WO2013011297A1

WO2013011297A1 - Method

Info

Publication number: WO2013011297A1
Application number: PCT/GB2012/051697
Authority: WO
Inventors: Erik ØSTRENG; Ola Nilsen; Helmer FJELLVÅG
Original assignee: Universitetet I Oslo; Golding, Louise
Priority date: 2011-07-15
Filing date: 2012-07-16
Publication date: 2013-01-24

Abstract

The present invention provides methods for the formation of a lithium-containing layer on a substrate by atomic layer deposition, which comprise the following steps: a) providing a substrate in a reaction chamber wherein said reaction chamber is arranged for gas-to-surface reactions; b) pulsing a lithium precursor through said reaction chamber whereby to cause said lithium precursor to deposit on and/or react with at least one surface of said substrate to form a lithium-containing layer on at least a portion of said surface; c) purging said reaction chamber; d) pulsing an additional precursor through said reaction chamber whereby to cause said additional precursor to react with said lithium-containing layer; and e) purging said reaction chamber; wherein said lithium precursor is a compound of general formula LiNR^aR^b (in which R^a and R^b are independently selected from H, C_1-6 alkyl and -SiR^c ₃; and each R^c is independently selected from H and C_1-6 alkyl; provided that at least one of R^a and R^b is a group -SiR^c ₃). Substrates carrying a lithium-containing layer produced by such methods are also provided by the invention and find particular use in batteries, for example as electrode materials or as electrolyte barriers.

Description

Method

The present invention relates to a method for the formation of a lithium-containing surface layer on a substrate.

Recently, there has been an emphasis on improving methods for the formation of thin material layers on substrates of various kinds, in particular methods for producing layers of lithium-containing materials which may be used in batteries, for example as electrode materials or as electrolyte barriers. WO 2009/084966 describes one such method in which atomic layer deposition (ALD) is used to deposit a thin film containing lithium. In this method, Li(thd) (lithium 2,2,6,6- tetramethylheptane-3,5-dionate) is used as a lithium precursor.

ALD (also known as atomic layer chemical vapour deposition, ALCVD, or atomic layer epitaxy, ALE) is a thin-film-deposition technique that relies on alternating self- terminating gas-to-surface reactions. The film is formed by sequential pulsing of two or more reactants (otherwise generally referred to as "precursors"), using purging with inert gas between the precursor pulses to avoid gas-phase reactions. Unlike most other deposition and crystal growth techniques, this method ensures an even growth of the film on all exposed surfaces.

The present inventors have now developed an improved method for the deposition of thin films containing lithium which involves the use of a novel lithium precursor. This method enables the formation of lithium-containing materials at higher growth rates than previously observed, e.g. when carrying out the method described in WO 2009/084966. Surprisingly, the novel lithium precursor has also been found to be capable of forming Li-N materials. Such materials have previously not been observed.

Viewed from one aspect the invention thus provides a method for the formation of a lithium-containing layer on a substrate by atomic layer deposition, said method comprising the following steps:

a) providing a substrate in a reaction chamber wherein said reaction chamber is arranged for gas-to-surface reactions; b) pulsing a lithium precursor through said reaction chamber whereby to cause said lithium precursor to deposit on and/or react with at least one surface of said substrate to form a lithium-containing layer on at least a portion of said surface;

c) purging said reaction chamber;

d) pulsing an additional precursor through said reaction chamber whereby to cause said additional precursor to react with said lithium- containing layer; and

e) purging said reaction chamber;

wherein said lithium precursor is a compound of general formula (I):

LiNR^aR^b (I)

(in which

R^a and R^b are independently selected from H, Ci_-6 alkyl (preferably d_-3 alkyl, e.g. methyl) and -SiR^c ₃; and

each R^c is independently selected from H and Ci_-6 alkyl (preferably Ci_-3 alkyl, e.g. methyl);

provided that at least one of R^a and R^b is a group -SiR^c ₃).

Each step of the process (e.g. steps b) to e)) may independently be repeated a desired number of times, e.g. until the desired film thickness is obtained. For example, steps b) and c) may be repeated as required in order to produce the desired layer of lithium-containing material. Alternatively, or in addition, steps d) and e) of the method may be repeated using the same or different additional precursors. Where two or more additional precursors are pulsed through the reaction chamber, it is generally preferred that these are not identical.

When two or more additional precursors are used in the method, these may be pulsed simultaneously or separately from one another. In the case where these are pulsed separately, additional purging steps will generally be employed after each pulse.

With each lithium precursor pulse cycle, a layer of lithium-containing material is deposited on the substrate. Preferred lithium precursors for use in the method of the invention are those compounds of formula (I) in which both R^a and R^b are groups -SiR^c ₃, preferably those in which at least one R^c is methyl. A particularly preferred lithium precursor for use in the invention is LiN(SiMe₃)₂ (where Me is methyl).

Examples of other lithium precursor materials which may be used include

LiN(SiEt₃)₂ (where Et is ethyl), LiNH(SiMe₃) and LiNH(SiEt₃)₂.

