This invention relates to a method of coating a substrate using atomic layer deposition.
Atomic layer deposition (ALD) is a thin film deposition technique whereby a given amount of material is deposited during each deposition cycle. Thus it is easy to control coating thickness. One downside is the speed at which a coating is built up.
ALD is based on sequential deposition of individual or fractional monolayers of a material. The surface on which the film is to be deposited is sequentially exposed to different precursors followed by purging of the growth reactor so as to remove any residual chemically active source gas or by products. When the growth surface is exposed to a precursor, it gets completely saturated by a monolayer of that precursor. The thickness of a monolayer depends on the reactivity of that precursor with the growth surface. This results in a number of advantages such as excellent conformality and uniformity, and easy and accurate film thickness control.
Two types of ALD are thermal and plasma enhanced (PEALD). ALD is very similar to chemical vapour deposition (CVD) based on binary reaction. A recipe for ALD is to find a CVD process based on binary reaction and then to apply two different kinds of reactants individually and sequentially. In ALD, the reactions occur spontaneously at various temperatures and will be referred to as thermal ALD because it can be performed without the aid of plasma or radical assistance. Single-element films are difficult to deposit using thermal ALD processes but can be deposited using plasma or radical-enhanced ALD. Thermal ALD tends to be faster and produce films with a better aspect ratio, and so it is known to combine thermal ALD and PEALD processes. The radicals or other energetic species in the plasma help to induce reactions that are not possible using just thermal energy. In addition to single-element materials, compound materials can also be deposited using plasma ALD. One important advantage is that plasma ALD can deposit films at much lower temperature than thermal ALD. Oxygen plasma ALD also can deposit metal oxides conformally on a hydrophobic surface.
In ALD, the growth of a film takes place in a cyclic fashion. Referring to
The deposition cycle is repeated as many times as necessary to obtain the desired film thickness.
According to a first aspect, the present invention provides a method of depositing a material on a substrate, comprising the steps of providing a substrate and depositing a coating on to the substrate using atomic layer deposition, wherein the deposition comprises a first deposition step, a pause in the deposition, followed by a second deposition step.
A deposition step comprises a plurality of deposition cycles. Each deposition cycle includes all the deposition stages required to make a layer of the coating. For example to produce an oxide, each deposition cycle includes one or more deposition stages for each of the metal precursor and the oxidising precursor as an example, for the production of hafnium oxide there is one deposition stage for each of the hafnium and oxidising precursors. The coating can be considered to have been produced by two deposition steps separated by a pause or a delay. Thus, the coating is produced by completing a number of deposition cycles, pausing and then completing a second set comprising a number of deposition cycles.
The pause is a break or delay in the deposition process which has been found advantageous to certain properties of the material deposited on the substrate. The delay preferably has a duration of at least one minute. Thus, in a second aspect the present invention provides a method of depositing a material on a substrate using an atomic layer deposition process, wherein the deposition process comprises a first deposition step, a second deposition step subsequent to the first deposition step, and a delay for a period of time of at least one minute between the first deposition step and the second deposition step.
The delay or pause between a first and a second deposition step is unlike a purge or exposure stage. A purge has to be followed after every exposure stage to evacuate the deposition chamber whether one atomic layer (i.e. metal oxide) is formed or not. On the other hand, the delay occurs only after one complete atomic layer deposition and it interrupts or intervenes with continuous deposition process flow. Thus the delay can be distinguished from a purge stage as the delay is not one of stages in a deposition cycle. Likewise the delay can be distinguished from an exposure stage where reactants are introduced into the chamber as the pressure in this stage increases and additionally this is one of the stages in a deposition cycle. In addition, it is preferred that the temperature within the chamber is maintained during the delay or pause. Thus, the temperature conditions for the delay or pause are substantially similar to those of the deposition steps. The delay or pause is not a post deposition annealing step where the temperature of the final coated substrate is increased it is rather an intermediate step between two deposition steps or two sets of deposition cycles.
The delay is preferably introduced to the deposition by maintaining constant base pressure in a process chamber for example by maintaining a constant flow of Argon gas in the process chamber in which the substrate is located for a period of time of at least one minute between the first deposition step and the second deposition step, and so in a third aspect the present invention provides a method of depositing a material on a substrate using an atomic layer deposition process, wherein the deposition process comprises a first deposition step, a second deposition step subsequent to the first deposition step, and for a period of time between the first deposition step and the second deposition step maintaining a substantially constant pressure in the chamber.
The duration of said period of time is preferably at least one minute and preferably in the range from 1 minute to 120 minutes, more preferably in the range from 10 minutes to 90 minutes. Each deposition step preferably comprises a plurality of consecutive deposition cycles. Each of the deposition steps preferably comprise at least fifty deposition cycles, and at least one of the deposition steps may comprise at least one hundred deposition cycles. In one example, each of the deposition steps comprises two hundred consecutive deposition cycles. The duration of the delay between the deposition steps is preferably longer than the duration of each deposition cycle. The duration of each deposition cycle is preferably in the range from 40 to 50 seconds.
