li-filters tsa khase tsa semiconductor

Hobaneng o hloka ho sebelisa li-filters tsa khase ts'ebetsong ea tlhahiso ea semiconductor? 

Li-filters tsa khase li bohlokoa ts'ebetsong ea tlhahiso ea semiconductor ka mabaka a 'maloa a bohlokoa:

1. Tloso e Silafetseng

Tlhahiso ea semiconductor e kenyelletsa mekhoa e mengata e hlokolosi moo le litšila tse nyane haholo,

joalo ka likaroloana tsa lerōle, mongobo, kapa masala a lik'hemik'hale, a ka ba le liphello tse kotsi. Li-filters tsa khase lia tlosa

particulate matter, litšila, le litšila tse tsamaisoang ke moea tse tsoang likhase tse sebetsang, ho netefatsa tikoloho e hloekileng.

le ho boloka botšepehi ba liphaphatha tsa semiconductor.

2. Ho boloka Maemo a Bohloeki a Phahameng

Indasteri ea semiconductor e hloka bohloeki bo phahameng haholo ba likhase tse sebelisoang, joalo ka ha litšila li ka khona

e lebisa ho bofokoli ho lisebelisoa tsa semiconductor. Li-filters tsa khase li thusa ho fihlela boleng ba khase e hloekileng haholo, ho thibela

tshilafatso le ho netefatsa botsitso le botshepehi ba dihlahiswa.

3. Thepa e Sireletsang

Lintho tse silafatsang tse ka har'a likhase li ke ke tsa lematsa liphaephe tsa semiconductor feela empa li ka senya le tse utloang bohloko

lisebelisoa tse sebelisoang ts'ebetsong ea tlhahiso, joalo ka li-chemical vapor deposition (CVD) reactors le

litsamaiso tsa etching. Lisebelisoa tsa khase li sireletsa mechine ena e theko e boima hore e se ke ea senyeha, e fokotsa kotsi ea

nako le tokiso e theko e boima.

4. Ho Thibela Tahlehelo ea Kotulo

Kotulo e bohlokoa tlhahisong ea semiconductor, moo liphoso li ka bakang tahlehelo e kholo tlhahisong.

Esita le karoloana e le 'ngoe kapa litšila tsa lik'hemik'hale li ka fella ka tahlehelo ea lihlahisoa, tsa ama tlhahiso le phaello.

Lisefe tsa khase li netefatsa hore likhase tsa ts'ebetso li hloekile, li fokotsa tšilafalo le ho fokotsa tahlehelo ea chai.

5. Ho netefatsa boleng ba Sehlahiswa

Ho lumellana le boleng ke tsa bohlokoa haholo tlhahisong ea semiconductor. Likhase tse silafetseng li ka baka

ho se lumellane, ho lebisang ho lisebelisoa tse sa tšepahaleng tsa semiconductor. Ka ho sebelisa li-filters tsa khase, bahlahisi ba ka

tiisetso ea hore sehlopha ka seng se kopana le litekanyetso tse thata tsa boleng tse hlokahalang, tse lebisang ho sesebelisoa se phahameng

tshebetso le bophelo bo bolelele.

6. Ho fokotsa nako ea ho phomola

Lintho tse silafatsang tse ka har'a likhase tse sebetsang li ka baka ho hloleha ha lisebelisoa, ho hloka tlhokomelo kapa ho nchafatsoa.

Ka ho sebelisa li-filters tsa khase, bahlahisi ba ka fokotsa nako e sa lebelloang, ho boloka ts'ebetso e sebetsang hantle, le

eketsa nako ea bophelo ea lisebelisoa tsa bohlokoa.

7. Ho lumellana ha Lik'hemik'hale

Likhase tse ngata tse sebelisoang mekhoeng ea semiconductor li sebetsa haholo kapa lia senya. Li-filters tsa khase li

e etselitsoeng ho mamella tikoloho ena e thata ea lik'hemik'hale ha e ntse e sefa litšila ka nepo, e netefatsa

ts'ebetso e sireletsehileng le e sebetsang.

Ka kakaretso, li-filters tsa khase li bohlokoa bakeng sa ho boloka bohloeki, ho ts'epahala le polokeho ea semiconductor.

ts'ebetso ea tlhahiso, e thusang ho fihlela lihlahisoa tsa boleng bo holimo, tse se nang sekoli tsa semiconductor ha li ntse li

hape ho sireletsa thepa ea bohlokoa.

