The effect of radio-frequency sputtering power on the structural properties of Chromium thin films

XXX International Conference on Surface Modification Technologies (SMT30) 29TH JUNE - 1ST JULY, 2016, MILAN, ITALY The effect of radio-frequency sputtering power on the structural properties of Chromium thin films Mahdie Parvizian 1 ; Fariba Rahimi-Ashtari 1 ; Amir Goodarzi 1 1. Iranian National Center for Laser Science and Technology, P.O.Box1439953951, Tehran, Iran Abstract To obtain a low stress and qualified Au/Cr/GaAs ohmic contact, nanostructural properties of chromium (Cr) thin film were studied. The residual stress was measured by substrate curvature method and the characterization of the films was carried out by SEM, AFM and XRD analysis while I-V measurement was used for electrical resistivity measurement. Cr thin films (40-120 nm) were deposited on GaAs and glass substrates by Radio-Frequency (RF) sputtering system. The achieved results revealed that 60 (nm) sample had the lowest residual stress on both glass and GaAs substrates. Furthermore Cr thin films (150-300 W) with 60 nm thickness were prepared and the effect of RF deposition power on the evolution of residual stress, surface roughness, grain size and resistivity of Cr thin films were investigated. Stress results, AFM analyze and I-V measurement proved that 250 W sample had the lowest stress, roughness and resistivity. Also XRD analyze demonstrated that mentioned sample had the largest grain size and minimum micro-strain stress that confirms the other results. Besides the sputtered Cr thin films were found to have [110] preferred orientation. Keywords: Chromium, RF sputtering power, thickness, resistivity, structural properties. 1. Introduction The investigation of metal-semiconductor contacts is a very important task from both a practical and a theoretical point of view. GaAs semiconductor compound has intrinsic electrical properties superior to silicon including higher electron mobility, direct energy gap and lower power dissipation [1].GaAs heterostructures are commonly applied for fabrication of semiconductor devices such as Heterojunction Bipolar Transistor (HBT), monolithic microwave integrated circuit, infrared light emitting diode [2] and laser diode [3]. Ohmic contacts are integral components of electronic devices and integrated circuits that stable and low-resistance ohmic contacts are critical for performance and reliability of the devices. The metallization should have smooth surface and good interface morphology and it s preparation and characterization require many technological efforts [4]. Cr/Au is a proper ohmic contact for p-gaas laser diode that Cr serves as an adhesive layer in this metallization [5]. The most important prerequisite for the preparation of high quality film is an understanding of its growth dynamics and structure in different phases of deposition. One important feature of sputtering deposition is its many parameters that can be controlled to influence the film characteristics such as residual stress, resistivity and surface roughness [6]. Intrinsic stress is the result of crystallographic defects in the film, and extrinsic stress is due to the thermal expansion mismatch between the film and substrate. Thin film delamination occurs when the intrinsic and extrinsic stress in the deposited metal films overcomes the interfacial energy. Generally, cracking is caused by tensile stress, and peel off by compressive stress [7]. Typical control parameters in RF sputtering include the gas pressure, substrate temperature, RF deposition power and substrate bias voltage [6]. In order to create a low stress, low resistance and smooth Cr thin film as an adherent layer in Au/Cr/GaAs ohmic contact we intended to study the effect of RF deposition power on the nanostructural properties of Cr thin film. Whereas thin-film properties usually depend on thickness [8], to obtain an ideal result, first we considered thickness effect on the characterization of Cr thin film Corresponding author. Tel.: +989188621575. E-mail address: mahdieparvizian@yahoo.com

