Conversion of a Patterned Organic Resist into a High Performance

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Conversion of a Patterned Organic Resist Into a High Performance Inorganic Hard Mask for High Resolution Pattern Transfer. Jean-Francois de Marneffe, Boon Teik Chan, Martin Spieser, Guy Vereecke, Sergej Naumov, Danielle Vanhaeren, Heiko Wolf, and Armin W Knoll ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b05596 • Publication Date (Web): 12 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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ACS Paragon Plus Environment

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Conversion of a Patterned Organi Resist Into a High Performan e Inorgani Hard Mask for High Resolution Pattern Transfer.

Jean-François de Marnee,∗ † Boon Teik Chan,∗ † Martin Spieser,‡ Guy Veree ke,† ,

,

Sergej Naumov,¶ Danielle Vanhaeren,† Heiko Wolf,§ and Armin W. Knoll§ †ime

v.z.w., Leuven, B-3001 Belgium

‡SwissLitho ¶Leibniz

AG, Zuri h, CH-8005 Switzerland

Institute of Surfa e Engineering - IOM, Leipzig, 04318 Germany

§IBM

Resear h - Zuri h, Rüs hlikon, CH-8803 Switzerland

E-mail: marneeime .be; BT.Chanime .be

Abstra t Polyphthalaldehyde is a self-developing resist material for ele tron beam and thermal S anning Probe Lithography (t-SPL). Removing the resist

in-situ

(during the

lithography pro ess itself) simplies pro essing and enables dire t pattern inspe tion, however, at the pri e of a low et h resistan e of the resist. To onvert the material into a et h resistant hard mask we study the sele tive y li inltration of tri-methylaluminum (TMA) / water into polyphthalaldehyde. It is found that TMA diuses homogeneously through the resist, leading to material expansion and formation of aluminum oxide on urrent to the exposure to water and the degradation of the polyphthalaldehyde polymer. The plasma et h resistan e of the inltrated resist is signi antly improved, as well as its stability. Using a sili on substrate oated with 13nm sili on nitride and 7nm ross-linked polystyrene, high resolution polyphthalaldehyde patterning 1


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is performed using t-SPL. After TMA/H2 O inltration, it is demonstrated that pattern transfer into sili on an be a hieved with good delity for stru tures as small as 10 nm, enabling >10x ampli ation and low surfa e roughness. The presented results demonstrate a simplied use of polyphthalaldehyde resist, targeting feature s ales at nanometre range, and suggest that tri-methyl-aluminum inltration an be applied to other resist-based lithography te hniques.

Keywords thermal s anning probe lithography, polyphthalaldehyde, sequential inltration synthesis, tri-methyl-aluminum, plasma et hing Over the past few de ades, dimension s aling in nanofabri ation was enabled by the progress of opti al photolithography, allowing to print ever smaller features sizes. To a hieve higher resolution, opti al photolithography has been using light sour es with wavelength redu ing from 365 nm, to 248 nm, 193 nm immersion and re ently 13.6 nm (Extreme Ultra Violet - EUV). Figure 1 illustrates the evolution of the thi kness of organi photoresist for dierent type of light sour es that were used in the last three de ades. Typi ally, 193 immersion may require photoresist lm thi kness in the range of 105 nm whi h be omes 80-90 nm after development. EUV resist thi kness is in the range of 45 nm and re ent EUV inorgani resist an be in the range of sub-20 nm. The photoresist lm thi kness needs to s ale with printed dimensions, so as to keep a reasonable aspe t ratio (resist prole) and be

ompatible with short depth of fo us required for high resolution. Despite major progress in

hemi ally amplied photoresists, nowadays their resolution remains limited to around 25 nm half-pit h at best, using highly omplex EUV lithography. Using opti al te hniques, a

ess to half-pit h dimensions smaller than 25 nm ne essitate to ombine 193 nm lithography with self-aligned doubling or quadrupling te hniques, requiring multiple pro essing steps (deposition of multiple masking lms, et hing, omplementary ut masks) and very areful optimization. 2


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10 Organic mask thickness (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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365nm I-Iine

1000

248nm

M

193i

100 DSA

Optical Lithography

193nm

EUV EUV - inorganic

10

Scanning Probe Lithography

t-SPL

1 10

100 1000 Feature dimension (nm) F

10

4

1

Figure 1: Evolution of the organi lithographi ally patterned resist lm thi kness (nm) with respe t to feature dimension that an be printed (nm) for various lithography methods (data from IMEC). In the past few years, several novel probe-based lithography methods were developed to a hieve single-digit nanometer patterning with high alignment a

ura y and low ost. Gar ia et al.

presented an overview of s anning probe lithography te hniques apable of sub-20 nm

pit h patterning. 1 Among these methods, thermal S anning Probe Lithography (t-SPL), 2,3 uses a heatable AFM antilever doing lo al thermal de omposition of a thermally sensitive resist. The AFM antilever omprises a heatable tip as small as 3 nm in radius that delivers high temperature lo ally. It is a robust method whi h ould re ently demonstrate 9 nm half-pit h patterning in resist and 14 nm half-pit h transfer into Si. 4 Until now, the optimal resist for t-SPL is polyphthalaldehyde (PPA), whi h shows self-amplied depolymerization around 150°C (the hemi al stru ture of PPA is shown as inset of Fig. 3). The PPA resist was rst introdu ed in 1983 as a rst generation of the then new hemi ally amplied resists. 5 Unfortunately, PPA was dismissed due to its high temperature instability, whi h is not suitable in the mass manufa turing setting. 3


