Low-temperature atomic layer deposition of boron nitride and BN structures

Abstract

Methods of the disclosure include a BN ALD process at low temperatures using a reactive nitrogen precursor, such as thermal N.sub.2H.sub.4, and a boron containing precursor, which allows for the deposition of ultra thin (less than 5 nm) films with precise thickness and composition control. Methods are self-limiting and provide saturating atomic layer deposition (ALD) of a boron nitride (BN) layer on various semiconductors and metallic substrates.

Claims

1. A method for atomic layer deposition (ALD) of boron nitride, the method comprising: placing a 2-dimensional semiconductor substrate in an ALD reactor, the 2-dimensional semiconductor substrate comprising highly ordered pyrolytic graphite (HOPG); heating the substrate to a deposition temperature of about 350 C. or less; and sequentially exposing the substrate to a reactive nitrogen containing precursor and a boron containing precursor.

2. The method of claim 1, wherein the reactive nitrogen containing precursor comprises hydrazine (N.sub.2H.sub.4).

3. The method of claim 2, wherein the boron containing precursor comprises one of BCl.sub.3, BBr.sub.3, BF.sub.3, B.sub.2H.sub.6, borazine (BH).sub.3(NH).sub.3, tris(dimethylamino)borane (TDMAB) and organometallic boron compounds.

4. The method of claim 1, wherein the boron containing precursor comprises one of BCl.sub.3, BBr.sub.3, BF.sub.3, B.sub.2H.sub.6, borazine (BH).sub.3(NH).sub.3, tris(dimethylamino) borane (TDMAB) and organometallic boron compounds.

5. The method of claim 1, wherein the substrate comprises a metal interconnect.

6. The method of claim 1, wherein the substrate comprises a ceramic.

7. The method of claim 1, comprising a plurality of sequential exposures at 350 C. or less followed by a plurality of sequential exposures at 400 C.

8. The method of claim 1, wherein 350 C. or less comprises temperatures below 350 C. sufficient to react the nitrogen containing precursor and a boron containing precursor.

9. A semiconductor device, the device comprising: a 2-dimensional semiconductor substrate comprising highly ordered pyrolytic graphite (HOPG); a thin, uniform and pin-hole free interfacial layer of boron nitride; and a dielectric deposited on the interfacial layer.

10. The device of claim 9, wherein the dielectric comprises one of Al.sub.20.sub.3, Hf0.sub.2, Zr0.sub.2, or HfZrO.

11. The device of claim 9, wherein the dielectric is a gate oxide material.

12. The device of claim 9, wherein the layer of boron nitride has a thickness of approximately 0.5 nm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 shows XPS spectra for clean SiGe followed by ALD cycles and subsequent saturation exposure of BCl.sub.3 and N.sub.2H.sub.4.

(3) FIGS. 2 and 3 show a SiGe wafer that ultimately had 60 total cycles of BN deposited.

(4) FIG. 4 shows AFM (atomic force microscopy) images of the 5 nm sample whose XPS data is shown in FIGS. 2 and 3.

(5) FIG. 5 shows the capacitance-voltage characteristics of the estimated 5 nm of BN on SiGe device, along with the corresponding leakage characteristics after a forming gas anneal.

(6) FIG. 6 shows the capacitance vs voltage performance of the 0.5 nm interfacial BN layer on SiGe with 40 cycles of Al.sub.2O.sub.3 versus a reference sample without an interfacial layer.

(7) FIG. 7 shows a Scanning Tunneling Microscopy (STM) 200 nm200 nm image of 15 cycles of BN on HOPG.

(8) FIG. 8 shows Scanning Tunneling Microscopy (STM) 500 nm500 nm (left) and 2000 nm2000 nm (right) images of 30 cycles of BN on HOPG.

(9) FIG. 9 shows XPS normalized peak areas of BN cycles on HOPG after 1, an additional 10, an additional 4 and an additional 15 cycles.

(10) FIG. 10 shows XPS corrected peak areas normalized to the sum of Cu 2p+N 1 s+B 1 s accounting for inelastic mean free paths of an ex-situ cleaned as loaded copper substrate, after a 350 C anneal, and after 30 BN cycles at 350 C.