The delay between the deposition steps has a duration that is greater than any delay between consecutive deposition cycles. It is preferred that there is substantially no delay between consecutive deposition cycles, but in any event the introduction of a pause between deposition steps is in addition to any delay between consecutive deposition cycles. In the event that there is a delay of any duration between consecutive deposition cycles, the invention may be considered to be a selective increase in the delay between a selected two deposition cycles.
Each deposition cycle preferably commences with the supply of a precursor to a process chamber housing the substrate. Each deposition cycle preferably terminates with the supply of a purge gas to the process chamber.
Each deposition cycle preferably terminates with the introduction of the purge gas into the chamber for a second period of time which is shorter than the duration of the period of time between the first deposition step and the second deposition step. The delay between deposition steps may be considered to be provided by a prolonged duration of a period of time for which purge gas is supplied to the process chamber at the end of a selected one of the deposition cycles. This selected deposition cycle may occur towards the start of the deposition process, towards the end of the deposition cycle, or substantially midway through the deposition process.
In a fourth aspect, the present invention provides a method of depositing a material on a substrate, wherein a plurality of atomic layer deposition cycles are performed on a substrate located in a process chamber to deposit the coating on the substrate, each deposition cycle comprising introducing a plurality of precursors sequentially into the chamber, and, after introducing each precursor into the chamber, introducing a purge gas to the chamber for a period of time, and wherein, for a selected one of the deposition cycles performed before a final deposition cycle, the duration of the period of time for which purge gas is supplied to the chamber immediately prior to the commencement of the subsequent deposition cycle is greater than the duration of that period of time for each of the other deposition cycles. For the selected one of the deposition cycles, the duration of said period of time is preferably at least one minute, and is preferably in the range from 1 to 120 minutes. During said period of time between deposition cycles that is greater, the pressure of the purge gas is preferably substantially in the chamber.
At least one of the deposition cycles is preferably a plasma enhanced atomic layer deposition cycle.
Preferably the substrate is a structured substrate. For example, the substrate may comprise a plurality of carbon nanotubes (CNTs), each preferably having a diameter of around 50-60 nm. The structured substrate may be provided as a regular array or as a random array. Alternatively, the substrate may be a non-structured substrate.
The substrate may comprise silicon or CNTs. A thin film, or coating, formed by the deposition process is preferably a metal oxide, for example hafnium oxide or titanium oxide.
Each deposition cycle preferably comprises the steps of (i) introducing a precursor to a process chamber, (ii) purging the process chamber using a purge gas, (iii) introducing an oxygen source as a second precursor to the process chamber, and (iv) purging the process chamber using the purge gas. The oxygen source may be one of oxygen and ozone. The purge gas may be argon, nitrogen or helium. To deposit hafnium oxide, an alkylamino hafnium compound precursor may be used. Each deposition cycle is preferably performed with the substrate at the same temperature, which is preferably in the range from 200 to 300° C., for example 250° C. Each deposition step preferably comprises at least 100 deposition cycles. For example, each deposition step may comprise 200 deposition cycles to produce a hafnium oxide coating having a thickness in the range from 25 to 50 nm. Where the deposition cycle is a plasma enhanced deposition cycle, step (iii) above preferably also includes striking a plasma, for example from argon or from a mixture of argon and one or more other gases, such as nitrogen, oxygen and hydrogen, before the oxidizing precursor is supplied to the chamber.
The introduction of a pause or a delay in an ALD process has been found to be beneficial to the electrical properties of a deposited material. One of the electrical properties that has been found to be unexpectedly improved by the introduction of a pause or delay in the ALD process is the dielectric constant of an oxide material. Another electrical property that has been improved is the leakage current of the deposited material.
The deposition step may comprise a first deposition step of PEALD followed by a second deposition step of thermal ALD. Some substrates, such as CNTs are hydrophobic for such materials, thus it is preferred that PEALD with an oxygen precursor is used for at least some of the cycles.
A fifth aspect of the present invention provides a coated substrate made using the aforementioned method.
A sixth aspect of the present invention provides a capacitor comprising a coated substrate made using the aforementioned method.
Features described above in connection with the first aspect of the invention are equally applicable to each of the second to sixth aspects of the invention, and vice versa.