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Conventional t-SPL lithography uses a thin PPA lm, into whi h lines are patterned 4-10 nm in depth. The PPA lines are then transferred into an underlying PMMA1 layer, then into 2-3 nm SiO2 and 20-50 nm spin-on arbon (HM8006). The et h sele tivity between PPA and PMMA is 1:1, due to similar omposition, while the et h sele tivity between PPA and SiO2 is 1:2. In order to in rease the resolution, thinner PPA is required to a hieve smaller pit h dimension without line distortion by the oni al upper part of the probe. The limited et h sele tivity between PPA and underlying layers (PMMA, SiO2) requires equivalent s aling down of the PMMA and SiO2 lm thi knesses, whi h is a real hallenge. In other words, a thinner PPA lm results in a lower et h resistan e budget for subsequent pattern transfer. Thus, t-SPL fa es te hni al hallenges to demonstrate very high resolution patterning due to the limitation of the PPA and tri-layer (PMMA/SiO2 /HM8006) sta k that is urrently used. It is therefore of high interest for t-SPL patterning to improve the et h resistan e of the PPA. One possibility to in rease the et h resistan e of the PPA is to use a onversion pro ess of PPA into a more et h resistant ompound. In addition, su h a onversion pro ess enables a purely polymeri resist sta k, whi h improves t-SPL resolution below 10 nm and in reases tip enduran e by avoiding onta t to the hard SiO2 interfa e. 4 Re ently published work relies on the inltration of a metal oxide into a thin polymer lm, forming a bi- omponent inorgani in-organi omposite material leading to a larger et h durability. The inltration of the metal oxide has been demonstrated on Dire ted Self-Assembly (DSA) of diblo k o-polymer (dBCP) omposite material. 611 In DSA, ommon dBCP used is Poly(styrene-blo k-methyl metha rylate) (PS-b-PMMA). The inltration pro ess, so- alled Sequential Inltration Synthesis (SIS), uses a y li exposure to tri-methyl-aluminum Al(CH3 )3 pre ursor (TMA) and water vapor in a modied atomi layer deposition (ALD) mode. It was demonstrated that the TMA does only rea t with the PMMA part of the BCP, but not with Polystyrene end. The TMA pre ursor diuses all the way down into the PMMA domain, and the metal oxide 1 The

PMMA polymer lm underneath the PPA lm a ts as heat buer, and allows to prote t the tip of the AFM antilever, extending its lifetime.

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formed in the bulk of the organi polymer redu es its degradation during plasma exposure. 12 Referring to the possible me hanism s heme for a Lewis-a idi TMA rea tion with the Lewis-basi arbonyl group in PMMA proposed by Parsons

et al.

, 13 ALD inltration of

TMA is also feasible for polymers that ontain oxygen in their polymer ba kbone. Thus, the polya etal ba kbone of PPA is expe ted to be prone to sele tive TMA inltration.

Results and Dis ussion Sequential Inltration Synthesis Applied to Polyphthalaldehyde Ultra-Thin Films The rst part of this study investigates the inltration of PPA resist using sequential exposures to TMA and H2 O vapors. The sample preparation onsisted rst in dissolving PPA (linear PPA powder from Sigma-Aldri h/Mer k ((C8 H6 O2 )n , produ t number 900029) into anisole at 0.75 % w, then stirring for 1 hour. After that, the solution was spin- oated onto 300 mm bare Si wafers, for 40s at 3000 rpm, without post-bake, leading to a nal thi kness of approximately 11-12 nanometers. SIS was performed in an ASM A412 ALD rea tor. Alumina was formed using binary rea tions of TMA and water at various temperatures, within the PPA lms. The SIS was performed following a sequen e inspired from literature 10,11,14 the rst pre ursor, TMA, was admitted into the rea tor in predetermined on entrations and for a predetermined exposure period. Afterwards, the hamber was purged with nitrogen for a few se onds. The above pro edure was repeated for H2 O. This sequen e was repeated

y li ally as in onventional ALD. However, higher pre ursor partial pressures are used in order to enhan e their diusion into the polymer bulk. Figure 2 shows the hange in thi kness and elemental omposition after sequential inltration synthesis on PPA lms, using an inltration temperature of 80°C. The thi kness growth observed in gure 2(a) is signi ant,

i.e.

the thi kness more than doubles after 8

y les. Also, the thi kness in rease is linear for the rst few y les, then starts to saturate for 6 and 8 y les indi ating a me hanism hange. The level of Aluminum in orporation an

5


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(a)

Element / bond (%)

(c)

50

Al2p

45

O1s

40

Si2p

35

C1s C-C/C-H

30

C1s C-O/C=O

25 20

2x cycles

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30

40

Bulk sensitive

(d)

