# Selective plasma etching of silicon-containing high chi block copolymer for directed self-assembly (DSA) application

Maria Gabriela Gusmão Cacho, Khatia Benotmane, Patricia Pimenta-Barros, Charlotte Bouet, Guido Rademaker, Maxime Argoud, Christophe Navarro, Raluca Tiron, Nicolas Possémé

► To cite this version:

Maria Gabriela Gusmão Cacho, Khatia Benotmane, Patricia Pimenta-Barros, Charlotte Bouet, Guido Rademaker, et al. Selective plasma etching of silicon-containing high chi block copolymer for directed self-assembly (DSA) application. Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, 2021, 39 (4), pp.042801. 10.1116/6.0001102. cea-04948502

HAL Id: cea-04948502

https://cea.hal.science/cea-04948502v1

Submitted on 14 Feb 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Selective plasma etching of silicon-containing high chi block copolymer for directed self-assembly (DSA) application

<figure><img src="image_2.png" /></figure>

Maria Gabriela Gusmão Cacho ID; Khatia Benotmane; Patricia Pimenta-Barros; Charlotte Bouet; Guido Rademaker ID; Maxime Argoud; Christophe Navarro ID; Raluca Tiron ID; Nicolas Possémé ID

<figure><img src="image_3.png" /></figure>

Check for updates

J. Vac. Sci. Technol. B 39, 042801 (2021)

https://doi.org/10.1116/6.0001102

<figure><img src="image_4.png" /></figure>

View
Online

<figure><img src="image_5.png" /></figure>

Export
Citation

## Articles You May Be Interested In

Study of plasma etching impact on chemoepitaxy directed self-assembly

J. Vac. Sci. Technol. A (March 2021)

Characterization of thin film deposits on tungsten filaments in catalytic chemical vapor deposition using 1,1-dimethylsilacyclobutane

J. Vac. Sci. Technol. A (August 2016)

Block copolymer selectivity: A new dry etch approach for cylindrical applications

J. Vac. Sci. Technol. B (July 2018)
# Selective plasma etching of silicon-containing high chi block copolymer for directed self-assembly (DSA) application

<figure><img src="image_6.png" /></figure>

Cite as: J. Vac. Sci. Technol. B 39, 042801 (2021); doi: 10.1116/6.0001102

Submitted: 27 April 2021 · Accepted: 4 June 2021 ·

Published Online: 22 June 2021

<figure><img src="image_7.png" /></figure>

View Online

<figure><img src="image_8.png" /></figure>

Export Citation

<figure><img src="image_9.png" /></figure>

CrossMark

Maria Gabriela Gusmão Cacho,<fnref n="1" /> ID Khatia Benotmane,<fnref n="1" />,ª Patricia Pimenta-Barros,<fnref n="1" /> Charlotte Bouet,<fnref n="1" /> Guido Rademaker,<fnref n="1" /> ID Maxime Argoud,<fnref n="1" /> Christophe Navarro,<fnref n="2" /> ID Raluca Tiron,<fnref n="1" /> ID and Nicolas Possémé<fnref n="1" /> ID

## AFFILIATIONS

## ABSTRACT

Directed self-assembly (DSA) of block copolymers (BCPs) is an advanced patterning technique being investigated to obtain small and dense patterns for future technological nodes. In order to demonstrate the potential of DSA to extend optical lithography, poly(styrene-*b*-*methyl methacrylate*) (PS-*b*-PMMA) has been the most commonly used block copolymer in different applications, such as line/space and contact hole patterning as well as uniformity repair. However, the minimum pitch for the PS-*b*-PMMA system is limited to around 24 nm due to its relatively weak segregation strength. Therefore, block copolymers with stronger microphase segregation have been developed to enable sub-10 nm patterning, thus obtaining the so-called “high chi” BCPs. In this article, the silicon-containing high chi system investigated is the poly(styrene-*b*-1,1-dimethylsilacyclobutane) (PS-*b*-PDMSB) presenting a pitch of 18 nm. A one-step top coat and PS removal based on H₂/N₂ plasma that presents good selectivity and profile has been studied. The H₂/N₂ gas ratio has been investigated to understand the trade-off between obtaining high selectivity and avoiding pattern collapse. Using this approach, the pattern transfer into different underlayers is demonstrated for the high chi PS-*b*-PDMSB.

Published under an exclusive license by the AVS. https://doi.org/10.1116/6.0001102

## I. INTRODUCTION

Directed self-assembly (DSA) of high chi block copolymers (BCPs) has been investigated over the last few years as a complementary next-generation lithography technique to continue pattern scaling.<fnref n="1" /> DSA is based on the self-assembling property of block copolymers, which are capable of microphase separating when annealed, thus forming a dense array of periodic nanostructures, like cylinders or lamellae, with domain dimensions below 10 nm. This technique also presents the advantage of using available industry processes, for example, 193 nm immersion lithography, therefore complying to manufacturing constraints such as cost and equipment availability.<fnref n="2" />

For several years, polystyrene-*b*-poly(methyl methacrylate) (PS-*b*-PMMA) was the most widely studied block copolymer for the DSA application.³ However, the minimum pitch for the PS-*b*-PMMA system is limited to around 24 nm due to its relatively

weak segregation strength, represented by its low Flory-Huggins parameter, known as “chi” or $χ$, which corresponds to around 0.06 at 298 K.⁴ For smaller technological nodes, block copolymers with stronger microphase segregation, and therefore smaller pitch, have been developed to enable sub-10 nm patterning, thus obtaining the so-called “high chi” block copolymers.⁵ Due to stronger repulsion between both blocks, these BCPs also present better Line Edge Roughness (LER) and Line Width Roughness (LWR) compared to PS-*b*-PMMA.⁶ The high chi BCP systems currently under investigation can be divided into two main groups: the purely organic and the silicon-based block copolymers.