and obtained a proper thickness. Then we investigated RF power effect on residual stress, morphology, electrical resistivity and structural properties of Cr layer at mentioned thickness. 2. Experimental procedure 2.1. Chromium thin films with different thicknesses Cr thin films with different thicknesses, 40-120 (nm), were deposited on p-gaas (100) wafer and glass substrates by RF sputtering technique. For this aim 4 GaAs wafers and 4 glass substrates with (1 1 cm) dimensions were carefully cleaned by chloroform, acetone, 2propanol alcohol and DI water alternatively and dried with (N 2) gas. Then the specimens were loaded in sputter chamber for deposition process with 99.999% Cr target purity. Table 1. Experimental conditions for chromium thin films deposition Pre-sputtering vacuum (Torr) 3 10-6 Sputtering pressure (Torr) 38 10-3 RF power (W) 200 Pre-sputtering time (min) 40 Before deposition, chamber was evacuated to 3 10-6 torr and then back filled with Ar gas to 38 10-3 torr. Sputtering conditions are summarized in Table 1. Deposition power was fixed at 200W and the samples were deposited by different deposition times. After deposition, thickness of thin films was measured by using DEKTAK Surface Profile measuring system. To investigate thickness dependence properties of Cr thin films, various analysis were applied. Residual stress of the films was studied using substrate curvature method analyze while AFM and FESEM analysis were applied for morphological characteristics. Finally I-V measurement was utilized for inspecting electrical resistance behavior versus layer thickness. 2-2- Chromium thin films different in RF deposition power Because of the vast application of Cr layer on GaAs, as an adhesive layer in microelectronic devices, our research was continued on GaAs substrate. The dependence of structural and electrical properties on the RF power variations was investigated at optimum thickness (60 nm), in the range of 150 W to 300 W in increments of 50 W. This range of RF deposition power was selected based on work experience and related articles. To make samples with the same 60 nm thickness at different deposition powers, it was indispensable to obtain deposition rate of Cr layer at various RF sputtering powers. For this purpose, 16 samples in 4 deposition run series were deposited and measured their thickness by DEKTAK system and calculated the deposition rate at each power. Resultant data are listed in Table 3. The final deposition for sample preparation was done at the same experimental conditions, which were mentioned in Table 1 and by obtained deposition rates. Then Cr layer thickness measured again for performance accuracy. Finally, the samples were prepared at different RF powers of 150, 200, 250 and 300 W at 60 nm Cr thickness. AFM, XRD, I-V measurement analysis and substrate curvature method were utilized for investigating Cr thin film characteristics versus RF deposition power. 3. Results and discussion 3.1. Results of Cr thin film with different thicknesses 3.1.1. AFM Results The thicknesses of the Cr films were varied from 40nm to 120nm at mentioned experimental conditions. Surface roughness (R a) of the samples was measured by AFM analyze. For each sample, roughness was measured at 4 different points on the surface at 10 10 (µm) 2 area and the average was considered as the surface roughness. From AFM results, roughness increased by increasing thickness on both substrates, that is a reasonable result. Roughness variation was from 0.81 nm to 4.36 nm on GaAs substrate and from 3.28 nm to 5.1 nm on glass substrate. It s apparent that roughness value for Cr layer on glass substrate is significantly more than GaAs substrate that refers to different in substrate surface morphology that affects film growth. Surface roughness for glass was 4.3 A while for GaAs was 2 A. Roughness data are listed in table 2. 2

Fig. 1 shows AFM image and surface roughness of 60 nm (figure 1 (a,b)) and 120 nm (figure 1 (c,d)) samples on glass and GaAs substrates. There is a distinct difference between surface morphology of Cr layer on glass and GaAs substrates with the same thickness. The maximum peak height for 60 nm Cr thickness on glass is about 27 nm while on GaAs is about 15 nm which leads to the higher roughness of the Cr layer on glass (R a= 4.1 nm) compare to GaAs (R a=1.64 nm). Also fig1(a,c) shows maximum peak height of about 35 nm for 120 nm Cr thickness on glass substrate while maximum height of 60 nm Cr on glass substrate is about 27 nm that a variation on the surface morphology of Cr layer with different thickness could be observed. The same variation is observable for the Cr layer on GaAs substrate with different thicknesses. Table 2. Electrical resistivity and roughness of Cr thin film on GaAs and glass substrates Thickness (nm) 40 60 100 120 Electrical resistivity on GaAs (Ω) 285± *2 116±3 41.5±2 27±1 Electrical resistivity on glass (Ω) 420±4 242±5 97±1 50±1 Roughness on GaAs (nm) 0.81±0.01 1.7±0.02 2.16±0.04 4.36±0.02 Roughness on glass (nm) 3.28±0.02 4.3±0.03 4.7±0.05 5.1±0.04 * Error bar = σ/ n ; σ : standard deviation ; n: number of measurements; n=4 for resistivity and 6 for roughness. a) b) c) d) R a=4.2 nm Ra=1.64 nm Ra=6 nm Ra=4.32 nm Fig. 1. AFM images of Cr layer at 60(nm) on a) glass and b) GaAs substrates and 120(nm) on c) glass and d) GaAs substrates 3.1.2. SEM Results a) b) 500 nm 500 nm c) d) 500 nm 500 nm Fig. 2. Top view FESEM images of a) 40 nm, b) 60 nm, c) 100 nm and d) 120 nm of Cr thin film on GaAs substrate 3