(b)

100

Bond/element percentage (%)

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50

60

70

80

90

Collection angle (o)

Surface sensitive

0

90

22.02

26.91

28.92

65.25

64.15

3.29 4.54

2.2 4.72

80

Al O C-O/C=O C-C/C-H

48.35

70 60 52.38

50

9.78

40 30 41.86

20 10 0

6.77

18.83

Pristine PPA

SIS 6x

SIS 6x+O2 pl. SIS 6x+O2 bake

Sample

Figure 2: Thi kness hange and evolution of elemental omposition as a fun tion of SIS

y les (80°C) applied to PPA resist. (a) full dots: thi kness as measured by spe tros opi ellipsometry using a single layer Cau hy model; stars: estimated Al2 O3 thi kness dedu ed from Al density as measured by elasti re oil dete tion (ERD) and assuming a stoi hiometri Al2 O3 with bulk density of 3890 kg/m3 (measured Al density is written next to ea h point). During SIS, the thi kness grows rst linearly up to 4 y les, then slows down indi ating a hange of me hanism; after post-inltration treatments, the thi kness de reases, lose to pristine value (annealing in air at 300°C). Ellipsometry data was onrmed by X-ray ree tometry. (b) evolution of the lm density and omparison with aluminum load (SIS at 25°C) as measured by XPS (21◦ exit angle). ( ) elemental omposition prole as measured by angle-resolved X-ray photoele tron spe tros opy, after 2 SIS y les. (d) elemental/bond density of pristine PPA resist, after SIS (6 y les, 80°C), followed by treatment with an Ar/O2 plasma for 20s and after annealing in air for 15 minutes at 300°C (21◦ XPS).

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be pre isely assessed by elasti re oil dete tion, as shown in gure 2(a). It shows also a linear in rease of aluminum areal density for the rst y les, then saturation starts after 6 y les, whi h orrelates with the thi kness trend. The equivalent Al2O3 thi kness an be al ulated from the areal density of Al atoms, assuming a nominal stoi hiometry and bulk density. The

al ulated equivalent Al2O3 lm thi kness in reases from 2.26 nm after 2 y les to 6.97 nm after 8 y les,

i.e.

an average growth rate of ∼ 1nm per y le, one order of magnitude larger

than what is typi ally measured for the onventional, two-dimensional ALD of Al2 O3 . An interesting observation is made in ref. 15 for the ALD of Al2O3 on 400nm thi k PMMA: during the rst 10-15 y les, high absorption and desorption of TMA is observed, orresponding to the formation of ∼ 9 TMA monolayers/ y le. Only after this initiation period of 15 y les, a linear Al2O3 ALD growth rate (lower) is rea hed. The level of TMA uptake is asso iated with the TMA diusion rate and hemi al solubility in the polymer. On e the bulk free volume is saturated, the Al2 O3 lm seals the surfa e, loses, and regular ALD layer-by-layer growth start. Coming ba k to the inltration of PPA, the density al ulated from the mass gain (see gure 2(b)) in reases by ∼ 50% after 8 y les, rea hing an absolute value of ∼ 2000 kg/m3 , signi antly below the bulk Al2O3 density (3890 kg/m3), whi h is orrelated with the still large arbon ontent of the lm, as observed by XPS (see Fig. 2(d)). The in rease in density orrelates very well with the aluminum intake, as shown by the overlap of the density and Al (%) urves. Figure 2( ) shows the angle-resolved XPS of PPA after 2 y les of SIS. The elemental proportion of aluminum, around ∼ 16%, is onstant irrespe tive of the probing angle, indi ating that Al diuses deep into the bulk of the PPA lm,

i.e.

for a low

number of SIS y les, the PPA has su ient free volume remaining to allow deep penetration of TMA. Similar proles were obtained for all (up to 8) SIS y les, without indi ation of a pure alumina layer on the surfa e. It an therefore be on luded that the saturation behavior observed here for the inltration of PPA by TMA an be explained by a similar me hanism as for PMMA: for the rst 1-4 y les, the PPA polymer is degraded, leading to the formation of Al2 O3 embedded nu leates; for further y les (up to 8) these nu leates grow inside the 7


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layer, i.e. TMA an still diuse into the bulk of the lm and rea t; although a larger number of y les was not tested, we an anti ipate that at some point, regular two-dimensional ALD will pro eed, onned to the surfa e, ertainly at a lower growth rate than initially. After inltration, a lot of arbon remains in the lm, as shown by gures 2( ) and 2(d). In order to get some insight into the topology of the residual arbon, after SIS, some treatments were performed on the samples, aiming at onsuming the arbon in a purely thermal (annealing in air) or rea tive ion assisted pro ess (plasma treatment in oxidizing Ar/O2 or redu ing N2 /H2 mixtures). Figure 2(a) shows that annealing leads to the most important thi kness redu tion, orrelated to the almost omplete removal of arbon from the lm (see 2(d)). Using an Ar/O2 plasma, the thi kness redu tion is lower, and more residual C is found; this is most likely aused by some sputter-indu ed sealing of the lm, slowing down the rea tion between oxygen radi als and C atoms. The almost omplete removal of C by annealing in air indi ates that the lm topology after SIS allows an easy degas of the C residues,

i.e.