Purely organic high chi block copolymers mostly present the advantage of having a neutral affinity to air, thus foregoing the need for a top layer in order to vertically self-assemble. Some examples of purely organic high chi BCPs include the modified PS-*b*-PMMA developed by Brewer Science and Arkema⁷ with a pitch of 18 nm and the “HC series” developed by EMD

<footnote n="1">¹Université Grenoble Alpes, CEA, Leti, F-38000 Grenoble, France</footnote>

<footnote n="2">²ARKEMA FRANCE, Route Nationale 117, 64170 Lacq, France</footnote>

<footnote>ªElectronic mail: khatia.benotmane@cea.fr</footnote>
Performance Materials⁸ with a pitch of 17–26 nm. However, the main limitation of these BCPs is the low etching selectivity between both organic phases which, combined with smaller dimensions (thickness and pitch), renders the pattern transfer into the under-layers more challenging. Indeed, for these purely organic high chi block copolymers, the main etching difficulty consists in developing a plasma process that presents high selectivity and no bridge formation.⁷

Therefore, a new family of high chi block copolymers with a silicon-containing phase has been greatly explored. Most of them present polydimethylsiloxane (PDMS) as one of the blocks, such as poly(styrene-*b*-dimethylsiloxane) (PS-*b*-PDMS) (χ = 0.26 at 300 K and pitch of 14 nm)⁴ and poly(lactic acid-*b*-dimethylsiloxane (PLA-*b*-PDMS) (χ = 1.41 at 300 K). Aissou *et al*.⁹ demonstrated a templated semicrystalline poly(1,1-dimethyl silacyclobutane-*b*-methyl methacrylate)(PDMSB-*b*-PMMA) thin film enabling the production of highly ordered cylinders of 9 nm diameter. These silicon-based BCPs present the advantage of higher etch selectivity between the organic and the silicon-containing phases. However, a higher incompatibility between the two blocks also presents some drawbacks: these BCPs usually have a high diffusive energy barrier and slow kinetics as well as a preferential wetting of one of the blocks to the air interface.¹⁰ Therefore, in order to obtain perpendicular lamellar morphologies, the control of surface affinities and wetting is mandatory, which often implies the addition of a top coat layer with neutral affinity to both phases of the BCP at the interface between BCP and air.¹¹

In this article, the silicon-containing high chi system investigated is a poly(styrene-*b*-1,1-dimethylsilacyclobutane) (PS-*b*-PDMSB) developed by Arkema. It presents a pitch of 18 nm, lamellar morphology, and a Flory-Huggins parameter χ in the range of 0.08–0.1 at room temperature. The PDMSB block presents a low glass transition temperature (T_g) to improve the self-assembly kinetics.¹¹ Our goal is to remove the top coat and PS selectively to PDMSB, conserving enough PDMSB budget to then transfer the line/space pattern into the subjacent layers.

We propose to evaluate and compare two plasma etching approaches. The first one is an Ar/O₂ plasma that has already been used in previous works to selectively etch the high chi PS-*b*-PMMA.⁷ The second approach is a reducing one-step top coat and PS removal based on H₂/N₂ that presents a better selectivity and profile. The H₂/N₂ ratio is investigated to understand the trade-off between obtaining high selectivity and avoiding bridge formation and pattern collapse. Using this second approach, the pattern transfer into the titanium nitride (TiN), silicon oxide (SiO₂), and silicon (Si) underlayers is demonstrated for the 18-nm pitch high chi PS-*b*-PDMSB.

## II. EXPERIMENTAL SETUP

### A. Materials

Figure 1 represents the material stack used to study the PS-*b*-PDMSB etching on 300 mm wafers. All the polymeric materials are provided by Arkema.

To neutralize the hard mask surface and obtain a vertical alignment of the block copolymer, a thin neutral layer is spin-coated and annealed at 200 °C for 75 s to be grafted to the hard mask. After a

<figure><img src="image_10.png" /><figcaption>FIG. 1. Material stack used on the study of high chi PS-*b*-PDMSB etching.</figcaption></figure>

propylene glycol methyl ether acetate (PGMEA) rinse, it presents a thickness of around 6 nm. The PS-*b*-PDMSB solution is then spin-coated and annealed at 60 °C for 60 s to obtain a thickness of approximately 30 nm. Next, the cross-linkable top coat is also spin-coated and then annealed at 90 °C for 180 s, presenting a final thickness of around 55 nm. Finally, the whole stack goes through final annealing at 200 °C for 5 min to assemble the block copolymer into vertical lamellae with period L₀ = 18 nm and CD = 9 nm.

Our goal is to remove the top coat and PS selectively to the PDMSB, conserving enough PDMSB budget to transfer the line patterns into a 15-nm layer of titanium nitride (TiN) hard mask deposited by physical vapor deposition (PVD).

To measure the top coat and PS etch rates and analyze their surface after etching, 300 nm silicon blanket wafers with either the top coat or the PS homopolymer deposited on top are used. Both materials present a thickness of around 55 nm.

### B. Etching setup

The plasma etching experiments have been carried out on two different 300 mm reactors from LamResearch*. The oxygen-based approach for the top coat and PS removal used a Kiyo FX* inductively coupled plasma (ICP) reactor, which is powered by an RF generator of 13.56 MHz. The gas ratio, pressure, RF power, and RF bias of the Ar/O₂ plasma are set at 50/5 SCCM, 10 mTorr, 220 W, and –100 V, respectively.

The reducing approach based on H₂/N₂ is evaluated on a FLEX CFE* capacitively coupled plasma (CCP) reactor powered by two RF generators (2 and 27 MHz) due to H₂ not being available in the ICP reactor. The gas ratio, pressure, RF 2 MHz power, and RF 27 MHz power of the H₂/N₂ plasma are set at 150/500 SCCM, 160 mTorr, 0 W, and 1000 W, respectively.

### C. Characterization setup

The top coat and PS etch rates are determined using the thickness measurements obtained by ellipsometry before and after
plasma etching of the PS homopolymer and top coat separately deposited on blanket silicon wafers. The ellipsometry measurements are conducted in an Atlas XP+™ tool from Nanometrics® using a Cauchy transparent model for wavelengths between 500 and 1000 nm, since these materials exhibit no absorption (k = 0) in this wavelength range. At 633 nm, as-deposited PS has a refractive index of 1.49, while as-deposited top coat has an index of 1.47. Both films present an as-deposited thickness of around 55 nm.

