Nicolas Posseme
email: nicolas.posseme@cea.fr

Thibaut David

Thierry Chevolleau

Maxime Darnon

Fanny Bailly

Régis Bouyssou

Julien Ducote

Hamed Chaabouni

Mohamed El Kodadi

Christophe Licitra

M El Kodadi

C Verove

O Joubert

Study of porous SiOCH patterning using metallic hard mask: challenges and solutions

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

One of the most serious challenges in semiconductor manufacturing is to produce low cost integrated circuits as well as to develop high performance devices. On a technical point of view, a serious problem is that electric signal propagation through metal interconnects is delayed by the resistance (R) of the metal lines and the capacitance (C) between adjacent metal lines. Nowadays, the chosen way to decrease the runtime delays and thus the RC product is to use a less resistive metal (ie. copper) and decrease the power consumption and cross-talk (given by the capacitance) by integrating lower dielectric constant insulators [1]. To achieve this, many low-k materials have been investigated. There are two categories of materials: silicon containing (silica-based and silsesquioxane) or non silicon containing [2][3][4][5] materials presenting different advantages and drawbacks in term of chemical and physical properties, thermal stability and expansion, etc.. Finally, from the 90 nm technological node, the semiconductor industry selected Si-CH 3 containing organosilicate materials close to the well known SiO 2 [START_REF] Grill | International Interconnect Technology Conference[END_REF]. These materials present k value of about 3 and can be reduced further by introducing porosity leading in the meantime to a weakening of its mechanical properties [2]. The introduction of porous SiOCH material was initially planned for the 65nm technological node, but the complexity of their introduction delayed their introduction for the 45nm interconnect technology node. For instance, previous study performed on blanket wafers, has shown that porous SiOCH materials are sensitive to fluorocarbon based plasma. These results obtained on spin-on p-SiOCH films presenting a porosity variation between 40% and 50% demonstrated the film degradation (methyl group depletion and moisture uptake) directly scales with porosity in the material [START_REF] Posseme | [END_REF].

In this paper, we propose to analyse the issues revealed during the integration of a porous low-k material (deposited by plasma enhanced chemical vapor deposition, with dielectric constant 2.35), and the associated solutions from an etching point of view. P-SiOCH line patterning can be achieved using different hard mask strategies (metallic or organic hard masks) [8][START_REF] Posseme | International interconnect technology conference (IITC) proceedings[END_REF][START_REF] Struyf | International interconnect technology conference (IITC) proceedings[END_REF], both presenting advantages and drawbacks in terms of etching performance etc. [START_REF] Darnon | [END_REF]. In this paper we will only focus on the metallic hard mask approach to discuss the etching challenges.

II. Experimental

The presentation of p-SiOCH integration difficulties with a metallic hard mask approach will be discussed and characterized using several techniques briefly described below.

II.1 Porous SiOCH materials

In this study, porous SiOCH material presenting a porosity of 28% with dielectric constant 2.35 is investigated. These materials are deposited by plasma enhanced chemical vapor deposition (PECVD) in a Producer TM mainframe from Applied Materials. Using a co-injection gas feed, two precursors are simultaneously vaporized (Diethoxymethylsilane (C 5 H 14 O 2 Si, DEMS) as a matrix precursor and a norbornadiene (NBD) component as a porogen precursor and injected in a capacitively coupled plasma chamber operated at 13.56 MHz. After depositing the hybrid film (SiOCH matrix plus the porogen), a thermal curing assisted by ultraviolet radiations is performed at 400 °C under He environment in a curing chamber to facilitate the porogen removal and generate the porous film [12,13].

II.2 Porous SiOCH integration with a metallic hard mask

In this work, the patterning of porous SiOCH trenches is performed using a metallic hard mask with the following stack on silicon substrate: porous carbon-doped silicon-oxide (PECVD p-SiOCH, with 28% porosity) / PECVD silicon dioxide (SiO 2 ) / titanium nitride deposited by physical vapor deposition (PVD TiN) and BARC/193 nm resist patterns. In the metallic hard mask (MHM) approach, a thin metallic hard mask layer is deposited on the top of a dense dielectric layer (SiO 2 ) which encapsulates the underlying p-SiOCH material. The trench photolithography is performed using a positive tone photoresist coupled with a bottom antireflective layer (BARC) (Fig. 1

(a)). (a) (b) (c)
The BARC and titanium nitride layers are etched in a chlorine-based chemistry (Cl 2 /BCl 3 ) using an inductively coupled plasma (ICP), followed by resist stripping in an O 2 micowave plasma (Fig. 1(b)). Then the SiO 2 capping and porous SiOCH layers are patterned in fluorocarbon (FC) based plasmas (C 4 F 8 /N 2 /Ar/O 2 ) using a multi-frequency capacitive coupled plasma (CCP) etcher (Fig. 1(c)).

II.3 Characterization techniques of p-SiOCH modification

In this study, the film modification will be presented through experiments performed on blanket wafers (to represent bottom p-SiOCH film modification) and on patterned wafers to analyse the sidewall film modifications. Different complementary techniques like Xray photoelectron spectroscopy (XPS), infrared spectroscopy, decoration method and ellipsometric porosimetry (EP) will be used.

II.3.1 X-ray Photoelectrons spectroscopy (XPS)

The surface composition of the p-SiOCH after FC based plasma exposure has been analyzed on 300mm blanket and patterned wafers using ex-situ XPS analyses. XPS analyses spectra are performed on blanket and patterned wafers using a Thermo Fisher Scientific Theta 300 spectrometer operating with a monochromatic Al KĮ x-ray source (hȣ = 1486.6 eV) and an electron energy analyzer operating in a constant pass energy mode of 20eV. Chemical compositions are derived from the areas of the different XPS spectra. Spectral decomposition is performed to extract the Si2p, O1s, C1s, N1s and F1s peak intensities. Individual line shapes are simulated with the combination of Lorentzian and Gaussian functions. The background subtraction is performed by using a Shirley function. After XPS analysis, the integrated intensities are divided by the theoretical Scofield (S) cross section (C1s:1; O1s:2.93; Si2p:0.82; F1s:4.43; N1s:1.8).

The sum of the concentration of the different elements present on the analysed surface is equal to 100%. The hydrogen content is not taken into account in this calculation since hydrogen cannot be detected by XPS. More details of the experimental characterization conditions can be found elsewhere [START_REF] Posseme | [END_REF]14].

The determination of the p-SiOCH sidewalls film composition is achieved thanks to the specific die where different zones exhibiting regular arrays of lines, blanket substrate and unpatterned mask material are present. From the concentrations of the different elements present on a blanket area (covered with the metal hard mask) and by recording XPS spectra in the perpendicular mode in dense features, we can obtain the chemical composition of the patterns sidewalls. However, the contributions originating from the SiO 2 sidewalls and those originating from the p-SiOCH sidewalls cannot be separated. The experimental protocol is described in more details elsewhere [15]. To overcome this issue we developed an improvement of the experimental procedure by using the subtraction between two analysed patterned areas (Fig. 2). Two XPS acquisitions are performed in the perpendicular mode with contributions originating from the top (TiN), SiO 2 and p-SiOCH sidewalls of the features in a large pitch (LP) (Fig. 2(a)) is performed first. A second XPS acquisition is then performed in a smaller pitch (SP) where contributions originate from the top (TiN) and SiO 2 sidewalls of the features only (Fig. 2(b)).

Assuming the compositions on top of metallic hard mask and sidewall of SiO 2 trenches are similar between LP and SP, the substraction of the signal (LP-SP) corrected to the ration P2/P1 provides the contribution of the p-SiOCH sidewalls only. For these specific measurements the TiN and SiO 2 thickness are 15 nm and 125 nm, respectively while the p-SiOCH thickness is set at 670 nm. Using this experimental protocol we can determine the p-SiOCH sidewall composition as follows:

-The concentrations of Ti, O, N, F, Si, and C atoms are extracted from the Ti2p, O1s, N1s, F1s, Si2p and C1s core-level energy regions, respectively.

-The background subtraction is performed using a Shirley function calculated from a numerical iterative method.

-The intensity of photoelectrons coming from hard mask and SiO 2 (I A(Mask) ) in large area is calculated as following: For a given element A : This method gives a rough estimate of the p-SiOCH composition on the feature sidewalls.