Fig.2 shows FESEM images of Cr layer with different thicknesses on GaAs substrate. By increasing thickness grain size growth and roughness evolution is evident that confirms AFM results. Cr layer with 40 nm and 60 nm thicknesses in figure 2 (a,b) are more uniform and compact while in 100 nm and 120 nm thicknesses in figure 2 (c,d) grains are connected to each other and hillocks are formed. Due to more uniformity and better grain size distribution, less cracks are observable in 60 nm thickness compare to the other. 3.1.3. I-V measurement Four point probe method was used for electrical resistivity measurement of Cr sputtered thin films. The electrical resistivity of chromium films on glass substrates range from 420 Ω to 50 Ω, whereas those deposited on GaAs substrates range from 285 Ω to 27 Ω. Achieved results are listed in Table 2. From table 2, electrical resistivity for both substrates decreases by increasing thickness. Comparing the resistivity data, it is apparent that the surface roughness of glass substrates dominates the scattering of the electrons reducing their mean free path lengths and thus increasing the electrical resistivity of thin metallic films [9]. 3.1.4. Stress results Stress reduction cause in limitation of crack or blister creation and leads to device life time improvement. Substrate curvature method is the most commonly used technique in thin film stress measurement that we applied it in our research. In this method, the stress measurement analysis uses the bending plate method to calculate stress in a deposited thin film layer, based upon the change in curvature and material properties of the film and substrate. Fig. 3 shows, residual stress variation versus thickness of Cr layer on GaAs and glass substrates. In both figures 3(a,b) either compressive or tensile stresses decrease by increasing thickness from 40 nm to 60 nm and get their minimum value at 60 nm Cr thickness, then start increasing behavior from 60 nm to 120 nm Cr thickness. Minimum value of residual stress for GaAs substrate is 60 Gpa (compressive) and -30 Gpa (tensile) and for glass substrate is 340 (compressive) Gpa and -250 Gpa (tensile), that the higher value of residual stress of Cr layer on glass compare to GaAs refers to different in the surface morphology of substrates. Based on obtained results from stress and SEM analysis the 60 nm Cr thickness because of better surface morphology and low residual stress is the optimum thickness for Cr layer. The power research continues to be done at this thickness. 3.2. Results of Cr thin film with different RF powers 3.2.1. AFM results Surface roughness of Cr thin films on GaAs substrate at various sputtering powers, 150-300 W, was measured by AFM analyze while for each specimen 6 different area 10 10 (µm) 2 were scanned and the average value of roughness was considered as the surface roughness of the sample. Obtained data are summarized in table 3. Considering results, roughness decreases by increasing power from 150 W to 250 W, then increases at 300 W. This shows that the 250 W sample has the lowest roughness. Fig. 4 shows AFM image and surface roughness (R a) of 60 nm Cr layer at different RF deposition powers. AFM images show that 150 W and 300 W samples, fig 4 (a,d), have rough and needle shape surface and their maximum peak height are 10.3 nm and 12.4 nm respectively that is more than maximum peak of 200 W and 250 W samples. Also Fig 4 demonstrates that the 250 W sample has a congeries shape and a better surface coverage. Maximum peak height at this sample is about 9 nm that is less than the others and makes it to have a smoother surface and lower roughness. 4

stress (Mpa) stress (Mpa) a) compressive tensile 3000 2000 1000 0-1000 -2000-3000 20 40 60 80 100 120 140 thickness (nm) b) compressive 450 tensile 300 150 0-150 -300-450 20 40 60 80 100 120 140 thickness (nm) Fig.3. Cr layer residual stress versus thickness for a) glass and b) GaAs substrates Table 3. Deposition rate, surface roughness, gain size and micro strain of Cr thin films at different RF sputtering powers Sample a b c d RF power (W) 150 200 250 300 Deposition rate (A /s) 1.88 2.29 2.9 3.13 Average roughness (nm) 1.04± *0.03 0.98±0.07 0.91±0.01 1.22±0.02 Grain size (nm) 7.83 9.53 21.72 14.40 Micro-strain 0.0117 0.0096 0.0042 0.0063 * Error bar = σ/ n ; σ : standard deviation ; n: number of measurements; n=6 a) b) c) d) R a=1.02 nm R a=0.98 nm R a=0.9 nm R a=1.2 nm Fig. 4. AFM image of Cr thin film (60 nm) on GaAs at a) 150 W, b) 200 W, c) 250 W and d) 300 W, RF sputtering power 3.2.2. XRD results X'Pert reflectivity was used to scan coated samples with Ni-filtered Cu-Kα radiation in 0.01 steps and counting time of 3s for scan range between 30 and 140 2θ. The X-Ray Reflectivity spectra of the Cr films on GaAs deposited at various RF sputtering powers are shown in Fig. 5. Only the Cr [110] peak was observed for all powers. In order to analyze the dependence of the grain size on the RF power, we calculated the size of the crystal based on the XRD results by the following equation 1. Also the relationship between the FWHM of the XRD data and micro-strain of the thin film can be expressed by equation 2. grain size = 0.9λ/FWHM. Cosθ (Scherrer formula) (1) < ε >= FWHM/4. tanθ (2) From equation 1 and 2: 5