Ex *situ* and quasi-*in situ* x-ray photoelectron spectroscopy (XPS) analysis are conducted to understand the etching mechanisms. A Theta 300™ spectrometer from Thermo Fisher Scientific® with a monochromatic Al Kα x-ray source (hν = 1486.6 eV) and a spot size of 400 µm is used. The concentrations of carbon, oxygen, nitrogen, fluor, silicon, and titanium atoms are extracted from the C1s, O1s, N1s, F1s, Si2p, and Ti2p core-level energy regions, respectively. The C1s C—C peak (285 eV) is used to calibrate the binding energies in all XPS spectra. Peaks are de-convoluted using the Avantage™ software and a combination of Lorentzian and Gaussian functions. Each element relative concentration is obtained by dividing the calculated peak areas by the corresponding Scofield factor (O1s: 2.88; C1s: 1.00; N1s: 1.68; F1s: 4.12; Si2p<sub>3/2</sub>: 0.60; Si2p<sub>1/2</sub>: 0.30; Ti2p<sub>3/2</sub>: 4.42; Ti2p<sub>1/2</sub>: 2.06). The sum of the relative concentrations for different elements is equal to 100%. For each element, the analysis consisted of five scans with a dwell time of 500 ms, a pass energy of 100 eV, and an energy step size of 0.1 eV.

The critical dimension SEM (CD-SEM) used for top-view morphological observation is the Verity 6i tool from Applied Materials®. SEM images are acquired with an accelerating voltage of 800 V and a probe current of 10 pA. After top-view observations, a cross-section SEM is used to determine the effectiveness of the top coat and PS removal and observe the morphology of the line/space patterns. The tool used is a Hitachi® S5500™ microscope. Cross-sectional samples are precoated with platinum metallization for 30 s to reduce the charging effect characteristic of the polymer reaction to the electron beam. All images are acquired with an accelerating voltage of 5.0 kV, a probe current of 15 µA, and using a tilt of -20°.

### III. RESULTS AND DISCUSSION

#### A. Comparison between oxidizing and reducing plasma chemistries for top coat and PS removal

For the PS-b-PMMA block copolymer, a variety of etching chemistries have been investigated for selective PMMA removal. Most of them are based on oxygen<sup>6,12</sup> or a combination of oxygen and an inert gas such as argon.<sup>13-15</sup> More passivating approaches based on CO/H<sub>2</sub><sup>16,17</sup> and CH<sub>4</sub>/N<sub>2</sub><sup>7,18</sup> have also been demonstrated.<sup>16</sup> The oxygen-based chemistries present low PMMA:PS selectivity around 2.0 due to their high reactivity,<sup>19,20</sup> while oxygen-free chemistries such as CH<sub>4</sub>/N<sub>2</sub> obtain high selectivity but may present bridge formation for smaller pitchs.<sup>7</sup> Since the top coat's composition consists of a poly(2,2,2-trifluoroethylmethacrylate-r-glycidylmethacrylate-r-2-hydroxyethylmethacrylate) statistical ter-polymer,<sup>21</sup> it should present etching mechanisms and selectivity similar to the PMMA with these same chemistries.

For the PS removal selectively to a silicon-containing phase such as PDMSB, the most commonly used approaches are oxygen-

based plasmas. Oxygen is known to etch the organic block while oxidizing the silicon-containing block, replacing Si—CH<sub>3</sub> bonds by Si—O<sub>x</sub> bonds, with good selectivity.<sup>4,9,11,22,23</sup>

#### 1. Top coat and PS removal with an oxidizing plasma

The Ar/O<sub>2</sub> plasma is applied to PS-b-PDMSB pattern wafers with the complete stack for 50 s, and the result is presented in Fig. 2. We observe some brighter spots where no line/space patterns are visible. Where the bright spots are not present, the lines are mostly interrupted and present a smaller CD. Ellipsometry measurements after etching give a block copolymer average thickness of 21.8 nm with a 3σ of 6.1. While this might indicate that the top coat has been fully removed and the Ar/O<sub>2</sub> plasma began to etch the block copolymer, it is important to notice that the ellipsometer measures an average thickness value over the whole wafer and, therefore, might not detect top coat residues. Varying the etching time or Ar/O<sub>2</sub> ratio did not eliminate these defects. These results indicate that the oxidizing approach using an Ar/O<sub>2</sub> plasma is not adapted to this PS-b-PDMSB stack.

Furthermore, the formation of a thicker TiO<sub>x</sub> layer on the surface of the TiN hard mask using the oxidizing plasma may induce some difficulties during pattern transfer due to the limited PDMSB budget available. Therefore, we decided to investigate a reducing plasma based on H<sub>2</sub>/N<sub>2</sub>, which has been shown to etch organic layers with good selectivity to silicon-containing materials without further modification of TiN hard mask composition.<sup>24,25</sup>

<figure><img src="image_11.png" /><figcaption>FIG. 2. Top-view SEM image of the PS-b-PDMSB block copolymer after Ar/O<sub>2</sub> etching for 50 s.</figcaption></figure>
## 2. Top coat and PS removal with a reducing plasma

The top coat and PS etch rates with H₂/N₂ plasma (ratio 150/500) on blanket wafers are 5.5 and 2.0 nm/s, respectively. Thus, etching times varying from 10 to 30 s have been applied on

patterned wafers in order to find the best etch durations on PS-b-PDMSB films.

The top-view and cross-section SEM images after top coat and PS removal for the studied etching times can be found in Fig. 3.