) ( ) ( ) ( Mask I LP I Sidewall I A A A - =

II.3.2 Infrared Spectroscopy

Infrared Spectroscopy analyses in transmission (T-FTIR) and in multi internal reflection (MIR-FTIR) are performed using a Bruker IFS-55 FTIR spectrometer. For the T-FTIR analyses, the IR beam passes trough the wafer and is focused on a DTGS detector. The T-FTIR mode covers a spectral range from 400 to 4000 cm -1 . For the MIR-FTIR analyses, the sample is coupled to the s-polarised IR beam with two silicon prisms whose spacing is adjusted to Z=6 cm. A s-polarized IR beam coming from FTIR spectrometer is directed on the coupling area of the input prism, which ensures optical tunneling inside the wafer.

After being internally reflected about 100 times in our conditions (with an incidence angle of 34° and a wafer thickness of about 600 µm), the IR beam is coupled out of the wafer by the second prism and focused onto a liquid-N 2 -cooled HgCdTe detector [16,17]. The MIR technique is 100 times more sensitive than the standard transmission mode (T-FTIR). The IR beam propagates inside the wafer so that the samples have to be double-side polished. The MIR-FTIR mode covers a spectral range from 2600 to 4000 cm -1 . The spectral range is lower than this with the T-FTIR mode due to absorption of the IR beam at higher wave numbers into the silicon wafer during the internal reflection. Spectra are recorded with a 2 cm -1 spectral resolution and an average of 32 scans and 200 scans with the T-FTIR and MIR-FTIR modes, respectively. Before the spectrum acquisition, 5 min nitrogen purge is performed in the spectrometer to remove H 2 O and CO 2 vapor. The baseline of IR spectra is removed with the software "Resolution Pro" using a spline curve. For T-FTIR acquisition, a reference spectrum is measured using a virgin silicon wafer in order to remove the spectral contribution of the silicon wafer and the optical bench.

II.3.3 Spectroscopic Ellipsometry (SE)

The refractive index, extinction coefficient and the thickness of the porous SiOCH films were measured using a UV-visible spectroscopic ellipsometry from Jobin Yvon. To determine those values, the ellipsometry parameters were fitted with an absorbent Cauchy model for the porous SiOCH.

II.3.4 HF decoration method

We can determine the thickness of the modified p-SiOCH film thanks to spectroscopic ellipsometry (SE) using the decoration technique. This technique relies on the principle that the pristine porous SiOCH film is not consumed during hydrofluoric acid (HF) dip while the modified layer (with moisture uptake and carbon depletion) is removed [18][19][20].

The thickness of the remaining p-SiOCH film is determined by spectroscopic ellipsometry (SE) before and after HF dip allowing the thickness of the modified layer to be determined. On patterned wafers, the wafers are cleaved and dipped into a 1% diluted HF solution during 15 s after p-SiOCH etching. Using this protocol, we can estimate the thickness of the damaged layer on the trench sidewalls by using scanning electron microscopy (SEM). Before SEM observations, the trenches are filled with resist in order to prevent potential feature collapse during SEM exposure.

II.3.4 Ellipsometric porosimetry (EP)

The Ellipsometric porosimetry technique has been used to characterize the impact of FC based plasma exposure on the mean pore radius, the pore size distribution and the open porosity of p-SiOCH. Ellipsometry porosimetric measurements are performed in the visible range on an EP12 ellipsometric porosimeter from SOPRALAB [21]. It consists of a rotating polarizer spectroscopic ellipsometer coupled with a vacuum chamber which is operated in a pressure range between 10 -3 Torr and the saturation vapor pressure (Ps) of the adsorptive. For a solvent partial pressure P/P S , the adsorptive penetrates into the lowk and condenses into the pores leading to an increase of the refractive index. Indeed the vapour of the adsorptive condenses in the pores for a vapor pressure (P a ) lower than a flat surface liquid (P s ) and the value of partial pressure P a /P s depends on the pore size, surface tension and molar volume of the solvent.

Two solvents are used as adsorptive: methanol (Ps~105 Torrs, nmet=1.329 at 633 nm) and water (Ps~20 Torrs, nWater=1.33 at 633 nm). The methanol solvent penetration kinetics over time through the damaged p-SiOCH surface gives information on the sealing of the surface [22]. The degree of hydrophilization associated to the plasma damage is determined with water vapors thanks to a dedicated bi-layer analysis. The spectra are saved with a CCD detector between 1.5 and 4 eV at an angle of incidence set at 60.25°. Thickness and refractive index of the layers are calculated from the ellipsometric data as a function of the relative pressure (P rel =P/P s , with P the chamber pressure) using a Cauchy law in adjunction with a Lorentz oscillator to take into account the absorption band in the ultraviolet region. On the basis of the Lorentz-Lorenz effective medium approximation, the volume fraction of solvent adsorbed in the pores is calculated as described in a previous paper [21].

The p-SiOCH sidewall modification of patterned structures differs from at the bottom of the trenches. An innovative non-destructive method has recently been developed to measure the porous properties of p-SiOCH patterned layers as integrated into circuit called Scatterometric Porosimetry (SP) [23,24]. It mainly consists in the use of an EP tool (previously described) to record the scatterometric response of periodic structures made of porous material as a function of the relative pressure of the solvent. The patterned structures are chosen to have a critical dimension which is equivalent to the interconnect line size except that they typically consist of parallel periodic lines. The porous properties are subsequently extracted from the SP measurement with the use of a specially-developed scatterometric modeling [23]. For such analyses, patterning of the porous SiOCH trenches is performed using a metallic hard mask (Figure 1) with the following film thickness: a titanium nitride layer (15 nm thick), an oxide capping layer (125 nm thick) and the porous dielectric material (670 nm thick and 28% of porosity).

II.3.5 Capacitance measurement: Mercury Probe

The dielectric constant of blanket films has been calculated before and after the different plasma exposures using capacitance-voltage (C-V) measurements at 0.1 MHz. C-V measurements have been performed using a mercury probe capacitance measurement (C-V) system (model SSM495).

II.5.1 Bottom line roughness

Atomic force microscopy (AFM) measurements are performed at ambient atmosphere with a Nanoscope III multimode microscope from Digital Instruments. The RMS roughness is determined by tapping mode AFM with the measurement of the standard deviation of the height distribution (RMS). More detail of the measurement can be found elsewhere [25].

III. P-SiOCH film modification induced by fluorocarbon based plasma

A low-k material must withstand different integration steps, in particular the etching step. However, p-SiOCH materials are much more sensitive than SiO 2 to plasma exposure (chemical and physical impact of the plasma may induce p-SiOCH damage which in turn alters the dielectric film properties) [2]. A large contribution of the sensitivity comes from porosity introduction [START_REF] Posseme | [END_REF]. In this section, we propose to evaluate the sensitivity of p-SiOCH (PECVD p-SiOCH with 28% porosity) material after trench etching under FC based plasmas exposure. The bottom (estimated thanks to experiment performed on blanket wafers) and sidewalls p-SiOCH film modifications are compared and the potential solutions are discussed.

III.1 Bottom p-SiOCH film modification after partial etching

III.1.1 Surface analyses

The XPS survey spectrum of as-deposited p-SiOCH (not shown), indicates the presence of silicon, oxygen and carbon (hydrogen is not detected by XPS). The surface composition, reported in Table 2, shows that p-SiOCH is composed of Si (36%), O (40%) and C (24%).

III.1.2 Volume analyses T-FTIR analyses

Fig. 3 shows a typical T-FTIR spectrum of the as deposited porous SiOCH. In the 1000 to 1500 cm -1 spectral range, T-FTIR spectrum exhibits three main absorption bands at 1022, 1065 and 1140 cm -1 corresponding to C-Si-O and Si-O-Si stretching vibration bands [21,26]. Two additional peaks are also observed at 1275 cm -1 and 2960 cm -1 , which are assigned to the Si-CH 3 and C-H 3 vibration bands, respectively. After p-SiOCH exposure to FC based plasma, the FTIR spectrum (see Fig. 3) exhibits the same vibration modes as those detected in the pristine material. Based on these spectra, the methyl content in the film can be monitored by calculating the peak area ratio from the T-FTIR spectra:

si O Si CH Si CH Si SiCH A A A R - - - - + = 3 3 3
where R Si-CH3 represents the methyl ratio, A Si-CH3 the area of the Si-CH 3 vibration bands and A Si-O-Si the area of the Si-O-Si vibration bands [14,34]. In these conditions, after partial etching, the methyl depletion is estimated at about 25% compared to the pristine material.