Mefuta ea li-filters tsa khase ts'ebetsong ea tlhahiso ea semiconductor

Ts'ebetsong ea tlhahiso ea semiconductor, mefuta e fapaneng ea li-filters tsa khase e sebelisoa ho sebetsana le tse fapaneng

methati le liphephetso tse amanang le bohloeki ba khase le ts'ireletso ea lisebelisoa.

Mefuta ea li-filters tsa khase tse sebelisoang hangata e kenyelletsa:

1. Li-Filters tsa Particulate

*Morero: Ho tlosa likaroloana, lerōle, le litšila tse ling tse tiileng likhase tse sebetsang.

* Tšebeliso: Hangata e kenngoa ka mekhahlelo e fapaneng ho sireletsa li-wafers, liphaposi tsa ho sebetsa le lisebelisoa ho tloha tšilafalo ea likaroloana.

*Lisebelisoa: Ka tloaelo e entsoe ka sintered stainless steel, PTFE, kapa lisebelisoa tse ling tse netefatsang ho tšoarella le ho lumellana ha lik'hemik'hale.

2. Lisefe tsa limolek'hule kapa tsa lik'hemik'hale (Lisefe tsa Getter)

*Morero: Ho tlosa litšila tse itseng tsa limolek'hule, tse kang mongobo, oksijene, kapa metsoako ea lintho tse phelang, tse ka 'nang tsa e-ba teng likhaseng tse sebetsang.

* Tšebeliso: E sebelisoa ha ho hlokahala khase e hloekileng haholo, joalo ka nakong ea deposition kapa etching process.

*Lisebelisoa: Hangata e hahuoa ka mashala a kentsoeng, zeolite, kapa lisebelisoa tse ling tse etselitsoeng ho ts'oara litšila tsa molek'hule.

3. Lisefa tsa Khase tse Hloekileng Haholo

*Morero: Ho finyella litekanyetso tsa khase tsa ultra-high purity (UHP), tse bohlokoa bakeng sa mekhoa ea semiconductor moo ho se hloeke ho fokolang ho ka amang boleng ba sehlahisoa.

* Tšebeliso: Lisefe tsena li sebelisoa lits'ebetsong tse kang Chemical Vapor Deposition (CVD) le Plasma Etching, moo litšila li ka bakang likoli tse tebileng.

*Lisebelisoa: E entsoe ka tšepe e sa hloekang e nang le li-membrane tse khethehileng ho boloka botšepehi tlas'a khatello e phahameng le maemo a feteletseng.

4. Li-Filters tsa Khase tse ngata

*Morero: Ho hloekisa likhase sebakeng sa ho kena kapa pele ho ajoa mecheng ea tlhahiso.

* Tšebeliso: E behiloe holimo ka har'a sistimi ea phano ea khase ho sefa likhase ka bongata pele li fuoa lisebelisoa kapa li-reactor ka bomong.

*Lisebelisoa: Hangata li-filters tsena li na le bokhoni bo phahameng ba ho sebetsana le likhase tse ngata.

5. Lisefe tsa Khase tsa Point-of-Use (POU).

*Morero: Ho etsa bonnete ba hore likhase tse fanoang ho sesebelisoa se seng le se seng sa ho sebetsa ha li na litšila leha e le life.

* Tšebeliso: E kentsoe pele likhase li hlahisoa lisebelisoa tsa ts'ebetso, joalo ka liphaposi tsa etching kapa deposition.

*Lisebelisoa: E entsoe ka thepa e lumellanang le likhase tse sebetsang tse sebelisoang mekhoeng ea semiconductor, joaloka tšepe e sintered kapa PTFE.

6. Li-Filters tsa Khase tse Inline

*Morero: Ho fana ka filthara ea inline bakeng sa likhase tse tsamaeang ka har'a sistimi ea kabo.

* Tšebeliso: E kentsoe ka har'a mela ea khase lintlheng tsa bohlokoa, e fana ka filtration e tsoelang pele ho pholletsa le tsamaiso.

*Lisebelisoa: Sintered stainless steel kapa nickel ho netefatsa hore lik'hemik'hale li lumellana le likhase.

