for each Ti load case. 206 CasePreliminary IV, which corresponds IH experiments to the were proposed conducted Ti load, using had a the water highest chiller heating system performance before generating because the of 207 theSHS, IH as property, shown in which Figure caused 14. The the experiment outer surface was ofcarried the heating out using body a heated to be heated object first.made Case of Ti IV for had four a 208 temperaturecases. The detailed rise rate experimental (∆T/∆t) that specifications was more than and 400 heating times higher object thanconditions that of are the presented other cases. in Table 209 3. The water to be passed through the Ti loads was cooled to 25 °C and supplied using the chiller 210 system at a flow rate of 9 L/min. Ti loads in all the four cases were fitted with a load current of 211 approximately 100 Arms. Figure 15 displays the preliminary experimental results for each Ti load case.

Water Chiller Output Water System ΔT measurement

High- Frequency Inverter

Input Water 9L/min Figure 14. IH test bed with water chiller system.

Energies 2020, 13, 6196 11 of 16 EnergiesEnergiesEnergiesEnergies 2020 202020202020 2020, 13,, , 13 ,,1313 13x, ,, FORx ,xx x FOR FORFOR FOR PEER PEER PEERPEER PEER REVIEW REVIEW REVIEWREVIEW REVIEW 11111111 of ofofof 16 161616

WaterWaterWaterWater Chiller Chiller ChillerChiller OutputOutputOutputOutput Water Water WaterWater SystemSystemSystemSystem ΔΔΔΔTTT Tmeasurement measurement measurementmeasurement

High-High-High- FrequencyFrequencyFrequency InverterInverterInverter

Energies 2020, 13, x FOR PEER REVIEW InputInputInputInput Water Water WaterWater 11 of 16 9L/min9L/min9L/min9L/min 25 25 2525℃℃℃℃ Table 3. Experimental specifications of heating objects. FigureFigureFigureFigureFigure 14. 14. 14.14. 14. IH IH IHIH IHIH test test test test testtest bed bed bed bed bedbed with with with with withwith water waterwater water waterwater chiller chiller chiller chillerchiller system. system. system. system.system.

TableTableTableTable 3. 3.3. 3.Experimental ExperimentalExperimental Experimental speciﬁcations specificationsspecifications specifications ofof of of heating heating heating objects.objects. objects. Picture of Table 3. Experimental specifications of heating objects. heating object PicturePicturePicturePicture of Heating ofof of Picture of HeatingHeatingHeatingObject ObjectObject Object HeatingCase Object I II III IV

Frequency 435 [kHz] 452 [kHz] 480 [kHz] 465 [kHz] CaseCaseCase I I I II II II III III III IV IV IV CaseCase I I II II III III IV IV CurrentFrequencyFrequencyFrequency 99.3 435 435 435 [A [kHz][kHz] [kHz]rms] 99.2 452 452 452 [A [kHz][kHz] [kHz]rms] 98.3 480 480 480 [kHz][A[kHz] [kHz]rms] 99.9 465 465 465 [kHz][kHz][A [kHz]rms] FrequencyFrequency 435 435 [kHz] [kHz] 452 452 [kHz] [kHz] 480 480 [kHz] [kHz] 465 465 [kHz] [kHz] CurrentCurrentCurrent 99.399.399.3 [A[A [Armsrmsrmsrms]]] ] 99.299.299.299.2 [A [A [A[Armsrmsrmsrms]]] ] 98.398.398.398.3 [A [A [A[Armsrmsrmsrms]]] ] 99.999.999.999.9 [A [A [A[Armsrmsrmsrms]]] ] HeatingCurrentCurrent time 99.3 99.334 3[A [A[s]rms ] ]99.2 99.2343 [A [A[s]rms ] ]98.3 98.3343 [A [A [s]rms ] ]99.9 99.94 [A[s] [Arms ] ] HeatingHeatingHeating timetime time 343 343 343 [s][s] rms[s] 343 343 343 [s][s] [s]rms 343 343 343 [s][s] [s]rms 4 4 4 [s][s] [s] rms HeatingHeating time time 343 343 [s] [s] 343 343 [s] [s] 343 343 [s] [s] 4 4[s] [s] MaxMax.Max.Max.. temperature temperaturetemperature temperature 36.3 36.3 36.3 36.3 [°C] [°C][°C] [°C] 43.4 43.4 43.4 43.4 [°C] [°C][°C] [°C] 37.8 37.8 37.8 37.8 [°C] [°C] [°C] 99.2 99.2 99.2 [°C][°C] [°C] Max.Max. temperature temperature 36.3 36.3 [°C] [ C] 43.4 43.4 [°C] [ C] 37.8 37.8 [°C] [ C] 99.2 99.2 [°C] [ C] ΔΔΔT/T/T/ΔΔΔttt t 0.030.030.03 [°C/s][°C/s] [°C/s]◦ 0.05 0.05 0.05 [°C/s][°C/s] [°C/s]◦ 0.04 0.04 0.04 [°C/s][°C/s] [°C/s]◦ 18.55 18.55 18.55 [°C/s][°C/s] [°C/s]◦ ΔΔT/Δt∆T/TΔ/∆tt 0.030.03 [°C/s] [◦C/s] 0.05 0.05 [°C/s] [◦C/s] 0.04 0.04 [°C/s] [◦C/s] 18.55 18.55 [°C/s] [◦C/s]

