Focused electron beam induced deposition (FEBID) is a direct-write 3D nanofabrication technique, which works on the principle of electron-induced molecular decomposition. In FEBID, a high-energy focused electron beam dissociates the precursor molecules adsorbed on a substrate into volatile and non-volatile species. The volatile species can be pumped away from the substrate by vacuum pumps and the non-volatile species will be deposited on top of the substrate. Although FEBID is a potentially useful 3-D direct write nanofabrication technique, it still faces some critical challenges such as deposit impurity and lateral broadening of deposited structures. These challenges are mainly due to the spatial distribution of secondary electrons (SEs, electrons with energy < 50 eV) outside the focal point of the primary electron beam and the incomplete electron- induced decomposition of precursor molecules. Several dissociation mechanisms are active in the low-energy SE energy range: dissociative electron attachment (DEA), dissociative ionization (DI), dipolar dissociation (DD) and neutral dissociation (ND). In particular, DEA and DI have been shown to be relevant to the deposition of several FEBID precursors. Understanding electron interactions with precursor molecules in this low-energy SEs range can therefore aid in addressing the challenges facing FEBID. One potential approach to realize this is combining gas phase and surface studies of FEBID precursor molecules.
In gas phase studies, an electron beam of variable energy (0 - 80 eV) is crossed with an effusive molecular beam of FEBID precursor molecules under single electron- molecule collision conditions, and the subsequently formed ionic species are detected using a quadrupole mass spectrometer. In surface studies, FEBID precursor molecules are adsorbed onto a cold substrate (at -120 0C) under ultrahigh vacuum (UHV) condi- tions and irradiated with a broad beam of electrons (energy ~500 eV) from a flood gun. Different changes occur in the adsorbed precursor film due to electron-induced reactions can be monitored using X-ray photoelectron spectrometry (XPS), while the species desorbed from the film can be detected using a Mass spectrometer (MS) attached to the UHV chamber.
This thesis focuses on gas phase and surface studies of the bimetallic precursor molecules HFeCo3(CO)12 and H2FeRu3(CO)13. The bimetallic nanostructures have many applications mainly in the semiconductor industry. The conventional method to fabricate bimetallic nanostructures in FEBID is mixing two different metal centered precursor molecules using a dual or multichannel gas injection system. However, this method has difficulties in reproducing the exact deposited structure and there is only limited control over the composition of the deposits. These difficulties in FEBID can be overcome by using bimetallic precursor molecules like HFeCo3(CO)12 and H2FeRu3(CO)13. Although these are both bimetallic and structurally similar precur-
sor molecules, their performance in FEBID is significantly different. This different behavior of HFeCo3(CO)12 and H2FeRu3(CO)13 in FEBID motivated us to conduct gas phase and surface studies of their electron-induced dissociation behavior. Gas phase studies presented in this thesis mainly contain DEA and DI of HFeCo3(CO)12 and H2FeRu3(CO)13. Gas phase studies also included quantum chemical calculations to identify most probable electron attachment fragmentation channels in HFeCo3(CO)12 and H2FeRu3(CO)13. The surface study part mostly discussed the electron induced bond breaking reactions of surface adsorbed HFeCo3(CO)12 and H2FeRu3(CO)13. The observations made from gas phase and surface study are used in this thesis to discuss why these two bimetallic precursor molecules behaved differently in FEBID reactions.
Furthermore, in this thesis, the gas phase study of trisilacyclohexane (TSCH) and the FEBID study of TSCH, dichlorosilacyclohexane (DCSCH) and silacyclohexane (SCH) are discussed in relation to their different behaviour towards DEA and DI. The precursor molecules SCH and TSCH are completely inert to DEA within the sensitivity of our experimental set up, however DCSCH is active to DEA and all these precursor molecules are active to DI. How the inert behaviour of SCH towards DEA compared to DCSCH reflects in the electron beam induced deposition of these precursor molecules will be discussed in this thesis. The questions like ’does this inert behaviour of SCH compared to DCSH influence significantly to the electron induced growth dynamics of these precursor molecules?, comparatively more silicon content in the TSCH precursor will have any advantage in FEBID?’ will be addressed in this thesis.