MIR-FTIR analyses

In the 2700 to 4000 cm -1 spectral range, the MIR-FTIR spectrum of porous SiOCH after partial etching shows vibration bands which are assigned to OH and CH x bonds (see Fig. 4) [16]. , respectively [17,20,27]. This broad OH vibration bands exhibits the same intensity as the OH vibration band originating from the Si bulk spectrum. Therefore the OH vibration is mainly attributed to the Si bulk coming from the native silicon oxide and it is then difficult to prove the presence of OH bonds in the as deposited materials. It has to be noticed that the OH vibration bands are hardly detected on the T-FTIR spectra due to the lower sensitivity compared to MIR-FTIR technique.

After partial etching, the absorption of the OH vibration bands is much higher than in the as deposited porous SiOCH. This result shows an increase in Si-OH bonds formation due to water uptake with free and bonded H 2 O. It has to be noticed that no OH vibration bands have been observed with FTIR measurement due to the lack of sensitivity. After partial etching, the CH x peaks (CH 3 and CH 2 ) are slightly lower than those of the as deposited porous SiOCH indicating lower methyl depletion in the remaining porous SiOCH (which is in good agreement with the FTIR analyses).

III.1.3 Depth modification of the p-SiOCH film Using decoration method

We have then determined the thickness of this modified p-SiOCH film thanks to spectroscopic ellipsometry (SE) using the decoration technique previously described.

Based on this protocol, after the etching process, the damaged layer is estimated to be about 30 nm thick. We can notice that after the HF dip which removes the modified layer, no methyl depletion is evidenced after partial etching (not shown) indicating that the methyl depletion is mainly localized in the modified layer.

Using EP

The hydrophilic properties of the modified layer have been also determined with EP measurement using water as solvent. Based on the protocol previously described, the thickness of the modified layers is estimated after FC based plasma exposure to be 29nm.

Both techniques show similar modified layer thickness confirming the correct estimation of modified layer thickness on blanket wafers.

III.1.4 Impact of the modified layer on pore sealing

The impact of the thick hydrophilic modified layer on open porosity and permeability has been investigated thanks to EP analyses with methanol as solvents (Fig. 5). The degree of porosity is followed as function of time at a fixed pressure (0.8 of vapour pressure) to determine the total porosity and permeability of the porous SiOCH modified layer (as described in the previous section). The open porosity which is measured at the plateau remains the same as the pristine material (of about 27%) with or without partial etching. This result demonstrates no significant change in pore density and pore size in the remaining porous SiOCH films. This also indicates that the porosity of the modified layer is similar to the pristine material. These experiments performed on blanket p-SiOCH show that the film is altered under FC based plasma exposure leading to the formation of a hydrophilic perturbed layer depleted in methyl groups. This perturbed layer induces an increase of the dielectric constant from 2.35 to 2.7 after partial etching. These results are similar than those observed for a spinon p-SiOCH material [START_REF] Posseme | [END_REF]. In this previous study, we demonstrated that the film modification could be assigned to fluorine reactive species diffusion and to the impact of the ion bombardment. In the first case, the diffusion of fluorine atoms into the porous material generates an important degradation of the material since fluorine is very reactive toward carbon and silicon. While in the second case, the ion bombardment can break Si-O, Si-CH 3 , and C-H bonds [START_REF] Posseme | [END_REF].

The study of the film modification performed on blanket wafer gives a good picture of the p-SiOCH film modification at the bottom of trenches. However, can we draw similar conclusion on the p-SiOCH sidewalls modification? These results are now compared with the film modification analyses performed on patterned structure.

III.2 p-SiOCH film sidewall modification after partial etching

After patterning of p-SiOCH trenches in CF 4 /C 4 F 8 / N 2 /Ar plasma, no sidewall modification is clearly evidenced using the decoration technique. This can be attributed to of a lack of sensitivity of this technique for thin damage layer (Fig. 6). Indeed the hydrophilic properties of the modified layer determined with SP measurement using water as solvent for EP analyses show different behaviour. The thickness of the damaged layer after etching is estimated in this case to 10 nm. This result tends to indicate the SP can be a more accurate technique to estimate the sidewall p-SiOCH film modification compared to decoration method.

Sidewall composition analyses performed using chemical topography analyses by XPS is summarized in Table 2. After etching, the presence of fluorocarbon species are detected on the p-SiOCH trench sidewalls while Ti is hardly detected. The presence of this small amount of Ti can be attributed either to the sputtering of the hard mask during etching or to the element concentration measurement error induced by the experimental protocol.

The impact of this FC layer formation on pore sealing was determined thanks to EP analyses using methanol as solvent. The methanol diffuses through the p-SiOCH sidewalls (not shown) indicating that p-SiOCH sidewalls are not pore sealed by the presence of the fluorocarbon layer. These results indicate that a CF x layer grows on the p-SiOCH sidewalls (porosity of 28% and k = 2.35) during FC based plasma and that no pore sealing is induced by this CFx layer.

These observations are similar to previous observations on blanket wafers. Therefore, we can propose similar mechanisms to explain the p-SiOCH sidewall film modification which can be assigned to fluorine reactive species diffusion on the sidewalls leading to the creation of a hydrophilic layer. The effect of the ion bombardment being lowered since the ions deflected on the hard mask have lower energy than ions directly striking the p-SiOCH blanket wafers.

The whole results show that p-SiOCH (bottom and sidewall) is very sensitive to FC based plasma leading to the formation of a hydrophilic layer. This modified layer cannot be neglected since it alters the dielectric film properties. Indeed, k value measurement performed on blanket and patterned wafers shows that the perturbed layer leads to an increase of the dielectric constant. The k degradation is more pronounced as the thickness of porous SiOCH decreased [28]. Furthermore, the modified layer can also degrade the reliability performance of the dielectric layer. Therefore solution must be found to limit this p-SiOCH film modification or to improve the hydrophobic properties of the dielectric film after etching.

III.3 Solutions to limit p-SiOCH film modification

As suggested, p-SiOCH film modification (bottom and sidewalls) can be induced by fluorine, ion bombardment. Based on these results, different solutions were proposed in the literature to limit the p-SiOCH film modification.

One approach is to reduce fluorine reactive species diffusion in the film and the ion bombardment impact. This can be achieved by using high polymerising chemistries [START_REF] Posseme | [END_REF]29]. Fig. 7 shows that higher CH 2 F 2 concentration in the CF 4 /Ar gas mixture enables a reduction of the p-SiOCH (spin-on with 45% porosity) sidewall film modification. We have shown that such polymerizing etch chemistries lead to the formation of thick fluorocarbon layers on the p-SiOCH sidewalls which act as protective layers against fluorine diffusion inside the porous SiOCH material. Less sidewall modification are observed in high polymerizing chemistries such as CF 4 /Ar/CH 2 F 2 which produce thicker FC layers on the sidewalls than in low polymerizing chemistries such as CF 4 /Ar [29]. Fig. 7: Sidewall film modification of porous SiOCH (containing 45% porosity) after etching as a function of the CH 2 F 2 concentration in CF 4 /Ar gas mixture. The sidewall modification is determined using the decoration method.

Finally an emerging solution, as proposed in the literature, is to restore the hydrophobic properties of the modified layer by using a curing process: a silylation based process [28]. For instance, for the 45nm interconnect technology node, Chaabouni et al [28] show that after p-SiOCH (porosity of 25% and k=2.5) integration using a metallic hard mask approach, the integrated k value is about 3 (see Fig. 8). After a post etching, restoration process using hexamethyldisilazane (HMDS: (CH 3 ) 3 -Si-NH-Si-(CH 3 ) 3 ) enables to decrease the integrated k value to 2.6 close to the dielectric constant of the as-deposited p-SiOCH [28]. HMDS molecules selectively react with the silanol species and convert them into hydrophobic trimethyl-siloxy (-O-Si-(CH 3 ) 3 ) groups leading to an increase of the carbon content that prevents water uptake.