(11) FIG. 11 shows XPS corrected peak areas normalized to the sum of Mo 3 d+S 2p+B 1 s+N 1 s of an as loaded MoS.sub.2 substrate, after a 400 C anneal to remove contamination, after 5 BN cycles at 350 C, and after 20 more cycles at 400 C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(12) Methods of the disclosure include a BN ALD process at low temperatures using a reactive nitrogen precursor, such as thermal N.sub.2H.sub.4, and a boron containing precursor, which allows for the deposition of ultra thin (less than 5 nm) films with precise thickness and composition control. Methods are self-limiting and provide saturating atomic layer deposition (ALD) of a boron nitride (BN) layer on various semiconductors and metallic substrates. The ALD occurs from the sequential exposures of a reactive nitrogen containing precursor such as hydrazine (N.sub.2H.sub.4) and a boron containing precursor. These precursors include, but are not limited to BCl.sub.3, BBr.sub.3, BF.sub.3, B.sub.2H.sub.6, borazine (BH).sub.3(NH).sub.3, as well as tris(dimethylamino)borane (TDMAB) and related organometallic compounds. The substrates that BN ALD can be performed on include, but are not limited to, semiconductors such as silicon, silicon germanium (SiGe), indium gallium arsenide (InGaAs), indium gallium antiminide (InGaSb), or indium gallium nitride (InGaN), with each compound substrate having varying alloy compositions. In addition, 2-dimensional semiconductor substrates such as highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS.sub.2), as well as metallic substrates, such as copper, can be used.

(13) BN (boron nitride) formed by a method of the disclosure can be an amorphous material. Amorphous BN acts as a diffusion barrier, and is especially useful on copper. BN can be formed via the disclosure in a crystalline form such as hexagonal or cubic. Crystalline BN has useful properties including high thermal conductivity, high chemical stability, a relatively low dielectric constant, and a wide bandgap.

(14) Embodiments will be discussed with respect to experiments. Artisans will appreciate broader aspects and variations of the disclosure from the description of experiments.

(15) Experiments have shown that depositing a thin (0.5 nm) interfacial layer of BN in a MOS architecture can lower Dit when compared to a similar device without the interfacial BN layer. The deposited films are uniform and pinhole free, a crucial requirement for ALD reactions, particularly in nm-sized devices. The deposited films are able to nucleate Al.sub.2O.sub.3 and are assumed to be able to nucleate any other gate oxides such as HfO.sub.2, ZrO.sub.2, and HfZrO. In effect, BN ALD has the ability to functionalize, passivate, and nucleate growth in each and every unit cell. Experiments also demonstrated a device with an interfacial layer of 0.5 nm BN with Al.sub.2O.sub.3 gate oxide. In addition, a device with 5 nm BN was fabricated on SiGe to see if a pinhole free film of BN was deposited. Atomic force microscopy (AFM) confirmed the presence of a uniform, pinhole free BN film. Deposition was also demonstrated on copper.

(16) FIG. 1: XPS spectra for clean SiGe followed by ALD cycles and subsequent saturation exposure of BCl.sub.3 and N.sub.2H.sub.4. The relatively unchanged percentages of boron after the additional 300 ML BCl.sub.3 and nitrogen after the last additional exposure of 300 ML N.sub.2H.sub.4 indicate the saturation behavior of the ALD process. The data is normalized to the intensity of the Si 2p+Ge 3 d+N 1 s+B 1 s signals, as well as all peaks being corrected by Schofield photoionization cross sectional relative sensitivity factors.

(17) FIGS. 2 and 3 show a SiGe wafer that ultimately had 60 total cycles of BN deposited. The thickness of this film was estimated at 5 nm using the attenuation of the substrate signals in XPS. The final H clean is merely to remove excess Cl from the surface after the completion of the cycles. This final removal of Cl may help prevent contamination and unwanted reactions with the surface when the sample is exposed to the air. The ratio of the N 1 s signal to the B 1 s signal is nearly 1:1 indicating the film has grown nearly stoichiometric. FIG. 3 shows the raw data of the B 1 s and N 1 s signal that increases as a function of the number of cycles.

(18) FIG. 4 shows AFM (atomic force microscopy) images of the 5 nm sample whose XPS data is shown in FIGS. 2 and 3. The film is very uniform and shows a root mean square surface roughness of 0.295 nm. The maximum feature height seen is 1.4 nm, which indicates the film, is pinhole free. A pinhole free film is required by ALD processes on nm-scale devices in order to achieve working devices with low gate leakage.

(19) FIG. 4: AFM images 22 um and 500 nm500 nm showing a uniform film of 5 nm BN was deposited. The corresponding line trace indicates a max height of 1.4 nm, which is below the 5 nm of BN deposited. This indicates the film does not contain any pinholes.

(20) FIG. 5 shows the capacitance-voltage characteristics of the estimated 5 nm of BN on SiGe device, along with the corresponding leakage characteristics after a forming gas anneal. Although there is a large frequency dispersion in the CV indicative of trap states, the I-V shows low leakage consistent with a pin-hole free film. Note these MOSCAP were fabricated with less than saturated ALD pulses so better MOSCAPs could be fabricated with properly saturated ALD pulses.

