ANALYSIS OF INTRINSIC STRESS IN DIAMOND FILMS BY X-RAY DIFFRACTION

Transcription

1 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol ANALYSIS OF INTRINSIC STRESS IN DIAMOND FILMS BY X-RAY DIFFRACTION Qiaoqin Yang*, Lihua Zhao, Hanning Xiao and Nanfang Zhao Materials Test & Research Center, Hunan University, Changsha, Hunan , P.R.China 1 Central South University of Technology, Changsha, Hunan , P. R. China ABSTRACT The relationships of the residual stress versus process parameters of diamond films grown by Hot Filament CVD were investigated by XRD. The intrinsic macrostress in the diamond films was tensile. The tensile stress decreased almost linearly with acetone concentration, and a minimum tensile stress was obtained at a deposition temperature of 9OOOC and 50 V bias voltage. The microstress in the film goes down with increasing deposition temperature and decreasing positive bias voltage. The relationships between stress and growth parameters were explained according to the microstructure of the diamond films. By optimizing the process parameters, the tensile intrinsic macrostress of diamond films can be reduced greatly without changing the quality of the films. The microstress in the films was released when the samples were heat-treated. 1. INTRUDUTION Diamond film has a wide range of potential applications such as fabrication of wear-resistant protective coatings, abrasives, heat sinks and high-temperature semiconducting and optical devices, due to its exceptional unique properties. However, the residual stress in diamond fnm generated during the process of diamond growth is mostly tensile. It makes the diamond fihn brittle and easy to peel off, and becomes one of the obstacles that hinder the wide application of diamond film. To understand the origin of the residual stress in diamond film and to find ways to relax them, it is important to analyze residual stress in diamond film. There are various kinds of stress measurement methods for diamond film such as X-ray diffraction (XRD) ( I, Raman spectroscopy I21 and the vibrating membrane method [31. XRD is a unique method to measure macrostress and microstress of diamond fihns at the same time. In this paper, the relationships of residual stress to the process parameters of diamond films grown by hot-filament chemical vapor deposition (HFCVD) were studied by XRD and a method to reduce these residual stresses is presented. 2. EXPERIMENTAL PROCEDURE The diamond films studied were grown on monocrystalline silicon (100) in a HFCVD system 14] from a mixture of hydrogen and acetone. The process parameters are summarized in Table 1. When one of the parameters was varied, the others were held at fixed value.

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol The intrinsic stress in the diamond fihns were measured using a Siemens DSOOO diffraction system. The method used for measurement of macrostress was the sin2w method. Lattice distortions due to contributions of microstress were calculated from the broadening of the diffraction peaks. All the diamond peaks were step scanned (at a step length of 0.02 deg. and a step time of 15 sec.) with Cu K, radiation generated at 40 kv and 45 ma. Table 1 Growth Parameters of Diamond Films Residual gas pressure 1.3 Pa Operating pressure 6.5 kpa Gas rate of flow 200 ml /min Filament temperature 2200 C Film growth rate nm/min Film thickness 8clm Acetone concentration O ~01% fixed 0.8 ~01% Substrate temperature C fixed 900 C Positive bias voltage O-200 v Fixed ov The measured macrostress is the total macrostress, o, which is the sum of two terms: intrinsic stress, Oi, and thermal stress, cr,. That is: Then we have: o*=o, + 0, (1) Thermal stress can be calculated by the equation c&=0;- 0, (2) cr,= E&z,-@AT/( l-v3 (3) where a, and ct,are the thermal expansion coefficient of substrate and film, respectively. AT is the temperature difference between deposition temperature of the film and the measurement temperature of the stress. E, /( l-v3 is the biaxial Young s modulus of the fihn. The value of the modulus of diamond is 1345 GPa r5]. When silicon was used as substrate and diamond films were deposited between 650 C and 1050 OC, the calculated value of o, of the samples is about 0.195, Gpa, compressive. Therefore, the intrinsic macrostress, crin of the samples can be calculated from the measured total macrostress, o, by Equation (2). The existence of microstress in materials results in lattice distortions of crystals, consequently, the diffraction peaks of the crystals are broadened, The relationship between the half width of the broadened diffraction peaks, B, and the distortion of lattice, Ad/d is described by the equation B, = 4(Ad/d)tan9 (4)

