Description

The present invention is in the field of processes for the generation of thin inorganic films on substrates, in particular atomic layer deposition processes.

With the ongoing miniaturization, e.g. in the semiconductor industry, the need for thin inorganic films on substrates increases while the requirements on the quality of such films become stricter. Thin metal films serve different purposes such as barrier layers, conducting features, or capping layers. Semiconductor products, for example computer chips, have a sophisticated ar- chitecture including different materials. For their production, films are usually made on struc- tured and cured photo resins which expose only parts of the underlying surface on which the film is supposed to be deposited. However, this approach requires a series of steps. It is there- fore much more desirable if films can be selectively deposited on certain regions of a substrate without using any masks.

WO 2013 / 070 702 A1 discloses a process for depositing metal films employing aluminum hy- dride which is coordinated by a diamine as reducing agent. However, only films uniformly cover- ing the whole substrate are disclosed, no selective depositions.

It was therefore an object of the present invention to provide a process for selectively depositing metal containing films on a substrate. It was aimed at a process which is easy, i.e. does not re- quire sophisticated masks. The process shall yield films of high quality with a high degree of re- producibility.

These objects were achieved by a process for preparing metal-containing films comprising

(a) depositing a metal-containing compound from the gaseous state onto a solid substrate hav- ing a surface comprising a first surface composition in a first region and comprising a second surface composition in a second region, wherein the first surface composition is selected from a noble metal, a metal or semimetal oxide, or hydrogen or alkyl terminated silicon and the second surface composition is selected from a non-noble metal, a hydroxide terminated substance, a metal or semimetal nitride, a metal or semimetal carbide, and a metal or semimetal silicide,

(b) bringing the solid substrate with the deposited metal-containing compound in contact with an aluminum hydride derivative.

Preferred embodiments of the present invention can be found in the description and the claims. Combinations of different embodiments fall within the scope of the present invention.

The process according to the present invention includes depositing a metal-containing corn- pound from the gaseous state onto a solid substrate. The metal-containing compound contains at least one metal atom. Metals include Li, Be, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os Ir, Pt, Au, Hg, Tl, Bi. Preferably, the metal-containing compound contains a metal which is more electropositive than Cu, more pref- erably more electropositive than Ni. In particular, the metal-containing compound contains Ti,

Ta, Mn, Mo, W, or Al. It is possible that more than one metal-containing compound is deposited on the surface, either simultaneously or consecutively. If more than one metal-containing corn- pound is deposited on a solid substrate it is possible that all metal-containing compounds con- tain the same metal or different ones, preferably they contain different metals.

Any metal-containing compound, which can be brought into the gaseous state, is suitable.

These compounds include metal alkyls such as dimethyl zinc, trimethylaluminum; metal alkox- ylates such as tetramethoxy silicon, tetra-isopropoxy zirconium or tetra-iso-propoxy titanium; metal cyclopentadienyl complexes like pentamethylcyclopendienyl-trimethoxy titanium or di(ethylcycopentadienyl) manganese; metal carbenes such as tris(neopentyl)neopentylidene tantalum or bisimidazolidinyliden ruthenium chloride; metal halides such as aluminum trichlo ride, tantalum pentachloride, titanium tetrachloride, molybdenum pentachloride, or tungsten hexachloride; carbon monoxide complexes like hexacarbonyl chromium or tetracarbonyl nickel; amine complexes such as bis(tert-butylimino)bis(dimethylamino)molybdenum, bis(tert-bu- tylimino)bis(dimethylamino)tungsten or tetrakis(dimethylamino)titanium; diketonate complexes such as tris(acetylacetonato)aluminum or bis(2,2,6,6-tetramethyl-3,5-heptanedionato) manga- nese. Metal halides are preferred, in particular aluminum chloride, aluminum bromide and alu- minum iodide. It is preferred that the molecular weight of the metal-containing compound is up to 1000 g/mol, more preferred up to 800 g/mol, in particular up to 600 g/mol, such as up to 500 g/mol.