If required, an inert purge gas is pulsed through the reaction chamber after each pulse of lithium precursor, or after each sequence of lithium precursor pulses, or even concurrently with the lithium precursor. Alternatively, several purge pulses may be performed between each lithium precursor pulse. Similar purging steps may also be performed after pulsing of the additional precursor (or additional precursors where more than one is used). The purging steps are intended to remove any unreacted reactants thereby ensuring that growth proceeds according to self-limiting gas-to-surface reactions.

Purging of the reaction chamber may be performed by flowing a purge gas through the chamber or, alternatively, by evacuating the chamber by reducing the pressure. Suitable purge gases include inert gases such as nitrogen, argon, etc. Preferred for use as a purge gas is nitrogen. Also suitable for use as purge gases are

nitrogen/argon mixtures, for example a mixture of 1 .5% Ar and 98.5% N₂, although any suitable gas or gas mixture may be used which will not react with the deposited layer of lithium-containing material or the precursors.

As will be readily understood, it is intended that the additional precursor for use in the method will be a non-lithium precursor. The choice of additional precursor will be dependent on the desired end product and may be selected from any

conventional precursor materials, such as those which are described in Puurunen (J. Appl. Phys. 97: 121301 , 2005). Preferred for use in the invention are additional precursor materials selected from the group consisting of oxygen precursors, metal precursors, nitrogen precursors, phosphorus precursors and halogen precursors.

Any suitable oxygen precursor may be used, such as for example, water, 0₃ or any other oxygen comprising gaseous compound such as C0₂. Combinations of oxygen precursors may also be used, either in separate pulsing steps or

simultaneously in the same pulsing step.

Suitable metal precursors include metal organic precursors such as metal silylamides, beta-diketonates, amidinates, halides, carbonyls, alkoxides, alkyls, cyclopentadienyls, etc.

Any suitable nitrogen precursor may be used, such as for example, ammonia and ammonia derivatives. Derivatives of ammonia which may be used include hydrazine, e.g. 1 ,1 -dimethyl-hydrazine and other hydrazine derivatives. A preferred nitrogen precursor is NH₃.

In the case where a nitrogen precursor is used, the method may further comprise the additional step of providing a capping layer using known deposition techniques. This step is carried out once each of steps b) to e) has been completed such that the capping layer is deposited on top of the final deposited Li-N layer. An additional purging step may be employed after deposition of the capping layer. Suitable for use as a capping layer are any materials which can be deposited without destroying the underlying Li-N layer and which protect against reaction with air, water and C0₂. Examples of such materials include TiN and Mo₂N.

Suitable phosphorus precursors include trimethyl phosphate, POCI₃, phosphoric acid and organophosphate esters of H₃P0₄.

Suitable halogen precursors include metal chlorides, preferably volatile chlorides, e.g. TiCI₄, NbCIs, or ZrCI₄.

In one aspect, the invention provides a method wherein more than one oxygen precursor is pulsed through the reaction chamber such that the resulting sequence comprises the following steps:

a) providing a substrate in a reaction chamber wherein said reaction chamber is arranged for gas-to-surface reactions;

b) pulsing a lithium precursor as herein defined through said reaction chamber whereby to cause said lithium precursor to deposit on and/or react with at least one surface of said substrate to form a lithium-containing layer on at least a portion of said surface;

c) purging said reaction chamber;

d) pulsing a first oxygen precursor through said reaction chamber whereby to cause said first oxygen precursor to react with said lithium-comprising layer;

e) purging said reaction chamber;

f) pulsing a second oxygen precursor through said reaction chamber whereby to cause said second oxygen precursor to react with at least one surface of the substrate; and

g) purging said reaction chamber.

In this embodiment, it is preferred that the first oxygen precursor is water and the second oxygen precursor is C0₂.

Each of steps b) to g) may independently be repeated a desired number of times in order for the formation of the desired thin film layer, e.g. until the desired film thickness is obtained. Alternatively, any of the groups of steps, for example b) and c), d) and e) and/or f) and g) may be independently repeated a desired number of times before continuing with the remaining steps of the method. Where water and C0₂ are used as the first and second oxygen precursors, respectively, the thin film layer will comprise Li₂C0₃ on said at last one surface of the substrate.

In a further aspect, the invention provides a method wherein a nitrogen precursor is pulsed through the reaction chamber such that the resulting sequence comprises the following steps:

c) purging said reaction chamber; d) pulsing a nitrogen precursor through said reaction chamber whereby to cause said nitrogen precursor to react with said lithium-comprising layer; and

e) purging said reaction chamber.

In this embodiment, it is preferred that the nitrogen precursor is ammonia.

Each of steps b) to e) may independently be repeated a desired number of times in order for the formation of the desired thin film layer, e.g. until the desired film thickness is obtained. Alternatively, any of the groups of steps, for example b) and c) and/or d) and e) may be independently repeated a desired number of times before continuing with the remaining steps of the method. In this embodiment of the invention, the resulting thin film layer will comprise a Li-N type material on said at last one surface of the substrate.