III.4 Summary

Based on these results, we have shown that the damages of porous SiOCH sidewalls induced by FC plasma exposure are inevitable. They can alter dielectric film properties and generate an increase of dielectric constant and/or reliability degradation. The film modification due to reactive fluorine species diffusion and ion bombardment impact can be limited by using high polymerizing FC chemistries. In addition, a complementary solution is the restoration of the hydrophobic properties of the modified layer using silylation process.

We have shown the porosity introduction into SiOCH material induces higher film sensitivity to FC based plasma. But porosity introduction also leads to new issue like roughening of bottom line. In this case, are the solutions proposed to limit p-SiOCH damage compatible with the reduction of bottom line roughness?

IV. Bottom line roughness

As illustrated in Fig. 9, increasing porosity of PECVD SiOCH material also leads to an increase of the bottom line roughness after line etching using a metal hard mask.

Fig. 9: Bottom line roughness evolution after line etching using a metal hard mask (with similar etching conditions) as a function of the p-SiOCH film porosity.

The impact of such a roughness on reliability performances has been evaluated by comparing two etching processes inducing either a low (Fig. 10(a)) or a high bottom line roughness (Fig. 10(b)). Whatever the etching chemistries, Fig. 10(c) shows that the electromigration trend is not impacted while in the meantime, the time dependent dielectric breakdown (TDDB) lifetime is degraded (Fig. 10(d)) when the bottom line roughness increase.

These results demonstrate that bottom line roughness strongly impact the TDDB. Therefore it's mandatory to limit the bottom line roughness in order to prevent TDDB degradation.

IV.1 Mechanism of bottom line roughness formation

The evolution of the roughness during plasma etch processes has already been studied [30 -39]. Plasma processes usually smooth the etched surfaces. The isotropic component of the etch process, [33,34] as well as the shadowing effect [40,41] and the preferential sputtering of the material for off-normal ions [33,42,[START_REF] Flamm | Plasma Etching: An Introduction[END_REF] contribute to this effect during plasma etch processes. Nevertheless, several groups have reported that etched surfaces can also be roughened when exposed to plasma etch processes. [39,[START_REF] Zhao | [END_REF][45][46][47]. Several explanations such as preferential etching by re-emitted species [42,[START_REF] Flamm | Plasma Etching: An Introduction[END_REF] or ion induced defects [46] have been proposed. However, recent studies [39,47] tend to prove that etch induced roughness rather originates from micromasking phenomena induced by particles sputtered form the chamber walls during the etch process. Etch induced roughness has also been observed with p-SiOCH materials [30][31][32][33]48]. These papers claim that, the fluorocarbon species deposited on the porous low-k surface induce p-SiOCH roughness. Tatsumi and Urata [49] suggest that low-k materials are roughened when a thin fluorocarbon layer (4 nm) is deposited on the material surface, while no fluorocarbon or thicker fluorocarbon layers do not lead to roughening. Yin et al. [31,48] attribute the etching induced roughness to fluorocarbon species micromasking the material surface. They attribute the non uniform coverage of the surface to the presence of pores in the material. Lazzeri et al. [30] propose that the larger surface of porous materials leads to an incomplete coverage of the surface of the dielectric by the fluorocarbon layer, compared to dense dielectric layers. Due to this incomplete coverage, plasma species can directly interact with the low-k material, leading to harsher etch conditions and roughness. In both cases, the models assume that the roughness is created by an incomplete or non homogeneous coverage of the porous SiOCH surface by fluorocarbon species. However in our experimental conditions close to the industrial ones, we demonstrated the roughness of porous SiOCH also occurs under fluorocarbon free plasma exposure ( SF 6 plasma) suggesting complementary mechanism [25]. We proposed that: (1) during the first few seconds of the etch process, the surface of porous SiOCH materials gets denser.

(2) Cracks are formed, leading to the formation of deep and narrow pits. (3) Plasma radicals diffuse through those pits and the pore network and modify the porous material at the bottom of the pits. (4) The difference in material density and composition between the surface and the bottom of the pits leads to a difference in etch rate and an amplification of the roughness [25].

IV.2 Solutions

Previous studies performed on blanket wafers indicate that the solution to smooth p-SiOCH during etching is to use highly polymerising FC chemistries [25]. The impact of the etch chemistry on porous SiOCH (p-SiOCH with 25% porosity) roughening is illustrated in Fig. 11 However, this solution does not take into account the impact of the metallic hard mask on the p-SiOCH roughness formation. Indeed, on patterned wafers, in addition to the roughening of the p-SiOCH induced by fluorocarbon plasmas, additional roughness also originates from the micromasking induced by TiFx compounds coming from the TiN mask sputtering [START_REF] Posseme | 6 th International Conference on Microelectronics and Interfaces[END_REF][START_REF] Darnon | [END_REF]. The two key parameters to limit this metallic hard mask impact on roughness formation are the substrate temperature and the polymerising rate of the chemistry as illustrated below.

IV.2.1 Impact of Substrate temperature on p-SiOCH roughness formation

The impact of the substrate temperature on porous SiOCH roughness is illustrated Fig. 12 by comparing the etching of p-SiOCH patterned structure using CF 4 based chemistry with two different substrate temperatures (higher or lower than 50°C). Increasing substrate temperature induces a lower bottom p-SiOCH roughness and straight profiles as evidenced by the SEM cross section in Fig. 12(b). This trend is correlated with a lower titanium (5%) concentration at the bottom of trenches (determined by XPS), compared to lower substrate temperature where 12% titanium is measured by XPS (Fig. 12(a)). Therefore increasing substrate temperature above 50°C favours the formation of more volatile TiFx etch products and therefore decrease the micromasking induced effect of Ti based compounds deposited on p-SiOCH surfaces. Another way to limit this metallic hard mask sputtering impact on p-SiOCH roughness formation is to protect it during etching. This can be achieved by using polymerizing chemistry.

IV.2.2 Impact of the etching chemistry on the p-SiOCH roughness

For substrate temperatures above 50°C, the impact of the etch chemistry on the porous SiOCH (p-SiOCH with 25% porosity) roughening is now presented in Fig. 13 by comparing two CF 4 based etching chemistries with and without C 4 F 8 addition. With C 4 F 8 addition to CF 4 , a lower bottom p-SiOCH roughness is clearly observed on top SEM-CD pictures (Fig. 13(b)) compared to CF 4 only (Fig. 13(a)). XPS analyses reveal that C 4 F 8 addition to CF 4 leads to the formation of a thick fluorocarbon layer on top of the metal hard mask (25% C and 13% F) decreasing its sputtering rate (5% titanium is detected at the bottom of trenches). Without polymerizing gas addition, the presence of higher titanium species concentration (10%) at the bottom of trenches correlated with a thinner fluorocarbon layer is detected on top of the metal hard mask (15% C and 10% F). These results show that the use of highly polymerizing chemistries protect the metal hard mask, and limit its sputtering rate. Consequently, Ti based micromasking is reduced limiting the development of rough p-SiOCH surfaces.

IV.2.3 Summary

In summary, we have shown that the bottom surface roughness is directly related to the degree of SiOCH porosity and such a rouhgness can lead to the degradation of the TDDB while the electromigration is not altered. The roughness formation is explained by different mechanisms such as the intrinsic p-SiOCH roughness developed under plasma exposure or micromasking coming from reactor wall conditioning, FC species or hard mask environment. With a metal hard mask approach, the solution to limit p-SiOCH formation is to develop etching processes at substrate temperature above 50°C (to limit sputtered metal species condensation leading to micromasking) and the use of highly polymerising chemistry (to protect MHM with FC layer and also playing the role of p-SiOCH smoothing).

Conclusions

The major difficulties of p-SiOCH material integration using a metallic hard mask integration have been presented from the etching point of view. The first problem encountered with porosity introduction into SiOCH material is the high sensitivity of p-SiOCH film during line etching with fluorocarbon based plasma. In this case, fluorine reactive species diffusion and ion bombardment leads to the formation of a hydrophilic layer (bottom and sidewalls), not sealing the pores, and altering the dielectric film properties. While the second problem presented with bottom line roughness formation and explained by intrinsic roughness or micromasking (coming from reactor wall conditioning or FC species and increased by the low volatile compounds (TiFx) coming from the sputtered hard mask) strongly impacts reliability performances like TDDB.