The solid substrate comprises at least two different surface compositions. Surface composition in the context of the present invention is the chemical composition of a layer extending from the surface of the substrate up to 10 nm into the substrate, preferably 5 nm, more preferably 1 nm, in particular 0.2 nm such as only the topmost atomic layer. It is generally accepted that surface compositions can only be approximated by bulk materials due to surface phenomena, for exam- pie due to the limited possibility of surface atoms to complete their thermodynamically preferred electronic structure. Therefore, all chemical formulae referring to surface compositions are in- tended to approximate the real composition, which generally means that a formula represents at least 80 % of the atoms present at the surface and that the atomic ratio of different elements in the formula closely resemble the atomic ratio at the surface. Nevertheless, it can never be ex- cluded that some atoms of lower layers diffuse into the surface composition. In some cases, this may even be a desired effect, for example the diffusion of silicon atoms from a silicon wafer into the topmost layer where the silicon atoms form metal silicides.

The first surface composition is selected from a noble metal, a metal or semimetal oxide, or hy- drogen or alkyl terminated silicon. The first region may comprise one of these surface composi- tions or more than one. Nobel metals in the context of the present invention are Ru, Rh, Pd, Os, Ir, and Pt, preferably Pt and Ru. Preferred metal or semimetal oxides are AI2O3, Zr0 ₂ , HfC>2, T1O2, MgO, S1O2, preferably S1O2. Preferred alkyl terminated silicon is methyl terminated silicon. It has been found that these surface compositions inhibit or at least significantly reduce the metal deposition such that the area of the first surface composition remains free of a continuous and thus conductive film of the deposited metal without any complex masking procedure.

The second surface composition is selected from a non-noble metal, a hydroxide terminated substance, a metal or semimetal nitride, a metal or semimetal carbide, and a metal or semi- metal silicide. The second region may comprise one of these surface compositions or more than one. Non-noble metals are all metals not mentioned as noble metal above, i.e. Li, Be, Na, Mg, Al, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge Rb, Sr, Y, Zr, Nb, Mo, Tc, Cd, In, Sn,

Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Hg, TI, Bi. Preferred non-noble metals are Ti, Co, Ni, Cu, Ge, Mo, Ta, W, in particular Co and Cu. Pre- ferred metal nitrides are nitrides of Ti, Co, Ni, Cu, Ge, Mo, Ta, W, in particular TiN, TaN and WN. Preferred metal or semimetal carbides are carbides of Ti, Co, Ni, Cu, Ge, Mo, Ta, W, in particular TiC and WC. A preferred hydroxide terminated substance is hydroxide terminated sili- con. It has been found that these surface compositions favor the metal deposition process ac- cording to the present invention such that the area of the second surface composition is well covered with the deposited metal-containing film.

It is possible that the substrate bulk is one material, for example silicon, which is treated in one part of its surface in one way, for example by an oxygen plasma to generate S1O2, while the other part is not treated or in a different way, for example by exposing it to HF. Such treatments preferably include plasma treatments, such as chlorine-, nitrogen-, or oxygen-containing plasma, electron beam bombardment, thermal treatment, ion bombardment or a corona dis- charge. It is also possible to treat the surface with acids, for example with HF or H2SO4/H2O2. Alternatively, it is possible that the substrate is of a certain material, for example Cu, and on parts of its surface a thin layer of a different material is deposited, for example Pt.

The solid substrate can have any shape. These include sheet plates, films, fibers, particles of various sizes, and substrates with trenches or other indentations. The solid substrate can be of any size. If the solid substrate has a particle shape, the size of particles can range from below 100 nm to several centimeters, preferably from 1 pm to 1 mm. In order to avoid particles or fi- bers to stick to each other while the metal-containing compound is deposited onto them, it is preferably to keep them in motion. This can, for example, be achieved by stirring, by rotating drums, or by fluidized bed techniques.

According to the present invention the solid substrate with the deposited metal-containing corn- pound is brought in contact with an aluminum hydride derivative. An aluminum hydride deriva- tive in the context of the present invention is any molecule which contains at least one Al-H bond. Aluminum hydride derivatives include alkyl aluminum hydrides, amine coordinated alumi- num hydrides, and carbene coordinated aluminum hydrides. Alkyl aluminum hydrides include dimethyl aluminum hydride and dibutyl aluminum hydride.

Amine coordinated aluminum hydrides include aluminum hydrides which are coordinated by an amine via one nitrogen atom, aluminum hydrides which are coordinated by an amine via two ni- trogen atoms and aluminum hydrides which are coordinated by an amine via three nitrogen at- oms.

Preferred examples for an aluminum hydride which are coordinated by an amine via one nitro- gen atom is trialkylamine alane such as trimethylamine alane, dimethylethylamine alane, tri- ethylamine alane, and N-methylpyrrolidine alane.