Advantageously, in any of the methods herein described, at least one of the precursors (e.g. at least the lithium precursor) is introduced into the reaction chamber in the gas phase. Particularly preferably, all precursors (i.e. the lithium precursor and all additional precursors) will be introduced in this way. Precursors which exist in solid or liquid form may be converted to the gaseous form by "direct vaporisation" prior to introduction into the reaction chamber. By "direct

vaporisation" is meant that a substantially pure (e.g. pure) precursor compound in solid or liquid form is converted directly to the gaseous state via sublimation or evaporation, without dispersion or dissolution in a liquid (e.g. a solvent or emulsion). Optionally, an inert gas may be employed (e.g. via a bubbler) to assist in the vaporisation and/or act as a carrier gas. Gaseous precursors may be introduced from a compartment which is separated from the reaction chamber, for example via a physical or inert gas valve. Such gaseous precursors may enter the reaction chamber by way of their own intrinsic vapour pressure or optionally by means of an inert carrier gas.

In comparison to conventional "liquid injection" techniques, where for example the precursor is mixed with a solvent and nebulised for injection into the reactor, the direct vaporisation technique permits simplification of the reaction system. Direct vaporisation also has the advantage that this minimises any pressure differences between precursor pulses and so eliminates the possibility of competing or interfering reactions between solvents and precursors.

Vaporisation of precursors should be carried out at an evaporation temperature capable of giving yield to a sufficient amount of precursor for use in the ALD process. Appropriate vaporisation temperatures will depend on the nature of the precursor and the ALD process conditions and may be readily determined by those skilled in the art. Such temperatures are typically from about 60 °C to about 200 °C. For example, for deposition of lithium-containing layers from LiN(SiMe₃)₂, a temperature of 65 'Ό or higher, e.g. 75 'Ό or higher, may be employed.

Suitable substrate materials include any materials which are capable of

withstanding the reaction conditions within the reaction chamber. Preferred substrate materials include electrode or electrolyte materials which may be used in a battery. Examples of substrate materials include semiconductors, for example Si (e.g. single crystalline Si(1 1 1 )), Si with native oxide, and Si-H; metallic surfaces, for example Si/Pt, Pt, or Ti; crystalline ceramics, for example Si0₂ and Al₂0₃; and amorphous glasses, for example glass (e.g. soda lime glass). Preferred substrate materials include Si, glass, Si0₂, Al₂0₃ and Ti.

Where required, a substrate surface may be pre-treated in order to provide an adhesion layer of Al₂0₃ using known deposition techniques. This layer may be up to 1 nm in thickness and, typically, will be deposited in situ. Also suitable for use as adhesion layers are Mo-N type materials (e.g. Mo₂N or MoN_x), which may be deposited using known deposition techniques. Alternatively an Mo-N type adhesion layer may be deposited by ALD using a Mo(CO)₆ precursor and a nitrogen precursor. Adhesion layers are particularly preferred where the desired thin film layer comprises an Li-N type material. Without wishing to be bound by theory, it is thought that such a barrier layer may prevent reactions with the substrate or assist in the formation of metal-nitrogen-metal bonds.

For deposition of LiN-type materials (e.g. Li₃N), it may be advantageous to deposit both an adhesion layer and a capping layer of an Mo-N type material (e.g. MoN_x) to assist uniform film growth and adhesion to the surface, and to prevent sample oxidation in ambient air. Deposition temperatures may be varied depending on the nature of the precursor materials used. For example, these may be adjusted to ensure that deposition occurs at an optimum rate. As would be readily understood, the temperature must not be too high that the substrate is damaged or that the precursor is decomposed to any appreciable extent. Typically, the methods herein described may be performed at a deposition temperature of up to 380 °C. A temperature of up to about 350 'Ό, e.g. a temperature of between about 89 °C and about 332 ^<Ό, is particularly preferred.

Pulse durations may be varied according to need, however, the pulse duration of each precursor will be such that there is sufficient time for a substantial proportion of the substrate surface (e.g. the entire substrate surface) to react with the precursor. Pulse durations can readily be determined by those skilled in the art; typical precursor pulse durations will be at least 0.1 seconds, preferably from about 0.1 to about 20 seconds. In the case of pulsing the lithium precursor, this will generally have a duration of at least 1 second, e.g. at least 4 seconds. Similarly, the purge pulse durations may be tailored to ensure that the purge is effective; typical purge pulse durations may range from about 0.8 to about 12 seconds, e.g. about 1 to 2 seconds. In the case where the additional precursor is NH₃, this will generally have a pulse duration of at least 2.5 seconds.