(21) FIG. 5: Capacitance vs voltage plot and current vs voltage plot shows the low leakage of the 5 nm BN sample on top of SiGe after a forming gas anneal.

(22) FIG. 6 shows the capacitance vs voltage performance of the 0.5 nm interfacial BN layer on SiGe with 40 cycles of Al.sub.2O.sub.3 versus a reference sample without an interfacial layer. The top contact is thermal Ni, and the back contact is sputtered Al. There is lower Dit bump seen along with a higher yield of devices. Note this was a non-optimized Al.sub.2O.sub.3 ALD, so better results could be expect with an optimized Al.sub.2O.sub.3 ALD process. In addition, the BN layer was formed with less than saturated ALD pulses, so better MOSCAPs could be fabricated with properly saturated ALD pulses

(23) FIG. 7: Scanning Tunneling Microscopy (STM) 200 nm200 nm image of 15 cycles of BN on HOPG. The corresponding line trace show feature heights on the order of 1 nm and feature widths on the order of 100 nm.

(24) FIG. 8: Scanning Tunneling Microscopy (STM) 500 nm500 nm (left) and 2000 nm2000 nm (right) images of 30 cycles of BN on HOPG. The corresponding line traces show feature heights on the order of 2 nm and feature widths on the order of 200 nm.

(25) FIG. 9: XPS normalized peak areas of BN cycles on HOPG after 1, an additional 10, an additional 4 and an additional 15 cycles.

(26) FIG. 10: XPS corrected peak areas normalized to the sum of Cu 2p+N 1 s+B 1 s accounting for inelastic mean free paths of an ex-situ cleaned as loaded copper substrate, after a 350 C anneal, and after 30 BN cycles at 350 C.

(27) FIG. 11: XPS corrected peak areas normalized to the sum of Mo 3 d+S 2p+B 1 s+N 1 s of an as loaded MoS.sub.2 substrate, after a 400 C anneal to remove contamination, after 5 BN cycles at 350 C, and after 20 more cycles at 400 C.

(28) The experimental self-limiting atomic layer deposition procedure is based upon sequential exposures to a reactive nitrogen-containing precursor (N.sub.2H.sub.4 in our case) and a boron containing precursor (BCI.sub.3, BBr.sub.3, BF.sub.3, B.sub.2H.sub.6, (BH).sub.3(NH).sub.3, TDMAB). During the first half-cycle reaction of the substrate with N.sub.2H.sub.4, the surface terminates in N-H.sub.x groups. During the second half cycle, exposure with the boron containing precursor (BCI.sub.3 is the only precursor we have tested), boron is added to the surface while a gaseous byproduct (HCI for BCI.sub.3) is formed. The surface reaction saturates when all surface hydrogen has reacted, and the surface is left terminated with B-CI groups. Upon the next exposure to N.sub.2H.sub.4, nitrogen is added to the surface again with the formation of a second gaseous byproduct (for example H-CI) that is pumped away. Similarly, the half-cycle reaction is completed when all surface CI has been reacted. Continued sequential exposures have shown that films upwards of 5 nm of nearly stoichiometric BN can be formed on SiGe with very minimal contamination. It is noted that on some substrates, the boron containing precursor will be more effective at nucleating the ALD (for example BCI.sub.3) than the N.sub.2H.sub.4 depending on if the substrate is electrophilic or nucleophilic. For example, on HOPG, the data is most consistent with ALD being nucleated by catalytic dissociation of BCI.sub.3 at step edges.

(29) Anhydrous N.sub.2H.sub.4, courteous of Rasirc, Inc. was employed in these studies to prevent unwanted oxygen contamination during film deposition. A source containing the anhydrous hydrazine was charged with ultra-pure nitrogen to a pressure of 760 Torr, with subsequent pulses using the headspace N.sub.2H.sub.4/N.sub.2 gas mixture. Similarly, semiconductor grade (99.999% pure, Praxair) BCl.sub.3 was used to deposit contamination-free films. The self-limiting atomic layer deposition procedure can be seen in FIG. 1 with the help of monochromatic aluminum channel x-ray photoelectron spectroscopy (XPS) spectrum collected on the surface at a glancing angle of 30. A SiGe wafer that has undergone a hydrogen clean to remove most surface contamination (oxygen and carbon), is shown in the first column. The clean surface was subsequently dosed with 5 cycles of alternating exposure of N.sub.2H.sub.4 and BCl.sub.3 at a sample temperature of 350 C. Each of these first 5 cycles consists of 150 MegaLangmuir (ML) exposure to N.sub.2H.sub.4 and 150 ML exposure to BCl.sub.3 per cycle; no carrier gas was employed but could be in the future. To prove saturation of the ALD cycles, additional BCl.sub.3 was dosed (additional 150 ML and additional 300 ML), which adds additional boron to the surface. The percentage of boron goes from 5.4% to 6.2% to 6.3%. The boron is virtually unchanged after the large exposure to BCl.sub.3 and can be concluded to saturate. In a similar fashion, the nitrogen goes from 11.6% to 12.5% to 12.8% after additional exposures to N.sub.2H.sub.4. This demonstrates the saturation of the N.sub.2H.sub.4 half cycle. A more perfect ALD cycle is achieved by the reduction of the Cl 1 s signal. This procedure outlines the nearly saturating ALD procedure: 450 ML of N.sub.2H.sub.4 and 300 ML of BCl.sub.3 per cycle.