The common solution to limit these issues is to develop high polymerising chemistry at high substrate temperature above 50°C. Developing etching processes at substrate temperature above 50°C allows limiting sputtered metal species condensation leading to micromasking. While the use of high polymerising chemistry leads to the formation of thick fluorocarbon layers which act as protective layers against fluorine diffusion inside the porous SiOCH material and ion bombardment impact but also protect the metallic hard mask and smooth the p-SiOCH film. In addition a complementary solution to limit the impact of the film modification is the restoration of the hydrophobic properties of the modified layer using silylation process.

Fig. 1 :

 1 Fig. 1: Metallic hard mask (MHM) description: (a) Line lithography with organic BARC, (b) HM etching and resist stripping, (c) Line etching.

Fig. 2 :

 2 Fig. 2: Desciption of XPS analyses on patterned wafers in large pitch (P1 = 800nm, feature dimension= 200nm) (a) and small pitch (P2= 340nm, feature dimension = 200nm) (b) areas to obtain p-SiOCH sidewall film composition.

-

  The sidewall porous SiOCH intensity is calculated for each element (extracted from the Ti2p, O1s, N1s, F1s, Si2p and C1s core-level energy regions).

-

  Then the concentration of the given element A is quantified with respect to the other elements detected as follows :

  In this part, experiments have been performed on blanket p-SiOCH films to estimate bottom p-SiOCH film modifications induced by fluorocarbon plasmas. First, the surface and structural composition of the pristine p-SiOCH are investigated. Then to mimic material consumption for 45 nm technology node, 120 nm of p-SiOCH has been etched in CF 4 /C 4 F 8 / N 2 /Ar used for p-SiOCH trench etching. P-SiOCH film has been analyzed on blanket wafers by XPS, T-FTIR and MIR-FTIR before and after partial etching.

Fig. 3 :

 3 Fig. 3: T-FTIR spectra of p-SiOCH before and after partial etching in FC based plasma

1 )Fig. 4 :

 14 Fig. 4: MIR-FTIR spectra of p-SiOCH before and after partial etching in FC based plasma

Fig. 5 :

 5 Fig. 5 : Evolution of porosity as a function of time at a fixed relative pressure (0.8Ps) with methanol thanks to EP measurement

Fig. 6 :

 6 Fig. 6: Estimation of the sidewall modification of p-SiOCH trenches after CF 4 /C 4 F 8 / N 2 /Ar etching using the decoration technique

Fig. 8 :

 8 Fig.8: Effect of sylilation process after etching on integrated k value[40]

Fig. 10 :

 10 Fig. 10: Impact of the bottom line roughness high (a) and low (b) on electromigration (EMG) (c) and Time Dependent Dielectric Breakdown (TDDB) (d)

Fig. 12 :

 12 Fig. 12: Illustration of p-SiOCH etching in a metallic hard mask environment with a CF 4 based etching chemistry and a substrate temperature lower (a) or higher (b) than 50°C, respectively.

Fig. 13 :

 13 Fig. 13: Illustration of p-SiOCH etching in a metallic hard mask environment with either a CF 4 (a) or C 4 F 8 (b) based etching chemistry for a substrate temperature higher than 50°C.

TiN

Table 2

 2 SiOCH partial etching in CF 4 /C 4 F 8 / N 2 /Ar plasma, XPS analyses reveal that the surface is composed of 25%Si, 30%O, 34% C, 8%F and 3%N (See Table2).

	: XPS surface composition of p-SiOCH (porosity of 28% and k=2.35) on
	blanket and patterned wafers after FC etching
	Material	Si(%) O(%) C(%) F(%) N(%) Ti
							(%)
	p-SiOCH as deposited (blanket wafer) 36	40	24	0	0	0
	p-SiOCH after FC etching (blanket	25	30	34	8	3	-
	wafer)
	p-SiOCH after FC etching (sidewall)	17	19	38	23	1	2
	After p-

  by comparing the RMS roughness on blanket wafers induced by two different fluorocarbon based etching chemistries (different degrees of polymerizing). Roughness evolution as a function of the etched depth for PECVD porous SiCOH (25% porosity) exposed to either CF 4 / Ar or CF 4 /Ar/CH 2 F 2 plasmas. RMS value of the roughness measured by AFM.

	CH 2 F 2 addition to CF 4 /Ar (more polymerizing chemistry) induces a decrease in
	roughness.
	Roughness rms (nm)	4 8 12				CF4/Ar CF4/Ar/CH2F2
		0	0	100	200	300	400
				Etch thickness (nm)
	Fig. 11:


N Chaix

C Gourgon

C Perret

S Landis

T Leveder

Nanoimprint lithography processes on 200 mm Si wafer for optical application: Residual thickness etching anisotropy

It is well known that one limitation of thermal nanoimprint lithography is the difficulty to imprint simultaneously nano-and microstructures because of the resulting different residual layer thicknesses, which induce a very poor control of the pattern sizes during the etching steps. Line gratings with densities varying from 1 to 15 have been imprinted on 8 in. wafers. The residual thickness varies from 38 to 158 nm. Different plasma chemistries have been studied for the etching of the residual layer. The patterns have been characterized after the imprint and the etching steps by scatterometry. The results show that some chemistries are very promising for the control of the patterns during the etching step. A O2/C12/Ar process has been particularly studied, and it has been demonstrated that it presents a very high anisotropy, which allows the use of long etching times in order to remove the residual layer in gratings with various densities with no variation of the critical dimension.

I. INTRODUCTION

Thermal nanoimprint lithography 1 ͑NIL͒ is a very promising nanopatterning technique for various domains of applications, especially for optical devices. Nevertheless, several issues still limit its use at an industrial scale. One of the important drawbacks of thermal NIL is the control of the residual thickness ͑hr͒ of polymer remaining at the bottom of the patterns after the imprint. 2 It is well established now that patterns with various densities and sizes result in a nonuniform residual thickness since the polymer flow is mainly driven by the ratio between the mold cavity volume and the pressed material volume. 3 This hr distribution is not a problem for applications using directly the imprinted polymer as a functional material. However, many applications require a transfer of the resist patterns into the underlying substrate. This transfer is usually done by plasma etching. The residual thickness is etched first. The etching time has to be long enough to guarantee a complete etching of hr even for large values. As a consequence this etching step is completed first, where hr is thinner. This may lead to a critical dimension ͑CD͒ change for printed feature due to the horizontal etch rate. Therefore the control of imprinted feature size with various densities may be very tricky. In this article we present a study of the control of the pattern size depending on the plasma chemistry during hr etching process, and a high anisotropy process is demonstrated. It is shown that a high control of the pattern size is obtained in structures with densities varying from 1 to 15.

II. EXPERIMENTAL DETAILS

Stamp has been manufactured by deep ultraviolet optical lithography ͑ = 248 nm͒ with a 450 nm thick positive resist ͑UV5͒ from Rohm & Haas Company. After the development, the patterns are transferred into the silicon using high density plasma etching. Stamps were coated with a fluorocarbonbased antisticking layer, Optool DSX ͑Daikin corporation 4 ͒, to reduce the surface free energy of the mold from 60 mJ/ m 2 down to 11 mJ/ m 2 .

Figure 1͑a͒ shows optical and scanning electron microscopy ͑SEM͒ views of 8 in. silicon stamps containing more than 400-180 nm deep optical devices made of a 250 nm dense line arrays. Another 260 nm deep mold with line size ranging from 200 nm up to 1 m and filling factor ranging from 1 to 15 was used in order to point out the influence of the feature density. The complete mold design is presented in Fig. 1͑b͒.

Printing processes were performed on 8 in. wafers in an EVG ® 520HE system. 5 The maximum force that can be applied by the piston is 40 kN, which is equivalent to a pressure of 12.7 bars over an 8 in. wafer. The printing process is performed under vacuum ͑7 ϫ 10 -2 mbar͒ with typical process conditions set at 5000 N, 130 °C, and a printing time set at 5 min ͑Fig. 2͒. A 200-nm-thick Sumitomo NEB 22 resist film was coated on a silicon substrate with native oxide. This resist is a high resolution, chemically amplified resist first designed for electron beam lithography. We have already shown 6 that this resist film is suitable for hot embossing processes, and thanks to its etching resistance, can be used to transfer the printed pattern into the substrate. Prior to the printing process, the resist film was first annealed at 110 °C for 2 min in order to remove the residual solvent. The resist is a blend of polymers based on poluhydroxysty-a͒ Author to whom correspondence should be addressed; electronic mail: nicolas.chaix@cea.fr ren, and it is a thermoplastic polymer with a glass transition temperature around 80 °C. The etching behavior of this resist is therefore similar to the one of standard NIL materials.