the

formed Al2 O3 is intimately mixed with remaining C in a o- ontinuous matrix. Finally, the starting PPA surfa e roughness, Ra ∼ 0.34 nm, was not signi antly modied by the SIS pro ess (Ra ∼ 0.38 nm after 8 y les). The hemi al hanges of the PPA lm during inltration are shown in Fig. 3, representing the infra-red response of the various bonds present in the lms, pristine PPA and after SIS. First, as ompared to the ba kground Si signal, the PPA does bring additional peaks whi h

an be all orrelated to spe tra from existing literature, see ref. 16. As a fun tion of the in reasing number of SIS y les, there is some in remental hange of the FTIR spe tra, whi h starts to saturate after 4 y les (for 6 and 8 y les, most FTIR peaks almost overlap). After peaks allo ation, it appears that the SIS pro ess lead to the step-by-step formation of Al2 O3 (see multiple peaks in the [700-1100℄ m−1 range 1719 ), along with PPA peaks fading out. The aromati ring present in PPA is responsible for the small peak present at ∼3050 m−1, whi h be omes invisible under the broad peak appearing at ∼3400 m−1 after more than one SIS y le. The presen e of phthalaldehyde monomer (PA) an be 8


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easily dete ted as a strong vibrational feature for arbonyl (C=O) at 1681 m−1 . 19 The small peak at ≈ 1680 m−1 in the PPA referen e spe trum reveils the presen e of some PA, whi h still remains visible as a shoulder during the rst two inltration y les and then vanishes. Together with Al2O3 , some physisorbed water be omes visible, namely in the [1350-1600℄ m−1 region orresponding to H2 O oordinatively bonded to Al ions 17 indi ating the presen e of AlOH spe ies. Also, the very large band entered at ∼3400 m−1 results from a superposition of vibration bands of bonded hydroxyl groups, isolated OH groups, and stret hing vibrations of adsorbed water mole ules (from moisture). Finally, some spe i C-H stret hing orresponding to methyl be omes visible at ∼2975 m−1 , indi ating the presen e of rea ted methyls (C-CH3 ) or unrea ted methyls (Al-CH3). 17 Separate annealing tests indi ate that moisture an be partially removed by annealing at 300°C, however it is known that temperatures around 600°C are required for omplete removal. Finally, the temperature dependen e of the SIS pro ess has been investigated in the range of 25-80°C, indi ating that the uptake of aluminum, forming Al2 O3 , is redu ed by half at 25°C ompared to higher temperatures; this is orrelated to a higher nal arbon ontent. XPS indi ates, for a xed number of y les, that the amount of dete ted C-O/C=O bonds is higher at low temperature, demonstrating a lower rea tivity at low T. A detailed explanation of the me hanisms of inltration would require some in-situ FTIR probing the PPA lm's omposition after ea h TMA and H2 O steps of ea h SIS y le, whi h was not available at our fa ility. Nevertheless, based on the nal produ t omposition (see Figs. 2 and 3), some rea tion path an be envisioned. The SIS rea tion, with PMMA, o

urs at a single arbonyl group. For PPA, there are two possible rea tive -C-O-C- sites in the polymer main hain, 13 one lo ated in the ve-membered ring (site A) and the other forming the link between monomer units (site B) - see inset of Fig. 3. Here it is assumed that, at start, the majority of rea tions o

ur at the entral parts of the polymer, due to their larger density ompared to polymer ends. The TMA is a Lewis a id, whi h an rea t with sites A and B. As shown by quantum hemi al al ulation (see supporting information), both 9


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orresponding rea tion paths are energeti ally favorable, with a slight theoreti al advantage to site A. Both rea tions (A and B) lead to hain s issioning and depolymerisation of one of the fragments after rea tion with water, while the other fragment is stable after methylation by TMA (see Fig. 2 in SI). This leads to the formation of phthalaldehyde monomers whi h

an rea t again with TMA as Lewis a id or leave the surfa e. Looking at the hemi al

hanges observed in gure 3 and in XPS data (gure 2( )), the degradation of PPA resist and loss of organi material is learly visible. The Lewis-a id rea tion des ribed here is dierent from the photoa id-indu ed PPA degradation as des ribed in referen e 20, whi h is based on a two-step protonation leading ultimately to the PPA unzipping rea tion by proton transfer along the hain. Finally, it is worth to note that the SIS rea tion des ribed here degrades the PPA polymer, ontrary to the ase of PMMA, whi h maintains a ontinuous main hain after SIS (only redu ing the ester group). The main benet of aluminum inltration is expe ted during pattern transfer,

i.e.

