Article pubs.acs.org/cm
Thermal Chemistry of Cu(I)-Iminopyrrolidinate and Cu(I)-Guanidinate Atomic Layer Deposition (ALD) Precursors on Ni(110) Single-Crystal Surfaces Taeseung Kim,† Yunxi Yao,† Jason P. Coyle,‡ Seán T. Barry,‡ and Francisco Zaera*,† †
Department of Chemistry, University of California, Riverside, California 92521, United States Department of Chemistry, Carleton University, Ottawa, Ontario, K1S5B6, Canada
‡
ABSTRACT: The thermal chemistry of tetrakis[Cu(I)-N-secbutyl-iminopyrrolidinate] and bis[Cu(I)-N,N-dimethyl-N′,N″di-iso-propyl-guanidinate], promising precursor for atomic layer deposition (ALD) applications, was investigated on a Ni(110) single-crystal under ultrahigh vacuum (UHV) conditions by using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). Both precursors, which exist as tetramers and dimers in the solid phase, respectively, undergo dissociative adsorption at temperatures below 200 K to produce adsorbed monomers on the surface. A β-hydride elimination step is then operative near 300 K that leads to the release of some of the ligands in dehydrogenated form. The remaining adsorbates obtained from either precursor undergo similar further decomposition between 350 K and 600 K as the Cu atoms are reduced from a Cu(I) oxidation state to metallic Cu(0). Hydrocarbons resulting from the elimination of the terminal moieties include ethene and acetonitrile from the Cu(I)-iminopyrrolidinate and propene from the Cu(I)-guanidinate, which are ejected at ∼420−490 K, and HCN in both cases at ∼570−580 K. These results shows several similarities with the surface chemistry previously reported for bis[Cu(I)N,N′-di-sec-butyl-acetamidinate], and they suggest a common behavior in the surface reactions of these families of Cu(I)amidinate, Cu(I)-iminopyrrolidinate, and Cu(I)-guanidinate ALD precursors. KEYWORDS: atomic layer deposition, copper films, surface chemistry, X-ray photoelectron spectroscopy, temperature-programmed desorption, amidinate complexes, guanidinate complexes
1. INTRODUCTION There has been an increasing interest in the deposition of thin films using chemistry-based methods to meet the requirements for the manufacturing of highly integrated microelectronic devices.1−3 The most prominent advantages of these chemical approaches are their tendency to deposit solid thin films conformally and their ability to successfully fill deep trenches in rough topographies with high aspect ratios. Atomic layer deposition (ALD), which is a version of chemical vapor deposition (CVD) that uses alternating doses of gas-phase metal−organic molecules and secondary reactants, is a particularly promising method, because it offers additional control over the thickness of the growing films at the monolayer level.4−8 ALD is already being used to deposit high-dielectric metal oxide thin films,9 and it has also shown great promise for the growth of electrical interconnects between nanometer-sized devices.10,11 As the width of interconnect lines shrink, there is a need to replace aluminum for copper, to lower resistivity and improve electromigration resistance,12−14 and current methods for growing copper films require the formation of a seed layer to be able to carry out further electrochemical deposition.15,16 The fact that the seed Cu layer must be grown conformally points to the choice of ALD for this process; however, to date, © 2013 American Chemical Society