A residual resist layer has been etched using several plasma processes in an Applied Materials IPC reactor ͑Table I͒. The main etching time t 0 was determined using end point detection. As soon as residual thickness ͑hr͒ is etched, emission of the CO bond decreases since a silicon surface appears, indicating that the etching step is finished.

To ensure a fine characterization of the etching process and its optimization, a nondestructive metrology technique should be used. As already shown 7 scatterometry appeared to be suitable since the whole geometry ͑height, CD, sidewall angle, hr͒ may be determined. 8 The diffraction figure was computed using rigorous electromagnetic simulation code ͓MMFE ͑modal method by Fourier expansion͒ 9 ͔. We used a library based approach for the extraction of the geometrical parameters from ellipsometric parameters measured on specific targets consisting on gratings larger ͑5m m ͒ 2 than the ellipsometer spot ͑typically close to 50 m͒. Therefore this measurement may be seen as an in line characterization.

III. RESULTS AND DISCUSSION

The lines imprinted with the two molds are shown on Fig. 2 and the CD was measured by scatterometry. The results show that the imprinted CDs are very close and independent of the position on the wafer ͑Fig. 3͒. For the same pattern, the difference between the CD values is less than 5%. The homogeneity of the CD values is good for different L ͑line size͒/S ͑spacing͒ ratio studied: 1 / 0.5; 0.2/ 0.2; 1 / 1; 0.5/ 1; 1 / 3; 1/ 7. The homogeneity control is possible with the very good imprint pattern quality.

The etching of the residual layer ͑hr͒ of polymer is the key point of the complete development of a NIL process for future industrial applications. It has been demonstrated that for a given pattern geometry, the residual layer is homogeneous on large surfaces such as 8 in. wafers, 10 but some small variations can also appear, and a etch process insensible to these differences would be of benefit to industrial processes. The key point of the hr etching is therefore the anisotropy of the process. Two etch rates can be determined during the plasma etching: one corresponds to the vertical etching and to the value commonly measured, and one is representative of the lateral etching. During the plasma etching process a time t 0 is needed to open the residual layer in the patterns having the smallest hr. If the next etching step is performed directly, only these patterns will be transferred into the substrate because some resist remains at the bottom of the other patterns. It is therefore necessary to increase the time of the hr etching to t 1 to get a complete opening of all the patterns. If the hr etching process is anisotropic, all the patterns will be transferred to the substrate with a good control of their size. That means that the etch rate in the lateral direction is very small. In the case of an isotropic process, the width of the first patterns will be changed during the overetching time because the lateral etch rate becomes higher as soon as the amount of resist that can be etched vertically decreases.

The anisotropy of several etching processes has been characterized. The imprinted patterns are 240/ 260 nm lines, as shown on the SEM picture of Fig. 4. The residual layer is 110 nm thick, and the pattern height is 180 nm, which is a small resist budget useful to characterize the profile fidelity during the hr etching. Five plasma chemistries have been tested. The etching rate was first measured for each process on a flat polymer surface to determine the etching time needed to open 110 nm. Then the patterned wafers were etched using this time and were characterized by SEM. The linewidth was measured at the top and at the bottom of the line to control the CD and the fidelity of the vertical profile. Figure 4 presents the corresponding SEM pictures. There is no picture for the process 5 since the higher content of oxygen leads to a huge lateral etching, and the patterns were completely stripped during the hr opening. FIG. 2. ͓͑a͒ and ͑b͔͒ Optical and SEM images of the 250 nm dense line imprint. The height of the feature is 180 nm. ͓͑c͒ and ͑d͔͒ SEM images of the second mold imprint for two samples: 0.2/ 0.2 and 1 / 7 ratios. It is clearly shown that the CF 4 / CH 2 F 2 process has a too low etch rate and leads to a very poor control of the profile due to etch products which are redeposited on the sidewalls during the etching. The values of the corresponding linewidth and height are given in Table I.

The O 2 / Cl 2 / Ar, CF 4 , and HBr/ O 2 ͑6 / 1͒ processes lead to better profiles and the resist height is still 180 nm after the etching. This demonstrates that the vertical etch time is well controlled. But some differences still appear. Despite a vertical sidewall, the lines etched with the CF 4 process present a smaller CD; this illustrates that the process is homogeneous, but not anisotropic enough. Finally, the O 2 / Cl 2 / Ar and HBr/ O 2 ͑6 / 1͒ processes exhibit a good anisotropy. Some corner rounding appears at the top of the line, but the vertical sidewall allows a direct transfer of the structures with a good control of the linewidth. The O 2 / Cl 2 / Ar process shows a higher etch rate ͑3n m/ s compared to 1.2 nm/ s͒ and would perhaps be preferred for industrial applications.

The O2/Cl2/Ar process was chosen to study the etching anisotropy in patterns with various densities. This choice is based on the previous results since this chemistry exhibits the best control of CD for one kind of structures. Several wafers have been imprinted using the mold with filling densities ranging from 1 up to 15. The residual thickness has been measured by scatterometry in several dies depending on the L / S ratio. The experimental results are presented on Fig. 5. The line is defined by the theoretical values of hr, determined by the conservation of the polymer volume which flows to the mold cavities. This leads to the following equation: hr= initial polymer thickness-mold depth ͑1 / ͑1+L / S͒͒.

It is clear on Fig. 5 that the experimental hr is in good agreement with the theoretical one. The wafers have been etched in order to remove the residual thickness using the O2/Cl2/Ar process. Three different etching times have been used. The first one corresponds to the time needed to etch 38 nm, which is the lowest hr measured in the 1 / 0.5 m lines. The residual thickness is therefore equal to zero only in this grating. The second is defined to etch 109 nm, and the last one is long enough to remove the largest hr of 158 nm. In this last case, the residual thickness is completely etched in all the patterns, but an overetch time was therefore applied to the gratings with a L / S ratio higher than 0.14. The resulting residual thickness has been characterized by scatterometry again after the etching process, and the results are summarized on Fig. 5. The hr values are in good agreement with the expected ones defined by the imprint hr minus the etched thickness, 38, 109, or 158 nm. In the case of a 158 nm etching, the overetching time corresponds to a virtual etching of 120 nm in the case of L / S = 2. That means that an isotropic etching process would lead to a complete stripping of the resist during the overetching. The CD was also measured after the etching step by scatterometry and compared to the imprinted CD on Fig. 6. With anisotropic etching process, one would expect no variation of the CD after the etching step, and this perfect behavior is illustrated by the dashed line. The experimental points corresponding to the three different etching times are reported on the graph, and it appears that there is a very good agreement between the CD achieved after the imprint and the etching step. This result demonstrates that the O2/Cl2/Ar process is highly anisotropic and that an overetching time guarantees nevertheless the control of the CD. This process is a potential solution for the pattern transfer in structures with nonuniform residual thickness. The remaining resist thickness after a 158 nm etching step is presented on Fig. 7. The theoretical value is defined as follows: when the etching time corresponds exactly to the removing of the residual layer, the remaining thickness is equal to the 260 nm mold depth. This is observed only for L / S = 0.14. As long as an overetching occurs for higher L / S, the CD is not changed because of the high anisotropy, but the vertical etching continues and decreases the resist thickness. The final value should then correspond to the difference between the mold depth and the overetched thickness. As the overetching ͑vertical etching͒ time corresponds to 120 nm for L / S = 2. The final resist budget is then 140 nm only. The graph shows that the experimental resist thickness is in good agreement with the expected one. This effect is a limitation to the use of the anisotropic process since the reduction of the resist budget for the following transfer into the underlying substrate limits the possible etching depth. A hard mask can be nevertheless used to overcome this drawback. Once again a trade off has to be done between the resolution and the mold depth allowing a high transfer depth.