A preferred example for an aluminum hydride which are coordinated by an amine via two nitrogen atoms is the compound of general formula (I)

wherein Rn to R17 are independent of each other hydrogen, an alkyl group, an alkenyl group, an aryl group, or a silyl group. Preferably, R14 to R17 is hydrogen. Preferably, Rn , R12, and R1 ₃ are alkyl groups, in particular Rn is tert-butyl and R12 and R1 ₃ is methyl.

Another preferred example for an aluminum hydride which are coordinated by an amine via two nitrogen atoms is the compound of general formula (II)

wherein Z is a C ₂ -C ₄ alkylene group, and

R is hydrogen, an alkyl group, an alkenyl group, an aryl group, or a silyl group.

Z in the compound of general formula (II) is a C2-C4 alkylene group including ethylene, i.e. -CH2- CH2-, 1 ,3-propylene, i.e. -CH2-CH2-CH2-, and 1 ,4-butylene, i.e. -CH2-CH2-CH2-CH2-, preferably ethylene. The alkylene group can be substituted by halogens, alkyl groups, alkenyl groups, aryl groups or silyl groups as described above. The compound of general formula (II) can be synthesized by heating a N-heterocyclic carbene with two equivalents of an aluminum hydride complex, for example aluminum hydride coordi- nated by trimethylamine. The synthesis of various N-heterocyclic carbenes is well known from the prior art. The following equation shows the synthesis.

The reaction can be done in a solvent of low polarity, for example aromatic hydrocarbons such as toluene. The reaction temperature is typically 25 to 150 °C, preferably 60 to 120 °C, the reac- tion time is typically 10 min to 2 h, preferably 20 to 40 min.

A preferred example for an aluminum hydride which are coordinated by an amine via

three nitrogen atoms is the compound of general formula (III)

wherein R31 to R311 are independent of each other hydrogen, an alkyl group, an alkenyl group, an aryl group, or a silyl group. Preferably, R34 to R311 are hydrogen.

A preferred example for carbene coordinated aluminum hydrides is the compound of general formula (IV)

wherein R41 to R46 are independent of each other hydrogen, an alkyl group, an alkenyl group, an aryl group, or a silyl group. Preferably, R43 to R46 are hydrogen. Another preferred example for carbene coordinated aluminum hydrides is the compound of gen- eral formula (V)

wherein R51 to R54 are independent of each other hydrogen, an alkyl group, an alkenyl group, an aryl group, or a silyl group. Preferably, R53 and R54 are hydrogen.

An alkyl group can be linear or branched. Examples for a linear alkyl group are methyl, ethyl, n- propyl, n-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl. Examples for a branched alkyl group are iso-propyl, iso-butyl, sec-butyl, tert-butyl, 2-methyl-pentyl, neo-pentyl, 2-ethyl- hexyl, cyclopropyl, cyclohexyl, indanyl, norbornyl. Preferably, the alkyl group is a C1 to Cs alkyl group, more preferably a C1 to C ₆ alkyl group, in particular a C1 to C4 alkyl group, such as me- thyl, ethyl, iso-propyl or tert-butyl.

An alkenyl group contains at least one carbon-carbon double bond. The double bond can in- clude the carbon atom with which R is bound to the rest of the molecule, or it can be placed fur- ther away from the place where R is bound to the rest of the molecule. Alkenyl groups can be linear or branched. Examples for linear alkenyl groups in which the double bond includes the carbon atom with which R is bound to the rest of the molecule include 1-ethenyl, 1-propenyl, 1- n-butenyl, 1-n-pentenyl, 1-n-hexenyl, 1-n-heptenyl, 1-n-octenyl. Examples for linear alkenyl groups in which the double bond is placed further away from the place where R is bound to the rest of the molecule include 1-n-propen-3-yl, 2-buten-1-yl, 1-buten-3-yl, 1-buten-4-yl, 1-hexen-6- yl. Examples for branched alkenyl groups in which the double bond includes the carbon atom with which R is bound to the rest of the molecule include 1 -propen-2 -yl, 1-n-buten-2-yl, 2-buten- 2-yl, cyclopenten-1-yl, cyclohexen-1-yl. Examples for branched alkenyl groups in which the dou- ble bond is placed further away from the place where R is bound to the rest of the molecule in- clude 2-methyl-1-buten-4-yl, cyclopenten-3-yl, cyclohexene-3-yl. Examples for an alkenyl group with more than one double bonds include 1 ,3-butadien-1 -yl, 1 ,3-butadien-2-yl, cylopentadien-5- yi-