there are no chemistry-based deposition methods that result in copper films with acceptable adhesion to the surface. Many copper precursors have been designed for CVD and ALD, including compounds with alkoxide,17 cyclopentadienyl,13 carboxylate,18 oxalate,19,20 and acetonate21−24 ligands, but most of those require high temperatures, leading to non-selflimiting deposition, possible precursor self-decomposition, and the agglomeration of the deposited copper. New β-ketiminates, β-diketiminates, amino-oxides, and similar N- and/or Obidentate coordinated compounds have recently been shown to perform better,25−28 but their full implementation in ALD processes has still not been accomplished. One promising family of precursors for copper ALD is that based on amidinate ligands.11,29−31 These compounds display low vapor pressures and reasonable stability, and also good resistance toward decomposition in air. A clean and simple surface chemistry has been identified for bis[Cu(I)-N,N′-di-secbutyl-acetamidinate] on silica surfaces, with the N,N′-di-secbutyl-acetamidinate ligand undergoing a displacement reaction at a surface −OH site during precursor adsorption and Received: May 27, 2013 Revised: July 8, 2013 Published: September 9, 2013 3630
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hydrogenation upon subsequent H2 exposure,32 and the growth of good-quality copper films with this compound has been reported as well.33,34 On the other hand, previous surfacescience studies in our laboratory have highlighted much more complex decomposition chemistry on metal surfaces.35−38 Specifically, on nickel (and cobalt) surfaces, the thermal decomposition of bis[Cu(I)-N,N′-di-sec-butyl-acetamidinate] starts with a C−N bond dissociation step at ∼200 K to produce a smaller N-sec-butyl-acetamidinate and adsorbed 2butene.37 Some of the former intermediates hydrogenate at ∼300 K to release N-sec-butyl-acetamidine into the gas phase, while the remainder dissociate at ∼400 K to form acetonitrile and a sec-butyl-amino surface species as the Cu atom is reduced to its metallic state.37 Further conversion releases 2-butene at 485 K, and by 800 K, only copper and a small amount of carbon remains on the surface.37 Not all this surface chemistry is detrimental to ALD: a temperature window between ∼350 K and ∼500 K was identified for the clean deposition of the precursor on the surface: lower temperatures are insufficient for activation of the dissociative adsorption, and higher temperatures lead to continuous decomposition beyond Cu monolayer saturation.36 Similar surface chemistry, although activated at higher temperatures, was seen on copper substrates.38 Based on those previous studies, it has become apparent that the trigger for the thermal decomposition of the copper acetamidinate precursors on metal surfaces is a C−N bond activation step involving one of the terminal alkyl moieties. Therefore, it seems reasonable to expect that modifications of the ligands aimed to hinder that step could provide added stability to this type of Cu ALD precursors. Here, we test this idea by studying the surface thermal chemistry of two related copper compounds, a Cu(I)-iminopyrrolidinate39,40 (1) and a Cu(I)-guanidinate41−43 (2), shown in Figure 1. In the Cu(I)iminopyrrolidinate case, the alkyl moiety in one end of the amidinate and the methyl group attached to the center carbon are tied together in a five-member ring, adding some geometrical constrains to prevent the C−N bond scission step seen in the Cu(I)-acetamidinate. With the Cu(I)
guanidinate, the additional exocyclic amido moiety offers the possibility of additional π-resonance structures, presumably increasing the stability of the ligand, and the ligand−copper plane, that is, the plane created by the two Cu and four N atoms in the guanidinate dimer, is also noticeably twisted, possibly offering some hindrance in terms of access of the R−N bond to the surface.40,41,44 Thanks to those changes, past studies with Cu(I)-iminopyrrolidinate and Cu(I)-guanidinate compounds have reported similar thermal stability to Cu(I) amidinates.40−42 On the surface, however, the results described below indicate that thermal decomposition is still facile with both compounds. Moreover, they appear to follow somewhat similar decomposition pathways, albeit starting with a β-hydride elimination step at ∼300 K, and to also display a similar transition temperature for the reduction of the copper center to its metallic state, between 350 K and 500 K. However, some differences are observed at higher temperatures, as discussed in more detail below.