while

exposing the inltrated resist to plasma pro essing. Figure 4 shows the hange in et h rate, as a fun tion of SIS y les, for two dierent plasma ompositions, and highlights the large gain in sele tivity. The N2 /H2 plasma aims at isotropi arbon removal and is mostly

hemi al, intended as photoresist strip and residues leaning. The CF4 /CHF3 /O2 plasma is biased, and targets sili on nitride et hing. The N2 /H2 stripping plasma requires more than two y les of inltration to see its rate ae ted, whi h is explained by the fa t that until a ertain level of Al2O3 is rea hed, the residual arbon in the lm remains a

essible to the a tive H∗ radi als. For the sili on nitride et h pro ess, from the rst two y les, a major gain in sele tivity o

urs. This is intrinsi to the nature of the CF4 /CHF3 plasma

hemistry, whi h tends to passivate Al2 O3 by the formation of non-volatile AlF3 and surfa e uoropolymers. The ee t of aluminum inltration on the PPA resist, after patterning by thermal s anning probe lithography, is shown in gure 5. Figure 5 (a) shows the ee t of temperature of 10


SiOx band

T-Al2O3 D-Al2O3

H2O coordinatively bonded to Al ions -CH3

CHx (PPA)

OH

Aromatic CHx (PPA)

Polyphthalaldehyde (PPA)

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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D-Al2O3

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PPA + 8 cycles PPA + 6 cycles PPA + 4 cycles PPA + 2 cycles PPA + 1 cycle PPA Ref. Si Ref.

PPA Wavenumber (cm-1)

Figure 3: Chemi al hanges as a fun tion of SIS y les (25°C) applied to PPA resist, as measured by grazing-angle attenuated total ree tan e (GATR) FTIR spe tros opy. GATR is blind below 700 m−1 due to IR absorption in the Ge ATR rystal. As there is no perfe t ba kground subtra tion in ATR-FTIR, some parasiti indentations at ∼748 and ∼840 m−1 are always noti eable, as well as indentations from ba kground CHx if the sample CHx peaks are too small. Inset: hemi al stru ture of the polyphthalaldehyde polymer.

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100

PPA etch rate (nm/min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90

N2/H2 plasma

80

CF4/CHF3/O2 plasma

70 60 50 40 30 20 10 0 0

2x

4x

6x

8x

SIS cycles

Figure 4: Ee t of the TMA-H2 O inltration on the et h rate of the PPA layer, for two illustrative plasmas. Et h rate is measured by spe tros opi ellipsometry using a single-layer Cau hy model, before and after pro essing the lm for a xed time. Lines are guides to the eyes. the TMA/H2 O SIS pro ess, for 60 nm lines patterned at 100 nm pit h. At an inltration temperature of around ∼ 80°C, the integrity of the patterned features is ompletely lost, indi ating the rossing of a glass transition line, i.e. melting of the resist lines, while around 25°C the pattern is largely maintained. This ee t an be as ribed with two phenomena: rst, the degradation of the PPA by the inltration pro ess into monomers, whi h are volatile at this temperature. Se ond, the heat release linked to the negative ∆H of the rea tion of TMA with PPA (see supplementary information). The heat may ause a temperature rise in the lm, approa hing or rossing the unzipping temperature of PPA (∼ 150°C). The level of this ontribution is hard to determine pre isely, due to the fa t that the lm is not thermally insulated, and therefore the substrate may a t as a heat sink. Figure 5 (b) indi ates the evolution of the PPA pattern as a fun tion of SIS y les, at T=25°C. The volume of the line de reases signi antly after the rst y le, then grows from two y les onwards, leading to a large redu tion of the spa ing between lines, rea hing ∼ 20 nm at four y les. The height of the lines does not vary mu h. If the width in rease orrelates qualitatively with

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the trend observed on blanket samples (see Fig. 2 (a)), the shrinkage after one y le and absen e of verti al expansion reveal some 3D ee ts that were not visible for a 1D geometry like the one shown in Figure 2 (a). It is also interesting to note the evolution of the smallest line (30nm, on the far right), whi h disappears after one y le but is restored after two and four y les. It shows that the shrinkage is signi ant and an, for the thinnest line, lead to pattern ollapse if not enhan ed further by more aluminum inltration. To demonstrate the stability of the PPA lm after inltration with Al2O3 , the lm thi kness was monitored for the PPA lms exposed to dierent SIS y les at 90°C and ben hmarked to non-SIS treated PPA lm - see supporting information. This onrmed that the inltration fully stabilized the PPA lm, within experimental errors, over a period of 36 days.

Pattern Transfer Using SIS Applied to Polyphthalaldehyde Ultra-Thin Films The sample sta k used in this study is shown in Figure 6. Thermal s anning probe lithography requires a buer between the PPA resist and the underlying sta k, whi h is normally

omposed of PMMA. 21 This buer layer is not patterned by the s anning heated probe and needs to be opened afterwards by a tailored N2 /O2 RIE plasma ('PPA thinning step'). Sin e PMMA an be inltrated by TMA during the SIS pro ess, it will form a ontinuous Al2 O3 blo king layer whi h needs to be avoided or minimized. Using our te hnique, ross-linked polystyrene (X-PS) was sele ted as a buer layer, with a thi kness of 7 nm, due to its inertness with respe t to TMA inltration. Another hara teristi of the onventional t-SPL approa h is the use, under the PMMA, of a dual hard mask omposed of a thin evaporated SiO2 hard mask, 2-3 nm thi k, whi h is used to amplify the pattern into a 50 nm thi k spinon, ross-linked arbon-ri h hard mask layer (HM8006, from JSR). 21 The SiO2 layer is et hed using a CHF3 RIE, followed by an O2 RIE to transfer the pattern into the HM8006. Su h a mask ombination exploits the high et h ontrast between the plasma hemistries used to et h ea h layer. However, it requires pre ise ontrol of multiple aspe ts of the pro ess, as des ribed in referen e 21. Parti ularly, the PPA thinning step and the SiO2 hardmask mask 13