<figure><img src="image_12.png" /><figcaption>FIG. 3. Top-view (a)–(c) and tilted cross section (d)–(f) images of the PS-b-PDMSB block copolymer after top coat and PS removal using H₂/N₂ with ratio 150/500 for (a) and (d) 10 s, (b) and (e) 20 s, and (c) and (f) 30 s.</figcaption></figure>
After 10 s, Figs. 3(a) and 3(d) show that the top coat has been completely removed and that around 6 nm of PS have been etched, while the PDMSB is not consumed. After 20 s [Figs. 3(b) and 3(e)], the PS has also been completely removed and around 23 nm of PDMSB remain, which means a PS:PDMSB minimum selectivity of 4.3. Indeed, since the PS has been completely removed, it is possible that the PS:PDMSB selectivity is greater than 4.3 for shorter etching durations. However, some collapsing of the lines can be observed in Fig. 3(e), probably due to the high aspect ratio of the lines, which is around 2.5.

Finally, after 30 s [Figs. 3(c) and 3(f)], the PDMSB presents an average thickness of 18 nm, translating to a PS:PDMSB selectivity of 2.5. No pattern collapse is observed due to the lower aspect ratio of 2. Moreover, the line patterns after the reducing plasma present an anisotropic profile and a CD bias of 1 nm when compared to the as-deposited material. However, there is some bridge formation between the lines, which has already been observed for such small and dense pitch in previous works and attributed to intensive sidewalls passivation and to a species confinement effect due to the small pitch.⁷

In order to understand the role of the H₂ and N₂ on the top coat and PS-b-PDMSB etching and passivation mechanisms, the gas ratio has been varied. The top-view and cross-section SEM images after top coat and PS removal during 30 s are presented in Fig. 4 for the following H₂/N₂ flow: 150/50, 150/150, 150/300, and 0/500 (N₂ only). The results for H₂/N₂ 150/500 have already been presented in Figs. 3(c) and 3(f). The estimated remaining PS and PDMSB thickness and the calculated PS:PDMSB selectivity for each H₂/N₂ ratio can be found in Table I. Once again, it should be noticed that since the PS has been completely removed, the PS: PDMSB selectivity presented corresponds to the minimum value and could be greater for shorter etching durations.

Figures 4(a) and 4(e) show the results for H₂/N₂ 150/50, which has the lower N₂ content and lower selectivity (1.7) when compared to other ratios, thus indicating that N₂ plays an important role on PDMSB passivation and protection. There are also no bridges observed on the top-view image [Fig. 4(a)], probably because this ratio is less passivating and has a smaller CD of around 8 nm. A few lines have collapsed, which might be related to the smaller CD and the slight difference in CD between the top and the bottom of the line that is shown in Fig. 4(e).

This difference in CD between top and bottom of the line pattern is even more significant for the ratio 1.0 (150/150) presented in Figs. 4(b) and 4(f), corresponding to 3 nm, which also explains the many collapsing lines visible in top-view [Fig. 4(b)]. The bigger gap in CD might be caused by the combination of two simultaneous phenomena: first, the larger CD observed on the top of the line is probably due to the higher N₂ content, which preferably passivates the top sidewall; whereas the smaller bottom CD may correspond to the lateral etching of the neutral layer under the PDMSB block, since this layer is purely organic and etches faster than the silicon-containing PDMSB.

As we observe the two next H₂/N₂ ratios—150/300 [Figs. 4(c) and 4(g)] and 150/500 [Figs. 3(c) and 3(f)]—as expected, the greater the N₂ content, the better the selectivity since N₂ seems to be the major passivating component of this etching chemistry. For both ratios, the CD difference between top and bottom of the

patterns still exist but is less important than the one observed for the 150/150 ratio and leads to less line collapsing. The higher N₂ content, and therefore passivation, also leads to the reappearance of bridges [Fig. 4(g)].

However, when attempting to etch the top coat and PS with N₂ only, we observe in Figs. 4(d) and 4(h) that while the top coat is completely removed, there is no selectivity between PS and PDMSB. This indicates that H₂ is necessary to etch the PS selectively to the PDMSB.

## B. Understanding the etching and passivation mechanisms

In order to understand the etching and passivation mechanisms involved during top coat and PS removal with reducing plasma, ex situ XPS analysis was carried out on the blanket top coat and PS homopolymers for the H₂/N₂ with ratio 150/500. The results are shown in Figs. 5 and 6.

Before etching, Fig. 5(c) indicates that PS consists of only carbon and hydrogen. Its C1s spectra presented in Fig. 6(b) show two peaks. The one at 284.8 eV, assigned to C—C/C=C, corresponds to two different bonds: aliphatic carbon (—CH₂ and —CH₃) at 284.7 eV and carbon from the phenyl cycle at 285.0 eV. Both peaks cannot be dissociated due to the spectral resolution. The second one is observed at 291.5 eV and represents a shake-up peak, which is the characteristic of the aromatic cycle, being therefore identified as pi → pi*.²⁶

As for the as-deposited top coat [Figs. 5(a) and 6(a)], it consists of a poly(2,2,2-trifluoroethylmethacrylate-r-glycidylmethacrylate-r-2-hydroxyethylmethacrylate) statistical terpolymer.²¹ The C1s spectra present four different peaks. The first one at 285.0 eV, also identified as C—C/C=C, represents both the aliphatic carbon (—CH₂ and —CH₃ bonds) at 284.7 eV and the acrylate compounds at 285.7 eV.²⁶ A second peak at 286.5 eV represents the methyl ester (C—O) bonds, while a third peak at 288.8 eV corresponds to the carboxyl group (C=O). Finally, the fourth one, at 292.7 eV, corresponds to carbon bound to three fluorine atoms (CF₃). The as-deposited top coat also presents around 24.3% of oxygen, and the O1s peaks (not represented) at 532.0 and 533.5 eV correspond to O=C and O—C bonds, respectively.²⁶ Same can be said for 12.2% of fluorine, which is bound to carbon (F1s peak at 688.6 eV).