2. EXPERIMENTAL SECTION All the experiments have been performed in an ultrahigh vacuum (UHV) chamber described elsewhere.45,46 The main stainless-steel chamber is turbopumped to a base pressure in the 10−10 Torr range. A Ni(110) single crystal, 10 mm in diameter and 1 mm in thickness, was mounted in the center of that chamber, on a manipulator capable of cooling to ∼80 K and of resistively heating to up to 1100 K. The temperature was measured by using a K-type thermocouple spotwelded to the side of the crystal, and was controlled by a homemade proportional−integral−derivative (PID) circuit. A constant heating rate of 5 K/s was employed in all temperature-programmed desorption (TPD) experiments. A quadrupole mass spectrometer (Extrel C-50) interfaced to a personal computer was used for the TPD experiments. The front of the quadrupole’s housing ends in an ionizer box with a 7-mm-diameter aperture, which is placed in front of the crystal to selectively sample the gases desorbed from the front surface. The evolution of the signals for up to 15 different masses can be recorded in a single TPD run. A 50-mm radius hemispherical electron energy analyzer (VSW HAC 5000), set at a 50-eV constant pass energy, was used to obtain the Xray photoelectron spectroscopy (XPS) data. An aluminum anode (hν = 1486.6 eV) was used as the X-ray source. The raw XPS data were deconvoluted and fitted using XPS PEAK, version 4.0. The surface of the crystal was cleaned before each TPD or XPS experiment by sequential cycles of Ar+ ion sputtering (using a Perkin−Elmer PHI 04−300 ion gun and an Ar+ ion energy of 2 kV) and annealing at 1100 K until deemed clean by XPS. The Cu(I)-iminopyrrolidinate (1) and Cu(I)-guanidinate (2) precursors were synthesized by following procedures provided in early publications. For the first compound, N-sec-butyl-iminopyrrolidine (3, Figure 1) was reacted with butyl lithium, and the resulting solution mixed with CuCl.40 The Cu(I)-guanidinate was prepared by room-temperature salt metathesis between CuCl and the lithium salt of N,N-dimethyl-N′,N″-di-iso-propyl-guanidine (4, Figure 1), made from lithium dimethyl amide and di-iso-propyl-carbodiimide, following standard guanidinate synthetic procedures.41 In both cases, the resulting solids were purified and characterized by 1H NMR. The precursors were transferred to the glass containers used for dosing inside a glovebox filled with argon to avoid their exposure to air, and the precursor housing was pumped before the experiments to remove any impurities generated during the transfer. To confirm the purity of the dosed precursors, repeated cycles of TPD and pumping on the precursor housing were performed until pure multilayer condensation could be achieved. Gas exposures are reported in units of Langmuirs (1 Langmuir = 10−6 Torr s), not corrected for differences in ion gauge sensitivities.
Figure 1. Cu(I)-iminopyrrolidinate (1) and Cu(I)-guanidinate (2) ALD precursors characterized in this study. Also shown are the structures of the respective free protonated ligands (3) and (4), tested for reference. 3631
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3. RESULTS The thermal chemistry of the ALD precursors on the clean Ni(110) single-crystal surface was tested by TPD first. Exhaustive surveys for most mass fragments between 2 amu and 140 amu were carried out to identify the major desorbing products generated upon thermal heating of the surface. A typical set of representative TPD traces for the case of tetrakis[Cu(I)-N-sec-butyl-iminopyrrolidinate] (1) is provided in Figure 2. The data correspond to a 20-L dose at 90 K.
The evolution of the TPD traces for the Cu(I)iminopyrrolidinate precursor (1) adsorbed on the Ni(110) surface is reported as a function of initial exposure in Figure 3
Figure 3. TPD for the main products detected from thermal activation of tetrakis[Cu(I)-N-sec-butyl-iminopyrrolidinate] (1) adsorbed on Ni(110) as a function of initial exposure. Traces are provided for (from left to right) molecular hydrogen (2 amu), hydrogen cyanide (27 amu), acetonitrile (41 amu), and 2-sec-butyleneamino-1-pyrroline (138 amu).