stress(mpa) Counts < ε > 1/grain size (3) Where λ is the X-ray wavelength, θ is the Diffraction angle, and FWHM is the Full Width at Half Maximum. By increasing power from 150 W, XRD graph peak (2θ) displaced from 44.25 to the maximum value of 44.42 at 250 W sample and then returned to a lower quantity at 300 W sample. Also FWHM decreased by increasing power from 150 W to 250 W and got it minimum value of 0.4 at 250 W and then increased. The size of the crystal was calculated to be in the range of 7.83-21.72 nm and the micro-strain was calculated to be from 0.0042 to 0.0117, while the largest crystal grain and the lowest micro-strain were obtained at 250 W that is a reasonable result considering relation 3.It can be concluded that at this power the effect of ion bombardment and atomic peening that causes too much stress for layer is significantly reduced [10]. The maximum value of grain size that related to 250 W sample was observable in AFM images too. The obtained values based on XRD are listed in table 3. 3.2.3. Stress results Residual stress evolution versus RF power of Cr thin films was studied by substrate curvature method which was entirely described in section 3.1.4. Stress graph in Fig. 6 shows that either compressive or tensile residual stresses decrease by increasing power and get their minimum value at 250 W, after that increasing behavior is observable. These results relatively well correspond with the obtained results of micro-strain. So 250 W can create a low stress layer with a better adherence. 150 W, FWHM: 1.1 200W, FWHM: 0.9 250W, FWHM: 0.4 300W, FWHM: 0.6 2Ѳ (deg.) Fig. 5. FWHM variation of XRD graph of Cr thin films versus RF power 1000 500 0-500 -1000 100 150 200 250 300 350 power(w) compressive tensile Fig. 6. Cr/GaAs residual stress at different sputtering powers 6

resistance (ohm) 3.2.3. I-V measurement Electrical contacts are the most basic and, at the same time, one of the most important parts of modem integrated circuitry. Specifically, lowered contact resistance decreases both the noise figure and the power dissipation. Electrical resistivity measurement of Cr films carried out by probe station system which revealed that 250 W sample had the lowest resistivity about 140 Ω.Electrical resistivity dependence to RF power is clearly obvious in Fig.7. This result confirms AFM images and stress outcome that showed the 250 W sample had the best surface coverage and minimum residual stress and predicted to have the lowest electrical resistivity. This can be explained by the fact that in this region the energy supplied to the growing film by the bombarding particles and the ion density current are optimal that reduces the applied stress to layer. This factors result in a denser packed crystalline structure and hence a lower resistance [11]. 280 240 200 160 120 100 150 200 250 300 350 power (w) Fig. 7. Cr thin film resistance versus RF sputtering power 4. Conclusion In summary, characteristics of chromium (Cr) thin film deposited by RF sputter technique on GaAs and glass substrates at various thicknesses, 40 nm to 120 nm and different RF sputtering powers, 150 W to 300 W, is investigated. As expected, the electrical resistance decreased by increasing thickness while surface roughness and grain size increased. Also the 60 nm sample had the minimum residual stress which obtained as the optimum thickness. According to RF power investigation (150, 200, 250 and 300 W) of Cr thin film at optimal thickness (60 nm) on GaAs substrate, only the [110] preferred crystal orientation observed for all samples. Based on analysis results the 250 W obtained as the best RF deposition power of Cr thin film. At this power minimal value was found for residual stress and micro strain makes it to create a suitable adherent layer. The maximum value for grain size and minimum value for roughness were obtained at mentioned power that enable Cr layer to be a smooth layer for the subsequent deposition of Au. Also minimum value for electrical resistivity was obtained at 250 W. Finally Cr thin film at 60 nm thickness and 250 W RF sputtering power which has a suitable morphological and structural properties can be applied as a proper adhesive layer in Cr/Au ohmic contact for optoelectronic devices. Acknowledgment The authors wish to thank P.Abbasi for supporting the project and B.Sabrloui for residual stress data. References: [1] R. V. Ghita, 2005. Studies of ohmic contact and schottky barriers on Au-Ge/GaAs and Au-Ti/GaAs, J. Optoelectron. Adv. Mat Vol. 7, No. 6, 3033 3037. [2] K. Fujita, H. Ohnishi, P.O.Vaccaro and T.Watanabe, 1997. J. Microelectron 28, 1019-1023. [3] HC. Ko, MW. Cho, JH. Chang and M. Yang, 1999. J. Appl. Phys A 68 467-470. [4] W.Macherzynski, M Wosko, B Paszkiewicz, B Sciana, R Paszkiewicz, M Tlaczala, 2009. Fabrication of ohmic contact based on platinum to p-type compositionally graded AlGaAs layer, J.Phys, Conference Series, 146, 012034. [5] P.R. Scheeper, B. Nordstrand, J.O. Gullov, B. Liu, T. Clausen, L. Midjord, T. Storgaard- Larsen, 2003. J. Microelectromech. Syst.12 880-891. 7

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