Aryl groups include aromatic hydrocarbons such as phenyl, naphthalyl, anthrancenyl, phenan- threnyl groups and heteroaromatic groups such as pyrryl, furanyl, thienyl, pyridinyl, quinoyl, benzofuryl, benzothiophenyl, thienothienyl. Several of these groups or combinations of these groups are also possible like biphenyl, thienophenyl or furanylthienyl. Aryl groups can be substi- tuted for example by halogens like fluoride, chloride, bromide, iodide; by pseudohalogens like cyanide, cyanate, thiocyanate; by alcohols; alkyl chains or alkoxy chains. Aromatic hydrocar- bons are preferred, phenyl is more preferred. A silyl group is a silicon atom with typically three substituents. Preferably a silyl group has the formula S1X3, wherein X is independent of each other hydrogen, an alkyl group, an aryl group or a silyl group. It is possible that all three X are the same or that two A are the same and the re- maining X is different or that all three X are different to each other, preferably all X are the same. Alkyl and aryl groups are as described above. Examples for siliyl groups include S1I-I3, methylsilyl, trimethylsilyl, triethylsilyl, tri-n-propylsilyl, tri-iso-propylsilyl, tricyclohexylsilyl, dime- thyl-tert-butylsilyl, dimethylcyclohexylsilyl, methyl-di-iso-propylsilyl, triphenylsilyl, phenylsilyl, di- methylphenylsilyl, pentamethyldisilyl.

Preferably, Rn , R21 and/or R22 bears no hydrogen atom in the 1-position, i.e. Rn , R21 and/or R22 bears no hydrogen atom which is bond to the atom which is bond to the nitrogen atom. Exam- pies are alkyl group bearing two alkyl side groups in the 1-position, i.e. 1 ,1-dialkylalkyl, such as tert-butyl, 1 ,1-dimethylpropyl; alkyl groups with two halogens in the 1-position such as trifluoro- methyl, trichloromethyl, 1 , 1 -difluoroethyl ; trialkylsilyl groups such as trimethylsilyl, triethylsilyl, dimethyl-tert-butylsilyl; aryl groups, in particular phenyl or alkyl-substituted phenyl such as 2,6- diisopropylphenyl, 2,4,6-triisopropylphenyl. Alkyl groups bearing no hydrogen atom in the 1-po- sition are particularly preferred.

Typically, the aluminum hydride derivative acts as a reducing agent on the deposited metal-con- taining compound. The metal-containing compound is usually reduced to a metal, a metal ni- tride, a metal carbide, a metal carbonitride, a metal alloy, an intermetallic compound or mixtures thereof. Therefore, the process for preparing metal-containing films is preferably a process for preparing metal films, metal nitride films, metal carbide films, metal carbonitride films, metal al- loy films, intermetallic compound films or films containing mixtures thereof. Metal films in the context of the present invention are metal-containing films with high electrical conductivity, usu- ally at least 10 ⁴ S/m, preferably at least 10 ⁵ S/m, in particular at least 10 ⁶ S/m.

The aluminum hydride derivative has a low tendency to form a permanent bond with the surface of the solid substrate with the deposited metal-containing compound. As a result, the metal-con- taining film hardly gets contaminated with the reaction products of the aluminum hydride deriva- tive. Preferably, the metal-containing film contains in sum less than 5 weight-% nitrogen, more preferably less than 1 wt.-%, in particular less than 0.5 wt.-%, such as less than 0.2 wt.-%.

The aluminum hydride derivative preferably has a molecular weight of not more than 1000 g/mol, more preferably not more than 800 g/mol, even more preferably not more than 600 g/mol, in particular not more than 500 g/mol. The aluminum hydride derivative preferably has a decomposition temperature of at least 80 °C, more preferably at least 100 °C, in particular at least 120 °C, such as at least 150 °C. Often, the decomposition temperature is not more than 250 °C. The aluminum hydride derivative has a high vapor pressure. Preferably, the vapor pres- sure is at least 1 mbar at a temperature of 200 °C, more preferably 150 °C, in particular 120 °C. Usually, the temperature at which the vapor pressure is 1 mbar is at least 50 °C. Both the metal-containing compound and the aluminum hydride derivative used in the process according to the present invention are used at high purity to achieve the best results. High purity means that the substance used contains at least 90 wt.-% metal-containing compound or alumi- num hydride derivative, preferably at least 95 wt.-%, more preferably at least 98 wt.-%, in partic- ular at least 99 wt.-%. The purity can be determined by elemental analysis according to DIN 51721 (Prufung fester Brennstoffe - Bestimmung des Gehaltes an Kohlenstoff und Wasserstoff - Verfahren nach Radmacher-Hoverath, August 2001).