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(a)

after t-SPL SIS 1x 80oC SIS 1x 65oC SIS 1x 50oC SIS 1x 25oC

X 50

Pristine PPA (nm)

40

a

30 20 10 0 -10 0.0

0.5

1.0

1.5

Scan length (Pm)

(b) 40

X

a

FWHM (nm) p

35 30

SIS 4x - 25oC

80.6

25

Heigth (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

SIS 2x - 25oC

60.5

15 10 5

SIS 1x - 25oC

37.9

0 -5

After t-SPL

60.3

-10 -15 0.0

0.5

1.0

1.5

Scan length (Pm) Figure 5: Evolution of the t-SPL patterns into PPA, after SIS treatments: (a) 100 nm pit h-30 nm FWHM, at various SIS temperatures, from 25°C to 80°C, and xed number of SIS y les (1x); (b) 100 nm pit h-65 nm FWHM, at xed temperature (25°C) but various number of y les. Note the right hand of the stru ture, showing a 30 nm wide line. Shapes measured by AFM probe are subje ted to a tip onvolution ee t. 14


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thi kness and roughness need to be arefully ontrolled. For the present work, it was de ided to opt for a single 12 nm Si3 N4 mask dire tly on top of bulk sili on, so as to mimi IMEC's FinFET patterning sta k. The pro ess ow for the hardening and transfer of the written stru tures is illustrated in Fig. 6. A summary of the et h parameters is given in table 1. Et hing was done in two stages: rst in a apa itive dis harge hamber (Tokyo Ele tron Vesta) for the hard-mask opening, then in a indu tive plasma hamber (Lam Resear h Kiyo C) for the sili on et h step. The rst step of the plasma patterning is a non-biased Ar/O2 step whi h aims at removing of residual arbon and densifying of the inltrated PPA resist, as shown in Fig. 2(a). This treatment also avoids material expansion during SIS treatment to keep the line width under

ontrol. The se ond step is des um and X-PS et h, using a O2 /Cl2 /He plasma hemistry, whi h aims at removing thin Al2 O3 formed during SIS at the bottom of the patterned PPA, and the transfer of the pattern into the 7 nm underlying X-PS lm. The hlorine allows removal of aluminum- ontaining residues and help formation of sidewall passivation during X-PS et h. The nal step of the hard-mask opening sequen e is the nitride et h, based on a CF4 /CHF3 based pro ess with little O2 addition, whi h is polymerizing and provide high sele tivity towards metal- ontaining masks su h as Al2 O3 . During pro ess development, it was found that the 'des um/X-PS et h' step is the most riti al sin e it an re ess the Al2 O3 mask if not well ontrolled. The nal step o

urs in a indu tive hamber, whose geometry is parti ularly suited for ondu tor et hing (high plasma disso iation, ne ontrol of wafer bias). The plasma hemistry utilisied relies on SF6 as prin ipal Si et hant, whi h is inherently isotropi . Some CF4 and N2 are used as additives for sidewall passivation and a hieve the required anisotropy,

i.e.

verti al Si et h without horizontal under ut; also arbon addition

by the CF4 enhan es polymerization and prote tion of the masking layers (Al2O3 , X-PS, Si3 N4 ) during patterning. The optimal inltration onditions require a number of y les high enough to ensure a omplete onversion of the PPA (i.e. maximum load of Al2 O3 ) and good enduran e to 15


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11nm

a) PPA patterned layer

7nm 13nm

X-PS layer Si3N4 layer

b) SIS, x cycles, 25qC then Ar/O2 hardening

c) Al2O3 descum and X-PS opening, O2/Cl2/He

d) Nitride etch, CF4/CHF3/O2

e) Silicon etch, 72nm

SF6/CF4/N2

Figure 6: S hemati of the pro ess ow used for high resolution Sili on pattern transfer: (a) sample sta k after writing into the PPA; the t-SPL pattern depth is ∼ 10 nm; (b) Sequential TMA/H2 O inltration (SIS) followed by Ar/O2 densi ation; ( ) Al2O3 des um and X-PS opening; (d) sili on nitride et h and (e) nal transfer of the pattern into bulk sili on.

Table 1: Plasma et h steps used for pattern transfer using SIS-hardened PPA resist. Purpose Chemistry Bias Hardening Ar/O2 No Des um & X-PS O2 /Cl2 /He Yes Sili on nitride CF4 /CHF3 /O2 Yes Sili on SF6 /CF4 /N2 Yes a CCP = Capa itive Coupled Plasma;

b

16

Time (s) Et h system & geometry 50 TEL Vesta, CCPa 7 TEL Vesta, CCPa 35 TEL Vesta, CCPa variable Lam Resear h Kiyo C, TCPb TCP = Transformer Coupled Plasma.


