Pulsed-N2 assisted growth of 5-20 nm thick β-W films

A technique to deposit 5-20 nm thick β-phase W using a 2-second periodic pulse of 1 sccm-N2 gas on Si(001) and SiN(5 nm)/Si(001) substrates is reported. Resistivity, X-ray photoelectron spectroscopy and X-ray reflectivity were utilized to determine phase, bonding and thickness, respectively. X-ray diffraction patterns were utilized to determine the crystal structure, lattice constant and crystal size using the LeBail method. The flow rate of Nitrogen gas (continuous vs. pulsing) had significant impact upon the crystallinity and formation of β-phase W.


I. INTRODUCTION
Thin β phase W films exhibit a giant spin Hall effect which can be integrated into a non-volatile spin logic device to interact with an adjacent magnetic layer. 1,2Tungsten films thicker than 5 nm naturally transform to bulk α phase, making it challenging to produce β-W thick enough for device fabrication under large scale manufacturing conditions. 3,4For example, the W film must be thick enough to survive the etch of the magnetic films and be under the spin diffusion length for effective device functioning which puts the desirable thickness in the 10-20 nm range. 5][8][9][10][11][12] In addition, it may be undesirable to introduce O 2 into deposition chamber which also grows ferromagnetic metals.
Introducing N 2 gas during deposition is a natural alternative to O 2 , however it forms tungstennitride crystals or an amorphous-like phase but not a β-phase. 8,10,13It appears that N 2 drastically inhibits the grain growth (similar to high concentrations of O 2 ), which is undesirable. 11This creates a need to form 10-20 nm thick β-W films without the assistance of O 2 gas flow and find the optimum conditions for N 2 flow during tungsten deposition.
In this article, β phase W films of thicknesses up to 20 nm have been fabricated using a 2 second periodic 1 sccm N 2 pulse.The growth kinetics are altered by changing the N 2 gas concentration in the chamber during the deposition which enables the deposition of up to 20 nm thick β-W.The crystal size of these films are ∼5 nm and resistivities are ∼160 µΩ cm, which show promise to be introduced into a fabrication facility to grow β-W for large scale device fabrication.

II. EXPERIMENTAL
Two substrates: single crystal Si(001), and 5 nm of SiN on Si(001) were chosen to deposit the W films. Four different thicknesses of W were deposited: 5, 7.5, 10, and 20 nm.Six 5 nm thick W films were grown, three on each substrate.Three films grown on the Si substrate with the three following different N 2 gas conditions: 1 sccm, 2 sccm, and a 2 second periodic-1 sccm pulse.Three  5 nm thick films were grown on the SiN(5 nm)/Si(001) substrate using the following three N 2 gas conditions: 1 sccm, 2 sccm and no N 2 during the deposition.Six 7.5 nm thick W films were grown on both substrates under the following three different N 2 gas conditions: 2 second periodic-1-sccm pulse, 1 sccm continuous flow and no N 2 during the deposition.
Four 10 nm thick films were grown on the two different substrates with two different flow conditions: 2 second periodic-1-sccm pulse and without N 2 .Two 20 nm thick films on the two substrates were grown using a 2 second periodic-1-sccm pulse of N 2 gas.
The resistivity of each film was measured using a four point probe technique with KLA Tencor's RS-100 instrument.Lattice parameters and crystal sizes were calculated from X-ray diffraction patterns using the LeBail method (Bruker TOPAS, version 4.2) to characterize the phases and estimate the lattice parameter which are displayed in Table I.A Bruker X-ray diffractometer was utilized and configured in a parallel-beam geometry with a fixed 0.5 • incidence angle (sealed Cu source).Beam conditioning consisted of a graded parabolic mirror with mechanical slits on the incident beam and a parallel plate collimator for the diffracted beam.Detector (2θ) scans covered a range of 20 • to 120 • in 0.05 • steps.Instrument function parameters were determined using NIST Standard Reference Material 1976 (α-Al 2 O 3 ).X-ray photoelectron spectroscopy was performed to understand the chemical state of the films using Thermoscientific Theta probe.X-ray reflectivity was measured to estimate the thickness of each film with a Jordan Valley BEDE Metrix-L (BML) X-ray diffractometer was used to measure thickness.A 2.2 kW Cu sealed tube source (0.154 nm) with a beam of a few millimeters (2 mm × 2 mm) was scanned from 0 to 20000 arcseconds at a step size of 10 arcseconds for XRR measurements.