In one embodiment of the method, at least two additional precursors may be employed. These materials may be selected from any of those herein described. A preferred method may involve separate or simultaneous (preferably separate) pulsing of H₂0 and C0₂ as hereinbefore described. A pulse duration of at least 0.25 seconds, e.g. at least 0.5 seconds, is preferred where the additional precursor is water. Where the additional precursor is C0₂, a suitable pulse duration will be at least 5 seconds. Particularly preferred pulse sequences include:

1 .5s/7.5s H₂0/C0₂ ; 1 s/1 s/7.5s H₂0/purge/C0₂; and 0.5s/7.5s H₂0/C0₂.

The products obtained or obtainable by any of the methods herein described form a further aspect of the invention. Such products will comprise a substrate carrying a layer containing lithium and at least one other element, e.g. an element selected from nitrogen, oxygen, phosphorus, silicon and carbon. The layer may further comprise at least one metal, such as one selected from Mo, Nb, Fe, Mn, Co, V, Ni, Ti, La and Al.

The obtained thin films will vary in stoichiometry depending on the precursor materials used and the conditions in the reaction chamber, but typically will be characterised by comprising lithium in the form of Li₂C0₃ or Li-CI, or Li-O(H), Li-N, Li-Si-O, Li-M-O, Li-M-N, Li-M-N-O, Li-P-O, Li-P-O-N, Li-M-P-0 or Li-M-P-O-N type materials (where M is a metal element other than Li). The films produced may have a layered structure or may comprise a monolayer.

In a preferred embodiment, the products according to the invention will be substantially free from (or contain only a limited number of) pin holes and/or defects.

The lithium-containing materials which are deposited on the substrate surface also form a further aspect of the invention. The Li-N type materials produced according to Examples 1 to 3 provided herein are particularly preferred materials and may be characterised by one or more of the following: (i) an X-ray diffraction pattern according to Figure 3 or Figure 15; (ii) a Raman spectrum according to Figure 5 or Figure 16; and (iii) an XPS profile according to Figure 6.

The resulting thin films find a variety of uses, including in particular, use in battery technology. These may, for example, be used to produce an electrolyte barrier or in the production of electrode (i.e. cathode or anode) materials suitable for use in a solid state lithium battery. For example, the Li carbonates may be used in SEI (solid electrolyte interface) layers of a battery. The Li nitrides are particularly suitable as electrolyte or electrode materials. Lithium nitride (Li₃N) may also find use in relation to imide-/amide-based hydrogen storage materials and is a very good lithium ion conductor.

The invention is illustrated further in the following non-limiting Examples and in the attached Figures, in which:

Figure 1 shows the growth rate (growth per cycle, in picometres) vs. Li pulse, water pulse and C0₂ pulse duration for an embodiment of the invention; Figure 2 shows the growth rate (GPC = growth per cycle) and refractive index vs. temperature for an embodiment of the invention;

Figure 3 shows XRD vs. deposition temperature for an embodiment of the invention;

Figure 4 shows the reaction of LiNSiMe₃ with NH₃ as measured by Quartz Crystal Microbalance in accordance with an embodiment of the invention;

Figure 5 shows the Raman spectra of a sample with deposited Li-N on top of Al₂0₃ capped with MoN in accordance with an embodiment of the invention; and

Figure 6 shows an XPS spectra of a sample with deposited Li-N on top of Ti capped with MoN in accordance with an embodiment of the invention.

Figure 7 shows the growth rate, in A, vs. Li pulse, water pulse and C0₂ pulse duration for an embodiment of the invention;

Figure 8 shows the growth rate and refractive index vs. temperature for an embodiment of the invention;

Figure 9 shows XRD vs. deposition temperature for an embodiment of the invention;

Figure 10 shows the composition of a sample with deposited Li₂C0₃ on top of Ti as a function of temperature as determined by XPS in accordance with an embodiment of the invention;

Figure 1 1 shows AFM images of Li₂C0₃ deposited in accordance with an embodiment of the invention at two different deposition temperatures;

Figure 12 shows the growth rate and density of Li-N vs Li pulse for an embodiment of the invention; Figure 13 shows the growth rate and density of Li-N vs NH₃ pulse for an embodiment of the invention;

Figure 14 shows the thickness of Li₃N as a function of the number of deposition cycles for an embodiment of the invention;

Figure 15 shows the XRD pattern of a sample with deposited Li-N in accordance with an embodiment of the invention;

Figure 16 shows the Raman spectra of a sample with deposited Li-N in accordance with an embodiment of the invention; and

Figure 17 shows an AFM image and representative line scan of a Mo-N adhesion layer deposited by ALD.

Examples Example 1

Lithium-containing thin films were deposited by ALD from lithium

bis(trimethylsilyl)amide (CAS: 4039-32-1 ). Reaction with water and C0₂, and with NH₃ , was carried out in order to deposit Li₂C0₃ and Li-N type compounds, respectively. Atomic layer deposition or atomic layer epitaxy (ALD/ALE) is a well known thin film technique related to CVD and described in the literature for instance in Puurunen (J. Appl. Phys. 97: 121301 , 2005).