The invention will now be described by example with reference to the accompanying drawings, in which:
a is a graph illustrating the relative permittivity of a hafnium oxide coating as a function of delay time;
b is a graph illustrating the fixed charge density (Qf) of a hafnium oxide coating as a function of delay time;
c is a graph illustrating the variation of Δk and ΔQf of a hafnium oxide coating as a function of delay time;
a and 13b show the hafnium oxide coating of
a and 15b show the hafnium oxide coating of
The invention utilises an atomic layer deposition process to form a thin film or coating on a substrate. The following examples describe a method for forming a coating of a dielectric material on a substrate, which may be a high-k dielectric material used in transistor and capacitor fabrication. The atomic layer deposition process comprises a plurality of deposition cycles. In this example, each deposition cycle is a plasma enhanced atomic layer deposition (PEALD) cycle, which comprises the steps of (i) introducing a precursor to a process chamber, in which a substrate is located, (ii) purging the chamber with a purge gas to remove any excess precursor from the chamber and, (iii) striking a plasma within the chamber and supplying an oxidizing precursor to the chamber to react with precursor adsorbed on the surface of the substrate to form an atomic layer on the substrate, and (iv) purging the chamber with the purge gas to remove any excess oxidizing precursor from the chamber.
Each PEALD process was conducted using a Cambridge Nanotech Fiji 200 plasma ALD system. Referring also to
Each deposition cycle commences with a supply of a hafnium precursor 720, 720a to the deposition chamber. The hafnium precursor was tetrakis dimethyl amino hafnium (TDMAHf, Hf(N(CH3)2)4). The hafnium precursor was added to the purge gas for a period of 0.25 seconds. Following the introduction of the hafnium precursor to the chamber, the argon gas flow purged 730, 730a for a further 5 seconds to remove any excess hafnium precursor from the chamber. A plasma was then struck 740, 740a using the argon purge gas. The plasma power level was 300 W. The plasma was stabilised for a period of 5 seconds before oxygen was supplied 750, 750a to the plasma at a flow rate of 20 sccm for a duration of 20 seconds. The plasma power was switched off and the flow of oxygen stopped, and the argon gas flow purged 760, 760a for a further 5 seconds to remove any excess oxidizing precursor from the chamber, and to terminate the deposition cycle.
Each coating was formed using a different respective deposition process. The first deposition process was a standard PEALD process comprising 400 consecutive deposition cycles, with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The second deposition process was a discontinuous PEALD process, comprising a first deposition step, a second deposition step, and a delay between the first deposition step and the second deposition step. The first deposition step comprised 200 consecutive deposition cycles, again with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The second deposition step comprised further 200 consecutive deposition cycles, again with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle. The delay between the end of the final deposition cycle 775 of the first deposition step and the start 780 of the first deposition cycle of the second deposition step was 30 minutes. During the delay, the pressure in the chamber was maintained 710a in the range from 0.3 to 0.5 mbar, the substrate was held at a temperature of around 250° C., and the argon purge gas was conveyed continuously to the chamber at 200 sccm. This delay between the deposition steps may also be considered to be an increase in the period of time during which purge gas is supplied to the chamber at the end of a selected deposition cycle. The thicknesses of coatings produced by both deposition processes were around 36 nm.
With reference to
Each thermal ALD process was conducted using the Cambridge Nanotech Fiji 200 plasma ALD system. Referring now to
Each deposition cycle commences with a supply of a hafnium precursor 620, 620a, 620b to the deposition chamber. The hafnium precursor was tetrakis dimethyl amino hafnium (TDMAHf, Hf(N(CH3)2)4). The hafnium precursor was added to the purge gas for a period of 0.25 seconds. Following the introduction of the hafnium precursor to the chamber, the argon gas flow purged 630, 630a, 630b for a further 5 seconds to remove any excess hafnium precursor from the chamber. The second precursor, water was then introduced 640, 640a, 640b into the chamber for a period of 0.06 seconds. Then, the argon gas flow purged 650, 650a, 650b for a further 5 seconds to remove any excess oxidizing precursor from the chamber, and to terminate the deposition cycle.
Each coating was formed using a different respective deposition process. Referring now to
Referring to
The graph of
Both the thermal and PEALD hafnium oxide coating produced on the antimony doped silicon substrate showed a similar improvement in dielectric constant when a pause was introduced into the ALD process. Thermal ALD has a slightly shorter cycle time as there is no plasma stage so for a given delay time thermal ALD is a more economical process.
Four titanium dioxide coatings were formed on respective silicon substrates, each using a different respective deposition process. The first deposition process was a standard PEALD process comprising 400 consecutive deposition cycles, with substantially no delay between the end of one deposition cycle and the start of the next deposition cycle, and the variation in dielectric constant of the resultant coating with voltage is indicated at 30 in
The variation of dissipation factor for both PEALD and thermal ALD hafnium oxide coatings was investigated. In both cases the dissipation factor was near zero, less than 0.1 across the voltage range of −2 to +2v. This lower value is due to the fact that hafnium oxide has a very low leakage current so is a close to perfect dielectric with close to perfect capacitor behaviour.