7. Lisefa tsa Khase tsa Mount Mount

*Morero: E tla kenngoa ka kotloloho holim'a likarolo tsa phanele ea khase ho tlosa likaroloana le litšila tsa limolek'hule.

* Tšebeliso: E tloaelehile libakeng tse thata, li-filters tsena li fana ka mokhoa o sebetsang oa ho hloekisa lits'ebetsong tsa bohlokoa.

*Lisebelisoa: Tšebeliso e phahameng ea tšepe e hloekileng bakeng sa ho tšoarella le ho lumellana le likhase tsa tlhahiso ea semiconductor.

8. Li-filters tsa Sub-Micron

*Morero: Ho sefa likaroloana tse nyane haholo, hangata tse nyane joalo ka li-sub-micron size, tse ntseng li ka baka mathata a bohlokoa lits'ebetsong tsa semiconductor.

* Tšebeliso: E sebelisoa lits'ebetsong tse hlokang boemo bo phahameng ka ho fetisisa ba ho hloekisa ho boloka phepelo ea khase e hloekileng haholo, joalo ka photolithography.

*Lisebelisoa: Lisebelisoa tsa tšepe tse sintered tse phahameng haholo kapa tsa ceramic tse ka tšoasang ka katleho esita le likaroloana tse nyenyane ka ho fetisisa.

9. Lisefa tsa Carbon tse kentsoeng

*Morero: Ho tlosa lintho tse silafatsang tsa tlhaho le likhase tse fetohang.

* Tšebeliso: E sebelisoa lits'ebetsong moo litšila tsa gaseous li hlokang ho tlosoa ho thibela tšilafalo ea liphaphatha kapa litšitiso tsa karabelo.

*Lisebelisoa: Lisebelisoa tsa khabone tse etselitsoeng ho khahla limolek'hule tsa tlhaho.

10.Sintered Metal Gas Filters

*Morero: Ho tlosa likaroloana le litšila ka katleho ha u ntse u fana ka matla a sebopeho le ho hanyetsa khatello e phahameng.

* Tšebeliso: E sebelisoa haholo ho pholletsa le mekhahlelo e mengata ea ts'ebetso ea semiconductor moo ho hlokahalang ho hloekisa ka matla.

*Lisebelisoa: Hangata e entsoe ka tšepe e sa hloekang ea sintered kapa lisebelisoa tse ling tsa tšepe ho mamella maemo a thata le lik'hemik'hale.

11.Lisefa tsa Khase tsa Hydrophobic

*Morero: Ho thibela mongobo kapa mouoane oa metsi hore o se ke oa kena ka har'a khase, e leng ntho ea bohlokoa lits'ebetsong tse itseng tse amehang esita le ho latela boholo ba mongobo.

* Tšebeliso: Hangata e sebelisoa lits'ebetsong tse kang ho omisoa ka sephaephe kapa ho chesoa ha plasma.

*Lisebelisoa: Li-membrane tsa Hydrophobic, tse kang PTFE, ho etsa bonnete ba hore likhase li lula li se na tšilafalo ea mongobo.

Mefuta ena e fapaneng ea li-filters tsa khase e khethoa ka hloko ho ipapisitse le thepa ea bona e ikhethileng, tšebelisano ea thepa, le ho tšoaneleha bakeng sa maemo a ikhethileng a lits'ebetso tsa tlhahiso ea semiconductor. Motsoako o nepahetseng oa li-filters o bohlokoa bakeng sa ho boloka boemo bo phahameng ka ho fetisisa ba bohloeki ba khase, ho netefatsa botsitso ba ts'ebetso, le ho thibela mefokolo ho lisebelisoa tsa semiconductor.

Lipotso tse ling ka li-filters tsa khase tsa semiconductor

FAQ 1:

Lisefa tsa khase ea semiconductor ke eng, 'me ke hobane'ng ha li le bohlokoa?

Li-filters tsa khase ea semiconductor ke likarolo tsa bohlokoa ts'ebetsong ea tlhahiso ea semiconductor.

Li etselitsoe ho tlosa litšila le litšila tse tsoang likhase tse sebetsang, joalo kaoksijene,

naetrojene, haedrojene, le likhase tse sa tšoaneng tsa lik’hemik’hale.

Litšila tsena li ka ama haholo boleng, chai, le ts'epahalo ea lisebelisoa tsa semiconductor.