Experimental

The samples were made in a ASM microchemistry F-120 ALD reactor from lithium bis(trimethylsilyl)amide (Aldrich, 97%) evaporated at 75^ and Dl-water, C0₂ (AGA), or NH₃ (AGA, 99.999%).

Results

Initial experiments were done with thermo gravimetric analysis (TGA) and decomposition in a sealed tube. The TGA showed an onset temperature of 75°C and the decomposition in a sealed tube showed a decomposition temperature of 375°C.

Deposition of Li₂C0₃:

A water pulse was following by pulsing of C0₂ to give the following reactions: LiN(SiMe₃)₂ + H₂0 -» LiOH + HN(SiMe₃)₂ (g)

2 LiOH + C0₂→· Li₂C0₃ + H₂0 (g)

This reaction was investigated with respect to pulse times and deposition temperature and the results are shown in attached Figure 1 . Figure 2 shows the growth rate and refractive index vs. temperature (of the instrument). The results show ALD-type growth from 80 °C up to 380 °C. Above 380 °C the precursor decomposes thermally and gives a black film with gradients.

Crystallinity vs. temperature was investigated using X-ray diffraction. The XRD shows the evolution of crystallinity with increasing deposition temperature (see attached Figure 3). XPS (X-ray photoelectron spectroscopy) showed the existence of Li-C-0 bonds.

Deposition of Li-N:

Li₃N is a compound notoriously difficult to handle and therefore the films were capped with another layer to avoid oxidation of the sample. In these experiments both TiN and Mo₂N were used as capping layers. The reaction of LiN(SiMe₃)₂ with NH₃ was proven by QCM (quartz crystal microbalance).

The reaction that is believed to occur is as follows:

3 LiN(SiMe₃)₂ + NH₃ Li₃N + 3 HN(SiMe₃)₂

Figure 4 shows the reaction of LiN(SiMe₃)₂ with NH₃ , as measured by QCM. Raman spectroscopy shows evidence of Li₃N (see Figure 5) and the XPS (see Figure 6) shows that the sample contains lithium and nitrogen. Example 2

Lithium-containing thin films were deposited by ALD from lithium

bis(trimethylsilyl)amide (CAS: 4039-32-1 ).

Thin films were deposited in a F-120 Sat ALD-reactor (ASM Microchemistry Ltd.) using LiN(SiMe₃)₂ (Aldrich 98%), Mo(CO)₆ (Aldrich 98%), deionised H₂0, C0₂ (95%, AGA) and NH₃ (Linde, anhydrous 99,999%) as precursors. LiN(SiMe₃)₂ was sublimated inside the reactor at 75 'Ό.

Nitrogen was used as a carrier gas in all experiments and supplied at 500 cm³/min. Nitrogen was generated with a Schmidlin UHPN3001 N₂ purifier which provided better than 99.999% N₂+Ar in the carrier gas. The carrier gas was further dried by P₂0₅ and purified of remaining 0₂ by a Mykrolis gas purifier.

Thin films were deposited on soda-lime glass, polished titanium plates and 2.5 x 2.5 cm² single crystal Si(100) wafers. The soda-lime glass substrates were cleaned with ethanol. The single crystals were blown dry with pressurized air, and otherwise used as supplied.

The LiN(SiMe₃)₂ compound, being orange to yellow-brown and sticky, reacts slowly with air and moisture. The compound must nevertheless be stored and handled under inert conditions to assure sufficient reproducibility of experiments. Fresh precursor was therefore transferred directly from a glovebox prior to each experiment.

Results

TGA experiments were done using a Perkin Elmer TGA 7 at a heating rate of 2 ^<C in N₂. Thermal decomposition studies of the precursor were done using the equipment described by Nilsen et al. (Thermochimica Acta, 404:187-192, 2003). The precursor was heated inside 8 mm sealed quartz tubes at 1 15°C for five days. TGA was used to determine a suitable sublimation temperature of 75 °C, at which the residue was about 1 wt%. The precursor decomposition test resulted in a white ring of decomposed precursor in the quartz tube at 375°C suggesting a potential maximum in the ALD-window around 375°C.

A. Reactions of LiN(SiMe₃)_? with H_?Q

The initial experiments using alternating pulses of LiN(SiMe₃)₂ and H₂0 resulted in films with rather uncontrolled growth and large gradients. These films were in addition highly reactive towards air which made characterization extremely difficult. In order to circumvent this problem, a pulse of C0₂ was introduced after each water pulse in order to form the stable and non-hygroscopic Li₂C0₃. This allowed studies of the temperature window and pulse parameters for the selected precursor.

The pulse and purge parameters were optimised at a deposition temperature of 186°C, as shown in Figure 7, resulting in a LiN(SiMe₃)₂ pulse of 4 s and a H₂0 pulse of 0.25 s in order to obtain surface limited growth.

A C0₂ pulse of 7.5 s was found optimal for providing samples which were stable in air. Samples made using 2.5 s and 5 s C0₂ pulses turned milky white over the course of a few days. Without wishing to be bound by theory, it is thought that this is probably due to reaction of unreacted Li₂0 or LiOH in the film with ambient C0₂.