Figures 5 and 6 also present the XPS results after etching these materials with H₂/N₂ plasma. For both materials, a C—N peak appears at 399.4 eV on the N1s spectra [Fig. 6(c)] and at 286 eV on the C1s spectra—in which due to spectral resolution, it cannot be differentiated from C—O (286.5 eV) bonds and is thus fitted together as C—O/C—N [Figs. 6(a) and 6(b)]. For the PS, this C—N bond indicates a nitridation of the PS surface.⁷ This CN-type layer then probably reacts with hydrogen in the plasma to form HCN as a volatile product. It should also be noticed that the presence of oxygen after etching is probably due to the air exposure between etching and the XPS analysis.

As for the top coat, as represented in Fig. 6(a), a decrease in the percentage of CF₃ and C—O simple and double bonds is observed, indicating that they are preferentially etched by the reactive species present in the H₂/N₂ plasma. The loss of C—O and C=O bonds is characteristic of methacrylate-based polymer
etching and similar to what is observed for PMMA etching with CH$_4$/N$_2$ as expected.

As the PDMSB homopolymer is not available, the PS-b-PDMSB block copolymer is deposited over a silicon wafer without

top coat and ex situ XPS analysis is conducted. Since there is no top coat layer, the top of the BCP layer will consist of horizontal PDMSB and PS lamellae, with the PDMSB on the interface with the air. This means that for the XPS analysis with an angle of

<figure><img src="image_13.png" /><figcaption>FIG. 4. Top-view (a)–(d) and tilted cross section (e)–(h) images of the PS-b-PDMSB block copolymer after top coat and PS removal using H$_2$/N$_2$ with different ratios for 30 s: (a) and (e) 150/50 SCCM, (b) and (f) 150/150 SCCM, (c) and (g) 150/300 SCCM, and (d) and (h) only N$_2$ (500 SCCM).</figcaption></figure>
<table><caption>**TABLE I.** Estimated remaining PS and PDMSB thickness and the calculated minimum PS:PDMSB selectivity for each H₂/N₂ ratio. These values were obtained by averaging the measured values on the SEM images presented in Figs. 3(f) and 4.</caption><thead><tr><th>H<sub>2</sub>/N<sub>2</sub> flow (SCCM)</th><th>PS remaining (nm)</th><th>PDMSB remaining (nm)</th><th>PS:PDMSB selectivity</th></tr></thead><tbody><tr><td>150/50</td><td>0</td><td>12.7</td><td>1.7</td></tr><tr><td>150/150</td><td>0</td><td>16.3</td><td>2.2</td></tr><tr><td>150/300</td><td>0</td><td>16.7</td><td>2.3</td></tr><tr><td>150/500</td><td>0</td><td>18.0</td><td>2.5</td></tr><tr><td>0/500</td><td>21.6</td><td>21.6</td><td>1.0</td></tr></tbody></table>

76.25°, which inspects around 2 or 3 nm of the top surface, it is mostly the PDMSB that will be analyzed. As we can see in Fig. 7(a), as-deposited PS-b-PDMSB consists of carbon (74.5%) and silicon (25.5%). Figure 8(a) presents the C1s scan for the as-deposited PS-b-PDMSB, where one peak is identified as C—C/C = C/Si—CH₃ at 284.5 eV: it represents both the carbon at CH₂ and CH₃ bonds (at 284.7 eV) and the carbon from Si—CH₃ bonds (at 284 eV).²⁶ The Si2p spectra in Fig. 8(b) confirm the presence of Si—CH₃ bonds at 100.7 eV.

In order to understand the etching and passivation mechanisms of the PDMSB lines during the reducing plasma, both *ex situ* and *quasi-in situ* XPS analyses have been conducted using patterned wafers after top coat and PS removal with an H₂/N₂ (150/500) plasma of 30 s. The surface composition of patterned

wafers after complete top coat and PS etching is presented in Figs. 7 and 8. Even though the results obtained cannot be considered quantitatively accurate, they can still be compared to mechanisms proposed by other authors.

Posseme *et al.*²⁷ propose the following reaction mechanism during downstream H₂/N₂ or reactive ion etching (RIE) NH₃ plasma for Si—CH₃ bonds:

$$ \equiv \text{Si}---\text{CH}_3 + \text{NH}_2 + \text{H} \rightarrow \text{Si}---\text{NH}_2 + \text{CH}_4. \quad (1) $$

For our PDMSB lines, the presence of Si—N bonds is confirmed by the quasi-*in situ* XPS analysis presented in Figs. 8(b) and 8(c), which indicates that the Si—CH₃ bonds in PDMSB are probably turned into Si—NH₂ during top coat and PS removal, as proposed in reaction 1.

It is also important to notice that the oxygen and nitrogen atomic percentages presented in Figs. 7(b) and 7(c) are the only ones that present a significant variation between quasi-*in situ* and *ex situ* XPS analyses. This difference can be explained by the reaction of the Si—NH₂ bonds with water molecules when the wafer is exposed to the atmosphere. The Si—NH₂ bonds are replaced by Si—OH bonds, according to the following reaction:

$$ \equiv \text{Si}---\text{NH}_2 + \text{H}_2\text{O} \rightarrow \text{Si}---\text{OH} + \text{NH}_3. \quad (2) $$

This reaction is confirmed by the XPS analysis shown in Figs. 8(b) and 8(d), where Si—N bonds are replaced by Si—O bonds after air exposure. Also according to Posseme *et al.*,²⁷ these

<figure><img src="image_14.png" /><figcaption>**FIG. 5.** Surface composition of (a) and (b) top coat (TC) and (c) and (d) PS homopolymer (hPS) determined by *ex situ* XPS with an incident angle of 23.75° (a) and (c) as-deposited (represented as “as dep”) and (b) and (d) after etching with H₂/N₂ ratio 150/500 for 5 s.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>FIG. 6. (a) and (b) C1s and (c) N1s spectra for (a) and (c) top coat (TC) and (b) and (c) PS homopolymer (hPS) determined by *ex situ* XPS with an angle of 23.75° before (represented as “as dep”) and after etching with H₂/N₂ ratio 150/500 for 5 s.</figcaption></figure>

different reactions lead to the formation of a modified layer composed of SiOₓNᵧH_z, which is denser and carbon depleted. This layer is observed for the PDMSB according to quasi-*in situ* XPS analysis in Fig. 8(d).