The metal-containing compound or the aluminum hydride derivative can be deposited or brought in contact with the solid substrate from the gaseous state. They can be brought into the gaseous state for example by heating them to elevated temperatures. In any case a tempera- ture below the decomposition temperature of the metal-containing compound or the aluminum hydride derivative has to be chosen. In this context, the oxidation of the aluminum hydride deriv- ative is not regarded as decomposition. A decomposition is a reaction in which the metal-con- taining compound or the aluminum hydride derivative is converted to an undefined variety of dif- ferent compounds. Preferably, the heating temperature ranges from 0 °C to 300 °C, more pref- erably from 10 °C to 250 °C, even more preferably from 20 °C to 200 °C, in particular from 30 °C to 150 °C.

Another way of bringing the metal-containing compound or the aluminum hydride derivative into the gaseous state is direct liquid injection (DLI) as described for example in US 2009 / 0 226 612 A1. In this method the metal-containing compound or the aluminum hydride derivative is typically dissolved in a solvent and sprayed in a carrier gas or vacuum. If the vapor pressure of metal-containing compound or the aluminum hydride derivative and the temperature are suffi ciently high and the pressure is sufficiently low the metal-containing compound or the aluminum hydride derivative is brought into the gaseous state. Various solvents can be used provided that the metal-containing compound or the aluminum hydride derivative shows sufficient solubility in that solvent such as at least 1 g/l, preferably at least 10 g/l, more preferably at least 100 g/l. Ex- amples for these solvents are coordinating solvents such as tetrahydrofuran, dioxane, diethoxy- ethane, pyridine or non-coordinating solvents such as hexane, heptane, benzene, toluene, or xylene. Solvent mixtures are also suitable.

Alternatively, the metal-containing compound or the aluminum hydride derivative can be brought into the gaseous state by direct liquid evaporation (DLE) as described for example by J. Yang et al. (Journal of Materials Chemistry, 2015). In this method, the metal-containing compound or the aluminum hydride derivative is mixed with a solvent, for example a hydrocarbon such as tetra- decane, and heated below the boiling point of the solvent. By evaporation of the solvent, the metal-containing compound or the aluminum hydride derivative is brought into the gaseous state. This method has the advantage that no particulate contaminants are formed on the sur- face. It is preferred to bring the metal-containing compound or the aluminum hydride derivative into the gaseous state at decreased pressure. In this way, the process can usually be performed at lower heating temperatures leading to decreased decomposition of the metal-containing corn- pound or the aluminum hydride derivative. It is also possible to use increased pressure to push the metal-containing compound or the aluminum hydride derivative in the gaseous state to- wards the solid substrate. Often, an inert gas, such as nitrogen or argon, is used as carrier gas for this purpose. Preferably, the pressure is 10 bar to 10 ⁷ mbar, more preferably 1 bar to 10 ³ mbar, in particular 1 to 0.01 mbar, such as 0.1 mbar.

It is also possible that the metal-containing compound or the aluminum hydride derivative is de- posited or brought in contact with the solid substrate from solution. Deposition from solution is advantageous for compounds which are not stable enough for evaporation. However, the solu- tion needs to have a high purity to avoid undesirable contaminations on the surface. Deposition from solution usually requires a solvent which does not react with the metal-containing corn- pound or the aluminum hydride derivative. Examples for solvents are ethers like diethyl ether, methyl-fe/7-butylether, tetrahydrofurane, dioxane; ketones like acetone, methylethylketone, cy- clopentanone; esters like ethyl acetate; lactones like 4-butyrolactone; organic carbonates like diethylcarbonate, ethylene carbonate, vinylenecarbonate; aromatic hydrocarbons like benzene, toluene, xylene, mesitylene, ethylbenzene, styrene; aliphatic hydrocarbons like n-pentane, n- hexane, cyclohexane, iso-undecane, decaline, hexadecane. Ethers are preferred, in particular tetrahydrofurane. The concentration of the metal-containing compound or the aluminum hydride derivative depend among others on the reactivity and the desired reaction time. Typically, the concentration is 0.1 mmol/l to 10 mol/l, preferably 1 mmol/l to 1 mol/l, in particular 10 to 100 mmol/l.