Spectroscopic ellipsometry data were collected with a Woollam Alpha SE ellipsometer between 380 and 900 nm and analyzed by fitting a Cauchy model to the whole dataset. Ellipsometry was used to determine refractive index and thickness of all Li₂C0₃ samples. A wide ALD window was observed experimentally, in line with the results of the thermal decomposition experiments described above. This is shown in Figure 8. Samples were deposited using 4 s Li pulse, 0.5 s water pulse and 7.5 s C0₂ pulse at temperatures between 89 'Ό and 429 ^<Ό. Uniform films with low silicon contents were achieved up to 380 °C. At 429 °C the resulting film was black, showed gradients and extraction of growth rate data were cumbersome. X-ray diffraction and reflectometry measurements

Characterization by X-ray diffraction (XRD) in θ-29-mode was performed with a Bruker AXS powder diffractometer equipped with a Ge(1 1 1 ) monochromator providing Cu-Και radiation and using a LynxEye detector. X-ray reflectometry (XRR) and ω-scans were performed using a Bruker AXS D8 diffractometer with a thin film stage and an asymmetric double bounce Ge(220) monochromator and 0.2 mm slits to provide Cu-Και radiation. The XRR-data were fitted using the GENX software package.

The formation of Li₂C0₃ films was further supported by the density as measured by XRR (not shown) of 2.01 g/cm³ compared to a theoretical density of 2.10 g/cm³.

The crystallinity of the films deposited during 2000 cycles on Si(100) at different temperatures was characterized with XRD (Figure 9). As expected, the crystallinity increased with increasing deposition temperature. However, surprisingly the samples deposited at 380 °C turned out to be X-ray amorphous.

All crystalline samples showed only the (002) of the Li₂C0₃ phase (zabuyelite) proving that this process yields an oriented film on silicon. Rocking curve measurements (ω-scan) of (002) for the sample deposited at 331 °C using 2000 cycles showed a FWHM (full width at half maximum) of 2.05°, supporting a highly oriented growth.

XPS analysis

XPS spectra were collected with a Kratos Axis UltraDLD instrument using monochromatic Al Ka X-ray radiation. The resolution was 0.54 eV as determined by the full width at half maximum of the Ag 3d_5/2 peak. Low energy electrons were used to compensate for surface charging on the Li₂C0₃ reference sample. Energy referencing was based on the C 1 s peak of adventitious carbon set to 285.0 eV binding energy (BE). Peak fitting was performed using Voight functions after subtraction of a Shirley type background in CASA XPS. Instrument manufacturer's sensitivity factors were employed for quantification. XPS analysis was undertaken in order to clarify the impurity level of silicon as a function of deposition temperature. 70 nm thick films of Li₂C0₃ were obtained on polished titanium substrates after 2000 deposition cycles, as shown in Figure 10.

Films deposited at temperatures up to 380 'Ό contained between 0.04 and 0.65 at% silicon impurities with no clear pattern in the variation with deposition temperature. However, the sample made at 428°C contained as much as 5.8 at% silicon. The carbon and oxygen levels in samples deposited between 89 and 380 'Ό were consistent with the lithium carbonate powder used as reference. The good correspondence of the oxygen contents between the reference and the deposited films indicated low hydrogen content in the film.

Surface topography

Surface topography was studied for selected samples with atomic force microscopy (AFM) using a Park Instruments XE-70 and analyzed using the XEI software.

Analysis of the topography by AFM showed that the morphology varies notably with deposition temperature. The sample deposited at 138°C was X-ray amorphous even though AFM analysis showed structures that resembled crystallites with dimensions of some 100 nm. At 331 'Ό larger grains were formed with sizes up to 1 μηι (Figure 1 1 ).

B. Deposition of Li₃N

Deposition of Li₃N was achieved on soda lime glass, silicon and titanium substrates by alternating pulses of LiN(SiMe₃)₂ and NH₃. The deposition was facilitated by first growing a 5 nm adhesion layer of MoN_x prior to the Li-N growth. The final Li₃N product was capped with a 20 nm layer of MoN_x in order to prevent reactions with the ambient air and to allow ex-situ characterization.

Initial studies proved that the Li₃N films were by far more reactive to ambient air than lithium oxide films. This sandwiching of the Li₃N film also amplified its signal in the XRR analysis. Depositions without the adhesion layer often resulted in powderlike depositions rather than a homogeneous film, regardless of the pulse and purge parameters; however, successful depositions were carried out on several different substrates without an adhesion layer (see Example 3). With an adhesion layer, a continuous layer was always formed.

The pulse parameters were optimised at 167°C using 200 cycles of 5s LiN(SiMe₃)₂ pulse and a 5s NH₃ pulse and modifying one parameter at a time (Figures 12 and

13) . The ammonia pulse could be as low as 1 s, however, then yielding products with reduced uniformity. For a 2.5s ammonia pulse, the uniformity was optimal throughout the whole reactor chamber. For longer ammonia pulses, a decrease in density was observed.