We also observe a slight variation of the TiO₂ and TiN composition when comparing the *ex situ* and quasi-*in situ* samples. Indeed, the *ex situ* sample probably presents a higher TiO₂ content due to the air exposure between etching and the XPS analysis,
PS-b-PDMSB surface composition

<figure><img src="image_16.png" /><figcaption>FIG. 7. PS-b-PDMSB surface composition determined by (a) and (b) ex situ and (c) quasi-in situ XPS with an incident angle of 76.25° on patterned wafers (a) as deposited (without top coat) and (b) and (c) after top coat and PS removal with H₂/N₂ ratio 150/500 for 30 s.</figcaption></figure>

which allows the TiN surface to be further oxidized. In order to minimize the oxidation of the TiN surface after top coat and PS removal, all experiments of pattern transfer into the TiN layer that shall be presented in Sec. III C are conducted without air exposure between the top coat and PS removal step and the TiN etching step. Therefore, we may conclude that the formation of Si—NH₂ and SiOₓNᵧH_z over PDMSB lines is responsible for their passivation during top coat and PS removal with H₂/N₂ plasma.

### C. Pattern transfer into the titanium nitride hard mask

For the pattern transfer of the 18-nm pitch high chi PS-b-PDMSB into the 18-nm hard mask (15 nm of TiN and around 3 nm of native titanium oxide, TiOₓ), we have selected three different combinations of H₂/N₂ ratio and etching times to evaluate the impact of high selectivity, aspect ratio, and bridge formation. These combinations are 150/50 ratio for 20 s, 150/500 ratio for 20 s, and 150/500 ratio for 30 s. The 150/50 ratio for 20 s, while not shown here, presents a minimum PS:PDMSB selectivity of 3.7 with around 21.9 nm of PDMSB remaining, as well as a straight line profile with a CD of 8 nm and no bridge formation.

Pattern transfer into the TiN hard mask is conducted in an ICP reactor without air exposure after the top coat and PS removal through H₂/N₂, in order to avoid further oxidation of the TiN and PDMSB exposed surfaces. The TiN etching consists of two steps. The first one is a breakthrough (BT) step using Cl₂/BCl₃ to remove the TiOₓ on the surface and initiate the TiN etching. This BT step

is followed by a main etch (ME) using Cl₂/CH₄/N₂, which presents better selectivity to PDMSB thanks to the presence of CH₄ and N₂ as passivating agents. The plasma parameters can be found in Table II.

Figure 9 shows the results for pattern transfer of high chi PS-b-PDMSB into the 15-nm TiN hard mask using three top coat and PS removal conditions evaluated. For H₂/N₂ with ratio 150/500 for 20 s, which is the condition with higher selectivity, and therefore higher aspect ratio, we observe from Fig. 9(b) that the line patterns have mostly collapsed. Figure 9(e) shows that, because of PDMSB collapse, pattern transfer into the TiN hard mask is not initiated.

For the 150/50 ratio for 20 s [Figs. 9(a) and 9(d)], which presents an intermediate PDMSB remaining thickness among the three conditions studied and no bridge formation, pattern collapse is still observed although in smaller intensity when compared to the previous condition [Fig. 9(b)]. Pattern transfer into the TiN hard mask is initiated: the TiOₓ layer has been opened and around 5 nm of TiN has been etched. However, pattern collapse probably prevented further TiN etching.

Finally, the best result is obtained with the third condition, H₂/N₂ with ratio 150/500 for 30 s [Figs. 9(c) and 9(f)]. For this combination, the TiOₓ layer has been opened and around 10 nm of TiN has been etched. Moreover, no pattern collapse is observed. However, the PDMSB mask has been completely etched, which is confirmed when we extend the main etch time to 10 s (not depicted) instead of 5 s and observe that the line patterns are no longer present because TiN has been overetched.
<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /><figcaption>FIG. 8. (a) C1s, (b) Si2p, (c) N1s, and (d) O1s spectra determined by ex situ and quasi-in situ XPS with an incident angle of 76.25° for the as-deposited PS-b-PDMSB (without top coat) and for the PDMSB lines after top coat and PS removal with H₂/N₂ ratio 150/500 for 30 s.</figcaption></figure>

These results lead to the conclusion that while good PS: PDMSB selectivity is desired in order to conserve enough PDMSB budget to completely transfer the line patterns into the hard mask, mechanical stability problems cause pattern collapse due to the high aspect ratio of the lines. Therefore, the smaller CD of the high chi block copolymer limits the maximum PDMSB budget allowed for pattern transfer, which in its turn limits the TiN thickness that can be etched using this block copolymer as a mask. Herein, we are able to transfer 18-nm pitch PDMSB lines into a TiN thickness of 10 nm. Further investigation into solutions to improve the block copolymer's mechanical stability are ongoing in order to increase the etching process window.

<table><caption>TABLE II. Plasma parameters of the chemistries used for the dry etching of the TiN hard mask.</caption><thead><tr><th>Plasma</th><th>Pressure (mTorr)</th><th>Power (W)</th><th>Bias (V)</th><th>Flow (ratio)</th><th>Time (s)</th></tr></thead><tbody><tr><td>BT:Cl<sub>2</sub>/BCl<sub>3</sub></td><td>8</td><td>400</td><td>-100</td><td>2:1</td><td>5</td></tr><tr><td>ME:Cl<sub>2</sub>/CH<sub>4</sub>/N<sub>2</sub></td><td>10</td><td>350</td><td>-200</td><td>4:1:3</td><td>5</td></tr></tbody></table>

### D. Pattern transfer into the silicon oxide hard mask and silicon bulk

Due to the limits imposed by the PDMSB budget to TiN etching, we decided to adapt the integration stack by replacing the TiN hard mask by a 10-nm SiO₂ hard mask. The top coat and PS are removed using the best result from the previous section: H₂/N₂ with ratio 150/500 for 30 s. The SiO₂ etching is conducted in the same CCP reactor as the top coat and PS removal without air exposure between both steps, with the following parameters: CF₄/H₂/Ar with ratio 3.6:1:14, pressure of 50 mTorr, RF 2 MHz at 600 W, and RF 27 MHz at 300 W for 7 s. Next, the silicon is partially etched in the ICP reactor without air exposure in between. The plasma parameters for Si etching are as follows: CH₂F₂/SF₆/N₂/He with ratio 3:2:3:7, pressure of 5 mTorr, RF power at 450 W, and RF bias at -70 V for 11 s.

