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Aluminum Nitride Thin Film Deposition Using DC Sputtering

Mohd H.S Alrashdan, Azrul Azlan Hamzah, Burhanuddin Yeop Majlis , Mohd Faizal Aziz . Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia

Selangor, 43600, Malaysia

E-mail: azlanhamzah@ukm.my

Abstract—Aluminum nitride thin film depositions at a low temperature become one of the most promising fields in micro-electro mechanical systems and in the semiconductor industry; because of its good compatibility with designs on silicon substrates, its mechanically strong, chemically stable, wide band-gap energy (≈6.2 eV), and has a large electro-mechanical coupling constant. An AlN thin film deposition using DC Magnetron sputtering have the advantage over other deposition methods due to its simplicity, better parameter control, cheapness, and requires a low deposition temperature . The NTI nano film DC sputtering system was used to deposit the AlN thin film with 99.99% pure aluminum target material and 100 silicon substrates ,the working temperature is at 20C°,there is a 10Cm separation distance between the target and the substrate , 335~351 V cathode voltage, the foreline and base pressures are 2×10-2 T, 4×10-5 T respectively, and uses 200W DC power. We vary the time and nitrogen/argon gas flow ratio. Deposited film was characterized by X-ray diffraction and (002) of wurtzite hexagonal phase of AlN thin film was found with beak intensity of 800 count per second for 50% nitrogen content. Field Emission Scanning Electron Microscopy was used to study thin film cross section, film thicknesses and deposition flow rate at different times and gas flow ratio ,there is inverse relationship between nitrogen gas percentage deposition and flow rate . Deposition flow rate are 4.12 nm/ min for 50% nitrogen and 2.217 nm/min for 75% of nitrogen content.

Keywords—Aluminum Nitride; DC Sputtering; Thin Film

Deposition.

I. Introduction Aluminum nitride (AlN) thin films deposition has been the topic

of research in micro-electro mechanical systems(MEMS) and in the semiconductor industry because of its good compatibility with designs on silicon (Si) substrate, also its mechanically strong, and has a high thermal conductivity. It is thermally and chemically stable, especially in an inert environment and oxidation starts at its surface in air at a temperature above 800 C°[56]. AlN has a high electrical resistivity (1016 Ω.cm) [1] which prevent AlN thin film from electrical failure and charge leakage, high Curie temperature (1150 C°), high melting point is at 2200 C° ,it has a good electro-mechanical coupling constant [2], a wide band-gap energy (≈6.2 eV) [3], therefore AlN thin film become one of the most promising piezoelectric material in MEMS power harvesting devices especially at a low frequency range, and the piezoelectric signal produced can overcome the DC leakage during its operating time in power harvester devices . AlN is covalently bonded, therefore it will not introduce any ionic conduction and due to the large band gap, the electronic conduction at high temperature will remain minimal.

AlN thin films deposition can be done by numerous techniques which include chemical vapor deposition (CVD) [4,5] , physical vapor deposition (PVD )[6], reactive molecular beam epitaxy [7], metal-organic chemical vapor deposition (MOCVD)[8], and DC/RF reactive sputtering [9].

DC sputtering have the advantage over other methods of AlN thin films deposition for electro mechanical system and VLSI applications due to its simplicity, better parameter control, cheapness, and requires a low deposition temperature compared to MBE and CVD techniques, which require a normally high substrate temperature of more than 500C° [3]. AlN thin films deposition at a low temperature is recommended due to a difference in the thermal expansion coefficients of the AlN thin films and silicon substrate thus can cause a film detachment during the cooling process to room temperature. This problem can be solved easily by using the sputtering method, since high-energy plasma overcomes the temperature limitations of other deposition methods so a variety of substrate materials with low melting point can be used. DC sputtering is cheaper than RF magnetron sputtering and also has a higher deposition rate, so it’s preferred because low cost and its thin film properties [9].

Mechanical, electrical ,thermal ,chemical, optical and electro mechanical properties of deposited AlN thin films using DC sputtering varies based on thin film microstructure, crystal structure, and chemical composition. These properties can be defined during deposition by controlling sputtering conditions such as distance between the source and target, sputtering power, gas flow ratio and its pressure [10], deposition time and temperature.