Each metal-insulator-semiconductor (Al/HfO2/n-Si) capacitor structure was made by applying dots of aluminum on top of the thermal ALD produced hafnium oxide coated antimony doped silicon substrate. The dots were 0.5 mm in diameter and were made by evaporation of aluminum. The four hafnium oxide-coated silicon substrates were formed using four different deposition processes. The first hafnium oxide-coated silicon substrate was formed using the first hafnium oxide deposition process described above with respect to
a shows a graph of the relative permittivity of the four capacitors discussed in relation to
The next set of Figures show TEM images of different hafnium oxide coatings. All the images were taken using scanning transmission electron microscopy high annular dark field imaging (STEM-HAADF) where a small probe is rastered across the specimen and the electronic radiation emerging from the sample is collected over a small solid angle in the far-field (Fraunhofer diffraction plane). Image intensity increases with specimen thickness, atomic number or density. Two microscopes were used for this investigation. An FEI Titan3 operated at 300 kV and an aberration corrector in the probe forming lens allowed an illumination angle of 18 milliradians, giving a (diffraction limited) probe size of 0.7 {acute over (Å)}. However, with the finite probe current (80 pA) this increases to about 0.92 {acute over (Å)}. Measurements here indicate transfer out to 1.02 {acute over (Å)}, i.e. about 10% broader than expected. Finally, a non-aberration corrected STEM (FEI Tecnai F20ST) was used for energy dispersive X-ray mapping. The probe size here was much broader: about 1 nm with a 1.3 nA probe current.
To prepare the cross-section of the films, a focussed ion beam microscope FEI Quanta single beam was used. Lamellae from a continuously grown PEALD hafnia film (
Both samples were about 10 μm wide and were thinned at the end to provide an electron transparent region. Both films could be tilted so that the silicon substrate was oriented along the [110] direction. All STEM imaging was undertaken in this condition on the assumption that the growth plane for the hafnia was (001)Si.
a and 13b show the hafnium oxide coating 510 of
a and 15b show the hafnium oxide coating of
Based on the TEM analysis presented above, there is no significant change in the crystallinity between the continuous and interrupted films. There is no significant difference in the thickness of the two films. However, the interrupted film is slightly rougher than the continuously deposited film. Importantly, there were dark bands towards the centre of the interrupted film obtained in the STEM ADF. These dark bands can mean that the film is less dense in that region or that the chemical composition in that region has a higher fraction of low atomic number (Z) elements. It is most likely if the hafnia has a large number of point defects (vacancies on either the Hf or O sites). It is suggested that the hafnia film incorporates vacancies in its structure during interruption (pausing the ALD cycle). The higher k could be due to increase in polarisation centres in these point defects at the midpoint region of the film where the dark bands are visible.
In summary, it is known that HfO2 exhibits a higher dielectric constant in the cubic (k˜29) or in the tetragonal (k˜70) structures than in a monoclinic one (k˜20). The cubic and the tetragonal phases of HfO2 are metastable and generally require high temperature (˜2700° C.) to achieve the monoclinic to tetragonal or tetragonal to cubic phase transformation. However, the cubic and tetragonal phases of HfO2 can be stabilised by the addition of rare earth metals. For example, Ce-doped HfO2 showed stabilised cubic or tetragonal phase and the dielectric constant of 32 [P. R. Chalker et al., Appl. Phys. Lett. 93, 182911 (2008)]. Meanwhile, a very simple modification in ALD process as discussed above can boost the dielectric constant as much as a doping technique. Electrical results showed that the dielectric constant of the interrupted film was at least 50 percent higher, with a value of around 30, than the continuously deposited film that had a k of 20. The leakage current of the two films were in the same order of magnitude (10-8 A/cm2). Physical characterisation techniques like transmission electron microscopy and X-ray analysis were performed to understand the reasons for the change in property of the two types of films. High resolution TEM showed dark bands in middle of film corresponding to the interruption of the process. EDX analysis showed a peak in Ga signal in the midpoint region indicating diffusion into vacancies. These bands are therefore attributed to defects and morphological changes due to annealing during the interruption. X-ray analysis did not show any presence of a high k cubic phase as both the films were monoclinic. Thus the vacancy related non-uniformity in the interrupted film could be the cause for the enhancement in the dielectric constant through increased polarization centres.
Thus, adding a delay between deposition cycles in an ALD process (both thermal and plasma enhanced) leads to the formation of a high quality oxide having a higher dielectric constant than that of conventional ALD formed oxide.
Number | Date | Country | Kind |
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1206096.8 | Apr 2012 | GB | national |
This application is a national stage application under 35 USC 371 of International Application No. PCT/GB2013/050873, filed Apr. 3, 2013, which claims the priority of United Kingdom Application No. 1206096.8, filed Apr. 5, 2012, the entire contents of which are incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/GB2013/050873 | 4/3/2013 | WO | 00 |