When depositing LiN(SiMe₃)₂ and NH₃ using 100, 200 and 500 cycles, a linear relationship between growth rate and thickness of the Li-N layers was found (Figure

14) . Linear regression gives a growth rate of 0.95 A/cycle.

Samples deposited at 167°C were amorphous as derived. In order to derive adequate annealing procedures, 95 nm thick films were annealed at 300 or 600 'Ό for 1 min. However, these samples turned milky after a few minutes exposure to air after the annealing. Crystalline samples were obtained by deposition of 1000 cycles of LiN(SiMe₃)₂ and NH₃ at 332^<€. X-ray diffraction shows the (1 10), (1 1 1 ) and (102) reflections of a-Li₃N, in addition to a few weak reflections attributed to β- Li₃N (Figure 15). The XRD-analysis also showed Li₂C0₃ and LiOH. These are believed to result from post deposition reactions with ambient air. The refined unit cell dimensions of hexagonal a-Li₃N were a = 3.696 A and c = 3.895 A, in good agreement with literature.

Raman spectroscopy was used as an additional tool for proving the chemical state of amorphous samples of lithium nitride. Raman spectroscopy was performed with a Spectra- Physics Millennia Pro 12sJS Nd:YV0₄ solid state laser using 532 nm wavelength operating at 200 mW.

Density functional perturbation theory (DFPT) was used for Raman calculations. For the Raman calculation norm-conserving pseudopotentials were used with 850 eV energy cut-off for all atoms together with a 15 x15 x12, mesh of k points, with the energy conversion threshold of 0.01 meV/atom, maximum displacement of 0.001 A and maximum force of 0.03 eV/A, yielding a high accuracy for the energy and atomic displacements. For Li and N atoms the valence states were modelled using the 2s and 2s², 2p³ electrons, respectively.

For amorphous samples, the Raman spectra confirmed the presence of both a- and β-ϋ₃Ν (Figure 16). The peak at ca 760 cm^"1 is attributed to a LiSi₂N₃ phase. The theoretically calculated modes are all within a Raman shift error of ± 5 %, in line with expectations. The relative intensities of calculated modes concur well with observations

The silicon content of a sample of Li₃N deposited at 186^ on MoN_x-buffered titanium was analyzed with XPS (not shown). The sample was deposited without a capping layer and allowed to oxidize before the analysis to avoid effects from sputtering through the capping layer. The silicon content was found to be ca. 6 atomic percent when assuming a film composition of Li₃N.

Example 3

Deposition of Li₃N was achieved on silicon with native oxide, Si-H, Si/Pt, titanium and glass substrates by alternating pulses of LiN(SiMe₃)₂ and NH₃ using the ALD techniques described in Examples 1 and 2. Deposition was achieved without the need to provide a MoN_x adhesion layer prior to Li film growth.

Growth was performed at 186°C using 500 cycles of 1 s LiN(SiMe₃)₂ pulse and a 1 s NH₃ pulse, with a 2s purge pulse in between.

The resulting Li-N layer was determined to have a uniform thickness of 48 nm ± 2 nm over the whole sample, independent of substrate.

Example 4

Molybdenum-containing thin films were deposited by ALD from Mo(CO)₆. Reaction with NH₃ was carried out in order to deposit Mo-N type compounds. Attached Figure 17 shows an AFM image of a Mo(CO)₆ adhesion layer deposited on Si at 167°C using 1 100 cycles of 0.5s Mo(CO)₆ and 2.5s of NH₃ and 2s purges. The samples were made in a ASM microchemistry F-120 Sat ALD reactor from molybdenum hexacarbonyl, Mo(CO)₆ (Strem, 98%) and NH₃ (Linde, 5.0). Mo(CO)₆ was delivered into the reactor from an external bubbler as described by Diskus et al. (Journal of Materials Chemistry: 21 (3): 705-710, 201 1 ). Nitrogen was used as a carrier gas and supplied at an overall flow rate of 500 cm³/min. The carrier gas was generated with a Schmidlin UHPN3001 N₂ purifier providing a mixture of N₂+Ar with a purity of 99.999% before being further purified by passing through P₂0₅ and a Mykrolis gas purifier.

Films were deposited on soda-lime glass and ca 2 x 2 cm² single crystals of Si(100) with native oxide. 1 x 1 cm² MgO single crystals were also used as substrates for selected depositions. The soda-lime glass substrates were cleaned with ethanol and the single crystals were blown with air and otherwise used as supplied.

ALD-type growth was observed from 1 14^<C up to 167^<Ό. The process proved to be self-limiting at 0.5 s per Mo(CO)₆ pulse. 2.5 s was found to be sufficient for NH₃ pulses. The growth rate increased slightly with longer NH₃ pulses but the difference was small. The pulse durations were therefore optimised at 0.5 s per Mo(CO)₆ pulse, 2.5 s per NH₃ pulse and 2.5 s per purge pulse. The growth rate was found to increase along the flow direction by about 0.02 A per cycle throughout all experiments.