50 Case I Case II Case III Case IV Power off Case I, II, III Case IV 45

Max. 99.2°C (4s) ] 40 °C 40

Temperature [ 30

20 0 2540 5080 12075 100160 125200 150240 175280 200320 225360 250400 275440 300480 325520 350560 375600 Time [s] Figure 15. Experimental results of temperature rise for each Ti load case. Figure 15. Experimental results of temperature rise for each Ti load case.

212 Case IV, which corresponds to the proposed Ti load, has the highest heating performance because of 213 the induction heating property, owing to which the outer surface of the heating body is heated first. 214 This case has a temperatureFigureFigureFigure 15.15. 15. ExperimentalExperimental Experimentalrise rate ΔT resultsresults /resultsΔt that ofof of temperaturetemperature istemperature more than riserise rise 400 forfor for eachtimeseach each TiTi Ti higher loadload load case.case. case. than those of other 215 cases. Figure 15. Experimental results of temperature rise for each Ti load case.

216 3.2. SHS Generation Heating Test Bed 217 A testbed was constructed using a 2.2 kW wafer-cleaning equipment, as shown in Figure 16, to 218 verify the SHS heating performance of the proposed system. The test conditions were as follows: 219 (1) Saturated steam that was heated to generate SHS was supplied by pre-heating the water using 220 a quartz heater. 221 (2) The instantaneous input and output steam temperatures of the Ti load were measured using a 222 k-type thermocouple (TC). 223 (3) The copper tubing coil of the Ti load and inverter switch were water-cooled using the chiller 224 system. 225 (4) The time at which high-temperature pure SHS to be used in the wafer-cleaning process was 226 generated by IH was measured.

Energies 2020, 13, x FOR PEER REVIEW 12 of 16 Energies 2020, 13, 6196 12 of 16 3.2. SHS Generation Heating Test Bed

3.2. SHSA test Generation bed was Heating constructed Test Bed using 2.2 kW wafer cleaning equipment, as shown in Figure 16, to verify the SHS heating performance of the proposed system. The test conditions were as follows: A test bed was constructed using 2.2 kW wafer cleaning equipment, as shown in Figure 16, to1. verifySaturated the SHS steam heating that performance was heated ofto thegenerate proposed SHS, system. supplied The by test preheating conditions the were water as follows: using a quartz heater. 1. Saturated steam that was heated to generate SHS, supplied by preheating the water using a 2. The instantaneous input and output steam temperatures of the Ti load were measured using a quartz heater. 2. k-typeThe instantaneous thermocouple input (TC). and output steam temperatures of the Ti load were measured using a 3. Thek-type copper thermocouple tubing coil (TC). of the Ti load and inverter switch were water-cooled using the chiller 3. system.The copper tubing coil of the Ti load and inverter switch were water-cooled using the chiller system. 4. TheThe time time at which the high-temperature pure SHS to be used in the wafer cleaning process was generated by IH was measured. generated by IH was measured.