IV. CONCLUSION

A study of the residual layer etching has been presented and it has been shown that special plasma chemistries are of benefit to the pattern transfer, compared to a pure oxygen process which usually requires a very small hr value. A O2/ Cl2/Ar plasma process has been developed and its high anisotropy has been demonstrated. It has been shown that overetching times can be used with no variation of the pat-tern sizes, and that this high control of the features allows a perfect transfer of patterns with various residual thicknesses induced by different densities. This etching process could be a solution for the well known limitation of thermal NIL, and it was proven that it allows the imprint and etching of nanoand microstructures simultaneously.

FIG. 1 .

 1 FIG.1. ͑a͒ First mold: Si wafer covered with 250 nm dense lines; ͑b͒ second mold: Si wafer with ϳ20 dies presenting various pattern sizes and densities.

FIG. 3 .

 3 FIG.3. ͑a͒ Measurement linewidth ͑CD͒ for several L / S ratios ͑1 / 0.5; 0.2/ 0.2; 1 / 1 ; 0.5/ 1;1/ 3;1/ 7͒ for three chips on 200 mm wafer. ͑b͒ Mapping of the scatterometry measurements on the 200 mm wafer.

FIG. 5 .

 5 FIG.5. Results of the mapping measurements: study of the residual thickness ͑hr͒, hr as a function of the ratio L / S for experimental and theoretical imprints and after three etching levels ͑38, 109, and 158 nm͒ for five L / S ratios ͑1 / 0.5; 0.2/ 0.2; 1 / 1 ; 0.5/ 1;1/ 3;1/ 7͒.

TABLE I .

 I Linewidth ͑CD͒ and height measured after the imprint and hr etching processes.

			O 2 / Cl 2 / Ar		CF 4 / CH 2 F 2	HBr/ O 2
		Imprint	͑3 / 4 / 3͒	CF 4	͑4 / 3͒	͑6 / 1͒
	CD top	240	210	200	130	210
	CD bottom	240	250	210	315	230
	Ei	180	180	180	154	180
	Vertical etch rate		3 nm/ s	2.3 nm/ s	0.6 nm/ s	1.2 nm/ s

J. Vac. Sci. Technol. B 25"6…, Nov/Dec 2007 1071-1023/2007/25"6…/2346/6/$23.00 ©2007 American Vacuum Society

J V S TB-M i c r o e l e c t r o n i c sa n dNanometer Structures

J. Vac. Sci. Technol. B, Vol. 25, No. 6, Nov/Dec 2007


Ha Vinh

Lam Nguyen

Kenneth Koziol

Tarek Trabelsi

Safa Khemissi

Martin Schwell

Joseph Francisco

Isabelle Kleiner

Discovery of a Missing Link: First Observation of the HONO-Water Complex

come


est destinée au dépôt

Methyl

  Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate Nhu-Ngoc Dang, Hoang-Nam Pham, Isabelle Kleiner, Martin Schwell, Jens-Uwe Grabow, Ha Vinh Lam Nguyen To cite this version: Nhu-Ngoc Dang, Hoang-Nam Pham, Isabelle Kleiner, Martin Schwell, Jens-Uwe Grabow, et al.. Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate. Molecules, 2022, 27 (9), pp.2639. 10.3390/molecules27092639. hal-03814357


Wenhao Sun

Isabelle Kleiner

Arne Senftleben

Melanie Schnell

Internal dynamics of methyl p -tolyl sulfoxide in the

published or not. The documents may come


Pascal Guillo

Olivier Hamelin

Jacques Pécaut

Stéphane Ménage

Complexation to [Ru(bpy) 2 ] 2+ : the trick to functionalize 3,3 0 -disubstituted-2,2 0 -bipyridine

Complexation to [Ru(bpy)_2]

2+ : the trick to functionalize 3,3'-disubstituted-2,2'-bipyridine

While many papers reported the synthesis and chemical transformations of x,x 0 -disubstituted-2,2 0 -bipyr-idine with x = 4, 5 or 6, this is not the case when x = 3. Though, such 3,3 0 -disubstituted-2,2 0 -bipyridines would be a useful building block for the design of new original complexes. Here we report the results of our investigation concerning the synthesis of members of this family. This study led to the formation of several [(2,2 0bipyridine) 2 Ru(3,3 0 -dialkylated-2,2 0 -bipyridine)] 2+ complexes highlighting a surprising chemical behaviour of the 3,3 0 -disubstituted-2,2 0 -bipyridine ligand compared to its 4,4 0 , 5,5 0 and 6,6 0 -homologues.

Bipyridine and in particular 2,2 0 -bipyridine is, to date, probably the most widely used ligand in coordination chemistry mainly for its availability, its robust redox stability and its relative ease of functionalization. 1 Literature contains countless examples of metal (Ru, 2,3 Fe, 4 Ir, 5 Os, 3,6 Cu 7 ...) complexes bearing one or more of this versatile building block. This enormous popularity of these classes of compounds is driven by the wide range of applications mainly related to their well defined redox and photophysical properties. 8,9 However, while many papers report the synthesis and/or chemical transformations of x,x 0 -dialkylated-2,2 0 -bipyridine with x =4, 10-13 5 14-16 or 6 17,18 , syntheses of 3,3 0 -disubstituted-2,2 0 -bipyridines are still extremely challenging. 19 The main difficulty is related to the relative proximity of the nitrogen atom and the substituent of the opposite ring. This often leads to side reactions when 3,3 0benzylic positions have to be functionalized.

In this Letter we report our investigations in the preparation of ruthenium bis(2,2 0 -bipyridine) complexes bearing various 3,3 0disubstituted-2,2 0 -bipyridine as the third bidentate ligand. The synthesized compounds could be useful building blocks to access new families of polynuclear complexes.

In order to synthesize 3,3 0 -di(hydroxymethyl)-2,2 0 -bipyridine (3), the commercially available 2,2 0 -bipyridine-3,3 0 -dicarboxylic acid (1) was first esterified using the standard procedure into its corresponding methyl diester in good yield (Scheme 1). 20 However, in the next reduction step, while 3 equiv of NaBH 4 efficiently reduced the 4,4 0 diester at room temperature in 3 h, 13 our attempts to reduce its 3,3 0 homologue with LiAlH 4 (LAH, 1.6 equiv room temperature) yielded to very low conversion. Consequently, more drastic conditions were required and the reduction of the 3,3 0 diester was achieved under reflux for 2.5 days using an excess of LAH (3.5 equiv). Surprisingly, the presence of substituents (phenyl, methyl and tert-butyl) at the 6,6 0 positions greatly enhanced the reduction of the diesters. Only 2 h at room temperature in the presence of 1.6 equiv of LAH was needed to reduce the 6,6 0 -diphenyl homologue of 2 as reported by Hayashi and coll. 19 Using sodium bis(2-methoxyethoxy)aluminium hydride as a more powerful reducing agent in diethyl ether at 0 °C, diol 3 was formed in 84% yield. 21 The substitution of the two hydroxyl groups by two potential leaving groups (chloride, tosylate, bromide) was also particularly difficult. After several unsuccessful attempts to introduce tosylate groups 22,23 or chloride substituents using SOCl 2 , the substitution of the two hydroxyl groups by bromide was investigated. Despite our efforts using NBS and PPh 3 24,25 or an aqueous solution of HBr at room temperature [26][27][28] E-mail address: ohamelin@cea.fr (O. Hamelin).