In this study we deposit (AlN) thin films using DC sputtering at different sputtering powers: Ar, N2 gas ratio, time for power harvesting devices.

II. Theory A. DC sputtering

DC Sputtering is a physical phenomenon based on ion acceleration using a high DC voltage source and the bombardment of a ‘target’ or cathode. Through momentum transfer, atoms close to the surface of the target metal become

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Fig 1: DC sputtering system.

volatile and are transported as a vapor to a (Anode) substrate. A thin film grows at the surface of the substrate via deposition.

Fig. 1 shows a classic DC sputtering system that consists of a high DC voltage as a power supply , a vacuum chamber, a sample holder, and a sputtering target of the desired film. After evacuating the chamber down to a pressure of 10−6 to 10−8 Torr, an inert gas such as argon is interred into the chamber at pressure of a few mTorr of plasma of the inert gas is then ignited. The accelerated ions with enough energy of the plasma bombard the surface of the conductive target such as Aluminum. In our case, Aluminum makes some of the conductive target atoms to be released from their position and react with other reactive gases in the chamber such as Nitrogen, producing new atoms or compounds. Some of these atoms land on the sample’s surface and makes a new thin film such as Aluminum Nitride.

III. Methodology

Silicon wafer with (100) orientation is cleaned for 5 minutes with acetone and ultrasonic, rinse it for 2 minutes with Deionized (DI) water, and another 5 minutes with methanol and ultrasonic and rinse it again for 2 minutes with deionized (DI) water, and finally 5 minutes with Hydrofluoric acid (HF) 10% to remove any native oxide on the silicon substrate.

NTI nano film DC sputtering system was used to deposit the AlN thin film with 99.99% pure Aluminum was used as a target material; the process parameter we use is illustrated in table 1. We vary DC power, deposition time nitrogen and argon gas flow ratio.

For the first group, which is composed of four samples; I used DC power of 200 W, and the same gas flow ratio between Ar: N2 equals to 20:20 sccm. The times vary from 20 min to 60 min.

TABLE 1. THE PROCESS PARAMETER

Process parameter Numerical value

Tempreture 20C°

Target substrate distance 10Cm

Cathode voltage 335~351 V

Foreline pressure 2×10-2 T

Base pressure 4×10-5T

The Second group of two samples using 200W of DC power and a time of 30 min and 60 min, the gas flow ratio between Ar:N2 equal 10:30 sccm.

The deposited film was characterized by X-ray diffraction (XRD) to study thin film orientation and film structure. Field Emission Scanning Electron Microscopy (FESEM) was used to study thin film cross section, film thicknesses, and construction details such as structure uniformity, determination small contamination feature geometry. Energy-dispersive X-ray spectroscopy (EDX) was used to study chemical characterization and elemental composition measurements.

IV. Results and Discussion The XRD pattern is shown in Fig. 2,The results show

AlN(002) thin film is deposited for all samples with a peak intensity for all samples about 800 count/ second. The Bragg angle equals 38, this indicates good crystal orientation of AlN (002) thin film with c axis normal to silicon substrate. It has been found that, for nitrogen/argon gas flow ratio of 20:20 sccm and 200 W, only the (002) reflection of Wurtzite hexagonal phase of AlN thin film was found with a very small amount of shift. This confirms the good crystalline of thin films with small amounts of residual stress, one of the samples have AlN(100) with a peak intensity of about 200 count/ second . This peak at a smaller diffraction angel indicates that there is a large spacing between the lattice plane and shows not good properties same other sample maybe due to its place inside the sputtering machine or some defect in the wafer. .Fig.3 show the XRD pattern for AlN, this film sputtered at 200 W and 30:10 sccm flow rate ratio between nitrogen and argon gas, we could not obtain (002) reflection at any time. Because of high nitrogen content and then decrease the momentum at silicon substrate due to smaller nitrogen mass compared to argon. And also low power and target temperature (20C°).