Quartz Heater for preheating

Oscilloscope Temperature recorder Laptop MCU, Inverter

Ti load

Quartz Heater Chiller Thermal Controller imager Slidax

Figure 16. Test bed for SHS generation. Figure 16. Test bed for SHS generation. 3.3. Experimental Results 3.3. Experimental Results Figure 17 shows the voltage and current waveforms of the inverters and coils of the wafer cleaningFigure equipment. 17 shows The the experimental voltage and waveform current wavefo showedrms that of the the inverter inverters operated and coils with ZVS.of the The wafer coil cleaning equipment. The experimental waveform showed that the inverter operated with ZVS. The current was 99.93 Arms and the coil voltage was 443.7 Vrms at a switching frequency of 482 kHz. coil current was 99.93 Arms and the coil voltage was 443.7 Vrms at a switching frequency of 482 kHz. The inverter current was 31.61 Arms, apparent power was 45.4 kVA, and active power was 2.15 kW. The inverter experimental current waveform was 31.61 shows Arms, apparent results that power are was similar 45.4 to kVA, those and obtained active power from mathematicalwas 2.15 kW. calculationsThe experimental and simulations. waveform shows results that are similar to those obtained from mathematical calculationsFigure 18 and plots simulations. the internal and external temperatures of the Ti load and the input and output temperatures of the ﬂowing steam measured using the TC; in this ﬁgure, the red line represents the SHS temperature, which is the steam output from the proposed cleaning equipment. In addition, Table4 shows the temperature values measured at 10 s intervals during the heating experiment performed with an SHS generator. When the inverter was operated with saturated steam supplied from the quartz heater, SHS with a temperature higher than 200 ◦C was generated in 10 s. After approximately 1 min, the SHS reached a temperature of 400 ◦C, and the modulated set temperature of the SHS could be controlled in order to perform TFT–LCD and wafer cleaning at the required temperature. Energies 2020, 13, x FOR PEER REVIEW 13 of 16

Inverter Current : [50 A/div.] Coil Voltage : [350 V/div.]

Energies 2020, 13, 6196 13 of 16 Energies 2020, 13, x FOR PEER REVIEW 13 of 16

Inverter Current : [50 A/div.] Coil Voltage : [350 V/div.] Coil Current : [100 A/div.] Inverter Pole Voltage : [200 V/div.] [1 us/div]

Figure 17. Experimental waveforms of the wafer cleaning equipment.

Figure 18 plots the internal and external temperatures of the Ti load and the input and output temperatures of the flowing steam measured using the TC; in this figure, the red line represents the SHS temperature, which is the steam output from the proposed cleaning equipment. In addition, Table 4 shows the temperature values measured at 10 s intervalsCoil duringCurrent the : [100heating A/div.] experiment performedInverter with Polean SHS Voltage generator. : When the inverter was operated with saturated steam supplied from the[200 quartz V/div.] heater, SHS with a temperature higher than 200 °C was generated[1 us/div] in 10 s. After approximately 1 min, the SHS reached a temperature of 400 °C, and the modulated set temperature of the SHS could Figurebe controlled 17. Experimental in order waveforms to perform of th theTFT–LCDe wafer cleaning and wafer equipment. cleaning at the required temperature. Figure 18 plots the internal and external temperatures of the Ti load and the input and output Switching Switching temperatures of Startthe flowing steam measured using the TC; in this figure, the red lineEnd represents the SHS temperature, which is the steam output Heating from thetime proposed68 s. cleaning equipment. In addition, 450 Table 4 shows the temperature values measured at 10 s intervals during the heating experiment performed with400 an SHS generator. When the inverter was operatedSteam Out with saturated steam supplied from the quartz heater, SHS with a temperature higher than 200 °C was generated in 10 s. After 350 approximately 1 min,Surface the SHS of Tireached Load a temperature of 400 °C, and the modulated set temperature of the SHS could300 be controlled in order to perform TFT–LCD and wafer cleaning at the required temperature. 250 Inside of Ti Load Switching200 Switching Start Steam In End

Temperature [°C] Temperature Heating time 68 s. 450150

400100 Steam Out

35050 0 10203040506070 Surface of Ti Load 300 Time [s]