Örprentun með skörpum rafeindageisla (e. Focused Electron Beam Induced Deposition,
FEBID) er aðferð sem nota má til að prenta þrívíða strúktúra á yfirborð með rafeindahvötuðu
niðurbroti svokallaðra forverasameinda. Það eru þó enn þónokkrar hindranir
sem standa ı vegi fyrir notkun FEBID, þar á meðal óhreinindi í útfellingum og og
breikkun þeirra. Þessar hindranir má að stúru leyti rekja til dreifingar lágorkurafeinda
(rafeinda með hreyfiorku undir 50 eV) út fyrir brennipunkt rafeindageislans og ófullkomið
niðurbrot á forverasameindunum. Nokkrar rafeindadrifnar niðurbrotsleiðir eru
mögulegar á lágorkusviðinu: rjúfandi rafeindarálagning (Dissociative Electron Attchment,
DEA), rjúfandi jónun (e. Dissociative Ionization, DI), tvískautssundrun (Dipolar
Dissociation, DD) og hlutlaus sundrun (e. Neutral Dissociation, ND). Við hagnýtingu á
FEBID getur vitneskja um hvörf drifin af lágorkurafeindum skipt sköpum. Ein möguleg
nálgun við rannsóknir á þessum ferlum er samtvinnaðar mælingar á forverasamendum í
gasfasa og á yfirborðum.
Tilraunir í gasfasa eru svokallaðar þvergeislatilraunir þar sem rafeindageisli (hreyfi-
orka rafeindanna er 0-80 eV) og sameindageisli mætast undir réttu horni. Þessar
mælingar eru gerðar við lágan þrýsting, sem tryggir að hver sameind víxlverkar einungis
við eina rafeind. Jónir sem myndast við þessa víxlverkan eru greindar með massagreini.
Í yfirborðsmælingum aðsogast forverasameindir á kalt yfirborð (við 153 K)
við mjög lágan þrýsting og eru síðan geislaðar með breiðum rafeindageisla (~500 eV).
Þær breytingar sem eiga sér stað á efnunum á yfirborðinu eru reyndar með Röntgen
ljósröfunarmæli (X-ray Photoelectron Spectrometer, XPS) og þau efni sem losna frá
yfirborðinu eru mæld með massagreini.
Í thessari ritgerð er sjónum beint að gasfasa- og yfirborðsmælingum á forverasameindunum
HFeCo3(CO)12 and H2FeRu3(CO)13. thó báðar sameindirnar séu tvímálmar,
með svipaða uppbyggingu og lögun, hefur reynslan af þeim í FEBID verið gjörólík.
Því er samanburður milli þeirra í gasfasa- og yfirborðsmælingum áhugaverður. Gasfasatilraunirnar
sem hér eru kynntar eru aðallega DEA og DI á HFeCo3(CO)12 and
H2FeRu3(CO)13. Að auki voru skammtafræðilegir útreikningar á sameindunum framkvæmdir
til að meta líklegustu niðurbrotsleiðir. Yfirborðsmælingarnar voru síðan
notaðar til að athuga hvaða áhrif yfirborðið hefur tengjarof á sameindunum. Niðurstöður
mælinganna sýna hvers vegna þessar tvær sameindir eru svo ólíkar í FEBID.
Að auki eru FEBID mælingar á trisilacyclohexane (TSCH), dichlorosilacyclohexane
(DCSCH) og silacyclohexane (SCH) og niðurbrot þeirra í DEA og DI borið saman.
Forverasameindirnar SCH og TSCH sýna ekkert niðurbrot í DEA en DCSCH sýnir
talsvert niðurbrot. Allar sameindirnar brotna niður við DI. Athyglinni er beint að
spurningum eins og "hefur óhvarfgirni TSCH m.t.t. DEA samanborið við DCSH áhrif á
FEBID?og "Hefur aukið kísilsinnihaldi í TSCH einhver áhrif í FEBID?".