Fig. 4 shows the relationship between the deposition rate of the ALN thin film and nitrogen gas concentration for different ratios of nitrogen and argon. The deposition rate in tables 2, 3 is calculated as thickness measured by FESEM divided by the deposition time. It is clear that there is an inverse relationship between the deposition rate and Nitrogen: Argon flow . The deposition rate

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decreased with an increasing N2:Ar + N2 ratio, when the gas ratio of N+ or N2 + ions increased compare to argon gas, the total mass decreased since the mass of Argon is higher than the mass of nitrogen, this leads to a lower momentum transferred to the target and causes a lower energy and efficiency of the target atoms. This causes a decrease in the ALN deposition rate [11], also high Nitrogen content in the chamber makes the chemical reaction faster at the surface of target and develop some AlN compounds at the surface of the Aluminum target. The surface binding energy of AlN is 9.1 ev while for Al it is 3.35 ev which lowers the atom released from the target and then lowers the deposition rate at the silicon wafer [12] .

Fig 2: XRD result at power of 200 w and gas flow rate 20:20 sccm.

Table 2: measured results for 200 w and gas flow rate 20:20 sccm

Wafer # Measured Results

Time (min)

Thickness (nm)

Deposition rate (nm/min)

1 20 80 4

2 30 130 4.33

3 50 190 3.8

4 60 207 4.35

Average 4.12

Fig 3: XRD result at power of 200 w and gas flow rate N2: Ar 30:10 sccm.

AlN thin films deposited using DC sputtering at different gases flow ratios had different colors because of the variation in the crystal structures and stereochemistry [13]. Thin Films deposited at a low flow ratio between Nitrogen and Argon are yellow , while the violet color is produced from films deposited at a higher flow ratio, and the blue color is produced from moderate flow ratios.

In general, the secondary electron emission coefficient for Aluminum’s target is lower than AlN, Fig.5 shows the relationship between gas flow ratio and the cathode voltage during the process. It was clear that when the Nitrogen gas content increases, it leads to an increase in Nitrogen’s partial pressure that causes a drop in the cathode voltage and an increase of the secondary electron emission coefficient of the original Al target as a result of AlNx compounds that was formed at the surface of the Al target. Figures 5 show the cathode voltages during AlN deposition as functions of the deposition time for different gas ratios.

A Strong relationship between the cathode voltage and the percentage of AlNx products covered the surface of the Al target during sputtering [14]. Initially the cathode voltage for 50% gas flow ratio was higher than that at 75% gas flow ratio because of more Al surface target purity, on the other hand, the initial cathode voltage decreased as a result of increasing gas flow ratio. Then normally for both gas flow ratio, the cathode voltage exhibit same manner and reaches an intermediate state and then increases slowly again with time to reach a steady state again. There is a small variation of cathode voltage during this process about 15 V.

Table 3: measured results for 200 w and gas flow rate 30:10 sccm

Wafer # Measured Results

Time (min)

Thickness (nm)

Deposition rate (nm/min)

1 30 75 2.5

2 60 116 1.93

Average 2.217

Fig 4: Deposition rates for the AlN thin films grown as functions of the N2/Ar

+ N2 ratio.

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Fig 5: Cathode voltage at synthesis mode 1 as a function of the sputtering time

for various N2/Ar + N2 ratios.

v. Conclusions AlN thin films were deposited on (100) silicon substrates

using DC sputtering. The deposited film has shown Wurtzite crystal structure with (002) reflection with a peak intensity of 800 counts per seconed when DC power is 200 W and a Nitrogen gas flow ratio of 50%. When flow ratio of nitrogen has been increased to 75% the thin film not deposited. There is an inverse relationship between nitrogen gas percentage deposition flow rates. Deposition flow rate are 4.12 nm/ min for 50% nitrogen and 2.217 nm/min for 75% of nitrogen content. The color of the thin film depends on the Nitrogen flow ratio. The cathode voltage decreased as a result of the Nitrogen flow rate increasing.

Acknowledgment The authors would like to thank University Kebangsaan

Malaysia for supporting this project under grant UKM-GUPNBT-08-25-084.

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