Improving optical properties of silicon nitride films to be applied in the middle infrared optics by a combined high-power impulse/unbalanced magnetron sputtering deposition technique Bo-Huei Liao and Chien-Nan Hsiao* Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan *Corresponding author: [email protected] Received 16 September 2013; revised 24 November 2013; accepted 3 December 2013; posted 7 January 2014 (Doc. ID 196684); published 24 January 2014

Silicon nitride films are prepared by a combined high-power impulse/unbalanced magnetron sputtering (HIPIMS/UBMS) deposition technique. Different unbalance coefficients and pulse on/off ratios are applied to improve the optical properties of the silicon nitride films. The refractive indices of the Si3 N4 films vary from 2.17 to 2.02 in the wavelength ranges of 400–700 nm, and all the extinction coefficients are smaller than 1 × 10−4 . The Fourier transform infrared spectroscopy and x-ray diffractometry measurements reveal the amorphous structure of the Si3 N4 films with extremely low hydrogen content and very low absorption between the near IR and middle IR ranges. Compared to other deposition techniques, Si3 N4 films deposited by the combined HIPIMS/UBMS deposition technique possess the highest refractive index, the lowest extinction coefficient, and excellent structural properties. Finally a four-layer coating is deposited on both sides of a silicon substrate. The average transmittance from 3200 to 4800 nm is 99.0%, and the highest transmittance is 99.97% around 4200 nm. © 2014 Optical Society of America OCIS codes: (310.3840) Materials and process characterization; (310.1210) Antireflection coatings; (310.6860) Thin films, optical properties. http://dx.doi.org/10.1364/AO.53.00A377

1. Introduction

Silicon nitride films are useful materials for functional device applications due to their excellent optical, chemical, and mechanical properties. Several examples include the gate dielectrics used in thin film transistors, the thin nitride-oxide stacked layers used in metal-oxide-semiconductor integrated circuits, the antireflection layers used in solar cells, and the gas and water vapor resistance applied in flexible electronics. More recently, interest has shifted to the application of Si3 N4 films for gradient and infrared (IR) optical multilayer coatings [1,2]. Silicon nitride films can be produced by a variety 1559-128X/14/04A377-06$15.00/0 © 2014 Optical Society of America

of methods [2–7], such as CVD, PECVD, reactive evaporation, ion beam sputtering, and reactive sputtering. Among these, magnetron sputtering has been proven to offer the advantages of high precision, good density, large-area deposition, and high rate of coating. High-power impulse magnetron sputtering (HIPIMS) is a newly developed sputtering technique [8–10] in which the pulse of power applied to the target has a low duty cycle (on/off time ratio) of less than 10% and a frequency of less than 10 kHz This leads to pulse power densities of several kW cm−2 on the target. This sputtering technique generates ultradense plasmas with a higher concentration of ionized sputtered atoms. New parameters are utilized to control the deposition process and to optimize the performance for the deposition of both elemental and compound deposited films. These unique features make 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A377

the deposited coatings dense and smooth, even on complex-shaped substrates, and make possible optical coatings with a high refractive index. In unbalanced magnetron sputtering (UBMS), some of the magnetic field lines are directed toward the substrate, and the secondary electrons are able to follow these field lines [11–13]. Consequently, this magnetic field configuration directs ions from the plasma toward the substrate where the resulting ion bombardment can be utilized to modify the optical properties of the growing films. This results in a reduction of absorption and an increase in the packing density of the thin films. Recently, several studies [14–18] have focused on the development of a combined HIPIMS/UBMS deposition technique for hard coating films. This deposition technique eliminates macro particle-induced defects and produces an extremely clean surface free of native oxides, so the resultant film is sharp and free of macro particles at the interface. Hence the films have excellent adhesion, a very smooth coating surface, high hardness, and high deposition rates. However, there has been very limited study of the application of this technique for optical coatings. In this research, a combined HIPIMS/UBMS deposition technique was designed and utilized in a homemade cosputtering system. Silicon nitride films were deposited using this technique, after which the optical properties and the microstructures of the films were characterized. Finally, a middle IR (MIR) antireflective (AR) coating was added. The process is a promising method for applications in MIR optics. 2. Experiment A.

Film Preparation

Figure 1 shows a schematic diagram of the homemade pulsed DC magnetron sputtering system. The system consisted of a deposition chamber and a quartz crystal microbalance with three cosputtering magnetron cathodes. A Si target (99.99% purity) was mounted on one cathode. The Si target was 101.6 mm (4 in.) in diameter and located approximately 16 cm below the substrate. The unbalance

level of a magnetron cathode can be estimated using the geometrical unbalance coefficient G defined as in Eq. (1) [11]: G  Z0 ∕2R;

(1)

where R is an average radius of the erosion zone, and Z0 is the distance from the target surface to the point on the axis of the magnetron where the magnetic field component takes a value of zero. In this study, Z0 was measured by a Gauss meter and two new targets were prepared and sputtered to get the average radius (R) of the erosion zone. A HIPIMS power supply system increased the kinetic energy of the deposition of the silicon nitride films by applying a high discharge voltage. A power supply of 350 W was used in this study. A pulsed on-time period of 45 μs was applied throughout the experiments. The repetition frequency of the pulse was varied from 1 to 20 kHz by changing the off-time periods of the pulse. The silicon nitride thin films were coated on both square B270 glass and silicon wafer substrates with each side being 2.54 cm (1 in) in length. The glass was 1 mm thick. The deposition chamber was pumped down to a base pressure of less than 6 × 10−6 Torr by a cryo-pump. All deposition operations were carried out at room temperature. B. Film Characterization