A. X-ray diffraction analysis
The X-ray diffraction patterns of 5 nm, 7.5 nm and 10 nm, 20 nm W films are displayed in Fig. 1, Fig. 2, Fig. 3, respectively.The four X-ray diffraction patterns of the 5 nm W films with 1 sccm and 2 sccm N 2 gas when grown on both, Si(001) and SiN(5 nm)/Si(001), substrates show amorphous-like diffraction patterns as displayed in Fig. 1.A strong crystalline diffraction is observed in the two 5 nm films grown on SiN(5 nm)/Si(001) without any N 2 gas and the 5 nm film deposited on the Si substrate with pulse N 2 gas.These have a lattice parameter of 5.04 Å when fit using LeBail method.Two 7.5 nm W films, deposited on Si(001) and SiN(5 nm)/Si(001) substrates form an α dominated phase with lattice constant of 3.15 Å and crystallite size around 8 nm.The two 7.5 nm W films with continuous flow of 1 sccm N 2 gas on SiN(5 nm)/Si(001) and Si(001) substrates show a smaller FIG. 2. X-ray diffraction patterns of the 7.5 nm W films deposited on the substrates as indicted.crystallite size of 2.3 nm and 4.7 nm and a β dominant phase with lattice constants of 4.9 Å and 5.04 Å respectively.The smallest crystal size is seen in the two films grown with a pulsed N 2 gas condition on both substrates.Lattice constants and crystal sizes are 5.00 Å, 4.7 nm and 5.03 Å, 3.9 nm for the SiN(5 nm)/Si(001) and Si(001) substrates respectively.The X-ray diffraction patterns for 10 nm and 20 nm films show a smaller crystal size, as seen in Fig. 3, also the β-W(200), β-W(211) peaks appear in films with 2s-1-sccm N 2 pulse samples.These films show a reduction in crystallite size in comparison to their α-W counter parts(similar thickness, Table I).Lattice parameter and crystallite size are 5.00 Å, 5.01 Å and 7.0 nm, 7.8 nm for 20 nm β-dominated W films.The 10 nm α dominated W films lattice constants and crystallite size are 3.14 Å, and 10.4 nm, 10 nm.The 10 nm β dominated films have a 5.00 Å and 5 nm lattice parameter and crystallite size, respectively.

B. X-ray photoelectron spectroscopy
The XPS results for all the films show a WO 3 peak in addition to a metallic peak of W as displayed in the Fig. 4 and Fig. 5.The 5 nm W films with different N 2 gas flow conditions on two different substrates are displayed in Fig. 4. The 5 nm W films grown on a SiN(5 nm)/Si(001) substrate with a 2 sccm of N 2 gas during the deposition cycle are displayed in Fig. 4(a).This shows significant W +0 peak at 30.7 eV and W +6 peak around 35.06 eV. 14,15A broad suboxide peak W +x (0 < x < 6) was necessary to obtain a better fit at 34.1 eV.The 5 nm W film grown during a N 2 FIG. 4. a.) X-ray photoelectron spectroscopy result of 5 nm amorphous-like W film. b.) X-ray photoelectron spectroscopy result of 5 nm βW film.FIG. 5. a.) X-ray photoelectron spectroscopy result of 10 nm α W film. b.) X-ray photoelectron spectroscopy result of 10 nm βW film.pulse was used during the deposition, as displayed in Fig. 4(b).This resulted in a increase in W +0 peak at 30.6 eV and W +6 peak around 35.3 eV with a suboxide of W +x (0 < x < 6) at 31.7 eV.A similar spectrum was observed for the rest of the 5 nm films with both metallic and oxide character of W in all the films.
The W film grown on a SiN(5 nm)/Si(001) substrate with W +0 peak at 30.9 eV and W +6 peak around 34.9 eV as displayed Fig. 5(a).No suboxide peak was necessary to obtain a better fit for this film.The 10 nm W film deposited during a N 2 pulse was used during the growth on a SiN(5 nm)/Si(001) substrate as displayed in Fig. 5(b).This resulted in W +0 peak at 30.5 eV and W +6 peak around 35.2 eV with a suboxide of W +x (0 < x < 6) at 31.9 eV was added to improve the fit.Similar spectrums were observed for the 7.5 nm and 20 nm films.