Ka ho sefa melapo ea khase ka katleho, li-filters tsa khase tsa semiconductor li thusa ho:

1.Ho boloka bohloeki bo phahameng:

Netefatsa hore likhase tse sebelisoang tšebetsong ea tlhahiso ha li na litšila tse ka nyenyefatsang ts'ebetso ea sesebelisoa.

2.Thibela tšenyo ea thepa:

Sireletsa lisebelisoa tse hlokolosi tsa semiconductor hore li se ke tsa silafatsoa ke likaroloana le lik'hemik'hale, tse ka lebisang ho theolelo ea nako le litokiso tse theko e boima.

3. Ntlafatsa tlhahiso ea lihlahisoa:

Fokotsa mefokolo le ho hloleha ho bakoang ke litšila tse bakoang ke khase, e leng se hlahisang chai e ngata ea tlhahiso.

4.Enhance ho tšepahala ha sesebelisoa:

Fokotsa ho senyeha ha nako e telele ha lisebelisoa tsa semiconductor ka lebaka la litaba tse amanang le tšilafalo.

FAQ 2:

Ke mefuta efe e tloaelehileng ea li-filters tsa khase ea semiconductor?

Ho sebelisoa mefuta e mengata ea li-filters tsa khase ha ho etsoa semiconductor, e 'ngoe le e' ngoe e etselitsoe ho e tlosa

mefuta e itseng ea litšila.

Mefuta e tsebahalang haholo e kenyelletsa:

1. Li-filters tse ikhethileng:

Lisefe tsena li tlosa likaroloana tse tiileng, tse kang lerōle, likhoele le likaroloana tsa tšepe, ho tsoa khase.

Hangata li entsoe ka lisebelisoa tse kang li-filters tsa tšepe tse sintered, ceramic kapa membrane.

2.Lisefa tsa lik'hemik'hale:

Lisefe tsena li tlosa litšila tsa lik’hemik’hale, tse kang mouoane oa metsi, li-hydrocarbon le likhase tse senyang.

Hangata li ipapisitse le melaoana ea adsorption kapa absorption, li sebelisa lisebelisoa tse kang activated carbon,

li-sieve tsa molek'hule, kapa sorbents ea lik'hemik'hale.

3. Li-filters tse kopantseng:

Lisefa tsena li kopanya bokhoni ba li-filters tsa particulate le tsa lik'hemik'hale ho tlosa mefuta e 'meli ea

litšila. Hangata li sebelisoa lits'ebetsong tsa bohlokoa moo bohloeki bo phahameng bo leng bohlokoa.

FAQ 3:

Lisefe tsa khase ea semiconductor li khethoa le ho etsoa joang?

Khetho le moralo oa li-filters tsa khase ea semiconductor li kenyelletsa lintlha tse 'maloa, ho kenyelletsa:

* Litlhoko tsa tlhoekiso ea khase:

Boemo bo lakatsehang ba tlhoeko bakeng sa phallo e itseng ea khase bo lekanya bokhoni ba ho sefa le bokhoni ba sefe.

* Sekhahla sa phallo le khatello:

Bophahamo ba khase e lokelang ho sefshoa le khatello ea ts'ebetso li susumetsa boholo ba sefa, thepa le tlhophiso.

* Mofuta oa litšila le khatello ea maikutlo:

Mefuta e khethehileng ea litšila tse teng mocheng oa khase e laela khetho ea mecha ea litaba le boholo ba pore.

*Mocheso le mongobo:

Maemo a ts'ebetso a ka ama ts'ebetso ea sefe le nako ea bophelo.

*Litšenyehelo le tlhokomelo:

Litšenyehelo tsa pele tsa sefe le litlhoko tsa eona tse tsoelang pele tsa tlhokomelo li tlameha ho nkoa.

Ka ho shebisisa lintlha tsena ka hloko, baenjiniere ba ka khetha le ho rala lihloela tsa khase tse lumellanang le tse ikhethileng

litlhoko tsa ts'ebetso ea tlhahiso ea semiconductor.

Lisefe tsa Khase li Lokela ho Emeloa Hangata Hakae Tlhahisong ea Semiconductor?