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plasma et h, still keeping a line dimension lose to the one written by t-SPL. Based on

hemi al and physi al hara terization of the SIS pro ess des ribed in Figs. 2, 4 and 5, it appears that four SIS y les would give the best result. Some pattern transfer tests were indeed performed on patterned samples on whi h one and two SIS y les were applied, and una

eptable mask surfa e roughness, pinholes and pattern ollapse were observed after sili on et h, indi ating that the aluminum inltration was not strong to maintain the mask integrity under the hosen plasma et h onditions, see Fig. 7(e). The Figures 7(a-d) show the line proles after TMA/H2 O inltration for four y les, followed by plasma et h onditions as spe ied in table 1. The AFM s ans after SIS, nitride opening and Sili on et h indi ate a good morphology after des um, X-PS and nitride et h. At that stage, one an see that the smallest lines (30nm width at 100nm pit h) do loose some height after nitride et h, but subsequent pattern transfer remains feasible. After pattern transfer into Si, the nal topbottom topography is around 80nm and therefore indi ate a su

essful 8x ampli ation of the pattern, starting from a 10nm pattern into PPA. TEM inspe tion of the nal pattern is shown on Figs. 7( ,d) and gives a better idea of the role of ea h layer. Residues of the PPA mask are visible as Al2 O3 , mainly visible on the largest (65nm wide) lines. Elemental analysis indi ates that no aluminum is present in the X-PS layer, onrming that this layer annot be inltrated by TMA/H2 O. Also, on the open areas morphology is ontinuous and indi ates a losed Al2 O3 lm that prevent further uorine et hing of the nitride and Si underlying layers. During Si3 N4 and sili on et h, the prin ipal masking layer was X-PS, whi h shows severe orner rounding. This means that the role of the TMA/H2 O inltration is mainly on preventing PPA loss during des um and X-PS opening. The bottom prole of the Si3N4 , even for the smallest line, remains rather straight indi ating that some further extension of the Si et h pro ess an be envisioned safely, rea hing > 10x ampli ation. The sili on prole is also straight and without under ut, indi ating that the used plasma hemistry is perfe tly anisotropi ; this is onrmed on image 7(e): despite a strong CD loss, the mask integrity would still allow a deep Si et h with verti al sidewalls. Con erning the line widths, some CD 17


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ACS Nano

loss is observed, from ∼65 nm to ∼59 nm for the largest lines, and ∼30 nm to ∼21 nm for the thinnest line; this ould be orre ted by using a larger number of SIS y les, as shown in Figure 5, leading to larger line expansion, or redu ing the Ar/O2 treatment time.

Con lusions In summary, this paper demonstrates that the sequential inltration synthesis with TMA pre ursor applies to the polyphthalaldehyde (PPA) polymer, forming a ontinuous Al2O3 C mask starting from four SIS y les. The me hanism of inltration relies on a hemi al rea tion between TMA, as Lewis a id atalyst, and the PPA polymer, leading to a step-bystep de omposition of PPA, on omitant with the step-by-step oxidation of TMA forming ultimately Al2 O3 embedded into a residual arbon matrix. The et h resistan e is signi antly in reased by the inltrated metal oxide and SIS inltration an notably enhan e the stability of the PPA resist, parti ularly after t-SPL lithography. By optimizing the SIS pro ess

onditions, parti ularly the pro ess temperature, the delity of t-SPL patterned PPA lines

an be preserved. SIS enables a safe pattern transfer of PPA into the X-PS, whi h is then transferred into underlying target substrates. The present paper demonstrates transfer into 13 nm nitride then into bulk sili on, a hieving a nal et h depth of ∼60 nm into Si, with a total ampli ation of 8x (from the original written PPA to the nal pattern depth); furthermore by further optimization of the pro ess parameters, larger ampli ation fa tors an be a hieved. The pro ess greatly simplies the onventional four-layer pattern transfer sta k for t-SPL to two polymeri layers, PPA on top of X-PS. X-PS provides a relatively thi k and stable stopping layer for the tip, whi h is expe ted to lead to signi antly higher tip enduran e in high resolution patterning. Moreover, the me hani al properties of su h a soft sta k were shown to lead to higher a hievable resolution of less than 10 nm half pit h. 4 Thus we expe t the te hnique des ribed here to further broaden the use of thermal s anning

18


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Figure 7: Morphologi al inspe tion at dierent stages of the pattern transfer for a test stru ture with line/pit h dimensions of 65/100, 30/100 and 65/140 nm respe tively (from left to right on the images): (a) AFM s an height after SIS, Si3N4 et h and Sili on et h, (fo us on the 65/140 stru ture); (b) orresponding AFM images; ( ) ross-se tion TEM images; (d) energy-dispersive elemental TEM analysis of the 65 nm FWHM / 100 nm pit h nominal stru ture; (e) morphology of a 30/100 nominal line after 2 SIS y les and full pattern transfer using onditions spe ied in table 1. Shapes measured by AFM probe are subje ted to a tip onvolution ee t.