For the deposition process, it is possible to sequentially contact the solid substrate with a metal- containing compound and with a solution containing an aluminum hydride derivative. Bringing the solid substrate in contact to the solutions can be performed in various ways, for example by dip-coating or spin-coating. Often it is useful to remove excess metal-containing compound or the aluminum hydride derivative, for example by rinsing with the pristine solvent. The reaction temperature for solution deposition is typically lower than for deposition from the gaseous or aerosol phase, typically 20 to 150 °C, preferably 50 to 120 °C, in particular 60 to 100 °C. In some cases it can be useful to anneal the film after several deposition steps, for example by heating to temperatures of 150 to 500 °C, preferably 200 to 450 °C, for 10 to 30 minutes.

The deposition of the metal-containing compound takes place if the substrate comes in contact with the metal-containing compound. Generally, the deposition process can be conducted in two different ways: either the substrate is heated above or below the decomposition temperature of the metal-containing compound. If the substrate is heated above the decomposition tempera- ture of the metal-containing compound, the metal-containing compound continuously decom- poses on the surface of the solid substrate as long as more metal-containing compound in the gaseous state reaches the surface of the solid substrate. This process is typically called chemi- cal vapor deposition (CVD). Usually, an inorganic layer of homogeneous composition, e.g. the metal oxide or nitride, is formed on the solid substrate as the organic material is desorbed from the metal M. This inorganic layer is then converted to the metal layer by bringing it in contact with the aluminum hydride derivative. Typically the solid substrate is heated to a temperature in the range of 300 to 1000 °C, preferably in the range of 350 to 600 °C.

Alternatively, the substrate is below the decomposition temperature of the metal-containing compound. Typically, the solid substrate is at a temperature equal to or slightly above the tem- perature of the place where the metal-containing compound is brought into the gaseous state, often at room temperature or only slightly above. Preferably, the temperature of the substrate is 5 °C to 40 °C higher than the place where the metal-containing compound is brought into the gaseous state, for example 20 °C. Preferably, the temperature of the substrate is from room temperature to 400 °C, more preferably from 100 to 300 °C, such as 150 to 220 °C.

The deposition of metal-containing compound onto the solid substrate is either a physisorption or a chemisorption process. Preferably, the metal-containing compound is chemisorbed on the solid substrate. One can determine if the metal-containing compound chemisorbs to the solid substrate by exposing a quartz microbalance with a quartz crystal having the surface of the sub- strate in question to the metal-containing compound in the gaseous state. The mass increase is recorded by the eigen frequency of the quartz crystal. Upon evacuation of the chamber in which the quartz crystal is placed the mass should not decrease to the initial mass, but up to a mono- layer of the residual metal-containing compound remains if chemisorption has taken place. In most cases where chemisorption of the metal-containing compound to the solid substrate oc- curs, the x-ray photoelectron spectroscopy (XPS) signal (ISO 13424 EN - Surface chemical analysis - X-ray photoelectron spectroscopy - Reporting of results of thin-film analysis; October 2013) of M changes due to the bond formation to the substrate.

If the temperature of the substrate in the process according to the present invention is kept be- low the decomposition temperature of the metal-containing compound, typically a monolayer is deposited on the solid substrate. Once a molecule of the metal-containing compound is depos- ited on the solid substrate further deposition on top of it usually becomes less likely. Thus, the deposition of the metal-containing compound on the solid substrate preferably represents a self- limiting process step. The typical layer thickness of a self-limiting deposition processes step is from 0.01 to 1 nm, preferably from 0.02 to 0.5 nm, more preferably from 0.03 to 0.4 nm, in par- ticular from 0.05 to 0.2 nm. The layer thickness is typically measured by ellipsometry as descri- bed in PAS 1022 DE (Referenzverfahren zur Bestimmung von optischen und dielektrischen Ma- terialeigenschaften sowie der Schichtdicke diinner Schichten mittels Ellipsometrie; February 2004). A deposition process comprising a self-limiting process step and a subsequent self-limiting re- action is often referred to as atomic layer deposition (ALD). Equivalent expressions are molecu- lar layer deposition (MLD) or atomic layer epitaxy (ALE). Hence, the process according to the present invention is preferably an ALD process. The ALD process is described in detail by George (Chemical Reviews 1 10 (2010), 1 11-131).