After SiO₂ etching, Fig. 10(a) shows no pattern collapse; however, some bridges are present, probably originated from top coat and PS removal steps. The pattern transfer into the 10-nm SiO₂ layer is complete [Fig. 10(c)], and there are around 7 nm of PDMSB left and the CD is slightly larger than the initial one (10 nm instead of 9 nm).
<figure><img src="image_21.png" /><figcaption>FIG. 9. (a)–(c) Top-view and (d)–(f) cross section SEM images of the pattern transfer of high chi PS-b-PDMSB into the TiN hard mask using three different conditions for the top coat and PS removal with H₂/N₂: (a) and (d) 150/50 ratio for 20 s, (b) and (e) 150/500 ratio for 20 s, and (c) and (f) 150/500 ratio for 30 s.</figcaption></figure>

After partially etching the Si bulk, the top-view and cross-section SEM images presented in Figs. 10(b) and 10(d) show that the line pattern has been transferred into 13 nm of Si and there are approximately 10 nm of SiO₂ left, thus indicating the possibility to

obtain very high aspect ratio silicon features with plasma etching with good Si:SiO₂ selectivity. Moreover, the good mechanical stability of silicon should prevent pattern collapse. The CD bias between the silicon oxide and silicon bulk etching steps is 2 nm and the
<figure><img src="image_22.png" /><figcaption>FIG. 10. (a) and (b) Top-view and (c) and (d) cross section SEM images of the pattern transfer of high chi PS-β-PDMSB (a) and (c) into the SiO₂ hard mask and (b) and (d) into the Si bulk. The top coat and PS removal was conducted using the H₂/N₂ plasma with ratio 150/500 during 30 s.</figcaption></figure>

14 February 2025 15:27:42

final CD is around 8 nm. Some bridges are still present, having been transferred from the previous etching steps. Furthermore, no pattern collapse is observed, though a slight wiggling of the lines may be observed with the top-view SEM image in Fig. 10(b). The silicon etching step could be optimized in order to better conserve the CD, possibly avoiding the wiggling effect.

Therefore, we demonstrate the pattern transfer of an 18-nm pitch high-χ silicon-containing block copolymer into a 23-nm thick SiO₂ and silicon stack using a reducing H₂/N₂ plasma etching for top coat and PS removal.

## IV. SUMMARY AND CONCLUSIONS

We investigated a reducing plasma based on H₂/N₂ to etch the organic phase (PS) of the block copolymer with good selectivity to the silicon-containing phase (PDMSB). For a H₂/N₂ ratio of 150/500, after 20 s, the PS is completely removed with a minimum PS: PDMSB selectivity of 4.3, but some pattern collapse is observed due to the high aspect ratio of the lines. Prolonging the etching time to 30 s leads to a PDMSB remaining thickness of 18 nm (PS: PDMSB minimum selectivity = 2.5), and no pattern collapse is
observed due to the lower aspect ratio of 2. However, some bridges are formed between the lines, which is attributed to intensive flank passivation and to a species confinement effect due to the small and dense pitch.⁷

The gas ratio was varied in order to understand the role of H₂ and N₂ on the top coat and PS-b-PDMSB etching and passivation mechanisms. We showed that higher N₂ content leads to higher selectivity but also more bridge formation, indicating that N₂ is the main passivating agent in this reducing plasma. Higher H₂/N₂ ratio, on the other hand, leads to lower selectivity and to an undercutting of the lines due to the neutral layer's lateral etching. However, when attempting to etch the top coat and PS with N₂ only, while the top coat is completely removed, there is no selectivity between PS and PDMSB. This indicates that H₂ is necessary to etch the PS selectively to the PDMSB.

Ex *situ* and quasi-*in situ* XPS analyses were conducted to further understand the etching and passivation mechanisms. Top coat presented a loss of C—O simple and double bonds as well as CF₃ bonds, which is a characteristic of other methacrylate-based polymers etching, such as PMMA. As for PS, the formation of a CN layer is observed on the surface, which is then removed by hydrogen reactive species by forming HCN as a volatile by-product. As for the PDMSB, the formation of Si-NH₂ and SiOₓNᵧHz is responsible for PDMSB passivation during top coat and PS removal with H₂/N₂ plasma.

Finally, we proceeded to the pattern transfer of PDMSB lines into 10 nm of the 15-nm TiN hard mask. The results led to the conclusion that while good PS:PDMSB selectivity is desired in order to conserve enough PDMSB budget to completely transfer the line patterns into the hard mask, mechanical stability problems cause pattern collapse due to the high aspect ratio of the lines. Therefore, the smaller CD of the high chi block copolymer limits the maximum PDMSB budget allowed for pattern transfer (and therefore the PS:PDMSB selectivity), which in its turn limits the TiN thickness that can be etched using this block copolymer as a mask. Due to the limits imposed by the PDMSB budget to TiN etching, we adapted the integration stack by replacing the TiN hard mask by a 10-nm SiO₂ hard mask. After complete pattern transfer into the silicon oxide layer with good profile and CD, there were around 7 nm of PDMSB left. We then demonstrated the partial pattern transfer into approximately 13 nm of the silicon bulk layer while conserving the SiO₂ hard mask. The silicon etching step could be further optimized by increasing Si:SiO₂ selectivity in order to obtain higher aspect ratio silicon features, such as the ones obtained with a HBr/O₂ synchronously pulsed plasma presented by Chevalier *et al.*²¹

Therefore, in this work, we validated the pattern transfer of an 18-nm pitch high-χ silicon-containing block copolymer into a 23-nm thick SiO₂ and silicon stack using reducing H₂/N₂ plasma etching for top coat and PS removal.