for the main desorbing species, namely, for molecular hydrogen, hydrogen cyanide, acetonitrile, and 2-sec-butyleneamino-1-pyrroline. The more prominent changes are seen in the H2 traces: hydrogen production, from dehydrogenation of the organic surface species, occurs mainly at ∼300 K at low coverages, but additional higher-temperature dehydrogenation steps become evident after higher precursor exposures. In terms of yields, those for the formation of HCN and CH3CN reach their maxima quite early on, by ∼5 L, whereas the production of 2-sec-butyleneamino-1-pyrroline starts only between 5 L and 10 L. Monolayer saturation may occur at ∼20 L, at which point the H2 TPD reaches its maximum yield and the molecular precursor desorption from condensation peak starts to grow. More details on the surface chemistry of the ALD precursors can be obtained from comparative studies with chosen reference compounds. In Figure 4, TPD data are reported for the main desorption products detected from decomposition of N-sec-butyl-iminopyrrolidine (3, left panel) and pyrrole (right). The surface chemistry of the iminopyrrolidine (3) shows some similarities with that of the Cu(I)-iminopyrrolidinate (1). For one, the hydrogen desorption profile is quite similar, with a number of features appearing between ∼300 K and ∼600 K. The production of ethene is also evident at ∼410 K by the sharp peaks in the 27 and 28 amu traces and the smaller feature in the 26 amu trace. Those traces also expose the formation of HCN at ∼560 K. Acetonitrile is produced in a broad temperature range at ∼450 K, and molecular nitrogen above 700 K. The similarities between the TPDs of the copper complex and the free ligand suggest that much of the surface chemistry of the first may take place once the iminopyrrolidinate ligand is displaced from the copper center and adsorbed on the Ni(110). On the other hand, no hydrogenation (protonation) of the ligand is seen with the copper precursor (its molecular desorption occurs at low temperatures, ∼230 K), and no 2-sec-butyleneamino-1-pyrroline production is observed at 290 K with adsorbed N-sec-butyl-iminopyrrolidine (3), suggesting that that chemistry is promoted by the copper center
Figure 2. Survey TPD traces for tetrakis[Cu(I)-N-sec-butyl-iminopyrrolidinate] (1) adsorbed on Ni(110). Twenty Langmuirs (20 L) of the Cu(I)-iminopyrrolidinate were dosed on the surface at 90 K. Desorption traces are shown for 2, 12, 14, 16, 26, 27, 28, 40, 41, 42, 43, 45, 56, 58, 69, 71, 72, 87, 137, 138, and 139 amu. The low-amu peaks observed in these data can be identified with the desorption of H2, ethene, propene, HCN, and N2 (left panel), whereas the lowtemperature high-amu signals are associated with molecular desorption and with the formation of 2-sec-butyleneamino-1-pyrroline, a product of β-hydride elimination from the ligand.
Molecular desorption at ∼230 K is evident by the signals seen at that temperature for several amus, including 139 amu (from a C8N2 H15+ ion, which has the stoichiometry of the iminopyrrolidinate ligand). Desorption of hydrogen is seen in multiple peaks starting below 300 K and extending beyond 600 K, an indication of extensive and stepwise decomposition of the ligands on the surface. The peaks seen at 490 K in the 40 and 41 amu traces are identified with acetonitrile (CH3CN) formation; assignment of these signals to propene is ruled out by the absence of the corresponding signal in the 42 amu trace. Similarly, the feature seen at 570 K in the 26 and 27 amu traces was determined to originate mainly from HCN desorption, although a limited amount of ethene may be produced as well, since a small peak is also seen at that temperature in the 28 amu trace (another earlier ethene feature is seen in the 27 and 28 amu trace at ∼420 K). Desorption of molecular nitrogen, perhaps in conjunction with other nitrogen-containing species, is observed as broad features in the 14 and 28 amu traces, with an onset at ∼700 K. Finally, clear sharp peaks are seen at 290 K in the 137 and 138 amu TPD data, which we assign to 2-sec-butyleneamino-1-pyrroline, the product of β-hydride elimination from the original iminopyrrolidinate ligand. The production of other gas-phase products, including butane, butene, pyrrole, pyrroline, and pyrrolidine, was monitored but not detected. 3632
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N,N-dimethyl-N′,N″-di-iso-propyl-guanidine free ligand (4) are shown in Figure 7. The main gas-phase products in this case are H2, propene, and HCN, which are identified by the broad peak in the 2 amu trace, covering the range of 300−600 K, by the peaks at ∼450 K for 41 and 42 amu, and by the peaks at ∼580 K for 26 and 27 amu, respectively. An additional product, N,Ndimethyl-N′-iso-propyl-N″-iso-propylidene-guanidine, resulting from β-hydride elimination from the ligand, was identified at 280 K by the signals in the traces for 132, 154, 155, 168, and 169 amu. Finally, molecular desorption at 200 K was seen in the traces for 2, 41, 42, 43, 45, 125, 126, 127, 156, and 171 amu. Notice that 171 amu is the molecular mass of the free (hydrogenated) guanidine ligand. However, we believe that, in this case, the signal corresponds to the copper