The transmittances of the thin films on the B270 glass and silicon wafer substrates were measured by two spectrometers with different spectra. The value of the transmittance in the visible region was measured by a Hitachi U4100 spectrometer, and the accuracy of the transmittance was 0.1%. The transmittance in the MIR region was measured by a Bruker tensor27 Fourier transform IR (FTIR) spectrometer, and the accuracy of the transmittance was 4 cm−1. From the spectral analysis, the refractive index, extinction coefficient, and physical thickness were determined by the envelope method [19]. X-ray diffractometry (XRD) was used to determine the crystalline phases in the deposited films. The cross-sectional morphology of the thin films was analyzed by scanning electron microscopy (SEM) to reveal the film structure. 3. Results and Discussion A. Enhancing the Optical Properties by UBMS

Fig. 1. Schematic diagram of the cosputtering system. A378

APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014

Figure 2 shows the transmittance spectra of silicon nitride thin films coated at room temperature under the different geometrical unbalance coefficients. A pulse generator operated at a frequency of 20 kHz is applied to decrease the target arcing. In this experiment, the flow rate of the working argon gas was 30 sccm, and that of the reactive nitrogen gas was 20 sccm. The thicknesses of silicon nitride films deposited under the 0.83 and 2 geometrical unbalance coefficients are 355 and 428 nm, respectively.

Fig. 2. Transmittance spectra of Si3 N4 films deposited with various unbalanced coefficients.

Fig. 3. Extinction coefficients of the Si3 N4 films deposited with various unbalanced coefficients.

The transmittance increases as the unbalance coefficients decrease. During the UBMS process, the plasma is not strongly confined to the target region, but flows out toward the substrate. Thus, the high ion currents extracted from the plasma react with the Si particles on the substrate to form an exactly stoichiometric Si3 N4 film. Figure 3 shows the extinction coefficients of silicon nitride films prepared with different geometrical unbalance coefficients at room temperature. The extinction coefficients decrease as the unbalance coefficients decrease. At a wavelength of 400 nm, the extinction coefficient of films deposited with an unbalance coefficient of 0.83 is five times smaller than that deposited with an unbalance coefficient of 2. This phenomenon is due to the higher ion current densities generated by the unbalanced magnetron, and this result is consistent with the results shown in Fig. 2.

Fig. 4. Transmittance spectra of Si3 N4 films deposited with various on/off times and an unbalanced coefficient of 0.83.

unbalanced coefficient. The thicknesses of silicon nitride films deposited under 45∕5 and 45∕955 pulse on/off time ratios are 407 and 278 nm, respectively. Increasing the pulse-off time can increase the difference between the maxima and minima in the transmittance curve within the 400–700 nm range. However, the maxima of the transmittance spectra will still match the corresponding transmittancespectrum values of the substrate well. The phenomena indicate that the films possess a denser structure and much lower optical loss. For optical coating applications, the extinction coefficient of stoichiometric dielectric films must be smaller than 1 × 10−3 . The dispersion of the refractive index and extinction coefficient of stoichiometric Si3 N4 films deposited by the different techniques are graphed in Fig. 5 and summarized in Table 1 [3–6]. The refractive indices of Si3 N4 films obtained in this study vary from 2.17 to 2.02 in the wavelength range of 400–700 nm. This indicates that the Si3 N4 films deposited by the combined HIPIMS/UBMS deposition technique possessed the highest packing density of all the deposition techniques [3–6]. In addition, the extinction coefficient obtained with our technique is the smallest compared to the other deposition techniques. The excellent optical properties arise from the high ion currents and the high ion bombarding energy provided by the combined HIPIMS/UBMS deposition technique.

B. Enhancing the Optical Properties by a Combined HIPIMS/UBMS Technique

Si3 N4 films deposited using an unbalance coefficient of 0.83 possess better stoichiometry. Hence a combined HIPIMS/UBMS technique is applied to improve the optical properties further using this same unbalance coefficient. Figure 4 shows the transmittance spectra of Si3 N4 films deposited at various pulsed on/off time ratios and with a 0.83

Fig. 5. Refractive indices of Si3 N4 films deposited with different techniques. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

A379

Table 1.

Refractive Indices and Extinction Coefficients of Si3 N4 Films Deposited with Different Techniques

Deposition Method HIPIMS/ UBMS IBSD [6] E-gun with IAD [3] Pulse dc sputtering [4] PECVD [5]

Refractive Index

Extinction Coefficient

400–700 (nm) 2.17–2.02 500 (nm) 2.07 400–700 (nm) 2.07–2.01 500 (nm) Unipolar 2.01 532 (nm) 1.9–1.95 Depends on NH3 concentration and plasma frequency

400–700 (nm)

unbalanced magnetron sputtering deposition technique.

Silicon nitride films are prepared by a combined high-power impulse/unbalanced magnetron sputtering (HIPIMS/UBMS) deposition technique. Different unba...
620KB Sizes 1 Downloads 0 Views