## ACKNOWLEDGMENTS

This work was financed by the REX-7 project (Grant Agreement No. DOS0075457/00) from the region Auvergne-Rhône Alpes, the DiSABlock chair of excellence of the Grenoble

Nanoscience Foundation, and the European ECSEL project MADEin4 (Grant Agreement No. 826589). It was supported by the French National Research Agency (ANR) within the IMPACT program under Contract No. ANR-10-EQPX-33.

## DATA AVAILABILITY

The data that support the findings of this study are available within the article and from the corresponding author upon reasonable request.

## REFERENCES

¹D. M. Tennant, *J. Vac. Sci. Technol. A* **31**, 050813 (2013).

²M. P. Stoykovich and P. F. Nealey, *Mater. Today* **9**, 9 (2006).

³M. Argoud *et al.*, *Proc. SPIE* **9049**, 904929 (2014).

⁴C. Girardot, S. Böhme, S. Archambault, M. Salaün, E. Latu-Romain, G. Cunge, O. Joubert, and M. Zelsmann, *ACS Appl. Mater. Interfaces* **6**, 18 (2014).

⁵H. Feng, X. Lu, W. Wang, N.-G. Kang, and J. W. Mays, *Polymers* **9**, 10 (2017).

⁶H. Tsai *et al.*, *Proc. SPIE* **9779**, 977910 (2016).

⁷M. G. Gusmão Cacho, P. Pimenta-Barros, M. Argoud, C. Navarro, K. Sakavuyi, R. Tiron, and N. Possémé, *Microelectron. Eng.* **230**, 111369 (2020).

⁸E. Hirahara *et al.*, *J. Photopolym. Sci. Technol.* **29**, 5 (2016).

⁹K. Aissou, M. Mumtaz, G. Fleury, G. Portale, C. Navarro, E. Cloutet, C. Brochon, C. A. Ross, and G. Hadziioannou, *Adv. Mater.* **27**, 2 (2015).

¹⁰P. Mokarian-Tabari, C. Cummins, S. Rasappa, C. Simao, C. M. Sotomayor Torres, J. D. Holmes, and M. A. Morris, *Langmuir* **30**, 35 (2014).

¹¹P. Bézard, X. Chevalier, A. Legrain, C. Navarro, C. Nicolet, G. Fleury, I. Cayrefourcq, R. Tiron, and M. Zelsmann, *Proc. SPIE* **10589**, 105890C (2018).

¹²H. Tsai *et al.*, *ACS Nano* **8**, 5 (2014).

¹³Y.-H. Ting, S.-M. Park, C.-C. Liu, X. Liu, F. J. Himpsel, P. F. Nealey, and A. E. Wendt, *J. Vac. Sci. Technol. B* **26**, 1684 (2008).

¹⁴F. Yamashita, E. Nishimura, K. Yatsuda, H. Mochiki, and J. Bannister, *Proc. SPIE* **8328**, 83280T (2012).

¹⁵B. T. Chan, S. Tahara, D. Parnell, P. A. Rincon Delgadillo, R. Gronheid, J.-F. de Marneffe, K. Xu, E. Nishimura, and W. Boullart, *Microelectron. Eng.* **123**, 180 (2014).

¹⁶H. Miyazoe, A. V. Jagtiani, H.-Y. Tsai, S. U. Engelmann, and E. A. Joseph, *J. Phys. D: Appl. Phys.* **50**, 204001 (2017).

¹⁷A. Sarrazin, N. Posseme, P. Pimenta-Barros, S. Barnola, A. Gharbi, M. Argoud, R. Tiron, and C. Cardinaud, *J. Vac. Sci. Technol. B* **34**, 6 (2016).

¹⁸A. Sarrazin, N. Posseme, P. Pimenta-Barros, S. Barnola, R. Tiron, and C. Cardinaud, *J. Vac. Sci. Technol. B* **37**, 030601 (2019).

¹⁹R. A. Farrell, N. Petkov, M. T. Shaw, V. Djara, J. D. Holmes, and M. A. Morris, *Macromolecules* **43**, 20 (2010).

²⁰M. Delalande, G. Cunge, T. Chevolleau, P. Bézard, S. Archambault, O. Joubert, X. Chevalier, and R. Tiron, *J. Vac. Sci. Technol. B* **32**, 051806 (2014).

²¹X. Chevalier *et al.*, *ACS Appl. Mater. Interfaces* **13**, 9 (2021).

²²J. Zhang, J. Wu, M. Li, V. V. Ginzburg, J. D. Weinhold, M. B. Clark, P. Trefonas III, and P. D. Hustad, *Proc. SPIE-Int. Soc. Opt. Eng.* **9051**, 905111 (2014).

²³A. Legrain, G. Fleury, M. Mumtaz, C. Navarro, J. Arias-Zapata, X. Chevalier, I. Cayrefourcq, and M. Zelsmann, *ACS Appl. Mater. Interfaces* **9**, 49 (2017).

²⁴G. Blachut *et al.*, *Proc. SPIE* **10589**, 1058907 (2018).

²⁵M. G. Gusmão Cacho, K. Benotmane, A. Le Pennec, C. Bouet, P. Pimenta-Barros, G. Rademaker, M. Argoud, R. Tiron, and N. Possémé, *J. Vac. Sci. Technol. A* **39**, 033004 (2021).

²⁶G. Beamson and D. Briggs, *High Resolution XPS of Organic Polymers: The Scientia ESCA300 Database* (Wiley, Chichester, 1992).

²⁷N. Posseme, T. Chevolleau, T. David, M. Darnon, O. Louveau, and O. Joubert, *J. Vac. Sci. Technol. B* **25**, 1928 (2007).