guanidinate, and that the 171 amu ion may form by ionization of the ligand followed by protonation in the ionizing region of the mass spectrometer. Indeed, a measurable signal at 171 amu is clearly visible in the mass spectrum of the gas-phase compound.42 No other gas-phase products were observed in the TPD of the adsorbed Cu(I)-guanidinate (1): in particular, no dimethylamine (from deinsertion of di-iso-propyl-carbodiimide)41 or N,N-dimethyl-N′-iso-propyl-guanidine (from hydrogenation/ protonation of the fragment left behind after propene elimination, in analogy with what has been observed with copper acetamidinates)37 could be detected in these TPD experiments. The data in Figure 6 show the evolution of the desorption profiles of the main gas-phase products as a function of initial Cu(I)-guanidinate (2) exposure, that is, for, from left to right, H2 (2 amu), HCN (27 amu), propene (42 amu), N,Ndimethyl-N′-iso-propyl-N″-iso-propylidene-guanidine (169 amu), and the molecular Cu(I)-guanidinate precursor (171 amu). No significant differences in the temperature profiles are seen as a function of the initial coverages for any of the products other than hydrogen, and even in that case, the changes are less marked than with the Cu(I)-iminopyrrolidinate. It is also noted that the yields for H2, HCN, and propene all increase approximately linearly with coverage, whereas the βhydride elimination product only becomes a major contributor at high coverages, near saturation, which happens at ∼10 L. In terms of the chemistry of the free guanidine ligand (4), it is interesting to note that the TPD traces obtained for its surface chemistry (Figure 7) are quite similar to those recorded for the Cu(I)-guanidinate (Figure 5), except for a couple of noticeable differences. First, dehydrogenation to produce H2 happens mostly at low temperatures with the guanidine; the trace for the Cu(I)-guanidinate shows an additional peak at ∼430 K not evident in the data for the guanidine. Second, no N,N-dimethylN′-iso-propyl-N″-iso-propylidene-guanidine (169 amu), the βhydride elimination product seen with the Cu(I)-guanidinate at 280 K, is detected in the case of the guanidine, arguing again for the coordination of the ligand to the copper center being responsible for this chemistry and for the displacement of the ligand to the surface only at higher temperatures. Finally, guanidine molecular desorption is seen at ∼170 K, as identified by a cracking pattern similar to that of the Cu(I)-guanidinate except for the additional intense peak seen for 169 amu (no significant signal was detected for that mass from molecular Cu(I)-guanidinate desorption; see Figure 5). Additional information on the thermal chemistry of both the Cu(I)-iminopyrrolidinate (1) and the Cu(I)-guanidinate (2) ALD precursors on the Ni(110) single-crystal surface was extracted from studies using XPS. Figures 8 (iminopyrrolidi-
Figure 4. TPD from saturation layers of N-sec-butyl-iminopyrrolidine (3) (left panel, 20 L dose at 90 K) and pyrrole (right, 10 L dose at 90 K) adsorbed on Ni(110). These are reference compounds studied to help with the understanding of the thermal surface chemistry of tetrakis[Cu(I)-N-sec-butyl-iminopyrrolidinate] (1).
of the complex and that the ligand is transferred to the nickel surface only at temperatures above ∼300 K. In terms of the surface chemistry of pyrrole, that adsorbate seems to dehydrogenate quite early on, mostly below 500 K, and to produce some ethene at ∼390 K and molecular nitrogen above ∼750 K. Given the similarities in the 28 amu trace behavior between the pyrrole and the iminopyrrolidine (3) and Cu(I)iminopyrrolidinate (1) compounds, it would appear that the pyrrolidine ring in our copper precursor may dehydrogenate at some point on the surface to form a pyrrole-like ring. Similar TPD studies were carried out with the bis[Cu(I)N,N-dimethyl-N′,N″-di-iso-propyl-guanidinate] precursor (2). The survey TPD for a saturation layer and the coverage dependence for the key desorbing products are provided in Figures 5 and 6, respectively, and TPD data for the reference
Figure 5. Survey TPD traces for bis[Cu(I)-N,N-dimethyl-N′,N″-di-isopropyl-guanidinate] (2) adsorbed on Ni(110). Twenty Langmuirs (20 L) of the Cu(I)-guanidimate were dosed on the surface at 90 K. Desorption traces are shown for 2, 16, 26, 27, 28, 41, 42, 43, 45, 125, 126, 127, 132, 154, 155, 156, 168, 169, and 171 amu. The low-amu peaks observed in these data can be identified with the desorption of H2, HCN, and propene (left panel), whereas the low-temperature high-amu signals are associated with molecular desorption and with the formation of N,N-dimethyl-N′-iso-propyl-N″-iso-propylidene-guanidine, a product of β-hydride elimination from the ligand. 3633
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Figure 6. TPD for the main products detected from thermal activation of bis[Cu(I)-N,N-dimethyl-N′,N″-di-iso-propyl-guanidinate] (2) adsorbed on Ni(110) as a function of initial exposure. Traces are provided for, from left to right, molecular hydrogen (2 amu), hydrogen cyanide (27 amu), propene (42 amu), N,N-dimethyl-N′-iso-propyl-N″-iso-propylidene-guanidinate (169 amu), and the original Cu(I)-guanidinate (171 amu).