Preferably, after deposition of a metal-containing compound on the solid substrate and before bringing the solid substrate with the deposited metal-containing compound in contact with a re- ducing agent, the solid substrate with the deposited metal-containing compound is brought in contact with an acid in the gaseous phase. Without being bound by a theory, it is believed that the protonation of the ligands of the metal-containing compound facilitates its decomposition and reduction. Suitable acids include hydrochloric acid and carboxylic acids, preferably, carbox- ylic acids such as formic acid, acetic acid, propionic acid, butyric acid, or trifluoroacetic acid, in particular formic acid.

Often it is desired to build up thicker layers than those just described. In order to achieve this the process comprising (a) and (b), which can be regarded as one ALD cycle, are preferably performed at least twice, more preferably at least 10 times, in particular at least 50 times. Usu- ally, the process comprising (a) and (b) is performed not more than 1000 times.

The deposition of the metal-containing compound or its contacting with a reducing agent can take from milliseconds to several minutes, preferably from 0.1 second to 1 minute, in particular from 1 to 10 seconds. The longer the solid substrate at a temperature below the decomposition temperature of the metal-containing compound is exposed to the metal-containing compound the more regular films formed with less defects. The same applies for contacting the deposited metal-containing compound to the reducing agent.

The process according to the present invention yields a metal film. A film can be only one mon- olayer of a metal or be thicker such as 0.1 nm to 1 pm, preferably 0.5 to 50 nm. A film can con- tain defects like holes. These defects, however, generally constitute less than half of the surface area covered by the film. The film preferably has a very uniform film thickness which means that the film thickness at different places on the substrate varies very little, usually less than 10 %, preferably less than 5 %. Furthermore, the film is preferably a conformal film on the surface of the substrate. Suitable methods to determine the film thickness and uniformity are XPS or ellip- sometry.

The film obtained by the process according to the present invention can be used in an electronic element. Electronic elements can have structural features of various sizes, for example from 100 nm to 100 pm. The process for forming the films for the electronic elements is particularly well suited for very fine structures. Therefore, electronic elements with sizes below 1 pm are preferred. Examples for electronic elements are field-effect transistors (FET), solar cells, light emitting diodes, sensors, or capacitors. In optical devices such as light emitting diodes or light sensors the film obtained by the process according to the present invention serves to increase the reflective index of the layer which reflects light.

Preferred electronic elements are transistors. Preferably the film acts as chemical barrier metal in a transistor. A chemical barrier metal is a material reduces diffusion of adjacent layers while maintaining electrical connectivity.

Brief Description of the Figures

Figure 1 : Scanning electron microscopy (SEM) image showing the Al deposition of example 1.

Figure 2: Scanning electron microscopy (SEM) image showing the Al deposition of example 2.

Figure 3: Scanning electron microscopy (SEM) image showing the Al deposition of example 3.

Figure 4: Scanning electron microscopy (SEM) image showing the Al deposition of example 4.

Figure 5: Scanning electron microscopy (SEM) image showing the Al deposition of example 5.

Examples

Example 1

A silicon wafer was treated with a 3:1 mixture of H ₂ SO ₄ and H ₂ O ₂ for 30 minutes at 80 °C to generate hydroxyl terminated silicon. This substrate was sequentially exposed to AICI ₃ vapor and vapor of compound 1-1

at 150 °C for 250 cycles. Figure 1 shows an SEM image of the deposited aluminum film with a film thickness of about 43 nm.

Example 2

A silicon wafer with a native oxide on its surface was sequentially exposed to AICI ₃ and 1-1 at 130 °C for 500 cycles. Figure 2 shows an SEM image of the deposited aluminum film with a film thickness of about 65 nm.

Example 3

The same process as in example 2 was made with a TiN substrate instead of the silicon wafer. Figure 3 shows an SEM image of the deposited aluminum film with a film thickness of about Example 4

A silicon wafer with Pt layer on its surface was sequentially exposed to AICI ₃ and 1-1 at 120 °C for 500 cycles. Figure 4 shows an SEM image of the surface with hardly any deposit.

Example 5

A silicon wafer was dipped for 60 s into a 1 % HF solution to generate a hydrogen terminated silicon surface. This surface was sequentially exposed to AICI ₃ and 1-1 at 140 °C for 50 cycles. Figure 5 shows an SEM image of the surface with hardly any deposit.