250 FigureFigure 18.18. TemperatureTemperature graphgraph ofof thethe steamsteam heatingheating test.test. Inside of Ti Load 200 TableTable 4.4. TemperatureTemperature resultsresults ofof thethe steam steam heating heating test. test. Steam In Temperature [°C] Temperature 150 Heating Time Time Case Case 10 [s] 20 [s] 30 [s] 40 [s] 50 [s] 60 [s] 100 10 [s] 20 [s] 30 [s] 40 [s] 50 [s] 60 [s] Inside of Ti load 183.6 [°C] 191.9 [°C] 211.1 [°C] 235.3 [°C] 262.2 [°C] 288.9 [°C] Inside of Ti load 183.6 [◦C] 191.9 [◦C] 211.1 [◦C] 235.3 [◦C] 262.2 [◦C] 288.9 [◦C] 50 Surface of Ti load 209.3 [°C] 281.2 [°C] 307.8 [°C] 326.1 [°C] 347.5 [°C] 363.7 [°C] Surface of Ti load 209.3 [◦C] 281.2 [◦C] 307.8 [◦C] 326.1 [◦C] 347.5 [◦C] 363.7 [◦C] 0Steam 10203040506070in 144.8 [°C] 145.6 [°C] 146.9 [°C] 147.9 [°C] 148.7 [°C] 149.5 [°C] Steam in 144.8 [◦C] 145.6 [◦C] 146.9 [◦C] 147.9 [◦C] 148.7 [◦C] 149.5 [◦C] Steam out 201.2 [°C] 262.9 Time[°C] 310.7 [s] [°C] 344.5 [°C] 370.8 [°C] 400.1 [°C] Steam out 201.2 [◦C] 262.9 [◦C] 310.7 [◦C] 344.5 [◦C] 370.8 [◦C] 400.1 [◦C] Figure 18. Temperature graph of the steam heating test. 4. Conclusions Table 4. Temperature results of the steam heating test. In this study, cleaning equipment with a capacity of 2.2 kW was designed and fabricated using Heating Time pure Ti to clean workCase pieces such as semiconductor wafers. Enthalpy steam characteristics were 10 [s] 20 [s] 30 [s] 40 [s] 50 [s] 60 [s] analyzed to perform the cleaning process with an eco-friendly method using SHS. IH was employed to Inside of Ti load 183.6 [°C] 191.9 [°C] 211.1 [°C] 235.3 [°C] 262.2 [°C] 288.9 [°C] rapidly and efficiently generate SHS with a high temperature and pressure. A heating object with a Surface of Ti load 209.3 [°C] 281.2 [°C] 307.8 [°C] 326.1 [°C] 347.5 [°C] 363.7 [°C] Steam in 144.8 [°C] 145.6 [°C] 146.9 [°C] 147.9 [°C] 148.7 [°C] 149.5 [°C] Steam out 201.2 [°C] 262.9 [°C] 310.7 [°C] 344.5 [°C] 370.8 [°C] 400.1 [°C] Energies 2020, 13, 6196 14 of 16 structure in which the inner fluid path was made of pure Ti and the fluid could stay within a limited length for a long time was proposed. The output power of the cleaning equipment for generating SHS was derived based on mathematical calculations and simulations. In order to effectively increase the current flowing through the coil used for IH, the LCL resonance network was applied to the proposed power converter, and the parameter values were analyzed. The steam heating performance of the proposed cleaning equipment was verified by the experimental results, which demonstrate that the designed cleaning equipment generates SHS with a temperature of 200 ◦C in 10 s and that the SHS reaches 400 ◦C after 60 s. The proposed cleaning system can contribute to the successful commercialization of wafer cleaning by satisfying the requirements of wafer cleaning processes.

Author Contributions: Conceptualization, S.M.P. and J.-H.C.; Data curation, S.M.P., E.J., J.-H.K. and J.-H.C.; Formal analysis, E.J. and J.S.P.; Investigation, J.-H.K. and J.-H.C.; Project administration, B.K.L.; Software, S.M.P.and J.-H.K.; Supervision, J.-H.C. and B.K.L.; Validation, E.J. and J.S.P.; Visualization, E.J. and J.-H.K.; Writing–original draft, S.M.P. and B.K.L.; Writing–review & editing, S.M.P., J.S.P. and B.K.L. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea (No. 2018201010650A and No. 20192010106690). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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