N

N R' R 1;R =R'=CO 2 H 2;R =R'=CO 2 Me 3;R =R'=CH 2 OH a b Scheme 1. Synthesis of 3,3 0 -dihydroxymethyl-2,2 0 -bipyridine. (a) N,N 0 -Dicyclohex- ylcarbodiimide (DCC), MeOH, 3 h, 0 °C, 85%; (b) LiAlH 4 , reflux, 2.5 days, 70%.
product was never obtained properly. A similar behaviour was observed by Meyer and coll. with 4,4 0 -di(bromomethyl)-2,2 0 -bipyridine. 30 The authors suggested that the problem was related to the close proximity of the nitrogen atom of one pyridine to the benzylic position of the second pyridine. In order to move away these functions from one to the other, the complexation of the ligand by its bipyridine moiety with a cation as template was envisaged. However Van der Waals repulsions between substituents in the 3,3 0 -positions might prevent chelation by avoiding a near cisplanar conformation required for an optimal coordination,. This is not the case with small substituents such as methyl groups, carboxylic acids and methyl esters as reported by Wallis and co-workers. 31 Notably, in these systems, pyridine rings are displaced from co-planarity by 30-35°and two diastereomeric forms (D,aR-K,aR/ K,a S-D,a S) could be obtained. Reaction of 3 with 1 equiv of Ru(bpy) 2 Cl 2 ,6H 2 O in refluxing ethanol resulted in the formation of complex [Ru(bpy) 2 3] 2+ (4) in reasonable yield (57%; Scheme 2). The reaction proved to be highly stereoselective since a unique diastereomer was detected by 1 H NMR spectroscopy at room temperature. The relative configuration K,a R-(D,a S) was attributed by X-ray crystallography of a single crystal obtained by the slow diffusion of diethyl ether into a solution of the complex in a 3:1 acetone-MeOH mixture (Fig. 1; Table 1). It is worth to note that the same relative configuration was observed by Ashby and coll. for the [(bpy) 2 Ru(3,3 0 -diamino-2,2 0 -bipyridine] 2+ complex. 32 In addition, an important dihedral angle of about 34°between the two pyridinyl moieties of the disubstituted ligand was observed, highlighting a noticeable steric strain between substituents. Moreover, two conformational forms, differing only by the position of one of the hydroxymethyl substituent, were observed in the crystal (65:35 ratio). Once again, the substitution of the two hydroxyl groups by bromide to form compound 5 was problematic. In the standard conditions (HBr 48% at reflux), the cyclic ether 6 was quantitatively formed as a result of a unique substitution of one hydroxyl group by a bromide followed by direct intramolecular cyclization due to the proximity of the hydroxyl and bromide groups. A 46:54 mixture of 6 and 5 respectively was however obtained when the reaction was carried out with PBr 3 in dichloromethane at room temperature. This problem was addressed by adding a large excess of PBr 3 (100 equiv) directly into a highly concentrated refluxing solution of 4 in dichloromethane. In such conditions, a 5:95 mixture was obtained in favour of the dibromo product. After purification, 5 was obtained in 70% yield. Single crystals suitable for X-ray analysis were obtained by the slow diffusion of diethyl ether in a solution of the complex in a mixture of acetone and dichloromethane with a little amount of ethanol (Fig. 1). Surprisingly, a K,a S-(D,a R) relative configuration with a dihedral angle of 39.5°was determined underlining an unexpected isomerization of the complex during the substitution reaction. One may hypothesize the decoordination of the disubstituted ligand followed by a subsequent recoordination after conformational isomerization around the C2-C2 0 axis to afford a more stable isomer. The observed important dihedral angle is without any doubt at the origin of the weak chelation of the disubstituted ligand even if Ru-N bond lengths involving the disubstituted ligand are nearly identical to the Ru-N bpy bonds (2.069 compared to 2.054 and 2.062 Å, respectively).

With the objective to synthesize compound 8 as an interesting metalloligand able to bind different metal cations, the substitution of the two bromide by 1,2-bis(4-pyridyl)ethane (BPA) was then investigated.

However, despite the use of a large excess of BPA (100 equiv), the cyclic ammonium product 7 was obtained as the unique product in good yield (78%). Despite the high concentration of BPA, cyclization occurred faster than intermolecular substitution of the remaining bromide by a second molecule of BPA. Similarly, when 1,4,11-triazacyclononane was used as nucleophile, the corresponding cyclic ammonium product 9 was formed, as detected by mass spectrometry. Compound 10 could not be detected. Consequently, in order to avoid such intramolecular cyclization, azide and phthalimide were used as nucleophile. In anhydrous DMF at 0 °C, the desired products 11 and 12 were obtained in 62 and 87% yields, respectively. Slow diffusion of diethyl ether into a solution of complex 11 in a 3:1 acetone:dichloromethane mixture afforded single crystals suitable for X-ray analysis. While an unexpected and unexplained isomerization was observed during the In conclusion, we report here the results of our investigation into the intriguing chemistry of 3,3 0 -disubstituted-2,2 0 -dipyridine and related ruthenium based-complexes yielding the formation of compounds of interest such as 7, 9, 11 and 12. To the difference with the 4,4 0 -, 5,5 0 and 6,6 0 -related systems, the close proximities between (i) the pyridinyl nitrogen atom and the benzylic position of the second aromatic moiety in the free ligand and, (ii) the two benzylic positions once coordinated to a metal cation, lead to an interesting reactivity. First, despite our efforts, we were unable to introduce a leaving group such as bromide, chloride or tosylate at the two benzylic positions in the free ligand from the corresponding diol. It is assumed that such benzylic position is particularly unstable and undergoes a nucleophilic substitution involving a neighbouring group participation by the pyridinyl nitrogen atom. Second, the coordination to a metal cation leads to move away these two functions but, on the other hand moves closer the two benzylic positions one to the other. Due to Van der Waals' repulsions, this generates an important dihedral angle which is not high enough to avoid the formation of cyclized products when the two leaving groups have to be simultaneously substituted. Thanks to this proximity, two new original metalloligands, namely 7 and 9, were synthesized in reasonable yields. Finally, without any doubt, the reported strategy will be of particular interest in the design of a new family of polynuclear complexes/catalysts.

Scheme 2 .

 2 Scheme 2. Complexes synthesis. (a) EtOH, reflux, 4 h, 57%; (b) PBr 3 (large excess), CH 2 Cl 2 , reflux, 87%; (c) 1,2-bis(4-pyridyl)ethane, K 2 CO 3 , acetone, reflux, 78%; (d) 1,4,11-triazacyclononane, K 2 CO 3 , acetone, reflux, 42%; (e) For 11: NaN 3 , anhydrous DMF, 0 °C, 62%; For 12: potassium phthalimide, DMF, rt, 24 h, 60%.

Figure 1 .

 1 Figure 1. ORTEP view of 4, 5 and 11 cations.

Table 1

 1 Selected crystallographic data for complexes[4][PF 6 ] 2 ,[ 5][PF 6 ] 2 and [11][PF 6 ] 2 Compound 2[4][PF 6 ] 2 ,0.39H 2 O 4 [ 5][PF 6 ] 2 ,8Et 2 O [ 11][PF 6 ] 2 Formula C 73 H 74.78 F 24 N 12 O 7.39 P 4 Ru 2 C 160 H 184 Br 8 F 48 N 24 O 8 P 8 Ru 4 C 32 H 26 F 12 N 12 P 2 Ru

	Molecular weight	2020.46		4774.63	969.66
	Colour		Orange		Orange	Orange
	Crystal system		Monoclinic	Monoclinic	Monoclinic
	Space group		P 2 1 /c		C 2/c	P 2(1)/n
	a [Å]		24.5602(8)	12.7396(4)	12.1165(2)
	b [Å]		12.6187(5)	20.6339(7)	24.5613(5)
	c [Å]		25.7537(9)	18.4614(5)	12.2965(2)
	a [°]		9 0		9 0	9 0
	b [°]		99.881		104.783	92.3794(16)
	c [°]		9 0		9 0	9 0
	V [Å] 3		7		4692.3(3)	3
				863.1(5)		656.25(12)
	Z			4		1	4
	R Indices		R1 = 0.0513, wR2 = 0.0997	R1 = 0.0309, wR2 = 0.0549	R1 = 0.0391, wR2 = 0.0780
	R1 =	P ||F0| À |Fc||/	P |F0| ; wR2 = {	P [w(F 2 0 À F 2 c ) 2 ]/	P [w(F 2 0 ) 2 ]} 1/2 with w = 1/[s 2 (F 2 0 ) + (aP) 2 + bP] and P = [2F 2 c + Max(F 2 0 , 0)]/3.

Acknowledgment

We thank C. Lebrun (CEA Grenoble, DRFMC) for ESI-MS analyses.

Supplementary data

Supplementary data (Crystallographic data for 4, 5 and 11 were deposited at the Cambridge Crystallographic Data Centre (CCDC) with the CCDC reference numbers: CCDC 858774 for [1][PF 6 ] 2 , CCDC 858775 for [5][PF 6 ] 2 and CCDC 858776 for [11][PF 6 ] 2 ) associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.tetlet.2012.11.089.