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol where 8 is half of the diffraction angle. The lattice distortion Add can be obtained Ii-om equation (4), if Bd is measured by XRD. However, the broadening of the diffraction peaks is usually caused by small crystal size as well as the existence of microstress. The crystal size, D is related to the half width of the broadened diffraction peaks, B, by the equation B, = khl(dcos~) (5) Assuming both peak profiles are Cauchy type, the total broadened width, B,is the sum of above two terms, that is: B,,= B, +- B, = khl(dcos0) + 4(Ad/d)tanO (6) Multiplying equation (6) by co@ we have: B,,co& = kh/d f 4(Ad./d)sine (7) From Equation (7), we can see that Btitcos9 varies linearly with sine. We can get a linear relationship between B,,cosB and sin0 from the results of XRD. Obviously, the slope of this line is 4(Ad/d). Therefore, the lattice distortion, Ad/d caused by microstress is equal to one quarter of the slope value of the line. 3. RESULTS AND DISCUSSION Fig.1 shows the variation of intrinsic macrostress in the diamond film with deposition temperature. From the figure, we can see that the intrinsic macrostress in the diamond films is tensile and the value has a minimum at a deposition temperature of about 900 C ?3 k 2 c/j 2 Cl B -2 E 2.0 t I I Deposition Temperature ( C) Acetone Concentration (WI %) Fig.1 Variation of Intrinsic Stress with Deposition Temperature Fig.2 Variation of Intrinsic Stress with Acetone Concentration

5 I, Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol The relationship of intrinsic macrostress to acetone concentration is illustrated in Fig.2. It indicates that the tensile stress decreases almost linearly with increase of acetone concentration. In terms of the grain boundary relaxation model [6-s1 tensile macrostress in diamond films increases linearly with the increase of the reciprocal of the grain size and with the decrease of impurity content. There are usually two kinds of impurities in diamond films: hydrogen and nondiamond carbon. The results of Windischmann Igl indicated that the content of hydrogen has no relation to the deposition temperature and increased linearly with the increase of concentration of CH,. Our SEM and Raman spectra results K1ol indicate that the content of non-diamond carbon goes up with increase in acetone concentration and decrease in deposition temperature. The grain size of the diamond film becomes smaller with decrease in deposition temperature, and is less than 1 pm when the deposition temperature is lower than 900 OC, and is bigger than 1 pm when the deposition temperature is higher than 900 C. Therefore, when the deposition temperature is lower than 900 C, the value of the tensile macrostress is mainly influenced by grain size, consequently goes down quickly with the rise of deposition temperature. When the deposition temperature is higher than 900 C, the value of the tensile stress was mainly decided by the content of non-diamond carbon, consequently, the tensile stress increased with the rise of the deposition temperature. Since the grain size has little relation to the concentration of acetone, the variation of intrinsic macorstress with acetone concentration was mainly influenced by the content of impurities in the film, which rises linearly with the increase of acetone concentration. Therefore, the stress decreased linearly with the increase of acetone concentration, B 2.0 t: _-- ---_/ _ ; j - -_-_-_ Bias Voltage (V) Fig. 3 Variation of Intrinsic Stress with Positive Bias Voltage 0.00 I Temperature ( C) Fig.4 Lattice Distortions versus Deposition Temperature Fig. 3 illustrates the effect of positive bias voltage on the intrinsic macrostress in the diamond film. It shows that the tensile stress was reduced from 2.6 to 0.6 GPa by applying 50 V positive voltage to the Si substrate during the growth process of the diamond film. The intrinsic stress increased when bias voltage applied was higher or lower than 50 V.