(30) All above data has discussed BN on SiGe. The following data discusses BN on HOPG. BN is isoelectronic to HOPG but has different electronic properties. Therefore, there has been research into depositing BN on HOPG to tune the electronic properties of devices, as well as gain the advantageous properties of BN. FIG. 7 shows a 200200 nm STM image and corresponding line trace of the surface after 15 BN ALD cycles at 350 C. with feature heights on the order of 1 nm and widths on the order of 100 nm. FIG. 8 shows two STM images with corresponding line traces after a total of 30 cycles of BN ALD on HOPG at 350 C. The line traces show that the feature heights are about 1.5-2.5 nm tall with the feature widths on the order of 100-300 nm. Note, saturation doses have not been performed yet. The BN seems to nucleate at the step edges of HOPG; the data is most consistent with the nucleation being caused by BCl3 dissociation since it is most reasonable the boron can freely diffuse across the surface. The STM image on the left shows row-like features that can be construed as a step edge. At the moment this is thought to be crystalline or polycrystalline BN that has deposited on HOPG. FIG. 9 shows the corresponding XPS results of the same sample. A BN film free of contamination was deposited

(31) BN deposition has also been performed on copper and MoS.sub.2 substrates in a very similar manner as on SiGe and HOPG discussed above. FIG. 10 shows the XPS of as loaded copper plus an anneal to remove contamination from the surface, as well 30 cycles of BN deposition at 350 C. The as-received copper sample has contamination, so it undergoes an ex-situ wet clean consisting of 5 minute sonications in acetone, IPA, and water before being loaded into the UHV chamber. The 15 minute anneal at 350 C removes significant oxygen, carbon, and chlorine contamination. After 30 cycles of deposition again nearly stoichiometric is formed. As of this time, saturation studies have not been performed to confirm the ALD. Similarly, FIG. 11 shows deposition on MoS.sub.2; however, the reaction did not appear to occur at 350 C. Thus, the temperature was increased to 400 C and 20 cycles were deposited, resulting in the attenuation of the substrate Mo 3 d and S 2 p signals, while increasing the B 1 s and N 1 s signals in XPS.

(32) BN ALD layers of the disclosure can be used as an interfacial layer in a gate oxide stack, or as a diffusion barrier on copper interconnects in devices, which can help prevent the interconnect form overheating. Due to its small bond length, a-BN should provide a better diffusion barrier than even Si.sub.3N.sub.4 or metal nitrides. While Si.sub.3N.sub.4 can readily be deposited by CVD and plasma enhanced ALD, the a-BN can be deposited by thermal ALD in accordance with the disclosure at low temperature, e.g., 350 C. or less, thereby providing a more conformal coating. Metal nitrides can be deposited by thermal ALD but these often require processing temperatures in excess of 350 C. while the ALD of a-BN can be done below 350 C. The-BN layer can be used in double, quadruple, etc. patterning since it can be deposited conformally on a feature and would have selective etching with respect to semiconductors or oxides (SiO.sub.2). While Si.sub.3N.sub.4 can readily be deposited by CVD and plasma enhanced ALD, the a-BN can be deposited by thermal ALD at 350 C. or less thereby providing a more conformal coating. There is also application for depositing BN as a low dielectric material, or it can serve as a selective etching material. It also has high mechanical resistance to chemical mechanical polishing (CMP) slurries, so depositing BN can help act as CMP stop layer. The ALD deposition of boron nitride is applicable for use as a semiconductor interfacial layer in MOSCAP, MOSFET, and FinFET device architectures. This interfacial layer can act as a surface protection layer from unwanted oxidation or surface contamination or act as diffusion barrier layer from substrate out-diffusion into the oxide. This application will serve useful during deposition and processing of gate stacks on FinFETs for MOSFETs.

(33) While specific embodiments of the present disclosure have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the disclosure.

(34) Various features of the disclosure are set forth in the appended claims.

REFERENCES

(35) The following references are hereby incorporated by reference in their entirety to the extent not inconsistent with the present disclosure.

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