The resulting Mo-N type films may be used as an adhesion layer prior to deposition of a Li-containing thin film in accordance with any of the methods herein described.

Claims

Claims:

1 . A method for the formation of a lithium-containing layer on a substrate by atomic layer deposition, said method comprising the following steps:

b) pulsing a lithium precursor through said reaction chamber whereby to cause said lithium precursor to deposit on and/or react with at least one surface of said substrate to form a lithium-containing layer on at least a portion of said surface;

c) purging said reaction chamber;

e) purging said reaction chamber;

wherein said lithium precursor is a compound of general formula (I):

LiNR^aR^b (I)

(in which

R^a and R^b are independently selected from H, Ci_-6 alkyl and -SiR^c ₃; and each R^c is independently selected from H and Ci_-6 alkyl;

provided that at least one of R^a and R^b is a group -SiR^c ₃).

2. A method as claimed in claim 1 , wherein each step of the process is independently repeated a desired number of times.

3. A method as claimed in claim 1 or claim 2, wherein two or more additional precursors are pulsed through said reaction chamber.

4. A method as claimed in claim 3, wherein said additional precursors are not identical.

5. A method as claimed in claim 3 or claim 4, wherein said additional precursors are pulsed simultaneously or separately from one another.

6. A method as claimed in any one of the preceding claims, wherein in formula (I) both R^a and R^b are groups -SiR^c ₃.

7. A method as claimed in claim 6, wherein in formula (I) at least one R^c is methyl, preferably wherein all groups R^c are methyl.

8. A method as claimed in any preceding claim, wherein purging of said reaction chamber is performed by flowing a purge gas through said reaction chamber or by evacuating said chamber.

9. A method as claimed in claim 8, wherein said purge gas is nitrogen.

10. A method as claimed in any one of the preceding claims, wherein at least one said additional precursor is an oxygen precursor, a metal precursor, a nitrogen precursor, a phosphorus precursor or a halogen precursor.

1 1 . A method as claimed in claim 10, wherein said oxygen precursor is H₂0, 0₃, C0₂, or a combination thereof.

12. A method as claimed in claim 10, wherein said nitrogen precursor is NH₃ or a hydrazine (e.g. 1 ,1 -dimethyl-hydrazine).

13. A method as claimed in any one of the preceding claims, wherein when at least one said additional precursor is a nitrogen precursor, said method further comprises the step of providing a capping layer after pulsing said nitrogen precursor through said reaction chamber.

14. A method as claimed in any one of the preceding claims, wherein said substrate is an electrolyte material for use in a battery.

15. A method as claimed in any one of the preceding claims, wherein said substrate is Si (e.g. single crystalline Si(1 1 1 )), Si with native oxide, Si-H, Si/Pt, Pt, glass (e.g. soda lime glass), Si0₂, Al₂0₃ or Ti, preferably Si, glass, Si0₂, Al₂0₃ or Ti.

16. A method as claimed in any one of the preceding claims, wherein said method is performed at a temperature of up to 380 'C.

17. A method as claimed in any one of the preceding claims, wherein the lithium precursor and additional precursor pulses each has a duration of at least 0.1 seconds.

18. A method as claimed in any one of the preceding claims, wherein the lithium precursor pulse has a duration of at least 4 seconds.

19. A method as claimed in any one of the preceding claims which comprises separately pulsing H₂0 and C0₂.

20. A method as claimed in any one of the preceding claims which further comprises the step of providing an adhesion layer on the surface of the substrate prior to carrying out step b).

21 . A substrate carrying a lithium-containing layer obtainable by a method as claimed in any one of the preceding claims.

22. A substrate carrying a lithium-containing layer as claimed in claim 21 , wherein said layer comprises lithium and at least one other element selected from nitrogen, oxygen, phosphorus, silicon and carbon.

23. A substrate carrying a lithium-containing layer as claimed in claim 22, wherein said layer further comprises at least one metal selected from Mo, Nb, Fe, Mn, Co, V, Ni, Ti, La and Al.

24. A substrate carrying a lithium-containing layer as claimed in claim 21 , wherein said layer comprises lithium in the form of Li₂C0₃ or Li-CI, or Li-O(H), Li-N, Li-Si-O, Li-M-O, Li-M-N, Li-M-N-O, Li-P-O, Li-P-O-N, Li-M-P-0 or Li-M-P-O-N type materials, preferably Li₂C0₃, Li-N or Li-P-O-N.

25. A Li-N type composition characterised by one or more of the following:

(i) an X-ray diffraction pattern according to Figure 3 or 15;

(ii) a Raman spectrum according to Figure 5 or 16; and

(iii) an XPS profile according to Figure 6.

26. Use of a substrate or composition as claimed in any one of claims 21 to 25 in the production of a battery, preferably a thin film battery.

27. The use as claimed in claim 26, wherein said substrate or composition forms an electrolyte layer or an electrode.