Author:Leppäniemi, Jarmo
Title:Stepper Lithography Optimization
Stepperin optimointi litografiaan
Publication type:Master's thesis
Publication year:2013
Pages:104 s. + liitt. 10      Language:   eng
Department/School:Kemian tekniikan korkeakoulu
Main subject:Soveltava Materiaalitiede   (MT3001)
Supervisor:Franssila, Sami
Instructor:Torkkeli, Altti
Electronic version URL: http://urn.fi/URN:NBN:fi:aalto-201401101108
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Location:P1 Ark Aalto  2152   | Archive
Keywords:stepper
optical lithography
process window
DOF
photoresist profile
Bossung plot
stepperi
optinen litografia
prosessi-ikkuna
fokustaso
photoresistiprofiili
Bossung-kuvaaja
Abstract (eng):In this work we found optimum exposure parameters and determined a process window for a 1X Stepper tool used in MEMS fabrication.
Compared to older full-wafer imaging, Stepper lithography has a smaller exposure field that enables to maintain high Depth of Focus (DOF) even at submicron dimensions.
Even though its resolution limit is far from the modern step-and-scan tools used in IC industry today, its higher depth of focus makes it a suitable tool for MEMS, where lithography over severe topography is often mandatory.

For the optimization of our Stepper tool, photoresist profiles obtained with different exposure parameters and test masks were analysed with Scanning Electron Microscope (SEM) and optical microscope delegated to measurement of Critical Dimensions (CD).
Both profiles on top of thermal oxide and on top of bare silicon were investigated.
A Swing Curve was constructed to determine photoresist thickness by RPM.
Photoresist profiles were analysed with different focus and exposure energies to construct Bossung plots.
These Bossung plots were used to determine our process window.
Different types of submicron structures were investigated with the optimum parameters found.

Optimum photoresist thickness obtained in Swing Curve analysis was 1,67 µm for planar surfaces.
The process window of our Stepper tool is quite large, with DOF of 4 µm.
The optimum exposure energy of our Stepper tool was 230 mJ and optimum focus was -1,0 µm.
The isofocal CD was about 150 nm lower than CD on mask.
The resolution limit of our Stepper tools is slightly lower than 1,0 µm: submicron lines are exposed and defined clearly, but they are sloped and thus quite narrow at top.
After final oxide etching, photoresist profiles were almost identical on top of bare silicon and oxide.

Our Stepper tool has sufficient resolution and high DOF suitable for MEMS fabrication.
The optimal exposure parameters found in this work allow for smaller safety tolerances in chip design and thus more efficient use of silicon area.
This allows reducing the size and cost of the MEMS without reducing device performance.
In a farther future with more strict resolution requirements, either creative solutions or new kind of lithography tools are needed.
Abstract (fin):Tässä työssä tutkimme 1X Stepperin optimivalotusparametreja, tarkoituksena määrittää laitteen prosessi-ikkuna.
Stepperilitografian koko kiekkoaluetta pienempi valotusalue mahdollistaa paremman fokustason alle mikrometrin dimensioiden rakenteille.
Vaikka laitteemme resoluutio ei vastaa moderneja step-and-scan valottimia, on sen tarjoama korkea fokustaso erittäin hyödyllinen topografiaa sisältävissä valmistusprosesseissa, kuten monissa MEMS-tuotteissa.

Tutkimme sähköelektronimikroskoopilla (SEM) photoresistiprofiileja eri valotusenergioilla ja testimaskeilla löytääksemme optimaaliset valotusparametrit.
Arvioimme optimaalista resistinpaksuutta Swing Curve -kuvaajista.
Eri valotusenergioiden ja fokuksien näytteiden SEM:llä tehtyjen profiilimittausten perusteella piirsimme Bossung-kuvaajat, joiden pohjalta arvioimme Stepperin prosessi-ikkunaa.
Vertailimme profiileja paljaan piin ja termisen oksidin välillä sekä tutkimme yleisesti alle mikrometrin rakenteiden valotuksen onnistumista.

Optimiresistipaksuudeksi tasaiselle pinnalle määritettiin 1,67 µm.
Stepperin prosessi-ikkunan huomattiin olevan varsin suuri, fokustason ollessa 4,0 µm.
Optimivalotusenergiaksi määritettiin 230 mJ ja optimifokukseksi -1,0 µm.
Stepperin resoluutio oli alle mikrometrin: tätä pienemmät rakenteet valottuivat hyvin, joskin ne olivat varsin kapeita yläosastaan.
Resistiprofiilien huomattiin olevan samanlaisia paljaan piin ja oksidin päällä oksidietsausprosessivaiheen jälkeen.

Stepperillämme on riittävä fokustaso ja resoluutiokyvykkyys MEMS-tuotantoon.
Tämän työn tuloksena saatiin optimivalotusparametrit, jotka mahdollistavat tehokkaamman piirisuunnittelun ja piikiekkopinta-alan käytön.
Tämä mahdollistaa MEMS-tuotteiden koon ja hinnan pienentämisen.
Kaukaisemmassa tulevaisuudessa kiristyvät resoluutiotarpeet vaativat joko luovia laitekehitysratkaisuja tai uuden, modernimman valotuslaitteen.
ED:2014-01-19
INSSI record number: 48440
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