Phetoho khafetsa ea li-filters tsa khase tlhahiso ea semiconductor e itšetlehile ka lintlha tse 'maloa, ho kenyelletsa le mofuta oa

ts'ebetso, boemo ba litšila, le mofuta o ikhethileng oa sefe e sebelisoang. Ka tloaelo, li-filters tsa khase li nkeloa sebaka kamehla

kemiso ea tlhokomelo ho thibela kotsi efe kapa efe ea tšoaetso,hangata likhoeling tse ling le tse ling tse 6 ho isa ho tse 12, ho itšetlehile ka maemo a tšebeliso

le likhothaletso tse tsoang ho moetsi oa sefe.

Leha ho le joalo, linako tsa ho fetola li ka fapana haholo ho latela maemo a ts'ebetso. Mohlala:

*Mekhoa ea Tšilafalo e Phahameng:

Lisefe li ka hloka ho nkeloa sebaka khafetsa haeba li pepesehetse maemo a holimo a

tšilafalo e itseng kapa molek'hule.

* Lits'ebetso tsa bohlokoa:

Mekhoeng e hlokang bohloeki bo phahameng haholo (mohlala, photolithography), li-filters li atisa ho nkeloa sebaka

ho etsa bonnete ba hore boleng ba khase ha bo senyehe.

Ho beha leihlo khatello ea phapang ho pholletsa le sefe ke mokhoa o tloaelehileng oa ho tseba hore na sefe se hloka ho nkeloa sebaka.

Ha litšila li ntse li bokellana, ho theoha ha khatello ho pholletsa le sefe ho ea eketseha, ho bontša ho fokotseha ha katleho.

Ho bohlokoa ho nkela li-filters sebaka pele ts'ebetso ea tsona e fokotseha, kaha tlolo efe kapa efe ea bohloeki ba khase e ka baka liphoso tse kholo,

fokotsa chai, esita le ho lebisa tšenyo ea thepa.

Li-filters tsa khase li entsoe ka lisebelisoa life bakeng sa lits'ebetso tsa semiconductor?

Lifilimi tsa khase tse sebelisoang lits'ebetsong tsa semiconductor li entsoe ka thepa e ka bolokang maemo a holimo a bohloeki

le ho mamella maemo a thata a fumanoang tlhahisong. Lisebelisoa tse tloaelehileng li kenyelletsa:

*Stainless Steel (316L): Thepa e sebelisoang ka ho fetisisa ka lebaka la ho hanyetsa lik'hemik'hale, matla a mochine, le

bokhoni ba ho etsoa ka boholo bo nepahetseng ba pore ho sebelisa theknoloji ea sintering. E loketse ho sefa ka bobeli reactive

le likhase tsa inert.

* PTFE (Polytetrafluoroethylene): PTFE ke thepa e inert ea lik'hemik'hale e sebelisetsoang ho sefa e sebetsang haholo kapa e senyang

likhase. E na le ts'ebeliso e ntle ea lik'hemik'hale le thepa ea hydrophobic, e etsang hore e be e loketseng bakeng sa ho se utloele mongobo

ditsamaiso.

*Nickel le Hastelloy:

Lisebelisoa tsena li sebelisoa bakeng sa lisebelisoa tsa mocheso o phahameng kapa bakeng sa mekhoa e amanang le lik'hemik'hale tse mabifi

moo tšepe e sa hloekang e ka senyehang.

* Ceramic:

Lisebelisoa tsa ceramic li sebelisoa bakeng sa lits'ebetso moo mocheso o feteletseng o hlokahalang, kapa bakeng sa sub-micron

tlhoekiso ea likaroloana.

Khetho ea thepa e itšetlehile ka mofuta oa khase, ho ba teng ha mefuta e sebetsang, mocheso, le

likarolo tse ling tsa ts'ebetso. Thepa e tlameha ho se sebetse ho netefatsa hore ha e hlahise litšila

kapa likaroloana ts'ebetsong, ka hona ho boloka maemo a bohloeki ba khase a hlokehang bakeng sa tlhahiso ea semiconductor.

Karolo ea Lisefe tsa Point-of-Use (POU) ke Efe ho Semiconductor Manufacturing?