IV. DISCUSSION
6][17][18][19] However, the films are less resistive than previous β-W films which are typically around ∼200 µΩ cm, which maybe beneficial for producing devices with lower resistances than films grown with O 2 . 5,6The α phase-W films have two peaks around W(110) these peaks were not commented for in earlier studies and fit well with a W 5 Si 3 .In addition, the β W(210) and α W(110) peaks are very close to each other which cannot be resolved using the Lebail method which does not take the site occupancy into account.However, the average grain size decreases whenever the β W(200) and (211) peaks appear around the β W(210)/α W(110).This characteristic smaller grain size is another indication of the formation of a β phase.
Depositing a 5 nm W film on SiN without any N 2 gas forms a β phase film.However depositing 5 nm on bare Si forms an α phase.This is similar to our previous findings where 5 nm of W deposited on SiO 2 forms a β phase, which was attributed to the uptake of O 2 causing the formation of β-W. 5 However the SiN substrate does not provide the oxygen and the roughness of the substrate and the thickness (5 nm) of W film constrain the average grain size to be around 2.8 nm, thus restricting the film to be in β phase.The Si substrate does not offer such a surface roughness, or source of oxygen, and the film readily forms an α phase.
A continuous N 2 gas flow renders the 5 nm thick W amorphous-like on both substrates similar to previous findings. 8,10,13However, when a N 2 pulse is introduced, the concentration is sufficient to kinetically constrain the grain growth to form β W. The XPS results of these films shows metallic character present in the all the W films.The native oxide peak of β-W is higher than the α-W of similar thickness.This can be interpreted as higher oxygen gettering capacity of β-W.The metallic peak of β-W is higher than amorphous-like W film of similar thickness and is attributed to the increased crystallinity in the β-W than the amorphous films.
As the thickness is increased to 7.5 nm and 10 nm there are more W atoms to nucleate and grain size increases to form thermodynamically stable α phase on Si and SiN substrates when no N 2 is introduced.When a continuous 1 sccm of N 2 gas is flown during deposition for the 7.5 nm W film on both substrates the film forms a β phase W but with a reduced grain size.However, a N 2 pulse during the 7.5 nm growth yielded much sharper βphase diffraction pattern with a larger grain size similar to the 5 nm thick film.For 10 nm and 20 nm thick W films on both Si(001) and SiN substrate only pulsing the N 2 resulted in β-W with larger average grain size and sharp diffraction patterns, indicating a high quality β-W film.
These findings demonstrate the importance of pulsing the N 2 to achieve β.When inert impurities such as N 2 are introduced into the deposition chamber, they interfere with the nucleation and grain growth of the deposited film.These impurities can either act as a roadblock to grain growth or act as a nucleation site resulting in smaller grain sizes.However if large concentrations of N 2 gas are introduced, about 1 or 2 sccm, it blocks W-metal from forming a crystalline phase and an amorphous-like film is formed.Upon lowering the concentration by pulsing, a kinetically grown phase which has much larger grain size is able to be produced.The 7.5 nm film still crystallizes to form a β phase in presence of 1 sccm continuous flow due to the availability of more W atoms but it has smaller grain size.The 5 nm thick W lies on the boundary of kinetic and thermodynamic stability offered by the SiN substrate or continuous N 2 gas flow.For films 10 nm and above pulsing the N 2 is required to form β-W films with larger grains.

V. CONCLUSION
In conclusion, a novel technique to deposit thick β phase films by kinetically controlling the growth by introducing a 2 second periodic pulse of 1 sccm N 2 gas from 5 to 20 nm thick films is demonstrated.The inert nature of N 2 gas will not effect other metal targets in the deposition chamber and offers a much more versatile alternative than using O 2 .These less resistive β phase films should produce devices with lower resistances which maybe beneficial for improved efficiency and power consumption.It would be interesting to explore these findings on other metals that have a β-phase such as Ta.

FIG. 1 .
FIG.1.X-ray diffraction patterns of the 5 nm W films deposited on the substrates as indicted.

FIG. 3 .
FIG.3.X-ray diffraction patterns of the 10 nm and 20 nm W films deposited on the substrates as indicted.

TABLE I .
W films deposited under different substrates and process conditions.