Figure 9. Cu 2p3/2 (left panel), C 1s (center), and N 1s (right) XPS from a Ni(110) single-crystal surface dosed with 20 L of the Cu(I)guanidinate precursor (2) at 100 K as a function of subsequent annealing temperature. The raw experimental data are shown as dots, and Gaussian fits to the spectra are shown as solid lines.
Figure 7. TPD from a saturation layer (after a 20-L dose at 90 K) of N,N-dimethyl-N′,N″-di-iso-propyl-guanidine (4) adsorbed on Ni(110), the free ligand associated with the Cu(I)-guanidinate precursor (2).
indicated temperatures. The experimental data are shown as dots and the appropriate fits to Gaussian peaks displayed as solid lines, and a summary of the results from the analysis of those data in terms of peak areas for each of the relevant species is provided in Figure 10 (in relative terms, but corrected for differences in XPS sensitivities).47 Many similarities are seen in the thermal evolution of the XPS signals for both compounds. In terms of the Cu 2p3/2 peak, two copper species are identified at 933.8 and 932.3 eV, which can be easily assigned to the Cu(I) center of the original precursors and to metallic copper, respectively.48 A transition between the two, which starts at ∼350 K and is completed by ∼550 K, indicates a cooperative redox reaction leading to the reduction of the metal center on the surface at the expense of the oxidation of the ligands. A similar transformation was previously reported for a Cu(I)acetamidinate.37 The total Cu 2p3/2 XPS signal increases with annealing temperature, presumably because the Cu atoms become more exposed as the ligands decompose and some of their fragments desorb from the surface. The C 1s and N 1s XPS traces are more complex, since they include several features reflective of C and N atoms in different chemical environments. The low-temperature traces were deconvoluted in a way consistent with these differences in
Figure 8. Cu 2p3/2 (left panel), C 1s (center panel), and N 1s (right panel) X-ray photoelectron spectra (XPS) from a Ni(110) singlecrystal surface dosed with 20 L of the Cu(I)-iminopyrrolidinate precursor (1) at 100 K, as a function of subsequent annealing temperature. The raw experimental data are shown as dots, and Gaussian fits to the spectra are shown as solid lines.
nate) and 9 (guanidinate) display the Cu 2p3/2 (left panel) C 1s (center), and N 1s (right) XPS spectra obtained for 20-L doses of each of the precursors at 100 K, after annealing to the 3634
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guanidinate (2), with three peaks at 284.9, 285.9, and 287.0 eV and area ratios of 4:4:1. For the N 1s XPS traces, two peaks of equal intensity were fitted to the spectra for the Cu(I)iminopyrrolidinate (1), at 398.6 and 400.1 eV, and three features with area ratios of 1:1:1 were used in the analysis of the Cu(I)-guanidinate (2), at 397.8, 399.2, and 399.95 eV. It is worth pointing out that the spectra in the former case are quite similar to those obtained previously for a Cu(I)-acetamidinate,37 and that in both cases the identification of two types of nitrogen atoms is indicative of the conversion of the gas-phase precursor oligomers into surface monomers. On the other hand, no indication was obtained for any further dissociation at low (