Li-filters tsa Point-of-Use (POU) li bohlokoa tlhahisong ea li-semiconductor, kaha li netefatsa hore likhase li hloekisoa hang pele.

ho kenya lisebelisoa tsa ts'ebetso. Lisefe tsena li fana ka tšireletso ea ho qetela khahlanong le litšila tse ka 'nang tsa kena ka har'a khase

nakong ea polokelo, lipalangoang, kapa kabo, ka hona ho matlafatsa botsitso ba ts'ebetso le boleng ba sehlahisoa.

Melemo ea Bohlokoa ea Lisefe tsa POU:

*E behiloe haufi le lisebelisoa tsa bohlokoa (mohlala, etching kapa deposition chambers) ho thibela tšilafalo hore e se fihle ho sephaphatha.

*Tlosa litšila tsa likaroloana le limolek'hule tse ka hlahisoang ke tsamaiso ea khase kapa ho pepeseha tikolohong.

* Netefatsa hore boleng ba khase bo phahameng ka ho fetesisa bo tlisoa sesebelisoa sa ts'ebetso, ho sireletsa lisebelisoa le ho ntlafatsa boleng ba lisebelisoa tse entsoeng.

* Fokotsa ho feto-fetoha ha tšebetso, eketsa chai, le ho fokotsa maemo a sekoli.

*E bohlokoa haholo libakeng tse tsoetseng pele tsa semiconductor moo le litšila tse nyane li ka amang tlhahiso le ts'epahalo ea sehlahisoa haholo.

Li-filters tsa Khase li Thibela Nako ea ho Theola lisebelisoa joang mekhoeng ea Semiconductor?

Li-filters tsa khase li thibela ho fokotseha ha lisebelisoa lits'ebetsong tsa semiconductor ka ho etsa bonnete ba hore likhase tsa ts'ebetso li lula li se na letho.

litšila tse ka bakang tšenyo ea thepa ea tlhahiso. Ho etsoa ha semiconductor ho kenyelletsa tšebeliso e phahameng haholo

lisebelisoa tse hlokolosi, ho kenyeletsoa liphaposi tsa polokelo, mechini ea plasma etching, le sistimi ea photolithography.

Haeba litšila tse joalo ka lerōle, mongobo, kapa litšila tse sebetsang li kena mochining ona, li ka baka mathata a mangata,

ho tloha ho li-valve tse koalang le li-nozzles ho isa ho bokaholimo bo senyehileng ba liphaephe kapa likarolo tse ka hare tsa reactor.

Ka ho sebelisa li-filters tsa khase tsa boleng bo holimo, bahlahisi ba thibela ho kenngoa ha litšila tsena, ho fokotsa monyetla oa ho

tlhokomelo e sa reroang le ho senyeha ha lisebelisoa. Sena se thusa ho boloka linako tse tsitsitseng tsa tlhahiso, ho fokotsa

nako e theko e boima haholo, le ho qoba litšenyehelo tse kholo tse amanang le tokiso kapa tse ncha.

Ntle le moo, li-filters tse hlokometsoeng hantle li thusa ho lelefatsa nako ea bophelo ea likarolo tsa bohlokoa, joalo ka li-control control, li-valve le li-reactors,

ka hona ho matlafatsa ts'ebetso e akaretsang le phaello ea ts'ebetso ea tlhahiso.

Kahoo ka mor'a ho hlahloba lintlha tse ling mabapi le li-filters tsa khase ea semiconductor, haeba u ntse u e-na le lipotso tse ling.

U itokiselitse ho ntlafatsa ts'ebetso ea hau ea tlhahiso ea semiconductor ka litharollo tsa boleng bo holimo tsa ho sefa khase?

Ikopanye le HENGKO kajeno bakeng sa tataiso ea litsebi le litharollo tse ikhethileng ho fihlela litlhoko tsa hau.

Kamora ho hlahloba lintlha tse ling mabapi le sefe ea khase ea semiconductor, haeba u na le lipotso tse ling?

U itokiselitse ho ntlafatsa ts'ebetso ea hau ea tlhahiso ea semiconductor ka litharollo tsa boleng bo holimo tsa ho sefa khase?

Ikopanye le HENGKO kajeno bakeng sa tataiso ea litsebi le litharollo tse ikhethileng ho fihlela litlhoko tsa hau.

Re romelle lengolo-tsoibila hoka@hengko.combakeng sa lintlha tse ling.

Sehlopha sa rona se teng ho u thusa ho ntlafatsa katleho ea hau ea tlhahiso le boleng ba sehlahisoa.