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol Fig.4 and Fig.5 illustrate the lattice distortion caused by microstress versus the deposition temperature and positive bias voltage, respectively. The lattice distortion goes down with the increase of deposition temperature and the decrease of applied positive bias voltage. The results of Fig. 4 are in agreement with our former results [ 11, that the microstress can be released when diamond films were heat-treated at higher temperature. The microstress resulted mainly from the impurities and defects in the fzm. Our former results show that the content of defects and impurities in diamond fihns goes down with the increase in deposition temperature [41. And when a positive bias voltage was applied, electrons emitted by the hot filament would bombard the Si substrate and the diamond film. The bombardment of electron would cause an increase of defects in the film. Consequently, the microstress in the diamond film decreases with increasing deposition temperature and decreasing applied positive bias voltage. Heat-treatment of the film can accelerate the diffusion of atoms, and decrease the content of defects in the film, therefore, the microstress in thi diamond film can be relaxed after heattreatment at higher temperature. II n Bias Voltage (V) Fig.5 Lattice Distortions versus Positive Bias Voltage Raman Shift, cm- Fig.6 Raman Spectra of Diamond Films Grown at 0 and 50 V Positiv? Bias Voltage Fg.6 shows Raman spectra of the diamond films grown at 0 and 50 V positive bias voltage. It indicates that the content of non-diamond carbon is similar. And the SEM micrographs of the films r41 show that the grain size of the films is similar too. Thus a 50 V applied bias voltage does not change the quality of the diamond film. Why is the macrostress reduced greatly by applying a 50 V bias voltage? We know that the applied positive bias voltage accelerates the formation of P-Sic at the interface, and that the lattice constant of P-Sic is between diamond and silicon. Therefore, the formation of P-Sic at the interface lowers the lattice distortion at the interface, resulting in the relaxation of macrostress at

7 Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol the interface. However, why does the macrostress go up when the applied bias voltage is higher than 50 V? The higher bias voltage may result in the increase of defects in the film and the defects increase the value of residual stress in the film. Fig.5 also shows that the lattice distortion caused by microstress increases with increasing applied positive bias voltage. 4. CONCLUSIONS The relationships of the residual stress to process parameters of diamond films grown by HFCVD were investigated by XRD, and the following can be concluded: (1) The intrinsic macrostress in the diamond films was tensile. The tensile stress decreased almost linearly with increasing acetone concentration. The minimum tensile stress is obtained at a deposition temperature of about 9OOOC and a 50 V bias voltage. (2) By optimizing the process parameters, the tensile intrinsic macrostress in diamond film could be reduced greatly without changing the quality of the diamond film. (3) The microstress in diamond films could be released when the samples were heat-treated. (4) The relationships between stress and growth parameters could be explained according to the microstructure of the diamond films. REFERENCES [l J Guo, H.; Alam, M., Thin SoEid Films, 1992,212,173. [2] Sail, S.R.; Gardiner, D.J.; Bowden M., Appl. Phys. Lett., 1994,65,43. [3] Berry, B.S.; Pritchet, W.C.; Cuomo, J.J., Appl. Phys. Lett., 1990,57,302. [4] Yang, Qiaoqin; Zhao, Nanfang; Xiao, Harming, Carbon (in Chinese), 1998, (I), 21. [5] Field, E., Theproperties of diamond, Academic Press: London, [6] d Heurle, F.M., Thin Solid Films, 1989,171,81. [7] Pulker, HK.; Maser, J., Thin Solid Films, 1979,59,65. [8] Doljak, F.A.; Hoffman, R.W.; Thin Solid Films, 1972,12,71. [9] Windischmann, H.; Epps, G.F., J. Appl. Phys., 1991,69, [lo] Yang, Q.Q.; Chen, B.J.; Wang, X.Q.; Su, Y.C:; Li, S. L., Vaccum, 1995,46, 185. [l l] Yang, Qiaoqin; Zhao, Lihua; Li, Shaolu; Du, Haiqing, Journal of Inorganic Materials (in Chinese), 1995,32,39 1,