19


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probe lithography as a versatile high-resolution patterning approa h in ele troni , opti al, and life-s ien e te hnologies. We also expe t this method to be appli able to other resistbased lithography te hniques, and to the use of other organo-metalli pre ursors, leading to formation of metal oxide patterns diering from Al2 O3 (ZnO, HfO2 ). Finally, the dire t SIS-based onversion of 3D-shaped polyphthalaldehyde is expe ted to greatly simplify the

reation of 3D patterns into metal oxide lms.

Methods Thermal s anning probe lithography was performed on a NanoFrazor system from SwissLitho. Pattern parameters were 20 nm pixelsize in y-axis and 10 nm in x-axis. Polyphthalaldehyde (PPA, Sigma Aldri h) was dissolved in anisole to form a 0.5 wt% solution and spin oated at 1800 rpm to a hieve a 6 nm thi k layer. A softbake was performed at 90°C for 3 minutes to remove residual solvent.

Pattern transfer by plasma et hing was performed in industry- ompliant 300mm platforms. Small samples ( oupons) were pla ed on a 2-mi ron thi k photoresist wafer with sili one-based thermal paste before pro essing. The patterning sequen e for mask opening was done in a TEL model Vesta CCP hamber. The sili on et h was done in a Kiyo C TCP

hamber from Lam Resear h Corp.

Thi kness measurements were performed with a Kla-Ten or F5-SCD spe tros opi ellipsometer using a single-layer Cau hy model. Mass measurements were performed using a high-pre ision 300mm Metior balan e from Metryx/Lam Resear h Corporation. AFM

measurements were done in Tapping mode with a Dimension ICON from Bruker using an OCML-AC160TS tip. A systemati tip onvolution ee t is present; this distortion is the same for all samples sin e all measurements were done with the same type of probe. In order to ompare lines from dierent samples (i.e. have the same amount of onvolution), the lines width was measured at the same height from the top of the lines (FWHM). X-ray

20


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Page 22 of 26

photoele tron spe tros opy (XPS) was performed in angle-resolved mode using a Theta300 system from ThermoInstruments. 16 spe tra were re orded at exit angles between 22◦ and 78◦ as measured from the normal of the sample. The measurements were performed using a mono hromatized Al Kα X-ray sour e (1486.6 eV) and a spot size of 400 mi rometers. Two artefa ts must be onsidered. Sin e the XPS measurements are done

, during sample

ex-situ

transfer some airborne ontamination o

urs, giving a apparent high C-C/C-H bond density; in order to minimize this ee t the measurement used in this paper are olle ted at 22◦ exit angle,

i.e.

maximizing signal from the bulk. Due to the very thin layers used in this study,

the underlying sili on substrate be omes visible for some samples, through the appearan e of Si metalli , Si-O and SiO2 ontributions; those were substra ted. Elasti Re oil Dete tion was performed with a home built ERD system (Giangrandi 2007) equipped with a 2MV tandem parti le a

elerator (National Ele trostati s Corporation, 6SDH). The beam energy is 8.0 MeV using

35

Cl4+ , at a s atter angle of 39.2◦ . The grazing-angle attenuated total

ree tan e (GATR) FTIR was done using a ThermoFisher Ni olet 6700 system.

A knowledgement The authors thank Thierry Conard, Werner Knaepen, Markus Heyne, Liping Zhang, Johan Meerss haut and Nadia Vandenbroe k for their help during preparation and pro essing of the samples. We a knowledge Ziad el Otell for his ontribution at the early stage of this work. The resear h leading to these results has re eived funding from the European Union's Seventh Framework Programme SNM (Single Nanometer Manufa turing) proje t under grant agreement no 318804.

21


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ACS Nano

Supporting Information Available Supporting Information Available: 1) the stability of PPA resist has been monitored over a period of 36 days, without and with SIS treatment; 2) DFT quantum hemi al al ulations were performed to understand the various rea tion pathways between PPA, TMA and H2 O. This material is available free of harge

via

the Internet at http://pubs.a s.org.

Corresponding Authors Jean-François de Marnee

Author Contributions The manus ript was written by Jean-François de Marnee and Boon Teik Chan, and was subsequently revised by all o-authors. All authors have given approval to the nal version of the manus ript. Martin Spieser performed the t-SPL lithography and sample preparation for it. Boon Teik Chan performed the pro essing at IMEC and oordinated the intera tions with the materials

hara terization & analysis group. Danielle Vanhaeren performed the AFM hara terization of all samples. Guy Veree ke performed GATR-FTIR measurements and subsequent data analysis. Sergej Naumov performed DFT-based quantum hemi al al ulations, whi h, with the support from Heiko Wolf, allowed to understand the rea tion me hanisms of PPA during the SIS pro ess. Jean-François de Marnee prepared the 300mm ows, optimized the ellipsometry measurements, and oordinated the whole proje t on the IMEC side. Armin W. Knoll oordinated the proje t on the IBM/SwissLitho side. Jean-François de Marnee and Boon Teik Chan ontributed equally.

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